EA Guidlines gas_treatment_jan_2003

Draft for Consultation: December 2002

Guidance on Gas TreatmentTechnologies for Landfill Gas Engines

Publishing OrganisationEnvironment Agency, Rio House, Waterside Drive, Aztec West, AlmondsburyBRISTOL, BS32 4UD

Tel: 01454 624400 Fax: 01454 624409Website: www.environment-agency.gov.uk

© Environment Agency 2002

All rights reserved. No part of this document may be produced, stored in a retrieval system, ortransmitted, in any form or by any means, electronic, mechanical, photocopying, recording orotherwise without the prior permission of the Environment Agency.The Environment Agency doesnot accept any liability whatsoever for any loss or damage arising from the interpretation or use ofthe information, or reliance upon the views contained herein.

Dissemination StatusInternal: ConsultationExternal: Consultation

Statement of UseThis guidance is one of a series relating to the management of landfill gas emissions. It providesguidance on determining the need for pre- and post-combustion gas cleanup for landfill gasutilisation plant, identifies current options available for treatment of landfill gas and exhaustemissions, and shows how cost benefit techniques should be used to assess whether and whattechnologies should be employed on an objective basis. It should be read in conjunction with theEnvironment Agency guidance for monitoring landfill gas engine emissions, and IPPC HorizontalGuidance Note H1 on ‘Environmental Assessment and Appraisal of BAT’.

This document constitutes best practice guidance issued by the Environment Agency and isprimarily targeted at its regulatory officers who may use it during their regulatory and enforcementactivities. It may also be of use to operators, contractors, consultants and Local Authoritiesconcerned with landfill gas emissions.

KeywordsLandfill gas, gas utilisation, gas cleanup, gas emissions, reticulation, dry scrubbing, enginemanagement system

ContractorThe majority of this document was produced under R&D Project P1-330 by:Land Quality Management Ltd, SChEME, University of Nottingham, University Park,Nottingham, NG7 2RD. Subcontractors supporting Land Quality Management on this contractwere Berwick Manley Associates Ltd, Diesel Consult, and Landfills+ Inc.

Environment Agency Project BoardThe Environment Agency’s Project Board for R&D Project P1-330 was comprised of:

Chris Deed (Project Manager) North WestJan Gronow Head OfficeRichard Smith Head Office, Centre for Risk & ForecastingAlan Rosevear ThamesPeter Braithwaite Head OfficePeter Stanley Wales

GUIDANCE ON GAS TREATMENT TECHNOLOGIES FOR LANDFILL GAS ENGINES

ENVIRONMENT AGENCY i Draft for Consultation: December 2002

EXECUTIVE SUMMARY

The bulk of emissions from modern landfills are through the gas management system and landfillsurface. The gas management system may include enclosed flares and/or utilisation plant, whichdestroy a significant proportion of the methane and volatile organic compounds within landfill gas,but can produce additional combustion products. The quality of engine emissions is dependent onthe landfill gas supply, design of the generating set and the engine management system.

This guidance document sets out the technical background for methods of gas cleanup and aconsistent approach for the cost benefit analysis to determine the level of cleanup required. Alldecisions should be transparent and made to demonstrate to the Agency that the technologychosen represents best practice. A number of case studies have been considered that are reportedelsewhere.

This guidance sets out an assessment procedure that follows a cost-benefit appraisal approach to deciding whethergas cleanup is necessary or practicable. The assessment procedure has the following steps:

1. define the objective of the assessment and the options for pollution control;2. quantify the emissions from each option;3. quantify the environmental impacts of each option;4. compare options to identify the one with the lowest environmental impact;5. evaluate the costs to implement each option; and6. identify the option which represents the optimum choice or best available technique.

If these steps are followed, the decision procedure for selecting or rejecting a particular cleanup technologyis transparent and an audit trail is apparent.

Gas cleanup is a multi-stage operation that can help reduce environmental emissions and reduce enginemaintenance costs. Gas cleanup involves both financial and environmental costs for the operator. Thisinvolves improving gas supply to conform with engine manufacturers requirements and achieve emissionstandards.

Pre-treatment processes fall into two groups:

• primary pre-treatment processes aimed at dewatering and particulate removal, and common toall landfills with gas collection and combustion facilities; and

• secondary pre-treatment processes aimed at removing a percentage of specific components ofthe supply gas e.g. halogens, sulphur or siloxane compounds.

Combustion treatment technologies are available for:

• in-engine technology for treating the effects of siloxanes and for NOx reduction; and

• post-combustion processes to reduce CO, unburnt hydrocarbons, HCl and HF emissions.

Changes in air quality regulation and tightening of emissions from all processes mean that landfilloperators will need to consider gas cleanup technologies in their PPC permit application.


ENVIRONMENT AGENCY ii Draft for Consultation: December 2002


ENVIRONMENT AGENCY iii Draft for Consultation: December 2002

CONTENTS

Executive SummaryList of Abbreviations

1. INTRODUCTION..................................................................................................... 1

1.1 Target Audience ......................................................................................................... 11.2 Structure of the Guidance ............................................................................................ 11.3 Relationship with other Guidance Documents ................................................................ 31.4 Technical Background................................................................................................. 31.5 Policy Background...................................................................................................... 6

1.5.1 Renewable energy drivers................................................................................. 61.5.2 Regulatory drivers ........................................................................................... 6

2. SUPPLY GAS QUALITY, EMISSIONS STANDARDS AND OPERATIONALREQUIREMENTS .................................................................................................... 9

2.1 Introduction................................................................................................................ 92.2 Engine Manufacturers’ Specifications ........................................................................... 9

2.2.1 Calorific value ................................................................................................ 92.2.2 Sulphur gases .................................................................................................102.2.3 Halogenated compounds..................................................................................102.2.4 Ammonia........................................................................................................112.2.5 Silicon compounds and Siloxanes ....................................................................112.2.6 Dust...............................................................................................................152.2.7 Lubricating oil ...............................................................................................15

2.3 Destruction Efficiencies of Gas Engines .......................................................................162.4 Engine Emissions and their Significance .......................................................................172.5 Crankcase Emissions..................................................................................................19

3. THE DECISION PROCESS FOR ASSESSING THE USE OF CLEANUPTECHNOLOGIES ...................................................................................................21

3.1 Approaches to Cleanup...............................................................................................213.2 Potential for Substitute Natural Gas as a Fuel for Landfill Gas Engines ...........................233.3 The Framework for Gas Cleanup Assessment ..............................................................243.4 Collating Basic Information for the Cost Appraisal ........................................................253.5 How to Perform a Cost Benefit Assessment for Gas Cleanup ........................................28

3.5.1 Step 1. Define the objective of the assessment and the options to be considered ..313.5.2 Step 2. Quantify the emissions from each treatment option ................................313.5.3 Step 3. Quantify the environmental impacts resulting from the emissions ...........333.5.4 Step 4. Compare options and rank in order of best environmental performance ..333.5.5 Step 5. Evaluate the costs to implement each option ..........................................363.5.6 Step 6. Identify the option which represents the cost-effective technique .............37

4. PRIMARY PRETREATMENT TECHNOLOGIES ...................................................41

4.1 Introduction...............................................................................................................414.2 Water/condensate Knockout .......................................................................................414.3 Liquid Water Capture.................................................................................................414.4 Foam Removal ..........................................................................................................424.5 Vapour Reduction......................................................................................................434.6 Contaminated Water Management...............................................................................454.7 Particulate Filtration ...................................................................................................464.8 Dealing with Wastes from Primary Cleanup Processes ..................................................46

5. SECONDARY PRETREATMENT TECHNOLOGIES..............................................47

5.1 Introduction...............................................................................................................47


ENVIRONMENT AGENCY iv Draft for Consultation: December 2002

5.2 Hydrogen Sulphide Pre-treatment................................................................................475.2.1 Hydrogen sulphide dry scrubbing ....................................................................475.2.2 Hydrogen sulphide wet scrubbing ....................................................................48

5.3 Pre-treatment of Halogenated Organic Species..............................................................495.3.1 Membrane separation techniques.....................................................................495.3.2 Pressure swing processes ................................................................................515.3.3 Liquid absorption/solvent scrubbing processes .................................................545.3.4 Water scrubbing processes ..............................................................................575.3.5 Cryogenic processes .......................................................................................58

5.4 Siloxane Pre-treatment ...............................................................................................605.5 Developmental Technologies.......................................................................................62

5.5.1 Hydrogen Sulphide scrubbing..........................................................................625.5.2 Halogenated organic scrubbing .......................................................................625.5.3 Humid Absorption Processes ...........................................................................62

5.6 Dealing with Wastes from Secondary Cleanup Processes...............................................625.6.1 Contaminated carbon dioxide off-gas...............................................................625.6.2 Contaminated aqueous condensates .................................................................635.6.3 Contaminated solids .......................................................................................63

6. ENGINE MANAGEMENT, IN-ENGINE AND EXHAUST GAS TREATMENTPROCESSES............................................................................................................65

6.1 Introduction...............................................................................................................656.2 Gas Engines and their Operation..................................................................................656.3 Engine Management Systems and NOx........................................................................666.4 In-engine Treatments .................................................................................................67

6.4.1 Water injection to reduce NOx.........................................................................676.4.2 Oxygen enrichment .........................................................................................676.4.3 Exhaust gas re-circulation...............................................................................676.4.4 Chemical injection..........................................................................................68

6.5 Exhaust After-treatments ............................................................................................696.5.1 Post combustion thermal oxidation of CO ........................................................696.5.2 Post combustion catalytic oxidation of NOx .....................................................696.5.3 Halide scrubbing ............................................................................................70

7. CONCLUSIONS......................................................................................................71

REFERENCES ..................................................................................................................73

GLOSSARY......................................................................................................................77


ENVIRONMENT AGENCY v Draft for Consultation: December 2002

LIST OF ABBREVIATIONS

ADA anthroquinone disulphonic acid

BAT best available technology

BOD biochemical oxygen demand

C&D construction and demolition

CBA cost benefit analysis

CFC chlorofluorocarbon

CFC-11 trichlorofluoromethane

CFC-12 dichlorodifluoromethane

CH4 methane

CHP combined heat and power

CLAIR™ clean air

CO carbon monoxide

CO2 carbon dioxide

COD chemical oxygen demand

CV calorific value

DCF discounted cash flow analysis

DEA diethanolamine

EAL Environmental Assessment Level

EMS engine management system

H2S hydrogen sulphide

HCl hydrogen chloride /hydrochloric acid (when damp)

HF hydrogen fluoride /hydrofluoric acid (when damp)

HMIP Her Majesty’s Inspectorate of Pollution

IPPC integrated pollution prevention (and) control

LEL lower explosive limit

LLD lower limit of detection (also as LDL, LOD)

MDEA methyldiethanolamine

MEA monoethanolamine

NMVOC non-methane volatile organic compound

NFFO Non-fossil fuel obligation

NOx nitrogen oxides

PCDD polychlorinateddibenzodioxin (dioxins)

PCDF polychlorinateddibenzofuran (furans)

PPC pollution prevention (and) control

ppm parts per million (usually by volume – ppm v/v)

RO Renewables obligation


ENVIRONMENT AGENCY vi Draft for Consultation: December 2002

RPI retail price index

SCR selective catalytic reduction

Selexol™ dimethyl ether polyethylene glycol

SNG synthetic (or substitute) natural gas

SOx sulphur oxides

TBN total base number (oil)

TCE trichloroethylene

TDS total dissolved solids

TRS total reduced sulphur

TEG triethylene glycol

TLV threshold limit value

TRS total reduced sulphur

UEL upper explosive limit

VOC volatile organic compound(s)

v/v by volume (i.e. volume for volume)

WDG waste derived gas

w/w by weight (i.e. weight for weight)


ENVIRONMENT AGENCY 1 Draft for Consultation: December 2002

1.1. INTRODUCTIONINTRODUCTION

1 . 11 . 1 Target AudienceTarget Audience

The guidance document is aimed primarily at Environment Agency (the Agency) staff tasked withthe regulation of landfill gas emissions from utilisation plant. The guidance document should onlybe applied if emissions exceed the current emission threshold values for individual components ofthe exhaust gases, or if emissions could pose a risk to an identified receptor following a site-specific risk assessment (Chapter 2 and Environment Agency, 2002a). The guidance documentshould be used to evaluate whether, on cost versus environmental benefit grounds, the secondarycleanup of landfill gas is:

• necessary; and

• practicable.

This guidance is also intended to be used by the waste management industry, utilisation plantoperators, and other interested parties to inform and help assess the merits, costs and benefits ofgas cleanup to minimise emissions (and/or achieve emission standards), and to maximise enginecomponent life. The staged assessment process should be used by operators if required by Agencyofficers to do so.

1 . 21 . 2 Structure of the GuidanceStructure of the Guidance

This guidance is accessible at various levels, but is intended to be used in the manner shown inFigure 1.1. In order to understand the setting in which the assessment process is carried out, somebackground information may be required. This is provided in Section 1.4 (Technical Background),Section 1.5 (Policy Background), and Chapter 2 which describes how the supply gas quality mayaffect emissions, and how manufacturers specify gas supply standards to help maintain the gasengine in good operational condition between service intervals; standards which may serve as asurrogate indicator of potential problems.

If it is considered that gas treatment may be required, the approach is outlined in Chapter 3. Thisapproach relies heavily on other Agency guidance, in particular Horizontal Guidance Note H1(Environment Agency, 2002d). Figure 1.1 shows which particular sections of Chapter 3 and thesupporting chapters are relevant to each stage of the decision making process.

Chapters 4 – 6 document the technologies which are currently considered to be applicable tolandfill gas engines. Chapter 4 covers primary pre-treatment. These technologies are generally incommon use. If additional gas treatment is indicated at a particular site, the technologies in thisChapter should be considered first, since they are the most straightforward to apply. Chapter 5covers secondary pre-treatment technologies, which are generally more complex and costly.Chapter 6 covers in-engine and post-combustion treatment technologies, which, unlike secondarypre-treatment technologies, tend to be cheaper than primary pre-treatment technologies. There isan element of “end-of-pipe” emissions management about some of these technologies which doesnot address the issue of managing the emission at the point of origin.



Figure 1.1 How to use this guidance in an assessment of cost-effective techniques

No

Identify the option (if any) whichrepresents the cost-effective technique

Evaluate the costs to implement eachoption

Compare options and rank in order ofbest environmental performance

Define the objective of the assessment

Quantify the emissions from eachtreatment option available

Quantify the environmental impacts

Chapter 3.5Chapter 5Secondary PretreatmentTechnologiesChapter 6Engine Management, In-Engine and Exhaust GasTreatment

Chapter 3.4Collating basicinformation for the costappraisalH1 Guidance

Chapter 3.5How to perform a CBAH1 Guidance



Do I need toknow about the supply gas quality or primary

pretreatments, engine operation oremission standards before evaluating

cleanupoptions?

Chapter 2Supply Gas Quality,Emission Standards andOperationalRequirements Chapter4Primary pretreatmenttechnologies

Do I need to knowabout the technical orpolicy background togas utilisation?

Chapter 1.4Technical background togas utilisation

Chapter 1.5Policy backgroundto gas utilisation

Yes Yes

No

Yes



1 . 31 . 3 Relat ionship wi th other Guidance DocumentsRelat ionship wi th other Guidance Documents

This is one of a series of linked documents which support the overarching document Guidance onthe Management of Landfill Gas (Environmnet Agency, 2002c). The full series comprises:

• Guidance for monitoring trace components in landfill gas;• Guidance on landfill gas flaring;• Guidance for monitoring enclosed landfill gas flares;• Guidance for monitoring landfill gas engine emissions;• Guidance for monitoring landfill gas surface emissions; and• Guidance on gas treatment technologies for landfill gas engines.

This document addresses the availability and cost of technologies for:

• pre-combustion landfill gas supply cleanup;

• gas cleanup using in-combustion and engine management system techniques; and

• post-combustion exhaust gas cleanup.

It sets out the formal decision making processes to proceed with when deciding whether gascleanup technologies in any form, including the use of engine management systems is a costeffective solution to managing combustion emissions.

This document will not necessarily provide definitive answers to gas management issues. Gascleanup needs to be assessed on a cost versus environmental and engine maintenance benefit basisand is therefore a site-specific matter. The guidance seeks to inform the user of the availabletechnologies and their application to landfill gas treatment. Due to the low take up of thesetechnologies and the poor demonstrable revenue performance of early cleanup methods, estimatesof capital and operating costs for gas engines are poor. It has been possible to estimate capitalcosts for some technologies, and also calculate the cost per tonne of the pollutant abated, or theapproximate cost of running such a cleanup plant for a year on a 1MWe gas engine. As moretechnologies become routinely available, cost benefit analysis (CBA) should be used to determinethe best solution for a particular problem.

Cleanup costs should be obtained on a site specific basis for a number of suitable technologies anda CBA performed as described in Chapter 3 below and in more detail in the Agency’s HorizontalGuidance Note H1, “Environmental Impact and Assessment of BAT” (Environment Agency,2002d). The CBA will give the costs versus the potential environmental and other benefits ofemissions management and reduction from using such technologies. When the cost per tonne ofpollutant abated (capital and operating costs considered) has been calculated, judgement can bemade on whether the process is cost-effective or not, based on the Agency’s interimrecommendations of cleanup cost thresholds.

1 . 41 . 4 Technical BackgroundTechnical Background

The bulk of atmospheric emissions from modern landfills are through the gas management systemand landfill surface. The gas management system may include enclosed flares and/or utilisationplant, which destroy a great proportion of the methane and volatile organic compounds withinlandfill gas, but can produce additional combustion products.



The quality of the exhaust emissions is dependent on:

• the quality of the landfill gas supply;

• the design of the generating set (dual fuel engines will have different emissions signatures tospark ignition engines); and

• the manner in which the engine management system is set up to manage and control the gas.

The Agency and industry research (Environment Agency, 2002e; Gillett et al, 2002) has providedinformation on both the emissions from gas utilisation plant and the effect of cleanup technologieson landfill gas prior to combustion or in-engine/post-combustion treatments. Gas cleanup has beenhistorically minimal in the UK. In the US and EU, and more recently in the UK, it has been usedsuccessfully to produce synthetic natural gas (SNG) to good effect. This guidance relies on muchwork that has been done internationally, though not necessarily for the purposes of emissionsreduction or maintenance of landfill gas engines.

In the context of this guidance, utilisation is power generation from landfill gas, although manycleanup technologies are more often used in similar biogas-fuelled projects or for reticulation(SNG) projects.

Gas cleanup can be justified through:

• the risk assessment of emissions, with the purpose of managing environmental impact, andwhich needs to be considered as part of the PPC application; and

• the potential reduction of gas engine downtime, balancing the cost of implementation ofcleanup technologies against savings in lost revenue during downtime, and the additionalmaintenance costs when engines fail due to contaminants in the gas supply.

Both objectives can be achieved with the right choice of cleanup technology if the choice of gascleanup technology is made on cost versus environmental/maintenance benefit grounds.

Simple practices may reduce the need or the extent of gas cleanup required. For example, thelocation of the engine(s) should be carefully considered and the exhaust outlet design should bevertically oriented to encourage dissipation and prevent early grounding of exhaust plumes.

Combustion destroys typically more than 99% of the volatile components in landfill gas. Pre-combustion gas cleanup should normally only be considered for landfill gas if any of the followingtypes of contaminants are present in the gas above the gas engine manufacturer’s recommendedmaximum concentration limits:

• hydrogen sulphide and other sulphur gases, because these lead to chemical corrosion of thegas engine (and resultant emissions of acidic gases);

• halogenated organics, because these lead to chemical corrosion of the gas engine, and due totheir subsequent potential contribution to emissions of acid gases HCl and HF, and PCDDs/PCDFs (dioxins and furans); and

• silicon compounds, because of the physical wear they cause to the gas engine.

In most cases, the decision to pretreat will be based on economic rather than environmentalfactors, since seldom will the resulting emissions of SOx, HCl and HF exceed emission standards(see Chapter 2 below). However some sites with atypical supply gas will justify gas cleanup onenvironmental grounds.



In-engine cleanup should be considered if silicon compounds are present in the gas above the gasengine manufacturer’s recommended maximum concentration limit. They should also beconsidered for NOx emissions reduction, if NOx exceeds the Agency’s emission standards (seeChapter 2).

Post-combustion exhaust gas cleanup should be considered if any of the following emissionsexceed the Agency’s emission standards or safe concentrations determined by risk assessment (seeChapter 2):

• carbon monoxide;

• methane and non-methane VOCs;

• hydrogen chloride;

• hydrogen fluoride; and

• sulphur oxides.

Engine management systems and post combustion gas cleanup systems are the only effectiveways to manage oxides of nitrogen (NOx) and carbon monoxide (CO) emissions, because thesegases are actually formed during the combustion process.

Gas engine management and emissions reduction are closely linked. Practices which may beemployed for the purposes of engine efficiency may reduce (or increase) specific emissions, andconsideration of the following inter-relationships is important:

(1) technologies or approaches for improving gas engine performance and reducingmaintenance costs; and

(2) technologies or approaches simply for achieving emissions reduction.

Established practices which already have a role in gas cleanup, include:

• aftercooling and pre-chilling;

• cyclone separation and other dewatering technologies;

• particle filtration; and

• gas engine modifications and other engine management techniques (both in engine and aftercombustion) for NOx, CO and particulate emissions.

Emerging and more specialist technologies include:

• wet or dry hydrogen sulphide scrubbing;

• activated charcoal/carbon/zeolites;

• liquid and/or oil absorption;

• cryogenic separation;

• solvent extraction;



• membrane separation for carbon dioxide, oxygen, and other gas scrubbing/ separationtechniques (which while predominantly used in the production of SNG may have applicationfor generating sets);

• thermal oxidation;

• catalytic conversion; and

• in-engine treatments.

Most of the more specialist techniques listed above have been used in combination on variouspilot/demonstration projects, but few have been applied regularly to landfill gas engines.

1 . 51 . 5 Pol icy BackgroundPol icy Background

1 . 5 . 11 . 5 . 1 R e n e w a b l e e n e r g y d r i v e r sR e n e w a b l e e n e r g y d r i v e r s

The key economic drivers for the continued increase in numbers of landfill gas utilisation schemesin the past decade have been twofold:

• the Non Fossil Fuel Obligation (NFFO), which drove the increase in renewable electricitygeneration capacity during the 1990s and continues to be significant due to the large numberof contracted projects still to be built; and

• the Renewables Obligation, which was introduced in April 2002, which will be a significanteconomic stimulus to utilise any landfill gas resources not already contracted under the NFFO.

The utilisation of landfill gas increased dramatically during the 1990s as a result of the NFFO. Asof September 2001, 400 MW of the 700MW capacity awarded had been constructed.

There will be no further NFFO orders made, as the Renewable Obligation (RO) has supersededthe NFFO as the driver for new renewable energy in the UK. The RO places an obligation onelectricity suppliers to source a certain percentage of their output from renewable sources. Theobligation for 2002 is set at 3% of total sales of electricity rising to 4.3% in 2003, 4.9% in 2004,increasing annually to 10.4% in 2010, and maintained at this level until 2027.

There is a shortfall in available power generated by renewable sources, and so this is a particularlypowerful economic incentive for landfill gas utilisation for electricity generation under the RO. Thepotential for higher prices has lead to increased interest in smaller landfill gas projects or projectswhich may be shorter lived and which would not have been economic under the NFFO system.

1 . 5 . 21 . 5 . 2 R e g u l a t o r y d r i v e r sR e g u l a t o r y d r i v e r s

The management of landfill gas at permitted or licensed landfills comes under two separate piecesof European legislation:

• IPPC Directive; and

• Landfill Directive.

The Integrated Pollution Prevention and Control Directive (Council of the European Union, 1996)requires that preventative measures are taken to manage the gas, through flaring and utilisation.This has been implemented in England and Wales through the Pollution Prevention and Control(England and Wales) Regulations 2000 and in Scotland through the Pollution Prevention and



Control (Scotland) Regulations 2000 SSI No 323 (the 2000 Regulations) under the PPC Act 1999.A separate system will be introduced to apply to the PPC Directive in Northern Ireland.

Landfills in England and Wales have been regulated under the Waste Management LicensingRegulations 1994. These Regulations still apply to landfills no longer accepting waste but whichcontinue to hold a waste management licence. All other landfills will be regulated under theLandfill (England and Wales) Regulations (the 2002 Regulations) which implements the LandfillDirective (council of the European Union, 1999).

Integrated Pollution Prevention and Control (IPPC) is a regulatory system that employs anintegrated approach to control the environmental impacts of certain industrial activities. One of themain features of IPPC is the requirement to use Best Available Technology (BAT). Whether alandfill gas utilisation plant is subject to BAT depends on the arrangement between the landfill siteoperator and the gas utilisation company.

Operators utilising landfill gas will have landfill site licences or permits that include utilisation plant.Utilisation under the Landfill Directive generally constitutes best practice. The application ofsecondary gas cleanup will constitute best practice only if it is required to meet current emission orair quality standards or to reduce the risk to specific receptors.

Utilisation companies operating on a “gas to flange” type agreement may have a PPC combustionpermit which will require BAT. The essence of BAT is that the selection of techniques to protectthe environment should achieve an appropriate balance between realising environmental benefitsand costs incurred.

These two conditions are subtly different. However, both should involve a comparable assessmentprocess, and one licensing arrangement should not result in a utilisation plant being required toemploy clean up technology whereas the other licensing arrangement would not require it.

It is clear that the drivers for renewable energy have to be acknowledged when consideringemissions limits and the need for gas cleanup to meet emissions limits. Many of the early NFFOschemes had higher prices paid per unit of electricity sold but the capital costs were comparativelymuch higher. None of the schemes commissioned to date have considered gas cleanup whenbidding for a utilisation contract. This guidance document should therefore be used to determinenot only whether a technology could be of benefit, but whether it is cost effective to implement.Whether the cost-effectiveness constitutes BAT only applies in the case of utilisation plant with aPPC permit provided under the 2000 Regulations.



2.2. SUPPLY GAS QUALITSUPPLY GAS QUALIT Y, EMISSIONS STANDARDS ANDY, EMISSIONS STANDARDS ANDOPERATIONAL REQUIREMENTSOPERATIONAL REQUIREMENTS

2 . 12 . 1 Introduct ionIntroduct ion

The calorific value of landfill gas is predominantly determined by the methane/carbon dioxideratio. Landfill gas has been found to contain as many as 577 trace components, includinghalogenated hydrocarbons, higher alkanes (≥ C2) and aromatic hydrocarbons (EnvironmentAgency, 2002f). Normally trace components constitute only about 1% by volume of landfill gas.Most higher hydrocarbons will burn, although if their calorific value is less than methane, they willlower the calorific value of the gas. Some of the aromatics (e.g. benzene) and chlorinatedhydrocarbons (e.g. chloroethene) give rise to health concerns, others are highly odorous (e.g.terpenes, esters and thiols) and some can damage gas utilisation plant (e.g. organohalogens,sulphur species and siloxanes).

Therefore, the overall trace component composition of landfill gas has important health andenvironmental implications and impacts on the gas engine performance. The enginemanufacturers’ specifications represent a gas quality standard at which supply gas cleanup mightneed to be considered. The Agency’s guidance on monitoring for landfill gas engines(Environment Agency, 2002a) provides factors for consideration of exhaust gas treatment or in-engine treatment (and in some cases, supply gas cleanup for some acid gas emissions).

2 . 22 . 2 Engine Manufacturers’ Speci f icat ionsEngine Manufacturers’ Speci f icat ions

When considering possible treatments for removal of trace components from landfill gas, it isimportant to take into account the requirements placed on supply gas by the engine manufacturers.Table 2.1 provides a comprehensive summary of current gas quality specifications from majorsuppliers of lean burn engines now being used in the EU and US. This includes two USmanufacturers (Caterpillar and Waukesha), an Austrian manufacturer (Jenbacher), and a Germanmanufacturer (Deutz).

These produce a useful starting point for site-specific calculations regarding gas quality and toassess the need for pre-combustion treatment. As engine manufacturers link these specificationsfor gas quality to their warranty agreements, it is important that the inlet gas is periodically testedusing a method and schedule approved by the manufacturer. The original measurement units, asprovided by the manufacturer, have been converted to SI units. It should also be noted that thesespecifications may vary with engine type and may be revised from time to time.

2 . 2 . 12 . 2 . 1 C a l o r i f i c v a l u eC a l o r i f i c v a l u e

The calorific, or heat value of the fuel is determined predominantly by the percentage of methanepresent. Typically this is 35% to 55% v/v for landfill gas in the UK. Pure methane, which has aheat value 9.97 kWe Nm-3, is the only significant hydrocarbon constituent in landfill gas that isconverted into mechanical/electrical energy by the engine combustion process. The lower themethane content, the greater the volume of gas that must pass through the engine to achieve thesame power output. This means that potentially more aggressive gas constituents enter the enginefor a lower methane value. It is for this reason that manufacturers’ limits for aggressive gasconstituents are defined “per 100% methane”.

Engine air to fuel ratio controllers have the ability to automatically adjust this ratio as the methanecontent of the supply gas changes, although system modification may be necessary for significantvariation outside the operating range of 45% ± 15% CH4 v/v.

The calorific value gives no indication of the aggressiveness of the supply gas or likely emissions.Bulking of supply gas typically occurs with low calorific value gas. The higher inlet pressure of thegas will generally result in increased emissions of methane, NMVOCs and other products of



incomplete combustion. In order to control and minimise this effect, continuous monitoring offlow rate and methane is necessary (Environment Agency 2002a).

2 . 2 . 22 . 2 . 2 S u l p h u r g a s e sS u l p h u r g a s e s

Landfill gas contains a variety of (often odorous) sulphur compounds. These include sulphides/disulphides (such as hydrogen sulphide, dimethyl sulphide, dimethyl disulphide, diethyl disulphideand carbon disulphide) and thiols (such as methanethiol (methyl mercaptan), ethanethiol andpropanethiol). Sulphur compounds are corrosive in the presence of free water or the moisture thatis found within the engine oil and/or landfill gas. These can lead to wear on engine piston rings andcylinder linings. Gas recirculation systems may also increase the availability of moisture within theengine system. This also impacts on oil quality leading to the need for increased frequency of oilchanges. For these reasons individual engine manufacturers recommend limits for the inlet landfillgas quality for total sulphur compounds (see Table 2.1) rather than individual compounds.

The primary mechanism for production of H2S in landfills is the reduction of sulphate underanaerobic conditions by sulphate-reducing microorganisms. Therefore, landfills expected to havehigher concentrations of H2S within landfill gas include:

• unlined landfills in sulphate-rich geological materials such as gypsum (CaSO4.2H2O) quarriesor gypsiferous soils;

• landfills where large quantities of gypsum plasterboard or sulphate-enriched sludges (from e.g.wastewater treatment or flue gas desulphurisation) have been buried;

• landfills where sulphate-rich soils have been used as intermediate cover materials; and

• landfills where construction and demolition (C&D) debris containing substantial quantities ofgypsum wallboard has been ground down and re-cycled as daily or intermediate cover.

Typically, landfill gas contains < 100 ppm v/v H2S, but on landfills where the sulphate loading ishigh, values for H2S can be as high as several thousand ppm v/v. Since combustion of H2S willachieve typically 99% destruction in the gas engine, on most landfill sites, it is likely that treatmentfor H2S in the supply gas will only need to be considered if non-routine maintenance periods arefrequent, and a cost saving can be envisaged. In the absence of an emissions limit value for SOx,emissions of SOx from combustion of H2S are likely to be below any local risk threshold but localair quality issues must be considered on a site-specific basis.

2 . 2 . 32 . 2 . 3 H a l o g e n a t e d c o m p o u n d sH a l o g e n a t e d c o m p o u n d s

Halogenated compounds containing chlorine, bromine and fluorine (e.g. carbon tetrachloride,chlorobenzene, chloroform and trifluoromethane) are broken down during the combustion processand can form the acid gases HCl and HF in the presence of moisture. These are responsible forcorrosion of metal piping and engine components. Combustion of halogenated compounds in thepresence of hydrocarbons within the landfill gas can also lead to the subsequent formation ofcompounds such as PCDDs and PCDFs (dioxins and furans), particularly as the combusted gasescool below 400°C.

Absorbtion of chlorine compounds into the engine oil is usually used to determine the frequency ofoil changes in landfill gas engines. The major engine manufacturers recommend limits for the inletlandfill gas quality for total chlorine and fluorine content as specified in Table 2.1.

Most halogenated species in landfill gas are the result of direct volatilisation from solid wastecomponents and their presence depends on vapour pressure relationships under landfill conditions.Temperatures within landfills are typically between 25 and 40 °C, with total pressure slightlyhigher than atmospheric pressure.



The most common trace components within landfill gas mirror the gaseous aromatic andchlorinated compounds produced in the largest quantities by the chemical industry for use inconsumer products. A notable exception is chloroethene (vinyl chloride). This compound andcertain di-chlorinated species can be produced in situ within landfills by anaerobic microorganisms through reductive dechlorination of higher chlorinated species such as trichloroethylene(TCE) (Molten et al., 1987).

The most common fluorinated species are the chlorofluorocarbons (CFCs), which were widelyused as refrigerants, propellants, and in insulating foams until their production was greatly reducedafter recognition of their role in stratospheric ozone depletion (Rowland and Molina, 1974; WorldMeteorological Organisation, 1998). The most abundant CFCs in landfill gas are CFC-12(dichlorodifluoromethane) and CFC-11 (trichlorofluoromethane). These appear to persist at lowconcentrations in landfills due to their slow volatilisation from old waste.

Landfill gas quality appears to be improving, in general, with the withdrawal of certain substancesfrom widespread use (e.g. HCFCs), and the future banning of liquids (e.g. solvents) to landfill.These practices are reducing the chlorine and fluorine content of landfill gas and the emissions arelikely to be concomitantly reduced in HCl and HF, and PCDDs/PCDFs (dioxins and furans).However, sites which had previously accepted large quantities of these wastes may continue toexhibit high total chlorine and fluorine concentrations in landfill gas and similarly exhaust emissionsof HCl, HF and PCDDs/PCDFs may be above the norm.

While a third of UK landfills have aggressive gas characteristics requiring high Total Base Number(TBN) lubricating oils, only a small percentage of these may produce HCl and HF in exhaustemissions at concentrations which may require treatment. These emissions might need to beaddressed at landfills where industrial waste has been accepted and where concentrations in theexhaust are shown to be potentially harmful as determined by a site specific riskassessment/emission standard.

2 . 2 . 42 . 2 . 4 A m m o n i aA m m o n i a

Ammonia is a problem for digester gas engines, and it is strictly limited by manufacturers onengines burning digester gas. It may also be found in landfill gas and the manufacturers may applysimilar limits. The combustion of ammonia leads to the formation of nitrogen oxide (NO) whichcan react to form NOx in the atmosphere.

2 . 2 . 52 . 2 . 5 S i l i c o n c o m p o u n d s a n d S i l o x a n e sS i l i c o n c o m p o u n d s a n d S i l o x a n e s

Discarded consumer products including cosmetics in the landfill tend to be the source of silicon inthe supply gas. Many consumer products (hair care, skin care, underarm deodorants) andcommercial lubricants contain silicones, a large group of related organosilicon polymers. The termsiloxane refers to a subgroup of the silicones containing Si-O bonds with organic radicals bondedto the Si; the organic radicals can include methyl, ethyl, and other organic functional groups.Siloxanes are present in landfills through both: a) the direct disposal of containers with smallamounts of remaining product; and b) through land filling of wastewater treatment sludges, inwhich siloxanes are retained through the process steps.

Organosiloxanes are semi-volatile organosilicon compounds which, while not an aggressive gascomponent in terms of emissions, can be converted to solid inorganic siliceous deposits within theengine combustion chamber. They form a coating or lacquer on all surfaces contacted by thelubricating oil and can alter the oil retaining surface finish of cylinder liners.



Table 2.1 Supply gas specifications from some commonly used landfill gas engine manufacturers1

Constituent Jenbacher Deutz Caterpillar Waukesha

Calorific value andvariability

Max variation:< 0.5% CH4 (v/v) per 30 seconds ≥ 14.4 MJ Nm-3 15.7 - 23.6 MJ Nm -3

(recommended range)>15.73 MJ Nm -3

Total S content 2000 mg Nm -3 CH4 (with catalyst)1150 mg Nm -3 CH4 (without catalyst)(total S as H2S)

< 2200 mg Nm -3 CH4 < 2140 mg H2S Nm -3 CH4

(total S as H2S)2

< 715 mg Nm -3 CH4

(total S bearing compounds)

H2S content - < 0.15% v/v - -

Ammonia < 55 mg Nm -3 CH4 (pertains mostly to anaerobicdigester gas – combined specifications for all biogassystems)

- < 105 mg NH3 Nm -3

(pertains mostly to anaerobicdigester gas – combinedspecifications for all biogassystems) 2

-

Total Cl content see Sum of Cl and F < 100 mg Nm -3 CH4 See Sum of Cl and F see Sum of Cl and F

Total F content see Sum of Cl and F < 50 mg Nm -3 CH4 See Sum of Cl and F see Sum of Cl and F

Sum of Cl and F Without catalyst 3

< 100 mg Nm -3 CH4 (weighted as 1 part Cl and 2parts F) without warrantyrestriction; 100–400 mg Nm -3 CH4 with warrantyrestriction; > 400 mg Nm -3 CH4 no warranty at allwith catalyst : 0 mg Nm -3 CH4

< 100 mg Nm -3 CH4 < 713 mg Cl Nm -3 CH4

(total halide compounds as Cl)2≥ 300 mg Nm -3 CH4

(total organic halides as Cl)4

Si old standardwithout catalyst2: < 20 mg Nm -3 CH4 withoutwarranty restriction; ( > 20 mg Nm -3 CH4 withrestriction)new standardwithout catalyst: see below 5

with catalyst : old or new standard 0 mg Nm -3 CH4

< 10 mg/Nm -3 CH4 < 21 mg Nm -3 CH4 2 < 50 mg Nm -3 CH4

total siloxanes(models with prechamberfuel system only)4

Dust < 50 mg Nm -3 CH4

(particles < 3µm)< 10 mg/Nm -3 CH4 < 30 mg Nm -3 CH4

(particles < 1 µm)2

removal of particles > 0.3 µm

Oil/ Residual Oil < 5 mg Nm -3 CH4 < 400 mg/Nm -3 CH4

(oil vapors > C5)< 45 mg Nm -3 CH4 (oil) < 2% v/v “liquid fuel hydro-carbons” at

coldest inlet temperature

Miscellaneous Project specific limits:“hydrocarbon solvent vapors”

no glycol

Relative humidity/Moisture

< 80%with zero condensate

< 60-80% < 80%at minimum fuel temperature

zero liquid water; recommend chillinggas to 4 °C followed by coalescing filterand then reheating to 29-35 °C; dewpoint should be at least 11 °C belowtemperature of inlet gas



Pressure at inlet Turbocharged engines: 80-200 mbarPre-combustion Chamber :Models 612-616: 2500-4000 mbarModel 620: 3000-4000 mbar

up to 2000 mbar - -

gas pressurefluctuations

< 10 mbar/sec < 10% of set value at a frequency of< 10 per hour

- -

Inlet gastemperature

< 40 °C 10-50 °C - > -29 °C and < 60°C

CH4 (% v/v) - ≥ 40% Recommended ratio of CH4:CO2 is1.1-1.2

-

Methane number - Approx. 140 for landfill gas - -

H2 (% v/v) - - - < 12%Notes:1 Dates of information as follows: Jenbacher, 2000 (TI 1000-0300); Deutz, 1999; Caterpillar, 1997; Waukesha, 2000.2 Manufacturers stated specification in mg MJ-1, converted to Nm3 CH4 assuming Calorific Value CH4 is 37.5 MJ Nm-3.3 Other conditions. A single exceedance of 30% above 100 mg Nm -3 CH4 is permissible out of four analyses per year. Limiting values for used oil and sump capacity must be

observed (see Jenbacher Technical Instruction No. 1000 – 0099).4 Manufacturers stated specification in µg l-1 landfill gas, converted to Nm3 CH4 assuming 50% CH4 (v/v).5 Relative limiting value of < 0.02 according to the following calculation (without catalyst):

Relative limiting value = (mg kg-1 Si in engine oil) X (total oil quantity in liters) (engine power in kW) X (oil service time in hours)



Siloxanes may enter the engine as insoluble matter in the gas fuel, forming a white deposit in thecombustion chamber, or be produced in the combustion chamber itself, or form a golden lacqueron components outside of the combustion chamber. This can be especially evident on the piston-ring wiped surface of the cylinder liner. The lacquer has a tendency to “fill” the oil retaininghoning pattern but rarely builds to the extent of requiring attention prior to routine overhaul (seePlate 2.1).

Plate 2.1. Golden lacquer of siloxane build up evident on cylinder liner

Silicon, silicon dioxide, and siloxanes all behave in different ways. Two “high silicon sites” usingan identical engine can result in widely varying effects and “trial and error” solutions are thecurrent norm. At the combustion conditions within landfill gas fuelled engines, organic siliconcompounds contained in the landfill gas may be deposited on the cylinder head as solid inorganicsilicon compounds. This deposited material is white to light grey, somewhat laminar, generallyopaque, and may exhibit a partial to poor crystalline structure. There have been few analyses ofthese deposits in the open literature and the existing data indicates that crystalline SiO2 is present,as well as other metals in solid forms (Niemann et al., 1997; Hagmann et al, 1999; Niemann,2001, personal communication).

These deposits severely reduce engine life, since the engine needs to be stripped down and thesolids manually scraped off the piston, cylinder head and valves. Through the combustionprocess, some silicon compounds are also partitioned to the engine oil, which needs to be changedmore frequently at sites with high siloxane levels in the inlet gas fuel. Engine manufacturers thusrecommend direct monitoring of silicon build-up in the engine oil. Judging by the increasing use ofthese compounds in consumer and commercial products, problems with volatile siloxanes inlandfill gas-fuelled engines are likely to increase.

At the present time, there is no standard method for analysis of volatile siloxanes in a gaseousmatrix and at least ten or more methods are being used at the present time (e.g. Aramata andSaitoh, 1997; Grumping et al., 1998; Hone and Fry, 1994; Huppman et al., 1996; Kala et al.,1997; Schweigkofler and Niessner, 1999; Stoddart et al., 1999; Varaprath and Lehmann, 1997;Wachholz et al., 1995). There is no general consensus within the landfill gas industry regardingwhich siloxane method to use, and there has been no rigorous comparison of several methods fora common set of samples.



Observations on individual well samples and composite landfill gas samples vary between <1 and>100 ppm v/v total organic Si, based on a gas chromatography/atomic emission detection method(GC/AED). For some applications, especially evaluation of potential treatment methods,determination of speciated siloxanes may be desirable using a combined GC/AED-MS (massspectrometry) method (e.g. Schweigkofler and Niessner, 1999).

Siloxanes do not directly cause problems with gas engine exhaust emissions, although theincreased wear may show itself as an increase in SOx emissions as lubricating oil is burnt.Typically, this is unlikely to exceed any risk-based criterion for emissions management and thedecision to implement gas cleanup for siloxane management purposes is entirely economic.

2 . 2 . 62 . 2 . 6 D u s tD u s t

Dust can be drawn into engines either in the landfill gas itself or in the combustion air.

Particulate filters or cyclones (see Chapter 4) are employed universally at the present time forremoval of particulates from the supply gas. However, because of the dusty external environment,attention is also paid to the combustion air drawn into the engine container or building andespecially to the air drawn into the engine. Two stages of inlet air filtration are therefore involved:

• on the engine enclosure inlet at filtration levels determined to prevent an unacceptable, visualbuild up of dust on engine and ancillary plant; and

• at the engine inlet itself. This filtration is of particular importance as abrasive silica is a majorculprit of premature component wear (down to 5 micron on the cell inlet filter and down to 2micron on the secondary engine mounted filtration).

Should the location have ‘desert’ conditions or should industrial processes be within the vicinity ofthe generating plant (such as cement production) ‘cyclone’ or ‘oil-wetted’ filters can be employed.

All utilisation plant should have dust filtration equipment installed if particulates in the supply gasare identified as a particular problem. Further information is given in Chapter 4 of this guidance.

2 . 2 . 72 . 2 . 7 L u b r i c a t i n g o i lL u b r i c a t i n g o i l

The most frequently encountered aggressive constituents of landfill gas, in the UK, are siloxanesand organohalogen compounds that introduce acids into the lubricating oil. It is known from thevolume of high total base number (TBN) oil formulations used on landfill gas engines thatapproximately one third of UK landfill gas users suffer from aggressive concentrations oforganohalogens (Hussein Younis, Exxon Mobil, personal communication 2002).

The acid forming chloride, fluoride and sulphur compounds contaminate the lubricating oil mostlyby bypassing the piston rings (blow-by) and to a lesser extent via the air and exhaust valve guides.The effect of these acids may be reduced by keeping the engine operating temperatures of jacketcooling water and associated lubricating oil temperatures high, to avoid dew points. Higher oiltemperature does however reduce crankshaft bearing oil film thickness and an optimum balancemust be achieved.

Corrosion prevention is achieved by both keeping the oil alkaline and by use of corrosion resistantcomponents, especially at the crankshaft, camshaft and other bearings. Aluminium-tin bearings arean example of bearings that may be used to replace ‘yellow metal’ bearings such as copper orphosphor bronze.

Alkalinity is retained by use of lubricating oil additives, additives that have non-combustibleproperties and hence produce more ash. Some ash can be beneficial as it serves as a lubricant forvalve seats. However, if there is too much and maintenance intervals become more frequent, in-



cylinder temperature sensors become less effective, with premature detonation owing to build-upof deposits.

An optimum balance has therefore to be achieved between very high alkalinity (high TBN) oil andthe frequency of oil replacement. Longer periods between oil changes may be achieved with largerengine sump capacities. An engine approaching overhaul will allow greater absorption owing toincreased blow-by. Typical oil replacement frequencies fall in the range 750 to 850 hours. Shuttingdown the engine to undertake oil replacement usually coincides with spark plug replacement.

2 . 32 . 3 Destruct ion Eff ic iencies of Gas EnginesDestruct ion Eff ic iencies of Gas Engines

The destruction of some components of landfill gas in the combustion chamber can be beneficialto the environment (particularly if the alternative is uncontrolled surface emission). However,because of the short residence time in the gas engine, no trace gas component can be destroyedwith 100% efficiency, and indeed other components are produced (particularly HCl, HF and SOx)to maintain the mass balance of chlorine, fluorine, and sulphur compounds in the landfill gas.

Table 2.2 provides a summary of the typical destruction efficiencies observed during themonitoring of a number of landfill gas engines (Gillett et al., 2002). Some of the minima presentedare only estimates, and the real destruction efficiency is greater than that stated due to reaching theanalytical detection limit for the compound within the engine exhaust. In many cases, therefore,the destruction efficiencies are expected to be much higher than the minima quoted.

Methane destruction (to carbon dioxide) is typically 96% to 99.6%. Longer chain alkanes aretypically destroyed with between 92% and > 99.9% efficiency, with some exceptions. Gillett et al.(2002) reported that butane was destroyed by only 70%, and some light alkanes appear to beformed. Simple substituted alkanes (alcohols, aldehydes and ketones) also showed a highdestruction (up to 99.9%), but again some destruction efficiencies appear low. The combustionchamber and exhaust system of a gas engine is a highly reactive chemical environment, and it ispossible that some simple compounds are preferentially formed from the destruction of othercomplex organic species.

The unburnt methane and other hydrocarbons leaving the exhaust is a relatively small fraction ofthe fuel, and the amount of methane “slippage” is a feature of principal engine design. Somemethane escapes the combustion chamber before it is “closed”, and some remains aftercombustion to be discharged on the non combustion stroke.

Aromatic compounds are destroyed at between 92% and 99.9% efficiency. Terpenes (responsiblefor some odour events on landfills) are destroyed at > 99.9% efficiency. Sulphur compounds,responsible for most odour complaints, are destroyed at between 8.7% and 96.6% efficiency.Hydrogen sulphide, the most common sulphur compound, has been found to undergo 70.6 – 96.6% destruction in a gas engine (an observation counter to claims that the gas is flammable and istherefore completely destroyed).

Halogenated compounds, potentially some of the most toxic compounds in landfill gas, aredestroyed at between 70% and 99.7%. However, existing research indicates some of theanomalous calculated destruction efficiencies are a result of very small amounts of thesecompounds being present



Table 2.2 Typical range of observed landfill gas engine destruction efficiencies forfunctional groups (after Gillett et al., 2002)

Functional group Minimum (%) Maximum (%)

Methane 96.0 99.6Alkanes 70.2 >99.9Alkenes 50.1 >99.6Alcohols 84.1 >99.8Aldehydes >42.4 95.9Ketones >87.4 99.9Aromatic hydrocarbons 92.0 >99.9Terpenes - >99.9Halogenated hydrocarbons >70.1 >99.7Sulphur compounds > 8.7 >96.6

These observed destruction efficiencies measured on UK plant tend to confirm that gas enginesare capable of destroying the trace components to high degrees of efficiency. While theobservations show actual performance, the theory states that the higher the peak combustiontemperature, the greater the efficiency of destruction of VOCs etc. However, this brings otherfactors into account.

The higher the thermal efficiency of an internal combustion engine, the lower the emission ofunburnt hydrocarbons, but the higher the thermal efficiency, the higher the peak combustiontemperature which is a direct function of NOx production.

NOx emissions can be reduced by principal engine design that effectively reduces thermalefficiency, by humidification of the inlet air/gas mixture prior to actual combustion thus loweringthe peak combustion temperature, or by constantly adjusting engine operationalparameters/thermal efficiency within a relatively small band. The latter is controlled by the enginemanagement system (EMS).

Most modern engines are designed and adjusted by the EMS, to retain design parameters, andmay be set, for example, to hold a NOx emission of 500mg Nm-3. Some engine types becomemore expensive to operate at this setting owing to a greater load being placed on the ignitionsystem, but the situation is manageable. Different engine types, having varying amounts ofadjustment, produce different levels of unburnt hydrocarbons at a given NOx setting.

2 . 42 . 4 Engine Emiss ions and their Signi f icanceEngine Emiss ions and their Signi f icance

The Agency has published indicative emission standards for the major exhaust gas emissions fromlandfill gas engines, and has provided guidance on which trace components should be assessed byrisk assessment (Environment Agency, 2002a). The emission standards are given in Table 2.3below (Environment Agency, 2002a).



Table 2.3 Emission standards for Landfill Gas Engines (after Environment Agency, 2002a).

Component in exhaust Emission standard for Spark Ignition Engines, mg Nm-3

Commissioned between01/01/98 – 01/11/04

Commissioned after 01/11/04

NOx 650 500

CO 1500 1400

Total VOCs(including CH4)

1750 1000

NMVOCs 150 75

Other component Determined by site specificrisk assessment

Determined by site specific riskassessment

Particulates,PCDDs/PCDFs, heavymetals, HCl, HF, H2S

Determined by site specificrisk assessment

Determined by site specific riskassessment

Note: these are minimum standards based on normal operating conditions and site-specific risk assessments may requirea stricter emissions standard to be applied. Risk assessment must be carried out for plant commissioned before 01/01/98.

If the concentration in the exhaust gas exceeds the indicated emission standard for any of thenamed components, then some action is needed. In the first instance this could be attention to theengine management system (EMS) or further emissions monitoring. If this is not considered to beappropriate action, then a more formal process of evaluation of why the emissions gave exceededthe emission standard should be made, and a decision on the need for gas cleanup should bemade.

In some locations there may be sensitive receptors close to or influenced by the exhaust stack. Ifthe concentration of particulates, PCDDs/PCDFs, heavy metals, HCl, HF or H2S in the emissionsis found, by site specific risk assessment, to be higher than the agreed tolerable concentration atthe site boundary, then again, a decision on the need for gas cleanup should be made. Theapproach for this site specific risk assessment is documented in the Agency’s Guidance on theManagement of Landfill Gas (Environment Agency, 2002c). In order to perform a site specificrisk assessment, the operator must first develop a conceptual model of the site, and then apply atiered risk assessment process, which will include dispersion modelling.

Development of the conceptual site model involves defining the nature of the landfill, the gasutilisation plant and the baseline environmental conditions, as well as identifying the source termreleases, pathways and receptors for the plant emissions, and the processes that are likely to occuralong each of the source-pathway-receptor linkages. In this situation, the pathway is most likely tobe atmospheric dispersion of the exhaust plume.

The hazard identification and risk screening stage should then consider the sensitivity of thereceptors, and there should be an initial selection of the appropriate environmental benchmark foreach receptor (e.g. Environmental Assessment Levels or EALs, for human health, or the nationalair quality objectives). Long term and short term EALs are provided in Horizontal Guidance NoteH1 (Environment Agency, 2002d).

An atmospheric dispersion model of the fate of the exhaust plume should form part of the PPCapplication, and this information will also be used in the risk assessment. The procedures whichthe entire CBA process should follow are given in Chapter 3 of this document..



2 . 52 . 5 Crankcase Emiss ionsCrankcase Emiss ions

The engine exhaust is not the only source of atmospheric emissions from gas engines. Combustionproducts that pass the piston rings (blow-by) and, to a lesser extent, escape past valve guideclearances, cause a positive pressure in the engine crankcase and contaminate the lubricating oil.

Historically, a crankcase vacuum of around 1 inch water gauge was used to counter this pressureand to assist prevention of lubricating oil leaks. However, extraction serves a further purpose forlandfill gas engines; namely a reduction in the rate of contamination of the lubricating oil andhence a direct saving in oil costs.

Exhaust from the extractor fan has a visual impact in that it is exhibited in the form of low volumeflow-rate smoke. This exhaust, or crankcase fumes are often passed through a length of pipeworkthat promotes condensation of the oil with the remaining vapour passed through a coalescer/filter.Simply exhausting below water has also been employed. Increasing the volume of flow topositively purge the crankcase may be considered to be a form of in-engine clean-up.

Gillett et al (2002) has quantified untreated crankcase exhaust as having high concentrations ofaggressive gases but at very low mass flow. This volume can be up to 30% of the total massemission rates of unburnt hydrocarbons and SOx from the engine and treatment is considered bestpractice. The direct release of crankcase exhaust emissions is no longer acceptable.

Options for management of this emission source are:

• re-circulation of the crankcase fumes into the combustion chamber inlet. This affectscomponent life and the emissions are combined and diluted in the exhaust;

• re-circulation by injection after combustion. This promotes longevity of engine componentsand the emissions are combined and diluted in the exhaust; and

• installation of coalescer and filter. This promotes component longevity but produces anadditional, low volume waste stream.

The cheapest option is to re-circulate; an option that is adopted by most engine makers.

A coalescer and filter could be fitted at a relatively low cost of between £1,500 and £3,000 capitalcost subject to flow rate and degree of reduction, plus the costs of disposal of the waste stream (ifa highly acidic supply gas is used).



3.3. THE DECISION PROCESS FOR ASSESSING THE USE OFTHE DECISION PROCESS FOR ASSESSING THE USE OFCLEANUP TECHNOLOGIESCLEANUP TECHNOLOGIES

3 . 13 . 1 Approaches to CleanupApproaches to Cleanup

Landfill gas in its raw form (i.e. as produced at the landfill site) is a complex and variable mixtureof gases and vapours. Any form of active management of such a gas mixture will be affected to agreater or lesser extent by the contaminants. The role of pre-combustion gas cleanup is to reducethe effects of the contaminants on the handling plant and promote a high degree of operationaleffectiveness. This in turn will improve the effectiveness of management of secondary wastestreams including emissions to atmosphere. Engine management systems and post-combustionactivities may also be employed to manage emissions to atmosphere.

Cleanup options range from commonly adopted simple water trapping and filtration to complexintegrated systems linking the energy exploitation plant (for example, a reciprocating engine drivingan alternator to produce electricity) to the landfill gas abstraction plant.

A typical utilisation scheme will generally include the basic features illustrated in Figure 3.1. Theraw gas will enter the utilisation compound via a de-watering and filtration knockout device toremove moisture and particulates to ensure flare burners do not become blocked and improvecombustion performance within the engine cylinders. A gas compressor (or booster) increases thelandfill gas pressure to ensure effective operation of the flare burners and adequate supply to thegas engine. Flow metering devices and a slam-shut valve, provide the volume flow-rate to the flareor engine and provide a final safety control device. The flame arrestors are used to preventflashback of a flame to the fuel feeder pipe.

Figure 3.1 Typical Utilisation Scheme with Basic Components Identified

Alternator

from landfill

Filter

Knockout vessel

Slam-shutvalve

Flow metering

Burners

PilotFlame arrestors

Slam-shutvalve

Flow meteringGas compressor / booster

High temperature flare

Engine



The simple systems may be defined as ‘Primary Processing’ and the more complex systems as‘Secondary Processing’. In broad terms, the options may be summarised as shown in the processblock diagram given as Figure 3.2. A summary table of examples of pre-combustion gas cleanupprocesses is provided in Table 3.1.

Figure 3.2 Basic Block Diagram for Cleanup Options and Emissions Management

The range of options for cleanup of landfill gas is quite extensive and some attempt at categorisingthe options has been made to cover the numerous examples which have been reported in theliterature. Table 3.1 shows that there are several systems which do not sit neatly in any onecategory - the so called ‘multiple systems’. In reality, all landfill gas cleanup processes are multiplein nature since there is no single process which takes raw landfill gas and produces a ‘clean fuel’product. Initial development of the processes followed a requirement to produce SNG, whichrequired the removal not only of trace contaminants but also all non-combustibles (principallycarbon dioxide and nitrogen). The fact that utilisation of the processed gas resulted in ‘cleancombustion’ minimising damage to the utilisation plant and lessening the atmospheric burden is abonus which has attracted the attention of some operators (and some regulators) of more recentsystems which utilise essentially unprocessed landfill gas. This approach has not been taken up inthe UK (see Section 3.2 below).

In this guidance, therefore, the applicability of a certain process may be for the cleanup of morethan one contaminant in the landfill gas, and so if the cost of abatement is calculated, this shouldbe shared by more than one contaminant (if present).

Raw Gas

Flue Gas

Exhaust

Clean Gas

Control

Pre -combustion

Post- combustion

Waste 1

SNG Boiler

Secondary

Waste 2

Waste 3

Waste 3 By Product (CO2 )

Engine

Engine Vehicle Compress ion

& Polishing

Exhaust

Primary Cleanup

Cleanup Pre -combustion

Post- combustion



Table 3.1 Some Examples of Secondary Pretreatment Cleanup Processes

GenericType

ProcessType

Example Location

Dates Size [m3 h-1]

CostsCapital

[£M]

CostsO&M

[£M pa]SolventScrubbing

Oil sprayDepogasPhytecHerbstSelexol™Selexol™Selexol™Selexol™Selexol™MDEADEA

Berlin-Wansee

Fresh KillsMountaingateOlindaMontereyCalumetPompanoScranton

UKGermanyGermanyGermanyUSAUSAUSAUSAUSAUSAUSA

1985

6004000600600

4170

1.590.190.90

0.110.010.05

WaterScrubbing

SMB TilburgSonzay

HollandFrance

2000 2.93

PressureSwingAdsorption(PSA)

NSR BiogasCarbiogasCarbiogasOxygen-Sulphuric acidNoritCirmacAC (Batch)Gemini V

FilbornaNeunenWijsterWijster-Beilen

BelgiumRumke

SwedenHollandHollandHolland

Belgium

15120012001000

6006001200

0.132.171.371.99

0.300.003

0.39

0.40

0.05

MembraneSeparation(Mem)

PolyamidePolyamideSeparexMonsanto Prism

VasseWeperpolderPuente HillsFlorence

HollandHollandUSAUSA

1992

1983

2001100

60

0.43 0.15

MolecularSieve

GSF Zeolite Palos VerdesMountain View

USAUSA

1975 590

MultipleSystems

ONSIGSF (Molecularsieve + Mem)Water wash +PSAGRS (PSA +Mem)GRS (PSA +Mem)Oxide bed

Flanders RoadMcCarty Road

Monteboro

Kilverstone

Coxhoe

Cinnaminson

USAUSA

Italy

UK

UK

USA 1978

350

1000

300

0.45

0.5 - 1.0

Note: there has been no full-scale long term use of secondary treatment processes in the UK.

3 . 23 . 2 Potent ia l for Subst i tute Natural Gas as Potent ia l for Subst i tute Natural Gas as a Fuel for Landfi l l Gas Enginesa Fuel for Landfi l l Gas Engines

The current global gas markets are such that SNG produced from landfill gas is likely to befinancially marginal at best – the case studies considered in Environment Agency (2002h) confirmthis – and therefore developing plant to exploit this market (at least in the UK) is unlikely to satisfyinvestment criteria. However, there are various options within the inventory of cleanup processeswhich are likely to be worthy of careful development to enhance the operation of existing andfuture systems aiming to utilise ‘raw landfill gas’. The focus of such development will be onremoval of trace contaminants (especially halogenated organics and siloxanes) without necessarilyhaving to remove the non-combustibles. However, the economics of the cleanup options iscurrently far from clear, and a thorough review could show that minimising the total mass flowprior to cleanup (i.e. firstly using a low cost process to remove the non-combustibles - essentiallycarbon dioxide) may offer significant operational and financial advantages.



Carbon dioxide removal processes (which effectively ‘upgrade’ the calorific value of the gas) fallinto four basic categories:

• absorption by a liquid (solvent);

• adsorption by a granular solid;

• differential transport (membrane separation); and

• cryogenic separation.

The underlying process principles defining these categories are described in Chapter 5. However,in the light of future applicability of landfill gas cleanup, the most appropriate (and by implicationthe lowest cost) option is likely to be liquid absorption using water as the solvent. However,further evaluation and financial analysis may show otherwise, and at this stage, none of theoptions described should be ruled out.

The principal requirement of gas cleanup technology when producing SNG is to remove (orminimise) the concentrations of reactive trace components. This can be achieved in part during the‘upgrade’ process (carbon dioxide removal) but to be fully effective, it requires additionalprocessing stages. These stages are likely to be derived from specialised development of sorptionprocesses which target individual or groups of reactive contaminants. Of the most attractiveoptions, activated carbon and proprietary compounds based on activated carbon show greatestpromise. However, solvent absorption offers the distinct advantage of enabling continuousprocessing and therefore, pending more detailed analysis, this option should not be ruled out.

3 . 33 . 3 The Framework for Gas Cleanup AssessmentThe Framework for Gas Cleanup Assessment

The basis for this approach is identified in the Horizontal Guidance Note IPPC H1, EnvironmentalAssessment and Appraisal of Best Available Techniques (BAT) (Environment Agency, 2002d).Rigorous cost benefit analysis (CBA) of the various gas cleanup options has not been carried outin this guidance for three reasons:

• lack of adequate cost and performance data for comparable systems;

• information on multiple systems was focussed on an SNG product and not landfill gas engineuse; and

• a certain reticence from the industry to discuss the costs of implementation of any technologyunless a real situation were involved.

The mechanism by which a rigorous CBA should take place is described, for situations whenthese data become available for a site specific requirement.

The aim of Horizontal Guidance Note H1 (Environment Agency, 2002d) is to provide informationon the preferred methods for quantifying environmental impacts to all media (air, water and land)and to calculate costs and provide guidelines on how to resolve any cross media conflicts. Themethods outlined in H1 can be used to conduct a costs/benefit appraisal of options to determinebest practice or BAT (where applicable) for selected releases from any installation. Spreadsheetsare provided within H1 in order to appraise the options or assess the overall environmental impactof emissions. In order to gain a PPC Permit, Operators will have to show that their proposalsrepresent best practice (or BAT where applicable) to prevent and minimise pollution from theirinstallation.



The following basic steps in the assessment methodology apply:

1. define the objective of the assessment and the options to be considered;

2. quantify the emissions from each option;

3. quantify the environmental impacts resulting from the emissions;

4. compare options and rank in order of best overall environmental performance;

5. evaluate the costs to implement each option; and

6. identify the option which represents the cost-effective technique (or BAT where applicable)by balancing environmental benefits against costs.

3 . 43 . 4 Col la t ing Bas ic Informat ion for the Cost Appraisa lCol la t ing Bas ic Informat ion for the Cost Appraisa l

This section is aimed at collecting all the information required to perform a cost benefit assessmentof gas cleanup options and providing a method for unambiguous presentation of the costs ofcleanup versus the environmental benefits to be gained by the cleanup process.

In order to understand the implementation of the cost appraisal it is necessary to define the termsused within the assessment.

Discount rate

The discount rate usually reflects the cost of the capital investment to the operator, typicallyvarying between 6% and 12% per annum depending on the level of risk associated with thecompany, industrial sector or particular project. The same discount rate should be used for alloptions being considered for the cleanup process, and the selection of a particular value should bejustified by the operator (particularly if it is outside the typical range). The discount rate should beexpressed as a decimal and not as a percentage value, e.g. 0.06 and not 6%.

Assumed life

The assumed life of the cleanup option should be based on the asset life. Current UK guidelinevalues for the different assets are given in Table 3.2. It is recognised that without cleanup anatypical gas will reduce asset life further and this should be factored into the cost benefit analysis.Operators should justify variations from the factors stated in Table 3.2.

Table 3.2 Current UK asset life guideline values to be used in cost appraisals

Asset LifetimeBuildings 20 years

Major components (e.g. landfill gas engines, generators,pollution control equipment)

15 years

Intermediate components (e.g. compressors, some filters, andground handling equipment)

10 years

Minor components (e.g. motors, servos, filters) 5 years



Capital costs

Capital costs include all costs required to purchase equipment needed for the pollution controltechniques, the costs of labour and materials for installing that equipment, costs of site preparation(including dismantling) and buildings and certain other indirect installation costs. Capital costsshould include not only those associated with stand-alone pollution control equipment, but alsocosts of making integrated process changes or installing control and monitoring systems.

The limits of the activity or components to which the costs apply should be described. Forexample, choice of one type of technology, which is inherently less polluting, would require allcomponents of that technology to be included in this limit.

Estimates of engineering costs are generally satisfactory for cost submissions, although anysignificant uncertainties should be described. This is especially relevant for those componentswhich could have a major influence on a decision between different options. Where available, thecost of each major piece of equipment should be documented, with data supplied by an equipmentvendor or a referenced source.

If capital costs are spread over more than one year, these should be reduced to the present valuein the first year as indicated in Table 3.3.

Table 3.3 Calculation of the present value of capital costs

Year 1 2 3Capital expenditure 2000 2000 2000Discount rate 0.1 0.1Value today 2000 2000 x 0.9 2000 x 0.9 x 0.9Equals 2000 1800 1620Present value in first year 5420

The template for the breakdown of capital and investment costs is presented in Table 3.4. Thesecosts should be provided as either £000's or % of total capital costs and the anticipated year ofexpenditure stated.

Operating costs and revenues

It is not anticipated that for the case of landfill gas cleanup for reciprocating engines any additionalrevenues would be forthcoming arising from the products of the gas cleanup. However, theinclusion of revenues would be appropriate for the case of gas cleanup for providing syntheticnatural gas (SNG) for selling to the national grid or for cases where improved energy productionand efficiency may arise from the benefits of cleanup.



Table 3.4 Breakdown of capital/investment costs

Specific cost breakdownIncluded in

Capital Costs33= Yes77= No

Costs£ 000’s/ % of totalcapital cost/ Other

(specify units)

Year

Pollution control equipment costs:• primary pollution control equipment• auxiliary equipment• instrumentation• modifications to existing equipmentInstallation costs:• Land costs• General site preparation• Buildings and civil works• Labour and materialsOther capital costs:• Project definition, design and planning• Testing and start-up costs• Contingency• Working Capital• End of Life - Clean up costs (note: this

cost would be typically discounted to apresent value)

The recurring annual costs for pollution control systems consist of three elements:

1. direct (variable and semi-variable) costs;

2. indirect (fixed) costs; and

3. recovery credits.

The recurring annual change in operating costs for options consists of the additional costs, minusany cost savings, resulting from the implementation of that option. This should include anychanges in production capacity.

The direct costs are those which tend to be proportional or partially proportional to the quantity ofreleases processed by the control system per unit time or, in the case of cleaner processes, theamount of material processed or manufactured per unit time. They include costs for rawmaterials, utilities (steam, electricity, process and cooling water etc.), waste treatment anddisposal, maintenance materials, replacement parts, and operating, supervisory, and maintenancelabour.

Indirect, or “fixed”, annual costs are those whose values are totally independent of the releaseflow rate and which would in fact be incurred even if the pollution control system were shutdown. They include such categories as overhead, administrative charges, insurance, and businessrates.

The direct and indirect annual costs may be partially offset by recovery credits, arising frommaterials or energy recovered by the control system. These may be sold, recycled to the process,or reused elsewhere at the site. These credits, in turn, should be offset by the costs necessary fortheir processing, storage, transportation, and any other steps required to make the recoveredmaterials or energy reusable or resaleable. They also include reduced labour requirements,



enhanced production efficiencies or improvements to product quality. For the case of gas cleanupfor landfill gas engines the increase in servicing intervals, reduction of oil consumption, andincrease in engine efficiency should be taken into account to offset the annual operating costs.

The template for the breakdown of operating and revenue costs is presented in Table 3.5. Thesecosts should be provided as either £000's or % of total capital costs and the anticipated year ofexpenditure stated.

Table 3.5 Breakdown of operating costs and revenues

Specific cost breakdownIncluded in

Operating Costs33= Yes77= No

Total Cost/£ 000’s per year

/ % of totaloperating cost/

Other(specify units)

Year

Additional Costs:• Additional labour for operation and

maintenance• Water/ Sewage• Fuel/ Energy costs (specify energy/

fuel type)• Waste Treatment and Disposal• Other materials and parts (specify

details)• Costs of any additional pollution

abatement equipment operation(specify details)

• Insurance• Taxes on Property• Other general overheadsCost Savings/ Revenues:• Energy savings• By-products recovered/sold• Environmental tax/charge savings• Other

The templates provided above are based on the guidelines issued by the European EnvironmentAgency (1999) and provide the basis for the operator to detail costs breakdown. They have beenadapted to show elements more appropriate to the waste management sector. The operator shouldat the minimum include a tick to indicate which elements have been included in the assessment ofcapital and operating costs.

3 . 53 . 5 How to Perform a CHow to Perform a C ost Benef i t Assessment for Gas Cleanupost Benef i t Assessment for Gas Cleanup

There are six key contaminants or contaminant groups which are potentially treatable, and there isalso the option of producing SNG. This last option is well described in the various Case Studies inEnvironment Agency (2002h). This section of the guidance is intended to deal with removal ofselected components from the supply gas (namely hydrogen sulphide, halogenated organics andsiloxanes) or the exhaust gas (NOx, carbon monoxide, and hydrogen chloride/hydrogen fluoride)for the purpose of achieving emissions reduction or improved economics of operation.

Figure 3.3 below lists the six groups of compounds and the most appropriate cleanup technologyto treat each group individually.



The technologies considered are reviewed and discussed in Chapters 4 - 6 of this guidancedocument. It is considered that primary treatment (Chapter 4) will be required on all landfills toone degree or another, and the relatively low cost of implementing primary gas cleanup means thatthese techniques should be used whenever and wherever they are needed.

The secondary treatment sector is an emerging industry and, as such, new information onavailable technologies will supersede the information contained within this guidance within a fewyears. While many of the technologies identified have been around since the start of the landfillgas industry itself, many others are new, and some are just reinventions and repackaging of oldchemistry. Availability, suitability and cost should be the deciding factors when short listingtechnology for further consideration.

The text below follows two examples through the process:

• removal of hydrogen sulphide; and

• removal of halogenated solvents.



Figure 3.3 The options of gas cleanup technology for particular components requiring treatment

Componentsrequiring treatment

Hydrogensulphide

Halogenatedorganics Siloxanes HCl/HFCONOx

Dry scrubbing

Water scrubbing

Solvent scrubbing

Water scrubbing

Solvent scrubbing

Membraneseparation

Pressure swingadsorption

Molecular sieve

Cryogenic

CO2 liquefaction

Activated carbon(AC)

AC + heat exchanger

AC + chilling

AC + cryogenictreatment

Water wash

In-engine chemicalinjection

EMS

Water injection

Exhaust gasrecirculation

Catalytic oxidation(also requires pre-treatment ofhalogenatedorganics)

Thermal oxidation

Oxygen enrichmentat inlet (potential forhigh NOx)

Catalytic scrubbingand solidification

PRE-COMBUSTIONTREATMENTS

IN-ENGINE AND POST-COMBUSTIONTREATMENTS



3 . 5 . 13 . 5 . 1 S t e p 1 . D e f i n e t h e o b j e c t i v e o f t h e a s s e s s m e n t a n d t h e o p t i o n s t o b e c o n s i d e r e dS t e p 1 . D e f i n e t h e o b j e c t i v e o f t h e a s s e s s m e n t a n d t h e o p t i o n s t o b e c o n s i d e r e d

The first step in the assessment process is to clearly identify the objective(s) of the assessmentand to state all the potential options which are going to be considered.

The objectives for the two examples considered, and the potential options for gas cleanup are asfollows:

Example 1.

Treatment of high hydrogen sulphide concentrations in landfill gas is required to reduce enginewear and to also reduce subsequent atmospheric emissions of SOx in a sensitive location – aneed which has been indicated by site-specific risk assessment.

Hydrogen sulphide cleanup may be achieved by dry or wet desulphurisation. Both techniquesare pre-combustion secondary cleanup technologies and information on these can be found inChapter 5 and Case Studies 1 – 4 in R&D Technical Report P1-330/TR (EnvironmentAgency, 2002h).

Example 2.

Treatment of high chlorine concentrations in the supply gas or treatment of high HClemissions in the exhaust – a need which has been indicated by site-specific risk assessment.

Cleanup of chlorine in the supply gas has been determined to be available by pressure waterscrubbing, pressure swing adsorption, or membrane separation techniques as pre-combustionsecondary cleanup techniques (see Chapter 5 and Case Studies 5 – 11), or by exhaust dryscrubbing (Chapter 6 and Case Study 18 in R&D Technical Report P1-330/TR).

3 . 5 . 23 . 5 . 2 S t e p 2 . Q u a n t i f y t h e e m i s s i o n s f r o m e a c h t r e a t m eS t e p 2 . Q u a n t i f y t h e e m i s s i o n s f r o m e a c h t r e a t m e n t o p t i o nn t o p t i o n

In the case of bulk emissions where emission standards have been set by the Agency, it is astraightforward task to ascertain the level of cleanup required to achieve the emission standard.Normally, any cleanup technology implemented should lower the emission of a particularcompound or substance to below its emission standard.

If the emissions are above the emission standard, or above the risk threshold agreed between theoperator and the Agency (e.g. in the case of site-specific risk assessments on trace componentswithout generic emission standards), then some decision on the efficiency of the cleanuptechnology will be required.

The degree of gas cleanup required will depend on the exceedance beyond the emission standard.For example, a technology offering 99.9% cleanup would not be justified if the emissions are only25% above the emission standards, assuming an alternative technology is available that can meetthe standard.



Example 1.

Table 3.6 shows that the efficiency of the dry desulphurisation process increases from 75% to98% with increasing sulphur load in the supply gas. This site has a total sulphur content ofapproximately 2700 mg S Nm-3 in the supply gas. This is equivalent to Case Study 2b inEnvironment Agency (2002h), and 98% cleanup could be achieved (giving a supply gasquality to the gas engine of 135 mg S Nm-3). Once combusted, the exhaust will probablycontain only ~1% of this as hydrogen sulphide, i.e. ~1.4 mg Nm-3.

Table 3.6 also shows that the wet desulphurisation process achieves 99% gas cleanup,although the information does not confirm whether this is achievable with the higher Sloadings found in our landfill gas supply.

Both technologies would be suitable for further consideration with these cleanup efficiencies.

Example 2.

Table 3.6 shows that the cleanup efficiencies vary for the component of interest. In this case,removal of chlorinated compounds should be achievable at 95% efficiency for all thesecondary pre-treatment options (pressure water scrubbing, pressure swing adsorption, ormembrane separation), and at 93% for post combustion dry scrubbing.

There is 560 mg Cl Nm-3 in the supply gas and so all technologies are appropriate.

In certain circumstances a “no action” baseline condition should be considered in addition to othertechnologies to help assess the environmental benefit of any cleanup treatment considered.

For components in the supply gas that affect the emissions of other gases, then the impact of thesupply gas quality on emissions must be calculated. For example, removal of chlorine in the supplygas in Example 2 may reduce the emissions of PCDDs and PCDFs from the exhaust stack.

Quantification of the emissions and emissions reduction from each treatment option can be madeon the basis of measurements at an existing installation, or (if this does not exist) on the basis ofmanufacturer’s information on the process (bearing in mind that both methods have inherentuncertainties). Table 3.6 gives some estimates of emissions reduction due to variations in stackheight for the processes described in some of the case studies (see Environment Agency, 2002h),and these data are used to illustrate the examples in this section..

The H1 method (Environment Agency, 2002d and 2002c) recommends calculation of the landfillgas engine contribution of emissions to air according to the following approach, if more complexmodelling is not required:

PCair = DF × RRWhere:

PCair = process contribution (µg m-3)RR = release rate of substance in g s-1

DF = dispersion factor, expressed as the maximum average ground level concentration perunit mass release rate (µg m-3 g-1 s-1) based on annual average for long term releases.

Conservative dispersion factors are given in Table 3.6 below.



Table 3.6 Dispersion factors for various effective release heights

Dispersion factor (µg m-3 g-1 s-1)Effective height of release (m)Long term: maximum annual

averageShort term: maximum hourly

average0 148 390010 32 580

Releases which warrant no further consideration (i.e. no action on emissions cleanup are required)are defined thus:

PClong term < 1% of the long term environmental benchmark.

PCshort term < 20% of the short term environmental benchmark.

The long-term emissions criterion is typically most applicable to landfill gas operations. Thecriterion for screening long term emissions that are unlikely to lead to significant environmentalimpacts is proposed as 1% of the relevant environmental benchmark. This is based on judgementof the level at which it is unlikely that an emission will make a significant contribution to anyimpact even if an EQS or EAL is exceeded.

Further guidance on discharge stack heights for polluting emissions is given in Technical GuidanceNote (Dispersion) D1 (HMIP, 1993).

3 . 5 . 33 . 5 . 3 S t e p 3 .S t e p 3 . Q u a n t i f y Q u a n t i f y t h e e n v i r o n m e n t a l i m p a c t s r e s u l t i n g f r o m t h e e m i s s i o n s t h e e n v i r o n m e n t a l i m p a c t s r e s u l t i n g f r o m t h e e m i s s i o n s

Each cleanup option identified will have its own environmental impact. Matter can neither becreated nor destroyed, and the pollutant abatement process will inevitably produce other wastestreams. The environmental impact of these (whether they are hazardous and costly to dispose, orwhether they create another environmental burden in another emissions pathway such asgroundwater) must be considered. Unfortunately, manufacturers do not highlight information onwaste streams produced by gas cleanup processes quite as readily as they do the benefits of thetechnologies.

It would not be appropriate to select a cleanup option which had a significant environmentalimpact in another media. For example, production of a hazardous waste oil or sludge as a result ofthe cleanup process might be harder to manage than the atmospheric emission already takingplace.

In the absence of much information at the present time on the likely wastes arising from thetechnologies in the two examples considered here, all technologies are considered equal in theirnegative environmental impact.

3 . 5 . 43 . 5 . 4 S t e p 4 . C o m p a r e o p t i o n s a n d r a n k i n o r d e r o f b e s t e n v i r o n m e n t a l p e r f o r m a n c eS t e p 4 . C o m p a r e o p t i o n s a n d r a n k i n o r d e r o f b e s t e n v i r o n m e n t a l p e r f o r m a n c e

Table 3.7 gives examples of a number of processes that can achieve gas cleanup for hydrogensulphide or halogenated organic compounds (or both). Nearly all have high cleanup efficiencies,indicating that any of the options could be employed and little noticeable difference in emissionsreduction between the technologies would be evident. In such a situation, the costs (both capitaland operation and maintenance costs) would be a driving factor in the choice of technology.

If full CBA is not carried out, comparison of the capital costs, the cost per tonne of pollutantabated, and annual operating cost are good indicators for assessing comparative costs, providedthe information for all options are comparable.



Interestingly, Table 3.7 also indicates that at lower pollutant loadings, the technology in case study2 has a lower cleanup efficiency. It is equally possible that other technologies would performbetter at lower pollutant loadings than higher ones. The capability of the technology to treat thegas quality of the site has an additional bearing on the comparison of technologies for theirenvironmental impact.


Table 3.7 Summary of factual information for Examples in text

ReferenceNo. of Case

Study inEnvironment

Agency(2002h)

Case Study technology Substances requiringabatement

Assumedcleanup

efficiency (%)

Raw inlet S or Cl gasconcentration

(mg Nm-3)(see note 1)

Quantity of Cl or Sabated

(mg Nm-3)

Operating cost to abate1 tonne substance(£)

(excl of SNG or S sales,rounded to the nearest

£100)

Annual operating costto treat lfg

supply/exhaust for1MWe engine @ 95%uptime, £ (excl of SNGor S sales, rounded to

the nearest £100)(see note 2)

Capital costs forplant (£million)

2a Dry desulphurisation Sulphur 98 6720 6585 2600 81200 0.0132b Dry desulphurisation Sulphur 95 2688 2553 2500 30000 0.0062c Dry desulphurisation Sulphur 75 134 101 21600 10300 0.0053 Wet desulphurisation Sulphur 99 376 373 11700 20600 5.0394 Pressure water

scrubbingCFCs (totalchlorine)

95 560 532 158500 400000 4.150

Sulphur 99 376 373 94300 166700 1.3707 Pressure SwingAdsorption CFCs (total

chlorine)95 376 358 98300 166700 1.370

Sulphur 99 376 373 108300 191600 2.1708 Pressure SwingAdsorption CFCs (total

chlorine)95 560 532 75900 191600 2.170

Sulphur 99 376 373 122500 216600 0.4309 Membrane separationCFCs (totalchlorine)

95 560 532 85900 216600 0.430

11 Multiple System Sulphur 99 376 373 481000 850200 1.00014 HCl/HF dry scrubbing

(exhaust)HCl/HF (as total

chlorine)93 440 410 1800 24500 0.015

Notes (1) maximum inlet gas concentrations for H2S and total Chlorine (as HCl) observed by Gillett et al. (2002) used for case studies 3-14,total Cl concentration would give exhaust concentration of 81 mg/Nm3 (@5% O2

For case study 2a,b,c various inlet H2S gas concentrations assumed as per quote provided by supplier(2) quantity of landfill gas required to maintain 1MWe engine assumed to be 570 m3 h-1



3 . 5 . 53 . 5 . 5 S t e p 5 . E v a l u a t e t h e c o s t s t o i m p l e m e n t e a c h o p t i o nS t e p 5 . E v a l u a t e t h e c o s t s t o i m p l e m e n t e a c h o p t i o n

When the appropriate technology(ies) have been selected for consideration, detailed capital,operational, maintenance, waste disposal and manpower costs are required. The operator isrequired to complete this process when more than one option exists for the mitigation of theenvironmental impact of the installation. However, if the operator proposes to implement theoption from Step 4, which represents the lowest environmental impact, then an evaluation of costsis not required.

The preferred method for appraising the various cleanup options is based on conventionaldiscounted cash flow (DCF) analysis, in which the future cash flows over the lifetime of an optionare converted to an equivalent annualised costs (dependent upon the discount rate chosen by theoperator). This facilitates the comparison of different cleanup options, which may be performedover different timescales and cost profiles. The information required to perform such an analysisshould be collated on the forms shown in Section 3.4 above.

As previously stated, the case study cost information in Table 3.6 does not loan itself to true DCFanalysis because of the estimated and aggregated nature of some of the cost estimates obtained.However, in Table 3.6 an attempt has been made to assess the capital cost of plant construction,cost per tonne of pollutant abated (excluding capital costs) and yearly operational costs for a1MWe landfill gas engine from the information obtained.

For the operator producing a cost evaluation, for each cleanup option, the annualised cost for eachoption must be determined. The various steps and calculations required are summarised in Table3.8.

The present value of the capital cost in the first year in Table 3.8 represents the sum of thediscounted capital costs spread over the term of the cleanup operation, as determined using thediscount rate. The average annual operating cost represents the average balance between operatingand revenue costs over the term of the cleanup operations. This is corrected to present value costsusing the present value factor.

In order to appraise the various cleanup options available the equivalent annual costs for eachoption should be summarised and presented as indicated in Table 3.9.

Table 3.8 Calculating the annualised cost for each cleanup option

Step Result UnitDiscount rate, r (Operator input) = FractionAssumed life of the option, n (Operator input) = YearsEquivalent annual cost factor =

rr

rn +

−+ 1)1(

=

Present value factor = 1 / equivalent annual costfactor

= £

Present value cost of the option = (Annual averageoperating costs x present value factor) + capitalcosts

= £

Equivalent annual cost = Present value cost of theoption x equivalent annual cost factor.

= £



Table 3.9 Comparison of the equivalent annual costs for each cleanup option

Cost category / factor Option 1 Option 2 … … Optionn

Capital Cost (£ ‘000s)

Operating costs (£ ‘000s year-1)

Life of option (n) (years)

Discount rate (r)

Equivalent annual cost (£ ‘000s)

In the case of the two examples given here, even without performing a true CBA, the followingconclusions can be drawn.

Example 1.

The hazardous nature of the treatment process chemicals and some process wastes makes the wetdesulphurisation (Stretford) process unacceptable in environmental impact terms. The inordinatelyhigh capital cost of the plant is too high, and the cost per tonne of S abated is significantly higherthan the dry desulphurisation process.

The dry desulphurisation process has a low capital investment cost and a moderate cost per tonneof S abated (£2500 tonne-1 S abated).

Example 2.

Most of the secondary pretreatment processes have inordinately high capital costs and equally highoperational costs. Post-combustion dry scrubbing technology has the lowest capital cost and againa moderate cost per tonne of Cl abated (£1800 tonne-1 Cl abated).

3 . 5 . 63 . 5 . 6 S t e p 6 . I d e n t i f y t h e S t e p 6 . I d e n t i f y t h e o p t i o n w h i c h r e p r e s e n t s t h e c o s t - e f f e c t i v e t e c h n i q u eo p t i o n w h i c h r e p r e s e n t s t h e c o s t - e f f e c t i v e t e c h n i q u e

The selection of the option which represents the most cost-effective technique involvesconsideration of both the economic and environmental appraisals. The option resulting in lowestimpact on the environment as a whole is considered to be the most appropriate one, unlesseconomic considerations make it unavailable.

If sufficient options are available to be appraised economically then it may be possible to generatea curve of the cost against the environmental benefit (Figure 3.4). This process may help toidentify the point (the cost effective point, or BAT point, where applicable) at which the cost ofabatement rises rapidly, indicative that value for money begins to decrease rapidly.



Figure 3.4 Cleanup option cost versus environmental benefit

Where only one technology is applicable, or only one technology survives the assessment processto this point, this method will not work. In these cases, calculating a cost per tonne of emissionsabated using the same cost information will give a number that can be assessed for costeffectiveness.

The Agency is developing a database of costs of pollutant abatement so that judgements onwhether a cleanup technology should be implemented or not can be made against this database ofcost-effectiveness. These are termed cost effectiveness benchmarks, and are used to judgewhether a given technique should be implemented (Environment Agency, 2002h).

At the current time, only NOx emissions have been assessed in detail, but values also exist forSOx, CO2, CH4, NMVOCs and PM10. Table 3.10 gives these indicative cost effectivenessbenchmarks (Note, these values will change with time).

Table 3.10. Indicative Cost Effectiveness Benchmarks (from Environment Agency, 2002g)

Emission Cost £ tonne-1 Study source

NOx 1400 Agency database

SOx 1600 Dutch database

CO2 25 - 30 DTI and ETSU

CH4 27 Dutch database

NMVOCs 3000 - 3100 Dutch/World Bank databases

PM10 1469 - 1600 AEA/Dutch database

CO has a typical persistence in the atmosphere of approximately two months. Post-combustionoxidation to carbon dioxide should only be considered necessary if the site is near a sensitive

Mass of Pollution avoided

Cost

£

“The Knee”

The cost

effective

point

Low cost techniques

High costtechniques

Options



receptor and modelled dispersion characteristics indicate a potential risk. In such circumstances, acost effectiveness benchmark of £350 per tonne is considered indicative. The lower pricingrelative to NOx reflects the availability of the technology as well as the rate at which CO oxidisesin the air if no sensitive receptor is present. This guidance cost per tonne is provided in theabsence of any actual costs and may be subject to negotiation between the operator and theregulator.

In other cases (such as siloxanes, which impact mainly on the longevity of engine componentsrather than emissions) the decision to implement abatement technology will be purely acommercial one, but nevertheless one which can be made on the basis of informed benefits versuscosts.

The need for gas cleanup will be determined by a number of factors, but the cost per tonne ofcomponent abated is the clearest indicator of whether a process should be implemented or not.

If the result of the cost benefit analysis suggests that of all the available technologies, none issufficiently cost effective in relation to the banding, then implementation of the cleanup technologywill not be required.



4.4. PRIMARY PRETREATMENT TECHNOLOGIESPRIMARY PRETREATMENT TECHNOLOGIES

4 . 14 . 1 IntrIntro d u c t i o no d u c t i o n

Primary pre-treatment technologies represent the first stage in reducing the amount ofcontaminants in the landfill gas and typically use simple physical process operations. The maincontaminants removed (or reduced) are water (albeit contaminated), referred to as ‘condensate’and particulates. These technologies have been in use for many years and are now relativelystandard fits to active landfill gas management plants. Typical equipment and its operation aredescribed in the following subsections.

4 . 24 . 2 Water /condensate KnockoutWater /condensate Knockout

The presence of liquid water in landfill gas pipework can have a detrimental effect on the plantperformance. Firstly, accumulation of water reduces the space available for gas flow whichmeans that the pressure loss will be raised, and, secondly, the unstable nature of two-phase flows(i.e. liquid and gas combined) gives rise to oscillations which in turn means that a steady andcontrollable operation cannot be achieved. The presence of contaminated water can also lead todeposit formation on the pipe walls which reduces the smoothness and further increases thepressure loss. Hence, the presence of liquid water in landfill gas pipes should be controlled andminimised.

There are three components which can be treated, depending both on the source of the gas andapplication or proposed usage of the treated landfill gas, namely:

• slugs of liquid;

• gas-liquid foam; and

• uncondensed water vapour.

The level of complexity (and therefore cost) increases down the list above and this has determinedthat many installations in the UK rely solely on passive ‘slug catching’ vessels. However, someschemes have adopted foam and droplet arresting systems to minimise the effects on engine intakeand control systems. Removal of uncondensed vapour is infrequently practised although there areexamples in the UK of plant which treat the landfill gas to yield a dew point of 2 °C. The basicprinciples of each of the treatment options are described below.

4 . 34 . 3 Liquid Water CaptureLiquid Water Capture

In-line dewatering features are frequently adopted by landfill operators and these are usuallyinstalled within the landfill gas collection network. However, there is invariably a need toincorporate additional control measures to prevent onward transmission of liquid water. In somecases, drains and water traps may be adequate for the supply gas specification.

A further common practice, usually forming the final element of dewatering is a knockout drum,often called a ‘condensate knockout pot’ (occasionally called a ‘slug catcher’). This is located asclose as practicable to the inlet to the gas booster. The purpose of the knockout drum is to lowerthe gas velocity sufficiently to enable ‘dropout’ of liquid which may then be drained or pumped todischarge. Such devices are simple and capable of handling large gas flows (up to 10 000 m3 h-1)and removing over 1 litre min-1 of water (see Figure 4.1).



4 . 44 . 4 Foam RemovalFoam Removal

An often adopted refinement to water control systems is the incorporation of coalescing (ordemisting) meshes in the gas pipes entering and leaving a condensate knockout drum whichcollapse entrained foam and prevent carryover. Typically the meshes are woven stainless steelpads which provide a large surface area to trap the foam and allow it to drain under gravity to thecollection drum.

As an alternative (or in addition) to the knockout drum, some equipment manufacturers providecyclones which impart swirl to the incoming gas flow and thereby enhance the rate of liquidremoval from the gas stream.

Often several elements (for example, dewatering manifold, knockout drum and secondary cyclonevessel) are built into a skid-mounted module which is linked directly to the landfill gas boosterinlet. Cyclones are reported by manufacturers to be able to capture 99% of droplets greater than10 µm1.

Water and condensates in landfill gas represent possibly the most intractable contaminant from thegas abstraction perspective, since accumulation in pipework is difficult to eliminate completely andthis can cause blockage. In addition, the acidic condensate can give rise to relatively high rates ofcorrosion of carbon-steel pipework. A simplified flowsheet for a more sophisticated (as comparedto that depicted in Figure 3.1) primary pre-treatment system is shown in Figure 4.1. The additions,compared to the typical primary pre-treatment system shown in Chapter 3 are a cyclone separatorand filter prior to the gas booster and an after cooler, chiller and secondary knockout pot betweenthe booster and the gas engine/flare.

Figure 4.1 More sophisticated primary processing system

1High efficiency knockout cyclone gas separators. Kelburn Engineering Ltd.

Burners

Pilot

High temperature flare

Alternator

Flow metering

Slam-shutvalve

Flow metering

SecondaryKnockout

Chiller

Cycloneseparator

Knockoutvessel

om landfill

Filter

AfterCooler

Gas compressor / booster

Engine

Condensate to landfill or treatment

Slam-shutvalve

Flame arrestors



4 . 54 . 5 Vapour Reduct ionVapour Reduct ion

Raising the pressure of a gas mixture leads to an increase in temperature. Whilst some of the heatof compression will be dissipated at source1, the delivery gas stream will inevitably be at atemperature significantly higher than ambient. This may give rise to the need to cool the gas toprotect control valve seats, prevent over-stressing of polyethylene (PE) pipework2 and meet othercriteria for reliable metering or consumer safety considerations.

For applications where gas conditioning is specified (to reduce the amount of water vapour andlower the dew point), the thermal load on the conditioning unit may be limited such that a pre-chilling step may be required. Pre-chilling and aftercooling, whilst carried out for different reasons,involve the same basic process, namely heat removal from the high pressure delivery gas stream.

The amount of heat to be removed will depend upon the specific heat capacity of the gas mixture,the booster exit temperature, the mass flowrate of gas and the specified final temperature. Fortypical primary cleanup processes, using for example a centrifugal gas booster, the heat load isunlikely to require specialist equipment and a length of 5 to 10 m of corrosion protected steelpipework may be sufficient. However, some cases where, for example, space is restricted, maybenefit from using a forced draught cooling stage.

It should be noted that in any instance of aftercooling, depending on the condition of the gasstream leaving the landfill (in terms of specific moisture content), compression will reduce therelative humidity which will be reversed on cooling. This can give rise to condensation in thedelivery line which can cause problems for the consumer. It is therefore essential to review andmeasure the temperature profile along the pipework, and if necessary, install insulation or lagging(or trace heating) of the downstream end of the pipe.

More complex (and much less widely used) types of gas cooling are available, these include: shelland tube heat exchangers; spray towers and chilled water recuperators.

For some applications, there is a requirement to reduce the moisture content of the gas streamsuch that at any point in the delivery pipework the relative saturation is always well below 100%.In order to achieve this, the gas stream requires ‘conditioning’ using a dehumidification process.

There are three basic options which may be adopted to achieve this function: refrigeration drying;deliquescent bed absorption; and glycol stripping. The former uses a refrigeration unit to chill thewet gas to around 2ºC, causing condensation of a proportion of the water vapour. This is followedby reheating of the cooled gas to between 10ºC and 15ºC. Greater levels of drying can beachieved by cooling to -18ºC, although to prevent pipeline icing-up, the gas stream has to bespiked with glycol, which is later removed from the product gas.

Deliquescent dryers involve passing the wet gas stream through a tower or vessel containing amoisture absorbent material (for example, common salt) which physically absorbs the moisture.

These techniques lead to a pressure loss in the supply that should be allowed for in thespecification of the gas booster and its operational settings. In addition, the techniques can add asignificant amount to the gas processing costs - refrigeration units have an electrical load(constituting a relatively large parasitic loss) whereas deliquescent dryers require regular ‘topping-up’ of the granular absorbent. The techniques, by their nature, give rise to a contaminated waterstream which should be treated or disposed appropriately.

1The heat of compression is described as adiabatic if there is no heat loss, isothermal if all of the heat ofcompression is dissipated and polytropic for situations between the two limits. Practical gas boostersand compressors operate polytropically.2Rated pressures for PE pipe fall off dramatically at temperatures above around 50 oC.



The glycol stripping process is more applicable to larger gas flow rates and involves passing thewet gas through a counter current contact tower employing for example triethylene glycol (TEG).Simplified process flowsheets for a refrigeration drying system and a TEG drying system areshown in Figure 4.2 and Figure 4.3, respectively. These may be compared with the basic primaryprocessing arrangement shown in Figure 4.1.

Figure 4.2 Typical Refrigeration-type Gas Conditioning System

Compressor

Heat Exchanger

Condensate

Pretreated Gas

'Conditioned' Landfill Gas

Primary

Air Blast Cooler

Refrigeration Cooler



Figure 4.3 Simplified Gas Drying Process using triethylene glycol (TEG)

4 . 64 . 6 Contaminated Water ManagemContaminated Water Managem entent

The dewatering steps described in the previous paragraphs give rise to a waste stream comprisinga slightly acidic contaminated water with many of the characteristics of landfill leachate. Thechemical composition of the waste water will not allow untreated discharge and therefore thefollowing treatment routes are likely: return to landfill; storage in a local storage tank (an openlagoon is unlikely to be acceptable owing to potential odour impacts); or piped to an on-siteleachate treatment facility. Analysis of condensates from field drainage points compared to plantdrainage points and from a range of landfill sites are given in Table 4.1 adapted from Knox (1991)with additional data from Robinson (1995).

Table 4.1 Characteristics of Landfill Gas Condensates

Component/parameter1

Plant

(Knox, 1991)

Field drains

(Knox, 1991)

Range

(Robinson, 1995)

Mean

(Robinson, 1995)

pH 4.0 - 7.6 3.1 - 3.9 3.5 - 7.5 5.04

Conductivity 76 - 5700 200 - 340 111 - 5190 1342

Chloride 1 - 73 <1 – 4 <2 – 10 9

Ammoniacal nitrogen <1 - 850 3 – 15 0.6 – 764 133

Total Organic Carbon 222 - 4400 720 - 9300 36 - 5080 1969

COD 804 - 14 000 4600 27 - 18000 6884

BOD5 446 - 8800 2900 30 - 11200 3757

Phenols 3 - 33 4 - 17

Total volatile acids 141 - 4021 730 - 4360 <5 - 2995 629

Notes: 1. All values in mg litre-1 except pH (dimensionless) and conductivity (mS cm-1)

Gly

col C

onta

ctor

Gly

col

Str

ippe

r

ChillerEvaporator

Dried Landfill Gas

Glycol

WaterVapourKnockout

DrumCompressor

Raw LandfillGas

Condensate



4 . 74 . 7 Part iculate Fi l trat ionPart iculate Fi l trat ion

Particulates can arise in a landfill gas stream for a variety of reasons, and if allowed to passdownstream to a supply plant or consumer can give rise to damage and wear of systems andequipment. Parry (1992) highlighted the need for vigilance whenever knockout drums are used insystems supplying gas engine generating sets. The issue of concern is bacterial growth in the vesselwhich leads to particulates that can seriously affect engine operation.

Particles can be controlled either by passing the gas stream through a filter pad (typically made ofstainless steel wire) which can also double as a foam coalescing mesh, or alternatively using acyclone separator. Cyclones are capable of removing particles down to 15 µm (or even 5 µm for ahigh efficiency cyclone) whereas filter pads are effective down to 2 µm. Both systems are proneto blockage and therefore require frequent maintenance to remove accumulated solids.

A further approach to filtration (used in Austria by Entec Environment TechnologyUmwelttechnik GmbH) is based on passing the raw landfill gas through a gravel pack or through aceramic filter pack. This removes particulates (down to 150 µm) and water droplets from the gasstream.

4 . 84 . 8 Deal ing with Wastes fDeal ing with Wastes f rom Primary Cleanup Processesrom Primary Cleanup Processes

The aims of primary treatment are to prevent transmission of liquid water and particulates to theenergy utilisation plant. It follows that the waste arisings from this treatment will comprisepredominantly contaminated water or condensate with similar characteristics and composition tothat of landfill leachate (Table 4.1). Assuming most of the liquids can be arrested within the gasfield network, the loading at the plant should be in the range 1 litre min-1 to 3 litre min-1 of liquidcondensate for every 1000 m3 h-1 of landfill gas flow (Robinson, 1995). This equates toapproximately 500 to 1500 tonne year-1 of contaminated water per 1000 m3 h-1 of landfill gasprocessed which would require treatment. The solid particulate material is likely to comprise amixture of ‘biomass’ and mineral deposits, the latter rich in iron, calcium and silicon. There are noknown reported data on actual compositions or arisings of this waste stream. A significantproportion of the waste stream may be removed in association with the condensate, while theremainder is likely to arise following plant maintenance in for example, replaceable filter cartridges.Spent filters will ultimately be disposed to landfill.



5.5. SECONDARY PRETREASECONDARY PRETREA TMENT TECHNOLOGIESTMENT TECHNOLOGIES


Currently in the UK, landfill gas is used in utilisation plant only after primary pre-treatment.However, there is now a growing body of evidence which indicates that in some instances thispractice can be detrimental both to the utilisation plant and to the environment. A range ofprocesses exist that are designed to provide a much greater level of gas cleaning than possibleusing primary systems only. Such processes, which include both physical and chemical treatmentsmay be defined collectively as secondary pre-treatment.

To date, there has been relatively sparse uptake of secondary pre-treatment in the UK, althoughoperators of electricity generating plant have considered advanced cleanup systems owing to theincreasing number of engine failures.

The lead for the development of secondary pre-treatment systems has come primarily from theUSA, which had large scale plant in operation over twenty years ago, primarily for the productionof Synthetic Natural Gas (SNG). Within the EU more recently, there has been interest inproducing SNG by cleaning-up landfill gas, and some of the techniques used in SNG manufactureare applicable to landfill gas pre-treatment for gas engines also.

A review of the experience in the US and Netherlands with cleanup technologies has enabled thefollowing information to be drawn together.

Pre-combustion cleanup of landfill gas trace constituents has no effect on bulk emissions of COand NOx and is therefore only of value in reducing aggressive gas constituents that either harm theengine or produce unacceptable emission levels. This section addresses those secondary pre-treatment options that are available for hydrogen sulphide, halogenated compounds and siloxanes.The gas engine operator might consider these treatment options if:

• hydrogen sulphide is causing engine wear;

• halogenated solvents are causing engine wear;

• siloxanes are causing engine wear; and/or

• emissions of H2S, SO2, HCl, HF, PCDDs and PCDFs exceed safe concentrations asdetermined by site-specific risk assessment.

5 . 25 . 2 Hydrogen Sulphide Pre-treatmentHydrogen Sulphide Pre-treatment

There are a number of methods of removing or stripping hydrogen sulphide from gas streams,involving both wet and dry scrubbing techniques. Wet scrubbing techniques are usually employedto remove not just hydrogen sulphide but a number of components.

5 . 2 . 15 . 2 . 1 H y d r o g e n H y d r o g e n s u l p h i d e d r y s c r u b b i n gs u l p h i d e d r y s c r u b b i n g

An early solid chemical treatment for H2S widely employed for coke-oven gas was the use of an‘iron sponge’ or a material of wood chips impregnated with hydrated ferric oxide (EnvironmentAgency 2002h: Case Study 1). The H2S within the gas reacts with the ‘iron sponge’ to form ironsulphide, with cleanup efficiencies up to 99.98%.

Early utilisation schemes made use of iron oxide boxes to reduce the concentration of hydrogensulphide. For example, in the late 1970s, Cinnaminson landfill in New Jersey, USA provided gas ata rate of around 300 m3 h-1 with 62% v/v methane to the Hoeganaes steel plant. The gas wastreated with partial success to reduce hydrogen sulphide by passing it through a bed of woodshavings impregnated with iron oxide. Different scales of operation have been employed ranging



from gas flow rates of ~2500m3 CH4 h-1 (e.g. Avenue Coking Works) down to much smaller scale

plants ~100m3 CH4 h

-1 (e.g. SCA paper recycling plant, Lucca, Italy; Camelshead Waste WaterTreatment Works, Plymouth, UK).

The SCA paper recycling plant uses two gas purifier units (Varec Vapor Control Inc.) which canreduce outlet H2S concentrations to 4.5 ppm v/v. The spent adsorption beds can be reactivated byair injection which converts the iron sulphide formed back to iron oxide and elemental sulphur,providing a 5 year life span for each unit.

A system marketed as Sulphur-Rite (Gas Technology Products) uses an unspecified iron-basedmedium to form iron sulphide to treat operations with H2S emission loads of <180 kg day-1 withpre-engineered units handling gas flow rates up to 4300 m3 h-1 (i.e. H2S gas concentrations < 1765mg m-3). This product claims to remove 3-5 times more H2S than a simple iron sponge system.The system consists of one or two vertical reacting vessels, and the spent material is sent tolandfill.

Activated carbon filters (as powder, granules or fibres) are generated chemically and/or in a hightemperature steam environment to produce an extensive network of impregnated pores. Thesepores provide the sites for the physical adsorption of H2S (and also water, CO2 and halogenatedcompounds). These are most effectively used for polishing gases after other treatment(s) and thehigh costs of replacement/regeneration and spent carbon disposal make such schemes expensivefor landfill gas operations.

Another system marketed as GAS RAP is described in Case Study 2 of Environment Agency(2002h). This system has the potential to cleanup H2S within the inlet supply gas down to levels ofbetween 25 and 50 ppm for landfill gas with typically 100 ppm v/v in the supply gas, and tobetween 100 and 200 ppm for a landfill gas supply at high H2S concentrations (> 2000 ppm v/v).The technology appears to be most cost-effective for landfill gas with high H2S concentrations(> 2000 ppm v/v).

5 . 2 . 25 . 2 . 2 H y d r o g e n s u l p h i d e w e t s c r u b b i n gH y d r o g e n s u l p h i d e w e t s c r u b b i n g

Chemicals used in the wet scrubbing of H2S can be solid or liquid and may be applied in batchcontactor towers or injected directly into the gas pipeline. The by-product of the reaction is usuallyseparated and disposed of as a waste. The chemical is consumed and the absorbent can beregenerated.

A liquid chemical process employed for removing H2S from numerous gas streams (including cokeoven gas) is the Stretford process (Environment Agency, 2002h - Case Study 3). This employeda caustic washing solution (containing sodium carbonate and pentavalent vanadium) to produceelemental sulphur. A catalyst of anthraquinone disulfonic acid (ADA) combined with air injectionwas used to regenerate (re-oxidise) the tetravalent vanadium and separate the sulphur. A removalefficiency of H2S of 99.99% was achieved using this process (Moyes et al., 1974), which atSmithy Wood was capable of treating gas at 8200 m3 h-1 with a H2S loading of 103 kg h-1.

A current proprietary liquid redox system that uses a chelated iron catalyst to convert H2S toelemental sulphur (LO-CAT process, Gas Technology Products) is designed for 99.9% removalof H2S. The iron catalyst is held in solution by organic chelating agents which prevent precipitationof iron sulphide or iron hydroxide so that the reduced (ferrous) iron can be reoxidised to ferric ironin the oxidiser and the catalyst regenerated for the absorber stage, with the formation of elementalsulphur. This system has been employed at the Central Sanitary Landfill (Broward County,Florida, US) to treat 11000 m3 h-1 of landfill gas containing up to 5000 ppm v/v of H2S, prior touse in gas turbines.

The landfill gas at Sonzay Landfill (Tours, France) has been treated using a fully operational waterscrubbing operation since 1994, with the capability to upgrade the gas for use as vehicle fuel



(Balbo, 1997). The landfill gas is initially compressed to 14 bars, water cooled and then passedthrough a water packed counter-current wet scrubber. This physically absorbs most of the H2Sand CO2 (both of which have higher relative aqueous solubility compared to CH4). The water isregenerated using ambient air, with the exhaust stream being cleaned using a biofilter. Thescrubbed gas is passed through dual adsorption columns (operational and regenerating) to dry thegas, prior to secondary compression. This process produces a compressed natural gas (CNG) ofbetween 86 and 97% (v/v) CH4, with oxygen < 0.5% (v/v) and H2S < 5 ppm v/v (Roe et al,1998). It is estimated that 2 litres of CNG can be produced per tonne of landfilled waste over a 15year period (Balbo, 1997).

Other liquid absorption techniques use proprietary solvents rather than water to selectively removethe H2S (plus CO2

and halogenated compounds) from the landfill gas stream, where the reducedsulphur species that have been stripped can be recovered as elemental sulphur (typically atbetween 95 and 99% recovery rates for H2S). The solvent Selexol™ (dimethyl ether ofpolyethylene glycol) has been used to upgrade landfill gas to pipeline quality at a number of landfillsites (Kohl and Nielsen, 1997) (see also Section 5.3 below).

A caustic wash process which relies on liquid absorption and salt formation to remove H2S (CO2

and mercaptans), uses either solutions of sodium or potassium hydroxide (NaOH or KOH) toform stable salts such as sodium carbonate (Na2CO3) and sodium sulphide (Na2S). However, thisis not a good choice for landfill gas operations which have high concentrations of H2S or CO2

(Kohl and Nielsen, 1997). A liquid absorption process, important for industrial processes at alarger scale than landfill gas operations (i.e. treatment of natural gas), uses various water solublesolvents or alkanolamines to selectively absorb H2S (and CO2). Alkanolamines such as mono-, di-,methyl-ethanolamine (MEA, DEA and MDEA) and di-isoproanolamine (DIPA) are widely used.The gas under high pressure is purified via contact with the DEA solution in an absorber column(trays or random packing). The DEA solution is released from the absorber under low pressure,allowing escape of dissolved hydrocarbons which are usually passed to the fuel gas system. TheDEA solution is regenerated upon contact with steam in a stripping column, with the solvent raisedto its boiling point (110 °C) and stripped by the steam. On cooling to 40 °C the DEA solution isre-circulated to the absorber column (typically up to 50 times per hour), whilst the H2S gas is fedto a sulphur recovery unit to remove 99.9% of the sulphur.

A wet scrubbing system developed by Q2 Technologies uses a patented amine-based materialreferred to as Enviro-Scrub. Company literature (Q2 Technologies, 1993) states that the amine-based scrubbing compound used has an advantage over other common scrubbing compounds(such as MEA, DEA, MDEA and NaOH) because the salts formed by the latter release H2S uponheating or acidification. The Enviro-Scrub systems uses a triazine compound called 1,3,5-tri(2-hydroxyethyl)-hexahydro-s-triazine (Environment Agency, 2002h - Case Study 4).

5 . 35 . 3 Pre- treatment of Halogenated Organic SpeciesPre- treatment of Halogenated Organic Species

A number of processes are available which are capable of treating most halogenated organiccompounds. These treatments also have an additional effect of scrubbing carbon dioxide and othertrace components. Historically, most of the operational experience to date has concentrated on theremoval of carbon dioxide and the information in the following paragraphs reflects this position.

5 . 3 . 15 . 3 . 1 M e m b r a n eM e m b r a n e s e p a r a t i o n t e c h n i q u e s s e p a r a t i o n t e c h n i q u e s

The basis of this process is the differential permeability of gases through polymeric membranes.The separation polymers typically comprise bundles of very large numbers of hollow fibresarranged in a pressure vessel. When landfill gas is introduced into the vessel, carbon dioxidepasses through whilst methane is held back. This gives rise to a high pressure methane-rich gas onthe outside of the fibres and a lower pressure carbon dioxide enriched gas inside the fibres.

A single stage separation unit cannot provide very complete separation of methane and carbondioxide and typically, the low pressure off-gas (carbon dioxide enriched) may contain as much as



12% v/v methane. The product gas contains around 88% v/v methane. However, multistageseparation processes can achieve 98% v/v methane though pressures required for this operationcan be as high as 4 MPa.

The technology was developed originally by Monsanto in the USA (initially to remove carbondioxide from natural gas) but has also been used in Holland and Japan. The process (known as‘Prism’) uses hollow silicone-coated polysulphone fibres contained within a steel pressure shell.Throughputs of up to 100 m3 h-1 have been achieved. Another process (known as ‘Separex’) wasdeveloped using spiral-wound cellulose acetate membranes packed in pressure tubes. This wasused for flows of up to 2360 m3 h-1 at Portland landfill in Oregon. The Puente Hills Separex plantis illustrated in Plate 5.1 below.

One of the first membrane separation plants to operate on landfill gas was at Florence, Alabamawhich, in 1983 produced approximately 60 m3 h-1 of SNG containing 90% v/v methane.



Plate 5.1 The Separex membrane separation plant at Puente Hills

In Holland, two plants have been set up to produce SNG from landfill gas. One at Vasse(Tubbergen) with a design capacity of 200 m3 h-1 of SNG commenced operation in May 1992 andthe other, with a slightly smaller capacity (150 m3 h-1) commenced operation in mid-1993. Furtherdetails of the Vasse plant are given in Environment Agency (2002h) Case Study 10, andWeberpolder in Case Study 11.

5 . 3 . 25 . 3 . 2 P r e s s u r e s w i n g p r o c e s s e sP r e s s u r e s w i n g p r o c e s s e s

Pressure swing processes rely on the selective adsorption of carbon dioxide on the surface ofspecial porous solid adsorbents. The adsorption takes place at elevated pressure and the separationtakes place when the pressure on the adsorbent is relieved - hence the name, ‘Pressure swing’adsorption or PSA. Cleanup plant utilising PSA operate in four steps:

• high pressure adsorption;

• depressurisation to ambient;

• vacuum stripping of carbon dioxide; and

• repressurisation of product.

There are two basic adsorbent types that have seen some use in the development of landfill gascleanup:

• molecular sieves; and

• activated carbon beds.

These are discussed in the following subsections.



Molecular sieve processes

A molecular sieve is essentially a packed bed of granular material that has special adsorptionproperties which vary depending on the type of gas. The granular materials which can be used aretypically aluminosilicate minerals called zeolites. These materials are characterised by large openstructures with numerous open channels which can effectively adsorb carbon dioxide.

The process can only be operated in a batch-wise way, so that an operational treatment plantrequires multiple cascaded vessels, some of which act to remove carbon dioxide and others (withspent zeolite) operate in a recharge mode. For a molecular sieve to be effective, the raw landfillgas must be pretreated to remove sulphides (especially hydrogen sulphide), dried to remove waterand water vapour and have a low concentration of nitrogen (nitrogen is not removed by themolecular sieve).

A simplified process flowsheet for a molecular sieve gas cleanup plant is shown in Figure 5.1.The process was developed by GSF Energy Inc. in the USA and operated for a time at the PalosVerdes site in California. A similar system was also developed for the Mountain View landfill, alsoin California.

Activated carbon beds

High pressure landfill gas is adsorbed on a bed of activated carbon. The bed is then depressurisedand methane and carbon dioxide desorb at different rates allowing a separation to be made. Inorder to provide a continuous flow product (since the process is batch-wise), a number of vesselsare configured such that some are adsorbing whilst others are yielding product in the desorptionphase.

A simplified flowsheet for this process is shown in Figure 5.2.

A scheme using this process was developed in Germany by Bergbau-Forshung GmbH comprisingthree separate beds made up with a proprietary ‘carbon molecular sieve' material.

Examples of pressure swing adsorption technologies are given in Environment Agency (2002h)Case Studies 7 – 9, and Case Studies 12 – 13 where the technology is used in series with othercleanup approaches.



Raw Landfill Gas

Knockout Drum

Compressor

Condensate

Drier & H2S Stripper S

ieve

1

Sie

ve 'n

'

Sie

ve 2

Sie

ve 1

Sie

ve 'n

'

Sie

ve 2

Compressor

Condensate

SNG Product

Operating Sieves

Regenerating Sieves

Figure 5.1 Simplified Flowsheet for a Molecular Sieve Gas Cleanup Plant

Regenerator Gas Heater

Knockout Drum

Compressor

Ra

w L

an

dfil

l Ga

s

Act

ive

an

d R

eg

en

era

tin

g B

ed

s

Cooler

Treated Gas

Re

cycl

e/F

ue

l Fe

ed

Condensate

Condensate

Figure 5.2 Simplified Flowsheet for a Pressure Swing Adsorption Plant using ActivatedCarbon beds



5 . 3 . 35 . 3 . 3 L i q u i d a b s o r p t i o n / s o l v e n t s c r u b b i n g p r o c e s s e sL i q u i d a b s o r p t i o n / s o l v e n t s c r u b b i n g p r o c e s s e s

There are a number of proprietary and developmental processes which use organic solvents totreat raw landfill gas to remove carbon dioxide, moisture and contaminants such as hydrogensulphide. The processes, which originated in the USA, all operate on the same principal and differfrom one another mainly in the solvent used. The basic objective of the process is to treat rawlandfill gas and produce a saleable SNG product.

One example, installed at Pompano landfill, Florida in 1985 had a throughput of over 4000 m3 h-1

of raw landfill gas giving around 2000 m3 h-1 of SNG. This plant used a 50% aqueous solution ofa tertiary amine as the solvent (MDEA, or methyldiethanolamine) which removed almost all of thecarbon dioxide and hydrogen sulphide.

The process was described by Dinsmore (1987) and may be summarised as comprising threebasic stages:

• compression of raw landfill gas to 2 mpa (around 20 bar);

• treatment in an amine contactor column; and

• drying and further compression (to 43 mg m-3 and 3.45 mpa, respectively) for onwardtransmission.

A simplified flowsheet for the process is shown in Figure 5.3.

2-Stage Compressor

Single-Stage Compressor

Drying Unit

Raw Landfill Gas

SNG Product

Wet SNG

Dry SNG

Am

ine

Con

tact

Tow

er

CO

2 F

lash

CO

2 S

tripp

er

Rich Amine

Lean Amine

CO2

Water vapour

Hydrocarbon Condensate Condensate

Figure 5.3 Simplified Flowsheet for Solvent (MDEA) Scrubbing Cleanup Plant

A variation on the theme of solvent scrubbing is a process developed to pilot scale in the UKwhich utilised a hydrocarbon oil as a solvent. The trace components in the landfill gas were partlyremoved in a counter current tower down which the solvent oil flowed. The contaminated oil wasregenerated in a vacuum stripping tower and the gaseous contaminants flared off. Pilot scale trials



showed successful reduction of concentrations of chlorinated compounds and siloxanes. However,removal of the complex mixture of halogenated compounds in the raw landfill gas at the designflow (600 m3 h-1) was not achieved. Further details of this process are given in Case Study 6 ofEnvironment Agency (2002h). Photographs of the decommissioned plant are given in Plate 5.2below.

Plate 5.2 A UK based pilot scale hydrocarbon oil-based scrubbing plant

The key characteristics of a solvent with potential use in gas cleanup are as follows:

• high affinity for acid gases (in particular carbon dioxide);

• low bond strength with absorbed gases;

• low affinity for alkanes (methane);

• low vapour pressure at ambient temperatures; and

• high motility (i.e. low viscosity).

There are several solvents which are reputed to meet these requirements and as a result have seenuse in gas cleanup, as outlined below.



Selexol™

Selexol™ is a proprietary solvent derived from a dimethyl ether of polyethylene glycol (originallydeveloped by Allied Chemical Corp. and later licensed by GSF). In addition to the properties listedabove, it is both non-toxic and non-corrosive and is suitable for removing carbon dioxide,hydrogen sulphide and water vapour. Its chief disadvantage is a relatively high cost (the equivalentof £4.40 (year 2000) litre-1). The 'Rich' solvent (i.e. that which has passed through the processand is saturated with the gas contaminants) can be regenerated using a series of flashdepressurisation and air stripping columns. The solvent has been used at a number of US landfills(for example, Monterey Park, California; Calumet City, Illinois; and Fresh Kills, NY). Somecolour plates illustrating the process plant at Brea Olinda and Mountain Gate (California) are givenin Plate 5.3 below.

Plate 5.3 Selexol™ plants at Brea Olinda and Mountain Gate, California

New plant costs are estimated at £500,000 for 3500 m3 h-1 plant including contactor, chiller,pumping, and above ground pipework. Existing U.S. plants (all owned and operated by GSFEnergy LLC) include Staten Island NY, Brea-Olinda CA, West Los Angeles CA, Kearney NJ andHouston TX. Typical feed gas flow rates on these sites is 5000 to 10000 m3 h-1 and is delivered tothe unit at approximately 2.8 Mpa. The Selexol™ circulation rate is approximately 30 litres s-1.

Kryosol

The Kryosol process uses methanol pressurised to 2.8 MPa and chilled to around -70 °C and canabsorb carbon dioxide, acid gases and water vapour. Typically, the process is split into twostreams, in one the gas is dehydrated and the other has carbon dioxide removed. The 'Rich'solvent streams (i.e. those which have passed through the process and are saturated with the gascontaminants) are regenerated by a combination of flash evaporation and light heating.

Urcarsol-CR (DEA)

This process was developed by the John Zink Co and uses a proprietary solvent called Ucarsol-CR. Alternatively, Diethanolamine (DEA) can be used. Both solvents remove carbon dioxide andthe process requires pre-treatment stages to remove moisture and other hydrocarbons by acombination of refrigeration and adsorption on activated carbon. The system has been used atScranton landfill, Pennsylvania.



MEA (Monoethanolamine)

Developed originally for processing natural gas by removing carbon dioxide and hydrogensulphide, the process uses an aqueous solution of Monoethanolamine (MEA) in a pressurisedscrubbing tower, followed by regeneration by flash evaporation and steam stripping. Use of thesolvent suffers a number of disadvantages, principally the high rate of loss of MEA duringregeneration; the high process thermal load; and breakdown by-product formation. In addition, the'Rich' solvent (saturated with carbon dioxide) is extremely corrosive. Further detail is given byZimmerman et al. (1985) and Henrich (1983).

5 . 3 . 45 . 3 . 4 W a t e r s c r u b b i n g p r o c e s s e sW a t e r s c r u b b i n g p r o c e s s e s

The basis of the process is high pressure scrubbing of the raw landfill gas with pressurised water.This removes a significant proportion of the acid gas contaminants including carbon dioxide whichcan be released from the wash water in an air or steam stripping tower. The resulting 'regenerated'water can be recirculated for further use. The chief disadvantage of water scrubbing is the verylarge power consumption associated with pumping and handling the circulating flows. Withoutsuch flows, the product yield would not have a suitably low concentration of carbon dioxide. Theprocess also removes hydrogen sulphide.

A simplified flowsheet for this process is shown in Figure 5.4 and a case study is discussed inEnvironment Agency (2002h) Case Study 5.

A proprietary process using water was developed by Central Plants Inc. and called the BinaxProcess. The product gas contains up to 98% v/v methane and less than 2% v/v carbon dioxide.Plant sizes range from 300 m3 h-1 to 1770 m3 h-1.

A variation on water scrubbing which could offer lower operational costs for landfill gas cleanup isscrubbing with an aqueous solution of potassium carbonate - the so-called ‘hot carbonate’ process.This process has been in existence for many years and was originally developed for use intreatment of ‘sour’ natural gas. The process is well-suited to treatment of gases with moderateconcentrations of carbon dioxide and low concentrations of hydrogen sulphide.



Col

umn

Stri

pper

KnockoutDrum

Compressor

Steam

Land

fill G

asR

aw 'Aci

d' G

as

Col

umn

Abs

orpt

ion

Heat Exchanger

Land

fill G

asT

reat

ed

Condensate

Figure 5.4 Simplified flowsheet for Cleanup Plant Using Water Scrubber

Typically, the carbonate solution is relatively stable although it can be neutralised by the presenceof sulphur dioxide and degraded by the presence of carbon monoxide. The overall efficiency ofthe process is not that high (unlikely to be able to yield a high grade SNG) although doping of thesolution with additives called promoters (typically amines which selectively enhance the rates ofsorption) can improve performance.

Benfield, Catacarb and Flexsorb HP are all examples of commercial developments of the processthat have seen use in the natural gas processing industry. These processes are capable of reducingcarbon dioxide concentration to less than 2% v/v and hydrogen sulphide to about 10 ppm v/v.

5 . 3 . 55 . 3 . 5 C r y o g e n i c p r o c e s s e sC r y o g e n i c p r o c e s s e s

Cryogenic processes involve gas cooling and liquefaction to afford separation and purification.There are two approaches to gas cleanup using cryogenic stages:

• liquefaction of methane; and

• liquefaction of carbon dioxide.

In the case of cleanup of landfill gas, the technique is most appropriately applied to theliquefaction of methane from a pretreated stream from which carbon dioxide has been removed.The basic aim is to remove nitrogen, which, when present in the raw landfill gas, passes directlythrough other cleanup techniques.

The boiling point of methane is –162°C and therefore, to achieve liquefaction, the process shouldhave adequate refrigeration and heat exchange capacity to achieve this temperature. Typically,several stages of counter current heat exchange are employed to cool the gas stream prior totreatment in a partial condensing rectification tower. The waste stream comprises a mixture ofmainly nitrogen with a small amount of residual methane which is either vented to atmosphere orblended and flared.

The outline flowsheet is shown in Figure 5.5.



Compressor

Ref

riger

atio

n T

ower

PrimaryHeat Exchanger

SecondaryHeat Exchanger

Pretreated Gas

Methane Gas Liquid Methane

Nitrogen-rich Offgas

Figure 5.5 Cryogenic Treatment to Remove Nitrogen

The alternative approach is to treat raw landfill gas, liquefy a proportion of the carbon dioxidecontent and fractionate the methane carbon dioxide mixture. This requires operation at a pressureof around 5 MPa and a temperature no lower than about -70 0C. Prior to the cooling stage, theraw gas is treated to remove water vapour using an adsorption technique (molecular sieve oractivated carbon bed). This gives rise to a product stream with 90% v/v methane. To increase themethane content further requires a second stage of purification which may be:

• further liquefaction using additional refrigeration;

• liquid absorption of carbon dioxide followed by secondary drying; or

• secondary adsorption of carbon dioxide.

A simplified flowsheet for this process is shown in Figure 5.6.



Col

umn

Dem

etha

nisi

ng

KnockoutDrum

Compressor

Land

fill G

asR

aw

'Aci

d' G

as

Ref

riger

atio

n

Heat Exchanger

Land

fill G

asT

reat

ed

Condensate

Col

umn

Dry

ing

RecoveryHeat

Figure 5.6 Simplified Flowsheet for Carbon Dioxide Liquefaction

5 . 45 . 4 Si loxane Pre-treatmentSi loxane Pre-treatment

Organic silicon compounds within landfill gas are predominantly in the form of organo-siloxanes orsilicones. Some of the major sources of these compounds within landfill gas include household andindustrial sources: cosmetics (carrier oils); detergents (anti-frothing agents); building materials(impregnating oils); paper coatings and textiles. The detrimental effect of siloxanes has beendescribed in Section 2.2.5. There is currently no standard method for treating landfill gas toeliminate or minimise siloxanes. Caterpillar and Waukesha, in the US, favour dropping the gastemperature to about 4 °C in a chilling step, followed by a coalescing filter/separator to removeadditional moisture. The analytical data reported from a consortium study in the US involvingCaterpillar, Dow Corning, and others were inconclusive regarding silicon reduction (Niemann etal., 1997). Effective activated carbon treatment systems reported by industry sources can becostly since the spent carbon cannot be regenerated, may incur expensive disposal costs, andactive sites in the carbon will retain water vapour and halogenated compounds which decreasecarbon life.

In the UK, Shanks experimented with a solvent liquid absorption system using a hydrocarbon oil,aimed primarily at scrubbing halogenated organics (see Section 5.3 above). This system alsoachieved 60% removal of siloxanes (Stoddart et al., 1999).

Prabucki et al. (2001) describe three methods for removing organic, stated as being capable ofreducing the treated gas to < 1mg m-3 siloxanes. A summary outlining the procedures,effectiveness of each method and the recommended range for each application is presented inTable 5.1.

For all three modules drying of the gas is required to prevent condensation of water vapour andblockage of the activated carbon sites. The first module uses a heat exchanger (from the watercooling system of the gas engine) to heat the gas to between 35 and 40 °C which prevents build upof moisture within the adsorption unit. No cleaning prior to the activated carbon unit is used,resulting is higher replacement costs compared to the other two processes. The second module



uses a compressor and heat exchanger to first cool and then warm up the gas prior to theadsorption unit. The cooling produces a watery condensate containing up to 25% of siloxanes,which also traps some hydrocarbons (olefins) increasing the useful life of the activated carbon.The gas is heated to 10 °C to take advantage of the observed increased siloxane loading capacityof the activated carbon. The third module uses a freezing procedure to initially remove up to 90%of siloxanes within the inlet gas. This saves considerable costs on the usage of the activated carbonbut requires additional electrical power for the compressors and a method of discharging the iceformed. This is a more economic method for inlet gases ranging in siloxane concentrations ofbetween 200 and 1000 mg m-3.

Table 5.1 Summary of the procedures and effectiveness of siloxane cleanup (after Prabuckiet al 2001)

Type Stage 1(gas drying)

Cleanupefficiency1

Stage 2(adsorbtion)

Siloxanelevel

Application range

GRW warm gas 35-40 °C

0% activated carbon < 1mg m-3 Siloxanes < 10 mg m-3

Gas flow rate < 150 m3 h-1

GRK Cool gas to 2°CPost-warm upto 10 °C

up to 25% activated carbon < 1mg m-3 Siloxanes < 30 mg m-3

Gas flow rate > 150 m3 h-1

GRTK Cool gas to <-30 °CPost-warm upto 10 °C

up to 90% activated carbon < 1mg m-3 Siloxanes 200 - 1000 mg m-

3

Note: 1 this is dependant upon the type of siloxane within the gas

For each of the three modules, two activated carbon adsorption units are arranged in series (for adesigned gas volume flow rate and pressure loss) with sampling valves used to monitor gas quality.Such an arrangement enables continuous operation of the module when siloxane breakthroughoccurs, as one container can be reloaded whilst the other is in operation.

Hagman et al. (2001) reported cleanup efficiencies, covering a range of volatile siloxanes foundwithin landfill gas, for a number of individual techniques: cooling to -25 °C (continuoustechnique), 25.9% cleanup efficiency; freezing to -70 °C (continuous technique), 99.3% cleanupefficiency; activated carbon (non-continuous technique), > 99.1% cleanup efficiency; and solventwashing (continuous technique), 60.0% cleanup efficiency.

In addition to those already described, there have been several studies, within Germany ,addressing the problem of removal of siloxanes. Schmidt (1997) discussed the use of a lightheating oil scrubbing system. Lenschow and Martens of Haase Energietechnik GmbH proposedthe use of a water wash process (Eur. Patent Appl. EP 955352, 1999) while Albertsen (1998)mentions an oxidation system (ESOF Process), a liquid absorption system (EASi-Wash), andactivated carbon as the three systems capable of removing > 70% of the total silicon.

American Purification, Inc. in the US has been testing a regenerative adsorption system forremoving siloxanes from landfill gas. This process consists of solid polymeric adsorbents that canbe regenerated using a microwave treatment system. However, according to company literature, infield testing at two Californian landfills, siloxane breakthrough was observed after only 12 hours.

At the present time, there is no consensus regarding siloxane treatment alternatives for landfill gas.There is no quantitative understanding of the mass balance and partitioning of silicon through alandfill gas combustion system, and there have been only a few investigations focusing on thechemistry of the gaseous, liquid (condensate, engine oil), and solid silicon phases. Case Study (18)



in Environment Agency (2002h) relates to in-engine siloxane treatment, which is an alternativeapproach.

5 . 55 . 5 Deve lopmenta l Technolog iesDeve lopmenta l Technolog ies

5 . 5 . 15 . 5 . 1 H y d r o g e n S u l p h i d e s c r u b b i n gH y d r o g e n S u l p h i d e s c r u b b i n g

A novel process for removal of hydrogen sulphide has been developed in Austria for cleanup ofdigester biogas and it is capable of adaptation for cleaning up landfill gas. The method, known asthe ‘Biosulfex’ process1, is based on the bacterial treatment in a packed bed reaction tower. Theprocess is aerobic and requires the addition of air to the landfill gas prior to treatment. This islikely to cause some difficulties in using the treated gas in a gas engine. However, high removalefficiencies are claimed, in the range of 90% to 95%, for H2S concentrations of up to 2% v/v (20000 ppm).

5 . 5 . 25 . 5 . 2 H a l o g e n a t e d o rH a l o g e n a t e d o r g a n i c s c r u b b i n gg a n i c s c r u b b i n g

A recently introduced Canadian process2 may also be suitable for treating landfill gas. The processutilises a bed of natural material (for example wood chips) appropriately seeded by micro-organisms which can remove VOCs and halogenated organics. The process appears to offersignificant cost advantages and is claimed to give a high performance (90 to 99% removal ofBTEX and halogenated compounds) whilst producing no hazardous waste products. However, theprocess was developed for treating contaminated air and it may require significant development tomake it suitable for landfill gas cleanup. A similar process is offered by Dessau-Soprin3. Again,this is an aerobic process and the treated gas may not be suitable for energy production.

5 . 5 . 35 . 5 . 3 H u m i d A b s o r p t i o n P r o c e s s e sH u m i d A b s o r p t i o n P r o c e s s e s

A further treatment option could be derived from ‘humid absorption’ technology4 which is basedon treating gases with a selected solvent in a packed tower. The process has been developed tosuit gaseous effluents from, for example, chemical plants and waste incinerators and is reported tobe able to remove acid gases, VOCs, NOx , SOx , H2S and Ammonia. Capital costs for the processare reported as in the range £0.2 million to £6.6 million for gas flows of 1700 m3 h-1 and 85 000m3 h-1, respectively.

5 . 65 . 6 Deal ing with Wastes from Secondary Cleanup ProcessesDeal ing with Wastes from Secondary Cleanup Processes

Wastes from secondary pre-treatment may be grouped into one of three categories:

• contaminated carbon dioxide offgas;

• contaminated aqueous condensates; and

• contaminated solids.

5 . 6 . 15 . 6 . 1 C o n t a m i n a t e d c a r b o n d i o x i d e o f f - g a sC o n t a m i n a t e d c a r b o n d i o x i d e o f f - g a s

The off-gas streams represent the largest arising and typically will amount to up to 5000 tonnesper annum of carbon dioxide and up to 1000 tonnes per annum nitrogen for every 1000 m3 h-1 of

1Entec Environment Technology Umwelttechnik GmbH2AirScience Technology, Montreal, Quebec H3K 1G63Dessau-Soprin, Montreal, Quebec H3G 1TZ4Mesar-Environair Inc. Quebec, Canada



raw landfill gas processed. Since the off-gas stream also contains some methane (5 - 10 % v/v),the usual approach to treatment would be to flare the stream blended with a proportion of rawlandfill gas. However, early process plant, especially those producing high grade SNG (methane~98% v/v) frequently vented the off-gas direct to atmosphere.

5 . 6 . 25 . 6 . 2 C o n t a m i n a t e d a q u e o u s c o n d e n s a t e sC o n t a m i n a t e d a q u e o u s c o n d e n s a t e s

The aqueous condensates arise in varying, although generally small rates and, typically, themajority is drained and returned direct to landfill. Condensates may also arise which may becontaminated with the process solvents and these may require specialised treatment or disposal.

The magnitude of such streams is unlikely to exceed ‘a few tonnes’ per year, since the processwill be designed to minimise loss of relatively costly solvents. However, in order that the sulphurand chlorine compounds should not be returned to the landfill, best practice is to have thesecondensates treated, or destroyed either chemically or by incineration. The sulphur and chlorineremoved should not be returned to the landfill to re-enter the gas management system.

5 . 6 . 35 . 6 . 3 C o n t a m i n a t e d s o l i d sC o n t a m i n a t e d s o l i d s

Contaminated solid wastes will comprise numerous materials, depending on the type of cleanupprocess. For example, pressure swing adsorption plant will produce batches of ‘spent’ adsorbent(of the order of 5 - 10 tonnes per year) which will typically be ‘regenerated’ to recover theadsorbent. The fate of the wastes that arise within the ‘regeneration’ part of the process is notknown, although it is likely that high temperature incineration would be used for disposal. Otherforms of solid wastes comprise:

• spent oxides (from desulphurising units), again of the order of 5 - 10 tonnes annually, whichcan also be regenerated;

• filter pads and meshes; and• other maintenance consumables.

The latter would have most likely been disposed to landfill. However, to avoid the recycling of thesulphur and chlorine compounds in this particular waste management cycle, these substances,which may well be hazardous in nature, should be separately monofilled in a non-gas generatingmonofill.



6.6. ENGINE MANAGEMENT, IN-ENGINE AND EXHAUST GASENGINE MANAGEMENT, IN-ENGINE AND EXHAUST GASTREATMENT PROCESSESTREATMENT PROCESSES


Secondary pre-treatment of landfill gas can be an expensive management option. In this section ofthe guidance document, engine management, in-engine and post combustion (exhaust) treatmentsare considered. Many of these are available at a much lower capital and operational cost and it islikely that these technologies will dominate future solutions to emissions management.

Engine management systems and in-engine treatments can be used to reduce NOx and siloxanes,and post-combustion systems can be used for NOx, CO, aldehydes, and acid halides.

6 . 26 . 2 Gas Engines and their OperGas Engines and their Opera t i o na t i o n

When considering engine management, it is first necessary to understand the operation of the gasengine.

The two principal methods used to ignite the gas and air mixture in the combustion chamber of areciprocating engine are by injection of a small quantity of diesel fuel (dual fuel engines) or withuse of a high voltage spark (spark ignition engines). More than 98% of engines used for powergeneration from landfill gas are of the spark ignition type, owing to the mass production ofnominal 1 MWe output generating sets that happen to be especially suitable for the quantity of gasthat is available from most landfill sites in the UK. These engines are also of a simplerconstruction and do not incur the added cost of diesel fuel or its on-site handling.

Gas turbines are in use, but are not currently considered a realistic option for power generationfrom landfill gas. This is because there is virtually no commercially viable applications for thesignificant waste exhaust heat that results from their lower thermal efficiency (25 – 28% in a gasturbine, compared to 38 – 42% in a spark ignition engine). Difficulties experienced in compressingthe gas to the required pressure, owing to condensation and corrosion and the fact that such unitstend to be sized at 3 MWe and above, further detracts from their selection.

Setting aside fuel constituents, the following principal engine design features influence poweroutput, efficiency and exhaust gas emissions:

• engine speed;

• bore diameter; and

• piston stroke.

Detailed design of combustion chamber related components, including cylinder heads, liners, andvalves have a secondary influence. All such design aspects are addressed when an engine typereceives certification to a defined emission standard, as in the TA-Luft approach. Once inproduction, with very significant numbers of a particular engine type in service, variation to thisprincipal design is not normally an option to significantly improve emission levels.

The most common variation on design is that of ‘wet’ or ‘dry’ exhaust manifolds. Use of ‘wet’(water cooled) exhaust manifolds means that energy is removed from the exhaust gases whichreduces the power of the turbocharger (that compresses the engine inlet air). The result is a lessthermally efficient engine, lower output and lower NOx.

Any form of thermal efficiency reduction poses penalties in the form of a higher capital cost perkWe installed and lost revenue. For example, a 5% reduction in output for plant earning 3p perunit equates to a nominal £12,000 per annum loss. When considering adoption of a cleanup



technology, the operator's main consideration relates directly to the need (engine longevity andenvironmental need) and overall cost that determines the commercial viability of the project forelectrical power generation.

Four key areas are addressed when considering the optimum balance between engine thermalefficiency, longevity and exhaust emissions:

• electronic engine management systems (EMS);

• in-engine treatment at pre-combustion or post combustion stages;

• exhaust gas after-treatment; and

• supply gas clean-up.

Supply gas cleanup has already been considered in Chapters 4 and 5. Here, the other options areconsidered.

6 . 36 . 3 Engine Management Systems and NOxEngine Management Systems and NOx

The combustion process in a modern gas engine is controlled and balanced by the electronicEngine Management System (EMS). Continuous, computer controlled adjustment of parameterssuch as engine ignition timing, air-flow from the turbocharger and cooling water temperature, isavailable on all modern gas engines fitted with analogue or digital transducers at key locations.Sampling of parameters is usually around once every 50 ms. The capability of these systems hassteadily improved and further advances are anticipated.

The complete EMS system is designed as an integrated suite of panels including control ofancillary motors, synchronising to the external electrical network, pre-detonation adjustment andtelemetry functions for remote monitoring, control and historic record. For the purpose of thisguidance, these functions are not of interest.

The operator interface is usually driven via a fascia mounted touch screen with full graphicrepresentation of the system. Faults, real time values and historic data can all be displayed. This isactually described as a Supervisory Control and Data Acquisition or SCADA system. Examplesinclude the Caterpillar “Lima”, Deutz “Tem”, and the Jenbacher “Diane” systems. These namesshould not be confused with major system components such as the air to fuel controller (e.g. theCaterpillar’s “Techjet”).

The operating conditions chosen on the EMS must achieve the balance between greater thermalefficiency (that brings with it high NOx), and the “lean-burn” condition with better destruction ofVOCs and lower NOx. Most gas engines are operated in “lean burn” mode.

Due to the ease of user interaction, setting the EMS of a typical 1MWe spark ignition engine tooperate at the level of 500 mg Nm-3 NOx rather than the currently typical value used in the UK of650 mg Nm-3 is a very cost-effective way to reduce NOx emissions, and it is recommended thatthis practice is carried out. Reducing NOx below this level can create other operational problemsand should not be deliberately encouraged. This will result in higher CO emissions than at 650 mgNm-3 NOx output and it is important to be aware that these gases tend to exhibit an antitheticrelationship and that a balance needs to be achieved between the two emissions.



It is preferential to hold NOx at 500 mg Nm-3 and allow CO emissions to rise, since NOx istypically the pollutant which requires primary control. CO will oxidise to CO2 although care shouldbe taken with high CO emissions in enclosed areas.

6 . 46 . 4 In-engine TreatmentsIn-engine Treatments

6 . 4 . 16 . 4 . 1 W a t e r i n j e c t i o n t o r e d u c e N O xW a t e r i n j e c t i o n t o r e d u c e N O x

Humidification or injection of de-ionised water (which quickly becomes steam), into the enginebefore combustion is a proven technology used extensively to reduce NOx in large industrial andmarine diesel engines. The technique has been tried with encouraging results on landfill gasburning engines and on those sites which need to reduce NOx, whilst retaining good thermalefficiency it is likely to attract greater interest. The ‘Biometer’ system that has been tested isreported to reduce NOx by 50% (see Case Study 17 of Environment Agency, 2002h)).

An added advantage of the system, if properly controlled, is to assist the removal of combustiondeposits (and potentially silica from siloxanes) from components and thus extend maintenanceperiods. Good control is essential as a moist atmosphere combined with hydrogen chloride andhydrogen fluoride produced from halogenated organic compounds in the supply gas may lead to arapid deterioration of engine components.

Capital costs are likely to be low if “humidification kits” are produced in quantity, although thefine balance currently achieved with lubricating oil formulation and other material influences wouldneed to be addressed.

6 . 4 . 26 . 4 . 2 O x y g e n e n r i c h m e n tO x y g e n e n r i c h m e n t

Increasing the oxygen content of the engine inlet air by a few percent, with associated enginetuning, can enable liquid and gaseous fuels to burn more efficiently. This has been well known fordecades, although has not been commercially viable owing to the relatively high cost ofcommercially produced oxygen and additional care for storage. Development in recent years onliquid fuel engines has enabled adequate air inlet enhancement with use of patented technologyresulting in a nominal 3m x 3m x 3m “air inlet filter” for a 1MWe engine. Refinement is expectedto enable smaller unit sizes, although most industrial installations can accommodate the additionalspace required.

Methane slippage, which tends to represent around 98% of VOCs in the exhaust, is expected to bereduced and CO levels to fall dramatically if this developing technology were implemented. Areduction in the peak cylinder pressure also assists in reducing the onset of pre-ignition (knock)and therefore a higher output may be anticipated.

The disadvantage to the process is the high levels of NOx generation, which is produced directlyas a result of higher peak combustion temperatures. Subsequent after-treatment of NOx to achieveacceptable emissions is unlikely to be commercially viable.

Capital costs are likely to be relatively low, if air inlet “kits” were to be produced in quantity.

6 . 4 . 36 . 4 . 3 E x h a u s t g a s r e - c i r c u l a t i o nE x h a u s t g a s r e - c i r c u l a t i o n

This system employs re-circulation of inert exhaust gas that reduces the peak combustiontemperature and engine efficiency. It is not currently used on landfill gas engines. If it were to beused, between 10 and 50% NOx reduction may be anticipated at minimal cost, with the addedlikely benefit of a reduction in methane slippage. However, the quantity of power loss involvedand the extent of accelerated engine deterioration as a result of re-circulating aggressive gasconstituents needs to be clarified.



6 . 4 . 46 . 4 . 4 C h e m i c a l i n j e c t i o nC h e m i c a l i n j e c t i o n

The presence of siloxanes in the supply gas can have a similar effect to the presence of acidforming gases in terms of engine wear, although unlike the ‘acid’ problems, keeping engineoperating temperatures well above dew points provides no benefits in this case. Extensive trialshave been undertaken with injection of a chemical formulation via stainless steel nozzles, locatedwithin the air inlet manifold. An automated dosing unit is programmed for frequency of applicationand spray duration to treat the particular characteristics of the contaminant, engine and operatingenvironment. The cleaning fluid is believed to be non-toxic and reacts to release a fine dry powderthat travels harmlessly through the engine and passes out with the exhaust gases.

Plate 6.1 Boroscope photographs of exhaust valves (1MWe landfill gas engine)

Trials have been carried out at six landfill site installations up to the end of 2001. It is claimed theyshow that siloxane build-up in a 1MWe engine can be effectively managed using a total of between1.5 to 2 litres of fluid per day administered in 6 doses.

The capital cost is approximately £5,500 and £5 per litre running cost per 1MWe engine. A casestudy is discussed in Environment Agency (2002h) (Case Study 18).



6 . 56 . 5 Exhaust After-treatmentsExhaust After-treatments

Engine exhaust after-treatment is not currently used for landfill engines in the UK. If and when itis, it will be necessary to determine which primary contaminants are to be treated.

6 . 5 . 16 . 5 . 1 P o s t c o m b u s t i o n t h e r m a l o x i d a t i o n o f C OP o s t c o m b u s t i o n t h e r m a l o x i d a t i o n o f C O

Post-combustion thermal oxidation is a process which is primarily intended for oxidation of CO toCO2. An additional benefit is oxidation of unburnt NMVOCs and particularly aldehydes such asformaldehyde (methanal). One such system produced by Jenbacher is the CLAIR systemdesigned specifically for biogas lean-burn engines. Further details can be found in Case Study 14of Environment Agency (2002h). Caterpillar market a similar system.

The exhaust gas is re-heated from around 540 to 800 °C, with the residual hydrocarbons (CH4

and NMVOCs) as well as CO oxidised to form water vapour and CO2 by the residual oxygenwithin the exhaust gas. It should be noted that NOx is not reduced. Reported emission levelsachievable are CO 150 mg Nm-3, total unburnt hydrocarbons 150 mg Nm-3 (as CH4) andformaldehyde 15 mg Nm-3. These are below the Agency’s emission standards (Table 2.3).

Dimensions of the entire system are around 6.7m in length by 4.2m width and 3.9m in height for anominal 1MWe generating set. Capital outlay is approximately 30% of the generating set packagewith efficiency savings that the manufacturers consider result in only a few percent additionaloverall cost.

Use of systems such as this for oxidation of CO to CO2 should be a matter of judgement, sinceCO will oxidise naturally over a two-month cycle in the atmosphere. They should only beconsidered if the Agency’s emission standard is otherwise unachievable.

6 . 5 . 26 . 5 . 2 P o s t c o mP o s t c o m b u s t i o n c a t a l y t i c o x i d a t i o n o f N O xb u s t i o n c a t a l y t i c o x i d a t i o n o f N O x

This process is known as the selective catalytic converter or SCR process. The technology isproven and consists of passing the oxygen bearing exhaust gas treated with a reactant ammonia orurea solution ((NH2)2CO) through a fine tubed honeycomb-patterned converter. The NOx isreduced to nitrogen, and water is liberated on the active SCR surface. HUG Engineering offer anadditional catalytic oxidation stage to reduce hydrocarbons and CO to CO2 and H2, by passing theSCR treated exhaust through a ceramic honeycomb coated with noble metals. It has beensuccessfully used in marine engines. This catalytic process is not generally applicable to landfill gasengines without additional pre-combustion gas cleanup, due to catalyst poisoning.

A 95% reduction in NOx can be achieved with heat recovery – a bonus if associated with a CHPscheme. The aspects of this technology which detract from its general use on landfill gas enginesare:

• landfill gas installations in the UK infrequently lend themselves to CHP;• the catalyst can be quickly poisoned by acid forming compounds within landfill gas; and• these would have to be removed (via the use of wet scrubbing pretreatment for example)

which would add significantly to the cost.



6 . 5 . 36 . 5 . 3 H a lH a l i d e s c r u b b i n gi d e s c r u b b i n g

On some landfills with very high chloride loadings and, if pre-combustion gas cleanup forhalogenated solvents has not been adopted, HCl and HF emissions may need to be the subject ofsite-specific risk assessment. If the risk assessment shows that treatment is required, there is aproduct which claims to be able to reduce HCl and HF emissions to sub-TA-Luft standards fromeven the highest observed concentrations of these components in exhaust gases.

The Absorption Modular System, described in Case Studies 15 and 16 of Environment Agency(2002h), has the potential for application to cleanup of hydrogen chloride (HCl) and hydrogenfluoride (HF) emissions from landfill gas engine exhausts. The exhaust gas containing the halidesflows through a reactor containing calcium hydroxide bonded to a honeycomb providing a largecontact surface area. The waste produced is a mixture of calcium chloride and calcium fluoride,and residual calcium hydroxide. A standard unit is capable of treating exhaust flow rates ofbetween 9000-10000 Nm3 h-1, and the replacement of the molecular monolith blocks is dependenton the inlet loading of HCl and HF in the exhaust.

It should be noted that this technology has not yet been applied to landfill gas engines.



7. 7 . CONCLUSCONCLUS IONSIONS

The treatment of landfill gas used in power generation will have both financial and environmentalcosts. A cost benefit analysis should be conducted on the financial and environmental factors thatwill arise from employing a particular treatment on the supply gas to or emissions from the engine.Although the financial issues such as reducing maintenance costs and economic viability of theutilisation scheme will be of concern to operators and power producers, the principal concern ofthe Agency is the cost benefit balance on environmental factors. Benchmark figures are given tojudge whether the cost of abatement of a particular emission will be beneficial.

The primary pretreament of the supply gas to remove particles, liquids and vapours enhances theperformance of the engines and normally gives significant financial benefits to the operator interms of reduced wear on the engine and better overall performance. The environmental costs aregenerally low and there will also be some environmental benefits. Hence, one or more methods forprimary pretreatment of landfill gas prior to use are typically installed.

Secondary pretreatment to remove chemical components in the supply gas has substantial financialcosts and will carry some environmental cost, particularly in the management of the secondarywaste arisings. A careful assessment of costs and benefits must be conducted following therecommended procedure in order to clarify the financial and environmental impact of a particulartreatment relative to the status quo.

In-engine and post-combustion emissions treatment removes some of the bulk (and trace) gasproducts of combustion that can impact on air quality. The cost of these processes should beconsidered in both the capital cost of the equipment and the reduced power output that normallyaccompanies employment of such methods. The environmental costs are generally low and theenvironmental benefits may be significant for some gas streams at particularly sensitive locations.A site-specific cost-benefit analysis is needed to determine whether in-engine treatment is justified.This should follow the systematic cost-benefit procedure recommended here to provide anauditable and common basis for comparison.

The benchmark figures for the introduction of particular abatement technology require lowprocess costs to warrant treatment for reducing methane and carbon monoxide emissions,intermediate process costs will justify treatment to reduce NOx emissions but much higher costprocesses are justified to abate elevated PM10 and NMVOC emissions.



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GLOSSARYGLOSSARY

1MWe 1 megawatt electrical output – the nominal output of most internal combustionspark ignition gas engines used in the industry today. Approximately 3MWthermal input is required to produce 1MW of electrical output.

abatement reducing the degree or intensity of, or eliminating, pollution

absorption removal of a pollutant from a liquid or gaseous supply by the uptake andretention of the pollutant into a solid or liquid

acidification continuing loss of capacity to neutralise acid inputs indicated by decliningalkalinity and increasing hydrogen ion concentration (i.e. the decrease in pH ofa solution resulting from increases in acidic anion inputs such as sulphate)

activated carbon a highly adsorbent form of carbon used to remove odours and toxic substancesfrom a liquid or gaseous supply

adsorption removal of a pollutant from a liquid or gaseous supply by collecting thepollutant on the surface of a solid material

aerobic in the presence of oxygen

after-burner a burner located so that the combustion gases are made to pass through itsflame in order to remove particulates, unburnt hydrocarbons and odours

after-cooling treatment process to separate and remove water vapour and liquids from acompressed gas supply to prevent condensation downstream

alkaline the condition of the gaseous or liquid supply which contains a sufficient amountof alkali substance to raise the pH above 7.0

alkalinity the capacity of bases to neutralize acids, for example the addition of limedecreases acidity

alkanes a group of straight chain, saturated hydrocarbons containing no double or triplebonds, includes methane

alkenes a class of unsaturated aliphatic hydrocarbons having one or more double bonds

anaerobic in the absence of oxygen

back-pressure a pressure that can cause gas or liquid to backflow into the upstream inlet gas orliquid supply if the system is at a higher pressure than that upstream

biodegradation breakdown by micro-organisms

biogas a methane-based fuel that is produced through the bio-digestion of organicmaterial

biomass term used to refer to the mass of biologically active material contained in areactor, such as a landfill

blow-down the cyclic or constant removal of water from a boiler to deter the collection ofsolids

borehole (also see wells) a hole drilled in or outside the wastes in order to obtain samples, also used as ameans of venting or withdrawing gas

boroscope a long thin rod-like device providing visual access into inaccessible areas (suchas engine combustion chambers), using a long narrow tube containing a highintensity light source and high definition optics system, which can be connectedto camera systems for obtaining images

bulk density the mass per unit volume of a substance

calorific value (CV) the number of heat units obtained by the complete combustion of a unit ofgaseous fuel, usually defined in terms of energy (MJ) released per unit volume(m3)

capital costs costs assigned to the setting up (commissioning), design, and construction of aplant for cleanup operations

capping the covering of a landfill, usually with low permeability material: permanentcapping is part of the final restoration following completion of landfill/tipping;temporary capping is an intermediate cap which may be removed onresumption of tipping

catalyst a substance that changes the speed or yield of a chemical reaction without beingconsumed or chemically changed by the chemical reaction



catalytic oxidation (1) an oxidation process that is speeded up by the presence of a substance that isitself not consumed in the reaction

catalytic oxidation (2) method of measurement for flammable gas: portable detectors measure thedifference in resistance of two pellistors, one control and one which lies in asample chamber. Flammable gas within the sample being monitored, will oxidiseclose to the surface of the pellistor changing its temperature and resistance; theamount of temperature change is proportional to the amount of flammable gasin the sample.

catalytic oxidation (3) a method for post-combustion gas cleanup, usually for CO and unburnthydrocarbons, but not normally used on landfill gas engines since the manyother components in landfill gas can poison the catalyst, rendering the cleanuptechnique ineffective

caustic any strongly alkaline material that has a corrosive or irritating effect on livingtissue, such as caustic soda (NaOH)

chiller a device that generates a cold liquid that is circulated through an air-handlingunit's cooling coil to cool a gas supply, used to dewater inlet supply gas

chlorinated solvent an organic solvent containing chlorine atoms (e.g., methylene chloride or 1,1,1-trichloromethane)

cleanup efficiency the quantity of the contaminant removed from the gas inlet due to the cleanupprocess relative to the quantity within the gas inlet, usually expressed as apercentage

coal gas gas produced in the old towns gas works by subjecting coal to heat and pressure

cogeneration the simultaneous generation of useful thermal and electric energy from thesame gaseous fuel source

combustion burning, or rapid oxidation, of a gas accompanied by the release of energy inthe form of heat and light, producing carbon dioxide and water, incompletecombustion will also produce intermediate and unburnt hydrocarbons, as wellas carbon monoxide

compression ratio an engine's static compression ratio is defined as the volume above the piston atbottom-dead-center (BDC), divided by the volume above the piston at top-dead-center (TDC). High compression ratios are generally associated withengine performance and efficiency.

condensate liquid formed when warm landfill gas cools as it travels through a collection orcleanup system, made up of mostly water with some trace hydrocarbonspresent

condensation the change of state of a substance from the vapour to the liquid (or solid) form,also a type of chemical reaction in which two or more molecules combine withthe separation of water, alcohol, or other simple substance

corrosion the electrochemical degradation of metals or alloys caused by reaction withtheir environment, which is accelerated by the presence of acids or bases

covalent bond sharing of electrons by a pair of atoms

cyclone a device that uses centrifugal force to remove large particles from a gas stream

cylinder the round hole in the engine block in which the piston(s) ride

cylinder block the main structural member of an engine in which is found the cylinders,crankshaft and other principal parts

cylinder head the detachable portion of the engine, usually fastened to the top of the cylinderblock and containing all or most of the combustion chambers

decomposition natural breakdown of organic materials by the action of micro-organisms,chemical reaction, or physical processes

degradation see decomposition

demineralising (deionising) a method for purifying water that first converts soluble salts into acids bypassing through a hydrogen exchanger and then removes them by an acidadsorbent or synthetic resin

demister cooled gas enters a demister which uses a fine spray or aerosol to separate outcontaminants which can discharged



desorption the process of removing an adsorbed material from the solid on which it isadsorbed, accomplished by heating, reduction of pressure, presence of anothermore strongly adsorbed substance, or a combination of these means

desulphurisation removal of sulphur from a gaseous fuel supply by means of wet or dryscrubbing

detonation the extremely rapid, self-propagating decomposition of an explosionaccompanied by a high-pressure-temperature wave moving at 1000-9000 m/s

diethyl sulphide chemical added to mains gas to give it an odour, also found within landfill gas

differential pressure a difference in pressure between two points, indicative of a pressure dropacross a section of piping, orifice or vessel

diffusion gas movement from a region of high concentration to a more dilute region dueto the difference in concentration

digester a closed tank or unit in which bacterial action is induced and accelerated inorder to break down organic matter and establish the proper carbon to nitrogenratio

drying the process of reducing or removing moisture from the gas supply

effluent the medium discharged from a process, such as landfill gas vented from carboncanisters or liquid released from a treatment system

emission a material which is expelled or released to the environment, usually applied togaseous or odorous discharges to atmosphere

end-of-pipe technologies such as scrubbers on exhaust stacks and catalytic converters thatreduce emissions of pollutants after they have formed

environmental impact the total effect of any operation on the environment

exhaust valve device to discharge the burned gases from the combustion chambers

exhaust manifold that part of the exhaust system that carries the exhaust gases from cylinders tothe exhaust pipe

exothermic reaction a chemical or biochemical reaction which results in the production of energy

filtration a treatment process for removing solid (particulate) matter from gases orliquids by means of porous media

flame ionisation detector detector based on the principle of conduction by ions produced when theanalyte is ionised in a hydrogen/air flame, the resulting voltage change isproportional to the concentration of the analyte

flammability limits limits of the flammable range

flammability range range over which the concentration of a flammable compound and oxygen aresuch that the mixture can burn

flammable a substance supporting combustion in air

flue gas exhaust gas coming out of a chimney or stack after combustion in the burner itis venting; can include nitrogen oxides, carbon oxides, water vapor, sulphuroxides, particles and many trace pollutants

fluorocarbon a number of organic compounds analogous to hydrocarbons in which thehydrogen atoms have been replaced with fluorine

freon trademark for a series of fluorocarbon products used in refrigeration and air-conditioning equipment, as blowing agents, fire-extinguishing agents, cleaningfluids and solvents

gas aggressiveness index an index commonly used to indicate the degree of acidic components (Cl and F)

of the landfill gas for a site, determined as ( )%4

%1002

CHFtotalCltotal + ,

where total Cl and total F are the chlorine and fluorine concentrations (mg/m3)and CH4% is the methane concentration (%v/v) of the inlet gas

gas chromatograph analytical method that utilises a gaseous mobile phase with either a liquid (GLC)or solid stationary (GSC) phase

halocarbon a hydrocarbon which contains a halogen functional group (e.g. fluoride,chloride, bromide, etc)



halon bromine-containing compounds with long atmospheric lifetimes whosebreakdown in the stratosphere causes depletion of ozone (often used infirefighting)

head pressure often used in the context of pressure exerted by a standing fluid,usually water

heat exchanger a reaction chamber in which the flow of hot exhaust gases can be reversed via aswitching unit in order to minimise heat losses and energy requirements

honeycomb structure or supported network within a container, providing a large surfacearea of a compound ensuring even flow of process gas with which it interacts toremove specific contaminants

hydration the reaction of molecules of water with a substance in which the H-OH bond isnot split, also the strong affinity of water molecules for particles of dissolved orsuspended substances

hydrocarbon a chemical compound containing hydrogen and carbon

hydrogen sulphide (H2S) gas emitted during organic decomposition, with an odour of rotten eggs andwhich, in high concentration, can kill or poison

ignitable capable of burning or causing a fire

infra-red detector an instrument that measures adsorption in the infra-red range of theelectromagnetic spectrum

in-situ in its original place, remaining at the site or in the subsurface

landfill gas all gases generated from landfilled waste

lean burn method of combusting weak fuel-air mixtures which are homogenous incharacter in order to reduce the emission pollutants of carbon monoxide, nitricoxide and hydrocarbons and at the same time improve fuel consumptionefficiency, thereby lowering carbon dioxide emissions

limit of detection (LOD) the minimum concentration of a substance being analysed that

lower detection limit (LDL) has a 99 percent probability of being identified; the smallest

lower limit of detection signal above background noise an instrument can reliably

(LLD) detect.

lower explosive limit (LEL) the lowest percentage by volume of a chemical with air which will allow anexplosion to occur in a confined space at 25oC and normal atmosphericpressure, and where an ignition source is present (% v/v)

mains gas a gas of natural origin distributed through underground pipes to domestic,commercial and industrial customers

mass spectrometer analytical method by which components within a mixture are separatedaccording to their molecular weight

membrane separation a gaseous fuel is compressed and filtered to remove carbon dioxide using aselective membrane (polyamide) unit

methane the hydrocarbon of typically highest concentration in landfill gas

micrometeorology atmospheric characteristics in the immediate vicinity of the study area

mist liquid particles measuring 40 to 500 µm, are formed by condensation of vapor

moisture content percentage of water or steam contained in a sample of landfill gas, usuallydetermined by sorption onto an inert absorbent medium

molecular sieve a microporous structure composed of either crystalline aluminosilicates (suchas zeolites), with an ability to selectively adsorb water or gaseous moleculeswithin the sieve cavities

multiple system a system consisting of a number of cleanup processes in sequence, oftenassociated with production of synthetic natural gas

neutralisation decreasing the acidity or alkalinity of a substance by adding alkaline or acidicmaterials, respectively

operating costs costs assigned to the operation of the plant which will include the cost ofreplacement or regeneration of reagents used in the cleanup process, labourcosts are not generally included unless otherwise stated



organic (strictly) pertaining to the chemistry of carbon, from a time when organicchemicals were synthesised from living matter; (broadly) any moleculecontaining a combination of carbon, hydrogen and possibly other elements

organosilicon an inorganic compound in which silicon is bonded to carbon (organosilane), thesilicon carbon bond is about as strong as the carbon-carbon bond and hassimilar properties to all-carbon compounds

osmosis the passage of a liquid from a weak solution to a more concentrated solutionacross a semipermeable membrane that allows passage of the solvent (water)but not the dissolved solids

oxidation the chemical addition of oxygen to break down pollutants or organic waste, e.g.destruction of chemicals such as cyanides, phenols, and organic sulphurcompounds by bacterial and chemical means

packed tower a device that forces dirty gas through a tower packed with e.g. crushed rock orwood chips and (in a dry tower) the solid reactant medium, or (in a wet tower)the reactant liquid. In a wet tower the liquid is sprayed downwards over thepacking material, with the gas flowing counter-current. Components of the gaseither dissolve or chemically react with the liquid or solid reactant medium.

parts per million (ppm) Method of measuring concentration: 10000 ppm v/v equates to 1% gas at STPby volume (ppm v/v = part per million by volume)

partial pressure refers to the pressure of an individual gas constituent as part of a mixture

pH an expression of the intensity of the basic or acid condition of a gas or liquid;may range from 0 to 14, where 0 is the most acid and 7 is neutral

piston ring an open-ended ring that fits into a groove on the outer diameter of the piston,chief function is to form a seal between the piston and cylinder wall

polycyclic (organic) chemical compound where the atoms form more than one ringstructure

pre-treatment processes used to reduce, eliminate, or alter the nature of gaseous pollutantssources before they are discharged into the main treatment system

pre-chilling treatment process to separate and remove water vapour and liquids from acompressed gas supply to prevent condensation prior to entry into a system

pressure swing adsorption a process whereby the landfill gas is compressed, dried and upgraded to removethe carbon dioxide to yield a product containing 95% to 98% methane

pressure water scrubbing separation of landfill gas, to yield a purified methane product, usually takingplace in a countercurrent water spray tower

primary treatment first steps in gas treatment, usually associated with the removal of particulates

purifier system which removes extraneous materials (impurities) by one or moreseparation techniques

radical an ionic group having one or more charges, either positive or negative (e.g. OH-

or NH4+)

reagent any substance used in a reaction for the purpose of detecting, measuring,examining or analysing other substances

reduction the addition of hydrogen, removal of oxygen, or addition of electrons to anelement or compound

regeneration restoration of a material to its original condition after it has undergonechemical modification

relative humidity ratio of the amount of water vapor actually in the air compared to the amountof water vapor required for saturation at that particular temperature andpressure, expressed as a percentage

reprocessing treatment of spent material after it has undergone chemical modification torecover the unconsumed fraction of the material

retail price index (RPI) used to convert costs between two different years, taking into account inflationand other factors

reticulation landfill gas filtered to (or approaching) standards of natural gas with a highmethane content (> 85% v/v) via the use of suitable cleanup technology(ies),for supply to the national gas grid

retrofit addition of a pollution control device on an existing facility without makingmajor changes to the generating plant, also called backfit



reverse osmosis a treatment process used in gaseous and liquid systems by adding pressure toforce the gas or liquid through a semi-permeable membrane, to removecontaminants

saturation the condition of a liquid or gas when it has taken into solution the maximumpossible quantity of a given substance at a given temperature and pressure

scrubber pollution device that uses a spray of water or reactant or a dry process to trapcomponents of the gas mixture

secondary treatment the second stage in a waste treatment system in which organic parts of the wasteare treated, accomplished by bringing together waste, bacteria, and oxygen intrickling filters or in an activated process to remove floating and settleablesolids and about 90 percent of the oxygen-demanding substances andsuspended solids

sewer gas gas produced by the decomposition of organic compounds in sewerage

silicone organosiloxane, any of a large group of siloxane polymers based on a structureconsisting of alternate silicon and oxygen atoms with various organic radicalsattached to the silicon

siloxane,oxosilane a straight chain compound consisting of silicon atoms single bonded to oxygenand arranged so that each silicon atom is linked with four oxygen atoms

solubility the amount of mass of a compound that will dissolve in a unit volume ofsolution, aqueous solubility is the maximum concentration of a chemical thatwill dissolve in pure water at a reference temperature

solvent a substance capable of dissolving another substance (solute) to form auniformly dispersed mixture (solution) at the molecular or ionic size level,includes water and organic solvents

sorption the action of soaking up or attracting substances, used in many pollutioncontrol systems

spray tower scrubber a device that sprays an alkaline solution into a chamber where acid gases arepresent to aid in neutralizing the gas

sour gas a gas containing hydrogen sulphide (H2S)

substitute or synthetic any gaseous fuel approximately equivalent in performance to natural gasnatural gas (SNG) that is created from other gases

sump a pit or tank that catches liquid runoff for drainage or disposal

surfactant a detergent compound that promotes lathering

tertiary treatment advanced cleaning of gas that goes beyond the secondary or biological stage,removing nutrients such as phosphorus, nitrogen, and most BOD and suspendedsolids

thermal conductivity meter detector which compares the thermal conductivity of a sample with an internalstandard: the higher the total flammable gas concentration the higher thethermal conductivity

thermal treatment use of elevated temperatures to treat exhaust emissions

total dissolved solids (TDS) the total mass of dissolved mineral constituents and chemical compounds inwater, which form a residue that remains after evaporation and drying

total reduced sulphur (TRS) total reduced sulphur comprises the sulphur present in hydrogen sulphide,mercaptans, dimethyl sulfide, dimethyl disulphide or other organic compounds,all expressed as hydrogen sulphide (does not include sulphur dioxide, sulphurtrioxide, or sulphuric acid)

turbocharger an exhaust driven pump that compresses intake air and forces it into thecombustion chambers at higher than atmospheric pressures, the increased airpressure allows more fuel to be burned and results in increased power output

upper detection limit the largest concentration that an instrument can reliably detect

upper explosive limit (UEL) the highest concentration of mixture of a compound and air which will supportan explosion at 25oC and normal atmospheric pressure and in the presence of aflame (%v/v)

uptime the period of time (stated as a percentage) over which the landfill gas engine isrunning continuously at full load or power output rating

valve a device that controls the pressure, direction of flow or rate of flow of thecombusted gas within an engine



valve seat on the valve inlet (orifice or seat) the disk (or plug or seal) that seals against theorifice

vapour pressure a measure of a substance's propensity to evaporate, vapour pressure is theforce per unit area exerted by vapour in an equilibrium state with surroundingsat a given pressure. It increases exponentially with an increase in temperature.A relative measure of chemical volatility, vapour pressure is used to calculatewater partition coefficients and volatilization rate constants.

volatile organic compounds organic compounds that are volatile or in a gaseous state at ambienttemperature and are found within landfill gas in trace quantities

well head the top portion of a well, usually containing a valve, and various monitoringparts

well a shaft installed in wastes or strata for the extraction of landfill gas (also seeborehole)

Wobbe Index ratio of the calorific value of a gas per unit volume and the square root of itsrelative density under the same reference conditions, it reflects the gas/airrelationship and combustion characteristics expressing the heat impact which aburner is exposed to during combustion

working area the area or areas of a landfill in which waste is currently being deposited

EA Guidlines gas_treatment_jan_2003

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