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Combustion Combustion -- Mazlan 2011Mazlan 2011UTM

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UNIVERSITI TEKNOLOGI MALAYSIAUNIVERSITI TEKNOLOGI MALAYSIA

Advanced Combustion Advanced Combustion MMJ 1443MMJ 1443

Assoc. Prof . Dr Mazlan Abdul WahidFaculty of Mechanical EngineeringUniversiti Teknologi Malaysia

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POLLUTANTPOLLUTANT

Objectives :

•to sensitize the student on the very importanttechnological and social problem of air pollution.

•to present the nature of pollutants emitted fromcombustion sources.

•to discuss some features of the chemistry of pollutantgeneration.

•to present typical techniques used to reduce pollution.

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The combustion of fossil fuels results in exhaust gases thatcontribute toair pollution. Air pollution is the presence of undesirable material in theair, in quantities large enough to cause harmful effects.

Some comments on air pollution

Why do we care?

There are various reasons why we must fight air pollution. Wecouldcategorize these reasons as:

•••• economic, e.g. destruction of property by a particular chemical;

•••• aesthetic, e.g. destruction of monuments from acid rain andtheunsightly haze in the atmosphere;

•related to sustainability: carbon dioxide contributes to thegreenhouse effect and acid rain destroys forests; and

•related to health: some pollutants cause serious harm, either immediately upon exposure or long-term, and even death.

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Smog•This word comes from “smoke” and “fog”

•Used to describe serious atmospheric pollution over cities.

•Smog is the manifestation of air pollution that more than anythingelse has forced society to act and control pollutants.

•Smog is usually a “cocktail”“cocktail” ofof chemicalschemicals

(NOx, organic vapours, ozone) and solid particles. If themeteorological conditions are such that the gases above thecities are not replenished, serious respiratory and otherhealth problems may result.

•The typical brown color over polluted cities with smog is dueto solidparticles and NO2.

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Examples of Catastrophic Air Pollution:

1911 in London. 1150 died from the effects of coal smoke. Author ofthe report coined the wordsmog for the mix of smoke and fog that hungover London.

1952in London. 4000 died from smog.

1948 in Donora, Penn. Town of 14,000 people. 20 died and 6000 wereill from smog from the community's steel mill, zinc smelter,and sulfuricacid plant.

1963in New York City. 300 people died from air pollution.

Costs of pollution:

Health: $36 billion in sickness annually—health care and lost work

Agriculture : up to 10% of nation's crops lost to all forms of pollution.

Materials: corrosion—$5.5 billion annually.

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Types of Air Pollution:

PrimaryPrimary airair pollutantspollutants: harmful chemicals that enterdirectly into the atmosphere.

SecondarySecondary airair pollutantspollutants: harmful chemicals that form fromother substances in the atmosphere.

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CarbonCarbon oxidesoxides:: TopTop twotwo productsproducts ofof pollutionpollution..

Carbon dioxide (CO2) and carbon monoxide(CO).

Produced by motor vehicles and industries, especially

power plants.

Carbon dioxide: most influential greenhouse gas.

Carbon monoxide: reduces the body’s ability to

transport oxygen. (Linked to Ozone production).

Hydrocarbons:Hydrocarbons:

Methane: From rice paddies, gas drilling, landfills,

cattle, termites. Important greenhouse gas, so

affects global climate change.

Non-Methane Hydrocarbons: Produced by vehicles

and other causes of fuel combustion. Causes

photochemical smog, cancer-causing, respiratory

tract irritants, and some mutagens.

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NitrogenNitrogen oxidesoxides

Nitric oxide (NO), nitrogen dioxide (NO2), and nitrous oxide (N2O).

Produced by vehicles and industries.

Involved in: (1) acid deposition

(2) global warming

(3) ozone depletion

(4) photochemical smog

Inhibits plant growth, aggravate health problems

SulfurSulfur oxidesoxides::

Produced by fossil fuel burning, industry, biomass burning.

Sulfur dioxide—involved in acid deposition; corrodes

metals and damages stone, irritates respiratory

system. (SO2 reacts with enzymes, impairing

their functions.)

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SuspendedSuspended ParticulateParticulate MaterialsMaterials (Dust)(Dust)::

Produced by construction, industry and vehicles.

Inhaled into respiratory system, it can cause health

problems, including scarring lungs and cancer.

Ozone: secondary pollutantOzone: secondary pollutant

Caused by nitrogen oxides and carbon monoxide,

hydrocarbons, and light.

Component of photochemical smog; reacts with cell

components, destroying cells—emphysema.

Industrial smog: smoke + fog.

Photochemical smog: sulfur oxides, nitrogen oxides, ozone, and suspended particulates, along with sunlight

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Inversion Layers: When air near the ground is colder than the air above it, thepollution stays trapped in near the ground. Occurs

(1) when a high pressure system stalls, (2) in cities in valleys, near thecoast or surrounded by mountains

Dust Domes: high concentration of fuel combustion and heat from concrete(urban heat islands). Causes particulates to be trapped in adome over the citybecause the air is warmer than the surrounding area.

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The total emissions of the regulated pollutants (i.e. controlled bylegislation) have been categorized according to sectors ofeconomicactivity of the percentage contribution

Transportation and electricity generation make the highest contributionfor almost all pollutants and most of it is related to combustion.

Estimates of the US emissions of the major regulated pollutants for1991 (million tons per year) per category of source. (From deNevers,1995).

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FuelFuel contributionscontributions

When we burn fuels, we have to deal with the impurities they mayalready contain, but also with pollutants that may be generated duringcombustion, for example nitric oxides (NOx) and soot.

Clean fuels like natural gas, propane, and good-quality gasoline orkerosene do not contain impurities. In contrast, some fuelscontainsubstances that, either directly or in modified form after passing throughthe flame, may cause harm when released.

For example:

•••• some heavy oils and diesels may containsulphur and traces ofmetals (e.g. from the refining process);

•••• coal contains sulphur andnitrogen (which contributes to NOx, seelater);

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•refuse-derived fuel (RDF), i.e. fuel briquettes derived frommunicipal waste, may containchlorine or heavy metals;

•various fuel-like substances used in incinerators (e.g. car tires,plastics, municipal waste, hazardous wastes) may releasehundreds of toxic substances if burned in the wrong way.

•Hence fuel-switching is an obvious method to reduce the amountof a particular pollutant emitted (e.g. from sulphur-containingcoal to natural gas). Of course, this may lead to the emissionofmore of another pollutant.

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CarbonCarbon dioxidedioxide

All the carbon in the fuel will eventually be transformed to carbondioxide in the atmosphere. Even if we have incomplete combustion(hence emitting CO from the flame), the CO will be oxidized toCO2 inthe atmosphere.

Carbon dioxide is a major contributor to the greenhousegreenhouse effecteffect, whichis a term denoting the warming of the atmosphere due to the CO2absorbing part of the radiation emitted by the earth surface.

This may then lead to globalglobal warmingwarming.

Using fossil fuels will invariably lead to CO2 production and hence, atthe very least, we should make sure we burn fuels as efficiently aspossible. Power generation is an important player in the public debateconcerning global climate change.

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CarbonCarbon monoxidemonoxide

Incomplete combustion results in CO formation.

Carbon monoxide is extremely dangerous and can cause death ifinhaled in large concentrations.

It is mostly emitted by cars and is caused by engines with richflames,incomplete mixing, flame quenching close to the cylinder walls, and ingeneral by combustion with too short residence times.

The use ofcatalysts has helped to reduce the problem considerably byensuring the oxidation of CO to CO2.

The CO emission from industrial sources is minimized by using excessair and ensuring good mixing between the fuel and the air. Because theCO-O2-CO2 equilibrium is attained quickly, an equilibrium calculati onof the combustion products will very often give a reasonableestimatefor the amount of CO emitted.

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SulphurSulphur oxidesoxides

When burning fuels with sulphur, like coal and diesel, all of thesulphur will be oxidized in the flame into SO2 and SO3, collectivelycalled SOx. This poses serious problems because:

(i) Sox dissolves in clouds to form sulphuric acid, which canthen bedeposited to the earth by rain. This is called “acid“acid rain”rain” and hascaused deforestation in Europe and North America and seriousdamage to structures (monuments, steel buildings).

(ii) SOx is a respiratory irritant and in large concentratio ns can causedeath. Sulphur-rich coal combustion for domestic use (e.g.forcooking or heating) has been responsible for thousands of deaths inLondon over the past centuries, notably during the“Great LondonSmog” in December 1952, in which about 4,000 people died.

Current emission standards on sulphur oxide emissions are verystrict and are met by post combustion treatment of the exhaust gases.This is based mostly onscrubbing: mixing the exhaust gases withwater droplets that contain limestone, which reacts with the oxideaccording to CaCO3 + SO2 + 0.5O2→→→→ CaSO4 + CO2. The cost of thescrubbers is a very large percentage of a modern coal power station.

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NitrogenNitrogen oxidesoxidesTwo of the most serious pollutants attributed to combustionsources arenitric oxide (NO)and nitrogen dioxide (NO2), collectivelycalled NOx.Usually, only NO is emitted, but this will react in the atmosphere to createNO2. Nitrogen oxides will form acid rain, by a similar mechanism to SOx,and will participate in the formation of solid particles during smog episodes.At ground level and under sunlight, NO2 will release an oxygen atomwhich can then form ozone (O3). Ozone is very irritating for the respiratorysystem and causes impaired vision. In addition, NO emitted by high-altitudeairplanes participates in ozone destruction and hence contributes to theozone hole problem. Nitrogen oxides are strictly regulatedand low−NOxcombustion equipment has become today a huge business.

There are three mechanisms by which NO is formed in combustion:

thefuelNO,

thepromptNO, and

thethermal or Zel’dovichNO. These are discussed next.

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11.. FuelFuel NONOVarious coals and oils may contain up to 2% N2 (by mass). Duringcombustion, from 10 to 50% of this nitrogen will react with hydrocarbonradicals like CH or CH3 and will form HCN, which will then lead to NO.There is not much we can do to stop the generation of the fuel NO.

22.. PromptPrompt NONOBy approximately the same mechanism as above, the nitrogen containedin the air will be attacked by the hydrocarbon radicals CH to form HCN,which then leads almost invariably to NO.

This is called “prompt” NO because it forms as quickly as thecombustion reactions and, again, we cannot do anything to limit itsgeneration.

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33.. ThermalThermal NONOThe Zel’dovich mechanism is a reduced model for the rate of

generation of NO by this mechanism was discussed. The mechanism iscomposed of the elementary reactions (I) and (II) and we supplementedit by the (sometimes) partially-equilibrated reaction (III):

N2 + O→ NO + N (I)

N + O2→ NO + O (II)

O2 = 2O (III)

Writing the thermal NO global reaction as N2 + O2→ 2NO, wederived the following expression for the net rate of NO formation(Lecture 2, Eq. 2.10):

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CombustionCombustion--Generated Pollution ControlGenerated Pollution Control

Uncontrolled Combustion-Generated Pollutants

Increasingly Deteriorating Atmospheric Environment

Increasingly More Stringent Emission Regulations

Development of Combustion-Generated Pollution Control Technologies

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Quantification of EmissionsQuantification of Emissions

• Emission Index for species i is the ratio of the mass ofspecies i to the mass of fuel burned by the combustionprocess

m

m EI

F, burned

i,emittedi ====

2

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F

i

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ii MW

MWx X X

X EI

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Quantification of EmissionsQuantification of Emissions

• Another frequently employed specific emission measure isthe mss of pollutant emitted per amount of fuel supplied

• In spark-ignition and diesel engines, emissions arefrequently expressed as

(((( ))))W

MSE

producedpower Brakepollutant of flow Mass

(MSE)

iif EIm

emissionSpecificMass

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ch

supplied energy Fuel

ipollutant of Mass

∆∆∆∆==== iEI

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Emissions Legislation / RegulationEmissions Legislation / Regulation-- Why ? Why ? --

• Concern over increasing regional and local emissions from mobile andstationary combustion equipment has led to action to reduce emissions bytheintroduction of legislation in many countries

– Ozone depletion– Photochemical smog– Acid rain– Human health– Global warming

• e.g. 1997 Kyoto Protocol - sets legally-binding greenhouse gas emissionobjectives for each industrialised country - achieve at least a 5% reduction inemissions from 1990 levels over the 2008-2012 period :- 8% cut from for theEuropean Union (EU), 7% for the USA, and 6% for Japan and Canada –however, Australia is allowed an 8% increase, while Russia has a target of 0%(i.e. 1990 levels)

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Examples of Automotive Emission Standards Examples of Automotive Emission Standards

• European Union (EU) emission standards

• USA emission standards

• Japanese emission standardsCCCCCCCC

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Examples of Automotive Emission Standards Examples of Automotive Emission Standards

• Comparison of USA emission standards from passenger carbetween 1950s and 1994

• Comparison of emission standards for motorcycle

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NOx Emissions fromNOx Emissions from Industrial Processes Industrial Processes and Power Generationand Power Generation

• Typical uncontrolled NOx levels for industrial processesCCCCCCCC

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Examples of Standard Emission Examples of Standard Emission for Domestic Appliancesfor Domestic Appliances

• NOx and CO emissions limits for domestic appliances

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Examples of NOx Standard Emission for Examples of NOx Standard Emission for Industrial Processes and Power GenerationIndustrial Processes and Power Generation

• California South Coast Air Quality Management DistrictStandard for industrial gas turbine and other NOxemissions sources

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Examples of NOx Standard Emission forExamples of NOx Standard Emission forPower GenerationPower Generation

• Japanese regulated NOx emissions standards

• Comparison of NOx emission regulations for coal-fired boilers

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CombustionCombustion--Generated Pollution Control Generated Pollution Control StrategiesStrategies

• Most important pollutant species (as far as environment is concerned) emitted from combustion system are UHC, CO and NOx, SOx and particulates

• The dilemma of controlling the combustion process is that enhancement of the reactions leads to the burn-up of CO and UHC while at the same time promotes those reactions leading to the formation of NOx – Achieving low emissions of all

three species simultaneously necessitates reaching an effective compromise between these conflicting requirements

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Control Strategies of Carbon Monoxide (CO) and Control Strategies of Carbon Monoxide (CO) and Unburnt hydrocarbon (UHC)Unburnt hydrocarbon (UHC) EmissionsEmissions

• Combustion products normally due to poor fuel atomisation (high local fuel-rich regions), inadequate burning rates or quenching of hydrocarbon reactions

• Approaches that have yielded worthwhile CO and UHC emission levels reduction include– Improvement in fuel atomisation– Improvement in air and fuel mixing quality– Increase in primary-zone volume– Increase in residence time– Increase in inlet air temperature and pressure (i.e. increased burning

rates)– Reduction of flame quenching (i.e. film-cooling in gas turbine

combustion)– Staged-combustion

• Some of the above approaches inevitably contribute to an increase in the NOx formation rates

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Control Strategies of Carbon Monoxide (CO) and Control Strategies of Carbon Monoxide (CO) and Unburnt hydrocarbon (UHC)Unburnt hydrocarbon (UHC) EmissionsEmissions

• In most premixed combustion system, unburned hydrocarbons are usually negligible except SI engines

– current simultaneous NOx/CO/UHC reduction technique in SI engine most widely used is catalytic after-treatment

• In non-premixed combustion system, two main additional sources are

• overly lean regions which do not support rapid combustion,and

• overly rich regions which do not mix with sufficientadditional air or do not have sufficient time for oxidationreactions to go to completion

– two important parameters are fuel injector characteristics and fuel-air mixing patterns

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Control Strategies of Oxides of Control Strategies of Oxides of Sulphur (SOx)Sulphur (SOx) EmissionsEmissions

• SOx emitted from combustion system burning sulphur containing fuels such as coal, diesel and heavy fuel oils

• Concentrations of oxides of sulphur in the exhaust can be minimisedby removing sulphur from the fuel prior to combustion, e.g. lowsulphur diesel

• Most SOx control technologies involve the addition of a calcium- orsodium-based sorbent to the system

– Under the proper conditions, these materials react with SO2 andsulfur trioxide (SO3) to form sulfite and sulfate salts

• Post-combustion treatment is the most common method of SOxremoval in the exhaust

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Post Combustion Treatment of SOx Post Combustion Treatment of SOx EmissionsEmissions

• Dry Sorbent Injection (DSI)

– Sorbent such as reactive calcium or sodium based is injecteddirectly into the furnace or flue gas duct, where the dry particlesreact with SO2 and are subsequently removed by the boiler'sparticulate control device

• Wet flue gas desulfurization (FGD )

– Sorbent is dissolved in or slurried with water, and the fluegascontacts the solution or slurry in a scrubber

• Oxidization of SO2to SO3 over a catalyst followed by absorption ofSO3 in water to form sulfuric acid.

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Example of Combined Control of SOx / NOx Example of Combined Control of SOx / NOx Emissions from Coal Fired BoilersEmissions from Coal Fired Boilers

SNOX™ Process

CCCCCCCC

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Control Strategies of Particulate MattersControl Strategies of Particulate Matters

• Soot is formed in the rich region of diffusion flames depending oncompetition between soot formation and oxidation processes– thus combustion system modifications to minimize other emissions

can act to reduce the amount of soot produced prior to oxidation

• In diesel engines particulate traps such as ceramic filter are usedas post treatment control devices

• Particulate emissions from premixed combustion results onlyfrom rich operation or from fuel additives – with the removal oftetraethyl lead from gasoline, this source of particulate matterfrom SI engine should be eliminated

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NOx Formation CharacteristicsNOx Formation Characteristics

• The formation of NOx depends

– flame temperature (strongly dependent on air/fuel mixing quality)

– Oxygen availability– Mixture strength (A/F ratio)– Pressure – Combustion residence time– Fuel types– Air-Fuel mixing quality �� hot spot NOx producing zones

• Basic reduction techniques– Pre-combustion– Combustion modification– Post combustion

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NOx Reduction Techniques in Stationary NOx Reduction Techniques in Stationary Combustion SystemCombustion System

• Pre-Combustion

– Fuel selectionFuel type NOx production potential

Methanol least

EthanolMethane

Propane

Butane

Fuel oilCoal greatest

– Fuel pre-treaatment• e. g. hydro-denitrogenation and hydro-desulphurisation

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• Combustion Modifications

– lower flame temperature in combustion zone

– decreasing oxygen availability in high temperature zones

– creating conditions under which NOx can be reduced to molecular nitrogen with hydrocarbon fragments

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• Premixed fuel-lean combustion

– all reactants are fully premixed prior to combustion

– pre-mixed combustion in homogeneous high excess air environment to reduce flame temperature and hence NOx ( ~ 5% of NOx produced from conventional burner)

– Gas turbine combustion (250% excess air) Fuel rich stoich fuel lean

NOx

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• Staged combustion

– NOx control techniques involving staged introduction of either fuel or air or both into combustion chamber so as to control flame temperature , oxygen availability and as well as combustion chemistry in flames

– Three established techniques

• Two staged combustion– Air staging

– Fuel staging• Three staged combustion

– Both fuel and air staging (reburning)

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• Staged combustion – Air staging– involves diverting a portion of combustion air from burners

and injecting it into combustor beyond burner– Reducing amount of air fed to burners to stoichiometric

requirement promotes a localised fuel-rich combustionregion, which reduces both thermal and fuel NOx

Fuel rich stoich fuel lean

conventionalNOx

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• Staged combustion – Fuel staging– Involves injecting part of fuel into bulk of combustion air

and remainder of fuel into primary and secondarycombustion zones

– Fuel mixed is lean in the primary zone, and generation ofthermal NOx in this zone is limited by the reduced flametemperature that results

NOx

Fuel rich stoich fuel lean

conventional

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• Three staged combustion (reburning) – a similar concept as staged fuel combustion– first stagemain fuel supply undergoes fuel lean combustion where NO is formed in

lean primary combustion zone– second stageis a reburning stage where additional fuel is injected to produce a zone of

fuel rich combustion - a fraction of the NO is getting converted to molecular nitrogen in rich high temperature reactions between NO and hydrocarbon fragments in post-flame gases

– third stage, downstream of reburning zone additional air is injected into thisfuel richzone to complete combustion

conventional

Fuel rich stoich fuel lean1

1

2

2

3

3

NOx

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GAS UTILISATION EQUIPMENT(Low NOx burners – NOx reduction stratergies)

• Catalytic combustion– a flameless process in which fuel and the combustion air are

premixed to produce a very homogeneous high excess air mixture which is converted at a relatively low temperature in the presence of a catalyst

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• Diluent injection

– Flue gas recirculation (FGR)• FGR involves recirculation some flue gas /

combustion products back into the combustor

• FGR can control NOx by reducing– oxygen concentration in the

combustion zone, reducing the formation of fuel NO

– the peak flame temperature, suppressing the generation of thermal NO

– Water injection• In concept, water injection is the same as

flue gas recirculation (FGR) in that both act as diluents

• Injecting water reduces the flame temperature because combustion energy is used to vaporise and superheat the water to combustion temperatures

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PostPost--Combustion NOx Reduction Techniques Combustion NOx Reduction Techniques in Stationary Combustion System in Stationary Combustion System

• Post-combustion NOx control techniques involve reduction of NOx-containing flue gases produced from main combustion zone

• Two most well known methods

– selective non-catalytic reduction (SNCR)

– selective catalytic reduction (SCR)

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• Selective Non-Catalytic Reduction (SNCR)

– Nitrogen-containing additives (NHi) such as ammonia, urea or cyanuric acid are injected and mixed with the exhaust gases to effect chemical reduction of NO to N2 without the aid of a catalyst – also known as “thermal de-NOx”

– Depends critically on the exhaust gas temperature and operation within a relatively narrow range of temperatures (900-1100oC) is required to achieve large NOx reductions

– The overall deNOx reaction is

– At higher temperatures, NH3 tends to oxidise to form NO via the following reaction

OH2

3 NO O

4

5 NH 223 +→+

OH2

3 N O

4

1 NH NO 2223 +→++

CCCCCCCC

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• Selective Non-Catalytic Reduction (SNCR) –cont.

– Effectiveness of deNOx agent and reaction temperature of NOx reduction efficiency

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• Selective Catalytic Reduction (SCR)

– A catalyst such as copper exchanged zeolites and Cu-ZSM-5, is used in conjunction with chemical injection often ammonia, to reduce NO to water and nitrogen at much lower reaction temperatures as compared with SNCR technique

– The temperature window for effective reduction depends upon the catalyst used, but is contained within the range of about 250oC to 500oC and the type of catalyst is usually titanium oxide

– The SCR technique has been used on very large power station and can remove 90-95% of NOx

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• Selective Catalytic Reduction (SCR) –cont.

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• Pollution control strategies for gas turbine system CCCCCCCC

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LowLow--NOx Burner TechnologyNOx Burner Technologyin Stationary Combustion Systemin Stationary Combustion System

• ABB Gas Turbine Combustor– Low-NOx burner designs technically involve the correct placement of fuel into the

correct turbulence level of combustion air so as to produce both low-NOxemissions and stable combustion operation - In most practical cases, thiscan beachieved by injecting all or portion of fuel into highly turbule nt swirl flow of air

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• Staged Combustion concept Gas Turbine CombustorCCCCCCCC

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– Combustion is initiated in a fuel rich primary zone, thereby reducing NOx formation by lowering both the flame temperature and the available oxygen

– Also, the initial fuel rich combustion discourages NOx formation from fuel-bound nitrogen– As the fuel-rich combustion products flow out of the primary zone, they encounter jets of air

that rapidly reduce their temperature to a level at which NOx formation is negligibly small– Transition from a rich zone to a lean zone must take place rapidly to prevent the formation of

near stoichiometric NOx formation.

Staged Combustion concept Gas Turbine Combustor - Rich-burn/Quick-quench/Lean-burn (RQL)-

CCCCCCCC

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• Flue gas recirculation (FGR)

Forced FGR Induced (Fuel-induced-recirculation (FIR)) FGR

CCCCCCCC

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• Clean Coal Technologies

Air staging Concept Coal Burner

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Reburning Concept

CCCCCCCC

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Selective Catalytic Reduction (SCR)

CCCCCCCC

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• Catalytic Gas Turbine Combustion– a flameless process in which fuel and the combustion air are premixed to

produce a very homogeneous high excess air mixture which is converted at a

relatively low temperature in the presence of a catalyst


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NOx Reduction Techniques NOx Reduction Techniques in Nonin Non--Stationary Combustion SystemStationary Combustion System

• Pre-combustion– Clean fuels– Fuel cleaning (pre-treatment)

• Combustion modification– variation in spark injection timing– exhaust gas recirculation– stratified charge engine

• Post-combustion– exhaust treatment

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PrePre--Combustion Techniques of NOx Reduction Combustion Techniques of NOx Reduction in Nonin Non--Stationary Combustion SystemStationary Combustion System

• Clean fuels– Electricity

– Ethanol

– Methanol

– Natural gas (methane)

– Propane

– Reformulated Gasoline

• Fuel cleaning– Desulphurisation (e.g. low sulphur diesel)

CCCCCCCC

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Combustion Modification Techniques Combustion Modification Techniques of of NOx Reduction NOx Reduction in Nonin Non--Stationary Combustion Stationary Combustion

SystemSystem

• Retarding ignition timing (delayedignition)

– Retarded ignition timing results inlower peak temperatures and pressuresresulting in

• reduced NOx due to lower flametemperature

• increased UHC/CO levels due tolower burning velocity (as a resultof lower temperature / pressure) –increased fuel consumption

CCCCCCCC

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SystemSystem

• Exhaust / Flue Gas Recirculation (EGR / FGR)

– Mixing some exhaust gases with fresh combustion air or fuelresultsin significant reduction in NOx emissions due to oxygen availabilityin the mixture is decreased and hence lower flame temperature

– Too high a level of EGR will increase CO and UHC due toinsufficient oxygen and/or combustion temperature is too low forcomplete combustion (see previous note on FGR) - In spark ignitionengines, EGR results in increases in CO and unburned hydrocarbonemissions, while in compression ignition engines, EGR results inincreases in CO emissions and exhaust smoke

CCCCCCCC

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SystemSystem

• Stratified Combustion (Staged Combustion)– improved fuel air mixture control through improvements infuel

injection and combustion chamber design – Better mixture controlresults in more uniformity of in-cylinder fuel-air distributions and,hence, better control over the temperature-time history duringcombustion

– Most common method – fuel staging• The basic NOx reduction concept of this so-called ‘rich/lean

combsution’ in SI engine is– to create a very rich mixture in the region of the spark plug so

that good stability and low NOx emissions associated with richcombustion are achieved

– and to subsequently complete the combustion of unburnedfuel, radical species and CO in a very lean mixture in theremainder of the combustion chamber where additional NOxproduction is also low

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SystemSystem• Stratified combustion (staged combustion) –cont.

– For staging to be effective, the mixing of rich combustion products andair must be very rapid and hence design of mixing control device iscrucial

• A separate carburetor and intake manifold feeds a fuel rich mixturethrough a separate small intake valve into the pre-chamber which containsthe spark plug

• A very lean mixture is fed to the main combustion chamber through themain carburetor and the intake manifold

• After combustion starts in the pre-chamber, rich burning mixtur e issues ajet through the orifice into the main chamber, entraining and igniting thelean main chamber charge

CCCCCCCC

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SystemSystem

• Stratified combustion (staged combustion) –example of two stagefuel mixing developed in Mitsubishi Gasoline Direct Injection (GDI)engine

1. Piston moves downward.2. Fuel is injected.3. Fuel is injected again during compression stroke.4. Sparkplug ignites mixture

1 2 3 4

-reducing fuel consumption by 20%, CO2 emissions are also reduced by 20%. NOx isreduced by 95% by a three-step process, including Exhaust Gas Recirculation

CCCCCCCC

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PostPost--Combustion Modification Combustion Modification of of NOx Reduction NOx Reduction in Nonin Non--Stationary Combustion SystemStationary Combustion System

• Catalytic after-treatment

– Catalytic after-treatment is the primarypost-combustion technique applied tocontrol NOx emissions from anautomotive exhaust at very much lowertemperature (~200-400oC )

– Common catalysts used are noble metalssuch as platinum (pt), palladium (pd),rhodium (rh) and combinations of thesemetals

– However, the use of catalyst reductiontechnique is limited by the presence ofhigh excess oxygen in the diesel exhaustand also the need for high UHC and COemissions (as reducing agents) if higherNOx conversion efficiencies are to beaccomplished

– Catalysts that can simultaneouslyeliminate NO, CO and UHC emissionsare termed as three-way catalyst (TWC)

CCCCCCCC

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PostPost--Combustion Modification Combustion Modification of of NOx Reduction NOx Reduction in Nonin Non--Stationary Combustion System Stationary Combustion System (continued)(continued)

• Catalytic converter

– A device (honeycomb of ceramic or metal whichis chemically treated and coated with smallamounts of precious / noble metals) made forcatalytic conversion of the automotive pollutants

– Two types of converter

• Three-way converters (TWC)– Use catalysts (e.g. noble metals, metal

ion-exchanged zeolites / ZSM-5 zeolites)which can oxidize CO, UHC and reduceNOx simultaneously from SI engines

• Dual-bed converters– Exhaust gases are passed through a TWC

to remove NOx (and some of UHC andCO) and subsequently a bed containingoxidizing catalyst to eliminate rest of CO andUHC

4/4/2011

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PostPost--Combustion Modification Combustion Modification of of NOx Reduction NOx Reduction in Nonin Non--Stationary Combustion System Stationary Combustion System (continued)(continued)

• Problems associated with TWC system

– In SI engines, conversion efficiency ishigh within a narrow band of air-fuelratios very close to the stoichiometricvalue – slightly higher fuel consumption

– In diesel or lean-burn engines, TWC arenot effective in reducing NOx as theyoperate at high excess air – require specialtreatment or SCR of NOx

– Catalytic converter has to reach a certain(light-off) temperature before it canachieve acceptable conversion efficiency– require 20-30 seconds warm-up period(e.g. using electrical heater) – additionalfuel penalty

– Only use unleaded petrol – catalyst willbe poisoned by lead if leaded petrol isused

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Effectiveness of Various NOx Control MethodsEffectiveness of Various NOx Control Methods((individually or when combined)individually or when combined)

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Example: Nitric Oxide ( NO) Formation

Thermal (Zeldovich) Mechanism:

kJ/mol 62 (2)

kJ/mol 315 (1)

2

2

=+→+=+→+

a

a

EONOON

ENNONO

Forward reaction (1) is endothermic and due to the high Ea requires high temperature (>1800K) to proceed and is very slow.

2/1221 ]][[2

][ON

TR

KPk

dt

NOd reff=

From above two reaction using basic arguments one c an derive thefollowing expression for the “initial” NO reaction rate for the global reaction:

NOON 222 →+ kG

kG

Global NO formation reaction:

nmG ONk

dt

NOd][][

][22=

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