REM TECHNOLOGY INC.

305 – 27 Street SE Calgary, Alberta T2A 7V2 CANADA

Telephone: (403) 207-0630 Fax: (403) 273-5835

www.remtechnology.com

A Comparison of Emissions Reduction Technologies

A White Paper - June 2015

Howard Malm Ph.D., P.Eng.

Chief Technical Officer

REM Technology Inc.

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Executive Summary

This white paper provides a brief review and comparison of the main options for achieving regulatory

emissions limits for stationary natural gas engines:

Stoichiometric with non-selective catalytic reduction

Lean Combustion with and without an oxidation catalyst

Exhaust Gas Recirculation (EGR) with and without non-selective catalytic reduction

The primary driving factors; the applicable regulatory limits, initial cost, running costs and reliability; are

evaluated for each of the technologies. The first two options are widely available with extensive field

and supplier experience, while the last option, EGR, which can achieve very low NOx emissions, has

barriers to adoption for high availability field applications.

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Table of Contents Executive Summary ........................................................................................................................................ i

1. Introduction .......................................................................................................................................... 1

2. Background ........................................................................................................................................... 2

3. Reduction of Natural Gas Engine Emission Concentrations ................................................................. 3

3.1. Catalytic Reduction ....................................................................................................................... 4

3.2. Combustion Temperature Reduction ........................................................................................... 4

3.3. Other Emission Reduction Technologies ...................................................................................... 5

4. Lean Operation ..................................................................................................................................... 6

5. Exhaust Gas Recirculation ..................................................................................................................... 8

6. Comparative Results ........................................................................................................................... 11

6.1. Stoichiometric Operation with NSCR .......................................................................................... 11

6.2. Lean Combustion ........................................................................................................................ 14

6.3. Exhaust Gas Recirculation ........................................................................................................... 15

7. Summary ............................................................................................................................................. 16

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1. Introduction Spark-ignited natural gas reciprocating engines have a number of exhaust emissions such as carbon

dioxide (CO2), nitrogen oxides (NOx), carbon monoxide (CO), volatile organic compounds (VOCs),

hazardous air pollutants (HAPs) and hydrocarbons (HCs) which may be subject to regulatory control.

Emissions reductions for these engines may be achievable with:

Exhaust catalysts

o Stoichiometric operation with non-selective catalytic reduction (NSCR)

o Selective catalyst reduction (SCR)

Combustion temperature reduction

o Very rich operation

o Lean operation

o Exhaust gas recirculation (EGR)

Catalytic reduction uses materials such as platinum and palladium on surfaces exposed to the hot

exhaust gases to promote chemical processes that reduce concentrations of specific emissions

components. NSCR (also known as 3-way catalytic reduction) requires the engine to operate with an air

to fuel mixture near to stoichiometric, where the amount of oxygen in the air nearly equals that

required for full fuel combustion. SCR refers to the concentration reduction of specific emission

components. An oxidation catalyst or a catalyst using urea is an example of an SCR.

Combustion temperature reduction offers improved engine efficiency (less CO2) compared to NSCR.

In very rich operation the extra fuel gas limits the temperature and the oxygen available for NOx

generation. In lean operation air, in excess of that required for fuel combustion, is supplied to the

engine. For EGR partly or fully cooled exhaust gases are added to the intake air to absorb some of the

combustion heat. Exhaust catalysts can be added to lean operation or EGR to reduce concentrations of

specific emission components.

In this paper the background section reviews the exhaust components that can be subject regulatory

limits, the main emission reduction methods and the methods of quantification for comparison and

regulatory purposes. Separate sections deal with each principal emission reduction method in terms of

the technology,

typical performance and emission values achievable, and

the effects on engine fuel efficiency.

In the final section the three methodologies are compared with respect to capabilities and reliability in

terms of advantages and disadvantages.

Since the allowed limits for emissions of NOx, CO, VOCs, HAPs and HCs, vary widely, as do the incentives

for engine efficiency (CO2 emissions), there is no best solution for all engines and locations.

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2. Background Combustion emissions produce a number of components that can be detrimental to human health and

the environment. These components include the nitrogen oxidesa (NO + NO2 = NOx), carbon monoxide

(CO), volatile organic componentsb (VOCs), unburned hydrocarbonsc (HCs) and sulfur dioxide (SO2). In

the USA there are also emission limits on hazardous air pollutantsd (HAPs).

Emissions limits may be expressed in terms of an exhaust concentration (% or ppm), a rate (g/HP-h,

g/kW-h), or a mass total over a defined period (g/h, t/y). For the purposes of this paper, the emissions

will be expressed as rates normalized to engine output power using grams divided by the product of

output horsepower or kilowatts and running hours (g/HP-h or g/kW-h).

In Canada provincial regulations limit one or more of the emissions components. Canadian federal

regulations are expected to come into force in the future to serve as an upper bound to provincial

regulatory limits. In the US the Environmental Protection Agency, (EPA), state and local authorities

place limits on emissions under the US Clean Air Act. In addition, the US Supreme Court has ruled that

carbon dioxide (CO2) is a pollutant (Ref. 1) and is subject to regulatory action by the EPA. Regulatory

bodies have used both regulations and incentives to achieve emissions goals. As such, there is a large

variation in the regulation specifications and in the limits assigned to various emissions components.

Figure 1 shows an example of the range of NOx regulatory limits that apply to natural gas engines.

a N2O, nitrous oxide, also known as laughing gas or nitro, is a very potent greenhouse gas. It is emitted in very small concentrations from natural gas engines with emission reduction technology in good working condition and is normally not subject to regulation. See also Section 3.1.

b Often the VOCs exclude the methane and ethane component c Sometimes a subset of HCs, known as non-methane hydrocarbons (NMHCs) or non-methane, non-ethane hydrocarbons

(NMEHCs), are regulated. d HAP examples are formaldehyde and BTEX (benzene, toluene, ethyl benzene and xylene)

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Figure 1 - NOx emissions limits

Most regulatory bodies establish limits on NOx because of its role in producing ozone and smog. NOx is

produced during high temperature combustion in the presence of the nitrogen component of air. For

reciprocating engines, NOx production occurs during the combustion portion of the engine cycle.

Many regulatory bodies also place emissions limits on CO, HCs, and VOCs, which are produced by

incomplete combustion. SO2 results from combustion of fuel components containing sulfur. Normally

sulfur compounds are removed or reduced to low levels in natural gas engine fuels to minimize long-

term engine damage.

3. Reduction of Natural Gas Engine Emission Concentrations The reduction of these emission components, except for SO2, from reciprocating engines can be

achieved by either of two main methods:

Catalytic reduction

o Stoichiometric operation with non-selective catalytic reduction (NSCR),

o Selective catalyst reduction (SCR)

Combustion temperature reduction

o Very rich operation

o Lean operation

o Exhaust gas recirculation (EGR)

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NOx Emissions Limits

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3.1. Catalytic Reduction NSCR, non-selective catalytic reduction at stoichiometric conditions, causes the chemical oxidation of

CO, VOCs and HCs and the chemical reduction of the NOx to much lower concentrations. The oxidation

and chemical reduction reactions occur in the presence of precious metal catalysts in the exhaust

catalyst assembly in the exhaust ducting. To effectively reduce the concentrations of all of the foregoing

components from natural gas fuel, NSCR requires that the exhaust gases come from combustion of a

near stoichiometric mixture. Effective NSCR requires very precise control of the ratio of air and fuel

supplied to the engine under all ambient and operating conditions. With NSCR the above exhaust

components can be reduced to less than 10% of the concentrations produced directly by the engine.

Engine fuel efficiency and, therefore, the CO2 emission rate is more than that with other operating

regimes. NSCR is possible only with 4 stroke-cycle engines that do not use extra air for exhaust

scavenging purposes. 2 stroke-cycle engines cannot use NSCR. Catalyst temperature must be

monitored as the catalysts are effective over a restricted temperature range. Catalyst deterioration due

to fouling, deactivation with contaminants, and over-temperature operation (due, in particular, to

excess periods with rich air-fuel ratios) require monitoring and scheduled maintenance. Aged catalysts

are also known to enable N2O, a potent greenhouse gas, to be released to the atmosphere.

Selective catalytic reduction (SCR) reduction of NOx in exhaust streams with more than 1% of unused

oxygen, requires the addition of a chemical agent such as ammonia or urea to the exhaust gases to

enable the chemical reduction of the NOx in the presence of a catalyst. Generally, NOx reduction in

exhaust gases having unused oxygen with SCR is complex, expensive, and used only when other

methods of NOx reduction cannot be employed or are insufficient to meet the regulatory limits in place.

3.2. Combustion Temperature Reduction NOx emissions can be reduced by reducing the peak combustion temperature, which can be achieved by

Extra fuel to absorb the heat produced – rich combustion

Extra air to absorb the heat produced – lean combustion

Exhaust gases added to the intake air to absorb the heat produced – EGR

Rich combustion, adding extra fuel to reduce NOx emissions, does work but consumes significantly more

fuel and markedly increases the CO and HC/VOC emissions. Because of the higher fuel cost, which can

be 20% or more compared to stoichiometric operation, and the very high CO/VOC/HC emissions, this

method of NOx reduction is not an allowed option in most jurisdictions.

Lean combustion, adding extra air to reduce the maximum combustion temperature, reduces the rate of

NOx generation and is effective in more completely combusting CO and VOCs. The resulting reduction

of heat loss results in improved engine efficiency and reduced CO2 emissions. Lean combustion is widely

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used with scavenged enginese (2- and 4-stroke) and is offered by many manufacturers of carbureted 4

stroke-cycle engines. To obtain NOx emission rates below 1.5 g/kW-h pre-combustion chambers (PCCs)

or chambered spark plugs are used to reliably ignite the very lean air-to fuel ratios.

EGR, the recirculation of partly or fully cooled exhaust gases, provides the extra mass necessary in the

intake stroke to reduce the maximum combustion temperature while not adding oxygen. The result is a

reduction in NOx and an improvement in engine efficiency. However the CO and VOCs are not reduced

as much as with lean combustion. NSCR can be used effectively with EGR to attain very low NOx, CO and

VOC emissions values.

A study of cost-effective emission control technologies suitable for meeting US emission controls

regulations was published in 2011 (Ref.2). Other studies relating to other jurisdictions are available in

the engineering and scientific literature.

Both lean combustion and EGR reduce the total power output of naturally aspirated (NA) engines,

where the amount of oxygen available for fuel combustion is limited by atmospheric pressure. Hence

these technologies are effectively restricted to turbo-charged or super-charged engines where the extra

gases can be added without engine power reduction. For NA engines where full power is not required,

very often, lean combustion and EGR can be used normally and then turned off for the high power

occasionsf.

EGR has been widely used in the transportation and small engine sector with varying degrees of success.

It has not yet received wide-spread adoption for larger, high use natural gas engines.

In 2004 the implementation of EGR was the subject of a publication from the Engine Manufacturer’s Association (Ref. 3). The 2004 publication contains the following statement “Until the necessary research, development and testing of EGR systems are completed, the very limited results regarding emissions from EGR equipped gaseous fueled engines must be viewed as preliminary and not satisfactory for widespread acceptance”. This publication pointed out the known difficulties with implementing EGR with continuous-duty natural gas engines.

3.3. Other Emission Reduction Technologies There are other emission reduction technologies that have been used or are in development. These technologies are discussed briefly here. NOx traps use a high absorbency material in the exhaust stream to temporarily absorb the NOx component of the exhaust gases. When the NOx trap approaches saturation the NOx is reduced to N2

e In a scavenged engine extra air is provided to help remove the cylinder exhaust gases. It is used on most 2 stroke-cycle engines and some 4 stroke-cycle engines.

f EGR is frequently used for automotive and truck engines where the full power of the engine is needed a small fraction of the total running time.

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and O2 by one of two methods. In one method the engine is switched for a short period from a lean air to fuel ratio to a rich ratio which supplies the CO necessary to react with the trapped NOx. In another method the exhaust gases are routed to a parallel unsaturated NOx trap while fuel is directed to the saturated NOx trap to reduce the trapped NOx. NOx trap technology has not gained wide commercial acceptance. Homogeneous Charge Compression Ignition (HCCI) is a new combustion method where the air-fuel mixture is compressed sufficiently to self-ignite as in a diesel engine. The characteristics are high efficiency and very low NOx emissions. Oxidation catalysts are needed to deal with incomplete combustion components. HCCI requires very tight engine control. HCCI engines for transportation are in advanced development but have not been introduced for industrial engines. The next sections will discuss lean combustion and EGR in more detail to provide a basis for comparison

of the two technologies. For further details on NSCR see Section 3.1.

4. Lean Operation For combustion of fuel, oxygen is required. The amount of oxygen depends on the amount and

composition of the fuel. The oxygen content in normal air is about 20.9% by volume, with the

remainder composed of nitrogen (78.1%) argon (0.9%) and smaller amounts of CO2 etc. (<0.1%). The air

may contain up to a few percent of water, depending on temperature and saturation level. When the

amount of air is just sufficient for complete combustion, the air-fuel mixture is known as stoichiometric.

For lean combustion, more air than is required for the fuel combustion is supplied to the engine. The

effects of increasing the amount of excess air are shown schematically in Figure 2.

Figure 2 - Typical Exhaust Emissions as a Function of Mass Air to Fuel Ratio

1.5

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Since, in most engines the air-fuel mixture is not homogeneousg, the extra oxygen ensures good

combustion in all parts of the air-fuel mixture, thereby reducing the exhaust emissions of CO, VOCs and

HCs and increasing engine fuel efficiency. This extra combustion heat from the improved combustion

and the increased availability of oxygen increases the NOx production rate as shown by the maximum in

NOx emissions with 10 to 20% excess air (λ = 1.1 to 1.2).

As the amount of extra air increases further, it absorbs more heat from the combustion and, thereby,

reduces the maximum temperature achieved during the fuel combustion stage. This reduces the

formation rates of NO and NO2 since these rates are strongly temperature-dependent.

The extra air necessary to sufficiently minimize NOx emissions can be supplied by using a turbocharger

or supercharger to compress the air fed to the engine. Since the compression heats the air, cooling is

needed to remove this heat. Engines designed to operate lean have turbochargers or superchargers,

and intercoolers with sufficient capacity for the required air. Turbochargers, which are powered by hot

exhaust gases and do not consume engine mechanical power, and inter-coolers are generally favored.

For engines originally designed as stoichiometric, the existing turbochargers and intercoolers may

require upgrades in order to operate sufficiently lean to achieve the desired NOx emission rate.

The amount of excess air is ultimately limited by the ability of a standard spark plug to reliably ignite the

air-fuel mixture. Common methods of extending the lean limit, and therefore reducing NOx emissions,

are pre-combustion chambers, chambered spark plugs and combustion chamber design. Stratified

charge methods are available but have not achieved wide commercial adoption.

While NOx emissions limits of 0.5 g/HP-h can be achieved with lean combustion technology, even lower

NOx emission values require urea SCR or NOx trap additions.

Often oxidation exhaust catalysts are added to lean combustion engines to meet regulatory limits for

HAPs.

g For a 4 stroke-cycle engine operating as 1200 RPM, incomplete mixing occurs as there is period of only 25 milliseconds (1-fortieth of a second) between the addition of the fuel to the air and the start of combustion.

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5. Exhaust Gas Recirculation With EGR a fraction of the total exhaust gases, which are depleted of oxygen in stoichiometric

combustion, are redirected and added to the intake air. The exhaust gases absorb some of the

combustion heat, thereby reducing NOx production. Typically, HC emissions increase with the fraction

of EGR. The behavior of NOx and exhaust HCs are shown in Figure 3 using data from Lavoie and

Blumberg (Ref. 4) .

Figure 3 - Dependence of Emissions of NOx and HC on EGR Percentage

There is an upper limit to the amount of EGR that will allow reliable ignition of the cylinder charge,

which depends on a number of parameters including the fuel, ignition and combustion zone design.

Operating conditions such as load and speed also affect the maximum allowable EGR fraction.

By combining EGR with a non-selective catalyst, the NOx, CO, and HC emissions can be reduced to very

small levels, below that possible with lean combustion or EGR alone. An example of test results for NOx

emissions for intake pressures of 3.8 and 6.8 atmospheres are shown in Figure 4 (Ref. 5).

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NO - Moderate burn

NO - Fast burn

HC moderate burn

HC - Fast burn

Moderate burn limit

Fast burn limit

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Figure 4 - NOx Emissions from a Test Engine - 1 g/kW-h = 1.33 g/HP-h

Clearly, the combination of EGR with NSCR can achieve very low exhaust emissions. The EGR serves to

improve the engine efficiency, compared to stoichiometric operation, by reducing the combustion

thermal losses. The NOx emissions are reduced since to NOx concentrations entering the catalyst are

lower, enabling the catalyst to reduce the NOx emissions to very low values.

EGR may be accomplished in several ways. In the simplest method, some of the hot exhaust gases are

added to the incoming air, using a valve to control the EGR percentage. The resulting air-exhaust gas

mixture temperature is increased compared to the air temperature. Either the engine uses this mixture

directly or some cooling of the air-exhaust mixture is performed. Alternately, the cooling may be

applied to just the exhaust gases before their addition to the engine intake air. For engines with

turbochargers the higher pressure exhaust gases from the exhaust system upstream of the turbocharger

may be directed, through a control valve and heat exchanger, into the compressed intake air after the

turbocharger. Cooling of this gas is necessary to minimize the increase to exhaust gas, air, and fuel

mixture entering the engine cylinders.

When EGR is used, the engine control system must be able to control the EGR flow, in addition to the

usual engine air, fuel, temperature, ignition timing and speed parameters.

Exhaust gas—largely carbon dioxide and water vapor—has a higher specific heat than air, so it is more

effective in lowering peak combustion temperatures. However, EGR reduces the specific heat ratio of

the combustion gases in the power stroke, which reduces the amount of power that can be extracted by

the piston. In all cases where EGR is employed, the combination of CO2, nitrates, organic acids and

water in the exhaust gases are acidic. Consequently, care is necessary to prevent condensation or the

corrosion of components in the intake system (Ref. 6). Figure 5 below shows components where the

possibility of corrosion must be considered.

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Figure 5 - Typical EGR Arrangement showing components that may be subject to corrosion.

Waukesha, Miratech, and Southwest Research (Ref. 7) reported in 2005 on tests with a 700 HP VGF test engine fitted with EGR. Low emissions were reported - 0.12 g/HP-h for NOx, 0.06 g/HP for CO, and 0.0257 g/HP-h for VOCs. The need for very precise control of the air-fuel ratio to maintain emissions values was noted. The test unit was operated for 500 hours under the test program. The authors note “Remaining challenges include the demonstration of consistent, long term emissions performance and the

long term durability of engine systems and components operating with EGR.”

Westport-Cummins currently supply spark-ignited natural gas engines using cooled EGR and NSCR to

meet 2010 EPA transportation standards. These engines are rated up to 400 HP and achieve NOx

emissions of < 0.2 g/HP-h for NOx, < 3.5 g/HP-h for CO and < 0.14 g/HP-h for NMHC. Field test

verification of EGR for NOx emission reductions to < 0.02 g/HP-h is underway (Ref. 8).

Fouling and corrosion of components where exhaust gases are present must be considered both in the

design and on-going maintenance. Other issues are the disposal of the acidic condensation and the

need for a more complex control system. It is worth noting that the running time for a vehicle that

travels 200,000 km is about 4000 hours compared to an expected interval of >20,000 hours between

overhauls for industrial natural gas engines. Hence potential maintenance issues must be evaluated

carefully.

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6. Comparative Results Most developed jurisdictions have regulations with respect to combustion engine emissions including

NOx and CO. It is generally true that the reduction of NOx emissions from natural gas engines costs

money, either in capital costs or in operating costs. Furthermore, the lower the NOx emissions levels,

the more it costs to achieve the specified emissions levels. Conversion of stoichiometric engines to lean

combustion or EGR is financially advantageous due to reduced fuel consumption.

Almost all emission reduction methods for natural gas engines require control of the air to fuel ratio of

the mixture reaching the combustion chambers. The tables below show the relative advantages and

disadvantages of

Stoichiometric operation with NSCR

Lean Combustion

Exhaust Gas Recirculation

6.1. Stoichiometric Operation with NSCR With NSCR, exhaust gas oxygen sensors in the engine exhaust measure the exhaust oxygen percentage

which is used to control the air to fuel ratio within narrow limits so the non-selective catalyst can oxidize

components such as CO and VOCs and reduce the NOx to N2 and O2. For proper operation the catalyst

must operate within a specific temperature range. Field studies on natural gas engines have shown that

the air to fuel ratio cannot be reliably maintained with the low-cost exhaust gas oxygen sensors (EGOs)

or heated exhaust gas sensors (HEGOs) over the normal range of ambient and operating conditions.

Improved control can be achieved with universal (wide-band) exhaust oxygen sensors (UEGOs)9.

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Table 1 - Stoichiometric with Non-Selective Catalytic Reduction (NSCR)

Stoichiometric with

NSCR

Pros Cons

Controls Simple feed-back control Exhaust sensors must be replaced

regularly

Not suitable for engines with cylinder

scavenging including all 2 stroke engines

Good control over all operating conditions

between sensor replacements; not good

with narrow band EGOs or HEGOs; better

with wide-band UEGOs

Maintenance Simple component

replacement

High emissions with mal-functions

Relatively inexpensive

replacement items

Regular catalyst service necessary

NOx emissions Can achieve < 1 g/HP-h

(higher cost catalyst for lower

limits)

High emissions with malfunctions

CO emissions Can achieve < 1 g/HP-h (see

above)

High emissions with malfunctions

NMHC, VOC emissions Can meet EPA HAPs limits High emissions with malfunctions

Only 50% reduction for unburned

methane

Fuel efficiency Better than no air-fuel

control or rich combustion

Relatively poor (compared to lean, EGR)

No Carbon credits available

Capital costs System relatively in-

expensive

Site catalyst housing installation can be

costly

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6.2. Lean Combustion In lean combustion air in excess of that required for stoichiometric combustion is supplied to the

combustion zone. The excess air ensures full fuel oxidation occurs even for non-homogeneous air-fuel

mixtures. The additional heat capacity limits the high temperatures at which NOx forms and reduces

the heat loss to the engine coolant, thereby improving mechanical efficiency.

Table 2- Lean Combustion

Lean Combustion Pros Cons

Controls Can be open loop or closed loop

control; standard systems

Closed loop systems need regular

UEGO replacement

Not available for most engines < 300

HP

Reduces power output on naturally

aspirated engines

Maintenance Proven reliability; little down-

time

PCC gas admission valves can be prone

to failure

Oxidation catalyst service

infrequent

Lower exhaust temperatures; less

head replacement

NOx emissions Can achieve < 1 g/HP-h

CO emissions Can achieve < 2 g/HP-h; ≈10x

lower with oxidation catalysts

NMHC, VOC emissions < 1g/HP-h ; lower with oxidation

catalysts

Fuel efficiency 10% or better compared to

stoichiometric; life cycle savings

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Capital costs Can be higher than stoichiometric

To reduce the NOx emissions from engines with oxygen in the exhaust gases selective catalytic reduction

(SCR) may be used for exhaust gas treatment. Carefully monitored amounts of ammonia or urea are

added to the exhaust gases to react with the NOx in the exhaust gases in the presence of a suitable

catalyst. While the urea SCR is widely used for vehicle engines to meet emission regulations, urea SCR is

used for large stationary engines only where other technologies cannot meet the regulatory limits. Urea

SCR systems require a continuous supply of ammonia or urea, which is consumed by the SCR, and

regular maintenance.

6.3. Exhaust Gas Recirculation Exhaust gas circulation, while used for many engines in the transportation sector, has not yet been

commercially adopted for large, high availability natural gas engines. The corrosive nature of exhaust

gases present considerable challenges for stationary engine application. The combination of EGR and

NSCR offers the opportunity for achieving very low NOx emissions rates.

Table 3 - Exhaust Gas Recirculation

EGR Pros Cons

Controls Does not require as much

turbocharger capacity as lean

More complex; no standard systems

for stationary NG engines

Maintenance Many intake components subject to

exhaust gas corrosion

Field reliability not proven

NOx emissions Similar to lean; much lower with

NSCR

CO emissions Higher than lean; much lower

with NSCR

NMHC, VOC emissions Higher than lean; much lower

with NSCR

Fuel efficiency Similar to lean

Capital costs Higher than lean

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7. Summary This white paper provides the basic information to aid in the selection of technology to control and, if

necessary, reduce undesired natural gas engine exhaust emissions. While many engine models can be

operated over a wide range of air to fuel ratios, some engine models have design features that limit this

air-to fuel ratio variation.

Stoichiometric operation with NSCR is the only available option for some engine models. The extra fuel

consumption and maintenance activity associated with stoichiometric engines is costly over the longer

term.

Currently lean combustion technology is offered by most engine manufacturers due to its reliability and

lower fuel consumption rates. Conversions of existing and new stoichiometric engines to lean

combustion with reduced fuel consumption can provide a revenue stream from the corresponding CO2

emission reduction. Carbon pricing changes may increase this revenue stream.

Exhaust gas recirculation, while common in smaller engines and those used in the transportation sector,

is not commercially accepted or field-proven for the larger stationary engines. EGR enables fuel

consumption reductions similar to that for lean combustion, with the opportunity to achieve very low

NOx emissions with the use of NSCR. If regulatory NOx limits for stationary engines are amended to well

below existing levels, it is to be expected that EGR technology for large stationary natural gas engines

will become more common in jurisdictions with very low NOx limits.

1 US Supreme Court – Decision Apr 7, 2007 MASSACHUSETTS ET AL. v. ENVIRONMENTAL PROTECTION AGENCY ET AL. 2 Cost-Effective Reciprocating Engine Emissions Control and Monitoring for E&P Field and Gathering Engines – K. Hohn, S. R.

Nuss-Warren – Kansas State University 2011. Report DE-FC26-02NT15464 http://www.netl.doe.gov 3 The Use of Exhaust Gas Recirculation (EGR) Systems in Stationary Natural Gas Engines – Aug 2004

www.enginemanufacturers.org 4 A Fundamental Model for Predicting Fuel Consumption, NOx and HC Emissions of the Conventional Spark-Ignited Engine –

G.A. Lavoie and P. N. Bloomberg, Combust Sci. Technol. Vol. 21, 225, 1980 5 Lean Burn Natural Gas Operation vs. Stoichiometric Operation with EGR and a Three Way Catalyst - Patrik Einewall, Per

Tunestål and Bengt Johansson SAE 2005-01-0250 6 Influence of EGR on Component Durability Hind - Abi-Akar Technical Services Division Caterpillar Inc.

http://www.osti.gov/scitech/servlets/purl/827861 7 DESIGN AND DEVELOPMENT OF WAUKESHA'S, STOICHIOMETRIC, COOLED EGR ENGINE FOR THE CALIFORNIA ARICE PROGRAM Robert W. Stachowicz David E. Watson Donald M. Newburry and Timothy J. Callahan ICEF2005-1329

8 http://www.westport.com/news/2015/near-zero-nox-emissions-isl-g-natural-gas-engine 9 Final Report: Cost-Effective Reciprocating Engine Emissions Control and Monitoring for E&P Field and Gathering Engines by K.

Hohn, S. R. Nuss-Warren DOE Award DE-FC26-02NT15464 Nov 2011