Gas Engine Emissions Technology 4th Edition

8/19/2019 Gas Engine Emissions Technology 4th Edition

1/16

Gas Engine

Emissions TechnologyFourth Edition

This document contains proprietary and trade secret informationand is given to the receiver in confidence. The receiver by recep-tion and retention of the document accepts the document in confi-dence and agrees that, except as with the prior expressed writtenpermission of Waukesha Engine, Dresser, Inc., it will; (1) not usethe document or any copy thereof or the confidential or tradesecret information therein; (2) not copy or reproduce the documentin whole, or in part without the prior written approval of WaukeshaEngine, Dresser, Inc.; and (3) not disclose to others either the doc-ument or the confidential or trade secret information containedtherein.

All sales and information herein supplied subject to StandardTerms of Sale, including limitation of liability.

WAUKESHA and DRESSER are registered trademarks of Dresser,Inc. The DRESSER logo is a trademark of Dresser, Inc. All othertrademarks, service marks, logos, slogans, and trade names (col-lectively “marks”) are the properties of their respective owners.Dresser, Inc. disclaims any proprietary interest in these marksowned by others.

!

FORM 536

Copyright 2004

Waukesha Engine

Dresser, Inc.

Waukesha, Wisconsin 53188

All rights reserved. Printed in U.S.A. 3/04


2/16

IMPORTANT NOTICE

This publication is designed to present information to professionals as an aid to independent research. It is

not to be regarded as providing opinion or advice for any individual case. Waukesha Engine, Dresser, Inc.

assumes no responsibility for use and application of the information contained herein. User accepts all

responsibility and risk for the use and application of the information contained herein.


3/16

GAS ENGINE EMISSIONS TECHNOLOGY

FORM 536 Fourth Edition 1

Deterioration of the atmosphere caused by gaseouspollutants is an important environmental issue. Local,state, and national governments continue to enact

stricter exhaust emissions legislation to reduce andpossibly reverse the atmospheric deterioration. This

legislation often affects natural gas engine installationsby limiting the horsepower allowed or requiring very

low emissions levels out of the engine. Natural gasengine manufacturers continue to develop productswhich help to meet these requirements. In addition

exhaust treatment companies have developed pro-cesses which reduce pollutants by converting them

into safe, naturally occurring compounds that are notdamaging to the atmosphere.

This paper discusses how air pollutants are formed innatural gas engines along with the health risks and

atmospheric deterioration which result from these pol-lutants. Some of the current emissions regulations arepresented and how they apply to gas engines is

addressed. Technology for reducing emissions is alsodiscussed.

DEFINITIONS

Air/Fuel Ratio (AFR): The ratio between the amountof air and the amount of fuel flowing into an engine. It

is commonly expressed two ways – on a mass basis oron a volume basis.

Ammonia: A chemical compound of nitrogen and

hydrogen with the formula NH3. It is used as a reduc-ing agent for nitrogen oxides, NOx, in a Selective Cat-alytic Reduction (SCR) system. Ammonia is

environmentally hazardous and toxic.

Carbon Monoxide (CO): A pollutant having the for-

mula CO.

Digester Gas: A gas suitable for fuelling an engineformed by the anaerobic decomposition of organicmatter in a digester. It is composed primarily of meth-

ane (CH4) and carbon dioxide (CO2). The saturatedlower heating value normally ranges from 500 – 600

Btu/ft3 (19.66 – 23.59 MJ/m3).

Dry Basis: A system for reporting engine exhaust

emission values based on the removal of all watervapor present in the exhaust. This is done either with

suitable instrumentation or mathematically.

Excess Air: The amount of air provided to a combus-

tion process over and above the amount needed forcomplete, chemically correct, burning of the fuel pro-

vided.

Excess Air Ratio (Lambda or “λ ”): A ratio of theamount of air provided to a combustion process to thechemically correct (stoichiometric) amount of air. It is

equivalent to the actual air/fuel ratio, AFR, divided bythe stoichiometric air/fuel ratio, AFRS.

Formaldehyde: A hazardous air pollutant with thechemical formula HCHO.

High (Higher) Heating Value (HHV): The total energyreleased from a standard volume – usually one cubic

foot – of a fuel gas when the products of combustionare cooled to the same pressure and temperature as

the original air and fuel mixture. This includes theheat of vaporization of the water formed during com-

bustion since the cooling causes the water vapor tobecome liquid.

Landfill Gas: A gas suitable for fuelling an engine

formed by the decomposition of landfill refuse. It iscomposed primarily of methane (CH4) and carbondioxide (CO2). The saturated lower heating value nor-mally ranges from 400 – 550 Btu/ft3 (15.73 – 21.62

MJ/m3).

Lean Operation: Operation of an engine with more airthan is necessary for complete combustion of the fuelsupplied to the cylinders. Lambda is greater than 1.0.

Low (Lower) Heating Value (LHV): The energyreleased from a standard volume – usually one cubic

foot – of a fuel gas when the products of combustionare cooled to the same pressure and temperature as

the original air and fuel mixture. This does not includethe heat of vaporization of the water formed during

combustion since this water is assumed to remaingaseous.

Natural Gas: A naturally occurring mixture of hydro-carbon and other gasses found beneath the earth’s

surface, often in connection with oil. The principle con-stituent is methane (CH4). Minor constituents areheavier hydrocarbons such as ethane (C2H6), propane

(C3H8), and butane (C4H10), and other gasses such asnitrogen (N2), carbon dioxide (CO2), helium (He),

argon (Ar), oxygen (O2), and hydrogen sulfide (H2S).

Nitrogen Oxides (NOx): A pollutant. The combination

of nitrogen oxide, NO, and nitrogen dioxide, NO2,expressed as NO2.

Non-Selective Catalytic Reduction (NSCR): A cata-lytic process allowing the simultaneous reduction of

NOx and oxidation of CO and unburned hydrocarbons.A precious metal catalyst is required for this process.

The AFR must be held within a very narrow band nearstoichiometry to permit all the reactions to occur at ahigh efficiency. Also known as a “three-way catalyst”.

A FR m as s

mass f low rate of air

mass f low rate of fuel -------------------------------------------------------------=

AFR vo l um e

vo lu m e f low rate of a i r

vo lu m e f low rate of fue l

---------------------------------------------------------------------=

λ actual air f low

stoichiom etr ic air flow ---------------------------------------------------------------

AFR

A FR s

--------------- = =


4/16


2 FORM 536 Fourth Edition

Ozone: A pollutant having the formula O3.

Particulate Matter: A pollutant composed of very

small natural and man-made solid or liquid particlessuch as dust, soot, carbon, and oils.

Parts-Per-Million (ppmv or ppmw): A ratio calculatedon the basis of the whole being divided into one million

equal parts. The ratio may be calculated on a volume(ppmv) or weight (ppmw) basis. For example, if 1 ft

3 ofcarbon dioxide is mixed with 99 ft3 of nitrogen, there is

10,000 ppmv of carbon dioxide in the mixture. Notethat 10,000 ppm equals 1%.

Percent (%v or %w): A ratio calculated on the basisof the whole being divided into a hundred equal parts.

The ratio may be calculated on a volume (%v) orweight (%w) basis. For example, if 1 pound of salt is

mixed with 99 pounds of sugar, the mixture contains 1%w salt. Note that 1% equals 10,000 ppm.

Rich Operation: Operation of an engine with less air

than is necessary for complete combustion of the fuelsupplied to the cylinders. Lambda is less than 1.0.

Selective Catalytic Reduction (SCR): A catalytic pro-

cess allowing the reduction of NOx in the presence ofhigh oxygen concentrations. The process requires theuse of a nitrogen bearing reducing agent, typically

ammonia or urea.

Stoichiometric Mixture: The chemically correct mix-ture of fuel and air that enables the complete burningof the combustible portion of the fuel present with zero

remaining oxygen. Lambda is equal to 1.0.

Sulfur Oxides (SOx):A pollutant, primarily SO2.

Urea: A chemical compound produced commercially

from ammonia and carbon dioxide with the formulaNH2 – CO – NH2. In the form of an aqueous solution itcan be used as a reducing agent for nitrogen oxides,

NOx, in a Selective Catalytic Reduction (SCR) system.When injected into the exhaust stream of an engine

the hot gasses over a catalyst cause the decomposi-tion of urea into ammonia that then reacts to reduce

the NOx. Urea does not have the environmental con-cerns associated with ammonia.

Waukesha Knock Index™ (WKI™): Waukesha’s pro-

prietary fuel knock resistance scale for gaseous fuelsdetermined from a basis of methane = 100 and hydro-gen = 0. It includes the knock resistance effects of cer-tain inert gases and is extended to values greater than

100 through the use of a nine gas mixture matrix.

Wet Basis: A system for reporting engine exhaust

emission values based on inclusion of all water vaporpresent in the exhaust. This is done with suitable

instrumentation.

ATMOSPHERIC POLLUTANTS

Pollutants which can be produced in natural gasengines are classified in six different categories:

NOx (oxides of nitrogen)

CO (carbon monoxide)

HC (hydrocarbons)

SOx (oxides of sulfur)

CHO (aldehydes, Formaldehyde is CH2O)

PM10 (particulate matter 10 microns and

smaller)

NOX

Oxides of nitrogen consist of NO (nitrogen oxide) mol-ecules and NO2 (nitrogen dioxide) molecules which

are formed when N2 (nitrogen) and O2 (oxygen), fromthe air, react with each other. This reaction requires a

high combustion temperature and the presence ofnitrogen and oxygen in the combustion chamber asthe fuel is burned.

NO2 harms humans and animals by reducing breath-ing capacity and limiting the blood’s ability to carry O2.

It is also harmful to vegetation. In the lower atmo-sphere NO2 and NO, when exposed to sunlight, act as

precursors in the formation of O3 (ozone). Ozone inthe lower atmosphere damages plants and synthetics,

and causes coughing, choking and headaches inhumans. Photochemical smog contains NO2, which isa yellowish-brown color, and ozone.1 These gasses

give smog its brownish color and irritate the lungs andcan weaken the respiratory system leading to

increased susceptibility to infections such as the flu,bronchitis, and pneumonia.

OZONE

Ozone forms when NOx and hydrocarbons combine

and chemically react in the presence of sunlight.Ozone irritates the eyes, reduces breathing capacity,

causes inflammation of the lungs, and may triggerasthma attacks. More sensitive people – such as the

elderly and children – can experience other symptoms

including chest pains, coughing, wheezing, labored

breathing, and nausea. Ozone can also reduce therespiratory system’s ability to fight infections.

CO

Carbon monoxide is formed by incomplete combustionof the fuel. Complete combustion of a methane mole-

cule is represented by the formula below:

CH 4 2O

2 + CO

2 2H

2 O + !


5/16



Incomplete combustion of a methane molecule willproduce CO instead of CO2 (carbon dioxide). Incom-plete combustion occurs when there is insufficient oxy-

gen near the hydrocarbon (fuel) molecule for completecombustion or when combustion is quenched near a

cold surface in the combustion chamber.

Carbon monoxide is a colorless and odorless poison-

ous gas. It replaces oxygen in the body’s red bloodcells. Exposure to high CO levels can cause nausea,

headache and fatigue, and, in heavy enough concen-trations, CO can even cause death. In the upper atmo-sphere CO reacts with O3 (ozone) producing CO2,

which depletes the ozone layer of the upper atmo-sphere. This ozone layer screens harmful sun rays

from reaching the Earth’s surface. Depleting the ozonelayer allows more harmful rays to reach the surface.1

HC

Natural gas is a fuel made up of several hydrocarbon

gases including: CH4 (methane), C2H6 (ethane), C3H8(propane), C4H10 (butane), and other heavier com-

pounds. A small fraction of these hydrocarbons willpass through the combustion chamber without react-

ing. Therefore these hydrocarbons will retain their formin the exhaust (ie. some methane, ethane, and pro-

pane etc. will be found in the exhaust). These hydro-carbon emissions are commonly broken down into twocategories and sometimes a third. These categories

are:

1. THC (Total Hydrocarbons) or TOC (Total

Organic Compounds)

2. NMHC (Non-Methane Hydrocarbons) orVOC (Volatile Organic Compounds) orROG (Reactive Organic Gases) or ROC

(Reactive Organic Compounds).

3. NM-NEHC (Non-Methane, Non-Ethane

Hydrocarbons)

THC

Total hydrocarbon emissions include all of the

hydrocarbon gases found in the exhauststream.

NMHC

Non-methane hydrocarbons are the portion of

the THC (total hydrocarbons) that does notinclude methane. For example an exhaust gas

contains:

1000 PPMV Methane

200 PPMV Ethane

100 PPMV Propane

+ 50 PPMV Butane

1350 PPMV THC

The non-methane portion of the THC is:

1350 PPMV THC

-1000 PPMV Methane

350 PPMV NMHC

Non-methane hydrocarbons are singled outfrom methane because they can react with

NOx in the lower atmosphere, acting as a pre-cursor in the formation of photochemical smog.Methane will not readily react in the lower

atmosphere in the smog reactions.1 There aretechnical differences between NMHC, VOC,

ROG, and ROC. However, Waukesha consid-ers all of the emissions the same when report-ing in technical publications. Consult area

regulation agency to determine potential differ-ences for area being considered.

NM-NEHC

Ethane is also disregarded in some controlled

areas because it has a much lower reactivitythan the heavier hydrocarbons. In these areas

the regulations are based on non-methane,non-ethane hydrocarbons.

SOX

Oxides of sulfur are formed when sulfur containingcompounds, in the fuel or lube oil, are oxidized in thecombustion chamber. In gaseous fuels sulfur can be

present in the form of H2S (hydrogen sulfide). Oxidesof sulfur enter the atmosphere and combine with water

in the air forming H2SO3 (sulfurous acid) and H2SO4(sulfuric acid). These acids return to Earth as acid rain.

H2S can be removed from gaseous fuels with propertreatments which will decrease the SOx exhaust emis-

sion levels.

In addition, sulfur dioxide, SO2, can narrow airway

passages and lead to difficult breathing – especiallyfor people that have asthma.

HCHO

Formaldehyde is a product of incomplete combustion

of hydrocarbon fuels and lube oil in an engine. It isalso a part of the resulting smog from photochemical

reactions between oxygen and hydrocarbons.

Starting in March 2004, formaldehyde emissions from

many engines over 500 bhp will be regulated by theU.S. Environmental Protection Agency.

Formaldehyde contributes to eye irritation, and poly-merizes to form visibility-reducing aerosols.4 Formal-

dehyde is one of several aldehyde emissions. For gasengines, formaldehyde is the primary aldehyde emis-

sion to consider.


6/16



Formaldehyde is a probable human carcinogen andcan cause irritation of the eyes and throat, coughing,tightening of the chest, headache, and heart palpita-

tions. Prolonged or severe exposure has caused bron-chitis, pulmonary edema, pneumonia, and even death

due to respiratory failure. Long term low level expo-sure can cause skin rash and respiratory problems.

PM/PM10 /PM2.5

Natural and man-made solid or liquid particles such as

dust, soot, carbon, oils, etc. that are finer than thediameter of a human hair. Particulate in the exhaust of

an engine is formed by incomplete combustion of liq-uid fuels and lubricating oil. Also, very fine silicon par-

ticulate can be ingested with landfill gas fuel and passthrough the engine to be emitted into the atmosphere.

Particulate is subdivided by effective aerodynamicdiameter. All suspended particulate is termed PM. Par-ticulate with an aerodynamic diameter of 10 microns (a

micron is a millionth of a meter) or less is PM10. Evensmaller particulate, with an aerodynamic diameter of

2.5 microns or less, is PM2.5. High levels of PM10 andPM2.5 are associated with increased respiratory infec-

tions, asthma attacks, decreased breathing ability, andincreased mortality rates. Fine particulate tends to

remain suspended in the air for longer periods andreduce visibility. PM2.5 particles are so small that nor-mal human lung clearing action is unable to remove

them leading to increased risk of lung and throat can-cer.

Engine out particulate levels from natural gas enginesare low when compared to diesel engines. Particulate

matter in diesel exhaust has been labeled as a proba-ble carcinogen (cancer causing agent) by some regu-

latory agencies.

EMISSIONS REGULATIONS

Regulations governing the quantity of pollutants whicha gas engine can discharge vary between different

regions due to the air quality in these regions. Regionswith poor air quality have much tighter restrictions on

exhaust emissions than areas where the air quality is

good. For this reason the local air quality board must

be contacted to determine emission requirementswhen engines are considered for new projects or re-

powers.

In the United States, there are four common methodsof specifying the amount of pollutant discharged by anengine or limits in permits or regulations. These are:

1. Pollutant per period, e.g., pounds/hour ortons/year

2. Pollutant per output energy, e.g.,

grams/bhp-hr3. Pollutant per unit volume, e.g., ppmvd*

(parts per million on a volume, dry basis)

4. Pollutant per energy consumed, e.g.,

lb/MMBtu** (pounds per million Btu)

* must be associated with a specific oxygen – dilution

– level, e.g., at 15% oxygen

** Need to specify if the energy is on a high or low

heating value basis

1. Pollutant Per Period

On a site that will use gas engines there can be a limiton how much mass of different pollutants can be dis-charged during a given time period. For example, in aPrevention of Significant Deterioration (PSD) area, the

federal Clean Air Act defines certain “major emittingsources” as those having the potential to emit 250

tons/year or more of any listed air pollutant. Beingclassified as a major source may necessitate addi-

tional permitting requirements including modeling, off-sets, monitoring, control, or other requirements notneeded for lower emitting, non-major, sites.

The amount of horsepower that can be installed at a

site without exceeding the major source threshold canbe determined from the engine exhaust emission lev-els, e.g., in lb/day or g/bhp-hr, and the number of hours

that the engine will operate in a year. Note, unlessthere is an enforceable limit on the number of operat-

ing hours per year, the “potential to emit” principlerequires that the number of hours in a full year, i.e.,8760, must be used in this calculation.

For example – an engine’s emission rates are 1.5

g/bhp-hr NOx, 2.65 g/bhp-hr CO, and 1.0 g/bhp-hrNMHC, the major source limit is 250 tons/year, andpotential to emit must be used. In this case, the CO

emission rate will govern since it is numerically thelargest. Then:

Horsepower 250 ton

year -------------------- -

907,200 grams

ton ----------------------------------------- ×

1 year

8760 hours -------------------------------

×

hp-hr

2.65 grams ------------------------------- × 9770 HP =

=


7/16



2. Pollutant Per Energy Unit Generated

Some regions limit pollution from a source based on

the amount of useful energy it is producing. For naturalgas engines the useful energy is horsepower-hours ofmechanical energy, therefore the engines are regu-

lated in grams/hp-hr. Limits are set for the pollutantswhich are causing the air quality deterioration.

3. Pollutant Per Unit Volume Of Exhaust

This method of specifying exhaust emissions uses theamount of pollutant in a given volume of exhaust pro-duced. In the United States this in commonly

expressed as ppmvd (parts per million on a volume,dry basis). In other parts of the world, e.g., Europe,

mass per volume of exhaust produced is commonlyused. The typical units are mg/Nm3 (milligrams per

normal cubic meter). To be complete and unambigu-ous, volume based measurements must be associatedwith a specific oxygen, or dilution, level. In the US this

is commonly 15% oxygen. Europe typically uses 5%oxygen while other parts of the world use different val-

ues.

4. Pollutant Per Energy Unit Consumed

This method involves regulating pollution based on the

amount of fuel consumed. Common units for this arelb/MMBtu of fuel.

Efficiency Adjustment Factors

Many regions will consider the efficiency of the engine

in the calculation to allow more efficient engines ahigher emissions limit. This is accounted for by the for-

mula:

Baseline efficiency is determined by the governing

agency.

Acronyms

The following definitions of emissions control technol-ogy follow federal EPA definitions. Note that the dis-

tinction between RACT, BACT, and LAER is oftenblurred on a state and local level. Some states may

use BACT generically to mean any, or all, of the threelevels of control as defined in the Clean Air Act.

BACT – Best Available Control Technology is appliedto major new or modified emissions sources in attain-

ment areas and applies to each regulated criteria pol-lutant.

EPA – The federal Environmental Protection Agency.

HAP – Hazardous Air Pollutant.

LAER – Lowest Achievable Emission Rate is appliedto new or modified sources in non-attainment areas.

MACT – Maximum Achievable Control Technology isthe maximum degree of reduction possible in HAPemissions. MACT may be a specified technology, e.g.,

oxidation catalyst, and/or may include revised proce-dures, fuel switching, operator training, etc. The MACT

“floor” level does not take cost or other factors intoconsideration. A more stringent, above the “floor”,

MACT takes into consideration cost, energy require-ments, and any non-air quality health and environmen-tal issues.

NA – A Non-Attainment area is one in which the

NAAQS standards for one or more of the criteria pol-lutants are not met. The state must have a SIP to bringthis region into compliance with the NAAQS at the ear-

liest possible date.

NAAQS – National Ambient Air Quality Standard.

One type of nation-wide, federal air pollution standard.It is applied to the six criteria pollutants, ozone, carbon

monoxide, total suspended particulate, sulfur dioxide,lead, and nitrogen oxide.

NESHAPS – The National Emissions Standards forHazardous Air PollutantS is a nation-wide, federal air

pollution program of standards for 188 specific hazard-ous air pollutants governed by Title III of the Clean Air

Act. It applies to new, reconstructed, and existingsources. A NESHAPS was enacted for reciprocating,internal combustion engines in February, 2003. It will

apply to engines at major sources of HAPs (formalde-hyde). A major source of HAPs is defined as any

source that emits 10 tons/year or more of any individ-ual HAP or 25 tons/year or more of any combination of

HAPs. Catalytic oxidation of formaldehyde is the tech-nology preferred by the EPA.

NSPS – New Source Performance Standard. A uni-form, national air standard set by the EPA for air emis-

sion from a specific type or class of source such asgas turbine, steel mill, acid plant, etc., for criteria pol-lutants. A NSPS has not been issued for reciprocating,

internal combustion engines.

RACT – Reasonably Available Control Technology is

applied to existing sources in non-attainment areas. Itis the lowest emission limit that a source can meet by

application of control technology that is reasonablyavailable taking technological and economic feasibility

into account.

SIP – A State Implementation Plan. This is an EPA

approved state plan for the establishment, regulation,and enforcement of air pollution standards and con-

trols. These state plans may incorporate regulationsmore stringent than those spelled out in federal EPAregulations or NSPS.

Allowable Pol lutant

Pol lutant Limit Engine Ef f ic iency

Basel ine Ef f ic iency ------------------------------------------------------ ×

=


8/16



THE 1990 CLEAN AIR ACT AMENDMENTS(CAAA) AND THE TITLE V OPERATINGPERMIT PROGRAM

The 1990 Clean Air Act Amendments arguably form

the most comprehensive and far-reaching federal envi-ronmental law. As amended, it now contains, in Title V,

the framework of a national operating permit systemthat will be administered by the states for many new,modified, and existing sources. Operating permits are

legally enforceable documents issued by regulatoryagencies to air pollution sources after the source has

begun to operate. Most large and some smallersources must now obtain a “Title V” permit to operate.

The purpose of a Title V permit is to reduce air pollu-tion violations and improve enforcement of air quality

regulations. A Title V permit does this by:

- recording in a single document all of the air pollution

control requirements that apply to the source. This

gives everyone – including the public and the source’smanagement – a clear picture of what the source isrequired to do to comply with its legal limits.

- requiring the source to make regular reports on howit is measuring its emissions and the performance ofthe controls it is using to limit emissions. These reports

are public documents.

- adding monitoring, testing, and/or record keepingrequirements, if needed, to assure that the sourceremains in compliance with its emission limits and

other pollution control obligations.

- requiring the source to certify each year that it hasmet the air pollution requirements in its Title V permit.The permits usually require self-reporting of any viola-

tions found by the source. These certifications are alsopublic information.

- making the terms of the Title V permit federallyenforceable. This means that the EPA and the general

public can bring suit to enforce the terms of the permitalong with the issuing state agency.

NATURAL GAS ENGINE EMISSIONS

In the past natural gas engines were commonly oper-ated at an air/fuel ratio which provided the most horse-

power for the amount of air being consumed. Thisair/fuel ratio is fuel rich of “Stoichiometry”. Stoichiome-

try is defined as: The chemically correct air/fuel ratiowhere all the fuel and all the oxygen in the mixture will

be consumed.

Another way in which air/fuel ratio is represented iswith an excess air ratio referred to as “Lambda” (λ ).Excess air ratio is determined with the following for-

mula:

In recent years, engines which operate at a much

leaner air fuel ratio have been utilized because of theirlow emissions and low fuel consumption characteris-tics. With a lean air/fuel ratio (λ > 1.0) there is more

oxygen in the combustion chamber than is required forcombustion which leaves a high concentration of oxy-

gen in the exhaust. Fuel consumption in a lean com-bustion engine is typically 5-12% lower than in a

similar stoichiometric combustion engine.

Figure 1 illustrates exhaust NOx output compared to

the air/fuel ratio.

Figure 1.

To the rich side (left side on the graph) of stoichiome-

try, NOx decreases significantly due to the lack of oxy-gen in the combustion chamber and lower combustion

temperatures. On the lean side (right) of stoichiometry

the NOx reaches a peak because combustion temper-ature remains high and there is an abundance of oxy-gen. At increasingly lean air/fuel ratios, the combustiontemperature continues to fall and NOx levels fall even

though excess oxygen exists in the cylinder. As statedearlier, NOx formation requires the presence of oxygen

and nitrogen in a high temperature environment, there-fore less NOx is formed at lower temperatures.

λ Operating air/ fuel rat io

Stoichiometr ic air /fuel rat io ----------------------------------------------------------------------------- =

λ 1.0 at the stoic hiom etric air/ fuel rat io =


9/16



Carbon Monoxide levels are also lower in a lean com-bustion engine than in a stoichiometric enginebecause there is now plenty of oxygen for the fuel mol-

ecules to react with. Figure 2 illustrates CO levelscompared to air/fuel ratio.

Figure 2.

Operating to the rich side of stoichiometry causes a sig-nificant increase in CO because of the lack of sufficient

oxygen to complete combustion of the fuel molecules.At a point slightly lean of stoichiometry, CO output hitsa minimum because there is sufficient oxygen and high

combustion temperatures. At leaner combustion air/fuelratios, CO increases due to poorer combustion from

low combustion temperatures and lower flammability ofthe fuel mixture. Emissions of CO, however, are still

lower at this point than at stoichiometry.

Levels of NMHCs also vary with air/fuel ratio as shown

in Figure 3.

Figure 3.

Like CO emissions, NMHC emissions also are higherat points rich of stoichiometry because of the lack ofoxygen for combustion. NMHC emissions are also

minimum at a point slightly lean of stoichiometry andincrease at further lean air/fuel ratios. The amount of

NMHCs are higher at the lean combustion air/fuel ratiothan at stoichiometry, also because of lower combus-

tion temperatures.Figure 4 sums up the emission levels for typical natu-

ral gas engines at various air/fuel ratios.

Figure 4.

Overall (with the exception of NMHCs) we can see that

a lean combustion engine provides much lower levels

of pollutants than a stoichiometric engine. The leancombustion engine does this without the aid ofexhaust after-treatment (catalytic converter) and with-out the need for electronic air/fuel ratio control.

Lean combustion engines have demonstrated lowemission levels consistently because these emissions

are not affected by deterioration of a catalyst or failureof electronic oxygen sensing devices.

Ignition of the high air/fuel ratio in a lean combustion

engine can be obtained fairly well with a high turbu-lence open chamber design. Another method, utilizing

a pre-chamber with a stoichiometric mixture to ignite a

lean main chamber, can produce better combustion atleaner air/fuel ratios. Open chamber and pre-chamberconfigurations are shown in Figure 5.


10/16



Figure 5.

Waukesha VGF-GL models are open chamber, leanburn engines, running at λ = 1.53 to 1.67. The VHPand AT-GL engines utilize a pre-chamber combustion

chamber for leaner operation at λ =1.74 to 2.0. SomeVHP engines utilize a high turbulence open chamber

design which operate at λ =1.52 to 1.54 (Refer to thecurrent WED Product Bulletins for emission levels on

all models.)The pollutants in exhaust gas comprise only a small

percentage of the total exhaust gas. The remainder ofthe exhaust gas consists of harmless, naturally occur-ring gases. Some of these gases are formed in the

combustion process while others are simply passingthrough the combustion chamber without chemically

reacting. The composition of air, and some typicalcompositions of exhaust gas, are given in the chart

below.

*Trace indicates less than 0.2%.

NOTE: Summary based on a wet volume basis.

EXHAUST EMISSIONS FOR ALTERNATEFUELS

Gaseous fueled engines are often operated on fuelsother than natural gas. Propane and waste recovery

fuels are the most popular of these: HD-5 propaneas a standby fuel and waste recovery fuels, such as

digester gas and landfill gas, as primary fuels. Thesefuels produce a noticeable difference in exhaust gasemissions when compared to natural gas. Before we

discuss the emissions levels of these fuels weshould have a better understanding of the fuels’ con-

tent. The chart below summarizes typical gas con-centrations in the fuel.

SUMMARY OF PRODUCTS OF COMBUSTION WITH NATURAL GAS FUEL*

Excess Air Ratio Setting = λ

GASES 0.97 1.00 1.06 1.53 1.74 2.00 AIR

N2 69.5% 71% 72.1% 73.8% 74.5% 75.3% 79%

H2O 20.9% 19% 17.1% 12.8% 10.9% 9.5% Trace

CO2 8.4% 9.2% 9% 6.4% 5.7% 4.9% Trace

O2 Trace 0.25% 1.15% 6.8% 8.8% 10.1% 21%

CO 0.9% 0.3% Trace Trace Trace Trace —

NOx Trace 0.25% 0.5% Trace Trace Trace —

HC Trace Trace Trace Trace Trace Trace —

GAS NATURAL HD-5 PROPANE DIGESTER LANDFILL COAL SEAM

Methane 95% — 65% 55% 95%

Ethane 3% 4% — — —

Propane 1% 95% — — —

Butane+ 1% 1% — — —

Carbon Dioxide — — 35% 45% 3%

Nitrogen — — — — 2%


11/16



Figure 6 illustrates NOx and CO output vs excess airratio for natural gas, HD-5 propane, and landfill gas.Note that air/fuel ratio has been removed.

Figure 6.

Stoichiometry for propane, digester gas, and landfillgas are at different air/fuel ratios than for natural gas,therefore we only use the “excess air ratio” designation

for this graph.

NOx emissions for natural gas and propane are nearly

the same while emissions for landfill gas are muchlower. This is because of the high concentration of

inert gas (CO2) in landfill gas which cools the peakcombustion temperature, reducing NOx.

HD-5 propane combustion at lean air fuel ratios is notas complete as methane. Therefore, CO concentration

is higher at lean air/fuel ratios,λ = 1.2 and above, thanwith natural gas or landfill gas.

Concentration of NMHCs also vary with the type offuel an engine is operated with. Figure 7 illustrates

NMHC concentration for the three fuels.

Figure 7.

HD-5 propane NMHCs are the highest since HD-5 pro-pane fuel is 100% non-methane hydrocarbon. Naturalgas contains about 5% non-methane hydrocarbon in

the fuel, therefore it has a lower level in the exhaust.Landfill gas and digester gas contain no non-methane

hydrocarbons in the fuel, therefore their NMHC emis-sion levels are much lower. The NMHCs which do exist

in landfill gas and digester gas exhaust are from com-bustion of lubricating oil in the engine.

EXHAUST GAS AFTERTREATMENT

The following discussion briefly covers commonly

available aftertreatment technologies and is not meantto be all inclusive.

Catalyst

Emissions from an engine can be reduced by chemi-

cally converting the pollutants into harmless, naturallyoccurring compounds. The most common method for

achieving this is through the use of a catalytic con-verter. A catalyst is a substance which promotes a

chemical reaction without being chemically changeditself. In a catalytic converter, the catalyst will eitheroxidize (oxidation catalyst) a CO or fuel molecule or

reduce (reduction catalyst) an NOx molecule. The gen-eral (not balanced) reducing reactions are shown

below:

These reactions are reducing the NOx to nitrogen and

oxidizing the fuel and CO molecules. These reactionsoxidize some of the CO and NMHC molecules, how-

ever further conversion is accomplished with an oxidiz-ing catalyst. The oxidizing reactions take place as

shown here:

NO x CO + N

2 CO

2 + !

NO x CH

4 + N

2 CO

2 H

2 O + + !

NO x

H 2

+ N 2

H 2

O + !

CO O 2

+ CO 2

!

CH 4 O

2 + CO

2 H

2 O + !

C nHm O 2

+ CO 2 H

2 O + !

H 2 O 2 +

H 2 O !


12/16



Three Way Catalyst

A 3-way catalyst contains both reduction catalyst

materials and oxidation catalyst materials and will con-vert NOx, CO, and NMHCs to N2, CO2, and H2O. Acatalyst process which causes reactions of several

pollutant components is referred to as Non SelectiveCatalyst Reduction (NSCR). Typical emission conver-

sion efficiencies for a three-way catalyst operating ona near stoichiometric engine are:

90+% decrease in NOx

80+% decrease in CO

50+% decrease in NMHC

The efficiency of a three way catalyst is highly depen-dent on the percentages of NOx, CO, O2, and NMHCs

in the reaction. A very narrow air/fuel ratio operatingrange is necessary to maintain these percentages.Electronic air/fuel ratio controls are often necessary to

maintain this range.

2

Oxidation Catalyst

An oxidation catalyst is often used on lean combustion

engines to oxidize CO and hydrocarbon molecules inthe exhaust. The lean combustion principle produces

very low NOx emission so this pollutant usually doesnot require further reduction. Since an oxidation cata-lyst eliminates CO and HC emissions it is considered

an NSCR.

Dual bed catalyst

A less common method for treating stoichiometric

engine exhaust emissions is with a dual bed catalyst.A dual bed catalyst utilizes separate reduction and oxi-dation sections with air introduced after the reduction

catalyst and before the oxidation catalyst. Figure 8illustrates a Dual Bed Catalyst.

Figure 8.

Exhaust from the engine first travels through thereduction catalyst where the following reactions takeplace.

Air is added to the exhaust stream before it enters theoxidation catalyst where these next reactions take

place.

A “Dual Bed” catalyst can convert up to 98% of bothNOx and CO and does not require the very narrow

air/fuel ratio operating range required for the 3-waycatalyst.2 Dual bed catalysts are losing popularity,however, because 3-way catalysts are now approach-

ing the same efficiencies.

Selective Catalytic Reduction

As discussed earlier, lean combustion, GL, engines

emit low NOx compared to similar stoichiometricengines without a catalyst. When GL engines were

first introduced this low NOx level was acceptable vir-tually everywhere without any exhaust aftertreatment.Many much more stringent emissions regulations cur-

rently – or soon to be – in force require NOx levels

lower than the best that lean combustion can provide.NOx from a lean combustion engine can sometimesbe reduced a small additional amount with engine

adjustments. These adjustments will compromiseother engine performance areas such as raising otheremission levels, raising fuel consumption, and affect-

ing power output and engine stability. A NSCR catalystcannot be used with a lean burn engine because of

the high levels of oxygen present in the exhauststream.

If further NOx reduction is needed from a lean com-

bustion engine, the most common way to obtain this

reduction is with a Selective Catalytic Reduction(SCR) system. SCR is selective in that it is effectiveonly on NOx and it avoids the problem of excess oxy-gen in the exhaust stream by the injection of an out-

side “reducing agent”.

NO x CO CO

2 N

2 + ! +

NO x H

2 H

2 O N

2 + ! +

NO x CH

4 CO

2 H

2 O N

2 + + ! +

CO O 2

+ CO 2 !

H 2 O

2 + H

2 O !

C nHm O 2 CO

2 H

2 O + ! +

CH 4 O

2 + CO

2 H

2 O + !


13/16



Early SCR technology involved injecting anhydrousammonia as the reducing agent into the exhaust gasupstream from the catalyst. As the hot exhaust gas

passes over the catalyst the NOx and ammonia com-bine to form nitrogen gas and water vapor. The

amount of ammonia must be carefully controlled sothat ammonia “slip” or “breakthrough” does not occur.

The system typically includes a NOx monitor upstreamof the catalyst and a feedback loop mechanism toensure that the proper quantity of ammonia is injected

to eliminate as much NOx as possible without emittingunreacted ammonia. Ammonia is, itself, a hazardous

compound requiring care in its use and storage andcan cause harmful effects if emitted.

Recently, SCR systems have been switching to anaqueous solution of urea as the reducing agent. Since

urea is not considered a hazardous material, as isammonia, handling costs and problems are reduced.Urea is a man-made compound produced commer-

cially from ammonia and carbon dioxide with the for-mula NH2 – CO – NH2.

For urea SCR systems, the first part of the catalystconverts the urea and water vapor from combustion to

ammonia and carbon dioxide. Then the ammonia andNOx react, as before, to form nitrogen gas and water

vapor. A final oxidation stage can be added to the cat-alyst housing where any excess ammonia is oxidized

to nitrogen gas and water vapor. At the same timehydrocarbons react with the oxygen present to formcarbon dioxide, water vapor, and a small amount of

carbon monoxide.

Figure 9 shows a basic ammonia SCR system. Ureacan be substituted for ammonia in this figure.

Figure 9.

The basic chemical reactions governing the SCR pro-cess are:

Urea to Ammonia:

NOx destruction:

or

Ammonia destruction:

This process can result in final stack emissions of NOxas low as 0.2 g/bhp-hr. Whether ammonia or urea is

used, it is a consumable that must be replaced.

ELECTRONIC AIR/FUEL RATIO CONTROL

Stoichiometric Combustion Engine

Maintaining low emissions in a stoichiometric combus-

tion engine using exhaust gas treatment often requiresa very closely regulated air/fuel ratio. Many control

devices are available for this and most use exhaustgas oxygen sensing to determine air/fuel ratio.

A more universal Custom Engine Control® Air/FuelModule (AFM) is offered by Waukesha. The AFM sys-tem is designed to function with all types of gaseous

fueled engines that Waukesha manufactureres includ-

ing near stoichiometric and lean burn, naturally aspi-rated and turbocharged.

Theory of Operation

The AFM system controls engine air/fuel ratio andconsists of three basic components: an oxygen sen-

sor, actuator, and AFM module. The AFM system is aclosed-loop process that looks at system outputs and

adjusts system inputs according to preprogrammedinstructions.

The AFM system functions by monitoring oxygen lev-els in the exhaust gases with an oxygen sensor

located in the engine’s exhaust stream (seeFigure 10). The oxygen level, detected by the sensor,

is then fed to the AFM module through an electricalsignal. If the oxygen level detected by the sensor is dif-ferent than the programmed oxygen set-point, the

AFM module directs the actuator to adjust the gas overair pressure of the fuel regulator.

NH 2 CO NH

2 H

2 O + 2NH

3 CO

2 +! ––

4NH 3 4 NO O

2 4N

2 6H

2 O +! + +

6NH 3 2NO

2 O

2 4N

2 6H

2 O +! + +

4NH 3 3O

2 2N

2 6H

2 O +! +


14/16



Figure 10.

The actuator adjusts the fuel regulator setting, within

programmed limits, by increasing or decreasing thespring pressure acting on the regulator diaphragm.

The design gives very accurate positioning capability.The regulator adjustment richens or leans out theair/fuel ratio.

A thermocouple is used to assure that temperaturesare high enough for correct operation of the sensor. A

programmed minimum temperature must be achievedbefore “closed-loop” control is enabled. A programmed

maximum temperature is also incorporated as a safetyto shut down operation on high exhaust temperature

conditions.

The oxygen sensor provides continuous feedback of

oxygen levels to the AFM module. The AFM module

makes the necessary actuator adjustments to cor-rectly control the engine’s air/fuel ratio.

Lean Combustion Engines

Electronic control of air/fuel ratio is not required onmany lean combustion engines because small

changes in air/fuel ratio have very little effect on theexhaust emissions. Figure 11 again illustrates emis-

sions levels vs. Air/fuel ratio. The boxes around “Sto-ich” and “Lean Combustion” indicate the air/fuel ratio

drift that might occur during operation of an enginewithout an air/fuel ratio control. At stoichiometry it is

apparent that a small change in air/fuel ratio can

cause a large change in NOx and CO which, whenused with a 3-way catalyst, can cause low conversionefficiency. At the lean combustion air fuel ratio a smallchange in air fuel ratio causes very little effect on the

emissions levels. Efficiency of an oxidation catalystoperating on a lean combustion engine is unaffected

by these small air/fuel ratio changes.

Electronic controls for air/fuel ratio on lean combustion

engines are often used for fuels which can have a wide

variance in heating value, such as some landfill gas

applications.

Figure 11.

Using air/fuel ratio controls on lean combustionengines which operate on steady heating value fuels

can add unnecessary complication and expense.

It’s apparent from the information presented in this

paper that most of the extremely low emissions levelsare obtained using some type of exhaust gas treatment.

These levels can be misleading however, because theyrely on very strict maintenance of the treatment cata-

lyst. Efficiency of catalytic converters will decrease ifthey are coated with contaminants from the fuel, orfrom lube oil additives. They can also be damaged by

overheating, or poisoning from fuel contaminants oroperating at an incorrect air/fuel ratio on stoichiometric

engines.


15/16



References

1. DeYoung, Richard, “GL and Emissions-Air Pollution ”, Waukesha Engines Service Training Center.

2. Mayer, Charles, “AT-GL Sales & Application-Gas Engine Exhaust Emissions Overview” , WaukeshaEngines Application Engineering, 1990.

3. Radian Corporation, Austin, TX.

4. Obert, Edward F., “Internal Combustion Engines and Air Pollution ”, Harper & Row, Publishers, Inc.,1973.

5. Stachowicz, Robert, “Near Term Emission Reduction Technologies For Stationary, Natural Gas Fuel Engines ”, Air & Waste Management Association, SP-83, 1992.


16/16


Documents

Gas Engine Emissions Technology 4th Edition