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Guidance on gas treatmenttechnologies for landfill gasengines

Landfill directive

LFTGN 06

Lfd

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www.environment-agency.gov.uk

The Environment Agency is the leading public body protectingand

improving the environment in England and Wales.

Its our job to make sure that air, land and water are lookedafter by

everyone in todays society, so that tomorrows generationsinherit a

cleaner, healthier world.

Our work includes tackling flooding and pollution incidents,reducing

industrys impacts on the environment, cleaning up rivers,coastal

waters and contaminated land, and improving wildlifehabitats.

Published by:Environment AgencyRio House, Waterside Drive, AztecWestAlmondsbury, Bristol BS32 4UD

Tel: 08708 506506

Environment Agency August 2004

All rights reserved. This document may be reproduced withpriorpermission of the Environment Agency.

This report is printed on Cyclus Print, a 100% recycledstock,which is 100% post consumer waste and is totally chlorinefree.Water used is treated and in most cases returned to sourceinbetter condition than removed.

Dissemination Status:

Internal: Released to RegionsExternal: Public Domain

Research Contractor: This document was based on researchundertaken as R&DProject P1-330 by:LQM Ltd, Berwick ManleyAssociates Ltd, Diesel Consult,Landfills +Inc and Golder Associate(UK) Ltd.

Environment Agencys Project Team: The following were involved inthe production of this guidance:

Chris Deed Head Office (Project Manager) Jan Gronow HeadOfficeAlan Rosevear ThamesPeter Braithwaite Head OfficeRichardSmith Head OfficePeter Stanley Wales

Statement o f Use This guidance is one of a series of documentsrelating to themanagement of landfill gas. It is issued by theEnvironmentAgency and the Scottish Environment Protection Agency(SEPA)to be used in the regulation of landfills. It is primarilytargeted atregulatory officers and the waste industry. It will alsobe of inter-est to contractors, consultants and local authoritiesconcernedwith landfill gas emissions. Environment Agency and SEPAoffi-cers, servants or agents accept no liability whatsoever forany lossor damage arising from the interpretation or use of theinforma-tion, or reliance on views contained herein. It does notconstitutelaw, but officers may use it during their regulatory andenforce-ment activities. Any exemption from any of the requirementsof

legislation is not implied.

Throughout this document, the term 'regulator' relates jointlytothe Environment Agency and the Scottish EnvironmentProtectionAgency. SEPA does not necessarily support and is notbound by theterms of reference and recommendations of otherdocumentationmentioned in this guidance, and reserves theright to adopt andinterpret legislative requirements and appro-priate guidance as itsees fit. The term 'Agency' should thereforebe interpreted asappropriate.

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Environment Agency Guidance on gas treatment technologies forlandfill gas engines 1

Executive summary

The bulk of emissions from modern landfills are through thelandfill gasmanagement system and the landfill surface.The gasmanagement systemmay include enclosed flares and/ or utilisationplant, which destroy asignificant proportion of the methane andvolatile organic compoundswithin landfill gas, but can produceadditional combustion products.Thecomposition of landfill gasengine emissions depends on the gas supply,the design of thegenerating set and the engine management system.

This guidance explains the technical background for landfill gasclean-up methods and describes a consistentapproach for determiningthe level of clean-up required. It sets out an assessment procedurethat follows a costbenefit analysis approach to deciding whethergas clean-up is necessary or practicable. The assessmentprocedurehas the following six steps:

define the objective of the assessment and the options forpollution control; quantify the emissions from each option;quantify the environmental impacts of each option; compare optionsto identify the one with the lowest environmental impact; evaluatethe costs to implement each option; identify the option thatrepresents the cost-effective choice or best availabletechnique.

If these steps are followed, the decision procedure forselecting or rejecting a particular clean-up technologyistransparent and an audit trail is apparent. The guidance alsoconsiders a number of case studies, which arereported inEnvironment Agency R&D Technical Report P1-330/TR.

This guidance will be used when:

specifying conditions in Pollution Prevention and Control (PPC)permits (including landfill permits) thatprovide all appropriatemeasures to be taken against pollution and to limit emissions andimpact on theenvironment;

setting appropriate conditions in waste management licences.

Gas clean-up is a multi-stage operation that can help reduceenvironmental emissions and reduce enginemaintenance costs. Itinvolves both financial and environmental costs for the operator,but it improves the gassupply to conform to the requirements laiddown by the engine manufacturer and/or to achieve emissionstandardsset by the regulator.

Pretreatment processes fall into two groups: primarypretreatment processes aimed at de-watering and particulate removal(common to all landfills with

gas collection and combustion facilities) secondary pretreatmentprocesses aimed at removing a percentage of specific components ofthe supply

gas, e.g. halogens, sulphur or siloxane compounds.

Combustion treatment technologies are available for:

in-engine technology to treat the effects of siloxanes and fornitrogen oxide reduction; post-combustion processes to reducecarbon monoxide, unburnt hydrocarbons, hydrogen chloride and

hydrogen fluoride emissions.

Changes in air quality regulation and the tightening ofemissions from all processes mean that landfill gasengine operatorsmay need to consider gas clean-up technologies in theirapplications for PPC permits(including landfill permits).

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ContentsExecutive summary 1

1 Introduction 4

1.1 Target audience 4

1.2 Structure of this document 5

1.3 Technical background 5

1.4 Policy background 7

2 Gas quality, emission standards and operational requirements10

2.1 Introduction 10

2.2 Engine manufacturers specifications 10

2.3 Destruction efficiencies of gas engines 16

2.4 Engine emissions and their significance 17

2.5 Crankcase emissions 18

3 Decision process: assessing the use of clean-up technolog ies19

3.1 Clean-up approaches 19

3.2 Potential for substitute natural gas as a fuel for landfillgas engines 22

3.3 The framework for assessing gas clean-up 22

3.4 Collating basic information for the cost appraisal 23

3.5 How to perform a cost benefit analysis for gas clean-up25

4 Primary pretreatment technolog ies 33

4.1 Water/condensate knockout 33

4.2 Particulate filtration 36

4.3 Dealing with wastes from primary clean-up processes 36

5 Secondary pretreatment technolog ies 37

5.1 Introduction 37

5.2 Hydrogen sulphide pretreatment 37

5.3 Pretreatment of halogenated organics 39

5.4 Siloxane pretreatment 46

5.5 Developmental technologies 47

5.6 Dealing with wastes from secondary clean-up processes 48

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6 Engine management, in-eng ine and exhaust treatment 49

6.1 Introduction 49

6.2 Gas engines and their operation 49

6.3 Engine management systems and NO x 50

6.4 In-engine treatments 50

6.5 Exhaust after-treatments 52

7 Conclusions 54

Glossary and acronyms 55

References 61

Index 63

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pollutant abated (capital and operating costs) hasbeencalculated, judgement can be made on whether theprocess iscost-effective based on the Agencys interimrecommendations ofclean-up cost thresholds.

1 .2 S tructure o f th is document

This guidance is accessible at various levels, but isintended tobe used as shown in Figure 1.1.

Some background information may be required inorder tounderstand the setting in which the assessmentprocess is carriedout. This is provided in:

Section 1.4 (technical background) Section 1.5 (policybackground) Section 2. This describes how the supply gas

quality may affect emissions and explains howmanufacturersspecify gas supply standards tohelp maintain the gas engine in goodoperationalcondition between service intervals. Suchstandards mayserve as a surrogate indicator ofpotential problems.

Section 3 outlines the approach to take if it isconsidered thatgas treatment may be necessary. Thisapproach relies heavily on IPPCHorizontal GuidanceNote H1 (Environment Agency, 2002a). Figure1.1indicates which parts of Section 3 and other sectionsarerelevant to the various stages of the decision-makingprocess.

Sections 46 document the technologies currentlyconsideredapplicable to landfill gas engines.

Section 4 covers primary pretreatmenttechnologies that are incommon use. If the needfor additional gas treatment is indicated ataparticular site, the technologies in this sectionshould beconsidered first as they are the moststraightforward to apply.

Section 5 covers secondary pretreatmenttechnologies, which aregenerally more complexand costly.

Section 6 covers in-engine and post-combustiontreatmenttechnologies. Unlike secondarypretreatment technologies, these tendto becheaper than primary pretreatment technologies.

1 .3 Te chnical b ackg ro und

The bulk of atmospheric emissions from modernlandfills arethrough the gas management system andlandfill surface. The gasmanagement system mayinclude enclosed flares and/or utilisationplant. Thesedestroy much of the methane (CH 4) and volatile

organic compounds (VOCs) within the landfill gas, butcan produceadditional combustion products.

The quality of the exhaust emissions depends on:

the quality of the landfill gas supply the design of thegenerating set (dual-fuel engines

have different emission signatures to spark ignition

engines) how the engine management system is set up.

Research by the Environment Agency and industry(Gillett et al,2002; Environment Agency, 2004c) hasprovided information on boththe emissions from gasutilisation plant and the effect of clean-uptechnologieson landfill gas prior to combustion orin-engine/post-combustion treatments. Historically, limited gasclean-up has occurred in the UK. In the USA and EU, andmorerecently in the UK, it has been used successfully toproducesynthetic natural gas (SNG) to good effect.

In the context of this guidance, utilisation is consideredto bepower generation from landfill gas althoughmany clean-uptechnologies are often used in similarbiogas-fuelled projects orfor reticulation (SNG) projects.

Gas clean-up can be justified through:

the risk assessment of emissions for the purpose ofmanagingenvironmental impact and which needsto be considered as part of anapplication for aPollution Prevention and Control (PPC) permit;

the potential reduction in gas engine downtime balancing thecost of clean-up technologiesagainst savings in lost revenue duringdowntime

and repair/maintenance costs when engines faildue tocontaminants in the gas supply.

Both objectives can be achieved with the right choice ofclean-up technology provided it is made on costversusenvironmental/maintenance benefit grounds.

Simple practices may reduce the need or the extent of gasclean-up required. For example, the exhaust outletdesign should bevertically oriented to encouragedissipation and to prevent earlygrounding of exhaustplumes. Alternatively, it may be useful toreconsider thelocation of a proposed utilisation compound.However,the relocation or dispersion of existing engines shouldonlybe considered after other options have beenexhausted.

Combustion destroys typically more than 99 per cent of thevolatile components in landfill gas. Pre-combustiongas clean-upshould normally only be considered forlandfill gas if any of thecontaminants listed in Table 1.1are present in the gas above themaximumconcentration limits recommended by theenginemanufacturer.

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Evaluate the costs to implementeach option

Environment Agency Guidance on gas treatment technologies forlandfill gas engines6

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

Section 1.3

Technical backgroundto gas utilisation

Section 1.4

Policy backgroundto gas utilisation

Section 3.5

Section 5Secondarypretreatmenttechnologies

Section 6Engine management,in-engine and exhaustgastreatment

Section 3.4Collating basicinformation for thecost appraisalH1Guidance

Section 2Supply gas quality,emission standards andoperationalrequirements

Section 4Primary pretreatmenttechnologies

Section 3.5How to perform a CBAH1 Guidance

Define the objective of the assessment

Quantify the environmental impacts

Quantify the emissions from eachtreatment option available

Compare options and rank in orderof best environmentalperformance

Evaluate the costs to implementeach option

Identify the option (if any) thatrepresents mostcost-effective

technique

Section 3.5How to perform a CBAH1 Guidance

Section 3.5How to perform a CBAH1 Guidance

Do I need toknow about the supply

gas quality, possible primarypretreatments, engine operationor

emission standards beforeevaluatingclean-up

options?

Do I need to knowabout the technical orpolicy background to

gas utilisation? Yes

Yes

Yes

No

No

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Environment Agency Guidance on gas treatment technologies forlandfill gas engines 7

Table 1.1 Contaminants whose presence may require pre-combustiongas clean-up

Hydrogen sulphide and other sulphur gases Leads to chemicalcorrosion of the gas engine(and resultant emissions of acidicgases)

Halogenated organics Leads to chemical corrosion of the gasenginePotential contribution to emissions of acid gaseshydrogenchloride (HCl), hydrogen fluoride (HF) and PCDDs/PCDFs(dioxins and furans)

Silicon compounds Physical wear caused to the gas engine

Category Reason

In most cases, the decision to pretreat will be based oneconomicrather than on environmental factors as theresulting emissions ofsulphur oxides (SO x), HCl and HFare unlikely to exceed emissionstandards (see Section2). However, some sites with an atypicalsupply gas willneed to examine gas clean-up onenvironmentalgrounds.

In-engine clean-up should be considered if siliconcompounds arepresent in the gas above the enginemanufacturers recommendedmaximum concentrationlimit. It may also be considered to reduceemissions of nitrogen oxides (NO x), if NO x exceed genericemissionstandards (see Section 2).

Post-combustion exhaust gas clean-up should beconsidered if anyof the following emissions exceedgeneric emission standards or thesafe concentrations

determined by risk assessment (see Section 2): oxides ofnitrogen carbon monoxide (CO) methane and non-methane VOCs (NMVOCs)hydrogen chloride hydrogen fluoride sulphur oxides.

Engine management and post-combustion gas clean-upsystems arethe only effective way of managing NO xand CO emissions becausethese gases are formedduring the combustion process.

Gas engine management and emissions reduction areclosely linkedas practices employed to improve engineefficiency may reduce (orincrease) specific emissions. Itis therefore important to considerthe following inter-relationships:

technologies or approaches for improving gasengine performanceand reducing maintenancecosts

technologies or approaches simply for achievingemissionsreduction.

Established practices that already have a role in gas

clean-up include:

after-cooling and pre-chilling cyclone separation and otherde-watering

technologies particle filtration gas engine modifications andother engine

management techniques (both in engine and aftercombustion) forNO x, CO and particulate emissions.

Emerging and more specialist technologies include:

wet or dry hydrogen sulphide scrubbing; activatedcharcoal/carbon/zeolites; liquid and/or oil absorption; cryogenicseparation; solvent extraction; membrane separation for carbondioxide (CO 2),

oxygen and other gas scrubbing/ separationtechniques (these arepredominantly used in the

production of SNG, but may have application forgeneratingsets);

thermal oxidation; catalytic conversion; in-enginetreatments.

Most of the more specialist techniques listed abovehave beenused in combination on variouspilot/demonstration projects, but fewhave beenapplied regularly to landfill gas utilisation schemes.

1.4 Policy b ackg round

1.4.1 Renewable energy drivers

There have been two key economic drivers for thecontinuedincrease in landfill gas utilisation schemes.

The Non-Fossil Fuel Obligation (NFFO) drove theincrease inrenewable electricity generationcapacity during the 1990s andcontinues to besignificant due to the large number ofcontractedprojects still to be built. The utilisation oflandfillgas increased dramatically during the 1990s dueto the NFFO.As of September 2001, 400 MW ofthe 700 MW capacity awarded hadbeenconstructed.

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Environment Agency Guidance on gas treatment technologies forlandfill gas engines8

The Renewables Obligation (RO) was introducedin April 2002 andis a significant economicstimulus to utilise any landfill gasresources notalready contracted under NFFO. No further NFFOorderswill be made as the Renewable Obligationhas superseded the NFFO asthe driver for newrenewable energy in the UK. The RO placesanobligation on electricity suppliers to source acertain percentageof their output from renewablesources. The obligation for 2002 wasset at 3 percent of total sales of electricity, rising to 4.3percent in 2003, 4.9 per cent in 2004 and thenincreasing annuallyto 10.4 per cent in 2010, andmaintained at this level until2027.

The shortfall in available power generated by renewablesourcesis a powerful economic incentive to use landfillgas for electricitygeneration. The potential for higherprices has led to increasedinterest in smaller landfill gasprojects or projects that may beshorter lived and whichwould not have been economic under theNFFOsystem.

1.4 .2 Regulatory drivers

The management of landfill gas at permitted or licensedlandfillsis covered by three pieces of Europeanlegislation:

Waste Framework Directive (Council of theEuropean Communities,1991)

Integrated Pollution Prevention and Control (IPPC)Directive(Council of the European Union, 1996)

Landfill Directive (Council of the European Union,1999).

Until recently, landfills were regulated under theWasteManagement Licensing Regulations (1994) asamended. Landfillsites that hold waste managementlicences will continue to beregulated under theseRegulations until such time as the regulatoraccepts thesurrender of the licence for either of thefollowing:

The landfill is deemed closed before the LandfillDirective wasimplemented on 16 July 2001.

The landfill has not been granted a PPC permitafter thesubmission and consideration of a SiteConditioning Plan and whereapplication for apermit has been made, or where anappropriateclosure notice has been served.

Sites that closed after 16 July 2001 have to comply withtheLandfill Directive and subsequent regulations inrelation to siteclosure and aftercare. Therefore, much of the guidance in thisdocument also applies to sitesregulated under a waste managementlicence.

The IPPC Directive has been implemented in England

and Wales through the Pollution Prevention andControl (Englandand Wales) Regulations 2000 (2002

Regulations). In Scotland, it has been implementedthrough thePollution Prevention and Control(Scotland) Regulations 2000.

The IPPC regime uses a permitting system to produce

an integrated approach to controlling theenvironmental impactsof certain industrial activities.Under the IPPC Directive, theregulator must ensure,through appropriate permit conditions, thatinstallationsare operated in such a way that all theappropriatepreventive measures are taken against pollutionandparticularly through application of Best Available

Techniques (BAT).

BAT is defined in Regulation 3 and those matters thatmust beconsidered when determining BAT are set outin Schedule 2 of the PPCRegulations. In respect of landfilling activities, however, thecondition-making

powers of the PPC Regulations are largely dis-applied bytheLandfill (England and Wales) Regulations 2002(LandfillRegulations). The relevant technicalrequirements of the LandfillRegulations, together withits condition-making powers, cover theconstruction,operation, monitoring, closure and surrender oflandfills.

Landfill gas utilisation plant in England and Wales mayalso beregulated individually by the Agency under thePPC Regulations as acombustion activity burning fuelmanufactured from or composed of awaste other thanwaste oil or recovered oil. The threshold forsuch

control is plant with a thermal input of greater than 3MW.Landfill gas utilisation plant may also be regulatedby the Agencythrough a landfill permit where it formspart of the installation.Although BAT cannot be appliedto the activity of landfilling, theprinciples of BAT shouldbe applied in the landfill permit todirectly associatedactivities and other listed non-landfillactivities.

The technical requirements of the Landfill Directive havebeenimplemented in England and Wales via the LandfillRegulations(England and Wales) 2002 and, in Scotland,via the Landfill(Scotland) Regulations 2003.

The general requirements of the Regulations demandthe followinggas control measures.

Appropriate measures must be taken to controlthe accumulationand migration of landfill gas.

Landfill gas must be collected from all landfillsreceivingbiodegradable waste and the landfill gasmust be treated and, to theextent possible, used.

The collection, treatment and use of landfill gasmust be carriedout so as to minimise the risk tohuman health and damage to ordeterioration ofthe environment.

Landfill gas that cannot be used to produce

energy must be flared.

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Environment Agency Guidance on gas treatment technologies forlandfill gas engines 9

It is important to acknowledge the drivers for renewableenergywhen considering emission limits and the needfor gas clean-up tomeet these limits. Many of the earlyNFFO schemes paid higher pricesper unit of electricitysold, but the capital costs werecomparatively muchhigher. None of the schemes commissioned todatehave considered gas clean-up when bidding for autilisationcontract. This guidance should therefore beused to determine notonly whether a technology couldbe of benefit, but also whether itis cost-effective toimplement. Whether the cost-effectivenessconstitutesBAT applies only in the case of utilisation plant withaPPC/landfill permit provided under the 2000Regulations.

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Gas quality, emission standards andoperational requirements

Environment Agency Guidance on gas treatment technologies forlandfill gas engines10

2

2.1 Introduction

The calorific value of landfill gas is predominantlydeterminedby the methane/carbon dioxide ratio. Inaddition, landfill gas hasbeen found to contain over

500 trace components, which normally constituteonly about 1 percent by volume. These includehalogenated hydrocarbons, higheralkanes andaromatic hydrocarbons (Environment Agency,2002b). Mosthigher hydrocarbons will burn but, if their calorific value is lessthan methane, theirpresence will reduce the calorific value of thelandfillgas. Some of the aromatics (e.g. benzene) andchlorinatedhydrocarbons (e.g. chloroethene) give riseto health concerns, whileothers are highly odorous(e.g. terpenes, esters and thiols) andsome candamage gas utilisation plant (e.g. organohalogens,sulphurspecies and siloxanes).

The overall trace component composition of landfillgas thus hasimportant health and environmentalimplications and impacts on gasengine performance.

The engine manufacturers specifications represent agas qualitystandard at which supply gas clean-upmight need to be considered.Guidance onmonitoring landfill gas engines (EnvironmentAgency,2004a) provides factors for consideration of exhaustgastreatment or in-engine treatment and, in somecases, supply gasclean-up for some acid gasemissions.

2.2 Engine manufacturers specificat ions

When considering possible treatments for the removalof tracecomponents from landfill gas, it is importantto take into accountthe requirements placed on thesupply gas by engine manufacturers.Table 2.1provides a summary of recommended gasqualityspecifications from major suppliers of lean burnengines nowbeing used in the EU and USA; theseinclude two US manufacturers(Caterpillar andWaukesha), an Austrian manufacturer (Jenbacher)and

a German manufacturer (Deutz). These gas quality specificationsprovide a usefulstarting point for site-specific calculationsregarding

gas quality and when assessing the need for pre-combustiontreatment. Because engine manufacturerslink these specifications totheir warranty agreements,it is important that the inlet gas istested periodicallyusing a method and schedule approved bythemanufacturer.

In Table 2.1, the original measurement units providedby themanufacturer have been converted to SI units.

The specifications given in Table 2.1 are providedforinformation purposes only. Specifications may varywith enginetype, be subject to revision from time totime, and may not reflectspecific agreements madebetween the engine manufacturer andengineoperator.

2 .2 .1 Ca lo ri fi c va lu e

The calorific or heat value of the fuel isdeterminedpredominantly by the percentage of methanepresent.Typically, this is 3555 per centvolume/volume (v/v) for landfillgas in the UK.

Pure methane, which has a heat value 9.97 kW e/m 3,is the onlysignificant hydrocarbon constituent inlandfill gas converted tomechanical/electrical energyby the engine combustion process. Thelower themethane content, the greater the volume of gas thatmustpass through the engine to achieve the samepower output. This inturn means that potentiallymore aggressive gas constituents couldenter the

engine. This is why manufacturers limits foraggressive gasconstituents are defined per 100 percent methane.

Engine air to fuel ratio controllers can adjust thisratioautomatically as the methane content of the supplygas changes,although it may be necessary to modifythe system for significantvariations outside theoperating range of 45 15 per cent CH 4v/v.

The calorific value (CV) gives no indication oftheaggressiveness of the supply gas or likely emissions.Bulking ofsupply gas (i.e. supplying the input gas athigher pressure)typically occurs with low calorificvalue gas. The higher inletpressure of the gas willgenerally result in increased emissions ofmethane,NMVOCs and other products of incomplete

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combustion (PICs). Continuous assessment of flowrate and methanecontent is necessary to control andminimise this effect(Environment Agency, 2004a).

2 .2 .2 S ulp hur g ase s

Landfill gas contains a variety of sulphur compounds,several ofwhich are highly odorous. These includesulphides/disulphides (e.g.hydrogen sulphide,dimethyl sulphide, dimethyl disulphide,diethyldisulphide and carbon disulphide) and thiols,e.g.methanethiol (methyl mercaptan), ethanethiolandpropanethiol.

Sulphur compounds are corrosive in the presence of free water orthe moisture found within the engine oiland/or landfill gas. Thesecompounds can lead towear on engine piston rings and cylinderlinings. Gasrecirculation systems may increase the availability ofmoisture within the engine system. This also affectsoil quality,leading to the need for more frequent oilchanges.

For these reasons, individual engine manufacturersrecommendlimits for total sulphur compounds in theinlet landfill gas (seeTable 2.1) rather than individualcompounds.

The primary mechanism for the production of hydrogen sulphide (H2S) in landfills is the reduction of sulphate under anaerobicconditions by sulphate-reducing micro-organisms. Landfills that areexpected

to have higher concentrations of H 2S within thelandfill gasinclude:

unlined landfills in sulphate-rich geologicalmaterials such asgypsum (CaSO 4.2H 2O) quarriesor gypsiferous soils;

landfills where large quantities of gypsumplasterboard orsulphate-enriched sludges (e.g.from wastewater treatment or fluegasdesulphurisation) have been buried;

landfills where sulphate-rich soils have been usedasintermediate cover materials;

landfills where construction and demolition (C&D)

debris containing substantial quantities of gypsumwallboard hasbeen ground down and recycled asdaily or intermediate cover.

Typically, landfill gas contains

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Environment Agency Guidance on gas treatment technologies forlandfill gas engines12

T a b l e 2 . 1

R e c o m m e n

d e d s u p p

l y g a s s p e c

i f i c a

t i o n s

f r o m s e

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l u e a n

d v a r i a

b i l i t y

M a x

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v a r i a

t i o n :

1 4 . 4 M J / N m

3

1 5 . 7 2

3 . 6 M J / N m

3

> 1 5 . 7 3 M J / N m

3

< 0 . 5

% C H

4 ( v / v ) p e r

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( r e c o m m e n d e d r a n g e )

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t

2 , 0 0 0 m g

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3 C H

4 ( w i t h c a

t a l y s t

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b e a r

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H 2 S

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< 0 . 1

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A m m o n

i a

< 5 5 m g / N

m 3 C H 4 ( a p p

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T o t a l

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t e n t

S e e :

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3 C H

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S e e :

S u m o f

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S e e :

S u m o f C

l a n d

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T o t a l

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S e e :

S u m o f

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S e e :

S u m o f C

l a n d

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S u m

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t 4 :

4 0 0 m g /

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a t a l

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( S i )

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t 3 : 2 0 m g

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3 C H 4 w

i t h r e s t r i c

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o n l y ) 5

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W i t h o u

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t : s e e

b e l o w

6

W i t h c a t a

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D u s t

< 5 0 m g / N

m 3 C H 4 ( p a r

t i c l e s < 3 m

)

< 1 0 m g /

N m

3 C H

4 ( p a r

t i c l e s < 3

0 m g /

N m 3 C H 4 ( p a r

t i c l e s < 1 m

) 3 R e m o v a l o f p a r t

i c l e s > 0 . 3

m

m a x i m u m

3 1 0 m

)

O i l / r e s

i d u a

l o i l

< 5 m g / N m

3 C H 4

< 4 0 0 m g /

N m

3 C H

4

< 4 5 m g /

N m 3 C H 4 ( o i l )

< 2 % v / v

l i q u i

d f u e l

( o i l v a p o r s > C 5 )

h y d r o c a r

b o n s

a t a t c o

l d e s

t

i n l e t t e m p e r a t u r e

C o n s t i t u e n t

J e n b a c h e r

D e u t z

C a t e r p i l l e r

W a u k e s h a

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Environment Agency Guidance on gas treatment technologies forlandfill gas engines 13

M i s c e l

l a n e o u s

P r o j e c t s p e c

i f i c

l i m i t s :

N o

G l y c o

l

h y d r o c a r

b o n s o

l v e n

t

v a p o u r s

R e l a t

i v e

h u m

i d i t y / m o i s t u r e

< 8 0 % w i

t h z e r o c o n d e n s a

t e

< 6 0 8 0 %

< 8 0 % a t m i n i m u m

f u e l

Z e r o

l i q u i

d w a

t e r ;

t e m p e r a

t u r e

r e c o m m e n

d c h i l l i n g g a s

t o

4 C f o l l o w e d

b y c o a l e s c i n g

f i l t e r a n

d t h e n r e

h e a t

i n g

t o 2 9

3 5 C ; d e w p o

i n t

s h o u

l d b e a t l e a s t

1 1 C

b e l o w

t e m p e r a t u r e o f

i n l e t

g a s

P r e s s u r e a t

i n l e t

T u r b o c

h a r g e d e n g i n e s :

8 0 2

0 0 m

b a r

U p t o 2 , 0 0 0 b a r

P r e - c o m

b u s t

i o n c h a m

b e r :

M o d e l s 6

1 2 - 6

1 6 : 2 , 5

0 0 4 , 0

0 0 m

b a r

M o d e l

6 2 0 : 3 , 0 0 0 4 , 0 0 0 m

b a r

G a s p r e s s u r e

f l u c t u a

t i o n

< 1 0 m

b a r / s e c o n d

< 1 0 % o f s e

t v a

l u e a t a

f r e q u e n c y o f < 1

0 p e r

h o u r

I n l e t g a s

t e m p e r a

t u r e

< 4 0 C

1 0 - 5

0 C

> - 2 9 C a n

d < 6 0 C

C H 4

( % v / v )

4 0 %

R e c o m m e n d e d r a

t i o o f

C H 4 :

C O

2 i s 1 . 1 1

. 2

M e t h a n e 7

~ 1 4 0 f o r

l a n d

f i l l g a s

H y d

r o g e n

( % v

/ v )

< 1 2 %

C o n s t i t u e n t

J e n b a c h e r

D e u t z

C a t e r p i l l e r

W a u k e s h a

N o t e s :

1 T h e s p e c

i f i c a

t i o n s g i v e n

i n t h i s t a b l e a r e p r o v

i d e d

f o r

i n f o r m a t

i o n p u r p o s e s o n

l y .

2 D a t e s o f

i n f o r m a t

i o n :

J e n b a c

h e r ,

2 0 0 0 ( T I 1 0 0 0

- 0 3 0 0 ) ; D e u

t z , 1

9 9 9 ; C a t e r p i

l l a r ,

1 9 9 7 ; W a u

k e s h a ,

2 0 0 0

.

3 S p e c i

f i c a t

i o n s s t a t e d

b y m a n u f a c

t u r e r s

i n m

g / M J w e r e c o n v e r

t e d t o m g /

N m

3 C H

4 a s s u m

i n g a c a

l o r i f

i c v a

l u e

f o r

C H

4 o f

3 7 . 5 M

J / N m

3 .

4 O

t h e r c o n d

i t i o n s .

A s i n g l e e x c e e d a n c e o f 3

0 p e r c e n t a b o v e

1 0 0 m g /

N m

3 C H

4 i s p e r m

i s s i b l e o u

t o f

f o u r a n a l y s e s p e r y e a r . L

i m i t i n g v a

l u e s

f o r u s e d o i

l a n d s u m p c a p a c i

t y m u s

t b e

o b s e r v e

d ( s e e

J e n b a c

h e r

T e c h n i c a

l I n s

t r u c t

i o n

N o .

1 0 0 0

0 0 9 9 )

.

5 S p e c i

f i c a t

i o n s s t a t e d

b y m a n u f a c

t u r e r s

i n m

g / l l a n

d f i l l g a s w e r e c o n v e r

t e d t o m g

/ N m

3 C H

4 a s s u m

i n g

5 0 p e r c e n t

C H

4 ( v / v ) .

6 R e l a t

i v e

l i m i t i n g v a

l u e o f < 0 . 0

2 a c c o r d

i n g

t o t h e

f o l l o w

i n g c a

l c u l a t

i o n

( w i t h o u

t c a

t a l y s

t ) :

R e l a t

i v e

l i m i t i n g v a

l u e =

( m g /

k g S i i n e n g

i n e o i

l ) x

( t o t a l o i

l q u a n t

i t y i n l i t r e s )

( e n g

i n e p o w e r

i n k W ) x

( o i l s e r v

i c e

t i m e

i n h o u r s )

7 M

e t h a n e n u m

b e r

f o r n a

t u r a

l g a s

i s t y p i c a l l y

b e t w e e n

7 0 a n

d 9 2

, m e t

h a n e

1 0 0 ( k n o c k

l e s s

) a n

d h y d r o g e n

0 ( k n o c k - f r

i e n d

l y ) .

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Environment Agency Guidance on gas treatment technologies forlandfill gas engines14

In general, landfill gas quality appears to beimproving with thewithdrawal of certain substancessuch as hydrochlorofluorocarbons(HCFCs) fromwidespread use. Sites that accept wastes withhighchlorine and fluorine concentrations are likely toproducelandfill gas and similarly exhaust emissions where HCl, HF andPCDDs/PCDFs may be abovethe norm.

While a third of UK landfills have aggressive gascharacteristicsrequiring high Total Base Number(TBN) lubricating oils, only asmall percentage of theexhaust emissions with HCl and HF mayrequiretreatment. These emissions might need to beaddressed atlandfills where industrial waste has beenaccepted and whereconcentrations in the exhaustare shown to be potentially harmful asdetermined bya site-specific risk assessment/emission standard.

2.2.4 Ammonia

Ammonia is a problem for digester gas engines andmanufacturersset strict limits for it for enginesburning digester gas. It isfound occasionally inlandfill gas and manufacturers may applysimilar limitsto landfill gas engines. The combustion ofammonialeads to the formation of nitric oxide (NO), which canreactto form other oxides of nitrogen in theatmosphere.

2.2.5 Sil icon compounds and si loxanes

Silicon, silicon dioxide and siloxanes all behave indifferentways. An identical landfill gas engine used attwo different siteswith a high silicon content canresult in widely varying effects,making trial anderror solutions the current norm.

Discarded consumer products (including cosmetics) inthe landfilltend to be the main source of silicon inthe supply gas. Manyconsumer products (hair care,skin care, underarm deodorants) andcommerciallubricants contain silicones (a large group ofrelatedorganosilicon polymers).

The term siloxane refers to a subgroup of siliconescontainingSi-O bonds with organic radicals bondedto the silicon atom; theorganic radicals can includemethyl, ethyl and other organicfunctional groups.Siloxanes are present in landfills through:

the disposal of containers with small amounts ofremainingsilicon-containing product

the landfilling of wastewater treatment sludges(siloxanes areretained during the process steps).

Organosiloxanes are semi-volatile organosiliconcompounds which,while not an aggressive gascomponent in terms of emissions, can beconverted

to solid inorganic siliceous deposits within theenginecombustion chamber. They form a coating or lacquer

Figure 2.1 Golden laquer of siloxane build-upevident on cylinderliner

At the combustion conditions within landfill gasengines, organicsilicon compounds present in thelandfill gas may be deposited onthe cylinder head assolid inorganic silicon compounds. Thisdepositedmaterial is white to light grey, somewhatlaminar,generally opaque, and may exhibit a partial topoorcrystalline structure. Few analyses of these depositsare givenin the literature; existing data indicate thatcrystalline SiO 2 ispresent alongside other metals insolid forms (Niemann et al .,1997; Hagmann et al ,1999; M. Niemann, personal communication,2001).

These deposits severely reduce engine life. The enginehas to bestripped down and the solids scrapedmanually from the piston,cylinder head and valves.

During the combustion process, some siliconcompounds are alsopartitioned to the engine oil,which needs to be changed morefrequently at siteswith high siloxane levels in the inlet gas fuel.Engine

manufacturers thus recommend direct monitoring of siliconbuild-up in the engine oil. The increasing use

on all surfaces contacted by the lubricating oil andcan alterthe oil retaining surface finish of cylinderliners.

Siloxanes can:

enter the engine as insoluble matter in the gasfuel, forming awhite deposit in the combustionchamber;

be produced in the combustion chamber itself; form a goldenlacquer on components outside the

combustion chamber. This lacquer can beespecially evident on thepiston-ring wiped surfaceof the cylinder liner. The lacquer has atendency tofill the oil retaining honing pattern but rarelybuildsto the extent of requiring attention prior toroutine overhaul (seeFigure 2.1).

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of these compounds in consumer and commercialproducts suggeststhat problems with volatilesiloxanes in landfill gas engines arelikely to increase.

There is currently no standard method for analysing

volatile siloxanes in a gaseous matrix; at least ten ormoremethods are being used (e.g. Aramata andSaitoh, 1997; Grumping etal. , 1998; Hone and Fry,1994; Huppman et al ., 1996; Kala et al .,1997;Schweigkofler and Niessner, 1999; Stoddart et al .,1999;Varaprath and Lehmann, 1997; Wachholz et al. ,1995). There is noconsensus within the landfill gasindustry regarding which method touse and therehas been no rigorous comparison of methods usingacommon set of samples.

Observations of individual well samples andcomposite landfillgas samples vary between 100 ppm v/v total organic silicon, basedon a gaschromatography/atomic emission detection method(GC/AED).For some applications and especially theevaluation of potentialtreatment methods,determination of speciated siloxanes may bedesirableusing a combined GC/AED-MS (mass spectrometry)method (e.g.Schweigkofler and Niessner, 1999).

Siloxanes do not directly cause problems with gasengine exhaustemissions, though the increased wearmay show itself as an increasein SO x emissions aslubricating oil is burnt. Typically, this isunlikely toexceed any risk-based criterion for emissions

management and the decision to implement gasclean-up forsiloxane management purposes isentirely based on cost.

2.2.6 Dust

Dust can be drawn into engines either in the landfillgas itselfor in the combustion air. Particulate filtersand cyclones (seeSection 4), which are relativelycommon, remove liquid droplets andparticulates(above a limiting threshold size) from the supplygas.However, due to the dusty external environment,attention shouldalso be paid to the combustion airdrawn into the engine containeror building andespecially to the air drawn into the engine.

Two stages of inlet air filtration are therefore involved. Theyare located:

on the engine enclosure inlet. The filtration level isthatnecessary to prevent an unacceptable, visualbuild-up of dust onengine and ancillary plant.

at the engine inlet. This filtration is particularlyimportant asabrasive silica is a major culprit ofpremature component wear (downto 5 mm onthe cell inlet filter and down to 2 mm on thesecondaryengine mounted filtration).

Environment Agency Guidance on gas treatment technologies forlandfill gas engines 15

Cyclone or oil-wetted filters can be used if thelocation hasdesert-like conditions or if dustyindustrial processes such ascement production arelocated near the generating plant.

All utilisation plant should have dust filtrationequipmentinstalled if particulates in the supply gasare identified as aparticular problem. Furtherinformation is given in Section 4.

2 .2 .7 Lub ri ca tin g oil

The combustion of landfill gas containing siloxanesandorganohalogen compounds introduces acids intothe lubricating oil ofthe engine. It is known from thevolume of high total base number(TBN) oilformulations used on landfill gas enginesthatapproximately one third of UK landfill gas generatorssufferfrom aggressive concentrations of organohalogens (Hussein Younis,Exxon Mobil,personal communication, 2002).

The acid forming chloride, fluoride and sulphurcompoundscontaminate the lubricating oil mostly bybypassing the piston rings(blow-by) and, to a lesserextent, via the air and exhaust valveguides. Keepingthe engine operating temperatures of jacketcoolingwater and associated lubricating oil temperatureshigh (toavoid dew points) may reduce the effect of these acids. However, ahigher oil temperature doesreduce the thickness of the crankshaftoil film and anoptimum balance must be achieved.

Corrosion is prevented by keeping the oil alkaline andby usingcorrosion resistant components (especially atthe crankshaft,camshaft and other bearings).Aluminium-tin may be used to replaceyellow metalbearings such as copper or phosphor bronze.

Lubricating oil additives are used to maintainalkalinity; theseadditives must be non-combustibleand thus produce more ash. Someash serves as alubricant for valve seats. However, if there istoomuch ash, maintenance intervals decrease and in-cylindertemperature sensors become less effective

due to premature detonation owing to a build-up of deposits.

A balance has therefore to be achieved between ahigh alkalinity(high TBN) oil and the frequency of oilreplacement. Longer periodsbetween oil changesmay be achieved with larger engine sumpcapacities.An engine approaching the need for overhaul willallowgreater absorption owing to increased blow-by.Oil replacementfrequencies are typically 750850hours. Shutting down engines toundertake oilreplacement usually coincides with sparkplugreplacement.

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Environment Agency Guidance on gas treatment technologies forlandfill gas engines16

2.3 Dest ruc tion effic ienc ies of gas engines

The environment can benefit from the destruction of somecomponents of landfill gas in the combustionchamber of an engineparticularly if the alternativeis uncontrolled surface emissions ofthesecomponents. However, the short residence time inthe gas enginemeans that no trace gas componentcan be destroyed with 100 per centefficiency.

Furthermore, other components such as HCl, HF andSO x will beproduced as a result of the combustion of chlorine-, fluorine- andsulphur-containingcompounds in the landfill gas.

Table 2.2 gives typical destruction efficiencies of varioustypes of organic compounds; these valueswere obtained by monitoringa number of landfill gasengines in he UK (Gillett et al. , 2002).The limit of detection of these compounds in the engineexhaustmeans that some of the minima are only estimatesand that theactual destruction efficiency will bemuch higher than that theminimum given in

Table 2.2. The destruction of methane to form carbon dioxideistypically 9699.6 per cent. Longer chain alkanes arenormallydestroyed at between 92 and >99.9 percent efficiency, butGillett et al. (2002) reported thatbutane was destroyed by only 70per cent and thatsome lighter alkanes appeared to be formed.

The unburnt methane and other hydrocarbonsleaving the exhaustrepresent a relatively smallfraction of the fuel, and the amount ofmethaneslippage is a feature of engine design. Somemethane escapesfrom the combustion chamberbefore it is closed, while some methaneremains

after combustion and is discharged on the non-combustionstroke.

In general, Gillet et al . (2002) also observed highdestructionefficiencies (up to 99.9 per cent) forsimple substituted alkanessuch as alcohols, aldehydesand ketones, but there were someexceptions. Thecombustion chamber and exhaust system of a gasengineis a highly reactive chemical environment and

some simple compounds may be formedpreferentially from thedestruction of other complexorganic species.

Aromatic compounds are destroyed at between 92and 99.9 per centefficiency. Terpenes, which areresponsible for some odour events onlandfills, aredestroyed at >99.9 per cent efficiency.Sulphurcompounds, which are responsible for most odourcomplaints,are destroyed at between 8.7 and 96.6per cent efficiency. Hydrogensulphide, the mostcommon sulphur compound, has been found toundergo70.696.6 per cent destruction in a gas

engine (this observation contradicts claims that thegas isflammable and thus will be completelydestroyed).

The destruction efficiency for halogenatedcompounds potentiallysome of the most toxiccompounds in landfill gas is between 70 and99.7per cent. However, research suggests that someanomalouscalculated destruction efficiencies are aresult of very smallamounts of these compoundsbeing present.

The observed values shown in Table 2.2 indicate thatgas enginesare capable of destroying tracecomponents to high degrees ofefficiency. These

Table 2.2 Typical destruction efficiencies for various types oforganic compound*

Methane 96.0 99.6

Alkanes 70.2 >99.9

Alkenes 50.1 >99.6

Alcohols 84.1 >99.8

Aldehydes >42.4 95.9

Ketones >87.4 99.9

Aromatic hydrocarbons 92.0 >99.9

Terpenes >99.9

Sulphur compounds >8.7 >96.6

Halogenated hydrocarbons >70.1 >99.7

Type of compound Minimum (% ) Maximum (% )

* Based on Gillett et al . (2002)

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Environment Agency Guidance on gas treatment technologies forlandfill gas engines 17

Table 2.3 Emission standards for landfill gas engines*

NO x 650 500

CO 1,500 1,400

Total VOCs 1,750 1,000(including CH

4)

NMVOCs 150 75

Other components Determined by site-specific risk assessmentDetermined by site-specificrisk assessment

Component in exhaustEmission standard for spark ignition engines (m g/ Nm 3)

Co mm issio ne d b etwe en 1 January 1 99 8 Co mm issio ne dafte rand 31 Decemb er 2005 31 Decemb er 2005

* Based on Environment Agency, 2004a.

Note: These are minimum standards based on normal operatingconditions and site-specific risk assessments may require astricteremissions standard to be applied. Risk assessment must becarried out for plant commissioned before 1 January 1998.

observations relate to actual performance;theoretically, thehigher the peak combustiontemperature, the greater the efficiencyof destructionof VOCs, etc. However, other factors areinvolved.

The higher the thermal efficiency of an internalcombustionengine, the lower the emission of unburnt hydrocarbons. However,the higher thermalefficiency results in a higher peakcombustiontemperature, which in turn increases NO x production.

NO x emissions can be reduced by an engine designthateffectively reduces thermal efficiency, either byhumidification ofthe inlet air/gas mixture beforeactual combustion (thus loweringthe peakcombustion temperature) or by constantly adjustingengineoperational parameters/thermal efficiencywithin a relatively smallband. The latter is controlled

by the engine management system (EMS).Most modern engines aredesigned and adjusted bythe EMS to retain design parameters, andmay be set,for example, to hold NO x emissions at 500 mg/Nm 3.Sometypes of engine become more expensive tooperate at this settingowing to the greater load onthe ignition system, but the situationis manageable.Different engine types have varying amounts ofadjustment and thus produce different levels of unburnthydrocarbons at a given NO x setting.

2.4 Engine emissions and the ir sign ificance

The Agency has published generic standards for themajor exhaustgas emissions from landfill gas enginesand guidance on the typicaltrace components in raw

landfill gas to be considered as part of any riskassessment(Environment Agency, 2004a). Theemission standards are given inTable 2.3.

Action is necessary if the concentration in the exhaustgas ofany of the named components exceeds thegeneric emission standard.Initially, this could beattention to the EMS or further emissionsmonitoring.If this is not appropriate, then a more formalevaluationof the emissions should be undertaken.

This should include a review of the need for gasclean-up.

In some locations, there may be sensitive receptorsclose to, orinfluenced by, the exhaust stack. If site-specific risk assessmentfinds that the concentration of particulates, PCDDs/PCDFs, heavymetals, HCl, HF orH2S in the emissions are higher than theagreedtolerable concentration at the site boundary, thenanassessment of the need for gas clean-up is necessary.A strategyfor site-specific risk assessment is describedin Guidance on th emanagement of landfill gas (Environment Agency, 2004b). Thisapproach involvessite-specific development of a conceptual model ofthe site and a tiered risk assessment process, whichmay includedispersion modelling.

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Environment Agency Guidance on gas treatment technologies forlandfill gas engines18

Development of the conceptual site model involves:

defining the nature of the landfill, the gasutilisation plantand the baseline environmentalconditions;

identifying the source term releases, the pathwaysand receptorsfor the plant emissions, and theprocesses likely to occur alongeach of thesourcepathwayreceptor linkages. In the case ofengines,the most likely pathway is atmosphericdispersion of the exhaustplume.

At the hazard identification and risk screening stage,thesensitivity of the receptors should be consideredand an initialselection of the appropriateenvironmental benchmark for eachreceptor shouldbe made. Suitable benchmarks includeEnvironmentalAssessment Levels (EALs) or air qualityobjectives).

Long-term and short-term EALs are given inHorizontal GuidanceNote H1 (Environment Agency,2002a).

An atmospheric dispersion model of the fate of theexhaust plumeis likely to form part of the PPCapplication; this information willalso be useful in therisk assessment. The procedures that shouldbefollowed in the cost benefit analysis of the need forgas clean-upare given in Section 3.

2 .5 Crankcase e missio ns

The engine exhaust is not the only source of atmosphericemissions from gas engines. Combustionproducts that pass the pistonrings (blow-by) and, toa lesser extent, escape past valve guideclearances,cause a positive pressure in the engine crankcaseandcontaminate the lubricating oil.

Historically, a crankcase vacuum of around 1 inchwater gauge wasused to counter this pressure andminimise lubricating oil leaks.However, extraction of the crankcase emissions reduces the rate ofcontamination of the lubricating oil producing adirect saving inoil costs.

Exhaust from the extractor fan takes the form of alow volume andflow rate smoke. This exhaust orthe crankcase fumes is often passedthrough alength of pipework to promote condensation of theoil; theremaining vapour is then passed through acoalescer/filter. Simplyexhausting the fumes belowwater is another method that has beenemployed.Increasing the volume of flow to positively purgethecrankcase could be considered a form of in-engineclean-up.

Gillett et al . (2002) found that untreated crankcase

exhaust had high concentrations of aggressive gases,but at verylow mass flow. This volume can be up to

30 per cent of the total mass emission rates of unburnthydrocarbons and SO x from the engine, andtreatment is consideredbest practice. The directrelease of crankcase exhaust emissions isgenerally nolonger acceptable and any crankcase emissions needto beincluded in any PPC reporting requirements.

Options for management of this emission source are:

Recirculation of the crankcase fumes into thecombustion chamberinlet this affectscomponent life, but the emissions are combinedanddiluted in the exhaust.

Recirculation by injection after combustion thisincreases thelife of engine components, while theemissions are combined anddiluted in theexhaust.

Installation of coalescer and filter this increasescomponentlife but produces an additional, lowvolume waste stream.

The cheapest option is to recirculate and mostenginemanufacturers (Deutz, Jenbacher and Caterpillar)have adoptedit. A coalescer and filter could be fittedat a cost of 1,5003,000(depending on flow rateand degree of reduction). If the supply gasis highlyacidic, then there will be to additional cost of disposingof the waste stream.

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Environment Agency Guidance on gas treatment technologies forlandfill gas engines 19

Decision process: assessing the use of clean-up technologies

3

3.1 Clean-up ap proache s

Raw landfill gas is a complex and variable mixture of gases andvapours. Active management of such a

mixture will be affected by the trace componentsandcontaminants. The role of pre-combustion gas clean-up is toreduce the effects of the contaminants on thehandling plant and topromote a high degree of operational effectiveness. This, in turn,may improvethe management of secondary waste streams,includingemissions to atmosphere. Enginemanagement systems andpost-combustion activitiescan also be used to manage emissions toatmosphere.

Clean-up options range from commonly adoptedsimple watertrapping and filtration to complexintegrated systems linked to theenergy utilisation

plant or landfill gas abstraction plant.

A typical gas combustion scheme generally includesthe featuresshown in Figure 3.1. The raw gas entersthe utilisation set-up via ade-watering and filtrationknockout device that removes moistureandparticulates. This ensures that flare burners do notbecomeblocked and improves combustionperformance within the enginecylinders. A gascompressor (or booster) increases the landfillgaspressure to ensure effective operation of the flareburners andadequate supply to the gas engine. Flowmetering devices and aslam-shut valve, provide thevolume flow rate to the flare orengine, and act as afinal safety control device. The flamearrestors preventflashback of a flame to the fuel feeder pipe.

Figure 3.1 Typical combustion scheme for landfill gas

From landfill

Filter

Knockout vessel

Gas compressor/booster

Flow metering

Flow metering

Slam-shut valve

Slam-shut valve

Flame arrestor

Engine

Alternator

High temperature flare

Burners

Pilot

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Environment Agency Guidance on gas treatment technologies forlandfill gas engines20

The simple systems can be defined as primaryprocessing and themore complex ones as secondaryprocessing. In broad terms, theoptions can besummarised as shown in Figure 3.2. Table 3.1givesexamples of pre-combustion gas clean-up processes.

The range of options for the clean-up of landfill gas isquiteextensive. This guidance attempts to categorisethese options andcover the numerous examplesreported in the literature. As shown inTable 3.1,there are a number of systems that do not sit neatlyinany one category; these are the so-called multiplesystems. Inreality, all landfill gas clean-up processesare multiple in naturebecause there is no singleprocess that takes raw landfill gas andproduces aclean fuel.

Figure 3.2 Clean-up options and emissions management

Early development of the processes was in responseto a need toproduce SNG. This required the removalnot only of tracecontaminants, but also all non-combustible components (principallycarbon dioxideand nitrogen). The fact that utilisation oftheprocessed gas resulted in clean combustion withminimal damage tothe utilisation plant and a loweratmospheric burden was a bonus.This attractedoperators of more recent systems, whichnormallyutilise unprocessed landfill gas. Nevertheless,thisapproach has not been taken up in the UK (seeSection 3.2).

A particular process may be applicable to the clean-up of morethan one contaminant in the landfill gas.

Therefore, if more than one contaminant is present,then thecalculated cost of abatement should beshared between them.

Raw gas

Primaryclean-up

Pre-combustion Boiler

Postcombustion

Vehicleengine

Clean gas

Control

Exhaust

Exhaust

Flue gas

Waste 3

Waste 3

By-product (CO 2)Waste 2

Waste 1

SNG

Compressionand

polishing

Pre-combustion Engine

Postcombustion

Secondaryclean-up

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Environment Agency Guidance on gas treatment technologies forlandfill gas engines22

3.2 Potent ia l for subs titute na tura l gas as afuel for landfill ga s eng ines

The current global gas market is such that SNGproduced fromlandfill gas is likely to be financially

marginal at best; this is confirmed by the casestudiesconsidered in recent Agency research (EnvironmentAgency,2004d). Developing plant to exploit thismarket in the UK is thusunlikely to satisfy investmentcriteria.

However, various other options for clean-up processesmay beworth developing to enhance the operationof existing and futuresystems for utilising raw landfillgas. The focus of suchdevelopment will be theremoval of trace contaminants(especiallyhalogenated organics and siloxanes) withoutnecessarilyhaving to remove the non-combustible

bulk gases. Nevertheless, the economics of the clean-up optionsare currently far from clear, but athorough review could show thatminimising the totalmass flow prior to clean-up (i.e. first using alow-costprocess to remove non-combustibles essentiallycarbondioxide) might offer significant operationaland financialadvantages.

Carbon dioxide removal processes effectivelyupgrade thecalorific value of the gas. Suchprocesses fall into four basiccategories:

absorption by a liquid (solvent) adsorption by a granular soliddifferential transport (membrane separation) cryogenicseparation.

The underlying principles defining these categoriesare describedin Section 5. However, the futureapplicability of landfill gasclean-up suggests that themost appropriate and, by implication, thelowestcost option is likely to be liquid absorption usingwater asthe solvent. However, further evaluation andfinancial analysis mayshow otherwise, and at thisstage, no options should be ruledout.

When producing SNG, the principal requirement of

gas clean-up technology is to remove (or minimise)theconcentrations of reactive trace components. Thiscan be partlyachieved during upgrading to removecarbon dioxide; but to be fullyeffective, it requiresadditional processing stages. These stagesare likely tobe sorption processes that target either individualorgroups of reactive contaminants. The optionsshowing the greatestpromise are activated carbonand proprietary compounds based onactivatedcarbon. However, solvent absorption offers theadvantage ofcontinuous processing and thus shouldnot be rejected until a moredetailed analysis has

been undertaken.

3 .3 The framework fo r a sse ssing gasclean-up

The basis for this approach is explained in HorizontalGuidanceH1 (Environment Agency, 2002a). Rigorous

cost benefit analysis of the various gas clean-upoptions has notbeen carried out in this guidance dueto:

a lack of adequate cost and performance data forcomparablesystems;

available information on multiple systems isfocussed on SNG asthe product and not onlandfill gas engine use;

a reticence within the industry to discuss the costsofimplementation of any technology unless a realsituation isinvolved.

However, the mechanism for conducting a rigorousCBA is describedfor situations when these databecome available for a site-specificrequirement.

The aim of Horizontal Guidance Note H1 is to:

provide information on the preferred methods forquantifyingenvironmental impacts to all media(air, water and land)

calculate costs provide guidelines on how to resolve anycross-

media conflicts.

The methods outlined in Horizontal Guidance NoteH1 can be usedto conduct a costs/benefits appraisalof options to determine bestpractice or BAT forselected releases from any installation.Spreadsheetsare provided in help users evaluate the optionsorassess the overall environmental impact of emissions.In order togain a PPC permit, operators have toshow that their proposalsrepresent best practice orBAT to prevent and minimise pollutionfrom theirinstallation.

The following six steps in the assessmentmethodology apply andare described in more detailin Section 3.5.

1 Define the objective of the assessment and theoptions to beconsidered.

2 Quantify the emissions from each option.3 Quantify theenvironmental impacts resulting from

the different options.4 Compare options and rank in order ofbest overall

environmental performance.5 Evaluate the costs to implement eachoption.6 Identify the option that represents the most cost-

effective technique or BAT by balancingenvironmental benefitsagainst costs.

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Environment Agency Guidance on gas treatment technologies forlandfill gas engines 23

3.4 Colla ting basic information for the costappraisal

This section describes how to collect the informationneeded toperform a CBA of gas clean-up options and

provides a method for unambiguous presentation of the costs ofclean-up versus the potentialenvironmental benefits.

In order to understand the implementation of thecost appraisal,it is necessary to define the terms usedwithin the assessment.

Discount rat e

The discount rate usually reflects the cost of thecapitalinvestment to the operator and typically variesbetween 6 and 12 percent per annum, dependingon the level of risk associated with thecompany,

industrial sector or particular project. The samediscount rateshould be used for all options underconsideration and the selectionof a particular valueshould be justified by the operator(particularly if it isoutside the typical range). In calculations,thediscount rate should be expressed as a decimal andnot as apercentage, e.g. 0.06 and not 6 per cent.

Table 3.2 Current UK asset life guideline values foruse in costappraisals

Buildings 20

Major components, e.g. landfill gas 15engines, generators,pollution controlequipment

Intermediate components, 10e.g. compressors, some filters andgroundhandling equipment

Minor components, e.g. motors, servos, 5filters

Asset Lifetime

(years)

Table 3.3 Calculation of the present value of capital costs

Capital expenditure 2,000 2,000 2,000

Discount rate 0.1 0.1

Value today 2,000 2,000 x 0.9 2,000 x 0.9 x 0.9

Equals 2,000 1,800 1,620

Present value in first year 5,420

Year 1 2 3

Assumed lif e

The assumed life of the clean-up option should bebased on theasset life. Current UK guideline valuesfor the different assets aregiven in Table 3.2.

Without clean-up, an atypical gas will reduce assetlife furtherand this should be factored into the costbenefit analysis.Operators should be able to justifyvariations from the values givenin Table 3.2.

Capital costs

Capital costs include the cost of:

purchasing equipment needed for the pollutioncontroltechniques

labour and materials for installing that equipment sitepreparation (including dismantling) and

buildings

other indirect installation demands.Capital costs should includenot only thoseassociated with stand-alone pollutioncontrolequipment, but also the cost of making integratedprocesschanges or installing control and monitoringsystems.

It is important to describe the limits of the activityorcomponents to which the costs apply. For example,the choice of atype of technology that is inherentlyless polluting would requireall components of thattechnology to be included in this limit.

Estimates of engineering costs are generallysatisfactory forcost submissions, although anysignificant uncertainties should beindicated. This isespecially important for components that couldhavea major influence on a decision between differentoptions. Whereavailable, the cost of each majorpiece of equipment should bedocumented, withdata supplied by an equipment vendor or areferencedsource.

If capital costs are spread over more than one year,these shouldbe reduced to the present value in thefirst year as indicated inTable 3.3.

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Table 3.4 gives a template for recording the breakdownof capitaland investment costs. These should beprovided either as pounds oras a percentage of totalcapital costs; the anticipated year ofexpenditure should

also be stated.Operating costs and revenues

No additional revenues are expected to arise from theclean-up oflandfill gas prior to its use in a reciprocatingengine. However, itis appropriate to include revenuesin the case of gas clean-up forthe provision of syntheticnatural gas (SNG) for selling to thenational grid or forcases where improved energy production andefficiencymay be a consequence of clean-up.

The recurring annual costs for pollution control systemsconsistof three elements:

direct (variable and semi-variable) costs indirect (fixed) costsrecovery credits.

The recurring annual change in operating costs foroptionsconsists of the additional costs minus any costsavings resultingfrom implementation of that option.

This should include any changes in production capacity.

Direct co sts are those that tend to be proportional orpartiallyproportional to the quantity of releasesprocessed by the controlsystem per unit time or, in thecase of cleaner processes, theamount of material

processed or manufactured per unit time. They includecostsfor:

raw materials utilities (steam, electricity, process andcooling

water, etc.) waste treatment and disposal

maintenance materials replacement parts operating, supervisoryand maintenance labour.

Indirect or fixed annual costs are those whosevalues are totallyindependent of the release flow rateand which would be incurredeven if the pollutioncontrol system were shut down. They includesuchcategories as:

overheads administrative charges insurance premiums businessrates.

The direct and indirect annual costs may be partiallyoffset byrecovery credits that arise from:

materials or energy recovered by the controlsystem which may besold, recycled to theprocess, or reused elsewhere on-site (butoffset bythe costs necessary for their processing, storageandtransportation, and any other steps requiredto make the recoveredmaterials or energyreusable or resaleable);

reduced labour requirements; enhanced production efficiencies;improvements to product quality.

Environment Agency Guidance on gas treatment technologies forlandfill gas engines24

Table 3.4 Breakdown of capital/investment costs

Pollution control equipment costs: Primary pollution controlequipment Auxiliary equipment Instrumentation Modifications toexisting equipment

Installation costs: Land costs General site preparationBuildings and civil works Labour and materials

Other capital costs: Project definition, design and planningTesting and start-up costs Contingency Working capital End-of-lifeclean-up costs (NB this cost would typically be discounted to apresent value)

Specific cost b reakdown Included in capital costs Cost in / % of total Year = yes capital cost/ other = no (specify units)

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Environment Agency Guidance on gas treatment technologies forlandfill gas engines 25

Table 3.5 Breakdown of operating costs and revenues

Additional costs: Additional labour for operation andmaintenance Water/sewage Fuel/energy costs (specify energy/fueltype) Waste treatment and disposal Other materials and parts (givedetails) Costs of any additional pollution abatement equipmentoperation (give details) Insurance premiums Taxes on property Othergeneral overheads

Cost saving/revenues: Energy savings By-products recovered/soldEnvironmental tax/charge savings Other

Specific cost b reakdown Included in operating cost Total annualcost in Year = yes / % o f total operating = no cost/ other(specify units)

In the case of gas clean-up for landfill gas engines,theincrease in servicing intervals, reduction of oilconsumption andincrease in engine efficiency shouldall be taken into account tooffset the annualoperating costs.

A template for recording the breakdown of operatingand revenuecosts is given in Table 3.5. These costsshould be provided eitheras pounds or as apercentage of total capital costs. The anticipatedyearof expenditure should also be stated.

The templates given in Tables 3.4 and 3.5 are basedon theguidelines issued by the EuropeanEnvironment Agency (EEA, 1999) andprovide a basisfor operators to detail the breakdown of costs.Thetemplates have been adapted to show elements moreappropriate tothe waste management sector. As a

minimum, operators should make a tick to indicatewhich elementshave been included in the assessmentof capital and operatingcosts.

3.5 How to perform a cost benefit analysisfor gas clean-up

Six key contaminants or contaminant groups arepotentiallytreatable. This section deals with removalof selected componentsfrom the supply gas(hydrogen sulphide, halogenated organicsandsiloxanes) or the exhaust gas (NO x, carbon monoxide

and hydrogen chloride/hydrogen fluoride) in order toreduceemissions or improve the economics of

operation. In addition, there is the option of producing SNG;this option is described in publishedcase studies (EnvironmentAgency, 2004d) and is notcovered in detail in this guidance.

Figure 3.3 shows the six groups of contaminants andthe mostappropriate clean-up technology for theindividual treatment of eachgroup.

The technologies indicated in Figure 3.3 are discussedinSections 46. The Agency considers that primarytreatment (Section 4)will be required at all landfills,and that its relatively lowimplementation cost meansthat these techniques should be usedwhenever andwherever necessary.

The secondary treatment sector is an emergingindustry and, assuch, new information on availabletechnologies will supersede theinformation given inthis guidance. While many of thetechnologiesidentified have been around since the beginning of thelandfill gas industry, many others are new andsome are justreinventions and repackaging of oldchemistry. Availability,suitability and cost should bethe deciding factors whenshortlisting a technologyfor further consideration.

The remainder of this section describes the six-stepassessmentprocess and illustrates its use through twoexamples:

the removal of hydrogen sulphide the removal of halogenatedsolvents.

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Environment Agency Guidance on gas treatment technologies forlandfill gas engines26

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Environment Agency Guidance on gas treatment technologies forlandfill gas engines 27

3.5.1 Step 1: define the objective and the options t o beconsidered

The first step in the assessment process is to:

identify the objective(s) of the assessment list the potentialoptions to be considered.

Example 1

Objective

Treatment of high hydrogen sulphideconcentrations in landfillgas is required to reduceengine wear and subsequentatmosphericemissions of SO x in a sensitive location aneedindicated by site-specific risk assessment).

Possible clean-up opt ions

Hydrogen sulphide clean-up can be achieved bydry or wetdesulphurisation. Both techniques arepre-combustion, secondaryclean-up technologiesand information on these can be found inSection5 and case studies 14 (Environment Agency,2004d).

Example 2

Objective

Treatment of high chlorine concentrations in thesupply gas ortreatment of high HCl emissions inthe exhaust a need indicated bysite-specific riskassessment.

Possible clean-up opt ions

Clean-up of chlorine in the supply gas can beachieved bypressure water scrubbing, pressureswing adsorption, or membraneseparationtechniques as pre-combustion, secondaryclean-uptechniques (see Section 5 and case studies 511described inEnvironment Agency, 2004d), or byexhaust dry scrubbing (see Section6 and case

study 18).

3.5.2 Step 2: quantify the emissions from eachtreatment option

When emission standards have been set by theregulator, it is astraightforward task to ascertain thelevel of clean-up required toachieve this limit.

The degree of gas clean-up required will depend onth