Guidance on Gas Treatment Technologies

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www.environment-agency.gov.uk

Guidance on gas treatmenttechnologies for landfill gas engines

Landfill directive

LFTGN 06

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www.environment-agency.gov.uk

The Environment Agency is the leading public body protecting and

improving the environment in England and Wales.

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

everyone in todays society, so that tomorrows generations inherit 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 wildlife habitats.

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

Tel: 08708 506506

Environment Agency August 2004

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

This report is printed on Cyclus Print, a 100% recycled stock,which is 100% post consumer waste and is totally chlorine free.Water used is treated and in most cases returned to source inbetter condition than removed.

Dissemination Status:

Internal: Released to RegionsExternal: Public Domain

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

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

Chris Deed Head Office (Project Manager) Jan Gronow Head OfficeAlan Rosevear ThamesPeter Braithwaite Head OfficeRichard Smith Head OfficePeter Stanley Wales

Statement o f Use This guidance is one of a series of documents relating to themanagement of landfill gas. It is issued by the EnvironmentAgency and the Scottish Environment Protection Agency (SEPA)to be used in the regulation of landfills. It is primarily targeted atregulatory officers and the waste industry. It will also be of inter-est to contractors, consultants and local authorities concernedwith landfill gas emissions. Environment Agency and SEPA offi-cers, servants or agents accept no liability whatsoever for any lossor damage arising from the interpretation or use of the informa-tion, or reliance on views contained herein. It does not constitutelaw, but officers may use it during their regulatory and enforce-ment activities. Any exemption from any of the requirements of

legislation is not implied.

Throughout this document, the term 'regulator' relates jointly tothe Environment Agency and the Scottish EnvironmentProtection Agency. SEPA does not necessarily support and is notbound by the terms of reference and recommendations of otherdocumentation mentioned in this guidance, and reserves theright to adopt and interpret legislative requirements and appro-priate guidance as it sees fit. The term 'Agency' should thereforebe interpreted as appropriate.


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Environment Agency Guidance on gas treatment technologies for landfill gas engines 1

Executive summary

The bulk of emissions from modern landfills are through the landfill gasmanagement system and the landfill surface.The gas management systemmay include enclosed flares and/ or utilisation plant, which destroy asignificant proportion of the methane and volatile organic compoundswithin landfill gas, but can produce additional combustion products.Thecomposition of landfill gas engine emissions depends on the gas supply,the design of the generating set and the engine management system.

This guidance explains the technical background for landfill gas clean-up methods and describes a consistentapproach for determining the level of clean-up required. It sets out an assessment procedure that follows a costbenefit analysis approach to deciding whether gas clean-up is necessary or practicable. The assessmentprocedure has the following six steps:

define the objective of the assessment and the options for pollution control; quantify the emissions from each option; quantify the environmental impacts of each option; compare options to identify the one with the lowest environmental impact; evaluate the costs to implement each option; identify the option that represents the cost-effective choice or best available technique.

If these steps are followed, the decision procedure for selecting or rejecting a particular clean-up technology istransparent and an audit trail is apparent. The guidance also considers a number of case studies, which arereported in Environment 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 appropriate measures to be taken against pollution and to limit emissions and impact on theenvironment;

setting appropriate conditions in waste management licences.

Gas clean-up is a multi-stage operation that can help reduce environmental emissions and reduce enginemaintenance costs. It involves both financial and environmental costs for the operator, but it improves the gassupply to conform to the requirements laid down by the engine manufacturer and/or to achieve emissionstandards set by the regulator.

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

gas collection and combustion facilities) secondary pretreatment processes aimed at removing a percentage of specific components of the 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 for nitrogen oxide reduction; post-combustion processes to reduce carbon monoxide, unburnt hydrocarbons, hydrogen chloride and

hydrogen fluoride emissions.

Changes in air quality regulation and the tightening of emissions from all processes mean that landfill gasengine operators may need to consider gas clean-up technologies in their applications 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 requirements 10

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 ies 19

3.1 Clean-up approaches 19

3.2 Potential for substitute natural gas as a fuel for landfill gas 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-up 25

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

Environment Agency Guidance on gas treatment technologies for landfill gas engines2


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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) has beencalculated, judgement can be made on whether theprocess is cost-effective based on the Agencys interimrecommendations of clean-up cost thresholds.

1 .2 S tructure o f th is document

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

Some background information may be required inorder to understand the setting in which the assessmentprocess is carried out. This is provided in:

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

quality may affect emissions and explains howmanufacturers specify gas supply standards tohelp maintain the gas engine in good operationalcondition between service intervals. Suchstandards may serve as a surrogate indicator ofpotential problems.

Section 3 outlines the approach to take if it isconsidered that gas treatment may be necessary. Thisapproach relies heavily on IPPC Horizontal GuidanceNote H1 (Environment Agency, 2002a). Figure 1.1indicates which parts of Section 3 and other sectionsare relevant to the various stages of the decision-makingprocess.

Sections 46 document the technologies currentlyconsidered applicable to landfill gas engines.

Section 4 covers primary pretreatmenttechnologies that are in common use. If the needfor additional gas treatment is indicated at aparticular site, the technologies in this sectionshould be considered first as they are the moststraightforward to apply.

Section 5 covers secondary pretreatmenttechnologies, which are generally more complexand costly.

Section 6 covers in-engine and post-combustiontreatment technologies. Unlike secondarypretreatment technologies, these tend to becheaper than primary pretreatment technologies.

1 .3 Te chnical b ackg ro und

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

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

The quality of the exhaust emissions depends on:

the quality of the landfill gas supply the design of the generating 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 both the emissions from gasutilisation plant and the effect of clean-up technologieson landfill gas prior to combustion or in-engine/post-combustion treatments. Historically, limited gas clean-up has occurred in the UK. In the USA and EU, andmore recently in the UK, it has been used successfully toproduce synthetic natural gas (SNG) to good effect.

In the context of this guidance, utilisation is consideredto be power generation from landfill gas althoughmany clean-up technologies are often used in similarbiogas-fuelled projects or for reticulation (SNG) projects.

Gas clean-up can be justified through:

the risk assessment of emissions for the purpose ofmanaging environmental impact and which needsto be considered as part of an application for aPollution Prevention and Control (PPC) permit;

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

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

Both objectives can be achieved with the right choice of clean-up technology provided it is made on cost versusenvironmental/maintenance benefit grounds.

Simple practices may reduce the need or the extent of gas clean-up required. For example, the exhaust outletdesign should be vertically oriented to encouragedissipation and to prevent early grounding of exhaustplumes. Alternatively, it may be useful to reconsider thelocation of a proposed utilisation compound. However,the relocation or dispersion of existing engines shouldonly be considered after other options have beenexhausted.

Combustion destroys typically more than 99 per cent of the volatile components in landfill gas. Pre-combustiongas clean-up should normally only be considered forlandfill gas if any of the contaminants listed in Table 1.1are present in the gas above the maximumconcentration limits recommended by the enginemanufacturer.



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Evaluate the costs to implementeach option


Figure 1.1 How to use this guidance in an assessment of cost-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 exhaustgas treatment

Section 3.4Collating basicinformation for thecost appraisalH1 Guidance

Section 2Supply gas quality,emission standards andoperational requirements

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 environmental performance

Evaluate the costs to implementeach option

Identify the option (if any) thatrepresents most cost-effective

technique



Do I need toknow about the supply

gas quality, possible primarypretreatments, engine operation or

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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Table 1.1 Contaminants whose presence may require pre-combustion gas clean-up

Hydrogen sulphide and other sulphur gases Leads to chemical corrosion of the gas engine(and resultant emissions of acidic gases)

Halogenated organics Leads to chemical corrosion of the gas enginePotential contribution to emissions of acid gases hydrogenchloride (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 oneconomic rather than on environmental factors as theresulting emissions of sulphur oxides (SO x), HCl and HFare unlikely to exceed emission standards (see Section2). However, some sites with an atypical supply gas willneed to examine gas clean-up on environmentalgrounds.

In-engine clean-up should be considered if siliconcompounds are present in the gas above the enginemanufacturers recommended maximum concentrationlimit. It may also be considered to reduce emissions of nitrogen oxides (NO x), if NO x exceed generic emissionstandards (see Section 2).

Post-combustion exhaust gas clean-up should beconsidered if any of the following emissions exceedgeneric emission standards or the safe concentrations

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

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

Gas engine management and emissions reduction areclosely linked as practices employed to improve engineefficiency may reduce (or increase) specific emissions. Itis therefore important to consider the following inter-relationships:

technologies or approaches for improving gasengine performance and reducing maintenancecosts

technologies or approaches simply for achievingemissions reduction.

Established practices that already have a role in gas

clean-up include:

after-cooling and pre-chilling cyclone separation and other de-watering

technologies particle filtration gas engine modifications and other engine

management techniques (both in engine and aftercombustion) for NO x, CO and particulate emissions.

Emerging and more specialist technologies include:

wet or dry hydrogen sulphide scrubbing; activated charcoal/carbon/zeolites; liquid and/or oil absorption; cryogenic separation; solvent extraction; membrane separation for carbon dioxide (CO 2),

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

production of SNG, but may have application forgenerating sets);

thermal oxidation; catalytic conversion; in-engine treatments.

Most of the more specialist techniques listed abovehave been used in combination on variouspilot/demonstration projects, but few have 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 thecontinued increase in landfill gas utilisation schemes.

The Non-Fossil Fuel Obligation (NFFO) drove theincrease in renewable electricity generationcapacity during the 1990s and continues to besignificant due to the large number of contractedprojects still to be built. The utilisation of landfillgas increased dramatically during the 1990s dueto the NFFO. As of September 2001, 400 MW ofthe 700 MW capacity awarded had beenconstructed.


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The Renewables Obligation (RO) was introducedin April 2002 and is a significant economicstimulus to utilise any landfill gas resources notalready contracted under NFFO. No further NFFOorders will be made as the Renewable Obligationhas superseded the NFFO as the driver for newrenewable energy in the UK. The RO places anobligation on electricity suppliers to source acertain percentage of their output from renewablesources. The obligation for 2002 was set at 3 percent of total sales of electricity, rising to 4.3 percent in 2003, 4.9 per cent in 2004 and thenincreasing annually to 10.4 per cent in 2010, andmaintained at this level until 2027.

The shortfall in available power generated by renewablesources is a powerful economic incentive to use landfillgas for electricity generation. The potential for higherprices has led to increased interest in smaller landfill gasprojects or projects that may be shorter lived and whichwould not have been economic under the NFFOsystem.

1.4 .2 Regulatory drivers

The management of landfill gas at permitted or licensedlandfills is 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 the WasteManagement Licensing Regulations (1994) asamended. Landfill sites that hold waste managementlicences will continue to be regulated under theseRegulations until such time as the regulator accepts thesurrender of the licence for either of the following:

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

The landfill has not been granted a PPC permitafter the submission and consideration of a SiteConditioning Plan and where application for apermit has been made, or where an appropriateclosure notice has been served.

Sites that closed after 16 July 2001 have to comply withthe Landfill Directive and subsequent regulations inrelation to site closure and aftercare. Therefore, much of the guidance in this document also applies to sitesregulated under a waste management licence.

The IPPC Directive has been implemented in England

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

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

The IPPC regime uses a permitting system to produce

an integrated approach to controlling theenvironmental impacts of certain industrial activities.Under the IPPC Directive, the regulator must ensure,through appropriate permit conditions, that installationsare operated in such a way that all the appropriatepreventive measures are taken against pollution andparticularly through application of Best Available

Techniques (BAT).

BAT is defined in Regulation 3 and those matters thatmust be considered when determining BAT are set outin Schedule 2 of the PPC Regulations. In respect of landfilling activities, however, the condition-making

powers of the PPC Regulations are largely dis-applied bythe Landfill (England and Wales) Regulations 2002(Landfill Regulations). The relevant technicalrequirements of the Landfill Regulations, together withits condition-making powers, cover the construction,operation, monitoring, closure and surrender of landfills.

Landfill gas utilisation plant in England and Wales mayalso be regulated individually by the Agency under thePPC Regulations as a combustion activity burning fuelmanufactured from or composed of a waste other thanwaste oil or recovered oil. The threshold for such

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

The technical requirements of the Landfill Directive havebeen implemented 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 following gas control measures.

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

Landfill gas must be collected from all landfillsreceiving biodegradable waste and the landfill gasmust be treated and, to the extent possible, used.

The collection, treatment and use of landfill gasmust be carried out so as to minimise the risk tohuman health and damage to or deterioration ofthe environment.

Landfill gas that cannot be used to produce

energy must be flared.


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It is important to acknowledge the drivers for renewableenergy when considering emission limits and the needfor gas clean-up to meet these limits. Many of the earlyNFFO schemes paid higher prices per unit of electricitysold, but the capital costs were comparatively muchhigher. None of the schemes commissioned to datehave considered gas clean-up when bidding for autilisation contract. This guidance should therefore beused to determine not only whether a technology couldbe of benefit, but also whether it is cost-effective toimplement. Whether the cost-effectiveness constitutesBAT applies only in the case of utilisation plant with aPPC/landfill permit provided under the 2000Regulations.


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Gas quality, emission standards andoperational requirements


2

2.1 Introduction

The calorific value of landfill gas is predominantlydetermined by the methane/carbon dioxide ratio. Inaddition, landfill gas has been found to contain over

500 trace components, which normally constituteonly about 1 per cent by volume. These includehalogenated hydrocarbons, higher alkanes andaromatic hydrocarbons (Environment Agency,2002b). Most higher hydrocarbons will burn but, if their calorific value is less than methane, theirpresence will reduce the calorific value of the landfillgas. Some of the aromatics (e.g. benzene) andchlorinated hydrocarbons (e.g. chloroethene) give riseto health concerns, while others are highly odorous(e.g. terpenes, esters and thiols) and some candamage gas utilisation plant (e.g. organohalogens,sulphur species and siloxanes).

The overall trace component composition of landfillgas thus has important health and environmentalimplications and impacts on gas engine performance.

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

2.2 Engine manufacturers specificat ions

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

a German manufacturer (Deutz). These gas quality specifications provide a usefulstarting point for site-specific calculations regarding

gas quality and when assessing the need for pre-combustion treatment. Because engine manufacturerslink these specifications to their warranty agreements,it is important that the inlet gas is tested periodicallyusing a method and schedule approved by themanufacturer.

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

The specifications given in Table 2.1 are provided forinformation purposes only. Specifications may varywith engine type, be subject to revision from time totime, and may not reflect specific agreements madebetween the engine manufacturer and engineoperator.

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

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

Pure methane, which has a heat value 9.97 kW e/m 3,is the only significant hydrocarbon constituent inlandfill gas converted to mechanical/electrical energyby the engine combustion process. The lower themethane content, the greater the volume of gas thatmust pass through the engine to achieve the samepower output. This in turn means that potentiallymore aggressive gas constituents could enter the

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

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

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


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combustion (PICs). Continuous assessment of flowrate and methane content 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 of which 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), ethanethiol andpropanethiol.

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

For these reasons, individual engine manufacturersrecommend limits for total sulphur compounds in theinlet landfill gas (see Table 2.1) rather than individualcompounds.

The primary mechanism for the production of hydrogen sulphide (H 2S) in landfills is the reduction of sulphate under anaerobic conditions by sulphate-reducing micro-organisms. Landfills that are expected

to have higher concentrations of H 2S within thelandfill gas include:

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

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

landfills where sulphate-rich soils have been usedas intermediate cover materials;

landfills where construction and demolition (C&D)

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

Typically, landfill gas contains


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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

l e c t e d m a n u

f a c t u r e r s o f

l a n d

f i l l g a s e n g i n e

1 , 2

C a l o r i f

i c v a

l u e a n

d v a r i a

b i l i t y

M a x

i m u m

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

3 0 s e c o n d s

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

T o t a l s u

l p h u r c o n t e n

t

2 , 0 0 0 m g

/ N m

3 C H

4 ( w i t h c a

t a l y s t

)

< 2 , 2

0 0 m g /

N m

3 C H 4

< 2 , 1

4 0 m g

H 2

S p e r

N m

3 C H 4

< 7 1 5 m g /

N m 3

C H 4 ( t o t a l

S

1 , 1 5 0 m g

/ N m

3 C H

4 ( w i t h o u

t c a

t a l y s t

)

( t o t a l

S a s

H 2

S ) 3

b e a r

i n g c o m p o u n d s )

( t o t a l

S a s

H 2

S )

H 2 S

c o n t e n

t

< 0 . 1

5 % v / v

A m m o n

i a

< 5 5 m g / N

m 3 C H 4 ( a p p

l i e s m o s

t l y t o

< 1 0 5 m g

N H

3 p e r

N m

3 ( a p p

l i e s

a n a e r o

b i c

d i g e s t e r g a s

c o m

b i n e

d

m a i n l y

t o a n a e r o

b i c

d i g e s t e r g a s

s p e c

i f i c a t

i o n s

f o r a l

l b i o g a s s y s t e m s )

c o m

b i n e d s p e c

i f i c a

t i o n s

f o r a l

l

b i o g a s s y s t e m s )

3

T o t a l

C l c o n

t e n t

S e e :

S u m o f

C l a n d

F

< 1 0 0 m g /

N m

3 C H

4

S e e :

S u m o f

C l a n d

F

S e e :

S u m o f C

l a n d

F

T o t a l

F c o n t e n

t

S e e :

S u m o f

C l a n d

F

< 5 0 m g /

N m

3 C H

4

S e e :

S u m o f

C l a n d

F

S e e :

S u m o f C

l a n d

F

S u m

o f C l a n d

F

W i t h o u

t c a t a l y s

t 4 :

4 0 0 m g /

N m

3 C H 4 n o w a r r a n t y

a t a l

l

W i t h c a t a

l y s t : 0 m g /

N m

3 C H

4

S i l i c o n

( S i )

O l d s t a n d a r

d

< 1 0 m g /

N m

3 C H

4

< 2 1 m g /

N m 3 C H 4

3

< 5 0 m g /

N m 3

C H

4 t o t a l

W i t h o u

t c a t a l y s

t 3 : 2 0 m g

/ N m

3 C H 4 w

i t h r e s t r i c

t i o n )

o n l y ) 5

N e w s t a n

d a r d

W i t h o u

t c a t a l y s

t : s e e

b e l o w

6

W i t h c a t a

l y s t

( o l d o r n e w s t a n

d a r d

) :

0 m g /

N m 3 C H 4

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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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

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< 8 0 % w i

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

t e

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P r e s s u r e a t

i n l e t

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8 0 2

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M o d e l s 6

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N o t e s :

1 T h e s p e c

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4 ( v / v ) .

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l u e o f < 0 . 0

2 a c c o r d

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f o l l o w

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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

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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

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( o i l s e r v

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t i m e

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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

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7 0 a n

d 9 2

, m e t

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l e s s

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0 ( k n o c k - f r

i e n d

l y ) .


16/72


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

While a third of UK landfills have aggressive gascharacteristics requiring high Total Base Number(TBN) lubricating oils, only a small percentage of theexhaust emissions with HCl and HF may requiretreatment. These emissions might need to beaddressed at landfills where industrial waste has beenaccepted and where concentrations in the exhaustare shown to be potentially harmful as determined bya site-specific risk assessment/emission standard.

2.2.4 Ammonia

Ammonia is a problem for digester gas engines andmanufacturers set strict limits for it for enginesburning digester gas. It is found occasionally inlandfill gas and manufacturers may apply similar limitsto landfill gas engines. The combustion of ammonialeads to the formation of nitric oxide (NO), which canreact to form other oxides of nitrogen in theatmosphere.

2.2.5 Sil icon compounds and si loxanes

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

Discarded consumer products (including cosmetics) inthe landfill tend to be the main source of silicon inthe supply gas. Many consumer products (hair care,skin care, underarm deodorants) and commerciallubricants contain silicones (a large group of relatedorganosilicon polymers).

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

the disposal of containers with small amounts ofremaining silicon-containing product

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

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

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

Figure 2.1 Golden laquer of siloxane build-upevident on cylinder liner

At the combustion conditions within landfill gasengines, organic silicon compounds present in thelandfill gas may be deposited on the cylinder head assolid inorganic silicon compounds. This depositedmaterial is white to light grey, somewhat laminar,generally opaque, and may exhibit a partial to poorcrystalline structure. Few analyses of these depositsare given in the literature; existing data indicate thatcrystalline SiO 2 is present 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 be stripped down and the solids scrapedmanually from the piston, cylinder head and valves.

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

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

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

Siloxanes can:

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

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

combustion chamber. This lacquer can beespecially evident on the piston-ring wiped surfaceof the cylinder liner. The lacquer has a tendency tofill the oil retaining honing pattern but rarelybuilds to the extent of requiring attention prior toroutine overhaul (see Figure 2.1).


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of these compounds in consumer and commercialproducts suggests that problems with volatilesiloxanes in landfill gas engines are likely to increase.

There is currently no standard method for analysing

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

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

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

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

2.2.6 Dust

Dust can be drawn into engines either in the landfillgas itself or in the combustion air. Particulate filtersand cyclones (see Section 4), which are relativelycommon, remove liquid droplets and particulates(above a limiting threshold size) from the supply gas.However, due to the dusty external environment,attention should also be paid to the combustion airdrawn into the engine container or building andespecially to the air drawn into the engine.

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

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

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


Cyclone or oil-wetted filters can be used if thelocation has desert-like conditions or if dustyindustrial processes such as cement production arelocated near the generating plant.

All utilisation plant should have dust filtrationequipment installed if particulates in the supply gasare identified as a particular problem. Furtherinformation is given in Section 4.

2 .2 .7 Lub ri ca tin g oil

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

The acid forming chloride, fluoride and sulphurcompounds contaminate the lubricating oil mostly bybypassing the piston rings (blow-by) and, to a lesserextent, via the air and exhaust valve guides. Keepingthe engine operating temperatures of jacket coolingwater and associated lubricating oil temperatureshigh (to avoid dew points) may reduce the effect of these acids. However, a higher oil temperature doesreduce the thickness of the crankshaft oil film and anoptimum balance must be achieved.

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

Lubricating oil additives are used to maintainalkalinity; these additives must be non-combustibleand thus produce more ash. Some ash serves as alubricant for valve seats. However, if there is toomuch ash, maintenance intervals decrease and in-cylinder temperature 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 periods between oil changesmay be achieved with larger engine sump capacities.An engine approaching the need for overhaul willallow greater absorption owing to increased blow-by.Oil replacement frequencies are typically 750850hours. Shutting down engines to undertake oilreplacement usually coincides with spark plugreplacement.


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2.3 Dest ruc tion effic ienc ies of gas engines

The environment can benefit from the destruction of some components of landfill gas in the combustionchamber of an engine particularly if the alternativeis uncontrolled surface emissions of thesecomponents. However, the short residence time inthe gas engine means that no trace gas componentcan be destroyed with 100 per cent efficiency.

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

Table 2.2 gives typical destruction efficiencies of various types of organic compounds; these valueswere obtained by monitoring a number of landfill gasengines in he UK (Gillett et al. , 2002). The limit of detection of these compounds in the engine exhaustmeans that some of the minima are only estimatesand that the actual destruction efficiency will bemuch higher than that the minimum given in

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

The unburnt methane and other hydrocarbonsleaving the exhaust represent a relatively smallfraction of the fuel, and the amount of methaneslippage is a feature of engine design. Somemethane escapes from the combustion chamberbefore it is closed, while some methane remains

after combustion and is discharged on the non-combustion stroke.

In general, Gillet et al . (2002) also observed highdestruction efficiencies (up to 99.9 per cent) forsimple substituted alkanes such as alcohols, aldehydesand ketones, but there were some exceptions. Thecombustion chamber and exhaust system of a gasengine is a highly reactive chemical environment and

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

Aromatic compounds are destroyed at between 92and 99.9 per cent efficiency. Terpenes, which areresponsible for some odour events on landfills, 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. Hydrogen sulphide, the mostcommon sulphur compound, has been found toundergo 70.696.6 per cent destruction in a gas

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

The destruction efficiency for halogenatedcompounds potentially some of the most toxiccompounds in landfill gas is between 70 and 99.7per cent. However, research suggests that someanomalous calculated destruction efficiencies are aresult of very small amounts of these compoundsbeing present.

The observed values shown in Table 2.2 indicate thatgas engines are capable of destroying tracecomponents to high degrees of efficiency. These

Table 2.2 Typical destruction efficiencies for various types of organic 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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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 assessment Determined by site-specificrisk assessment

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

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

* Based on Environment Agency, 2004a.

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

observations relate to actual performance;theoretically, the higher the peak combustiontemperature, the greater the efficiency of destructionof VOCs, etc. However, other factors are involved.

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

NO x emissions can be reduced by an engine designthat effectively reduces thermal efficiency, either byhumidification of the inlet air/gas mixture beforeactual combustion (thus lowering the peakcombustion temperature) or by constantly adjustingengine operational parameters/thermal efficiencywithin a relatively small band. The latter is controlled

by the engine management system (EMS).Most modern engines are designed and adjusted bythe EMS to retain design parameters, and may be set,for example, to hold NO x emissions at 500 mg/Nm 3.Some types of engine become more expensive tooperate at this setting owing to the greater load onthe ignition system, but the situation is manageable.Different engine types have varying amounts of adjustment and thus produce different levels of unburnt hydrocarbons at a given NO x setting.

2.4 Engine emissions and the ir sign ificance

The Agency has published generic standards for themajor exhaust gas emissions from landfill gas enginesand guidance on the typical trace components in raw

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

Action is necessary if the concentration in the exhaustgas of any of the named components exceeds thegeneric emission standard. Initially, this could beattention to the EMS or further emissions monitoring.If this is not appropriate, then a more formalevaluation of 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, or influenced by, the exhaust stack. If site-specific risk assessment finds that the concentration of particulates, PCDDs/PCDFs, heavy metals, HCl, HF orH2S in the emissions are higher than the agreedtolerable concentration at the site boundary, then anassessment of the need for gas clean-up is necessary.A strategy for site-specific risk assessment is describedin Guidance on th e management of landfill gas (Environment Agency, 2004b). This approach involvessite-specific development of a conceptual model of the site and a tiered risk assessment process, whichmay include dispersion modelling.


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Development of the conceptual site model involves:

defining the nature of the landfill, the gasutilisation plant and the baseline environmentalconditions;

identifying the source term releases, the pathwaysand receptors for the plant emissions, and theprocesses likely to occur along each of thesourcepathwayreceptor linkages. In the case ofengines, the most likely pathway is atmosphericdispersion of the exhaust plume.

At the hazard identification and risk screening stage,the sensitivity of the receptors should be consideredand an initial selection of the appropriateenvironmental benchmark for each receptor shouldbe made. Suitable benchmarks include EnvironmentalAssessment Levels (EALs) or air quality objectives).

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

An atmospheric dispersion model of the fate of theexhaust plume is likely to form part of the PPCapplication; this information will also be useful in therisk assessment. The procedures that should befollowed in the cost benefit analysis of the need forgas clean-up are given in Section 3.

2 .5 Crankcase e missio ns

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

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

Exhaust from the extractor fan takes the form of alow volume and flow rate smoke. This exhaust orthe crankcase fumes is often passed through alength of pipework to promote condensation of theoil; the remaining vapour is then passed through acoalescer/filter. Simply exhausting the fumes belowwater is another method that has been employed.Increasing the volume of flow to positively purge thecrankcase 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 very low mass flow. This volume can be up to

30 per cent of the total mass emission rates of unburnt hydrocarbons and SO x from the engine, andtreatment is considered best practice. The directrelease of crankcase exhaust emissions is generally nolonger acceptable and any crankcase emissions needto be included in any PPC reporting requirements.

Options for management of this emission source are:

Recirculation of the crankcase fumes into thecombustion chamber inlet this affectscomponent life, but the emissions are combinedand diluted in the exhaust.

Recirculation by injection after combustion thisincreases the life of engine components, while theemissions are combined and diluted in theexhaust.

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

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


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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 and vapours. Active management of such a

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

Clean-up options range from commonly adoptedsimple water trapping and filtration to complexintegrated systems linked to the energy utilisation

plant or landfill gas abstraction plant.

A typical gas combustion scheme generally includesthe features shown in Figure 3.1. The raw gas entersthe utilisation set-up via a de-watering and filtrationknockout device that removes moisture andparticulates. This ensures that flare burners do notbecome blocked and improves combustionperformance within the engine cylinders. A gascompressor (or booster) increases the landfill gaspressure to ensure effective operation of the flareburners and adequate supply to the gas engine. Flowmetering devices and a slam-shut valve, provide thevolume flow rate to the flare or engine, and act as afinal safety control device. The flame arrestors 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


22/72


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

The range of options for the clean-up of landfill gas isquite extensive. This guidance attempts to categorisethese options and cover the numerous examplesreported in the literature. As shown in Table 3.1,there are a number of systems that do not sit neatlyin any one category; these are the so-called multiplesystems. In reality, all landfill gas clean-up processesare multiple in nature because there is no singleprocess that takes raw landfill gas and produces aclean fuel.

Figure 3.2 Clean-up options and emissions management

Early development of the processes was in responseto a need to produce SNG. This required the removalnot only of trace contaminants, but also all non-combustible components (principally carbon dioxideand nitrogen). The fact that utilisation of theprocessed gas resulted in clean combustion withminimal damage to the utilisation plant and a loweratmospheric burden was a bonus. This attractedoperators of more recent systems, which normallyutilise 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 more than one contaminant in the landfill gas.

Therefore, if more than one contaminant is present,then the calculated 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


23/72


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

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

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

However, various other options for clean-up processesmay be worth developing to enhance the operationof existing and future systems for utilising raw landfillgas. The focus of such development will be theremoval of trace contaminants (especiallyhalogenated organics and siloxanes) withoutnecessarily having to remove the non-combustible

bulk gases. Nevertheless, the economics of the clean-up options are currently far from clear, but athorough review could show that minimising the totalmass flow prior to clean-up (i.e. first using a low-costprocess to remove non-combustibles essentiallycarbon dioxide) might offer significant operationaland financial advantages.

Carbon dioxide removal processes effectivelyupgrade the calorific value of the gas. Suchprocesses fall into four basic categories:

absorption by a liquid (solvent) adsorption by a granular solid differential transport (membrane separation) cryogenic separation.

The underlying principles defining these categoriesare described in Section 5. However, the futureapplicability of landfill gas clean-up suggests that themost appropriate and, by implication, the lowestcost option is likely to be liquid absorption usingwater as the solvent. However, further evaluation andfinancial analysis may show otherwise, and at thisstage, no options should be ruled out.

When producing SNG, the principal requirement of

gas clean-up technology is to remove (or minimise)the concentrations of reactive trace components. Thiscan be partly achieved during upgrading to removecarbon dioxide; but to be fully effective, it requiresadditional processing stages. These stages are likely tobe sorption processes that target either individual orgroups of reactive contaminants. The optionsshowing the greatest promise are activated carbonand proprietary compounds based on activatedcarbon. However, solvent absorption offers theadvantage of continuous processing and thus shouldnot be rejected until a more detailed analysis has

been undertaken.

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

The basis for this approach is explained in HorizontalGuidance H1 (Environment Agency, 2002a). Rigorous

cost benefit analysis of the various gas clean-upoptions has not been carried out in this guidance dueto:

a lack of adequate cost and performance data forcomparable systems;

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

a reticence within the industry to discuss the costsof implementation of any technology unless a realsituation is involved.

However, the mechanism for conducting a rigorousCBA is described for situations when these databecome available for a site-specific requirement.

The aim of Horizontal Guidance Note H1 is to:

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

calculate costs provide guidelines on how to resolve any cross-

media conflicts.

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

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

1 Define the objective of the assessment and theoptions to be considered.

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

the different options.4 Compare options and rank in order of best overall

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

effective technique or BAT by balancingenvironmental benefits against costs.


25/72


3.4 Colla ting basic information for the costappraisal

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

provides a method for unambiguous presentation of the costs of clean-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 thecapital investment to the operator and typically variesbetween 6 and 12 per cent per annum, dependingon the level of risk associated with the company,

industrial sector or particular project. The samediscount rate should be used for all options underconsideration and the selection of 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 a percentage, e.g. 0.06 and not 6 per cent.

Table 3.2 Current UK asset life guideline values foruse in cost appraisals

Buildings 20

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

Intermediate components, 10e.g. compressors, some filters and groundhandling 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 the asset life. Current UK guideline valuesfor the different assets are given in Table 3.2.

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

Capital costs

Capital costs include the cost of:

purchasing equipment needed for the pollutioncontrol techniques

labour and materials for installing that equipment site preparation (including dismantling) and

buildings

other indirect installation demands.Capital costs should include not only thoseassociated with stand-alone pollution controlequipment, but also the cost of making integratedprocess changes or installing control and monitoringsystems.

It is important to describe the limits of the activity orcomponents to which the costs apply. For example,the choice of a type of technology that is inherentlyless polluting would require all components of thattechnology to be included in this limit.

Estimates of engineering costs are generallysatisfactory for cost submissions, although anysignificant uncertainties should be indicated. This isespecially important for components that could havea major influence on a decision between differentoptions. Where available, the cost of each majorpiece of equipment should be documented, withdata supplied by an equipment vendor or areferenced source.

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


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Table 3.4 gives a template for recording the breakdownof capital and investment costs. These should beprovided either as pounds or as a percentage of totalcapital costs; the anticipated year of expenditure should

also be stated.Operating costs and revenues

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

The recurring annual costs for pollution control systemsconsist of three elements:

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

The recurring annual change in operating costs foroptions consists of the additional costs minus any costsavings resulting from implementation of that option.

This should include any changes in production capacity.

Direct co sts are those that tend to be proportional orpartially proportional to the quantity of releasesprocessed by the control system per unit time or, in thecase of cleaner processes, the amount of material

processed or manufactured per unit time. They includecosts for:

raw materials utilities (steam, electricity, process and cooling

water, etc.) waste treatment and disposal

maintenance materials replacement parts operating, supervisory and maintenance labour.

Indirect or fixed annual costs are those whosevalues are totally independent of the release flow rateand which would be incurred even if the pollutioncontrol system were shut down. They include suchcategories as:

overheads administrative charges insurance premiums business rates.

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

materials or energy recovered by the controlsystem which may be sold, recycled to theprocess, or reused elsewhere on-site (but offset bythe costs necessary for their processing, storageand transportation, and any other steps requiredto make the recovered materials or energyreusable or resaleable);

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


Table 3.4 Breakdown of capital/investment costs

Pollution control equipment costs: Primary pollution control equipment Auxiliary equipment Instrumentation Modifications to existing equipment

Installation costs: Land costs General site preparation Buildings and civil works Labour and materials

Other capital costs: Project definition, design and planning Testing and start-up costs Contingency Working capital End-of-life clean-up costs (NB this cost would typically be discounted to a present value)

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


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Table 3.5 Breakdown of operating costs and revenues

Additional costs: Additional labour for operation and maintenance Water/sewage Fuel/energy costs (specify energy/fuel type) Waste treatment and disposal Other materials and parts (give details) Costs of any additional pollution abatement equipment operation (give details) Insurance premiums Taxes on property Other general overheads

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

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

In the case of gas clean-up for landfill gas engines,the increase in servicing intervals, reduction of oilconsumption and increase in engine efficiency shouldall be taken into account to offset the annualoperating costs.

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

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

minimum, operators should make a tick to indicatewhich elements have been included in the assessmentof capital and operating costs.

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

Six key contaminants or contaminant groups arepotentially treatable. This section deals with removalof selected components from the supply gas(hydrogen sulphide, halogenated organics andsiloxanes) or the exhaust gas (NO x, carbon monoxide

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

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

Figure 3.3 shows the six groups of contaminants andthe most appropriate clean-up technology for theindividual treatment of each group.

The technologies indicated in Figure 3.3 are discussedin Sections 46. The Agency considers that primarytreatment (Section 4) will be required at all landfills,and that its relatively low implementation cost meansthat these techniques should be used whenever andwherever necessary.

The secondary treatment sector is an emergingindustry and, as such, new information on availabletechnologies will supersede the information given inthis guidance. While many of the technologiesidentified have been around since the beginning of the landfill gas industry, many others are new andsome are just reinventions and repackaging of oldchemistry. Availability, suitability and cost should bethe deciding factors when shortlisting a technologyfor further consideration.

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

the removal of hydrogen sulphide the removal of halogenated solvents.


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3.5.1 Step 1: define the objective and the options t o be considered

The first step in the assessment process is to:

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

Example 1

Objective

Treatment of high hydrogen sulphideconcentrations in landfill gas is required to reduceengine wear and subsequent atmosphericemissions of SO x in a sensitive location a needindicated by site-specific risk assessment).

Possible clean-up opt ions

Hydrogen sulphide clean-up can be achieved bydry or wet desulphurisation. Both techniques arepre-combustion, secondary clean-up technologiesand information on these can be found in Section5 and case studies 14 (Environment Agency,2004d).

Example 2

Objective

Treatment of high chlorine concentrations in thesupply gas or treatment of high HCl emissions inthe exhaust a need indicated by site-specific riskassessment.

Possible clean-up opt ions

Clean-up of chlorine in the supply gas can beachieved by pressure water scrubbing, pressureswing adsorption, or membrane separationtechniques as pre-combustion, secondary clean-uptechniques (see Section 5 and case studies 511described in Environment Agency, 2004d), or byexhaust dry scrubbing (see Section 6 and case

study 18).

3.5.2 Step 2: quantify the emissions from eachtreatment opt ion

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

The degree of gas clean-up required will depend onth

Guidance on Gas Treatment Technologies

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