Anaerobic Digestion of Agro-Industrial

8/10/2019 Anaerobic Digestion of Agro-Industrial

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ANAEROBIC DIGESTION OF AGRO-INDUSTRIAL

WASTES: INFORMATION NETWORKS

Technical Summary on Gas Treatment

AD-NETT

Project FAIR-CT96-2083 (DG12-SSMI)

FINAL VERSION

August 2000

Barbarossastraat 35P.O. Box 1516500 AD NIJMEGEN00 31 24 328 42 8400 31 24 323 93 46


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AD-NETT Project FAIR-CT96-2083 (DG12-SSMI)Technical Summary on Gas Treatment

H0489.A0/R002/TS/GR - i - 25 January 2000

DISCLAIMER

Contract number: 355299/4010

This project is cofunded by the Dutch energy from waste biomass program(EWAB).

Control and co-ordination of the EWAB program are at:NovemCatharijnesingel 59PO Box 84243503 RE UtrechtThe Netherlandstelephone : +31-30-239.36.31telefax : +31-30-231.64.91

The project has been executed by:HASKONING Consulting Engineers and ArchitectsPO Box 1516500 AD Nijmegen

Date of report: August 1999


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CONTENTS

Page

ABSTRACT 1

1. INTRODUCTION 31.1 AD-NETT 31.2 Anaerobic digestion 31.3 Gas treatment 5

2. BIOGAS COMPOSITION 72.1 Biogas components 72.2 Biogas composition 8

3. STANDARDS ON GAS QUALITY AND EMISSIONS 93.1 Standards applicable gas quality for biogas utilisation. 93.2 Standards applicable on flaring of biogas 103.3 Standard emission for biogas fuelled internal

combustion engines 11

4. TREATMENT OF RAW BIOGAS 134.1 Introduction 134.2 Removal of water 144.3 Removal of H2S 154.4 Special gas engine lubricants 184.5 Removal of dust 18

4.6 Removal of CO2 18

5. TREATMENT OF FLUE GAS 195.1 Lean burn engines 195.2 DeNOxsystems 19

6. UPGRADING OF BIOGAS 216.1 Pressure Swing Adsorption (PSA) 216.2 Membrane separation 226.3 Physical (water, methanol) or chemical CO2 -absorption

techniques 226.4 Evaluation of CO2removal techniques 23

7. CONCLUSIONS 24

REFERENCES 25

SUPPLIERS OF GAS TREATMENT EQUIPMENT 27


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ABSTRACT

This technical summary on gas treatment in relation to anaerobic digestion (AD)

deals with the treatment processes for biogas in all stages of AD. Three majortopics are discussed: the treatment of raw biogas in order to produce a biogas quality which is ac-

cording to the required standards for driving gas engines or gas fired boilers; the treatment of flue gasses produced by gas engines or gas fired boilers in

order to meet the emissions standards in a certain country; the upgrading of biogas to natural gas quality or a required synthesis gas

quality by the removing of macro components like CO2.The context for the discussion of these three topics is presented in the firstthree chapters of this technical summary.Chapter 1 presents a short introduction of the AD-NETT context; the basic technology of anaerobic digestion; the topic of gas treatment with the identification of the three main concepts

i.e. treatment of raw biogas, treatment of flue gas and the upgrading of bio-gas;

the principles of gas treatment i.e. removal or conversion, application of ad-ditives, catalysts, bacteria or based on a physical principle;

identification of specific elements in AD and gas treatment i.e. scale (small,big) and budget (low, high), ease of operation, ease of monitoring, ease ofmaintenance, fail safety, production of hazardous or toxic pollutants.

Chapter 2 gives a review on the composition of biogas. The description of thecomposition concerns: available components in the biogas; treatment objectives for specific components i.e. reduction of toxicity, acidifi-

cation, harmfulness for equipment or compliance with emission standards; effect of the digester feed and the AD-process itself on the composition of

the biogas; variation of the biogas composition under normal operating conditions.In chapter 3 the emission standards dealing with AD in the countries participat-

ing in the AD-NETT are discussed. First of all the policy and intentions of theEuropean Community and the available standards are presented. Furthermoreattention is paid to: emission of components which have high priority in reduction; standards in relation to biogas flaring; standards in relation to biogas utilisation in gas engines, gas boilers or in re-

lation to the delivery of upgraded biogas to the natural gas grid.Chapter 4 focuses on the treatment of the raw biogas in the form as the digesterproduces it. Attention will be given to: the removal of water;

the reduction of H2S by aerobic bacteria;


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wet gas treatment systems; gas treatment by utilising special gas engine lubricants; removal of dust particles.

In chapter 5 the possibilities for treatment of the flue gasses that are formed bythe gas engine or gas boilerare reviewed. In this respect flue gasses are thosespecifically generated from: lean burn gas engines; catalytic and non-catalytic DeNOxsystems.Chapter 6 deals with the possibilities to upgrade biogas to a quality required bythe natural gas grid. Upgrading techniques discussed are: membrane separation; wet treatment;

active carbon; pressure swing adsorption.

In chapter 7 conclusions on the maturity of gas treatment for AD are presentedbased on the present state of the art.

Chapter 8 presents a list of the main literature references with short descriptionsof the scope of each reference and relevant ordering information. In this chapteran overview is given of the main suppliers of gas treatment equipment.


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1. INTRODUCTION

1.1 AD-NETT

AD-NETT is a network of professionals working in anaerobic digestion (AD)ofagro-industrial wastes in Europe and Canada. AD-NETT is sponsored by the ECFAIR programme in the EU and by national programmes in Switzerland andCanada. The network began in January 1997 and will continue to December1999.

The prime motive of AD-NETT is to enable exchange of information and experi-ence and to disseminate this information to relevant organisations, such as po-tential users and developers. In this respect AD-NETT publishes amongst oth-ers on its Internet site http://www.ad-nett.org - news on developments in thenetwork, information about technical developments on AD and reports from da-tabases of plant and contacts.

AD-NETT aims to examine and compare successes and failures of the worldwide application of AD technology in order to gain from past experience and topass this information on to potential users of the technology.

The information presented in this report is restricted to the application of AD fortreatment of agro-industrial wastes, although many of the gas treatment tech-niques described are also suited for anaerobic wastewater treatment, treatmentof landfill gas, and digestion of VGF and other organic waste streams.

The technical summary on gastreatment is the Dutch contribution to the AD-Nett, and was prepared by HASKONING, a Dutch engineering and consultancyfirm, in close co-operation with Novem, the Dutch Agency for Energy and theEnvironment.

1.2 Anaerobic digestion

AD is applied in particular in the agricultural sector all over the world in the formof small on-farm digesters producing biogas to heat farmhouses, dairies andother farm buildings. Larger scale centralised anaerobic digesters (CentralisedAD) have also been developed, using feedstock imported from a number ofsources, specifically residues from livestock farming (such as dairy, beef and pigslurry, or poultry litter) and the food processing industries (including vegetablepreparation and dairy food processing). Figure 1 presents a basic layout of anAD-plant.

Anaerobic digesters produce conditions that encourage the natural breakdownof organic matter by bacteria in the absence of air-oxygen. Figure 2 shows thedegradation routes in the AD-process, during which organic material is con-verted by various types of micro-organisms into methane (CH4) and carbon di-oxide (CO2), the main components of the biogas |ref 1|.


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Feedstock

Storage and

pre-treatment(mixing, screening)

DigesterBiogas

storage

DigestateBiogas cleaning

Upgrading

(natural gas)

Engine

turbine

Gas burner

boiler

Flue gas cleaning Fluegas cleaning

Gas treatment techniques reviewed

Flue gas Natural gasFlue gas

Figure 1: Basic Layout of an AD-plant

PROCESS MATERIAL BACTERIA

PARTICULATE ORGANIC MATERIAL

Hydrolysis

Fermentation

(acidogenesis)

Fermentation(acetogenesis)

Methanogenesis

AMINO ACIDS / SUGARS

FATTY ACIDS

ACETATE / HYDROGEN

METHANE / CARBONDIOXIDE

Lipolytic, proteolytic

and cellulytic bacteria

Fermentative bateria

Hydrogen producing

bacteria

Methanogenic bacteria

PROTEINS CARBOHYDRATES LIPIDS


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Table 1: Overview of techniques used for biogas treatment

COMPOUND REMOVED TECHNIQUE PRINCIPLE

Water Demister Cyclone separatorMoisture trapWater tap

Adsorption to silicaGlycol drying unit

physicalphysicalphysicalphysicalphysicalphysical

H2S Air oxygen dosingFeCl3dosing to digester slurry

Adsorption to Fe2O3pelletsAbsorption with caustic solutionAbsorption with iron solutionAbsorption closed loop systemsMembrane separationBiological filters

Activated carbonMolecular sieves

biologicalchemicalphysical-chemicalphysical-chemicalphysical-chemicalphysical-chemicalphysicalbiological

physical-chemicalphysical

CO2 Pressure swing adsorptionMembrane separation

Absorption techniques

physical-chemicalphysicalphysical-chemical

Table 2: Importance of gas treatment aspects in relation to the sizeof the AD-plant (+ = important, - = less important)

PARAMETER SMALL SCALE AD-PLANT1) LARGE SCALE AD-PLANT2)

Low budget + +/-

Ease of operation + +/-Unattended operation + -

Ease of monitoring + -

Ease of maintenance + +/-

Safety standards + +

Health standards + +

1) AD-plant with capacity lower than 5.000 tons feedstock per year (arbitrarily set)2) AD-plant with capacity higher than 5.000 tons feedstock per year (arbitrarily set)

Table 3: Possible application of biogas in relation to the size of the AD-plant (+ = yes, - = no)

END-USE SMALL SCALE PLANTS1) LARGE SCALE PLANTS2)

Direct gas use (e.g. heating) + +

Production electricity +/- +

Combined heat power generation - +

Upgrading to natural gas - +

1) AD-plant with capacity lower than 5.000 tons feedstock per year (arbitrarily set)2) AD-plant with capacity higher than 5.000 tons feedstock per year (arbitrarily set)


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2. BIOGAS COMPOSITION

2.1 Biogas components

Biogas produced in AD plants is primarily composed of methane (CH 4) and car-bon dioxide (CO2), with smaller amounts of hydrogen sulphide (H2S) and am-monia (NH3). Slight concentrations of hydrogen (H2), nitrogen (N2), carbon mon-oxide (CO) and oxygen (O2) are occasionally present in the biogas. Finally thebiogas is usually saturated with water and might contain dust particles.The properties of the main gas components mentioned above are outlined be-low: CH4gas is considered as a valuable fuel. The gas is non-toxic, non-smelling,

and is lighter than air. When burned CH4is converted into a molar equivalentamount of CO2and water.

CO2 is an inert colourless, odourless gas and is heavier than air. CO2 ismildly toxic, is an asphyxiant and has an occupational exposure standard(OES) of 5.000 ppm. A higher CO2concentration in the biogas results in alower calorific value of the biogas.

H2S is a colourless gas. Since H2S is heavier than air, it might cause extradanger at low levels. At low concentrations this gas has the typical smell ofrotten eggs. At higher, more dangerous concentrations is has no smell. Dueto its toxic properties hydrogen sulphide has an OES of 10 ppm. In additionto its toxicity H2S is corrosive which can cause problems during combustionof the biogas. When burned the H2S in the gas is converted into SO2, also atoxic product, which can cause acidification.

NH3is a pungent and lachrymatory gas that is lighter than air. The OES is 10

ppm. When burned in a flare or a gas engine, NOx-products are formed.Normally, NH3concentrations in the biogas are rather low.

Water vapour, although a harmless product, becomes corrosive in combina-tion with the NH3, CO2and especially the H2S of the biogas. The maximumwater content of the biogas is governed by the gas temperature. When wa-ter saturated biogas leaves the digester, cooling of the gas will result in con-densation of water.

In view of the properties of the main biogas components listed above, treatmentof biogas will often be required. The treatment is commonly focussed on: Removal of water, which is needed because of potential accumulation of

moisture in equipment in the gasline, the formation of a corrosive acidic so-lution when water is combined with H2S and in order to create optimal condi-tions for the end-use of the biogas.

Removal of H2S, which is required because it is toxic, corrosive and since itcan damage gas-equipment. The H2S concentration in the gas should bekept below the recommendations of the manufacturer of the equipmentused. Furthermore, burning of biogas results in the oxidation of H 2S to SO2.In order to comply with environmental regulations for SO2emissions the H2Slevel in the biogas should be kept sufficiently low.

Removal of CO2will be required if the biogas needs to be upgraded to thequality of natural gas (in terms of Wobbe-index, corrosion, etc).


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2.2 Biogas composition

The biogas production and composition in AD-plants is dependent on the type

of feedstock and the operational parameters used in the digestion process.The feedstocks used for anaerobic digestion vary considerably in composition,homogeneity and biodegradability. Table 4 shows some characteristics and op-erational parameters for different agricultural feedstocks.

Table 4: Characteristics and operational parameters for different agricul-tural feedstocks |ref. 2|.

FEEDSTOCK TOTALSOLIDS (TS),

%

VOLATILESOLIDS (VS),

% of TS

BIOGAS YIELD,

m3/Kg VS added

METHANECONTENT,

VOL. %

RETENTIONTIME,days

Pig slurry 3-8 70-80 0.25-0.50 70-80 20-40

Cow slurry 5-12 75-85 0.20-0.30 55-75 20-30Chicken slurry 10-30 70-80 0.35-0.60 60-80 > 30

Garden waste 60-70 90 0.20-0.50 n.a. 8-30

Fruit waste 15-20 75 0.25-0.50 n.a. 8-20

Food remains 10 80 0.50-0.60 70-80 10-20

Tables 5 show a comparison between biogas from an average AD plant andnatural gas.

Table 5: Composition of biogas from an AD-plant and natural gas in theNetherlands |ref. 3, 4|.

COMPONENT DIMENSION NATURAL GAS BIOGASCH4 vol-% 85 55-70

CO2 vol-% 0,89 30-45

C2H6 vol-% 2,85 ---

C3H8 vol-% 0,37 ---

C4H10 vol-% 0,14 ---

N2 vol-% 14,35 ---

O2 vol-% < 0,5 ---

H2S mg/m3 < 5 0-15.000

NH3 mg/m3 --- 0-450

Humidity -- dew point at 10C saturated

Caloric value, lower-upper MJ/m3 32-35 20-28

Wobbe index, lower-upper MJ/m

3

40-44 20-30


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3. STANDARDS ON GAS QUALITY AND EMISSIONS

3.1 Standards applicable gas quality for biogas utilisation.

The biogas as produced in AD plants can be used for several purposes: direct-gas use; fuel for an engine connected to a generator to produce electricity; combined heat and power generation; upgrade of biogas to natural gas quality to run motor vehicle engines or to

supply to the local gas net (pipeline or substitute natural gas quality).

In the case of direct-gas use water and H2S removal will be the appropriatetreatment steps. If the gas is to be used by gas engines, or if it is upgraded to anatural gas quality, the biogas composition should comply with the appropriate

requirements. Table 6 list typical requirements for gas engines.Table 6: Typical requirements for gas engines |ref. 3|.

COMPONENT DIMENSION RANGE

Energy content MJ/m3 13-21

Variation in energy content MJ/m3 0-2

Maximum temperature feed C 40-60

Minimum delivery pressure mbar 25-80

Humidity biogas % < 70-80

H2S mg/m3 < 1000-2000

Chloride and Fluor (total) mg/m3 < 60-80

For upgrading biogas to natural gas quality removal of CO2, H2S, NH3, waterand dust is essential in order to achieve the required quality. Table 7 lists therequired gas quality in order to be used as natural gas substitute in the Nether-lands. At the moment no European standard has been defined, but it can beassumed that differences in the European countries will be small.

Table 7: Typical requirements for biogas to be injected in natural gasline applicable the Netherlands |ref. 3|.

PARAMETER DIMENSION VALUE

Caloric value MJ/m3 34,7-34,9

Wobbe-index MJ/m3

43,5-44,4H2S mg/m

3 < 5

H2O (dew point at 1 bar) C - 35

For utilisation of biogas as vehicle fuels the same upgrade technique is used asfor natural gas. In practice the upgrading of biogas up to vehicle fuel quality in-volves removal of CO2, H2S, NH3, particles and water. The methane contentsshould be at least 95 % |ref. 5|. Quality requirements for vehicle fuel of biogasare different in different countries.


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3.3 Standard emission for biogas fuelled internal combustion engines

Guidelines in the Netherlands

In the Netherlands, the emission of NOx in exhaust gasses of biogas fuelled in-ternal combustion engines is restricted |ref. 7|: new engines < 50 kW: 800 g NOx/GJ * 1/30 of the engine efficiency; new engines > 50 kW and < 50 MW: 140 g NOx/GJ * 1/30 of the engine effi-

ciency.

This means that if the engine efficiency increases, higher emissions are ac-cepted. The engine efficiency is the percentage of power (energy) output thatcan be obtained from the energy input.

Guidelines in other countries

There can be considerable differences in the regulations between the differentEuropean countries. In addition to the restrictions for emission during biogas-combustion used in the Netherlands, the standards as applied in Spain, Den-mark, Italy and Austria are given as examples for the various regulations andstandards which are applied within Europe.

AustriaTable 9 gives the data regarding emission restriction during the combustion ofbiogas as applied in Austria. The emission restriction are based on:- Lufteinhaltverordnug fr Kesselanlagen BGBI Nr. 19/1989 idgf (LVR-K);

- Deutsche Technische Anleitung zur Reinhaltung der Luft (TA-Luft 1986)

|ref 8 |;- Richtlinienvorschlages Entgasung von Deponiekrpern der WAV

(WAV, 1997) |ref 9 |.

Table 9: Maximal emission values as applied in Austria based on the LVR-K, TA-Luft 1986, and WAV 1997.

WAV 1997PARAMETER LVR-K1) TA-LUFT19862) < 2.000 m3/h > 2.000 m3/h

DIMENSION

DustNOx

SO2CO

50-

-100

5200

500100

5-

100100

5200

100100

mg/m3

mg/m3

mg/m3

mg/m3

HClHFPb, Zn, Cr

As, Co, NiCdHgTotal organic carbonPCDD/PCDFCO:CO2H2SO2 content

300,751

0,10,1200,1

0,002-

11

20551

0,20,2---53

20551

0,10,1200,1

0,00253

20551

0,10,1200,1

0,00253

mg/m3

mg/m3

mg/m3

mg/m3

mg/m3

mg/m3

mg/m3

ng/m3

-mg/m3

%1)small plants, 2)landfill gas

Denmark


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4. TREATMENT OF RAW BIOGAS

4.1 Introduction

The raw biogas produced in a digester is normally treated in order to removewater, H2S, dust and/or CO2. The choice of the cleaning method employed andthe compound to be removed depends on the type of end use of the gas.

Table 10 shows the compound(s) of the biogas that needs to be removed in re-lation to the end-use of the gas.

Table 11: Removal of specific compounds present in biogas, in relation to thegas utilisation (+ = yes, - = no).

BIOGAS UTILISATION WATER H2S DUST CO2Direct heating (gas boilers/burners) + +1) + -

Electricity (gas engine) + +1) + +/-2)

Upgrade for natural gas + + + +1)removal required if input limits are exceeded2)dependent of CO2content biogas and manufacturers specification.

The treatment methods for the removal of water, H2S and dust are reviewedbelow. The more sophisticated removal methods for CO2are discussed in detailin chapter 6 that deals with biogas upgrading techniques to natural gas quality.Figure 3 show the different removal techniques which will be discussed in moredetail in the chapter.

DIGESTERRemoval of water, foam

and dust

Removalof H2S

Specificremoval of dust

Removalof CO2

Figure 3 Treatment techniques for biogas.

Removal of water and foam is always needed for the prevention of corrosion inthe biogas line. At present very simple and cheap methods exist that will removewater in sufficient quantities. Water removal is normally accomplished with theremoval of foam and dust. If the biogas is to be used removal of H2S is neededin most cases. CO2removal and specific methods for the removal of dust is onlyneed when a high quality gas is required. This can be the case when the biogasis to be used as a substitute for natural gas or when sensitive equipment isused.


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4.2 Removal of water

In AD plant technology the removal methods for water are commonly based on

separation of condensed water or gas drying. In addition to water also otherimpurities in the biogas such as foam and dust are removed.

Condensation methods

Methods employed using separation of condensed water are based on the prin-cipal that water droplets are caught or entrapped and subsequently removed.The removal can be either manually or automatic. Techniques using separationof condensed water include the application of: demisters, in which liquid particles are separated with a wired mesh (micro

pores); cyclone separators, in which water drop(let)s are separated using centrifugal

forces equal to several hundred gravities; moisture traps, in which condensation of water by expansion of the biogas

takes place; water tapsin the gaspipe, from which condensed water from the biogas can

be removed from the gas.

Of the methods mentioned above the simplest method is based on using watertaps in the gas-pipe. Normally this will give a sufficient removal of water for thegas to be used in gasengines.

Drying methods

Methods based on gas drying include:

Cooling of the gas and leading it through a demister with micro pores(cooldryer)

When the gas is cooled the gas becomes over-saturated with water. Dewpoints of 20 tot 2 C can be reached. As a result, condensation of the waterin the gas occurs. The condensed water is entrapped in a demister. In orderto have a high efficiency the demister is equipped with micro pores.

Adsorption of the gas to silica(adsorption dryer)

Using adsorption dryers a high removal of water can be achieved. The gas isled through a column filled with silica in which dew points of -10 tot -20 Ccan be reached. Usually two columns are used simultaneously: one columnfor adsorption of water, while the other is regenerated in that stage. Regen-eration can be done by evaporation of the water by heating. As alternativesfor silica also activated carbon or molecular sieves can be used for this dry-ing process.


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Glycol drying unit(glycol dryer)

Drying takes place using tri-ethylene-glycol (a water binding component).

Dew points of -5 tot -15 C can be reached. Used glycol is pumped into a re-generation unit, where regeneration takes place with a temperature of 200C.

Removal of water is essential in preventing potential accumulation of moisture inequipment installed and the formation of a corrosive acidic solution. However,as indicated in paragraph 1.3 and tables 2 and 3 the appropriate technique isstrongly dependent on the size of the AD-plant and the end-use of the gas. Forsmall scale AD-plants water removal techniques are limited to the application ofmoisture traps or water taps in the gas line. These methods are commonly usedand are sufficient for using of gas in gas-engines. The other techniques men-tioned are not justified economically or do not fulfil the criteria mentioned in ta-bles 2 and 3. Only if the objective of the biogas treatment is upgrading to anatural gas or vehicle fuel quality, water removal based on drying methods canbe considered.

4.3 Removal of H2S

A number of techniques have been developed over the years for the removal ofH2S from the biogas. These techniques are outlined in this section.

Air/oxygen dosing to the biogas system

This technique is based on the biological aerobic oxidation of H2S to elementalsulphur by a group a specialised micro-organisms. In order to have the bacteriadeveloped in the biogas system, liquid-manure-wetted surface is required. Thefollowing reaction occurs in the biogas:

2 H2S + O22 S + 2 H2O

The small amount of oxygen (approximately 5 %) required in this method is in-troduced in the biogas system e.g. by using an air pump. As a result the sul-phide in the biogas is oxidised into sulphur and the H2S concentration of thebiogas is lowered. The results obtained with this method to date are very prom-

ising. A reduction of H2Slevels down to 20-100 ppm H2S which equals 30- 150mg/m3 H2S, and removal efficiencies of H2S between 80-99 % have beenachieved |ref. 13|. The H2S removal obtained with this method is sufficient for adirect use of biogas in gas-engines. On the other hand, the H2S concentrationremains too high for the use of biogas as a substitute of natural gas. The majoradvantages of this removal method are: Investment and exploitation costs are very low compared with other H2S re-

moval methods. Operation, monitoring, maintenance and unattended operation are very easy

to realise. No special chemicals or equipment are required.


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Chemical absorption with iron salts solution. In this process an iron chloridesolution is used. The process is based on the formation of insoluble precipi-tates that needs to be removed.

Chemical absorption using closed systems. In closed systems the absorp-tion phase is followed by regeneration. An example is the Gluud-process. Inthis process, H2S is removed using iron(III)hydroxide resulting in the forma-tion of Fe2S3. Regeneration is done with oxygen-air. Another example is thedirect formation of elemental sulphur using an iron(III)chloride solution. Byadding complexing chemicals the formation of FeS or Fe(OH)3is prevented.Regeneration again is done with oxygen-air.

The physical and chemical absorption processes mentioned above in generalhave the disadvantage of a relative high consumption of water and/or chemi-cals, which makes the method less attractive for small-scale AD-plants. Forlarge-scale AD-plants the method may become economically more feasible.

Other methods

Other methods that can be used include: Membrane separation. H2S can be separated from the gas using a semi-

permeable membrane. In this process the biogas is led through a mem-brane. H2S can pass the membrane whereas CH4and CO2cannot pass themembrane.

Biological treatment. Biological treatment methods are based on using spe-cific bacteria that are able to oxidise the H2S. In this method the biogas is ledthrough a special biological filter. In this filter H2S is in a first step absorbed

to a liquid phase and subsequently oxidised by the bacteria present in thefilter.

Activated carbon processes. Application of molecular sieves.

Evaluation of H2S removal techniques

Table 12 shows a comparison between the different techniques for H2S removal.Table 12 illustrates clearly that air dosing or FeCl3dosing are the best solutionsto remove H2S, especially for small-scale low budget AD plants.However, when a natural gas quality is needed, techniques like chemical ab-

sorption have to be applied, In this case H2S removal is normally combined withCO2removal.


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Table 12: Comparison of different available H2S removal techniques

REMOVAL TECHNIQUE 1 2 3 4 5 6 7

Air dosing + + + + + + ++FeCl3dosing + + + + + + +

Fe2O3pellets + + + +-/ + +/- +/-

Chemical absorption using caustic +/- + +/- +/- + +/- +/-

Chemical absorption using iron solution +/- + +/- +/- + +/- +/-

Chemical absorption based on closed loop system +/- + +/- +/- + +/- +/-

Membrane separation - - - - + + -

Biological treatment - +/- +/- +/- + + +/-

Activated carbon - +/- +/- +/- + +/- +/-

Molecular sieve - +/- +-/ +/- + +/- +/-

1. Application on small scale (+ = yes) 5. H2S treated gas < 250 ppm (+ = yes)

2. Application on large scale (+ = yes) 6. environmental impact (+ = low)

3. Simplicity (+ = simple) 7. costs (+ = low)

4. operation and maintenance (+ = little)

4.4 Special gas engine lubricants

Special gas engine lubricants are available that can deal with a relatively largeamount of contaminants, and research is on going. Engine suppliers are wellknown with the different types of lubricants. The choice of the type of lubricantdepends mainly on the biogas quality and combustion conditions, which areboth specific for feedstock and location. If this information is known, a choice

can be made. Other factors to be considered include the frequency of lubricantanalyses and maintenance.

4.5 Removal of dust

As mentioned in paragraph 4.1 removal of water is accomplished with dust re-moval. Normally the dust removed in this way is sufficient. Only if the gas isused in sensitive equipment additional specific dust removal is required.If specific dust removal is needed, dust filters have to be used. For this purposeair filter type filters are needed and are often placed before regulator valves orgas utilisation equipment. To prevent dust filters from clogging, the gas must befree of any oil or water droplets. Dust filters must therefore be placed behind thecondensation traps and oil separators. The use of dust filters has consequencesfor the choice of extraction machines, due to pressure losses. As mentioned inparagraph 1.3 and tables 2 and 3 the appropriate technique is strongly depend-ent on the size of the AD-plant and the end-use of the gas. This means thatdust removal will only be considered for large-scale AD plants.

4.6 Removal of CO2

As indicated in the introduction of this chapter ( 4.1) treatment techniques forthe removal of CO2are reviewed in chapter 6.


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5. TREATMENT OF FLUE GAS

5.1 Lean burn engines

The emission of flue gas from internal combustion (IC) engines can be con-trolled using lean burn engines. Lean burn engines use a lambda sonde, whichmeasures the oxygen contents of the exhaust gas. With this information theoxygen inlet, and in this way the oxygen-fuel mixture, is regulated. The lambdacontrol regulation results in optimal combustion conditions and low NOx emis-sion.

The lambda sonde occasionally causes problems. Due to contaminants in thebiogas, the lambda sonde gets contaminated after a while, and gives inaccurateresults. Minimising the lambda (richer fuel mixture) causes a low NOxemission,but results in an undesired increase of CO and hydrocarbons emission, and re-sults in lower engine efficiency. A high lambda (lean mixture) leads generally toa low NOxemission too.

Frequent maintenance is therefore necessary to keep the lambda sonde andengine in optimal condition and to fulfil the required emission standards. Re-placement of the oxygen measurement device is required at least every year.

5.2 DeNOxsystems

DeNOx can take place by the use of non-catalytic and catalytic systems. Thesystems described in this paragraph are usually applied at waste incineration

plants |ref. 14| and might be used at large scale utilisation of biogas from AD-plants.

Non-catalytic systems

Non-catalytic DeNOx-systems are referred to as SNCR-systems. SNCR standsfor Selective Non Catalytic Reduction. SNCR-systems use ammonia or urea asa reactant to reduce NOx.

The reactant is injected with steam into the fluegas just above the fire zone attemperatures of 800 to 1000 C.

Catalytic systems

Two types of catalytic DeNOxare being discussed |ref. 15|: non-selective catalyst, which reduces the emission of NOx, but also reduces

the emission of CO and hydrocarbons; selective catalyst, which reduces NOx by using ammonia or urea, also re-

ferred to as the SCR, Selective Catalytic Reduction-system.


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Non-selective catalyst

With the non-selective catalyst system NOxis reduced using CO as reactant:

NO + CO N2+ CO2

The amount of CO is essential for optimisation of the DeNOx. The lambda of theengine therefore has to be regulated. With this method also hydrocarbons (ex-cluded CH4) are oxidised to water and carbon dioxide.

Selective catalyst (SCR)

NOx is changed into N2 by using ammonia or urea as a reactant. Large scaleplants use ammonia (gaseous or in solution):

4 NO + 4 NH3+ O2 4 N2+ 6 H2O8 NO2+ 8 NH3 7 N2and 12 H2O

Small scale DeNOxplants use urea for safety reasons.

The catalyst material in SCR-systems generally consists of a carrier of titaniumoxide (TiO2) with added active substances like vanadium oxide (V2O5) and tung-sten oxide (WO3).

For the catalytic reaction, the lambda has to be higher than with the non-catalytic system, and an exhaust gas temperature of at least 300 C is needed.

In the case of biogas fuelled engines, this type of catalysts is being used if avery high DeNOxremoval efficiency (> 90%) is required. For example if the ex-haust gas is used as CO2 source for greenhouses. The costs of a selectivecatalyst DeNOxsystem are relatively high compared to the non-selective DeNOxsystem.


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6. UPGRADING OF BIOGAS

Upgrading of biogas to substitute natural gas (SNG) involves a number of steps

|ref. 16, 17,18, 19, 20, 21|.At first, water (vapour) and H2S have to be removed. Subsequently CH4/CO2separation must be carried out, using one of the following techniques for theremoval of CO2: Pressure swing adsorption (PSA); Membrane separation; Physical or chemical CO2-absorption.

If the gas is meant for input in a natural gas piping system, the gas has to becooled and has to be compressed. Dust and (halogenated) hydrocarbons haveto be removed, and the gas needs to be odorised for safety reasons.

The three techniques focussed on the removal of CO2are reviewed below. Thetechniques for the removal of water, H2S, dust and hydrocarbons are discussedin chapter 4.

6.1 Pressure Swing Adsorption (PSA)

The PSA unit is schematically shown in figure 4. The unit exists of at least threeactive carbon beds. The process in each bed (successively) is as follows:The bed is fed with biogas under pressure. In this step the bed will be loadedwith carbon dioxide upon saturation, while methane is released. If the breakthrough point is reached, the process will be switched to the next bed. After

saturation the bed is depressurised to ambient pressure. In this step a CH4/CO2mixture is released with a high content of CH4and will be recycled into the feed.The next step is to regenerate the active carbon bed by putting it under vacuum,in which the carbon dioxide is released.The PSA technique gives high efficiency rates (to 98%), but it is a relatively ex-pensive process due to capital costs (control system).

Methane rich recycle

Biogas

Molecular sieves Mixing vessel

Submitted

natural gas

CO2

H2S CFC

1 2 3

Figure 4: Pressure Swing Adsorption process


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6.2 Membrane separation

Membrane separation is based on selective permeability of membranes for dif-

ferent components. Figure 5 shows the process scheme of this separation pro-cess. The membrane efficiency varies between 73 % and 83%. Advantages arethe simple techniques and operation. Disadvantages are the efficiency and theenergy input for high pressures that are needed (20-35 bar) and high tempera-ture.

Biogas

H2S CFC

CO2

Substitute

Natural gas

Membranes

Figure 5: Upgrading biogas using gas separation membranes

A pilot study has been carried out |ref. 16| with a low-pressure (8 bar) mem-brane separation system. An active carbon filter is needed in the feed line toremove trace components. Relatively to high-pressure membrane separationtechniques, a reduction of costs could be achieved of 10 % per m 3natural gasequivalents. The major lack of knowledge during the pilot study was the long-term behaviour of the membranes. Research on this has been initiated.

6.3 Physical (water, methanol) or chemical CO2-absorption techniques

These absorption techniques are based on the principle of separation of CO2and CH

4by using an absorbent. Figure 6 shows the technique schematically.

One of the methods is the use of water as absorbent liquid. Biogas is fed into avessel. In this vessel the water is sprayed. During the flushing the concentrationof carbon dioxide decreases with distance and the gas becomes more concen-trated with methane. The washing liquid is generated in two depressurisationsteps. At a pressure drop from 10 bar down to 4 bar, methane and part of thecarbon dioxide is released and is recycled. After the next pressure drop CO2 isreleased. The efficiency is about 95 %.


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6.4 Evaluation of CO2removal techniques

In Table 13 the three reviewed CO2removal processes are compared.

Biogas

H2S

Substitute

Natural gas

CFC

CO2

DryingAbsorber

Washing liquid

regeneration

Figure 6: Upgrading biogas using physical absorption techniques

Table 13: Comparison of different available CO2removal techniques

UPGRADING BY PSA

Application Large scale plants (CDA)

Stage in the development Proven technique

Experience (in years) 12 years

Emission to the air Chlorinated and fluor hydrocarbons (can be flared)CO2reduction and CH4reduction

Energy efficiency Energy input : energy output rate: 1:15

Gas pretreatment Gas drying, H2S removal

UPGRADING BY MEMBRANE SEPARATION


Stage in the development Proven technique (high pressure separation)Pilot stage (low pressure separation)

Experience (in years) Appr. 10 years (high pressure separation)Emission to the air Chlorinated and fluor hydrocarbons (can be flared)

CO2reduction and CH4reduction

Energy efficiency Energy input : energy output rate: 1:10(high pressure)


UPGRADING BY PYSICAL/CHEMICAL ABSORPTION


Stage in the development Proven technique

Experience (in years) Appr. 10 years

Emission to the air Chlorinated and fluor hydrocarbons (can be flared)CO2reduction and CH4reduction

Energy efficiency Information not available



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7. CONCLUSIONS

This technical summary has dealt with different aspects of gas in AD plants. The

topics have been: production and composition of biogas, treatment of biogas, treatment of flue gas.

On an average base the biogas formed in AD plants consist of: 55-80 vol.-% CH4 20-45 vol.-% CO2 0-1,5 vol.-% H2S 0-0,05 vol.-% NH3 saturated with water.

Prior to utilisation the biogas needs to be treated in order to: prevent corrosion of equipment installed. fulfil demands on gas quality either for gas equipment or in case of gas up-

grading to natural gas quality.

Biogas treatment can involve the removal of water, H2S and/or CO2.For the removal of water several methods based on separation of condensedwater or on gas drying are available. For small scale AD plants water removal islimited to moisture traps and water taps in the gas line. If upgrading of biogas isthe treatment objective gas drying techniques become necessary.In order to remove H2S several methods have been developed. Based on theeconomics and simplicity in terms of (unattended) operation and maintenance,air-oxygen dosing in the biogas and iron chloride dosing to the digester slurryare the most suitable, especially for small-scale farms. For large-scale farms orwhen upgrading of biogas to natural gas is the objective, chemical absorption ofH2S might become more feasible.When biogas is upgraded to natural gas quality, in addition to water and H2Sremoval, CO2 removal is one of the gas treatment objectives. CO2 removaltechniques that can be applied are a pressure swing adsorption unit, a high orlow-pressure membrane separation unit, and physical or chemical absorptiontechniques.

Flue gases as produced at the site of AD plants by gas engines or boilers needto be treated. Emission control can be done using lean burn engines or catalyticor non-catalytic DeNOxsystems.


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REFERENCES

|1|Biogas technology in The Netherlands; Anaerobic waste and waste water

treatment with energy production, Netherlands Agency for Energy and theEnvironment (NOVEM), Utrecht, The Netherlands, 1988.

|2|Feedstocks for anaerobic digestion.AD-Nett report.

|3| Guidebook on landfill gas extraction and utilisation. Netherlands Agencyfor Energy and the Environment (NOVEM), Utrecht, The Netherlands,1997.

|4| Manure digestion in the Netherlands. 10 years of knowledge and experi-ence. Netherlands Agency for Energy and the Environment (NOVEM),Utrecht, The Netherlands, 1990 (in Dutch).

|5|Upgrading of biogas to vehicle fuel standard. Sweco 1998.

|6| Clampdown on emissions from landfill gas flares.UK Policy, ENDS Report290.

|7| Besluit emissie-eisen stookinstallaties (BEES), recalculated to ISO stan-dard air conditions, Ministry of Housing, Spatial Planning and Environ-ment, The Netherlands, 1998 (in Dutch).

|8| Ta-LUFT Technische Anleitung zur Reinhaltung der Luft vom 27.2.1986.

Erste Allgemeine Verwaltungsvorschrift zum Bundes-Immissionsschutzgesetz (BlmSchG) vom 15.3.1974. Deutsches BGBI IS721,1986.

|9| WAV Entgasung von Deponiekrpern. Heft 110 sterreichischerWasser- und Abfallwirtschaftsverband, Marc-Aurelstrae 5, 1010, Wien,1997.

|10| H. Ortenblad, Hernig Kommunale Vaerker. Hernig, Denmark. personalcommunication. 1999.

|11| N Carreras. CIEMAT, Instituto de Energias Renovalbes, Lubia, Spain per-sonal communication. 1999.

|12| S Piccinini. Centr Ricerche Produzioni Animali, Reggio Emilia, Italy. per-sonal communication. 1999.

|13| Desulphurization of biogas. Practical experience with the biocatalytic De-sulphurization process.Biogas forum.

|14| NOx-removal in waste incineration, full scale demonstration and researchin the Netherlands,Novem report DV3.5.92 97.03, The Netherlands, July1995.


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|15| DeNOx-installation gas engines oil production site at Berkel, NAM/BUO,The Netherlands, July 1998 (in Dutch).

|16| Feasibility study low pressure (8 bar) landfill gas separation system(Haal-baarheid toepassing nieuw membraan voor opwerking stortgas naar aard-gaskwaliteit bij lage druk (8 bar)), Novem 355220/0440, The Netherlands,1997.

|17| Landfill Gas, from environment to energy, Commission of the Europeancommunities, 1992.

|18| Listing of Dutch landfill gas projects (1983-1991)(Overzicht stortgasprojec-ten in Nederland (1983-1991)), Dutch Landfill Gas Advisory Centre (Advi-escentrum Stortgas), The Netherlands, March 1993.

|19| Evaluation of Landfill Gas Upgrading Project Carbiogas B.V. at Nuenen,Novem /Gastec, The Netherlands, December 1992.

|20| Evaluation of Landfill Gas Upgrading Project COGAS N.V. at Vasse, No-vem/Gastec, The Netherlands, July 1993.

|21| Guidebook on landfill gas extraction and utilisation, NOVEM/Thermie, TheNetherlands, 1997.


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SUPPLIERS OF GAS TREATMENT EQUIPMENT

Carbiogas B.V.PO Box 908500 AB JoureThe Netherlands

Carbotech Anlagenbau GmbHPO Box 130140D-45291 EssenGermany

Cirmac B.V.PO Box 9957301 BE ApeldoornThe Netherlands

Dynaf Energietechniek B.V.PO Box 54800 AB AlkmaarThe Netherlands

EltacomPO Box 2762700 AG Zoetermeer

The Netherlands

Petrogas Gas systems B.V.PO Box 202800 AA GoudaThe Netherlands

Spruyt Energiesystemen B.V.PO Box 83493503 UtrechtThe Netherlands

Zanthingh Energie systemen B.V.PO Box 2551430 AG AalsmeerThe Netherlands

Documents

Anaerobic Digestion of Agro-Industrial