biogas upgrading

IEA BioenergyTask 24: Energy from biologicalconversion of organic waste

BIOGASUPGRADINGANDUTILISATION

2 BIOGAS UPGRADING AND UTILISATION

Table of Contents

Introduction 3

Biogas Composition 4

Gas Utilisation 5Heating 5CHP-engines 5Vehicle fuel 6Fuel cells 7

Gas upgrading technologies 10

Carbon dioxide removal 10

Water scrubbing 10Polyethylene glycol scrubbing 11Carbon molecular sieves 11Membranes 12High pressure gas separation 12Gas-liquid absorption membranes 13

Hydrogen sulphide removal 13Biological desulphurisation 14Iron chloride dosing to digester slurry 15Iron oxide 15Iron oxide wood chips 16Iron oxide pellets 16Impregnated activated carbon 16Water scrubbing 17Selexol scrubbing 17Sodium hydroxide scrubbing 17

Halogenated hydrocarbon removal 17

Siloxane removal 17

Removal of oxygen and nitrogen 17

List of reference plants 18

UPGRADING AND UTILISATION OF BIOGAS

Flotec waterscrubbing

Kompogascarbonmolecularsieves

CirmacMembraneGas upgrading

Selexolscrubbing

3BIOGAS UPGRADING AND UTILISATION

Anaerobic digestion (AD) has successfully been used for many applications that haveconclusively demonstrated its ability to recycle biogenic wastes. AD has been suc-cessfully applied in industrial waste water treatment, stabilisation of sewage sludge,landfill management and recycling of biowaste and agricultural wastes as organic ferti-lisers. Increasingly the AD-process is applied for degrading heavy organic pollutantssuch as chlorinated organic compounds or materials resistant to aerobic treatment.

Today, farm-based manure facilities are perhaps the most common use of AD-techno-logy. Six to eight million family-sized low-technology digesters are used in the far East(Peoples Republic of China and India) to provide biogas for cooking and lighting. The-re are now over 800 farm-based digesters operating in Europe and North America.Thousands of digesters help to anaerobically stabilise and thicken sewage sludge be-fore it is either used on agricultural land, dried and incinerated or landfilled. More than1 000 high-rate anaerobic digesters are operated world-wide to treat organic pollutedindustrial waste water including processors of beverages, food, meat, pulp and paper,milk among others.Gas recovery from landfills has become a standard technology in most of the indu-strialised countries for energy recovery, environmental and safety reasons. Increasinglythe gas is used in combined heat and power (CHP) engines or as a supplement tonatural gas.There are more than 120 AD plants operating or under construction using the organicfraction of source separated municipal solid waste to produce a high quality compostor mechnically separated MSW to stabilise the organic fraction before landfilling. Thetotal installed capacity is close to five million tonnes.The product of anaerobic digestion is a mixed gas primarily composed of methaneCH4 and carbon dioxide which commonly is called biogas. In small scale installationsthe gas is primarely utilised for heating and cooking. In larger units CHP’s are fueledwith biogas. In any case, the driving force for the gas utilisation was to economise fos-sil fuels or wood as in developing countries.More recently, as discussed at the conferences of Rio and Kyoto, various airborneemissions have caused serious concern about climatic, environmental and health im-pacts. Discharges of acid and green house gases are actually at levels that require im-mediate actions to counter severe future problems. This is particularly true for thetransport sector. Alternative fuels might help considerably to reduce emissions. Thishas been recognised by a number of governements which brought forward programs(such as the EU ZEUS project which stands for Zero Emission vehicles in Urban So-ciety) and legislations (e.g. the Californian clean fuel act).In particular, biogas as a fuel could bring substantial reductions in green house gases,particles and dust or nitrogen oxide emissions. This has been acknowledged amongothers by by the Swedish Ministry of Environment who suggest to fully free biogas fromfuel taxes as the only biofuel before methanol and ethanol from bio-based origin or ra-pemethylester (RME). In Switzerland biogas is exempt of all fuel taxes for a limited pilotperiod. There are good chances that it will be fully freed in the near future.When biogas is used as a vehicle fuel it has to be upgraded and compressed. A num-ber of technologies have been developed during the passed ten years. This brochureprovides an insight of the 1999 status of biogas upgrading and utilisation.

Introduction

From Arthur Wellinger, Switzerlandand Anna Lindberg, Sweden.

1 000 high-rate anaerobic digestersare operated world-wide to treat

organic waste water and 120 ADplants to digest totally 5 million tons

of MSW and otherbiogenic solid wastes.


Biogas produced in AD-plants or landfillsites is primarily composed of methane(CH4) and carbon dioxide (CO2) withsmaller amounts of hydrogen sulphide(H2S) and ammonia (NH3). Traceamounts of hydrogen (H2), nitrogen (N2),carbon monoxide (CO), saturated or ha-logenated carbohydrates and oxygen(O2) are occasionally present in the bio-gas. Usually, the mixed gas is saturatedwith water vapour and may contain dustparticles and siloxanes.

Biogas Composition

Characteristics of different fuel gases

Parameter Unit Natural Gas Town Gas Biogas

Calorific value (lower) MJ/m3 36.14 16.1 21.48

Density kg/m3 0.82 0.51 1.21

Wobbe index (lower) MJ/m3 39.9 22.5 19.5

Max. ignition velocity m/s 0.39 0.70 0.25

Theor. air requirement m3air/m3gas 9.53 3.83 5.71

Max. CO2-conc. in stack gas vol% 11.9 13.1 17.8

Dew point °C 59 60 60-160

(60% CH4, 38%

CO2, 2% Other)

The characteristics of biogas are some-where in-between town gas (derivingfrom cracking of cokes) and natural gas.The energy content is defined by theconcentration of methane. 10 % of CH4

in the dry gas correspond to approx. onekWh per m3.

For many applications the quality of bio-gas has to be improved. The main pa-rameter that may require removal in anupgrading systems are H2S, water, CO2

and halogenated compounds:• Desulphurisation to prevent corrosionand avoid toxic H2S concentrations (the

maximal workplace concentration is 5ppm). When biogas is burned SO2/SO3

is emitted which is even more poiso-nous than H2S. At the same time SO2 lo-wers the dew point in the stack gas. Thesulphurous acid formed (H2SO3) is high-ly corrosive.

• Removal of water because of potentialaccumulation of condensate in the gasline, the formation of a corrosive acidicsolution when hydrogen sulphide is dis-solved or to achieve low dew pointswhen biogas is stored under elevatedpressures in order to avoid condensati-on and freezing.

• Removal of CO2 will be required if thebiogas needs to be upgraded to naturalgas standards or vehicle fuel use. It dilu-tes the energy content of the biogas buthas no significant environmental impact.

• Landfill gas often contains significantamounts of halogenated compoundswhich need to be removed prior to use.Occasionally the oxygen content is highwhen too much air is sucked in duringcollection of the landfill gas.


Biogas can be used for all applicationsdesigned for natural gas. Not all gas ap-pliances require the same gas stan-dards. There is a considerable diffe-rence between the requirements of sta-tionary biogas applications and fuel gasor pipeline quality.

Heating

Boilers do not have a high gas qualityrequirement. Gas pressure usually hasto be around 8 to 25 mbar. It is recom-mended to reduce the H2S concentrati-ons to values lower than 1.000 ppmwhich allows to maintain the dew pointaround 150°C. The sulphurous acid for-med in the condensate leads to heavycorrosion. It is therefore recommendedto use stainless steel for the chimneysor condensation burners and high tem-perature resistant plastic chimneys.Most of the modern boilers have tin-la-minated brass heat exchangers whichcorrode even faster than iron chimneys.Where possible, cast iron heat ex-changers should be utilised.It is also advised to condense the watervapour in the raw gas. Water vapour cancause problems in the gas nozzles.Removal of water will also remove a lar-ge proportion of the H2S, reducing thecorrosion and stack gas dew point pro-blems.

CHP-engines

The utilisation of biogas in internal com-bustion engines is a long establishedand extremely reliable technology. Thou-sands of engines are operated on se-wage works, landfill sites and biogas in-stallations. The engine sizes range from45kW (which corresponds to approx. 12kWel ) on small farms up to several MWon large scale landfill sites.

Gas utilisationRequirements to remove gaseous components depending onthe biogas utilisation

Application H2S CO2 H2O

Gas heater (boiler) < 1000 ppm no no

Kitchen stove yes no no

Stationary engine (CHP) < 1’000 ppm no no condensation

Vehicle fuel yes recommended yes

Natural gas grid yes yes yes

Gas engines do have comparable requi-rements for gas quality as boilers exceptthat the H2S should be lower to guaran-tee a reasonable operation time of theengine. Otto engines designed to run onpetrol are far more susceptible to hydro-gen sulphide than the more robust die-sel engines. For large scale applicati-ons (> 60 kWel) diesel engines are the-refore standard. Occasionally, organicsilica compounds in the gas can createabrasive problems. If so, they should beremoved.

A diesel engine can be rebuilt into aspark ignited gas engine or a dual fuelengine where approx. 8-10 % of dieselare injected for ignition. Both type of en-gines are often applied. The dual fuelengine has a higher electricity efficiency.The requirements for the gas upgradingare the same; small CHP (< 45 kWel)

Boilers and CHP-engines are reliablegas utilisers. They do not have a high

gas quality requirement. Usuallyupgrading is limited to water

condensation and eventually H2Scontrol.


achieve practical electric efficiencies of29 % (spark ignition) and 31 % (dualfuel). Larger engines have efficiencies ofup to 38 %.In biogas engines NOx emissions areusually low because of the CO2 in thegas. CO-concentration is often more of aproblem. Catalysts to reduce the CO aredifficult to use because of the H2S in thegas. However, from an environmentalpoint of view CO is a far smaller pro-blem than NOx because it is rapidly oxi-dised to CO2 which makes part of the

natural carbon cycle.Best results are achieved with lean burnengines. At air-fuel ratios (λ) of 1.5, NOx

and CO concentrations of less than 500ppm can be achieved.

A promising application of electrical ge-neration is the use of gas turbines. Mo-dern engines are equally efficient as in-ternal combustion engines and very ro-bust. They allow recovery of the heat inform of valuable steam. Unfortunately,the efficient turbines are available onlyat scales greater than 800 kWel. Theirgas requirements are comparable tothose of CHP engines.

Vehicle fuel

The utilisation of biogas as vehicle fueluses the same engine and vehicle con-figuration as natural gas. In total thereare more than 1 million natural gas ve-hicles all over the world, this demon-strates that the vehicle configuration isnot a problem for use of biogas as ve-hicle fuel. However, the gas quality de-mands are strict. With respect to thesedemands the raw biogas from a dige-ster or a landfill has to be upgraded.Through upgrading we obtain a gaswhich:• has a higher calorific value in order toreach longer driving distances,• has a regular/constant gas quality toobtain safe driving,• does not enhance corrosion due tohigh levels of hydrogen sulphide, am-monia and water,• does not contain mechanically dama-ging particles,• does not give ice-clogging due to ahigh water content,• has a declared and assured quality.

In practice this means that carbon dioxi-de, hydrogen sulphide, ammonia, partic-

Transportation of the biowaste withbiogas operated trucks allows to fullyclose the natural carbon cycle: No nonrenewable energy is required

Replacement of diesel or petrol bybiogas reduces the emissions andalso the engine noise considerably.


The four steps of biogas upgrading tovehicle fuel.

Fuel cells

Fuel cells (FC)are power generating sy-stems that produce DC electricity bycombining fuel and oxygen (from the air)

Different methods of environmentalrating gave natural gas a 75% over

all advantage over diesel and a 50%advantage over petrol.

les and water (and sometimes other tra-ce components) have to be removed sothat the product gas for vehicle fuel usehas a methane content above 95 vol%.In different countries different qualityspecifications for vehicle fuel use of bio-gas and natural gas are applied.

Upgraded biogas is actually the clea-nest vehicle fuel possible with respect toenvironment, climate and human health.A 1995 Swedish report on alternativefuels classified biogas on top long befo-re bio-based methanol and ethanol(resp. their tertiary butylesters) as wellas rapemethylester (RME).In 1998 two Swiss studies confirmedthe Swedish findings. Different methodsof environmental rating gave naturalgas a 75 % over all advantage over die-sel and a 50 % advantage over petrol.Human toxicity gave a 70 % lower value,the ozone potential was reduced by 60to 80 %, acid formation by more than 50%. Parallel monitoring of comparablecar engines fueled with either petrol, die-sel or natural gas in a town cycle (EUstandard) demonstrated a reduced NOx

emission for gas of 57 % resp. 88 %when compared to petrol resp. diesel, a96 % reduction of the ozone potentialand virtually no emission of canceroge-nic compounds. Non methanogenic hy-drocarbon emission was reduced by 73%. Only the methane emission wasincreased with the gas fueled engineswhich reduced the advantage of greenhouse gas emission to 25 %.

in an electrochemical reaction. There isno intermediate process which first con-verts fuel into mechanical energy andheat. Therefore fuel cells have extremelylow emissions. The reaction is similarto a battery however, fuel cells donot store the energy with chemicals in-ternally.

In a first step the fuel is transformed intohydrogen either by a catalytic steam re-forming conversion or by a (platinum)catalyst. The H2 is converted to directelectrical current. The by-products of thereaction are water and CO2.

Conversion efficiency to electricity is ex-pected to exceed 50 %. FC’s demon-strate relatively constant efficiencies overa wide range of loads. There are five ty-pes of fuel cells, classified by the type ofelectrolyte: Alkaline (AFC), PhosphoricAcid (PAFC), Molten Carbonate (MCFC),Solid Oxide (SOFC) and Proton Ex-change Membrane (PEM) fuel cells.

Alkaline fuel cell technology is used ex-tensively in NASA’s space shuttle pro-gram but is more difficult to use for terre-strial applications because of its intole-rance to carbon oxides.


Quality demands in different countries for utilisation of bio-gas as vehicle fuel

Unit France1) Switzerland1) Sweden

Wobbe indexlower MJ/nm3 45,5

Wobbe indexupper MJ/nm3 48,2

Water dewpoint °C 5° lower than the lowestambient temperature

Energy content upper kWh/nm3 10.7

Water content, maximum mg/nm3 100 5 32

Methane minimum vol% 96 97

Carbon dioxide, maximum vol% 3

Oxygen, maximum vol% 3.5 0.5 1

Carbon dioxide, + oxygen +nitrogen, maximum vol% 3 3 3

Hydrogen, maximum vol% 0,5

Hydrogen sulphide, maximum mg/nm3 7 5 23

Total sulphure mg/nm3 14.3

Particles or other solidcontaminants, max. diameter mm 5

Halogenated hydrocarbons mg/m3 1 0

1) Basically there are no specificrequirements for fuel gas. Therequirements are valuable for biogasintroduction into the grid from where itis also used for fuel gas. In Switzer-land the gas quality corresponds tonatural gas quality type H.

All the other technologies could be app-lied with upgraded biogas.

Currently, phosphoric acid fuel cells arethe only commercialised technology. Uti-lity-scale PACF have been in operationsince 1983. A number of power plants inthe 200 kW to 2 MW range are operatedin Japan and the USA. A 200 kW plant isalso running in Switzerland. The largestunit of 11 MW is operated by the TokyoElectric Power Company. The practicalelectric efficiency is 41 %.A number of PAFCs have been installedon water treatment plants, such as Port-land, USA. A feasibility study has shown

that a PACF can be operated on biogaswithout CO2 removal however, with care-full cleaning of halogens and hydrogensulphide.

Molten-Carbonate Fuel Cells are a typeof direct fuel cell that eliminate externalfuel processors. Methane (from naturalgas) and steam are converted into hy-drogen-rich gas in the reforming anodewhich is part of the fuel cell stack.MCFC pilot plants have demonstratedup to 50 % electric efficiency. They canoperate from 25 up to 125 % of the no-minal plant design. They are far morecompact than PAFC’s.At the end of 1999 a first demonstrationproject of an MCFC using digester gasas a fuel will be completed near Seattleat the King County’s waste water treat-ment plant.

Solid Oxide Electrolyte Fuel Cellsusually are utilising doped zirconia or yt-trium as the electrolyte. It operates at at-mospheric pressure or slight overpres-sure at temperatures above 900°C.There are several features of SOFC thatmake the technology attractive: One isthe high tolerance to fuel contaminantand two, the high temperature does notrequire expensive catalyst and permitsdirect fuel processing.Sulzer Hexis has developed a ZrO2-Ce-ramic fuel cell. One of two test stacks of1 kW electrical power has over 10.000hours of operation with H2. 10 pilotplants of 2 kW are in operation at poten-tial clients. With an expected electric ef-ficiency of 40 % and low emissions, theyare thought to have a role as heat andpower plants in living areas.

Proton Exchange Membrane Fuel Cellsare the most compact technology and


Types of fuel cells

Fuel Cell and PAFC MFC SOFC PEMCharacteristics

Electrolyte Phosphoric Molten Carbo- Solid Oxide MembranesAcid H3PO4 nate LiKCO3 Y2O3 and ZrO2

Operating Temperature 200°C 650°C 1000°C 50-120°C

System Efficiency (%) 40-45% 50-57% 45-50%

Module Size 200 kW- 2 MW 3-100 kW2 MW

Fuel type Natural, coal or landfill gas, etc. Gases MeOH

Commercial Availability now 1999 2001 2004

Gazelle, a 200 kW natural gasoperated phosphoric acid fuel cell has

been in operation since 1995 with anelectric efficiency of 41 %.

the only ones which operate at tempera-tures below boiling point of water.They are therefore particularly interestingfor vehicles. All the major car industriesare heavily investing in the developmentof the PEMFC technology. Two majorimprovements made the success pos-sible:One, the amount of the platinum catalystcould be reduced by a factor of 30 andtwo, new membranes are boosting PEMperformance at lower cost.


A number of gas upgrading technolo-gies have been developed for the treat-ment of natural gas, town gas, sewagegas, landfill gas etc. However, not all ofthem are recommended for the applica-tion with biogas because of price and/orenvironmental concerns.

Carbon dioxide removal

For an effective use of biogas as vehiclefuel it has to be enriched in methane.This is primarily achieved by carbon di-oxide removal which then enhances theenergy value of the gas to give longer

Gas upgrading technologiesdriving distances with a fixed gas stora-ge volume. Removal of carbon dioxidealso provides a consistent gas qualitywith respect to energy value. The latter isregarded to be of great importance fromthe vehicle manufacturers in order to re-ach low emissions of nitrogen oxide.At present four different methods areused commercially for removal of car-bon dioxide from biogas either to reachvehicle fuel standard or to reach naturalgas quality for injection to the naturalgas grid.These methods are:• water absorption,• Polyethylene glycol absorption,• carbon molecular sieves,• membrane separation.Below the methods are described inmore detail.

Water scrubbingWater scrubbing is used to remove car-bon dioxide but also hydrogen sulphidefrom biogas since these gases aremore soluble in water than methane.The absorption process is purely physi-cal. Usually the biogas is pressurisedand fed to the bottom of a packed co-lumn where water is fed on the top andso the absorption process is operatedcounter-currently.

Water scrubbing can also be used forselective removal of hydrogen sulphidesince hydrogen sulphide is more solu-ble than carbon dioxide in water. Thewater which exits the column with absor-bed carbon dioxide and/or hydrogensulphide can be regenerated and recir-culated back to the absorption column.The regeneration is made by de-pressu-rising or by stripping with air in a similarcolumn. Stripping with air is not recom-mended when high levels of hydrogensulphide are handled since the waterwill soon be contaminated with elemen-tary sulphur which causes operational

Water scrubbing of carbon dioxideand hydrogen sulphide is oftenapplied in Sweden, France and theU.S.A. The process is especiallysimple and usefull in sewage andindustrial waste water treatment plantswhere the water does not have to beregenerated and recycled. The pictureshows the Renton plant (Seattle,U.S.A.) with a capacity of 4.000 nm3

per day.


Schematic flow sheet for waterabsorption with recirculation for

removal of carbon dioxide and/orhydrogen sulphide from biogas.

Pressure swing adsorbtion of biogason activated carbon with removal of

H2S (right), halogenated hydrocarbons(middle) and 4-bed CO2

adsorption (left).

problems. The most cost efficient me-thod is not to recirculate the water ifcheap water can be used, for example,outlet water from a sewage treatmentplant.

Polyethylene glycol scrubbingPolyethylene glycol scrubbing is like wa-ter scrubbing a physical absorption pro-cess. Selexol is one of the trade namesused for a solvent. In this solvent, like inwater, both carbon dioxide and hydrogensulphide are more soluble than metha-ne. The big difference be-tween waterand Selexol is that carbon dioxide andhydrogen sulphide are more soluble inSelexol which results in a lower solventdemand and reduced pumping. In addi-tion, water and halogenated hydrocar-bons (contaminants in biogas fromlandfills) are removed when scrubbingbiogas with Selexol.

Selexol scrubbing is always designedwith recirculation. Due to formation ofelementary sulphur stripping the Selexolsolvent with air is not recommendedbut with steam or inert gas (upgradedbiogas or natural gas). Removing hydro-gen sulphide on beforehand is an alter-native.

Carbon molecular sievesMolecular sieves are excellent productsto separate specifically a number of dif-ferent gaseous compounds in biogas.Thereby the molecules are usually loo-sely adsorbed in the cavities of the car-bon sieve but not irreversibly bound. Theselectivity of adsorption is achieved bydifferent mesh sizes and/or applicationof different gas pressures.When the pressure is released the com-pounds extracted from the biogas aredesorbed. The process is therefore of-ten called “pressure swing adsorption”(PSA). To enrich methane from biogasthe molecular sieve is applied which is

produced from coke rich in pores in themicrometer range. The pores are thenfurther reduced by cracking of the hydro-carbons.In order to reduce the energy consumpti-on for gas compression, a series of ves-sels are linked together. The gas pres-sure released from one vessel is sub-sequently used by the others. Usuallyfour vessels in a row are used filled withmolecular sieve which removes at thesame time CO2 and water vapour.After removal of hydrogen sulphide, i.e.using activated carbon and water con-densation in a cooler at 4°C, the biogasflows at a pressure of 6 bars into the ad-sorption unit. The first column cleansthe raw gas at 6 bar to an upgraded bio-gas with a vapour pressure of less than

Absorbtion tower

upgraded biogas~90 % CH4

Dryer

Water

Water pumpDigester

Desorption tower(regeneration ofwater)

Compression 10 bar

CO2 + H2S


Schematic flow sheet for upgrading ofbiogas to vehicle fuel standards withcarbon molecular sieves.

10 ppm H2O and a methane content of96 % or more.In the second column the pressure of 6bar is first released to approx. 3 bar bypressure communication with column 4,which was previously degassed by aslight vacuum. In a second step thepressure is then reduced to atmosphe-ric pressure. The released gas flowsback to the digester in order to recoverthe methane. The third column is eva-cuated from 1 bar to 0.1 bar. The desor-bed gas consists predominantly of car-bon dioxide but also some methane andis therefore normally released to the en-vironment. In order to reduce methanelosses the system can be designed withrecirculation of the desorbed gases.

The product gas of column 1 is monito-red continuously for CH4 by an infraredanalyser. If the required Wobbe index isnot maintained the gas flows back toPSA. If the methane content is highenough, the gas is either introduced intothe natural gas net or compressed in a3 stage compressor up to 250 bar.Continuous monitoring of a small-scaleinstallation (26 m3/hr) demonstrated ex-cellent results of gas cleaning, energyefficiency and cost.

MembranesThere are two basic systems of gaspurification with membranes: a highpressure gas separation with gas pha-ses on both sides of the membrane,and a low-pressure gas liquid absorpti-on separation where a liquid absorbsthe molecules diffusing through themembrane.

High pressure gas separationPressurised gas (36 bar) is first cleanedover for example an activated carbonbed to remove (halogenated) hydrocar-bons and hydrogen sulphide from theraw gas as well as oil vapour from thecompressors. The carbon bed is follo-wed by a particle filter and a heater.The membranes made of acetate-cellu-lose separate small polar moleculessuch as carbon dioxide, moisture andthe remaining hydrogen sulphide. The-se membranes are not effective in sepa-rating nitrogen from methane.The raw gas is upgraded in 3 stages toa clean gas with 96 % methane or more.The waste gas from the first two stagesis recycled and the methane can be re-covered. The waste gas from stage 3(and in part of stage 2) is flared or usedin a steam boiler as it still contains 10 to20 % methane.First experiences have shown that themembranes can last up to 3 yearswhich is comparable to the lifetime ofmembranes for natural gas purification -a primary market for membrane techno-logy - which last typically two to fiveyears. After 1½ years permeability hasdecreased by 30 % due to compaction.The clean gas is further compressed upto 3.600 psi (250 bar) and stored insteel cylinders in capacities of 276 m3

divided in high, medium and low pres-sure banks.

The membranes are very specific for gi-ven molecules, i.e. H2S and CO2 are se-

upgraded biogas

(CH4)

4-bedadsorberCompressor

Biogas

CondensationCooler

CH4/CO2-Seperation

Vacuum pump

CO2

H2O

First experiences have shown thatthe high pressure gas seperationmembranes can last up to 3 yearswhich is comparable to the liefetimeof membranes for natural gaspurification.


parated in different modules. The utilisa-tion of hollow-fibre membranes allowsthe construction of very compact modu-les working in cross flow.

Gas-liquid absorption membranesGas-liquid absorption using membra-nes is a separation technique whichwas developed for biogas upgradingonly recently. The essential element is amicroporous hydrophobic membraneseparating the gaseous from the liquidphase. The molecules from the gasstream, flowing in one direction, whichare able to diffuse through the membra-ne will be absorbed on the other side bythe liquid flowing in counter current.The absorption membranes work at ap-prox. atmospheric pressure (1 bar)which allows low-cost construction.The removal of gaseous components isvery efficient. At a temperature of 25 to35°C the H2S concentration in the rawgas of 2 % is reduced to less than 250ppm. The absorbent is either Coral orNaOH.

H2S saturated NaOH can be used in wa-ter treatment to remove heavy metals.The H2S in Coral can be removed byheating. The concentrated H2S is fedinto a Claus reaction or oxidised to ele-mentary sulphur. The Coral solution canthen be recycled.CO2 is removed by an amine solution.The biogas is upgraded very efficientlyfrom 55% CH4 (43 % CO2) to more than96% CH4. The amine solution is regene-rated by heating. The CO2 released ispure and can be sold for industrial app-lications.

Hydrogen sulphide removal

Hydrogen sulphide is always present inbiogas, although concentrations varywith the feedstock. It has to be removed

absorbtionliquid

landfill-biogas

microporousmembrane

in order to avoid corrosion in compres-sors, gas storage tanks and engines.Hydrogen sulphide is extremely reactivewith most metals and the reactivity is en-hanced by concentration and pressure,the presence of water and elevated tem-peratures. Due to the potential problemshydrogen sulphide can cause, it is re-commended to remove it early in theprocess of biogas upgrading. Experi-ence has also shown that two of the

H2S

H2S

inlet

outlet

inlet

outlet

Gas-liquid absorption usinghydrophobic membranes is a recentdevelopment working at atmospheric

pressures which allows a low-costconstruction.

The cross-flow gas absorptionmembrane is particularly well adaptedfor the removal of H2S with NaOH orcoral as an absorbant. The latter canbe regenerated.


most commonly used methods for hy-drogen sulphide removal are internal tothe digestion process: 1) air/oxygen do-sing to digester biogas and 2) iron chlo-ride dosing to digester slurry. The mostcommon commercial methods for hy-drogen sulphide removal are describedbelow:• air/oxygen dosing to digester biogas,• iron chloride dosing to digester slurry,• iron sponge,• iron oxide pellets,• activated carbon,• water scrubbing,• NaOH scrubbing,• biological removal on a filter bed,• air stripping and recovery.

Biological desulphurisationDesulphurisation of biogas can be per-formed by micro-organisms. Most of thesulphide oxidisin�g micro-organismsbelong to the family of Thiobacillus. Most

of them are autotrophic i. e. they areusing carbon dioxide from the biogas tocover their carbon need. The productsformed are predominantly elementarysulphur but also sulphate. The latterforms in solutions sulphuric acid whichmay cause corrosion.

For the microbiological oxidation of sulp-hide it is essential to add stoichiometricamounts of oxygen to the biogas. De-pending on the concentration of hydro-gen sulphide this corresponds to 2 to 6% air in biogas.Air/oxygen dosing to digester biogas.The simplest method of desulphurisati-on is the addition of oxygen or air directlyinto the digester or in a storage tank ser-ving at the same time as gas holder.Thiobacilli are ubiquitous and thus sy-stems do not require inoculation. Theygrow on the surface of the digestate,which offers the necessary micro-aero-philic surface and at the same time thenecessary nutrients. They form yellowclusters of sulphure. Depending on thetemperature, the reaction time, theamount and place of the air added thehydrogen sulphide concentration can bereduced by 95 % to less than 50 ppm.Measures of safety have to be taken toavoid overdosing of air in case of pumpfailures. Biogas in air is explosive in therange of 6 to 12 %, depending on themethane content). In steel digesters wit-hout rust protection there is a small riskof corrosion at the gas/liquid interface.

Biological filtersIn large digesters there is often a combi-ned procedure of water scrubbing (ab-sorption) and biological desulphurisati-on applied. Either raw waste water orpress-separated liquor from digestate isdispensed over a filter bed. In the bed, li-quor and biogas meet in counterflowmanner. In the biogas 4 to 6 % air is ad-ded before entering the filter bed. The fil-ter bed offers the required surface forscrubbing as well as for the attachmentof the desulphurisation micro-orga-nisms.The system is applied in several instal-lations for industrial waste water treat-ment and in many of the Danish agricul-tural and co-digestion plants.

Addition of 2 to 6 % of air to the biogasallows the indigenous Thiobacilli tooxidise the H2S to natural sulfureadhering to the digester surface or thedigestate.

The simplest method of desulphuri-sation is the addition of air into astorage tank serving at the sametime as gas holder. Microorganismsreduce the H2S concentration by95 % to less than 50 ppm.


Iron chloride dosing to dige-ster slurryIron chloride can be fed directly to the di-gester slurry or to the feed substrate in apre-storage tank. Iron chloride then re-acts with produced hydrogen sulphideand form iron sulphide salt (particles).This method is extremely effective in re-ducing high hydrogen sulphide levelsbut less effective in attaining a low andstable level of hydrogen sulphide in therange of vehicle fuel demands. In thisrespect the method with iron chloridedosing to digester slurry can only be re-garded as a partial removal process inorder to avoid corrosion in the rest of theupgrading process equipment. The me-thod need to be complemented with a fi-nal removal down to about 10 ppm.The investment cost for such a removalprocess is limited since the only invest-ment needed are a storage tank for ironchloride solution and a dosing pump.On the other hand the operational costwill be high due to the prime cost for ironchloride.

Iron oxideHydrogen sulphide reacts easily withiron hydroxides or oxides to iron sulphi-de. The reaction is slightly endothermic,a temperature minimum of approximate-ly 12°C is therefore required to providethe necessary energy. The reaction isoptimal between 25 and 50°C. Since thereaction with iron oxide needs water thebiogas should not be too dry. However,condensation should be avoided becau-se the iron oxide material (pellets,grains etc.) will stick together with waterwhich reduces the reactive surface.

The iron sulphides formed can be oxi-dised with air, i. e. the iron oxide is reco-vered. The product is again iron oxide orhydroxide and elementary sulphur. Theprocess is highly exothermic, i.e. a lot ofheat is released during regeneration.

Therefore, there is always a chance thatthe mass is self-ignited. The elementarysulphur formed remains on the surfaceand covers the active iron oxide surface.After a number of cycles depending onthe hydrogen sulphide concentration theiron oxide or hydroxide bed has to be ex-changed.

Usually an installation has two reactionbeds. While the first is desulphurisingthe biogas, the second is regeneratedwith air.The desulphurisation process workswith plain oil free steel wool covered withrust. However, the binding capacity forsulphide is relatively low due to the lowsurface area.

Iron oxide wood chipsWood chips covered with iron oxide havea somewhat larger surface to volume ra-tio than plain steel. Their surface toweight ratio is excellent thanks to the lowdensity of wood. Roughly 20 grams ofhydrogen sulphide can be bound per100 grams of iron oxide chips.

The application of wood chips is very po-pular particularly in the USA. It is a lowcost product, however, particular carehas to be taken that the temperaturedoes not rise too high while regenera-ting the iron filter.

Iron oxide pelletsThe highest surface to volume ratios areachieved with pellets made of red mud,a waste product from aluminium pro-duction. However, their density is muchhigher than that of the wood chips. At hy-drogen sulphide concentrations bet-ween 1.000 ppm and 4.000 ppm totally50 grams can be loaded on 100 gramsof pellets. Most of the German andSwiss sewage treatment plants withoutdosing of iron chloride are equippedwith an iron oxide pellet installation.

Iron chloride dosing to digester slurryis an extremely efficient method to

reduce hydrogen sulphide levels, butdoes not allow to achieve vehicle

fuel demands.

Hydrogen sulphide reacts with ironoxide (rust) to iron sulphide. The

latter can be reoxidised with air. Theproduct is again iron oxide and

elementary sulphure.


Impregnated activated carbonWith PSA systems H2S usually is remo-ved by activated carbon doted with po-tassium iodide (KI). Like in biological fil-ters in presence of air which is added tothe biogas, the hydrogen sulfide is cata-lytically converted to elementary sulphurand water. The sulphur is adsorbed bythe activated carbon. The reaction worksbest at a pressure of 7 to 8 bar and atemperature of 50 to 70°C. The gas tem-perature is easy to achieve through theheat formed during compression.Usually, the carbon filling is adjusted toan operation time of 4.000 to 8.000hours. If a continuous process is requi-red the system consists of two vessels.At H2S concentrations above 3.000 ppmthe process is designed as a regenera-tive system.

Water scrubbingWater scrubbing is a purely physical ab-sorption process, described above,which can be used for selective removalof hydrogen sulphide. The cost for sel-ective removal has not yet been shownto be competitive with other hydrogensulphide removal methods.Thus water scrubbing probably only will

be considered for the simultaneousremoval of carbon dioxide in order tomeet vehicle fuel demands on biogasquality.

Selexol scrubbingSelexol scrubbing is like water absorpti-on a purely physical absorption processdescribed above. Selexol is one of thetrade names for a solvent mainly consti-tuting of a dimethylether of polyethyleneglycol (DMPEG). The cost for selectivehydrogen sulphide removal has not yetshown to be competitive and so Selexolscrubbing will probably only be conside-red for simultaneous removal of carbondioxide and hydrogen sulphide in orderto meet vehicle fuel demands on biogasquality.

Sodium hydroxide scrubbingAbsorption in a water solution of sodiumhydroxide (NaOH) enhances the absorp-tion capacity of the water and the ab-sorption process is no longer purelyphysical but chemical. Sodium hydroxi-de reacts with hydrogen sulphide toform sodium sulphide or sodium hydro-gen sulphide. Both these salts are inso-luble and the method is not regenerati-ve. Since the absorption capacity of wa-ter is enhanced lower volumes are nee-ded and pumping demands are redu-ced. The main disadvantage is the dis-posal of the large volumes of water con-taminated with sodium sulphide.

Hydrogen sulphide reacts easily withiron oxide or hydroxide which isusually bound on wood chips or redmud pellets to increase the reationsurface. In a two-column plant onecolumn binds H2S where as the otheris regenerated.


Halogenated hydrocar-bon removal

Higher hydrocarbons as well as haloge-nated (FHC) hydrocarbons, particularlychloro- and fluoro-compounds are pre-dominantly found in landfill gas. Theycause corrosion in CHP engines, in thecombustion chamber, at spark plugs,valves, cylinder heads, etc. For this rea-son CHP engine manufacturers claimmaximum limits of halogenated hydro-carbons in biogas.

They can be removed by pressurisedtube exchangers filled with specific acti-vated carbon. Small molecules like CH4,CO2, N2 and O2 pass through while lar-ger molecules are adsorbed. The size ofthe exchangers are designed to purifythe gas during a period of more than 10hours. Usually there are two parallelvessels. One is treating the gas whilethe other is desorbed. Regeneration iscarried out by heating the activated car-bon to 200°C, a temperature at which allthe adsorbed compounds are evapora-ted and removed by a flow of inert gas.

Siloxane removal

Organic silicon compounds are occa-sionally present in biogas which cancause severe damage to CHP engines.During incineration they are oxidised tosilicon oxide which deposits at sparkplugs, valves and cylinder heads abra-ding the surfaces and eventuallycausing serious damage. Particularly inOtto engines this might lead to major re-pairs. Dual fuel engines are less su-ceptible because the temperature of theentire motor body is much higher thanwith Otto engines.Because of the increased wearing ofcombustion chambers caused by silicadeposits nowadays manufacturers of

CHP engines claim maximum limits ofsiloxanes in biogas.It is known that the organic silicon com-pounds in biogas is in the form of linearand cyclic methyl siloxanes.These compounds are widely used incosmetics and pharmaceutical pro-ducts, and as anti-foaming agents in de-tergents.Siloxanes can be removed by absorpti-on in a liquid medium, a mixture of hy-drocarbons with a special ability to ab-sorb the silicon compounds. The absor-bent is regenerated by heating and des-orption. A full scale removal plant for bio-gas from a landfill is in operation in Dort-mund-Huckarde since 1993.

Removal of oxygenand Nitrogen

Oxygen and in part also nitrogen in thebiogas is a sign that air has been su-cked in. This occurs quite often in land-fills where the gas is collected throughpermeable tubes by applying a slightunderpressure. Low concentrations ofoxygen is not a problem. Higher concen-tration however bear a risk of explosion.Biogas with a methane content of 60 %,the rest being predominantly carbon di-oxide, is explosive in concentrationsbetween 6 and 12 % in air.Oxygen and nitrogen can be removed bymembranes or low temperature PSAhowever, removal is expensive. Preven-ting the introduction of air by carefullymonitoring the oxygen concentration isfar cheaper and more reliable than gastreatment.

Halogenated hydrocarbonspredominantly found in landfill gascause corrosion in CHP engines.

They have to be removed byspecific activated carbon.

Organic silicon compounds areoccasionally present in biogas.

They are used in cosmetics,pharmaceutical products and

detergents. They can be removedby liquid hydrocarbons.


List of selected reference plants with full gas

Country City Product Biogas CH4 CO2- removal H2S- removal Raw gas In operationgas Production Requi- (technique) (technique) f low sinceutilisation rementgas grid or (landfill/ sewage %vehicle fuel sludge/waste/

manure)

Czech Rep. Bystrany/ Vehicle fuel Sewage sludge 95 Water scrub. Water scrub. 368 1985TepliceBystrica Vehicle fuel Sewage sludge 95 Water scrub. Water scrub. 186 1990Chanov/Most Vehicle fuel Sewage sludge 95 Water scrub. Water scrub. 186 1990Liberec Vehicle fuel Sewage sludge 95 Water scrub. Water scrub. 368 1988Zlin/Tecovice Vehicle fuel Sewage sludge 95 Water scrub. Water scrub. 186 1990

France Chambéry Vehicle fuel Sewage sludge 96,7 Water scrub. Biol. filter/ 30water scrub.

Lille Vehicle fuel Sewage sludge Water scrub. Water scrub. 100 1995Tours Vehicle fuel Landfill Water scrub. Water scrub. 200 1994

The Nether- Collendorn Natural gas Landfill 88 Membranes Activated 375 1991lands carbon

Gorredijk Natural gas Landfill 88 Membranes Activated 400 1994carbon

Nuenen Natural gas Landfill 88 Carbon Activated 1 500 1990molecular carbonsieves

Tilburg Natural gas Sewage sludge 88 Water scrub. Iron oxide 2 100 1987Landfill pelletsGreen waste

Wijster Natural gas Landfill 88 Carbon Activated 1 150 1989molecular carbonsieves

New Zeeland Christchurch Vehicle fuel Water scrub.

Sweden Eslöv Vehicle fuel Sewage sludge 97 Water scrub. Water scrub. 40 1998Vegetable waste

Göteborg Vehicle fuel Sewage sludge 97 Carbon Activated 6 1992molecular carbonsieves

Helsingborg Vehicle fuel Slaughterhouse waste 97 Carbon Activated 16 1996molecular carbonsieves

Kalmar Vehicle fuel Sewage sludge 97 Water scrub. Water scrub. 30 1998 + manure + slaugh-terhouse waste

Linköping Vehicle fuel Sewage sludge 97 Water scrub. Iron chloride 700 1997+ manure + slaugh- dosing +terhouse waste water scrub

Vehicle fuel Carbon 200 1991molecularsieves

Stockholm Vehicle fuel Sewage sludge 97 Water scrub. Water scrub. 45 1997Trollhättan Vehicle fuel Sewage sludge 97 Water scrub. Water scrub. 200 1996

+ fish waste


ImpressumText: Arthur Wellinger and

Anna Lindberg,Member IEA, Task 24

Layout: Gaby Roost,Nova Energie GmbH,Switzerland

Pictures: ManufacturersNova Energie GmbHSwecoVerband derSchweiz. Gasindustrie

Printer: Sailer Druck, Winterthur

Country City Product Biogas CH4 CO2- removal H2S- removal Raw gas In operationgas Production Requi- (technique) (technique) flow sinceutilisation rementgas grid or (landfill/ sew. %vehicle fuel sludge/waste/

manure)

Sweden Uppsala Vehicle fuel Sewage sludge 97 Water scrub. Water scrub. 200 1997 + manure

Switzerland Bachenbülach Vehicle fuel Biowaste 96 Carbon Activatedmolecular carbonsieves

Otelfingen Vehicle fuel Biowaste 96 Carbon Activatedmolecular carbonsieves

Rümlang Vehicle fuel Biowaste 96 Carbon Activatedmolecular carbonsieves

Samstagern Natural gas Biowaste 96 Carbon Activated molecular carbon sieves

USA Croton landfill, Vehicle fuel Landfill 90 Selexol Selexol 120 1993 (reusedWestchester scrubbing scrubbing from TorkCo. (NY) Landfill Wisc.

1996)Fresh Kills, Natural gas Landfill Selexol Selexol 13 000 1981Staten Island scrubbing scrubbing(NY)Puente Hill Vehicle fuel Landfill 96 Membranes Activated 2 600 1993Landfill, Los carbonAngeles (CA)Renton (WA) Natural gas Sewage sludge 98 Water scrub. Water scrub. 4 000 1984 + 1998Mc Carty Natural gas Landfill Selexol Selexol 9 400 1986Road (NY) scrubbing scrubbing

Cartoon by JTI

upgrading to natural gas/vehicle fuel standards


Task 24: Energy from biologicalconversion of organic waste

Task 24 Participants

Task LeaderPat WheelerAEA Technology EnvironmentE6 Culham LaboratoryAbingdonOxfordshireOX14 3DBUKTel. +44 1235 463135Fax +44 1235 463010e-mail: [email protected]

FinlandTerho JaatinenEco-Tecnology JVV OYValkärventie 2SF-02130 EspooFinlandTel. +358 9 4357 7477Fax +358 9 4357 7488e-mail: [email protected]

SwedenAnna LindbergSweco/VBB ViakP.O Box 34044S-100 26 StockholmSwedenTel +46 8 695 62 39Fax +46 8 695 62 30e-mail: [email protected]

Simon LundebergRVF/Swedish Association of Waste

DenmarkJens Bo Holm-NielsenThe Biomass Institute, SUCNiels Bohrs vej 9DK 6700, EsbjergDenmarkTel +45 79 14 11 11Fax +45 79 14 11 99e-mail: [email protected]

SwitzerlandArthur WellingerNova EnergieElggerstr. 368356 EttenhausenSwitzerlandTel. +41 52 368 34 70Fax +41 52 365 43 20e-mail: [email protected]

UKAlastair PettigrewOnyx Waste ManagementOnyx House401 Mile End Rd.London, E3 4PBUKTel. 0181 983 5945Fax 0181 983 0100e-mail: [email protected]

biogas upgrading

Documents

gas utilisation

gas recovery

mixed gas

increasinglythe gas

supplement tonatural

high pressure gas separation

carbon dioxide removal

organic fraction