nachwachsende-rohstoffe.de

Biogas – an introduction

Published by:Fachagentur Nachwachsende Rohstoffe e.V. (FNR)Hofplatz 1 · 18276 Gülzow · GermanyPhone: +49 (0)38 43/69 30-0Fax: +49 (0)38 43/69 30-102E-Mail: info@fnr.de Website: www.fnr.de

With financial support of the Federal Ministry of Food,Agriculture and Consumer Protection (BMELV)

Text:Department of Public Relations, FNR

Design and production:nova-Institut GmbH · www.nova-institut.de

Printing and handling:Media Cologne Kommunikationsmedien GmbHwww.mediacologne.de

1st edition - April 2008

FNR 2008

Table of contents

Renewable energy from biogas 4

How much energy could be produced from biogas? 5

Renewable raw mat erials and sus tainability 5

What are the environmental benefits of biogas production? 8

How is biogas produced? 8

Which substrates can biogas be made from? 9

How does a biogas plant work? 11

How is biogas utilised? 16

What is the applicable regulatory framework? 20

How does a biogas plant become cost-effective? 22

Further information 23

Useful figures 24

Important process parameters 24

3

Renewable energy frombiogas

There is no disputing the dependency of the industrialised nations on finite resources and the consequences of theuse of fossil energy sources can alreadybe discerned. A U-turn can only comeabout through the increased use of su-stainable energy sources. In 2006, re -new able energy provided 11.5 % of total elec tricity consumption and 6 % of hea-ting (cf. Fig. 1).

The German Government intends to re-duce greenhouse gas emissions substan-tially by 2020 and to increase re newableenergy’s share of electricity sup ply to 25– 30 % and that of heat production to14 %. Biogas use can make an importantcontribution to this. After all, energyfrom biomass has the advantage that itcan be produced in a CO2-neutral wayand used in line with demand.

Energy production through the utlisa -tion of biogas has long been well-known.

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Figure 1: Electricity and heat production from renewable energy sources in Germany in2006 (Source: Federal Ministry for Environment (BMU), 2007)

Biogenic liquid fuel1.9 %

Electricity production from renewable resources in 2006 (70.4 TWh)

Heat production from renewable resources in 2006 (89.5 TWh)

Yet it is only since the beginning of the1990s that any major use has been madeof it in the more than 3,700 mainly farm-based plants that presently exist in Ger -many. There has been enormous growthsince the revised Renewable EnergySource Act (EEG) came into effect in Au-gust 2004.

How much energy could beproduced from biogas?

The biogas, sewage and landfill gas po-tential in Germany is approximately 23 – 24 billion m3/year. The largest con-tribution, roughly 85 %, comes from thepotential biogas production in the agri-cultural sector. This gives a theoreticallyavailable annual energy source potentialfor biogas, sewage gas and landfill gas

of 417 petajoules (PJ/a). In relation tothe total primary energy consumption of 13.842 PJ in Germany in 2007, thiswould represent a share of about 3 %.

Ren ewable raw mat erialsand sus tain ability

Fossil energy sources are running lowand in order to spare them and halt cli-mate change, we will have to graduallyswitch to renewable energy over the coming decades. Bioenergy, which alrea-dy is already the largest contributor a -mong regenerative energy sources inGermany with about 70 % of the total,will continue to play a central role in thefuture. Re newable raw materials are al -ready of great importance in the materi-als sector too.A fundamental prerequisi-

5

Figure 2: Number of biogas plants with their total installed electrical capacity in Germany(Source: Federal Ministry for Environment (BMU), 2007; German Biogas Association,2007; provided FNR)

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te for the in crea sed utilisation of plantraw materials and energy sources is ho-wever that they are produced and usedsustainably. Sus tainability, as defined inthe 1987 Brundtland Report, meansmeeting the needs of the present genera-tion without compromising the abilityof future generations to meet their ownneeds1). Sustainability therefore has anenvironmental, an econo mic and a soci-al dimension. When ap plied to renewa-ble raw materials, this means that theirutilisation needs to strike a balance bet-ween what is economically necessary,such as high and guaranteed biomassyields, and what nature can be expectedto tolerate. The social component refersamong other things to peo ple’s workingconditions, new income op por tunitiesand a share of value-added processes.There are many different approaches to

sustainable production in Europeanagri culture and forestry. One of the Fe-deral Ministry of Agriculture’s (BMELV)main funding domains is to test theseapproaches through research projectsand to further develop them. Some ofthe strategies that are being pursued are:• Increasing the species diversity used

in energy crop production;• Breeding new varieties;• New production methods using lower

doses of pesticides and fertilisers aswell as year-round vegetative cover onfields;

• The use of especially efficient conver-sion processes;

• The recycling of residues as fertiliser.

The BMELV’s task is to fund research inan appropriate and consistent mannerso as to develop the most suitable me-

Figure 3: Utilisable energy potential (Hartmann/Kaltsschmitt, 2002, reworked by FNR)

Utilisable energy potential of Biogas

Energy crops(on 2 mio. hectares)

236 PJ/a

Agricultural residues13.7 PJ/a

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thods for a sustainable energy and rawmaterial industry. It will then require thewhole of society to put these methodsinto practice: it is business and consu-mers who have to integrate these newprocesses and products into their dailylives.

Agricultural markets have long beenglobalised. The needs for bioenergy andrenewable raw materials are thus increa-singly satisfied by international marketsand this cannot help but have an impacton questions of sustainability. In the tro-pics, there are different problems than inEurope, as areas of rainforest are clearedto grow food, feed and energy cropssuch as oil palm and soya, the workfor-ce is exploited and indigenous peoplesare displaced. A pilot project for a certi-fication system, funded by the BMELV,intends to remedy this situation. Its aimis, as a first step, to authorise only bio-mass with a certificate of sustainabilityto be used in the production of biofuels.Later, such certificates will be applied toall possible methods of using agricultu-ral raw materials so as to avoid displa-cement effects. The certification project,which is now beginning a two-year testphase, is thus also a suitable instrumentto test the sustainability requirements ofdraft Federal and EU-wide laws.2)

After all, the situation in Southern coun-tries is exactly the same as in the North:bioenergy can be as much a threat as anopportunity for the ecosystems and peo-ple who live there. If in the South, for ex-ample, small farmers can be establishedas biomass producers and the large

areas of uncultivated land can be takenback into production for energy crops,then the advantages outweigh thedisadvan tages. Extremely drough-resi-stant plants such as jatropha (also called“physic nut”) offer possibilities to reve-getate desert-like areas.

In the North, new varieties of energycrops and new production methods canensure greater diversity and sustainabi-lity. What is more, in rural areas,bioenergy is a first-rate instrument forstruc tural de velopment: it offers newsources of in come, new economic confi-gurations and greater independence toregions that are presently often some ofthe more structurally disadvantaged,problem areas.

Paths towards a sustainable, renewablesociety do exist. It is now a question ofchoosing them and developing them not

1) In 1983, the World Commission on Environment and

Development (Brundt land Commission) chaired by the

former Norwegian Prime Minister Gro Harlem Brundt-

land used the term “sustainability”, which originally ca-

me from forestry, for the first time in a development

context. This definition is quoted from the Brundtland

Commission’s final report “Our Common Future” (also

known as the Brundtland Report) from 1987.

2) Draft sustainability directive by the German Federal

Government (2007), which prescribes that only sustaina-

bly produced biofuels may be considered for the quotas

(Biofuel Quota Act), as well as draft guidelines from the

European Commission to promote renewable energy

(2008).

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only economically but also environmen -tally and socially. The BMELV is makingits contribution by funding research andraising awareness.

What are the environ-mental benefits of biogasproduction?

The most important contribution of bio-gas technology to environmental protec-tion is that it avoids additional carbondioxide (CO2) emissions compared withfossil energy sources. Producing energyfrom biogas is largely CO2-neutral, i.e.the CO2 released by burning biogas waspreviously removed from the atmos -phere during the generation of biomassthrough photosynthesis.

The fermentation of manure also redu-ces emissions of methane, a gas that hasan effect on the climate and wouldother wise escape uncontrolled from rawliquid manure with far more damagingeffects for the climate than CO2. New re-search suggests that emissions of laug-hing gas – which also has an effect onthe climate – can also be reduced by fer-mentation. Moreover, fermentation re-duces the development of odours du-ring liquid manure storage and sprea-ding since the odours contained in the li-quid manure are broken down and neu-tralised during the fermentation pro-cess. In addition, fermentation improvesthe quality of man ure as pathogens andweed seeds are killed and nutrients ma-de more available for plants, enablingthe manure to be applied in a more tar-

geted fashion as a substitute for inorga-nic fertilisers.

How is biogas produced?

Biogas occurs widely in nature. Biogasforms wherever organic material accruesunder exclusion of oxygen (called anae-robic digestion), e.g. in bogs, on the bot-tom of lakes or in ruminants’ stomachs.The organic matter is almost entirely con -verted into biogas in these conditions.The actual process by which biogas formsinvolves the complex interac tion of vari -ous microorganisms and takes place inbasiclly four separate phases (cf. Fig. 4).

The first stage of decomposition in met-hane producing fermentation is the li-quefaction phase, which splits long-chain organic compounds (e.g. fats, car-bohydrates) into simpler organic com-pounds (e.g. amino acids, fatty acids, su-gars) through bacterial action.

The products of hydrolysis are sub se -quently metabolised in the acidificationphase (acidogenesis) by acidogenic bac-teria and broken down into short-chainfatty acids (e.g. acetic, propionic and bu-tyric acid). Acetate, hydrogen and carb-on dioxide are also created and act as initial products for methane formation.

In the acetic acid phase (acetogenesis),the organic acids and alcohols are brokendown into acetic acid, hydrogen and car-bon dioxide. These products act as a sub-strate for methanogenic microorganisms.In the fourth and finale phase, during

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which methane is formed (methanoge-nesis), the products from the previousphases are converted into methane bymethanogenic microorganisms (archaea).The end product of fermentation is thecombustible biogas that is mainly com-posed as follows:

50 – 75 % methane (CH4)25 – 45 % carbon dioxide (CO2)

2 – 7 % water (H20)< 2 % oxygen (O2)< 2 % nitrogen (N2)< 1 % ammonia (NH3)< 1 % hydrogen sulphide (H2S).

The energy content of the biogas is di-rectly dependent on the methane con-tent. The higher the content of sub-stances such as fats and starch that are easy to break down in the fermentedmass, the greater the gas yield. One cu-bic metre (m3) of methane has an energycontent of about ten kilowatt hours (9.97kWh). E.g. if the biogas contains 60 %methane, then the energy value of onecubic metre of biogas is about six kilo-watt hours. In this case, the heating va-lue of one cubic metre of biogas is roug-hly 0.6 litres of heating oil.

Which substrates can bio-gas be made from?

Many kinds of organic substrate can beused to produce biogas. In farm-basedplants, it is mainly animal excrementthat is used (e.g. cattle and pig liquidmanure) as the basic substrate. Other or -ganic materials can also be fermented forbiogas to increase the biogas production.

Plants that are grown for energy pro-duction are known as energy crops.With their help, new biomass can be ma-de available year after year to produceelectricity, heat and fuel. Energy cropscan also be grown on set-aside. A marketfor biogas substrates from renewableraw materials is already emerging inGermany and use of these substrates isprogres sively increasing. Renewable rawmaterials include cereal crops, grass, mai-ze, millet, sunflower and many others.

Figure 4: Simplified diagram of how orga-nic matter is broken down during biogaspro duc tion

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Figure 5: Biogas yield and methane content of various substrates (Source: HandreichungBiogas, FNR, 2006; Energiepflanzen, KTBL, 2006)

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Maize sila

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Sudan g

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Along with renewable raw materials,non-agricultural substrates are also sui-table for producing biogas, such as resi-dues from the food industry (e.g. poma-ce, distiller’s wash, grease separator re-sidues), vegetable waste from wholesalemarkets, food waste or grass clippingsand organic waste from municipal wa-ste disposal. The fermentation of residu-es material (called co-fermentation) pro-vides a possibility of closing the cycleand dealing with them in a way thatproduces few emissions and is hygienic.

Fig. 5 shows the comparative biogasyields of various substrates (in relationto their fresh mass) with their average methane content.

The substrate used in biogas plants a -cross Germany is composed of about48 % animal excrement, 26 % organic wa-

ste and industrial and agricultural resi-dues, and 26 % renewable raw materials.

Fermentation inhibitor

The anaerobic digestion is very sus -ceptible to disturbances. These can eit-her be due to technical reasons or to in-hibitors. Inhibitors, even in small quan-tities, can have a negative effect on bac-teria and therefore on the process of de-composition. They enter the fermentereither with the substrate itself or alter-natively stem from intermediate pro-ducts of individual stages in decomposi-tion.

For example, adding excessive amountsof substrate to the fermenter can inhibitthe fermentation process because, inprinciple, the presence of an excess con-centration of any ingredient in a sub-

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strate can have a harmful effect on thebacteria.

There are however also substances thatare not conducive to the production ofbiogas. If present in too high a concen-tration, these can be toxic to bacteria sin-ce microorganisms even have a certaintolerance towards these substances. Thisis particularly true for disinfectants anddetergents, antibiotics, solvents, herbici-des, salts or heavy metals, even smallquantities of which can inhibit the fer-mentation process.

Hydrogen sulphide, on the other hand, isa product of the fermentation processthat can be poisonous to cells in solutionand can also hinder the process of de-composition. Sulphur is nevertheless anessential trace element and therefore animportant mineral compound for met-hanogenic bacteria. Too high a concen-tration of ammonia can also inhibit met-hane production, which is why poultrydroppings and occasionally pig liquidmanure are diluted or mixed with co-substrates that have a low nitrogen con-tent.

How does a biogas plantwork?

Farm biogas plants generally consist of aliquid manure store, a feed-in unit forsolid substances, a digester where theactual fermentation takes place and a di-gestate storage tank for the fermen -mented biomass. Depending on the typeof substrate, co-fermentation plants may

also require a receiving pit, disintegrati-on, the removal of contraries and past-eurisation. The gas produced for utilisa-tion then continues towards gasholders,gas cleaning and its respective uses.

Substrate-bearing components of abiogas plant

The liquid manure store is used for in-termediate storage of liquid manure as afermentation substrate. Solid substratesrequire a suitable metering device. The-se must be large enough to even out va-riations in the amount of substrate avai-lable. If co-substrates are being used inthe plant, then additional buildings maybe required to receive and treat the sub-strates depending on the latters’ pro-perties. Along with disintegration, theremoval of contraries is especially im-portant for the process to be able runsmoothly as well as for the quality of thedigestate.

When substrates that could potentiallyspread epidemics, such as organic wa-ste, animal processing and food wasteamong others, are co-fermented, thesub strate receiving area and substrateprocessing area have to be kept separateby maintaining an unclean and a cleanside. Furthermore, pasteurisation equip -ment is required to heat the substrate upto 70°C for a minimum of 60 minutes.This prevents pathogens that represent ahealth risk from persisting in the sub-strate.

The digester or reactor, the heart of a bio -gas plant, is supplied with fermentation

substrate from the liquid manure store.De pending on how the substrate flows in-to the fermenter, these can be divided into: • continuous and• discontinuous plants.

In the discontinuous biogas process, alsoknown as batch process, the fermenter iscompletely filled with fresh substrateand hermetically closed. The substrateremains in the container until the end ofthe selected retention time without nosubstrate added or removed. The fer-menter is then emptied and filled withnew substrate. Gas production beginsslowly after filling and subsides againafter the maximum value has been rea-ched. Discontinuous feeding is the pro-cess most widely used for dry fermenta-tion (see page 15).

Continuous processes are characterised byregular feeding of the digester. The dige-ster also acts as a digestate storage tankin which the substrate is kept until it isspread. The disadvantage is the high ener -gy consumption necessary to heat thelarge reactor room; the advantages arethe low investment cost and the use of bio -gas from secondary fermentation. Semi-continous fermentation is the most wi-dely used process in Germany. The sub-strate is pumped into the digester severaltimes daily from the holding tank/liquidmanure store. A quantity of fresh sub-strate equivalent to that added to thefermenter is expelled or removed into adownstream fermenter. This results infairly regular gas – and therefore electri-city – production. When the retention ti-me has elapsed, the fermentation sub-

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Figure 6: Scheme of processes in a farm-based biogas plant using co-substrates

Biogas plant with co-fermentation

Digester withfoil covering

Electricity Heat Substrate

CHP

Biogas

GasLiquid

manure

Fermentationsubstrate

Agricultural useReceiving area

Residential house

Stable Digestate

Legend:

Storage tank

Injection intoelectricity grid

Liquid manurestore

Co-substrates

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Figure 7: A biogas plant with a foil roof acting as an integrated gasholder

strate is introduced into the covered di-gestate storage tank.

There are many different possible mo-dels of fermenter (steel or concrete, rec-tangular or cylindrical, horizontal or ver-tical). The crucial thing is that the con-tainer is gas- and watertight as well ascompletely opaque. A stirring device en-sures that the substrate remains homo-geneous as, depending on the primarymaterial, the substrate will have a grea-ter or lesser tendency to separate into afloating layer and a sedimentation layer.

A stirring device in the fermenter ensuresby its rotations that the substrate isequally distributed throughout the reac-tor and that the gas that forms can esca-pe from the substrate. If sedimentation

layers form, e.g. when chicken manureor organic waste is fermenting, thenthey must be regularly removed by sui-table dischargers.

As bacteria produce only small quanti-ties of sensible heat through their own“work” and this is insufficient to attainthe necessary ambient temperature, thefermenter has to be insulated and exter-nally heated to create the ideal tempera-ture conditions for the bacteria that arenecessary for the fermentation process.The fermentation temperature is an im-portant process factor that influencesthe speed of anaerobic digestion. Essen-tially two temperature ranges can be di-stinguished:• between 32 and 42°C (mesophilic)• between 50 and 57°C (thermophilic).

Around 85 % of German’s farm biogasplants operate in the mesophilic range.Thermophilic fermentation can be anadvantage if forms of biomass that arerisky from a sanitary point of view are tobe fermented or if the aim is to achieve ahigh throughput in the plant.

One further important parameter is thehydraulic retention time (HRT). This in-dicates the average time the added sub-strate remains in the fermenter be forebeing removed. This is calculated fromthe utilisable volume of the fermenterand the amount of biomass loaded daily.

The aim when operating a biogas plantis to attain the maximum rate of gas pro-duction or the complete digestion of theorganic matter contained in the substra-te. If the organic ingredients are to com-pletely decompose, then one has toreckon with a long retention time for thesubstrate in the fermenter and thereforeensure that the reactor is of an appro-priate size, since some substances areonly broken down – if at all – after a con-siderable length of time. Volume load isalso an important operating parameterin this respect. It indicates how many ki-logrammes of organic dry matter can beloaded into the fermenter per m3 of vo-lume and unit of time. When it has finis-hed digesting, the substrate goes intothe digestate storage tank. This shouldbe covered so as to utilise any furtherbiogas that is produced, as well as pre-venting emissions and smells. The sizeof the digestate storage tank is determi-ned by the required storage time whichin turn depends on specifications for the

environmentally friendly use of this re-sidue in plant production.

Dry fermentation

Almost without exception, the biogasplants currently operating in Germanyare based on the principle of wet fer-mentation. Yet the use of solid substan -ces (e.g. renewable raw materials) is on-ly possible to a limited extent.From a biological point of view, it is ac-tually misleading to divide processesstrictly into wet or dry fermentation asin both cases the bacteria involved in thefermentation process require a liquidphase to survive. Nevertheless, the fer-mentation process during dry fermenta-tion does not need a basic liquid sub-strate. However, “dry fermentation” isof special interest to operations that ha-ve no liquid manure nor any other li-quid primary substrates at their dispo-sal, but do have enough stackable bio-mass.

In contrast to wet fermentation, the sub-strate used in dry fermentation is neit-her pumpable nor capable of flowing,nor does any constant mixing take placeduring biogas production. However, asin wet fermentation, the biological fer-mentation process requires a moist envi-ronment. The processes to fermentstackable organic biomass were original-ly developed to utilise organic wasteand residual waste and have now foundan application in the agricultural sector.Biomass with a dry matter content ofbetween 20 and 40 % can thus be fer-mented. The substrates used include so-

14

lid manure, renewable raw materials(such as silage made from maize, cerealcrop or grass) and crop residues (such asstraw and cereal debris), as well as greenand organic waste.

A wide range of alternative processesare currently being applied and they canbasically be subdivided into continuous(e.g. plug-flow fermenter) and disconti-nuous (e.g. percolation reactor) systems.Dry fermentation processes represent analternative to the widespread wet fer-mentation process and have future po-tential mainly because they make fer-mentation both technically simpler andpossible without liquid manure.

Measurement and control technology

It is well-known that the anaerobic dige-stion of organic substrates is a highlycomplex, multi-stage process that is in-

fluenced by a great number of micro-biological, chemical and physical fac-tors. Process control therefore requires therecording of various indicators. A furt-her complication for process control isthat the individual stages of decompo -sition take place at very different speedsand the dynamic behavour of the pro-cess thus depends to a great extent onthe material composition of the substratesused. Some of the main para meters foroperating a biogas plant are:• The temperature in the fermenter• The pH value in the fermenter• The amount of gas produced• The methane, hydrogen sulphide and

oxygen contents during the gas phase• The VOA/TIC value (ratio of volatile

organic acids (VOA) to total inorganiccarbon (TIC))

• The ratio of volatile organic acids tototal inorganic carbon in the fermenter(VOA/TIC value).

It is also important to have data aboutthe chemical substances in the substratemix that is used including the quantityof biomass added as well as its dry mat-ter content and organic dry matter con-tent. Due to the serious impact of met-hane on the climate, plants in Germanyproducing more than 20 m3 gas/h musthave a second gas consumer installation(e.g. a gas burner) or a gas flare whichcan burn off the biogas if there is a faultin the block heat and power plant. Bio-gas is inflammable and explosive inmixtures with 6 – 12 % air. The securityregulations for farm biogas plants andthe relevant comprehensive body of le-gislation (DIN standards, etc.) must the-

15

Figure 8: Dry fermentation biogas plant

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refore be observed. If these guidelinesare respected, then handling biogas pre-sents no greater risk than handling natu-ral gas.

Gas-bearing components of a biogasplant

Gasholders are used to smooth out thefluctuations between gas productionand gas consumption. For storing gas(low pres sure), integrated storage (un-der an inflated roof on the fermenter)have proved their worth. There is a pre-ference for using cheap, balloon-shapedfoil tanks as exter nal gasholders (separa-te enclosures) (Fig. 9).

Before the gas is used, particles andcondensate have to be removed. It is alsoimportant to desulphurise the gas to pro-tect the CHP engines from corrosion. Aninexpensive desulphurisation pro cess,by which 3 – 5 percent air is added to the

fermenter, has gained ac ceptance for usein farm biogas plants. If this is correctlymanaged, removal rates of up to 95 %can be achieved.

How is biogas utilised?

Desulphurised and cleaned biogas canbe used in as many different ways as na-tural gas. One cubic metre of biogas canreplace about 0.6 l of heating oil. Themost common use of biogas in Germanyat the moment is to produce electricityin a combined heat and power plant(CHP). However, biogas is a versatilesource of ener gy, as shown by the follo-wing diagram.

Utilisation for combined heat and power

Biogas can be used in a CHP to produceelectricty and heat. CHPs consist of acombustion engine running on biogasthat drives a generator to produce elec-trical energy (Fig. 10). The surplus heatfrom cooling and exhaust fumes is usedto heat the digester and, if possible, toheat residential houses and other consu-mers of heat. There are several availableengine models and combustion proces-ses. Engines that have been specially developed to run on gas (the Otto engi-ne principle) are used as well as sparkig nition units (the diesel engine princi-ple). Gas-Otto engines are capable ofburning biogas with a methane contentof at least 45 % directly. Spark ignitionengines, on the other hand, require igni-tion oil that may not make up more than

Figure 9: Foil biogas storage tanks in a ven-tilated loft above a digester

17

10 % of the fuel capacity supplied toburn the biogas. Moreover, since the be-ginning of 2007, new plants are no lon-ger authorised to use fossil based igniti-on oils.

When choosing a CHP engine, the focus should be on a high degree of effi-ciency and low proneness to defects. Especially in the case of co-fermentationplants, there can be variations in thequality and the quantity of gas and thiscan cause damage to the engine. Thiscan be remedied by electronic enginecontrol systems.

A biogas plant can operate particularlyeconomically in situations where a cu-stomer can be found for the surplus heatfrom the combined heat and powerplant. With conventional technology, upto 40 % of the energy contained in bio-gas can be converted into electricity.When the resulting surplus heat is also

used, the overall degree of efficiency (el-ectrical and thermal) can be raised toabout 90 %. The surplus heat can beused to heat residential houses, schoolsor as process heat and replace fossil fuel.

Micro gas turbines

The micro gas turbine represents a newalternative to motor-driven gas utilisati-on that has been favoured up until now.Micro gas turbines or micro turbines aresmall, fast-running gas turbines withlow temperatures and pressures in thecombustion chamber and an electricalcapacity up to 200 kW.

In gas turbines, compressed air and ad-ded biogas are burned in a combustionchamber. The resulting increase in tem-perature causes the gas to expand befo-re this relaxes again inside the turbineand thus drives the generator so as toproduce electricity. Impurities in the bio-

Figure 10: Scheme of the various uses of biogas and stages of its conditioning (Source: ATB(2003), modified)

18

gas can damage the micro turbine, the-refore the gas must be cleaned anddried. Micro gas turbines require a mini-mum methane content of 35 % in the gas.

The electrical efficiency of micro gas tur-bines is relatively low at about 28 %. Theoverall degree of efficiency is about82 %. As there is continuous combustionwith excess air and at low pressures inthe combustion chamber, micro gas tur-bines have considerably lower exhaustemissions than engines. The intervalsbetween maintenance are, at least in thecase of micro gas turbines that run onnatural gas, far longer than for engines.For the time being though, there is rela-tively little experience of micro gas tur-bines in actual use.

Fuel cells

In order to achieve higher electricityyields, one subject researchers are cur-rently working on, is the use of biogas infuel cells that are capable of convertingthe chemical energy from the processedbiogas directly into electricity. In a fuelcell, H2 from the biogas is the “fuel” thatreacts with O2 to give water (H2O) whi-le producing electrical energy and heat.Biogas has to be conditioned for this byremoving the H2S and increasing themethane concentration.

So far, fuel cells are expensive but theydo run quietly and can reach degrees ofelectrical efficiency of up to 50 percent.Fuel cell technology (with the exceptionof portable systems) is still being resear-ched and could play a role in the use ofbiogas in the future.

Biomethane as a substitute for naturalgas

Alongside its conventional role in elec-tricity and heat production, biogas canalso be used as a substitute for naturalgas. This requires costly conditioning ofthe biogas until it has the same qualityas natural gas so that it can be fed intothe natural gas grid as “biomethane”.This method is a good alternative to thedecentralised utilisation in CHP that hasbeen common up to now and is especi-ally interesting for biogas plants that donot have a suitable method of utilisingthe surplus heat on the site where thebiogas is converted into electricity. Bio-methane can be transported any di-

Figure 11: Assembling a “hot module“ fuelcell

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stance using the existing gas grid infra-structure and for example be convertedinto electricity where the surplus heatthis produces is actually needed.

The order of processing steps to obtainthe minimum quality required dependsmainly on the choice of technology andthe obtainable quality of the gas in the

local grid. The main processing stepalongside desulphurisation and remo-ving contraries is the methane enrich-ment of the gas from about 60 % to over87 %. The biomethane is delivered to thenetwork at injection stations. This iswhere the composition of the gas is de-termined and its compatibility with thelocal network established.

Although this process works, it is on ly incertain cases the best means of usingbiogas. For it is not everywhere that the-re is a connection to the gas grid, thatconstant demand is guaranteed or thatthe extra technical expense makes eco-nomic sense. This means that injectingbiogas into the natural gas grid is inte-resting above all for large biogas plants,for, as a general rule, it is only in suchcases that it is profitable to invest in thenecessary conditioning technology.The main possibilities for utilisation thatresult from injecting biomethane in tothe natural gas grid are location-un -specific use for combined heat and po-wer, as well as a substitute for naturalgas, a mong other things for natural gasboi lers and natural gas filling stations.Biomethane is widely used as fuel inSwitzerland and in Sweden. In Ger-many, this type of use is still in its infan-cy. The first German biogas filling stati-on was inaugurated in 2006. In princi-ple, biogas is also suitable as an energysource for fuel cells, Stirling engines andmicro gas turbines. It will be severalyears before the necessary develop-ments lead to their being widely used.

Figure 12: Scheme of the various uses andstages of biogas conditioning

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Figure 13: Legal specifications in Germany

What is the applicableregu latory framework?

Constructing a plant

Depending on the size of the plants inGermany or the type of substrate to beprocessed, the construction of a biogasplant is subject to construction law orenvironmental immissions law. A majordecision criterion here is the dailythroughput of substrate. If this is morethan 10 t of non special supervisions wa-ste per day requiring no special supervi-sion per day, the approval proceduremust take place according to the FederalImmission Control Act (BImSchG). In-formation about the approval procedureas well as the necessary documents canbe requested from the competent autho-

rities in the Federal States and from theTrade Supervisory Offices.A biogas plant operator who wishes touse animal by-products other than li-quid manure has to meet a comprehen-sive catalogue of requirements. The pro-visions of the Ordinance on Biowastes(BioAbfV) only apply to plants in whichbiowaste is fermented. In principle, anymaterial listed in Biowaste Ordinancemay be used in the biogas plant. Accor-ding to BioAbfV, all digestate to bespread on the ground that contains plantwaste must be phytohygienically harm-less. According to the Use of FertilisersOrdinance, materials that are put into cir-culation must be hygienically harmless.

Fig. 13 shows the various legal specifica-tions that are to be complied with for

Legal specifications Substrates concernedRegulations on nutrientsOrdinance on Fertilisation all substratesUse of Fertilisers Ordinance all substrates that are not spread on the

farm’s own landRegulations on pollutantsOrdinance on Biowastes all organic waste that is not subject to the EU

Hygiene Directive, digestate using organic waste as co-fermenting agent

Regulations on product hygieneEU Hygiene Directive substrates of animal originUse of Fertilisers Ordinance all substrates that are not spread on the

farm’s own landOrdinance on Biowastes all organic waste that is not subject to the EU

Hygiene Directive, digestate with organic waste as co-fermenting agent

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each type of digestate utilisation. If theUse of Fertilisers Ordinance is applied,the legislator places restrictions on thecomposition of the digestate and thefeed material.

Renewable Energy Source Act

The most important legal instrument inGermany to support the production ofelectricity from renewable sources is theRenewable Energy Source Act (EEG),which first came into action in the year2000 and was revised in 2004.

The EEG regulates the preferentialconnection of plants that produce elec-tricity from renewable energy sourcesand the purchasing, transmission andpayment of electricity by the operator ofthe grid. The EEG defines payment ratesfor every kilowatt hour of renewable electricity that is fed into the public grid.The basic payments differ according tothe type of renewable energy source, theconversion technology and the size ofthe plant (see table one).

The basic payment is subject to an an-nual degression of 1.5 % based on thebasic rate applicable in the previousyear (cf. Tab. 1). The basic fee of a plantis derived from the payment in the yearoperations started and is valid for a 20-year period.

The EEG also stipulates an increase inthe electricity payments through vari -ous bonuses that are linked to specificconditions. There are additional pay -ments for the utilisation of renewable

raw materials specifically grown forenergy production (biomass bonus), forthe external utilisation of the heat pro-duced (combined heat and power) andfor the use of innovative technologiessuch as Stirling engines, fuel cells or up-grading biogas to natural gas quality(biomethane).

In the last four years, the EEG’s pay -ment regulations have led to a consider-able increase in electricity productionfrom biomass. With the improvement ofthe payment rates as part of the revisionof the EEG in 2004, energy production infarm biogas plants expanded noticeablyand this is reflected by the quantity ofplants (Fig. 2).

The present version of the RenewableEnergy Source Act (EEG) is currentlyundergoing a second revision by the Fe-deral Government. For biogas produc-tion, there are plans to increased basicpayments for biogas plants up to 150kW, to introduce a bonus for greater uti-lisation of liquid manure in small biogasplants and to increase the bonus for theuse of renewable raw materials and ofsurplus heat.

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How does a biogas plantbecome cost-effective?

Reducing investment costs

Building a small biogas plant (under 100kW) for renewable raw materials and li-quid manure involves specific invest-ment costs of 5,000 to 3,000 euros perkW of installed electrical capacity. Ho-wever, these decrease as plant capacityincreases. Larger wet fermentation plantscan cost about € 2,500/kWel to purchase.Possibilities for realising economies ofscale lie in serial production (industrialprefabrication) for which a prerequisitewould be the standardisation of themain plant components and a simplifiedand optimised process. In contrast tothis, plants that are specially designedand built for individual farms and aredependent on individual services resultin comparatively high investment costs.

Lowering operating costs

Along with substrate costs, which canmake up about 50 % of operating costs ifrenewable raw materials are used, main-tenance and repair costs represent a large share of operating costs. Furtheritems are lubricants such as ignition oilfor a spark ignition CHP, as well as ex-penses for laboratory tests and insuran-ce. The costs for maintenance and re-pairs of CHPs can be minimised by elec-tronic engine control and regulation, es-pecially for variations in the quality andquantity of the gas in co-fermentationoperations, i.e. a changing substratecomposition. The required working timeper day for a biogas plant can – depen-ding on the size of the plant – be between0.5 and 5 hours.

Table 1: Payments for electricity from bio-mass according to the Renewable EnergySource Act

Payments[ct/kWh]

up to 150 kW 10.99 10.83up to 500 kW 9.46 9.32up to 5MW 8.51 8.38up to 20 MW 8.03 7.91

Biomass bonusup to 500 kW 6from 500 kW to 5 MW 4Combined heat and power bonus 2Technology bonus (up to 5 MW) 2

2007Basic payment 2008

Further information

Fachagentur Nachwachsende Rohstoffe e.V. (FNR)Agency for Renewable Resources e.V.FNR “Bioenergy Consulting Service”E-mail: info@bio-energie.deWebsite: www.bio-energie.de

Bundesministerium für Ernährung, Landwirtschaft und Verbraucherschutz (BMELV)Federal Ministry of Food, Agriculture and Consumer Protection (BMELV)Website: www.bmelv.de

Bundesministerium für Umwelt, Naturschutz und Reaktorsicherheit (BMU)Federal Ministry for the Environment, Nature Protection and Reactor Security (BMU)Website: www.erneuerbare-energien.de

Johann Heinrich von Thünen-Institut (vTI)Federal Research Institute for Rural Areas, Forestry and Fisheries (vTI)Website: www.vti.bund.de

Kuratorium für Technik und Bauwesen in der Landwirtschaft e. V. (KTBL)Association for Technology and Structures in Agriculture (KTBL)Website: www.ktbl.de

Leibniz-Institut für Agrartechnik Potsdam-Bornim e.V. (ATB)Leibniz Institute for Agricultural Engineering Potsdam-Bornim e.V. (ATB)Website: www.atb-potsdam.de

Fachverband Biogas e.V. German Biogas Association e.V.Website: www.biogas.org

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Useful figures

1 m3 biogas 5.0 – 7.5 kWhoverall

1 m3 biogas 1.5 – 3 kWhel

1 livestock unit (GV) 500 kg of animal body mass

1 ha maize silage 7,800 – 8,300 m3 biogas

1 m3 methane (CH4) 9.97 kWh

1 kWh 3.6 MJ (3.6 x 106 joule)

1 billion kWh 3.6 PJ (3.6 x 1015 joule)

Efficiency rate CHPel 30 – 40 %

Efficiency rate CHPth 40 – 60 %

Efficiency rate CHPoverall 85 – 90 %

Operating time CHP per year 7,500 – 8,000 OH/a

Specific investment costs

CHP (gas engine) 150 kWel 900 €/kWel

CHP (gas engine) 250 kWel 740 €/kWel

CHP (gas engine) 500 kWel 560 €/kWel

Biogas plant up to 100 kWel 5,000 – 3,000 €/kWel

Biogas plant from 100 to 350 kWel 3,000 – 2,500 €/kWel

Biogas plant above 350 kWel ≤ 2,500 €/kWel

Working time 3 – 7 h/kWela

Temperature variation in fermenter ≤ ± 2 °C per day

Optimal VOA/TIC range 0.4 – 0.6

Dry mass (DM) [kg]= fresh mass [kg] – water content [kg]

Organic dry mass (ODM) [kg]= dry mass [kg] – ash residue [kg]

Biogas yield [m3]= FM substrate [t] • DM [%] • ODM [%] • yield [kg/t ODM]

Required fermenter volume [m3]= substrate added daily [m3/d] • average retention time [d]

Retention time [d]HRT = capacity fermenter [m3]

substrate added [m3/d]

Loading rate [kg org. dry matter/m3 • d]= ODM added [kg/d]

capacity fermenter [m3]

Important process parameters

(Source: Handreichung Biogas, FNR, 2006; Federal Research Institut for Rural Areas, Forestry and Fisheries (vTI))

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Literature

You can find detailed information about the utilisation of renewable raw materialsamong others in the following FNR publications:

nachwachsende-rohstoffe.de

Bioenergy

PlantsRaw materialsProducts

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Published byFachagentur Nachwachsende Rohstoffe e.V. (FNR)Hofplatz 1 · 18276 Gülzow · GermanyWebsite: www.fnr.deE-Mail: info@fnr.de

FNR Bioenergy Consulting ServicePhone: +49 (0) 38 43/69 30-1 99Fax: +49 (0) 38 43/69 30-1 02Website: www.bio-energie.deE-Mail: info@bio-energie.de

With financial support of the Federal Ministry of Food, Agriculture and Consumer Protection

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