Biogas Production Current State and Perspectives

MINI-REVIEW

Biogas production: current state and perspectives

Peter Weiland

Received: 2 September 2009 /Revised: 2 September 2009 /Accepted: 3 September 2009 /Published online: 24 September 2009# Springer-Verlag 2009

Abstract Anaerobic digestion of energy crops, residues, andwastes is of increasing interest in order to reduce thegreenhouse gas emissions and to facilitate a sustainabledevelopment of energy supply. Production of biogas providesa versatile carrier of renewable energy, as methane can be usedfor replacement of fossil fuels in both heat and powergeneration and as a vehicle fuel. For biogas production,various process types are applied which can be classified in wetand dry fermentation systems. Most often applied are wetdigester systems using vertical stirred tank digester withdifferent stirrer types dependent on the origin of the feedstock.Biogas is mainly utilized in engine-based combined heat andpower plants, whereas microgas turbines and fuel cells areexpensive alternatives which need further development workfor reducing the costs and increasing their reliability. Gasupgrading and utilization as renewable vehicle fuel or injectioninto the natural gas grid is of increasing interest because the gascan be used in a more efficient way. The digestate fromanaerobic fermentation is a valuable fertilizer due to theincreased availability of nitrogen and the better short-termfertilization effect. Anaerobic treatment minimizes the survivalof pathogens which is important for using the digested residueas fertilizer. This paper reviews the current state andperspectives of biogas production, including the biochemicalparameters and feedstocks which influence the efficiency andreliability of the microbial conversion and gas yield.

Keywords Anaerobic digestion . Biogas . Biogasupgrading . Biomethanation . Biomass . Co-digestion .

Digestate . Dry fermentation . Energy crops .Methanepotential .Wet fermentation

Introduction

The global energy demand is growing rapidly, and about88% of this demand is met at present time by fossil fuels.Scenarios have shown that the energy demand will increaseduring this century by a factor of two or three (IEA 2006).At the same time, concentrations of greenhouse gases(GHGs) in the atmosphere are rising rapidly, with fossilfuel-derived CO2 emissions being the most importantcontributor. In order to minimize related global warmingand climate change impacts, GHG emissions must bereduced to less than half of global emission levels of1990 (IPCC 2000). Another important global challenge isthe security of energy supply, because most of the knownconventional oil and gas reserves are concentrated inpolitically unstable regions.

In this context, biogas from wastes, residues, and energycrops will play a vital role in future. Biogas is a versatilerenewable energy source, which can be used for replace-ment of fossil fuels in power and heat production, and it canbe used also as gaseous vehicle fuel. Methane-rich biogas(biomethane) can replace also natural gas as a feedstock forproducing chemicals and materials.

The production of biogas through anaerobic digestionoffers significant advantages over other forms of bioenergyproduction. It has been evaluated as one of the mostenergy-efficient and environmentally beneficial technologyfor bioenergy production (Fehrenbach et al. 2008). It candrastically reduce GHG emissions compared to fossil fuelsby utilization of locally available resources. The digestate isan improved fertilizer in term of its availability to cropswhich can substitute mineral fertilizer.

The European energy production from biogas reached 6million tons of oil equivalents (Mtoe) in 2007 with a yearlyincrease of more than 20% (EurObserv’er 2008). Germanyhas become the largest biogas producing country in the

P. Weiland (*)Johann Heinrich von Thünen-Institute,38116 Braunschweig, Germanye-mail: [email protected]

Appl Microbiol Biotechnol (2010) 85:849–860DOI 10.1007/s00253-009-2246-7

world, thanks to the strong development of agriculturalbiogas plants on farms. At the end of 2008, approximately4,000 agricultural biogas production units were operated onGerman farms (Fachverband Biogas 2009). Within theagricultural sector in the European Union (EU), 1,500million tons of biomass could be digested anaerobicallyeach year, and half of this potential is accounted for byenergy crops (Amon et al. 2001). The different aspects ofagricultural biogas production and utilization are reviewedin this paper.

Biochemical process

Methane fermentation is a complex process, which can bedivided up into four phases: hydrolysis, acidogenesis,acetogenesis/dehydrogenation, and methanation (Fig. 1).The individual degradation steps are carried out by differentconsortia of microorganisms, which partly stand in syntro-phic interrelation and place different requirements on theenvironment (Angelidaki et al. 1993). Hydrolyzing andfermenting microorganisms are responsible for the initialattack on polymers and monomers and produce mainlyacetate and hydrogen and varying amounts of volatile fattyacids such as propionate and butyrate. Hydrolytic micro-organisms excrete hydrolytic enzymes, e.g., cellulase,cellobiase, xylanase, amylase, lipase, and protease. Acomplex consortium of microorganisms participates in thehydrolysis and fermentation of organic material. Most ofthe bacteria are strict anaerobes such as Bacteriocides,

Clostridia, and Bifidobacteria. Furthermore, some faculta-tive anaerobes such as Streptococci and Enterobacteriaceaetake part. The higher volatile fatty acids are converted intoacetate and hydrogen by obligate hydrogen-producingacetogenic bacteria. The hydrogen-producing acetogenicbacteria are not well characterized. Typical homoacetogenicbacteria are Acetobacterium woodii and Clostridium ace-ticum. The accumulation of hydrogen can inhibit themetabolism of the acetogenic bacteria. The maintenanceof an extremely low partial pressure of hydrogen is,therefore, essential for the acetogenic and H2-producingbacteria. Although many microbial details of metabolicnetworks in a methanogenic consortium are not clear,present knowledge suggests that hydrogen may be alimiting substrate for methanogens (Bagi et al. 2007). Thisassumption is based on the fact that addition of H2-producing bacteria to the natural biogas-producing consor-tium increases the daily biogas production. At the end ofthe degradation chain, two groups of methanogenic bacteriaproduce methane from acetate or hydrogen and carbondioxide. These bacteria are strict anaerobes and require alower redox potential for growth than most other anaerobicbacteria. Only few species are able to degrade acetate intoCH4 and CO2, e.g., Methanosarcina barkeri, Metanono-coccus mazei, and Methanotrix soehngenii, whereas allmethanogenic bacteria are able to use hydrogen to formmethane. The first and second groups of microbes as wellas the third and fourth groups are linked closely with eachother (Schink 1997). Therefore, the process can beaccomplished in two stages.

A balanced anaerobic digestion process demands that inboth stages the rates of degradation must be equal in size. Ifthe first degradation step runs too fast, the acid concentra-tion rises, and the pH drops below 7.0 which inhibits themethanogenic bacteria. If the second phase runs too fast,methane production is limited by the hydrolytic stage.Thus, the rate-limiting step depends on the compounds ofthe substrate which is used for biogas production. Undis-solved compounds like cellulose, proteins, or fats arecracked slowly into monomers within several days whereasthe hydrolysis of soluble carbohydrates takes place withinfew hours. Therefore, the process design must be welladapted to the substrate properties for achieving a completedegradation without process failure.

It is difficult to describe the whole process by reliablekinetics since hydrolysis of complex insoluble substratedepends on many different parameters such as particle size,production of enzymes, pH, and temperature. A systematicdescription of the complex kinetics models is given in fewpioneering works on organic waste digestion (Angelidaki etal. 1999; Gavala et al. 2003). For solid wastes, severalkinetic models were developed for mesophilic and thermo-philic digestion (Andara and Esteban 1999; Linke 2006;

Complex Polymers(polysacch, proteins, lipids)

Monomers and Oligomers(sugars, amino acids,long chain fatty acids)

VolatileFatty Acids

(C > 2)

Acetate H + CO2 2

Biogas(CH + CO )4 2

Fig. 1 The stages of the methane fermentation process (accordingGujer W and Zehnder AJB 1983)

850 Appl Microbiol Biotechnol (2010) 85:849–860

Biswas et al. 2007). The kinetic of biogas production fromenergy crops and manure was studied recently in detail byMähnert (2007). Results from quasi continuous digestionexperiments have shown that the degradation can bedescribed by a simple first order reaction. For theapplication of this simple model, only the maximum gasyield of the substrate and the specific reaction rate must beknown from a continuously digestion test.

Much is known about the basic metabolism in differenttypes of anaerobic digestion processes, but little is knownabout the microbes responsible for these processes. Onlyfew percent of bacteria and archaea have so far beenisolated, but little is known about the dynamics andinteractions between these microorganisms. The lack ofknowledge results sometimes in malfunctions and unex-plainable failures of biogas fermenters. With moleculartechniques, more information can be received about thecommunity structures in anaerobic processes (Elferink etal. 1998; Yu et al. 2005; Karakashev et al. 2005; Klocke etal. 2008). Fluorescence in situ hybridization has beensuccessfully used as a simple and rapid technique suitablefor quantification of methanogens and the assessment of awide range of samples from agricultural biogas plants(Stabnikova et al. 2006). With 16SrDNA analysis tech-nique, Klocke et al. (2009) detected 68 taxonomic groupsof methanogens in samples from ten agricultural biogasplants. He has shown that hydrogenotrophic methanogensdominate in most agricultural biogas plants. A high shareof acetogenic methanogens can be found obviously onlyin biogas plants which are operated at low ammoniaconcentrations.

The digestion process takes place at mesophilic (35–42 °C) or thermophilic (45–60 °C) temperature conditions. Itis important to keep a constant temperature during thedigestion process, as temperature changes or fluctuations willeffect the biogas production negatively. In most instances,methanogenic diversity is lower in plants operating atthermophilic temperatures (Karakashev et al. 2005; Levenet al. 2007). Therefore, thermophilic processes are moresensitive to temperature fluctuations and require longer timeto adapt to a new temperature. Mesophilic bacteria toleratetemperature fluctuations of +/−3 °C without significantreductions in methane production. The growth rate ofmethanogenic bacteria is higher at thermophilic processtemperatures making the process faster and more efficient.Therefore, a well-functioning thermophilic digester can beloaded to a higher degree or operated at a lower hydraulicretention time (HRT) than at mesophilic conditions. But thethermophilic process temperature results in a larger degree ofimbalance and a higher risk for ammonia inhibition.Ammonia toxicity increases with increasing temperature,and washout of microbial population can occur (Angelidakiet al. 2003; Dornack 2009). Especially the undissociated

form of ammonia is considered to be responsible for processinhibition at concentrations above 80 mg/l (Kroiss 1985).When the process is inhibited by ammonia, an increase in theconcentration of volatile fatty acids (VFA) will lead to adecrease in pH which will partly counteract the effect ofammonia. Strategies for recovery the biogas efficiencyfollowing ammonia inhibition were studied and evaluatedfor anaerobic digestion of manure together with organicindustrial waste (Nielsen and Agelidaki 2008). The moststable recovery process was observed when biomass wasdiluted with reactor effluent.

Methane formation takes place within a relatively narrowpH interval, from about 6.5 to 8.5 with an optimum intervalbetween 7.0 and 8.0. The process is severely inhibited if thepH decreases below 6.0 or rises above 8.5. The pH valueincreases by ammonia accumulation during degradation ofproteins, while the accumulation of VFA decreases the pHvalue. The accumulation of VFA will often not alwaysresult in a pH drop, due to the buffer capacity of thesubstrate. Animal manure has a surplus of alkalinity whichstabilizes the pH value at VFA accumulation. VFA are akey intermediate in the process and are capable ofinhibiting methanogenesis in high concentrations. Aceticacid is usually present in higher concentration than otherfatty acids, but propionic and butyric acids are moreinhibitory effective to methanogens (Wang et al. 1999;Mösche and Jördening 1999). The inhibition is clearlyassociated with the undissociated form. Therefore, theinhibiting effect of VFAs is much higher in systems oflow pH value.

For the growth and survival of the specific groups ofmicroorganisms, several macro- and micronutrients arenecessary. Macronutrients are carbon, phosphor, and sulfur.The need of nutrients is very low due to the fact that notmuch biomass is developed, so that a nutrient ratio of C:N:P:S=600:15:5:1 is sufficient. Trace elements like iron,nickel, cobalt, selenium, molybdenum, and tungsten areimportant for the growth rate of microorganisms and mustbe added if, e.g., energy crops are used for biogasproduction as the only substrate (Abdoun and Weiland2009; Jarvis et al. 1997). Nickel is generally required for allmethanogenic bacteria because it is necessary for thesynthesis of the cell component cofactor F430, which isinvolved in the methane formation. For optimal growth, thecells require cobalt to build up the Co-containing corrinoidfactor III. The function of selenium, molybdenum, andtungsten is not completely clear, and the growth of only fewmethanogens depends on these trace elements. The neces-sary concentration for the micronutrients is very low and inthe range between 0.05 und 0.06 mg/l. Only iron isnecessary in higher concentration between 1 and 10 mg/l(Bischoff 2009). For monofermentation of energy crops,addition of micronutrients is absolutely necessary in order

Appl Microbiol Biotechnol (2010) 85:849–860 851

to achieve stable process conditions and high loadings(Friedmann and Kube 2008). Addition of manure reducesthe lack of micronutrients, but even in processes with ashare of 50% manure, the adding of micronutrients canincrease the anaerobic conversion rate (Preißler et al.2009).

Feedstocks

All types of biomass can be used as substrates for biogasproduction as long as they contain carbohydrates, proteins,fats, cellulose, and hemicelluloses as main components.The composition of biogas and the methane yield dependson the feedstock type, the digestion system, and theretention time (Braun 2007). The theoretical gas yieldvaries with the content of carbohydrates, proteins, and fats(Table 1). Only strong lignified organic substances, e.g.,wood, are not suitable due to the slowly anaerobicdecomposition. The real methane content in practice isgenerally higher than the theoretical values shown inTable 1 because a part of CO2 is solubilized in thedigestate.

Historically, anaerobic digestion has mainly been asso-ciated with the treatment of animal manure and sewagesludge from aerobic wastewater treatment. Nowadays, mostof the agricultural biogas plants digest manure from pigs,cows, and chicken with the addition cosubstrates toincrease the content of organic material for achieving ahigher gas yield. Typical cosubstrates are harvest residues,e.g., top and leaves of sugar beets, organic wastes fromagriculture-related industries, and food waste, collectedmunicipal biowaste from households and energy crops.The biogas yield of the individual substrates variesconsiderably dependent on their origin, content of organicsubstance, and substrate composition (Fig. 2). Fats providethe highest biogas yield, but require a long retention timedue to their poor bioavailability. Carbohydrates andproteins show much faster conversion rates but lower gasyields. All substrates should be free of pathogens and otherorganisms; otherwise, pasteurization at 70 °C or steriliza-

tion at 130 °C is necessary prior fermentation. The contentsof nutrients, respectively the C/N ratio should be wellbalanced to avoid process failure by ammonia accumula-tion. The C/N ratio should be in the range between 15 and30 (Braun 1982; Zubr 1986). The composition of thefermentation residue should be such that it can be used asfertilizer.

The most important cosubstrates are energy crops whichhave the highest potential in the EU. Figure 3 shows theusable biogas potential of organic wastes and energy cropsin Germany (FNR 2008). More than 50% of the biogaspotential result from energy crops, if 2 million hectares(11% of agricultural land) are used for crop cultivation.Together with animal manure and harvesting residues, morethan 80% of the potential feedstocks come from theagricultural sector.

The most important parameter for choosing energy cropsis their net energy yield per hectare. Many conventionalforage crops produce large amounts of easily degradablebiomass which is necessary for high biogas yields (Braun2009). The highest gross energy potential has maize andforage beets but also different cereal crops and perennialgrasses have potential as energy crops (Table 2).

Forage crops have the advantage of being suitable forharvesting and storing with existing machinery and meth-ods. The specific methane yield is affected by the chemicalcomposition of the crop which changes as the plant matures(Döhler et al. 2006; KTBL/FNR 2007). Harvesting timeand frequency of harvest are, thus, important for thesubstrate quality and biogas yield. Amon et al. (2007) haveshown that maize crops were harvested after 97 days ofvegetation at milk ripeness produced up to 37% greatermethane yields when compared with maize at full ripeness.Crops can be grown as preceding crop, main crop, orsucceeding crop. Also mixed cultivation of different crops,e.g., maize and sunflower, can be applied (Karpenstein-Machan 2005).

Energy crops and crop residues can relatively easily bestored by ensiling. Ensiling is a biochemical process whichconverts the soluble carbohydrates contained in the plantmatter to lactic acid, acetate, propionate, and butyrate whichinhibit the growth of detrimental microorganisms by a strongdrop in pH to values between 3 and 4 (Weinberg et al. 2003).Optimal ensiling results in rapid lactic acid (5–10%) andacetic acid (2–4%) formation within few days. Butyric acidformation is usually prevented by the rapid pH decrease. Theensiling process can be controlled and accelerated bystimulating the acid formation by addition of starter cultures,enzymes, or easily degradable carbohydrates. For optimalensiling conditions, the energy crops should be cut to particlelength of 10–20 mm. Crop materials for ensilage shouldhave total solid contents (TS) between 25% and 35%. TScontents below 25% result in poor silage qualities, high

Table 1 Maximal gas yields and theoretical methane contents(Baserga U 1998)

Substrate Biogas (Nm3/t TS) CH4 (%) CO2 (%)

Carbohydratesa 790–800 50 50

Raw protein 700 70–71 29–30

Raw fat 1,200–1,250 67–68 32–33

Lignin 0 0 0

a Only polymers from hexoses, not inulins and single hexoses


leachate formation, and subsequently poor biogas yields. Thestorage by ensiling can be considered as a pretreatmentprocess, because the structural polysaccharides of plantmaterial, which is quite resistant to anaerobic digestion, arepartly degraded during storage. The ensiling process resultsin energy losses between 8% and 20% which are caused byundesirable aerobic degradation processes (Banemann andNelles 2009). Therefore, it is necessary to compact theharvested crops in silos and to cover it by a plastic wrap forreducing biomass losses by aerobic degradation. Energy lossby aerobic attack after opening the bunker silo can belowered if heterofermentative starter cultures are appliedwhich form acetic and lactic acids simultaneously. The aceticacid formation improves the aerobic stability of silagebecause acetic acid inhibits the growth of specific speciesof yeasts that are responsible for heating upon exposure tooxygen (Driehuis et al. 1999). As a consequence of the lowpH value of ensilaged crops, a quasi continuously feeding ofthe fermenter is necessary in order to avoid processinstabilities and fluctuating gas qualities. Semicontinuouslyfeeding of high amounts of silage results in a sudden changeof the gas quality because CO2 is stripped out due to thelocal reduction of the pH in the fermenter.

For increasing the degradation rate of substrates, apretreatment by mechanical, thermal, chemical, or enzy-matic processes can be applied (Müller et al. 2003). Thedecomposition process is faster with decreasing particlesize but does not necessarily increase the methane yield(Mshandete et al. 2006). Feedstock crushing is usuallydirectly connected to the feeding system by application ofan extruder or by ultrasonic treatment of a side stream ofthe fermenter (Kim et al. 2003; Lehmann 2008; Nickel2008). Treatment by thermal pressure hydrolysis (230 °C,20–30 bars) results in the splitting of organic polymers byhydrolysis into short chain, biologically good availablecompounds which increases the biogas yield while theretention time in the digester can be reduced drastically(Prechtel et al. 2004; Mladenovska et al. 2006). Theaddition of hydrolytic enzymes can improve the decompo-sition of structural polysaccharides resulting in an increasedbiogas yield of up to 20% (Gerhardt 2007; Kaiser 2004;Schimpf and Valbuena 2009). Batch experiments withwheat grass have shown that the rate of biogas productionis significantly affected by the addition of enzymes,however, at the end of the digestion period, there were nosignificant improvement of the methane yield or solidsdegradation (Romano et al. 2009). The addition of enzymesreduces the viscosity of the substrate mixture in the digestersignificantly and avoids the formation of floating layers.But the effect of enzymes can be strong reduced if proteasesof anaerobic microorganisms degrade the added enzymes(Morgavi et al. 2001).

Process technology

For biogas production, various process types are appliedwhich can be classified in wet or dry fermentation

Total energy potential: 417 PJ/a

Energy crops

59%

Manure

24%

Landfill

2%Wastewater

5%

Harvesting

residues

3%Landscape

2%Municipal

wastes

3%

Industrial

wastes

2%

Fig. 3 Usable biogas potential in Germany

25 C

ow

manure

30 P

ig m

anure

500

400

300

200

100

0

Agricultural

Wastes

Agricultural

Raw Materials

Non-Agricultural

Wastes

Biogas yield [m3 /t FM]

11

0 F

od

de

r b

ee

ts

12

5 S

ud

an

gra

ss

200 M

aiz

e

120 B

iow

aste

240

400

800

Food r

esi

dues

Fat tr

ap

Used g

rease

10

2 G

rass

63

0 W

he

at

co

rn

Fig. 2 Mean biogas yield ofvarious substrates


processes. Wet digestion processes are operated with totalsolids concentrations in the fermenter below 10% whichallows the application of completely stirred tank digesters.The digested material is pumpable and can be spread onfields for fertilization. For the treatment of solid substrates,e.g., energy crops, the input must be mixed with liquidmanure or recycled process water in order to achievepumpable slurries. Dry digestion processes are operatedwith a total solids content inside the fermenter between15% and 35%. All wet digestion processes are operatedcontinuously whereas for dry fermentation batch andcontinuously operated processes are applied. Today, wetdigestion processes dominate in the agricultural sector(Weiland 2008a).

Many types and concepts of agricultural biogas plantsare applied (Schulz and Eder 2001). The most commonreactor configuration employed for wet fermentation is thevertical continuously stirred tank fermenter which isapplied in nearly 90% of modern biogas plants in Germany(Gemmeke et al. 2009). Quite often, the fermenter iscovered with a gas tight single or double membrane rooffor storing the gas in the fermenter top before utilization.Active stirring must be implemented, using mechanical,hydraulic, or pneumatic mixing in order to bring themicroorganisms in contact with the new feedstock, tofacilitate the upflow of gas bubbles, and to achieve constanttemperature conditions in the whole fermenter. Up to 90%of biogas plants use mechanical stirring equipment.According to their rotation speed, mechanical stirrers canbe divided in slow and fast running mixers. Fast runningstirrers run mostly in sequences with several times per daywhereas slow rotating paddles run mainly continuously.Submerged motor propeller stirrers are most often appliedwhich can be adjusted to the height, tilt, and to the side(Gemmeke et al. 2009). Dependent on the fermenter sizeand substrate type, up to four stirrers are necessary toprevent swimming layers and sediments. If the fermenter is

operated at high TS, slow rotating paddle stirrers arepreferred with a horizontal, vertical, or diagonal axis andlarge scale paddles. The motor is positioned outside thefermenter. Axial stirrers are mounted on shafts that arecentrally installed on the digester ceiling. They create asteady stream in the digester that flows from the bottom upto the walls which results in a very efficient homogeniza-tion of solid substrates with manure or recycled processwater. Pneumatic stirring uses the produced biogas formixing, which is blown to the bottom of the digester. Thesystem has the advantage that the necessary equipment isplaced outside the fermenter, but it is not frequently used inagriculture, because the destruction of floating layers isdifficult (Gerardi 2003). Hydraulic stirring by pumps areused only for few specific reactor types. The typical size ofcompletely mixed fermenter is in the range from 1,000 to4,000 m3 reactor volume.

Horizontal digesters are typical plug flow systems whichare equipped with a low rotating horizontal paddle mixer.They are mainly applied for the first stage of two-stagereactor configurations because they can be operated athigher total solids content of the input. The reactor volumeis limited to a maximum of about 700 m3 due to technicaland economical aspects.

For energy crops digestion, two-stage digester systemsare preferred which consist of a high-loaded main fermenterand a low-loaded secondary fermenter in series which treatsthe digestate from the first stage. The evaluation of 61 farmplants has shown that two-stage digestion results in highergas yields and a reduced residual methane potential of thedigestate (Gemmeke et al. 2009). A typical flow chart of atwo-stage plant is shown in Fig. 4. In two-stage digestion,hydrolysis and methanation take place in both reactors.For achieving a better metabolization of solid organiccompounds into readily biodegradable carbonic acids, theapplication of two-phase reactors with a separate hydro-lysis stage can be advantageous, because the ideal pH

Crop Crop yield (t FM/ha) Biogas yield (Nm3/(t VS) Methane content (%)

Sugar beet 40–70 730–770 53

Fodder beet 80–120 750–800 53

Maize 40–60 560–650 52

Corn cob mix 10–15 660–680 53

Wheat 30–50 650–700 54

Triticale 28–33 590–620 54

Sorghum 40–80 520–580 55

Grass 22–31 530–600 54

Red clover 17–25 530 -620 56

Sunflower 31–42 420–540 55

Wheat grain 6–10 700–750 53

Rye grain 4–7 560–780 53

Table 2 Gross crop yield andbiogas potential of differentcrops


range for hydrolysis (5.5–6.5) and methanation (6.8–7.2)is different (Vieitez and Gosh 1999; Parawira et al.2008). This technology is often applied for municipal andindustrial organic wastes and solid manure but only fewresults are available from energy crops digestion. Adisadvantage of two-phase digestion is the difficultcontrol of operation and process parameters. In amalfunctioning hydrolysis stage, methane and hydrogencan be formed in a large extent, which causes energylosses and has a negative climate effect when thehydrolysis gas is emitted to the atmosphere (Oechsnerand Lemmer 2009). Therefore, a gas tight covering of thehydrolysis fermenter is generally necessary in order toavoid energy losses and emissions of climate relevantgases and odorous substances.

Most wet fermenter are operated at mesophilic temper-atures with optima between 38 and 42 °C, and only fewbiogas plants use thermophilic conditions between 50 and55 °C. At higher temperatures, the degradation rate is faster,and thus, shorter HRTs and smaller reactor volumes arerequired but the ultimate methane yield from organic matteris not influenced. Decreasing the temperature to 50 °C orbelow reduces the toxicity of ammonia, but the growth rateof the thermophilic microorganisms will drop drastically,and a risk of washout of the microbial population can occur,due to a growth rate lower than the actual HRT (Angelidaki

et al. 2003). The increased energy requirement for main-taining the reactor at thermophilic temperatures is not animportant factor at present because surplus heat from thecombined heat and power station (CHP) is often wastedtoday. But with increasing selling of heat to local residentialhouses and industry or by injection of upgraded biogas intothe natural gas grid, thermophilic processes become lowerefficient.

Energy crops digestion requires prolonged hydraulicretention times of several weeks to month to achievecomplete fermentation with high gas yields and minimizedresidual gas potential of the digestate (Gemmeke et al.2009). Hence, the typical loading rate of organic dry matter(ODM) for wet fermentation processes is only between 2and 4 kg ODM/(m3*d).

For dry fermentation, several batch processes withpercolation and without mechanical mixing are appliedmainly for monofermentation of energy crops. The solidsubstrate is loaded batchwise in a gas tight fermenter boxby a wheel loader and mixed with inoculum from aprevious batch digestion. The necessary share of solidinoculum has to be determined individually for eachsubstrate (Weiland 2006). While yard manure from cowsrequires only small ratios of solid inoculum, up to 70% ofthe input is necessary for energy crops (Kusch et al. 2005).During the digestion period, process water is recycled and

Air

Stage 1

41°CStage 2

40°CStorage tank

24°C

Manure

Maize silage

Gasholder Gasholder Gasholder

Solids dosing unit

el

el

Ignition oil

2 CHP

Fig. 4 Typical two-stage agricultural biogas plant


sprinkled over the substrate to facilitate start-up andinoculation as well as to control moisture content anddigestion temperature. When the digestion is complete after3–4 weeks, the digested material is unloaded, and a newbatch is initiated. To achieve a constant gas production, atleast three fermenters must be operated in parallel run withdifferent start-up times. The gas yields are nearly in thesame order of magnitude as achieved by wet fermentation(Heiermann et al. 2007). Applying a second stage meth-anogenic fermenter in combination with the leach bedprocess is advantageous, because the methane yields can beincreased and the residual methane potential of the digestatecan be lowered (Lethomäki 2006). It is also possible to usethe leach bed only for hydrolysis with subsequent treatmentof the leachate in a fixed bed methane reactor if therecycled process water is aerated slightly in order tosuppress methane bacteria that have been washed out ofthe packings (Parawira et al. 2008; Busch et al. 2009).

Another approach is to apply continuous dry fermenta-tion processes for substrates that contain more than 25%dry matter (Weiland et al. 2009). For continuous dryfermentation, horizontal mechanically mixed fermenter orvertical plug flow fermenter are applied, which are knownfrom anaerobic treatment of municipal organic solids(Schön 1994; De Baere and Mattheeuws 2008). Thevertical digester type needs no mixing inside the fermenter,and the substrate flows from the top to the bottom bygravity only. Before the substrate is fed to the fermentertop, it must be mixed with digestate coming from thebottom of the digester. The high rate of mixing digestatewith fresh feedstock prevents the accumulation of VFA andallows a high organic loading rate of up to 10 kg ODM/(m3*d). The loading rate of wet fermentation processes ismuch lower and normally between 2 and 4 kg ODM/(m3*d).

Process control is difficult because only few parameterscan be measured online, and the complexity of the processmakes it difficult to find a simple and suitable controlparameter which reflects the metabolic state of the wholeprocess. Methane production is normally the only contin-uously measured parameter at agricultural biogas plants,but this parameter cannot reflect a process imbalance, if thebiogas plant treats substrates with changing composition.Hydrogen and redox potential could be an obvious controlparameter, but complicated dynamics and variability forgiven reactors and substrates make the interpretation of theresults difficult (Brauer and Weiland 2009). Only VFA canserve as an efficient indicator of process imbalances.Weiland (2008b) suggested a ratio of propionic acid:aceticacid >1 as an indicator for digester failure, if the propionicacid concentration is higher than 1,000 mg/l. Ahring et al.(1995) suggested that the concentration of both butyrateand isobutyrate could be a reliable tool for indication ofprocess failure, and Nielsen et al. (2007) suggested

propionate as the key parameter for process control andprocess optimization. VFA measurement has the disadvan-tage that manual sampling and subsequent analysis by gaschromatography or high pressure liquid chromatography isa slow procedure. Online measuring of VFA by headspacechromatography is possible but difficult in practice (Boe etal. 2005). A fast control of the process stability is possibleby determining the ratio of VFA to total inorganiccarbonate by a simple titration test (Rieger and Weiland2006). If the ratio is <0.3, the process is stable, and ananalysis for determining the individual VFAs is notnecessary (Lossie and Pütz 2008).

Biogas utilization

Biogas is primarily composed of methane and carbondioxide, contains smaller amounts of hydrogen sulfide andammonia, and is saturated with water vapor. Biogas mustbe desulfurizated and dried before utilization to preventdamage of the gas utilization units. Biogas produced bycofermentation of manure with energy crops or harvestingresidues can contain levels of H2S between 100 and3,000 ppm. CHPs which are mainly used for the utilizationof biogas need mostly levels of H2S below 250 ppm, inorder to avoid excessive corrosion and expensive deterio-ration of lubrication oil. Removal of H2S is done nowadaysmainly by biological desulfurization (Schneider et al.2002). The process is based on the oxidation of H2S byinjection of a small amount of air (2–5%) into the rawbiogas. For this kind of desulfurization, Sulfobacter oxy-dans bacteria must be present, to convert H2S intoelementary sulfur and sulfurous acid. For the desulfuriza-tion inside the digester, S. oxydans does not have to beadded, because it is present inside the digester. The air canbe injected directly in the headspace of the digester, and thereaction occurs on the floating layer, on reactor wall, and onother surfaces in the gas room. For achieving an efficientbiological desulfurization, specific supports of wood orfabric must be installed in the fermenter top in order toachieve enough surface area for microorganisms' fixation.

For biological desulfurization outside the fermentertrickling filter, installations filled with plastic supportmaterials on which the microorganisms can grow, are used(Schneider et al. 2002). Raw biogas and air are injected inthe bottom of the column, and an aqueous solution ofnutrients is circulated from the top to the bottom in order towash out acidic products and to supply nutrients to themicroorganisms. The process needs mesophilic temperatureconditions of around 35 °C. The support material must beshowered with an air/water mixture at regular intervals inorder to prevent clogging by sulfur deposits. The directinjection of air in the trickling filter results in a reduction of


the methane concentration due to the accumulation ofnitrogen in the desulfurized biogas which impedes theupgrading to biomethane. This can be prevented bywashing the gas with water with subsequent biologicaldesulfurization of the aqueous phase in a separate fixed bedreactor (Polster and Brummack 2009). Desulfurization canalso be done by adding a commercial ferrous solution to thedigester. Ferrous compounds bind sulfur in an insolublecompound in the liquid phase, preventing the production ofgaseous hydrogen sulfide, but the method is expensive.

The gas is usually used in CHPs using gas or dual fuelengines. Electric efficiencies of up to 43% can be achieved.Alternatives to the common motor CHP are microgasturbines and fuel cells. Microgas turbines result in a lowerelectric efficiency (25–31%) but have a good part loadingefficiency and long maintenance intervals. A big advantageover reciprocal engines is the availability of the exhaustheat which still has at least 270 °C after the recuperator.This opens new ways of using the heat for process steamproduction (Schmid et al. 2005). Fuel cells result in a higherelectric efficiency but need an efficient gas cleaning, becausethe catalyst for converting methane into hydrogen and thecatalyst inside the fuel cell are very sensitive to impurities(Ahrens and Weiland 2007). The various fuel cell types areoperated at temperatures between 80 and 800 °C. Theinvestment costs are much higher than for engine-driven CHP.

Upgrading of biogas with injection into the grid or forthe utilization as vehicle fuel has become increasingimportance because the gas can be used in a more energy-efficient way throughout the whole year. In many EUcountries, the access to the grid is guaranteed by stateordinances. Countries like Germany, Sweden, and Switzer-land have defined quality standards for biogas injection intothe natural gas grid. All gas contaminants as well as carbondioxide must be removed, and the upgraded gas must havea methane content of more than 95% in order to fulfill thequality requirements of the different gas appliances. Inorder to ensure that biomethane does not contain bacteriaand molds that could create unacceptable risks for humanhealth and equipment, the application of HEPA filters isdiscussed (Wempe and Dumont 2008). Utilization of methanein the transport sector is widely distributed in Sweden andSwitzerland. The upgraded biogas is stored at 200 to 250 barsin gas bottles. Various technologies can be applied forincreasing the methane content (Persson et al. 2006).

The most common methods of removing carbon dioxidefrom biogas are water scrubbing or scrubbing with organicsolvents like polyethylene glycol (Kapdi et al. 2005) as wellas pressure swing adsorption using activated carbon ormolecular sieves (Schulte-Schulze Berndt 2005). Lessfrequently used are chemical washing by alkanol amineslike monoethanolamine or dimethylethanolamine (Wünsche2008) as well as membrane technologies (Miltner et al.

2009) and cryogenic separation at low temperature(Petersson 2008). When removing carbon dioxide fromthe gas stream, small amounts of methane are alsoremoved. These methane losses must kept low for bothenvironmental and economical reasons since methane is agreenhouse gas 23 times stronger than CO2.

Digestate utilization

The anaerobic digestion process results in a mineralizationof organically bounded nutrients, in particular nitrogen andin a lowering of the C/N ratio. Both effects increase theshort-term N fertilization effect. The digestate allows anaccurate dosage and integration in a fertilization plan with areduced application of additional mineral nitrogen fertil-izers. The ammonia nitrogen content increases in somecases by a factor of three if energy crops are used as theonly substrate (Gemmeke et al. 2009). Due to the improvedflow properties, the digestate can penetrate faster in the soilwhich reduces the risk for nitrogen losses by ammoniaemissions. Anaerobic digestion results also in a significantreduction of odors and in a positive change in thecomposition of odors. Measurements have shown that upto 80% of the odors in the feedstock can be reduced.

The anaerobic digestion process is able to inactivateweed seeds, bacteria (e.g., Salmonella, Escherichia coli,Listeria), viruses, fungi, and parasites in the feedstockwhich is of great importance if the digestate is used asfertilizer (Sahlström 2003; Strauch and Philipp 2000). Thedecay rate is dependent on temperature, treatment time, pH,and volatile fatty acids concentration. Temperature is themost important factor concerning survival of pathogensduring anaerobic digestion. The best sanitation effect isobtained at thermophilic temperatures above 50 °C andlong retention times. A 90% reduction of a Salmonellapopulation is achieved at thermophilic temperature (53 °C)within only 0.7 h whereas at mesophilic conditions (35 °C)at least 2.4 days are necessary (Bendixen 1999). Forspecific wastes, a separate pasteurization before or afteranaerobic digestion at 70 °C for 60 min is stipulated by theEuropean Union Animal By-Products Regulation (EC1774/2002). Pasteurization is an effective way of heattreatment; however, bacterial spores are not reduced.Pasteurization after digestion is more effective but thedigestate is particularly prone to recontamination.

Outlook

Biogas production in the agricultural sector is a very fastgrowing market in Europe and finds increased interest inmany parts of the world. In the next few decades, bioenergy


will be the most significant renewable energy source,because it offers an economical attractive alternative tofossil fuels. The success of biogas production will comefrom the availability at low costs and the broad variety ofusable forms of biogas for the production of heat, steam,electricity, and hydrogen and for the utilization as a vehiclefuel. Many sources, such as crops, grasses, leaves, manure,fruit, and vegetable wastes or algae can be use, and theprocess can be applied in small and large scales. Thisallows the production of biogas at any place in the world.

For an increased dissemination of biogas plants, furtherimprovements of the process efficiency, and the developmentof new technologies for mixing, process monitoring, andprocess control are necessary. Furthermore, the influence ofthe microbial community structure on process stability andbiogas yield requires further efforts and must be analyzed inmore detail. Recent research results have demonstrated thatstrong variations in the community structures occur during theongoing fermentation process which influences the processefficiency. Molecular analyses have shown the presence ofnumerous recently unknown bacteria which may have animportant influence on the degradation process. A majorpotential for increasing the biogas yield has also thepretreatment of substrates and the addition of micronutrients.Important for the future is also a better process control. Today,there are only few sensors available that are sufficiently robustto monitor online. With the increasing number of biogasplants, also an improvement of the effluent quality isnecessary, in order to avoid a contamination of ground waterwith pathogens and nutrients.

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Biogas Production Current State and Perspectives

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