Present and Future Prespectives Ali WaqasGhulam MohiuddinMSc. Energy Environmental EngineeringMSc. Energy Environmental EngineeringRoll No. MSCE-F-17-01Roll No. MSCE-F-017-17Center for Coal Technology (University of the Punjab)Center for coal technology (University of the Punjab)[email protected] Abstract Anaerobic digestion of energy crops, residues and wastes is of increases interest in order to reduce the greenhouse gas emissions and to facilitate a sustainable development of energy supply. Production of biogas provides a versatile carrier of renewable energy, as methane can be used for replacement of fossil fuels in both in both heat and power generation and as a vehicle fuel. For biogas production, various process types are applied which can be classified in wet and dry fermentation system. Most often applied are wet digester system using vertical stirred tank digester with different stirrer types dependent on the origin of the feedstock. Biogas is mainly utilized in engine-based combined heat and power plants, whereas micro gas turbines and fuels cells are expensive alternative which need further development work for reducing the costs and increasing their reliability. Gas upgrading and utilization as renewable vehicle fuel or injection into the natural gas grid is of increasing interest because the gas can be used in a more efficient way. The digestate from anaerobic fermentation is a valuable fertilizer due to the increased availability of nitrogen and the better short-term fertilization effect. Anaerobic treatment minimizes the survival of pathogens which is important for using the digested residue as fertilizer. This paper reviews the current state and perspectives of biogas production, including the biochemical parameters and feedstocks which influence the efficiency and reliability of the microbial conversion and gas yield. KeywordsBio gas formatting style styling insert (key words) INTRODUCTION The global energy demand is growing rapidly, and about 88 of this demand is met at present time by fossil fuels. Scenarios have shown that the energy demand will increase during 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 fossil fuel-derived CO2 emission being the most important contributor. In order to minimize related globing warming and climate change impacts, GHS emission must be reduced to less than half of global emission levels of 1990 (IPCC 2000). World thanks to the strong development of agriculture biogas plants on farms. At the end of 2008, approximately 4,000 agricultural biogas production units were operated on German farms (Fachverband Biogas 2009). Within the agricultural sector in the European Union (EU), 1,500 million tons of biomass could be digested anaerobically e4ach year and half of his potential is accounted for by energy crops (Amon et al.2001). the different aspects of agricultural biogas production and utilization are reviewed in this paper. Biochemical process. Methane fermentation is complex process which can be divided up into four phases hydrolysis, acidogensis, acidogensis/dehydrogenation, and methnation. The individual degradation steps are carried out by different consortia of microorganisms, which partly stand in syntro-phic interrelation and place different requirements on the environment (angelidaki et al.1993). hydrolyzing and fermenting micoorganisms are responsible for the initial attack on polymers and monomers and produce mainly acetate and hydrogen and varying amounts of volatile fatty acids such as propionate and butyrate. A balanced anaerobic digestion process demands that in both stages the rates of degradation must be equal in size. If the first degradation step runs too fast, the acid concentra-tion rises, and PH drops below 7.0 which inhibits the methanogenic 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 of the substrate which is use for biogas production. Undis-solved compounds like cellulose proteins, or fats are cracked slowly into monomers within several days whereas the hydrolysis of soluble carbohydrates takes place within few hours. Much is known about the basis metabolism in different types of anaerobic digestion processes, but little is known about the microbes responsible for these processes. Only few percent of bacteria and archaea have so far been isolated, but little is know about dynamics and interactions between these microorganisms. The lack of knowledge result sometimes in malfunctions and unex-plainable failures of biogas fermenters. With molecular techniques, more information can be received about the community structures in anaerobic processes (elferink et al.1998 Yu et al.2005 Karakashev et al.2005 klocke et al.2008). All types of biomass can be used as substrates for biogas production as long as they contain carbohydrates, proteins, fats, cellulose, and hemicelluloses as main components. The biochemical processes as well include the esterification and many complex organic reactions which are catalyzed as well controlled by enzyme concentration as well ph change according to system and feedstock parameters. Feedstocks Composition of biogas and the methane yield depends on the feedstock type the digestion system and the retention time (Braun 2007). The theoretical gas yield varies with the content of carbohydrates proteins, and fasts (table.1) only strong lignified organic substance, e.g., wood are not suitable due to the slowly anaerobic decomposition. The real methane content in practice is generally higher than the theoretical values shown in Table.1 because a part of CO2 is solubilized in the digestive. The most important parameter for choosing energy crops is their net energy yields per hector. Many conventions frag crops produce large amount of easily degradable biomass which is necessary for high biogas yield (Braun 2009). The highest gross energy potential has maize and forge beets but also different cereal crops and perennial grasses have potential as energy crops (table 2). Forage crops have a advantage of being suitable for harvesting and storing with existing machinery and meth-ods. The specific methane yield is affected by the chemical composition of the crop which changes as the planet matures (Dohler et at. 2006 KTBL/FNR 2007). Harvesting time and frequency of harvest are, thus, important for the substrate quality and biogas yield. Amon et al. (2007) have shown that maize crops were harvested after 97 days of vegetation at milk ripeness produced up to 37 greater methane yield when compared with maize at full ripeness. Crops can be grown as preceding crop, main crop, or succeeding crop. Also mixed cultivation of different crops, e.g., maize and sunflower, can be applied (Karpenstein-Machan 2005). Table 1 maximal gas yields and theoretical methane contents (baserga U 1998) SubstrateBio GasCH4 ()CO2 ()(Nm3/t )Carbohydrates790-8005050Raw Protein70070-7129-30Raw Fat1,200-67-6832-331,250Lignin000 processes dominate in the agriculture sector (Weiland 2008a). Horizontal digesters are typical plug flow system which are equipped with a low rotating horizontal paddle mixer. They are mainly applied for the first stage of two-stages reactor configurations because they can be operated at higher total solids contents of the input. The reactor volume is limited to a maximum of about 700 m3 due to the technical and economic aspects. Energy crop digestion required prolonged hydraulic retention times of several weeks to month a achieve complete fermentation with high gas yields an minimized residual gas potential of the digestate (Gemmeke et al. 2009). Hence the typically loading rate of organic dry matter (ODM) for wet fermentation processes is only between 2 and 4 kg ODM (M3 /d). Table .1 Process technology For biogas production, various process types are applied which can be classified in wet or dry fermentation processes. Wet digestion processes are operated with total solids concentrations in the fermenter below 10 which allows the application of completely stirred tank digesters. The digested material is jumpable and can be spread on fields for fertilization. For the treatment of solid substrates, e.g., energy crops, the input must be mixed with liquid manure or recycled process water in process water in order to achieve pumpable slurries. Dry digestion processes are operated with a total solids content inside the fermenter between 15 and 35. All we digestion processes are operated continuously whereas for dry fermentation batch and continuously operated processes are applied. Today, wet digestion Biogas Utilization Biogas is primarily composed of methane and carbon dioxide, contain smaller amounts of hydrogen sulfide and ammonia and is saturated with water vapor. Biogas must be desulfurizated and dried before utilization to prevent damage of the gas utilization units. Biogas produced by the confermention of manure with energy crops or harvesting residues can contain levels of H2 S between 100 and 3,000 ppm. CHPs which are mainly used for the utilization of biogas need mostly levels of H2 H below 250 ppm, in order to avoid excessive corrosion and expensive deterio-ration of lubrication oil. Removal of H2 S is done nowadays mainly be biological desulfurization (Schneider et al. 2002). The process is based on the oxidation of H2 S by injection of a small amount of air (2-5) into the raw biogas. For this kind of desulfurization, sulfobacter oxy-dans bacteria must be present, to convert H2 S into elementary sulfur and sulfurous acid. For the desulfuriza-tion inside the digester, S. oxdans does not have to be added, because it is present inside the digest. The air can be injected directly in the headspace of the digester, and the reaction occurs on the floating layer, on reactor wall and on other surface in the gas room. For achieving an efficient biological desulfurization, specific supports of wood or fabric must be installed in the fermenter top in order to achieve enough surface area for microorganisms fixation. The most common methods of removing carbon dioxide from biogas ae water scrubbing or scrubbing with organic solvent like polyethylene glycol (Kapdi et al. 2005). Less frequently used are chemical washing by alkanol amines like monoethanolamide or dimethylethanolamine (Wunshe 2008) as well as membrane technologies (Miltner et al.2009) and cryogenic separation at low temperature (Petersson 2008). when removing carbon dioxide from the gas stream, small amounts of methane are also removed. These methane losses must have kept low for both environmental and economical reason since methane is a greenhouse gas 23 times stronger than CO2. Digestive Utilization The anaerobic digestion process result in a mineralization of organically bounded nutrients. In particular nitrogen and in a lowering of the C/N ration. Both effect increase the short-term N fertilization effect. The digestate allows an accurate dosage and integration in a fertilization plan with a reduced application of additional mineral nitrogen fertile-izers. The ammonia nitrogen content increase in some cases by a factor of three if energy crops are used the only substrate (Gemmeke et al. 2009). Due to the improved flow properties the digestate results also in in a significant reduction of odors and in a positive change in the composition of odors. Measurements have shown that up to 80 of the odors in the Feedstocks can be reduced. The anaerobic process is able to inactivate weed seeds, bacteria (e.g. Salmonella, Escherichia coli, listeria,), viruses fungi and parasites in the feedstock which is of great importance if the digestive is used a fertilizer (Sahlstrom 2003 Strauch and Philipp 2000). The decay rate is dependent on temperature treatment time, ph, and volatile fatty acids concentration. Temperature is the most important factor concerning survival of pathogens during anaerobic digestion. The best sanitation effect is obtained at the thermophilic temperature above 50 oC and long retention times. Outlook Biogas production in the agricultural sector is a very fast growing market in Europe an and finds increase interest in many 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 to fossil fuels. The success of biogas production will come from the availability at low costs and the broad variety of usable forms of biogas for the production of heat, steam, electricity and hydrogen Hydrogen and for the utilization as a vehicle fuel. Many sources such as crop grasses leaves manure, fruit and vegetable wastes or algae can be use, and the process can be applied in small and large scales. This allows the production of biogas at any place in the world. References Angelidaki I, Ellegard L, Ahring BK (1999) A comprehensive model of anaerobic bioconversion of complex substance to biogas, bioethanol Bioeng 63363-372 Angelidaki I Ellegaard L, Ahring B (2003) Application of the anaerobic digestion process. In Biomethanation II Adv. Bio-chem Eng/biotehnol,springer,pp 2-33 Andara AR, Esteban JMB (1999) Kinetic study of the anaerobic digestion of the solid fraction of piggery slurries. Biomass Bioenergy 17435443 Angelidaki I, Ellegaard L, Ahring BK (1993) A mathematical model for dynamic simulation of anaerobic digestion of complex substrates focusing on ammonia inhibition. Biotechnol Bioeng 42159166 Driehuis F, Elferink SJ, Spoelstra SF (1999) Anaerobic lactic acid degradation during ensilage of whole crop maize inoculated with Lactobacillus buchneri inhibits yeast growth and improves aerobic stability. J Appl Microbiol 87583594 FNR (2008) Biogas Basisdaten Deutschland. Fachagentur Nachwachsende Rohstoffe, Gulzow Fehrenbach H, Giegrich J, Reinhardt G, Sayer U, Gretz M, Lanje K, Schmitz J (2008) Kriterien einer nachhaltigen Bioenergienutzung im globalen Mastab. UBA-Forschungsbericht 20641112 Jarvis A, Nordberg A, Jarlsvik T, Mathisen B, Svensson BH (1997) Improvement of a grass-clover silage-fed biogas process by the addition of cobalt. Biomass Bioenergy 12453460 Schimpf U, Valbuena R (2009) Increase in efficiency of biomethanation by enzyme application. Bornimer Agrartechnische Berichte 684456 Schink B (1997) Energetics of syntrophic cooperation in methanogenic degradation. Microbiol Mol Biol Rev 61262280 Leven L, Eriksson ARB, Schnrer A (2007) Effect of process temperature on bacterial and archael communities in two methanogenic bioreactors treating organic household waste. FEMS Microbiol Ecol 59683693 Y, dXiJ(x(I_TS1EZBmU/xYy5g/GMGeD3Vqq8K)fw9
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