Anaerobic digestion (AD) transform organic compounds (biomass wastes) to methane biogas by microbes.
Important elements ideal for AD Large amount of water Large amount of degradable organic substances such as starch and cellulose High content of nitrogen (N) and phosphorus (P) or nutrients
Potential feedstocks Municipal sludge Animal manures Organic fraction of municipal solid wastes (OFMSW) Food and food- processing waste Agricultural residues and energy crop
Municipal sludge Includes primary and waste activated sludge derived from centralized waste water treatment plants High water content, little readily fermentable substrate but plenty of nutrients (nitrogen and phosphorus), high density of bacterial cells 6.2 million dry tons of sludge = 6 million m3 of methane biogas
Animal manures High water content Have very little readily degradable substances (Carbohydrates and proteins) low biochemical methane potentials (BMP) & slow AD process High concentration of nitrogen and large pH buffering capacity against acids High concentration of ammonia cause toxicity to methanogens Contains large amounts of microbial biomass (does not need external digested sludge as starter culture)
Food and Food-Processing Wastes High water content and volatile solid (VS) contents Large amount of readily fermentable substrates 348 m3 of methane can be produced per dry ton of food wastes within 10 days of AD. Most food processing waste are poor in nitrogen except meat processors Food processing wastes have been codigested with nitrogen-rich feedstocks (municipal sludge & animal manures) to enhance system stability and methane production
The Organic Fraction of Municipal Solid Wastes (OFMSW)
Includes paper, grass mowings, food scraps Little moisture and readily fermentable substrates Deficient in nitrogen and phosphorus but large methane potential if digested adequately Pretreatment such as grinding is required Codigestion with other nutrient-rich biomass
Agricultural Residues and Energy Crops Low moisture content, high VS content and variable content of readily fermentable substrates Nonleguminous crops have little nitrogen Codigestion with animal manures or municiple sludge Biomass rich in starch/proteins is easier to digest than cellulosic biomass Reduced particle size of insoluble feedstock can enhance AD N and P are important as nutrients for microbes to grow
Biotechnologies - biomass is converted by microbe to produce methane (CH4) biogas. Anaerobic process – through a series of complex microbiological process in the absence of oxygen. Process is applied universally in hot anaerobic digester – using heterotrophic bacteria. Advantage – can generates energy in the form of methane by produce small amount of sludge (10%).
The anaerobic treatment is characterized by the production of “biogas” consisting mainly of methane (60-80%) & carbon dioxide (40-20%). Used as fuel for generating thermal energy / electric. Used only small amount of COD (5-10%) to form new bacteria. Used complex process involving several groups of bacteria – strictly anaerobic & facultative. Every stage of process is used different group of bacteria.
Process 1)Hydrolysis 2)Acidogenesis 3)Acetogenesis 4)Methanologies
Hydrolysis Bacteria hydrolyze the biomass polymers (feedstock) to monomers/oligomers. It catalyzed by the extracellular hydrolytic enzymes that secreted by the hydrolytic bacteria. Acidogenesis Resulting hydrolytic products are immediately fermented to short chain fatty acid (SCFA), CO2 &H2.
Acetogenesis Product of acidogenesis are converted into the final precursors for methane generation; acetate, hydrogen & carbon dioxide. Methanogenesis Methane is produced from:- i.Acetate - acetotrophic ii.Reduction of carbon dioxide by hydrogen – hydrogentrophic
Consists of an U shaped tank reactor. Influent enters the reactor at one end and flows forward and loops back, exiting at the other end. Influent Effluent Settling compartment
Digester content is mixed by gas or water jets in direction perpendicular to the plug flow of the reactor. Solids separated from effluent can be recycled if increased microbial biomass is needed. Biogas
MPFLR has been built at several dairy farms in US. Examples: √ Herrema Dairy located at Fair Oaks, Indiana operates a MPFLR that digests more than 400 m 3 of manure slurry (8% solids generated daily by 3800 cattle). √ The biogas produced fuels two Hess engine-generators (375 kWh)
√ Separated solids are being dried and subsequently used as bedding in the barns. √ Heat from the engine-generators is recovered and used to heat the digester, barns and alleyways.
H 2 S, and moisture 40% CO 2 60% CH 4 trace amounts of ammonia Methane biogas
Natural gas -energy value of 5.8–7.8 kWhm3 whereas the calorific value of typical methane biogas (60% CH 4 and 40% CO 2 ) ranges from 5.5 to 6.5 kWhm3. biogas -as a substitute for natural gas. Biogas produced from nearly all large-scale AD reactors -used to power CHP systems to generate heat and electricity. heat and electricity generated - used to operate AD reactors and associated facilities with excess heat and electricity perhaps being provided to nearby communities or utility companies. CHP systems -very efficient in utilizing methane biogas.
Direct electricity generation from biogas using fuel cells - appealing alternative. conventional fuel cells - based on precious metal catalysts can only use pure H2 or H2-rich gas as fuel Example: biogas with 60% methane can be processed to H 2 And CO in a steam reformer, -such that the resultantH 2- rich gas (H 2 >50% of the total gases) is sufficient for efficient and stable operation of a polymer electrolyte membrane (PEM) fuel cell stack.
the solid oxide fuel cell (SOFC) – that uses solid oxide catalysts can use biogas directly without prior reformation The SOFC uses a hard, ceramic compound of metal (e.g., calcium or zirconium) oxides as an electrolyte, and operates at temperatures ranging from 900 to 1000 C (Singhal and Kendall, 2003). high temperatures - a reformer is not required to extractH2 from biogas, and the waste heat can be recycled to produce additional electricity. Pilot-scale SOFCs have achieved an efficiency of approximately 60%. Because SOFCs operate at high temperatures, they have the greatest fuel flexibility and can use biogas without prior cleansing. In theory, the ammonia and H2S in biogas can be used by the SOFC as fuels
References Vertex, A., Qureshi, N., Blaschek, H. P., Yukawa, H. (2010). Biomass to Biomass to Biofuels: Strategies for Global Industries.