Biochemical conversion processes include anaerobic digestion or decomposition and anaerobic fermentation. These processes occur at lower temperatures and lower reaction rates compared to thermochemical processes.
High moisture feedstocks, such as food waste and green waste, are generally good candidates for biochemical processes.
Energy products from biochemical processes include biogas (primarily methane and carbon dioxide) and ethanol (bioethanol). Biogas can be used to produce electricity, fuel, or renewable natural gas. According to the U.S. Department of Energy, other biofuels, such as biobutanol and dimethyl ether, are being investigated and could become important biofuels.
The anaerobic digestion (AD) process operates without free oxygen and results in a biogas containing mostly methane and carbon dioxide, but frequently carrying impurities such as moisture, hydrogen sulfide, ammonia, siloxane, and particulate matter. AD occurs in landfills, manure lagoons (covered or not), controlled reactors, or in-vessel digesters.
Biogas is the principal energy product from AD processes. Biogas can be burned directly for heat or steam, or converted to electricity in reciprocating or gas turbine engines, steam turbines, or fuel cells. Biogas can also be upgraded to biomethane and used as a vehicle fuel, injected into the natural gas pipeline, or reformed into hydrogen fuel. The liquid and solid digestate from AD may be suitable for use as a compost product, soil amendment or as a liquid or dry fertilizer.
Fugitive greenhouse gas (GHG) emissions (primarily methane) resulting from the anaerobic decomposition of organic wastes in landfills has been identified as a significant source of emissionsPDF download contributing to global climate change. Reducing the amount of organic materials sent to landfills by diverting those materials to anaerobic digestion and compost facilities is an integral part of both the AB 32 (California Global Warming Solutions Act of 2006) Scoping Plan and SB 1383 (Short-Lived Climate Pollutants). For more information on the connection between the waste sector and California’s GHG emission reduction goals, please see CalRecycle’s Climate Change page.
AD systems have been used in Europe for over 30 years to treat the biodegradable fraction of solid waste prior to landfilling in order to reduce future methane and leachate emissions and recover energy. As a consequence of the European Commission Landfill Directive, installed AD capacity in EuropePDF download has increased sharply and in 2013 there were 244 AD plants with more than 8 million tons of annual capacity.
There are a growing number of AD facilities in California that use the organic fraction of municipal solid waste (MSW) as feedstock, including some Waste Water Treatment Plants (WWTPs) that co-digest food waste and wastewater. With the passage of AB 341 , AB 1826, and SB 1383, the development of a viable anaerobic digester infrastructure in California that uses our food waste and other urban organic wastes is one of CalRecycle’s highest priorities. To find AD facilities in California, refer to the List of Anaerobic Digesters in California.
For an in depth review and discussion of AD systems for MSW, see CalRecycle report Current Anaerobic Digestion Technologies Used for Treatment of Municipal Organic Solid WastePDF download, March 2008. For information on permitting AD facilities in California, see CalRecycle’s Anaerobic Digestion permitting website.
Anaerobic fermentation (i.e., hydrolysis followed by fermentation to alcohols) converts substrates such as cellulose into glucose and then to ethanol for use in beverages, fuels, and to other chemicals (e.g., lactic acid used to produce renewable plastics) and products (e.g., enzymes for detergents).
While fermentation of starch and sugar-based feedstocks (i.e., corn and sugar cane) into ethanol is common, cellulosic biomass feedstocks are more difficult to process with fermentation. Cellulosic feedstocks, including a large portion of the organic fraction of MSW, need hydrolysis pretreatment (acid, enzymatic, or hydrothermal hydrolysis) to break down cellulose and hemicellulose to simple sugars needed by the yeast and bacteria for the fermentation process.
The lignin portion of biomass is also difficult to convert via biochemical processes, and is typically considered for use as boiler fuel or as a feedstock for thermochemical conversion to other fuels and products. Alternately, hydrolysis of lignocellulosic feedstocks is the subject of intense research and may prove to be a viable use for these feedstocks.
Alcohols, such as ethanol and butanol, are the primary energy product from hydrolysis and fermentation processes. There are no known hydrolysis and fermentation systems operating on MSW feedstock but research has been done showing potential for this process.
Additional information on anaerobic digestion and fermentation can be found in New and Emerging Conversion Technologies: Report to the LegislaturePDF download, June 2007.