Conversion Technologies

Biochemical Conversion Processes

Biochemical conversion processes include aerobic conversion (i.e., composting), anaerobic decomposition or digestion (which occurs in landfills and controlled reactors or digesters), and anaerobic fermentation (for example, the conversion of cellulose derived sugars to ethanol).

Biochemical conversion proceeds at lower temperatures and lower reaction rates (compared to thermochemical processes). Higher-moisture feedstocks, such as food waste, are generally good candidates for biochemical processes.

For biomass feedstocks, the lignin fraction currently can not be converted biochemically,¹ although research is currently investigating lignin fermentation processes.

Energy products from biochemical routes include biogas (also landfill gas) and ethanol (sometimes referred to as bioethanol). Biobutanol and other fermented alcohols are being investigated and could become important biofuels. 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 be upgraded to biomethane (by stripping carbon dioxide and minor contaminants) and used as a vehicle fuel, injected to the natural gas transmission system, or reformed into hydrogen fuel.

The lignin and other stabilized residue from biochemical conversion may be suitable as a compost product or could be used for energy by burning or gasifying, perhaps to supply the energy needs for the biochemical conversion plant. If the residue is of poor quality (i.e., the feedstock came from mixed waste rather than clean source separated material), it may not have a market value and would likely end up in the landfill.

Anaerobic Digestion

Anaerobic digestion offers the citizens of California multiple benefits. Its simple biological processes use our waste as resources to make renewable energy or low carbon biofuels along with a compost that can be used to restore the health of our precious agricultural soils. Anaerobic digesters can provide distributed energy, have a small footprint, and typically are enclosed, so they can be good neighbors in virtually any community setting. Because they produce electricity or fuel, they can help reduce our dependence on foreign sources of fossil fuel and significantly reduce greenhouse gas emissions that contribute to global warming. 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.

--Margo Reid Brown, Acting Director, California Department of Resources Recycling and Recovery (CalRecycle)

Anaerobic digestion (AD) is a fermentation technique that operates without free oxygen and results in a biogas containing mostly methane and carbon dioxide but frequently carrying impurities such as moisture, hydrogen sulfide (H2S), ammonia, siloxane, and particulate matter. Anaerobic digestion occurs in manure lagoons (covered or not), controlled reactors, or digesters and is the principal process occurring in landfills.

Biogas, primarily methane and carbon dioxide, 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 be upgraded to biomethane and used as a vehicle fuel, injected to the natural gas transmission system, or reformed into hydrogen fuel. The digestate from AD (lignin and other stabilized residue) may be suitable as a compost product.

AD systems are employed in many wastewater treatment facilities for sludge degradation and stabilization, and used in engineered anaerobic digesters to treat high-strength industrial and food processing wastewaters prior to disposal.

AD systems are used, primarily in Europe, to treat the biodegradable fraction of solid waste prior to landfilling in order to reduce future methane and leachate emissions and recover some energy.^2,3 As a consequence of the European Commission Landfill Directive, installed AD capacity in Europe has increased sharply and now stands at more than 4 million tons annual capacity.⁴

There are no commercial-scale AD systems operating on municipal solid waste (MSW) in the United States. A few have been operating in Canada. There are a few pilot-scale systems in operation in California. AD technologies are among those appearing on technology review “short lists” for several California jurisdictions exploring alternatives to landfills.

For an in depth review and discussion of AD systems for MSW, see the CIWMB report Current Anaerobic Digestion Technologies Used for Treatment of Municipal Organic Solid Waste, March 2008.⁵

Anaerobic Digestion facility permitting information is available.

Anaerobic Fermentation

Anaerobic fermentation (i.e., hydrolysis followed by fermentation to alcohols) is generally used industrially to convert substrates such as glucose to ethanol for use in beverage, fuel, and chemical applications and to other chemicals (e.g., lactic acid used to produce renewable plastics) and products (e.g., enzymes for detergents).⁶

Fermentation of starch- and sugar-based feedstocks (i.e., corn and sugar cane) into ethanol is fully commercial but not yet for cellulosic biomass because of the expense and difficulty in breaking down (hydrolyzing) the materials into fermentable sugars.

Cellulosic feedstocks, including the majority 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. With the possible exception of acid recycling and recovery, acid processes are technologically mature, but enzymatic processes are projected to have a significant cost advantage once improved.⁷

Lignin in biomass is a byproduct of fermentation processes and is typically considered for use as boiler fuel or as a feedstock for thermochemical conversion to other fuels and products. Hydrolysis of lignocellulosic feedstocks is the subject of intense research.

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 feedstocks in the world. A facility that would use acid hydrolysis on MSW residuals is undergoing permitting in Southern California.

Additional information on technology vendors is available on this site.

Footnotes

1. Lignin fraction does not decompose anaerobically. It undergoes aerobic fungal decay over long time periods.
2. Rapport, J., R. Zhang, B.M. Jenkins, and R.B. Williams, (2008). Current Anaerobic Digestion Technologies Used for Treatment of Municipal Organic Solid Waste. University of California, Davis: Contractor Report to the California Integrated Waste Management Board.
3. See the EC Landfill Directive (Council Directive 99/31/EC) which became enforceable in 2001. It required the biodegradable portion of MSW be reduced by 25 percent of that disposed in 1995 within five years, 50 percent within eight years, and 65 percent within 15 years
4. De Baere, L., (2006). Will anaerobic digestion of solid waste survive in the future? Water Science and Technology. 53(8): p. 187-194.
5. Adapted from Hackett, C., Williams, R. B., Durbin, T. D., Welch, W., Pence, J., Aldas, R., Jenkins, B. M., and Salour, D. (2004). " Evaluation of conversion technology processes and products, Draft Final Report." University of California. (PDF, unknown file size)
6. Lynd, L., R,. "Overview and Evaluation of Fuel Ethanol from Cellulosic Biomass: Technology, Economics, the Environment, and Policy." Annual Review of Energy and the Environment, Vol. 21, 1996, pp. 403-465.