Conversion challenges

Cellulose? is the world’s most abundant organic material and its use in production of biofuels could improve longer term outlooks for energy supply in a world of growing demand. However, conversion of cellulosic materials to biofuels is more difficult and costly compared to use of sugars and starches from grains. Cellulose is the primary source of sugar in fibrous biomass?, but it is tightly bound with hemicellulose and lignin which form a rigid protective sheath. For fermentation technologies pretreatment of cellulosic biomass is necessary to break apart and release the sugars for fermentation to ethanol. Mechanical (e.g., crushing) and thermochemical (e.g., hydrolysis) pretreatments, typically applied in a sequential process (figure 1), can successfully break up hemicellulose and lignin, but they also result in formation of by-products which inhibit fermentation processes. Detoxification is typically necessary to remove the inhibiting by-products, adding to overall costs of cellulosic ethanol? production. Additionally, many pretreatments are reportedly ineffective on forest biomass (i.e., woody feedstocks with high lignin content).

Figure 1. Cellulosic ethanol production involves a multi-step process for releasing sugars from hem-cellulose and lignin which are then subject to fermentation before further refinement into biofuels. Lignin residues from the fermentation contain carbon and can be recovered for use in thermal and thermochemical bioenergy? conversion technologies. (CL Williams, 2011).

A key pretreatment technology for cellulosic ethanol fermentation is hydrolysis - the transformation of cellulose to glucose (figure 1). In chemical hydrolysis acids are used to break down cellulose, sometimes accompanied with application of heat and increased atmospheric pressure. These inputs and processes increase the costs of biofuel production as well as time required for producing the biofuel end product. Enzymatic hydrolysis involves use of cellulase enzymes like those present in the stomachs of ruminant animals. These enzymes are produced by bacterias. Cellulase enzymes are currently in too limited supply (and costly) for cellulosic ethanol to move beyond pilot projects to commercial scale? production. However, researchers are seeking technological breakthroughs for mass production of hydrolysis enzymes at competitive prices.

Thermochemical technologies for conversion of cellulosic materials are alternatives to fermentation in the production of liquid and gaseous fuels. However, gasification? and pyrolysis? do not directly produce liquid fuels suitable for current light vehicle fleets (e.g., U.S. passenger vehicles). Therefore, additional processing is necessary, adding to overall biofuel production costs. Much effort is being directed toward technological advances necessary to reduce costs in order to move cellulosic biofuel production from pilot scales to commercial scales.

References:


American Chemical Society. 2010. More economical process for making ethanol from nonfood sources. ScienceDaily. Available at: http://www.sciencedaily.com/releases/2010/03/100325121952.htm.

National Renewable Energy Laboratory. 1994. Technology Brief: cellulose conversion key to fuel of the future. Available at: http://www.nrel.gov/docs/legosti/old/5679.pdf.

Wyman, CE. 1999. Biomass ethanol: technical progress, opportunities, and commercial challenges. Annual Review of Energy and the Environment 24:189-226.

Anerobic Digestion and Biogas

UW Extension have created seven modules focused on the use of anaerobic digestion technologies. Details of the process are introduced, as well as factors that influence start-up, operation and control of anaerobic digesters at different scales.

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Carol Williams clwilliams4@wisc.edu
(608) 890-3858 (office)
(515) 520-7494 (mobile)
Department of Agronomy
1575 Linden Dr.
University of Wisconsin, Madison, WI 53706

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Use of contour buffer strips in commodity crop systems in southwestern Wisconsin helps reduce soil loss and traps nutrients on slopes. Photo courtesy of Wisconsin Farm Bureau Federation.