Turning Plants Into Biofuels
Plant biomass has a low energy density (MJ kg–1) in comparison with fossil fuels. In specific, maize kernels for starch, sugarcane shoots for sucrose, and switchgrass shoots for cellulose contain less than one-third of the energy in the equivalent weight of gasoline. It is economically unviable to transport materials such as biomass over long distances. Consequently, biofuel processing facilities are located near the sites of biomass production. For example, bioethanol processing facilities in the United States are concentrated in the midwestern corn belt. Transport of biomass to the processing facility, even over short distances, contributes between 2% and 4% to the total greenhouse gas emissions resulting from bioethanol production. [1]
Starting a fire using wet wood is very difficult. This is because vaporizing any water in the biomass dissipates 2.27 MJ of energy per kg of H2O vaporized. For this reason, drying biomass before combustion enhances its energy density as well as diminishes its transport weight. Unfortunately, drying biomass without the use of ovens or kilns, which themselves consume energy, proceeds slowly in many regions of world. Wood pellets (typically, cylinders 6 mm in diameter and 10 mm in length) are a form of biofuel that travels well. They require only simple processing, have moderate energy densities easily conform to packaging, and are safe to store. Processing entails harvesting the aboveground portions of trees, stripping off leafs and twigs, drying the raw wood from about 55% water to less than 10%, and forming the pellets in a die and roller press. [2] These processes consume, on average, 6.4 MJ per kg, or around 30% of the energy in the final product. [3]
Some procedures for converting biomass to biofuels are as old as human civilization, whereas others still await technological breakthroughs. The procedures for direct burning of biomass, fermentation of sugars to ethanol, and cellulose purification are well established. Still under development are efficient procedures to convert cellulose into ethanol.
The burning of leaves, twigs, underbrush, and other so-called “waste” biomass can offset some of the energy requirements of pellet processing. Removal of such material for energy purposes, however, does not compensate for the loss of nutrients and carbon from the soils from which the biomass sprang. Longterm sustainability of biomass plantations is doubtful without periodic restoration of the soils. [4]
[1] Farrell, A. E., R. J. Plevin, B. T. Turner, A. D. Jones, M. O'Hare, and D. M. Kammen (2006a) Energy and Resources Group Biofuels Analysis Meta-Model. Release 1.1. Regents of the University of California, http://rael.berkeley.edu/EBAMM/, accessed Aug. 26, 2007.
[2] Leaver, R. H. (2007) Wood Pellet Fuel and the Residential Market Northeast Regional Biomass Program, Washington, D.C., http://www.nrbp.org/papers/032.pdf.
[3] Patzek, T. W. and D. Pimentel (2005) Thermodynamics of energy production from biomass. Critical Reviews in Plant Sciences 24:327–364.
[4] Patzek, T. W. and D. Pimentel (2005) Thermodynamics of energy production from biomass. Critical Reviews in Plant Sciences 24:327–364.
This is an excerpt from the book Global Climate Change: Convergence of Disciplines by Dr. Arnold J. Bloom and taken from UCVerse of the University of California.
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