Crops for biodiesel contain between 4% and 62% extractable vegetable oil by weight. Oil is extracted physically, in a mechanical press, or chemically, by the use of an organic solvent such as hexane. Chemical extraction produces higher yields and is faster and less expensive, but it is less suitable to rural areas because it requires sophisticated equipment. Production of biodiesel involves the reaction of extracted vegetable oil with methanol or ethanol in the presence of a strong base and then separation of the resulting biodiesel from the unreacted alcohol and base and from the glycerol that forms.
The efficiency of extraction and conversion is on the order of 89% , but processors must deal with the 38% to 97% of plant biomass that remains after conversion (96% to 38% of the plant biomass that is not oil plus 11% of the oil that is not converted). The economic viability of biodiesel depends on the value of this leftover biomass.
An alternative process for producing biodiesel is Fischer-Tropsch synthesis, in which gasification of carbon from natural gas, biomass, or coal with steam under heat and pressure produces carbon monoxide and hydrogen. This reaction requires the input of energy that usually is generated by combusting a portion of the natural gas, biomass, or coal. The products of this reaction, carbon monoxide and hydrogen, then react in the presence of a catalyst (typically, iron or cobalt) to produce hydrocarbons and water. This reaction is allowed to proceed until it synthesizes long-chain hydrocarbons that are liquid at room temperatures. The overall result of this process is to convert natural gas, coal, or biomass into liquid fuels of high energy density that are suitable for use in vehicle engines. During World War II, petroleum-poor but coal-rich Germany produced diesel fuel for its vehicles via Fischer-Tropsch synthesis. Today, this process is relatively uncommon because its energy efficiencies are lower and costs are higher than fuel production through other means.
 Balat, M. (2007) Production of biodiesel from vegetable oils: A survey. Energy Sources Part a-Recovery Utilization and Environmental Effects 29:895-913.
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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