Page 297 - Synthetic Fuels Handbook

FUELS FROM CROPS 283

Biodiesel is made through a chemical process (transesterification) whereby the glycerin
is separated from the fat or vegetable oil. The process leaves behind two products: (a) methyl
esters (the chemical name for biodiesel) and (b) glycerin (a valuable by-product usually sold
to be used in soaps and other products).
Biodiesel (fatty acid methyl esters; FAME) is a notable alternative to the widely used
petroleum derived diesel fuel since it can be generated by domestic natural sources such
as soybeans, rapeseeds, coconuts, and even recycled cooking oil, and thus reduces depen-
dence on diminishing petroleum fuel from foreign sources. In addition, because biodiesel
is largely made from vegetable oils, it reduces life cycle greenhouse gas emissions by as
much as 78 percent (Ban-Weiss et al., 2007).
Vegetable oils and animal fats belong to an extensive family of chemicals called lipids.
Lipids are bioproducts from the metabolism of living creatures. As a result, they can be
found widely distributed in nature. Their biofunctions are diverse, but they are most known
for their energy storage capacity. Most lipids can easily dissolve in common organic sol-
vents, meaning that they are hydrophobic. If a lipid is a solid at 25°C, it is classified as a
fat; otherwise, it is oil. Typically, fats are produced by animals and oils by plants, but both
are mainly made of triglyceride (TG) molecules, which are triesters of glycerol (a triol)
and free fatty acids (long alkyl chain carboxylic acids). Other glyceride species, such as
diglycerides and monoglycerides, are obtained from triglycerides by the substitution of one
and two fatty acid moieties, respectively, with hydroxyl groups (Lotero et al., 2006).
Biodiesel production is a very modern and technological area for researchers due to the
relevance that it is winning everyday because of the increase in the petroleum price and the
environmental advantages (Marchetti et al., 2005). The successful introduction and com-
mercialization of biodiesel in many countries around the world has been accompanied by
the development of standards to ensure high product quality and user confidence (Knothe,
2005). In general, biodiesel compares well to petroleum-based diesel (Lotero et al., 2006).
Pure biodiesel fuel (100 percent esters of fatty acids) is called B100. When blended with
diesel fuel the designation indicates the amount of B100 in the blend, for example, B20 is
20 percent B100 and 80 percent diesel, and B5 used in Europe contains 5 percent B100 in
diesel (Pinto et al., 2005).
Biodiesel is biodegradable and nontoxic, and typically produces about 60 percent less
net carbon dioxide emissions than petroleum-based diesel, as it is itself produced from
atmospheric carbon dioxide via photosynthesis in plants. This figure can actually differ
widely between fuels depending upon production and processing methods employed in
their creation. Pure biodiesel is available at many gas stations in Germany.

9.5.1 Feedstocks for Biodiesel
Soybeans are widely used as a source of biodiesel. However, a variety of oils can be used
to produce biodiesel. These include:
1. Virgin oil feedstock; rapeseed, and soybean oils are most commonly used, soybean oil
alone accounting for about 90 percent of all fuel stocks. It also can be obtained from
field pennycress and Jatrophaother crops such as mustard, flax, sunflower, canola, palm
oil, hemp, and even algae show promise.
2. Waste vegetable oil (WVO).
3. Animal fats including tallow, lard, yellow grease, chicken fat, and the by-products of the
production of omega-3 fatty acids from fish oil.
Worldwide production of vegetable oil and animal fat is not yet sufficient to replace
liquid fossil fuel use. Furthermore, there are objections to the vast amount of farming and