Page 261 - Synthetic Fuels Handbook

FUELS FROM BIOMASS 247

the list of alternatives to gasoline that includes ethanol (E85), methanol (M85), natural gas,
propane, and electricity.

Biodiesel. Biodiesel is the generic name for fuels obtained by esterification of vegetable
oil (Knothe et al., 2005; Bockey, 2006). The esterification can be done either by methanol
or by ethanol. Biodiesel can be used in a diesel engine without modification and is a clean
burning alternative fuel produced from domestic, renewable resources. The fuel is a mix-
ture of fatty acid alkyl esters made from vegetable oils, animal fats, or recycled greases.
Where available, biodiesel can be used in compression-ignition (diesel) engines in its pure
form with little or no modifications.
Biodiesel is biodegradable, nontoxic, and essentially free of sulfur and aromatics. It is
usually used as a petroleum diesel additive to reduce levels of particulates, carbon monoxide,
hydrocarbons, and air toxics from diesel-powered vehicles. When used as an additive,
the resulting diesel fuel may be called B5, B10, or B20, representing the percentage of
the biodiesel that is blended with petroleum diesel.
Biodiesel is produced through a process in which organically derived oils are combined
with alcohol (ethanol or methanol) in the presence of a catalyst to form the ethyl or methyl
ester (Fig. 8.7). The biomass-derived ethyl or methyl esters can be blended with conven-
tional diesel fuel or used as a neat fuel (100 percent biodiesel). Biodiesel can be made
from any vegetable oil, animal fats, waste vegetable oils, or microalgae oils. Soybeans and
Canola (rapeseed) oils are the most common vegetable oils used today.

H H
H C O(O)CR RC(O)OMe H C OH
Catalyst
H C O(O)CR´ + 3MeOH R´C(O)OMe + H C OH
R´´C(O)OMe
H C O(O)CR´ H C OH
H Methylesters H
Triglyceride (Biodiesel) Glycerol
FIGURE 8.7 The chemistry of biodiesel production.

Biodiesel is usually made from soybean oil or recycled cooking oils (Chap. 9). Animal
fats, other vegetable oils, and other recycled oils can also be used to produce biodiesel,
depending on their availability.
The production of biodiesel from vegetable oil represents another means of producing
liquid fuels from biomass, and one which is growing rapidly in commercial importance.
Commercially, biodiesel is produced from vegetable oils, including rapeseed, sunflower,
and soybean oil, and animal fats (Fig. 8.8) (McNeil Technologies Inc. 2005). These oils and
fats are typically composed of C to C fatty acid triglycerides (constituting approximately
14
20
90–95 percent by weight of the oil). In order to produce a fuel that is suitable for use in
diesel engines, these triglycerides are converted to the respective alkyl esters and glycerol
by base-catalyzed transesterification with short-chain alcohols (generally methanol). Thus,
for every 10 lb of biodiesel produced, approximately 1 lb of glycerol is formed. Glycerol
finds application in a wide range of industries (cosmetics, pharmaceuticals, as a plasticizer,
etc.), although as biodiesel production grows, new uses will have to be developed to avoid
a surplus of glycerol.
Fuel-grade biodiesel must be produced to strict industry specifications (ASTM D6751)
in order to ensure proper performance. Biodiesel is the only alternative fuel to have fully
completed the health effects testing requirements of the 1990 Clean Air Act Amendments;