Page 258 - Synthetic Fuels Handbook

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in constructing hydrogen collection and transport channels of low cost. A second pathway
uses plant material such as agricultural residues in a fermentation process leading to biogas
from which the desired fuels can be isolated. This technology is established and in widespread
use for waste treatment, but often with the energy produced only for on-site use, which often
implies less than maximum energy yields. Finally, high-temperature gasification supplies a
crude gas, which may be transformed into hydrogen by a second reaction step. In addition to
biogas, there is also the possibility of using the solid by-product as a biofuel.
The technologies for gas production from biomass include: (a) fermentation, (b) gasifi-
cation, and (c) direct biophotolysis.

8.4.2 Liquid Fuels

Generally, liquid fuels are those fuels which flow readily under ambient conditions.
However, for the present purpose, liquid fuels also include those fuels that flow with dif-
ficulty under ambient conditions but will flow to the fuel chamber in heated pipes.

Alcohols. Alcohols are oxygenate fuels insofar as the alcohol molecule has one or more
oxygen, which decreases to the combustion heat (Minteer, 2006, Chap. 1). Practically, any
of the organic molecules of the alcohol family can be used as a fuel. The alcohols which can
be used for motor fuels are methanol (CH OH), ethanol (C H OH), propanol (C H OH), and
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butanol (C H OH). However, only methanol and ethanol fuels are technically and economi-
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cally suitable for internal combustion engines (Bala, 2005).
Ethanol (ethyl alcohol, CH CH OH), also referred to as bioethanol, is a clear, color-
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less liquid with a characteristic, agreeable odor. Currently, the production of ethanol by
fermentation of corn-derived carbohydrates is the main technology used to produce liquid
fuels from biomass resources (McNeil Technologies Inc., 2005). Furthermore, amongst
different biofuels, suitable for application in transport, bioethanol and biodiesel seem to
be the most feasible ones at present. The key advantage of bioethanol and biodiesel is that
they can be mixed with conventional petrol and diesel respectively, which allows using the
same handling and distribution infrastructure. Another important strong point of bioethanol
and biodiesel is that when they are mixed at low concentrations (= 10 percent bioethanol in
petrol and = 20 percent biodiesel in diesel), no engine modifications are necessary.
Ethanol can be blended with gasoline to create E85, a blend of 85 percent ethanol and
15 percent gasoline. E85 and blends with even higher concentrations of ethanol, E95; pure
bioethanol (E100-fuel) has been used mainly in Brazil (Davis, 2006; Minteer, 2006, Chap. 7).
More widespread practice has been to add up to 20 percent to gasoline (E20-fuel or gasohol)
to avoid engine changes.
Ethanol has a higher octane number (108), broader flammability limits, higher flame
speeds, and higher heats of vaporization than gasoline. These properties allow for a higher
compression ratio, shorter burn time, and leaner burn engine, which lead to theoretical
efficiency advantages over gasoline in an internal combustion engine.
On the other hand, the disadvantages of ethanol include its lower energy density than
gasoline, its corrosiveness, low flame luminosity, lower vapor pressure, miscibility with
water, and toxicity to ecosystems.
Ethanol from cellulosic biomass materials (such as agricultural residues, trees, and
grasses) is made by first using pretreatment and hydrolysis processes to extract sugars,
followed by fermentation of the sugars. Although producing ethanol from cellulosic bio-
mass is currently more costly than producing ethanol from starch crops, several countries
(including the United States) have launched biofuels initiatives with the objective of the
economic production of ethanol from biosources. Researchers are working to improve
the efficiency and economics of the cellulosic bioethanol production process.