Page 29 - Synthetic Fuels Handbook

FUEL SOURCES 17

Biofuel is derived from biomass (recently living organisms or their metabolic byproducts)
(Hudson, 2004) and has the potential to produce fuels that are more environmentally benign
than petroleum-based fuels (American Coalition for Ethanol, 2004). In addition, ethanol,
a crop-based fuel alcohol (Chaps. 8 and 9), adds oxygen to gasoline thereby helping to
improve vehicle performance and reduce air pollution. Biodiesel, an alternative or addi-
tive to petroleum diesel, is a nontoxic, renewable resource created from soybean or other
oil crops (Pacific Biodiesel, 2004). Agricultural products specifically grown for use as
biofuels include crops (Chap. 9) such as corn, soybeans, flaxseed, rapeseed, wheat, sugar
beet, sugar cane, palm oil, and Jatropha (Chap. 10). Biodegradable outputs from industry,
agriculture, forestry, and households can be used as well; examples include straw, timber,
manure, rice husks, sewage, biodegradable waste, and food leftovers. They are converted
to biogas through anaerobic digestion. Biomass used as fuel often consists of underutilized
types, like chaff and animal waste. The quality of timber or grassy biomass does not have
a direct impact on its value as an energy source.
Unlike other forms of renewable energy, biofuels do not reduce the amount of green-
house gases in the atmosphere. The combustion of biofuels produces carbon dioxide and
other greenhouse gases. The carbon in biofuels is often taken to have been recently extracted
from atmospheric carbon dioxide by plants as they have grown. The potential for biofuels
to be considered carbon neutral depends upon the carbon that is emitted being reused by
plants for further growth. Clearly however, cutting down trees in forests that have grown
for hundreds or thousands of years for use as a biofuel, without the replacement of this
biomass would not have a carbon-neutral effect.
It is generally believed that a way to reduce the amount of carbon dioxide released
into the atmosphere is to use biofuels to replace nonrenewable sources of energy. Dried
compressed peat is also sometimes considered a biofuel. However, it does not meet the
criteria of being a renewable biofuel. Though more recent than petroleum or coal, on the
time scale of human industrialization, peat is a fossil fuel and burning it does contribute to
atmospheric carbon dioxide.
The production of biofuels to replace oil and natural gas is in active development, focus-
ing on the use of cheap organic matter (usually cellulose, agricultural and sewage waste)
in the efficient production of liquid and gas biofuels which yield high net energy gain. One
advantage of biofuel over most other fuel types is that it is biodegradable, and so relatively
harmless to the environment if spilled.
Direct biofuels are biofuels that can be used in existing unmodified petroleum engines.
Because engine technology changes all the time, direct biofuel can be hard to define; a
fuel that works well in one unmodified engine may not work in another. In general, newer
engines are more sensitive to fuel than older engines, but new engines are also likely to be
designed with some amount of biofuel in mind.
Straight vegetable oil can be used in many older diesel engines (equipped with
indirect injection systems), but only in the warmest climates. Usually it is turned into
biodiesel instead. No engine manufacturer explicitly allows any use of vegetable oil in
their engines.
Biodiesel can be a direct biofuel. In some countries manufacturers cover many of
their diesel engines under warranty for 100 percent biodiesel use. Many people have run
thousands of miles on biodiesel without problem, and many studies have been made on
100 percent biodiesel.
Butanol is often claimed as a direct replacement for gasoline. It is not in wide spread
production at this time, and engine manufacturers have not made statements about its use.
While on paper (and a few laboratory tests) it appears that butanol has sufficiently similar
characteristics with gasoline such that it should work without problem in any gasoline
engine, no widespread experience exists.