Page 31 - Synthetic Fuels Handbook

FUEL SOURCES 19

Methanol is the lowest molecular weight and simplest alcohol, produced from the natu-
ral gas component methane. It is also called methyl alcohol or wood alcohol, the latter
because it was formerly produced from the distillation of wood.
Ethanol, also known as grain alcohol or ethyl alcohol, is most commonly used in alco-
holic beverages. However, it may also be used as a fuel, most often in combination with
gasoline. For the most part, it is used in a gasoline-to-ethanol ratio of 9:1 to reduce the nega-
tive environmental effects of gasoline. Ethanol can be readily produced by fermentation
of simple sugars that are converted from starch crops. Feedstocks for such fermentation
ethanol include corn, barley, potato, rice, and wheat. This type of ethanol may be called
grain ethanol, whereas ethanol produced from cellulose biomass such as trees and grasses
is called bioethanol or biomass ethanol. Both grain ethanol and bioethanol are produced via
biochemical processes, while chemical ethanol is synthesized by chemical synthesis routes
that do not involve fermentation.
There is increasing interest in the use of a blend of 85 percent fuel ethanol with
15 percent gasoline. This fuel blend called E85 has a higher fuel octane than premium
gasoline allowing in properly optimized engines increase both power and fuel economy
over gasoline.
Butanol, also known as butyl alcohol, may be used as a fuel with the normal combustion
engine, typically as a product of the ferment of biomass with the bacterium Clostridium
acetobutylicum.
The advantages of butanol are the high octane rating (over 100) and high energy con-
tent, only about 10 percent lower than gasoline, and subsequently about 50 percent more
energy-dense than ethanol, 100 percent more so than methanol. The major disadvantage of
butanol is the high flashpoint (35°C, 95°F).

1.3 SYNTHETIC FUELS

The production of synthetic fuels is essentially a hydrogen-addition process. Fuels such as gaso-
line and natural gas have an atomic hydrogen/carbon ratio on the order of 2.0 while the
fuel sources have lower atomic hydrogen/carbon ratio (on the order of 1.0 to 1.5). The
source of hydrogen can be intramolecular in which a carbonaceous low-hydrogen residue
(e.g., coke) is produced or intermolecular in which hydrogen is added from an external
source.
On the one hand, that is, intramolecular hydrogenation, pyrolysis of the feedstock in the
absence of any added agent produces volatile (high-hydrogen) products and a nonvolatile
(low-hydrogen) coke. In pyrolysis the carbon content is reduced by heating the raw hydro-
carbon until it thermally decomposes to yield solid carbon, together with gases and liquids
having higher fractions of hydrogen than the original material.
On the other hand, that is, hydrogenation from an external source, the hydrogenation is
either direct or indirect. Direct hydrogenation involves exposing the raw material to hydro-
gen at high pressure. Indirect hydrogenation involves reaction of the feedstock with steam,
and the hydrogen is generated within the system.
Thus, gaseous or liquid synthetic fuels are obtained by converting a carbonaceous material
to a gaseous or liquid form, respectively. In the United States and many other countries,
the most abundant naturally occurring materials suitable for this purpose are coal and oil
shale. Tar sands are also suitable, and large deposits are located in Canada. The conversion
of these raw materials is carried out to produce synthetic fuels to replace depleted, unavailable,
or costly supplies of natural fuels. However, the conversion may also be undertaken to remove
sulfur or nitrogen that would otherwise be burned, giving rise to undesirable air pollutants.
Another reason for conversion is to increase the calorific value of the original raw fuel by