Page 296 - Synthetic Fuels Handbook

282 CHAPTER NINE

presence of low levels of oxygen (i.e., less than required for complete combustion to carbon
dioxide and water). Above certain temperatures the biomass will break down into a gas
stream and a solid residue. The composition of the gas stream is influenced by the operat-
ing conditions for the gasifier, with some gasification processes more suited than others to
producing a gas for methanol production. In particular, simple gasification with air creates
a synthesis gas stream that is diluted with large quantities of nitrogen. This nitrogen is
detrimental to subsequent processing to methanol and so techniques using indirect gasifica-
tion or an oxygen feed are preferred. For large-scale gasification, pressurized systems are
considered to be more economic than atmospheric systems.
Once the economic optimum synthesis gas is available the methanol synthesis takes
place. This typically uses a copper-zinc catalyst at temperatures of 200 to 280°C and pres-
sures of 50 to 100 atm.
The crude methanol from the synthesis loop contains water produced during synthesis
as well as other minor by-products. Purification is achieved in multistage distillation, with
the complexity of distillation dictated by the final methanol purity required.

9.4.2 Propanol and Butanol
Propanol and butanol are considerably less toxic and less volatile than methanol. In par-
ticular, butanol has a high flash point of 35°C (95°F), which is a benefit for fire safety, but
may be a difficulty for starting engines in cold weather.
The fermentation processes to produce propanol and butanol from cellulose are fairly
tricky to execute, and the Clostridium acetobutylicum currently used to perform these con-
versions produces an extremely unpleasant smell, and this must be taken into consider-
ation when designing and locating a fermentation plant. This organism also dies when the
butanol content of whatever it is fermenting rises to 7 percent. For comparison, yeast dies
when the ethanol content of its feedstock hits 14 percent. Specialized strains can tolerate
even greater ethanol concentrations—so-called turbo yeast can withstand up to 16 percent
ethanol. However, if ordinary Saccharomyces yeast can be modified to improve its ethanol
resistance, scientists may yet one day produce a strain of the Weizmann organism with
a butanol resistance higher than the natural boundary of 7 percent. This would be useful
because butanol has a higher energy density than ethanol, and because waste fiber left over
from sugar crops used to make ethanol could be made into butanol, raising the alcohol yield
of fuel crops without there being a need for more crops to be planted.

9.5 BIODIESEL

Biodiesel is a diesel-equivalent fuel derived from biologic sources (such as, vegetable oils)
which can be used in unmodified diesel-engine vehicles. It is thus distinguished from the
straight vegetable oils or waste vegetable oils used as fuels in some diesel vehicles. In the
current context, biodiesel refers to alkyl esters made from the transesterification of veg-
etable oils or animal fats.
Biodiesel fuel is made from the oil of certain oilseed crops such as soybean, canola,
palm kernel, coconut, sunflower, safflower, corn and hundreds of other oil producing crops.
The oil is extracted by the use of a press and then mixed in specific proportions with
other agents which cause a chemical reaction. The results of this reaction are two products,
biodiesel and soap. After a final filtration, the biodiesel is ready for use. After curing, the
glycerin soap which is produced as a by-product can be used as is, or can have scented oils
added before use.