Page 292 - Synthetic Fuels Handbook

278 CHAPTER NINE

water adjusts the proportion of solids in the manure slurry to the optimal consistency. The
digester is a long, rectangular container, usually built below-grade, with an airtight, expand-
able cover.
New material added to the tank at one end pushes older material to the opposite end.
Coarse solids in ruminant manure form a viscous material as they are digested, limiting sol-
ids separation in the digester tank. As a result, the material flows through the tank in a plug.
Average retention time (the time a manure plug remains in the digester) is 20 to 30 days.
Anaerobic digestion of the manure slurry releases biogas as the material flows through the
digester. A flexible, impermeable cover on the digester traps the gas. Pipes beneath the
cover carry the biogas from the digester to an engine-generator set.
A plug-flow digester requires minimal maintenance. Waste heat from the engine-generator
can be used to heat the digester. Inside the digester, suspended heating pipes allow hot water
to circulate. The hot water heats the digester to keep the slurry at 25 to 40°C (77 to 104°F),
a temperature range suitable for methane-producing bacteria. The hot water can come from
recovered waste heat from an engine generator fueled with digester gas or from burning digester
gas directly in a boiler.

9.3 ETHANOL

Ethanol is the predominant fuel produced form crops and has been used as fuel in the
United States since at least 1908. Although early efforts to sustain an ethanol program
failed, oil supply disruptions in the Middle East and environment concerns over the use of
lead as a gasoline octane booster renewed interest in ethanol in the late 1970s. At present,
extending the volume of conventional gasoline is a significant end use for ethanol, as is
its use as an oxygenate. To succeed in these markets, the cost of ethanol must be close to
the wholesale price of gasoline, currently made possible by the federal ethanol subsidy.
However, in order for ethanol to compete on its own merits the cost of producing it must
be reduced substantially.
The production of ethanol from corn is a mature technology that holds much poten-
tial (Nichols et al., 2006). Substantial cost reductions may be possible, however, if
cellulose-based feedstocks are used instead of corn. Producers are experimenting with
units equipped to convert cellulose-based feedstocks, using sulfuric acid to break down
cellulose and hemicellulose into fermentable sugar. Although the process is expensive
at present, advances in biotechnology could decrease conversion costs substantially.
The feed for all ethanol fermentations is sugar—traditionally a hexose (a six-carbon
or “C6” sugar) such as those present naturally in sugar cane, sugar beet, and molasses.
Sugar for fermentation can also be recovered from starch, which is actually a polymer
of hexose sugars (polysaccharide).
Biomass (Chap. 8), in the form of wood and agricultural residues such as wheat straw,
is viewed as a low cost alternative feed to sugar and starch. It is also potentially available
in far greater quantities than sugar and starch feeds. As such it receives significant atten-
tion as a feed material for ethanol production. Like starch, wood and agricultural residues
contain polysaccharides. However, unlike starch, while the cellulose fraction of biomass is
principally a polymer of easily fermented six-carbon sugars, the hemicellulose fraction is
principally a polymer of five-carbon sugars, with quite different characteristics for recovery
and fermentation the cellulose and hemicellulose in biomass are bound together in a com-
plex framework of crystalline organic material known as lignin.
These differences mean that recovery of these biomass sugars is more complex than
recovery of sugars from a starch feed. Once recovered, fermentation is also more complex
than a simple fermentation of six-carbon sugars. The current focus is on the issues of releasing