Alternative Fuel Sources - Hydrogen and Fuel Cells: Advances in Transportation and Power - page 85

monoxide and dioxide. Smaller amounts of methane, ethane, and other
hydrocarbons are also produced. The solids left are carbon and ash. The
liquids are similar to crude oil and must be refined before they can be used
as fuels.
In liquefaction systems wood and wood wastes are the most com-
mon fuelstocks. They are reacted with steam or hydrogen and carbon
monoxide to produce liquids and chemicals. The chemical reactions that
take place are similar to gasification but lower temperatures and higher
pressure are used. Liquefaction processes can be direct or indirect. The
product from liquefaction is pyrolytic oil which has a high oxygen con-
tent. It can be converted to diesel fuel, gasoline or methanol.
BIOCHEMICAL CONVERSION/FERMENTATION
Biochemical conversion, or bioconversion, is a chemical reaction
caused by treating moist biomass with microorganisms such as enzymes
or fungi. The end products may be liquid or gaseous fuels. Anaerobic di-
gestion and fermentation are the two processes used for biochemically
converting biomass to energy.
Anaerobic digestion involves limiting the air to moist biomass such
as sewage sludge, MSW, animal waste, kelp, algae, or agricultural waste.
The feed stock is placed in a reaction vessel with bacteria. As the bacteria
break down the biomass, they create a gas that is 50 to 60% methane. Small
scale digesters are used on Asian and European farms. Sewage treatment
plants use this process to generate methane and digesters are used to com-
post municipal organic waste. Anaerobic systems range from large sys-
tems that can handle 400,000 cubic feet of material and produce 1.5 million
cubic feet of biogas per day to small systems that handle 400 cubic feet of
material and produce 6,000 cubic feet of biogas a day.
Fermenting grains with yeast produces a grain alcohol. The process
also works with other biomass feedstocks. In fermentation, the yeast de-
composes carbohydrates which are starches in grains, or sugar from sugar
cane juice into ethyl alcohol (ethanol) and carbon dioxide. The process
breaks down complex substances into simpler ones.
Microalgae and oilseed crops can provide diesel fuel. The use of
these alcohol fuels can reduce air pollution. Methane made from anaero-
bically digested manure was used to light streets in England as early as
1895. Anaerobic digestion is also used to produce fertilizers.