Alternative Fuel Sources - Hydrogen and Fuel Cells: Advances in Transportation and Power - page 84

more and its energy density is low. A cubic meter contains less than
1/30th of the energy of the same volume of coal. Refuse-derived fuel
(RDF) refers to a range of products resulting from the separation of un-
wanted components, shredding, drying and treating of raw material to
improve its combustion properties. Relatively simple processing can
involve separation of large items, magnetic extraction of ferrous met-
als and rough shredding. The most fully processed product is known as
densified refuse-derived fuel (d-RDF). It is the result of separating out
the combustible part which is then pulverized, compressed and dried to
produce solid fuel pellets with about 60% of the energy density of coal.
THERMOCHEMICAL CONVERSION PROCESSES
Thermochemical conversion processes use heat in an oxygen con-
trolled environment that produce chemical changes in the biomass.
The process can produce electricity, gas, methanol and other products.
Gasification, pyrolysis, and liquefaction are thermochemical methods for
converting biomass into energy.
Gasification involves partial combustion to turn biomass into a mix-
ture of gases. Gasification processes may be direct or indirect. The di-
rect processes uses air or heat to produce partial combustion in a reactor.
Indirect processes transfer the heat to a reactor its walls using heat ex-
changers or hot sand. This process produces low- or medium-Btu gases
from wood and wood wastes, agricultural residues and MSW. Processing
these synthetic gases with water can produce ammonia, methanol, or hy-
drogen. Commercial gasification systems exist, but their widespread use
has been limited by hauling distances for the feedstock.
PYROLYSIS AND LIQUEFACTION
Pyrolysis is a type of gasification that breaks down the biomass in
oxygen deficient environments, at temperatures of up to 400°F. This pro-
cess is used to produce charcoal. Since the temperature is lower than oth-
er gasification methods, the end products are different. The slow heating
produces almost equal proportions of gas, liquid and charcoal, but the
output mix can be adjusted by changing the input, the temperature, and
the time in the reactor. The main gases produced are hydrogen and carbon