Alternative Fuel Sources - Hydrogen and Fuel Cells: Advances in Transportation and Power - page 83

grains require processing to convert the starch to sugar. This is done by en-
zymes. Biodiesel fuel is derived from vegetable oils or waste animal fats.
It is a renewable, clean burning fuel that can be used for diesel engines or
oil heating.
When biomass is transformed into energy by burning, it releases
CO 2 that was previously sequestered or held in the atmosphere, for some
time, so the net CO 2 emitted is zero. Biomass provides the potential of a
sustainable way of providing energy.
BIOMASS PROCESSES
Plants create energy through photosynthesis using solar radiation
and converting carbon dioxide and water into energy crops. Technology
allows us to take that energy and transform it through a variety of process-
es for our uses. The three basic types of bioenergy conversion are direct
combustion, thermochemical conversion, and biochemical conversion.
The direct combustion of wood and other plant matter has been a
primary energy source in the past. Any type of biomass can be burned to
produce heat or steam to turn a generator or perform mechanical work.
Direct combustion is used in large power plants that produce up to 400
megawatts. Most direct combustion systems can use any type of biomass
as long as the moisture content is less than 60%. Wood and wood residues
are commonly used along with a number of other agricultural residues.
Bioenergy can be derived from biomass products such as energy
crops, forestry and crop residues and even refuse. One characteristic of
these biofuels is that three fourths or more of their energy is in the volatile
matter or vapors, unlike coal, where the fraction is usually less than half.
It is important that the furnace or boiler ensure that these vapors burn and
are not lost. For complete combustion, air must reach all the char, which
is achieved by burning the fuel in small particles. This finely-divided fuel
means finely-divided ash particulates which must be removed from the
flue gases. The air flow should be controlled. Too little oxygen means in-
complete combustion and leads to the production of carbon monoxide. Too
much air is wasteful since it carries away heat in the flue gases. Modern
systems for burning biofuels include large boilers with megawatt outputs
of heat.
Direct combustion is one way to extract the energy contained in
household refuse, but its moisture content tends to be high at 20% or