Environmental Engineering Reference
In-Depth Information
Probably the most pressing disadvantages or shortcomings of biodiesel, at least
at the present time, are its lack of availability or accessibility and high cost relative
to petroleum diesel. This trend in non-availability and non-accessibility is bound to
change as the less expensive, more accessible petroleum diesel becomes more expen-
sive and difficult to find. Additional disadvantages to consider are that biodiesel
requires special handling, storage and transportation management as compared to
petroleum diesel. With regard to environmental considerations, biodiesel produces
more nitrous oxide emissions when combusted than an equal amount of petroleum
diesel. Another potential problem with the production of biodiesel is its dependence
on soybeans as its primary feedstock; there is some concern that the widespread use
of biodiesel as fuel will contribute to higher food prices and indirectly to world hun-
ger. There is a slight reduction in performance and mileage per gallon with biodiesel
as compared to petroleum diesel. Biodiesel can also act as a solvent in some diesel
engines, causing loosened deposits that may clog filters.
BIOGAS (METHANE)
Primarily known as a fuel for interior heating systems, methane or biogas can also
be used as a replacement for natural gas—a fossil fuel for electricity generation and
for cooking and heating—and as an alternative fuel to gasoline. Methane is a natu-
ral gas produced by the breakdown of organic material in the absence of oxygen in
termite mounds and wetlands and by some animals. Humans are also responsible
for the release of methane through biomass burning, rice production, cattle raising,
and releases from gas exploration. Methane can also be obtained directly from the
earth; however, other methods of production have been developed, most notably the
fermentation or composting of plant and animal waste. The reasons for considering
biogas (methane) as a possible biofuel include the following:
•
It is viable because of its potential use as an alternative fuel source.
•
It is a viable alternative fuel to use to improve air quality.
•
It can be produced locally, reducing the need to use imported natural gas.
Methane is produced under anaerobic conditions where organic material is biode-
graded or broken down by a group of microorganisms. The three main sources of
feedstock material for anaerobic digestion are given in
Figure 5.10
and are described
in the following.
a
naeroBic
d
igestion
Anaerobic digestion is the traditional method of managing waste, sludge stabiliza-
tion, and releasing energy. It involves using bacteria that thrive in the absence of
oxygen and is slower than aerobic digestion, but it has the advantage that only a
small portion of the wastes is converted into new bacterial cells. Instead, most of the
organics are converted into carbon dioxide and methane gas.
