Bio Gas-Source for Cheap Energy.

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Bio Gas-Source for Cheap Energy.

Post by khandairies on Wed Sep 16, 2009 9:39 pm

Bio Gas-produce your own Energy

Energy conservation, coupled with concern for the management of
livestock wastes, has revived an interest in generating methane from livestock
Converting organic materials, such as animal wastes, to an easily used
form of energy can be accomplished by several methods. The process with the
greatest potential is anaerobic fermentation or digestion.
The extraction of energy from wastes using anaerobic digestion to
produce bio-gas is not new and the general technology is well known. Bio-gas,
which is methane and other gases, has been known as swamp gas, sewer gas and
fuel gas. Sewage treatment plants generate bio-gas from the sewage sludge as
part of the sewage treatment processes. Many small units were used in Europe
and India after World War II.
Characteristics of Bio-Gas
Bio-gas usually contains about 60 to 70 percent methane, 30 to 40
percent carbon dioxide, and other gases, including ammonia, hydrogen sulfide,
mercaptans and other noxious gases. It also is saturated with water vapor.
The heat value of the raw gas at typical Colorado atmospheric pressures
is about 400 to 600 British thermal units (Btu) per cubic foot. In comparison,
natural gas has a heat value of 850 Btu per cubic foot and gasoline contains
approximately 120,000 Btu per gallon. Partial removal of the impurities may be
required. This is not necessarily difficult, but it does complicate the system.
Basic Digester Process
Methane is produced by bacteria. The bacteria are anaerobes and operate
only in anaerobic environments (no free oxygen). Constant temperature, pH and
fresh organic matter promote maximum methane production. Temperatures
usually are maintained at approximately 95 degrees F. Other temperatures can be
used if held constant. For each 20 degrees F decrease, gas production will be cut
approximately one half or will take twice as long. A constant temperature is
critical. Temperature variations of as little as 5 degrees F can inhibit the methaneformers
enough to cause acid accumulation and possible digester failure.
Anaerobic digestion is a two-part process and each part is performed by
a specific group of organisms. The first part is the breakdown of complex organic
matter (manure) into simple organic compounds by acid-forming bacteria. The
second group of microorganisms, the methane-formers, break down the acids
into methane and carbon dioxide. In a properly functioning digester, the two
groups of bacteria must balance so that the methane-formers use just the acids
produced by the acid-formers.
Total bio-gas production varies depending on the organic material
digested, the digester loading rate, and the environmental conditions in the
digester. Under ideal conditions (95 degrees F temperature and proper pH), it is
possible to produce about 45 cubic feet of gas at atmospheric pressure from one
day’s manure from a 1,000 pound cow. Not all of the bio-gas energy is available
for use. Energy is required to heat and mix the digester, pump the effluent, and
perhaps compress the gas. Table 2 summarizes the estimated gas production from
various animal wastes.
Basic Elements
A well-insulated, three-bedroom home takes
about 900,000 Btu per day for heating during cold
weather. Because 50 percent of the bio-gas goes back into
maintaining the necessary temperature of the digester, it
would take the manure from 50 cows to produce enough
bio-gas each day for home heating.
Bio-gas is produced on a relatively constant basis.
Most applications are somewhat intermittent; therefore,
storage is required. The amount of storage depends on the
storage time and pressure. High demand applications,
such as grain drying, normally are impractical due to the
excessive storage capacity required.
Methane in a concentration of 6 to 15 percent
with air is an explosive mixture. Since it is lighter than
air, it will collect in rooftops and other enclosed areas. It
is relatively odorless and detection may be difficult.
Extreme caution and special safety features are necessary
in the digester design and storage tank, especially if the
gas is compressed.

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