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BIOGAS CONTRIBUTES TO THE "TRIPLE BOTTOM LINE"

          1. Benefiting the environment
          2. Reducing costs
          3. Contributing to the social structure

Biogas is Not New

People have been using biogas for over 200 years. In the days before electricity, biogas was drawn from the underground sewer pipes in London and burned in street lamps, which were known as "gaslights." In many parts of the world, biogas is used to heat and light homes, to cook, and even to fuel buses. It is collected from large-scale sources such as landfills and pig barns, and through small domestic or community systems in many villages

"Traditionally, biogas is produced from dung. Around 40 kg of dung needs to be fermented for 40 days to produce 250 grams of methane. Due to this highly inefficient process, biogas has not emerged as an alternative to LPG. However, the CARE  system requires just 1 kg of waste or equivalent in biodegradable substance to be fermented for 24 hours, to produce the same amount of methane." It is this efficiency and uniqueness, as compared to other biogas

There are ways you can contribute to the environment even as you stand in the kitchen. Rather than throw the banana peels or spinach stems or other biodegradable waste into the dustbin, you could toss them into the Compact Biogas Plant to create pollution-free methane gas and, in effect, help conserve planet earth's depleting fossil fuels

 It's been a wild, exciting ride... but our blindly wasteful squandering of the planet's fossil fuels will soon be a thing of the past. In the United States alone (the worst example, perhaps, but not really unusual among "modern" nations), every man, woman and child consumes an average of three gallons of oil each day. That's well over two hundred billion gallons a year.

If we continue burning off petroleum at only this rate -- which isn't very likely since population is climbing and the big oil companies remain chained to "sell-more-tomorrow" economics -- experts predict the world will run out of refine able oil within (are you ready for this?in 30 years.

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So where does that leave us? 

Well, number one, we obviously must get serious about population control and per capita consumption of power and, number two, if we don't want to see brownouts and rationing of the power we do use, we'd better start looking around for ecologically-sound alternative sources of energy.

And there are alternatives. One potent reservoir that's hardly been tapped is methane gas.

Hundreds of millions of cubic feet of methane -- sometimes called "swamp" or bio-gas -- are generated every year by the de- composition of organic material. It's a near-twin of the natural gas that big utility companies pump out of the ground and which so many of us use for heating our homes and for cooking. Instead of being harnessed like natural gas, however, methane has traditionally been considered as merely a dangerous nuisance that should be gotten rid of as fast as possible. Only recently have a few thoughtful men begun to regard methane as a potentially revolutionary source of controllable energy.

And with good reason. Population pressure has practically eliminated India's forests, causing desperate fuel shortages in most rural areas. As a result, up to three-quarters of the country's annual billion tons of manure (India has two cows for every person) is burned for cooking or heating. This creates enormous medical problems -- the drying dung is a dangerous breeding place for flies and the acrid smoke is responsible for widespread eye disease -- and deprives the country's soil of vital organic nutrients contained in the manure.

  • Cow dung gas is 55-65% methane, 30-35% carbon di- oxide, with some hydrogen, nitrogen and other traces. Its heat value is about 600 B.T.U.'s per cubic foot.
  • A sample analyzed by the Gas Council Laboratory at Watson House in England contained 68% methane, 31% carbon dioxide and 1% nitrogen. It tested at 678 B.T.U.
  • This compares with natural gas's 80% methane, which yields a B.T.U. value of about 1,000.
  • Gobar gas may be improved by filtering it through limewater (to remove carbon dioxide), iron filings (to absorb corrosive hydrogen sulphide) and calcium chloride (to extract water vapor).
  • Cow dung slurry is composed of 1.8-2.4% nitrogen (N), 1.0-1.2/a phosphorus (P2O5), 0.6-0.8% potassium (K2O) and from 50-75% organic humus.
  • About one cubic foot of gas may be generated from one pound of cow manure at 75 F. This is enough gas to cook a day's meals for 4-6 people.
  • About 225 cubic feet of gas equals one gallon of gasoline. The manure produced by one cow in one year can be converted to methane which is the equivalent of over 50 gallons of gasoline.
  • Gas engines require 18 cubic feet of methane per horse- power per hour. *Hindi for "cow dung"

This comprehensive eleven-year-long research program has yielded designs for five standardized, basic gobar plants that operate efficiently under widely varying conditions with only minor modifications (see construction details of 100 cubic foot digester that accompany this article)... and a treasure trove of specific, field-tested principles for methane gas production.

There are two kinds of organic decomposition: aerobic (requiring oxygen) and anaerobic (in the absence of oxygen). Any kind of organic material -- animal or vegetable -- may be broken down by either process, but the end-products will be quite different. Aerobic fermentation produces carbon di- oxide, ammonia, small amounts of other gases, considerable heat and a residue which can be used as fertilizer. Anaerobic decomposition -- on the other hand -- creates combustible meth- ane, carbon dioxide, hydrogen, traces of other gases, only a little heat and a slurry which is superior in nitrogen content to the residue yielded by aerobic fermentation. 

Anaerobic decomposition takes place in two stages as certain micro-organisms feed on organic materials. First, acid- producing bacteria break the complex organic molecules down into simpler sugars, alcohol, glycerol and peptides. Then -- and only when these substances have accumulated in sufficient quantities -- a second group of bacteria converts some of the simpler molecules into methane. The methane-releasing microorganisms are especially sensitive to environmental conditions.

BASIC DESIGN 

Central to the operation and common to all gobar plant designs' is an enclosed tank called a digester. This is an airtight tank which may be filled with raw organic waste and from which the final slurry and generated gas may be drawn. Differences in the design of these tanks are based primarily on the material to be fed to the generator, the cycle of fermentation desired and the temperatures under which the plant will operate.

Tanks designed for the digestion of liquid or suspended- solid waste (such as cow manure) are usually filled and emptied with pipes and pumps. Circulation through the digester may also be achieved without pumps by allowing old slurry to overflow the tank as fresh material is fed in by gravity. An advantage of the gravity system is its ability to handle bits of chopped vegetable matter which would clog pumps. This is quite desirable, since the vegetable waste provides more carbon than the nitrogen-rich animal manure.

CONTINUOUS FEEDING (LIQUIDS) 

Complete anaerobic digestion of animal wastes, such as cow manure, takes about fifty days at moderately warm temperatures. Such matter -- if allowed to remain undisturbed for the full period -- will produce more than a third of its total gas the first week, another quarter the second week and the remainder during the final six weeks.

A more consistent and rapid rate of gas production may be maintained by continuously feeding small amounts of waste into the digester daily. The method has the additional advantage of preserving a higher percentage of the nitrogen in the slurry for effective fertilizer use.

If this continuous feeding system is used, care must be taken to insure that the plant is large enough to accommodate all the waste material that will be fed through in one fermentation cycle. A two-stage digester -- in which the first tank produces the bulk of the methane (up to 80%) while the second finishes the digestion at a more leisurely rate -- is often the answer.

GAS COLLECTION 

Gas is collected inside an anaerobic digester tank in an inverted drum. The walls of this upside down drum extend down into the slurry, forming a "cap" which both seals in the gas and is free to rise and fall as more or less gas is generated.

The drum's weight provides the pressure which forces the gas to its point of use through a small valve in the top of the cap. Drums on larger plants must be counter-weighted to keep them from exerting too much pressure on the slurry. Care must also be taken to insure that such a cap is not counter-weighted to less than atmospheric pressure, since this would allow air to travel backwards through the exhaust line into the digester with two results: destruction of the anaerobic conditions inside the tank and possible destruction of you by an explosion of the methane-oxygen mixture.

The radius of an inverted drum should never be less than three inches smaller than the radius of the tank in which it floats, so that minimal slurry is exposed to the air and maximum gas is captured.

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copyright 2007 @ Centre for Application of Renewable Energy
email : care@care-india.com