How to build a 100 cu.ft. day methane gas plant.
KIM F. ZARNEY
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 refinable oil within (are you ready for this?) 30 years.
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 decomposition 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.
The Gobar (Hindi for "cow dung") Gas Research Station—established in 1960 as the latest of along series of Indian experimental projects dating back to the 1930's—has concentrated its efforts, as the name suggests, on generating methane gas from cow manure. At the station, Ram Bux Singh and his coworkers have designed and put into operation bio-gas plants ranging in output from 100 to 9,000 cubic feet of methane a day. They've installed heating coils, mechanical agitators and filters in some of the generators and experimented with different mixes of manure and vegetable wastes. Results of the project have been meticulously documented and recorded.
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.
Ram Bun Singh has compiled much of this information into two booklets, BIO-GAS PLANT and SOME EXPERIMENTS WITH BIO-GAS. The following information has been adapted, by permission, from the handbooks:
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 dioxide, 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 methane, 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.
Anaerobic digestion of waste material will occur at temperatures ranging from 32° to 156° F. The action of the bacteria responsible for the fermentation decreases rapidly below 60° F, however, and gas production is most rapid at 85-105° and 120-140° F. Different bacteria thrive in the two ranges and those active within the higher limits are much more susceptible to environmental changes. Thus, a temperature of 90° to 95° F. is the most nearly ideal for stable methane gas generation.
The proper pH range for anaerobic fermentation is between 6.8 and 8.0 and an acidity either higher or lower than this will hamper fermentation. The introduction of too much raw material can cause excess acidity (a too-low pH reading) and the gas-producing bacteria will not be able to digest the acids quickly enough. Decomposition will stop until balance is restored by the growth of more bacteria. If the pH grows too high (not enough acid), fermentation will slow until the digestive process forms process forms enough acidic carbon dioxide to restore balance.
Although bacteria responsible for the anaerobic process require both elements in order to live, they consume carbon about 30 to 35 times faster than they use nitrogen. Other conditions being favorable, then, anaerobic digestion will proceed most rapidly when raw material fed into a goba plant contains a carbon-nitrogen ratio of 30-1. If the ratio is higher, the nitrogen will be exhausted while there is still a supply of carbon left. This causes some bacteria to die, releasing the nitrogen in their cells and eventually restoring equilibrium. Digestion proceeds slowly as this occurs. On the other hand, if there is too much nitrogen, fermentation (which will stop when the carbon is exhausted) will be incomplete and the "left over" nitrogen will not be digested. This lowers the fertilizing value of the slurry. Only the proper ratio of carbon to nitrogen will insure conversion of all available carbon to methane and carbon dioxide with minimum loss of available nitrogen.
Percentage of Solids
The anaerobic decay of organic matter proceeds best if the raw material consists of about 7 to 9 percent solids. Fresh cow manure can be brought down to approximately this consistency by diluting it with an equal amount of water.
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.
Batch Feeding (Solids)
Bio-gas plants may be designed to digest vegetable wastes alone but, since plant matter will not flow easily through pipes, it's best to operate such a digester on a single-batch basis. With this method the tank is opened completely, old slurry removed and fresh material added. The tank is then resealed.
Depending on the fermenting material and temperature, gas production from a batch-feeding will begin after two to four weeks, gradually increase to a maximum output and then fall off after about three or four months. It's best, therefore, to use two or more batch digesters in combination so that at least one will always be producing gas.
Because the carbon-nitrogen ratio of some vegetable matter is much higher than that of animal wastes, some nitrogen (preferably of organic origin) usually must be added to the cellulose digested this way. On the other hand, vegetable waste produces—pound for pound—about seven times more gas than animal waste, so proportionally less must be digested to maintain equal gas production.
Some means of mixing the slurry in a digester is always desirable, though not absolutely essential. If left alone, the slurry tends to settle out in layers and its surface may be covered with a hard scum which hinders the release of gas.
This is a greater problem with vegetable matter than with manure, since the animal waste has a somewhat greater tendency to remain suspended in water and, thus, in intimate contact with the gas releasing bacteria. Continuous feeding also helps, since fresh material entering the tank always induces some movement in the slurry.
Although it's relatively easy to hold the temperature of a digester at ideal operating levels by shading a gobar plant located in a hot region, maintaining the same ideal temperature in a cold climate is somewhat more difficult.
The first and most obvious provision, of course, is insulating the tank with a two or three-foot thick layer of straw or similar material that is, in turn, protected with a waterproof seal. If this proves insufficient, the addition of heating coils must be considered.
When hot water is regulated by a thermostat and circulated through coils built into a digester, the fermenting process may be kept at an efficient gas producing temperature quite easily. In fact, circulation only for a couple of hours in the morning and again in the evening should be sufficient in most climates. It is especially interesting to note that using a portion of the gas generated to heat the water is entirely feasible . . . the resulting enormously-increased rate of gas production more than compensates for the gas thus burned .
Methane 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.
Above vs Below Ground Digesters
Gobar tanks built above ground must be made of steel to withstand the pressure of the slurry and it's simpler and less expensive to construct underground methane plants. It's also easier to gravity-feed a tank built at least partially beneath the earth's surface. On the other hand, above-surface models are easier to maintain and, if painted black, may be partially heated by solar radiation.
Click on the image gallery link at the top of the article for a graphic of the instructions for an underground, single-stage, double-chamber plant, "How to Build a 100 cu. ft./day Methane Gas Plant".
These brief excerpts from Ram Bux Singh's books should make it obvious that methane gas production from manure and vegetable waste is no armchair visionary's dream. It's being done right now and over 2,500 gobar plants are currently operating in India alone.
Here, in the U.S. our more than four hundred million cattle, pigs and chickens produce over two billion tons of manure a year . . . enough to spread four feet deep over an area of five hundred square miles! This valuable natural resource can be used to generate both combustible gas — thus relieving part of our reliance on fossil fuels — and a fertilizer richer in nitrogen than raw manure.
Instead of contributing mightily to our water pollution crisis as feedlot runoff, this bountiful end-product of animal life could be turned to our advantage . . . as an economical and ecologically-sound power source!
Facts About Gobar* Gas
- Cow dung gas is 55-65% methane, 30-35% carbon dioxide, 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), 1.01.2%
phosphorus (P 2 0 5 ), 0.6-0.8% potassium (K 2 0) and from 50-75%
- 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
- 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 horsepower per hour.
*Hindi for "cow dung"