DURING
early 1970s the government decided to give thrust to the generation of gobar
gas (renamed after 1982 as biogas) by anaerobic digestion of gobar (cow dung)
for meeting fuel needs of rural population. This mode was expected to bring
economic uplift and social welfare of the rural sector. Both family size and
community size models were launched and appropriately subsidised. The task for
the development and propagation of the mode was entrusted to PAU and the
Department of Agriculture, Punjab.
During the
initial 17 years (up to 1988) after the launch of the project, the total
number of biogas plants built were 16,878. The September 1988 heavy rain and
consequent floods damaged more than 3,000 biogas plant which needed total
rebuilding.
In March,
1990, an assessment of the community-size biogas plant (CBPs) was carried out
and it was found that till then 120 CBPs were sanctioned and out of which only
68 had been completed and commissioned. The performance efficiency of the 68
commissioned since was eight CBPs "closed" or "non-functional", 15 operating
at "connection capacity (CC)" of less than 25 per cent, 43 operating at the CC
of 50 per cent. For this plight of the CBPs, as obtained, not only the poor
management but also non-existence of suitable infrastructure needed for
technology improvement, were considered to be responsible. Thereafter, there
has not been any significant change in the situation even though the
"management of CBPs" was passed on to PEDA (Punjab Energy Development Agency).
Now the
Punjab Government was to put a CBP at Kaljharani village in Bathinda district
which would be part of the total rural employment project. For the project at
present Rs 50 lakh has been earmarked and the project comprises setting up a
modern dairy complex and a centralised solar power plant. The CBP will be fed
by the dung resulting from the dairy complex. It is hoped PEDA will put up the
CBP of improved technology than hitherto adopted for the CBPs so that it gives
the targeted performance. It is proposed to examine the improved models which
have been found successful in India.
Biogas
technology
Anaerobic digestion of cow dung is essentially a two-phase process. In the
first phase, the acid-forming bacteria hydrolyse and ferment the organic
compounds (such as carbohydrates, lipids, proteins, etc.) to form organic
acids, alcohols and gases of carbondioxide, nitrogen and traces of hydrogen
sulphide. The environment in which the above activity occurs is "acidic" with
pH ranging from 5.1 to 6.8 and bulk of digestion resulting in the reduction in
volume takes place during this phase. The bacteria of this phase are "less
sensitive to changes in temperatures".
In the
second phase, a "consortium of bacteria" work upon the organic acids produced
during the earlier phase to produce methane gas. This group of bacteria is
relatively slow performing and gives optimum performance i.e. amount of
methane gas production when the prevailing temperature is around 30°C and the
environment alkaline (pH ranging from 7.2 to 7.4). If there is rise or fall of
temperature more than 10°C from the optimum value listed above, the bacteria
stop working and methane gas production stops altogether.
In the
model adopted for the CBPs so far i.e. the KVIC (Khadi Village Industries
Corporation) Model, which comprises a masonry tank to accommodate cow dung
slurry, and a floating steel cylinder for storage of biogas produced. The set
up provides anaerobic digestor with Œdetention period‚ of around 60 days. In
this model both phases take place in one vessal. The gas produced is at low
pressure and theoretically should have mix ratio of methane (60 per cent) and
carbondioxide (40 per cent). The peak efficiency of the "best designed and
operated model" seldom exceeds 40 per cent. The effluent from the digestor is
black in colour, having offensive odour and large percentage of moisture (more
than 80 per cent) and it is very difficult to remove water from the slurry or
its "drying" in pits. The methane gas content has been found to vary with feed
material and also during a day. It may go down to 50 per cent even. The
chlorofic value of the gas has been found to be around 650 BTUs per ft3 or
5735 K Cal per m3.
As already
mentioned, both phases are accommodated in a single chamber and it is presumed
that in most of the area and for most of the time alkaline environment is
obtained and acidic activity is limited near the inlet of the cow-dung feed or
slurry. Each time fresh slurry is added a shock results in the alkaline
environment. So it is essential to carefully regulate the frequency of loading
(to once per day) and also to control the dose, mix ratio and rate of loading.
The dose is kept between 2 and 3 per cent of the total volume of slurry held
in the digester so that alkaline environment is not disturbed and an
equilibrium state prevails for methanogenic stage activity to take place.
If the
equilibrium is disturbed in the digester foaming trouble develops in the
digester. This may be on account of the presence of inhibitors in the feed
(like ammonia, antibiotics, etc.). Excess ammonia may be obtained if pig dung
is added and the antibiotics may be from the dung of sick animals under
treatment.
Though some
attempts have been made to prevent drop of temperature in the digestor during
winter to prevent a drop in the methane gas production, yet these have been
with limited success.
Characteristics of a good CBP
A good biogas plant must optimise the following four functions:
i) gas production rate
ii) methane gas concentration in the biogas produced
iii) stability of the process
iv) lower overall cost of production of biogas.
For this
two-phased digester system in which phases I and II are achieved in two
separate digesters, which has been found to optimise all the four functions,
is considered next.
Conclusion
It is hoped that PEDA learns a lesson from previously obtained performance of
the CBPs, and would not go in for the same i.e. KVIC model, but should
introduce a "state-of-the-art" design at the Kaljharani complex.
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
refineable oil within (are you ready for this?) n30 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 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.
One such
man is Ram Bux Singh, director of the Gobar Gas Research Station at Ajitmal in
northern India. Although some basic research into methane gas production was
done in Germany and England during World War II's fuel shortages, the most
active exploration of the gas's potential is being done today in India.
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 a long 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 co- workers
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.
Facts about
gobar* <http://ww2.green-trust.org:8383/2000/biofuel/methane.htm#gobar> gas
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.
Ram Bux
Singh has compiled much of this information into two booklets, BIO-GAS PLANT
and SOME EXPERIMENTS WITH BIO-GAS. The set of two manuals is available Air
Mail for $5.00 from Ram Bux Singh, Gobar Gas Research Station, Ajitmal, Etawah
(U.P.), India. The following information has been adapted, by permission, from
the handbooks:
FERMENTATION
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.
TEMPERATURE ACIDITY
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 enough acidic carbon
dioxide to restore balance.
CARBON-NITROGEN RATIO
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 gobar 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.
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.
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.
AGITATION
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.
TEMPERATURE CONTROL
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.
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.
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 2,600 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!
(These
instructions are for an underground, single-stage, double-chamber plant
designed to digest 100 pounds of manure every 24 hours -- five cows' worth --
but may be scaled upward to construct a plant capable of producing 500 feet of
gas a day).
Dig a hole
13 feet deep and 12 feet in diameter, cutting away trenches for the inlet and
outlet pipes to angle down through.
In the
center of the hole, pour a slab of concrete six inches thick and six feet in
diameter. The composition of the concrete should be 1 part cement, 4 parts
sand and 8 parts of 1" stone aggregate.
The
digester will be built on this base from 1:2:4 concrete using 1/2" aggregate.
The floor and walls will be 3" thick, giving an inside diameter of 5'6". The
walls will be 16' high and reinforced with eight 3/8" machine steel vertical
rods and 15 horizontal rings of the same material.
Inlet and
outlet pipes of 4" galvanized iron should be positioned before pouring the
walls so that the pipes are positioned 1-1/2' above the digester floor and in
from the walls. This is so that when the dividing wall is built across the
center of the digester, each pipe will be centered in its chamber. The
concrete must be tightly packed around the pipes to prevent leakage.
Another
wall of brick or concrete will be built three feet outside the digester wall
and to the same height (i.e. four feet above ground level). This space will be
filled with an insulating material: straw, sawdust, shavings, etc.
Provide
some means of descending into this space -- perhaps rungs of machine steel rod
extending from the digester wall to the brick retaining wall -- in case it
should ever become necessary to empty the insulation. Seal the top of this
area to prevent water from getting in, and leave bare earth in the bottom for
drainage.
Bisecting
the digester will be a wall of 4" reinforced concrete eight feet high, at the
top of which an iron support structure with a guide pipe for the gas collector
will be placed. This structure is made of angle iron and the guide pipe is
eight feet of 3" galvanized iron pipe. The structure will be set in the
digester walls and solidly fixed atop the chamber-dividing wall. The pipe must
be in the exact center of the digester, allowing the gas collector to descend
into the slurry when empty and rise to ground level when full. This requires
4' of vertical travel, thus the top eight feet of the digester are left for
the gas collector while the bottom eight feet contain the dividing wall.
The gas
collector is a roofed cylinder five feet in diameter and four feet high
constructed of 12-gauge machine steel sheeting. It is braced internally with
angle irons fitted at different heights so that when the collector is rotated
around its guide pipe the scum on the surface of the slurry will be broken.
The cylinder will first be riveted, welded, tested for leaks by filling with
water and finish-welded. After all leaks are sealed it should be given two
coats of enamel paint inside and out. The top will be covered with an
insulating material.
The top of
the gas collector is also fitted with a 1" tap and valve, and to this is
connected a flexible pipe leading to your gas appliances. Inside the tap a
piece of wire mesh is attached to serve as a flame arrester. The actual
capacity of the gas holder is less than 100 cubic feet, but if the gas is
being used regularly there's no need to make it larger.
The mixing
tank is a cylinder 2'4" in diameter and two feet high. Its floor is one foot
above ground level to provide hydraulic head to feed the plant. The inlet pipe
opening is flush with the bottom of the mixing tank and is covered with a
coarse screen to prevent large pieces of waste from being ingested. The tank
may be built of bricks or concrete and is about 8-1/2 cubic feet in volume,
sufficient for the daily charge of waste matter.
The
discharge pit should be large enough to accommodate all the spent slurry that
is expected to accumulate at a time. It's made of bricks or concrete and the
discharge end of the outlet pipe should be just even with ground level.
An earth
walkway at least three feet wide and level with the top of the plant should be
raised outside the brick wall for support and additional insulation.
Approximate
cost of materials for this plant in the United States is $400.
rying
amounts of "unscrubbed" methane at 540 to 700 BTU per cubic food, within
an initial detention time period of 36 to 48 hours.