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.
Gobar
Gas Methane top
