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re: methane questions
7 apr 1997
amaznbeast wrote:
>...i want to get off of the grid completely.
a common form of mental illness :-)
>...i do have space, means and experience to raise pigs, and was wondering
>if the rumors i have heard about methane generators with methane coming
>from pig poop are true?
my impression is that a family of 4 might have trouble making enough methane
for cooking, not to mention house heating or electrical power, but this might
work with lots of pigs. is it worth raising pigs just to be off the grid?
the northeast regional agricultural engineering service (nraes)/151 riley-robb
hall/cooperative extension/ithaca, ny 14853-5701 can send you a copy of their
$6 10-year-old booklet on how to make and use methane on a farm...
expired us patent no. 3,933,628 ($3 from superintendent of patents/washington,
dc 20231) describes a "method and apparatus for the anaerobic digestion of
decomposable organic materials," issued to inventor frederick t. varani of
golden, co on jan 20, 1976, assigned to bio-gas of colorado.
this patent describes a 100,000-cow feedlot with 2 epdm-rubber-lined trenches,
each 700 feet long x 80 feet wide x 40 feet deep. the trenches have self-
inflated translucent "solar covers" and cost $0.02 per gallon, including
excavation. the feedlot generates 3.3 million pounds of manure each day,
along with 6 million pounds of water and 200,000 pounds of carbon, which
the digesters turn into about 7 million cubic feet of methane per day with
a heating value of about 277 million btu per hour, along with 2 1/2 million
cubic feet of co2 per day.
the digesters contain heat exchangers for temperature control. the patent says:
the fermentation reaction will proceed satisfactorily at any temperature
between approximately 90 f and 115 f, however, between these limits many
different species of bacteria become active, each in its own particular
temperature zone carved out of this broader range. in other words, the
digestion process is basically an equilibrium between many species of
bacteria that live upon various substrates (food) and on one another.
changes in temperature cause this equilibrium to shift and some of the more
temperature-sensitive species die off or become less active while others
assume a more active role... ideally, methanogenic bacteria should be kept
at about 95 f and the temperature range should not be allowed to vary more
than +/- 2 f per day... if temperature shock is to be avoided.
this might an on-site or municipal sewage treatment system, without the cows,
or an efficient way to combine sewage treatment and long term passive solar
thermal storage, for a single house, on a smaller scale. one might build a
house that needs 250k btu/day to stay warm in the winter with a solar heated
digester with a minimum volume of about 64k pounds of water changing 4 f per
day, ie about 1000 cubic feet of water, eg a 10 foot cube.
pages 825-826 of metcalf and eddy's 1991 _wastewater engineering_ say
typical values [of gas production] vary from 12-18 ft^3/lb of volatile solids
destroyed... gas production can also be crudely estimated on a per capita
basis. the normal yield is 0.6 to 0.8 ft^3/person/day (15 to 22 m^3/1000
persons/day) in primary plants treating normal domestic wastewater. in
secondary plants, the gas production is increased to 1.0 ft^3/person/day...
because digester gas is typically about 65% methane, the low heating value
of digester gas is approximately 600 btu/ft^3 (22,400 kj/m^3.)...
so we might make about 1000 btu per person or pig per day, from methane: 100
250 pound pigs or 50 1000 pound horses or cows might generate heat equivalent
to a gallon of oil per day, in methane, and another 24hrx600btu/hrx100 = 1.4
million btu/day in sensible heat (according to the ashrae hof), ie the heat
equivalent of about 10 gallons of oil per day, sufficient to heat a house, if
they live in the basement. one might make about 130kx30%/3410 = 11kwh/day of
electrical energy, enough for an electrically-frugal household.
the book goes on...
in large plants digester gas may be used as fuel for boiler and internal
combustion engines, which are in turn used for pumping wastewater, operating
blowers, and generating electricity. because digester gas contains hydrogen
sulfide, particulates and water vapor, the gas frequently has to be cleaned
in dry or wet scrubbers before it is used in internal combustion engines.
nick
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