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re: fuel value / composition of hog manure
10 nov 1995
>robert tomenchok jr (retjr@ix.netcom.com) wrote:
 
> i heard a story on national public radio the other day about the plight
> of north carolina hog farmer's disposal of manure. it seems that they
> are raising hogs in very high density and as such, generate a lot of
> hog poopies. i was thinking that perhaps the sludge might have a fairly
> good fuel value, and could be dried out to a high enough solids content
> for incineration in either a power or recovery boiler.
 
they might well add water, not remove it, as in expired us patent no. 3,933,628
(us patents are available for $3 each from the superintendent of patents and
trademarks, washington, dc 20231) "method and apparatus for the anaerobic
digestion of decomposable organic materials," issued to inventor frederick
t. varani of golden, co on jan 20, 1976, and assigned to bio-gas of colorado.

this patent describes a way to make methane in conjunction with a 100,000-cow
feedlot, using 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 from this base temperature if temperature shock
  is to be avoided.

this could be an interesting 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.

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.)... 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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