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re: combustion value of wood
24 jun 1997
bruce hamilton wrote:
>the net calorific value on an oven dry basis can vary because of
>the higher lignin content of softwoods.
>softwoods 19.2 mj/kg
>hardwoods 18.2 mj/kg
>bark 19.7 mj/kg
the heat output of compost varies from about 14 to 28 mj/kg of volatile
solids, depending on the materials composted and the conditions of the
composting process, according to my informant:
>under ideal conditions (55-60 degrees c, >15% o2, 45-55% h2o) highly
>degradable materials like food waste will quickly ramp up to a peak
>degradation rate of 100 - 200 g co2/kg volatile solids (vs) per day.
i gather food wastes are close to 100% volatile solids. how about leaves?
>after this peak there will be a gradual decrease to 5 to 10 g co2/kg vs-day.
>the decrease can happen over as little as 3 weeks with a highly degradable,
>low cellulose substrate, or can last 6 months with a less degradable,
>lignicellulose feedstock (in the latter case the initial peak would be
>considerably lower, however).
i guess leaves are closer to "lignicellulose."
>converting from degradation rate to heat evolution requires assumptions
>about the reaction stoichiometry. assuming an aerobic reaction,
>measurements of the heat of combustion of different composting feedstocks
>range from 14.2 to 28.5 kilojoules (kj) per gram of material.
about 14 to 28 btu/gram or 6-12k btu/lb, with a rate of heat production of
something like 10k btu/(3 monthsx30daysx24hours)-lb = 5 btu/hr-lb, so a
50k btu/hr "compost furnace" might require 10,000 lb of compost at 30 lb/ft^3
(50% h2o), ie 333 ft^3, a 7 foot cube, or a 4' high by 10' diameter cylinder.
with 50% water, i guess it would be a fairly good thermal conductor, so heating
a house with the pile might be easy, and it might also serve as a thermal
store, with the airflow input cut down to reduce the rate of heat production.
i wonder how much of the heat is latent, and condensable out of the exhaust
gas. are any of the gas products toxic to humans?
>when degradation is at its peak, the degradation rate or decay coefficient
>can be as high as 15% per day, but is obviously usually much lower (0.01 to
>0.05/day is typical).
is this the decay of an exponential process, once composting is well-developed?
power(time) = p0exp(-days/tau), where tau is 20 to 100 days?
>with this much variation, it helps to know the specifics of a situation to
>hone in on a realistic range.
i'm thinking about mostly leaves for starters, with maybe some human bathroom
and kitchen waste, in an indoor compost pile. some sort of yurt with a warm
cylindrical center and chimney, a wooden or cement block box lined with epdm
rubber or plastic film, and maybe some perforated pvc aeration pipes running
through the pile, and outer walls made with tires filled with bags of leaves,
tied in horizontal rings?
>composting web pages: http://www.cals.cornell.edu/dept/compost/
nick
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