[Greenbuilding] A composting furnace?

[Nick Pine]

Composting guru Tom Richards writes:

>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
>
>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.
>
>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).
>
>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.
>
>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).

About 14 to 28 Btu/gram or 6-12K Btu/lb. With a heat evolution of
0.025x9K = 225 Btu/lb per day, a 50K Btu/h "composting furnace"
might require 5K lb of compost at 30 lb/ft^3 (50% H2O), ie 200 ft^3,
eg a 6 foot cube, or a 4' high by 8' diameter cylinder, eg a stack of old
tires tied in rings with an oxygen sensor with a relay output ($400) for
a small blower and a $30 humidistat and a solenoid valve from a dead
washing machine and a soaker hose to water the compost as needed
to increase the heat output through a simple condensing heat exchanger.

Where I live in PA, gas stations and tire stores pay people $1 each
to take away old tires :-)

Nick