[Gasification] Energetics of pyrolysis

[Anthony Nekut]

To familiarize myself with pyrolysis, I heated 100g of low MC wood chips
in a vented vessel in an electric kiln at 500C flaring the gases until
gas production ceased.  The char yield was 30g.  The heating value of
the wood is about 8000 BTU/lb while the heating value of the charcoal is
about 13000 BTU/lb so I presume that the heating value of non-charcoal
pyrolysis products is 8000-0.3*13000 = 4000 BTU/lb of input wood. The
remaining 4000 BTU/lb could be released by buring the charcoal.
Correct?

However, I read (in http://www.fao.org/docrep/X5555E/x5555e00.HTM):

"An ideal carbonising process would be one which required no external
heat to carry out the carbonisation. The exothermic heat of the process
would be captured together with the heat produced by burning off-gas and
liquid by-products and this in total would be sufficient to dry out the
residual moisture in the wood, raise it to spontaneous pyrolysis
temperature and then heat it to a temperature sufficient to drive-off
residual tars. In practice due to losses of heat through the walls of
the carboniser and poor drying of the feedstock it is >>almost
impossible<< to achieve this aim. However some systems particularly the
large hot rinsing gas retorts come close to the ideal where the climate
of the locality permits proper drying of the wood raw material."

I am interested in producing biochar on a small scale for addition to
soil while recovering any excess process heat to make hot water (see
diagram "continuous automated pyrolysis rig" posted 4/19/08 to
http://gasifiers.bioenergylists.org/).  Can someone comment on the
thermodynamics here or direct me to a good source of quantitative
information?

Thanks,
Tony Nekut