[Terrapreta] Long term durability of Low Temp Chars

[AJH]

On Thu, 17 May 2007 16:48:17 -0500, Sean K. Barry wrote:

>Pyrolysis reactions above, I think 250C, are exothermic, i.e. they produce heat.  It is enough heat for the reactions to be self sustaining,

Products start being given off by wood quite early on but the initial
ones are volatile aromatic compounds rather than species given off by
the wood splitting, this starts from about 230C but doesn't really
kick off till 270C. The initial products are highly oxygenated, like
acetic acid. Whilst it may be weakly exothermic from 270 I doubt it is
self sustaining until >300 and the perceived wisdom is that this
exothermic phase exists to about 440C, after which it is endothermic
again, so the carbonising is self limiting.


> as long as additional feedstock and some oxygen (air) are continuously added.  So, there is no need to add energy to the process, once ignition has occurred. 

OK but "ignition" suggests some combustion, so while the energy is
from within the rawstock releasing it to heat the remainder means that
we are now harnessing the exothermy of combustion to drive the
pyrolysis. 

> If the air flow remains low, then the heated charcoal will pyrolize (partial combustion) the biomass chemicals into producer gas.

In fact pyrolise simply means to split with heat, there is a blurred
line between gasification and pyrolysis in a kiln. Gasification uses
the heat from partial combustion of carbon, to carbon monoxide, to
drive the pyrolysis and this consumes the char. With charcoaling we
aim to maximise the return of charred solids so we don't want the char
to burn to provide the heat to start the process.

>  If the air flow is increased too much, then the chemicals in the biomass will fully combust, ejecting mainly CO2 and H2O.

Plus of course the nitrogen that's always associated when you use air
to provide the oxidant.

Pyrolysis can take place in the absence of combustion if you have a
heat source and I suspect given a high temperature energy source and
low pressure you could get syn gas and near enough no char.

For purposes of terra preta for sequestering and soil building it
looks like we should be aiming at a high volatiles charcoal without
using any resulting char as the energy source.

AJH