[Terrapreta] Farm Scale Batch Charcoal Furnace for Homestead or Farm

[andrew]

On Tuesday 08 April 2008 13:04, David Stewart wrote:

>
> 1. In an oxidative burn of wood, the wood is reduced to ash and
> any gases evolved from the wood are burned in the process,
> resulting in emissions of CO2, NOx etc...  and of course, heat.

Wood is oxidised to CO2 and Water plus ash if the combustion is 
complete, at the lower temperatures achieved in most stoves I doubt 
NOx is an issue.


> 2. In a reduced oxygen level burn, the wood is reduced to charcoal
> with varying chemistries based on the temp of the burn, among
> other variables. 

OK this may be partial oxidation but it's less a reduced level of 
oxygen and more how that the oxygen that does react with the wood 
functions, it basically burns some of the wood or wood products and 
produces heat, this heat then pyrolyses the remaining wood, leaving 
a char residue. Any free oxygen will tend to react with hot char in 
preference to offgas.

> Also produced are offgasses like hydrogen, 
> methanol, acetic acid, carbon monoxide, among others, which can be
> used to power an engine/generator.

Yes but the methanol and acetic acid are small parts of the offgas, 
in fact only broadleaved wood yields methanol. As to powering an 
engine: yes it was done years ago but engine life between services 
was short. We read tales of workers having to take pick axes to the 
deposits in the engine. This pyrolysis offgas is a rich fuel but it 
is a sol of acid tarry vapours not well suited to a reciprocating
engine, that's why gasifiers aim to refine and crack this offgas 
using the heat from oxidation of the char, evenso gas cleanup is a 
big issue for them.

> 3. It is possible to harvest charcoal from a burn chamber, if the
> charcoal has not been allowed to burn to completion.

Yes, this would be a kiln.

> My guess is 
> that this charcoal would be produced at a fairly high temp, given
> the open flame. 

Not necessarily, the tlud type small char producing stoves seem to 
make char at around 600C, the open flame will reach much higher 
temperatures, say 1200C but this is above the char making stage.

> However, any offgas produced in the way would be 
> burned in the fire.

Yes otherwise the process would be smoky and polluting, not to 
mention wasteful of the wood's energy.

> I also understand that low temp charcoal is 
> the best for soil amendment.

I'm still looking for a cite for that because the lower the pyrolysis 
temperature the higher the yield of char in a conventional 
atmospheric slow carbonistaion prcess.


> 4. It is possible to make charcoal in an indirectly heated box, if
> you will, which would enable you to isolate the offgas from any
> direct contact with flame and therefore would make it collectible.

Yes, this would be a retort system

>  This temperature could also be controlled to insure that the
> charcoal produced is low temp.

In practice this will be more difficult than it seems because there 
is a range over which the process is exothermic which tends to 
tempratures of over 400C being reached.

<snipped complicated idea>

> 7. It also sounds like there is a need to balance two competing
> interests... optimizing the production of charcoal for soil
> amendment or optimizing for the production of offgas.   In this
> case I would choose to optimize the production of charcoal and and
> sacrifice some of the offgas production.  My true goal here is to
> improve soils and sequester carbon.

Not a problem if you have enough dry plant material and burn the 
offgas cleanly, choose your  carbonisation temperature and you can 
work out the calorific value of the residual char, substract this 
from the calorific value of the wood and what you have is the gross 
heat output of the process, you may get about 70% of this into your 
hydronic system.

> Finally, has anyone developed a method for pulverizing the chunks
> of charcoal into fines on a scale fit for hundreds of pounds of
> charcoal?

This is largely a non problem, char the wood completely and it's just 
about self comminuting from thermal shock, just run over the 
remaining lumps with a tractor.

AJH