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

[David Stewart]

Forgive my relative lack of knowledge but let me explain what I think
I know from this discussion so far.

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.
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.  Also produced are offgasses like hydrogen, methanol,
acetic acid, carbon monoxide, among others, which can be used to power
an engine/generator.
3. It is possible to harvest charcoal from a burn chamber, if the
charcoal has not been allowed to burn to completion.  My guess is that
this charcoal would be produced at a fairly high temp, given the open
flame.  However, any offgas produced in the way would be burned in the
fire. I also understand that low temp charcoal is the best for soil
amendment.
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.  This
temperature could also be controlled to insure that the charcoal
produced is low temp.
5. The sketch that I put up would have two separate chambers, filled
with two different charges of wood.  The wood in the firebox would be
allowed to oxidatively burn and go to ash, providing the heat for the
house and for the charring chamber.  The wood in the charring chamber
would be "baked" by the heat from the firebox, turn to charcoal, and
evolve offgas.
6. In fact, my thoughts on the furnace run to three chambers, two for
charcoal and one for the firebox.  With two charring chambers one
could be baking while the other is cooling or being quenched in some
way.  This would allow me to reload the chamber and harvest the
charcoal in a less dramatic way.
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.

Several folks have identified the need to calculate heat requirements.
 My intuition is that if the furnace is adequately sized for heating
my house using hydronics, then capturing heat lost up the stack and
radiating off of the furnace itself is actually part of the heat
calculation.  In effect, this lost heat is reflected in the efficiency
rating of the furnace.  If the furnace is rated at 70%, then 30% is
lost primarily through radiation or up the stack.  I'm also assuming
that on all but the coldest days, the furnace will have periods of
time where heat is not being sent to the house for heating and the
firebox is damped down to slow down the burn.  Slowing down or damping
the burn essentially switches it to charcoal mode, whereby the evolved
gases/smoke are sent up the chimney into the atmosphere.  By keeping
the burn going in an oxidized environment, I will increase my fuel use
in the burn chamber by some amount, but it will be a cleaner burn and
I will harvest the energy from radiant and exhaust stack and get
charcoal from the charring chambers.

I'm not trying to convince anyone of anything, I just want to make
sure that I am understanding the processes correctly before I spend
too much time and, more importantly, money on the project.

Additionally, I should mention that the furnace will be housed in a
greenhouse of a solar design as described in "The Solar Greenhouse
Book" written by McCullagh in  the 70s.  It will be highly insulated
on 4 of the 5 exterior surfaces (the 6th is the ground).  The radiant
heat from the furnace itself would be used only to temper the interior
above freezing, not actually heat the house to tropical temps.  The
heat from the exhaust stack would be counterflowed into a 3000 gallon
tank located in the greenhouse for additional tempering.  My hope is
only to keep the greenhouse above freezing in the darkest days of
winter and therefore will not have a separate zone on the furnace
system.  This will keep my wood requirement down to the amount
necessary for heating the house and making charcoal.  It would still
be more than just heating the house, but perhaps it would be done more
efficiently due to using the heat from the firebox when it would
otherwise be idling waiting for the house to call for heat.

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

Forgive me if this is too rudimentary or involved for the normal scope
of the discussion, but I am really appreciative of the comments that I
have gotten so far and want to make sure that I use them to their best
effect.  Would it be possible to get a poll of those reading this as
to whether it will be successful or a complete waste of time?  Please
factor into your thought process that I am a farmer who milks cows.
Having a daily responsibility that will never allow me to leave home
in the winter months is not something that is a problem... the cows
already provide that for me.

Best

Dave