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

[AJH]

On Mon, 7 Apr 2008 06:47:59 -0700, Dave Stewart wrote:

>I've posted a very simplified sketch of the furnace that I'm looking
>for on my google page.

I've had a look and the comments I make are:

In this case you can have your cake and eat it, there is no need to
divert heat from the fire to the carboniser as it can happen all at
once if you run continuously or batch sequentially.

You don't actually need much heat to carbonise dry wood.

I think it can be made much simpler than you envisage.

Think of the carboniser and flare simply exhausting via the hydronic
heat exchanger but all the heat for pyrolysis being recycled within
the carboniser.

You might be able to visualise the genesis of a system by starting
with an open fire. Once it's burning well, there is little smoke if
the wood is dry and where there is smoke is mostly from the edges
where cool air quenches the flame. In the middle a pyramid of
pyrolysing wood forms shielded by the flames and it's only once that
wood has become carbonised that the offgas evolution diminishes and
air is able to burn the char. You can see a white ash film starts
forming on the char.

Now at this stage if you either add dry wood or invert a dish over the
charcoal you can prevent the char being burned away. Adding dry wood
is what I call the modified pit method.

The next step would be to fill a container with dry wood and invert it
in the fire plus put a shield around the fire to control air supply.
This is what ADKarve does in order to carbonise sugar cane trash.

I have successfully done just the same by inverting a 205 litre oil
drum in my wood chip furnace.

Yury takes it one stage further and inverts the cassettes of wood over
a common manifold that feeds the offgas up stream of the cassettes and
flares it, so the heat from the flare continually heats the cassettes
as they are sequentially changed. There are expenses and drawbacks to
this indirect heating method.

The thing about making char for terrapreta is that you want a good
yield but how good? You want to sequester carbon but does that have to
be fixed carbon or high volatiles carbon? You want to add growth
potential to the soil (as opposed to simple fertility) but what is the
best char for that?

What makes terrapreta char easier to do is that it doesn't need to be
big lumps and it needn't be dry, which takes away one of the time
constraints put on normal charcoal making.

I don't intend to do more of a brain dump so come back with specific
questions on what *I* have posted and I'll attempt to answer them.

I don't manage to read all message to this list.

AJH