[Stoves] Ron Larson's Charcoal Making Stoves

[AJH]

On Fri, 20 Oct 2006 20:09:40 -0400, Alex and Christine English wrote:

>I think it is safe to say that the particulate matter doesn't all stay in the charcoal.

I agree, it may be that some tars stay in the charcoal, depending on
how hot an environment the charcoal is made in, but tars given off in
the pyrolysis offgas do not necessarily lead to particulates. Tom
Miles pointed out that the burner conditions can lead to particulates
and I have asked a number of times how the particulate chemistry
varies between different types of cookers. On the large scale burners
I deal with I suspect we have fairly clean combustion, so little soot,
but that the air velocities tend to produce fly ash, that which is not
trapped in the flue system is presumably small particulates of ash.

So I think there may be two types of particulates, ashy types from
forced draught or high velocities through the fuel bed and sooty types
which are actually created by conditions in the secondary flame.

One thing about the tlud type burner is that there is very little
primary air, this has two effects velocity through the bed is low and
there is little dilution from nitrogen and CO2 as there would be with
a conventional updraught fire. Also  for the device to work at all the
wood has to be fairly dry, so little losses die to water.

>My Charcoal maker burns a thick and pungent smoke. I sampled the smoke with a 
>dilution tunnel and glass fibre filter and plugged it in minutes. I the rough calculation 
>put it at at least 100grams/kg of dry fuel. These would be almost all soluble in acetone
>or organic carbon with relatively little elemental carbon or black soot.

I can understand that, I think it's because the soot is formed after
the "thick and pungent smoke" starts burning, I think the cv of this
smoke is so high and concentrated that the fuel and air burn out well,
it's when there are other gases diluting this offgas that means the
tars start burning and the heat cracks them but subsequently the
dilution prevents the PICs from completing combustion, so they are
emitted as sooty particles.

IIRC the offgas from a gasifier has a cv of around 5MJ/kg and will be
around 1100C, the offgas from an updraught stove will be less than
this but at a higher temperature. I suspect the offgas from a tlud
stove will be around 500C and 12MJ/kg so the gas particles will still
have a higher chemical energy plus they will be closer together, which
means lower mean free paths to intercept an oxygen molecule.

AJH