Re: flash carbonization of biomass

[AJH <andrew.heggie@dtn.ntl.com>]

On Mon, 7 Jan 2002 11:18:27 EST, Reedtb2@cs.com wrote:

>Dear Ron and Andrew:
>
>Ron said..

In fact the bit below this was my thoughts.
>
>> I think Tom Reed agrees with me the IDD pyrolysis
>> front is powered by charcoal burning, plainly this detracts from
>> charcoal yield. With the advanced clean charcoal making I, and Mike
>> Antal, are playing with any heat necessary for maintaining pyrolysis
>> comes from burning some of the offgas and recirculating this hot gas
>> stream through the char. There are a number of ways of configuring
>> this, none patentable IMO but nonetheless the actual designs will be
>> proprietary and subject to confidentiality."
>> 
>Not quite.  It is powered by the combustion of the volatiles of each particle 
>burning in insufficient air and yielding a combustible gas.  However, in 
>order to move DOWN to the next layer it needs to ignite it and if it is wet 
>it needs additional heat to move down.  Therefore, the wetter the fuel the 
>more charcoal is also burned.  With bone dry wood we get 25% charcoal yield;  
>with 30% moisture we get 5% charcoal yield.  But the gases for cooking burn 
>remarkably similarly across this range.  

Tom I hope we can revisit this and your other post in the thread "Re:
"Formula for Biomass" and Charcoaling...."  where you say:

"Pyrolysis to make gas is endothermic, but the charcoal producing 
reactions are exothermic"

I have previously posted, but without comment from this peer group,
about the mechanisms for char forming in IDD burning. I will try and
sum up my reasons for my position. I will not be upset if I am wrong,
it does not affect any process I am playing with. However it will
cause me to re experiment with the idd as a co-generstyor of clean
heat and charcoal. My playing with this largely stopped after I burned
coal, following Ronal's visit, and the interest seemed to die out.

My reasons for thinking idd was at a cost of carbon yield was:

1) To maximise char yield whilst maintaining a clean secondary flame
primary air must be well controlled. In the absence of good primary
air control char yield goes down but power goes up and length of burn
reduces.

2) When primary air is too low offgas is insufficient to maintain a
secondary flame. Pyrolysis continues but the offgas remains as smoke.
I wonder if this is because its cv is diluted too much by the small
amounts of CO2 being formed.

3) Because the primary air is low so is the superficial velocity
through the pyrolysis front. Hence the flow is near laminar and there
is little scope for turbulence or micromixing at the pyrolysis front.

4) For pyrolysis to occur temperatures of >270C are required to stay
in the exothermic region, these small stoves lose a lot of heat
through their walls.

5) We know that organic gases like propane will only burn (even in the
presence of a flame?) within specific air:fuel ratios, is this not
also true of our mixture of offgas? This coupled with 6 and 8 below
make me think consumption of char will be favoured over offgas.

6) Freshly formed char is highly reactive, will cause an oxygen atom
to be adsorbed and dissociate on its surface at temperatures <270C
even in the absence of a spark or flame. This is *not* a flaming
reaction. Further oxidation of the CO so formed to CO2 *is* a flaming
reaction.

7) The downward progression of the pyrolysis front is dependant on
heat transfer from this pyrolysis zone downward. If this is delayed
because the layer below is not readily ignitable (e.g. if it is too
moist) then the hot char at this point continues to burn in the
primary air, thus reducing char yield.

8) The only draught present is the low primary air below the pyrolysis
front and added to this is the evolved offgas above this point. There
is thus a constant upward flow of gas. Unless there is some micro
circulation of gases at the pyrolysis front the offgas is swept away
from the oxygen, which in general is consumed in the carbon to CO or
CO2 reaction, there being an equilibrium depending on the temperature
and depth of the pyrolysis front.


None of the above are statements of fact, just bits of my
understanding so far, which I am reviewing.

I am sorry if this is a bit garbled, I should have sat and thought it
through a bit but I just picked up your message, it is late and I will
not be able to sleep whilst pondering it.

I hope someone feels free to take potshots at these 8 points or
provides a few more.

AJH

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