[Stoves] Biomass Pyrolysis chemistry

[Thomas Reed]

Dear Andrew and All:

Good discussion below of a very specific experiment.

Coal contains no oxygen, and so gasification must proceed at 800-1000C 
in order to achieve a thermodynamic stability for the CO and H2 that it 
is converted to.  However, biomass is almost half oxygen by weight.  A 
great deal of CO, CO2, H2 and CH4 gases as well as thousands of 
monomers, oligomers and fragments of the cellulose, hemicellulose and 
lignin are made as "prompt" pyrolysis products at quite low 
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which summarizes 10 years of research at NREL on pyrolysis products.)
------------------------------------------------------------------------
In the carbohydrates all the carbons are attached to oxygen with a very 
strong bond that is hard to break during pyrolysis.  However, lignin is 
based on a benzene ring with hooks, and there are carbon-carbon bonds 
that are also difficult to break.  So lignin is the precursor to most of 
the tars resulting from biomass pyrolysis. 

Low T and slow pyrolysis tends to favor formation of charcoal and CO2 
according to

C H1.4 O0.6 (typical biomass) ==> 0.7 C + 0.3 CO2 + 0.7 H2 (etc. etc.)

Medium temperature and faster pyrolysis favors less charcoal and more CO 
according to

C H1.4 O0.6 (typical biomass) ==> 0.4 C + 0.6 CO2 + 0.7 H2 (etc. etc.)

(these are idealized for illustration purposes.)

The whole picture is clouded by the fact that water is typically a part 
of the equation in varying degrees, so don't look for specific answers 
unless you specify the specific conditions. 

Onward...

TOM REED                 BEF   BEC






------------------------------------------------------------------------
So while it would be nice to have a simple answer to pyrolysis results,

AJH wrote:
> On Fri, 1 Jun 2007 22:43:51 +0200, Crispin Pemberton-Pigott wrote:
>
>> so I would like you to add a little if you can on one issue.
>
> I'll try but I admit to not fully understanding the configuration you
> are describing.
>
>> When I have a low O2 concentration fire and it is gasifying, burning the gas 
>> where I allow secondary air to enter, and I measure the concentration of the 
>> CO in the gases immediately below the flames, 
>> I get humongous amounts of 
>> CO - up to and even above 95% of carbon emitted.
>
> CO is a pyrolysis product
>> Similarly if the flame is out and it is making charcoal 'down there' with no 
>> burning of the gases emitted, the CO accounts for nearly all the carbon 
>> coming out of the fire.
>
> That surprises me as you should get a cocktail of CO, H2,CH4, Acetic
> acid, tars, and "pyrolysis oils" plus CO2 and nitrogen if oxygen is
> still getting to the char.
>> According to your explanation, if I understand it correctly, this should not 
>> occur because the burning of the fuel (whether volatiles or some char) 
>> should be producing CO2 which is too stable and cold to split.  I find the 
>> idea interesting but I have not yet observed it.
>
> See above, the CO2 from low temperature char burning will be present
> but the heat that produces will have split the other products from the
> wood.
>> It is my observation that when the fire is choked and the excess air drops 
>> below zero, the CO production leaps.
>
> Yes and there will always be some CO from the char burning because an
> equilibrium exists, it's just that the low temperature favours CO2
> when char burning.
>> Examples:
>> I have measured very low excess air in a Daewon paraffin space heater a week 
>> ago: 7.5% with a CO/CO2 level of 0.0011 but it was burning completely, not 
>> producing a combustible gas. This demonstrates that low excess air and low 
>> CO are possible simultaneously.
>
> I'm not familiar with paraffin stoves but what you show is good
> stoichiometry and complete combustion, natural gas appliances
> regularly burn premixed with as little as 5% excess air, that's why it
> is such a good fuel and wood is poor in comparison.
>> I have found CO/CO2 in a roaring Lion Stove at 0.010 with 25% excess air 
>> burning wood.  Ditto the demonstration of a clean burn with low air.
>>
>> However, when I choke a charcoal stove like a JIKO, I get very high CO, 
>> going up from 15% COr to nearly pure CO in terms of the form of carbon 
>> emitted.
>
> Sorry Crispin you are really confusing me, I thought the Jiko was a
> simple updraught device, we would expect to see much higher
> temperature in this burning charcoal if there is a dancing blue flame
> above the coals, then of course it is gasifying, this has little
> relation to what happens in tlud where the char is preserved.
>> When the flames go out on a wood burning (charcoal making) Vesto in gasifier 
>> mode, the CO also jumps from below 0.0005 to the point where it swamps the 
>> cell.
>>
>> The only difference I can find between what you describe as a TLUD and these 
>> three examples is perhaps that the fire is not necesarily descending on a 
>> horizontal front.  I can't guarantee that it does, but it appears from the 
>> testing I have done that there is a strong tendency to form CO in an oxygen 
>> starved environment with biomass fuels.
>
> Well at least we can agree on that even if we disagree on the source
> of the CO.
>> I have measured the CO level in the centre bottom of pot sitting on a Maputo 
>> Ceramc Stove (charcoal fuelled).  The CO/CO2 ratio was very low in the very 
>> centre of the flame, but higher when sampled towards the outer 
>> circumference.  This to me indicated a classic reaction as described by Dr 
>> Tom where CO2 breaks down to CO.  That is the only clear example I have seen 
>> of that.  There is lots of heat present to accomplish this.  I increased the 
>> secondary air supply and the CO formation basically stopped.
>
> Or the CO was burned out in the presence of extra oxygen.
>> Am I able to blame the CO formation on the fact that (perhaps) the burning 
>> of the fuels is not in a TLUD fashion? Is it a special condition that 
>> promotes CO2 formation?  
>
> There's not doubt in my mind that the tlud stove is a special case,
> possibly along with the low temperature charcoal heaters in some
> samovars.
>
> AJH
>
>
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