[Gasification] Biomass Pyrolysis chemistry

[Bruno M.]

Hi Mr Reed,

you stated:
" Coal contains no oxygen, and so gasification must proceed at 800-1000C ..."

I believe this is not true, most coals contain a fair amount of oxygen :

See e.g. :    www.aie.org.au/national/factsheet/FS3_COAL.pdf
------------->
Australian Institute of Energy :
CHEMICAL COMPOSITION OF COAL
Coal varies widely in its composition. It is composed chiefly of rings of six
carbon atoms joined together in an extremely complex composition of
layered arrangements that have in them, not only hydrogen
          but significant amounts of oxygen and nitrogen.
The structure also includes varying
amounts of sulphur and other environmental pollutants. Up to one tenth of
the total mass of coal can be material with no fuel value.
Coal is usually analysed for moisture, volatile matter fixed carbon and ash
(proximate analysis - Figure 1). The sulphur and nitrogen content are
important as emissions of their chemical oxides during coal burning can
cause acid rain.
...
--------------<
If you were referring with "coal" to charcoal :
C21H32O14       ----->          C7H4O   +       9 CO    +       5 
CH4   +       4H2O
Wood                              Charcoal 
carbon monoxide gas + methane gas +water ( steam)
(simplified formula)
Oxygen content is not high in charcoal, but not absent.
and high temp BBQ charcoal still contains up to 15% volatiles, which 
are also not oxygen free.
Biochar and NTP, the lower temp charcoals contains even more volatiles
means less Carbon and more other stuff, incl more oxygen ( about 15%).

Hope this helps some and didn't messed up the conversation.
;-)  Grts
Bruno M.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
At 15:36 3/06/2007, T Reed wrote:
>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
>temperature.  (See our book available at  WoodGas.com....
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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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