[Stoves] Comments about T-LUDs: Types of carbon

[Kevin Chisholm]

Dear Paul

Ash can be inert, but it could also be very influential on the operation of 
a biomass gasification or combustion system.

Consider Silica, or SiO2, as a simple case. It has a melting point, as a 
pure compound, of about 3,200 degrees F. As pure silica, it is inert to the 
combustion process. Quicklime, or CaO, has a melting point of about 4,000 
degrees F, and as a pure compound, it also is inert to the combustion 
process. HOWEVER, when mixed together, in an equimolar ratio, the melting 
point drops to about 2,100 degrees. Add the right amount of Al2O3, FeO, and 
K2O, and you can end up with a slaggy mess with a melting point in the 1,300 
to 1,500 degree F range.

K2O has a catalytic effect that promotes the reaction of S with CaO; I don't 
know, but if it can exert a catalytic effect on one reaction, it just might 
catalyze others.

There is also the phenomenon of "alkali pumping"... when K2O reaches the 
lower and hotter zones of an updraft gasifier, it is reduced to metallic 
Potassium, which is then carried upward to the cooler fuel bed, where it 
"back reacts" with CO2 to again form K2O. This imposes a significant 
endothermic heat load on the reaction zone.. If it recycles sufficiently, it 
can interfere with gas passage through the bed, and cause channeling.

Lime in wood would normally be as a carbonate or silicate, or perhaps even 
an organo-metallic compound. It would not be found as CaO per se. When the 
char is burned, if the Ca was there as a Calcium Carbonate, CaCO3, it could 
have a significant effect on the combustion process. CaCO3 requires in the 
order of about 4,000 BTU per pound of Calcium Carbonate that is converted to 
quicklime, CaO. If there was a significant presence of CaCO3 in the char, it 
could impose a very significant endothermic heat load on the reaction zone.

A very simple test will tell if CaO, Na2O, and K2O are present in the ash... 
simply cool the ash in a sealed tin can, and when at room temperature, add 
water. If there is significant quantities of these components present, they 
will hydrate, with the liberation of noticeable heat.

Best wishes,

Kevin
----- Original Message ----- 
From: <psanders at ilstu.edu>
To: <frank at compostlab.com>
Cc: <stoves at listserv.repp.org>
Sent: Sunday, October 08, 2006 12:44 PM
Subject: [Stoves] Comments about T-LUDs: Types of carbon


> Frank and all,
>
> Your comments about carbon arise questions to me.
>
> Quoting frank at compostlab.com:
>> As I see it there are two types of carbons left in the ash. One
>> carbon that is
>> left from the unburned wood and the other carbonates.
>      [and]
>> Ash, even in wood, is much more than just unburned organic material.
>
> I could easily be wrong, but to me ash is the absolutely inert,
> non-combustible
> residuals from combustion, and does NOT contain unburned organic
> material. Silica and clinkers etc are all part of the inert "ash."
> Maybe my terms are
> incorrect.
>
> Is a "carbonate" inert, meaning, are the carbon atoms LOCKED into
> carbonates? If so, then carbon-in-carbonate is
> carbon-not-available-for-combustion.
>
> Hydrocarbons are the backbone of combustion.  And there is
> carbon-in-unattached-form. (Is that pure carbon, or fixed carbon or 
> elemental
> carbon, or all of those names?)    The unattached carbon also has great 
> fuel
> value.
>
> I know that good-quality char (quite pure carbon) can be pulverized and
> added to
> soil as a great soil builder.  But is that true for "carbonates" also?
>
> Yes, this relates to T-LUDs because those devices deal with the various
> forms of
> carbon compounds and element at distinct times in the T-LUD pyrolytic
> gasification.
>
> Paul
>
>
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