[Stoves] Stove comparison coming

[ajheggie at gmail.com]

On Sunday 20 March 2011 16:35:54 Crispin Pemberton-Pigott wrote:
> I am suspicious that high BC emissions do not arise from fixed carbon.

First we'd have to define "fixed carbon" in terms of how it is derived, 
i.e. which bits of the original biomass molecules contribute to the 
remaining char. We know that high lignin biomass tends to produce more 
char and interestingly lignin has phenolic like (6 carbon atoms in a 
ring)structures, whereas cellulose and hemicellulose are basically 
chains, aliphatic.

I  thought we took that soot was formed in the secondary combustion as 
read for a few years now?

When Tami used to post to the list I inferred from her messages that soot 
was formed in the secondary combustion area.

Since then we have had a few discussions when suggestions have been made 
for the mechanism for this.

My take has been that the pyrolysis offgas from oxygen starved primary 
combustion, as in a tlud, is almost all fuel with very little dilution 
from primary air combustion products and when either premixed or in a 
diffuse flame as long as the right conditions existed it burned out 
completely, hence no soot. Alter the conditions by quenching the flame 
and the combustion doesn't get to completion, hydrogen atoms from offgas 
are stripped and burn preferentially leaving a carbon rich particles 
exposed to high temperatures glowing in the flame. If these particles 
stay hot enough and meet an oxygen molecule then combustion is clean but 
if combustion isn't completed what transformations do these carbon 
bearing particles undergo?  We know from pyrolysis that many secondary 
reactions take place, such that if the feedstock is large sections  
carbon is redeposited within the char. Also that as the pyrolysis 
temperature increases so aliphatic compounds morph into phenolic 
compounds.

Further I hypothesise that groups of carbon rings are more resistant to 
oxidation than simple chains of carbon.

So my bet is that in a diffuse flame (and most of our stoves flames are 
diffuse to some extent, witness the yellow flame, if you want to see a 
good premixed woodgas flame visit some of the [Gasification] pages and 
see the pure blue flames they produce, because their offgas has been 
subject to a temperature of between 1100C and 850C at its exit to, render 
almost all the wood into simple gases, CO,H2 and N2,[1]) if you don't 
burn the offgas out hot and quickly you end up producing a graphite like 
soot that is difficult to burn.

Another point is that many stoves lend themselves to running with poor 
combustion conditions, excess air and moisture will particularly cause 
this as will cold masses in the stove walls. The TLUD avoids this because 
it plain won't work with high mc fuel and if you add too much primary air 
total combustion occurs at the pyrolysis front and no offgas is produced. 
So to some extent the tlud self limits poor combustion.

[1] Alexis Belonio also posted pictures of a perfect blue flame from his 
most recent rice husk gasifier.

AJH