[Stoves] Stove comparison coming

Crispin Pemberton-Pigott crispinpigott at gmail.com
Sun Mar 20 14:49:03 CDT 2011


Thanks Andrew

Lots to think about. I am looking for common elements in the clean
combustion and it make take a while to decide much. At the moment it is
clear that if the environment is hot enough, and the excess air low enough,
and the flame path long enough, very little PM is detected. That a wide
variety of stove shapes can achieve this is exciting. 

>From your description I conclude that it is the high temperature
coke-surfaced burns that are providing the best result.

Fixed carbon seems to be a loosely defined term that is, in the field of
coal analysis, carbon that remains after the sample is heated to 400 C, or
420. It is usually determined by heating a sample in a closed container and
watching what is left and what leaves. If it disappears, it is part of the
volatile portion.

After this heating is finished, the top of the container is opened and air
is allowed to burn with the remaining 'fixed' carbon. What remains is ash.

Charcoal is mostly but not entirely fixed carbon. The char remaining in
stoves is likely to be highly variable in content, but tending towards the
low volatile end of the spectrum.

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

What I observe is that particles are formed far more when there is badly
burning whole fuel than when there is badly burning carbon. Why is that?

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

There is a lot of support for that. Harold Annegarn often demonstrates this
using a saucer, passing it slowly over a candle with the tip of the flame
touching the china surface. Lots of black particles immediately accumulate.

>We know from pyrolysis that many secondary reactions take place, such that
if the feedstock is large sections carbon is redeposited within the char.

I hear there are about +450 reactions involving +150 species of chemicals.

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

Quite possibly.

>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.

This is true within the limits you describe. The main factor seems to be the
cracking of complex molecules in the red hot zone. This is akin to a coke
layer on a downdraft stove and works the same way, though at a lower
temperature in the gasifier.

So, what is the reason that a gasifier should be built? Why not burn the
fuel properly in the first place, rather than trying to make a burnable gas
to combust at a point different from where the gases are evolving? What's
the attraction?

One clear advantage of the gasifiers built so far is that they are usually
much cleaner on PM than the wood stoves built so far. That does not mean the
gasifiers are inherently cleaner burning that other stoves, it just means
they are cleaner than the stoves they with which they were compared, and
nothing more. As TLUD's have inherent and difficult to deal with issues,
namely difficulty refuelling them for continuous operation and the
production of unburned char (which can be viewed as a waste of
already-harvested energy).  

It is becoming clear to me that seeing as coal, and pretty crummy coal at
that, can be burned with such low particulate matter in the exhaust stream,
why can't wood or any biomass be burned just as cleanly? What's the big
deal? Why go through the process of gasifying biomass? Because it is fun?

So maybe the problem is that biomass doesn't burn with low PM if it is damp,
or cold, or both. What I am reaching for here is an understanding of why
most stoves have such ordinary PM performance, when the fuels that are
supposed to be really difficult to burn clean - lignite and bituminous coal
- are in fact able to be burned so cleanly if given a decent combustion
environment?

We need to give more though to this apparent inconsistency.

Regards
Crispin






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