[Stoves] Stove comparison coming

ajheggie at gmail.com ajheggie at gmail.com
Sun Mar 20 15:56:37 CDT 2011


On Sunday 20 March 2011 19:49:03 Crispin Pemberton-Pigott wrote:

> 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, 

OK this is re stating 2 of the 3 Tees

Hotness=Temperature
flame path/velocity=Time

Excess air is also related to temperature as it increases mass flow which 
has to be heated by the fire and thus lowers temperature
>
> From your description I conclude that it is the high temperature
> coke-surfaced burns that are providing the best result.

I'm not sure I follow that but one thing is apparent and that is coke (and 
coal) has a higher cv/kg than even dry wood so less mass to distribute 
the heat over.
>
> 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.

Yes that's what I thought, the question still remains which bits of the 
original feedstock is it? With cellulose it looks like a CH2OH group 
would be the first to split off but after that...

>
> 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?

There's simply no scope for these secondary compounds to exist if char is 
burned, they have to be formed during the combustion of the pyrolysis 
offgas.
>
> >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.

Yes because the cold surface is in the flame, a flame is an area of 
combining gases and the yellow flame is glowing carbon that has already 
been split from the gases that are burning, so the cold surface inhibits 
further combining and hence they deposit as soot.

>
> >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.

Actually I suggest that the temperatures would be similar if you could 
keep the layers stratified. It's the limits of what heat is released in 
the various zones and the massflow that makes clean gasification of wood 
far more difficult than gasifying coal, coke or char.
>
> 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?

When I first joined these lists in ~1995 a chap gave a rather elegant 
answer to that but I can't remember it.

Basically a gasifier operates within the bounds of stoichiometry for 
producing producer gas. The gas is then cool and can be mixed with a near 
stoich amount of air and burned as a premixed open flame. Apart from 
thermal losses in the gasifier ( and cold gas efficiencies can be as good 
as 80% of the original fuel calorific value) all the heat is delivered in 
a more controlled way and less need for excess air.
>
> 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.

Precisely because all the wood has been converted to CO, H2 and some N2 
from the gasifying air so the gas can be premixed and burn out 
completely. There is no scope for larger pyrolysis products being 
incompletely burned because they were all cracked out in the oxidation 
zone of the gasifier throat at above 850C.



> 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?

For one thing because even poor coal has a higher cv than dry biomass and 
hence adiabatic combustion temperatures are higher. This is why wood was 
able to make bronze but charcoal was necessary to produce iron,


> So maybe the problem is that biomass doesn't burn with low PM if it is
> damp, or cold, or both.

Actually these relate to the stove rather than the fuel. Depending on ones 
definition of clean, and I think you posted some figures from EPA for 
what is acceptable from a US wood heater, it's pretty certain that 
there's a temperature and retention time which pretty much guarantees 
clean combustion if air and fuel are right. I'd suggest a couple of 
seconds above 850C but others may prove different. This sort of 
temperature can be reaches with as harvested wood in a large well 
insulated combustor with getting on for 50% excess air.

> 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?

Again because the fuel defines some of the combustion environment. Dry 
biomass is already partially oxidised ( think of it as 50% carbon and 50% 
water) so it has less heat to give up and more molecules having a free 
ride through the system interfering with oxygen finding a carbon molecule 
to oxidise.
>
> We need to give more though to this apparent inconsistency.

There's no inconsistency, its just the hand biomass gets dealt.

Crispin, please remember I don't type or think as fast as you ;-)

AJH




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