[Stoves] Particles and particle types (was Charcoal MakingStove)

[AJH]

On Sat, 28 Oct 2006 17:11:53 +0200, Crispin Pemberton-Pigott wrote:

>Would you call stoichiometric + 150% the needed air for bulky wet fuel, or
>stoichiometric +50%?
>
>I want everyone to be on the same page.  Stoichiometric + 150% = 250% total,
>right?  Then a meter reading 'Excess Air' = 150% really means stoichiometric
>amount + another 150%.

One of the readers with a technical background will need to answer
this, as you say my untrained usage may be incorrect. I say that 50%
excess air supplies 150% of the stoichiometric air required. If
combustion is clean this results in 2/3 of this air becoming oxygen
depleted. For wood burning you have about 6 parts by mass of air for
every one part of wood, so 150% excess air would mean 9 parts of air
for every 1 part of wood. The oxygen in 3 parts of this air remains
unreacted, as air is only 20% oxygen 3/5 parts of oxygen remains in
the exhaust by mass, about 6%.
>
>This means we should, in the field, opt for an air supply that is over what
>is theoretically required by 150% and not much more.  This implies air
>control in some form in a lot of cases!  Many other things being perfect, it
>will still be necessary to limit the air supply to get a hot fire.

I agree but supplying too much air is often safer.
>
>In field measurements of large stoves I have seen excess air below 50%
>occassionally but not often.  I have seen fuel at 50% moisture content as
>recently as last week.  As we are involved with getting baseline data it
>would be useful to have a target moisture content that is the result of the
>training intervention planned.

I see large woodburners burning quite wet wood and often this means
10% oxygen by volume in the exhaust, this is quite wasteful to my
mind.
>
>By that I mean we have to teach people to dry their wood, but it has to be a
>practical limit.  It people dry their wood to 20% mc on a wet-weight basis
>(WWB) what does that mean for excess air? 

I am confident that all other things being equal it means excess air
can be reduced. In UK the equilibrium moisture content of wood is such
that I think it is not worth drying beyond this point.


> If we can get a decent controlled
>air supply, basically choked in some way, it will tend to deliver the best
>possible temperature at the pot with the cleanest burn in the circumstances.
>
>For a baseline, we can check the moisture content of the fuel and the
>consumption, and probably predict the increase in efficiency a stove would
>gain when using drier wood. 

I think so.
<snipped bits that I feel unable to comment on>
>
>Suppose we find in Swaziland people are using 25% mc wood and we train them
>to dry it to 18% (average).  This will allow us to plan for a lower excess
>air ratio as well as a slightly smaller fire, tighter spaces, smaller
>chamber and so on.  It is all related.  The moisture content of the fuel
>changes all the stove dimensions so we need good models before making a
>recommendation.

Bear in mind drying wood always has some cost associated with it.


AJH