[Stoves] RE Particles and particle types (was Charcoal Making Stove)

[AJH]

On Mon, 30 Oct 2006 01:33:37 +0200, Crispin Pemberton-Pigott wrote:

>>Perhaps I haven't followed the discussion about the air needed for
>>burning wet fuel. The concentration of oxygen in the mixture is reduced
>>when water vapour is present but has it been experimentally found that
>>increasing the air factor contributes to clean burning?
>
>Well, that is the sort of question we should answer first.  Andrew notes 
>that about 150% EA is equired to get 'clean burning' of damp wood.  Define 
>'clean.  Define 'damp'.

I think Peter's point needs further investigation. My observations
were subjective and my conclusion of cause and effect may be wrong. I
deal with a number of large installations and some small pellet
burning ones. I note I often see residual oxygen levels of ~5% in the
pellet burners but the large chip burners running on ~35%mc wwb whole
tree chips seldom run below 10% residual oxygen. Now it may be that
the excess air is needed to counter the dilution or possibly that
higher air flows mean higher velocities and better mixing/turbulence.
>
>If excess air beyond 150% is not required for clean combustion, then this 
>needs particular attention.  What I have found difficult to do is to make a 
>stove which can automatically control air while having a power variation. 
>The excess air in an updrafting stove is highly variable and certainly far 
>beyond 150% at low power level.

One of the reasons I like the toplit forced air concept is that fan
power seems to control the heat output and the way the air enters the
fire means primary air and secondary air control themselves.

>This week we seem to have arrived at a consensus that we should be designing 
>stoves for 15-20% MC wood and an excess air supply of 150%. 

My guess would be that if you have wood this dry you should be able to
reduce air below this.

AJH