[Stoves] Pressed biofuel - and Fire balls. Testing

[Crispin Pemberton-Pigott]

Dear David

Thanks for boldness. This is the place for it, eh?

>Might I be so bold as to suggest that examining the combustion with both 
>ambient and preheated air would make sense?
>
>Given that the preheated air might make the fuel hotter and combust more 
>fully, would it suggest better combustion?

In general preheating the air _can_ make the combustion better, and should,
but there are limitations. One is that if the fuel has a high surface area
to volume ratio, it tends to pyrolyse too quickly to burn properly in most
stoves. In a gasifier this is done deliberately to create a thick
combustible smoke which is 'burned elsewhere'.

If the fuel is extremely hard, like some Ironwoods and Acacias, it will not
burn unless the air is preheated, both primary and secondary.

Burning hotter is more a function of limiting excess air than it is
preheating it.  A good example of this is the Rocket Stove where there is
usually high excess air (600-800%) which is at least partially compensated
for by insulating the combustion chamber to keep the heat in.  If large
amounts of excess air are heated (which cools the whole stove) it is a loss
as the cooler gases pass by the pot at a lower heat transfer efficiency. 

The best situation is to have limited or limitable excess air and
'contained' heat, meaning that either heat which goes out through the walls
should be recycled into the stove, or it must be insulated to keep all heat
in. 

In practise, it is hard to make a durable and well insulated combustion
chamber in a low tech environment. For that reason it is easier to preheat
the air and limit the amount if it entering the fire.

Once this principle is understood, you can make rational decisions about how
much heat you could generate, capture and direct to the correct points
inside the stove. There are constant trade-offs.

As a general rule, the higher the excess air, the higher the CO generated
per kg of fuel burned (per MJ of heat released) so clean stoves usually have
some form of air control or inverted draft to improve the combustion
efficiency and limit the burning rate.

Unfortunately most large cast iron space heating stoves like those Grandma
had paid no attention to CO at all and are quite dangerous, actually. They
just choked the fire ('banked down for the night...') and they smouldered
for hours generating masses of CO and particulates.

Most stoves seen discussed on this list are FAR better than those old ones.

Good question.
Regards
Crispin