[Stoves] Fuelwood moisture and wood stove

[Crispin Pemberton-Pigott]

Dear Otto

 

>Is such a stove to complicatied or expencive for the low income households
of Africa?

 

A stove with preheated primary necessarily means controlling the air. If the
stove has a door, or a system of down-drafting the primary air in a
preheater (usually made of metal) then it is possible. With no control, any
amount of air can access the fire so fuel moisture will be a problem.

 

The simplest method I know of is to put a metal shell around a combustion
chamber. That is not complicated and is accessible to low income families.

 

If you put a natural draft gasifier in an enclosure to get the primary air
prehated using dry fuel, I would guess that you could use fuel of higher
moisture content, later on. 

 

Yes, you have to start the fire with kindling. If in the case of the Peko Pe
the fuel it directly ignited, and it was wet, it would mean putting some
other fuel on too first to get the top layer dried.

 

All the natural draft gasifiers that I have ever seen are controlled-air
stoves, either by design of the air holes, or by the fuel ad how it admits
air because of how it packs into the chamber.

 

I understand that very dry fuel might burn so fast, that PM might enter the
indoor air.

 

That is true if the primary air is allowed to enter freely, or if the
combustion chamber => air supply has a positive feedback character. For
example if you have a self-heating retort where the fire heats the fuel
more, making a bigger fire, heating the fuel more, that is going to be a
problem because it is self-accelerating. At some point it will run out of
primary or secondary air. Whatever the device, either dry fuel or wet fuel
will cause a problem if it cannot be adjusted.

 

Thus designing for the available fuel is important. You can't develop a
stove using dry fuel and expect it to perform predictable with wet fuel.
There are many examples of that in the field, unfortunately. Stoves that
have some form of air control are able to burn wet and dry fuel on condition
that enough wet fuel can be put into the chamber at once. You need to have
more fuel involved if it is wet - a physically larger fire. If the chamber
is designed for dry wood it can be much smaller, doesn't need much air
preheating, and needs better excess air control, and can outperform any wet
fuel stove.

 

I also think a fan driven gasifier might speed up that process and make it
even worse?

 

A fan is just a power driven chimney so include fans and chimneys. Does
increasing the draft make combustion worse? Depends on the stove! A fan is
no panacea, that is for sure. 

 

Regards

Crispin

 

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