[Stoves] Just add a fan.

[Crispin Pemberton-Pigott]

Dear Friends

I concur with Tom M on this.  The counterflowing-flow air can work with a 
fan or not, depending on the draft available.

Another often overlooked benefit with counterflowing air is that the wood 
can be burned at the tips more than on that part of the fuel that is just a 
bit away from the tips.

What this accomplishes is that there should be less 'drop off' of burned 
wood falling into the base of the combustion chamber.  Something noticeable 
in a Rocket Stove is that there is quite a lot of charcoal left at the end 
of a fire compared with many other stove types.  This is mostly because the 
fuel is not supported when it is burning and eventually pieces fall off 
either when they are rammed into the back of the chamber, or by gravity as 
it burns through.  One of the defining characteristics of a Rocket Stove is 
overhanging fuel and no grate.

The air passing through the fuel magazine and under the shelf can tend to 
keep the fire at the ends of the fuel, but also this tends to burn the 
'available' surface which is not the end of the stick, the tip being 
somewhat sheltered from the air by the rest of the fuel.

Running the air from the other side tends to make the tips glow brightly and 
burn faster than the fuel behind (and sheltered by) the material at the 'hot 
end'.

As the entry of air with the fuel is almost inevitable, there is a 'meeting 
of two airs' at the fire, the direction being opposite and mostly cancelling 
each other above the fire.  But at the tips of the sticks, the reverse air 
is preferred so as to burn it brightly and to limit the drop-off of 
partially burned wood.

Further, making a lot of charcoal raises stove testing questions.  Should 
the charcoal remaining be subtracted from the 'fuel used' for the test?  If 
it is, where does the charcoal end up?  If it cannot be burned in the same 
stove, has it not to be thrown out and more wood used for the next fire?  My 
point is that subtracting charcoal from the fuel used can be misleading when 
reporting how much fuel a stove uses.  Limiting the production of charcoal 
with counterflowing air necessarily means limiting charcoal losses with a 
stove that can't burn charcoal well.

Ever further, making a lot of charcoal byproduct means that wood gases were 
produced and used by the stove and that affects the emissions rating.  The 
more charcoal produced, the lower the emissions because it is really easy to 
burn wood gas cleanly and really hard to do the same with charcoal.  If the 
charcoal is left over, the wood consumption is actually higher then it 
'appears to be' and the emissions are actually lower.

If the charcoal _can_ be burned in the same stove, then the leftover 
charcoal from a previous fire should be used in the new tests, and the 
emissions noted.  Burning the old charcoal in the course of a test provides 
a more realistic emissions profile than not doing so.  If the stove can't 
burn the charcoal, then the char left at the end should be added to the 
consumption figure, and the emissions figure stands.

There is no free lunch.

Regards
Crispin

----- Original Message ----- 
From: "Tom Miles" <tmiles at trmiles.com>
To: "'Discussion of biomass cooking stoves'" <stoves at listserv.repp.org>
Sent: Wednesday, November 29, 2006 7:46 PM
Subject: Re: [Stoves] Just add a fan.


Kyle,

I think this is a very sensible idea.

It seems like the opposed flow designs would work with or without the fan.
There are probably other designs that would work with either natural or
forced draft.  People who use the fans will find that they can burn a wider
variety of fuel quality with the fan.

I don't know if any of the fans tested at Stoves camp this summer were
designed to also work with just natural draft. I wasn't there for that
exercise.

Tom Miles