Re: CO emissions response to Andrew

["Dean Still" <dstill@epud.net>]

Dear Andrew,

Great to hear from you.

 Batch loading has advantages and a few disadvantages. I hear from Crispin
that batch loading can be much preferred but our experience in Central
America is that pushing the sticks into the fire, as into a three stone
fire, is traditional. I don't think that we'll find that batch loading is
necessarily lower in CO emissions, my very limited experience so far is that
even smoke free fires can be spewing a lot of CO into the room. It would be
great if the money gods were to give each of us a combustion analyzer...
This is a complicated problem...to tell when the stove is really clean
burning is not evident to eyes or nose."

I wanted to try various clean burning strategies in the August class, like:
>
>1.) Have smoke go through coals

Why? The coals will still need to burn and thence will require excess
air in the stove part.

"The Rocket stove forces a lot of air through the coals so that they are
very hot. Directing all hot flue gases to go down through the bed of coals
might force the uncombusted particles to catch fire, i.e., downdraft through
coals. A hot bed of coals under the fire is in the worst place to scrub hot
flue gases, they rise up and away."

>2.) Force smoke to scrape against surfaces above 1200F.

I guess you mean flue gas rather than smoke, or are you intending the
surface to feed back some energy to the "smoke" to burn it out?

" Directing unburnt combustibles between two glowing very hot surfaces  (or
variations of same idea) should help to ignite them."


>4.) Preheat primary or secondary air

I do not think it likely you can afford the dubious benefit of this
with only natural draught, unless it also serves another purpose.

"Larry agrees."

>5.) Increase the time that fire is inside combustion chamber

Yes

>6.) Keep the portion of the wood that is not burning cool

This to prevent "runaway" pyrolysis upsetting the fuel:air ratio?

"We want to burn all wood at very high temperatures. Having too much air
isn't a problem, I think, if it doesn't cool the fire."
>
>And we did experiment with several of these ideas.
>
>IDEA ONE We built a experimental stove in which the hot flue gases went
down
>into the bed of coals on a screen before passing under a fence and going up
>a 24" insulated chimney. We agreed that: 1.) more draft is better than less
>draft in cutting CO

Yes both from excess air and turbulence

>  2.) that the coals need to be close to the fire for
>good performance

Otherwise heat losses are too great and they cannot sustain burning in
the low oxygen flue gas?

"Tests show that there is plenty of oxygen in the flue gas even at the top
of the combustion chimney. As you say, the coals sustain burning if close to
the wood, not if too far away."

.
>
>IDEA THREE We built four types of vanes placed in the Rocket chimney above
>the fire to see if we could create turbulence. We agreed that: 1) Because
>the air in the chimney has low velocity the small vanes we created did not
>make much swirl or mixing. A larger vane set up was too large and
suppressed
>the draft creating a lot of smoke. Needs more work...

Am I correct in thinking the vanes were creating turbulence in the
flue i.e. post the combustion area? If so why? creating turbulence
uses power, consuming power in the chimney is wasted. That is why I do
not advocate chimney dampers, if you need a chimney damper it means
your air controls are inadequate and you are squandering air movement
power that should be available to create turbulence in the combustion
zone.

"I agree, except that: the easiest way to produce less smoke in a Rocket
stove is to make the insulated chimney above the fire higher. But the excess
draft created by the taller chimney pulls too much air past the sticks
lowering exit temperatures. Bad for cooking efficiency...So, using up the
excess draft by tumbling hot flue gases in the chimney seemed a possible way
to make the taller chimney (which gives more time for combustion to occur)
practical."
>
>
>Best,

Dean


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