August Stove Course: CO emissions

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

Dear Friends,

Just a quick report while it's fresh in my mind...I'm leaving for vacation
and will be on the river for a week which might erase my all too faulty
circuits.

Rob Bailis, Marian Grebanier, Michael Kunashko, Miho, Martin, and Tami Bond
met with Larry Winiarski, Ken Goyer, Peter Scott, Mike Hatfield, Damon Ogle
and myself ( the Aprovecho stove crew) this week to build stoves and see if
we could lower the CO created. Rob brought a Enerac 2000 combustion analyzer
which pretty much agreed with the humble HOBO CO data logger and meter that
we use. Emissions were checked at the top of a 6' chimney above the stove
hood where it was cool enough for the HOBO that can't stand high
temperatures. Using our set up we are limited to comparing stoves and can't
make guesses about what the stove emissions would do to a kitchen. The
numbers/stoves are only comparable to eachother.

It was only on the last day that I could predict what effect feeding the
wood into the stove would have on the amount of CO flying up the chimney.
Very small changes tending the fire resulted in large changes in CO. We saw
numbers in the thousands (parts per million) which very quickly could go
down to the teens. Putting a stick on the fire could shoot the numbers up
hundreds of points. I was truly amazed at the variability.

I intended that we would try to test the following approaches to lowering CO
emissions:

1.) Have smoke go through coals
2.) Force smoke to scrape against surfaces above 1200F.
3.) Natural draft is used to create turbulence
4.) Preheat primary or secondary air
5.) Increase the time that fire is inside combustion chamber
6.) Keep the portion of the wood that is not burning cool

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  2.) that the coals need to be close to the fire for
good performance 3.) the screen has to have 1/4" holes or larger to remain
unclogged. Whether the approach has merit needs more experimentation to
determine.

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...

IDEA SIX Blocking most of the air entering the fuel magazine above the
sticks while allowing air to pass under the sticks through the coals
resulted in a controllable rate of CO production. It was possible to keep
the stove between 20 to 30 PPM by: 1.) using four sticks at a time to make a
big fire that shot flames out of the 17" high internal chimney above the
fire 2.) Using less wood and creating a less active fire made more CO. We
agreed that a hot, fierce fire with lots of draft seemed to produce less CO.
3.) Allowing air to pass over the sticks may raise the levels of CO but more
tests need to be done.

Many other topics were discussed, light weight bricks were made, we enjoyed
making food for staff using Lanny's wok stove and a submerged two pot Rocket
stove, Larry made excellent grilled chicken on his beautiful wood fired
downfeed barbecue (like Peter Verhaart's) at a party, I had a great time
with such wonderful, productive folks.

And I'm already looking forward to continuing this fascinating look at CO.
Then on to particulates!

Best,

Dean





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