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  • From: Toby Hemenway <toby@patternliteracy.com>
  • To: permaculture <permaculture@lists.ibiblio.org>
  • Subject: Re: [permaculture] small cooking woodstove
  • Date: Tue, 09 Oct 2007 19:55:49 -0700

Healing Hawk wrote:
I was wrong about wood burning contributing to global
warming. It contributes to air pollution, not to global warming.

True; most wood stoves are awful. It's surprising that we're still using 18th-century technology. And industry then takes a poor design and sticks a bunch of toxic catalytic converter afterthoughts on it to clean up the mess it makes, instead of asking why it is so dirty in the first place (just as it does with internal combustion engines).

But I once again must tout the glorious rocket stove. I have my rocket-stove heated cob bench warming in my Portland back yard as I write, and the fire is utterly clean-burning. The chimney at the moment opens right under the eave of the sauna, since I haven't bought the roof-penetration hardware nor cut the opening, but it doesn't matter. There is no smoke and almost no heat leaving the chimney, so the underside of the eaves is not blackened at all. And during a burn, steam from the chimney condenses on the eaves and they drip water, showing the near-perfect conversion of fuel to water and CO2.

The genius of the rocket stove is that it separates combustion from heat exchange. The reason fireplaces and wood stoves are so dirty is because the fire is constantly radiating heat and sucking in cold air, which cools it, resulting in a partial, smoky burn. You need a hot burn to completely combust the fuel. The key is to do the burn in an insulated firebox that is designed to mix the burning gases well, so they get ridiculously hot. Then the hot CO2 and steam that result rise into a heat exchanger, which can be a metal barrel, large earthen mass like a bench, or a water heater. When the system is sized right, the gas leaving the chimney is 150-200 degrees F or less (60-90C or so) and you can put your hand on the end of the stovepipe, or more impressively, stick your face over the opening and just get a little damp from the water. It's awesome--and clean, outrageously efficient, and you can build it yourself cheaply. Again, I recommend Ianto's book, "Rocket Mass Heaters." A download pdf is available for US$10 at the rocketstove website, below. There is no excuse for dirty wood stoves; wood can be burned cleanly using very simple technology.

But words don't do it justice. Links to see pictures are at
http://www.countryplans.com/cgi-bin/yabb2/YaBB.pl?num=1167119392
and Ianto Evans's website (he's the co-inventor)
http://www.rocketstoves.com/index.htm
(that site is a little clunky and you have to click around a bit to find everything)

and since none of these has a good schematic of a rocket stove, I put one up at
http://patternliteracy.com/rocketstove.html

Toby
http://patternliteracy.com








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