permaculture

[Toby Hemenway <toby@patternliteracy.com>]

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