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.