Outdoor Wood Burning Gasification Boilers/Furnaces
How the gasification process works
When you burn wood under "open" conditions (which includes most wood stoves), it
burns at about 1200 degrees. This is hot enough to burn only the volatile oils and
some of the gases in the wood, but not the wood itself - which is why relatively large
amounts of unburned wood is deposited as ash or escapes up the chimney as smoke. .
The gasification process burns wood under such conditions that it cooks out the carbon
monoxide, hydrogen, and other trace gases. This occurs in the primary combustion
chamber very similar to a wood stove, except the oxygen is limited so it produces
carbon monoxide instead of carbon dioxide.
From there, the wood gas travels through a ceramic nozzle to a secondary combustion
chamber. At some point in, or just after the nozzle, more air is injected which combusts
the wood gas at about 2000 degrees and burns any remaining ash/smoke particles
suspended in the stream of gas and smoke.
The super heated flue gas then travels through a heat exchanger in the furnace
transferring the heat to water used to heat the house.
Outdoor furnaces sold in Maine after April 1, 2008 must comply with Phase I of Maine's
rule (0.60 lbs/MMBtu) and furnaces sold after April 1, 2010 must comply with Phase II.
Phase II has an emissions limit of 0.32 lbs/MMBtu.
The new, EPA compliant gasification boilers can be distinguished from traditional
outdoor furnaces by the "EPA Orange Tag Approved" label. However, not all "EPA
Orange Tag Approved" outdoor furnaces meet the stricter standards imposed by
By the way, the outdoor wood furnace has been
officially renamed "Hydronic Wood Furnace" by the
American Society for Testing and Materials to more
accurately describe what has also been known as a
The term "boiler" suggests these devices operate
under pressure and produce steam for heating, when
in fact, most outdoor wood boilers operate under
zero pressure and produce no steam, only hot water.
That being said, the terms "furnace" and "boiler" are
still used interchangeably on various manufacturers'
websites. Back to gasification; here's how it works:
|Central Boiler E-2300 Classic
Wood Gasification Boiler
With an efficiency rating of 85%, wood gasification is about 30% more fuel efficient
than the typical wood burning stove or furnace. To put it another way, you need 30%
less wood to produce the same amount of heat.
Some manufacturers of outdoor wood burning gasification boilers also offer indoor
models. These are basically the same as the outdoor model minus the shed. Prices
range from $7,000 - $10,000, which compares favorably with traditional outdoor wood
Here's a link to the current list of EPA Orange Tag Approved outdoor wood burning
gasification boilers and the Phase 2 White Tag Hydronic Heaters.
Ash and some small chunks of wood from the primary burn chamber get
blown through the nozzle into the secondary chamber. This type of furnace
does not need an ash box.
Outdoor wood burning
gasification boilers and furnaces
are now being offered in response
to the backlash against traditional,
smoky, outdoor furnaces.
The gasification process practically
eliminates smoke and reduces
emissions to comply with current
Traditional outdoor wood furnaces
average 1.8 lbs of particulate
emissions (smoke) per MMBtu (one
million BTU of heat input). The
current standard set by the EPA is