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EPA Approved Wood Stoves: How do they do it?

Q: Call me a skeptic, but I just want to know exactly how an EPA approved wood stove reburns the exhaust. Your site explains very well WHAT happens, but doesn't go into detail about HOW it is happening. I am referring to the chemical reactions that are taking place during the "reburn". I have an Engineering degree and have had many classes in Physics and Chemistry. Is there a website somewhere where I could read a white paper on exactly how this reburn works?

Thanks,
Kirk Seddon

Sweepy How about a Black paper?

Before we get into the chemical reactions involved, let's establish three "givens":

  • The goal of a modern woodstove is to reburn the exhaust from the primary fire, to extract the heat value and reduce emissions.

  • The reaction that accomplishes this is called secondary combustion.

  • For secondary combustion to occur, three things must be present in the right quantities: fuel, oxygen and heat.

In a typical wood stove of bygone days, there was plenty of fuel available (the wood exhaust from the fire), but very little oxygen (most of it was burned up by the primary fire), and insufficient ignition temperature (too much of the heat from the fire radiated into the room or escaped up the flue). These old "smoke dragons" belched the exhaust from the primary fire directly into the admosphere.

Briefly stated, the job of today's thermal design engineers is to overcome these oxygen and temperature deficiencies, to enable combustion of the exhaust gases.

In modern stoves, the insufficient oxygen problem has been solved by incorporating additional air intakes into the design of the stove, to deliver combustion air directly into the secondary burn area. We refer to this infusion as secondary air.

The insufficient ignition temperature problem has been solved by incorporating one of three different designs:

Technique #1: The catalytic reburn (lower the ignition temperature of the fuel)
By definition, a catalyst causes a reaction to occur that would not occur in its absence. In catalytic woodstoves, the desired reaction is the combustion of wood exhaust at lower temperatures than would ordinarily be required. To accomplish this, a substrate (typically honeycomb ceramic) is coated with a catalyst (typically a blend of platinum and palladium), and installed in the stove in such a way that the rising exhaust gases from the primary fire must pass through it (and come in contact with the catalyst) on their way out the flue. In operation, the wood exhaust hits the catalyst in the presence of secondary air (introduced as described above), and ignites and burns at about 500 degrees. Catalytic stoves are the easiest to design, although some means must be incorporated to bypass the converter until a freshly loaded stove reaches lightoff temperature. Wood stoves equipped with catalytic converters are a bit more complicated to operate, as they must be manually engaged when the stove reaches lightoff temperature, and require periodic cleaning to remove flyash that coats the catalyst. Further, care must be taken not to burn anything that might foul the catalyst, including but not limited to colored ink, certain metals, plastic products, etc. Catalytic converters become progressively less effective over time, and most manufacturers recommend they must be replaced every 3-5 years. Catalytic stoves are known for relatively long-duration, relatively low-temperature fires.

Technique #2: The trapped heat reburn (raise the temperature inside the stove)
A secondary combustion chamber is incorporated into the design of the stove, at the top of the firebox. Space-age insulation is installed on top of the chamber, to "trap" rising heat from the primary fire in the chamber below. The secondary air inputs in this design are fed by passageways that are exposed to the heat from the primary fire, thus preheating the secondary air to help ensure that secondary ignition temperature is achieved and maintained. In operation, the rising exhaust from the primary fire encounters the pre-heated air in the superheated secondary combustion chamber and ignites, at a temperature of about 1100 degrees. Trapped heat stoves are more difficult to design, but easier and less expensive to operate and maintain, as there is no need to engage a catalytic converter, or clean it and replace it as needed. Trapped heat stoves are known for higher heat output and a livelier, more attractive fire view. All of the woodburners we sell use this technique.

Technique #3: The downdrafter reburn (achieve direct ignition via contact with the burning coal bed)
Downdrafting stoves are designed so the exhaust from the primary fire is drawn downward, through the hot coals, before rising up the flue. Again, secondary air is introduced, but in this case, it enters the firebox below or adjacent to the coal bed. In operation, the wood exhaust ignites when it contacts the burning coals, and burns at about 1100 degrees. Downdrafter stoves seem to be the hardest to design (we know of less than a handful of current models), and also the most difficult to operate at maximum efficiency, as the necessary hot coal bed must be created at startup, then maintained throughout the burn. Downdrafters require more frequent tending than either of the other two designs, and reportedly need a fairly strong, fairly constant chimney updraft to prevent "stalling" (extinguishing secondary combustion).

To read about the possibility of creating one of these designs yourself, click here.

To read about why we prefer the trapped heat reburn technique, click here.

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8/12/08: Terminology Correction?

Q: Catalytic CONVERTER? Didn't you mean a catalytic COMBUSTOR? If not I do have a '89 Ford Mustang that needs exhaust parts.

John Doe

Sweepy A: Hi John,

Nice to hear from you! Hope Jane is well.

The catalytic element in a wood stove is indeed a combustor, as it causes the combustion of unburned exhaust particles and volatiles from the primary fire.

It is also a converter, as it converts the thermal energy in those particles and volatiles into heat.

Thus, in the hearth product industry, the terms "catalytic combustor" and "catalytic converter" are used interchangeably. Google "wood stove catalytic converter" and you'll see what I mean.

 

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