Defiant Woodburning Stove
Appendix: Catalytic Combustor
In any chemical reaction, including the combustion process, there are certain conditions which must be met before the reaction can take place. For example, a reaction may require a certain temperature, or a certain concentration of the reactants (the combustion gases and oxygen), or a certain amount of time. Catalysts act at a molecular level to change these requirements, though they are not changed themselves during the re- action. In the Defiant’s secondary combustion chamber the catalyst reduces the temperature at which second- ary combustion can start from the 1000° - 1200° F. (540° - 650° C) range to the 500° - 600°F. (260° - 315°
C)range, increasing efficiency, and reducing creosote and emissions.
The catalytic reaction does have some limitations of its own. Primary among these is that the reactants (the gases) come into close physical contact with the catalyst itself.
To ensure the necessary contact, the catalytic element in your Defiant is a ceramic base in the shape of a honeycomb. Each of the honeycomb’s many surfaces carries a coating of the catalyst (usually a noble metal such as platinum or palladium). The large surface area exposed in this way ensures that the combustion gases have the greatest opportunity to come in contact with the catalyst.
Loss of catalytic activity will be apparent in several ways. First you may notice an increase in fuel con- sumption. Second, there will be a visible increase in the rate at which creosote builds up in your chim- ney connector system. You may also notice a heavy discharge of smoke from the chimney. There are a number of catalytic problems which can cause loss of activity:
Blockage
While the honeycomb pattern ensures good contact, it also increases the resistance to flow of the combustion gases, and, because of the many surfaces, provides more places for creosote and fly ash to deposit. It is important to follow the operating instructions in order to minimize these deposits, and to periodically inspect your catalyst for signs of blockage.
Masking and Poisoning
While the catalyst itself does not enter into the combus- tion process, certain elements, such as lead and sulfur, can attach to the active sites on the surface of the hon- eycomb. Though the catalyst is still there, it is covered, or masked, by the contaminant, and cannot function.
To avoid this situation, it is important not to burn any- thing in your Defiant that is a source of these contami- nants. Particularly avoid painted or treated wood, coal,
household trash, colored papers, metal foils, or plastics. Chemical chimney cleaners may also contain harmful elements. The safest approach is to burn only untreat- ed, natural wood.
Flame Impingement
The catalytic element is not designed for exposure to direct flame. If you continually overfire your Defiant, the chemistry of the catalyst coating may be altered, inhibit- ing the combustion process. Do not use your Defiant with the baffle removed or damaged, as it shields the catalytic element from direct flame.
Thermal degradation of the ceramic base may also oc- cur, causing the element to disintegrate. Stay within the recommended guidelines of the Operation section.
Overfiring
At low firing rates, especially with a large load of fuel, the catalytic element can become overloaded with gases which do not burn in the firebox. This puts increased strain on the catalytic element, causing it to operate near or even beyond the high end of its nor- mal temperature range. An immediate sign of this is a low rumbling sound, or a sound like a small gas torch, coming from the stove’s rear chamber. If you frequently hear such a sound, or continually see a bright glow through the baffle, open up the stove’s primary air valve to make the fire burn more cleanly in the firebox, and cut back on the size of your firewood load. Essentially the catalyst is an afterburner, and will last longer under lighter loads.
Mechanical Damage
If the element is mishandled, damage may occur. Always treat the element carefully. Remember the catalyst is made of a ceramic material; treat it as you would fine china. Hairline cracks will not affect the performance of the catalyst, as long as the steel sleeve holds the element in the proper position.
Peeling
The surface coat may peel if the catalytic element is frequently subjected to excessive temperatures. Follow the operating instructions carefully to avoid this type of damage.
General
Every Vermont Castings’ Defiant is equipped with either
aCorning
If you must ship your catalytic element for any reason, remember its fragile nature. Place the element in a plastic bag, and package it with a generous amount of shock absorbing material.
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