29
Defiant Woodburning Stove
30003841
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 540° - 650° C (1000°
- 1200° F) range to the 260° - 315° C (500° - 600° F)
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
a Corning “Long-Life”
®
or an Applied Ceramics combus-
tor. The products are equivalent.
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.
