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Vermont Castings Defiant
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 reaction. In the Defiant's secondary com-
bustion chamber the catalyst reduces the temperature
at which secondary 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 chimney
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
combustion process, certain elements, such as lead
and sulfur, can attach to the active sites on the surface
of the honeycomb. 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 anything in your Defiant that is a source of these
contaminants. 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 untreated, 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,
inhibiting 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
occur, 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 normal
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. Essen-
tially 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
combustor. 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.