Note: Descriptions are shown in the official language in which they were submitted.
IMPR~VED CATALYTIC WOOD S'l'OVE
Background of the Invention:
This invention re1ates to the field oE wood
burning stoves. More particularly, this invention
relates to wood burning stoves incorporating a
catalytic combustor for combustion (oxidation) of
unburned components (typically carbon monoxide and
hydrocarbons) normally present in exhaust smoke.
The use of catalytic combustors is a fairly
recent developmen~ in the field of wood burning
s~oves. Cataly~ic combustors have been incorporated
in a number of wood burning stoves sold commercially;
and the subject of catalytic combustors has been
discussed in various papers, such as the paper
entitled "Catalytically Assisted Combustion In
~5 Residential Wood-Fueled Heating Appliances" and
"Catalytic Combustion In Residential Wood Stoves"
pre~ented respectively by J.W. Shelton and R.V. Van
Dewoestine at the E.P.A. conducted conference on Wood
Combustion Environmental Assessment held in
cooperation with The Wood Heating Alliance l981
International Trade Show and Wood Heating Seminar in
New Orleans9 Louisiana, February ~1-24, 1981 and the
paper entitled "Catalytic Wood Stoves Utilizing The
Corning Wood Stove Combustor (Operation and Design)",
January l982 by F.E. Noll.
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It has been known in the field of wood burning
stoves that a considerable amountl perhaps as much as
30~, of the chemical energy available from wood fuel
leaves a typical wvod stove unburned as smoke. This
creates not only air pollution problems~ but ~150
results in the build up of creosote inside the
chimney or the connector from the stove to the
chimney. The escape of unburned combustibles is both
an economic problem (the cost of heating is
increased) and an air pollution problem (which has
the potential of becoming of serious magnitude with
the increased use of wood burning stoves). Tile
problem of creoso-te build up is of concern because it
can result in dangerous chimney f1res, and the
creosote can cause corrosion in chimney systems and
can create odor problemsO
The use of catalytic combustors in wood stoves,
as has been proposed in the past, offers the
theoretical po~entlal advantages o increases in
energy efficiency of the stoves, reduction in
pollution, and reduction in creosote build up
problems. However, despite apparent indications to
the contrary in some literature, practical experience
with catalytic combustors has shown the existence of
several problems. Practice has shown that high
primary combustion flame temperature in the firebox
and direct impingement of the flame on the combustor
is required in order to fire off the combustor and
keep it in full operation. If t~le primary combustion
flame temLoerature has not been very high and did not
implnge directly on the catalytic combustor then the
catàlytic combustor will not ignite or will not
function properlyO The dlrect impingement of high
temperature flame heretofore required to liyht of
the cataly~ic combustor resulted in thermal shock to
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the combustor, also contributed to burn out of the
combustor and high fuel consumption to generate the
necessary primary flame temperature to set off the
combustor. As a result, combustors (which are
expensive) typically burn out or disintegrate in less
than one season. This has led to schemes such as
flame shields (a metal plate) in front of the
combustor to protect against flame impingement; but
such shields are counterproductive in that they make
it more difficult to light off the combustor. Also,
to combat disintegration, elaborate "canning" schemes
have been proposed to encase the combustor in a metal
case.
Conventional prior art practice for use of
catalytic combustors has also required the use of
secondary airflow delivered to the catalytic
combustor. The secondary air has been necessary in
prior systems in order to have sufficient oxygen for
combustion in the combustor. However, it is known
that secondary airflow can be counterproductive in
that too much secondary airflow can quench the
catalytic combustor. Furthermore, the secondary air
is at a lower tempera~ure than combustion gases, thus
aggravating the problem of developing a sufficiently
high temperature to set off the combustor.
Summary of the Invention:
The above discussed and other problems of the
prior art are reduced or overcome by the present
invention which produces a more optimizPd catalytic
combustor stove structure than heretofore known. In
accordance with the present invention, a combustor
ignitor plate is disposed in the firebox in the
vicinity of tne combustor~ l'he combustor ignitor
plate is positioned in the path of gas and airflow to
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the combustor and is spaced from the combustor~ The
combustor ignitor plate defines an air space above
the plate between the plate and the combustor and
functions to create one or more zones of high turbu-
lence and mixing of combustion yases from theprimary burning of the wood and oxygen from the
primary air supply to both the main volume of the
firebox and at the entrance to the combustor to
produce a thoroughly mixed gas which is delivered
to the combustor with sufficient amounts of primary
air oxygen for combustion.
In accordance with a particular embodiment
of the invention a stove includes a firebox for hold-
ing and burning a load of fuel. Air inlet means
deliver a primary air supply to the firebox for
combustion. Exhaust means discharge the gases
from the firebox. Catalytic combustor means are
provided in the firebox for combustion of gaseous
products emanating from the fuel. Combustor ignitor
plate means are mounted in the firebox to produce a
mixed gas of the gaseous products and at least part
of the primary air supply for delivery to the com-
bustor and also effect ignition of the combustor~
The present invention also incorporates a
manifold plate across the front of the stove to
deliver the primary air supply to the stove in a
screen flow across the front of the stove. This
primary airflow serves both to feed the primary
combustion of the wood in the stove, may also form
a secondary burning zone in the nature of a stand-
ing flame, and delivers unburned oxygen to the
combustor ignitor plate for thorough mixing with
the primary combustion gases for delivery to the
catalytic combus-tor.
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Tests of a stove built in accordance with the
'present invention have shown that the catalytic com-
bustor of a conventional prior art type will be fired
or set off at low firebox temperatures without direct
impingement of the flame and almost simultaneously
with the initiation of primary wood burning in the
stove, and the catalytic combustor operates continu-
ously along with normal primary wood combustion in
the stove. Significantly, primary airflow can be
greatly cut back to reduce primary wood combustion
to almost a smoldering state, and yet the combustor
will operate at increased temperatures to burn
essentially all of the combustibles in the
smoke at a high temperature in the combustorO Thus,
fuel consumption can be significantly reduced while
maximum available energy is useably extrac~ed from
the fuelO
Brief Description of the Drawinys:
Referrring now ~o the drawings, wherein like
elements are numbered alike in the several FIGURES:
FIGURE 1 i5 a front elevation view of a wood
burning stove incorporating the pres~nt lnvention.
FXGURE 2 is a sectional side el.evation view of
t~e stove of FIGURE 1.
FIGURE 8 is a partlal perspe~tive view of the
stove of the present invention with the front doors
removed and the side partially segmented ~o show the
combustion ignitor plate and airflow manifold of the
present invention.
Description of the Preferred Embodiment:
Referring to the three FIGURES jointly, a stove
10 of ~he fireplace insert type is shown, although it
will, of course, be understood that the invention is
equally applicable to Eree standing stoves. Stove 10
has an insert frame or box 12 which is intended to
fit into the openlny of a fireplace, the front
opening of which would then be sealed off by a face
or cover plate (not shown) attached to the front of
insert 12. Stove 10 has a generally cubicle firebox
10 which defines an interior combustion space 16 in
which the wood charge is placed for burning. The
bottom interior of firebox 14 is lined with
refractory brick 18 in the customary manner, and the
wood load to be burned would be placed on the brick
(on a raised grate if desired). The stove has a pair
of pivo~ally connected doors 20 which may have glass
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windows 22. The doors have been removed from FIGURE
3 for purposes of clearer illustration of the
interior of the firebox. The stove also has a bypass
damper 24 operated by a projecting operating handle
26 and a flue damper 28 also operated by a projecting
operating handle 30. The stove str~cture described
immediately above in this paragraph is a conventional
fireplace stove, and i~ lS descrlbed ror the purposes
of setting the environment for the present invention.
A catalytic combustor 32 is positioned ln the
stove toward the front of the fireb~x, as best shown
in FIGURE 2. This catalytic combustor may be any
catalytic combustor known in the art, such asS for
example, a one piece cellular ceramic honeycomb uni~
made o~ Corning Celcor Code 9475 Cordierite coated
with a Nobel metal catalyst of the platinum metal
family. This combustor is available from Corning
Glass Works, Corning, New York. The combustor is
mounted in the stove and held in place in accordance
with known techniques for mounting this catalytic
combustor. In the preferred embodiment of this
invention, a screen is placed in front of the
entrance to the sombustor to act as a filter to
prevent paper particles, etc. from entering and
physically clogging combustor passages. However~ a
~lame shield located below the combustor to protect
it from direct flame impingement from the primary
combustion flame in the firebox (as suggested in the
prior art) is not needed, because the usual direct
flame impingement is not encountered.
In accordance with the present invention, a
combustor ignitor plate and a primary air delivery
system are included in the stove and provide
extremely unexpected and lmproved results in
operation. The primary air supply system includes an
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air supply manifold box or tube 34 with an array of
spaced airflow holes 35 distributed along the length
of the manifold. Manifold 34 is positioned across
the width of the stove immediately inside the front
door area. Manifold box 34 communicates with a
primary air draf~ control valve 38 (there being one
each of the drafts 38 at each side of the firebox).
Drafts 38 communicate directly wi~h the hollow
interior of manifold 34, and the drafts may be
manually adjusted to control airflow. A sight glass
39 may be provided to view combustor 32.
The present invention also irlcorporates as its
key element a combustor ignitor plate 40 which
cooperates and interacts principally with catalytic
combustor 32 and possibly also with the primary air
supply from manifold 34 tO produce totally surprising
and unexpected results. Plate 40 extends between the
sides of the firebox along substantially the entire
front width of the firebox and it may be removably
mounted in a friction clamp bracket 42 adjacent to
each side of the stove ~or it could be an integral
part of the stove). It is preferred that the plate
40 extend the full front width of the stove for
optimum performance, but a lesser width of the plate
may also produce acceptable improvement in operation
of tne stove. As can best be seen from a combined
consideration of FIGURES 2 and 3, plate 40 is
posltioned at an angle to the vertical axis of
combustor 32 and extends from a position (starting at
the left as shown in FIGURE 2) beneath the combustor
and behind the frontmost portion of the combustor to
the right to a position above the bottom of the
combustor and forward of the frontmost portion of the
combustor. Thus, plate 40 overlaps a portion of that
part of the combustor into which the combustion gases
flow and extends beyond the combustor toward the
front of the stove above where the primary airflow
enters the stove. While ~he exac~ configuration
shown in FIGURES 2 and 3 may not be the only operable
arrangement, it is believed to be important that the
combustor ignitor plate 40 be in a positlon where it
both overlaps the combustor to project into the gas
stream flowing into ~he combustor and also extends
into the primary airflow of air entering the firebox
to deflect the major portion of primary air in a
swirling and turbulent pattern into the main fire
portion of the firebox (i.e., that portion where the
wood to be burned is located). It is also important
to insure airflow above plate 40 into the space
52(a), either by an airflow space 46 defined between
the forwardmost edge of plate 40 and the front wall
44 of the firebox or airflow passages in plate 40 or
otherwise. It is believed that airflow with space
52(a) plays an important part in the success of the
present invention, probabLy by providing a warm
unburned air supply for the combustion gases at the
entrance to the combustor.
In the operation of the stove of the present
invention, primary air is introduced into the firebox
through drafts 38 and manifold 34 through the
marlifold holes 36. The major portion of this primary
air flows upward (arrow A) to the plate 40 and is
deflected back down by plate 40 to form a swirling
and turbulent air flow (a main mixing zone) for the
main burning portion of the firebox, as indicated by
the arrows A to feed and support combustion of a wood
load (not shown) located in the firebox. Bypass
damper 24 is closed during normal operation of the
stove, so the gaseous products vf combustion from the
burniny wood in the stove flow through the honeycomb
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passage structure of combus~or 32 and are then
delivered via flow passage 48 to exhaust s~aclc 50
which is connected tc> a chimney or other suitable
exhaust structure. While all of the reasons and
details are not fully understood at this time, ~ests
of the presen~ invention have shown that the presence
of combustor ignitor pla~e 40 produces ignition or
light off of the combustor almost immediately upon
the beginning of wood burning in the stove and at the
rela~ively low temperatures of normal primary
combustion in the stove and without direct flame
impingemnet on the combustor. While the reasons are
not fully understood, it is theorized that the
airflow from manifold 34, in addition to delivering
air to support combustion in the center of the
firebox, se~s up a screen of air B which washes the
interior of the front wall of the firebox and moves
toward the top of the firebox. This air stream, lt
is theoriæed, flows through airflow space 46 and into
what may be referred to as second mixing or
turbulence spaces or zones 52(a) and 52(b). Zone
52(a) is defined by the cooperation of plate 40 and
combustor 32 and the stove walls at the top front of
the stove while zone 52(b) is between the plate 40
and the entrance to the combustor. Simultaneously,
the gases of combustlon which contain large amounts
of unburned combustible products flows, as generally
indicated by arrows C, toward the bottom of the
combustor to enter the combustorO Wnile the causes
and reasons are not fully understood, it appears that
the structure of the present invention results in
significant turbul~nce and mixing in zones 52(a)
and/or 52(b) (perhaps much in the nature of the
effects of an automobile carburetor~ of the
combustion gases from the fire and the primary air
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from air flow B flowing along the front of the stove
into space 52(a). In one way or another, the
structure produces significant turbulence and mixing
(as indicated by the small circularly shaped arrows D
and E, which are shown only for purposes of
illustration) to provide a gas flow at the entrance
to the combustor which is a thorough mixture of the
products of com~ustion from the fire and primary air
oxygen which has been warmed by its passage along the
front of the stove and the swirling action induced by
plate 40. The result is the delivery to the
combustor of a thoroughly mixed and warm gas stream
which has a high content of combustible material and
an adequate supply of heated but unburrled oxygen
whereby catalytically induced combustion immediately
occurs in the combustor to burn substantially all of
the unburned combustion products.
I~ is significant to note that with the structure
of the present invention, light-off or ignition of
the combustor occurs almost immediately upon the
ignition of fire in the firebox and occurs at
relatively low temperatures of the combustion gases
(such as on the order of 600 F) encountered in
normal primary combustion in the firebox and, most
significantly, without direct Elame impingement on
the combustor~ Indeed, for reasons not fully
understood, with the construction shown, direct flame
impingement (i.e~, the flame being drawn or sucked
into the combustor) does not occur. It is also
important to note that with the structure of the
present invention there is no need for secondary
airflow to the combustor. Generally, prior art
cataly~ic combustor structures have required a
secondary airflow to the catalytic combustor; and
this secondary airflow has actually created problems
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for proper operation of the combustorO The secondary
airflow is cool~ while the combustor normally
requires high temperatures to operate, and the
secondary airflow may result in oxygen rich quenching
5 of the combustor operation.
While it is recognized that the structure of the
present invention is relatively simple, the simple
structure produced significantly unexpected results
in a synergistic combination. The combustor ignitor
plate 40 cooperates with ~he combustor 3~ to produce
a synergistic effect whereby the combustor may be lit
off and maintained in continuous operation throughout
normal burning operation of the stove at relatively
low temperatures of normal combustion and without
direct flame impingemen~ on the combustor.
Furthermore, while it is not known whether the air
supply manifold structure 36 is essential to the
operation of the device or whether other primary air
supply systems may be used, the air supply manifold
structure is known to also contribute to the
synergistic effects to produce essentially complete
combustion. Tests have shown that with the use of
the air supply manifold, air and combustion gas
mixing occurs so thoroughly in the firebox that a
standing flame may be created immedi~tely behind the
front door irl a part of air stream 51 which
constitutes, in effect, a secondary burn of the
combustlon gases.
Tests have also shown the synergistic result ~hat
main airflow and combustion can be reduced while
increasing the catalytic combustor operation and
temperature. Thus, in one such test, the drafts 38
were first operated essentially fully open to produce
maximum airflow to the wood load in the firebox. In
that situation, ~he wood burned with a vigorous flame
(which, however, did not get drawn or sucked up in~o
combustor 32), and the combustor also lit off
indicating the occurrence of catalytic combustion.
In this first instance, firebox internal temperature
was in the range of 600-700 F, the catalytic
combustor temperature was about 1000 F, and the
exhaust stack temperature was about 350 F. Then,
draf~s 38 were closed down to reduce the primary
airflow to about 25% of its original value. In this
second instance, the flame in the firebox died way
down (almost, but not completely extinguishing). I'he
wood then smoldered and yenerated large amounts of
smoke (containing large amounts of unburned
combustion products) which were delivered to
combustor 32. The combustor 32 not only continued to
operate, but its temperature rose significantly to
the range of 1200 F (thus indicating increased
burning in the combustor), while the firebox
temperature was reduced to the range of about 400
F and the stack temperature was reduced to about
200 F~ Therefore, with reduced airflow more heat
was generated from the catalytic combustor while at
the same time thee rate of consumption of fuel (wood)
was reduced. This was achieved by the more efficient
burning of combustion products in the catalytic
combustor realized by the present invention.
To es~ablish the synergistic effects oE the
combustor ignitor plate 40 (which causes the
combustor to light off and continue operation without
direct flame impingement), plate 40 was removed/ and
the stove operated. However, without plate 40
present, the catalytic combustor 32 would not light
off without direct flame impingement.
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The ability to operate the catalytic combustor at
high efficiency and without direc~ flame impingement
means that the operating life of the catalytic
combustorn can ~e greatly extended, tnus making the
catalytic combustor ~seful in a practical an
economical sense for the first time.
Whi3e preferred embodiments have been shown and
described, various modifications and substitutions
may be made thereto without departiny from the spirit
and scope of the inventiorl. Accordingly, it is to be
understvod that the present invention has been
described by way of illustration and not limitation.
