CA1225554A - Solid fuel heating appliance and catalytic converters therefor - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
This specification discloses solid fuel heating appliances, and more particularly, appliances which burn wood, densified or compacted wood products, coal, charcoal, peat, compacted trash or similar solid fuels and utilize a combustor or catalytic converter. The specification discloses a combuster device for oxidizing oxidizable species in the exhaust from the combustion chamber of a solid fuel heating appliance. The device comprises a structure having a plurality of axially extending cells for passing oxidizable species therethrough and at least one leakage path for the oxidizable species through said structure, or around and adjacent said structure. The structure may preferably comprise a honeycomb arrangement of cells. In one embodiment the leakage path may have a substantially larger cross-sectional area.
The leakage path may extend through the interior of the combustor. By the use of this device there is provided a combustor or catalytic converter for a solid fuel heating appliance wherein the plugging of the cells in the catalytic converter will not result in the leakage of smoke into the living space adjacent the wood burning stove.

Description

~225S5~

SOLID FUEL HEATING APPLIANCE
AND CATALYTIC CONVERTERS THEREFOR

Background of the Invention This invention relates to solid fu~l heating appliances, and more particularly, to appliances which burn wood, densified or compacted wood products, coal, charcoal, peat, compacted tra~h or similar solid fuels and utilize a combustor or catalytic converter.
U.S. Patent 4,373,452 filed July 28, 1980 (assigned to the assignee of this application) discloses the use of a catalytic converter in a wood burning stove. The catalytic converter which 3erves a~ a combu3tor -~ provides more complete burning or oxidation of the volatile and particulate organic substances present in gases arising from burning wood in a wood burning stove and especially ; those solid particles and resinous and oily droplets that cause the dense smoke which upon deposition on the inside surface of the flue pipe or chimney are generally known as creosote. More particularly, a catalytic converter which comprises noble-metal catalysts on a suitable sub3trate - reduces the ignition temperatures of carbon monoxide and the lower boiling, more volatile hydrocarbons present in the exhaust issuing from the combustion of wood. As the hydrocarbons and carbon monoxide burn, the temperature of the catalyst and its substrate is raised which increase~ its catalytic activity. The elevated temperature pyrolyzes and crack~ the - higher molecular weigh~ hydrocarbons occuring in the ~moke as solid particles and oily droplets, converting them to volatile compounds which readily mix with oxygen present and thereby leading to their rapid oxidation. Temporature con~inu2s to rise until the system reaches a temperature at which ~22555~
there is equilibrium between the inlet gas temperature, flow rate and the amount of oxidizable material. This temperature is typically 60~C to ~0~C for a properly sized catalyst system. At these temperatures, oxidation proceeds very rapidly to completion if the catalytic device has the ap-propriate volume and internal surface area. As converter temperatures increase, the exhaust gas temperature ris~s above the ignition point of an increasing number of its constituents so that the catalytic combustion process is augmented by thermal combustion. The high temperatures also break the complex hydrocarbons and other combustibles (including solid particular entrained in the combustion gases) into com-pounds which will burn more easily.
As disclosed in the aforesaid U.S. patent ~,373,452 the catalytic converter comprises a number of cells which extend axially through the substrate so as to permit the stove exhaust to pass therethrough. Since the cells may be relatively small, e.g., 16 cells per square inch (about

2.5 cells/cm.2), it is possible that some or many of the cells may become plugged with exhaust materials including creosote and other hydrocarbon compositions. This, in turn, may produce a safety hazard since the smoke from the stove which is unable to pass properly through the catalytic converter may enter any living space surrounding the stove and create an asphyxiation hazard. It will be understood that such a hazard can occur when a combustor or catalytic converter is utilized in any solid fuel heating appliance which utilizes a solid fuel such as wood, densified or com-pacted wood products, coal, charcoal, peat, compacted trash ...

i225SS~

and/or garbage and the like which may give off, during burning,solid particles and vapor that may lead to some temporary plugging of the catalytic converter.
Adjustable and closeable bypasses have recently been utilized or proposed between a combustion chamber and an exhaust flue ox chamber of a wood burning stove as disclosed in co-pending U.S. application Serial No. 136,687, of Albertsen filed April 2, 1980, (in Canada Serial No. 374,510; in E~O
Serial No. 81-301,389.3 published 7 October, 1981), and U.S.
patents 4,345,528 and 4,330,503 of Allaire et al filed July 28, 1980, all of which are assigned to the assignee of this invention.
The main purpose of these bypasses is to minimize back pressure within the stove having a combustor or catalytic converter when the door of the stove is open which can result in the intro-duction of smoke into the living space around the stove. Such a bypass is spaced a substantial distance from the catalytic converter.
Summary of the Invention It is an object of this invention to provide a combus-tor or catalytic converter for a solid fuel heating appliancewherein the danger of asphyxiation is minimized.
It is a further object of this invention to provide a combustor or catalytic converter for a solid fuel heating appliance wherein the plugging of the cells in the catalytic converter will not result in the leakage of smoke into the living space adjacent the wood burning stove.
It is a further specific object of this invention to provide a combustor or catalytic converter in a solid fuel heating appliance wherein the h~zard of asphyxiation is ~,- 3 -`` ~ZZS5S4 minimized without adversely affecting the efficiency of the stove.
It is a still further object of this invention to provide a combus~or or catalytic converter for a solid fuel heating appliance wherein the hazard of asphyxiation is minimized while at the same time optimizing oxidation of carbon monoxide, hydrocarbons and other combustibles ~including solid particles entrained in the combustion gases).
Thus the present invention provides in a broad aspect a solid fuel heating appliance of the type comprising a combustion chamber, a flue for removing exhaust from said chamber, and a combustor having a plurality of cells extending therethrough for oxidizing oxidizable species in the exhaust.
The improvement comprises provision of at least one leakage path around the cells located immediately adjacent the com-bustor. The leakage path extends through the interior of the combustor.
In another aspect the present invention provides a combustor for oxidizing oxidizable species in the exhaust of a solid fuel heating appliance comprising a honeycomb structure having a plurality of axially extending cells for passing oxidizable species therethrough, at least one leakage path for the oxidizable species through said structure, said leakage path having a substantially larger cross-sectional area than any of said cells, and wherein the leakage path extends through the interior of the combustor.

_ 3a -In accordance with these and other objects of the invention, a preferred embodiment of the invention comprises a solid fuel heating appliance in the form of a wood burning stove of the type including a combustion chamber, a flue for removing exhaust from the combustion chamber and a combustor or catalytic converter means having a plurality of cells extending therethrough for oxidizing oxidizable species in the exhaust. Other -~olid fuel heating appliances which may embody the invention include boilers, incinerators and the like.
In accordance with this invention, at least one leakage path, as a passive bypass, is provided around the cells and is located immediately adjacent the combustor or catalytic converter. In several embodiments of the invention, the leakage path is loca~ed at the periphery of the catalytic converter. In one of the several embodiments, the leakage path may be provided a spaced distance from the catalytic converter but nevertheless in the vicinity immediately adjacent the catalytic converter. In another of these several embodiments of the invention, a leakage path may be provided between a container for the catalytic converter and the periphery of the catalytic converter. In still another of these several embodiments, the catalytic converter i5 movably mounted within an opening and provided with the leakage path adjacent to the periphery, which may also permit a variation in the size of the leakage path. In yet another embodiment of the invention, the leakage path may be provided by one or more axially extending and enlarged openings within the interior of the catalytic converter itself.
In order to provide sufficient leakage, the overall transverse cross-sectional area of the leakage path must be 122555~

substantially greater than the transverse cross-sectional area of any of the individual cells. Preferably, the trans-verse cross-sectional ~rea of the leakage path is substantially greater than the averase cell transverse cross-sectional area of substantially all of the cells. For example, it is desirable to have a transverse cross-sectional area of the leakage path which is at least two and preferably four times as great as the average cell transverse cross-sectional area. In any event, the leakage path should provide for at least 10% and preferably 10% to 40% of the overall flow of exhaust through the converter itself and the lea~age path.
The invention provides an embodiment in which a leakage path around said cells is located immediately adjacent the combustor and the leakage path extends through the interior of the combustor.
Brief Description of the Drawings FIG. 1 is a sectional view of a solid fuel heating appliance comprising a wood burning stove representing a preferred embodiment of the invention including a combustor in the form of a catalytic converter mounted in accordance with this invention;
FIG. 2 is an enlarged fragmentary view of the cata-lytic converter and mounting shown in FIG. l;
FIG. 3 is a bottom view of a catalytic converter and mounting taken along line 3-3 of FIG. 2;
FIG. 4 is a sectional view of a catalytic converter and mounting in yet another embodiment of the invention;
FIG. 5 is a bottom view of the catalytic converter and mounting taken along line 5-5 of FIG. 4;
FIG. 6 is a sectional view of a catalytic converter and mounting comprising yet another embodiment of the invention;
FIG. 7 is a bottom view of the catalytic converter and its mounting taken along line 7-7 of FIG. 6;

lZ2~54 FIG. 8 is a sectio~al view of a catalytic converter and mounting in still another embodiment of the invention; and " FIG. 9 is a bottom view of the catalytic converter and mounting taken along line 9-9 of FIG. 8.

S Detailed Description of a Preferred Embodiment Referring to F~G. 1, a solid fuel heating appliance comprising a wood burning stove 10 is shown including a ' primary wood combustion chamber 12 in the lower portion of the stove and the exhaust or secondary combustion chamber 14 in the upper portion of the stove. A combustor in the form of a catalytic converter 16 is located between the primary combustion chamber 12 and the exhaust or secondary combustion ~, , chamber 14 to promote more complete burning or oxidation of ; the carbon monoxide, hydrocarbons and other combustibles ; 15 (including solid particles entrained in combustion gases) exiting from the primary combustion chamber 12.
The stove 10 includes a grate 18 within the primary combustion chamber for supporting the wood to be burned therein. Air to the primary combustion chamber 12 is supplied through a primary combustion air inlet 20. A secondary - combustion air inlet 22 is coupled to a manifold 2~ which provides air or oxygen to the inlet face of the catalytic converter 16 so as to optimize the oxidation or burning with the catalytic converter. The manifold 24 is positioned and designed in the appliance so as to provide adequate premixing of the secondary air from the manifold 24 with the combustion gases and fumes before they enter the converter 16. Additional combustion or oxidation and/or heat exchange to living space occurs within the chamber 14 before the exhaust gases leave through a flue 26.
An adjustable or closeable bypass to the flue 26 is preferably provided by a damper 28 which is attached at a pivot or hinge position 30 at a wall 31 of the stove 10 (similar to copending Canadian a~plication Serial ~o. 37~,510, although it may optionally be like the damper in U.S. Patent 4,330,503).

In accordance with this invention, the catalytic converter 16 is mounted between the primary combustion chamber 12 and the chamber 14 so as to provide the leakage path around the axially extended cells of the catalytic converter ?. 16 as best shown in FIGs. 2 and 3. More specifically, the catalytic converter 16 comprises an open-ended honeycomb structure with a plurality of axially extending cells 32 supported on rods 34 which extends transversely across a cylindrical structure 36 extending downwardly from a wall 38 separating the primary combustion chamber 12 from the chamber 14. Since the catalytic converter 16 is of lesser diameter than the opening through the tubular or cylindrical structure 36, exhaust f-om the primary combustion chamber is permitted to flow around the periphery of the catalytic converter 16 - as depicted by arrows 40. This flow of exhaust gases as depicted by arrows 40 assures that, even if the axially extending cells 32 of the catalytic converter 16 should become plugged preventing the flow of exhaust gases through the axially extending cells 32 as depicted by arrows 42, exhaust gases will still be permitted to pass from the primary combustion chamber 12 to the chamber 14 and hence to the flue 26 without backing up and entering the living space surrounding the stove 10.

lZ25554 As also shown in FIGs. 1 and 2, ~he mounting of the catalytic converter 16 includes a cylindrical insulating member 44 having a central opening 45 substantially corresponding in diameter with the opening of the cylindrical structure 36.
The insulating material 44 in conjunction with the cylindrical member 36 provides a container for the catalytic converter which permits the establishment of a leakage path 40 between the catalytic converter and the container. By utilizing the insulation material 44, the exhaust gases which pass through the leakage path 44 are maintained in close proximity to the catalytic converter 16 and heat generated at the catalytic convert~r 16 is retai-.ed aO as to create an elevated tempera-ture at the outlet of the catalytic converter 16 to assure at least some oxidation or combustion of the carbon monoxide, hydrocarbons and other combustibles (including solid particles entrained in the combustion gases) flowing through the leakage path even though that exhaust does not pass through the small cells of the catalytic converter 16 itself.
In order to fabricate the structure shown in FIGs.
1 through 3, a refractory insulating material 44 such as FIBERFRAX0 refractory fiber products, may be secured to the wall 38 by means of a refractory of cement, such as QF 180 cement made and sold by the Carborundum Company. The rods 34 pass through openings in the metallic cylindrical structure 36. Although the rods 34 need not be fastened in place within those openings, it may be desirable to provide fasteners at the ends of the rods 34, e.g., nuts received by threaded portions of the rods 34.
Another embodiment of the catalytic converter mounted in a wood burning stove with a leakage path for i2~5554 exhaust gases is shown in FIGs. 4 and S. In this e~bodiment, a cylindrical structure 136 is provided having an interior opening which substantially corresponds with the dimension of the catalytic converter 16 so that there is no leakage path between the catalytic converter and the container 136 for the catalytic converter. The leakage path around the catalytic converter 16 is provided by a vent structure 138 located immediately adjacent the catalytic converter 16.
The vent structure 138 includes an elbow 140 which directs the leaked exhaust gas back to the high temperature area above the catalytic converter 16 so as to assure, to the degree possible, combustion and oxidation of the carbon monoxide, various hydrocarbons and other combustibles (including solid particles entrained in the combustion gases) which are leaked around the catalytic converter 16. In order to mount the catalytic converter 16 within the cylindrical structure 136, the structure 136 includes small flange segments 142 which are of insufficient length or depth to substantially block the cells 32 while at the same time having sufficient length and depth to assure that the catalytic converter 16 is retained within the cylindrical structure 136.
In the embodiment of the invention shown in FIGs. 6 and 7, the leakage around the cells of the catalytic con-verter 16 occurs as a result of enlarged openings extending through the interior of (and thereby being adjacent to) the converter itself. For this purpose, the catalytic converter 16 includes a plurality of openings 2no having substantially larger transverse cross-sectional areas than the cell transverse cross-sectional area of the average or even the largest of ~22S554 the cells 32 through the catalytic converter 16. If so desired, the converter 16 may include only one opening 200 where such a single opening will provide adequate leakage.
The converter 16 is mounted within a cylindrical structure 236 as shown in FIGs. 6 and 7 using a refractory cement such as Super 3000 cement made and sold by Combustion Engineering, Inc.
FIGs. 8 and g disclose yet another embodiment of the invention wherein the catalytic converter 16 is mounted within a metallic ring 300 which is pivotally supported on a cylindrical structure 336 by means of pins 338 which extend through openings in the cylindrical structure 336. In the embodiments of FIGs. 8 and 9, the leakage flow 340 is provided by the annular opening between the ring 300 and the structure 336 and may be adjusted by change in the angle of rotation of the ring 300 about pins 338. As the catalytic structure 16 and the supporting metallic ring 300 is pivoted out of the horizontal plane, the size of the leakage path 340 increases.
In the embodiment of FIGs. 8 and 9, it will be appreciated that the ceramic structure 16 may be mounted within the ring 300 by suitable means including a refractory cement such as Combustion Engineering Super 3000 cement. It will also be appreciated that it may be desirable to displace the manifold 24 (Fig. 1) for the secondary combustion air so as 25 to permit substantial and free rotation of the catalytic converter 16 within the ring 300.
It will be a~preciated that it is important that the leakage path around the cells 32 of the converter 16 be in the vicinity of the converter such that the elevated temperatures in the vicinity of the converter can assist as 1~5554 much as possible in oxidizing or burning the carbon monoxide, hydrocarbons and other combustibles ~including solid particle within the exhaust gases which flow through the leakage path. It is also important that the volume of flow through the leakage path be consistent with the objective that most of the exhaust gases flow through the catalytic converter while at the same time providing sufficient flow through the leakage path to avoid the backing up of smoke if the cells of the catalytic converter should become blocked. In th~s regard, it is desirable that the tran3verse cros~-sectional area of the leak~se path(s) be such th~t at least 10~ and preferably 10% to 40~ of the overall flow of exhaust ga~e~
goes through the leakage path(s~ instead of the axially extending cells. Where the leakage paths are provided ~y enlarged openings of square transverse cro~s-section in the catalytic converter 16 having square transverse cross-sectional cells 32 as shown Ln FIGs. 6 and 7, the amount of : leakage for any given cell distribution can be calculated from the following equation for the total flow through the :20 combustor:
~`
56 8~L t Nl Xl + N2 X42 ~ + N X4 where Q = total volume flow through the com~ustor;
~P = pressure drop across combustor;
~ 25 ~ = gas vi~cosity;
L = co~llbus~or length;
Nl = number of holes of insàde dimension Xl;
N2 = number of holes of in~ide dimension Xz;
Nn = number of holes of inside dimension Xn;

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It will be appreciated that A = combustor frontal area - Nl(Tl + Xl)2 +
N2 (T2 + X2) + ... I Nn (Tn ~ Xn)2 where Tl, T2, Tn are wall thicknesses.
From the foregoing equation for Q, the percentage of flow through the leakage path of a given size opening may be expressed as:
Nn X4n x 100%
(N, X4 ~ N2 X4 -- + Nn Xn) Assuming, for example, a catalytic converter of 14.38 cm.
(5.66 inches) in diameter with 4 square transverse cross-sectioned cells per square cm. (16 square transverse cross-sectioned cells per square inch), the percent of flow through the leakage path using various sized holes of square transverse cross-section in various numbers may be calculated:
Large Hole Number of % Flow Through Size Xn Large Holes Large Holes 2.54 cm. (1.0 inch) 1 60%
1.91 cm. (0.75 inch) 1 29%
1.52 cm. (0.60 inch) 2 25~
1.27 cm. (0.50 inch) 3 20%
As noted in the foregoing, it is preferred that the percen-tage of leakage flow be 10~ to 40% and the hole sizes of 1.91 cm., 1.52 cm. and 1.27 cm. with the number of holes indicated falling within this range. A hole size of 2.54 cm. (1.0 inches) in this particular catalytic converter structure produces a percentage of flow which is somewhat higher than the preferred range. On the other hand, the smaller size holes and the numbers indicated provide sufficient flow without any substan-12~5~

tial risk of hole plugging. In general, it is desirable to provide leakage paths having transverse cross-sectional areas which are at least two and preferably four ~imes as large as the average cell transverse cross-sectional area of the cells.
It will be appreciated by skilled persons in this field of technology ~hat the converter structure may op~ionally be formed with cells of transverse cross-section o~her than square. Thus, the configuration of transverse cross-section for converter cells may be circular, oval, any polygon, etc.
Such persons can reasonably formulate a suitable formula for percentage fiow through large holes with respect to any selection of any other ~ransverse cell cross-sectional configuaration, based on analogy and guidance of the preceding formula for square cells.
In the catalytic converters 16, it is, of course, ex-tremely important to assure proper combustion for oxidation of the carbon monoxide, hydrocarbons and other combustables (in-cluding solid particles entrained in the combustion gases) ex,tir.g the primary com~ustion chamber 12. Details concerning the catalytic converter 16 are set forth in the aforesaid U.S. patent 4,373,45~

In all of the various embodiments of the invention, it will be appreciated that the leakage path(s) around the axially extending cells of the catalytic converter is (are) located in the immediate vicinity of the catalytic converter.
This is, of course, important, as set forth in the foregoing in order to su~ject the leaked exhaust gases to high tempera-tures thereby assuring, to the degree possible, combustion ~22555~, or oxidation of carbon monoxide, hydrocarbons and other combustibles (including solid particles) within the leakage exhaust gases themselves. As shown in FIG. 1, a leakage path within the vicinity of the catalytic converter 16 is to be distinguished from any leakage resulting from a bypass of exhaust gases provided at a position more remote from the catalytic converter. The function served by the bypass opened and closed by the damper 28 is entirely different from the function served by the leakage path in the immediate vicinity of the catalytic converter 16. See the aforesaid Albertsen U.S. application Serial Mo. 136,687, (Canadian Serial No. 374,510).
Although a wood burning stove has been shown and described in detail, it will be appreciated that the invention may be embodied in other solid fuel heating appliances which can utilize various solid fuels including densified or compacted wood products, coal, charcoal, peat and compacted trash and/or garbage and the like which may give off during burning, solid particles and vapors that may lead to some temporary plugging of the catalytic converter. Such appliances may take the form of stoves as well as boilers, incinerators - and the like, especially those well-suited for residential use.
Although particular embodiments of the invention have been shown and described, it will be understoo~ that various modifications may be made which will fall within the true spirit and scope of the invention as set forth in the ` appended claims.

Claims (13)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. In a solid fuel heating appliance of the type comprising:
a combustion chamber;
a flue for removing exhaust from said chamber;
a combustor having a plurality of cells extending therethrough for oxidizing oxidizable species in said exhaust, the improvement comprising:
at least one leakage path around said cells located immediately adjacent said combustor; and wherein the leakage path extends through the interior of the combustor.

2. The appliance of claim 1 including a plurality of said paths.

3. The appliance of claim 1 wherein the cross-sectional area of the leakage path is substantially greater than the average cell cross-sectional area of the cells.

4. The appliance of claim 3 wherein the volume flow through the leakage path comprises at least 10% of the overall exhaust.

The appliance of claim 3 wherein the volume flow through the leakage path comprises 10% to 40% of the overall exhaust.

6. The appliance of claim 1 wherein the volume flow through the leakage path comprises at least 10% of the overall exhaust.

7. The appliance of claim 1 wherein the volume flow through the leakage path comprises 10% to 40% of the overall exhaust.

8. A combustor for oxidizing oxidizable species in the exhaust of a solid fuel heating appliance comprising:
a honeycomb structure having a plurality of axially extending cells for passing oxidizable species there-through;

at least one leakage path for the oxidizable species through said structure;
said leakage path having a substantially larger cross-sectional area than any of said cells; and wherein the leakage path extends through the interior of the combustor.

9. The combustor of claim 8 wherein the leakage path has a cross-sectional area at least two times as large as the average cell cross-sectional area of said cells.

10. The combustor of claim 8 including a plurality of leakage paths.

11. The combustor of claim 8 wherein said at least one leakage path is adapted to accommodate a flow of at least 10% of the overall flow through the converter.

12. The combustor of claim 8 wherein the flow through said at least one leakage path is 10% to 40% the overall flow through said structure.

13. The combustor of claim 8 further comprising a container around the periphery of said structure.