CA1202539A - Wood burning stove - Google Patents

Abstract

Abstract of the Disclosure A multi-stage combustor communicates with the combustion chamber and the flue of a wood burning stove. A combustcr includes a plurality of serially arranged honeycomb segments having open-ended cells extending therethrough wherein a downstream segment of the combustor has a substantially larger cell density than an upstream segment.

Description

53g IMPROVED WOOD BURNING STOVE
Background of the Invention --This invention relates to wood burning stoves, and moreparticularly, to wood burning stoves utilizing a combustor or a catalytic converter.
Canadian Patent Application Serial No. 374,510, van Dewoestine, ~iled April 2, :L981, corresponding in part to U.S.
patent application Serial No. 173,155 filed July 28, 1980, and assigned to the assignee of this application discloses the use of a catalytic converter in a wood burning stove~ The catalytic converter which serves as a combustor provides more complete burning or oxidation of the volatile and particulate organic sub-stances present in gases rising 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 con~erter which com-prises noble-metal catalysts on a suitable substrate 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 increases its catalytic activity. The elevated temperature pyrolyzes and cracks the higher molecular weight hydrocarbons occurring in the smoke as solid particles and oily droplets, converting them to volatile compounds which readily mix with oxygen present and thereby leading to their rapid oxidation.
Temperature continues to rise until the system reaches a tempera-ture at which there is equilibrium between the inlet gas tempera-ture, flow rate and the amount of oxidizable material. Thistemperature is typically 600C to 900C for a properly sized Y~

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c:atr~lyst systemO At these tempexatuxes;, oxidatio~ proc:~ds very rapiLaly to completion if ~he catalytic detrice ha~

approprlate volume nd internal surface area.
It is impo~ta~t that the combu~tor or catalytic converter }: e of th~ approp~iate val~ne and in~ernal geometry to provide both satisfactory.s::ataly~ic p~3r~:~n2Lnce and a mir~imal pressure drop so as not to ad~r~rqely 2ffect: the stove operation~ However, to some ex~n~, perormance and the minimizing of back pressur~ have been diff icult to 10 achieve. If the cell density of t:h~ combustor or ;:atalytic ~on~erter i increased, the Pfficiency of the eombu tor or catalytic converter i5 increased. EIcwe~rex, higher cell densities increase the pxessure d~op go as to ad~?er~ely affect s~ove operation. Moreover, ~igher cell densities are 15 prone to plugging i ~ a wocd burning stove e~v ironment due to the presence of the abo~re-di~cus~ed volatile and suspended solid particulate organic substancQs in the ~lue gas whic~
condense and collect on the cell walls of the converte~, thus reducing the trans~erse dimension3 of the gas flow passage~ of the con~erter cells and ur~er increasing pressure drop across the axial ~gas flo~ length of the converter. On the other hand, decreasi~g the cell d~nsity minimizes pressure drops and the pos5i~ility of plugsing ~y the volatile combustible substances and unburned solid particles. However, converter efficiency utilizing these lower cell densities is impai~e2.

Summarv of the In~ention ~ .
It is an object of t~is invention to provids a Gombustor fox a wood burning stove having a high degree o efficiency.

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It is a furtller ob ject of this înventio;l to p~o~id~
a combustox ~or a wood burnir~g s~ov~ which min~mize~ ~he pressure drop across the catalytic con~rerter there~y permi~ing normal wood burning stove operation..
:It i5 a fur~her ~3b j~c~ of thi~ ~nvention ~o p~ovide a combu~tor for a wood burning s ~ove wk:Lt::h i~ no~ 3usc:~ptible to pluygin~.
In accordance with these an~ o t~x o~ jects, a pre-~con~inusd on page 3 ~
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~erred embodiment of the inv~ntion comprises a wood burning ~tove including a combustion chamber fc7r con~aining wood, a flue for exhaust and a multi-stage combustor communicating with the com-bustion ch~mber and the ~lue. The combustor includes a plurali~y of s~rially-arranged or ~equential honeycomb segmen~s having open-er~led axially extending cells. In accordance with this inv~n~ion, a downstream segment has a substan~ially larger cell density than an ups~ream segment where cell den~i~y is the number of cells per unit of transverse cross-sectional a ea of the combustor , i .e ., the area in a plane substantially perpendicular to the axial length or flow direction through the cells of the combustor .
In the preerred embodiment of the invention, the cell de ns .i ty o the downs tre am segme nt i s more tha n 4 0 % g re a te r tha n the cell density of the upstream ~gmen~ and preferably at leas~:
5096 greater.
In ~ccordance with another important a~pect of the invention, the axial length of the downstream segment is less than the axial length of the upstream segment.
In a particularly preferred embodiment of the inven-tioni each downstream segment has a cell densi~y greater than the adjacent ups~ream segment and an axial length less than the adjacent upstream segment. In the preferred embodiment wherein the combustor comprises three segmen~s, the cell density o~ each segment increases relative to the adjacen1: upstream 5egment, and the axial length of each segmen~ decreases relative to the adjacent ups~ream segment.
In the preferred embodiment of the invention, at least one of the segments includes a cal:alyst. Preferabl~r, all of the segments include a catalys~.

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In on~ embodimen~ of ~he invention~ the combuskor is loeated in the flue., In another embodiment of the ir~vention, the ~ood burnin3 stove includes a seco nd chamber fsr heat exchange and/or secondary combusl:ion and ~he combustor is located be~ween the first or primary combus~cion chamber and khe second chamber.

E3rief ~escription of the_Drawln~Ls FIG. 1 is a sectional view o a wood burning stove representing one preferred embodiment of the invention;
FIGo 2 i5 an enlar~ed sectional ~riesJ of the flue of the stove shown in FIG. 1 showing the multi-stage combustor in greater detail;
FIGs. 3-5 are sectiorlal views of the various se~ments irl the mul~i-s~age combus~or taken along lines 3-3, 4-4 and 5-5 of Fig. 2;
FIG. 6 is a sectional view o~ a wood burning stove representiny another preferred embodiment of ~he invention;
FIG. 7 is an enlarged sectional view of the combustor in the stove shown in FIG, 6 and FIGs. 8 and 9 are sec~ional views of the segments in the combustor shown in FI~:;. 7 taken along lines 8-8 and 9-9.

D~talled Description of Preferred ~mbodimenl:s Referring to FIGo 1~ a wood burning stove 10 is shown comprising a combustion chamber 12 in the lower region of the stove 10 and a flue 14 at the upper reyion of the 51 ove 10 for exhausting the combustion products from the stove 10. A damper 16 i5 provided for controlling air intake into the combustiorl chamber 12 thereby controlling the rate of combustion of wood sup-ported on a grate 18.
In accordance with this invention, a multl-stage com-bustor 20 communicates with the combustion chamber 12 and ~he -- 4 ~

:flue 14 to promote combustion and oxida~ion within the chamber 12 so a~ to remove volatile substances a~d solid particles in the exhaust from the stove 10. In accordance with this invention, the combustor 20 comprises A plurality of serially-arranged honeycomb segments havin~ open-ended cells extending axially therethrough as more clearly ~hown in FIG. 2~
R~ferring now to FIG. 2, the cc~mbllstor 20 as shown comprises 8tages or segments 22, 24 and 26 which are supported wi~hin the flue 14 adjacen~ the combustion chamber by rods 2~
and 30 which e:cter,d through the flue 14 and are retained within the flue 14 by enlarged heads 3~ and 34 respectively. Spacing ~e tween the segments 22, 24 an~ 26 is maintained by rings 36 and 38.
In accordance with this inventionS ~he density of the cells in ~he downstream segment 22 is substantially larger than the density of the adjacent upstream segment 24. Similarly, the ~ell density of the segment 24 is substantially larger than the cell density of the adjacent upstream segment 26.
Preferably, at leas~ one of the segments 22 r 24 or 26 includes a catalyst such as the Engelhard Catalyst No. 10902-8.
In general catalyst~ comprising precious metals such as palladium or pl~tinum or a combination thereof may be u~ilized. For the sake of re~uced cost, the catalys~ can be limited to a single segment. However, or optimal efficiency, each of the segments

2~, 24 and 26 includes a catalyst for promoting combustion of the volatile materials in the exhaust.
In accordance wi'ch another important asp~ct of the inven~ion, the overall axial len~th ( i .e., length in direc~ion o gas flow through cells) of a downstream segment i5 less than the axial length of an upstream segment. More specifically, ~z~

the axial leng~h o~ the segment 22 is less than the axial len~h of the se~ment 24 and the axial length of the segmen~ 24 is less than the axlal length of the segment 26.
By,utilizin~ the mul~i-stage combus~or as shown in FIGs. 1 and 2, combustion of the volatile substances within the exhaust gas is optimized without adversely a~ectin~ the operation of the s~ove. In this connection, i~ will be apprecia~ed ~ha~
~he increas:ing cell density from ups~ream to downs~ream provides ever-increasing surface area within the combustor or catalytic converter so a ~o increaslngly promote combus~ion within the combustor as the exhaust gases mov.e from upstream to downstream At the same time, the risk of plu~gin~ is minimized since the upstream segments have a lesser cell density but a substantial amc~unt of the volatlle ub~tances and burnable particles which tend t~ result in plugging are oxidized beore the exhaust can reach the segmen~ of higher cell density. Moreover, ~he pressure dr~p across the combustor i9 minimi2ed without sacrificir~ ~he eEectiveness of the combus~or in oxidiæing the volatile substances and burnable particles since the total length o~ the combu~tor is not filled with the high cell density segments.
Reerrin~ to FIGs. 3-5, the dif~erence in cell den-sity between the variolls segmen~s 22, 24 and 26 may be more readily appreciated. A~ shown in FIG. 3, the cell walls 40 provide cell~ 42 of a much higher density than the cells 46 pro-vided by cell walls 44 in the segment 24 as shown in FI~. 4.
Similarly, the cell walls 44 provide a density of the cells 46 in ~he segment 24 which is substantially greater than the deQsity of the cells 50 provided by cell walls 48 in the segment 26 as shown in FIG. 5. Preferably, the downs~ream cell density is more than 40% greater than tAe adjac~n~ upstream cell density and S~3 an increase of a~ least 5096 in cell d~nsi~y from ~he upstream to ~he downstream segmenl: is even more preferred.
The follo~ing example~ illustrate a variety of multi-s~age combustors which may be employed in the stove shoT.~n i n FIGs. 1 and ~ with various ::ell ~ensities ar~ various axial leng~hs and with variou~ segmeFlt~ c~ alyzed~

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am~le 1 Cell 3~ensl~v 1 ~xlal Len~tn Catalvst I Dlame~er ~ . ~
5egmen 22 7 . 75 cell~/cm . 2 2 . 54 cm ~ yes14 . 61 cm .
(S0 cells/in.2) (1 inch) (S.75 inches) , _ _ ~ _ ..
Segment 24 3.88 cells/cm.2 5.08 cm. yes14.61 cm.( 25 cells/in . 2 ) ( 2 inches ) t 5 . 75 inches ) _ _ _ ~ _ .
Seglllent 26 1.40 cells/cm.2 7.62 cm. yes 14.61 cm.
(9 cells/in.2) ~3 inches) (5.75 inches) , _ _ , _ _ I_ __ _ ~ = . _ _ _ _.
ample 2 Cell Densi~y _ Axlal Len~th _ ~ Dlameter _ Segment 22 7.75 cells/cm.2 2~54 cm. yes 14.61 cm.
( 25 cells~in.2) (1 inch) ( 5.75 inches) __ . . _ ~ ~ ~
Se~ment 24 2 . 48 cells/cm . 2 5 . 08 cm . yes 14 . 61 cm .
( 16 cells/in . 2 )( 2 inches ) ( s . ~5 inches ~ .~ __ _ . ~ __ .
S~gment 26 1.40 cells/cm.2 7.62 cm. yes 14.61 cm.
~ 9 cells/in2. ~ ~3 inches) ( s.75 inches) ,. . _ , , _. _ ___ _ ~ , _ _ _~ _ __ ~3~ Axlal Length Catalyst Diameter Segm~nt 22 7.75 cell~/cm.2 5.08 cm. yes 14~61 cm.
(25 cells/in.~) (2 inche~) (5.75 inches) _~ ~----~1 Segment 24 2.48 cells/cm.2 5.08 cm. yes 14.61 .
( 16 cells/in. 2 ) ( 2 inches ) ( 5 f 75 inches ) . . . . _ ___ Segment 26 1. 40 cells/cm . 2 cm yes cm ( 9 cells/in. 2 ) ( 2 lnches ) ( 5 . 7S lnches ) . _ . .. , _, , . _ ___ ~)ZS39 -- . . .. , .. _. . . , . _ _ Ex~mple ~ cell DensitY Axlal Lellq~h cat~lyst Diame~er . _ . ~ . .. _ _ _ Segment 22 7.75 cells/c~.2 2.54 cm. no 14.61 cm.
(25 cells/in.2) (1 inches) (5.75 inches) ~ . _ . _ . _ .. ~
Segment 243088 cells/cm.2 5.08 cm. no 14.61 cm.~ ~ 16 celLs/in. 2 ) ( 2 inches 3 ( 5 . 75 inches ) , .. ,. . . . , ~ . . _ . , _ _ .
Segment 261.4Q cells/cm.2 7.62 cm~ yes 14.61 cm.( 9 cells/in. 2 ) ( 3 inches ) ( 5 . 75 inches ) .. . _ ..., ~ . _ __ , ExamPle :5 Cell Densltv ~ Axlal Lenq~h I Catalyst Dlameter .. I ~ _ _ . _ Segmerlt 227.75 c~lls/cm.2 2.54 cm. no 14.61 cm.( 25 cells/in, 2 ) ( 1 inches ) ( S . 75 inches ) _ _ , ., _ Segment 24 3.88 cells/.2 SoO8 cm. yes14.61 cm.
( 16 cells/in . 2 ) ( 2 inches ) ( S . 75 inches ) . ., ,_ _ __ _ _ _ Se~ment 261.40 cells/cmO2 7.62 cm~ no 14.61 cm.(9 cells/in.2) (3 inches) (5.75 inches) .. , _ _ --ExamDle 6 Cell DensltY Axial Lenqth Catal~st I Dlameter ~._. . ~ . . ~
Se~ment 227.75 cells/c~l~.2 2.54 cm. yes 14.61 cm.( 25 cells/in. 2 ) ( 1 inches ) ( 5 ~ 75 inches ) _ ___ . _ Segment 243.38 cells/cm.2 5.08 cm. no 14.61 cm.
(16 cells/in.~) (2 inches) (s.75 inches) _ __ _ _ ~ .
Segment 261.40 cells/cm.~ 7~62 cm. yes 14.61 cm.
(9 cells~in.~) (3 inehes) (s.75 inches) ~ ,. - . .. . . , ~ .

Exam~le 7Cell Densit~r I~al Length Catalyst Diame~:er _ _ ~ ~ _ ~
Segment 227.75 cells/cm.2 2.54 cm. no 14.61 cm.
(25 cells/in~2) (1 inches) (5.75 inches) _ ~. . . _. _ _ _ , . _ , ., Segment 24 3.88 cells/cm.2 5.08 cm. no 14.61 cm.
( 16 cel 1 s/ in . ~ ) ( 2 inche s 3 ( 5 ~ 7 5 i nche s ) . . . .
Segment 261.40 cells/cm.~ 7.62 cm. no 14.61 cm.
¦ (9 cells/in.23 (3 inches) (s.75 inches) _ ~ , _ . . _ ~

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Referring now ~o the e3nbodiment of FIGo 6 ~ a stove 110 includes a E~ri~ary c:ombustion chamber 112 which is lined with a firebox refractory material lld~ at ~che bot~om and along the back.
The front of the combust;on chamber 112 includes an opening covered by a door 116 with an adjustable damper 118 for providing air within the combu s tion chamber 112, A second combustion chamber and heat exchan~er 120 is located immediately above he combus~ion chamber 112. The chamber 120 communicates with ~he chamber 112 through a mul~i-stage combustor or catalytic converter 122 located in an opening 1~4 between the cha;nber 112 and the chamber 120~ An impingement refractory material 1~6 is located above the downstream or exit end of the converter 122 and secured to a horizontally extendir wall 128 within ~he chamber 120 and forming part of ~he heat exchan~e ~tructure of the stove 110 for ~ransferring additional heat released by combustion facilitated by the converter 122 to the living space around the s ove 110. Æxhaust ln ~he chamber 120 pa3ses upwardly to the rear of the stove 110, then along the lower horizontal surface of an additional heat exchange baffle 20 130 so as to be advarlced forward in the s~ove 110 and then rearward at the uppermost extremity of the stove which communicates with a flue 13~ ~or exhaust purposes.
In accordance wit~ this invention, the multi~stage combustor 1~2 as best shown in FIGs . 7 through 9 comprises s ~ages or segmen~ 134 and 136 of differing cell d~n~ities. Segments 134 and 136 are ret~ined wi~hin a cylindrical member 138. The lowermost:
segment 136 re ts on the flan~e 140 which extends radially inwardly rom the periphery of the cylindrical member 138. A rod 142 termi~

nated in heads 144 extends through diam2tric~11y-Qpposed openings in the cylindrical member 138 so as to re~ain the uppermost segmen~

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134 in place~ The spacing between ~he segments 134 and 136 i~
establi~hed by a spacer ring 146.
A~ shown in -FIGs. 8 and 9, there is a substantial di~far-ence in he density of the cell5 148 ~ormad by cell walls 150 in the segment 134 and the density o~ the cells 152 for~ed by the walls 154 in ~h~ segmen~ 136. More sp~cifically, the density of he cells 148 is substantially grea~er than the density o th~
cells 152~ As poin~ed out previously, the density of the cells 148 should be more than 40% greater ~han the density of the cell~ 152 and prefe~a~ly at leas~ 50% greater. The particular cell densities which are u~ilized in the embodiment shown in ~IGs. 6 through g may, for example ~ include cell densit.ies of 25 cells per square inch or the segment 134 and cell densities of 16 cells per square inch for ~he segment 136. In the alternative, the cell den.~ity for the s~gmenk 134 may be 2.48 cells per square cm. (16 cells per square inch) and the cell density for the segment 136 may be l o 40 cells per square cm . ( 9 cell5 per square inch). ~s shown in FIGs. 6 and 7, the axial length of the segments 134 and 136 are sub~tantially identical. These axial lengths may rang~ ~rom 2.54 ~o 12.70 cm. ~1 to 5 inches) in length for each se~ment with 7 .62 cm. ( 3 inches~ in length beiIig preferred.
Th~ out~ide diameter of the segments 134 and 136 may vary ~rom 7.~2 to 20~32 cm. (3 to 8 inches) with 12.70 to 15.24 cm. (5 to 6 inches) preferred. It will also be appreciated that the segments need not be circular in cross-sec~ional shapes, e.y. o~al or rectan~ular. Regardless o the cross-sectional shape, the overall cro~s-sectional area may be in the range of 45~53 to 259.54 cm.2 t7.06 ~-o 40.24 in.2) with 126.54 to 182.27 cm.2 (19.62 to 28.26 in.2) preferred.

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~ he u~e of a mul~i~stage combu~tor where a downstream se~men~ has a substantially larger cell density than an ups~ream segmen~ i~ advantageous for a m~mber of important reasons. During start up of a ~tove, the stove and the combustor is :~ela~ively cool. As a consequence, ~he combus~or is unable to burn the heavy resinou~ and oily vapors of the creosote in th~ combustion gases being exhausted from ~he stove. However, the relatively cold lower density Ce!ll5 of the upstream segment is believed to serve as a filter on which the creosote is deposited while the combustor is rela~ively cocl. However, the downs~ream segment with the higher cell density has a ~ufficient catalyzed surface area so as ~o promote combustion of partially oxidized, volatile constituents of tile combustion gases such as carbon monoxide, methyl alcohol and methaneO The burning of ~hese volatile constituents of the exhaust ga5 increases the temperature at the high cell density downstream segment to the poin~ that higher molecular wei~h~ constituents of the exhaust gases including polycyclic organic materials and non-polycyclic organic materials begin to burn. As the high cell density, downstream segments be~in to heat up, the heat is radiated upstream toward the lower cell density downstream segment or segments so as to promote combustion t those segments. As temperature rises at the down-stream segmentJ the creosote which is deposited on the cell walls burns. In this manner, start up of he stove is accomplished without ~he plugging of the holes in the upstream segment by ~he creosote.
The multi-stage combustor with different cell densities i5 also advantageous during a slow burn mode of operation character-istic of fall and spring operation~ ~uring ~his slow burn, the primary combustion chamber of the stove is at a lower temperature.

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However, due to th~ presence of a high c~ll den~ity and large surface area in the downstream segment of the combustor, overall combustion e~ficiency within ~he stove is substan~ially increased.
0~ ~ourse, creosote within ~he exhaus~ gases may be burned off in the low density of ~he upstream segmen~ of the combustor without risX of plugging.
~ he multi-sta~e com~ustor is also advantageous during the final burn of the 5~0ve operation, i.eO, after a number of hours have elapsed in the ~urning o wood ~n~ the organic ma~erials have bsen burned of~ leaving essentially charcoal. Through the use of the high surface area i~ the high density downstream s~gment, compIe~e combustion of combustible materials is achieved i nclud i ng subs tan~ial amou nts of carbon monoxide. As a consequence, pollutants from the stove may be subs~antially minimized.
Finall~y, a multi-stage combustor is par~icularly advan-.
tageous late in the lie of the catalyst on the combus~or. ~ueto the high surface area of the downstream segment, relatively low efficiency of the catalyst may be accommodated. In other words, a ufficient surface area of low cataly~iG efficiency is available to Assure the necessary burning of the ~arious combustible con~tituents within the exhaus~ gases.
~ rc~m the foregoing, it will apprecia~e~l that multi-stage combustors is particularly impor~ant in a w~od burning stove.
. The effect o~ the multi stage combus~or having segments i of differi~ cell den~ity is cl arly demonstrated with the ollowing example. A multi-stage combustor comprising an upstream segment havin~ a cell densi~y of 1.40 cells/cm.2 (9 cell/in.2) and a down-stream segmen~ of 2.~8 cells/cm~ ~16 cell/in.~). Both the upstream segment and the down~tr2am segment have circular cross-sections of a diameter of 14~61 cm. (5.75 inches3 arld an axial ~Z~ j;39~

length of 7.62 cm. (3 inches) each. Both the upstream segment and the downstream se~ment were catalyzed with 55 grams of platinum-base catalyst per cubic ft. of substrate. Twenty minutes after a stove had begun burning with the multi-stage com-bustor located in the flue pipe, the amount of creosote present immediately downstream and immediately upstream was measured by sampling the wood fire combustion gases. I~ was found that immediately upstream of the multi-stage combustor that creosote was present in the amount of 4.25 milligrams per liter of com-bustion gases while downstream only 0.3 milligrams per liter ofcreosote was present. It will therefore be understood that better than 90~ of the creosote was removed by the multi-stage combustor. Moreover, there was no trace of tar and oil down-stream of the multi-stage combustor.
The selection of cell density for a multi-stage com-bustor may vary as suggested above. However, the choice in cell densities should be guided in accordance with the disclosure of copending application Serial No. 374,510 filed April 2, 1981. A
cell density of substantially less than 200 cells/sq. inch is desirable. In addition, it will be appreciated that the particu-lar stoves may desirably incorporate certain features such as an exhaust gas bypass for bypassing the converter cells, such as disclosed in the above copending Canadian application.
Although particular embodiments and examples of the invention have been shown and described, it will be understood that other embodiments, examples and modifications will occur to those of ordinary skill in the art and such embodiments, examples and modifications will fall withi~ the true spirit and scope of the invention as set forth in the appended claims.

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Claims (20)

PATENT CLAIMS:

1. A wood burning stove comprising:
a combustion chamber for containing wood;
a flue for exhaust; and a multi-stage combustor for communicating with the combustion chamber and the flue, said combustor comprising a plurality of serially arranged honeycomb segments having open-ended cells extending therethrough, said combustor having a downstream segment with substantially larger cell density than an upstream segment.

2. The wood burning stove of claim 1 wherein the cell density of said downstream segment is at least 40% greater than the cell density of said upstream segment.

3. The wood burning stove of claim 1 wherein the axial length of said downstream segment is less than the axial length of said upstream segment.

4. A wood burning stove comprising:
a combustion chamber for containing wood;
a flue for exhaust; and a multi-stage combustor communicating with said combustion chamber and said flue, said combustor comprising a plurality of honeycomb segments having open-ended cells ex-tending axially therethrough, each of said segments being charac-terized by a larger cell density than an adjacent upstream segment.

5. The wood burning stove of claim 4 wherein the cell density of each segment is at least 40% greater than any adjacent upstream segment.

6. The wood burning stove of claim 4 wherein each of said segments is characterized by a lesser axial length than any adjacent upstream segment.

7. The wood burning stove of claims 1 or 4 wherein at least one of said segments includes a catalyst.

8. The wood burning stove of claims 1 or 4 wherein all of said segments include a catalyst.

9. The wood burning stove of claims 1 or 4 further com-prising a second chamber for heat exchange and/or secondary combustion, said combustor being located in an area of communica-tion between said combustion chamber and said second chamber.

10. The wood burning stove of claims 1 or 4 wherein said combustor is located in said flue.

11. The wood burning stove of claims 1 or 4 wherein the cell density of each segment is substantially less than 200 cells per square inch.

12. The wood burning stove of claims 1 or 4 wherein the cell density of each segment is in the range of 9-50 cells per square inch.

13. The wood burning stove of claims 1 or 4 wherein the cell density of each segment is in the range of 9-25 cells per square inch.

14. A multi-stage combustor for a wood burning stove, which stove has a combustion chamber and a flue for exhaust from said combustion chamber, said multi-stage combustor comprising:
an exhaust path for communicating with the combustion chamber and the flue, a plurality of honeycomb segments having open-ended cells extending axially therethrough, said segments mounted in said exhaust path so that the exhaust passes through the segments, each of said segments being characterized by a larger cell density than an adjacent upstream segment.

15. The combustor of claim 14 wherein the cell density of each segment is at least 40% greater than any adjacent upstream segment.

16. The combustor of claim 14 wherein each of said segments is characterized by a lesser axial length than any adjacent upstream segment.

17. The combustor of claim 14 wherein at least one of said segments includes a catalyst.

18. The combustor of claim 14 wherein the cell density of each segment is substantially less than 200 cells per square inch.

19. The combustor of claim 14 wherein the cell density of each segment is in the range of 9-50 cells per square inch.

20. The combustor of claim 14 wherein the cell density of each segment is in the range of 9-25 cells per square inch.