Note: Descriptions are shown in the official language in which they were submitted.
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~
;'`~
~Z6~i3~
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.
;: '',
~Lf~(~253~
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 ~
.
- 2a -
S3~
~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~.
-- 3 --
~LZ~5~Y~
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,
-- 5 --
~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
- 6 -
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~
~ _-- - ~
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 )
. _ . .. , _, , . _ ___
-- 7 --
~)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)
_ ~ , _ . . _ ~
$Z~s435~
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~
~2~253~
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.
-- 10 --
~Z~ 53~
~ 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.
v~
.~
53g
., .
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
-- 12 --
~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.
,~ ~
