Secondary combustion system for wood burning stove

A secondary combustion system for a wood burning stove employs an insulated passageway which is configured to substantially increase the turbulence of an exhaust stream passing therethrough. The insulated passageway comprises a generally transversely extending portion and a generally vertically extending portion. The passageway is configured to impart a rotation to the passing exhaust stream in one direction and to successively impart a rotation in the opposite direction. The insulated passageway comprises a plurality of helically extending channels. The passageway is also configured to have an increasing cross-sectional area to insure sufficient residence time for effecting the secondary combustion of the exhaust gases.

BACKGROUND OF THE INVENTION 
This invention relates to a system for controlling the emission of 
pollutants from a wood burning stove. More particularly, this invention 
relates to a system for effecting the secondary combustion of exhaust 
gases generated by the primary combustion of combustibles in a wood 
burning stove. 
The increased popularity of wood burning stoves and wood burning technology 
has been tempered by the increased focus on the potential adverse 
environmental effects from pollutants and exhaust gases generated by wood 
burning stoves. A number of proposals have been advanced for reducing the 
pollutants generated by the wood burning process such as passing the 
exhaust gases through a catalytic combustor and/or various systems for 
effecting a secondary combustion of the exhaust gases. The necessity of 
improving the means for removing pollutants from the exhaust gases of wood 
burning stoves has been made evident by the increasing number of 
governmental regulations which legally restrict the atmospheric emissions 
of exhaust gases generated by wood burning stoves. The present invention 
is a new and improved secondary combustion system which is readily 
incorporated into a wood burning stove for combusting pollutants and the 
exhaust gases generated by the primary combustion. 
BRIEF SUMMARY OF THE INVENTION 
Briefly stated, the invention in a preferred form is an improved secondary 
combustion system for combusting the exhaust gases in a wood burning 
stove. The system employs an insulated conduit forming an insulated 
exhaust passageway having an intake opening from the fire box and an exit 
opening which leads to the exhaust opening of the stove. A dispersing 
member, which is preferably a generally flat screen having a plurality of 
apertures is interposed across the exhaust passageway in the vicinity of 
the intake opening to disperse exhaust gases which are injected through 
the intake opening from the fire box. The dispersing member functions to 
accelerate the exhaust gases and to facilitate the mixing of unconsumed 
oxygen from the primary combustion. The insulated conduit is configured so 
that a rotation is imparted to the gas stream in a first portion of the 
exhaust passageway. Secondary air is injected into the first portion of 
the passageway to add additional oxygen and to further increase the 
turbulence of the gas stream. A second portion of the conduit is 
configured so that a counter-rotation is imparted to the gas stream. In 
the second portion of the exhaust passageway, helically extending channels 
are formed in the conduit for promoting the counter-rotation. The second 
portion of the conduit is also structured so that the cross-sectional area 
of the formed exhaust passage increases thereby providing a means for 
decelerating the gas stream so that there is sufficient residence time in 
the exhaust passageway to effect secondary combustion. Direct flame 
impingement on the screen and into the chamber formed in the first portion 
of the passageway increases the temperature of the formed chamber 
sufficiently so that a self-sustaining secondary flame front is formed. 
In a preferred embodiment of the invention, the exhaust passageway extends 
in a generally vertical orientation with a small transverse intake opening 
leading from the fire box. The intake opening portion of the passageway is 
rounded and the central vertical axis of the exhaust passageway is 
laterally offset from the intake opening to facilitate the imparting of 
the rotational motion to the exiting gas stream. The secondary air source 
is preferably injected at substantially a right angle to the advancing gas 
stream. 
A method in accordance with the invention for removing pollutants from an 
exiting exhaust stream of unconsumed oxygen and exhaust gases produced by 
the combustion of combustibles in a wood burning stove involves injecting 
the exhaust gas stream into an insulated passageway. The exhaust stream is 
dispersed into a plurality of small stream segments flowing in divergent 
directions by passing the stream through a dispersion screen interposed in 
the passageway. The dispersed gases are recombined and inter-mixed in the 
passageway to form a more homogeneous mixture of the unconsumed oxygen and 
exhaust gases. The gas stream is accelerated due to the relatively small 
cross-sectional area of the initial portion of the passageway. A 
turbulence is imparted to the accelerated gas stream thereby facilitating 
the combusting of the exhaust gases in the presence of the unconsumed 
oxygen and secondary air which is injected into the gas stream. The step 
of imparting turbulence to the gas stream is accomplished by imparting a 
counter-clockwise rotation to the gas stream, changing the direction of 
the gas stream imparting a clockwise rotation to the gas stream and 
injecting secondary air at an angle to the direction of flow of the 
advancing air stream. 
An object of the invention is to provide a new and improved secondary 
combustion system for combusting the exhaust gases produced by a wood 
burning stove. 
Another object of the invention is to provide a new and improved secondary 
combustion system which is effective and may be efficiently incorporated 
into a wood burning stove. 
A further object of the invention is to provide a new and improved 
secondary combustion system for a wood burning stove which system does not 
require catalytic combustor means. 
Other objects and advantages of the invention will become apparent from the 
drawing and the specification.

DETAILED DESCRIPTION OF THE INVENTION 
With reference to the drawing wherein like numerals represent like parts 
throughout the several FIGURES, a wood burning stove incorporating a 
secondary combustion system of the present invention is generally 
designated by the numeral 10. Wood burning stove 10, excepting for the 
secondary combustion system hereinafter described, may assume any of a 
number of forms. Stove 10 generally includes a housing 12 of cast iron, 
soapstone, or other suitable material. Housing 12 is supported on four 
legs 14. A door 16 or a pair of doors is located at the front of the 
housing for accessing a centrally located fire box chamber 18. An exhaust 
opening at the upper rear of the housing leads from an exhaust plenum 22 
and communicates with an exhaust flue 24 in a conventional manner. Stove 
10 is preferably of a compact construction which provides for an efficient 
controlled combustion of fuel received in fire box chamber 18 and for the 
exhaust of the exhaust gases to the exhaust flue 24. 
With reference to FIG. 2 and FIG. 5, a secondary combustion system in 
accordance with the invention is generally designated by the numeral 30. 
Secondary combustion system 30 is located in the illustrated embodiment at 
the central rear interior of stove 10. The secondary combustion system may 
also be mounted at the central rear exterior of a wood burning stove. A 
conduit 32 extends vertically from the base of the housing interior to 
forwardly open into a generally transversely extending exhaust plenum 34. 
Exhaust plenum 34 forms in an upper forward portion an exhaust opening 36 
which communicates via a second exhaust plenum 22 and exhaust opening 20 
with exhaust flue 24. A forwardly protruding knob (not illustrated) may be 
employed for regulating a by-pass damper for controlling the exhaust path 
into the secondary combustion system 30. 
An intake opening 38 is formed at a lower front portion of conduit 32 to 
provide direct communication between the fire box chamber 18 and the 
passageway formed by the conduit. Conduit 32 is lined with a thermal 
insulation layer 40 to form a generally vertically extending insulated 
passageway and a relatively short transverse passageway leading from 
intake opening 38 to exhaust plenum 34. Insulation layer 40 is a specially 
configured vacuum formed ceramic fiber material formed of allumina-silica 
ceramic fiber such as that marketed by Fire Line Incorporated of 
Youngstown, Ohio. The thermal insulation layer 40 may be precasted into 
sections and assembled to form the illustrated exhaust passageway. Other 
refractory-type materials capable of maintaining a high temperature level 
and capable of being configured as described below such as materials 
employed for lining kilns may also be suitable for insulation layer 40. 
Insulation layer 40 forms a lower mixing chamber 42 which leads from intake 
opening 38 to a generally vertically extending exhaust passageway 44 which 
is also formed by the insulation layer. Mixing chamber 42 is partially 
defined by an upper surface 46 and a rounded side surface 48 which 
cooperate to direct exhaust gases exiting the fire box toward the central 
vertically extending exhaust passageway 44. The central vertical axis of 
exhaust passageway 44 is laterally offset from intake opening 38 so that 
exhaust gases entering the mixing chamber 42 through the intake opening 
traverse a quasi-lateral path before exiting vertically through exhaust 
passageway 44. A plurality of secondary air passages 50 open through the 
rear wall of mixing chamber 42. The secondary air passages 50 are disposed 
so that secondary air may be introduced or injected into the mixing 
chamber at an angle which is substantially perpendicular to the vertical 
axis of exhaust passageway 44. 
A plurality of helically extending channels 52 are formed in the 
intermediate portion of insulation layer 40 which defines exhaust 
passageway 44. The helical channels or grooves 52 are substantially 
equidistantly spaced and are directionally oriented so that the vertical 
height at a location along the helical channels increases in accordance 
with the clockwise angular location as viewed in FIG. 4. In addition, the 
exhaust passageway 44 formed by the insulation layer gradually tapers from 
a substantially circular opening at the lower portion of the exhaust 
passageway to an enlarged oval-shaped opening at the top of the passageway 
as best illustrated in FIG. 4 so that the cross-sectional area of exhaust 
passageway 44 increases in area in accordance with the vertical height of 
the cross-section. The upper interior rear walls of insulation layer 40 
are contoured toward exhaust plenum 34 so that the exhaust stream is 
directed to the exhaust plenum. 
A generally flat screen 60 is mounted at intake opening 38 so that the 
exhaust gases exiting the fire box impinge and/or pass through the screen 
into mixing chamber 42. Screen 60 has a plurality of apertures which are 
preferably of a uniform size and are uniformly distributed. In preferred 
form, screen 60 may be manufactured from a 300 series stainless steel 
sheet with the apertures being dimensioned on the order of approximately 
1/4 inch in diameter. 
A high temperature probe 70 may be positioned in the vertical exhaust 
passageway 44. Probe 70 is adapted for detecting relatively high 
temperatures and for providing an output signal to a controller for 
regulating an air inlet to the stove and for regulating the injection of 
secondary air through passages 50. 
In operation, an exhaust stream of unconsumed oxygen and the exhaust gases 
from the combustion of the combustibles in the fire box chamber 18 exits 
the fire box through intake opening 38 in a general direction of the 
arrows in FIG. 1. The exhaust stream encounters screen 60 which functions 
as a dispersing element for dispersing the exhaust stream into a plurality 
of small stream segments flowing in divergent directions in the mixing 
chamber 42. Screen 60 also functions as a constricting element for 
accelerating the exhaust stream. Secondary air is also injected into the 
formed mixing chamber through the inlet passages 50. The unconsumed oxygen 
and the exhaust gases are re-combined and inter-mixed in the mixing 
chamber to form a more homogenous exhaust stream mixture which is also 
mixed with the secondary air. The resulting exhaust stream mixture is 
vertically propelled in a generally upward direction in a very turbulent 
fashion. The high degree of turbulence results from the gas stream 
encountering screen 60, the shape of the mixing chamber which functions to 
impart a counter-clockwise helical rotation to the upwardly propelled 
exhaust stream, the directional change from essentially a transverse 
direction in the mixing chamber to essentially a vertical direction as the 
stream leaves the mixing chamber, and the impingement of the secondary air 
at an angle which is generally perpendicular to the advancing exhaust 
stream. The helical rotation is generally tangential to the general 
direction of flow of the advancing gas stream. 
The turbulence of the gas stream is further exaserbated by the passing of 
the gas stream through the exhaust passageway 44. The helical channels 52 
function to impart a generally clockwise helical rotation to the advancing 
gas stream which has a generally counter-clockwise helical rotation prior 
to encountering the channel. The inter-mixed exhaust stream ignites as a 
result of direct flame impingement into mixing chamber 42 and the heating 
of screen 60. Under suitable conditions, a sustaining secondary flame 
front forms on the downstream side of screen 60. The secondary flame front 
is self-sustaining as temperatures within the insulated passageway reach 
1200.degree. F. The exhaust stream from the secondary combustion within 
the insulated passageway is relatively clean and eventually exits via 
exhaust plenum 34 and plenum 22 to the exhaust flue 24 in a general 
direction indicated by the arrows in FIG. 1. The entire insulated 
passageway essentially forms a secondary combustion chamber. 
A temperature probe 70 may be located in the formed secondary combustion 
chamber for sensing the local temperature and transmitting a signal 
indicative of the sensed temperature to a controller unit. The controller 
unit functions to adjust the air inlets to the fire box and to control the 
injection of secondary air through the air passages 50 in accordance with 
the detected temperatures. A suitable probe is the VT Group High-Temp 
Probe marketed by Vermont Technology Group, Inc. It should be noted that 
the formed secondary combustion passageway is insulated to retain the high 
temperatures of the exhaust gases within the exhaust passageway so that 
the exhaust stream is essentially dispersed, inter-mixed, accelerated, 
imparted with turbulence and secondarily combusted in a substantially 
thermally isolated environment. 
The intake opening 38 is dimensioned to provide a relatively restricted 
opening in relation to the fire box chamber 18 so that a relatively high 
velocity exhaust stream exits into the secondary combustion system. For 
example, in a preferred stove embodiment wherein the fire box chamber has 
dimensions on the order of 18".times.10".times.13", intake opening 38 has 
an opening area of approximately 14 sq. inches. In order to maintain the 
initial relatively high velocity of the exhaust stream within the 
secondary combustion system, the insulated passageway formed by insulation 
layer 40 is a relatively narrow passageway having initially a small 
substantially uniform cross-sectional area which is approximately equal to 
the area of the intake opening. The cross-sectional area of the intake 
opening increases along the exhaust path to allow more residence time for 
the exhaust stream within the passageway to thereby facilitate the 
secondary combustion. 
In preliminary tests employing the foregoing described secondary combustion 
system 10 wherein the exhaust gases within the fire box were between 
700.degree. F. and 1100.degree. F., the exhaust temperatures in the mixing 
chamber 42 were on the order of 1500.degree. F. to 2000.degree. F. due to 
the insulation of the chamber, the acceleration of the exhaust stream and 
the increased turbulence of the exhaust stream. Under such conditions a 
secondary flame front was formed at the downstream side of screen 60. 
Temperatures between 1600.degree. F. and 2000.degree. F. were detected by 
a high temperature probe 70 which was located within the formed secondary 
combustion chamber. At a location approximately 4 feet up into the exhaust 
flue, the temperatures of relatively clean exhaust gases were measured at 
approximately 450.degree. F. The oxygen content of the flue exhaust gases 
was as low as 3-5%. The flue exhaust gas stream also had a carbon dioxide 
content on the order of 18% and a carbon monoxide content on the order of 
0-0.025%. 
The foregoing description of the secondary combustion system for a wood 
burning stove has been set forth for purposes of illustration and should 
not be deemed a limitation of the invention herein. Accordingly, various 
modifications, adaptations, and alternatives may occur to one skilled in 
the art without departing from the spirit and scope of the present 
invention.