WO2014063200A2

WO2014063200A2 - Wood fuelled heating stove

Info

Publication number: WO2014063200A2
Application number: PCT/AU2013/001237
Authority: WO
Inventors: Neal Alexander BOWERS
Original assignee: Bowers Neal Alexander
Priority date: 2012-10-26
Filing date: 2013-10-24
Publication date: 2014-05-01
Also published as: US20150292746A1; CN104870903A; AU2013334488A1; WO2014063200A3; CA2886781A1; EP2912382A2; EP2912382A4

Abstract

A wood fuelled heating stove including: a stove portion having a stove chamber therein and an fine mesh charcoal basket dividing the stove chamber into upper and lower chambers; a reduction portion having a primary combustion chamber therein and at least partly within the upper chamber of the stove chamber and having a primary grating vertically separating the primary combustion chamber from the upper chamber; an air jacket surrounding at least some of the primary combustion chamber and having a plurality of ports providing fluid communication between the primary combustion chamber and the air jacket and a primary grating providing limited flow of mobile solids and fluid between the primary combustion chamber and the stove chamber; an air passage providing fluid communication from the outside of the stove chamber to the air jacket and to the inside of the stove chamber; and a flue passage providing fluid communication from the lower chamber of the stove chamber to a flue outlet at a level above the primary combustion chamber and upper level of the stove chamber.

Description

WOOD FUELLED HEATING STOVE

This invention relates to a wood fuelled heating stove. The invention has particular application to a wood fuelled heating stove which uses "reverse flow progressive oxidation" such as that described in my Australian Provisional Patent Application Nos. 2012904682 filed 26 October 2012 and 2013901350 filed 17 April 2013, the contents of which are incorporated herein by reference. However, the invention is not necessarily limited to such stoves nor necessarily to the theoretical operation described in the specifications of the aforesaid applications.

A basic open fire has an ash zone at the base above which the wood is "baked" to release carbon-based gases which ignite when mixed with air coming in from the sides. The hot gases rise to heat newly added wood fuel. Cool air flows in from the sides, starting near the base but continuing up the sides. Fire places and wood fuel burning stoves have this basic open fire structure. The intrinsic problem with this kind of structure is that the physical movement of the gaseous fuels and the inflow of cooler air is conducive to the production of unburnt carbon particles, tars and creosotes, some of which may become air pollutants. The highest average temperatures are located in the burning charcoal region of the open fire as well as in prior art stoves. The flaming gases above are from the wood above releasing hydrocarbon and other carbon-based gases which undergo some combustion with inflowing cooler air. More readily reactive hydrocarbons tend to be those of the lower densities and lower vaporising temperatures, the combination of which sometimes leaves tars and creosotes as unburnt air pollutants in some circumstances and often also producing undesirable smoke.

Smoky fires can result from insufficient heating of the air passing through the bed of coals, particularly where a bed of coals has not been established, or where the amount of new fuel exceeds what can be heated and burned with little or no smoke. In a stable wood fire, some of the oxygen content of the air is used in the combustion of carbonaceous material in the coals with consequential heating of the air to a temperature sufficient to ignite part of the wood fuel above the bed of coals. Traditional wood fuelled heating stoves often do not completely combust the wood or combustion can be inefficient. Standard enclosed stoves crudely direct air to the general area of wood placement with up to 50% of the air not used in the combustion process and effectively cooling the combusted gases which reduces the incidence of further combustion .

The present invention aims to provide a wood fuelled heating stove which addresses one or more of the shortcomings of the prior art. Other aims and advantages of the invention may become apparent from the following description.

With the foregoing in view, in one aspect the present invention resides broadly in a wood fuelled heating stove including: a stove chamber having a floor, a side wall extending upwardly from the periphery of the floor and a top wall extending across the top of the side wall; a primary combustion chamber at least a part of which extends at least part way into the stove chamber through an opening penetrating the top wall, the primary combustion chamber having a base wall interposed between the floor and the top wall of the stove chamber and having a primary grating provided comprising at least a part of the base wall, a curtain wall extending upwardly from the base wall through the opening in the top wall and having a plurality of ports penetrating the curtain wall and a closure wall extending across the top of the curtain wall to enclose the primary combustion chamber; an air jacket wall circumscribing at least part of the curtain wall at least partly below the opening to provide an air jacket circumscribing the primary combustion chamber but separated therefrom by the curtain wall, the curtain wall having a plurality of ports distributed about and penetrating therethrough to provide fluid communication between the air jacket and the primary combustion chamber, the air jacket wall and/or the curtain wall substantially sealing the opening through the top wall and separating the air jacket from the stove chamber; an air inlet opening through the floor or the side wall at or near the floor and a stove chamber air inlet into the stove chamber through or near to the top wall; a primary air duct, a secondary air duct and a tertiary air duct, the primary air duct extending along the floor of the stove chamber and providing fluid communication from the air inlet to the secondary air duct, the secondary air duct extending upwardly from the primary air duct and the tertiary air duct providing fluid communication between the secondary air duct and the air jacket and the stove chamber air inlet; a charcoal grating partition interposed between the floor of the stove chamber and the base wall of the primary combustion chamber; and a flue passage extending from a flue opening to the stove chamber lower than the grating wall and upwardly to a flue outlet higher than the top and closure walls. In another aspect, the present invention resides broadly in a wood fuelled heating stove including: a stove portion having a stove chamber therein and an fine mesh charcoal basket dividing the stove chamber into upper and lower chambers; a reduction portion having a primary combustion chamber therein and at least partly within the upper chamber of the stove chamber and having a primary grating vertically separating the primary combustion chamber from the upper chamber; an air jacket surrounding at least some of the primary combustion chamber and having a plurality of ports providing fluid communication between the primary combustion chamber and the air jacket and a primary grating providing limited flow of mobile solids and fluid between the primary combustion chamber and the stove chamber; an air passage providing fluid communication from the outside of the stove chamber to the air jacket and to the inside of the stove chamber; and a flue passage providing fluid communication from the lower chamber of the stove chamber to a flue outlet at a level above the primary combustion chamber and upper level of the stove chamber. In such form, a portion of the grating wall comprises a grating through which ash or such like may fall to rest upon the floor of the stove chamber. Preferably, the side wall includes four side wall portions, each substantially planar which, together with the floor and top wall, provide a substantially rectangular prism form for the outside walls of the stove chamber. Preferably, one of the side wall portions has a door opening through and a the door being moveable between an open attitude permitting the access to the stove chamber and a closed attitude substantially sealing the stove chamber against air flow through the door opening as well as sealing the upper chamber from the lower chamber.

Preferably, the primary combustion chamber is of similar form to that of the stove chamber, but is smaller in size, and arranged to depend downward from the top wall. In similar fashion, the air jacket surrounds the lower portion of the primary combustion chamber in rectangular prism form, part of the air chamber wall provided as an extension of the base wall or extending outwardly from the chamber wall. In such form, the air jacket wall extends upwardly from its attachment to the base wall or curtain wall to the inside edges of the opening through the top wall of the stove chamber. Alternatively, the air jacket wall ends short of the top wall and rejoins the curtain wall. In such form, the chamber wall provides the sealing arrangement about the inner edge or edges of the opening through the top wall.

For convenience, the side wall portion with the door opening will be referred to as the front wall and, observing the stove chamber from the outside of the front wall, a right side wall is to the right and a left side wall is to the left of the front wall, and a rear wall is opposite the front wall adjoining the right and left side walls. Preferably, the air inlet to the primary air duct is provided substantially centrally through the front wall of the stove below the door. In such form, the primary air duct extends horizontally along the floor and front of the stove chamber to the secondary air duct, which comprises two rising ducts which run along each side of the front wall from the ends of the primary air duct. Preferably, the primary air duct has a damper or such like interposed between the air inlet and the secondary air duct for damping or restricting airflow through the primary air duct. Preferably, the air inlet forms an orifice to the primary air duct and the damper is provided to selectively occlude, or partly occlude, the orifice.

Preferably, the tertiary air duct provides fluid communication between the two secondary air ducts, though in operation, air flow through the secondary air ducts is substantially parallel, flow from the primary air duct being split between the two secondary air ducts and joining together again in the tertiary air duct. However, the tertiary air duct splits the air flow again, some of the air passing to and through the stove chamber inlet and the remainder of the air passing to and through the air jacket.

The plurality of ports are arranged to provide limited air flow to the primary combustion chamber, but the majority of the pressure drop thereto. The remainder of the air flow is directed to the stove chamber. The air flowing to the primary combustion chamber in use is at least partly deoxygenated with the combustion of wood fuel and hot coals in the primary combustion chamber to produce hot gas, but the draft of hot gases from combustion is downward through the grates and then upward through the flue passage by virtue of the enthalpic rise of lower density hot gases in the flue passage.

The present invention relates to a reverse flow, progressive oxidation wood fuel heating stove encompassing an improved combustion process which may be effected in the stove herein described. The oxidation of wood fuel in the stove is progressive, the wood moving by gravity to locations of increasing oxidation with a gas and flame flow which is the reverse of that which may be regarded as "normal". It has surprisingly been discovered that if the fire gases are made to flow downward, combustion of the problem hydrocarbons and carbon particles occurs more readily because the vertical temperature gradient may also be reversed. Sustaining the flow of air for the reverse flow movement is by way of the chimney draft. The natural tendency for air to rise through the fire is overcome by the strength of the downward draft which is enhanced by locating the chimney uptake close to the hot gases exiting the tertiary burning charcoal and the chimney or flue outlet well above the level of the fire. This also permits the chimney to be used more effectively as a heat radiator .

The progressive oxidation has been tested and observed to operate in an enclosed box (the primary combustion chamber), open only at the base and providing a combustion pathway of length sufficient to allow the oxidation processes to occur in an ordered way. Air intake ports of predetermined size are strategically located to penetrate the sides of the primary combustion chamber. The size and location of the air ports are selected to control air without a need to use the damper as well as to introduce a kinetic or momentum component to the air flow to create, or at least to encourage the creation of, turbulence . When the system is combusting and the draft flowing, some first air, limited in amount, is drawn through the upper holes for an initial lower temperature burning, which progressively heats the wood fuel lower down and preheats the wood above by radiation. As more air enters lower down, the air becomes at least partly depleted of oxygen with progressively higher temperatures. Because the temperatures are higher at the base, final disintegration of the wood can occur at the base which creates space for the upper level to progressively move down to the next oxidation position.

A wood feed door is provided at the front top of the primary combustion chamber with a rotating locking handle on the side. A locking handle is coupled to a chimney bypass door at the rear by a rod. The lower portion of the primary combustion chamber has a predetermined number ports of predetermined size. This portion of the primary combustion chamber is enclosed by an air jacket.

In an alternative form, a basket is suspended at the open end of the primary combustion chamber. Grating rods extend across the bottom of the primary combustion chamber and the grating basket is suspended internally by vertical rods. The primary combustion chamber may be provided as a primary combustion chamber module fitted through the top of the main stove body which may be specially constructed, or may be an existing wood fuelled stove body of the prior art converted to a wood fuelled heating stove according to the invention.

In operation ash falls through the recessed fine mesh charcoal basket into the lower chamber, which acts as a "final flaming area" where the newly introduced oxygen from the air curtain flow is mixed with the oxygen depleted gases exhausting the primary combustion chamber and has passed through the flaming charcoal in the fine mesh charcoal basket. The lower chamber also leads to the start of a vertical exhaust box chamber which forms a lower portion of the chimney. A chimney bypass door can be rotated or otherwise moved across the exhaust box chamber to block off or reduce normal chimney flow, but open flow to the chimney from the upper end of the reduction chamber. The other end of the vertical exhaust box chamber receives the vertical portion of chimney .

In another aspect, the present invention resides in an improved combustion process for domestic wood heating stoves using a form of stratified downdraft where the oxidation of the wood fuel is progressive with the vertical movement of the wood feed starting with an oxygen limited combustion and progressing to combustion which is not so limited at the base. This invention has two advantages that result in higher stove temperatures per unit of wood burnt. Firstly, it is able to combust high yielding fuels such as tars, creosotes and carbon particles that standard stoves often fail to burn. Secondly, the invention uses a system of limited, but specifically targeted air. This approach increases the stove temperature because unused cooler air does not unnecessarily increase total gas volume or cool the combusted gases. The combination of these two advantages results in a stove that operates at a temperature which, based on testing conducted by the inventor, is up to 30% higher without any increase in wood consumption .

In order that the invention may be more readily understood and put into practical effect, the basic open fire structure of the prior art and an example of the present invention will now be described with reference to the following drawings, and wherein:

Fig. 1 is a diagrammatic side elevation of a basic open fire structure according to the prior art; Fig. 2 is a pictorial view of a wood fuelled heating stove according to the invention;

Fig. 3 is a vertical cross section of the stove of Fig. 2 along line 2-2 of Fig. 5; Fig. 4 is a horizontal cross section of the stove of Fig. 2 along line 3-3 of Fig. 5 ;

Fig. 5 is a vertical cross section of the stove of Fig. 2 along line 4-4 of Fig. 4 ;

Fig. 6 is a diagrammatic oblique view of part of a grate for the stove of Fig. 2 ;

Fig. 7 is a diagrammatic exploded view of some of the parts of the grate of Fig. 5; and

Fig. 8 is a diagrammatic pictorial view of part of an adjustment system for the grate of Figs. 6 and 7;

Fig. 9 is diagrammatic sectional side view of part of the combustion process in the primary combustion chamber of the stove of Fig. 2 ;

Figs. 10 is a diagrammatic sectional side view of the reduction portion of Fig. 9 showing its disposition to with respect to the stove portion;

Fig. 11 us a diagrammatic sectional side view of the reduction portion of Fig. 9 showing its disposition with respect to the flue;

Fig. 12 is a diagrammatic sectional front view of the reduction portion of Fig. 9 showing an optional basket for raising and lowering fuel in the reduction portion; Figs. 13 and 14 are diagrammatic sectional side view of the stove of Fig. 2 with the upper portion of the flue removed;

Fig. 15 is another diagrammatic sectional side view of the stove of Fig. 2 ;

Fig. 16 is a diagrammatic front elevation of part of the flue for the stove of Fig. 2; and

Fig. 17 is a graphical representation comparing combustion temperatures of an open fire of the prior art with combustion temperatures of a fire in the stove of

Fig. 2.

In Figs. 2 to 16 of the drawings, because different parts or portions of the wood fuelled stove are exemplified in different drawings, not all of the elements are shown in all of the drawings and consequently, some of the reference numerals are provided in some of the drawings. Additionally, some of the description will refer to the way the stove operates when it is in use.

A basic open fire structure can be divided into zones "A", "B", "C" and "D" for convenience of explanation. At the base of the fire is an ash zone "A" containing ash and charcoal. Burning of the charcoal to ash "bakes" the wood above to release carbon-based gases which ignite when mixed with air in zone "B" which contains flames, burning gas and baking wood. The flames stream upwards to heat newly added wood fuel in zone "C" which also contains released gases for burning to produce the flames above in zone "D". Air flows in from the sides, starting near the base of the fire and also up the sides, the inflowing air being cool, that is, at or not much above ambient temperature.

The wood fuelled heating stove 10 illustrated in Figs. 2 to 5 includes a stove portion 11 and a reduction portion 12 partly penetrating into the stove portion and partly extending upwardly therefrom. The stove portion is supported on a plinth or base 13. The stove portion has a floor 14, a right side wall 15, a rear wall 16, a left side wall 17, a front wall 18 and a top wall 27 which enclose a stove chamber 41, the reduction portion having enclosed therein a reduction chamber 42. The front wall has a door 19 providing access to the stove chamber. It can be seen that when closed the door seals against the partition at 19a.

An air inlet 20 proceed from the front wall and leads to a primary air duct 22. The primary air duct is a horizontal duct extending transversely along the front bottom corner of the stove chamber. Two rising secondary air ducts 21 and 23 extend from the ends of the primary air duct and extend upwardly along the front left and right corners of the stove chamber to a tertiary air duct 24 above the top wall of the stove chamber.

The tertiary air duct provides fluid communication from the secondary air duct to an air jacket 26. Once stable combustion is established in the wood fuelled heating stove, inlet air passes through the inlet represented by arrows 29. The inlet air passes through the horizontal portion of the primary air duct and into the secondary air ducts as represented by arrows 30. The inlet air then passes upwardly through the secondary air ducts and into the tertiary air duct has represented by arrows 31.

Some of the air passing through the tertiary air duct passes through a stove chamber inlet 40 into the stove chamber as represented by arrows 32. The remainder of the air passing through the tertiary air duct passes therethrough as represented by arrows 33 and 34. For convenience, the air passing from the tertiary air duct to the air jacket will be referred to as primary combustion air. The primary combustion air passes from the tertiary air duct to the air jacket, proceeding around the air jacket as represented by arrows 35 and 36 including downwardly throughout the air jacket as represented by arrows 37. The primary combustion air from the air jacket passes through a plurality of airports 39 and into the primary combustion chamber as represented by arrows 38. The airports are provided in a curtain wall 51 which surrounds the reduction chamber. The air jacket is separated from the stove chamber by a curtain wall 50. The stove chamber is divided into an upper stove chamber 54 and a lower stove chamber 55 by a charcoal grating partition 52 having a fine mesh charcoal basket 53 disposed somewhat centrally in the charcoal grating partition. The reduction chamber is separated from the upper stove chamber by a primary grating 56 having a plurality of openings 57 through which hot coals may fall to land on the fine mesh charcoal basket for combustion to an ash .

The primary grating has a variable aperture or spacing as illustrated in more detail in Figs 6 to 8. The primary grating includes two side channel sections 58 which are arranged opposite one another with the channels facing towards one another. Only one of the side channel sections is illustrated in Figs. 6 and 7, but the arrangement is substantially symmetrical. Two sets of bars extend between the opposed side channels with the ends of each bar inserted at least part way into the channel of each side channel section.

Each set of bars includes a plurality of bars shown typically at 61 substantially parallel to one another within each set as well as the bars of one set being substantially parallel to the bars of the other set. One set is a fixed set 59 and the other set is a moveable set 60 which may be slid along the channel, the direction of sliding being along the channels and transverse to the bars. Each set of bars has substantially the same spacing. The fixed set is fixed to the side channel sections by welds 62 at or near the ends of each fixed rod and at regularly spaced intervals along the lips 58a of the side channel sections. The ends of the bars of the moveable set of bars are fastened in spaced disposition to a sliding bar 63 which has cross-sectional dimensions selected for sliding engagement with the channel of the side channel sections. The ends of the fixed bars are shorter than the moveable bars so that the sliding bar can slide past the ends of the fixed bars. The moveable bars are also fixed to a push-pull bar 64, a portion of which extends away from the remainder of the moveable bars to a connecting rod 65 and the remainder is welded to at least one, but preferably all, of the moveable bars by welds at 73.

The connecting rod is pivotally connected to a lever portion of a crank assembly 67. The lever portion is fixed to a torsion rod 68 having an axis parallel to the axis of the pivot to which the connecting rod is connected. A crank portion 69 extends at rights angles to the torsion rod and has a crank handle 70 attached or fastened thereto. Rotation of the torsion rod in the direction of a rotational arrow 71 by application of a tangential force to the crank handle causes linear movement of the connecting rod and push-pull bar, whereupon the moveable rods are moved in the direction of a linear arrow 72. It can be seen that the rotational and linear arrows are bi-directional, indicating that the moveable rods may be moved from against one side of the fixed bars to against the other side of the fixed bars, and at any point therebetween. ' In the diagrammatic sectional side view of Fig. 9, new wood at 111 is added at the top and as it falls through the chamber, undergoes limited burning at 112, burning at 113 and intense burning at 114. At 115, the wood has been converted to charcoal and is flaming charcoal at 115. Air is introduced, as indicated by arrows at 116, but in a limited amount in accordance with the invention. The graph to the right of the diagrammatic sectional side view illustrates the relationship between the oxygen availability for combustion and the temperature, also with respect to the relative position in the combustion chamber.

The reverse combustion process will now be described with reference to Figs. 10 to 15. To attain control of the reverse flow fire process^', stove according to the present invention is constructed using two basic modules. The first module is the primary combustion chamber constituting the reduction portion 12, which is enclosed at the top and sides but open at the bottom. The primary combustion chamber uses the primary grating 56 at the bottom to retain the feed wood 43 therein. The second module is the stove portion, and is substantially in the form of a common wood combustion stove which has been modified to include the primary combustion chamber half set into the roof of the stove. The flue receives gases only from the lower stove chamber. It is suggested that the reverse flow fire process has threefold elements: Firstly, suction is provided by the draft from the flue, thereby enabling the down draft through the combustion module into the upper stove chamber, through the fine mesh charcoal grating into the lower stove chamber with the exhaust gases exiting via the flue.

Secondly, splitting the air and targeting the major portion thereof to the combustion chamber and the remainder to the upper stove chamber results in higher temperatures being concentrated in smaller volumes of gases that move more slowly through the system, thereby providing pre-heating of air before entering the combusting areas of the stove and maximising the oxidation of the wood fuel in the primary combustion chamber. Air is directed around the sides of the stove body to use the heat radiating from the sides of the stove to pre-heat the air used for combustion of the wood and associated volatile gases, tars and creosotes. Air flowing through the lower air ports flames the charcoal on the coarse mesh grate used and enables secondary combustion of unburnt gases and also helps with the tertiary combustion of any remaining volatile gases passing through the charcoal on the fine mesh charcoal basket.

The primary combustion chamber is set into the top wall of the stove portion such that approximately half of the primary combustion chamber is positioned above the stove roof, while the lower half extends into the upper portion inside the stove. To assist in safe operation, the fire is dampened when the adding additional wood by constricting or closing off the draft through the chimney by a connecting a linkage and hinge that connects the wood feed access door locking handle to the chimney diversion door.

Within the upper stove chamber, the primary combustion chamber air manifold or air jacket surrounds the lower half of the primary combustion chamber receives pre-heated air and further heats air from' the air supply system. The pre-heated air is drawn in such volumes that oxygen limited burning in the upper part of the primar combustion chamber progresses to oxygen rich burning in the lower part. In an alternative form, a basket is provided which can be raised or lowered within the primary combustion chamber to alter the combustion of the wood fuel and its relation to the relative position of the air entry holes. The height of the basket is set by a hollow course pitch screw thread which is rotated by a crank handle. A rod passes through the hollow screw, then passes through the length of the combustion chamber and is attached to the centre of a blade which can be rotated by a handle to break up and agitate the large pieces of charcoal sitting on the grate. A spring placed between the handle and the top of the hollow screw positions the blade or stirrer in a raised position. The rod extends through locating holes in the grate and terminates with a second stirring blade, the rotation of which, and height variation, is used to smooth and distribute charcoal that has dropped through the coarse grating.

The partition between the upper stove chamber and lower stove chamber has soft, but airtight seal against the stove access door. The fine mesh charcoal basket is recessed down into the partition to receive charcoal falling from the course mesh charcoal grate at the base of the combustion chamber. The flue may be is split as shown in Fig. 16 into two (or more) parallel pipes to increase the radiating surface area. Being a reverse flow (downdraft) fire, a downdraft is required before the main fuel body can be ignited. To achieve this the following procedure is required:

The Chimney is preheated by igniting a tray of conunercial alcohol (40-50 ml) under the chimney opening in the lower stove chamber to allow the chimney to be heated to about 100 °C in approximately 40 seconds.

With the main stove access door open, the charcoal and kindling in the fine mesh charcoal tray separating the upper and lower stove chambers is lit, access thereto being via the stove body door which is closed after the lighting operation. The charcoal strongly burns within 60 seconds with a chimney temperature of about 150°C and rising with a strong reverse flow through the stove. After the burning of charcoal in the fine mesh charcoal tray has stabilised, the air supply system control is opened. Two minutes after lighting the charcoal in the fine mesh charcoal tray the chimney temperature reaches approximately 200°C and a strong downdraft is evident. Using kindling on top, a moderate volume of the main feed wood in the primary combustion chamber is lit, with the primary combustion wood feed access door open, and the air supply system open. As the wood catches on fire and combustion stabilises, the wood feed access door is gradually closed and the air supply system is adjusted to optimise combustion.

After the feed wood has been ignited in the base of the combustion chamber, the volume of wood in the combustion chamber is gradually Increased to give time for charcoal to form on the course mesh charcoal grate and the fine mesh charcoal basket enabling secondary and tertiary combustion. All air in use in the primary combustion chamber and the upper stove chamber is supplied from and controlled by the air supply system. With the ignition operating in a stabilised state, the preheated air delivered for the air curtain on. the inside face of the stove body door is of the order of 200 °C, and in the in the primary combustion chamber air manifold entering the primary combustion chamber at about 250 °C at the top and about 300 *C at the bottom. By partially capturing the heat radiating from the stove body and circulating this heat back into the primary combustion chamber via the air supply system and the primary combustion chamber air manifold, the temperature of the unburnt wood is raised which assists in the removal of moisture thus enabling the primary combustion of the wood and the release of volatile gases.

In Fig. 14, the progressive oxidisation of the feed wood begins at the level XI of the first air inlet. At this level the air is limited and supports only weak oxygen-limited combustion. The movement of gas is subject to the strong draft and the overall movement is downwards, accelerating as more air inlet holes are passed, more fuel combusted, the temperature rises and the combustion process progressively changes from oxygen limited at XI to become less oxygen limited at X2.

At about this point the hydrocarbons have mostly been subject to pyrolysis to be cooked out of the original wood feed to leave charcoal as the dominant residue. In an oxygen rich environment the charcoal at X3 is the dominant fuel with a temperature in excess of about 1,0QO°C. This is sufficient to combust all of the hydrocarbons provided that oxygen has been able to mix from the sides and past obstacles at the base. The combusting charcoal reduces in size, fractures with the weight above or by mechanical crushing. Charcoal then falls through the grate to replenish the . charcoal in the fine mesh, charcoal basket or tray.

The progressive oxidation of the system within the primary combustion chamber requires a column of wood to be baking and releasing gases above the lower oxygen rich combusting area. If the wood is withheld with the thought of slowing the combusting rate, the fire will tend towards being a common fire that happens to be downdraft, the lower charcoal beds will diminish, combustion will be incomplete and the process will deteriorate with pollution deposits internally and externally.

Additional air at X4 from the air curtain is added and drawn through the smaller charcoal in the basket. This action of drawing gases through the basket containing smaller charcoal pieces creates extra mixing as it passes through and the extra air flames the charcoal. The gases emerging into lower stove chamber are different, produce a flames of a different colour and have been all but purged of tars and creosotes as can be seen by the lack of condensate deposits o the lower stove glass door which has no protective air curtain.

Ash falls to the floor, the gases are drawn down and over the ash pile and the combusted gases are drawn up the rear flue space to egress into a large diameter thin walled chimney flue. To slow down the velocity of the flue gases the flue is duplicated by a union. This configuration significantly increases the time for heat energy to be radiated and distributed by convection into the domestic space before being released into the atmosphere. The stripping of heat can be done without adversely affecting the workings of the stove because of the high temperatures of the gases at the base of the chimney (approximately 500 °C) . It has been found that the wood fuelled heating stove according to the present invention when used in the manner described herein arranges for both the depleted wood solids and various gases of the process to arrive at the final oxidation position at the highest temperature of the process and in an environment of controlled availability of oxygen. The wood and air proceed in the same direction downwards, the wood by gravity and the air by the chimney draft. It is suggested that chimney temperature be monitored and maintained as it provides the driving force of the reverse flow action.

Referring to the graph in Fig< 17, in a conventional fire the volatile gases released in the primary combustion rise vertically from the burning charcoal, not through it. As these flaming gases rise they cool while- mixing with inflowing air. The resulting cooled air/gas stoichiometric mixture restricts the maximum temperature at the top of the flame to approximately 300 ^eC as represented by line 101 in Fig. 17.

The available oxygen or the temperature are often insufficient to complete the secondary combustion of the volatile gases and particles released from the burning wood when mixed with air.

In the reverse flow system of the present invention, the combustion temperatures of the gas mixture increase as the flow moves downwards culminating with the gases passing through the hottest area of the fire burning charcoal at approximately 1,000 °C as represented by line 102 in Fig. 17. The gases released from the burning wood reac their maximum temperature just prior to exiting the system as exhaust gases. Mixing an additional and pre-heated oxygen supply to where the gas has reached its maximum temperature creates optimum conditions for the tertiary combustion of remaining combustible gases and particles as an additional fuel source. This enables the near complete combustion of the wood and all by-product gases expelled during the primary combustion phase, thereby increasing the efficiency of stove while also reducing the quantity and nature of the pollutants finally released through the chimney flue.

Primary combustion takes place where the wood is burned in both an oxygen limited and an oxygen rich environment in the primary combustion chamber. Secondary combustion takes place in the charcoal resting on the coarse mesh charcoal grate at the base of the primary combustion chamber and burns the majority of the volatile gases (typically methane and methanol), tars and creosotes expelled from the burning wood. Tertiary combustion of the remaining volatile gases takes place in the charcoal resting on the fine mesh charcoal^' basket which ultimately achieves a temperature of approximately l,00O^eC. Although the invention has been described with reference to a specific example, it will be appreciated by persons skilled in the art that the invention may be embodied in other forms within the broad scope and ambit of the invention as herein set forth and defined by the following claims.

Claims

THE CLAIMS DEFINING , HE INVENTION ARE AS FOLLOWS:

1. A wood fuelled heating stove including: a stove chamber . having a floor-, a side wall extending upwardly from the periphery of the floor and a top wall extending across the top of the side wall; a primary combustion chamber at least a part of which extends at least part way into the stove chamber through an opening penetrating the top wall, the primary combustion chamber having a base wall interposed between the floor and the top wall of the stove chamber and having a primary grating provided comprising at least a part of the base wall, a curtain wall extending upwardly from the base wall through the opening in the top wall and having a plurality of ports penetrating the curtain wall and a closure wall extending across the top of the curtain wall to enclose the primary combustion chamber; an air jacket wall circumscribing at least part of the curtain wall at least partly below the opening to provide an air jacket circumscribing the primary combustion chamber but separated therefrom by the curtain wall, the curtain wall having a plurality of ports distributed about and penetrating therethrough to provide fluid communication between the air jacket and the primary combustion chamber, the air jacket wall and/or the curtain wall substantially sealing the opening through the top wall and separating the air jacket from the stove chamber; an air inlet opening through the floor or the side wall at or near the floor and a stove chamber air inlet into the stove chamber through or near to the top wall; a primary air duct, a secondary air duct and a tertiary air duct the primary air duct extending along the floor of the stove chamber and providing fluid communication from the air inlet to the secondary air duct, ti e secondary air duct extending upwardly from the primary air duct and the tertiary air duct providing fluid communication between the secondary air duct and the air jacket and the stove chamber air inlet; a charcoal grating partition interposed between the floor of the stove chamber and the base wall of the primary combustion chamber; and

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a flue passage extending from a flue opening to the stove chamber lower than the grating wall and upwardly to a flue outlet higher than the top and closure walls.

2. A wood fuelled heating stove including: a stove portion having a stove chamber therein and an fine mesh charcoal basket dividing the stove chamber into upper and lower chambers; a reduction portion having a primary combustion chamber therein and at least partly within the upper chamber of the stove chamber and having a primary grating vertically separating the primary combustion chamber from the upper chamber; an air jacket surrounding at least some of the primary combustion chamber and having a plurality of ports providing fluid communication between the primary combustion chamber and the air jacket and a primary grating providing limited flow of mobile solids and fluid between the primary combustion chamber and the stove chamber; an air passage providing fluid communication from the outside of the stove chamber to the air jacket and to the inside of the stove chamber; and a flue passage providing fluid communication from the lower chamber of the stove chamber to a flue outlet at a level above the primary combustion chamber and upper level of the stove chamber.

3. The wood fuelled heating stove according to Claim 2, wherein a portion of the grating wall comprises a grating through which ash or such like may fall to rest upon the floor of the stove chamber.

4 . A wood fuelled heating stove according to an one of the preceding claims, wherein the side wall includes four side wall portions, each substantially planar which, together with the floor and top wall, provide a substantially rectangular prism form for the outside walls of the stove chamber, one of the side wall portions having a door opening through and a door being moveable between an open attitude permitting the access to the stove chamber and a closed attitude substantially sealing the stove chamber against air flow through the door opening as well as sealing the upper chamber from the lower chamber.

5. The wood fuelled heating stove according to Claim 4, wherein the air jacket surrounds the lower portion of the primary combustion chamber in rectangular prism form, part of the air chamber wall being provided as an extension of the base wall or extending outwardly from the chamber wall.

6. The wood fuelled heating stove according to Claim 4 or Claim 5, wherein the air jacket wall extends upwardly from its attachment to the base wall or curtain wall to the inside edges of the opening through the to wall of the stove chamber .

7. The wood fuelled heating stove according to Claim 4 or Claim 5, wherein the air jacket wall ends short of the top wall and rejoins the curtain wall.

8. A wood fuelled heating stove according to any one of the preceding claims, wherein the air inlet to the primary air duct is provided substantially centrally through the front wall of the stove below the door such that the primary air duct extends horizontally along the floor and front of the stove chamber to the secondary air duct, which comprises two rising ducts which run along each side of the front wall from the ends of the primary air duct.

9. The wood fuelled heating stove according to Claim 8, wherein the primary air duct has a damper interposed between the air inlet and the ^* secondary air duct for damping or restricting airflow through the primary air duct .

10. A wood fuelled heating stove according to any one of the preceding claims, wherein the plurality of ports are arranged to provide limited air flow to the primary combustion chamber, but the majority of the air pressure drop thereto.

11. A wood fuelled heating stove according to any one of the preceding claims which, in use, provides a reverse flow, progressive oxidation wood fuel heating stove encompassing an improved combustion process.

12. The wood fuelled heating stove according to Claim 11, wherein oxidation of wood fuel in the stove is progressive, the wood moving by gravity to locations of increasing oxidation with a gas and flame flow which is the reverse^" of that which may be regarded as "normal".