NZ515943A

NZ515943A - Solid fuel stove with means to maintain secondary combustion chamber temperature for clean burning of fuel

Info

Publication number: NZ515943A
Application number: NZ515943A
Authority: NZ
Inventors: Roger John Best
Original assignee: Clean Comb Technologies Ltd
Priority date: 2001-12-05
Filing date: 2001-12-05
Publication date: 2004-06-25
Also published as: WO2003048645A1; AU2002365742A1

Abstract

A solid fuel stove, which includes, a receiver adapted to receive fuel burnt and provided with a grate spaced above the base thereof. Means for supplying primary combustion air to the upper portion of the fuel receiver such that the primary combustion air is drawn through the fuel in the receiver to a primary combustion zone in the vicinity of the grate. A secondary combustion zone opens out from the first, which includes a chamber lined with heat retaining refractory material and being connected to one end of a gas outlet flue. Also included is means for supplying secondary combustion air to the secondary combustion zone and means to heat the zone to, or greater than a predetermined temperature.

Description

r' 515943 Intellectual Property Office of NZ 0 5 DEC 2002 RECEIVED Patents Form No. 5 No. 515943 Date: 5th December 2001 Patents Act 1953 COMPLETE SPECIFICATION IMPROVED STOVE We, Clean Combustion Technologies Limited, a New Zealand company of 287 Mound Road, Broomfield, Amberley 8251, New Zealand, hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: 1 Technical Field The present invention relates to an improved stove which is suitable for burning wood, coal or other solid fuels and which can be produced on a scale suitable for domestic/light industrial space heating or scaled up for larger scale industrial heating.

Background Art Although electrical space heating has been heavily promoted as efficient and "clean", (i.e. does not pollute the immediate environment), nevertheless there is a widespread preference for solid fuel heating, which is perceived as being significantly less expensive than electrical heating. In addition, the consumer preference for solid fuel 10 heating has been greatly reinforced by electrical power shortages and power cuts.

Over the last century or so, hundreds of different designs of solid fuel stoves have been proposed, but none of the stoves currently available for solid fuel combustion on a domestic or light industrial scale is sufficiently clean burning to make the stove 15 acceptable in areas such as Christchurch, New Zealand where the city's position and winter weather conditions have a tendency to produce a temperature inversion layer, with consequent air pollution.

A number of existing stove designs are said to be clean burning, but in fact are clean 20 burning only when the stove and flue have reached optimum temperature and the stove is operating efficiently:- at the start of the combustion process, or if the stove is damped down to burn slowly, substantial quantities of smoke are produced.

Disclosure of Invention It is therefore an object of the present invention to provide a solid fuel stove which is substantially clean burning during all stages of combustion.

The present invention provides a solid fuel stove which includes:- a fuel receiver adapted to receive fuel to be burnt and provided with a grate spaced 30 above the base thereof; means for supplying primary combustion air to the upper 1a portion of the fuel receiver, such that in use primary combustion air is drawn down through the fuel in the receiver to a primary combustion zone in the vicinity of the grate; a secondary combustion zone opening out of the primary combustion zone, said secondary combustion zone including a chamber lined with heat retaining refractory 5 material and being connected to one end of a gas outlet flue; means for supplying secondary combustion air to said secondary combustion zone; and means for heating said secondary combustion zone to temperature equal to or greater than a predetermined temperature.

Preferably, said means for heating said secondary combustion zone comprises a burner for gaseous fuel associated with a secondary air supply.

Brief Description of Drawings By way of example only, a preferred embodiment of the present invention is described 15 in detail with reference to the accompanying drawings, in which: Fig.s 1 and 2 are diagrammatic longitudinal sections through stoves in accordance with first and second embodiment of the present invention; and Fig. 3 is a view from below the grate of Fig. 2, on a larger scale.

Best Mode for Carrying out the Invention Referring to Fig. 1, a stove 2 includes a fuel receiver 3, the lower surface of which is formed by a grate 4, an ash pit 5, a secondary fuel inlet channel 6, and a reburning section 7. The ash pit 5 can be accessed through a separate door 5a, for cleaning.

The fuel receiver 3 comprises a metal shell partially or completely lined with fire brick and formed with a closed top 8 and a front door 9 through which solid fuel can be placed in the fuel receiver 3. Although the fuel receiver 3 has the appearance of a firebox, in fact primary combustion starts at the base of the receiver 3, just above or just below the grate 4, and proceeds upwards through the fuel.

The grate 4 lies in a horizontal plane, separating the bottom of the receiver 3 from the 2 top of the ash pit 5 and secondary combustion zone 7. The base of the ash pit 5 is formed by a fire brick liner 10. The secondary fuel inlet channel 6 extends vertically down the back of the receiver 3, and opens into the top of the ash pit 5. A pipe 11 connected to a supply of secondary, gaseous, fuel extends down the channel 6 and 5 supports a burner nozzle 12 at a level slightly higher than the grate 4.

The rear of the ash pit 5 opens into the reburning section 7, which comprises a vertical flue 13 the sides of which are lined with fire brick. Fire brick bridges 14 (of which two only are shown) are formed across the flue 13, for the purpose hereinafter described. 10 The top of the vertical flue 13 opens into a conventional chimney flue 15, of which only the lower portion is shown. The flue 15 may be water jacketed.

A pair of parallel ducts 16 forming the primary air inlet extend horizontally through the upper portion of the flue 13, into the upper portion of the receiver 3, as indicated in 15 broken lines in the drawing.

The above described stove is used as follows:- suitable kindling material (such as small logs) is placed on the grate 4 through the open door 9, and larger logs are stacked in the receiver 3 on top of the kindling. In addition, kindling material may be 20 placed around the lower end of the secondary fuel inlet 6, just below the nozzle 12. The door 9 is then closed.

Gaseous fuel is supplied to the burner nozzle 12 through the pipe 11, and this fuel is ignited by any suitable means, (e.g. matches, lighter, or (preferably) a built-in electrical 25 ignition coil) to burn the kindling material and preheat the ash pit/secondary combustion area 5 and the reburning section 7.

The burner nozzle 12 is supported in the pipe 11 with an air space around it, so that combustion air is drawn in to the secondary fuel combustion zone 17 both through a 30 Venturi inlet 18 around the sides of the nozzle and through the gap 19 between the pipe 11 and the sides of the channel 6. With an ample air supply of this type, the gaseous fuel is substantially clean burning, and rapidly raises both the primary and 3 secondary combustion zones to the optimum temperature for efficient burning.

The burning kindling material sets fire to the fuel in the receiver 3:- the portion of fuel closest to the grate 4 burns first, since it is closest to the kindling. During start-up, the 5 flame from the secondary fuel provides a wall of fire through which any gasses distilled from the burning solid fuel have to pass. Because secondary air is provided, these gases also burn, to give a temperature typically in excess of 1200°C. The primary combustion air follows the route indicated by the double headed arrows - air surrounding the stove is drawn through the ducts 16, thereby pre-heating the air, and 10 then drawn down through the fuel stacked above the grate in the receiver 3, to the primary combustion zone adjacent the grate 4. The solid fuel is burned from the bottom up - the most efficient mode.

The gases given off from the primary combustion zone pass into the secondary 15 combustion zone at the base and the rear of the ash pit 5, which is heated to a higher temperature by the burning of the gaseous fuel. Secondary combustion takes place here. The combustion gases then pass up the flue 13, where further secondary combustion occurs as the gases contact the fire brick lining of the flue 13 which has been preheated by the combustion of the gaseous fuel. The bridges 14 help to 20 provide a labyrinth path through the flue 13, to ensure a thorough mixing and complete combustion of the gaseous combustion products.

Once the temperature of the ash pit/secondary combustion zone 5 and the flue 13 have reached their optimum operating temperature, the combustion of the secondary 25 gaseous fuel is progressively reduced to zero, since the solid fuel combustion is sufficient to maintain that optimum operating temperature, (at or above 900°C, typically 1000 - 1100°C) ensuring that all of the primary combustion products are efficiently reburnt. If the temperature drops below 900°C, combustion of the secondary gaseous fuel is re-started, to lift the secondary combustion temperature to about 1000°C and 30 ensure complete combustion.

It has been found in practice that the above described stove burns without any visible smoke at all. 4 It is envisaged that the flue 13 or the chimney flue 15 could be fitted with a wet scrubber or dry particulate collector and/or heat recovery unit, to further improve the standard of air cleanliness and to optimize thermal efficiency. With these additions close to 100% heat recovery is possible, and particulate emissions as low as 0.006 5 gm/kg of wood burnt have been achieved. However, it will be noted that all heat recovery apparatus is located downstream of the secondary combustion zone, so as not to compromise efficient secondary combustion, and so that the exhaust gases cooled by heat recovery are relatively clean. This is important, because if exhaust gases containing toxic, corrosive or tarry products are cooled, the products are 10 deposited in the flue or on the out side of the chimney.

Referring to Fig.s 2 and 3, the stove 20 is in many respects similar to the stove shown in Fig. 1, except that the fuel receiver 3 is shaped differently and the secondary fuel and secondary air are supplied through a different route.

The stove 20 includes a fuel receiver 21 which consists of a metal shell partially or completely lined with fire brick and formed with a heat resistant glass panel 22 across its front wall and a sloped upper portion 23 which incorporates a hinged, lockable lid 24 through which solid fuel can be placed in the fuel receiver 21.

A grate 25 (shown in detail in Fig. 3) extends horizontally, spaced above the base 26 of the stove 20. The grate 25 separates the bottom of the receiver 21 from the top of the secondary combustion zone 27:- there is only a small gap between the inner end 28 of the grate and a ridge of refractory liner 29 which extends across the full width of 25 the lower portion of the receiver 21, protecting the base of the flue 30.

As shown in detail in Fig. 3, the grate 25 is formed from two spaced tubes 31, each closed at its inner end and formed with a series of holes 32 on its underside, and a gas burner 33 located between the tubes 31. The gas burner 33 consists of two connected 30 pots with curved outer walls; each pot has a closed upper surface 34 and an open lower surface 35. The gas burner 33 is connected to a source of gaseous fuel by an air/gas supply tube 36 which extends parallel to the tubes 31.

A gas pipe 37 which is substantially smaller in diameter than the interior diameter of the tube 36 is arranged to supply gas part way down the length of the tube 36; the outer end of the tube 36 is open so that when gas is supplied to the burner 33 through 5 the tube 37, (Arrow A) air is drawn into the outer end of the tube 36 (arrows B) to mix with the gas.

The grate 25 shown in Fig. 3 is of a size suitable for use in a small domestic stove. For larger stoves, the grate consists of two or more pairs of tubes 31, each with a 10 corresponding tube 36 and gas burner 33.

An ash pit 40 is located immediately below the grate 25, and opens into the reburning section 27. For cleaning ash out of the stove, the ash pit 40 can be accessed from the front of the stove through a lower door 41.

As in the stove shown in Fig. 1, the base of the ash pit 40, the reburning section 27, and the flue 30 all are lined with fire brick, with fire brick projections 42 extending into the flue so that gases passing out of the flue follow a labyrinth path, maximizing the heat exchange between the gases and the flue. The top of the flue 30 opens into a 20 conventional chimney flue 43, of which only part is shown.

A heat exchanger 44 may be located at the top of the flue 30. The heat exchanger 44 may be of any suitable known type and the water circulated through the heat exchanger may be used directly or indirectly for water or space heating.

One or more inlet ducts 45 for primary combustion air extend horizontally through the upper portion of the flue 30, so that incoming primary combustion air is preheated by the heat in the flue.

The stove shown in Fig.s 2 and 3 is used in the same general manner as the stove shown in Fig. 1:- suitable kindling material (e.g. small logs) is placed on the grate 25 through the door 24, and larger logs are stacked in the receiver 21 on top of kindling. 6 The door 24 is then closed. Gaseous fuel is supplied (Arrow A) through the tube 37, entraining air with it (Arrow B), and the gas is ignited by any suitable means to burning in the gas burner 33. Additional secondary combustion air is supplied through the holes 32 in the tubes 31.

The upper surface 34 of the gas burner 33 is closed, so the gas flame is directed downwards into the start of the secondary combustion zone 27, preheating that zone. At start-up, the secondary combustion zone is heated to a temperature generally in excess of 1200°C. The gas burner 33 also heats the grate 25 and ignites the kindling 10 material, which in turn ignites the fuel stacked on top of it. During start-up, the flame from the secondary fuel provides a wall of fire through which any gases from the burning solid fuel have to pass, as described with reference to the Fig. 1 embodiment.

The primary combustion air follows the route indicated by the double headed arrows:-15 air surrounding the stove is drawn through the ducts 45, preheating the air, and then drawn down through the fuel stacked in the receiver 21 to the primary combustion zone adjacently top of the grate 25. Thus, the fuel is burned from the bottom up, which is the most efficient method and as with the Fig. 1 stove, gases given off from the primary combustion zone pass into the secondary combustion zone 27 at the base 20 and in the rear of the ash pit 40, where secondary combustion takes place.

The combustion gases then pass into the flue 30, through the labyrinth path provided by the projections 42. Further secondary combustion occurs here, since the fire brick lining of the flue has been preheated by the gaseous fuel.

Once the temperature of the ash pit, secondary combustion zone and flue have reached their optimum operating temperature, the combustion of the secondary gaseous fuel is progressively reduced to zero, since the solid fuel combustion is sufficient to maintain an optimum operating temperature, typically in the range 900 -30 1100°C.

The supply of secondary gaseous fuel preferably is automatically controlled by 7 temperature sensors in the ash pit and/or the flue, so that if the temperature in these areas falls below a preset temperature, ignition of the gaseous fuel is automatically restarted.

Either of the above described stoves may be operated with an induced draft, provided by an extractor fan mounted in the flue.

Either of the above described stoves may be provided with the following additional feature, if a particularly clean burning stove is essential:- one or more optical sensors 10 may be mounted at the top of the gas outlet flue 13/30 or in the chimney flue 15/43, and tertiary combustion air at above ambient pressure is introduced into the upper part of the secondary combustion chamber, to provide an excess of air to burn off carbon/volatile fuel components which may appear in the secondary combustion chamber during periods of over stoking (i.e. supplying excessive fuel) or the use of 15 highly volatile fuel materials (e.g. plastics materials). The tertiary combustion air may be introduced into the secondary combustion chamber by any convenient means, e.g. through the gap 19 in the stove shown in Fig. 1.

Preferably, the tertiary combustion air is preheated. The tertiary air may be raised in 20 pressure by any suitable means, e.g. a fan or a blower.

Instead of using a secondary gaseous fuel for the preheating of the ash pit, reburning section, and the flue, it may be possible to substitute electrical preheating of these areas. However, it is believed that it may be advantageous to maintain a continuous 25 supply of gaseous fuel, because such fuels produce a hydrogen rich atmosphere in the secondary combustion areas; the hydrogen combines with any available chlorine to form hydrogen chloride and thus prevents toxic chlorinated PCBs from forming. A further drawback of electrical preheating is that it would need to be turned on well in advance of burning the solid fuel, because (unlike gaseous fuel) the electrical heating 30 does not provide a wall of flame through which combustion gases are drawn, and hence is ineffective until the secondary combustion zone reaches the desired operating temperature. 8 The above described stove may be manufactured in a wide range of sizes, to suit particular applications, and may be freestanding or built in.

It is envisaged that existing stoves or open fireplaces could be adapted to form a stove in accordance with the present invention by supplying a kit of parts which consists of a refractory lining for the stove/fireplaces and flue, and a grate which incorporates a gas burner as described with reference to the Fig.s 2/3 embodiment.

Since existing stoves and fireplaces come in a multitude of different sizes and shapes, the kit of parts may need to be varied widely to suit particular applications. However, the same principle applies to each kit of parts, in that the necessary components must be provided to allow for a downward movement of the primary combustion air, the provision of a secondary combustion chamber, preheating of the primary and 15 secondary combustion air, and the use of a secondary fuel to provide optimum combustion conditions when the stove/fireplace is first started up. 9 4

Claims

Claims 5 10 15
2. 20
3.
4. 25
5. 30
6. A solid fuel stove which includes:- a fuel receiver adapted to receive fuel to be burnt and provided with a grate spaced above the base thereof; means for supplying primary combustion air to the upper portion of the fuel receiver, such that in use primary combustion air is drawn down through the fuel in the receiver to a primary combustion zone in the vicinity of the grate; a secondary combustion zone opening out of the primary combustion zone, said secondary combustion zone including a chamber lined with heat retaining refractory material and being connected to one end of a gas outlet flue; means for supplying secondary combustion air to said secondary combustion zone; and means for heating said secondary combustion zone to temperature equal to or greater than a predetermined temperature. The stove as claimed in claim 1, wherein said means for supplying primary combustion air is such that in use, primary combustion air is preheated by the exhaust gases from the stove. The stove as claimed in claim 2, wherein said means for supplying primary combustion air is arranged to pass through the gas outlet flue. The stove as claimed in any one of the preceding claims wherein the fuel receiver is formed with a closable door in an upper portion of the receiver, for placing fuel into the receiver. The stove as claimed in any one of the preceding claims wherein the gas outlet flue is lined with heat retaining refractory material. The stove as claimed in claim 5 wherein said heat retaining refractory material is arranged to provide a labyrinth path for exhaust gases through said flue. 10 Intellectual Property Office of NZ 0 1| MAY 200*1
7. The stove as claimed in any one of the preceding claims further including a heat exchanger mounted in said gas outlet flue, at a level in said flue such that in use secondary combustion is substantially completed before exhaust gases reach said heat exchanger. 5
8. The stove as claimed in any one of the preceding claims wherein said means for heating said secondary combustion zone is mounted at the top of the secondary combustion zone. 10
9. The stove as claimed in any one of the preceding claims, wherein said means for heating said secondary combustion zone comprises a gas burner connectable to a supply of gaseous fuel by a gas supply tube.
10. The stove as claimed in claim 9, wherein said means for supplying secondary 15 combustion air to said secondary combustion zone includes an open ended tube arranged coaxially with said gas supply tube, one end of said tube being open to the air outside the stove.
11. The stove as claimed in claim 9 or claim 10 wherein said gas burner extends 20 substantially parallel to the gas outlet flue and opens into the interior of the stove at the top of the secondary combustion zone.
12. The stove as claimed in claim 9 or claim 10 wherein the gas burner is incorporated in the grate. 25
13. The stove as claimed in claim 12 wherein the grate comprises one or more units, each unit consisting of a pair of parallel spaced tubes adapted to supply secondary combustion air in use and a gas supply tube mounted between said parallel spaced tubes, one end of said gas supply tube extending outside 30 the stove and being connectable to a supply of gaseous fuel, and the other end of said gas supply tube being in communication with the gas burner. 11
14. The stove as claimed in claim 12 wherein said other end of said gas supply tube terminates inside and is surrounded by a secondary air supply tube.
15. The stove as claimed in claim 8 wherein said means for heating said 5 secondary combustion zone comprises electrical heating means.
16. The stove as claimed in any one of the preceding claims, further including means for supplying tertiary combustion air at above ambient pressure to said secondary combustion zone. 10
17. A kit of parts for constructing a stove as claimed in any one of the preceding claims.
18. A solid fuel stove substantially as hereinbefore described with reference to 15 and as shown in Fig. 1 or Fig.s 2 and 3 of the accompanying drawings. 12