GB2265384A

GB2265384A - Method and apparatus for producing heat from combustible material

Info

Publication number: GB2265384A
Application number: GB9124205A
Authority: GB
Inventors: Edward Weiching Ling; George Sungching Ling
Original assignee: Individual
Current assignee: Individual
Priority date: 1991-11-14
Filing date: 1991-11-14
Publication date: 1993-09-29
Also published as: GB9124205D0

Abstract

A solid material strip of the combustible material 16 having a series of spaced holes 22 is burnt in an oven to produce heat. A mixture of coal, clay, and lime may be used as the combustible material, and the products of the combustion then include a structurally intact, solid residual strip composed substantially only of dehydrated clay and calcium sulfate, and a flue-gas substantially free of carbon monoxide, nitrogen oxides, sulfur oxides, and micro fly-ash. Because of the stable and uniform combustion characteristic, it is also suitable for the co-combustion and reduction of most toxic and solid wastes into safe, recyclable residues. <IMAGE>

Description

METHOD & APPARATUS FOR EXTRACTING HEAT FROM A COMBUSTIBLE MATERIAL BACitGROUND OF THE INVENTION 1. Field of the Invention The present invention relates generally to methods and apparatus for extracting heat from combustible materials andq more particularly, to a method and apparatus for combusting a solid material strip at a controlled temperature.

2. Description of the Prior Nrt Historically, solid fuel was burnt mainly by the flat-bed combustion process. In the past two decades, solid fuel was replaced by liquid fuel because liquid was less labour intensive and was cleaner in operation. The liquid fuel was burnt by spraying the liquid fuel through a high-speed nozzle to atomize the fuel. In recent years, as liquid fuel became more costly, many power plants were modified for both liquid and powdered solid fuel combustion using the spraying technique originally designed for the liquid fuel. For home heating, liquid fuel is still in wide use, mainly due to the handling and pollution problems of solid fuel.

The fundamental problem with both liquid and powdered solid fuel combustion is that both type uf combustion are unstable runwawy forms of combustion which generally take place within 10 milliseconds, since it is near-ly impossible to control the reaction rates inherent in these types of combustions.The resultant high temperatures of combustion promote excessive production of carbon monoxide, nitrogen oides and, sulfur oxides and the production of corrosive and coating micro fly--ash. These byproducts of combustion are the cause of many environmental and operational problems, i.e. the pollution of the atmosphere and the coating of heat exchangers, resulting in the reduced heat exchange efficiency, whose avoidance greatly increases the cost of combusting liquid and powdered solid fuels.

ni so, the processes used in the prior art combustion methods often result in combustion temperatures of over ,00 oC. At such high temperature the combustion product is mainly CO instead of Con2.

Consequently, a large power plant required a mammoth combustion chamber to provide sufficient resident time for the CO gas to complete a secondary combustion into C02 at a lower temperature of about 1,000 oC, thus increasing the cost of operating the power plant. It is clear that a form of combustion is needed which avoids these problems.

OBJECTS NND SUMMARY OF THE INVENTION It is therefore an object of the invention to overcome the disadvantages of the prior art methods by providing a method and apparatus of etracting heat from combustible material which includes combusting a solid material strip having regularly shaped holes of a predetermined size therein.

Another object of the present invention is to provide a heat extraction method and apparatus which includes combusting a solid material strip at a desired temperature such that a flue-gas produced by the combustion is substantially free of carbon monoxide, nitogren oxides, sulfur oxides and micro fly-ash.

another object of the present invention is to provide a heat extractiun method and apparatus in which one product of a combustion is structurally intact, solid residual strip which may be easily removed from a combustion oven after combustion.

Another object of the present invention is to provide a heat extraction method and apparatus which is relatively rel at vel y i inexpensive to operate and maintain.

It is yet another object of the present invention to mix chemical compounds with the solid fuels for the purpose of complete and safe dissociation of toxic compounds either within the solid fuels or the additives into non-toxic solid residuals or flue gas. The stable and uniform combustion characteristics of the present invention is also ideal for the safe disposal of most garbage and toxic wastes.

The method of the present invention of extracting heat from the combustible material includes the steps of forming a solid material strip of the combustible material, forming a plurality of spaced holes in the material strip, and combusting the material strip in a combustion chamber.

The method of the present invention may also include the steps of forming a solid material strip of the combustible material and providing a plurality of spaced holes punched into the material strip, and preheating the material strip in the absence of ambient air. The material strip is combusted in a combustion oven by two independent controls. The desired temperature of combustion is regulated by forcing air or oxygen into the spaced holes at a controlled rate and, a fluid is circulated at a controlled rate to carry heat from the combustion of the material strip to a heat exchanger at a desired rate. During the combustion, a structurally intact solid residual strip is produced, and the residual strip is removed from the combustion oven.

The apparatus of the present invention for extracting heat from a combustible material includes a means for forming a solid material strip of the combustible material, a means of forming purality of spaced holes of predetermined diameters and spacings required for different rates of combustion in the material strip. The material strip combusting means includes a combustion oven.

The apparatus of the present invention may include an extrusion screw for extruding a solid material strip of the combustible material and a conveyor extending away from the extrusion screw for carrying the material strip. Located along the conveyor are a hole punch for punching a plurality of spaced holes into the material strip, and a preheating oven for preheating the material strip. A combustion oven located along the conveyor for combusting the material strip includes an inlet for the material strip, an outlet for a structurally intact solid residual strip, means for intaking air into the combustion oven, and means for removing a hot flue gas from the combustion oven.

BRIEF DESCRIPTION OF THE DPhWINGS The preferred embodiment of the invention is described in the detailed description which should be considered in connection with the figures in the accompanying drawings, in which: FIG.1 is a schematic view of the apparatus of the present invention.

FIS.2 is a cut away view of the apparatus of the present invention.

FIG.: is å sectional view through one of the plurality of holes in the material strip as the material strip undergoes combustion at a low combustion rate.

FIG.4 is a sectional view through one of the plurality of holes in the material strip as the material strip undergoes combustion at a high rate of combustion.

DETAILED DESCRIPTION OF THE INVENTIOIN With reference to FIG.1. the heat extracting apparatus of the present invention includes a hopper 10. and extrusion screw 14. located at the lower end of the hopper 10. and a conveyor 18. extending away from the extrusion screw 14. which runs the length of the apparatus. Located along the length of the conveyor are a hole punch 20., a preheating oven 24., a combustion oven 2b.! and an air-intake preheating chamber 27. The combustion oven Zb. is connected to the preheating chamber 24. by a first conduit 2. and includes a material strip inlet 28., a material strip outlet 30., an inlet 32. for air/oxygen and a heat exchanging fluid/fluegas and first and second outlets 34.,35. for a fluegas produced in the combustion oven ob. fi heat exchanger 36, is also located within the combustion oven 26.

Piping 38. connects the first fluegas outlet 34. with a first blower 40.

which in turn is connected by piping 42. to the air and heat exchanging fluid inlet 32. Piping 44. connects the second fluegas outlet 3. with the preheating oven 24.

The preheating chamber 27. is -conncected to the combustion oven 26 by conduit 46. and includes a preheating chamber residual strip inlet 48., a preheating chamber residual strip outlet 50. and air/oxygen inlet 52. and an air outlet 54. Rir outlet 54. is connected to a second blower 56. by piping 58. and the second blower 56. is in turn connected to the air and heat exchanging fluid inlet 32. by piping 60. Both of the first and second blowers 40., 56. are controlled by a computer 62. as is indicated generlly at 64. and 66.

In operation, basic data input to the computer 62. are the system heat output 9. and the heat exchanger 36. and the combustion temperature 92.

of the gas at A. The computer 62. compares these parameters with the system heat output rate command 91. and combustion temperature 93. to operate the respective blowers 40. and 56. for an automatic regulation of the combustion system.

in operaton, the hopper 10. holds a supply of a combustible material 12.

generally in a granular or pulverized form. In the preferred embodiment, the combustible material 12. includes a mouldable mixture of coal, clay, water and lime. Rs the extrusion screw 14. is turned, it extrudes a solid material strip 16. of the combustible material 12. and the preferred form of the material strip 16. is that of a continuous strip of uniform recctangular cross section. However, a material strip which is of a short finite length, or which has a non-rectangular cross section, may be used to achieve at least some of the advantages of the present invention.

The extrusion screw 14. extrudes the material strip 16. onto the conveyor 18. and the conveyor carries the material strip lb. along the length of the apparatus. 5 the material strip 16. is carried along the conveyor 18. the hole punch 20. operates to punch a plurality of spaced holes 22. into the material strip 16. As will be described below, the diameter a nd spacing of the holes 22. is critical for achieving the desired combustion of the material strip 16. the feed rate for the material strip 16. will be controlled by the computer 62. depending on the power demand 91 and other secondary factors relating to the composition of the fuels.

The material strip 16 then enters the preheating chamber 24. wherein hot flue gas supplied by the piping 44. from the combustion oven 26 is circulated over the material strip 16. The preheating of the material strip 16. occurs in the absence of ambient air'oxygens and so the temperature of the material strip 16 is raised to about 800 oC in the preheating chamber 24. without combustion of the material strip 16. tThile the material strip 16. is being preheated, the volatile components of the coal in the material strip 16. are extracted with the circulating flue gas and are removed from the preheating chamber 24. by piping 6B. so that the volatile components may be processed to obtain side products.

The preheated material strip 16. is next carried into the combustion oven 26. by the conveyor 18 through the first conduit 25. and the material strip inlet 28. Ns will be discussed in more detail below with reference to Fl(3.2 air supplied by the air and heat exchanging fluid inlet 32. below the conveyor 18, flows into the spaced holes 22. in the material strip 16.

and the material strip 16. begins to combust. The rate at which air is supplied from the preheating chamber 27. to the air and heat exchanging fluid inlet 32. by the second blower 56. is controlled by the computer 62.

such that the combustion of the material strip 16. occurs at a desired temperature. In the preferred embodiment, this desired temperture is sub- stantially between 800 oC and 1,200 oC.

rks the material strip 16. is carried through the combustion oven 26. it continues to combust at the desired temperature until the material strip 16. has completely combusted. tRt this point the material strip 16. is in the form of a structurally intact, solid residual strip 70. which, in the preferred embodiment, is composed substantially only the dehydrated clay and calcium sulfate.During combustion, a flue gas is produced as shown by arrows A and, due to factors which are discussed herein below, the flue gas is substandtially free of carbon monoxide, nitrogen oxides, sulfur oxides, and micro fly-ash. The flue gas circulates over the heat exchanger 36. and is then removed from the combustion oven 26. by the first and second flue gas outlets 34., 35. Flue gas which is removed from the combustion oven 26 through the first flue gas outlet 34. is recirculated into the combustion oven 2t. by the first blower 4cox. through piping 38., piping 4. and the air and heat exchanging fluid inlet 32.The first blower 4f9. ! is controlled by the computer 62. such that the rate of the heat exchange between the fluegas produces a desired rate of heat exchange between the flue gas and the heat exchanger 36.

The residual strip 70. which remains on the conveyor 18. after complete combustion of the material strip 16. is then carried out of the combustion oven 26. through the residual strip outlet 0. The residual strip 70. is carried through the second conduit 46. and into the preheating chamber 97.

through the preheating chamber residual strip inlet 48. While the residual strip 70. is carried through the preheating chamber 27. air is cycled over and through the residual strip 70. from the air inlet 52. to the air outlet 54. thereby preheating the air before it is supplied to the combustion oven 26. The residual strip 70. is then carried out of the preheating chamber 27. through the preheating chamber residual strip outlet after which it can be removed for disposal or for further processing.

In particular, the residual strip 7, is especially suitable to be reclaimed for the manufacture of cement, building block, and road pavement, since as discussed below it is substantially free of residual carbon.

FIG. 2. shows a cutaway view of the material strip 16. as it is combusted in the combustion oven 26. 5 seen from FIG.2 the combustion oven 26.

includes inside walls 72. made of, for example, heat insulating brick. The material strip 16. is carried on the conveyor 18. (not shown) through combustion oven 26. such that the walls 72. of the combustion oven 26.

closely contact the edges of the material strip 16. This contact prevents air from passing between the walls 72. and the material strip 16. 5 a result, as air and recirculated flue gas is supplied to the combustion oven 26. as shown by arrows B. The air and flue gas mixture is thereby forced into and through the combusting holes 22.

As mentioned above, the flow of air is regulatedby the computer 62.

through the second blower 56. such that the combustion temperature of the material strip 16. is between 800 oC and 1,200 oC In the preferred embodiment, the combustion temperature is maintained substanstially at 1,000 oC which is measured by the radiating colour of the material strip 16 This combuston temperature is in contrast to atomized fuel combustion process, which often have a combustion temperatures of over 2,0 oC Such a high combustion temperature results in the production of large amounts of carbon monoxide, which requires a very large combuston chamber to allow the carbon monoxide to undergo a secondary combustion to carbon dioxide.Due to the combustion process of the present invention occuring at a much lower temperature and at a much slower and steady rate, carbon monoxide produced during the combustion undergoes a secondary combustion to carbon dioxide substantially only within the spaced holes 22. and immediately there-above, thus resulting in blue tongues of flame 80. extending from each of the holes 2 Rlso, a minimum stoichiometry of t is all that is needed to maintain ideal combustion in the present invention, as opposed to a minimum stofchiamtry of 1. to 1.5 required for conventional processes. As a result, flue heat loss is greatly reduced.

Because the rate of combustion of the material strip 16 is controlled such that combustion occurs more slowly and steady than atomized combustoin processes, the combustion process of the present invention is much more complete and produces far less toxic by-products than the atomized combustion processes. Substantially, all of the carbon in the material trip 16. is combusted in the combustion oven 26. Rlsos substantially all of the sulfur present in the coal of the preferred embodiment is captured in the material strip 16. because of the intimate physical contact between the lime and the coal particles in the material strip 16. and substantially none of the nitrogen present in the air forms nitrogen oxides during the combustion process.As a result the products of the combustion include a flue gas which is substantially free of carbon monoxide, nitrogen oxides, sulfur oxides, and micro fly-ash, and a structurally intact solid residual strip composed substantially only of dehydrated clay and calcium sulfate.

Because the residual strip 70. is substantially free of residual carbon, it is particularly suitable for reclamation for the manufacture of cement, building block, and road pavement. On the other hand, the ash from present power plants is not suitable for these applications due to its high carbon content.

FIGS.3. & 4. give sectional views of one of the plurality of spaced holes 22. as the material strip 16. undergoes combustion at a low rate, and at a high rate respectively. The combustion process of the present invention is necessarily ablative in nature, and so as the combusition progresses the effective reactive surface available within the hole 22.

will increase. This tendency for the combustion rate to increase is counter-balanced however, because the diffusion path which oxygen from the air entering the hole must travel to leach the reacting surface and which carbon monoxide from the reacting surface must travel to reach the hole 22.

increases as the combustion progresses. It provides a stable combustion environment for further regulation of its combustion rate and temperature by a computer.

Rs will be seen from FIGs.3 & 4, the diameter to length ratio of the hole 2. is critical to the achievement of proper combustion of the material strip 16. With reference to FIG. 3 when the material strip 16 undergoes combustion at a low combustion rate the gas flow speed in the hole 22. is very slow. Consequently the CO gas formed in the wall of the hole 22. will have sufficient time to diffuse across the centre line D of the hole 22. to react with the oxygen gas in the hole 22. The 02 gas is therefore nearly exhausted near the exhaust end 74. of the hole 22.Thus, to achieve proper combustion at a low combustion rate, the diameter to length ratio of the hole 22. should be sufficiently large to prevent the complete depletion of oxygen in the hole 22. 50 that at least some oxygen will be present at the exhaust end 74. of the hole 22.

With reference to FIG.4 under a high combustion rate, there will be a much faster gas flow speed in the hole 22. and the boundary layers of the oxygen and the CO will be very thin. Consequently, there will be a high concentraton of oxygen at the exhaust end 74. of the hole 22. and most of the secondary combustion of CO and C02 will take place immediately above the hole 22, which gives rise to a long blue tongue of flame SCa. Therefore for a high combustion rate, more holes with a small diameter to length ratio must be punched in the material strip 16 to ensure complete secondary combustion of the CO to C09.

While this invention has been illustrated and described in connection with the preferred embodiments, it is recognized that variations and changes may be made and equivalents may be employed herein without departing from the scope of the invention as set forth in the claims. For example,instead of carrying away the volatile components of the coal in the material strip 16. for procesing after they have been extracted in the preheating chamber 24. the volatile components may be separately carried to the combustion oven 26.to be combusted. Also the heat exchanger 36.need not be immediately within the combustion oven 26. as the flue gas may be carried from the flue gas outlet 34. to a separate heat exchanger before the flue gas is recirculated into the combustiron oven 26.

Additionally! the flue gas from the combustion oven 26. need not necessarily be used as the heat exchanging fluid which is supplied to the air and heat exchanging fluid inlet 32. as one skilled in the art would realize that other fluids may be substituted for the flue gas while retaining at least some of the advantages of the present invention. Also, in the preheating chamber 27. the air need not be directly physically passed over and through the residual strip 70. to preheat the air, though it is an efficient method

Claims

WHAT IS CLAIMED IS:

1. n METHOD OF EXTRACTING HEAT FROM A COMBUSTIBLE MATERIAL, COMPRISING FORMING A SOLID t1ATERIAL STRIP OF 5ID COMBUSTIBLE MATERIAL;; FORMING A PLURALITY OF SPACED HOLES IN SAID MATERIAL STRIP; COMBUSTING SAID klATERIhl STRIP IN 1k COMBUSTION OVEN hND REMOVING A STRUCTURALLY INTACT RESIDUAL STRIP FROM SAID COMBUSTION OVER RFTER SAID MATERIAL STRIP HAS BEEN COMPLETELY COMBUSTED.

2. n method of extracting heat from a combustible material as claimed in claim 1. further comprising circulating a fluid at a controlled rate to carry heat from the combustion of said material strip to a heat exchanger at a desired rate.

3. A method of extracting heat from a combustible material as claimed in claim 2. wherein said circulating step includes circulating a fluid which includes hot flue gas recycled from said combustion oven.

4. A method of extracting heat from a combustible material as claimed in claim 2. further comprising of supplying air to said combustion oven at a controlled rate to maintain the temperature of combustion of said material strip at a predetermined temperature.

5. A method of extracting heat from a combustible material as claimed in claim 4. wherein said air supplying step includes maintaining said desired temperature substantially in the range of 800 to 1,200 oC.

6. n method of extracting heat from a combustible material as claimed in claim 4, further comprising the removal of structurally intact solid residual strip from said combustion oven after said material strip has been completely combusted; and preheating said air by circulating said air past said residual strip prior to supplying said air to said combustion oven.

7. A method of extracting heat from a combustible material as claimed in claim 4. wherein said air supplying step includes supplying said air tn said spaced holes in said material strip during combustion so that a secondary combustion of carbon mopnoxide to carbon dioxide occurs substan- tially only within said holes and immediatley there-above.

8. A method of extracting heat from a combustible material as claimed in claim 7. wherein said combustion step includes producing a structurally intact solid residual strip, and a flue gas substantially free of carbon monoxide, nitrogen oxides, sulfur oxides, micro fly-ash, and molten iron sulfide.

9. n method of extracting heat from a combustible material as claimed in claim 1. wherein said material strip forming step includes forming said material strip from a mixture including coal, clay, lime and water.

10. A method of extracting heat from a combustible material as claimed in claim 9. wherein said combustion step includes the production of a structurally intact solid residual strip composed substantially nf dehydrated clay and calcium sulfide, and a flue gas substantially free of carbon monoxide, nitrogen oxides, sulfur oxides, and micro fly-ash.

11. n method of extracting heat from a combustible material as claimed in claim 1. further comprising of preheating said material strip before combustion, said preheating including circulating a hot flue gas from said combustion oven past said material strip in the absence of ambient air.

12. n method of extracting heat from a combustible material as claimed in claim 1. wherein said material strip forming step includes extruding said material strip.

13. 1k method of extracting heat from a combustible material as claimed in claim 1. wherein said hole forming step includes punching said spaced holes into said material strip.

14. A method of extracting heat from a combustible material as claimed in claim 1. further comprising : wherein said combusting step is performed under independent control of heat production rate and temperature.

15. A method of extracting heat from a combustible material comprising forming a solid material strip of said combustible material; punching a plurality of spaced holes into said combustible material strip; preheating said material strip in the absence of ambient air; combusting said material strip in a combustion oven at a desired temperature by supplying air to said plurality of spaced holes at a controlled rate, said combustion step including producing a structurally intact solid residual strip; circulating a fluid at a controlled rate to carry heat from the combustion of said material strip to a heat exchanger at a desired rate; and removing said solid residual strip from said combustion oven.

1 A method of extracting heat from a combustible material as claimed in claim 15. wherein said material strip forming step includes extruding said material strip.

17. R method of extracting heat from a combustible material as claimed in claim 15. wherein said material strip forming step includes forming said material strip from a mixture of coal, clay, and lime and said combustion step includes producing a residual strip composed substantially of a mixture of dehydrated clay and calcium sulfide, and a flue gas substantially free of carbon monoxide, nitrogen oxides, and sulfer oxides.

18. n method of extracting heat from a combustible material as claimed in claim 15. wherein said preheating includes circulating a hot flue gas from said combustion oven over a portion of said material strip.

19. A method of extracting heat from a combustible material as claimed ion claim 15. wherein said fluid circulating step includes recirculating hot flue gas from said combustion oven back into said combustion oven.

20. n method of extracting heat from a combustible material as claimed in claim 15. wherein said combustion step includes a secondary combustion of carbon monoxide which occurs substantially only within said spaced holes and immediately there-above.

21. A method of extracting heat from a combustible material as claimed in claim 15. further comprising: preheating said air by circulating said air past said residual strip prior to supplying said air to said plurality of spaced holes;

22. A method of extracting heat from a combustible material as claimed in claim 15. further comprising: means of forming a solid material strip of said combustible material; means of forming a plurality of spaced holes of variable size and shape in said material strip; means of combusting said material strip, said means for combusting including a combustion oven; and means for removing a structurally intact solid residual strip from said combustion oven

23. n apparatus for extracting heat from a combustible material as claimed in claim 22. further comprising: a heat exchanger; and means for circulating a fluid at a controlled rate to carry heat from the combustion of said material strip to said heat exchanger at a desired rate

24. Nn apparatus for extracting heat from a combustible material as claimed in claim 23. wherein said fluid circulating means includes means for recirculating a hot flue gas from said combustion oven back into said combustion oven.

25 Nn apparatus for extracting heat from a combustible material as claimed in claim 23. further comprising: means for supplying air to said combustion oven at a controlled rate to maintain the temperature of combustion of said material strip at a desired temperature.

26. An apparatus for extracting heat from a combustible material as claimed in claim 25. wherein said air supply means maintains said material strip combustion temperature between 800 and us200 Q

27. hn apparatus for extracting heat from a combustible material as claimed in claim 25. further comprising: means for removing a structurally intact, soild residual strip from said combustion oven; and means for preheating said air, said preheating means including means for circulating said air past said residual strip prior to supplying said air to said combustion oven.

28. Nn apparatus for extracting heat from a combustible material as claimed in claim 25. wherein said air supply means includes means for supplying said air to said spaced holes in said material strip during combustion such that a secondary combustion of carbon monoxide to carbon dioxide occurs substantially only within said spaced holes and immediately there-above.

29. An apparatus for extracting heat from a combustible material as claimed in claim 22. further comprising: means for preheating said material strip before combustion thereof, said preheating means including means for circulating a hot flue gas from said combustion oven past said material strip without ambient air.

30. An apparatus for extracting heat from a combustible material as cl;aimed in claim 22. wherein said material strip forming means includes means for extruding said material strip.

31. An apparatus for extracting heat from a combustible material as claimed in claim 22. wherein said holes forming means includes means for punching said holes into said material strip.

32. An apparatus for extracting heat from a combustible material comprising: an extrusion screw for extruding a solid material strip of said combustible material; a hole punch located along said conveyor for punching a plurality of spaced holes into said material strip; a preheating oven located along said conveyor for preheating said material str-ip; and a combustion oven located along said conveyor for combusting said material strip, said combustion oven including a material strip inlet for inletting said material strip, a residual strip outlet for outletting a structurally intact, solid residual strip, means for intaking air into said combustion oven, and means for removing a hot flue gas from said combustion oven.

An An apparatus for extracting heat from a combustible material as claimed in claim 32. further comprising: means for recycling said hot flue gas back into said combustion oven; and wherein said flue gas recycling means includes means for controlling the circulation rate of said flue gas based on a desired rate of heat exchange between said recycled flue gas and said heat exchanger.

34. fln apparatus for extracting heat from a combustible material as claimed in claim 32. further comprising: means for controlling the rate of flow of said air into said combustion oven based on a desired temperature of combustion of said material strip

35. An apparatus for extracting heat from a combustible material as claimed in claim 32, further comprising: a preheating chamber for preheating said air before said air enters said combustion oven, said preheating chamber including a preheating chamber residual strip inlet for inletting said residual strip, a preheating chamber residual strip outlet for outletting said residual strip, and means for circulating said air past said residual strip.

s An apparatus for extracting heat from a combustible material as claimed in claim 32. wherein the volatile gas released from the material strip in the preheating chamber is collected at the top and directed to join recycling hot flue gas to be burned in the combustion oven.

37. n method of extracting heat from a combustible material as claimed in claim 1. wherein the size and numbwer of holes and the spacing between holes are intimately related to the burning rate of the material strip.

Increasing number of holes per unit area and reducing the spacing between holes will reduce the diffusion path-length in the porous material for supply of air and heat to effect combustion; this will lead to increasing burning rate. The porosity of the material will also affect the diffusion of air and heat that determine the burning rate.