WO1990000238A1

WO1990000238A1 - Furnace

Info

Publication number: WO1990000238A1
Application number: PCT/AU1989/000278
Authority: WO
Inventors: John K. Cannell; Ivan P. Chugg; Anthony G. Leitch
Original assignee: AIRWALL PTY Ltd
Current assignee: AIRWALL PTY Ltd
Priority date: 1988-06-29
Filing date: 1989-06-28
Publication date: 1990-01-11
Anticipated expiration: 1990-12-29

Abstract

This invention relates to a furnace and more particularly to a furnace adapted to heat a gaseous fluid passed through the furnace for use elsewhere. The furnace includes a combustion chamber (4) formed at least in part by side walls (6). The combustion chamber is adapted to have a solid fuel combusted therein and that fuel is supported on a grate (5) which forms the floor of the combustion chamber. A flow passage (9) for gaseous fluid is in contact with the combustion chamber, that flow passage being formed at least in part from a stainless steel membrane which separates the interior of the flow passage from the combustion chamber. The flow of gas within the flow passage causes heat to be extracted from the combustion chamber and thus the furnace is able to supply clean heated air for use in other processes or applications. The stainless steel material from which the membrane is made is preferably an austenitic steel having silicon as one of its constituents. A preferred material for the membrane is Avesta 253 MA sheeting. The flow passage (9) is preferably formed within one of the side walls of the combustion chamber. That side wall preferably has inner and outer skins which are spaced apart to define the flow passage. The inner skin is formed of the stainless steel material. Optionally, the inner skin may be formed of a corrugated material, the corrugations each defining individual sub-passages which can be individually shut-off to thereby vary the cross-sectional area of the flow passage.

Description

FURNACE This invention relates to a furnace and more particularly although not exclusively to a furnace adapted to heat a gaseous fluid passed through the furnace for use elsewhere. The invention will be described with reference to a furnace suitable for heating air but clearly other gasses may be heated using a furnace of similar design and the invention is thus not to be limited to an air heating furnace.

For a furnace to operate efficiently it is important that a high percentage of the energy released during combustion of fuel within the combustion chamber of the furnace be extracted. It is also important that a high percentage of the combustible material which is introduced into the combustion chamber actually undergoes combustion. It has been found that many prior art furnaces do not efficiently combust fuels having a high moisture content. Inefficiency in these circumstances frequently is as a result of inferior furnace design. Further problems with prior art furnaces arise as a result of an inability to properly control and modulate temperature of gasses being heated in the furnace.

Also, to properly combust solid fuels, particularly those fuels having a high moisture content, it is necessary to maintain a high temperature level within the combustion chamber. Conventional technology utilizes a combustion chamber with either a refractory lined or a steel lined water cooled chamber wall in order to accommodate these high temperatures. Both the refractory lining and water jacket arrangements suffer from disadvantages generally arising as a result of the high capital cost of those arrangements and additional problems arise in extracting energy in usable form from those linings.

A further disadvantage with water jacket type furnaces is that they can be dangerous in that high pressure build-up occurs in the jacket which can lead to escape under pressure of heated water or steam. The jacket and associated feed conduits must thus be of robust construction and this leads to the high cost.

Advantages of using water as the heat transfer medium are that the density and nature of water result in rapid heat absorption by the water. The water also keeps the furnace wall relatively cool (say up to 200°C) which minimises scaling of the metal from which the furnace wall is formed, and the low temperature means there is a high temperature differential between the water and the furnace which means that heat is rapidly extracted from the furnace. However, the problems associated with the high pressure at which the system must operate negatives to a significant extent those advantages and has induced the applicant to seek alternative solutions. It is an object of this invention to provide an efficient and effective gas heating furnace. A further object of the invention is to provide a furnace which can be easily modulated and automatically operated. A yet further object of the invention is to provide a furnace which can supply, simultaneously from different positions on the furnace, gaseous fluid heated to different temperatures. The furnace may be used for timber curing.

A furnace according to the present invention comprises a combustion chamber formed at least in part by side walls, a combustion air inlet into said chamber, and an exhaust outlet from said chamber, a fuel inlet into said chamber, and a combustion grate on which solid fuel admitted to the chamber is supported during combustion, characterised in that a flow passage for gaseous fluid is in contact with said chamber, said flow passage being formed at least in part from a stainless steel membrane which separates the interior of the flow passage from the combustion chamber, means being provided for causing gas to flow in said flow passage to thereby extract heat from said combustion chamber in use. Preferably at least one of the side walls is formed having inner and outer skins and said flow passage is formed between said skins. Said stainless steel membrane is preferably an austenitic steel stabilized by the addition of nitrogen thereto and is preferably a stainless steel such as "Avesta 253 MA" (trade mark) manufactured by Axel Johnson Corporation AB of Sweden. The furnace may be used for curing timber.

An embodiment of the invention will now be described by way of example, reference being made to the accompanying drawings in which: Figure 1 shows a cross-sectional side view through a furnace according to the invention, taken along line II-II of Figure 2,

Figure 2 shows a perspective view of a furnace and accessory apparatus according to the invention, Figure 3 shows a cross-sectional view through a portion of the side wall of the furnace along line III-III of Figure 1, and

Figure 4 shows an enlarged sectional view of portion of the grate for the furnace of Figure 1 as indicated by circle IV in Figure 1. Figure 5 shows diagramatically a furnace coupled to a timber curing chamber.

In a preferred embodiment of the invention as shown in the drawings the furnace 1 is comprised of two sections, namely a radiant energy section 2 and a convective heat transfer section 3. The radiant energy section 2 will comprise that part of the furnace 1 where combustion actually takes place and includes a combustion chamber 4^" having a furnace floor 5 and side walls 6. The walls 6 are preferably formed having an inne skin 7 and an outer skin 8. The two skins 7 and 8 are sμaced apart to form a flow passage 9 therebetweenT The inner skin 7 is preferably formed from a corrugated sheet 10 as depicted in Figure 3, with the axes of the corrugations 12 being aligned substantially vertically and the corrugated sheet 10 defining the inner face of the furnace wall 6. The inner skin 7 will be made from a material suitable for withstanding the high temperatures generated in the furnace and will also have a high coefficient of thermal conductivity. The crests of the corrugated sheet 10 may be bonded or joined to the outer skin 8 at the points of contact between the crests and the outer skin so that the flow passage 9 is divided into a number of sub-passages 15. Each sub-passage 15 is formed by a single corrugation. Preferably those sub-passages can be individually shut off by means of a valve closure arrangement indicated diagrammatically at numeral 16. The valve closure arrangement may include a valve closure gate 17 for each sub-passage 15 and each of those gates 17 may be independently controllable so they can be selectively closed off as required. The valve closure arrangement will thus allow the total cross-sectional area of the flow passage 9 to be varied in accordance with combustion rates, heated air demand rates, temperature requirements and like factor.

The upper cover or roof 18 of the combustion chamber 4 is also of hollow wall constructions, and the central passage 19 in the roof 18 forms a continuation of the passage 9 in the walls 6. Thus, air which is moving in the roof passage 19 will also be heated by the high temperatures in the combustion chamber in use. The radiant energy section 2 of the furnace described is adapted to heat air which is forced through the flow passage 9 and is conveyed to apparatus (not shown) wherein heated air is required. That apparatus may, for example, be a wood drying chamber which requires a source of heated air for effective and controlled drying, and wherein the temperature of the heated air can be carefully controlled. The heated air form the furnace passes out through ducts depicted at numeral 14 in Figure 2. The air to be heated is channelled into the flow passage 9 through a header 20 which is located below the furnace walls 6 and feeds the air directly into the passage 9 between the inner and outer skins. The air which passes through the furnace in this manner serves to carry heat from the radiant energy section 2 for use elsewhere as well as serving to cool and thereby protect the furnace walls 6.

The floor 5 of combustion chamber 4 can take any convenient form but in a preferred form of the invention the floor is substantially horizontal grate 21 and is comprised of a plurality corrugated and perforated sheets 22. The corrugations 23 are preferably of triangular shape in cross-section with the apexes 24 of the corrugations 23 uppermost, and the perforations 25 may be formed in the sloping sides of those corrugations 23. Fuel to be combusted may be fed into the combustion chamber 4 by any suitable means. In one form of the invention the fuel is fed by means of a rotatable screw feed 26 through an elevated opening 27 in the floor 5. This screw type of feed 26 is particularly suitable where the fuel to be used in combustion is of particulate solid form such as wood pulp or chip or coal pieces.

Combustion air is fed into the interior of the furnace through pipes 30 located below the floor 5, and from the pipes

30 into a manifold 31 and then into the perforations 25 in the grate 21. The perforations 25 are angled so as to direct incoming combustion air in different directions defined by the axis of the individual perforations 25. The incoming combustion air serves to at least partially fluidize the fuel within the combustion chamber 4. However, the form and position of those perforations 25 on the sloping sides of the corrugations 23 serves to ensure that when combustion air is switched off, the fuel does not fall through the grate of the furnace. In this condition the fuel will slump down onto the grate 21 but will not pass through the perforations 25 into the area below the grate 21. The elevated opening 27 may be located in the centre of the floor 5 and the elevated nature of the opening serves to more evenly distribute the fuel over the floor 5. Also, fuel being fed through the cylinder 28 which forms the opening 27 will have a chance to heat up, and if necessary dry out, prior to actually being deposited on the floor 5. Combustion air can be provided either by a controlled induced draft fan downstream of the combustion chamber 4 which will provide for the delivery and flow of most or all of the combustion air, and for a slightly below atmospheric combustion chamber 4. Such a fan will thus be located an exhaust flue 50 intermediate the combustion chamber 4 and an outlet stack 51. Alternatively, a forced draft fan can be located upstream of the combustion chamber 4, slightly pressurising the chamber 4. Both such fans may be used in appropriate circumstances.

It is preferred that a controlled induced draft fan is used since this will mean, as mentioned above, that the combustion chamber 4 operates at a pressure slightly below atmospheric pressure. This reduced pressure chamber will mean that, should any leaks occur in the wall of the chamber, rather than gas from the combustion chamber being forced out of the combustion chamber, ambient air will be sucked into the combustion chamber. This is particularly important where a leak may occur between the flow passage 9 and the combustion chamber 4. A reduced pressure combustion chamber will ensure that combustion gasses are not inadvertently forced into the flow passage 9 which would thereby contaminate the air or other gas in that flow passage 9.

Optionally secondary combustion air may be directed into the combustion chamber at a position above the fluidized bed to thereby enhance combustion within the combustion chamber. Pipes 52 running along the side walls 6 of the radiant energy section 2 through a plurality of nozzles 53 placed at intervals along the length of the pipes 52. The secondary combustion air will serve to provide additional combustion oxygen for any combustible products which remain unburnt after combustion has taken place in the fluidized bed.

The grate 21 is preferably supported on a plurality of spaced joists 32 which are situated at an elevated position to define a gap or gaps 33 below the grate 21. This gap 33 can provide a space in which a fuel feed mechanism is located as well as providing an area through which combustion air can be supplied.

Air heated in the radiant energy section 2 of the furnace 1 may be directed towards and collect in a plenum 35 mounted above the radiant energy section 2. The plenum 35 may then carry the heated air towards the outlet ducts 14 of the furnace 1. The plenum 35 is basically a large manifold mounted on top of the combustion chamber 4 and the passage 9 in the walls 6 and the passage 19 in the roof 18 all lead into this plenum which in turn leads into a duct 36 which connects the passages 9 and 19 with the outlet duct 14. This duct 36 and the outer skin 8 of the combustion chamber are both preferably formed of or covered by a heat insulating material so that heated air retains its heat on its passage to the outlet ducts 14. It is envisaged that air will be heated by the chamber to between 250°C and 400°C, but it may be significantly higher than this, up to approximately 1000°C.

The inner skin 7 is preferably made from a stainless steel material which is a fully austenitic steel. The austenite is stabilized by the addition of nitrogen. This type of material is known and is manufactured by Axel Johnson Corporation AB of Sweden under the trade mark "Avesta 253 MA" . Such a material has the following chemical composition: Chemical Composition %

Cmax Si Cr Ni N Other metals

0.10 1.7 21 11 0.17 Ce and other rare earth metals

The relatively high level of chromium and silicon in this material would normally give rise to a two phase microstructure - an austenitic-ferritic steel. However, due to the high carbon content for an austenitic steel, the microstructure contains carbides. The material thus possess a very high stability against sigma phase precipitation. It therefore does not become embrittled, even after a long period of high temperature operation.

The material contains rare earth metals added in very small but carefully controlled amounts. They provide resistance to oxidation and good creep strength. The rare earth metals consist primarily of lanthanides, i.e., cerium, lanthanum, neodymium, praseodymium, etc.

It is * found that the material has a very high scaling temperature in air, and scaling does not occur to any significant extent below approximately 1150°C. The oxide which forms in the material has a good adhesive power which is responsible for the resistence to scaling in the face of large and rapid temperature fluctuations. Growth of the oxide layer is inhibited by the high silicon content and the addition of the rare earth metals. As is common with many stainless steel materials 253 MA possesses a coefficient of thermal conduction which is less than that of steel. At 20°C the material will have a thermal conduction of approximately 15 /m C. This is not significantly different from other stainless steel materials. It is envisaged that the inner wall 7 will be made from a relatively thin sheet of material, approximately 1.5 to 1.6 millimetres thick. Heat transfer for a material with a wall thickness as thin as that is not too critical.

One problem with this type of steel is that material has a reasonably high coefficient of thermal expansion and this can cause difficulties particularly in situations where uneven heating occurs within the combustion chamber. Thus, it is preferred that a relatively even combustion takes place over the entire area of the combustion chamber. For reasons of thermal expansion difficulties it may be preferable for the inner skin not to be bonded to the outer skin 8. It is not, for example, critical that the individual sub-passages 15 are absolutely air tight relative to each other and if some leakage between the passages does occur this should not effect the process to any great extent. Thus, it may be preferable if some form of sliding or relatively movable arrangement between the skins is possible in order to accommodate this thermal expansion. The 253 MA material has a thermal expansion of approximately 18 x 10^~ mm/°K. Also for reasons of thermal expansion it may be preferable if the walls 6 of the combustion chamber 4 are not directly connected to the grid 5 in order that the walls can move under conditions of thermal expansion without stresses being set up as a result of connection to the grid 5. It is envisaged that AVESTA 253 material will be particularly suitable for use in a combustion chamber since with its relatively low nickel content, 253 MA possesses a better resistence to sulphur attacks than grades of stainless steel having higher nickel contents. The degree of sulphur penetration increases with a rising nickel content of the material. The properties of the oxide layer also seem to have some influence on the degree of oxidation. A thin, adhesive oxide layer does not exhibit as much tendency to form cracks as a thicker layer which is less elastic. The penetration of sulphur is usually initiated through cracks in the oxide. Once inside the metal, sulphur reacts with nickel, forming nickel sulphide, among other compounds. Nickel sulphide combines with the nickel in the base material to form a low temperature eutectic, which spoils the protective capacity of the outer oxide.

253 MA has physical and mechanical properties which are more than satisfactory for the requirements of a furnace as described above. This is so, particularly since the flow passage 9 is a relatively low pressure passage and the combustion chamber 4 is also a low pressure chamber.

The convective section 3 of the furnace 1 is adapted to transfer heat, carried by exhaust gasses being conveyed from the combustion chamber 4 to a chimney stack 51, to auxiliary air which in turn may be mixed with the heated air from the flow passage 9. The convective section 3 may be divided into a plurality of hermetically sealed zones 37 and those zones 37 may be separate from each other in order that heated air extracted from one of the zones 38 is at a different temperature to the heated air in another zone 39. An auxiliary air blower 40 is used to furnish the auxiliary air, and this blower 40 is operable separately from the blower (not shown) which supplies air through the header 20 into the passage 9.

The conduits 41 carrying air in the convective section 3 of the furnace 1 will preferably be in contact with the flue 50 carrying exhaust gasses from the combustion chamber 4. Transfer of heat from the exhaust gasses to the auxiliary air to be heated will then be effected by conduction taking place through the walls of the conduits 41. The conduits 41 will thus be made of a material which facilitates this transfer of heat and specifically envisaged is a stainless steel or like material.

The exhaust flue 50 will connect into the combustion chamber 4 so that exhaust gasses and combusted material will be carried through the flue 50 to a suitable outlet stack 51. Preferably there will be one or more hoppers (not shown) in the flue 50 in which particle matter carried by the exhaust gasses will be deposited, and the flue 50 will preferably be internally configured so as to induce that deposition. The outlet stack 51 may have an ash and particle separator (not shown) installed therein.

Clearly, it is not essential that the flow passage 9 be formed between two skins which make up the side walls 6 of the combustion chamber 4. Another arrangement will be for the flow passage to be formed by a series of tubes which pass through the combustion chamber. Gas to be heated will then be passed through those tubes. It is envisaged that this tubular arrangement will tend to be expensive and sealing difficulties are envisaged at the points where the tubes pass through the wall of the combustion chamber. The tubes, if used, will preferably be made from a stainless steel material.

The furnace will preferably be controlled by a suitable automatic control facility (not shown) to regulate combustion air flow, fuel consumption and energy extraction in accordance with demand. The automatic control facility may be in the form of a programmable computing device which will be operable to accommodate differing circumstances which arise in individual situations.

It will be appreciated that a furnace as described above is advantageous in that it heats air, or other gas which may be used, directly, that is, combustion within the furnace serves to heat the gas which can then be ,used where necessary. Thus the arrangement of prior art systems in which a transfer medium, such as oil which is heated and then in turn is used through a heat exchanger to heat air, is not employed in this system. The direct heating of air in the furnace allows for a simple and efficient system of heating to be employed. In addition, the hollow wall construction enables lightweight metal components to be used which are able to withstand the high temperatures generated in the combustion chamber. This is possible because the air which is being heated in that hollow wall serves to remove heat continuously from the combustion chamber directly, ensuring that the sheet material from which the hollow walls 6 are made does not reach unacceptably high temperatures.

One particular application which is envisaged for the furnace as described hereinbefore is in the curing of high grade timber. High grade timber -needs to be dried under conditions which ensure that its internal stresses do not exceed as certain limit which, if exceeded, would cause the timber to check, or crack. If timber can be dried at just under this stress level, minimum drying times can be achieved. To achieve this, timber will require various atmospheric conditions within the drying chamber which will continually change during the drying cycle. Thus, it is important to be able to vary the temperature, humidity and air flow within the chamber as required from time to time by the drying timber. The airwall furnace, can be used to achieve all of the above requirements. For example, the temperature

10. of the gas produced by the chamber can be varied as required. Also, it is envisaged that a moisture content variation can be provided for the chamber. It is preferred that the convective section 3 of the furnace be used to heat water to just below boiling temperature. For this purpose, a tank of water (not

15; shown) can be heated in the convective section 3 and the water from the tank will be sprayed into hot air ducts in the timber curing chamber to produce an abundance of non-pressurised steam as required for processing. Thus, the water which is pre-heated will be sprayed into the chamber together with the

20 high temperature air carried by duct 14 thereby providing a steam supply into the chamber which can be varied as required to change the humidity within the chamber.

Such a system will be employed together with an appropriate control system which measures the timber chamber

25 conditions and sensors within the timber itself will also feed information to that control system. A continuous analysis can be conducted to determine what changes are required in the temperature, humidity and air flow supply. The same control system can be used for controlling the furnace so that the

30 required drying conditions are always achieved. The kiln thus forms an integral part of the furnace system for drying timber.

Figure 5 of the drawings illustrates diagrammatically one possible application for the furnace 1. As shown, a combustion chamber 4 is coupled to a timber curing chamber 60. Ducting 14 links the two chambers 4 and 60 and supplies heated air generated in the flow passage 9 of the furnace to the curing chamber 60. A control valve 61 controls the supply of heated air to' the curing chamber 60. Combustion air is supplied to the chamber through pipes 30 and a control facility 62 controls the supply of combustion air. Fuel is fed to the combustion chamber 4 through fuel supply passage 26 and fuel supply is controlled by fuel control means 63. Water is supplied to the curing chamber 60 through spray nozzles 64. Those spray nozzles 64 are fed heated water from a tank 65 which is heated to just below boiling point in the convection section 3 of the furnace 1. A water control facility 66 is provided for controlling the supply of water to the spray nozzles 64. A monitor 67 monitors the internal conditions within timber 68 located in the curing chamber 60. An atmospheric monitor 69 monitors conditions of temperature and humidity within the chamber 60. A central control means 70 controls the supply of fuel, combustion air, water and heated air in accordance with information obtained from monitors 67 and 69. The central control means thus adjusts the controls 61, 62, 63, 65 and 66 in accordance with conditions as determined by monitors 67 and 69 so that no unacceptably great stresses will occur within the timber 68 which could lead to cracking of the timber 68.

The whole system operates at substantially atmospheric temperature ensuring the furnace 1 is safe during operation. Also, the simple manner by which the combustion chamber to air heat exchange takes place within the hollow furnace wall 6 enables the furnace to be of compact construction.

It is to be understood that various alterations, modifications and/or additions may be introduced into the constructions and arrangements of parts previously described without departing from the spirit or ambit of the invention as defined by the appended claims.

Claims

CLAIMS :

1. A furnace for heating a gaseous fluid comprising, a combustion chamber formed at least in part by side walls, a combustion air inlet into said chamber, and an exhaust outlet from said chamber, a fuel inlet into said chamber, and a combustion grate on which solid fuel admitted to the chamber is supported during combustion, characterised in that a flow passage for gaseous fluid is in contact with said chamber, said flow passage being formed at least in part from a stainless steel membrane which separates the interior of the flow passage from the combustion chamber, means being provided for causing gas to flow in said flow passage to thereby extract heat from said combustion chamber in use.

2. A furnace according to claim 1 wherein said stainless steel membrane is formed from an austenitic steel having silicon as one of its constituents.

3. A furnace according to claim 2 wherein the silicon content is from 1.5% to 2.0% of the chemical composition of the stainless steel.

4. A furnace according to any preceding claim wherein the stainless steel has a chemical composition which includes a small percentage of rare earth metals.

5. A furnace according to any preceding claim wherein the stainless steel membrane comprises Avesta 253 MA sheeting with a thickness of 1.0 to 2.0 mm.

6. A furnace according to any preceding claim wherein at least one of said sidewalls is formed having inner and outer skins spaced apart and said inner skin comprises said stainless steel membrane, the flow passage being formed between said inner and outer skins.

7. A furnace according to claim 1 wherein one of said skins is formed of a corrugated material having a pluality of vertically aligned corrugations, the crests of the corrugations being bonded to the other skin to divide said flow passage into a plurality of substantially discrete sub-passages.

8. A furnace according to claim 7 wherein the inner skin is corrugated.

9. A furnace according to claim 7 or 8 wherein one or more of said sub-passages is adapted to be selectively shut off so that the overall cross-sectional area of the flow passage can be varied as required.

10. A furnace according to any preceding claim wherein the grate is horizontal and is of a corrugated configuration wherein the corrugations are substantially triangular in cross-section, said corrugations having sloping sides which meet at an apex which points in an upward direction, perforations being formed in said sloping sides through which combustion air is introduced into the combustion chamber.

11. A furnace according to any preceding claim wherein a rotatable screw fuel feed device is arranged to feed solid fuel into said combustion chamber.

12. A furnace according to any preceding claim wherein a plenum is mounted to an outlet from said flow passage and fluid heated in use in said flow passage collects in said plenum from where it is conveyed for use elsewhere.

13. A furance according to any preceding claim wherein said combustion chamber is at least substantially surrounded by side walls having inner and outer skins, and an upper cover extends across the top of said combustion chamber, said upper cover being formed of inner and outer skins which form a continuation of the inner and outer skins of said side walls, and an outlet from said flow passage is provided on said upper cover through which heated fluid in said flow passage exhausts.

14. A furnace according to any preceding claim wherein an auxiliary gaseous fluid supply is provided for mixing with the gaseous fluid heated in said flow passage, said auxiliary supply being operable to mix heated gas with auxiliary gas to thereby provide mixed gas at a selected temperature.

15. A furnace according to any preceding claim wherein heated air from said furnace is adapted to be used for curing timber, said furnace being coupled to a timber curing chamber, ducting linking said flow passage with said timber curing chamber, water sprays arranged to spray finely divided water into said timber curing chamber, monitoring means for monitoring the condition of timber in said curing chamber and the temperature and humidity within said curing chamber, and control means for controlling the temperature and humidity in said chamber in accordance with the changing condition of thetimber being monitored by the monitoring means.

16. A furnace substantially as hereinbefore described with reference to the accompanying drawings.