COMBUSTION APPARATUS

Abstract: A kiln for calcining minerals and/or incinerating waste preferably using a solid fuel, including a vertical shaft 32, an inlet 2 at the upper part of the shaft for a solid charge 3 to be calcined and an outlet 28 at the bottom of the shaft for calcined material, the kiln including a fan 17 for drawing combustion air downwardly through the charge, a fan 20 for drawing cooling air through the calcined material in the shaft to cool the calcined material and thereby heat the cooling air, and inlets 30 for introducing the heated cooling air free from gaseous combustion products into the upper part of the shaft 32 to serve as combustion air for the calcination process. An outlet 15 for gaseous products of the combustion/­ calcination reaction, is positioned above the cooling air outlet, the fan 17 draws gaseous products of the combustion/­ calcination reaction through outlet 15 and to a heat exchan­ ger 21 where it further heats air supplied as combustion air through conduit 22. A heater 11 is used on start-up, and for supplemental heating. In place of the solid fuel mixed with charge 3, a gaseous or liquid fuel can be used by incorporating suitable burners 39 immediately below combustion air inlets 30.

Description: COMBUSTION APPARATUS This invention relates to combustion apparatus and in particular to kilns for calcining materials, particularly minerals. The invention also relates to processes of calcination using such kilns. The process may be used for the whole range of calcinable materials, such as limestone, dolomite, magnesite, or gypsum, and such materials are referred to generically herein as "stone". The kiln may also be used for the destruction of waste materials, particularly noxious industrial waste, either simultaneously with stone calcination, in which case the waste may serve in whole or part as fuel for the calcination process, or without simultaneous stone calcination, in which case the kiln functions in effect as an incinerator for the waste, either with or without the use of additional fuels. A very large number of types of kiln are in use throughout the world; most being designed to use natural gas or gasified liquid hydrocarbons as fuel. As hydrocarbon fuels are becoming increasingly difficult to obtain with continuity of supply, there is a growing need for multi-purpose kilns capable of burning solid fuels such as coke, and particularly coal, in an efficient manner to yield a good quality product, and which give rise to the minimum production of smoke. A further problem arises in the calcination of minerals in that the calcined mineral product is very sensitive to the presence of traces of pollutants such as sulphur dioxide, particularly from solid fuels e.g. coal, in the process gases into which they come into contact. In British Patent Specification No 1301502, a shaft furnace is disclosed which is designed for the integrated calcining and sintering of a mineral. Here a special design is used in order to attain efficient sintering at near maximum temperatures by means of extensive heat recovery and the utilisation of sintering zone exhaust gases in the calcination stage. The disclosed furnace would be difficult to adapt to the use of a solid fuel in the calcination stage, and further the recycling of the sintering zone exhaust gases is detrimental to the quality of the calcined product. Another kiln is disclosed in British Patent Specification No 1081006 in which undesirable direct recycling of exhaust gases is used, and wherein adapt'iron to solid fuel firing is difficult. This invention provides combustion apparatus suitable for solid fuel firing, including a vertical shaft, an inlet at the upper part of the shaft for a charge of solid material, an outlet at the bottom of the shaft for burned material, and means for removing gaseous combustion products from the shaft, the apparatus including means for drawing combustion air downwardly through the charge, means for drawing cooling air through the burned material in the lower part of the shaft to oc;1 the burned material and thereby to heat the cooling air, and means for introducing the heated cooling air substantially free from gaseous combustion products nto the upper part of the shaft to serve as combustion air for the combustion process. The term "burned material" as used above should be construed as including calcined material as well as the residue of burned fuel. In a preferred embodiment, the heated cooling air is further heated by heat exchange with the gaseous products of the combustion/calcination reaction, before being introduced into the upper part of the shaft as combustion air. The shaft preferably has two gas outlets in its lower part. The upper of the two draws combustion/ calcination reaction products downwardly through the shaft, and thereby draws combustion air downwardly through the charge. The combustion/calcination reaction products may then be passed by means of a fan to a heat exchanger where they are cooled and then vented to the atmosphere. The lower of the two outlets is used to draw air upwardly through the burnt material (e.g. calcined stone) in the bottom part of the shaft, thereby to cool the burnt material, and in the process heat the air. The cooling air is preferably passed to the heat exchanger, to be further heated by heat exchange with the gaseous combustion/calcination reaction product, before being passed to the upper part of the shaft, to serve as combustion air for the combustion process. A heater may be provided to provide auxiliary heating for the combustion air both at start-up, and if under particular conditions of operation the heating provided by heat exchange should not be sufficient to raise the combustion air to the desired temperature. The air supplied to the upper part of the shaft is preferably introduced at a point in the shaft spaced from the shaft walls, so as to avoid so called "channell ing", which tends to cause the gas flow to be down the walls of the shaft, and leads to uneven combustion and uneven calcination in the centre of the charge. the most desirable gas-flow profile is obtained with air introduced across the whole width of the shaft, more being introduced at the centre part than close to the walls. Similarly, the gas outlets in the lower part of the shaft are preferably taken from points spaced from the walls of the shaft. When solid fuel firing is required, the present apparatus includes means for charging a solid fuel, e.g. coal, with the solid material into the shaft. On the other hand if a gaseous or liquid fuel is to be used, appropriate combusting means are provided in the upper part of the shaft. Typically these means comprise a plurality of hydrocarbon oil gasifying/burning units e.g. those described in British Patent Specification No 1282553, disposed immediately below the heated cooling air introducing means. The present invention also provides a number of methods of utilising the present apparatus. One provides a method of calcining a solid material, which method comprises supplying the material together with a solid fuel to the charge inlet of the apparatus, and combusting the fuel to calcine the material. A second provides a method of calcining a solid material, which method comprises supplying the material to the charge inlet of the apparatus, and combusting a gaseous or liquid fuel in the upper part of the shaft to calcine the material. A third provides a method of incinerating a waste material in solid form or coated onto a solid material, which method comprises supplying the material to the charge inlet of the apparatus, and combusting the waste material, optionally with a fuel. A preferred embodiment of the invention will now be described with reference to the accompanying drawing, which is a diagrammatic cross section of a kiln according to the invention. The kiln comprises a vertical shaft having walls 32, and fed with a charge 3 including calcinable stone and a solid fuel (e.g. lignite, coal) via charging bell 4. An airtight charging door 1 in charging chute 2 ensures that air drawn downwardly through the kiln is substantially all heated air supplied by the various gas ducting arrangements described below. The kiln includes a wall 5, supported at its ends in the walls of the kiln, and from its base by beams 6, for dividing the charge into two parts. Both wall 5 and beams 6 include ducts for supplying combustion air to the charge from a ring main 7. A temperature sensing device 9 is provided in the wall of the kiln to facilitate measurement of the temperature of calcination. A fan 17 draws the gaseous products of combustion and calcination through an inverted bell outlet 15, via ash and fines trap 16. The low pressure region thus created causes combustion air to flow downwardly through the charge through a preheat region 8 to the main calcination region 10, where calcination of the charge by burning of the fuel takes place. From fan 17, the gaseous products of the combustion/calcination reaction are passed to heat exchanger 21, and thus to chimney stack 24, via conduit 23. Discharge doors 28 are opened periodically so that the whole of the charge moves progressively down the kiln as calcination takes place. The charge passes through an equilibration or "soaking" zone 29, before being cooled in region 27, the cooling being assisted by cooling air drawn through tuyeres 26 via outlets positioned across the length of a beam 18, and an ash and fines trap 19. Air is supplied to tuyeres 26 via a ring main 33 supplied via conduit 37 equipped with valve 34. Valve 34 is operated manually or automatically in conjunction with flow control valves 35 and 36 in order to obtain the desired balance of' air flows through fans 17 and 20. The cooling air is drawn by fan 20, and is passed to heat exchanger 21, for heat exchange with the gaseous combustion/calcination reaction products. Additional cooling air can be added, if required, via valve 38. The thus heated cooling air is passed through conduit 22 to a standby heater 11, which can be brought into operation either on start-up of the kiln, or if necessary to effect rapid temperature changes of the combustion air to modify rapidly the combustion pattern in the kiln. A fuel supply 14, preferably for liquid or gaseous hydrocarbon fuel, is provided for introducing fuel to the heater 11. A further conduit 25 is provided by which additional air may be introduced into the upper part of the kiln via heater 11. Conduit 25 is finned to improve heat exchange with hot flue gases in chimney stack 24. Heated air from heater 11 is passed to the upper part of the shaft by means of air inlets in beams 6, and inlets 30 in wall 5. Additionally, a conduit 12 is provided for passing heated air to the void space surrounding charging bell 4, to assist in drying and preheating the charge. Heated air may also be taken off through conduit 31 to preheat the charge before it is supplied to chute 2. Flow through these various conduits is controlled via valves 13. The arrangement above described allows accurate control of combustion by facilitating control of the temperature of combustion air fed to the upper part of the shaft. Because of the down draught arrangement, the kiln can be made to run with substantially no production of smoke. Furthermore, the down draught arrangement enables the kiln to be shut down, without the need to discharge the entire contents of the kiln when it is desired to recommence calcination. The kiln shown in the drawing may be operated in the same fashion as described above but fed with a charge of waste material, for example noxious industrial waste, either alone, or together with another solid fuel, and the kiln may be used with waste material as the sole or partial fuel either for the calcination of stone, or simply to incinerate the waste material. Liquid waste may be incinerated by coating onto solid particles, for example of pebbles, refractory material or even stone, which particles may be recycled after passage through the kiln. The described kiln can readily be adapted for gaseous or liquid fuel firing by the incorporation of suitable burners immediately below the point in the shaft where the heated cooling air is introduced. In the drawing these burners are indicated in dotted outline and marked 39. By providing both these burners and a solid fuel charging system, a kiln of the present invention can readily be adapted to use a wide variety of different fuels.

Claims: CLAIMS: 1. Combustion apparatus suitable for solid fuel firing, including a vertical shaft, an inlet at the upper part of the shaft for a charge of solid material, an outlet at the bottom of the shaft for burned material, and means for removing gaseous combustion products from the shaft, the apparatus including means for drawing combustion air downwardly through the charge, means for drawing cooling air through the burned material in the lower part of the shaft to cool the burned material and thereby to heat the cooling air, and means for introducing the heated cooling air substantially free from gaseous combustion products into the upper part of the shaft to serve as combustion air for the combustion process. 2. Apparatus as claimed in claim 1, including means for further heating the heated cooling air by heat exchange with the gaseous combustion products. 3. Apparatus as claimed in claim 2, including an outlet for the cooling air in the lower part of the shaft, a fan for passing the cooling air through a heat exchanger and introducing it as heated combustion air into the upper part of the shaft, an outlet for gaseous combustion products positioned above the cooling air outlet, and a fan for drawing gaseous combustion products throuyh the latter outlet, and passing them to the heat exchanger, thereby to cause a downward flow of combustion air through the charge. 4. Apparatus as claimed in claim 3, wherein an inlet for cooling air is provided below the cooling air outlet, thereby to cause an upward flow of cooling air through the burned material. 5. Apparatus as claimed in claim 3 or claim 4, wherein the outlet for gaseous combustion products is spaced from the walls of the shaft. 6. Apparatus as claimed in claim 5 wherein the said outlet includes a plurality of outlet ports spaced from the walls of the shaft. 7. Apparatus as claimed in any one of claims 2 to 6, including a heater for initial or supplementary heating of the air supplied to the upper part of the shaft. 8. Apparatus as claimed in any one of the preceding claims, wherein the means for introducing heated combustion air into the shaft is arranged to introduce the said air into a point in the shaft spaced from the shaft walls. 9. Apparatus as claimed in claim 8, wherein the means for introducing heated combustion air into the shaft is arranged to introduce the said air into the shaft at a plurality of points spaced from the shaft walls. 10. Apparatus as claimed in any one of the preceding claims, including means for charging a solid fuel with the solid material into the shaft. 11. Apparatus as claimed in any one of claims 1 to 9, including means for combusting a gaseous or liquid fuel in the upper part of the shaft. 12. Apparatus as claimed in claim 11, wherein the combustion means comprises a plurality of hydrocarbon oil gasifying/burning units disposed immediately below the heated cooling air introducing means. 13. A method of calcining a solid material, which method comprises supplying the material together with a solid fuel to the charge inlet of apparatus as claim'ed in any one of the preceding claims, and combusting the fuel to calcine the material. 14. A method of calcining a solid material, which method comprises supplying the material to the charge inlet of apparatus as claimed in any one of claims 1 to 12, and combusting a gaseous or liquid fuel in the upper part of the shaft to calcine the material. 15. A method of incinerating a waste material in solid form or coated onto a solid material, which method comprises supplying the material to the charge inlet of apparatus as claimed in any one of claims 1 to 12, and combusting the waste material, optionally with a fuel.

Inventors: JOPE, EDWARD CHARLES, MIDDLEMAS, JAMES WILLIAM
Applicants: CATAGAS LIMITED
Application year: 1982
Main classification: F27B1
IPC codes: F27B1, F23G5