POWER PLANT FOR BURNING FUEL IN A FLUIDIZED BED

Abstract: Power plant for burning fuel, primarily coal, in a bed vessel (12) with a fluidized bed (22) of particulate material at a pressure exceeding atmospheric pressure. Consumed bed material and/or ash separated from the combustion gases are removed via a pressure-reducing discharge means (50). A part (50b) of this discharge means is located in a drier for crushed coal in the form of a bed (56). This is arranged as a bed vessel (54) where crushed coal to be dried forms a fluidizable bed (56). The bed (56) is fluidized by gas introduced through nozzles (66) in the bottom (64) of the bed vessel. The discharge part (50b) constitutes a heater for the bed (56) and a cooler for the material to be discharged by the discharge means (50).

Description: The invention relates to a power plant for burning fuel in a fluidized bed according to the precharacterising part of Claim 1. The power plant is built up as a PFBC power plant, the name showing the initial letters of the term P ressurized F luidized B ed C ombustion. The invention relates particularly to a power plant having a bed vessel located inside a pressure vessel and surrounded by compressed combustion air. Fuel, such as crushed coal, often has such a high moisture content that it is not suitable for pneumatic transportation. The high moisture content entails a considerable risk of clogging in pneumatic transport pipes. For this reason it is advisable to dry the fuel before transporting it pneumatically to a combustion chamber. Particulate material is preferably dried in a bed vessel where it forms a bed which is maintained in fluidized state by a gas blown in through nozzles in a bottom supporting the bed. Heat is supplied to the bed either by the fluidizing gas or by heaters in the bed. US-A-4 304 049 describes a method and equipment for drying moist material in a fluidized bed. EP-A-0 192 073 describes a PFBC power plant where dust separated in dust separators is continously transported away in a pneumatic transport system. For cooling the dust and transporting gas, a first cooler is arranged in a channel which conducts combustion air from the space in the pressure vessel to the combustion chamber. Outside the pressure vessel a second cooler is arranged in a channel through which cooling air is injected by means of a fan. One disadvantage of this plant is that the heat exchange from the second cooler is not exploited. The invention aims at a design of a power plant of the above-mentioned kind in which the afore-mentioned problem of rendering crushed moist fuel pneumatically transportable is solved in a very economical way simultaneously enabling increased recovery of heat energy contained in the hot ashes or bed material to be discharged. To achieve this aim the invention suggests a power plant for burning fuel in a fluidized bed according to the introductory part of Claim 1, which is characterized by the features of the characterizing part of Claim 1. Further developments of the invention are characterized by the features of the additional claims. According to the invention a PFBC power plant includes a second bed vessel to dry a particulate fuel, usually crushed coal, in a fluidized bed. The bed consists wholly or partially of fuel. This is fluidized by gas supplied to the bed through openings or nozzles in a bottom supporting the bed. At least a part of a pneumatic pressure-reducing discharge means for ash or bed material is located in this second bed vessel and provides a source of heat for the bed and a cooler for ash and the transport gas therefore. This pressure-reducing discharge means may comprise a number of series-connected, parallel pipe sections joined to turning chambers which deflect the material/gas flow 180° between the downstream end of one pipe and the upstream end of the next pipe. Such a pressure-reducing discharge means is described in EP-A-0 108 505. The gas for fluidization of the second bed and removal of vaporized moisture may be air but should preferably consist of an inert gas or a gas poor in oxygen in view of the risk of the fuel igniting. Combustion gases leaving the gas turbine associated with the plant may be used. Another possibility is to circulate gas within the drying installation. A compressor for circulating the gas may be connected to the outlet from the drier and is suitably connected via a cooler which removes moisture from the gas before it is compressed. In a PFBC power plant with a bed vessel inside a pressure vessel and discharge means for ash or bed material in a channel or shaft conducting compressed combustion air to the bed vessel, cooling is limited by the temperature of this air. The air temperature is so high, up to 300° C, that cooling to a temperature below 350° C is scarcely possible. By placing a portion of the discharge means in a bed vessel for drying fuel, additional heat can be exploited as the ash or bed material is further cooled by the surrounding temperature. The accompanying drawing shows schematically a PFBC power plant which forms an embodiment of the present invention. In the drawing, 10 designates a pressure vessel surrounding a bed vessel 12 and a gas-cleaning plant 14. In the latter dust is separated from the combustion gases leaving the free-board 16 of the bed vessel through the outlet pipe 18. The gas-cleaning plant 14 generally comprises a number of parallel-connected groups of series-connected cyclones. The lower part of the bed vessel 12 forms a combustion chamber 20 housing a fluidizable bed 22 of a particulate material consisting at least partially of a sulphur-absorbent such as limestone or dolomite. Tubes 24 are located in the combustion chamber 20 which, dependent on the load condition of the plant, are either wholly or partially immersed in bed material 20. The tubes are utilized both to generate steam to drive a steam turbine, not shown, and to cool the bed so that the bed temperature is kept within permissible limits, i.e. about 800-950°C. Fuel is supplied to the combustion chamber 20 through a pipe 26 with its orifice positioned in the lower part of the combustion chamber 20 just above the bottom 28 of the bed vessel. This bottom is provided with nozzles 30 through which compressed air is supplied from the space 32 via shaft 34 to the combustion chamber 20 to effect fluidization of the bed 22 and combustion of the fuel supplied. The combustion gases from the cleaning plant 14 are conducted through pipe 38 to the turbine 40, which drives a generator 42 and a compressor 44. Compressed air from the compressor 44 is transported via conduit 46 to the space 32. The compressed combustion air may have a pressure of up to 20 bar. Dust separated in the cleaning plant 14 is continuously removed through a pressure-reducing discharge means 50 which also cools the dust and transport gas passing through the discharge means. The discharge means 50 is divided into two parts 50a and 50b. Both these parts consist of a number of pipes physically arranged in parallel and functionally connected in series via chambers joining the downstream end of one pipe to the upstream end of the next pipe. The part 50a is located in the shaft 34 and is cooled by the combustion air on its way to the nozzles 30 in the bottom 28. During this cooling process the combustion air is pre-heated and energy thus recovered. The temperature of the combustion air is first increased by compression in the compressor 44 and then further increased in the space 32 due to heat losses from the bed vessel 12 and other equipment in the cleaning plant 14. As mentioned above, the temperature of the air entering the shaft 34 may be 300° C or more, which limits cooling of the dust and transport gas to about 350° C. The dust/gas flow is conveyed through conduit 52 to part 50b of the dust discharge means, this part being located in a separate bed vessel 54 outside the pressure vessel 10. Here the dust/gas flow is further cooled and the heat thus extracted is used to dry crushed coal forming the bed 56. The dust/gas flow is conducted from the discharge part 50b through the conduit 58 to the container 60 where the dust is separated from the transport gas. Container 60 is under atmospheric pressure. The transport gas is released via filter 62 into the atmosphere. The lower part of the bed vessel 54 has a bottom 64 with nozzles 66 and an air-distributing chamber 68 for gas to fluidize the bed 56 and to remove vaporized moisture. Crushed coal from container 70 is supplied to the bed vessel 54 via a cell-feeder 72 and a conduit 74. An outlet 76 for dried coal is arranged in the bed vessel 54, at the side immediately opposite the supply pipe 74. A cell-feeder 80 to control the material flow is provided in the outlet pipe 78 from said outlet 76. The gas is conducted from the free-board 82 in the bed vessel 54 of the drying plant, to the dust-separator 84, the lower end of which is connected to outlet 76. The fluidization gas and water vapour generated are withdrawn via filter 86 and conduit 88. The supply of moist coal should be regulated so that the moisture content does not fall below the dew point of gases in the free-board 82 and the dust separator. In view of the risk of fire and explosion, the gas used to effect fluidization of the bed 56 of coal should be poor in oxygen or inert. To ensure this, one method is to circulate gas in the drying plant as shown in the drawing. A compressor 90 for fluidization gas may be connected to conduit 88 via a cooler 92 and a conduit 94. Water vapour is condensed in the cooler 92 and the condensate removed through conduit 96. Due to the temperature increase during compression in the compressor 90, the relative moisture falls so that the fluidization gas is relatively dry when it is supplied to the bed vessel 54. Gas leaving the gas turbine 40 may also be used as fluidization gas in the bed vessel 54. The compressor is then connected to an outlet pipe from the turbine 40 by the conduit 96' indicated in broken lines.

Claims: Power plant for burning fuel, primarily coal, in a fluidized bed of particulate material at a pressure exceeding atmospheric pressure, comprising a first bed vessel (12) having a bottom (28) with nozzles (30) and a shaft (34) below the bottom to supply the bed vessel (12) with air for fluidization of the bed (22) and combustion of the fuel supplied to the bed (22), a turbine (40) adapted to be driven by combustion gases generated in the bed vessel (12), a compressor (44) adapted to compress air for the fluidization and combustion processes, a gas cleaner (14) for separating ash from the combustion gases prior to their supply to the turbine (40), and a pressure-reducing pneumatic ash discharge means (50) having a number of series-connected pipe sections and turning chambers for conveying the gas/ash flow with deflection of the flow direction between the pipe sections, a first part (50a) of this pressure reducing pneumatic ash discharge means (50) being located in the shaft (34) and a second part (50b) of this pressure reducing pneumatic ash discharge means (50) being located outside the bed vessel (12), characterized in that the plant includes a second bed vessel (54) to dry a particulate fuel in a fluidized bed (56) which consists entirely or partly of said fuel, that the second part (50b) of the pneumatic pressure-reducing discharge means for blown-off ash or bed material is arranged in said second bed vessel (54) providing a heat source for the bed (56) and a cooler for the ash or bed material and transport gas transported in the discharge means (50). Power plant according to Claim 1, characterized in that it comprises a compressor (90) connected to an air-distributing chamber (68) in the second bed vessel (54) to fluidize and dry fuel forming the bed (56) in the second bed vessel (54) and to remove vaporized moisture. Power plant according to Claim 2, characterized in that the gas for fluidizing and drying the fuel in the bed (56) in the second bed vessel (54) consists of air. Power plant according to Claim 2, characterized in that the gas for fluidizing and drying the fuel in the bed (56) in the second bed vessel (54) consists of an inert gas or a gas poor in oxygen, such as combustion gases which have left the turbine (40).

Inventors: BRAENNSTROEM, ROINE, Brännström, Roine
Applicants: ASEA STAL AKTIEBOLAG
Application year: 1988
Main classification: F23C11
IPC codes: F23C10, F22B31, F23K1