POWER PLANT BURNING FUEL IN A FLUIDIZED BED

Abstract: Power plant for burning fuel, primarily coal, in a fluidized bed (18), preferably a PFBC power plant. A bed vessel (12) is divided into a combustion chamber (22) and an ash chamber (24) by an air distributor (16) with nozzles (42) for the supply of air for fluidization of the bed (18) and combustion of the fuel. The air distributor (16) is provided with openings (44), through which bed material can pass from the combustion chamber (22) to the ash chamber (24). Tubes (54) for generating steam are also provided in the combustion chamber (22). Tubes (56) for superheating steam are arranged in the ash chamber (24). The plant is provided with a transport system (66) for conveying bed material from the ash chamber (24) to the combustion chamber (22). During transport from the ash chamber (24), the bed material will circulate between combustion chamber (22) and ash chamber (24). Regulating the quantity of bed material circulating also regulates the supply of heat to the superheating tubes (56), and thus superheating of the steam passing through the tubes (56).

Description: The invention relates to a power plant for burning fuel in a fluidized bed according to the precharacterising part of Claim 1. Such a power plant is known from the DE-A-2 935 542. The particulate bed material may consist of limestone or dolomite, serving also as sulphur absorbent. The bed is located in a bed vessel and steam is generated in tubes arranged in the bed. The invention can be applied in a plant operating at approximately atmospheric pressure and generating steam for heating or to drive a steam turbine. However, it is primarily designed for a power plant operating at a pressure considerably exceeding atmospheric pressure, known as a PFBC power plant, the name being derived from the term Pressurized Fluidized Bed Combustion . The bed vessel and a gas cleaning plant comprised in a PFBC power plant are usually enclosed in a pressure vessel. Combustion gases from the bed vessel drive one or more gas turbines and the steam generated in said tubes in the bed of the bed vessel, drives one or more steam turbines. The DE-A-2 935 542 discloses a power plant with a combustion chamber for burning a combustible substance such as waste material. There are no steam-generating tubes in the fluidized bed of the combustion chamber. To prevent the fluidized bed from becoming to hot, a heat exchanger is arranged in the ash chamber below the air distributor. The only purpose of this heat exchanger is to extract heat from the bed material. The bed material cooled by this heat exchanger is recirculated into the fluidized bed or the free-board of the combustion chamber, thus decreasing the temperature in these spaces. From the SE-B-441 698 it is well known to provide steam-generating tubes in the fluidized bed of a combustion chamber. The object of the invention is to reduce in a power plant of the above mentioned kind the thermal stresses in the superheating tubes, thus enabling superheating of the steam to extremely high temperatures of about 550-600° C. A further object of the invention is to achieve improved control properties, particularly at partial load. To achieve this aim the invention suggests a power plant 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 at least the last portion of an superheater or an intermediate superheater between two turbine stages is placed in the ash chamber below the air distributor of the bed vessel. The transport system effects a circulation of the bed material to such an extent that the steam temperature in the superheating tubes is prevented from exeeding a value that could cause damage to these tubes. In this way also the supply of heat to the superheating tubes is regulated. Since the bed material around the superheating tubes is not in fluidized state, the heat-transfer coefficient will be considerably lower than in fluidized bed material. The surface temperature of the tubes in the ash chamber will be lower and thus also the thermal stresses. This is particularly of value when superheating to high temperatures. Overheating here shall be taken to mean superheating to about 550-600° C. The invention will now be described more fully with reference to the accompanying drawing showing schematically an embodiment of the invention applied in a PFBC power plant. In the drawing, 10 designates a pressure vessel. A bed vessel 12 and a gas-cleaning plant symbolized by a cyclone 14 are arranged in the pressure vessel. In reality the gas cleaning plant is built up of parallel-connected groups of series-connected cyclones. In the lower part of the bed vessel 12 is a distributor 6 to disperse air for fluidization of a bed 8 of particle material and combustion of a fuel supplied to the bed 18 through a fuel-supply pipe 20. The air distributor divides the bed vessel into an upper combustion chamber 22 and a lower ash chamber 24. The upper part of the combustion chamber 22 forms a free-board 22a where combustion gases from the bed 18 collect. These gases are conducted from the free-board 22a via conduit 26 to the cyclone 14. Dust separated from the gas in the cyclone 14 is removed through pipe 50 and a pressure-reducing dust discharge device 52 for collecting the ashes in a container outside the pressure vessel. The cleaned gas is conducted through the pipe 28 to a gas turbine 30 which drives a generator 32 and a compressor 34. This compressor compresses combustion air which is supplied through the pipe 36 to the space 38 that surrounds the bed vessel 12 inside the pressure vessel 10. The air distributor is constructed of elongate distribution chambers 40 with nozzles 42. Air for fluidization and combustion is supplied to these distribution chambers 40 from the space 38 via valve members or dampers, not shown, which determine the air flow. The distribution chambers 40 are formed with slits 44 through which bed material can flow from the bed 18 in the combustion chamber 22 to the ash chamber 24. The bed material, sulphur absorbent and residual combustion products are removed from the ash chamber through the cell-feeder 46 in discharge pipe 48. Tubes 54 are provided in the bed 18 in the combustion chamber 22, to generate steam and to cool the bed, and tubes 56 are provided in the ash chamber 24 for superheating this steam. The steam drives a steam turbine 58 and a generator 60 connected thereto. The stream leaving the turbine 58 is condensed in a condenser 62 and the condensate is returned by the feed-water pump 64 to the tubes 54 in the bed 18. Alternatively the tubes 56 may constitute an intermediate superheater between two stages in the turbine 58. A pneumatic transport means 66 is provided in the installation to carry bed material up from the ash chamber 24 to the combustion chamber 22 so that it can be circulated between these chambers. This transport means 66 includes a suction nozzle 68, an ejector 70, a pipe 72 and a supply nozzle 74 with its orifice in the bed 18, or in the free-board 22a as indicated by broken lines. Due to the pressure drop in the nozzles 42 and in the bed 18, the pressure in the space 38 is higher than at the orifice of the supply nozzle 74. The ejector nozzle 76 can therefore obtain transport air directly from the space 38 through a control valve or, as shown in the drawing, via a booster compressor 80 driven by the motor 78. In the pipe 82 connecting the compressor 80 to the ejector nozzle 76 is a throttle-valve 84 for regulating the gas flow and transport of material. Alternatively, the flow may be regulated by controlling the speed of the compressor 80. The valve 84 is operated by a control device 86. In the steam pipe 88 is a thermoelement 90 which measures the temperature of the steam supplied to the turbine 58. This thermoelement communicates via cable 92 with a control equipment 94 where the actual valve of the signal from the thermoelement is compared with a set valve. The control equipment is connected either to the operating device 86 of the valve 84 by a cable 96a or to speed-control equipment in the motor 78 by a cable 96b. The flow of transport gas to the transport means 66 is controlled either by varying the through-flow area of valve 84 or by varying the speed of the motor 78 and compressor 80. In a fluidized bed the temperature is usually T B 800-900°C. Steam generated in the tubes 54 in the bed can be given a temperature of up to about 500°C. In a fluidized bed the heat-transfer coefficient α ₁ between bed and tubes is extremely high, α = 300-500 W/m²K. This causes a high surface temperature on the tubes, high thermal flux in the tube wall and high specific effect. It also entails high thermal stress in the tubes and certain control problems. The steam temperature should therefore be limited to below 500° C. Desired superheating to 550-600° C therefore entails particular problems and these problems are especially noticeable at partial load. The bed material in the ash chamber 24 is not in fluidized state. The heat-transfer coefficient α ₂ between bed material and tubes 56 is therefore considerably lower than the heat-transfer coefficient α ₁ in the fluidized bed 18 in the combustion chamber 22 above the air distributor 16. The heat-transfer coefficient α ₁ 300-500 W/m²K and α ₂ = 30-100 W/m²K. This lower value for α ₂ results in lower thermal stress in the tubes 56. The steam temperature can simply be regulated by regulating circulation of the bed material between the combustion chamber 22 and the ash chamber 24. Bed material supplied to the ash chamber 24 has a temperature T B = 800-900°C. The flow of bed material past the tubes 56 determines the heat supply to the over-heating part of the ash chamber. Bed material which has passed the tubes 56 is cooled to a temperature T a . Cooling to about 600°C or lower is possible. Coolers are provided in the ash chamber for additional cooling of the material prior to its removal to be removed through the cell feeder 46.

Claims: Power plant for burning fuel in a fluidized bed (18) of particle material comprising a bed vessel (12) with an air distributor (16) with nozzles (42), which air distributor divides the bed vessel into an upper combustion chamber (22) and a lower ash chamber (24) and is adapted to guide air for fluidizing the bed material in the upper combustion chamber (22) and burn fuel supplied to said combustion chamber, openings (44) in said air distributor (16), permitting bed material to flow from the combustion chamber (22) to the ash chamber (24), heat extracting tubes (56) arranged in said ash chamber (24), a pneumatic transport system (66) for conveying material from the ash chamber (24) to the combustion chamber (22), and a temperature-measuring element (90) for controlling the amount of material conveyed by said transport system (66), characterized in that steam generating tubes (54) are arranged in said combustion chamber (22), that said heat extracting tubes (56) arranged in the ash chamber (24) are adapted to superheat or reheat steam generated in said tubes (54) in the combustion chamber (22), that said temperature-measuring element (90) is adapted to sense the temperature of the superheated steam in the tubes (56) in the ash chamber, and that the control equipment for said transport system (66) is arranged to be controlled in dependence of the sensed temperature in such a way that the temperature of the tubes (56) is kept below a value not harmful for these tubes. Power plant according to Claim 1, characterized in that the bed vessel (12) is enclosed in a pressure vessel (10) and that combustion takes place at a pressure considerably exceeding atmospheric pressure. Power plant according to Claim 1 or 2, characterized in that the system (66) for conveying material from the ash chamber to the combustion chamber includes an ejector (70) arranged to draw bed material from the ash chamber (24) and a pipe (72) leading to the combustion chamber (22). Power plant according to Claim 3, characterized in that the orifice of the pipe (72) is connected to the bed vessel (12) at a level occupied by the bed. Power plant according to Claim 3, characterized in that the orifice of the pipe (72) is connected to the bed vessel (12) at a level occupied by the free-board (22a) above the bed (18). Power plant according to any of claims 2 to 5, characterized in that compressed combustion air from the space (38) that surrounds the bed vessel (10) inside the pressure vessel (12) is used as transport gas in the pneumatic transport system (66) for conveying bed material from the ash chamber (24) to the combustion chamber (22). Power plant as claimed in Claim 6, characterized in that it contains a booster compressor (80) arranged to increase the pressure of the transport gas.

Inventors: PILLAI, KRISHNA K.
Applicants: ASEA STAL AB
Application year: 1988
Main classification: F23C11
IPC codes: F22B31, F23C10, F22B1