POWER PLANT FOR COMBUSTION OF FUEL IN A FLUIDIZED BED

Abstract: Power plant for combustion of fuel in a fluidized bed, primarily a PFBC power plant. The plant includes a multistage steam turbine (13) and an intermediate superheater (12) for superheating steam between the turbine stages. The combustion chamber (2) is divided into a first and a second part (2a, 2b) by a wall (4) having one or more openings (42) which take/takes up a minor part of the cross-section in the bed region and makes possible a limited exchange of bed material. The first part (2a) includes a nest of boiler tubes (11) for generating steam. The second combustion chamber part (2b) includes a nest of boiler tubes (12) for intermediate superheating of steam between the turbine stages. The combustion chamber parts (2a, 2b) are each connected to a fuel supply system (20, 21, 22 and 23, 24, 25, respectively). The control of the superheating takes place by control means (62) which control the bed temperature in the second combustion chamber part (2b) by controlling the fuel supply to said combustion chamber part (2b). (Figure 2)

Description: The invention relates to a power plant for combustion of fuel in a fluidized bed according to the precharacterising part of Claim 1. A combustion chamber with such features is known from the GB-A-20 72 524. The invention is especially intended for a PFBC power plant. The term "PFBC" is formed by the initial letters in the English expression "Pressurized Fluidized Bed Combustion". For power plants of the kind referred to here, no satisfactory technique exists for superheating of steam between two turbine stages or between a high pressure turbine and a low pressure turbine. A choice between two principles is possible: 1. A separate nest of boiler tubes for intermediate superheating of steam is located along with the proper steam generating boiler tubes in a common bed vessel. This embodiment gives insufficient possibilities of obtaining optimum steam data. The superheating tube nest can be dimensioned so as to obtain optimum steam data at full load. The tubes in the tube nest can be distributed in horizontal layers in such a way that the tube area above the bed and in the bed, respectively, is of such a magnitude as to obtain as suitable a superheating as possible at partial load. However, the dimensioning and the distribution of the tubes make it impossible to obtain optimum intermediate superheating of the steam. This applies particularly to the partial load condition. Maladjustment between steam flow and tube area means that it is necessary either to inject water to prevent an inadmissible increase in temperature in the tube nest, or that it must be accepted that no optimum superheating is obtained. In both cases, the efficiency of the power plant is poor. 2. A tube nest for intermediate superheating of steam is located in a separate bed vessel. This embodiment makes it possible separately to control the intermediate superheating and obtain optimum steam data for different turbine stages under all operating conditions. The plant is complicated by the fact that each one of the beds has to be provided with complete control systems for air supply, fuel supply and bed depth control, i.e. a doubling of the control systems is required. From the GB-A-20 72 524 a combustion chamber is known the inner space of which is divided by a partition wall in two combustion chamber parts each part having a seperate freeboard for collecting gases. One of the parts is the proper combustion chamber while the other part serves to regenerate consumed desulferating agent transferred from the combustion chamber proper into the regenerating chamber. Since the consumed desulferating agent (CaSO₄) has a higher specific gravity than the unconsumed agent (CaO), the former collects at the bottom of the fluidized bed and is then, via a sloped bottom in the combustion chamber part and an opening at the lower end of the partition wall, transferred into the regenerating chamber. In the regenerating chamber the material is fluidized by a regenerating gas that re-converts the CaSO₄ into lighter CaO which is then, through a second opening in the partition positioned above the level of the fluidized bed in the combustion chamber proper, transferred back into the combustion chamber proper. The invention aims at a power plant for combustion of fuel in a fluidized bed which is capable of simultaneously generating steam of two different optimum conditions without resorting to two separate bed vessels. 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. In the first combustion chamber part there is a first tube nest for generating and superheating steam for a high pressure turbine or a first turbine stage, and in the second combustion chamber part there is a second tube nest, separated from the first tube nest, for intermediate superheat ing of the steam supplied to a low pressure turbine or a second turbine stage. In addition to the normal measuring and control devices for power, bed depth, bed temperature and air quantity, etc., the plant is provided with a temperature sensor to sense the temperature of the intermediately superheated steam, a further temperature sensor to sense the temperature in the second combustion chamber part, and a signal processing and control equipment which receives output signals from said sensors and controls the fuel supply to a separate fuel supply system for the second combustion chamber part. The temperature of the intermediately superheated steam is controlled by controlling the temperature of the bed between a highest and a lowest value through adjusting the fuel supply. By dividing the combustion chamber into two parts by means of a wall with one or more openings enabling a limited exchange of bed material, and by supplying the combustion chamber parts with separately controlled fuel supply systems, different bed temperatures can be achieved in the two combustion chamber parts when the same bed depth and the same specific air flow prevail. For controlling the temperature of the intermediately superheated steam, only an additional, separate fuel supply system and a separate control system therefore are required. Thus, sufficient possibilities of controlling the intermediate superheating of steam can be obtained in a simple way at only slightly increased investment and operating costs. The invention will now be described in greater detail with reference to the accompanying drawings showing - by way of example - in Figure 1 the invention as applied to a PFBC power plant with a combustion chamber and a cleaning equipment enclosed within a pressure vessel, Figure 2 a longitudinal section through a combustion chamber, Figure 3 a cross section through the combustion chamber taken along A-A in Figure 2. In Figure 1 a pressure vessel 1 surrounds a combustion chamber 2 and a gas cleaning plant symbolized by a cyclone 3. The combustion chamber 2, as shown in the longitudinal section in Figure 2, is divided by a partition wall 4 into two parts 2a and 2b. The combustion chamber 2 is provided with a bottom 5 with air nozzles 6 and with fuel nozzles 7 in part 2a and fuel nozzles 8 in part 2b. The combustion chamber 2 accommodates a fluidizable bed 10 of particulate material containing or consisting of a sulphur absorbent such as lime or dolomite. As shown in Figure 2, the first combustion chamber part 2a contains a nest of tubes which is divided into a first tube nest 11a and a second tube nest 11b for respectively generating and superheating steam for a turbine 13 which drives a generator 14. The turbine 13 contains a high pressure part 13a, which is supplied with superheated steam from the superheater tube nest 11b, and a low pressure part 13b, which is supplied with steam which has passed through the high pressure part 13a of the turbine 13 and has been superheated in the intermediate superheater 12. Steam leaving the low pressure part 13b of the turbine 13 is passed through the conduit 15 to the condensor 16. The condensate is returned to the tube nest 11a via the conduit 17 with the feed water pump 18 which is driven by the motor 19. Fuel is supplied to the combustion chamber part 2a from a fuel storage 20 via a rotary vane feeder 21, the conveying pipe 22 and the nozzles 7. To the combustion chamber part 2b fuel is supplied from a fuel storage 23 via a rotary vane feeder 24, the conveying pipe 25 and the nozzles 8. Air for fluidization of the bed 10 and for combustion of supplied fuel is supplied to the combustion chamber 2 via the nozzles 6 in the bottom 5 thereof from the space 26 between the pressure vessel 1 and the combustion chamber 2 (Figure 1). Bed material is supplied to the bed 10 through a conduit 27 and is removed through a conduit 28 (Figure 1). Transport gas is compressed in the compressors 30 and 31, respectively. The combustion gases are collected in the freeboard 32, which is common to both combustion chamber parts 2a, 2b, above the bed 10 and is passed via the conduit 33 to a cyclone 3, in which dust is separated from the gases. This separated dust is transported away through the conduit 34 to the collecting container 33. Between the conduit sections 34a and 34b there is a pressure reducing cooler 35 for the dust and its transport gas. The cleaned combustion gases are passed through the conduit 36 to a gas turbine 37 which drives the compressor 38 supplying compressed combustion air to the space 26 in the pressure vessel 1. The turbine 37 also drives a generator 40. The gases leaving the turbine 37 are brought to a feed water preheater (not shown). As shown in Figure 3, the partition wall 4 is water-cooled. It does not completely separate the combustion chamber p arts 2a, 2b from each other. It has a height somewhat exceeding the highest bed depth. A free connection is provided between the parts 2a, 2b in the freeboard 32 through the opening 41 above the partition 4. Further, in the shown embodiment there are an opening 42 between the bottom 5 and the partition and gaps 43 between the partition 4 and the side walls 44 of the combustion chamber 2. The total area of the opening 42 and the gaps 43 is chosen such that, on the one hand, sufficient material exchange can take place between the parts 2a and 2b as to obtain the same bed level in both parts while, on the other hand, the exchange between the parts 2a, 2b is so low that at the same time different temperature levels can be maintained. Through the opening 42 and the gaps 43, the combustion chamber parts 2a, 2b act as communicating vessels in the bed region. The bed level is therefore the same in both combustion chamber parts 2a, 2b. In the case of uniform operation, a very limited transfer of bed material is obtained between the parts 2a and 2b. Therefore, it will be possible, to a certain extent and in a simple manner, to control the temperature in the bed in the second combustion chamber part 2b such that the temperature deviates from the temperature in the first combustion chamber part 2a only by controlling the fuel supply. Thus independent control is achieved in superheating the steam from the high pressure turbine 13a, which is intermediately superheated in the tube nest 12 before being supplied to the low pressure turbine 13b. Because the parts 2a and 2b communicate with each other and because a fluidized bed 10 behaves like a liquid, the level of the entire bed can be changed with one single bed controlling system. By injecting gas through suitably horizontally orientated nozzles close to the openings 42, 43, the material exchange between the parts 2a and 2b can be increased, for example to rapidly reduce the temperature difference. The appropriate bed temperature is to a certain extent dependent on the fuel and its tendency to form major slag lumps. A bed temperature of about 850° is usually suitable and there may be possibilities of operating the bed within the range of 750-900°. If the temperature drops to below a certain temperature, combustion cannot be maintained. If the temperature rises to above a certain level, the formation of slag may render continued operation impossible. For controlling the superheating, the possibility of raising the temperature in the bed in the second combustion chamber part 2b by 25°C above or lowering it by 50° below the temperature in the bed in the first combustion chamber part 2a is fully sufficient. The first combustion chamber part 2a includes a temperature sensor 50. This is connected to a signal processing and control equipment 51 which receives the output signal of the sensor 50 and compares the actual temperature value with a reference value and, in dependence of the temperature deviation, controls the speed of a motor 52 which drives a rotary feeder 21 which determines the fuel supply to the combustion chamber part 2a. Further there are measuring means (not shown) for measuring the bed depth, the air excess, and so on, as well as signal processing and operating means for controlling the bed depth and the air supply in dependence on the power requirement. The second combustion chamber part 2b includes a temperature sensor 60. In the conduit 12a from the tube nest 12 to the turbine stage 13b, there is a temperature sensor 61 which measures the temperature of the outgoing steam. These two sensors 60, 61 are connected to a signal processing and control equipment 62 which compares the sensed actual temperature values and controls the speed of a motor 63 which drives the rotary feeder 24 which controls the fuel supply to the combustion chamber part 2b. By the control equipment 62, the fuel supply to the combustion chamber part 2b is controlled so as to maintain such a temperature in the bed as to obtain the desired steam temperature. The control possibility is limited by the maximum and minimum permissible temperatures in the bed with respect to the risk of slag formation and to the possibility of maintaining the combustion. With a suitable dimensioning of the tube nest 12, a sufficient control of the steam temperature can be obtained within the permissible temperature variation within the bed.

Claims: Power plant for combustion of a fuel, primarily carbon, in a fluidized bed (10) of particulate material inside a combustion chamber (2), which is provided with a partition (4) that divides the combustion chamber (2) into a first and a second part (2a, 2b), each part having a freeboard for collecting gases, said partition (4) being provided with at least one opening (42, 43) through which said two combustion chamber parts (2a, 2b) communicate with each other, and with boiler tubes arranged in the fluidized bed, characterized in that the partition is arranged such that the freeboards above said first and second parts of the combustion chamber are combined to form one common freeboard (32) for collecting combustion gases generated in said two combustion chamber parts, that the total area of said at least one opening in the partition is arranged such that sufficient material exchange can take place between the two combustion chamber parts as to obtain the same bed level in both parts while the exchange between said parts is so low that at the same time different temperature levels can be maintained, that in the first combustion chamber part (2a) there is a first tube nest (11) for generating steam and in said second combustion chamber part (2b) there is a second tube nest (12) for superheating of steam, and that means are provided for separate control of the temperature in said first and second combustion chamber parts. Power plant according to Claim 1, characterized in that the power plant includes a steam turbine (13) with a high pressure part (13a) and a low pressure part (13b) and that said second tube nest (12) is connected between said high pressure part (13a) and said low pressure part (13b) of said turbine (13) and forms an intermediate superheater. Power plant according to Claim 1 or 2, characterized in that in said first tube nest (11) is adapted to superheat the generated steam. Power plant according to any of Claims 1 to 3, characterized in that the plant comprises a first fuel supply system (20, 21, 22) which supplies said first combustion chamber part (2a) with fuel, and a second fuel supply system (23, 24, 25) which supplies said second combustion chamber part (2b) with fuel. Power plant according to Claim 4, characterized in that the plant includes a temperature measuring means (61) which measures the temperature of the steam superheated in said second tube nest (12), and a temperature measuring means (60) which measures the temperature in the bed (10) in said second combustion chamber part (2b), that a signal processing and control equipment (62) is provided adapted to receive output signals from said measuring means (60, 61), compare the actual value of the steam temperature with a reference value and the actual value of the bed temperature with permissible maximum and minimum values of the bed temperature and, in dependence on the deviations from reference values, deliver a control signal to a supply unit (63, 24) for supplying fuel to said second combustion chamber part (2b). Power plant according to any of the preceding Claims, characterized in that the combustion chamber (2) is enclosed within a pressure vessel (1) and surrounded by compressed combustion air. Power plant according to any of the preceding Claims, characterized in that close to the openings (42, 43) in the partition 4 there are substantially horizontally oriented nozzles for injecting gas intended to influence the material exchange between the parts (2a, 2b) of the combustion chamber.

Inventors: NILSSON, KARL-JOHAN, PILLAI, KRISHNA
Applicants: ASEA STAL AKTIEBOLAG
Application year: 1987
Main classification: F23C11
IPC codes: F22B31, F23C10