METHOD AND DEVICE FOR TEMPERATURE CONTROL IN A COMBUSTION PLANT

Abstract: A method and a device for limiting temperature variations in a plant for combustion in a pressurized fluidized bed, a PFBC - Pressurized Fluidized Bed Combustion - plant, with a combustor (10) in the form of a pressurized fluidized bed, air paths (1) in which a compressor (13) is arranged to pressurize air supplied to the fluidized bed, flue gas paths (2) in which energy contained in the flue gases is extracted with a gas turbine (14), and flue gas economizers (15, 16) by means of which residual heat is extracted from the flue gases, and a feedwater/steam system (3) in which heat is utilized. Temperature variations in pressurized air supplied to the fluidized bed are essentially eliminated with a heat exchanger (17) arranged in the air paths (1). The heat exchanger is connected to the high temperature section of the feedwater/steam system (3) of the PFBC plant.

Description: Method and device for temperature control In^a combustion plant TECHNICAL FIELD The invention relates to limitation of temperature varia¬ tions in flowing gases in a combustion plant in which heat transfer surfaces are arranged in the gas paths to limit the temperature of the gas which is supplied to a combustor located in the plant and of the flue gases emitted from the plant. The invention- is especially valuable in a power plant with combustion in a pressurized fluidized bed, a PFBC - Pressurized Fluidized Bed Combustion - plant, in which it permits limitation of temperature variations in pressurized air supplied to the combustor and flue gases emitted from the plant, which means that power output or efficiency remains essentially. unaffected by variations in ambient temperature and compression ratios. BACKGROUND ART During combustion in a fluidized bed, the fluidized bed is supplied with air for fluidization of the bed material and for combustion of fuel supplied to the fluidized bed. If the fluidized bed is part of a plant for combustion in a pressurized fluidized bed, a PFBC - Pressurized Fluidized Bed Combustion - plant, the fluidized bed contained within a bed vessel is enclosed in a pressure vessel and the air supplied to the fluidized bed is pressurized, for example in a compressor driven by a gas turbine. The mass flow of pressurized air supplied to a PFBC plant is controlled within an interval of 40-105% of nominal flow. The pressurization is normally carried out in a gas turbine- driven compressor. From the point of view of capital cost, high compression ratios are desirable. A gas turbine-driven compressor provides different possibilities of controlling the mass flow, depending on the type of gas turbine. A single-shaft unit may control the mass flow by varying the • adjustment of compressor guide vanes and inlet valves, and, in addition, compressed air may be recirculated through the compressor. Moreover, in a multi-shaft unit, adjustable turbine guide vanes and nozzles as well as variable rotor speed are utilized. The temperature of the air supplied from the compressor via the pressure vessel to the fluidized bed must be limited, both when the air is used for cooling of pressure vessel, bed vessel, cyclones and other supporting components arranged in the pressure vessel, and when temperature variations, caused by compression ratios and ambient temperature, in air supplied to the fluidized bed affect the output power from the plant and the efficiency of the plant. The temperature of air supplied to the pressure vessel is not limited in normal PFBC plants, and thus there is no equalization of the temperature variations which occur in the pressurized air. Temperature variations occur as a consequence of variations in the ambient temperature and varying compression ratios and are compensated for in a normal PFBC plant by a change in the output power from the plant and in the efficiency of the plant. The residual heat in flue gases emitted from a combus¬ tion plant is delivered to flue gas economizers, which are arranged in the flue gas paths. SUMMARY OF THE INVENTION The influence from variations in the ambient temperature, compression ratios in air pressurized in the compressor, etc, which in a plant for combustion in a pressurized fluidized bed, a PFBC - Pressurized Fluidized Bed Combustion - plant, is reflected in the output power from the plant and in the efficiency of the plant, is essentially eliminated when temperature variations in incoming combustion air are limited according to the present invention. The plant comprises a combustor in the form of a pressurized fluidized bed, air paths in which air supplied to the flui¬ dized bed is pressurized, flue gas paths in which energy contained in flue gases emitted from the plant is partially extracted with a gas turbine arranged in the flue gas paths, and a feedwater/steam system comprising heat transfer surfaces arranged in the air and flue gas paths. According to the invention, the temperature variations of pressurized air supplied to the fluidized bed are limited by means of heat transfer surfaces, preferably in the form of a heat exchanger, arranged in the air paths According to a preferred embodiment of the invention, the temperature of flue gases discharged from the plant is simultaneously limited with heat transfer surfaces, arranged in the flue gas paths, in the form of cold and hot flue gas economizers. In addition, heat transfer surfaces arranged in the hot and cold sections of the flue gas paths and in the air paths are interconnted in the high temperature section of the feedwater/steam system of the combus¬ tion plant. By this interconnection and by the arrangement of control valves adjacent to the heat transfer surfaces, for control and distribution of the heat work in and between the heat transfer surfaces, the temperature of air supplied to the pressure vessel may be limited and maintained inde¬ pendent of temperature variations of air pressurized in the compressor while at the same time the flue gas temperature is limited. The heat work in the heat transfer surfaces may be controlled from outside with temperature sensors, for example thermocouples, measured temperatures of air and flue gas, respectively. Measured temperatures are compared, in conventional temperature regulators, with a desired value and the deviation gives a control signal out from the temperature regulator to the control valves arranged adjacent to the heat transfer surfaces. Based on the received control signal, the heat work in the heat-transfer surfaces is controlled. Thus, according to the invention, the necessary limitation of the variations of air supplied to the fluidized bed is obtained, so that the output power from the combustion plant or the efficiency of the plant remains unaffected by ambient temperature and compression ratios while at the same time heat absorbed in the heat transfer surfaces is utilized in the feedwater/steam system of the plant. In addition, during start-up and shutdown of a PFBC plant with contro.1 of air and flue gas temperatures according to the invention, possibilities are provided of reducing the heating and cooling times. The heating time during start-up can be reduced and hence the corrosion, caused by flue gas condensate in the gas paths, be reduced by the heat transfer surfaces upon start¬ up being traversed by steam from an external source, for example from an existing auxiliary boiler intended to supply the plant with de-aired water. The cooling times can be reduced by the heat transfer surfaces, upon shutdown, being traversed by water, for example by being connected to a condenser circuit. BRIEF DESCRIPTION OF THE DRAWINGS The features and advantages of the invention will be explained in greater detail with reference to functional and schematic flow diagrams. The limitation, according to the invention, in a combustion plant with gas turbine-driven pressurization of air supplied to the combustor, of temperature variations of pressurized air supplied to the fluidized bed is illustrated func¬ tionally in Figure 1. The parts of the air and flue gas paths, the feedwater/steam system and other components of the plant, which are necessary for the invention, are schematically shown in Figure 2. Figure 3 illustrates alternative solutions to the supply of the pressurized air to the pressure vessel. The design and connection of the feedwater/steam system to an auxiliary boiler during start-up and to a condenser circuit during cooling are shown in Figures 4 and 5, respectively. An alternative connection which under special circumstances, especially when only part of the pressurized air passes the heat transfer surfaces in the air paths, provides increased efficiency is shown in Figure 6. DESCRIPTION OF THE PREFERRED EMBODIMENTS Limitation of temperature variations of pressurized air supplied to the fluidized bed according to the invention is illustrated in Figure 1. The air is supplied to a combustor 10, in the form of -a fluidized bed, through air paths 1, flue gases formed during the combustion 10 are discharged through flue gas paths 2 and heat is extracted from the plant and utilized through a feedwater/steam system 3. In a power plant with combustion in a pressurized fluidized bed, a PFBC - Pressurized Fluidized Bed Combustion - plant, the combustion takes place in a fluidized bed 10 contained within a bed vessel 12 enclosed in a pressure vessel 11. Air is introduced into the plant at A, is pressurized in a compressor 13, the temperature being raised to a temperature which depends on the prevailing compression ratio and the ambient temperature. The pressurized air is used for fluidization of the fluidized bed 10 and for combustion of fuel supplied to the fluidized bed 10. The flue gases formed during the combustion pass through a gas turbine 14 arranged in the flue gas paths 2 of the plant, in which at least part of the energy contained in the flue gases is extracted. The compessor 13 is suitably driven by the gas turbine 14. In addition, to increase the efficiency of the plant, the residual heat is extracted from the flue gases in heat transfer surfaces 15, 16, arranged in both the hot and cold sections of the flue gas paths 2, for example flue gas economizers, designated the hot 15 and the cold 16 flue gas economizer, respectively, before the flue gases are discharged from the plant at B. In order not to subject the pressure vessel 11 or other " components, arranged in the pressure vessel 11 or the bed vessel 12,. to high temperatures, these are cooled by supplied pressurized air. To limit the temperature of the supplied pressurized air and to correct for temperature variations, caused by ambient temperature and compression ratios, the pressurized air passes through heat transfer surfaces 17, for example a heat exchanger, arranged in the air paths 1 beween the compressor 13 and the pressure vessel 11. The temperature variations, which are caused by fluc¬ tuating ambient temperature or compression ratios, are corrected according to the invention in the heat exchanger 17, which means that the efficiency of the plant is not affected by these temperature fluctuations while at the same time energy extracted in the heat exchanger 17 is utilized in the feedwater/steam system 3 of the plant. The temperature of the pressurized air is measured in conventional manner, for example by thermocouples, in the air paths downstream of the compressor 13. The measured temperature is compared with the desired temperature in a conventional temperature regulator (not shown) . The deviation gives rise to an output signal, control signal, to a control valve 18. The control valve 18 controls the heat work in the heat exchanger 17 by varying the flow of feedwater/steam through the heat exchanger l~f, for example via the by-pass duct 19. Variations in the feedwater/steam temperature arising downstream of the heat exchanger 17 are measured in conventional manner and corrected when the hot flue gases, in the hot flue gas economizer 15, pass through the feedwater/steam system 3 resulting in the flue gas temperature downstream of the hot flue gas economizer 15 being influenced. The influence on the flue gas temperature downstream of the hot flue gas economizer 15 is measured in conventional manner and, after treatment in a conventional temperature regulator (not shown) , supplies a control signal to a control valve 20. The control valve 20 then controls the heat work in the cold flue gas economizer 16, for example by distributing the feedwater/steam flow between the two branches 21 of the feedwater/steam circuit 3, comprising the cold flue gas economizer 16, and 22, comprising heat transfer surfaces 23 for heating another medium, for example high pressure feedwater. Where necessary or if the branch 22 is missing, feedwater/steam is conducted, at least partially, past _the cold flue gas economizer 16, preferably via a by-pass duct 24. By integration of the heat transfer surfaces 15, 16, 17, arranged in the air paths 1 and the flue gas paths 3, into the feedwater/steam system 3 of the power plant, the invention provides a limitation of the temperature of compressed air supplied to the pressure vessel and the bed vessel while at the same time temperature variations in this air are essentially eliminated. This means that the efficiency and power output of the plant remain essentially unaffected by variations in ambient temperature and com¬ pression ratios. Energy extracted from air and flue gases is transferred to • the feedwater/steam system 3 of the power plant. The heat transfer surfaces 15, 16, 17, which are necessary according to the invention, are connected at the point C, for example to a feedwater tank, and at the point D, for example to a boiler arranged in the fluidized bed 10, to the high tempe¬ rature section of the feedwater/steam system 3. In certain situations, for example during start-up and shutdown of the power plant, the heat transfer surfaces may be connected to a circuit by being interconnected at C and D. If the circuit is then provided with steam or cold water, heating and cooling, respectively, of air paths 1 and flue gas paths 2 may be obtained. Figure 2 schematically shows how the heat transfer surfaces, which are necessary for the invention, are arranged in the air paths 1, flue gas paths 2 and feedwater/steam system 3 of the power plant. In a PFBC plant pressurized air is supplied to a fluidized bed 10 enclosed in a pressure vessel 11. The air is supplied to the fluidized bed 10 for fluidization of the bed material and for combustion of fuel supplied to the fluidized bed 10. The air, which is admitted from the environment via at least one controllable throttle valve 25, is pressurized in a compressor 13, suitably driven by a gas turbine 14 arranged in the flue gas paths. The gas turbine 14 also drives a generator 26. The gas turbine 14 and the compressor 13 are often integrated into one unit and may be of an arbitrary type with a variable number of shafts. The figures show no intermediate cooling of the pressurized air, which occurs in multi-shaft units. The mass flow of pressurized air to the pressure vessel 11 in a PFBC plant is controlled within an interval of 40-105% of nominal flow. The mass flow from the compressor 13 may, depending on the type of gas turbine/compressor unit 14/13, be controlled in different ways. A single-shaft gas tur- bine/compressor unit 14/13, as indicated in Figure 2, may be controlled by adjusting the throttle valve 25, the com¬ pressor guide vanes 27 and via a recirculation circuit 28 for pressurized air. For a multi-shaft gas turbine/com¬ pressor unit, the possibilities of varying turbine guide vanes, turbine nozzles and rotor speed are added. The temperature of the pressurized air usually amounts to 350-450°C, depending on compression ratio and ambient tem¬ perature. . Before the pressurized ' air is supplied to the pressure vessel 11, it is cooled to a temperature suitable for the pressure vessel 11 and the parts enclosed in the pressure vessel 11, usually 200-300°C, in at least one heat exchanger 17 arranged in the air paths . According to the invention, the heat exchanger 17 is arranged in the high temperature section of the feedwater/steam .system 3, up¬ stream of a flue gas economizer 15 arranged in the hot part of the flue gas paths 2. To maintain the temperature of pressurized air supplied to the pressure vessel 11 essentially independent of com¬ pression ratio and ambient temperature, the feedwater/steam flow through the heat exchanger 17 is controlled in a control valve 18. The control valve 18 distributes the feedwater/steam flow, between the heat exchanger 17 and a by-pass duct 19, based on the deviation between desired and measured temperature of the pressurized air. With the by- ' pass duct 19, the feedwater/steam flow is adapted to the measured temperature of the pressurized air. Without the by-pass duct 19, there would be a risk of the feedwater temperature and hence the temperature of air supplied to the pressure vessel 11 dropping towards the ambient temperature. The control in the heat exchanger 17 gives rise to variations of the feedwater/steam temperature downstream of the heat exchanger 17, which are essentially eliminated in at least one flue gas economizer 15 arranged in the hot section of the flue gas paths 3, resulting in the flue gas temperature downstream of the hot flue gas economizer 15 being affected. The influence on the flue gas temperature is essentially eliminated in at least one flue gas econo¬ mizer 16 arranged in the cold section of the flue gas paths 3 by adapting the feedwater/steam flow therethrough to correct, in conventional manner, any deviation, measured in the flue gas paths 3 downstream of the hot flue gas eco¬ nomizer 15, of the flue gas temperature relative to the desired flue gas temperature. The control of the feedwater/steam flow through the cold flue gas economizer is performed with the control valve 20 which controls the distribution between the two parallel branches 21 and 22 in the feedwater/steam system 3, inclu¬ ding the cold flue gas economizer 16 and the heat exchanger 23, respect-ively, connected for heating of another medium, for example high-pressure feedwater. With heat transfer surfaces comprising at least one heat exchanger 17 arranged in the air paths, in which the temperature of air supplied to the pressure vessel 11 and the fluidized bed 10 is limited and temperature variations in the air are essentially eliminated, at least one flue gas economizer 15 arranged in the hot section of the flue gas paths, in which simultaneously with the flue gas temperature being reduced temperature variations of the feedwater/steam are essentially eliminated by allowing the flue gas tempe¬ rature downstream of the hot flue gas economizer 15 to vary, at least one flue gas economizer 16 arranged in the cold section of the flue gas paths, in which variations of the flue gas temperature are essentially eliminated, and the by¬ pass ducts 18 and 24 for control of the heat work in the heat exchanger 17 and the cold flue gas economizer 16, respectively, according to the invention a limitation of the temperature of air supplied to the pressure vessel 11 and of flue gases emitted from the PFBC plant is obtained while at the same time the influence from ambient temperature and compression ratios on the efficiency or the power output of the plant is essentially eliminated. The heat exchanger 17 can be dimensioned for two cases: I Maximum heat work for the operation at the maximum air temperature and full air flow; II Only part of the heat work of the operation, which means that part of the pressurized air is conducted past the heat exchanger 17 in a pipe 29 direct to the air inlet to the fluidized bed 10. The two cases are illustrated in Figure 3. Case I corresponds well with the previous description whereas in case II only part of the air quantity from the compressor 13 passes through the heat exchanger 17 ' . The remaining air quantity is supplied, via a pipe 29, to the cooled air flow near the air inlet to the fluidized bed 10. The distribution of air is controlled such that the heat work in the heat exchanger 17 is maintained constant, that is, an increased ambient temperature entails an increased flow via the pipe 29. Case II means that the temperature of vital components such as pressure vessel 11, bed vessel 12 and cyclones 30 may be limited with a heat exchanger 17 of limited power. During start-up of a PFBC plant, air paths 1 and flue gas paths 2 are preheated according to Figure 4. Preheating is usually performed by burning fossil fuels in the air paths 1 upstream of the fluidized bed 10. To avoid corrosion connected with flue gas condensate, components included in the air paths 1 and the flue gas paths 2 must be preheated, for example with dry hot air, to a temperature exceeding the dew point of the flue gases which occur during the pre¬ heating. This first phase of the preheating is achieved in a favourable way by connecting the heat transfer surfaces - the heat exchanger 17, the hot flue gas economizer 15 and the cold flue gas economizer 16 -, which according to the invention are interconnected and arranged in the air paths 1 and the flue gas paths 2, to an external source (not shown) with hot medium, for example a boiler present in the plant and intended to supply the plant with de-aired water during the start-up stage. During the starting perod the gas turbine 14 is driven by a starting device 31, which may consist of a frequency con- vertor which permits the gas turbine 14 to be run as a syn¬ chronous motor, but may also consist of a motor connected to any of the shafts of the gas turbine 14, or other starting equipment for gas turbines . The air is heated in the heat exchanger 17, the hot flue gas economizer 15 and the cold flue gas economizer 16 and transfers the heat to walls and other components in the air paths 1 and the flue gas paths 2. If the bed vessel 12 is empty and the valve 32 shown in Figures 2 and 3 is open, the air will flow through the pressure vessel 11 and the bed vessel 12 thus heating these. The heat exchanger 17, the hot flue gas economizer 15 and the cold flue gas economizer 16 are connected in a starting circuit, which is illustrated in Figure 4. As before, the heat transfer surfaces 15, 16, 17 are connected to the high temperature section of the feedwater/steam system 3 of the plant, for example at an existing feedwater tank 33. The feedwater tank 33 is provided with steam, for example from an auxiliary boiler (not shown) present in the plant. The feedwater/steam circulates during the starting stage from the feedwater tank 33 through the two flue gas economizers 15 and 16 and the heat exchanger 17 and back to the feed¬ water tank 33 via the open return pipe 34. During shutdown of the plant, the cooling period can be shortened by utilizing the heat transfer surfaces 15, 16 and 17 arranged in the air paths 1 and the flue gas paths 2 according to the invention. This makes the plant more rapidly available for, for example, maintenance work. The heat transfer surfaces 15, 16 and 17 are connected (see Figure 5) to an external source with a coolant, for example a condenser circuit located in the plant for hot water production, via a valve 35. This causes the heat transfer surfaces 15, 16 and 17 arranged in the air paths 1 and the flue gas paths 2 to be traversed by a cold medium and the temperature in air and flue gas paths to be rapidly reduced. An alternative solution of the arrangement of the heat exchanger 17 in the system, in relation to the hot flue gas economizer 15, is shown in Figure 6. The heat exchanger 17 is connected in parallel with the hot flue gas economizer 15, which reduces the temperature difference between air and feedwater/steam in the heat exchanger 17. Especially when dimensioning the heat exchanger 17 in accordance with the above case II, this solution may further increase the efficiency of the plant.

Claims: CLAIMS 1. A method for limiting temperature variations in a plant for combustion in a pressurized fluidized bed, a PFBC - Pressurized Fluidized Bed Combustion - plant, in which air is supplied to a combustor (10) , in the form of a pressu¬ rized fluidized bed, through air paths (1), the air is pressurized in a compressor (13), heat is utilized in a feedwater/steam system (3) comprising heat transfer surfaces (15, 16, 17) arranged in the air and flue gas paths, and energy contained in the flue gases is partially extracted with a gas turbine (14) arranged in the flue gas paths (2) of the plant, characterized in that temperature variations in pressurized air supplied to the fluidized bed are essentially eliminated with heat transfer surfaces (17) , arranged in the air paths (1) , preferably in the form of a heat exchanger connected to the high temperature section of the feedwater/steam system (3) of the PFBC plant. 2. A method according to claim 1, characterized in that temperature variations in discharged flue gases are limited with heat transfer surfaces (15, 16) , arranged in the flue gas paths (2) , in the form of cold and hot flue gas economizers, that the heat transfer surfaces are inter¬ connected in the high temperature section of the feed¬ water/steam system (3) of the PFBC plant, and that the heat work is controlled and distributed in and between the heat transfer surfaces (15, 16, 17) arranged in the air and flue gas paths. 3. A method according to claim 2, characterized in that the heat work in the heat exchanger (17) is controlled to limit temperature variations in pressurized air supplied to the fluidized bed (10) , variations in the feedwater/steam temperature downstream of the heat exchanger thus arising, which are essentially eliminated in a hot flue gas eco¬ nomizer (15) arranged in the hot section of the flue gas paths, the flue gas temperature thus being influenced and this influence being essentially eliminated in a cold flue gas economizer (16) , arranged in the cold section of the flue gas paths, by controlling the feedw_ater/steam flow through the cold flue gas economizer. 4. A method according to claim 3, characterized in that at least part of the pressurized air supplied to the flui¬ dized bed (10) is supplied to the fluidized bed without passing through the heat exchanger (17) arranged in the air paths . 5. A device for limiting temperature variations in a plant for combustion in a pressurized fluidized bed, a PFBC plant, comprising a combustor (10) in the form of a fluidized bed enclosed in a pressure vessel, air paths (1) in which a compressor (13) is arranged for pressurizing air supplied to the fluidized bed, flue gas paths (2) in which a gas turbine (14) is arranged for partially extracting energy contained in the flue gases, and a feedwater/steam system (3) com¬ prising heat transfer surfaces (15, 16, 17) adapted for utilizing heat, characterized in that at least one heat transfer surface, preferably in the form of a heat exchanger (17) , is arranged in the air paths to essentially eliminate temperature variations in pressurized air supplied to the fluidized bed (10) , and that the heat exchanger is connected to the high temperature section of the feedwater/steam system of the PFBC plant . 6. A device according to claim 5, characterized in that at least one flue gas economizer (15) arranged in the hot section of the flue gas paths and at least one flue gas economizer (16) arranged in the cold section of the flue gas paths, together with a heat exchanger (17) arranged in the air paths (2), are arranged interconnected in the high temperature section of the feedwater/steam system of the PFBC plant, and that distributing valves (18, 20) are arranged adjacent to the heat transfer surfaces to control and distribute the heat work in and between the heat transfer surfaces so that temperature- variations in air supplied to the combustor and in flue gases discharged from the combustion plant are essentially eliminated. 7. A. device according to claim 6, characterized in that the heat exchanger (17) arranged in the air paths is provided with a control valve (18) to distribute the feedwater/steam flow between the heat exchanger and a by¬ pass duct (19) , arranged adjacent to the heat exchanger, for controlling the heat work, that a flue gas economizer (16) provided in the hot section of the flue gas paths is adapted to allow the temperature of the flue gases to vary to essentially eliminate variations in the feedwater/steam temperature and that a flue gas economizer (15) arranged in the cold section of the flue gas paths is provided with a control valve (20) to control the feedwater/steam flow through the flue gas economizer and control the heat work. 8. A device according to claim 7, characterized in that the heat exchanger (17) , arranged in the air paths, in the feedwater/steam circuit is arranged upstream of and series- connected to the flue gas economizer (15) arranged in the hot section of the flue gas paths. 9. A device according to claim 7, characterized in that the heat exchanger (17) , arranged in the air paths, in the feedwater/steam circuit is arranged in parallel with the flue gas economizer (15) arranged in the hot section of the flue gas paths .

Inventors: KEMMER LEIF, MUKHERJEE DILIP, TJELLANDER GOERAN, KEMMER, LEIF, MUKHERJEE, DILIP, TJELLANDER, GOERAN
Applicants: ABB STAL AB
Application year: 1991
Main classification: F22B31
IPC codes: F01K23, F22B1, F22B31, F23C10