METHOD OF CONTROLLING A PFBC POWER PLANT AND A PFBC POWER PLANT WITH EQUIPMENT FOR SUCH CONTROL

Abstract: The invention relates to a method of controlling a PFBC power plant in the event of an operational disturbance af­ fecting a turbine unit driven by the plant, for example a load drop out, said plant comprising a bed vessel (12) en­ closed within a pressure vessel (10) and surrounded by com­ pressed combustion air, said bed vessel containing a bed (22) of fluidizable particulate material, and a gas turbine-­ compressor unit (34,38,48) driven by combustion gases from the bed vessel (12). In a hot gas conduit (32) between the bed vessel (12) and a turbine (34) there is a first cut-off valve (54), and in a conduit (50) between a compressor (48) and the bed vessel (12) there is a second cut-off valve (58). Between said conduits (32,50) there is a short-circuit conduit (62) with a third cut-off valve (64). In case of an operational disturbance as mentioned above, the turbine-com­ pressor unit (34,38,48) is cut off from the bed vessel (12) and the pressure vessel (10), respectively, by shutting off said first and second valves (54,58) in the conduits (32,50) between the bed vessel (12) and the turbine (34) and between the compressor (48) and the pressure vessel (10), respec­ tively. At the same time, the bypass valve (64) in the short-circuit conduit (62) is opened. In the event of a lengthy operational disturbance requiring shutdown of the power plant, the combustion gases in the bed vessel (12) are dumped into the atmosphere via a dump conduit (68) and a dump valve (72). When dumping the combustion gases, cooling water is injected into the dump conduit (68) upstream of the dump valve (72) so that the gases are cooled before reaching the dump valve (72). The invention also relates to a power plant having a device (70) for injection of cooling water into a conduit (68) upstream of a dump valve (72) for cool­ ing the hot combustion gases from the bed vessel (12) before they reach the dump valve (72).

Description: The invention relates to a method of controlling a Pressurized Fluidized Bed Combustion (PFBC) power plant according to the introductory part of claim 1, in the event of an operational disturbance affecting the gas turbine unit. The invention also relates to a PFBC power plant with equipment for such control. Such a method and power plant are known from the US-A-4 498 286. Several forms of operational disturbance may occur, for example excessive speed in the case of a load drop out, vibrations, pumping in a compressor, faults in an auxiliary system, or other stability problems resulting in a GT-trip (gas turbine trip, i.e. shutdown of the gas turbine) by opening of a short circuit valve between the gas conduit to the turbine and the air conduit from the compressor to the pressure vessel and by closing of valves in the afore-identified gas and air conduits. The intention is to protect the gas turbine and the compressor, that is, to reduce as fast as possible the speed as well as the pressure and temperature levels to harmless values. If the gas turbine unit of the plant cannot rapidly regain normal operating conditions, for example by re-synchronizing of the generator of a power turbine, the very large energy contents of the PFBC plant in the hot bed, in the surrounding pressure vessel and in unburnt fuel in the bed entails special and difficult problems to solve. When a rapid reconnection of the generator of a power turbine to its power network is not possible, the energy contents in the bed and in the unburnt fuel have to be removed in a unharming manner. When shutting off the turbine, the gas flow through the bed vessel and the turbine is reduced to a level determined by the unavoidable leakage flow through the valve in the hot gas conduit between the bed vessel and the turbine. The air supply to the bed becomes insufficient for fluidization of the bed and for complete combustion of the fuel present in the bed. This means that the fluidized bed collapses with the ensuing risk of the bed material sintering together and of formation of carbon monoxide (CO), which entails a risk of explosion and also means that the leakage flow contains energy-rich combustible gas. Combustion of this gas downstream of the cut-off valve may entail an impermissibly high gas temperature for the gas turbine and that the energy contents may give an impermissibly high speed of turbines in the plant. It has been proposed to blow off the hot combustion gases from the bed vessel of the plant into the atmosphere. The gases have a temperature of 800-900°C and are mixed with about 200 ppm dust. It is difficult - if not impossible - to keep a valve operating at such a high temperature and in such a severe environment to seal. In addition, it is very difficult to satisfactorily clean the very large flow of gas at such a high temperature. A suitable valve would be expensive and its working life would be short. A proposal for reducing the inconvenience of leakage in a valve for blowing off combustion gases from the bed vessel is to blow off, at the same time, the compressed combustion air in a pressure vessel surrounding the bed vessel in the manner disclosed in US-A-4 498 285. The combustion gases are then mixed with air and cooled so that a valve in the pressure vessel wall is not subjected to gases of such extremely high temperatures. A disadvantage with this method is that compressed combustion air is to a large extent consumed for cooling of the combustion gases which leave the bed vessel and do not pass through the bed. This may mean that the fuel in the bed is not completely burnt and that the bed material will not be cooled to the desired extent. The combustible gases may entail a risk of explosion. A high bed temperature after a blow-off may result in the bed material sintering. In a valve in the bed vessel wall, a certain leakage into the bed vessel may be tolerated. However, the method is not completely satisfactory. EP-A-0 234 265 discloses a method of overcoming the problems by supplying an inert gas, suitably nitrogen, to the bed vessel when the turbine is being shut off. This interrupts the supply of oxygen to the bed and hence the combustion, thus preventing the formation of carbon monoxide and eliminating the risk of explosion. The invention aims at a method to discharge the energy alive in the power plant, in case of a disturbance of the kind explained above, in a manner not suffering from the above-mentioned disadvantages of the known methods and without having other detrimental effects. The invention also aims to develop a plant able to function in accordance with the method according to the invention. To achieve this aim the invention suggests a method 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 2 to 6. A power plant for implementing the method according to the invention is characterized by the features of claim 7. Further developments of the power plant according to the invention are characterized by the features of the additional claims 8 to 11. In the case of an operational disturbance resulting in a load drop out, the gas turbine of the plant or the gas turbine and the combustion air compressor is/are shut off in a known manner from the bed vessel or the pressure vessel, surrounding the bed vessel, by shutting off the valve in the hot gas conduit between the bed vessel and the turbine or the turbines and by shutting off the valve in the air conduit between the compressor and the pressure vessel. At the same time a short circuit valve is opened in a short-circuit conduit between the air conduit and the hot gas conduit, so that air from the compressor is passed directly to the turbine. A certain residual flow through the valve in the hot gas conduit is obtained owing to the fact that a complete sealing cannot be achieved in a valve which is subjected to hot gas at a temperature of 800-950°C. To shut down the power plant in the case of an operational disturbance, the remaining fuel in the bed must be burnt and the bed be cooled. Gas from the bed vessel is dumped to the atmosphere via a damping conduit extending directly from the bed vessel, or via a conduit extending from the hot gas conduit to the turbine the damping conduit being provided with a blow-off valve. The damping conduit is provided with an injection device for a coolant through which, when the combustion gases are dumped, coolant is injected in such an amount that the gases are efficiently cooled before they reach the blow-off valve. Suitably, coolant is injected in such an amount that the gases are cooled to a temperature of 400°C. When the gases are cooled to below 400°C, valves with satisfactory sealing, function, and a satisfactory working life of the valves can be obtained at reasonable costs. Water is a suitable coolant, but also other liquids, for example liquid nitrogen, may be used. Cooling water for injection into the blow-off or dump conduit is suitably stored in a container pressurized by gas. The container communicates with the injection device via a conduit with a valve which is operatable by means of an operating device connected to the control equipment of the power plant. The invention will now be described in greater detail with reference to the accompanying drawings showing - by way of example - in Figure 1 an embodiment of a power plant according to the invention provided with water injection devices in the freeboard of the bed vessel and in gas conduits arranged downstream of cyclones, Figure 2 a power plant similar to that in Figure 1 with a water injection device in a blow-off conduit extending directly from the bed vessel. In the Figures, 10 designates a pressure vessel. The pressure vessel accommodates a bed vessel 12 and a cleaning plant, symbolized by two series-connected cyclones 14 and 15. In reality, the cleaning plant comprises a number of parallel-connected groups of series-connected cyclones. The space 16 between the pressure vessel 10 and the bed vessel 12 contains compressed combustion air. The pressure may amount to 2 MPa or more. Combustion air is supplied to the bed vessel 12 via nozzles 18 at the bottom 20. This air fluidizes the bed 22 in the lower part of the bed vessel 12 and burns a fuel, usually coal, which is supplied to the bed 22 from a fuel storage (not shown) via the conduit 24. The bed 22 contains tubes 26 in which steam is generated and fed to a steam turbine (not shown) driving a generator. The combustion gases are collected in the freeboard 28 and are conducted in the conduits 30 and 31 to the cyclones 14 and 15, which represent a usually employed plurality of parallel-connected groups of series connected cyclones. From the cyclone 15 the hot gases are conducted via the conduit 32 to a high pressure turbine 34, from there via a conduit 36 to a low pressure turbine 38 and from there further via the conduit 40 to the economizer 42, the filter 44 and further, through the chimney 46, into the atmosphere. The high pressure turbine 34 drives the compressor 48. Via the conduit 50, the space 16 is supplied with compressed combustion air. The low-pressure turbine 38 drives a generator 52. The hot gas conduit 32 includes a cut-off valve 54 with an operating device 56. The conduit 50 from the compressor 48 is provided with a cut-off valve 58 with an operating device 60. Between the conduits 32 and 50 there is a short-circuit conduit 62 with a by-pass valve 64 operatable by an operating device 66. In the embodiment according to Figure 1, a passage way for dumping combustion gases from the bed vessel 12 is provided by a conduit 68 extending from the hot gas conduit 32 between cyclone 15 and turbine 34 and opening out into the economizer 42. The conduit 68 is provided with at least one dump valve 72 with an operating device 74. For security reasons two such valves are usually connected in series. A device 70 with nozzles 71 for injection of cooling water is provided at the outlet side of cyclone 15 or in the dump conduit 68. In addition there are a water injection device 70' with nozzles 71' in the conduit 31 between the cyclones 14 and 15 and a water injection device 83 with nozzles 87 in the freeboard 28 of the bed vessel 12. Injection of cooling water into the freeboard 28 of the bed vessel 12 entails a long dwell time of injected water in the hot gases for evaporation and good mixing in the cyclones. The injection of cooling water in the injection device 70 can thereby be limited. A plant can be constructed with one or more of the water injection devices 70,70' and 83 shown. The water injection devices 70,70' and 83 are connected to a pressurized cooling water tank 78 via the conduit 80, the valve 82 with the operating device 84 and the conduits 81,81' and 85. The water tank 78 communicates with a pressure gas source 88 via the conduit 86. Nitrogen gas is suitably used. The conduit 86 comprises a valve 90 with an operating device 92. The operating devices 56,60,66,74,84,92 of the valves 54,58,64,72,82,90 are connected by conduits 100-105 to an operating unit 110, which forms part of the central control equipment for the plant. A temperature sensor 112 in the dump conduit 68 is connected to the operating unit 110 via the conduit 114. In the embodiment according to Figure 2, the dump conduit 68 is connected directly to the freeboard 28 of the bed vessel 12. Between the dump valve 72 and the economizer 42 there is a gas cleaner 76 for separation of dust. In the case of an operational disturbance, for example a gas turbine trip following a load drop out, the fuel supply to the bed 22 is stopped and the valves 54 and 58 in the hot gas conduit 32 and in the air conduit 50, respectively, are shut off so that the turbines 34 and 38 and the compressor 48 are cut off from the bed vessel 12 and from the pressure vessel 10, respectively. At the same time, the by-pass valve 64 in the short-circuit conduit 62 is opened so that the air from the compressor 48 flows directly into the turbine 34. By interrupting the energy supply from the bed vessel 12, the power turbine 38 is prevented from racing. Thereafter, the valve 72 is opened and the combustion gases from the bed vessel 12 are dumped. The valve 82 is at the same time opened, and cooling water is injected into the hot gas stream through nozzles 71,71',87 in the water injection device 70,70',83 in the embodiment shown in Figure 1. The water is evaporated and lowers the temperature of the gases before they reach the blow-off valve 72. Dust is separated in the cyclones 14,15 or in the cleaner 76. The gases are cooled in the economizer 42 to a temperature tolerable for the filter 44, whereupon the gases are led to the chimney 46. If the same amount of water is injected per unit of time during the whole pressure reduction process, the gas temperature drops as the gas flow successively decreases. For this reason, the flow of cooling water should be controlled. The water injection is suitably controlled by the valve 82 so that the temperature of the gases when they enter the economizer 42 is equal to or somewhat lower than the temperature of the waste gases from the turbine during normal operation, i.e. about 400°C. This results in the lowest temperature stress on the economizer 42 when blowing off the combustion gases from the bed vessel 12. The capacity in the dump conduit 68 is chosen such that a sufficient fluidization of the bed 22 is obtained. During the first part of the dumping operation, fuel remaining in the bed 22 is burnt. When the fuel has been consumed, the air flowing through the bed 22 cools the bed material. Also the tubes 26, through which water and steam flow, contribute at the same time to the cooling of the bed material. When the compressed combustion air in the space 16 has been consumed, the bed material has been cooled to such an extent that the risk of it sintering together is eliminated. It is readily appreciated that the embodiment of Figure 1 and 2 can also be combined in many various ways.

Claims: Method of controlling a PFBC power plant in the event of an operational disturbance affecting a turbine unit driven by the plant, for example a load drop out, said plant comprising a bed vessel (12) enclosed within a pressure vessel (10) and surrounded by compressed combustion air, said bed vessel containing a bed (22) of fluidizable particulate material, and a gas turbine-compressor unit (34,38,48) driven by combustion gases from the bed vessel (12), whereby on the occurrence of a disturbance of the above-mentioned kind a first cut-off valve (54) in the supply conduit (32) for propellant gas from the bed vessel to the turbine (34) and a second cut-off valve (58) in the conduit (50) for the compressor to the pressure vessel are closed, a third cut-off valve (64) in a short circuit conduit (62) between the turbine (34) and the compressor (48) is opened, combustion gases from the bed vessel are dumped into the atmosphere via a blow-off valve (72) in a conduit (68), and said combustion gases, prior to their damping through said blow-off valve (72), are mixed with a coolant, characterized in that said coolant is a liquid. Method according to claim 1, characterized in that coolant is supplied in such a quantity that the combustion gases are cooled to a temperature below 400°C. Method according to claim 1 or 2, characterized in that the coolant flow is controlled such that the gases downstream of the coolant injection point attain a constant temperature during the blow-off process. Method according to any of the preceding claims, characterized in that the coolant flow is controlled such that the gases, downstream of the coolant injection point, attain a temperature below about 400°C but exceeding the acid dew point (120-180°C). Method according to any of the preceding claims, characterized in that the coolant is in the form of water. Method according to any of the preceding claims, characterized in that the coolant is supplied from a permanently pressurized coolant container. PFBC power plant comprising a pressure vessel (10), a bed vessel (12) enclosed within the pressure vessel (10), at least one gas turbine (34), a conduit (32) between the bed vessel (12) and a turbine (34) having a first valve (54), a compressor (48) driven by the turbine, a conduit (50) between the compressor (48) and the pressure vessel (10) with a second valve (58), a short-circuit conduit (62) between the compressor (48) and the turbine (34) with a third valve (64), a blow-off valve (72) for dumping gases from the bed vessel (10) in the case of an operational disturbance, and a device (70,70') for injecting a coolant into the gas stream upstream of said blow-off valve (72), characterized in that said device (70,70') is designed for the injection of a liquid coolant and that there is a device (78) for supplying a liquid coolant under a high pressure to the injection device (70,70'). PFBC power plant according to claim 7, characterized in that the blow-off valve (72) is arranged in a conduit (68) directly connected to the bed vessel. PFBC power plant according to claim 7, characterized in that the blow-off valve (72) is arranged in a conduit (68) connected to the conduit (32) supplying combustion gas to the gas turbine (32,38). PFBC power plant according to any of claims 7 to 9, characterized in that it comprises a device (83) for injection of a coolant into the freeboard (28) of the bed vessel (12). PFBC power plant according to any of claims 7 to 10, characterized in that a water injection device (70,70') for coolant is connected, via a conduit (80) with a valve (82), to a water container (78) pressurized by gas.

Inventors: BRAENNSTROEM, ROINE, Brännström, Roine
Applicants: ABB STAL AB
Application year: 1988
Main classification: F01D21
IPC codes: F23C10, F02C9, F02C3, F02C6