PROTECTING COMPONENTS OF FLUIDISED BED BOILERS

Abstract: A system for protecting components, including heat exchangers (10, 11), of a fluidised bed boiler against thermal mismatch, includes an injection tank (1) containing an emergency supply of heated and pressurised feedwater. A heater (7) is associated with the injection tank (1) to maintain the temperature of the feedwater in the tank at or about the same temperature as that of the feedwater in the heat exchangers (10, 11). A pressurised gas is supplied (45) to the injection tank (1) to cause feedwater to flow from the injection tank (1) to the heat exchangers (10, 11) during thermal mismatch.

Description: This invention relates to protecting components of fluidised bed boilers. Once-through circulation of fluidised bed boilers requires an inventory system to maintain coolant in the event of a loss of normal coolant flow. A standby pump and storage tank for supplying coolant to an economiser of a boiler has been proposed in the past. However, rapid depressurisation and thermal shocking when using low temperature coolants present a problem in that damage is likely to result to heat exchanger tubes and attached components. A Babcock and Wilcox Technical Paper entitled "The Fast Fluidized Bed - a True Multi-Fuel Boiler" by L Stromberg et al, presented to the Eight International Conference of Fluidized-Bed Combustion, Houston, Texas, U.S.A, 18 to 21 March 1985, discloses the structure and operation of a fast fluidised bed boiler utilising enclosure wall, bed, superheater and economiser heat exchangers. US Patent No US-A-4 563 267 (J.J Graham et al) discloses the problems of thermal shock for steam generator coils of a fluidised bed reactor when the reactor is subjected to load changes. A Babcock and Wilcox Technical Paper entitled "The Babcock & Wilcox Atmospheric Fluidized Bed Combustion Development Program" by J.W. Smith, presented to the Southeastern Electric Exchange, 1982 Annual Conference, Kissimmee, Florida, U.S.A, 21 to 23 April 1982, discloses the structure and operation of atmospheric fluidised bed combustors. According to this technical paper, the fluidised bed in such combustors is at a temperature range of 816° to 871° (1500°F to 1600°F. Fluidised bed combustors having tubular heat exchanges at various locations throughout the combustion gas flow path, as well as on enclosure walls of the combustor, are disclosed in US Patents Nos US-A-4 542 716 (J. Dreuilhe et al) and us-A-4 614 167 (J. Bergkvist). Document VGB Kraftwerkstechnik, vol. 67, June 1987, pages 556-559; K. Maintok, P. Winklhofer, reveals an emergency cooling system for a fluidised bed boiler. According to a first aspect of the invention there is provided a system for protecting components of a fluidised bed boiler against thermal mismatch during transient operations, the components including at least one heat exchanger communicating at one end with a feed coolant for supplying feed coolant to be heated and vapourised under pressure, and communicating at an opposite end with a separator for separating liquid coolant from vapourised coolant, the system comprising a coolant injection tank communicating with the feed coolant line for supplying coolant to the heat exchanger; characterised by valve means for opening and closing communication between the injection tank and the feed coolant line; heating means in the injection tank for heating the coolant therein to about the temperature of the feed coolant in the heat exchanger, the valve means being operable upon the occurrence of a thermal mismatch condition in the heat exchanger; and pressurisation means for pressurising the injection tank to cause the coolant therein to flow through the heat exchanger during said thermal mismatch condition. According to a second aspect of the invention there is provided a method of protecting components of a fluidised bed boiler against thermal mismatch during transient operations, the components including at least one heat exchanger communicating at one end with a feed coolant line supplying feed coolant to be heated and vapourised under pressure, and communicating at an opposite end with a separator for separating liquid coolant from vapourised coolant, the method comprising: connecting an injection tank with the separator for filling the injection tank with warmed liquid coolant during start-up of the fluidised bed boiler; and connecting the injection tank with the feed coolant line ahead of the heat exchanger; characterised by heating the coolant in the injection tank to maintain the temperature of the coolant in the injection tank at about the temperature of the feed coolant in the heat exchanger; and pressurising the injection tank so that, during the occurrence of a thermal mismatch condition, the coolant in the injection tank is caused to flow from the injection tank to the heat exchanger. A preferred embodiment of the invention described in detail hereinbelow provides an apparatus (system) and method for supplying cooling liquid to the interior of heat exchanger tubes for a fluidised bed boiler under emergency conditions to avoid rapid depressurisation and thermal shock. The preferred system protects heat exchanger tubes of a fluidised bed boiler against thermal mismatch during transient operations, such as start-up and shutdown. The fluidised bed boiler has at least one tubular heat exchanger which is supplied at one end with a coolant such as feedwater to be heated under pressure. The opposite end of the tubular heat exchanger is connected to a separator, preferably of vertical orientation, for separating the steam-water mixture discharging from the tubular heat exchanger. An injection tank storing a supply of water is connected to the tubular heat exchanger through piping fitted with valves which can open and close communication between the injection tank and the tubular heat exchanger. The injection tank can be filled, warmed and pressurised as the fluidised bed boiler is started up using feedwater from the steam-water separator. At higher loads, a heater provided in or around the injection tank maintains the temperature of the feedwater in the tank at about the temperature of the feedwater in the tubular heat exchanger. A source of pressurised gas maintains the necessary pressure to cause the water in the injection tank to flow through the tubular heat exchanger under emergency conditions. When emergency conditions occur that prevent protection of the heat exchanger tubes through the normal feedwater supply, the preferred emergency bed cooling system will activate after a selected time delay to establish a flow of pressurised and preheated feedwater from the injection tank to the heat exchanger tubes. Thermal shock is avoided by maintaining the temperature of feedwater in the injection tank at about the temperature of the feedwater in the tubular heat exchanger. Advantages of the preferred system include the fact that major components of the system are used during start up operations to improve operating characteristics. Thermal shock and rapid depressurisation are much less severe on boiler components. Immediate injection ability for high flow demand, as well as lower flow rates that are required later during the operation of the boiler, are both provided by the system. The injection tank can be initially warmed up and matched with boiler feedwater temperature and pressure with less wasted energy. At high loads, the maintenance of thermal conditions for the emergency feedwater has much smaller energy requirements and does not need additional costly equipment. The invention will now be further described, by way of illustrative and non-limiting example, with reference to the accompanying drawing, the sole figure of which is a schematic block diagram of the above-mentioned preferred system embodying the invention for protecting components of a fluidised bed boiler. The drawing shows a system embodying the invention for protecting a fluidised bed boiler against thermal mismatch during transient operations, such as start-up, shutdown and emergency conditions. The boiler has at least one tubular heat exchanger which is shown as a boiler enclosure surface 10 and an evaporation surface 11. A control valve 12 regulates the quantity of feedwater being supplied to an economiser 13. Heated feedwater discharged from the economiser 13 is conveyed through a feedwater line 24 for further heating as it passes through the boiler enclosure surface 10 and the evaporation surface 11. A steam-water mixture discharging from the evaporation surface 11 is conveyed through a steam-water line 25 to a steam-water separator 6. THe steam is separated out of the mixture and is conveyed to one or more superheaters 18. The superheated steam is then conveyed through a steam line 26 to branch lines 26A and 26B, the former conveying the steam to a steam turbine (not shown) and the latter bypassing the steam turbine and including a control valve 21 which regulates the steam flow during turbine start-up or shutdown. The steam line 26 connects to a vent line 27 which includes a pressure control valve 17 for regulating the depressurisation and evaporative cooling of the superheater(s) 18. A valve 6A is located in the steam line 26 at the discharge side of the steam-water separator 6. The valve 6A can be throttled during start-up and shutdown of the fluidised bed boiler to increase the steam pressure in the separator 6. In the event that the valve 17 becomes inoperative, the valve 6A can be used to regulated the depressurisation and evaporative cooling of the superheater(s) 18. A bypass line 25A connects the steam-water line 25 with the steam line 26 and includes a valve 6B which can be regulated to bypass steam-round the separator 6 during high load operation, thereby reducing pressure loss. A condensate line 30 connects the lower end of the separator 6 with a condenser (not shown) and provides means for discharging feedwater from the separator 6 to the condenser. A feedwater filling line 29 is connected to the condensate line 30 and includes a control valve 8 which operates to ensure that the separator 6 will be supplied with the minimum feedwater required to maintain a net positive suction head for a boiler circulation pump 9. The feedwater level in the separator 6 is monitored by a controller 42 through a transducer 40. The controller 42 may be connected to the control valve 8 to supply feedwater to the separator 6 when required. The condensate line 30 includes valves 4 and 5 and is connected with an injection feedwater supply line 36 and a feedwater injection tank 1 through a crossover line 32 and a tank overflow line 34. The lines 32 and 34 include valves 2 and 3, respectively. A bypass line 32A is provided around the valve 2 and includes a non-return valve 2A which admits feedwater flow to the injection tank 1 from the separator 6 at all loads, thereby maintaining the injection tank pressure at or near the vertical separator pressure. The valves 2, 3, 4 and 5 provide means for selectively routing the flow of feedwater and condensate to and from the separator 6 and the injection tank 1, and the flow of condensate from the separator 6 to the condenser (not shown). The injection tank 1 is activated by introducing a pressurised gas such as nitrogen through a gas line 45. The pressure in the injection tank 1 is regulated by a gas control valve 15 to cause the feedwater to flow from the injection tank 1 through the boiler enclosure surface 10 and the evaporation surface 11 when a control valve 14 in the injection feedwater supply line 36 is opened due to emergency conditions. A heater 7 is located within the injection tank 1 so that, at higher boiler loads, the temperature of the feedwater within the injection tank 1 is maintained at or about the same temperature as the temperature of the feedwater in the boiler enclosure surface 10 and the evaporation surface 11. A feedwater fill pump 19 delivers make-up water from one or more storage tanks (not shown) to the injection tank 1. A valve 20 is situated on the discharge side of the pump 19 to admit make-up water to the injection tank 1. The bottom of the separator 6 is connected to a circulation line 28 which branches into a natural circulation line 28A and a boiler circulation pump inlet line 28B, the latter discharging to the boiler circulation pump 9, which is powered by the plant electrical system or by a standby diesel generator 16. The pump 9 is connected by way of a discharge line 44, the injection feedwater supply line 36 and the feedwater line 24 to the boiler enclosure surface 10 and the evaporation surface 11 to circulate vertical separator water therethrough during cool-down of the bed. The line 28A includes a natural circulation valve 22 which, when opened, allows thermally induced (natural) circulation between the separator 6 and the boiler enclosure surface 10 and the evaporation surface 11 after shutdown of the pump 9. A line 28C includes a valve 9C and interconnects the condensate line 30 and the boiler circulation discharge pump line 44 to accommodate the minimum recirculation flow required to protect the pump 9. The discharge line 44 includes a control valve 9A and a non-return valve 9B to regulate the output from the pump 9. A line 46 interconnects the injection feedwater supply line 36 with the line 44 at the discharge end of the pump 9 to circulate feedwater for warming the pump 9 when the latter is out of service. The line 46 includes a control valve 9D and a non-return valve 9E. The boiler enclosure surface 10 comprises heat exchanger tubes disposed in side-by-side fashion to form an enclosure which contains the fluidised bed. The evaporation surface 11 comprises bundles of heat exchanger tubes immersed in the fluidised bed. The boiler enclosure surface 10 and the evaporation surface 11 are of conventional design, well known in the field of fluidised bed boilers. In use of the system, the injection tank 1 is filled, warmed and pressurised as the fluidised bed boiler is started up. The valves 2A, 3 and 5 are opened to allow feedwater to flow from the separator 6 to the injection tank 1. The valve in opens at cold start-up to allow flow to the condenser via the valve 5 without flooding the vertical separator 6. During operation of the fluidised bed, the valves 2 and 3 are normally closed and, at higher loads, the heater 7 is activated to maintain the feedwater temperature in the injection tank 1 at substantially the same temperature as that of the feedwater flowing through the boiler enclosure surface 10 and the evaporation surface 11. As feedwater flow approaches a minimum requirement at low loads or under transient operating conditions, the separator 6 begins to run dry. Under such conditions, the control valve 8 will open to supply feedwater to the separator 6, thereby maintaining the required net positive suction head pressure for the circulation pump 9. The control valve 8 may also be opened at higher loads to maintain the feedwater in the separator 6 at the level required to allow starting of the boiler circulation pump 9, when necessary. The emergency bed cooling system will activate after a selected time delay upon the occurrence of conditions which prevent protection of the boiler enclosure surface 10 and the evaporation surface 11 by the normal means of feedwater flow from the economiser 13 as regulated by the control valve 12. When the emergency bed cooling system is activated, firing of the fluidised bed will be stopped, the injection feedwater control valve 14 will open, and the gas control valve 15 will regulate the gas pressure in the injection tank 1 to maintain up to 100% maximum continuous rated feedwater flow for about one minute or until feedwater cooling demand is reduced. If the circulation pump 9 is not in service and the separator 6 does not contain the required level of feedwater, the valve 2 is opened to establish the feedwater level in the separator 6 which will allow the starting of the pump 9. As soon as the pump 9 is able to deliver the selected feedwater flow to the boiler enclosure 10 and the evaporation surface 11, the flow of feedwater from the injection tank 1 through the control valve 14 is discontinued. When the boiler enclosure 10 and the evaporation surface 11 have been cooled down to safe temperature levels, the pump 9 may be shut down and the natural circulation valve 22 opened. In the event of a total plant shutdown condition, the normal flow paths are stopped, causing the entire fluidised bed to be isolated. During this condition, the pressure control valve 17 will open to regulate the depressurisation and evaporative cooling of the superheater(s) 18. The feedwater lost during the evaporative cooling of the superheater(s) 18 will be replaced through the emergency bed cooling system by activating the condensate fill pump 19 to deliver make-up feedwater to the injection tank 1 from the one or more storage tanks (not shown). The flow of make-up feedwater to the injection tank 1 is provided by the valve 20.

Claims: A system for protecting components of a fluidised bed boiler against thermal mismatch during transient operations, the components including at least one heat exchanger (10, 11) communicating at one end with a feed coolant line (24) for supplying feed coolant to be heated and vapourised under pressure, and communicating at an opposite end with a separator (6) for separating liquid coolant from vapourised coolant, the system comprising a coolant injection tank (1) communicating with the feed coolant line (24) for supplying coolant to the heat exchanger (10, 11); characterised by valve means (14) for opening and closing communication between the injection tank (1) and the feed coolant line (24); heating means (7) in the injection tank (1) for heating the coolant therein to about the temperature of the feed coolant in the heat exchanger (10, 11), the valve means (14) being operable upon the occurrence of a thermal mismatch condition in the heat exchanger (10, 11); and pressurisation means (15, 45) for pressurising the injection tank (1) to cause the coolant therein to flow through the heat exchanger (10, 11) during said thermal mismatch condition. A system according to claim 1, wherein the heating means includes an injection coolant filing line (32) connecting the separator (6) with the injection tank (1) for supplying warmed liquid coolant from the separator to the injection tank. A system according to claim 2, wherein the heating means includes a heater (7) associated with the injection tank (1) for heating the coolant in the injection tank. A system according to claim 1, claim 2 or claim 3, wherein the pressurisation means includes a source of pressurised gas, a gas line (45) connecting the source of pressurised gas with the injection tank (1), and a control valve (15) disposed in the gas line (45) for regulating the supply of pressurised gas from the source to the injection tank. A system according to any one of the preceding claims, including an injection coolant supply line (36) connecting the injection tank (1) with the feed coolant line (24), the valve means (14) comprising an injection feedwater control valve in the injection coolant supply line (36) for regulating the flow of coolant between the injection tank (1) and the feed coolant line (24). A system according to claim 5, including a condensate line (30) connecting the separator (6) with the injection coolant supply line (36) for delivering liquid coolant from the separator (6) to the injection tank (1). A system according to claim 6, including a coolant filling line (29) connected to the condensate line (30) and a control valve (8) disposed in the filling line (29) for regulating the supply of coolant to the separator (6) upon the occurrence of a low liquid coolant condition in the separator. A system according to claim 5, claim 6 or claim 7, wherein the heat exchanger (10, 11) comprises a boiler enclosure surface (10) and an evaporation surface (11) connected in series with the boiler enclosure surface, the injection coolant supply line (36) being connected to the feed coolant line (24) ahead of the boiler enclosure surface (10). A system according to any one of the preceding claims, including a circulation pump (9) communicating with the separator (6) and the heat exchanger (10, 11) for circulating liquid coolant from the separator to the heat exchanger. A system according to claim 9, including a standby generator (16) for generating electricity, the standby generator (16) being connected to the circulation pump (9) for supplying electrical power to the pump under emergency conditions. A system according to any one of the preceding claims, including a natural circulation line (28A) connected between the separator (6) and the feed coolant line (24), and a valve (22) disposed in the natural circulation line (28A), the valve being opened to permit natural circulation between the separator (6) and the heat exchanger (10, 11) when such circulation is possible and desired. A system according to any one of the preceding claims, including a fill pump (19) connected to the injection tank (1) for supplying coolant to the injection tank. A system according to claim 1, including means for supplying coolant from the separator (6) to the injection tank (1). A method of protecting components of a fluidised bed boiler against thermal mismatch during transient operations, the components including at least one heat exchanger (10, 11) communicating at one end with a feed coolant line (24) supplying feed coolant to be heated and vapourised under pressure, and communicating at an opposite end with a separator (6) for separating liquid coolant from vapourised coolant, the method comprising: connecting an injection tank (1) with the separator (6) for filling the injection tank with warmed liquid coolant during start-up of the fluidised bed boiler; and connecting the injection tank (1) with the feed coolant line (24) ahead of the heat exchanger (10, 11); characterised by heating the coolant in the injection tank (1) to maintain the temperature of the coolant in the injection tank (1) at about the temperature of the feed coolant in the heat exchanger (10, 11); and pressurising the injection tank (1) so that, during the occurrence of a thermal mismatch condition, the coolant in the injection tank is caused to flow from the injection tank to the heat exchanger (10, 11).

Inventors: CRISPIN, LARRY G., WEITZEL, PAUL S.
Applicants: THE BABCOCK & WILCOX COMPANY
Application year: 1990
Main classification: F22D11
IPC codes: F22D11