A METHOD AND A DEVICE FOR MAINTAINING A HIGH GAS TEMPERATURE IN A PFBC POWER PLANT

Abstract: The present invention relates to a method and a device for maintaining a high gas temperature to the gas turbine in a PFBC power plant, wherein gas is fed out from a fluidized bed (2) in a combustor (1) which also includes tubes (3) for cooling the bed (2), said tubes (3) being completely or partially covered by the fluidized bed (2) in dependance on the degree of load. The method is characterized in that hot combustion gases are discharged from the combustor (1) via outlets (5a) arranged at higher levels or via outlets (6a) arranged at lower levels in dependence on the degree of partial load.

Description: A method and a device for maintaining a high gas temperature in a PFBC power plant TECHNICAL FIELD The present invention relates to a method and a device for maintaining a high gas temperature to a gas turbine in connection with partial-load operation of a PFBC plant, in which gas is fed out from a fluidizable bed in a combustor which also includes tubes for generating steam, the tubes being completely or partially covered by the fluidized bed in dependence on the degree of load. BACKGROUND ART In operation of a PFBC (Eressurized Eluidised £ed Combus¬ tion) plant, a varying part of the tubes which are to gene¬ rate steam for a steam turbine in the plant will be covered by the fluidized bed in dependence on the degree of partial load. In case of full-load operation, the tubes are comple- tely covered, and in case of partial-load operation they are partially covered. One problem in this connection is that the non-covered tube portions cool the combustion gases which leave the bed at a temperature of about 850°C. A reduced temperature of the gases to the gas turbine entails, inter alia, a deteriorated efficiency of the plant, less good conditions for NOχ reduction, and an increased CO content in the waste gases from the plant. SUMMARY OF THE INVENTION The invention aims to provide a solution to the above- mentioned problems and other problems associated therewith and is characterized in that hot combustion gases are discharged from outlets located at different levels in the combustor in dependence on the degree of partial load. What characterizes the device according to the invention will become clear from the appended device claims. By discharging combustion gas via outlets at different levels, outlets at lower levels during low-load operation and outlets at higher levels during high-load operation, the cooling of the combustion gases is reduced because of the cooling effect of exposed tube portions. By means of this device the waste gas temperature can be maintained at the desired, high value. BRIEF DESCRIPTION OF THE DRAWING The invention will be exemplified with reference to the accompanying figures, wherein Figure 1 is a view showing the principle of a PFBC power plant according to the invention, and Figure 2 is a side view of the same plant. Figures 3 and 4 show features of the invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Figures 1 and 2 show a combustor 1 for a PFBC power plant, wherein the combustor 1 comprises a fluidizable bed 2 of a kind known per se (cf. EP 108 505) . This bed 2 accomodates tubes 3 for cooling the bed 2, which generate steam for steam turbines. In operation of the plant, in case of full load, the tubes 3 are normally fully covered by the fluidized bed 2. The combustion gases leaving the bed 2 to a freeboard 2a above the bed surface, have a high temperature, preferably 850°C. In partial-load operation of the plant, the bed level is lowered, parts of the tubes 3 thus being exposed such that portions of the tubes 3, tube portions 3a, will be located above the surface of the bed 2. Valve members 4 are arranged at the upper part of the com¬ bustion chamber in the combustor 1. For discharging combustion gases, two separate flue gas channels are provided. These consist primarily of a centrally arranged flue gas channel, a central channel 5, which may, per se, consist of and is suitably designed as several channels separated from each other. The central channel 5 is provided with a higher outlet 5a for flue gases from the freeboard 2a, which outlet 5a is located at a higher level in the combustor 1 than the tube portions 3a. Other channels for flue gases, side channels 6, are arranged adjacent to the. walls of the combustor 1. These side channels 6 are provided at their lower part with lower outlets 6a for flue gases from the freeboard 2a, which lower outlets 6a are located at a lower level in the combustor 1 than the outlets 5a, namely at the side of the upper tube portions 3a. This means that the flue gases which are forced out from the combustor 1 via the side channels 6 do not pass the tube portions 3a to any major extent and consequently are not cooled by these tube portions 3a. In full-load operation, the valve members 4 are arranged such that the central channel 5 is open and the side channels 6 closed, which means that the flue gases which leave the bed surface, which in case of full load is located at a higher level than the tube portions 3a, are passed out via the higher outlet 5a to the central channel 5. In partial-load operation, the surface of the fluidized bed 2 is lowered, whereby tube portions 3a are exposed and will be positioned above the bed surface of the freeboard 2a. The valve members 4 are adjusted such that the central channel 5 is closed completely or partially in dependence on the degree of partial load, while at the same time the side channels 6 are opened completely or partially to a corresponding extent, in dependence on the degree of partial load, the whole of or part of the flue gas flow thus being discharged from the combustor 1 at the side of the tube portions 3a via the lower outlets 6a to the side channels 6. In this way, the flue gases are passed out at the side of the tube portions 3a, the cooling from these tube portions 3a thus being reduced or completely stopped. A double set of central channels 5 and side channels 6, one channel of each type being located along the long sides of the combustor 1, is used in the embodiment shown in Figure 1 in a version of a combustor 1 having rectangular cross section. Between the central channel 5 and the side channel 6 a partition 7 is arranged. The tube portions 3a cover a smaller area of the horizontal cross section area of the combustor 1 than that cross section area which is occupied by the other tubes 3 in the entire set of steam tubes in the bed 2. At certain partial-load levels between low-load opera¬ tion and full-load operation of the plant, flue gases are removed both via the central channel 5 and via the side channels 6. Such operating modes occur where this is advantageous because of the limitations imposed on the gas turbine. Both central channels 5 and side channels 6 open out into one (or more) coarse cleaner cyclones 8, in which dust is separated from the flue gases and removed via the cyclone leg 11. Dust in the cyclone leg 11 is returned to the bed, for example via a so-called L-valve 12, by means of which dust is injected into the bed 2. Cleaned flue gases are discharged from the cyclone 8 at outlets in the uppermost section 13 of the cyclone. In the flue gas channels 5, 6 in the gas paths, there are arranged beams, such as U-beams 10, suitably arranged in a labyrinth to perform a coarse cleaning of the waste gases, coarser particles being captured by the labyrinth and returned to the bed. The coarse-cleaned waste gases are thereafter passed to the coarse cleaner cyclone 8. Figure 3 illustrates an embodiment with a combustor 1 designed with rectangular cross section, indicating a proposed location of the valve members . The flue gases are cleaned further by means of conventional dust separators, shown schematically in Figure 1 in the form of cyclones 9, adjacent the flue gas outlet from the coarse cleaner cyclone 8. Via nozzles 14 shown in Figure 1, NH 3 may be injected for reducing NO x in flue gases. Sorbent for sulphur cleaning (e.g. lime) can also be injected through corresponding nozzles 14. By adjustment of the simple valve members 4, the flow of waste gas from the bed 2 can thus be completely or partially passed by the upper tube portions 3a, which are exposed during partial-load operation, to the coarse cleaner cyclone 8. This coarse cleaner cyclone 8 returns more or less unburnt dust/ashes emanating from the bed 2 to the bed via the cyclone leg 11. Figure 4 illustrates how the valve member 4 is controlled from a drive member 16 positioned outside a partially shown pressure vessel wall in the pressure vessel which surrounds the combustor 1. The method and the device according to the above can be varied in many ways within the scope of the following claims.

Claims: CLAIMS 1. A method of maintaining a high gas temperature to the gas turbine in partial-load operation of a PFBC power plant, wherein gas is fed out from a fluidized bed (2) in a combustor (1) which also includes tubes (3) for cooling the bed (2) and for generating steam, said tubes (3) being completely or partially covered by the fluidized bed (2) in dependence on the degree of load, characterized in that hot combustion gases are discharged via outlets (5a, 6a) arranged at different levels of the combustor (1) in dependence on the degree of partial load. 2. A method according to claim 1, characterized in that in case of a low bed height, i.e. lower partial load, the gases are discharged from outlets (6a) located at lower levels, and that at certain intermediate-load levels, between low load and full load, the gases are discharged at both higher and lower outlets (5a, 6a) . 3. A method according to claim 1, characterized in that. at a high bed height, i.e. at full load, the gases are discharged via outlets (5a) located at a higher level. 4. A method according to claim 1 or 2, characterized in that at a low bed height, i.e. at partial load, the gases are discharged from the combustor (1) at the side of the upper tube portions (3a) exposed by the tubes (3) . 5. A method according to claim 1, characterized in that the gases are coarse-cleaned in a labyrinth of U-beams (10) before they leave the combustor (1) . 6. A method according to claim 5, characterized in that particles from the combustion gases, which are separated during the coarse cleaning, are returned to the fluidized bed (2) . 7. A method according to one or more of the preceding claims, characterized in that the outlets (5a, 6a) and the adaptation of the flue gas flow to the central channels (5) and the side channels (6), respectively, are controlled by valve members (4) , which are regulated in dependence on the degree of partial load. 8. A device for carrying out the method for maintaining a high gas temperature to a gas turbine in partial-load operation of a PFBC power plant, wherein gas is fed out from a fluidized bed (2) in a combustor (1) which also includes tubes (3) for cooling the bed (2) and for generating steam, said tubes (3) being completely or partially covered by the fluidized bed (2) in dependence on the degree of partial load, characterized in that gas can be discharged from the combustor (1) via outlets (5a, 6a) arranged at different levels of the combustor (1) . 9. A device according to claim 8, characterized in that lower outlets (6a) for flue gases are arranged at the side of the upper tube portions (3a) exposed by the tubes (3) , whereas higher outlets (5a) are arranged at a higher level than the highest possible surface of the bed (2) . 10. A device according to claim 8, characterized in that valve members (4) are arranged for opening and closing, respectively, completely or partially, the different gas outlets (5a, 6a) in dependence on the degree of partial load. 11. A device according to claim 8, characterized in that the outlets (5a, 6a) for gas channels open out into at least one coarse cleaner cyclone (8) . 12. A device according to claim 11, characterized in that the coarse cleaner cyclone (8) , or the cyclone leg (11) thereof, is connected to the bed space.

Inventors: ANDERSSON MATS, WEATHERBY JOHN, ANDERSSON, MATS, WEATHERBY, JOHN
Applicants: ABB CARBON AB
Application year: 1992
Main classification: F01K23
IPC codes: F01K23, F23C10