A PFBC-POWER PLANT

Abstract: A PFBC-power plant comprises a pressure vessel (2), which encloses a combustor (1) for combustion of a particle-like fuel in a pressurised fluidised bed, and at least two gas turbines (14, 20) located successively in the path of the flue gases produced by said combustion. The plant comprises also an arrangement (9-11) for supplying a fuel to a chamber (23) located in the flue gas path between the two gas turbines and a burner (35) for combustion of the fuel in this chamber for mixing the hot gases resulting therefrom with the gases expanded in the first gas turbine (14) so as to increase the temperature thereof before they are fed to the second gas turbine (20).

Description: A PFBC-power plant FIELD OF THE INVENTION AND PRIOR ART The present invention relates to a PFBC-power plant comprising a pressure vessel enclosing a combustor for combustion of a particle-like fuel in a pres¬ surised fluidised bed, and at least two gas tur¬ bines located successively in the path of the flue gases produced by said combustion. Such a power plant is already known through for ex¬ ample the Swedish patent 469 039 of the applicant, and the advantage of a PFBC-plant resides primarily in a small volume of the plant with respect to the useful power created in comparison with other types of plants in which the fuel is combusted in a flui¬ dised bed under atmospheric pressure conditions. Furthermore, the efficiency of a PFBC-plant is high and the combustion takes place under conditions be- ing favourable from the environmental and economi¬ cal point of view. However, a problem burdening the PFBC-technique has been the fact that a PFBC-power plant is difficult to control, especially depending on the large amounts of energy contained in the hot bed and which may not that easy be taken care of when rapid changes of the load are required. Inspite of the high efficiency there is still a de¬ sire to further increase it, primarily in partial load operation, and it is desired to provide means for improving the controllability of such a power plant. A suggestion to an improvement of the efficiency of the plant is described in inter alia the Swedish patent publication SE 8704075-4 and relates to a temperature increase of the flue gases deriving from the bed before these are supplied to the gas turbine located upstream thereof. The bed chemistry of a PFBC-power plant does namely not make any com¬ bustion of the fuel while obtaining flue gases hav- ing a temperature exceeding about 850-950°C possi¬ ble, and it is in principle this temperature that the propellant gases, i.e. the flue gases, have when they reach the gas turbine located upstream. However, the turbine power increases remarkably with an increased temperature of the propellant gas, which makes it desirable to obtain a higher gas temperature, most preferred within the region 1200-1500°C. This is obtained by arranging a so called topping combustor in the flue gas path up- stream of the gas turbine located upstream, to which a fuel is supplied and combusted while mixing the hot gases resulting therefrom with the flue gases for raising the temperature thereof before they reach the gas turbine. The flue gases will expand in the gas turbine lo¬ cated upstream and have the temperature thereof lowered. These gases are after that supplied to said second gas turbine for further expansion and recovering of further energy from the flue gases. The high pressure gas turbine may then drive a gen¬ erator for producing useful energy and a compressor for pressurising the bed, while the second gas tur¬ bine could drive only a compressor for pre-com- pressing air for the compressor first mentioned for pressurising the bed. SUMMARY OF THE INVENTION The object of the present invention is to provide a PFBC-power plant of the type defined in the intro¬ duction, in which the efficiency may be further in¬ creased while simultaneously increase the possi¬ bilities to control the power generation of such a plant. This object is according to the invention obtained by providing such a power plant with an arrangement for supplying a fuel to a chamber located in the path of the flue gases between the two gas turbines and a burner for combustion of the fuel in this chamber for mixing the hot gases resulting there¬ from with the flue gases expanded in the first gas turbine for increasing the temperature thereof be- fore they are fed to the second gas turbine. Thanks to the provision of such a reheating combus¬ tor in the flue gas path between the two gas tur¬ bines the power generation of the second gas tur- bine may be increased considerably and by that a raise of the total efficiency of the entire plant be obtained. The possibilities to control the power generation of the device may in this way also be increased, since this may in such a plant now also be controlled by controlling the supply of said fuel to the reheating combustor. According to a preferred embodiment of the inven- tion such a power plant is provided with a topping combustor arranged in the path of the flue gases upstream of said first gas turbine, whereby the possibilities to control the plant and the effi¬ ciency thereof, primarily at partial load, increase further. According to another preferred embodiment of the invention said arrangement comprises a gasifier adapted to gasify a liquid or solid fuel and means for supplying the combustible gases so produced as fuel to said chamber for reheating the flue gases. The supply of the fuel, the combustible gases, may thanks to the arrangement of such a gasifier be easily controlled. It is advantageous that the gasifier is arranged to generate combustible gases having a pressure substantially exceeding the pres¬ sure of the flue gases in the path thereof down¬ stream of the combustor, especially in the reheat¬ ing chamber, since this results in an optimum regu- lating freedom independent of the size of the load presently existing. According to another preferred embodiment of the invention, which is directed to a power plant co - prising a compressor adapted to generate compressed air for pressurising the bed, the compressor is ar¬ ranged to also provide said chamber for reheating with compressed air for combustion of the fuel sup¬ plied thereto, whereby the reheating is ensured by simple means. According to another preferred embodiment of the invention the power plant comprises a high pressure gas turbine arranged on a first axle and a low pressure gas turbine arranged on a second separate axle in the path of the flue gases downstream of the high pressure gas turbine, and the high pres¬ sure gas turbine forms said first turbine. Such a division of the gas turbine on different axles fur- ther improves the possibilities to rapidly control the power delivered by the plant, in which this may take place by arranging flow regulating devices, such as controllable guide vanes, for instance at the inlet of the low pressure gas turbine. According to a further, very preferred embodiment of the invention the second gas turbine is formed by an intermediate pressure gas turbine arranged on the same axle as the first gas turbine, said first gas turbine being called high pressure gas turbine. This second gas turbine receives an intermediate pressure thanks to the existence of the reheating chamber according to the invention and contributes to a higher power that may be delivered by the plant, in which the power is increased further in the case of a low pressure gas turbine arranged on a second separate axle in the flue gas path down¬ stream of the intermediate pressure gas turbine. According to a preferred embodiment, which consti¬ tutes a combination of the features of the pre¬ ferred embodiments described above, the power plant has a topping combustor defined above for moving the temperature level of the flue gases arriving to said first gas turbine to an optimum value, a gasi- fier adapted to gasify a liquid or solid fuel and means for supplying the combustible gases so pro¬ duced as fuel to said chamber for reheating the flue gases, said first gas turbine being a high pressure gas turbine arranged on a first axle, and the plant comprises further a low pressure gas tur¬ bine arranged on a second axle and downstream of the high pressure gas turbine in the flue gas path, said low pressure gas turbine being at the inlet thereof provided with a flow regulating device, so that the number of revolutions of the second axle may be varied. A PFBC-power plant that may be driven with a high efficiency over an extended load range and a controllability being excellent consid- ering the inertness of PFBC-power plants is achieved in this way. The experience has shown that these features make a load change of as much as 5% per minute possible. Further advantages and advantageous characteristics of the invention will appear from the following de¬ scription and the other dependent claims. BRIEF DESCRIPTION OF THE DRAWING With reference to the appended drawing, below fol¬ lows a description of a preferred embodiment of the invention cited as an example. In the drawing: The single Fig 1 illustrates schematically a PFBC- power plant having a combined gas and steam cycle (the latter is not shown) according to a preferred embodiment of the invention. DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION A PFBC-power plant, i.e. a plant for combustion of a particle-like fuel in a pressurised fluidised bed, is shown schematically in the single figure, and it comprises a combustor 1, which is received in a pressure vessel 2, which may have a volume in the order of 10 4 m^ and which may be pressurised to for example about 16 bars. Compressed air 3 for pressurising the combustor 1 and for fluidising a bed 4 in the combustor is supplied to the pressure vessel. The compressed air is supplied to the com- bustor through fluidising nozzles 5 schematically indicated in the bottom of the combustor for flui¬ dising the bed enclosed in the combustor. The bed is made of bed material, granule-shaped absorbent and a particle-like fuel, preferably crushed car- bon, which is combusted in the fluidising air sup¬ plied to the bed. The combustion gases, hereafter called flue gases, from the bed are then led through a purification apparatus 6, which in the example is made of a high temperature filter in- tended for high pressures, and an intercept valve 7 furtheron to a topping combustor 8. A combustible gas is also supplied to the topping combustor 8 through a conduit 9 from a gasifier 10 of known type through a high temperature filter 11. The com- bustible gases are combusted in the topping com¬ bustor 8 in connection with a supply of compressed air through the conduit 12 from a high pressure compressor 13 through influence of a burner 34 and mixed with the flue gases from the combustor 1 for raising the temperature thereof, so that the gases leaving the topping combustor have a temperature ( 1200-1500°C) making them well suited as propellant gases for propelling a first gas turbine 14 in the form of a high pressure gas turbine. The tempera- ture of said combusting gases or flue gases has through the topping combustor been increased from about 850-950°C to about 1200-1500°C. The high pressure turbine and the high pressure compressor are arranged on the same axle as a gen¬ erator 15, from which useful energy may be taken. The high pressure compressor 13 delivers also com¬ pressed air to the PFBC-combustor 1 through the conduit 16 from which the conduit 12 is branched. An intercept valve 17 is arranged between the high pressure compressor and the combustor 1. It deliv¬ ers also air through the conduit 18 for gasifying in the gasifier 10, wherein in this conduit 18 a compressor not shown may be arranged so as to in- crease the gas pressure further in the gasifier, since it is desired that the gas flow delivered thereby has a higher pressure then the flue gas flow arriving to the topping combustor, so that the combustible gases in every given pressure situation may easily be supplied to the topping combustor and/or the reheating combustor described below. A liquid or solid fuel, in this example a particle¬ like carbon, is gasified in the gasifier, through which in a known way combustible gases are produced through a sub-stoichiometric process. The remaining fuel from the gasifier 10 may be supplied to the bed 4 in the combustor 1 through a fuel conduit 19. The reason for arranging a separate gasifying ar¬ rangement in this way, which operates under a higher pressure than the PFBC-bed 4, is that it is simply necessary to have a higher pressure of the gas in the topping combustor, and also in the re¬ heating combustor described further below, than the pressure in the combustors so as to be able to regulate the fuel flow and distribute the fuel flow uniformly in these combustors. Thus, the pressure of 26 bars may be achieved in the gasifier 10 at a possible pressure of 16 bars at the PFBC-bed. The PFBC-power plant shown in the figure is of an advanced type, since it has a further gas turbine 20, in the form of an intermediate pressure tur¬ bine, which is arranged on the same axle 21 as the high pressure turbine 14. The gas expanded and ex- perienced a temperature lowering in the high pres¬ sure turbine 14 is led through a conduit 22 to a second room 23, which is called reheating combustor or combustion chamber. The reheating combustor 23 receives a flow of said combustible gases deriving from the gasifier 10 and compressed air from the high pressure compressor 13 in the same way as the topping combustor 8, which is shown in the drawing through the conduit 24 and 25, respectively, whereby these combustible gases are combusted there through a burner 35 and the hot gases so produced are mixed with the flue gases from the high pres¬ sure turbine so as to raise the temperature thereof again before they are led through the conduit 26 to the intermediate pressure turbine 20. The power tapped from the intermediate pressure turbine may in this way be increased considerably. The flue gases expanded in the intermediate pres¬ sure turbine 20 are led to a low pressure turbine 27. The inlet thereof is preferably provided with a flow regulating device 36, such as in the form of controllable guide vanes on a guide rail ring, so that the number of revolutions of the second axle may be varied. The exhausted gases leaving the low pressure turbine still contain energy, which an economiser 28 may take care. On the axle 29 of the low pressure turbine 27 there is also a low pres¬ sure compressor 30, to which atmospheric air is supplied through a filter 31. Thus, the low pres- sure compressor is driven by the low pressure tur¬ bine and provides from the outlet thereof the high pressure compressor 13 with air compressed in a first step. An intercooler 32 is connected between the low pressure compressor and the high pressure compressor so as to lower the temperature of the air supplied to the inlet of the high pressure com¬ pressor 13. Furthermore, the power plant has a steam turbine side, which is not shown here, but which is indi¬ cated by a set of tubes 33 immersed into the flui¬ dised bed 4, in which tubes water is circulated and evaporated and superheated through heat exchange between the tubes and the bed material for absorb- ing heat generated during the combustion carried out in the bed. Thus, by arranging a reheating combustor 23 the power output from the flue gases expanded in the first gas turbine and experienced a temperature re¬ duction therein may be increased considerably and the efficiency of the power plant may by that be improved. The invention is of course not in any way re¬ stricted to the preferred embodiment described above, but several possibilities to modifications thereof would be apparent to a man skilled in the art without departing from the basic idea of the invention. It would for example be possible that the plant only has two gas turbines, i.e. the intermediate gas turbine shown in the figure is omitted, and the reheating combustor will then increase the tempera¬ ture of the flue gases coming from the high pres¬ sure gas turbine for arriving to the low pressure gas turbine, which in such a case will receive gases having a higher pressure than described above and could be called intermediate gas turbine. It is neither necessary, although advantageous, that the PFBC-plant according to the invention has a topping combustor, even if it has to be mentioned that it will be taken fully advantage of the advan¬ tages of the reheating combustor or chamber when the topping combustor is also there. It is of course also possible to arrange a reheat¬ ing combustor between the second and the third gas turbine as seen in the path of the fluid gases in the case of more than two gas turbines, such as in the embodiment described above, should this be de- sired.

Claims: Claims 1. A PFBC-power plant comprising a pressure vessel (2) enclosing a combustor (1) for combustion of a particle-like fuel in a pressurised fluidised bed, and at least two gas turbines (14, 20) located suc¬ cessively in the path of the flue gases produced by said combustion, characterized in that it comprises an arrangement (9-11) for supplying a fuel to a chamber (23) lo¬ cated in the path of the flue gases between the two gas turbines and a burner (35) for combustion of the fuel in this chamber for mixing the hot gases resulting therefrom with the flue gases expanded in the first gas turbine (14) for increasing the tem¬ perature thereof before they are fed to the second gas turbine (20). 2. A power plant according to claim 1, characterized in that said arrangement comprises a gasifier (10) adapted to gasify a liquid or solid fuel and means (9, 11) for supplying the combusti¬ ble gases so produced as the fuel to said chamber for reheating the flue gases. 3. A power plant according to claim 1 or 2, said plant comprising a compressor (13) adapted to gen¬ erate compressed air for pressurising the bed (4), characterized in that the compressor is adapted to also provide said chamber (23) for reheating with compressed air for combustion of the fuel supplied thereto. 4. A power plant according to claim 2, said plant comprising a compressor (13) adapted to generate compressed air for pressurising the bed (4), characterized in that the compressor is adapted to also deliver compressed air to a gasifier (10) for gasifying said liquid or solid fuel. 5. A power plant according to claim 2, characterized in that the gasifier (10) is adapted to produce combustible gases having a pressure es¬ sentially exceeding the pressure of the flue gases in the path thereof downstream of said combustor (1). 6. A power plant according to any of the preceding claims, characterized in that it also comprises a topping combustor (8) arranged in the path of the flue gases upstream of said first gas turbine ( 14 ) , a gasifier (10) adapted to gasify a liquid or solid fuel and means for supplying the combustible gases so produced to the topping combustor for combustion thereof therein and mixing the hot gases resulting therefrom with the flue gases for increasing the temperature thereof to an optimum level for a pro¬ pellant gas for said first gas turbine (14). 7. A power plant according to claim 6, said plant comprising a compressor (13) adapted to generate compressed air for pressurising the bed (4), characterized in that the compressor is adapted to also provide a topping combustor (8) with com¬ pressed air for combustion of the combustible gases supplied thereto. 8. A power plant according to any of the preceding claims, characterized in that it comprises a high pressure gas turbine (14) arranged on a first axle (21) and a low pressure gas turbine (27) arranged in the flue gas path downstream of the high pressure gas turbine on a second separate axle (29), and that the high pressure gas turbine forms said first gas turbine. 9. A power plant according to claim 8, characterized in that the low pressure gas turbine (27) is at the inlet thereof provided with a flow regulating device (36), so that the number of revo- lutions of said second axle (29) may be varied. 10. A power plant according to any of the preceding claims, characterized in that said second gas turbine is formed by an intermediate pressure gas turbine (20) arranged on the same axle (21) as said first gas turbine (14), said first gas turbine being called high pressure gas turbine. 11. A power plant according to claim 3 or 7 and any of claims 8-10, characterized in that the compressor (13) is ar¬ ranged on the same first axle (21) as and driven by said first gas turbine (14). 12. A power plant according to claim 3 or 7 and 8 or 9 and possibly 10 or 11, characterized in that it comprises a second com¬ pressor (30), a low pressure compressor, and that this is arranged on the same second axle (29) as and driven by said low pressure gas turbine (27) and adapted to supply pre-compressed air to the high pressure compressor (13). 13. A power plant according to any of claims 8-12, characterized in that a generator (14) for tapping useful energy is arranged on the axle (21) of said first gas turbine.

Inventors: BRAENNSTROEM ROINE, LOEVGREN ANDERS, VEENHUIZEN DIRK, BRAENNSTROEM, ROINE, LOEVGREN, ANDERS, VEENHUIZEN, DIRK
Applicants: ABB CARBON AB, BRAENNSTROEM ROINE, LOEVGREN ANDERS, VEENHUIZEN DIRK, BRAENNSTROEM, ROINE, LOEVGREN, ANDERS, VEENHUIZEN, DIRK
Application year: 1996
Main classification: F02C3
IPC codes: F02C3, F23C6, F23C10