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 increasing the total efficiency of a PFBC power plant with a gas turbine cycle and a steam turbine cycle. Low grade heat is utilized for generation of steam which is injected into the combustor (10) and increases the mass flow through the gas turbine (30) and hence the gas turbine power generated. Preferably, waste gases from the gas turbine (30) of the plant are utilized for generation of steam in a steam generator (84) separate from the main steam cycle (38, 40). The invention also relates to a PFBC power plant, in which low grade steam is injected into the combustor (10).

Description: Method of controlling a PFBC power plant and a PFBC power plant with equipment for such control. TECHNICAL FIELD The present invention relates to a method of increasing the total efficiency of a PFBC power plant with a gas turbine cycle and a steam turbine cycle by the injection of steam into the combustor of the plant. Further, the invention relates to a PFBC power plant for application of the above- mentioned method. SUMMARY OF THE INVENTION According to the invention, the total efficiency is increased by utilizing low grade heat in a PFBC power plant for generation of steam which is injected into the bed vessel and increases the mass flow through the gas turbine cycle and hence the gas turbine power. Conventionally, waste gases from the gas turbine are utilized for generation of this steam, but also other low grade heat within the power plant can be utilized. The steam is generated in a steam . generator or waste heat boiler separate from the steam turbine cycle. This boiler is supplied with waste gases with such a temperature that the steam which is generated attains a pressure which exceeds the working pressure in the combustor so that it can be injected directly into the combustor. The steam pressure is, however, considerably lower than the steam pressure in the steam cycle. It is also possible to utilize steam extraction flows from the steam turbine. This steam is taken at a pressure which somewhat exceeds the working pressure in the combustor and is injected into the combustor. The object of the invention is primarily to utilize the heat contents of waste gases from a gas turbine in a more efficient way than before. Instead of utilizing the entire heat quantity in the waste gases from the gas turbine in the steam cycle only for feedwater preheating, intermediate superheating of steam, etc., only part of the heat quantity is used for increasing the power in the steam turbine cycle. In a plant with an operating pressure of about 16 bar in the combustor, the waste gases are first utilized in a high temperature heat exchanger, for example for preheating of feedwater, and then in a steam boiler for generation of the steam which is injected into the combustion bed such that the increased mass flow through the gas turbine increases the power generated in the gas turbine cycle. The temperature of the low grade source of heat must be so high that steam with a pressure suitable for injection, about 20 bar, is obtained. The injected steam takes up energy from the bed. In case of unchanged thermal firing rate in the power plant, the steam injection results in the steam generation in the bed tubes, and hence the steam flow in the steam turbine cycle, being reduced. In total terms, an increase of efficiency and of electric power in the plant is obtained by ensuring that the power increase in the gas turbine cycle is greater than the power reduction in the steam cycle. An embodiment of the invention will be described in the following with reference to the accompanying figure. In the figure, 10 designates a combustor with combustion of a fuel, say coal, in a fluidized bed 12. The combustor 10 and a cleaning plant for separation of dust in the combustion gases, symbolized by a cyclone 14, are contained within a pressure vessel 16 and surrounded by compressed combustion air in the space 18 between the pressure vessel 16 and the combustor 10 and the cyclone 14. Fuel is supplied to the bed 12 of the combustor 10 through the inlet conduit 20. For fluidization of the bed 12 and combustion of supplied fuel, the combustor 10 is supplied with air from the space 18 via an air distributing bottom 22 with nozzles 24. The combustion gases are collected in the freeboard 26 and are passed, after cleaning in the cyclone 14, through the conduit 28 to a gas turbine 30. This drives a compressor 32 which feeds the space 18 in the pressure vessel 1 with compressed combustion air and a generator 34. The gas turbine may be of single-shaft design as in the figure, or or two-shaft design in which case one shaft drives the compressor 32 and the other shaft drives the generator 34. The compressor 32 is provided with an intermediate cooler 36. Heat from this cooler can be utilized for preheating feedwater. The combustor 10 accomodates nests of boiler tubes 38 and 40 for cooling of the bed and for generation and superheating of steam and intermediate superheating of steam, respectively. Steam generated and superheated in the tubes 38 is passed in a conduit 42 to the high pressure turbine 44, from there in the conduit 46 to the intermediate superheater 40 and from there in the conduit 48 to the low pressure turbine 50. The turbines 44 and 50 drive the generator 52. Steam for feedwater preheating can be taken out from the turbine 44, 50 through the steam extraction conduit 54. The low pressure turb ine 50 is connected to the condenser 56 through the conduit 58. Condensate is collected in the condensate tank 60. The feedwater pump 62 pumps the condensate through the feedwater preheaters 64, 66 and 68 and the conduit 70 to the steam tubes 38 in the combustor 10. Heat from the intermediate cooler 36 of the compressor and heat from the steam extracted from the turbine 44, 50 can be utilized for the preheating in the feedwater preheaters 64 and 66. The waste gases from the gas turbine 30 are passed to the feedwater preheater 68 via the conduit 71 and from there to the steam boiler 72, the feedwater preheater 74 for heating feed water to the steam boiler 72, the gas filter 76, the economizer 78 and from there to the chimney 80. In the embodiment shown, also an ash cooler 82 is utilized as preheater for the feedwater of the steam boiler 72. Water from the steam drum 84 of the boiler 72 is circulated through the tubes 86 by means of a circulation pump 88. The steam drum is connected to the steam injection nozzles in the combustor by means of the steam conduit 90. Dilution water to the plant is supplied via the conduit 92. The economizer 78 is connected, by a conduit 94, to the feedwater conduit 96 downstream of the feedwater pump 62 and is utilized for feeding the tubes 38 in the combustor 10 as well as for feeding the steam boiler 72. The feedwater flow to the steam boiler 70 is controlled by a valve 98 in the conduit 100 between the economizer 78 and the ash cooler/feedwater preheater 82. In a PFBC power plant, which at full power operates with a pressure of about 16 bar, the high temperature economizer/the feedwater preheater 68 is dimensioned such that the waste gases from the steam turbine which leave the feedwater preheater 68 have such a temperature that steam with a pressure exceeding 16 bar can be generated. In the steam boiler 72 a suitable quantity of steam with a pressure of about 20 bar can be generated. In a plant with a power of 800 MW t , the suitable quantity is about 25 kg/s . The steam is injected into the combustor and increases the mass flow through the turbine and hence the gas turbine power generation. The heating of the steam injected into the combustor results in reduced steam generation in the steam cycle and reduced power. The power gain in the gas turbine cycle is at least twice as great as the power loss in the steam turbine cycle. The net power gain may amount to 10-15 MW e in an 800 plant. This corresponds to an increase of the total efficiency of the plant of about 1-2%. A total net efficiency of approximately 46% is obtainable. The power gain is achieved without any increase in the cost of installation. The cost increase for the gas turbine cycle is insignificant and is compensated for by reduced costs in the steam turbine cycle. This results in smaller and hence less expensive steam turbines and condenser. The increased consumption of feedwater for generation of steam in the waste heat boiler 72 for bed injection entails a certain increased operating cost for preparation of the feedwater.

Claims: CLAIMS 1. A method of increasing the total efficiency of a power plant with combustion of a fuel, primarily coal, in a fluidized bed of particulate material, at a pressure exceeding atmospheric pressure, a so-called PFBC power plant comprising a combustor (10) with a bottom (22) with nozzles (24) for supplying the combustor with air for fluidization of a bed (12) and combustion of fuel supplied to the bed (12) , a gas turbine (30) driven by combustion gases from the combustor (10) , a compressor (32) which is driven by the gas turbine (30) and which compresses fluidization and combustion air, a generator (34) driven by the gas turbine (30) , tubes (30,40) arranged in the combustor (10) for cooling the bed and generating steam for a steam turbine (44, 50) which drives a generator (52) , c h a r a c t e r i z e d in that the mass flow through the gas turbine (30) and hence the gas turbine power generation are increased by steam being injected into the combustor (10) and steam with the necessary pressure for injection into the combustor (10) being either, generated by low grade heat in a separate steam generator (72,84) or being discharged from a steam turbine (50) which is driven by the steam generated in the combustor (10) . 2. A method according to claim 1, c h a r a c t e r i ¬ z e d in that the waste gases from the gas turbine (30) are utilized as a heating agent in a low pressure steam generator (72,84) for generation of steam with a pressure exceeding the operating pressure in the combustor (10) and that the generated steam is injected into the combustor (10) and increases the mass flow through the gas turbine (30) and hence the gas turbine power. 3. A power plant with combustion of a fuel, primarily coal, in a fluidized bed of particulate material, at a pressure exceeding the atmospheric pressure, a so-called PFBC power plant comprising a combustor (10) with a bottom (22) with nozzles (24) for supplying the combustor (10) with air for fluidization of a bed (12) and combustion of fuel supplied to the bed (12) , a gas turbine (30) which is driven by combustion gases from the combustor (10) , a compressor (32) which is driven by the gas turbine (30) and which compresses the fluidization and combustion air, a generator (32) driven by the gas turbine (30) , tubes arranged in the combustor for cooling the bed and generating steam for a steam turbine (44,50) which drives a generator (52) , c h a r a c t e r i z e d in that nozzles for injection of steam into the combustor (10) are either connected to a low pressure steam generator (72,84) which is heated by waste gases from the gas turbine (30) , or to an outlet (54) for discharging steam in the steam turbine (50) at a point where the pressure of the steam somewhat exceeds the pressure of the combustor (10) .

Inventors: KEMMER LEIF, PILLAI KRISHNA, KEMMER, LEIF, PILLAI, KRISHNA
Applicants: ABB STAL AB
Application year: 1990
Main classification: F02C3
IPC codes: F01K21, F01K23, F02C3, F22B1, F23C10