A METHOD AND A DEVICE FOR CONTROLLING OUTPUT OF POWER BETWEEN A GAS AND A STEAM TURBINE IN CONNECTION WITH A PFBC PROCESS

Abstract: By slightly oversizing the heat transfer area of the steam generator (18a) in the bed (16) of a PFBC power plant, a surplus of steam is generated in the steam system of the power plant. The steam surplus is extracted from the steam system downstream of the bed and is supplied to preheter (3) for the combusiton air (32) upstream of the bed, a higher temperature thus being imparted to the combustion air (32) included, which results in an increase of the output of power from the gas turbine (27) in the gas system. A transmission of power from the steam system to the gas system is made possible by simple means. By varying the quantity of extracted steam from the steam system and thus varying the preheating of the combustion air (32) into the bed (16), the output of power between the steam turbine (19) and the gas turbine (27) in a PFBC power plant may be balanced to full nominal power magnitudes from the respective turbine.

Description: A method and a device for controlling output of power between a gas and a steam turbine in connection with a PFBC process TECHNICAL FIELD A PFBC power plant (Pressurized Fluidized Bed Combustion) is provided with a steam turbine for utilizing steam energy generated in a tube system immersed into a fluidized bed. A gas turbine is employed at the same time for utilizing the energy contents in flue gases formed during combustion in the bed. The invention relates to the problem of controlling the output of power in a steam turbine and a gas turbine, respectively, to the nominal power magnitudes to which the respective turbine type is dimensioned. BACKGROUND ART During combustion of particulate fuel, usually coal, in a pressurized fluidized bed in a PFBC power plant, the bed is supplied with combustion air in the form of compressed air from the pressure vessel which surrounds a bed vessel, in which the fluidized bed is stored, via fluidization nozzles below the bed. The combustion gases which are generated during the combustion process pass a freeboard above the bed surface, are cleaned and supplied to a gas turbine. The combustion gases drive the gas turbine, which in turn drives a generator as well as a compressor which supplies the pressure vessel with compressed air. In the following, the combustion air, the combustion gases and the devices which handle these gases are assumed to belong to and are referred to as the gas system of the plant. Thus, the gas system comprises, inter alia, the gas turbine. For cooling the bed to a temperature of the order of magni¬ tude of 850°C, a steam generator in the form of a tube bundle is located therein, which constitutes one component in a steam system. Steam is generated in the tube bundle, whereby energy is taken out from the bed via the steam turbines to which the steam is passed in the steam system. In case of full load, the entire tube bundle is within the bed. The cooling capacity of the tube bundle must be dimen- sioned according to the output of power from the bed to be able to maintain the correct bed temperature. In case of reduced output of power, the fuel supply and the power development in the bed must be reduced but the cooling capacity of the tube bundle remains unchanged, which entails an undesired reduction of the bed temperature. The cooling capacity of the tube bundle in the bed can be reduced by lowering the bed level, part of the tube bundle then being situated above the surface of the bed. Upon a change of load in the plant, the cooling capacity of the tube bundle is adjusted by varying the bed level, whereby a certain part of the tubes in the tube bundle may be located above the bed surface so that they thus do not cool the bed. On the other hand, the exposed tubes cool the combustion gases flowing from the bed to the gas turbine, whereby the temperature of the combustion gases is reduced, which entails a reduced output of power from the gas turbine, that is to say that the measures taken to change the output of power from the •steam turbine lead to a corresponding change of the output of power from the gas turbine, which means that, for example, the above-mentioned control method for controlling the output of power cannot be utilized for trimming the output of power from the steam and gas turbines, respec¬ tively, in relation to each other. During power control, the fuel and the air quantity supplied to the bed must be regulated in dependence on power taken out from the plant and at the same time the bed level be regulated such that the cooling capacity of the tube bundle is adapted to the energy supply, whereby the correct bed temperature is maintained. In case of a constant load output, the height of the bed is kept constant. In case of full load output from the plant, it is desired that both steam turbine and gas turbine be supplied with nominal power values, which are dimensioned for the plant, for the respective turbine type. On the other hand, it presents a difficult technical problem to dimension the steam turbine for the exactly correct nominal power magni¬ tude when designing a PFBC plant, that is, the design of the steam turbine. The dimensioning of the steam turbine is dependent on the fuel fineness, the moisture content of the fuel, the type of fuel, etc., that is, the dimensioning of the steam turbine means an adaptation to each individual plant taking into consideration, inter alia, the properties of the fuel. A fundamental and difficult question to answer is how large a heat transfer surface is required in the tube bundle in order to attain a certain desired steam produc¬ tion. In case of full load, the heat transfer to the tube bundle shall be such that the calculated desired nominal temperature of the combustion gases, which are to drive the gas turbine, in case of full bed height at the same time provides a nominal steam production in the tube bundle such that a 100 per cent output of power from the gas turbine is obtained while at the same time obtaining a 100 per cent output of power at the steam turbine. The fundamental problem is that one of the types of turbine, the gas or steam turbine, can reach full power before full power is attained in the other unit. According to the available technique, the possibilities of trimming the energy produc¬ tion between the two units is limited. The heat absorption in the bed takes place by the generation of steam in the steam generator in the form of a tube bundle. The steam production is controlled by the relation¬ ship Q = k * A * ΔT where Q is the transferred quantity of heat ΔT is the locally driving temperature difference A is the installed area of the steam generator k is the coefficient of heat transfer For the steam cycle in a PFBC power plant, the temperature difference cannot be freely influenced. Factors concerning combustion engineering and, especially, the environment are the reasons for this . A constitutes the installed area of the steam generator in the PFBC plant and cannot be influenced without interfering with the plant. The third parameter k can, to a certain extent, be influenced. However, it must be pointed out that, from an engineering point of view, the parameter k or that part of the parameter k which is related to the external heat transfer coefficient is a difficult parameter to calculate with satisfactory accuracy and thus to take particular account of when dimensioning the steam generator. This means that in designing the steam generator in a PFBC plant, a certain inaccuracy in the surface dimensioning must be accepted. In addition, the dimensioning of the steam generator requires that variations in the steam production, for example because of the quality of the fuel, are to be taken into consideration. Another problem which should be noted is the uncertainty which arises because of the properties of the bed, over which technicians have moderate control. This includes, for example, variations in the particle fineness or particle properties in case of fluctuations of the quality or composition of the fuel, or when switching to firing of a somewhat different type of coal than what was originally intended when firing, for example, particulate coal. All of these examples lead to a changed k-value, resulting in incorrectly dimensioned heat transfer. The normal way to reduce the output of power to the steam turbine is to lower the bed height. A consequence of this, as mentioned, is that exposed steam tubes above the bed surface will then cool combustion gases flowing out to the gas turbine, the gas turbine power thus being reduced. This rigid compliance of the gas turbine power in relation to the desired steam turbine power constitutes a problem. Both in case of full load and in case of partial load, it is desirable to have a possibility of redistribution of the output of power between the steam turbine and the gas turbine to allow trimming of the output of power in both the steam and the gas turbine individually to the optimal values. SUMMARY OF THE INVENTION By slightly oversizing the heat transfer area of the steam generator in the bed of a PFBC power plant, a surplus of steam is generated in the steam system of the power plant . The surplus steam is extracted from the steam system downstream of the bed and is supplied to a preheater for the combustion air upstream of the bed, thus giving the combustion air included a higher temperature, which results in an increase of the power output from the gas turbine in the gas system. A transfer of power from the steam system to the gas system is made possible by simple means. By varying the quantity of extracted steam from the steam system and thus varying the preheating of the combustion air into the bed, the output of power between the steam turbine and the gas turbine in a PFBC power plant can be balanced to full nominal power magnitudes in the respective turbine. In the path of the combustion air to the bed, an air pre¬ heater is installed which is suitably dimensioned to receive a steam surplus in case of a steam flow which is of the order of magnitude of 10% too great in relation to what is nominally calculated for the steam system, and at the same time intentionally oversizing the steam generator with a heat transfer area which is 5% too large. In this way, a variation of the steam production within ± 5% can be accepted. The suggested dimensioning values for the steam generator and the air preheater can be regarded as recom- mended values for obtaining reasonable trimming possibili¬ ties of the power balance between the steam and gas tur¬ bines. Extraction of steam from the steam system can be performed by means of an ordinary steam valve located in the steam conduit from the tube bundle to the steam turbine, preferably outside the pressure vessel. The preheater for the combustion air consists of a heat exchanger traversed by the extracted steam and located in the path of the combustion air to the bed, suitably in an air duct ahead of the fluidization nozzle of the bed. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic view of a PFBC power plant according to the prior art, and Figure 2 is a diagram showing the principle of a PFBC power plant with an installed steam extraction valve and a pre¬ heater for combustion air according to the invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the method and the device according to the invention will be described with reference to the accompanying figures . Figure 1 schematically shows a combustor 10 in a PFBC power plant, in which combustion takes place at a pressure considerably exceeding the atmospheric pressure. The figure shows a combustor 1Q placed inside a pressure vessel 12. The space 13 is supplied with compressed combustion air from a compressor 14 through the conduit 14a. The combustor 10 comprises a fluidizable bed of particulate material 16, in which a fuel is burnt. In addition, the combustor 10 also comprises a steam generator 18 composed of tubes to form a tube bundle for generating steam for a steam turbine 19 and for cooling the bed 16. Combustion gases leaving the bed 16 are collected in the freeboard 20, are passed through the conduit 22 to a cleaning plant 24, symbolized by a cyclone 24, in which dust is separated and falls down into a cyclone leg 24b, and are passed on from the cyclone through a conduit 26 to a gas turbine 27, from where the combus¬ tion gases after, inter alia, cooling and possible filtering are returned to the atmosphere. Separated dust is discharged from the cyclone 24 via the conduit 28 to an ash discharge and pressure reducing system (not shown) . Fuel to the bed is fed to the combustor 10 via the conduit 30. The combustor 10 is supplied with air 32 from the space 13 via an air duct 35 and further via nozzles 38 for fluidization of the material in the bed 16 and combustion of the supplied fuel. A preferred embodiment of the invention is illustrated in Figure 2. A steam generator 18a, oversized in relation to the power required in an installed steam turbine 19, is located in the combustor 10. At the connecting conduit of the steam genera¬ tor 18a to primarily a high-pressure turbine, which is one of the stages in the steam turbine 19, there is arranged an extraction valve 1. This extraction valve 1 is preferably located outside the pressure vessel 12 and allows extraction of a partial amount of steam generated in the steam genera¬ tor 18a to a preheater 3 for the combustion air 32 for the purpose of preheating the combustion air 32 to the bed 16 in order thus to supply additional power to the gas system. The preheater 3 may consist of steam tubes, which serve as heat exchangers, that is, heat the combustion air 32 passing by, whereas at the same time the extracted steam is cooled. The cooled or condensed steam is passed out from the preheater 3 via a conduit 4 and is utilized further in the process, for example for preheating high-pressure steam or for some other purpose. The quantity of extracted steam from the steam system via the valve 1 can be controlled by means of that valve 1, in which case the power transfer from the steam system to the gas system can be balanced so that full nomi¬ nal output of power can be obtained from both the steam tur¬ bine and the gas turbine at the same time. A redistribu¬ tion of the power from the steam turbine to the gas turbine becomes possible, thus obtaining a method for trimming the plant. The trimming method is useful both during full-load operation and partial-load operation of the plant. Alternative locations of the extraction valve 1 and/or the preheater 3 are possible. The condition is that the device according to the invention extracts steam from the steam system downstream of the steam generator 18a, whereas pre¬ heating of the combustion air 32 in the gas system by means of extracted steam takes place in the flow of combustion air 32 upstream of the fluidization nozzles 38.

Claims: CLAIMS 1. A method for balancing the output of power between a steam turbine (19) and a gas turbine (27) in a PFBC power plant in which combustion of a particulate fuel takes place in a fluidized bed (16) in a combustor (10) enclosed in a pressure vessel (12) , in which combustion air (32) is supplied to the bed (16) via an air duct (35) from the pressure vessel (12) , whereafter, in a gas system, the combustion air (32) transformed into flue gas is passed to and drives a gas turbine (27), which in turn drives a compressor (14) which supplies the pressure vessel (12) with compressed combustion air (32) and where, in a steam system, a steam generator (18a) absorbs heat from the bed (16) , which heat, transformed into steam, is passed to a steam turbine (19) , characterized in that a somewhat oversized steam generating area is installed in the steam generator (18a) and that the available steam power surplus thus obtained is transferred, where necessary, from the steam system to the gas system by extracting steam downstream of the steam generator (18a) and supplying it to a preheater (3) for preheating of the combustion air (32) upstream of the bed (16) , whereby power is supplied to the gas system. 2. A method according to claim 1, characterized in that by controlling the quantity of steam extracted to the preheater (3) of the combustion air (32), full output of power from both the steam turbine (19) and the gas turbine (27) is balanced to a position of equilibrium. 3. A method according to claim 1, characterized in that by controlling the quantity of steam extracted to the preheater (3) of the combustion air (32) , the output of power between the steam turbine (19) and the gas turbine (27) is balanced in case of partial-load operation of the plant. 4. A device for carrying out the method of balancing the output of power between the steam turbine (19) and the gas turbine (27) in a PFBC power plant, comprising: - a combustor (10) for combustion of a particulate fuel in a fluidized bed (16) contained within a pressure vessel (12) , - a gas turbine (27) driven by flue gases generated during combustion and the gas turbine in turn driving a compressor (14) which supplies the pressure vessel (12) with compressed combustion air (32) which is passed on, via an air duct (35) and fluidization nozzles (38) , to the bed (16) , - in the bed (16) a steam generator (18a) which generates steam which drives a steam turbine (19) , characterized in that steam can be extracted downstream of the steam generator (18a) via an extraction valve (1) and be supplied to a preheater (3) for heating the combustion air (32) , whereby the output of power between the steam turbine (19) and the gas turbine (27) can be balanced. 5. A device according to claim 4, characterized in that the heat transfer area of the steam generator (18a) is oversized, steam surplus thus being obtained in the steam system. 6. A device according to claim 5, characterized in that the preheater (3) for preheating the combustion air (32) is located in the air duct (35) . 7. A device according to claim 5, characterized in that the preheater (3) for preheating the combustion air (32) is located upstream of the fluidization nozzles (38) . 8. A device according to claim 5, characterized in that the preheater (3) consists of steam tubes, which may be supplied with steam from the steam system of the plant. 9. A device according to claim 5, characterized in that the steam surplus can be extracted from the steam system via an extraction valve (1) and be supplied to the preheater (3), whereby the gas turbine power can be varied within the order of magnitude of ± 5% in dependence on the quantity of extracted steam.

Inventors: KULLENDORFF ANDERS, KULLENDORFF, ANDERS
Applicants: ABB CARBON AB
Application year: 1992
Main classification: F01K23
IPC codes: F01K23, F22B1, F22B31