REHEAT TYPE WASTE HEAT RECOVERY BOILER AND POWER GENERATION PLANT USING THE SAME

Abstract: The power generation plant comprises a gas turbine GT , a waste heat recovery boiler (1) in which fed water is heat-exchanged with exhaust gas F from the gas turbine to produce vapour. A steam turbine system (10) incorporating a reheat turbine MPT, LPT and a high pressure turbine HPT to which the vapour is supplied from the waste heat recovery boiler, a generator G driven by the steam turbine system and condenser (11) for condensing vapour from the steam turbine system into condensate and for supplying the condensate to the waste heat recovery boiler as the feed water. The waste heat recovery boiler is provided with a secondary reheater (2), a super heater (5), a primary reheater (4) re­ heating vapour to be supplied to the secondary reheater and an evaporator (7) for evaporating the feed water from the condenser into vapour and for supplying the vapour to the super heater, in order with respect to the exhaust gas flow. The vapour produced in the super heater is introduced into the high pressure turbine and the vapour produced in the secondary reheater is introduced into the reheat turbine.

Description: The present invention relates to a combined power generation plant combining gas turbine means, waste heat recovery boiler means, steam turbine system and generator means, and more particularly to a reheat type combined power generation plant in which a reheat cycle is applied to the steam turbine system. As compared with the common thermal power generation plant, the combined power generation plant incorporating the gas turbine means therewith is capable of conducting load changes and starting-up/stopping rapidly and effectively. In order to operate such combined power generating plant more effective, it is performed to apply the reheat cycle thereto. Namely, not only the evaporator and the super heater, but also the reheater is incorporated with the waste heat recovery boiler which recovers thermal energy from the exhaust gas from the gas turbine means and generates vapour. The vapour generated in the evaporator is superheated in the super heater into superheated vapour which is to be introduced into the high pressure turbine. The superheated vapour does the work on the high pressure turbine and thereafter is exhausted therefrom. The exhaust vapour from the high pressure turbine is reheated by the reheater in the recovery boiler means to high temperature vapour which is to be introduced into the medium or the low pressure turbine. The reheat cycle is effective in improving the efficiency in the combined power generation plant in which combustion temperature in the gas turbine means is high. The reheat type combined power generation plant can obtain a high efficiency than that of the conventional combined power generation plant, for example, one incorporating multi-pressure turbine system. The prior art waste heat recovery boiler incorporated in the reheat type combined power generation plant is disclosed, for example, in JP-A-61-289201. In such recovery boiler, the exhaust gas from the gas turbine means is once separated into two flows. In one flow, a secondary super heater and a primary reheater are disposed in order with respect to such flow. On the contrary, in the other flow, a secondary reheater and a primary super heater are disposed in order with respect to such flow. Evaporator means are disposed in a combined exhaust gas flow which is integrated by the separated exhaust gas flows. The vapour generated in the evaporator means flows through the secondary super heater, the primary super heater, high pressure turbine means, the secondary reheater, the primary reheater and the medium/low pressure turbine means. Such prior art waste heat recovery boiler causes the following disadvantages. At first, since any consideration is not paid to the change of the temperature of exhaust gas which has been heat exchanged in the super heaters and the reheaters, there is a difference between the exhaust gas temperatures in the respective separated exhaust gas flows. Accordingly, the amounts of vapour generated in the respective evaporators differ from each other, which are disposed down stream side of the super heaters and of the reheaters. In order to make the temperature of the exhaust gas which has flowed through one of the separated exhaust gas flows identical to that through the other one, it is required to change the thermal load distribution on the respective super heaters and the reheaters in accordance with the difference between such temperatures. However, it is impossible in fact to change such distribution in proper within all over load range. Secondary, in the combined plant in which heat is recovered from the exhaust gas from the gas turbine means, and in which vapour is generated by such recovered heat, as different from the thermal plant (e.g., steam plant), it is difficult to maintain the temperature and the pressure of the vapour in high level. Further, the vapour has a tendency to readily become wet vapour after conducting the work on the steam turbine. In particularly, the reheat type turbine has the thermodynamic disadvantages and the problems of erosion generated in the final stage of the steam turbine due to the wetness therein. Further, in a partial load operation of the gas turbine, the temperature of the exhaust gas decreases. Such decrease exerts worse influences on the above disadvantage. JP-A-58-167811 discloses a combined plant having a gas turbine unit; a heat recover boiler which recover heat from the exhaust gas produced by the gas turbine unit; a steam turbine system operated by steam provided by the heat recovery boiler, which produces low temperature vapour and high temperature vapour; and a condenser which condenses the steam from the steam turbine system. This document corresponds to the pre-characterising part of claims 1 and 5 of the present application. EP-A-0129167 discloses another combined plant having a coal gasification furnace for producing gas, which has high-pressure and low-pressure steam generators for recovering heat from the furnace wall; a gas turbine operated by the gas from the furnace; a heat recovery boiler for further recovering heat from the exhaust gas of the gas turbine by the steam from the high-pressure and low-pressure steam generators; a steam turbine system operated by the steam provided by the high-prssure and low-pressure steam generator and reheated in the boiler; and a condenser which condenses the steam from the steam turbine system. According to the present invention, there is provided a power generation plant comprising: a gas turbine means ( GT ); waste heat recovery boiler in which feed water is heat-exchanged with exhaust gas from said gas turbine means to produce vapour; steam turbine system including high pressure turbine means (HPT) and reheat turbine means (MPT,LPT) to both of which said vapour is supplied from said waste heat recovery boiler; generator means ( G ) driven by said steam turbine system; and condenser means for condensing vapour from said steam turbine system into condensate and for supplying said condensate to said waste heat recovery boiler as said feed water; wherein said waste heat recovery boiler comprises a secondary reheater, a super heater a primary reheater for heating vapour to be supplied to said secondary reheater, and an evaporator for evaporating feed water from said condenser means to vapour to be supplied to said super heater, which are disposed in order, with the secondary reheater in an upper most side with respect to flow of said exhaust gas, and in that vapour produced in said super heater is introduced into said high pressure turbine means and vapour produced in said secondary reheater is introduced into said reheater turbine means. According to the present invention, the reheat cycle is applicable to the combined power generation plant combining the gas turbine means and the steam turbine system, whereby improving the thermal efficiency over wide range of load of the gas turbine means. In the drawings: Fig. 1 is a diagrammatic view showing one embodiment of the present invention; fig. 2 is a graph showing characteristics between the specific enthalphy and the specific entropy; Fig. 3 is a graph showing characteristics of heat exchange between the exhaust gas and the vapour/feed water; Fig 4 is a graph showing characteristics of exhaust gas temperature in the respective positions in the heat recovery boiler shown in Fig. 1 with respect to load of the gas turbine means; and Fig. 5 is a diagrammatic view showing another embodiment of the present invention. Fig. 1 shows a waste heat recovery boiler 1 in a power generation plant according to one embodiment of the present invention, which is associated with a gas turbine system including a compressor C , a gas turbine GT and a combustor B . The waste heat recovery boiler 1 exchanges heat in exhaust gas F from the gas turbine GT with feed water flowing through the waste heat recovery boiler 1. The boiler 1 includes a secondary reheater 2, a secondary super hater 3, a primary reheater 4, and a primary super heater 5, which are disposed in order with respect to a flow direction of the exhaust gas F . The boiler 1 further includes an economizer 6 and vapour generator means (evaporator) 7 disposed between the economizer 6 and the primary super heater 5. During load operation of the gas turbine GT , the vapour generated in the evaporator 7 flows into a high pressure turbine (HPT) of a steam turbine system 10 through the primary super heater 5 and the secondary super heater 3. Such vapour does the work on the high pressure turbine (HPT) and then is introduced into a medium pressure turbine (MPT) of the steam turbine system 10 through the primary reheater 4 and the secondary reheater 2. After the vapour does the work on the medium pressure turbine (MPT), the vapour is introduced into a low pressure turbine (LPT) and does the work thereon. Thereafter, the vapour is condensed into condensate in a condenser 11. Since the boiler 1 is a type of plural-stage recovery boiler, the boiler 1 further includes a lower pressure side economizer 8 and a lower pressure side evaporator 9. A part of feed water flowing through the lower pressure side economizer 8 is evaporated into vapour of lower pressure in the lower pressure side evaporator 9 and is introduced into the low pressure turbine (LPT). The lower pressure vapour together with the vapour from the medium pressure turbine (MPT) does the work on the low pressure turbine (LPT). The rest of feed water bypasses the lower pressure side evaporator 9 and is returned back to the higher pressure side evaporator 7 through the economizer 6. In generally, an exhaust vapour temperature from the high pressure turbine (HPT) is about 300°C. The exhaust vapour from the high pressure turbine (HPT) is reheated by the reheaters 2 and 4, so that a wetness fraction of the vapour in a final stage of the low pressure turbine (LPT) can be reduced. With referring to Fig. 2, the work done on the steam turbine system 10 will be described hereinunder. A line ls is a saturation line. Under the line ls , the lower region the vapour resides, the higher the wetness fraction thereof becomes. To the contrary, the higher region the vapour resides, the higher the dryness fraction thereof becomes, i.e. the vapour becomes a superheated vapour. A line li is an intake pressure line which is determined by the pressure of vapour introduced into the steam turbine system 10. the values of specific enthalpy h and of specific entropy s of the vapour introduced into the high pressure turbine (HPT) from the secondary super heater 3 shown in Fig. 1 are represented by the point a in Fig. 2. A line le is an exhaust pressure line which is determined by the pressure of vapour introduced into the condenser 11 from the low pressure turbine (LPT). The point e '' represents values of specific enthalpy h and of specific entropy s of the vapour without employing a reheat cycle. The point f represents values of specific enthalpy h and of specific entropy s of the vapour with employing the reheat cycle. In Fig. 2, the segment abe represents change of vapour condition in the steam turbine without reheating the vapour. According as the work done on the steam turbine increase, the vapour condition becomes under the point e , so that the vapour includes the wet vapour. If the steam turbine exhausts the vapour at the point e ' on a characteristic line le ' which does not reside in the wet vapour region, it is impossible to recover heat from the exhaust vapour sufficiently. On the contrary, if the steam turbine exhausts the vapour at the point e '' on the exhaust pressure line le in the wet vapour region, it becomes possible to recover heat from the exhaust vapour sufficiently. However, the steam turbine may be damaged by wet vapour. The segment abcdf represents change of vapour condition in the steam turbine with reheating the vapour. The vapour at the point a does the work on the high pressure turbine (HPT) and declines to the point b . Thereafter, the vapour is reheated to rise to the point c . According this, such reheated vapour can do the work cdf on the medium (MPT) and the low pressure turbine (LPT). Even though, under the saturation line ls , i.e. between the point d and the point f , the vapour includes wet vapour, the metal material selection in the final stage in the turbine makes it somewhat proof against such wet vapour. Since the vapour is reheated and can be maintained in a higher temperature level, the wetness fraction in the final stage of the steam turbine can be reduced. Namely it can be possible to reduce the wetness fraction from a degree e-e'' to a degree d-f . In a power generation plant combining steam turbine system and gas turbine means, the temperature in exhaust gas from the gas turbine means is low on a lower load operation of the gas turbine means. In the plant shown in Fig. 1, since the reheater is disposed in an upper most stream side of the exhaust gas, the vapour can be reheated to a high temperature close approximate to the exhaust gas temperature. According this, optimum vapour can be obtained, which is optimized with respect to wetness fraction in the final stage of the steam turbine. In this regard, the reheat cycle is effective. However, since the low rate of the primary vapour is substantially identical to that of the reheated vapour, if the reheater is simply disposed upstream side of the super heater, the temperature of vapour in such super heater is lowered. To avoid this, according to the present invention, the separate reheaters are so disposed as to interpose the super heater therebetween in order to balance the heat absorption. According this, the heat exchange characteristics shown in Fig. 3 can be obtained, so that outlet temperatures of vapour in the super heater and the reheater are balanced. Fig. 3 shows the relationship between the vapour/feed water temperature in the respective apparatus in the heat recovery boiler 1 and the exhaust gas temperature therein. As shown in Fig. 3, the exhaust gas from the gas turbine means exchanges heat with the apparatus in the heat recovery boiler 1 and then the temperature of the exhaust gas is lowered. On the contrary, the vapour/feed water is heated in the economizers 8, 6, the evaporator 7, the primary super heater 5 and the secondary super heater 3, in order. Namely, the feed water of temperature To is heated and then changed into the vapour of temperature T1. The vapour of temperature T1 from the secondary super heater 3 does the work on the high pressure turbine (HPT) and then is changed into the vapour of temperature T2. The vapour of temperature T2 is introduced into the primary reheater 4 and is reheated in the primary reheater 4 and the secondary reheater 2 to be changed into the reheated vapour of temperature T3. According this, it is possible to obtain the reheated vapour of the highest temperature and it is also possible to maintain the primary vapour in a high temperature level by means of selecting heat capacities of the super heaters and the reheaters proper. The vapour generated in the lower pressure side evaporator 9 has a temperature corresponding to the saturation temperature T4 in Fig. 3. To the contrary, in the prior art, since the super heater and the reheater are parallel to flow of exhaust gas from the gas turbine so as to balance the temperatures in the super heater and the reheater, the energy in the exhaust gas is substantially separated into two halves. If the heat-transfer area is changed, the exhaust gas energy is separated into two parts which are not identical to each other. However, when the temperatures in the super heater and the reheater are balanced, the exhaust gas energy is separated into two parts which are approximately equal to each other. Further, in the prior art, the flow rate of the vapour flowing through the super heater is substantially identical to that through the reheater. Accordingly, when the temperature of the exhaust gas from the gas turbine is lowered, or the gas turbine operates in a low load level, the reheater can only use no more than a half of exhaust gas flow rate due to countermeasure for wet vapour in the final stage of the steam turbine. Accordingly, in the prior art, in order to obtain a heat balance between the super heater and the reheater the same as that of the present invention, it is required to prepare the super heater and the reheater, each of which has a cross section with respect to the exhaust gas flow direction twice that of the present invention. Further, in order to restrict the exhaust gas speed in the heat transfer surface of each of the super heater and the reheater within a predetermined range, i.e. in generally from 20 m/s to 30 m/s, the exhaust gas receiving part (the super heater and the reheater) of the waste heat recovery boiler must be considerably enlarged, so that disadvantages with respect to the arrangement and the cost are raised. The present invention can solve such disadvantages. Fig. 4 shows the temperature characteristics of the exhaust gas in the respective parts of the waste heat recovery boiler with respect to the load of the gas turbine. The temperature of the exhaust gas from the gas turbine means decreases in accordance with the load decrease. The reheater is disposed in an upper-most stream side with respect to the exhaust gas flow direction. The reheated vapour can be readily maintained in maximum temperature level close to the exhaust gas temperature, and is most effective with respect to the wet vapour in the final stage in the steam turbine. Further, the temperature characteristics in the super heater and the evaporator which are disposed in a down stream side are also improved the same as the reheater, so that the temperature distribution is well balanced. As apparent from the above-mentioned explanation, since the reheater is disposed in the upper-most stream side, the reheated vapour having a maximum temperature can be obtained even though the temperature of the exhaust gas from the gas turbine is lowered, or the gas turbine operates in a low load level, so that it is possible to avoid the erosion in the final stage of the steam turbine due to wet vapour. Further, referring back to Fig. 4, the following explanation will be made to the vapour in the outlet of the high pressure turbine (HPT) in the plant in which the super heater is interposed between the separate reheaters. In generally, it is required to maintain the super heat degree of the vapour in the outlet of the high pressure turbine in a predetermined level. e.g. 50°F, in order to prevent the erosion due to the wet vapour from generating in the final stage of the turbine, and so on. Further, when the temperature of the exhaust gas from the gas turbine is low or the gas turbine operates in a low load, the temperature distribution in a down stream side of the exhaust gas flow is flat and the temperature difference is smaller than that on the high load operation. The degree of temperature decrease in the down stream side becomes small. Namely, due to decrease in the exhaust gas temperature, the heat exchange in the secondary reheater is not fully performed, and then the amount of vapour generation therein is reduced. However, the heat exchange in the secondary super heater, which is disposed in a down stream side of the secondary reheater, is performed fully to supplement the heat exchange loss. The lower the load becomes, the smaller the temperature decrease in the down stream side becomes, whereby maintaining the temperature. In other words, when the temperatures in the outlets of the reheater and the super heater on full (100%) load operation are setted to any desired ones, the temperature change in the super heater becomes gradual on the load decrease, which is disposed in the down stream side of the reheater, which is disposed in the down stream side of the reheater, whereby being capable of maintaining the degree of super heating in the outlet of the high pressure turbine fully. The other embodiment of the present invention shown in Fig. 5 additionally includes a bypass passage 100 for controlling the vapour temperature, which bypasses the secondary reheater 2 and connects the inlet of the secondary reheater 2 to the outlet thereof. Further, any other varieties of the recovery boiler can be readily obtained by providing the bypass passage similar to the bypass passage 100 to the super heater for controlling the vapour temperature therein, which is disposed between the separate reheaters. According to the present invention, it can be possible to apply the reheat cycle to the plant combining the steam turbine system and the gas turbine means, whereby obtaining a safe operation of the plant having a high thermal efficiency over a wide range of the load. Firstly, the reheater is disposed in an upper most stream side in the waste heat recovery boiler, so that the vapour in the reheater can be reheated up to a temperature close to the temperature of the exhaust gas from the gas turbine. Therefore, it becomes possible to maintain the wetness fraction of the vapour in the final stage of the low pressure turbine within a proper range. Secondary, the separate reheaters are so provided that the super heater is interposed between such reheaters, so that the primary vapour is maintained in a high temperature level even though the gas turbine operates in a partial load. Therefore, it is also possible to maintain the wetness fraction of the vapour in the outlet of the high pressure turbine within a proper range.

Claims: A power generation plant comprising: gas turbine means ( GT ); waste heat recovery boiler (1) in which feed water is heat-exchanged with exhaust gas from said gas turbine means to produce vapour; steam turbine system including high pressure turbine means (HPT) and reheat turbine means (MPT,LPT) to both of which said vapour is supplied from said waste heat recovery boiler; generator means ( G ) driven by said steam turbine system; and condenser means (11) for condensing vapour from said steam turbine system into condensate and for supplying said condensate to said waste heat recovery boiler as said feed water; characterised in that: said waste heat recovery boiler comprises a secondary reheater (2), a super heater (3), a primary reheater (4) for heating vapour to be supplied to said secondary reheater, and an evaporator (7) for evaporating feed water from said condenser means to vapour to be supplied to said super heater, which are disposed in order, with the secondary reheater (2) in an upper most side with respect to flow of said exhaust gas, and in that the superheated vapour produced in said super heater is introduced into said high pressure turbine means and vapour produced in said secondary reheater is introduced into said reheater turbine means. A power generation plant according to claim 1, wherein said waste heat recovery boiler comprises a secondary reheater (2), a secondary super heater (3), a primary reheater (4) heating vapour to be supplied to said secondary reheater, a primary super heater (5) heating vapour to be supplied to said secondary super heater (3) and an evaporator (7) evaporating feed water from said condenser means to vapour to be supplied to said primary super heater, which are disposed in order with respect to flow of said exhaust gas. A power generation plant according to claim 1 or 2, wherein bypass valve means (100) is provided in passage means conneccting said secondary reheater (2) and said primary reheater for bypassing said secondary reheater.

Inventors: ARAKAWA, TADAO, HASHIMOTO, TSUGUO, KAWAUCHI, AKIHIRO
Applicants: HITACHI, LTD.
Application year: 1988
Main classification: F01K23
IPC codes: F01K23, F22G7, F22B1