Abstract:
A multi-pressure reheat combined cycle configuration in which the HP steam turbine exhaust temperature is colder than the HP steam saturation temperature and the coldest section of the reheater is placed downstream of the high pressure evaporation section in the heat recovery steam generator gas path with respect to the direction of exhaust gas flow. When configured in this manner, the optimum reheat pressure for the cycle is lower then for a cycle with all reheating taking place upstream of the HP evaporation section in the HRSG gas path, and the cycle output and efficiency are improved.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a multi-pressure reheat combined cycle configuration and, in particular, such a combined cycle configuration in which cycle output and efficiency are improved. 
     The optimization of steam cycle conditions for a combined cycle (CC) steam plant is a strong function of the constraints placed on the evaluation. A key constraint is the configuration of the surfaces within the heat recovery steam generator (HRSG), which relate not only to the gross cycle configuration, that is one pressure vs. two pressure vs. three pressure cycle, or reheat vs. non-reheat, etc., but also finer scale details of achievable steam conditions and cost/performance trade-off studies. 
     Traditionally, studies of optimal reheat pressure for a three pressure reheat bottoming cycle have been performed with the reheat sections of the HRSG constrained to be upstream of the HP evaporator section with respect to the exhaust gas flow. An exemplary such HRSG is shown and described, for example, in U.S. Pat. No. 5,628,179, the disclosure of which is incorporated herein by this reference. These studies showed that cycle output peaked at approximately 20-25% P CRH /P THROTTLE . This result was obtained for a three pressure reheat cycle wherein the IP superheater discharge steam is combined with the cold reheat (CRH) steam from the steam turbine and sent to the reheater. A similar result was obtained as well with two and three pressure cycle variations wherein the IP steam was generated at a pressure less than P CRH  and admitted to an IP turbine admission. 
     BRIEF SUMMARY OF THE INVENTION 
     A more recent study demonstrated that combined cycle performance could be improved by reducing reheat pressure and placing some of the reheater surface downstream of the HP evaporator section in the heat recovery steam generator. This result overturned all previous studies of optimal reheat pressure which had been erroneously constrained to perform of all the steam reheating in an HRSG reheater section upstream of the HP evaporator, which adversely impacts HP steam production when the cold reheat pressure is less than approximately 20% of throttle pressure. 
     The invention is thus embodied in an improved HRSG surface arrangement which in combination with appropriate cycle steam conditions yields a cost effective performance improvement over current steam bottoming cycle practice. A key feature of the proposed arrangement is placement of the coldest section of the reheater downstream of the HP evaporation section with respect to the direction of exhaust gas flow. This improves steam bottoming cycle performance because it allows use of lower reheat pressures without a penalty in HP steam production. Lower reheat pressure as compared to current practice improves cycle output by reducing steam turbine exhaust moisture which improves steam turbine efficiency. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These, as well as other objects and advantages of this invention, will be more completely understood and appreciated by careful study of the following more detailed description of the presently preferred exemplary embodiments of the invention taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a schematic illustration of a conventional combined cycle system; 
     FIG. 2 is a schematic illustration of a combined cycle system in accordance with an exemplary embodiment of the invention; 
     FIG. 3 is a schematic illustration of a combined cycle system in accordance with an alternate embodiment of the invention; and 
     FIG. 4 is a graph of net bottoming cycle output versus cold reheat pressure for a range of throttle pressures. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention is incorporated in a single pressure or a multi-pressure reheat combined cycle power generation system. A schematic of a conventional three pressure reheat combined cycle power generation system is shown in FIG.  1 . In this schematic illustration steam flow is indicated by a solid line, water flow is indicated by a dashed line, and air and gas flow are indicated by a long and short dash line. 
     This example includes a gas turbine system  10  comprising a compressor  12 , a combustion system  14  and a gas turbine  16 , and a steam turbine system  18  including a high pressure section  20 , an intermediate pressure section  22 , and one or more low pressure sections  24  with multiple steam admission points at different pressures. The low pressure section  24  exhausts into a condenser  26 . The gas turbine  10  and steam turbine  18  drive the generator  28  (or other load). The gas turbine  10 , steam turbine system  18 , and generator  28  may be arranged in tandem, on a single shaft  30  as shown in FIG. 1, or in a multi-shaft configuration wherein the gas turbine and steam turbine drive separate loads. 
     The steam turbine system  18  is associated with a multi-pressure HRSG  32  which includes low pressure (LP), intermediate pressure (IP) and high pressure (HP) economizers  34 ,  36 ,  38 , respectively, an LP evaporator  40 , further HP and IP economizers  42 ,  44 , an IP evaporator  46 , an LP superheater  48 , a final HP economizer  50 , an IP superheater  52 , an HP evaporator  54 , an HP superheater section  56 , a reheater  58 , and a final HP superheater section  60 . 
     Condensate is fed from condenser  26  to the HRSG  32  via conduit  62  with the aid of condensate pump  64 . The condensate subsequently passes through the low pressure (LP) economizer  34  and into the LP evaporator  40 . Steam from the LP evaporator  40  is fed via conduit  66  to the LP superheater  48  and then returned to the low pressure section  24  of the steam turbine  18  via conduit  68  and appropriate LP admissions stop/control valve(s) (not shown). 
     Feedwater with the aid of feedwater pump(s)  70  passes (1) through the IP economizers  36 ,  44  via conduit  72  and to the IP evaporator  46 , and (2) through the HP economizers  38 ,  42  via conduit  74  and then on to the final HP economizer  50  via conduit  76 . At the same time, steam from the IP evaporator  46  passes via conduit  78  to the IP superheater  52  and thereafter flows via conduit  80 , is combined with the cold reheat steam  82  from the HP section  20  of the steam turbine  18  and sent through one pass  84  of the reheater  58  and through an attemperator  86 . After flowing through a second pass  88  of the reheater  58 , the reheated steam is returned to the IP section  22  of the steam turbine  18  via conduit  90  (and appropriate stop/control valves not shown). 
     Meanwhile, condensate in the final HP economizer  50  is passed to the HP evaporator  54 . Steam exiting the HP evaporator  54  passes through the HP superheater sections  56  and  60  and is returned to the HP section  20  of the steam turbine  18  by way of conduit  92  and appropriate stop/control valves (if required, not shown). 
     Heat is provided to the HRSG  32  by the exhaust gases from gas turbine  16  introduced into the HRSG via conduit  94  and which exit the HRSG to a stack (not shown) via conduit  96 . 
     As mentioned above, FIG. 1 illustrates the conventional arrangement with regard to the placement of the coldest reheater section  84  within the HRSG  32 . Exhaust from the gas turbine  16  enters the HRSG  32  where it encounters high temperature superheater  60  and  56  and reheater  58  sections  88 ,  84  disposed upstream of the HP evaporator  54  with respect to the direction of gas flow. Thus, in this conventional arrangement, the coldest section  84  of reheater  58  is upstream of HP evaporator  54  and, as mentioned above, the IP superheater  52  discharge is combined with the cold reheat steam  82  from the HP section  20  of the steam turbine  18  and sent through the reheater  58 . 
     The multi-pressure reheat configuration provided in accordance with the invention is a modification of a conventional combined cycle system of the type illustrated in FIG.  1  and described above with respect thereto. Those components of the inventive system that correspond to components of the conventional system are identified with corresponding reference numbers incremented by a factor of 100. However, a detailed discussion of the components of the embodiments of the inventive system will be generally limited to those that differ from the conventional configuration. Reference numbers shown in FIGS. 2 and 3 but not discussed hereinbelow are substantially identical to the corresponding components of the conventional system and are labeled to provide a frame of reference. 
     As noted above with respect to FIG. 1, in the conventional arrangement, the coldest section  84  of the reheater  58  is disposed upstream of the HP evaporator  54 . In the configuration proposed in accordance with the present invention, schematically illustrated in FIG. 2, the coldest section  184  of the reheater  158  is downstream of the HP evaporator  154 . In the presently preferred embodiment, the exhaust  182  from the HP section  120  of steam turbine  118  will be mixed with IP steam of equal temperature that may be supplied either directly from the IP steam drum  198 , if present, or an IP superheater  152  itself downstream of the HP evaporator  154  and upstream of the IP evaporator  146 . The presence or absence of the IP superheater  152  will be governed by the economic and performance trade-offs of achieving a temperature match with the HP steam turbine exhaust  182 . 
     FIG. 3 shows a further alternate embodiment of the invention in which the IP steam  280  is not mixed with the HP steam turbine exhaust  282 . Rather, in this embodiment, the IP stream  280  is admitted to the IP section of the steam turbine  218  at a pressure lower than the hot reheat pressure and at a temperature determined through evaluation of the economic and performance trade-offs associated with superheating this steam. 
     FIGS. 2 and 3 show embodiments of the invention adapted to a three pressure system, but as will be appreciated from a review of the foregoing discussion, the same principle of placing the coldest reheater downstream of the high pressure evaporation section can be applied to a cycle with any number of pressure levels (1 or more). The figures also show a drum type HRSG which is also not necessary to the implementation of the invention and realization of its benefits. The HRSG could be of once through design with no steam drums or even supercritical in which case the reheating of steam downstream of the HP pinch would be covered. 
     FIG. 4 shows the results of the cycle study intended to identify optimum steam cycle conditions for a three pressure reheat steam cycle. In these figures, 10% and 13% P CRH /P THROTTLE  lines are with a cycle in accordance with the invention. As is particularly clear in FIG. 4, bottoming cycle performance improves with reduced reheat pressure (P CRH ) when the coldest reheater  184 ,  284  is allowed to be downstream of the HP evaporator  154 ,  254  in accordance with the invention. 
     As will be understood from the foregoing disclosure, this invention is applicable to all reheat combined cycles with sufficiently low hot reheat pressure constraints. Some systems may have constraints limiting the minimum practical reheat steam pressure (e.g. IGCC). These systems may also benefit from the proposed invention but will would generally favor high throttle pressures. 
     While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.