Multi-pressure reheat combined cycle with multiple reheaters

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.

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.sub.CRH /P.sub.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.sub.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.

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.sub.CRH /P.sub.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.sub.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.