Multi-pressure reheat combined cycle with multiple reheaters

Details Multi-pressure reheat combined cycle with multiple reheaters Multi-pressure reheat combined cycle with multiple reheaters

1999-09-13

2001-04-24

Kim, Ted (Department: 3746)

Power plants

Combustion products used as motive fluid

C060S039182, C110S219000

Reexamination Certificate

active

06220013

ABSTRACT:

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.


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