Multiple Pressure Systems

Steam turbine Rankine efficiency is improved when the heat drop across the turbine is increased. Therefore, since the exit condition is set by ambient temperature (and cost considerations related to the condenser, final pressure, etc.), efficiency can be improved by operating the steam generator at higher temperature and pressure. With combined-cycle operation, a balance must be struck between gas turbine (or reciprocating engine), HRSG, and steam turbine efficiency.

As HRSG temperature and pressure are increased, the rate of exhaust gas heat energy utilization is decreased. Conversely, lower steam pressure and lower exhaust gas exit temperature increase the overall utilization of the exhaust gas heat. An effective way to incorporate higher steam pressures and temperatures into combined-cycle systems is to use a multi-pressure HRSG. This requires a steam turbine with two or more steam admissions: one at high pressure and one or more at lower pressures. These configurations allow for greater thermodynamic efficiency of the steam turbine because higher-pressure steam can be used in the first stage without compromising HRSG efficiency.

For example, with a multiple-pressure system that produces 39 MW from the steam turbine, overall net thermal efficiency would be increased from 45.4% single-pressure efficiency to 48.1% (LHV) and heat rate would be reduced to 7,096 Btu/kWh (7,483 kJ/kWh). The thermal efficiency of this combined-cycle plant is:

Today's larger capacity combined-cycles featuring high-efficiency gas turbines and triple-pressure steam cycles can achieve total thermal fuel efficiencies in excess of 50% (LHV basis). Figures 12-4 and 12-5 show combined-cycle power generation systems with capacities of about 250 MW. Both feature gas turbines, triple-pressure steam cycles, and single-shaft power blocks. Both systems guarantee NOx emissions of below 25 ppm when operating on natural gas and are believed capable of achieving single-digit ppm NOx emissions levels.

Figure 12-4 shows a labeled layout of a 251 MW system featuring a 160 MW ABB sequential combustion gas turbine, with dual combustion chambers. The gas turbine features a 30:1 pressure ratio, which is unusually high for such an efficient combined-cycle system. The steam turbine is part of the single power train, with the common generator located in the middle. The three variable stators at the compressor inlet allow the gas turbine to be operated with a relatively flat efficiency curve in the part-load range in combined-cycle operation. By adjusting the stators, the mass flow is reduced linearly to 60% of the full-load figure, allowing the turbine exhaust to be maintained at almost its design point of about 1,130°F (610°C).

Figure 12-5 shows a basic heat balance for a 254 MW system featuring a Siemens 170 MW gas turbine. The common hydrogen-cooled generator is solidly coupled to the gas turbine, with a synchronous clutch used for the steam turbine connection. The two-casing steam turbine features a high-pressure turbine and a combined intermediate- and low-pressure turbine with an axial exhaust to the condenser. The gas turbine features a pressure ratio of 16.6:1 and an exhaust flow of 3,600,000 lbm/h (454 kg/sec) at a temperature of 1,004°F (562°C).

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