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Chapter 2
Thermal Cycle and Layout of a Pulverized-Coal—Fired Power Plant

Excerpt

The thermal cycle of a 750-MW size pulverized-coal—fired reference power plant is shown in the heat balance diagram of Figure 2-1. The pulverized-coal—fired steam generator provides main steam of 4133 psia/1112°F (285 bar/600°C) and reheat steam of 870 psia/1112°F (60 bar/600°C). The supercritical steam is generated in a once-through Benson-type boiler with a steam generator efficiency of 95%. The turbine is a single-reheat steam turbine with 2 double-flow LP turbine sections. The HP turbine is designed as a single-flow turbine with an extraction at a pressure of about 1360 psia (94 bar) to provide a final feedwater temperature of 581°F (305°C). The main steam expands in the HP turbine to cold reheat steam parameters of 935 psia (64.5 bar) and 684°F (362°C). Cold reheat steam is fed back to the reheater section of the boiler and after being reheated to 1112°F (600°C), enters the double-flow IP turbine, which features 2 extractions for feedwater heating and for steam supply to 2 boiler feedwater pump turbines. The IP turbine exhaust steam with a pressure of 67 psia (4.6 bar) enters the 2 double-flow LP turbines. The LP turbines are equipped with 3 extractions for the LP feedwater heaters. With the extraction steam supply from the HP and IP turbine exhausts, the thermal cycle features 8 extraction points for the 8 feedwater heaters.

Two 50% boiler feedwater pumps with each driven by a 12.8-MW boiler feedwater pump turbine. Since the plant is operated in a variable or sliding pressure mode, the feedwater pumps only have to provide the full main steam pressure at full load. At variable pressure operation one of the 50% pumps can supply feedwater up to about 70% load at the reduced sliding pressure level of roughly 2900 psia (200 bar).

The LP turbine backpressure of 1.5 in.Hg.abs. (51 mbar) has been selected for the freshwater application. This relatively low backpressure requires a large LP turbine annulus exhaust area and a cooling water temperature of about 15 to 20°C (59°F to 68°F). For the 750-MW rating, 2 double-flow LP turbine sections with titanium last-stage blades have been selected. The annulus area of each of the 4 exhaust sections covers 119 sq ft (11.1 m2). The titanium last-stage blades for this 750-MW/60-Hz application are about the largest last-stage blades available today. For higher backpressures, smaller LP turbine sections with shorter last-stage blades can be applied. For larger units or lower backpressures, even a 6-flow instead of 4-flow LP turbine arrangement could be considered. The net efficiency of power plants is highly affected by the selected LP turbine backpressure. Figure 2-2 shows this effect when smaller or larger LP turbines are selected to keep the exhaust velocity about constant.

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