COMPLEX SYSTEM: COAL-FIRED ELECTRICAL GENERATING STATION

Energy and exergy analyses are applied to a coal – fired electrical generating station. The coal-fired Nanticoke Generating Station is selected as the representative station for the analysis. The station has a net unit electrical output of approximately 500 MWe and is operated by the provincial electrical utility, Ontario Power Generation (formerly Ontario Hydro), in Ontario, Canada. This example clearly illustrates how exergy analysis allows process in­efficiencies to be better determined than does an energy analysis and efficiencies to be more rationally evaluated. Consequently, the example identifies areas in which the potential for performance improvement is high and trends that may aid in the design of future stations.

2.1 Process Description

A detailed flow diagram for a single unit of the station is shown in Fig. 2. The symbols identifying the streams are described in Table I, in which data are provided for the streams. The diagram in Fig. 2 is divided into four main sections:

• Steam generation: Heat is produced and used to generate and reheat steam. In the station, eight pulverized coal-fired natural circulation steam gen­erators each produce 453.6 kg/s steam at 16.89 MPa and 538°C and 411.3 kg/s of reheat steam at 4.00 MPa and 538°C. Air is supplied to the furnace by two 1080-kW, 600-rpm motor-driven forced-draft fans. Regenerative air preheaters are used. The flue gas passes through an electrostatic precipitator rated at 99.5% collection efficiency and exits the plant through two multiflued, 198-m-high chimneys.

• Power production: The steam produced in the steam generation section is passed through a series of turbine generators that are attached to a transformer. Extraction steam from several points on the turbines preheats feedwater in several low – and high-pressure heat exchangers and one spray-type open deaerating heat exchanger. The low-pressure turbines exhaust to the condenser at 5 kPa. Each station unit has a 3600­rpm, tandem-compound, impulse-reaction turbine generator containing one single-flow high-pressure cylinder, one double-flow intermediate-pressure cy­linder, and two double-flow low-pressure cylinders. Steam exhausted from the high-pressure cylinder is reheated in the combustor.

• Condensation: Cooling water from Lake Erie condenses the steam exhausted from the turbines. The flow rate of cooling water is adjusted so that a specified temperature increase in the cooling water is achieved across the condenser.

• Preheating: The temperature and pressure of the feedwater are increased in a series of pumps and feedwater-heater heat exchangers.

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