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Plant Performance from 40 to 105% of Base Load for a Medium-sized Coal-fired Steam Plant

Whereas in STEAM PRO you often run many cases with different configurations or different design assumptions to optimize a design for a possible project, in STEAM MASTER, you often want to run many different operational scenarios using a single plant configuration.  This sample focuses on using a single plant model to compute expected performance under different loading situations at a fixed ambient condition.

This is a model of a 250 MW single reheat steam plant with a pulverized coal boiler, and a cooling tower servicing the wet condenser.  The plant was designed for summer ambient conditions in Manila, Philippines where the ASHRAE 0.4% evaporation ambient is 32.8 °C mean dry bulb with a 28.4 °C mean coincident wet bulb.

Thermoflow’s E-LINK utility lets you run batches of cases from MS Excel.  This model is run in E-LINK at various conditions to develop the results described below.

The marketplace for power and fuels has recently become much more dynamic. Consequently, many power plants designed for baseload operation are now operating at part loads when power prices drop overnight and on weekends, or at overload conditions when power prices peak in response to increased heating/cooling demands. STEAM MASTER can predict plant performance at the baseloaded design point, and at any other loading condition you specify. For example several runs were made in Excel (using E-LINK) based on the plant model described above.

This chart shows plant net LHV efficiency at loads above and below baseload. To reduce power below baseload, the STEAM MASTER model was configured to reduce fuel input at constant throttle pressure. The green line on the chart shows performance under these conditions. The HPT valves close as the flow to the turbine is reduced at lower boiler loads. This maintains delivery pressure so the boiler operates near its design pressure. To increase load, fuel input to the boiler is increased above the baseload level, until the steam turbine inlet pressure rises to its maximum, assumed to be 5% above rated pressure. This region is shown as the orange line, and is characterized by a slight increase in efficiency with load. Assuming the generator has enough margin, additional comes from maintaining steam production and reducing bleed steam used to heat feedwater. In this region, shown as the red line, feedwater is bypassed around the high pressure feedwater heaters which reduces steam drawn from the turbine, thereby allowing it to expand and make power. However, this reduces final feedwater temperature, requiring more fuel to be burnt in the boiler, thus reducing plant efficiency.