Page 137 - Handbook of Energy Engineering Calculations

combined-cycle plant has a heat rate of 700 Btu/kWh (738.5 kJ/kWh) less
than the steam plant. This is a saving of 6.7 percent. Such a saving can be
significant in a time of rising fuel prices and increased concern over
atmospheric pollution.
At about 30 percent of capacity the heat rates for the two types of plants

are equal. But once this capacity is exceeded, the combined-cycle plant is
more efficient. This is why the gas turbine is proving so popular in today’s
central stations for both topping and combined-cycle duty. Figure 7 is from

Skrotzki and Vopat—Power Station Engineering and Economy, McGraw-
Hill 1960.

FIGURE 7 Combined gas-turbine–steam-turbine plant works with
superior efficiency compared with conventional steam-turbine plant.

SELECTING MOST EFFICIENT METHOD FOR

INCREASING COMBINED-CYCLE PLANT OUTPUT

Select the best option to boost the output of a 230-MW facility based on a
155-MW natural-gas-fired gas turbine (GT) featuring a dry low NO,

combustor (Fig. 8). The plant has a heat-recovery steam generator (HRSG)
which is a triple-pressure design with an integral deaerator. A reheat