Cogeneration Cycle Comparison

Figure 3-12 is a comparison of FCP heat rate for three advanced cogeneration cycle alternatives:

1. A simple-cycle system featuring a gas turbine and HRSG with bypass stack and supplementary firing.

2. A combined-cycle system featuring a gas turbine, HRSG, back-pressure steam turbine cycle with bypass stack, and supplementary firing.

3. A combined-cycle system featuring a gas turbine, HRSG, condensing-extraction steam turbine cycle with supplementary firing, but no bypass stack.

The comparisons are based on a basic power cycle featuring steam operating conditions of 900 psig/900°F (63 bar/482°C) and 150 psig (11.4 bar) saturated process steam, 2.5 in. (6.4 cm) Hg condenser pressure, 2% blowdown, and

Fig. 3-11 Basic Heat Balance Diagram for Combined-Cycle Cogeneration System Featuring Gas Turbine, Heat Recovery Steam Generator and Non-Condensing Steam Turbine.

180°F (82°C) condensate return. The displaced boiler energy conversion efficiency taken into account in the FCP expression is assumed to be 83%.

For each system, the FCP heat rate varies as the amount of steam sent to process (actually steam per MWh of power generated) varies. In the first two alternatives, zero steam sales represents a gas turbine with all exhaust vented to the atmosphere (a peaking turbine), and the indicated 11,000 Btu/kWh (11,600 kJ/kWh) heat rate is similar to those of conventional steam cycle plants.

Alternative 3, at 8,000 Btu/kWh (8,440 kJ/kWh), is roughly 25% more efficient than the average conventional steam cycle plant, since the listed condensing-extraction steam turbine remains in operation at zero steam sales. The FCP heat rate drops with increased process steam use, until a ratio of about 3 Mlb/MWh (1,360 kg/MWh) is reached. This is the point at which no more steam can be taken from the condensing section of the steam turbine (below the extraction port) without overheating, and additional steam must come from supplemental firing with a duct burner or from a fired heat recovery boiler. Because this "fired" steam is used less efficiently, the heat rate curve starts to climb back slowly with greater firing, but the cycle remains roughly twice as efficient as conventionally produced power purchased from the utility grid.

The back-pressure cycle (Alternative 2) break point allows slightly greater steam export before firing, since all steam passes completely through the steam turbine. None of the fired steam ever goes to condenser, so this cycle keeps getting more efficient as supplemental firing increases.

Fig. 3-12 Fuel-Chargeable-to-Power Comparison of Three Cogeneration Cycles. Source: Cogen Designs, Inc.
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