One of the variations in cogeneration systems using gas turbines is the Cheng cycle. This system is ideal for plants with varying electrical and steam loads. It consists of a gas turbine with an HRSG, which has a superheater, evaporator, and economizer (Fig. 1.19). A duct burner is located between the superheater and evaporator. The HRSG generates saturated steam, which is superheated in the superheater and injected into the gas turbine, which increases its electrical power output significantly. The figure shows an Allison 501K machine, which normally generates 3.5MW, in injection mode about 6 MW. The superheater is capable of running dry, that is, without steam. When only process steam is required, saturated steam from the evaporator is used. When additional process steam is required, the duct burner is fired. Hence the HRSG can operate in a variety of modes and at various points as shown in the figure by varying the amount of steam injected into the gas turbine and by varying the amount of fuel fired in the duct burner. Thus the plant can vary the ratio of power to process steam significantly according to the cost of fuel or electricity and thus optimize the overall efficiency. Cogeneration plants with fluctuating steam and power demands are ideal candidates for the Cheng cycle. The system’s proven success in small — scale plants is now being applied to midsized gas turbines ranging from 50 to 125MW. Cheng cycle systems are in operation in over 50 installations worldwide.

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