Abstract
An overall picture of combined gas and steam cycle options, without dependence on numerical analysis, is long overdue. A single equation is derived for plant thermal efficiency. This is based upon component efficiencies and the ratio of steam plant size to gas turbine size. Therefore, a unique map can be graphically presented covering the whole steam range from power generation through to cogeneration (that is combined heat and power) in association with gas turbines.
Understanding of the map leads to recognition of an underutilized area. The gas turbine is normally integrated with the main steam boiler. Conversely, the unfixed combined cycle plant may be integrated with a conventional boiler and back-pressure turbine. The correct steam turbine-gas turbine size ratio must be chosen.
A practical example is given of how a gas turbine and waste heat boiler may be introduced part-way along a large power station's main steam turbine. The integration is achieved in the steam cycle and not at the main boiler. This approach allows low-risk, low capital cost, simple repowering of large power stations. It is not a new thermodynamic cycle, but has the following advantages:
The tie between the extent of after-firing and waste heat boiler efficiency is avoided. The conventional boiler is fired and the waste heat boiler is unfired. Integration of the plant occurs at the intermediate pressure level of the steam turbine. The above integration gives high thermal efficiency. It also retains the conventional boiler, or equivalent, which is free to burn a range of fuels. The gas turbine waste heat boilers and the main boiler together fill a common steam turbine of an economic size.
Two types of plant are considered to demonstrate the practical usefulness of the integrated steam cycle. Examples are given based on coal-fired, or heavy-fuel-oil-fired combined cycle plant, integrating conventional boiler, gas turbine plus unfired boiler and steam turbine. The data are based on real equipment, which is either at existing UK installations or plant of known performance.
An explicit analysis is also presented to compare fuel running costs for the integrated steam cycle with costs for both unfired combined cycle plant and conventional steam plant.
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