A Comprehensive Classification of Combined Cycle and Cogeneration Plant: Part 2: Introducing the Integrated Steam Cycle in Practice

Abstract

A single equation for plant thermal efficiency was derived in Part 1 of this paper. This equation was based upon component efficiencies and the ratio of steam plant size to gas turbine size. The analysis resulted in a unique map being graphically presented. The map covered the whole steam range from power generation through to combined heat and power, in association with gas turbines. Understanding of the map leads to recognition of an underutilized area. Part 2 of this paper explores the type of plant within that area of classification. The gas turbine is normally integrated with the main steam boiler. The natural tendency is to fully after-fire the gas turbine's exhaust. The resulting plant then overshoots the area of the map in question. Instead, the unfired 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. This particular option is suitable for a medium-scale power plant. Conversely, 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. The concept of the integrated steam cycle is developed by this paper first considering turbines and boilers as components in isolation. Next the characteristics of primary combinations of those components are considered as ‘building blocks’, each with an inherent performance and cost. Finally, the required practical blend of standard components is derived by concentrating on the predetermined size ratio. The end result is simple and, with the benefit of hindsight, may seem obvious. Nonetheless, the integrated steam cycle offers a compromise between combined cycle and conventional steam plant, with favourable performance, cost and environmental impact. 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.