Optimization of boiler cold-end and integration with the steam cycle in supercritical units

In order to gain an extra increment of efficiency to compensate for capital costs, one of the main issues in the design of advanced supercritical power plants is the reduction of boiler exit gas temperature below typical values of conventional, subcritical units. Currently, the use of heat exchange surfaces made of plastic has become feasible, thereby avoiding corrosion and fouling problems derived from cold-end acid condensate. In this manner, flue gas temperature can be reduced down to typically 90 °C, which obviously leads to an increase of boiler efficiency. Besides, there is an additional energy available for heating the main condensate flow of the power cycle. If modification of air-gas rotary heaters is also considered, a manifold of possibilities opens up for plant optimization and integration of components. The objective of this paper is to analyze this class of schemes for increasing power output and net efficiency of a reference supercritical plant. A complete simulation of the steam cycle is assembled using Aspen Plus and different plant configurations are examined under reduced exit gas temperatures. Several uses of flue gas energy are considered, taking into account limits of temperature and realistic efficiencies of heat exchangers. Mass flow rates, point of extraction of condensate, pressures and temperatures are selected heuristically to optimize performance. Finally, required exchange areas are estimated, and a cost analysis is carried out in order to economically assess the new configurations and estimate the additional profit for the plant.