Process optimization and revamping of combined-cycle heat and power plants integrated with thermal desalination processes

Abstract

Optimal revamping, sizing, and operation of an existing gas-turbine combined-cycle dual-purpose power/desalination plant – simultaneous electricity and freshwater generation – which operates with a heat recovery steam generation with one-pressure level (1P-HRSG) and a multi-stage flash desalination process, is addressed. The sizes and configurations of the gas turbine and desalination unit are kept the same as in the existing plant through the study. However, the 1P-HRSG is conveniently extended to two- or three-pressure levels with different exchanger arrangements, including steam reheating. To this end, a superstructure-based representation of the HRSG simultaneously embedding several candidate structures was proposed and a mixed-integer nonlinear programming model was derived from it. One revamping case consisted in maximizing the ratio between the freshwater production rate and the heat transfer area of HRSG, keeping unchanged the electricity generation rate (around 73 MW). It was found that the inclusion of a 3P-HRSG resulted in an increase of 13.782 kg⋅s−1 in the freshwater production, requiring 22753 m2 of heat transfer area less in the HRSG. Another revamping case consisted in maximizing the profit, contemplating the possibility to sell extra amounts of electricity and freshwater. Sale prices, for which producing extra electricity and freshwater is beneficial, were determined.