A new method to boost performance of heat recovery steam generators by integrating pinch and exergy analyses

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Combined cycle power plants are gaining more attention throughout the world and their usage is increasing. This is because their efficiency is higher than simple Rankine or Brayton cycles. However, their efficiency could still increase. In this article, the driving force plot and exergy analysis are used to study the behavior of heat exchangers of a heat recovery steam generator in base case and off-design conditions. Based on the obtained results, a new method is proposed to improve performance of a heat recovery steam generator during the summers in which higher ambient temperature results in lower efficiency of plant. In this new method, hot flue gas at the heat recovery steam generator inlet is split which results in higher steam production in heat recovery steam generator. The results showed that the new proposed method is very beneficial and increases net output power of the plant. When split ratio increases from 2.5% to 15%, the power produced in steam turbine increases from 0.15 to 0.9 MW. It also increases exergy destruction of the heat recovery steam generator. This is due to higher mass flow rate of the produced steam and higher temperature difference between hot and cold streams. It is shown that this new method can reduce the negative effects of consuming more fuel in duct burner during the summers.