EFFICIENCY IMPROVEMENT OF COMBINED CYCLE POWER PLANT USING TRIPPLE PRESSURE HRSG

Abstract

By combining a Brayton cycle used as topping cycle and a Rankine cycle Bottoming it we can achieve a solution that allow efficiency and power of installation. Such an arrangement when used to produce power by electric generator is called combined cycle power plant (CCPP). It utilizes the gas turbine for Brayton cycle and steam turbine for Rankine cycle with air and water as working fluids to achieve efficient, reliable, and economic power generation. Currently overall thermal efficiencies up to 60% are confirmed by the foremost manufacturers from the sector as state of the art and special modifications have been proposed to improve the overall thermal efficiencies more than 60%. The exhaust gas temperature entering the HRSG is in the range of 600 °C and gradually reduces to 150 °C or less as it flows over the various heat transfer sections. For Rankine cycles to be more efficient the steam pressure has to be high. Currently many of the steam turbines in Combined Cycle plants operate at 140 bar - the High Pressure (HP) circuit. At pressures of 140 bar the saturation temperature will be above 335 °C. The gas exiting the evaporative section will be in the order of 335 °C. This means absorption of the balance heat of the flue gas has to be in the economizer which is not feasible. The high temperature exhaust is let to atmosphere and energy is wasted hence to absorb more heat two additional evaporators are used. One operating at lower pressure 40 bar - Intermediate Pressure (IP) circuit with gas exiting at 255 °C. The second one at 7 bar - Low Pressure (LP) circuit with gas exiting at 170 °C is used. This triple pressure HRSG together with a combination of Re-heated Rankine cycle help improve cycle Rankine efficiency together with absorbing more heat.