Improved design of supercritical CO2 Brayton cycle for coal-fired power plant

Abstract

As an advanced power cycle, supercritical CO2 (sCO2) Brayton cycle has been considered as a promising alternative of conventional steam Rankine cycle for coal-fired power plants. The sCO2 power cycle must be improved to deal with coal-fired system integration constraints since coal-fired boiler is significantly different from nuclear and CSP heaters. The inlet temperature of the working fluid entering coal-fired boiler in sCO2 cycle is much higher than that in steam cycle, as it can be preheated more sufficiently in recuperators. Hence, the exhaust heat of coal-fired boiler flue gas cannot be fully utilized itself. It is of great importance to study on how to make good use of the exhaust heat of flue gas. An in-house code of sCO2 Brayton cycle tailored for coal-fired power plant was developed at first. Then, three improved cycle layouts for better utilization of the exhaust heat of flue gas were proposed, which were assessed in depth based on comprehensive analyses of both sCO2 boiler and cycle layout. The improved cycle with a second split flow to the boiler was proved to be the most effective one. With parameters of 31MPa/600 °C/620 °C, the maximum net efficiency was improved from a based value of 45.96% to an optimized value of 50.71%. It was also higher than that of a state-of-the-art ultra-supercritical steam power plant with same paremeters (about 46%–47%). Finally, the effects of the second split flow ratio on net efficiency were analyzed, and the equations to calculate the optimal range of second split flow ratio was derived.