Effect of flue gas cooler and overlap energy utilization on supercritical carbon dioxide coal fired power plant

Abstract

Supercritical carbon dioxide (sCO2) cycle is suitable for high temperature heat source, but introduces challenging in absorbing moderate/low temperature flue gas energy for coal fired power plant. Here, we explore the effect of flue gas cooler (FGC) and overlap energy utilization (OEU) on sCO2 cycle. FGC and OEU extract moderate temperature flue gas energy by a splitting CO2 flow rate from the cycle and a combined cycle, respectively. Recompression cycle plus reheating (RC + RH) and tri-compression cycle plus reheating (TC + RH) are two basic cycle types for coal fired power plant. A thermodynamics model coupling with thermal–hydraulic characteristic is developed. The analysis is performed for a 100 MW rated power capacity. For RC + RH, we show that OEU yields an electric power efficiency increment of 0.13% compared to FGC, which is caused by smaller pressure drops in boiler components when using OEU. Even though TC + RH introduces difficulty in absorbing moderate/low temperature flue gas energy, OEU still can decrease outlet flue gas temperature (Tfg,ex) to 126 °C, which is acceptable. However, FGC achieves a higher Tfg,ex of 172.6 °C by a pinch temperature difference limit of 30 °C, which deteriorates boiler efficiency thus it is not acceptable. We conclude that OEU is better than FGC, no matter for RC + RH or TC + RH. The double-channel-tail-flue concept is proposed to not only increase thermal efficiency of the system, but also elevate boiler efficiency. This paper presents an important clue to design thermodynamics cycle for small capacity coal fired power plant, which is expected to have fast response with respect to load variations.