Exergy analysis of supercritical CO2 coal-fired circulating fluidized bed boiler system based on the combustion process

Abstract

A process simulation on a 600 MW supercritical CO2 (S–CO2) coal-fired circulating fluidized bed (CFB) boiler system was established with the detailed combustion reaction and heat exchange being involved, based on which the detailed exergy distributions of the boiler, as well as its dependence on the varying operating conditions were comprehensively studied. The results showed that the boiler accounting for the largest exergy loss has the exergy efficiency of about 57.7%, which is higher than that of pulverized coal boilers with steam cycle or S–CO2 cycle by 3–5% points. As the excess air coefficient increases, the exergy efficiency of the boiler increases first and then decreases, and the better performance is obtained under 1.1–1.3 of excess air coefficient. When the load decreases, the heat proportion of the cooling wall increases significantly while that of the high-temperature heater accordingly decreases. The exergy efficiency of the boiler decreases with decreasing load especially under the load of 80%∼60%. Additionally, the combustion simulation also effectively predicted the decreases in the SO2, NO and CO emissions with the increasing excess air, and the sharp decrease in SO2 emission as the load is below 80%.