How to Construct a Combined S-CO₂ Cycle for Coal Fired Power Plant?

It is difficult to recover the residual heat from flue gas when supercritical carbon dioxide (S-CO₂) cycle is used for a coal fired power plant, due to the higher CO₂ temperature in tail flue and the limited air temperature in air preheater. The combined cycle is helpful for residual heat recovery. Thus, it is important to build an efficient bottom cycle. In this paper, we proposed a novel exergy destruction control strategy during residual heat recovery to equal and minimize the exergy destruction for different bottom cycles. Five bottom cycles are analyzed to identify their differences in thermal efficiencies (η), and the CO₂ temperature entering the bottom cycle heater (T₄) etc. We show that the exergy destruction can be minimized by a suitable pinch temperature between flue gas and CO₂ in the heater via adjusting T₄. Among the five bottom cycles, either the recompression cycle (RC) or the partial cooling cycle (PACC) exhibits good performance. The power generation efficiency is 47.04% when the vapor parameters of CO₂ are 620/30 MPa, with the double-reheating-recompression cycle as the top cycle, and RC as the bottom cycle. Such efficiency is higher than that of the supercritical water cycle power plant.