Thermodynamics analysis of S-CO<sub>2</sub> recompression-reheating cycle for coal fired power plant

Abstract

As a kind of fossil energy, coal plays an important role in global energy structure. In China, coal fired power plants provide more than 60% of the total electricity supply. Compared to the water-steam Rankine cycle, the supercritical carbon dioxide (S-CO2) Brayton cycle exhibits advantages in simpler cycle layout and higher efficiencies. The objective of this paper is to explore the possibility that the S-CO2 Brayton cycle can be used for large scale coal fired power plant. Such application presents challenges in the thermodynamic cycle construction, arrangement of heat transfer components and residual heat utilization of flue gas. Here, the S-CO2 recompression cycle, incorporating reheating/double-reheating technique, is integrated with the coal fired power plant. The performance of the power plant is analyzed by coupling the thermodynamics cycle, boiler pressure drops in heat transfer components, and thermal energy distribution of flue gas in the boiler tail flue. The residual heat of flue gas is absorbed by the S-CO2 cycle, an additional flue gas cooler (FGC) and two air preheaters to satisfy the energy cascade utilization principle. The results show that, if the S-CO2 recompression cycle is used alone without reheating, the low-temperature flue gas energy is difficult to be absorbed to cause non-acceptable outlet flue gas temperature such as higher than 120°C. By neglecting pressured drops in heat transfer components, the S-CO2 recompression cycle incorporating reheating increases the system thermal efficiency, but makes it difficult to recover the residual heat of flue gas. When pressure drops are considered, we show that, for the first time, the curves of thermal efficiencies versus reheater pressure drops are crossed between the S-CO2 recompression-reheating cycle and the S-CO2 recompression-double-reheating cycle. Based on this finding, the criterion is proposed to judge at what condition a single-reheating cycle is needed, and at what condition a double-reheating cycle is necessary. It is demonstrated that the CO2 temperature entering the boiler can be decreased by elevating the main vapor pressure. Thus, the adjusting main vapor pressure method is proposed to absorb the residual heat of flue gas. The system efficiency is sharply increased with increase of main vapor pressures. Because the main vapor temperature T 5 and pressure P 5 are strongly coupled, for example, P 5 reaches 43.6 MPa at T 5=615°C and the secondary air temperature of 500°C, the adjusting main vapor pressure method challenges the material pressure tolerance limit. Adding a flue gas cooler (FGC) not only recovers the residual heat of flue gas, but also ensures that the pressure is within the material pressure tolerance limit. The optimal scheme of the FGC integrated with the thermal cycle is investigated, among which, a smaller CO2 flow rate stream consecutively extracted from the main compressor outlet, being heated by the low temperature flue gas, and entering the outlet of the low temperature recuperator heat exchanger, is the best. By using the S-CO2 recompression-reheating cycle together with a FGC, the system thermal efficiency and boiler efficiency attain 50.82% and 94.43%, respectively. This paper gives a clue to design and operate high efficiency, simple layout and compact S-CO2 coal fired power plant.