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A new type of improved coal-fired recompression SCO2 Brayton cycle with a second split flow to the boiler was introduced. The design parameters of the high and low pressure turbines of the above SCO2 Brayton cycle were determined based on a consideration of the economics of the SCO2 system and the realization of the equipment. The self-compiled mean-line design program for two-stage SCO2 axial turbines was used to design the high and low pressure turbines for the 5 MW SCO2 Brayton cycle, with the important design data of the two turbines being given. RANS equations and the SST Turbulence Model were chosen for numerical simulation of the aerodynamic performance of two turbines in the design point and variable operating conditions that used ANSYS CFX and the real physical property data file of SCO2 from NIST software. Numerical results indicated that the isentropic efficiency of the SCO2 high pressure turbine and low pressure turbine with a good variable operating performance are 82.88% and 82.26% in the design point, respectively. The output power of the high pressure turbine and low pressure turbine are 3,251 kW and 6,156 kW, respectively, in the design point, which can meet the design requirements. The Cps curves, limited streamline spectrums, and the Mach number contours of the root, middle and top flow field of the turbine blades is reasonable.

The concept of the supercritical CO2 (S-CO2) power cycle has been widely proved to be effective by several small scale test loops. However, more specific system layout for fossil-fired power generation and key components design assessment is still imperative to demonstrate the technology feasibility of commercial scale utility. The S-CO2 power cycle must be optimized to deal with fossil-fired system integration constraints. In the present study the technology adaption of S-CO2 power cycle for fossil-fired power plant has been evaluated in terms of both the whole thermodynamic cycle layout and the preliminary assessment of key components. The design considerations and selections of key parameters such as turbine inlet parameters, compressor inlet parameters and split flow ratios were analyzed by a self-developed code for the purpose of optimization design of a 5 MW fossil-based S-CO2 pilot test loop. The proposed recompression and reheat cycle with two split ratios tailored for fossil-fired power plants can achieve 33.49% net efficiency. As a first step to the final recompression cycle, the design of simple recuperated and reheat cycle after carefully considerations of the specific design assessment of core components such as boiler, turbines, compressor and compact heat exchangers has been accomplished and currently under construction.

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.

An improved recompression S–CO2 cycle for coal-fired power plant is proposed in order to improve boiler thermal efficiency, increase cycle exergy efficiency, and resolve issues of regulating the working fluid temperature of the boiler rapidly and effectively. The models for energy and exergy analyses are built. Systematic comparisons between the improved S–CO2 cycle system and the original S–CO2 cycle system are carried out based on the detailed energy and exergy analyses. The results suggest that the improved S–CO2 cycle is excellent in both energy and exergy efficiencies, the thermal efficiency and exergy efficiency of the boiler in the improved S–CO2 cycle system are 94.68% and 53.41%, respectively, which are 1.04% and 2.76% higher than those of the original S–CO2 cycle system. The power efficiency of the improved S–CO2 cycle system is higher than that of the original one, which are 48.06% and 47.89%, respectively. The exergy efficiency of the entire cycle system of the improved S–CO2 cycle is 48.24%, which is 2.32% higher than that of the original S–CO2 cycle. In addition, the spray attemperator performance is investigated. The boiler in the improved S–CO2 cycle can regulate the working fluid temperature effectively.

Supercritical CO2 (sCO2) Brayton cycle usually shows high efficiency and also has compact structure, which can be considered as a promising alternative in future coal-fired power plants. However, sCO2 boiler has different structures and configurations compared with the conventional steam boiler, especially for the arrangement of radiative heat transfer surfaces. Proper design of vertical gas wall and reheat gas wall is crucial to the success design of sCO2 boiler. In this study, a thermohydrodynamic model of radiative heat transfer surfaces was developed to predict pressure drops and wall temperature distributions. A sCO2 power cycle calculation model was employed to comprehensively analyze the influences of pressure drops on the thermal efficiency. A 300 MW sCO2 boiler was taken into account as the baseline case and thermohydrodynamic analyses of gas wall and reheat gas wall were carried out. The influences of flow directions and mass fluxes on wall temperature, pressure drops and thermal efficiency were obtained by the present models. Then, optimized structures of radiative heat transfer surfaces were proposed, i.e., vertical downward gas wall with tubes of φ34 × 4 mm and vertical upward reheat gas wall with tubes of φ57 × 6 mm. Accordingly, the net cycle efficiency achieved an optimized value of 50.07% under the conditions that the wall temperatures of radiative heat transfer surfaces did not exceed the allowable temperature.