Replacing steam Rankine cycle with supercritical CO2 Brayton cycle (S-CO2 cycle) has been demonstrated to be an effective way to improve the energy efficiency of oxy-coal power plants. Several improved S-CO2 cycles and boiler configurations for fluidized bed oxy-coal power plants are proposed in this paper to utilize the flue gas heat in whole temperature zones more efficiently and increase the thermal and exergy efficiency of the whole system. A detailed exergy analysis and energy analysis model is built to evaluate the performance of different S-CO2 oxy-coal fluidized bed power systems. First, the system with S-CO2 oxy-coal fluidized bed boiler operated under different flue gas recirculation modes was compared, the results suggest that the boiler under wet mode achieves the highest boiler efficiency (92.69%) because of the lower exhaust heat loss, next is the half-dry mode boiler (91.77%) and dry mode boiler (90.14%). Then, different S-CO2 cycles driven by the wet mode boiler were analyzed. Among those S-CO2 cycle configurations for medium-temperature flue gas heat utilization, the thermal efficiency of the configuration with working fluid splitting at the inlet of high-temperature recuperator cold side(SHTR) achieves 52.21%, which is 2.15% and 0.61% higher than those of low-temperature recuperator cold side inlet split case(SLTR) and connected-top-bottom cycle case(CTB), due to lower energy lost in cooler and exergy destructed in high-temperature recuperator and cooler. For different improved S-CO2 cycles based on SHTR, the results show that the cycle with the partial intercooling and double recompression processes is more effective to improve the cycle efficiency of SHTR by 1.24%. Although the boiler thermal efficiency with this improved cycle decreases slightly due to rising exhaust flue gas temperature, the net power efficiency of this system can still achieve 37.59%(LHV). Furthermore, to reduce its exhaust flue gas temperature, a further improved cycle configuration was proposed and finally, the net efficiency of the S-CO2 oxy-coal fluidized bed combustion system reaches 38.01%(LHV).

Energy and exergy analyses of supercritical CO2 cycle driven by oxy-coal fluidized bed boiler

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