Proposal and comparison of two heat recovery measures for the coal-based Allam cycle: Double expansion and lower turbine backpressure

Improving the thermodynamic efficiency of the coal-fired semi-closed supercritical CO2 cycle (coal-based Allam cycle) can be achieved by implementing self-heat recuperative coal drying and recompression with more syngas heat integrated with the efficient power cycle. To prevent the heat exchanger from overtemperature during syngas heat recovery, two measures are considered: double expansion and lower turbine backpressure. For the double expansion, a portion of the recycle CO2 is fed into an expander and the exhaust is mixed with the flue gas in the main recuperator to raise the recycle flow rate. For the lower turbine backpressure, the turbine backpressure is decreased to reduce exhaust flue gas temperature. Thermodynamic analysis indicated that self-heat recuperative coal drying and recompression could enhance the net efficiency by 4.28 % points compared to the basic cycle, but the heat exchanger for raw syngas heat recovery faces overtemperature problem with the cold stream exceeding 777 °C. Both the measures proposed above can effectively reduce the temperature to 730–760 °C with slightly decreased efficiencies of 46.93 %∼47.68 % for double expansion and 47.12 %∼47.81 % for lower turbine backpressure. Furthermore, economic assessment shows that compared to the basic cycle, the cost of electricity decreases by 2.49 % and 5.04 % for the schemes with double expansion and lower turbine backpressure, respectively. Therefore, the proposed design with lower turbine backpressure demonstrates advantages in both thermodynamic and economic performance.