Thermodynamic analysis of exhaust waste heat recovery in a compound open absorption heat pump system

To recover waste heat from flue gas in coal fired power plants and mitigate the environmental problems caused by wet flue gas, a novel compound open absorption heat pump (COAHP) system was proposed in this paper. COAHP combined the open absorption heat pump (OAHP) and the closed absorption heat pump (CAHP). The CAHP recovered the sensible heat of the flue gas before desulfurization, while the OAHP recovered the latent heat of the flue gas after desulfurization. The evaporator of the CAHP served as a cooling source for the inlet solution of the absorber of the OAHP. This system enables efficient waste heat recovery, reduces moisture levels, and eliminates wet plumes. A thermodynamic model using Aspen Plus was developed to evaluate the waste heat recovery from a 350 MW coal fired unit. In addition, the system's performance was evaluated from the perspectives of energy analysis and exergy analysis. Liquid-gas ratio, solution inlet temperature, and secondary regeneration pressure impacted on system performance parameters and exergy efficiency. The results showed that superior water and thermal recovery efficiencies of the COAHP compared to the typical OAHP. Energy consumption for eliminating the flue gas plume decreased from 41.97 MW to 13.59 MW. When the liquid-gas ratio in the COAHP increased from 2.2 to 3.8, the water recovery efficiency increased to 72 %, the heat recovery efficiency raised to 75 %, and the coefficient of performance (COP) increased to 1.7. When the solution inlet temperature is between 40 C-degrees and 45 C-degrees, the heat recovery efficiency decreases to 62 %. However, the energy consumption of the system components decreases slightly, leading to a gradual increase in the COP to 1.9. The COP is maximized at a regeneration pressure of 70 kPa. At a regeneration pressure of 70 kPa, the COP reached maximum, significantly reducing the energy consumption of the vacuum pump. This work provided a feasible solution for recovering and utilizing the high-humidity flue gas waste heat following wet desulfurization in coal fired power plants.