Comprehensive analysis and optimization of an efficient combined power and refrigeration cycle suitable for recovering low/mid-grade waste flue gas

To effectively recover low/mid-grade waste flue gas and provide larger refrigeration capacity, an efficient combined power and refrigeration cycle only using ammonia water as the working fluid is proposed and studied in this paper. Parametric analysis is conducted from the perspectives of thermodynamics, exergoeconomics, and environment, the results demonstrate that the low/mid-grade waste flue gas within a wide temperature range can be sufficiently recovered by the proposed cycle, and the cycle performance is significantly affected by the turbine inlet pressure, work/basic concentration and separation temperature of work solution. Based on the heat source condition of 300 °C/10 kg·s−1, the results of single-objective optimization show that the maximum effective exergy efficiency, reduction amount of CO2 emission and waste heat recovery ratio that can be achieved are 0.5989, 262 kg/h and 0.97 respectively, and the achievable minimum unit cost of produced exergy is 8.432 $·GJ−1. Besides, the multi-objective optimization indicates that the optimal comprehensive thermodynamic efficiency is 0.5458 with net power output of 372.5 kW and larger refrigeration capacity of 545.7 kW, and the associated effective exergy efficiency, unit cost of produced exergy, reduction amount of CO2 emission and waste heat recovery ratio are 0.5701, 9.187 $/GJ, 258.3 kg/h and 0.9574, respectively.