Performance analysis of an open absorption heat pump and thermal desalination system for heat and water recovery of coal-fired flue gas based on zero liquid discharge

Abstract

An innovative combined system based on an open absorption heat pump and low-temperature multi-effect desalination system is proposed to provide fresh water and domestic hot water in order to achieve zero liquid discharge from coal-fired power plants and recover the waste heat of flue gas. This system takes into consideration the evaporation of desulfurization wastewater by flue gas. By using a 600 MW unit as an example, energy and exergy analysis is used to examine the impacts of flue gas humidity, temperature, mass, generator pressure, solution concentration, and boiler load on system performance. The findings showed that because coal-fired power plants produce a significant amount of dry flue gas, an increase in gas humidity has little effect on product supply. As a result, little variation in the combined system is caused by the desulfurization wastewater spray evaporation, which allows power plants to achieve zero liquid discharge with only a small loss in exergy efficiency of about 6%. The recovery of sensible heat rather than latent heat benefits from an increase in the bulk of dry flue gas. The recovery of heat and water benefits more from an increase in operating load or flue gas temperature than the open absorption heat pump system itself. The generator pressure and rich solution concentration have negligible effects on heat and water recovery. The open absorption heat pump system's increasing exergy efficiency trend and the boiler operating loads are also related. The exergy efficiency of the generator is the highest by about 75%, that of the desalination system is the lowest by about 29%, and the exergy destruction of the absorber is the highest by about 43%. The findings are likewise applicable to coal-fired power stations with various levels of electrical output.