Performance analysis of a novel two-stage membrane waste heat recovery system

Abstract

• A two-stage membrane system is proposed to recover the total heat of flue gas. • The system realizes the self-humidification of boiler fresh air. • The system thermal efficiency based on net caloric value reaches 105.4% at highest. • The total heat recovery efficiency reaches 77.0% at highest. • The coupled effect between two membrane exchangers stabilizes recovery performance. There is a tremendous waste heat recovery potential for the gas boiler exhaust flue gas. Conventional condensing recovery and state-of-the-art membrane (one-stage membrane more accurately) recovery technologies have upper limit of the total heat recovery efficiency, depending on cooling source temperature. This paper innovatively proposes a two-stage membrane waste heat recovery (2sMWHR) to enlarge the heating capacity of district heating network (DHN). The flue gas exchanges the total heat with the DHN return water and boiler fresh air in two membrane total heat exchangers (MTHX) successively. The system realizes the self-humidification of the boiler fresh air, and therefore the mass transfer potential of the boiler flue gas is elevated, without any additional energy consumption. The 2sMWHR system yields the total heat recovery efficiency of 77.0% and the system thermal efficiency (based on net caloric value of fuel) of 105.4% at the DHN water temperature of 40 °C. The waste heat exhausted finally to the ambience accounts for 3.6% only, in comparison with 15% when no heat recovery is employed. Owning to the coupled effect between the two MTHX, the total heat recovery performance is retained at a high level to a certain extent as the DHN water temperature rises from 40 °C to 60 °C. The total heat recovery efficiency just decreases from 77.0% to 53.5%. In comparison, both the condensing system and one-stage membrane system perform dramatic decrease from 41.8% to 14.9% and from 45.3% to 15.1%, respectively.