Exploring low-grade heat in exhaust gases with moisture via power generation cycles

Abstract

Low-grade exhaust gases from industrial processes can be reutilised for electricity generation by ways of various thermodynamic cycles. Previous studies assumed that only sensible heat is available in the heat source. However, the presence of moisture is common, and as the moisture condenses, a significant amount of additional heat will be released for reutilisation. To gain further understandings to the influence of moisture in heat recovery, comparative analyses between a simple organic Rankine cycle and alternative cycles, including organic flash cycle, Kalina cycle, transcritical ORC and dual-pressure organic Rankine cycle has been carried out. The results show that with the presence of moisture, the net power output per kg/s of heat source flow increases significantly for all cycles examined. It is due to the large amount of latent heat release during moisture condensation process, that is not seen for dry gases. Among all cycles considered, the dual-pressure organic Rankine cycle shows the most convincing performance enhancement compared to a simple organic Rankine cycle, in terms of the cycle net power output. This is attributed to its flexibility to cope with both high and low-temperature heat sources. With a moisture content of 0.1 (mass fraction of moisture in the heat source), a dual-pressure organic Rankine cycle can achieve a performance enhancement up to 58%. As the moisture content further increases or decreases, its performance enhancement tends to reduce, although still noticeably higher than the simple organic Rankine cycle throughout the range of moisture content examined.