Exergy cascade release pathways and exergy efficiency analysis for typical indirect coal combustion processes

Abstract

Indirect coal combustion can solve some problems such as low efficiency and air pollution during traditional coal combustion, this research investigates energy release characteristics of indirect combustion and aims to achieve maximum energy utilisation. Firstly, Energy Utilization Diagrams Methodology was used to explore exergy cascade release pathways and exergy losses for typical indirect combustion such as supercritical water gasification re-combustion, steam reforming re-combustion, and chemical-looping combustion, which are compared with direct combustion. It is pointed out that indirect combustion has changed release pathways of chemical exergy, and decreased energy level difference between chemical exergy in fuel and thermal energy in combustion, then exergy losses is reduced and exergy in fuel is released step by step. Secondly, to prove that indirect combustion can indeed reduce exergy losses and improve exergy efficiency in the combustion process, supercritical water gasification re-combustion with least exergy losses was selected and exergy analysis model was established by energy quality coefficient. Results show that thermal efficiency and exergy efficiency of supercritical water gasification re-combustion reaches as high as 94.8% and 79.5%, respectively, 2.23% and 19.61% higher than those of traditional coal-fired boilers. As a result, energy matching and conversion are more reasonable in indirect coal combustion processes.