Simultaneous optimization and heat integration of the coal-to-SNG process with a branched heat recovery steam cycle

The coal-to-SNG process is an energy-intensive process, and optimizing the heat recovery network can improve the economy and energy efficiency. This study proposes a branched, triple pressure level heat recovery steam cycle (HRSC) to recover waste heat, in which one branch is responsible for recovering the waste heat from the water gas shift (WGS) unit, and the other branch is responsible for the methanation (METH) unit. The extended Duran-Grossmann model is used to optimize two heat exchanger networks to match the branched HRSC superstructure. The temperature/pressure/flow rates of the HRSC streams and the operating temperature of the WGS and METH units are optimized. The optimal bypass ratio of the WGS unit as well as the recycle ratio and split ratio of the METH unit, are 0.506, 0.681 and 0.456, respectively. The exergy efficiency of the coal-to-SNG plant is improved by 1.28% compared with the industrial plant, which can reach 54.17%.