The synergetic effects of carbon and hydrogen metabolism on the exergy destruction and loss in a coal-to-SNG process

Abstract

The carbon utilization and exergy efficiency in coal to synthetic natural gas (coal-to-SNG) industry have always been the focuses of researchers and engineers. But few studies concern the linkage between carbon metabolism, hydrogen metabolism, and exergy destruction and loss, which significantly dominates the productivity and environmental performances of coal-to-SNG. Taking a typical coal-to-SNG project with 4 billion Nm3SNG/y as a case study, the effects of carbon and hydrogen metabolism on exergy destruction and loss are explored in detail. Coal gasification unit as the core of the entire coal-to-SNG process, results in 4.52% of total carbon emissions, 45.93% of total water consumption, and 46.83% of total exergy destruction and loss. The pursuit of high material utilization does not necessarily bring about less exergy destruction and loss, taking coal gasification and methanation units as typical representatives. While low carbon and hydrogen utilization efficiencies often mean certain exergy loss, taking rectisol and phenol ammonia recovery units as examples. The exergy loss per unit of carbon emission and wastewater discharge are 1.58 MJ/kg and 0.05 MJ/kg, respectively. According to regression analysis, the unilateral impact of carbon or hydrogen utilization on exergy destruction and loss is not so obvious, but synergetic effects between them play important roles in causing exergy destruction and loss. Raising exergy efficiency of each unit to the optimization point will lead to reduction of coal consumption by 158.61 t/h (0.32 kg/Nm3SNG), water consumption by 255.12 t/h (0.51 kg/Nm3SNG) and carbon emission by 54.76 t/h (0.11 kg/Nm3SNG). The way of exergy analysis coupled with material metabolism is benefit for comprehensively analyzing resource and energy utilization of industrial processes.