Thermodynamic comparison and efficiency enhancement mechanism of coal to alternative fuel systems

Coal to alternative fuels is an important path to enforce energy security and to provide clean energy. In this paper, we use exergy analysis and energy utilization diagram (EUD) methods to disclose the internal reasons for performance differences in typical coal to alternative fuel processes. ASPEN plus software is used to simulate the coal-based energy systems, and the simulation results are verified with engineering data. Results show that coal to substitute natural gas (SNG) process has a higher exergy efficiency of 56.56%, while the exergy efficiency of traditional coal to methanol process is 48.65%. It is indicated that three key factors impact the performance enhancement of coal to alternative fuel process: (1) whether the fuel is distillated, (2) the synthesis temperature and the amount of heat release from reactions, and (3) whether the chemical purge gases from synthesis and distillation units are recovered. Distillation unit is not recommended and synthesis at high temperature and with large heat release is preferable for coal to alternative fuel systems. Gasification is identified as the main source of exergy destruction, and thereby how to decrease its destruction is the key direction of plant efficiency improvement in the future. Also, decreasing the power consumption in air separation unit by seeking for advanced technologies, i.e. membrane, or using another kind of oxidant is another direction to improve plant performance.