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 of56.56%, 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.
Conclusions
In this paper, we compared the exergy efficiencies of three coal to alternative fuel processes. There are three key factors affecting the performance of coal to alternative fuel systems. (1) The distillation unit to get high purity fuel. (2) The temperature and heat release of the synthesis unit. (3) The recovery of chemical purge from synthesis and distillation units. Take traditional coal to methanol and coal to SNG processes for example, the coal to SNG process has a higher exergy efficiency of 56.56% compared to traditional coal to methanol process (48.65%). It’s mainly due to the high synthesis temperature and the high conversion rate of synthesis reaction of coal to SNG process which leads to higher power recovery rate, while for traditional coal to methanol process, a large capacity captive power plant is needed which leads to large exergy destruction. When using methanol as vehicle alter native fuel, distillation unit is not necessary to get fuel with too high purity. Besides, by reusing purge gas in captive power plant rather than in the torch system, we can further improve the exergy efficiency of coal to methanol process to around 56.58% which is close to that of coal to SNG process.
For both coal to SNG and coal to methanol processes, gasification is the main source of exergy destruction, and power consumption in ASU accounts for about 50% of the total utility consumption. Thereby how to decrease exergy destruction in gasification unit is the key direction of plant efficiency improvement in the future, and decreasing the power consumption in air separation unit or using another kind of oxidant (CO2 separated from syngas is an ideal oxidant)is another direction to improve plant performance.
The results have been published on Applied Thermal Engineering 103 (2016) 1251–1260.
