1. Taiyuan Institute of Technology, Taiyuan 030008, P. R. China
2. State Key Laboratory Breeding Base of Coal Science and Technology Co-founded by Shanxi Province and the Ministry of Science and Technology, Taiyuan University of Technology, Taiyuan 030024, P. R. China
3. Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, P. R. China