1. Key Laboratory of Coal Science and Technology (Taiyuan University of Technology), Ministry of Education and Shanxi Province, Taiyuan 030024, P. R. China
2. Training Base of State Key Laboratory of Coal Science and Technology Jointly Constructed by Shanxi Province and Ministry of Science and Technology, Taiyuan 030024, P. R. China
3. State Key Laboratory of High-efficiency Utilization of Coal and Green Chemical Engineering, Ningxia University, Ningxia 750021, P. R. China