1. College of Safety Science and Engineering, Xi’an University of Science and Technology, Xi An, Shaanxi 710054, P. R. China
2. Key Laboratory of Coal Resources Exploration and Comprehensive Utilization, MLR., Xi An, Shaanxi 710021, P. R. China
3. School of Safety Engineering, China University of Mining and Technology, Xu Zhou, Jiangsu 221116, P. R. China
4. School of Mining Engineering, Guizhou Institute of Technology, Gui Yang, Guizhou 550003, P. R. China