1. College of Safety and Environmental Engineering, Shandong University of Science and Technology, Qingdao 266590, P. R. China
2. Mine Disaster Prevention and Control-Ministry of State Key Laboratory Breeding Base, Shandong University of Science and Technology, Qingdao 266590, P. R. China
3. National Key Laboratory of Gas Disaster Detecting, Preventing and Emergency Controlling, Chongqing 400037, P. R. China