Funding for this research was provided by:
National Natural Science Foundation of China (71673270)
The Think Tank of Energy Mining Economy (2018WHCC01)
Fundamental Research Funds for the Central Universities (2242019K40171)
China Scholarship Council (201806425016)
Article History
Received: 4 June 2019
Accepted: 29 August 2019
First Online: 4 September 2019
Compliance with ethical standards
:
: The authors declare that they have no conflict of interest.