Ecological efficiency of coal cities in China: evaluation and influence factors

China's energy consumption and environmental problems are becoming increasingly urgent issues as the nation's economy continues its rapid development. A further investigation of ecological efficiency would assist the healthy development of China's "coal cities.'' In this study, the stochastic frontier analysis based on the assumptions of the translog production function is employed to assess the ecological efficiency and potential promotion of 28 typical coal cities in China. The Tobit method is used to analyze the influence factors in ecological efficiency. The results show that the ecological efficiency of coal cities is not high but increased overall from 2007 to 2014. In regional terms, coal cities in the east have the highest ecological efficiency while coal cities in the west have the highest potential promotion; coal cities in the northeast have the lowest ecological efficiency and potential promotion. Moreover, ownership structure and environmental regulations have significant inhibitory effects on the improvement in ecological efficiency, while marketization level and technological innovation have significantly positive impacts.