China’s booming coal production is running well ahead of demand at the onset of winter, leading to swelling inventories and plunging prices that analysts forecast will fall further. A gauge of power-station coal prices in China has dropped around 9% since the end of September to an 18-month low of 790 yuan ($108) a ton. While demand for the fossil fuel would typically rise as the approach of winter boosted power demand, brimming stockpiles and flagging economic growth are weighing on prices.