During the same period, sales reached 2.47 billion tons, up 4 percent. However, the growth rate also slumped 10.7 percentage points, according to CNCA data.
As of the end of August, coal firms' stocks stood at 87.85 million tons, up 69 percent year on year. The figure represented a 6.8-percent increase from July.
The CNCA said downward pressure in the coal industry also weighed on prices. Coal prices at the Qinhuangdao port stood at 620 to 630 yuan per ton as of the end of August, down 200 yuan year-on-year.
It predicted that supply will continue to outweigh demand for the full year, adding that a recovery for the sector has yet to arrive.