Sorry, I need to correct that. They have recently become one of the larger importers of US coal. Historically they were small.
http://www.eia.gov/coal/production/quarterly/pdf/t7p01p1.pdf
For comparison, we exported about 1% of our total coal production to China.
http://www.eia.gov/coal/production/quarterly/pdf/t1p01p1.pdf
About 13% of our total production is exported to several countries.