煤炭 is the general term for coal as a resource or commodity. 煤 alone also means coal but is less formal. 煤炭 is more common in written and news contexts (煤炭工业 coal industry, 煤炭资源 coal resources).
memory
Both characters 煤 and 炭 relate to burning: 煤 has the fire radical 火 on the left (coal burns), and 炭 has 火 at the bottom (charcoal). Together they reinforce 'something that burns.'
