Specific varieties of coal are essential in the production of steel. Steel is essentially an alloy of coal (specifically carbon) and iron. And Democrats are certainly not opposed to all coal for every purpose. But burning coal as fuel is harmful to human beings as well as to the environment.
There is no real chance that coal mining jobs will ever return to the Appalachian coal fields of VA, WV, KY because (1) coal mining is highly mechanized and one mining machine can do the work of many, many miners with picks ad shovels, and (2) the coal in this area has already been mined. It was never an eternal resource.