Super Sacks, as mentioned above by John Kokas, may not be a bad idea. There are versions that can be dumped through the bottom. Pick them up by their straps with a forklift or crane, move over the coal space, pull the rip cord, and Voila! coal is unloaded instantly. If we had, say, half a dozen of these, they could be filled with coal and positioned ready to go. They could be filled by hand shoveling or with a loader. Anthracite coal is available from coal dealers in super sacks.