For safe storage of a few tons up to a few dozen tons, I would proceed as follows.
if the coal was purchased in small bags on pallets store it thus, but do not stack the pallets atop each other and allow a free air space on every side of every pallet.
For coal tipped loose, pile it as convenient but avoid more than 5 tons in one pile.
Why?
if you do a search, you can see pallets of coal stacked 2 high,
Truck loads of 20-24 tons commonly stored in a pile, or in a truck sized bin....
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