bituminous coal will lose it's volitile content and thus it's heat content over time. perhaps the more important issue is that it's properties change over time; a high coking coal (hard to burn easily in a residential situation) will become a free burning coal and vice versa. these things cannot be predicted unless extensive testing has been done on the coal for long-term storage purposes. a coal that may have burned fine in your appliance 5 or ten years ago may not burn well at all today in a long-term storage situation, of course, it may burn just fine and nothing may change at all save for slight loss of btu value. If you would like to do it, do it, but if it were me, i would probably not buy more than a 5 year supply. I do not forsee any inability to obtain bituminous coal in the near future, however, should something catastrophic happen, by all means stock up. Having said all the above, i will tell you the absolute best way to store coal to prevent almost all deterioration and loss of btu content. The ideal way to store coal is to dig a pond, put a liner in it, fill it will coal, then with water, put another liner on top of it and prevent it from going dry and especially from cycling: dry/wet, dry/wet. basically your building a stagnant pond but with a liner to keep out soil minerals and excessive surface/ground water flow which causes mineral deposition and deterioration of the coal. This is the ideal way to store all coals/ anthracite or bituminous and will prevent almost any changes or degradation to the coal over time.