Well i was hearing alot about and i am a supporter of making a destroyed version of coal!! this would be cool so people could move it around and feed it into coal power plants along with other cool stuff! say aye if you agree nay if your a horse that doesnt agree!
use dust or gun powder. if neither of them are appropriate, mix the together, they have the same density.
But they don't burn as slowly as coal. :[ Gun powder just EXPLODES i want a realistic constant heat source without resorting to thermal nuclear contraptions.