Coal dust (also known as coal chips) is a byproduct of normal coal, but sometimes it accumulates in large quantities, making it a shame to throw it away. The problem is that it's quite difficult to burn.
Firstly, because there's little space between the grains, the heat doesn't reach them, and the chips only burn on the sides. Secondly, if you sprinkle these chips on burning wood, they'll simply fall between the grains and into the ash pan without burning.