Coal dust is a fine powdery substance made up of tiny particles of coal, which is a combustible black or brownish-black sedimentary rock. CULM is an acronym for Coal Utilization By-Product Material, which is the material left over after coal has been processed. SOOT is a black powdery or flaky substance composed primarily of amorphous carbon, produced by the incomplete combustion of coal, oil, wood, or other substances.