Campaign of the Month: January 2015
Skies of Glass
Sub-bituminous is one of the types of coal whose properties range from those of lignite to those of bituminous coal. It may be dull, dark brown to black, soft and crumbly at the lower end of the range, to bright, jet-black, hard, and relatively strong at the upper end. Subbituminous coal contains 20 to 30 percent inherent moisture by weight. It is more popular than lignite because it has a lower sulfur content and therefore burns cleaner. Since it has relatively low density and high water content, it does pose an additional danger. If it is not packed correctly, it has a tendency to spontaneously combust. While this is a danger of coal dust in general, letting air circulate around this type of coal makes ignition likely even without a spark.
Often, sub-bituminous coal is obtained through strip mining since it is often found near the surface in coal seams.