Coal has four main types, or ranks—lignite, sub-bituminous, bituminous, and anthracite—in order of increasing carbon and energy content. Most of the coal burned in U.S. power plants is bituminous or sub-bituminous coal.
A fifth type, called metallurgical, or “coking” coal, is used for steelmaking. Coke is a residue remaining after certain types of coals are heated to a high temperature until substantially all of the volatile constituents have been driven off. The resulting residue is largely carbon, with minor amounts of hydrogen and sulfur.
All types of coal also contain sulfur, which when burned, releases toxic air pollution. When these materials are combusted sulfur will be released in the form of sulfide and hydrogen sulfide, which will impact water environment, acidify the soil and do great harm to plants and health.
Levels of sulfur in coal/coke can be determined with the use of a high temperature tube furnace in an oxygen rich environment. The analysis is done in accordance with ASTM D4239 and under ISO/IEC 17025 and 17034 certifications to ensure the quality of products.