Formation of Coal
All coal has been formed from biomass. Over time this biomass has been turned into peat. When covered under a layer of overburden, the influence of time, pressure, and temperature convert this material into browncoal or lignite. Subsequently, the latter material will turn into sub-bituminous coal, then into bituminous coal, and finally into anthracite. Coal is often classified in terms of its rank, which increases from browncoal to anthracite. The classification of coal by rank for ash and moisture-free coal is given in Tables 4-2 and 4-3. Figure 4-1 provides an alternative presentation. Browncoal, lignite, and sub-bituminous coals are called low-rank coals, whereas higher-rank coals are often called hard coals. The terms browncoal and lignite are essentially synonymous, lignite being used more often in the United States and browncoal in Europe and Australia.
