All coal is not cooked at the same temperature and for the same time, so we find on the earth that the peat material changes to varying degrees. To go back to the cake analogy, cakes not cooked enough are raw on the inside and resemble the dough, cakes that are overcooked are burnt and resemble charcoal. The
same is true of coals. Even plant materials from the same seam can be cooked to different degrees, depending on the thermal history. The metamorphosis of peat to coal leads to the idea of coal rank. This is a scale of the maturity, or time and temperature history of coal. We rank coals into several major categories:
There are further
subdivisions, such as high volatile A or high volatile B bituminous. This
way of ranking coal is based on parameters useful for the industrial users
of coal. This ranking is not really good for scientific purposes.
There is no single scale of rank that is universally used by scientists
studying coal. Carbon
content, H/C and O/C ratios, or reflectance of light from polished coal
samples are some parameters used to plot coal data against. None of them
are completely adequate.
Information provided by: http://chemistry.anl.gov