The types of coal

During the conversion of vegetable matter to rock it is possible to follow the virtually continuous order from the original biological material with low carbon content and high share of impurities to materials with almost 100% carbon content in the form of graphite (black-lead):

• wood
• turf
• lignite (the youngest brown coal)
• brown coal
• transitional types
• hard coal
• anthracite
• graphite (black-lead)
  

In fact, there is a difference in origin between the younger and older types of coal: hard coal of the palaeozoic era originated from horsetails, club-moss and ferns; young brown coal originated from trees and other higher plants.

Carbon content and calorific value of the main coal types

typ - carbon share - calorific value

lignite/ - 30 to 50% - approx. 13 MJ/kg

brown coal - 50 to 80% - 15 to 20 MJ/kg

hard coal - 80 to 90% - 18 to 30 MJ/kg

anthracite - over 90% - 26 to 30 MJ/kg

Calorific value is heat released by burning of the fuel to produce gaseous carbon dioxide (CO₂), sulphur dioxide (SO₂), nitrogen (N) and water vapour (H₂O).

Technically speaking, coal is also distinguished according to its possible uses, for example gasifiable coal, or coking coal, coal for chemical production, etc. In the Ostrava-Karviná district the best hard coal (including the types suitable for coke production) and anthracite is extracted.