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This is a brief overview of the Origins of Coal from the World Coal Institute. As you read, please note that coal can be very different depending on its maturation (extent of maturing). Also, note that we classify coal according to its properties (the classification being called "rank"). It's important that you understand that the term "rank" here does not mean a true "ranking" but rather a classification. All coals are used for combustion within electricity generation, select bituminous coals are more valuable and are used for coke making (used in the manufacturing of steel from iron ore).
Read up on coal here with the goal of understanding coal rank and how coal forms (it is fair game for the exam — see the learning objectives).