Why is shale oil, a possible substitute for petroleum in some parts of the world, considered to be a pyrolysis product? List some coal ranks and describe what is meant by coal rank. Why was it...

Why is shale oil, a possible substitute for petroleum in some parts of the world, considered to be a pyrolysis product? List some coal ranks and describe what is meant by coal rank. Why was it necessary to add hydrocarbons to gas produced by reacting steam with hot carbon from coal in order to make a useful gas product?