what is produced when coal is burned
Before the coal is burned, it is pulverized to the fineness of talcum powder. It is then mixed with hot air and blown into the firebox of the boiler. Burning in suspension, the coal/air mixture provides the most complete combustion and maximum heat possible.
The youngest coal is not even coal yet -- peat. Peat is a traditional fuel in parts of the world, like Ireland, where it is cut from the earth, dried, and burned for heat. The energy content of peat is quite low. Young coal is called lignite, and is soft and brown, not much different than dried peat.