Europe's early crematories were coal or coke fired furnaces. The oven or furnace which is used to cremate corpses is properly termed a retort. Early retorts were merely ovens which cooked all the moisture out of the corpse and reduced it to ash. Bones cannot be burned and must be pulverized, even today. The early mortar and pestle has been replaced by a crushing machine, however.
Modern retorts are mostly gas fired, although some are still supplied for oil.
None are still fired by coke or coal in the United States or Canada. Earlier retorts were simply a drying or baking kiln and simply dried the human remains. Modern retorts of brick-lined steel actually blow fire from a nozzle onto the remains setting them afire, causing combustion and rapid burning. Modern retorts also have a second or afterburner for reburning all the pollutants in the combusted gaseous material. This second burner is a requirement set by the various state agencies responsible for air pollution.