Cremation

 

Cremation
QUOTE
The cremation occurs in a crematory (or crematorium), consisting of one or more cremator furnaces or cremation retorts for the ashes. A cremator is an industrial furnace capable of generating temperatures of 870–980 °C (1,598–1,796 °F) to ensure disintegration of the corpse. A crematorium may be part of chapel or a funeral home, or part of an independent facility or a service offered by a cemetery. Modern cremator fuels include natural gas and propane. However, coal and coke were used until the early 1960s. Modern cremators have adjustable control systems that monitor the furnace during cremation. These systems automatically monitor the interior to tell when the cremation process is complete, after which the furnace shuts down automatically. The time required for cremation thus varies from body to body, and in modern furnaces may be as fast as one hour per 45 kilograms (99 lb) of body weight.
UNQUOTE
This is quite hot. A typical domestic oven for cooking would use about 3 kilowatts. Crematoria would have to use a good deal more. 10 - 20 kW sounds like a practical minimum with an hour and half to do what it takes. Doing Jews in Auschwitz implies coal. 6 million implies implies 6,000,000 as much.