Centrifugal fans with electric motor drives are used to supply combustion air. Since the fans for high-velocity air must operate at higher tip speeds and therefore with considerable noise, the fans should be located where their noise will not be a nuisance. Low velocity combustion air can also be admitted through louvers, doors, or other openings as a result of the negative pressure or draft within the furnace chamber. The quantity of combustion air admitted through such openings can be controlled by dampers. Combustion air for suspension burning of shredded and air-classified refuse or other finely divided waste material that does not use grates as the principal site of waste gasification requires a different combustion air concept than that appropriate for a grate system.