Abstract: Fuel is burned in a primary combustion chamber with less than the air required for stoichiometric combustion so that the combustion gases have a high carbon monoxide (CO) and hydrocarbon content and the temperature of the gases is held below that at which significant nitrogen oxides (NO.sub.x) would be produced. The combustion gases are then passed through a secondary combustion zone in which more air is injected into the gas stream to oxidize the CO and hydrocarbons to carbon dioxide (CO.sub. 2). The secondary burner comprises a plurality of foraminous tubes through which secondary air is emitted. Combustion in the secondary zone is maintained at a temperature below that at which nitrogen oxides (NO.sub.x) will be produced in significant quantities.