An apparatus and method for reducing nitrogen oxide emissions from furnace flue gas is provided in which natural gas or other fluid fuel which has little or no fixed nitrogen is introduced into the upper portion of the furnace. The fuel diffuses by laminar or turbulent diffusion into oxygen-rich flue gas where it reacts with oxygen and nitric oxide to form CO.sub.2, H.sub.2 O and N.sub.2, thus reducing the nitrogen oxide emissions from the furnace. In this manner, the amount of nitrogen oxide in the flue gas is reduced. The apparatus and method can be easily applied to new furnaces or retrofitted on existing furnaces.