NOVELTY - Nitrogen oxide emission in a heating furnace is reduced by injecting a fluid fuel and a nitrogenous compound downstream of the combustion zone to form an overall fuel-lean nitrogen oxide-reduction zone.    USE - For reducing nitrogen oxide emissions from industrial heating furnaces, such as utility boilers, fired by carbonaceous fuels or fuels with fixed nitrogen, e.g., coal.    ADVANTAGE - The invention results in higher NOx reduction than either natural gas re-burn or selective non-catalytic reduction, and at a lower cost. It is well matched to the NOx reduction requirements of ozone non-attainment areas where reduction greater than 75% from uncontrolled levels may be required. Natural gas re-burn or selective non-catalytic reduction alone cannot achieve such a level of NOx control. It also offers a flexible approach which is ideal for seasonal NOx control. Raw natural gas injection with controlled mixing can be used when up to 50% NOx reduction is required. The nitrogenous compound may be co-injected with natural gas during summer when greater NOx reductions are required. The chemistry of the invention occurs over a wider temperature window than the selective non-catalytic reduction process. It also requires less natural gas than standard re-burn. Capital costs are also lower than standard re-burn because flue gas recirculation and overfire air duct work are not required. Because the nitrogenous compound may be co-injected with the fluid fuel, the use of a carrier medium is optional. Also, there is a reduced risk of ammonia or other reagent leakage or ammonium sulfate or related deposit formation because the fluid fuel and nitrogenous compound injection occurs at higher temperatures than selective non-catalytic reduction.    DETAILED DESCRIPTION - Reduction of nitrogen oxide emissions generated in a primary combustion zone of a heating furnace involves injecting fluid fuel(s) and nitrogenous compound(s) downstream of the combustion zone to form an overall fuel-lean NOx-reduction zone which is at 2100-3000 degrees F. The fluid fuel can be natural gas, hydrogen, CxHy compounds, and/or CxHyOz compounds present at 2-25% of a total heat input to the furnace. The nitrogenous compound can be ammonia, amine, urea, and/or cyanuric acid.