Methods for control of NOx emission in the incineration of tail gas are provided wherein tail gas that comprises NOx, NOx precursors, or both is introduced into a combustor and diluent is introduced into the combustor for controlling the combustor temperature to a temperature of from about 950° C. to about 1100° C. Methods also are provided for reducing NOx emissions by controlling air-to-fuel ratio in a tail gas combustor while controlling the combustor flame temperature through diluent injections. A boiler unit for carrying out these methods also is provided. A system for carbon black production using the boiler unit also is provided.