A method and system for reducing the amount of nitrogen oxides (NOx) in a combustion gas waste stream by (1) analyzing the waste stream to determine the amount of NOx; (2) determining the stoichiometric amount of ammonia required to reduce the NOx concentration down to a required level or less; (3) determining the flow rate profile of NOx components across the combustion gas waste stream upstream of an ammonia injection grid; (4) selecting specific locations within the ammonia injection grid to activate ammonia valves; (5) injecting controlled amounts of ammonia vapor into the gas stream at grid locations corresponding to the location of NOx in the gas stream; and (6) treating the gas stream using a selective catalytic reduction unit to reduce the amount of NOx down to acceptable levels.