A method for reducing NOx emissions from a lean burn combustor equipped with a NOx reducing exhaust catalyst, includes at least the following steps: (i) generating a computer based model of the geometry of an exhaust system of the combustor; (ii) computing at least one of flue gas velocity profiles and mass flow stream lines for exhaust gas flow through the exhaust system; (iii) inputting injector data comprising at least droplet size and velocity; (iv) modeling droplet trajectories for a plurality of injector locations; (v) modeling at least one flow conditioning device in the exhaust system; and (vi) manipulating the computer based model until an injector location is identified that provides a predicted root mean square (RMS) of reagent at the face of the catalyst that is less than 15%.