NOx and SO3 emissions from the combustion of a sulfur containing carbonaceous fuel are reduced simultaneously. The combustion gases comprising NOx and SO2 are mixed with a NOx control agent. The NOx control agent is mixed with the combustion gases at an upstream point of a selective catalytic reduction catalyst for the reduction of NOx. Following an SCR catalyst or other equipment that can oxidize SO2 to SO3 and prior to contact with an air heater for heating incoming combustion air, magnesium hydroxide is introduced in amounts and with droplet sizes and concentrations effective to form nano-sized particles in the effluent and reduce SO3 caused by the oxidation of SO2 in the catalyst. Computational fluid dynamics is employed to determine flow rates and select reagent introduction rates, reagent introduction location(s), reagent concentration, reagent droplet size and/or reagent momentum.