NO.sub.x and SO.sub.3 emissions from combustion of a sulfur containing carbonaceous fuel are reduced simultaneously. The combustion gases comprising NO.sub.x and SO.sub.2 are mixed with a NO.sub.x control agent into the combustion gases at a point upstream of a selective catalytic reduction catalyst for reduction of NO.sub.x. Following an SCR catalyst or other equipment that can oxidize SO.sub.2 to SO.sub.3 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 SO.sub.3 caused by the oxidation of SO.sub.2 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.