A method is provided for reducing the fouling during the removal of sulfur trioxide from a flue gas stream by maintaining the reagent (i. e. sodium sesquicarbonate) in contact with the flue gas for a sufficient time and temperature to react a portion of the reagent with a portion of the sulfur trioxide to substantially avoid formation of liquid phase NaHSO.sub.4 reaction product that combines with the fly ash so as to not form a sticky ash residue that adheres to the flue gas duct, wherein the reaction product of the reagent and the sulfur trioxide is selected from the group consisting of Na.sub.2SO.sub.4, Na.sub.2S.sub.2O.sub.7 and mixtures thereof.