Formation of strong primary acidity in combustion particulate: causes and remedies

Having observed great variations in the strong primary acidity (SPA) content of the ash of fossil fuels with similar sulphur contents, we studied the chemical causes of the formation of SPA in ashes. The main factors behind these variations were found to be: (a) combustion conditions and the extent to which they favour or do not favour the formation of SO3 (excess of oxygen, presence of suitable catalysts, etc.); (b) the amount of basic oxides present in the mineral fraction of the fuel in relation to the SO3 produced; and (c) the basic strength of these oxides. The variable but, nevertheless, high SPA contents common only to oil ashes can be explained in the light of the modest values for factors (b) and (c) and the conditions favourable to factor (a) in this fuel. Remedies involve the in-flame addition of highly basic oxides in order to artificially increase factors (b) and (c).