Mass Transfer Aspects Governing Magnesia Additive Performance in Oil Fired Boilers

Abstract

Powdered magnesia based additives injected at the flue gas airheater inlets on oil fired plant are being evaluated as possible palliatives to acid smut emission and surface corrosion problems. This paper considers some important mass transfer aspects which govern the effectiveness of these “back end” additives together with factors which influence the “scaling” of injection rates as functions of plant size. The relevance of additive particle size, duct diameter and surface temperature on additive and acid deposition on surfaces is discussed and the potential for gas phase reaction between sulphuric acid vapour and suspended additive is quantified. The overall conclusion substantiates facts already observed in recent full scale plant trials— namely that prediction of additive performance is very difficult and that scaling factors are complex functions of many variables.