Fluidized-Bed Combustion of Coal with Lime Additives: Catalytic Sulfation of Lime with Iron Compounds and Coal Ash

Iron oxide (Fe₂O₃) is catalytic to the sorption of SO₂ by CaO in a chemical environment similar to that in fluidized-bed combustion of coal. Four percent by weight of Fe₂O₃ physically mixed with CaO approximately doubles the sorption (sulfation) rate. The kinetic mechanisms of the catalytic reaction are discussed based on the view that SO₃ formation is the rate-limiting step. Coal ash also catalyzes the sulfation reaction, and Fe₂O₃ appears to be the active constituent in such catalytic effects. Other iron-containing compounds existent in coal and fluidized-bed combustion systems are oxidized rapidly in the combustion gases to form Fe₂O₃ and in turn catalyze the sulfation reaction.