The Effect of Calcination on the Reactions of Ammonia over Different Carbonates and Limestones in Fluidized Bed Combustion Conditions

Abstract

In this work, the reactions of ammonia (NH 3 ) in the presence of carbonate and corresponding oxide surfaces were studied in an oxygen-free, as well as in an oxygen-containing atmosphere at temperatures between 450 and 950 °C. In the temperature range below the carbonate dissociation temperature, NH 3 was decomposed to N 2 and H 2 in an oxygen-free atmosphere. The reactions of NH 3 during the dissociation of the carbonate led to formation of additional products, namely H 2 O, CO 2 , CO, hydrocyanic acid (HCN), and isocyanuric acid (HNCO). The rate of NH 3 decomposition decreased with increasing release of CO 2 during the calcination, which may be due to adsorption of H 2 O formed in various reactions. Calcium carbonate, dolomite, and potassium carbonate had similar, but not quantitatively the same, effects. The presence of barium carbonate did not give formation of HCN or HNCO, probably due to a too low amount of reactive carbonate ions in the temperature interval studied. Possible reactions leading to formation of these products are discussed. No formation of nitrogen oxides was observed during these experiments, which may be due to adsorption of H 2 O on surface sites active in the heterogeneous NH 3 oxidation mechanism. In an atmosphere containing O 2 , decomposition and oxidation reactions removed NH 3 more effectively in the presence of CaO (calcined limestone) than in the presence of CaCO 3 (uncalcined limestone), whereas when O 2 was present, NH 3 was removed by oxidation to NO and N 2 O as well.