The effect of calcination on the reactions of ammonia over different carbonates and limestones in fluidized bed combustion conditions

In this work, the reactions of ammonia (NH3) in the presence of carbonate a nd corresponding oxide surfaces were studied in an oxygen-free, as well as in an oxygen-containing atmosphere at temperatures between 450 and 950 degr eesC. In the temperature range below the carbonate dissociation temperature , NH3 was decomposed to N-2 and H-2 in an oxygen-free atmosphere. The react ions of NH3 during the dissociation of the carbonate led to formation of ad ditional products, namely H2O, CO2, CO, hydrocyanic acid (HCN), and isocyan uric acid (HNCO). The rate of NH3 decomposition decreased with increasing r elease of CO2 during the calcination, which may be due to adsorption of H2O formed in various reactions. Calcium carbonate, dolomite, and potassium ca rbonate 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 st udied. Possible reactions leading to formation of these products are discus sed. No formation of nitrogen oxides was observed during these experiments, which may be due to adsorption of H2O on surface sites active in the heter ogeneous NH3 oxidation mechanism. In an atmosphere containing O-2, decompos ition and oxidation reactions removed NH3 more effectively in the presence of CaO (calcined limestone) than in the presence of CaCO3 (uncalcined limes tone), whereas when O-2 was present, NH3 was removed by oxidation to NO and N2O as well.