NH3 oxidation catalysed by calcined limestone -: a kinetic study

The oxidation of NH3 catalysed by calcined limestone was studied in a small fixed bed reactor at fluidized bed combustion temperatures (750-950 degreesC) using three types of limestones: Faxe Bryozo, Stevns Chalk and Ignaberga. It was shown that the limestones are very active catalysts for the oxidation of NH3. Experiments were carried out with O-2 concentrations between 0 and 4.5 vol%. At low O-2 concentrations (<1 vol%), a strong increase in the NH3 oxidation rate was observed with increasing O-2 concentration. At O-2 concentrations above 1 vol% no further effect of increasing the O-2 concentration was observed. The selectivity for NO formation found in most experiments is about 0.65-0.70 for all limestones used when the O-2 concentration is above 0.5 vol%. This means that the oxidation of NH3 to NO is about two times faster than the oxidation to N-2. Below 0.5 vol% O-2, the selectivity for NO decreases with decreasing O-2 concentration. The selectivity for NO is not influenced by inlet concentrations of NH3 or NO. The experimental observations are explained by a reaction scheme initially proposed by de Soete [Proc. of the 5th International Workshop on Nitrous Oxide Emissions, 1992] and modified in this study. The experiments showed that the NH3 oxidation is first order in NH3 and that the reaction rate is not influenced by NO. The influence of O-2 is modelled by a Langmuir model for O-2 adsorption on the CaO surface. Based on the kinetic model, reaction rate expressions for NH3 oxidation to NO and N-2 were obtained from the experimental data. To our knowledge, this is the first time that validated reaction rate expressions, which can describe the NH3 oxidation over calcined limestone over a broad O-2 range and a broad temperature range, are reported in the literature. The kinetics obtained in this work may be used to improve the modelling of NO emissions from fluidized bed combustors. (C) 2002 Elsevier Science Ltd. All rights reserved.