The influence of H2O and CO2 on the reactivity of limestone for the oxidation of NH3

Although it is known that both H2O and CO2 reduce the catalytic activity of CaO, the kinetics of NO formation catalysed by CaO are often obtained with out the presence of H2O or CO2. In this work, the catalytic activity for NH 3 oxidation with three types of calcined limestone was tested under fluidis ed bed combustion conditions by adding H2O (0-12 vol%) and CO2 (0-16 vol%). All three types of limestones are active catalysts for the oxidation of NH 3. When water is added the activity decreases sharply and already at 3 vol% water the NH3 conversion is reduced by 50%. When the water addition is sto pped the water desorbs and the activity is restored. Addition of CO2 did no t result in a decrease in the oxidation of NH3. Blocking of the active sites by adsorption of H2O is the main cause of the deactivation. A model with a Langmuir adsorption type was developed and bot h NO and NH3 exit concentrations were modelled correctly over a wide range of reaction conditions. This study shows that kinetic investigations concerning NH3 oxidation over CaO should be carried out in the presence of water. (C) 2000 Elsevier Scien ce Ltd. All rights reserved.