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 without the presence of H2O or CO2. In this work, the catalytic activity for NH3 oxidation with three types of calcined limestone was tested under fluidised 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 NH3. 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 stopped the water desorbs and the activity is restored. Addition of CO2 did not 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 both 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 Science Ltd.

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