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.