Kinetics of Heterogeneous NO and N2O Reduction at FBC Conditions

A new method for reduction of N2O at the same time as the emissions of NO and SO2 are kept low has been proposed by Chalmers University of Technology. The method involves a reversing of the conventional air staging strategy, i.e. running at almost stoichiometric conditions in the bottom of the combustion chamber and adding secondary air in the top or in the cyclone. The change in reaction pathways for N2O and NO formation and reduction is very complex, and the catalytic activity of the solid material in the boiler may vary with the air staging. Samples of solids taken from large scale tests in a 12 MW CFB boiler under different operating conditions (no, severe and reversed air staging) have been tested in small scale laboratory fixed bed reactors. The activity of char and bed material (a mixture of sand, ash and partly sulfated limestone) for decomposition of N2O and simultaneous catalytic reduction of N2O and NO was measured. The char was found to be very active compared to bed material under inert conditions. There was no influence of operating conditions on the activity of the char. Pore diffusion influenced the reaction rate for particle sizes above 0.2 mm at 1076 K, and the larger particles above 1 to 2 mm were in the strong pore diffusion regime. The presence of CO had only minor effect and the increase in reduction rate levelled off at concentrations above about 2 vol%. Bed material is an active catalyst for N2O decomposition, but the activity is lower than for char when compared on a mass basis. Rate equations for the decomposition of N2O over bed material and reduction of N2O and NO over char were estimated. It was not possible to determine rate constants for the reduction of N2O and NO with CO over bed material because of reductive decomposition of partly sulfated limestone during the experiments. Under these transient conditions the reduction of N2O and NO over bed material was very fast. The final conclusion was that char and bed material will both play a role for the reduction of N2O and NO in a CFB boiler and bed material will be the most important solid material for at reducing conditions when reductive decomposition of sulfated limestone takes place.