Decomposition and reduction of N2O over Limestone under FBC Conditions

The addition of limestone for sulfur retention in FBC has in many cases been observed to influence the emission of N2O. The catalytic activity of N2O over calcined Stevns Chalk for decomposition of N2O in a laboratory fixed bed quartz reactor was measured. It was found that calcined Stevns Chalk is a very active catalyst for N2O decomposition in an inert atmosphere, and the presence of 3 vol% CO increased the rate of N2O destruction by a factor of 5 due to the catalytic reduction of N2O by CO. The activity decreased with increasing CO2 concentration, and uncalcined or recarbonated limestone had negligible activity. Sulfation of the calcined limestone under oxidizing conditions lowered the activity, however sulfidation under reducing conditions showed that CaS is an active catalyst for the reduction of N2O by CO. Without CO present a gas solid reaction between N2O and CaS takes place and SO2 is released. Kinetic rate expressions for the decompostion and reduction of N2O were developed and used to simulate the addition of Stevns Chalk to a 12 MWth CFBC burning bituminous coal. The rate of N2O destruction was doubled when limestone was added at a Ca/S ratio of 1.8 with 90% desulfurization.