Decomposition and reduction of N_2O over limestone under FBC conditions

摘要

The addition of limestone for sulfur retention in FBC has in many cases been observed to influence the emisison of N_2O. The catalytic activity of N_2O over calcined Stevns Chalk for decomposition of N_2O in a laboratory fixed bed quartz reactor was measured. It was found that calcined Stevns Chalk is a very active catalyst for N_2O decomposition in an inert atmosphere, and the presence of 3 volpercent CO increased the rate of N_2O destruction by a factor of 5 due to the catalytic reduction of N_2O by CO. The activity decreasd with increasing CO_2 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 N_2O by CO. Without CO present a gas solid reactionb etween N_2O and CaS takes place and SO_2 is released. Kinetic rate epxressions for the decomposition and reduction of N_2O were developed and used to simulate the addition of Stevns Chalk to a 12 MW_(th) CFBC burning bituminous coal. The rate of N_2O destruction was doubled when limestone was added at a Ca/S ratio of 1.8 with 90percent desulfurization.