CATALYTIC AND GAS-SOLID REACTIONS INVOLVING HCN OVER LIMESTONE

In coal-fired combustion systems solid calcium species may be present as ash components or limestone added to the combustion chamber. In thi s study heterogeneous reactions involving HCN over seven different lim estones were investigated in a laboratory fixed-bed quartz reactor at 873-1,173 K. Calcined limestone is an effective catalyst for oxidation of HCN. Under conditions with complete conversion of HCN at O-2 conce ntrations above about 5,000 ppmv the selectivity for formation of NO a nd N2O is 50-70% and below 5%, respectively. Nitric oxide can be reduc ed by HCN to N-2 in the absence of O-2 and to N-2 and N2O in the prese nce of O-2. At low O-2 concentrations or low temperatures. HCN may rea ct with CaO, forming calcium cyanamide, CaCN2. The selectivities for f ormation of NO and N2O from oxidation of CaCN2 is 20-25% for both spec ies. The catalytic activity of limestone for oxidation of HCN decrease s with increasing degree of sulfation. Simultaneously the selectivity for formation of NO decreases while that for N2O increases. The cataly tic activity of sulfated limestone increases with decreasing SO2 conce ntration indicating a competition between SO2 and HCN for sites on the surface. The results indicate that heterogeneous oxidation of HCN is important in calciners and fluidized-bed combustors with limestone add ition or when burning coals with an ash with a high catalytic activity .