CATALYTIC REDUCTION OF NO AND N2O ON LIMESTONE DURING SULFUR CAPTURE UNDER FLUIDIZED-BED COMBUSTION CONDITIONS

sulfur retention by limestone (CaCO3) under fluidized bed combustion conditions is the net effect of a competition between sulfur capture and sulfur release during which the composition of the Ca surface changes continuously between CaO, CaS and CaSO4, the ultimate product being CaSO4. The presence of limestone in fluidized bed combustors interacts with the nitrogen chemistry; it acts as a catalyst for volatile-N oxidation to NO and as a catalyst for the reduction of NO and N2O, the latter being the subject of this paper. The results show that CaS and CaO are active catalysts for reduction of NO and N2O, whereas CaSO4 and CaCO3 are poor catalysts. N2O decomposes readily over CaO under oxidizing conditions which leave NO unaffected. Under reducing conditions, both NO and N2O are reduced simultaneously but apparently not at the same active sites. The catalytic activity of limestone decreases during sulfation due to a loss in porosity and a lower activity of the CaSO4 formed. The highest catalytic activity for NO and N2O reduction was found for limestone with a high sulfur capacity. The presence of H2O appears not to affect the heterogeneous reactions involving NO and N2O.