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. N₂ 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.