The Effect of Chlorine on the formation of nitrogen oxides in a fluidized bed combustor

Abstract

The effect of chlorine on NOx and N2O emissions has been studied in an electrically heated bed of fluidized sand. Pyridine (C5H5N) was burnt as a model compound in a mixture of O2 and N2 to produce NO and N2O. The experiments were done at temperatures 750, 825 or 900°C. The concentrations of O2 and pyridine in the fluidizing gas were maintained at 3.2 or 11.6%, and 500, 1500 or 2000 ppmv, respectively. When pyridine was burnt in the fluidized bed, the concentrations of NO, N2O, CO2 and CO were measured continuously in the freeboard. Chlorine was introduced to the fluidized bed as HCI at concentrations up to 6000 ppmv. The effect of CaCl2 on the formation of NOx, N2O and CO was also studied. The addition of HCI to the fluidizing gas mixture decreased the concentration of NOx, and increased the concentration of CO. However, the addition of HCl caused the concentration of N2O to decrease or to increase, depending on the temperature. At 750 °C, the addition of HCl reduced the concentration of N2O, while it increased the concentration at 825 and 900°C. Temperature has a significant impact on the effect caused by the addition of HCl on the formation of NOx, N2O and CO. On the other hand, the concentrations of pyridine and O2 do not have a significant effect on the percentage decrease or increase in the concentrations of NOx and CO. The addition of CaCl2 also seems to decrease the concentration of NOx, while it increases the concentration of CO. However, the presence of calcium based compounds in the fluidized bed also tends to catalyze the formation of NOx. The increase in the concentration of CO caused by HCl is due to the suppression of OH radicals, which are the main precursor for the formation of CO2 from CO. During pyridine combustion, the radical pool of 0, Hand .OH plays a major role in converting HCN and NCO to NOx. Thus, the decrease in NOx caused by HCl is due to the suppression of 0, H and OH radicals. Again the increase in the concentration of N20 at 825 and 900°C is due to the suppression of OH radical by HCI. However, the decrease in N2Oat 750°C is probably due to the inhibition of pyridine combustion by HCl.