INFLUENCE OF HCL ON THE HOMOGENEOUS REACTIONS OF CO AND NO IN POSTCOMBUSTION CONDITIONS - A KINETIC MODELING STUDY

The influence of hydrogen chloride (HCl) on homogeneous gas-phase reac tions of carbon monoxide (CO) and nitric oxide (NO) was studied in typ ical postcombustion conditions of industrial furnaces using detailed k inetic modeling. A well-established reaction mechanism (203 reactions) describing the oxidation of moist CO, as well as of NH, and HCN was e xtended by a recently published subset of 36 reactions for the oxidati on of HCl. Validation of modeling predictions was achieved in that the effect of HCl on the CO burnout showed excellent agreement with avail able independent laboratory data. The modeling results led to the conc lusion that the presence of HCl (100-600 ppmv) has a strong effect on the CO oxidation at low temperatures of approximately 1023 K. The effe ct is dependent on the H2O concentration and the presence of NO. Very interestingly, at high concentrations of H2O (7 vol %) and without any NO, HCl led to a totally unexpected acceleration of the CO burnout at residence times longer than 0.5 s. According to the reaction path ana lysis, CO is oxidized by OH radicals via CO + OH --> CO2 + H. The acce leration of the CO burnout is explained by the reactions HO2 + Cl --> HCl + O-2 and HO2 + Cl --> OH + ClO decreasing the concentration of th e HO2 radical and, consequently, also the rate of the reaction HO2 + O H -> H2O + O-2, which competes with CO for the OH radicals. Thus, when HCl is present, more OH will be available for CO oxidation and also m ore H radicals are formed via the CO burnout reaction. This enhances O H formation further via the reactions H + O-2 --> O + OH and H2O + O - -> OH + OH under these conditions. At lower concentrations of H2O (1 v ol %) without any NO, and always if NO was present (150 ppmv), a decel eration of CO burnout was predicted, in agreement with available labor atory studies and most findings in practical combustors. This decelera tion is explained by a decrease of the radical pool (OH). Around and a bove 1123 K the influence of HCl on the CO burnout was found to be ver y small for all conditions investigated. Furthermore, it was predicted that in the presence of ammonia, HCl extends the temperature window f or NO reduction, particularly on the low temperature side. (C) 1998 by The Combustion Institute.