Influence of HCl on the Homogeneous Reactions of CO and NO in Postcombustion Conditions—A Kinetic Modeling Study

Abstract

The influence of hydrogen chloride (HCl) on homogeneous gas-phase reactions of carbon monoxide (CO) and nitric oxide (NO) was studied in typical postcombustion conditions of industrial furnaces using detailed kinetic modeling. A well-established reaction mechanism (203 reactions) describing the oxidation of moist CO, as well as of NH 3 and HCN was extended by a recently published subset of 36 reactions for the oxidation of HCl. Validation of modeling predictions was achieved in that the effect of HCl on the CO burnout showed excellent agreement with available independent laboratory data. The modeling results led to the conclusion that the presence of HCl (100–600 ppmv) has a strong effect on the CO oxidation at low temperatures of approximately 1023 K. The effect is dependent on the H 2O concentration and the presence of NO. Very interestingly, at high concentrations of H 2O (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 analysis, CO is oxidized by OH radicals via CO + OH → CO 2 + H. The acceleration of the CO burnout is explained by the reactions HO 2 + Cl → HCl + O 2 and HO 2 + Cl → OH + ClO decreasing the concentration of the HO 2 radical and, consequently, also the rate of the reaction HO 2 + OH → H 2O + 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 more H radicals are formed via the CO burnout reaction. This enhances OH formation further via the reactions H + O 2 → O + OH and H 2O + O → OH + OH under these conditions. At lower concentrations of H 2O (1 vol %) without any NO, and always if NO was present (150 ppmv), a deceleration of CO burnout was predicted, in agreement with available laboratory studies and most findings in practical combustors. This deceleration is explained by a decrease of the radical pool (OH). Around and above 1123 K the influence of HCl on the CO burnout was found to be very small for all conditions investigated. Furthermore, it was predicted that in the presence of ammonia, HCl extends the temperature window for NO reduction, particularly on the low temperature side.