Detailed Kinetic Modeling of Moist CO Oxidation Inhibited by Trace Quantities of HCL

A detailed kinetic model for moist CO oxidation chemistry inhibited by HCI is developed and analyzed by sensitivity and reaction flux analyses. The model is validated against experimental data obtained from an atmospheric pressure flow reactor. The kinetics were studied experimentally and numerically with dilute mixtures of CO (~ 1%), H₂0 (~ 0.5%), O₂and HCI reacting in N₂at a temperature near 1000 K. The effect of increasing the Cl/H ratio was investigated by increasing HCI concentrations from 0 to 200 ppm while the effect of excess O₂was studied by varying the fuel oxidizer equivalence ratio from 1.0 to 0.33. The results showed that small quantities of HCI inhibit CO oxidation and that increasing _O2concentrations to stoichiometric mixtures further decreases the oxidation rate, a counter intuitive result. The model predictions for CO, O₂, CO₂and temperature profiles agree well with the experimental results (within 10%). Thus the inhibiting characteristics of HCI are well described. However, the model predictions of the HCI profiles were found to be only within 30% of the experimental data. Both experimental and model deficiencies may be responsible for this larger discrepancy. Sensitivity and reaction flux analyses were used to determine the rate-controlling inhibitory steps and major inhibitory pathways created by HCI respectively. At high temperatures the chain termination reactions in the inhibitory cycles generated by HCI have been reported to be primarily H + Cl + M→ HCI + M and Cl + Cl + M→ Cl₂+ M. The principal chain terminating step at 1000 K was found to be the reaction Cl +HO₂→HCI + O₂. Reactions involving COCI were also found to generate chain terminating cycles not previously reported in the literature. Extension of the modeling calculations to higher temperatures and Cl/H ratios showed the relative importance of these two additional inhibitory mechanisms to decrease and increase, respectively.