An experimental and kinetic calculation of the promotion effect of hydrocarbons on the NO-NO2 conversion in a flow reactor

Experimental and detailed chemical kinetic modeling work has been performed to investigate the role of hydrocarbon oxidation in NO-NO₂ conversion. An atmospheric pressure., quartz flow reactor was used to examine the dependence of NO oxidation to NO₂ by hydrocarbon type, reaction temperature, and residence time. The five hydrocarbons examined were methane, ethylene, ethane, propene, and propane. In the experiment, probe measurement of the species concentrations was performed in the flow reactor using a mixture of NO(20 ppm)/air/hydrocarbon(50 ppm) at residence times from 0.16 to 1.46 s and temperatures from 600 to 1100 K. In the chemical kinetic calculation, the time evolution of NO, NO₂, hydrocarbons, and reaction intermediates were evaluated for a series of the hydrocarbons and the temperatures. The chemical mechanism consisted of 639 reversible reactions and 126 species.

Experimental results indicate that, in general, ethylene and propane effectively oxidize NO to NO₂ while methane is less effective. The calculation indicates the important chemical kinetic features that control NO-NO₂ conversion for each hydrocarbon type. The dependence of NO-NO₂ conversion with hydrocarbon type and temperature is qualitatively reproduced by the calculation. The calculation indicates that all five hydrocarbons oxidize NO to NO₂ predominantly through NO+HO₂ ahNO₂+OH and that the contribution of oxidation by RO₂ and HORO₂ is minor. Highest effectiveness comes from hydrocarbons that produce reactive radicals (i.e., OH, O atom) that promote hydrocarbon oxidation and lead to additional HO₂ production. On the other hand, if hydrocarbons produce radicals, such as methyl and allyl, which resist oxidation by O₂, then these radicals tend to reduce NO₂ to NO. Experimental results show that the effectiveness of hydrocarbons varies appreciably with temperature and only within the low-temperature range. Propane shows the greatest NO-NO₂ conversion for the lowest temperatures. This ability is primarily due to the hydroperoxy-propyl plus O₂ reactions as indicated by the sensitivity analysis results.