The effect of low-concentration fuels on the conversion of nitric oxide to nitrogen dioxide

Abstract

The effect of low-concentration fuels on the conversion of NO to NO2 is studied by an experiment and a chemical kinetic calculation. In the experiment, the NO and NO2 concentrations are measured for the mixing process of hot combustion gas with cold air in which nine types of fuel (seven hydrocarbons from C1 to C4, H2 and CO) are added. The hot combustion gas contains from 6 ppm to 79 ppm NO. The experimental results show that the conversion of NO to NO2 in the mixing process is strongly promoted by the addition of small amount of all nine types of fuel. For example, the addition of only 40 ppm of C3H8 raises the proportion of NO2 to NOx from 0.24 to 0.90. The effectiveness of promotion strongly depends on the type of fuel. For seven types of fuel showing a similar pattern of promotion, the effectiveness increases as H2<CH4<C2H6<C2H4<C3H8<i-C4H10<n-C4H10. In the chemical kinetic calculation, in order to understand the fundamental chemical kinetic aspect of the effect of fuel on the conversion of NO to NO2, a constant-temperature air premixed with NO and a low concentration of hydrocarbons is assumed as a simple model. The reaction mechanism includes 115 reactions for C3 hydrocarbons and NO-NO2. The conversion of NO to NO2 is indicated to proceed mainly through the HO2 mechanism (NO+HO2=NO2+OH) even in the presence of fuel. Therefore, high effectiveness is obtained by the fuels which are most easily decomposed to produce HO2. Also, the promotion is shown to proceed strongly in a certain temperature range. It is suggested from the results that the presence of fuel in the cool region of the flow in a combustion system should be prevented in order to reduce NO2 exhaust emission.