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The formation of Nitrous oxide (N2O) over urea-based selective catalytic reduction (SCR) systems was scrutinized using exhaust gases from a 3.4-liter non-road diesel engine. Four catalysts were used for the SCR systems; Cu-zeolite (Cu/Fe = 4), Fe-zeolite (Fe only), vanadia and mixed (vanadia and Cu-zeolite) one. For all SCR systems, the N2O concentration at the SCR inlet (before SCR) was below 1 ppm, except for the low-load and low-speed engine conditions, where it was below 3 ppm. However, at the SCR outlet, the N2O concentration had risen to nearly 20 ppm, except for a few conditions. That is, N2O increased more than a dozen times by passing through the SCR systems. The amount of N2O formed within the SCR systems depended on the SCR catalysts; the vanadia catalyst had the lowest amount of N2O, while the Fe-zeolite catalyst as well as the Cu-zeolite catalyst had the highest. In addition, the formation of N2O was strongly involved in the ratio of nitrogen dioxide (NO2) to nitrogen oxides (NOx) at the SCR inlet, which was coupled to SCR inlet temperatures. The higher ratio of NO2 to NOx led to higher N2O at the SCR outlet. It was found that the start of N2O formation corresponded to the moment when nitric oxide (NO) was close to 0 ppm as the ratio of an ammonia (NH3) in aqueous urea solution to NOx increased. In other words, NO depletion played a crucial role in the start of N2O formation when NO and NO2 were simultaneously consumed. Therefore, the difference in N2O formation among the four catalysts for the SCR systems depended on how quickly NO was depleted. Results from α values of intersections of N2O and NO (α@N₂O = NO) for Cu- and Fe-zeolite catalysts showed that α@N₂O = NO for the Fe-zeolite catalyst are much smaller than that for the Cu-zeolite catalyst, and this implies that the Fe-zeolite catalyst can start N2O formation earlier or have faster NO depletion than the Cu-zeolite catalyst. |
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