Reactivity of Soot With Nitrogen Oxides in Exhaust Stream 2002-01-1683
Exhaust stream from automotives and trucks will be subjected to new and more stringent emissions standards. In the case of Diesel engine the lowering of carbon particulates emission (soot) is based on a filtration system in which trapped soot is periodically removed by controlled oxidation with O2 at 550°C or above. It is known that in presence of NO2 the carbon-O2 reaction is significantly speeded-up. Since NO2 may be formed in the gas stream by catalytical oxidation of NO present in the exhaust stream, it is thought that small amounts of NO2 (500 ppm by vol) will promote the continuous oxidation of the deposited carbon particulates in the temperature range 200 - 500°C. The behaviour of the CRT™ system is based on this principle.
Very little is known about the reactivity of soot with NO2. Moreover, the effect of other species present in the exhaust stream like O2 and H2O is also not well established. The present study investigates the reactivity of carbon black particles having properties similar to soot with small amount of NO2 (450 ppm by volume) in the gas phase. The rate of carbon consumption in presence of NO2 becomes significant at 300°C. Moreover, the reactivity is significantly improved in the presence of O2 which is not reacting alone with carbon at that temperature. The simultaneous presence of NO2 and O2 leads to an enhanced gasification rate of the carbon. The investigation of the reaction mechanism suggests the build-up of a transient NO2 complex on the carbon surface which desorbs as NO and CO2 or CO. Furthermore, it appears that the main effect of NO2 is to promote the desorption rate below 400°C of oxygen complexes built-up on the carbon surface.