Analysis of carbon black oxidation

University dissertation from Chalmers University of Technology

Abstract: Diesel engines are known to be a major source of a highly pollutant material known as particulate matter, PM, which is a strongly threatening agent to human health. Therefore, diesel particulate filters are used to reduce PM emissions by trapping soot. Regenerating the particulate filter is necessary to keep the exhaust back pressure below a certain limit and, thereby, to maintain efficient fuel consumption. Regeneration can be run in two modes: active or passive. The former mode is performed by injecting additional fuel into the exhaust to raise the temperature above 550 C and, thereby, burn the accumulated soot with oxygen. In contrast, passive regeneration can be performed at the exhaust temperature, i.e. ~350-450 C, which is more efficient. The key compound for passive regeneration that reacts with soot is NO2, which is more reactive than oxygen. It is formed by oxidizing NO over an oxidation catalyst.
The goal of this thesis is to develop both experimental and theoretical methodologies to study the soot-NO2 reaction from a fundamental perspective. To this end, a thorough characterization study of the experimental setup was conducted with respect to heat transfer and residence time distribution, which resulted in a number of guidelines to help increase the quality of experimental measurements. Moreover, a novel sample preparation method is proposed that ensures a highly controlled carbon deposition in each and every channel of the monolithic reactors, which was found to be crucial for obtaining reliable and reproducible results. Furthermore, a deconvolution algorithm was developed to decrease the unresolved time span of transient measurements. Numerical simulations, in particular computational fluid dynamics, were extensively employed to both aid as simulation tools and to gain a deeper insight into the subject of study.
Significant promoting effects were identified for both water vapor and molecular oxygen, which indicates the importance of studying automotive soot oxidation under realistic exhaust conditions. A global kinetic model was formulated that can express the observed rate of oxidation in the presence of water vapor, oxygen, and NO2 with very high accuracy over the entire 0-100% conversion interval. It thus serves as a predictive tool for optimizing both the operation of diesel engines and the design of soot traps.

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