Soot oxidation

Several researchers have focused their attention on the application of oxide materials to lower the oxidation temperature of soot particulates. It was reported that active soot oxidation catalysts are PbO, C03O4, V2O5, M0O3, CuO, and perovskite type oxides[3]. 

The main reactions, which have to be considered on SCR catalysts, are the standard-SCR, fast-SCR, and the N02-SCR reactions, beside the ammonia oxidation and the formation of N20. The fast-SCR reaction is promoted by N02 in the feed that can be generated from NO in a pre-oxidation catalyst. However, the right dimensioning of the oxidation catalyst is critical in order to prevent the production of an excess of hazardous N02. This problem is further aggravated if a continuous regenerating DPF is installed in front of the SCR system, as part of the N02 produced by the oxidation catalyst is always consumed in the filter for soot oxidation. 

The reaction probabilities for O and OH with soot particles have been measured by Roth and co-workers in a series of shock tube experiments [58-60], They have found that both radicals react with soot particles with a collision efficiency of between 0.10 and 0.20. In contrast, the reaction probability with 02 is at least an order of magnitude lower [55], Of course, at lower temperatures and sufficiently lean mixtures, soot oxidation by radical species becomes small and oxidation by 02 is important (though slow). Consequently, soot that passes through or avoids the primary reaction zone of a flame (e.g., due to local flame quenching) may experience oxidation from 02 in the post-flame gases. Analysis of soot oxidation rates in flames [54-57] has supported the approximate value of the OH collision efficiency determined by Roth and co-workers. 

Unfortunately, OH and O concentrations in flames are determined by detailed chemical kinetics and cannot be accurately predicted from simple equilibrium at the local temperature and stoichiometry. This is particularly true when active soot oxidation is occurring and the local temperature is decreasing with flame residence time [59], As a consequence, most attempts to model soot oxidation in flames have by necessity used a relation based on oxidation by 02 and then applied a correction factor to augment the rate to approximate the effect of oxidation by radicals. The two most commonly applied rate equations for soot oxidation by 02 are those developed by Lee el al. [61] and Nagle and Strickland-Constable [62],... 

With appropriate choices of kinetic constants, this approach can reproduce the NSC experimental data quite well. Park and Appleton [63] oxidized carbon black particles in a series of shock tube experiments and found a similar dependence of oxidation rate on oxygen concentration and temperature as NSC. Of course, the proper kinetic approach for soot oxidation by 02 undoubtedly should involve a complex surface reaction mechanism with distinct adsorption and desorption steps, in addition to site rearrangements, as suggested previously for char surface combustion. 

Chughtai, A. R., W. F. Welch, Jr., M. S. Akhter, and D. M. Smith, A Spectroscopic Study of Gaseous Products of Soot-Oxides of Nitrogen/Water Reactions, Appl Spectrosc., 44, 294-298 (1990). 

Current research efforts are concentrating on computationally efficient implementations of the energy equation within the MicroFlowS framework to allow realistic simulations of soot particle reaction in the porous structures. The next section shows a parallel line of development that started in Konstandopoulos and Kostoglou (2004), which tries to extend continuum models of soot oxidation to account for microstructural effects. 

For a= 1, soot in the catalytic layer is oxidized fast leaving the soot in the thermal layer unreacted. This has been observed with some early catalytic filters. As a decreases the soot from the top layer replaces more rapidly the soot oxidized in the catalytic layer increasing the global oxidation rate. The corresponding soot layer thickness evolution is shown in Fig. 22. For values of a close to 1 (e.g. 0.9) the catalytic layer is totally depleted from soot at some instances, followed by sudden penetration events from the soot of the thermal layer. These events are clearly shown in the thickness evolution for oc = 0.9 in... 

Fig. 22. For smaller values of a, equilibrium between the soot entering the catalytic layer and the soot oxidized in it is established leading to a constant global oxidation rate.

The two-layer model for soot oxidation with N02 in a catalytic filter can be written as follows ... 

In this layer only soot oxidation by N02 happens ... 

Based on the N02-soot oxidation data promoted by the catalytic coating tested in Konstandopoulos et al. (2000) an R value of about 3.4 has been computed. This represents a significant enhancement. 

AH heat of soot oxidation reaction (per unit mass of soot)... 

Konstandopoulos, A. G., and Kostoglou, M. A mathematical model of soot oxidation on catalytically coated ceramic filters . Advances in Vehicle Control and Safety (AVCS 98), Amiens, France, July 1-3, 1998. 

Konstandopoulos, A. G., Kostoglou, M., Housiada, P., Vlachos, N., and Zarvalis, D. Multichannel simulation of soot oxidation in diesel particulate filters. SAE Technical Paper No. 2003-01-0839 (2003). 

Appendix. Microstructural Model of Soot Oxidation The Effect of Catalyst... 

In the presence of a catalyst and with a finite selectivity for CO production, soot oxidation is described with the following global reactions, one for the thermal and one for the catalytic path (i.e. oxidation of soot by oxygen transferred from the catalyst by a redox and/or spill-over mechanism). Konstandopoulos and Kostoglou (1999b, 2000) ... 

Chemical characteristics of the oil condition monitoring By using the description and data given in Chapter 6.2, p. 231, specify the analytical techniques measuring the changes of oil deterioration soot, oxidation, nitration and sulfation. 

More recent investigations have been conducted into NOx-assisted soot oxidation [55, 56]. NO2 was suggested as an oxidation agent to assist the combustion of particulate matter in the presence of oxygen in the exhaust gas [57]. NO2 can be produced by catalytic oxidation of NO prior to the catalytic filter. 

PYROLYSIS SOOT FORMATION SOOT OXIDATION GAS PHASE OXIDATION NOx FORMATION... 

Simonsen SB, Dahl S, Johnson E, Helveg S. Ceria-catalyzed soot oxidation studied by environmental transmission electron microscopy. J Catal. 2008 255 1. 

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