(227c) Combined Filtration and Catalytic Combustion of Diesel Particulate: Secondary Nanoparticle Emissions during Trap Regeneration

There is increasing concern about health effects of particulate matter, PM, emitted from Diesel engines for automotive applications. PM is suspected of causing detrimental acute and chronic damage to the pulmonary and the cardiovascular system. Based on scientific proofs, air pollution legislations of several countries classifies Diesel soot as carcinogenic. Moreover, several health effects are associated with the ultra-fine particles with diameters below 100 nm [1]. Recent research shows that these particles can penetrate the cell membranes, enter the blood and even reach the brain [2]. Besides, some investigations indicate that particles can induce inheritable mutations [3]. The most promising particulate abatement technology is based on wall-flow catalytic traps, periodically regenerated by a peculiar use of last generation Common-Rail Diesel engines (i.e. post injection of HCs burned out by a specific catalytic converter so as to heat up the downstream trap up to ignition of the trapped soot). The role of the catalyst is to enhance the combustion rate of the soot collected on the filter by reducing its ignition temperature [4]. The particle trap efficiency is very high also with the ultra-fine particles and does not significantly vary with particle size. However, during the regeneration phase itself secondary particulate emissions were clearly noticed at our labs (Fig. 1). This paper aims at elucidating this unexpected phenomenon. For this purpose, a trap based on a novel catalyst, CoCr₂O₄, synthesised and deposited on a wall-flow monolith via in situ combustion synthesis, underwent the phases of a typical bench test performance analysis described in [4]. The distribution of the ultra-fine particles has been measured via a Scanning Mobility Particle Sizer (SMPS by TSI Inc.). This analyser can measure a wide range of diameters (3 nm ? 1000 nm). Specific tests for both a catalytic and non catalytic wall-flow trap gave us the evidence of ultra-fine particles formation only during the regeneration of the catalytic trap. Particularly, in Fig.1 a bi-modal distribution of the particles downstream the catalytic trap during the regeneration can be noticed. Several evidences of the chemical and physical nature of the nanoparticles emitted were derived by X-ray and surface area analysis. The SMPS capability of extracting from a heterogeneous stream a narrow range of particles was extremely useful to study the phenomena in depth. There are suggestions in the open literature about the non-carbonaceous nature of the ultra fine particles emitted by the diesel engine [5]. The most narrow particles were indeed found to be characterised by significant levels of sulphates, as a likely consequence of the oxidative properties of the soot oxidation catalysts. As for the larger secondary particles emitted they seem to be mostly composed of soot particles. Their emission seems to be primarily caused by the comparatively faster regeneration kinetics enabled by the catalytic trap compared to the non-catalytic one. In the former case, the fast release of heat and gases, as a consequence of the catalysed soot combustion process, brings about the disintegration of some soot agglomerates into smaller particles that can eventually find their way throughout the filter. This secondary particle emissions partly counterbalances the positive effect of the presence of a soot combustion catalyst in terms of regeneration effectiveness and time reduction.

References

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