Biomorphic Silicon Carbide as porous substrate for automotive Diesel Particulate Filters

Resumen

Given the amount of diesel vehicles worldwide and their overall environmental impact, particulate emission control is an increasingly important issue. Particles control systems based on improvements of the combustion process or on the design of the engines are not sufficient to comply with current regulations. Therefore, the use of aftertreatment systems has become necessary. The most popular aftertreatment device for the abatement of particles, the wall-flow Diesel Particulate Filter (wall-flow DPF), still has a considerable scope for improvement. The high backpressure they introduce in the exhaust pipe, and the technical issues associated with the on-board regeneration process are some of the most concerning aspects. The aim of this work is to develop a new substrate for wall-flow Diesel articulate Filters based on a novel porous ceramic material: biomorphic silicon carbide (bioSiC). BioSiC is a particular type of silicon carbide obtained from the pyrolysis of wood and its subsequent infiltration with silicon. It is characterized by having a porous microstructure that replicates the cellular biological tissue of the wood precursor used for its manufacture. The main advantage of this material is the possibility of manipulating its microstructural features through the adequate selection of the precursor. With a suitable combination of porosity, pore size, and microstructural arrangement, the resulting filter could have enough filtration efficiency, with lower pressure drop, and better response to regeneration. In the first part of this research work, bioSiC is characterized in terms of its suitability as substrate for particle filter. A filtration-focused characterization study was carried out in which the main physical and microstructural features involved in a proper behaviour in a particulate filter for automotive applications were measured. Up to five different wood precursors were chosen to make the laboratory bioSiC samples. In the case of natural woods, the anisotropy of the material was also taken into account; in this sense, the two possible cutting directions were considered and analyzed. As a result, measurements of density, porosity, pore size distribution, thermal expansion coefficient, thermal conductivity, compression strength, permeability and intrinsic filtration efficiency were provided for the nine bioSiC specimens. The characterization study was complemented with a deeper study on the relationships between the different functional properties of bioSiC and its microstructure. Crucial parameters in the filtration performance of a DPF substrate such as permeability, thermal conductivity or compressive strength, were correlated with relevant microstructural features. The purpose of this analysis is to extrapolate the measurements obtained in this thesis with the nine selected specimens to any other bioSiC sample, and to foresee the potential of other wood precursors with different microstructure for their use as substrate in DPFs for the same application. After selecting a suitable precursor for the target application, small bioSiC wall-flow DPF prototypes were manufactured. The chosen precursor was MDF. A complete and customized procedure for the manufacturing of these systems is proposed including the mechanization of the channels, the sticking of the sections, and the plugging of the channels. The resulting prototypes were tested at laboratory scale under real operating conditions similar to those produced by an internal combustion engine. Their performance in terms of efficiency and pressured drop was measured through accurate and repeatable tests. Then, the experimental results were scaled up to a real-size MDF-bioSiC wall-flow DPF by means of a renowned and validated numerical model. A possible geometry was proposed for the full scale DPF in terms of length, diameter, cell density and wall-thickness, and it was simulated under real driving operating conditions, but implementing the physical and microstructural measured features of MDF-bioSiC in the definition of the substrate. The results were promising and encouraging. The resulting system widely complies with the current European standards in terms of number and mass of released particles. Furthermore, a comparative study was carried out with other commercial DPFs, and the results show that bioSiC DPFs may be clearly competitive in the aftertreatment systems industry for the automotive sector.