Biomorphic Silicon Carbide as porous substrate for automotive Diesel Particulate Filters.

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

Numerical Simulation of a Wall-Flow Particulate Filter Made of Biomorphic Silicon Carbide Able to Fit Different Fuel/Biofuel Inputs

To meet the increasingly strict emission limits imposed by regulations, internal combustion engines for transport applications require the urgent development of novel emission abatement systems. The introduction of biodiesel or other biofuels in the engine operation is considered to reduce greenhouse gas emissions. However, these alternative fuels can affect the performance of the post-combustion systems due to the variability they introduce in the exhaust particle distribution and their particular physical properties. Bioceramic materials made from vegetal waste are characterized by having an orthotropic hierarchical microstructure, which can be tailored in some way to optimize the filtration mechanisms as a function of the particle distribution of the combustion gases. Consequently, they can be good candidates to cope with the variability that new biofuel blends introduce in the engine operation. The objective of this work is to predict the filtration performance of a wall-flow particulate filter (DPF) made of biomorphic silicon carbide (bioSiC) with a systematic procedure that allows to eventually fit different fuel inputs. For this purpose; a well-validated DPF model available as commercial software has been chosen and adapted to the specific microstructural features of bioSiC. Fitting the specific filtration and permeability parameters of this biomaterial into the model; the filtration efficiency and pressure drop of the filter are predicted with sufficient accuracy during the loading test. The results obtained through this study show the potential of this novel DPF substrate; the material/microstructural design of which can be adapted through the selection of an optimum precursor.Ministerio de Economía y Competitividad de España (MINECO) MAT2013-41233-RMinisterio de Economía y Competitividad de España (MINECO) BES-2014-069023Ministerio de Economía y Competitividad de España (MINECO) EEBB-I-17-1233

Preliminary study on the performance of biomorphic silicon carbide as substrate for diesel particulate filters

This paper presents the results of a preliminary experimental study to assess the performance of biomorphic silicon carbide when used for the abatement of soot particles in the exhaust of Diesel engines. Given its optimal thermal and mechanical properties, silicon carbide is one of the most popular substrates in commercial diesel particulate filters. Biomorphic silicon carbide is known for having, besides, a hierarchical porous microstructure and the possibility of tailoring that microstructure through the selection of a suitable wood precursor. An experimental rig was designed and built to be integrated within an engine test bench that allowed to characterizing small lab-scale biomorphic silicon carbide filter samples. A particle counter was used to measure the particles distribution before and after the samples, while a differential pressure sensor was used to measure their pressure drop during the soot loading process. The experimental campaign yielded promising results: for the flow rate conditions that the measuring devices imposed (1 litre per minute; space velocity = 42,000 L/h), the samples showed initial efficiencies above 80%, pressure drops below 20 mbar, and a low increase in the pressure drop with the soot load which allows to reach almost 100% efficiency with an increase in pressure drop lower than 15%, when the soot load is still less than 0.01 g/L. It shows the potential of this material and the interest for advancing in more complex diesel particle filter designs based on the results of this workMinisterio de Economía y Competitividad (España) MAT2013-41233-R DPI2013-46485-C3-3-RFondos FEDER MAT2013-41233-R DPI2013-46485-C3-3-RUniversidad de Sevilla VI Plan Propio I.3B - C.I. 24/05/2017 MAT2016-76526-

Permeability and mechanical integrity of porous biomorphic SiC ceramics for application as hot-gas filters

Biomorphic SiC is a biotemplated material fabricated by Si melt-infiltration of carbon preforms from wood pyrolysis. In this work, porous bioSiC ceramics from five different wood precursors, with porosities between 45 and 72% were studied for their feasibility in filtering applications.Gas permeability and mechanical stability were investigated as a function of the microstructure of the starting wood precursor. Air-permeation performance at room temperature was measured for a range of flow rates, and the permeability constants were assessed by fitting of Forchheimer's equation to the experimental data. Darcian permeabilities were achieved in the range 10-10 m, while inertial terms were in the range 10-10 m, showing a correlation with the average pore size and orientation of the larger channels. Regarding the mechanical stability, maximum compressive strength values were reached in the range of 3-115 MPa.These results improve our understanding of the ways in which the microstructure influences permeability and mechanical robustness, enabling the device requirements to be tailored by selecting the wood precursor. It was also shown that these materials are promising for hot-gas filtering applications.Ministerio de Economía y Competitividad MAT2013-41233-