Ceramic Diesel Particulate Filters

Twenty-five years of diesel particulate filter (DPF) developments have shown that ceramic materials are well-suited candidates to fulfill the harsh requirements of exhaust after treatment. The introduction of DPF in passenger cars in Europe in 2000 was a real breakthrough from both a scientific and a commercial point of view. Different systems and filter materials can be used as DPF; however, at the moment silicon carbide wall flow filters seem to be at advantage. There is a continual demand for cost-effective and reliable materials and systems forced by increasing legal emission standards.     

Comparison of Different Diesel Particulate Filters

The present investigation concerns the development of a catalytic wall-flow trap for the simultaneous removal of particulate, CO and marginally for NO _x . Three different geometries (200/17, 300/12, and 300/12-HAC) of commercial SiC filters were catalyzed with a nanostructured perovskite catalyst (La–K–Cu–FeO₃ + 2%Au) via the in situ Solution Combustion Synthesis method, and tested on real diesel exhaust gases on a commercial vehicle engine bench. The pros and cons of all the filters were evaluated on the ground of numerous loading and regeneration cycles. In all filter geometries tested, the La–K–Cu–FeO₃ + 2%Au catalyst entails a much more complete regeneration and lower fuel penalties compared to those of a non catalytic trap.     

Filtration Efficiency and Pressure Drop of Miniature Diesel Particulate Filters

A method was developed to evaluate miniature diesel particulate filters (DPFs). To validate the performance of the instrumentation and test apparatus, measurements were made using silicon carbide (SiC) and cordierite miniature filters with representative microstructures. Filtration efficiency (FE), the most penetrating particle size (MPPS), and pressure drop were measured for catalyzed and uncatalyzed advanced ceramic material (ACM) acicular mullite and representative commercial filters to determine the impact of substrate morphology, the formation of a soot cake, and the presence of a catalyst coating on filtration properties. FE measurements demonstrated that filter geometry and microstructure significantly influence initial filtration performance. ACM filters had high initial FE and the MPPS near ～200 nm. Reduction of the ACM pore size in the absence of a reduction in porosity increased initial FE even more, but its influence on MPPS was not resolvable. The presence of a catalyst and washcoat on the ACM increased the pressure drop but increased initial FE and reduced MPPS to <100 nm. The addition of a washcoat allowed the rapid buildup of a soot cake, which resulted in a more rapid rate of increase in FE compared to uncatalyzed ACM. The similarity in the ACM and cordierite soot cakes after a long loading time is consistent with theory that suggests the formation of the soot cake depends primarily on the Péclet (Pe) number, which is influenced only by macroscopic filter geometry and prevailing test conditions.

Copyright 2013 American Association for Aerosol Research     

In-Operation Monitoring of the Soot Load of Diesel Particulate Filters: Initial Tests

Diesel particulate filters (DPF) are indispensable parts of modern automotive exhaust gas aftertreatment systems due to the stringent emissions legislation. For a fuel-efficient control strategy, it would be beneficial to determine directly and in-operation their actual trapped soot mass. Two novel approaches—based on the electrical conductivity of trapped soot particles—emerged recently. By measuring the electrical resistance between different single walls inside the filter, the soot load is determined with local resolution. The microwave-based technique is a contactless approach that gives an integral value depending on the soot mass in the DPF. We present investigations on loading and regeneration of DPFs in a dynamometer test bench applying both methods. The results are compared with each other and correlated with the differential pressure and the soot mass. Especially the microwave-based technique has a potential for serial application.     

Development of a Surface Area Dependent Rate Expression for Soot Oxidation in Diesel Particulate Filters

We collected soot from diesel engine exhaust on miniature particulate filter samples and evaluated soot oxidation rates on an automated flow reactor system. A series of isothermal pulsed oxidation experiments quantified reaction rates as a function of gas composition, temperature, flow rate, and soot consumption. An O₂ chemisorption method measured the soot active surface area as filter regeneration progressed. We developed a rate law with an explicit dependence on carbon surface area and estimated the associated kinetic parameters from the pulsed oxidation data. The resulting rate expression successfully captures the soot oxidation behavior over a wide range of operating conditions.     

Analysis of Thermal Processes in Catalytic Particulate Filters

Mathematical simulation of accumulation of soot particles and oxidative regeneration of a particulate filter based on catalytically active materials is performed. The influence of parameters of the model considered on dynamics of these processes is studied.     

Mechanical Characterization of Diesel Particulate Filter Substrates

Test procedures for the mechanical evaluation of diesel particulate filter (DPF) substrate materials have been developed and applied for the characterization of porous cordierite under ambient conditions. Specifically the double-torsion test method was employed to characterize fracture toughness and slow crack behavior while resonant ultrasound spectroscopy (RUS) was used to determine the elastic properties of the substrate walls. A dry grinding procedure was developed to fabricate test specimens for these tests. The fracture behavior of porous cordierite was related to the pore structure inside the filter wall. Implications of the test results on the mechanical reliability of DPFs are discussed.     

High‐Temperature Flexural Strength and Thermal Stability of Near Zero Expanding doped Aluminum Titanate Ceramics for Diesel Particulate Filters Applications

A substantial increase in sinterability, high‐temperature flexural strength, thermal stability in combination with an average thermal expansion of 0.42 × 10^?6/^°C (30–1000°C) is achieved through magnesium silicate (Mg₃Si₄O₁₀(OH)₂) doping of Aluminum Titanate (Al₂TiO₅) ceramics. Doped specimens exhibited the sintered density of 99% of theoretical density at 1550°C and a maximum enhancement of 169.23% (70 MPa) in flexural strength at 1200°C as compared with 26 MPa measured at 30°C. Enhancement of flexural strength at elevated temperature can be attributed to the increasing extent of thermally activated crack blunting with increasing temperature, which is further evident from the dilatometric hysteresis curve recorded for these samples. XRD investigations of undoped (Al₂TiO₅, AT) samples annealed at 1100°C for 5 and 10 h have shown clear evidence of decomposition to precursor oxides by 7% and 21.13%, respectively. However, the samples of magnesium silicate–doped Al₂TiO₅ (TAT) under identical conditions have shown no sign of decomposition, indicating significantly high thermal stability. TAT formulations were also extrusion processed to investigate the suitability of forming cellular honeycomb structures. TAT formulation with superior thermo‐mechanical properties and excellent adaptability for extrusion processing can be explored for the development of next generation diesel particulate filters (DPF).     

Performance Evaluation of Electrostatically Augmented Air Filters Coupled with a Corona Precharger

An experimental study of electrostatically augmented air (EAA) filters coupled with a corona precharger has been conducted using Arizona road dusts and tobacco smoke. The measurements of filter efficiency and pressure drop across the EAA filter have been made using an ASHRAE 52.1-1992 filter test system and an opacity meter to measure the particle concentration upstream and downstream of the test filter. The two-stage EAA filter unit consists of the positive corona precharger upstream of a filter, to precharge particles with the electrical strength of 4.7 kV/cm, and an electrified filter collector, which has folded media with meshy metal separators, in the upstream and downstream side gaps. DC voltage of +1,000 V (1.4 kV/cm) is applied between the upstream and downstream separators to produce an electric field between the separators and media as well as across the media in a polarity so that most of the precharged particulates are collected on the upstream filter collector. A conventional filter was measured and had 70.0% efficiency with dusts of 1.96 w m in mass median diameter and 2.5 m/s face velocity, while the EAA filter had 92.9% efficiency. An electrical effect on the EAA filter was evaluated to both improve the filter efficiency and reduce the pressure drop across the filter. Also, the performance evaluation of the EAA filter using an air handling chamber system in occupied space was investigated with tobacco smoke particles.     

Study of Catalytic Filters for Soot Particulate Removal from Exhaust Gases

Two ceramic supports (sintered and foam) were employed for the preparation of catalytic filters for soot removal at diesel exhausts. Laboratory tests showed that while the foam filter is appropriate for small size and low engine backpressure, the sintered filter is more suitable for achieving high filtration efficiency. Tests carried out at the exhaust of a diesel engine showed that the catalytic filter can be continuously regenerated at operating conditions typical of diesel exhaust.     

Construction of Integrated Software for Drying Calculations

Problems associated with construction of a computer program that can serve as a workhorse for drying calculations and provide a basis for Future expansion to full CAPD dryer simulator are described. The program at present is a set of routines based on common data libraries and functions Fully integrated to save computer code volume and provide consistency of all calculations.     

Development of a Novel Design for Diesel Particulate Filter

A novel design for diesel particulate emission filter based on wall structure of pore channels of honeycomb filter is proposed. Our anticipation in this new design is that, if whiskers are coated on walls of the pore channels of honeycomb filter, the trapping efficiency of honeycomb filter will be improved and also the pressure drop will be substantially reduced. In present study, cordierite whiskers were coated on cordierite based honeycomb support (diameter 13 cm, and height 14.5 cm, 600 cells/inch²) by in-situ method. Whisker coating was carried out on walls of the pore channels of cordierite support from aqueous slurry containing a mixture of kaolin, talc, alumina and silica. Coating was performed by wash coat method. Firing was carried out at 1350C for 4 h under flowing air atmosphere. XRD measurements revealed us complete formation of cordierite phase. SEM analysis on the coating showed the formation of three-dimensional network of cordierite whiskers on cordierite support. In order to evaluate the particulates trapping efficiency of the newly developed honeycomb filter, smoke test was carried out with a 2L diesel engine for the both un-coated and coated cordierite honeycomb samples and the results were discussed with respect to their microstructures.     

Development of Catalysts for Diesel Particulate NO x Reduction

While diesel vehicles feature high fuel economy with low CO₂ emissions, further suppression of particulate matter (PM) and NO _x in the exhaust stream is demanded worldwide. We have been working to develop a new diesel particulate-NO _x reduction (DPNR) system to decrease both PM and NO _x emissions by combining the NO _x storage-reduction catalyst for direct injection gasoline engines with the most advanced engine control technologies. This paper describes the development of the DPNR system, a post-treatment technology for PM and NO _x , which was achieved through a combination of catalysis and engine control technologies.     

Effect of Biofuels on Catalyzed Diesel Particulate Filter Regeneration

Under the terms of the Renewable Energy Directive, EU member states are required to use 10 % of transport energy sourced from renewable sources, mainly biofuels, by 2020. The purpose is to reduce greenhouse gas (GHG) emissions from the transport sector. However, biodiesel used as fuel has a significant impact on emissions, as related by most of the literature on the subject. In particular, nitric oxides (NOx) and particulate matter (PM) emissions from current diesel technologies are critical factors because they are already close to the limits permitted by regulations and both limits will be even more stringent in the near future. Soot particles are trapped on a diesel particulate filter (DPF). If the DPF is catalyzed like in this study, the soot is then burned by reaction with NO₂ (CDPF continuous regeneration) which occurs at lower temperatures than reaction with O₂ (active regeneration). Tests of ultra-low sulfur diesel blended with rapeseed-biodiesel at 30 % (B30) and Fischer–Tropsch diesel (FT30) were conducted. The Fischer–Tropsch diesel was chosen to represent a biomass-to-liquid fuel. This work investigated the impact of these two biofuels on engine polluting emissions and the resulting CDPF ability to regenerate. When compared with similar inlet conditions on a synthetic gas bench, an impact of fuel was observed on soot reactivity: the CDPF loaded with FT30 soot regenerated slightly faster. Engine bench tests were also performed to combine the effects of fuel on engine emissions and soot reactivity and to evaluate the CDPF. The increase in NOx and decrease in PM emissions observed for B30 appeared to significantly improve CDPF continuous regeneration by NO₂.     

Low-Velocity Pneumatic Conveying Calculations for a High Concentration of the Particulate Material

The assumption that a moving solid–gas mixture is quasi- homogeneous and is characterized by some pseudoviscosity allows one to use conventional hydrodynamic methods and to generalize numerous experimental data on the low-velocity pneumatic conveying of various particulate materials. Formulas are derived for calculating the friction pressure drop criterion and the local flow-resistance coefficient in bent pipes. These formulas take into account the relative velocity, concentration, and properties of the solids. Basic principles are developed for the design of stepped-diameter pipelines of complicated configuration for long-distance pneumatic conveying.     

Cs–V Catalysts for the Combustion of Diesel Particulate

The soot combustion process, promoted by some promising diesel particulate combustion catalysts based on Cs and V oxides (Cs₂O, Cs₃VO₄, Cs₄V₂O₇, Cs₂O · V₂O₅, CsVO₃, Cs₂V₄O₁₁, V₂O₅), is discussed on the grounds of an experimental test campaign involving reaction runs in micro-reactors (temperature-programmed conditions) and in a differential scanning calorimeter apparatus. Such investigations led to define the roles of the Cs and V species in these catalysts as well as to identify the Cs₃VO₄ catalyst as the most active.     

Influences on the Particle Size Distribution of Diesel Particulate Emissions

Particulates give great concern for mankind health. Especially the nano size particles are under discussion. Therefore, the particle size distribution from the combustion chamber to tail pipe emissions are of great interest. With the aim of scanning mobility particle sizer the number weighted particle size distributions were measured in the combustion chamber as well as in the exhaust gas up and downstream of aftertreatment systems. Using the identical particle measurement technique results can be compared without changing the particle size definition. The particles in the cylinder of a modern serious DI diesel engine were sampled with a time resolved fast gas sampling valve. The Soot particles formed in the cylinder during the early combustion phase are oxidized by about 99% in the late combustion/early expansion phase, whereas the soot particle sizes distribution in the cylinder at the end of the expansion phase are equal to that in the tail pipe. DI diesel engines with high pressure injection system emit less numbers of particle with in tendency greater sizes compared to IDI diesel engines. Oxidation catalysts do not influence particle size distribution but particulate traps reduce particle number by up to two orders of magnitude. Detail analysis shows that an increase of nano size particle number downstream of an aftertreatment device results from artifacts.     

New Tool for Experimental Analysis of Diesel Particulate Filter Loading

The loading of a diesel particulate filters (DPFs) entails the need of trap regeneration by particulate combustion, whose efficiency and frequency are somehow affected by the way soot is deposited along the channels. Great efforts are thus spent to improve the understanding of the filtration process of DPFs, aimed at obtaining a deeper insight into the relationship between engine performance and filter loading so as to take advantage of this insight for DPF design and optimization purposes. Small lab-scale 300 cpsi DPF samples were loaded downstream the Diesel oxidation catalyst (DOC) in an ad hoc designed reactor capable of hosting five samples with part of the entire flow produced by an automotive diesel engine at the 2500 × 8 BMEP operating condition, selected to be representative as one of the critical engine points of the New European Driving Cycle (NEDC). Soot layer thickness was estimated by means of Field emission scanning electron microscope (FESEM) observations after sample sectioning at progressive locations, obtained through a procedure defined not to affect the distribution of the soot inside the filter and to enable estimation of the actual soot thickness along the channel length. This is a pre-requisite to get suitable data for the validation of the DPF models required for trap design and optimisation.     

Sensitivity of Diesel Particulate Material Emissions and Composition to Blends of Petroleum Diesel and Biodiesel Fuel

A number of investigations have examined the impact of the use of biodiesel on the emissions of carbon dioxide and regulated emissions, but limited information exists on the chemical composition of particulate matter from diesel engines burning biodiesel blends. This study examines the composition of diesel particulate matter (DPM) emissions from a commercial agriculture tractor burning a range of biodiesel blends operating under a load that is controlled by a power take off (PTO) dynamometer. Ultra-low sulfur diesel (ULSD) fuel was blended with soybean and beef tallow based biodiesel to examine fuels containing 0% (B0), 25% (B25), 50% (B50), 75% (B75), and 100% (B100) biodiesel. Samples were then collected using a dilution source sampler to simulate atmospheric dilution. Diluted and aged exhaust was analyzed for particle mass and size distribution, PM_2.5 particle mass, PM_2.5 organic and elemental carbon, and speciated organic compounds. PM_2.5 mass emissions rates for the B25, B50, and B75 soybean oil biodiesel mixtures had 20%–30% lower emissions than the petroleum diesel, but B100 emissions were about 40% higher than the petroleum diesel. The trends in mass emission rates with the increasing biodiesel content can be explained by a significant decrease in elemental carbon (EC) emissions across all blending ranges and increasing organic carbon (OC) emissions with pure biodiesel. Beef tallow biodiesel blends showed similar trends. Nevertheless, it is important to note that the study measurements are based on low dilution rates and the OC emissions changes may be affected by ambient temperature and different dilution conditions spanning micro-environments and atmospheric conditions. The results show that the use of biodiesel fuel for economic or climate change mitigation purposes can lead to reductions in PM emissions and a co-benefit of EC emission reductions. Detailed speciation of the OC emissions were also examined and are presented to understand the sensitivity of OC emissions with respect to biodiesel fuel blends.

Copyright 2012 American Association for Aerosol Research