Ceria‐coated diesel particulate filters for continuous regeneration

The potential of diesel particulate filters wash‐coated with highly dispersed nano‐metric ceria particles for continuous regeneration has been investigated. To this end, catalytic filters were prepared, soot‐loaded (avoiding the formation of the cake layer), and regenerated—under isothermal conditions—at temperature ranging from 200–600°C. Results have shown that catalytic oxidation of soot starts from 300°C and, at all temperatures, the selectivity to CO₂ is higher than 99%. 475°C is the minimum temperature at which the filter is regenerated via catalytic path. At this temperature, the catalytic filter maintains substantially the same performance over repeated cycles of soot loading and regeneration, indicating that the thermal stability of ceria is preserved. This has been further confirmed by comparison between the outcomes obtained from characterization (X‐ray powder diffraction, N₂ adsorption at 77 K, Hg intrusion porosimetry, and scanning electron microscope/energy dispersive X‐ray analysis) of fresh filter and filter subjected to repeated regeneration tests. © 2017 American Institute of Chemical Engineers AIChE J, 63: 3442–3449, 2017     

Combined effects of soot load and catalyst activity on the regeneration dynamics of catalytic diesel particulate filters

The combined effects of soot load and catalyst activity on the regeneration dynamics of a catalytic diesel particulate filter have been investigated through transient CFD‐based simulations of soot combustion in a single‐channel configuration. The soot load was changed by varying the amount of soot accumulated as cake layer, while keeping the amount of soot trapped inside the catalytic wall constant. Substantially uniform soot combustion that allows reasonably fast regeneration of the filter under controlled temperature conditions has been simulated only in the absence of cake and at relatively low catalyst activity. Conversely, in the presence of cake, numerical predictions have shown that, regardless of both soot load and catalyst activity, fast regeneration always occurs by propagation of sharp reaction fronts that result in high temperature rises. These findings highlight the importance of avoiding the cake formation, while properly optimizing the catalyst activity, to conduct a safe and effective regeneration of catalytic filters. © 2017 American Institute of Chemical Engineers AIChE J, 64: 1714–1722, 2018     

Out-of-plane mechanical characterization of acicular mullite and aluminum titanate diesel particulate filters

This study characterizes the flexural and compressive behavior of two porous ceramic honeycombs commonly used in diesel particulate filtration, acicular mullite and aluminum titanate. Compression along the axis normal to the honeycomb cross-section, referred to as out-of-plane compression, is compared to in-plane flexure. The relationship between these loading modes is assessed using the failure strength and elastic modulus of the honeycomb structure and the constituent wall material. Weibull analyzes showed that flexure and out-of-plane compression exhibit similar behavior in cases where failure is governed by a single flaw, such as in acicular mullite. However, in heavily microcracked systems like aluminum titanate, compressive failure occurs by damage accumulation rather than growth of a single flaw, so compressive failure strengths are higher than flexural ones. Buckling was also shown to occur in both systems, but the geometries required are unlikely to be encountered in practical application. In the context of filter life assessment, failure in flexure occurs at much lower stresses for systems that rely on microcracking to accommodate thermal strains, so flexure is better suited as an estimate of filter strength.     

Hot zones formation during regeneration of diesel particulate filters

A diesel particulate filter (DPF) is used to remove particulate matter (PM) from the diesel engine exhaust. The accumulated PM is periodically removed by combustion, which sometimes leads to excessive temperature excursions that melt the ceramic filter. This behavior cannot be explained by operation under stationary feed conditions. We propose that these temperature excursions are a dynamic effect following a rapid change in the driving mode while the DPF is being regenerated. Specifically, a rapid decrease in the exhaust temperature can lead to a counterintuitive large transient temperature rise above that which would exist under a higher stationary feed temperature. This unexpected behavior is similar to the well‐known wrong‐way behavior in packed‐bed reactors, even though the axial‐dependent flow through the filter in a DPF is rather different from the constant axial flow through a packed bed. We present simulations that provide insight about the dependence of the amplitude of this wrong‐way temperature rise on the filtration velocity, the PM loading, dimensions of the DPF, and the amplitude of the rapid temperature decrease and when it occurs after the start of the regeneration. The insight provided by these simulations will help develop operation and control protocols that circumvent or at least decrease the probability of the occurrence of the destructive melting of the DPF. © 2010 American Institute of Chemical Engineers AIChE J, 2011     

Adaptation of the chevron‐notch beam fracture toughness method to specimens harvested from diesel particulate filters

The apparent fracture toughness of a porous cordierite ceramic was estimated using a large specimen whose geometry was inspired by the ASTM‐C1421‐standardized chevron‐notch beam. Using the same combination of experiment and analysis used to develop the standardized chevron‐notch test for small, monolithic ceramic bend bars, an apparent fracture toughness of 0.6 and 0.9 MPa√m were estimated for an unaged and aged cordierite diesel particulate filter structure, respectively. The effectiveness and simplicity of this adapted specimen geometry and test method lends itself to the evaluation of (macroscopic) apparent fracture toughness of an entire porous‐ceramic, diesel particulate filter structure.     

Chemical composition of diesel particulate matter and its control

In this review, we have systematically discussed diesel particulate composition and its formation, understanding of which is essential to design the effective catalyst compositions. The most commonly used after treatment strategies such as diesel oxidation catalysts, diesel particulate filters, and partial flow filters are described followed by chronological and category-wise discussions on various groups of reported soot oxidation catalysts. A detailed review is also presented on mechanistic and kinetics aspects of non-catalytic direct particulate matter (PM) or soot oxidation in air/O₂ and NO₂. Recent progress in catalyst development with a focus on the low-cost catalyst for diesel PM oxidation has been given more emphasis considering their renewed importance.     

Ni、Pt对柴油车排气颗粒物去除催化剂的修饰作用

以改性整体式堇青石蜂窝陶瓷为载体,用Ni和Pt修饰Cu-K-V/TiO₂/γ-Al₂O₃ /cordierite催化剂,制备了用于消除柴油车排气颗粒物的催化剂;采用DSC/TG方法确定催化剂的活性及BET、TG-IR等方法对催化剂进行了表征,探讨了Ni和Pt对Cu-K-V/TiO₂/γ-Al₂O₃ /cordierite催化剂的修饰作用及老化对催化剂的影响.研究发现经Ni和Pt修饰后的催化剂能有效降低柴油车排气颗粒物的最大燃烧速率时的温度,特别是对颗粒物中最难燃烧的石墨化部分的去除;而且Ni和Pt的添加能提高颗粒物的去除效率.对含Pt催化剂,老化会降低催化剂的活性;而对含Ni的催化剂,老化使催化剂的活性增加,这可能是Ni向催化剂非孔道的外表面迁移的结果.     

Promotion effect of Au on perovskite catalysts for the regeneration of diesel particulate filters

Four LaBO₃ perovskite catalysts (B = Cr, Mn, Fe and Ni), also supporting 2% by weight of gold, were prepared via the so-called Solution Combustion Synthesis (SCS) method, and characterized by means of XRD, BET, FESEM-EDS, TEM, O₂-TPD and CO-TPR analyses. The performance of these catalysts, in powder form, was evaluated towards the simultaneous oxidation of soot and CO. The 2 wt.% Au–LaNiO₃ showed the best performance with a peak carbon combustion temperature of 431 °C and a half CO conversion of 156 °C. The same nano-structured catalyst, deposited by in situ SCS directly over a SiC filter and tested on real diesel exhaust gases, fully confirmed the encouraging results obtained with catalyst powder.     

Influence analysis of monolith structure on regeneration temperature in the process of microwave regeneration in the diesel particulate filter

Based on models A, B, and C of three kinds of diesel particulate filter (DPF) with microwave regeneration, a DPF microwave regeneration model is established according to the laws of conservation of mass, momentum, and energy. The trends of internal temperature under different velocities of exhaust gas in channels are simulated and analyzed. The results show that: (1) Regeneration temperature in the process of microwave regeneration will begin to increase from the front to the rear end of along the axial direction, and the maximum temperature value will appear in the rear end of the monolith. (2) The internal flow velocity in the DPF of model C is the most uniform and the temperature gradient is the smallest among the three models. Therefore, it is the most useful for DPF regeneration. (3) The minimal thermal stress is exerted on the DPF of model C. Therefore, this model is most useful for prolonging the service life of a DPF.     

Catalytic wall-flow filters for the abatement of diesel particulate: regeneration parameters study

A 1% Pt on CeO₂-promoted PrCrO₃ perovskite catalyst has been synthesized over a wall-flow monolith by the in situ solution combustion synthesis (SCS) method. The role of the catalyst, highly active towards diesel particulate combustion, has been studied during the regeneration phase as a function of three different operating parameters: the inlet trap temperature at which the regeneration is induced, the residual oxygen concentration in the exhaust gases and the load of particulate at the start of the regeneration. The final aim of this study is to improve the knowledge on the catalytic regeneration process in order to derive information suitable for designing an optimized catalytic soot trap entailing minimal fuel penalties.     

Structured glass catalysts for diesel particulate filters

Efforts to create a structured glass catalyst coating for use in diesel particulate filters (DPF) are described. Several methods for producing porous ceramics were investigated as possible routes to increase the active surface area of a K–Ca–Si–O (KCS) glass catalyst coating applied via a sol-gel route. A carbon pitch template yielded a desirable fibrous glass catalyst structure, but the fibers lay predominantly parallel to the substrate surface, minimizing their effectiveness in capturing soot particles and blocking the cordierite pore structure. A mesophase carbon microbead template yielded a structure that was too fragile for practical use. A chemical blowing agent (CBA) based on ethylene cellulose encapsulated KHCO₃ was developed that offered a means to orient glass catalyst fibers away from the surface. However the viscosity of the CBA/fiber loaded sol makes it unsuited for applying into DPF channels. Finally, phase separation by addition of polypropylene glycol (PPG) was successful for creating a microporous K–Ca–Si–O glass catalyst coating on a cordierite filter. A PPG/KCS coated cordierite filter provides 2.8 times higher active surface area than a non-porous KCS coated cordierite filter, and a T₅₀ (the temperature when half of the carbon is oxidized) that is 30°C lower than a non-porous KCS-coated cordierite filter. The excellent performance of the PPG/KCS coated cordierite filter is attributed to its higher catalyzed surface area and more intensive soot-catalyst interaction.     

金属氧化物催化剂在消除柴油车排气颗粒物中的应用

介绍了用于柴油车排气颗粒物后处理的金属氧化物催化剂，包括过渡金属氧化物、主族金属氧化物和碱土金属氧化物。较详细地讨论了催化剂组成及性能、相关催化反应、控制因素和反应机理。     

Novel cordierite-acicular mullite composite for diesel particulate filters

Cordierite-acicular mullite composites containing 0, 25, 50, 75 and 100 wt% of mullite were fabricated from waste MoSi₂ and commercial powders of Al₂O₃ and spinel (MgAl₂O₄). Careful oxidation of pulverised waste MoSi₂ rendered a precursor mixture of MoO₃ and amorphous SiO₂, which served as pore forming agent and SiO₂ source, respectively. Evaporation of MoO₃ at ～750 °C allowed production of highly porous cordierite-mullite ceramic composite after sintering in air at 1350 °C for 4 h. The combination of equiaxed cordierite grains and elongated (prism-like) mullite grains, resulted in unique microstructure with open porosity between 53.3 and 55.6 vol% which makes the obtained composite convenient for application as diesel particulate filter material. The presence of mullite affected four key thermo-mechanical properties which determine the thermal shock resistance of cordierite-mullite composite. The best thermal shock resistance was measured in composite containing 75 wt% of mullite. It was a result of improved thermal conductivity (1.081 W/mK) and bending strength (3.62 MPa) and relatively low values of coefficient of thermal expansion (3.8 × 10^?6 K^?1) and elastic modulus (2.27 GPa).     

Highly porous hybrid particle-fibre ceramic composite materials for use as diesel particulate filters

This paper describes an investigation into the potential of a novel type of ceramic composite for use as Diesel Particulate Filters (DPFs). Materials for this application need to be highly permeable to gas flow, capable of filtering and retaining fine particulate and be stable under demanding thermo-mechanical conditions. Novel materials have been produced, via a conventional blending, cold pressing and sintering route, containing coarse alumina particles and/or fine alumina fibres. Several thermal and mechanical properties are measured, for a range of fibre contents. Thermal Shock Resistance (TSR) has been investigated both experimentally and via evaluation of a TSR merit index, based on a combination of properties. This type of material shows promise in terms of thermo-mechanical stability, particularly with high fibre content. There are also indications that such composites would be suitable for DPF usage in terms of permeability levels and the filtration of fine particulate.     

Development of a semi-analytical model to predict the pressure drop of clean pleated high-efficiency particulate air filters

The present article outlines the development of a semi-analytical model devoted to predict the pressure drop induced by clean pleated high-efficiency particulate air (HEPA) filters. Both experimental measurements and numerical simulations are used to characterize the velocity field in the pleat channel. On this basis, a semi-analytical model is derived to determine the gas flow within the pleat channel. This analytical formulation is used to predict the air pressure evolution according to filtration velocity in the pleat. This model is then validated on the basis of comparisons with measurements found in the scientific literature for different kinds of HEPA filters with different pleat geometries. This model is easy to use, fast to run compared to standard computational fluid dynamics (CFD) approaches, and is in good agreement with the experimental results.     

Experimental Techniques for the Development of the Turbulent Precipitator as a Diesel Particulate Filter

A novel type of diesel particulate filter is introduced: the turbulent precipitator. The aim is to develop a catalytically active filter, based on Cs₂SO₄V₂O₅ molten salt catalyst or cerium fuel-borne catalyst. The novel filter type is developed to circumvent obvious problems like plugging and high pressure drop. In addition to that, it should be flexible, robust and possible to tune for different diesel engines. Its main features are an open flow channel (to prevent plugging and high pressure drops) and soot collection plates (to trap diesel soot). Two filter geometries are described, one with metal collector plates and one with ceramic foam collector plates. Results show that different geometries have different capabilities, making tuning for different diesel engines possible. An engine test bench was designed to measure filter efficiencies, both by particle numbers and particle mass. The diesel soot aerosol is measured with an electrical low-pressure impactor (ELPI). These measurements are not straightforward. For evaluation purposes, the engine test bench was divided into three major components to test it for aerosol measurements: diesel setup, aerosol sampling setup, and ELPI. Each part is restricted by a maximum time on stream.     

Catalytic oxidation for carbon-black simulating diesel particulate matter over promoted Pt/Al2O3 catalysts

Catalytic oxidation activity of carbon-black (CB) simulating the soot of diesel particulate matters to CO₂ over 3Pt/Al₂O₃, 3Pt5Mn/Al₂O₃ and 3Pt/30Ba–Al₂O₃ catalysts is investigated with model gases of diesel emission. In case of the large amount of CB compared to the amount of catalyst (3/1, w/w) in the mixture sample, insufficient oxygen at the point of sudden increase in the amount of CO₂ is leaded to the partial oxidation using the lattice oxygen of the catalyst. And the peaks of CO₂ after the first peak were attributed to the regional combustion of the CB, which was not in contact with catalyst particles. The fresh 3Pt5Mn was estimated to the oxidation states on the catalyst surface by XPS. For used sample at 700 °C, the BEs of Pt 4d5 was revealed to metallic state Pt(0) (314.4 eV) in a predominant levels compared with Pt(II) (317.3 eV). While BEs of Mn 2p were similar to that obtained from the fresh 3Pt5Mn. It is suggested that Pt is in charge of the roles in CB-oxidation, using the lattice oxygen of the catalyst. Two-stage catalytic system with the strategies of promoting the soot oxidation and NO_x reduction, simultaneously, were composed of the CB oxidation catalyst and the diesel oxidation catalyst. The catalytic oxidation of CB was accelerated by activated oxidants and exothermic reaction resulted from the diesel oxidation catalyst, which lies in upstream of two-stage. But the system with the CB oxidation catalyst sited in the upstream showed the initiation of CB oxidation at a lower temperature than the other case. Two-stage catalytic system composed of 3Pt5Mn with CB in the upstream and DOC in the downstream showed high oxidation activity with 95% consumption rate of CB to the total loaded CB in the range of 100–500 °C during the TPR process.     

纤维材料过滤理论的研究进展

以单纤维模型为基础,介绍了纤维过滤理论的研究方法,阐述了单纤维过滤机理,综述了国内外纤维过滤理论的发展及其最新进展,给出了过滤材料的效率与压降计算公式,分析了理论研究过程中存在的问题,并对纤维理论的发展趋势做了初步预测。