Bench Scale Experiments of Diesel Soot Oxidation Using Pr0.7Sr0.2K0.1MnO3 Perovskite Type Catalyst Coated on Ceramic Foam Filters

Potassium and strontium substituted praseodymium manganate type perovskite catalyst coated on ceramic foam filters have been studied for diesel particulate removal. The synthesized catalyst coated filter pieces have been characterized by using XRD, SEM and TG analysis, whereas their catalytic activity towards soot oxidation was tested using a bench scale facility with real diesel engine exhaust. The catalyst coated filters decrease the soot oxidation T_initial value by 150 °C and T_final by 100 °C as compared to bare soot oxidation reaction, which can be considered as high activity under the actual conditions of diesel engine. The catalytic materials show good thermal stability, while their low cost will also add to their potential for practical applications. Although perovskites have been studied for laboratory evaluations of catalytic soot oxidation, present results further substantiate the possibility of using low-cost, supported, non-noble metal based catalysts for diesel exhaust emission control applications, especially for the cost-effective retrofitment of in-use vehicles with old generation engines.     

Science and technology of catalytic diesel particulate filters

During the last few decades, concerns have grown on the negative effects that diesel particulate matter has on health. Because of this, particulate emissions were subjected to restrictions and various emission-reduction technologies were developed. It is ironic that some of these technologies led to reductions in the legislated total particulate mass while neglecting the number of particles. Focusing on the mass is not necessarily correct, because it might well be that not the mass but the number of particles and the characteristics of them (size, composition) have a higher impact on health. To eliminate the threat of diesel particulate matter, essentially absolute filtration in combination with the oxidation of all emitted hydrocarbons is what will be required.

After two decades of development, the first filters will soon be introduced on a large scale. Many different problems had to be overcome; it was especially important that the filter was robust and its regeneration was controllable. The key technology to controllable regeneration is oxidation catalysis, which is the main area of focus in this review. Catalytic filter regeneration is very complex, something which is apparent in the main aspects of catalysis (i.e., activity, stability, and selectivity). Complications are that the process conditions can be very transient and that the temperatures are usually low. It is shown that the oxidation catalyst cannot be examined isolated from the total system. Within the margins of size restrictions and an engine's service life, essentially all particulate matter should be trapped, the filter should be regenerated safely, no toxic by-products should be formed, and the catalyst should not alter the filtration characteristics, and vice versa.

The exhaust conditions of passenger cars are not favorable for continuous regeneration strategies, because the best strategy seems to be periodic regeneration with the aid of a catalyst. This concept is not passive, which makes it complex and expensive. The best technology for filter regeneration with trucks and buses seems to be continuous regeneration. Using the NO_x present in the exhaust gas for soot oxidation amounts to a simple and robust concept. A future limitation might be the minimal required NO_x:soot ratio; it is not sure if this will be met in future engines. Alternatively, a low-temperature catalyst may be developed that does not require NO_x. Developing such an advanced catalytic trap will be one of the major challenges of catalytic filter engineering.     

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.     

Diesel particulate emission control

This paper reviews the emission control of particulates from diesel exhaust gases. The efficiency and exhaust emissions of diesel engines will be compared with those of otto engines (petrol engines). The formation of particulates (or “soot”), one of the main nuisances of diesel exhaust gases, will be briefly outlined. The effects of various emission components on human health and the environment will be described, and subsequently the emission standards for particulates and for NO_x, which have been introduced worldwide, will be summarized. Possible measures for reducing exhaust emissions of particulates and NO_x will be discussed, such as the use of alternative fuels, modifications to the engine and the use of aftertreatment devices. It will be made clear that aftertreatment devices may become necessary as diesel emission standards become more stringent, in spite of important progress in the other fields of reducing exhaust emissions. Selective catalytic reduction via hydrocarbons, ammonia or urea, a possible aftertreatment method for NO_x emission control, will be discussed briefly. Filters for collecting particulates from diesel exhaust gases will be examined in more detail and aftertreatment control systems for particulate removal will be reviewed. These can be divided into (i) non-catalytic filter based systems which use burners and electric heaters to burn the soot once it has been collected on the filter; (ii) catalytic filter-based systems which consist of filters with a catalyst coating, or filters used in combination with catalytically active precursor compounds added to the diesel fuel; and (iii) catalytic non-filter-based systems in which gaseous hydrocarbons, carbon monoxide and part of the hydrocarbon fraction of the particulates are oxidized in the exhaust gases. Finally, recent trends in diesel particulate emission control will be discussed, indicating the growing importance of the catalytic solutions: the fast introduction of non-filter-based catalysts for diesel engines and the possible application of filters in combination with catalytically active precursor compounds added to diesel fuel.     

The influence of a 2-component model on the computed regeneration behaviour of an uncoated diesel particulate filter

The present work deals with the modelling of diesel particulate filters. Model extensions as well as further development regarding soot monitoring during regeneration are investigated. A slip flow model, leading to an improvement of computed pressure drop profiles, a shape correction term, considering an increasing perimeter which changes the wall flow through the particle layer, and a 2-component model, which distinguishes between soot as cake upon and soot within the filter wall, have been implemented and the effects on previous results discussed. However, the main focus of the work is laid upon a sensitivity analysis regarding soot combustion within the pore combined with a discussion upon judging the accuracy of computed results. In order to compare experimental and computed results an uncoated SiC filter with 200 CPSI and 15 mil wall thickness was used. All experiments were conducted under real-world conditions on an engine test bench, which includes filter loading and regeneration. It can be shown, that the 2-component model effects positively the simulation of filter loading, as the filtration can be divided into deep bed and cake filtration. Due to the sensitivity analysis on kinetic parameters different effects on pressure drop and soot loading profiles during regeneration have been investigated. A faster soot combustion within the filter wall shows only a slight effect on the total soot loading curve. However, there is a strong influence on the pressure drop profile, which lowers the deviation between experiment and simulation.     

Distribution characteristics of exhaust gases and soot particles in a wall-flow ceramics filter

An inhomogeneous soot distribution in a diesel particulate filter may deteriorate its behavior and result in higher pressure drops and fuel consumption. This will cause mechanical stresses on the filter due to temperature gradients resulting from the non-uniformly burning of soot during regeneration. The purpose of this paper is to investigate the flow distribution of the exhaust gas entering into a diesel particulate filter, the turbulent motion of diesel soot particles in the inlet header, and their deposition and distribution in the front surface of a diesel particulate filter. A Lagranian continuous random walk (CRW) model is developed to simulate soot particulate motion, which considers a succession of uncorrelated random forcing and drift corrections. The effects of particle inertia, turbulent fluctuation, and lift on the particle motion and trajectory are analyzed. Correlations of the uniformity index of the exhaust gas and soot particles with the flow rate, soot loading, and inlet expansion angle are evaluated. The results show that there is a two-peak phenomenon in the soot distribution at the front entrance of the filter, which is comprised of a peak in the central area due to inertia and a second peak in the periphery owing to diffusion and recirculation action. Exhaust flow rates and the inlet expansion angle have a major influence on the flow uniformity and soot uniformity, while soot loading has a slightly smaller effect on soot uniformity.     

Reduction of diesel particulate emissions by catalytic filtration

Several kinds of diesel soot filters and filter materials of high melting points with eleven different catalytic coatings were prepared. They were calcined at a maximum temperature of 1000°C. The reaction rate and the kinetic parameters of the combustion of diesel soot were determined in a closed loop laboratory reactor. A packed bed of alumina particulates was the most active filter in soot combustion. Catalytic coatings with oxides of vanadium, copper or cerium accelerated the reaction. In each case the effect of the coating was dependent on specific interactions between the catalyst and the filter material. The manner of filtration and the nature of the soot filter are found to be more important for the overall result than the nature of the catalytic coating.     

Kinetics of catalyzed and non-catalyzed oxidation of soot from a diesel engine

To comply with the new regulations on particulate matter, car manufacturers more and more commonly use diesel particulate filters (DPF). The working of these systems needs to periodically burn soot that has been accumulated during the loading of the DPF. This paper describes the kinetics of the non-catalytic and catalytic oxidation of real diesel soot with oxygen. From these experiments, mechanisms for catalyzed and non-catalyzed soot oxidation have been proposed.     

Innovative means for the catalytic regeneration of particulate traps for diesel exhaust cleaning

Catalytic traps for diesel particulate removal are multifunctional reactors coupling filtration and catalytic combustion of soot. This paper reviews the most recent developments carried out at Politecnico di Torino concerning two different trap types: zirconia-toughened-alumina foams catalysed with Cs–V catalysts, operating according to a deep filtration mechanism, and cordierite or SiC wall-flow filters catalysed with perovskite catalysts (e.g. LaCr_0.9O₃), enabling shallow-bed filtration. The preparation and characterisation of these two trap types are described and the performance of the traps (filtration efficiency, pressure drops, etc.) evaluated on a diesel engine bench under various operating conditions. A final critical assessment points out that most chances of practical application in mobile sources lie in wall-flow type traps for their superior filtration efficiency (>95%) and their compatibility with active trap regeneration means (e.g. fuel post-injection) that can occasionally rise on purpose the exhaust gas temperature to accelerate the catalytic combustion of trapped soot. Conversely, completely passive solutions based on deep filtration catalytic traps show only promise for stationary applications at temperatures higher than 350°C, due to insufficient catalyst activity at lower temperatures.     

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.     

DIESEL SOOT COMBUSTION WITH PEROVSKITE CATALYSTS

Diesel soot emissions from stationary or mobile sources can be reduced through physical trapping in particulate filters until periodical in situ combustion takes place. This study focuses on the development of several perovskites for the catalytic combustion of diesel particulates in multifunctional catalytic reactors. Several perovskites, with BET surface areas of 20–30 m²/g, were prepared by the solution combustion synthesis method and were characterized by XRD, SEM, TEM, and TPD techniques. Catalytic activity tests have shown that the most promising catalysts, namely, perovskite catalysts with Cr in the B site and Tb or Pr in the A site, can ignite soot combustion well below 400°C, i.e., at a temperature 200°–250°C lower than that of noncatalytic diesel soot combustion. The best catalytic formulation was deposited on a full-scale wall-flow filter and tested against the soot emissions of a diesel engine, resulting in reduced regeneration time and substantial fuel consumption saving compared to the corresponding bare filter performance.     

Study of the PM Gas-Phase Filter Artifact Using a Setup for Mixing Diesel-Like Soot and Hydrocarbons

The filter artifact is a significant source of error in gravimetric measurements of particulate matter (PM) exhaust. However, only a few studies on the subject exist. Results from these studies show a large discrepancy mainly because the experiments were performed using real diesel vehicle exhaust with varying exhaust composition. In this study, a setup for mixing diesel-like soot and hydrocarbon vapor was constructed for generating a stable exhaust aerosol with adjustable composition. The particle size distribution of the diesel-fueled soot generator (GMD [geometric mean diameter] adjustable between 27 and 164 nm) was found to represent “real” exhaust particulate emission. This setup was applied for studying the filter artifact on Teflon-coated glass fiber filters using pentadecane as the hydrocarbon vapor. Experiments were performed using particle and hydrocarbon concentrations of 130–700 μg/m³ and 10–12 ppm, respectively. It was found that the particle concentration of the aerosol affects the filter artifact. At lower particle concentrations, more hydrocarbon adsorption was detected. In the absence of particles, the adsorption was highest. Furthermore, filter soot load, corresponding to 0.13%–0.66% of the clean filter mass, was found to affect adsorption. Sooty filters adsorbed less vapor than clean filters. However, increasing the soot load resulted in more adsorption. Moreover, it was found that the backup filter serves as a reasonable estimate of the filter artifact only for low particle concentrations and filter soot loads. These results indicate that the filter soot load is an important parameter influencing the filter artifact, and therefore, it should be considered when performing gravimetric sampling. The setup was proven to be a unique tool for quantitative studies of the filter artifact.

Copyright 2012 American Association for Aerosol Research     

Numerical simulation of soot filtration and combustion within diesel particulate filters

The numerous benefits offered by diesel engines, compared to gasoline ones, are balanced by a drawback of increasing concern, namely soot emissions. Nowadays, soot emissions can be reduced by physically trapping the particles within on-board diesel particulate filters (DPF). The filter gets progressively loaded by filtering the soot laden flue gases, thus causing an increasing pressure drop, until regeneration takes place. The aim of this work is to develop a fully predictive three-dimensional mathematical model able to accurately describe the soot deposition process into the filter, the consequent gradual modification of the properties of the filter itself (i.e. permeability and porosity), the formation of a soot filtration cake, and the final regeneration step. The commercial computational fluid dynamics (CFD) code Fluent 6.2.16, based on a finite-volume numerical scheme, is used to simulate the gas and particulate flow fields in the DPF, whereas particle filtration sub-models and regeneration kinetics are implemented through user-defined-subroutines (UDS).Model predictions highlight uneven soot deposition profiles in the first steps of the filtration process; however, the very high resistance to the gas flow of the readily formed cake layer determines the evolution into an almost constant layer of soot particles. The ignition of the loaded soot was simulated under different operating conditions, and two regeneration strategies were investigated: a “mild regeneration” at low temperature and oxygen concentration, that operated a spatially homogeneous ignition of the deposited soot, and a “fast regeneration”, with an uneven soot combustion along the axial coordinate of the filter, due to strong temperature gradients inside the filter itself. These findings are supported by comparison and validation with experimental data.     

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.     

Molten Salts Are Promising Catalysts. How to Apply in Practice?

A diesel soot filter with a Cs₂SO₄V₂O₅ molten salt diesel soot oxidation catalyst has been developed. An engine test-bench was used to test it in diesel exhaust gas with ELPI analysis and to deposit diesel soot on filters for temperature programmed oxidation experiments. Molten salt (Cs₂SO₄V₂O₅) based catalytic foam has an onset temperature for catalytic oxidation of 320°C. This is a promising temperature for continuous filter-regeneration applications. Unfortunately the liquid state of the catalyst makes it unfit for the very effective wall-flow monolith filter, and necessitates the use of a foam filter as support. The onset temperature of the catalytic foam of 320°C is still too high to justify a change from wall-flow monolith to foam, as ceramic foam is a less effective filter than the wall-flow monolith. Foams are no absolute filters, and should be optimized for each application.     

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.     

Co3O4–CeO2 mixed oxide-based catalytic materials for diesel soot oxidation

Co₃O₄–CeO₂ type mixed oxide catalyst compositions have been prepared by using co-precipitation method and, their catalytic activity towards diesel particulate matter (PM)/carbon oxidation has been evaluated under both loose and tight contact conditions. These catalysts show excellent catalytic activity for PM/carbon oxidation, despite their low surface area. The activation energy observed for non-catalyzed and catalyzed reactions are 163 kJ/mol and 140 kJ/mol, respectively, which also confirm the catalytic activity of catalyst for carbon/soot oxidation. The promotional effects of an optimum amount of cobalt oxide incorporation in ceria and presence of a small amount of potassium appears to be responsible for the excellent soot oxidation activity of this mixed oxide type material. The catalytic materials show good thermal stability, while their low cost will also add to their potential for practical applications.     

Study on regeneration of diesel particulate filter using a laboratory reactor

Regeneration of diesel particulate filters can be accomplished by complete combustion of a collected particulate. A reactor has been developed for study of the regeneration in the presence of catalysts, additives and ignition aids. This reactor allows an accurate measurement of soot ignition temperatures and a kinetic study of soot oxidation with an undisturbed soot layer and under a defined gas composition. Results of various investigations carried out with this reactor are presented.     

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     

Measurements of diesel soot oxidation kinetics in an isothermal flow reactor–catalytic effects using Pt based coatings

Despite the significant progress in soot oxidation studies, there is still high uncertainty regarding the rate expressions to model the reactions in diesel particulate filters (DPF). This uncertainty arises from inherent difficulties in sampling and measuring the reaction rate in a realistic way, as well as different properties of the examined soot. In this context, the scope of this study is the development of a novel experimental set-up capable of overcoming existing experimental difficulties. The developed set-up allows for real diesel soot oxidation studies in an isothermal flow reactor. The reaction of soot with oxygen and NO₂ is studied with synthetic gas and with real diesel exhaust and the reaction kinetics are derived for both bare and Pt-based catalyzed substrates by combining experimental and model results.