Soot Filtration Research Articles

High-Porosity Acicular Mullite Ceramics For Multifunctional Diesel Particulate Filters

The advantage of acicular mullite over other ceramic materials is its ability to produce highly porous structures while maintaining high mechanical integrity. This is due to controlled acicular grain growth that affects the connectivity of individual mullite crystals forming a 3D network. High porosity, in turn, enables coating with large amounts of washcoat and catalyst enabling efficient NOx control and other multifunctional applications without penalizing back pressure. This paper describes the development of the experimental ultra-high porosity acicular mullite ceramic to be used for combined NOx reduction and filtration of soot in diesel engines. Catalyzed acicular mullite honeycombs having porosities between 64% and 80% showed excellent NOx reduction and pressure drop that is comparable with state-of-art commercial filters without any catalyst coating. The ability to combine several functions into one substrate provides the promise of a lower cost for the entire emission treatment and creates an opportunity for other applications requiring highly porous but mechanically strong scaffold as a carrier for functional materials.

Sequence of monolithic converters DOC–CDPF–NSRC for lean exhaust gas detoxification: A simulation study

With increasingly strict automotive emission regulations the exhaust gas aftertreatment becomes more complex and expensive. Mathematical modelling and simulations play an important role in design of the aftertreatment systems consisting of multiple catalytic devices, reducing the time and cost demands of the system design. In this paper a combined exhaust gas aftertreatment system for diesel engines is studied. It consists of a diesel oxidation catalyst (DOC) for CO and hydrocarbons oxidation, a catalyzed diesel particulate filter (CDPF) for soot filtration, and an NO x storage and reduction catalyst (NSRC, also called lean NO x trap, LNT) for NO x abatement. Effective mathematical models of the individual converters are presented and used first to demonstrate the functionalities of the system, and then to conduct a parametric simulation study. The aim of this study is to map the influence of the individual components on the performance of the entire system in standard test driving cycle. The sizes of the DOC, CDPF, and NSRC converters are varied while the overall volume of the combined system is kept constant. The resulting maps of pressure drop, CO, HC, particulate matter, and NO x conversions show non-linear dependences on the sizes of individual converters. Co-operative and competitive effects occurring in the combined system are discussed. Suitable reactors sizes are found that enable high conversions of all controlled exhaust gas components.

Performance of radial-type metal foam diesel particulate filters

An analytical study of the performance of a radial-type, metal foam diesel particulate filter is reported. A mathematical model for the filtration and regeneration of soot in a metal foam filter was developed. Nickel foam was selected for the filter medium due to its large specific area, high porosity, and high thermal resistance. For various metal foams, the filtration efficiency and the pressure drop through the filter were calculated, as was the deposition of soot. The results from the analytical model were compared with experimental data. In comparison with a conventional wall flow filter, the metal foam diesel particulate filter (DPF) is effective in utilizing the volume of material, due to the porous structures. As the size of the metal foam pores in the DPF increases from 580 µm to 800 µm, the filtration efficiency decreases from 90% to 50%, and the pressure drop decreases from 380 mbar to 20 mbar. The metal foam DPF with a large pore size is effective in utilizing the volume of material with a small pressure drop. The regeneration is completed within four minutes by the flow of hot exhaust gases under full load conditions.

Modelling of soot oxidation by [formula omitted] in various types of diesel particulate filters

Diesel particulate filters (DPF) form significant part of the diesel exhaust aftertreatment systems. Collected soot particles can be burned off continuously at intermediate temperatures by nitrogen dioxide NO 2 (“passive regeneration”) or periodically at increased temperature by oxygen O 2 (“active regeneration”). NO 2 for passive regeneration of the DPF is generated from nitrogen monoxide (NO) and O 2 in the diesel oxidation catalyst (DOC) located upstream, or in the catalytic coating of DPF (CDPF), or by a combination of both. Two-dimensional nonstationary model of the filter, soot filtration and regeneration with detailed kinetics of soot combustion, and spatially 1D plug-flow model of the DOC are presented and utilized to compare various catalytic systems for soot oxidation by NO 2 and O 2 (DPF, CDPF, CRT®, CCRT®). Pressure drop, soot conversion and other properties of the systems during the passive and active regeneration are compared.

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.

Combining Biodiesel and Exhaust Gas Recirculation for Reduction in NOx and Particulate Emissions

The coupled elimination of NOx and particulate matter (PM) in diesel exhaust was investigated by combining biodiesel, exhaust gas recirculation (EGR), and diesel particulate filtration (DPF). The combustion of biodiesel fuels, being oxygenated hydrocarbons, produces less partial oxidation products such as CO, volatile organic hydrocarbons, and particulate soot. EGR requires auxiliary particulate filtration due to the increased soot that is produced while the reliability of the DPF regeneration requires a minimum exhaust temperature and NOx/PM ratio. We employed a heavy-duty chassis dynamometer system to quantify the synergistic effects of biodiesel, EGR, and soot filtration for a vehicle having a drive cycle with a low exhaust temperature. The biodiesel/EGR/DPF system performance was assessed in terms of NOx and PM emissions for low-temperature operation. The combination of biodiesel and EGR was shown to produce significantly less NOx and PM for a refuse truck operated at low load. The combination of biodiesel and EGR replaces gas phase O2 from air with nascent oxygen in the fuel, resulting in up to a 40% reduction in both PM and NOx compared to the use of ultralow sulfur diesel (ULSD) without EGR. The decrease in PM and corresponding increase in the NOx/PM ratio has a beneficial effect on DPF operability due to a more favorable oxidation stoichiometry. Another advantage of this system is the possibility of optimizing NOx reduction by increasing the amount of EGR, while staying within the prescribed limits of favorable DPF operation.

Modeling of Filtration for a Metal Foam Diesel Particulate Filter

The filtration of soot in the metal foam DPF has been studied. INCOFOAM®HighTemp is selected for DPF material for its large specific area for filtration. The structural properties of the foam such as pore diameter, strut diameter and porosity are determined from the pictures by 3D Xray scope and SEM. The permeability and Forchheimer coefficient obtained by clean filter experiments are correlated with the structural properties. During the filtration process, the soot particles deposited inside the filter affect the local strut diameter, porosity and permeability, which determine the filtration efficiency and pressure drop. By the analytic model developed, the actual pressure drop in the engine operation can be predicted.

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 0.9O 3), 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.

Soot filtration and regeneration behaviour of particulate filters for diesel passenger cars

The economic diesel engine with it’s low fuel consumption as vehicle powertrain is an important contribution to the intensively discussed CO2 challenge. Unfortunately, diesel engines typically have high NOx and particulate emissions. Apart of improvements in engine technology, those emissions can only be reduced aplying complex exhaust aftertreatment devices. In the last years, DeNOx systems and particulate filter systems have been intensively developed and partially been applied in series. Tenneco Automotive in Edenkoben (Germany) is currently running an investigation programm for optimisation of particulate filter systems for passenger cars.

Deep filtration and catalytic oxidation: an effective way for soot removal

Catalytic and uncatalytic axial flow ceramic foam traps for soot removal were prepared and tested at the exhaust of a gas oil burner. Soot filtration efficiencies of uncatalytic and catalytic traps were comparable and depended on the burner operating conditions. A threshold temperature of the catalyst of about 330 °C was determined. Above such a temperature while the pressure drop through the uncatalytic trap increases continuously because of soot load, that through the catalytic trap reaches a steady-state value where the soot amounts captured and burned on the trap were equivalent. Also radial flow ceramic foam traps were prepared and tested at the exhaust of a common rail diesel engine. Their behaviours reflected those of the corresponding axial flow traps. A model descriptive of the relevant phenomena (filtration, combustion and pressure drop time evolution) occurring all over the traps was formulated. It is able to yield the overall trap performances although further improvements are needed in the catalytic case.

Polynuclear aromatic hydrocarbon and particulate emissions from two-stage combustion of polystyrene: the effects of the secondary furnace (afterburner) temperature and soot filtration.

Laboratory experiments were conducted in a two-stage horizontal muffle furnace in order to monitor emissions from batch combustion of polystyrene (PS) and identify conditions that minimize them. PS is a dominant component of municipal and hospital waste streams. Bench-scale combustion of small samples (0.5 g) of shredded styrofoam cups was conducted in air, using an electrically heated horizontal muffle furnace, kept at Tgas = 1000 degrees C. Upon devolatilization, combustion of the polymer took place in a diffusion flame over the sample. The gaseous combustion products were mixed with additional air in a venturi and were channeled to a secondary muffle furnace (afterburner) kept at Tgas = 900-1100 degrees C; residence time therein varied between 0.6 and 0.8 s. At the exits of the primary and the secondary furnace the emissions of CO, CO2, O2, NOx, particulates as well as volatile and semivolatile hydrocarbons, such as polycyclic aromatic hydrocarbons (PAH), were monitored. Online analyzers, gravimetric techniques, and gas chromatography coupled to mass spectrometry (GC-MS) were used. Experiments were also conducted with a high-temperature barrier filter, placed just before the exit of the primary furnace to prevent the particulates from entering into the secondary furnace. Results demonstrated the beneficial effect of the afterburner in reducing PAH concentrations, including those of mutagenic species such as benzo[a]pyrene. Concentrations of individual PAH exhibited a pronounced after burner temperature dependence, typically ranging from a small decrease at 900 degrees C to a larger degree of consumption at 1100 degrees C. Consumption of PAH was observed to be the dominant feature at 900 degrees C, while significant quantities of benzene and some of its derivatives, captured by means of carbosieve/Carbotrap adsorbents, were formed in the afterburner at a temperature of 1000 degrees C. In the primary furnace, about 30% of the mass of the initial polystyrene was converted into soot, while the total mass of PAH represented about 3% of the initial mass of combustible. The afterburner reduced the particulate (soot) emissions by only 20-30%, which indicates that once soot is formed its destruction is rather difficult because its oxidation kinetics are slow undertypical furnace conditions. Moreover, increasing the afterburnertemperature resulted in an increasing trend of soot emissions therefrom, which might indicate competition between soot oxidation and formation, with some additional formation occurring at the higher temperatures. Contrary to the limited effect of the afterburner, high-temperature filtration of the combustion effluent prior to the exit of the primary furnace allowed for effective soot oxidation inside of the ceramic filter. Filtration drastically reduced soot emissions, by more than 90%. Limited soot formation in the afterburner was again observed with increasing temperatures. The yields of both CO and CO2 were largely unaffected by the temperature of the afterburner but increased at the presence of the filter indicating oxidation therein. A previously developed kinetic model was used to identify major chemical reaction pathways involving PAH in the afterburner. The experimental data at the exit of the primary furnace was used as input to these model computations. A first evaluation of the predictive capability of the model was conducted for the case with ceramic filter and a temperature of 900 degrees C. The afterburner was approximated as a plug-flow reactor, and model predictions at a residence time of 0.8 s were compared to experimental data collected at its exit. In agreement with the experimental PAH concentration, only a minor impact of the afterburner treatment was observed for most species at 900 degrees C. OH was deduced to be the major reactant with a mole fraction about 4 orders of magnitudes higher than that of hydrogen radicals. Evidence for the need of further work on the quantitative assessment of oxidation of PAH and their radicals is given.

Selection and development of a reactor for diesel particulate filtration

The diesel engine is an efficient power generator but its exhaust gas needs to be cleaned. A diesel particulate filter, in fact a multi-phase multi-purpose chemical reactor for environmental protection, can lower the emission of diesel soot particles. The aim is to develop a diesel particulate filter that is robust, dependable, energy efficient and resistant to plugging. Moreover, ultrafine particles should be trapped and no poisoning gases should be emitted. A strategic approach is used for the design of a diesel particulate filter, inspired by the method of Krishna and Sie (1994) Chemical Engineering Science 49, 4029–4065, for multiphase reactor selection. The method involves three strategic levels for reactor selection: catalyst design, heat and mass injection and dispersion, and hydrodynamic flow regime; in this paper, the emphasis is on hydrodynamics. This approach has led to the design of a novel filter type for diesel soot filtration: the turbulent precipitator with foam collector plates. In this filter the gas flow is divided over two zones with different hydrodynamic characteristics: fast gas flow in an open channel and slow gas flow in stagnant zones. The open channel enables low pressure drop and prevents plugging, the stagnant zones enable deposition of diesel soot particles and, if desired, the placement of the catalytic material. The results for two different geometries of the turbulent precipitator are presented, they indicate that 90 ppi ceramic foam collector plates perform the best and that it is possible to tune different turbulent precipitators for different diesel engines. Computational fluid dynamics can be used to optimize the turbulent precipitator because it identifies the two hydrodynamic zones in the filter.