Influence of the cell geometry on the conversion efficiency of oxidation catalysts under real driving conditions
Graphical abstract
Introduction
Current and future emissions standards are pursuing zero local pollutant emissions. Therefore, the elements involved in the emissions control, from the source to the final stage of abatement in the aftertreatment system, need to be revisited to make them efficient contributors to the pollutants abatement under the new boundaries [1]. In particular, the aftertreatment systems have become standard and embedded both in compression and spark ignition engines to reduce the tailpipe pollutant emissions [2]. In this context, improving the aftertreatment performance by means of optimised catalyst formulation [3], impregnation and carrier [4] for faster light-off is not effectively enough [5] to fulfil incoming emissions regulations applied to real driving conditions [6]. The complexity of emission reduction under realistic transient conditions demands the conjoint design of exhaust systems and thermal management strategies, what actively involves the combustion process [7], turbocharger [8] and advanced exhaust components. Such a scenario demands a comprehensive and precise understanding of the processes governing the pollutants depletion by means of experimental [9] and modelling tools [10].
With this approach, the monolith design can bring relevant improvements to the conversion efficiency, as required by new combustion concepts [11]. It can be promoted by means of modifications of the monolith physical properties, adapting the substrate [12] or cell size [13] to the flow properties, considering the washcoat loading effect on the cross-section geometry [14] or attending to the porous substrate properties [15]. The arrangement of the channels has a primary potential to alter the conversion efficiency [16]. It is possible to increase the exhaust gas to catalytic area by increasing the cell density and enhance the heat and mass transfer in the channels by reducing the cell hydraulic diameter [17]. Combined with the use of thinner walls also minimises the substrate mass so that the warm-up is accelerated [18] and can provide benefits in inertial pressure drop contributions [19]. However, if the channel walls become too thin, they become extremely fragile and eventually break. An alternative solution is the use of triangular or wave channels to improve the thermal strength [20]. In that sense, metallic substrates with triangular cells [21] allow the use of higher cell densities, whilst maintaining mechanical strength and thermal durability. However, this is done so at the expense of higher thermal conductivity. In addition, some concerns have been raised about washcoat adhesion on metallic walls [22] due to the reduced roughness of the non-porous surface and the differences in thermal expansion between substrate and washcoat. From a cost point of view, these substrates are also more expensive on average than their ceramic counterparts.
This study investigates the potential of varying cell geometries and substrates to enhance pollutant conversion efficiency under real driving conditions represented by the Worldwide harmonized Light vehicles Test Cycle (WLTC). A commercial cordierite oxidation catalyst with square cells was taken as baseline design. This selection simplifies the discussion on the chemical mechanism to put the focus on the geometry effects while considering the different fashion in raw pollutant emissions under highly dynamic operation. This way, the characteristic CO emission spikes during accelerations and the HC accumulation capability are identified as phenomena determining the trends in cell geometry and washcoat loading optimization at the same time that involve limitations to the expected benefits in conversion efficiency. For this discussion, a catalytic converter model for flow-through monoliths [23] solving heat transfer and chemical species transport was used. Its use provided flexibility to adapt the numerical solver to square and triangular cells with accurate control on the boundary conditions. The model formulation is described with special detail in explicit expressions for outlet pollutants mole fraction calculation and the definition of the specific surfaces and heat and mass transfer parameters. Their expressions are provided for square and triangular cell cross-sections including the sensitivity to the washcoat loading. This theoretical background supports the discussion on the role of the geometric parameters on the mass and heat transfer, which then govern the pollutant conversion efficiency. In particular, variations in cell shape and density, washcoat loading and substrate material were simulated. The contributions of the involved abatement mechanisms were also evaluated considering their sensitivity to the exhaust flow properties. As a result, the application of the model provides new knowledge on the design of efficient catalytic converters in the context of real driving operation. Particularly, the effects of cell design parameters on the residence time and diffusion towards the active sites as well as the inhibition or the thermal response are understood.
Section snippets
Catalytic converter model
A lumped model for flow-through catalytic converters [23] was applied in this work to describe the impact of the channel geometry, washcoat loading and cell density on the CO and HC conversion efficiency. The mass flow, the inlet gas composition, pressure and temperature are imposed as boundary conditions. Based on the lumped approach, the solution of the mass and energy balances between the inlet and outlet sections of the monolith provide the outlet gas temperature, velocity and composition
Cell cross-section geometry influence on model parameters
The following assumptions have been considered to study the influence of channel geometry and washcoat loading on the catalytic conversion of pollutant emissions for square and triangular cells (sketched in Fig. 3):
- •
The lumped flow and monolith thermal solution in Section 2 imply that all monolith channels have the same cross-section geometry concerning substrate and washcoat layer as well as they have the same thermofluid behaviour.
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The fillet radius of the substrate is zero. Consequently,
Definition of the study
The performance of a commercial oxidation catalyst with square cells, whose main characteristics are shown in Table 3, was taken as baseline for the theoretical analysis of the cell geometry influence on the pollutant conversion efficiency. The catalytic converter was installed in a passenger car diesel engine. As summarized in Table 4, the engine was equipped with variable geometry turbine (VGT) and high- and low-pressure cooled exhaust gas recirculation (EGR). The high-pressure EGR line
Results and discussion
Fig. 8 shows the effect of cell density, shape and washcoat loading on the main catalyst geometrical parameters. Plot (a) in Fig. 8 depicts the OFA. As observed, the triangular cells show higher penalty in effective open frontal area, specially as the fillet radius increases. The corresponding channel width and wall thickness are represented in Fig. 8(b) and (c) respectively. As expected, both parameters decrease with the cell density, with higher channel width at the expense of thinner wall in
Conclusions
The influence of the cell geometry on the conversion efficiency of honeycomb catalytic converters was analysed under real driving conditions. A model with sensitivity to the flow properties and cell geometry in terms of mass and heat transfer was employed for this purpose. In particular, the effect of washcoat loading and cell density was analysed for ceramic and metallic monoliths with square and triangular cells taking as reference experimental data obtained from a commercial oxidation
CRediT authorship contribution statement
Pedro Piqueras: Funding acquisition, Supervision, Software, Methodology, Data curation, Writing - original draft. María José Ruiz: Software, Investigation, Visualization, Writing - original draft. José Martín Herreros: Conceptualization, Methodology, Writing - review & editing. Athanasios Tsolakis: Conceptualization, Methodology, Writing - review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors acknowledge FEDER and Spanish Ministerio de Economía y Competitividad for partially supporting this research through project TRA2016-79185-R. Additionally, the Ph.D. student María José Ruiz has been funded by a grant from Universitat Politècnica de València with reference number FPI-2018-S2-10.
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