Influence of the cell geometry on the conversion efficiency of oxidation catalysts under real driving conditions

Highlights

• Understanding of cell and washcoat geometry on catalyst conversion efficiency.

• Square and triangular cells are evaluated under dynamic driving conditions.

• Positive effect of high gas and low washcoat specific surfaces on conversion.

• Best conversion efficiency is found for triangular cells and ceramic substrates.

• Cell geometry optimization is not enough to offset CO emission spikes penalties.

Abstract

Worldwide pollutant regulations applied to the transportation sector are progressively tightening the emission limits and widening the operating conditions of the type approval tests. As a result, the layout and thermal management of the exhaust system is becoming highly complex looking to achieve early catalytic converter activation. On this regard, the monolith meso-geometry plays a primary role to optimise the pollutants conversion efficiency. The geometrical characteristics simultaneously affect and trade-off multiple flow phenomena as the exhaust gas is transported through the channels. These include the bulk gas and internal pore diffusion towards the active sites in addition to the heat transfer including convection, radial conductivity and thermal capacitance. In this work, the impacts of the cell size, cross-section shape, washcoat loading and substrate material on CO and HC conversion efficiency have been investigated under representative real driving conditions. From the real driving conditions experimental data, the study decouples the influence of the washcoat loading from the cell size and material applying a catalytic converter model. Detailed expressions are provided for the calculation of the specific surfaces and heat and mass transfer parameters as a function of the cell and washcoat meso-geometry in square and triangular cells. Therefore, this work enables to identify the processes which govern the catalytic abatement of pollutant emissions. In particular, the role of the gas and washcoat specific surfaces is highlighted because of its importance on the optimization of the mass transfer process by means of a proper cell geometry selection. In parallel, the differences in the change of the CO and HC abatement patterns, which are explained by the characteristic CO emission spikes in accelerations and the HC accumulation, contribute to evidence the limitations on the conversion efficiency benefit that the optimum cell geometry and washcoat loading can provide.

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.