Wall-flow Particulate Filters Research Articles

Catalytic oxidation in wall-flow reactors with zoned coating

Modern diesel engine technology has recently adopted the wall-flow particulate filter technology. Most commercial filters are also catalyst coated, which gives the potential option to combine oxidation and filtration functionalities in a single reactor. Due to high precious metal costs, it is desirable to make optimal usage of the catalytic coating by applying catalyst zoning. The present study presents and applies a mathematical model that is able to describe the governing heat, mass transfer and chemical reaction phenomena occurring in a wall-flow monolithic reactor with axially variable catalytic activity. The results show that a zoning scheme with more catalysts in the frontal part is beneficial for CO and HC conversion in cold-start transients. The trends are inversed in the case of a simulated cool-down scenario. The effect of different zoning schemes is investigated in a fully transient legislated driving cycle corresponding to a passenger car application. The reactors’ performances are analyzed by examining the transient temperature and species profiles in the inlet and outlet channels. It was shown that the conversion efficiency is a complex function of combined thermal, species transport and reaction phenomena. The results highlight the increased engineering flexibility provided by the catalyst zoning technology and the challenges faced in applications where transient operating conditions prevail.

Catalytic Oxidation Performance of Wall-Flow versus Flow-Through Monoliths for Diesel Emissions Control

Catalytic exhaust after-treatment for the reduction of carbon monoxide and hydrocarbons, using flow-through monolithic reactors, is a well-proven technology in commercial diesel engines. However, recently, most diesel exhaust systems have started to be additionally equipped with wall-flow particulate filters. Most commercial filters are also catalyst-coated, which gives the potential option to combine oxidation and filtration functionalities in a single reactor. The present study compares the steady-state and transient performance characteristics of catalyzed wall-flow diesel particulate filters to the respective flow-through oxidation catalysts. The comparison is based on a theoretical basis, using mathematical models previously published and experimentally validated. It is shown that the conversion efficiency of the wall-flow catalyzed filter is a complex function of thermal, mass-transport, and chemical phenomena that occur simultaneously. Under steady-state conditions, the wall-flow reactor has a higher conversion efficiency, compared to a respective flow-through with the same dimensions and catalytic loading, under mass-transfer-limited conditions (high temperature, high flow rate). Under kinetically limited conditions, both systems perform identically. The transient performance during a simulated cold-start operation of the wall-flow reactor is shown to be inferior, compared to the flow-through one. The phenomena are analyzed and explained in detail by examining the time-dependent profiles of flow distribution, temperature, and species concentration in both reactors.