Catalytic Diesel Filters

W. Addy Majewski

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Abstract: Most catalytic diesel filters utilize a combination of an upstream oxidation catalyst and a wall-flow filter, often coated with a catalyst as well. The catalyst allows the filter to regenerate passively during periods of high exhaust gas temperature and shortens the duration of active regenerations. A number of diesel filter catalysts have been developed, including both noble and base metal formulations. Catalyzed ceramic filters exhibit very good PM and PN filtration efficiencies, but can be used only with diesel fuels of low sulfur content and can increase tailpipe NO2 concentrations.

Introduction

Catalytically regenerated filters, commonly referred to simply as catalytic filters, employ an oxidation catalyst coated onto the filter body itself and/or onto a flow-through substrate located upstream of the filter for the purpose of regenerating the filter. We will narrow the term ‘catalytic filter’ to mean a filter that utilizes a coated catalyst by excluding filters using fuel borne catalysts (FBC) from this discussion. This nomenclature is not entirely consistent—some authors extend the term ‘catalytic filter’ to also include FBC systems. Filters coated with other types of catalyst such as an SCR catalyst are not commonly referred to as catalytic filters.

Catalytic ceramic filters have been established as the most important diesel particulate filter technology for both OEM and retrofit/aftermarket applications. The purpose of the catalyst is to lower the soot oxidation temperature and facilitate passive DPF regeneration. Passive regeneration is preferred over active regeneration, because it occurs at a lower temperature (with a beneficial effect on filter durability) and involves a lesser fuel economy penalty. Catalytic filters utilizing wall-flow monolithic substrates are by far the most common type of diesel particulate filters, in both OEM and retrofit applications.

Three basic configurations are possible, depending on the location of the catalyst:

Catalytic filters have been used in filter systems that utilize both passive and active regeneration. Most retrofit DPF systems rely exclusively on passive regeneration. In filter systems for OEM applications, passive regeneration occurs at high engine load conditions, while various means of thermal management are typically used to increase the exhaust gas temperature and ensure reliable, actively supported regeneration at lower engine loads.

In these latter “passive-active” filter systems, the role of the catalyst—in addition to lowering the soot ignition temperature—is to accelerate the soot oxidation rate to minimize the fuel economy penalty. Shorter duration regeneration also minimizes the chances for unforeseen and unwanted interruptions (e.g., due to changed engine operating conditions where the engine management strategy to increase temperature can be no longer sustained). In catalyzed filters, the oxidation catalyst may also help control any excessive CO/HC emissions that could otherwise occur during active regeneration, when large amounts of soot are burned during a short time period.

Configurations of Catalytic Filters

Catalyzed Diesel Particulate Filter

In the catalyzed diesel particulate filter, a catalyst is applied onto the filter media to promote chemical reactions between components of the gas phase and the soot collected in the filter. The main purpose of the catalyst is to facilitate passive regeneration of the filter by enabling the oxidation of the carbonaceous particulate matter under exhaust temperatures experienced during regular operation of the engine/vehicle, typically in the 300-400°C range. In the absence of a catalyst, particulates can be oxidized at appreciable rates only at temperatures around 550-650°C, which can occur only at full load conditions in the diesel engine and in most cases are rarely seen during real-life operation.

In the most common design, the CDPF utilizes a ceramic wall-flow monolith made of either cordierite or silicon carbide, packaged into a steel housing, as shown in Figure 1. The porous walls of the monolith are coated with the catalyst. As the diesel exhaust aerosol permeates through the walls, the soot particles are deposited within the wall pore network and over the inlet channel surface. The catalyst, through a combination of NO2 and oxygen based reaction mechanisms discussed later, facilitates PM oxidation under the lean conditions in diesel exhaust.

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Figure 1. Catalyzed diesel particulate filter utilizing a wall-flow ceramic substrate

The application of a catalyst may be also possible with filter substrates other than wall-flow monoliths, such as with wire mesh, ceramic foams, ceramic fibers, and other media.

Oxidation Catalyst + Uncatalyzed DPF

In this two-stage DPF configuration, an uncatalyzed filter is regenerated using NO2 generated over a diesel oxidation catalyst (DOC) positioned upstream of the filter. By using NO2 to oxidize diesel soot, filters can be regenerated at relatively low exhaust temperatures. Indeed, on suitable applications this type of filter is capable of passive regeneration at temperatures as low as 250-300°C.

A schematic of the dual stage DOC + DPF configuration is shown in Figure 2. The filter system is composed of two devices—an oxidation catalyst (upstream) and a ceramic wall-flow diesel filter (downstream). This two stage DPF configuration has been also known as the CRT® filter.

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Figure 2. Two stage DOC + DPF configuration a.k.a. CRT filter

“CRT”—an abbreviation for the “Continuously Regenerating Technology” trade name, originally introduced as “Continuously Regenerating Trap”—is a registered trademark of Johnson Matthey, whose researchers first described the use of NO2 for soot oxidation [470]. This type of filter was also referred to as the CR-DPF, which stands for “continuously regenerating DPF”. The CRT filter was patented by Johnson Matthey in the USA [111] and in other countries, followed by market introduction in retrofit applications in the late 1990s [112]. As the patents expired, this type of filter has been available from many suppliers, and the term CRT is rarely used anymore as a general reference for the DOC + DPF configuration.

Application Limits. The exhaust gas NOx:PM ratio and fuel sulfur content—in addition to the exhaust gas temperature—are important factors that influence the filter regeneration as well as emission performance. The passively regenerated CRT filter had the following application limits:

It should be noted that the above application limits are examples from early retrofit applications. OEM filter systems, introduced later, may require much higher NOx:PM ratios and/or higher exhaust temperatures to rely on passive regeneration alone (compare Figure 13). These differences may be due to the different engine technology and engine-out PM level, as well as to the fact that the maximum pressure drop observed in many retrofit DPFs was higher than would be acceptable in OEM engines.

Filters deactivated by operation with high sulfur fuels regain their ability to regenerate when operated for a period of time on ultra low sulfur fuels [455]. Periodic exposure to high temperatures can also restore the catalyst activity. A gradual deterioration of NO conversion rate leading to slow regeneration was attributed to sulfur in applications with prolonged periods of low exhaust temperature, such as in urban buses, even with ultra low sulfur fuels [1117].

As long as ultra-low sulfur fuel is used, the DOC + DPF configuration has a balance temperature advantage over the catalyzed filter. A relatively high average balance temperature advantage of 60°C was found by the DECSE study [455]. A much smaller advantage of only 15°C was reported by others [619]. Since balance temperatures depend on a number of engine and aftertreatment specific parameters, generalization is difficult.

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