Diesel Particulate Filters

W. Addy Majewski

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Abstract: Diesel particulate filters capture particle emissions through a combination of filtration mechanisms, such as diffusional deposition, inertial deposition, or flow-line interception. Collected particulates are removed from the filter, continuously or periodically, through thermal regeneration. Diesel filters are highly effective in controlling solid particulate emissions—including solid particle numbers—but may be ineffective in controlling liquid fractions of PM emission. Filters were first commercialized as retrofit devices, followed by a wide scale adoption on new light-duty and heavy-duty diesel engines in both highway and nonroad applications.

Introduction

Definition. Diesel particulate filters (DPF) are devices that physically capture diesel particulates to prevent their release to the atmosphere. Diesel particulate filter materials have been developed that show impressive filtration efficiencies, in excess of 90%, as well as good mechanical and thermal durability. Diesel particulate filters have become the most effective technology for the control of diesel particulate emissions—including particle mass and numbers—with high efficiencies.

Due to the particle deposition mechanisms in these devices, filters are most effective in controlling the solid fraction of diesel particulates, including elemental carbon (soot) and the related black smoke emission. Filters may have limited effectiveness, or be totally ineffective, in controlling non-solid fractions of PM emissions—SOF and sulfate particulates. To control total PM emissions, DPF systems are likely to incorporate additional functional components targeting the SOF—typically oxidation catalysts—while ultra low sulfur fuels may be required to control sulfate particulates.

The term “diesel particulate trap” is sometimes used as a synonym for “diesel particulate filter”, especially in older literature. The term “trap” covers a wider class of particle separation devices. Several particle deposition mechanisms other than filtration are commonly employed in industrial dust separation equipment. Examples include gravity settling, centrifugal separation, or electrostatic trapping. None of these techniques could be adopted to control diesel PM emissions, due to the small particle size and low density of diesel soot.

It may be noted that particle oxidation catalysts (POC)—sometimes called partial filters—can also capture diesel particulates, but provide a much lower overall efficiency than diesel particulate filters. In their common designs, POCs capture particulates only from a fraction of the flow, whereas the total flow is filtered in diesel particulate filters. In the case of some filter media, however, the distinction may not be very clear and the devices can be classified as either a POC or a (depth) particulate filter.

Collection & Regeneration. Due to the low bulk density of diesel particulates, which is typically below 0.1 g/cm3 (the density depends on the degree of compactness; as an example, a number of 0.056 g/cm3 was reported by Wade [497]), diesel particulate filters can quickly accumulate considerable volumes of soot. Several liters of soot per day may be collected from an older generation heavy-duty truck or bus engine. The collected particulates would eventually cause excessively high exhaust gas pressure drop in the filter, which would negatively affect the engine operation. Therefore, diesel particulate filter systems have to provide a way of removing particulates from the filter to restore its soot collection capacity. This removal of particulates, known as the filter regeneration, can be performed either continuously, during regular operation of the filter, or periodically, after a pre-determined quantity of soot has been accumulated. In either case, the regeneration of filter systems should be “invisible” to the vehicle driver/operator and should be performed without his intervention. Thermal regeneration of diesel particulate filters is typically employed, where the collected particulates are oxidized—by oxygen and/or nitrogen dioxide—to gaseous products, primarily to carbon dioxide. Thermal regeneration, schematically represented in Figure 1, is undoubtedly the cleanest and most attractive method of operating diesel particulate filters.

[schematic]

Figure 1. Schematic of Particulate Filter with Thermal Regeneration

To ensure that particulates are oxidized at a sufficient rate, the filter must operate at a sufficient temperature. In some filter systems, the source of heat is the exhaust gas stream itself. In this type of filter system, referred to as a passive filter, the filter regenerates continuously during the regular operation of the engine. Passive filters usually incorporate some form of a catalyst, which lowers the soot oxidation temperature to a level that can be reached by exhaust gases during the operation of the vehicle. Another approach which may be needed to facilitate reliable regeneration involves a number of active strategies for increasing the filter temperature (engine management, fuel combustion in the exhaust system, electric heaters, etc.). Regeneration of such devices, known as active filters, is usually performed periodically, as determined by the vehicle’s control system. The classification of particulate filter systems is presented in more detail in the paper on filter systems.

An alternative strategy involves the use of disposable filter cartridges, which are replaced with new units once filled with soot. Particulate filters of this kind are used in some occupational health environments. Such maintenance intensive filter systems are clearly not acceptable in highway vehicle applications.

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