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Definition. Particle oxidation catalysts (POC) are devices that can capture and store carbonaceous PM material for a period of time sufficient for its catalytic oxidation, while having open flow-through passages that allow exhaust gases to flow, even if the PM holding capacity is saturated. In other words, the particle oxidation catalyst is a specialized diesel oxidation catalyst with a capacity to hold solid soot particles. The captured particles must be removed from the device through oxidation to gaseous products, in a process called regeneration. The POC regeneration is typically accomplished via reactions between soot and nitrogen dioxide, generated in an upstream NO2 catalyst. Contrary to the diesel particulate filter (DPF), the POC will not plug once filled with soot to its maximum capacity in the absence of regeneration. Rather, the PM conversion efficiency will gradually decrease, allowing the PM emissions to pass through the structure.
Particle oxidation catalysts—a relatively new PM emission control technology—have a particulate control efficiency higher than that of the DOC, but lower than diesel particulate filters. These devices may use different types of substrates, and are known by several names, including:
The term “particle oxidation catalyst” properly describes the operating principle of this type of catalyst and is applicable to devices utilizing any type of substrate. The name “flow-through filter” is consistent with the nomenclature used in California for verification of retrofit diesel emission control strategies. The term “open filter” is relatively common in Europe.
To describe the POC operation, we make references to various aspects of the DPF technology throughout this paper. Readers who are not familiar with diesel particulate filters are referred to the introductory DPF paper and—most importantly—the paper on DPF regeneration.
Historical Background. Examples of traditional POC substrates are ceramic or metallic foams, metal fleece, and wiremesh. It may be noted that with this type of substrates, the distinction between a POC device and a deep-bed DPF is not always well defined. A ceramic foam of small pores may perform as a deep-bed DPF, exhibit 90%+ filtration efficiency, and plug with soot if not regenerated. Another foam structure of larger pores, on the other hand, may perform as a POC, where gas passages still exist once the maximum thickness of the soot layer is formed.
In the early 1980’s, Texaco developed an alumina-coated steel wool filter that can be considered an early POC example. The PM mass collection efficiency in the Texaco filter ranged between 50-70% . The collection efficiency initially increased as the particle mass was accumulated, reached a peak, and then decreased. The decrease in efficiency was explained by re-entrainment of agglomerates of collected particles; particle size measurements indicated that the coarse particle mass fraction leaving the filter increased markedly with time. The developers in the 1980’s and 90’s were seeking DPF materials of high, 90%+ efficiency, not a POC type of device. Since most filters utilizing ceramic foams, metal fleece, wiremesh and similar materials showed low filtration efficiency and/or had other problems, they were eventually replaced with the wall-flow monolith design.
Renewed interest in POC substrates was triggered by new engine applications which required modest PM emission reductions—higher than those that could be provided by the DOC, but lower than those of the DPF. An advanced POC design, based on a metallic honeycomb catalyst substrate with an added sintered metal fleece layer, was developed by Emitec. This substrate was used in the first commercial POC system—named PM-Kat—launched in 2005 on Euro IV heavy-duty truck engines by MAN, followed by certain non-SCR engine models by Scania. It was also used on some models of diesel passenger cars including some Euro 3 vehicles by Volkswagen with a Generation 2 2.0 TDI engine intended for India, Russia and South America.
Several POC devices were approved for retrofitting diesel passenger cars under the 2007 German incentive program. The substrates included the Emitec design (in the TwinTec system) as well as ceramic and metallic foams from various suppliers. The devices were often incorrectly referred to as “particulate filters”, while technically most of them were either particle oxidation catalysts or DOCs. The official regulation used the term Partikelminderungssystem (PMS) and required a minimum PM emission reduction of 30% over the NEDC test.
In the North American diesel retrofit market, development of flow-through filters was stimulated by the introduction of a Level 2—a minimum of 50% PM emission reduction—verification category in California. Several FTFs using various types of substrates received California Level 2 verification and/or the EPA diesel retrofit technology verification. California emission regulations for in-use diesel engines created some demand for the verified FTF Level 2 systems, especially in applications where Level 3 devices (85% PM emission reduction) were not yet verified. However, a number of Level 2 FTF/POC devices lost their verifications as they were unable to comply with California NO2 emission requirements that became effective in 2007 (maximum 30% NO2 over baseline) or 2009 (maximum 20% NO2 over baseline).
In spite of earlier technology predictions, particle oxidation catalysts found only very limited application on new Tier 4i/Stage IIIB and Tier 4/Stage IV nonroad diesel engines.