Ceramic Fibers and Cartridges

W. Addy Majewski

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• Introduction
• Fiber Materials
• Filtration Mechanism
• Filter Cartridges
• 3M Nextel Fiber Cartridges

Introduction

Fiber filters utilize depth filtration over high temperature resistant ceramic fiber cartridges. The filters can be regenerated by fuel additives (fuel bone catalysts, FBC), electrically, or using fuel burners. Various types of ceramic fiber filters were commonly used in the 1990s, in the early years of DPF technology, Table 1 [899]. With time, fiber filters lost their market share to wall-flow filters and their use became limited to some niche applications such as retrofit filters for industrial engines.

Performance, design parameters, and other features of fiber filters can be compared to those of wall-flow monoliths, which have become the benchmark in diesel filter materials. The advantages of fiber filters include:

• Resistance to damage by thermal or mechanical stress.
• Less risk of clogging with soot and/or ash.
• Tolerance to ash accumulation when used with FBC regeneration. The ash, uniformly deposited throughout the entire media, would not cause high pressure drop or clogging.
• Good efficiency for particle number reduction—under some conditions, fiber filters could outperform monolithic filters [475].
• Good noise attenuation [478].

On the negative side, fiber filters have showed a number of disadvantages:

• Incompatibility with catalysts. Catalyst coating of fiber filters was attempted [474], but no satisfactory results have been reported. Catalyzing fibers may require more sophisticated methods than washcoating, such as chemical vapor deposition (CVD) [182]. Even if CVD coating was successful, washcoated catalyst formulations would not be applicable to fiber filters.
• Vulnerability to damage by sintered ashes, which may coat and bond fibers together [461]. Such coated and aggregated fibers are subject to vibrational failure, resulting in voids in the fiber bed and gas leakage. Fibers may also undergo chemical attack by some additives [102].
• Lower filtration efficiency. Fiber filters show somewhat lower gravimetric filtration efficiency than monolithic filters. They may be also susceptible to blow-off of accumulated soot. This may result in visible black smoke during accelerations and further reduced efficiency on transient test cycles.
• Secondary fiber emissions. In case of fiber breakage, fiber fragments are emitted as a secondary emission.
• Large size. Fiber filter systems are typically larger than monolithic filters.
• High cost. Fiber filters were typically more costly than cordierite monoliths.

Fiber Materials

Diesel particulate filter cartridges have been developed utilizing high-temperature continuous ceramic fibers. A number of such fibers—typically polycrystalline metal oxide fibers made from alumina and silica—have been used in textiles intended for high temperature operating environments. Typical applications include insulation for thermocouple wires, belts for high temperature furnaces, filter bags for hot gas filtration, components of space shuttle tiles, and flame curtains. Traditional fiber materials include leached silica, fused silica, asbestos or glass. In general, these fibers are characterized by high strength, high rigidity, low coefficients of thermal conductivity and expansion, and large active surface area.

Nextel, a brand of synthetic fiber developed by 3M, is an example high temperature fiber that had been extensively tested for diesel filter applications. Typical properties of Nextel 312 fibers [304], compared with commercial SiO₂ fibers [473], are listed in Table 2. The data indicates that Nextel fibers exhibit favorable strength and flexibility.

The fibers have a virtually round cross-section and their surface is very smooth and uniform. In filter applications, the adhesion of filtered particles to the fibers can be greatly enhanced through special treatment to increase the surface roughness and the active surface, such as leaching the fibers. A special texturization process had been developed for the manufacturing of the Nextel 312 fibers used for diesel filtration [476]. In the process, the raw fiber bundles were separated and the individual fibers were spread in a controlled manner. Spreading and separation produced “expanded” material with a number of 10-12 micron continuous fibers. The degree of expansion could be controlled, which allowed manufacture of consistent material.

Fiber diameter is one of the most important parameters that determine the filter performance. Generally, small diameter fibers give better filtration efficiencies. Coarse fibers tend to show low efficiency, critical overloading phenomena, and blow-off [473]. However, very small fiber diameters may present a health hazard, as well as increased costs and problems with handling. A typical diameter of diesel filter fibers is in the range of 10 µm.

Fibers of 10 micron diameter are generally considered not to be in the respirable range and to present a minimal health hazard. This opinion was also confirmed by the Nextel fiber manufacturer through experiments with animals [304]. Figure 1 shows Nextel fibers compared with asbestos in 1000× magnification. The Nextel fibers are very uniform in diameter. In contrast, the asbestos sample represents an inhomogeneous collection of fibrils with diameters 100 to 1000 times smaller than those of Nextel fibers.

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