Diesel Filter Materials

W. Addy Majewski

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• Material Requirements
• Ceramic Filters
  • Wall-Flow Monoliths
  • Ceramic Fibers
  • Ceramic Foams
• Metal Filters
  • Sintered Metal
  • Metal Fiber Filters
  • Other Metal Filters
• Pleated Paper Filters

Material Requirements

Filter substrate is the key component of the diesel filter system, affecting both its performance and durability. Its task is to physically capture solid particulates and hold them until they can be removed in the regeneration process (periodic or continuous). Design targets for diesel particulate filter materials include:

• High filtration efficiency:
  • particle mass
  • particle number
• High maximum use temperature
• High soot holding capacity
• Thermal shock resistance
• Strength and mechanical integrity
• Low pressure drop:
  • empty
  • loaded with particulates and ashes
  • low statistical variation of pressure drop (i.e., repeatable ΔP at a given gas flow rate and soot load)
• Chemical stability:
  • resistance to exhaust gas components including sulfur
  • low reactivity with ash compounds
  • oxidation resistance
• Compatibility with regeneration methods:
  • coatability with a catalyst
  • compatibility with fuel additives
  • other methods
• Low mass and low heat capacity (thermal mass)
• Compact size
• No generation of secondary emissions (such as fiber loss in fibrous media)
• Long life and high durability
• Low cost

Depending on the regeneration method, diesel filter materials may be exposed to very high temperatures, in excess of 1000°C, as well as rapid temperature changes. These thermal conditions are caused by the release of heat during rapid oxidation of soot accumulated in the filter. Since the distribution of soot is not necessarily uniform throughout the filter, the thermal stress frequently has a local character. Thermal phenomena, both high temperature and thermal stress, are responsible for most instances of filter failure, such as melting or cracking.

An important aspect of filter durability is the reactivity of filter material with the accumulated ash. Various metal oxide ashes originating from lubrication oil additives and other sources are always present in diesel particulates. Much higher quantities of ash may be present in filters that utilize fuel borne catalysts (FBC) to facilitate regeneration. These ashes can either chemically attack and corrode the filter media through solid-state reactions (e.g., creating pinholes) or form eutectic-type systems, which may result in dramatic deterioration of the material melting point. Ash related problems were reported with many types of filter media, including wall-flow monoliths [434] and ceramic fiber cartridges [461]. In many cases, reactions with ash—rather than the material melting temperature—determine the maximum operating temperature of a given filter material.

Typical reactions of ash with a filter material are illustrated in Figure 1 [462]. Phenomena occurring at lower temperatures, such as ash sintering, depend primarily on the properties of the particular ash, rather than the filter media. Phenomena in the mid-temperature range, such as solid-state corrosion or eutectic melting, depend on the interaction between ash and the particular material. In the case of cordierite, a common wall-flow filter material with a melting point of 1450°C, the maximum operating temperature is limited to about 1200°C, largely due to ash reactions.

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