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Education: Diesel

Application of Conventional Filtration Technologies

Catalyzed Diesel Particulate Filters: DPF’s can achieve PM filtration rates of ≥ 90% given specific, controlled operating conditions. Moreover, DPF’s reduce each sub-category of particulate material, i.e. SOL, SOF, and Sulfate Particulates. It is necessary to note however, the application and effectiveness of DPF technology is significantly constrained by the following limitations:

  • DPF’s are very expensive.
  • The California Air Resources Board provides cost-range information for DPF’s corresponding to the following engine capacitates:

    • 100 horsepower: US$5,000 – US$7,000
    • 275 horsepower: US$6,900 – US$9,000
    • 400 horsepower: US$10,000 average
    • 1,400 horsepower: US$32,000 + (18)
  • DPF’s are incapable of affecting PM emissions reductions when using fuels that exceed 150 PPM Sulfur (19).
  • DPF’s performance is adversely affected by insufficient operating temperatures.
  • In less-than-optimal conditions, DPF’s are prone to clogging and failure. When failure occurs, the potential for engine damage or destruction is significant (2).
  • Because DPF’s can create significant engine back pressure, expensive engine recalibrations are often required upon their installation.
  • DPF’s often need to be equipped with expensive electronic back pressure monitoring devices, such as data loggers (21).
  • Because passive DPF regeneration is entirely dependent on operating temperature, passive DPF’s do not work under “low load” conditions.
  • “Active” components in DPF technologies significantly increase DPF unit price and complexity.
  • DPF’s do not work well on older engines (22).
  • DPF’s can become a source of hazardous zinc, sulfuric, calcium, and phosphorus ash particulate (23).
  • DPF’s can reduce engine performance (24).
  • DPF’s often produce fuel economy penalties (25).
According to the United States Department of Energy (USDOE), “fuel sulfur has significant effects on post-DPF total PM emissions”, and, as fuel sulfur levels increase, DPF’s reduction efficiencies decreases to a point where DPF’s actually becomes a source of PM emissions when using fuels with sulfur concentrations ≥ 150 PPM (26).

Tests conducted by the USDOE report that DPF’s that achieved 95% reductions of PM emissions when using fuels with 3-PPM sulfur concentrations had their filtration efficiencies reduced to only 74% when using fuels with 30-PPM. Further, these same devices were reduced to PM filtration rates of 0% to -3% when using fuels with 150-PPM sulfur, and they experienced TPM emissions increases of 122% to 155% when using fuels with sulfur concentrations ≥ 350-PPM.

The Natural Resources Defense Council (NRDC) states that “[Catalytic] technologies can not work properly if there is sulfur in the fuel—and in some cases, sulfur in the fuel will render the [catalytic filtration] equipment and even the vehicle inoperable” (28).

Diesel Oxidation Catalysts: DOC technologies are generally less expensive than DPF technologies, and because DOC’s are “flow through”, instead of “wall flow” devises, they do not have the same propensity to create engine back pressure, clog and/or cause potential engine damage like their DPF counterparts. DOC’s can achieve PM filtration rates between 19% and 50% (29). Unfortunately, the application of DOC technology is constrained by the following:

  • DOC’s are too expensive for ubiquitous application.
  • The California Air Resources Board provides cost average information for DPF’s corresponding to the following engine capacitates:

    • 275 horsepower: US$2,100
    • 1,400 horsepower: US$20,000 +
    • The Everett School District in Washington state reported an average per-unit-cost of US$2,500 per DOC for each bus in its fleet (30)
  • DOC’s reduction of TPM is significantly reduced when using fuels with high sulfur fuels (31).
  • DOC’s do not filter Solid Organic Fraction [“dry”] particulate (32,33) and dry particulates typically comprise the majority of Total Particulate Material (34).
  • DOC’s do not work well on older engines.
  • DOC effectiveness is extremely dependent upon operating temperatures.
  • When operating at higher temperatures, DOC’s oxidize sulfur oxides, and in doing so become generators of sulfuric acid. When this occurs, DOC’s create a net increase TPM emissions by increasing production of sulfate particulates at rates that offset Soluble Organic Fraction reductions

The University of Washington’s Extension Energy Program confirms that “Diesel Oxidation Catalysts can oxidize sulfur dioxide to form sulfate particulates (sulfuric acid (H2SO4)). Therefore, high sulfur content fuels can increase total particulate emissions via[the production of sulfuric acid, which can offset soluble organic fraction [“Wet PM”] reductions” (35).

The United States Department of Energy found “statistically significant increases in PM with high sulfur fuel due almost exclusively to the increase in the SO4 fraction of the total PM. At this high exhaust temperature (405°C at catalyst inlet), the DOC accelerates the conversion of SO2 to SO3, thereby increasing the SO4 fraction of the PM. As expected, the effect is seen only with the higher sulfur (150-PPM and 350-PPM S) fuels. With the 350-PPM S fuel, post catalyst PM emissions were approximately 200% higher than those measured without an active catalyst” (36).
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