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Environmental implications of iron fuel borne catalysts and their effects on diesel particulate formation and composition
Nash, D., N. Swanson, W. Preston, T. Yelverton, W. Roberts, J. Wendt, AND W. Linak. Environmental implications of iron fuel borne catalysts and their effects on diesel particulate formation and composition. JOURNAL OF AEROSOL SCIENCE. Elsevier Science Ltd, New York, NY, 58:50-61, (2013).
Diesel particulate emissions (DPEs) are a major contributor of primary ambient particulate, and have long been implicated in the onset and exacerbation of adverse respiratory and cardiovascular health outcomes. DPEs are comprised primarily of chain agglomerates of soot with surface coatings of condensed organics. To meet new emissions standards, technologies are being developed and implemented for both new and retrofit applications. One approach is the use of fuel borne metal catalysts to promote additional in-cylinder soot oxidation. Besides yielding lower engine out mass emissions, fuel borne metal catalysts also have the reported advantages of providing increases in fuel economy and engine performance. Fuel borne metal catalysts are intended for use in combination with diesel particulate filters (DPFs) where they are effectively trapped with the DPEs. The catalyst is then used to regenerate the filter by lowering the activation energies and ignition temperatures associated with soot oxidation. However, they may also be used without DPFs, where the metals contribute to ambient air emissions. The goals of this study were to evaluate the potential environmental consequences of using metal fuel borne catalysts in diesel engines without the accompanying use of DPFs. We characterize the particle size distributions (PSDs) of DPEs to understand the effect of Fe concentration on both the particle mass and number emissions. This also includes Fe partitioning and speciation. Another goal of this work was to examine the effect of an Fe catalyst on the elemental and organic carbon composition of DPEs, and evaluate the evolving particle size distribution of an Fe containing DPE and its implication for on-road exposures. Finally, an overall mechanism for Fe reduction of soot in a diesel engine is proposed. This work represents a first step toward future studies to characterize the effect of metal fuel additives on health effects associated with DPEs.
Metal fuel borne catalysts can be used with diesel fuels to effectively reduce engine out particle mass emissions. Mixed with the fuel, the metals become incorporated as nanometer-scale occlusions with soot during its formation and are available to promote in-cylinder soot oxidation. Used in this manner, they also have the purported benefits of increasing engine performance and fuel efficiency. Metal fuel borne catalysts are intended to be used with diesel particulate filters (DPFs) where they are effectively trapped to oxidize additional soot and regenerate the filter. However, they are sometimes used without DPFs, and contribute to ambient air emissions. The work presented here used a small diesel generator to examine the emission characteristics from an iron fuel borne catalyst. Experiments using ferrocene, varying fuel Fe concentrations from 0 to 200ppm, indicate ~30-40% decreases in particle mass, total particle volume, and black carbon emissions, and increases (approaching a factor of 5) in particle number concentrations associated with 10-30nm Fe particles liberated during soot oxidation. Fe concentrations in the particle emissions increase from 0.1 to 7.5% as the Fe catalyst is increased from 0 to 200ppm. The Fe is primarily in the elemental form. While PAHs are reduced with increasing Fe, emissions of alkanes and organic acids show no clear trend. This is consistent with a proposed mechanism whereby the Fe acts to oxidize soot-related PAH species, but does not affect organic compounds associated with unburned fuel and lubrication oil that avoid flame processes. Calculations performed to predict the evolution of the particle size distribution (PSD) associated with the emitted particles suggest that once diluted along the roadway, the Fe-rich nuclei mode is likely to persist for some time. This has implications related to exposures associated with these particles for vehicle occupants as well as those living in near-road environments.