Multi-catalytic soot filtration in automotive and marine applications

• Future catalytic particulate filtration systems for treating diesel engine exhaust.
• Catalysts’ role in passive and active soot combustion for filter regeneration.
• Integration of SCR catalysts in diesel particulate filters.
• New catalytic soot removal process for marine engines using heavy-fuel oil.

The emission of particulate matter (PM) and NOx from diesel engines has proven to have a negative influence on health, and future regulations will require more than 90% PM and NOx removal. For land-based trucks using low-sulfur fuels, a sequential catalyst system is foreseen consisting of an upstream diesel oxidation catalyst (DOC), a catalyzed diesel particulate filter (cDPF), and a downstream SCR NOx removal catalyst together with an ammonia slip catalyst (ASC). Urea is injected as a precursor for ammonia: engine → DOC → cDPF → Ureainj → SCR → ASC → out. The SCR function can with process advantage be integrated into the filter and reduce volume. The trapped soot in the cDPF must currently be combusted away. Three different soot combustion mechanisms are used: (1) passive soot regeneration by NO2, (2) combustion by direct catalyst soot contact, and (3) active soot regeneration with O2 at around 600 °C. Full soot combustion by direct catalyst soot contact is not needed in a system as soot combustion by NO2 and O2 gases plays major roles. For marine vessels, no PM emission regulation exists today. The emission problem is especially severe for vessels using heavy-fuel oil (HFO) with up to 3.5% sulfur and high (heavy) metal content, fuel properties that until now have prevented a reliable PM removal process. Recently, however, a catalyst-assisted passive soot regeneration process above 350 °C was developed including an “in situ” ash removal. This new process was validated on a ship using a sulfur-resistant Pd,V2O5 filter catalyst that combusts soot, CO, and HC including PAHs, and it is well suited as front end for an SOx scrubber.

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