Investigation of the Role of Nitrogen Dioxide and Diesel Particulate Filters in the Production of Nitro Polycyclic Aromatic Hydrocarbons in Diesel Exhaust, 03-R9657

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Principal Investigators
Patrick M. Merritt
Joseph C. Pan

Inclusive Dates:  10/1/06 – Current

Background - Dramatic reductions in the allowable rates of emission of oxides of nitrogen (NOx) and particulate matter (PM) from diesel engines are prompting the development of novel combustion processes and aftertreatment technologies for compliance with the stringent regulations taking effect through 2010 in the United States, Europe, and Asia. Unprecedented R&D expenditures have been made by engine manufacturers and exhaust treatment suppliers towards achieving the difficult, and often contradictory, objectives of reducing emissions of NOx and PM.

A major component of the comprehensive effort to reduce diesel emissions of NOx and PM is the diesel particulate filter (DPF). The DPF is known under a number of trade names and is often referred to as a trap. A DPF undergoes a periodic regeneration cycle to burn accumulated material. DPFs are sometimes catalyzed. Regeneration cycles can be triggered by an alteration of engine operating parameters or the introduction of fuel into the exhaust. A major concern during implementation of new strategies and technologies is whether new problems are created while trying to achieve goals such as the simultaneous reduction of PM and NOx. Formation of nitro-aromatic compounds is a concern in the environment of diesel engine exhaust. The moderately low temperatures and conditions that can lead to partial oxidation reactions, such as the lightly catalyzed surface of a DPF, are of particular interest.

Approach - The primary objective of the project is to determine if NPAH formation (through nitration of polycyclic organic hydrocarbons, PAH) occurs at the DPF, and if so, to what degree and under what conditions does nitrogen dioxide (NO2) influence such formation. The approach has been to operate a light-duty diesel engine equipped at five representative speed and load conditions and characterize the exhaust emissions for PAH and NPAH compounds, with the engine exhaust flowing through, respectively, a non-catalyzed DPF, a lightly catalyzed DPF, and a heavily catalyzed DPF. The test matrix will then be repeated with enrichment of the exhaust stream NO2 concentration.

Accomplishments - The initial sampling matrix has been completed. Three DPF substrates, one with no catalyst and two with precious metal catalyst loadings of 25 g/ft3 and 50 g/ft3, respectively, were used to generate samples for subsequent analysis of PAH and NPAH. Particulate and gas-phase samples were collected using mini-dilution tunnels upstream and downstream of the DPF. In addition, after each sampling interval, the DPF was removed from the exhaust system and particulate matter was harvested from the DPF itself. A clear trend was evident, indicating that NPAH concentrations in the DPF-captured soot decrease with increasing precious metal loading. This trend was observed at all power and speed levels. Future efforts will repeat this work with supplemental NO2 to raise the concentration of NO2 upstream of the DPF.

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