Impact of biodiesel blending on diesel soot and the regeneration of particulate filters

A critical requirement for the implementation of diesel particulate filters on diesel-powered vehicles is having a low break-even temperature (BET), which is defined as the temperature at which particulate deposition on the filter is balanced by particulate oxidation on the filter. This balance point needs to occur at sufficiently low temperatures, either to fit within the exhaust temperature range of the typical duty cycle for a diesel vehicle or to require a minimum of active regeneration. Catalytic coating on the diesel particulate filter, the use of a fuel-borne catalyst, and oxidation catalysts placed upstream of the particulate filter can all reduce the BET. Another important factor in reducing the BET is the sulfur content of the fuel, because the sulfur dioxide generated during combustion can poison catalyst activity. However, fuel formulation factors other than sulfur content can also have significant effects on the BET. Considered in this work were low sulfur diesel fuel (LSD, 325 ppm sulfur), ultralow sulfur fuel (ULSD, 15 ppm sulfur), and blends of both diesel fuels with 20 wt % biodiesel. The lowest observed BET was for the 325 ppm sulfur fuel that was blended with 20 wt % biodiesel, due, in part, to increased engine-out NO,, emissions with the B20 blend, which shows that the engine-out exhaust composition can be as or more important than sulfur content. Furthermore, examination of the soot generated with these fuels shows a variation in the nanostructure and the oxidative reactivity for soots derived from the different fuels. There exists evidence of correlation between reactivity and structure in the case of carbon blacks or coal chars that are synthesized from different hydrocarbons and at different temperature conditions. Soot nanostructures of particulates produced from different fuels in a commercial direct injection (DI) diesel engine were compared by means of high-resolution electron microscopy imaging. The crystalline information, such as the graphene layer size and orientation, is used to interpret the quantitative reactivity differences measured in an idealized thermogravimetric analysis/differential scanning calorimetry (TGA/DSC) oxidation experiment. Together, these results show the potential impact of biodiesel blending on the low-temperature oxidation characteristics of soot and the impact these soot characteristics can have on particulate filter regeneration.