Active Regeneration Characteristics in Diesel Particulate Filters (DPFs) 2011-24-0185

Particulate matter (PM) captured in diesel particulate filters (DPF) consists of: (a) soot, the product of incomplete combustion of the fuel and (b) ash, produced by combustion of lubricating oil plus minor amounts of metal components in the fuel. Among the various types of DPFs, most efficient are the so-called wall flow filters, where the exhaust gas is forced to pass through porous walls of adjacent channels, which are plugged alternately at their opposite ends. Accumulation of PM in DPFs leads to increasing pressure drop across the filter. Since increased PM load in the filter and thus increased pressure drop across the filter deteriorates the engine performance, the filter load of the DPF has to be periodically removed during a process referred to as regeneration. During the regeneration process, soot PM captured in the DPF is expected to be oxidized. The temperature needed for oxidation of PM is usually exceeding ca. 550°C. Since diesel exhaust temperatures seldom reach these levels, oxidation of soot is promoted by the so-called passive regeneration by means of different technologies: (1) the continuous regenerating trap (CRT) technology, which takes advantage of the NO₂ produced by oxidation of engine-out NO over a Pt catalyst preceding the DPF; (2) incorporation of a catalyst precursor in the fuel, so that PM and catalyst are built together and facilitate PM combustion in the presence of oxygen at lower temperatures. Both of these techniques are, however, only partly successful. Higher degrees of soot oxidation are achieved during the so-called active regeneration, whereby higher exhaust temperatures are enforced.

In this study we have measured and computed the soot oxidation rates during active and passive regeneration in a small heavy-duty truck. By means of the measured species mass flow balances over the diesel oxidation catalysts (DOC) and the DPF we are able to compute the soot burning rates and to compare them with the weight decrease of the DPF. In addition, we have examined in detail the emission characteristics of the entire aftertreatment system during defined active regenerations. Particulate emissions have been measured by particle number counting. Moreover, soot was measured optoacoustically.

The emissions during active regenerations deviate substantially from those in normal operation. Tailpipe CO and unburnt hydrocarbons, soot mass and particulate matter are significantly higher. Only NO_x is rather unaffected. The overall emission profile is not severely influenced given the rare occurrence of active regenerations.

Based on the species balances over the DPF, the soot oxidation rate and the oxidized soot mass during active regeneration were computed. The obtained remaining soot mass in the filter was in good agreement with the weight of the filter. Based on the soot oxidation rate and the temperature measurement characteristic, kinetic parameters for soot oxidation have been computed. The activation energies have been in reasonable agreement with comparable values reported in the literature lying between 80 and 170 kJ/kmol in the different regeneration phases. The computed pre-exponential factors are also in agreement with reported values, their high variations though renders interpretation more difficult. Targeted optimization in analytics and measurement techniques are expected to improve the accuracy of the kinetic parameters.