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 NOx 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.