Diesel Particulate Filter (DPF)
Diesel particulate filters are an emerging technology
that can reduce over 90% of particulate emissions. Several
types are under development, including catalyzed DPF (CDPF),
continuously regenerating DPF (CR-DPF), microwave-regenerated
DPF. All DPFs filter the exhaust to remove the particulate,
but differ in the means of regenerating the filter. The CDPF
is washcoated with a catalyst to promote oxidation of the stored
particulate. The CR-DPF employs an oxidation catalyst upstream
to convert exhaust NO to NO2, and the NO2 is the primary oxidant
for the stored PM. The microwave-regenerated DPF uses microwaves
to heat the filter for combustion of the stored PM. ORNL researchers
have examined all three types of DPFs. The CDPF was examined
as part of the light-duty DVECSE project.
The CR-DPF and CDPF were examined in a complementary DECSE effort,
in which ORNL measured particle size and unregulated emissions
from the devices on a heavy-duty engine to
Storey's DECSE complementary). A microwave-regenerated DPF
has been evaluated at ORNL on light-duty engines.
NOx adsorber
The oxidizing environment in lean-burn engine exhaust
makes reduction of NOx difficult. The three-way catalyst in
wide use on homogeneous-charge, stoichiometric engines uses
unburned fuel as the reducing agent for NOx reduction. However,
in a lean exhaust, hydrocarbons have proven to be only marginally
effective at NOx reduction, despite considerable R&D efforts
at so-called lean NOx catalysis. The NOx adsorber catalyst
(a.k.a. Lean NOx trap, NOx storage/reduction catalyst) is a
flow through device that will store (adsorb) NOx emissions
during lean operation. This technology is attractive as it
has the potential to enable lean burn engines to meet the same
stringent emissions standards as their stoichiometric counterparts,
with improved fuel economy. The exhaust must periodically be
taken to a rich condition to purge and reduce the stored NOx.
Considerable R&D is being conducted to determine reliable
and cost-effective means to generate the rich exhaust conditions
in (normally lean) diesel engines. ORNL researchers are involved
in several NOx adsorber projects: DVECSE ,
NOx Control and Measurement Technology for heavy-Duty Diesel
Engines (Cummins CRADA), NOx adsorber
modeling, Precompetive Catalyst R&D,
and NOx Adsorber Engineering.
NOx adsorbers have also been shown to be extremely sensitive
to sulfur. As such, R&D efforts are also focused on managing
this sulfur sensitivity with periodic desulfation or with the
use of sulfur traps.
Selective Catalytic Reduction
(SCR)
Reduction of diesel NOx emissions
is difficult due to the presence of oxygen
in the exhaust. In the oxidizing environment of lean exhaust, fuel
has proven to be only a marginally effective reductant. Urea SCR
systems utilize aqueous urea as a means of introducing ammonia reductant
in the NOx reduction catalyst. These systems have
been shown to be less sulfur sensitive than NOx adsorbers.
The addition of a small amount of urea into the lean exhaust
gas, upstream of an SCR catalyst, has the potential to greatly
reduce tailpipe NOx emissions. Stationary power applications
have used SCR for years. Potential barriers to the successful
implementation of urea-SCR in transportation applications include
the production of unwanted by-products of the urea and irregular
behavior of the system under transient conditions. Emissions
by-products can include ammonia, N2O, urea decomposition products
and carboxylic acids. Irregular catalyst behavior can result
from unpredictable storage and release of ammonia during transients
between high and low engine loads.
