Tuning the standard SCR reaction kinetics to model NO conversion in a diesel engine exhaust SCR catalyst system under steady state conditions in 1D and 3D geometries using ammonia gas as the reductant

Removal of NOx from lean Diesel exhaust can be achieved
by the use of selective catalytic reduction technology. The
supplied reductant is often ammonia, either as urea or as
ammonia gas released from a storage medium. Experiments
have been carried out on an engine test rig run to steady state
conditions using NOx composed mainly of NO, with
ammonia gas as the reductant. This was essentially a 1D
study because a long 10 degree diffuser was used to provide
uniform temperature and velocity profile to the SCR catalyst
brick in the test exhaust system. Tuning of the standard
reaction, the NO SCR reaction, in a kinetic scheme from the
literature and adjustment of the ammonia adsorption kinetics
achieved improved agreement between the measurements and
CFD simulations. This was carried out for studies at exhaust
gas temperatures between 200 and 300 °C. The effect of
diffuser geometry upstream of the SCR catalyst on NOx
conversion was then investigated experimentally using a 180
degree sudden expansion as a 3D diffuser. These were also
steady state studies with the exhaust NOx composed mostly
of NO. The SCR brick was short, 45 mm in length, to provide
a rigorous test of the kinetics. Observed NOx conversion
profiles for ammonia supplied in quantities ranging from
deficient to excess showed that the combined influence of
temperature and velocity profiles upstream of the SCR was
apparent in this 3D case. 2D axially symmetric CFD
simulations have been carried out to model the 3D case and
the predictions are discussed and compared with engine test
data in this paper.