HC-SCR is more convenient when compared to urea-SCR, since for
HC-SCR, diesel fuel can be used as the reductant which is already
available onboard the vehicle. However, the DeNOX
efficiency for HC-SCR is lower than that of urea-SCR in both low
and high temperature windows. In an attempt to improve the
DeNOX efficiency of HC-SCR, the effect of hydrogen were
evaluated for the fresh and aged catalyst over 2 wt.% Ag/Al₂O₃
using a Euro-4 diesel engine.
In this engine bench test, diesel fuel as the reductant was
injected directly into the exhaust gas stream and the hydrogen was
supplied from a hydrogen bomb. The engine was operated at 2,500 rpm
and BMEP 4 bar. The engine-out NOX was around 180
ppm-200 ppm. H₂/NOX and HC₁/NOX ratios were
5, 10, 20, and 3, 6, 9, respectively. The HC-SCR inlet exhaust gas
temperatures were around 215°C, 245°C, and 275°C. The catalyst
volumes used in this test were 2.5L and 5L for both fresh and aged
catalysts.
The DeNOX efficiency of the 5L fresh catalyst was in
the range of 0-25% without hydrogen, but it increased to 14-79%
when the hydrogen was added to the feed stream. The final
efficiency was a function of the hydrocarbon and hydrogen
concentrations in the exhaust gas mixtures and the HC-SCR inlet
temperatures. The NOX conversion efficiency of the 5L
aged catalyst, which had been aged thermally at 750°C for 25 hrs in
an electrical furnace, was in the range of 6-75% with hydrogen and
2-22% without hydrogen. In case of the 2.5L catalyst, the
DeNOX efficiency was in the range of 8-35% for the fresh
catalyst and 3-9% for the aged catalyst with hydrogen. In the
absence of hydrogen, the NOX conversion rate was in the
range of 3-8% and 3-5% for the fresh and aged catalyst,
respectively.