Case 721G Stage IV, Operator'S Manual

Page: 273 / 522

7 - MAINTENANCE
Selective Catalytic Reduction (SCR)
Is composed by:
Selective Catalytic Reduction on Filter
(SCRoF) (1)
The Selective Catalytic Reduction on Filter (SCRoF) (1) is
located at the inlet of the SCR catalyst (2) . In the SCRoF
occurs the physical filtration of the the Particulate Matter
(PM) and the reaction of the exhaust gas.
Selective Catalytic Reduction (SCR) (2)
The main reaction involved in the SCR catalyst is the con-
version of the NO
x
to N
2
and water (H
2
O). An average
NO
2
/NO
x
ratio around 50% is required to give the opti-
mum NO
x
reduction efficiency in the exhaust system.
Excess of ammonia (NH
3
) is often used to facilitate the
NO
x
reduction to nitrogen (N
2
), resulting in NH
3
slip. This
ammonia must be removed by a secondary step in the
Clean Up Catalyst (CUC).
Clean Up Catalyst (CUC) (3)
The Clean Up Catalyst (CUC) is located at the end of the
SCR catalyst. The CUC converts the ammonia (NH
3
) slip
to nitrogen (N
2
) and water vapor (H
2
O). The CUC ensures
that the ammonia slip to ambient is minimal.
LEIL18WHL1225AA
4
721G
LEIL18WHL1103AA
5
821G
Exhaust flap
The exhaust flap is a device located between the turbine
and the exhaust system. The exhaust flap is equipped
with a throttle valve which manages the rate and the “light
off” temperature of the exhaust gas.
The “light off” temperature indicates the temperature at
which the catalytic process starts.
LEIL18WHL1104AA
6
7-35