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Selective
Catalytic Reduction (SCR)
Selective Catalytic Reduction (SCR) of NOx using
an ammonia compound as reductant has been used for many years in stationary
diesel engine applications, as well as for mobile applications. In the SCR
process, NOx reacts with the ammonia, which is injected into the exhaust gas
stream before a special SCR Catalyst. SCR is the only technology capable of
reducing diesel NOx emissions to levels required by future
emission standards.
Three forms of reductants can be used in SCR systems:
(1) pure anhydrous ammonia, (2) aqueous ammonia and (3) urea.
The chemical reaction which occurs in an ammonia SCR system is
shown by this Equation: 4NO+ 4NH3 + O2 = 4N2 + 6H2O
The SCR process requires precise control of
the ammonia injection rate. An insufficient injection may result in
unacceptably low NOx conversions. An injection rate that is too high results
in release of undesirable ammonia to the atmosphere. These ammonia emissions
are known as ammonia slip. The ammonia slip increases at higher NH3/NOx
ratios. The stoichiometric NH3/NOx ratio is about 1.
SCR remains the only proven catalyst technology which is capable of reducing
diesel NOx emissions to levels required by future emission regulations.
SCR System Configuration
A SCR system for mobile engines works in an
open loop configuration where a pre-programmed map of engine NOx emissions is
used to control the ammonia injection rate. This open loop configuration is
capable of some 95% NOx reductions. The automotive industry recognizes that
ammonia injection offers the most effective SCR solution for mobile
applications.
The System is comprised of sensors for exhaust
gas temperature, intake air temperature, engine load information, throttle
position, engine rpm, exhaust back-pressure, the electronic control unit (ECU)
and a SCR converter. The ECU is responsible for controlling the amount of
ammonia being injected for maximum NOx reduction and includes circuitry for
the redundant fail-save and OBD (On Board Diagnostic) system.
The ECU calculates the correct amount of Ammonia needed, by "reading"
the information supplied by the various sensors and compares these values with
the appropriate point of the factory-programmed ECU Map. The fail-save system
assures that possible ammonia leaks do not go undetected and
the OBD system alarms the vehicle operator of any problems. High quality
electromagnetic valves assure auto-shutoff of the ammonia supply in case of
accidents or system malfunctions.
The prevention of ammonia slippage and a high NOx conversion rates is
guarantied by the advanced Electronic Control Unit (ECU) and high-sensitivity
engine and ambient parameter sensors.
Ammonia SCR systems have several benefits over urea SCR systems. Low
temperature climate compatibility, an advantage concerning infrastructure
issues, a higher conversion rate and a smaller less complex system are only
a few.
Without an advanced ECU, low to no ammonia slippage and high NOx reduction
cannot be guarantied.
Layout
of SCR
System
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