21 calibrated t o g enerate a mmonia f or t he S CR w as als o discussed. Despite p romising N Ox
conversion with the SCR system, many other grey areas need attention to further improve the system.
2.5.1 Ammonia slip
Ammonia slip re mains the u ndesired e mission in th e S CR s ystem. It c an b e described as ammonia that exits the SCR system unreacted. Huennekes et al., 2006 suggested 3 ways the
injected urea can lead to NH
3
slipping out of the SCR catalyst. It involved the incomplete SCR reaction due to NH
3
: NOx ratio higher than NOx conversion efficiency, the released o f stored ammonia from SCR catalyst and the incomplete decomposition of urea before reaching the SCR
catalyst. Girard et al., 2007 also reported NH
3
slippage as a result of high NH
3
: NOx ratio called alpha. It was suggested reducing the alpha value less than one at low temperature where the
ideal alpha is equal to one.
2.5.2 Uniform mixing of Urea.
The urea injection quality and mixing are complex and critically important. In real engine testing such as in this study, uncertainties existed over the uniform mixing of the urea spray with the
exhaust gases. Gorbach et al., 2009 introduced urea mixers for mixing of urea droplets from sprays and saw s ystem efficiencies v ary fr om 60 t o 9 5 d epending o n a mmonia d istribution
across the catalyst. The urea mixer comes in a variety of types ranging from wire mesh designs to vanes and honeycomb. Breedlove et al., 2008 suggested the use of different nozzle designs
to provide different droplet quality with range of characteristics at different injection stages.
2.5.3 Spray effect on temperature Johannessen et al., 2008
reported that th e s prayed u rea i n exhaust s tream reduced the exhaust g as te mperature b y 1 0-15
O
C t herefore d iminished t he N Ox c onversion e fficiency
especially in the low temperature region. Way 2008 also reported problem with urea injection
22 at low temperature less than 190
O
C where incomplete evaporation of urea and solid deposit build-up occurred in the exhaust system.
2.5.4 Space velocity Koebel et al., 2001
described problem faced by the SCR system in automotive application due to low exhaust gas temperature and short resident time due to space constraints in LD Diesel
application. T he p roblem le ads t o t he re duced p erformance o f S CR s ystems re sulting fro m incomplete thermolysis of urea before entering the SCR catalyst. It is reported that only 50 of
urea decomposed at 400
O
C and even lower than 15 at 255
O
C.
2.5.5 Light duty diesel engine study Fisher et al., 2004
