In
a urea selective catalytic reduction (SCR) system, the urea solution
is injected into hot exhaust gas, after which the urea solution becomes
ammonia that acts as a reducing agent for de-NO
x
through evaporation, thermolysis, and hydrolysis. The formation
of the reducing agent from urea decomposition is closely connected
with thermofluid dynamics as well as various chemical reactions. An
experimental study was performed to investigate urea decomposition
in a low-temperature environment that is similar to the emission gas
temperature of a large marine diesel engine. Also, this study investigated
urea decomposition in conjunction with thermofluid dynamics related
to the urea SCR system driving conditions. The modeled exhaust pipe
was designed to control the inflow gas temperature and velocity. The
urea solution injector was chosen to obtain almost identical spray
performance, regardless of the urea solution flow rate, to exclude
the effect of the spray on urea decomposition. A multicomponent Fourier
transform infrared spectroscopy gas analyzer was used to measure the
concentrations of ammonia and isocyanic acid (HNCO) in the modeled
exhaust pipe. This study showed that the conversion efficiencies of
ammonia and HNCO were different under the experimental conditions
of this study, although there is no difference between the conversion
efficiencies of ammonia and HNCO in theoretical urea thermolysis.
Also, it showed that there is no need for a long residence time to
improve the total conversion efficiency at a low temperature.
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