Numerical analysis of mixing chamber non-uniformities and feed conditions for optimal performance of urea SCR

The ammonia used for NOx reduction in urea SCR is formed from the decomposition of urea in the mixing chamber. The main challenges in urea SCR are the incomplete decomposition of urea to NH3 and its subsequent non-uniform distribution at the SCR inlet. A uniform profile at the SCR inlet without accounting for the non-uniformities from the mixing chamber may lead to an error in the system design. For symmetrical injection of urea inside the mixing chamber, the insufficient exhaust gas temperature and lower residence time of urea are two important factors that lead to the incomplete conversion of urea and the non-uniform distribution of ammonia at the SCR inlet. A CFD analysis of the mixing chamber with a symmetrical injection of urea is carried out to study these factors. The analysis showed that by simultaneously lowering the flow rate of urea and reducing the velocity of the exhaust gas, which is at a sufficiently high temperature, the NH3 generation is improved considerably. The CFD analysis was further extended to model the SCR with different inlet conditions. This analysis revealed that for SCR with non-uniform inlet conditions arising from the mixing chamber, the NOx reduction achieved is lower compared to that in the case of a uniform inlet profile for the SCR. The analysis shows that the radial variation in NOx reduction arising from the non-uniformities in the mixing chamber diminishes with a rise in the exhaust gas temperature. As compared to that with uniform inlet profiles, the NOx conversion in SCR with non-uniform inlet profiles exhibit a sharp rise with an increase in NO2 concentration.