Experimental Investigation of Urea Deposit Formation in Selective Catalyst Reduction System to Guide Product Development

The urea-selective catalyst reduction system implemented in commercial vehicles facilitates ensuring compliance with the NOx regulation limit. A significant challenge in urea injection is to comprehend its decomposition chemistry that often leads to the formation of unfavorable deposits in the exhaust system unit. Due to the complex interaction of the multiphase fluid flow and transport processes, a significant degree of uncertainty is associated with the identification of the interacting factors that control the deposit initiation and their growth. A systematic investigation was conducted through numerous experiments to study the factors controlling the urea deposit that guide innovation for new product development. For the first time, the effect of pressure on urea deposits was investigated by heating an aqueous urea solution in a closed system maintained between 30 and 200 psi. Chemical characterization procedure was conducted using liquid chromatography-multiple reaction monitoring (LC-MRM). Additionally, deposit test was conducted on a typical after-treatment system layout. Deposit-initiating temperature were identified at different operating conditions. The results indicated that the deposit initiation occurred in a narrow temperature range. The critical diesel exhaust fluid injection rate, as a function of temperature, was plotted, and the effect of gas flow rate on urea deposit formation was studied. Further, experiments were conducted on a liquid film to identify the timescale and chemical composition of the generated deposits. The experiment concluded that higher temperatures within a liquid film facilitated early deposit initiation and formed compounds that require extremely high temperature to decompose.