Development of Urea-water Solution with Incorporated Isocyanic Acid Hydrolysis Catalyst for Deposit Mitigation in Selective Catalytic Reduction Aftertreatment Systems
Formation of urea-derived deposits in selective catalytic reduction (SCR) aftertreatment systems continues to be particularly problematic at temperatures of 215 °C and below. Several consequences of deposit formation include: NOX and NH3 emission, exhaust flow maldistribution, increased engine backpressure, and corrosion of aftertreatment components. Many methods have been developed to modestly reduce deposit formation, but to date, there has been no solution for continuous low-temperature dosing of urea-water solution (UWS). This manuscript presents a novel methodology for reducing low-temperature deposit formation in SCR aftertreatment systems. The methodology described herein involves incorporation and dissolution of an HNCO hydrolysis catalyst directly into the UWS. HNCO is a transient species formed by the thermolysis of urea upon injection of UWS into the aftertreatment system. Ideally HNCO undergoes hydrolysis to form NH3 and CO2, but under certain conditions HNCO may polymerize or react with other constituents in the exhaust. Reaction of HNCO with species other than water generally results in the formation of deposits in the aftertreatment system. In this research, the coordination complex ammonium titanyl oxalate (ATO) was combined with urea and thermolyzed under controlled conditions such that gaseous products could be quantified using IR spectroscopy. It was found that ATO significantly reduced HNCO evolution from urea decomposition. Burner-based deposit evaluations of UWS with incorporated ATO resulted in a 57 % reduction in deposit mass over conventional UWS at a 200 °C experimental condition. Further deposit reduction was conferred by incorporating both a surfactant and ATO to UWS. By combination of ATO and surfactant to UWS, the deposit mass was reduced by 89 % over conventional UWS at the same 200 °C experimental condition.