Optimization of Surfactant and Catalyst Modified Urea-Water Solution Formulation for Deposit Reduction in SCR Aftertreatment Systems

Selective Catalytic Reduction is the primary method of NO_X emission abatement in lean-burn internal combustion. This process requires the decomposition of a 32.5 wt. % urea-water solution (UWS) to provide ammonia as a reducing agent for NO_X, but at temperatures < 250 °C the injection of UWS is limited due to the formation of harmful deposits within an aftertreatment system and decreased ammonia production. Previous work has sufficiently demonstrated that the addition of surfactant and a urea/isocyanic acid (HNCO) decomposition catalyst to UWS can significantly decrease deposit formation within an aftertreatment system. The objective of this work was to further optimize the modified UWS formulation by investigating different types and concentrations of surfactants and titanium-based urea/HNCO catalyst. Because there is a correlation between surface tension and water evaporation, it was theorized that minimizing the surface tension of UWS would result in decreased deposit formation. Eighteen surfactants underwent a surface tension evaluation study to determine what type and concentration of surfactant is required to maximize surface tension reduction. Surfactant concentration was able to be reduced by 94% while maintaining surface tension reduction of over 50%. Six distinct catalysts were evaluated for urea decomposition activity utilizing SwRI’s Universal Synthetic Gas Reactor® (USGR®) to measure CO₂ and NH₃ evolution from solid mixtures of catalyst and urea. At a minimum, CO₂ production increased by 298% when a titanium-based catalyst was present with urea with the highest performing catalyst increasing CO₂ production by 540 % compared to pure urea. Deposit generation evaluations were then conducted utilizing the optimized catalyst and surfactant to determine the impact of catalyst and surfactant in UWS on deposit formation. Tailpipe NOx and PM emissions of the optimized UWS were compared against a conventional UWS on a 15 L low NO_X engine. Both formulations met the CARB 2027 PM standards, but only the modified UWS met the CARB 2027 low NO_X 0.02 g/hp-hr standard. The NO_X composite FTP of the modified UWS and conventional UWS were 0.019 g/hp-hr NO_X and 0.051 g/hp-hr respectively. The improved performance of the modified UWS was attributed to a more aggressive dosing strategy, which could be implemented due to reduced risk of urea deposit formation.