Background
Selective Catalytic Reduction (SCR) is a process commonly used to remediate pollutants from diesel engines. This process incorporates an externally supplied reductant, typically ammonia (NH3), to convert NOX (oxides of nitrogen) to N2 (nitrogen) and H2O (water). The NH3 is generated through the decomposition of a 32.5 wt.% Urea-Water Solution (UWS), known in industry as Diesel Exhaust Fluid (DEF). Three steps take place for complete decomposition of UWS to NH3, including water evaporation, urea thermolysis and isocyanic acid (HNCO) hydrolysis.
At exhaust gas temperatures <200 °C the injection of UWS is limited due to the formation of undesired deposits in the aftertreatment system, which coincide with decreased ammonia production. Generally, engine manufacturers elect to stop dosing of DEF at temperatures below 180 °C due to the risk of formation of these deposits. This reduction of DEF dosing results in decreased NOX conversion performance of the aftertreatment system. The formation of these deposits under low temperature operating conditions is due to decreased water evaporation rates and reduced kinetics for the hydrolysis of HNCO. There is a direct correlation between surface tension and water evaporation, whereby minimizing the surface tension of DEF will result in more efficient water evaporation. At moderate temperatures of 200 – 250 °C, water evaporation rates improve, but the HNCO hydrolysis reaction is relatively slow and can result in the formation of undesired reaction products that are deposit forming. The addition of a catalyst precursor selectively accelerates the desired reaction (hydrolysis of HNCO) which decreases the undesired by products that form during HNCO hydrolysis. In accordance with this, strategic targeting of the desired reaction significantly increases the NH3 available for NOX reduction and reduces the formation of undesired solid deposits.
Approach
In this work, both the surfactant and catalyst type and concentration were optimized such that low-temperature deposit generation would be reduced while balancing the cost and commercial producibility of the solution. Eighteen surfactants underwent a surface tension evaluation study to determine the type and concentration impact of surfactant on surface tension reduction of UWS. Six distinct catalysts were evaluated for urea decomposition activity using SwRI’s Universal Synthetic Gas Reactor® (USGR®) to measure CO2 and NH3 evolution from solid mixtures of catalyst and urea. The best performing catalysts and surfactants from these studies were incorporated into UWS and evaluated for deposit generation. All urea deposit testing was conducted on the Exhaust Composition Transient Operation Laboratory® (ECTO-Lab®). The surfactant and catalyst concentrations for the modified UWS formulation were then optimized using a response surface optimization routine using data generated from deposit testing results under different operating conditions and surfactant and catalyst concentrations. The tailpipe NOX and PM emissions from the modified UWS formulation were compared against conventional DEF on a Cummins X15 research engine equipped with a Low NOX dual SCR system.
Accomplishments
A surfactant and catalyst modified UWS was successfully optimized in this program, and sufficiently reduced deposits while simultaneously ensuring the fluid can be produced commercially at a viable price point of less than $0.10 per gallon. The surfactant concentration was reduced by 94% (from initial evaluations) while maintaining surface tension reduction of over 50%. When optimizing the catalyst type, it was found that CO2 production increased by 3-6x compared to pure urea. Deposit generation evaluations were conducted utilizing the optimized catalyst and surfactant concentrations 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 the 15 L low NOX engine. The modified UWS met the CARB 2027 low NOX 0.02 g/hp-hr standard without the need for electrical heating or supplemental energy. The NOX composite FTP of the modified UWS and conventional UWS were 0.019 g/hp-hr NOX 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. The performance of the final catalyzed DEF solution was able to meet 2027+ NOX regulations without the need for auxiliary heating devices.