Numerical simulation study on urea-SCR system of diesel engine

Abstract

Nitrogen oxides (NO_x) emitted by diesel engines represent a major category of atmospheric pollutants. As the most sophisticated and efficient technology for controlling NO_x emissions from diesel engines, Urea-SCR (Urea-Selective Catalytic Reduction) technology necessitates complex engineering during its development and matching processes. Simulation calculations offer an effective approach to reducing the time and cost involved in Urea-SCR system development. Currently, commercial software dominates the computational research on Urea-SCR systems. Although commercial software boasts powerful capabilities, it poses challenges for users to understand and expand models, accompanied by high costs for usage and upgrades. This study aims to develop a one-dimensional flow model and simulation program for Urea-SCR systems, verifying their accuracy and effectiveness through experimental validation. An unsteady one-dimensional flow model for engine exhaust pipelines was established, solved using the finite volume method in conjunction with the Runge–Kutta method. The Rosin–Rammler empirical equation was employed to fit the droplet size distribution of an injected urea aqueous solution, while the Lagrangian method was applied to calculate the state variations of droplets throughout their lifecycle. The program was utilized to compute urea decomposition efficiency, and the results showed favorable agreement when compared with the experimental data reported by Kim et al. A simplified one-dimensional flow model for the SCR reactor was constructed, solved via the SIMPLE algorithm, with the under-relaxation method adopted to enhance the convergence of implicit format iterative calculations. A one-dimensional Urea-SCR system simulation program was developed using C++. Leveraging an SCR small-scale performance evaluation test bench, the impacts of different operating conditions on NO_x conversion efficiency were tested. The results indicate that the program's computational outcomes exhibit close consistency with experimental data. In the low-temperature range, a higher space velocity corresponds to a lower NO_x conversion rate. The addition of NO₂ improves NO_x conversion efficiency, with the optimal effect achieved when the NO₂/NO ratio is 1 : 1. An ammonia–nitrogen ratio below 1 imposes limitations on NO_x conversion. D2 and E3 test cycle evaluations were conducted on a medium-speed diesel engine test bench, and simulations were performed using the developed program.