A Computational Investigation of Industrial Selective Catalytic Reduction Systems for NO_x Control

The selective catalytic reduction (SCR) is a promising NO_x (a mixture of NO and NO₂) reduction technology for various applications. The SCR process entails the conversion of NO_x by the use of a reducing agent such as ammonia and a suitable catalyst. Due to increasingly stricter NO_x emission regulations, the SCR technology for NO_x control needs continuous improvement. The improvement requires better understanding of complex processes occurring in the SCR system. The current study employs a mathematical model to elucidate the effect of key operating and geometric parameters on the performance of SCR systems. The model considers both standard and fast SCR reaction processes. The model was used to investigate the effects of NH₃/NO_x and NO₂/NO_x ratios in the exhaust on the SCR performance and the effect of using a dual layer SCR system. Furthermore, the effect of different operating parameters and the interdependence of parameters is analyzed by using a factorial approach. The results show that the SCR performance is very sensitive to NH₃/NO_x ratio. The SCR performance is also affected by the NO₂/NO_x ratio particularly at low temperatures. The optimal NO_x conversion performance requires a combination of NH₃/NO_x ratio of 1.0, NO₂/NO_x ratio of 0.5, low space velocities, and high inlet temperature. The results depict that adding a second catalyzed layer results in increased reaction activity especially when the concentration is still high after the first layer.