Development of an Ammonia-SCR Reaction Computation Model and Experimental Studies of Zeolite Catalysts

Abstract

The urea-SCR system has been used commercially due to the high performance of NOx reduction, and much research has been conducted to invent low-noble metal catalysts effective at low exhaust temperatures. The CFD (computational fluid dynamics) with a reaction model is effective for SCR catalyst development and many SCR simulation models have been proposed. However, experimental feedback of surrogate gas tests is required in the simulations and fitting parameters in the models are adjusted by the experiment results. This is a cause of the low versatility of the reaction models.
To develop a versatile ammonia-SCR reaction model, the reaction rate of Standard-SCR reactions, molecular diffusion in catalyst coated layers, and structural parameters of honeycomb catalysts are introduced in the simulation in the report. These chemical and material parameters are not based on engine emission test data, and the reaction rate coefficients and order of reactions are measured with the catalyst in powder form by flow reactor experiments, and the mass-transport is described by the mass transfer coefficient and Knudsen diffusion in the model. The NOx conversion in the model is calculated for several copper zeolite catalysts, ZSM5, Beta, SSZ13, and P-CHA at an 88,000 h^-1 space velocity (SV) condition, and the calculated results are compared with the NOx conversion data from engine emission tests and surrogate gas (mixture of nitrogen oxide [NO], oxygen [O₂], water vapor [H₂O], and nitrogen [N₂] as the balance) tests. The calculated results of the ammonia SCR model correlate well with the surrogate gas tests up to 350°C with all catalysts, but the correlation is poor above 400°C because the ammonia oxidation is not considered in the simulation.