Unsteady Analysis of NO Reduction over Selective Catalytic Reduction−De-NOx Monolith Catalysts

Abstract

A heterogeneous 1D dynamic model of SCR−de-NOx (SCR = selective catalytic reduction) monolith reactors is developed, based on the concept of the “active” catalyst region. Its predictions are practically equivalent to those of a previous 2D model, but its numerical solution is faster by 1 order of magnitude, making it suitable for predictive control applications. The model is successfully fitted to transient NO reduction data at different temperatures, space velocities, and NH3/NO feed ratios over two commercial SCR monolith catalysts. The data indicate characteristic times up to several minutes. The model analysis shows that: (i) SCR dynamics are determined by buildup/depletion of the catalyst NH3 coverage, which result primarily from a competition between adsorption of ammonia and its reaction with NO, the rate of NH3 desorption being markedly slower; (ii) the dynamic response of NO conversion is slow compared to changes in the NH3 feed content, which is unfavorable for a feedback control system; and (iii) positive/negative peaks of NO emissions vs progressive changes of the NH3 slip are predicted upon sudden load variations.