UNSTEADY ANALYSIS OF NO REDUCTION OVER SELECTIVE CATALYTIC REDUCTION-DE-NOX MONOLITH CATALYSTS

A heterogeneous 1D dynamic model of SCR-de-NOx (SCR = selective cataly tic reduction) monolith reactors is developed, based on the concept of the ''active'' catalyst region. Its predictions are practically equiv alent to those of a previous 2D model, but its numerical solution is f aster by 1 order of magnitude, making it suitable for predictive contr ol applications. The model is successfully fitted to transient NO redu ction data at different temperatures, space velocities, and NH3/NO fee d 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 cata lyst NH3 coverage, which result primarily from a competition between a dsorption of ammonia and its reaction with NO, the rate of NH3 desorpt ion being markedly slower; (ii) the dynamic response of NO conversion is slow compared to changes in the NH3 feed content, which is unfavora ble for a feedback control system; and (iii) positive/negative peaks o f NO emissions vs progressive changes of the NH3 slip are predicted up on sudden load variations.