Kinetic model comparison and elucidation of mass transfer limitations in NH3-SCR reactors using Vanadia based washcoats with different thicknesses

Kinetic models of NH3-SCR of NO based on modified redox (MR) and modified Eley-Rideal (ER) mechanisms calibrated using monoliths with Vanadia based washcoats with different thicknesses were cross compared. Both models were satisfactory for the washcoat thicknesses investigated at steady conditions in 120–540 °C range. MR mechanism better represented NO transients at 192 °C. Dependence of limiting regions on the washcoat thickness was determined via the analysis of washcoat concentration profiles and the concentration ratios of the fluid, fluid-washcoat interface and washcoat. Both models generally agreed with the borders of the limiting regions. During Standard SCR, the limits of the kinetic regime was below 309 and 271 °C for thinner and thicker washcoats, respectively. This was followed by mixed limitation regime upto 488 °C at the reactor entrance. Above 488 °C, transition to kinetically controlled regime was observed at the center of the monolithic reactors.