A method for removing urea deposits from a catalyst

A method is disclosed for operating an aftertreatment device 20 having a urea-based selective catalyst reduction (SCR) catalyst 14 coupled downstream of an internal combustion engine 10. The method comprises establishing a threshold urea deposit accumulation for regeneration of the SCR catalyst 14, determining that the threshold urea deposit accumulation in the SCR catalyst 14 has been met and, in response to the determination, regenerating the SCR catalyst 14 by maintaining the SCR catalyst 14 at a predetermined regeneration temperature for a predetermined time interval. An oxidation catalyst 13 may be used to raise the temperature of the SCR catalyst and the method may be carried out in combination with particulate filter 15 regeneration,, in which case the amount of urea released during filter regeneration is subtracted from the estimate of urea deposition. In addition excess urea is injected into the exhaust following regeneration to maintain optimum urea deposition in the SCR catalyst. The estimation may include taking account of operating temperature, NOx concentrations, urea flow rate, exhaust flow rate, exhaust pressure and pressure drop across the SCR catalyst.