Method for NOx adsorber desulfation in a multi-path exhaust system

A method of treating exhaust gas stream which is in a lean state, fuel-lean of stoichiometric, and contains NOX and SO2, said method comprising:

• a) splitting the exhaust gas stream into major and minor exhaust gas portions for flow through at least first and second separate flow paths, each of the flow paths containing a particulate trap and, downstream of the particulate trap, a NOX absorber containing a NOX oxidation catalyst and a nitrate adsorbent;
• b) passing the major exhaust gas portion in the lean state and at a first temperature, for a first period of operation, along at least one of said flow paths and through, in succession, the particulate trap and the NOX absorber to convert the NOX to nitrate, to convert the SO2 to sulfate, and to adsorb the nitrate and sulfate on the nitrate adsorbent;
• c) monitoring temperatures in the particulate trap and in the NOX adsorber;
• d) after the first period of operation, switching the flows of the exhaust gas portions so that the one flow path receives the minor exhaust gas portion for a second period of time;
• e) during at least a portion of the second period of time, introducing fuel into the one flow path, upstream of the particulate trap, for combustion in the particulate trap, thereby producing a fuel-rich, reducing exhaust flow;
• f) during said second period of operation, diverting a bypass portion of exhaust from the other flow path at point upstream of the particulate trap and introducing the bypass portion into the one flow path upstream of the particulate trap;
• g) responsive to the temperature of the particulate trap in the one flow path reaching a first predetermined temperature, discontinuing feed of fuel to the particulate trap in the one flow path and increasing flow of exhaust gas through the one flow path to transfer heat from the particulate trap to the NOX adsorber to raise the temperature of the NOX adsorber to a second predetermined temperature for desulfation;
• and
• h) periodically switching the major and minor exhaust gas portions between flow paths so that second period operation is effected in a flow path while first period operation is effected in at least one other flow path.