Exhaust emission control method and system

An exhaust emission control system for treating exhaust gas emitted from an internal combustion engine, comprising:

• an emission control device disposed in an exhaust gas passage of the internal combustion engine, the emission control device storing SOx when an air/fuel ratio of exhaust gas flowing through the emission control device is lean, and releasing the stored SOx when a temperature of the emission control device is raised to a desulfurization temperature or higher and the air/fuel ratio of the exhaust gas flowing through the emission control device becomes substantially equal to a stoichiometric air/fuel ratio or rich;
• and
• a controller that:
• performs a temperature control process to control the temperature of the emission control device to be within a predetermined temperature range whose lower limit is substantially equal to or higher than the desulfurization temperature and to raise the temperature of the emission control device only when the air/fuel ratio of exhaust gas is leaner than or equal to the stoichiometric air/fuel ratio;
• and
• performs a desulfurization process to release sulfur from the emission control device by controlling the air/fuel ratio of the exhaust gas flowing through the emission control device to be substantially equal to the stoichiometric air/fuel ratio or rich when the temperature of the emission control device is within the predetermined temperature range, wherein
• the controller repeats the temperature control process and the desulfurization process when sulfur is to be released from the emission control device.