Method for purifying exhaust gas and system therefor

A system for purifying exhaust gas from an engine comprising,

• a selective reducing catalyst provided in an exhaust passage of the engine;
• an oxidation catalyst provided upstream of the selective reducing catalyst in the exhaust passage;
• an additive agent supplying device provided in the exhaust passage to supply an additive agent to the selective reducing catalyst;
• a controller configured to:
• calculate an amount of NOx that flows into the selective reducing catalyst;
• set an amount of the additive agent to be supplied to the selective reducing catalyst based on the calculated amount of the NOx that flows into the selective reducing catalyst;
• determine a degradation degree of the oxidation catalyst;
• determine whether a correction is necessary to be made to the amount of the additive agent to be supplied based on the determined degradation degree of the oxidation catalyst;
• correct the amount of the additive agent to be supplied when the correction is determined to be necessary;
• and
• control the additive agent supplying device to supply the additive agent to the selective reducing catalyst in the corrected amount if the amount of the additive agent is determined to be corrected and in an uncorrected amount if the amount of the additive agent is determined not to be corrected.