METHOD OF EXHAUST GAS PURIFICATION AND EXHAUST GAS PURIFICATION SYSTEM

摘要

In exhaust gas purification system (10) including catalyst unit (50) carrying a NOx occlusion-reduction type catalyst, a first-stage rich control having a target air-fuel ratio lower than theoretical air-fuel ratio is conducted through addition of the amount of reducing agent meeting the amount of oxygen emitted in the initial stage of regeneration control. In the first-stage rich control, the completion of oxygen emission is judged on the basis of the oxygen concentration on the downstream side of catalyst unit (50). Upon determination of the completion of oxygen emission, a later-stage rich control close to the theoretical air-fuel ratio with the target air-fuel ratio increased over that of the first-stage rich control is carried out to thereby accomplish regeneration of the catalyst unit (50). As a result, during the regeneration control of the catalyst unit (50), there can be prevented not only any outflow of unpurified NOx occurring in the initial stage of regeneration but also any outflow of virgin reducing agents, such as HC and CO, occurring in the later stage of regeneration.