Exhaust gas purifying method and exhaust gas purifying system

Abstract

In an exhaust gas purifying system ( 1 ) for applying NOx purification by a NOx occlusion reduction type catalyst ( 42 ) and PM purification by a DPF ( 41 ) to the exhaust gas of an internal combustion engine, when it is judged that both regeneration of the DPF ( 41 ) and sulfur purge of the NOx occlusion reduction type catalyst ( 42 ) are necessary, the DPF regeneration control for raising the temperature of the DPF ( 41 ) is performed and the sulfur purge control for decreasing the oxygen concentration of the exhaust gas flowing into the NOx occlusion reduction type catalyst ( 42 ) is intermittently repeated. Thereby, it is possible to efficiently purge the sulfur accumulated in the NOx occlusion reduction type catalyst while preventing deterioration of fuel efficiency and discharge of NOx, HC, and CO into atmospheric air.

1 . An exhaust gas purifying method using an exhaust gas purifying system having a DPF and an NOx occlusion reduction type catalyst in order from the upstream side in an exhaust passage of an engine and a control means; 
 said control means comprising;    an exhaust gas component detection means to detect the oxygen concentration and NOx concentration of the exhaust gas passing through the NOx occlusion reduction type catalyst;    a DPF regeneration control means for controlling regeneration of the DPF;    a NOx catalyst regeneration control means for recovering the NOx occlusion ability of the NOx occlusion reduction type catalyst; and    a sulfur purge control means for controlling the sulfur purge of the NOx occlusion reduction type catalyst;    wherein, when it is judged that both regeneration of the DPF and sulfur purge of the NOx occlusion reduction type catalyst are necessary,    the DPF regeneration control for raising the temperature of the DPF is performed and    the sulfur purge control to decrease the oxygen concentration in the exhaust gas flowing into the NOx occlusion reduction type catalyst is intermittently repeated.    
     
     
         2 . The exhaust gas purifying method according to  claim 1 , wherein 
 the air-fuel ratio state of the exhaust gas flowing into the NOx occlusion reduction type catalyst is brought into a stoichiometric air-fuel ratio state in the sulfur purge control.    
     
     
         3 . An exhaust gas purifying system having a DPF and a NOx occlusion reduction arranged in order from the upstream side in an exhaust passage of an engine, and a control means; 
 said control means comprising;    an exhaust gas component detection means to detect the oxygen concentration and NOx concentration of the exhaust gas passing through the NOx occlusion reduction type catalyst:    a DPF regeneration control means for controlling regeneration of the DPF;    a NOx catalyst regeneration control means for recovering the NOx occlusion ability of the NOx occlusion reduction type catalyst; and    a sulfur purge control means for controlling the sulfur purge of the NOx occlusion reduction type catalyst;    wherein when it is judged that both regeneration of the DPF and sulfur purge of the NOx occlusion reduction type catalyst are necessary,    said control means performs the DPF regeneration control for raising the temperature of the DPF and executes intermittently the sulfur purge control to decrease the oxygen concentration in the exhaust gas flowing into the NOx occlusion reduction type catalyst,.    
     
     
         4 . The exhaust gas purifying system according to  claim 3 , wherein 
 the air-fuel ratio state of the exhaust gas flowing into the NOx occlusion reduction type catalyst is brought into a stoichiometric air-fuel ratio state in the sulfur purge control.