Exhaust emission purifying apparatus

Abstract

The present invention provides an exhaust emission purifying apparatus. In the exhaust emission purifying apparatus, while an ammonia adsorbent is coated on a predetermined length portion on the exhaust inflow side of a carrier of a catalytic converter which reduces and purifies NOx in exhaust gas, a NOx reduction catalyst is coated on the remaining portion on the exhaust outflow side of the carrier of the catalytic converter. On the other hand, a first layer containing the ammonia adsorbent and the NOx reduction catalyst in a mixed state and a second layer containing a hydrolysis catalyst are coated in that order on a carrier surface of the catalytic converter. In a transition of an engine operation state, NOx is reduced and purified with the use of ammonia adsorbed to the ammonia adsorbent.

Description

[0001]This application is a continuation of PCT / JP2009 / 066134, filed on Sep. 16, 2009.BACKGROUND OF THE INVENTION[0002]1. Field of the Invention[0003]The present invention relates to an exhaust emission purifying apparatus which reduces and purifies nitrogen oxide (NOx) in exhaust gas with the use of ammonia generated from a urea aqueous solution by hydrolysis.[0004]2. Description of Related Art[0005]As a catalytic purification system that removes NOx contained in exhaust gas of an engine, there has been proposed an exhaust emission purifying apparatus described in Japanese Laid-open (Kokai) Patent Application Publication No. 2000-27627. In the exhaust emission purifying apparatus, a urea aqueous solution corresponding to an engine operation state is injection-supplied to the exhaust upstream side of a NOx reduction catalytic converter arranged in an exhaust pipe, and ammonia that is a reducing agent is generated from the urea aqueous solution by hydrolysis using exhaust heat and wa...

AI Technical Summary

Benefits of technology

[0007]Thus, in view of the above related art problems, an object of the present technique is to provide an exhaust emission purifying apparatus which devises a coating structure of an ammonia adsorbent, a hydrolysis catalyst, and a NOx reduction catalyst coated on a carrier of the NOx reduction catalytic converter to thereby enhance the NOx purification efficiency.

[0009]According to the first aspect of the present technique, when an engine performs a steady operation, ammonia generated from the urea aqueous solution by hydrolysis is adsorbed to the ammonia adsorbent coated on the exhaust inflow side of the carrier of the catalytic converter. In a transition of the engine operation, when the generation of ammonia may not follow a rapid SV change, ammonia adsorbed to the ammonia adsorbent is released to be supplied to the NOx reduction catalyst coated on the exhaust outflow side of the carrier. Thus, even if hydrolysis for generating ammonia from the urea aqueous solution is not satisfactorily carried out, the reduction reaction between NOx and ammonia occurs in the NOx reduction catalyst, and especially a NOx purification efficiency in the transition of the engine operation state can be enhanced.

[0010]According to the second aspect of the present technique, among the urea aqueous solution injection-supplied from the injection nozzle, the urea aqueous solution that is not converted to ammonia by hydrolysis is in contact with the hydrolysis catalyst to be converted to ammonia in the second layer coated on a carrier surface of the catalytic converter. When the engine performs the steady operation, ammonia is adsorbed to the ammonia adsorbent contained in the first layer. In the transition of the engine operation state, when the generation of ammonia may not follow a rapid SV change, ammonia adsorbed to the ammonia adsorbent is released to be supplied to the NOx reduction catalyst contained in the same first layer. Thus, the NOx purification efficiency can be enhanced in the transition of the engine operation state.

Problems solved by technology

However, even if the hydrolysis function or the ammonia adsorption function is added to the NOx reduction catalytic converter, it may not be concluded that hydrolysis for generating ammonia from a urea aqueous solution is satisfactorily carried out in a wide range of the engine operation state, and it has been a fact that the NOx purification efficiency that would be expected may not be obtained.

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