Patent ID: 8176731

Claim:
Exhaust gas aftertreatment system for reducing nitrogen oxides and particulates in a stream of exhaust gas from an internal combustion engines operated with a lean mixture, the system comprising: an oxidation catalyst arranged in a main stream of exhaust gas, the oxidation catalyst converting at least a portion of the nitric oxide present in the exhaust gas main stream to nitrogen dioxide, a reservoir containing a reducing agent comprising an aqueous urea solution; a metering device for metering the reducing agent into an exhaust gas split stream at a supply point parallel to the oxidation catalyst, wherein the exhaust gas split stream is split from the exhaust gas main stream upstream of the oxidation catalyst and returned to the exhaust gas main stream downstream of the oxidation catalyst, the reducing agent splitting off ammonia; a thermolysis catalyst located in the exhaust gas split stream downstream of the supply point of the reducing agent, the thermolysis catalyst reacting with the urea to form isocyanic acid and vaporizing the water component of the aqueous urea solution at an exhaust gas temperature above 135° C., the thermolysis catalyst being thermally stable at exhaust gas temperatures occurring near the engine; a hydrolysis catalyst located in the exhaust gas stream downstream of the return of the exhaust gas partial stream into the main stream of the exhaust gas, wherein the hydrolysis catalyst converts the isocyanic acid to ammonia and carbon dioxide with the aid of the water vapor formed in the thermolysis catalyst; a particle separator located in the exhaust gas stream downstream of the hydrolysis catalyst, the separator reacting with soot particles to form carbon monoxide, carbon dioxide, nitrogen, and nitric oxide by means of the nitrogen dioxideformed by the oxidation catalyst; and an SCR catalyst located downstream of the particle separator reducing the nitrogen oxides contained in the exhaust gas stream by selective catalytic reduction to nitrogen and water vapor using the ammonia formed by the hydrolysis catalyst.