Method and apparatus for selective catalytic reduction of NOx

1. An apparatus, comprising:

parallel first and second exhaust gas passageways,

a urea injection system configured to inject urea into the first exhaust gas passageway,

a hydrolysis catalyst positioned in the first exhaust gas passageway to generate ammonia at least partially from urea injected into the first exhaust gas passageway by the urea injection system, and

an SCR catalyst downstream from the first and second exhaust gas passageways to reduce NOx with the generated ammonia.

2. The apparatus of claim 1 , further comprising an exhaust gas valve operable to control flow of exhaust gas between the first and second exhaust gas passageways.

3. The apparatus of claim 2 , further comprising a controller electrically coupled to the valve to control operation of the valve.

4. The apparatus of claim 3 , further comprising an engine and at least one sensor, wherein the controller is configured to control operation of at least one of the urea injection system and the valve in response to at least one of an engine map associated with the engine and operation of the at least one sensor.

5. The apparatus of claim 1 , further comprising a mixer positioned in the first exhaust gas passageway fluidly between the urea injection system and the hydrolysis catalyst.

6. The apparatus of claim 1 , wherein each of the first exhaust gas passageway and the hydrolysis catalyst is annular so as to surround the second exhaust gas passageway.

7. The apparatus of claim 1 , further comprising a first conduit and a second conduit, wherein:

the first and second conduits are secured to one another at upstream and downstream locations so as to extend outside one another between the upstream and downstream locations,

the first conduit defines the first exhaust gas passageway in which the hydrolysis catalyst is positioned, and

the second conduit defines the second exhaust gas passageway.

8. The apparatus of claim 1 , wherein:

the urea injection system is not fluidly coupled to the second exhaust gas passageway, and

there is no hydrolysis catalysis in the second exhaust gas passageway.

9. The apparatus of claim 1 , further comprising an engine, an exhaust gas supply passageway, and a downstream exhaust gas passageway, wherein:

the exhaust gas supply passageway is positioned fluidly between the engine and the first and second exhaust gas passageways to supply exhaust gas of the engine to the first and second exhaust gas passageways, and

the downstream exhaust gas passageway is fluidly coupled to and downstream from the first and second exhaust gas passageways and contains the SCR catalyst.

10. An apparatus, comprising:

an exhaust gas supply passageway dividing into first and second exhaust gas passageways to supply exhaust gas to the first and second exhaust gas passageways,

a urea injection system configured to inject urea into the first exhaust gas passageway,

a hydrolysis catalyst positioned in the first exhaust gas passageway to generate ammonia from urea injected into the first exhaust gas passageway by the urea injection system and from water present in the exhaust gas, and

an SCR catalyst that is positioned in a downstream exhaust gas passageway fluidly coupled to and downstream from the first exhaust gas passageway and that is configured to reduce NOx with ammonia received from the first exhaust gas passageway.

11. The apparatus of claim 10 , wherein the first and second exhaust gas passageways are parallel so as to be fluidly coupled to and positioned fluidly between the exhaust gas supply passageway and the downstream exhaust gas passageway.

12. The apparatus of claim 10 , including an exhaust gas valve operable to control flow of exhaust gas between the first and second exhaust gas passageways.

13. The apparatus of claim 10 , including a mixer positioned in the first exhaust gas passageway fluidly between the urea injection system and the hydrolysis catalyst.

14. The apparatus of claim 10 , wherein each of the first exhaust gas passageway and the hydrolysis catalyst is annular so as to surround the second exhaust gas passageway.

15. The apparatus of claim 14 , including an exhaust gas valve operable to control flow of exhaust gas between the first and second exhaust gas passageways, and wherein the exhaust gas valve is positioned within the second exhaust gas passageway.

16. The apparatus of claim 15 , including a housing having an inlet port that receives exhaust gas from the exhaust gas supply passageway and an outlet portion that discharges exhaust gas and ammonia to the downstream exhaust gas passageway, and including an inner tube that defines the second exhaust gas passageway with the first exhaust gas passageway being defined between an outer surface of the inner tube and an inner surface of the housing.

17. The apparatus of claim 16 , wherein the urea injection system includes a urea dispenser secured to the housing and the inner tube.

18. The apparatus of claim 10 , further comprising a first conduit and a second conduit, wherein:

the first and second conduits are secured to one another at upstream and downstream locations so as to extend outside one another between the upstream and downstream locations,

the first conduit defines the first exhaust gas passageway in which the hydrolysis catalyst is positioned, and

the second conduit defines the second exhaust gas passageway.

19. The apparatus of claim 10 , wherein the first and second exhaust gas passageways recombine at a downstream location and then flow directly into the SCR catalyst.

20. The apparatus of claim 1 , wherein the first and second exhaust gas passageways recombine at a downstream location and then flow directly into the SCR catalyst.