System and Method of LIquid Injection into a Gas Stream

Abstract

A system for use in injection of a liquid reductant into an exhaust gas (20) and for evaporating the liquid reductant includes an exhaust pipe (12) having an interior area (15). Exhaust gas (20) flows through the interior area (15) of the exhaust pipe (12), the flow of exhaust gas being at an elevated temperature from the ambient. A deflection plate (118) has a non-planar shape and is disposed within the interior area (15) of the exhaust pipe (12). An injector (14) injects the liquid reductant into the exhaust pipe (12) and directs the liquid reductant at the deflection plate (118).

Description

FIELD OF THE INVENTION

The present invention relates generally to a system and method of injecting an emission liquid reductant into a gas stream, and more particularly, to a system and method for injecting an emission reductant, such as urea, into a gas stream of an aftertreatment system, such as an SCR system.

BACKGROUND OF THE INVENTION

Typically, urea selective catalytic reduction systems (urea SCR systems) are used to reduce oxides of Nitrogen (NO_x) from engines. Urea SCR systems rely on injection of 32.5% aqueous urea solution into the exhaust line of a vehicle upstream of an SCR catalyst. In the SCR catalyst, the NO_x is reduced by the ammonia, and the emission from the catalyst is N₂, H₂O and CO₂.

At the moment of injection, the urea solution temperature is typically close to ambient, and is preferably less than 60° C. The SCR reaction requires gaseous ammonia. To produce the gaseous ammonia, the injected urea solution must be heated, preferably to over 150° C., to evaporate the water and decompose the remaining urea into ammonia and isocyanic acid. If the evaporation and the decomposition are not complete, the SCR catalyst performance is reduced due to insufficient availability of reductant.

For efficient performance of the SCR catalyst, the urea should be injected into the engine exhaust gas, vaporized and decomposed before the inlet of the catalyst. The urea is sprayed directly into the exhaust pipe, but is corrosive when the urea contacts the metal of the exhaust pipe. Solid deposits of urea can be formed in the internal surface of the exhaust pipe, particularly when the pipe is cold. The corrosive urea can also cause rust and complete failure of the SCR system.

To prevent the direct contact of the urea with the internal surface of the exhaust pipe, a flat, metal plate is installed inside the pipe to deflect the urea spray. When the urea is sprayed directly at the metal plate, the urea is deflected down towards the bottom of the pipe. The longer, deflected path of the urea spray provides increased time within which the urea can vaporize and decompose. However, some urea remains that is not vaporized and decomposed. While the deflection plate impedes the corrosion of the pipe slightly, the urea that deflects off the metal plate to the sides of the exhaust pipe progressively damages the exhaust pipe, progressively plugging the pipe with solid deposits, and eventually leading to plugging and failure of the exhaust pipe.

SUMMARY OF THE INVENTION

A system for use in injection of a liquid reductant into an exhaust gas and for evaporating the liquid reductant includes an exhaust pipe having an interior area. Exhaust gas flows through the interior area of the exhaust pipe, the flow of exhaust gas being at an elevated temperature from the ambient. A deflection plate has a non-planar shape and is disposed within the interior of the exhaust pipe. An injector injects the liquid reductant into the exhaust pipe and directs the liquid reductant at the deflection plate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of a prior art plate in an exhaust pipe of an SCR system.

FIG. 2 is a schematic of a deflection plate in the exhaust pipe of the SCR system in accordance with the invention.

DESCRIPTION OF A PREFERRED EMBODIMENT

While the following description will describe one application of the present system and method, it should be appreciated that the present system and method is applicable to any liquid reductant into any gas stream. The following will describe the present system and method with respect to injecting urea, a liquid reductant, into the gas stream of an SCR system. When an engine combusts diesel, nitrogen oxides form in the flame, and released with the exhaust gas. Nitrogen oxides, No_x, are a pollutant that are reduced in SCR systems by ammonia, NH₃, resulting in the emission of less harmful nitrogen, N₂, water, H₂O, and carbon dioxide, CO₂.

Ammonia is formed when urea decomposes as it is sprayed into a hot exhaust mixture in the exhaust pipe. The urea SCR systems rely on injection of 32.5% aqueous urea solution into the exhaust line of a vehicle upstream of an SCR catalyst, where the temperature of the exhaust gases is preferably in the range of about 130 to 700° C., with the minimum limit of about 130 to 200° C., and more preferably at least about 150° C. for the urea decomposition to occur. If the urea solution is not evaporated and decomposed soon after leaving the injection nozzle, the urea will hit the interior surface of the exhaust pipe. Since the pipe is usually colder than the exhaust gas, the urea will not decompose, and upon evaporation of water, will form a solid deposit on the interior surface of the exhaust pipe. Solid urea deposition can decrease the flow area of the exhaust pipe, resulting in an increased pressure drop and higher exhaust gas velocity in the pipe, which can in turn, result in urea deposition at the downstream catalyst.

Referring to FIG. 1, an SCR system 10 has an exhaust pipe 12 and an injector 14 mounted at the exhaust pipe such that the sprayed urea is in fluid communication with the interior area 15 of the exhaust pipe. Inside the exhaust pipe 12, and spaced from an interior surface 16 of the exhaust pipe, is a planar, metal plate 18.

Inside the exhaust gas pipe 12, exhaust gases 20 flow in the direction from the inlet 22 to the outlet 24. Downstream of the outlet 24 is a catalyst (not shown). The temperature of the exhaust gases is preferably about 150° C. to enable vaporization of the aqueous urea solution.

When the injector 14 sprays the urea solution into the exhaust pipe 12, an incident spray 26 hits the metal plate 18 resulting in a reflected spray 28. If the urea spray 28 is not fully decomposed within a short time, the urea will remain aqueous and will accumulate on the interior surface 16 of the exhaust pipe 12. The accumulated urea 30 will slowly vaporize by the heat of the exhaust gas 20, leaving rust and urea deposits that will progressively damage the exhaust pipe 12.

Additionally, there is another physical phenomenon that occurs when the urea solution spray 26 hits the plate 18. Under certain conditions (relatively low temperature and flow rate of the exhaust gas, and high urea solution flow rate), the spray 26 cools down the plate 18 so that a liquid film is formed on the front surface of the plate. Due to the force of gravity, the liquid film slowly flows downwards, eventually dropping to the bottom of the pipe. This adds to the formation of solid deposits on the bottom surface of the pipe 12.

Referring now to FIG. 2, an SCR system 110 has an exhaust pipe 12 and an injector 14 mounted at the exhaust pipe such that the sprayed aqueous urea solution is in fluid communication with an interior area 15 of the exhaust pipe. Inside the exhaust pipe 12, and spaced from an interior surface 16, is a deflection plate 118. Preferably, the deflection plate 118 is made of a corrosion-resistant material. The deflection plate 118 redirects the spray of urea from the injector 14 in a direction generally parallel with the exhaust pipe 12 to minimize contact of the sprayed urea with the interior surface 16 of the exhaust pipe.

Inside the exhaust gas pipe 12, exhaust gases 20 flow in the direction from the inlet 22 to the outlet 24 (as seen in FIG. 1). Downstream of the outlet 24 is a catalyst (not shown). The temperature of the exhaust gas flow 20 is about at least 150° C. to enable vaporization of the aqueous urea solution.

When the injector 14 sprays the urea solution into the exhaust pipe 12, an incident spray 126 hits the deflector plate 118 resulting in a reflected spray 128. The deflector plate 118 is preferably concave with respect to the direction of flow of exhaust gas, and more preferably, the deflector plate 118 has a parabolic shape.

When the incident urea spray 126 hits the deflector plate 118, the reflected urea spray 128 is directed away from the interior surface 16 of the exhaust pipe 12. Preferably, the reflected spay 128 is directed upstream of the exhaust gas flow 20, generally parallel with the exhaust pipe 12.

The deflector plate 118 is preferably located generally centrally in the exhaust pipe 12, with the spray plume from the injector 14 generally centered on the center of the deflector plate, however other configurations are contemplated. Further, the deflector plate 118 is preferably sized and arranged to permit adequate exhaust flow around the deflector plate and within the interior surface 16 of the exhaust pipe 12.

With the urea spray being redirected away from the internal surface 16 of the exhaust pipe 12, the urea is provided with a longer residence time in the exhaust gas flow 20. The result of the longer residence time in the exhaust gas flow 20 is that there is improved evaporation of the urea, improved efficiency of the SCR system 110, reduced solid urea buildup at the internal surface 16, and reduced corrosion of the exhaust pipe 12. In contrast to the prior art plate 18, the curved design of the deflector plate 118 creates a tangential flow that forces the liquid film to move upwards, thus counterbalancing gravity and minimizing or preventing the dripping.

The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes that come within the meaning and range of equivalency of the claims are to be embraced within their scope.

Claims

1. A method for injection of a liquid reductant into an exhaust gas and for evaporating the liquid reductant, the method comprising:

providing an exhaust pipe having an interior area;

providing a flow of exhaust gas through the interior area of the exhaust pipe, the flow of exhaust gas having an elevated temperature compared to an ambient;

disposing a deflection plate having a non-planar shape within the interior area of the exhaust pipe and disposed generally transversely to the direction of exhaust gas flow; and

injecting the liquid reductant into the exhaust pipe and directing the injected liquid reductant at the deflection plate.

2. The method of claim 1 wherein the liquid reductant is injected upstream of the deflection plate.

3. The method of claim 1 wherein the injected liquid reductant is deflected off of the deflection plate and has a path that is generally parallel to the exhaust pipe.

4. The method of claim 1 wherein the deflection plate has a generally concave shape with respect to the flow of exhaust gas.

5. The method of claim 1 wherein the deflection plate has a generally parabolic shape.

6. The method of claim 1 wherein the exhaust gas is an exhaust gas from combustion of a diesel engine.

7. The method of claim 1 wherein the deflection plate is disposed upstream of a catalyst.

8. A system for use in injection of a liquid reductant into an exhaust gas and for evaporating the liquid reductant, the system comprising:

an exhaust pipe having an interior area;

a flow of exhaust gas through the interior area of the exhaust pipe, the flow of exhaust gas having an elevated temperature compared to an ambient;

a deflection plate having a non-planar shape disposed within the interior area of the exhaust pipe and disposed generally transversely to the direction of exhaust gas flow; and

an injector for injecting the liquid reductant into the exhaust pipe and directing the liquid reductant at the deflection plate.

9. The system of claim 8 wherein the liquid reductant is injected upstream of the deflection plate.

10. The system of claim 8 wherein the injected liquid reductant is deflected off of the deflection plate and has a path that is generally parallel to the exhaust pipe.

11. The system of claim 8 wherein the deflection plate has a generally concave shape with respect to the flow of exhaust gas.

12. The system of claim 8 wherein the deflection plate has a generally parabolic shape.

13. The system of claim 8 wherein the exhaust gas is an exhaust gas from combustion of a diesel engine.

14. The system of claim 8 wherein the deflection plate is disposed upstream of a catalyst.

15. A system for use in injection of a liquid reductant into an exhaust gas and for evaporating the liquid reductant, the system comprising:

an exhaust pipe having an interior area;

a flow of exhaust gas through the interior area of the exhaust pipe, the flow of exhaust gas having an elevated temperature from an ambient;

a deflection plate having a parabolic shape disposed within the interior area of the exhaust pipe; and

an injector for injecting the liquid reductant into the exhaust pipe and directing the liquid reductant at the deflection plate.

16. The system of claim 15 wherein the exhaust pipe is generally cylindrical.

17. The system of claim 16 wherein the deflection plate is generally concentrically mounted in the exhaust pipe.

18. The system of claim 15 wherein the deflection plate deflects a reflected liquid reductant spray generally parallel to the exhaust pipe.

19. The system of claim 18 wherein the reflected liquid reductant spray does not contact interior surfaces of the exhaust pipe.

20. The system of claim 18 wherein the deflection plate is made of a corrosion-resistant material.