WO2013022516A1

WO2013022516A1 - Method and system for mitigating n2o output from exhaust gas systems

Info

Publication number: WO2013022516A1
Application number: PCT/US2012/041496
Authority: WO
Inventors: Brad J. Adelman; Russell P. Zukouski; Matthew Albert TYO; Dean Alan Oppermann
Original assignee: International Engine Intellectual Property Company, Llc
Priority date: 2011-08-09
Filing date: 2012-06-08
Publication date: 2013-02-14
Also published as: WO2013022517A1

Abstract

An exhaust system is disclosed that includes a selective catalytic reduction volume that is configured to receive exhaust gas from an engine. The selective catalytic reduction volume uses NH3 as a reductant to convert NOx to nitrogen and water. An NOx trap is disposed to receive a downstream flow of exhaust gas from the selective catalytic reduction volume. The NOx trap includes a NOx adsorber for storing residual NOx in the downstream flow of exhaust gas. The NHS slip in the downstream flow of exhaust gas from the selective catalytic reduction volume reacts with the adsorbed NOx and is converted to nitrogen and water.

Description

METHOD AND SYSTEM FOR MITIGATING N₂O OUTPUT

FROM EXHAUST GAS SYSTEMS

RELATED APPLICATIONS

[1] This application makes reference to, claims priority to, and claims the benefit of

United States Provisional Patent Application Serial No. 61/532,637 which was filed on

September 9, 2011, and is entitled "NITROUS OXIDE ABATEMENT IDEAS." The disclosure of the above-identified Provisional Patent Application is hereby incorporated by reference in its entirety.

BACKGROUND

[2] Emissions from engines and their impact on the environment are of increasing concern given the number of engines currently deployed. Such engines are used in electric generators, vehicle engines, and the like. With this concern comes increasing government regulations that limit the amount of various emission gases that may be exhausted to the environment. Three such undesirable emission gases are NO_x, NH₃, and N₂O.

[3] NOx gas emissions from an engine can be reduced using a selective catalytic reduction volume (SCR) disposed in-line with the exhaust exhaust pipe of the engine. The SCR volume uses ammonia to break down NO_x emissions into nitrogen and water. In vehicle applications the SCR delivers, for example, a urea solution, which is sprayed into the exhaust stream by an injection system. The urea solution is used as a precursor to generate ammonia (NH₃) on a carrier supporting a catalyst. Additionally, or in the alternative, NH₃ gas and/or NH₃ liquid may be provided as the reduction agent to the exhaust stream without an intermediate precursor.

[4] The SCR volume may be used in both transient and steady state systems. In steady state systems, a sufficient amount of NH₃ is used to provide a stoichiometric with the desired level of NO_x removal. Under these conditions, the level of reductant stored on the catalyst surface is relatively constant and, as a result, only a limited amount of NH₃ gas passes through the SCR volume and into the environment. Passage of the ammonia reductant to the outlet of the exhaust is known as "NH₃ slip." This NH₃ slip is undesirable since the NH₃ gas may be considered a pollutant.

[5] Highly transient conditions may result when the engine is used, for example, in a vehicle. SCR technologies used in many such transient systems do not seek to obtain high NO_x conversion efficiencies. Rather, the exhaust transients are managed by buffering the variances in NO_x to NH₃ stoichiometry by dosing a relatively limited amount of NH₃ into the exhaust stream compared to the amount of emitted NO_x. As a result, only a limited amount of NH₃ slip occurs.

[6] Control of NH₃ slip may also be difficult to maintain when the engine operates in a lean burn range, where the air-to-fuel ratio of the mixture provided is high compared to the typical stoichiometric ratio for engine operation. Operation in the lean burn range results in higher temperatures. The higher temperatures, in turn, cause larger amounts of NO_x emission to pass through the SCR volume.

[7] In both transient and steady state operation of such engines, the exhaust system may be unable to limit NO_x emissions to an acceptable level given the limited amount of NH₃ employed in the SCR. A reduction in the NO_x may be obtained if a larger amount of NH₃ is used in the SCR. However, when the amount of NH₃ is increased, the amount of NH₃ slip may also increase to an undesirable level.

[8] Some engine exhaust systems may include an NH₃ oxidation catalyst that is disposed downstream of the SCR volume. The NH₃ oxidation catalyst may be separate or overlapping with the rear of the SCR volume and often contains low levels of Pt. When the Pt reacts with the NH₃ slip it is possible to form N₂0 or NO_x as a byproduct, although N₂ is the desired product. N₂0 gas, however, is a known potent greenhouse gas and its emission from the exhaust is likewise undesirable. SUMMARY

[9] Apparatus described herein relate to an exhaust system for an engine. The exhaust system comprises a selective catalytic reduction volume configured to receive exhaust gas from the engine and uses NH₃ as a reductant to convert NO_x to nitrogen and water. An NO_x trap is disposed to receive a downstream flow of exhaust gas from the selective catalytic reduction volume. The NO_x trap includes an NO_x adsorber for storing residual NO_x in the downstream flow of exhaust gas. The NH₃ slip in the downstream flow of exhaust gas from the selective catalytic reduction volume reacts with the adsorbed NO_x and is converted to nitrogen and water.

[10] Further apparatus described herein relate to a method for use in an exhaust system of an engine. The method comprises providing a flow of an exhaust gas from the engine, wherein the flow of exhaust gas includes NO_x. A portion of the NO_x in the exhaust gas is reduced to nitrogen and water using NH₃ as a reductant and produces a downstream flow of exhaust gas having a residual amount of NO_x and an amount of NH₃ slip. The residual amount of NO_x in the downstream flow is trapped on an NO_x adsorber. The adsorbed NO_x is used to convert the NH₃ slip in the downstream flow to nitrogen and water.

[11] Still further apparatus herein relate to a diesel engine. The diesel engine comprises a diesel engine core that generates NO_x as an exhaust gas. A selective catalytic reduction volume is configured to receive the exhaust gas from the diesel engine core and uses NH₃ as a reductant to convert the NO_x in the exhaust gas to nitrogen and water. An NO_x trap is disposed to receive a downstream flow of exhaust gas from the selective catalytic reduction volume. The downstream flow includes residual NO_x and residual NH₃. The NO_x trap includes an NO_x adsorber for storing an amount of the residual NO_x in the downstream flow. The residual amount NH₃ reacts with the adsorbed NO_x and is converted to nitrogen and water. BRIEF DESCRIPTION OF THE DRAWINGS

[12] FIG. 1 illustrates a diesel engine having an exhaust system that includes a lean NO_x trap downstream from a selective catalytic reduction volume, where the selective catalytic reduction volume is configured to receive exhaust gas from a diesel engine core.

[13] FIG. 2 illustrates the diesel engine of FIG. 1, wherein an NH₃ slip oxidizer is disposed downstream of the lean NO_x trap.

[14] FIG. 3 illustrates the diesel engine of FIG. 2, wherein a diesel particle filter is disposed downstream of the NH₃ slip oxidizer.

[15] FIG. 4 illustrates a diesel engine having a bypass system that selectively directs a downstream flow of exhaust gas to a flow path that includes a lean NO_x trap.

[16] FIG. 5 is a flow diagram of operations that may be executed to process an exhaust gas of a diesel engine.

DETAILED DESCRIPTION

[17] FIG. 1 illustrates a diesel engine 10 having an exhaust system 15 and a diesel engine core 20. The diesel engine core 20 is configured to burn fuel which, in turn, generates a flow of exhaust gases having NO_x. The exhaust gases are provided to the exhaust system 15 through a conduit, such as a pipe 30.

[18] The exhaust system 15 includes a selective catalytic reduction volume 25 (SCR) that receives the exhaust gas from the diesel engine core 20 through pipe 30. The SCR 25 uses NH₃ as a reductant in converting NO_x to nitrogen and water. The NH₃ may be provided in a number of different forms. For example, the SCR 25 may use a urea solution which is sprayed into the exhaust stream by an injection system and then converted into NH₃ on a carrier supporting a catalyst. Additionally, or in the alternative, NH₃ gas and/or NH₃ liquid may be provided directly as the reduction agent in the exhaust stream. In each instance, NH₃ is the ultimate reductant used to convert the NO_x. [19] The SCR 25 may be configured to remove a predetermined amount of NO_x in the exhaust gases when the diesel engine core 20 operates in a steady state during base operation. While in base operation, a stoichiometric amount of NH₃ can be used by the SCR 25 to reduce the NO_x to a desirable level in which, for example, substantially all of the NO_x in the exhaust gases from the diesel engine core 20 are reduced. Even during base operation of the diesel engine core 20, the SCR 25 may be provided with an amount of NH₃ that is greater than that required to maintain the stoichiometric relationship. As a result, an amount of NH₃ that has not reacted with the NO_x is provided from the SCR 25 in a downstream flow of exhaust gas. The presence of NH₃ in the downstream flow is known as "NH₃ slip."

[20] NH₃ slip is also an issue during transient operation of the diesel engine core 20. For example, when the diesel engine 10 is used in a vehicle, any acceleration or deceleration of the vehicle results in a corresponding change in the speed of operation of the diesel engine core 20. When the speed of operation increases, the flow of exhaust gases to the exhaust system 15 likewise increases. In such instances, the SCR 25 increases the amount of NH₃ that it utilizes in order to reduce the NO_x to acceptable levels. This increased introduction of NH3, however, may result in significant NH₃ slip if too much NH₃ is employed.

[21] Similarly, when the speed of operation decreases, the flow of exhaust gases to the exhaust system 15 also decreases. In such instances, the SCR 25 decreases the amount of NH₃ that it utilizes to compensate for the fact that there is a smaller flow of NO_x in the exhaust. However, the decrease is not immediate and takes place over a period of time. During this time, excess NH₃ in the SCR 25 passes to the downstream flow as NH₃ slip.

[22] The amount of NH₃ slip ultimately exhausted from the diesel engine 10 may be minimized using the exhaust system 15 shown in FIG. 1. To this end, the exhaust system 15 includes an NO_x trap 35 that receives the downstream flow of exhaust gas, including NH₃ slip from the SCR 25. The NO_x trap 35 includes an NO_x adsorber 37 that stores residual NO_x that is present in the downstream flow of exhaust gas. The NO_x adsorber 37 may be formed, for example, from a basic metal oxide (such as BaO, K₂0, Ce0₂) or other adsorber material. [23] In operation, NO_x slip from SCR 25 is trapped onto the NO_x adsorber 37 of NO_x trap

35. The NO_x slip can be held in the NO_x trap 35 and stored over a large temperature range. This temperature range is greater than that of the temperature range associated with optimal NH₃ reaction with SCR 25 formulations. Instead of slipping past the SCR 25, the NH₃ encounters the NO_x stored in the NO_x trap 35 (usually in the form of a metal nitrate complex). This interaction results in the conversion of NH₃ and nitrate into N₂ and H₂0.

[24] FIG. 2 shows the diesel engine 10 with an alternative exhaust system 15. The alternative exhaust system 15 includes an NH₃ slip oxidizer 45 configured to receive a further downstream flow of exhaust gas from the NO_x trap 35. The NOx trap 35 may be added to oxidize residual NH₃ in the further downstream flow of gas received from the NO_x trap 35.

However, since a substantial amount of the NH₃ is already removed from the further downstream flow by the NO_x trap 35, there is only a small amount of residual NH₃ available to react with the Pt of the NH₃ slip oxidizer 45 and, as such, the amount of N₂0 is reduced when compared to other exhaust system configurations.

[25] FIG. 3 shows the diesel engine 10 with a still further alternative of exhaust system 15.

In this exhaust system, a diesel particle filter 50 is disposed downstream of the NH₃ slip oxidizer 45. The diesel particle filter 50 is designed to remove diesel particulate matter or soot from the exhaust gas of the diesel engine 10. A catalyzed diesel particulate filter, which contains Pt, may afford the same functionality as the NH₃ slip oxidizer and may allow the NH₃ slip oxidizer 45 to be removed from the system 15.

[26] FIG. 4 illustrates a diesel engine 10 having a bypass system 55. The bypass system

55 selectively directs a downstream flow of exhaust gas to a flow path 60 that includes the NO_x trap 35 or to a flow path 65 that bypasses the NO_x trap 35.

[27] In FIG. 4, the bypass system 55 directs the downstream flow of gas from the SCR 25 to either flow path 60 or flow path 65 based on the amount of NH₃ slip from SCR 25. The amount of NH₃ slip may be measured by an ammonia sensor 70, which provides an electrical signal output, either in digital or analog form, to a controller 75. The electrical signal may be either directly or indirectly correlated to the NH₃ slip. Controller 75 is configured to receive the electrical signal and actuate valves 80 and 85 to inhibit/regulate the downstream flow of gas through each of the flow paths 60 and 65.

[28] The bypass system 55 may be programmed to operate in various modes. For example, controller 75 may actuate valves 80 and 85 to direct the downstream flow of exhaust gas to the flow path 65 while inhibiting/regulating the flow through flow path 65. This mode of operation may be desirable when the NH₃ slip measured at NH₃ sensor 70 is at or below a predetermined threshold. In such instances, the NO_x trap 35 and, if used, the NH₃ slip oxidizer 45, are bypassed, which may result in a corresponding increase in their useful lives. To place the bypass system 55 in this state, the controller 75 actuates valve 80 to direct the downstream flow of exhaust gas to port 90 while inhibiting/regulating flow of the exhaust gas to port 95. Further, exhaust gas received at port 100 of valve 85 is provided to portl05 and does not exit from port 110.

[29] When the NH₃ slip measured at NH₃ sensor 70 is above the predetermined threshold, the bypass system 55 directs the downstream flow of exhaust gas to flow path 60. In this state, the downstream flow is directed to the NO_x trap 35 and, if used, through NH₃ slip oxidizer 45. To accomplish this, the controller 75 actuates valve 80 to direct the downstream flow of exhaust gas to port 95 while inhibiting/regulating the flow of the exhaust gas to port 90. Further, exhaust gas received at the port 110 of valve 85 is passed to outlet port. Valve 85 is also controlled to inhibit exhaust gas flow through port 100.

[30] Other programmable modes may provide for a controlled flow rate of the downstream exhaust gas through either flow path 60 or 65. For example, a regulated flow rate of the downstream exhaust gas may be provided to flow path 60 while inhibiting flow through flow path 65. As another example, a regulated flow rate of the downstream exhaust gas may be provided to flow path 65 while inhibiting flow through flow path 60. In each instance, the back pressure to SCR 25 may be controlled to increase its efficiency since the amount of time that the exhaust gas spends in the SCR 25 increases with an increase in the back pressure. Still further, the downstream exhaust gas may be concurrently distributed between flow paths 60 and 65 to optimize efficiency of the exhaust system 15.

[31] FIG. 5 is a flow diagram 120 of operations that may be executed to process an exhaust gas of a diesel engine. As shown, a flow of an exhaust gas having NO_x is provided at operation 125. At operation 130, at least a portion of the NO_x in the exhaust gas is reduced to nitrogen and water using NH₃ as a reductant. Also, an amount of NH₃ slip is generated during this reduction operation.

[32] At operation 135, a portion of any non-reduced NO_x is trapped using an NO_x adsorber. The adsorbed NO_x is used to reduce the NH₃ slip to nitrogen and water at operation 140. A residual amount of NH₃ slip remaining after operation 140 may optionally be oxidized at operation 145.

[33] While various examples of the methods and apparatus have been illustrated and described, it should be appreciated that the principles associated with each of the disclosed examples may be extended while still falling within the scope of the following claims.

Claims

1. An exhaust system for an engine comprising:

a selective catalytic reduction volume configured to receive exhaust gas from the engine, wherein the selective catalytic reduction volume uses NH₃ as a reductant in converting NO_x to nitrogen and water; and

an NO_x trap disposed to receive a downstream flow of exhaust gas from the selective catalytic reduction volume, wherein the NO_x trap includes an NO_x adsorber for storing residual NO_x in the downstream flow of exhaust gas, and wherein NH₃ slip in the downstream flow of exhaust gas from the selective catalytic reduction volume reacts with the adsorbed NO_x and is converted to nitrogen and water.

2. The exhaust system of claim 1, wherein the selective catalytic reduction volume is configured to convert substantially all of the NO_x during base operation of the engine.

3. The exhaust system of claim 1, further comprising a bypass system, wherein the bypass system is configured to direct the downstream flow of exhaust gas to the NO_x trap during transient operation of the engine, and wherein bypass system is further configured to inhibit flow of the downstream flow of exhaust gas to the NO_x trap during base operation of the engine.

4. The exhaust system of claim 1, further comprising an NO_x slip catalyst configured to receive a further downstream flow of exhaust gas from the NO_x trap to oxidize residual NH₃ in the further downstream flow of exhaust gas.

5. The exhaust system of claim 1, wherein the selective catalytic reduction volume and NO_x trap form a serial flow path through the exhaust system.

6. The exhaust system of claim 1, wherein the NO_x trap is disposed in parallel with a further flow path that is also connected to the downstream flow of gas from the selective catalytic reduction volume.

7. The exhaust system of claim 1, wherein the NOx adsorber comprises a metal nitrate complex.

8. A method for use in an exhaust system of an engine, the method comprising:

providing a flow of an exhaust gas from the engine, wherein the exhaust gas includes NOx;

reducing a portion of the NO_x in the exhaust gas to nitrogen and water using NH₃ as a reductant and produces a downstream flow of exhaust gas having a residual amount of NO_x and an amount of NH₃ slip;

trapping the residual amount of NO_x in the downstream flow of exhaust gas on an NOx adsorber; and

using the adsorbed NO_x to convert the NH₃ slip in the downstream flow of exhaust gas to nitrogen and water.

9. The method of claim 8, wherein substantially all of the NO_x in the exhaust gas is reduced during base operation of the engine.

10. The method of claim 8, further comprising oxidizing an amount of NH₃ in the downstream flow of the exhaust gas after using the adsorbed NO_x to convert the NH₃ slip in the downstream flow to nitrogen and water.

11. The method of claim 8, further comprising providing the downstream flow of exhaust gas to the NO_x adsorber during base operation of the engine, and bypassing the downstream flow of exhaust gas away from the NO_x adsorber during transient operation of the engine.

12. A diesel engine system comprising:

a diesel engine core generating NO_x as an exhaust gas;

a selective catalytic reduction volume configured to receive the exhaust gas from the diesel engine core, wherein the selective catalytic reduction volume uses NH₃ as a reductant to convert the NO_x in the exhaust gas to nitrogen and water; and

an NO_x trap disposed to receive a downstream flow of exhaust gas from the selective catalytic reduction volume, wherein the downstream flow includes residual NO_x and residual NH₃, wherein the NO_x trap includes an NO_x adsorber for storing an amount of the residual NO_x in the downstream flow, and wherein the residual amount of NH₃ reacts with the adsorbed NO_x and is converted to nitrogen and water.

13. The diesel engine of claim 12, wherein the selective catalytic reduction volume is configured to convert substantially all of the NO_x during base operation of the engine.

14. The diesel engine of claim 12, further comprising a bypass system, wherein the bypass system is configured to direct the downstream flow to the NO_x trap during transient operation of the engine, and to inhibit provision of the downstream flow to the NO_x trap during base operation of the engine.

15. The diesel engine of claim 12, wherein a further downstream flow of exhaust gas is provided at an output of the NO_x trap, and wherein the diesel engine further comprises an NO_x slip catalyst configured to receive the further downstream flow of exhaust gas.