Aftertreatment System Using LNT and SCR

Abstract

A multiple catalyst system for the treatment of exhaust gases generated by a combustion engine in an after treatment system. The system includes a substrate and a plurality of catalysts. The plurality of catalysts may provide a plurality of catalyst sections. A variety of different catalysts, as well as a variety of different combinations of catalysts, may be employed. Catalysts may include both catalysts that assist in the removal of nitrogen oxides from the exhaust gas during cold start operation of the engine, as well as catalysts that are generally more effective in removing nitrogen oxides after the temperature of the after treatment system and/or exhaust gas is elevated to normal operating temperatures.

Description

BACKGROUND

Combustion engines may employ emission controls or systems that are configured to reduce the amount of nitrogen oxides (NO_x), carbon dioxide (CO₂), and/or iron (Fe), among others, which are present in the engine's exhaust gas. For example, one aspect of controlling such emissions may include the use of a Selective Catalytic Reduction (SCR) system. The SCR typically uses a catalyst and a reductant to convert NO in the exhaust gas into at least nitrogen gas and water. The reductant may be a liquid or gas, such as, for example, urea, anhydrous ammonia, or aqueous ammonia, among others.

In some systems, the ammonia reductant is molecularly bounded to a solid host salt that is placed inside a metallic vessel, which may be referred to as a main unit. Through an exothermic reaction, the ammonia reductant is released from the host salt in a gaseous state. Initiation of such an exothermic reaction may be provided by the flow of hot engine coolant, which may circulate through a heating mantle surrounding the main unit. Moreover, in such systems, engine heat transferred to coolant through conduction may provide heat that aids the exothermic reaction that releases the ammonia reductant from the solid host salt.

Such SCR systems may also be used with an ammonia oxidizing catalyst (AMO_x). The AMO_x system is typically configured to prevent ammonia (NH₃) that had been injected into the exhaust gas, but was not used by the SCR system in the conversion of NO_x, from slipping out of the after treatment system. For example, the AMO_x system may include a catalyst, such as, for example, a zeolite-based and alumina-supported metal or metal oxide catalyst, that converts at least a portion of the ammonia remaining in the exhaust gas into nitrogen gas (N₂) and water (H₂O). However, in at least some systems, the effectiveness of the AMO_x system may be impacted by the temperature of the exhaust gas, and more specifically, whether the exhaust gas has been heated to temperatures that promote the ability of the AMO_x system to convert the ammonia into nitrogen gas and water.

For example, SCR systems are not typically effective in controlling NO_x emissions at temperatures below 200° Celsius, which may present issues with compliance with emissions regulations. Additionally, the dosing capability of the SCR system may require exhaust gas temperatures to be above 190° Celsius so as to prevent reductants injected into the exhaust gas from being deposited on the SCR system. Therefore, after treatment systems that rely on an SCR system alone for the removal of NO_x generally require a relatively quick elevation of exhaust gas temperature in order to be effective.

To attain the elevated temperatures needed for the SCR system, engines used with after treatment systems having only SCR systems for the treatment of NO may operate in a warm up mode in which the engine is running in richer condition with more fuel consumption so as to generate heat necessary for the effective operation of the SCR system. However, higher fuel consumption is directly related to an increase in CO₂ production, and may also increase the fuel consumption rate of engine.

Yet, with SCR systems, there may be a delay before the engine coolant reaches temperatures needed for the engine coolant to aid in the above-discussed exothermic reaction. For example, a cold internal combustion engine may require sufficient operation time following cold start-up before the engine coolant is elevated to sufficient temperatures (e.g. greater than 200° Celsius) to aid in this exothermic reaction. This delay in time may be further extended due to the temperature of the surrounding environment, such as, for example, during extreme cold weather conditions. Another type of catalyst based system for reducing the amount of NO present in the engine's exhaust gas is a lean NO trap (LNT). The LNT is typically configured to remove NO from the exhaust gas by trapping or absorbing the NO_x, such as, for example, through the use of zeolite. The ability of the LNT to trap NO typically is not dependent on the temperature of the exhaust gas being generated by the operation of the associated engine. Thus, the LNT may be at least partially effective during cold engine start-up conditions and/or during operation of the engine in relatively cold environments.

BRIEF SUMMARY

According to certain embodiments, a multiple catalyst system is provided for the treatment of exhaust gases by after treatment system. The system includes a common substrate and a plurality of catalysts. The plurality of catalysts may provide a plurality of catalyst sections.

Additionally, according to certain embodiments, a multiple catalyst system is provided for the treatment of exhaust gas in an after treatment system. The multiple catalyst system includes a first catalyst applied to a portion of a substrate, the first catalyst including a lean NO_x trap catalyst. Further, a second catalyst is applied to a portion of the substrate, the second catalyst including a selective catalytic reduction catalyst.

Embodiments also provide a multiple catalyst system for an exhaust gas after treatment system that includes a common substrate having a plurality of catalyst sections. Each of the plurality of catalyst sections includes at least one catalyst formulated to treat exhaust gas generated by a combustion engine.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 illustrates a multiple catalyst system that includes a single common substrate having multiple catalysts.

FIG. 2 illustrates a multiple catalyst system that includes substrate portions that are joined together and which have multiple catalysts.

DETAILED DESCRIPTION

FIG. 1 illustrates a multiple catalyst system 10 that includes a single common substrate 18 having multiple catalysts 12, 14, 16, and which are housed in a housing 20. The housing 20 may be operably connected to an exhaust gas treatment system. Additionally, the housing 20 may have a first end 28 that is operably connected to a pipe or line, such as by a clamp or press fit, among others, that allows exhaust gas to be delivered to the interior 32 of the housing 20. The second end 30 may also be operably connected to the exhaust gas treatment system so that exhaust gas may exit the interior 32 and continue flowing to other components of the exhaust treatment system, such as through piping or lines that are operably connected to the second end 30.

At least a portion of the catalysts 12, 14, 16 used with the substrate 18 may be different than other catalysts 12, 14, 16 used with the substrate 18. The catalysts 12, 14, 16 may provide, either alone or in combination, one or more catalyst sections 20, 22, 24 on the substrate 18. For example, referencing FIG. 1, a first catalyst 12 applied to a portion of the substrate 18 provides a first catalyst section 24, while a second catalyst 14 applied to at least another portion the substrate 18 provides a second catalyst section 24, and a third catalyst 16 applied to at least another portion of the substrate 18 provides a third catalyst section 26. The catalysts may be a catalyst and/or a catalyst washcoat.

The catalysts 12, 14, 16, and their associated catalyst sections 22, 24, 26, may be arranged on portions of the substrate 18 such that the catalyst sections 22, 24, 26 abut against, overlap, and/or are spaced apart from the adjacent catalyst section(s) 22, 24, 26. For example, for at least purposes of illustration, the three catalyst sections 22, 24, 26 shown in FIG. 1 are in an abutting arrangement on the substrate 18. However, application of the catalysts 12, 14, 16 to the substrate 18 may result in at least some degree of overlapping of at least some of the catalysts 12, 14, 16, and thus overlapping of the associated catalyst sections 22, 24, 26.

According to certain embodiments, the catalysts 12, 14, 16 may be arranged in a particular order on the substrate 18 so that exhaust gas from the operation of an internal combustion engine flows over the catalysts 12, 14, 16 in a particular order. For example, the arrangement, order, and selection of catalysts 12, 14, 16, may be suitable for after treatment of the exhaust gases during different types of engine operating conditions or periods of operation. Accordingly, a variety of different catalyst formulations and combinations of catalyst may be applied to the substrate 18. For example, the catalysts 12, 14, 16 may be selected for after treatment of NO_x in the exhaust gas by at least some of the catalysts 12, 14, 16 during cold starting conditions, such as when engine exhaust gas temperatures are below approximately 200° Celsius, as well as treatment of the exhaust gases by the same and/or different catalysts 12, 14, 16 on the substrate 18 during normal operating conditions, such as when the temperature of the exhaust gas is above 250° Celsius. For example, the embodiment illustrated in FIG. 1 provides a first catalyst section 22 that that has an LNT catalyst as the first catalyst 12, an SCR catalyst as the second catalyst 14 of the second catalyst section 24, and a third catalyst 16 that is an AMO_x catalyst for the third catalyst section 26.

According to such embodiments, during cold starts, the temperature of the exhaust gas and/or temperature of the after treatment system is typically relatively low. Such low temperature conditions may cause SCR catalysts to be generally less effective than when operating under normal, elevated exhaust gas temperature conditions. However, under such low temperature operating conditions, an LNT catalyst may still be effective in removing NO_x from the exhaust gas to meet a desired maximum level of NO_x reduction. Accordingly, the system 10 may be arranged such that the LNT catalyst is positioned on the substrate 18 upstream of an SCR catalyst, such as, for example, the LNT catalyst being the first catalyst 12 shown in FIG. 1 to be exposed to exhaust gas delivered through the first end 28 of the housing, while the SCR catalyst is the second catalyst 14. According to such an arrangement, the SCR catalyst may be able to remove at least a portion of the NO_x in the exhaust gas that was not removed by the LNT catalyst. Therefore, according to certain embodiments, the LNT catalyst and the SCR catalyst are sized and proportioned to provide, together, sufficient NO_x reduction during cold start conditions. Further, the quantity of NO_x removed by the SCR catalyst may increase as the temperature of the exhaust gas and/or the temperature of the after treatment system increases.

According to such embodiments, as the temperature of the exhaust gas and/or after treatment system increases, NO_x that had been stored by the LNT catalyst for the first catalyst 12 may begin to be released from the first catalyst 12. However, typically, when such temperature increases are attained, the SCR catalyst, such as the second catalyst 14 in the illustrated embodiment, has warmed up sufficiently that the SCR catalyst is able to provide the requisite NO_x reduction to reduce both the NO_x being released from the LNT catalyst and the NO_x that is present in the exhaust gas that is passing by the second catalyst 14.

As previously discussed, generally, SCR systems are generally not effective in NO_x conversion until exhaust gas temperatures and/or temperatures in the interior 32 of the housing are above 250° Celsius. However, the use of one or more substrates that contain both a catalyst that is effect at treating NO_x in exhaust gases at cold temperatures, such as an LNT catalyst, and a catalyst that is effective in converting NO_x at normal operating temperatures, allows the system 10 to effectively remove and/or convert NO_x in exhaust gases at both cold and normal operating temperatures. As an AMO_x catalyst may be used to remove ammonia (NH₃) that had been injected into the exhaust gas but was not used by the SCR catalyst, the AMO_x catalyst may be positioned downstream of the SCR catalyst on the substrate 18, such as the third catalyst 16 being the AMO_x catalyst. Additionally, the AMO_x catalyst may be configured to reduce the ammonia passing by the AMO_x catalyst to sufficiently low levels at all expected operating temperatures.

FIG. 2 illustrates an embodiment of a multiple catalyst system 50 that includes a substrate 52 having first and second substrate portions 54, 56 that are housed within an interior 64 of a housing 58. Similar to the housing 20 discussed above with respect to FIG. 1, the housing 58 shown in FIG. 2 may also include first and second ends 60, 62 that are operably connected to an exhaust gas after treatment system.

According to certain embodiments, the first and second substrate portions 54, 56 may be part of a common, single substrate 52. According to other embodiments, the first and second substrate portions 54, 56 may be from separate substrates that are either physically adjoined together after being coated with one or more catalysts or positioned in relative close proximity to each other, such as, for example, in an abutting, non-abutting, or overlapping orientation.

According to certain embodiments, the first substrate portion 54 includes a first catalyst section 66 that contains one or more catalysts, such as, for example, first and second catalysts 68, 70. In the illustrated embodiment, the first catalyst 68 is shown as being an LNT catalyst, while the second catalyst 70 is shown as being an SCR catalyst. However, the first catalyst section 66 may include a variety of different catalysts and/or combinations of catalysts. Similar to the catalysts 14, 16, 18 discussed above with respect to FIG. 1, the first and second catalysts 68, 70 shown in FIG. 2 may also be positioned in the first substrate portion 54 in overlapping or abutting orientations, or may be separated from each other on the first substrate portion 54.

Similar to the first substrate portion 54, the second substrate portion 56 may also include a second catalyst section that includes one or more catalysts. For example, the embodiment shown in the FIG. 2 illustrates the second catalyst section 72 as having third and fourth catalysts 74, 76.

The use of separate substrate portions 54, 56 the second and third catalysts 54, 56 may facilitate the coating of the substrate 52 with one or more of the catalysts, such as facilitating the coating of the main SCR catalysts, illustrated in FIG. 2 as the second and third catalysts 70, 74.

After being coated with their respective catalysts, the first and second substrate portions 54, 56 may be joined together, such as, for example, by physically joining the first and second substrate portions 54, 56, or positioning the first and second substrate portions 54, 56 in relative close proximity to each other.

Claims

1. A multiple catalyst system for the treatment of exhaust gas in an after treatment system, the multiple catalyst system comprising: a common substrate; anda plurality of catalysts applied to at least a portion of the common substrate to provide a plurality of catalyst sections.

2. The multiple catalyst system of claim 1, wherein the multiple catalyst system includes a plurality of common substrates joined together, each of the plurality of common substrates having a plurality of catalysts.

3. The multiple catalyst system of claim 1, wherein the plurality of catalysts include a first catalyst, a second catalyst, and a third catalyst.

4. The multiple catalyst system of claim 3, wherein the first catalyst is a lean nitrogen oxide trap catalyst, the second catalyst is a selective catalytic reduction catalyst, and the third catalyst is an ammonia oxidizing catalyst.

5. The multiple catalyst system of claim 4, wherein the lean nitrogen oxide trap catalyst is positioned upstream of the selective catalytic reduction catalyst on the substrate, and the selective catalytic reduction catalyst is positioned upstream of the ammonia oxidizing catalyst on the substrate so that exhaust gas in the after treatment system first encounters the lean nitrogen oxide trap catalyst before encountering the selective catalytic reduction catalyst.

6. The multiple catalyst system of claim 4, wherein at least at least two of the plurality of catalyst sections abut each other.

7. The multiple catalyst system of claim 1, wherein the substrate includes a first substrate portion and a second substrate portion, a portion of the plurality of catalysts being applied to at least a portion of the first substrate portion, and the remaining plurality of catalysts applied to at least a portion of the second substrate portion.

8. A multiple catalyst system for the treatment of exhaust gas in an after treatment system, the multiple catalyst system comprising: a substrate;a first catalyst applied to a portion of the substrate, the first catalyst having a lean nitrogen oxide trap catalyst; anda second catalyst applied to a portion of the substrate, the second catalyst having a selective catalytic reduction catalyst.

9. The multiple catalyst system of claim 8, wherein the first catalyst is positioned upstream of the second catalyst so that exhaust gas passes the first catalyst before reaching the second catalyst.

10. The multiple catalyst system of claim 9, further including a third catalyst applied to a portion of the substrate and a fourth catalyst applied to another portion of the substrate, the third catalyst being a selective catalytic reduction catalyst, the fourth catalyst being an ammonia oxidizing catalyst.

11. The multiple catalyst system of claim 10, wherein the substrate includes a first substrate portion and a second substrate portion, wherein the first and second catalysts are applied at least a portion of the first substrate portion, and the third and fourth catalysts are applied to at least a portion of the second substrate portion.

12. A multiple catalyst system for an exhaust gas after treatment system comprising: a common substrate having a plurality of catalyst sections, each of the plurality of catalyst sections having at least one catalyst formulated to treat exhaust gas generated by a combustion engine.

13. The multiple catalyst system of claim 14, wherein the plurality of catalysts sections include a first catalyst section having a first catalyst and a second catalyst section having a second catalyst, the first and second catalysts formulated for the removal of nitrogen oxides from exhaust gas, wherein the first catalyst is more effective than the second catalyst in removing nitrogen oxides from exhaust gas generated during cold start engine conditions, and wherein the second catalyst is more effective than the first catalyst in removing nitrogen oxides from exhaust gas during normal engine operating temperatures.

14. The multiple catalyst system of claim 13, further including a third catalyst, and wherein the first catalyst is a lean nitrogen oxide trap catalyst, the second catalyst is a selective catalytic reduction catalyst, and the third catalyst is an ammonia oxidizing catalyst.

15. The multiple catalyst system of claim 14, wherein the lean nitrogen oxide trap catalyst is positioned upstream of the selective catalytic reduction catalyst on the substrate, and the selective catalytic reduction catalyst is positioned upstream of the ammonia oxidizing catalyst on the substrate so that exhaust gas in the after treatment system first encounters the lean nitrogen oxide trap catalyst before encountering the selective catalytic reduction catalyst.