COMPRESSED NATURAL GAS COMBUSTION AND EXHAUST SYSTEM

Abstract

The present invention relates to a compressed natural gas combustion and exhaust system comprising: (i) a natural gas combustion engine; and (ii) an exhaust treatment system, the exhaust treatment system comprising a intake for receiving an exhaust gas from the combustion engine and a catalyst article arranged to receive and treat the exhaust gas, wherein the catalyst article comprises: a substrate having at least first and second coatings, the first coating being free from platinum-group-metals and comprising a copper-containing zeolite having the CHA framework-type and the second coating comprising a palladium-containing zeolite, wherein the first coating is arranged to contact the exhaust gas before the second coating. The present invention further relates to a method and a use.

Claims

1. A compressed natural gas combustion and exhaust system comprising: (i) a natural gas combustion engine and (ii) an exhaust treatment system, the exhaust treatment system comprising a intake for receiving an exhaust gas from the combustion engine and a catalyst article arranged to receive and treat the exhaust gas, wherein the catalyst article comprises: a substrate having at least first and second coatings, the first coating being free from platinum-group-metals and comprising a copper-containing zeolite having the CHA framework-type and the second coating comprising a palladium-containing zeolite, wherein the first coating is arranged to contact the exhaust gas before the second coating.

2. The system of claim 1, wherein the first coating is provided as a washcoat on the substrate and has a washcoat loading of 1 to 50 g/ft.sup.3 and/or wherein the second coating is provided as a washcoat on the substrate and has a washcoat loading of 1 to 50 g/ft.sup.3.

3. The system according to claim 1, wherein the copper-containing zeolite having the CHA framework-type has: (i) a SAR of from 15 to 30; and/or (ii) a Cu loading of from 1 to 5 wt %.

4. The system of claim 1, wherein the first coating is upstream of the second coating in a zoned configuration.

5. The system of claim 4, wherein the substrate has an inlet end and an outlet end, optionally wherein the first coating extends from the inlet end and the second coating extends from the outlet end.

6. The system of claim 4, wherein the first coating extends from 20 to 80%, preferably 60 to 80% of an axial length of the substrate and/or wherein the second coating extends from 20 to 80%, preferably 20 to 40% of an axial length of the substrate, and/or wherein the first coating and the second coating together substantially cover the substrate.

7. The system of claim 5, wherein the first coating and the second zone overlap by at least 10% of an axial length of the substrate.

8. The system of claim 1, wherein the first coating is arranged on the second coating in a layered configuration.

9. The system of claim 1, wherein the substrate is a flow-through monolith

10. The system of claim 1, wherein the palladium-doped zeolite has a SAR of at least 1500, preferably at least 2000, more preferably at least 2200.

11. The system of claim 1, wherein the exhaust gas has an SOx content of less than 10 ppm.

12. The system of claim 1, further comprising an SCR catalyst downstream of the catalytic article.

13. The system of claim 1, wherein the natural gas combustion engine is a stationary engine.

14. A method for the treatment of an exhaust from a natural gas combustion engine, the method comprising: contacting the exhaust with a catalyst article, wherein the catalyst article comprises: a substrate having at least first and second coatings, the first coating comprising a copper-doped zeolite having the CHA framework-type and the second coating comprising a palladium-doped zeolite, wherein the first coating is arranged to contact the exhaust gas before the second coating.

15. Use of a copper-doped CHA zeolite in an exhaust system as a sulphur-trap to protect a downstream palladium-containing zeolite catalyst.

Description

FIGURES

[0053] The invention will be described further in relation to the following non-limiting FIGURES, in which:

[0054] FIG. 1 shows the improvement in light-off performance achieved with the present invention.

EXAMPLES

[0055] The invention will now be described further in relation to the following non-limiting examples.

[0056] A synthetic gas mixture was flowed through a packed bed of pelletised catalyst beads. In the embodiment representative of the system described herein, 0.1 g of beads comprising a copper-containing zeolite were placed upstream of 0.1 g of palladium-containing-zeolite beads. In a comparative example, the copper-containing zeolite beads were replaced with 0.1 g of inert cordierite beads.

[0057] The copper-containing zeolite contained 3 wt % Cu. The zeolite of the copper-containing zeolite was a CHA zeolite with a SAR of 22.

[0058] The palladium-containing zeolite contained 3 wt % Pd. The zeolite of the palladium-containing zeolite was a ZSM-5 zeolite having a SAR of 2120.

[0059] The synthetic gas mixture comprised ˜2 ppm SO.sub.2, 4000 ppm CH.sub.4, 100 ppm C.sub.2H.sub.6, 35 ppm C.sub.3H.sub.8, 1000 ppm CO, 500 ppm NO, 10% O.sub.2, 10% H.sub.2O, 7% CO.sub.2, balance N.sub.2 at a space velocity of 100,000 h.sup.−1 Notably, the synthetic gas mixture has a SO.sub.2 content of ˜2 ppm.

[0060] FIG. 1 is a plot of temperature (X-axis) against % conversion of CO, CH.sub.4 and NO. The dashed lines indicate the CO, CH.sub.4 and NO activity of the comparative example. The solid lines indicate the CO, CH.sub.4 and NO activity of the inventive examples.

[0061] As can be seen from FIG. 1, the % conversion for CO, CH.sub.4 and NO is significantly greater for the inventive example than the comparative example. For example, the light-off temperature (the temperature at which 50% conversion is achieved) for CO conversion for the inventive example is approximately 170° C. and the light-off temperature for CO conversion for the comparative example is approximately 235° C. Similarly, the light-off temperature for CH.sub.4 conversion for the inventive example is 370° C. and the light-off temperature for CH.sub.4 conversion for the comparative example is approximately 440° C. It can be seen that the peak NO conversion for the inventive example is 15% that is reached at approximately 410° C. Substantially 0% NO conversion was demonstrated for the comparative example.

[0062] FIG. 1 therefore demonstrates improved light-off performance for treatment of CO and CH.sub.4 and improved NO activity achieved by the catalyst of the present invention.

[0063] The catalyst of the present invention demonstrates such improved performance due to the presence of the upstream copper-containing zeolite, which is particularly effective for trapping sulphur present in the exhaust gas that would otherwise deactivate the downstream palladium-containing catalyst.

[0064] The improved performance demonstrated by the catalyst of the present invention is particularly relevant for treatment of an exhaust gas from a CNG engine. Exhaust gas generated by a CNG engine contains significant quantities of methane (so-called “methane slip”) and rely on palladium-containing zeolites for effective methane treatment. However, as demonstrated by the comparative example, such palladium-containing zeolites are susceptible to poisoning by sulphur present in the exhaust stream from a CNG engine (as sulphur is typically present in the lubricant of the CNG engine). The catalyst of the present invention reduces sulphur poisoning of the downstream palladium-containing zeolite, which in turn achieves improved light-off performance for treatment of methane and CO and improved NO activity.

[0065] As used herein, the singular form of “a”, “an” and “the” include plural references unless the context clearly dictates otherwise. The use of the term “comprising” is intended to be interpreted as including such features but not excluding other features and is also intended to include the option of the features necessarily being limited to those described. In other words, the term also includes the limitations of “consisting essentially of” (intended to mean that specific further components can be present provided they do not materially affect the essential characteristic of the described feature) and “consisting of” (intended to mean that no other feature may be included such that if the components were expressed as percentages by their proportions, these would add up to 100%, whilst accounting for any unavoidable impurities), unless the context clearly dictates otherwise.

[0066] It will be understood that, although the terms “first”, “second”, etc. may be used herein to describe various elements, layers and/or portions, the elements, layers and/or portions should not be limited by these terms. These terms are only used to distinguish one element, layer or portion from another, or a further, element, layer or portion. It will be understood that the term “on” is intended to mean “directly on” such that there are no intervening layers between one material being said to be “on” another material. Spatially relative terms, such as “under”, “below”, “beneath”, “lower”, “over”, “above”, “upper” and the like, may be used herein for ease of description to describe one element or feature's relationship to another element(s) or feature(s). It will be understood that the spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if a device as described herein is turned over, elements described as “under” or “below” other elements or features would then be oriented “over” or “above” the other elements or features. Thus, the example term “under” can encompass both an orientation of over and under. The device may be otherwise oriented and the spatially relative descriptors used herein interpreted accordingly.

[0067] The foregoing detailed description has been provided by way of explanation and illustration, and is not intended to limit the scope of the appended claims. Many variations of the presently preferred embodiments illustrated herein will be apparent to one of ordinary skill in the art, and remain within the scope of the appended claims and their equivalents.