Compressed Natural Gas Combustion and Exhaust Systems

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 comprising an inlet for receiving exhaust gas from the combustion engine and a catalytic article arranged to receive and treat the exhaust gas, the catalytic article comprising a substrate having at least a first coating and a second coating, the first coating being free of platinum group metals and comprising a copper-containing zeolite having a CHA framework type, and the second coating comprising a palladium-containing zeolite, the first coating being arranged to contact the exhaust gas prior to the second coating. The present invention further relates to methods and uses.

Description

[Technical field]

[0001] The present invention relates to compressed natural gas combustion and exhaust systems, particularly those having a zeolite supported Pd catalyst with improved light-off performance for NOx, CO, and HC. Specifically, the present invention relates to systems having an upstream sulfur trap to realize these performance benefits, which is necessary in light of the sulfur present in natural gas. [Background technology]

[0002] Natural gas is of increasing interest as an alternative fuel for vehicles and stationary engines that traditionally use gasoline and diesel fuels. Natural gas is composed primarily of methane (typically 70-90%) with varying percentages of other hydrocarbons such as ethane, propane and butane (up to 20% in some deposits) and other gases. It can be produced commercially from oil or natural gas fields and is widely used as a combustion energy source for electricity generation, industrial cogeneration and domestic heating. It can also be used as a vehicle fuel.

[0003] Natural gas can be used as a transportation fuel in the form of compressed natural gas (CNG) and liquefied natural gas (LNG). CNG is carried in tanks pressurized to 3600 psi (about 248 bar) and has an energy density of about 35% of that of gasoline per unit volume. LNG has 2.5 times the energy density of CNG and is used mainly in heavy vehicles. LNG is cooled to liquid form at -162 °C, resulting in a 600-fold reduction in volume, which means that LNG is easier to transport than CNG. Bio-LNG is a potential alternative to natural (fossil) gas and is produced from biogas, obtained by anaerobic digestion from organic matter such as landfill waste or manure.

[0004] Natural gas has many environmental advantages: it is a cleaner burning fuel that typically contains fewer impurities, has a higher energy per carbon (Bti) than traditional hydrocarbon fuels, resulting in lower carbon dioxide emissions (25% lower greenhouse gas emissions), and has lower PM and NO emissions compared to diesel and gasoline. x Biogas can reduce these emissions even further.

[0005] Further drivers for the adoption of natural gas include its high abundance and lower cost compared to other fossil fuels.

[0006] Natural gas engines have significantly lower PM and NO emissions compared to heavy-duty and light-duty diesel engines. x NG engines emit less than 95% and 70%, respectively. However, the exhaust gases produced by NG engines often contain significant amounts of methane (so-called "methane slip"). Regulations limiting emissions from these engines currently include Euro VI and the U.S. Environmental Protection Agency (EPA) Greenhouse Gas Act. These impose emission limits for methane, nitrogen oxide (NOx), and particulate matter (PM).

[0007] The two main operating modes used for methane-fueled engines are stoichiometric (λ=1) and lean-burn (λ≧1.3) conditions. Palladium-based catalysts are known as the most active type of catalyst for methane oxidation under both conditions. Regulated emission limits for both stoichiometric and lean-burn compressed natural gas engines can be met by applying either a palladium-rhodium three-way catalyst (TWC) or a platinum-palladium oxidation catalyst, respectively.

[0008] The development of this Pd-based catalyst technology depends on overcoming challenges related to cost and catalyst deactivation due to sulfur, water and thermal aging.

[0009] Methane is the least reactive hydrocarbon, and high energy is required to break the primary C-H bond. The ignition temperature of alkanes generally decreases with increasing fuel-air ratio and with increasing hydrocarbon chain length, which correlates with C-H bond strength. It is known that Pd-based catalysts have a higher light-off temperature for methane conversion than other hydrocarbons ("light-off temperature" means the temperature at which the conversion rate reaches 50%).

[0010] When operated under stoichiometric conditions (λ=1), the TWC can be used as an effective and cost-effective aftertreatment system for combusting methane. -3 Most bimetallic Pd-Rh catalysts with high total platinum group metal (pgm) loadings, exceeding 10000 ppm, are required for high levels of methane conversion to meet end-of-life total hydrocarbon (THC) regulations due to the very low reactivity of this hydrocarbon and catalyst deactivation by thermal and chemical effects. The use of high pgm loadings improves total HC conversion in stoichiometric CNG engines. However, high methane conversion can be achieved at relatively low pgm based on engine calibration, i.e., by controlling the air-fuel ratio to operate near or richer than stoichiometric. The pgm loading can also be modified to accommodate local legislative requirements for methane and non-methane conversion.

[0011] NO x The reduction of ethane and the oxidation of methane are also more difficult under highly oxidizing conditions. Lean-burn CNG applications require higher total pgm loadings (>200 gft 3) due to the lower temperature combustion of methane. -3 ) is required. Unlike stoichiometric engines, NO is generated in the presence of excess oxygen. xA reductant must also be injected into the exhaust stream so that NO can be reduced. This is usually in the form of ammonia (NH3), and thus lean-burn applications require a completely different catalytic system than stoichiometric, where efficient NO x Reduction can be accomplished using CO or HC at slightly rich or stoichiometric conditions.

[0012] Due to the non-reactive (or less reactive) characteristics of methane at low temperatures, increased methane emissions occur during cold start and idle conditions, primarily in lean burn situations where exhaust temperatures are lower than stoichiometric. One option to improve methane reactivity at lower temperatures is to use higher pgm loadings, but this increases cost.

[0013] Natural gas catalysts, especially Pd-based catalysts, can suffer from poisoning by water (5-12%) and sulfur (<0.5 ppm SO2 in lubricating oil), especially under lean conditions, which results in a dramatic decrease in catalyst conversion over time. Water deactivation is significant due to the formation of hydroxyls, carbonates, formates and other intermediates on the catalyst surface. Activity is reversible and can be fully restored when the water is removed. However, this is impractical since methane combustion feeds always contain high levels of water due to the high content of H in methane.

[0014] H2O can be either an inhibitor or promoter, depending on the air-fuel ratio, i.e., λ. Under stoichiometric reducing conditions (λ>1), H2O can act as a promoter for the oxidation of hydrocarbons through steam reforming reactions in both CNG and gasoline engines. However, in lean-burn CNG operating at λ>1, H2O acts as an inhibitor of methane oxidation. It is important to understand the water-inhibiting effect and design catalysts that are more tolerant to the presence of H2O. This will allow for improvements when trying to control methane emissions from lean-burn CNG.

[0015] Although sulfur levels in engine exhaust are very low, Pd-based catalysts are severely deactivated upon sulfur exposure due to the formation of stable sulfates. Regeneration of the catalyst to restore activity after sulfur poisoning is difficult and usually requires high temperatures, rich operation, or both. This is easily achievable under stoichiometric operation but is more difficult under lean burn. Lean-burn vehicles operate at much higher air-fuel ratios than stoichiometric vehicles and require injection of much higher concentrations of reductant to switch to rich operation. Thermal deactivation resulting from poor engine transient control and high levels of misfire events due to ignition systems destroys the catalyst and results in correspondingly high levels of exhaust emissions.

[0016] Palladium-containing catalysts deactivate under both lean and stoichiometric conditions, but sulfur poisoning has a more dramatic effect than heat aging in lean operation. Sulfur poisoning can be ameliorated by adding small amounts of Pt to the Pd catalyst. This is because sulfur inhibition due to the formation of palladium sulfate can be significantly reduced by the addition of Pt. However, the addition of Pt further increases costs. Summary of the Invention

[0017] It is therefore desirable to provide an improved system for natural gas combustion and exhaust gas treatment to reduce methane emissions by addressing catalyst deactivation due to sulfur, water, thermal aging, etc., without increasing the cost of the catalyst. It is an object of the present invention to address this problem and address the shortcomings associated with the prior art, or at least provide a commercially useful alternative thereto.

[0018] According to a first aspect, there is provided a compressed natural gas combustion and exhaust system, comprising: (i) a natural gas burning engine; (ii) an exhaust gas treatment system comprising an inlet for receiving exhaust gas from said combustion engine and a catalytic article positioned to receive and treat said exhaust gas, said catalytic article comprising: a substrate having at least a first and a second coating, the first coating being free of platinum group metals and comprising a copper-containing zeolite having a CHA framework type, and the second coating comprising a palladium-containing zeolite; an exhaust gas treatment system, the first coating being positioned to contact the exhaust gas prior to the second coating; A system is provided, comprising:

[0019] In the following passages, different aspects / embodiments are defined in more detail. Each aspect / embodiment so defined can be combined with any other aspect / embodiment or aspects / embodiments, unless otherwise expressly indicated to the contrary. In particular, any feature indicated as being preferred or advantageous can be combined with any other feature or features indicated as being preferred or advantageous.

[0020] Sulfur components in the exhaust gas, coming from the fuel and lubricants (which in CNG are the lubricants), poison the catalyst. Pd supported on very high SAR zeolites shows superior activity in the presence of water compared to alumina, but is highly deactivated when exposed to even small amounts of sulfur. The use of sulfur trap materials upstream of the oxidation catalyst is an alternative option to prevent catalyst deactivation over a wide range of operating temperatures. The present invention relates to the use of copper-containing zeolites as sulfur traps for palladium-containing zeolite catalysts to maintain high oxidation performance in the presence of sulfur. Copper-containing zeolites are particularly effective in capturing sulfur under wet and sulfur-containing conditions such as those found in CNG systems. In fact, the copper-containing zeolites effectively capture sulfur to protect the downstream palladium-containing zeolites, which would otherwise be highly susceptible to deactivation.

[0021] The present invention relates to a compressed natural gas combustion and exhaust system that includes a natural gas combustion engine and an exhaust treatment system.

[0022] A natural gas combustion engine is an engine used to burn natural gas. Preferably, the natural gas combustion engine is a stationary engine, preferably a gas turbine or a power generation system. In stationary applications, natural gas combustion can be configured to always operate under lean or stoichiometric conditions. In such systems, the combustion conditions and fuel composition are generally kept constant over long operating times. This means that there is less opportunity to have a regeneration step to remove sulfur and moisture contaminants compared to mobile applications. Therefore, the benefits described herein can be particularly beneficial for stationary applications. That is, it is particularly desirable to provide a catalyst with high sulfur and moisture resistance when there is limited opportunity to regenerate the catalyst. It should be understood that both types of lean and stoichiometric systems can be used across a range of different applications.

[0023] The exhaust treatment system is a system suitable for treating exhaust gases from a combustion engine. The exhaust treatment system includes an inlet for receiving exhaust gases from the combustion engine and a catalytic article positioned to receive and treat the exhaust gases.

[0024] Catalytic articles are components suitable for use in exhaust gas systems. Typically, such articles are honeycomb monoliths, sometimes referred to as "bricks". They have a high surface area configuration suitable for contacting the gas to be treated with the catalytic material to effect conversion or transformation of the exhaust gas components. Other forms of catalytic articles are also known, including plate configurations, as well as wrapped metal catalytic substrates. The catalytic articles described herein are suitable for use in all of these known forms, but are particularly preferred in the form of honeycomb monoliths, as they offer a good balance between cost and simplicity of manufacture.

[0025] The catalytic article is for the treatment of exhaust from a natural gas combustion engine. That is, the catalytic article is for the catalytic treatment of exhaust gas from a natural gas combustion engine to convert or transform components of the gas prior to release to the atmosphere to meet exhaust gas regulations. When natural gas is combusted, it produces both carbon dioxide and water, but the exhaust gas also contains additional amounts of methane (and other short chain hydrocarbons) that need to be catalytically removed before the exhaust gas is released to the atmosphere. The exhaust gas also typically contains significant amounts of water and sulfur that can accumulate and deactivate the catalyst.

[0026] The catalytic article includes a substrate having at least a first and a second coating. Preferably, the first coating is provided as a washcoat on the substrate and / or the second coating is provided as a washcoat on the substrate. Preferably, the substrate is a flow-through monolith. Alternatively, the substrate may include a first flow-through monolith and a second flow-through monolith arranged in series, the first flow-through monolith having a first coating and the second flow-through monolith having a second coating.

[0027] The first coating is free of platinum group metals and comprises a copper-containing zeolite having a CHA framework type. Preferably, the copper-containing zeolite having a CHA framework type is (i) an SAR of 15 to 30, preferably 20 to 25, and / or (ii) having a Cu loading of 1-5 wt%, preferably 2-4 wt%, and most preferably about 3 wt%.

[0028] This particular zeolite effectively captures sulfur present in the exhaust gases received from CNG engines.

[0029] Preferably, the first coating has a thickness of 1 to 50 g / ft 3 , more preferably 5 to 40 g / ft 3 , and most preferably 10 to 30 g / ft 3 has a washcoat loading of .

[0030] The second coating comprises a palladium-containing zeolite. Preferably, the palladium-doped zeolite has an SAR of ≧1200, preferably ≧1300, such as ≧1500 (e.g. ≧1700), more preferably ≧2000, such as ≧2200. Such palladium-containing zeolites show excellent activity for treating exhaust gases from CNG engines, despite the presence of water in the exhaust gases, but are highly susceptible to sulfur inhibition.

[0031] Preferably, the second coating has a thickness of 1 to 50 g / ft 3 , more preferably 5 to 40 g / ft 3 , and most preferably 10 to 30 g / ft 3 has a washcoat loading of .

[0032] The first coating is positioned in contact with the exhaust gas prior to the second coating, which allows the first coating to capture sulfur present in the exhaust gas such that the exhaust gas received by the second coating has a reduced sulfur content, resulting in reduced sulfur deactivation of the palladium-containing zeolite in the second coating.

[0033] Preferably, the first coating is upstream of the second coating in a zoned configuration, which allows the first coating to contact the exhaust gases before the second coating.

[0034] Preferably, the substrate has an inlet end and an outlet end, and optionally, the first coating extends from the inlet end and the second coating extends from the outlet end.

[0035] Preferably, the first coating extends over 20-80%, preferably 60-80%, of the axial length of the substrate, and / or the second coating extends over 20-80%, preferably 20-40%, of the axial length of the substrate, and / or the first and second coatings together substantially cover the substrate.

[0036] Preferably, the first coating and the second zone overlap by at least 10% of the axial length of the substrate. Preferably, the first coating and the second zone overlap by no more than 25% of the axial length of the substrate.

[0037] Alternatively, the first coating may be disposed over the second coating in a layered configuration, allowing the first coating to contact the exhaust gases before the second coating.

[0038] The exhaust gas may have a SOx content of less than 10 ppm.

[0039] Preferably, the system further includes an SCR catalyst downstream of the catalyst article, which serves to further treat other components of the exhaust gas.

[0040] According to a further aspect, a method for treating exhaust from a natural gas burning engine, comprising: contacting the exhaust with a catalytic article, the catalytic article comprising: a substrate having at least a first coating and a second coating, the first coating comprising a copper-doped zeolite having a CHA framework type and the second coating comprising a palladium-doped zeolite; A method is provided in which the first coating is placed in contact with the exhaust gas before the second coating.

[0041] Preferably, the method described in this aspect may be applied to the system described herein, and therefore all features described as preferred for the system apply equally to the method aspect.

[0042] According to a further aspect, there is provided the use of a copper-doped CHA zeolite in an exhaust system as a sulfur trap to protect a downstream palladium-containing zeolite catalyst.

[0043] Preferably, the use described in this aspect can be applied to the methods and systems described herein, and therefore all features described as preferred for the systems and methods apply equally to the use aspect. [Brief description of the drawings]

[0044] The invention is further described in relation to the following non-limiting figures. [Figure 1] 1 illustrates the improvement in light-off performance achieved by the present invention. EXAMPLES

[0045] The invention will now be further described with reference to the following non-limiting examples.

[0046] The synthesis gas mixture was flowed through a packed bed of pelletized catalyst beads. In an 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.

[0047] The copper-containing zeolite contained 3 wt. % Cu. The zeolite of the copper-containing zeolite was a CHA-type zeolite with an SAR of 22.

[0048] The palladium-containing zeolite contained 3 wt. % Pd. The zeolite of the palladium-containing zeolite was a ZSM-5 zeolite with a SAR of 2120.

[0049] The synthetic gas mixture is heated to a space velocity of 100,000 h -1 The synthesis gas mixture contained approximately 2 ppm SO2, 4000 ppm CH4, 100 ppm C2H6, 35 ppm C3H8, 1000 ppm CO, 500 ppm NO, 10% O2, 10% H2O, 7% CO2, and the balance N2. The synthesis gas mixture has a SO2 content of approximately 2 ppm.

[0050] 1 is a plot of temperature (X-axis) versus % conversion of CO, CH4, and NO. The dashed lines show the CO, CH4, and NO activity of the comparative example. The solid lines show the CO, CH4, and NO activity of the inventive example.

[0051] As can be seen from Figure 1, the percent conversion of CO, CH4, and NO is significantly higher for the inventive examples than for the comparative examples. For example, the light-off temperature for CO conversion (the temperature at which 50% conversion is achieved) for the inventive examples is about 170°C, while the light-off temperature for CO conversion for the comparative examples is about 235°C. Similarly, the light-off temperature for CH4 conversion for the inventive examples is 370°C, while the light-off temperature for CH4 conversion for the comparative examples is about 440°C. It can be seen that the peak NO conversion for the inventive examples is 15%, which is reached at about 410°C. The comparative examples demonstrated essentially 0% NO conversion.

[0052] Thus, FIG. 1 demonstrates the improved light-off performance and improved NO activity for the treatment of CO and CH4 achieved by the catalyst of the present invention.

[0053] The catalysts of the present invention exhibit such improved performance due to the presence of an upstream copper-containing zeolite which is particularly effective at trapping sulfur present in the exhaust gas that would otherwise deactivate the downstream palladium-containing catalyst.

[0054] The improved performance exhibited by the catalyst of the present invention is particularly relevant for the treatment of exhaust gas from CNG engines. The exhaust gas produced by CNG engines contains a significant amount of methane (so-called "methane slip") and relies on palladium-containing zeolites for effective methane treatment. However, as demonstrated by the comparative example, such palladium-containing zeolites are susceptible to poisoning by sulfur present in the exhaust stream from CNG engines (as sulfur is typically present in the lubricant of CNG engines). The catalyst of the present invention reduces the sulfur poisoning of downstream palladium-containing zeolites, thereby achieving improved light-off performance for the treatment of methane and CO as well as improved NO activity.

[0055] As used herein, the singular forms "a," "an," and "the" include plural referents unless the context clearly dictates otherwise. Use of the term "comprising" is intended to be interpreted as including such features but not excluding other features, and is intended to include options of features that are necessarily limited to those recited. In other words, this term also includes the limitations "consisting essentially of" (intended to mean that certain additional components may be present provided they do not materially affect the essential properties of the recited feature) and "consisting of" (intended to mean that when components are expressed as percentages by their proportions, other features may not be included so that they add up to 100%, but any unavoidable impurities are taken into account), unless the context clearly dictates otherwise.

[0056] Although terms such as "first", "second", and the like may be used herein to describe various elements, layers, and / or portions, it is understood that the elements, layers, and / or portions should not be limited by these terms. These terms are used only to distinguish one element, layer, or portion from another or further element, layer, or portion. It will be understood that the term "on" is intended to mean "directly on", such that there is no intervening layer between one material that is said to be "on" another material. For ease of description to describe the relationship of an element or feature to another element(s) or feature(s), spatially relative terms such as "under", "below", "beneath", "lower", "over", "above", "upper", and the like may be used herein. It will be understood that the spatially relative terms are intended to encompass different orientations of the device during use or operation in addition to the orientation depicted in the drawings. For example, if a device described herein were inverted, elements described as being "below" or "below" other elements or features would then be oriented "above" or "above" the other elements or features. Thus, the exemplary term "below" can encompass both an orientation of above and below. The device may be otherwise oriented, and the spatially relative descriptors used herein interpreted accordingly.

[0057] The foregoing detailed description has been provided for purposes of explanation and illustration and is not intended to limit the scope of the appended claims. Many variations of the presently preferred embodiments described herein will be apparent to those of ordinary skill in the art and remain within the scope of the appended claims and their equivalents.

Claims

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

2. The first coating is provided as a washcoat on the substrate and has a coating weight of 1 to 50 g / ft. ３ and / or the second coating is provided as a washcoat on the substrate and has a washcoat loading of 1 to 50 g / ft. ３ 13. The system of claim 1 having a washcoat loading of

3. The copper-containing zeolite having a CHA framework type is (i) an SAR of 15 to 30, preferably 20 to 25, and / or 3. The system of claim 1 or 2, having (ii) a Cu loading of 1-5 wt.%, preferably 2-4%, and most preferably about 3 wt.%.

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

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

6. 6. The system of claim 4 or claim 5, wherein the first coating extends over 20-80%, preferably 60-80%, of the axial length of the substrate, and / or the second coating extends over 20-80%, preferably 20-40%, of the axial length of the substrate, and / or the first and second coatings together substantially cover the substrate.

7. 7. The system of claim 4, 5, or 6, wherein the first coating and the second zone overlap by at least 10% of the axial length of the substrate.

8. The system of any one of claims 1 to 3, wherein the first coating is disposed on the second coating in a layered configuration.

9. The system of any one of claims 1 to 8, wherein the substrate is a flow-through monolith.

10. The system according to any of claims 1 to 9, wherein the palladium doped zeolite has a SAR of at least 1500, preferably at least 2000, more preferably at least 2200.

11. A system according to any preceding claim, wherein the exhaust gas has a SOx content of less than 10 ppm.

12. The system of any preceding claim, further comprising an SCR catalyst downstream of the catalyst article.

13. A system according to any preceding claim, wherein the natural gas combustion engine is a stationary engine, preferably a gas turbine.

14. 1. A method for treating exhaust from a natural gas burning engine, comprising: contacting the exhaust with a catalytic article, the catalytic article comprising: a substrate having at least a first coating and a second coating, the first coating comprising a copper-doped zeolite having a CHA framework type and the second coating comprising a palladium-doped zeolite; The method, wherein the first coating is disposed in contact with the exhaust gas before the second coating.

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

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