Compressed Natural Gas Combustion and Exhaust Systems

Abstract

The present invention relates to a compressed natural gas combustion and exhaust system including (i) a natural gas combustion engine and (ii) an exhaust treatment system, the 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 including a substrate having at least first and second coatings, the first coating comprising a palladium-containing alumina and / or a base metal-containing alumina 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 system being configured to selectively operate in (a) a lean first mode in which sulfur is captured on the first coating and (b) a rich second mode in which sulfur is released from the first coating. The present invention further relates to a method.

Description

[Technical Field]

[0001] The present invention relates to compressed natural gas combustion and exhaust systems, and more particularly to systems with improved sulfur tolerance. [Background technology]

[0002] Natural gas is gaining interest as an alternative fuel for vehicles and stationary engines that traditionally use gasoline and diesel fuel. Natural gas is composed primarily of methane (typically 70-90%), with varying proportions of other hydrocarbons such as ethane, propane, and butane (up to 20% in some deposits) and other gases. It can be commercially produced from oil or natural gas fields and is widely used as a combustion energy source for power generation, industrial cogeneration, and home 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 (approximately 248 bar) and has an energy density per unit volume that is approximately 35% of that of gasoline. LNG has 2.5 times the energy density of CNG and is primarily used in heavy vehicles. LNG is cooled to liquid form at -162°C, resulting in a 600-fold reduction in volume, meaning that LNG is easier to transport than CNG. Bio-LNG, a potential alternative to natural (fossil) gas, is produced from biogas, obtained by anaerobic digestion of organic matter such as landfill waste or manure.

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

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

[0006] Natural gas engines produce significantly lower PM and NO emissions compared to large and small diesel engines. x (up to 95% and less than 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 US 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 in methane-fueled engines are stoichiometric conditions (λ = 1) and lean-burn conditions (λ ≥ 1.3). Palladium-based catalysts are known to be 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] Advances in this Pd-based catalyst technology depend on overcoming challenges related to cost and catalyst deactivation by sulfur, water, and heat aging.

[0009] Methane is the least reactive hydrocarbon, requiring high energy 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 the light-off temperature for methane conversion on Pd-based catalysts is higher than that for other hydrocarbons ("light-off temperature" refers to 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 1000 MPa 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 varied to accommodate local legislative requirements for methane and non-methane conversion.

[0011] NO x The reduction of NO and oxidation of methane are also more difficult under highly oxidizing conditions. For lean-burn CNG applications, Pd-Pt with high total pgm loadings (>200 gft-3) is required for methane combustion at lower temperatures. Unlike stoichiometric engines, NO is reduced in the presence of excess oxygen. x A reductant must also be injected into the exhaust stream so that NO can be reduced. This is usually in the form of ammonia (NH3). Therefore, lean-burn applications require a completely different catalytic system than the stoichiometric one, where efficient NO xReduction can be achieved using CO or HC under slightly rich or stoichiometric conditions.

[0012] Methane emissions can increase during cold starts and idle conditions due to the non-reactivity (or insufficient reactivity) of methane at lower temperatures. To improve methane reactivity at lower temperatures, one option is to use higher pgm loadings, but this increases costs.

[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 decline 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 because methane combustion feed always contains high levels of water due to the high H content in methane.

[0014] HO can be either an inhibitor or promoter, depending on the air-fuel ratio, i.e., λ. Under stoichiometric and reducing conditions (λ > 1), HO can act as a promoter for hydrocarbon oxidation via steam reforming reactions in both CNG and gasoline engines. However, in lean-burn CNG operating at λ > 1, HO acts as an inhibitor of methane oxidation. Understanding the water-inhibitory effect and designing catalysts that are more tolerant to the presence of HO is important. This allows 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 significantly deactivated upon sulfur exposure due to the formation of stable sulfates. Regenerating catalysts to restore activity after sulfur poisoning is difficult and typically requires high temperatures, rich operation, or both. This is easily achievable under stoichiometric operation but is more difficult under lean-burn conditions. Lean-burn vehicles operate at much higher air-fuel ratios than stoichiometric vehicles and require much higher concentrations of reductant injection to switch to rich operation. Thermal deactivation, resulting from a high level of misfire events due to poor engine transient control and ignition systems, destroys the catalyst and results in correspondingly high exhaust emissions.

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

[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 to address the shortcomings associated with the prior art, or at least to provide a commercially useful alternative thereto. Summary of the Invention

[0018] According to a first aspect, there is provided a compressed natural gas combustion and exhaust system, comprising: (i) a natural gas combustion engine; (ii) an exhaust treatment system, the exhaust treatment system comprising an intake for receiving exhaust gases from the combustion engine and a catalytic article positioned to receive and treat the exhaust gases, the catalytic article comprising: a substrate having at least a first and a second coating, wherein the first coating comprises a palladium-containing alumina and / or a base metal-containing alumina, and the second coating comprises a palladium-containing zeolite; The first coating is placed in contact with the exhaust gas before the second coating, and the system comprises: (a) a lean first mode in which sulfur is trapped on the first coating; (b) a second mode in which sulfur is richly released from the first coating.

[0019] In the following sections, 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 expressly indicated otherwise. 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] The present invention relates to compressed natural gas combustion and exhaust systems.

[0021] A natural gas combustion engine is an engine used to combust natural gas. The natural gas combustion engine may be a mobile engine, generally meaning suitable for use in an automobile or other vehicle (e.g., an off-road vehicle), where such a system may have varying fuel supply and demand during operation depending on operator requirements such as acceleration. In mobile applications, the natural gas combustion may be configured to operate in a lean or stoichiometric configuration. In mobile applications, it is generally possible to temporarily operate the system in a rich mode. It should be understood that while the engine is described as being "mobile," it may be used across a variety of different applications.

[0022] The exhaust treatment system is 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 arranged to receive and treat the exhaust gases. The inlet may be a conduit or pipe configured to direct exhaust from the combustion engine to the catalytic article.

[0023] Catalyst 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 components of the exhaust gas. Other forms of catalyst articles are known, including plate configurations, as well as wrapped metal catalyst substrates. While the catalyst articles described herein are suitable for use in all of these known forms, the honeycomb monolith form is particularly preferred, as it offers a good balance between cost and simplicity of manufacture.

[0024] The catalytic article is for the treatment of exhaust from a natural gas combustion engine. That is, the catalytic article is for catalytic treatment of exhaust gas from a natural gas combustion engine, converting or transforming components of the gas before it is released into the atmosphere to meet emission regulations. When natural gas is combusted, it produces both carbon dioxide and water, but the exhaust gas also contains amounts of additional methane (and other short-chain hydrocarbons) that need to be catalytically removed before the exhaust is released into the atmosphere. The exhaust gas also typically contains significant amounts of water and sulfur, which can accumulate and deactivate the catalyst.

[0025] The catalyst 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, both coatings are provided as washcoats on the substrate. Preferably, the substrate is a flow-through monolith.

[0026] The first coating comprises palladium-containing alumina and / or base metal-containing alumina. The first coating is effective at absorbing or capturing sulfur present in exhaust gas received from a CNG engine under lean conditions (λ>1) and can be regenerated (i.e., sulfur is released therefrom) for a short period of time under rich conditions (λ<1). Advantageously, the release of sulfur from the first coating, and thus the regeneration of the first coating, can be chemically induced by reducing the oxygen concentration of the exhaust gas received by the catalytic article, and optionally by injecting hydrocarbons into the exhaust gas received by the catalytic article, as discussed with respect to the data shown in FIG. 2 of the present application. In other words, the rich second mode rich conditions can be achieved by reducing the oxygen concentration of the exhaust gas received by the catalytic article, and optionally by injecting fuel or hydrocarbons into the exhaust gas received by the catalytic article.

[0027] The alumina may be any type of alumina, such as alpha or gamma alumina. Additionally, the alumina may be doped as known in the art. For example, doping with Si or La in an amount of up to 15 wt%, e.g., 1-10 wt%, preferably about 5 wt%, can provide improved thermal stability. Preferably, the alumina in the first coating is gamma alumina. Preferably, the first coating has a coating density of 1-50 g / ft 3 , more preferably 5 to 40 g / ft 3 , most preferably 10 to 30 g / ft 3 The washcoat loading is

[0028] The second coating includes a palladium-containing zeolite. Such palladium-containing zeolites exhibit excellent activity for treating exhaust gas from CNG engines, despite the presence of water in the exhaust gas, but are susceptible to sulfur inhibition. Such palladium-containing catalysts have high stability when exposed to rich conditions during the rich second mode and can withstand periodic exposure to high concentrations of sulfur, such as during the rich second mode. As shown by the data in Figure 1 of the present application and described later herein, the presence of SO2 in the exhaust gas during the rich second mode temporarily reduces the activity of the palladium-containing zeolite. However, because the palladium-containing zeolite does not significantly store SO2 when exposed to SO2 for the short period the system operates in the second rich mode, the activity of the palladium-containing zeolite is substantially restored when the lean first mode resumes. In other words, the palladium-containing zeolite is not destabilized by sulfur released from the first coating during intermittent operation of the system in the rich second mode. This is in contrast to other palladium-containing catalysts, such as palladium supported on alumina, as shown by the data in Figure 1, which is discussed in more detail below.

[0029] Preferably, the zeolite has an SAR of ≧1200, preferably ≧1300, such as ≧1500 (e.g. ≧1700), more preferably ≧2000, such as ≧2200. Preferably, the second coating has an SAR of 1 to 50 g / ft 3 , more preferably 5 to 40 g / ft 3 , most preferably 10 to 30 g / ft 3 The washcoat loading is

[0030] The first coating is positioned to contact the exhaust gas before the second coating, allowing the first coating to capture sulfur in the exhaust gas so that the exhaust gas received by the second coating has a reduced sulfur content during the first lean mode, thereby reducing deactivation of the palladium-containing zeolite of the second coating by sulfur during the first lean mode.

[0031] 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.

[0032] 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, which allows the first coating to contact the exhaust gas before the second coating.

[0033] 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.

[0034] 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 at most 25% of the axial length of the substrate.

[0035] 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.

[0036] The system is configured to selectively operate between (a) a lean first mode in which sulfur is trapped on the first coating and (b) a rich second mode in which sulfur is released from the first coating, during the lean first mode the system operates under lean burn conditions, i.e., (λ>1), and during the rich second mode the system operates under rich burn conditions, i.e., λ<1.

[0037] During the lean first mode, the first coating captures sulfur present in the exhaust gas so that the exhaust gas received by the second coating has a reduced sulfur content. This reduces deactivation / poisoning of the palladium-containing zeolite catalyst present in the second coating, thereby maintaining its high oxidation performance during operation of the system in the lean first mode. To regenerate the catalyst, particularly the first coating of the catalyst, the system periodically / intermittently operates in a rich second mode, during which sulfur is released from the first coating.

[0038] During the rich second mode, the oxygen concentration in the exhaust gas received by the catalytic article can be reduced by injecting an inert gas, such as argon, for example. The system can include an injector upstream of the catalytic article configured to inject an inert gas, such as argon, into the exhaust gas during operation of the system in the rich second mode.

[0039] During the rich second mode, the concentration of hydrocarbons in the exhaust gas received by the catalytic article may be increased by injecting one or more hydrocarbons into the exhaust gas. For example, the system may include an injector upstream of the catalytic article configured to inject one or more hydrocarbons, such as propylene, into the exhaust gas during operation of the system in the rich second mode.

[0040] By reducing the oxygen concentration in the exhaust gas and optionally increasing the hydrocarbon content, a rich condition can be achieved that releases sulfur from the first coating, thereby regenerating the first coating. In other words, operation of the system in the rich second mode can be achieved by reducing the oxygen concentration in the exhaust gas received by the catalytic article and, optionally, by injecting one or more hydrocarbons. Advantageously, such chemically induced sulfur release does not require an increase in the exhaust gas temperature. In other words, the exhaust gas temperature can be maintained substantially the same between the lean first mode and the rich second mode. In contrast, typical techniques for regenerating known sulfur trap devices involve the thermal release of stored sulfur, which requires a substantial temperature increase (i.e., several hundred degrees Celsius) above its operating temperature. This can be extremely difficult to apply due to the increased engine control required, the potential for thermal deactivation of the catalytic article, and the potential for thermal degradation of engine components such as the turbocharger.

[0041] As noted above, the temperature of the exhaust gases may remain substantially the same during the lean first mode and the rich second mode. For example, the temperature of the exhaust gases during the lean first mode may be within 20°C, preferably within 10°C, and more preferably within 5°C of the temperature of the exhaust gases during the second rich mode. The temperature of the exhaust gases during the lean first mode and the rich second mode may be less than 750°C, preferably less than 650°C, and most preferably between 500 and 600°C.

[0042] In an alternative, but less preferred, arrangement, the temperature of the exhaust gases may be increased during the rich second mode. During the first lean mode, the temperature of the exhaust gases may be less than 750°C. Preferably, during the rich second mode, the temperature of the exhaust gases does not exceed 850°C.

[0043] Preferably, during one cycle, the system is configured to operate in the second rich mode for less than 5 minutes, preferably less than 3 minutes, more preferably 1 minute or less, and in the first lean mode for more than 10 minutes, preferably more than 30 minutes, preferably 60 minutes or more. In other words, the system may intermittently switch to operate in the second rich mode. By intermittently operating in the second rich mode, the first coating, which functions as a sulfur trap, can be regenerated without destabilizing the performance of the palladium-containing zeolite of the second coating. In other words, the downstream second coating can withstand the rich conditions and high sulfur concentrations experienced during the second rich mode during these short periods. Because the system enables effective regeneration of the first coating over such short periods, the time between regeneration intervals (i.e., the time the system operates in the first lean mode) can be increased.

[0044] Preferably, the system further includes a sulfur trap downstream of the catalyst article to receive sulfur released from the first coating during the rich second mode. The sulfur trap can capture sulfur released from the first coating during the rich second mode, thereby reducing sulfur emissions from the system.

[0045] Optionally, the system includes an SO sensor downstream of the second coating and a controller configured to switch the system to operate in a rich second mode when the sensor detects an exhaust SO level above a threshold. This is advantageous because by measuring the SO level in the exhaust gas downstream of the second coating, the SO sensor can monitor SO absorption by the first coating and determine whether the first coating needs to be regenerated. For example, if the SO level in the exhaust gas increases significantly, this indicates that sulfur in the exhaust gas is not being sufficiently captured / absorbed by the first coating, and as a result, the first coating needs to be regenerated by operating the system in a rich second mode. The SO sensor may be positioned downstream of the catalyst article.

[0046] According to a further aspect of the present invention, there is provided a method for treating exhaust gas from a natural gas burning engine, the method comprising: contacting the exhaust gas with a catalytic article of an emission treatment system; The catalytic article a substrate having at least a first and a second coating, wherein the first coating comprises a palladium-containing alumina and / or a base metal-containing alumina, and the second coating comprises a palladium-containing zeolite; the first coating is placed in contact with the exhaust gas before the second coating; The method further includes operating the exhaust system in a lean first mode in which sulfur is trapped on the first coating, and intermittently operating the exhaust system in a rich second mode in which sulfur is released from the first coating.

[0047] Preferably, the method described in this aspect can be applied to the system described herein, and therefore all features described as being preferred for the system apply equally to the method aspect. [Brief explanation of the drawings]

[0048] The invention is further described with respect to the following non-limiting figures. [Figure 1] The top graph in Figure 1 shows the methane conversion performance of the catalysts of Example 1 and Example 2 under simulated steady-state reactor conditions at 550°C, where a 50 ppm SO pulse having a 1 minute duration was introduced into the syngas mixture flowing through the catalyst every hour. The middle graph in Figure 1 shows the SO slip from the catalyst of Example 2, which is a palladium-containing zeolite, under simulated steady-state reactor conditions at 550°C, where a 50 ppm SO pulse having a 1 minute duration was introduced into the syngas mixture flowing through the catalyst every hour. The bottom graph in Figure 1 shows the SO slip from the catalyst of Example 1, which is palladium supported on alumina, under simulated steady-state reactor conditions at 550°C, where a 50 ppm SO pulse having a 1 minute duration was introduced into the syngas mixture flowing through the catalyst every hour. [Figure 2] 1 is a graph showing the release of stored sulfur from the catalyst of Example 2, which is palladium supported on alumina, upon reduction in oxygen concentration and injection of polypropylene into the syngas mixture flowing past the catalyst. [Figure 3] The graphs show the methane and ethane conversion performance of the catalysts of Examples 1 and 2 under simulated steady-state reactor conditions at 550°C. Rich pulses with 1-minute durations occurred at 4.5, 10.5, and 16.5 hours. The NO gas supply to the syngas mixture was turned off at 7.5 hours and turned back on at 14 hours, ensuring that no NO was present in the syngas mixture from 7.5 to 14 hours. The top graph shows the methane and ethane conversion for the catalyst of Example 2, which is palladium supported on alumina, and the bottom graph shows the methane and ethane conversion for the catalyst of Example 1, which is a palladium-containing zeolite. [Example]

[0049] The invention will now be further described with reference to the following non-limiting examples.

[0050] Example 1 The catalyst of Example 1 is a catalyst having a palladium-containing high-siliceous (SAR>1500) zeolite having a palladium content of 2 to 3 wt %.

[0051] The catalyst of Example 1 was prepared by impregnating a powder sample of siliceous zeolite (SAR>1500) with a solution of palladium nitrate by the conventional incipient wetness technique. After impregnation, the sample was dried at 80°C for 5 hours and calcined in air at 500°C for 2 hours in a static oven.

[0052] A washcoat containing preformed Pd-containing siliceous zeolite, an alumina binder, and a silica binder was applied to a ceramic substrate, and then the washcoat was pulled down onto the substrate using a vacuum. The article was dried and calcined at about 500°C for about 1 hour. The loading of Pd on the article was 120 g / ft. 3 It was.

[0053] Example 2 The catalyst of Example 2 is a catalyst in which palladium is supported on alumina, and has a palladium content of 3 wt %.

[0054] A washcoat containing palladium nitrate, gamma-alumina, an alumina binder, and citric acid was applied to a ceramic substrate, and then a vacuum was used to pull the washcoat down onto the substrate. The article was dried and calcined at about 500°C for about 1 hour. The Pd loading on the article was 120 g / ft 3 It was.

[0055] The sulfur tolerance of the catalysts of Examples 1 and 2 was tested by measuring their methane conversion under simulated steady-state reactor conditions at 550°C, where a pulse of 50 ppm SO2 having a duration of 1 minute was introduced hourly into the syngas mixture flowing through the catalyst. The syngas mixture was operated at a space velocity of 100,000 h -1The composition of the test mixture was 1000 ppm CH, 25 ppm C2H, 5 ppm C3H, 1000 ppm CO, 500 ppm NO, 12% O, 8% H2O, 7.2% CO, and the balance N. Data from the sulfur tolerance test are shown in the top graph of Figure 1.

[0056] This test simulated the sulfur buildup that occurs during cyclic operation of the system of the present invention in the rich second mode due to sulfur release from the first coating, with each cycle operating in the rich second mode for 1 minute and the lean first mode for 59 minutes.

[0057] The catalyst of Example 1 has the same composition as the second coating of the present invention (palladium-containing zeolite). Therefore, this test compares the sulfur tolerance of the second coating of the present invention during cyclic operation of the system in the rich second mode (1 minute per cycle) with Example 2, which instead contains palladium supported on alumina.

[0058] In Figure 1, the methane conversion performance of the Example 1 catalyst, a palladium-containing zeolite, is shown by a series of peaks all between 90 and 100%. The methane conversion performance of the Example 2 catalyst, a palladium-supported alumina catalyst, is shown by a series of peaks that continuously decrease to less than 80%. As shown in Figure 1, for the Example 1 catalyst, a sharp drop in performance occurred during each SO2 pulse due to deactivation of the palladium-containing zeolite by sulfur in the exhaust gas. However, the performance drop rapidly recovered upon removal of sulfur from the gas stream, and performance stabilized at over 95% methane conversion after multiple cycles. In contrast, for the Example 2 catalyst (palladium-supported alumina), catalyst performance continuously decreased with each SO2 pulse and did not stabilize after multiple cycles. The contrasting behavior in performance recovery can be explained by monitoring the SO2 slip from the Example 1 and Example 2 catalysts during the sulfur tolerance test, which is shown in the middle and bottom graphs of Figure 1. As shown in the middle graph of Figure 1, palladium supported on alumina stored a significant amount of SO during the SO pulse. In contrast, as shown in the bottom graph of Figure 1, SO passes through the palladium-containing zeolite of Example 1 during the SO pulse, as evidenced by the greater SO slip. The palladium-containing zeolite of Example 1 does not store significant sulfur during the SO pulse and is therefore not deactivated by sulfur over these short periods of high sulfur concentration.

[0059] 2 is a graph showing the chemically induced release of stored sulfur from the catalyst of Example 2 (palladium supported on alumina), which has the same composition as the first coating of the catalyst of the present invention. The catalyst of Example 2, which has 0.5 ppm SO and a composition of 1000 ppm CH, 25 ppm C2H6, 5 ppm C3H8, 1000 ppm CO, 500 ppm NO, 12% O2, 8% H2O, 7.2% CO2, and the balance N2, was subjected to a space velocity of 100,000 h2. -1A synthesis gas mixture was flowed at 550°C. The oxygen concentration of the gas mixture was reduced by periodically injecting propylene into the gas mixture in 1-minute / hour cycles. In other words, in each cycle, a lean gas mixture was fed to the catalyst for 59 minutes, and a rich gas mixture was fed to the catalyst for 1 minute.

[0060] Figure 2 shows that palladium supported on alumina effectively absorbs sulfur during lean operation and releases it during periodic rich pulses. The concentration of sulfur released during each rich pulse increased with time (i.e., a greater amount of sulfur was released during the 15.5-hour rich pulse than during the 9.5-hour rich pulse). This is expected because the longer exposure of the palladium supported on alumina to the gas mixture allowed it to absorb more sulfur, which could then be released during the rich pulse.

[0061] Figure 2 demonstrates that periodic rich pulses can be created by simultaneously decreasing the oxygen concentration and increasing the hydrocarbon concentration of the gas received by the catalyst. In other words, Figure 2 demonstrates that the release of SO2 from palladium on alumina can be chemically induced, thus maintaining the exhaust gas temperature at 550°C during regeneration of the palladium on alumina.

[0062] Thus, Figure 2 demonstrates that the palladium-on-alumina composition of the first coating of the present invention can absorb sulfur during lean operation without a substantial temperature increase, but instead can be regenerated by chemical induction. In other words, the data in Figure 2 demonstrates that reducing the oxygen concentration and increasing the hydrocarbon concentration of the exhaust gas received by the catalyst during operation of the system of the present invention in a rich second mode releases sulfur from the first coating without the need to increase the exhaust gas temperature. This is advantageous because thermal release of stored sulfur typically requires a substantial temperature increase (i.e., several hundred degrees Celsius) above its operating temperature, which can be extremely difficult to implement due to engine control issues, potential catalyst thermal deactivation, and potential thermal degradation of components such as turbochargers.

[0063] The effect of rich pulses on the catalysts of Examples 1 and 2 was tested by measuring their methane and ethane conversions under simulated steady-state reactor conditions at 550°C when a syngas mixture was flowed through the catalyst. The syngas mixture was heated at a space velocity of 100,000 h -1 The synthesis gas mixture contained 1000 ppm CH, 25 ppm C2H, 5 ppm C3H, 1000 ppm CO, 500 ppm NO, 12% O, 8% H2O, 7.2% CO, and the balance N. Rich pulses, in which the oxygen concentration in the synthesis mixture was reduced via injection of propylene, occurred for 1 minute at 4.5, 10.5, and 16.5 hours. The NO gas supply to the synthesis gas mixture was turned off at 7.5 hours and turned back on at 14 hours, ensuring that no NO was present in the synthesis gas mixture from 7.5 to 14 hours. Data from this test are shown in Figure 3.

[0064] In Figure 3, the ethane conversion for both Example 1 and Example 2 remains at 90-100%. The methane conversion for the Example 1 catalyst (palladium-containing zeolite) starts at 100% and remains at 90-100% during lean operation, but shows a sharp decrease of approximately 20-25% during each rich pulse. The methane conversion for the Example 2 catalyst (palladium-containing zeolite) starts at 100% and gradually decreases before and after each rich pulse. For both Example 1 and Example 2, the exhaust gas temperature at the inlet decreases from 700-710°C after 2 hours and then remains at approximately 560°C, while the exhaust gas temperature at the outlet decreases from 650°C after 2 hours and remains at approximately 570-580°C, and then suddenly increases to approximately 610°C during the rich pulse.

[0065] As shown in Figure 3, the conversion achieved by the Example 2 catalyst, a palladium supported on alumina, gradually decreased over time both before and after each rich pulse. This is expected to be due to water-induced catalyst deactivation. However, after the rich pulse, when NO was not present in the syngas mixture, the deactivation rate of the Example 2 catalyst showed a sharp increase. This suggests that a significant change had occurred to the palladium on the catalyst and that its methane oxidation activity is dependent on the presence of NO in the feed. The change to palladium on the Example 2 catalyst was expected to be the reduction of PdO to metallic Pd, an assessment supported by visual observation of the sample color. In contrast, the graph in Figure 3 demonstrates that the performance of the Example 1 catalyst, a palladium-containing zeolite, was extremely stable both before and after the rich pulse and was not affected by the presence / removal of NO in the feed. Overall, these results suggest a much greater stability of the palladium-containing zeolite, regardless of the rich pulse, compared to palladium supported on alumina. It should be noted that this greater stability is particularly advantageous when such catalysts are used as the second coating of the present invention, since the system of the present invention can withstand rich conditions when operated in a rich second mode to regenerate the first catalytic coating.

[0066] 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 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 features) and "consisting of" (intended to mean that when components are expressed as percentages by their proportions, no other features may be included such that they add up to 100%, taking into account any unavoidable impurities), unless the context clearly dictates otherwise.

[0067] Although terms such as “first,” “second,” and the like may be used herein to describe various elements, layers, and / or portions, it will be understood that the elements, layers, and / or portions are not 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 and another material that is said to be “on” that material. Spatially relative terms such as “under,” “below,” “beneath,” “lower,” “over,” “above,” and “upper” may be used herein for ease of description to describe the relationship of one element or feature to another. It will be understood that spatially relative terms are intended to encompass different orientations of the device during use or operation in addition to the orientation depicted in the figures. 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 would be interpreted accordingly.

[0068] The foregoing detailed description has been provided for purposes of illustration and example, and is not intended to limit the scope of the appended claims. Many variations of the presently preferred embodiments set forth herein will be apparent to those skilled 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 treatment system, the exhaust treatment system comprising an intake for receiving exhaust gases from the combustion engine, and a catalytic article positioned to receive and treat the exhaust gases, the catalytic article comprising: a substrate having at least a first and a second coating, wherein the first coating comprises a palladium-containing alumina and / or a base metal-containing alumina, and the second coating comprises a palladium-containing zeolite; the first coating is disposed to contact the exhaust gas before the second coating, and the compressed natural gas combustion and exhaust system comprises: (a) a lean first mode in which sulfur is trapped on the first coating; (b) a second mode in which sulfur is released from said first coating, said second mode being rich in sulfur.

2. The compressed natural gas combustion and exhaust system of claim 1 , wherein during the rich second mode, the oxygen concentration in the exhaust gas received by the catalytic article is decreased.

3. 3. The compressed natural gas combustion and exhaust system of claim 1, further comprising an injector upstream of the catalytic article, the injector configured to inject one or more hydrocarbons into the exhaust gas during operation of the compressed natural gas combustion and exhaust system in the rich second mode.

4. 2. The compressed natural gas combustion and exhaust system of claim 1, wherein the temperature of the exhaust gas remains substantially the same during the lean first mode and the rich second mode.

5. The compressed natural gas combustion and exhaust system of claim 1 , wherein the temperature of the exhaust gas increases during the rich second mode.

6. The compressed natural gas combustion and exhaust system of claim 1 , wherein the first coating is disposed upstream of the second coating in a zoned configuration.

7. The compressed natural gas combustion and exhaust system of claim 6 , wherein the substrate comprises an inlet end and an outlet end.

8. 8. The compressed natural gas combustion and exhaust system of claim 6 or 7, wherein the first coating extends between 20 and 80% of the axial length of the substrate, and / or the second coating extends between 20 and 80% of the axial length of the substrate, and / or the first coating and second coating together substantially cover the substrate.

9. The compressed natural gas combustion and exhaust system of claim 6 , wherein the first coating and second zone overlap by at least 10% of the axial length of the substrate.

10. The compressed natural gas combustion and exhaust system of claim 1 , wherein the first coating is disposed on the second coating in a layered configuration.

11. 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 10. The compressed natural gas combustion and exhaust system of claim 1, having a washcoat loading of

12. 10. The compressed natural gas combustion and exhaust system of claim 1, wherein the compressed natural gas combustion and exhaust system further comprises a sulfur trap downstream of the catalyst article for receiving sulfur released from the first coating during the rich second mode.

13. SO downstream of the second coating ２ a sensor for detecting SO in the exhaust flow above a threshold value by said sensor; ２ 10. The compressed natural gas combustion and exhaust system of claim 1, further comprising: a controller configured to switch the compressed natural gas combustion and exhaust system to operate in the rich second mode when a level is detected.

14. The compressed natural gas combustion and exhaust system of claim 1 , wherein the substrate is a flow-through substrate.

15. 10. The compressed natural gas combustion and exhaust system of claim 1, wherein the zeolite has an SAR of at least 1500 and / or the alumina is gamma alumina.

16. 10. The compressed natural gas combustion and exhaust system of claim 1, wherein during one cycle, the compressed natural gas combustion and exhaust system is configured to operate in the second rich mode for less than 5 minutes and in the first lean mode for more than 10 minutes.

17. 1. A method for treating exhaust gas from a natural gas burning engine, comprising: contacting the exhaust gas with a catalytic article of an emission treatment system; the catalyst article comprising: a substrate having at least a first and a second coating, wherein the first coating comprises a palladium-containing alumina and / or a base metal-containing alumina, and the second coating comprises a palladium-containing zeolite; the first coating is disposed in contact with the exhaust gas before the second coating; The method further includes operating the compressed natural gas combustion and exhaust system in a lean first mode in which sulfur is trapped on the first coating, and intermittently operating the compressed natural gas combustion and exhaust system in a rich second mode in which sulfur is released from the first coating; the compressed natural gas combustion and exhaust system comprises: (i) a natural gas burning engine; (ii) an exhaust treatment system.

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