Methods of regenerating a methane oxidation catalyst

Abstract

Disclosed herein is a method of regenerating a methane oxidation catalyst positioned in a methane-containing gas stream, wherein the method comprises two or more CO addition steps; wherein each CO addition step comprises adding CO into the gas stream for a duration of time, wherein the methane oxidation catalyst positioned in the gas stream is contacted with CO added from the two or more CO addition steps; wherein the method further comprises allowing a further duration of time to elapse between each CO addition step, wherein CO is not added into the gas stream over the duration of time elapsing between each CO addition step; wherein the duration of each CO addition step, and the duration of time elapsing between each CO addition step, define a CO dosing %, the CO dosing % being defined as: CO dosing % = [ton / (ton+toff)] × 100 wherein ton is the duration of each CO addition step, wherein toff is the duration of time elapsing between each CO addition step; wherein the CO dosing % is greater than 0%, and less than or equal to 70%.

Description

[0001] METHODS OF REGENERATING A METHANE OXIDATION CATALYST

[0002] FIELD OF THE INVENTION

[0003] The present disclosure relates to methods of regenerating a methane oxidation catalyst positioned in a methane-containing gas stream.

[0004] BACKGROUND

[0005] There is increasing concern regarding pollutant emissions from the use of fossil fuels and other processes, which contribute to poor air quality, environmental damage, and harm to human health. This has caused increased focus on air quality and emissions regulations and a need to provide means to eliminate or reduce the concentration of pollutant emissions into the atmosphere.

[0006] Hydrocarbons represent one pollutant where it is desirable to minimise or eliminate release into the atmosphere. In particular, there is a desire to minimise methane (CH4) emissions because methane is a potent greenhouse gas and, as such, is of concern due to its contribution to rising global temperatures.

[0007] Methane emissions originate from a variety of different sources. For example, methane may be a constituent in the gaseous mixture emitted from engines, such as those found in vehicles, power plants, or industrial machinery. Such gaseous mixtures are often referred to as “exhaust gas”, particularly when the gases are emitted from an internal combustion engine. Methane may also be a constituent in the gaseous mixture emitted from a flue or exhaust stack following industrial processes, particularly those of power plants or heating systems. Such gaseous mixtures are often referred to as “flue gas”. Methane may also be a constituent in the gaseous mixture emitted from industrial processes or manufacturing, chemical processes or reactions, waste treatment operations such as wastewater treatment operations, landfill operations, or biogas production. Such gaseous mixtures are often referred to as “waste stream gas”.

[0008] Methane may be a constituent in the gaseous mixtures emitted from such sources due to methane being produced during the process in question as a byproduct, or as a result of “methane slip”. Methane slip occurs when methane forms part of a fuel used to power an engine, and unburned methane passes through the engine, thereby being emitted along with the remainder of the exhaust gases. For example, natural gas (NG), compressed or liquified, which has methane as its main component, has attracted attention as an alternative fuel to petroleum and light oil and has been used as a fuel to power engines. From an air quality perspective, NG fuel has many advantages compared to traditional fuels. The emissions of sulphur dioxide (SO2) are low due to low or non-existing sulphur content of the gas. The low sulphur content and the absence of fuel aromatics also contribute to low particulate formation levels. Furthermore, NG combustion results in less carbon dioxide (CO2) emissions in comparison to diesel and gasoline. The use of NG as a fuel has increased significantly since the start of the 21 st century. However, many NG engines that are produced today have problems with “methane slip”, i.e. unburned methane passing through the engine and being emitted along with the remainder of the exhaust gases. Methane slip results from incomplete combustion of the fuel injected in the pre-chamber or cylinders of the engine.

[0009] Irrespective of the source, and of the reason why methane is a constituent in the gaseous mixture emitted from a source, removing methane upstream of release into the atmosphere is desirable due to the impact of methane on the environment, climate, and human health.

[0010] Various means of removing methane upstream of release into the atmosphere exist. For example, one means of removing methane upstream of release into the atmosphere is by engine design measures. Another means is by using a methane oxidation catalyst positioned in a methane-containing gas stream.

[0011] Methane oxidation catalysts are widely used and include materials such as palladium, platinum, rhodium, manganese, or combinations thereof. Methane oxidation catalysts reduce the methane content of a methane-containing gas stream by oxidising the methane to CO2 and water. Methane oxidation catalysts have therefore proved useful in reducing methane emissions, but nevertheless suffer various drawbacks. For example, the performance of catalysts are generally temperature dependent, with performance improving at higher temperatures. It will also be appreciated that even small changes in temperature can cause significant impacts on catalyst activity. This is particularly relevant when addressing methane emissions, as methane is a relatively stable molecule, and so typically requires higher temperatures than other less stable molecules to oxidise. Often, the temperature of a gas stream by the time the gas stream contacts the methane oxidation catalyst is lower than desired for efficient methane removal. For example, when the gas stream originates from an internal combustion engine, not only will the temperature of the gas stream leaving the engine vary depending on the specifics of the engine in question (for instance, the number of strokes), but further the methane oxidation catalyst is generally positioned downstream of the engine to the extent that heat losses will cause the operating temperature of the catalyst to be even lower. In addition to the temperature dependency, methane oxidation catalysts also suffer from “catalyst poisoning”, due to species such as SOx (e.g. SO2 and SO3, predominantly SO2) and water. It is thought that such species occupy the active sites on the catalyst, thereby reducing the number of active sites available for methane removal, and so reducing the effectiveness of the catalyst.

[0012] The drawbacks suffered by methane oxidation catalysts result in the catalyst activity decreasing over time. Various methods for regenerating methane oxidation catalysts exist which seek to reverse and / or counteract the decrease in catalyst activity that occurs in use over time, thereby improving their longevity. These methods can be “ex-situ” (which require the methane oxidation catalyst to be removed from the gas stream) or “in-situ” (which allow the methane oxidation catalyst to remain positioned in the gas stream). Ex-situ methods include removing the catalyst from the gas stream, and subjecting it to extreme heat treatment to remove catalyst poisoning impurities (such as SO2 and water), using N2as a carrier gas to carry such impurities away. Such ex-situ methods have their benefits, but bring the inconvenience of having to interrupt operation in order to remove the methane oxidation catalyst from the system. Meanwhile, in-situ methods offer more convenient and efficient methods of regenerating a methane oxidation catalyst, as such methods allow the methane oxidation catalyst to remain positioned in the gas stream. Examples of in-situ methods of regenerating a methane oxidation catalyst include adding heat to the system using an electrical heater, thereby increasing the operating temperature of the catalyst and so increasing its activity. However, doing so brings energy inefficiencies, and also requires the site in question to have the electrical set up to install and power the electrical heater. Other examples of in-situ methods of regenerating a methane oxidation catalyst include adding particular constituents to the methane-containing gas stream which regenerate the methane oxidation catalyst, as disclosed in US11459926B2, W02014191060A1 , and

[0013] US10184374B2. Such constituents include combustible gases and reducing agents, with examples including methanol, natural gas, very low sulphur diesel, alcohols, ethers, hydrogen, carbon monoxide (CO) or ammonia. Such methods offer benefits over in-situ methods that require the addition of an electrical heater, but on the other hand, bring the drawback of needing to source and supply the constituent in question that is added to the gas stream. Furthermore, depending on the constituent in question and the associated hazards, there may be additional concerns around operating safety.

[0014] Consequently, there remains a need for new methods of regenerating a methane oxidation catalyst.

[0015] SUMMARY OF INVENTION

[0016] In a first aspect there is a method of regenerating a methane oxidation catalyst positioned in a methane-containing gas stream, wherein the method comprises two or more CO addition steps; wherein each CO addition step comprises adding CO into the gas stream for a duration of time, wherein the methane oxidation catalyst positioned in the gas stream is contacted with CO added from the two or more CO addition steps; wherein the method further comprises allowing a further duration of time to elapse between each CO addition step, wherein CO is not added into the gas stream over the duration of time elapsing between each CO addition step; wherein the duration of each CO addition step, and the duration of time elapsing between each CO addition step, define a CO dosing %, the CO dosing % being defined as: 100 wherein tonis the duration of each CO addition step wherein toff is the duration of time elapsing between each CO addition step; wherein the CO dosing % is greater than 0%, and less than or equal to

[0017] 70%.

[0018] It has surprisingly been found that methods according to the first aspect offer an improved method of regenerating a methane oxidation catalyst.

[0019] For example, the catalyst is positioned in a methane-containing gas stream, which provides improved convenience over ex-situ methods that require removal of the catalyst from the gas stream. Without wishing to be bound by theory, it is thought that the addition of CO to the gas stream regenerates the methane oxidation catalyst by releasing heat locally to the catalyst. It is thought that the CO, upon contact with the catalyst, is oxidised to CO2, and in doing so releases heat locally to the catalyst, thereby increasing the operating temperature of the catalyst and so enhancing methane removal. In addition, without wishing to be bound by theory, it is also thought that the addition of CO to the gas stream regenerates the methane oxidation catalyst by reacting with the catalyst poisoning species that would otherwise occupy and therefore block active sites on the catalyst. It is thought that by scavenging the catalyst poisoning species, methane removal is enhanced. As such, the methods disclosed herein enhance the methane removal from a methane-containing gas stream. Furthermore, CO offers benefits over alternative catalyst regenerative constituents that might otherwise be added to the gas stream, such as hydrogen for instance, which brings health and safety implications due to its explosive nature.

[0020] Moreover, it has surprisingly been found that the method disclosed herein allows less CO to be used, whilst still offering the methane removal benefits associated with CO addition. Previously, it was thought that a continuous dose of CO to the gas stream would be appropriate for catalyst regeneration. Meanwhile, the methods disclosed herein comprise two or more CO addition steps, and also comprise allowing a further duration of time to elapse between each CO addition step, wherein the duration of each CO addition step, and the duration of time elapsing between each CO addition step, define a CO dosing %. By allowing time to elapse between each CO addition step, the quantity of CO added to the gas stream is reduced compared with scenarios where CO is added continuously to the gas stream. Despite the quantity of CO used being lower, it has surprisingly been found that the associated methane removal is in fact comparable to that achieved when adding CO continuously to the gas stream. Due to this surprising realisation, the methods disclosed herein achieve a desirable balance between enhancing methane removal from a methane-containing gas stream, whilst reducing the quantities of CO required. Reducing the quantities of CO required provides benefits in terms of materials savings, and overall process efficiency. The methods disclosed herein therefore provide an improved method of catalyst regeneration.

[0021] BRIEF DESCRIPTION OF DRAWINGS

[0022] Figure 1 shows a schematic flow diagram illustrating the addition of CO into a gas stream, the methane oxidation catalyst being positioned in the gas steam.

[0023] Figure 2 illustrates the concept and calculation of the CO dosing %. CO is added into a gas stream in a pulsed manner, with a period of time elapsing between each CO addition step. The duration of each CO addition step, ton, and the duration of time elapsing between each CO addition step, toff, define the CO dosing %.

[0024] Figures 3-10 display experimental data results as described in the Examples. DETAILED DESCRIPTION

[0025] Disclosed herein is a method of regenerating a methane oxidation catalyst positioned in a methane-containing gas stream. As will be appreciated, “methane- containing gas stream” refers to a gaseous mixture comprising methane. It will be appreciated that the composition of the methane-containing gas stream may vary, for example depending on the source from which the methane-containing gas stream has been emitted. The methane-containing gas stream can comprise methane at various levels. For example, prior to treatment by the methods disclosed herein, the methane-containing gas stream can comprise at least 100 ppmv methane, at least 200 ppmv methane, at least 300 ppmv methane, at least 400 ppmv methane, or at least 500 ppmv methane, in relation to the total volume of gas. Prior to treatment by the methods disclosed herein, the methane- containing gas stream can for example comprise at most 5000 ppmv methane, at most 4000 ppmv methane, at most 3000 ppmv methane, or at most 2000 ppmv methane, in relation to the total volume of gas. The methane-containing gas stream generally comprises one or more additional constituents further to methane, and can for example comprise one or more of nitrogen, oxygen, carbon dioxide, water, nitrogen monoxide, nitrogen dioxide, sulphur dioxide, hydrogen sulphide, and carbon monoxide (CO). Example amounts of each of these constituents, when present, are 60-80 vol% nitrogen, 5-20 vol% oxygen, 1-10 vol% carbon dioxide, 5-20 vol% water, 100-300 ppmv nitrogen monoxide, 10-50 ppmv nitrogen dioxide, 0.01-13ppmv sulphur dioxide, 10-400ppmv hydrogen sulphide, and 50-200 ppmv carbon monoxide (CO), in relation to the total volume of gas. In one scenario, the methane-containing gas stream comprises 5-20 vol% water and 0.01-13ppmv sulphur dioxide, in relation to the total volume of gas. Such ranges are understood as including the stated end points of the range in question. Given the methane-containing gas stream may already contain CO (in the instance that CO is comprised by the gaseous mixture emitted by the source in question) the CO added in the two or more CO addition steps is to be understood as CO added in addition to any pre-existing CO in the methane- containing gas stream. It will also be appreciated that the methane-containing gas stream may further comprise volatile organic compounds (VOCs), and may also comprise particulate matter in addition to the gaseous constituents. The composition of the gas can be verified and the amounts of individual components measured using instruments such as an FTIR (Fourier transform infrared) spectrometer at 180 °C and GC (gas chromatography).

[0026] It will be understood that the methane-containing gas stream treated by the methods disclosed herein can be generated from a variety of different sources. It will be understood that the source is positioned upstream of the methane oxidation catalyst. The methods disclosed herein have applicability to methane-containing gas streams generated from a variety of different sources. The methane- containing gas stream can for example be generated from an engine (such as those found in vehicles, power plants, or industrial machinery), a flue or exhaust stack (such as those of power plants or heating systems), an industrial process, a chemical process or reaction, a waste treatment operation such as a wastewater treatment operation, a landfill operation, or biogas production. The methods disclosed herein have particular applicability to methane-containing gas streams generated from an engine. Therefore, the methods disclosed herein preferably comprise generating the methane-containing gas stream from an engine, more preferably an internal combustion engine, positioned upstream of the catalyst. In such a scenario, the internal combustion engine may either be rich bum (an excess of fuel in the combustion chamber during combustion) or lean bum (an excess of air in the combustion chamber during combustion). It will be appreciated that such an engine will have a variety of different applications. For example, the method disclosed herein may further comprise driving a compressor unit using the engine, and compressing natural gas in a natural gas pipeline using the compressor unit.

[0027] Disclosed herein is a method of regenerating a methane oxidation catalyst. The term “regenerating” as applied to the methane oxidation catalyst refers to reversing and / or counteracting the decrease in catalyst activity that occurs in use over time. Methane oxidation catalysts at any stage in their life cycle can benefit from the methods disclosed herein. For example, the method disclosed herein may be applied immediately after the catalyst is first put to use, in order to guard against the decrease in catalyst activity that would otherwise occur in use over time. Alternatively, the methods disclosed herein may be applied after a decrease in catalyst activity has begun, with the view of restoring the catalyst activity back to the same (or similar) levels of activity observed when the catalyst was first put to use.

[0028] The method disclosed herein has applicability to any methane oxidation catalyst. The skilled person will be familiar with such catalysts, which are publicly available and widely used. The term “methane oxidation catalyst” takes its usual definition in the art, and so refers to a catalyst capable of oxidising methane. The methane oxidation catalyst generally comprises a metal. The methane oxidation catalyst may for example comprise palladium, platinum, rhodium, manganese, or combinations thereof. Preferably, the methane oxidation catalyst comprises platinum, palladium, rhodium, or combinations thereof, on account of the popularity of such catalysts in reducing / removing methane. More preferably, the methane oxidation catalyst comprises palladium, on account of such catalysts gaining particular benefit from the method disclosed herein. To save on material costs, the methane oxidation catalyst may comprise the metal(s) referred to herein supported on a substrate. By having the metal(s) referred to herein supported on a substrate, the quantity of metal(s) required are reduced, which saves on the associated material costs. The substrate preferably has a high surface area, to maximise the surface area available for methane oxidation. The substrate can for example be comprised of the oxide of a further metal, such as iron oxide (Fe2C>3, Fe3C>4) and / or aluminium oxide (AI2O3). Preferably, the substrate is comprised of aluminium oxide (AI2O3).

[0029] The methane oxidation catalyst is positioned in the methane-containing gas stream. In other words, the method disclosed herein is an in-situ method of catalyst regeneration, and so allows the methane oxidation catalyst to remain in the methane-containing gas stream while the method is carried out. This allows the methane-containing gas stream to continue flowing throughout the duration that the method is carried out. Preferably, the flow rate of the methane-containing gas stream remains substantially the same throughout the duration that the method is carried out. Example flow rates for the methane-containing gas stream can include a gas hour space velocity (GHSV) in the range of 15k-95k IT1, preferably in the range of 15k-70k h'1, more preferably in the range of 20k-50k h’ 1

[0030] Disclosed herein, the method comprises two or more CO addition steps, wherein each CO addition step comprises adding CO into the gas stream for a duration of time. The methane oxidation catalyst positioned in the gas stream is contacted with the added CO (i.e. with the CO added from the two or more CO addition steps). The method may comprise adding CO into the gas stream upstream of, or at the same point as, the methane oxidation catalyst. The method preferably comprises adding CO into the gas stream upstream of the methane oxidation catalyst. Adding CO into the gas stream upstream of the methane oxidation catalyst provides improved efficiencies. As used herein, the terms “upstream” and “downstream” take their usual definitions in the art, and so define a location relative to the direction of the flow of the gas stream. As such, when the method comprises adding CO into the gas stream upstream of the methane oxidation catalyst, the CO is added into the gas stream, and the gas stream together with the added CO then flows to the catalyst.

[0031] As will be appreciated by the skilled person, “CO” refers to carbon monoxide. It will be understood that the CO is added as a gas. The skilled person will be familiar with suitable means and methods with which to add the CO into the gas stream. Preferably, each CO addition step comprises adding CO into the gas stream using an injector which injects the CO into the gas stream. The injector generally comprises a nozzle through which the CO is injected into the gas stream.

[0032] Prior to addition into the gas stream, the CO may be stored in a container, with suitable containers being familiar to the skilled person. Suitable containers include for example high-pressure cylinders, tube trailers or ISO modules. Often, the CO is stored as a compressed gas at pressures up to 2000 psig (138 bar). The method may comprise adding the CO into the gas stream directly from such suitable containers. Alternatively, the method may comprise generating CO from a CO conversion device, and adding CO into the gas stream from the CO conversion device. The term “CO conversion device” refers to a device capable of converting one or more reagents into CO.

[0033] In the methods disclosed herein, each CO addition step comprises adding CO into the gas stream for a duration of time. The duration of time of each CO addition step is the time over which CO is added into the gas stream. In each CO addition step, CO may be added as pure (i.e. 100 vol%) CO, or the CO may be added as part of a bulk gas containing other constituents. Such other constituents may include impurities, and / or other combustible gases or reducing agents (but preferably not H2). The composition of the bulk gas may vary depending for example on the purity of the CO grade in question. The CO may be added as part of a bulk gas with a CO content of 70-99.99 vol%, preferably 80-99.99 vol%, more preferably 90-99.99 vol%, in relation to the total volume of the bulk gas. Most preferably, the CO is added as part of a bulk gas with a CO content of 95-99.99 vol%, in relation to the total volume of the bulk gas. Preferably, the CO is added as part of a bulk gas that is substantially free from H2. Such ranges are understood as including the stated end points of the range in question.

[0034] Disclosed herein, the method comprises two or more CO addition steps, wherein each CO addition step comprises adding CO into the gas stream for a duration of time, and further comprises allowing a further duration of time to elapse between each CO addition step, wherein CO is not added into the gas stream over the duration of time elapsing between each CO addition step. As a result, the CO is added into the gas stream in a pulsed manner which involves alternating between adding CO to the gas stream, and ceasing to add CO to the gas stream. The duration of each CO addition step, and the duration of time elapsing between each CO addition step, define a CO dosing %, the CO dosing % being defined as: 100 wherein tonis the duration of each CO addition step wherein toff is the duration of time elapsing between each CO addition step; wherein the CO dosing % is greater than 0%, and less than or equal to 70%.

[0035] It will be understood that the CO dosing % is a measure of the relative time periods associated with the CO addition step and the duration of time elapsing between each CO addition step. Disclosed herein, the CO dosing % is greater than 0%. A CO dosing % of 0% corresponds with a scenario where tonis zero i.e. where no CO is added into the gas stream, and accordingly falls outside of the methods disclosed herein. A CO dosing of 100% corresponds with a scenario where tOff is zero, and so corresponds with a continuous CO dose, and accordingly falls outside of the methods disclosed herein. Disclosed herein, the CO dosing % is less than or equal to 70%. It has been found that a CO dosing % of less than or equal to 70% offers a particularly desirable balance between achieving comparable methane removal benefits to that achieved using a continuous CO dose, whilst saving significant quantities of CO. Generally speaking, the higher the CO dosing %, the closer the scenario is to a continuous CO dose, and the lower the CO dosing %, the closer the scenario is to no CO being added to the gas stream.

[0036] As used herein, tonis the duration of each CO addition step, and toff is the duration of time elapsing between each CO addition step. It will be understood by the term “each” that tonis the duration of any one CO addition step, and toff is the duration elapsing between any two CO addition steps. It will be understood from this that ton and toff generally do not vary between different CO addition steps, with the respective durations of the individual CO addition steps being substantially the same as one another, and similarly with the duration of time elapsing between each CO addition step remaining substantially the same. As such, tonremains substantially the same throughout the method, and toff remains substantially the same throughout the method. This regularity in the CO dosing allows for convenient implementation and control.

[0037] Preferably, the CO dosing % is less than or equal to 50%, preferably less than or equal to 35%, more preferably less than or equal to 10%, most preferably less than or equal to 6%. Preferably, the CO dosing % is at least 1 %, preferably at least 2%, more preferably at least 3%, most preferably at least 4%. Such preferred CO dosing %s have achieved particularly good results, as can be seen in the experimental data. Particularly optimal results are achieved when the CO dosing % is 1-10%, preferably 1-6%, said ranges being understood as including the stated end points of the range in question.

[0038] The duration of each CO addition step, and the duration of time elapsing between each CO addition step, can be tailored as desired whilst arriving at a CO dosing % disclosed herein. The skilled person will be familiar with appropriate means and methods of tailoring the respective durations. For example, each CO addition step may comprise adding CO into the gas stream via a valve that alternates between an open and closed state, such that the CO is added into the gas stream when the valve is in an open state, and such that no CO is added into the gas stream when the valve is in a closed state. The valve may alternate between being in an open and closed state using well-known means in the art. For example, the valve may alternate between being in an open and closed state in response to an electrical signal, for example one sent from a thermal mass flow controller. Thermal mass flow controllers are widely available, and are generally comprised of a thermal mass flow sensor, a control valve, and a microprocessor based pc board with signal and fieldbus conversion. One such example of a thermal mass flow controller being an IN-FLOW F-203AI available from Bronkhorst. The duration of each CO addition step, and the duration of time elapsing between each CO addition step, can be tailored by adjusting the time periods for which the valve is in its open and closed states.

[0039] Preferably, the duration of each CO addition step (ton) is less than or equal to 100s, preferably less than or equal to 80s, preferably less than or equal to 50s, more preferably less than or equal to 20s, more preferably less than or equal to 10s, most preferably less than or equal to 3s. Preferably, the duration of each CO addition step (ton) is at least 0.01s, more preferably at least 0.1s, most preferably at least 0.5s. Examples of potential ranges for each CO addition step (ton) include 0.01 s-100s, preferably 0.1s-20s, more preferably 0.1s-10s, most preferably 0.1s- 3s, said ranges being understood as including the stated end points of the range in question.

[0040] Preferably, the duration of time elapsing between each CO addition step (toff) is less than or equal to 100s, preferably less than or equal to 75s, more preferably less than or equal to 50s, most preferably less than or equal to 25s. Preferably, the duration of time elapsing between each CO addition step (toff) is at least 0.01s, more preferably at least 1s, more preferably at least 5s, more preferably at least 10s, most preferably at least 15s. Examples of potential ranges for the duration of time elapsing between each CO addition step (toff) include 0.01 s-100s, more preferably 1s-50s, most preferably 15s-25s, said ranges being understood as including the stated end points of the range in question.

[0041] The total duration of the method disclosed herein can be varied as desired. It will be appreciated that the total duration can vary widely and need not be specifically limited for the purposes of the method disclosed herein. The total duration of the method disclosed herein can for example be at least 100hrs, preferably at least 500hrs, preferably at least 1000hrs, more preferably at least 4000hrs, most preferably at least 7000hrs. The method disclosed herein can for example be less than or equal to 25000hrs, preferably less than or equal to 20000hrs, or preferably less than or equal to 16000hrs. The methods disclosed herein may be applied for only part of the time during which the methane oxidation catalyst is in use in the methane-containing gas stream. In such scenarios, the method has a total duration, which is less than the duration that the methane oxidation catalyst is in use in the methane-containing gas stream. However, to maximise the regenerative benefits, the method disclosed herein is preferably applied throughout the entirety of the time during which the methane oxidation catalyst is in use in the methane-containing gas stream. Accordingly in this preferable scenario, the method has a total duration, which is substantially the same as the duration that the methane oxidation catalyst is in use in the methane-containing gas stream. The number of CO addition steps will vary depending on the total duration of the method disclosed herein, and the respective values for tonand toff. It will be appreciated that the number of CO addition steps can vary widely and need not be specifically limited for the purposes of the method disclosed herein. As an example, the number of CO addition steps can be in the region of 1000- 10,000,000, or in the region of 10,000-5,000,000, said ranges being understood as including the stated end points of the range in question.

[0042] For a given CO dosing %, the resulting concentration of CO in the gas stream can be tailored as desired. The skilled person will be familiar with appropriate means and methods of tailoring the CO concentration in the gas stream. For example, each CO addition step may comprise adding CO into the gas stream via a valve, wherein the size of the valve opening through which the CO is added is adjusted to achieve the resulting concentration of CO in the gas stream. A wider valve opening achieving a higher resulting concentration of CO in the gas stream, and a narrower valve opening achieving a lower resulting concentration of CO in the gas stream. The skilled person will be familiar with appropriate methods and means to adjust the size of the valve opening. For example, the size of the valve opening may be adjusted in response to an electrical signal, for example one sent from a thermal mass flow controller. Thermal mass flow controllers are widely available, and are generally comprised of a thermal mass flow sensor, a control valve, and a microprocessor based pc board with signal and fieldbus conversion. One such example of a thermal mass flow controller is an IN-FLOW F-203AI available from Bronkhorst.

[0043] Preferably, the two or more CO addition steps result in a concentration of CO in the gas of at least 0.5vol%, more preferably at least 1 vol%, in relation to the total volume of gas inclusive of added CO. Preferably, the two or more CO addition steps result in a concentration of CO in the gas of less than or equal to 5vol%, more preferably less than or equal to 3vol%, most preferably less than or equal to 2.5vol%, in relation to the total volume of gas inclusive of added CO. Examples of potential ranges for the concentration of CO in the gas include 0.5-5 vol%, preferably 0.5-3 vol%, more preferably 0.5-2.5 vol%, in relation to the total volume of gas inclusive of added CO, said ranges being understood as including the stated end points of the range in question. It will be understood that “total volume of gas inclusive of added CO” includes added CO, in addition to any pre-existing CO in the methane-containing gas stream. The concentration of CO in the gas is measured by FTIR (Fourier transform infrared) spectroscopy using a FTIR spectrometer at 180 °C, involving taking a sample of gas downstream of the point at which CO is added to the gas stream, but upstream of the methane oxidation catalyst.

[0044] Preferably in each CO addition step, CO is added via a valve that controls the flow of the CO. More preferably, wherein the valve alternates between an open and closed state, such that the CO is added into the gas stream when the valve is in an open state, and such that no CO is added into the gas stream when the valve is in a closed state, and wherein the size of the valve opening through which the CO is added is adjusted to achieve a resulting concentration of CO in the gas stream.

[0045] The temperature of the gas stream prior to (i.e. upstream of) the methane oxidation catalyst may vary depending on the source from which the gas stream originates. The temperature of the gas stream upstream of the methane oxidation catalyst may for example be 50-900 °C, preferably 100-800 °C, more preferably 150-700 °C, most preferably 150-600 °C, said ranges being understood as including the stated end points of the range in question. The temperature of the gas stream can be measured by widely used methods familiar to the skilled person.

[0046] Although the methods disclosed herein are not dependent on requiring an electrical heater in order to achieve catalyst regeneration, they may be compatible with the presence of such a heater. For example, the gas stream may pass through an electrical heater positioned upstream of the catalyst, wherein the catalyst and the electrical heater are positioned within a housing.

[0047] The following non-limiting examples illustrate the invention.

[0048] EXAMPLE 1

[0049] Methane-containing exhaust gas was generated and passed through a palladiumbased catalyst. The exhaust stream contained 10-12 vol% H2O, and no SO2. CO was added into the gas stream at different CO dosing %s and at different volume concentrations (%vol) controlled using an IN-FLOW F-203AI available from Bronkhorst. Constituents of the exhaust stream were measured using an FTIR spectrometer at 180 °C.

[0050] The results at 0% CO dosing correspond with scenarios where no CO is added to the gas stream, and the results at 100% CO dosing correspond with a continuous CO dose. The remaining CO dosing %s correspond with the following durations of ton (the duration of each CO addition step) and toff (the duration of time elapsing between each CO addition step, where no CO is added to the gas stream):

[0051] 5%: 1s ton / 19s toff

[0052] 10%: 2s ton / 18s toff

[0053] 18%: 4s ton / 18s toff 31 %: 8s ton / 18s toff 47%: 16s ton / 18s toff

[0054] For each CO dosing %, the valve through which the CO was delivered was adjusted to achieve the three CO volume concentrations of 1.5 vol%, 1.0 vol% and 0.5 vol% in the gas, measured using an FTIR spectrometer at 180 °C, involving taking a sample of gas downstream of the point at which CO is added to the gas stream but upstream of the methane oxidation catalyst. The delta methane removal efficiency % was measured in each instance, the delta methane removal efficiency % being the amount of methane removed as a % of total methane originally in the exhaust gas. The respective quantities of methane were measured using an FTIR spectrometer at 180 °C, involving taking a sample of gas immediately upstream of the catalyst (to assess the amount of methane originally in the gas stream) and a sample of gas immediately downstream of the catalyst (to assess the amount of methane remaining in the gas stream after the gas passes through the catalyst). The difference between the methane originally in the gas stream, and the amount of methane remaining in the gas stream after the gas passes through the catalyst, being the amount of methane removed. The results are shown in Figure 3 (for an exhaust gas that is at a temperature of 450 °C) and Figure 4 (for an exhaust gas that is at a temperature of 420 °C), with the results for the three different CO volume concentrations being represented by three different bars for each CO dosing %. It will be appreciated that the presence of three different bars for the results at 0% CO dosing is for purely nominal purposes given at 0% CO dosing, no CO is added to the gas stream, and the delta methane removal efficiency for 0% CO dosing can be read by looking at any one of the three bars.

[0055] As can be seen from Figures 3 and 4, for a given CO dosing %, some increase in the delta methane removal efficiency was observed with increasing CO volume concentration. However, the delta methane removal efficiency for CO dosing %s less than 100% (here, 5%, 10%, 18%, 31 %, and 47%) were comparable to the delta methane removal efficiency for a CO dosing % of 100% i.e. a continuous CO dose. Even the delta methane removal efficiency for the CO dosing % of 5% was comparable to the delta methane removal efficiency for a CO dosing % of 100%.

[0056] Figures 3 and 4 therefore demonstrate that by employing a CO dosing % disclosed herein, the benefits of enhancing methane removal can be achieved whilst saving on significant quantities of CO, this being the case across a number of different CO volume concentrations.

[0057] EXAMPLE 2

[0058] Methane-containing exhaust gas was generated and passed through a palladiumbased catalyst, the catalyst being sourced from a different supplier to Example 1 . The exhaust stream contained 10-12vol% H2O, and no SO2. CO was added into the gas stream at different CO dosing %s and at different volume concentrations (%vol) controlled using an IN-FLOW F-203AI available from Bronkhorst. The exhaust gas was 450 °C. Constituents of the exhaust stream were measured using an FTIR spectrometer at 180 °C.

[0059] The results at 0% CO dosing correspond with scenarios where no CO is added to the gas stream. The remaining CO dosing %s correspond with the following durations of ton(the duration of each CO addition step) and toff (the duration of time elapsing between each CO addition step, where no CO is added to the gas stream):

[0060] 2.5%: 1s ton / 39s toff

[0061] 4%: 1 s ton / 25s toff 5%: 1 s ton / 19s toff 10%: 2s ton / 18s toff 18%: 4s ton / 18s toff 31 %: 8s ton / 18s toff

[0062] For each CO dosing %, the valve through which the CO was delivered was adjusted to achieve the three CO volume concentrations of 1.5 vol%, 2.0 vol% and 3 vol% in the gas, measured using an FTIR spectrometer at 180 °C in the same manner as in Example 1. The delta methane removal efficiency % was measured in each instance, the delta methane removal efficiency % being the amount of methane removed as a % of total methane originally in the exhaust gas, the results of which are shown as a dashed line in Figure 5 (1.5 vol% CO), Figure 6 (2.0 vol% CO), and Figure 7 (3 vol% CO). The delta methane removal % increase is also shown in Figures 5-7 as a solid line. The delta methane removal % increase is the increase in methane removal relative to the scenario when the CO dosing % is 0% i.e. when no CO is added to the gas stream. The respective quantities of methane were measured using an FTIR spectrometer at 180 °C in the same manner as in Example 1 . Across Figures 5-7, the bars are merely a visual representation of the difference in the CO dosing %.

[0063] As can be seen from Figures 5, 6, and 7, although some improvement in the methane removal was observed with increasing CO dosing %, diminishing returns were observed above CO dosing %s above 5%. Figures 5, 6, and 7 therefore demonstrate that the benefits of enhancing methane removal can be achieved whilst saving on significant quantities of CO, this being the case across a number of different CO volume concentrations.

[0064] EXAMPLE 3 Methane-containing exhaust gas was generated and passed through a palladiumbased catalyst, the catalyst being sourced from the same supplier as Example 2. The exhaust stream contained 10-12vol% H2O, and no SO2(Figure 8), 1 ppmv SO2 (Figure 9) and 4 ppmv SO2 (Figure 10). CO was added into the gas stream at different CO dosing %s and at a CO volume concentration of 1.5 vol% (measured in the same manner as in Example 1) controlled using an IN-FLOW F- 203AI available from Bronkhorst. The exhaust gas was 450 °C. Constituents of the exhaust stream were measured using an FTIR spectrometer at 180 °C.

[0065] The results at 0% CO dosing correspond with scenarios where no CO is added to the gas stream. The remaining CO dosing %s correspond with the following durations of ton(the duration of each CO addition step) and toff (the duration of time elapsing between each CO addition step, where no CO is added to the gas stream):

[0066] 5%: 1 s ton / 19s toff

[0067] 10%: 2s ton / 18s toff 18%: 4s ton / 18s toff 31 %: 8s ton / 18s toff

[0068] The delta methane removal efficiency % was measured, the delta methane removal efficiency % being the amount of methane removed as a % of total methane originally in the exhaust gas, the results of which are shown as a dashed line in Figure 8 (no SO2), Figure 9 (1 ppmv SO2), and Figure 10 (4 ppmv SO2). The delta methane removal % increase is also shown in Figures 8-10 as a solid line. The delta methane removal % increase is the increase in methane removal relative to the scenario when the CO dosing % is 0% i.e. when no CO is added to the gas stream. The respective quantities of methane were measured using an FTI R spectrometer at 180 °C in the same manner as in Example 1 . Across Figures 8-10, the bars are merely a visual representation of the difference in the CO dosing %.

[0069] As can be seen from Figures 8-10, although some improvement in the methane removal was observed with increasing CO dosing %, diminishing returns were observed above CO dosing %s above 5%. Figures 8-10 therefore demonstrate that the benefits of enhancing methane removal can be achieved whilst saving on significant quantities of CO, even in the presence of varying quantities of SO2 as well as H2O.

Claims

CLAIMS1. A method of regenerating a methane oxidation catalyst positioned in a methane-containing gas stream, wherein the method comprises two or more CO addition steps; wherein each CO addition step comprises adding CO into the gas stream for a duration of time, wherein the methane oxidation catalyst positioned in the gas stream is contacted with CO added from the two or more CO addition steps; wherein the method further comprises allowing a further duration of time to elapse between each CO addition step, wherein CO is not added into the gas stream over the duration of time elapsing between each CO addition step; wherein the duration of each CO addition step, and the duration of time elapsing between each CO addition step, define a CO dosing %, the CO dosing % being defined as: 100wherein tonis the duration of each CO addition step wherein toff is the duration of time elapsing between each CO addition step; wherein the CO dosing % is greater than 0%, and less than or equal to 70%.

2. The method of claim 1 , wherein the CO dosing % is less than or equal to 50%, preferably less than or equal to 35%, more preferably less than or equal to 10%.

3. The method of claim 1 or claim 2, wherein the CO dosing % is at least 1 %, preferably at least 2%, more preferably at least 3%.

4. The method of any preceding claim, wherein the duration of each CO addition step is 0.01 s-100s.

5. The method of any preceding claim, wherein the duration of time elapsing between each CO addition step is 0.01 s-100s.

6. The method of any preceding claim, wherein the duration of each CO addition step is 0.1s-20s, preferably 0.1 s-10s, more preferably 0.1s-3s.

7. The method of any preceding claim, wherein the duration of time elapsing between each CO addition step is 15s-25s.

8. The method according to any preceding claim, wherein the two or more CO addition steps result in a concentration of CO in the gas of at least 0.5vol%, preferably at least 1 vol%, in relation to the total volume of gas inclusive of added CO.

9. The method according to any preceding claim, wherein the two or more CO addition steps result in a concentration of CO in the gas of less than or equal to 5vol%, preferably less than or equal to 3vol%, more preferably less than or equal to 2.5vol%, in relation to the total volume of gas inclusive of added CO.

10. The method according to any preceding claim, wherein the method has a total duration which is substantially the same as the duration that the methane oxidation catalyst is in use in the methane-containing gas stream.11 . The method according to claim 10, wherein the total duration of the method is at least 100hrs, preferably at least 1000hrs, more preferably at least 4000hrs.

12. The method according to any preceding claim, the method further comprising generating the methane-containing gas from an engine positioned upstream of the catalyst.

13. The method according to claim 12, wherein the method further comprises driving a compressor unit using the engine, and compressing natural gas in a natural gas pipeline using the compressor unit.

14. The method according to any preceding claim, wherein the methane oxidation catalyst comprises palladium, platinum, rhodium, manganese, or combinations thereof, preferably wherein the catalyst comprises palladium.

15. The method according to any preceding claim, wherein the temperature of the gas steam upstream of the catalyst is 150-700 °C, preferably 150-600 °C.

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