A photochemical method for removing methane in a photoreactor

Abstract

The invention discloses a method for removing methane (CH4) content in a fluid with a photoreactor system. The invention enables unprecedented high efficiency and fast removal by non-catalytic photochemical oxidation of methane by conveying a flow of a methane containing fluid through a mixing zone with water, which results in a mixed fluid, and illuminating the mixed fluid with UV radiation to generate photochemical oxidation of methane. The conveying and UV illuminating is performed so that the photochemical oxidation has reaction kinetics with a half-life (T50) of 60 minutes or less. This has been successfully demonstrated across a broad range of methane concentrations and operation conditions, and the invention may therefore provide a significant technological step forward for UV mediated photooxidation of the otherwise stable CH4 molecule to products with less radiative forcing potential and thereby reduce climate changes from methane.

Description

[0001] 83910PC01 1 A PHOTOCHEMICAL METHOD FOR REMOVING METHANE IN A PHOTOREACTOR Technical field of the inventionThe present invention relates to a method for removing methane content in a fluid(liquid or gas) with a photoreactor system. In particular, the present inventionrelates to a corresponding photoreactor with ultraviolet light (UV) illuminationmeans for such photochemical oxidation. The present invention enablesunprecedented high efficiency and fast removal by non-catalytic photochemical oxidation of methane. Background of the invention Methane (CH4 / CH4) is the most dominant anthropogenic greenhouse gas in the atmosphere after carbon dioxide (CO2) and current levels are at a record high. A CH4 molecule has a much greater global warming potential compared to a CO2 molecule and it has been estimated that CH4 is >80 times more potent than CO2 during the first 20 years after release and about 28 times more potent after a century. CH4 therefore accounts for about one third of the estimated global warming since pre-industrial times. Anthropogenic activities are responsible for more than half of the annual methane emission to the atmosphere of about 600 Tg CH4 / yr, and major sources include agriculture, waste disposal, biomass and biofuel burning and fossil fuel production and use. There is currently an imbalance of 5-10% between global CH4 sources and sinks which is why the atmospheric concentration is increasing. The global sinks for atmospheric CH4 are limited to 2 main processes: microbiological oxidation in upland soils (<10%), and photochemical oxidation in the atmosphere (>90%). It has proven extremely difficult to stimulate or control the biological sink for atmospheric CH4 in soils, and this has motivated a recent political and scientific interest in anthropogenic CH4 removal principles. More than 100 countries have recently signed a Global Methane Pledge to reduce emissions of CH4 by at least 30% before 2030. Decreasing or stabilizing CH4 concentrations in the atmosphere would have a more immediate effect on climate than targeting 83910PC01 2 CO2, and it has been suggested that CH4 removal could be the strongest instrument to affect global temperatures over the next decades. CH4 is an inherently stable and inert trace gas with high C–H bond strengths, low electron and proton affinity, and low polarizability. Oxidation of CH4 to CO2 isthermodynamically favourable (ΔHr = -803 kJ / mol) but is challenging at typicalconditions at atmospheric temperature and pressure. Although catalytic transformation processes have been established for CH4, very few will proceed efficiently at ambient conditions and at low mixing ratios. Hence, only a limited number of methods have been suggested for oxidation of CH4 from point and non- point sources and no sustainable concepts have so-far been established for efficient removal or capture of methane from many sources. In contrast, several technological solutions have been proposed for capture of CO2 to combat global warming, but the time may have come to focus more on CH4. CH4 conversion can be more expensive per molecule than conversion of CO2, but this cost could be offset by a greater climate impact and economic value because of methane’s greater radiative forcing potential. Homogenous and heterogenous advanced oxidation processes (AOPs) are promising technologies for removal of organic chemicals from water and air. MostAOPs are based on in-situ production of highly reactive radicals that oxidize thetarget molecule(s) in a gas or water phase. The radicals are unselective and will fragment the target molecule into smaller organic or inorganic molecules. However, many AOPs require addition of various compounds to allow efficient generation of reactive species able to react with the target molecule(s) (e.g., radical precursors and / or metal catalysts). Common radical precursors includehydrogen peroxide (H2O2), ozone (O3), persulfate (e.g., HSO5- and S2O8--),chlorine (Cl2), and common catalysts include ferrous iron (e.g., FeSO4), titanium dioxide (TiO2) and zinc oxide (ZnO). Addition of these supplementary compounds requires exact dosages of reaction mixtures to obtain maximum and sustained activity. In addition, some of these AOP processes require elevated temperatures to proceed efficiently, and some processes will only proceed in a gas phase or in aliquid phase. The need for special phases, addition of chemical reactants andcatalysts, and / or increasing temperatures has some drawbacks in real world operations including cost effectiveness, decreasing activity and need for 83910PC01 3 regeneration of catalysts, and unsustainable waste production with inherent toxicity.Hence, an effective method for removing methane from both gas or liquid withoutadded chemicals or catalysts would be advantageous, and in particular a moreefficient and / or faster method would be advantageous / attractive.Summary of the invention Thus, an object of the present invention relates a simplified and / or more effective method for removing methane from gas and / or liquids, preferably without adding chemicals or catalysts. In particular, it is an object of the present invention toprovide a method and a system that solves the above mentioned problems of theprior art with removing methane from various sources, preferably converting the methane to other chemical compounds with less radiative forcing potentials.Thus, one aspect of the invention relates to a method for removing methanecontent in a fluid with a photoreactor system, the photoreactor system comprising: -an inlet for receiving a flow of a methane containing fluid, preferably from a point source of methane emission, -a source of water, -a mixing zone wherein the flow of methane containing fluid and water is arranged for being combined into a mixed fluid, -a container wherein said mixed fluid is arranged for being conveyed from a first end to a second end in an internal fluid pathway, optionally an intermittent flow of said mixed fluid, and -ultraviolet (UV) illumination means arranged for UV illumination of at least part of said internal fluid pathway, the method comprising:- conveying a flow of a methane containing fluid through said mixing zoneresulting a mixed fluid being conveyed in said internal fluid pathway, and 83910PC01 4- illuminating said mixed fluid with UV radiation so as to generate photochemicaloxidation of methane in the mixed fluid, wherein said conveying and UV illuminating is performed so that said photochemical oxidation has reaction kinetics with a half-life (T50) of 60 minutes or less. Another aspect of the present invention relates to a photoreactor system for removing methane content in a fluid, the photoreactor system comprising: -an inlet for receiving a flow of a methane containing fluid, preferably from a point source of methane emission, -a source of water, -a mixing zone wherein the flow of methane containing fluid and water is arranged for being combined into a mixed fluid, -a container wherein said mixed fluid is arranged for being conveyed from a first end to a second end in an internal fluid pathway, and -ultraviolet (UV) illumination means arranged for UV illumination of at least part of said internal fluid pathway, wherein the photoreactor system is arranged for 1) conveying a flow of a methane containing fluid through said mixing zone resulting a mixed fluid being conveyed in said internal fluid pathway, optionally an intermittent flow of said mixed fluid, and 2) illuminating said mixed fluid with UV radiation so as to generate photochemical oxidation of methane in the mixed fluid, the 1) conveying and 2) UV illuminating being performed so that said photochemical oxidation has reactionkinetics with a half-life (T50) of 60 minutes or less. The conveying and UVillumination is preferably controlled with a process controller, such as a computer or similar. Yet another aspect of the present invention is related to use of UV light to remove methane content in a fluid, wherein the use is in a photochemical oxidation process having reaction kinetics with a half-life (T50) of 60 minutes or less. 83910PC01 5Another aspect of the present invention is providing a method for removingmethane content in a fluid with a photoreactor system, the method comprising:- receiving a flow of a methane containing fluid, preferably from a point source ofmethane emission, -providing a source of water,- mixing the flow of methane containing fluid and water to provide a combinedmixed fluid,- optionally, conveying said mixed fluid from a first end to a second end,optionally an intermittent flow of said mixed fluid, such as in an internal fluid pathway, and- illuminating said mixed fluid with UV radiation so as to generate photochemicaloxidation of methane in the mixed fluid, and wherein said UV illuminating, or said conveying and UV illuminating, is performed so that said photochemical oxidation has reaction kinetics with a half-life (T50) of 60 minutes or less.The present invention is advantageous in that a simplified as compared to state ofthe art technology and / or more effective method and / or system for removingmethane from gas and / or liquids, preferably without adding chemicals or catalysts, is obtained.More specifically, the invention has been successfully demonstrated with two UVfacilitated catalyst-free photochemical processes for removal of CH4 at ambient conditions: a) a combined vacuum UV (185 nm) and UV-C (254 nm) photochemical process for CH4 (VUMOX), and b) a combined ozone (O3) and UV- C photochemical process for CH4 (UMOX).Both processes may proceed with e.g. rainwater as the only added reactant andresulted in rapid CH4 photochemical oxidation. CH4 oxidation may proceed in a gas-water matrix as a wet air oxidation process at ambient pressure (ca. 1 bar) and at ambient temperature (15-40 °C). Laboratory prototypes of the corresponding VUMOX and UMOX systems accomplished >99% removal inrecirculated modes and 70% to >99% CH4 removal efficiency when operated inflow through modes. 83910PC01 6 CH4 concentration between 1.9 ppm (atmospheric levels) and 100,000 ppm CH4was successfully oxidized by the present invention. In some other embodiments,500,000 ppm CH4 was successfully oxidized by the present invention. Transienttransformation product may include methanol (CH3OH) formaldehyde (CH2O),and dihydrogen (H2), and the final oxidation product was carbon dioxide (CO2). The UV based photochemical CH4 oxidation followed second order kinetics, and rate coefficients (k-1) were concentration dependent. Time for 50% removal (T50)of CH4 may be as low as <1-10 minutes (min) depending on the initial CH4concentrations. Ambient CH4 may be oxidized to sub-atmospheric levels (<1.9ppmv) in <2 min. Thus, the invention provides a significant technological potential at ambient conditions for UV mediated photooxidation of the otherwise stable CH4molecule to products with less radiative forcing potential so as to reduce ormitigate climate changes from methane.Still another aspect of the present invention is to provide a method for removingmethane and other volatile organic compounds (VOCs) including hydrocarbons,preferably short-chain hydrocarbons, content in a fluid with a photoreactor systemaccording to the first aspect of the invention. Thus, in the context of the presentinvention, the application of the photoreactor is primarily intended and tested for removing methane in a gas or liquid, but some initial tests and experimentsperformed by the present inventors suggest that other short-chain hydrocarbons,such as ethane, propane, butane, ethene, methanol, formaldehyde etc. and anyisomers, and combinations thereof, may also be removed by the present invention. It is further contemplated that other volatile organic compounds (VOCs) and ammonia (NH3) may be removed by the present invention. Brief description of the figuresFigure 1 shows schematic flow charts with two embodiments, abbreviated VUMOXand UMOX, of the photoreactor system according to the present invention,Figure 2 shows two photographs of the laboratory setup for testing the present invention, 83910PC01 7 Figure 3 shows results of an embodiment of UV mediated methane oxidation in aphotochemical reactor being operated in recirculation mode until completedegradation with section A: Time course; B: illustration of initial first-orderkinetics, and C: illustration of initial second-order kinetics,Figure 4 shows the effect of the reaction matrix, liquid water or humid air, on therelative oxidation of CH4,Figure 5 shows illustrations of some contemplated key processes in the UVmediated photochemical oxidation of methane, where the reactions in panel A are particularly important in the aqueous phase, whereas reactions in Panel B are more important in the gas phase, and reactions in Panel C are with an ozone generating device,Figure 6 shows the effect of initial CH4 concentrations of 100, 1000 and 10000ppmv on the UV mediated photochemical methane oxidation in a photoreactor operated in recirculation mode,Figure 7 shows the relationship between the initial CH4 loading and the half-life(T50) of CH4 in the photoreactor system (Panel A) and the volumetric removalrate of CH4 (Panel B),Figure 8 shows an embodiment of UV mediated methane oxidation during start-upof a photochemical reactor being operated in one-pass (flow-through) mode untilsteady state, where approximate steady-state was obtained after about 40 min with >70% removal, Figure 9 shows the effect of reactor turbulators on UV mediated photochemical methane oxidation in a photoreactor operated in one-pass (flow-through) mode with different residence times, Figure 10 shows the effect of relative humidity (RH) on UV mediated photochemical methane oxidation in reactors operated in one-pass (flow-through) mode, where Low: RH < 10%; Medium: 10% >RH <80%; and High: RH >80%, 83910PC01 8 Figure 11 shows the effect of dry and humid air on oxidation of atmospheric methane in air (Panel A), and oxidation of atmospheric methane and elevated methane concentrations in water (Panel B), Figure 12 shows the effect of partial recirculation on UV mediated photochemical methane oxidation in a photoreactor operated in one-pass (flow-through) mode, Figure 13 shows the effect of the present invention on removal of methane in a natural gas source with a mixture of hydrocarbons from the North Sea, and Figure 14 is a flow chart of the method according to first aspect of the invention. The present invention will now be described in more detail in the following. Detailed description of the invention Definitions Prior to discussing the present invention in further details, the following terms and conventions will first be defined: In the context of the present application, it will be understood by the skilledperson in chemistry that the concept of “removing methane” from the fluid – gasor liquid - may be understood as decreasing or lowering the methane content,partially or completely, by the photochemical oxidation in the present invention. Itmay also mean abatement, eradication, destruction, etc. and / or conversion ofmethane so as to lower the methane concentration in the fluid as a result of thephotochemical oxidation in the present invention.In the context of the present application, it will also be understood by the skilledperson in chemistry that the concept of photochemical oxidation may be broadlydefined as a process where electromagnetic irradiation, such as ultraviolet light,initiates chemical reactions resulting in oxidation of a substance, i.e. in thecontext of this invention primarily methane. The skilled person will furtherunderstand that photochemical oxidation in connection with water often involves the formation of the hydroxyl radical, conventionally named “•OH”, which is highly 83910PC01 9reactive. This will be further explained below in relation with Figure 5 and Figure5B, where some possible hydroxyl radical generating processes will be discussed, but it should be emphasized that the principle and teaching of the presentinvention is not limited to the suggested reaction scheme for hydroxyl radicalgenerating. Thus, the present invention is not necessarily limited to thesehypothetical hydroxyl radical generating processes. In the context of the present application, it will also be understood by the skilledperson in chemistry that the concept of UV light can be defined broadly aselectromagnetic irradiation in the range of approximately 10-400 nanometres(nm), i.e. next to visible light (above ca. 400 nm) at the upper end of the range,and close to X-rays at the lower end of the range. Historically, UV light has beensubdivided into UV A (315-400 nm), UV B (280-315 nm) and UV C (100-280 nm),or alternatively into near UV (300-400 nm), middle UV (20-300 nm), far UV (112-200), and extreme UV (10-121 nm). The wavelength interval of 10-200 nm is alsoknown as “vacuum UV” because of the relative strong absorption in air, and thisinterval has been advantageously applied in the context of the present invention,presumably due to its efficiency in hydroxyl radical formation. The skilled personin chemistry will however understand that various wavelengths of UV light may be applied in the context of the present invention. Additionally, the skilled person will readily understand that the source of the UV light may vary depending on the circumstances and / or application of the present invention. Thus, the presentinvention has been thoroughly experimentally tested with low-pressure highoutput amalgam VUVs lamps (5, 50 and 200 W), but the invention is not limitedto this particular kind of UV source or UV lamp. On the contrary, the skilled personwill readily understand that any commercial implementation and exploitation ofthe present invention may alternatively be performed by UV light emitting diodes(LEDs), metal halide lamps, and / or excimer lamps.In the context of the present application, it will also be understood that aphotoreactor system is an entity used to carry out photochemical reactions, whichare chemical reactions initiated by light, especially UV light. Such photoreactorsystems are designed to provide controlled light exposure to the reactants,ensuring consistent and reproducible results as the skilled person will readilyunderstand. 83910PC01 10 In the context of the present application, it will also be understood by the skilledperson in chemistry that the properties of the mixed fluid combined from waterand the fluid with methane depending on circumstances, such as pressure, temperature, humidity etc. in and / or around the mixing zone. In someembodiments, the mixed fluid may be described as humid air with a certainrelative humidity (RH) relative to the maximum amount. Thus, relative humidity(RH) is generally a measure of how much moisture is in the air compared to the maximum amount of moisture the air can hold at a given temperature. It istypically expressed as a percentage. In context of the present application, theterm “wet air” can be used synonymously with humid air in this application,though wet air is typically considered as having a rather high humidity. As it willbe explained in more detail below, the presence and amount of water has been found to be an important feature of the present invention. In the context of the present application, it will further be understood that the mixing zone may have various shapes, sizes, and / or working regimes. The skilled person working with fluid dynamics will thus readily contemplated that mixing oftwo or more fluids can be performed overall divided into passive (using geometryand flow) and active (using external energy input) methods. Here is a structuredoverview of various mixing types:1. Mechanical Mixing These methods rely on moving parts to stir or agitate fluids. ^Impellers and Stirred TanksRotating blades generate turbulence, which enhances mixing by stretching and folding fluid elements. ^Static MixersStationary baffles or helical elements inside a pipe force the fluid to split and recombine repeatedly. Turbolators have been tested with the presentinvention, cf. Figure 9. ^Paddle or Propeller MixersCommon in industrial reactors for liquid–liquid or liquid–gas mixing. 83910PC01 11 2. Turbulent Mixing When the flow is in the turbulent regime (high Reynolds number), chaotic eddies and vortices distribute fluid components quickly. ^Jet MixingOne fluid is injected into another at high velocity, creating strong shear and turbulent eddies. ^Shear Layer InstabilitiesVelocity differences at fluid interfaces (Kelvin–Helmholtz instabilities) cause rapid entrainment and mixing. 3. Laminar (Diffusive) Mixing When Reynolds numbers are low (microfluidics or viscous fluids), mixing is dominated by molecular diffusion. ^Chaotic AdvectionEven in laminar flows, certain flow patterns (e.g., serpentine microchannels) can stretch and fold fluid parcels to enhance diffusion. 4. Density and Buoyancy-Driven Mixing When fluids have different densities or temperatures: ^Rayleigh–Taylor InstabilitiesOccur when a denser fluid lies above a lighter one in a gravitational field, leading to complex interpenetration patterns. 5. Active External Energy Methods Energy is supplied from outside the fluid system. ^Acoustic MixingUltrasound or vibrations create microstreaming and cavitation, enhancing mixing at small scales. ^Pulsed Pressure or Oscillatory FlowCreates periodic accelerations that promote mixing without mechanical stirrers. 6. Multiphase Interactions When mixing involves gas–liquid or liquid–solid phases: 83910PC01 12 ^Bubble Column MixersRising gas bubbles create circulation and interfacial mixing. ^Fluidized BedsSolid particles suspended in a fluid cause intense mixing due to particle– fluid interactions. In the context of the present application, it will thus be understood that mixingcan be performed in various ways as outlined above. The term mixing zone canaccordingly be broadly interpreted due to the variety of possible ways of mixing two fluids. In a particular embodiment, the mixing zone can be an integrated part of the container, where the UV illumination means is arranged for photochemical oxidation, e.g. the mixing zone may be arranged in a portion of the container. Inother embodiments, the container itself can be used as a mixing zone, e.g. beforethe UV illumination is started mixing can take place in the container, especially ifthe UV illumination is performed in batch like way with an intermittent flow through the photoreactor system.In other embodiments, the fluid containing methane may also contain a sufficientamount of water, which can be used as an inherent source of water for thephotochemical oxidation according to the present invention. Thus, if the fluidcontaining methane is for example humid air from a pig stable, or an undergrounddrilling liquid with methane also containing water, the present invention may beimplemented if such fluids initially contain a sufficient level of water for generatingphotochemical oxidation and removal of methane from such fluids. In theseembodiments, the source of water is thus omitted because sufficient water is thusalready present in the methane containing fluid according to the various aspects of the invention.In some embodiments, a liquid containing both methane and sufficient water maypossibly be vapourised before being UV illuminated according to the presentinvention. In the context of the present application, it will also be understood by the skilledperson in chemistry that the photochemical oxidation can be performed in a way,where the flow of the methane containing fluid, and after that the flow of the 83910PC01 13mixed fluid are both continuously, or substantially continuously, to provide anoverall continuously, or substantially continuously, flow through the photoreactorsystem according to the present invention. However, in some embodiments, thephotoreactor system may have an intermittent flow i.e. where the flow is temporarily interrupted, preferably to allow more time for the photochemicaloxidation of methane in the photoreactor system, more preferably to improve orincrease the removal of methane in the received fluid, as it will be demonstratedin below. Thus, it is contemplated that the photoreactor system may be operatedin a batch mode, where the flow through the photoreactor system is paused for some time, such as 5, 10, 15, 20, or 30 minutes.In one embodiment, said conveying and UV illuminating may be performed so thatsaid photochemical oxidation has reaction kinetics with a half-life (T50) of 30 minutes or less, such as 15 minutes or less, such as 7 minutes or less, such as 3 minutes or less, such as 1 minute or less, such as even 0.5 minutes or less.In another embodiment, said reaction kinetics may be approximately described bya first order rate equation with respect to the methane concentration, the corresponding pseudo first order rate coefficient k being at least 0.05 min-1corresponding to a half-life (T50) of approximately 14 minutes or less, at a methane concentration below or around 2000 ppm, preferably the correspondingpseudo first order rate coefficient k being at least 0.1 min-1 corresponding to ahalf-life of approximately 7 minutes or less, at a methane concentration below or around 2000 ppm. In yet another embodiment, said photochemical oxidation may proceed at concentrations covering a range from ambient methane concentrations, preferablyless than 2 ppm CH4, up to at least 500,000 ppm CH4 in air. Thus, the presentinvention has been successfully tested across a quite broad range of methane concentrations.In other embodiments, said reaction kinetics may be related to the methaneconcentration, and wherein said reaction kinetics may be selected from the groupconsisting of: 83910PC01 14• ambient methane concentrations about 2 ppm CH4 or less, and the pseudofirst order rate coefficient being at least 0.05 min-1corresponding to half-life of up to 14 minutes, such as less than 5 minutes, or the pseudo first order rate coefficient being at least 0.2 min-1corresponding to a half-life of up to 4 minutes, such as less than 1 minute;• low methane concentrations about 2-1000 ppm, and the pseudo first orderrate coefficient being at least 0.2 min-1 corresponding to a half-life of less than 4minutes, such as less than 2 minutes; and• high methane concentrations about 1000 ppm CH4 or above, such as evenat least 500,000 ppm, and the pseudo first order rate coefficient being at least 0.01 min-1corresponding to a half-life up to 60 minutes, such as less than 5minutes, or the pseudo first order rate coefficient being at least 0.1 min-1corresponding to a half-life of up to 10 minutes, such as less than 2 minutes.In some particularly advantageous embodiments, the received flow of methanecontaining fluid may be a gas, such as air comprising methane, and said gas and said water is arranged for being combined into a mixed fluid, such as by appropriate nozzles or vaporizers etc., wherein the mixed fluid can be described as humid air with a relative humidity (RH) above zero promoting saidphotochemical oxidation of methane, preferably above 2%, preferably above20%, more preferably above 40%, most preferably above 60% or even above80%. The presence and amount of water is an important parameter asconvincingly demonstrated by the present invention. Thus, the photochemical oxidation described in this invention can procced in fluids consisting of air withrelative humidity >0%, but alternatively the photochemical oxidation described inthis invention can also take place in fluids comprising one or more liquids havingwater-like structures, functions and / or properties, like for example liquidscomprising water, such as oil-gas condensates, manure from agricultural sites,municipal waste stream, ground water resources etc. In another alternative, thephotochemical oxidation described in this invention may in addition to thepresence of water comprising one or more liquids having a synergistic effecttogether with water, hydrogen peroxide and peroxy-monosulfate already beingtested for use in this context. 83910PC01 15 In other advantageous embodiments, wherein the received flow of methane containing fluid may be a liquid with dissolved methane, and said liquid and saidwater is arranged for being combined into a mixed fluid, such as by valvescontrolling the relative proportions of the constituents, wherein the mixed fluidcan be described as a mixed liquid. The liquid with methane may originate fromfor example oil and / or gas off-shore plants, where methane may be present inhigh concentrations, and where the present invention may be applied for removing at least partly the methane from the liquid. In other embodiments, the method according the is also capable of removing methane emitted from animal husbandry including livestock farming in agriculture. Such fluids may include air and liquids from cattle, sheep, pigs, poultry and other animals. The present invention has successfully been tested for removing methane in a stable with pigs, cf. Fig. 13B below. In yet other advantageous embodiments, the photochemical oxidation may be performed at ambient temperature, preferably 10-40 deg. C, more preferably 15-30 deg. C. Thus, the present invention is relatively simple to implement becausetypically no heating is required, like in some prior art AOP processes and systems.Preferably, the photochemical oxidation may alternatively or additionally beperformed at ambient pressure, such as 1 atm (101,325 Pa). Alternatively oradditionally, the photochemical oxidation of methane is not assisted by any solid-state catalyst in the internal fluid pathway like in some prior art AOP processesand systems. Thus, the present invention is relatively simple to implement.Further, the invention may also be implemented without addition of oxidants oroxidant precursors like in some prior art AOP processes and systems, whichmakes the present invention very suitable for upscaling to an industrial scale.In some embodiments, though some amount of hydrogen peroxide (H202),persulfates (e.g., oxone), or similar or comparable oxidants, could be added toenhance the photochemical oxidation, preferably in water, e.g. for offshore oil orwastewater. Furthermore, the present invention may be implemented by various sources of water, i.e. the present invention is not critical dependent on the purity of thewater. Thus, nearly all water sources have been tested by the inventors and can 83910PC01 16 be successfully applied: rain, sea, ground water, demineralized water, tap water etc. Furthermore, the incoming fluid need not have a high purity and could be any methane containing fluids, like natural gas, biogas, and other methane containing gas, based on air, or liquid streams, like sewers or similar waste streams. In someembodiments, the fluid containing methane could originate from a farm, anoffshore oil / gas plant, a drinking water plant, wastewater plant, or another kind ofindustrial site. Preferably, the source is a point source of methane, but in other embodiments the fluid with methane may not be a point source of methane, for example the present invention may remove methane from atmospheric air as has been convincingly demonstrated, cf. Figure 11.In some embodiments, the ultraviolet (UV) illumination means may have awavelength below approximately 320 nm, 250 nm, 200 nm, or 172 nm, preferably with a local maxima intensity around 185 nm or 254 nm for ozone generation. Thus, various standard bands of UV; such as vacuum UV 10-200 nm etc.UVC (254 nm) and / or VUV (185 nm) at a 4:1 ratio hereafter referred to as VUV-UVC may be applied.In a preferred embodiment, it is contemplated that the method may comprise the steps of reacting water in a liquid phase with UV light to produce •OH and H+, and allowing the methane to react with said •OH to provide transformationproducts including CO2, cf. Scenario A in Figure 5 for more details.In another preferred embodiment, the method may comprise the steps of reacting water in gas phase with UV light to produce O3, said O3 reacting with water to produce •OH, and allowing the methane to react with said •OH to providetransformation products including CO2 cf. Scenario B in Figure 5 for more details.In yet another preferred embodiment, the method may comprise the steps of reacting water in gas phase with O3, said O3 reacting with water to produce •OH, and allowing the methane to react with said •OH to provide transformation products including CO2. cf. Scenario C in Figure 5 for more details, where in oneembodiment O3 could be from an ozone generating device. 83910PC01 17 In some advantageous embodiments, the methane content may be reduced by converting at least 50% of said methane into another chemical compound, such as at least 60%, such as at least 70%, such as at least 80%, such as at least 90%of said methane into another chemical compound, such as carbon dioxide. Thus,the invention may significantly remove the methane content from a gas or liquidsupplied to the photoreactor system, where a photochemical oxidation accordingto the invention is taking place. Even though carbon dioxide is a greenhouse gas,carbon dioxide has a significantly lower radiation forcing potential than methane,and therefore the net contribution of the present invention in combating climate change could be really significant.In some advantageous embodiments, the quantum yield of the UV illuminationmay be at least 1 %, such as even 2 %, preferably at least 3 %. Apparentquantum yield (methane molecule oxidized per photon) may be defined asfollows: where : ^^^^= molecules of methane oxidizede = plug efficiency of the light sourcePinput = power input to the light source ^= wavelength of light sourceh = Planck’s constant c = speed of lightThe plug efficiency of a Low pressure VUV lamps typically range from 30-35%^ ^^^^^^^= ^^^^^^^^^^^where: 83910PC01 18 Puv is the radiant power output in the UV wavelength range. Pelectrical is the total electrical power input to the lamp.Thus, apparent quantum yield is generally a measure of photons absorbed or usedthrough the photochemical reaction taking place in the oxidation. Estimates andinitial calculations / models overall suggest quantum yield of around at least 1%, atleast 2%, at least 3%, at least 4%, at least 5%, at least 6%, at least 7%, or atleast 8%, or even higher, could be required for optimum methane removal, andespecially for viable economic methane removal in general. The present inventionhas achieved an apparent quantum yield values from 0.05% to ca 5%, cf. Table 2,and in some cases up to ca. 8% quantum yield. It is contemplated that for further improvement in the quantum yield, and therefore economic feasibility, of the present invention, the UV illumination may be further improved, in particularapplying UV LEDs could be an advantageous option for further optimisation in thisrespect.In other advantageous embodiments, the volumetric methane removal of thephotochemical oxidation may be at least 5 g / (m3h), such as at least 20 g / (m3h),such as even at least 20 g / (m3h), preferably at least 40 g / (m3h), more preferablyat least 50 g / (m3h), most preferably at 70 g / (m3h). Thus, the invention isbeneficial for quickly removing a significant amount of methane, cf. Table 2 for anoverview of the obtained results. According to the photoreactor system according to the second aspect, theinvention may comprise an ozone generator (reference 25 in Figure 1B)generating device arranged for conveying ozone into said internal fluid pathway. Preferably, the ozone generator generating device may further comprise auxiliary UV illumination means, preferably said auxiliary UV illumination means being arranged for UV illumination with an intensity maximum around 254 nm. Thus,the invention may beneficially be implemented with a dedicated ozone generator,cf. Figure 1B for further details below.In advantageous embodiments, the photoreactor system may comprise one ormore turbulator(s) arranged along a longitudinal part of said internal fluid 83910PC01 19pathway for prolonging the pathway through the internal fluid pathway. Resultsprovided below demonstrate this effect, cf. Figure 9. In other beneficial embodiments, the photoreactor system may comprise additional fluid conveying means arranged for complete or partial recycling said flow of methane containing fluid at least twice through said container, such asthree, four, five, six, or even higher times. Thus, the invention may furtherremove methane in these embodiments. In some embodiments, the photoreactor system may comprise additional fluid confining means, such as valves, taps, and / or regulators, arranged for confining said mixed fluid without flow in said container, such as operating the photoreactorsystem in a batch mode also called an intermittent flow, where the mixed fluid iscontained for some predetermined time in the internal fluid pathway, as opposedto a continuous flow mode of operation through the internal fluid pathway.Methane removal according to the present invention can also be accomplished in acompartmentalized photoreactor system with multiple reaction zones where different light regimes and UV wavelength are applied in different compartmentsof the photoreactor system. A compartmentalized photoreactor system can resultin a more efficient methane removal under some conditions compared to a singlecompartment photoreactor system. A compartmentalized photoreactor may alsobe able to remove ozone (O3) from the exhaust gas, for example by having one or more dedicated compartments for removal of ozone. It should be noted that embodiments and features described in the context of one of the aspects of the present invention also apply to the other aspects of the invention. All patent and non-patent references cited in the present application, are hereby incorporated by reference in their entirety. The invention will now be described in further details in the following non-limiting examples. 83910PC01 20 Examples: Materials and Methods Gases, water and chemicals Ambient air with 1.9 ppm CH4 was collected in Aalborg, Denmark. Compressedgases from by Air Liquid Denmark were also included: CH4 (4.0 - 99.99% purity);N2 (5.0 - 99.999% purity); He (5.0 - 99.999% purity) and O2 (3.5 - 99,95%purity). Natural gas from the North Sea was collected at Lille Torup Gas StorageFacility, Denmark.Rainwater was collected in Northern Jutland in Aalborg Municipality, Denmark.Seawater from the North Sea was collected in the Bay of Woe, Denmark.Groundwater was obtained from Aalborg Municipality and consisted of hardgroundwater (12°dH) with pH 7.6 and a NVOC concentration of 0.99 mg / L. MilliQ water was used as reference for experiments examining the effects of water matrices. Analytical methods CH4 concentrations in gas and water samples were determined by gas chromatography and by using a tunable diode laser analyzer (Los Gatos Research, ABB). CH4 concentrations in liquid samples were quantified based on a headspace equilibration technique (Cedric et al. 2014). Headspace CH4 concentrations were determined using a Thermo 1300 trace gas chromatograph equipped with a Hyasep Q column and an FID detector (GC-FID). Nitrogen was used as carrier gas and the oven, injector, and detector temperatures were 80 °C, 120 °C, and 240 °C, respectively. Methanol (CH3OH) in water samples were determined by direct injection of liquid samples using the GC-FID system described above.Concentrations of formaldehyde (CH2O) were determined based on the Hantzschreaction with β-diketone, 4-amino-3-pentene-2-one. The product 3,5-diacetyl-1,4- dihydroluthydine was detected by spectrometry and fluorimetry using a UV / VIS spectrophotometer (Thermo ScientificTM GENESYSTM 20) and a Perkin Elmer Victor X2 multilabel plate reader and by liquid chromatography using a KnauerASM 2.2L HPLC system equipped with a C18 column. CO2, H2 and O2 in gas 83910PC01 21 samples were quantified on an Agilent 7890A gas chromatograph equipped with a TCD detector (GC-TCD), and a parallel set of HP-PlotQ and HP-Plot 5A columns. Helium was used as carrier gas and the oven, injector, and detector temperatures were 60°C, 150°C, and 250°C respectively. UV photoreactor Prototypes of the UV based methane oxidation systems (VUMOX and UMOX) were constructed for examination of the potential for photochemical CH4 oxidation(Figure 1A and 1B, and Figure 2). The VUMOX process was evaluated in two VUVphotoreactors: i. a tubular stainless-steel reactor with an inner diameter of 53 mm, a length of 1270 mm, and a reactor volume of 1.7 L equipped with a 200 Wlow-pressure high output amalgam VUV Hg lamp; ii. a tubular stainless-steelreactor with a length of 523 mm a diameter of 204 mm and a reactor volume of 15 L equipped with a 50 W low-pressure high output amalgam VUV Hg lamp(Figure 1 and Figure 2). The UV lamp simultaneously emitted UVC (254 nm) andVUV (185 nm) at a 4:1 ratio hereafter referred to as VUV-UVC irradiation. The UMOX process was evaluated in the same two UV reactors as described above butin this setup the inlet was connected to an O3 generator, and the systems wereequipped with a 200 W and 50 W low-pressure high output amalgam UV lamps that only emitted UV-C (254 nm). This is hereafter referred to as UV-O3. The UMOX process was also evaluated in a setup with 5W LED lamps emitting 254 nm. Samples for gas and water analytes were withdrawn with needle and syringe from different sampling ports before and after UV irradiation. The photoreactor was connected to a Huber Minichiller cooling system operated at 10 °C. The reactor was equipped with temperature, flow and UV sensors and an energy meter to allow on-line monitoring of relevant parameters. UV irradiation experiments were conducted by loading the photoreactor with water or air mixtures and operating the reactor with and without UV light (UV irradiation vs Dark controls). The UVphotoreactor was operated at a temperature between 20 and 40 °C and pressureof 1 bar without addition of catalysts. Control experiments consisted of a time course and sampling but without UV irradiation. Different water, methane, ozoneand oxygen concentrations (H2O - CH4 – O3 - O2 mixing ratios) were evaluatedas well as different flow regimes including intermittent flow and partial recirculation. 83910PC01 22 Reaction matrixExperiments were conducted to examine the effects of water and air concentrationand water source on the photochemical CH4 oxidation in the UMOX and VUMOX systems. MilliQ water was used as reference in these experiments that also included groundwater, seawater and rainwater. The experiments employed water vapor generated by different diffusors, humidifiers, and membranes to generate different “air-in-water” and “water-in-air” matrices. The relative humidity in inlet and outlet samples of humid air experiments were determined using a relative humidity meter. Statistics Comparisons of results were carried out using the nonparametric Kruskal-Wallis H test for evaluating differences among multiple conditions, and the Mann-Whitney U test (Wilcoxon rank sum test) for evaluating differences between two defined conditions. Statistical analyses were carried out using KaleidaGraph 5.0 (Synergy Software, USA) with a significance level of p < 0.05.Results and discussionThe ambition of the current study was to establish a wet air oxidation process that could facilitate efficient CH4 oxidation at ambient temperature and pressure without addition of catalysts. Initial experiments were conducted with different UV configurations and UV mediated photochemical removal of CH4 was achieved, cf. Figure 3. The process was initiated at room temperature 20-22 °C and the finaltemperature was <45 °C. The process proceeds without addition reactants(oxidant precursors) in the VUMOX process and with added O3 in the UMOX process. The absence of added catalysts and heating in the current process is highly attractive because photocatalytic methane oxidation often suffers from low service life of catalysts, potential poisoning of the catalyst by intermediates resulting in higher costs, and a requirement for expensive heating to activate thecatalyst. The successful UV mediated methane oxidation process according to thepresent invention proceeded as a second order process, cf. Panel C, but the initialoxidation activity could also be represented as pseudo first order reaction as shown in Figure 3, Panel B. 83910PC01 23The CH4 oxidation process proceeds efficiently in both liquid water and in watersaturated air (wet air), cf. Figure 4. However, the volumetric oxidation rates were greater in wet air due to the relatively low solubility of CH4 in water. UV mediated photochemical removal in air is more effective than in an water for several reasons : (1) longer penetration of the UV light in air as fewer photons are absorbed by gases than water molecules; (2) increased exposure between photons and targets; (3) increased levels of oxidative species generated in the presence of O2; (4) the lower levels of carbonates as the scavengers of free radicals in air; (5) higher mobility of dissociated species in air. UV mediated oxidation without catalysts can generate energetic photons and highly oxidative species including hydroxyl radical (•OH), O3 , excited state oxygen atom (O(1D)), and ground state oxygen atom (O(3P))) by the photodissociation of H2O and O2. A key step in the oxidation of CH4 in the photochemical reactors was the initial reaction between CH4 and the OH radical,cf. reaction [1] below. This step is a potential rate limiting step in thephotochemical oxidation of methane due to the relatively low reaction rate coefficient of this reaction. An overview of potential reactions involved in the wet air photochemical CH4 oxidation is illustrated in Figure 5. The overview includes reactions that are particularly important in the aqueous phase and reactions that are more important in the gas phase. CH4 + •OH → •CH3 +H2O [1]The exact reaction mechanisms for oxidation of CH4 in the UV photoreactors arecurrently being investigated further, but different radicals and transformationproducts have been identified including hydroxyl radicals (•OH), dihydrogen (H2) and formaldehyde (CH2O). It is well-known that CH4 can be photochemically oxidized by hydroxyl radicals in the troposphere. Transformation of the otherwise stable CH4 molecule occurs in series of reactions that include i. oxidation of CH4 and formation of formaldehyde (CH2O); ii. oxidation of CH2O and formation of CO; iii. oxidation of CO and formation of CO2. In the present study, a screening of rection products from the UMOX and VUMOX systems identified methanol,formaldehyde, dihydrogen (H2) and CO2 as some of the transformation products 83910PC01 24 after photochemical oxidation of CH4. These products have a lower radiative forcing and global warming potential than that of CH4. Low concentrations of higher hydrocarbon such as ethane, propane and butane may also be generated. These hydrocarbons also have a lower radiative forcing and global warming potential than that of CH4.In a preferred embodiment, the •OH may facilitate the methane to react with said•OH to provide transformation products including CO2. The •OH can be producedfrom UV radiation of water or humidified air, cf. Scenario A in Fig. 5, or from UVradiation of air to produce O3, said O3 reacting with water to produce •OH , cf.Scenario B in Fig. 5, or from UV radiation of O3 supplied to the reactor from anozone generating device to produce •OH, cf. Scenario C in Fig. 5.Fig. 5B shows more detailed reaction equations:UV mediated oxidation of methane is well-known from the lower atmosphere where reactive species in the troposphere oxidize trace gases including carbon monoxide (CO) and methane. Reactive species that are necessary for oxidation ofsomewhat inert molecules can also be generated in photochemical reactors withshort-wave UV irradiation of aqueous matrices. For example, vacuum UV irradiation of gas containing O2 and H2O at wavelengths <200 nm cause photodissociation of H20 and O2 resulting in generation of oxidizing species such as •OH (6.7 eV), and ozone decomposition at wavelengths<310 nm can contribute to •OH production in photoreactors as indicated in eq. [1-6].As also seen in eq. [1–6], short-wave UV irradiation leads to generation of severalreactive species including excited state oxygen atom (O(1D)), ground stateoxygen atom (O(3P)), hydroxyl radicals (•OH) and ozone (O3). O3 cannot oxidizethe stable CH4 molecule whereas O(1D) and •OH with very high oxidationpotentials have an inherent capacity (E0 =2.42 V and E0 =2.8 V at 25C). Asindicated in eq. [1–6], •OH can be produced in gas phase processes by severalphotodissociation reactions involving H2O and O2. •OH radicals are among thestrongest known oxidizing agents and can oxidize many persistent organic molecules by hydrogen abstraction. 83910PC01 25The process includes combined irradiation at 185 nm and 254 nm and the reactivespecies and transformation products identified during methane oxidation includedO3, •OH, H2, CH2O, and CO2 whereas CO was not detected (<10 ppm).Reactions involving such products are also observed in atmospheric reactionsduring methane oxidation. The inventors therefore suggest that reactionsin the process likely include some or all of the reactions shown in eq. [1-17].Eqs. [1–17] in Figure 5B suggest that the ability of the process to successfullyoxidize the stable methane molecule likely depended on multiple redox reactions and transformation products, but significant oxidation could nonetheless be achieved within minutes. The UV-C irradiance (mW / cm2) at 254 nm measured during oxidation process increased when the lamp was turned on but then decreased over time. A similar pattern was observed in both recirculatedmode and in one pass mode or flow-through mode. The decreases in measuredUV-C signal were likely related to generation of transformation products withsignificant UV-C absorption. Methane has negligible absorption at 254 nm butpotential reactants and transient transformation products such as O3, CH3OH,CH2O, and H2O vapor have significant absorption coefficients at 254 nm. Incomparison, the global warming potential of a compound is a measure of howmuch infrared thermal radiation a molecule can absorb (>780 nm). Interestingly,all the potential end products generated in the process from methane[Eqs. 7–17] have a lower radiative forcing and global warming potential compared to that of the parent molecule.However, O3 is generated in the process, and this compound is an importantgreenhouse gas and air pollutant. In the current version of the invention,transformation products remaining in the exhaust air such as CH3OH, CH2O andO3 were removed by passing air from the reactor outlet through a polishing filterwith activated carbon and granular Carulite 200. Carulite 200 consists of CuMnO3,and this material removed volatile organic transformation products (VOCs) andsurplus O3 to below detection (<1 ppm). However, it should be noted that thepolishing filter did not remove CH4, and methane oxidation only occurred in the photochemical reactor after UV irradiation. 83910PC01 26 Oxidation of CH4 in the photochemical reactors were affected by the initial CH4 concentrations, cf. Figure 6 with a broad interval of concentrations. As a result, the pseudo first order rate coefficient decreased with increasing methane concentration. However, it should be noted that although the rate coefficients were lower at elevated CH4 concentrations, the volumetric CH4 removal was greater due to greater initial CH4 concentrations. The time for 50% removal of the added CH4 (T50) was generally 2-10 min depending on the initial CH4 concentration. Interestingly, the photochemical reactors also rapidly removed CH4 at atmospheric levels (1.9 ppm) to below ambient in <2 min. Hence, the T50 for atmospheric CH4 removal in the UV photoreactors were then many thousand times lower than the half-life for CH4 in the atmosphere. Figure 7 shows the relationship between the initial CH4 loading and the T50 of CH4 in the photoreactor system (Panel A) and the volumetric removal rate of CH4(Panel B). The T50 decreased with decreasing initial CH4 loading (g / m3) whereasthe volumetric CH4 removal (g / m3 / h) increased non-linearly with increasing CH4 loading (g / m3). An example of UV mediated methane oxidation during start-up of a photochemical reactor operated in flow-through mode is shown in Figure 8. The reactor was operated at 1 L / min at RH >90% until steady state. Approximate steady-stateconcentrations were obtained after about 40 min with 74% CH4 removal. Recentexperiments have shown that a methane removal of 90% or higher may be possible. The reactor geometry affected the UV mediated CH4 oxidation particularly at low flow rates when operated in flow-through modes, cf. Figure 9. The decreasing flow rates corresponded to increasing mean residence times and the greatest methane during removal (>50%) was observed when the reactor was equipped with turbulators (bafflers) to increase air mixing at low flows (Figure 9). Greater residence time and increased mixing results in higher number of photons absorbed by H2O and more reactive species and longer reaction time for the initialoxidation of CH4 (Equation 1 above). 83910PC01 27 CH4 oxidation in the UV based photochemical process was affected by humidity and the greatest removal in flow-through operations was obtained at a RH >80%, cf. Figure 10. The best overall removal efficiencies >70% in a UV reactor operated in flow-through mode occurred at relatively low CH4 concentrations (Figure 7), long reactor residence times (Figure 9), and a high relative humidities / RHs (Figure 10). Figure 11 (Panel A) shows the effect of dry and humid air on oxidation of atmospheric methane in air, whereas Figure 11 (Panel B) shows oxidation of atmospheric methane and elevated methane concentrations in water. In the leftPanel A, it is clearly demonstrated that the effect of humid air is significant whenoperating on atmospheric air with methane, e.g. initially around 2 ppmv. In the right Panel B, the effect of the invention is convincingly demonstrated in liquid water with various concentrations of methane. The removal of methane is faster with decreasing methane concentration, atmospheric air in water being inserted for comparison. Figure 12 shows another graph with the effect of partial recirculation through the photoreactor on the methane concentration in one-pass (flow-through) mode. Thus, the effect of the invention can be significantly enhanced by partial (here) or full recirculation (not shown here). Figure 13 shows the effect of the present invention on removal of methane in a natural gas source with a mixture of hydrocarbons from the North Sea.The almost complete removal of methane in just 20 minutes is compellinglyshown (Figure 13), and other hydrocarbons present in the gas mixture were alsoremoved. Figure 13B shows additional experiments performed: A (left) Removal of methanefrom diluted North Sea natural gas and pig stable air by in the VUMOX system. B(right) Removal % at ambient temperature. Photooxidation and Removal% for puremethane (99.99%) is shown for comparison. 83910PC01 28 Figure 14 is a flow chart of the method according to first aspect of the invention.Thus, the invention relates to a method for removing methane content in a fluidwith a photoreactor system, cf. reference 100 in Figure 1A called VUMOX and reference 200 in Figure 1B called UMOX for two embodiments. The photoreactorsystem comprising:-an inlet for receiving a flow of a methane containing fluid, such as air, preferablyfrom a point source of methane emission, such as a farm,-a source of water, such as an inlet connected to the humidifier,-a mixing zone wherein the flow of methane containing fluid and water is arranged for being combined into a mixed fluid, such as a humidifier,-a container, cf. reference 10 in Figure 1A or reference 20 in Figure 1B, whereinsaid mixed fluid is arranged for being conveyed from a first end to a second end inan internal fluid pathway (not shown here), i.e. inside the container, and-ultraviolet (UV) illumination means arranged for UV illumination of at least part of said internal fluid pathway, the method comprising:S1 conveying a flow of a methane containing fluid through said mixing zoneresulting a mixed fluid being conveyed in said internal fluid pathway, optionally an intermittent flow of said mixed fluid,S2 illuminating said mixed fluid with UV radiation so as to generate photochemicaloxidation of methane in the mixed fluid, andS3 the conveying and UV illuminating is performed so that said photochemical oxidation has reaction kinetics with a half-life (T50) of 60 minutes or less.The invention may be implemented in one or more separate steps in time, i.e. S1,S2, and / or S3 at the same time, or consecutively in time, depending on thedesired operation of the photoreactor system, e.g. a continuous flow operation oran intermittent flow or a batch operation, as the skilled person will readilyunderstand.In short, the invention discloses a method for removing methane (CH4) content ina fluid with a photoreactor system cf. Figure 1A and 1B. The invention enables 83910PC01 29 unprecedented high efficiency and fast removal by non-catalytic photochemical oxidation of methane by conveying a flow of a methane containing fluid through a mixing zone with water, which results in a mixed fluid, and illuminating this mixed fluid with UV radiation to generate photochemical oxidation of methane. The conveying and UV illuminating is performed so that the photochemical oxidation has reaction kinetics with a half-life (T50) of 60 minutes or less. This has been successfully demonstrated across a broad range of methane concentrations and operation conditions, and the invention may therefore provide a significant technological step forward for UV mediated photooxidation of the otherwise stable CH4 molecule to products with less radiative forcing potential and thereby reduceclimate changes from methane, cf. Figures 3 and 6.

[0002] 83910PC01 30TABEL 1 (next page)Summary of main results for UV mediated methane oxidation (VUMOX and UMOX). Mode: B = Batch; R = Recirculated; F = Flow through (one pass).Humidity: Low Humidity <5% RH; Medium humidity 5 – 60 % RH; High humidity>60% RH. Initial CH4 concentration: Atm = Ambient CH4 (≤10 ppm); Low CH4 (10< CH4 <1000 ppmv); High initial CH4 (≥1000 ppmv). Grey shading: The most electrical energy efficient conditions (kWh / g CH4) are indicated with grey shading.

[0003] 83910PC01 31 5 Pseudo first T50 order (appar Volumetric Initial CH ent 4 tor Mode Matrix Humidityconcentration r CH4 removalProcess Reacate half-life) coefficient3 -1[min-1] [min] g [m x h]VUMOX Photoreactor 1 R Water High Atm 0.851 0.8 0.002VUMOX Photoreactor 1 R Water High Low 0.544 1.3 0.080VUMOX Photoreactor 1 R Water High High 0.28 2.5 0.041VUMOX Photoreactor 1 R Air Low Atm 0 0.000VUMOX Photoreactor 1 R Air High Atm 0.237 2.9 0.022VUMOX Photoreactor 1 R Air High Low 0.286-0.435 1.6-2.4 1.348-2.051VUMOX Photoreactor 1 R Air High High 0.11-0.57 1.2-6.3 5.186-26.872VUMOX Photoreactor 1 R Air High High 0.17 3.95 75.55VUMOX Photoreactor 1 F Air High Atm 0.18 3.9 0.017VUMOX Photoreactor 1 F Air High High 0.08 8.7 3.772VUMOX Photoreactor 1 R Air (natural gas) High Low 0.314 2.21 7.402VUMOX Photoreactor 1 R Air (natural gas) High High 0.266 2.61 91.202VUMOX Photoreactor 2 R Air High Low 0.05-0.076 9.1-13.9 0.23-0.349VUMOX Photoreactor 2 R Air High High 0.014-0.015 46.2-49.5 0.643-0689VUMOX Photoreactor 2 F Air High Low 0.03-0.08 8.7-23.1 0.138-0.367VUMOX Photoreactor 2 F Air High High 0.013-0.059 11.7-53.3 0.06-0.271Photoreactor 1 VUMOX +F Air High Low 0.08 8.7 0.367Photoreactor 2UMOX Photoreactor 3 B Air Medium Low 0.024 28.9 0.11UMOX Photoreactor 3 F Air Medium Low 0.030-0.048 14.6-23.3 0.138-0.22UMOX Photoreactor 1 F Air High Low 0.08-0.26 2.6-8.6 0.367-1.194UMOX Photoreactor 1 F Air High Atm 0.214 3.2 0.02UMOX Photoreactor 2 R Air High Low 0.0318 21.80 0.146UMOX Photoreactor 2 R Air High High 0.0133 52.12 0.611UMOX Photoreactor 2 R Air High High 0.0044 157.53 2.020UMOX Photoreactor 2 F Air High Low 0.06 11.55 0.253-0.294UMOX Photoreactor 2 F Air High Low 0.04 17.33 0.303-0.367UMOX Photoreactor 2 F Air High High 0.0234 29.62 1.075UMOX Photoreactor 2 F Air High High 0.0128 54.15 5.878 83910PC01 32TABEL 2 – Quantum YieldQuantum yield (Φ) of vacuum UV mediated methane oxidation.Mode: R = Recirculated; F = Flow through (one pass).Initial CH4 concentration: Low CH4 (CH4 <1000 ppmv); High CH4 (≥1000ppmv). Reactor Mode MatrixIni^al CH4concentra^on Quantum Yield (%) Photoreactor 1 F Air Low 0.05Photoreactor 1 F Air High 1.6-2.1Photoreactor 2 R Air Low 0.07-0.55Photoreactor 2 R Air High 0.71-1.61Photoreactor 2 F Air Low 0.09-0.68Photoreactor 2 F Air High 1.21-4.76

[0004] 83910PC01 33 References Bustillo-Lecompte, C et al. 2022. Advanced Oxidation Processes: Applications,Trends, and Prospects. IntechOpen, United Kingdom.Jackson R.B., Solomon E.I., Canadell J.G. Cargnello M., & Field C.B. 2019. Methane Removal and Atmospheric Restoration. Nature Sustainability 2: 436. Sun X., Li C., Yu B., Wang J., and Wenhui Wang W. 2023. Removal of gaseous volatile organic compounds via vacuum ultraviolet photodegradation: Review and prospect. Journal of Environmental Sciences 125: 427. Zhang J., Wang Y., Wang Y., Bai Y., Feng X., Zhu J., Lu X., Mu L., Ming T., de Richter R., and Li W. 2022. Solar Driven Gas Phase Advanced Oxidation Processesfor Methane Removal - Challenges and Perspectives. Chemistry. 16: 28.H. Akimoto. Atmospheric Reaction Chemistry (Springer Atmospheric Sciences).1st ed. 2016 Edition. ISBN-13: 978-4431558682, ISBN-10: 4431558683 Ming, T., Li, W., Yuan, Q., Davies, P., de Richter, R., Peng, C., Deng, Q., Yuan, Y., Caillol, S., and Zhou, N.: Perspectives on removal of atmospheric methane, Advances in Applied Energy, 5, 100085, https: / / doi.org / 10.1016 / j.adapen.2022.100085, 2022.

Claims

83910PC01 34 Claims1. A method for removing methane content in a fluid with a photoreactor system,the photoreactor system (100, 200) comprising:-an inlet for receiving a flow of a methane containing fluid, preferably froma point source of methane emission, -a source of water, -a mixing zone wherein the flow of methane containing fluid and water isarranged for being combined into a mixed fluid,-a container (10, 20) wherein said mixed fluid is arranged for beingconveyed from a first end to a second end in an internal fluid pathway, and-ultraviolet (UV) illumination means arranged for UV illumination of at leastpart of said internal fluid pathway,the method comprising:- conveying a flow of a methane containing fluid through said mixing zoneresulting a mixed fluid being conveyed in said internal fluid pathway, optionally anintermittent flow of said mixed fluid, and- illuminating said mixed fluid with UV radiation so as to generate photochemicaloxidation of methane in the mixed fluid,wherein said conveying and UV illuminating is performed so that saidphotochemical oxidation has reaction kinetics with a half-life (T50) of 60 minutesor less.

2. The method according to claim 1, wherein said conveying and UV illuminating is performed so that said photochemical oxidation has reaction kinetics with a half- life (T50) of 30 minutes or less, such as 15 minutes or less, such as 7 minutes or less, such as 3 minutes or less, such as 1 minute or less, such as even 0.5 minutes or less.83910PC01 353. The method according to claim 1 or 2, wherein said reaction kinetics can beapproximately described by a first order rate equation with respect to themethane concentration, the corresponding pseudo first order rate coefficient kbeing at least 0.05 min-1corresponding to a half-life (T50) of approximately 14minutes or less, at a methane concentration below or around 2000 ppm,preferably the corresponding pseudo first order rate coefficient k being at least0.1 min-1 corresponding to a half-life of approximately 7 minutes or less, at amethane concentration below or around 2000 ppm.

4. The method according to claim 1 or 2, wherein said photochemical oxidation proceeds at concentrations covering a range from ambient methaneconcentrations, preferably less than 2 ppm CH4, up to at least 500,000 ppm CH4in air.

5. The method according to claim 4, wherein said reaction kinetics are related tothe methane concentration, and wherein said reaction kinetics are selected from the group consisting of: ^ambient methane concentrations about 2 ppm CH4 or less, and the pseudofirst order rate coefficient being at least 0.05 min-1corresponding to half- life of up to 14 minutes, such as less than 5 minutes, or the pseudo firstorder rate coefficient being at least 0.2 min-1 corresponding to a half-life ofup to 4 minutes, such as less than 1 minutes;^ low methane concentrations about 2-1000 ppm, and the pseudo first orderrate coefficient being at least 0.2 min-1 corresponding to a half-life of lessthan 4 minutes, such as less than 2 minutes; and^ high methane concentrations about 1000 ppm CH4 or above, such as evenat least 500,000 ppm, and the pseudo first order rate coefficient being atleast 0.01 min-1 corresponding to a half-life up to 60 minutes, such as lessthan 5 minutes, or the pseudo first order rate coefficient being at least 0.1min-1 corresponding to a half-life of up to 10 minutes, such as less than 2minutes.83910PC01 366. The method according to any of claims 1-5, wherein the received flow ofmethane containing fluid is a gas, and said gas and said water is arranged forbeing combined into a mixed fluid, wherein the mixed fluid can be described ashumid air with a relative humidity (RH) above zero promoting said photochemicaloxidation of methane, preferably above 20%, more preferably above 40%, mostpreferably above 60% or even above 80%.

7. The method according to any of claims 1-5, wherein the received flow of methane containing fluid is a liquid with dissolved methane, and said liquid and said water is arranged for being combined into a mixed fluid, wherein the mixed fluid can be described as a mixed liquid.

8. The method according to any of the preceding claims, wherein thephotochemical oxidation is performed at ambient temperature, preferably 10-40deg. C, more preferably 15-30 deg. C.

9. The method according to any of the preceding claims, wherein thephotochemical oxidation is performed at ambient pressure, such as 1 atm(101,325 Pa).

10. The method according to any of the preceding claims, wherein thephotochemical oxidation of methane is not assisted by any solid-state catalyst inthe internal fluid pathway.

11. The method according to any of the preceding claims, wherein the ultraviolet(UV) illumination means has a wavelength below approximately 320 nm, 250 nm,200 nm, or 172 nm, preferably with a local maxima intensity around 185 nm or254 nm for ozone generation.83910PC01 37 12. The method according to any of the preceding claims, wherein the methodcomprises the steps of reacting water in liquid phase or humidified air with UVlight to produce •OH and H+, and allowing the methane to react with said •OH to provide transformation products including CO2.

13. The method according to any of the preceding claims, wherein the method comprises the steps of reacting water in gas phase with UV light to produce O3, said O3 reacting with water to produce •OH, and allowing the methane to react with said •OH to provide transformation products including CO2.

14. The method according to any of the preceding claims, wherein the method comprises the steps of reacting water in gas phase with O3, said O3 reacting with water to produce •OH, and allowing the methane to react with said •OH to provide transformation products including CO2.

15. The method according to any of the preceding claims, wherein the methanecontent is reduced by converting at least 50% of said methane into anotherchemical compound, such as at least 60%, such as at least 70%, such as at least 80%, such as at least 90% of said methane into another chemical compound.

16. The method according to any of the preceding claims, wherein the quantum yield of the UV illumination is at least 1 %, such as even 2 %, preferably at least 3 %.

17. The method according to any of the preceding claims, wherein the volumetricmethane removal of the photochemical oxidation is at least 5 g / (m3h), such as atleast 20 g / (m3h), such as even at least 20 g / (m3h), preferably at least 40g / (m3h), more preferably at least 50 g / (m3h), most preferably at 70 g / (m3h).83910PC01 3818. A photoreactor system (100, 200) for removing methane content in a fluid,the photoreactor system comprising: -an inlet for receiving a flow of a methane containing fluid, preferably from a point source of methane emission, -a source of water, -a mixing zone wherein the flow of methane containing fluid and water is arranged for being combined into a mixed fluid, -a container (10, 20) wherein said mixed fluid is arranged for beingconveyed from a first end to a second end in an internal fluid pathway, and -ultraviolet (UV) illumination means arranged for UV illumination of at least part of said internal fluid pathway,wherein the photoreactor system is arranged for 1) conveying a flow of a methanecontaining fluid through said mixing zone resulting a mixed fluid being conveyed in said internal fluid pathway, optionally an intermittent flow of said mixed fluid,and 2) illuminating said mixed fluid with UV radiation so as to generatephotochemical oxidation of methane in the mixed fluid, the 1) conveying and 2)UV illuminating being performed so that said photochemical oxidation has reactionkinetics with a half-life (T50) of 60 minutes or less.

19. The photoreactor system according to 18, wherein the photoreactor systemcomprises an ozone generating device (25) arranged for conveying ozone intosaid internal fluid pathway.

20. The photoreactor system according to 19, wherein the ozone generating device comprises auxiliary UV illumination means, preferably said auxiliary UV illumination means being arranged for UV illumination with an intensity maximum around 254 nm.

21. The photoreactor system according to any of claims 18-20, wherein the photoreactor system comprises one or more turbulator(s) arranged along alongitudinal part of said internal fluid pathway for prolonging the pathway through83910PC01 39 the internal fluid pathway.

22. The photoreactor system according to any of claims 18-21, wherein thephotoreactor system comprises additional fluid conveying means arranged forcomplete or partial recycling said flow of methane containing fluid at least twicethrough said container.

23. The photoreactor system according to any of claims 18-21, wherein thephotoreactor system comprises additional fluid confining means, such as valves,arranged for confining said mixed fluid without flow in said container.

24. Use of UV light to remove methane content in a fluid, wherein the use is in aphotochemical oxidation process having reaction kinetics with a half-life (T50) of60 minutes or less.

25. A method for removing methane content in a fluid with a photoreactor system, the method comprising: -receiving a flow of a methane containing fluid, preferably from a pointsource of methane emission, -providing a source of water, -mixing the flow of methane containing fluid and water to provide acombined mixed fluid, -optionally, conveying said mixed fluid from a first end to a second end,optionally an intermittent flow of said mixed fluid, such as in an internal fluid pathway, and -illuminating said mixed fluid with UV radiation so as to generatephotochemical oxidation of methane in the mixed fluid, andwherein said UV illuminating, or said conveying and UV illuminating, is performedso that said photochemical oxidation has reaction kinetics with a half-life (T50) of60 minutes or less.

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