Method and plasma reactor for the production of hydrogen gas

Abstract

A method for the production of hydrogen gas comprising (i) providing a DC electrical power supply, (ii) providing a plasma reactor with chamber 105, plasma torch 135 with a plasma cathode extending in

Description

The present invention relates to a method and plasma reactor for the production of hydrogen gas. More particularly, the method relies on a plasma arc for the thermolytic breakdown of a hydrogencontaining feedstock to form hydrogen gas at commercial scale. The major world economies are under pressure to transition from fossil-based energy generation to renewable sources, thus reducing anthropogenic impacts on the environment. Hydrogen is anticipated to play an important role in this shift, predominantly as a green fuel for energy intensive sectors such as aviation, international shipping, and large energy consuming foundation industries as a fuel source, such as in glass, cement and steel production. Hydrogen is also a storable energy source which may smooth out the transient nature of energy supply from renewable energy sources such as photovoltaic panels and wind / tidal turbines. Hydrogen is also expected to be used for a wide range of vehicles, as is presently found in fuel cell electric vehicles, with heavy-duty transportation being the prime user. There are several available technologies for hydrogen production. Presently, the most widely used technology is steam reformation of natural gas, which at the same time is the least environmentally friendly. The other commercially available technologies consist of different permutations of electrolysis such as alkaline electrolysis and proton exchange membrane (PEM) electrolysis. Due to the environmental and commercial (increasing cost of carbon as CO2 emissions) implications, hydrogen production based on natural gas will need to be replaced by technologies driven by renewable energy sources, which will produce environmentally friendly hydrogen, so-called “green hydrogen” (the standard given by the Green Hydrogen Organisation being <1 kg COa-eq per kg H2 taken as an average over a 12-month period). The production cost of green hydrogen constitutes the main barrier for its deployment One of the components driving the cost up is the efficiency and operating costs of hydrogen production processes, i.e. the cost of consumed electrical energy and chemical conversion efficiency. This invention relates to a process (and apparatus) for achieving enhanced dissociation rates of hydrogen-containing raw materials and efficiency, exploiting the thermal, electric, and chemical effects of plasma, for the large-scale commercial production of hydrogen gas. Various examples of laboratory testing and small-scale hydrogen production with plasma are given in the following literature publications: Journal of Energy Engineering 132(3) 2006 “Hydrogen Production by Plasma Electrolysis” relates to a small-scale investigation of the feasibility of producing small stable atmospheric plasmas between DC electrodes and a water surface. IOP Conf. Series: Materials Science and Engineering 162, 2017, 012010 “Hydrogen production by plasma electrolysis reactor of KOH-ethanol solution” relates to an electrolysis reactor of 1 litre capacity and the effects of voltage and cathode depth on plasma electrolysis were studied. The Chinese Journal of Process Engineering 6(3), 2006, 396 “Experimental Study of Plasma Underliquid Electrolysis in Hydrogen Generation” discloses a contact glow discharge electrolysis (CGDE) in which plasma is sustained by DC or pulsed DC glow discharges between an electrode and the surface of the surrounding electrolyte. Jpn. J. Appi. Phys. 44(1 A), 2005, 396 “Hydrogen Evolution by Plasma Electrolysis in Aqueous Solution” discloses an electrolysis cell of 1 litre capacity for plasma electrolysis. A tungsten cathode is separated from a platinum mesh anode by an inverted quartz glass funnel. Journal of The Electrochemical Society 166(6), 2019, E181 “Effect of Competing Oxidizing Reactions and Transport Limitation on the Faradaic Efficiency in Plasma Electrolysis” studies the plasma electrolysis reduction of chloroacetate and ferricyanide. WHEC13-16 June 2006 - Lyon France “Hydrogen production by thermal water splitting using a thermal plasma” presents a brief state of the art of water thermal plasmas, showing the temperatures and quench velocity ranges technologically achievable. Thermodynamic properties of a water plasma are presented and discussed and a kinetic computational model is presented, describing the behaviour of splitted products during the quench in a plasma plume for various parameters, such as the quench rate. J. Electrochem. Soc. 167, 116504 “On the Non-Faradic Hydrogen Gas Evolution from Electrolytic Reactions at the Interface of a Cathodic Atmospheric-Pressure Microplasma and Liquid Water Surface” provides a quantitative study of the H2 gas evolved from a cathodic atmospheric-pressure microplasma formed in argon gas contacting the surface of an acidic aqueous solution. “Confirmation of anomalous hydrogen generation by plasma electrolysis” in 4th Meeting of Japan CF Research Society, 2003 provides a study of hydrogen generation by an electrolysis cell. Energy Sci. Eng. 9, 2021,267 “Comprehensive assessment of hydrogen production in argon-water vapors plasmolysis” reports a simultaneous investigation of a theoretical and experimental analysis of hydrogen production from an atmospheric pressure argon-water vapor mixture as a function of DBD plasma applied voltage. CN 102502487 A relates to plasma-enhanced photocatalytic hydrogen production. WO 2008 / 141369 A1 relates to electrolysis of water for producing hydrogen and oxygen gas. WO 2019 / 096880 A1 relates to a method and device for plasma-induced water splitting. WO 2023 / 222903 A2 is the Applicant’s own prior disclosure of a method and plasma treatment unit for the commercial scale production of hydrogen plasmolysis. The method comprises arranging a first electrode comprised within a plasma torch above a surface of a reservoir of aqueous electrolyte, with a second electrode submerged in the aqueous electrolyte. The Applicant’s video entitled “Tetronics Hydrogen Plasmolysis Animation” published online 24 June 2024 (https: / / www.youtube.com / watch?v=9DUSsjxSMz8&t=77s&ab channel=tetronicsplasma) discloses a schematic of plasmolysis module apparatus for producing hydrogen gas made up of two sections: a plasmolysis chamber and an electrolyser. Despite the various developments in the prior art, there remains a need for an improved method and apparatus for the commercial production of hydrogen gas. There are significant longevity, control, and therefore safety, considerations for commercial production due to the large volume of mixtures of hydrogen and oxygen gas being formed at elevated temperatures. There is also an ongoing need to increase the efficiency of hydrogen production and therefore, the overall energy efficiency. The inventors developed the present invention with the aim of addressing these issues with the prior art or at least to provide a commercially viable alternative thereto. Thus, a first aspect of the present invention provides a method for the production of hydrogen gas, the method comprising: (i) providing a DC electrical power supply; (II) providing a plasma reactor comprising: (a) a plasma chamber, (b) a plasma torch comprising a plasma cathode extending into the plasma chamber, (c) a plasma anode extending into the plasma chamber, and (d) first and second spray systems, each extending into the plasma chamber; (ill) establishing a DC electric potential between the plasma cathode and the plasma anode to generate and sustain a reaction zone about a plasma arc therebetween; (iv) providing a spray of a hydrogen-containing feedstock into the reaction zone from the first spray system, whereby a mixture of gases comprising hydrogen gas is formed in the plasma chamber by decomposition of the hydrogen-containing feedstock; and (v) providing a spray of water into the plasma chamber adjacent the reaction zone from the second spray system, whereby the spray of water cools and dilutes the mixture of gases formed in step (iv). The present disclosure will now be described further. In the following passages, different aspects / embodiments of the disclosure are defined in more detail. Each aspect / embodiment so defined may be combined with any other aspect / embodiment or aspects / embodiments unless clearly indicated to the contrary. In particular, any feature indicated as being preferred or advantageous may be combined with any other feature or features indicated as being preferred or advantageous. A second aspect provides a plasma reactor for the production of hydrogen gas comprising: (a) a plasma chamber; (b) a plasma torch comprising a plasma cathode extending into the plasma chamber; (c) a plasma anode extending into the plasma chamber whereby, in use, a plasma arc can form between the plasma cathode and the plasma anode providing a reaction zone; (d) a first spray system configured to provide, in use, a spray of a hydrogen-containing feedstock into the reaction zone; and (e) a second spray system configured to provide, in use, a spray of water into the plasma chamber adjacent the reaction zone. The apparatus is therefore suitable for carrying out the method of the first aspect, and equally, the method is preferably performed in the apparatus. Features described in respect of the method may therefore be combined with the apparatus, and vice versa. A particularly preferred embodiment of the reactor for the production of hydrogen gas comprises: (a) a plasma chamber; (b) a plasma torch comprising a plasma cathode extending into the plasma chamber; (c) a multi-functional device comprising: (c1) a plasma anode extending into the plasma chamber whereby, in use, a plasma arc can form between the plasma cathode and the plasma anode providing a reaction zone, (c2) a first spray system comprising a first annular passage surrounding the plasma anode for providing, in use, a spray of a hydrogen-containing feedstock into the reaction zone, and (c3) a second spray system comprising a second annular passage surrounding the first annular passage for providing, in use, a spray of water around the reaction zone. A further aspect of the present invention provides an apparatus for the production of hydrogen gas, the apparatus comprising a plasma reactor as described herein, which further comprises an electrolyser having one or more electrolyser cathodes and one or more electrolyser anodes each spaced apart by a membrane in a reservoir for a hydrogen-containing electrolyte; wherein the plasma anode is in electrical communication with the one or more electrolyser cathodes. The present invention relates to a method for the production of hydrogen gas, more preferably the combined electrolytic and thermal production of hydrogen gas. Such a method may simply be referred to herein as plasmolysis or the plasmolytic production of hydrogen gas. The method generates a first gas stream comprising hydrogen gas in the plasma chamber (alternatively referred to as a plasmolysis chamber or plasma vessel) by decomposition of a hydrogen-containing feedstock that is introduced via a spray system. In preferred embodiments, the method simultaneously and synergistically generates a second gas stream comprising hydrogen gas in an electrolyser by decomposition of the hydrogen-containing electrolyte. The method thus involves a first step of providing a DC electrical power supply. It is generally particularly preferred that the power supply provides power from a renewable energy source. Preferably the power supply provides a substantially constant current and variable voltage. The present invention advantageously allows for use of the apparatus to convert excess renewable energy into hydrogen on-site (e.g. as part of a wind or solar / photovoltaic farm / power station). The method is performed using a plasma reactor. As described herein, the plasma reactor comprises: (a) a plasma chamber, (b) a plasma torch comprising a plasma cathode extending into the plasma chamber, (c) a plasma anode extending into the plasma chamber, and (d) first and second spray systems, each extending into the plasma chamber. The plasma chamber therefore provides a container within which the plasma reactions occur for the plasmolytic production of hydrogen gas. As such, the plasma electrodes (i.e. plasma cathode and plasma anode) extend into the plasma chamber with a gap provided between their tips in which a plasma arc is formed during use. The electrodes and other current carrying components may be formed from any conventional conductive material, typically copper or the like, and are preferably provided with a refractory metal tip, preferred refractory metals including tungsten and molybdenum. More preferably, the tip is a refractory ceramic, lanthanum hexaboride being particularly preferred. Such materials are more resistant to etching under the conditions of the plasmolysis reducing the risk and need for reactor down-time. The electrodes may generally be “rod” shaped and housed within a body. The plasma cathode, for example, is comprised within a plasma torch. Suitable plasma torches for use in the present invention may be known in the fields of extractive metallurgy. One suitable plasma torch assembly is described in WO 2019 / 092416 A1. The method comprises establishing a DC electric potential between the plasma cathode and the plasma anode to generate and sustain a reaction zone about a plasma arc therebetween (i.e. “transferred arc” mode where the arc bridges the plasma cathode and plasma anode). The voltage is applied whilst providing a flow of inert ionisable gas through the plasma torch and past the tip of the plasma cathode to the gap between the tips of the electrodes. Preferably the plasma arc is a thermal plasma. That is, the plasma is a thermodynamic equilibrium and is not a non-equilibrium “cold plasma”. The inert ionisable gas that is used is preferably argon or nitrogen. Preferably the electric potential has a voltage of from 100 to 2,000 V, preferably from 650 to 1,500 V, and a current of from 10 to 200 A, preferably from 50 to 150 A. Such ranges are particularly suitable for a 50kW supply. The ranges allows for operational phases and transience. The inventors have found that such high-voltage and scaled-current is particularly suitable for large-scale hydrogen production by plasmolysis since these parameters are found to favour hydrogen gas production and minimise energy losses which is essential for commercial hydrogen gas production.. The plasma anode and plasma cathode are generally arranged co-axially and are preferably oriented vertically. That is, the rod shaped electrodes are aligned along the same axis. Preferably the distance between the tip of the plasma cathode and the plasma anode is from 1 to 75 mm (which may be regarded as the length of the plasma arc when in use). With a co-axial arrangement, the plasma torch and plasma anode extend through opposite faces of the plasma chamber. Preferably, the plasma cathode extends into the plasma chamber through an upper surface (providing a downward flow of the inert ionisable gas) and the plasma anode extends through a bottom surface of the plasma chamber. Preferably the spray system extends through the bottom surface of the chamber, and as described herein, preferably is integrated into a device which comprises the plasma anode. Preferably the plasma cathode and / or plasma anode is liquid-cooled. The liquid coolant is not particularly limited with various suitable coolants commercially available. Water however may preferably be used for simplicity. More specifically, the tip of the plasma anode / cathode may be liquid-cooled so to actively cool the tip during the production of hydrogen gas. This further helps to reduce etching of the tip. Cooling may be provided by coolant inlet and outlet channels integrated within the electrode. One specific unique aspect of the present invention that has been developed by the present inventors is the use of liquid sprays. Despite the advantages afforded by prior solutions to improving the production of hydrogen, such as the Applicant’s WO 2023 / 222903 A2, these involve the plasmolysis of a reservoir of feedstock and generating a plasma arc between the cathode and surface of the reservoir of liquid. One such problem, particularly when generating a mixture of gases comprising hydrogen within a sealed vessel rather than the open atmosphere, is the effect of the variable pressure within the vessel during gas production. The inventors have found that, without careful control, pressure changes within the vessel affect the level of the feedstock reservoir. This results in a change in the length of the plasma arc which in turn affects the rate of plasmolysis (e.g. due to a spontaneous change in current / voltage) leading to change in the rate of gas production which therefore feedback further pressure changes in the reaction vessel. The inventors have found that the configuration of the present invention and use of a spray leads to a more consistent and steady-state production of gases (hydrogen) which greatly enhances the stability of the plasma arc, avoids localised high voltages which can result in arc tracking, and improves the overall efficiency of the production process. More specifically, the method comprises providing a spray of a hydrogen-containing feedstock into the reaction zone from the first spray system, whereby a mixture of gases comprising hydrogen gas is formed in the plasma chamber by decomposition of the hydrogen-containing feedstock, whilst also providing a spray of water into the plasma chamber adjacent the reaction zone from the second spray system, whereby the spray of water cools and dilutes the mixture of gases formed in step (iv). The plasma reactor comprises first and second spray systems which extend into the reaction chamber. That is, the outlet of each spray system is arranged within the plasma chamber and provides its own independent spray of liquid, each spray system further comprising conduits or channels for delivering the necessary liquid to the spray outlet from an external reservoir or tank. Each spray system may comprise one or more nozzles at the outlets to shape and direct the spray. As described further herein in respect of a particularly preferred embodiment, the spray systems may be integrated within the same device and may then share a common nozzle to direct the sprays from each system. The thermal decomposition of water is not predicted to occur until about 3,000°C at atmospheric pressure, and this is also predicted to increase as the pressure of the vessel increases, for example, in order to overcome kinetic limitations. As described herein, a preferred pressure within the vessel may be up to about 5.0 barg such that the thermal decomposition temperature for water is predicted to increase, at which point water is expected to be fully decomposed at about 4,000°C. A plasma arc itself is of a very high temperature (capable to separate water into hydrogen and oxygen), but the highest temperatures are only expected in very localised spots in close proximity to the arc. As a result of this, it is expected that water may only thermally decompose under very localised conditions. However, the feedstock may also decompose through interaction with highly reactive free radicals and photons generated through the ‘plasma effect’ providing a reaction zone which typically greatly reduce energy barriers for reactions I cracking to occur by offering alternative reaction pathways. The reaction zone is a localised region around the plasma arc which enables the thermolytic decomposition of the feedstock. The first spray system therefore provides a spray of a hydrogen-containing feedstock into this reaction zone. The heat of the reaction zone is expected to rapidly vaporise the small droplets of the liquid feedstock at which point the hydrogen-containing feedstock is decomposed producing hydrogen gas and other gaseous by-products. Preferably the hydrogen-containing feedstock is water, methanol or ammonia, methane, or a combination thereof. As such, in some embodiments the hydrogencontaining feedstock may be fed as a gas into the plasma chamber and into the reaction zone (e.g. ammonia or methane). Some preferred combinations of feedstock include liquid aqueous solutions with water (e.g. aqueous methanol or aqueous ammonia), such solutions may include at least 50 wt%, such as from 60 to 80 wt% water. These feedstocks are rich in hydrogen, chemically simple and readily available. Methanol and ammonia have lower breakdown temperatures helping to ensure breakdown temperatures are achieved in the reaction zone and contribute to catalytic decomposition. Preferably, any water present in the feedstock is substantially pure (e.g. deionised water). The feedstock consisting of water is particularly preferred as this allows for the production of so-called “green hydrogen” with essentially zero greenhouse gas emissions (when combined with power from renewable energy sources). When compared to the prior art, it is also particularly advantageous and therefore preferred that the feedstock does not comprise salts which are typically required to increase the electrical conductivity of the liquid for electrolysis, e.g. sodium and / or potassium salts of hydroxide and / or halides and / or carbonates. The decomposition of the feedstock increases the total moles and therefore volume of gases. For example, 1 mole of water produces 1 mole of hydrogen gas and 0.5 mole of oxygen gas. The vaporisation, and increase in temperature and pressure about the plasma arc, provides a driving force for the product gases to move away from the reaction zone. A preferred further step of the method involves continuously removing and recovering the mixture of gases from the plasma chamber via a gas outlet. One or more gas outlets are typically provided in an upper surface of the chamber. The expansion of gases and thermal gradient from the plasma arc allows for the continuous spray of new feedstock into the reaction zone. This approach of using a spray to introduce the feedstock into the reaction zone is, without wishing to be bound by theory, believed to minimise the formation of a gas boundary layer around the plasma which is likely to have resulted in energy and mass transfer limitations in known methods for the plasmolytic decomposition of water. Preferably the pressure in the plasma chamber is from 1.5 to 5.0 barg (i.e. bar gauge), preferably about 3 barg. Whilst a lower pressure may be useful to reduce the predicted thermal decomposition temperature of the feedstock thereby increasing decomposition rate, the increased pressure above 1.5 barg is particularly useful for improving the efficiency of hydrogen separation in a subsequent gas purification system as described further herein. Whilst there is not specific upper limit, there are safety considerations with operating the reactor with pressures that are too great, with up to 5.0 barg being generally acceptable. At least where oxygen is present in the feedstock (particularly water as this consists of hydrogen and oxygen), both hydrogen gas and oxygen gas can form as the gaseous products. This produces a high temperature, highly flammable and explosive mixture of gases. The flammability of hydrogen gas is especially high when compared to other known fuels (e.g. hydrocarbons). There are therefore major safety considerations to be considered when producing hydrogen gas, particularly at increased pressure. The spray of water from the second spray system provides a second distinct function from the spray of feedstock. This spray of water into the plasma chamber is provided adjacent the reaction zone. As such, the water from the second spray system essentially does not enter the reaction zone and is not sprayed close enough to the plasma arc to result in thermolysis, but instead is sprayed into the hot mixture of product gases which are formed and expelled from the reaction zone. The spray of water serves to cool and dilute such a mixture, with the water forming a vapour and mixing with the product gases. This results in quenching of the product gases and minimises the occurrence of back, or reformation, reactions to improve hydrogen production efficiency. In particular, the present invention advantageously allows for the production of hydrogen gas at a specific energy consumption of <40 kWh / kg of hydrogen produced. In addition the spray of water can reduce the concentration of oxygen in the resulting mixture of gases to below the Limiting Oxygen Concentration (LOG) which is particularly important where the feedstock comprises or consists of water. The LOC is defined as the limiting concentration of oxygen below which combustion is not possible, independent of the concentration of fuel (i.e. hydrogen). As such, it is generally preferred that the spray of water provides sufficient water such that water forms at least 30 wt% of the mixture of gases recovered from the plasma chamber. In embodiments comprising greater amounts of water as the hydrogen-containing feedstock, a greater volume of quenching spray is preferred to reduce the concentration of oxygen in the mixture of gases generated by the decomposition of water. As such, in some embodiments, the spray of water provides sufficient water such that water forms at least 70 wt% of the mixture of gases recovered from the plasma chamber (particularly where the hydrogen-containing feedstock is water), preferably at least 80 wt% and / or up to 95 wt%. Preferably, the concentration of oxygen in the mixture of gases recovered from the plasma chamber is less than 10 wt% and / or the concentration of hydrogen is less than 5 wt%. The use of a second spray (which may be referred to as a quench spray) can therefore be used to avoid the formation of a flammable atmosphere within the plasma chamber whilst in the presence of the plasma arc as an ignition source. The water spray reduces the temperature of the mixtures of gases as they are expelled from the reaction zone providing a steep thermal gradient away from the reaction zone, reducing recombination rate and increasing yield of hydrogen gas. The autoignition temperature of the mixture of gas is the temperature at which the gas spontaneously ignites (for a given pressure - e.g. the preferred about 3 barg of the present method). It is preferred that the spray of water is configured to maintain a temperature of the mixture of gases within the plasma chamber of less than about 90% of the autoignition temperature of the mixture. As will be appreciated, this is the temperature outside of the reaction zone following mixing with the spray of water. Preferably, the temperature is maintained less than about 80% of the autoignition temperature, more preferably less than about 75%. As such, it is preferred that the temperature of mixture of gases outside the reaction zone is maintained at about 450°C or less, preferably 425°C or less, and more preferably 390°C or less. More preferably, the temperature of the mixture of gases as they are recovered at the gas outlet may be about 150°C or less (e.g. about 100°C) as measured at the gas outlet of the plasma chamber. The dynamic process of hydrogen production may be controlled through various parameters and are dependent on the feedstock and electric potential and current which determines the amount of power and energy going into the system. The specific shape and direction of the first and / or second sprays may be varied. It is generally preferred that a feed rate of the water via the second spray system is at least 0.3 times (i.e. 30%) the feed rate of the hydrogen-containing feedstock via the first spray system, preferably at least 0.5 times (i.e. 50%). Such a feed rate can be suitable for forming a mixture of gases comprising at least 30 wt% water as described above. In other embodiments such as those comprising water as a hydrogen-containing feedstock, the feed rate of the water via the second spray system may be much greater such as at least 1 times (i.e. 100%) the feed rate of the hydrogencontaining feedstock, preferably at least 5 times, or even at least 10 times greater when hydrogencontaining feedstock is water. These feed rates are desired in order to provide the desired temperature quench to reduce back reactions and improve hydrogen production efficiency, as well as to dilute the concentration of oxygen when present. The inventors have found that it is preferred that the first and second spray systems are integrated within a multi-functional device which further comprises the plasma anode. As such, the spray outlets will be arranged at the end of the device proximate to the tip of the plasma anode. Preferably the first spray system comprises one or more first channels for providing the spray of the hydrogen-containing feedstock and the second spray system comprises one or more second channels for providing the spray of water, wherein each first channel is arranged between the plasma anode and a second channel. As such, the first spray of the hydrogen-containing feedstock is positioned between the second spray of water and the plasma anode. Multiple channels for each spray system may be provided with those of the first spray system preferably evenly distributed around the plasma anode, and those of the second system evenly distributed around the first. Such an arrangement provides the feedstock spray from various angles around the plasma anode and the plasma arc, and into the reaction zone. The channels of the second spray system may then be concentrically arranged around the first to provide effective quench spray cooling and dilution of the gases liberated by decomposition of the feedstock. By extension, it is particularly preferred that the first spray system comprises a first annular passage surrounding the plasma anode for providing the spray of the hydrogen-containing feedstock, and the second spray system comprises a second annular passage surrounding the first annular passage for providing the spray of water. Such an embodiment may be seen as one first channel being in the form of a first annular passage (or a large number or infinite number of channels evenly distributed about the anode coalesced to form an annular passage) and the one or more second channels in the form of a second annular passage. The annular passages are generally concentric sharing the same centre point of the plasma anode, and are generally circular. This preferred embodiment ensures that the second spray of water fully surrounds the feedstock spray into the reaction zone. These arrangements of the spray system introducing the spray from the bottom of the chamber adjacent the plasma anode are preferred in that the spray system directs the water generally in the co-axial direction, parallel to the plasma anode and cathode, surrounding the plasma arc. In a preferred embodiment, the first spray from the annular passage is convergent. That is, the shape of the first spray is generally conical directing the feedstock into the centre of the reaction zone and into the plasma arc, this being directly above the plasma anode which the annular passage surrounds. In another preferred embodiments, the first and / or the second annular passages contain a helical flow guide. This directs the flow of liquid through the passage around the plasma anode providing the flow of liquid with a rotational momentum which, without wishing to be bound by theory, is believed may help to stabilise the shape of the spray, and preferably focus the spray into a convergent form. The resulting diluted mixture of gases comprising hydrogen gas and gaseous water (vapour) is recovered from the plasma chamber via a gas outlet. Preferably the method further comprises continuously recovering the mixture of gases from the plasma chamber and separating hydrogen therefrom in a gas separation apparatus. Prior to separation of hydrogen, it is preferred that water in the recovered mixture of gases is condensed in a heat exchanger upon exiting the plasma chamber (thereby forming what may be referred to as a dried mixture of gases). The heat exchanger will therefore cool down the gases, preferably to 40°C or lower, or to about ambient temperature (e.g. about 25°C). Additionally, the condensed water may then be recycled into the process as the feedstock or further quench spray. Advantageously, the heat recovered in the heat exchanger may be used to pre-heat the hydrogencontaining feedstock that is feed to the first spray system. It is generally preferred that the hydrogencontaining feedstock is fed at a heated temperature before being sprayed into the plasma chamber, preferably to greater than 40°C, such as from 60°C to 80°C, with the optimum temperature being a function of hydrogen carrier feedstock. This heating helps to avoid quenching the plasma arc through too great of a reduction in the temperature of the reaction zone, though this may be modulated through other means such as spray shape and flow rate. Returning to the cooled dried mixture of gases, the inventors have found that the removal of the water from the mixture - which is a major component of the mixture leaving the chamber - generally results in the dried gas mixture being in the flammable range in terms of the concentration of hydrogen and oxygen gas. This is a problem for downstream gas separation apparatuses and appropriate purifications systems. To overcome this problem, the inventors have found that a diluent gas, such as nitrogen or argon, may be added to the dried mixture before separating hydrogen therefrom. It is preferred to maintain an oxygen gas concentration below the LOC, preferably less than 4 vol%. The diluent gas may be the same as that used for the plasma torch and arc formation. The gas may be recovered and recycled accordingly. In a particularly preferred embodiment, the hydrogen is separated using pressure swing absorption. A pressure of the reaction chamber of from 1.5 to 5.0 barg, preferably about 3 barg, is particularly preferred when combined with a compressor ahead of pressure swing adsorption separation. This reduces the pressure increase needed for pressure swing absorption which may involve a pressure of greater than 10 barg, such as from 12 to 18 barg. A compressor may be used to increase the pressure of the dried and diluted gas mixture prior to pressure swing absorption. The preferred chamber pressure therefore provides an ideal balance of high pressure to improve compression efficiency in view of safety considerations which escalate rapidly at higher pressures. In particularly preferred embodiments of the present invention, the method further comprises providing an electrolyser having one or more electrolyser cathodes and one or more electrolyser anodes each spaced apart by a membrane and a hydrogen-containing electrolyte; wherein the plasma anode is in electrical communication with the one or more electrolyser cathodes and wherein the electrical power supply is in electrical communication with the plasma cathode and the one or more electrolyser anodes, whereby hydrogen gas is formed in the electrolyser by decomposition of the hydrogen-containing electrolyte. The electrolyser is not particularly limited and commercially available electrolysers may be used together with the plasma reactor in order to carry out the method of hydrogen production described herein. Various suitable types of electrolysers will be known to those skilled in the art, including alkaline electrolysers, proton exchange membrane electrolysers and solid oxide electrolysers. The separation of the electrolyser cathode(s) and the electrolyser anode(s) permit recovery of hydrogen and oxygen gas as separate pure gas streams from the electrolyser. An electrolyser may comprise a plurality of cells, i.e. combinations of electrolyser anodes and cathodes. Preferably the hydrogencontaining electrolyte is water (i.e. pure water) and optionally alkali (typically sodium or potassium hydroxide). The plasma reactor is wired in series electrical communication with the electrolyser, specifically with the plasma anode in electrical communication with the one or more electrolyser cathodes. As such, step (Hi) of the method involves establishing a DC electric potential between the plasma cathode and the one or more electrolyser anodes, to generate and sustain a reaction zone about a plasma arc between the plasma cathode and the plasma anode. The current is used to simultaneously power the electrolyser and the inventors have found that the combination with the plasma reactor advantageously yields a greater than expected volume of hydrogen based on the operation of the two components separately resulting from the use of the same current for the two operations and the plasma effect. In some embodiments, there are provided two or more plasma reactors wired independently but in fluid product communication. That is, each plasma reactor will be wired to one electrical power supply, with the plasma anodes being in series with the electrolyser cathode(s) of the electrolyser. In some preferred embodiments, the multi-functional device further comprises an adjustable nozzle for directing the spray of hydrogen-containing feedstock and the spray of water. The nozzle can be used to adjust the size and shape of the spray system channel outlet so as to direct and shape either spray. In embodiments with a multi-functional device in which the spray systems are formed of annular channels, a common nozzle may be included which may generally take the form of a ring whereby the first spray of feedstock is directed from the inside of the ring and the second spray of water is directed from the outside of the ring. Such a nozzle may comprise a thread which may screw into an end of the body of device with the plasma anode extending through the nozzle. In some embodiments, the plasma chamber as described herein is housed within a larger outer plasma vessel (each of which may be referred to herein as a plasma chamber or vessel). The plasma reactor then further comprises internal cooling coils which surround the inner plasma chamber between an outer wall of the plasma chamber and an inner wall of the vessel. Preferably, an inner wall of the inner plasma chamber is coated with a thermal shield since this serves to absorb or deflect heat back into the chamber (especially irradiated heat / photons) and protect the walls of the larger sealed vessel and the internal cooling coils. The thermal shield may be formed of stainless steel and preferably has a high temperature high emissivity coating. Examples of such coatings are Aremco’s HiE-Coat™ 840-Series line of black-body formulations (typically ceramic-based, black-pigmented coatings). It is preferred that the plasma chamber further comprises a drain for collecting and recycling liquid condensate. The plasma reactor may further comprise one or more ceramic discs, preferably formed of magnesium aluminosilicate (cordierite). These ceramic discs are arranged within the plasma vessel, and optionally within the inner plasma chamber or between the inner plasma chamber and the outer plasma chamber. The discs are arranged to surround the plasma torch and / or the multi-functional device where the torch and / or device each enter the inner or outer chamber and help to protect the outer vessel from the heat generated during the plasmolysis reaction and hydrogen production. The ceramic discs also electrically isolate the electrodes from the plasma vessel chamber(s) so that the plasma arc occurs in the defined location between the anode and cathode. In further preferred embodiments, the plasma torch is mounted on an actuator assembly, whereby the distance between the plasma anode and plasma cathode can be varied by moving the plasma torch further into or out of the plasma chamber. The multi-functional device described herein comprising both first and second spray systems also has the advantage in that it allows for the design of the plasma torch to be kept simple (i.e. electrode and liquid cooling plus gas shroud) which keeps the weight of the torch much lower than typical devices. This allows for rapid positioning with the actuator assembly and excellent control of the plasma arc gap improving overall operability / efficiency (when compared to mounting the heavier multi-functional device to the actuator). The reactor may preferably comprise one or more viewing ports. The reactor may further comprise an infra-red temperature sensor for measuring the temperature within the plasma chamber via a viewing port. The present invention further relates to an apparatus for the production of hydrogen gas, the apparatus comprising the plasma reactor described herein, and further comprising an electrolyser having one or more electrolyser cathodes and one or more electrolyser anodes each spaced apart by a membrane in a reservoir for a hydrogen-containing electrolyte. The plasma anode of the plasma torch is wired in electrical communication with the one or more electrolyser cathodes. In such embodiments wherein the plasma reactor is used in combination with an electrolyser, the plasma cathode and the one or more electrolyser anodes are connected to the DC electrical power supply. Such a configuration provides the plasma reactor and electrolyser is series electrical communication. Thus one preferred embodiment of the method comprises: (I) providing a DC electrical power supply; (II) providing an electrolyser having one or more electrolyser cathodes and one or more electrolyser anodes each spaced apart by a membrane and a hydrogen-containing electrolyte; (III) providing a plasma reactor comprising: (a) a plasma chamber, (b) a plasma torch comprising a plasma cathode extending into the plasma chamber, (c) a plasma anode extending into the plasma chamber, and (d) first and second spray systems, each extending into the plasma chamber; wherein the plasma anode is in electrical communication with the one or more electrolyser cathodes and wherein the electrical power supply is in electrical communication with the plasma cathode and the one or more electrolyser anodes. the method further comprising: (IV) establishing a DC electric potential between the plasma cathode and the one or more electrolyser anodes, to generate and sustain a reaction zone about a plasma arc between the plasma cathode and the plasma anode; (V) providing a spray of a hydrogen-containing feedstock into the reaction zone from the first spray system, whereby a mixture of gases comprising hydrogen gas is formed in the plasma chamber by decomposition of the hydrogen-containing feedstock; and (VI) providing a spray of water into the plasma chamber adjacent the reaction zone from the second spray system, whereby the spray of water cools and dilutes the mixture of gases formed in step (iv). Preferably, the method comprises: (I) providing a DC electrical power supply; (II) providing an electrolyser having one or more electrolyser cathodes and one or more electrolyser anodes each spaced apart by a membrane and a hydrogen-containing electrolyte; (III) providing a plasma reactor comprising: (a) a plasma chamber, (b) a plasma torch comprising a plasma cathode extending into the plasma chamber, (c) a plasma anode extending into the plasma chamber, and (d) first and second spray systems, each extending into the plasma chamber; wherein the plasma anode is in electrical communication with the one or more electrolyser cathodes and wherein the electrical power supply is in electrical communication with the plasma cathode and the one or more electrolyser anodes. the method further comprising: (IV) establishing a DC electric potential between the plasma cathode and the one or more electrolyser anodes, to generate and sustain a reaction zone about a plasma arc between the plasma cathode and the plasma anode; (V) providing a spray of a hydrogen-containing feedstock into the reaction zone from the first spray system, whereby a mixture of gases comprising hydrogen gas is formed in the plasma chamber by decomposition of the hydrogen-containing feedstock; and (VI) providing a spray of water into the plasma chamber adjacent the reaction zone from the second spray system, whereby the spray of water cools and dilutes the mixture of gases formed in step (iv) wherein the first and second spray systems are integrated within a multi-functional device which further comprises the plasma anode; wherein the first spray system comprises a first annular passage surrounding the plasma anode for providing the spray of the hydrogen-containing feedstock, and the second spray system comprises a second annular passage surrounding the first annular passage for providing the spray of water around the reaction zone; and wherein the method further comprising one or more (preferably all) of the following: (1) the spray of water provides sufficient water such that water forms at least 30 wt% of the mixture of gases recovered from the plasma chamber; (2) the spray of water is configured to maintain a temperature of the mixture of gases within the plasma chamber of less than about 90% of the autoignition temperature of the mixture; and (3) a feed rate of the water via the second spray system is at least 50% of a feed rate of the hydrogen-containing feedstock via the first spray system. Such a method includes an effective spray system for the highly efficient production of hydrogen gas at commercial scale in a safer manner through the rapid quench of mixture gases. For the production of green hydrogen, it is particularly preferred that the hydrogen-containing feedstock consists of water. The method preferably further comprises continuously recovering the mixture of gases from the plasma chamber and separating hydrogen therefrom by pressure swing absorption. The pressure in the chamber for hydrogen production is ideally between 1.5 and 5.0 barg. Figures The present invention will now be described further with reference to the following non-limiting Figures, in which: Figure 1 is a perspective view of a plasma reactor according to the present invention. Figure 2 is a perspective view of a cross-section of a plasma vessel. Figure 3 is a cross-sectional view of a plasma torch for use in the present invention. Figure 4 is a close-up cross-sectional view of the tip of the plasma torch shown in Figure 3. Figure 5 is a cross-sectional view of a multi-functional device for use in the present invention. Figure 6 is a close-up cross-section view of the tip of the multi-functional device shown in Figure 5. Figure 7 is a cross-sectional view of the plasma reactor shown in Figure 1. Figure 1 is a perspective view of a plasma reactor 100 according to the present invention. The plasma reactor 100 comprises a plasma vessel (plasma chamber) 105 which comprises an upper flange 110a and a lower flange 100b and a cylindrical side wall (tube) 110c. The upper flange 110a and the lower flange 110b are secured together with vessel tie rods 110d to seal the vessel 105. The plasma vessel 105 further comprises one or more gas outlets 115 in the upper flange 110a, together with a cooling liquid inlet 120a and cooling liquid outlet 120b to provide cooling to internal cooling coils (not shown). The plasma vessel 105 further comprises one or more viewing ports 125 which permits measuring of the temperature within the plasma chamber 105, for example with an infra-red temperature sensor (not shown). The plasma reactor further comprises a drain manifold 130 for collecting and recycling liquid condensate which forms within the plasma vessel 105. This condensate may be feed to a heat exchanger to recover heat before recycling. The plasma reactor 105 further comprises a plasma torch 135 which comprises a rod shaped electrode extending vertically through the upper flange 110a and into the plasma chamber 105. The plasma torch 135 is mounted on an actuator assembly 145 which is itself secured to the plasma vessel body (e.g. to the upper flange 110a and the lower flange 110b). The actuator assembly 145 controls the vertical movement of the plasma torch 135 and its electrode into and out of the plasma vessel 105 thereby varying the distance to the counter electrode. The counter electrode is provided by a multi-functional device 140 which extends through the opposite facing lower flange 110b into the plasma vessel 105. Figure 2 is a perspective view of a cross-section of the plasma vessel 105 shown in Figure 1. Housed within the larger plasma vessel is a smaller plasma chamber 150 mounted centrally within the vessel 105. The plasma chamber is cylindrical and is coated on the inside with a thermal shield which can either reflect or absorb heat. This protects a series of internal cooling coils 155 which surround the inner plasma chamber 150, between an outer wall of thereof and an inner wall of the side wall 110c of the vessel 105. The plasma vessel further comprises two ceramic disc reflectors 160 formed from cordierite. An upper ceramic disc is mounted between the upper flange 110a of the vessel 105 and the inner plasma chamber 150 and is arranged to surround the plasma torch which extends into the plasma chamber 150 through the upper flange 110a. A lower ceramic disc 160 is mounted within the inner plasma chamber 150 and is arranged to surround the plasma anode which extends through the lower flange 110b of the vessel 105 and into the plasma chamber 150. The ceramic disc reflectors 160 help to protect the outer vessel 105 from the heat generated during plasmolysis. Figures 3 and 4 provide a cross-sectional view of a plasma torch 200 suitable for use in the present invention. The torch 200 comprises a generally rod shaped electrode 205 which comprises a refractory metal or refractory ceramic tip 205a (lanthanum hexaboride for example). During hydrogen production, the electrode 205 is used as a plasma cathode 205 which is liquid coolable. The electrode 205 comprises a coolant blade separator 210 which permits a flow of coolant to the electrode tip 205a. The torch 200 comprises a coolant inlet 220a and a coolant outlet 200b which provides a flow of coolant 220’ (as illustrated by the directional arrows in Figure 4). The cooling of the electrode tip 205a reduces wear on the electrode 205 reducing plasma reactor 100 downtime. The torch further comprises a gas shroud 215 which surrounds the electrode 205 and provides an annular passage for the flow of gas 225’ (as illustrated by the directional arrows in Figure 4). A suitable ionisable gas (preferably nitrogen or argon) is provided via the gas inlet 225 and exits the torch around the tip 205a to help sustain a plasma arc during hydrogen production. Figures 5 and 6 provide a cross-sectional view of a multi-functional device 300 suitable for use in the present invention. The device 300 comprises a generally rod shaped electrode 305 which like the electrode 205 of the plasma torch 200, is liquid coolable and comprises a coolant blade separator 310 which permits a flow of coolant (not shown) to the electrode tip 305a from coolant inlet 320a to coolant outlet 320b. During hydrogen production, the electrode 305 is used as a plasma anode 305. Integrated within the body of the device 300 are first and second spray systems. The first spray system comprises a feedstock inlet 325 connected to a first annular passage 340, the full length of which surrounds the plasma anode 305 to a first spray outlet 340a. The second spray system comprises a water inlet 330 connected to a second annular passage 345, the full length of which surrounds the first annular passage 340 to a second spray outlet 345a such that the first channel 340 is between the plasma anode 305 and the second channel 345. The feedstock inlet 325 and water inlet 330 in use may be connected to a tank or reservoir of the feedstock and water, respectively. A flow 325’ of suitable feedstock (such as pure water) is provided through the first inner annular passage 340 and a flow 330’ of water is provided through the second outer annular passage 345 (as illustrated by the directional arrows in Figure 6). The device 300 further comprises a nozzle 350 arranged at the end of the device 300 adjacent the anode tip 305a to shape and direct the sprays 235”, 330” from each spray system. The nozzle 350 may be screwed into the body of the device 300 which forms the electrode 305 and spray systems, and the nozzle 350 is adjustable allowing the size of the annulus gaps at the first spray outlet 340a and the second spray outlet 345a to be adjusted, and therefore the spray patterns. Each channel 340, 345 further comprises a helical flow guide 355, 360 which directs the flows 325’, 330’ of the feedstock and water, respectively, around the plasma anode 305. The helical flow guides 355, 360 help to shape the sprays 325”, 330”. As illustrated by the directional arrows in Figure 6, the first spray 325” of a hydrogen-containing feedstock is convergent and directed to the space directly above the tip 305a of the plasma anode 305 (and therefore into the reaction zone about a plasma arc when in use). The second spray 330’ of water is directed adjacent the plasma arc, the annular passage ensuring that the spray 330’ completely surrounds the reaction zone in use such that the water can cool and dilute the gases which form upon decomposition of the feedstock in the reaction zone. The multi-functional device 300 further comprises outer cooling channels for an outer flow 335' of a cooling liquid from the outer coolant inlet 335a to the outer coolant outlet 335b. This helps to keep the body of the device, and the spray systems, cool. Figure 7 is a cross-sectional view of the plasma reactor 100 shown in Figure 1 which illustrates the co-axial arrangement of the plasma torch 135 and the multi-functional device 140, with the tips of the plasma cathode and the plasma anode arranged within the inner plasma chamber of a plasma vessel 105. In use, the plasma cathode and plasma anode are connected to a DC electrical power supply and the actuator 145 can move the plasma torch 135 to vary the distance between the tips of the electrodes whilst sustaining a plasma arc therebetween. As used herein, the singular form of “a”, “an” and “the” include plural references unless the context clearly dictates otherwise. The use of the term “comprising” is intended to be interpreted as including such features but not excluding other features and is also intended to include the option of the features necessarily being limited to those described. In other words, the term also includes the limitations of “consisting essentially of” (intended to mean that specific further components can be present provided they do not materially affect the essential characteristic of the described feature) and “consisting of” (intended to mean that no other feature may be included such that if the components were expressed as percentages by their proportions, these would add up to 100%, whilst accounting for any unavoidable impurities), unless the context clearly dictates otherwise. It will be understood that, although the terms "first", "second", etc. may be used herein to describe, for example, various elements, layers and / or portions, the elements, layers and / or portions should not be limited by these terms. These terms are only used to distinguish one element, layer or portion from another, or a further, element, layer or portion. Spatially relative terms, such as “under”, "below", "beneath", "lower", “over”, "above", "upper" and the like, may be used herein for ease of description to describe one element or feature's relationship to another element(s) or feature(s). It will be understood that the spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if a device as described herein is turned over, elements described as "under” or “below" other elements or features would then be oriented “over” or "above" the other elements or features. Thus, the example term "under" can encompass both an orientation of over and under. The device may be otherwise oriented and the spatially relative descriptors used herein interpreted accordingly. Numerical lower and upper limits of features described herein may preferably be combined to provide a closed range. The foregoing detailed description has been provided by way of explanation and illustration, and is not intended to limit the scope of the appended claims. Many variations of the presently preferred embodiments illustrated herein will be apparent to one of ordinary skill in the art, and remain within the scope of the appended claims and their equivalents. For the avoidance of doubt, the entire contents of all documents acknowledged herein are incorporated herein by reference. The present invention will now be described further in the following numbered clauses: 1. A method for the production of hydrogen gas, the method comprising: (i) providing a DC electrical power supply; (ii) providing a plasma reactor comprising: (a) a plasma chamber, (b) a plasma torch comprising a plasma cathode extending into the plasma chamber, (c) a plasma anode extending into the plasma chamber, and (d) first and second spray systems, each extending into the plasma chamber; (Hi) establishing a DC electric potential between the plasma cathode and the plasma anode to generate and sustain a reaction zone about a plasma arc therebetween; (iv) providing a spray of a hydrogen-containing feedstock into the reaction zone from the first spray system, whereby a mixture of gases comprising hydrogen gas is formed in the plasma chamber by decomposition of the hydrogen-containing feedstock; and (v) providing a spray of water into the plasma chamber adjacent the reaction zone from the second spray system, whereby the spray of water cools and dilutes the mixture of gases formed in step (iv). 2. The method according to clause 1, wherein the method produces hydrogen at an efficiency of <40 kWh / kg of hydrogen. 3. The method according to clause 1 or clause 2, further comprising providing an electrolyser having one or more electrolyser cathodes and one or more electrolyser anodes each spaced apart by a membrane and a hydrogen-containing electrolyte; wherein the plasma anode is in electrical communication with the one or more electrolyser cathodes and wherein the electrical power supply is in electrical communication with the plasma cathode and the one or more electrolyser anodes, whereby hydrogen gas is formed in the electrolyser by decomposition of the hydrogen-containing electrolyte. 4. The method according to any preceding clause, wherein the first and second spray systems are integrated within a multi-functional device which further comprises the plasma anode. 5. The method according to clause 4, wherein the first spray system comprises one or more first channels for providing the spray of the hydrogen-containing feedstock and the second spray system comprises one or more second channels for providing the spray of water, wherein each first channel is arranged between the plasma anode and a second channel. 6. The method according to clause 4 or clause 5, wherein the first spray system comprises a first annular passage surrounding the plasma anode for providing the spray of the hydrogencontaining feedstock, and the second spray system comprises a second annular passage surrounding the first annular passage for providing the spray of water around the reaction zone. 7. The method according to clause 6, wherein the first and / or the second annular passages contain a helical flow guide. 8. The method according to clause 6 or clause 7, wherein the spray of the hydrogen-containing feedstock from the first annular passage is convergent. 9. The method according to any preceding clause, wherein the hydrogen-containing feedstock is water, methanol, methane or ammonia, or a combination thereof. 10. The method according to any preceding clause, wherein the spray of water provides sufficient water such that water forms at least 30 wt% of the mixture of gases recovered from the plasma chamber. 11. The method according to any preceding clause, wherein the hydrogen-containing feedstock comprises water, and wherein the spray of water provides sufficient water such that water forms at least 70 wt%, at least 80 wt% and / or up to 95 wt% of the mixture of gases recovered from the plasma chamber. 12. The method according to any preceding clause, wherein the spray of water is configured to maintain a temperature of the mixture of gases within the plasma chamber of less than about 90% of the autoignition temperature of the mixture, preferably about 390°C or less. 13. The method according to any preceding clause, wherein a feed rate of the water via the second spray system is at least 50% of the feed rate of the hydrogen-containing feedstock via the first spray system, preferably at least 10 times greater. 14. The method according to any preceding clause, wherein, at a power of from about 40 kW to about 60 kW, such as about 50 kW, the electric potential has a voltage of from 100 to 2,000 V, preferably from 650 to 1,500 V, and a current of from 10 to 200 A, preferably from 50 to 150 A. 15. The method according to any preceding clause, wherein the power supply provides a substantially constant current and variable voltage. 16. The method according to any preceding clause, wherein the method further comprises continuously recovering the mixture of gases from the plasma chamber and separating hydrogen therefrom, preferably by pressure swing absorption. 17. The method according to clause 16, wherein water in the recovered mixture of gases is condensed in a heat exchanger to form a dried mixture of gases, and then a diluent gas, preferably nitrogen, is added to the dried mixture before separating hydrogen therefrom. 18. The method according to clause 17, wherein the heat recovered in the heat exchanger is used to pre-heat the hydrogen-containing feedstock. 19. The method according to clause 17 or clause 18, wherein the mixture of gases are cooled to 40°C or lower in the heat exchanger. 20. The method according to any preceding clause, wherein the pressure in the plasma chamber is from 1.5 to 5.0 barg, preferably about 3 barg. 21. The method according to any preceding clause, wherein the plasma anode and / or plasma cathode has a refractory metal or refractory ceramic tip, preferably tungsten or molybdenum metal or lanthanum hexaboride. 22. The method according to any preceding clause, wherein the plasma cathode and / or plasma anode is liquid-cooled. 23. The method according to any preceding clause, wherein a distance between a tip of the plasma cathode and a tip of the plasma anode is from 1 to 75 mm. 24. The method according to any preceding clause, wherein the plasma arc is a thermal plasma. 25. The method according to any preceding clause, wherein the power supply provides power from a renewable energy source. 26. The method according to any preceding clause, in which two or more plasma reactors are provided, with each reactor wired in parallel electrical communication. 27. A plasma reactor for the production of hydrogen gas comprising: (a) a plasma chamber; (b) a plasma torch comprising a plasma cathode extending into the plasma chamber; (c) a plasma anode extending into the plasma chamber whereby, in use, a plasma arc can form between the plasma cathode and the plasma anode providing a reaction zone; (d) a first spray system configured to provide, in use, a spray of a hydrogen-containing feedstock into the reaction zone; and (e) a second spray system configured to provide, in use, a spray of water into the plasma chamber adjacent the reaction zone. 28. A plasma reactor for the production of hydrogen gas comprising: (a) a plasma chamber; (b) a plasma torch comprising a plasma cathode extending into the plasma chamber; (c) a multi-functional device comprising: (c1) a plasma anode extending into the plasma chamber whereby, in use, a plasma arc can form between the plasma cathode and the plasma anode providing a reaction zone, (c2) a first spray system comprising a first annular passage surrounding the plasma anode for providing, in use, a spray of a hydrogen-containing feedstock into the reaction zone, and (c3) a second spray system comprising a second annular passage surrounding the first annular passage for providing, in use, a spray of water around the reaction zone. 29. The plasma reactor according to clause 28, wherein the multi-functional device further comprises an adjustable nozzle for directing the spray of hydrogen-containing feedstock and the spray of water. 30. The plasma reactor according to any one of clauses 27 to 29, wherein the plasma chamber is housed within a larger plasma vessel, the plasma reactor further comprising internal cooling coils surrounding the inner plasma chamber between an outer wall of the plasma chamber and an inner wall of the vessel. 31. The plasma reactor according to any one of clauses 27 to 30, wherein the plasma reactor further comprises ceramic discs, preferably formed of magnesium aluminosilicate (cordierite), which surround the plasma torch and / or the multi-functional device where the torch and / or device each enter the chamber. 32. The plasma reactor according to any one of clauses 27 to 31, wherein an inner wall of the plasma chamber is coated with a thermal shield. 33. The plasma reactor according to any one of clauses 27 to 32, wherein the plasma chamber further comprises a drain for collecting and recycling liquid condensate. 34. The plasma reactor according to any one of clauses 27 to 33, wherein the reactor comprises one or more viewing ports and an infra-red temperature sensor for measuring the temperature within the plasma chamber via a viewing port. 35. The plasma reactor according to any one of clauses 27 to 34, wherein the plasma torch is mounted on an actuator assembly, whereby the distance between the plasma anode and plasma cathode can be varied by moving the plasma torch further into or out of the plasma chamber. 36. The plasma reactor according to any one of clauses 27 to 35, wherein the plasma anode and plasma cathode are arranged co-axially. 37. An apparatus for the production of hydrogen gas, the apparatus comprising the plasma reactor according to any one of clauses 27 to 36, and further comprising an electrolyser having one or more electrolyser cathodes and one or more electrolyser anodes each spaced apart by a membrane in a reservoir for a hydrogen-containing electrolyte; wherein the plasma anode is in electrical communication with the one or more electrolyser cathodes and wherein the plasma cathode and the one or more electrolyser anodes are connectable to an electrical power supply.

Claims

1. A method for the production of hydrogen gas, the method comprising:(i) providing a DC electrical power supply;(II) providing a plasma reactor comprising:(a) a plasma chamber,(b) a plasma torch comprising a plasma cathode extending into the plasma chamber, (c) a plasma anode extending into the plasma chamber, and(d) first and second spray systems, each extending into the plasma chamber;(Hi) establishing a DC electric potential between the plasma cathode and the plasma anode to generate and sustain a reaction zone about a plasma arc therebetween;(iv) providing a spray of a hydrogen-containing feedstock into the reaction zone from the first spray system, whereby a mixture of gases comprising hydrogen gas is formed in the plasma chamber by decomposition of the hydrogen-containing feedstock; and(v) providing a spray of water into the plasma chamber adjacent to the reaction zone from the second spray system, whereby the spray of water cools and dilutes the mixture of gases formed in step (iv).

2. The method according to claim 1, further comprising providing an electrolyser having one or more electrolyser cathodes and one or more electrolyser anodes each spaced apart by a membrane and a hydrogen-containing electrolyte;wherein the plasma anode is in electrical communication with the one or more electrolyser cathodes and wherein the electrical power supply is in electrical communication with the plasma cathode and the one or more electrolyser anodes, whereby hydrogen gas is formed in the electrolyser by decomposition of the hydrogen-containing electrolyte.

3. The method according to claim 1 or claim 2, wherein the first and second spray systems are integrated within a multi-functional device which further comprises the plasma anode;wherein the first spray system comprises one or more first channels for providing the spray of the hydrogen-containing feedstock and the second spray system comprises one or more second channels for providing the spray of water, wherein each first channel is arranged between the plasma anode and a second channel.

4. The method according to claim 1 or claim 2, wherein the first and second spray systems are integrated within a multi-functional device which further comprises the plasma anode;wherein the first spray system comprises a first annular passage surrounding the plasma anode for providing the spray of the hydrogen-containing feedstock, and the second spray system comprises a second annular passage surrounding the first annular passage for providing the spray of water around the reaction zone.

5. The method according to claim 4, wherein the first and / or the second annular passages contain a helical flow guide.

6. The method according to any preceding claim, wherein the spray of water provides sufficient water such that water forms at least 30 wt% of the mixture of gases recovered from the plasma chamber, preferably at least 80 wt% and / or up to 95 wt%.

7. The method according to any preceding claim, wherein the spray of water is configured to maintain a temperature of the mixture of gases within the plasma chamber of less than about 90% of the autoignition temperature of the mixture, preferably about 390°C or less.

8. The method according to any preceding claim, wherein a feed rate of the water via the second spray system is at least 30% of the feed rate of the hydrogen-containing feedstock via the first spray system, preferably at least 10 times greater.

9. The method according to any preceding claim, wherein the power supply provides a substantially constant current and variable voltage.

10. The method according to any preceding claim, wherein the hydrogen-containing feedstock is water, methanol, methane or ammonia, or a combination thereof.

11. The method according to any preceding claim, wherein the method further comprises continuously recovering the mixture of gases from the plasma chamber and separating hydrogen therefrom by pressure swing absorption.

12. The method according to claim 11, wherein water in the recovered mixture of gases is condensed in a heat exchanger to form a dried mixture of gases, and then a diluent gas, preferably nitrogen, is added to the dried mixture before separating hydrogen therefrom.

13. The method according to any preceding claim, wherein the pressure in the plasma chamber is from 1.5 to 5.0 barg, preferably about 3 barg.

14. The method according to any preceding claim, wherein the plasma anode and / or plasma cathode has a refractory metal or refractory ceramic tip, preferably tungsten or molybdenum metal or lanthanum hexaboride.

15. The method according to any preceding claim, wherein the power supply provides power from a renewable energy source.

16. A plasma reactor for the production of hydrogen gas comprising:(a) a plasma chamber;(b) a plasma torch comprising a plasma cathode extending into the plasma chamber;(c) a multi-functional device comprising:(c1) a plasma anode extending into the plasma chamber whereby, in use, a plasma arc can form between the plasma cathode and the plasma anode providing a reaction zone,(c2) a first spray system comprising a first annular passage surrounding the plasma anode for providing, in use, a spray of a hydrogen-containing feedstock into the reaction zone, and(c3) a second spray system comprising a second annular passage surrounding the first annular passage for providing, in use, a spray of water around the reaction zone.

17. The plasma reactor according to claim 16, wherein the multi-functional device further comprises an adjustable nozzle for directing the spray of hydrogen-containing feedstock and the spray of water.

18. The plasma reactor according to claim 16 or claim 17, wherein the plasma chamber further comprises a drain for collecting and recycling liquid condensate.

19. The plasma reactor according to any one of claims 16 to 18, wherein the plasma torch is mounted on an actuator assembly, whereby the distance between the plasma anode and plasma cathode can be varied by moving the plasma torch further into or out of the plasma chamber.

20. An apparatus for the production of hydrogen gas, the apparatus comprising the plasma reactor according to any one of claims 16 to 19, and further comprising an electrolyser having one or more electrolyser cathodes and one or more electrolyser anodes each spaced apart by a membrane in a reservoir for a hydrogen-containing electrolyte;wherein the plasma anode is in electrical communication with the one or more electrolyser cathodes and wherein the plasma cathode and the one or more electrolyser anodes are connectable to an electrical power supply.

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