Method and apparatus for nitrogen fixation

Abstract

A method of forming NOx species from nitrogen and oxygen. The method comprising: a) directing an incoming gas stream comprising nitrogen gas and oxygen gas into a reaction chamber comprising a first electrode and a second electrode to generate a vortex flow of the incoming gas stream within the reaction chamber; b) generating a plasma from the vortex flow of the incoming gas stream by applying a voltage difference between the first electrode and the second electrode; and c) collecting an outgoing gas stream comprising NOx species from the reaction chamber. The method may produce NOx species with high efficiency and low energy consumption, compared to known methods. A method of synthesising nitric acid and a method of synthesising ammonia are also disclosed. A plasma reactor and an apparatus for carrying out said methods are also disclosed.

Description

[0001] Method and Apparatus for Nitrogen Fixation

[0002] Field

[0003] The present invention relates to nitrogen fixation, specifically to a method of forming NOx species from nitrogen and oxygen and to a plasma reactor and associated apparatus for forming NOx species.

[0004] Background

[0005] Nitrogen is a vital building block for all living organisms, being a fundamental component of nucleic acids. Despite comprising about 78% of the Earth's atmosphere in its elemental form, the inert properties of nitrogen render it largely inaccessible to most life forms. The process of nitrogen fixation is crucial for atmospheric nitrogen to be made chemically available to support life. While natural phenomena such as lightning can convert atmospheric nitrogen into NOx species (for example NO and NO2) and biological processes involving nitrogenase enzymes can produce ammonia from nitrogen, these processes fall short of providing a sufficient supply of fixed nitrogen to meet the need created by the growing global human population.

[0006] Throughout the past century, the industrial Haber-Bosch (HB) process has been the primary method used for large-scale nitrogen fixation. This process involves the synthesis of ammonia from nitrogen and hydrogen. However, despite its large scale adoption, the HB process comes with significant drawbacks. It is highly energy intensive, consuming about 1-2% of global energy due to its operation at high temperatures (400-600 °C) and pressures (200-400 atm). Moreover, the HB process accounts for over 1 % of global CO2 emissions as it relies on hydrogen derived from fossil fuels such as natural gas. Consequently, there is a pressing need for greener and more sustainable technologies which may allow carbon-neutral nitrogen fixation, which operate under milder conditions and are preferably powered by renewable energy sources.

[0007] To achieve this goal, various technologies such as electrocatalytic, photocatalytic, and plasma-based approaches have emerged as promising alternative processes. Among these, non-thermal plasma (NTP) technology has garnered significant attention for its potential for providing decentralised, on- demand nitrogen fixation, owing to several key advantages. Firstly, NTP processes can operate under ambient pressure and low temperature conditions, resulting in reduced reactor sizes and capital costs. Additionally, the rapid reaction kinetics of plasma systems enable instantaneous on / off switching, offering flexibility in coupling with renewable energy sources like wind and solar power, which may be intermittently available. Moreover, nitrogen fixation using plasma technology exhibits a lower theoretical energy consumption limit compared to the conventional HB process. This is attributed to the ability of energetic electrons generated by NTP to activate inert N2 molecules through electron impact excitation and dissociation, facilitating the conversion of N2 into nitrogen compounds (such as ammonia and nitric oxides) without the need for additional heating. Presently, most research into the use of NTP for nitrogen fixation has focussed on directly synthesizing ammonia from nitrogen and hydrogen. However, this research has revealed a challenging trade-off between achieving low energy consumption and high ammonia yield. For instance, while a high ammonia yield of 6.4% demands substantial energy consumption at 81 MJ mol-1NH3, achieving low energy consumption of 2 MJ mol-1NH3 results in an extremely low NH3 yield (<0.1 %). Also, the separation of ammonia from such a diluted gas mixture has proven to be highly energy-intensive. Additionally, this process requires expensive green hydrogen to produce carbon- neutral ammonia. An alternative strategy to circumvent this trade-off involves converting N2 into a more reactive form such as nitrogen oxide species (NOx species).

[0008] Until now, a wide range of NTP types have been utilised to generate NOx species, including dielectric barrier discharge, plasma jet, gliding arc discharge, spark discharge, and microwave discharge. However, the energy consumption varies significantly among these different plasma methods. Among atmospheric pressure NTPs, for instance, a pulsed gliding arc has demonstrated the capability to generate 2% NOx at an energy cost of 2.8 MJ mol-1, representing one of the lowest energy costs achieved to date. Nevertheless, the current best energy cost achieved by the gliding arc method falls short of surpassing the energy efficiency of traditional HB process, let alone reaching the theoretical energy limit of NTP. Consequently, there remains a need to reduce the energy consumption associated with NOx production in order to realise the potential for environmentally benign and energy-efficient nitrogen fixation directly from air.

[0009] Summary of the Invention

[0010] It is one aim of the present invention, amongst others, to provide an apparatus and method for forming NOx species from nitrogen and oxygen gas that addresses at least one disadvantage of the prior art, whether identified here or elsewhere.

[0011] For instance, it may be an aim of the present invention to provide a method of plasma-assisted synthesis of NOx directly from air, with a high NOx concentration and low energy consumption, compared to known methods of NOx production.

[0012] It may be an additional or alternative aim of the present invention to provide a method of plasma- assisted synthesis of NOx directly from air using a compact, catalyst-free system, which is easy to manufacture.

[0013] It may be an additional or alternative aim of the present invention to provide an apparatus and method for forming NOx species that does not require additional heating, can be conducted at ambient pressure, and can operate across a wide range of flow rates, including using an economically low gas flow rate. It may be an additional or alternative aim of the present invention to provide an apparatus and method for forming NOx species that may be integrated with renewable energy sources, particularly for use with intermittent renewable energy sources during peak load for localised or distributed energy storage, for example wind and solar power sources.

[0014] According to aspects of the present invention, there is provided an apparatus and method as set forth in the appended claims. Other features of the invention will be apparent from the dependent claims, and from the description which follows.

[0015] According to a first aspect of the present invention, there is provided a method of forming NOx species from nitrogen and oxygen, the method comprising; a) directing an incoming gas stream comprising nitrogen gas and oxygen gas into a reaction chamber comprising a first electrode and a second electrode to generate a vortex flow of the incoming gas stream within the reaction chamber; b) generating a plasma from the vortex flow of the incoming gas stream by applying a voltage difference between the first electrode and the second electrode; c) collecting an outgoing gas stream comprising NOx species from the reaction chamber.

[0016] By “NOx species” we mean nitrogen oxides, including mixtures of different nitrogen oxides, suitably nitric oxide (NO) and nitrogen dioxide (NO2).

[0017] The inventors have found that the method of this first aspect which involves generating a plasma from the vortex flow of the incoming gas stream, as further described herein, can produce NOx species with relatively high efficiency and relatively low energy consumption, compared to known methods. For example, an energy consumption of less than 2.0 MJ mol-1, less than 1 .5 MJ mol-1or less than 1 .3 MJ mol-1of NOx species produced may be achieved by the method of the present invention.

[0018] Furthermore, the inventors have found that the NOx species can be produced with high selectivity for NO over NO2, which may be advantageous for the further capture and utilisation of the NOx species. For example, the method may provide NOx species comprising at least 90 vol% NO, suitably at least 95 vol% NO.

[0019] The inventors have also found that the method of this first aspect can provide these benefits when using air as the incoming gas stream. The use of air may provide further cost and energy consumption reductions for an overall nitrogen fixation process, compared to the use of purified nitrogen and oxygen gas, which may further reduce the environmental impact of such a process. The inventors have also found that the method of the first aspect can provide further benefits when using air enriched with oxygen in the incoming gas stream. The use of additional oxygen may improve the yield of NOx produced, thus providing additional cost and energy consumption reductions for an overall nitrogen fixation process, which may further reduce the environmental impact of such a process.

[0020] The inventors have found that the method of this first aspect wherein the incoming gas stream is directed into the reaction chamber to generate a vortex flow can operate in either rotating glow plasma discharge mode or in a gliding arc plasma discharge mode to efficiently produce NOx species from the nitrogen and oxygen gases. The plasma discharge mode generated in the method may depend on the voltage difference applied between the first electrode and the second electrode and therefore the power of plasma discharge produced.

[0021] The method of this first aspect is suitably carried out using a plasma reactor and / or apparatus as described herein.

[0022] In some embodiments, step b) involves generating a rotating glow discharge plasma from the vortex flow of the incoming gas stream. Such a rotating glow discharge plasma may be generated when a voltage difference is applied between the first electrode and the second electrode sufficient to produce a plasma discharge with a power of at least 35 W, for example a power of from 35 to 50 W.

[0023] In some embodiments, step b) involves generating a gliding arc discharge plasma. A gliding arc discharge plasma may be formed in the method when a voltage difference is applied between the first electrode and the second electrode sufficient to produce a plasma discharge with a power of at least 27 W, for example a power of from 27 to 35 W.

[0024] Suitably the steps a), b) and c) of the method are carried out sequentially with respect to a portion of the incoming gas stream. Suitably the method is a continuous process and therefore steps a), b) and c) are carried out simultaneously, during operation of the method using a suitable apparatus.

[0025] The incoming gas stream directed into the reaction chamber in step a) comprises nitrogen gas and oxygen gas. The incoming gas stream may be provided by separate sources of nitrogen gas and oxygen gas, for example a container of pure nitrogen and a container of pure oxygen. The incoming gas stream may be dry nitrogen gas and dry oxygen gas, provided from separate gas cylinders. The ratio of nitrogen gas and oxygen gas in the incoming gas stream is suitably controlled. Suitably the ratio of nitrogen gas and oxygen gas is controlled by changing the gas flow rates of nitrogen and oxygen while keeping the total flow rate of the incoming gas stream the same. Suitably, the ratio of nitrogen gas and oxygen gas may be controlled by suitable gas flow control known in the art. The ratio of nitrogen gas and oxygen gas may be controlled for example by using at least one mass flow controller, suitably using separate mass flow controllers for each of the sources of nitrogen gas and oxygen gas. The ratio of nitrogen and oxygen (N2:C>2) used in the incoming gas stream is suitably from 1 :20 to 20:1 , suitably from 1 :1 to 10:1 , from 2:1 to 5:1 or from 3:1 to 4:1 . An excess of nitrogen gas compared to oxygen gas may be used. Such an excess of nitrogen gas may provide a favourable selectivity for the production of NO over NO2 in the method. Therefore, the incoming gas stream may comprise at least 70 vol% N2, suitably at least 80 vol%, at least 90 vol% or at least 95 vol% N2. Suitably the incoming gas stream comprises at least 2 vol% O2, at least 3 vol% or at least 5 vol% O2. An excess of oxygen gas compared to nitrogen gas may be used. Such an excess of oxygen gas may improve the yield of NOx species produced in the method. Therefore, the incoming gas stream may comprise at least 50 vol% O2, suitably at least 60 vol%, at least 70 vol% or at least 75 vol% O2. Suitably, the incoming gas stream may comprise at least 25 vol% N2, at least 30 vol% N2, at least 40 vol% N2, or at least 50 vol% N2.

[0026] The inventors have found that controlling the ratio of nitrogen gas and oxygen gas in this manner may allow the yield of NOx species and / or the selectivity for NO production to be optimised.

[0027] In some embodiments, the method may comprise using, as the incoming gas stream, a mixture of gases comprising nitrogen, oxygen, argon, carbon dioxide, neon, helium, methane, hydrogen and / or water vapour.

[0028] In some embodiments, the incoming gas stream is provided by air, for example ambient air, which contains oxygen gas and nitrogen gas (approximately 78 vol% N2 and 21 vol% O2). Therefore, in step a), the incoming gas stream may be air comprising nitrogen and oxygen. The incoming gas stream may be dry air, suitably comprising less than 1 wt.% water, suitably substantially free of water. In such embodiments, the incoming gas stream of air may be provided by a pressurized source of air, suitably dry air. Alternatively, the incoming gas stream of air may be drawn from the atmosphere using suitable air moving equipment such as a pump.

[0029] In some embodiments, the incoming gas stream may be provided by air which is enriched with oxygen, suitably to increase the oxygen content in the incoming gas stream to the ratios discussed above. Enriching the incoming gas stream of air with oxygen may have beneficial effects on the process, as discussed above. Suitable equipment and arrangements for enriching air with oxygen may be known in the art. The oxygen content of the incoming gas stream may be monitored by a sensor and the flow rate of oxygen entering the incoming gas stream may be adjusted accordingly, to achieve a target oxygen content.

[0030] The incoming gas stream provided in step a) of the method suitably has a gas flow appropriate for the scale of the apparatus on which the method of the present invention is carried out. The experimental scale apparatus described herein may use a flow rate of the incoming gas stream of from 0.1 to 10 standard litres per minute (SLM), suitably a gas flow rate of 0.2 to 5 SLM. In embodiments wherein the incoming gas stream is air provided by a single source of air, the method may involve controlling the flow rate of the incoming gas stream. In embodiments wherein the incoming gas stream is provided by separate sources of nitrogen gas and oxygen gas, the method may involve controlling the overall flow of the incoming gas stream by controlling the flow of incoming nitrogen and incoming oxygen separately, for example by using a first mass flow controller associated with the nitrogen gas source to control the flow of nitrogen gas into the incoming gas stream and a second mass flow controller associated with the oxygen gas source to control the flow of oxygen gas into the incoming gas stream.

[0031] Controlling the flow rate in this manner may further contribute to the control and optimisation of the selectivity of NO production in the method. For example, a higher flow rate may provide a higher selectivity for NO production over NO2 production. Without being bound by theory, this effect may be attributed to the higher flow rate providing a shorter residence time of the NO in the plasma and therefore a reduced likelihood of conversion of the NO to NO2.

[0032] In embodiments wherein the method is carried out on a larger, industrial scale apparatus, it will be appreciated that the flow rate of the incoming gas stream may be far higher, for example at least 100 SLM, at least 1 ,000 SLM, at least 10,000 SLM or at least 100,000 SLM, depending on the scale at which the method is implemented, and in particular considering the use of an array of the plasma reactors described herein. As discussed above, the flow rate of the incoming gas stream may be controlled to optimise the efficiency of the process, according to the principles discussed above, implemented at the appropriate scale. Controlling the flow rate in this manner may improve the economic efficiency of the process.

[0033] In the method of the present invention, the temperature and pressure of the incoming gas stream, and therefore the temperature and pressure of the gas stream within the reaction chamber, may be controlled, for example by heating the incoming gas stream to a set temperature and pressurising the incoming gas stream to a set pressure. Suitably the incoming gas stream has a temperature of at least 300 K, suitably at least 350 K or at least 375 K. The temperature of the incoming gas stream may be from 300 to 500 K, from 350 to 450 K or from 375 to 425 K. For example, the temperature of the incoming gas stream may be approximately 398 K. Suitably said temperatures are maintained in the reaction chamber.

[0034] It is believed that the method of this first aspect can be effective when operating with incoming gas streams having low, high or ambient pressures. Suitably, the incoming gas stream has a pressure of at least 15 kPa, at least 75 kPa or at least 100 kPa. The pressure of the incoming gas stream may be from 15 kPa to 3,000 kPa, from 50 kPa to 1 ,000 kPa or from 75 kPa to 150 kPa. For example, the pressure of the incoming gas stream may be from 90 to 110 kPa, or approximately 101 kPa. Suitably, the pressures are maintained in the reaction chamber.

[0035] In some embodiments, the incoming gas stream has a pressure approximate to ambient pressure.

[0036] For example, the incoming gas stream may have a pressure of approximately 101 kPa. In some embodiments, the incoming gas stream may have a pressure higher than ambient. The inventors have found that higher pressures can advantageously favour NO2 production and capture when producing nitric acid. Higher pressures may also improve NOx yield. Therefore, the incoming gas stream may have a pressure of at least 101 kPa, suitably at least 110 kPa or at least 150 kPa. The incoming gas stream may have a pressure of from 101 kPa to 3,000 kPa, suitably from 110 kPa to 2,000 kPa or from 150 kPa to 1 ,000 kPa.

[0037] In some embodiments, the method comprises maintaining the temperature and pressure of the reactor at 398 K and 101 kPa respectively.

[0038] Step a) of the method involves directing the incoming gas stream into the reaction chamber. Suitably the incoming gas stream is directed into the reaction chamber through at least one gas inlet. In some embodiments, the incoming gas stream is directed into the reaction chamber through more than one gas inlet, for example two gas inlets or more than two gas inlets. The use of more than one gas inlet may improve the vortex flow of the incoming gas stream in the reaction chamber. The improvement of the vortex flow may lead to an increase in the yield of NOx species produced by the method, which may be due to increasing the amount of gas stream within the plasma at any one time.

[0039] Suitably step a) of the method involves directing the incoming gas stream into the reaction chamber in a manner which creates a vortex-type flow of the incoming gas stream in the reaction chamber, i.e. a gas flow which revolves around an axis line in the reaction chamber. Therefore, step a) suitably involves generating a vortex flow of the incoming gas stream in the reaction chamber. The incoming gas stream therefore suitably flows in a rotating manner around the reaction chamber from the at least one gas inlet to a gas outlet from which the gas stream exits the reaction chamber as the outgoing gas stream. Optimizing the gas inlet configuration may enhance the performance of the reactor.

[0040] Said vortex flow of the incoming gas stream may be created by directing the incoming gas stream into the reaction chamber at an obtuse angle relative to the direction of main axis of the reaction chamber, suitably at an angle approximate to perpendicular to the direction of the main axis of the reaction chamber, and suitably by directing the incoming gas stream away from the centre and / or main axis of the reaction chamber, suitably at a tangent to the main axis of the reaction chamber. As such, the incoming gas stream is induced to flow in a vortex or spiral around the reaction chamber from the at least one gas inlet to the gas outlet.

[0041] The generation of the vortex flow of the incoming gas stream in the reaction chamber may be assisted by the shape of the reaction chamber. The reaction chamber may have an inner surface is shaped to induce the incoming gas stream to flow around said inner surface in a spiral I vortex, for example by having curved or angled side walls, suitably curved side walls. The reaction chamber may be a cylinder, an elliptical cylinder or a prism of a polyhedron having at least four sides. Suitably the reaction chamber is cylindrical. Suitably the incoming gas stream is directed away from the central axis of the cylinder, elliptical cylinder or prism shaped reaction chamber and at an obtuse angle to said central axis, in order to induce the vortex flow of the incoming gas stream.

[0042] Suitably the at least one gas inlet is arranged in an upper portion of the reaction chamber and the gas outlet is arranged in a lower portion of the reaction chamber. In embodiments wherein the reaction chamber is cylindrical, the at least one gas inlet and the gas outlet are suitably arranged at or proximate to opposite ends of the cylinder. Suitably the at least one gas inlet and the gas outlet are arranged and configured (i.e. directed) in the reaction chamber, shaped as described above, in order to generate the vortex flow of the incoming gas stream from the at least one gas inlet, through the reaction chamber and to the gas outlet. For example, in some embodiments wherein the reaction chamber is cylindrical, two gas inlets may be located opposite each other, equidistantly spaced apart on the circumference of the cylinder.

[0043] The inventors have found that directing the incoming gas stream in the vortex flow through the reaction chamber may optimise the amount of nitrogen and oxygen gases which comes into contact with the plasma and reacts to form NOx species, which may lead to a greater activation of the nitrogen and oxygen gases and a higher yield of NOx species.

[0044] Furthermore, the inventors have found that the vortex flow of the incoming gas stream may assist with the formation of the glow discharge plasma in the reaction chamber, which is a rotating glow discharge plasma due to the vortex flow. The rotating glow discharge plasma may provide an advantageous improvement in plasma stability, which may improve the efficiency and reliability of the method.

[0045] The reaction chamber used in the method of this first aspect comprises a first electrode and a second electrode. The first and second electrodes are arranged and configured to create a plasma discharge from the incoming gas stream when a voltage difference is applied between the first electrode and the second electrode.

[0046] Suitably the first electrode comprises an active discharge surface and step a) involves directing the incoming gas stream to contact the active discharge surface. The active discharge surface may have a convex, concave or substantially planar surface. Suitably the active discharge surface is substantially planar. Suitably the active discharge surface is substantially circular. The active discharge surface may be referred to as a plate portion of the first electrode. Suitably the active discharge surface of the first electrode is spaced apart from the side walls of the reaction chamber to allow the incoming gas stream to flow, suitably in a vortex, around and over the substantially planar surface.

[0047] Suitably, the active discharge surface has a diameter of at least 5 mm, suitably at least 10 mm, suitably at least 50 mm. The active discharge surface may be a disc-shaped (i.e. circular) plate arranged in the reaction chamber, suitably aligned with the central axis of the reaction chamber. Suitably the active discharge surface of the first electrode faces the second electrode. The active discharge surface is suitably a continuous surface, i.e. not containing pores or apertures.

[0048] Suitably the configuration and arrangement of the first electrode in the reaction chamber provides a free volume of the reaction chamber (for receiving the incoming gas stream) which has a toroid shape (i.e. a volume of a solid toroid). In embodiments wherein the reaction chamber is cylindrical and the first electrode is cylindrical, the free volume of the reaction chamber suitably has a square toroid shape.

[0049] The first electrode suitably comprises a cylindrical portion. In some embodiments, the first electrode comprises rod portion and a disc-shaped plate (which comprises the active discharge surface) wherein the disc-shaped plate is suspended in the chamber from the rod portion of the first electrode and spaced apart from the side walls of the reaction chamber, suitably wherein the central axis of the rod portion and disc-shaped plate are approximately aligned with the central axis of the reaction chamber.

[0050] In some embodiments, the first electrode is a cylinder comprising the active discharge surface, which is suitably substantially planar, at one end and is suspended in the reaction chamber, spaced apart from the side walls of the reaction chamber, suitably wherein the central axis of the cylindrical first electrode is approximately aligned with the central axis of the reaction chamber.

[0051] In such embodiments, the parts of the first electrode which are not the active discharge surface may be referred to as the non-active discharge portion of the first electrode. Suitably the rod portion or the walls of the cylinder portion of the embodiments described above are the non-active discharge portions of said first electrodes.

[0052] Optimizing the configuration of the non-active discharge portion of the first electrode may improve the flow dynamics in the operation of the method. For example, the flat, upper electrode active discharge surface may minimize gas bypass and may provide greater contact with the plasma, and promote the formation of a stable glow discharge. Additionally, a relatively large flat, active discharge surface may provide an increased surface area for enhanced heat dissipation through passive cooling via the gas stream, which may further promote formation of a glow discharge of plasma.

[0053] The second electrode suitably comprises an aperture and step a) suitably involves directing the incoming gas stream through the aperture. The second electrode therefore suitably comprises a ring portion which forms and surrounds the aperture. The ring portion is suitably the active discharge portion of the second electrode. Therefore step b) of the method suitably involves applying a voltage difference between the active discharge surface of the first electrode and the ring portion of the second electrode. Suitably the second electrode is arranged in the reaction chamber such that the only pathway for the incoming gas stream to pass from the at least one gas inlet to the gas outlet is through the aperture in the second electrode. The second electrode suitably defines an upper portion and a lower portion of the reaction chamber, wherein the upper portion and the lower portion of the reaction chamber communicate through the aperture of the second electrode. Suitably the at least one gas inlet is arranged in the upper portion of the reaction chamber and the gas outlet is arranged in the lower portion of the reaction chamber. Suitably the first electrode is arranged in the upper portion of the reaction chamber. Therefore the first and second electrodes are suitably arranged and configured such that the incoming gas stream is directed to contact the substantially planar surface of the first electrode and then flow through the aperture in the second electrode, with the discharge of plasma being generated between the first electrode and the second electrode.

[0054] The first electrode and the second electrode are suitably spaced apart in the reaction chamber, to define a working gap in which the plasma discharge is generated. The first electrode and the second electrode are suitably configured and positioned such that the vortex flow of the incoming gas stream is forced to flow through the working gap and so be exposed to the plasma generation. Suitably substantially all of the vortex flow of the incoming gas stream is exposed to the plasma reaction in the working gap. Therefore substantially none of the vortex flow of the incoming gas stream can bypass the working gap and flow towards the gas outlet without being exposed to the plasma.

[0055] The aperture of the second electrode has an opening on the surface of the second electrode which faces the active discharge surface of the first electrode.

[0056] The first and second electrodes are suitably spaced apart by at least 1 mm, suitably at least 3 mm, suitably at least 5 mm. Said distance is suitably the shortest distance between the active discharge surface of the first electrode and the plane of the opening of the aperture which faces the active discharge surface of the first electrode. The first and second electrodes are suitably spaced apart sufficiently to establish a working gap which allows for efficient plasma generation, suitably to allow the formation of a rotating glow discharge of plasma. The distance of separation of the first and second electrodes, and so the size of the working gap, may be set according to the particular scale of the plasma reactor used to carry out the method of this first aspect of the present invention, in order to promote and preferably optimise efficient plasma generation, suitably generation of a glow discharge of plasma.

[0057] The active discharge surface of the first electrode and the aperture of the second electrode are suitably arranged such that the centre of the active discharge surface is substantially aligned with the centre of the aperture of the second electrode. In embodiments wherein the active discharge surface of the first electrode and the aperture of the second electrode are circular, the central axis of the active discharge surface of the first electrode and the central axis of the aperture of the second electrode are suitably substantially aligned. In such embodiments, the reaction chamber is cylindrical and the central axis of the reaction chamber is substantially aligned with the central axis of the active discharge surface of the first electrode and the central axis of the aperture of the second electrode.

[0058] The second electrode may be circular, elliptical, triangular, quadratic or other suitable polygonal shape. Suitably the second electrode is substantially circular. The aperture of the second electrode may be circular, elliptical, triangular, quadratic or other suitable polygonal shape. Suitably the aperture of the second electrode is circular.

[0059] The active discharge surface of the first electrode has an area which can be measured. The opening of the aperture which faces the active discharge surface of the first electrode also has an area that can be measured. Suitably, the ratio of the area of the active discharge surface on the first electrode to the area of said opening of the aperture of the second electrode is equal to or greater than 0.5:1 . Suitably the area of the active discharge surface on the first electrode is equal to or larger than the area of said opening of the aperture of the second electrode. Suitably the ratio of the area of the active discharge surface on the first electrode to the area of said opening of the aperture of the second electrode is at least 1 :1 , at least 2:1 or at least 4:1 .

[0060] Said ratio may be up to 20:1 , up to 15:1 or up to 10:1 .

[0061] Said ratio of is suitably from 0.5:1 to 20:1 , or from 1 :1 to 10:1 .

[0062] Suitably both the active discharge surface of the first electrode and the opening of the aperture of the second electrode which faces the active discharge surface of the first electrode are circular. In such embodiments, the ratio of the diameter of the active discharge surface on the first electrode to the diameter of said opening of the aperture of the second electrode is equal to or greater than 0.75:1 . Suitably the diameter of the active discharge surface on the first electrode is equal to or larger than the diameter of said opening of the aperture of the second electrode. Suitably the ratio of the diameter of the active discharge surface on the first electrode to the diameter of said opening of the aperture of the second electrode is at least 1 :1 , at least 2:1 or at least 4:1 .

[0063] Said ratio may be up to 20:1 , up to 15:1 , up to 10:1 or up to 5:1 .

[0064] Said ratio of is suitably from 0.75:1 to 20:1 , or from 1 :1 to 5:1 .

[0065] In such embodiments, the central axis of the active discharge surface of the first electrode and the central axis of the aperture of the second electrode are suitably substantially aligned.

[0066] The size and configuration of the active discharge surface of the first electrode and the opening of the aperture of the second electrode may be adjusted and set for any particular scale of plasma reactor in order to promote and preferably optimise efficient plasma generation, suitably generation of a glow discharge of plasma.

[0067] The second electrode may have a funnel shape wherein the aperture is wider at an upper end of the second electrode, nearest the first electrode, and wherein the aperture is narrower at a lower end of the second electrode, farthest from the first electrode. Said funnel shape may assist with directing the incoming gas stream into the plasma discharge and may improve the performance of the method.

[0068] The second electrode may comprise a magnetic material. For example the second electrode may comprise a neodymium or samarium-cobalt magnetic material. Such a magnetic material may improve the rotation of the plasma discharge, which may provide an improvement in the performance of the method. In such embodiments, a protective coating may be provided on the second electrode to prevent the magnetic material coming into contact with the incoming gas stream and the plasma discharge.

[0069] In some embodiments, the method involves actively cooling the first electrode and / or the second electrode. The first electrode may be configured to enable cooling of the electrode when in use. Suitably, the first electrode may comprise an insert which may be placed inside the first electrode, wherein the insert is configured to allow transfer of heat away from the first electrode. The insert may be made of a ceramic or other material suitable for enabling the transfer of heat from the first electrode to a cooling liquid whilst insulating the cooling liquid from the electrical current passing through the electrode when the electrode is in operation.

[0070] The second electrode may be configured to allow coolant to flow into the electrode for cooling. The coolant may be water, an oil or a refrigerant. The method may comprise providing the electrodes with a flow of liquid cooled using a refrigeration unit or heat exchanger for cooling the electrodes; and wherein the method comprises controlling the flow of liquid. Suitably, the liquid may be water, an oil or a refrigerant. Suitably, the flow of liquid may be controlled using a pump or similar suitable device.

[0071] The cooling of the first and / or second electrode may help to prevent NOx species formed in the plasma from decomposing back to nitrogen and oxygen gas and / or may increase the yield of NO2 relative to NO, which may be beneficial in the production of nitric acid from the outgoing gas stream. The cooling of the first and / or second electrode may also help to prevent degradation of parts of the plasma reactor used to carry out the method, for example stainless steel parts of the reactor which may otherwise degrade during long term usage for NOx production.

[0072] In some embodiments, the second electrode comprises a catalyst. Such a catalyst is suitably an active catalyst for the formation of NOx species from nitrogen gas and oxygen gas. Suitably the second electrode comprises one or a plurality of mesh inserts for holding the catalyst. Suitably, one mesh insert may be located below the catalyst and one mesh insert may be located above the catalyst to prevent excessive movement of the catalyst. Suitably, the catalyst may comprise at least one of metals, oxides of molybdenum, tungsten or vanadium, and / or polyoxometalates of molybdenum, tungsten or vanadium or a mixture thereof.

[0073] In the method of this first aspect, step b) involves generating a discharge of plasma from the incoming gas stream by applying a voltage difference between the first electrode and the second electrode, suitably between a substantially planar surface of the first electrode and a ring portion of the second electrode, as described above.

[0074] Suitably the discharge of plasma is generated between the first electrode and the second electrode. The discharge of plasma may extend through the aperture of the second electrode.

[0075] The discharge produces reactive species from the nitrogen and oxygen gases in the incoming gas stream which react to form the NOx species of the outgoing gas stream.

[0076] The voltage difference applied between the first electrode and the second electrode may be a DC, pulsed or AC voltage. In some embodiments, the voltage is an AC voltage. The voltage is suitably selected to provide a desired discharge power of the plasma. The discharge power may be determined from the integral of the applied voltage multiplied by the arc current produced in the plasma, as defined below by Equation (1):

[0077] Where T is a time period of discharge, V(t) is the arc voltage and l(t) is the discharge current.

[0078] The power of the plasma discharge, as defined by Equation (1), may be in the range of from 27 W to 60 W, preferably in a range of from 30 Wto 50 W, for the experimental scale apparatus and method described below. As noted above with respect to incoming gas flow rates, the voltage difference applied and the power of plasma discharge produced by the method may depend on the scale at which the method is carried out. When the method is carried out on a larger, industrial scale apparatus, it will be appreciated that higher voltage differences may be used and higher powers of plasma discharge may be achieved. Suitably, depending on the scale at which the method is implemented, the voltage difference may be selected and controlled to optimise the efficiency of the process. When the method is carried out on a larger scale, for example a scale appropriate for industrial production of NOx, the power of the plasma discharge may be increased to a level appropriate for the scale of the plasma reactor. For example, the power of the plasma discharge may be up to 1 kW.

[0079] The energy consumption of NOx production may be defined by Equation (2): Where CNOxis the concentration of NOx, Fgasis the feed gas flow rate, and 24.5 L mol-1is the molar volume of ideal gas at 1 atm, 298 K.

[0080] Suitably step b) involves applying a sufficient voltage difference between the first electrode and second electrode to provide a desired power of the plasma discharge. The inventors have found that a stable, diffuse, glow discharge of plasma can be obtained in the present method with a voltage difference which provides a power of plasma discharge which meets a minimum threshold. In the experimental scale process discussed below, this minimum threshold of power of the plasma discharge was at least 35 W. Such a glow discharge of plasma may advantageously allow the method of forming NOx species to operate with the relatively low energy consumption and / or the high NO selectivity discussed above, compared to known methods. As discussed above, the minimum threshold of power of the plasma discharge required to form the advantageous glow discharge may vary according to the scale of the apparatus used to carry out the method.

[0081] Suitably step b) involves applying a sufficient voltage difference between the first electrode and second electrode to provide a desired specific energy input, in order to generate a stable plasma discharge and provide an efficient process for NOx formation. For example, the voltage difference is suitably set to provide a specific energy input to the reaction chamber of from 0.05 to 3 kJ / L, suitably from 0.1 to 2 kJ / L, from 0.25 to 1 .5 kJ / L or suitably from 0.42 to 1 .2 kJ / L.

[0082] The outgoing gas stream resulting from the glow discharge of plasma and subsequent reaction of the reactive species produced therein is collected in step c) of the method. The outgoing gas stream comprises product gas and unreacted incoming gas. The product gas comprises mainly the NOx species NO and NO2. Other gaseous products may be formed, for example, N2O, N2O5 and / or ozone. However, these gaseous products will be present in low amounts and NOx species are the major products formed.

[0083] The outgoing gas stream suitably comprises at least 1 vol% product gas, suitably at least 2 vol% product gas or at least 3 vol% product gas. The outgoing gas stream may comprise up to 50 vol%, suitably up to 25 vol%, suitably up to 13 vol% product gas or suitably up to 12 vol% product gas.

[0084] The product gas suitably comprises at least 90 vol% NOx species, suitably at least 95 vol% NOx species, at least 98 vol%, at least 99 vol% or at least 99.9 vol% NOx species.

[0085] The major component of the NOx species in the product gas is suitably NO. Suitably at least 75 vol% of the NOx species in the product gas are NO, suitably at least 80 vol%, suitably at least 85 vol% or at least 90 vol%.

[0086] Step c) of the method may involve cooling the outgoing gas stream, and therefore the NOx species contained therein. The outgoing gas stream may be cooled using a condenser or a heat exchanger. Suitably, the condenser may be any type of condenser including but not limited to a coil condenser, a Leibig condenser, a Graham condenser, and Allihn condenser. Suitably, the liquid in the condenser may be cooled using a refrigeration unit or heat exchanger. Suitably, the liquid in the condenser may be water, an oil or a refrigerant. Suitably, the flow of liquid in the condenser may be controlled using a pump or similar suitable device.

[0087] The cooling of the condenser can be provided through the flow of a liquid such as water, oil or a refrigerant. The liquid may be cooled using a refrigeration unit or heat exchanger. The flow of the liquid can be controlled by a pump or a similar suitable device.

[0088] The inventors have found that cooling the outgoing gas stream from the plasma reactor may enhance NOx production and may avoid decomposition of NO to nitrogen and oxygen.

[0089] Step c) may involve collecting the outgoing gas stream in a post-discharge container.

[0090] Step c) may involve adjusting a NO2 / NO ratio in the product gas using the post-discharge container. The post-discharge container may be configured to allow adjusting of the NO2 / NO ratio in the formed NOx to from 1 :2 to 2:1 . In one example the NO2 / NO ratio is 1 :1 .

[0091] The inventors have established that the NO2 / NO ratio in the product gas may be controlled by changing the dimensions of the post-discharge container. Suitably, the post-discharge container is a cylindrical post-discharge container. Suitably, the post-discharge container is a cylindrical container made from a plastics material, for example acrylic. However, any suitable material can be used.

[0092] In one example, the cylindrical post-discharge container has a total volume of 1 L. In one example, the cylindrical post-discharge container has a total volume of 5 L. In one example, the cylindrical post-discharge container has a total volume of 10 L.

[0093] In one example, the post-discharge container is a cylindrical container made from a plastics material, for example acrylic. However, any suitable material can be used.

[0094] In some embodiments, the post-discharge container comprises a micro-porous membrane dividing the container into two parts. Suitably a solution, for example an aqueous solution, may be added to one part of the post-discharge container to absorb the produced NOx species in the outgoing gas stream. The second part of the post-discharge container comprises the produced NOx. The micro- porous membrane suitably allows the penetration of gaseous NOx into the solution while prohibiting the penetration of aqueous solution into the gaseous part. Any suitable membrane material may be used. In one example, the membrane is metal, such as nickel, cobalt or stainless steel.

[0095] In one example, the diameter of the micro-porous membrane is from 10 to 100 mm, for example from 20 to 80 mm, such as 60 mm. In one example, the thickness of the membrane is from 0.05 to 0.5 mm, for example from 0.1 to 0.3 mm, such as 0.2 mm.

[0096] In one example, the pitch of the micro-porous membrane is from 350 to 500 pm, for example from 380 to 480 pm, such as 450 pm or 480 pm.

[0097] In one example, the pore size is from 5 to 100 pm, for example from 10 to 70 pm, such as 20 pm or 50 pm.

[0098] In one example, the diameter of the micro-porous membrane is 60 mm; the thickness of the membrane is 0.2 mm; the pitch of the micro-porous membrane is 450 pm, and the pore size is 50 pm. In one preferred example, the diameter of the micro-porous membrane is 60 mm; the thickness of the membrane is 0.2 mm; the pitch of the micro-porous membrane is 480 pm, and the pore size is 20 pm.

[0099] In one example, the diameter of the container is 70 mm, and the height of the container is 120 mm.

[0100] According to a second aspect of the present invention, there is provided a method of synthesising nitric acid, the method comprising the steps of:

[0101] 1) forming NOx species from nitrogen and oxygen using a method according to the first aspect or according to the second aspect of the present invention; and

[0102] 2) contacting the outgoing gas stream comprising NOx species formed in step 1) with an aqueous solution to form nitric acid.

[0103] Step 2) of the method of this third aspect may involve collecting the outgoing gas stream in a postdischarge container comprising an aqueous solution, as further described above in relation to the first aspect.

[0104] In step 2) of the method, the aqueous solution may be alkaline solution, suitably comprising hydroxide, for example potassium hydroxide. Such methods of forming nitric acid from an outgoing gas stream are described in W02023105230A, which is incorporated herein by reference.

[0105] In an alternative embodiment, step 2) may involve feeding the outgoing gas stream comprising the NOx species, suitably at elevated pressure, for example from 5-10 atm, up through a, suitably cooled, column filled with inert material whilst water is fed from the top down. This method can produce a solution of nitric acid having a concentration of about 45-60%. Such a step is part of known processes for producing nitric acid, for example the Ostwald process. Producing nitric acid from the NOx species of the outgoing gas stream may provide a more energy efficient way of obtaining useful nitrogen species compared to methods which convert said NOx species to ammonia, which may involve significant further energy inputs.

[0106] According to a third aspect of the present invention, there is provided a method of synthesising ammonia, the method comprising the steps of:

[0107] A) forming nitric acid by a method according to the third aspect of the present invention; and

[0108] B) reducing the nitric acid formed in step A) to form ammonia.

[0109] Step B) may involve electrochemically reducing the nitric acid formed in step A) to ammonia. An H- type cell may be used for such an electrochemical reduction. The H-type cell may be arranged as a divided electrochemical cell. Alternatively, an electrocatalytic nitrate reduction may be carried out using a flow cell. Such a flow cell may allow the method of this third aspect to be carried out as a continuous process for the production of ammonia from nitrogen gas and oxygen gas.

[0110] According to a fourth aspect of the present invention, there is provided a plasma reactor comprising; a reaction chamber; a first electrode arranged in the reaction chamber; a second electrode arranged in the reaction chamber, the second electrode comprising an aperture; at least one gas inlet configured to direct an incoming gas stream into the reaction chamber; and a gas outlet configured to direct an outgoing gas stream out of the reaction chamber.

[0111] The plasma reactor, and in particular the reaction chamber, the first electrode, the second electrode, the at least one gas inlet and gas outlet, may have any of the suitable features and advantages described in relation to the first aspect. The plasma reactor suitably operates as described in relation to the method of the first aspect.

[0112] Suitably the first electrode comprises an active discharge surface, suitably as further described above. Suitably the active discharge surface is arranged to face the second electrode. Suitably the active discharge surface is substantially planar. Suitably the active discharge surface is substantially circular. Suitably, the active discharge surface has a diameter of at least 5 mm, suitably at least 10 mm, suitably at least 50 mm. Suitably the reaction chamber is cylindrical, suitably as further described above.

[0113] Suitably the first electrode comprises a cylindrical portion, suitably as further described above.

[0114] Suitably the first electrode has a cylindrical shape, wherein the central axis of the cylindrical first electrode is approximately aligned with the central axis of the reaction chamber.

[0115] Suitably the reaction chamber comprises an upper portion and a lower portion, wherein the second electrode is arranged between the upper portion and the lower portion of the reaction chamber, and wherein the upper portion and the lower portion of the reaction chamber communicate through the aperture of the second electrode.

[0116] Suitably the at least one gas inlet is arranged in the upper portion of the reaction chamber and the gas outlet is arranged in the lower portion of the reaction chamber. The at least one gas inlet and the gas outlet are suitably arranged at or proximate to opposite ends of the reaction chamber. Suitably the at least one gas inlet and the gas outlet are arranged and configured (i.e. directed) in the reaction chamber shaped as described above, in order to generate a vortex flow of an incoming gas stream from the at least one gas inlet, through the reaction chamber and to the gas outlet.

[0117] Suitably the plasma reactor comprises two gas inlets configured to direct an incoming gas stream into the reaction chamber, wherein the two gas inlets are arranged on opposite sides of the reaction chamber. Said arrangement is believed to provide an improvement in the vortex flow of the incoming gas stream which may provide an increase in the efficiency of the formation of NOx species of the present invention.

[0118] In some embodiments, the second electrode comprises a catalyst, suitably wherein the catalyst is retained by at least one mesh insert in the second electrode.

[0119] According to a fifth aspect of the present invention, there is provided an apparatus for forming NOx species from nitrogen and oxygen, the apparatus comprising; a plasma reactor according to the fourth aspect of the present invention; at least one source of gas arranged in communication with the at least one gas inlet of the plasma reactor; and a post-discharge container arranged in communication with the gas outlet of the plasma reactor.

[0120] The plasma reactor, source of gas and post-discharge container may have any of the suitable features and advantages described in relation to the first, second, third and fourth aspects of the present invention. In some embodiments, the post-discharge container comprises a condenser for cooling an outgoing gas stream from the plasma reactor, suitably as further described in relation to the first aspect.

[0121] The post-discharge container may comprise a micro-porous membrane which divides the postdischarge container into a first part and a second part, suitably as further described in relation to the first aspect.

[0122] The apparatus of this fifth aspect may further comprise means for converting NOx species to nitric acid, suitably as further described in relation to the second aspect.

[0123] The apparatus of this sixth aspect may further comprise means for converting NOx to ammonia, suitably as further described in relation to the third aspect.

[0124] The means for converting NOx to ammonia may be an electrochemical means. A suitable electrochemical means may be an H-type cell arranged as a divided electrochemical cell.

[0125] Alternatively, a suitable means for converting NOx to ammonia may be provided by a flow-cell.

[0126] The means for converting NOx to nitric acid and / or ammonia are suitably configured to enable the production of nitric acid / and or ammonia using renewable energy sources.

[0127] The apparatus of this fifth aspect may be configured to allow integration and operation with existing means for converting NOx species to nitric acid and / or existing means for converting NOx to ammonia. The this may facilitate the adoption of the apparatus and methods of the present invention by reducing the costs associated with establishing new equipment and methods for said steps.

[0128] Brief Description Of The Drawings

[0129] For a better understanding of the invention, and to show how example embodiments may be carried into effect, reference will now be made to the accompanying drawings in which:

[0130] Figure 1 shows a schematic diagram of an apparatus according to the fifth aspect of the present invention for forming NOx species from nitrogen and oxygen in a method according to the first aspect of the invention.

[0131] Figure 2 shows a diagram and cross-section of a plasma reactor according to the fourth aspect of the present invention.

[0132] Figure 3a shows a graph illustrating the effect of the N2 fraction on the NOx concentration, for a gas flow rate of 5 SLM, a power of 45 W, in a method according to the first aspect of the present invention. Figure 3b shows a graph illustrating the effect of N2 fraction on energy consumption of NOx production and NO selectivity, for a gas flow rate of 5 SLM, a power of 45 W, in a method according to the first aspect of the present invention.

[0133] Figure 4a shows a graph illustrating the effect of the discharge power on the NOx concentration, for a gas (air) flow rate of 5 SLM, in a method according to the first and / or second aspects of the present invention.

[0134] Figure 4b shows a graph illustrating the effect of the discharge power on NO selectivity, and energy consumption of NOx production, for a gas (air) flow rate of 5 SLM.

[0135] Figure 5 shows the discharge voltage, current, instantaneous discharge power, and energy of the RGD over a time scale of (a) 100 ms and (b) 1 ms, at a discharge power of 27 W and gas flow rate of 5 SLM;

[0136] Figure 6 shows discharge voltage, current, instantaneous discharge power, and energy of the RGD at (a) 30 W, (b) 35 W, (c) 40 W, (d) 45 W(e) 50 W, and (f) 60 W for a gas flow rate of 5 SLM;

[0137] Figure 7 shows the effect ofthe flow rate on (a) the NOx concentration, (b) NO selectivity, and energy consumption of NOx production using RGD at a fixed power of 45 W.

[0138] Figure 8 shows the discharge voltage, current, instantaneous discharge power, and energy of the RGAD at (a) 3 SLM, (b) 3.5 SLM, and (c) 4 SLM, for a discharge power of 45 W.

[0139] Figures 9 shows the effect of post container volume on NOx composition;

[0140] Figure 10 shows a cross-section of an embodiment of a plasma reactor according to the fourth aspect of the present invention.

[0141] Figure 11 shows a cross-section of an embodiment of a plasma reactor according to the fourth aspect of the present invention, including a reflux condenser outlet.

[0142] Figure 12 shows an alternative quartz glass housing for the rotating glow discharge reactor.

[0143] Figure 13 shows an alternative second electrode design.

[0144] Detailed Description Of The Example Embodiments

[0145] Figure 1 shows an apparatus 100 according to the fifth aspect of the present invention for forming NOx species from nitrogen and oxygen in a method according to the first aspect of the invention. The apparatus comprises a plasma reactor 200 as further described below in relation to Figure 2. The apparatus 100 also comprises a source of nitrogen gas 1 10 and a source of oxygen gas 120, which are pressurized containers of said gases connected to the plasma reactor 200 through mass flow controllers 111 and 121 , respectively. Said mass flow controllers can be used to control the proportion of nitrogen gas and oxygen gas entering the plasma reactor and also the overall flow rate of the gas stream (comprising the nitrogen gas and the oxygen case) into the plasma reactor. In embodiments which utilise air as an incoming gas stream, the sources of nitrogen gas 110 and oxygen gas 120 are replaced with a source of air, for example a pressurized container comprising air or a means for compressing and / or impelling air into the apparatus 100. The apparatus 100 also comprises equipment 130 for applying a voltage difference to the plasma reactor and to measure and control the properties I parameters of said voltage and the current produced. The equipment 130 includes high voltage power supply 131 , a high voltage probe 132, a current probe 134, and an oscilloscope 133. The apparatus 100 further comprises a post-discharge container 150 for receiving an outgoing gas stream from the plasma reactor, and an FTIR spectrometer 140 for analysing said outgoing gas stream.

[0146] Figure 2 shows the plasma reactor 200 of the apparatus 100 of Figure 1 , which is according to the fourth aspect of the present invention. The plasma reactor comprises a first gas inlet 201 and a second gas inlet (not shown), a gas outlet 202, a reaction chamber 210, a first electrode 220 and a second electrode 230. The reaction chamber 210 is cylindrical in shape and is divided into an upper portion 211 and a lower portion 212, relative to the orientation of the plasma reactor shown in Figure 2. The first gas inlet 201 is arranged in the upper portion 211 of the reaction chamber 210, adjacent to the top of the reaction chamber. The first gas inlet 201 is directed perpendicular to and at a tangent to the central axis of the reaction chamber 210 to direct an incoming gas stream in a vortex flow around the free space inside the cylindrical reaction chamber 210. The second gas inlet is arranged on the opposite wall of the reaction chamber 210 and is directed as described above for the first gas inlet. The gas outlet 202 is arranged in the lower portion 212 of the reaction chamber, at a bottom end of the reaction chamber 210.

[0147] The reaction chamber 210 may be formed of any suitable material having sufficient strength and chemical resistance. In some embodiments, the reaction chamber is formed of stainless steel. In some embodiments, the reaction chamber is formed of a polymeric material, suitably a non- electrically conductive polymeric material, for example polytetrafluoroethylene (PTFE). Such polymeric materials may provide the advantages of lower cost, lower risk of chemical degradation and lower risk of the electric shock for a user. Such polymeric materials may have a lower embodied energy cost associated with their manufacture.

[0148] The first electrode 220 is arranged in the upper portion 211 of the reaction chamber and is aligned with the central axis of the reaction chamber 210. The first electrode 220 comprises a rod portion 221 and a disc-shaped plate 222. The disc-shaped plate 222 comprises a planar bottom surface which faces the second electrode 230. The rod portion 221 is present to attach and position the discshaped plate 222, which is an active discharge portion of the first electrode, in the reaction chamber 210. The lower surface of the disc-shaped plate 222 faces the second electrode and provides an active discharge surface. The disc-shaped plate 222 is 20 mm in diameter, 5 mm in thickness. The first electrode 220 is configured as a high voltage electrode and is connected to an AC high voltage power supply with a maximum peak-to-peak voltage of 30 kV and a fixed frequency of 1 1 kHz in the apparatus 100. The shape, size and position of the first electrode 221 in the upper portion 21 1 of the reaction chamber 210 provides a free space within reaction chamber which has a substantially toroidal shape, the free space being bounded by the walls of the reaction chamber and the first electrode. The free space receives the incoming gas stream from the gas inlets 201 .

[0149] The second electrode 230 is arranged in the reaction chamber 210 between the upper portion 211 and the lower portion 212. The second electrode 230 is ring-shaped, comprising ring portion 231 which defines an aperture 232. The ring portion 231 is the active discharge portion of the second electrode 230. The second electrode 230 has a diameter (of the ring portion 231) of 80 mm and the aperture 232 has a diameter of 15 mm. The thickness of the second electrode 230 is 15 mm. The second electrode is aligned with the central axis of the reaction chamber 210. The ring portion 231 is the active discharge part of the second electrode 230. The aperture 232 provides a passageway for the incoming gas stream to pass from the upper portion 211 to the lower portion 212 of the reaction chamber 210. Therefore the arrangement of the reaction chamber described above provides a pathway for an incoming gas stream from the gas inlets to pass, in a vortex flow, over and around the disc-shaped plate 222 of the first electrode 220, through the aperture 232 of the second electrode 230 and out of the gas outlet 202. The second electrode 230 is configured to be a ground electrode and is grounded in the apparatus 100.

[0150] The first electrode 220 and the second electrode 230 are arranged such that both the centre of the disc-shaped plate 222 and the centre of the aperture 232 are aligned with each other and with the central axis of the reaction chamber 210, such that the planar bottom surface of the disc-shaped plate 221 of the first electrode 220 faces the aperture 232 of the second electrode 230. The first electrode 220 and the second electrode 230 are spaced apart at a suitable distance such that a current can flow therebetween, forming a plasma, suitably a rotating glow discharge plasma, when a sufficient voltage difference is applied. In plasma reactor 200, the first electrode 220 and the second electrode 230 are spaced 5 mm apart.

[0151] It will be appreciated that the scale of the apparatus 100 and plasma reactor 200 can be varied according to the desired scale of NOx production, for example in order to implement the process of the present invention on an industrial scale.

[0152] In use, the sources of nitrogen gas 110 and oxygen gas 120, and the mass flow controllers 111 and 121 , are manipulated to provide an incoming gas stream comprising nitrogen gas and oxygen gas having a set composition and flow rate, which passes into the reaction chamber 210 of the plasma reactor 200 through gas inlets 201 . The incoming gas stream flows through the upper chamber 21 1 of the reaction chamber 210 in a vortex flow around the cylindrical / toroidal-shaped free-space in the reaction chamber and towards the gas outlet 202. The incoming gas stream flows over and around the disc-shaped plate 221 of the first electrode 220, contacting the planar bottom surface (active discharge surface) of the disc-shaped plate 221 . A voltage is applied to first electrode, as further discussed below, to form a plasma from the incoming gas stream. The plasma so formed is between the first electrode 220 and the second electrode 230 and also through the aperture 232 in the second electrode 230 into the lower chamber 212 of the reaction chamber 210. The plasma comprises reactive species formed from the nitrogen gas and oxygen gas in the incoming gas stream which subsequently react to form NOx species, including NO and NO2, as further discussed below, in an outgoing gas stream which passes through the gas outlet 202. The outgoing gas stream is collected in post-discharge container 120 and analysed by FTIR spectrometer 140.

[0153] Figure 10 shows plasma reactor 300 according to an embodiment of the fourth aspect of the present invention. Plasma reactor 300 is as described in relation to Figure 2 with respect to the reaction chamber 310, gas inlets 301 (only one shown), the second electrode 330 and the arrangement of the first and second electrodes. In the plasma reactor 300, the first electrode 320 has a cylinder shape comprising a substantially planar bottom surface 322, which is the active discharge surface of the first electrode 320. As with the plasma reactor 200, the central axis of the cylindrical first electrode 320 is approximately aligned with the central axis of the reaction chamber 310. The first electrode 320 is also configured to enable cooling of the electrode when in use. The first electrode 320 is hollow and comprises an insert 340 inside the first electrode 320. The insert 340 comprises a coolant inlet 341 and a coolant outlet 342 configured for the flow of a coolant liquid through the insert 340 to enable cooling of the first electrode 320 when in use. The insert 340 is formed of a material suitable for enabling the transfer of heat from the first electrode 320 to a cooling liquid whilst insulating the cooling liquid from the electrical current passing through the first electrode 320 during operation. For example, the insert 340 may be formed of a suitable ceramic material.

[0154] The second electrode 330 is arranged in the reaction chamber 310 as described in relation to Figure 2. The second electrode 330 is ring-shaped, comprising ring portion 331 which defines an aperture 332. The aperture is able to fit a coolant pipe (not shown) for cooling the second electrode 330 in use. One or a plurality of metal meshes 333 are placed inside the aperture 332. One or a plurality of metal meshes 333 are configured to hold a catalyst. The catalyst is suitably active with respect to promoting the reactions involved in forming NOx species from oxygen gas and nitrogen gas. Examples of catalyst that could be used include, but are not limited to metals and oxides of molybdenum, tungsten and vanadium, including polyoxometalate variations thereof.

[0155] Figure 11 shows plasma reactor 400 according to a further embodiment of the fourth aspect of the present invention. Plasma reactor 400 is as described in relation to Figure 10 with respect to the reaction chamber 410, gas inlets 401 a and 401 b, gas outlet 402, the first electrode 420, the second electrode 430 and the arrangement of the first and second electrodes. The lower portion of the reaction chamber 412 comprises a condenser 450 which replaces the lower portion of the reactor shown in Figure 2. The condenser 450 is configured to enable immediate cooling of the outgoing gas from the plasma reactor 400 to enhance NOx production and avoid NO decomposition to N2 and O2. In Figure 11 , a coil condenser 450 has been used, although the skilled person will understand that any type of condenser may be used for this purpose, as described above. The condenser 450 comprises a coolant inlet 452 and a coolant outlet 451 . Cooling of the condenser can be provided through the flow of a liquid such as water, oil or a refrigerant. The liquid may be cooled using a refrigeration unit or heat exchanger (not shown). The flow of the liquid can be controlled by a pump or a similar suitable device (not shown).

[0156] Figure 12 shows an embodiment of a quartz glass housing 500 for the plasma reactor (200, 300). The quartz glass housing is cylindrical and hollow. The housing 500 has a side wall 501 which provides the outer walls of the reaction chamber 210, 310, 410 of the plasma reactor 200, 300, 400. Each end 520, 530 of the housing 500 is open and comprises a flange 521 , 531 to facilitate alignment and retention of the housing 500 in a plasma reactor 200, 300, 400.

[0157] Figure 13 shows an embodiment of a second electrode 630 as described in Figure 2, Figure 10 and Figure 11. The second electrode 630 is ring-shaped, comprising ring portion 631 which defines an aperture 632. The ring portion 631 comprises a guide channel 634 which is configured to fit a coolant pipe (not shown) for cooling the electrode. The aperture is configured to fit the coolant pipe for electrode cooling (not shown). The aperture is configured to fit one or a plurality of metal meshes (not shown). Said one or a plurality of metal meshes may be configured to hold a catalyst. Examples of catalyst, as discussed above in relation to Figure 10.

[0158] Examples

[0159] The following procedures were used in the examples which follow.

[0160] The discharge voltage was measured by a high voltage probe (Tektronix P6015A), while the current was measured by a current monitor (Pearson 2877). The electrical signals (discharge voltage, current) were recorded by a four-channel digital oscilloscope (Tektronix MDO 3054, 500 MHz, 2.5 GS / s) at a sampling rate of 5 Mpts per record to ensure precise measurement.

[0161] The gaseous reaction products were analyzed online using a Fourier transform infrared (FTIR) spectrometer (Jasco 4600) at a wavenumber resolution of 2 cm1, and each spectrum was obtained by averaging 16 scans. The absorption spectra were recorded 10 minutes after the discharge ignition to ensure a stable discharge, and each measurement was repeated at least three times. For quantitatively analyzing the concentrations of NOx, precise calibration gas mixtures (NO or NO2 in Argon) with a wide range of concentrations were introduced to the gas cell by mass flow controllers. The concentrations of aqueous nitrate, nitrite and ammonia were measured by spectrophotometric method using a microplate reader (Thermo Scientific Varioskan® Flash Reader). For the detection of nitrite, 100 pL Griess reagent was added into 100 pL sample, and the absorbance was measured at 540 nm. For the detection of nitrate, 100 pL saturated VCh was added into the sample, after which 100 pL Griess reagent was added into the above solution, and the absorbance was measured at 540 nm after the solution was incubated at 37 °C for 12h to insure full reduction of nitrate by VCh. Finally the nitrate concentration was obtained by subtracting the nitrite concentration from the total concentration of nitrate and nitrite. For detection of ammonia, 100 pL potassium sodium tartrate was added into 100 pL sample and then 100 pL Nessler reagent was added into the above solution, and the absorbance was measured at 420 nm. All the measurements were calibrated by using the standard curves.

[0162] Example 1 : NOx production at different N2 / O2 ratios

[0163] The effect of the N2 / O2 feed ratio was investigated at a fixed power of 45 W and flow rate of 5 SLM. The NO and NO2 concentrations both followed parabolic trends upon increasing N2 fraction (Figure 3 (a)). At a N2 fraction of 5 vol%, the concentrations of NO and NO2 were 2351 .7 ppm and 501 .7 ppm, respectively. The increase in N2 fraction led to an increase in both NO and NO2 concentrations. NO2 concentration reached its maximum (3089.1 ppm) at a N2 fraction of 40 vol%, while NO favoured a higher N2 fraction with an optimum N2 fraction of 70 vol%. Further increasing the N2 fraction above 70 vol% led to a rapid decrease in NO concentration. The concentration of NOx reached its maximum (11944.3 ppm) at a N2 fraction of 60 vol%.

[0164] Figure 3 (b) shows the selectivity of NO as a function of N2 fraction. The high NO selectivity could be attributed to the higher flow rate, which indicates a shorter residence time of the NO in the plasma and therefore less likely to be converted to NO2. At the highest N2 fraction of 95 vol%, the NO selectivity reached 96.7%.

[0165] The energy consumption of NOx production at different N2 fraction was shown in Figure 3 (b). An exceptionally low energy consumption of NOx production (0.94 MJ mol-1) was achieved at the optimum N2 fraction of 60 vol%. Indeed, at the N2 fraction of between 40 vol% and 70 vol%, the energy consumption stayed all below 1 MJ mol-1. Note at a N2 fraction of 80 vol%, like the composition of air, the energy consumption (1 .06 MJ mol-1) was only 10% higher than the best condition, making air a suitable feed gas as no additional energy is required to prepare pure O2 and N2 as mentioned before. Further increasing the N2 fraction above 80 vol% led to notable increase in energy consumption.

[0166] Example 2: NOx production at different discharge powers

[0167] The effects of discharge power and flow rate on NOx production were also investigated using air as the feed gas. The minimum discharge power to maintain a stable discharge was 27 W, at which the discharge was composed of many arcs with high current values (~2 A) (Figure 5). Increasing the discharge power from 27 Wto 35 W resulted a fast increase in NOx concentrations, which could be attributed to the discharge mode transition to a more diffuse discharge as can be seen in the discharge signals. As shown in Figure 6, the high current peaks dropped significantly compared to that at a discharge power of 27 W.

[0168] Further increasing the power above 35 W led to an almost linear increase in NOx concentration (Figure 4 (a)) and the discharge all operated in diffuse glow mode as can be seen from the voltage and current signals (Figure 6). The NO selectivity dropped as power increased. As seen in Figure 4 (b), the energy consumption of NOx production dropped quickly from 1.4 MJ mol-1to the minimum energy consumption of 1.04 MJ mol-1as the discharge transited to diffuse glow mode. Further increasing the power led to a slight increase in energy consumption, which may be due to higher NOx decomposition rates at higher NOx concentrations.

[0169] Example 3: NOx production at different flow rates

[0170] Higher flow rates led to lower NOx concentrations because of lower specific energy input, which also resulted in higher NO selectivity as shown in Figure 7. However, increasing the flow rate only had a minor effect on energy consumption, which stayed around 1 .1 MJ mol-1(Figure 7 (b)). The discharge operated in diffuse glow mode at all investigated flow rates at a power of 45 W. This can be seen from the voltage and current profiles (Figure 8), in which no high value current was observed at all tested flow rates.

[0171] The exceptional NOx production performance with RGD may be attributed to the intrinsic characteristics of glow discharge, which enables NOx production mainly by vibrational excitation of N2 and thus overcome the high energy barrier for N2 dissociation. In addition, the unique design of our RGD reactor enabled a larger proportion of the feed gas to traverse the discharge in comparison to conventional two-dimensional gliding arcs, thereby improving energy efficiency. In the investigated range, discharge power and flow rate both significantly affected the NOx concentration but only had a limited effect on energy consumption.

[0172] Example 4: NOx composition manipulation with a post-discharge container

[0173] Figure 9 shows the impact of post-discharge container volume on NO and NO2concentrations. For these experiments, air was used as the incoming gas stream at a flow rate of 5 SLM. The discharge power of the plasma was 35 W. The use of this post-discharge container enabled tuneable NO2selectivity within a range of 13% to 87%. Notably, this manipulation was achieved without affecting the total NOx concentration. These results demonstrate that the methods, plasma reactor and apparatus of the present application can provide an efficient and effective production of useful NOx species which can be carried out, advantageously, at ambient pressure using air at economically low gas flow-rates.

[0174] Although a few preferred embodiments have been shown and described, it will be appreciated by those skilled in the art that various changes and modifications might be made without departing from the scope of the invention, as defined in the appended claims.

[0175] Throughout this specification, the term “comprising” or “comprises” means including the component(s) specified but not to the exclusion of the presence of other components. The term “consisting essentially of’ or “consists essentially of’ means including the components specified but excluding other components except for materials present as impurities, unavoidable materials present as a result of processes used to provide the components, and components added for a purpose other than achieving the technical effect of the invention. Typically, when referring to compositions, a composition consisting essentially of a set of components will comprise less than 5% by weight, typically less than 3% by weight, more typically less than 1 % by weight of non-specified components.

[0176] The term “consisting of’ or “consists of’ means including the components specified but excluding addition of other components.

[0177] Whenever appropriate, depending upon the context, the use of the term “comprises” or “comprising” may also be taken to encompass or include the meaning “consists essentially of’ or “consisting essentially of’, and may also be taken to include the meaning “consists of’ or “consisting of’.

[0178] The optional features set out herein may be used either individually or in combination with each other where appropriate and particularly in the combinations as set out in the accompanying claims. The optional features for each aspect or exemplary embodiment of the invention as set out herein are also to be read as applicable to any other aspect or exemplary embodiments of the invention, where appropriate. In other words, the skilled person reading this specification should consider the optional features for each exemplary embodiment of the invention as interchangeable and combinable between different exemplary embodiments.

[0179] Attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference. All of the features disclosed in this specification (including any accompanying claims, and drawings), and / or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and / or steps are mutually exclusive.

[0180] Each feature disclosed in this specification (including any accompanying claims, and drawings) may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

[0181] The invention is not restricted to the details of the foregoing embodiment(s). The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

Claims

Claims1 . A plasma reactor comprising; a reaction chamber; a first electrode arranged in the reaction chamber; a second electrode arranged in the reaction chamber, the second electrode comprising an aperture; at least one gas inlet configured to direct an incoming gas stream into the reaction chamber; and a gas outlet configured to direct an outgoing gas stream out of the reaction chamber.

2. The plasma reactor according to claim 1 , wherein the first electrode comprises an active discharge surface arranged to face the second electrode.

3. The plasma reactor according to claim 2, wherein the active discharge surface is a continuous surface.

4. The plasma reactor according to claim 2 or claim 3, wherein the active discharge surface is substantially circular; and wherein the active discharge surface has a diameter of at least 5 mm, suitably at least 10 mm, suitably at least 50 mm.

5. The plasma reactor according to any one of claims 2 to 4, wherein the ratio of the surface area of the active discharge surface on the first electrode to the area of the aperture of the second electrode is equal to or greater than 0.5:1 .

6. The plasma reactor according to any one of the preceding claims, wherein the first electrode has a cylindrical shape, wherein the central axis of the cylindrical first electrode is approximately aligned with the centre of the aperture of the second electrode.

7. The plasma reactor according to any one of the preceding claims, wherein the reaction chamber is cylindrical.

8. The plasma reactor according to any one of the preceding claims, wherein the second electrode comprises a catalyst, suitably wherein the catalyst is retained by at least one mesh insert in the second electrode.

9. The plasma reactor according to any one of the preceding claims, wherein the reaction chamber comprises an upper portion and a lower portion, wherein the second electrode is arranged betweenthe upper portion and the lower portion of the reaction chamber, and wherein the upper portion and the lower portion of the reaction chamber communicate through the aperture of the second electrode.

10. The plasma reactor according to claim 9, wherein the at least one gas inlet is arranged in the upper portion of the reaction chamber and the gas outlet is arranged in the lower portion of the reaction chamber.11 . The plasma reactor according to any one of the preceding claims, comprising two gas inlets configured to direct an incoming gas stream into the reaction chamber, wherein the two gas inlets are arranged on opposite sides of the reaction chamber.

12. An apparatus for forming NOx species from nitrogen and oxygen, the apparatus comprising; a plasma reactor according to any any one of the preceding claims; at least one source of gas arranged in communication with the at least one gas inlet of the plasma reactor; and a post-discharge container arranged in communication with the gas outlet of the plasma reactor.

13. The apparatus according to claim 14, wherein the post-discharge container comprises a condenser for cooling an outgoing gas stream from the plasma reactor.

14. The apparatus according to claim 12 or claim 13, wherein the post-discharge container comprises a micro-porous membrane which divides the post-discharge container into a first part and a second part.

15. The apparatus according to any one of claims 12 to 14, wherein the apparatus further comprises means for converting NOx species to nitric acid.

16. The apparatus according to any one of claims 12 to 15, wherein the apparatus further comprises means for converting NOx to ammonia.

17. The apparatus according to claims 16, wherein the means for converting NOx to ammonia comprises a flow-cell.

18. A method of forming NOx species from nitrogen and oxygen, the method comprising: a) directing an incoming gas stream comprising nitrogen gas and oxygen gas into a reaction chamber comprising a first electrode and a second electrode to generate a vortex flow of the incoming gas stream within the reaction chamber;b) generating a plasma from the vortex flow of the incoming gas stream by applying a voltage difference between the first electrode and the second electrode; c) collecting an outgoing gas stream comprising NOx species from the reaction chamber.

19. The method according to claim 18, wherein step b) involves generating a rotating glow discharge plasma from the vortex flow of the incoming gas stream.

20. The method according to claim 18, wherein step b) involves generating a gliding arc discharge plasma from the vortex flow of the incoming gas stream.21 . The method according to any one of claims 18 to 20, wherein in step a) the incoming gas stream comprises air.

22. The method according to any one of claims 18 to 21 , wherein the reaction chamber is cylindrical.

23. The method according to any one of claims 18 to 22, wherein the first electrode comprises an active discharge surface arranged to face the second electrode and step a) involves directing the incoming gas stream to contact the active discharge surface.

24. The method according to any one of claims 18 to 23, wherein second electrode comprises an aperture and step a) involves directing the incoming gas stream through the aperture.

25. The method according to any one of claims 18 to 24, wherein the voltage difference is set to provide a specific energy input to the reaction chamber of from 0.05 to 3 kJ / L.

26. The method according to any one of claims 18 to 25, wherein the method involves actively cooling the first electrode and / or the second electrode.

27. The method according to any one of claims 18 to 26, wherein step c) involves cooling the outgoing gas stream.

28. The method according to any one of claims 18 to 27, wherein step c) involves collecting the outgoing gas stream in a post-discharge container.

29. The method according to claim 28, wherein step c) involves adjusting a NO2 / NO ratio in the outgoing gas stream using the post-discharge container.

30. The method according to any one of claims 18 to 29, wherein the incoming gas stream comprises at least 50 vol% O2.31 . A method of synthesising nitric acid, the method comprising the steps of:1) forming NOx species from nitrogen and oxygen using a method according to any one of claims 19 to 30;2) contacting the outgoing gas stream comprising NOx species formed in step 1) with an aqueous solution to form nitric acid.

32. A method of synthesising ammonia, the method comprising the steps of:A) forming nitric acid by a method according to claim 31 ;B) reducing the nitric acid formed in step A) to form ammonia.

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