Pollutant abatement system

Abstract

There is provided a reactor for reducing a quantity of a pollutant in a gas. The reactor comprises a plasma generator, catalyst and a gas flow path along which gas passes in use. The gas flow path passes through a plasma region and over the catalyst, plasma being formed in the plasma region in use by the plasma generator. The plasma generates active species that are entrained in the gas and decay in a decay region downstream of the plasma region. The catalyst is downstream of the plasma generator and at least partially inside the decay region.

Description

[0001] POLLUTANT ABATEMENT SYSTEM

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to plasma-catalysis reactors, typically for use in treatment of gas being passed through the reactor, such as to provide methane abatement.

[0004] BACKGROUND

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

[0006] In addition to sulphur oxides (SOx), nitrogen oxides (NOx) and carbon dioxide (CO2, CO2), hydrocarbons represent one pollutant where it is desirable to minimise or eliminate release into the atmosphere. In particular, there is a desire to minimise methane (CH4, CH4) emissions. This is because methane is a potent greenhouse gas and, as such, is of concern due to its contribution to rising global temperatures.

[0007] Methane may be a constituent in gaseous mixtures emitted from various sources due to methane being produced as a byproduct, or as a result of the known occurrence of “methane slip”. Irrespective of the source, and the reason why methane is a constituent in the gaseous mixture emitted from a source, removing methane upstream of release into the atmosphere is desirable due to the impact of methane on the environment, climate, and human health.

[0008] Various means of removing methane upstream of release into the atmosphere exist. For example, one means of removing methane upstream of release into the atmosphere is by engine design measures. Another means is by using a Methane Oxidation Catalyst (MOC) positioned in a methane-containing gas stream. Another means is by using plasma-based technologies, such as Electron Beam Flue Gas Treatment (EBFGT) and Dielectric Barrier Discharge (DBD).

[0009] MOCs include materials such as palladium, platinum, rhodium, manganese, or combinations and alloys thereof. These are widely used to reduce the methane content of a methane-containing gas stream. This is achieved by oxidising methane to CO2 and water. MOCs have proved useful in reducing methane emissions, but have drawbacks. For example, the performance of catalysts is generally temperature dependent, with performance improving at higher temperatures. This is particularly relevant when addressing methane emissions, as methane is a relatively stable molecule, and so typically requires higher temperatures than other, less stable, molecules to oxidise.

[0010] However, often, by the time the gas stream contacts the MOC, the temperature of a gas stream is lower than desired for efficient methane removal, such as when the gas stream originates from an Internal Combustion Engine (ICE). This is because ICEs have variable temperature output and, by the time the gas stream reaches the MOC, generally have output temperatures of lower than the optimum MOC operating temperature.

[0011] In addition to the temperature dependency, methane oxidation catalysts also suffer from “catalyst poisoning” and water-induced sintering, due to species such as SOx (for example SO2 and SO3) and water.

[0012] The drawbacks suffered by MOCs result in the catalyst activity decreasing over time. However, there remains a need for methane slip removal while avoiding limitations of precious metal based catalyst materials.

[0013] Alternative, catalyst-free, technology includes EBFGT and DBD. However, EBFGT, the details of which are set out in WO 2017 / 055881 A1 , which is incorporated by reference, has various negative factors. These include cost, implementation, maintenance, safety requirements, radiation shielding needs and lack of mobility. These make EBFGT undesirable. For DBD, achieving a homogeneous plasma density allows chemical reactions to be driven throughout the volume between the electrodes. This is difficult to achieve satisfactorily, however. This is due to even slight imperfections in electrodes focusing the electric field and disrupting the distribution of discharge, which negatively affects the homogeneity of the plasma density. This limits the value of DBD as a viable catalyst-free option.

[0014] There have been attempts to provide systems that combine multiple types of pollutant reduction techniques in a staged manner. These include providing systems that implement a plasma generation process and a catalysis process. These either provide a distinct separation between a plasma generation stage and a catalysis stage, or provide a catalyst in the plasma generator, such as by coating electrodes with a catalyst. Various examples can be seen in WO 2004 / 085038 A1 , CN 108798842 A, US 2008 / 0173534 A1 and EP 1968740 B1.

[0015] In configurations where a catalyst is provided in a plasma generator, there is evidence of generation of unwanted nitrogen species, such as nitrogen oxide (NO), and more generally of NOx (so including at least NO2 in addition to NO). This is also compounded by a more limited ability to remove these from the gas stream. It is, however, understood that the best ability to reduce the quantity of pollutants in the gas stream is to provide this configuration over having plasma stage and catalysis stage separated, even though this would remove the chance of the catalyst being damaged by discharge in the plasma generator.

[0016] There is therefore a need for a means of enhanced gas stream pollutant reduction while limiting generation of unwanted by-products and component damage.

[0017] SUMMARY OF INVENTION

[0018] Plasma-catalysis is the integration of a catalyst with a plasma to generate desired products at desired rates and at desired efficiencies that are otherwise inaccessible via conventional catalytic means. In a non-thermal plasma (NTP), also referred to as a non-equilibrium plasma, the electron temperature is much greater than the temperature of the heavy species (ions and neutrals), and thus the radicals and excited species are formed at temperatures closer to ambient temperature. This non-thermal distribution of energy offers a potential avenue to overcoming both the kinetic and thermodynamic limitations on chemical transformations of reactants into desired products. This energy, appropriately directed, can, in principle drive, equilibrium-limited reactions at conditions at which equilibrium conversions are small. Similarly, the energy can accelerate reaction pathways that are kinetically slow at prevailing conditions. The highly energetic electrons in an NTP produce (rotationally, vibrationally and electronically) excited species, ions and radicals through inelastic collisions with feedstock molecules, yielding a plethora of new species and states that are inaccessible at the bulk thermal temperature. Since NTPs can contain a diverse mix of highly reactive species, they are difficult to operate in such a way as to produce single products in high yield and at high selectivity, however.

[0019] If it were not for the undesirable factors set out in the previous section, integration of plasma and catalysts together promises to combine the advantages of the two, to effect transformations that are currently difficult or impossible to achieve in the frame of thermodynamic equilibrium conditions.

[0020] In order to address this, we have developed a form of plasma-catalysis that avoids the detrimental factors while minimising the decrease in the effectiveness of using plasma generation and catalysis within the same overall system. This is achieved by providing catalysis at the edge of plasma generation, such as (immediately) adjacent. This is due to this (immediate) sequential arrangement avoiding the catalyst from being damaged by discharge during plasma generation and providing capacity to remove unwanted species generated. This is achieved while still allowing for the plasma to enhance pollutant conversion by the catalyst as well as the plasma providing pollutant conversion itself, providing synergistic effects.

[0021] Thus, according to an aspect, there is provided a reactor for (i.e. suitable for) reducing a quantity of a pollutant in a gas, the reactor comprising: a plasma generator, catalyst and a gas flow path along which gas passes in use, the gas flow path passing through a plasma region and over the catalyst, plasma being formed in the plasma region in use by the plasma generator, the plasma generating active species that are entrained in the gas and decay in a decay region downstream of the plasma region, the catalyst being downstream of the plasma generator / plasma region and at least partially inside (or overlapping with) the decay region.

[0022] As a manner to address disadvantages set out above, as noted, this provides integrated plasma and catalysis, providing, not only the benefits of the two separately, but we have found this provides a synergistic effect of enhancing reduction in pollutant quantity beyond the sum of the two independent from each other. This thus provides a desired practical means for gas purification capable of favourably reducing a quantity of pollutants, for example by oxidizing unburned components (such as methane), of a gas source, for example, a gas fuel (such as CNG, Compressed Natural Gas, and / or LNG).

[0023] The reactor according to any one of the preceding claims, wherein the plasma region is aligned with the plasma generator.

[0024] The plasma generator and the catalyst may be immediately sequential to each other, i.e. with no gap between the plasma generator and the catalyst. Typically, however, there is a separation between the plasma reactor and the catalyst, the separation being larger than a length along the gas flow path of the plasma reactor. This offset between the plasma generator and the catalyst limits electrical creepage between these features and provides ability for species formed as a result of the plasma generation to mix between the plasma generator and the catalyst.

[0025] The catalyst may be up to 160 millimetres (mm) from the plasma generator or plasma region. This may be a distance between an upstream end of the catalyst and a downstream end of the plasma generator or plasma region. This provides flow to stabilise after having passed through the plasma generator while providing an ability for the components in the flow to mix.

[0026] If wanted, the distance between an upstream end of the catalyst and a downstream end of the plasma region is able to be kept to a minimum. For example, the (upstream end of the) catalyst may be up to 10 mm from the (downstream end of the) plasma generator or plasma region. This allows the plasma region to be kept small and minimises the energy needed to generate plasma, which the limited size of the region permits. Further, we have found that the benefits of the plasma-catalysis synergy increase as proximity between the plasma region and catalyst. As such, typically, the separation may be 0 mm, or, in other words, there being no separation.

[0027] This may result in the upstream end of the catalyst and the (upstream end of the) decay region being aligned on the gas flow path. In other words, the upstream end of the catalyst and the downstream end of the plasma region may abut. This may result in the plasma region and catalyst being sequential, such as without intervening space or a gap therebetween. We have found the co-location of the decay region and catalyst can maximise reduction in quantity of pollutant in some circumstances.

[0028] In other circumstances, we have found that providing a separation between the catalyst and plasma generator can allow for mixing of components in the gas and species resulting from the plasma generation. As such, the catalyst may be at least 30 mm from the plasma generator.

[0029] The decay region may comprise a primary decay region and a secondary decay region.

[0030] The primary decay region may be a region in use in which plasma particles / active species recombine and / or interact with components of the gas.

[0031] The secondary decay region may be a region in use in which reduced species breakdown that are formed by interaction with plasma particles / active species and / or with the reduced species.

[0032] The active species may include primary species and secondary species.

[0033] The primary species may include charged particles. The primary species may include ions (ionised species).

[0034] The primary species may include (free) electrons.

[0035] The secondary species may include components of the gas oxidised in reactions resulting from presence of, and / or interaction with, the primary species.

[0036] The primary species may only be generated in the plasma region.

[0037] The secondary species may be generated in the active region and decay region.

[0038] The plasma generator may generate (electromagnetic) plasma through any suitable means. Typically, this is achieved through use of electrodes. Further typically, the plasma generator includes electrodes arranged coaxially relative to the gas flow path, an electric field being establishable between the electrodes in use, discharge being generated when an electric field strength is at least at a discharge ignition threshold (the discharge causing plasma to be formed in use). While electrodes that are separated from each other along a length of the gas flow path would equally be able to generate plasma, providing the coaxial arrangement allows more reliable, repeatable and homogeneous establishment of plasma across the (width, such as over the whole width, for example, perpendicular to, the) gas flow path.

[0039] A decay region length along the gas flow path may be determined by a gas flow rate (of the gas along the gas flow path); and / or the length of the decay region along the gas flow path may be determined by a temperature of the gas. Additionally or alternatively, typically, a length of the decay region along the gas flow path may be longer than a length of the plasma region along the gas flow path.

[0040] By the term “discharge”, we intend to mean electrical discharge of some form, such as plasma-generating discharge. Typically, this means release and transmission of electricity in an applied electric field through a medium such as a gas. A flow of electrons in the form of a filament passing from one location to another or between two points typically achieves this. The flow of electrons is typically a transient flow of electrons in the form of a filament. By this, we intend to mean that the flow of electrons in a microdischarge / filament during electrical discharge lasts for only a short time per individual discharge ignition event. There may of course be many filaments over time if suitable conditions are maintained. The electrical discharge allows transmission of electricity in an applied electric field through the gas.

[0041] The electrical discharge may be for use in removing a pollutant, such as methane, by converting that pollutant into one or more other substances. The high-energy electrons generated during the staged discharge have been found to remove methane and other pollutants from gases containing methane and / or other pollutants. This provides an enhanced process by which methane and / or other pollutants are able to be removed from a gas over known techniques. The process reduces the amount of methane and / or other pollutants present in the gas after having been processed.

[0042] Any form of electrical discharge may be suitable for removing a pollutant from a gas, such as pulse, corona, electron beam, radio frequency, microwave, ultraviolet light radiation electrical discharge, brush, electric glow, electric arc, electrostatic, partial, streamer, vacuum arc, Townsend, field emission of electrons, or electric discharge in gases, leader (or spark), St. Elmo’s fire or lightning. Typically, however, the electrical discharge may be barrier electrical discharge. We have found that barrier electrical discharge is able to be used to reduce pollutant content in gas, thereby allowing it to be used to reduce pollutant from air and / or point sources (such as exhaust gases). The presence of the barrier limits, and, in most cases, does not allow, arcs or sparks to occur (i.e. discharge that generates sustained current between the electrodes). Instead, it only allows microdischarges to occur, which typically only last for microseconds. This provides the necessary energy and components to contribute to the chemical reaction pathway by which methane and other pollutants are able to be broken down while limiting the amount of power needed to provide sustained discharge.

[0043] Typically, the electrical discharge is dielectric barrier electrical discharge. In using dielectric barrier electrical discharge, the discharge is more controllable since less sparking occurs, meaning there is less wear and damage caused by the discharge.

[0044] Consequently, there may be a dielectric barrier between the electrodes, the discharge thereby being dielectric barrier discharge. While various dielectric barriers are able to be used, typically, the dielectric barrier is provided by a conduit providing the gas flow path. In view of this, typically, the reactor further comprises a conduit arranged in use to provide the gas flow path.

[0045] Typically, a length of the decay region along the gas flow path may be longer than a length of the plasma region along the gas flow path. While the length of the decay region is typically proportional to the gas flow rate, this keeps the length of the plasma region short, limiting the energy requirements for generating the plasma.

[0046] Typically, the plasma generator is arranged in use to form a plasma veil across the gas flow path. By this, we intend to mean that a (relatively) thin layer of plasma is formed across the gas flow path, and by the term “across”, we intend to mean perpendicular to the gas flow path, such as extending radially from a central axis of the gas flow path with minimal axial extension. This causes formation of active species and allows sufficient interaction of the gas passing along the flow path to reduce the pollutants while keeping energy use to generate the plasma efficient.

[0047] As an implementation of this, the plasma veil may be a sheet of plasma in use. This is intended to provide the relatively thin layer of plasma. This provides focused plasma generation, maintaining energy efficiency and high levels of active species generation.

[0048] As noted above, the plasma generator may include electrodes, an electric field being establishable between the electrodes in use, discharge being generated when an electric field strength is at least at a discharge ignition threshold.

[0049] The sheet of plasma could have any thickness appropriate for a sheet, namely considerably larger length and width than thickness. The sheet may have a thickness of up to three times the thickness of a plasma generator electrode (or a part thereof).

[0050] By thickness, we intend to mean the distance through the relevant object. This can be the breadth of depth of the object, as a third dimension relative to width or length / height. However, the thickness can also be the same as, for example the width of a (circular) cylinder where both are typically considered to be the diameter of the cylinder. In some circumstances, the thickness can be the length in a particular direction. For example, the thickness may be the length of a part of component of the electrode along the gas flow path.

[0051] The thickness of the veil or sheet mat be achieved though any suitable means. Typically, at least one of the electrodes may be arranged in use to confine a length of the plasma region along the gas flow path of up to the shortest distance between the electrodes. This allows the thickness of the plasma sheet to be controlled by arrangement of the component(s) from which the discharge that forms the plasma is emitted. Typically, the arrangement is achieved through the shape or shaping of at least one of the electrodes.

[0052] At least one electrode of the plasma generator may have a node (such as one or more, a plurality, or only one node). The node may be feature on the at least one electrode, such as a field intensifying structure. Typically, only one electrode has a node. Typically, there is only a single (i.e. only one) node.

[0053] The node or nodes, typically when a field intensifying structure, form a location or locations of higher intensity electric field in use, providing one or more areas preferential for discharge. This provides shaping of the electrode to help confine the plasma generation to a (thin) veil or sheet.

[0054] In such a case, and the sheet may have a thickness of up to three times the thickness of the node or field intensifying structure. This thickness may be the thickness along the gas flow path. In some cases, the length of the plasma region along the gas flow path may be up to the length of the shortest length of the node (such as the node thickness) along the gas flow path. While typically the thickness of the sheet of plasma may be about the thickness of the node or field intensifying structure (such as along the gas flow path), there can be fluctuations, such as due to prevailing or transient conditions that cause instantaneous, momentary, or temporary changes in thickness. We have found that providing the node or field intensifying structure to provide the stated confinement of the plasma provides energy efficient plasma formation while allowing a high rate of active species creation for reduction of pollutant in the gas.

[0055] The node may have a thickness appropriate for plasma generation therefrom. Typically, the thickness of the node along the gas flow path is a length up to the shortest distance between the electrodes. This limits the thickness of the node along the gas flow path, limiting material use and limits any blocking effect of the node on the flow.

[0056] Since, when a node or electric field intensifying structure is used are present, or plasma veil or sheet of plasma are implemented through arrangement of the plasma generator, the plasma is typically only generated in that location. This means that the plasma generator can be considered to only extend along the length of the node or electric field intensifying structure along the gas flow path. This may be considered the case for the purposes of identifying the proximity of the plasma generator and catalyst.

[0057] The node may be a structure for electric field intensification for (i.e. suitable for) use in a dielectric barrier discharge device. The structure may comprise: a body, such as a ring, including at least one tip, the tip extending along a first radial axis passing through the centre of the body, wherein in use, the body is arranged around or as part of a first electrode of a discharge device, there being a gap between the structure and an opposing electrode of the discharge device, the at least one tip limiting a minimum gap between the structure and the opposing electrode, thereby increasing a probability of an electric breakdown occurring at the tip when an electric field is applied between the first electrode and the opposing electrode. The provided structure for electric field intensification provides various benefits in terms of increasing the efficiency at which a discharge device, for example a dielectric discharge device, removes pollutants from exhaust gas. In use, the body or ring (which are used interchangeably in relation to the structure for electric field intensification), and by extension, the structure is arranged around a first electrode in such a way that a gap is formed between an opposing electrode and the structure. The presence of the structure introduces asymmetry in an electric field applied between the first and opposing electrodes. This leads to a higher concentration of active species near the structure relative to a conventional discharge device. The structure therefore promotes oxidation of a gas passing through the discharge device in use and, due to its form contributes to confining plasma generation into a veil or sheet across a plane in the reactor. Specifically, the radially extending tip reduces the gap between the structure and the opposing electrode at the location of the tip. This concentrates the applied electric field at the tip. This increases the production of active species at the tip, and so gas in the vicinity of the tip is therefore more likely to be oxidised.

[0058] Further, because of the inherent geometry of the tip, that is, because the tip typically terminates in a sharp point, the tip assists in the formation of plasma streamers when a pollutant is passed through a discharge device in use.

[0059] It has been observed that in use, a discharge device including such a structure removed pollutants at a higher efficiency compared to a similar device without the structure.

[0060] All of these benefits are achieved without the need to provide any extra energy. That is, the electric field usually applied to a conventional discharge device can be applied to a staged device with a structure to provide an improved efficiency of pollutant removal.

[0061] In advantageous examples, the at least one tip of the electric field intensifying structure comprises a plurality of tips, each tip in the plurality of tips being arranged on the outer edge of the ring. A plurality of tips may provide further improvements to the efficiency of pollutant removal in use. That is, a plurality of tips provides additional sites for electric breakdown of gas to occur. This evidently means that more gas can be treated compared to if the structure has a single tip.

[0062] In some further examples, the plurality of tips may each extend along a respective radial axis not aligned with the second radial axis.

[0063] This effectively means that each of the plurality of tips extends along a different radial direction. This is intended to mean that the tips will be distributed to some extent on the outer edge of the ring. The effect of the tips is therefore less likely to overlap with each other, and therefore the tips complement each other. That is, in use, the tips each encounter a different part of the gas flowing through the discharge device.

[0064] Moreover, it is advantageous for the plurality of tips to be uniformly distributed on the outer edge of the ring in some examples.

[0065] A uniform distribution of the tips provides further improvements to the efficiency of pollutant removal in use. By requiring a plurality of uniformly distributed tips, it is ensured that each part of the gas flowing through the discharge device in use has some chance to be treated.

[0066] Note that by uniformly distributed we intend to mean that the tips are uniformly distributed around the material of the ring, rather than the full 360 degrees of the structure. For example, a channel may provide a break in the ring.

[0067] In some examples, the plurality of tips may comprise 3 tips.

[0068] In other examples, the plurality of tips may comprise 21 tips.

[0069] A particular number of tips may be effective for removing a pollutant in use.

[0070] The plurality of tips may typically comprise 6 tips in certain advantageous examples. The plurality of tips may also consist of 6 tips,

[0071] A structure with 6 tips has been found to be optimal for removal of a pollutant, such as methane, from gas flowing through the discharge device in use.

[0072] Further details relating to the structure may be found in the claims, summary of invention, detailed description and figures of WO 2024 / 165601 , which is herein incorporated by reference.

[0073] The reactor may further comprise a drive circuit for (i.e. suitable for) a dielectric barrier discharge device. The circuit may comprise: a power supply connectable in use across a dielectric discharge gap, the dielectric discharge gap providing a capacitance; and an inductance between the power supply and the dielectric discharge gap when connected thereby establishing a resonant tank in use, wherein power is provided in use to the tank in pulse-trains and only during a pulse-train, a pulse frequency of each pulse-train being tuneable in use to a resonant frequency of the tank, power provided by each pulse-train charging and maintaining the tank to a threshold at which discharge ignition occurs (at the dielectric discharge gap), discharge ignition events per pulse-train (such as discharge ignition events occurring during the period of any one pulse-train) being limited to a maximum number based on the drive circuit being arranged in use to prohibit each pulse-train transferring power to the resonant tank after the maximum number has occurred.

[0074] We have found that using the drive circuit in combination with the other components of the reactor according to the first aspect has a synergistic effect, benefiting the overall rate of removal of pollutants from a gas flow.

[0075] By providing pulse-trains of power to the resonant tank, the amount of energy stored in the resonant tank increases, also referred to as “charging” the resonant tank, over the duration of each pulse-train. Dielectric barrier electrical discharge occurs across the dielectric discharge gap when the potential difference across the gap reaches a threshold (Vth). By tuning the pulse frequency (by which we intend to mean the reciprocal of the period between individual pulses or cycle period of pulses within a pulse-train) of the pulse-trains to a resonant frequency of the tank the charging process causes a rapid increase in the amplitude of the potential difference. This increases the potential difference amplitude to the threshold over, for example, less than ten cycles, to reach a threshold at which dielectric barrier electrical discharge occurs (which can also be referred to as an “ignition threshold”).

[0076] A limitation on current imposed stress is provided by using the device of the first aspect. Limitation on current imposed stress is achieved using such a device by the build up to the potential difference to the threshold occurring over several cycles (i.e. individual pulses) during the pulse-train by means of the resonant tank voltage gain resulting in reduced power losses in the driving circuit. In conventional pulsed-plasma systems, plasma discharge is provided by use of a single pulse, requiring a high step-up transformer, resulting in a higher current, and thereby raising current imposed stress on the primary winding side.

[0077] Further, the power supply is protected from short-circuits without needing overcurrent detection. This is due to the inductance of the resonant tank providing enough impedance to limit currents if the output terminal of the power supply is shorted, for example, due to a short circuit failure at the dielectric barrier.

[0078] Additionally, by limiting the number of discharge ignition events, there is a reduction in dissipation of energy simply to heat or to generation of less reactive species. Indeed, we have found that by implementing such a hybrid of resonant AC and limited pulse excitation effective pollutant reduction is providable while also having high power conversion efficiency.

[0079] Accordingly, overall, in the drive circuit, power transfer to the dielectric barrier discharge device with a high efficiency is achieved (due to the resonance operation) while also limiting current imposed stress and protecting against short- circuits so as to protect circuit components.

[0080] The dielectric discharge gap is intended to be a gap between electrodes of a dielectric discharge device. This typically provides a capacitance due to the gap, with a further capacitance being provided by the dielectric. Of course, when the drive circuit according to the first aspect is connected across the discharge gap, since the edges / sides of this gap are provided by the electrodes, it is intended the drive circuit is connected (i.e. electrically connected) to at least the electrodes in a manner that allows the drive circuit to provide current to the electrodes and establish a potential difference across the electrodes. In various examples, the drive circuit may still be connected across the dielectric discharge gap by being connected to wires or cabling connected to the electrodes that form a closed circuit that includes the drive circuit and dielectric discharge gap.

[0081] The cycle period of power being supplied by the resonant tank is intended to refer to the period taken for the current and / or voltage to pass through a single oscillation cycle (only) as determined by the frequency. In other words, this is intended to be the time taken for the current and / or voltage to pass through a single wavelength (only).

[0082] Turning to a process by which discharge caused by a drive circuit according to the first aspect can be thought of as there initially being an absence of discharge occurring before an ignition threshold is reached. This means gas in the discharge gap (such as between electrodes) has not been ionized, and there is no electric discharge, and, of particular relevance, power is not delivered to the gas. Once the threshold is reached discharge occurs however. This results, from a single point (such as some form of sub-macroscopic structure on the surface of an electrode defining a side of the discharge gap), in innumerable transient filaments (each representing a micro-discharge) being formed. Each filament’s lifetime (i.e. the period of time during which a respective filament exists) is of the order of tens of nanoseconds. It is only during the lifetime of these transient micro-discharges that high energy electrons are formed in the discharge gap, allowing power to be delivered to the medium in the gap. The power delivered by high energy electrons that are generated is able to initiate pollutant breakdown due to the energy levels being of a sufficient amount to initiate chemical reactions.

[0083] Maintaining a discharge gap at the voltage threshold indefinitely causes charge accumulation on the surface of the electrodes and dielectric barrier of a dielectric discharge gap of a DBD device. This can be avoided by the use of pulses. Pulses can be thought of, due to the alternating polarity provided by pulses, as limiting the amount of time the instantaneous voltage at the discharge gap is maintained at the ignition threshold to a period in the order of a few microseconds. This means that transient filaments are only able to be produced for this period. As such, the period in which microdischarges can occur can be thought of as limited to the amount of time the instantaneous voltage at the discharge gap is maintained at the ignition threshold, and the summation of those transient filaments may be considered to be a “macro-discharge” or “discharge event”.

[0084] In view of the preceding four paragraphs, the term “discharge ignition event” is therefore intended to be the start of a macro-discharge or discharge event; or, in other words, the start of the period during which micro-discharges in the form of transient filaments are able to occur, which is when a threshold is reached. This threshold is typically a voltage threshold, such as a voltage threshold at the dielectric discharge gap, for example in the form of a potential difference (e.g. AV) across the el ectrodes / di electric layer and electrode delimiting the gap.

[0085] The pulse frequency of the pulse-train being tuneable in use to a resonant frequency (also able to be referred to as a “resonance frequency”) of the tank, is intended to mean that the pulse frequency may be tuned to one or more of a number of frequencies that is able to be considered the resonant frequency. These include the theoretical resonant frequency (i.e. the frequency that would be calculated as being the resonant frequency when not accounting for real-world effects), or a practically applicable resonant frequency, such as a frequency that takes account of real-world effects, which may include one or more of inductance and / or resistance in wiring and / or other components, damping or impedance. As such, as detailed further below, a zero voltage switching frequency.

[0086] The maximum number of discharge ignition events may typically be between one and five events, such as between one and three events, including (only) one event, two events or three events. By limiting to so few discharge events, we have found this produces the most energy efficient and effective breakdown of pollutants. This is due to the energy transfer that occurs due to the discharge ignition event(s) limiting transfer to the medium in the discharge gap, and thereby directing a higher proportion of the energy to cause breakdown of compounds in the medium.

[0087] The drive circuit may further comprise a phase meter in communication with the tank and arranged in use to identify (such as by monitoring) a phase shift in power provided to the tank during each pulse-train, the phase shift corresponding to occurrence of discharge ignition events, and wherein the drive circuit may be further arranged in use to determine when the maximum number of discharge ignition events has occurred based on the number of pulses in the respective pulse-train since each respective discharge ignition event.

[0088] We have found that such a phase shift represents the start of discharge, and, as such, it is possible to identify the number of discharge ignition events that occur from that point (such as by counting or being aware of the number of pulses in the pulse train from that point onwards). This means it is possible to determine when a maximum number of discharge ignition events has been reached to stop further discharge ignition events occurring. By monitoring a voltage-current phase-shift at, for example, an input to the resonant tank (such as a voltage-current phaseshift measured at the H-bridge terminal, relevance of which H-bridge being detailed further below) a first discharge ignition event may be detected. During charging of the resonant tank (e.g. the rapid voltage built-up) there is typically close to zero phase-shift (excited at resonance). However, once the plasma is ignited as part of the discharge ignition event, there is typically a shift in the resonance frequency because of the increase in capacitance imposed by the “ignited” discharge gap. When monitored, this resonance frequency shift may be detected immediately by monitoring the phase-shift.

[0089] Such a phase meter (e.g. a phase detection unit) as mentioned above may be provided by a controller, processor, microprocessor or microcontroller or another such device capable of monitoring phase of at least two signals.

[0090] Additionally or alternatively to phase monitoring or using a phase meter, each pulse-train may have a pre-tuned or optimised pulse-number (i.e. number of pulses within the pulse-train). It is typically possible to calculate or model how many pulses will be needed to charge the resonant tank, and typically there is (only) a single discharge ignition event per pulse, or at least it is possible to calculate how many discharge ignition events will be caused per pulse. This allows it to be possible to set the number of pulses in a pulse-train to at least the maximum number of discharge ignition events wanted plus the number pulses needed to charge the tank. If such an approach is used, there may of course be further pulses included in a respective pulse-train, such as when pulses are used to discharge the resonant tank. These may also be included in calculation of how many pulses are needed per pulse-train if this approach is used.

[0091] The circuit may further comprise a power storage device connected across the power supply arranged in use to accept and store power discharge (i.e. power drained) from the tank after each pulse-train (or after the maximum number of discharge ignition events has occurred). This provides a means for storing / recouping power within the circuit that would otherwise be lost due to energy in the resonant tank dissipating. This reduces energy loss between pulsetrains and allows the stored energy to contribute in forming the next high voltage pulse-train, which results in increased efficiency.

[0092] Energy or power recuperation is able to be achieved through passive or active means. Typically, an active means is used, such as the drive circuit typically being arranged in use to shift the phase of (pulses in) the pulse-train by 180 degrees (°) after the maximum number of discharge ignition events has occurred. By implementing this mechanism, energy recovery is able to be achieved when passive means for energy recovery (and potentially any other active means) are not possible, such as due to use of a loosely coupled air-core transformer. This thereby allows the efficiency gains achievable from energy recovery to still be achieved The phase shift may be in place for the same number of pulses as the number of pulses used in the pulse-train to charge the resonant tank to the threshold, although it would be possible to apply the phase shift for a different number of pulses. This maintains similar power flows when charging and discharging the resonant tank. Further details relating to the drive circuit may be found in the claims, summary of invention, detailed description and figures of WO 2022 / 106622, which is herein incorporated by reference.

[0093] The catalyst may be provided as a block or powder. Typically though, the catalyst is located on a support. This allows for the catalyst surface area to be increased over scenarios where no support is used.

[0094] The catalyst support may be a wire structure or a frame, such as a coated wire or frame. Instead, typically, the support is a monolithic catalyst support, and preferably (i.e. optionally) the support is a cordierite monolithic support. This provides an inert, electrically non-conductive support for the catalyst, with cordierite, when used having minimal thermal expansion, and therefore being beneficial in a varying temperature environment. The catalyst may be a coat or layer, such as a wash coating on the support.

[0095] The catalyst may be any suitable form of catalyst. Typically, the catalyst may be an oxidation catalyst. This may result in the catalyst being a methane oxidation catalyst.

[0096] According to a second aspect, there is provided a method of reducing a quantity of a pollutant in a gas, the method comprising: passing gas along a gas flow path; generating plasma in a plasma region through which the gas flow path passes, active species being formed by the plasma; entraining the active species in the gas, the active species decaying in a decay region downstream of the plasma region; and passing the gas and active species over a catalyst, the catalyst being downstream of the plasma generator / plasma region and at least partially inside the decay region.

[0097] The method of the second aspect may implement any feature of the first aspect. For example, typically, the upstream end of the catalyst is aligned with the upstream end of the decay region, thereby aligning the upstream end of the catalyst with the downstream end of the plasma region. The method of the second aspect may be implemented with a separation between the plasma reactor and the catalyst, the separation being larger than a length along the gas flow path of the plasma reactor.

[0098] In a third aspect, there is provided plasma-generated active species for use in reduction of a quantity of a pollutant in a gas by catalysis.

[0099] In a fourth aspect, there is provided sequential plasma and catalysis with plasma- generated active species for use in reducing a quantity of a pollutant in a gas.

[0100] In a fifth aspect, there is provided a method of reducing quantity of a pollutant in a gas with plasma, catalysis and plasma-generated active species.

[0101] In a sixth aspect, there is provided use of (plasma-generated) active species for use in reduction of a quantity of a pollutant in a gas by catalysis.

[0102] In a seventh aspect, there is provided use of sequential plasma and catalysis with plasma-generated active species for use in reducing a quantity of a pollutant in a gas.

[0103] In an eight aspect, there may be provided use of sequential plasma and catalysis with plasma-generated active species at a separation of plasma generation and catalysis of greater than a length of a plasma generator for use in reducing a quantity of a pollutant in a gas.

[0104] In relation to any of the above aspects, the reduction of a quantity of a pollutant, is typically removal of a quantity of methane in a gas.

[0105] The following features may be applicable to any aspect:

[0106] While the gas passed through the reactor in use may be any gas from any source or may simply be gas available locally, such as air, the gas may be a waste gas. In other words, the gas may be air or gas from any local, remote, ambient, environmental or man-made source. The gas may be a gas containing methane. Additionally or alternatively, the gas may be a gas from an engine, such as a gas fuel engine, including a CNG and / or LNG engine.

[0107] The electrodes may be any suitable material for providing electrodes that allow an electrical field to be established between the electrodes. Typically, the electrodes may be made of an electrically conductive metal, such as steel and / or aluminium, or electrically conductive alloys thereof.

[0108] The dielectric barrier may be connected to one electrode and an electric field intensifying structure may be connected to or formed as part of a further second electrode allowing application of the dielectric barrier and electric field intensifying structure to the respective electrodes to be independent. This avoids the possibility of the processes for applying the dielectric portion to the electrode and for applying the electric field intensifying structure to the electrode damaging the electric field intensifying or dielectric respectively. Accordingly, this simplifies the process of manufacturing the apparatus and reduces the failure rate in manufacture.

[0109] The dielectric barrier may provide a form of covering of at least part of the or each electrode to which it is connected. The dielectric barrier may be a coating, layer or powder on at least part of a surface of the or each electrode to which the dielectric portion is connected. For example, the dielectric portion may coat the entire surface of the or each electrode to which it is connected. Typically, however, the dielectric barrier may be abutting the electrode with one or both of the dielectric barrier or electrode held to the other or to each other.

[0110] The dielectric barrier may have a thickness of between about 0.1 mm and 10 mm, such as about 2 mm.

[0111] By the dielectric barrier being connected to at least one electrode, we intend to mean that each electrode to which the dielectric barrier is connected to a dielectric barrier independently of each other dielectric barrier and electrode. This means there may be a plurality of dielectric barriers. Each dielectric barrier may be connected to only a single electrode.

[0112] The dielectric barrier may be one or more of mica, quartz, fused silica, alumina, titania, barium titanate, fused silica, titania silicate, silicon nitride, hafnium oxide, polymer or a ceramic. By the phrase “one or more of’ in this case, we intend to mean a combination of two or more of the named materials when two or more of these are used.

[0113] Typically, the dielectric barrier is quartz. This is because quartz as this material is readily available, low cost, can be processed in large quantities and can have a high resistance to thermal stress. The dielectric barrier may alternatively be mica. Mica is beneficial because it has a slightly higher dielectric constant than other dielectric materials, such as glass.

[0114] The electric field intensifying structure may be any form of suitably sized structure that provides a point or sharp edge.

[0115] The catalyst may be any formulation of oxidation catalyst. The catalyst may be one or more of chromium, molybdenum, manganese, rhenium, iron, ruthenium, cobalt, rhodium, iridium, nickel, palladium, platinum, copper, silver, gold, vanadium oxide, zinc oxide, titanium dioxide, cerium oxide, zirconium oxide, aluminium oxide, and tungsten trioxide.

[0116] BRIEF DESCRIPTION OF DRAWINGS

[0117] Examples of an apparatus and example methods are described in detail below with reference to the drawings, in which:

[0118] Figure 1 A shows a schematic of a first example apparatus;

[0119] Figure 1 B shows a schematic of an alternative form of the first example apparatus;

[0120] Figure 2 shows an example electric field intensifying structure arranged in an Example apparatus;

[0121] Figure 3A shows a schematic of a second example apparatus; Figure 3B shows a schematic of an alternative form of the second example apparatus;

[0122] Figure 4 shows a plot of methane change against temperature when applying plasma only, catalysis only and plasma-catalysis methane abatement;

[0123] Figure 5 shows a plot of methane change against temperature when applying catalysis only and plasma-catalysis methane abatement;

[0124] Figures 6A and 6B show plots of methane change against power and specific energy input change;

[0125] Figure 7 shows an example circuit able to be used with the example apparatus;

[0126] Figure 8 shows an example method of operating an example circuit;

[0127] Figure 9 shows example plots of voltage, current and power applied in an example circuit;

[0128] Figure 10 shows a further example plot of voltage and current over time during an example pulse-train;

[0129] Figure 11 shows example plots of voltage against time comparing applied gap voltage to output voltage and a corresponding plot with a magnified portion of output current against time;

[0130] Figure 12 shows an example plot of switching sequence over time and resulting voltage over time;

[0131] Figure 13 shows example plots for voltage over time for power transfer rates;

[0132] Figures 14A and 14B show plots of methane change against power and specific energy input change for different plasma and catalyst separations at a first temperature;

[0133] Figures 15A and 15B show plots of methane change against power and specific energy input change for different plasma and catalyst separations at a second temperature;

[0134] Figures 16A, 16B, 16C and 16D show plots of nitrogen species change against power for different plasma and catalyst separations at a first temperature; and Figures 17A, 17B, 17C and 17D show plots of nitrogen species change against power for different plasma and catalyst separations at a second temperature. DETAILED DESCRIPTION

[0135] Methane emissions originate from a variety of different sources. For example, methane may be a constituent in the gaseous mixture emitted from engines, such as those found in vehicles, power plants, or industrial machinery. Such gaseous mixtures are often referred to as “exhaust gas”, particularly when the gases are emitted from an internal combustion engine. Methane may also be a constituent in the gaseous mixture emitted from a flu e or exhaust stack following industrial processes, particularly those of power plants or heating systems. Such gaseous mixtures are often referred to as “flue gas”. Methane may also be a constituent in the gaseous mixture emitted from industrial processes or manufacturing, chemical processes or reactions, waste treatment operations such as wastewater treatment operations, landfill operations, or biogas production. Such gaseous mixtures are often referred to as “waste stream gas”. This equally applies to other pollutants.

[0136] Methane slip occurs when methane forms part of a fuel used to power an engine, and unburned methane passes through the engine, thereby being emitted along with the remainder of the exhaust gases. For example, natural gas (NG), compressed or liquified, which has methane as its main component, has attracted attention as an alternative fuel to petroleum and light oil and has been used as a fuel to power engines. From an air quality perspective, NG fuel has many advantages compared to traditional fuels. The emissions of sulphur dioxide (SO2, SO2) are low due to low or non-existing sulphur content of the gas. The low sulphur content and the absence of fuel aromatics also contribute to low particulate formation levels. Furthermore, NG combustion results in less CO2 emissions in comparison to diesel and gasoline. The use of NG as a fuel has increased significantly since the start of the 21st century. However, many NG engines that are produced today have problems with “methane slip”, i.e. unburned methane passing through the engine and being emitted along with the remainder of the exhaust gases. Methane slip results from incomplete combustion of the fuel injected in the pre-chamber or cylinders of the engine. In order to remove and reduce pollutants, such as methane slip in a gas stream, there is provided electrical discharge and catalyst. In some examples, this is for use in removing methane from a gas, by so-called “plasma-catalysis”.

[0137] The principle behind an example plasma-catalysis reactor is the application of energetic electrons to a gas and a synergistic interaction with a catalyst. This allows removal of pollutants from a gas due to the electrons driving reduction of the pollutants by making reaction sequences possible, which is also aided by the catalyst.

[0138] In various examples, gas is passed along a gas flow path. In some examples, this is provided by a conduit or pipe.

[0139] Electron generation on the gas flow path is achieved, in several examples, through the use of opposing electrodes. In various examples, one electrode has a dielectric layer or dielectric barrier intervening between that electrode and the other electrode. In some examples, the opposing electrode has an electric field intensifying structure. An example of such a structure is defined in the claims of, and disclosed in the description of, WO 2024 / 165601 A1 , which is incorporated by reference. In other examples, structures of alternative shapes are used.

[0140] The electrodes are upstream of a catalyst in a number of examples. In some examples, this is provided by a cordierite monolith on which a catalyst is located. At times, the catalyst has been applied to the monolith during fabrication.

[0141] On application of an electric field of at least a discharge ignition threshold, microdischarges occur, which generate electrons. Due to the positioning of the electric field intensifying structure and catalyst, this forms an ionisation region adjacent the structure forming plasma (and so a plasma region) between the electrodes (and dielectric barrier) in which active species are generated. Further, a decay region is also formed. This at least partially overlaps with the catalyst, allowing species in the gas to be ionised and then to recombine either in the catalyst or upstream of the catalyst to then pass resulting species into the catalyst, steadily reducing the pollutant quantities in the gas. The electric field is generated in various examples by any means capable of raising the electric field strength to the discharge ignition threshold is used. In some examples, a drive circuit as defined in the claims of WO 2022 / 106622 A1 , which is incorporated by reference and disclosed in the corresponding description, is used.

[0142] The interaction of the gas and plasma as generated by the discharge typically provides a transfer of real power to the gas. In some examples, the amount of real power provided is adjustable.

[0143] By the phrase “real power”, we intend to mean the instantaneous power (p(t)) provided by a plasma generator averaged over the period (TO) of the voltage applied due to the electric field.

[0144] Real power (P) can be calculated as: t0+T0

[0145] P =rn p(t)dt

[0146] 1 uo

[0147] By real power, we also intend to mean, in some examples, the rate of generating high energy electrons in the gas to be present between electrodes and the unwanted losses involved in this process. There is a desire to minimize losses to have a maximal rate of production of high energy electrons.

[0148] By an electric field intensifying structure being connected to at least one electrode or dielectric barrier, we intend to mean that at least one electric field intensifying structure is (electrically) connected to, or in communication with, at least one electrode or dielectric. An example is set out in more detail below.

[0149] This means that more than one electrode and / or the dielectric barrier may have one or more electric field intensifying structures connected thereto. There may, of course, be a plurality of electric field intensifying structures, each electric field intensifying structure being connected to one of an electrode or the dielectric barrier, such as all the electric field intensifying structures being connected to only a single electrode or only the dielectric barrier, or one or more electrodes and / or the dielectric barrier having one or more electric field intensifying structures connected thereto. In various examples, it is intended that, when an electric field intensifying structure is connected to an electrode or the dielectric barrier, that electric field intensifying structure is only connected to that respective electrode or the dielectric barrier, and not also connected to an or another electrode or the dielectric barrier (when connected to an electrode).

[0150] In some examples, there is a power supply as part of the plasma generator. The power supply, or plasma generator in general, may maintain real power through any suitable means, such as by providing a constant supply of power at a set amount, from a DC power supply of some form, or by providing a constant or modulated AC power supply or continuous supply of power in a sinusoidal waveform at a predetermined frequency.

[0151] Typically, there is a separation or gap between electrodes with the dielectric barrier therebetween in some examples. This can be referred to as a “dielectric discharge gap”. The dielectric discharge gap is intended to be a gap between electrodes of a dielectric discharge device. This typically provides a capacitance due to the gap, with a further capacitance being provided by the dielectric. Of course, when the plasma generator, as part of the various aspects disclosed herein, is connected across the discharge gap, since the edges / sides of this gap are provided by the electrodes, it is intended the plasma generator has components connected (i.e. electrically connected) to at least the electrodes in a manner that allows the power supply to provide current to the electrodes and establish a potential difference across the electrodes. In various examples, the power supply may still be connected across the dielectric discharge gap by being connected to wires or cabling connected to the electrodes that form a closed circuit that includes the power supply and dielectric discharge gap.

[0152] The presence of the dielectric barrier at the dielectric discharge gap typically limits the occurrence of arcs or sparks (i.e. discharge that generates sustained current between the electrodes). Instead, it typically only allows microdischarges to occur, which typically only last for microseconds. This provides the necessary energy and components to contribute to a chemical reaction pathway to break down compounds in the medium through which the discharge is passing while limiting the amount of power needed to provide sustained discharge. This form of discharge is often referred to as “dielectric barrier discharge” or “DBD”.

[0153] A process by which discharge caused by a power supply can be thought of is that there initially is an absence of discharge occurring before an ignition threshold is reached. This means gas in the discharge gap (such as between electrodes) has not been ionized, and there is no electric discharge, and, of particular relevance, power is not delivered to the gas. Once the threshold is reached, discharge occurs, however. This results, from a single point (such as some form of electric field intensifying structure on the surface of an electrode defining a side of the discharge gap), in transient filaments (each representing a micro-discharge) being formed. Each filament’s lifetime (i.e. the period of time during which a respective filament exists) is of the order of tens of nanoseconds. It is only during the lifetime of these transient micro-discharges that high energy electrons are formed in the discharge gap. The electrons acquire kinetic energy when accelerated by the applied electric field. Then the electrons transfer this energy through collisions that can be elastic (kinetic energy conservation) or inelastic (transfer to internal energy of a molecule and to react). The energy delivered by high energy electrons that are generated is able to initiate pollutant breakdown due to the energy levels being of a sufficient amount to initiate chemical reactions.

[0154] The term “discharge ignition event” is therefore intended to be the start of a macrodischarge or discharge event; or, in other words, the start of the period during which micro-discharges in the form of transient filaments are able to occur, which is when a threshold is reached. This threshold is typically a voltage threshold, such as a voltage threshold at the dielectric discharge gap, for example in the form of a potential difference (for example, AV) across the electrodes / dielectric layer and electrode delimiting the gap.

[0155] Turning to the catalyst, in conventional catalysis, proportions and quantities of products formed (i.e. selectivity and yield) are controlled by the conditions (bulk temperature, pressures, concentrations) prevailing in the reactor and the rates of the underlying catalytic processes at those conditions. Since a non-thermal plasma is not characterized by a single temperature, chemical conversions are not bound by the thermodynamic equilibrium constraints of the bulk gas temperature and pressure. In addition to the lower barriers and alternative pathways provided by plasmas, other interactions are also possible between plasmas and catalysts, such as electric field effects, surface charging, hot spot formation, and morphological changes of the catalyst, as well as effects of the catalyst on the plasma characteristics, which may also lead to plasma-catalyst synergies. Thus, the use of the dielectric portion, the electric field intensifying structure and catalyst provide a synergistic effect of lowering the power and voltage needed to establish electrical discharge while allowing pollutant to be removed from gas.

[0156] The catalyst may be positioned relative to the plasma region as pre-plasma (where the catalyst is upstream of the plasma region), post-plasma (where the catalyst is downstream of the plasma region), or in-plasma (where the catalyst is located in the electric discharge device). We have found that, in a specific arrangement, locating the catalyst at an edge of the plasma region or downstream of the plasma region is advantageous to provide a synergistic effect to remove pollutant from gas.

[0157] Additionally, using the dielectric portion allows the discharge to be more controllable by reducing the amount of sparking and thereby the amount of wear and damage caused by electrical discharge. If the electric field intensifying structure was used without the dielectric portion, the larger amount of sparking would limit the usefulness of the electric field intensifying structure since this is typically more susceptible to damage from sparking than other parts of the apparatus. Additionally, the use of the catalyst increases the overall efficiency. As such, the combined effect of using the dielectric, the electric field intensifying structure and catalyst has a greater benefit than the benefits offered by using each independently.

[0158] With regard to the examples shown in the figures, Figure 1A and 1 B, and Figure 3A and 3B show two example reactors 1 , T. These each have a path along which gas is able to be passed during use of the reactor, providing a gas flow path. In various examples, a conduit 10 provides the gas flow path. In the examples shown in Figure 1A and 1 B, and Figure 3A and 3B, the conduit is shown as a circular cylinder. In other examples, this can be a cylinder of another shape, such as square or oval. Further, the gas flow path is able to be provided by another component. This can be a pipe, tube or exhaust passage.

[0159] In use, in the examples shown in Figure 1A and 1 B, and Figure 3A and 3B, gas is passed along the conduit 10 in the direction indicated by the arrows. This is from left to right on the page when landscape, the direction from which the gas arrives being an upstream end and the direction by which the gas leaves being a downstream end.

[0160] Within the conduit 10, each of the examples shown in Figure 1A and 1 B, and Figure 3Aand 3B, includes a plasma generator. In various examples, this includes electrodes 20, 20’, to which a power source 22 is electrically connected. In Figure 1 Aand 1 B, the electrodes 20 are represented by electrodes at reference numerals 20a and 20b. In Figure 3A and 3B, the electrodes 20’ are represented by electrodes at reference numerals 20a’ and 20b’.

[0161] In the examples shown in Figure 1A and 1 B, and Figure 3A and 3B, a catalyst 30 is located downstream of the plasma generator (or at least downstream of the electrodes 20, 20’ of the plasma generator). In several examples, the catalyst is provided on a support 32. The support, in the examples shown in Figure 1 A and 1 B, and Figure 3A and 3B, is a catalyst monolith. In various examples, this is a cordierite catalyst monolith, such as in a honeycomb, with, for example, square, hexagonal, round, or other shaped channels oriented to allow gas to pass through the monolith along the gas flow path.

[0162] Returning to the plasma generator, in various examples, there is a dielectric barrier located between the electrodes 20, 20’. Depending on the arrangement of the electrodes, the dielectric barrier is coated onto at least one of the electrodes, or is physically in contact with one electrode (or, if there are two dielectric barriers, possibly in contact with each electrode). In some of the arrangements of the example shown in Figure 1A and 1 B, the dielectric barrier is provided by at least a portion of the conduit 10, which, at the relevant portion is any suitable dielectric, such as quartz or any dielectric identified above. In various arrangements of the example shown in Figure 3A and 3B, the dielectric barrier coated onto one of the electrodes.

[0163] With respect to the arrangement of the electrodes, in some examples, such as the example shown in Figure 1A and 1 B, the electrodes are arranged coaxially. In other examples, such as in the example shown in Figure 3Aand 3B, the electrodes are arranged at an offset from each other along the gas flow path.

[0164] In the example shown in Figure 1Aand 1 B, one electrode 20a is located along the central longitudinal axis of the conduit 10. The other electrode 20b is located around and against the outside of the conduit.

[0165] In some examples, the electrode 20a located along the central axis of the conduit may extend along the whole conduit. In other examples, this electrode may only extend along part of the central axis, or may only be located at a point in alignment with the other electrode 20b. The only need for this electrode to fulfil its purpose is to be in an electric circuit with the other electrode and power supply 22.

[0166] In Figure 1A and 1 B, only a portion of the electrode around the outside of the conduit is shown so as not to impede visibility of the other components depicted. In practice, in various examples, this electrode extends around the whole circumference of the conduit.

[0167] In another form of the example shown in Figure 1A and 1 B, the electrode 20b located around the outside of the conduit 10 may be located around the inside of the conduit in a similar position to that shown in Figure 1. In such an example, since the conduit would not then provide the dielectric barrier, an alternative dielectric barrier is provided between the electrodes.

[0168] In the example shown in Figure 1A and 1 B, the electrode 20a in the conduit 10 has an electrical field intensification structure 24 in electrical communication with the electrode. In Figure 1 , this is depicted as a thin disk. The shape of this structure is able to vary from example to example. Typically, it includes features, such as projections, recesses or textured portions that provide inhomogeneities in the outer edge of the structure. In other words, the outer edge is intended not to be a smooth, featureless and / or continuous surface.

[0169] An example of such a variance is shown in Figure 2. This illustrates an example electric field intensifying structure 3000 together with a first electrode 3040 and an opposing electrode 3100. These would respectively correspond to the electrical field intensification structure 24 together with the electrode 20a and the electrode 20b located around the outside of the conduit 10 of Figure 1A and 1 B.

[0170] The electric field intensifying structure 3000 of the example of Figure 2 comprises a ring 3010 and at least one tip 3020. It will be understood that although six tips are shown in Figure 2, the structure 3000 requires only one tip to operate.

[0171] In use, the ring 3010 is arranged around a first electrode 3040. This may be achieved by means of a slot or channel which allows the electric field intensifying structure 3000 to be deformed to fit firmly around a first electrode. Other means of arranging the ring around the first electrode are possible. For example, the first electrode may be integral with the structure. Such a structure could be formed for example by cutting or milling a material.

[0172] It can be seen that the at least one tip 3020 extends along a radial axis passing through the centre of the structure 3000. In use, the tip limits the minimum distance between the structure and an opposing electrode. That is, the tip effectively creates a region where the distance between the opposing electrode and the structure is smaller than it would be in the absence of the tip. In practice, this is can be achieved by using a cylindrical opposing electrode with a dielectric barrier between the structure and the opposing electrode.

[0173] In some examples, the first electrode 3040 has a diameter of 8 mm, as is the case for the electrode 20a in some examples of reactor 1 in Figures 1 A and 1 B. The opposing electrode 3100 may be formed of a suitable conducting material 3120. A dielectric material, such as a glass tube 3110 may provide a support for the conducting material consistent with the example shown in Figures 1A and 1 B. Figure 2 illustrates that the presence of at least one tip 3020 limits the minimum gap between the structure 3000 and the opposing electrode 3100. It can be seen that the distance d1 , between the end of the tip and the opposing electrode is smaller than the distance d2 between the opposing electrode and a location on the structure which is devoid of a tip.

[0174] In some examples, the size of the distance d1 , which is the gap between the sharp point of a tip 3020 and the opposing electrode 3100, may be 8 mm.

[0175] In use, when an electric field is applied between the first electrode 3040 and the opposing electrode 3100, the probability of electric breakdown occurring at the tip 3020 is increased compared at another location between the two electrodes where there is no tip. The electric field strength is concentrated in the immediate vicinity of the tip, which in turn increases the likelihood of electric breakdown occurring. The resulting electrical discharge leads to oxidation of gases, such as pollutants passing through the device.

[0176] As discussed above, the electric field intensifying structure 3000 provides a technical advantage by facilitating higher production of reactive species to effect pollutant removal. The underlying concept is that an asymmetry has been provided in the applied electric field to encourage electrical breakdown at the tip 3020. The inherent geometry of the tip, in that the tip is sharp, is another factor that encourages electric field breakdown at the tip.

[0177] As can be seen from Figure 6 of WO2024 / 165601 , we have found that by implementing a tip of the form shown in Figure 2 in the arrangement shown in Figures 1A and 1 B, there is a significant benefit to pollutant removal.

[0178] Returning to the example shown in Figure 1A and 1 B, the structure 24 is the point at which filaments form when the discharge ignition threshold is reached. As such, discharge is more favourable between the structure and the conduit at the point at which the other electrode 20b is located.

[0179] In use, when discharge occurs, this generates plasma in the space between the structure 24 and conduit 10 to form a plasma region. As set out above, the structure is shaped with the intention of evenly distributing discharge around its perimeter and around the (inner circumference of the) conduit. As such, the plasma of the plasma region forms a veil, sheet or curtain across the gas flow path, through which the gas passes in use.

[0180] In some examples, the structure 24 is thin, such as, within mechanical tolerances and possible manufacture techniques, between about 0.01 mm and about 2.00 mm. Since discharge is more typical and more favourable along the shortest path between the two electrodes, the discharge, and consequently plasma, occurs and extends radially between the structure and axially adjacent inner perimeter of the conduit 10 in various examples.

[0181] While in many examples, the electrode 20b around the outside of the conduit 10 is wider than the structure 24, this is not required, and this electrode can be a similar width to the structure. Further, this does not alter the region in which the plasma forms for the reasons stated above. As such, in several examples, the plasma region is of a similar thickness to the structure. While this thickness may be consistent, the specific shape of the plasma region can vary due to prevailing conditions, such as gas flow rate and fluctuations in discharge path. This can cause undulations or curving in the shape while maintaining the veil or sheet-like shape. Depending on the discharge, the plasma thickness along the gas flow path can vary and, while still being of a comparable thickness to the thickness of the structure, can be thicker than the structure, such as up to about three to five times the thickness of the structure.

[0182] In the example in Figure 1A and 1 B, the high voltage electrode is the electrode 20a in the conduit and the low voltage electrode is the electrode 20b around the outside of the conduit. In other examples, this may be switched.

[0183] Turning to the example in Figure 3A and 3B, this has electrodes 20a’, 20b’ each in the form of a mesh. These are separated from each other along the gas flow path. By establishing an electric field between the electrodes of at least the discharge ignition threshold, discharge occurs. This generates plasma in the space between the electrodes, to provide a plasma region. As set out above, the example shown in Figure 1 A and 1 B includes a structure 24 to cause one or more favourable locations for discharge to occur. The mesh of the electrodes 20a’, 20b of the example shown in Figure 3A and 3B provide a similar function.

[0184] In the example shown in Figure 3A and 3B, while any arrangement is possible, typically, the downstream electrode 20b has a dielectric coating. In various examples, the downstream electrode is also the low voltage electrode. However, in some examples, this is the high voltage electrode.

[0185] In the examples shown in Figure 1A and 1 B, and Figure 3A and 3B, in use, active species are formed in the plasma region when plasma is generated. As gas passes through the conduit 10, the active species are entrained in the gas. Outside of the plasma region, no further active species are generated other than from other active species as they react with each other, the gas, or become unstable. As such, immediately downstream of the plasma region, there is a decay region, in which the quantity of active species decreases as the active species react, decay and / or recombine. As such, in various examples, the plasma region and decay region are (immediately) sequential. By this we intend to mean there is nothing between these two regions, or, in other words, there is no intervening space.

[0186] The length of the decay region, i.e. the distance along the gas flow path the decay region extends is dependent on the flow rate and the temperature of the gas. This is due to the decay region length being determined by presence of active species, or at least presence of active species above an ambient or environmental normal or threshold.

[0187] Since the active species are entrained in the gas as it passes through the conduit 10, if the gas is passing through the conduit at one rate, the length of the decay region will be shorter than when the gas is passing through the conduit at a faster rate. Regardless of this, in several examples, the decay region is longer relative to the gas flow path than the plasma region. For the example shown in Figure 1 A and 1 B, this is easily identifiable due to the plasma region being of similar thickness to the thickness of the structure 24. In the example shown in Figure 3A and 3B, this means the electrodes 20a’, 20b’ are located sufficiently close together to achieve this arrangement. This may be achieved differently in different examples.

[0188] In various examples, the catalyst 30 overlaps with the decay region. In the examples shown in Figure 1A and Figure 3A, the catalyst, and thus the catalyst support 32 has an upstream end that is aligned with the upstream end of the decay region.

[0189] In practice, in some examples similar to the example shown in Figure 1A, this means the catalyst support 30 is located against (i.e. abuts) the structure 24. For the example shown in Figure 3A, this would mean the catalyst support abuts the downstream side of the downstream electrode 20b’. In other examples, there may be a separation, which may be 1 mm, 2 mm, 4 mm, 5 mm or 10 mm or other distances. As in the examples of Figure 1A and 3A, there can, however, be no separation, i.e. the separation is 0 mm.

[0190] In several examples, this results in the decay region being fully contained by the catalyst 30. By this, it is intended to mean that the length of the decay region is fully within the length of the gas flow path over which the catalyst extends. In other words, the decay region is not longer than the catalyst or not longer than the combination of the catalyst length and the separation between the catalyst and the plasma region or structure, and so, in various examples, fits within the length of the catalyst, such as within only a portion of the length of the catalyst. In several examples, the decay region may, of course, be partially outside of the catalyst, such as the decay region starting more upstream than the upstream end of the (support for the) catalyst.

[0191] In examples where there is no gap between the plasma region and catalyst, the decay region is typically of about 1 mm to 2 mm in length, such as up to about 5 mm. In some examples, this is the length in a temperature range of about 360 °C and 440 °C, such as between about 380 °C and 420 °C or 400 °C. Additionally or alternatively, in various examples, this is at a gas flow rate of between 1 litres per second (l / s) and 4 l / s, such as 2 l / s. This may be in a pipe diameter of between 30 mm and 40 mm, such as 34 mm. This permits an active species decay time (also able to be referred to as a “lifetime”) as about 1 millisecond (ms).

[0192] In other examples, such as the examples of Figure 1 B and Figure 3B, the catalyst 30 (still) overlaps with the decay region, but at an offset from the downstream end of electrodes and the transition from the plasma region to the decay region. This means the upstream end of the catalyst is not aligned with the upstream end of the decay region. Due to this, the decay region is not fully contained by the catalyst.

[0193] In such examples, the (upstream end of the) catalyst 30 may be up to 160 mm from the (downstream end of the) electrodes along the length of the gas flow path, such as being at least 30 mm from the electrodes along the gas flow path. By this, and considering the example of Figure 1 B, we intend the separation to be measured between the electrical field intensification structure 24 (such as the downstream end) and the (upstream end of the) catalyst (support), as the location at which plasma is generated and where the catalysis is possible. We have found that this still allows a benefit from the dual plasma-catalysis arrangement, but avoids discharge occurring in the catalyst, preserving the catalyst and reducing potential for shorting or electrical creepage.

[0194] In terms of potential reaction pathways, the major chemical reaction in the conversion of the methane into carbon monoxide and carbon dioxide are:

[0195] (1) N2+ e ^ N2++ N2* + e

[0196] (2) O2+ e" — > O2”

[0197] (3) N2++ 2H2O H3O++ OH- + N2

[0198] (4) H3O++ O2- ^ HO2' + H2O

[0199] (5) CH4+ OH- H2O + CH3-

[0200] (6) CH3- + O2CH3O2-

[0201] (7) CH3O2- + HO2- CH3OOH + O2

[0202] (8) CH3OOH + hv CH3O- + OH- (9) CH3O' + OH' H2CO + H2O

[0203] (10) H2CO + OH' HCO' + H2O

[0204] (11) HCO' + O2— ► CO + HO2'

[0205] (12) CO + OH' ^ HCO2'

[0206] (13) HCO2' + O2CO2+ HO2'

[0207] The notation “+” indicates the relevant molecule has a positive charge, the notation indicates the relevant entity has a negative charge, the notation indicates the relevant molecule is at an excited state, and the notation indicates the relevant molecule has a free radical. The notation “hv” is intended to indicate energy (specifically, the notation represents the product of Planck’s constant, h, and frequency, Greek letter nu, v), which in this example is achieved by the gas interacting with the electrical discharge.

[0208] To initiate the above process, typically a minimum of about 4 electron Volts per molecule (eV / molecule) of methane is generally required. This can be achieved due to the plasma generated by the plasma generator. In view of the lifetime of the plasma related species, however, some of the components need further energy or alternative routes to complete the breakdown to carbon dioxide.

[0209] Intermediate components can include CH3OH (CH3OH), CH2O (CH2O), carbon monoxide and molecular hydrogen, H2(H2). Reactions needed to breakdown these components into carbon dioxide typically have a lower activation energy than reactions involving methane.

[0210] In some circumstances, such as certain gas flow rates and / or other conditions, in examples where the separation between the plasma region and catalyst is sufficiently large, a negligible quantity of active species from the plasma region reach the catalyst. However, the above intermediate components, and others, such as at least some of those above, do not decay without intervention.

[0211] That intervention is able to be provided by the catalyst in some examples. As such, the catalyst is able to provide the ability for such intermediate components as well as methane to be broken down into carbon dioxide. As such, we have found that the effect demonstrated below is able to implemented with a separation between the plasma generator (i.e. the most downstream part of the generator at the location at which plasma is generated or present) and the catalyst as set out above is able to be implemented. In other words, a separation of up to about 160 mm allows this effect to be demonstrated.

[0212] Turning to the functionality of an example according to an aspect, we have found that by implementing arrangements in the form of the examples shown in Figures 1 or 3 or similar arrangements, the ability to decrease a quantity of a pollutant in a gas passing along the gas flow path is enhanced. This is typically achieved by passing gas along the conduit 10.

[0213] Plasma is generated in the plasma region between the electrodes 20, 20’ by establishing an electric field between the electrodes with the power supply 22. Once a discharge ignition threshold (of electric field strength or potential between the electrodes) is achieved, at least one (electrical) discharge event occurs, generating the plasma. This is typically a non-thermal plasma. The plasma generates active species, and together with electrons in the plasma, drive reactions in the gas to cause pollutants to react. The active species then pass into the catalyst support with the gas, where further reactions occur as the active species decay through reaction, recombination or due to being unstable. This causes a further decrease in the quantity of pollutant in the gas, which then passes out of the catalyst support and passes further downstream.

[0214] An example of what happens when gas is passed through the reactor 1 , T of various examples, including those of Figures 1A and 1 B, Figures 3A and 3B, is shown in Figure 4.

[0215] Figure 4 shows a plot 300 of change (decrease) in methane quantity present in a gas in percentage (%) terms on the y-axis against temperature of gas at an inlet to a reactor according to an aspect on the x-axis. This is thus the decrease in quantity of methane able to be achieved when applying specific methane decrease measures. The plot 300 of Figure 4 shows four lines. This includes the change in methane relative to temperature for a plasma-only line 320, catalysis-only line 340, a sum line 360 (i.e. the simple addition of the methane change value of the plasma-only line at a temperature to the methane change value of the catalysis-only line at that temperature), and a plasma-catalysis line 380 (which is the result when implementing a reactor according to an aspect).

[0216] The plasma-only line 320 shows a methane decrease range of about 2% at 350 degrees Celsius (°C) increasing to about 10% at about 391 °C with the increase from about 4% to 10% being in the 385 °C to 391 °C range. The catalysis-only line 340 shows a methane decrease of about 28% at about 362 °C to about 52% at 377 °C.

[0217] The sum line 360 consequently shows a methane decrease of about 31 % at 362 °C to about 60% at 380 °C. Comparably, the plasma-catalysis line 380 shows a methane decrease of about 50% at 367 °C to about 62% at 380 °C. From this, it can be seen that applying plasma-catalysis according to an aspect has a synergistic effect providing a greater methane decrease than when applying plasma and catalysis separately from each other, even if provided in the same overall reactor.

[0218] As a further demonstration of the difference between catalysis-only and plasmacatalysis according to an aspect, a further plot 400 is shown in Figure 5. This shows a solid line as a catalysis-only line 420 and a dashed line as the plasmacatalysis line 440.

[0219] Over a temperature range of 300 °C to close to 460 °C, the catalysis-only line 420 progresses from a methane removal amount of about 2% to about 98% in an S- bend curve. The section with the steepest gradient is in the temperature range of 380 °C and 420 °C, which corresponds to a methane removal range of about 26% and 88%. The 98% methane removal occurs at about 440 °C. This curve shape is caused, at least in part, by the activation temperature for the catalyst, which, in some examples is a MOC. Over the same temperature range, the plasma-catalysis line 440 achieves the same overall range of methane removal of about 2% to about 98% in an S-bend curve. However, the whole curve is shifted to achieve the same methane removal at a temperature about 40 °C lower than the catalysis-only line 420. The 97% removal amount is reached at about 420 °C, with 90% removal being achieved at about 390 °C by applying plasma-catalysis according to an aspect instead of at about 420 °C by applying catalysis only.

[0220] As well as the clear increase in methane removal ability at lower temperatures, this also combats catalyst ageing, since this typically causes the curve to shift in the opposite direction as the catalyst sustains greater use.

[0221] For the scenarios in which a reactor according to an aspect is likely to be implemented, such as in a ship exhaust system, the temperature of the gas that will be received is about 400 °C, but may be in the range of about 380 °C to 420 °C. As can be seen from Figure 5, at a temperature of about 400 °C, the catalyst- only line 420 has a methane reduction rate of about 58% compared to a more favourable rate of about 93%. Should temperatures dip to 380 °C, the plasmacatalysis still offers a removal rate of more than 80 % (such as about 82%) compared to the catalysis-only removal rate of less than 30% (such as about 28%) at the same temperature.

[0222] In part, this shift in the curve is because, while the plasma is non-thermal plasma, in some examples, there is local heating in the catalyst caused by the plasma generation, boosting the temperature (of the gas) in the catalyst (even when the separation between the plasma generator and catalyst is as set out above). Additionally, in some examples, there are also some photo-catalytic effects, further boosting the catalytic effect due to the photons produced in the plasma generation.

[0223] As noted above, the amount of power able to be provided to generate the discharge, and thus plasma, is variable, which provides more energetic plasma. In a comparison of catalysis-only and plasma-catalysis according to an aspect, Figure 6A and Figure 6B each show a plot 500, 500’. Figure 6A shows a plot 500 of reduction in methane content in parts per million (ppm) against power provided or power input, measured in watts (W), at a temperature of 350 °C. Figure 6B shows a plot 500’ for the same temperature of reduction in methane content on a percentage (%) basis against specific energy input (SEI), measured in joules per litre (J / l), since watts are joules per second, and flow rate is litres per second. The SEI measure is provided as a means of normalising results to make them comparable to other systems.

[0224] The plots 500, 500’ show reduction in methane over a power range of 0 W and 0 J / l (so without plasma, and thus catalysis-only) up to a power of 50 W and an SEI of about 41 J / l. The line 520 in the plot of Figure 6A shows a roughly linear increase from about 250 ppm to about 1050 ppm. The line 540 in the plot of Figure 6B shows the same roughly linear increase from about 18 % to about 72%. If a higher power / SEI is used, the reduction in methane increases up to about 100% methane removal.

[0225] As a comparison, the plot 400 of Figure 5, corresponds to a fixed SEI of about 10 J / l to 13 J / l. While another power or SEI can be used, typically, according to various examples, the power implemented by the plasma generator of a reactor according to an aspect is about 10 J / l to 13 J / l, but could be within a larger range, such as 8 J / l to 15 J / l, or about 5 J / l to 20 J / l, or up to 25 J / l. Typically, higher SEI values are avoided because the generation of the power to achieve this produces as much carbon dioxide equivalent (CO2e) emissions as is saved by the reduction in methane emissions. Correspondingly, if implemented on a ship, the power draw from the ship’s engines is only up to about 4% of the operation / electrical power output of the engine.

[0226] To assist in providing this efficiency, and further efficiency, we have found that using a drive circuit to establish an electric field between the electrodes and raise the voltage across the electrodes to a discharge ignition threshold is beneficial in some examples. In various examples, the reactor includes a drive circuit, such as the drive circuit system generally illustrated at 100 in Figure 6. This shows a circuit diagram of an example drive circuit 200 suitable for providing dielectric barrier discharge. This includes a model of the plasma generator section of the reactor at 110, also referred to as a dielectric barrier discharge (DBD) reactor.

[0227] The model representing the plasma generator 110 in Figure 6 is a diode bridge with a power input (also referred to as a power source) providing a voltage of Vth in use. The electrodes of the plasma generator are shown in the model as being connected across the diode bridge.

[0228] The electrodes (specifically the dielectric discharge gap between the electrodes) and the dielectric barrier mounted to one of the electrodes are represented in Figure 6 by capacitors 12. This is because the electrical functionality the gap and dielectric barrier provide to the plasma generator when represented as a circuit is capacitance.

[0229] The capacitance provided by the dielectric discharge gap is shown as being connected directly across the diode bridge. The capacitance provided by the dielectric barrier itself is shown as being connected at one end to the diode bridge in parallel with the capacitance provided by the gap. The other end of the capacitance provided by the dielectric barrier is not connected to the diode bridge. This is instead connected to the drive circuit 200 arranged to drive dielectric barrier electrical discharge across the gap between the electrodes.

[0230] While represented by a model in Figure 6, the plasma generator 110 capacitance is determined predominantly by the capacitance of the medium (typically gas, such as air) in the dielectric discharge gap. This is typically due to the dielectric constant of the medium being about 1 and the dielectric material being significantly higher than 1 , such as between about 3 and 6 (when measured at about 20 degrees Celsius at about 1 kHz). As the medium and dielectric are connected in series, it is the smaller capacitance that is dominant, and therefore, due to these relative dielectric constants, the effective capacitance of the plasma generator is governed by the medium.

[0231] Further, the contribution from the capacitance of the medium in the gap is approximately constant and does not depend on temperature of composition of the medium in the gap. This “air-gap” capacitance is therefore approximately constant because, as explained in more detail below, the pulse-trains used in examples limit the number of discharge ignition events to the extent that minimal change occurs to this capacitance. The same cannot be said however for previous resonant systems. This is either due to the extended nature of the discharge causing a shift in the capacitance of the medium, or the medium is of a different nature, such as when surface dielectric barrier discharge devices are used.

[0232] As mentioned above, the drive circuit is illustrated at 200 in Figure 6. The drive circuit has a power source 220 connected to an inverter 300. The power source is provided by a DC power supply in the examples of these figures. This is a DC link voltage supply, Vdc, in the examples shown.

[0233] In the example shown in Figure 6, the inverter 30 has a circuit loop connected across it. This circuit loop has a connection to the electrodes of the plasma generator 110 connecting in series across the capacitance provided by the dielectric discharge gap and dielectric barrier. This closes the circuit loop connected across the inverter.

[0234] In other examples, the inverter 300 has a transformer connected across it. In such arrangements it is the primary side of a transformer that is connected across the inverter. The secondary side of the transformer has a connection to the electrodes of the plasma generator 110 connecting in series across the capacitance provided by the dielectric discharge gap and dielectric barrier.

[0235] The connection across the capacitance of the plasma generator 110, and the ability to connect across this capacitance in the example of Figure 6 causes the drive circuit 200 to be a separate, and in some examples separable, circuit from the plasma generator.

[0236] In the example shown in Figure 6, when the drive circuit 200 is connected as set out above to the plasma generator 110, a resonant tank 40 is formed between the inverter 300 and the capacitors 12 provided by the dielectric discharge gap and the dielectric barrier. The inductance of the resonant tank is provided in this example by an inductor 42 connected in series with the capacitance. Some inductance will also be provided by the wiring of the resonant tank. The inverter provides the power source for the resonant tank.

[0237] In other examples, when the drive circuit 200 is connected, as set out above, to the plasma generator 110, a resonant tank 40 is formed between the transformer and the capacitance 12 provided by the dielectric discharge gap and the dielectric barrier. The inductance of the resonant tank is provided by an inductor 42 connected in series with the secondary side of the transformer and the capacitance in combination with stray / leakage inductance of the transformer. Such an inductance can be considered as being an inductor connected in series with the transformer between the output from the inverter 300 and the input to the primary side of the transformer.

[0238] In examples where a transformer is used, this may further have magnetisation induction. When represented as an inductor in a circuit diagram, this would be an inductor connected in parallel with the primary side of the transformer.

[0239] In further examines where a transformer is used, in addition to providing a step change in voltage and current based on the turns ratio in the transformer, the transformer also provides galvanic isolation. This suppresses electromagnetic interference across the transformer from the inverter 300 to the resonant tank.

[0240] A conventional magnetic core transformer is able to be used in various examples. In other examples, an Air-Core Transformer (ACT) is able to be used.

[0241] Compared to a regular (i.e. magnetic core) transformer, an ACT can have a very low coupling (such as 40% instead of 98% as would typically in a magnetic core transformer) between the windings. This results in higher leakage inductance than in a regular transformer. However, this is desirable in some examples, since it allows several desirable functions for the drive circuit as a whole to be incorporated in a single component, namely galvanic isolation for safety and EMI suppression (since the transformer provides a noise barrier), voltage step-up and resonance inductance (as is discussed in more detail below). These functions are also able to be provided by a regular transformer but to a lesser extend in some examples.

[0242] Examples that include a transformer can be seen in Figure 7 of WO 2022 / 106622, the content of which, as noted above, is incorporated herein by reference.

[0243] Turning to the inverter 300 in more detail, in the example shown in Figure 6, the inverter is provided by an H-bridge. The H-bridge has four switches 320 providing two high-side switches, Si+ and S2+, and two low-side switches, Si- and 52-

[0244] In other examples (one of which can be seen in Figure 6 of WO 2022 / 106622), the inverter is provided by a half bridge. This has two switches 32 and two capacitors, with the switches providing one high-side, S1+, and one low-side, S1-, switch.

[0245] The switches 320 of the inverter 300 are, in the example shown in Figure 6 provided by transistors. These are silicon carbide MOSFETs in the example shown in this figure. In other examples, each switch is able to be provided by a MOSFET, such as an n-type MOSFET, silicon MOSFET; or other types of electronic switches, such as Insulated Gate Bipolar Transistors (IGBTs), such as a silicon IGBT, Junction Field Effect Transistors (IFETs), Bipolar Junction T ransistors (BJTs), or High Electron-Mobility T ransistors (HEMTs), such as gallium nitride (GaN) HEMTs.

[0246] In the examples shown in Figure 6, a capacitor 240 is connected in parallel with the inverter 300 and voltage supply 22. This provides a DC link capacitance for the drive circuit 200. In the example implementing a half-bridge inverter, this capacitance is provided by the capacitors of the half-bridge inverter.

[0247] In terms of functionality, as shown in Figure 7, the drive circuit 200 is used to provide an electrical pulse-train to the resonant tank and to prohibit power transfer to the resonant tank after the pulse-train. There are also steps of modulating power properties in order to modify the pulse-train before a further pulse-train is provided and to recover energy from the resonant tank after the discharge ignition event(s) and store the energy. While there are examples where energy recovery is not included in this process, typically energy recovery is included in this process. The step of modulating power properties is optional, however. The details of the process are set out in more detail below along with further details of power modulation and energy recovery processes.

[0248] During use of the drive circuit system 100, the power supplied to the plasma generator 110 needs to reach at least the dielectric barrier electrical discharge voltage level (Vth). This is needed in order to stimulate dielectric barrier electrical discharge across the discharge gap. The model circuit shown in Figure 6 for the plasma generator shows the ability of the device to accept power and voltage clamping across the gap when Vth is reached. The power absorbed by the DBD voltage source shown in these figures is given by the product of Vth and the current impressed in the resonant tank (when the diodes are conducting). As such, when the voltage across the gap exceeds Vth, the corresponding pair of diodes in the model circuit of the plasm generator are conducting, and power is being transferred to the (model) Vth voltage source depicted in the figures, representing a power transfer to the plasma. In this model, the voltage across the gap is clamped to Vth whenever dielectric barrier electrical discharge occurs.

[0249] The power to provide the dielectric barrier electrical discharge voltage is provided by the drive circuit 200 as a pulse-train. The power provided by the pulse-train is drawn from the DC link voltage source 220 at a level of about 800 V. This is fed to the inverter 300. In other examples, the voltage provided by the DC link voltage source is up to 900 V when using a silicon carbide MOSFET, and can be higher, such as 1.2 kV to 1 .3 kV when using a 1 .7 kV rated silicon carbide transistor.

[0250] To initiate the pulse-train, when using the drive circuit system 100 in the example shown in Figure 6, as power is drawn from the DC link voltage source 220, the Flbridge is then used to excite the resonant tank 40. In this example this is achieved by the H-bridge outputting a 100% duty-cycle square wave voltage over the duration of the first two modes of the pulse-train as set out below in relation to Figure 8. The switches 320 of the H-bridge are arranged to provide output at a switching frequency tuned to excite the resonant tank 40 at the resonance frequency of the tank. This causes only real power to be processed by the H-bridge. In order to minimize switching losses, operation slightly above the resonance frequency is feasible to achieve ZVS of the switches.

[0251] As set out below in relation to Figure 8, the excitation of the resonant tank 40 causes dielectric barrier electrical discharge once the voltage level in the resonant tank 40 reaches Vth. This transfers power into the plasma between the electrodes in the plasma generator 110.

[0252] When the second mode of the pulse-train is to be ended, the switches 320 are turned off. When using transistors as in the examples shown in Figure 6, this is achieved either by turning the transistors off apart from the transistor body diodes (or external anti-parallel diodes), which are left active, or the bridge voltage (VFB) across the inverter 300 is phase-shifted by 180 degrees (°) in order to respectively passively or actively recover the remaining energy stored in the resonant tank 40.

[0253] The recovered energy is transferred to the DC link capacitor 240 (this corresponds to the capacitors of the inverter 30 when a half-bridge inverter is instead of the H- bridge inverter of the example drive circuit 200 shown in Figure 6). This is achieved by the reversal of the power flow through the passive or active recovery described in the previous paragraph. This allows this energy to contribute to the energy used for the next pulse-train.

[0254] Passive power recovery is achieved by the transistors in the inverter 300 simply being switched off at the end of the second mode (i.e. when dielectric barrier electrical discharge is to be ended), as referred to below. Due to the arrangement of the circuit in an H-bridge or half bridge, this removes all circuit paths through the transistors and leaves a path through the transistor body diodes (which, as shown in Figure 6 provides a connection across the transistors). The connection of the resonant tank across the inverter as shown in Figure 6 relative to the diodes allows energy to flow through the diodes and into the DC link capacitor 240 when the transistors are switched off. Active power recover is instead achieved by making use of the transistors to provide a 180° phase shift in the output of the inverter 300 from the phase of the output in the second mode. Instead of allowing energy to flow into the DC link capacitor 240, as occurs during passive power recovery, this drives the energy into the DC link capacitor.

[0255] The quality factor (Q) of the resonant tank equates to the voltage gain of voltage across the dielectric discharge gap (vdbd) to the bridge voltage (i.e. Q = vdbd / vFB) at the resonance frequency (without transformer or unity turns-ratio, which would make the quality factor as Q = vdbd / (vFB / n), where n is the turns ratio of the transformer; the total gain when using a transformer would also be determined from the transformer step-up plus the resonance gain). The effective voltage gain of the resonant tank is determined by the power losses imposed by the parasitic resistances of the magnetic components and the wires connecting the electrodes of the plasma generator, which provide damping to the circuit. Unlike known systems that use resonant converters, in examples according to an aspect disclosed herein the effective voltage gain is not determined by the actual power being delivered to the plasma since there is no discharge occurring during charging of the resonant tank. For this reason, practical values of Q of greater than 40 allow dielectric barrier electrical discharge voltages above 30 kV from the 800 V DC link input voltage without the explicit need of a step-up transformer.

[0256] It can therefore be appreciated that once power is being absorbed by the onset of discharge ignition events in the plasma generator, a lower voltage gain may cause a self-quenching effect due to the damping this causes and the Q value shift. However, since only a few discharge ignition events are wanted from each pulsetrain (such as between one and about five discharge ignition events) and because there is enough momentum in the resonant tank (stored energy much larger than energy absorbed by electric discharges), this does not impose any practical challenges for the examples according to an aspect disclosed herein. On the other hand, known resonant converters are configured for comparably low voltage gains resulting from continuous power absorption by the plasma and therefore need, and are designed with, high step-up transformer turns-ratios. The voltage across the dielectric discharge gap is determined by the capacitance of the dielectric discharge gap. This is made up of the capacitance of the dielectric and the capacitance of the gap itself. In the example in Figure 6, the capacitance of the dielectric (Cdiei) is typically much larger than the capacitance of the gap (Cgap). For example, Cdiei is typically at least ten times larger than Cgap. This also gives a voltage ratio of voltage across the gap (Vgap) compared to the voltage across the dielectric (Vdiei) of at least 10.

[0257] The process of recovering energy can be applied in a corresponding manner using a drive circuit example using a half-bridge. When using a drive circuit with a transformer, the same process as is able to be applied for the drive circuit 200 of the example shown in Figure 6 can be used.

[0258] The power being provided by the DC link power supply is the power provided to the drive circuit averaged over the pulse-train repetition interval. The energy exchanged between the DC-link capacitor and the resonant tank during resonant tank charging, power transfer during dielectric barrier electrical discharge, and resonant tank discharging typically causes a voltage ripple across the DC link capacitors. The interval where power is transferred to the plasma by dielectric barrier electrical discharge also contributes to the DC-link voltage ripple.

[0259] In various examples using a transformer, the transformer provides a step up ratio of between about 1 : 1 and 1 :10. This lower step up ratio that those of conventional pulsed-power circuits (example step-up ratios of which are set out above), allows the current passing through the primary side of the transformer to be limited.

[0260] When a ratio of 1 :1 is used for the transformer, this only provides galvanic isolation instead of providing galvanic isolation and step up in voltage when a higher step- up ratio, such as a step up ration of 1 :10, is used.

[0261] The inductor 240 used in some example drive circuits implementing a transformer can be located on either the primary side or secondary side of the transformer. However, by locating the inductor on the secondary side (and therefore high voltage side), as mentioned above, the kVA rating of the transformer is able to be reduced. The reactive power of the plasma generator 110 can then be directly compensated. Under such a reactive load matching condition, only the real power is processed by the transformer.

[0262] The galvanic isolation imposed by the transformer reduces ground currents, which are currents flowing in the parasitic capacitance between electrodes of the plasma generator 110 and any surrounding metallic housing. This assists in meeting electromagnetic compatibility (EMC) limits.

[0263] Considering wavelet pulse-trains, when using a DBD device, such as one plasma generator described above, we have developed a process that implements a high frequency sinusoidal waveform with varying amplitude, resembling a wavelet-type waveform. In various examples, the wavelet is generated by connecting an inductor in series with a plasma generator, which, as set out above provides a capacitance.

[0264] The series resonance circuit or a series resonant tank is capable of being excited at a resonance frequency. When excited at a resonance frequency repeatedly for several cycles using bipolar voltage pulses, this allows the plasma generator to be excited with a high voltage slew rate while substantially reducing current stress, and which lowers the peak power processed by the power electronics. As such, voltage gain achieved in the resonant tank provides the high ignition voltage levels for the plasma generator, instead of using a pulse-transformer with a high turns ratio to provide the voltage gain. Relevant attributes of the resonant tank are therefore the achievable voltage gain and the ability to compensate for the reactive power of the plasma generator.

[0265] Applying several consecutive bipolar voltage pulses to form a pulse-train allows low power loss (demonstrated by the high efficiency noted below) and a higher pulse repetition frequency to be applied, and therefore the capability of average power transfer is substantially increased over a system using a single pulse. As an example, by applying this process, the pulse repetition frequency is able to be increased by at least ten times over such a system. This is achievable in combination with the use of silicon carbide semiconductor technology as described in more detail below.

[0266] Repetition frequency of pulse-trains is limited by a maximum operating temperature of power electronics. In general, pulse-power converter designs take advantage of the slow thermal response. This means that if a high pulse repetition frequency were used in a conventional pulsed system, dissipated peak power would be too large to stay within safer operating temperatures of the power electronics. This is avoided in the examples described herein by using the pulsetrain modulation described below. Additionally, this is avoided by limiting the maximum number of discharge ignition events produced from a single pulse-train and then having a period that allows cooling to occur before the next pulse-train.

[0267] By implementing a pulse-train of several consecutive bipolar voltage pulses as described in relation to the examples set out herein, even if the number of discharge ignition events is limited to between one and five, this is achieved while providing energy transfer at very high efficiency, such as at about 90% efficiency or greater.

[0268] As shown in Figure 8, the use of consecutive bipolar voltage pulses creates three modes of operation induced at a DBD device, such as the plasma generator. The first mode, which occurs between 0 ps and time A in Figure 8, is the charging of the resonance circuit. This builds up the potential difference across the electrodes in the DBD device. As set out above, this is achieved by applying consecutive bipolar voltage pulses at the resonant frequency of the resonant tank.

[0269] In the plots shown in Figure 8 this can be seen to be a sinusoidal wave at consistent frequency that steadily increases in amplitude for both voltage and current. This results in an instantaneous power level of a rectified sine wave (as the multiplication of rectangular voltage and sinusoidal inductor current) with a steadily increasing amplitude. The duration of the mode in the example shown in Figure 8 is around 2.5 voltage cycles, 2.5 current cycles and 5 power cycles (one power cycle being the transition from zero to a peak and back to zero). In this example, the current waveform leads the voltage waveform by about 90°. The second mode takes place between time A and time B in the example plots of Figure 8. This mode is reached when the voltage reaches the ignition or breakdown voltage (Vth) causing dielectric barrier electrical discharge between the electrodes of the plasma generator. This delivers power to the plasma and should last only a few discharge cycles for most efficient pollutant reduction. During this mode the voltage amplitude remains above the Vth level due to continued excitation of the resonant tank at the resonant frequency. In the plots it can be seen that the voltage and current continue in a sinusoidal wave with consistent frequency. The amplitude of the waves varies slightly over the duration of this period (increasing to approximately the halfway point of the mode’s duration and then begins to decrease).

[0270] The example shown in Figure 8 is based on a plasma generator having a capacitance of approximately 3.0 nF. The voltage has a peak at about ±24 kV (positive-negative 24 kV) and a current of ± 80 A. In other examples the capacitance of approximately 1 .0 nF, but could also be approximately 45.0 nF or higher.

[0271] The voltage and current amplitude pattern is the same for the instantaneous power, which continues to be the rectified sine wave. The peak instantaneous power is about 180 kilo-Watts (kW) in the example shown in Figure 8.

[0272] The duration of the second mode is about 1.5 voltage cycles, about 1.5 current cycles and about 3 power cycles.

[0273] During the first and second mode the resonant tank is excited by having power provided to it. During the third mode the excitation is stopped and the resonant tank discharges by draining. In some examples the tank is actively discharged by recovering the energy from the tank. A passive discharge is also possible.

[0274] Due to the excitation being stopped and a discharge path being provided, in the third mode the voltage, current and power reduce to zero. In the example plots in Figure 8, the third mode is shown from time B onwards. The voltage and current follow a sinusoidal waveform with a consistent frequency as in the first and second modes. The power continues to be a rectified sine wave. The amplitude of the voltage and current decrease towards zero over the period of about 2.5 cycles for the voltage and about 2.5 cycles for the current.

[0275] The power plot shown in Figure 8 is consistent with an example in which the resonant tank is passively discharged. This can be seen by the instantaneous power being inverted so as to be the rectified sine wave, but with the peaks being negative values instead of positive as in the first and second mode. The amplitude of the power decreases to zero over about five cycles.

[0276] The three modes form a wavelet pulsed power process in the form of a pulse-train implemented by excitation of the resonant tank. The duration of the power transfer achieved using this process is determined by the length of time over which this excitation pulse-train is provided to the resonant tank. This is just one parameter of the excitation pulse-train that is determined by circuit by which the pulse-train is implemented. As set out above, Figure 6 shows an example circuit capable of being used to implement one or more pulse-trains.

[0277] An example of the excitation applied to the resonant tank is shown in Figure 9 below. As can be seen in that figure, in various examples, the excitation takes the form of a square wave voltage waveform, the waveform comprising multiple consecutive individual pulses that together form a pulse-train. This induces a sinusoidal current in a resonant tank (the current waveform shown in Figure 9), and provides the waveforms at the plasma generator shown in Figure 8.

[0278] While Figure 9 does not show the dielectric barrier electrical discharge threshold, or specific include markings separating the first, second and third modes, it is possible to see in these figures where the third mode begins. At time D in Figure 9, it can be seen that the voltage waveform has a peak at a maximum positive value that has a shorter duration than the other peaks in the waveform. This occurs due to the transition from the second mode to the third mode. At this point, the excitation is stopped, meaning voltage is no longer actively provided to the resonant tank and plasma generator. Depending on the action taken at that stage, such as whether active or passive energy recovery is used, this causes a phase shift in the voltage waveform. Passive energy recovery is used in the simulation used to produce Figure 9, and as such, the change in the applied waveform is caused by means of freewheeling of current in H-bridge diodes. An alternate active energy recovery means applied in some examples is 180 degree phase shift causing power to be drained instead. These processes are described in more detail below along with an example inverter providing the H-bridge.

[0279] In various examples, the transition to the third mode in examples according to an aspect disclosed herein is applied after a maximum number of discharge ignition events. A number of examples limit the maximum number of discharge ignition events to only a single discharge ignition event, or to up to about five discharge ignition events. When only a single discharge ignition event is used as the maximum number, or after the last discharge ignition event at a larger maximum number, the third mode is transitioned to directly after (such as immediately after) the maximum number of discharge ignition events have occurred.

[0280] In terms of how an example excitation applied to the DBD device provided by the plasma generator translates into discharge, this is demonstrated by the plots shown in Figure 10. This shows an upper plot and a lower plot. The upper plot is a plot of voltage against time and the lower plot is a plot of current against time.

[0281] The upper plot of Figure 10 shows a solid line and a dashed line. The solid line is in the form of a sinusoidal wave that is at a minimum at time zero. In this example, this line corresponds to a voltage applied across a plasma generator. The dashed line is in the form of a sinusoidal wave with its maximum and minimum peaks truncated to a plateau. As with the applied voltage curve, this is at a minimum at time zero, and, in this example, corresponds to a voltage across the discharge gap.

[0282] The amplitude of the gap voltage is less than the applied voltage amplitude. As the applied voltage transitions towards positive, the gap voltage increases. After about an eighth of a cycle of the applied voltage, the gap voltage turns positive. Just before the end of a second eighth of said cycle, the amplitude of the gap voltage reaches a threshold. In Figure 10 this occurs at time a. This plateau is maintained until the applied voltage reaches a maximum, at time y, in Figure 10. At time y, the process repeats itself, but with the polarities reversed, and continues to switch between movements in the positive and negative directions as long as the applied voltage continues.

[0283] As a comparison to the first, second and third modes set out above, the rise in the gap voltage corresponds, for example, to the rise in voltage during the second mode after the first fall in voltage during the second mode. From this it can be understood that discharge is able to occur during this period, and as such, the plateau in the gap voltage curve is due to the threshold voltage being reached.

[0284] The current plot of Figure 10 shows the current at the gap induced by gap voltage. At time zero this has an amplitude of approximately zero. This increases in the form of a sinusoidal wave. Should the gap voltage not reach the threshold voltage (such as if the plots of Figure 10 represented voltage and current during the first or third modes), then, as shown by the dashed line in the current plot in Figure 10, the sinusoidal wave would proceed uninterrupted. However, at time a, due to the threshold voltage having been reached, ignition occurs. This causes ionisation of the medium in the discharge gap and electrical discharge to begin.

[0285] From time a, the gap current rapidly increases to a peak at time p, which corresponds to the zero-cross point of the applied voltage. Since time a is almost at the end of a quarter cycle of the applied voltage cycle, this is a very short period relative to the cycle of the current curve. From time p, the current then, in a sinusoidal manner, decreases to zero at time y, at which point it returns to its original form and amplitude range. This cycle continues in parallel with the gap voltage and applied voltage.

[0286] As can be seen from this, the amplitude of the current is simply increased to an amplified level. The main current plot of Figure 10 shows a continuous curve between time a and time y. As noted above this is the time during which discharge occurs. This period is therefore able to be considered to be a macro-discharge period, and time a is when a discharge ignition event occurs. As is shown by the magnified section of the current plot of Figure 10, the current curve does not have a continuous form, however. Instead, the curve is made up of many current spikes that are so close together that they cause the curve to appear continuous. Each spike represents a micro-discharge or transient filament, which is initiated from a single point on one of the electrodes. It is the connection each of these filaments provide between the opposing electrodes (one electrode 20b of course having the dielectric layer 10 thereon as shown in Figures 1Aand 1 B) that causes the current spike because the filament provides a current path across the discharge gap. Due to these microdischarges ionising the medium in the gap and passing high energy electrons into the medium, enough energy is present to drive chemical reactions that, for example, breakdown pollutants in the medium.

[0287] Returning to the functionality of the drive circuit, the duration of each wavelet pulse-train determines the number of dielectric barrier electrical discharge ignition events. As can be seen from Figure 11 , for a given Vdc, the number of excitation periods np (i.e. frequency cycles) defines the effective duration of the wavelet pulse-train and the number of dielectric barrier electrical discharge ignition events once Vth has been reached in the resonant tank. This therefore determines the amount of energy transferred to the plasma per pulse-train.

[0288] The real power is adjusted by moving the bridge-leg switching frequency away from the resonance frequency. This can be achieved by increasing the switching frequency above the resonance frequency or lowering the switching frequency below the resonance frequency. This causes a phase-shift between the VFB and the bridge current SFB, and thus lowers the real power being transferred to the plasma generator.

[0289] By taking this approach the high voltage gain is lowered and processing of reactive power increases. In order to maintain the high voltage gain and minimise the processing of reactive power, instead, in accordance with aspects of the present disclosure, the inverter 300 is able to be arranged in use to provide excitation close to the resonance frequency. This is achieved by keeping the phase shift between VFB and iFB close to zero. The average power is adjusted by varying the repetition frequency of the wavelet pulse-trains (i.e. how frequently a wavelet pulse-train is used to excite the resonant tank to cause dielectric barrier electrical discharge). This allows very high partial load efficiency to be achieved since the resonant tank is always operated at its resonance and therefore there is little to no processing of reactive power.

[0290] As mentioned above, the length of a pulse-train is variable. A pulse-train of one duration can be seen in Figure 11. The pulse-train illustrated in Figure 11 is a short pulse-train, such as one that is able to be used with an example according to an aspect disclosed herein due to it producing between two and four discharge ignition events.

[0291] In Figure 11 , the pulse-train is generated by an example drive circuit such as the one shown in Figure 6. Of the two plots shown in this figure, one plot shows the state of the switches 320 within the H-bridge inverter 300. These are either in an off state (a “0” state) or an on state (a “1” state). By operating these switches in pairs, the wave pattern shown in the lower plot of Figure 9 is producible at the plasma generator.

[0292] The switch pairs are the Si+ switch paired with the S2- switch, and the Si- switch paired with the S2+ switch. During the first two modes of a pulse-train, the switches of each pair (i.e. the two switches within the respective pairs) are operated in phase, causing each switch to be in the same state as the other switch of the pair. In the first two modes of a pulse-train, the pairs are operated out of phase, meaning that when the switches of one pair are in one state, the switches of the other pair are in the other state.

[0293] As is conventional with an inverter, there is a “dead-time” or “interlocking time” between the switches S1+ and Si- being switched from one state to the opposing state. This dead-time is a period of time where both the switches are turned off. This period is typically several hundred nanoseconds. This period is provided as a safety interval to avoid the DC-link power supply being accidentally shorted, since this would cause a catastrophic failure within the system.

[0294] By having the switch pair Si+ and S2- in the on state and the switch pair Si- and S2+ in the off state, this causes a positive voltage increase. By reversing the states, so having the switch pair S1+ and S2- in the off state and the switch pair S1- and S2+ in the on state, this causes a negative voltage increase. By alternating this arrangement, a sinusoidal waveform as shown in the lower plot of Figure 11 is produced with the frequency of the waveform being determined by the length of time each switch pair is in an on and off state.

[0295] In Figure 11 each switch pair is operated for seven on-off cycles, with the S1+ and S2- pair being the first pair to be in the on state. This generates a pulse-train with a duration of around 40 ps and a voltage of at least Vth for about 1.75 cycles. When the switch pair on-off cycles are stopped, the third mode of the pulse-train occurs until the voltage returns to 0 V. Additionally, in the pulse-trains illustrated in Figure 9 the first mode and third mode of each pulse-train have approximately the same duration.

[0296] Figure 12 shows a mechanism for varying the amount of power transferred to the plasma. As mentioned above, a further mechanism for altering the amount of power transferred to the plasma is to vary the frequency of pulse-trains (i.e. the number of pulse-trains per unit of time). This is referred to as the repetition frequency (fr). Three different power transfer levels are shown in the three plots of Figure 12.

[0297] Each plot in Figure 12 illustrates about a 200 ps period. At a low power transfer rate, such as in the bottom plot of Figure 10, there may be one pulse-trains thereby defining an frof about 5 kHz (equivalent to the reciprocal of 200 ps) with each pulse-train having a duration of about 40 ps. In the plot above this in Figure 12, the fris about 10 kHz (equivalent to the reciprocal of 100 ps) with a pulse-train duration of about 40 ps. This second plot provides a medium power transfer rate. A (very) high power transfer rate is exemplified by the plot at the top of Figure 12 (a third plot). In this third plot the fris about 18 kHz (equivalent to the reciprocal of 55 ps) with a pulse-train duration of about 40 ps. In each of these three plots the pulse-trains are distinguishable from each other due to the increase and then decrease in voltage amplitude of each pulse-train being determinable. With each pulse-train, dielectric barrier electrical discharge occurs when the voltage increases to at least Vth. Dielectric barrier electrical discharge then stops as the voltage decreases below Vth.

[0298] Turning to the other components of the reactor 1 , as identified above, we have found that the distance between the catalyst and the plasma region affects the removal of pollutants from the gas passing through the reactor. In some examples, this is due to the change in the overlap of the catalyst with the decay region.

[0299] This effect is shown in Figures 14A and 14B. These show plots 600, 600’ with the same axes as the plots 500, 500’ of Figures 6A and 6B (namely methane removal in ppm against power and methane removal in % against SEI). The plots in Figures 14A and 14B show results of use of a reactor according to an aspect with a gas input temperature of about 380 °C.

[0300] Each plot 600, 600’ shows a solid line curve 620, 620’ (with round data points) when the catalyst (and thus catalyst support) is located about 100 mm from the plasma region. In practice, this is a distance of 100 mm from the downstream electrode or electric field intensification structure.

[0301] In the plot 600 of Figure 14A, the curve 620 has a methane removal rate of about 430 ppm at 0 W increasing, at a decreasing gradient, to a removal rate of about 630 ppm at 50 W. In the plot 600’ of Figure 14B, the curve 620’ has a methane removal rate of about 61 % at 0 J / l increasing, at a decreasing gradient, to a removal rate of about 89 % at about 37 J / l.

[0302] A dashed line curve 640, 640’ (with X-shaped data points) is shown in each plot 600, 600’. This shows the methane reduction when the catalyst (and thus catalyst support) is located with no separation (so 0 mm) from the plasm region. In practice, this is when the (upstream end) of the catalyst support is abutting the (downstream side) of the downstream electrode or electric field intensification structure.

[0303] In the plot 600 of Figure 14A, the curve 640 has a methane removal rate of about 480 ppm at 0 W increasing, at a decreasing gradient, to a removal rate of about 660 ppm at 50 W. In the plot 600’ of Figure 14B, the curve 640’ has a methane removal rate of about 68 % at 0 J / l increasing, at a decreasing gradient, to a removal rate of about 94 % at about 37 J / l.

[0304] By comparison, it can be seen that having no separation results in higher methane reduction rates than when there is a 100 mm separation. However, there are still advantages of having a separation that extend beyond just the methane abatement performance.

[0305] While more limited, the same results can be seen at the higher temperature of 416 °C. Results of the same arrangements as set out above for the plots of Figures 14A and 14B are used for the plots 700, 700’ of Figures 15A and 15B.

[0306] Each plot 700, 700’ shows a solid line curve 720, 720’ (with round data points) when the catalyst (and thus catalyst support) is located about 100 mm from the plasma region. In practice, this is a distance of 100 mm from the downstream electrode or electric field intensification structure.

[0307] In the plot 700 of Figure 15A, the curve 720 has a methane removal rate of about 671 ppm at 0 W increasing, in a roughly linear manner, to a removal rate of about 680 ppm at 50 W. In the plot 700’ of Figure 15B, the curve 720’ has a methane removal rate of about 96.0 % at 0 J / l increasing, at a decreasing gradient, to a removal rate of about 98.5 % at about 35 J / l.

[0308] A dashed line curve 740, 740’ (with X-shaped data points) is shown in each plot 700, 700’. This shows the methane reduction when the catalyst (and thus catalyst support) is located with no separation (so 0 mm) from the plasm region. In practice, this is when the (upstream end) of the catalyst support is abutting the (downstream side) of the downstream electrode or electric field intensification structure. In the plot 700 of Figure 15A, the curve 740 has a methane removal rate of about 672 ppm at 0 W increasing, at a decreasing gradient, to a removal rate of about 694 ppm at 50 W. In the plot 700’ of Figure 15B, the curve 740’ has a methane removal rate of about 96.0 % at 0 J / l increasing, at a decreasing gradient, to a removal rate of about 99.0 % at about 35 J / l.

[0309] In terms of other pollutants, while various examples are intended to be used to reduce quantity of methane, other pollutants have been analysed. This includes nitrogen species. This includes nitrogen oxide (NO), nitrogen dioxide (NO2, NO2) and dinitrogen oxide (N2O, N2O).

[0310] At a temperature of 416 °C, using the same reactor configuration as the examples used to produce the results for the plots 600, 600’, 700, 700’ of Figures 6A, 6B, 15Aand 15B, Figures 16Ato 16D shows plots 80, 82, 84, 86 of results for a reactor with a 100 mm catalyst-plasma region separation and when there is no separation. This is in the same arrangements as set out above.

[0311] The curves that relate to the 100 mm separation are the solid line curves with round data points, illustrated at 802 in Figure 16A, 822 in Figure 16B, 842 in Figure 16C and 862 in Figure 16D. The curves that relate to no separation are the dashed line curves with X-shaped data points. These are illustrated at 804 in Figure 16A, 824 in Figure 16B, 844 in Figure 16C and 864 in Figure 16D.

[0312] All of Figures 16A to 16D show change in the relevant nitrogen species in ppm against power. Figure 16A relates to change in NO. The two curves 802, 804 of the plot 80 of Figure 16A are similar, with both having a reduction rate at about - 18 ppm (so NO generation of about 18 ppm) at 0 W, which increases in a linear manner to a reduction rate of about 6 ppm at 50 W.

[0313] Figure 16B relates to change in NO2. The two curves 822, 824 of the plot 82 of Figure 16B are similar from 0 W to 20 W and then diverge slightly. Each curve is roughly linear. Each curve has a reduction rate of about 14 ppm at 0 W, which increases to about 17 ppm to 18 ppm at 20 W. The 100 mm curve 822 then increases to a reduction of about 24 ppm at 50 W with the no separation curve 824 instead increasing to a reduction of about 21 ppm at 50 W.

[0314] Figure 16C relates to change in N2O. The two curves 842, 844 of the plot 84 of Figure 16C are linear and show no change in N2O across the power range of 0 Wto 50 W.

[0315] Figure 16D relates to change in NOx, so the sum of the changes of the plots of NO and NO2. The two curves 862, 864 of the plot 86 of Figure 16D are roughly linear and match each other between 0 W and 30 W. There is a slight divergence in the curves from 30 W to 50 W. At 0 W, the reduction rate of NOx is about -4 ppm (so an increase in NOx of about 4 ppm). This increases to a reduction of about 15 ppm at 30 W. The 100 mm separation curve 862 then increases to about 28 ppm at 50 W. The no separation curve instead increases to about 26 ppm at 50 W.

[0316] At the lower temperature of 380 °C, the results vary from this. These are shown, using the same reactor configuration as the examples used to produce the results for the plots 600, 600’, 700, 700’, 80, 82, 84, 86 of Figures 14A, 14B, 15A, 15B and Figures 16A to 16D in Figures 17A to 17D. These show plots 90, 92, 94, 96 of results for a reactor with a 100 mm catalyst-plasma region separation and when there is no separation. This is in the same arrangements as set out above.

[0317] The curves that relate to the 100 mm separation are the solid line curves with round data points, illustrated at 902 in Figure 17A, 922 in Figure 17B, 942 in Figure 17C and 962 in Figure 17D. The curves that relate to no separation are the dashed line curves with X-shaped data points. These are illustrated at 904 in Figure 17A, 924 in Figure 17B, 944 in Figure 17C and 964 in Figure 17D.

[0318] All of Figures 17A to 17D show change in the relevant nitrogen species in ppm against power. Figure 17A relates to change in NO. The two curves 902, 904 of the plot 90 of Figure 17A differ from each other. The 100 mm curve 902 is a curve that increases in reduction rate increase in power, but at a decreasing gradient. This has a reduction rate of about -9.5 ppm (so an increase of about 9.5 ppm) at 0 W, which increases to a maximum at 40 W of a reduction of about 2.5 ppm, which then decreases to a reduction of about 2.0 ppm at 50W. The no separation curve 904 is roughly linear, with a slight decrease in gradient between 0 Wand 20 W, which then increases again to 50 W. At 0 W, the reduction is about -14.5 ppm (so an increase of about 14.5 ppm), which increases to a reduction rate of about 1.5 ppm at 50 W.

[0319] Figure 17B relates to change in NO2. The two curves 922, 924 of the plot 92 of Figure 17B are similar to each other, but with an offset between 0 W and 40 W, but which then converge at 50 W. Each curve is roughly linear (other than the convergence). The 100 mm separation curve 922 has a reduction rate of about 6 ppm at 0 W, which increases to about 25 ppm at 50 W. The no separation curve 924 has a reduction rate of about 12 ppm at 0 W, which increases to about 25 ppm at 50 W.

[0320] Figure 17C relates to change in N2O. The two curves 942, 944 of the plot 94 of Figure 17C are linear and show no change in N2O across the power range of 0 Wto 50 W.

[0321] Figure 17D relates to change in NOx, so the sum of the changes of the plots NO and NO2. The two curves 962, 964 of the plot 96 of Figure 17D are a curve with increasing reduction rate with temperature at a decreasing gradient. The curves approximately match each other between 0 Wand 10 W, and then diverge slightly at 20 W and re-converge to 50 W. At 0 W, the reduction rate of NOx is about -4 ppm (so an increase in NOx of about 4 ppm). This increases to a reduction of about 5 ppm at 10 W. The 100 mm separation curve 862 then increases to about 12 ppm at 20 W and then increases to about 26 ppm at 50 W. The no separation curve instead increases to about 8 ppm at 20 W (and has a reduction rate of about 4 ppm at 10 W) and then increases to a reduction rate of about 26 ppm at 50 W.

[0322] The plots of Figures 16A to 16D and Figures 17A to 17D, show how a reactor according to an aspect processes a pollutant other than methane. While this shows more variable results, any increase in NOx is minimal, with any increase potentially being smaller than an in-plasma catalysis arrangement. As such, and regardless, we note that the reactor according to an aspect has been found to be beneficial in removal of pollutant from gas.

Claims

1. CLAIMS1. A reactor for reducing a quantity of a pollutant in a gas, the reactor comprising: a plasma generator, catalyst and a gas flow path along which gas passes in use, the gas flow path passing through a plasma region and over the catalyst, plasma being formed in the plasma region in use by the plasma generator, the plasma generating active species that are entrained in the gas and decay in a decay region downstream of the plasma region, the catalyst being downstream of the plasma generator and at least partially inside the decay region.

2. The reactor according to claim 1 , wherein the plasma generator is arranged in use to provide the plasma region as a plasma veil across the gas flow path.

3. The reactor according to claim 2, wherein the plasma veil is a sheet of plasma in use.

4. The reactor according to any one of the preceding claims, wherein the plasma generator includes electrodes, an electric field being establishable between the electrodes in use, discharge being generated when an electric field strength is at least at a discharge ignition threshold.

5. The reactor according to claim 3 and claim 4, wherein the sheet has a thickness of up to three times the thickness of a plasma generator electrode.

6. The reactor according to any one of claims 4 or claim 5, wherein at least one of the electrodes is arranged in use to confine a length of the plasma region along the gas flow path of up to the shortest distance between the electrodes.

7. The reactor according to claim 6, wherein at least one electrode has a node, the sheet having a thickness of up to three times the thickness of the node along the gas flow path.

8. The reactor according to claim 7, where the thickness of the node along the gas flow path is a length up to the shortest distance between the electrodes,9. The reactor according to any one of claims 4 to 8, wherein the electrodes are arranged coaxially relative to the gas flow path.

10. The reactor according to any one of claims of 4 to 9, wherein there is a dielectric barrier between the electrodes, the discharge thereby being dielectric barrier discharge, the dielectric barrier being provided by a conduit providing the gas flow path.

11. The reactor according to any one of the preceding claims, wherein a length of the decay region along the gas flow path is determined by a gas flow rate.

12. The reactor according to any one of the preceding claims, wherein a length of the decay region along the gas flow path is determined by a temperature of the gas.

13. The reactor according to any one of the preceding claims, wherein a length of the decay region along the gas flow path is longer than a length of the plasma region along the gas flow path.

14. The reactor according to any one of the preceding claims, wherein there is a separation between the plasma reactor and the catalyst, the separation being larger than a length along the gas flow path of the plasma reactor.

15. The reactor according to claim 14, wherein the catalyst is up to 160 millimetres (mm) from the plasma generator.

16. The reactor according to claim 14, wherein the catalyst is up to 10 mm from the plasma generator.

17. The reactor according to claim 16, wherein the upstream end of the catalyst and the decay region are aligned on the gas flow path.

18. The reactor according to claim 14 or claim 15, wherein the catalyst is at least 30 mm from the plasma generator.

19. The reactor according to any one of the preceding claims, wherein the catalyst is located on a support.

20. The reactor according claim 19, wherein the support is a monolithic catalyst support.

21. The reactor according to claim 20, wherein the support is a cordierite monolithic support.

22. The reactor according to any one of the preceding claims, wherein the catalyst is an oxidation catalyst.

23. The reactor according to any one of the preceding claims, wherein catalyst is a methane oxidation catalyst.

24. The reactor according to any one of the preceding claims, further comprise a structure for electric field intensification for use in a dielectric barrier discharge device, the structure comprising: a body including at least one tip, the tip extending along a first radial axis passing through the centre of the body, wherein in use, the body is arranged around a first electrode of a discharge device, there being a gap between the structure and an opposing electrode of the discharge device, the at least one tip limiting a minimum gap between the structure and the opposing electrode, thereby increasing a probability of an electric breakdown occurring at the tip when an electric field is applied between the first electrode and the opposing electrode.

25. The reactor according to claim 24, wherein the at least one tip of the structure comprises a plurality of tips, each tip in the plurality of tips being arranged on the outer edge of the body.

26. The reactor according to claim 25, wherein the plurality of tips are uniformly distributed on the outer edge of the body.

27. The reactor according to claim 25 or claim 26, wherein the plurality of tips comprises 6 tips.

28. The reactor according to any one of the preceding claims, further comprising: a drive circuit for a dielectric barrier discharge device, the circuit comprising: a power supply connectable in use across a dielectric discharge gap, the dielectric discharge gap providing a capacitance; and an inductance between the power supply and the dielectric discharge gap when connected thereby establishing a resonant tank in use, wherein power is provided in use to the tank in pulse-trains and only during a pulse-train, a pulse frequency of each pulse-train being tuneable in use to a resonant frequency of the tank, power provided by each pulse-train charging and maintaining the tank to a threshold at which discharge ignition occurs, discharge ignition events per pulse-train being limited to a maximum number based on the drive circuit being arranged in use to prohibit each pulse-train transferring power to the resonant tank after the maximum number has occurred.

29. The reactor according to claim 28, wherein the maximum number of discharge ignition events is between 1 and 5 events.

30. The reactor according to claim 28 or claim 29, wherein the drive further comprises a power storage device connected across the power supply and arranged in use to accept and store power discharge from the tank after each pulse-train.31 . The reactor according to claim 30, wherein the drive circuit is arranged in use to shift the phase of the pulse-train by 180 degrees (°) after the maximum number of discharge ignition events has occurred.

32. A method of reducing a quantity of a pollutant in a gas, the method comprising: passing gas along a gas flow path; generating plasma in a plasma region through which the gas flow path passes, active species being formed by the plasma; entraining the active species in the gas, the active species decaying in a decay region downstream of the plasma region; andpassing the gas and active species over a catalyst, the catalyst being downstream of the plasma generator and at least partially inside the decay region.

33. The method according to claim 32, wherein the upstream end of the catalyst is aligned with the upstream end of the decay region, thereby aligning the upstream end of the catalyst with the downstream end of the plasma region.

34. The method according to claim 32, wherein there is a separation between the plasma reactor and the catalyst, the separation being larger than a length along the gas flow path of the plasma reactor.

35. Plasma-generated active species for use in reduction of a quantity of a pollutant in a gas by catalysis.

36. Sequential plasma and catalysis with plasma-generated active species for use in reducing a quantity of a pollutant in a gas.

37. A method of reducing quantity of a pollutant in a gas with plasma, catalysis and plasma-generated active species.

38. Use of active species for use in reduction of a quantity of a pollutant in a gas by catalysis.

39. Use of sequential plasma and catalysis with plasma-generated active species for use in reducing a quantity of a pollutant in a gas.

40. Use of sequential plasma and catalysis with plasma-generated active species at a separation of plasma generation and catalysis of greater than a length of a plasma generator for use in reducing a quantity of a pollutant in a gas.

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