GB2634029A

GB2634029A - Air purification

Info

Publication number: GB2634029A
Application number: GB2314752.3A
Authority: GB
Inventors: Mark Armstrong Kenneth
Original assignee: Dyson Technology Ltd
Current assignee: Dyson Technology Ltd
Priority date: 2023-09-26
Filing date: 2023-09-26
Publication date: 2025-04-02
Also published as: WO2025068827A1; GB202314752D0

Abstract

A system for treating ambient air to reduce air pollutants comprises a plasma generation unit to electrically charge the air pollutants. The plasma generator may be a dielectric barrier discharge (DBD) unit for generating a micro-plasma using a low voltage (typically < 1kV). The DBD may contain a dielectric coating comprising metal oxides and noble metal catalysts to aid in the degradation of the volatile organic compounds (VOCs) and other air contaminants. The plasma generation unit may not generate ozone (O³) or nitrogen oxides (NOx). The charged air pollutants may be electrostatically attracted and collected in a collection unit comprising an electrostatic precipitator. An electrostatic filter may be present which avoids pressure drop. The system may be used to remove or degrade dust, particulate matter, VOCs or other pollutants from ambient air.

Description

Air Purification

BACKGROUND

The problem of air pollution is becoming more and more serious and a variety of air pollutants are known or suspected to be harmful to human health. The negative effects that can be caused by air contaminants depend on the type and concentration of the contaminant, as well as the length of time of exposure to the contaminated air. For example, high levels of air pollution may immediately lead to health problems, such as exacerbation of cardiovascular and respiratory diseases, while long term exposure to polluted air may have permanent health effects, such as loss of lung capacity and reduced lung function, as well as the development of diseases, such as asthma, bronchitis, emphysema and possibly cancer.

Many people have recognized the benefit of minimizing their exposure to these pollutants and use air purification systems. These systems are for air treatment which removes airborne particulates, including pollutants, from the air. Known examples of air purifiers use particulate filters that physically capture airborne particles by size exclusion, with a high-efficiency particulate air (HEPA) filter removing at least 99.97% of particulates with a size of 0.3 micrometres. Other known examples of air purifiers ionise the air and generate negatively charged airborne particulates, allowing particulate filtration from the air to be improved by electrostatic attraction.

The negative impact of gas-phase pollutants, such as VOCs, on human health in indoor environments is also known. Many indoor pollutants found in residential settings are organic chemicals and span a wide range of functional groups, size and potential chemistries. These can be categorised as volatile organic compounds (VOCs) and volatile inorganic compounds (VICs). VOCs that humans are commonly exposed to in indoor environments include alcohols, aldehydes, aromatic compounds, ketones, alkenes and alkenes.

Various techniques have been investigated for the removal of VOCs from air, such as adsorption, catalytic thermal oxidation, cold plasma and photo-catalysis.

Ultimately, the aim is to improve performance of air purifiers and ensure fast and efficient removal of solid, liquid and gaseous air pollutants from the air.

SUMMARY

A disadvantage with known air purification systems that utilise mechanical filtration media to remove solid-and liquid-phase air pollutants from the ambient air is that higher efficiency with respect to the removal of air pollutants is correlated with increased pressure-drop across the system, making the system more energy intensive and expensive for a given processing rate of ambient air.

Accordingly, in a first aspect there is provided an air purification system for purification of ambient air, the air purification system comprising: a plasma-generation unit configured to generate a plasma in the ambient air to electrically charge air pollutants therein to provide electrically-charged air pollutants; and a collection unit configured to collect the electrically-charged air pollutants by electrostatic attraction between at least a portion of the collection unit and the electrically-charged air pollutants.

By providing an air purification system in which the collection unit is configured to collect air pollutants by electrostatic attraction between at least a portion of the collection unit and the air pollutants, the pressure drop across the collection unit can be made lower than is achievable with purely mechanical filtration systems for the same filtration efficiency. From an alternative viewpoint, the filtration efficiency for a given pressure drop is improved compared to purely mechanical filtration systems. By generating the electrically-charged air pollutants for the collection unit to collect with a plasma-generation unit, it is possible to omit an ioniser from the air purification system and consequently reduce the quantity of ozone molecules produced by the air purification system compared to if an ioniser was used. Accordingly, there is a reduced likelihood of ozone concentrations in the ambient air being treated by the system exceeding the WHO guideline values for ozone of 100 pg/m3 (51 ppb) 8 hr daily maximum and 60.tg/m3 (31 ppb) as an average of daily maximum time-weighted average over 8 hours in six months of peak season 03 concentration (World Health Organization. (2021). WHO global air quality guidelines: particulate matter (PM2.5 and PM10), ozone, nitrogen dioxide, sulphur dioxide and carbon monoxide: executive summary. World Health Organization.). Moreover, VOCs contained in the ambient air can be decomposed by reactive species within the plasma generated by the plasma-generation unit, thereby also reducing the concentration of VOCs in the ambient air alongside enhancing the removal of solid-and/or liquid-phase air pollutants.

The electrostatic attraction may include ion-dipole forces or ion-ion forces. It is not intended that the electrostatic attraction includes ion-induced dipole forces.

The electrically-charged air pollutants generated by the plasma-generation unit and collected by the collection unit may comprise solid-and/or liquid-phase air pollutants. The particulate air pollutants present in the ambient air in the plasma-generation unit to be electrically-charged may include solid-and/or liquid-phase particulates having an aerodynamic diameter of at least 0.1 micrometres.

The particulate air pollutants present in the ambient air in the plasma-generation unit to be electrically-charged may include solid-and/or liquid-phase particulates having an aerodynamic diameter of at least 0.2 micrometres, at least 0.3 micrometres, at least 0.4 micrometres, or at least 0.5 micrometres.

The ambient air may contain one or more VOCs. The plasma-generation unit may be configured to decompose the one or more VOCs contained in the ambient air. The plasma-generation unit may be configured to decompose the one or more VOCs contained in the ambient air such that VOC decomposition products and plasma by-products are formed (e.g. 03 and/or NOR).

The system and/or collection unit may not comprise a HEPA filter. The use of the plasma-generation unit to generate electrically-charged air pollutants and the use of a collection unit configured to collect the electrically-charged air pollutants by electrostatic attraction can provide sufficient removal of solid-and liquid-phase air pollutants from the ambient air passing through the system that a HEPA filter is not required, thereby allowing the pressure drop across the system and/or collection unit to be reduced compared to if a HEPA filter was present in the system and/or collection unit.

The plasma-generation unit may be configured such as to not generate ozone. This can be beneficial in reducing the exposure of users of the system to ozone, which is harmful to humans in high concentrations (see WHO exposure guidelines above).

The plasma-generation unit may be configured to generate a microplasma. As a result of the low voltages (typically around 1 kV) and small discharge gaps (typically < 1 mm) used to generate microplasmas compared to larger plasmas, the rate of formation of NOx and 03 by the plasma-generation unit is reduced compared to plasma-generation units generating larger plasmas.

A microplasma may be defined as a plasma having a discharge gap of less than or equal to (about) 1 mm and/or a plasma voltage (i.e. the voltage required to generate the plasma) of approximately 1 kV. Optionally, the discharge gap of a microplasma may be greater than or equal to 0.01 mm. Optionally, the discharge gap of a microplasma may be less than or equal to 0.5 mm, less than or equal to 0.25 mm, or less than or equal to 0.15 mm. Optionally, the discharge gap of a microplasma may be greater than or equal to 0.05 mm, greater than or equal to 0.09 mm, greater than or equal to 0.11 mm, greater than or equal to 0.20 mm, or greater than or equal to 0.4 mm. Optionally, a microplasma may be defined as a plasma having a plasma voltage of greater than or equal to 0.3 kV. The plasma voltage may be less than or equal to 1.9 kV. The plasma voltage may be greater than or equal to 0.8 kV. The plasma voltage may be less than or equal to 1.0 kV. The plasma voltage may be greater than or equal to 0.88 kV. The plasma voltage may be less than or equal to 0.92 kV.

The plasma generation unit may comprise a plurality of electrode members. The discharge gap may be measured between the opposing and adjacent surfaces of two adjacent electrode members. The plasma voltage may be the potential difference between two adjacent electrode members.

The plasma-generation unit may be a dielectric barrier discharge (DBD) plasma-generation unit. This facilitates generating plasma at lower voltages (about 1 kV) than typical and in small discharge gaps (< 1 mm) with low free-electron energy. Generating high-energy free electrons increases the possibility of generating various reactive species within the plasma-generation unit that then lead to NOx and 03 formation. Therefore, microplasma DBD plasma-generation units have the potential to reduce NOx and 03 formation when using plasma to treat ambient air in comparison to if the plasma-generation unit was not a microplasma DBD unit.

Where the plasma-generation unit is a DBD plasma-generation unit, the DBD plasma-generation unit may comprise: a first electrode member comprising an electrically conducting core and a dielectric coating; and a second electrode member disposed with respect to the first electrode member, so as to generate a plasma between the first and second electrode members on the application of a plasma generation voltage between the first and second electrode members. In this way, in use, VOCs in the ambient air in the plasma generation unit may be decomposed to form VOC decomposition products, and plasma byproducts may be formed. Such a microplasma-generation unit is able to effectively decompose VOCs in the ambient air whilst limiting the quantity of plasma by-products and partially-oxidised VOCs released by the plasma-generation unit compared to conventional plasma-generation units.

The dielectric coating may comprise a catalyst that catalyses the decomposition of one or more of: the VOCs; the VOC decomposition products; and the plasma by-products. This allows air pollutants to be removed from the airflow through the system via plasma-decomposition, catalyst-decomposition and electrostatic filtration, thereby increasing the efficiency of the air purification system compared to a system relying on only one of these mechanisms. Moreover, such a plasma-generation unit is able to effectively decompose VOCs in the ambient air whilst limiting the quantity of plasma by-products and partially-oxidised VOCs released by the plasma-generation unit compared to conventional plasma-generation units. The dielectric coating may comprise a dielectric material that is itself a catalyst (e.g. Mn02, A1203, CeO2, SiO2 and TiO2) and/or a catalyst (e.g. a Noble metal) may be provided on (e.g. impregnated in) the dielectric material of the dielectric coating.

The catalyst contained within the dielectric coating may comprise one or more of a metal oxide, a mixed metal oxide, a noble metal, a noble metal-metal oxide composite, and a noble metal-mixed metal oxide composite. This allows the composition of the catalyst within the dielectric coating to be tailored to the quantity and/or type of VOCs contained within the ambient air that require decomposition and/or the quantity and/or type of plasma by-products generated within the plasma-generation unit The catalyst may comprise one or more of Ag, Pt, Pd, Rh, Ni, Cu, Mo, Co, Mg and Ti. A catalyst comprising one or more of the above elements may be effective in catalysing the decomposition of one or more of the VOCs, the VOC decomposition products, and the plasma by-products.

The dielectric coating may comprise one or more metal oxides, optionally one or more of Mn02, A1203, CeO2, SiO2 and TiO2. Alternatively, the dielectric coating of an electrode member may comprise activated carbon or a zeolite. Such materials have a high surface area and can also act as a catalyst in the presence of a plasma. Noble metal catalysts, for example, one or more of Au, Ag, Tu, Rh, Pd and Pt, can be impregnated in the dielectric layer (e.g. the metal oxide coating layer).

The first electrode and second electrode may be arranged upstream and downstream of each other with respect to an airflow path through the plasma-generation unit. A plasma-generation unit having such a geometry is able to generate a plasma that spans a range of residence times within the plasma-generation unit and therefore catalyse a range of compounds within the plasma.

The second electrode member of the first aspect may comprise an electrically conducting core and a dielectric coating. Both electrodes having the structure of an electrically conducting core and a dielectric coating reduces the likelihood of arcing between the electrode members and thus reduces the likelihood of formation of a large quantity of plasma arc by-products compared to if no dielectric coating was provided on the second electrode member.

The dielectric coating of the second electrode member may also comprise a catalyst that catalyses the decomposition of one or more of: the VOCs; the VOC decomposition products; and the plasma by-products. This provides a greater surface area of catalyst to catalyse the decomposition of these compounds compared to if no catalyst was provided on the dielectric coating of the second electrode member. The dielectric coating may comprise a dielectric material that is itself a catalyst (e.g. Mn02, A1203, CeO2, SiO2 and TiO2) and/or a catalyst (e.g. a Noble metal) may be provided on (e.g. impregnated in) the dielectric material of the dielectric coating.

The dielectric coating of the first electrode member and/or the dielectric coating of the second electrode member may comprise a first catalyst and a second catalyst. The presence of two catalysts on an electrode member may allow each catalyst to be more specialised in catalysing the decomposition of a certain compound without overly compromising the catalysis of other compounds within the plasma-generation unit.

Alternatively, where both the first and second electrode members comprise a dielectric coating comprising a catalyst, the dielectric coating of the first electrode member may comprise a first catalyst and the dielectric coating of the second electrode may comprise a second catalyst. The first catalyst may be different from the second catalyst. This can allow the catalytic efficiency of the plasma-generation unit to be improved by tailoring the catalyst on a given electrode to the compounds in the vicinity of that electrode during operation of the plasma-generation unit.

Where the first and second electrode members are arranged upstream and downstream of each other with respect to the airflow path through the plasma-generation unit, the first catalyst may only be applied to the downstream electrode. Such a configuration may allow for efficient use of the first catalyst as, with respect to the midpoint between the two electrode members, the catalyst is positioned in the direction the reactive species generated in the plasma will travel through the plasma-generation unit.

The electrically conducting cores of each electrode member may comprise one or more of stainless steel, aluminium, brass, iron or copper. The composition of the electrically conducting cores can be tailored based on the desired conductivity, cost, weight, and ductility of the electrically conducting cores.

The dielectric coating of an electrode member may be porous or non-porous. For porous coating, any catalyst that the dielectric coating further comprises (e.g. an additional catalyst in the case that the dielectric material is itself a catalyst) may be disposed, at least in part, on internal surfaces of the pores of the dielectric coating. Such a structure can provide an increased surface area for the catalysis of the decomposition reactions occurring within the plasma-generation unit compared to if the dielectric coating is not porous.

Where the dielectric coating of an electrode member is porous and a catalyst is to be disposed, at least in part, on internal surfaces of the pores of the dielectric coating, the catalyst may be disposed on internal surfaces of the pores using an impregnation technique, such as, incipient wetness impregnation technique. The incipient wetness impregnation technique is technically simple, low cost, and produces limited amounts of waste, whilst also ensuring that catalyst is disposed within the pores of the substrate, meaning the overall thickness and uniformity of the coating is not impacted by the deposition of catalyst.

Porous dielectric coatings may present inhomogeneity in terms of wide range of cracks and pores into the film, may connected internally down to metal electrode, which may alter the discharge characteristics and breakdown mechanism. The impregnation of dielectric film with a catalyst may provide an additional benefit of pore/crack sealing, in this way, the porosity of coating film can be controlled from a relatively porous to non-porous, which is useful in maintaining the desired electrical, mechanical, and chemical characteristics.

The plasma-generation unit may comprise a plurality of first electrode members and a plurality of second electrode members, with each pair of adjacent first electrode members interposed by a second electrode member and each pair of adjacent second electrode members interposed by a first electrode member. Such a plasma-generation unit can process a larger volume of air than a plasma-generation unit having only a single first electrode member and a single second electrode member whilst maintaining the conversion rate of the plasma-generation unit by increasing the residence time of the air within the plasma-generation unit compared to a plasma-generation unit comprising only one pair of first and second electrode members.

The plasma-generation unit may further comprise a power supply, the power supply connected to each first electrode member and/or each second electrode member. The power supply so connected may be configured to deliver pulsed DC to each electrode member to which the power supply is connected such as to generate a plasma between a first electrode member and that first electrode's adjacent second electrode member. By providing pulsed DC to the electrode members connected to the power supply, the energy consumption of the plasma-generation unit can be reduced in comparison to using non-pulsed DC and additional control variables are provided for controlling the plasma discharge within the plasma-generation unit.

The plasma-generation unit may further comprise a third electrode member, the third electrode member being interposed between the first electrode member and the second electrode member. The third electrode member may incorporate one or more of the optional features of the first and/or second electrode members set out above.

The power supply may be configured such that the parameters of the pulsed DC that the power supply is configured to deliver to each electrode member to which the power supply is connected can be adjusted independently of the other electrode members to which the power supply is connected. This allows the plasma discharge between a pair of adjacent electrode members to be controlled independently of the plasma discharge between other pairs of adjacent electrode members.

Where the power supply is configured to deliver pulsed DC to each electrode member to which the power supply is connected, the pulsed DC received by each electrode member may: generate an electric field strength with applied voltage greater than or equal to 0.3 kV and less than or equal to 1.9 kV between that electrode member and an adjacent electrode member; have a pulse frequency greater than or equal to 0.5 kHz and less than or equal to kHz; and have a pulse width greater than or equal to 0.05 [is and less than or equal to 50 Rs. The plasma-generation unit being configured to provide such pulsed DC to each electrode member that the power supply is connected to facilitates the effective decomposition of VOCs in the ambient air whilst limiting the quantity of plasma by-products and partially-oxidised VOCs released by the plasma-generation unit compared to a case where the power supply does not provide such pulsed DC to the electrode members to which the power supply is connected.

In a plasma-generation unit comprising a power supply configured to deliver pulsed DC to each electrode member to which the power supply is connected, the pulsed DC received by each electrode member may have a voltage greater than or equal to 0.3 kV. The pulsed DC received by each electrode member may have a voltage less than or equal to 1.9 kV. The pulsed DC received by each electrode member may have a voltage greater than or equal to 0.8 kV. The pulsed DC received by each electrode member may have a voltage less than or equal to 1.0 kV. The pulsed DC received by each electrode member may have a voltage greater than or equal to 0.88 kV. The pulsed DC received by each electrode member may have a voltage less than or equal to 0.92 kV. Providing pulsed DC with such a voltage to each electrode member can be beneficial in generating a plasma to catalyse the decomposition of VOCs whilst limiting the presence of plasma by-products in air exiting the plasma-generation unit compared to a case where the power supply does not provide such pulsed DC to the electrode members to which the power supply is connected.

In a plasma-generation unit comprising a power supply configured to deliver pulsed DC to each electrode member to which the power supply is connected, the pulsed DC received by each electrode member may have a current amplitude greater than or equal to 0.01 A. The pulsed DC received by each electrode member may have a current amplitude less than or equal to 8 A. The pulsed DC received by each electrode member may have a current amplitude greater than or equal to 0.2 A. The pulsed DC received by each electrode member may have a current amplitude greater than or equal to 0.3 A. The pulsed DC received by each electrode member may have a current amplitude greater than or equal to 0.4 A The pulsed DC received by each electrode member may have a current amplitude less than or equal to 0.5 A. Providing pulsed DC with such a current amplitude to each electrode member can be beneficial in generating a plasma to catalyse the decomposition of VOCs whilst limiting the presence of plasma by-products in air exiting the plasma-generation unit compared to a case where the power supply does not provide such pulsed DC to the electrode members to which the power supply is connected.

In a plasma-generation unit comprising a power supply configured to deliver pulsed DC to each electrode member to which the power supply is connected, the pulsed DC received by each electrode member may have a pulse width greater than or equal to 0.05 Rs. The pulsed DC received by each electrode member may have a pulse width greater than or equal to 3 Rs. The pulsed DC received by each electrode member may have a pulse width less than or equal to 10 ps. The pulsed DC received by each electrode member may have a pulse width less than or equal to 50 Rs. The pulsed DC received by each electrode member may have a pulse width less than or equal to 15 Rs. The pulsed DC received by each electrode member may have a pulse width less than or equal to 5 ris. Providing pulsed DC with such a pulse width to each electrode member can be beneficial in generating a plasma to catalyse the decomposition of VOCs whilst limiting the presence of plasma by-products in air exiting the plasma-generation unit compared to a case where the power supply does not provide such pulsed DC to the electrode members to which the power supply is connected.

In the plasma-generation unit, each electrode member may be a perforated plate. Providing the electrode members as perforated plates facilitates a large amount of contact between the ambient air and electrode members, increasing the conversion of the plasma-generation unit. Moreover, perforated plate structures are easy to manufacture and provide electrode members with a high surface-area-to-volume ratio, reducing redundant material within the electrode member. A perforated plate may comprise a regular array of perforations or may comprise an irregular arrangement of perforations. The perforated plates may have an open area percentage greater than or equal to 10 %. The perforated plates may have an open area percentage less than or equal to 35 %. The perforated plates may have an open area percentage greater than or equal to 22 %. The perforated plates may have an open area percentage less than or equal to 90%. The perforated plates may have an open area percentage greater than or equal to 24.5 %. The perforated plates may have an open area percentage less than or equal to 25.5%. The open area percentage may be set such as to balance the pressure drop across the plasma-generation unit with the surface area of the electrode members that can be used for catalysis and plasma generation.

Where the electrode members are plate-shaped and offset from each other in an axial direction of the plate-shaped electrode members, the first catalyst may only be applied to the face of an electrode member that is adjacent another electrode member within the electrode group. This positioning of catalyst makes efficient use of the catalyst by positioning the catalyst close to the centre of a discharge zone in which plasma is generated in the plasma-generation unit.

Where the electrode members are perforated plates, those perforated plates may be curved and/or bent. Having curved and/or bent perforated plates can facilitate different plasma-generation unit geometries.

The plasma-generation unit may further comprise a non-conductive separator positioned between adjacent electrode members. The non-conductive separator may extend around at least a portion of the circumference of the electrode members. Such a non-conductive separator can electrically isolate the electrode members from each other, and from the remainder of the plasma-generation unit. The non-conductive separator may comprise PTFE or another electrically-insulating polymer, e.g. other polymers with about the same hardness and/or electrical resistance as PTFE.

In a plasma-generation unit in which each electrode member is a perforated plate, adjacent plate electrode members may be substantially parallel to each other and offset from each other by a distance of 1 mm or less between adjacent surfaces of the adjacent electrode members. The offset distance between adjacent electrode members may be greater than or equal to 0.01 mm. The offset distance between adjacent electrode members may be less than or equal to 1 mm. The offset distance between adjacent electrode members may be less than or equal to 0.5 mm. The offset distance between adjacent electrode members may be less than or equal to 0.25 mm. The offset distance between adjacent electrode members may be less than or equal to 0.15 mm. The offset distance between adjacent electrode members may be greater than or equal to 0.05 mm. The offset distance between adjacent electrode members may be greater than or equal to 0.09 mm. The offset distance between adjacent electrode members may be less than or equal to 0.11 mm. The offset distance between adjacent electrode members may be greater than or equal to 0.20 mm. The offset distance between adjacent electrode members may be greater than or equal to 0.4 mm. Providing such an offset distance between adjacent plate electrode members facilitates the generation of a plasma with a voltage range of approximately 0.3 to 1.9 kV and with low electron energy, resulting in the formation of fewer plasma by-products than at higher voltages and electron energies.

The system may further comprise an adsorption unit connected to the plasma-generation unit. The adsorption unit may be connected to the plasma-generation unit such that the adsorption unit is directly downstream of the plasma-generation unit and thus the air exiting the plasma-generation unit flows directly into the adsorption unit (i.e. without entering the ambient air). The adsorption unit may be configured to adsorb one or more of: the VOCs, the VOC decomposition products, and the plasma by-products, contained in an airflow out of the plasma-generation unit during use of the plasma-generation unit. Advantageously, including an adsorption unit downstream of the plasma-generation unit can reduce the concentration of VOCs, VOC decomposition products and/or plasma by-products contained within the airflow released into the ambient air having passed through the plasma-generation unit. Moreover, long-lived plasma species reaching the adsorption unit from the plasma-generation may decompose VOCs and VOC decomposition products adsorbed onto an adsorbent within the adsorption unit, thereby increasing the lifetime of the adsorbent.

The system may further comprise a catalysis unit that is connected to the plasma-generation unit. The catalysis unit may be connected to the plasma-generation unit such that the catalysis unit is directly downstream of the plasma-generation unit and thus the air exiting the plasma-generation unit flows directly into the adsorption unit (i.e. without entering the ambient air). The catalysis unit may be configured to catalyse the decomposition of one or more of: the VOCs, the VOC decomposition products, and the plasma by-products, contained in an airflow out of the plasma-generation unit during use of the plasma-generation unit. Advantageously, the inclusion of a catalysis unit downstream of the plasma-generation unit can allow the VOC conversion achieved by the air purification device to be further increased beyond that achieved in the plasma-generation unit. Moreover, long-lived plasma species reaching the catalysis unit from the plasma-generation unit may enhance decomposition of VOCs and VOC decomposition products on the surface of a catalyst in the catalysis unit, thereby further reducing the concentration of VOCs and VOC decomposition products contained within the airflow released into the ambient air having passed through the plasma-generation unit. The catalysis unit may be operated at room temperature or at a higher temperature, depending on the balance of energy consumption and conversion desired.

The system may comprise both the adsorption unit and the catalysis unit in combination with the plasma-generation unit. Where both the adsorption unit and catalysis unit are present, the catalysis unit may be positioned directly downstream of the plasma-generation unit and directly upstream of the adsorption unit, such that the air exiting the plasma-generation unit flows directly into the catalysis unit (i.e. without entering the ambient air) and the air exiting the catalysis unit flows directly into the adsorption unit (i.e. without entering the ambient air). Alternatively, where both the adsorption unit and catalysis unit are present, the adsorption unit may be positioned directly downstream of the plasma-generation unit and directly upstream of the catalysis unit, such that the air exiting the plasma-generation unit flows directly into the absorption unit (i.e. without entering the ambient air) and the air exiting the adsorption unit flows directly into the catalysis unit (i.e. without entering the ambient air).

The adsorption unit and catalysis unit may be combined into a single unit by doping catalyst onto an adsorbent contained within the adsorption unit. In this way, the oxidation of adsorbed compounds (including VOCs, VOC decomposition products and plasma by-products) can be accelerated compared to if the adsorbent was not doped with catalyst.

The adsorption and/or catalyst unit(s) may be configured not to remove the electrically-charged air pollutants generated by the plasma-generation unit from the airflow therethrough, for example, by the adsorption and/or catalyst unit being charge neutral and 30 non-electrostatic.

Where the system comprises an adsorption unit and/or catalysis unit, the system may be configured to be operated under a continuous storage-discharge operation in which air flows through the plasma-generation unit and the adsorption unit and/or catalysis unit once before exiting to the ambient air. Advantageously, this can increase the flowrate of ambient air that can be introduced into the plasma.

Alternatively, where the system comprises an adsorption unit and/or catalysis unit, the air purification device may be configured to be operated under a cyclic storage-discharge operation in which a proportion of the air exiting the adsorption unit and/or catalysis unit is recycled back into the plasma-generation unit before exiting to the ambient air.

Advantageously, this can increase the overall VOC conversion achieved by the system.

Within the air purification system, the collection unit may comprise an electrostatic precipitator. Thus charged air pollutant particles generated by the plasma-generation unit can be collected by the collection unit and removed from the ambient air and such a collection unit provides a low pressure drop for the corresponding collection efficiency compared to purely mechanical filtration media.

Where the collection unit of the air purification system comprises an electrostatic precipitator, the electrostatic precipitator may comprise: one or more primary collection elements; and a primary voltage source configured to supply a positive DC voltage to the one or more collection elements. In this way, in use, negatively-charged air pollutants can be electrostatically attracted to the primary collection elements. The plasma-generation unit will preferentially generate negatively charged air pollutants, and accordingly it may be beneficial for the electrostatic precipitator to include positively charged primary collection elements.

The electrostatic precipitator may comprise one or more secondary collection elements and a secondary voltage source configured to supply a negative DC voltage to the one or more collection elements. In this way, in use, positively-charged air pollutants can be electrostatically attracted to the secondary collection elements. This can allow the electrostatic precipitator to collect positively charged air pollutants in addition to negatively charged air pollutants. Alternatively, the electrostatic precipitator may not comprise the one or more secondary elements and the secondary voltage source. Omitting the secondary elements and the secondary voltage source facilitates making the electrostatic precipitator more compact compared to if the secondary elements and secondary voltage source are present.

The electrostatic precipitator may not comprise a charging section (e.g. an air ioniser). This facilitates making the electrostatic precipitator more compact. Typically, a charging section / air ioniser is unnecessary in the system according to the first aspect because charged air pollutant particles are already generated by the plasma-generation unit.

The primary collection elements may comprise a plurality of substantially parallel plates and the electrostatic precipitator may be configured such that an airflow through the collection unit is in a direction substantially parallel to the plates. Where present, the secondary collection elements may comprise a plurality of substantially parallel plates, and the electrostatic precipitator may be configured such that an airflow through the collection unit is in a direction substantially parallel said plates. The secondary collection elements may be interspaced with the primary collection elements (e.g. for each pair of primary collection elements, there may be a secondary collection element interposed therebetween). Where the electrostatic precipitator comprises a single primary collection element and a single secondary collection element, the primary and secondary collection elements may be substantially parallel plates, and the electrostatic precipitator may be configured such that an airflow through the collection unit is in a direction substantially parallel to said plates.

The collection unit of the air purification system according to the first aspect may comprise an electrostatic filter medium. The electrostatic filter medium may be an electret filter medium, for example, a space charge electret or a dipolar electret. The electret filter medium may be an electrostatically charged synthetic non-woven material. The electret filter medium may be produced using thermal charging, the corona effect or the triboelectric effect. By making use of an electrostatic filter medium within the collection unit, a means for collecting electrically-charged air pollutants by electrostatic attraction that does not consume power is provided.

The electrostatic filter medium may comprise one or more positively statically-charged regions separated from one or more negatively statically-charged regions. Accordingly, the electrostatic filter medium may be effective in collecting both positively-electrically-charged and negatively-electrically-charged air pollutants by electrostatic attraction between at least a portion of the collection unit and the electrically-charged air pollutants.

In the air purification system according to the first aspect, the plasma-generation unit may be comprised in a plasma-generation device; the collection unit may be comprised in a collection device; and the plasma-generation device may be separate to the collection device such that said devices can be positioned independently of each other. That is, the system may comprise said two devices to position independently in the volume of ambient air (e.g. a room) to be purified by the system. By the plasma-generation unit and collection unit being provided in separate devices, the electrically-charged air pollutants from the plasma-generation unit enter the surroundings before passing through the collection unit. This provides the possibility for electrically-charged air pollutants to be removed from the ambient air in the surroundings by agglomeration and settling or attraction to surfaces within the surroundings rather than passing into the collection unit. Accordingly, the required capacity of the collection unit is reduced compared to if the airflow from the plasma-generation unit flows directly into the collection unit, because fewer electrically-charged air pollutants from the plasma-generation unit subsequently enter the collection unit, allowing the collection unit to be made more compact.

In a second aspect there is provided an air purification apparatus comprising: an air inlet; an air outlet; and an air purification system according to the first aspect; wherein, in use, ambient air flows along an air pathway passing into the apparatus via the air inlet, through the air purification system, and out of the apparatus via the air outlet.

By providing an air purification apparatus in which the collection unit is configured to collect air pollutants by electrostatic attraction between at least a portion of the collection unit and the air pollutants, the pressure drop across the collection unit can be made lower than is achievable with purely mechanical filtration systems for the same filtration efficiency. From an alternative viewpoint, the filtration efficiency for a given pressure drop is improved compared to purely mechanical filtration systems. By generating the electrically-charged air pollutants for the collection unit to collect with a plasma-generation unit, it is possible to omit an ioniser from the air purification system and consequently reduce the quantity of ozone molecules produced by the air purification system. Moreover, VOCs contained in the ambient air can be decomposed by reactive species within the plasma generated by the plasma-generation unit, thereby also reducing the concentration of VOCs in the ambient air alongside enhancing the removal of solid-and/or liquid-phase air pollutants.

Any one or more of the optional features set out with respect to the first aspect may be incorporated into the second aspect, except where such a combination of optional features is clearly impermissible or expressly avoided.

The air purification apparatus may comprise a housing containing the air purification system according to the first aspect and defining the air inlet and air outlet.

The air purification apparatus may further comprise an airflow generation unit (e.g. a fan assembly or a compressor, such as a motor-driven impeller) configured to drive an air flow along the air pathway. Consequently, a greater flowrate of air can be processed by the apparatus in comparison to where air flow along the air pathway is driven by natural convection, diffusion or the like.

The plasma-generation unit may be positioned upstream of the collection unit along the air pathway through the apparatus. This arrangement results in electrically-charged air pollutants from the plasma-generation unit entering the collection unit before said electrically-charged air pollutants reach the air outlet. Accordingly, the probability of an electrically-charged air pollutant generated by the plasma-generation unit being collected by the collection unit is increased compared to an arrangement where the plasma generation unit is positioned downstream of the collection unit along the air pathway through the apparatus.

Alternatively, the plasma-generation unit may be positioned downstream of the collection unit along the air pathway through the apparatus. This arrangement results in electrically-charged air pollutants from the plasma-generation unit exiting the apparatus via the air outlet into the surroundings and the electrically-charged air pollutants entering the collection unit being drawn into the apparatus through the air inlet from the surroundings. This arrangement provides the possibility for electrically-charged air pollutants to be removed from the ambient air in the surroundings by agglomeration and settling and/or attraction to surfaces in the surroundings rather than being drawn into the collection unit. Accordingly, the required capacity of the collection unit is reduced compared to a configuration where the plasma-generation unit is positioned directly upstream of the collection unit along the air pathway, because fewer electrically-charged air pollutants from the plasma-generation unit subsequently enter the collection unit, thereby allowing the collection unit to be made more compact.

The apparatus may further comprise a particulate filter positioned upstream of the plasma-generation unit and/or collection unit along the air pathway through the apparatus (e.g. a non-electrostatic particulate filter). This can reduce the quantity of large (e.g. aerodynamic diameter > 5 pm) solid and/or liquid particles entering these units. Where the collection unit is upstream of the plasma-generation unit along the air pathway and the collection unit comprises an electrostatic filter medium, the particulate filter may be omitted from the apparatus without the quantity of large solid and/or liquid particles entering the plasma-generation unit increasing substantially. However, the inclusion of a particulate filter (for example, a coarse pre-screen filter or a mesh grille, but typically not a HEPA filter (e.g. a filter with a lower filtration efficiency than a HEPA filter)) may substantially reduce the quantity of large solid and/or liquid particles entering the plasma-generation unit where either the plasma-generation unit is positioned upstream of the collection unit along the air pathway or the collection unit comprises an electrostatic precipitator rather than an electrostatic filter medium.

The air purification apparatus may be a fan assembly, e.g. an indoor fan, such as a desk fan or floor fan.

In a third aspect there is provided a method of treating ambient air to reduce a concentration of air pollutants using an air purification system, the system comprising: a plasma-generation unit; and a collection unit; wherein the method comprises the steps of: passing an airflow containing air pollutants through the plasma-generation unit; generating a plasma in the ambient air using the plasma-generation unit to electrically charge the air pollutants to provide electrically-charged air pollutants; and passing an airflow containing the electrically-charged air pollutants through the collection unit to collect the electrically-charged air pollutants by electrostatic attraction between at least a portion of the collection unit and the electrically-charged air pollutants.

The method according to the third aspect is able to provide higher efficiency of solid-and/or liquid-phase air pollutant removal for a given pressure drop than purely mechanical filtration by utilising electrostatic attraction, whilst al so avoiding the use of an ioniser to generate the electrically-charged air pollutants, thereby reducing the quantity of ozone molecules produced when executing the method. Moreover, VOCs contained in the ambient air can be decomposed by reactive species within the plasma generated by the plasma-generation unit, thereby also reducing the concentration of VOCs in the ambient air alongside enhancing the removal of solid-and/or liquid-phase air pollutants.

Any one or more of the optional features set out with respect to the first aspect and/or second aspect may be incorporated into the third aspect, except where such a combination of optional features is clearly impermissible or expressly avoided.

The air pollutants contained in the airflow passed through the plasma-generation unit may comprise solid-and/or liquid-phase air pollutants.

The electrically-charged air pollutants generated by the plasma-generation unit and collected by the collection unit may comprise solid-and/or liquid-phase air pollutants.

The air pollutants contained in the airflow passed through the plasma-generation unit may comprise one or more VOCs.

In a fourth aspect there is provided a method of treating ambient air to reduce a concentration of air pollutants using an air purification system, the system comprising a plasma-generation unit, the method comprising the steps of: passing an airflow containing particulate air pollutants through the plasma-generation unit; generating a plasma in the ambient air using the plasma-generation unit to electrically charge the particulate air pollutants to provide electrically-charged air pollutants; wherein the particulate air pollutants present in the airflow include solid-and/or liquid-phase particulates having an aerodynamic diameter of at least 0.1 micrometres.

The method according to the fourth aspect is able to utilise a plasma both for the purpose of generating electrically-charged solid-and/or liquid-phase air pollutants so that these pollutants can subsequently be removed from the airflow by electrostatic attraction, and for the purpose of promoting the decomposition of VOCs in the airflow via reaction with reactive species present in the plasma, thereby reducing the concentration of VOCs in the airflow.

The median mass aerodynamic diameter of the solid-and/or liquid-phase particulates present in the airflow may be at least 0.1 micrometres.

The airflow containing particulate air pollutants passed to the plasma generation unit may have a concentration of particulate air pollutants of diameter 2.5 micrometres or less (PM2.5) of 12 µg/m3 or more, 21 µg/m3 or more, 29 µg/m3 or more, 36 µg/m3 or more, 43 pg/m3 or more, 49 µg/m3 or more, 54 pg/m3 or more, 63 pg/m3 or more, 71 µg/m3 or more, 1 1 1 p.g/m3 or more, 151 µg/m3 or more, 201 µg/m3 or more, 251 µg/m3 or more, or 376 pg/m3 or more.

The airflow containing particulate air pollutants passed to the plasma generation unit may have a concentration of particulate air pollutants of diameter 2.5 micrometres or less (PM2.5) of 500 µg/m3 or less, 375 µg/m3 or less, 250 p.g/m3 or less, 200 µg/m3 or less, 150 µg/m3 or less, 110 µg/m3 or less, 70 µg/m3 or less, 62 µg/m3 or less, 53 pg/m3 or less, 48 µg/m3 or less, 42 pg/m3 or less, 35 µg/m3 or less, 28 pg/m3 or less or 20 µg/m3 or less.

The airflow containing particulate air pollutants passed to the plasma generation unit may have a concentration of particulate air pollutants of diameter 10 micrometres or less (PM10) of 17 pg/m3 or more, 29 pg/m3 or more, 40 Rg/m3 or more, 51 pg/m3 or more, 60 µg/m3 or more, 68 pg/m3 or more, 76 µg/m3 or more, 89 pg/m3 or more, 101 g/m3 or more, 226 pg/m3 or more, 351 pg/m3 or more, 368 pg/m3 or more, 421 pg/m3 or more, or 511 pg/m3 or more.

The airflow containing particulate air pollutants passed to the plasma generation unit may have a concentration of particulate air pollutants of diameter 10 micrometres or less (PM10) of 600 pg/m3 or less, 510 pg/m3 or less, 420 µg/m3 or less, 385 µg/m3 or less, 350 pg/m3 or less, 225 gg/m3 or less, 100 itg/m3 or less, 88 µg/m3 or less, 75 pg/m3 or less, 67 itg/m3 or less, 59 µg/m3 or less, 50 gg/m3 or less, 39 pg/m3 or less, or 28 pg/m3 or less.

The diameter of particulate air pollutants may be defined as the aerodynamic diameter of the air pollutants.

Any one or more of the optional features set out with respect to the first aspect, second aspect, and/or third aspect may be incorporated into the fourth aspect, except where such a combination of optional features is clearly impermissible or expressly avoided.

In a fifth aspect there is provided use of a plasma generated in ambient air by a plasma-generation unit to electrically charge air pollutants to provide electrically-charged air pollutants for subsequent collection in a collection unit, the air pollutants being entrained in a flow of ambient air incident at the plasma-generation unit and the electrically-charged air pollutants being conveyed by the flow to the collection unit.

The method according to the fifth aspect is able to use a plasma both for the purpose of generating electrically-charged solid-and/or liquid-phase air pollutants so that these pollutants can subsequently be removed from the airflow by electrostatic attraction, and for the purpose of promoting the decomposition of VOCs in the airflow via reaction with reactive species present in the plasma, thereby reducing the concentration of VOCs in the airflow.

Any one or more of the optional features set out with respect to the first aspect, second aspect, third aspect and/or fourth aspect may be incorporated into the fifth aspect, except where such a combination of optional features is clearly impermissible or expressly avoided.

The anhydrous gaseous room temperature composition of the ambient airflow containing air pollutants into the plasma-generation unit may be: 21% by mass 02, and 78% by mass N2, the balance comprising VOCs, Ar, CO2 and further comprising trace elements and molecules. Ambient air (e.g. having a temperature greater than or equal to 0°C and less than or equal to 40°C, more typically greater than or equal to 15°C and less than or equal to 25°C, having an absolute pressure greater than or equal to 80 kPa and less than or equal to 120 kPa, typically about 100kPa) can alternatively be referred to as atmospheric air and is distinguished from e.g. exhaust gases from a vehicle by its composition. The system and/or any air purification apparatus it forms part of is preferably not configured for use in vehicle architecture (e.g. not configured for use in treating exhaust gases from an engine of a vehicle). For example, in an air purification apparatus that the system forms part of, the airflow generator may be positioned downstream of the system (i.e. the opposite configuration to an exhaust treatment system of a vehicle). Additionally, or alternatively, the catalyst module and/or adsorbent modules may be configured to catalyse and adsorb, respectively, VOCs at low temperatures (e.g. less than or equal to 40°C). The system and/or air purification apparatus may be configured for use in a building environment, e.g. inside a building. By way of example, the system and/or air purification apparatus may be for, or form part of, a fan, e.g. a desk fan or floor fan.

The present disclosure includes the combination of the aspects and preferred features described except where such a combination is clearly impermissible or expressly avoided.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 illustrates a first air purification system comprising a plasma generation unit and a collection unit; Figure 2 illustrates a first plasma-generation unit that may be incorporated into the system of Figure 1; Figure 3 illustrates a second plasma-generation unit that may be incorporated into the system of Figure 1; Figures 4 illustrates a third plasma-generation unit that may be incorporated into the system of Figure 1; Figure 5 illustrates the different controllable parameters of pulsed DC provided by a power supply to a plasma-generation unit on a voltage-time graph; Figure 6 illustrates a first electrostatic precipitator that may be incorporated into the system of Figure 1 within the collection unit; Figure 7 illustrates a second electrostatic precipitator that may be incorporated into the system of Figure 1 within the collection unit; Figures 8A -8C illustrate first and second air purification apparatus that may incorporate the air purification system of Figure 1 and an alternative configuration of the air purification system, respectively; Figure 9 is a schematic of an air purification apparatus; Figure 10 provides a flowchart for a method of treating ambient air to reduce a concentration of air pollutants therein; Figure 11A is a process flow diagram for a plasma-generation unit combined with an adsorption unit; Figure 11B is a process flow diagram for a plasma-generation unit combined with a catalysis unit; Figure 11C is a process flow diagram for a plasma-generation unit combined with a catalysis unit and an adsorption unit.

Figure 12 contains chromatograms corresponding to the plasma-generation unit feed and effluent streams for Comparative Reference Example 1 in Table 3; and Figure 13 contains chromatograms corresponding to the plasma-generation unit feed and effluent streams for Reference Example 2 in Table 3.

DETAILED DESCRIPTION

Aspects, embodiments and experiments relating to the present disclosure will now be discussed with reference to the accompanying figures. Further aspects and embodiments will be apparent to those skilled in the art. All documents mentioned in this text are incorporated herein by reference.

Figure 1 is a schematic diagram of a first air purification system 10. The air purification system comprises a plasma-generation unit 1100 and a collection unit 1200. Figure 1 illustrates an airflow of ambient air through each of said units by the arrows 1 leading into and out of each unit. Typically, the airflow through each unit is provided by an airflow generation unit (e.g. a fan assembly) that drives an airflow through the unit. The use of an airflow generation unit within the system is discussed further in relation to Figure 8B.

However, it is also envisaged that the airflow may be driven by natural convection, diffusion or the like, rather than by an airflow generation unit.

The ambient air flowing into each unit will typically comprise a combination of solid-and/or liquid-phase air pollutants (e.g dust, aerosols, etc.) and also gas-phase air pollutants such as 30 VOCs.

The plasma-generation unit 1100 is configured to generate a plasma in the ambient air flowing through the plasma-generation unit 1100. The generation of a plasma in the ambient air has two effects with regard to the air pollutants contained in the ambient air.

Firstly, the plasma contains several highly-reactive species that are effective in reacting with, and thereby decomposing, VOCs contained in the ambient air in the plasma-generation unit 1100. Consequently, the plasma-generation unit 1100 reduces the concentration of VOCs in the ambient air flowing through the plasma-generation unit 1100.

Secondly, the plasma contains several different charged species (e.g. free electrons, hydroxide ions, 02 etc.) that can interact with solid-and/or liquid-phase air pollutants within the ambient air in order to form electrically-charged solid-and/or liquid-phase air pollutants that are sufficiently long-lived to exit the plasma-generation unit. Consequently, the plasma-generation unit 1100 discharges an airflow containing a plurality of electrically-charged air pollutants. Typically, because of the high mobility of free electrons in comparison to larger charged species such as helium nuclei (which are positively charged), the electrically-charged air pollutants generated by the plasma-generation unit 1100 are predominantly negatively charged. The plasma-generation unit 1100 is discussed in more detail in relation to Figures 2 to 5.

The collection unit 1200 is configured to collect the electrically-charged air pollutants produced by the plasma-generation unit 1100.The collection unit 1200 does so by the airflow into the collection unit 1200 containing electrically-charged air pollutants and the collection unit 1200 being configured to generate electrostatic attraction between at least a portion of the collection unit 1200 and those electrically-charged air pollutants. By way of example, this electrostatic attraction may be generated by the collection unit comprising an electrostatic precipitator or an electrostatic filter medium, both of which are discussed further in relation to Figures 6 and 7. By the collection unit 1200 separating out the electrically-charged air pollutants from the airflow through the unit by electrostatic attraction to the collection unit 1200, the collection unit 1200 reduces the concentration of solid-and/or liquid-phase air pollutants in the ambient air flowing through the collection unit 1200. By utilising electrostatic attraction, the collection unit 1200 can provide the same reduction in concentration of such air pollutants as purely mechanical filtration media for a lower pressure drop across the collection unit 1200. Where the airflow through the collection unit 1200 is driven by a powered airflow generation unit, this reduction in pressure drop reduces the power consumption of the airflow generation unit for a given flowrate of air.

The system 10 may be configured such that the plasma-generation unit 1100 and collection unit 1200 are provided in a unitary air purification apparatus/device. However, the plasma-generation unit 1100 and collection unit 1200 may alternatively be provided in separate devices to each other, e.g. a plasma-generation device and a collection device, respectively.

This is discussed further in relation to Figures 8A -8C.

Figure 2 provides a schematic of a first plasma-generation unit 2100 in a cross-section lying substantially parallel to an air pathway though the unit 2100 (illustrated by the thick arrows 1 in Figure 2). The first plasma-generation unit 2100 may be incorporated into the system 10 of Figure 1. The plasma-generation unit 2100 has a first electrode member 2110 and a second electrode member 2120 separated by a discharge gap 2140 between their adjacent surfaces. The first electrode member 2110 comprises an electrically conducting core 2111 and a dielectric coating 2112. In the plasma-generation unit 2100 of Figure 2, the second electrode member 2120 also comprises a respective electrically conducting core 2121 and a dielectric coating 2122. Accordingly, the plasma-generation unit 2100 is a dielectric barrier discharge (DBD) unit. With both electrode members 2110, 2120, the dielectric coating 2112, 2122 of the electrode member 2110, 2120 fully coats the electrically conducing core 2111, 2112 of that electrode member 2110, 2120, such that no portion of the core 2111, 2112 is exposed on an exterior surface of that electrode member 2110, 2120. As is described below in greater detail, the dielectric coating of the first electrode member 2110 and the second electrode member 2120 may optionally comprise one or more catalysts.

A power supply 2150 is connected to the electrically conducting core 2111 of the first electrode member 2110, and the electrically conducting core 2121 of the second electrode member 2120 is connected to earth 2155. Current is provided from the power supply 2150 to the first electrode member such that a potential difference is provided between the electrode members 2110, 2120 and an electric field is generated across the discharge gap 2140. The power supply 2150 and the current provided to the electrode members 2110, 2120 is discussed further in relation to Figure 5. The potential difference applied between the electrode members 2110, 2120 and the strength of the resulting electric field in the discharge gap 2140 can be made great enough that a plasma is generated in the discharge gap 2140 and in the vicinity of the electrode members 2110, 2120 (i.e. adjacent the surfaces of the electrode members 2110, 2120 not adjacent the discharge gap 2140), which together form a plasma discharge zone of the plasma-generation unit 2100. The highly reactive species generated within the plasma are effective in decomposing VOCs contained in ambient air in the plasma-generation unit and the electrically-charged species generated within the plasma can interact with solid-and/or liquid-phase air pollutants flowing through the plasma discharge zone in order to generate electrically-charged air pollutants.

Although the core 2121 of the second electrode member 2120 in Figure 2 is connected to earth 2155, other configurations are possible provided that a potential difference can be generated between the first electrode member 2110 and second electrode member 2120 in order to generate a plasma, for example, it is also possible for the cores 2111, 2121 of both the first electrode member 2110 and the second electrode member 2120 to be connected to opposite terminals of a power supply 2150, or for the core 2111 of the first electrode member 2110 to be connected to earth 2155 and the core 2121 of the second electrode member 2120 to be connected to a power supply 2150.

In Figure 2, both the first electrode member 2110 and second electrode member 2120 take the form of perforated plates, with the first electrode member 2110 having a plurality of perforations 2113 arranged in a regular array across the first electrode member's planar surface and the second electrode member 2120 also having a plurality of perforations 2123 arranged in a regular array across the second electrode member' s planar surface. In the case of Figure 2, the perforations 2113 in the first electrode member 2110 are aligned with the perforations 2123 in the second electrode member 2120 across the discharge gap 2140 that separates the electrode members 2110, 2120. Where there is an airflow through the electrode members 2110, 2120 in the direction in which the perforations 2113, 2123 extend through the electrode members 2110, 2120 (as illustrated by the thick arrows 1 in Figure 2) the alignment of the perforations 2113, 2123 of adjacent electrode members 2110, 2120 can reduce the pressure drop across the unit 2100. However, by misaligning the perforations 2113, 2123 of adjacent electrode members 2110, 2120, the flow through the unit 2100 can be made more turbulent and the residence time and mixing of air in the discharge gap 2140 can be increased. When a plasma is generated in the unit 2100 in the vicinity of the electrode members 2110, 2120, increasing the residence time and mixing of air in the discharge gap 2140 can increase the VOC conversion achieved by the plasma-generation unit 2100 and can increase the proportion of the solid-and/or liquid-phase air pollutants that become electrically charged. For similar reasons, it is desirable that the feed airflow into the plasma-generation unit 2100 is controlled such that the flowrate of air through the perforations 2113, 2123 is uniform across the area of the electrode members 2110, 2120, as opposed to the flowrate through the perforations 2113, 2123 closer to the centre of the electrode members 2110, 2120 being substantially larger than the flowrate through the perforations 2113, 2123 closer to the edge of the electrode members 2110, 2120.

In a plasma-generation unit 2100 such as that illustrated in Figure 2, the cores 2111, 2121 of the plate electrode members 2110, 2120 may have a thickness of approximately 1 mm and the pitch of the apertures may be approximately 5 mm, with a 3 mm hole diameter, providing the cores 2111, 2112 with an open area of approximately 32.7 %. The dielectric coatings 2112, 2122 are then uniformly applied to the cores 2111, 2121 and may have a thickness of approximately 200 p.m, reducing the open area of the resulting electrode member 2110, 2120 to approximately 24.6 %. The width of the discharge gap 2140 (i.e. the distance between adjacent surfaces of the adjacent electrode members) is typically less than 1 mm, and preferably approximately 0.1 mm, in order to allow a plasma to be formed at a low voltage and with a low electron energy such that the formation of plasma by-products is limited (discussed below in relation to the Tests and Examples in Tables 1 -3).

The cores 2111, 2121 of the electrode members 2110, 2120 can be formed from any electrically conductive material; however, it is preferable that the core 2111, 2121 comprises one or more of stainless steel, aluminium, brass, iron or copper. These metals and metal alloys have a high conductivity and are readily available. Aluminium, copper and brass may be particularly advantageous in being lightweight and highly ductile in comparison to stainless steel and iron. Another factor in the selection of the material for the cores 2111, 2121 is the fraction of the power supplied to the electrode member that is lost as thermal energy. Of the materials listed above, stainless steel provides the highest process efficiency with respect to heat loss, with aluminium, brass, iron and copper becoming progressively less efficient. The external surfaces of the cores 2111, 2121 are ground (i.e. undergo a grinding process, for example, to make them flat) prior to coating to increase surface uniformity and reduce plasma concentrated areas and/or arc formation within the plasma-generation unit 2100.

Depending on the operation of the plasma-generation unit 2100, VOC decomposition within the plasma can result in the formation of partially-oxidised VOCs (VOC decomposition products) and formation of long-lived plasma by-products such as 03 and NO,. By providing a catalyst on the dielectric coating 2112, 2122 of one or both of the electrode members 2110, 2120 the decomposition of one or more of the VOCs, the VOC decomposition products, and the plasma by-products can be catalysed in the vicinity of the plasma such as to reduce the concentration of these species in the airflow leaving the plasma-generation unit 2100.

The dielectric coatings 2112, 2122 applied to the cores 2111, 2121 of the electrode members 2110, 2120 can have a substrate formed from a metal oxide such as MnO2, A1203, CeO2, SiO2 and TiO2. Alternatives to metal oxides include activated carbon and zeolites.

Advantageously, these metal oxide materials and alternative materials can be provided as a substrate with a high surface area onto which to dispose the catalytic material and can also themselves act as a catalyst in the presence of a plasma. Where a dielectric coating 2112, 2122 is porous, the catalyst may be disposed on the substrate, at least in part, on internal surfaces of the pores of the dielectric coating. In this way, the porosity of the coating film can be controlled from a relatively-porous to a non-porous one by filling the pores or cracks with a catalyst.

In the present plasma-generation unit 2100, at least the first electrode member 2110 has a dielectric coating 2112 comprising a catalyst that catalyses the decomposition of one or more of the VOCs, the VOC decomposition products; and plasma by-products contained within the ambient air flowing through the unit 2100. If, in the plasma-generation unit 2100 illustrated in Figure 2, only the first electrode member 2110 comprises a catalyst, it is preferable for the first electrode member 2110 to be located downstream of the second electrode member 2120, as is the case in Figure 2, because then the catalyst is located downstream of the discharge gap 2140 in which plasma generated by the unit 2100 is concentrated. This then means that VOCs and plasma species are more likely to interact on a surface on which a catalyst is present, since this is the direction in which the VOCs and plasma species will travel through the plasma-generation unit 2100 after being generated.

The catalyst may comprise one or more of: a metal oxide, a mixed metal oxide, a noble metal, a noble metal -metal oxide composite, and a noble metal -mixed metal oxide composite. Such a catalyst may comprise one or more of Ag, Pt, Pd, Rh, Ni, Cu, Mo, Co, Mg and Ti. An incipient wetness impregnation (1W1) technique may be used to dispose the selected catalyst on the surface of material forming the remainder of the dielectric coating 112, 122. The use of an IWI technique means that the macro-scale thickness and uniformity of the dielectric coating 2112, 2122 is not impacted by the deposition of catalyst.

Whilst the plasma-generation unit 2100 may only comprise a single electrode member 2110 having a dielectric coating 2112 comprising a catalyst, it is typical for several, or all, of the electrode members 2110, 2120 within the unit 2100 to have a dielectric coating 2112, 2122 and for each dielectric coating 2112, 2122 to comprise catalyst. However, the catalyst(s) that the dielectric coating 2112, 2122 of each electrode member 2110, 2120 comprises may differ between the electrode members 2110, 2120, such that different catalysts can be provided at different residence times within the plasma-generation unit 2100, i.e. the choice of catalyst for each electrode member 2110, 2120 can be tailored to the species present in the plasma at the position of that electrode member 2110, 2120 in the plasma-generation unit 2100.

In Figure 2 both the first electrode member 2110 and second electrode member 2120 comprise a dielectric coating 2112, 2122 as this can help to reduce the likelihood of arcing between the electrodes and the formation of a large quantity of plasma by-products and damage to the coating caused by the arc formation.

Moreover, disposition of catalyst on the substrate of the dielectric coating 2112, 2122 increases the capacitance of the electrode members 2110, 2120, such that the plasma generation unit 2100 can operate at lower current levels whilst still generating a plasma, thereby reducing the energy consumption of the plasma-generation unit 2100.

Figure 3 provides a schematic of a second plasma-generation unit 3100 in a cross-section lying substantially parallel to an air pathway though the unit (illustrated by the thick arrows 1 in Figure 3). The second plasma-generation unit 3100 may be incorporated into the system 10 of Figure 1. The plasma-generation unit 3100 in Figure 3 can be considered a modification of the first plasma-generation unit 2100 illustrated in Figure 2, and much of the foregoing description of Figure 2 is applicable, mutatis mutandis, to Figure 3.

However, the second plasma-generation unit 3100 in Figure 3 differs from the first plasma-generation unit 2100 illustrated in Figure 2 in that the second plasma-generation unit 3100 further comprises a third electrode member 3130 interposed between the first electrode member 3110 and the second electrode member 3120. The electrode members 3110, 3120, 3130 are spaced apart from each other such that a discharge gap 3140 is provided between adjacent electrode members 3110, 3120, 3130 as discussed in relation to Figure 1 (i.e. the distance between the first electrode member 3110 and second electrode member 3120 in the plasma-generation unit 3100 in Figure 3 is made greater such as to accommodate the third electrode member 3130 interposed between them, with the discharge gaps 3140a, 3140b provided between adj acent electrode members 3110, 3120, 3130 being within the same range of sizes as discussed in relation to the discharge gap 2140 of the first plasma-generation unit 2100). The electrode members 3110, 3120, 3130 of the second plasma-generation unit 3100 of Figure 3 have much the same structure as set out in the forgoing description in relation to the first electrode member 2110 and/or second electrode member 2120 of the first plasma-generation unit 2100. However, the catalyst(s) that the dielectric coating 3112, 3122, 3132 of each electrode member 3110, 3120, 3130 comprises may differ between the electrode members 3110, 3120, 3130, such that different catalysts can be provided at different residence times within the plasma-generation unit 3100 such that the choice of catalyst for each electrode member 31 10, 3120, 3130 can be tailored to the species present in the plasma at the position of that electrode member 3110, 3120, 3130 in the unit 3100.

An additional difference between the second plasma-generation unit 3100 and the first plasma generation unit 2100 is the connection of the power supply 3150 to the electrode members 3110, 3120, 3130. In the first plasma-generation unit 2100, the power supply 2150 was only connected to the core 2111 of the first electrode member 2110, with the core 2121 of the second electrode member 2120 being connected to earth 2155. However, with the second plasma-generation unit 3100, the cores 3111, 3121 of both the first electrode member 3110 and the second electrode member 3120 are connected to the power supply 3150 and the core 3131 of the third electrode member 3130 is connected to earth 3155. The connection of the power supply 2150, 3150 differs between Figures 2 and 3 because the electrode members 3110, 3120, 3130 of the second plasma-generation unit 3100 need to be provided with current in such a manner that there is a potential difference generated between a given electrode member and the electrode member(s) adjacent that given electrode member, such that a plasma is then generated by the resulting electric field.

It can be appreciated that by interposing the third electrode member 3130 between the first and second electrode members 3110, 3120 as illustrated in Figure 3, the distance in the direction of the air pathway over which plasma is generated within the plasma-generation unit 3100 is increased such that, for a given flowrate of air into the unit 3100, the residence time of air within the unit 3100 is increased whilst still maintaining a discharge gap and a potential difference between adjacent electrode member 3110, 3120, 3130 that facilitates the generation of plasma with minimal plasma by-products. Thus, the conversion of VOCs and VOC decomposition products within the plasma-generation unit 3100 is increased as a result of those compounds spending a greater amount of time within a plasma and proximate to a catalyst for the reactions to decompose those compounds. Moreover, the proportion of the solid-and/or liquid-phase air pollutants in the airflow that become electrically charged increases.

An additional feature of the second plasma-generation unit 3100 is the presence of a nonconductive separator 3160 positioned between the electrode members 3110, 3120, 3130. The non-conductive separator extends circumferentially around the electrode members 3110, 3120, 3130 such as to isolate adjacent electrode members 3110, 3120, 3130 from each other and from any other conductive structures within the plasma-generation unit 3100, for example a casing of the plasma-generation unit 3100. Typically, the separator 3160 comprises a hard, insulating plastic (e.g. PTFE or other polymers with about the same hardness and/or electrical resistance as PTFE). In order to provide adequate insulation, the separator 3160 has a thickness greater than or equal to 25 um. Generally, the separator thickness is less than or equal to 1000 pm.

Figure 4 provides a schematic of a third plasma-generation unit 4100 in a cross-section lying substantially parallel to an air pathway though the unit (illustrated by the thick arrows 1 in Figure 4). The second plasma-generation unit 4100 may be incorporated into the system 10 of Figure 1. The plasma-generation unit 4100 in Figure 4 can be considered a modification of the first plasma-generation unit 2100 illustrated in Figure 2, and much of the foregoing description of Figure 2 is applicable, mutatis mutandis, to Figure 4.

However, the third plasma-generation unit 4100 in Figure 4 differs from that illustrated in Figure 2 in that the third plasma-generation unit 4100 comprises plural first electrode members 4110 and plural second electrode members 4120. Specifically, the third plasma-generation unit 4100 comprises two first electrode members 4110a and 4110b and two second electrode member 4120a and 4120b. The pair of adjacent first electrode members 4110a-b is interposed by the second electrode member 4120b, and the pair of adjacent second electrode members 4120a-b is interposed by the first electrode member 4110a. The electrode members 4110, 4120 are spaced apart from each other such that a discharge gap 4140 is provided between adjacent electrode members as discussed in relation to Figure 2 (i.e. the discharge gaps 4140a, 4140b and 4140c provided between adjacent electrode members are within the same range of sizes as discussed in relation to the discharge gap 140 of the first plasma-generation unit 2100 in Figure 2). The cores 4121 of both the second electrode members 4120a, 4120b are connected to the power supply 4150, whilst the cores 4111 of both the first electrode members 4110a, 4110b are connected to earth 4155. It can be appreciated that by increasing the number of first and second electrode members 4110, 4120 within the plasma-generation unit 4100 as illustrated in Figure 4, the distance in the direction of the air pathway over which plasma is generated within the unit 4100 is increased, such that, for a given flowrate of air, the residence time of air within the plasma-generation unit 4100 is increased whilst still maintaining a discharge gap and a potential difference between adjacent electrode member 4110, 4120 that facilitates the generation of plasma with minimal plasma by-products. Thus, the conversion of VOC s and VOC decomposition products within the plasma-generation unit 4100 is increased as a result of those compounds spending a greater amount of time within a plasma and proximate to a catalyst for the reactions to decompose those compounds. Moreover, the proportion of the solid-and/or liquid-phase air pollutants in the airflow that become electrically charged increases.

In the third plasma-generation unit 4100 in Figure 4, the catalyst(s) that the dielectric coating 4112, 4122 of each electrode member 4110a-b, 4120a-b comprises may differ between the electrode members, such that different catalysts can be provided at different residence times within the unit 4100, i.e. the choice of catalyst for each electrode member 4110a-b, 4120a-b can be tailored to the species present in the plasma at the position of that electrode member 4110a-b, 4120a-b in the plasma-generation unit 4100.

It can be appreciated that the plasma-generation unit 3100, 4100 arrangements illustrated in Figures 3 and 4 can be expanded upon in a similar manner such as to further increase the residence time of a plasma-generation unit 3100, 4100. For example, the plasma-generation unit 4100 illustrated in Figure 4 could be scaled up such that there were three, four or five first electrode members 4110 and second electrode members 4120, respectively, with each pair of adjacent first electrode members 4110 interposed by a second electrode member 4120 and each pair of adjacent second electrode members 4120 interposed by a first electrode member 4110.

As discussed in relation to Figures 2 and 3, a power supply 2150, 3150, 4150 is connected to one or more electrode members in the plasma-generation units 2100, 3100, 4100 of Figures 2 and 3 such as to provide a current to the electrode members to which the power supply is connected that creates a potential difference between adjacent electrode members that then generates an electric field across the discharge gap between adjacent electrode members. The potential difference applied between the electrode members and the strength of the resulting electric field in the discharge gap can be made great enough that a plasma is generated in the discharge gap and in the vicinity of the electrode members (i.e. adjacent the surfaces of the electrode members not adjacent the discharge gap), which together form a plasma discharge zone of the plasma-generation unit. The energy of free electrons within the plasma generated in the unit 2100, 3100, 4100 influences likelihood of plasma by-product formation and also influences the conversion of VOCs contained within the plasma and the generation of solid-and/or liquid-phase electrically-charged air pollutants. Typically, the higher the free electron energy, the more plasma by-products are formed, the higher the conversion of VOCs within the plasma-generation unit 2100, 3100, 4100 and the greater the number of solid-and/or liquid-phase electrically-charged air pollutants generated. Thus, a balance must be struck between these factors. Free electron energy within the plasma is strongly dependent on the electric field strength, and thus the potential difference, between adjacent electrode members. The power supply 2150, 3150, 4150 is configured to provide pulsed DC to the electrode members to which the power supply is connected; by providing pulsed DC to the electrode members connected to the power supply, the energy consumption of the plasma-generation unit 2100, 3100, 4100 can be reduced in comparison to using non-pulsed DC and also additional control variables for the electric field strength between adjacent electrode members, is provided.

Figure 5 provides an illustrative voltage-time graph containing a waveform for the pulsed DC provided by the power supply to an electrode member of a plasma-generation unit of the present disclosure. The pulsed DC waveform comprises pulse-ON phases, when the voltage of the power supply is non-zero, and pulse-OFF phases, when the voltage of the power supply is approximately zero. There are three parameters of the pulsed DC waveform illustrated in Figure 5 that can be controlled in order to adjust the potential difference between adjacent electrode members in the plasma-generation unit. Firstly, the voltage amplitude 4 of the pulsed DC is adjustable, the voltage amplitude 4 being the amplitude of the waveform during pulse-ON phases. Secondly, the pulse width 2 is adjustable, the pulse width 2 being the duration of a pulse-ON phase. Thirdly, the period 3 of the waveform is adjustable, the period 3 being the duration for a full pulse-ON-pulse-OFF cycle to complete. The period 3 is the reciprocal of the frequency of the pulsed DC.

The influence of these parameters of the pulsed DC on the performance of the plasma-generation unit will be explained further in relation to the data in Tables 1 -3 of the Examples.

Figure 6 provides a schematic of a first electrostatic precipitator that the collection unit 1200 of the system 10 in Figure 1 may comprise. As discussed in relation to Figure 1, the collection unit 6200 typically comprises an electrostatic precipitator and/or an electrostatic filter medium.

An electrostatic precipitator is a filterless device that can be used within the system of the first aspect of the present disclosure for removing electrically-charged air pollutants from an airflow passing through the collection unit 6200. It can be appreciated that Figure 6 illustrates a specific electrostatic precipitator, but that other configurations of electrostatic precipitator may be used within the system of the first aspect of the present disclosure.

The first electrostatic precipitator in Figure 6 comprises three primary collection elements 6210a-c and a primary voltage source 6250 configured to supply a positive DC voltage to all three of the primary collection elements 6210a-c such as to generate an electric field around each of the primary collection elements 6210a-c. Consequently, on providing an airflow through the collection unit 6200 containing negatively-electrically-charged air pollutants that have been generated by the plasma-generation unit of the system, those air pollutants will be electrostatically attracted to the primary collection elements 6210a-c and will move to come into contact with the collection elements 6210a-c and remain adhered thereto by electrostatic attraction. As discussed in relation to Figure 1, the electrically-charged air pollutants generated by the plasma-generation unit 1100 are typically predominantly negatively charged, and accordingly, the electrostatic precipitator may omit negatively-charged collection elements without substantially effecting the collection efficiency of the electrostatic precipitator and thus allowing the electrostatic precipitator to be made more compact (since space does not need to be provided for negatively-charged collection elements within the electrostatic precipitator).

In the first electrostatic precipitator, the primary collection elements 6210a-c take the form of plate-like elements, each with two major surfaces (i.e. the largest, planar surface of each collection element), at least one of which faces an adjacent primary collection element 6210a-c. The primary collection elements 6210a-c are disposed such as to lie substantially parallel to each other. They are generally overlapping with each other but are in spaced arrangement to each other in a direction perpendicular to their major surfaces such that airflow ducts that are bounded on two sides by primary collection elements 6210a-c are provided between adjacent primary collection elements 6210a-c.

The primary collection elements 6210a-c are orientated with respect to the airflow through the collection unit 6200 (illustrated by the thick arrows 1 in Figure 6) such that the airflow is substantially parallel to the major surfaces of the primary collection elements 6210a-c. This orientation may provide the advantage that it increases the time that the airflow spends adjacent a surface of a primary collection element 6210a-c and therefore exposed to the electric field of the primary collection element 6210a-c in comparison to, for example, if the airflow was perpendicular to the major surfaces of the primary collection elements 6210a-c. Consequently, the present arrangement increases the probability of electrically-charged air pollutants in the airflow being attracted to the primary collection elements 6210a-c and thus removed from the airflow.

Following the build-up of air pollutants on the primary collection elements 6210a-c, a regeneration cycle may be performed in order to clear the build-up of air pollutants on the collection elements 6210a-c. This regeneration cycle typically involves halting the airflow through the electrostatic precipitator and subsequently temporarily disconnecting the primary voltage source 6250 from the primary collection elements 6210a-c to discharge the collection elements and allow the air pollutants to fall from the collection elements under gravity for removal.

Figure 7 provides a schematic of a second electrostatic precipitator that the collection unit 1200 of the system 10 in Figure 1 may comprise. The second electrostatic precipitator can be considered a modification of the first electrostatic precipitator and much of the foregoing description of Figure 6 is applicable, mutatis mutandis, to Figure 7.

However, the second electrostatic precipitator differs from the first electrostatic precipitator in that the second electrostatic precipitator comprises both negatively-charged and positively-charged collection elements. Specifically, in the electrostatic precipitator of Figure 7 there are two primary collection elements 7210a-b that are positively charged by a primary voltage source 7250 and a single secondary collection element 7220 that is negatively charged by a secondary voltage source 7260. The primary and secondary voltage sources 7250, 7260 are positive and negative terminals, respectively, of a cell in Figure 7.

As with the primary collection elements of the first electrostatic precipitator in Figure 6, the primary and secondary collection elements 7210a-b, 7220 of the second electrostatic precipitator are plate-like elements, with the secondary collection element 7220 interposed between the two primary collection elements.

By comprising both positively-charged primary collection elements 7210a-b and a negatively charged collection element 7220, the second electrostatic precipitator is able to collect both negatively-electrically-charged air pollutants and positively-electricallycharged air pollutants from the airflow through the collection unit 7200.

Additionally, or typically alternatively, to the collection unit comprising an electrostatic precipitator as discussed in relation to Figures 6 and 7, the collection unit may comprise an electrostatic filtration medium. Electrostatic filtration media generate electric fields by either uncompensated space charge or aligned dipoles being present within the media, and these electric fields allow the media to electrostatically-interact with air pollutants, in particular electrically-charged air pollutants. A range of techniques for forming electrostatic filtration media can be used, including thermal charging, the corona effect or the triboelectric effect. Typically the electrostatic filtration medium is an electrostatically charged synthetic nonwoven material.

In a second aspect of the present disclosure, the above-described air purification system is incorporated into an air purification apparatus. Such an air purification apparatus comprises an air inlet through which ambient air can be provided into the system and an air outlet through which the air treated by the system can exit the air purification apparatus. In other words, the air inlet, air purification system and air outlet are all provided along an air pathway along which ambient air can flow.

Figures 8A and 8B are schematics of first and second air purification apparatus that may incorporate the above-described air purification system. In both the first and second air purification apparatus, the plasma-generation unit 8100 and collection unit 8200 are provided within a unitary air purification device 800. The apparatus have an air inlet 810 and an air outlet 820 through which ambient air can flow into and out of the device 800, thereby providing an air pathway through the apparatus via the air inlet 810, through the air purification system and out of the apparatus via the air outlet 820. In both the air purification apparatus of Figures 8A and 8B, the plasma-generation unit 8100 and collection unit 8200 are contained within a housing 830 of the device 810. The housing 830 may define the air inlet 810 and/or air outlet 820 and may further define the air pathway through the apparatus.

The first and second air purification apparatus in Figures 8A and 8B differ from each other in the ordering of the plasma-generation unit 8100 and collection unit 8200 along the air pathway. In the first air purification apparatus, the plasma-generation unit 8100 is provided upstream of the collection unit 8200, whereas in the second air purification apparatus the plasma-generation unit 8100 is provided downstream of the collection unit 8200.

In the first air purification apparatus, by positioning the plasma-generation unit 8100 upstream of the collection unit 8200 within the device 800, the electrically-charged air pollutants generated in the plasma-generation unit 8100 flow into the collection unit 8200 without passing through the air outlet 820 of the device and thus without entering the ambient air surrounding the device 800 (e.g. the ambient air contained in a room that the device 800 may be positioned in). In other words, where there are no other units interposed between the plasma-generation unit 8100 and the collection unit 8200 within the first apparatus, the electrically-charged air pollutants flow directly from the plasma-generation unit 8100 into the collection unit 8200.

In contrast, in the second air purification apparatus, by positioning the plasma-generation unit 8100 downstream of the collection unit 8200 within the device 800, the electrically-charged air pollutants generated in the plasma-generation unit 8100 flow out of the device 800 via the air outlet 820 and into the ambient air surrounding the device 800 rather than flowing directly into the collection unit 8200. Thus, with the second air purification apparatus, for the electrically-charged air pollutants generated by the plasma-generation unit 8100 to reach the collection unit 8200, the electrically-charged air pollutants must first exit the device 800 into the ambient air surrounding the device 800 via the air outlet 820, and subsequently re-enter the device via the air inlet 810 to flow along the air pathway into the collection unit 8200.

The ordering of the units in the first air purification apparatus provides a higher probability of an electrically-charged air pollutant generated by the plasma-generation unit 8100 being collected by the collection unit 8200 compared to the ordering of the units in the second air purification apparatus. This is because the residence time of an electrically-charged air pollutant passing between the plasma-generation unit 8100 and the collection unit 8200 in the first apparatus is much shorter than the equivalent residence time in the second apparatus (because of the dilution of the electrically-charged air pollutants that occurs by emitting them into the surroundings with the second apparatus). Consequently, with the second apparatus there is a higher probability of the electrically-charged air pollutants being neutralised or removed from the ambient air (e.g. by settling on a surface) before reaching the collection unit 8200. Thus, the first apparatus can provide a higher efficiency of solid-and/or liquid-phase air pollutant removal by the collection unit 8200 with respect to the number of electrically-charged air pollutants generated.

However, there are also advantages associated with the second apparatus despite the second apparatus's probable lower efficiency. By emitting the electrically-charged air pollutants form the plasma-generation unit 8100 into the surroundings of the device 800 before they reach the collection unit 8200, the possibility for electrically-charged air pollutants to be removed from the ambient air in the surroundings by agglomeration and settling and/or attraction to surfaces in the surroundings.

Agglomeration and settling involves electrically-charged air pollutants being attracted to other air pollutants (likely either oppositely charged, or neutrally charged, but with the ability to electrically interact with the electrically-charged air pollutant via van der walls forces) within the ambient air and forming a collective mass that is then large enough to settle out of the air under gravity.

Alternatively, electrically-charged air pollutants may be attracted to surfaces within the surroundings (e.g. walls, ceilings, floors and other surfaces within a room that the device 800 may be positioned in) by van der walls forces between the electrically-charged air pollutant and the atoms or molecules of the surface, causing the electrically-charged air pollutants to become adhered to such surfaces and thus be removed from the ambient air.

The occurrence of agglomeration and settling and/or adherence to surfaces of electrically-charged air pollutants within the surroundings of the device 800 reduces the quantity of electrically-charged air pollutants in the ambient air that need to be collected by the collection unit 8200 to be removed from the ambient air. Accordingly, the required capacity of the collection unit 8200 is reduced because fewer electrically-charged air pollutants from the plasma-generation unit 8100 subsequently enter the collection unit 8200, allowing the collection unit to be made more compact and/or have a longer lifespan. Where the collection unit 8200 comprises an electrostatic filter medium, for example, a smaller quantity of that medium may be included in the device 800 and/or the medium requires replacement or regeneration less frequently.

Figure 8C is a schematic of an alternative configuration of the air purification system of Figure 1 to that of Figures 8A and 8B. Rather than the plasma-generation unit 8100 and collection unit 8200 being provided in a unitary device (as they are in Figures 8A and 8B), in Figure 8C the system is configured such that the plasma-generation unit 8100 is comprised in a plasma-generation device 800a and the collection unit 8200 is comprised in a collection device 800b, the plasma-generation device 800a and collection device 800b being separate from each other.

The plasma-generation device 800a and the collection device 800b each comprise their own air inlet 810a, 810b, air outlet 820a, 820b and housing 830a, 830b.

By providing the plasma-generation unit 8100 and collection unit 8200 in their own separate devices 800a, 800b, the air purification system functions in much the same way as the second air purification apparatus 800 in Figure 8B in that the electrically-charged air pollutants generated by the plasma-generation unit 8100 enter the ambient air surrounding the devices 800a, 800b before they reach the collection unit 8200. Thus, the possibility of agglomeration and settling and/or adherence to surfaces of the electrically-charged air pollutants is provided.

However, an additional benefit of the configuration of the system in Figure 8C may be that the plasma-generation device 800a can be positioned independently of the collection device 800b, for example, in different locations within a room containing ambient air to be purified. Moreover, because each of the plasma-generation device 800a and collection device 800b in Figure 8C can be made smaller than a unitary device 800 that incorporates both the plasma-generation unit 8100 and collection unit 8200, such as those in Figures 8A and 8B, the configuration of the system in Figure 8C can be advantageous when using the system in locations where space for a single larger device 800 is not available or limited.

Figure 9 illustrates an embodiment of an air purification apparatus that takes the form of a unitary device 900. An air inlet 910 is provided at the base of the air purification device 900 and an air outlet 920 is provided at the top of the device 900, with the plasma-generation unit 9100 and collection unit 9200 of the air purification system interposed between the air inlet 910 and the air outlet 920 along the air pathway and contained within a housing 930 of the device 900. The plasma-generation unit 9100 and collection unit 9200 are placed in series along the air pathway, such that the air passing through the air inlet 910 (as illustrated by the thick arrows 1 in Figure 9) first flows through the plasma generation unit 9100 and subsequently through the collection unit 9200.

Although in the device 900 of Figure 9 the plasma generation unit 9100 is positioned upstream of the collection unit 9200, it can be appreciated that the ordering of these units along the air pathway can be reversed as discussed in relation to Figures 8A and 8B.

The air purification device 900 in Figure 9 further comprises an air propulsion unit 9300 (e.g. a compressor, a fan, or a blower) to drive an airflow along the air pathway through the device 900. In Figure 9 the air propulsion unit 9300 is located downstream of the collection unit 9200 along the air pathway such that air is sucked through the plasma-generation unit 9100 and collection unit 9200 by the air propulsion unit 9300. However, it is also possible to provide an air propulsion unit 9300 upstream of the plasma-generation unit 9100 and collection unit 9200 along the air pathway. The position of the air propulsion unit 9300 relative to the plasma-generation unit 9100 and collection unit 9200 influences the characteristics of the airflow through those units: locating the air propulsion unit 9300 upstream of the plasma-generation unit 9100 and collection unit 9200 results in more turbulent and higher pressure airflow through those units, whilst locating the air propulsion unit 9300 downstream of the plasma-generation unit 9100 and collection unit 9200 provides a more laminar and lower pressure airflow through those units. Although an air propulsion unit 9300 can be advantageous in driving an airflow through the air purification device 900, it is also possible for the device 900 to operate absent an air propulsion unit 13, for example, if the device 900 is configured and/or positioned such that an airflow is naturally convected through the device 900, and/or if airflow is convected due to temperature gradients arising from heat generated at the plates of the plasma-generation unit and/or by the power electronics, and/or if airflow is convected due to an ionic wind induced by electrostatic forces acting on particles within the plasma. Although not illustrated in Figures 8A-8C, equivalent air propulsions unit(s) to that in Figure 9 can be included in the devices 800, 800a, 800c illustrated therein.

Although not illustrated in Figures 8A -8C and 9, these air purification apparatus/devices may further comprise a particulate filter positioned upstream of the plasma-generation unit 8100, 9100 and/or collection unit 8200, 9200 along the air pathways through the devices in order to reduce the quantity of large (e.g. aerodynamic diameter > 5 i_tm) solid and/or liquid particles entering the these units. In particular, it is desirable to reduce the quantity of these large particles entering the plasma-generation unit 8100, 9100, where they can, for example, clog the electrode members used to generate the plasma and interfere with plasma generation. Various types of particulate filter can be used, including coarse pre-screen filters or mesh grilles, but typically not HEPA filters.

Figure 10 provides a flowchart for a method of treating ambient air to reduce a concentration of air pollutants therein using an air purification system. The method in Figure 10 utilises an air purification system comprising at least a plasma-generation unit, and optionally a collection unit.

At step S100, an ambient airflow containing air pollutants is passed through the plasma generation unit. Typically, the air pollutants contained in the ambient air includes solid and/or liquid particulates having an aerodynamic diameter of at least 0.5 micrometres.

At step 5200, the plasma-generation unit is used to generate a plasma in the ambient air passed through the plasma generation unit in step S100. Generating a plasma in the ambient air firstly generates highly-reactive species (e.g. H, 0, NO, -OH, -NO2, ON00-, -02-, OOH) that are effective in reacting with, and thereby decomposing, VOCs contained in the ambient air in the plasma-generation unit. Consequently, the generating a plasma in the ambient air reduces the concentration of VOCs in the air passing through the plasma-generation unit.

Additionally generating the plasma in the ambient air produces several different charged species (e.g. free electrons" hydroxide ions, (1),-etc.) that can interact with solid-and/or liquid-phase air pollutants within the ambient air in order to form electrically-charged solid-and/or liquid-phase air pollutants that are sufficiently long-lived to exit the plasma-generation unit.

Consequently, the plasma-generation unit discharges an airflow containing a plurality of electrically-charged air pollutants. Typically, because of the high mobility of free electrons in comparison to larger charged species such as helium nuclei (which are positively charged), the electrically-charged air pollutants generated by the plasma-generation unit are predominantly negatively charged.

Where these electrically-charged air pollutants are discharged from the plasma-generation unit into the ambient surroundings, they may be removed from the ambient air by agglomeration and settling and/or attraction to surfaces in the surroundings, as discussed above in relation to the second air purification apparatus in Figure 8B, thereby reducing the concentration of air pollutants in the ambient air.

However, where the air purification system further comprises a collection unit, step 5300 can optionally be included in the method. At step 5300, ambient air containing electrically-charged air pollutants pass into the collection unit, which is configured to generate electrostatic attraction between at least a portion of the collection unit and those electrically-charged air pollutants. By way of example, this electrostatic attraction may be generated by the collection unit comprising an electrostatic precipitator or an electrostatic filter medium, as discussed in relation to Figures 6 and 7. Thus, on passing into the collection unit, the electrically-charged air pollutants are electrostatically attracted to the collection unit. separated out from the airflow and retained in the collection unit, thereby reducing the concentration of solid-and/or liquid-phase air pollutants in the ambient air passing out of the collection unit.

Figures 11A -11C provide process flow diagrams for several different configurations of the plasma-generation unit and an adsorption unit and/or a catalyst unit that can be connected to the plasma-generation unit. In the flow diagrams of Figures 11A -11C the plasma-generation unit is provided with a test airflow that is humid, VOC-containing, ambient airflow containing 02, N2, H2O, toluene, solid and/or liquid air pollutants, and other trace elements and molecules. The anhydrous gaseous room temperature composition of the VOC-containing ambient airflow into the plasma-generation units in Figures 11A - 11C is: 21% by mass 02, 78% by mass N2 and 1ppm Toluene, the balance comprising Ar, CO2 and other trace elements and molecules. Connecting the adsorption unit and/or catalysis unit to the plasma-generation unit such that the adsorption unit and/or catalysis unit is directly downstream of the plasma-generation unit and thus the air exiting the plasma-generation unit flows directly into the adsorption unit (i.e. without entering the ambient air) may be useful where the concentration of VOCs and/or the concentration of plasma by-products in the air leaving the plasma generation unit remains high. For example, there may be a limit on the number of electrode members within the plasma-generation unit due to factors such as the pressure drop across the plasma-generation unit and the space available for the plasma generation unit and/or additional electrode members.

In Figure 11A, the system further comprises an adsorption unit connected to the plasma-generation unit directly downstream of the plasma-generation unit, the airflow path being illustrated by the thick arrows 1 in Figure 11A. Thus, the air exiting the plasma-generation unit flows directly into the adsorption unit (i.e. without entering the ambient air). The plasma-generation unit acts to decompose the toluene molecules contained in the feed airflow as described above with reference to Figures 1 -4, with the plasma-generation unit effluent airflow comprising a mixture of toluene not decomposed by the plasma-generation unit, partially oxidised VOCs produced by the decomposition of toluene, long-lived plasma species (e.g. 0, 02(A1A), OH, HO2) and plasma by-products (e.g. 03, NOx). The plasma-generation unit also acts to electrically-charge the solid and/or liquid air pollutants within the feed airflow by interactions of the solid and/or liquid air pollutants with charged species (e.g. free electrons, hydroxide ions, oxygen radicals, etc) formed by the plasma-generation unit. Within the plasma-generation unit's effluent airflow, the concentration of toluene, the concentration and identity of the VOC decomposition products and the concentration of plasma by-products is a function of the operating conditions of the plasma-generation unit (as discussed above in relation to Figure 5 and below in relation to Tables 1 -3 and Figures 12 and 13).

The plasma-generation unit's effluent airflow in Figure 11A then passes to the adsorption unit. The adsorption unit is provided in order to adsorb toluene, toluene decomposition products and plasma by-products from the air treated by the plasma-generation unit before the air exits to the ambient air. An adsorbent within the adsorption unit positioned downstream of the plasma-generation unit is able to be (at least partially) regenerated in-situ by the long-lived plasma species (e.g. 0, 02(A1A), OH, HO2) that sustain beyond the plasma discharge zone of the plasma-generation unit and react with the toluene, toluene decomposition products and plasma by-products that adsorb onto the adsorbent of the adsorption unit. The plasma-generation unit and adsorption unit thus act synergistically to provide an air stream with a reduced concentration of VOCs and that meets WHO guidelines for ozone and nitrous oxides. This air stream still contains the electrically charged liquid and/or solid air pollutants to be collected by the collection unit but may now be released into the ambient air.

In Figure 11B, the system further comprises a catalysis unit connected to the plasma-generation unit directly downstream of the plasma-generation unit, the airflow path being illustrated by the thick arrows 1 in Figure 11B. Thus, the air exiting the plasma-generation unit flows directly into the catalysis unit (i.e. without entering the ambient air). The plasma-generation unit acts to decompose the toluene molecules contained in the feed airflow as described above with reference to Figures 1 -4, with the plasma-generation unit effluent airflow comprising a mixture of toluene not decomposed by the plasma-generation unit, partially oxidised VOCs produced by the decomposition of toluene, long-lived plasma species (e.g. 0, 02(A1A), OH, 1102) and plasma by-products (e.g. 03, NOX). The plasma-generation unit also acts to electrically-charge the solid and/or liquid air pollutants within the feed airflow by interactions of the solid and/or liquid air pollutants with charged species (e.g. free electrons, hydroxide ions, oxygen radicals, etc) formed by the plasma-generation unit. Within the plasma-generation unit's effluent airflow, the concentration of toluene, the concentration and identity of the VOC decomposition products and the concentration of plasma by-products is a function of the operating conditions of the plasma-generation unit (as discussed above in relation to Figure 5 and below in relation to Tables 1 -3 and Figures 12 and 13).

The plasma-generation unit's effluent airflow in Figure 11B then passes to the catalysis unit.

The catalysis unit is provided in order to catalyse the decomposition of one or more of the toluene, the toluene decomposition products and the plasma by-products contained within the plasma-generation unit's effluent airflow b efore the air exits to the ambient air. Similarly to the arrangement in Figure 11A, by positioning the catalysis unit downstream of the plasma-generation unit, long-lived plasma species (e.g. 0, 02(A1A), OH, H02) that sustain beyond the plasma discharge zone of the plasma-generation unit can reach the catalysis unit.

These long-lived plasma species, in the presence of the catalyst in the catalysis unit can increase the conversion of toluene and other partially oxidised VOCs on the catalyst surface and decompose plasma by-products on the catalysts surface. The catalysis unit may be operated at room temperature or at a higher temperature, depending on the balance of energy consumption and conversion desired. The plasma-generation unit and catalysis unit thus act synergistically to provide an airflow with a reduced concentration of VOCs and that meets WHO guidelines for ozone and nitrous oxides. This airflow still contains the electrically charged liquid and/or solid air pollutants to be collected by the collection unit but may now be released into the ambient air.

In Figure 11C, the system further comprises a catalysis unit connected to the plasma-generation unit and an adsorption unit connected to the plasma-generation unit, the catalysis unit is directly downstream of the plasma-generation unit and the adsorption unit is then downstream of the catalysis unit, the airflow path being illustrated by the thick arrows 1 in Figure 11C. It can be appreciated that the ordering of the catalysis unit and adsorption unit in Figure 11C can be switched such that the adsorption unit is interposed between the plasma-generation unit and the catalysis unit. The plasma-generation unit acts to decompose the toluene molecules contained in the feed airflow as described above with reference to Figures 1 -4, with the plasma-generation unit effluent airflow comprising a mixture of toluene not decomposed by the plasma-generation unit, partially oxidised VOCs produced by the decomposition of toluene, long-lived plasma species (e.g. 0, 02(A1A), OH, HO2) and plasma by-products (e.g. 03, NOx). The plasma-generation unit also acts to electrically-charge the solid and/or liquid air pollutants within the feed airflow by interactions of the solid and/or liquid air pollutants with charged species (e.g. free electrons, hydroxide ions, oxygen radicals, etc) formed by the plasma-generation unit. Within the plasma-generation unit's effluent airflow, the concentration of toluene, the concentration and identity of the VOC decomposition products and the concentration of plasma by-products is a function of the operating conditions of the plasma-generation unit (as discussed above in relation to Figure 5 and below in relation to Tables 1 -3 and Figures 12 and 13).

The functionality of the catalysis unit and adsorption unit in Figure 11C is as described above in relation to Figures 11 A and 11B, respectively. Advantageously, by the effluent airflow from the catalysis unit flowing into the adsorption unit, any toluene not decomposed by the plasma-generation unit, partially oxidised VOCs produced by the decomposition of toluene, long-lived plasma species (e.g. 0, 02(A1A), OH, HO2) and plasma by-products (e.g. 03, NOx) remaining in the airflow can be adsorbed onto the adsorbent contained within the adsorption unit before the airflow is released into the ambient air. This airflow still contains the electrically charged liquid and/or solid air pollutants to be collected by the collection unit when the airflow is released into the ambient air.

Although illustrated as separate units in Figure 11C, the adsorption unit and catalysis unit may be combined into a single unit by doping catalyst onto an adsorbent contained within the adsorption unit. In this way, the oxidation of adsorbed compounds (including VOCs, VOC decomposition products and plasma by-products) can be accelerated compared to if the adsorbent was not doped with catalyst.

Alternatively, where the system comprises an adsorption unit and/or catalysis unit, the air purification device may be configured to be operated under a cyclic storage-discharge operation in which a proportion of the air exiting the adsorption unit and/or catalysis unit is recycled back into the plasma-generation unit before exiting to the ambient air. Advantageously, this can increase the overall VOC conversion achieved by the system.

Reference Examples

Table 1 contains data on experiments using a microplasma-generation unit supplied with pulsed DC to study the formation of plasma by-products in the plasma-generation unit. The plasma-generation unit comprised two perforated plate electrode members having an alumina dielectric coating 200 um thick, separated by a discharge gap of 100 pm, with no catalyst within the dielectric coating. The electrode members had a diameter of 58.4 mm and a thickness of 1.4 mm. The perforations were 2.6 mm in diameter and arranged in a regular hexagonal array with a pitch of 5 mm, providing an open area of 24.6 %. Dry ambient air (relative humidity < 1 %) free of VOCs was supplied to the plasma-generation unit at a flowrate of 5 L/min.

Tests 1 -15 in Table 1 correspond to differing pulsed DC parameters, with voltage, frequency and pulse width. The concentration of ozone (03), nitrogen monoxide (NO) and nitrogen dioxide (NO2) (plasma by-products) in the plasma-generation unit effluent airflow was measured, alongside the current amplitude and the power consumption of the plasma-generation unit. The power consumption is calculated by the time integration of the product of voltage and current pulses over one cycle by using the correlation, P = -T1 f (v(01 (t)) dt, where T is the period time, V(t) and I(t) are the applied voltage and current, respectively.

The tests in Table 1 can broadly be divided into three operational regions. There is a first region covering tests 1 -9, in which the voltage of the DC power supply was within the range 0.9 -1.1 kV and a plasma was generated without the formation of plasma by-products (i.e. no 03, NO or NO2 was formed within the plasma-generation unit). The second region covers tests 10 and 12, in which pulsed DC with a voltage of 1.2 kV and frequencies of 1 kHz and 50 kHz, respectively, were supplied and plasmas were generated with the formation of 03 but without NOx formation. The third region covers tests 11 and 13 -15 and resulted in the formation of 03 and NOx within the plasma due to the high free electron energy generated by the pulsed DC parameters of tests 11 and 13 -15, which more readily caused nitrogen dissociation and lead to NOx formation. The presence of the second region illustrates that it is possible to selectively form 0 radicals within the plasma-generation unit whilst suppressing the formation of N radicals.

Table 2 contains data on experiments using a microplasma-generation unit supplied with pulsed DC and a catalytic microplasma-generation unit supplied with pulsed DC to study the formation of plasma by-products in the plasma-generation units. Humid ambient air (relative humidity -50%) free of VOCs was supplied to the plasma-generation unit at a flowrate of 5 L/min. The microplasma-generation unit used in tests 16, 18, 20, 22, 24, 26, 28, 30, 32, 34, 36, 38, 40, 42 and 44 was the same as that used in tests 1 -15 of Table 1. The catalytic microplasma-generation unit (used in tests 17, 19, 21, 23, 25, 27, 29, 31, 33, 35, 37, 39, 41 and 43) was the same as that used in tests 1 -15 of Table 1, but with the dielectric coating further comprising Mn02 catalyst. Manganese dioxide catalyst was incorporated onto the alumina coated electrodes of the catalytic microplasma-generation unit by an incipient wetness impregnation technique. The catalyst precursor, manganese (II) nitrate tetrahydrate, was dissolved in water with volume equivalent to the pore volume of alumina coating. The amount of precursor in a solution was based on the mass of the metallic element (10 wt.% Mn) per mass of A1203 desired. Capillary action draws the precursor solution into the pores. The impregnated electrodes were dried in oven at 110 °C for 30 minutes to drive off the water, followed by calcination at 450 °C for 2 hr with heating and cooling rates of 2 °C/min. This calcination process transformed the manganese nitrate precursor to manganese dioxide.

Plasma discharges in humid ambient air have a strong tendency towards instabilities and thus by-product formation occurs much more readily than with dry air under the same pulsed DC parameters. First considering the microplasma-generation unit test results, it is evident that, under the same pulsed DC parameters, the humid air tests result in much higher concentrations of 03, NO and NO2 than with the dry air tests in Table 1. Moreover, for humid air, the first and second regions in Table 1 are no longer present within the range of voltages investigated in the tests of Table 2: 03 and NOx formed at voltages as low as 0.7 kV. In contrast, with dry air the plasma-generation unit only generated 03 and NOx at voltages greater than or equal to 1.2 kV. An explanation for the behaviour seen with humid air is that the presence of water in the discharge gap alters the reaction mechanisms and rates of the reactions that form 03 and NOx within the plasma. In particular, the formation of radical species 0 and OH within the plasma, which can act as a precursor to 03 and NOx, is expected to be contributing to the greater concentration of 03 and NOx in the air leaving the plasma-generation unit. Many of the test in Table 2 using the catalytic microplasma-generation unit resulted in the 03 concentration in the plasma-generation unit effluent airflow being in excess of the WHO guidelines for 03 exposure levels of 100 µg/m3 (51 ppb) 8 hr daily maximum and 60 µg/m3 (31 ppb) as an average of daily maximum time-weighted average over 8 hours in six months of peak season 03 concentration (World Health Organization.

(2021). WHO global air quality guidelines: particulate matter (PM2.5 and PM10), ozone, nitrogen dioxide, sulphur dioxide and carbon monoxide: executive summary. World Health Organization.).

However, the tests in Table 2 using the catalytic microplasma-generation unit resulted in 30 much lower concentrations of plasma by-products than with the microplasma-generation unit under the same or similar conditions. In tests 17, 19, 21, 23, 25, 27, 29, 31, 33, 35, 37, 39, 41, and 43, no 03 was detected within the plasma-generation unit effluent airflow, even up to a voltage of 1 kV. Whilst NO and NO2 were detected in the plasma-generation unit effluent airflow, the concentrations were always smaller than the concentrations seen with the microplasma-generation unit operating under the same or similar conditions, typically by an order of magnitude or more. A conclusion of the comparison of the tests in Table 2 using the microplasma-generation unit and catalytic microplasma-generation unit is that the MnO2 catalyst within the dielectric layer is catalysing the decomposition of 03, NO and NO2 that forms within the plasma when feeding humid air into the catalytic microplasmageneration unit.

Table 3 contains data on Reference Examples 1 -5 using a catalytic mi croplasma-generati on unit to decompose toluene contained in a plasma-generation unit feed airflow and data on Reference Examples 6 -9 using a microplasma-generation unit to decompose toluene contained in a plasma-generation unit feed airflow. The catalytic microplasma-generation unit and microplasma-generation unit used in the Reference Examples, were the same as those described above in relation to Tables 1 and 2. Humid ambient air (relative humidity - 50%) containing 1 ppm of Toluene was supplied to the plasma-generation units at a flowrate of 5 L/min. The electrode members were provided with pulsed DC from a power supply, with the Reference Examples in Table 3 covering a range of pulsed DC parameters (voltage, frequency, pulse width).

In a low-energy plasma discharge such as that generated in a microplasma-generation unit, toluene is oxidised to CO2 and water by a series of reactions with electrons and active radicals (0, H, OH) and N2. However, by supplying catalyst within the plasma discharge, an alternative reaction mechanism for toluene oxidation is provided.

Reference Examples 6 -9 of Table 3 show that a microplasma-generation unit can achieve toluene decomposition in excess of 70 % and minimised (<1 ppb) formation of 03, NO and NO2 whilst operating at ambient conditions. However, Figure 12 provides chromatograms of plasma-generation unit feed and effluent airflows for Reference Example 6 (0.9 kV, 25kHz, 1 ps, 97.9 % toluene conversion) and these illustrate that a range of partially-oxidised VOC compounds are present in the effluent airflow. The partially-oxidised VOC compounds present includes benzene, phenol, and ortho-xylene, which are approximately as toxic as the toluene contained in the plasma-generation unit feed, and acetone and ethanol, which are less toxic. The formation of these partially-oxidised VOCs, present in concentrations up to 0.1 ppm, indicates that integration of an adsorbent bed into an air purification device downstream of the microplasma-generation unit would be desired if a catalyst is not incorporated into a dielectric layer of the microplasma-generation unit.

Considering Reference Examples 1 -5 of Table 3, which relate to a catalytic micropl asmageneration unit according to the present disclosure, it can be seen that improved toluene conversion with reduced plasma by-product fon-nation is realised. Since plasma processes are non-selective, the presence of a catalyst can improve reaction selectivity by favouring certain reaction pathways. Considering 03 formation within the plasma for instance, 03 does not react with toluene directly, but is readily decomposed in the presence of an Mn02 catalyst to produce oxygen radicals that have a higher oxidation activity and will react with toluene.

Moreover, the energy consumption of the catalytic microplasma-generation unit is approximately an order of magnitude lower than that of the microplasma-generation unit. This reduction in power can be attributed to both the increased capacitance of the electrode members when the alumina substrate of the dielectric coating is impregnated with catalyst (increasing the capacitance of the electrode members to 100 pF, in comparison to 20 pF with the electrode members of the microplasma-generation unit) and the reduced activation energy of the reaction mechanisms occurring in the presence of the catalyst. The Reference Examples in Table 3 indicate that there is a wide operating window at 0.9 kV and 25 kHz pulsed DC, with pulse widths ranging from 0.6 to 30 p.s.

Moreover, Figure 13, which provides chromatograms of plasma-generation unit feed and effluent airflows for Reference Example 1 (0.9 kV, 25kHz, 1 ns, 99.8 % toluene conversion) illustrates the concentration (indicated by peak size) and range of partially oxidised VOCs present in the effluent airflow of the catalytic microplasma-generation unit are much lower. Only benzene, ethanol, acetone and phthalic anhydride are identified, and all are at very low concentrations (<0.01 ppm).

Contrasting Reference Examples 1-5 and Reference Examples 6 -9 of Table 3, it is evident that a catalytic microplasma-generation unit can provide higher conversion, lower byproduct formation (plasma by-products and partially-oxidised VOCs) and lower power consumption than a microplasma-generation unit operating under the same conditions.

Suitable embodiments take, for instance, a catalytic plasma-generation unit according to one of Reference Examples 1 -5 or a plasma-generation unit according to one of Reference Examples 6 -9 and combine such a (catalytic) plasma-generation unit with a collection unit, such as an electrostatic precipitator or an electrostatic filter medium, positioned downstream of the (catalytic) plasma-generation unit along an airflow path.

Test Catalyst Relative Voltage / Frequency Period / has Pulse Current Power 1W 03! ppb NO.1 ppb NO2! ppb Humidity / °A kV f kHz Width / ps Amplitude / A N <1 0.9 1 1000 4 7.8 0.2 0 0 0 2 N <1 0.9 25 40 4 8.1 4.8 0 0 0 3 N c1 0.9 50 20 4 7.1 8.6 0 0 0 4 N <1 1 1 1000 4 8.7 02 0 0 0 N <1 1 25 40 4 8.9 5.5 0 0 0 6 N <1 1 50 20 4 6.3 8.1 0 0 7 N 1 1000 4 9.2 02 0 0 0 8 25 40 4 10,0 6.6 0 0 0 9 N 50 20 4 7.2 8.6 0 0 0 N <1 1.2 1 1000 4 9.9 02 7.3 0 0 11 N f.2 25 40 4 9.9 6.3 7.2 183 8 12 N 2 50 20 4 7.9 7.6 6.8 0 0 13 N 1.3 1 1000 4 9.2 0.1 7.2 875 303 14 N <1 1.3 25 40 4 7.6 5.4 7.4 231 48 N 50 20 4 6.4 8.1 7.3 55 2 Test catalyst Rative vogage i Frequency /kHz Period P:Se Current Power C): I ppb NO;Pmt? NO2! ppb Eiuntity 1 s kv µa Width I Na Amplitude IA 1W 16 N 50 0.7 1 1000 4 0.36 0.013 2.4 0 2 17 V 50 0,75 1 1000 4 0.33 0.02 0 0.2 0.9 18 N 50 0.7 25 40 4 0.36 0.332 15.4 6 16 19 Y 50 0.75 25 40 4 0,42 0.39 0 0.1 1.0 N 50 0.7 50 20 4 0.36 0.658 15.9 20 23 21 Y 50 0,75 50 20 4 0.42 0.78 0 0.1 1.1 22 N r 50 50 1- 1 1 1000 4 4 0,40 0.015 1.9 0 1 2 18 23 0.8 0.8 1000 0.39 0.02 0.4 24 N 50 0.B 25 40 4 0.40 0 391 34.1 6 25 1" 50 0.8 25 40 4 0.44 0.44 0 0.4 2.1 26 N 50 0,8 50 20 4 0.40 0.887 22.8 32 73 7 Y 5 0 8 50 20 4 0A4 0 88 0 0.66 2.5 26 N 50 1 1000 0,44 0 019 6,4 2 29 Y 50 0.85 1 1000 4 0.33 0.02 0.2 1.1 N 50 0.9 25 40 4 0.45 0.498 42 a 13 55 31 50 0.85 25 40 4 043 0.47 0 02 1,1 32 N 60 0.9 50 20 4 0 0,936 31.3 13 44 33 Y 50 0.85 50 20 4 0.46 0.96 0 0.1 1.1 34 N 50 1 1 1000 4 0.50 0.023 14.1 38 16 Y 50 0.9 1 1000 4 0.44 0.02 0 0.1 J, 0.7 38 N 50 1 25 40 4 0.50 0.580 50.1 32 121 37 Y 50 0.9 25 40 4 0.48 0.51 0 0.5 0.7 38 N 50 1 50 20 4 0.42 0.338 24.8 29 33 39 'V 50 0.9 50 20 4 0 44 0 88 0 0 2 0.7 N 50 1 1:000 4 0.82 0.020 0 942 162 41 V 50 1 1 "L. 1000 4 0.39 0.02 _ 0 4.6 2.5 42 N 50 25 40 4 0.38 9.445 0 4233 340 43 Y 50 1 25 20 4 0.45 0.90 0 2.6 9.6 44 N 50 1 50 20... 4 0 43 0.941 0 2237 38 co C' SS 0 LC '0 9"a " graiuieK9 graualajad 0 C2. 616...TOOL S.0 S.0 8.0 0 a - t 013 SZ 60, O tt 0 0 VO Sg DEl OP SZ 6:0 wartutoca -, 01 OP,, gatta.i.apd V&A, 5 0 I. Adi.UPK.A OP, axitantad ci q a ueKa gottantrau aldwexia a ft.arfluexig earamaki WO 1:0 0 9 0 0'0 w itiNIA Gattatajad Et 1, I 1 0 Sto /9t o CO, t. op Amato $j St z aidittex9 amalai:ad 1.orkrna LA rd i ( .0) - , A WO OP A. u 1. grid i '0: - It MCI.. P.,.v. $.11 t _ POOGri i, ,:nua.i.aystl WM 1 q r'I' The features disclosed in the foregoing description, or in the following claims, or in the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for obtaining the disclosed results, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof.

While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention.

For the avoidance of any doubt, any theoretical explanations provided herein are provided for the purposes of improving the understanding of a reader. The inventors do not wish to be bound by any of these theoretical explanations.

Any section headings used herein are for organizational purposes only and are not to be construed as limiting the subject matter described.

Throughout this specification, including the claims which follow, unless the context requires otherwise, the word "comprise" and "include", and variations such as "comprises", "comprising", and "including" will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.

It must be noted that, as used in the specification and the appended claims, the singular forms "a," "an," and "the" include plural referents unless the context clearly dictates otherwise.

Ranges may be expressed herein as from "about" one particular value, and/or to "about" another particular value. When such a range is expressed, another embodiment includes from the one particular value and/or to the other particular value. Similarly, when values are expressed as approximations, by the use of the antecedent "about," it will be understood that the particular value forms another embodiment. The term "about-in relation to a numerical value is optional and means for example +1-10%.

Claims

CLAIMS1. An air purification system for purification of ambient air, the air purification system comprising: a plasma-generation unit configured to generate a plasma in the ambient air to electrically charge air pollutants therein to provide electrically-charged air pollutants; and a collection unit configured to collect the electrically-charged air pollutants by electrostatic attraction between at least a portion of the collection unit and the electrically-charged air pollutants.
2. The air purification system according to claim 1, wherein the plasma-generation unit is configured such as to not generate ozone.
3. The air purification system according to any preceding claim, wherein the plasma-generation unit is configured to generate a microplasma.
4. The air purification system according to any preceding claim, wherein the plasma-generation unit is a dielectric barrier discharge plasma-generation unit.
5. The air purification system according to claim 4, wherein the dielectric barrier discharge plasma-generation unit comprises a first electrode member comprising an electrically conducting core and a dielectric coating; and a second electrode member disposed with respect to the first electrode member, so as to generate a plasma between the first and second electrode members on the application of a plasma generation voltage between the first and second electrode members such that, in use, VOCs in the ambient air in the plasma generation unit are decomposed to form VOC decomposition products, and plasma by-products are formed.
6. The air purification system according to claim 5, wherein the dielectric coating comprises a catalyst that catalyses the decomposition of one or more of: the VOCs; the VOC decomposition products; and the plasma by-products.
7. The air purification system according to claim 6, wherein the dielectric coating comprises metal oxide, optionally one or more of Mn02, A1203, CeO2, SiO2 and TiO2.
8. The air purification system according to claim any of claims 5 to 7, wherein: the plasma-generation unit further comprises a power supply connected to the first electrode member or the second electrode member; and the power supply is configured to deliver pulsed DC to the electrode member to which the power supply is connected such as to generate a plasma between the first electrode member and the second electrode member.
9. The air purification system according to claim 8, wherein the power supply is configured to deliver pulsed DC to each electrode member to which the power supply is connected, the pulsed DC received by each electrode member: generating an electric field strength with applied voltage greater than or equal to 0.3 kV and less than or equal to 1.9 kV between that electrode member and an adjacent electrode member; having a pulse frequency greater than or equal to 0.5 kHz and less than or equal to kHz; and having a pulse width greater than or equal to 0.05 ps and less than or equal to 50 ps.
10. The air purification system according to any preceding claim, wherein the collection unit comprises an electrostatic precipitator.
11. The air purification system according to claim 10, wherein the electrostatic precipitator comprises: one or more primary collection elements; and a primary voltage source configured to supply a positive DC voltage to the one or more primary collection elements; such that, in use, negatively-charged air pollutants are electrostatically attracted to the primary collection elements.
12. The air purification system according to any of claims 1 to 9, wherein the collection unit comprises an electrostatic filter medium.
13. The air purification system according to claim 12, wherein the electrostatic filter medium comprises one or more positively statically-charged regions separated from one or more negatively statically-charged regions.
14. The air purification system according to any preceding claim, wherein: the plasma-generation unit is comprised in a plasma-generation device; the collection unit is comprised in a collection device; and the plasma-generation device is separate to the collection device such that said devices can be positioned independently of each other.
15. An air purification apparatus comprising: an air inlet; an air outlet; and an air purification system according to any of claims 1 to 13; wherein, in use, ambient air flows along an air pathway passing into the apparatus via the air inlet, through the air purification system, and out of the apparatus via the air outlet.
16. The air purification apparatus according to claim 15, wherein the plasma-generation unit is positioned upstream of the collection unit along the air pathway through the apparatus.
17. The air purification apparatus according to claim 16, wherein the apparatus further comprises a particulate filter positioned upstream of the plasma-generation unit along the air pathway through the apparatus.
18. The air purification apparatus according to claim 15, wherein the plasma-generation unit is positioned downstream of the collection unit along the air pathway through the apparatus.
19. A method of treating ambient air to reduce a concentration of air pollutants therein using an air purification system, the system comprising: a plasma-generation unit; and a collection unit; wherein the method comprises the steps of: passing an airflow containing air pollutants through the plasma-generation unit; generating a plasma in the ambient air using the plasma-generation unit to electrically charge the air pollutants to provide electrically-charged air pollutants; and passing an airflow containing the electrically-charged air pollutants through the collection unit to collect the electrically-charged air pollutants by electrostatic attraction between at least a portion of the collection unit and the electrically-charged air pollutants.
20. A method of treating ambient air to reduce a concentration of air pollutants therein using an air purification system, the system comprising a plasma-generation unit, the method comprising the steps of: passing an airflow containing particulate air pollutants through the plasma-generation unit; and generating a plasma in the ambient air using the plasma-generation unit to electrically charge the particulate air pollutants to provide electrically-charged air pollutants; wherein the particulate air pollutants present in the airflow include solid-and/or liquid-phase particulates having an aerodynamic diameter of at least 0.1 micrometres.
21. Use of a plasma generated in ambient air by a plasma-generation unit to electrically charge air pollutants to provide electrically-charged air pollutants for subsequent collection in a collection unit, the air pollutants being entrained in a flow of ambient air incident at the plasma-generation unit and the electrically-charged air pollutants being conveyed by the flow to the collection unit.