High enthalpy non-transferred direct current plasma torch

Abstract

The present invention relates to a non-transferred DC plasma torch, comprising: a first cylindrical tubular body having an inner volume and a central axis; a second cylindrical tubular body provided coaxially within the inner volume of the first cylindrical tubular body; a ring-shaped first electrode joined to a proximal end of the first cylindrical tubular body, having a first electrode upper surface; a second electrode joined to a proximal end of the second cylindrical tubular body, having a second electrode upper surface; first and second cooling means provided within the first and second cylindrical tubular body, respectively; a gas supply configured to, in use, provide a gas in a first annular space between the first and the second electrode; a DC power supply; plasma ignition means and a magnetic field generating means provided within the first cylindrical tubular body.

Description

High enthalpy non-transferred direct current plasma torchTechnical field

[0001] The present invention is related to a non-transferred direct current(DC) plasma torch, more particularly a high enthalpy non-transferred DC plasma torch, and to its use for uniformly distributing heat to both inert and oxidative environments.Background art

[0002] There is growing interest in non-transferred direct current (DC) plasma torches as industries seek ways to decarbonize and reduce reliance on fossil fuels for high-temperature processes. Many industrial heating applications, such as metallurgy, waste treatment, and cement, lime and pulp industrial processes, rely on fossil fuels for heating, releasing significant greenhouse gases and pollutants. By using plasma torches, which operate through electrical energy rather than combustion, these industries can potentially cut CO2emissions and decrease their environmental impact, in particular if the electricity is generated through renewable energy sources.

[0003] Non-transferred DC plasma torches work by generating an arc between electrodes and passing a gas through this arc, thereby ionizing the gas and creating a directional hot plasma jet that can reach temperatures exceeding 3000 K and going as high as 30000 K in atmospheric pressure environments, depending on the design of the plasma torch, the power (or DC) supplied and the gas and gas flow used. This intense heat can be directed to an environment or a material without direct contact, providing a versatile and powerful heating solution.

[0004] Limitations of existing non-transferred DC plasma torches include a high gas consumption, a limited heat efficiency, electrode erosion, and intensive cooling requirements. Plasma torches require a continuous supply of gas(es) to sustain the plasma, which leads to substantial operational costs and demands infrastructure for gas handling. Existing torches are known to require a gas flow of more than 200 Nm3, even almost 300 Nm3, per MWh energy input. Typical heat efficiencies are in the range of 50 % to 80 %, with some torches going as high as 85 % to 90 %.

[0005] Additionally, the intense heat causes electrodes to erode over time, impacting efficiency and necessitating frequent maintenance, which adds to downtime and costs. Except for tungsten cathodes with thermionic emission of electrons, the cathode spot and anode spots must be moved rapidly to reduce electrode wear. Commonly used methods for arc and electrode spots movement are use of magnetic fields and / or tangential plasma gas injection.

[0006] The high temperatures also require robust cooling systems to prevent overheating. This cooling requirement further increases complexity, as well as the energy needed to pump and circulate coolant.

[0007] A further characteristic of existing plasma torches is that the plasma, generated by gas passing through the arc, is formed in a concentrated jet. This plasma jet heats only the area, material, or article directly in front of it, making these torches highly effective for applications requiring focused, directional heating of a specific area or spot. However, this feature limits the efficiency of plasma torches in applications requiring broader, multi-directional heating, reducing their suitability as replacements for fossil fuel heating in such scenarios.

[0008] EP0157407 provides a method for heating and broadening the plasma jet exhausting from a plasma spray gun. The plasma jet exiting the plasma spray gun is directed along the central axis of a separate, downstream induction coil. A high frequency alternating current is passed through the coil, which heats the outer layers of the jet more than the centre area, and increases the average temperature of the jet. The plasma jet comprises a plasma gas and particles, and the method results in improved heating and acceleration of the particles, which reach a target while still in the molten state. This results in improved finished characteristics of the target.

[0009] A disadvantage of this method and set-up is its scalability and the need for a separate, downstream induction coil to broaden the plasma jet. Further, even though the plasma jet is broadened, there is still no uniform heat distribution in all directions (multi-directional heating).

[0010] US11 ,304,288 discloses a plasma torch, particularly suited for the production of carbon black, comprising two coaxially positioned cylindrical electrodes, one or both having a barrel stave design for improved thermal resistance. The electrodes can have an upper and a lower section connected at a junction. The electrodes are made of a material having a high thermal conductivity of > 100 W / m.K, an electrical resistivity of < 10e-2 Q.m and a resistance to chemical erosion in a highly reactive H2 free radical atmosphere, for example graphite or silicon carbide. A magnetic field at the tip of the electrode with an axial component between 10 to 100 mT is generated by magnet coils located outside the electrodes. The magnetic field pushes the plasma outside of the confines of the immediate space between the electrodes.

[0011] A disadvantage of the foregoing plasma torch is that it is not suitable for use in an oxidizing atmosphere. In addition, the graphite or silicon carbide electrodes are continuously consumed during the process, i.e., they have a limited lifespan. Inaddition, consumed electrode material can end up in the atmosphere surrounding the plasma torch. Further, the external magnetic field requires special precaution for the system design, for example furnace or kiln, in which the plasma torch is used.

[0012] Other prior art attempts to address electrode wear through different structural and magnetic means.

[0013] For instance, CN 118973067 A discloses a plasma torch with a first anode, a second anode, and a cathode arranged therebetween. In CN 118973067 A, the second anode is designed to separate the arc channels to reduce interference and distribute the current, thereby reducing electrode wear and avoiding frequent replacement. While this configuration aims to mitigate electrode erosion, it does so through a specific multi-electrode structure and does not disclose the use of a magnetic field to control the plasma arc. A disadvantage of this approach is that it still results in a concentrated plasma jet and does not solve the problem of providing uniform, multidirectional heat distribution.

[0014] The use of a magnetic field to prevent electrode erosion is known in the art. For example, KR 101249457 B1 discloses a plasma torch comprising a rear torch part and a front torch part. A magnetic coil is provided in the rear torch part, wound around a rear electrode housing. This external coil generates a magnetic field to rotate the plasma arc, preventing it from concentrating in one place and thereby reducing erosion. However, this arrangement, where the magnetic coil is located in a rear housing and external to the active front torch components, adds significant complexity and bulk to the overall system design. Such an external arrangement teaches away from a compact design where the magnetic field generating means is integrated internally within the main body of the torch, close to the electrodesSummary of the invention

[0015] The present invention aims to overcome one or more of the above drawbacks. It is an aim to provide a non-transferred direct current (DC) plasma torch requiring a significantly reduced gas flow per MWh energy input when compared to nontransferred DC plasma torches known in the art, thereby significantly reducing the need for backend equipment for gas storage, gas supply (such as pumps and compressors), and gas recycling, and thus reducing the environmental footprint compared to prior art non-transferred DC plasma torches.

[0016] A further aim is to provide a plasma torch which generates a uniform heat distribution in all directions (uniform multi-directional heating) of an atmosphere surrounding the plasma torch, i.e. which is capable of uniformly distributing heat to thesurrounding atmosphere, in particular generating temperatures of at least 5000 K. This renders the plasma torch very suitable for processes and systems requiring high temperatures and benefitting from a more even (i.e. uniform, multi-directional) temperature distribution, for example (rotary) kilns, without being limited thereto.

[0017] The present invention solves the drawbacks of the prior art by providing a magnetic field generating means within the first cylindrical tubular body. This internal and integrated placement is a significant departure from prior art solutions, such as that disclosed in KR 101249457 B1 , which rely on external magnetic coils. The inventive configuration results in a more compact, less complex, and more robust plasma torch. The close proximity of the internal magnetic field generating means to the plasma arc allows for efficient arc control and stabilization, effectively reducing electrode wear without the need for bulky and complex external components. This makes the torch easier to integrate into industrial systems, such as furnaces or kilns, where space is often limited and external magnetic fields can be problematic.

[0018] A further aim is to provide a non-transferred DC plasma torch which has both a high heat efficiency, i.e. at least 85 %, and a high enthalpy.

[0019] Yet another aim is to provide a versatile non-transferred DC plasma torch. More particularly, an aim is to provide a plasma torch where the power input can be adjusted to the process needs, and can go down to less than 20 % of the maximum plasma torch power. More particularly, an aim is to provide a plasma torch which can operate with and is thus suitable for use with a wide range of plasma gases.

[0020] Yet another aim is to provide a non-transferred DC plasma torch which can be used in various environments, including an oxidizing atmosphere, a reducing atmosphere and an inert atmosphere.

[0021] In the light of the present disclosure, the term “inner volume of a tubular body” is used for the space defined by the through-opening of the tubular body, i.e. the space defined by an inner surface of the tubular body.

[0022] The term “within a tubular bod refers in the present disclosure to a position in the body of the tubular body, i.e. in the space between the outer surface and the inner surface of the tubular body, said differently in the space comprised within the thickness of the tubular body itself.

[0023] According to a first aspect of the present disclosure, there is provided a non-transferred direct current (DC) plasma torch as set out in the appended claims.

[0024] The non-transferred DC plasma torch comprises a first cylindrical tubular body having an inner volume and a central axis, and a second cylindrical tubular body provided coaxially within the inner volume of the first cylindrical tubular body.

[0025] The inner diameter of the first cylindrical tubular body is larger than the outer diameter of the second cylindrical tubular body. In other words, there is a gap, i.e. a second annular space, between the first and the second cylindrical tubular bodies.

[0026] The first and second cylindrical tubular bodies can be electrically insulated from one another. This electrical insulation can be provided by techniques known in the art.

[0027] The plasma torch further comprises a ring-shaped first electrode, i.e. an electrode having a through opening. The first electrode is joined to a proximal end of the first cylindrical tubular body. The first electrode has a first electrode upper surface, which is distally arranged from the first cylindrical tubular body, i.e. which is at a side of the first electrode opposite to the surface of the first electrode joined to the first cylindrical tubular body.

[0028] The plasma torch further can comprise a disk-shaped second electrode. The second electrode is joined to a proximal end of the second cylindrical tubular body. The second electrode has a second electrode upper surface distally arranged from the second cylindrical tubular body, i.e. which is at a side of the second electrode opposite to the surface of the second electrode joined to the second cylindrical tubular body.

[0029] In the light of the present disclosure, the term “disk-shaped electrode” is used for electrodes which have an electrode upper surface, a surface opposite thereto and joined to a cylindrical tubular body, and a peripheral edge extending circumferentially around the electrode, and which optionally have a through opening extending from the electrode upper surface to the surface opposite thereto. In other words, a disk-shaped electrode can have a through opening, in which case the electrode has a ring-shape.

[0030] In certain embodiments, the first electrode upper surface and the second electrode upper surface define a common plane. The common plane can be inclined at an angle between 80° and 100°, preferably between 85° and 95°, more preferably between 88° and 92°, most preferably substantially 90° (substantially perpendicular) from (to) the central axis. In other words, the common plane has an inclination angle equal to or lower than 10°, preferably equal to or lower than 5°, morepreferably equal to or lower than 2°, most preferably substantially 0°, with regards to the central axis.

[0031] The inner diameter of the first electrode is larger than the outer diameter of the second electrode. In other words, there is a gap, i.e. a first annular space, between the first and the second electrodes.

[0032] The first cylindrical tubular body further comprises first cooling means, i.e. first cooling means are provided within the first cylindrical tubular body. The first cooling means are configured to, in use, cool the first electrode.

[0033] The second cylindrical tubular body further comprises second cooling means, i.e. second cooling means are provided within the second cylindrical tubular body. The second cooling means are configured to, in use, cool the second electrode.

[0034] More particularly, the first and the second cooling means are configured to, in use, cool the respective electrode to a predetermined level, i.e. temperature, advantageously over an extended period of time. This significantly reduces, and can even substantially prevent, damage to the respective electrode, thereby improving the lifetime of the electrode.

[0035] The plasma torch further comprises a gas supply. The gas supply is configured to, in use, provide a plasma gas (.i.e., a gas that will be brought in its plasma state of matter) in the first annular space via the second annular space. Advantageously, the gas supply is provided at a distal end of the first and / or the second cylindrical tubular body.

[0036] The plasma torch further comprises a DC power supply. The DC power supply is configured to, in use, generate a plasma arc. In an embodiment, the DC power supply is configured to, in use, provide a potential from the first electrode to the second electrode or vice versa. Upon providing this potential, a plasma arc is generated. It will be understood that the plasma arc has its both extremities at each one of the first and the second electrode.

[0037] The plasma torch further comprises plasma ignition means.

[0038] The first cylindrical tubular body further comprises magnetic field generating means, i.e. a means for generating a magnetic field is provided within the first cylindrical tubular body.

[0039] The inventors have surprisingly discovered that the technical effect of the combination of the positioning of the first and the second electrode, the magneticfield generating means provided within the first cylindrical tubular body, and the first and the second cooling means provided within the first and the second cylindrical tubular body, respectively, is that the plasma arc is positioned in front of the first and the second electrodes, i.e. outside of the plasma torch. This results in heat being radiated directly from the plasma arc to the surroundings, leading to an efficient heat transfer (i.e. a high enthalpy) and uniform heat distribution in all directions, contrary to prior art plasma torches where the plasma jet is a directional heat source.

[0040] The inventors have further surprisingly discovered that the plasma torches of the present invention are not dependent on plasma gas for heat transfer, and that high heat efficiencies can be achieved with a very low gas flow, much lower than what was considered possible based on what is known in the field.

[0041] Advantageously, the first electrode is an anode and the second electrode is a cathode. Consequently, the cathode is advantageously provided coaxially in the inner volume of the anode defined by the through opening of the anode’s ring shape.

[0042] Advantageously, the magnetic field generating means comprises or substantially consists of a permanent magnet, i.e. one or a plurality of permanent magnets. Alternatively or additionally, yet advantageously, the magnetic field generating means comprises or substantially consists of a magnet coil, i.e. one or a plurality of magnet coils.

[0043] Advantageously, the first cooling means comprises a first cooling fluid circuit. Advantageously, the second cooling means comprises a second cooling fluid circuit. The term “cooling fluid circuit” is used in the present disclosure for a cooling circuit suitable for receiving, transferring and / or passing a cooling fluid. The cooling fluid can be any fluid that is capable of cooling the respective electrode to a predetermined level, i.e. temperature, advantageously over extended periods of time. Advantageously, the cooling fluid is a liquid, preferably water.

[0044] Advantageously, the first and the second cooling circuit, when present, are fluidly connected to one another, i.e. they form a single cooling circuit. This renders the cooling of the electrodes less complex in that only one entrance and one exit for the cooling fluid is to be foreseen.

[0045] Advantageously, the first cooling circuit comprises first vortexinducing means. Advantageously, the second cooling circuit comprises second vortexinducing means. The term “vortex-inducing means” is used in the present invention for means, for example a component provided in the cooling circuit, or a design feature ofthe cooling circuit, that, in use, induces a vortex-shaped flow to the cooling fluid. Advantageously, this vortex-shaped flow accelerates the cooling fluid, thereby providing a more efficient cooling of the electrode cooled by the respective cooling circuit.

[0046] Advantageously, the second annular space comprises directional flow guidance means. The directional flow guidance means is configured to, in use, supply a gas flow, advantageously provided by the gas supply, to the first annular space at an inclined direction with respect to the central axis. Advantageously, the inclined direction can be a tangential direction, i.e. a direction having an inclination angle between 1 ° and 60° with regards to the central axis. Alternatively, yet advantageously, the directional flow guidance means are configured to direct the gas flow in a rotational direction about the central axis, i.e. a swirling direction. Advantageously, the directional flow guidance means does not accelerate the gas flow.

[0047] Advantageously, the second electrode upper surface is concave.The inventors have surprisingly discovered that when the second electrode upper surface is concave, the stability of the plasma arc is improved, thereby reducing fluctuations in arc current and arc voltage.

[0048] Advantageously, at least a portion of an exposed surface of the first electrode comprises a high temperature resistive electrically insulating shielding. Advantageously, at least a portion of an exposed surface of the second electrode comprises a high temperature resistive electrically insulating shielding.

[0049] In the light of the present invention, “an exposed surface of an electrode" refers to any surface of the electrode that is visible from outside the plasma torch.

[0050] The shielding can comprise or substantially consist of any material that has a high temperature resistance and that is electrically insulating. Non-limiting examples of suitable materials for the shielding include alumina and boron nitride.

[0051] In the present disclosure, with “high temperature resistance" is meant a resistance to at least the temperature to which the environment or atmosphere surrounding the plasma torch is heated. A “high temperature resistive material" is used in the present disclosure for a material having a high temperature resistance.

[0052] Advantageously, the shielding comprises or substantially consists of a coating, i.e. a high temperature resistive electrically insulating coating. Advantageously, the coating comprises or substantially consists of alumina.

[0053] Alternatively or additionally, yet advantageously, the shielding is provided in the respective electrode, i.e. is a portion of the electrode that is provided in a suitable material as described hereinabove. A particularly preferred material for such a shielding is boron nitride.

[0054] It will be understood that the shielding can comprise a coating as described hereinabove at a first surface area of an exposed surface of the first and / or the second electrode, and can further comprise a shielding provided in the respective electrode at a second surface area.

[0055] Advantageously, a high temperature resistive and thermally insulating layer is provided to at least a portion of an outer surface of the first cylindrical tubular body. The layer can be made of any material that has a high temperature resistance (as defined hereinabove), is thermally insulating and can (chemically) resist the atmosphere surrounding the plasma torch (i.e., any one of an inert, oxidative or reductive environment). Such a layer allows to reduce heat loss to the first cylindrical tubular body when the atmosphere surrounding the plasma torch is heated to high temperatures during the use of the plasma torch. More particularly, heat loss to the cooling fluid (e.g. water) in the first cooling means can be limited.

[0056] Advantageously, the high temperature resistive and thermally insulating layer is a high temperature resistive and thermally insulating coating. Advantageously, the coating comprises or substantially consists of zirconia (ZrCh).

[0057] Alternatively or additionally, yet advantageously, the high temperature resistive and thermally insulating layer is a third cylindrical tubular body provided coaxially around the first cylindrical tubular body. Such a third cylindrical tubular body can be made of any high temperature resistant material capable of resisting the high temperatures of the atmosphere surrounding the plasma torch, having low heat conductivity properties and being capable of (chemically) resisting the atmosphere surrounding the plasma torch. A particularly suited material is zirconia, i.e. the third cylindrical tubular body advantageously comprises or substantially consists of zirconia. It will be understood that the material of the third cylindrical tubular body is selected depending on surrounding temperature and the surrounding atmosphere. For inert or reducing atmospheres, thermally insulating materials which are not resistive to oxidation can also be used, for example carbon-based materials, for example graphite.

[0058] Advantageously, the first and / or the second electrode comprises or substantially consists of copper and / or silver. For example, the first and / or the second electrode consist of substantially pure copper, substantially pure silver or a copper alloys(for example a copper-silver alloy). It will be understood that the first electrode and the second electrode can have the same or different compositions.

[0059] Optionally, the first and / or the second electrode can further comprise an alloying element for copper and / or silver. It will be understood that when the electrode comprises or substantially consists of copper, the alloying element, when present, is an alloying element for copper. Likewise, when the electrode comprises or substantially consists of silver, the alloying element, when present, is an alloying element for silver. And when the electrode comprises or substantially consists of copper and silver (e.g. as a copper-silver alloy), the alloying element, when present, is an alloying element for both copper and silver. Advantageously, the alloying element induces magnetic conductivity to the electrode, i.e. it is an alloying element imparting magnetic properties to the electrode. Non-limiting examples of suitable alloying elements for copper and / or silver include cobalt, iron, nickel and manganese. A particularly preferred alloying element for copper and / or silver is cobalt.

[0060] When the first and / or the second electrode comprise(s) such an alloying element for copper and / or silver, they, individually, comprise between 0.1 and 5 % by weight, preferably between 0.25 and 2.5 % by weight, more preferably between 0.5 and 1.5 % by weight of this alloying element, for example 1 .0 % by weight, based on the weight of the respective electrode.

[0061] The present disclosure is further related to the use of the nontransferred DC plasma torch of the first aspect for uniformly distributing heat to the atmosphere surrounding the plasma torch directly from the plasma arc at temperatures equal to or higher than 5000 K, preferably temperatures between 5000 K and 30000 K. The surrounding atmosphere can be an inert, a reducing or an oxidizing atmosphere, i.e., the plasma torch of the present disclosure is versatile in the environment where it can be used.

[0062] The inventive plasma torch is particularly suited for use in kiln processes, i.e. processes performed in a kiln, in particularly rotary kiln processes and for processes performed in fluidized beds. Non-limiting examples include the production of lime (including the calcination of limestone), the production of cement (including clinkering and carbonation processes), and pulp treatment processes. It will be understood that the plasma torch can be used in any application and process requiring a large quantity of thermal energy (heat), in particularly benefitting from uniform heat distribution, such as, without being limited thereto: melting furnaces (as heat source), such as dry hearth or metal heel furnaces, ore reduction processes.

[0063] Non-limiting advantages of the inventive plasma torches include: low gas flow per MWh energy input, versatility in power supply, ranging from low kW to tens of MW, and even higher, non-directional plasma heat, i.e. uniform heat distribution in all directions, high heat efficiency of 85 % and more, efficient heat transfer, i.e. high enthalpy (high temperature and high energy density), high temperatures of more than 5000 K, to even 30000 K can be achieved, versatility in suitable atmosphere, more particularly suitable for use in an inert, a reducing, as well as an oxidizing atmosphere, the dimensions, and in particular the length, of the plasma torch are adaptable to the system wherein the plasma torch is to be installed and used,- wide range of plasma gases can be used, and- wide range of applications.Brief description of the figures

[0064] Aspects of the invention will now be described in more detail with reference to the appended drawings, wherein same reference numerals illustrate same features and wherein:

[0065] Figure 1 schematically illustrates a first embodiment of a nontransferred DC plasma torch according to the invention.

[0066] Figure 2 schematically illustrates a second embodiment of a nontransferred DC plasma torch according to the invention.

[0067] Figures 3 and 4 provide detailed views of the plasma torch depicted in Figure 2.

[0068] Figure 5 shows the plasma current, plasma voltage as used during operation of an inventive plasma torch, as well as the measured plasma power and thermal efficiency.Description of embodiments

[0069] Fig. 1 shows a schematical representation of an inventive nontransferred DC plasma torch 1. The plasma torch comprises a first cylindrical tubular body 2 and a second cylindrical tubular body 3 provided coaxially in the inner volume of the first cylindrical tubular body 2. Both cylindrical tubular bodies 2, 3 share the same central axis 20.

[0070] The inner diameter of the first cylindrical tubular body 2 is larger than the outer diameter of the second cylindrical tubular body 3, thereby defining a second annular space 9 between them.

[0071] The length of the first 2 and the second 3 cylindrical tubular body can be varied. This allows to adapt the dimensions and design of the plasma torch to the environment or set-up wherein the plasma torch is to be used.

[0072] The first 2 and the second 3 cylindrical tubular body are preferably electrically insulated from one another. This can be realised by means known in the field of plasma physics. Advantageously, the first cylindrical tubular body 2 comprises an electrically insulating material at an inner surface facing the second cylindrical tubular body 3. Alternatively or additionally, yet advantageously, the second cylindrical tubular body 3 comprises an electrically insulating material at an outer surface facing the first cylindrical tubular body 2. Non-limiting examples of suitable electrically insulating materials include polytetrafluoroethylene (PTFE, also known under the commercial name Teflon®), perfluoroalkoxy alkanes (PFA), perfluorinated ethylene propylene (FEP), ethylene tetrafluoroethylene (ETFE) and polyether ether ketone (PEEK). A particularly preferred electrically insulating material is PTFE.

[0073] First cooling means 7 is provided within the first cylindrical tubular body 2, and second cooling means 8 is provided within the second cylindrical tubular body 3. Both cooling means 7, 8 are a cooling fluid circuit. Advantageously, the cooling fluid used in the plasma torch is water. Advantageously, the first 7 and second 8 cooling means are fluidly connected to a cooling fluid supply. The cooling fluid supply can be thermally insulated to limit heating of the cooling fluid prior to entering the cooling means 7, 8 to a minimum, thereby ensuring adequate electrode cooling.

[0074] A temperature resistive and thermally insulating coating 16 is provided at the outer surface of the first cylindrical tubular body 2, for example a coating comprising or substantially consisting of ZrC>2. Such a coating reduces the loss of cooling fluid in the first cooling means 7.

[0075] A ring-shaped anode 4 is joined (attached, connected) to the first cylindrical tubular body 2 at a proximal end thereof. The anode 4 has an anode upper surface 40. A disk shaped cathode 5 is joined (attached, connected) to the second cylindrical tubular body 3 at a proximal end thereof. The cathode 5 has a cathode upper surface 50.

[0076] The anode 4 and the cathode 5 are metal electrodes. More particularly, they comprise or substantially consists of copper and / or silver and optionallyan alloying element for copper and / or silver, such as cobalt. The inventors have discovered that upon adding between 0.1 and 5 % by weight, preferably between 0.5 and 1.5 % by weight, for example 1 % by weight, of such an alloying element, in particular cobalt, based on the weight of the respective electrode, magnetic properties are imparted to the respective electrode. Such magnetic properties allow to improve arc stability and reduce the electrode consumption of the plasma torch, and thus the operating cost.

[0077] The cathode 5 is thus provided coaxially with the anode 4. The inner diameter of the anode 4 is larger than the outer diameter of the cathode 5, thereby defining a first annular space 10 between them. It will be understood that the width of the annular space 10 depends on the dimensions of the plasma torch, the voltage level to be achieved, the power level to be achieved and the plasma gas(es) used. Advantageously, but without being limited thereto, the width of the annular space 10, i.e. , the gap, is between 2 mm and 30 mm. Advantageously, the width of the annular space 10 varies from the side of the electrodes adjacent to the cylindrical tubular bodies to the opposite side (i.e., the outside in front of the plasma torch). More particularly, the gap advantageously widens towards both the side adjacent to the cylindrical tubular bodies and towards the outside front of the plasma torch, i.e. the gap is the smallest somewhere between both extremities of the electrodes.

[0078] In the embodiment shown in Fig. 1 , the anode upper surface 40 and the cathode upper surface 50 define a common plane 60, which is positioned substantially perpendicular to the central axis 20.

[0079] A magnetic field generating means, in Fig. 1 a magnet coil 6, is provided within the first cylindrical tubular body 2. It will be understood that the same is valid for a permanent magnet. It will further be understood that one or a plurality of magnet coils and / or one or a plurality of permanent magnets can be used. During use, the magnetic field generating means is supplied with electricity by a power source, generating a magnetic field, advantageously a static magnetic field. This magnetic field interacts with the plasma and the arc at the electrodes, creating a rapid movement of the cathode spot (i.e. the spot on the surface of the cathode where the plasma arc is in contact with the cathode) and the anode spot (i.e. the spot on the surface of the anode where the plasma arc is in contact with the anode) over the electrode surfaces, which reduces wear of the electrodes. The inventors have discovered that by positioning the magnetic field generating means and the electrodes as shown in Fig. 1 , the plasma arc is generated in front of the plasma torch, i.e. outside the plasma torch. Said differently, the plasma arc is “pushed out” of the plasma torch.

[0080] The magnetic field generating means can be powered by means known in the art, for example a DC power supply. The power supply of the magnetic field generating means is advantageously separate (individual) from the power supply of the electrodes, the latter generating the plasma arc. By using a separate power supply for the magnetic field generating means and the arc generation, it is possible to adjust the strength of the magnetic field independently from the arc current, i.e. the current used to generate the plasma arc.

[0081] Optionally the DC current to the plasma arc can be arranged to pass through the magnet coil. The inventors have found that by doing so, the DC current supplied by the power supply contributes to creating the magnetic field.

[0082] The magnet coil 6 as shown in Fig. 1 can be any arrangement of a magnet coil. It will be understood that the thickness and geometry of any of the wire, the tube and the beam, whether the coil is hollow or massive, and the number of windings / turns is chosen in function of the dimensions of the required magnetic field, the dimensions of the plasma torch and all operational parameters, such as the supplied power and the environment or atmosphere wherein the plasma torch is used. Advantageously, the magnet coil comprises a pure, electrically insulated copper wire, for example having a thickness between 0.5 mm and 2 mm, for example between 1 mm and 1.5 mm, such as 1.2 mm.

[0083] The first cylindrical tubular body 2, more particularly its outer surface, advantageously comprises or substantially consists of a non-magnetic material, such as austenitic stainless steel, thereby avoiding influencing of the magnetic field created by the permanent magnet(s) and / or the magnet coil((s) comprised within the first cylindrical tubular body 2.

[0084] Optionally, the second cylindrical tubular body 3 further comprises a compound imparting or improving the magnetic field, for example ferritic stainless steel. The inventors have found that the presence of such a compound improves the arc stability and reduces the electrodes consumption, thereby improving the lifetime of the electrodes and reducing the operating cost of the plasma torch.

[0085] When using the plasma torch 1 , a plasma gas is supplied from a gas supply to the first annular space 10 via the second annular space 9. Optionally, the gas supply is thermally insulated. A wide range of plasma gases can be used, including, without being limited thereto, argon, air and CO2.

[0086] Advantageously, the gas flow per MWh energy input is equal to or lower than 25 %, preferably equal to or lower than 20 %, more preferably equal to orlower than 15 %, most preferably equal to or lower than 10 %, for example equal to or lower than 5 % of the gas flow per MWh energy input of plasma torches known in the art. Advantageously, the gas flow per MWh energy input is equal to or lower than 50 Nm3, preferably equal to or lower than 40 Nm3, more preferably equal to or lower than 30 Nm3, most preferably equal to or lower than 20 Nm3.

[0087] A plasma arc 11 is ignited by means of a plasma ignition means, for example a high voltage, low current, fast pulse igniter. The plasma arc 11 is then maintained by the power supplied by means of the DC power supply (not shown), which can be any DC power supply known in the art.

[0088] An advantage of the plasma torches of the present invention is that the power supply can be adapted according to need, and can thus go as low as 60 kW and lower (i.e. “low kW’), to tens of MW and even higher.

[0089] Said differently, for a given maximum power input of a plasma torch of the invention, the power input can be varied between 20 % of this maximum power input and the maximum power input.

[0090] Advantageously, the DC power supply is configured to vary the power supply to the plasma torch during use of the plasma torch. This allows to optimize the power supply at every moment during use of the plasma torch. For example, the power supply can be varied in a cyclic pattern. Varying the power that is supplied during the use of the plasma torch allows to optimize the power consumption, thereby reducing the overall power consumption.

[0091] According to a particular embodiment, argon is used as gas to ignite the plasma arc. It is known that argon requires a lower voltage to ignite the plasma arc compared to other gases. Once the plasma arc is ignited, argon can be replaced or complemented by the gas or gases capable of generating the plasma species (ions) required for the intended process and use of the plasma torch, for example, without being limited thereto, air or CO2 , which give a higher voltage, a higher power and an improved plasma torch heat efficiency when compared to argon.

[0092] The cathode 5 as shown in Fig. 1 is a “solid” electrode, i.e. a disk without a central through opening. Alternatively (not shown), the cathode 5 can have a through opening, i.e. can be hollow or ring-shaped. Such a through opening allows for providing further materials, such as an additional gas or particles, to the plasma arc 11. Alternatively, the through opening can be filled with an electrically insulating material. The inventors have surprisingly found that when the through opening is filled with an electrically insulating material, an improved plasma arc rotation and better electricstability is obtained. Improved plasma arc rotation is known to give a lower and more uniform wear of the electrodes 4, 5, thereby reducing so-called “hot spots” or spots showing significantly higher wear. Consequently, the lifetime of the electrodes 4, 5 is significantly increased.

[0093] Advantageously, the design of the upper surface of the first 4 and / or the second 5 electrode, individually, can be varied. For example, the curvature can be varied so at to increase the surface area of the upper surface. A larger surface area contributes to a more even movement of the plasma arc. Additionally or alternatively, the upper surface can have a low surface roughness, which can reduce the wear of the respective electrode and can give a more even movement of the plasma arc.

[0094] Fig. 2 shows a schematical representation of a further inventive nontransferred DC plasma torch 100. The cathode upper surface 50 of this plasma torch 100 is concave.

[0095] Both the anode 4 and the cathode 5 further comprise a high temperature resistive electrically insulating shielding 15.

[0096] A first preferred surface area to which such a shielding 15 (coating) is applied is the upper surface 50 of the cathode 5, more preferably a central portion of the upper surface 50. The inventors have found that by providing a shielding 15 at a central portion of the upper surface 50 of the cathode 5 allows for a more stable plasma arc. At this position, the shielding 15 advantageously is a coating, preferably a coating comprising or substantially consisting of alumina. As mentioned hereinabove, a more stable plasma arc is known to give a more uniform wear of the electrodes 4, 5, thereby reducing so-called “hot spots” or spots showing significantly higher wear. Consequently, the lifetime of the electrodes 4, 5 is significantly increased.

[0097] A second preferred surface area for such a shielding 15 are surface areas of both electrodes 4, 5 defining the annular space 10 between the electrodes 4, 5. The inventors have found that applying or providing a shielding 15 to these areas allows to reduce the risk or chance for unwanted arcing happening there, and thus contributes to a more stable arc in front of the electrodes 4, 5 and thus the plasma torch. At this position, the shielding 15 is advantageously provided in the electrode 4, 5, i.e. is part of the electrode 4, 5. Preferably, the shielding 15 provided in the electrode 4, 5 comprises or substantially consists of boron nitride.

[0098] Fig. 3 represents a detailed view of the plasma torch 100 of Fig. 2, more particularly the first 7 and the second 8 cooling means. Both the first 2 and the second 3 cylindrical tubular body comprises a cooling fluid circuit as first 7 and second8 cooling means, respectively. In addition, both cooling fluid circuits 7, 8 are fluidly connected and form a single cooling fluid circuit.

[0099] In use, a cooling fluid, preferably water, is supplied to the central portion of the cooling fluid circuit 8 of the second cylindrical tubular body 3, cools the cathode 5, and is then guided towards the anode 4, and, after cooling the anode 4, is guided through the first cylindrical tubular body 2. It will be understood that the flow direction can be inversed.

[0100] The cooling fluid circuit further comprises vortex-inducing means 12 to induce a vortex-shaped flow to the cooling fluid in the portion of the cooling fluid circuit where the anode 4 is being cooled. Similarly, the cooling fluid circuit also comprises similar vortex-inducing means 13 to induce a vortex-shaped flow to the cooling fluid in the portion of the cooling fluid circuit where the cathode 5 is being cooled. With a “vortexshaped flow of the cooling fluid” is meant in the present invention that the cooling fluid is brought in a swirling motion, creating vortices within the fluid and increasing its velocity in the regions of the vortices. Such a vortex-shaped flow of the cooling fluid contributes to an enhanced heat transfer and improves the cooling rate, thereby providing a more efficient cooling of the respective electrode 4, 5.

[0101] It will be understood that such vortex-inducing means 12, 13 can also be provided to the cooling fluid circuit of the anode 4 and the cathode 5, respectively, when these cooling fluid circuits are not fluidly connected, i.e. are two separate cooling fluid circuits. It will further be understood that it is possible that only one of both cooling fluid circuits comprises such vortex-inducing means.

[0102] Any means or components capable of inducing a vortex-shaped flow to the cooling fluid and being capable of being integrated in the cooling fluid circuit(s) of the plasma torch of the present disclosure, can be used. A particularly preferred example, and as shown in Fig. 3, are threads provided in such a way that they give the cooling fluid the swirling movement mentioned hereinabove.

[0103] Fig. 4 represents a further detailed view of the plasma torch 100 ofFig. 2, more particularly the first 10 and the second 9 annular space, wherein the plasma gas is supplied. The second annular space 9, i.e. the annular space between the first 2 and the second 3 cylindrical tubular body comprises directional flow guidance means 14. The directional flow guidance means 14 are configured to let the supplied gas follow a spiral movement around the second cylindrical tubular body 3 prior to entering the first annular space 10 between the anode 4 and the cathode 5.

[0104] The inventors have discovered that such directional flow guidance means 14 positively contribute to the positioning, rotation and stabilization of the plasma arc.

[0105] Advantageously, the directional flow guidance means do not increase the velocity of the gas. Alternatively yet advantageously, the directional flow guidance means act as vortex-inducing means to the gas, thereby bringing the gas in a swirling motion and increasing its velocity.

[0106] Any means or components capable of directing the gas flow can be used as the directional flow guidance means. Non-limited examples are components that guide the gas flow in a certain direction different from a direction, parallel to the central axis of the first and the second cylindrical tubular body, a Venturi-shaped flow channel, or threads.

[0107] Advantageously, the directional flow guidance means 14 electrically insulates the electrodes 4, 5. It advantageously is the electrically insulating part situated closest to the plasma arc and thus shields any electrical insulation provided further inside the plasma torch from direct heat radiation from the plasma arc. To this end, the directional flow guidance means 14 is advantageously made of (comprises or substantially consists of) a high temperature resistant and thermal shock resistant electrically insulating material A particularly suited material for the directional flow guidance means 14 is boron nitride.Example

[0108] A plasma torch according to Figs. 2-4 of the present invention was operated in a rotary kiln and the plasma current, the current of the magnet coil, and the flow of CO2 used as plasma gas were varied. Arc current, arc voltage, magnet coil current, plasma gas flow rate and plasma torch cooling water heat loss were recorded during the testing. The plasma power, thermal efficiency and the gas to enthalpy ratio were calculated by averaging the recorded measurements for periods of stable operation lasting at least 5 minutes. Table 1 shows the different process parameters that were used, and the values that were calculated.Table 1: Thermal efficiency, plasma power and gas to enthalpy ratio for varying plasma arc current, magnet coil current, CO2 plasma gas flow rate and arc current.

[0109] From Table 1 it is clear that high thermal efficiency at high power, very high gas to enthalpy ratios and stable operation for varying power input have been obtained.

[0110] Figure 5 shows the logged data for the plasma power and the thermal efficiency after running the plasma torch at a CO2 flow of 50 Ipm, a magnet coil current of 3 A and the shown plasma current and plasma voltage, in addition to during which times the operational parameters were obtained from.

Claims

CLAIMS1 . A non-transferred direct current (DC) plasma torch (1), comprising: a first cylindrical tubular body (2) having an inner volume and a central axis (20), a second cylindrical tubular body (3) provided coaxially within the inner volume of the first cylindrical tubular body (2), a ring-shaped first electrode (4) joined to a proximal end of the first cylindrical tubular body (2), having a first electrode upper surface (40) distally arranged from the first cylindrical tubular body (2), a second electrode (5) joined to a proximal end of the second cylindrical tubular body (3), having a second electrode upper surface (50) distally arranged from the second cylindrical tubular body (3), first cooling means (7) provided within the first cylindrical tubular body (2) and configured to, in use, cool the first electrode (4), second cooling means (8) provided within the second cylindrical tubular body (3) and configured to, in use, cool the second electrode (5), a gas supply configured to, in use, provide a gas in a first annular space (10) between the first electrode (4) and the second electrode (5) via a second annular space (9) between the first (2) and the second (3) cylindrical tubular bodies, a DC power supply, configured to, in use, generate a plasma arc (11), and plasma ignition means, and a magnetic field generating means (6) provided within the first cylindrical tubular body (2).

2. The plasma torch (1) according to claim 1 , wherein the first (2) and the second (3) cylindrical tubular bodies are electrically insulated from one another.

3. The plasma torch (1) according to claim 1 or 2, wherein the second electrode (5) is disk-shaped.

4. The plasma torch (1) according to any one of the preceding claims, wherein the first electrode upper surface (40) and the second electrode upper surface (50) define a common plane (60).

5. The plasma torch (1) according to claim 4, wherein the common plane (60) is inclined at an angle between 80° and 100° from the central axis (20).

6. The plasma torch (1) according to claim 5, wherein the common plane (60) is substantially perpendicular to the central axis (20).

7. The plasma torch (1) according to any one of the preceding claims, wherein the DC power supply is configured to provide a potential from the first electrode (4) to the second electrode (5) or vice versa8. The plasma torch (1) according to any one of the preceding claims, wherein the first electrode (4) is an anode and the second electrode (5) is a cathode.

9. The plasma torch (1) according to any one of the preceding claims, wherein the magnetic field generating means (6) comprises a permanent magnet and / or a magnet coil.

10. The plasma torch (1) according to any one of the preceding claims, wherein the second electrode upper surface (50) is concave.

11. The plasma torch (1) according to any one of the preceding claims, wherein the second annular space (9) comprises directional flow guidance means (14) configured to supply a gas flow to the first annular space (10) in a rotational direction about the central axis (20).

12. The plasma torch (1) according to any one of the preceding claims, wherein the second electrode (5) comprises a central through opening.

13. The plasma torch (1) according to any one of the preceding claims, wherein the first cooling means (7) comprises a first cooling fluid circuit and / or the second cooling means (8) comprises a second cooling fluid circuit.

14. The plasma torch (1) according to claim 13, comprising the first and a second cooling fluid circuit are fluidly connected to one another.

15. The plasma torch (1) according to claim 14, wherein the first cooling fluid circuit comprises first cooling fluid vortex-inducing means (12) and / or wherein the second cooling fluid circuit comprises second cooling fluid vortex-inducing means (13).

16. The plasma torch (1) according to any one of the preceding claims, wherein at least a portion of an exposed surface of the first electrode (4) and / or at least a portion of an exposed surface of the second electrode (5) comprises a high temperature resistive electrically insulating shielding (15).

17. The plasma torch (1) according to claim 16, wherein the shielding comprises a coating, preferably wherein the coating comprises alumina.

18. The plasma torch (1) according to any one of the preceding claims, wherein a high temperature resistive and thermally insulating layer (16) is provided to at least a portion of an outer surface of the first cylindrical tubular body (2).

19. The plasma torch (1) according to any one of the preceding claims, wherein the first electrode (4) and the second electrode (5), individually, comprises copper and / or silver, and optionally an alloying element for copper and / or silver, respectively, preferably cobalt.

20. The plasma torch (1) according to any one of the preceding claims, wherein the second annular space (9) comprises directional flow guidance means (14) configured to, in use, supply a gas flow to the first annular space (10) at an inclined direction with respect to the central axis (20).

21. Use of the plasma torch of any one of the preceding claims, for uniformly distributing heat to an atmosphere surrounding the plasma torch directly from the plasma arc at temperatures equal to or higher than 5000 K.

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