Improved heat induction system

Abstract

Provided herein is an induction heating system for heating a flowing gas, the system comprising: an induction coil for generating an alternating electromagnetic field; a susceptor for converting the alternating electromagnetic field into heat, said susceptor providing an electrically closed path around an inner cavity; a magnetic flux concentrator located within said inner cavity, said magnetic flux concentrator being for providing a low reluctance path for magnetic flux through said inner cavity; a first thermally insulating region for thermally insulating the susceptor from the magnetic flux concentrator, and a second thermally insulating region for thermally insulating the induction coil from the susceptor, said first and second thermally insulating regions defining an annular heating duct that contains the susceptor; and a cooling system for maintaining at least a portion of the magnetic flux concentrator at or below its Curie temperature when the system is in use.

Description

[0001] IMPROVED HEAT INDUCTION SYSTEM

[0002] FIELD OF THE INVENTION

[0003] The present invention relates generally to a heat induction system, and more particularly to a heat induction system for heating a flowing gas.

[0004] BACKGROUND OF THE INVENTION

[0005] Induction heaters are effective systems for the contact- less heating of a variety of electrically conductive materials. Conventional induction heaters include an inductor component (e.g. a conductive coil) that is used to generate an alternating electromagnetic field. An electrically conductive target component placed within said alternating electromagnetic field will spontaneously heat due to the effect of eddy-current heating from resistive losses. If the target component is also ferromagnetic, hysteretic heating may also arise from the alternate changing of magnetic polarity. In these arrangements, the induction-heated component used to generate the heat is conventionally termed a "susceptor".

[0006] Conventional induction heaters may also be used to heat non-conducting target components, such as plastics or glass. In those instances, the susceptor can be induction-heated to generate heat which is then transferred to the non-conducting material by any of conduction, convection, and / or radiation.

[0007] Heat induction systems have also been used for the heating of gases. In those instances, a susceptor located within a heating chamber would generate heat that can be transmitted to a gas filling or flowing through said chamber, primarily by convective heat transfer between the flowing gas and the susceptor surface. By being confined to heat transfer at the susceptor surface, conventional gas induction heating systems are typically limited to small scale applications that achieve heating of small volumes of gas. For instance, gas induction heating is commonplace in chemical vapour deposition systems, where they are used to achieve precise heating of a target substrate (acting as the susceptor) and reactive gas molecules in contact with the susceptor. Conventional gas induction heating systems are also limited by poor power transfer between the alternating electrical current and the target gas, which limits both the efficiency and achievable output heating power of the system.

[0008] There remains therefore an opportunity to address limitations that characterise conventional gas induction heaters.

[0009] SUMMARY OF THE INVENTION

[0010] The present invention provides an induction heating system for heating a gas, the system comprising:

[0011] an induction coil for generating an alternating electromagnetic field,

[0012] a susceptor for converting the alternating electromagnetic field into heat, said susceptor providing an electrically closed path around an inner cavity,

[0013] a magnetic flux concentrator located within said inner cavity, said magnetic flux concentrator being for providing a low reluctance path for magnetic flux through said inner cavity,

[0014] a first thermally insulating region for thermally insulating the susceptor from the magnetic flux concentrator, and a second thermally insulating region for thermally insulating the induction coil from the susceptor, said first and second thermally insulating regions defining an annular heating duct that contains the susceptor, and

[0015] a cooling system for maintaining at least a portion of the magnetic flux concentrator at or below its Curie temperature when the system is in use.

[0016] At least a portion of the susceptor is designed to provide, when the system is in use, one or more closed electrically conducting closed paths (e.g. loops) which enclose at least a portion of the alternating electromagnetic field generated by the coil. When the system is in use, the susceptor is therefore coupled to the induction coil by alternating magnetic flux enclosed within the susceptor. In that context, placement of a magnetic flux concentrator within the cavity of the susceptor affords significant increase of said inductive coupling. As a result, increasing the total magnetic flux enclosed by the susceptor enhances the induced total current flowing in the susceptor and thereby the total resistive heating power. Due to its position relative to the susceptor, the magnetic flux concentrator located within the cavity of the susceptor is referred herein also as “inner magnetic flux concentrator”.

[0017] In some embodiments, the susceptor comprises an assembly of stacked discrete electrically conductive tubular sections. The adoption of discrete electrically conductive tubular sections stacked together ensures the required closed electrical loops around at least a portion of electromagnetic field generated by the coil, for example a loop enclosing a planar region that is perpendicular to a main axis of the induction coil when the coil and the susceptor are coaxial. Arrangements in which the susceptor comprises stacked discrete electrically conductive tubular sections also limit the mechanical effects of thermal expansion and / or stress on the assembly during use. Those arrangements also provide redundancy in case of cracking of one or more tubular sections, prolonging the lifetime of the susceptor assembly.

[0018] In alternative arrangements, the susceptor may be made of stacked electrically conductive units. Said units may have any shape conducive to their intended function. That is, said units may be any set of solid shapes that packed together form an open porous assembly comprising a network of interconnected pores through which gas can flow, and a network of electrically conductive paths through the solid components. For example, said units may be in the form of balls, disks, cylinders, and the like.

[0019] For example, the susceptor may be made of stacked electrically conductive balls which, collectively, provide the required electrically closed path(s) around an inner cavity for at least a portion of the susceptor. An example susceptor in that regard is shown in Figure 9. From the functional standpoint, an assembly of stacked electrically conductive balls can provide the required closed electrical loops around at least a portion of electromagnetic field generated by the coil. Stacked balls would form electrical paths through the stack via current passing across the points of surface contact between each ball. These arrangements are also characterised by high surface area for improved heat exchange with gas flowing in contact with the susceptor. The balls do not need to be spherical. Due to the inherently high temperatures generated by the system, the implementation of a dedicated cooling system for the inner magnetic flux concentrator has been found to be effective to maintain at least a portion of the inner magnetic flux concentrator at or below its Curie temperature when the system is in use. In some embodiments, the cooling system comprises piping thermally coupled to the inner magnetic flux concentrator and adapted to circulate a cooling medium. In those instances, cooling of the inner magnetic flux concentrator is particularly effective. Said piping may be metal piping. If metal piping is used, said metal piping may be inserted within the inner magnetic flux concentrator to ensure that heat-dissipating eddy-currents are not induced within the metal piping itself.

[0020] The system of the invention can be effectively used for fast heating of a gas to temperatures that can exceed 1,000°C. The present invention can therefore be particularly useful in the industrial processing needing fast heating of large gas streams to a significantly high temperature. Accordingly, the system of the invention may also be a system for in-line heating of a gas stream. For instance, the present invention may be successfully applied as an in-line heating system to heat a hydrogen gas stream used in iron or steel manufacturing plants. In that context, the induction heating system of the present invention may be used to heat a flowing stream of hydrogen gas to temperatures up to ~1,100°C. Once heated, the hydrogen stream can be supplied to a reactor and used for the reduction of iron ore to direct reduced iron (DRI), a key intermediate in the production of steel.

[0021] This method has a number of key benefits over conventional gas heating systems, including (1) the possibility to operate combustion-free, i.e. with no CO2 emissions from fossil fuel combustion, (2) much longer lifetime than either 'resistive element', or plasma units, as the susceptor can be much more robust (and has redundancy) compared to high current elements of electrodes, (3) service replacements of active electronics components is straightforward, as these are located in a physically separate cold module outside the heated volume, and (4) induction systems are high efficiency and readily available at MW-scale. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Embodiments will be now described with reference to the following non-limiting drawings, in which:

[0023] Figure 1 shows a cross-sectional view of an embodiment induction heating system in which the susceptor has a circular cross-section and comprises a stacked assembly of circular rings of susceptor material, the system being provided with a rod-like inner magnetic flux concentrator arranged co-axial with the susceptor,

[0024] Figure 2 shows a cross-sectional view of an embodiment induction heating system of the kind depicted in Figure 1, further having an outer magnetic flux concentrating shield,

[0025] Figure 3 shows a cross-sectional view of an embodiment induction heating system of the kind depicted in Figure 2, further being provided with a magnetic flux concentrating base plate,

[0026] Figure 4 shows a cross-sectional view of an embodiment induction heating system of the kind depicted in Figure 3, in which the outer magnetic flux shield presents an inward tapered end portion,

[0027] Figure 5 shows a cross-sectional view of an embodiment induction heating system of the kind depicted in Figure 3, having a cooling circuit flowing internally and externally to the rod-like inner magnetic flux concentrator,

[0028] Figure 6 shows cross-sectional views of example cooling circuits for the rod-like inner magnetic flux concentrator, with (a) channels drilled in ferrite for fluid cooling, (b) internal inlet and external return around the inner magnetic flux concentrator, and (c) internal inlet and internal return, Figure 7 shows an example susceptor with circular cross-section in the form of stacked round rings of susceptor material,

[0029] Figure 8 shows an example variant of the susceptor of Figure 7, in which the rings of susceptor material are provided with inner and outer ribs,

[0030] Figure 9 shows a susceptor with circular cross-section obtained by stacking balls of susceptor material in a hollow cylindrical geometry,

[0031] Figure 10 shows an example susceptor with circular cross-section where stacked rings of susceptor material are surrounded internally and externally by stacked balls of susceptor material, resulting in an overall hollow cylindrical geometry,

[0032] Figure 11 shows an embodiment susceptor with circular cross-section obtained by stacking serpentine shaped susceptor rings, resulting in alternating grooves and ribs running parallel to the central axis of the susceptor,

[0033] Figure 12 shows a vertical cross-section view of simulated electromagnetic field lines generated by the induction coil using (a) a conventional tubular inductor heater with no inner magnetic flux concentrators and (b) an embodiment induction heating system of the invention having a rod-like inner magnetic flux concentrator co-axial with both the induction coil and the susceptor,

[0034] Figure 13 shows a vertical cross-section view of simulated electromagnetic field lines generated by the induction coil using (a) an embodiment induction heating system of the invention having a rod-like inner magnetic flux concentrator co-axial with both the induction coil and the susceptor, and an outer magnetic shield, and (b) an embodiment system further having a magnetic flux concentrating base plate,

[0035] Figure 14 shows a vertical cross-section view of simulated electromagnetic field lines generated by the induction coil using an embodiment induction heating system of the invention having a rod-like inner magnetic flux concentrator co-axial with both the induction coil and the susceptor, a base magnetic flux concentrator, and an outer magnetic shield presenting an inward tapered end portion,

[0036] Figure 15 shows performance obtained using different arrangements of inner magnetic flux concentrators in terms of (a) coupling efficiency and (b) corresponding overall efficiency,

[0037] Figure 16 shows simulated output power obtainable at increasing coil currents (or apparent power) for different arrangements of inner magnetic flux concentrators,

[0038] Figure 17 shows the time profile of gas temperature measured at the outlet of the example system described in Example 2,

[0039] Figure 18 shows examples of the determination of the radial thickness of an embodiment ring-shaped susceptor of (a) thin, (b) medium, and (c) large thickness,

[0040] Figure 19 shows an example assembly of an induction heating system having multiple induction coils, and

[0041] Figure 20 shows (a) a cross-sectional and (b) top-down schematic of the arrangement shown in Figure 19.

[0042] DETAILED DESCRIPTION OF THE INVENTION

[0043] The present invention provides an induction heating system for heating of a gas.

[0044] In some embodiments, the system is adapted for in-line heating of a gas stream. By "in-line" heating means that the system can heat a gas stream as the gas flows through a flow line, for example a pipeline. Accordingly, the induction heating system may be integral to a gas line, for example integral to a gas pipeline. The overall design of the induction heating system can therefore be adapted to fit to, or be integral with, a gas pipeline. In alternative configurations, the induction heating system may be located in-line with the pipeline at the inlet to a reactor vessel or chamber, such that the heated gas is injected directly into the body of that vessel.

[0045] For instance, if the gas pipeline has a cylindrical geometry, the elements forming the induction heating system may themselves have a cylindrical geometry that fits dimensionally with the pipeline. Examples of induction heating systems with overall cylindrical geometry are depicted in Figures 1-5. In those instances, containment shell 1 may itself be part of a gas pipeline or be designed to fit to an existing gas pipeline.

[0046] The induction heating system of the invention comprises an induction coil.

[0047] As a skilled person would know, in the context of induction heating an "induction coil" is a component which function is to generate an alternating electromagnetic field when alternating electric current flows through it, such that alternating magnetic flux passes through an element (i.e. a so-called “susceptor”) so that high current densities are induced to flow within that element and hence causes high local heat dissipation. Essentially, the electromagnetic field extends through the susceptor and induces eddy currents to flow in the susceptor, which generate heat due to the inherent resistance of the susceptor.

[0048] Typically, an induction coil is a passive two-terminal electrical component that generates an electromagnetic field as a result of an applied electric current. If the applied electric current alternates at a given frequency, the polarity of the corresponding electromagnetic field also alternates at that frequency. When an electrically conducting susceptor material is placed within the alternating electromagnetic field, for example by being located within the gas line, the alternating electromagnetic field induces electrical current to flow in the susceptor material induction coil according to the well-established principles of electromagnetic induction described by Faraday’s Law. As the susceptor has a non-zero resistivity, power is locally dissipated as heat within the susceptor, according to the equation:

[0049] ^QHeat= ρJ2· ΔV where QHeatis the instantaneous heat dissipated within a susceptor volume AT, that has a resistivity p, and through which an induced electrical current density J is flowing.

[0050] In the system of the invention, the induction coil may have any configuration that is conducive to achieving heating of a susceptor material.

[0051] In some embodiments, the induction coil is wound around a susceptor of the kind described herein, such that the coil windings enclose the susceptor.

[0052] Examples of suitable induction coil configurations in that regard include helical wound coils (or solenoids consisting of a number of turns of an electrical conductor, for example a copper tube or Litz wire wound around a mandrel), and precision-machined coils obtained from a solid electrically conductive material (e.g. copper) and brazed together.

[0053] In some embodiments, the induction coil is in the form of a multi-turn helical coil. In those arrangements, the helical (i.e. solenoid) is the most common and efficient coil with the number of turns defining the width of the heating pattern.

[0054] In some embodiments, the induction coil is in the form of a single-turn coil.

[0055] In some embodiments, the induction coil is in the form of parallel coils. Those configurations afford lower inductance.

[0056] In some embodiments, the induction coil is in the form of a split helical coil. This configuration includes single or multiple turn split helical coils, and is advantageous when it is not possible to get access to the entire heating area using a single helical coil.

[0057] It will be understood that the induction heating system may comprise one or more inductions coils, provided they are assembled to provide the intended function. Accordingly, in some embodiments the induction heating system comprises one induction coil.

[0058] In some embodiments, the induction heating system comprises a plurality of induction coils. For example, the induction heating system may have two induction coils, or four induction coils, or more.

[0059] When there are multiple induction coils, the heating may be operated such that the coils are driven at different phases with respect to each other to generate or control the heating of the susceptor assembly. Multiple induction coils operated with varying phases and currents can advantageously produce a time varying magnetic field in a relatively cold temperature (e.g. less than 150°C).

[0060] An example assembly of an induction heating system having multiple coils is shown in Figure 19. This example assembly includes four induction coils 4, each wound around a magnetic flux concentrator bar 2 (e.g. ferrite bars), which are connected to the inner magnetic flux concentrator 7 (e.g. forming a unitary assembly). In use, the magnetic flux generated by each coil 4 is concentrated through the flux concentrator bars 2 and the inner magnetic flux concentrator 7 toward the susceptor 5, where it induces current and generates heat. The base magnetic flux concentrator 11 may be shaped in any manner than links the outer magnetic flux concentrators 2 to the inner 7. This approach allows for easier cooling of the coils 4 and outer magnetic flux concentrator whilst achieving a high coupling efficiency. In Figure 19, thermal insulation between the induction coils and susceptor is not shown, for convenience.

[0061] Figure 20 shows (a) a cross-sectional and (b) top-down schematic of the arrangement shown in Figure 19, highlighting the specific arrangement of the magnetic flux concentrators, the induction coils, and the susceptor.

[0062] An induction coil for use in the invention may be wound in accordance to any low-loss AC winding arrangement known to those in the art. For instance, the induction coil may be wound as a solid coil, water cooled coil, a Litz wire coil, or a water-cooled Litz wire coil. A solid induction coil would be used when significant heating is not expected, since a solid coil may not effectively dissipate heat generated from resistive heating in the coil itself. In that regard, the amount of heat generated is expected to be proportional to the resistance of the coil and the magnitude of the current. The application of an alternating electrical current on the coil may result in losses due to skin effects. Skin effects cause current to flow primarily on the surface of the conductor, increasing the current density and therefore losses.

[0063] If substantial heat is generated, active cooling of the induction coil may be necessary. In those instances, for example, removal of the core of the wound conductor may be performed to allow liquid or gas to pass through and actively cool the conductor coil. Accordingly, in some embodiments the induction coil is in the form of a hollow tube of electrically conductive material, which is designed for internal passage of a cooling medium. The cooling medium may be any cooling medium known to those skilled in the art, for example a liquid or gas cooling medium. In use, the cooling medium may be introduced at a temperature around room temperature (i.e. 15°C-35°C) or below.

[0064] The inductor coil may be made of any material that is capable of conducting an electrical current. Examples of suitable inductor materials in that regard include metals and corresponding alloys, for example copper, nickel, iron, molybdenum, aluminium, stainless steel, silicon steel, and an alloy thereof.

[0065] In some preferred embodiments, the induction coil is in the form of a hollow double helical copper coil designed for water cooling and connected in parallel.

[0066] The induction coil in the heating system of the invention may be designed for operation with alternating current at a frequency of at least 1kHz. In some embodiments, the induction coil is designed for operation with alternating current at a frequency in the range of 1kHz – 1 MHz, for example 1kHz – 750kHz, 1kHz – 500MHz, 1kHz – 250MHz. In some embodiments, the induction coil is designed for operation with alternating current at a frequency of 10-40 kHz. The electrical current may be in the range of 10-2, 000A, with a voltage between 50-2, 000V, whereby the apparent power will be in the range of 1-4,000kVA.

[0067] The axial length of the coil may be in the range of 0.1-1 m.

[0068] When the coil is a circular coil, the induction coil may be characterised by an inner diameter of at least 50 mm, at least 100 mm, at least 200 mm, or at least 400 mm. For example, the induction coil may be characterised by an inner diameter of in the range of 50-500 mm.

[0069] The system of the invention comprises a susceptor for converting electromagnetic energy generated by the inductor into heat.

[0070] As it is known in the field of induction heaters, the term "susceptor" denotes the characteristic of a component that can absorb electromagnetic energy and convert it into heat. In the context of the system of the invention, the susceptor converts the alternating electromagnetic field energy generated by the induction coil into heat by being positioned relative to the induction coil such that the susceptor is at least partially immersed in the alternating electromagnetic field generated by the inductor, when in use. In other words, when the device is in use conducting loops are formed which are ‘linked’ by lines of alternating magnetic flux. Advantageously, the device can form a conducting loop path which encloses a large area, but for which the susceptor ‘ring’ is relatively thin radially (as this increases the electrical resistance of the available electrically conducting path whilst still linking all the incident flux).

[0071] The susceptor is designed to provide an electrically closed path around an inner cavity. That is, the structure of at least a portion of the susceptor is such that it provides for at least one closed material loop around a centre space. Functionally, the susceptor provides formation of one or more closed electrical loops around its inner cavity when the susceptor is inductively coupled with the coil. Any susceptor design providing the required electrically closed path around the inner cavity may be adopted.

[0072] In some embodiments, the susceptor has an annular cross-section. By the susceptor having an “annular” cross-section, the structure of at least a portion of the susceptor is such that it provides for at least one closed material loop around the inner cavity. Said annular crosssection may have any annular geometry. Examples of suitable geometries in that regard include circular, square, rectangular, triangular, elliptic, or any other simple or complex geometry known in the art which may characterise an annulus.

[0073] In some embodiments, the susceptor has a tubular geometry. By having a “tubular” geometry, the susceptor presents as an elongated hollow tube developing along a main axis.

[0074] Without wanting to be limited by theory, the rate of heat transfer to the flowing gas may be determined by the total surface area of the susceptor that is in contact with the flowing gas. Typically, an axially longer susceptor may enable higher flows of gas to be heated to a specified temperature, provided there is also sufficient electrical power available. From a practical standpoint, in some instances the susceptor tube may be of a similar length, or slightly longer than, the coaxial electromagnetic coil. Those arrangements may be favourable to improve the coupling efficiency between the coil and susceptor.

[0075] Accordingly, the susceptor may have any length conducive to its intended function. For example, the susceptor may have a length of at least 100 mm, at least 500 mm, or at least 1,000 mm along a main axis. In some embodiments, susceptor has a length of from about 100 mm to about 1,500 mm.

[0076] The thickness of the susceptor may also be tailored in relation to the skin effect. As a skilled person would know, the skin effect is the tendency of an alternating electric current to become distributed within a conductor such that the current density is largest near the surface of the conductor and decreases exponentially with greater depth into the conductor. In those instances, the thickness of the susceptor may be tailored to the skin depth of the applied current to improve heating effects. When the electromagnetic field induces a current in the susceptor, the current flows primarily at the surface of the susceptor.

[0077] In general, the higher the operating frequency the shallower the skin depth, whilst the lower the operating frequency the deeper the skin depth and hence penetration of the heating effect. For a given susceptor material, skin depth or penetrating depth is dependent on the operating frequency and material properties at the temperature of the part.

[0078] Skin depth is a function of susceptor resistivity and operating frequency, according to the equation:

[0079] 1

[0080] 8 = >

[0081] √(σπfμ)

[0082]

[0083] Skin depth

[0084] material conductivity

[0085] ■<: Frequency of the AC signal

[0086] Magnetic permeability

[0087] On that basis, as skilled person may design the susceptor according to steps such as (1) choosing a susceptor material which can be machined and has desirable mechanical and heat transfer properties at operating temperature (e.g. graphite), (2) consider a desired operating frequency (typically a function of power and cost for available componentry) so that skin depth is sensible (e.g. in the range 3-50mm), and (3) set the radial thickness of the susceptor to be less than said skin depth. While an optimum value may depend on the specific geometry of the susceptor, one may consider a value usually in the range of 0.2xskin depth to 0.8xskin depth.

[0088] Example determinations of the radial thickness of embodiment susceptors are provided in Example 3.

[0089] In some embodiments, the susceptor has a thickness, for example an annular thickness, of at least about 0.5 mm, at least about 1 mm, at least about 5 mm, at least about 10 mm, or at least about 25 mm. For example, the susceptor may have a thickness, for example an annular thickness, of from about 0.5 mm to about 50 mm.

[0090] In some embodiments, the susceptor comprises walls having a thickness of at least about 0.5 mm, at least about 1 mm, at least about 5 mm, at least about 10 mm, or at least about 25 mm. For example, the susceptor may comprise walls having a thickness of from about 0.5 mm to about 50 mm.

[0091] The susceptor is made of any material making it suitable to operate as a susceptor in the context of an inductive coupling with the coil. Typically, the susceptor would be made of an electrically conductive material. Suitable susceptor materials in that regard include materials having an electrical conductivity in the range of 100 to 300,000 S / m. The susceptor material may also be selected to have a melting temperature that is at least 200°C higher than the target output temperature of the gas.

[0092] In some embodiments, the susceptor material is selected from graphite, partially graphitized carbon, steel, tungsten, tungsten carbide, carbon foam, molybdenum, stainless steels, niobium, nickel, nickel-chromium alloy, nickel-containing ferrous alloy, chromium-containing ferrous alloy, iron-chromium-aluminium alloy, titanium, and titanium alloy. Any of those alloys may be a nanocrystalline alloy, or an amorphous alloy.

[0093] In some embodiments, the susceptor has a foam structure, for example an open-cell foam, obtained using a suitable susceptor material. For example, the susceptor may be made of electrically conductive porous susceptor foam material. In those instances, the susceptor may present significant contact surface area for improved heat transfer rates with gas flowing in contact with the susceptor.

[0094] In some embodiments, the susceptor is in the form of a unitary body. By the susceptor being in the form of a "unitary body" is meant that the susceptor is a single and discrete physical entity, as opposed to resulting from the union of discrete units. In some embodiments, the susceptor is an assembly of discrete susceptor units, for example an assembly of discrete electrically conductive units. Those configurations accommodate any heat-induced expansion of each unit and ensure structural integrity of the susceptor during use.

[0095] In some embodiments, the susceptor comprises a plurality of stacked tubular sections. Said sections could be made of a susceptor material of the kind described herein. For example, each tubular section may be an electrically conductive tubular section. Those configurations accommodate any heat-induced expansion of each unit and ensure structural integrity of the susceptor during use. Axial segmentation also provides redundancy in case of cracking of a susceptor section, as the operation of the heating system can continue even if single or multiple sections crack, provided at least some sections remain intact to form an electrically conducting loop.

[0096] In some embodiments, each tubular section has an axial length of from 5 mm to 100 mm.

[0097] The tubular sections may be directly stacked on top of each other.

[0098] In alternative configurations, the tubular sections may be stacked at a distance from one another. Such distance may be in the range of, for example, 0.1-10 mm. In these configurations, each section can move freely and independently of the others, allowing each unit to expand or contract independently from the others, minimising build-up of thermal stress between sections.

[0099] In some embodiments, the susceptor comprises a plurality of hollow cylindrical sections coaxially aligned. An example in that regard is depicted in Figure 7. This configuration can advantageously limit adverse mechanical effects of thermal expansion and / or stress, and provide redundancy in case of cracking.

[0100] In some embodiments, the susceptor comprises an assembly of stacked balls. Each ball could be made of a susceptor material of the kind described herein. For example, each ball may be an electrically conductive ball. This configuration ensures the required closed electrical loops around a central space while offering extended surface area for heat transfer to gas flowing through the susceptor. An example arrangement of this kind is shown in Figure 9. While the Figure depicts the thermal transfer elements as balls, any other shape which packs in a porous arrangement while providing the required closed electrical loops (e.g. cylinders, tubes, etc.) may be used instead of balls.

[0101] Balls stacked to form the susceptor may have any dimension that is conducive to their intended use. In some embodiment, each ball has a main dimension of at least about 3 mm, at least about 10 mm, or at least about 20 mm. For example, each ball may have a main dimension from about 3 mm to about 20 mm.

[0102] In some embodiments, the susceptor comprises a combination of stacked tubular sections and stacked balls, collectively arranged in an overall hollow cylindrical geometry to provide the required electrically closed path around the inner cavity. An example configuration in that regard is shown in Figure 10.

[0103] In some embodiments, the discrete electrically conductive sections forming the susceptor comprise geometric features which, once the sections are stacked to form the susceptor, align to generate grooves and / or ribs along the susceptor outer surface. Examples in that regard are shown in Figures 7 and 11. Figure 8 shows an example susceptor obtained by stacking rings of susceptor material having inner and outer spiral ribs. Figure 11 shows an embodiment susceptor with circular cross-section obtained by stacking serpentine shaped susceptor rings, resulting in alternating grooves and ribs running parallel to the central axis of the susceptor.

[0104] Geometric features on the susceptor (such as grooves, ribs, fins, etc.) can contribute to generate turbulence in the gas flowing along the susceptor, resulting in improved heat transfer from the susceptor surface to the gas. For instance, the spiralling ribs in the assembly of Figure 8 may induce swirling motion to gas flowing along the susceptor, improving heat exchange and, ultimately, increasing the heating rate of the gas. The cross-section of the susceptor and its location relative to the induction coil ensure formation of ring-shaped conductive paths that enclose a region of alternating magnetic flux when the induction coil is activated.

[0105] In some embodiments, the induction coil and the susceptor are positioned relative to one another such that the induction coil wounds around the susceptor, such that the induction coil encloses the susceptor.

[0106] In some embodiments, the induction coil and the susceptor have a concentric and coaxial geometry. Examples of arrangements in that regard are shown in Figures 1-5. From the functional standpoint, in those arrangements the geometry of the susceptor and its location relative to the induction coil are intended to ensure formation of closed electrical loops around the main axis of the resulting assembly.

[0107] In the system of the invention, the susceptor converts electromagnetic energy provided by the induction coil into heat. Typically, the alternating electromagnetic field generated by the induction coil induces a corresponding current distribution to flow in the susceptor. Without wanting to be confirmed by theory, according to a first heating principle the induced current flowing through the susceptor generates heat within the susceptor from resistive losses. According to a second heating principle, hysteretic heating may also be in effect when the susceptor is a made of a ferromagnetic material (such as carbon steel). In those instances, heat is also generated within the susceptor by the alternating electromagnetic field changing the magnetic polarity within the susceptor itself. Hysteretic heating occurs only up to the Curie temperature of the ferromagnetic material (e.g. 750°C for steel), as above this temperature the material’s magnetic permeability decreases to 1.

[0108] In the system of the invention, the susceptor can be designed to maximise coupling efficiency with the induction coil. In general, coupling efficiency is a measure of the apparent power in the inductor circuit versus the real power dissipated in the susceptor. Whilst coupling efficiency is not directly related to the overall efficiency, it influences the magnitude of the apparent power required for a given heating power. The greater the apparent power within the electrical system, the larger the parasitic electrical losses.

[0109] The location and geometry of the susceptor relative to the induction coil plays a role in modulating the coupling efficiency between the susceptor and the induction coil. In general, the susceptor and the induction coil may be shaped and mounted to minimise the distance between them, since the intensity of the electromagnetic field is strongest near the induction coil and decreases with the distance from the induction coil.

[0110] In some embodiments, a distance between the inner boundary of the coil and the outer boundary of the susceptor is at least about 3 mm, at least about 15 mm, at least about 25 mm, at least about 50 mm, or at least about 100 mm. For example, the distance between the inner boundary of the coil and the outer boundary of the susceptor may be from about 3 mm to about 100 mm.

[0111] The system of the invention comprises a magnetic flux concentrator located within the inner cavity of the susceptor. As a result, magnetic flux concentrator is surrounded at least in part by the susceptor. This magnetic flux concentrator is referred herein also as the “inner magnetic flux concentrator”.

[0112] As it is known in the art, a magnetic flux concentrator is an element made of low-loss soft magnetic material (i.e. a material with high permeability, low coercivity, and low eddy current losses) that can provide a low reluctance magnetic path for magnetic flux generated by the induction coil in a defined space. Having higher permeability than air, high permeability magnetic materials can form a path of least reluctance which concentrates magnetic flux within the material. Flux density can therefore be increased in desired areas, and this can be used to increase the inductive coupling between a pair of coils. This principle is used in power transformers, where primary and secondary coils are both wound onto a single ferromagnetic core. High permeability materials can also be used to shield flux from areas where unwanted flux leakage would lead to parasitic energy losses or other effects. When the system is in use, the magnetic flux concentrator provides a low reluctance path for magnetic flux through said inner cavity. Advantageously, a low reluctance magnetic circuit can channel / guide the magnetic field to intercept the susceptor or a number of susceptors where part of this low reluctance magnetic circuit may be in a medium temperature range between 150°C and 300°C.

[0113] From the functional standpoint, the proposed arrangement of the inner magnetic flux concentrator essentially ensures that the inner magnetic flux concentrator is located inside the closed conductive loops provided by the susceptor, when in use. In the proposed arrangement, the susceptor (for example each tubular section of conductive material forming a susceptor) is coupled to the induction coil by alternating magnetic flux enclosed within the susceptor’ s inner cavity. Presence of the inner magnetic flux concentrator located within said inner cavity advantageously enhances that inductive coupling by concentrating magnetic flux inside the susceptor (for example inside each tubular section of conductive material forming a susceptor). As a result, increased flux through the susceptor increases induced currents flowing in the closed conductive loops, thereby enhancing the total available heating power.

[0114] In some embodiments, the inner magnetic flux concentrator is an elongated element, for example located along a main length axis of the susceptor.

[0115] In some embodiments, the inner magnetic flux concentrator is a rod-like shaped element located along a main length axis of the susceptor. Example arrangements in that regard are shown in Figures 1-5 (element 7).

[0116] The inner magnetic flux concentrator may have any cross-section area and geometry conducive to the concentrator performing its intended function.

[0117] For instance, when the inner magnetic flux concentrator and the coil are in a co-axial arrangement,,1 / r v > ACOii where AM is the cross-sectional area of the inner magnetic flux concentrator,,uris the relative permeability of the inner magnetic flux concentrator, and ACOii is the cross-sectional area enclosed by the induction coil. In those instances, the minimum cross-sectional area of the inner magnetic flux concentrator may be determined according to:

[0118] Magnetic flux inside coil Minimum cross sectional area = — - - — -; - - Saturation flux density

[0119] where Magnetic flux inside coil = Coil current x Inductance.

[0120] In some embodiments, the inner magnetic flux concentrator has a cross-section area of at least 15 mm2, at least 100 mm2, at least 500 mm2, at least 1,000 mm2, at least 10,000 mm2, at least 50,000 mm2, at least 100,000 mm2, or at least 200,000 mm2. For example, the inner magnetic flux concentrator may have a cross-section area of from about 15 mm2to about 130,000 mm2. Further, the inner magnetic flux concentrator may have a cross-section geometry that includes circular, squared, rectangular, elliptical, etc.

[0121] From the dimensional standpoint, the inner magnetic flux concentrator may have any axial length conducive to the concentrator performing its intended function. Typically, the inner magnetic flux concentrator has an axial length that is about 1.2 times or less than the total susceptor length.

[0122] In some embodiments, the inner magnetic flux concentrator has an axial length of about 0.1-1 m.

[0123] In some embodiments, the inner magnetic flux concentrator is in the form of a unitary body.

[0124] In some embodiments, the inner magnetic flux concentrator is made of discrete magnetic flux concentrator units. In those instances, magnetic flux concentrator units may be stacked together to form the inner magnetic flux concentrator. The units may have any geometry conducive to formation of the intended inner magnetic flux concentrator. For example, the inner magnetic flux concentrator units may be in the form of disks, rings, plates, cylinders, hollow cylinders, etc. In some embodiments, the inner magnetic flux concentrator is made of stacked magnetic flux concentrator disks.

[0125] In some embodiments, the length of the inner magnetic flux concentrator is approximately equal to the length of the induction coil.

[0126] The inner magnetic flux concentrator may be made of any material suitable to interact and concentrate magnetic flux. In some embodiments, the inner magnetic flux concentrator is made of a material having an electrical conductivity of a factor of at least 10 less than that of the susceptor.

[0127] Suitable materials for the inner magnetic flux concentrator include materials that combine a relative magnetic permeability greater than 10, and an electrical conductivity less than 5,000 S / m.

[0128] In some embodiments, the inner magnetic flux concentrator is made of a ferromagnetic or ferrimagnetic material. Below their respective Curie temperature, atomic dipole moments within the crystal lattice of these materials cooperatively align, which results in a net magnetic moment when an external magnetic field is applied and hence concentrates magnetic flux within these materials.

[0129] Examples of suitable magnetic flux concentrator materials include ferrite, permalloy, NiFeMo, supermalloy, nickel alloys, iron alloys, cobalt alloys, and manganese alloys. In some embodiments, the inner magnetic flux concentrator is made of ferrite.

[0130] Typically, where an electrically conducting metal flux-concentrating material is chosen, it may be a component within a composite body arranged to prevent bulk eddy currents being generated within the inner magnetic flux concentrator body, e.g. in laminate or particulate form. Magnetic flux concentrators may be made from magnetic materials which only exhibit magnetic flux concentrating properties below their curie temperature. As used herein, the expression "magnetic material" is used to refer to a soft magnetic material having a relatively high magnetic permeability, low coercivity, and low remanence. Examples of suitable magnetic materials in that regard include one or more of ferrite, permalloy, NiFeMo, supermalloy, a nickel alloy, an iron alloy, a cobalt alloy, and a manganese alloy. In some cases the magnetic material may be nanocrystalline.

[0131] In some preferred embodiments, the inner magnetic flux concentrator is an assembly of axially stacked ferrite rings.

[0132] To effectively provide magnetic flux concentration when the system is in use, at least a portion of the inner magnetic flux concentrator should be maintained at or below the Curie temperature of the inner magnetic flux concentrator material. By way of reference, ferrite materials have Curie temperatures typically in the range of 100°C-300°C.

[0133] Accordingly, the system of the invention also comprises a cooling system for maintaining at least a portion of the inner magnetic flux concentrator at or below its Curie temperature when the system is in use. It will be understood that for the cooling system to perform its intended function, the cooling system should be thermally coupled with the inner magnetic flux concentrator. By being "thermally coupled" with the inner magnetic flux concentrator, the cooling system and the inner magnetic flux concentrator are positioned relative to one another such that thermal energy can transfer from the inner magnetic flux concentrator to the cooling system.

[0134] In some embodiments, the cooling system is designed for use with a cooling medium. Suitable cooling media include fluids that can absorb and remove thermal energy from the inner magnetic flux concentrator. Examples of suitable cooling media in that regard include gases (e.g. air, nitrogen, etc.), water, oil, water-soluble coolants, salt baths, poly-siloxanes, fluorinated hydrocarbons, poly-glycol ethers, etc. Provided thermal contact is achieved, the cooling system may be located internally or externally to the inner magnetic flux concentrator.

[0135] In some embodiments, the cooling system comprises piping thermally coupled to the inner magnetic flux concentrator and adapted to circulate a cooling medium.

[0136] In some embodiments, said piping is provided in a cavity of the inner magnetic flux concentrator. This arrangement ensures that magnetic flux is shielded, in part or fully, from impinging on the cooling pipe walls.

[0137] In some embodiments, the piping thermally coupled to the inner magnetic flux concentrator and adapted to circulate a cooling medium is metal piping.

[0138] In some embodiments, the piping forming the cooling system is in the form of a double pipe made up of two concentric tubes located inside a cavity of the inner magnetic flux concentrator. The piping is designed to allow circulation of a cooling medium through the concentric pipes, for example by introducing the cooling medium into the inner pipe and making it flow out through the outer pipe. An example arrangement in that regard is shown in Figures 1-4 (element 8). In those instances, the cooling system is a metal double pipe running through a central axial cavity of a rod-like magnetic flux concentrator.

[0139] In some embodiments, the cooling system comprises a pipe running through an inner cavity of the inner magnetic flux concentrator, with an outlet on the outer surface of the concentrator such that cooling medium exits from the outlet and wets the outer surface of the concentrator. An example arrangement in that regard is shown in Figure 5 (element 8).

[0140] Figure 6 shows cross-sectional views of additional example cooling circuits for the rod-like inner magnetic flux concentrator. The arrangement of Figure 6(a) is made of channels drilled in the inner magnetic flux concentrator (e.g. ferrite) for fluid cooling. Figure 6(b) depicts an arrangement having internal inlet and external return around the inner magnetic flux concentrator. Figure 6(c) shows an arrangement having an internal inlet and internal return. The system of the is invention employs a combination of thermal insulation and cooling to ensure that at least a portion of the inner magnetic flux concentrator remains below its Curie temperature during gas heating operation. Thermal insulation is also needed to protect components of the system from overheating during use. Any of the thermally insulation regions may be made from layers of different insulation types.

[0141] Accordingly, the system of the invention comprises a first thermally insulating region for thermally insulating the susceptor from the inner magnetic flux concentrator.

[0142] The first thermally insulating region is intended to thermally shield the inner magnetic flux concentrator from the susceptor, thereby assisting to prevent the inner magnetic flux concentrator from overheating.

[0143] In addition, the first thermally insulating region is intended to define a gas boundary to confine gas flow close to the inner surface of the susceptor, thus ensuring effective heat transfer between the susceptor and flowing gas.

[0144] The system of the invention also comprises a second thermally insulating region for thermally insulating the induction coil from the susceptor.

[0145] The purpose of the second thermally insulating region is to suppress the heat flux transmitted from the susceptor to the induction coil, as well as to any other element external to the susceptor (e.g. gas containment shell, etc.). This thermal insulting region serves to minimise parasitic heat leakage thus increasing the overall thermal efficiency of the system.

[0146] In addition, the second thermally insulating region is intended to define a boundary to confine gas flow close to the outer surface of the susceptor, thus ensuring effective heat transfer between the susceptor and gas. The second thermally insulating region may have any design suitable to thermally insulate the induction coil from the susceptor. For example, the second thermally insulating region may be provided to envelop at least a portion of the coils, for example the entirety of the coils. An example in that regard is shown in Figures 1-5. In alternative configurations, the second thermally insulating region may be provided as a thermal insulation layer interposed between the coil and the susceptor.

[0147] The first and / or second thermal insulating regions may be made of a thermally insulating material that is chemically and physically stable at temperatures of 1,000°C or above, for example l,200°C or above. Suitable materials in that regard include ceramics.

[0148] In some embodiments, the first or second thermal insulating region is made of alumina fibreboard, alumina silicate, calcium silicate, zirconia, or silica aerogel.

[0149] In some embodiments, the first thermal insulating region is made of alumina fibreboard.

[0150] In some embodiments, the second thermal insulating region is made of alumina silicate.

[0151] Collectively, the first and second thermally insulating regions define an annular heating duct that contains the susceptor. Specifically, the annular heating duct has an outer boundary defined by the inner boundary of the second thermal insulating region, and an inner boundary defined by the outer boundary of the first thermal insulating region. As a result, gas can flow through said annular heating duct with annular flow and in physical contact with the susceptor for heat exchange.

[0152] By the annular heating duct “containing” the susceptor is meant that the susceptor is located within the annular volume of the heating duct, for example in a co-axial arrangement. An example in that regard is shown in Figures 1-5.

[0153] The annular heating duct may have any annular cross-sectional geometry conducive to effective thermal contact between gas flowing through said duct and the susceptor. For example, the annular heating duct may be a cylindrical annular heating duct, a square annular heating duct, etc.

[0154] The annular heating duct may have any dimensions conducive to effective thermal contact between gas flowing through said duct and the susceptor. In general, said annular heating duct may be characterised by an annular space that is sufficiently narrow to promote close thermal contact between the flowing gas and the hot susceptor surface, resulting in high convective heat transfer to the gas. In some embodiments, the annular space is designed to provide at least 10 W / m²·K convective heat transfer rate to the gas.

[0155] In some embodiments, the annular heating duct defines an annular space having a width of 50 mm or less, for example 25 mm or less. In some embodiments, the annular heating duct have an inner wall and an outer wall each at a distance of about 25 mm or less from the susceptor.

[0156] In some embodiments, the system of the invention further comprises a thermally insulating base, for example in the form of a plate of thermal insulation material located upstream of the susceptor (relative to the direction of gas flow when the system is in use) and extending laterally to join with the second thermally insulating region. An example in that regard is shown in Figures 1-5, with reference to element 9. The thermally insulating base may be made of a thermally insulating material of the kind described herein, for example alumina fibreboard or alumina silicate.

[0157] In some embodiments, the system of the invention further comprises a thermally insulating end portion, for example in the form of a thermally insulating cap placed downstream of the susceptor (relative to the direction of gas flow when the system is in use), and extending laterally to join with the second thermally insulating region. An example in that regard is shown in Figures 1-5, with reference to element 3 A. The thermally insulating end portion may be made of a thermally insulating material of the kind described herein, for example alumina fibreboard or alumina silicate. In some embodiments, the system further comprises an outer magnetic flux shield surrounding the induction coil. The outer magnetic flux shield is designed to concentrate fringing fields, preventing eddy currents from being generated in the gas containment shell. Similar to the magnetic flux concentrator, the outer magnetic flux shield may also be designed to act as a flux guide that reduces the reluctance of the magnetic path followed by returning flux. In this way the inductive coupling between the coil and susceptor can be increased.

[0158] The outer magnetic flux shield should be made from a high permeability magnetic material which, in use, should be kept below its Curie temperature. To that effect, the outer magnetic flux shield may be provided with a cooling system, for example designed to force cool the shield, to ensure at least a portion of the shield can be maintained below its Curie temperature during use.

[0159] In some embodiments, the outer magnetic flux shield is in the form of a unitary body.

[0160] In some embodiments, the outer magnetic flux shield is formed by discrete flux concentrating units. For example, the outer magnetic flux shield may be formed by an array of high permeability magnetic rods, for example an array of ferrite rods, positioned to form an outer cage, such as a cylindrical cage.

[0161] The outer magnetic flux shield may be shaped to taper or angle inwards to reduce the reluctance of the magnetic path between the inner magnetic flux concentrator and the outer magnetic flux shield, thus increasing the total magnetic flux linked by the susceptor assembly. Accordingly, in some embodiments the outer magnetic flux shield presents an inward tapered end portion. An example arrangement in that regard is shown in Figure 4, with reference to elements 2.

[0162] Suitable materials for outer magnetic flux shield include ferri / ferro-magnetic materials typically have a relative permeability greater than 10. In the absence of said outer magnetic flux shield, the system of the invention may be provided with an outer gas containment shell made from an electrical insulating material to prevent parasitic losses due to eddy current heating of the outer shell material.

[0163] In some embodiments, the system further comprises a magnetic flux concentrating plate located upstream of the susceptor and arranged to provide a low reluctance path between the outer magnetic shield and the inner magnetic flux concentrator. As a result, the flux density passing through the susceptor is increased and thereby coupling efficiency can be further enhanced. In some embodiments, the magnetic flux concentrating base plate is in the form of a circular ferrite plate with holes drilled for gas inlets, the cooling system, and the induction coil leads. An example system that comprises a magnetic flux concentrating base plate is shown in Figures 3-5.

[0164] The system of the invention may include a gas inlet for introducing gas into the annular heating duct, and a gas outlet for collecting heated gas from the annular heating duct. The specific inlet positioning, type and setup may be any known to those in the art which will achieve gas flow through the annular duct and in contact with the susceptor. Examples of inlet arrangements in that regard are shown in Figures 1-5. In those instances, the system presents four gas inlets (two shown) at the bottom of the assembly through the base plate that allow introduction of gas into the annular duct to contact the susceptor. The annular duct tapers downstream of the susceptor into a single outlet for extraction of heated gas.

[0165] The inlets may be made of any material suitable for the intended purpose. In that regard, suitable materials for the inlets would be those that are impermeable to the gas, can withstand operational gas temperatures and pressures required for gas flow, and are chemically inert to the gas. In some embodiments, the inlets are in the form of pipes, for example metal pipe, providing fluid connection with the annular duct. The pipes may be made of metal or polymeric material. Practically, the total cross-sectional area of all pipework may be designed to be sufficient to ensure that the gas velocity within these pipes does not exceed ~ 50% of the speed of sound for the gas mixture being used. For the outlet, the outlet material should additionally be capable to handle gas exiting the annular duct at high temperature. Suitable materials in that regard include metal alloys such as Inconel, nickel-based superalloys, cobalt based superalloys, stainless steel, titanium alloys, etc.

[0166] When in use, the induction heating system described herein can bring a gas flowing within the system to a temperature of 1,000°C or above. For example, the outlet gas temperature may be in the range of 750-1, 100°C, heated from a starting inlet gas temperature of 20-700°C. Gas may flow through the system at a flow rate in the range of 100 to 50,000 SLPM. As such, the present invention can advantageously be performed to heat streams of any gas requiring heating, for example within an industrial process.

[0167] In some embodiments, the gas comprises one or more reducing gases. By "reducing gas" is meant a gaseous agent which acts as electron donor in a redox reaction. Examples of reducing gases include hydrogen, carbon monoxide, methane, and ammonia.

[0168] In some embodiments, the gas comprises one or more inert gases. Examples of inert gases include nitrogen, argon, helium or any other noble gas.

[0169] Accordingly, the present invention can be particularly useful in industrial processes which need fast heating of large gas streams.

[0170] For instance, the present invention may be successfully applied as in-line heating system to heat hydrogen gas streams used in iron manufacturing plants. In that context, the induction heating system of the present invention may be used to heat a flowing stream of hydrogen gas to temperatures up to ~1,100°C. Once heated, the hydrogen stream can be supplied to a reactor and used for the reduction of iron ore to direct reduced iron (DRI), a key intermediate in the production of steel. Additional applications of interest may include heating hydrogen to produce ammonia via the Haber-Bosch process, or to make Sustainable aviation fuel (SAF) via the Fischer-Tropsch process or similar. Alternatively, a gas (e.g. nitrogen) could simply be heated to provide gas heating to a secondary process vessel. In some embodiments, the system has an outer gas containment shell for preventing gas leakage. The purpose of the gas containment shell is to provide a hermetically sealed outer layer that prevents gas leakage to the surrounding environment and to prevent ingress of gas from the surrounding environment into the heated gas flow, and to house the other components. The containment shell may be made from a metal or a ceramic. In some embodiments, the containment shell is made from stainless steel.

[0171] The system may further include a base sealing plate located upstream of the annular heating duct. The sealing plate can be designed to ensure the system is gas tight and to provide hermetic feedthroughs for the delivery of gas, cooling fluid and electrical current to the assembly. For instance, the sealing plate may seal to the gas containment shell using a layer of sealing rubber (e.g. viton, neoprene) or make use of an O ring. If the plate is made from an electrically conductive material, electrical feedthroughs should be electrically insulated from the plate. The sealing plate itself may also be force-cooled. In some embodiments, the sealing plate is made of stainless- steel or brass. An example sealing plate is shown in the schematics of Figures 1-5 (element 12).

[0172] Figure 1 shows a cross-sectional schematic view of an induction heating system in accordance with an embodiment of the present invention. The system assembly depicted in the Figure is characterised by a cylindrical geometry. The system assembly is adapted to be connected to an existing pipeline through connecting plates 12 and 13.

[0173] The assembly is contained within a gas impermeable containment shell 1. The containment shell 1 may be made of an electrically conducting material. In those instances, the outer magnetic flux shield may prevent any induction heating, and / or a cooling system (not shown) may have to be associated with shell 1 to dissipate excess induction heat generated during use. The system includes an induction coil 4, which in the schematic is in the form of a water cooled double 4.5 coiled hollow tube. Cooling water can flow along the inner cavity of the coil.

[0174] Susceptor 5 is in the form of hollow cylindrical sections (or rings) stacked along a main vertical axis. Collectively, the stacked sections result in a susceptor. A schematic of the susceptor coil is shown in Figure 7.

[0175] The system of Figure 1 is provided with a rod-like inner magnetic flux concentrator 7 located within the inner cavity of the susceptor 5, concentric and co-axial with the susceptor 5. As the coil generates magnetic flux it is extended through the susceptor toward the inner magnetic flux concentrator 7. Concentrating more flux through the susceptor increases coupling efficiency (CE).

[0176] When the system is in use, the inner magnetic flux concentrator concentrates magnetic flux at the centre of the assembly, thus increasing the magnetic flux which links each hollow cylindrical section of the susceptor. Increasing flux through the susceptor increases the coupling efficiency between the induction coil and the susceptor rings, which leads to an increase in the total dissipated power within the susceptor.

[0177] As the flux density increases down toward the bottom of the flux concentrator its radius may increase to avoid saturation (not shown).

[0178] To maintain the inner magnetic flux concentrator 7 below its curie temperature an inner cooling circuit 8 is used. In Figure 1 the inner cooling circuit only cools internal to the inner magnetic flux concentrator 7. The cooling circuit 8 is a metal double pipe running through a central axial cavity of the rod-like magnetic flux concentrator 7.

[0179] Outer thermal insulation region 3 is intended to restrict the heat transfer from the susceptor 5 and gas to the gas containment shell 1 and the induction coil 4. Central insulation 6 is used to restrict heat transferred from the susceptor 5 and the gas to the inner magnetic flux concentrator 7.

[0180] The system of Figure 1 includes a thermal insulation base 9 to restrict the heat transfer from the susceptor to the sealing plate 12. The thermal insulation base 9 has a circular groove on its top surface to accommodate the base of susceptor 5.

[0181] One or more gas inlets 10 are used to introduce gas into the annular duct containing the susceptor 5. In the embodiment schematic of Figure 1, four gas inlets are used (two of which are shown in the cross-sectional plane).

[0182] Sealing plate 12 provides a gas tight seal to the gas containment shell 1 while allowing various feedthroughs including induction coil 4, gas inlets 10, cooling circuit 8.

[0183] The output connection 13 is an example of how subsequent processes could connect to the unit downstream of the system.

[0184] Figure 2 shows a cross-sectional view of an induction heating system embodiment which has an inner magnetic flux concentrator 7 and outer magnetic flux shield 2. In this embodiment, outer insulation 3 and base insulation 9 have cavities to accommodate the outer magnetic flux shield 2. Both inner magnetic flux concentrator 7 and outer magnetic flux shield 2 encourage the flux to pass through the electrical loop formed by the susceptor thus increasing CE. The gas containment shell 1 may now be made from an electrically conductive material as fringing fields are suppressed by the outer magnetic flux shield 2.

[0185] Figure 3 shows a cross-sectional view of an induction heating system embodiment which has an inner magnetic flux concentrator 7, an outer magnetic flux shield 2, and a magnetic flux concentrating base plate 11. In this arrangement, the thickness of base insulation 9 is reduced relative to the embodiment shown in Figure 2 to make room for the magnetic flux concentrating base plate 11. Elements 2,7, and 11 work together to guide the magnetic flux in a low reluctance path which passes through the susceptor, significantly increasing coupling efficiency.

[0186] Figure 4 shows a cross-sectional view of an induction heating system embodiment which has an extended outer magnetic flux shield 2 with an inward tapered end portion, which functions to guide more magnetic flux through the susceptor and the top of the outer magnetic flux shield. In this arrangement, outer insulation 3 has an additional cavity for the extended outer magnetic flux shield.

[0187] Figure 5 shows a cross-sectional view of an induction heating system embodiment which has a different inner cooling circuit 8. In this arrangement, elements 6 and 9 have increased internal diameters to accommodate for a larger inner cooling circuit 8. The cooling fluid is fed up through inside the inner magnetic flux concentrator 7 and back down on the outside, ensuring the entire body of the inner magnetic flux concentrator 7 is kept below its curie temperature.

[0188] Figure 6 shows cross-sectional views of example cooling circuits for a rod-like inner magnetic flux concentrator made of ferrite material. The diagrams show: (a) two co-axial channels internal to the ferrite body, with the outer channel in direct thermal contact with the ferrite body; (b) an internal cooling fluid channel connected to an external cooling fluid channel that directly cools the outer surface of the inner magnetic flux concentrator; and (c) two internal channels within the ferrite body that are both in thermal contact with the ferrite body.

[0189] Figure 7 depicts a susceptor 5 made of a plurality of annular susceptor segments co-axially aligned and stacked. Axially aligned and separated susceptor segments 5 assist with limiting thermal stress on the susceptor and can provide redundancy in case of cracking. This arrangement ensures that the operation of the heating system can continue if a segment cracks, extending the lifetime of the susceptor. Axial segments may be 5-100 mm sections with 0.1-10 mm spacing or be directly stacked upon.

[0190] Figure 8 depicts a cross-sectional view of a susceptor 5 made of a plurality of annular susceptor segments co-axially aligned and stacked. In this embodiment threads are added to induce a swirling motion into the gas stream which will increase the heat transfer from the susceptor 5 to the gas stream.

[0191] Figure 9 shows a cross-sectional view of a susceptor obtained by stacking balls of susceptor material (other system features not shown). The Figure shows the staked ball in a spatial configuration they would assume, for example, when used to fill the annular heating duct formed between the outer insulation and the central insulation. To act as a susceptor these balls must be electrically conductive. Balls may be thermally conductive to effectively transfer heat throughout the internal volume, whilst simultaneously providing a high surface area to convectively heat the flowing gas. The balls may have a diameter in the range of 3-20 mm and offer extended surface area for heat transfer. While the Figure depicts the thermal transfer elements as balls, any other shape which packs in a porous arrangement (e.g. cylinders) may be used instead of balls.

[0192] Figure 10 shows a cross-sectional view of a susceptor with stacked balls surrounding susceptor rings, which may be of the kind as those shown in Figure 7.

[0193] Figure 11 shows a cross-sectional view of a susceptor obtained by stacking serpentine shaped susceptor rings, resulting in alternating grooves and ribs running parallel to the central axis of the susceptor. These geometric features may also be described as radial fins along the outer and inner edge. These features can advantageously provide additional surface area for heat exchange between the flowing gas and susceptor.

[0194] Figure 12 shows a vertical cross-section view of simulated electromagnetic field lines generated by the induction coil using (a) a conventional tubular inductor heater with no inner magnetic flux concentrators and (b) an embodiment induction heating system of the invention having a central rod-like inner magnetic flux concentrator co-axial with both the induction coil and the susceptor. For each geometry the electromagnetic behaviour was simulated using COMSOL Multiphysics to compute the magnetic flux distribution and produce the corresponding schematic. Simulation conditions used were: susceptor conductivity 20,000 S / m; coil driving current of 500 A; frequency of 20 kHz; and 5 coil turns. The same simulation conditions were used for the schematics shown in Figures 13-14.

[0195] Figure 13 shows a vertical cross-section view of simulated electromagnetic field lines generated by the induction coil using (a) an embodiment induction heating system of the invention having a central rod-like inner magnetic flux concentrator co-axial with both the induction coil and the susceptor, and an outer magnetic shield, and (b) an embodiment system further having a base magnetic flux concentrator,

[0196] The use of an outer magnetic shield is advantageous in that it forms a return path for magnetic flux passing through the inner magnetic flux concentrator. Said outer magnetic shield may have any design conducive to its intended function. For example, the outer magnetic shield may have a cylindrical shape. Also, the outer magnetic shield may be a unit body (e.g. a single cylinder), or be made of multiple components (e.g. multiple rods positioned around the whole assembly).

[0197] Figure 14 shows a vertical cross-section view of simulated electromagnetic field lines generated by the induction coil using an embodiment induction heating system of the invention having a central rod-like inner magnetic flux concentrator co-axial with both the induction coil and the susceptor, a magnetic flux concentrating base plate, and an outer magnetic shield presenting an inward tapered top portion,

[0198] Embodiments of the invention will now be described with reference to the following nonlimiting Examples. EXAMPLES

[0199] EXAMPLE 1

[0200] Coupling efficiencies were also determined for the arrangements depicted in the figures. Coupling efficiency (CE) is a measure of the apparent power in the coil versus the real power in the susceptor. It defines a relationship between the overall efficiency and coupling efficiency.

[0201] „ >> >-. / • / •■ ■ (. P^^^Susceptor ~ P^^^Thermal heat leakage) Electrical Overall Efficiency = - - - - - - coil PowerSusceptor

[0202] Calculation conditions:

[0203] Current = 500 A

[0204] Coil AC resistance = 0.00405 Ohms (Assuming a 5-meter copper coil)

[0205] Efficiency of the electronic driving circuit: T] Eiectricai=95%

[0206] Heat loss due to parasitic heating of cooling circuits and surrounding air: Power Heat leakage = 2% of Power susceptor

[0207] COMSOL simulation results: PowerApparent, PowerSusceptor

[0208] Figure 15 shows performance obtained using different arrangements of inner magnetic flux concentrators in terms of (a) coupling efficiency and (b) corresponding overall efficiency, and Figure 16 shows simulated output power obtainable at increasing coil currents (or apparent power) for different arrangements of inner magnetic flux concentrators.

[0209] The simulated data allows to appreciate that increasing the magnitude of the coil current or the apparent power significantly increases the output power. These results show how the system would scale up. Simulation conditions were as follows: Susceptor conductivity 20,000 S / m; Coil driving current ramped from 50 - 1000 A in 50A steps; Coil driving frequency: 20 kHz, and number of coil turns = 5. The data also allows to appreciate that as current in the coil increases, the susceptor power increases, and that geometries of the susceptor can significantly increase the coupling efficiency and thereby both the susceptor power and overall system efficiency.

[0210] EXAMPLE 2

[0211] A heat induction prototype from the embodiment in Figure 3 was built and tested.

[0212] The gas containment shell was made from 316 stainless steel. The insulation consisted of alumina and calcium silicate. The coil was a double helical water-cooled copper coil with 4 turns. The susceptor rings were made from graphite with an OD of 95mm, ID of 85mm, an individual height of 20mm, and a total stack height of 200mm.

[0213] Magnetic flux concentrators corresponding to elements 2, 7, 11 were made from a manganese-zinc ferrite with a curie temperature of 220°C. A thermocouple was used to measure the outlet temperature.

[0214] Two gas bottles with 8.3m3of nitrogen are fed into a mass flow controller capable of setting and measuring the flow, during this run the flow was set at 200SLPM. A current clamp on the coil was used to measure the current as 340A rms at a frequency of 15kHz.

[0215] Figure 17 shows that after approximately 13 minutes the outlet gas temperature reached 1000°C.

[0216] EXAMPLE 3

[0217] The susceptor may be designed according to steps such as (1) choosing a susceptor material which can be machined and has desirable mechanical and heat transfer properties at operating temperature (e.g. graphite), (2) consider a desired operating frequency (typically a function of power and cost for available componentry) so that skin depth is sensible (e.g. in the range 3-50mm), and (3) set the radial thickness of the susceptor to be less than said skin depth. While an optimum value may depend on the specific geometry of the susceptor, one may consider a value usually in the range of 0.2xskin depth to 0.8xskin depth.

[0218] Example determination of radial thicknesses for a ring-shaped susceptor are shown in Figure 18. For Figure 18(a) the radial thickness is 10 mm for a total power dissipation of 929 W. For Figure 18(b) the radial thickness is 30 mm for a total power dissipation of 477 W. For Figure 18(c) the radial thickness is 90 mm for a total power dissipation of 471 W.

[0219] Throughout this specification and the claims which follow, unless the context requires otherwise, the word ‘comprise’, and variations such as ‘comprises’ and ‘comprising’, 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.

[0220] The reference in this specification to any prior publication (or information derived from it), or to any matter which is known, is not, and should not be taken as an acknowledgment or admission or any form of suggestion that that prior publication (or information derived from it) or known matter forms part of the common general knowledge in the field of endeavour to which this specification relates.

[0221] 1 Gas Containment Shell

[0222] 2 Outer Magnetic Flux Shield

[0223] 3 Second Insulating Region

[0224] 3A Top Insulation

[0225] 4 Induction coil

[0226] 5 Susceptor (stacked circular ring sections)

[0227] 6 First Insulating Region

[0228] 7 Inner Magnetic Flux Concentrator

[0229] 8 Cooling System

[0230] 9 Base Insulation

[0231] 10 Gas Inlet

[0232] 11 Base Magnetic Flux Concentrator Sealing Plate Output Connection Gas Outlet

Claims

- 41 -THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS1. An induction heating system for heating a flowing gas, the system comprising: an induction coil for generating an alternating electromagnetic field,a susceptor for converting the alternating electromagnetic field into heat, said susceptor providing an electrically closed path around an inner cavity,a magnetic flux concentrator located within said inner cavity, said magnetic flux concentrator being for providing a low reluctance path for magnetic flux through said inner cavity,a first thermally insulating region for thermally insulating the susceptor from the magnetic flux concentrator, and a second thermally insulating region for thermally insulating the induction coil from the susceptor, said first and second thermally insulating regions defining an annular heating duct that contains the susceptor, anda cooling system for maintaining at least a portion of the magnetic flux concentrator at or below its Curie temperature when the system is in use.

2. The system of claim 1, wherein the annular heating duct defines an annular space having a width of 50 mm or less.

3. The system of claim 1 or 2, wherein the annular heating duct has an inner wall and an outer wall each at a distance of 25 mm or less from the susceptor.

4. The system of any one of claims 1-3, wherein the magnetic flux concentrator comprises stacked discrete units of a material with a relative permeability of more than 10.

5. The system of any one of claims 1-4, wherein the cooling system comprises piping thermally coupled to the magnetic flux concentrator and adapted to circulate a cooling medium.

6. The system of any one of claims 1-5, wherein the susceptor comprises an assembly of stacked discrete electrically conductive tubular sections.- 42 -7. The system of claim 6, wherein each tubular section has, independently from the others, an axial length of from 5 mm to 50 mm.

8. The system of any one of claims 1-5, wherein the susceptor comprises electrically conductive porous susceptor foam material.

9. The system of any one of claims 1-5, wherein the susceptor comprises a stacked assembly of electrically conductive balls.

10. The system of any one of claims 1-9, further comprising an outer magnetic flux shield surrounding the induction coil.

11. The system of claim 10, further comprising a base magnetic flux concentrator plate located upstream of the susceptor and arranged to provide a low reluctance path between the outer magnetic shield and the magnetic flux concentrator.

12. The system of any one of claims 1-11, comprising an outer gas containment shell for preventing gas leakage.

13. The system of any one of claims 1-12, wherein the induction coil is designed to have integrated cooling.

14. The system of any one of claims 1-13, wherein the induction coil is designed for sustaining an alternating electrical current at a frequency between 1 kHz and 1 MHz.

15. The system of any one of claims 1-14, wherein the induction coil, the susceptor, the magnetic flux concentrator, and the annular heating duct have a circular cross-section geometry.- 43 -16. The system of any one of claims 1-15, wherein the induction coil, the susceptor, the magnetic flux concentrator, and the annular heating duct have concentric co-axial cylindrical geometry.

17. The system of any one of claims 1-16, wherein the magnetic flux concentrator comprises one or more of ferrite, permalloy, NiFeMo, supermalloy, a nickel alloy, an iron alloy, a cobalt alloy, and a manganese alloy.

18. The system of any one of claims 1-17, wherein the susceptor comprises one or more of graphite, partially graphitized carbon, steel, tungsten, tungsten carbide, carbon foam, molybdenum, stainless steels, niobium, nickel, nickel-containing ferrous alloy, titanium, and titanium alloy.

19. The system of any one of claims 1-18, wherein the thermally insulating regions are designed to provide sufficiently low thermal conductivity such that 80% or more of the heat delivered by the susceptor is transferred to gas flowing through the annular heating duct.

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