Hydrocarbon pyrolysis apparatus

Abstract

A Hydrocarbon pyrolysis apparatus (100) suitable for the endothermic decomposition of hydrocarbon gas into hydrogen and carbon by conduction of electricity between electrodes using an alternating current electrical supply. The apparatus (100) includes a reactor vessel (102), which includes: walls defining an interior which includes a fluidised bed zone, a plurality of electrodes (4) extending into the fluidised bed zone, a fluidising feed gas inlet for feeding hydrocarbon gas into the interior, and a product gas outlet located at or towards an uppermost part of the interior. In use, carbon-rich particles (11) are located in the fluidised bed zone and a hydrocarbon gas travels upwardly from the fluidising feed gas inlet through the fluidised bed zone to fluidise the carbon-rich particles. The electrodes are supplied with electricity which passes between the electrodes via the carbon-rich particles, thereby heating the carbon-rich particles to cause endothermic decomposition of the hydrocarbon gas into hydrogen and carbon.

Description

Technical Field The present invention relates to hydrocarbon pyrolysis apparatus. Background to the Invention There is a requirement for industrial users of fossil fuels to plan for a transition to net zero, as part of international climate treaties and nationally implemented targets. This requires the decarbonisation of industrial processes, including the use of natural gas in package boilers and in furnaces. One important response to this challenge is to replace the use of fossil fuels with electricity in industrial processes. Unfortunately, there will be insufficient electrical generation and distribution capacity available in the medium term to allow the elimination of all industrial natural gas applications. The continued industrial use of natural gas in the medium term means that there is a requirement to develop processes that mitigate the carbon emissions of these processes. The pyrolysis of natural gas allows the hydrogen so generated to be used as a fuel and for the carbon so generated to be captured and used as a raw material. By capturing the carbon instead of releasing it as carbon dioxide there is an opportunity to reduce the carbon impact of using the natural gas. Suitable uses for the carbon are, for example, in tyre and rubber manufacture, for metal smelting electrodes, and in building products. The pyrolysis process is endothermic and requires an energy input. If the energy source is renewably generated electricity, then the carbon emissions of the use of natural gas as a fuel source can be effectively mitigated. An effective design for this process is known, involving an ohmically heated and slowly downward moving carbon bed. The disadvantage of this approach is that it involves significant equipment to support the reaction and is suitable only for operation at large scale, such as on large integrated industrial sites or by energy utilities. Unfortunately, however, despite the development of the concept of piped hydrogen networks the majority of industrial users of natural gas will not have access to piped supplies of low carbon hydrogen for the foreseeable future. In summary there is no suitable technology available that will be able to mitigate the carbon emissions from the continued industrial use of natural gas at locations that are beyond the limited reach of future piped hydrogen networks. A solution is required for the many industrial applications operating at thermal outputs in the approximate thermal power output range 1 MWto 10 MW. In this specification, the term “ohmic heating” is used to refer to heat which is generated in a material by an electrical current flowing through the material. In this specification, the terms inner, outer, inwardly and outwardly are used with respect to a vessel interior, in which inward is directed into the interior and outward is directed away from the interior. The terms upward and downward are used in relation to the in-use orientation of a vessel in which downward means down into the ground and upward means away from the ground. In this specification, the word “substantially" is used to include the meaning of “exactly or for practical purposes, as will be understood by a skilled person in the technical field”. This includes some variation from “exactly” because of practical considerations. For example, in the technical field of this invention, “substantially horizontal” might include ±10° from the horizontal. Background Art Johnson et al. (US patent 2,921,840 January 19, 1960, Shawinigan Chemicals Limited) disclose an ohmically heated fluidised bed reactor for the conversion of carbon dioxide to carbon monoxide. Carbon dioxide is passed up through a fluidised bed of catalyst impregnated carbon particles which is ohmically heated to between 1000 °C and 1200 °C. A single phase AC electrical supply is used. This patent establishes the principle of ohmically heated fluidised bed reactors using AC current, but there is no provision for the generation and collection of solid products. Jahnig et al. (US patent 2,982,622 May 2, 1961, Esso Research and Engineering Company) disclose a process for the conversion of a gaseous hydrocarbon feed into hydrogen and coke by means of an ohmically heated fluid bed pyrolysis reactor. A pre-heated hydrocarbon feed gas passes upwards through a relatively dense and slowly downwardly moving particle bed. An electric current passes through at least a portion of the solids and the resultant ohmic heating raises the temperature of the solids to between 1800 °F and 3000 °F. The carbon pyrolysis product is deposited onto the downward moving carbon bed, and hydrogen is collected from the top of the reactor. This arrangement is not suitable for small-scale use because of the requirement for bulk solids handling and conveying machinery, with a fraction of the discharged coke being conveyed from the bottom exit to the top feed of the reactor. Meiers et al. (US 3,254,957 June 7, 1966, Exxon Research and Engineering Co.) define an ohmically heated fluidised bed for the decomposition of hydrocarbons in a reaction vessel with current passed between pairs of electrodes. Pairs of electrodes are positioned on opposite sides of the reaction vessel, the polarities of electrodes at one side all being the same. The use of pairs of electrodes allows the use of either DC or single-phase AC current, but not three phase. The hydrocarbon gas is introduced into a zone positioned above the electrodes, and an inert gas is injected below the electrodes to effect fluidisation of the bed. The system overcomes previous problems associated with electrical breakdown of insulation around the electrodes and vessel wall, but at the expense of requiring the use of an inert fluidising gas. Goldberger et al. (US patent 3,448,234 03 June 1969, The Battelle Development Corporation) disclose the use of vibrations applied to an ohmically heated fluidised bed reactor in order to increase and control the electrical resistivity of the bed. Heinz et al. (European patent EP 3 980 174 B1, 19 July 2023, Linde GmbH, BASF SE and others) disclose an ohmically heated methane pyrolysis reactor with a downward moving dense bed of carbon particles, and with electrical current flowing radially between the inner and outer walls of an annular reaction chamber. The advantage of the annular design is a more uniform electric field strength within the carbon bed, reducing the propensity for inhomogeneous heating. One disadvantage of the design is the requirement for bulk solids handling equipment to return material from the bottom to the top of the reactor. Another disadvantage is the additional mechanical complexity involved in an annular reactor configuration. These features make the design unattractive for smaller scale use. Thomson et al. (WO2022 / 232942 A1, 06 May 2022 Aurora Hydrogen Inc.) disclose the invention of a microwave heated fluidised bed reactor operating in excess of 1200 K with carbon heated by the microwave field to initiate the decomposition of hydrocarbons to form carbon and hydrogen. Microwave heating provides a solution for smaller scale operations but the low penetration of microwaves into a carbon bed requires additional engineering that increase complexity, and the electrical inefficiency involved in microwave generation reduces the overall process efficiency. In summary the background art establishes the principle using ohmically heated fluidised beds for carrying out the pyrolysis of methane. There are designs available suitable for large scale use such as by energy utilities and on integrated industrial sites. There is no design suitable for smaller scale package use however, with the existing designs suffering from various drawbacks including the need for bulk solids handling machinery, the inherent electrical inefficiency of microwave generation, and the need for the use of inert fluidisation gas streams in addition to the hydrocarbon feed gas. Statements of Invention According to a first aspect of the present invention there is provided a hydrocarbon pyrolysis apparatus for the endothermic decomposition of hydrocarbon gas into hydrogen and carbon by conduction of electricity between electrodes using an alternating current electrical supply, the apparatus including a reactor vessel, the vessel including: a wall arrangement defining an interior, the interior including a fluidised bed zone, a plurality of electrodes extending into the fluidised bed zone, a feed gas inlet arrangement for feeding hydrocarbon gas into the interior, the feed gas inlet arrangement including a fluidizing feed gas inlet located at or towards a lowermost part of the interior below the fluidised bed zone, a product gas outlet arrangement, the product gas outlet arrangement including a product gas outlet located at or towards an uppermost part of the interior; wherein, in use, carbon-rich particles are located in the fluidised bed zone; a hydrocarbon gas travels upwardly from the fluidising feed gas inlet through the fluidised bed zone to fluidise the carbon-rich particles; the electrodes are supplied with electricity which passes between the electrodes via the carbon-rich particles, thereby heating the carbon-rich particles to cause endothermic decomposition of the hydrocarbon gas into hydrogen and carbon; and the hydrogen travels upwardly to the product gas outlet. Possibly, the apparatus is arranged so that as the hydrocarbon gas travels upwards, a proportion of the carbon-rich particles also travel upwards in the fluidised bed zone. Possibly, the carbon-rich particles travel upwards in an inner part of the fluidised bed zone and may travel downwards in an outer part of the fluidised bed zone. Possibly, the apparatus includes one or more isolating transformers that provide the AC supply to the electrodes. Possibly, the number of electrodes is no less than the number of phases of the AC supply. Possibly, the apparatus includes at least three electrodes which extend into the fluidised bed zone. Possibly, the at least three electrodes are connected to separate phases of a multiphase alternating current (AC) electrical supply. Possibly, the frequency of the alternating current is between 1 Hz and 1000 Hz. Possibly, the AC supply comprises at least 3 and possibly no more than 30 separate phases connected to at least an equivalent number of electrodes. Possibly, the AC supply is controlled by a thyristor or multiple thyristors. Possibly each phase of a primary side of the transformer is controlled by one of the thyristors. Possibly, the apparatus includes five or more electrodes. Possibly, at least one electrode is polarised as an anode. Possibly, at least one electrode is polarised as a cathode. Possibly, the electrodes extend vertically into the fluidised bed zone. Possibly the wall arrangement includes an inner side wall, which may include one or more electrically conductive areas. Possibly, in use, current paths form both between the electrodes and also between the electrodes and the inner side wall electrically conductive area(s). Possibly, the or each electrically conductive area is formed of an electrically conductive material. Possibly, substantially the entire inner side wall comprises one inner side wall electrically conductive area, which may be formed of the electrically conductive material. Possibly, the or each inner side wall electrically conductive area is not electrically connected by wiring to any circuit. Possibly, the or each inner side wall electrically conductive area does not form one of the electrodes. Possibly, the or each inner side wall electrically conductive area is not electrically connected to the or any AC transformer. Possibly, the electrically conducting material includes carbon or a metallic material, which may be resistant to the operating temperature and, possibly, in which the electrical resistivity of the material is less than 200 pQ.m, more preferably below 50 pQ m, and optimally below 20 pQ m. Possibly, the vessel has a reference axis, which may be a central axis, which may extend vertically. Possibly, the inner side wall has a central axis, which may be coaxial with the vessel reference axis and may extend vertically. Possibly, the inner side wall is circular in horizontal cross section and may have a radius. Possibly, the shortest electrical conduction path length between the surface of each electrode and the inner side wall is less than half the radius of the inner side wall. Possibly, the shortest electrical conduction path length between the surface of each electrode and the inner side wall is more than one tenth of the radius of the inner side wall, and may be more than one fifth of the radius of the inner side wall, and optimally may be more than one quarter of the radius of the inner side wall. Possibly, the electrodes are spaced apart from each other. Possibly, the electrodes are equispaced apart from each other. Possibly, all of the electrodes are equispaced radially from the reference axis. Possibly, all of the electrodes are equispaced radially from the conducting inner side wall. Possibly, each electrode is equispaced between the reference axis and the conducting inner side wall. Possibly, a shortest electrical conduction path length between the surfaces of nearest neighbouring electrodes is greater than the radius of the inner surface wall divided by the total number of electrodes in the reactor. Possibly, the fluidised bed zone includes a parallel region, in which the inner side wall is parallel to the reference axis. Possibly, the electrodes extend into the parallel region. Possibly, in the parallel region, the shortest distance between the surface of each electrode and the inner surface of the reaction vessel is less than half the radius of the inner surface of the reaction vessel. Possibly, in the parallel region, the shortest distance between the surfaces of adjacent electrodes is greater than the radius of the inner surface of the reaction vessel divided by the number of electrodes. Possibly, the electrodes are spaced such that the current passing through the fluidised bed directly between the electrodes is less than twice the current passing from the electrodes to the conductive wall. Possibly, the electrodes are spaced such that that the current passing through the fluidised bed directly between the electrodes is more than half the current passing from the electrodes to the conductive wall. Possibly, the apparatus is arranged so that the impedance between any two electrodes is greater than the impedance between the any single electrode and the conducting inner side wall. Possibly, when three electrodes are located in the fluidised bed connected to three phases of an isolating transformer the separation of the electrodes is such that the direct electrical impedance between the two electrodes passing current is similar to the impedance between the same two electrodes when current is passed from one of the electrodes to the inner side wall and thence to the other second electrode. Possibly, when six electrodes are located in the fluidised bed connected to six separate phases of an isolating transformer the separation of the electrodes is such that the direct electrical impedance between the two electrodes passing current is similar to the impedance between the same two electrodes when current is passed from one of the electrodes to the inner side wall and thence to the other electrode and where the electrodes are connected to the isolating transformer such that the direct current path to form the circuit is not connected to adjacent electrodes. Possibly, the vessel includes a cooling arrangement for cooling the electrodes. Possibly, the cooling arrangement keeps the electrodes at a lower temperature than the carbon-rich particles in the fluidised bed zone. Possibly, the feed gas is at a lower temperature than the carbon-rich particles in the fluidised bed zone. Possibly, the feed gas inlet arrangement includes a plurality of cooling conduits, which may extend into the fluidised bed zone and may direct feed gas around or towards the electrodes to cool the electrodes and thus comprise the cooling arrangement. Possibly, at least a part, possibly a lowermost part, of each electrode is located within a different one of the cooling conduits so that, in use, the feed gas in the cooling conduit provides a cooling layer around the electrode. Possibly, the cooling layer is turbulent. Possibly, each electrode comprises the part within the respective conduit (the cooled part) and an exposed part, which may extend clear of the respective conduit. Possibly, each part has a respective length, namely, a cooled part length and an exposed part length. Possibly, the vessel includes an adjustment arrangement for adjusting the relative lengths of the cooled part and the exposed part, possibly by moving the respective electrode along the respective conduit. Possibly, the adjustment arrangement includes an actuator for moving the electrode relative to the respective conduit. The adjustment movement may be a linear movement and / or a rotational movement. Possibly, the adjustment arrangement permits adjustment of the lengths of the exposed parts in the fluidised bed zone. Possibly, the feed gas arrangement includes a plenum chamber, and the cooling conduits may extend from the plenum chamber. Possibly, each of the electrodes extends into the plenum chamber. Possibly, the vessel includes a plurality of seals. Possibly, each seal seals between one of the electrodes and a wall of the plenum chamber as it passes therethrough, and may be arranged to permit a relative adjustment movement of the electrode, which may be a linear movement (eg a sliding movement) and / or a rotational movement. Possibly, one or more of the electrodes is hollow in cross section so that the respective electrode comprises one of the cooling conduits. Possibly, the wall arrangement defines a carbon-rich particle feed inlet. Possibly, the vessel includes a carbon-rich particle outlet arrangement. Possibly, the carbon-rich particle outlet arrangement includes an overflow outlet, which may be located adjacent an upper part of the fluidised bed zone. Possibly, the carbon-rich particle outlet arrangement includes a settlement outlet which may be located in a lower part of the fluidised bed zone, and which, in use, captures heavier carbon particles which settle out of the fluidised bed zone. Possibly, the feed gas inlet arrangement includes tangential gas inlets, which are arranged to impart, in use, a vorticular gas flow in the fluidised bed zone. Possibly, the vessel includes a magnet arrangement, which may be mounted externally to the reaction vessel, and which, in use, may impose a magnetic field on the interior of the vessel which may impart a force on the carbon-rich particles conducting electricity in the fluidised bed zone. Possibly, the magnet arrangement includes a plurality of magnets and / or one or more electromagnetic coils. Possibly, the electromagnetic coil or coils use direct current. Possibly, the electrical supply to the electrodes is arranged to provide a time averaged net electrical current in the fluidised bed zone in one direction in the magnetic field leading to a net movement of the carbon-rich particles conducting the current. Possibly, the electrical supply includes a DC component. Possibly, in use, the fluidised bed zone has a temperature of between 700 K and 2,500 K. Possibly, the wall arrangement includes an external pressure containment wall. Possibly, the wall arrangement includes a layer of insulation between the external and the internal walls. Possibly, in use, the product gas includes methane, ethane and hydrogen, and may include entrained carbon-rich particles. Possibly, the interior includes a separation zone, which may be located above the fluidised bed zone, for reducing the amount of entrained carbon-rich particles in the product gas. Possibly, the separation zone has a larger horizontal cross-sectional area than the fluidised bed zone. Possibly, the parallel region of the fluidised bed zone has a length and may be constant in horizontal cross-sectional area along its length. Possibly, the fluidised bed zone has a lower region, which may be located below the parallel region, and which may reduce in horizonal cross-sectional area, possibly towards the or one of the fluidising feed gas inlet. Possibly, the separation zone has a lower part, which may reduce in horizonal cross-sectional area towards the fluidising bed zone. Possibly, the separation zone has an upper part above the lower part, which may have a length, and may be constant in horizontal cross-sectional area along its length. Possibly, the carbon-rich particles are primarily comprised of carbon, and may be more than 50% carbon, desirably more than 75% carbon and optimally more than 90% carbon by weight. Possibly, the carbon-rich particles have an average particle size of at least 0.02 mm and may possibly be no more than 10 mm, desirably less than 5 mm and optimally less than 2 mm. Possibly, the conducting inner side wall is formed of a material with a low magnetic susceptibility which may comprise carbon, metal and / or a metallic alloy which may comprise any of Cr, Mo, Ta, W, V, Os, Ti, Si, Mn, Ha, Au, Re, Rh, or Pt. Possibly, the apparatus includes additional fixed or variable circuit elements which improve the power factor of the AC supply to the electrodes. Possibly, each electrode is connected to the electrical supply by a connector. Possibly, the apparatus includes capacitors, which may be located between the electrodes’ electrical supply connection. Possibly, the capacitors have a value which may be variable or selectable. Possibly, the capacitors may have a capacitance value of between 1 pF and 1 F. Possibly, the electrodes are formed of carbon, metal and / or a metallic material that comprises any of Cr, Mo, Ta, W, V, Os, Ti, Si, Mn, Ha, Au, Re, Rh, or Pt and in which the diameter of each electrode may be between than 1% and 25% of the internal diameter of the fluidised bed zone. According to a second aspect of the present invention, there is provided a method of pyrolising a hydrocarbon gas, the method including endothermically decomposing the hydrocarbon gas into hydrogen and carbon by conduction of electricity between electrodes using an alternating current electrical supply, providing a hydrocarbon pyrolysis apparatus, the apparatus comprising a reactor vessel, the vessel including: a wall arrangement defining an interior, the interior including a fluidised bed zone, a plurality of electrodes extending into the fluidised bed zone, a feed gas inlet arrangement for feeding hydrocarbon gas into the interior, the feed gas inlet arrangement including a fluidising feed gas inlet located at or towards a lowermost part of the interior below the fluidised bed zone, a product gas outlet arrangement, the product gas outlet arrangement including a product gas outlet located at or towards an uppermost part of the interior; wherein, in use, carbon-rich particles are located in the fluidised bed zone; a hydrocarbon gas travels upwardly from the fluidising feed gas inlet through the fluidised bed zone to fluidise the carbon-rich particles; the electrodes are supplied with electricity which passes between the electrodes via the carbon-rich particles, thereby heating the carbon-rich particles to cause endothermic decomposition of the hydrocarbon gas into hydrogen and carbon; the hydrogen travels upwardly to the product gas outlet. Possibly, the apparatus includes any of the features described in any of the preceding statements or following description. Possibly, the method includes any of the steps described in any of the preceding statements or following description. Figures Embodiments of the present invention will now be described, by way of example only, and with reference to the accompanying drawings, in which: Fig. 1 is a side cross-sectional schematic view of hydrogen pyrolysis apparatus comprising a reaction vessel; Fig. 2 is a plan cross-sectional schematic view along a section line A-A indicated in Fig. 1 showing a reactor vessel with three electrodes; Fig. 3 is a plan cross-sectional schematic view along a section line A-A indicated in Fig. 1 showing a reactor vessel with six electrodes; Fig. 4 is a perspective schematic view of the vessel of Fig. 2; Fig. 5 is a diagram of an electrical circuit arrangement for the reactor vessel of Figs. 2 and 4 with three electrodes; Fig. 6 is a diagram of an electrical circuit arrangement for the reactor vessel of Fig. 3 with six electrodes; Fig. 7 is a diagram of the electrical circuit arrangement of Fig. 2 showing indicative current paths with the electrodes located in preferred positions; Fig. 8 is a diagram of the electrical circuit arrangement of Fig. 2 showing indicative current paths with the electrodes located in non-preferred positions; Fig. 9 is another diagram of the electrical circuit arrangement of Figs. 2, 4 and 7 with the electrodes located in the preferred positions. In the drawings, where multiple instances of the same or similar features exist, only a representative one or some of the instances of the features have been provided with numeric references, for clarity. It is well known to those skilled in the art that natural gas contains methane and a range of other gaseous hydrocarbons, the reactor described is suitable for operating with this range of gaseous hydrocarbons. The present invention is a fluidised bed hydrocarbon gas pyrolysis reactor that has an increased conversion efficiency through both improved temperature distribution and improved electrical energy transfer efficiency. The improvement is partly by the use of multiple phase AC current conducted to the carbon fluidised bed by means of multiple electrodes that are connected to separate phases of an isolated AC electrical supply and where each electrode entry into the reactor is designed such that there is a suitable gas flow local to the electrode surface. Surprisingly the electrical distribution is further improved by the use of an electrically conductive inner surface of the reactor in contact with the fluidised bed which is not connected to any of the alternating current phases and where the impedance between any two electrodes is greater than the impedance between any one electrode and the conductive inner surface of the reactor. The improvement may also be due in part to active control of inlet distribution of gas in response to signals from the electrical feed circuits. Additionally the improvement may be partly due to an externally applied magnetic field which exerts a force on the particles in the bed proportional to the local current, when such a field is applied. Additionally, the improvement in reactor efficiency may also result from the use of additional variable and or fixed circuit elements such as capacitors that improve the power factor of the applied alternating current. Additionally, the improvements may result from electrodes that can be increased in length or rotated whilst in situ. ’It is surprisingly found that this arrangement of three phase ohmic heating in a fluidised bed gives rise to an improvement in the homogeneity of the heating effect compared to previous designs. Without wishing to be bound by theory, the use of three phase alternating current and a particular reactor geometry is thought to give rise to three constantly changing current paths, so that hot-spots or tracks in the bed are reduced and the heating effect is evenly distributed. The use of more than 3 phases may be preferable in some cases, depending on the reactor dimensions and the flow rate of gas. The preferred number of phases could be 6 or 9 or other multiples of 3 upto 30. It is known to those skilled in the art that the predominant industrial electrical distribution method uses three phase electricity and that for processes that consume large amounts of electricity it is important that the load between each of the phases is balanced. Power is preferentially controlled by means of a thyristor or thyristors in each phase. The characteristic AC waveform that thyristors provide is with a sharp attack and more gradual decay across each half cycle of the AC waveform. The rapid change in electric field associated with the sharp attack is advantageous because it gives rise to more current paths compared to a slower attack. This reduces the propensity for tracking and the formation of hot-spots. The electrical current from the thyristor is supplied to an isolation transformer. The use of an isolating transformer is known to those skilled in the art for the purpose of isolating the primary and secondary circuits. As used in this invention this device allows the conductive wall of the reactor to be become an additional current path between electrodes and be connected to ground potential without the transfer of electricity through the wall causing a short circuit. The instantaneous electrical properties of the reactor in terms of impedance and capacitance between each of the phases can vary in response to the distribution and temperature of material in the fluid bed. Without wishing to be bound by theory, changes of impedance arise due to fluctuations in the lengths of the varying current paths in the bed, and changes in capacitance arise due to fluctuations in the separation between particles in the current paths, and both impedance and capacitance are also functions of the temperatures of the material in the current paths. The measurement of instantaneous current and voltage on each phase, between pairs of electrodes, can be used to control the reactor. The electrical properties of the reactor are important for the power factor or efficiency of electrical power transfer. Surprisingly it has been found that the electrical power transfer efficiency can be improved by making parts of the reactor variable, and to this end the reactor may optionally contain features for increasing or decreasing the exposed length of the electrodes or by adding additional capacitance between the electrical connections to the electrodes. The change in electrode exposed length may be achieved by the use of a sliding gas tight seal on the electrode to increase or decrease the exposed area of the electrode or by changing the length of insulating gas conduit surrounding part of the electrode. The avoidance of the need for any mechanical recirculation of solids or mechanical stirring of the bed makes the pyrolysis reactor design suitable for application at smaller scale than previous designs, and in particular suitable for distributed plants supporting hydrogen boilers or furnaces on individual industrial sites. Surprisingly it has been found that mixing effects with a similar outcome to mechanical mixing of the fluid bed can be achieved by means of electric coils or permanent magnets mounted externally to the reactor and used to generate a magnetic field within the reactor which imparts a force on the current paths in the fluid bed and thereby provides additional homogeneity of heating. This additional effect is of use in extending the range of operating parameters for which homogeneous heating is achieved, again without the need for any moving mechanical components. In this case the reactor chamber wall is of a metal or carbon material with relatively high electrical conductivity and with low relative magnetic susceptibility. This effect works at the operational frequency of the AC supply. Description Figure 1 shows a vertical cross section of hydrogen pyrolysis apparatus 100 for the endothermic decomposition of hydrocarbon gas into hydrogen and carbon by conduction of electricity between electrodes using an alternating current electrical supply. The apparatus 100 includes a reactor vessel 102. The vessel 102 includes: a wall arrangement defining an interior, the interior including a fluidised bed zone, a plurality of electrodes extending into the fluidised bed zone, a feed gas inlet arrangement for feeding hydrocarbon gas into the interior, the feed gas inlet arrangement including a fluidizing feed gas inlet located at or towards a lowermost part of the interior below the fluidised bed zone, a product gas outlet arrangement, the product gas outlet arrangement including a product gas outlet located at or towards an uppermost part of the interior; wherein, in use, carbon-rich particles 11 are located in the fluidised bed zone; a hydrocarbon gas travels upwardly from the fluidising feed gas inlet through the fluidised bed zone to fluidise the carbon-rich particles; the electrodes are supplied with electricity which passes between the electrodes via the carbon-rich particles, thereby heating the carbon-rich particles to cause endothermic decomposition of the hydrocarbon gas into hydrogen and carbon; and the hydrogen travels upwardly to the product gas outlet. The conductive wall (101) of the reactor, in contact with the fluidised bed (11), is constructed of a metallic or carbon material. Thermal insulation (2) is located between the inner conductive wall and the pressure retaining wall of the reactor (3). The pressure retaining reactor wall (3) is preferentially constructed of stainless steel. Gas enters a plenum chamber (7) by means of a gas inlet (8). There are multiple gas inlets to the reactor from the plenum chamber. There is a central gas inlet (9). There are additional gas inlets to the reactor (18), some or all of which may be angled so as to impart a tangential velocity to the gas stream entering the reactor. There are inlets (10) to insulating conduits around the electrodes (4). For clarity only one electrode is shown (4), but there will be a minimum of three electrodes. Each electrode passes through the insulating gas conduit (5) and through the plenum chamber (7) to the seal arrangement (6). The seal arrangement allows both rotational and translational movement of the electrode, so that the length of electrode in contact with the fluid bed (11) may be varied, and the electrode may be rotated about its long axis. Rotation and translation of the electrode is achieved by means of an actuator (19).The insulating gas conduit (5) is hollow in cross section and allows gas to flow from the plenum chamber (9) via the opening (10) between the inner wall of the insulating gas conduit and the electrode (4). As solid material is generated in the bed during operation the amount of material in the bed can be controlled by the removal of material through conduit (12) to a receiver chamber (13) or by overflow of the bed through the conduit (14) into receiver chamber (15) or by transport with motive gas via tube (16). The product gas exits the reactor vessel through exit pipes (17). Electric coils (20) are mounted externally to the reactor. These generate a magnetic field within the reactor which acts on the transient electric current paths in the fluid bed and serves to impart a stirring motion to the bed, assisting with bed homogeneity. Figure 2 shows a horizontal cross section A-A of the reactor shown in Figure 1. The section shows the substantially fluidised bed of carbon particles (11). The conductive wall (101) ofthe reactor, in contact with the fluidised bed (11). Thermal insulation (2) is located between the inner conductive wall and the pressure retaining wall ofthe reactor (3). The figure shows three electrodes (21,22,23) arranged with uniform electrode to electrode spacing, uniform radial position, and with each electrode located within the centre of an insulating gas conduit (5). Operation with three electrodes and three phases of alternating current requires a uniform electrode to electrode spacing, with a corresponding uniform radial positioning. The electrode to electrode spacing and the radial separation between the electrodes (21,22,23) and the wall (101) can vary depending on the properties ofthe material of construction, the conductivity of the bed and the gas distribution. Figure 3 shows the same horizontal cross section A-A ofthe reactor shown as shown in Figure 2, but in this case there are six electrodes (21,22,23,24,25,26), each located within an insulating gas conduit (5). Operation with six electrodes and six phases of alternating current again requires a uniform electrode to electrode spacing, with a corresponding uniform radial positioning. The electrode to electrode spacing and the radial separation between the electrodes (21,22,23,24,25,26) and the wall (101) can vary depending on the properties ofthe material of construction, the conductivity ofthe bed and the gas distribution. Figure 4 is a schematic perspective drawing of a reactor vessel with three-electrodes. For clarity only an outline ofthe reactor vessel and the electrodes and conduits are shown. The three electrodes (21,22,23) are each enclosed in conduits (5). The conduits extend from the plenum chamber beneath the reactor (not shown), through the conical base ofthe reactor, and extend upwards to a point in the fluid bed (11). Figure 5 shows an electrical arrangement for three electrodes using an isolating transformer. The incoming phases 27,28,29 enter a transformer which isolates the primary and secondary circuits using an isolation barrier 31. The three secondary phases are connected to the electrodes (21,22, 23). Capacitors are situated between the electrode connections 32,33, 34. The capacitors may be variable or may be selected based on the operation. Figure 6 shows an electrical arrangement for six electrodes using an isolating transformer. The incoming phases 27,28,29 enter a transformer which isolates the primary and secondary circuits using an isolation barrier 30. The six secondary phases are connected to the electrodes (21,22,23,24,25,26). In this embodiment the electrodes are connected to the isolating transformer such that the electrical current is not conducted between adjacent electrodes but crosses a significant portion ofthe diameter of the fluidised bed. In this manner the volume of conductive particles heated in each longer current path is greater. Other electrical connection ofthe electrodes are possible. Fig. 7 is a diagram ofthe electrical circuit arrangement of Fig. 2 showing indicative current paths with the electrodes located in preferred positions relative to the conductive inner side wall. In Figure 7, isosurface current density distribution is illustrated with dashed lines 35. In this embodiment the spacing of the electrodes and the conductivity are such that the distribution of current both directly between electrodes and through the wall is similar. Fig. 8 is a diagram ofthe electrical circuit arrangement of Fig. 2 showing indicative current paths with the electrodes located in non-preferred positions relative to the conductive inner side wall. In the non-preferred positions, the electrodes are located such that the shortest distance between closest neighbouring electrodes is smaller than the shortest distance between each electrode and the conducting inner side wall. The isosurface current density distribution is again illustrated with dashed lines 35, and shows that a significant preferential current path between the electrodes lies only in the region between the electrodes. Fig. 9 shows the arrangement ofthe three electrodes of Fig. 7 in the preferred positions. The conducting inner side wall has a reference axis 38 and a radius Rw 40. The electrodes are located on a circle of radius Re 42. The shortest electrical conduction path Pew 44 between the surface of each electrode and the inner side wall has a length Lew 46. In one example ofthe preferred positions, the length Lew 46 could be less than half the radius Rw 40 of the inner side wall. In some examples the length Lew 46 could be more than one tenth of the radius Rw 40; more preferably more than one fifth of the radius Rw 40, and optimally more than one quarter of the radius Rw 40. In another example of the preferred positions, the electrodes could be equispaced apart from each other. The shortest path between the electrodes Pee 52 has a length Lee 54. In further examples, all of the electrodes could be equispaced radially from the reference axis. In still further examples, all of the electrodes could be equispaced radially from the conducting inner side wall. In another example, the shortest electrical conduction path length Lee 54 between the surfaces of nearest neighbouring electrodes is greater than the radius Rw 40 of the inner surface wall divided by the total number of electrodes in the reactor. Thus in the example of three electrodes: Lee 54 >Rw 40 3 The shortest radial distance of the electrodes from the reference axis Rea is indicated by reference numeral 48. The Applicant has surprisingly found that the presence of the conducting inner side wall 101 enables a significant improvement in the evenness of the current density distribution with the electrodes in the preferred positions. This improves heat transfer efficiency and thus reduces reaction time, improving output and increasing efficiency. In one embodiment of the invention natural gas or methane flows up through the fluidised carbon particle bed (11) which is heated by alternating current passing between electrodes (4). Heat is generated in the substantially fluidised bed (11). As the reaction proceeds hydrogen exits at the top of the reactor (17) and carbon pyrolysis product is deposited on the carbon fluid bed particles. At the same time particle fragmentation is occurring due to collisions, and the result is that an approximately steady state particle size is reached. As the reaction proceeds the mass of the fluid bed and the number of particles increases, and particles overflow across a barrier (14) at the top of the fluidised bed and are thereby removed from the reactor as product. There is no requirement for the external transport of solids from the bottom to the top of the reactor, so that the need for the associated solids handling equipment is avoided. Power is supplied using a thyristor or thyristors that control each phase and the voltage and current response for each phase can be recorded separately and used to control the gas inlet distribution via additional gas entry conduit (12). The gas seals on the electrodes (6) are preferably in the plenum chamber located below the reactor chamber. Gas flows from the plenum chamber into the reaction chamber through a central entry point (9) and also through the annular spaces (10) between the electrodes (6) and insulating gas conduits (5). The gas seals (6) allow the electrode length in contact with the fluidised bed of carbon (11) to be increased or decreased based on the electrical response of the reactor. Advantageously, locating the electrodes through the plenum chamberand the cooling conduits allows preheating of the feed gas while cooling the electrodes. Each electrode enters the reactor chamber through an insulating gas conduit (5) which also provides a gas connection into the reactor. The electrodes preferably enter the reactor in the lower half of the reactor and are aligned with the flow of gas. Additional gas entry points (12) may be included other than the insulating gas conduits and the flow of gas in these may be controlled separately in response to the measured electrical variance between phases of the alternating current supply. The gas distribution and flow volume in the reactor may be adjusted in response to current variation in parts of the reactor. Optionally additional gas may be added to the reactor at points that will change the distribution of the particles and affect the electric properties of the bed. In this case the additional gas may be in the form of a hydrocarbon gas or another gas. A proportion of the gas may enter the reactor via inlets (12) that direct the flow radially or tangentially into the reactor. Inlets that direct a tangential flow generate a swirl as the gas moves through the reactor and promote a net rotational movement of the particles. In one embodiment three electrodes (4) are used, and the reactor chamber material (1) in contact with the fluidised bed is electrically conductive and provides a continuous current path circumferentially. Preferably this may be constructed from a metallic material or more preferentially with a lining constructed of rings of graphitic carbon that are stacked to form the inner wall of the reactor in contact with the fluidised bed (11). The electrodes (4) are orientated vertically within the reactor chamber, parallel to the upward gas flow and the chamber walls, and equidistant from one another. The distance between any two electrodes and between each of the electrodes and reactor wall is such that the electrical resistance from one electrode to another passing directly through the fluid bed is approximately equal to that from any electrode to the reactor wall, along the shortest path around the reactor wall, and then to any second electrode through the fluid bed. The insulating gas conduits (5) may preferentially be formed of a hollow section tube constructed from a nonconductive thermally and chemically stable material. Preferentially this material is a ceramic, more preferentially this is alumina or zirconia or aluminosilicate type ceramic. Optionally there is provided measurement of the instantaneous voltage and current in each of the phases and corresponding adjustment of the power of each phase. This allows control of the relative heating effect between three regions of the reactor where each pair of electrodes passing current dominate. Optionally thermal energy contained in the output gas stream may be recovered by use of a heat exchanger or series of heat exchangers and used to pre heat gas entering the plenum chamber (8). In operation the gas entering the reactor is preheated to a temperature below its decomposition temperature. This serves to increase the energy efficiency of the process. The preheating can be from energy recovered from the exit gas using heat exchange over a surface or from other heat sources. The pre heated hydrocarbon containing gas enters the plenum chamber (7) via a gas connection (8), then flows to the reaction chamber via the insulating gas conduits (5) and gas inlets (9) and causes a proportion of the solid carbon particulate material present in the reactor to move. The gas entering the reactor is heated and continues to increase in volume and additionally as the gas decomposes and hydrogen is formed, increasing the volume of the motive gas. The size and density of the particles are preferably suitable to form a Geldart type B fluidised bed where bubbling behaviour is a primary means of solid material movement. These bubbles 50 (as shown in Fig. 1) rise through the bed which generates contact between the gas and the particles. The bubbling behaviour is concentrated in the region between the electrodes and causes a net upward movement of particulates along the central axis of the reactor and a net downwards flow around the perimeter. The flow of particles and the velocity profile reduce wear on the inner surface of the conductive wall (101). To promote this distribution the bottom of the reactor expands from the gas inlet (9) forming a conical region of the reactor. The particles are heated by the transfer of current between the electrodes and the hot surface of the particles initiates the reaction to release hydrogen. The gas entering the reactor via the insulating gas conduits is at a lower temperature than the fluidised bed and acts to reduce the surface temperature of the electrodes and increase particle velocity in the region surrounding the electrodes. The current density will be highest at the surface of the electrode so good gas flow and material transport is required to distribute the heat generated into the fluidised bed. Optionally an external magnetic field produced by either permanent magnet material or electromagnetic coils may be applied to generate an additional force on the charge carrying material in the reactor. Preferentially the direction of the magnetic field is aligned parallel to the gas flow up the reactor. Optionally the electric coil may have a constant direct current or a pulsed electric current. The direction of the current in the coil may be reversed periodically. In a second embodiment, six phase alternating current electrical supply is used. Preferably the electrodes are connected as a double delta arrangement. In this arrangement the six uniformly separated electrodes pass through six separate insulating gas conduits (5) into the reactor and make contact with the carbon particulate in the fluidised bed (11). The use of a double delta arrangement in six phase alternating current is a possible arrangement but other arrangements that do not utilise a neutral electrode connection are possible. In a further embodiment additional power supplies may be connected to additional electrodes in excess of the number of alternating phases supplied to the reactor, and this may be direct current. Optionally the use of an external magnetic field may be linked with the use of a direct current between electrodes in resulting in a in a net movement of the bed of solid materials. Optionally the electrode may form a conduit to allow gas to enter the reaction chamber, being constructed of a hollow tube with either an open end or with multiple holes. Other Modifications Various other modifications could be made without departing from the scope of the invention. The embodiment of the reactor could be of any suitable size and shape, and could be formed of any suitable material (within the scope of the specific definitions herein). The various components including reactor electrodes or gas supply could be of any suitable size and formed of any suitable material (within the scope of the specific definitions herein). In one example, the inner side wall could comprise one or more electrically conductive areas, ratherthan being formed entirely of an electrically conductive material. The areas could be in any suitable form and could be in the form of strips, wires, a mesh. Any of the features or steps of any of the embodiments shown or described could be combined in any suitable way, within the scope of the overall disclosure of this document. Final Remarks There is thus provided an embodiment of the reactor with a number of advantages over conventional arrangements. In particular, the current density distribution is improved, enabling increased efficiency and reduced power consumption.

Claims

1. Hydrocarbon pyrolysis apparatus for the endothermic decomposition of hydrocarbon gas into hydrogen and carbon by conduction of electricity between electrodes using an alternating current electrical supply, the apparatus including a reactor vessel, the vessel including:a wall arrangement defining an interior;the interior including a fluidised bed zone,at least three electrodes extending into the fluidised bed zone,a feed gas inlet arrangement for feeding hydrocarbon gas into the interior, the feed gas inlet arrangement including a fluidizing feed gas inlet located at or towards a lowermost part of the interior below the fluidised bed zone,a product gas outlet arrangement,the product gas outlet arrangement including a product gas outlet located at ortowards an uppermost part of the interior;wherein, in use, carbon-rich particles are located in the fluidised bed zone, a hydrocarbon gas travels upwardly from the fluidising feed gas inlet through the fluidised bed zone to fluidise the carbon-rich particles,the electrodes are supplied with electricity which passes between the electrodes via the carbon-rich particles, thereby heating the carbon-rich particles to cause endothermic decomposition of the hydrocarbon gas into hydrogen and carbon;the hydrogen travels upwardly to the product gas outlet;wherein the at least three electrodes are connected to three separate phases of a multi-phase alternating current (AC) electrical supply,and wherein the wall arrangement includes an inner side wall which includes one or more electrically conductive areas,so that, in use, current paths form both between the electrodes and also between the electrodes and the inner side wall electrically conductive area(s).

2. Apparatus according to claim 1, in which the apparatus includes one or more isolating transformers that provide the AC supply to the electrodes, and in which the number of electrodes is no less than the number of phases of the AC supply.

3. Apparatus as in claims 1 or 2, in which the or each inner side wall conductive area is not electrically connected by wiring to any circuit, does not form one of the electrodes and is not electrically connected to the or any AC transformer.

4. Apparatus as in any previous claim, in which the or each inner side wall conductive area is formed of an electrically conducting material which includes carbon or a metallic material that is resistant to the operating temperature and in which the electrical resistivity of the material is less than 200 pQ.m.

5. Apparatus as in any previous claim in which the AC electrical supply comprises at least 3 and no more than 30 separate phases connected to at least an equivalent number of electrodes.

6. Apparatus as in any previous claim in which the apparatus includes five or more electrodes and at least one electrode is polarised as an anode and at least one electrode is polarised as a cathode.

7. Apparatus according to any of the preceding claims, in which the electrodes extend vertically into the fluidised bed zone.

8. Apparatus as in any previous claim, in which the inner side wall has a central vertically extending axis; is circular in horizontal cross-section with a radius; the shortest electrical conduction path length between the surface of each electrode and the inner side wall is less than half the radius of the inner side wall.

9. Apparatus as in any previous claim in which the electrodes are equispaced apart from each other and all of the electrodes are equispaced radially from the inner side wall.

10. Apparatus as in any previous claim in which a shortest electrical conduction path length between the surfaces of adjacent electrodes is greater than the radius of the inner surface wall divided by the total number of electrodes in the reactor.

11. Apparatus according to any of the preceding claims, in which the vessel includes a cooling arrangement for cooling the electrodes; the feed gas is at a lower temperature than the carbon-rich particles in the fluidised bed zone; the feed gas inlet arrangementincludes a plurality of cooling conduits; the feed gas conduits extend into the fluidised bed zone and direct feed gas around or towards the electrodes to cool the electrodes and thus comprise the cooling arrangement.

12. Apparatus as in claim 11, in which a part of each electrode is located within a different one of the cooling conduits so that, in use, the feed gas in the cooling conduit provides a turbulent cooling layer around the electrode.

13. Apparatus according to claim 12, in which each electrode comprises the part within the respective conduit (the cooled part) and an exposed part, which extends clear of the respective conduit; each part has a respective length, namely, a cooled part length and an exposed part length; and the vessel includes an adjustment arrangement for adjusting the relative lengths of the cooled part and the exposed part.

14. Apparatus as in claim 13 in which the feed gas arrangement includes a plenum chamber; the cooling conduits extend from the plenum chamber and each of the electrodes extends into the plenum chamber.

15. Apparatus as in claim 14 in which the vessel includes a plurality of seals; each seal seals between one of the electrodes and a wall of the plenum chamber as it passes therethrough, and is arranged to permit the relative adjustment movement of the electrode, which may be a linear and / or a rotational movement.

16. Apparatus according to any of claims 11 to 15, in which one or more of the electrodes is hollow in cross section so that the respective electrode itself comprises one of the cooling conduits.

17. Apparatus according to any of the preceding claims, in which the feed gas inlet arrangement includes tangential gas inlets, which are arranged to impart, in use, a vorticulargas flow in the fluidised bed zone.

18. Apparatus as in any previous claim, in which the apparatus includes a magnet arrangement which, in use, imposes a magnetic field on the interior of the vessel which imparts a force on the carbon-rich particles conducting electricity in the fluidised bed zone; the magnet arrangement may include a plurality of permanent magnets and / orone or more electromagnetic coils, which may be mounted externally to the reaction vessel.

19. Apparatus according to claim 18, in which the magnet arrangement includes one or more electromagnetic coils which use direct current.

20. Apparatus as in any previous claim in which the fluidised bed zone has a temperature of between 700 K and 2,500 K.

21. Apparatus as in any previous claim in which the carbon-rich particles are comprised of more than 90% carbon by weight.

22. Apparatus as in any previous claim in which the carbon-rich particles have an average particle size of at least 0.02 mm and less than 2 mm.

23. Apparatus as in any previous claim in which the conducting inner side wall is formed of a material with a low magnetic susceptibility which may comprise carbon, metal and / or a metallic alloy that comprises any of Cr, Mo, Ta, W, V, Os, Ti, Si, Mn, Ha, Au, Re, Rh, or Pt.

24. Apparatus as in any previous claim in which the apparatus includes additional fixed or variable capacitors between the AC supply to the electrodes and in which the capacitors may have a capacitance value of between 1 pF and 1 F.

25. Apparatus as in any previous claim in which the electrodes are formed of carbon, metal and / or a metallic material that comprises any of Cr, Mo, Ta, W, V, Os, Ti, Si, Mn, Ha, Au, Re, Rh, or Pt and in which the diameter of each electrode is between than 1 % and 25% of the internal diameter of the fluidised bed zone.28

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