Plasma torch and plasma torch electrode

Abstract

A plasma torch for use in a chemical reactor comprises: a substantially cylindrical torch chamber 300 with an open end for outflow of reaction products and a closed end opposite to the open end; a fir

Description

Field of Invention The invention relates to a plasma torch and a plasma torch electrode. It is particularly relevant to a plasma torch for use in a plasma torch reactor. Background to Invention Plasma torches may be used for a variety of purposes, with one particular area of interest being for use in reactors. A reactor design in which a plasma torch is used for pyrolysis of hydrocarbons such as methane is described in the applicant’s International Patent Application No. PCT / GB2022 / 050938 entitled “Plasma Torch Reactor and Reaction Method” filed on 13th April 2022. This describes a hyperbaric system in which a plasma torch operating at high temperature as well as elevated pressure is used to pyrolyse methane, with the reaction products output into a liquid metal circulating system which is adapted to support separation of reaction products as well as in some cases enabling further reaction steps to take place. Under these conditions, it is challenging to achieve satisfactory electrode life. It would be desirable to develop an electrode solution that achieves effective performance over a longer electrode lifetime. Summary of Invention In a first aspect, the invention provides a plasma torch for use in a chemical reactor, the plasma torch comprising: a substantially cylindrical torch chamber with an open end for outflow of reaction products and a closed end opposite to the open end; a first electrode disposed in the torch chamber; a second electrode disposed in the torch chamber between the first electrode and the open end; and an input system for input of one or more gaseous feedstocks into the torch chamber; wherein at least the second electrode comprises a porous refractory metal. It is found that use of a porous refractory metal for at least the second electrode - in embodiments, a similar construction may be used for the first electrode - materially improves electrode life, particularly in the context of a chemical reactor with a liquid metal circulating system. While the electrode may be eroded during plasma torch process, with this design the electrode can be flooded with liquid metal during cycling of the plasma torch, and this liquid metal can then form a protective layer which will be eroded before the main body of the electrode itself, therefore significantly extending the life of the electrode. In embodiments, the second electrode is substantially cylindrical. In embodiments, at least a part of the input system is configured to input one or more gaseous feedstocks into the torch chamber through the second electrode, and at least said part of the input system is provided with means to prevent ingress of material from the torch chamber into the input system. In embodiments, the plasma torch is adapted for hyperbaric operation. In a second aspect, the invention provides a plasma torch electrode formed from a porous refractory metal and adapted for use as a first electrode or as a second electrode in the plasma torch of the first aspect. Such a plasma torch electrode may be formed by low temperature sintering, and may be formed of porous tungsten. In a third aspect, the invention provides a plasma torch reactor, comprising the plasma torch of the first aspect and a liquid metal circulation system, wherein the plasma torch is disposed so that an output of the plasma torch discharges into the liquid metal circulation system. This liquid metal circulation system may be adapted for at least partial filling of the torch chamber when the plasma torch is not in operation. In a fourth aspect, the invention provides a method of operating a plasma torch in a chemical reactor, wherein the plasma torch comprises a substantially cylindrical torch chamber with an open end for outflow of reaction products, a first electrode disposed in the torch chamber, and a second electrode disposed in the torch chamber between the first electrode and the open end, wherein in operation of the chemical reactor the plasma torch discharges into a liquid metal circulation, the method comprising: operating the plasma torch to discharge plasma torch reaction products into the liquid metal circulation; stopping operation of the plasma torch, upon which liquid metal from the liquid metal circulation at least partly fills the torch chamber; wherein the second electrode comprises a porous refractory metal, and the liquid metal at least partly filling the torch chamber coats and at least partly fills pores of the second electrode; and restarting operation of the plasma torch, expelling a plug of liquid metal from the torch chamber, while retaining a sacrificial coating of liquid metal on the second electrode. Brief Description of Figures General and specific embodiments of the invention will be described below with reference to the Figures. Figure 1 shows a longitudinal cross-section of an existing plasma torch adapted for use in a plasma torch reactor for pyrolysis of hydrocarbons; Figure 2 shows a side elevation view of a plasma torch reactor using a plasma torch as shown in Figure 1; Figure 3 shows a high-level schematic diagram of the main functional elements of a plasma torch reactor of the type shown in Figure 2; Figure 4 shows flow of reaction gases through the plasma torch of Figure 1; Figures 5A and 5B show side elevation and sectional views respectively of a ring for inlet of feedstock gases for use in the plasma torch of Figure 1; Figure 6 shows a side elevation of a plasma torch suitable for using an embodiment of the invention; Figure 7 shows an exemplary cathode structure for use in a plasma torch according to an embodiment of the invention; Figure 8 illustrates the effect of flooding with liquid metal with a torch with the cathode structure of Figure 7; Figure 9 illustrates an exemplary reaction process for a first reactor system; Figure 10 illustrates a liquid metal pyrolysis reactor system driven by the plasma torch system and including a housing for the plasma torch system; Figure 11 is a system diagram of a second reactor system. Description of Specific Embodiments An existing plasma torch for use in a plasma torch reactor is shown in Figure 1, which provides a longitudinal sectional view of plasma torch 30 is provided. The plasma torch 30 is generally cylindrical, and in the arrangement described in more detail below, it extends into a liquid metal circulation system 40 (discussed further below) where it jets directly into the liquid metal. The plasma torch has a central chamber 300 containing a cathode 31 and an anode 32. These may be of any conductive material suitable for the conditions in the central chamber 300 - carbon (graphite) could be used, or any suitable metal or alloy, either uniform or with suitable inserts - for example, copper with hafnium inserts would be a possible choice. Here, the cathode 31 is located towards the end of the plasma torch 30 remote from the liquid metal reactor 39, with a ceramic cup-shaped end section 38 terminating the plasma torch. In alternative torch designs, the electrodes may be disposed the other way around, or an alternating current plasma torch may be used in which it is only meaningful to talk of electrodes, rather than anode and cathode. The anode is generally cylindrical, but it has a shaped inner surface 34 which comprises a nozzle 35 and a diffusing section 36, which will be described in greater detail below. A protective electrode 33 may be disposed between the cathode 31 and the anode 32 - the skilled person will appreciate that again the electrode structure may be varied to achieve a desired field pattern within the plasma torch chamber, and may involve none, one or multiple intermediate electrodes - multiple protective electrodes may be cascaded to help stabilisation of the spark, for effective ignition, or to prevent wear on the anode. Gas inputs 37 are provided to admit gaseous feedstock into the reactor - in the arrangement shown in Figure 1, methane is admitted in the gas input 37 disposed in the protective electrode 33. As will be indicated in further detail below, different gas input positions are provided for different feedstock gases in different embodiments of the invention. While the discussion below will refer primarily to methane, it should be appreciated that other hydrocarbons may equally well be used - for example, propane can be transported in liquid form but will vaporise easily for reaction in a plasma torch reactor, so will be another particularly suitable choice for processing. Figure 2 provides a perspective view of a reactor according to an embodiment of the invention. The reactor 1 is formed as a pressure vessel 2 with electrical inputs to power a plasma torch (not shown here, though Figure 10 shows how this is integrated into the system) and gas inputs 4 for gaseous feedstocks to be admitted to the system. These inputs are each directed to a revolver assembly adapted to fit into an assembly aperture 5 -the revolver assembly houses a set of plasma torches and which also provides gaseous feedstock to each plasma torch. The reactor shown here has multiple stages - the plasma torches act as a first reactor stage, with a liquid metal reactor as a further reactor stage consuming heat generated by the plasma torches (in alternative arrangements, a liquid metal stage may be used primarily for separation rather than for further reaction). The reaction products include heated gas - hydrogen in the main example discussed below - and a heat exchanger may use the heated gas to bring feedstock gases to the correct temperature for reaction, and in such a case the heat exchanger may effectively act as a preliminary reactor stage. The elements of the reactor system are shown schematically in Figure 3. Gaseous inputs 11 - for example, hydrocarbons such as methane, and additional hydrogen for cooling (though this may be recirculated from the output products) - are admitted into the plasma torch 12, and the plasma torch 12 consumes the input feedstock gases providing a first set of output products, such as carbon and hydrogen. These first output products pass at high temperature as inputs 13 into a liquid metal system 14, which then in embodiments provides pyrolysis of further feedstock gas. Final output products 15 - such as carbon, which may depending on the design be extracted through the liquid metal or from the gaseous output, and hydrogen, output as a gas - are provided from liquid metal reactor 14 after a separation process - these final output products include the first output products from the plasma torch 12 and may be supplemented by further output products produced from pyrolysis in the liquid metal reactor 14. The pyrolysis reaction is endothermic, but there is still sufficient heat present that the gaseous final output products are at significantly greater temperature than desired for storage, so there is excess heat to be used. Here, this heated gas output is used by a heat exchanger 16 which controls the temperature of feedstock gases for different stages of the reactor process. As will be noted further below, embodiments of the invention may not require all the features shown in the Figure 3 arrangement to operate - the Figure 3 arrangement is a synergistic combination of a series of processes for particularly effective production of carbon and hydrogen from hydrocarbons such as methane. As will be indicated further below, such processes may also be adapted to produce other output products, such as syngas. Where plasma torch 30 is used to break down methane, a practical consideration is management of carbon. In this reaction, methane decomposes at high temperature into hydrogen gas and carbon through action of the plasma torch spark, which may have a temperature of 6000 degrees Centigrade, resulting in instant decomposition. A practical issue is that this may result in carbon deposits 41 which would clog the torch, which will significantly affect the efficiency of the process and which could lead to significant downtime for maintenance. It would be desirable to prevent such carbon build up, and for both reaction products to exit the plasma torch 30. One feature to achieve this is to protect the anode with a gas that will inhibit build up. This can be achieved by making the anode 32 porous, with anode gas outputs 42 delivering gas - in this case, hydrogen, through the anode to provide a protective curtain along the inside of the anode, inhibiting carbon build up. The gas is delivered at an angle to the anode such that it has a component of velocity towards the plasma torch output to achieve this protective curtain - alternatively, a component of velocity can be provided away from the plasma torch output, as this will still provide a protective curtain to the electrode. In addition to providing a protective curtain, there may also be active erosion of deposited carbon by the hydrogen - the hydrogen can react with the carbon in a back reaction back to methane, thus further eroding any carbon deposited. The hydrogen also serves to cool the anode, preventing it from being degraded. In addition to using a porous anode in this way, the cathode can also be made porous and cooled in a similar way. Further strategies are used to prevent carbon build-up. The shaping of the anode can also be arranged such that a likely deposition point for carbon would be on the anode in the region of the spark gap with the torch in operation - spark action can then further erode any carbon build-up. Another feature that prevents carbon build up is shown in Figure 4, which illustrates the passage of gas through the plasma torch structure. Here, methane enters the plasma torch tangentially through the gas input 37 in the protective electrode, and this input methane travels towards the cathode following a generally helical path - such a vortex may be provide stable flow. The gas input 37 here is provided through a ceramic ring 51, shown in more detail in Figures 5a and 5b. The ceramic ring 51 has a gallery 52 for circulation of the input gas around the ring, allowing the input gas to pass into a number (four in the design shown) of channels 53 which deliver input gas tangentially into the chamber, establishing both a helical path in the output gas adjacent to the wall of the chamber and also a vortex within the plasma torch chamber. This may be optimised taking into account gas type, flow conditions, pressure and temperature to achieve the desired flow pattern. The wall structure (in particular wall roughness and geometry promotes the outer helix of gas maintaining its momentum and separating from the faster rotating inner helix of gas, with the torch geometry forcing the gas into an inner returning helix at a greater speed and with a tighter inner circle. The gas adopts this tighter helix on travelling back between cathode and anode, and it maintains this on heating as it is broken down into carbon and hydrogen in the spark gap between the cathode and the anode. Plasma formation is rapid - it will typically take less than a microsecond. For a gas, proper tuning allows this to be tuned (by pressure, temperature and density) to minimise exchange of energy between the helices, similarly to a tornado. This configuration already gives the output gas - in this case, hydrogen -significant velocity towards the output of the plasma torch, and it will also prevent carbon condensation and deposition, as the carbon is formed in the centre of the plasma torch chamber rather than at the walls. The plasma comprises ions and electrons in energetic balance in a state of near thermal equilibrium, with molecules largely decomposed into atoms - under operating conditions of temperature and pressure in the plasma torch, the stable state of carbon is as a gas, reducing likelihood of carbon deposition. The plasma torch design is generally arranged so as to promote the reaction in the centre of the chamber and to inhibit it at the walls, so that the reaction products are preferentially driven out of the plasma torch into the liquid metal reactor. The outer helix cools and insulates the wall, while preventing atomic carbon in the inner helix from condensing on the walls. The hydrogen from the reaction passes through the nozzle 35, which results in an increase in speed and a decrease of pressure according to the Venturi effect. The gas is then output from the plasma torch 30 through the diffuser 36 with high temperature (and kinetic energy) - the plasma is ejected from the torch at supersonic speeds. By the cumulative effect of these features, carbon is generally carried through into the plasma torch output without significant build-up of deposit on the walls of the anode. The role of the diffuser 36 is to match the pressure of the output of the plasma torch with the next reactor stage, as will be described in more detail below. As noted here, the embodiment described in detail here, the next reactor stage is a liquid metal reactor - the liquid metal here may also be used to interact directly with the torch, as will also be discussed further below. This arrangement in the torches described above allows for operation at high temperature (above 6000 degrees Centigrade at the point of reaction) and hyperbaric pressure in the torch, with a very high throughput of gaseous feedstock. For an input of 200kW of power into the plasma torch, and with operating temperatures within the torch chamber in the region of 6000 degrees Centigrade at the point of reaction and pressures of 50 bar, approximately 72kg / hour of methane can be processed using this design. The voltage across the electrodes will typically be between 150V and 600V, typically about 250V, with operating current between 100A and 500A, typically about 200A. Feedstock gases can be pre-heated by using a heat exchanger - taking advantage of the heat given out in the pyrolysis reaction (see further discussion below), though hydrogen used to cool the anode will be provided at a lower temperature. Hyperbaric operation of the plasma torch is common to embodiments of the invention described here, and the reaction system is typically contained within a pressure vessel as shown in Figure 2. However, an effective reaction can be achieved at a variety of pressure regimes - while the system described here is particularly suitable for operation at 50 barg (50 bars above atmospheric pressure), it is differentiated from conventional approaches to use of plasma torches in connection with pyrolysis by hyperbaric operation and use of a system at lower but still elevated pressures (20 barg, 10 barg, or even 1 barg) allows for more efficient operation than in temperatures operating at atmospheric pressure. As will be noted further below, operation at lower temperatures and with lower power torch operation is also possible (this is discussed further below with respect to Figure 11). As indicated previously, it is challenging to work at both elevated pressure and elevated temperature, and torch cooling becomes a significant problem. A further issue is the use of a liquid metal system, which can provide benefits - with some complications - for torch operation. If the diffuser of the plasma torch extends sufficiently far into the liquid metal, then the liquid metal will act to clean the diffuser and prevent carbon build-up there - in embodiments, the diffuser section may be porous in part to support liquid metal flow. If the torch arrangement is so designed, the liquid metal can even be driven up to flood the plasma torches, rapidly quenching the reaction and stopping their operation. Liquid metal can thus be used to flood - and hence clean - the porous anode (and where used, cathode) structures. This has consequences for the operation of the system on shutdown and startup. Once the plasma torch is stopped, some degree of backfilling of the plasma torch structure from the liquid metal can be expected - liquid metal will enter the torch chamber, and (depending on the choice of metal) may solidify as the torch cools. If porous electrodes are used, the electrodes, or at least the electrode closest to the torch aperture may flood with liquid metal. This can be used beneficially for effective operation of the torch, and for extending its working lifetime. The electrodes may be directly replenished by the solidified metal, which may compensate for erosion during use. Restarting a plasma torch is normally achieved using a high voltage pulse - this will typically have a significant ageing effect on the electrodes and on the torch structure generally. If liquid metal has entered the torch chamber, and particularly if it has formed a solid metal plug, the effect of starting the torch is significantly softened. Such a solid metal plug will typically form a link between the electrodes of the plasma torch, so the high voltage pulse will typically result in a high current (perhaps 200A) through the plug which will heat and melt it very rapidly - the combination of plug melting with the supply of feedstock gas to the torch will result in rapid expulsion of the metal plug while also providing a soft start to the plasma torch to reduce the ageing effect of power cycling it - the result is a more effective autoignition process assisted by the liquid metal system. To make expulsion of the plug more rapid, this may be stimulated either by injection of gas behind the plug or vacuum in advance of it to pressure the plug forward into the liquid metal system. A mechanical system for engaging with and ejecting the plug is also possible. In this approach, the spark will have a very high temperature and will create local hot spots -these are likely to occur in the region of the (ceramic) swirl plate or at the electrode edges -this could result in significantly limiting torch life, or the period of torch operation before torch maintenance is required. Figure 6 shows an alternative plasma torch arrangement- this arrangement is adapted to support effective gas cooling and to prevent damage to the electrode and to ceramic elements. Gas enters the torch through main gas inlet 601, passes through a main gas channel 602 to the front of the torch, adjacent to the exit chamber 614 of the torch. Here, the gas passes into a front commuter 604 acting as a circular connecting cavity and out into a front swirl plate 605. From the front swirl plate, the gas passes at an angle - hence establishing rotation, and a vortex - into the injection chamber 610. In contrast to the arrangement of Figure 1, this way of introducing the gas creates a “reverse vortex” in the tube. Such a “reverse vortex” design has been used in microwave plasma generators (as described in A Gutsol and JA Bakken, “A new vortex method of plasma insulation and explanation of the Ranque effect”, J. Phys. D: Appl. Phys 31 (1998) 704-711), but not in plasma torches of this type. Here, it is found that a reverse vortex design is particularly beneficial, as it addresses both cooling and spark location issues at the same time. From the front swirl plate 605, the gas circles into the injection chamber 610 and serves to push the spark backwards, into the plasma chamber 611 and into rear electrode 622 (here the cathode) and away from the front electrode 621 (here the anode). The second vortex - carrying hot gas and with high angular speed - passes out through the restriction 613 and the exit chamber 614 into the liquid metal system. Taking this approach, the system is protected - the insulator does not see the spark at all, and the spark is pushed away from both anode and cathode. Using this approach, anode and cathode walls are cooled with relatively cool input gas, and the “hot” gas and reaction products have limited interaction with the elements of the torch that are susceptible to damage. One consideration in providing the gas circulation near the front of the torch is that this may need protection from liquid metal ingress. This can be achieved by having a small ball barrel in the connecting holes, and locking this in place (for example, with a small scrub screw bored through the centre). This may then act as a one-way valve to prevent backfill into the gas supply lines. A separate main inlet 601 and front inlet 603 may alternatively be provided (this is shown in Figure 6). Another possibility is to have separate gas injection at the back -this is shown in Figure 6 through gas back inlet 606, gas back commuter 607, gas back channel 608 and back swirl plate 609. This can be used to suck out superfluous liquid metal on start up - for example, by brief opening of the system to ambient to vent it into a container. While the arrangement of Figure 6 may provide more protection for the electrodes - and in particular, the cathode - than the Figure 1 arrangement, electrode life will still be an issue for plasma torches operating in a high temperature, high pressure environment. One approach is to make the electrode out of a refractory metal relatively resistant to erosion in such way (for example, Molybdenum or Wolfram, or perhaps Zirconium) - these may be suitable choices for the rear electrode 622 in the Figure 6 design, or an insert of one of these materials could be provided in the regions of the electrode where erosion from the spark is particularly significant. Figures 7 and 8 illustrate a further strategy for maintaining electrode life - using a porous refractory material for the electrode, and using liquid metal flooding of the torch at the end of operation for rejuvenation of the cathode. Figure 7 shows an exemplary cathode structure, and Figure 8 illustrates the effect of flooding of the torch. Figure 7 illustrates a porous tungsten cathode 701 suitable for use in the torch design of Figure 6. Solid tungsten for tungsten cathodes is typically formed by sintering at temperatures of above 2000°C, but porous tungsten be formed instead by sintering at much lower temperatures. As shown in Figure 7, this creates a solid with a number of voids and porosity of up to 50%. Similar approaches can be used to produce porous versions of other practical metals. Figure 8 shows the position when the torch is flooded with liquid metal on being turned off-the plasma chamber 611 fills with liquid metal that cools to form a plug 801, but the liquid metal also reaches the tungsten electrode 701 and fills a substantial number of the voids within it to form a sacrificial coating 702. This liquid metal - lead, bismuth and even copper, or alloys thereof, are all suitable for use here - has significantly lower melting and boiling points than the tungsten, and in operation will be consumed before the refractory material itself will be damaged. This increases the life of the refractory material electrode significantly (typically from 100-200 hours up to about 10,000-20,000 hours), thus increasing very significantly the normal interval between major maintenance events for the torch. It should be noted that care is needed generally to limit the flooding by liquid metal to volumes where the flooding is beneficial (plasma chamber for plug formation, electrodes for electrode rejuvenation) and not into regions where it is not (gas inlets). Gas inlets may be protected by one-way valves, as discussed above, or other design elements (such as boron nitride disks for masking off system elements). Other strategies for ensuring effective torch operation are described in the applicant’s copending application of even date entitled “Plasma Torch”. One example of an overall reaction flow will now be described with respect to Figure 9. This reaction flow is specific to decomposition and pyrolysis of methane, but it is used here more generally to illustrate the different reaction processes taking place in different parts of the composite reactor. For example, other hydrocarbons such as propane may be used as a feedstock hydrocarbon, rather than methane, in not only the plasma torch reactor but also the liquid metal reactor. Two inputs to the system are shown: electricity 1101 and hydrocarbon 1102 (in this case, methane). Two outputs are shown: hydrogen 1103 (though for other reactions, other output gases may be provided as well or instead - note also that some of the hydrogen generated is recirculated for use in the reaction processes) and carbon black 1104. Both inputs are provided to the plasma torch 1105 - in addition to electrical power and the hydrocarbon feedstock, hydrogen is provided as an input. In the arrangement shown, a low temperature hydrogen input 1111 (shown here in the 200-400 degree Centigrade range) is provided to the plasma torch 1105 for cooling the anode, for example, with high temperature hydrocarbon 1112 (shown here at around 700 degrees centigrade), used as a reaction feedstock and also to maintain the temperature and pressure of the reaction chamber and to promote the flow of material through the plasma torch. As the plasma torch consumes electrical energy and generates a high temperature output, this is partially consumed by the pyrolysis reaction in a second reactor 1122, from which heated output gases can be used in a heat exchanger 1121 to circulate the hydrocarbon feedstock so that it is elevated from low temperature hydrocarbon 1113 at about 200 degrees Centigrade to high temperature hydrocarbon 1112 at a plasma torch reaction temperature of about 700 degrees centigrade -the heat exchanger 1121 can also provide hydrogen at cooler temperatures to the plasma torch. This heat exchanger 1121 thus effectively acts as a first reactor process, absorbing the heat of the end process and using it to bring gases required for reaction stages to the correct temperature. The plasma torch 1105 itself acts as a second reactor 1122, providing high temperature hydrogen and (primarily) gasified carbon as outputs 1114. The plasma torch 1105 through its reaction products operates on the next reactor stage, which is a liquid metal pyrolysis reactor 1123. The plasma torch 1105 provides heat for this reaction, heating up the metal (here, lead) to reaction temperature, and also providing rotation to the lead, allowing the carbon to be extracted at the centre of the reactor. More high temperature hydrocarbon 1115 is provided from the heat exchanger 1121 as a feedstock for the liquid metal pyrolysis reactor 1123. The hydrogen output 1116, provided at very high temperature (approximately 1200 degrees Centigrade) from the exothermic reaction in the pyrolysis reactor, is returned to the heat exchanger 1121 and partly recirculated to the plasma torch 1105 while mainly provided (at a lower temperature) at the hydrogen gas output 1104. The liquid metal pyrolysis reactor is shown in more detail in Figures 10. Figure 10 illustrates the main elements of the reactor assembly. The torch mounting 121 is directed into a liquid metal racetrack 122 which feeds into the main reactor volume 123. There are also gas inputs 124 to the main reactor volume 123, which contains a swirl chamber 125. The liquid (molten) metal is delivered into the swirl chamber 125 so as to give rotation to the liquid metal column, allowing the liquid metal both to initiate a pyrolysis reaction in the input gas and to act as a centrifugal separator, separating reaction products towards the centre of the rotating column. Carbon is then extractable from the base of the reactor in a carbon output 126. Hydrogen rises from the liquid metal and is released through a hydrogen output 127 from the top of the reactor. The reaction is carried out at elevated temperature and pressure (typically 800-1000 degrees Centigrade and 50 bar). This functionality may be usefully combined with that of the plasma torch even if the liquid metal system is not itself a reactor - in that case, it only acts as a separator to separate the reaction products from the plasma torch, powered by the energy of the plasma torch output. This leaves significant excess heat, however, and it is found that making the liquid metal system itself a reactor, used for endothermic pyrolysis of further hydrocarbon, leads to a particularly effective reactor system. Plasma from the plasma torch is jetted on to the liquid metal. This heats the liquid metal up to a sufficient temperature to initiate a pyrolysis reaction in hydrocarbons such as methane, and also carries the reaction products of the plasma torch reaction into the liquid metal reactor so that they can be collected from the system (methane passing into the liquid metal from the plasma torch jet may also be pyrolyzed at this point). The heated metal passes along the rest of the liquid metal racetrack and enters the liquid metal reactor chamber from the bottom. The parts of the racetrack structure as a result need to withstand high temperatures from the heated liquid metal, and they will also need to be adapted for expansion from the significant difference between temperatures during reaction processes and outside reaction processes. The plasma torch is designed so that it will jet effectively into the liquid metal racetrack 122 -in particular, the diffuser of the plasma torch is designed to match pressures with the outside of the torch. This will have the benefit of supporting linear rather than turbulent flow in the liquid metal racetrack. The liquid metal may be brought into a swirl or vortex which will act to stabilize the plasma jet. Reaction products from the plasma torch - in the example shown, hydrogen and carbon - will be carried in the liquid metal for subsequent separation in and output from the liquid metal reactor, as described below. The liquid metal system may also serve to purge the outputs of the plasma torch reactor from impurities. For example, ethylene may be produced as a by-product, but then be broken down again in the liquid metal system. Lead, or a mixture containing lead, may be used as the liquid metal in the liquid metal system. Lead is a suitable choice as it is liquid at reaction temperatures without having a high vapour pressure, and it creates fewer toxicity issues than most other suitable metals. Gallium is another possible choice, as is bismuth. One alloy used in embodiments of the system is WR58, comprising bismuth and lead, available from William Rowland Ltd. While the term “liquid metal” is used throughout this description, in embodiments the circulating liquid may not itself be a metal, providing that it is a liquid at reactor temperatures and supports separation of the reaction products (and where acting as a reactor, supports further pyrolysis) but does not itself have a further chemical reaction with feedstock gases or pyrolysis reaction products. A number of salts also have appropriate properties. As noted above, where a cooled plug is used for autoignition of the torch it will be necessary for the circulating liquid to become solid when the reactor is cold and for it to be electrically conductive to some degree. Figure 11 is a system diagram of a further reactor system design. In this design, the temperature of operation is lower, the liquid metal system conveys product away from the torch for separation rather than providing an additional reactor stage, and the heat exchanger stage is not used. While this reactor system is designed for lower temperature operation (here for 250°C normal temperature with an operating range of 200-300°C through the reactor as a whole - though clearly the temperature locally at the plasma spark will be significantly higher) it is also designed for high pressure operation (with 50barg as the normal reactor pressure, with an operating range of 40-60barg). In this arrangement, a hydrocarbon feed 2202 - methane, for preference - is provided at ambient temperature and pre-heated to 80°C before being provided to the plasma torch 2205. In this embodiment, a single plasma torch is used (there is no revolver). The system is designed for 18kg / hr throughput with the whole system operating at around 50barg. The plasma torch 2205 operates at 50kW (up to 800VDC, 300 Amps). The reaction products of hydrogen and carbon black are jetted out into the liquid metal handling system (which may also be termed a quench reactor - this is equivalent to the “plasma reactor” of the Figure 9 arrangement) -in this case, the liquid metal system uses WR58 alloy heated to at least 80°C, with the liquid metal system 2214 supplied from a metal supply tank 2214a. The reaction products are extracted from the liquid metal handling system 2214 by an extraction system 2230 here comprising a first and a second cyclone 2231, 2232 and a pulse jet filter 2233 (with an alternate pulse jet filter than can be switched in). Each of these systems deposit carbon black as a solid while allowing a gas comprising predominantly hydrogen to progress to the next stage. In the arrangement shown, the first cyclone 2231 accepts a stream from the reactor at 18kg / hr, 48barg and 200°C). The solid deposited by the first cyclone 2231 will be carbon black with some metal alloy, so a separator is needed at this point. The second cyclone 2232 accepts an input at 17.66kg / hr, 47barg, 190°C, and outputs gas at 4.96kg / hr, 46barg and 185°C while shedding more carbon black. This output is received by the pulse jet filter 2233, which sheds more carbon black and outputs hydrogen gas, which is cooled to ambient temperature in a cooler 2234. The carbon black will be deposited into intermediate bulk containers 2235 - the arrangement shown optimises deposition into the second and third bulk containers such that most of the carbon black produced does not require separation from the liquid metal. Further embodiments can be produced with higher operating temperatures up to the high temperature operation (which may be at 1200°C) of the Figure 9 arrangement. Such designs may use features from either or both of the Figure 9 and Figure 11 arrangements, as appropriate - for example, an intermediate design may have a heat exchanger similar to that shown in Figure 9 while using a separation arrangement of the type shown in Figure 11. As the skilled person will appreciate, other embodiments of the plasma torch and the reactor technology and reaction processes set out here may be provided within the scope of the claims provided, without limitation to specific features set out in the embodiments but not required by the claims.

Claims

1. A plasma torch for use in a chemical reactor, the plasma torch comprising: a substantially cylindrical torch chamber with an open end for outflow of reaction products and a closed end opposite to the open end;a first electrode disposed in the torch chamber;a second electrode disposed in the torch chamber between the first electrode and the open end; andan input system for input of one or more gaseous feedstocks into the torch chamber; wherein at least the second electrode comprises a porous refractory metal.

2. The plasma torch of claim 1, wherein the second electrode is substantially cylindrical.

3. The plasma torch of claim 1 or claim 2, wherein at least a part of the input system isconfigured to input one or more gaseous feedstocks into the torch chamber through the second electrode, and wherein at least said part of the input system is provided with means to prevent ingress of material from the torch chamber into the input system.

4. The plasma torch of any preceding claim, wherein the plasma torch is adapted for hyperbaric operation.

5. A plasma torch electrode formed from a porous refractory metal and adapted for use as a first electrode or as a second electrode in the plasma torch of any of claims 1 to 4.

6. The plasma torch electrode of claim 5, wherein the plasma torch electrode is formed by low temperature sintering.

7. The plasma torch electrode of claim 5 or claim 6, wherein the plasma torch electrode is formed of porous tungsten.

8. A plasma torch reactor, comprising the plasma torch of any of claims 1 to 4 and a liquid metal circulation system, wherein the plasma torch is disposed so that an output of the plasma torch discharges into the liquid metal circulation system.

9. The plasma torch reactor of claim 8, wherein the liquid metal circulation system is adapted for at least partial filling of the torch chamber when the plasma torch is not in operation.

10. A method of operating a plasma torch in a chemical reactor, wherein the plasma torch comprises a substantially cylindrical torch chamber with an open end for outflow of reaction products, a first electrode disposed in the torch chamber, and a second electrode 5 disposed in the torch chamber between the first electrode and the open end, wherein inoperation of the chemical reactor the plasma torch discharges into a liquid metal circulation, the method comprising:operating the plasma torch to discharge plasma torch reaction products into the liquid metal circulation;10 stopping operation of the plasma torch, upon which liquid metal from the liquid metalcirculation at least partly fills the torch chamber;wherein the second electrode comprises a porous refractory metal, and the liquid metal at least partly filling the torch chamber coats and at least partly fills pores of the second electrode; and15 restarting operation of the plasma torch, expelling a plug of liquid metal from thetorch chamber, while retaining a sacrificial coating of liquid metal on the second electrode.

Images