Electrode heating system for high-temperature industrial processing

Abstract

The present application provides a use of electric plasma as the heating process in pyroprocessing, for example in cement kilns, to replace the fuel burner currently employed in cement kilns. The electric heating system provided comprises an electrode pair for use in pyroprocessing which is designed to overcome the limitations of traditional graphite electrodes used, e.g., in electric arc furnaces. The electrode pair comprises a core cylindrical electrode (the inner electrode) made of a high-temperature resistant material such as metal alloy, e.g., high chromium steel, molybdenum or tungsten alloys, surrounded by a second hollow cylindrical electrode (the outer electrode) also made of a high-temperature resistant material such as the aforementioned metal alloys.

Description

ELECTRODE HEATING SYSTEM FOR HIGH-TEMPERATURE INDUSTRIAL PROCESSINGRELATED APPLICATION

[0001] This application claims benefit of priority to U.S. Provisional Application No. 63 / 614,042, filed December 22, 2023, which is incorporated by reference herein in its entirety.TECHNICAL FIELD

[0002] This disclosure generally relates to the field of industrial processing requiring high-power heating, e.g., pyroprocessing, such as cement production and, more particularly, to the field of low carbon clinker production in rotary kilns using electric energy. More particularly, the present application relates to non-consumable electrodes for use in ovens, furnaces or kilns, which are designed to provide thermal energy through electrical plasma arcs, while improving lifespan and reducing maintenance costs over current commercial systems.BACKGROUND OF THE ART

[0003] Apart from sand and gravel, steel and concrete are the most used construction materials in the world by mass. The steel and concrete industries each release approximately 8% of the global human-generated CO2emissions, which is very significant and must be reduced to meet greenhouse gas reduction goals to mitigate climate change.

[0004] Concrete is typically formed using a blend of Portland cement, aggregate, and water. Portland cement is manufactured through a closely controlled chemical combination of calcium, silicon, aluminum, iron and other ingredients. Common materials used to manufacture cement include limestone, shells, and chalk or marl combined with shale, clay, slate, blast furnace slag, silica sand, and iron ore. These ingredients, when heated at high temperatures in a rotary kiln form a rock-like substance (clinker) that is ground into the fine powder that we commonly think of as cement.

[0005] The high temperature (1 ,300°C-1 ,600°C) in the kiln required to initiate the chemical reactions forming clinker is currently obtained by burning fossil fuel in a large flame from the downstream end of the kiln. The combustion of the fossil fuel produces CO2in the offgas of the kiln which is then released in the atmosphere. The chemical reactions leadingto the formation of clinker, such as calcination in the calciner and the kiln, also release a lot of carbon as CO2and CO in the offgas and the atmosphere.

[0006] Yet, concrete, as it cures, absorbs carbon from the atmosphere for decades. Therefore, the lifecycle of concrete could be made low-carbon, or even carbon negative, if one were able to make the calcining and clinker processes low-carbon ones.

[0007] The production of steel and other metals such as copper, gold, aluminum and nickel or materials such as glass by pyroprocessing also requires high temperatures to melt the ore, oxides or metal scraps. The required temperatures in production furnaces to melt the feed are obtained either by burning fossil fuel or through electric arc plasma using consumable graphite electrodes or using plasma torches. While the latter process may not be releasing much CO2, all of the other processes do and contribute to the massive carbon footprint of metals and minerals processing.

[0008] A review of the literature, including patents, indicates the following disclosures which are relevant to the present application:

[0009] US 4,333,766 CALCIUM PROCESS FOR MANUFACTURING SILICATE AND / OR ALUMINATE BASED PRODUCTS; Relates to a method of manufacturing of cement clinker containing more than 50% of one of C3S, C2S, CS, CA, (using the cement chemist notation) or slag is described in which the solid starting material containing CaCO3is (a) heated to remove CO2, at least partly by hot gas, obtained, e.g., by combustion, circulating in countercurrent in contact with the solid material; (b) the temperature is further raised in the presence of a reduced quantity of gas; and (c) the product is cooled; the hot gases leaving the third step being passed over the product to heat it during the first step without passing through the chamber where the second step occurs, while the gas leaving the second step is not passed through into the first heating zone. Specifically, in the second heating step, the temperature is raised to above 1500 °C (above 1800 °C) especially above 1950 °C using a plasma in a rotary inclined plasma kiln to allow fusion of more than 80% of the material.

[0010] This patent describes a process starting from a typical clinker kiln operation where the flame in the kiln generated by combustion can be enhanced or replaced by a plasma produced in a fusion furnace downstream of the kiln by a plasma oven, including an electric arc furnace with process gas being candidates to produce the plasma. It also mentions that graphite electrodes coated with silicon or copper carbide can reduce theiroxidation (consumption) rate. This expired patent provides the main background for the current disclosure.

[0011] US 6,794,600B2 PLASMA TORCH USED FOR HEATING MOLTEN STEEL; This patent relates to a plasma burner comprising an electrode holder which carries a tubular outer electrode and a coaxially arranged central electrode, and an electrical current supply source, which can be coupled via current feeders to the outer and central electrodes respectively. Also a device is provided for feeding gas into the region of the electrodes, between which an arc is produced for plasma formation. The tubular outer electrode has a first cooled, arc-inactive tube section, consisting of electrically conductive metal, and an adjoining second arc-active uncooled tube section, which consists of heat-resistant and electrically conductive material, and which is releasably connected to the first tube section. The arc passes between the second tube section and the neighboring arc-active section of the coaxial central electrode.

[0012] This patent describes a conventional water-cooled lance-type DC plasma torch whereby the plasma is ejected from the tip of the lance operation. It is proposed for heating molten steel in a ladle. A range is provided for the ratio of electrical conductivity of the inner electrode (anode) versus the outer electrode (cathode) for better arc stability.

[0013] US 2022 / 0177366 A1 MANUFACTURING PORTLAND CEMENT WITH THERMAL PLASMA; This application provides various examples related to manufacturing Portland cement with thermal plasma. In one example, a method includes providing a raw kiln feed to a plasma arc gasification kiln; and forming clinker by heating the raw kiln feed in the plasma arc gasification kiln. The raw kiln feed can be heated with a plasma plume supplied with argon gas to a temperature in a range from about 1800 °C. to about 3000 °C. In another example, a system includes a kiln feed system and a plasma arc gasification kiln that receives raw kiln feed from the kiln feed system and heats the raw kiln feed with a plasma plume to form clinker. The system can include a clinker processing system configured to process the formed clinker to produce portland cement.

[0014] This patent application describes a process in which a high temperature plasma gasification kiln is used to produce clinker as a precursor for Portland cement. The experimental setup was based on a Pyrogenesis argon plasma arc gasification batch furnace that can produce temperatures from 1800 °C to 3000 °C and most of the application describes this batch approach to producing clinker. The application of a thermal plasma to existing rotary cement kilns is suggested, but only in general terms and without any teaching as to how to implement the use of thermal plasma in a rotary cement kiln.

[0015] Kilns are used to produce a variety of minerals-based and cement products, including Portland cement, a material that is widely used for concrete production throughout the world. These kilns typically operate at very high temperatures, ranging from 1300 to 1600 °C, and require specialized equipment to withstand these conditions. Current kilns are powered by fossil fuel burners, which produce an enormous amount of CO2emissions released in the atmosphere.

[0016] One of the key components of an electric kiln are the electrodes, which are used to conduct electricity to generate heat into the furnace to bring the raw materials to a high temperature to turn it into, e.g., clinker. Traditional electrodes for, e.g., electric arc furnaces for steel production, are made of graphite, which, despite its good thermal and electrical conductivity, has a number of limitations, including a relatively short lifespan due to the high temperatures and corrosive environments in the furnace, and its oxidation resulting in gaseous CO2being emitted as the electrodes are consumed.

[0017] To address these limitations, there remains a need for a new type of electrode that can provide improved performance, durability, and cost-effectiveness of pyroprocessing.

[0018] The above information is provided for the purpose of making known information believed by the applicant to be of possible relevance to the present invention. No admission is necessarily intended, nor should be construed, that any of the preceding information constitutes prior art against the present invention.SUMMARY OF THE INVENTION

[0019] The present inventors have developed a device and system that facilitates replacement of the fossil fuel burner typically found in kilns with a pair of electrodes that generate radiative heat through plasma arcs conducting electric current between the electrodes in order to initiate and sustain the chemical reactions producing clinker from the meal, whether pre-calcined or not. This electric arc process releases much less CO2than current rotary kilns, particularly when the consumed electricity is produced from renewable energy.

[0020] In one aspect, there is provided a use of an electric plasma as the heating process in pyroprocessing, for example in cement kilns, to replace the fuel burner currently employed in cement kilns. The electric heating system described herein comprises an electrode pair for use in pyroprocessing which is designed to overcome the limitations of traditional graphite electrodes used, e.g., in electric arc furnaces. The electrode pair comprises a core cylindrical electrode (the inner electrode) made of a high-temperatureresistant material such as metal alloy, e.g., high chromium steel, molybdenum or tungsten alloys, surrounded by a second hollow cylindrical electrode (the outer electrode) also made of a high-temperature resistant material such as the aforementioned metal alloys. The electrodes may be coated by refractory material to improve their resistance to heat and oxidation.

[0021] The inner and outer electrodes of the electrode pair provide excellent thermal and electrical conductivity, as well as high resistance to heat and corrosion. Both electrodes are cooled by a fluid that is circulating in channels within the electrodes to remove heat that would otherwise melt the alloy. This excess heat can be transported and used in other processes in the plant for, e.g., meal pre-calcining. The cylindrical inner electrode has a number of protruding rings that can strike and maintain plasma arcs when the rings are positioned close to corresponding rings protruding inside of the outer electrode. The inner electrode can be moved axially with respect to the outer electrode to control the arc lengths between the two, and hence the radiative power release, between the inner and outer ring pairs. Holes underneath the outer cylindrical electrode and optionally at its tip allow the radiative power of the plasma arcs to heat the pre-calcined meal at the bottom of the kiln, furnace or oven to transform it into, e.g., clinker in a standard rotary kiln and prevent temperature buildups between the two electrodes.

[0022] The electrodes can be manufactured using a variety of methods, such as powder metallurgy, extrusion, forming, machining and welding, and additive manufacturing. The electrodes can be sized according to the specific requirements of the kiln, furnace or oven while the rings can be tailored to provide the desired arc power. Process gas can be circulated between the electrodes to further cool the rings so as to prevent their melting or excessive oxidation.

[0023] The electrodes of the present invention allow the electrification of the material production process and offer numerous advantages over traditional fossil-fuel burners, including: improved radiative thermal energy distribution, leading to more efficient and effective heating and melting of the raw materials; reduced cost of consumable fuels, resulting in lower operating costs and improved productivity; resistance to heat and corrosion, ensuring consistent performance and reliability over time; and / orreduced C02emissions during heating operation.

[0024] The electrodes are suitable for use in a variety of existing continuous rotary kilns, including those used in the production of cement and the processing of other types of industrial minerals.

[0025] Furthermore, the gases released in the offgas system by the chemical reaction forming the product can be recirculated back through the electrode arcs for processing and partial electrode cooling.

[0026] The recirculated offgas can also be used to operate torch plasmas out of the holes close to the ring pairs located at the bottom and optionally at the tip of the outer electrode. The offgas would then ionize in the electric arcs between the electrode rings to generate plasma torches directed at the bed and improve heating efficiency.

[0027] The electric plasma arc heating system for the pyroprocessing of materials and metals, e.g., clinker and cement production, described herein provides a novel and innovative solution to the limitations of kilns, furnaces or ovens heated by a fuel-based burner. The electrodes offer superior environmental performance and cost-effectiveness, and are suitable for use in a variety of kilns, furnaces and ovens. The present invention is therefore considered to be both novel and non-obvious, and a provisional patent application is hereby filed.

[0028] An electric heating system for high-temperature minerals and metals processing, e.g., rotary kilns, is described herein, which is composed of long cantilevered high-current electrodes sustaining powerful electric arcs that can heat the feed material and turn it into product using preferably renewable electricity. This heating system can be used to replace traditional fuel burners used, for example, in rotary kilns.

[0029] The recirculated offgas can also be processed or used to operate torch plasmas out of the holes close to the ring pairs to improve heating efficiency. Other types of gases can be used to the same effect.

[0030] In accordance with one aspect, there is provided an electric heating system for high-temperature pyroprocessing of a feed, said system comprising: an electrode pair for generating radiative heat through plasma arcs conducting electric current between each electrode of the electrode pair, wherein: the electrode pair comprises an outer, hollow cylindrical electrode and an inner, core cylindrical electrode at least partially positioned concentricallywithin the outer electrode, there being an annular space therebetween to permit circulation of a process gas, the inner electrode comprises a plurality of outwardly protruding rings and the outer electrode comprises a plurality of inwardly protruding rings, such that each outwardly protruding ring of the plurality of outwardly protruding rings can strike and maintain plasma arcs when positioned close to a corresponding inwardly protruding ring of the plurality of inwardly protruding rings protruding inside of the outer electrode, the outer electrode comprises a wall having channels therein and the inner electrode comprises interior channels, wherein the channels of the outer and inner electrodes are for circulation of a cooling fluid, the wall of the outer electrode comprises a plurality of holes therethrough that are aligned along a length of the outer electrode parallel to the axis of the outer electrode, said holes facilitating ionization of the process gas circulating in the annular space and exiting through said holes, and facilitating release of radiative power from the plasma arcs to a surrounding environment to provide heating; the outer and inner electrodes are made of a first and a second high- temperature resistant material, respectively, wherein the first and the second high-temperature resistant materials are the same or different, means for rotating or distributing the plasma arcs around the protruding rings of the inner and outer electrodes, and means for moving the inner electrode axially within the outer electrode to modulate the arc lengths between the inner and outer electrode and thereby modulate the radiative power release.

[0031] In some embodiments, the means for rotating or distributing the plasma arc are: an electromagnetic field produced by a coil, coils or other means to make the arcs rotate around the rings, a rotational gas flow injected between the electrodes to make the arcs rotate around the rings, or a design of the electrode rings comprising protruding teeth such that multiple arcs are distributed around the rings, each producing a fraction of the power that would otherwise radiate for a single arc between the rings.

[0032] Optionally, the means for rotating or distributing the plasma arc functions to avoid localized overheating and destructive melting of the electrodes by having excessive current and radiation where the arcs operate for too long a time.

[0033] In some embodiments, means for moving the inner electrode axially within the outer electrode is a linear actuation system that can move the inner electrode with respect to the outer electrode to adjust the arc length which in turn changes its resistance R and radiative power P=V2 / R (the longer the arc, the higher the resistance and the lower the radiative power for a fixed voltage V).

[0034] Optionally, the electrode rings are tailored to provide a desired arc power distribution in the kiln.

[0035] In some embodiments, the electric heating system further comprises: a cooling system that extracts heat from the electrodes by circulating a cooling fluid, said cooling system comprising: cooling channels in both electrodes wherein a cooling fluid is circulated to carry the excess heat away through a circulating cooling fluid. cooling fluid entering input channels in the electrodes and coming out as hotter cooling fluid through the output channels of the electrodes, a heat exchanger that can extract the heat carried out of the electrodes by the cooling fluid, and a pump or blower to circulate the cooling fluid through the electrode channels and the heat exchanger.

[0036] In some embodiments, the first and second high-temperature resistant materials are independently a metal alloy, such as, a high chromium steel, molybdenum or tungsten alloy.

[0037] In some embodiments, the first and second high-temperature resistant material can withstand a temperature of up to about 2000°C.

[0038] In some embodiments, the system additionally comprises a high-power voltage source applied to the inner and outer electrodes whereby: the voltage source is of constant voltage (DC source) and:the inner electrode is connected to the positive lead of the voltage source and the outer electrode is connected to the negative lead of the voltage source, which may or may not be grounded, the outer electrode is connected to the positive lead of the voltage source and the inner electrode is connected to the negative lead of the voltage source, which may or may not be grounded, the voltage amplitude of the DC voltage source can be adjusted to modify the current in the electrodes and the plasma arcs, or the voltage source is alternating (AC source) and: the inner electrode is connected to one lead of the AC voltage source (labeled as positive for circuit voltage reference) and the outer electrode is connected to the other lead of the AC voltage source (labeled as negative), which may or may not be grounded, the outer electrode is connected to one lead of the AC voltage source (labeled as positive for circuit voltage reference) and the outer electrode is connected to the other lead of the AC voltage source (labeled as negative), which may or may not be grounded, the voltage amplitude of the AC voltage source can be adjusted to modify the AC current in the electrodes and the plasma arcs.

[0039] In some embodiments, a power control system is applied to the electrodes wherein: a heating power setpoint is chosen by a control system or an operator, the power setpoint divided by the voltage gives the desired electrode current, the means for moving the inner electrode axially within the outer electrode are used to modulate the electrode current through a feedback loop such that the power released in the plasma arcs is regulated to the power setpoint, and the voltage amplitude of the voltage source can also be used to modulate the electrode current.

[0040] In accordance with another aspect, there is provided system for materials manufacture, comprising: a kiln, oven or furnace; a kiln feed system for providing feed to the kiln, oven or furnace; andan electric heating system as described herein.

[0041] In some embodiments, the system for materials manufacture additionally comprises a freeboard temperature control system, wherein: a freeboard temperature setpoint is chosen by a control system or an operator, a measurement of the freeboard temperature is taken, a higher arc power setpoint for the power controller is chosen if the freeboard temperature is lower than the measured one, a lower arc power setpoint for the power controller is chosen if the freeboard temperature is higher than the measured one, and a temperature control algorithm repeats this process regularly to effectively regulate the freeboard temperature to the setpoint.

[0042] In some embodiments, the system for materials manufacture comprises a kiln and is used for manufacture of cement.

[0043] Without loss of generality, the electrode heating system is exemplified below by its potential application to Portland cement production in a clinker kiln. As would be readily appreciated by the skilled person, the electrode heating system can be applied to other processing equipment; the following description relating to the clinker kiln is provided as an example only.

[0044] Many further features and combinations thereof concerning the present improvements will appear to those skilled in the art following a reading of the disclosure.BRIEF DESCRIPTION OF THE DRAWINGS

[0045] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate various embodiments. In the drawings:

[0046] Fig. 1 is a schematic view of a typical cement kiln;

[0047] Fig. 2 is a schematic three-dimensional view of an electrified model cement kiln;

[0048] Fig. 3 is a three-dimensional view of an embodiment of an electrode pair ((a) inner electrode, (b) outer electrode) of Fig. 2;

[0049] Fig. 4 is a photograph of a small-scale prototype of the electrode pair of Fig. 3

[0050] Fig. 5 is a flow chart illustrating steps for operating the electric kiln of Fig. 2, to regulate the freeboard temperature to a favorable range for clinkering, where Tdesis the freeboard setpoint temperature and Tfbis the measured freeboard temperature;

[0051] Fig. 6 is a cross-section view of the electrodes of Fig. 3 showing the cooling channels; and

[0052] Fig. 7 is a schematic view of an embodiment of a drive system that can be used to control the relative position of the inner electrode with respect to the outer electrode of Fig. 3 to control the arc lengths and therefore their radiative power.DETAILED DESCRIPTION

[0053] Definitions

[0054] Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs.

[0055] As used in the specification and claims, the singular forms “a”, “an” and “the” include plural references unless the context clearly dictates otherwise.

[0056] The term “comprising”, as used herein, will be understood to mean that the list following is non-exhaustive and may or may not include any other additional suitable items, for example one or more further feature(s), component(s) and / or ingredient(s) as appropriate.

[0057] The term “or” as used herein, refers to a non-exclusive or, unless otherwise indicated.

[0058] The term “and / or” as used in a phrase such as “A and / or B” herein is intended to include “A and B”, “A or B”, “A”, and “B”.

[0059] Reference throughout this specification to “one embodiment,” “an embodiment,” “another embodiment,” “a particular embodiment,” “a related embodiment,” “a certain embodiment,” “an additional embodiment,” or “a further embodiment” or combinations thereof means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, the appearances of the foregoing phrases, or the like, in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features,structures, or characteristics may be combined in any suitable manner in one or more embodiments.

[0060] The embodiments herein and the various features and advantageous details thereof are explained more fully with reference to non-limiting embodiments that are illustrated in the accompanying drawings and detailed in the following description. Descriptions of well-known components and processing techniques are omitted so as to not unnecessarily obscure the embodiments herein. Also, the various embodiments described herein are not necessarily mutually exclusive, as some embodiments can be combined with one or more other embodiments to form new embodiments. The examples used herein are intended merely to facilitate an understanding of ways in which the embodiments herein can be practiced and to further enable those skilled in the art to practice the embodiments herein. Accordingly, the examples should not be construed as limiting the scope of the embodiments herein.

[0061] Referring to Figure 1 , a typical cement kiln used to produce clinker, which is the main component of Portland cement, is shown schematically ((1) steel shell, (2) refractory lining, (3) feeder) in which the main energy input is provided by a fossil fuel burner (4) sustaining a flame (5). The main chemical process in the calciner which is fed a pre-heated raw meal of various compositions (typically, limestone, sand, clay) follows calcination of calcium carbonate CaCO3into calcium oxide CaO (quicklime) at about 900C in the precalciner which releases CO2. The main components of the raw meal coming from the precalciner into the kiln are CaO=C, SiO2=S, AI2O3=A, Fe2O3=F. Higher temperatures of 900°C to 1400°C in the kiln make these oxides react to form compounds in the clinker that give Portland cement its material properties, such as alite (Ca3SiO5= 3(CaO).SiO2= or 3CS in the cement chemist notation) and belite (Ca2SiO4= 2(CaO).SiO2= or 2CS).

[0062] Cement plants release enormous amounts of CO2mainly because of three processes: (i) fossil fuel combustion, (ii) calcination of the meal in the cyclone calciner upstream of the kiln, or in the kiln itself, and (iii) clinkering in the kiln which is the combination of high-temperature chemical reactions producing clinker. These plants are continuous in nature and therefore the fuel burner in the kiln must maintain the material at a high temperature around 1300°C by continuously burning oil, gas, rubber or other waste material. Technologies that can significantly reduce greenhouse gas emissions are needed.

[0063] Figure 2 shows an electric model kiln in which electrodes (6) maintaining electric plasma arcs replace the heating power obtained from the classical fuel burner. Assuming that the electricity powering the heating electrodes is generated from renewable sources,then such a kiln would be much less polluting than a classical fuel-burning one. It should be noted that the electrode heating system could be designed to be used in parallel with a fuel burner to enable a hybrid heating operation of the kiln or to continue operation of the kiln in fuel burning mode in case of electrical power outages so as not to jeopardize cement production during such outages.

[0064] Figure 3 shows an embodiment of the electrodes (inner electrode (7) located inside the outer electrode (8) without touching it) in a three-dimensional view of the electrode pair. The electrodes are positioned axially in the center of the kiln (see Figure 2) such that their rings are close to each other and are supported in a cantilevered fashion by a structure affixed outside of the output end of the kiln (see Figure 7). The outer electrode is attached to this structure, and therefore it is fixed, whereas the inner electrode can be moved axially to adjust the relative position of its rings with respect to the rings of the outer electrode. This allows the control of the lengths of the arcs between the electrode rings and therefore the control of the power radiating from their plasmas to adjust the temperature inside the kiln. Control of the arc lengths was tested on a scaled-down prototype of the electrode pair (Figure 4). These tests showed that the arcs could be struck by axially moving the inner electrode such that its rings got closer to the outer electrode rings. The arcs could then be elongated by moving the inner electrode in the opposite direction, showing that arc length and electrode current could be controlled in this fashion.

[0065] The flow chart in Figure 5 shows how the electrode system is operated in a periodic mode to control the temperature inside the kiln. A real-time automatic feedback control system can be implemented to maintain the freeboard temperature to a setpoint (9) in which the controller aims to reduce the temperature error by activating the actuator moving the inner electrode or by changing the supply voltage. If the freeboard temperature is too low, then either the supply voltage is increased or the inner electrode is moved in the direction that brings the ring pairs closer together resulting in shorter arcs of lower impedance that produce more power (10). Both of these control actions can be used at the same time. Conversely, if the freeboard temperature is too high, then either the supply voltage is decreased or the controller moves the inner electrode in the direction that takes the ring pairs further apart resulting in longer arcs of higher impedance that produce less power (11). Both of these control actions can be used at the same time.

[0066] The electric arcs between the rings release a lot of heat locally which could damage the electrode material. Cooling channels are therefore built in both electrodes to carry the excess heat away through a circulating cooling fluid. Figure 6 shows a view of the electrodes of Figure 3 showing these cooling channels. In the inner electrode, the coolingfluid enters two opposite channels (12) and comes out through the other two channels (13). Similarly, the outer electrode is cooled by fluid entering two opposite channels (14) and coming out through the other two channels (15). The heated fluid is then circulated through a heat exchanger to provide recovered heat to other parts of the plant, such as the precalciner. The cooled fluid can then be recirculated in the electrodes in a continuous process.

[0067] An additional embodiment includes a method to avoid overheating damage of the rings of the inner electrode (7) and the outer electrode (8) where the arcs impinge is to make the arcs constantly move over them. This can be done by applying an axial magnetic field which, according to the Lorentz law, will apply a force to the ionized particles and electrons in the plasma producing the current in the arcs which will make them rotate between the two rings. This principle has been tested for a constant DC magnetic field applied axially along a pair of scaled-down inner and outer electrodes (Figure 4). These tests have shown that under such a magnetic field (of a magnitude of 50 mT for the prototype of Figure 4), the electric arcs rotate between the ring pairs, preventing a quick erosion of the ring alloys.

[0068] According to one embodiment, there is provided a drive system that is used to control the relative position of the inner electrode with respect to the outer electrode is shown in Figure 7. This drive system controls the arc lengths between the electrode rings and therefore their radiative power by engaging a rack (16) attached to the inner electrode through a pinion (17) connected to a motor (18). Also show in Figure 7 is the voltage source (19) applying voltage to the electrodes.

[0069] All publications, patents and patent applications mentioned in this Specification are indicative of the level of skill of those skilled in the art to which this invention pertains and are herein incorporated by reference to the same extent as if each individual publication, patent, or patent applications was specifically and individually indicated to be incorporated by reference.

[0070] The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.

Claims

WHAT IS CLAIMED IS:

1. An electric heating system for high-temperature pyroprocessing of a feed, said system comprising: an electrode pair for generating radiative heat through plasma arcs conducting electric current between each electrode of the electrode pair, wherein: the electrode pair comprises an outer, hollow cylindrical electrode and an inner, core cylindrical electrode at least partially positioned concentrically within the outer electrode, there being an annular space therebetween to permit circulation of a process gas, the inner electrode comprises a plurality of outwardly protruding rings and the outer electrode comprises a plurality of inwardly protruding rings, such that each outwardly protruding ring of the plurality of outwardly protruding rings can strike and maintain plasma arcs when positioned close to a corresponding inwardly protruding ring of the plurality of inwardly protruding rings protruding inside of the outer electrode, the outer electrode comprises a wall having channels therein and the inner electrode comprises interior channels, wherein the channels of the outer and inner electrodes are for circulation of a cooling fluid, the wall of the outer electrode comprises a plurality of holes therethrough that are aligned along a length of the outer electrode parallel to the axis of the outer electrode, said holes facilitating ionization of the process gas circulating in the annular space and exiting through said holes, and facilitating release of radiative power from the plasma arcs to a surrounding environment to provide heating; the outer and inner electrodes are made of a first and a second high- temperature resistant material, respectively, wherein the first and the second high-temperature resistant materials are the same or different, means for rotating or distributing the plasma arcs around the protruding rings of the inner and outer electrodes, and means for moving the inner electrode axially within the outer electrode to modulate the arc lengths between the inner and outer electrode and thereby modulate the radiative power release.

2. The electric heating system of claim 1 , wherein the means for rotating or distributing the plasma arc are: an electromagnetic field produced by a coil, coils or other means to make the arcs rotate around the rings, a rotational gas flow injected between the electrodes to make the arcs rotate around the rings, or a design of the electrode rings comprising protruding teeth such that multiple arcs are distributed around the rings, each producing a fraction of the power that would otherwise radiate for a single arc between the rings.

3. The electric heating system of claim 1 or 2, wherein the means for rotating or distributing the plasma arc functions to avoid localized overheating and destructive melting of the electrodes by having excessive current and radiation where the arcs operate for too long a time.

4. The electric heating system of any one of claims 1 to 3, wherein the means for moving the inner electrode axially within the outer electrode is: a linear actuation system that can move the inner electrode with respect to the outer electrode to adjust the arc length which in turn changes its resistance R and radiative power P=V2 / R (the longer the arc, the higher the resistance and the lower the radiative power for a fixed voltage V).

5. The electric heating system of any one of claims 1 to 4, wherein the electrode rings are tailored to provide a desired arc power distribution in the kiln.

6. The electric heating system of any one of claims 1 to 5, wherein the system further comprises: a cooling system that extracts heat from the electrodes by circulating a cooling fluid, said cooling system comprising: cooling channels in both electrodes wherein a cooling fluid is circulated to carry the excess heat away through a circulating cooling fluid. cooling fluid entering input channels in the electrodes and coming out as hotter cooling fluid through the output channels of the electrodes,a heat exchanger that can extract the heat carried out of the electrodes by the cooling fluid, and a pump or blower to circulate the cooling fluid through the electrode channels and the heat exchanger.

7. The electric heating system of any one of claims 1 to 6, wherein the first and second high-temperature resistant materials are independently a metal alloy, such as, a high chromium steel, molybdenum or tungsten alloy.

8. The electric heating system of any one of claims 1 to 7, wherein the first and second high-temperature resistant material can withstand a temperature of up to about 2000 °C.

9. The electric heating system of any one of claims 1 to 8, wherein the system additionally comprises a high-power voltage source applied to the inner and outer electrodes whereby: the voltage source is of constant voltage (DC source) and: the inner electrode is connected to the positive lead of the voltage source and the outer electrode is connected to the negative lead of the voltage source, which may or may not be grounded, the outer electrode is connected to the positive lead of the voltage source and the inner electrode is connected to the negative lead of the voltage source, which may or may not be grounded, and the voltage amplitude of the DC voltage source can be adjusted to modify the current in the electrodes and the plasma arcs, or the voltage source is alternating (AC source) and: the inner electrode is connected to one lead of the AC voltage source (labeled as positive for circuit voltage reference) and the outer electrode is connected to the other lead of the AC voltage source (labeled as negative), which may or may not be grounded, the outer electrode is connected to one lead of the AC voltage source (labeled as positive for circuit voltage reference) and the outer electrode is connected to the other lead of the AC voltage source (labeled as negative), which may or may not be grounded, andthe voltage amplitude of the AC voltage source can be adjusted to modify the AC current in the electrodes and the plasma arcs.

10. The electric heating system of claim 9, wherein a power control system is applied to the electrodes wherein: a heating power setpoint is chosen by a control system or an operator, the power setpoint divided by the voltage gives the desired electrode current, the means for moving the inner electrode axially within the outer electrode are used to modulate the electrode current through a feedback loop such that the power released in the plasma arcs is regulated to the power setpoint, and the voltage amplitude of the voltage source can also be used to modulate the electrode current.

11. A system for materials manufacture, comprising: a kiln, oven or furnace; a kiln feed system for providing feed to the kiln, oven or furnace; and the electric heating system of any one of claims 1 to 10.

12. The system of claim 11 , additionally comprising a freeboard temperature control system, wherein: a freeboard temperature setpoint is chosen by a control system or an operator, a measurement of the freeboard temperature is taken, a higher arc power setpoint for the power controller is chosen if the freeboard temperature is lower than the measured one, a lower arc power setpoint for the power controller is chosen if the freeboard temperature is higher than the measured one, a temperature control algorithm repeats this process regularly to effectively regulate the freeboard temperature to the setpoint.

13. The system of claim 11 or 12, wherein the system comprises the kiln and is used for manufacture of cement.

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