Electric furnace with multiple floors

Abstract

The present invention relates to a multi-deck or multi-stage oven (1) for carrying out at least one heat treatment at a temperature above 1000°C, characterized in that the oven heating device comprises at least one or more radiant tubes (16) incorporating a high-temperature electrical resistance (14) in cantilever and fixed at a first end rigidly to said outer casing (3) below an upper deck (2).

Description

Object of the invention

[0001] The present invention relates to the technical field of industrial reactors for carrying out heat treatments, or combinations of heat treatments, of solid, powdery or pasty materials, such as in particular drying, pyrolysis, incineration, roasting, carbonization, activation, regeneration, calcination or toasting of such materials.

[0002] The present invention relates more particularly to a multi-deck oven, also called a multi-level oven ( multiple hearth furnace or MHF). Technological background and state of the art

[0003] There figure 1 This schematically represents a multi-hearth furnace 1, according to the prior art. This is the common design for this type of furnace equipped with burners and refractories.

[0004] Such an oven 1 may for example take the form of a series of floors or baking plates 2 generally circular, arranged parallel to one another in a steel casing 3 generally cylindrical and lined with refractory material (not shown).

[0005] A vertical rotating shaft 4, positioned along the axis of the kiln, carries arms equipped with rakes 5, which stir the charge fed into the kiln at its upper end 6 and move it through each hearth 2 in a spiral pattern. The material moves through any two adjacent hearths in opposite directions, respectively towards the center and towards the outside of the kiln, and vice versa. To achieve this, successive hearths 2 are equipped with openings located respectively near the vertical shaft and the diametrical end of the hearths.

[0006] The biomass, or feed in general, is supplied to the upper plate and forced through it via the aforementioned openings leading to the plate immediately below, and so on. The material thus passes over and through each plate 2 towards the bottom of the installation at an outlet 7 where the product is discharged.

[0007] Hot gases 13, circulating either counter-currently to the charge or in the same direction, raise the furnace to the desired temperature and induce the desired heat treatment reaction(s) of the charge. Traditionally, the heat is therefore produced by the combustion of either components of the charge itself or auxiliary fuel. For example, liquid or gaseous fuel burners 8 may be provided through the outer casing 3 at the level of some of the hearths 2.

[0008] Steam can also be injected into the furnace to improve control or to participate in process reactions. The installation generally operates under a controlled atmosphere and includes means for controlling the temperature and residence time of the charge in the furnace. The charge feed can be continuously adjusted to maintain a constant thickness of the wire bed inside the furnace.

[0009] This type of furnace, capable of reaching temperatures of 1000°C or more, generally incorporates significant amounts of refractory material supported by the outer shell. This material is typically made of bricks assembled in self-supporting structures known as "Chinese hat" structures, familiar to those skilled in the art. The refractory thickness is typically 10 to 30 cm, or more. Furthermore, the central shaft of the furnace must be cooled by a device also familiar to those skilled in the art.

[0010] According to an alternative design not shown, the furnace is a metal furnace with indirect heating by heat transfer fluid. This alternative to the MHF design described above is that specified in patent EP 3 411 458 B1: this distinct MHF design, lacking refractories and a combustion system, but with a heat transfer fluid exchanger, can also serve as a basis for prior art within the scope of the present invention.

[0011] Furthermore, if one is looking for heating methods other than those presented above, and in the current context of seeking to reduce the use of fossil fuels, one is of course familiar with electric heating elements that provide heat through the Joule effect and are used in numerous industrial applications. First and foremost are ovens, baking tunnels, and muffle furnaces, heated by electric heating elements embedded in the walls or in a hearth.

[0012] Thus, the annealing and galvanizing lines for steel strips include electric or combustion radiant tubes in certain annealing, temperature holding or aging sections.

[0013] Finally, Joule effect heating is also used in some glass furnaces, thanks to the conduction of high-temperature glass between two immersion electrodes.

[0014] Document WO 2000 / 017404 A1 discloses a multi-hearth furnace comprising several superimposed hearths for the production of directly reduced metal from metal oxides, characterized by electric heating elements arranged under the individual hearths or on the furnace shell. The process heat required for the reduction of the metal oxides is generated by indirect heating of the metal oxides solely by the electric heating elements, the heating elements being advantageously independent of each other. The electric heating elements may also include a protective sheath.

[0015] Document WO 2014 / 107132 A1 discloses a pressurized reactor in which a flowing pressurized gas is heated by being transported through a space between two concentric tubes. The inner tube is heated by radiant heat from heating elements and is kept open to the gas flow path in the pressure vessel so that pressure equalization is achieved between the inside and outside of the inner tube without the tube becoming part of the gas flow path. Objectives of the invention

[0016] The present invention aims to eliminate the use, or at least to reduce the use, of liquid or gaseous fuels for heating the furnace.

[0017] The invention aims in particular to avoid, totally or partially, the use of burners and combustion air fans, the absence of a burner in the MHF making it possible to avoid any CO2 emissions related to combustion, in particular of fossil origin.

[0018] The present invention also aims to provide the use of a specific oven control system. Main characteristic elements of the invention

[0019] A first aspect of the present invention relates to a multi-hearth or multi-stage furnace for carrying out at least one heat treatment at a temperature above 1000°C, comprising an upper inlet for the material to be heat-treated, a lower outlet for the treated material, a rotating central shaft, a heating device for the material to be heat-treated, comprising electrical resistors, an outer casing to which a plurality of hearths are attached, at least one raking arm above each hearth, configured to be rotated by the rotating central shaft, said hearths having alternately and successively a material discharge opening proximal to the shaft and a material discharge opening distal to the shaft, so as to give the material to be treated a spiral path along the entire height of the furnace,said multi-deck oven being characterized in that the aforementioned heating device comprises at least one or more radiant tubes incorporating a high-temperature electrical resistance in a cantilevered position and rigidly fixed at one end to said outer casing below an upper deck.

[0020] According to preferred embodiments of the invention, the multi-deck oven further comprises at least one of the following features or an appropriate combination thereof: The heating device comprises a combination of radiant tubes incorporating an electrical resistance allowing combustion by indirect heating with one or more burners with a flame directly produced in the oven, allowing combustion by direct heating, and / or possibly radiant pipes containing a heat transfer fluid attached to one face of one or more hearths; high-temperature electrical resistances are further fixed to the internal walls of the oven between the hearths, said electrical resistances being provided with a protective muffle; each radiant tube incorporating an electrical resistance in cantilever and rigidly fixed to said outer casing is positioned between said upper hearth and the framing arm associated with the immediately lower hearth;Radiant tubes incorporating an electrical resistance are made of high-temperature stainless steel, nickel-chromium or ni-chromium alloy, iron-chromium-aluminum alloy, metallic ceramic alloy or cermet, ceramic such as silicon carbide or silicon nitride or refractory alloy such as molybdenum or tungsten; high-temperature electrical resistances are made of iron-chromium-aluminum alloy, nickel-chromium or nichrome alloy, molybdenum, silicon carbide or SiC, or molybdenum disilicon or MoSi2; protective muffles are made of iron-chromium-aluminum alloy, heat-resistant stainless steel, nickel-chromium or Inconel alloy, silicon carbide or silicon nitride, or refractory ceramic such as alumina, mullite or yttrium-stabilized zirconia;Support is provided for suspending the radiant tubes at the level of the upper hearth, at a second end, while allowing for the free expansion of the radiant tubes; the length of the radiant tubes is at least equal to half, preferably 75%, of the radius of the cylindrical furnace; the furnace includes control means, in particular individual and independent control for the heating elements; the furnace includes means for reinjecting hot post-combustion gases, produced in a first part of the furnace, into a second part of the furnace having a lower temperature than that of the first part of the furnace; the furnace includes means for injecting either oxidizing or reducing gases to maintain the oxidizing or reducing character of a stage, respectively.

[0021] A second aspect of the invention relates to the use of a multi-hearth furnace as described above, as an industrial reactor for carrying out a heat treatment or a combination or succession of heat treatments of solid, powdery or pasty materials, such as drying, pyrolysis, incineration, roasting, carbonization, activation, regeneration, calcination or toasting of such materials.

[0022] A third aspect of the invention relates to a method for controlling and piloting a multi-hearth furnace as described above, characterized in that said electrical resistances are controlled individually and independently, and in that the temperature monitoring at each of the furnace levels is carried out by means of distributed measurements, with the aid, under certain operating conditions, of infrared imaging techniques and / or a digital twin, making it possible to optimize heat transfer, or even to flexibly approach the operating conditions achieved in a furnace with one (or more) direct flame burner(s). Brief description of the figures

[0023] There figure 1 shows a schematic cross-sectional view of a multi-hearth oven according to the prior art.

[0024] There figure 2shows a schematic cross-sectional view of a multi-hearth oven according to one embodiment of the present invention.

[0025] There figure 3 represents a detailed cross-sectional view of a cantilevered radiant tube positioned radially in a stage of the furnace according to a preferred embodiment of the invention. Description of preferred embodiments of the invention

[0026] The present invention proposes to provide a use of electrical resistances inside a multi-stage furnace, as well as the combination of this use of electrical resistances with either radiant tubes for burners (indirect heating combustion) or burners with flame directly produced in the furnace (direct heating combustion), or possibly radiant coils heated by heat transfer fluid.

[0027] According to one embodiment of the present invention shown in the figures 2 And 3, the electrical resistors 14 are integrated inside a radiant tube 16 positioned and supported stably through the enclosure 3 and below the soles 2, more particularly between the soles 2 and the rails 5.

[0028] This technique involves using a specific material for the tube, capable of withstanding high temperatures, exceeding 1000°C, and oxidizing and potentially aggressive and corrosive atmospheres inside the furnace, such as an atmosphere containing water vapor, sulfur compounds, etc. Preferred materials include high-temperature stainless steels, nickel-chromium (Ni-chromium) alloys, iron-chromium-aluminum alloys, metallic ceramic alloys (cermet), ceramics (silicon carbide or nitride), or refractory alloys (molybdenum, tungsten). The choice of material depends on factors such as the operating temperature, the atmosphere inside the furnace (oxidizing, reducing, corrosive), and the desired furnace lifespan.

[0029] High-temperature heating elements are preferably made of iron-chromium-aluminum alloy, nickel-chromium alloy (nichrome), molybdenum, silicon carbide (SiC), or molybdenum disilicon (MoSi2). The choice of material will be dictated primarily by the operating temperature, since the heating elements will not be in direct contact with the furnace atmosphere.

[0030] As schematically represented on the Figure 3 , the radiant tubes 16 containing the resistors 14 are installed radially in cantilever, with a rigid fixing on the side of the metal casing 3 of the MHF as well as preferably with a support 17 of specific design by suspending the end of the radiant tube 16, known in itself to the man skilled in the art, allowing to relieve the cantilever forces while allowing the differential expansion between this tube and the upper base which supports it.

[0031] This cantilevered arrangement of the radiant tubes offers several advantages: a positioning that does not significantly obstruct the workspace within the oven for the passage of materials; homogeneous heating with better heat distribution at the core of the oven, particularly for processes requiring uniform high temperatures; structural advantages insofar as the cantilevered elements reduce stress on the walls and insulation of the oven, which also increases the oven's lifespan.

[0032] One design variant, when service conditions require it, involves fixing electrical resistors 15 to the internal walls of the MHF ( figure 2 also), with a protective mitten of specific design (not shown).

[0033] Protective muffs for heating elements in high-temperature furnaces are essential devices that surround or encapsulate the heating elements to ensure greater durability, improve thermal efficiency, and protect the elements from potentially aggressive environmental conditions inside the furnace.

[0034] More specifically, protective muffs act as a physical and thermal barrier between an electric heating element and the oven's internal atmosphere. They help extend the lifespan of the heating elements by protecting them from oxidation, chemical corrosion, reactive gases, and deposits of heated materials. Finally, they reduce the deformation of the heating elements under the effect of high temperatures and minimize heat loss.

[0035] Advantageously, these muffles have a tubular or semi-tubular shape so as to completely surround the heating element or create a curved plate for partial protection. The tubular structure promotes uniform heat transfer while ensuring adequate insulation. The thickness of the muffle depends on the operating conditions. Greater thickness offers better thermal protection but can affect the heating time.

[0036] Advantageously, protective mittens will be made of iron-chromium-aluminum alloy, heat-resistant stainless steel, nickel-chromium alloy or Inconel, silicon carbide or nitride, or refractory ceramic such as alumina, mullite or yttrium-stabilized zirconia.

[0037] The choice of material depends on factors such as the operating temperature, the atmosphere in the furnace (oxidizing, reducing, neutral), and the presence of frequent thermal cycles. In highly aggressive environments, ceramics (SiC) or nickel-chromium alloys are preferred for their durability and chemical stability.

[0038] The invention allows for improved temperature uniformity across each floor of the MHF (Magnetic Heat Facility). It also reduces the volume of gas circulating within the furnace and the volume of fumes emitted outside. This also results in reduced dust emissions and more precise temperature control at different furnace levels through the use of independently controlled heating elements. Consequently, the flue gas treatment line and its component equipment, such as afterburners, filters, fans, etc., can be smaller and more energy-efficient.

[0039] With better heating efficiency, the total energy consumption of the oven, which is electric rather than fuel-based, is also drastically reduced, by up to 50%.

[0040] Individual control of the different electrical resistances and monitoring of the temperature at each stage through distributed measurements, with, in certain operating conditions, infrared imaging techniques and / or a digital twin, advantageously allows for the optimization of heat transfer and even approaches the operating conditions achieved with one (or more) burner(s), and this with great operational flexibility.

[0041] Replacing direct heating with indirect heating also offers the advantage of avoiding thermochemical interactions between the furnace atmosphere and the product(s) to be treated.

[0042] In the case of radiant tubes, the invention also allows for easier maintenance in the event of a heating system failure. The electric heating element can be designed to be removed from the oven, leaving the sealed radiant tube inside, thus preventing gas exchange with the outside. In contrast, troubleshooting or replacing a burner requires the oven to be completely shut down and cooled.

[0043] The invention eliminates the risk associated with leaks of gaseous or liquid fuel that could potentially generate explosive atmospheres, both inside the reactor (shut-off valve seals) and outside (pipes and joints). Similarly, equipment operation and maintenance are optimized by eliminating the need for inerting procedures.

[0044] According to a preferred embodiment of the invention, the furnace includes means for reinjecting the hot post-combustion gases, produced in a first part of the furnace, into a second part of the furnace having a lower temperature than the first part of the furnace (not shown).

[0045] According to another embodiment of the invention, the oven includes means for injecting either oxidizing or reducing gases to maintain the oxidizing or reducing character of a hearth, respectively. List of reference symbols

[0046] 1 Multi-hearth furnace (MHF) 2 Refractory or metal hearth 3 Outer casing 4 Central shaft 5 Loading arm 6 Upper charge inlet 7 Lower charge outlet 8 Fossil fuel burner 9 Central shaft cooling 14 Radiant tube heating element 15 Side wall heating element with muffle 16 Cantilevered radiant tube with integrated heating element 17 Supporting element

Claims

1. A multi-hearth or multi-stage furnace (1) for carrying out at least one heat treatment at a temperature exceeding 1000°C, comprising an upper inlet (6) for the material to be heat-treated, a lower outlet (7) for the treated material, a rotating central shaft (4), a heating device (8, 14, 15) for the material to be heat-treated, comprising electric heating elements (14, 15), an outer casing (3) to which a plurality of hearths (2) are attached, at least one raking arm (5) above each hearth (2), configured to be rotated by the rotating central shaft (4), said hearths (2) having alternately and successively a material discharge opening proximal to the shaft and a material discharge opening distal to the shaft, so as to impart to the material to be treated a spiral path along the entire height of the furnace, said multi-hearth furnace (1) being characterized in thatthe aforementioned heating device comprises at least one or more radiant tubes (16) incorporating a high-temperature electrical resistance (14) in cantilever and fixed at one end rigidly to said outer casing (3) below an upper base (2).

2. Multi-deck oven (1) according to claim 1, characterized in that the heating device includes a combination of radiant tubes (16) incorporating an electrical resistance (14) allowing combustion by indirect heating with one or more burners (8) with a flame directly produced in the oven, allowing combustion by direct heating, and / or possibly radiant pipes containing a heat transfer fluid attached to one face of one or more hearths (2).

3. Multi-deck oven (1) according to claim 1, characterized in thatHigh-temperature electrical resistances (15) are further fixed to the internal walls of the oven between the floors (2), said electrical resistances (15) being provided with a protective muffle.

4. Multi-deck oven (1) according to claim 1, characterized in that each radiant tube (16) incorporating an electrical resistance (14) in cantilever and rigidly fixed to said outer casing (3) is positioned between said upper sole (2) and the framing arm (5) associated with the directly lower sole (2).

5. Multi-deck oven (1) according to claim 1, characterized in that radiant tubes (16) incorporating an electrical resistance (14) are made of high temperature stainless steel, nickel-chromium or ni-chromium alloy, iron-chromium-aluminium alloy, metallic ceramic alloy or cermet, ceramic such as silicon carbide or nitride or refractory alloy such as molybdenum or tungsten.

6. Multi-deck oven (1) according to any one of claims 1 or 3, characterized in that High temperature electrical resistors (14, 15) are made of iron-chromium-aluminum alloy, nickel-chromium or nichrome alloy, molybdenum, silicon carbide or SiC, or molybdenum disilicon or MoSi2.

7. Multi-deck oven (1) according to claim 3, characterized in that Protective mittens are made of iron-chromium-aluminum alloy, heat-resistant stainless steel, nickel-chromium alloy or Inconel, silicon carbide or nitride, or refractory ceramics such as alumina, mullite or yttrium-stabilized zirconia.

8. Multi-deck oven (1) according to claim 1, characterized in that a support (17) is provided for the suspension of the radiant tubes (16) at the level of said upper sole (2), at a second end, while allowing the free expansion of the radiant tubes (16).

9. Multi-deck oven (1) according to claim 1, characterized in that the length of the radiant tubes (16) is at least equal to half, preferably 75%, of the radius of the cylindrical furnace (1, 3).

10. Multi-deck oven (1) according to any one of the preceding claims, characterized in that It includes means of control, in particular individual and independent control for the resistors (14, 15).

11. Multi-deck oven (1) according to any one of the preceding claims, characterized in that the furnace includes means for reinjecting hot post-combustion gases, produced in a first part of the furnace, into a second part of the furnace having a lower temperature than that of the first part of the furnace.

12. Multi-deck oven (1) according to any one of the preceding claims, characterized in thatThe furnace includes means for injecting either oxidizing or reducing gases to maintain the oxidizing or reducing character of a stage respectively.

13. Use of a multi-hearth furnace (1) according to any one of the preceding claims, as an industrial reactor for carrying out a heat treatment or a combination or succession of heat treatments of solid, powdery or pasty materials, such as drying, pyrolysis, incineration, roasting, carbonization, activation, regeneration, calcination or toasting of such materials.

14. Method for controlling and operating a multi-deck oven according to any one of claims 1 to 12, characterized in that said electrical resistances (14, 15) are controlled individually and independently, and in thatTemperature monitoring at each stage of the furnace is carried out using distributed measurements, with the help, under certain operating conditions, of infrared imaging techniques and / or a digital twin, allowing for optimization of heat transfer, or even for flexibly approaching the operating conditions achieved in a furnace with one (or more) direct flame burner(s).

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