Fedorov S.S., Sibir A.V., Gubinsky S.M., Gubinsky M.V., Gogotsi A.G.
July 11, 2019

Conventionally, the working chamber can be divided into several elements that determine the efficiency of the furnace: the working space of the furnace, where the fluidized bed with a height Hrab and a diameter Drab is located, the superlayer separation space with a height Hns and a diameter Dns, the distance from the electrode with a diameter De to the gas distribution grate Nre.

Fig. 11 Schematic of the working space of a furnace with an electrothermal fluidized bed, 1 – central electrode, 2 – feeding raw materials into the fluidized bed, 3 – exhaust gas removal, 4 – furnace lining – electrode, 5 – thermal insulation, 6 – water-cooled body, 7 – gas distribution grate.

The determining dimensions are the dimensions of the electrothermal fluidized bed, in the volume of which heat is released during the passage of current: diameter of the working zone Drab, diameter of the electrode De, height of the working zone Hrab. The choice of these parameters is determined by the following characteristics of the furnace, which must be coordinated

• furnace performance;
• the electrical resistance of the fluidized bed, which determines the voltage and current at the electrodes;
• permissible current density per electrode cross-section;
• permissible current density on the electrode surface along the height of the working area;
• uniformity of heat generation and temperature field in the gap between the central electrode and the furnace lining;
• stability of the central electrode.

Based on the mathematical modeling of the processes of heating carbon material in an electrothermal fluidized bed [27, 29], the height of the furnace working zone is recommended as 2-5 diameters of the central electrode Hrab=(2-5)Dе. In [25], the height of the working zone is determined by the diameter of the working zone and is recommended as Hrab=(0.5-2)Drab.

To select the diameter of the working zone, the authors of [29] recommended a dependence that links the main design and technological parameters of the furnace:

D equ is the equivalent diameter of the working area [29], m,
N is the electric power of the furnace, kW,
D e is the electrical conductivity of the fluidized bed, Ohm*m,
U is the voltage between the center and the lining, V,
V is the volume of the fluidized bed of the working zone, m3.

In [30], the authors recommend a ratio of the diameters of the inner and outer electrodes of De/Dab = 0.55…0.66 in order to increase the reliability of the furnace and increase the stability of the electrode. This ratio corresponds to the minimum electric field intensity on the surface of the central electrode. This reduces the likelihood of spark discharges on the electrode surface characteristic of an electrothermal fluidized bed.
It is known that spark discharges, in which the temperature can reach up to 10,000˚C, cause increased anode erosion and the need for periodic replacement. The solution to this problem is possible by shielding the working surface of the anode with a bulk layer of fluidized bed material (Fig. 5, 12) [22, 31]. The latter option can be used only for “low-temperature” furnaces.

Fig. 12 Scheme of anode shielding by fluidized bed material
1 – gas distribution grate, 2 – electrothermal fluidized bed, 3 – superlayer space, 4 – furnace lining made of dielectric material, 5 – protective layer of material, 6 – electrode.

The choice of the dimensions of the superlayer Hns and Dns is determined by the ability to separate the particles ejected from the fluidized bed and reduce their mechanical removal from the furnace. The diameter of the superlayer space is usually larger than the diameter of the furnace working space (Figs. 6, 7, 11), which makes it possible to reduce the gas velocity and reduce the removal of material from the furnace [9, 10, 15, 21, 25-27]. In [25], it is recommended to choose the height of the superlative space one and a half times higher than the height of the fluidized bed (Hrab + Hre).
However, in “low-temperature” furnaces, the dimensions of the working zone and the supernatant space may coincide (Figs. 8-10) [9,18,19, 22]. The height of the over-layer space should minimize the removal of material and, according to recommendations [32], should be at least 1 m for particles with a diameter of 100-200 µm.
One of the important dimensions of the furnace working space is the distance from the gas distribution grate to the beginning of the furnace working area (Fig. 10). The value of this distance should provide the main heat generation in the radial gap between the electrodes and minimize current leakage to the grid, which will allow controlling the process by changing the immersion height of the central electrode.

The authors of [27] recommend a value of Nre of at least one diameter of the central electrode, which guarantees the release of at least 90% of the thermal energy in the gap between the lining and the central electrode.
The design of the gas distribution grate must ensure uniform distribution of inert gas across the furnace working space, intensive mixing of the material, which ensures a uniform temperature field and the absence of stagnant zones in which the accumulation of untreated material is possible. In addition, the grate usually has a channel for unloading the processed material from the working chamber.
In practice, two types of gratings are used: flat gratings with uniform [9, 15, 16, 18, 20, 21, 22, 23] (Figs. 4, 5, 7-10) and uneven [25] (Fig. 11) distribution of holes for gas passage, as well as conical gratings that determine the movement of material to the discharge opening [7, 10, 24, 26]. The grates can be of falling [18, 19, 23, 25, 27] and non-falling [7, 15, 16, 21, 22, 24, 26] types, including cap grates (Fig. 7, 8), which are typical for “low-temperature” furnaces.

The design of the gapless grid by the authors [24] is a stepped structure of graphite rings, between which the gas supply to the layer is organized, while the rings are electrically decoupled from each other. This makes it possible to control the voltage applied to them and thus control the distribution of heat sources in a vertical fluidized bed. At the same time, it is possible to organize the gas supply tangentially (Fig. 15), which ensures the rotational movement of the layer and improves the mixing of the material.

Fig. 13 Schematic of an electrothermal fluidized bed furnace for the production of silicon carbide [10].
1 – raw material supply, 2 – graphite lining of the furnace, 3 – central electrode,
4 – furnace body, 5 – electrothermal fluidized bed, 6 – discharge opening, 7,9 – cone rod for controlling the discharge of the finished material, 8 – gas distribution grate, 10 – inert gas supply.

Small.14 Furnace with electrothermal fluidized bed [24]
1 – exhaust gas removal spigot, 2 – lined furnace body, central electrode, 4 – gas supply to the central electrode, 5 – graphite rings forming a spill-free grid, 6 – channel for removal of treated material, 7 – raw material loading spigot, 8 – electrothermal fluidized bed, 9 – electric power supply, 10 – inert gas supply.

Small.15 Scheme of gas distribution grid with tangential inert gas supply [24]
1- lined furnace body, 2 – graphite ring, 3 – tangential channel for inert gas supply, 4 – electrothermal fluidized bed

Fig. 16 An electrothermal fluidized bed furnace for processing carbonaceous material [25].
1 – nozzle for loading raw materials, 2 – furnace body, 3 – thermal insulation,
4 – graphite lining – electrode, 5 – exhaust gas outlet, 6 – central electrode, 7 – conical gas distribution grate, 8 – inert gas supply, 9 – finished product discharge pipe.

Conical grids with uneven distribution of holes in height are used [7, 25] (Fig. 6, Fig. 16). Gas is supplied to the top of the cone grate through one row of nozzles. In [25], it is recommended to choose the central angle of the cone grate in the range of 40-60˚, and the height of the nozzles is 0.5-0.75 of the cone height. Such a solution ensures the operation of the fluidized bed with a given circulation of material in the working chamber similar to the operation of devices with a flowing bed. Raising the material near the central electrode and lowering the material near the furnace lining. This ensures complete processing of the entire material and guaranteed mixing.

Similarly to the design of [24], the authors of the furnace [25] provided for the possibility of tangential injection of gas jets into the furnace working space to ensure vortex movement of the layer (Fig. 17). In this case, the nozzles form an angle to the tangent equal to β=10˚-20˚, and the total area of the nozzles is 0.15-0.5 of the cross-sectional area of the furnace working zone with a diameter Drab.

Fig. 17 Scheme of tangential gas supply through a conical grate [25].
1 – thermal insulation, 2 – gas distribution chamber, 3 – conical gas distribution grate with nozzles for inert gas supply

A fundamentally different solution for supplying inert gas to the fluidized bed, typical for tuyere blowing of melts in metallurgical units, was proposed in [33]. The distribution grate is combined with a central electrode through which inert gas is supplied and distributed in the bed through a series of nozzles distributed along the bottom of the electrode. This option simplifies the design of the lower part of the furnace and makes it possible to organize the movement of material in the working area similarly [25], as well as to eliminate the operation of replacing the grid in case of its failure, which requires complete cooling of the furnace.

Fig. 18 Electrothermal fluidized bed furnace for high-temperature treatment of carbon materials [33].
1 – furnace body, 2 – thermal insulation, 3 – graphite lining, 4 – central electrode, 5 – outlet pipe for the processed material, 6 – pipe for loading raw materials, 7 – flue for removing exhaust gases, 8 – vertical channel for inert gas supply, 9 – nozzles for inert gas supply to the bed.

In addition, the inert gas cools the electrode and is heated before being fed into the bed. A similar solution was used in [30] (Fig. 19), where it was proposed to heat the reagent in the channels of the central electrode and at the same time increase its stability. The difference from [33] is that the heated gas is supplied to the working space through an annular gas distribution grate.

Fig. 19 Schematic of a synthesis reactor with an electrothermal fluidized bed [30].
1 – body, 2 – lid, 3 – bottom, 4 – supply pipe for the volatile component, 5 – gas distribution grid, 6 – central electrode, 7 – under-grid space, 8 – supply pipe for the heavy component, 9 – outer electrode, 10 – reaction space

Preheating the inert gas before it is fed into the electrothermal fluidized bed allows solving several problems simultaneously

reduce inert gas consumption;
reduce heat losses associated with its heating;
to maintain a uniform gas velocity along the height of the furnace, which ensures the maintenance of a given hydraulic mode of operation of the fluidized bed.
This is especially important for high-temperature furnaces. Thus, the authors of [25] solve this issue by organizing the supply of inert gas through an annular channel, in which the gas is heated before being fed into the fluidized bed (Fig. 20).

Fig. 20 High-temperature furnace for processing carbonaceous material in an electrothermal fluidized bed [25].
1 – central graphite electrode, 2 – graphite lining of the working chamber, 3 – thermal insulation, 4 – water-cooled furnace body,
5 – gas distribution grate; 6 – channel for unloading the finished product from the working chamber; 7 – distribution chamber of the finished material; 9 – refrigerator of the finished material of the first stage; 10 – inert gas supply; 11 – annular channel for inert gas supply.

Many of the considered furnace designs are continuous furnaces, in which the flow process of material processing is implemented [7,9,10,16,17, 21-28,33]. It requires constant loading of raw materials and unloading of the finished product.
The raw materials are loaded into the furnace through a separate channel in the furnace top cover. At the same time, the raw material enters the working space of the furnace and under the influence of gravity falls into the layer [7, 9, 10, 21, 22, 24, 25] (Figs. 6-8, 11, 13-14, 16).
In the upper part of the furnace, exhaust gases are organized, so the particles and gas move in a countercurrent, which can cause the removal of untreated material particles from the furnace along with the gases. To eliminate this disadvantage, the authors of [25, 27, 33] load the raw material through a pipe directly into the fluidized bed or onto its surface (Figs. 16, 18). In [20] (Fig. 5), the feedstock is fed directly into the fluidized bed through electrodes, thus simplifying the design of the furnace.

In most designs of continuous furnaces, unloading is carried out through a channel in the gas distribution grate, in which it moves in a dense layer due to gravity. After the processed material is unloaded from the furnace chamber, it goes to the refrigerator, while a dense layer of material is formed, which is a hydraulic resistance (gate) that prevents the movement of gases from the working space through the refrigerator (Fig. 20).
In [25], a distribution chamber is provided in which the material is averaged, distributed between refrigerators, and held at high temperatures, which ensures the processing of particles that have transited through the fluidized bed. The movement of material in the refrigerator is determined either by feeders (Fig. 11) or by gates of various types (Fig. 21) [9].

Fig. 21 Electrothermal furnace for the production of carbides [9].
1 – central electrode, 2 – exhaust gas exhaust pipe, 3 – furnace lining, 4 – gas distribution grate, 5,6 – raw material supply, 7 – inert gas supply, 8 – finished product discharge gate.

Electrothermal fluidized bed – the basis for the development of high-temperature thermal processes (Part 1)

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