Jul 12, 2019

The working chamber can be divided conventionally into several elements witch determine the efficiency of the furnace operation:

    •  the furnace working space, where the fluidized bed is located with the height Нраб and the diameter Dраб;
    •  the over-layer separation space with height Ннс and diameter Dнс;
    •  the distance from electrode with diameter Dе to gas distribution grate Нре.

Scheme of the electrothermal fluidized bed furnace working space
Fig.11 Scheme of the electrothermal fluidized bed furnace working space,

1- the central electrode, 2 - the supply of raw materials to the fluidized bed, 3 - the exhaust gases removal , 4 - the furnace lining is the electrode, 5 - thermal insulation, 6 - the water cooled body, 7 - gas distribution grate.

The key factor is the dimensions of the electrothermal fluidized bed (working zone diameter Dраб, electrode diameter Dе, working zone height Нраб), in which when current passes, heat generation are occurs.The choice of these parameters is determined by the following furnace characteristics:

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

The height of the furnace working zone is recommended to choose as 2-5 diameters of the central electrode Нраб=(2-5)Dе. This expression is based on mathematical modeling of the heating process of carbon material in an electrothermal fluidized bed [27, 29]. In [25], the height of the working area is related to the working area diameter and it is recommended as Нраб=(0,5-2)Dраб.

The authors of [29] recommended the dependence linking the main structural and technological parameters of the furnace to choose the working area diameter.


       furnace working area diameter (1)

Dэкв - equivalent working area diameter [29], m,
N - furnace electric power, kW,
Dе - electric conductivity of the fluidized bed, Ohm * m,
U - voltage between the central electrode and the lining, V,
V - volume of the working area fluidized bed, m3

In [30], the authors recommend the ratio of the diameters of the inner and outer electrodes is Dе /Dраб =0,55…0,66 for order to increase the reliability of the furnace and increase the resistance of the electrode. This ratio corresponds to the minimum electric field strength on the central electrode surface. This reduces the likelihood of the formation of spark discharges on the electrode surface, typical of an electrothermal fluidized bed.

As is known, it is spark discharges that lead to increased erosion of the anodes and the need for their periodic replacement, the temperature in them can reach 10,000 ° C. The solution to this problem is possible by shielding the working surface of the anode with a bulk layer of fluidized bed material (Figure 4, 12) [22, 31]. This option can only be used for "low-temperature" furnaces.


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

The choice of the over-bed space sizes Ннс and Dнс is determined by the possibilities of separating the particles ejected from the fluidized bed and the reduction of their mechanical entrainment from the furnace. The diameter of the over-bed space is usually larger than the diameter of the working space of the furnaces (Fig.6, 6.10), which allows to reduce the gas velocity and reduce the material entrainment from the furnace [9,10,15, 21, 25-27]. In [25], it was recommended to choose the over-bed space height in one and a half times higher than the fluidized bed height (Нраб+Нре). However, the dimeters of the working area and the over-bed space can coincide in the “low-temperature” furnaces (Fig.8-10) [9,18,19, 22]. The over-bed space height should minimize material entrainment 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 working zone of the furnace (Fig. 11). The value of this distance should provide the main heat dissipation in the radial gap between the electrodes and minimize the current leakage to the gas distribution grate, which will allow to control the process when changing the immersion height of the central electrode. The authors of [27] recommend to choose the value of Нре more than one central electrode diameter, which guarantees the release of at least 90% of thermal energy in the gap between the lining and the central electrode.

The design of the gas distribution grate should ensure uniform distribution of inert gas throughout the furnace working space, intensive mixing of the material, which ensures a uniform temperature field and the absence of stagnant zones in which there can be raw material. In addition, the grate, as a rule, has a channel for unloading the treated material from the working chamber. Two types of flat grates are used in practice with a uniform [9, 15, 16, 18, 20, 21, 22, 23] (Fig.4,5,7-10) and non-uniform [25] (Fig.11) holes distribution for the gas passage, as well as the conical grate material which directs the material to the discharge opening [7,10,24, 26]. Еhe grates can be discharge [18, 19, 23, 25, 27] and non- discharge [7, 15, 16,21,22, 24, 26] types, including caps type (Fig. 7,8), typical of «low - temperature» furnaces.

A conical discharge grate (Fig. 13) [10] with a gas supply holes uniform distribution and a central discharge opening has a taper angle of 5˚-10, which ensures the material movement to the central opening for unloading.

Non-discharge grate design of the [24] authors is a stepped construction of graphite rings, between which the gas supply to the layer is organized, while the rings are electrically isolated from each other. This allows you to control the voltage applied to them and thus control the distribution of heat sources in the vertical fluidized bed. In this case, it is possible to organize the tangential gas supply (Fig. 15), which provides rotational movement of the layer and provides improved material mixing.


Scheme of a furnace with an electrothermal fluidized bed for the production of silicon carbide 
Fig.13 Scheme of a furnace with an electrothermal fluidized bed for the production of silicon carbide [10]
1 - raw materials supply, 2 - furnace graphite lining, 3 - central electrode, 4 - furnace body, 5 - electrothermal fluidized bed, 6 - unloading opening, 7,9 - conical rod to regulate the treaed material unloading, 8 - gas distribution grate, 10 - inert gas supply.




Furnace with electrothermal fluidized bed
Fig.14 Furnace with electrothermal fluidized bed [24]
1 - exhaust gas outlet, 2 - lined furnace body, central electrode, 4 - gas supply to the central electrode, 5 - graphite rings forming a discharge grate, 6 - channel for removing the treated material, 7 - raw material loading pipe, 8 - electrothermal fluidized bed, 9 - electricity supply, 10 - inert gas supply.


Scheme of gas distribution grate with a tangential inert gas supply

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


Electrothermal fluidized bed furnace for treating carbon material

Fig. 16 Electrothermal fluidized bed furnace for treating carbon material [25]
1 - pipe for loading raw materials, 2 - furnace body, 3 - thermal insulation, 4 - graphite lining - electrode, 5 - nozzle for exhaust gas removal, 6 - central electrode, 7 - cone gas distribution grate, 8 - inert gas supply, 9 - nozzle for unloading treated products.

The use of conical gratings with uneven holes distribution in height [7, 25] (Fig. 6, Fig. 16). The gas supply is carried out in the upper part of the cone grate through one row of nozzles. In [25], the cone grate central angle is recommended to choose 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 fluidized bed operation with a given material circulation in the working chamber, similar to the operation of devices with a spouting bed - raising the material at the central electrode and lowering the material at the furnace lining. This ensures complete treatment of the entire material and guaranteed mixing.

The authors of the furnace [25], wich has similar to the design of [24], provided for the possibility of tangential injection of gas jets into the working space of the furnace, to ensure the vortex motion 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 cross-sectional area of ​​the furnace working area with a diameter Dраб.

Scheme of tangential gas supply through a conical grate

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 a fluidized bed is typical for tuyere stock blowing of melts in metallurgical units and was proposed in [33]. The distribution grate is combined with the central electrode, through which inert gas is supplied and distributed in the layer through a series of nozzles, which are located along the generatrix in the lower part of the electrode.

This option simplifies the design of the furnace lower part and allows you to organize the material movement in the working area similarly to [25], and also to eliminate the operation of replacing the grating when it fails, which requires complete cooling of the furnace. In addition, the inert gas cools the electrode and it is heated before it entering to 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 simultaneously increase its durability. The heated gas is fed into the working space through an annular gas distribution grid in the design [33].


Electrothermal fluidized bed furnace for high-temperature processing of carbon materialsFig. 18. Electrothermal fluidized bed furnace for high-temperature processing of carbon materials [33]
1 - furnace body, 2 - thermal insulation, 3 - graphite lining, 4 - central electrode, 5 - exhaust pipe for the treated material, 6 - pipe for loading raw materials, 7 - flue gas duct, 8 - vertical channel for feeding inert gas, 9 - nozzle for supplying inert gas to the bed.


Scheme of a synthesis reactor with an electrothermal fluidized bed 
Fig.19 Scheme of a synthesis reactor with an electrothermal fluidized bed [30]
1 - body, 2 - cover, 3 - bottom, 4 - nozzle for supplying a volatile component, 5 - gas distribution grate, 6 - central electrode, 7 - sub grating space, 8 - nozzle for feeding a volatile component, 9 - outer electrode, 10 - reaction space

Preheating of the inert gas before being fed into the electrothermal fluidized bed allows several problems to be solved at the same time:

    • to reduce the inert gas consumption;
    • to reduce heat losses associated with its heating;
    • to maintain a uniform gas velocity throughout the furnace, which maintains the desired hydraulic mode of the fluidized bed.

This is especially important for high-temperature furnaces. The solution of this issue by the authors [25] is to organize the supply of inert gas through the annular channel in which the gas is heated before being fed into the fluidized bed (fig.20).


High-temperature furnace for treating carbon material in an electrothermal fluidized bed 
Fig. 20 High-temperature furnace for treating carbon 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 - a channel for unloading the treated product from the working chamber, 7 - a distribution chamber of the treated material, 9 - first stage refrigerator for the treated material, 10 - inert gas supply, 11 - an annular channel for inert gas supply.

A lot of the considered furnace structures are continuous modes with implemented material treatment continuous process [7, 9, 10, 16, 17, 21–28, 33]. Continuous process requires constant loading of raw materials and unloading the treated product.

The raw material is loaded into the furnace through a separate opening in the furnace upper lid. In this case, the raw material enters the furnace working space and under the action of gravity is lowered into the bed [7, 9, 10, 21,22,24,25] (Fig. 6-8,11,13-14,16). Tap of exhaust gases are organized in the furnace upper part. Thus particles and gas move in countercurrent, which may cause carry-over of untreated particles of material along with gases from the furnace. The authors [25, 27, 33] load the raw material through a pipe directly into the fluidized bed to eliminate this drawback (Fig.16, 18). In [20] (Fig. 5), the supply of raw material directly into the fluidized bed is carried out through electrodes, thus simplifying the furnace design.

In most designs of continuous furnaces, unloading is carried out through a channel in the gas distribution grate, where material moves in a dense layer due to gravity. After unloading the treated material from the furnace working chamber, it is sent to the refrigerator. A dense layer of material is formed, which is the hydraulic resistance (gate) that impedes the movement of gases from the working space to the refrigerator (Fig. 20). In [25], a distribution chamber is provided in which the material is averaged, distributed between refrigerators, and exposed at high temperatures, providing additional particles treatment. The movement of the material in the refrigerator is determined either by feeders (Fig. 10) or by various types of (Fig. 20) [9]. The material moves in the refrigerator with the help of conveyors (Fig.11) or various types of valves (Fig.21) [9].


Electrothermic furnace for the production of carbides

Fig.21 Electrothermic furnace for the production of carbides [9].
1 - central electrode, 2 - pipes for exhaust gases removal,
3 - furnace lining, 4 - gas distribution grate, 5,6 - raw material supply, 7 - inert gas supply, 8 - treated product unloading valve.

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Electrothermal fluidized bed is the basis for the high-temperature heat engineering processes development (Part I)

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July 11, 2019


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