EP0098771A1 - Process for the production of form coke in a shaft furnace, and shaft furnace for carrying out the process - Google Patents

Abstract

1. Process for producing moulded coke in a shaft oven with a vertical axis (1), which is supplied in its upper part (3) with a burden of coal for coking in the form of ovoids, forming a movable bed descending in the shaft (1) in counter flow to gas circulating from the bottom upwards and passing successively from the top downwards through a preheating zone, a coking zone and a cooling zone, the rising stream of gas being formed by the gases released by the coal during its heating and coking and by some of the gases recovered in the upper part of the oven and recycled in the lower part after washing and dust extraction, heat being supplied by circulation of electrical current through the bed of solid materials between electrodes (8) located on two opposite sides of the wall (2) of the oven (1), characterized in that the electrical heating is carried out in a controlled way over a specific height in the middle part (C) of the oven (1), electric currents, the intensities of which can be regulated, being passed through the burden in several staggered horizontal planes, and in that, by adjustment of the circulation of the gases and of the electrical power distributed to the various levels of electrodes according to the heat requirements and the electrical resistance of the burden, a progressive and uniform heating of the latter is controlled during its descent in the oven according to a law of heating selected as a function of the type of coal, to preserve the shape of the ovoids, in such a way that the temperature of the bed of solid material is between 600 degrees C and 850 degrees C at the entrance of the middle zone (C) of electrical heating and that coking is carried out completely in this zone (C) at a temperature remaining between 950 degrees C and 1150 degrees C, the coke ovoids subsequently being cooled in the lower part of the oven (1).

Description

The invention relates to a method for manufacturing coke molded in a shaft furnace and the corresponding shaft furnace.

A process is known for manufacturing coke molded in a shaft furnace where coal balls are introduced into the shaft furnace, at its upper part, to form a bed circulating from top to bottom through the furnace over its entire height.

During their circulation inside the oven, these coal balls are in contact with hot gases passing through the oven from bottom to top.

The upper part of the oven constitutes a balanced exchanger in which the bed of solids is dried and heated to a certain temperature and the circulating gases cooled before their exit to the upper part of the oven.

The middle part of the oven constitutes the coking zone in which heat is supplied to the bed of circulating solid materials, for example by means of burners.

This addition of heat can be carried out by combustion of part of the gases circulating in the shaft furnace, by means of oxidizing air introduced at the level of the central zone.

In this case, at least part of the gases escaping from the top of the shaft furnace are recycled after taring and dusting, by re-injecting these gases at the lower end of the outlet for solid products, at the base of the shaft furnace .

The lower part of the shaft furnace, below the middle zone, constitutes a second balanced heat exchanger where the solid materials coked in the middle zone are cooled by the gases injected into the lower part of the shaft furnace. The gases are therefore at high temperature when they reach the central heating and coking zone.

Coal balls liberate in particular at the time of their coking combustible gases coming from volatile materials of coal.

These gases have great industrial value, since they can be recovered, treated and reused either in the shaft furnace itself, or for other uses.

However, the injection of combustion air to cause the combus tion of part of the gases circulating in the shaft furnace is the cause of the presence in the gases recovered from the furnace top of the furnace of a significant proportion of nitrogen which reduces the calorific value of the gas. It is also necessary to treat larger quantities of gas, which entails a higher cost of this recovered gas. Thus, for each tonne of coke produced, we are in the presence of an excess of gas due to the volatile matter of the coal and the injected air, of approximately 680 m ³ to 800 m ³ depending on the nature treated coals.

On the other hand, control of the process, as far as thermal adjustment is concerned, is relatively difficult to obtain.

It is indeed necessary to adjust the temperature in the coking zone in a relatively precise manner and to prevent the air introduced for the combustion of the gas and the heat supply in the central zone of the furnace from oxidizing part of the carbon of the balls, this which would be at the expense of the efficiency and effectiveness of the coking operation.

It has therefore been proposed for the simultaneous production of coke and combustible gas to carry out the supply of heat in the shaft furnace by means of electrodes passing through the walls of the furnace and coming into contact with the bed of circulating solid materials. A strong electric current can thus cross the bed of solid materials and produce a heat release by Joule effect.

Such a process is described for example in French patent 917,058 where an electric heating zone is provided in the oven, below the coking zone. The coke produced is thus brought to very high temperature by electrical heating and the gases coming from the base of the furnace which pass through this zone heat up strongly in contact with the coke at very high temperature and are capable of causing the coking of coal in the zone. oven located above the electric heating zone.

This process has the drawback of requiring the coke to overheat up to a temperature in the region of 1,400 °.

By way of comparison, the temperature of the coke in conventional coke ovens does not exceed 1,200 °.

It has also been proposed to use electrically heated ovens, with gas circulation against the current of solid materials, for the calcination of coal or other carbonaceous materials and in particular for the manufacture of electrodes or other parts. graphite.

However, such processes require temperatures much higher than the temperatures necessary for coking.

Also known are processes for treating coke, for example for its desulphurization, which use both electric heating and the passage of gas through the mass of coke brought to high temperature.

However, such processes are carried out very differently from a coking process and are only intended for the treatment of the coke itself, however, there is no point in bringing the coke to temperatures which can exceed 1.400 ° because the metallurgical quality of coke is affected and its reactivity, among other things, decreases.

The object of the invention is to propose a process for the manufacture of molded coke in which the heating of the charge is carried out by observing a heating law determined according to the nature of the coal so as to avoid the formation of clusters and to keep the shape of the balls until the end of the treatment, in particular by avoiding their bursting and their overheating.

For this purpose, a known kiln is used, at the top of which coking charcoal balls are introduced which constitute a moving bed descending into the furnace against the current of gas passing through the furnace from bottom to top. , part of which consists of gases recovered at the top of the oven and recycled at its bottom and another part of which consists of the gases released by the coal during its heating and coking, a supply of heat being effected by passing electric currents through the bed of solict materials.

In accordance with the invention, electrical heating is carried out in a controlled manner, in the central zone of the furnace over a determined height, by passing electrical currents, the intensities of which can be individually adjusted, in several horizontal planes of the load, by adjusting the heat exchanges between the solid charge and the gases and the heat supply in the electric heating zone, the heating of the charge is controlled according to a heating law chosen according to the nature of the coal, such so that the temperature of the bed of solid material is between 600 ° and 850 ° at the entrance to the electric heating zone and reaches in this zone a value remaining between 950 ° and 1,150 °, the coking reaction being carried out completely in the middle zone of electric heating and the coke formed is then cooled in the lower part of the oven.

According to another important characteristic, in the upper preheating zone of the furnace, an adjustable flow rate of the rising current of hot gases is derived, so as to control a gradual rise in temperature of the materials at a speed chosen according to the nature of the coal for avoid melting or sticking of the balls.

Preferably, the electrical resistance of the charge is increased after leaving the central coking zone by decompressing the charge in the cooling zone, capable of increasing the contact resistances between the balls.

To encourage the passage of electric current in the middle coking zone, make the electric heating homogeneous and facilitate the electric adjustment of the power consumed, the charcoal balls introduced into the upper part of the oven are mixed with particles of small coke or any other electrically conductive product, unalterable at temperatures reached in the oven and being in the form of grains of dimensions smaller than those of the coal balls, said grains being distributed homogeneously in the interstices between the balls.

The cell furnace for implementing the method according to the invention comprises at least two pairs of superposed electrodes, placed in horizontal planes spaced from one another and whose electrical supply voltages are individually adjustable by pair.

In a preferred embodiment, the electrodes placed in at least one of the horizontal planes are mounted movable horizontally and each connected to alternate advancing and reversing means, each electrode exerting constant pressure on the charge by progressive advancement towards the interior of the oven to a determined position from which a rapid reverse is controlled accompanied by a descent of the load and followed by a new advancement.

In order to clearly understand the invention, we will now describe, by way of nonlimiting example, with reference to the attached figures, an example of implementation of the method according to the invention in the case of an oven with tank fitted with a double set of electrodes.

• Figure 1 shows schematically a tank furnace according to the invention in a sectional view through a vertical plane.
• FIG. 2 represents a section of the furnace along II II of FIG. 1.
• Figure 3 shows the lower part of the electrodes of the pre mier ensemble shown in Figure 1, in an enlarged view.
• FIG. 4 represents a more improved embodiment of a cell oven according to the invention.
• FIG. 5 is a diagram representing the heating law of the load.

In Figure 1, we see the tank furnace 1 having a double insulating wall 2 and the upper part of which constitutes a chute 3 for charging charcoal balls.

This blower is of a type similar to that of blast furnace blowers and allows, thanks to a device 4, the recovery of gases at the top of the tank furnace. The gueulard is completely waterproof and avoids the entry of atmospheric air.

At the bottom of the shaft furnace, hoppers and rotary evacuators 5 allow a regular outlet of the coke reaching the bottom of the furnace.

A movable bed 6 of solid materials moves continuously in the oven. The upper surface 60 of this bed of solids is maintained at a substantially constant level by the introduction of coal balls thanks to the mouthpiece 3 at a flow rate identical to the coke outlet flow rate.

In the example shown in Figure 1, a first set of electrodes 7 is introduced into the furnace through its upper part. These electrodes can be adjusted in height, so that the level of the plane AA in which electric currents of horizontal direction circulate between the electrodes can be adjusted by vertical displacement of all of the upper electrodes.

As can be seen in FIGS. 2 and 3, these electrodes comprise a conductive part 10 constituted by a graphite plate crimped in a piece of refractory steel 11. The pieces 11 of refractory steel are connected to support tubes 12 also in refractory steel constituting conductors serving to bring the current to the end of the electrodes 7. Cooling air possibly circulates inside these tubes and comes to cool, at their lower part, the part 11 for fixing the conductive part 10.

Blocks of refractory material 14 make it possible to isolate the suspension and supply tubes from the stream 12 from the bed of solid materials and high temperature gases circulating in the furnace.

The electrodes 7 plunge into the bed of solids at a depth of approximately 1.50 meters below the surface of level 6. This insertion depth of the electrodes can be adjustable.

In Figure 2, we see the rectangular section of the oven in which are arranged six electrodes 7 allowing a flow of current between the electrodes in horizontal directions. The electric current passes from one electrode to another following a horizontal path by crossing the bed of solids having at the level of the electric heating zone a certain conductivity.

A second set of electrodes 8 disposed at a level B lower than the level of the plane A, pass horizontally through the double wall 1, 2 of the furnace. These electrodes consist of blocks of graphite housed in the refractory located between the two walls 1 and 2 of the furnace and slightly projecting in the interior space of the furnace.

Preferably, these electrodes are movable horizontally so as to exert a constant thrust on the load.

To this end, as shown diagrammatically in FIG. 1, each electrode 8 can be mounted on a carriage 81 which can be moved horizontally, for example by means of a jack 82 at constant pressure. The forward movement towards the inside of the furnace is limited in its course so that the electrodes 8 extend a few centimeters beyond the internal face of the wall 2 of the furnace. The jack 82 then commands a rapid retreat of the electrode which is accompanied by a descent of the materials lying above and the advance movement can resume. An easy-to-design system compensates for the wear of electrode 8.

Outside the oven, a gas recovery and treatment circuit 15 is connected on the one hand to the gas recuperator 4 at the upper part of the oven and on the other hand to the lower part of the oven by an injection pipe. 18.

On this gas circuit are arranged a gas washing unit 16 where the gas is dusted and tared as well as a circulation and injection pump 17. In addition, the circuit can advantageously include equipment 19 limiting the vapor content water from the recycled gas and constituted for example by an exchanger lowering the dew point of the gas by cooling.

A valve 20 makes it possible to regulate the injection of gas at the base of the furnace and to direct a part of this gas towards a storage tank or a circuit of use.

In a more sophisticated embodiment shown in the FIG. 4, the oven comprises several levels of electrodes 8 placed in horizontal planes staggered along the middle zone C and the power supply of which, not shown in the figure, can be adjusted individually in each plane, depending on the electrical power desirable for the realization of the heating law.

The required area of electrodes is determined by the intensity of the electric current to be circulated in the mass so as to avoid overheating.

The total surface area of the electrodes is therefore significant. But it is necessary that a ball does not stay too long under a strong intensity. There will therefore be a number of electrodes constituting horizontal planes and of small thickness. The current voltage can be adjusted by group of electrodes according to the heating law.

Furthermore, the furnace 1 has a rectangular section allowing the modular production of an installation made up of adjoining cells.

In the upper preheating zone P the furnace is provided with means making it possible to divert part of the ascending current of hot gases and which can for example consist of a double side wall 21 providing a space for the circulation of gases which is effected by the difference in pressure drop between the two internal and peripheral circuits respectively thus formed.

A shutter system makes it possible to adjust the pressure drop and therefore the gas flow in the main circuit.

In the lower cooling part R, the oven has a larger width and is further crossed by horizontal bars 22 lined with refractory and possibly internally cooled and which extend from one wall to the other. The extraction of the products at the base 5 of the oven therefore ensures a decompression of the charge at the outlet of the coking zone C. This results in an increase in contact resistances which reduces the passage of electric current from the start of the zone cooling. This limits downward zone C of electric heating.

We will now describe the operation of the furnace for the production of molded coke and high calorific gases.

FIG. 5 gives an example of a heating law and of the respective temperatures of the materials (indicated on the abscissa) according to the height of the oven (indicated on the ordinate). The solid line curve gives the temperature of the solids and the dotted line the temperature of the gas.

Is introduced into the oven, via the gueulard 3 a charcoal charge consisting of briquettes or balls of usual dimensions (for example: 40 x 25 x 20 mm) mixed with small coke with a particle size ranging from 5 to 15 mm . This small coke is previously distributed homogeneously in the balls, in a suitable proportion, for example: 10% by weight or 19% by volume. (The small coke could be replaced by any equivalent product, of the same dimension, that is to say conductive of electricity and unalterable at the temperatures practiced in the gaseous medium considered).

This small coke is partially lodged between the coal balls, occupying the interstices of the charge.

The charcoal balls consist of a mixture of lean or flaming dry coal associated with fatty or flaming coals mixed with a binder consisting of pitch (possibly mixed with tar).

The homogenized mixture descends into the oven against the flow of gases and reaches the lower part of the upper exchanger of the oven at a temperature close to 850 °. The coal balls are therefore dried and then heated so that their temperature is close to 800 ° at the outlet of the upper exchanger from the shaft furnace.

The gas flow rate and the heating of the furnace are regulated according to the flow rate of solid materials to obtain the adequate heat exchanges.

As can be seen in FIG. 5, by deriving an adjustable portion of the hot gases between the levels E and F of the upper preheating zone, it is possible to control the heating law of the load, in particular between 400 and 700 ° so as to avoid the fusion of the balls.

Under operating conditions, the temperature of the balls at the outlet of the upper exchanger P must, in practice, be greater than 700 ° C. so that the flow of current occurs in a suitable manner and not exceed 850 ° C. for that the thermal efficiency of the operation is good.

When the bed of solids comprising the agglomerated coal balls and the coke reaches plane A of the first set of electrodes, a current flows through this bed of materials and produces a rise in temperature inside the balls by the Joule effect.

The particles of small coke inserted between the coal balls favor the passage of the current in the bed of solid matter circulating in the furnace by multiplying the points of contact.

Raw balls as introduced into the shaft furnace are not very conductors of electricity. However, from a certain degree of devolatilization, the internal resistivity of these balls decreases rapidly. For example at 800 °, the measurements have shown that these balls have an internal resistivity which does not exceed 1,500 ùJ / cm. Thus, by controlling the temperature rise of the balls in the preheating zone P, one limits upwards the zone C of electric heating.

It is also necessary to avoid excessive local temperature increases which would cause excessive cracking of the gaseous hydrocarbons circulating in the furnace and a carbon-black deposit between the electrodes, which would create short circuits, this carbon-black being conductive.

However, the method according to the invention makes it possible to reduce the risk of overheating since it is easier to regulate the temperature of the electric heating by distributing it in several successive horizontal sections of the oven.

The small coke mixed with the balls of raw coal before being introduced into the oven is recovered at the base of the oven by screening the coke balls produced. This small coke has not undergone any transformation throughout its passage through the furnace. Its role is limited to reducing the contact resistances between the carbon balls and obtaining a more homogeneous heating of the load.

On entering the central electric heating zone of the furnace lying essentially between the two planes A and B of the electrodes, the bed of solids has a very uniform temperature, on the one hand because of the movement of the charge of solids crossing the electrode area and subjected to Joule heating of the currents which cross it, resulting from the multiple contacts established by the grains of small coke distributed in the balls and which constantly change position, and on the other hand because of the circulation gases in this area.

In the case of a shaft furnace having a square section whose side measures 3.50 meters, and whose coke production is approximately 350 tonnes per day, electrodes arranged as shown in FIG. 4 are used. ie about 3.5 m apart.

These electrodes are supplied with a voltage which is preferably continuous and regulated for a power kept constant and give off a total power of 1,500 kw for example. This power is distributed between the various levels of electrodes so as to obtain the desired heating law.

This thermal power of electrical origin enables the temperature of the solid materials to drop from approximately 800 to 1,050 ° in zone C of electric heating.

The complete coking of the coal takes place in this zone, the: oke at 1000 ° penetrating into the lower exchanger by the upper part thereof. The gases circulating in the furnace heat up in contact with: oke and coal during coking in the electric heating zone and their flow increases by volatilization of a part of the products contained in the balls of coal.

The gases injected at the base of the furnace through line 18 allow it to cool the coke produced from the exit temperature from the middle zone, that is to say a temperature close to 1000 ° to a temperature close to 150 °.

For the oven mentioned above, about 1,000 m ³ of gas per tonne of solid matter flowing in the Eour is injected at the base of the oven, which ensures balanced exchanges at the bottom and at the top of the oven.

The gas produced in the furnace is recovered at the top, dusted and tared before being introduced into the furnace via line 18.

An additional quantity of combustible gas is produced, depending on the nature of the charcoals used, for example 500 m ³ per tonne, ie coke produced. The calorific value of this gas is approximately 4,500 calories per _m 3 _.

Electric power consumption is around 150 kWh per tonne of coke produced.

Coking therefore occurs at a relatively moderate temperature and generally a little lower than the coking temperature in conventional coke ovens.

The setting of the electric heating according to the flows of solids and gases may vary somewhat but to obtain a good thermal efficiency and optimal coking conditions, the maximum temperature of the bed of solids in the electric heating zone should not exceed a value between 950 and 1150 ° C.

By distributing the electric power on several levels, we obtain a progressive heating and adapted to all types of coal, which would not be the case if we used a single set of electrodes allowing a passage of current in a single plane horizontal.

The balance concerning the energy consumption in the process according to the invention is entirely favorable if it is compared to what it is for conventional coke ovens.

Indeed, such coke ovens consume approximately 800 therms per tonne of coke produced while the consumption of electrical energy in the shaft furnace according to the invention would correspond approximately to a consumption of 150 kWh per tonne of coke produced.

It can therefore be seen that the main advantages of the process according to the invention are to allow a reduction in the energy consumption for the production of coke and the recovery of a gas with high calorific value which can be produced continuously.

In addition, the thermal adjustment of the process can be carried out in a simple manner, so that the exchanges between the gases and the solid materials are balanced and that the calorific contribution by electrical energy is used practically only to compensate for the heat losses of the furnace and the heat of endothermic reactions that can occur in the oven. The flexibility of this type of oven makes it possible to modulate the consumption of electric current with load shedding during peak hours.

Finally, the complete tightness of the oven makes it possible to avoid any gas leaks, which avoids any pollution of the environment.

It is obvious that the invention is not limited to the process and to the device described in a nonlimiting manner, but that they include all variants thereof.

Thus, depending on the operating conditions and the quality of the coals used, the thermal adjustment conditions may vary within the intervals mentioned.

The shape of the straight section of the oven is not necessarily square or rectangular but can also be circular.

The shape of the electrodes, their arrangement and their spacing can be variable depending on the shape of the oven and the desired heating conditions.

The gases recovered at the top of the furnace undergo treatments which depend on their end use.

Claims (12)

1. Method for manufacturing coke molded in a vertical axis tank furnace (1) fed at its upper part (3) with a charge of coking coal in the form of balls, forming a moving bed descending into the tank (1) , against the current of gas flowing from bottom to top, passing successively from top to bottom, through a preheating zone, a coking zone and a cooling zone, the ascending gas stream being constituted by the gases released by the coal during its heating and its coking and by a part of the gases recovered at the upper part of the furnace and recycled at the lower part after washing and dedusting, an addition of heat being effected by passage of electric current in the bed of solids between at least one pair of electrodes (8) placed on two opposite sides of the wall (2) of the oven (1), characterized in that the electric heating is carried out in a controlled manner over a determined height in the middle part e of the furnace (1) by passing electric charge currents, the intensities of which can be individually adjusted, through several staggered horizontal planes and, by adjusting the heat exchanges between the solid charge and the gases and the heat supply in the electric heating zone (C), the heating of the load is controlled according to a heating law chosen according to the nature of the coal, so that the temperature of the bed of solid material is between 600 ° and 850 ° at the entry of the electric heating zone (C) and reaches in this zone (C) a value remaining between 950 ° and 1.150 ° C, the coking reaction being completed at the exit of the heating zone (C) electric and the coke formed then being cooled in the lower part (R) of the oven. 2. A method of manufacturing molded coke according to claim 1, characterized in that, in the preheating zone (P), an adjustable flow rate of the ascending stream of hot gases is derived so as to control a gradual rise in temperature of the materials to be a speed chosen according to the nature of the coal to avoid melting or sticking of the balls. 3. A method of manufacturing molded coke according to one of claims 1 and 2, characterized in that the electrical resistance of the charge is increased after leaving the coking zone (C) by decompressing the load in the cooling zone (R) likely to increase the contact resistances between the balls. 4. A method of manufacturing molded coke according to claim 1, characterized in that the coal balls introduced into the upper part (3) of the furnace (1) are mixed with particles of small coke or of any other product in the form of electrically conductive grains, unalterable at temperatures reached in the oven and of dimensions smaller than those of the balls, said grains being distributed homogeneously in the interstices between the coal balls and promoting the passage of electric current in the middle zone ( 2) coking, making heating uniform and facilitating the electrical adjustment of the power consumed. ¹ 5. A method of manufacturing molded coke according to claim 4, characterized in that the size of the small coke particles is between 5 and 15 mm. 6. Coke oven for implementing the method according to one of claims 1 to 5, consisting of a tank (1) with a vertical axis comprising, at its upper part, a mouth (3) for introducing a charge of coal in the form of balls and gas recovery means and, at its lower part a solids evacuation member and gas injection means and provided, in its middle part, with at least one pair of electrodes (8) for passing an electric current through the charge, characterized in that it comprises at least two pairs of electrodes (8) placed one below the other in two planes horizontal (A, B) spaced from each other and whose electrical supply voltages are individually adjustable in pairs. 7. Coke oven according to claim 6, characterized in that the load supplied by the jaw (3) is added with a conductive product such as small coke consisting of grains of dimensions smaller than those of the balls and distributed so homogeneous between them. 8. Coke oven according to claim 6, characterized in that it comprises at least one pair of electrodes (8) mounted movable horizontally and each connected to means (81, 82) for alternating advancement and retreat, each electrode (8) exerting a constant pressure on the load by progressive advancement towards the interior of the furnace (1) to a determined position from which is controlled a rapid retreat accompanied by a descent of the load and thus after. 9. Coke oven according to claim 6, characterized by the fact that it comprises a first set of electrodes (10) placed at the upper level (A) and suspended from vertical conductors (12) whose position according to the height of the oven is adjustable, and at least a second set electrodes (8) spaced apart and extending horizontally at a level (B) through the side wall of the oven (1). 10. Coke oven according to claim 9, characterized in that the vertical conductors allowing the suspension and movement of the mobile electrodes (10) consist of tubes (12) surrounded by refractory material and connected at their upper part to a device blowing air for cooling the electrodes. 11. Coke oven according to one of claims 6 to 10, characterized in that the wall (2) of the oven (1) widens in cross section in the cooling zone (R), below the coking middle zone (C). 12. Coke oven according to claim 11, characterized in that it comprises means for slowing down the load such as cross members extending horizontally in the load and placed at the start of the cooling zone (R).

Images