Title of Invention

METHOD AND PLANT FOR PRODUCING LOW-TEMPERATURE COKE

Abstract

The present invention relates to a method and a plant for producing low 15 temperature coke, in which granular coal and possibly further solids are heated to a temperature of 700 to 1050˚C in a fluidized-bed reactor (2) by means of an oxygen – containing gas. To improve the utilization of energy it is proposed to introduce a first gas or gas mixture from below through at least one gas supply tube (3) into a mixing chamber region (8) of the reactor (2), the gas supply tube (3) being at least partly surrounded by a stationary annular fluidized bed (6) which is fluidized by supplying fluidizing gas. The gas velocities of the first gas or gas mixture and of the fluidizing gas for the annular fluidized bed (6) are adjusted such that the Particle-Froude-Numbers in the gas supply tube (3) are between 1 and 100, in the annular fluidized bed (6) between 0.02 and 2 and in the 25 mixing chamber (8) between 0.3 and 30.

Full Text

FORM - 2 THE PATENTS ACT, 1970 (39 of 1970) & THE PATENTS RULES 2003 Complete (See section 10 and rule 13) METHOD AND PLANT FOR PRODUCING LOW-TEMPERATURE COKE OUTOKUMPU TECHNOLOGY OY a Finnish Public Limited Company of Riihitontuntie 7, FIN-02200 Espoo, Finland THE FOLLOWING SPECIFICATION PARTICULARLY DESCRIBES THE INVENTION AND THE MANNER IN WHICH IT IS TO BE PERFORMED. The present invention relates to a method for producing low-temperature coke, in which granular coal and possibly further solids are heated to a temperature of 700 to 1050°C in a fluidized-bed reactor by means of an oxygen-containing gas, and to a corresponding plant. Such methods and plants are used for instance for producing low-temperature coke or for producing a mixture of low-temperature coke and ores, for instance iron ores. In the latter case, granular ore is supplied to the low-temperature carbonization reactor apart from granular coal. The low-temperature coke produced in this way, or the mixture of low-temperature coke and ore, can then be processed for instance in a succeeding smelting process. From DE 10101 157A1 there is known a method and a plant for producing a hot, granular mixture of iron ore and low-temperature coke, in which granular coal and preheated iron ore are charged to a low-temperature carbonization reactor, and in which temperatures in the range from 800 to 1050°C are generated by supplying oxygen-containing gas and by partial oxidation of the constituents of the coal, the granular solids being maintained in a turbulent movement and being supplied from the upper region of the reactor to a solids separator. The low-temperature carbonization reactor can constitute a fluidized-bed reactor, and it is left open whether the method can be performed with a stationary or a circulating fluidized bed. To minimize the energy demand of the plant, it is furthermore proposed to preheat the iron ore before supplying the same to the low-temperature carbonization reactor with the hot exhaust gases of the solids separator. However, the product quality to be achieved with this method, 2 which in particular depends on the mass and heat transfer conditions, needs improvement. In the case of the stationary fluidized bed, this is chiefly due to the fact that although very long solids retention times are adjustable, the mass and heat transfer is rather moderate due to the comparatively low degree of fluidization, and dust-laden exhaust gas, e.g. from the product cooling, can hardly be integrated in the process. Circulating fluidized beds, on the other hand, have better mass and heat transfer conditions due to the higher degree of fluidization, but are restricted in terms of their retention time because of this higher degree of fluidization. Summary of the Invention Therefore, it is the object of the present invention to provide a method for producing low-temperature coke, which can be performed more efficiently and is characterized in particular by a good utilization of energy. In accordance with the invention, this object is solved by a method as mentioned above, in which a first gas or gas mixture is introduced from below through a gas supply tube (central tube) into a mixing chamber region of the reactor, the central tube being at least partly surrounded by a stationary annular fluidized bed which is fluidized by supplying fluidizing gas, and in which the gas velocities of the first gas or gas mixture as well as of the fluidizing gas for the annular fluidized bed are adjusted such that the Particle-Froude-Numbers in the central tube are between 1 and 100, in the annular fluidized bed between 0.02 and 2 and in the mixing chamber between 0.3 and 30. 3 In the method of the invention, the advantages of a stationary fluidized bed, such as a sufficiently long solids retention time, and the advantages of a circulating fluidized bed, such as a good mass and heat transfer, can surprisingly be combined with each other during the heat treatment, while the disadvantages of both systems are avoided. When passing through the upper region of the central tube, the first gas or gas mixture entrains solids from the annular stationary fluidized bed, which is referred to as annular fluidized bed, into the mixing chamber, so that due to the high slip velocities between solids and gas an intensively mixed suspension is formed and an optimum heat transfer between the two phases is achieved. As a result of the reduction of the flow velocity of the first gas or gas mixture upon leaving the central tube and/or as a result of the impingement on one of the reactor walls, a large part of the solids is precipitated from the suspension in the mixing chamber and falls back into the stationary annular fluidized bed, whereas only a small amount of non-precipitated solids is discharged from the mixing chamber together with the first gas or gas mixture. Thus, a solids circulation is obtained between the reactor regions of the stationary annular fluidized bed and the mixing chamber. Due to the sufficient retention time on the one hand and the good mass and heat transfer on the other hand, a good utilization of the thermal energy introduced into the low-temperature carbonization reactor and an excellent product quality is thus obtained. Another advantage of the method of the invention consists in the possibility of operating the process under partial load without a loss in product quality. 4 To ensure a particularly effective mass and heat transfer in the mixing chamber and a sufficient retention time in the reactor, the gas velocities of the first gas mixture and of the fluidizing gas are preferably adjusted for the fluidized bed such that the dimensionless Particle-Froude-Numbers (Frp) in the central tube are 1.15 to 20, in the annular fluidized bed 0.115 to 1.15 and/or in the mixing chamber 0.37 to 3.7. The Particle-Froude-Numbers are each defined by the following equation: Pf with u Ps Pf dp effective velocity of the gas flow in m/s density of a solid particle in kg/m effective density of the fluidizing gas in kg/m3 mean diameter in m of the particles of the reactor inventory (or the particles formed) during operation of the reactor gravitational constant in m/s . When using this equation it should be considered that dp does not indicate the grain size (d50) of the material supplied to the reactor, but the mean diameter of the reactor inventory formed during the operation of the reactor, 5 which can differ significantly in both directions from the mean diameter of the material used (primary particles). From very fine-grained material with a mean diameter of 3 to 10 urn, particles (secondary particles) with a grain size of 20 to 30 urn are formed for instance during the heat treatment. On the other hand, some materials, e.g. certain ores, are decrepitated during the heat treatment. In accordance with a development of the invention it is proposed to recirculate part of the solids discharged from the reactor and separated in a separator, for instance a cyclone, into the annular fluidized bed. The amount of the product stream recirculated into the annular fluidized bed preferably is controlled in dependence on the pressure difference above the mixing chamber. In dependence on the solids supply, the grain size and the gas velocity a level is obtained in the mixing chamber, which can be influenced by splitting the withdrawal of product from the annular fluidized bed and from the separator. To achieve a good fluidization of the coal, coal with a grain size of less than 10 mm, preferably less than 6 mm, is supplied to the low-temperature carbonization reactor as starting material. Highly volatile coals, such as lignite, which can possibly also contain water, turned out to be particularly useful starting materials for the method in accordance with the invention. 6 As fluidizing gas, air is preferably supplied to the low-temperature carbonization reactor, and for this purpose all other gases or gas mixtures known to the expert for this purpose can of course also be used. It turned out to be advantageous to operate the low-temperature carbonization reactor at a pressure of 0.8 to 10 bar and particularly preferably between 2 and 7 bar. The method in accordance with the invention is not restricted to the production of low-temperature coke, but in accordance with a particular embodiment can also be used for producing a mixture of ore and low-temperature coke by simultaneously supplying other solids to the low-temperature carbonization reactor. The method in accordance with the invention turned out to be particularly useful for producing a mixture of iron ore and low-temperature coke. In this embodiment, the iron ore is expediently first preheated in a preheating stage, comprising a heat exchanger and a downstream solids separator, for instance a cyclone, before being supplied to the low-temperature carbonization reactor. With this embodiment, mixtures of iron ore and low-temperature coke with an Fe:C weight ratio of 1:1 to 2:1 can be produced. In accordance with a development of the invention it is proposed to heat the iron ore in the suspension heat exchanger by means of exhaust gas from a cyclone downstream of the reactor. In this way, the total energy demand of the process is further reduced. 7 Furthermore, the present invention relates to a plant which is in particular suited for performing the method described above. In accordance with the invention, the plant includes a reactor constituting a fluidized-bed reactor for the low-temperature carbonization of granular coal and possibly further solids. In the reactor, a gas supply system is provided, which extends into the mixing chamber of the reactor and is formed such that gas flowing through the gas supply system entrains solids from a stationary annular fluidized bed, which at least partly surrounds the gas supply system, into the mixing chamber. Preferably, this gas supply system extends into the mixing chamber. It is, however, also possible to let the gas supply system end below the surface of the annular fluidized bed. The gas is then introduced into the annular fluidized bed e.g. via lateral apertures, entraining solids from the annular fluidized bed into the mixing chamber due to its flow velocity. In accordance with the invention, the gas supply system has a gas supply tube (central tube) extending upwards substantially vertically from the lower region of the reactor preferably into the mixing chamber of the reactor, which gas supply tube is at least partly surrounded by a chamber in which the stationary annular fluidized bed is formed. The central tube can constitute a nozzle at its outlet opening and have one or more apertures distributed around its shell surface, so that during the operation of the reactor solids constantly get into the central tube through the apertures and are entrained by the first gas or gas mixture through the central tube into the mixing chamber. Of course, two or more gas supply tubes with different or identical dimensions may also be provided in the reactor. 8 Preferably, however, at least one of the gas supply tubes is arranged approxi¬mately centrally with reference to the cross-sectional area of the reactor. In accordance with a preferred embodiment, a cyclone for separating solids is provided downstream of the reactor. To provide for a reliable fluidization of the solids and the formation of a stationary fluidized bed, a gas distributor is provided in the annular chamber of the low-temperature carbonization reactor, which divides the chamber into an upper annular fluidized bed and a lower gas distributor, the gas distributor being connected with a supply conduit for fluidizing gas and/or gaseous fuel. The gas distributor can constitute a gas distributor chamber or a gas distributor composed of tubes and/or nozzles, where part of the nozzles can each be connected to a gas supply for fluidizing gas and another part of the nozzles can be connected to a separate gas supply of gaseous fuel. In accordance with a development of the invention it is proposed to provide a preheating stage including a suspension heat exchanger and a cyclone downstream of the same upstream of the low-temperature carbonization reactor. In the annular fluidized bed and/or the mixing chamber of the reactor, means for deflecting the solid and/or fluid flows can be provided in accordance with the invention. It Is for instance possible to position an annular weir, whose diameter lies between that of the central tube and that of the reactor wall, in the annular fluidized bed such that the upper edge of the weir protrudes beyond the solids level obtained during operation, whereas the 9 lower edge of the weir is arranged at a distance from the gas distributor or the like. Thus, solids separated out of the mixing chamber in the vicinity of the reactor wall must first pass by the weir at the lower edge thereof, before they can be entrained by the gas flow of the central tube back into the mixing chamber. In this way, an exchange of solids is enforced in the annular fluidized bed, so that a more uniform retention time of the solids in the annular fluidized bed is obtained. Developments, advantages and possible applications of the invention can also be taken from the following description of embodiments and the drawing. All features described and/or illustrated form the subject-matter of the invention per se or in any combination, independent of their inclusion in the claims or their back-reference. Brief Description of the Drawings Fig. 1 shows a process diagram of a method and a plant in accordance with a first embodiment of the present invention; Fig. 2 shows the process diagram of a plant as shown in Fig. 1 with a temperature control of the reactor; and Fig. 3 shows a process diagram of a method and a plant in accordance with a further embodiment of the invention. Detailed Description of the Preferred Embodiments In the method for producing low-temperature coke without further solids, which is shown in Fig. 1, fine-grained coal with a grain size of less than 10 10 mm is charged into the low-temperature carbonization reactor 2 via conduit 1. In its lower central region, the reactor 2 has a vertical central tube 3 which is surrounded by a chamber 4 which is annularly formed in cross-section. The chamber 4 is divided into an upper part and a lower part by a gas distributor 5. While the lower chamber acts as gas distributor chamber for fluidizing gas, a stationary fluidized bed 6 (annular fluidized bed) of fluidized coal is located in the upper part of the chamber, the fluidized bed extending a bit beyond the upper orifice end of the central tube 3. Through conduit 7, air is supplied to the annular fluidized bed 6 as fluidizing gas, which flows through the gas distributor chamber and the gas distributor 5 into the upper part of the annular chamber 4, where it fluidizes the coal to be subjected to low-temperature carbonization by forming a stationary fluidized bed 6. The velocity of the gases supplied to the reactor 2 preferably is chosen such that the Particle-Froude-Number in the annular fluidized bed 6 is between 0.12 1. Through the central tube 3 air is likewise constantly supplied to the low-temperature carbonization reactor 2, which air upon passing through the central tube 3 flows through the mixing chamber region 8 and the upper duct 9 into the cyclone 10. The velocity of the gas supplied to the reactor 2 preferably is adjusted such that the Particle-Froude-Number in the central tube 3 is between 6 and 10. Due to the high velocity, the air flowing through the central tube 3 entrains solids from the stationary annular fluidized bed 6 into the mixing chamber region 8 upon passing through the upper orifice region, so that an intensively mixed suspension is formed. As a result of the reduction of the flow velocity by the expansion of the gas jet and/or by 11 impingement on one of the reactor walls, the entrained solids quickly lose velocity and fall back into the annular fluidized bed 6. Only a small amount of non-precipitated solids is discharged from the low-temperature carbonization reactor 2 together with the gas stream via the duct 9. Thus, between the reactor regions of the stationary annular fluidized bed 6 and the mixing chamber 8 a solids circulation is obtained, by means of which a good mass and heat transfer is ensured. The solids retention time in the reactor can be adjusted within wide limits by the selection of height and outside diame¬ter of the annular fluidized bed 6. Solids separated in the cyclone 10 are fed into the product discharge conduit 12 via conduit 11, whereas the still hot exhaust gas is supplied via conduit 13 into another cyclone 14, separated there from possibly remaining solids, and withdrawn via an exhaust gas conduit 15. Solids separated in the cyclone 14 are supplied again to the reactor 2 via conduit 16 for low-temperature carbonization. Optionally, as shown in Fig. 1, part of the solids discharged from the reactor 2 and separated in the cyclone 10 can be recirculated to the annular fluidized bed 6. The amount of the product stream recirculated to the annular fluidized bed 6 can be controlled in dependence on the pressure difference above the mixing 10 chamber 8 (APMC). The process heat required for low-temperature carbonization is obtained by partial oxidation of the constituents of the coal. Part of the low-temperature coke is continuously withdrawn from the annular fluidized bed 6 of the low-temperature carbonization reactor 2 via 12 conduit 19, mixed with the product discharged from the cyclone 10 via conduit 11, and withdrawn via the product conduit 12. As shown in Fig. 2, the temperature of the reactor can be controlled by varying the volume flow of the fluidizing air. The more oxygen (Oa) is supplied, the more reaction heat is produced, so that a higher temperature is obtained in the reactor. Preferably, the volume flow through conduit 7 is kept constant, whereas the volume flow supplied to the central tube 3 is varied by conduit 18, for instance by means of a blower 22 with spin controller. In contrast to the apparatus described above, the plant shown in Fig. 3, which can in particular be used for producing a mixture of low-temperature coke and iron ore, includes a suspension heat exchanger 20 upstream of the reactor 2, in which granular iron ore introduced through conduit 21, preferably exhaust gas from the cyclone 10 downstream of the low-temperature carbonization reactor 2, is suspended and heated, until a large part of the surface moisture of the ore is removed. By means of the gas stream, the suspension is subsequently introduced via conduit 13 into the cyclone 14, in which the iron ore is separated from 5 the gas. Thereupon, the separated preheated solids are charged through conduit 16 into the low-temperature carbonization reactor 2. The pressure-controlled partial recirculation shown in Fig. 1 and 2 and the temperature control can of course also be employed in the plant as shown in Fig. 3. On the other hand, the pressure and/or temperature control can also be omitted in the plant as shown in Fig. 1 and 2. 13 In the following, the invention will be explained with reference to two examples demonstrating the invention, but not restricting the same. Example 1 (Low-temperature carbonization without addition of ore) In a plant corresponding to Fig. 1, 128 t/h coal with a grain size of less than 10 mm with 25.4 wt-% volatile components and 16 wt-% moisture was supplied to the low-temperature carbonization reactor 2 via conduit 1. Through conduits 18 and 7, 68,000 Nm /h air were introduced into the reactor 2, which air was distributed over conduit 18 and conduit 7 (fluidizing gas) in a ratio of 0.74:0.26. The temperature in the low-temperature carbonization reactor 2 was 900° C. From the reactor 2, 64 t/h low-temperature coke were withdrawn via conduit 12, which coke consisted of 88 wt-% char and 12 wt-% ash. Furthermore, 157,000 Nm3/h process gas with a temperature of 900°C were withdrawn via conduit which process gas had the following composition: 11 vol-%CO 10 vol-% C02 24 vol-% H20 20 vol-% H2 1 vol-% CH4 34 vol-% N2. 14 Example 2 (Low-temperature carbonization with preheating of ore) In a plant corresponding to Fig. 3, 170 t/h iron ore were supplied to the suspension heat exchanger 20 via conduit 21 and upon separating gas in the cyclone 14 charged into the low-temperature carbonization reactor 2 via conduit 16. Furthermore, 170 t/h granular coal with 25.4 wt-% volatile constituents and 17 wt-% moisture were supplied to the reactor 2 via conduit 1. Via conduits 18 and 7, 114,000 Nm7h air were introduced into the reactor 2, which air was distributed over conduits 18 and 7 (fluidizing gas) in a ratio of 0.97:0.03. The temperature in the low-temperature carbonization reactor 12 was adjusted to 950° C. From the reactor 2, 210 t/h of a mixture of low-temperature coke and iron ore were withdrawn via conduit 2, which mixture consisted of 16 wt-% Fe2O3 49 wt-% FeO 28 wt-% char, and 7 wt-% ash. Furthermore, 225,000 Nm3/h process gas with a temperature of 518°C were withdrawn from the plant via conduit 15, which process gas had the following composition: 11 vol-%CO 11 vol-%CO2 22 vol-% H2O 15 vol-% H2 1 vol-% CH4 40 vol-% N2. 15 List of Reference Numerals: 1 solids conduit 2 low-temperature carbonization reactor 3 gas supply tube (central tube) 4 annular chamber 5 gas distributor 6 annular fluidized bed 7 supply conduit for fluidizing gas 8 mixing chamber 9 duct 10 first cyclone 11 solids discharge conduit 12 product discharge conduit 13 conduit 14 second cyclone 15 exhaust gas conduit 16 supply conduit for preheated solids 18 gas stream conduit 19 solids discharge conduit 20 suspension heat exchanger 21 supply conduit for ore 22 blower 16 5. The method as claimed in any of the preceding claims, characterized in that part of the solids discharged from the reactor (2) and separated in a separator (10) are recirculated to the annular fluidized bed (6). 6. The method as claimed in claim 5, characterized in that the amount of the product stream recirculated to the annular fluidized bed (6) is controlled in dependence on the pressure difference above the mixing chamber (8). 7. The method as claimed In any of the preceding claims, characterized In that coal with a grain size of less than 10 mm is supplied to the reactor (2) as starting material. 8. The method as claimed in any of the preceding claims, characterized in that highly volatile coal is supplied to the reactor (2) as starting material. 9. The method as claimed in any of the preceding claims, characterized in that air is supplied to the reactor (2) as fluidizing gas. 10. The method as claimed in any of the preceding claims, characterized in that the pressure in the reactor (2) is between 0.8 and 10 bar. 11. The method as claimed in any of the preceding claims, characterized in that iron ore is additionally supplied to the reactor (2). 12. The method as claimed in claim 11, characterized in that the iron ore is preheated before being supplied to the reactor (2). 13. The method as claimed in any of claims 10 to 12, characterized in that from the reactor (2) a product of iron ore and low-temperature coke is withdrawn, which has a weight ratio of iron to carbon of 1:1 to 2:1. 14. A plant for producing low-temperature coke, in particular for performing a method as claimed in any of claims 1 to 13, comprising a reactor (2) which constitutes a fluidized- bed reactor, characterized in that the reactor (2) has a gas supply system which is formed such that gas flowing through the gas supply system entrains solids from a stationary annular fluidized bed (6), which at least partly surrounds the gas supply system, into the mixing chamber (8). 15. The plant as claimed in claim 14, characterized in that the gas supply system has at least one gas supply tube (3) which in the lower region of the reactor (2) extends upwards substantially vertically into the mixing chamber (8) of the reactor (2), the gas supply tube (3) being surrounded by a chamber which at least partly annularly extends around the gas supply tube (3) and in which the 10 stationary annular fluidized bed (6) is formed. 16. The plant as claimed in claim 15, characterized in that the gas supply tube (3) is arranged approximately centrally based on the cross- sectional area of the reactor (2). 17. The plant as claimed in any of claims 14 to 16, characterized in that downstream of the reactor (2) there is provided a separator (10) for separating solids, which preferably has a solids return conduit (11a) leading to the annular fluidized bed (6) of the reactor (2). 19 18. The plant as claimed in any of claims 14 to 17, characterized in that in the annular chamber (4) of the reactor (2) a gas distributor (5) is provided, which divides the chamber (4) into an upper fluidized bed region (6) and a lower gas distributor chamber, and that the gas distributor chamber is connected with a 25 supply conduit (7) for fluidizing gas. 19. The plant as claimed in any of claims 14 to 18, characterized in that upstream of the reactor (2) a preheating stage is provided, which consists of a heat exchanger (20) and a separator (14). Dated this 5th day of July, 2005. MOHAN DEWAN OF R.K.DEWAN & COMPANY APPLICANTS" PATENT ATTORNEY 20

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