GB2158088A - Process and apparatus for the production of calcined coke - Google Patents

Abstract

Coke containing volatile materials (e.g. petroleum coke containing residual hydrocarbons) is converted, in a relatively high proportion, to high quality calcined coke by subjecting the volatiles-containing coke in a first stage to a temperature in the range 650 to 850 DEG C (e.g. about 750 DEG C) in one stage, and then subjecting the calcined coke from the first stage to calcination in a subsequent stage at a temperature in the range 1000 to 1600 DEG C (e.g. 1300 to 1500 DEG C). The first stage and the second stage may be in the same or different items of equipment. Preferably, the first stage is in an outer zone (21, Figure 1) or a rotary hearth calciner, and the second zone is in an inner zone (20, Figure 1) of the rotary hearth calciner, the outer zone being separated from the inner zone by an annular baffle (19, Figure 1) which substantially shields the outer zone from radiation and other effects of the high temperature in the inner zone. <IMAGE>

Description

1

SPECIFICATION

Process and apparatus for the production of calcined coke GB 2 158 088 A 1 The present invention relates to a process and apparatus for the production of calcined coke.

Coke which is intended for use in the production of certain metals from raw materials by electrolysis of melts containing compounds of the metals or which is intended for the production of graphitic carbon for ultrahigh power electrodes in electric are furnaces must be substantially free of volatile materials, e.g. hydrocarbons. Accordingly, raw coke for such purposes is calcined at temperatures which are suitably high to produce a volatiles-free coke calcinate. In the case of petroleum coke derived by coking hydrocar- 10 bon materials of petroleum origin, calcination is performed at temperatures of about 1300 to 1500'C, e.g. about 1400'C. Similar or different calcination temperatures may be used to clevolatilize coke obtained from other sources (e.g. coal, lignite).

The present invention will be more particularly described below with reference to petroleum coke, but it is to be understood that it applies equally to coke derived from other sources such as coal, lignite and 15 other carbonaceous and hydrocarbonaceous materials.

Coke which is to be used for the aforesaid purposes is preferably not only substantially free of volatile material (e.g. the residual hydrogen content is preferably not greater than 0.01 to 0.03 wt.%), but prefera bly also meets other stringent quality specifications, including a specified range of particle size distribu tion and/or a specified range of absolute density.

Two processes and types of equipment are presently employed to produce high quality calcined coke, namely:

(a) rotary kilns, in which the raw uncalcined coke (the so-called green coke) is passed through the kiln in counter-current flow to hot flue gases. The calcined coke product is of an acceptable standard, but compared to rotary hearth furnaces, mentioned below, coke yields are lower by 4 to 6%, there are penal- 25 ties due to higher utility consumption, and the investment cost is higher.

M rotary hearth furnaces, which comprise a large circular combustion chamber above a circular ro tatable hearth having a hole at the centre of the hearth. A soaking pit beneath the central hole in the hearth receives coke from the hearth via the central hole. The hearth may be substantially flat or it may slope downwardly towards the central hole either from the outer rim of the hearth or from a location between the rim and the central hole. Cold green coke is fed to the outer rim of the hearth and rabbles cause the coke to execute a spiral path across the hearth to the hole at the centre thereof. The green coke is exposed to the high temperatures of the combustion chamber, and a calcined coke product is recovered from the soaking pit. Although the rotary hearth furnace has advantages compared to the ro tary kiln, the coke product tends to be less uniform in quality so far as size distribution and size stability 35 are concerned.

The present invention provides a process for producing calcined coke from coke containing volatile materials, the process comprising the steps of subjecting the volatiles- containing coke to a temperature in the range of from 650 to 850'C in one stage, subjecting the heated coke from said one stage to a temperature of at least 1000'C in a subsequent stage, and recovering calcined coke from said subsequent 40 stage.

Preferably, the said volatiles-containing coke contains less than 14 wt.% of volatile material, more pref erably less than 10 wt.%, still more preferably less than 8.5 wt,%. In preferred embodiments, the volatiles content is in the range of from 5 to 8 wt.%, e.g., about 7 wt.%.

The invention further provides apparatus for producing calcined coke from coke containing volatile ma- 45 terials comprising one discrete heating device or zone wherein, during operation, the volatiles-containing coke is subjected to a first stage temperature in the range of from 650 to 850'C, and another discrete heating device or zone is arranged to receive heated coke from the said one device or zone wherein, during operation, the heated coke is subjected to temperatures of at least 1000'C in the other discrete heating device or zone thereby to calcine the coke to a product which is substantially non-friable and substantially free of volatile material.

It has unexpectedly been found that by conducting the calcination in two (or more) discrete heating stages as aforesaid, a high proportion of a volatiles- containing coke, e. g. a raw coke, can be converted to a high quality calcined coke meeting the size, size-stability and density specifications of the highest quality calcined cokes.

The invention will now be further described with reference to nonlimitative examples thereof and with reference to the accompanying drawings (not to scale) in which:

Figure 1 is a cross-sectional elevation of one type of rotary hearth cokecalcining furnace according to the invention; Figure 2 is a cross-sectional elevation of another type of rotary hearth furnace according to the inven- 60 tion; Figure 3 is a cross-sectional elevation of yet another type of rotary hearth furnace according to the invention; and Figure 4 is a cross-sectional elevation of a further type of rotary hearth furnace according to the inven tion.

2 GB 2 158 088 A 2 The same reference numeral is used to denote the same or similar features in all of the Figures of the drawings.

In the embodiment of Figure 1, the furnace 10 comprises a rotary table or hearth 11 which inclines downwardly towards a hole at its centre. Beneath the hole is a soaking pit which receives coke material which falls from the hearth 11 and from which calcined coke material may be recovered, as indicated by 5 arrow 23. Above the hearth is a stationary roof member 13. The outer rim of the roof member 13 has a downwardly- extending lateral wall 14 which is sealed (e.g., by means of water seals or sand seals, not shown) against a lateral wall 15 which extends upwardly from the rim of the hearth 11. The hearth 11 is supported on, and rotated, by suitable wheels 16. A hole 17 is provided in the roof member 13 for the discharge of flue gas from the interior of the furnace to a flue gas exhaust duct 18. An annular baffle 19 10 extends downwardly from the roof 13 to define the outer boundary of a central high temperature combustion chamber 20. The bottom of the baffle 19 is spaced from the upper side of the hearth 11 by sufficient distance to permit movement of coke under the baffle 19 from the radially outer regions of the hearth to the hole at the centre of the hearth while substantially preventing radiation and hot gases from the combustion chamber 20 affecting the region radially outward of the baffle 19. A conduit 320 supplies 15 raw petroleum coke (e.g. from a delayed coker) to an outer region of the hearth. Preferably rabbles (not shown) are provided to cause coke deposited on the hearth at the outer rim to execute an inward spiralling movement towards the central hole in the hearth. Preferably other rabbles (not shown) are provided to turn over the coke layer so as to expose substantially all the coke in the coke layer to the atmosphere above the coke layer. Fuel gas and air are supplied to the annular space 21 between the outer face of the 20 baffle 19 and the inner faces of the lateral walls 14, 15 to maintain a temperature therein in the range of from 650 to 850'C, preferably from 700 to 800'C, more preferably about 750'C so that the raw coke is preheated, while passing via space 21, at a bulk heating rate preferably of up to 100'Orninute, more preferably up to 60'C/minute (e.g. about 40'C per minute). The coke may have a residence time in space 21 of from 0.25 to 1.5 hours, preferably 0.6 to 1.3 hours (e.g. 0.9 to 1.25 hours), and passes under the baffle 25 19 at a bulk temperature which may be in the range of from 600 to 850'C, preferably 700 to 850'C, e.g.

about 750'C, when initially exposed to the conditions of the high temperature combustion chamber 20.

The chamber 20 is supplied with suitable amounts of air and fuel to maintain a temperature of at least 1000'C, preferably 1000 to 1600'C, e.g. about 1400 to 1500'C. The turning and propelling action of the rabbles (not shown) ensures that coke is exposed to the radiation and other high temperature effects 30 within chamber 20 so that the volatile-matter-content of the coke is reduced to very low levels, and cal cined coke, at a temperature of 1100 to 1470'C (e.g. about 1400 to 1450'C) is received in the soaking pit 12 from where the last residues of labile volatile material may escape. The calcined coke ("calcinate") withdrawn via line 23 from the soaking pit 12 has a very low content of residual volatile material (e.g.

0.01 to 0.03 wt.% hydrogen), a good size distribution and high size stability which meet the standards for 35 the higher grades of calcined coke (e.g. needle coke).

Preferably, the operating conditions in the annular space 21 are such that no more than 90 wt.%, but preferably at least 50 wt,% (more preferably 55 to 85 wt.%, e.g. 60 to 80%) of the volatile material content of the green coke feed is removed in the first heating stage which is constituted by the preheat section in space 21, the remainder being removed in the subsequent heating stage constituted by the section in cluding the high temperature combustion chamber 20.

Reference is now made to Figure 2 which is largely the same as Figure 1 except for the arrangement of the roof and annular baffle. Items which are common to Figures 1 and 2 have the same reference numer als. As will be seen, the fixed roof 113 is circular and slopes downwardly and inwardly towards a central aperture 114 in much the same way as the rotary hearth 11 slopes towards the central hole at its juncture 45 with the soaking pit 12. An annular baffle 119 extends upwardly from the edge of the aperture 114 to form the lateral boundary of a high temperature combustion section 120, the top of which is closed by a cap member 121 having a central hole 17 for the escape of flue gas to the flue duct 18.

Fuel gas and air are supplied to the spaces 21 and 120 in amounts sufficient to maintain their tempera- tures within the ranges 650 to 8500C and 1000 to 1600'C respectively. It will be appreciated that the configuration of the furnace of Figure 2 is such that coke in the preheat section 21 is shielded from radiation and other high temperature effects until it approaches the notional downward geometric projection of the baffle 119.

The absolute and relative amounts of volatilizable material removed -in the preheat section 21 and the high temperature section are arranged to be about the same for the Figure 2 embodiment as for the Figure 1 embodiment by expedients which will be readily apparent to those skilled in the art.

Refrence is now made to Figure 3 of the drawings. It will be seen that the embodiment of Figure 3 is largely similar to that of Figure 1 except that the rotary hearth 211 of Figure 3 is substantially flat (instead of downwardly sloping towards the central hole as in Figure 1). The passage of coke across the rotating hearth 211 from the outer region where the coke enters the furnace from the feed duct 320 to the central 60 hole above the soaking pit 12 is promoted by rabbles (not shown). Additional rabbles (not shown) are preferably furnished to turn over the coke layer so that all the coke in the coke layer is exposed to the conditions in the furnace.

With reference to Figure 4, it will be seen that the furnace which is depicted is largely similar to that of Figure 2, except that the rotary hearth 311 is substantially flat and resembles the hearth 211 of Figure 3 65 3_ GB 2 158 088 A 3 in this respect. As in the Figure 3 embodiment, rabbles (not shown) are provided to promote the passage of coke across the hearth from the outer region to the central hole, and preferably, additional rabbles (noll shown) are provided to turn over the coke layer in order to expose all the coke to the conditions in the combustion chamber. As depicted, the soaking pit 312 is frusto-conical and downwardly tapering and differs in this respect from the cylindrical soaking pits illustrated in Figures 1 to 3. The roof 221 of the central high temperature combustion chamber 120 is similar to the roof 121 of the furnace of Figure 2 except that the flue opening 217 and the flue duct 218 are eccentrically located with respect to the vertical axis of the chamber 120 whereas in Figure 2 they are concentric with respect to the vertical axis of the chamber 120.

It is to be appreciated that features and/or items of equipment which are illustrated in connection with 10 one embodiment may be employed in feasible combinations with features and/or items of equipment shown in connection with other embodiments. Moreover, although the embodiments described each have a single annular baffle to separate the higher and lower temperature zones, it is contemplated that more than one baffle may be provided so as to separate the interior of the furnace into more than two heating stages.

Although the description refers particularly to a rotary hearth calciner in which a baffle is provided to prevent exposure of the coke in the outer preheat zone to radiation and other high temperature effects of the high temperature central zone, the invention also comprises other equipment for heating and calcin ing the feed coke in at feast two stages. For example, the feed coke may be calcined at temperatures of 650 to 850'C in a first stage provided by a rotary kiln and the thus calcined coke may then be calcined at 20 1000 to 1600'C in a subsequent stage provided by a rotary hearth furnace. When a rotary kiln is em ployed to perform the first stage calcination, part of the subsequent calcination stage can also be per formed in the rotary kiln at temperatures in the range 850 to 1600'C. The rotary hearth furnace need not be divided into inner and outer zones by an annular baffle. Alternatively, the first stage heating may be effected in one rotary hearth kiln and the second stage calcination in another rotary hearth kiln. In the 25 latter arrangement, it is not essential that one or both of the kilns should be divided by an annular baffle into zones of different temperatures.

In yet other arrangements, one or both of the heating stages can be effected in a tunnel kiln or a shaft furnace, and the skilled technologist will appreciate that the invention may be performed in various com binations of kilns and/or furnaces to achieve the benefits of the invention without departing from the scope of the invention.

By the practice of the invention, it is surprisingly found that a high quality calcined coke can be ob tained in equipment which operates or can be operated so as to subject the coke to relatively rapid heat up rates.

The following comparative simulations illustrate the benefits afforded by the practice of the invention. 35 Test 1 A sample of a typical green petroleum coke at room temperature was exposed in a kiln to a temperature of about 1000'C for about 20 minutes to simulate the conditions in a conventional rotary hearth furnance. After 20 minutes, the sample had attained a temperature of about 1000'C. It was removed from 40 the kiln and the particles of calcined coke were found to be severely cracked and highly friable.

Test 2 Test 1 was repeated with another sample of the same green coke but with the kiln temperature at about 11 OOOC. The calcined coke was found to be somewhat inferior in quality to the calcined coke prod- 45 uct of Test 1.

Test 3 Samples of the same green coke used in Test 1 were exposed first to a relatively lower temperature in a first heating stage and cooled to room temperature (about 20'C) before being exposed to a temperature 50 of 1 1000C in a second heating stage. In each heating stage, the sample was exposed for sufficiently long to attain approximately the temperature to which it was exposed. The results are given in Table 1.

TABLE 1

55 Stage No. 1 Product Stage No. 2 Product Run No. Temperature OC Quality Temperature OC Quality 1 -900 friable -1100 high friability 2 -800 hard -1100 low friability 60 3 -700 hard -1100 low friability 4 -600 hard -1100 increased friability Runs 2 and 3 illustrate the invention.

4 GB 2 158 088 A

Claims (23)

1. A process for producing calcined coke from coke containing volatile materials, the process comprising the steps of subjecting the volatilescontaining coke to a temperature in the range of from 650 to 850'C in one stage, subjecting the heated coke from said one stage to a temperature of at least 10000C in a subsequent stage, and recovering calcined coke from said subsequent stage.

2. A process as in claim 1 in which the coke has an average residence time in the first stage of from 0.2 to 2.0 hours.

3. A process as in claim 2 in which the average residence time in the first stage is in the range of from 0.25 to 1.5 hours.

4. A process as in any one of claims 1 to 3 in which the coke is heated in the first stage at an average bulk heating rate of up to 100'C/minute.

5. A process as in any one of claims 1 to 4 in which no more than 90 wt.% volatizable material contained in the coke is removed in the said one stage, and more than 50 wt.% of the remaining volatilizable material is removed in the said subsequent stage.

6. A process as in claim 5 in which at least 50 wt.% of the volatilizable material contained in the coke entering said one stage is removed in the said one stage.

7. A process as in claim 5 or claim 6 in which from 55 to 85 MA of the volatilizable material con tained in the coke entering the said one stage is removed in the said one stage.

8. A process as in any one of claims 1 to 7 in which the temperature of the said subsequent stage is 20 in the range of from 1000 to 1600'C.

9. A process as in any one of claims 1 to 8 in which the said one stage is contained in one type of heating device and the subsequent stage is contained in a diferent type of heating device.

10. A process as in any one of claims 1 to 8 in which the said stages are contained in a single rotary hearth furnace, the said one stage being radially outwards of the said subsequent stage.

11. A process as in claim 10 in which the said one stage is separated from the subsequent stage by an annular baffle defining, at its lower side, a port or gap permitting at least coke to pass from said one stage to said subsequent stage.

12. A process for producing calcined coke substantially as described.

13. Apparatus for use in performing the process of any one of claims 1 to 12 in which the said one processing stage is contained in one heating device or zone, the subsequent processing stage is con tained in another heating device or zone, and means are provided for the passage of coke from the exit of said one device or zone to the entrance of said other device or zone.

14. Apparatus as in claim 13 in which the said one heating device and said other heating device are each selected from a shaft furnace, a tunnel kiln, rotary kiln and a rotary hearth furnace.

15. Apparatus as in claim 14 in which said one heating device or zone is comprised in or by a rotary kiln and the said other heating device or zone is comprised in or by a rotary hearth furnace.

16. Apparatus as in claim 14 in which the said one zone and the said other zone are provided as respective zones of a single rotary hearth furnace, the said one zone being an annular zone radially out- wards of the other zone, the two zones being operable at different temperatures and arranged so that the 40 outer zone is substantially shielded andlor insulated from the operating conditions of radiation and high temperature in the other, inner, zone, during operation.

17. Apparatus as in claim 16 in which the outer zone is substantially shielded from the operating conditions of the other, inner, zone by an annular baffle which defines at least one port or gap at or near the bottom of the baffle for the passage of at least coke from the outer zone to the inner zone.

18. Apparatus as in claim 16 or claim 17 in which the roof of the outer zone slopes inwardly and downwardly to the base of the annular baffle, the latter bounding the sides of a high temperature region above the inner zone whereby, during operation, substantially no radiant heat can be directly received in the outer zone from the high temperature region.

19. Apparatus as in claim 18 in which the high temperature region has a roof which reflects radiant 50 heat onto coke in the inner zone.

20. Apparatus as in any one of claims 16 to 19 in which the operating temperatures in the outer and inner zones are separately regulatable.

21. Apparatus for producing calcined coke from coke containing volatile materials substantially as herein described.

22. Apparatus substantially as described with reference to the accompanying drawings.

23. Calcined coke made by the process of any one of claims 1 to 12 or made in apparatus according to any one of claims 13 to 22.

4 1 Printed in the UK for HMSO, D8818935, 9 85, 7102. Published by The Patent Office, 25 Southampton Buildings, London, WC2A lAY, from which copies may be obtained.