DEVICE AND METHOD FOR SORTING A PARTICULATE STREAM

Abstract

The invention relates to a device for continuously sorting a particulate stream, comprising: a) a screw conveyor (1) for conveying the particulate stream to a separation device, b) a sieve (7) for separating the particulate stream ac cording to size, characterized in that it further comprises c) a pre-sieve (3) comprising elongated sieve openings formed by fingers extending towards an end portion of the pre-sieve with the tips of the fingers not being connected in that end portion, the pre-sieve (3) being located between the conveyor outlet and the sieve, d) wherein the pre-sieve (3) encloses part of the circumference of the conveyor screw in an axial end portion of the conveyor screw, and e) the radial distance between the conveyor screw (1) and the pre-sieve (3) is less than 10%, preferably less than 5% of the conveyor screw diameter. The pre-sieve allows to separate elongated objects like threads or wires from the particulate stream which would otherwise clog or block subsequent separation devices including the sieve.

Claims

1. Device for continuously sorting a particulate stream, comprising: a) a screw conveyor (1) for conveying the particulate stream to a separation device, b) a sieve (7) for separating the particulate stream according to size, characterized in that it further comprises c) a pre-sieve (3) comprising elongated sieve openings formed by fingers extending towards an end portion of the pre-sieve with the tips of the fingers not being connected in that end portion, the pre-sieve (3) being located between the conveyor outlet and the sieve, d) wherein the pre-sieve (3) encloses part of the circumference of the conveyor screw in an axial end portion of the conveyor screw, and e) the radial distance between the conveyor screw (1) and the pre-sieve (3) is less than 10%, preferably less than 5% of the conveyor screw diameter.

2. Device according to claim 1, characterized in that the pre-sieve (3) comprises elongated sieve openings including an angle of −40° to 40° with the conveying direction of the screw conveyor.

3. Device according to claim 1 or 2, characterized in that the width of the elongate openings increases from the base to the tip of the fingers.

4. Device according to claim 3, characterized in that the increase in width is by a factor of 2 to 6, preferably 3 to 5.

5. Device according to any of the claims 1 to 4, comprising at least one of: a) the width of the elongate openings at the base is between 1 and 5 mm, preferably 2 and 4 mm; b) the width of the elongate openings at the tip of the fingers is between 4 and 20 mm, preferably 8 and 16 mm, further preferred 10 and 14 mm; c) the length of the fingers from base to tip is between 100 and 500 mm, preferably 100 and 400 mm, further preferred 150 and 250 mm. d) the mesh size of the sieve (7) is between 200 and 1,000 μm, preferably 300 and 800 μm.

6. Device according to any of the claims 1 to 5, characterized in that it further comprises a mechanical impact device for providing mechanical impacts to the presieve (3) .

7. Device according to claim 6, characterized in that the mechanical impact device is a hammer or piston-vibrator.

8. Device according to claim 6 or 7, characterized in that the impacting force and impacting frequency of the mechanical impact device is controllable.

9. Device according to any of the claims 6 to 8, characterized in that the mechanical impact device provides impacts in the area of the base of the fingers of the presieve (3) .

9. The use of a device of any of the claims 1 to 8 for the separation of a particulate ash stream from a fluidized bed boiler.

10. A method for operating a fluidized bed boiler, comprising the steps of: a) carrying out a fluidized bed combustion process; b) removing at least one ash stream from the fluidized bed boiler; c) separating the ash stream into at least two fractions, wherein the separation includes a separation step using a device according to any of the claims 1 to 8; d) recirculating a separated particle fraction into the bed of the fluidized bed boiler.

11. The method of claim 10, characterized in that the boiler is a circulating fluidized bed boiler (CFB) or a bubbling fluidized bed boiler (BFB) .

12. The method of claim 10 or 11, characterized in that the fluidized bed and the ash stream from the fluidized bed comprise ilmenite particles and that the separated recirculated particle fraction is enriched in ilmenite.

13. The method of 12, characterized in that the separation includes a step of using a magnetic separator (12) comprising a field strength of 2,000 Gauss or more, preferably 4,500 Gauss or more.

14. The method of claim 12 or 13, characterized in that the fraction of ilmenite in the bed material is 25 wt. % or more, preferably 30 wt. % or more.

Description

[0066] Embodiments of the invention are now shown by way of example with reference to the figures.

[0067] It is shown in:

[0068] FIG. 1: metal threads that have got stuck in the sieve of a prior art device;

[0069] FIG. 2: schematically a device according to the invention;

[0070] FIG. 3: in view from above the end portion of the conveyor screw and a division wall;

[0071] FIG. 4: different views of a pre-sieve according to the invention;

[0072] FIG. 5: three different pre-sieve geometries, straight, angled to the right, and angled to the left;

[0073] FIG. 6: schematically a test setup for testing different embodiments of the pre-sieve;

[0074] FIG. 7: a mechanical impact device in the form of a piston-vibrator.

[0075] First, the existing problem in prior art ash and recirculation systems for fluidized bed boilers is explained.

[0076] The boiler P14 at the plant Händelöverket is located in Östergötland County. It is operated by E.ON, an international utility company. Boiler P14 was constructed in 2002 by Kvaerner. It is a circulating fluidized-bed boiler with a nominal thermal power of 75 MW, usually fired with a mix of household waste and light industrial waste. It is operated all year around, and part of the produced steam is usually used to produce electricity. The cross section of the furnace is 2.5 m×8.4 m at the level of the fluidizations nozzles and expands to 3.9 m×8.4 m higher up. The height of the furnace from fluidization grid to roof is about 23 m. The boiler has two cyclones and two loop seals with in-bed superheaters. After the cyclones, the flue gases pass an empty pass, convective heat exchangers and several flue gas cleaning units before they are released to the atmosphere through the stack.

[0077] Boiler P14 is operated with an ilmenite containing fluidized bed and an Improbed Loop™ system as schematically disclosed in WO 2018/188786 A1.

[0078] Improbed Loop™ results in an increased concentration of ilmenite in the boiler, hence an improved distribution of oxygen, which increases the boiler efficiency, which can be utilized to increase the fuel throughput, which gives increased gate fee income, increased production of steam, electric power and heat, i.e. an improved process economy.

[0079] However, the availability of Improbed Loop™ has been reduced by mechanical blockages of the mechanical sieve by metallic objects, e.g. threads and wires.

[0080] FIG. 1 visualizes the problem. Metal threads make their way into the sieve and get stuck. The threads build up nests that eventually block the sieve inlet and holes in the sieve. Consequently, the recovery of the fine-grained part of the bottom ash will cease and the ilmenite is lost through the ash discharge.

[0081] The threads originate from the waste fuel and have managed to pass the various magnets installed in the fuel preparation and transportation system. The reason is that pieces of e.g. copper, aluminium and stainless steel are not magnetic. This is a typical situation for waste fired boilers.

[0082] FIG. 2 schematically shows a device according to the invention.

[0083] A screw conveyor 1 conveys bottom ash from the boiler towards an end portion 2 in which the lower part of the circumference of the screw is circumferentially surrounded by a pre-sieve 3. Particulate material falling through the pre-sieve 3 is falling through the front part 4 of a chute comprising a division wall 5 separating front part 4 and rear part 6. This particulate material is then falling onto a sieve 7 as in the existing prior art system to mechanically separate coarse and fine particulate fraction. The fine particulate fraction is then further separated in a magnetic separator 8 as disclosed in WO 2018/188786 A1.

[0084] Threads and other elongated objects are moved by the conveyor screw over the pre-sieve 3 and fall off at the end of this pre-sieve into the rear part 6 of the chute leading towards an ash elevator and are being discarded.

[0085] The location of the pre-sieve and the principal form is shown in FIG. 3. It is a view from inside the classifier screw 2 looking down to the chute with the division wall 5 separating the path 4 to the existing sieve in the Improbed Loop™ system to the right and the path 6 to the ash elevator to the left. The pre-sieve 3 has the geometry of multiple fingers oriented in the axial direction of the screw. The pre-sieve is mounted just below the last part of the screw. Small particles, i.e. the accept fraction, are supposed to fall between the finger-type pre-sieve down to the sieve in the Improbed Loop™ system, whereas elongated objects and metal threads, i.e. the reject fraction, are moved by the screw over the pre-sieve to the chute leading to the ash elevator. The pre-sieve will hence be continuously cleaned by the screw. Therefore, no nesting of threads or other types of blockages caused by the ash can occur.

[0086] FIG. 4 shows the pre-sieve from different angles. The fingers are tapered, and the width of the elongated openings increases from 3 mm at the bottom to 12 mm at the top or end of the fingers.

[0087] Three different pre-sieve designs were tested, distinguished by different orientation of the fingers; directed to the right, straight forward and to the left, when viewing in the axial direction of the classifier screw (FIG. 5) .

[0088] In all three alternatives, the fingers are tapered in the flow direction in order to achieve an increasing gap between the fingers. This decreases the risk for blockages, e.g. due to formation of nests of metal threads. Also, the sieve is formed to fit the screw diameter and is located just below the end part of the screw so that the screw continuously moves the material over the pre-sieve and prevents it from blocking the pre-sieve. The size of the gap between the screw and the pre-sieve preferably should be less than 5% of the conveyor screw diameter. The pre-sieve is designed for easy replacement.

[0089] An experimental test set up for testing the effectiveness of the pre-sieve according to the invention is shown in FIG. 6.

[0090] The screw 1 corresponds to the bottom ash screw in boiler P14. It was operated during the test with similar rpm (speed) as the P14 screw. The screw motor is controlled by a frequency converter. The casing of screw 1 has the inner diameter 231.9 mm and the diameter of the screw threads is 200.0 mm.

[0091] An inspection box2, with two of the sides made of plexiglass, was mounted at the end of the screw. The screw was arranged with a 12° inclination upwards, i.e. like the inclination of the P14 screw. The pre-sieve 3 was mounted under the screw end, in the inspection box. In the series of tests, the two last tests were performed with the screw in horizontal position.

[0092] Two plastic boxes, the accept box and the reject box 4 and 6, with a steel plate 5 in between, were placed below the inspection box. The plate is mounted perpendicular to the screw and so that the edge of the plate is in line with the edge of the pre-sieve, 3 cm below the screw. This arrangement simulates the ash chute in the P14 system. The idea is that the fine-grained ash shall fall between the sieve fingers to the accept box and the coarse fraction, including metal pieces, stones, gravel, etc. shall be forced by the screw along the sieve to the reject box.

[0093] A bulk of 75 kg of bottom ash from the waste-fired boiler P14 at Handelo was used for the test. Moreover, an extra bucket filled with 5 kg metal scrap (metal threads, steel plate pieces, copper wires, etc) was used for the test.

[0094] A sieve with the mesh 0.71 mm was used for dividing a fraction of the bottom ash sample into two fractions; a finer passing the sieve and a coarser remaining on the sieve. The mass of the fractions was measured; 11400 g fine fraction and 15200 g coarse fraction. The two fractions were mixed again and approximately 300 g of metal scrap from the extra bucket, mainly metal threads, was added to the mixture.

[0095] The operating parameters and the settings in eight trials are presented in Table 1. The ash was reused in all trials and prior to each trial the fine and the coarse fractions were mixed, and the metal scrap was added to the mixture.

[0096] In trials 1 through 6 the pre-sieve to be tested was mounted approximately 1 cm below the screw thread. In trials 7 and 8 the pre-sieve was moved down to a distance from the screw of 15-20 cm and bent to the angle 30° downwards. The idea was to test the function of the pre-sieve with the material falling by gravity onto and along the pre-sieve instead of being pushed upwards be the screw.

TABLE-US-00001 TABLE 1 Operating parameters and settings in the 8 trials. Screw incli- Screw nation down- speed Trial Sieve streams (RPM) 1 Straight Upwards 0.92 2 Right Upwards 0.92 3 Right Upwards 0.92 4 Left Upwards 0.92 5 Left Upwards 0.92 6 Left Upwards 1.33 7 Straight/ Horizontal 0.92 + higher inclined and lower position 8 Straight/ Downwards 0.92 inclined and lower position

[0097] The trial commenced by starting the screw and adjusting the speed to the chosen value. The ash was feed continuously from a bucket through a chute down to the screw for further transport along the screw over the pre-sieve.

[0098] The operation was visually observed and recorded by a video camera. The mass of the fine and coarse fractions was measured.

[0099] Trials 1 through 4 used the same operating conditions and settings except that the three different pre-sieves were used.

[0100] One fundamental idea of the pre-sieve is that it shall be self-cleaning. By locating the pre-sieve close to the screw and by tapering the pre-sieve fingers, the screw is supposed to remove fastened threads or nests from the pre-sieve and transport it to the end of the screw. Trial 5 aimed at testing this idea by simulating a case in which metal threads get stuck in the pre-sieve. A nest of threads was manually made and fastened in the pre-sieve.

[0101] Trial 6 was like trial 4 except that the rotation speed of the screw was increased by 45%.

[0102] Trials 7 and 8 were carried out in order to test if the self-cleaning function observed in trials 1 through 6 was necessary for the sieving performance or if similar good results could be obtained if the ash and metal scrap mixture was accelerated by gravity down onto the pre-sieve.

[0103] The removal of scrap and threads is one of the important goals with installing the pre-sieve. Besides, it is important that the loss of fine material, potentially containing ilmenite, is low. The latter aspect was evaluated by measuring the mass of fine material in the reject box, after each trial, and comparing it with the total mass of fine material used in the trial. The loss, η, was defined as by eq. 1 where m.sub.fine, reject is the mass of fine material in the reject and m.sub.fine, accept is the mass of fine material in the accept.

[00001]$\begin{array}{cc}\eta =\frac{m_{\mathrm{fine},\mathrm{rejekt}}}{m_{\mathrm{fine},a\mathrm{ccept}}+m_{\mathrm{fine},\mathrm{reject}}}*100& \left(1\right)\end{array}$

[0104] In trials 1 through 6, the separation of metal scrap and metal threads was very good. The metal scrap and threads end up in the reject, as intended.

[0105] The trials also show that the pre-sieve separates 15% of other coarse ash, which transferred to P14 conditions, would mean a significant reduction of the flow to the existing sieve in the ash recirculation system, which would lead to less wear and maintenance cost.

[0106] In trial 5 a big metal thread nest was intentionally fastened on the pre-sieve. After one minute of operation of the screw, the nest was withdrawn by the screw and transported to the reject box.

[0107] The pre-sieve arrangement in trials 7 and 8 did not work well. The performance with only gravel and sand type ash was acceptable but as soon as metal threads appeared in the ash stream problems occurred. The threads got stuck in the pre-sieve and blocked the ash flow. The threads contribute to form blockages with the ash particles, even with the finer ash. The threads act as reinforcement in the blockages. Eventually, the entire pre-sieve surface is covered by blockages. The sieving function is lost, and the whole material flow goes to the reject box.

[0108] The other trials (1-6) on the other hand, were successful and demonstrate that the design of the self-cleaning pre-sieve has a great potential to remediate the blockage problems in the P14 system.

[0109] Table 2 contains the measured accept and reject mass fractions in each of the eight tests and the loss of the fine particles to the reject, η, defined in Eq.1.

[0110] The table shows that the best results were obtained with the pre-sieve Right in trial 3 in which only 1% of the fine fraction was lost with the reject in this trial. Similar good results were obtained in trials 2 (pre-sieve Right) and 4 (pre-sieve Left) . Hence, the pre-sieves Right and Left appears to be equally good. The Straight pre-sieve however, gave 12% loss of the fine fraction, i.e. a significantly worse result.

TABLE-US-00002 TABLE 2 Results from the eight trials Fine frac- Fine tion in fraction Loss Accept the accept Reject in the η Trial (g) (g) (g) reject (g) (%) 1 3085 1482 1142 200 12 2 7952 3667 2487 144 4 3 7470 3682 1154 42 1 4 8830 3642 1710 79 2 5 Na Na Na Na Na 6 7746 3781 2125 200 5 7 Na Na Na Na Na 8 Na Na Na Na Na Na: no data available

[0111] Visual observations during the trials showed that when pre-sieve Right was used, somewhat more material was accumulating on the right-hand side of the screw. This is because the material follows the thread of the screw, which is a reasonable explanation why pre-sieve Left gave a somewhat more even distribution of the material.

[0112] Trial 6 shows that even an increase of the screw speed by 45% does not affect significantly the loss of the fine fraction.

[0113] FIG. 7 shows the mechanical impact device in the form of a piston-vibrator (9). It comprises a body (10) and a piston head (14) . The piston-vibrator (9) is mounted to the casing of the bottom ash screw (not shown in FIG. 7) by means of the mounting (11). A rubber cushion (12) surrounds the body (10) of the piston-vibrator (9) in order to minimize vibrations induced from the casing of the bottom ash screw. The piston head (14) protrudes through a hole in the casing (15) and hits the pre-sieve on the inside of the casing (not shown). It is operated continuously. The pre-sieve can be inclined downwards in the direction of the end portion.

[0114] The body (10) of the piston-vibrator (9) can be circular or square shaped. It can consist for example of cast iron or aluminum. The piston-vibrator (9) is operated pneumatically. The body (10) comprises a mounting for an air supply (13) . The pressure of the air supplied is in the range of 1.5 to 2 bar.

[0115] The piston head (9) vibrates with a frequency of 1860 Hz at 2 bar and 2220 Hz at 6 bar pressure. It hits with a force of 50 to 200 N, preferably 80 to 150 N. The maximum sound pressure level is 80 dB (A), which is acceptable in a boiler house.

[0116] As the piston head (9) hits the pre-sieve with high frequency, it induces a vibration of the pre-sieve. This vibration prevents finer particles such as bottom ash from forming a layer of material on the pre-sieve.