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SYSTEM AND PROCESS FOR RECYCLING FLUIDIZED BOILER BED MATERIAL

20200292169 ยท 2020-09-17

    Inventors

    Cpc classification

    International classification

    Abstract

    The invention relates to a system for recycling fluidized bed boiler bed material, comprising: a. a bottom ash removal device for removing bed material from a fluidized bed boiler, b. a mechanical classifier (10) comprising a mesh size from 200 to 1,000 m designed to separate a coarse and a fme particle size fraction, c. a magnetic separator (12) designed to magnetically classify the fine particle fraction from the mechanical classifier, d. a device for recirculating the magnetic particle fraction into the boiler. The invention allows effective recirculation and reuse of ilmenite bed material.

    Claims

    1. A system for recycling fluidized bed boiler bed material, comprising: a) a bottom ash removal device for removing bed material from a fluidized bed boiler, b) a mechanical classifier (10) comprising a mesh size from 200 to 1,000 m designed to separate a coarse and a fine particle size fraction, c) a magnetic separator (12) designed to magnetically classify the fine particle fraction from the mechanical classifier, d) a device for recirculating the magnetic particle fraction into the boiler.

    2. The system of claim 1, wherein the mechanical classifier (10) comprises a mesh size from 300 to 800 m.

    3-15. (canceled)

    16. The system of claim 2, wherein the mechanical classifier (10) comprises a mesh size from 400 to 600 m.

    17. The system of claim 1, wherein the mechanical classifier (10) comprises a rotary sieve.

    18. The system of claim 17, wherein the mechanical classifier further comprises a primary sieve prior to the rotary sieve (10) to separate coarse particles having a particle size of 2 cm or greater.

    19. The system of claim 1, further comprising a device for separating elongate ferromagnetic objects from the ash stream prior to the magnetic separator (12).

    20. The system of claim 19, wherein the device for separating elongate ferromagnetic objects from the bed material prior to the magnetic separator (12) comprises a slot mesh.

    21. The system of claim 1, wherein the magnetic separator (12) comprises a field intensity of 2,000 Gauss or more on the surface of the transport means of the bed material.

    22. The system of claim 21, wherein the magnetic separator (12) comprises a field intensity of 4,500 Gauss or more on the surface of the transport means of the bed material.

    23. The system of claim 1, wherein the magnetic separator (12) comprises a rare earth roll (RER) or rare earth drum (RED) magnet.

    24. The system of claim 23, wherein the magnetic field is axial.

    25. The system of claim 23, wherein the magnetic field is radial.

    26. The system of claim 1, wherein the separation efficiency for ilmenite bed material is at least 0.5.

    27. A process for recycling fluidized bed boiler bed material comprising ilmenite, the process comprising the steps of: a) removing bed material from a fluidized bed boiler, b) mechanically classifying the bed material using a mesh size from 200 to 1,000 m to separate a coarse and a fine particle size fraction, c) magnetically classifying the fine particle fraction from the mechanical classifier, d) recirculating the magnetic particle fraction into the boiler.

    28. The process of claim 27, wherein the separation efficiency of step c) is at least 0.7 by mass for ilmenite.

    29. The process of claim 27, wherein the average residence time of ilmenite in the system is 20 h or more.

    30. The process of claim 27, wherein the average residence time of ilmenite in the system is 30 h or more.

    31. The process of claim 27, wherein the average residence time of ilmenite in the system is 40 h or more.

    32. The process of claim 27, wherein the average residence time of ilmenite in the system is 100 h or more.

    33. The process of claim 27, wherein the fraction of ilmenite in the bed material is 25 wt. % or more

    34. The process of claim 27, wherein the fraction of ilmenite in the bed material is 30 wt. % or more.

    Description

    [0044] Embodiments of the invention are now shown by way of example with reference to the figures.

    [0045] It is shown in:

    [0046] FIG. 1: a schematic illustration of a system according to the invention in cooperation with a boiler,

    [0047] FIG. 2: a schematic illustration of magneticdrum separator,

    [0048] FIG. 3: a schematic illustration to show the mass streams in an embodiment of the process according to the invention.

    EXAMPLE 1

    [0049] In this example the composition and particle size distribution of bottom ash is analyzed. The bottom ash was taken from a 75 MW municipal solid waste fired boiler operating with the bed material comprising silica sand and 16 wt. % ilmenite.

    [0050] The bottom ash was sieved through a 500 m mesh which removed the particle fraction coarser than 500 m (about 50 wt. % of the original sample).

    [0051] The bottom ash sample, excluding particulates coarser than 500 m, of 8.3 kg was analyzed for ranges of material content of bed materials (ilmenite, silica oxide, calcium oxide, aluminum oxide) and particle size distribution.

    [0052] Material composition (ranges, wt. %):

    TABLE-US-00001 Ilmenite: 10-20% Silica oxide: 40-60% Calcium oxide: 5-10% Aluminum oxide: 5-10%

    [0053] Particle size distribution (wt. %):

    TABLE-US-00002 355-500 m: ~7% 250-355 m ~17% 125-250 m: ~69% <125 m: ~7%

    [0054] This analysis shows typical percentages of ilmenite in the bottom ash which can be retrieved according to the invention and also shows that the particle size distribution of the bottom ash does allow an initial mechanical classification to remove coarse particles with e.g. a mesh size of 500 m.

    EXAMPLE 2

    [0055] In this example the effectiveness of magnetic separation processes is tested. The following test equipment was used:

    [0056] Eriez 305 mm dia.305 mm wide model FA (Ferrite Axial) magnetic drum. Field strength ca. 2000 Gauss (drum #1).

    [0057] Eriez 305 mm dia.305 mm wide model RA (Rare Earth Axial) magnetic drum. Field strength ca. 4500 Gauss (drum #2).

    [0058] Eriez 305 mm dia.305 mm wide model RR (Rare Earth Radial) magnetic drum. Field strength ca. 4000 Gauss (drum #3).

    [0059] FIG. 2 shows an arrangement of two magnetic separation drums or rolls in sequential order.

    [0060] Material is fed through a feed 3 on a magnetic drum 1 rotating into the direction indicated by the arrow (counterclockwise). Magnetic particles tend to adhere to the drum longer than nonmagnetic particles which is indicated by the arrows nonmagnetics 1 and magnetics 1 in the drawing. A mechanical separator blade 4 helps to separate the magnetic and nonmagnetic particle fractions.

    [0061] When using a two-stage process, the nonmagnetic particle fraction from the first drum 1 can be fed to a second drum 2 for a second magnetic separation step.

    [0062] Three tests were carried out, the first test using a two-step separation process and the second and third test using single step separation processes. The tests were carried out with bottom ash as analyzed in example 1.

    [0063] Test 1

    [0064] A 2.5 kg bottom ash sample was passed over a ferrite magnetic drum (drum #1) with an axial magnet arrangement. This causes the strongly magnetic material to tumble as it passes from north to south poles, releasing any entrapped nonmagnetic or paramagnetic materials, thus providing a cleaner magnetic fraction.

    [0065] The nonmagnetic fraction from this first separation step was then passed over a second drum (drum #2), with a stronger Rare Earth axial magnetic field.

    [0066] Test 2

    [0067] A 1.25 kg bottom ash sample was passed over a drum (drum #2), with a strong Rare Earth axial magnetic field.

    [0068] Test 3

    [0069] A 1.25 kg bottom ash sample was passed over a drum (drum #3), with a strong Rare Earth radial magnetic field.

    [0070] Both tests 2 and 3 utilized single step magnetic separation.

    [0071] The test results are shown in the following table. The table also indicates the splitter position in terms of the distances A and B of the leading edge of the mechanical splitter from the rotational axis of the drum (see FIG. 2) and the drum speed in terms of min.sup.1 and surface speed in m/min. The table also indicates the results of the magnetic separation.

    TABLE-US-00003 Feed Splitter % of Test Drum Rate Position Drum Speed Sample Weight Feed No. Type (t/hr) A B RPM M/Min. Description No. (g) Weight 1 FA 1.5 125 mm 140 mm ~63 60 Feed 100 2498 Magnetics 1 101 716 28.7 Non Magnetics 1 102 1782 71.3 RA 1.5 70 mm 160 mm ~63 60 Magnetics 2 103 236 16.8 Non Magnetics 2 104 764 54.5 2 RA 1.5 70 mm 160 mm ~63 60 Feed 1248 Magnetics 1 201 593 47.5 Non Magnetics 1 202 655 52.5 3 RR 1.5 115 mm 170 mm ~63 60 Feed 1247 Magnetics 1 301 736 59.0 Non Magnetics 1 302 511 41.0

    EXAMPLE 3

    [0072] FIG. 1 shows schematically an embodiment of the system of the invention connected to a boiler.

    [0073] A boiler 6 is fed with fuel (waste) at 7 and ilmenite bed material at 8.

    [0074] Bottom ash is retrieved via 9 and fed to a rotary sieve 10 having a mesh size of 500 m. The coarse fraction comprising mostly ash and some lost ilmenite material is discarded at 11.

    [0075] The fine particle size fraction is fed to a magnetic separator 12 comprising a rare earth roll magnet (as shown above). The nonmagnetic fraction from the magnetic separator 12 is discarded at 13. The magnetic fraction is recirculated as bed material (ilmenite) to the boiler at 14.

    EXAMPLE 4

    [0076] This example serves to illustrate material stream calculations in a further embodiment of the invention shown in FIG. 3.

    [0077] The system of FIG. 3 corresponds to that of FIG. 1 but additionally comprises a classifier 15 wherein the finer particles from the bottom ash are entrained by an airflow and carried back to the boiler.

    [0078] A bottom ash mass balance, taking into account coarse ash, fine ash, and ilmenite was constructed for the system shown in FIG. 3.

    [0079] Coarse ash components (A) include large particles that are easily separated by the existing recirculation system and are not accumulated, fine ash components (As) include inert sand and small agglomerates of ash that can be accumulated by the existing recirculation system, the ilmenite (I) can also, of course, be accumulated by the existing recirculation system.

    [0080] For the purposes of this example, the boiler is a 75 MW municipal solid waste fired boiler with a classifier that operates at 95% separation efficiency for ilmenite and fine ash. The material streams of interest are denoted in FIG. 3. Another material stream, not included in the model, consists of the very fine particles that are carried out of the furnace by the flue gas and separated as fly ash in the flue gas treatment plant, e.g. a bag-house filter or an electrostatic precipitator. This material stream consists of very fine particles from the fuel, very fine particles of fresh bed material and very fine bed material particles formed by attrition in the furnace.

    [0081] C denotes the classifier 15, B the boiler 6, R the rotary sieve 10, and M the magnetic separator 12. The indexes e and r denotes exiting and returning respectively. The separation efficiencies of the classifier and rotary sieve are assumed to be equal for ilmenite and fine ash while the magnetic separator is described using two different efficiencies for ilmenite and fine ash (optimally 0% for ash). The separation efficiency is varying in relation to the inflow for all separators of the system: classifier, mechanical and magnet.

    [0082] The mass balances for ilmenite and fine ash are similar and therefore only that of ilmenite is described as follows, m.sub.i denotes the mass of ilmenite inside the boiler.

    [00001] I i = 225 .Math. .Math. kg h As i = 1000 .Math. .Math. kg h A i = 4000 .Math. .Math. kg h c = 0.95 T = 0 - 0.9 M , I = 0 - 0.9 M , As = 0 - 0.1 m tot = 25 .Math. .Math. ton dm i dt = I i + I C , r + I M , r - I B , e ( 1 ) I B , e = ( A i + I i + As i + I C , r + I M , r + As C , r + As M , r ) * m i m tot ( 2 ) I C , r = I B , e * C ( 3 ) I C , e = I B , e - I C , r ( 4 ) I R , r = I C , e * R ( 5 ) I R , e = I C , e - I R , r ( 6 ) I M , r = I R , r * M , I ( 7 ) I M , e = I R , r - I M , r ( 8 )

    [0083] Upon deriving a matching set of equations for the fine ash (As), the system is calculated to yield the fraction of ilmenite in the boiler and the average time that the ilmenite spends inside the system.

    [0084] For the base case (comparative example not according to the invention), the efficiencies of the rotary sieve and magnet are set to 0% while the case describing the system according to the invention uses efficiencies of 0.8, 0.8, and 0 for the rotary sieve, magnet on ilmenite and magnet on fine ash respectively.

    [0085] The calculated data describe the fraction of ilmenite in the boiler, the average residence time of ilmenite within the system (including the effects of recirculation), and the possible reduction in the amount of introduced ilmenite that maintains the ilmenite fraction of the base case. The deduced data is presented in Table 2.

    TABLE-US-00004 TABLE 2 derived data for the base case and for operation with the proposed system. Fraction Average resi- Possible of dence time reduction in ilmenite of ilmenite ilmenite in the in the feed [kg/h] Case bed [%] system [h] (new flow) Base case 15.8 17.5 Inventive 34.2 38.0 140 (85) system