Apparatus and method for screening a stream of particles

Abstract

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

Description

[0001] This invention relates to an apparatus for continuously screening particle streams, to using the apparatus for separating particulate ash streams from fluidized bed boilers, and to a method for operating fluidized bed boilers.

[0002] Fluidized bed combustion is a well-known technology in which fuel is suspended in a hot fluidized bed of solid particulate material (typically silica sand and / or fuel ash). Other bed materials are also possible. In this technology, fluidizing gas passes through the solid particulate bed material at a specific fluidization velocity. The bed material acts as a mass and heat carrier to facilitate rapid mass and heat transfer. At very low gas velocities, the bed remains stationary. Once the velocity of the fluidizing gas rises above the minimum fluidization velocity at which the force of the fluidizing gas balances the gravity acting on the particles, the solid bed material behaves in many ways like a fluid, and the bed is considered fluidized. In bubbled fluidized bed (BFB) boilers, the fluidizing gas passes through the bed material to form bubbles in the bed, thereby facilitating gas transport through the bed material and allowing for better control of combustion conditions (better temperature and mixing control) compared to grate combustion. In circulating fluidized bed (CFB) boilers, the fluidizing gas passes through the bed material at a fluidization velocity, where most of the particles are carried away by the fluidizing gas flow. The particles are then separated from the gas stream, for example by a cyclone separator, and are typically recirculated back into the furnace via a loopseal. An oxygen-containing gas, usually air or a mixture of air and recirculated flue gas, is typically used as the fluidizing gas (so-called primary oxygen-containing gas or primary air) and passes through the bed material from below or under the bed, thus serving as the oxygen source for combustion. A portion of the bed material fed to the burner exits the boiler with various ash streams, particularly with bottom ash. Removal of bottom ash, i.e., the ash at the bottom of the bed, is usually a continuous process aimed at removing alkali metals (Na, K) and coarse inorganic particles / lumps from the bed, as well as any agglomerates formed during boiler operation, and maintaining adequate pressure differential in the bed. In typical bed management cycles, the bed material lost with the various ash streams is replenished with fresh bed material.

[0003] According to existing technology, it is known to replace part or all of the silica sand bed material with ilmenite particles in CFB processes (H. Thunman et al., Fuel 113 (2013) 300-309). Ilmenite is a naturally occurring mineral mainly composed of iron-titanium oxide (FeTiO3) and can be repeatedly oxidized and reduced. Due to the reduction / oxidation properties of ilmenite, this material can be used as an oxygen carrier in fluidized bed combustion. Compared with inactive bed materials, such as 100 wt.% silica sand or fuel ash particles, the combustion process can be carried out at a lower air-fuel ratio when using a bed containing ilmenite particles.

[0004] The problem with this invention is to provide an apparatus and method that allows for improvements in bed management cycles.

[0005] The apparatus for continuously screening particle streams according to the present invention comprises:

[0006] a) A screw conveyor (1) is used to transport the particle stream to a separation device.

[0007] b) A sieve (7) for separating particle streams according to size.

[0008] The device for continuously screening particle streams is characterized by further comprising...

[0009] c) A pre-screen (3) comprising an elongated screen opening formed by fingers extending toward the end portion of the pre-screen, wherein the ends of the fingers are not connected in the end portion, the pre-screen (3) being located between the conveyor outlet and the screen.

[0010] d) Wherein, the pre-screen (3) surrounds the outer periphery of the screw of the conveyor in the axial end portion of the screw, and

[0011] e) The radial distance between the screw (1) of the conveyor and the pre-screen (3) is less than 10% of the screw diameter of the conveyor, preferably less than 5% of the screw diameter of the conveyor.

[0012] First, several terms will be explained in the context of this invention.

[0013] Screw conveyors are used to transport particle streams, typically those with particles ranging in size from a few μm to a few mm. A preferred example of such a particle stream is the ash stream from a fluidized bed boiler, as explained below. The invention is not limited thereto and can be used in cases of other particle streams. The term "particle stream" is intended to encompass streams of microparticles or other particulate matter. In the context of this invention, this particular stream may be contaminated by elongated objects, as will be explained below.

[0014] Screw conveyors are well known to those skilled in the art.

[0015] The term "sieve" as used is known to any person skilled in the art. The sieves used in this invention typically have a sieve aperture size adapted to the particle size of the particle stream, thereby separating the particle stream into coarse and fine portions.

[0016] The apparatus according to the invention further includes a pre-screen with elongated screen openings. The pre-screen is located between the conveyor outlet and the screen. The pre-screen being located between the two means that the particle stream from the screw conveyor outlet first passes through the pre-screen, and then a portion or some of the particle stream passes through the pre-screen before entering the screen.

[0017] The ratio of the length to the maximum width of the elongated sieve opening is 4 or greater, preferably 10 or greater. A common upper limit for this ratio is 100, and a preferred upper limit is 80, 60, 40 or 20.

[0018] This invention is based on the discovery that particulate streams (particularly ash streams from fluidized bed boilers) can be contaminated by elongated objects (particularly metallic objects such as filaments or threads). This is a particularly prominent problem for boilers burning waste, wood residue, or the like. Such objects tend to clog and block screens used to separate ash streams according to size. The pre-screening according to the invention allows such elongated objects to be separated from the particulate stream before it is fed to the screen. The elongated screen openings allow particulate material to fall through them, while elongated objects typically travel over the area of ​​the elongated openings and fall off the pre-screen at the distal end. Thus, elongated objects can be effectively separated from the particulate stream. Meanwhile, as with standard screens in the prior art, the elongated openings are generally not clogged by elongated objects.

[0019] The pre-screen surrounds a portion of the outer periphery of the conveyor screw at its axial end. This means the pre-screen is close to the end portion of the screw conveyor, causing the screw to transport all material that does not fall through the narrow opening toward the end of the pre-screen, thus facilitating the separation of filaments and threads, in particular, from the particulate material falling through the narrow opening. This close proximity also prevents clogging or material nesting on the pre-screen.

[0020] To achieve such close proximity, the radial distance between the conveyor screw and the pre-screen is less than 10% of the conveyor screw diameter, preferably less than 5% of the conveyor screw diameter.

[0021] The pre-screen includes an elongated opening formed by fingers extending toward the end portion of the pre-screen, wherein the ends of the fingers are not connected in this end portion. This structure effectively prevents clogging and material accumulation on the pre-screen because any material reaching the end portion of the pre-screen can easily fall off without any structural elements that could potentially obstruct the material in the transverse direction of transport. In particular, threads and filaments cannot be blocked or tangled at this end portion.

[0022] Fluidized bed boiler is a well-known term in the art. This invention can be used, in particular, for bubbling fluidized bed (BFB) boilers and circulating fluidized bed (CFB) boilers.

[0023] In a preferred embodiment, the pre-screening includes an elongated screen opening at an angle of -40° to 40° relative to the conveying direction of the screw conveyor. For the screw conveyor, the conveying direction corresponds to the axis of the screw. The acute angle of -40° to 40° is defined between the conveying direction and the longitudinal axis of the elongated opening. This angle relative to the conveying direction allows for effective separation of the particle stream from elongated objects with minimal or no clogging.

[0024] Preferably, the width of the elongated opening increases from the base to the tip of the finger. This helps prevent material from accumulating on the pre-screen or forming material nests on the pre-screen.

[0025] Preferably, the width is increased by 2 to 6 times, and more preferably by 3 to 5 times.

[0026] The features listed below are particularly preferred embodiments of the device according to the invention. Each of these features may be used alone or in combination with one or more of the other listed features:

[0027] a) The width of the elongated opening at the base is between 1 mm and 5 mm, preferably between 2 mm and 4 mm;

[0028] b) The width of the elongated opening at the end of the finger is between 4 mm and 20 mm, preferably between 8 mm and 16 mm, and more preferably between 10 mm and 14 mm;

[0029] c) The length of the finger from base to tip is between 100 mm and 500 mm, preferably between 100 mm and 400 mm, and more preferably between 150 mm and 250 mm;

[0030] d) The sieve aperture size is between 200 μm and 1000 μm, preferably between 300 μm and 800 μm.

[0031] In a preferred embodiment, the apparatus further includes a mechanical impact device for providing mechanical impact to the pre-screen.

[0032] Mechanical impact devices effectively prevent finer particles (especially bottom ash) from forming a layer of material on the pre-screen. The mechanical impact device will cause any layer to break down, with fine particles falling through the pre-screen to the mechanical screen, and coarser material passing through the pre-screen to continue into the waste container.

[0033] Preferably, the mechanical impact device is a hammer or a piston vibrator.

[0034] Preferably, the impact force and impact frequency of the mechanical impact device are controllable and can be set in the electronic control system of the entire device. The impact device or hammer may include, for example, a pneumatic or electric drive mechanism. During the continuous operation of the impact device, no ash blocking layer forms on the pre-screen.

[0035] Preferably, the mechanical impact device provides impact in the region at the base of the pre-screened fingers. More than one mechanical impact device may be used, optionally at different locations in the pre-screening process.

[0036] Another aspect of the invention is a method of using the previously disclosed apparatus to separate particulate ash streams from a fluidized bed boiler.

[0037] Another aspect of the present invention is a method for operating a fluidized bed boiler, comprising the following steps:

[0038] a) Conducting a fluidized bed combustion process;

[0039] b) Remove at least one ash stream from the fluidized bed boiler;

[0040] c) Separating the ash stream into at least two parts, wherein the separation includes a separation step using the previously disclosed apparatus;

[0041] d) The separated particulate portion is recycled back into the fluidized bed boiler bed.

[0042] The boiler can be, for example, a bubbling fluidized bed boiler (BFB) or a circulating fluidized bed boiler (CFB), preferably a CFB boiler.

[0043] The method of the present invention is particularly advantageous for boilers in which some or all of the standard silica sand bed material is replaced by ilmenite particles. This method allows the ilmenite particles from the ash stream to be recycled back into the fluidized bed, as will be explained in more detail below.

[0044] Therefore, in a preferred embodiment, the fluidized bed and the ash stream from the fluidized bed contain ilmenite particles, and the separated recycled particles are partially rich in ilmenite.

[0045] This method may include additional separation steps, as will be explained below. Preferably, the separation includes the use of a magnetic separator with a field strength of 2000 gauss or greater, preferably 4500 gauss or greater. The field strength of the magnetic separator is preferably determined on the surface of the conveying device for the bed material used to perform the magnetic separation.

[0046] During boiler operation, the proportion of ilmenite in the bed material can be maintained at 25 wt.% or more, preferably at 30 wt.% or more. In another embodiment of the invention, the preferred concentration of ilmenite in the bed material is between 10 wt.% and 95 wt.%, more preferably between 50 wt.% and 95 wt.%, and even more preferably between 75 wt.% and 95 wt.%.

[0047] Ilmenite particles can be conveniently separated from boiler ash using a three-stage separation process, which includes screening, subsequent magnetic separation, and a previously disclosed pre-screening. Even after extended use as bed material in fluidized bed boilers, ilmenite retains excellent oxygen-carrying capacity and reactivity in oxidizing carbon monoxide (CO) to carbon dioxide (CO2), a process known as "gas conversion," and also exhibits good mechanical strength. The wear rate of ilmenite particles decreases with extended residence time in the boiler, and their mechanical strength remains excellent even after prolonged use as bed material.

[0048] Given the excellent wear resistance and oxygen-carrying capacity of used ilmenite particles, they can be utilized by recycling the separated particles back to the boiler bed. This reduces the need to supply fresh ilmenite to the boiler, which in turn significantly reduces the overall consumption of natural ilmenite resources and makes the combustion process more environmentally friendly and economical. Furthermore, separating and recycling ilmenite from ash allows for control of the ilmenite concentration in the bed and simplifies operation. Moreover, this bed management cycle further enhances fuel flexibility by allowing the feed rate of fresh ilmenite to decouple from the rate of ash removal (especially bottom ash removal). Therefore, variations in ash content in the fuel become less significant because a higher bottom bed regeneration rate can be applied without losing ilmenite in the system.

[0049] The fresh ilmenite particles fed into the bed can be rock or sandy ilmenite.

[0050] Hard, massive ilmenite can be found in igneous sediments, such as those in Canada, Norway, and China. The TiO2 content in rock ilmenite is relatively low (typically 30-50 wt%), but its iron content is relatively high (typically 30-50 wt%). Rock ilmenite is mined and refined through crushing and impurity separation. This results in a lower sphericity than, for example, natural silica sand. The shape factor of Norwegian rock ilmenite (provided by Titania A / S) is approximately 0.7.

[0051] For example, ilmenite sands (less preferred) can be found in heavy mineral sand sediments in South Africa, Australia, North America, and Asia. Typically, sandy ilmenite originates from weathered rock sediments. Weathering reduces the iron content while increasing the TiO2 concentration. Due to the oxidation and dissolution of native iron, it is also known as altered ilmenite, and the TiO2 content can be as high as 90 wt.%. The shape factor of sandy ilmenite is typically in the range of 0.8-1, with an average factor of approximately 0.9.

[0052] Preferably, the maximum particle size of the fresh ilmenite particles is distributed in the range of 100 μm to 400 μm, more preferably in the range of 150 μm to 300 μm.

[0053] To determine the particle size distribution, sieving is performed using a suitable sequence of sieve aperture sizes. Sieve plates with the following aperture sizes can be used: 355 μm, 250 μm, 180 μm, 125 μm, and 90 μm, and a base plate can be used for the portion below 90 μm.

[0054] Preferably, at least one ash stream is selected from the group consisting of bottom ash stream and fly ash stream. Most preferably, at least one ash stream is a bottom ash stream. In advantageous embodiments, any combination of two or more ash streams is possible. Bottom ash is one of the main causes of bed material loss in fluidized bed boilers, and in a particularly preferred embodiment, at least one ash stream is a bottom ash stream. Fly ash refers to the portion of ash carried out of the fluidized bed by gas, and this portion flies out of the furnace along with the gas or gaseous ash compounds to condense into solid particles after exiting the furnace.

[0055] Preferably, the sieve for separating particle streams according to size includes mesh openings with a size of 200 μm to 1000 μm, preferably 300 μm to 800 μm, and more preferably 400 μm to 600 μm.

[0056] Most of the ilmenite in the bottom ash has a particle size of 500 μm or smaller, so the sieve provides a fine-sized fraction with a more uniform size distribution, which still contains most of the ilmenite particles. Magnetic separation in the second step can then be performed more efficiently.

[0057] Using a pre-screen that includes narrow openings for initial pre-screening helps protect the screen and magnetic separator from long, thin objects such as nails, wires, or threads that could clog the screen or damage the magnetic separator or its components.

[0058] The magnetic separator incorporates an electric field strength of 2000 Gauss or greater, preferably 4500 Gauss or greater, on the surface of the bed material conveying device. This has been found to effectively separate ilmenite from ash and other non-magnetic particles in a particle stream.

[0059] Preferably, the magnetic separator comprises a rare-earth roller (RER) or rare-earth drum (RED) magnet. The corresponding magnetic separators are known in the art and are available, for example, from Eriez Manufacturing Co. (www.eriez.com). The rare-earth roller magnetic separator is a high-intensity, high-gradient permanent magnet separator used to separate magnetic and weakly magnetic iron-containing particles from a dried product. The ash stream is transported on a belt running around a roller or drum comprising rare-earth permanent magnets. During transport around the roller, ilmenite remains attracted to the belt, while the non-magnetic particle portion detaches. Mechanical separator blades facilitate the separation of these two particle portions.

[0060] In one embodiment of the invention, the magnetic field is axial, i.e., parallel to the axis of rotation of the roller or drum. An axial magnetic field with magnets of a fixed orientation causes strongly magnetic materials to tumble as they pass from the North Pole to the South Pole, thereby releasing any entrained nonmagnetic or paramagnetic materials.

[0061] In another embodiment of the invention, the magnetic field is radial, i.e., radially oriented relative to the axis of rotation. Generally, radial orientation has the advantage of providing higher recovery rates for all weakly magnetic materials, but this can lead to a decrease in purity due to entrained non-magnetic materials.

[0062] Two-stage magnetic separation can also be used, where the first step uses axial orientation to help release entrained nonmagnetic material, and the second step uses radial orientation to improve recovery rate. Using radial orientation in the first step and axial orientation in the second step is also within the scope of this invention.

[0063] Preferably, the average residence time of ilmenite particles in the fluidized bed boiler is at least 100 hours, more preferably at least 200 hours, and even more preferably at least 300 hours. Even after the fluidized bed boiler has been running continuously for about 300 hours, the ilmenite particles still exhibit very good oxygen carrying capacity, gas conversion, and mechanical strength, thus clearly demonstrating that a longer residence time can be achieved.

[0064] In a preferred embodiment, the average residence time of the ilmenite particles can be less than 600 h, more preferably less than 500 h, more preferably less than 400 h, and even more preferably less than 350 h. All combinations of the lower and upper limits of the average residence time are possible within the scope of this invention and are explicitly disclosed herein.

[0065] Preferably, the separation efficiency of the method for using ilmenite bed feed is at least 0.5 by mass, and more preferably at least 0.7 by mass. This means that at least 50 or 70 wt.% of ilmenite contained in the ash stream can be separated from the ash and recycled back to the boiler. In the context of this invention, the term wt.% is used as a synonym for mass%.

[0066] Recycling capacity and separation efficiency are also affected by ash flow temperature, with a trade-off between the two. Higher temperatures reduce the efficiency of magnetic separation and necessitate the use of more expensive heat-resistant materials in the system used to implement the method of this invention. The negative impacts of high temperatures on separation efficiency and material requirements can be eliminated by taking measures to cool the ash flow. The system can also be equipped with temperature sensors and ash flow separators that can redirect the airflow and bypass the separation system in the event of temporary high temperatures.

[0067] Embodiments of the invention will now be shown by way of example with reference to the accompanying drawings.

[0068] As shown below:

[0069] Figure 1 Metal wire stuck in the sieve of existing technology devices;

[0070] Figure 2 : A schematic diagram of the device according to the present invention;

[0071] Figure 3 Top view of the end portion of the screw of the partition wall and conveyor;

[0072] Figure 4 Different views of the pre-screening according to the present invention;

[0073] Figure 5 Three different pre-screening geometries: straight, tilted to the right, and tilted to the left.

[0074] Figure 6 : A schematic diagram of the test setup for different implementation methods of pre-screening;

[0075] Figure 7 Mechanical impact device in the form of a piston vibrator.

[0076] First, the problems existing in the ash and recycling system for fluidized bed boilers in the prior art are explained.

[0077] The factory's boiler P14 is located in East Gyotland. County. The boiler is operated by the international utility company E.ON. Boiler P14 was built in 2002 by Kvaerner. It is a circulating fluidized bed boiler with a nominal thermal capacity of 75MW, typically burning a mixture of municipal solid waste and light industrial waste. The boiler operates year-round, and a portion of the steam produced is usually used for power generation. The furnace cross-section is 2.5m x 8.4m at the height of the fluidizing nozzles, extending upwards to 3.9m x 8.4m. The furnace height from the fluidizing grid to the top is approximately 23m. The boiler has two cyclone separators and two annular seals with in-bed superheaters. After the cyclone separators, the flue gas passes through an empty channel, a convection heat exchanger, and multiple flue gas cleaning units before being released into the atmosphere through the chimney.

[0078] Boiler P14 utilizes a fluidized bed containing ilmenite and an improved loop. TM (Improbed Loop TM The system is operating as illustrated in WO 2018 / 188786 A1.

[0079] Improved loop TM This increases the concentration of ilmenite in the boiler, thereby improving oxygen distribution, which in turn improves boiler efficiency. This can be used to increase fuel throughput, which increases gate fee revenue and the production of steam, electricity and heat, thus improving process economy.

[0080] However, the improved loop TM The availability of the mechanical screen is reduced due to mechanical clogging caused by metal objects (such as wires and threads).

[0081] Figure 1 This illustrates the problem. Metal wires enter the screen and become stuck. These wires form nests that eventually clog the screen inlets and orifices. Consequently, the recovery of the finer particles from the bottom ash stops, and ilmenite is lost through the ash discharge.

[0082] The filaments originate from waste fuel and have been managed to pass through various magnets installed in the fuel preparation and transportation system. This is because materials such as copper, aluminum, and stainless steel are non-magnetic. This is a common situation in waste incineration boilers.

[0083] Figure 2 The device according to the invention is shown schematically.

[0084] A screw conveyor 1 transports bottom ash from the boiler toward an end section 2, in which the lower outer periphery of the screw is circumferentially surrounded by a pre-screen 3. The particulate material falling through the pre-screen 3 passes through a front section 4 of a chute, which includes a partition wall 5 separating the front section 4 and the rear section 6. The particulate material then falls onto a screen 7, as in prior art systems, to mechanically separate coarse and fine particles. The fine particles are then further separated in a magnetic separator 8, as disclosed in WO 2018 / 188786A1.

[0085] Wires and other slender objects move on the pre-screen 3 via the screw of the conveyor, and at the end of the pre-screen, the wires and other slender objects fall into the rear part 6 of the chute leading to the ash elevator and are discarded.

[0086] The location and main forms of pre-screening are as follows: Figure 3 As shown. This is a view looking down into the inclined groove from inside the classifier screw 2, where the partition wall 5 leads to the improved loop. TM The existing screen path 4 of the system is split to the right, and the path 6 to the ash elevator is split to the left. The pre-screen 3 has a geometry of multiple fingers oriented in the axial direction of the screw. The pre-screen is installed directly below the tail of the screw. Small particles (i.e., the receiving section) should fall between the finger-shaped pre-screens into the modified loop. TM In the system, slender objects and wires (i.e., waste) are moved by a screw on a pre-screen to a chute leading to the ash elevator. Therefore, the pre-screen is continuously cleaned by the screw. Consequently, nests of filaments or other types of blockages caused by ash will not occur.

[0087] Figure 4 The pre-screening is shown from different angles. The fingers gradually taper, and the width of the elongated opening increases from 3 mm at the bottom of the fingers to 12 mm at the top or end of the fingers.

[0088] Three different pre-screening designs were tested and distinguished by different finger orientations; when viewed in the axial direction of the grading screw, the three different pre-screening designs pointed to the right, directly forward, and to the left. Figure 5 ).

[0089] In all three alternatives, the fingers taper gradually in the flow direction to increase the gap between them. This reduces the risk of clogging, such as due to nesting formed by the wires. Furthermore, the screen is designed to match the screw diameter and be located directly below the screw end, allowing the screw to move the material continuously across the pre-screen and preventing clogging. The gap between the screw and the pre-screen should be less than 5% of the conveyor screw diameter. The pre-screen is designed for easy replacement.

[0090] The experimental setup for testing the pre-screening effectiveness according to the present invention is as follows: Figure 6 As shown.

[0091] Screw 1 corresponds to the bottom ash screw in boiler P14. During testing, this screw operates at a similar rpm (speed) as the screw in boiler P14. The screw motor is controlled by a frequency converter. The inner diameter of the casing of screw 1 is 2319 mm, and the screw diameter is 2000 mm.

[0092] Observation boxes 2, made of plexiglass on both sides, are installed at the end of the screw. The screw is arranged at an upward tilt of 12°, similar to the tilt of the screw in boiler P14. The pre-screen 3 is installed below the end of the screw in the observation box. In a series of tests, the last two tests were conducted with the screw in a horizontal position.

[0093] Two plastic boxes are placed below the observation box. These two plastic boxes are a receiving box and a waste box (4 and 6) with a steel plate (5) in the middle. The plate is installed perpendicular to the screw 3 cm below it, so that the edge of the plate is aligned with the edge of the pre-screen. This arrangement simulates the ash trough in the boiler P14 system. The idea is that fine ash should fall into the receiving box between the screen fingers, and coarse particles (including metal flakes, stones, gravel, etc.) should be pushed along the screen by the screw into the waste box.

[0094] Used from A large sample of 75 kg of bottom ash from the P14 waste incineration boiler was used for testing. Additionally, an extra bucket filled with 5 kg of metal scrap (metal wire, steel sheets, copper wire, etc.) was used for testing.

[0095] A sieve with a 0.71 mm mesh was used to separate a portion of the base ash sample into two parts: a finer portion that passed through the sieve and a coarser portion that remained on the sieve. The masses of these parts were measured: 11,400 g of the fine portion and 15,200 g of the coarse portion. The two portions were then mixed again, and approximately 300 g of metal scrap (mainly metal wire) from an additional bucket was added to the mixture.

[0096] Table 1 shows the operating parameters and settings for the eight tests. The ash was reused in all tests, and the fine and coarse portions were mixed and metal scrap was added to the mixture before each test.

[0097] In tests 1 through 6, the pre-screen to be tested was installed approximately 1 cm below the screw thread. In tests 7 and 8, the pre-screen was moved downwards to a distance of 15-20 cm from the screw and bent downwards at an angle of 30°. The idea was to test the function of the pre-screen in such a way that the material falls onto and along the pre-screen due to gravity, rather than being pushed upwards by the screw.

[0098] Table 1. Operating parameters and settings in the 8 experiments

[0099]

[0100] The experiment was initiated by starting the screw and adjusting the speed to the selected value. Ash was continuously fed from the bucket to the screw via a chute for further transport along the screw on a pre-screen.

[0101] The operation is visually observed and recorded using a camera. The quality of both fine and coarse parts is measured.

[0102] Experiments 1 through 4 used the same operating conditions and settings, the difference being the use of three different pre-screening methods.

[0103] A fundamental idea behind prescreening is that it should be self-cleaning. By positioning the prescreen close to the screw and making the fingers of the prescreen gradually taper, the screw should remove any fastened wires or nests from the prescreen and transport the wires or nests to the end of the screw. Experiment 5 aimed to test this idea by simulating the situation where wires get stuck in the prescreen. Nests of wires were handmade and secured in the prescreen.

[0104] Experiment 6 was similar to Experiment 4, except that the screw speed was increased by 45%.

[0105] Experiments 7 and 8 were conducted to test whether the self-cleaning function observed in experiments 1 through 6 was necessary for screening performance, or whether similar good results could be obtained when the mixture of ash and metal waste was accelerated down onto the pre-screen by gravity.

[0106] Removing waste and filaments is one of the key objectives of installing pre-screening. Furthermore, it is important to minimize the loss of fine material that may contain ilmenite. This latter aspect is assessed by measuring the mass of fine material in the waste bin after each test and comparing it to the total mass of fine material used in the test. The loss η is defined by Equation 1, where m 细，废弃 It is the mass of the waste fine materials, m 细，接受 It refers to the quality of the fine materials accepted.

[0107]

[0108] In tests 1 through 6, the separation of metal scrap and metal wire was excellent. As expected, the metal scrap and wire were ultimately discarded.

[0109] The tests also showed that pre-screening separated 15% of the other coarse ash transported to the boiler P14 environment, which means that the flow of existing screens to the ash recycling system will be significantly reduced, thereby reducing wear and maintenance costs.

[0110] In Experiment 5, a large nest of metal wires was intentionally fixed to the pre-screen. After the screw had been running for one minute, the nest was detached by the screw and transported to the waste bin.

[0111] The pre-screening arrangements in Tests 7 and 8 were ineffective. Performance was acceptable with only gravel and sand-type ash, but problems arose once metal wires entered the ash stream. These wires became stuck in the pre-screen and clogged the ash stream. These wires formed blockages that impeded ash particles, even finer ones. The wires reinforced the blockages at the points of clogging. Eventually, the entire pre-screen surface was covered in blockages. Screening function was lost, and all material flowed into the waste bin.

[0112] On the other hand, other tests (1-6) were successful and demonstrated that the self-cleaning pre-screen design has great potential in solving the clogging problem in the boiler P14 system.

[0113] Table 2 includes the received mass and discarded mass measured in each of the eight tests, as well as the loss η of the discarded fine particles as defined by Equation 1.

[0114] The table shows that the best results were obtained using a right-facing pre-screen in Experiment 3, where only 1% of the fine particles were lost as waste. Similar good results were obtained in Experiments 2 (right-facing pre-screen) and 4 (left-facing pre-screen). Therefore, right-facing and left-facing pre-screens appear to be equally good. However, straight pre-screening resulted in a 12% loss of fine particles, a significantly worse result.

[0115] Table 2: Results of the eight trials

[0116]

[0117] Na: No data available.

[0118] Visual observations during the experiment showed that when using a right-hand pre-screen, more material accumulated on the right side of the screw. This is because the material follows the screw thread, which is a reasonable explanation for why a left-hand pre-screen results in a more uniform material distribution.

[0119] Experiment 6 shows that even a 45% increase in screw speed does not significantly affect the loss of fine parts.

[0120] Figure 7 A mechanical impact device in the form of a piston vibrator (9) is shown. The piston vibrator includes a body (10) and a piston head (14). The piston vibrator (9) is mounted to a bottom screw via a mounting member (11). Figure 7On the housing (not shown). A rubber pad (12) surrounds the body (10) of the piston vibrator (9) to minimize vibration caused by the housing of the bottom gray screw. The piston head (14) extends through a hole in the housing (15) and strikes a pre-screen (not shown) inside the housing. The piston vibrator operates continuously. The pre-screen can be tilted downwards in the direction of the end portion.

[0121] The body (10) of the piston vibrator (9) can be circular or square. The body can be made of, for example, cast iron or aluminum. The piston vibrator (9) is pneumatically operated. The body (10) includes a mounting for an air supply unit (13). The pressure of the supplied air is in the range of 1.5 bar to 2 bar.

[0122] The piston head (9) vibrates at a frequency of 1860 Hz under a pressure of 2 bar and at a frequency of 2220 Hz under a pressure of 6 bar. The piston head is impacted with a force of 50 N to 200 N, preferably 80 N to 150 N. The maximum sound pressure level is 80 dB(A), which is acceptable in a boiler room.

[0123] When the piston head (9) strikes the pre-screen at a high frequency, it causes the pre-screen to vibrate. This vibration prevents finer particles, such as bottom ash, from forming a layer of material on the pre-screen.

Claims

1. An apparatus for continuous sieving of a flow of particles, comprising: a) a screw conveyor (1) for conveying the flow of particles to a separating apparatus, b) a sieve (7) for separating the flow of particles according to size, characterized in that the apparatus for continuous sieving of a flow of particles further comprises c) a pre-sieve (3) comprising elongated sieve openings formed by fingers extending towards end portions of the pre-sieve, wherein the tips of the fingers are unconnected in the end portions, the pre-sieve (3) being located between the outlet of the conveyor and the sieve, d) wherein the pre-sieve (3) encloses a part of the circumference of the screw of the conveyor in an axial end portion of the screw of the conveyor, wherein, when the flow of particles is conveyed to the pre-sieve (3), particle material in the flow of particles falls through the pre-sieve (3), threads and other elongated objects in the flow of particles are moved over the pre-sieve (3) by the screw of the conveyor, falling off at the end portions of the pre-sieve (3); and e) the radial distance between the screw of the conveyor and the pre-sieve (3) is less than 10% of the diameter of the screw of the conveyor.

2. The apparatus of claim 1, wherein, the radial distance between the screw of the conveyor and the pre-sieve (3) is less than 5% of the diameter of the screw of the conveyor.

3. The apparatus of claim 1, wherein, the elongated sieve openings comprised by the pre-sieve (3) have an angle of -40° to 40° to the conveying direction of the screw conveyor.

4. The apparatus of any one of claims 1-3, wherein, the width of the elongated sieve openings increases from the base to the tip of the fingers.

5. The apparatus of claim 4, wherein, the width increases by a factor of 2 to 6.

6. The apparatus of claim 5, wherein, the width increases by a factor of 3 to 5.

7. The apparatus according to claim 4, comprising at least one of: a) the width of the elongated sieve openings at the base is between 1 mm and 5 mm; b) the width of the elongated sieve openings at the tip of the fingers is between 4 mm and 20 mm; c) the length of the fingers from the base to the tip is between 100 mm and 500 mm; d) the mesh size of the sieve (7) is between 200 pm and 1000 pm.

8. The apparatus according to claim 7, comprising at least one of: a) the width of the elongated sieve openings at the base is between 2 mm and 4 mm; b) the width of the elongated sieve openings at the tip of the fingers is between 8 mm and 16 mm; c) the length of the fingers from the base to the tip is between 100 mm and 400 mm; d) the mesh size of the sieve (7) is between 300 pm and 800 pm.

9. The apparatus according to claim 8, comprising at least one of: b) the width of the elongated sieve openings at the tip of the fingers is between 10 mm and 14 mm; c) the length of the fingers from the base to the tip is between 150 mm and 250 mm.

10. A method of use of the apparatus according to any one of claims 1 to 9 for separating a flow of particulate ash from a fluidized bed boiler.

11. 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 comprises a separation step using the apparatus according to any one of claims 1 to 9; d) recycling the separated particulate fraction into the bed of the fluidized bed boiler.

12. The method of claim 11, wherein, The boiler is a circulating fluidized bed boiler (CFB) or a bubbling fluidized bed boiler (BFB).

13. The method according to claim 11 or 12, characterized in that, The fluidized bed and the ash stream from the fluidized bed comprise ilmenite particles, and the separated particulate fraction is enriched in ilmenite.

14. The method of claim 13, wherein, The separation comprises a step using a magnetic separator (12) comprising a field strength of greater than or equal to 2000 Gauss.

15. The method of claim 14, wherein, The separation comprises a step using a magnetic separator (12) comprising a field strength of greater than or equal to 4500 Gauss.

16. The method of claim 13, wherein, The proportion of ilmenite in the bed material is greater than or equal to 25 wt.%.

17. The method of claim 16, wherein, The proportion of ilmenite in the bed material is greater than or equal to 30 wt.%. The proportion of ilmenite in the bed material is greater than or equal to 25 wt.%. The proportion of ilmenite in the bed material is greater than or equal to 30 wt.%. The proportion of ilmenite in the bed material is greater than or equal to 25 wt.%. The proportion of ilmenite in the bed material is greater than or equal to 30 wt.%. The proportion of ilmenite in the bed material is greater than or equal to 25 wt.%. The proportion of ilmenite in the bed material is greater than or equal to 30 wt.%. The proportion of ilmenite in the bed material is greater than or equal to 25 wt.%. The proportion of ilmenite in the bed material is greater than or equal to 30 wt.%. The proportion of ilmenite in the bed material is greater than or equal to 25 wt.%. The proportion of ilmenite in the bed material is greater than or equal to 30 wt.%. The proportion of ilmenite in the bed material is greater than or equal to 25 wt.%. The proportion of ilmenite in the bed material is greater than or equal to 30 wt.%. The proportion of ilmenite in the bed material is greater than or equal to 25 wt.%. The proportion of ilmenite in the bed material is greater than or equal to 30 wt.%. The proportion of ilmenite in the bed material is greater than or equal to 25 wt.%. The proportion of ilmenite in the bed material is greater than or equal

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