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Particulate flow characteristics in a novel moving granular bed
Accepted Manuscript Particulate flow characteristics in a novel moving granular bed
Shaowu Yin, Ying He, Li Wang, Chuanping Liu, Lige Tong, Yulong Ding PII: DOI: Reference:
S0032-5910(18)30745-9 doi:10.1016/j.powtec.2018.09.018 PTEC 13692
To appear in:
Powder Technology
Received date: Revised date: Accepted date:
25 June 2018 27 August 2018 6 September 2018
Please cite this article as: Shaowu Yin, Ying He, Li Wang, Chuanping Liu, Lige Tong, Yulong Ding , Particulate flow characteristics in a novel moving granular bed. Ptec (2018), doi:10.1016/j.powtec.2018.09.018
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ACCEPTED MANUSCRIPT Particulate flow characteristics in a novel moving granular bed Shaowu Yina,b,*,1, Ying Hea,1, Li Wanga,b, Chuanping Liua,b, Lige Tonga,b, Yulong Dingc a
School of Energy and Environmental Engineering, University of Science and Technology Beijing, Beijing 100083,
Beijing Key Laboratory of Energy Saving and Emission Reduction in Metallurgical Industry, University of Science
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b
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PR China;
c
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and Technology Beijing, Beijing 100083, PR China;
College of Chemical Engineering, University of Birmingham, Birmingham B15 2TT, UK)
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Corresponding author at: School of Energy and Environmental Engineering, University of Science and Technology Beijing, Beijing 100083, PR China. Tel.: +86-010-62332741; Fax: +86-010-62332741.
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E-mail address: [email protected] (S. W. Yin).
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Abstract In this work, we studied the flow characteristics of particulates in a novel moving granular bed by using experiments and numerical simulations. The new moving granular bed was equipped with one adjustable baffle and two slopes to adjust filter thickness and increase the gas-particle contacting
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surface. Doing so overcomes the poor adaptability and low collection rate of fine dust during the
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filtration, and improved the filtering performance of filter devices. In the experiment, the movement
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of particulates in a moving bed was recorded using a camera, and the effects of the device structure and the filter moving speed on the flow of filter particles were examined. Combined with Discrete
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Element Method (DEM) to simulate the velocity variation of particles in the granular bed, the
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mechanism of particulate flow was analyzed. The inclined surface was found to affect the flow of filter particles, and a small angle of inclination was not conducive to the particle flow. Therefore, a
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layer of dead particles was easily formed. The dead zone particle layers on the left slope could
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protect the insulation layer and the heat insulation layer outside the particle bed. Increasing the moving speed of the filter increased the velocity difference between the particles in the wall surface
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and the middle part. The fluidity of particles also increased to make the particle layer of dead area
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become thinner. The trajectory of a single particle was greatly affected by the overall moving speed, and the single particle tended to move towards the faster direction in the same plane with the increase of the filter moving speed. Keywords: particulate flow, moving granular bed, discrete element simulation, bulk material transport
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1 Introduction
Particulate matter is widely found in the natural world and belongs to the category of soft condensed matter physics. It is closely related to people's daily lives and production [1-2]. Granular
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materials are widely and often used as raw materials or catalysts in chemical metallurgy and material
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production industries. Particulate matter has extremely rich physical connotations, such as discrete,
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nonlinear response, high dissipation, and non-uniform internal force chains due to different atoms or molecules [3-6]. The production, processing, storage, and transportation of particulate matter
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each year consume about 10% of the earth’s energy [7]. Therefore, an in-depth study of particulate
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matter is conducive to energy conservation and economic development. Granular bed that has particulate matter as medium is considered as one of the most promising
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dust removal technologies. It has the advantages of high temperature resistance, corrosion resistance,
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low running resistance and high dust removal efficiency [8]. It is suitable for industrial boilers and other industrial furnaces. Granular bed mainly includes the fixed bed which needs the back blowing
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regeneration process, and the moving bed which can be continuously operated [9]. Relative to the fixed bed, the moving granular bed dedusting system has good performance in dedusting efficiency
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and pressure drop. It can also achieve the integration of filtration and dedusting, which is easy to be large-scaled and suitable for dust removal of high temperature and high-pressure gas [10]. Domestic and foreign scholars and technical personnel have conducted in-depth researches and discussion on the particle layer filtering theory, optimization of filtration parameter and equipment structure [11]. The moving granular bed was found to not have the ability to adjust the filter thickness due to its fixed size during operation resulting in the poor adaptability, and the filtration
ACCEPTED MANUSCRIPT efficiency of fine dust was not as good as the fixed bed [12]. And due to the high temperature environment, the protective material outside the granular bed was easily damaged and needed to be replaced periodically. These factors affected the filtration and operating performance of the moving granular bed. Combining the advantages and disadvantages of moving bed and fixed bed [13], the
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problems and improve the filtration performance of granular layer.
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author proposes a new type of granular bed with adjustable filter thickness, which can eliminate the
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Particulate flow means that the particulate material resembles the fluid motion state when external forces and internal stress conditions change. Nedderman et al. [14] proposed that the movement of
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particles in the moving model was entirely caused by gravity, and the voids generated by the particle
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flow were immediately supplemented by the adjacent particles. Many scholars extended it in later studies. Among them, Waston et al. [15] emphasized that the finite-element analysis method could
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be used to calculate the velocity field of solid particles with stable flow. The aforementioned study
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provided a new method for the simulation of subsequent particle velocity field. Due to the complexity and particularity of the particles themselves, the particle system would exhibit different
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moving behaviors under the action of the outside world. Zhang et al. [16] studied the effect of
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particle characteristics on the flow law using Discrete Element Method (DEM). The angle of repose was found to be related to the friction between particles and the fluidity of particles, which had great influence on the size and shape of the dead zone in the moving bed. In order to study the mechanism of particle movement in the moving bed further, this paper uses 3–5 mm hollow corundum balls as the filter material, and uses experimental and DEM simulation methods to study the effect of the new granular bed structure and the moving speed of the filter material on the particle flow characteristics. In addition, the experimental data was processed using Image-Pro to obtain the
ACCEPTED MANUSCRIPT velocity distribution of the particles under various conditions. The result was compared with the DEM simulation results.
2 Experimental apparatus and method
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2.1 The novel moving granular bed
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The moving granular bed with adjustable filter thickness absorbs the advantages of fixed bed and
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moving granular bed. It can also adjust the thickness of the particle layer and increase the gas-
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particle contacting surface. The schematic diagram of its structural design is shown in Fig. 1. Based on the granule inlet and the granule outlet, a longitudinal filter area and two diagonal filter areas
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were sequentially distributed, and the longitudinal filter area was the main dust removal zone.
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During the operation of the granular bed, the flue gas passed through the flue gas inlet, the diagonal
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filter area 1, the longitudinal filter area, the diagonal filter area 2, and finally exited through the flue gas outlet. The feed channel and the discharge channel had a cross-sectional size of 150 mm x 150
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mm square, the cross-sectional size of the particle filter layer in the longitudinal filter area was 200 mm x 200 mm square, and the flue gas inlet and the flue gas outlet had a cross-sectional size of 200
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mm x 200 mm square. The whole device was made of organic glass, and the filter particles are 3-5 mm diameter corundum balls. The adjustable baffle was located between the granule inlet and the flue gas outlet. Changing the height of the baffle adjusts the thickness of the granular layer on the longitudinal filter area to adapt to different filtration environments. The slope of the diagonal filter area was determined by the angle of repose of the selected filter material. The angle of repose of the 3-5 mm corundum ball was 27°. Therefore, the angle of inclination is selected to be 30°, which is beneficial to the particle flow and
ACCEPTED MANUSCRIPT is less likely to form a blockage. The inclined surface formation increases the contact area between the flue gas and the filter particles. This formation is favorable for the collection of fine dust. The discharge control mechanism adopts a conveyor belt, and the speed of the particles can be changed by adjusting the rotation speed of the conveyor belt.
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From the filtration process of the granular bed, the filter device can be seen to have the following
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characteristics: (1) the thickness of the granular layer can be adjusted and the application range of
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the moving granular bed is expanded; (2) the use of the diagonal filter area can increase the contact area between the flue gas and the filter particles, avoiding the difficulty of trapping fine dust in the
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filtration process of the moving granular bed; (3) the moving speed of particles can be adjusted to
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eliminate the fatigue damage of the filter medium and ensure the stability of the filtration
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2.2 Research method
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performance of the granular layer.
The experiments were conducted to observe the entire bed and different areas of the granular bed
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using a high-definition camera by changing the moving speed of the filter particles. To facilitate the tracking of particle trajectories, a portion of the white corundum balls were stained to obtain red
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corundum balls, which were mixed into the granular bed. In the study of the flow of particles in different regions, 12 shots were taken every 10 seconds for each group, and the total duration was two minutes. The obtained photos were imported to Image-Pro and converted into a photo album, and then the TRACK function was used to trace the red corundum balls to obtain the particle velocity distributions. The simulation uses DEM, which is a new numerical method for analyzing and solving dynamic
ACCEPTED MANUSCRIPT problems of complex discrete systems. It establishes a parametric model of the solid particle system to simulate and analyze the behavior of particles, providing a platform for solving many complex problems involving particles, structures, fluids, electromagnetics, and their coupling [17]. The particle filling process was simulated based on the discrete element method (DEM). During the
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particle flow, elastic collisions occur between the particles and the walls and particles [18]. The
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collision process between particles adopts the Hertz-Mindlin non-sliding contact model [19]. The
is expressed as follows:
(1)
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∑ 𝐹 = 𝑚 𝑖 𝑎𝑖
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calculation time step was 5 x 10-7 s. The motion equation of particle I during the motion of the filter
(2)
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∑ 𝑀 = 𝐼𝑖 𝜃𝑖
where 𝑎𝑖 is the acceleration of particle I, 𝜃𝑖 is the angular acceleration of particle I, 𝑚𝑖 is the
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mass of particle I, and 𝐼𝑖 is the moment of inertia of particle I. Using the center difference method
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to perform numerical integration to the expression, the update rate expressed by the middle point of
(𝑎𝑖 )𝑁+1 = (𝑎𝑖 )𝑁−1 + [∑ 𝐹 ⁄𝑚𝑖 ]𝑁 ∆𝑡
(3)
(𝜃𝑖 )𝑁+1 = (𝜃𝑖 )𝑁−1 + [∑ 𝑀⁄𝐼𝑖 ]𝑁 ∆𝑡
(4)
2
2
2
2
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the two iteration time steps was obtained as follows:
where ∆𝑡 is the time step, and N corresponds to time t. Integrating equations (3) and (4), we get the equation about displacement as follows: (𝑎𝑖 )𝑁+1 = (𝑎𝑖 )𝑁 + (𝑎𝑖 )𝑁+1 ∆𝑡
(5)
(𝜃𝑖 )𝑁+1 = (𝜃𝑖 )𝑁 + (𝜃𝑖 )𝑁+1 ∆𝑡
(6)
2
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The new displacement value of the particle can be obtained, and the new force can be calculated by substituting the new displacement value into the relationship. Thus, the cycle was repeated to
ACCEPTED MANUSCRIPT determine the tracking of particle movement at any time. The model was imported into EDEM using Hertz-Mindlin no-slip contact model to track the motion of each particle. The detailed motion process and the motion properties of the particle group
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were recorded. The velocity distribution of the particles in the bed was obtained after the treatment.
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3 Results and discussion
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The moving speed of the filter particle is in the range of 0.01-0.5 m/s in the industrial moving
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bed [20]. After the calculation, the moving speeds of the filter granular in the experiment were 200 g/min, 400 g/min, and 600 g/min.
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3.1 Flow characteristics of particles in the moving granular bed
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The experimental device adopts the designed moving granular bed, and is filled with the hollow
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corundum balls having a particle size of 3-5 mm, a bulk density of ρ=1950 kg/m3, a void ratio of ε = 0.502, and an angle of repose of φ = 27°. Corundum ball has high temperature resistance, corrosion
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resistance and non-breaking properties, and is suitable for filtration of high temperature flue gas.
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Some of the hollow corundum balls were dyed red and used as tracer particles to observe the flow characteristics of the filter media. In the moving granular bed, the adjustment baffle was located in the lower position, the thickness of the particle layer was 30 cm, and the three groups of particles were selected to move at 200 g/min, 400 g/min, and 600 g/min. A high-definition camera was used to observe and record the changes in the position of the filter media in the granular bed. Throughout the experiments, we found that the change in particle moving speed leads to changes in the dead zone in the granular bed with the movement of the filter particles and the distribution of
ACCEPTED MANUSCRIPT particle velocity in the same section. When the particle moving speed was 200 g/min, the flow process of particles in the moving bed is shown in Fig. 2. Comparing different particle flow processes, the filter particle moving near the right wall at the longitudinal filter area can be seen to be moving at a higher speed in the same plane. This
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phenomenon is caused by the large flow speed at the angle between the longitudinal filter area and
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the flue gas inlet. In the lower part of the moving bed, the flow velocity of the filter particle between
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the two slopes located on the upper part of the granule outlet channel was greater than that on the two walls. In the two slopes, the dead particle zones were formed due to the small velocity of the
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filter particles, and the distribution of the dead particle zone at the right slope was wider than that
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of the left side. The tilt angle of the right slope was 45°, and the tilt angle of the left slope was 60°. Therefore, the filter particles flow more easily on the left slope. The granule outlet was too small,
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so the total amount of filter particle flowing out at each time had a certain limit. Therefore, the filter
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particle with a slower flow rate at the inclined plane was more likely to form a layer of dead zone. The dead zone particle layer near the left side wall collected few dust in the dust filtration process,
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and basically did not need to be replaced. And its presence could protect the thermal insulation layer
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and the protective layer outside the device to avoid wear and tear. Therefore, the dead zone particle layer on the left side wall was an effective particle layer. While the dead zone particle layer near the right side wall was the main part of the flue gas inlet passage, and this part of the particle layer was the first to contact with the dust and captured the most dust. However, it formed a dead zone, which caused the particles to not flow, and was not conducive to replacement, which may lead to a decrease in the filtration efficiency of the granular bed. Therefore, this part of the dead zone particle layer was not conducive to improving the filtration performance of the granular bed.
ACCEPTED MANUSCRIPT The particle distribution changes when the particle moving speed changes. When the particle moving speed was 400 g/min, the flow process of the particles in the moving bed is as shown in Fig. 3. When the particle moving speed was 600 g/min, the flow process of the particles in the moving bed is as shown in Fig. 4.
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Compared with the flow process at different particle moving speeds, the velocity difference
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between the particles in the middle part and near the right side wall of the longitudinal filter area
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was found to increase as the particle moving speed increases. In the lower part of the moving bed, the granular layer of dead zone at the left slope was thinned as the increase of particle moving speed,
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and the pit between the dead zone at the right slope and the active granular layer on its left side
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became large. The main reason for this phenomenon is that the total amount of filter particle flowing out of the granular bed at each time increased with the increase of particle moving speed, the filter
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particles in contact with the active granular layer in the dead zone were changed into active filter
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particles, and the dead zone granular layer was thinned. The increase of the flow rate of the particles in the active area caused the particles in the upper side to not be supplemented, so that pits are
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formed between the active area and the dead zone at the right slope.
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3.2 Movement of particles in the moving granular bed 3.2.1 Effect of bed structure on the particle flow
To study the particle velocity distribution, the flow states of particles in four different parts were recorded using a high-definition camera according to the three kinds of particle moving speeds mentioned above. The four parts were the longitudinal filter channel, the end of longitudinal filter channel, the junction between the longitudinal filter channel and the diagonal filter channel, and the
ACCEPTED MANUSCRIPT middle part between the two slopes. Twelve shots were taken every 10 seconds for each group, and the total duration was two minutes. To track easily, seven to eight particles selected on the same section after the shots were imported into the software Image-Pro, and then the average speed of each particle was calculated to get the law of particle movement. When the particle moving speed
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was 200 g/min, three cross-sections were selected respectively at the longitudinal filter channel and
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the end of it to analyze the particle velocity distribution, as shown in Fig. 5. One cross-section was
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selected at the junction between the longitudinal filter channel and the diagonal filter channel, and the middle part between the two slopes, whose filter trajectory and particle velocity distribution are
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shown in Fig. 6.
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At the same particle moving speed, the filter particles move toward the lower right in the same plane. The particles near the wall move slowly, in which the speed of particles on the left side was
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the slowest, and the filter particles near the middle right part moved the fastest. When the particles
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moved downwards at different planes, the filter particles near the left wall surface became slower, and the filter particles near the right wall surface became faster. At the end of the longitudinal filter
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channel, the particle velocity gradually increased from the left to the right in the same plane, and
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the velocity of the particles at the top was greater, which is consistent with the velocity distribution law of particles in the upper part of the longitudinal filter channel. At the junction between the longitudinal filter channel and the diagonal filter channel, the particle speed gradually increased from left to right, and the particle speed near the slope was close to 0, the state of not moving. The particle velocity in the middle part of the two slopes had a peak-shaped distribution, and the particle located at the middle right position which was located directly above the discharge channel had the highest speed. The particle speed on the left and right sides were close to 0, forming a particle dead
ACCEPTED MANUSCRIPT zone. Among the four structures, the slope with large inclination angle had greater influence in the flow state of the particles. The lower portion of the granular bed had an effect on the overall particle flow, and the effect of the different oblique angles on the particle flow was different. The existing of the
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slopes caused the formation of dead zone particle layers, and the effect of the particle layers
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depended on the situation, which were not all unfavorable.
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3.2.2 Effect of granular moving speed on the particle flow
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The filter moving speed in the moving bed also affects the movement of the filter particles and
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the distribution of the dead zone particle layer. Three groups of particles were selected at the speed of 200 g/min, 400 g/min, and 600 g/min to analyze the particle movements in the four parts
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mentioned above. The filter trajectory and particle velocity distribution are shown in Fig. 7.
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In the same part of the moving granular bed, the velocity distribution of the filter particles was the same under different particle moving speeds, and the velocity had always increased from small
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to large. As the particle moving speed increased, the difference in the filter particle speed in the same part increased, and the filter particles located directly above the discharge channel were more
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likely to flow out of the granular bed. The slopes were located on the lower part of the moving granular bed. This part of the filter material captured the most dust and needed the largest displacement. It can be seen from the particle velocity distribution that as the particle moving speed increased, the velocity difference of the particles on the same plane gradually increased. When the particle moving speed was 200 g/min, the relative maximum speed of the particles was 5.2 mm/s, and when the particle moving speed was
ACCEPTED MANUSCRIPT 400 g/min, the relative maximum speed of the particles was 10 mm/s, which was 1.9 times that of the former. When the particle moving speed was 600 g/min, the relative maximum speed of the particles was 17 mm/s, which was 3.3 times that of the particle moving speed of 200 g/min, and the increase rate was larger than that of the previous working condition. The reason was that as the
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particle moving speed increased, the thickness of the active area and the dead zone particle layer
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did not change significantly, and the total number of particles in the active area remained basically
area became large, and the particle moved faster.
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unchanged. However, the particle moving speed was increased, so that the particle flow in the active
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On the left slope, the particle velocity increased with the particle moving speed, and the granular
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layer of dead zone became thinner. This phenomenon also existed on the right slope. The increase in the moving speed of the filter particle increased the relative velocity between the dead zone and
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the active zone, so that part of the dead zone particles were converted into active particles, and the
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particle layer of the dead zone got thinner.
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3.3 Particle motion simulation by discrete element method
The calculation area was 800 mm x 720 mm x 200 mm. The wall material of the bed was an
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acrylic plate. In order to facilitate the simulation, the selected filter material had a larger particle size than the actual one. The particle material was a corundum ball whose particle size was 6 mm, and the density was 4000 kg/m3. The Poisson's ratio of the granular material was 0.24; the shear modulus was 1.24*1011 Pa; the elastic recovery coefficient between particles was 0.5; the sliding slip coefficient was 0.3; and the kinetic friction coefficient was 0.1. The collision between the particle and the wall surface could be approximated as a collision between the particle and the plane,
ACCEPTED MANUSCRIPT because the bed size was much larger than the particle. The simulation still adopted the HertzMindlin non-sliding contact model, and the coefficient of elastic recovery between the particles and the wall was 0.45; the sliding friction coefficient was 0.26; and the dynamic friction coefficient was 0.01.
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During the filling stage, the granule outlet of the moving bed was closed, so that the particles
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could accumulate in it. When the particles filled up the entire device, the granule outlet was opened,
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and the particles flow out, while the flow state of the particles during this period was observed. The distribution of the particles after filling up the bed is shown in Fig. 8. The results are the same as
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the experimental phenomena. After opening the 200 mm x 200 mm granule outlet, filter particles
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flowed down through the channel, and the speed distribution of the filter particles is as shown in Fig. 9. The speed of particle can be differentiate by color, in which red means high moving speed,
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green means small moving speed, and the speed of the blue portion is almost zero. The specific
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particle speed values are shown on the left side of Fig. 9. In accordance with the actual situation, the size of the granule outlet was changed to 50 mm x 50 mm, and the velocity distribution of the
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filter particle is shown in Fig. 10.
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The simulation result is in accordance with the experimental result. The filter particles located directly above the granule outlet had the highest moving speed, and preferentially discharged the granular bed, which were easy to replace. The moving speed of the filter particles on both sides of the slope was close to 0, and was not easy to flow. The filter particles on the right slope were less likely to flow, because the inclination angle of the right slope was smaller than that of the left slope. Also, the particle layer with a speed of 0 was thicker than the left side. When the particles flowed, except for the influence of the device structure, the rear particles were also affected by the movement
ACCEPTED MANUSCRIPT of the front particles. When the front particles move faster, the rear particle moving speed may increase. When the size of the granule outlet was large, filter particles flowed more easily. Only the filter particles near the right slope had a velocity of 0, and the rest of the particles in the granular bed were active. When the granule outlet size was small, the number of particles that can move
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fluently became less. Only the filter particles located directly above the granule outlet had a certain
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movement speed, and the speed of the rest filter particle was almost close to 0, forming a dead zone
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particle layer which is not conducive to the replacement of the filter particles in the device. To reduce the disadvantageous particle dead zone, the following optimizations are recommended
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for the moving granular bed: (1) Change the structure of the inlet of flue gas, increase the angle of
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inclination of the slope, such as it can be changed to a straight plate, so that the filter particles will not form a stack at this place and to avoid the formation of particle dead zone; (2) Increase the angle
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of inclination of the slop in the diagonal filter area in the upper part of the moving bed to facilitate
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the flow of the filter particles, where the inclined angle of the inclined surface is 30°, which is only slightly larger than the angle of repose of the corundum ball 27°; (3) Increase the size of the granule
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outlet to increase the flow rate of the filter particles, thereby increasing the replacement rate of the
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filter particles and reducing the number of particles in the dead zone. The improved moving granular bed is shown in Fig. 11.
4 Conclusions
Through the experiment, simulation, and analysis of the test results, the following conclusions have been reached:
ACCEPTED MANUSCRIPT (1) The accumulation of the filter particles in the new type of moving granular bed is determined by the structure of the device. The inclined surface will affect the flow of the filter particles, and the slop with a small inclination angle is not conducive to the flow of particles. As the speed of particle increases, the velocity difference between the particles at the wall surface and the middle part
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increases, and some of the dead-zone particles are converted into active particles, thus thinning the
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dead-zone. The dead zone particle layer which was far away from the flue gas inlet could protect
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the insulation layer and the heat insulation layer outside the granular bed to improve the performance of the particle bed.
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(2) According trajectories study of individual particles, the velocity of the particles was found to
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increase from left to right in the same plane, and the velocity of the filter particle in the lower part of the granular bed showed a peak-shaped distribution. Increasing the moving speed of the filter
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will bias the single particle trajectory in the direction of the faster speed in the same plane and
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increases the flow in the faster area. The smaller the moving speeds of particle, the more uniform the velocity distribution is in the horizontal direction, and the closer to the overall flow.
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(3) The particle velocity distribution obtained by the discrete element method is consistent with
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the experimental results that the particle velocity is greatly affected by the structure. The moving speed of the filter particle near the wall surface was close to 0, and the particles located directly above the granule outlet had the fastest moving speed. The stationary state of the particles at the slope is related to the inclination angle of the slope, and the issue of how to replace them needs further study.
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Acknowledgement
This work is supported by the National key research and development plan of China (No.2016YFB0601101) and the Fundamental Research Funds for the Central Universities (FRF-
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BD-18-009A). The authors would also like to express their sincere thanks to the anonymous
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reviewers for their detailed and insightful comments that helped in improving the quality of this
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paper.
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[16] Zhang Z P, Liu L F, Yuan Y D, et al. A simulation study of the effects of dynamic variables on the packing of spheres [J], Powder Technology, 2001, 116(1): 23-32.
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[17] Mishra B K, Rajamani Raj K. The discrete element method for the simulation of ball mills
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[J], Applied Mathematical Modelling, 1992, 16(11): 598-604. [18] Kruggel-Emden H, Simsek E, Rickelt S, et al. Review and extension of normal force models for the discrete element method [J], Powder Technology, 2007, 171(3): 157-173.
[19] Alberto Di Renzo, Francesco Paolo Di Maio. Comparison of contact-force models for the simulation of collisions in DEM-based granular flow codes [J], Chemical Engineering Science, 2004, 59(3): 525-541. [20] Chen Y S, Chyou Y P, Li S C. Hot gas clean-up technology of dust particulates with a
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moving granular bed filter [J], Applied Thermal Engineering, 2015, 74: 146-155.
ACCEPTED MANUSCRIPT Fig. 1. Schematic diagram of the novel moving granular bed. Fig. 2. The flow process of particles in the moving speed of 200 g/min. Fig. 3. The flow process of particles in the moving speed of 400 g/min. Fig. 4. The flow process of particles in the moving speed of 600 g/min.
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Fig. 5. The filter trajectory and particle velocity distribution in the longitudinal filter channel
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and the end of it.
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Fig. 6. The filter trajectory and particle velocity distribution in the junction between the longitudinal filter channel and the diagonal filter channel and the middle part between the two
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slopes.
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Fig. 7. The filter trajectory and particle velocity distribution in four parts. Fig. 8. Particle accumulation in moving granular bed.
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Fig. 9. The speed distribution of the filter particles when the size of granule outlet was 200 mm x
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200 mm.
Fig. 10. The speed distribution of the filter particles when the size of granule outlet was 50 mm x
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50 mm.
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Fig. 11 The improved moving granular bed.
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Highlights The dead zone particle layer on the left slope can protect the granular bed. The slop with a small inclination angle is not conducive to the flow of particles. The particle velocity is greatly affected by the structure. The increased moving speed biases the single particle trajectory. The moving speed has effect on the velocity difference between particles.
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Figure 7A
Figure 7B
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Shaowu Yin, Ying He, Li Wang, Chuanping Liu, Lige Tong, Yulong Ding PII: DOI: Reference:
S0032-5910(18)30745-9 doi:10.1016/j.powtec.2018.09.018 PTEC 13692
To appear in:
Powder Technology
Received date: Revised date: Accepted date:
25 June 2018 27 August 2018 6 September 2018
Please cite this article as: Shaowu Yin, Ying He, Li Wang, Chuanping Liu, Lige Tong, Yulong Ding , Particulate flow characteristics in a novel moving granular bed. Ptec (2018), doi:10.1016/j.powtec.2018.09.018
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ACCEPTED MANUSCRIPT Particulate flow characteristics in a novel moving granular bed Shaowu Yina,b,*,1, Ying Hea,1, Li Wanga,b, Chuanping Liua,b, Lige Tonga,b, Yulong Dingc a
School of Energy and Environmental Engineering, University of Science and Technology Beijing, Beijing 100083,
Beijing Key Laboratory of Energy Saving and Emission Reduction in Metallurgical Industry, University of Science
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b
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PR China;
c
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and Technology Beijing, Beijing 100083, PR China;
College of Chemical Engineering, University of Birmingham, Birmingham B15 2TT, UK)
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Corresponding author at: School of Energy and Environmental Engineering, University of Science and Technology Beijing, Beijing 100083, PR China. Tel.: +86-010-62332741; Fax: +86-010-62332741.
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E-mail address: [email protected] (S. W. Yin).
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Abstract In this work, we studied the flow characteristics of particulates in a novel moving granular bed by using experiments and numerical simulations. The new moving granular bed was equipped with one adjustable baffle and two slopes to adjust filter thickness and increase the gas-particle contacting
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surface. Doing so overcomes the poor adaptability and low collection rate of fine dust during the
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filtration, and improved the filtering performance of filter devices. In the experiment, the movement
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of particulates in a moving bed was recorded using a camera, and the effects of the device structure and the filter moving speed on the flow of filter particles were examined. Combined with Discrete
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Element Method (DEM) to simulate the velocity variation of particles in the granular bed, the
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mechanism of particulate flow was analyzed. The inclined surface was found to affect the flow of filter particles, and a small angle of inclination was not conducive to the particle flow. Therefore, a
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layer of dead particles was easily formed. The dead zone particle layers on the left slope could
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protect the insulation layer and the heat insulation layer outside the particle bed. Increasing the moving speed of the filter increased the velocity difference between the particles in the wall surface
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and the middle part. The fluidity of particles also increased to make the particle layer of dead area
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become thinner. The trajectory of a single particle was greatly affected by the overall moving speed, and the single particle tended to move towards the faster direction in the same plane with the increase of the filter moving speed. Keywords: particulate flow, moving granular bed, discrete element simulation, bulk material transport
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1 Introduction
Particulate matter is widely found in the natural world and belongs to the category of soft condensed matter physics. It is closely related to people's daily lives and production [1-2]. Granular
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materials are widely and often used as raw materials or catalysts in chemical metallurgy and material
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production industries. Particulate matter has extremely rich physical connotations, such as discrete,
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nonlinear response, high dissipation, and non-uniform internal force chains due to different atoms or molecules [3-6]. The production, processing, storage, and transportation of particulate matter
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each year consume about 10% of the earth’s energy [7]. Therefore, an in-depth study of particulate
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matter is conducive to energy conservation and economic development. Granular bed that has particulate matter as medium is considered as one of the most promising
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dust removal technologies. It has the advantages of high temperature resistance, corrosion resistance,
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low running resistance and high dust removal efficiency [8]. It is suitable for industrial boilers and other industrial furnaces. Granular bed mainly includes the fixed bed which needs the back blowing
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regeneration process, and the moving bed which can be continuously operated [9]. Relative to the fixed bed, the moving granular bed dedusting system has good performance in dedusting efficiency
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and pressure drop. It can also achieve the integration of filtration and dedusting, which is easy to be large-scaled and suitable for dust removal of high temperature and high-pressure gas [10]. Domestic and foreign scholars and technical personnel have conducted in-depth researches and discussion on the particle layer filtering theory, optimization of filtration parameter and equipment structure [11]. The moving granular bed was found to not have the ability to adjust the filter thickness due to its fixed size during operation resulting in the poor adaptability, and the filtration
ACCEPTED MANUSCRIPT efficiency of fine dust was not as good as the fixed bed [12]. And due to the high temperature environment, the protective material outside the granular bed was easily damaged and needed to be replaced periodically. These factors affected the filtration and operating performance of the moving granular bed. Combining the advantages and disadvantages of moving bed and fixed bed [13], the
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problems and improve the filtration performance of granular layer.
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author proposes a new type of granular bed with adjustable filter thickness, which can eliminate the
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Particulate flow means that the particulate material resembles the fluid motion state when external forces and internal stress conditions change. Nedderman et al. [14] proposed that the movement of
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particles in the moving model was entirely caused by gravity, and the voids generated by the particle
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flow were immediately supplemented by the adjacent particles. Many scholars extended it in later studies. Among them, Waston et al. [15] emphasized that the finite-element analysis method could
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be used to calculate the velocity field of solid particles with stable flow. The aforementioned study
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provided a new method for the simulation of subsequent particle velocity field. Due to the complexity and particularity of the particles themselves, the particle system would exhibit different
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moving behaviors under the action of the outside world. Zhang et al. [16] studied the effect of
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particle characteristics on the flow law using Discrete Element Method (DEM). The angle of repose was found to be related to the friction between particles and the fluidity of particles, which had great influence on the size and shape of the dead zone in the moving bed. In order to study the mechanism of particle movement in the moving bed further, this paper uses 3–5 mm hollow corundum balls as the filter material, and uses experimental and DEM simulation methods to study the effect of the new granular bed structure and the moving speed of the filter material on the particle flow characteristics. In addition, the experimental data was processed using Image-Pro to obtain the
ACCEPTED MANUSCRIPT velocity distribution of the particles under various conditions. The result was compared with the DEM simulation results.
2 Experimental apparatus and method
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2.1 The novel moving granular bed
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The moving granular bed with adjustable filter thickness absorbs the advantages of fixed bed and
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moving granular bed. It can also adjust the thickness of the particle layer and increase the gas-
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particle contacting surface. The schematic diagram of its structural design is shown in Fig. 1. Based on the granule inlet and the granule outlet, a longitudinal filter area and two diagonal filter areas
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were sequentially distributed, and the longitudinal filter area was the main dust removal zone.
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During the operation of the granular bed, the flue gas passed through the flue gas inlet, the diagonal
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filter area 1, the longitudinal filter area, the diagonal filter area 2, and finally exited through the flue gas outlet. The feed channel and the discharge channel had a cross-sectional size of 150 mm x 150
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mm square, the cross-sectional size of the particle filter layer in the longitudinal filter area was 200 mm x 200 mm square, and the flue gas inlet and the flue gas outlet had a cross-sectional size of 200
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mm x 200 mm square. The whole device was made of organic glass, and the filter particles are 3-5 mm diameter corundum balls. The adjustable baffle was located between the granule inlet and the flue gas outlet. Changing the height of the baffle adjusts the thickness of the granular layer on the longitudinal filter area to adapt to different filtration environments. The slope of the diagonal filter area was determined by the angle of repose of the selected filter material. The angle of repose of the 3-5 mm corundum ball was 27°. Therefore, the angle of inclination is selected to be 30°, which is beneficial to the particle flow and
ACCEPTED MANUSCRIPT is less likely to form a blockage. The inclined surface formation increases the contact area between the flue gas and the filter particles. This formation is favorable for the collection of fine dust. The discharge control mechanism adopts a conveyor belt, and the speed of the particles can be changed by adjusting the rotation speed of the conveyor belt.
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From the filtration process of the granular bed, the filter device can be seen to have the following
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characteristics: (1) the thickness of the granular layer can be adjusted and the application range of
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the moving granular bed is expanded; (2) the use of the diagonal filter area can increase the contact area between the flue gas and the filter particles, avoiding the difficulty of trapping fine dust in the
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filtration process of the moving granular bed; (3) the moving speed of particles can be adjusted to
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eliminate the fatigue damage of the filter medium and ensure the stability of the filtration
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2.2 Research method
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performance of the granular layer.
The experiments were conducted to observe the entire bed and different areas of the granular bed
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using a high-definition camera by changing the moving speed of the filter particles. To facilitate the tracking of particle trajectories, a portion of the white corundum balls were stained to obtain red
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corundum balls, which were mixed into the granular bed. In the study of the flow of particles in different regions, 12 shots were taken every 10 seconds for each group, and the total duration was two minutes. The obtained photos were imported to Image-Pro and converted into a photo album, and then the TRACK function was used to trace the red corundum balls to obtain the particle velocity distributions. The simulation uses DEM, which is a new numerical method for analyzing and solving dynamic
ACCEPTED MANUSCRIPT problems of complex discrete systems. It establishes a parametric model of the solid particle system to simulate and analyze the behavior of particles, providing a platform for solving many complex problems involving particles, structures, fluids, electromagnetics, and their coupling [17]. The particle filling process was simulated based on the discrete element method (DEM). During the
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particle flow, elastic collisions occur between the particles and the walls and particles [18]. The
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collision process between particles adopts the Hertz-Mindlin non-sliding contact model [19]. The
is expressed as follows:
(1)
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∑ 𝐹 = 𝑚 𝑖 𝑎𝑖
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calculation time step was 5 x 10-7 s. The motion equation of particle I during the motion of the filter
(2)
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∑ 𝑀 = 𝐼𝑖 𝜃𝑖
where 𝑎𝑖 is the acceleration of particle I, 𝜃𝑖 is the angular acceleration of particle I, 𝑚𝑖 is the
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mass of particle I, and 𝐼𝑖 is the moment of inertia of particle I. Using the center difference method
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to perform numerical integration to the expression, the update rate expressed by the middle point of
(𝑎𝑖 )𝑁+1 = (𝑎𝑖 )𝑁−1 + [∑ 𝐹 ⁄𝑚𝑖 ]𝑁 ∆𝑡
(3)
(𝜃𝑖 )𝑁+1 = (𝜃𝑖 )𝑁−1 + [∑ 𝑀⁄𝐼𝑖 ]𝑁 ∆𝑡
(4)
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2
2
2
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the two iteration time steps was obtained as follows:
where ∆𝑡 is the time step, and N corresponds to time t. Integrating equations (3) and (4), we get the equation about displacement as follows: (𝑎𝑖 )𝑁+1 = (𝑎𝑖 )𝑁 + (𝑎𝑖 )𝑁+1 ∆𝑡
(5)
(𝜃𝑖 )𝑁+1 = (𝜃𝑖 )𝑁 + (𝜃𝑖 )𝑁+1 ∆𝑡
(6)
2
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The new displacement value of the particle can be obtained, and the new force can be calculated by substituting the new displacement value into the relationship. Thus, the cycle was repeated to
ACCEPTED MANUSCRIPT determine the tracking of particle movement at any time. The model was imported into EDEM using Hertz-Mindlin no-slip contact model to track the motion of each particle. The detailed motion process and the motion properties of the particle group
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were recorded. The velocity distribution of the particles in the bed was obtained after the treatment.
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3 Results and discussion
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The moving speed of the filter particle is in the range of 0.01-0.5 m/s in the industrial moving
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bed [20]. After the calculation, the moving speeds of the filter granular in the experiment were 200 g/min, 400 g/min, and 600 g/min.
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3.1 Flow characteristics of particles in the moving granular bed
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The experimental device adopts the designed moving granular bed, and is filled with the hollow
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corundum balls having a particle size of 3-5 mm, a bulk density of ρ=1950 kg/m3, a void ratio of ε = 0.502, and an angle of repose of φ = 27°. Corundum ball has high temperature resistance, corrosion
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resistance and non-breaking properties, and is suitable for filtration of high temperature flue gas.
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Some of the hollow corundum balls were dyed red and used as tracer particles to observe the flow characteristics of the filter media. In the moving granular bed, the adjustment baffle was located in the lower position, the thickness of the particle layer was 30 cm, and the three groups of particles were selected to move at 200 g/min, 400 g/min, and 600 g/min. A high-definition camera was used to observe and record the changes in the position of the filter media in the granular bed. Throughout the experiments, we found that the change in particle moving speed leads to changes in the dead zone in the granular bed with the movement of the filter particles and the distribution of
ACCEPTED MANUSCRIPT particle velocity in the same section. When the particle moving speed was 200 g/min, the flow process of particles in the moving bed is shown in Fig. 2. Comparing different particle flow processes, the filter particle moving near the right wall at the longitudinal filter area can be seen to be moving at a higher speed in the same plane. This
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phenomenon is caused by the large flow speed at the angle between the longitudinal filter area and
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the flue gas inlet. In the lower part of the moving bed, the flow velocity of the filter particle between
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the two slopes located on the upper part of the granule outlet channel was greater than that on the two walls. In the two slopes, the dead particle zones were formed due to the small velocity of the
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filter particles, and the distribution of the dead particle zone at the right slope was wider than that
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of the left side. The tilt angle of the right slope was 45°, and the tilt angle of the left slope was 60°. Therefore, the filter particles flow more easily on the left slope. The granule outlet was too small,
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so the total amount of filter particle flowing out at each time had a certain limit. Therefore, the filter
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particle with a slower flow rate at the inclined plane was more likely to form a layer of dead zone. The dead zone particle layer near the left side wall collected few dust in the dust filtration process,
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and basically did not need to be replaced. And its presence could protect the thermal insulation layer
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and the protective layer outside the device to avoid wear and tear. Therefore, the dead zone particle layer on the left side wall was an effective particle layer. While the dead zone particle layer near the right side wall was the main part of the flue gas inlet passage, and this part of the particle layer was the first to contact with the dust and captured the most dust. However, it formed a dead zone, which caused the particles to not flow, and was not conducive to replacement, which may lead to a decrease in the filtration efficiency of the granular bed. Therefore, this part of the dead zone particle layer was not conducive to improving the filtration performance of the granular bed.
ACCEPTED MANUSCRIPT The particle distribution changes when the particle moving speed changes. When the particle moving speed was 400 g/min, the flow process of the particles in the moving bed is as shown in Fig. 3. When the particle moving speed was 600 g/min, the flow process of the particles in the moving bed is as shown in Fig. 4.
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Compared with the flow process at different particle moving speeds, the velocity difference
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between the particles in the middle part and near the right side wall of the longitudinal filter area
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was found to increase as the particle moving speed increases. In the lower part of the moving bed, the granular layer of dead zone at the left slope was thinned as the increase of particle moving speed,
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and the pit between the dead zone at the right slope and the active granular layer on its left side
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became large. The main reason for this phenomenon is that the total amount of filter particle flowing out of the granular bed at each time increased with the increase of particle moving speed, the filter
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particles in contact with the active granular layer in the dead zone were changed into active filter
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particles, and the dead zone granular layer was thinned. The increase of the flow rate of the particles in the active area caused the particles in the upper side to not be supplemented, so that pits are
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formed between the active area and the dead zone at the right slope.
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3.2 Movement of particles in the moving granular bed 3.2.1 Effect of bed structure on the particle flow
To study the particle velocity distribution, the flow states of particles in four different parts were recorded using a high-definition camera according to the three kinds of particle moving speeds mentioned above. The four parts were the longitudinal filter channel, the end of longitudinal filter channel, the junction between the longitudinal filter channel and the diagonal filter channel, and the
ACCEPTED MANUSCRIPT middle part between the two slopes. Twelve shots were taken every 10 seconds for each group, and the total duration was two minutes. To track easily, seven to eight particles selected on the same section after the shots were imported into the software Image-Pro, and then the average speed of each particle was calculated to get the law of particle movement. When the particle moving speed
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was 200 g/min, three cross-sections were selected respectively at the longitudinal filter channel and
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the end of it to analyze the particle velocity distribution, as shown in Fig. 5. One cross-section was
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selected at the junction between the longitudinal filter channel and the diagonal filter channel, and the middle part between the two slopes, whose filter trajectory and particle velocity distribution are
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shown in Fig. 6.
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At the same particle moving speed, the filter particles move toward the lower right in the same plane. The particles near the wall move slowly, in which the speed of particles on the left side was
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the slowest, and the filter particles near the middle right part moved the fastest. When the particles
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moved downwards at different planes, the filter particles near the left wall surface became slower, and the filter particles near the right wall surface became faster. At the end of the longitudinal filter
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channel, the particle velocity gradually increased from the left to the right in the same plane, and
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the velocity of the particles at the top was greater, which is consistent with the velocity distribution law of particles in the upper part of the longitudinal filter channel. At the junction between the longitudinal filter channel and the diagonal filter channel, the particle speed gradually increased from left to right, and the particle speed near the slope was close to 0, the state of not moving. The particle velocity in the middle part of the two slopes had a peak-shaped distribution, and the particle located at the middle right position which was located directly above the discharge channel had the highest speed. The particle speed on the left and right sides were close to 0, forming a particle dead
ACCEPTED MANUSCRIPT zone. Among the four structures, the slope with large inclination angle had greater influence in the flow state of the particles. The lower portion of the granular bed had an effect on the overall particle flow, and the effect of the different oblique angles on the particle flow was different. The existing of the
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slopes caused the formation of dead zone particle layers, and the effect of the particle layers
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depended on the situation, which were not all unfavorable.
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3.2.2 Effect of granular moving speed on the particle flow
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The filter moving speed in the moving bed also affects the movement of the filter particles and
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the distribution of the dead zone particle layer. Three groups of particles were selected at the speed of 200 g/min, 400 g/min, and 600 g/min to analyze the particle movements in the four parts
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mentioned above. The filter trajectory and particle velocity distribution are shown in Fig. 7.
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In the same part of the moving granular bed, the velocity distribution of the filter particles was the same under different particle moving speeds, and the velocity had always increased from small
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to large. As the particle moving speed increased, the difference in the filter particle speed in the same part increased, and the filter particles located directly above the discharge channel were more
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likely to flow out of the granular bed. The slopes were located on the lower part of the moving granular bed. This part of the filter material captured the most dust and needed the largest displacement. It can be seen from the particle velocity distribution that as the particle moving speed increased, the velocity difference of the particles on the same plane gradually increased. When the particle moving speed was 200 g/min, the relative maximum speed of the particles was 5.2 mm/s, and when the particle moving speed was
ACCEPTED MANUSCRIPT 400 g/min, the relative maximum speed of the particles was 10 mm/s, which was 1.9 times that of the former. When the particle moving speed was 600 g/min, the relative maximum speed of the particles was 17 mm/s, which was 3.3 times that of the particle moving speed of 200 g/min, and the increase rate was larger than that of the previous working condition. The reason was that as the
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particle moving speed increased, the thickness of the active area and the dead zone particle layer
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did not change significantly, and the total number of particles in the active area remained basically
area became large, and the particle moved faster.
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unchanged. However, the particle moving speed was increased, so that the particle flow in the active
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On the left slope, the particle velocity increased with the particle moving speed, and the granular
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layer of dead zone became thinner. This phenomenon also existed on the right slope. The increase in the moving speed of the filter particle increased the relative velocity between the dead zone and
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the active zone, so that part of the dead zone particles were converted into active particles, and the
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particle layer of the dead zone got thinner.
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3.3 Particle motion simulation by discrete element method
The calculation area was 800 mm x 720 mm x 200 mm. The wall material of the bed was an
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acrylic plate. In order to facilitate the simulation, the selected filter material had a larger particle size than the actual one. The particle material was a corundum ball whose particle size was 6 mm, and the density was 4000 kg/m3. The Poisson's ratio of the granular material was 0.24; the shear modulus was 1.24*1011 Pa; the elastic recovery coefficient between particles was 0.5; the sliding slip coefficient was 0.3; and the kinetic friction coefficient was 0.1. The collision between the particle and the wall surface could be approximated as a collision between the particle and the plane,
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During the filling stage, the granule outlet of the moving bed was closed, so that the particles
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could accumulate in it. When the particles filled up the entire device, the granule outlet was opened,
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and the particles flow out, while the flow state of the particles during this period was observed. The distribution of the particles after filling up the bed is shown in Fig. 8. The results are the same as
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the experimental phenomena. After opening the 200 mm x 200 mm granule outlet, filter particles
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flowed down through the channel, and the speed distribution of the filter particles is as shown in Fig. 9. The speed of particle can be differentiate by color, in which red means high moving speed,
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green means small moving speed, and the speed of the blue portion is almost zero. The specific
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particle speed values are shown on the left side of Fig. 9. In accordance with the actual situation, the size of the granule outlet was changed to 50 mm x 50 mm, and the velocity distribution of the
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filter particle is shown in Fig. 10.
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The simulation result is in accordance with the experimental result. The filter particles located directly above the granule outlet had the highest moving speed, and preferentially discharged the granular bed, which were easy to replace. The moving speed of the filter particles on both sides of the slope was close to 0, and was not easy to flow. The filter particles on the right slope were less likely to flow, because the inclination angle of the right slope was smaller than that of the left slope. Also, the particle layer with a speed of 0 was thicker than the left side. When the particles flowed, except for the influence of the device structure, the rear particles were also affected by the movement
ACCEPTED MANUSCRIPT of the front particles. When the front particles move faster, the rear particle moving speed may increase. When the size of the granule outlet was large, filter particles flowed more easily. Only the filter particles near the right slope had a velocity of 0, and the rest of the particles in the granular bed were active. When the granule outlet size was small, the number of particles that can move
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fluently became less. Only the filter particles located directly above the granule outlet had a certain
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movement speed, and the speed of the rest filter particle was almost close to 0, forming a dead zone
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particle layer which is not conducive to the replacement of the filter particles in the device. To reduce the disadvantageous particle dead zone, the following optimizations are recommended
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for the moving granular bed: (1) Change the structure of the inlet of flue gas, increase the angle of
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inclination of the slope, such as it can be changed to a straight plate, so that the filter particles will not form a stack at this place and to avoid the formation of particle dead zone; (2) Increase the angle
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of inclination of the slop in the diagonal filter area in the upper part of the moving bed to facilitate
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the flow of the filter particles, where the inclined angle of the inclined surface is 30°, which is only slightly larger than the angle of repose of the corundum ball 27°; (3) Increase the size of the granule
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outlet to increase the flow rate of the filter particles, thereby increasing the replacement rate of the
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filter particles and reducing the number of particles in the dead zone. The improved moving granular bed is shown in Fig. 11.
4 Conclusions
Through the experiment, simulation, and analysis of the test results, the following conclusions have been reached:
ACCEPTED MANUSCRIPT (1) The accumulation of the filter particles in the new type of moving granular bed is determined by the structure of the device. The inclined surface will affect the flow of the filter particles, and the slop with a small inclination angle is not conducive to the flow of particles. As the speed of particle increases, the velocity difference between the particles at the wall surface and the middle part
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increases, and some of the dead-zone particles are converted into active particles, thus thinning the
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dead-zone. The dead zone particle layer which was far away from the flue gas inlet could protect
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the insulation layer and the heat insulation layer outside the granular bed to improve the performance of the particle bed.
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(2) According trajectories study of individual particles, the velocity of the particles was found to
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increase from left to right in the same plane, and the velocity of the filter particle in the lower part of the granular bed showed a peak-shaped distribution. Increasing the moving speed of the filter
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will bias the single particle trajectory in the direction of the faster speed in the same plane and
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increases the flow in the faster area. The smaller the moving speeds of particle, the more uniform the velocity distribution is in the horizontal direction, and the closer to the overall flow.
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(3) The particle velocity distribution obtained by the discrete element method is consistent with
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the experimental results that the particle velocity is greatly affected by the structure. The moving speed of the filter particle near the wall surface was close to 0, and the particles located directly above the granule outlet had the fastest moving speed. The stationary state of the particles at the slope is related to the inclination angle of the slope, and the issue of how to replace them needs further study.
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Acknowledgement
This work is supported by the National key research and development plan of China (No.2016YFB0601101) and the Fundamental Research Funds for the Central Universities (FRF-
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BD-18-009A). The authors would also like to express their sincere thanks to the anonymous
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reviewers for their detailed and insightful comments that helped in improving the quality of this
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paper.
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References
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moving granular bed filter [J], Applied Thermal Engineering, 2015, 74: 146-155.
ACCEPTED MANUSCRIPT Fig. 1. Schematic diagram of the novel moving granular bed. Fig. 2. The flow process of particles in the moving speed of 200 g/min. Fig. 3. The flow process of particles in the moving speed of 400 g/min. Fig. 4. The flow process of particles in the moving speed of 600 g/min.
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Fig. 5. The filter trajectory and particle velocity distribution in the longitudinal filter channel
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and the end of it.
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Fig. 6. The filter trajectory and particle velocity distribution in the junction between the longitudinal filter channel and the diagonal filter channel and the middle part between the two
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slopes.
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Fig. 7. The filter trajectory and particle velocity distribution in four parts. Fig. 8. Particle accumulation in moving granular bed.
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Fig. 9. The speed distribution of the filter particles when the size of granule outlet was 200 mm x
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200 mm.
Fig. 10. The speed distribution of the filter particles when the size of granule outlet was 50 mm x
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50 mm.
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Fig. 11 The improved moving granular bed.
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Highlights The dead zone particle layer on the left slope can protect the granular bed. The slop with a small inclination angle is not conducive to the flow of particles. The particle velocity is greatly affected by the structure. The increased moving speed biases the single particle trajectory. The moving speed has effect on the velocity difference between particles.
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Figure 11