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Annular Carbon Stripper for Chemical-looping Combustion of Coal Mao Cheng, Hongming Sun, Zhenshan Li, and Ningsheng Cai Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b03168 • Publication Date (Web): 18 Jan 2017 Downloaded from http://pubs.acs.org on January 25, 2017

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Industrial & Engineering Chemistry Research

Annular Carbon Stripper for Chemical-looping Combustion of Coal Mao Chenga, Hongming Sunb, Zhenshan Lia*, Ningsheng Caia a) Key Laboratory for Thermal Science and Power Engineering of Ministry of Education, Beijing Municipal Key Laboratory for CO2 Utilization & Reduction, Department of Thermal Engineering, Tsinghua University, Beijing 100084, China

Department of Thermal Engineering, Tsinghua University, Beijing 100084, China b) Institute of Engineering Thermal Physics, Chinese Academy of Sciences, Beijing 100190 *Corresponding author: [email protected] Abstract Carbon stripper (CS), which is a fluidization bed aimed to separate char particles from oxygen carriers during coal-fired chemical looping combustion (CLC), is vital to achieve high carbon capture efficiency of a CLC system. An effectively designed CS could transport most char particles back to the fuel reactor and simultaneously allow most oxygen carriers to reach the air reactor. An annular carbon stripper was designed, and a cold model apparatus was built for operation and optimization. The CS consists of an annular fluidized bed and a center riser. The riser was inserted into the annular fluidized bed, and the fluidized bed was divided into the annular zone and the cylindrical zone. Plastic beads were used to simulate char particles, and ilmenite was used as the oxygen carrier. The effect of operational parameters (solid feeding rate, gas velocities) and particle properties (the average size of plastic beads, the mass concentration of plastic beads) on the separation efficiencies of plastic beads and ilmenites was investigated in detail. The main parameters of the CS structure (the length of the annular zone, the diameter of the riser and annular fluidized bed) were studied and optimized. The axial distribution of the solid volume fraction and the mass concentration of light particles along the annular fluidized bed were measured, and the fluidization behavior in the CS was analyzed. The separation process in the annular CS and the important factors influencing the separation of binary particles were discussed. Under the optimized structure and operational conditions, the annular CS could be an effective apparatus to completely separate char particles from oxygen carriers, which could greatly improve the carbon capture efficiency during the operation of a coal-fired CLC. Key words: binary particle separation; cluster; annular fluidized bed; carbon stripper; chemical looping combustion; carbon capture efficiency 1 Introduction Chemical-looping combustion (CLC) is a novel technology of fossil fuel combustion with inherent separation of CO2. In CLC, metal oxides such as Mn, Fe, Ni, and Cu are generally used as the oxygen carriers (OC) to transfer oxygen from air to fuel, and hence, direct contact between fuel and air is avoided1. The CLC system mainly consists of two interconnected reactors: fuel reactor (FR) and air reactor (AR)2. Coal-fired CLC has become an important development in low-carbon technology3. The normal temperature and pressure in an FR are approximately 900°C and 1 bar, respectively, which causes the gasification of coal char in the FR to usually be a slow process4, 5. Therefore, the residence time of char in the FR should be long enough to completely convert char to CO2 to prevent char particles from burning in the air reactor. A so-called carbon stripper (CS) is implemented after the FR6, in which char particles are separated from OC particles

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and carried back to the FR by gas flow. The separation efficiency of char particles in the CS has great influence on the carbon capture7, 8. The separator of binary particles should tolerate high temperature and wear from the particle stream. Therefore, CS should be a fluidization bed instead of a mechanical particle separator. Particle separation in a fluidization bed is mainly due to the difference in particle size and density. Heavy/coarse particles tend to sink to the bottom of the bed through gravity, and light/fine particles tend to be entrained by fluid flowing to the top of the bed. The fluid could be liquid phase or gas phase9. Liquid phase fluidization has been used to separate coal particles, washing coal free of the minerals10. However, this technique is restricted to low temperatures, and the separation process could be slow. An air dense medium fluidized bed (ADMFB) was proposed by the China University of Mining and Technology11. The technology used dense fluidized beds, and the separating medium was formed using fluidizing gas and medium solids in a certain size range. The feedstock is segregated and separated according to the density of the bed. This technology is difficult to apply to CS. The CLC operates continuously, during which the medium solids can be easily lost, and it is difficult to transport char particles back to the FR by controlling the gas flow in ADMFB. In the CLC systems of Chalmers 10 kWth12, Chalmers 100 kWth6, 13, Darmstadt 1 MWth14, 15 and CSIC 20/50 kWth16, the CS was normally operated in the bubbling or turbulent regime. Few studies have been conducted on the performance of CS. Sun et al17 studied the turbulence-based CS in a 70 kWth cold model of a CLC system. The results indicated that the turbulence-based CS could not efficiently separate the light/fine particles from heavy/coarse particles and that the baffles in the CS were not important. Instead, the turbulence-based CS was like another fuel reactor if using CO2/H2O as the fluidization gas. The operational results from the CLC system of CSIC 50 kWth showed that the turbulence-based CS could not reach high char separation efficiency even if the solid circulation rate was comparatively low (20–40 g/s) and the gas velocity of CS was comparatively high (0.5–0.7 m/s)18. Rowe et al.19 studied the mechanisms of the segregation and mixing of binary particles in the dense fluidization bed, which suggested that rising bubbles were the vehicle for particle segregation and mixture. Both heavy and light particles are carried up the bed by the bubble wakes. In the dense fluidization bed, the high gas velocity enhances the mixing of the binary particles, and at gas velocity that close to the minimum fluidization velocity, the binary particles tend to be segregated20. The segregation and mix pattern of binary particles in the dense fluidization bed is the possible reason for the low char separation efficiency of the carbon stripper used in the CLC system. The binary particle segregation in the fast fluidized bed was experimentally studied by Palappan and Sai21-24. The particle size, particle density, solid feed rate and gas velocity in the fast fluidized bed were the important factors in the separation of the binary particles. A riser-based CS was operated by Sun et al25, in which binary particles containing approximately 5 wt.% plastic beads were processed. More than 90% of the plastic beads (d50=94 μm, ρp=960 kg/m3) were carried up, and approximately 88% of ilmenite particles (d50=257 μm, ρp=4260 kg/m3) sank to the bottom at a solid feed rate of 40 g/s and gas velocity of 2 m/s. The separation ability of riser-based CS was greatly improved compared with the turbulence-based CS17, 25. However, the separation efficiency of plastic beads significantly decreased with increasing solid feed rate, and the gas velocity in the riser was confined to approximately 2 m/s. The downwards-inclined inlet of the particle stream is probably a negative


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structure in which the initial particle cloud could directly carry the plastic beads down and enhance the particle collision approximately the particle inlet. Various forms of solid separators aimed to separate particles of different sizes using air as the medium were introduced in the review by Shapiro and Galperin26. However, those solid separators did not show high separation efficiency. It is important to avoid intensifying collisions between particles, and the particle stream should be dispersed as much as possible. An annular carbon stripper was designed and experimentally studied in this paper, which showed better performance in the separation of binary particles. The effect of operational parameters (solid feed rate, gas velocities) and particle properties (the average size of light particles, the mass concentration of light particles) on the separation efficiencies of light particles and heavy particles from the solid mixture was investigated in detail. The main parameters of the CS structure (the length of the annular zone, the diameter of the riser and the annular fluidized bed) were studied and optimized. The axial distribution of the solid volume fraction and the mass concentration of light particles along the annular fluidized bed were measured, and the fluidization behavior in the CS was analyzed. The separation process in the annular CS and the important factors influencing the separation of binary particles were discussed. 2 Design idea of the annular carbon stripper Based on fluidization theory, three forces mainly determine a particle’s motion in a fluidized bed if ignoring the wall effects: gas–solid drag (Fd), particle gravity and collision force between particles (Fc). For the dilute phase in a fluidized bed, the solid volume fraction is small, and the gas drag force dominates the motion of particles27. The separation of binary particles in the dilute phase should satisfy the following:

Fc, L ≈ 0 (1) Fc , H ≈ 0 (2)

Fd ,L > mL g (3) Fd , H < mH g (4) where Fc,L, Fc,H, Fd,L, and Fd,H are the collision force of light particles, collision force of heavy particles, gas–solid drag of light particles, and gas–solid drag of heavy particles, respectively, and mL and mH are the masses of the light particles and heavy particles, respectively. Therefore, the gas velocity in the CS is chosen between the terminal velocity of light particles and heavy particles for satisfying formulas (3) and (4).

ut ,L < ug < ut ,H

(5)

If assuming only gas–solid drag on the particles and neglecting the collision force, the light particles could be separated from the heavy particles completely in theory. However, in reality, a certain circulation rate of the binary particle mixture is processed by the CS with limited volume, and the fluidization behavior inside the CS is complicated and cannot be simply regarded as the dilute phase. Therefore, the collision force between particles cannot be ignored. The design is focused on reducing the collision force between particles in the CS by controlling the fluidization



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behavior of the gas–solid mixtures. By optimizing the operational and structural parameters of the CS, formulas (6) and (7) could be applied to most particles to achieve a high separation extent of binary particles.

Fd ,L > mL g + Fc,L (6) Fd ,H < mH g + Fc, H (7) Based on the force analysis above, the basic design idea of the annular CS was proposed, as shown in Figure 1. A center riser is used to inject the particle mixture into the annular fluidized bed. The gas velocity in the center riser (u1) is chosen to be several times the terminal velocity of the heavy particle (ut,H).

u1 = 2 ~ 5 ut , H (8) Fast fluidization in the center riser is formed at high gas velocity (u1), which is assumed to be an effective way to disperse the particle stream. The solid volume fraction decays exponentially along the center riser, which is described by equation (9)20.

ε s =ε s* + ( ε sd -ε s* ) e − aL

(9)

where ɛs* is the saturated carrying capacity of the gas flow; ɛsd is the solid volume fraction in the dense region of the center riser; a is a constant related to the gas velocity. The solid volume fraction at the exit of center riser (ɛse), which could be far less than ɛsd, can be estimated by equation (10).

ε se ≈ where

m& mix ,in

S1 ρ H ( u1 - ut , H )

=

4m& mix ,in

π D ρ H ( u1 - ut , H ) 2 1

(10)

m& mix,in is the mixture solid feed rate; S1 is the cross-sectional area of the center riser, and

D1 is the diameter of the center riser; ρH is the heavy particle density. The particle velocity at the outlet of the center riser is estimated as u1- ut,H.


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Figure 1. Design schematic of annular carbon stripper The particles injected into the annular fluidized bed behave as a “fountain” shape, and the distance between particles is assumed to be larger because the space in the cylindrical zone is larger. The heavy particles fall into the bottom of the annular fluidized bed, and most of the light particles are entrained into the cylindrical zone by the gas flow. The gas velocities in the annular and cylindrical zones of the annular fluidized bed (u2 and u3) are chosen to be close to the average terminal velocity of the heavy particles to achieve maximum separation of the light particles from heavy particles without entraining up many heavy particles.

u2 ≈ 0.8 ~ 1 ut , H (11) u3 ≈ 0.8 ~1 ut ,H (12) The relation of the three velocities (u1, u2 and u3) should satisfy mass balance and can be expressed as

u1S1 + u2 S2 = u3 S3 (13) where S1, S2 and S3 are the cross-sectional areas of the center riser, the annular zone and the cylindrical zone of annular fluidized bed, respectively. The design value of the solid feed rate is determined by the design power of the CLC system. The solid circulation rate in the CLC system is usually approximately 5 g/(s·kWth) for ilmenite1. The distribution of the solid volume fraction in the carbon stripper is directly influenced by the feed rate of the solid mixture. It is assumed that the collision force would be significantly increased when processing a high solid feed rate. The light and heavy particles are chosen as plastic beads and ilmenite, respectively, to simulate



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the char particle and oxygen carrier in CLC. The particle gravity, the gas–solid drag, and the collision force between particles are closely related to the particle size and density. The separation would be easier for two types of particle with larger differences in particle size and density. The mass concentration of light particles in the solid mixture is selected as 3–10%, which is more than the actual concentration of char particles in the FR (normally less than 2%). This is to reduce the measurement error of the mass concentration of light particles. The main parameters of the annular CS structure are the diameter of the annular fluidized bed (D); the diameter of the center riser (D1); the length of the cylindrical zone (L2); the annular zone (L1) and the length of the center riser (L), as Figure 1 shows. D1 and D must satisfy equation (13), which is rewritten in equation (14):

π D12

u1

4

+ u2

π ( D 2 − ( D1 + δ ) 2 ) 4

= u3

π D2 4

(14)

where δ is the wall thickness of the center riser. D is determined by the design value of the solid feed rate

m& mix,in,design .

D = f ( m& mix ,in , design ) (15) D is first selected as the diameter of the riser-based CS operated by Sun25 for comparison. The distributions of the solid volume fraction and the pressure drop in the center riser are closely related to D1 and L, as shown in equations (9) and (10). The selection of D1 and L should satisfy two constraints: 1) The solid volume fraction at the exit of the center riser (εse) should not exceed an upper limit, which ensures that the particle stream is dispersed when it is injected into the annular fluidized bed.

ε se < ε se,max

(16)

2) The pressure drop in the center riser should not exceed a maximum pressure drop. A large pressure drop in the center riser would destabilize the operation of the CLC system. L

∆P = ∫ ρ s gε s dL < ∆Pmax (17) 0

L1 is a part of L and should be long enough. It is assumed here that a longer L1 could provide more time for light particle separation from heavy particles. The length of the cylindrical zone (L2) mainly must consider 1) providing enough space for the separation of the binary particles and 2) preventing the heavy particles from reaching the top of the cylindrical zone, in which L2 should be more than the maximum distance reachable by the heavy particles. The two conditions can be together estimated by formula (18).

L2

(u - u ) ≥ 1

t ,H

2g

2

(18)

3 Cold apparatus and experimental procedures 3.1 Cold apparatus


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The annular CS used in the experimental study was made of transparent Plexiglas, as shown in Figure 2, which mainly consisted of two parts: the center riser (12) and the annular fluidized bed (14). The binary particles were mixed well in the feed hopper (11). The particles flowing into the center riser were controlled by the gate valve (10), and the solid feed rate was controlled by the pre-calibrated ball valve (9). The fast fluidization in the center riser was formed, and the particle mixture was injected into the cylindrical zone of the annular fluidized bed. The heavy particles fell into the bottom and were collected by the bottom product collection tank (7). The light particles were carried into the cyclone (17), separated from gas and collected by the top product collection tank (16). The air was pumped into the pressurized air storage tank (2) by the air compressor (1). The gas flows from the pressurized air storage tank into the center riser and the annular fluidized bed were controlled by the mass flow controllers (5). The oil, water, and dust were removed from the gas by the air cleaner (3) and depressurized to the working pressure of the mass flow controllers by the pressure reduction valve (4). The gas distributors (6) and (8) were used to distribute the gas flows. The sampling port (15) was used to take samples of particles from the annular fluidized bed as described in the previous work25. The pressure port (13) was mounted to measure the pressure drop.



Figure 2. Schematic of the experimental systems of the annular carbon stripper.

3.2 Materials In the experiments, several types of ilmenite and plastic beads were used. The main physical properties of the ilmenites and plastic beads are listed in Table 1. ω indicates the weight loss ratio of particles after heating at 800°C in a muffle furnace for 0.5 h. The particle density was measured using a true density meter (ONE MICROMERITICS DR, NORCROSS, USA). The porosity of fresh ilmenite without activation is small28 and thus we could use the true density instead of particle density. The distribution of particle size was measured using a Malvern Mastersizer (Malvern Instruments Ltd. Spring Lane South, Malvern Worcestershire, UK). The distribution of the particle sizes is shown in Figure 3. Different groups of plastic beads and ilmenite were chosen as the binary particle mixture to study the effect of the plastic bead size on the separation. The properties of the binary particle mixtures used in the experiments are listed in Table 2. Table 1. Properties of ilmenite particles and plastic beads. The particles

ρpa (kg/m3)

d50b (μm)

utc (m/s)

ω %

Mozambique ilmenite Vietnam ilmenite Sri Lanka ilmenite Plastic beads I Plastic beads II Plastic beads III

4470 4530 4246 960 940 1226

194 220 257 90 120 200

1.43 1.56 1.82 0.176 0.275 0.819

-0.955 -1.776 -0.636 100 100 100

a. The particle density was estimated by the true density while ignoring the particle porosity. b. The average size of the particles was estimated by the size at which approximately 50 vol. % of particles were smaller. c. The average terminal velocity of the particles was calculated by the method from Haider and Levenspiel29. Plastic beads I Plastic beads II Plastic beads III Mozambique ilmenite Vietnam ilmenite Sri Lanka ilmenite

25

dV/15/d(log(dp))

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20

15

10

5

0 10

100

1000

Particle size (μm)

Figure 3. Volume fraction distribution of particle size. Table 2. Properties of the binary particle mixtures. Heavy particle

Light particle

cpb, mix

Size


Density

ut

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I II III IV V

Sri Lanka ilmenite Mozambique ilmenite Mozambique ilmenite Mozambique ilmenite Vietnam ilmenite

Plastic beads I Plastic beads I Plastic beads II Plastic beads III Plastic beads I

0.07 0.07 0.07 0.07 0.03, 0.07

ratio

ratio

ratio

2.86 2.16 1.62 0.97 2.44

4.42 4.66 4.76 3.65 4.72

10.3 8.1 5.2 1.8 8.9

3.3 Experimental procedures The gas flows in the annular zone and the center riser were settled by the mass flow controllers first. When the gas flows were stable, the gate valve was opened, and the particle mixture flowed into the center riser from the hopper feed. Loose gas flowing through the feed hopper caused the particles to flow easily. The solid feed rate was mainly determined by the open extent of the ball valve and the flow of loose gas, whose relationship was calibrated. The gate valve was used to control the starting and stopping of the particle flow, and the time interval was approximately 30–100 s which was given in the supporting information. Nearly all particles flowed out of the annular fluidized bed after 2–3 min when the gate valve was closed, and then the gas flows were shut off. The pressure differences were measured online during the stable operation of the CS, and the pressure drop data were averaged. The particles in the top tank are called top particles, and the particles in the bottom tank are called bottom particles. The mass of the bottom particles and the top particles were measured. The samples of bottom particles and top particles and the samples taken from the particle sampling ports were collected, and the mass concentration of plastic beads in these samples were measured through the burning method25. The ranges of operational and structural parameters of each test are listed in Table 3. The effects of the solid feed rate and the size of plastic beads were studied in Test 1. The effect of gas velocities was studied in Tests 2–5 using two types of binary mixtures. The effect of plastic bead concentration was studied in Test 6. The study and optimization of the structural parameters were conducted in Tests 7–9. The separation efficiencies of the plastic beads and the ilmenites were measured. The axial distributions of plastic bead concentration and solid volume fraction along the annular fluidized bed were measured in Tests 6–9. Table 3. Range of operational parameters and the main structure sizes of the CS. Test Num.

& mix,in m

u1

u2

m/s

m/s

cpb,mix

Mixture

CS

name

name

L1

L2

L

D1

D

m

m

m

mm

mm

CS1

1.15

2

1.5

30

70

CS2

1.5

2

1.85

30

70

CS3

2

2

2.35

30

70

g/s 1

0–100

3

1.4

0.07

I, II, III, IV

2

100

7.4–3

0.2–1.4

0.07

I

3

100

3

1–2

0.07

I

4

100

3–7

1.6

0.07

I

5

50

3; 3.5; 4

1–1.8

0.07

IV

6

40–130

4

1.4

0.03;

V

0.07 7

40–200

4

1.4

0.07

V



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8

40–200

6.6

1.4

0.07

V

CS4

1.15

2

1.5

20

70

9

40–200

4–6

1.4

0.07

V

CS5

1.15

2

1.5

30

100

3.4 Evaluation methods The mixture solid feed rate is calculated by equation (19).

m& mix ,in =

mtop + mbottom

(19)

t

where mbottom and mtop are the masses of the bottom particles and top particles, respectively; t is the time of the mixture solid feeding. The separation efficiency of plastic beads describes the fraction of plastic particles flowing into the top particles, which is vital for CLC because it indicates the amount of char particles slipping into the air reactor. Equation (20) gives the calculation method of the separation efficiency.

η=

m pb ,top m pb ,top + m pb ,bottom

=

mtop c pb ,top mtop c pb ,top + mbottom c pb,bottom

(20)

where cpb,top and cpb,bottom are the mass concentrations of plastic beads in the top particles and bottom particles, respectively. The ilmenite separation efficiency describes the fraction of ilmenite particles falling into the bottom particles, which is determined by equation (21). The separation efficiency of ilmenite particles (α) is also important, which indicates the energy consumption of the CS. If a large amount of ilmenites flows up, the CS will consume extra energy to transport these ilmenites.

α=

milm ,bottom milm ,bottom + milm,top

=

mbottom (1 − c pb ,bottom )

mbottom (1 − c pb ,bottom ) + mtop (1 − c pb ,top )

(21)

The average solid volume fraction between two pressure ports can be estimated from the pressure drop as shown in equation (22).

ε s ,i − j ≈

∆Pi − j

ρ p ghi − j

(22)

Equation (22) is based on the assumption that the particle acceleration and the wall frictional resistance are negligible. Such assumptions are valid only at some position of the CS. However, this is a general and good approximation of the solid volume fraction30. In this study, we use this method to estimate the solid volume fraction. The mass concentration of the plastic beads in the samples taken from the annular fluidized bed is determined by equation (23).

c pb = (ωmix - ωilm ) (1- ωilm ) (23) where ωmix and ωilm are the weight loss ratio of the samples from annular fluidized bed and the pure ilmenites after heating in the furnace at 800°C for 0.5 h, respectively.

4 Results 4.1 Effects of the operational parameters and the particles properties 4.1.1 Solid feed rate


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In CLC, the solid circulation rate is an important parameter, which determines the amount of oxygen and heat transferred by OC between AR and FR. As shown in Figure 4, the solid feed rate has a large effect on the separation efficiency of CS. It was found in Figure 4(a) that more than 95% of plastic beads can be separated from ilmenites when the solid feed rate was smaller than a threshold, which was approximately 35 g/s for Mixtures I and II, 25 g/s for Mixture III and 10 g/s for Mixture IV. The threshold decreased with increasing size of the plastic beads. As the solid feed rate was increased beyond the threshold, the separation efficiency (η) linearly decreased, and the decreasing rate was greatly increased by the size of plastic beads. Figure 4(b) shows that the separation efficiency of ilmenite particles (α) increased with increasing solid feed rate, which was the opposite trend of η. The coarser ilmenite particles showed higher α compared with Mixtures I and II. In addition, α increased with coarser plastic beads compared with Mixtures II, III and IV. 1.0

go to 100%

Mixture I Mixture II Mixture III Mixture IV

0.9

0.90

The ilmenite particles separation efficiency

The plastic beads separation efficiency

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0.8 0.7 0.6 0.5

0.85 0.80 0.75

Mixture I Mixture II Mixture III Mixture IV

0.70 0.65

0.4 0

10

20

30

40

50

60

70

80

90

100

0

20

The solid feed rate (g/s)

40

60

80

100


(a) separation efficiency of plastic beads (η) (b) separation efficiency of ilmenites (α) Figure 4. Effect of solid feed rate on (a) separation efficiency of plastic beads and (b) separation efficiency of ilmenites for Mixtures I, II, III and IV in Test 1. (u1=3 m/s, u2=1.4 m/s, cpb,mix=0.07, CS1). The solid feed rate in CS1 should be in the range of 10–40 g/s to achieve high η and acceptable α. In a CLC system, the solid circulation rate corresponds to the mass of ilmenites falling to the bottom of the CS in the present experimental apparatus, and the proper solid circulation rate for CS1 should be in the range of 8–32 g/s if assuming that the average α is 0.8. The circulation rate is normally approximately 5 g/s/kWth for ilmenite, so CS1 could be applied to a 1–6 kWth CLC system to reach 100% carbon capture efficiency. It is necessary to enlarge the CS to reach high carbon capture efficiency if applied to a larger CLC system. 4.1.2 Gas velocity The gas flow in the CS is the vehicle of separation of the plastic beads from the ilmenites. The gas velocities in the CS should be optimized for higher separation efficiency. The total gas flow in the CS is dependent on the gas velocity (u3) in the cylindrical zone and was first kept the same with a constant u3 (u3=1.5 m/s) at the cylindrical zone. It was found that the separation was greatly influenced by the division of the gas flow between the center riser and the annular zone, as shown in Figure 5. η is increased and α was nearly unchanged as u2 increased and u1 decreased with constant u3. η decreased from 80% to 60%; α remained at approximately 88% when decreasing u2 from 1.4 m/s to 0.4 m/s, although u1 increased from 3 m/s to 7 m/s and the total gas flow was unchanged. Therefore, u2 probably played an important role in the separation of plastic beads from ilmenites.



The separation efficiency

0.9 0.8 0.7 0.6

plastic beads ilmenite particles

0.5

The gas velocity at the center riser u1 (m/s)

0.4 7.4

0.3

0.2

6.7

5.9

5.2

4.5

3.7

3.0

0.4

0.6

0.8

1.0

1.2

1.4

The gas velocity at the annular zone u2 (m/s)

Figure 5. Effect of u1 and u2 on the separation efficiency of the binary particles with constant u3 in Test 2. (u3=1.5 m/s,

m& mix,in is approximately 100 g/s, Mixture I, CS1)

To obtain more information about the role of u2, the separation efficiency was measured with increasing u2 while u1 was held constant, and u3 was changed with u2, as shown in Figure 6(a). It is indicated from Figure 5 and Figure 6(a) that u2 is highly important for the separation of the plastic beads from the ilmenites. As Figure 6(a) shows, η increased from approximately 52% to 80% when increasing u2 from 1 m/s to 1.6 m/s. The gas velocity at the center riser (u1) was mainly related to the fast fluidization, and the effect on the separation with constant u2 was shown in Figure 6(b). The increase in η with increasing u1 was possibly due to the increasing u3 in which more particle mixture was carried up as was shown in Figure 6(b). 0.9

0.9 0.8 0.7


0.6 0.5


1.0


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 27

0.8 0.7 0.6 0.5


0.4 0.3

1.0

1.2

1.4

1.6

1.8

2.0

The gas velocity at the annular riser u2 (m/s)

3

4

5

6

7

The gas velocity at the center riser u1 (m/s)

(a) Effect of u2 with constant u1 in Test 3 (b) Effect of u1 with constant u2 in Test 4 Figure 6. Effect of the gas velocity on the separation efficiency of binary particles.

& mix,in is approximately 100 g/s, Mixture I, CS1) (m The effect of the gas velocities on the separation of particle mixture IV, which contains the coarsest plastic beads, was studied. η significantly decreased owing to the coarser plastic beads as shown in Figure 4(a). By increasing u2, η could be improved as shown in Figure 7(a). Especially at u2 of 1.35–1.5 m/s, η could be significantly increased by u2. It was assumed that there existed a threshold of u2 for the separation of Mixture IV. When u2 was increased near the threshold, the separation of the plastic beads from ilmenites was greatly enhanced. When increasing u2, α decreased linearly as shown in Figure 7(b). At the same time, u3 increased with increasing u1 or u2. It was found in Figure 5 that α was nearly unaffected by the various groups of u1 and u2. It is assumed that α was mainly determined by u3 in CS1 at the constant solid feed rate.


Page 13 of 27

The total gas flow used in the CS is related to the power of the CLC system. Higher power of the CLC system requires coupled larger CS, and the gas flow should be sufficient to maintain the gas velocity in the CS. The gas could be recirculated gas from the outlet of CS and FR. 0.9

0.95

u1=3 m/s



u1=3.5 m/s

0.8

u1=4 m/s

0.7 0.6 0.5 0.4

0.90 0.85 0.80

u1=3 m/s u1=3.5 m/s

0.75

u1=4 m/s 0.70

0.3 1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.1

1.8

1.2

1.3

1.4

1.5

1.6

1.7

1.8



(a) η (b) α Figure 7. Effect of u2 on the separation efficiency in Test 5 at u1=3 m/s; 3.5 m/s; 4 m/s.

& mix,in is approximately 50 g/s, Mixture IV, CS1) (m 4.1.3 Particle properties In CLC, the size of char particles in FR decreases with time owing to the char gasification, fragmentation and attrition. Therefore, it is important to study the effect of the size of light particles on the separation of binary particles. The smaller plastic beads were more easily separated from OC as shown in Figure 8. η linearly decreased because the size of plastic beads and the decreasing rate were higher at the higher solid feed rate. For each 20 g/s increase in the solid feed rate, the decreasing rate nearly doubled. Based the analysis above, it is assumed that the evolution of char particles in the FR could enhance the separation efficiency of char particles in the CS. However, fine char particles are easily lost from the cyclone when the particle size is smaller than the cut diameter of the cyclone at the outlet of FR or CS. Therefore, the initial fuel particle size should be optimized considering both the separation efficiency of the CS and the capture efficiency of the cyclone. 1.0


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.9 0.8 0.7

20 g/s 40 g/s 60 g/s

0.6 0.5 0.4 80

100

120

140

160

180

200

220

The size of the plastic beads (μm)

Figure 8. Effect of the size of plastic bead on η at the solid feed rates of 20 g/s; 40 g/s and 60 g/s in Test 1. (u1=3 m/s, u2=1.4 m/s, cpb,mix=0.07, CS1). In addition to the particle size, the composition of the mixture was also important for the separation. Figure 9 (a) shows the separation efficiency of plastic beads significantly increased by decreasing cpb,mix from 7% to 3%, and the separation efficiency of ilmenites slightly increased as



shown in Figure 9(b). η could be more than 95% at a solid feed rate less than 85 g/s when cpb,mix was 3%. 1.0

3% 7%

0.9 0.8 0.7 0.6 0.5 40

50

60

70

80

90 100 110 120 130 140 150



1.0


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 27

0.9 0.8

3% 7%

0.7 0.6 0.5 40

50

60

70

80

90

100 110 120 130


(a) η (b) α Figure 9. Effect of the mass concentration of plastic beads on (a) η and (b) α at different solid feed rates in Test 6. (u1=4 m/s, u2=1.4 m/s, Mixture V, CS1) It is found in Figure 8 and Figure 9(a) that the effects of plastic bead size and the concentration of plastic beads on the separation were similar, and both effects could be enlarged by increasing the solid feed rate at 0–60 g/s. It is possible that a higher concentration of plastic beads could form a cluster of plastic beads, which is similar to the increase in the size of plastic beads.

4.2 Effects of CS structural parameters The main parameters of the CS structure were experimentally studied, which showed better performance in the separation of binary particles after optimizing the parameters, as shown in Figures 10–12. By increasing the length of the annular zone (L1) from 1.15 m to 1.5 m, η increased by approximately 15% as shown in Figure 10(a). However, when increasing L1 to 2 m, the separation efficiency was slightly improved. Therefore, there was an optimized L1 for the separation. The diameter of the center riser (D1) also played an important role as shown in Figure 11. η significantly increased with decreasing D1 from 30 mm to 20 mm. η was approximately 60% at D1=30 mm and could be more than 90% at D1=20 mm at a solid feed rate of 100 g/s as shown in Figure 11(a). The total gas flow was constant during the optimization of L1 and D1, and u2 was maintained at 1.4 m/s. u1 increased with decreasing D1 to 20 mm. α decreased by approximately 10% as shown in Figure 11(b), which was probably ascribed to the higher injecting velocity at the center riser. In addition, α also decreased by approximately 7% with increasing L1 from 1 m to 1.5 m or 2 m as shown in Figure 10(b). To maintain a similar velocity in the CS, the total gas flow in the CS was not changed when optimizing L1 and D1. However, the total gas flow was nearly doubled when increasing the diameter of the annular fluidized bed (D) from 70 mm to 100 mm. As Figure 12 shows, the separation was greatly enhanced after increasing D. At the same η, the solid feed rate could be increased by a factor of 2–3 after increasing D to 100 mm.


Page 15 of 27

1.0



1.0 0.9 0.8 0.7

2m 1.5 m 1.15 m

0.6

0.9 0.8 0.7

2m 1.5 m 1.15 m

0.6 0.5

0.5 40

50

60

70

80

90

50

100 110 120

60

70

80

90

100

110

120



(a) η (b) α Figure 10. Effect of L1 on (a) η and (b) α at different solid feed rates in Test 7. (u1=4 m/s, u2=1.4 m/s, u3=1.68 m/s, cpb,mix=7%, Mixture V, CS3, CS2, CS1)

1.0



1.0 0.9 0.8 0.7

20 mm 30 mm

0.6 0.5

0.9 0.8 0.7

20 mm 30 mm 0.6 0.5

50

60

70

80

90 100 110 120 130 140 150

50

60

70

80

90 100 110 120 130 140 150



(a) η (b) α Figure 11. Effect of D1 on (a) η and (b) α at different solid feed rates in Test 8. (u1=6.6 m/s, u2=1.4 m/s, u3=1.68 m/s, cpb=7%, Mixture V, CS4, CS1)

1.0

0.9 D=100 mm u1=4.7 m/s u2=1.4 m/s u3=1.6 m/s

0.8

D=100 mm u1=5.5 m/s u2=1 m/s u3=1.3 m/s

0.7

D=70 mm u1=4 m/s u2=1.4 m/s u3=1.68 m/s

0.6


1.0


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.9 0.8 0.7 0.6 0.5

0.5 40

60

80

100

120

140

160


180

200

40

60

80

100

120

140

160

180

200


(a) η (b) α Figure 12. Effect of D on (a) η and (b) α at different solid feed rates in Test 9. (cpb,mix=7%, Mixture V, CS5, CS1)

4.3 Fluidization behavior in the annular fluidized bed of the CS The gas–solid drag and the collision force between particles were significantly influenced by the fluidization behavior, which is assumed to be a vital factor of the separation of the binary particles. Therefore, it is necessary to study the fluidization behavior in the annular fluidized bed of the CS.


3000

m& mix ,in = 141 g / s

2500

m& mix ,in = 97 g / s 3000

2000

2500 2000

1500

1500 1000

1000

500 0 0.000 0.005 0.010 0.015 0.020

500

0 0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16

The average solid volume fraction Figure 13. The axial distribution of the solid volume fraction in the annular fluidized bed of CS4 at solid feed rates of 97 g/s and 141 g/s in Test 8 (The location of the gas distributor of annular zone is set as zero height). One of the important indicators of the fluidization behavior is the solid volume fraction in the annular fluidized bed. As Figure 13 shows, the distribution of solid volume fraction (ɛs) along the annular fluidized bed generally decreased with increasing height. ɛs significantly decreased above the height of Hdense. The solid volume fraction at the bottom of the annular zone, for which the height was between 0 and Hdense, was much higher than ɛs above the height of Hdense. It was also found in Figures 13 that ɛs along the annular fluidized bed increased with increasing solid feed rate, and at the bottom of the annular zone, the increased extent of ɛs was the largest. By studying the change in the fluidization behavior after optimizing the CS structure, it was possible to understand the mechanism and process of the binary particle separation in the annular CS and the means of enhancing separation. ɛs along the annular zone decreased with decreasing D1 from 30 mm to 20 mm, and the decrease extent of ɛs at the bottom of the annular zone was the largest, as shown in Figure 14. A similar phenomenon occurred when increasing D from 70 mm to 100 mm as shown in Figure 15. This phenomenon was ascribed to the fact that the volume of the annular fluidized bed increased with decreasing D1 or increasing D. 3000

20mm-110 g/s 30mm-112 g/s

2500

Height (mm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Height (mm)


2000 1500 1000 500 0 0.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

The average solid volume fraction

Figure 14. The axial distribution of the solid volume fraction in the annular fluidized bed of CS4 and CS1 at solid feed rates of 110 g/s in Test 6 and Test 8.


Page 16 of 27

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3000 2500

Height (mm)

Page 17 of 27

D=100 mm D=70 mm

2000 1500 1000 500 0 0.00

m& mix ,in =112 g / s

m& mix ,in =138 g / s 0.01

0.02

0.03

0.04

0.05

0.06

0.07

The average solid volume fraction

Figure 15. The axial distribution of the solid volume fraction in the annular fluidized bed of CS5 and CS1 in Test 6 and Test 9. In addition to the solid volume fraction, the mass concentration of plastic beads (cpb) in the sample taken from the annular fluidized bed is another important indicator of the fluidization behavior. In the annular zone, the cpb of the sample collected from the position near the wall was measured, whereas in the cylindrical zone, the concentration in the sample collected from the position near the center was measured. cpb was higher than the true concentration because the plastic beads were more easily collected. However, the measured concentration of plastic beads was still a good indicator of the amount of plastic beads around the sampling ports. The concentration at the height of 0 mm (cpb(H=0 mm)) was nearly equal to the concentration of plastic beads in the bottom particles. The segregation of the plastic beads in the annular fluidized bed is shown in Figure 16. cpb generally increased along the annular fluidized bed. cpb at 500 mm above the exit of the center riser was usually lower, which was ascribed to the inlet effect. This inlet effect also existed in Sun’s work 25, but cpb was higher at the particle inlet position in the riser-based CS. When reducing D1 from 30 mm to 20 mm, cpb(H=0 mm) could be decreased as shown in Figure 17. Therefore, η was improved as shown in Figure 11. However, the cpb distribution was similar and did not decrease obviously. Even when cpb(H=0 mm) was zero at a small solid feed rate, the cpb distribution at the height of 0–500 mm was still maintained at a high level as shown in Figure 16(b) and Figure 17. This indicates that many plastic beads sank to the bottom of the annular zone, whereas cpb(H=0 mm) could be reduced to nearly zero at the optimized conditions. It is assumed that the separation at the bottom of the annular zone was important.



3500

m& mix,in ≈ 110 g / s

m& mix ,in ≈ 50 g / s

Height (mm)

3000 2500 inlet

2000

inlet

1500

inlet 1000

L1=2 m L1=1.5 m

500

L1=1.15 m

0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 (a)

0.0 0.1 0.2 0.3 0.4 0.5 0.6 (b)

The plastic bead concentration Figure 16. The axial distribution of the plastic bead concentration in the annular fluidized bed of CS3, CS2 and CS1 at solid feed rates of 110 g/s and 50 g/s in Tests 6 and 7 (The location of the gas distributor of annular zone is set as zero height). 3500

m& mix ,in ≈ 70 g / s

D1=20 mm

3000

D1=30 mm

2500 1000

2000

800

Height (mm)

Height (mm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1500

600

D1=20 mm m& mix ,in =73 g / s m& mix ,in =97 g / s m& mix ,in =141 g / s

400

1000 200

500

0 0.00

0.05

0.10

0.15

0.20

The plastic bead concentration

0 0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

The plastic bead concentration Figure 17. The axial distribution of the plastic bead concentration in the annular fluidized bed of CS4 and CS1 in Test 6 and Test 8.

5 Discussion 5.1 Process analysis of plastic bead separation from ilmenites In the fast fluidization bed, the axial solid distribution is denser at the bottom region and dilute in the upper region; the radical solid distribution is dilute in the core with a denser falling annulus20. The particle cluster is the key feature of gas–solid fast fluidization, which greatly influences the gas–solid drag and the gas–solid slip velocity31, 32. The solid volume fraction in the cluster is significantly greater than the time-averaged solid volume fraction in the local position32. Based on the experimental results of the present work, this non-uniformity of the solid distribution in the scale of the riser and the local position is assumed to also exist in the annular fluidized bed of the CS. The possible fluidization structure of the annular fluidized bed is shown in Figure 18. Figure 18 shows that the dilute phase and cluster phase co-exist in the annular fluidized bed.


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The dilute phase is the gas–solid drag dominant zone, and the plastic beads in the dilute phase could be separated from ilmenites. However, the cluster phase is the solid contact dominant zone32, and the plastic beads in the cluster phase could be carried by the falling particle cluster owing to the momentum exchange inside the cluster, which could explain the phenomenon. 1) The separation of plastic beads from ilmenites was not enhanced obviously with increasing length of the annular zone as shown in Figure 10. 2) Even in the case of nearly complete separation of the plastic beads from ilmenites, the measured average plastic bead concentration in the annular zone was still high, as shown in Figures 16–17. Therefore, clusters carrying the plastic beads could be the main reason why a fraction of the plastic beads is finally mixed with ilmenites. The plastic bead concentration (cpb) significantly decreased at the bottom of the annular zone, as shown in Figures 16-17. This indicates that the cluster is broken at the bottom of the annular zone. The bottom area is co-controlled by the solid collision and the gas–solid drag, in which the plastic beads have the possibility of being carried up by the gas flow, which is presented in Figure 18. The separation of plastic beads from ilmenites at the bottom of the annular zone is highly important for the separation efficiency of plastic beads (η).



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 18. Schematic of the binary particle fluidization structure in the annular fluidized bed. In the radial direction, cpb at the center of the annular zone was higher than cpb at the wall area in the annular zone, as shown in Figure 19. The fluidization structure of the plastic beads in the annular zone is assumed to be the annular-core structure, which is presented in Figure 18.


Page 20 of 27

600 500

Height (mm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


wall center

400 300 200 100 0 0.00

0.02

0.04

0.06

0.08

0.10

The plastic beads concentration

Figure 19. The axial distribution of plastic bead concentration in the annular zone at two radial positions (wall and center) in Test 9. The cluster size, shape and solid fraction of the cluster are changing with time, and the cluster is also probably changing through the processes of growing, breaking and merging with other clusters33. During the dynamic change of clusters, the plastic beads are possibly transferred from the inside of the cluster to interface of the cluster and then carried by gas to the core area. The opposite process is also likely to occur in which the dynamic cluster absorbs the plastic beads transferred from the core area. The two processes could be balanced after some residence time of clusters in the annular zone. 5.2 Factor analysis of plastic bead separation from ilmenites Based on the discussion above, it is assumed that the solid volume fraction (ɛs) at the bottom of the annular zone is highly important and that a lower ɛs would enhance the binary particle separation at the bottom area of the annular zone. The solid volume fraction at the bottom of the annular zone could be significantly decreased after optimizing the structure of the annular CS, as Figure 20 shows. Reducing ɛs at the bottom of the annular zone is probably the key to improving the separation extent of the binary particles. The average solids volume fraction at the height from 0 mm to 500 mm

Page 21 of 27

0.06 CS1 CS2 CS4 CS5

0.05 0.04 0.03 0.02 0.01 0.00 60

80

100

120

140

160

180

200

The solid feed rate (g/s) Figure 20. Effect of solid feed rate on the average solid fraction at heights between 0 mm and 500 mm for CS1–4 in Tests 6–9. (cpb,mix=0.07, u2=1.4 m/s, u3=1.68 m/s, Mixture V) The separation efficiency of the plastic beads (η) decreases linearly with increasing solid feed rate, as Figure 4(a) shows. This may be because the cluster size and solid volume fraction in the cluster increase with increasing solid feed rate32. Larger and denser clusters would carry higher



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

fractions of plastic beads down. In addition, the solid volume fraction at the bottom of the annular zone also increases with increasing solid feed rate as shown in Figure 20, which increases the average collision force acting on the plastic beads at the bottom area. The size and density of the binary particles determine the potential of the binary particle separation. The size of the plastic beads significantly influences η as shown in Figure 8. This could be explained by equations (24) and (25) 27; increasing size will cause a longer moment response time and shorter average time between particle–particle collisions. The moment (velocity) response time is

ρ p d p2 τV = 18µc

(24)

The average time between particle–particle collisions is

τC =

1 1 = fC 4 π nd p 2 v '

(25)

The ratio of the momentum response time of a particle to the average time between collisions is 4 τV 4n π ρ p d p v ' = τC 18µc

(26)

where μc is the gas viscosity; ρp and dp are the particle density and diameter, respectively; fC is the particle collision frequency; n is the number density of particles; v’ is the mean particle velocity. The ratio of the two response times shown in equation (26) is used to estimate the “collision force vs gas drag”. The larger ratio of the two response times indicates that the collision is more important. We can find that the time ratio would be significantly increased by increasing the size of the plastic beads. Based on equation (26), it is assumed that the influence of the particle size on the separation could be much larger than that of the particle density, which is opposite to the conclusion that the particle density plays a more important role in the segregation in a bubbling fluidized bed19. The plastic beads at lower concentrations are more easily separated from ilmenites as shown in Figure 9. This is beneficial to the operation of annular CS in CLC, in which the mass concentration of char particles in the bed materials of FR is usually less than 2%7. The plastic beads at higher concentrations possibly tend to agglomerate to larger clusters, and η decreases owing to the larger “particle” size as shown in Figure 8, which is the possible reason for the lower η at higher cpb. 6 Conclusion An annular carbon stripper for chemical-looping combustion of coal was designed, built and optimized, which can separate the binary particle mixture efficiently. The annular carbon stripper coupled into the CLC system has the ability to greatly enhance the carbon capture efficiency. The separation efficiency of plastic beads is improved at a lower solid feed rate, higher gas velocity, smaller size of the plastic beads and lower concentration of plastic beads. The separation efficiency of ilmenite particles is improved at a higher solid feed rate, lower gas velocity and lower concentration of plastic beads. The gas velocity in the annular zone is highly


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important to the separation of plastic beads from ilmenites. The solid distribution in the annular fluidized bed is dense at the bottom region and dilute in the upper region. A fraction of plastic beads were carried down by the falling cluster. The plastic bead separation from ilmenites at the bottom of the annular zone is vital to the annular CS. The plastic bead motion structure in the annular zone is the annular-core structure. Reducing the solid volume fraction at the bottom of the annular zone is the key to enhancing the separation of binary particles in the annular CS. Supporting Information The operation process of CS was included in the supporting information file, which could give operation time interval. In addition, more information about the axial distribution of solid volume fraction and plastic bead concentration along the annular fluidized bed were presented in the supporting information file. This information is available free of charge via the Internet at http://pubs.acs.org/. Acknowledgments This work was supported by the National Natural Science Foundation of China (51376105, 91434124, 51561125001), Tsinghua University Initiative Scientific Research Program, Program for New Century Excellent Talents in University (NCET -12-0304) and the National Key Research and Development Project (2016YFB0600802-A). Nomenclature a= constant in equation (9), which is related to the gas velocity (m-1) cpb= plastic bead mass concentration in samples, dimensionless cpb,top= plastic bead mass concentration in top particles, dimensionless cpb,bottom= plastic bead mass concentration in bottom particles, dimensionless cpb, mix= plastic bead mass concentration in binary mixtures, dimensionless d50=particle size (μm) dp= particle diameter (m) dH= heavy particle diameter (m) dL= light particle diameter (m) D1= diameter of the center riser (m) D= diameter of the annular fluidized bed (m) e= natural exponent, dimensionless fC= particle collision frequency (s-1) Fd= gas–solid drag (N) Fd,L= gas–solid drag acting on light particles (N) Fd,H= gas–solid drag acting on heavy particles (N) Fc= collision force between particles (N) Fc,L= collision force between particles acting on light particles (N) Fc,H= collision force between particles acting on heavy particles (N) g = gravity acceleration rate (m/s2) hi-j= distance between pressure ports i and j (m) L= length of center riser (m) L1= length of annular zone (m) L2= length of cylindrical zone (m)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

m& mix,in = rate of solid feeding into the annular CS (kg/s) mH = mass of heavy particles (kg) mL = mass of light particles (kg) mtop = mass of top particles (kg) mbottom = mass of bottom particles (kg) mpb,top = mass of plastic beads in the top particles (kg) mpb,bottom = mass of plastic beads in the bottom particles (kg) milm,top = mass of ilmenites in the top particles (kg) milm,bottom = mass of ilmenites in the bottom particles (kg) n= number density of particles (m-3);

∆P = pressure drop of the center riser (Pa) ∆Pi - j = pressure drop between pressure ports i and j (Pa) S1= cross-sectional areas of the center riser (m2) S2= cross-sectional areas of the annular zone (m2) S3= cross-sectional areas of the cylindrical zone (m2) t= time of the mixture solid feeding (s) ug= operational gas velocity in CS (m/s) u1=operational gas velocity in center riser (m/s) u2=operational gas velocity in annular zone (m/s) u3=operational gas velocity in cylindrical zone (m/s) ut= terminal velocity of particles (m/s) ut,L= terminal velocity of light particles (m/s) ut,H= terminal velocity of heavy particles (m/s) v’=mean particle velocity (m/s) vH=heavy particle velocity (m/s) vL=light particle velocity (m/s) ρp= particle density (kg/m3) ρH= heavy particle density (kg/m3) ρL= light particle density (kg/m3) ω= weight loss ratio of particles after heating at 800°C in a muffle furnace for 0.5 h, dimensionless ωilm= weight loss ratio of the samples from the pure ilmenite after heating in the furnace at 800°C for 0.5 h, dimensionless ωmix = weight loss ratio of the samples from annular fluidized bed after heating in the furnace at 800°C for 0.5 h, dimensionless ɛs= solid volume fraction in the annular CS, dimensionless ɛs*= saturated carrying capacity of the gas flow in the center riser, dimensionless ɛsd= solid volume fraction in the dense region of the center riser, dimensionless ɛse= solid volume fraction at the exit of the center riser, dimensionless ɛs,i-j= average solid volume fraction between pressure ports i and j, dimensionless ƞ= separation efficiency of plastic beads, dimensionless


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α= separation efficiency of ilmenites, dimensionless δ= wall thickness of the center riser (m) τV= moment (velocity) response time (s) τC= average time between particle–particle collisions (s) μc= gas viscosity (kg/(m·s)) References 1.

Abanades, J. C.; Arias, B.; Lyngfelt, A.; Mattisson, T.; Wiley, D. E.; Li, H.; Ho, M. T.; Mangano, E.;

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Figure. For Table of Contents Only


Annular Carbon Stripper for Chemical-Looping ... - ACS Publications

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