Hydrodynamic simulation and optimization of the

Powder Technology 321 (2017) 336–346

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Hydrodynamic simulation and optimization of the feeding system of a bubbling fluidized-bed gasifier in a triple-bed circulating fluidized bed with high solids flux Zhongkai Zhao a, JingXuan Yang a, Wei Zhang a, Peng Li b, Wenhao Lian a, Zhonglin Zhang a, Yuming Huang a, Xiaogang Hao a,⁎, Chihiro Fushimi c, Guoqing Guan d,⁎ a

Department of Chemical Engineering, Taiyuan University of Technology, Taiyuan 030024, China School of Chemical and Biological Engineering, Taiyuan University of Science and Technology, Taiyuan 030024, China c Department of Chemical Engineering, Tokyo University of Agriculture and Technology, 2-24-16 Naka-cho, Koganei, Tokyo 184-8588, Japan d North Japan Research Institute for Sustainable Energy, Hirosaki University, 2-1-3 Matsubara, Aomori 030-0813, Japan b

a r t i c l e

i n f o

Article history: Received 28 June 2017 Received in revised form 11 August 2017 Accepted 14 August 2017 Available online 15 August 2017 Keywords: Triple-bed circulating fluidized bed Bubbling fluidized bed High solids mass flux Feeding tube

a b s t r a c t A bubbling fluidized bed (BFB) with a continuous feed/discharge of solids was used as a gasifier in a high-density triple-bed circulating fluidized bed (TBCFB). To carry the heat from the combustor to the pyrolyzer/gasifier effectively, a high-density and high solids-mass flux circulating system is necessary. In this study, the gas-solid flow behavior of a BFB with high solids continuous feeding/discharging mass flux was simulated by an Eulerian– Eulerian model incorporating the Lagrangian (discrete phase model) properties of particles as a tracer to analyze particle motion and determine the optimal feeding tube location and diameter. It was observed that a U-shaped track of particles from the inlet to the outlet and double vortices resulted in particle back-mixing and extended particle residence time. According to the results, some feeding-tube design principles were proposed to achieve smooth solids flow and improve the particle residence time in the BFB in solids mass circulating fluxes ranging from 800 to 1200 kg/m2 s. © 2017 Elsevier B.V. All rights reserved.

1. Introduction The circulating fluidized bed (CFB) has been widely used in chemical industrial processes such as combustion, gasification, and fluid catalytic cracking because of its higher gas-solids contact efficiency, enhanced mass, and heat transfer characteristics [2–10]. However, in the traditional CFB gasifier, when pyrolysis and gasification occur in the same reactor, the tar, light hydrocarbon gas, and hydrogen produced by coal pyrolysis seriously hinder the gasification of char [11,12]. To maintain the activity of the catalyst and improve the gasification efficiency of char, the pyrolysis process and gasification process should be separate [13–15]. A new concept of triple-bed circulating combined fluidized bed (TBCFB) was proposed by Tsutsumi et al. [10]. A TBCFB (Fig. 1) is mainly composed of a downer (pyrolysis reactor), a bubbling fluidized bed (BFB) (gasifier), and a riser (combustor) [1,11,16–19]. A typical TBCFB first pyrolyzes the coal quickly in the downer pyrolyzer, and then, the produced gases and tar are separated from the char by a gassolid separator. The produced char enters the BFB gasifier for steam gasification. The non-reacting char flows out of the BFB and moves into the riser combustor to be combusted by oxygen. Many heat carrier particles ⁎ Corresponding authors. E-mail addresses: [email protected] (X. Hao), [email protected] (G. Guan).

http://dx.doi.org/10.1016/j.powtec.2017.08.046 0032-5910/© 2017 Elsevier B.V. All rights reserved.

circulate in the system to transfer heat from the riser combustor to the downer pyrolyzer for coal pyrolysis and to the BFB gasifier for char gasification [11]. In the entire cycle, the system must be operated under the condition of high solid particle circulating flux to efficiently utilize the heat produced by char combustion in the riser [17]. Guan et al. [11] reviewed the studies related to TBCFBs and proposed that the following research is still needed: (1) theoretical and experimental studies on the binary solids system, (2) heat and mass transfer in the downer with high solids density, (3) residence time distributions of solids and gas to reaction modeling and structure design, and (4) effects of inlet and exit structures on the hydrodynamics of the system. To solve these problems, some fundamental studies on TBCFB structures and hydrodynamics operated at high density and high solids mass flux have been performed in recent years. Guan et al. [16] obtained the effects of solids inventory and the seals between the three reaction zones on the solid particles circulating flux. They found that the height of the gas seal between the downer and BFB (seal DB) and the seal between the BFB and the riser (seal BR) should be designed carefully to guarantee a smooth passage of particle flow with high solids mass flux [17]. In the work of Fushimi et al. [18], a gas-sealing bed (GSB) was installed between the BFB and the riser bottom to increase the pressure head to transport solids to the riser. It was found that the static bed height of the GSB had a great effect on the solid particles circulating

Z. Zhao et al. / Powder Technology 321 (2017) 336–346

Nomenclature

△t γΘs

time steps, s collisional dissipation of energy, m2 s−2

drag coefficient, dimensionless CD Cμ, C1ε, C2ε coefficients in turbulence model, dimensionless particle diameter, m ds DPM particle diameter, m dp D diameter of feeding tube, m F DPM particle force, N other interphase forces, N Fother particle-particle restitution coefficient, dimensionless es particle-wall restitution coefficient, dimensionless ew probability density function of DPM particle residence E(tp) time, dimensionless cumulative distribution function of DPM particle F(tp) residence time, dimensionless g gravitational acceleration, m/s2 g0 radial distribution coefficient, dimensionless production of turbulent kinetic energy, dimensionless Gk, m circulating flux of solid particles, kg/m2s Gs circulating flux of DPM, kg/m2s Gt H height of BFB, m height of BFB outlet, m Houtlet height of seal between downer and bubbling fluidized Hdb bed, m

τg

shear stress of gas phase, N m−2

τs λs ρg ρp ρs ρi ρm

shear stress of solids phase, N m−2 solids bulk viscosity, Pa s gas density, kg m−3 DPM particle density, kg m−3 solids density, kg·m−3 density of phase i, kg m−3 density of system m, kg m−3

I I2D

stress tensor, dimensionless second invariant of the deviatoric stress tensor, dimensionless k turbulence kinetic energy tensor, dimensionless diffusion coefficient for granular energy, kg/s m kθs interphase exchange coefficient, kg m2/s Kgs L length of BFB, m minimum distance between opposite side wall of BFB Lt outlet and feeding tube, m Mdischarging solid particle discharging mass flow rate, kg/s Mfeeding solid particle feeding mass flow rate, kg/s DPM particle mass flow rate, kg/s mp number of entrained DPM particles in dtp, dimensionless Ndtp number of DPM particles, dimensionless N1 p pressure, Pa particulate phase pressure, Pa ps Re relative Reynolds number, dimensionless particle Reynolds number, dimensionless Res t time, s time of DPM particle injection, s tp gas velocity, m s−1 vg ! νm velocity vector of system m, m s−1 vp DPM particle velocity, m s−1 vs solids velocity, m s−1 W width of BFB, m volume fraction of gas phase, dimensionless αg volume fraction of phase i, dimensionless αi volume fraction of solids phase, dimensionless αs ε turbulence dissipation rate, m2 s−3 φgs dissipation of granular energy resulting from the fluctuating forcer, m2 s−2 μg viscosity of gas phase, Pa s solids shear viscosity, Pa s μs solids collisional viscosity, Pa s μs, col. solids kinetic viscosity, Pa s μs, kin solids frictional viscosity, Pa s μs, fr frictional viscosity of system m, Pa s μt, m granular kinetic theory parameter (kinetic viscosity), Pa s σε θ angle of internal friction, degrees granular temperature, m2 s−2 Θs

337

flux. They also studied the influence of GSB on the solids holdup of the riser and downer [18,19]. Fushimi et al. [1] investigated the flow behaviors of a binary mixture of silica sand and nylon shot (coal char substitute) in a TBCFB as a cold coal-gasifier model. Cheng et al. [20] conducted numerical simulations to research heat transfer in the downer and quantify the mixing characteristics under normal and tangential arrangements of the lateral injection nozzle near the inlet of the downer. Shu et al. [21] developed a multifluid CFD numerical framework to simulate the coal pyrolysis in a downer reactor, which utilized the particle radiation mechanism and combined species transport equations, the reaction kinetic model, and the water evaporation model. The BFB is an important part of the TBCFB system and plays an important role in improving the overall reaction rate of the TBCFB because the gasification reaction is a rate-determining step. In addition, BFB affects solids circulation flux of the entire system. Guan et al. [1,17–19] used a small-scale BFB to measure the solids holdup and analyze the change in pressure in the local area of a BFB. However, research on the BFB with continuous feed/discharge flow characteristics, optimization of flow field, and structure and residence time distributions combined with a downer/drop tube pyrolyzer and riser have not yet been performed. The initial solid particle circulating flux of a TBCFB can only reach 114 kg/m2 s. Through the addition of GSB between the BFB and riser, Fushimi et al. [18] solved the limitation of the BFB outlet to solid particles circulating flux and then successfully obtained values above 500 kg/m2 s; however, it is difficult to continue increasing solid particle circulating flux. In this case, the gas-solid flow behavior in the BFB and the design of the feeding tube were key factors in restricting the solids circulation of the entire system. The BFB feeding tube is the channel connected with the downer; therefore, a change in the feed rate will affect the flow behavior of the gas and the solid phases in the BFB [22]. At the same time, the position of the feeding tube will influence the flow-field structure. Flow behavior and flow-field structure affect the residence time distributions which are essential for the reaction. In this study, the numerical simulation of the BFB with continuous feeding and discharging as shown in Fig. 1 was used to determine the optimum feeding tube parameters. 2. Methods FLUENT 15.0 commercial software, a 2D grid, and the Eulerian– Eulerian two-fluid model were applied in this study. A BFB was constructed as a cold model for a gasifier and ambient air was used instead of steam in the simulation. The gas-solid flow behaviors in the BFB reactor were simulated by solving the mass and momentum conservation equations combined with other equations related to the specific process. At the same time, the Lagrangian (discrete phase model - DPM) properties of particles were coupled in an Eulerian–Eulerian two-fluid model as a tracer, which was used to determine the residence time distributions of solid particles in the BFB [23–26]. The characteristics of the DPM particles based on a random-walk model were chosen, assuming they had the same properties as the solid particles. In this study, Gt was the mass flux of tracer particles. It was found that when Gs/Gt =

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1000, the particles of the DPM flowed only when accompanied by solid particles, and their effect on the gas-solid flow behavior was negligible. 2.1. Eulerian–Eulerian two-fluid model and the kinetic theory of granular flow The Eulerian–Eulerian two-fluid model incorporated with the kinetic theory of granular flow was used to simulate the gas-solid flow behavior in a BFB. The k-ε model was used to describe the gas turbulence, and the kinetic theory of granular flow was used to describe the particle collision behavior. The governing equations can be summarized as follows [27–36]: mass conservation equations of gas (g) and solid (s) phases,
 ∂ α g ρg ∂t

 
  μ t;m ∂ ε ! ðα i ρm εÞ þ ∇  α i ρm ε ν m ¼ ∇  α i ∇ε þ α i C 1ε Gk;m −C 2ε ρm ε σε k ∂t ð6Þ þα i ρm Π ε ;

and for the assumption of excellent mixing in the reactor, ρm ¼ Σα i ρi ; N X

! νm ¼

ð7Þ

! α i ρi ν m

i¼1 N X

ð8Þ α i ρi ;

i¼1 2


 ! þ ∇  α g ρg v g ¼ 0;

ð1Þ


 ∂ðα s ρs Þ ! þ ∇  α s ρs v s ¼ 0; ∂t

ð2Þ

and momentum conservation equations of gas (g) and solid (s) phases,

μ t;m ¼ ρm C μ

k : ε

ð9Þ

The kinetic theory of granular flow, which considers the conservation of solids fluctuation energy, was used for the closure of the solids stress terms [37]. According to the particle phase fluctuation of the laminar flow mechanism, the concept of “particle temperature” was proposed and applied to Gidaspow's model [26].



 ∂
!! ! ! α g ρg vg þ ∇  α g ρg ν g ν g ¼ −α g ∇p þ ∇  τg þ K gs ν s − ν g ∂t ! ð3Þ þ α g ρg g ;



   * * 3 ∂ ðρs α s Θs Þ þ ∇  ρs α s ν s Θs ¼ −ps I þ τ s : ∇ν s −∇  kΘs ∇Θs −γθs −φgs : 2 ∂t


 ∂ !! ðα s ρs vs Þ þ ∇  α s ρs ν s ν s ¼ −α s ∇p−∇ps þ ∇  τs ∂t
 ! ! ! þ K gs ν g − ν s þ α s ρs g :

The particle energy was assumed to be in a steady state while dissipation was considered to occur only in the local region. Furthermore, convection and diffusion were ignored, and only the generation and dissipation terms were preserved [38]. Eq. (12) was simplified as an algebraic expression of the particle temperature:

ð4Þ

The k-ε model is written as follows:  
 ∂ μ ! ðα i ρm kÞ þ ∇  α i ρm k ν m ¼ ∇  α i ∇k þ α i Gk;m −α i ρm ε σk ∂t þ α i ρm Π k ;

ð10Þ

! −ps I þ τs Þ : ∇ ν s−γ Θs ¼ 0: ð5Þ

ð11Þ

In this study, the momentum transfer between gas and solids was described by Gidaspow's drag force model [26]. The drag coefficient of

Fig. 1. Diagram of experimental equipment of a typical TBCFB from Fushimi et al. [1] and BFB structure.

Z. Zhao et al. / Powder Technology 321 (2017) 336–346

gas and solids was solved by using this model. The corresponding equations are:

Table 1 Simulation model parameters. Description

Value

Comment

Circulating flux of solid particles

1000 kg/m2 s 280 kg/m2 s for a 218 kg/m2 s for b 394 kg/m2 s for c 349 kg/m2 s for d 200–1200 kg/m2 s 2600 kg/m3 2009 kg/m3 for a and c 1875 kg/m3 for b and d 2600 kg/m3 128 μm 209.6 μm for a and c 264 μm for b and d 128 μm 0.5 m/s 2600 kg/m3 128 μm 0.2–1.2 kg/m2 s 1.225 kg/m3 1.7894 × 10−5 kg/m s 0.75 m 1.9 m 0.37 0.28,0.37,0.46,0.56 1m 0 0.14 0,0.14,0.27,0.41,0.64 0.05 0.05,0.25,0.45 1000 0.54 0.1 m/s 0.041 m/s for a and c 0.082 m/s for b and d 0.1 m/s Velocity-inlet Pressure-outlet No slip for air, specularity coefficient 0.01 for solid phase 0.63 Gidspow [26] Lun et al. [27] Schaeffer [33] 30° Algebraic Gidspow [26] 0.95

SL.1.,2 SL.3.

Particles density

Particles diameter

DPM injections particles velocity DPM injections particles density DPM injections particles diameter DPM injections particles mass flux Gas density Gas viscosity L H D/W Static bed height Hdb/Houtlet

Lt/L Gt/Gs Initial solids packing Superficial gas velocity

Inlet boundary conditions Outlet boundary conditions Wall boundary conditions Packing limit Granular viscosity Granular bulk viscosity Frictional viscosity Angle of internal friction Granular temperature Drag force Coefficient of restitution for particle–particle collisions Time steps Convergence criteria

339

0.0005 s 10−3

Fig. 2. Ratio of BFB discharging and feeding rates.

SL.4. SL.1,2 SL.3. SL.4. SL.1.,2 SL.3. SL.4. For all cases For all cases For all cases For all cases For all cases For all cases For all cases For all cases SL.1.,2,3 SL.4. For all cases SL.1.,2 SL.3. SL.4. SL.1.2,3, SL.4. For all cases For all cases SL.1.,2 SL.3.

at α g N 0:8;

K gs

and at α g ≤ 0:8;



! !

3 α s α g ρg ν s − ν g −2:65 ¼ CD αg ; ds 4

K gs ¼ 150

  α s 1−α g μ g 2

α g ds



! !

7 α s ρg ν s − ν g

þ : ds 4

"  0:687 # 24 3 CD ¼ ; for Res N 1000; 1þ α g Res α g Res 20 C D ¼ 0:44; for Res N 1000;

SL.4. For all cases For all cases For all cases For all cases For all cases For all cases For all cases For all cases For all cases For all cases For all cases For all cases For all cases

Fig. 3. Grid sensitivity analysis.

ð12Þ

ð13Þ

ð14Þ

ð15Þ

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Z. Zhao et al. / Powder Technology 321 (2017) 336–346

and Res ¼



! !

ρg ds ν s − ν g

μg

:

ð16Þ

and momentum transfer between DPM particles and solids phase,

F¼ Nowadays, these constitutive equations for the solids phase stress based on the kinetic theory concepts of Lun et al. [29] are widely accepted. In the two-fluid model, constitutive equations were used to describe the rheology of the solids phase. The constitutive equations are as follows:

" X 18βμ C D Re  s 2

ρp dp 24



#

vp −vs þ F other mp Δt:

2.2. Reactor geometry and simulation conditions The following reasonable assumptions regarding the BFB were set to simplify the calculation model:

gas-phase stress tensor, 
 2 ! ! T ! − μg∇  ν g; τg ¼ μ g ∇ ν g þ ∇ ν g 3

ð17Þ

solid-phase stress tensor,   
 2 ! ! ! T þ α s λs − μ s ∇  ν s  I; τs ¼ α s μ s ∇ ν s þ ∇ ν s 3

1) the gas distributor was able to create a uniform distribution of gas, and 2) the sphericity of solid particles was 1, and the particle diameters were the same.

ð18Þ

bulk viscosity, λs ¼

 1=2 4 2 Θs ; α s ρs ds g0 ð1 þ es Þ π 3

ð19Þ

radial distribution function, "  1=3 #−1 αs ; g 0 ¼ 1− α s; max

ð20Þ

solids shear viscosity, μ s ¼ μ s;col þ μ s;kin þ μ s;fr ;

ð21Þ

solids collision viscosity, μ s;col ¼

 1=2 4 Θs ; α s ρs ds g 0  ð1 þ es Þ  π 5

ð22Þ

kinetic viscosity, μ s;kin ¼

 2 10ρs ds ðΘs πÞ1=2 4 1 þ g0 α s  ð1 þ es Þ ; 96α s ð1 þ es Þg 0 5

ð23Þ

solids frictional viscosity, μ s;fr ¼

ps sin θ 1=2

2I2D

;

ð24Þ

collision dissipation energy, γ Θs ¼

  12  1−e2s  g0 pffiffiffi ρs α 2s Θ3=2 s ; ds π

ð25Þ

and solids pressure, ps ¼ α s ρs Θs þ 2ρs ð1 þ es Þα 2s g 0 Θs :

ð26Þ

The discrete phase particle in Fluent 15.0 software can integrate the following force balance on a particle in a Lagrangian reference frame: momentum transfer between DPM particles and gas phase, F¼

" X 18βμ g C D Re  2

ρp dp 24

ð28Þ



#

vp −vg þ F other mp Δt;

ð27Þ Fig. 4. DPM sensitivity analysis.

Z. Zhao et al. / Powder Technology 321 (2017) 336–346

The main fluid and particle properties, the BFB structure, and other parameters used in the simulation are summarized in Table 1. The parameters marked SL.1, SL.2, SL.3, and SL.4 were used for the sensitivity analysis of the grid, the sensitivity analysis of the DPM, the simulation verification, and the simulation of the BFB structure optimization, respectively. When the solids circulating flux in the feeding tube was more than 1200 kg/m2 s, too many bed materials resulted and some particles moved out from the gas outlet on the top of the BFB but not from the particle outlet in the middle. As such, in our simulations, the maximum circulating flux of solids was set to 1200 kg/m2 s. As shown in Fig. 2, it was found that the feeding and discharging in the BFB achieved balance after 20 s and the flow field was in a relatively stable state. Therefore, the data obtained were derived from calculation results between 20 and 70 s. 2.3. Sensitivity analysis of grid Considering the balance of accuracy and the workload, we first analyzed the grid size sensibility to find the appropriate grid scale. The simulations were carried out with four grid dimensions: 10 × 10 mm, 5 × 10 mm, 5 × 5 mm, and 3 × 4 mm, and as shown in Fig. 3, the simulation results included pressure drop, solids holdup, and bed height between 25 s and 35 s. It can be seen that shrinking the grid scale improved the stability of the results. It should be noted that the results derived from the 5 × 5 mm and 3 × 4 mm grids were almost identical. The fluctuations of pressure drop, bed height, and solids volume fraction with time were related to either the number of grids or the turbulence.

341

In order to eliminate the influence of turbulence, the averages of pressure drop, bed height, and solids volume fraction were used for grid sensitivity study. The averages were found to be not change with the mesh, which indicated that the flow characteristics were no longer related to the size of mesh, and the results of simulation should be independent on the mesh. Therefore, to reduce the workload, the 5 × 5 mm grid was used in this study. 2.4. DPM sensitivity analysis The DPM particles were coupled with solid particles with the same initial velocity as a tracer to determine the residence time of solid particles in the BFB. The momentum exchange between the two existed in a two-fluid model. To ensure the veracity of the simulation results, the influence of the DPM particle injection on the flow distribution calculated by the two-fluid model should be negligible. As such, the ratio of the solid particle flux Gs to the DPM particle flux Gt was taken as a variable, and the sensitivity analysis of the DPM particle injection was performed. Fig. 4 shows the results of ten cases with different values of Gt. Obviously, with an increase in Gs/Gt, the results in the cases with DPM particle injection were closing to 18.13 kPa (pressure drop of gas), 0.4905 (solids volume fraction), and 1.511 m (bed height) which were the results of the case without DPM particle injection. When Gs/Gt was 1000, the results in the cases with and without DPM particle injection were almost identical. Therefore, it can be argued that the effects of the DPM particles on the flow distribution calculated by the two-fluid model can be ignored when Gs/Gt = 1000.

Fig. 5. Verification of model correctness.

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Z. Zhao et al. / Powder Technology 321 (2017) 336–346

2.5. Verification of model correctness In the initial stage of the study, we used the methods mentioned above to simulate the flow field in the BFB reported by Fushimi [1] and compared the simulation results with the experimental results. The experiment was carried out without chemical reactions, material consumption, or product generation, and the two types of particles mixed evenly. In Fushimi's work, the main analytical term was the pressure generated by the mixture, and a newly developed pressure balance model successfully predicted the Gs of the binary mixtures in the TBCFB. However, in our work, to simplify the simulation, two types of particles were weighted averaged as homogeneous particles. As shown in Fig. 5, a downward pressure trend existed along the axial position from the bottom to the top, and the maximum difference was less than 2 kPa. Compared to the pressure drop reported by Fushimi [1], there was an approximate one order of magnitude difference. Fig. 5 indicates the similarities between the experimental and simulation results. On this basis, another simulation was performed to optimize the structure of the BFB so that the entire system could run normally under the high circulating flux of solid particles. Here, inside the BFB, a U-shaped particle track from inlet to outlet and a double-vortex flow structure were observed. 3. Results and discussion 3.1. Horizontal position and length of feeding tube In general, char particles need sufficient residence time in a BFB to facilitate the gasification reaction in the BFB of the TBCFB, and studies on the residence time distributions of solids are essential for BFB structure design. Here, we investigated the effect of the horizontal position and the length of the feeding tube on the flow pattern in a BFB. In the simulation, we injected DPM particles instantaneously (a pulse input) as a tracer according to the proportion of Gs/Gt = 1000, and the number of DPM particles flowing to the outlet of the BFB per second was monitored to determine the particle residence time. First, the effect of the horizontal position of the feeding tube indicated by Lt/L was investigated. The case1, case2, and case3 structures in Fig. 1 were numerically simulated, and the cumulative distribution function curves of the DPM injected particle residence times were drawn by Eqs. (29) and (30).   E t p ¼ Ndtp =N0 :   F tp ¼

Z

Z t ave ¼

0

  E t p dt p :

ð30Þ

  t p E t p dt p :

ð31Þ

tp 0

∞

ð29Þ

As shown in Fig. 6(a), the amounts of entrained DPM particles before tp = 3 were 86.1%, 68.5%, and 53.2%. The average time based on Eq. (31) required for a particle to flow out of the BFB was the same time for the particles to remain in the BFB. From Fig. 6(a), the average residence times of three cases were 2.2, 2.87, and 3.59 s. The particle residence times were extended with a decrease in Lt/L. Although the feeding tube was as far from the outlet as possible, the particle residence times were still not long enough. As shown in Fig. 7, it is obvious that the particles flowed horizontally from the inlet to the outlet, which resulted in a short residence time, and therefore the particles did not make full contact with the gas in the reactor. To extend the particle residence times, we simulated the gas-solid flow behavior at different axial feeding positions. Here, in order to maintain the activity of the catalyst and improve the gasification efficiency of char, the pyrolysis process and gasification process should be separated. As such, a sealing system between gasifier and pyrolyzer is necessary. In this study, the feeding tube was inserted into the bed to make some

Fig. 6. Particle residence time distributions of DPM particles in BFB: (a) feeding-tube location, and (b) feeding-tube length.

particles accumulated inside to form a gas sealing. In other words, when the axial position of the inlet is lower than that of the outlet, a gas seal can be achieved. As shown in Fig. 6(b), the particle residence times in the BFB increased when the length of the feeding tube increased. The average residence times were 3.59, 4.45, and 5.18 s when Hdb/Houtlet was 0, 0.14, and 0.27, respectively. However, the residence time distributions of the DPM particles in the cases of Hdb/Houtlet = 0.27, 0.41, and 0.64 were very similar. Further lengthening of the feeding tube failed to obtain longer residence times, and the average residence time remained at approximately 5.2 s. However, it was found that the entire system cannot be operated at a high solid-particle circulating flux in our TBCFB system if too long a Hdb tube was used [16]. This may have resulted from the high resistance time of particle flow in the long feeding tube, which could slow down the particle moving rate and form a bottleneck for particles flowing from the downer into the BFB at high solids mass flux conditions [17]. Considering these two aspects, we determined that the optimal numerical value of Hdb/Houtlet should be 0.27. In a BFB with Lt/L = 0.05 and Hdb/Houtlet = 0.27, the flow pattern was different from that in the flow structure of an ordinary BFB. The internal circulation gas-solid flow behavior of traditional BFBs follows a radial arrangement. However, as shown in Fig. 8 (vector shown in the SCALE and SKIP of FLUENT 15.0 was set to 10 for image clarity.), the internal circulation attained an axial arrangement in this kind of structure. The parts ①, ②, ③, and ④ show a clockwise vortex at the upper part of the BFB, and ①, ⑤, ⑥, and ⑦ show a counterclockwise vortex at the lower part of the BFB. Under the dominant action of the double-vortex flow structure, the solid particles belonged to the two vortexes horizontally flowed with the same direction at the confluence of the two

Z. Zhao et al. / Powder Technology 321 (2017) 336–346

343

Fig. 7. Local flow-field vector of case1, case2, and case3 at Gs = 800 kg/m2 s and t = 50 s.

vortices. The special flow pattern resulted in the residence time distribution of particles as shown in Fig. 6. Owing to the U-shape flow, a few injected particles quickly reached the exit position and were entrained from the BFB, resulting in a short residence time. However, most particles were carried back by the two vortexes to the region near the feeding tube and mixed with newly injected particles. This back-mixing phenomenon extended the particle residence time. In the BFB, because of the coalescence of bubbles and the pressure change, small bubbles grew up and rose accelerative. The bubbles contained too much gas and weakened the gas-solid contact producing adverse effects on chemical reactions [39]. As shown in Fig. 9, in the BFB, the large number of small bubbles that formed at the bottom passed through the region of the U-shaped particle flow (region marked by the black line) before they coalesced and became larger, which increased the gassolid contact and the efficiency of the mass and heat transfer. 3.2. Diameter of feeding tube As shown in Fig. 10(a), the horizontal velocity in the central axis of the BFB was analyzed, and the horizontal-right was taken as the positive direction of the velocity vector. The region marked by the red line was the part of the U-shaped trajectory that flowed through the BFB central axis. The solids holdup in this region was statistically analyzed. As shown in Fig. 10(b), when the diameter of feeding tube reduced, the

solids holdup increased continuously. However, when the solids holdup increased to about 0.5, it almost ceased to increase; i.e., the maximum solids holdup of the U-flow in the BFB was about 0.49–0.5. As shown in Fig. 8, we selected the 400-cm2 square region next to the inlet of the BFB. Because it was the initial region of the flow of particles in BFB, the inlet condition directly determined the gas-solid flow in the region, and the region was studied to optimize the diameter of the feeding tube. Fig. 11 shows the average solids holdup in the regions from 20 to 70 s. It can be observed that when D/W was 0.46 or 0.56, the solids holdup increased linearly with an increase in Gs. When D/W was reduced to 0.28 or 0.37, there was a critical solids mass flux with respect to the solids holdup. When the Gs was smaller than the critical value, the solids holdup increased linearly with an increase in the Gs; when the Gs exceeded the critical value, the solids holdup exhibited an accelerating rise instead of a linear rise. At a low Gs, the particles entered into the square region and then smoothly flowed into region ① (Horizontal segment of the U-shape flow trajectory as shown in Fig. 10(a)). Due to the U-flow, they cannot be discharged rapidly. Thus, in both regions, the solids holdup increased linearly with an increase in Gs. However, when the solids holdup of region ① increased to about 0.5, it almost ceased to increase. The highest solids holdup indicated the carrying capacity of U-flow reached the maximum. It is obvious that the knee point of the curve shown in Fig. 11 located in the solids holdup of 0.5. Because the amount of injected particles exceeded the maximum carrying

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Fig. 8. Special flow behavior in BFB at Gs = 800 kg/m2 s and t = 50 s.

capacity of U-flow, more particles retained in the square region. Thus, the solids holdup increased more rapidly. Increasing the diameter of the feeding tube solved this problem, and the solids holdups in these two regions were almost identical. To ensure the uniformity of the solids holdup in the entire U-shaped flow trajectory, the Gs should be limited to 650 kg/m2 s for D/W = 0.28 or 810 kg/m2 s for D/W = 0.37. Obviously, decreasing the diameter of the feeding tube in the BFB reduced the solid particle circulating flux of the TBCFB system. From a design point of view, the limiting value of the Gs yielded the minimum feeding tube diameter. In addition, the effect of the feeding tube diameter on the particle residence time was investigated. As shown in Fig. 12, enlarging the feeding tube diameter decreased the average particle residence time. Considering the smooth flow and the residence time, the diameter corresponding to the limiting value of the Gs was optimum. According to these results, the feeding tube diameter should be D/W = 0.46 in the Gs range from 810 to 1200 kg/m2 s. The feeding tube diameter should be D/W = 0.37 when Gs is in the range of 650 to 810 kg/m2 s. 4. Conclusions

Fig. 9. Solids holdup distribution at Gs = 600 kg/m2 s and t = 30 s.

In this study, an Eulerian–Eulerian model incorporating the kinetic theory of granular flow was applied to simulate the gas-solid flow behavior in a BFB. DPM particles were coupled as the tracer to analyze particle motion in the BFB. To extend the particle residence time, the horizontal position of the feeding tube should be Lt/L = 0.05 and the length of feeding tube should be Hdb/Houtlet = 0.27 to create a U-shaped particle track from the inlet to the outlet and the formation of a double vortex above and below the U-shaped track. The doublevortex structure resulted in particles back-mixing, which extended the particle residence time. The uniformity of the solids holdup in the U-shaped flow was obviously affected by the diameter of the feeding

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Fig. 12. Particle residence time in BFB with different feeding tube diameters.

Acknowledgments This work was supported by the International Joint Research Project of Shanxi Province (Nos. 2015081051 and 2015081052), Special Talents Project of Shanxi Province (No. 201605D211005), and the Strategic International Collaborative Research Program of the Japan Science and Technology Agency. References

Fig. 10. Solids holdup in the horizontal segment of the U-shaped flow trajectory.

tube and the particle flux. A high small-diameter particle flux resulted in the accumulation of injected particles below the inlet; however, increasing the diameter of the feeding tube solved this problem. Considering both the smooth flow and the residence time, the feeding tube diameter should be D/W = 0.46 when the particle flux ranges from 810 to 1200 kg/m2 s for optimal performance.

Fig. 11. Effect of Gs and feeding-tube diameter on solids holdup in delineated region.

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