Simulation of coal pressurized pyrolysis process in an industrial-scale spout-fluid bed reactor

Advanced Powder Technology xxx (xxxx) xxx

Contents lists available at ScienceDirect

Advanced Powder Technology journal homepage: www.elsevier.com/locate/apt

Original Research Paper

Simulation of coal pressurized pyrolysis process in an industrial-scale spout-fluid bed reactor Guanwen Zhou a,b, Wenqi Zhong a,b,⇑, Aibing Yu b,c, Jun Xie a,b a

Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education, School of Energy and Environment, Southeast University, Nanjing 210096, PR China Centre for Simulation and Modelling of Particulate Systems, Southeast University – Monash University Joint Research Institute, Suzhou 215123, PR China c ARC Research Hub for Computational Particle Technology, Department of Chemical Engineering, Monash University, Clayton, VIC 3800, Australia b

a r t i c l e

i n f o

Article history: Received 22 November 2018 Received in revised form 1 July 2019 Accepted 22 September 2019 Available online xxxx Keywords: Pressurized grade conversion Coal pyrolysis Spout-fluid bed Industrial-scale MP-PIC

a b s t r a c t A three-dimensional Eulerian-Lagrangian model, facilitated with multiphase particle-in-cell (MP-PIC) method, was developed to simulate gas-solid flow and pyrolysis characteristics of coal (with the capacity of 500 thousand tons per year) in an industrial-scale spout-fluid bed reactor (H = 16.6 m and D = 3.1 m), aiming at providing guidance for industrial application of pressurized grade conversion of coal. The performance of the reactor and the effects of operating parameters such as coal feeding rate, semicoke to coal ratio, and particle sizes were numerically investigated. It was found that the flow pattern in this case is a ‘‘jet in the fluidized bed with bubbling”. The raise of pressure has a positive impact on the spouting structure and the flow uniformly. The increase of the semi-coke to coal ratio is beneficial to the coal pyrolysis, but the improvement of the pyrolysis is limited and the number of particles in the reactor will be sharply increased. With the increase of particle sizes, the flow pattern in the pyrolysis reactor tends to be stable while the mixing effectiveness is getting worse. It is suggested that the particle size of the material should range within 0–6 mm. Ó 2019 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.

1. Introduction Pressurized grade conversion of coal is attracting more and more attention from both research and industrial sectors, due to its remarkable contribution to the highly efficient and clean utilization of coal. Within this conversion process, the thermochemical reaction of coal is treated as a combination of low temperature pyrolysis and semi-coke combustion/gasification [1,2]. And the pyrolysis gas and tar are obtained as high-grade raw materials, while the semi-coke can be used for power generation, gasification and heat supply for coal pyrolysis [3,4]. Pyrolysis, as the initial step of coal grade conversion, has a significant effect on the thermochemical conversion of coal such as combustion and gasification. Some progress has been made in terms of the efficient coal pressurized pyrolysis techniques [5]. One of the promising attempts is to use semi-coke as the heat carrier and coal gas as the fluidizing agent, respectively. Studies have shown that by applying this technique, the yields of tar can be significantly increased and the heat can be recycled [6]. In our previous work, we have further demon-

⇑ Corresponding author at: Sipailou 2#, Nanjing 210096, Jiangsu, PR China. E-mail address: [email protected] (W. Zhong).

strated that under the coal gas atmosphere, the yields of tar increased with the pressure [7]. For the application of this technology, the most vital step is to develop an applicable reactor which can ensure the heat transfer between semi-coke and coal particles is quick and uniform. Spout-fluid beds have attracted increasing interest in the petrochemical, chemical, and metallurgical industries, benefiting from the efficient particle motion and sufficient contacts between multicomponent particle inside the reactor [8–10]. In recent years, there have been a number of theoretical and experimental studies on both atmospheric pressure and pressurized spouted-fluid beds. Its excellent performance in dealing with non-spherical particles has been demonstrated by Ren et al. [11] and Liu et al. [12]. The minimum spouting velocity, axial pressure drop and fountain height in a cylindrical spout-fluid bed with elevated pressure were investigated by Zhong et al. [13–15]. Pressurized spout-fluid bed coal gasifiers at a pressure of 0.5 MPa have been developed for a second-generation pressurized fluidized bed combustion combined cycle (PFBC-CC) system in China [16]. These studies show enormous potential for spout-fluid beds to deal with various particles under different conditions. However, these studies are mainly focused on the bench-scale or pilot-scale, the spout-fluid bed technique has not been used to an industrial process yet, especially

https://doi.org/10.1016/j.apt.2019.09.021 0921-8831/Ó 2019 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.

Please cite this article as: G. Zhou, W. Zhong, A. Yu et al., Simulation of coal pressurized pyrolysis process in an industrial-scale spout-fluid bed reactor, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.09.021

2

G. Zhou et al. / Advanced Powder Technology xxx (xxxx) xxx

Nomenclature U cp Cd dp D Ds F G h H k P q Q r R Sh

velocity vector (m s
1) heat capacity (J kg
1 K
1) drag coefficient particle diameter (mm) turbulent mass diffusion coefficient for gas (m2 s
1) interphase momentum transfer coefficient inter-phase momentum transfer rate feeding rate (kg s
1) specific enthalpy (J kg
1) reactor height (m) chemical reaction rate coefficient pressure (MPa) heat transfer flux (J m
2 s
1) gas flowrate (Nm3 h
1) reaction rate (mol m
3 s
1) universal gas constant (J mol
1 K
1) inter-phase energy exchange rate

for the coal pressurized pyrolysis. In fact, to date, how to identify the optimum operating conditions of a large scale spout-fluid bed is not well investigated. Numerical simulation has been accepted as an efficient tool to design and optimize dense particulate reactors of various types. The Eulerian-Lagrangian approach, in which the motion of particles is described by Newton’s second law of motion whilst the fluid flows by solving the Navier-Stokes equations, is well-suited to study the fluid flow, heat and mass transfer in a dense particulate reaction system. However, the application of the traditional discrete element method (DEM) is restricted by the computing capacity, particularly when the number of particles is large. If coupled with heat transfer and chemical reactions, the computation becomes more complicated and difficult. Therefore, there have been few studies on the simulation of dense gas-solid flow coupling with chemical reactions using the Eulerian-Lagrangian approach in a large-scale reactor. In order to improve the computational efficiency, the multiphase particle-in-cell (MP-PIC) method has been used for the modelling and simulation of coupled dense granular flow and chemical reactions. This method was proposed by Andrews and O’Rourke [17] in 1996. The heat transfer and heterogeneous reactions were added into the model and were applied to fluidized bed coal gasification by Snider et al. [18] in 2011. Xie et al. [19] then successfully applied this method to the spouted bed gasification. According to this method, the particle impact force and the heterogeneous reaction rate can be calculated on the Eulerian grid, and then interpolated back to discrete particles [20]. In the meantime, the considered particles are used to estimate a large number of real particles so that the gas solid reaction system can be efficiently simulated. In this study, the MP-PIC method was chosen to simulate the pressurized pyrolysis of coal in an industrial-scale spout-fluid bed reactor, and semi-coke acts as the bed material and heat carrier. In particular, an original industrial-scale spout-fluid bed reactor with the capacity of 500 thousand tons per year (63 t/h) was proposed for pressurized pyrolysis of coal, and the corresponding three-dimensional Eulerian-Lagrangian approach, facilitated with the MP-PIC method, was developed to simulate gas-solid flow and pyrolysis characteristics of coal in the reactor. The performance of the reactor and effects of different operating parameters such as coal feeding rate, coal to semi-coke ratio, flowrate of spouting gas and fluidized gas, and particle sizes were numerically

T Yi

temperature (K) mass fraction

Greek letters a volume fraction q density (kg m
3) l viscosity (kg m
1 s
1) s stress tensor (Pa) k thermal conductivity (W m
2 K
1) n grid cell Subscripts g gas phase s solid phase i the ith species coke Semi-coke coal coal

investigated. The results provide some theoretical basis and useful guidance for the commercialization of the pressurized grade conversion technology. 2. Model description and numerical simulation 2.1. Computational domain and model setup The computational domain of a three dimensional spout-fluid bed pyrolysis reactor with a diameter 3.1 m and height of 16.6 m shown in Fig. 1 (a). The reactor mainly contains four parts: the freeboard area (A), transition area (B), reaction area (C) and distribution area (D). The semi-coke and coal inlets are located at the middle of reaction area (C), with an angle of 90°. The fluidized gas inlets are distributed on the cone-shaped distributor while the spout gas inlet is a central orifice at the bottom of distribution area (D). The volatiles, including coal gas and tar, get out the reactor via an upper volatile outlet located on the top lateral side of the freeboard area (A). The semi-coke is designed to leave from a semicoke outlet at the bottom of the reactor. The main characteristic lengths of each part are shown in Fig. 1 (b). 2.2. Numerical model 2.2.1. Governing equations The 3D Eulerian–Lagrangian numerical model is based on the MP-PIC method. Complex particulate flow, mass and heat transfer, coal pyrolysis reactions are taken into account. In this method, the fluid dynamics described by average Navier-Strokes equations has strong coupling with the particle phase. The particle momentum equation follows the multiphase particle-in-cell formulation with the addition of a relaxation-to-the-mean term to represent the damping of particle velocity fluctuations due to particle collisions. A numerical or trends particle is accounted for via an ensemble of particles displaying the same properties such as chemical composition, size and density. The numerical particle is established within a numerical control volume where the properties of the fluid are considered constant. Mass, momentum and energy of the two-phase mixture are conserved by exchange term in the fluid phase mass, momentum and energy equations, respectively. The governing equations for gas and solid phases are listed in Table 1 and more details can be found in Refs. [18,21,22].

Please cite this article as: G. Zhou, W. Zhong, A. Yu et al., Simulation of coal pressurized pyrolysis process in an industrial-scale spout-fluid bed reactor, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.09.021

G. Zhou et al. / Advanced Powder Technology xxx (xxxx) xxx

3

Fig. 1. Industrial-scale spout-fluid bed pressurized pyrolysis reactor: (a) schematic of the reactor; (b) main sections and sizes of the reactor.

Table 1 Governing equations for gas and solid phases [18]. Gas phase Continually equation   @ðag qg Þ þ r  ag qg ug ¼ dm_ s @t Momentum equation   @ðag qg ug Þ þ r  ag qg ug ug ¼
ag rp þ F þ ag qg g þ r  ðag sg Þ @t Constitutive equation   @uj @ui @uk 2 @xj þ @xi
3 ldij @xk

sg;ij ¼ l

Transportation equation     @ðag qg Y gi Þ _ i;chem þ r  ag qg Y gi ug ¼ r  qg Dg ag rY gi þ dm @t Energy equation       @ðag qg hg Þ _ _ þ r  ag qg hg ug ¼ ag @p @t þ lg  rp þ u
r  ag q þ Q þ Sh þ qD @t

Coal ! H2 O, CO, CO2 , H2 , CH4 , C2 H4 , tar, semi-coke(s)

Mass source item RRR dms _s¼ dm f dt dms dus dT s

s

s

Particle normal stress equation b s


 dV i
E ðV 0
V i Þ ¼ Aexp RT dt

ss ¼ max½ðacp
Psaas Þ;dð1
as Þ Particle energy equation dT s dt

¼ m1s

kg Nug;p 2rs

As ðT f
T s Þ

Particle volume fraction RRR

ð2Þ

where Vi and V0 are the instantaneous and total amounts of volatile material for the gaseous component, respectively. The parameters A and E in this model were calculated from the pyrolysis kinetic experimental data, which are listed in Table 2.

s as ¼ fm qs dms dus dT s as þ ag ¼ 1

Interphase momentum transfer equation i o  RRR n h  p dms F¼
f ms Ds ug
us
r q þ us dt dm dus dT s s

ð1Þ

The quantities of these compositions are determined based on the experimental data in our previous work [7]. The kinetic model for the gas release during devolatilization is the single-step firstorder reaction [23]:

Solid phase Particle motion equation   s A ¼ du ¼ b ug
us
q1 rp
as1q rss þ g þ F s dt

Cv

2.2.2. Chemical reaction model In this study, the cell-average gas-solid chemistry calculation is taken into consideration. The cell chemistry used properties are mapped from the properties of particle to the grid, and the cell particle properties varied in cells depending on the particle distribution. The reaction rates are calculated at each computational time-step for every grid cell. Mass, momentum and energy are transferred between the solid and gas phases through heterogeneous reactions. For the sake of simplification, it was assumed that the coal devolatilization takes place instantaneously. Pyrolysis process causes the coal to thermally decompose into the following components:

s

Conservative energy exchange

i h 2 k Nu  2 io RRR n h  s Sh ¼ f ms Ds us
ug
g ds gs As ðT g
T s Þ
dm hs þ 12 us
ug dm dus dT s dt

2.3. Computational parameters

s

Interphase drag coefficient (Based on Wen-yu & Ergun model) 8 < 3 as ag qg jmg
m s j C D ag
2:65 ag > 0:8 4 ds 2 : 150 as l2g þ 1:75 as qg jm g
ms j ag  0:8 ds ag ds ( 0:44 Res > 1000 h i 0:687 24 Res  1000 ag Res 1 þ 0:15ðag Res Þ Res ¼

ag qg jmg
ms jds ls

The computational parameters including physicochemical parameters of coal and semi-coke are listed in Table 3. Initially calculation is set up by assuming that the semi-coke is filled up to the height of 5 m with a valid volume fraction of 0.55. The calculation is initialized with a uniform gas (873.15 K) and semi-coke particle temperature (1173.15 K) throughout the reactor and the entire reactor is filled with N2. The initial velocity of gas and particle is

Please cite this article as: G. Zhou, W. Zhong, A. Yu et al., Simulation of coal pressurized pyrolysis process in an industrial-scale spout-fluid bed reactor, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.09.021

4

G. Zhou et al. / Advanced Powder Technology xxx (xxxx) xxx

and properties are interpolated to the grid cell. More details can be referred to Refs. [21,26].

Table 2 The value of parameters A and E for pyrolysis under different pressures. Pressure (MPa)

E/(kJ/mol)

A/S-1

0.1 0.3 0.5

79.689 75.561 70.749

509.223 224.126 114.331

Table 3 Properties of materials and initial computational parameters. Properties

Unit

Proximate analysis of coal Moisture Ash Volatile matter Fixed carbon Ultimate analysis of coal C H O N S Coal density Coal inlet temperature Coal feeding rate Semi-coke density Semi-coke inlet temperature Semi-coke feeding rate Coal gas component N2 CO CO2 CH4 H2 C2H4 Coal gas inlet temperature Coal gas density Spouted gas flowrate Fluidized gas flowrate Particle size distribution

wt%

Value 10.18 6.11 33.19 50.52

wt %

kg/m3 K t/h kg/m3 K t/h vol %

K kg/m3 kg/s kg/s mm

70.39 5.18 6.11 1.01 1.03 1200 293.15 63 800 1173.15 252 10 15 35 10 20 10 673.15 1.06

0–6

set to zero while those at inlets are specified. The particle normalto-wall momentum retention coefficient is 0.3 and the tangent-towall retention coefficient is 0.99. The time step is 1  10-4 s. 2.4. Numerical solution procedure The equations of gas phase are solved by a numerical control volume method. The numerical methodology approximates conservation by finite volumes with momentum nodes and staggered scalar [24,25]. A SIMPLE solution scheme is adopted to adjust pressure and gas velocity to satisfy gas continuity. The momentum, pressure and energy equations are solved with a conjugate gradient solver, and particle momentum, motion and energy ordinary differential equations are directly solved. A stiff, sparse ODE solver is used to calculate the chemistry ordinary differential equations. In the MP-PIC method, particle properties are mapped to and from the Eulerian grid to obtain grid-properties for particles. Gas properties are reflected to discrete particle positions. The interpolation operator is the product of interpolation operators in the three orthogonal directions. Particle-to-particle collisions are simulated by the particle normal stress which is an approximation of collective effects of neighbor particles on a particle. The MP-PIC approach utilizes spatial gradients as they are readily calculated on the Eulerian grid and then apply the gradient to discrete particles. The particle stress is acquired from the particle volume fraction in turn calculated from particle volume that is mapped to the grid. The gas momentum equation implicitly couples gas and particles via the interphase momentum transfer. The particle drag

3. Results and discussion 3.1. Performance of a typical pyrolysis process A typical operating process is obtained under the pressure (P) of 0.1 MPa. The ratio of semi-coke to coal (Gcoke / Gcoal) is 4, and the spouted gas Qs is 741.5 Nm3/h while the fluidized gas Qf is 3707.5 Nm3/h. 3.1.1. Flow patterns and temperature distribution Fig. 2(a) shows the typical flow patterns of semi-coke particles and coal particles. The spouting gas and fluidizing gas are respectively introduced from the central tube and lateral distributor, forming an initial jet above the spouted gas inlet. When the jet extends upward, it continues to diffuse into the annular area on both sides, and the jet diameter increases but the penetration capability decreases. Hence, the first bubble is formed. The bubble is deformed and expanded by the action of particles in the process of rising, and finally breaks when it rises to the surface of the bed. Owing to the first bubble reduces the resistance of the central jet area, the rising velocity of subsequent bubbles increases and the bubble size decreases, and then gradually forming the granular fountain (jet flow). Due to the disturbance of fluidization, the jet flow is irregular in the process of injection. When bubbles moving up to the surface, the trailing vortex takes the particles into the upper part of the bed. The particles move upward with the effect of spouting gas and fluidizing gas until they penetrate into the bed and fall into the annular region under the action of gravity, and then enter a new cycle with the help of fluidization and spouting gas. Currently, 6 typical flow patterns of spout-fluid bed have been reported [27]. They are fixed bed, internal jet, spouting with aeration, jet in fluidized bed with bubbling, jet in fluidized bed with slugging and spout-fluidizing. In this study, it can be seen that except the large bubbles in the spout zone, there are also some bubbles in the annular region between the spout and wall. The processes of bubble formation, rising and bursting make particles in the bed circulate and mix. Therefore, the flow pattern in this case is a jet in fluidized bed with bubbling. Mixing behavior is one of the most important phenomenon for pyrolysis reactors. This behavior may affect the particle temperature distribution in the reactor, and further affect the pyrolysis of coal. Fig. 2 (b) shows the mixing performance of coal and semicoke particles. As can be seen from Fig. 2(b), the heat carrier semi-coke particles (900 ) enter into the reactor from semi-coke inlet and they immediately encounter the coal particles which are fed from the coal inlet. Some particles interact with the bubbles on the surface of the bed and fall into the bed as the bubbles break while other particles will move directly downwards in the annulus region and mix with the initial particles in the reactor. During this process, coal particles and semi-coke particles mix with each other by bubbles or jets and move back to the upper part of bed level, and then go through one or more internal loops below the bed surface. A small group of particles will directly fly up to the top of the reactor and leave without experiencing any mixing and circulation process. Inevitably, a very small proportion of coal particles may directly leave the reactor from the semi-coke outlet without any devolatilization process. Fig. 3 shows the particle temperature distribution in the reactor. It can be seen from this figure that an obvious temperature rise occurs from 300 K near the coal inlet to about 900 K at the central of the bed while another temperature rise can be observed along the jet from 673 K near the spouted gas inlet to about 1100 K at

Please cite this article as: G. Zhou, W. Zhong, A. Yu et al., Simulation of coal pressurized pyrolysis process in an industrial-scale spout-fluid bed reactor, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.09.021

G. Zhou et al. / Advanced Powder Technology xxx (xxxx) xxx

5

Fig. 2. Typical flow patterns and mixing behaviors.

the bed surface. In the annulus region, the average temperature near the semi-coke inlet is higher than near the coal because of the incoming raw coal of 293 K. However, the particle temperature distribution in annulus region is quite uniform owing to the mixing of semi-coke and coal particles, and the high concentration of coal particles results in the intensive endothermic pyrolysis reaction, so the temperature in this region is lower than the semi-coke inlet temperature. In the lower part of the reactor, the particle temperature is about 873 K after the pyrolysis reaction. Some particles have higher temperature in the upper part of bed where the inlet semi-coke with the temperature of 1173 K firstly enters the reactor. The distribution of gas temperature in the dense flow region

area is similar to the particle temperature while that in the freeboard area is quite uniform due to the mix of volatiles and fluidizing or spouting gas. 3.1.2. Distribution of gas compositions Fig. 4 presents the molar fraction of the six volatile species in the pyrolysis reactor. As the coal gas is supplied from the bottom, the CO persists with a high molar fraction near the two coal gas inlets. In annulus region, the concentration of CO is lower than that at the bottom area due to the existence of a large amount of particles. However, the molar fraction of CO near the coal inlet is relatively higher than that at the annulus region as a result of

Please cite this article as: G. Zhou, W. Zhong, A. Yu et al., Simulation of coal pressurized pyrolysis process in an industrial-scale spout-fluid bed reactor, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.09.021

6

G. Zhou et al. / Advanced Powder Technology xxx (xxxx) xxx

Fig. 3. Particle temperature distribution in the reactor.

pyrolysis reaction which takes place in this area. In the freeboard area there is no apparent variation of CO concentration. Similar composition distributions are found by other species except tar. As for tar, there is no tar content in the spouting and fluidizing coal gas, so it persists a low molar fraction of tar near the coal gas inlet. In the dense area, especially near the coal inlet, the concentration of tar is higher due to the existence of a large number of coal particles, where the pyrolysis reaction is intensified in this region. In this research, no secondary reaction was considered, thus the distribution of tar in the freeboard area presents with a similar characteristics to other components. 3.1.3. Evaluation of reactor operation The operation process of the pyrolysis reactor is evaluated with an annual coal treatment capacity of 500,000 tons (63 t/h). For a reactor, the evaluation index of its operation should be made considering whether the material balance can be reached after the flow pattern is stable, and whether the outlet gas-solid temperature can meet the design requirements. Fig. 5(a) shows the monitoring results of mass flowrate of semi-coke and coal at semicoke outlet from 60 s to 90 s. Due to the unsteady process in the spout-fluid bed reactor, the mass flowrates of both coal and semi-coke always fluctuate with time at semi-coke outlets, and the fluctuation amplitude is reduced and becomes stable after 80 s. The average total discharging quantity at the semi-coke outlet

is 78.31 kg/s, and the total reactor feeding rate is 87.5 kg/s. After removing the volatile matter, the semi-coke outflow rate should be about 80 kg/s, which is basically equal to the monitoring results. Inevitably, a very small part (the average mass flowrate is 1.186 kg/ s) of raw coal particles directly leave the reactor without pyrolysis. The proportion of these particles in total particle outflow is 1.49%, which is acceptable in industrial application. Fig. 5(b) shows the monitoring results of particles and volatiles outflow temperature from 60 s to 90 s. Similar to the particle outflow rate, the particle and volatile outflow temperature fluctuate all the time, but the fluctuation decreases after 80 s. It can be seen from the figure that the particle temperature at the outlet is basically maintained at about 873 K, which meets the design requirements of the process. The volatiles temperature is lower than the particle temperature, and the overall temperature is lower than 600 , which effectively prevents tar from the so called secondary cracking due to heating in the actual operation process. 3.2. Effects of operating parameters 3.2.1. Pressure For a pressurized pyrolysis reactor, the operating pressure is one of the most important factors which affects flow patterns and production yields. To investigate their influences on the reactor performance, the pressure is increased from 0.1 to 0.5 MPa with

Fig. 4. The molar fraction of the six major gas species.

Please cite this article as: G. Zhou, W. Zhong, A. Yu et al., Simulation of coal pressurized pyrolysis process in an industrial-scale spout-fluid bed reactor, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.09.021

G. Zhou et al. / Advanced Powder Technology xxx (xxxx) xxx

7

spouting structure is more stable. This is mainly because when the pressure rises, the gas viscosity increases, the gas-solid interaction is strengthened, and the minimum fluidization velocity decreases [13,15]. Therefore, compared with atmospheric pressure conditions, the particles are easier to fluidized and present a more intense and stable flow at the same operating velocity [28]. As shown in Fig. 6(b), the mixing of coal and semi-coke is less satisfac-

Fig. 5. Monitoring results of (a) mass flowrate of particles and (b) particles and volatiles outflow temperature.

the interval of 0.2 MPa, while the spouting and fluidizing speeds remain the same. The different flow patterns and mixing behaviours in the spout-fluid reactor with different pyrolysis pressures are shown in Fig. 6. It can be seen from Fig. 6(a) that the spout-fluid bed always keeps the state of the jet in the fluidized bed with bubbling (JFB). When the pressure rises, the bubbles in the central jet area become larger, the motion of particles is more intense and the

Fig. 7. Production yields and particle outflow under different pressures (Qg = 4449 Nm3/h, Gcoal = 63 t/h, Gcoke/Gcoal = 4/1, Qs/Qf = 1/5, dp = 0–6).

Fig. 6. Flow patterns and mixing behaviors with different pressures under different pressure (Qg = 4449 Nm3/h, Gcoal = 63 t/h, Gcoke/Gcoal = 4/1, Qs/Qf = 1/5, dp = 0–6).

Please cite this article as: G. Zhou, W. Zhong, A. Yu et al., Simulation of coal pressurized pyrolysis process in an industrial-scale spout-fluid bed reactor, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.09.021

8

G. Zhou et al. / Advanced Powder Technology xxx (xxxx) xxx

Fig. 8. Flow patterns and mixing behaviors of coal and semi-coke under different Gcoke/Gcoal ratio (P = 0.1 MPa, Qg = 4449 Nm3/h, Gcoal = 63 t/h, Qs/Qf = 1/5, dp = 0–6).

Therefore, the time of coal and particle mixing is relatively longer, and the mixing effectiveness is not as good as that of atmospheric pressure. The results are consistent with the observation of Liu et al. [29]. Fig. 7(a) shows the production yields of tar and coal gas versus pyrolysis pressure. Since the influence of the secondary reactions such as the secondary cracking is neglected in the model, the only factor affecting the yield is the adequacy of the reaction. As shown in Fig. 7(a), with the increase of pyrolysis pressure, both tar and coal gas yields are increased. This suggests that although the mixing of coal and semi-coke becomes worse due to the increase of pressure, it can still promote the full pyrolysis of coal particles. Relevant studies show that with the increase of system pressure, spouting capacity of spout-fluid bed increases, which brings a better flow state and gas-solid contact, so it is easier to promote pyrolysis reactions. Furthermore, when the gas-solid contact is strengthened, the residence time of coal particles in the bed is prolonged to a certain extent, which correspondingly lengthens the reaction time and makes the coal particles to react fully. Fig. 7(b) illustrates the particle outflow rates under different pressures. The figure indicates that the mass flowrate of unreacted coal particles decreases with the increase of pressure, which supporting the fact that increasing the pressure can promote coal pyrolysis.

Fig. 9. Production yields and particle outflow under different Gcoke/Gcoal (P = 0.1 MPa, Qg = 4449 Nm3/h, Gcoal = 63 t/h, Qs/Qf = 1/5, dp = 0–6).

tory when the pressure increases. This shows that stable spouting structure in spout-fluid bed is not conducive to the uniform mixing of coal and semi-coke. The reason for this phenomenon is that the stable flow structure will form two stable and clear regions: annulus region and jet region, which means the trajectories of particles in the bed are more regular. In this case, coal particles will tend to move in a relatively fixed trajectory when they enter the reactor.

3.2.2. Semi-coke to coal ratio As the heat carrier of the pyrolysis reactor, semi-coke not only plays the role of heating coal particles, but also has a great effect on fluidizing the coal particles and making it uniformly distributed in the reactor. Therefore, the input of the semi coke is very important to the operation of the pyrolysis reactor. To investigate the effects of the semi-coke to coal ratio on the pyrolysis reactor performance, different ratios of semi-coke to coal (Gcoke / Gcoal = 3/1, 4/1, and 5/1) are studied in this section. Fig. 8 demonstrates the flow patterns and mixing behaviors of coal and semi-coke in the reactor when the ratio of semi-coke to coal is different. Two phenomena can be observed in Fig. 8. Firstly, the overall height of the bed increases slightly with the increase of the semi-coke, which is caused by the increase of particle feeding rate [27]. Secondly, the mixing effectiveness in the reactor becomes worse. This is because a large number of semi-coke particles enter the reactor, resulting in a longer time for coal and semi-coke to be fully mixed. It should be noted that this does not indicate that the mixture of coal and semi-coke will affect the

Please cite this article as: G. Zhou, W. Zhong, A. Yu et al., Simulation of coal pressurized pyrolysis process in an industrial-scale spout-fluid bed reactor, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.09.021

G. Zhou et al. / Advanced Powder Technology xxx (xxxx) xxx

9

Fig. 10. Flow patterns and mixing behaviors under different particle sizes(P = 0.1 MPa, Qg = 4449 Nm3/h, Gcoal = 63 t/h, Gcoke/Gcoal = 4/1, Qs/Qf = 1/5).

pyrolysis of coal, because semi-coke is excessive relative to the coal which has obtained the heat required for pyrolysis. Fig. 9(a) shows the production yields of gas and tar at different semi-coke to coal ratio. As shown in Fig. 9(a), when the semi-coke to coal ratio increases, the yield of gas and tar increases correspondingly. This is because a large quantity of semi-coke enters the reactor generating a lot of heat, which improves the efficiency of coal particles to obtain heat, thus promoting coal pyrolysis. However, it should be noted that the increase in the yield of the coal pyrolysis products is limited because of the constant coal feeding rate. Fig. 9(b) shows the particle outflow rates at the semi-coke outlet under different semi-coke to coal ratio. It can be seen from the figure that with the increase of the semi-coke to coal ratio, the semi-coke outflow increases while that of unreacted coal is reduced. This is because the increase in the amount of the semicoke directly leads to a significant increase in the amount of particles in the reactor, which makes the discharge of semi-coke to increase, and a large amount of semi-coke means that coal particles will get more heat sources, thus promoting coal pyrolysis to a certain extent. Furthermore, a large number of particles are accumulated in the reactor to a certain extent, and affect the fluidization and mixing of particles in the reactor, which reduces the particles residence time in the reactor and also affects the pyrolysis of coal. Therefore, we can also observe that when the semi-coke continues to increase, the reduction of coal export volume decreases. Based on the above analysis, it is suitable to maintain the semi-coke to coal ratio at 4/1.

3.2.3. Particle size In order to understand the effect of particle size on reactor operation, the operation processes of the reactor with three different particle sizes (dp = 0–3 mm, 0–6 mm, and 0–10 mm) were simulated. Fig. 10 shows the flow patterns and mixing behaviors in pyrolysis reactors at different particle sizes dp. Compared with the case of dp = 0–6 mm, when the particle size range is reduced to 0–3 mm, there is no steady spouted fluidization in the reactor. The morphology of the bed becomes chaotic and no obvious bubbles can be observed. However, when the particle sizes range in 0–3 mm, the mixture of coal particles and semi-coke particles in the reactor is more uniform. This is due to the decrease of particle sizes results in unstable flow patterns in the bed, promoting the mixing to a certain extent. When particle size range is 0–10 mm, the bubble size in the reactor increases, but the mixing effectiveness is a bit worse than the other two cases. Moreover, from the

Fig. 11. Production yields and particle outflow under different particle sizes (P = 0.1 MPa, Qg = 4449 Nm3/h, Gcoal = 63 t/h, Gcoke/Gcoal = 4/1, Qs/Qf = 1/5).

view point of industrial operation, the power consumption required for grinding coal particles into fine powder is increased, and it is meaningless for the steady flow of material in the reactor. Fig. 11(a) shows the production yields of gas and tar under different conditions. As shown in Fig. 11(a), the yield of gas and tar is decreased after the particle size increases. This also means that for this study, the stable flow patterns may not helpful to the mixing of coal and semi-coke which is a key factor of the production yield.

Please cite this article as: G. Zhou, W. Zhong, A. Yu et al., Simulation of coal pressurized pyrolysis process in an industrial-scale spout-fluid bed reactor, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.09.021

10

G. Zhou et al. / Advanced Powder Technology xxx (xxxx) xxx

Table 4 Particle outflow rate of the volatile outlet (kg/s). Particle size (mm)

0–10 0–6 0–3

Coal (mm)

Semi-coke (mm)

210–336

336–462

110–242

242–374

0 0 4.0  10-4

0 0 0

9.1  10-2 8.8  10-2 9.3  10-2

2.11  10-4 2.9  10-3 9.5  10-4

It should be pointed out that under the premise of constant amount of coal and total gas volume, the change of coal pyrolysis product yields is limited. Fig. 11 (b) and Table 4 show the coal and semi-coke particle outflow at different outlets. It can be seen from Fig. 11(b) that when the particle size decreases, the semi-coke output at semicoke outlet increases significantly, while the unreacted coal decreases. This is because when the particle size decreases, the regularity of bed motion decreases and the movement of particles tends to be more disordered, thus improving the motion of particles to a certain extent. This phenomenon not only makes the semi-coke particles more easily discharged from the bottom, but also helps the coal completely pyrolysis, thus reducing the amount of coal at the bottom outlet. It is found from Table 4 that the particle size has an obvious effect on the particle size at the volatile outlet. When the reactor runs under the particle size range 0– 3 mm, the export of semi-coke powder increases, unreacted coal also increases. This is because the proportion of semi-coke powder increases when particle size is reduced, so more semi-coke powder is blown out of the reactor at the same gas velocity. Therefore, it is suggested that the particle sizes should better range in 0–6 mm. 4. Conclusions An original industrial-scale spout-fluid bed reactor has been proposed for pressurized pyrolysis of coal (with the capacity of 500 thousand tons per year), and its corresponding threedimensional Eulerian-Lagrangian approach, facilitated with the MP-PIC method, has been developed to simulate the gas-solid flow and pyrolysis characteristics of coal in this reactor. The performance of the reactor and the effects of different operating parameters such as pressure, semi-coke to coal ratio, and particle size have been numerically studied. The following conclusions can be drawn from the current investigation: (1) The flow pattern in the reactor considered is a ‘‘jet in the fluidized bed with bubbling”. The designed reactor can achieve mass balance, and the outlet particle temperature is stable at around 600 °C. (2) The pressure can promote full pyrolysis because it brings a better flow state and gas-solid contact. The raise of pressure has a positive impact on the spouting structure and the flow uniformly. (3) The increase of the semi-coke to coal ratio is beneficial to the coal pyrolysis, but the improvement in pyrolysis is limited and the number of particles in the reactor will be sharply increased. (4) With the increase of particle size, the flow pattern in the pyrolysis reactor tends to be stable while the mixing of particles becomes unsatisfactory. It is suggested that the particle sizes should be in the range of 0–6 mm.

Acknowledgements This work was supported by The National Key Research and Development Program of China, China (NO. 2016YFB0600802),

the Key Research and Development Program of NSFC, China (NO. 51736002) and the Scientific Research Foundation of Graduate School of Southeast University, China (YBJJ1604). AY is also grateful to the Australian Research Council, Australia for the financial support for the research hub. References [1] M. Fang, Z. Luo, X. Li, Q. Wang, M. Ni, K. Cen, A multi-product cogeneration system using combined coal gasification and combustion, Energy 23 (3) (2014) 203–212. [2] M. Ni, C. Li, M. Fang, Q. Wang, Z. Luo, K. Cen, Research on coal staged conversion poly-generation system based on fluidized bed, Int. J. Coal Sci. Technol. 1 (1) (2014) 39–45. [3] Z. Gong, T. Zhou, Q. Lu, Y. Na, Y. Sun, Combustion and NOx emission characteristics of shenmu char in a circulating fluidized bed with postcombustion, Energ Fuel. 30 (1) (2016) 31–38. [4] C. Zhou, Y. Wang, Q. Jin, Y. Zhou, Mechanism analysis on the pulverized coal combustion flame stability and NOx emission in a swirl burner with deep air staging, J. Energy Inst. (2018). [5] W. Fan, Y. Li, Q. Guo, C. Chen, Y. Wang, Coal-nitrogen release and NOx evolution in the oxidant-staged combustion of coal, Energy 125 (2017) 417– 426. [6] B. Fidalgo, D.V. Niekerk, M. Millan, The effect of syngas on tar quality and quantity in pyrolysis of a typical South African inertinite-rich coal, Fuel 134 (9) (2014) 90–96. [7] G. Zhou, W. Zhong, A. Yu, Y. Dou, J. Yin, Experimental study on characteristics of pressurized grade conversion of coal, Fuel 234 (2018) 965–973. [8] H. Xu, W. Zhong, Y. Shao, A. Yu, Experimental study on mixing behaviors of wet particles in a bubbling fluidized bed, Powder Technol. 340 (2018) 26–33. [9] H. Xu, W. Zhong, Z. Yuan, A. Yu, CFD-DEM study on cohesive particles in a spouted bed, Powder Technol. 314 (2016) 377–386. [10] Y. Shao, X. Liu, W. Zhong, B.S. Jin, M. Zhang, Recent advances of spout-fluid bed: a review of fundamentals and applications, Int J Chem React Eng. 11 (1) (2013) 243–258. [11] B. Ren, W. Zhong, B. Jin, Y. Shao, Z. Yuan, Numerical simulation on the mixing behavior of corn-shaped particles in a spouted bed, Powder Technol. 234 (2013) 58–66. [12] X. Liu, W. Zhong, X. Jiang, B. Jin, Spouting behaviors of binary mixtures of cylindroid and spherical particles, AIChE J. 61 (1) (2014) 58–67. [13] W. Zhong, X. Chen, M. Zhang, Hydrodynamic characteristics of spout-fluid bed: pressure drop and minimum spouting/spout-fluidizing velocity, Chem. Eng. J. 118 (1) (2006) 37–46. [14] W. Zhong, Q. Li, M. Zhang, B. Jin, R. Xiao, Y. Huang, et al., Spout characteristics of a cylindrical spout-fluid bed with elevated pressure, Int J Chem React Eng. 139 (1) (2013) 42–47. [15] W. Zhong, Q. Li, M. Zhang, B. Jin, R. Xiao, Y. Huang, et al., Spout characteristics of a cylindrical spout-fluid bed with elevated pressure, Chem. Eng. J. 139 (1) (2008) 42–47. [16] R. Xiao, M. Zhang, A. Baosheng Jin, Y. Huang, H. Zhou, High-temperature air/ steam-blown gasification of coal in a pressurized spout-fluid bed, Energ Fuel. 20 (2) (2014) 715–720. [17] M.J. Andrews, P.J. O’Rourke, The multiphase particle-in-cell (MP-PIC) method for dense particulate flows, Int. J. Multiphas. Flow. 22 (2) (1996) 379–402. [18] D.M. Snider, S.M. Clark, P.J. O’Rourke, Eulerian-Lagrangian method for threedimensional thermal reacting flow with application to coal gasifiers, Chem. Eng. Sci. 66 (6) (2011) 1285–1295. [19] J. Xie, W. Zhong, B. Jin, LES-Lagrangian modelling on gasification of combustible solid waste in a spouted bed, Can. J. Chem. Eng. 92 (7) (2014) 1325–1333. [20] D.M. Snider, An incompressible three-dimensional multiphase particle-in-cell model for dense particle flows, J. Comput. Phys. 170 (2) (2001) 523–549. [21] W. Zhong, A. Yu, G. Zhou, J. Xie, H. Zhang, CFD simulation of dense particulate reaction system: approaches, recent advances and applications, Chem. Eng. Sci. 140 (50) (2016) 16–43. [22] J. Xie, W. Zhong, B. Jin, Y. Shao, L. Hao, Simulation on gasification of forestry residues in fluidized beds by Eulerian-Lagrangian approach, Bioresour. Technol. 121 (7) (2012) 36–46. [23] R. Radmanesh, J. Chaouki, C. Guy, Biomass gasification in a bubbling fluidized bed reactor: experiments and modeling, AIChE J. 52 (12) (2010) 4258–4272. [24] P.J. O Rourke, D.M. Snider, An improved collision damping time for MP-PIC calculations of dense particle flows with applications to polydisperse

Please cite this article as: G. Zhou, W. Zhong, A. Yu et al., Simulation of coal pressurized pyrolysis process in an industrial-scale spout-fluid bed reactor, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.09.021

G. Zhou et al. / Advanced Powder Technology xxx (xxxx) xxx sedimenting beds and colliding particle jets, Chem. Eng. Sci. 65 (22) (2010) 6014–6028. [25] D.M. Snider, S. Banerjee, Heterogeneous gas chemistry in the CPFD EulerianLagrangian numerical scheme (ozone decomposition), Powder Technol. 199 (1) (2010) 100–106. [26] P.J. O’Rourke, D.M. Snider, Inclusion of collisional return-to-isotropy in the MPPIC method, Chem. Eng. Sci. 80 (10) (2012) 39–54.

11

[27] N. Epstein, J.R. Grace, Spouted and Spout-fluid Beds: Fundamentals and Applications, Cambridge University Press, 2010. [28] W. Zhong, X. Liu, J.R. Grace, N. Epstein, B. Ren, B. Jin, Prediction of minimum spouting velocity of spouted bed by CFD-TFM: scale-up, Can. J. Chem. Eng. 91 (11) (2013) 1809–1814. [29] X. Liu, W. Zhong, A. Yu, B. Xu, J. Lu, Mixing behaviors in an industrial-scale spout-fluid mixer by 3D CFD-TFM, Powder Technol. 314 (2016) 455–465.

Please cite this article as: G. Zhou, W. Zhong, A. Yu et al., Simulation of coal pressurized pyrolysis process in an industrial-scale spout-fluid bed reactor, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.09.021