CFD modeling of the flow dynamics and gasification in the combustor and gasifier of a dual fluidized bed pilot plant

Energy 198 (2020) 117366

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CFD modeling of the flow dynamics and gasification in the combustor and gasifier of a dual fluidized bed pilot plant Zhanghao Wan , Shiliang Yang *, Yonggang Wei , Jianhang Hu , Hua Wang State Key Laboratory of Complex Nonferrous Metal Resources Clean Utilization, Kunming University of Science and Technology, Kunming, 650093, Yunnan, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 19 January 2020 Received in revised form 6 March 2020 Accepted 10 March 2020 Available online 14 March 2020

Numerical simulation of the dense gas-solid reactive flow in a pilot-scale dual fluidized bed for biomass gasification is conducted with large eddy simulation coupled with multiphase particle-in-cell approach. After validating the numerical results with the experimental data, the hydrodynamics of different components operating under different flow regimes are explored. The results show that dense bottom and dilute upper exist in the combustor, while the elutriation of fine particles appears in the freeboard of gasifier. Strong swirling flow together with the fishtailing phenomenon of gas phase appear in the cyclone. Gas density is large in the lower part of combustor, and decreases along the horizontal direction. Large thermal conductivity and specific heat capacity of gas phase exist in the gasifier due to the large mass fraction of low-temperature steam. Biomass inlet results in a large mass fraction of gaseous species in the right part of gasifier. The size distribution of heat carrier negligibly affects the gas thermal property in the lower part of both the combustor and gasifier. © 2020 Elsevier Ltd. All rights reserved.

Keywords: Dual fluidized bed Gasification Hydrodynamics Multiphase particle-in-cell Renewable energy

1. Introduction The utilization of fossil fuels gives rise to the global warming and climate change, which drive the attention towards the wide exploitation of clean energy resources [1,2], such as solar, biomass [3], and wind. Among them, biomass energy occupies nearly 14% of the total energy supply over the world. As one of the thermochemical conversion processes, gasification efficiently converts the biomass feedstock into the gaseous products for generating heat and electricity, biochar, high-grade fuels for transportation, and syngas for many chemical syntheses (e.g., methanol, fertilizer). This complicated process is carried out within several types of reactors, among which the fluidized bed gasifier has been widely selected due to its excellent heat and mass transfer efficiency and the ability to maintain a constant gasification temperature. As an advanced fluidizing reactor, dual fluidized bed gasifier (DFBG) consisting of a gasification reactor, a combustor reactor, and the loop seal connecting them, recently draws more and more attention due to its remarkable advantages (e.g., high-quality gaseous products, and low tar contents [4], highest heat flux with

* Corresponding author. E-mail address: [email protected] (S. Yang). https://doi.org/10.1016/j.energy.2020.117366 0360-5442/© 2020 Elsevier Ltd. All rights reserved.

relatively small gasifier reactor [5]). The underlying principle of DFBG is the inherent separation of endothermic gasification and exothermic combustion. In the gasifier, steam is introduced as gasification agent for generating high-quality gaseous products with the absence of nitrogen. In the combustor, the combustion of gas fuel releases the heat required for the endothermic gasification reactions, which is achieved with the internal circulation of heat carrier. Until now, several laboratory infrastructures together with some commercial implementations of DFBG have been put into practice over the world [4]. Indeed, despite the utilization for biomass gasification, this kind of geometrical design has been widely selected for the chemical processes with high-temperature solid looping cycles [5], such as calcium-looping process with CO2 capture [6], chemical loop combustion [7], and chemical looping reforming [8]. However, the multi-scale structures and multiphysiochemical processes in the combustor and gasifier of the complex DFBG give rise to the essential role of deeply understanding the complicated multiphase reactive flow for the further design and optimization of industrial apparatus. The experimental measurement provides the first-hand data for the processes involved in the DFBG, such as the mixing behavior of biomass and char [9], the tar formation [10,11], the effect of bed material [12,13], the impact of biomass type [14], the geometry optimization [15], and the CO2 gasification [16]. In general, the

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experiments mainly focus attention on assessing the effect of operating parameters (e.g., gasification temperature, gasification agent) on the macroscopic property of gasification process (e.g., the quality of gaseous products). Despite some work for the cold apparatus [ [6,17e19]], no reports regarding the internal flow dynamics of DFBG under high temperature are available since the high temperature and opaque nature of dense reactive flow challenge the experimental device. Besides the experimental approach, computational fluid dynamics (CFD) simulation has been a powerful tool for efficiently and economically exploring the detailed local information and multiscale flow structures in the fluidizing apparatus [20]. The previous numerical efforts are mainly carried out for the cold fluidizing apparatus with simple geometry. During these several years, growing numerical work for the reactive gas-solid flow in the fluidizing apparatus have been reported with the significant progress on the numerical models [21,22]. The current available approaches for modeling the reactive gas-solid flow are the twofluid model (TFM) [23,24], CFD coupled with discrete element method (CFD-DEM) [25e27], and the multiphase particle-in-cell (MP-PIC) [28,29]. Numerically simulating the dense reactive flow in the DFBG via TFM approach has been reported in the literature [30,31]. However, this approach cannot provide the microscale solid information at the particle level, and cannot handle the bed material with wide size distribution. For the CFD-DEM, the study on the reactive two-phase flow in the full-loop DFBG with this approach has not been reported until now, due to the extremely complex bed structure and the huge computational resources needed. The MP-PIC approach recently proposed by Andrews and O’Rourke [32] becomes more and more popular for exploring the reactive fluidizing bed due to its ability of providing a fast solution for the industrial-scale apparatus. This approach has been selected to examine the deacidification of waste incinerator flue gas [33], biomass gasification [34,35], coal gasification [36], and CO2 capture [37]. Regarding the DFBG, Yan et al. [38] used MP-PIC to simulate the biomass gasification in the DFBG and observed a higher concentration of gaseous products in the conical transition section; Liu et al. numerically explored the impacts of operating parameters on the gasification performance [39] and solid circulation rate [40] in a DFBG. They found that the effect of air supply on the produced gas is minor [39], and the solid circulation rate can be enhanced with enlarging the solid inventory in the bed [40]. Recently, Kraft et al. [41] numerically explored the effect of drag laws on the pressure and material circulation rate in a cold DFBG with MP-PIC. They further [42] studied the biomass mixing in the industrial-scale DFBG and found that a bypass stream of hot bed material presents between the loop seal and the chute. The complicated internal flow hydrodynamics in different components of the complex DFBG are far away from been well understood. For example, the spatial distribution of gas thermal property has not been reported, but it is critical for solid motion as the viscosity has a strong relationship with the drag force exerted on solid phase. Thus, this work conducts a numerical simulation of the reactive gas-solid flow in a pilot-scale 1MWth dual fluidized bed biomass gasifier to explore the bed hydrodynamics, the spatial distribution of gas thermal property together with the impact of wide size distribution of heat carrier in the different components of the apparatus. Specifically, the multiphase particle-in-cell (MP-PIC) is chosen and the lognormal particle size distribution (PSD) is adopted for heat carrier. The mass fraction of gas species monitored in the gasifier outlet is compared with the experiment for model validation. Then, the general flow dynamics and the gas-solid flux distribution are assessed, followed by exploring the spatial distribution of thermal property (density, viscosity, thermal conductivity, and specific heat capacity) of gas phase in the DFBG. Finally, the

spatial distribution of gas species is discussed. 2. Numerical models MP-PIC is a numerical method of tracking the gas and solid phase under the Eulerian and Lagrangian framework, respectively. Specifically, the gas phase is treated as a continuous medium, while particles are tracked as discrete phase. For the gas motion, large eddy simulation is chosen due to the strong swirling gas flow in the cyclone. For the solid phase, instead of accurately resolving the collision process, this method assesses the collision forces between particles through the solid concentration. Moreover, the concept of numerical parcel is introduced to significantly speed up the computational speed. The exchange of the momentum, energy and species between the gas and solid phase is explicitly considered with introducing the related sinking terms in the governing equations. 2.1. Continuous-phase governing equations For the reactive flow, the mass transfer from solid phase to gas due to chemical reactions should be considered. The continuity equation of gas phase can be formulated as



 v qg rg þ V , qg rg ug ¼ dm_ s vt

(1)

where qg is the gas volume fraction; rg is the gas density; ug is the gas velocity; and dm_ s is the production rate of gas in the chemical reactions. t is the time variable. With introducing the gas voidage and sinking term, the momentum equation of gas phase can be written as




  v qg rg ug þ V , qg rg ug ug ¼  Vpg þ rg qg g þ V , qg tg þ F gs vt (2) where pg is the gas pressure; g is the gravitational acceleration; Fgs is the interphase momentum exchange with solid phase. The gas stress tensor tg is assessed with the constitutive correlation described as

vui vuj tg;ij ¼ ðml þ mt Þ þ vxj vxi

!

2 vu  ðml þ mt Þ k dij 3 vxk

(3)

where ml is the laminar viscosity. mt is the turbulent viscosity evaluated with the Smagorinsky model [43] formulated as:

vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi !2 u u vu vuj i mt ¼ Cs D rg t þ vxj vxi 2

(4)

where Cs and D represent the Smagorinsky constant and the subgrid length scale, respectively. The transportation equation of gas species can be expressed as

  v qg rg Yg;k vt

    þ V, qg rg ug Yg;k ¼ V, qg rg Dgk VYg;k þ dm_ k;react (5)

where Yg;k is the mass fraction of gas species; Dgk is the turbulent mass diffusivity; dm_ k;react is the generation rate of gas species k due to the physiochemical reactions. With considering the energy exchange with solid phase and

Z. Wan et al. / Energy 198 (2020) 117366

local wall, the energy equation of gas phase can be expressed as












  v qg rg hg vpg þ ug , Vpg  V, qg q þ V, qg rg ug hg ¼ qg vt vt þ Q_ þ Sgs þ Sgw  DHrg D

(6) where hg is the gas enthalpy; Sgs is the energy exchange with particles; Sgw is the convective heat flux of gas phase with local wall; D Hrg is the heat generation due to chemical reactions. The enthalpy of gas mixture is calculated from the multi-component gas species as

hg ¼

XN

h Y i¼1 g;i g;i

¼

XN

Tðg

Y i¼1 g;i

! Cp;i dT þ Dhg;i

(7)

where T0 stands for the reference temperature; Tg is the actual gas temperature; Cp,i represents the specific heat capacity of gas species i; Dh is the heat of formation. Q_ is the enthalpy diffusion term. q D

g;i

represents the gas heat flux. The equations for these two quantities can be written as

Q_ D ¼

XN i¼1


 V$ hi qg rg Dg VYg;i

q ¼ lg VTg

The drag function Ds in the particle acceleration equation is calculated with the Gidaspow drag model [46]. It can be described as


 8 3 rg εg 1  εg ug  us 2:65 > > CD εg εg  0:8 > > ds < 4 bgs ¼
2
 > > > > 150 1  εg mg þ 1:75rg 1  εg ug  us εg < 0:8 : 2 ds εg d s (14) 8   > < 24 1 þ 0:15Re0:687 Res < 1000 s CD ¼ Res > : 0:44 Res  1000

T0

(8)

3

(15)



rg εg ug  us ds Res ¼ mg

(16)

where ds is the particle diameter. Res represents the particle Reynolds number. For a solid particle, the considered heat transfer mechanisms consist of the convective heat transfer with gas phase, the radiative heat transfer with local environment, and that due to chemical reactions. The temperature equation for a particle can be formulated as

(9) ms CV

where lg represents the gas thermal conductivity.

dTs ¼ Qsg þ Qradi þ Qreact dt

(17)

2.2. Discrete-phase governing equations

Here, Cv stands for the specific heat capacity of the particle. The convective and radiative heat exchanging quantities for a particle are evaluated as

MP-PIC method uses the numerical parcel to represent a fixed number of identical particles with the same properties (e.g., size, density, and chemical species) at a specific position. The acceleration equation of a particle can be described as [28, 29]

Qsg ¼


 1 du 1 us  us Gs ¼ s ¼ Ds ug  us  Vpg  Vt þ g þ dt rs qs rs s 2tD

g

(11)

Here, Ps, qcs and g are the model parameters. In the MP-PIC approach, the probability distribution function (PDF) of solid phase is introduced to track the solid phase. The transportation equation of PDF can be formulated as [28, 29]

vfs vðfs us Þ vðfs Gs Þ fD  fs þ þ ¼ vx vus vt tD

ms

rs

dms dus dTs

where ms is the solid mass; Ts is the solid temperature.

As Tg  Ts



  4  Ts4 Qradi ¼ sεs As Tb;local

(18)

(19)

where As is the particle surface area. Tb,local is the local bed temperature. To evaluate the convective heat transfer, the RanzeMarshall correlation [47] is chosen to calculate the Nusselt number Nugs. With the information of heat exchange of solid phase with environment, the energy source term in the governing equation of gas phase can be written as




2 2 dTs dms 1
 Es þ ug  us Sgs ¼ ∭ fs ms Ds ug  us  CV 2 dt dt

 dms dus dTs (20)

(12) 2.3. Combustion and gasification reactions

where fD is the PDF obtained by collapsing the velocity dependence [45]. With the information of PDF, solid concentration can be evaluated as

qs ¼ ∭ fs

ds

(10)

where us is the local mass-average velocity of solid phase; us is the instantaneous velocity; rs is the solid density. tD is the damping time of particle collision [29]. ts represents the particle stress, which is evaluated with the Harris and Crighton model [44] described as

Ps qs ts ¼ max½ðqcs  qs Þ; að1  qs Þ

lg Nugs


(13)

2.3.1. Dehydration and devolatilization model Biomass gasification undergoes a series of physicochemical processes. The moisture in the biomass particles is released when biomass particles are heated up to a specific temperature. The drying process of a biomass particle is described as Moisture in biomass(s) / H2O(g)

(R1)

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Z. Wan et al. / Energy 198 (2020) 117366

The evaporation rate of the moisture can be formulated as [48,49]. R1 ¼ 5.13  1010exp(-10585/Ts)mmoisture

(21)

Biomass pyrolysis can be described as Dry biomass / Volatile þ Char þ Ash

(R2)

The corresponding reaction rate is modeled with the empirical correlation as [49,50]. 5

R2 ¼ 1.49  10 exp(-1340/Ts)mvolatile

C þ O2/CO2

(R12)

The corresponding reaction rates for them are expressed as [49, 50] R9 ¼ 6.36Tsmcexp(-29645/Ts)[CO2]2

(29)

R10 ¼ 6.63Tsmcexp(-29645/Ts)[H2O]

(30)

R11 ¼ 0.00638Tsmcexp(-8078/(Ts-7.087) [H2]

(31)

R12 ¼ 4.34  107Tsmcexp(-13590/Ts) [O2]

(32)

(22)

where mmoisture and mvolatile stand for respectively the mass of moisture and mass of volatile in this biomass particle. 3. Computational setups and model validation 2.3.2. Gas-gas homogeneous reactions After the pyrolysis process, gas species such as CO, CO2, H2, CH4 and C2H4 react with the surrounding gases via homogeneous reactions. Moreover, the combustion of gas fuel is achieved in the combustor. In this work, the considered homogeneous reactions for gas species are described as CO þ 0.5O2 / CO2

(R3)

H2 þ 0.5O2 / H2O

(R4)

CH4 þ 2O2 / CO2 þ 2H2O

(R5)

CO þ H2O / CO2 þ H2

(R6)

C2H4 þ 3O2 / 2CO2 þ 2H2O

(R7)

C3H8 þ 5O2 / 3CO2 þ 4H2O

(R8)

The dual fluidized bed gasifier (DFBG) contains a gasifier reactor for biomass gasification, a cyclone for recycling the hot bed material, a combustion reactor for heating the bed material, a loop seal and chute transferring the heat carrier within them. Fig. 1 gives the three-dimensional (3-D) illustration of the explored pilot-scale dual fluidized bed gasifier [49]. The upper diameter and height of the gasifier is 1.067 m and 6.45 m, respectively. The combustor is a riser with a diameter of 0.356 m and a cyclone height of 7.9 m. To stabilize the bed temperature, the 1st, 2nd, 3rd air supply inlets are designed in the combustor with a height of 0 mm, 600 mm and

The reaction rates of these homogeneous reactions are evaluated as [49,51e56] R3 ¼ 1.3  1011exp(-15155/Ts)[CO][O2]0.5

(23)

R4 ¼ 2.2  109exp(-13110/Ts)[H2][O2]

(24)

R5 ¼ 5.01  1011exp(-24417/Ts)[CH4]0$7[O2]0.8

(25)

R6 ¼ 2.75qcexp(-10079/Ts)[CO] [H2O]

(26)

R7 ¼ 1  1015exp(-20808/Ts)[C2H4][O2]

(27)

R8 ¼ 8.6  1011exp(-15000/Ts)[C3H8]0$1[O2]1.65

(28)

2.3.3. Heterogeneous chemical reactions After the biomass pyrolysis, the residual char reacts with gases in the gasifier reactor, and part of them are circulated to the combustor to participate in the combustion reaction. The heterogeneous reactions considered in this work are described as C þ CO2/2CO

(R9)

C þ H2O/CO þ H2

(R10)

Cþ2H2/CH4

(R11)

Fig. 1. Geometry of the pilot-scale dual fluidized bed gasifier explored in the current study.

Z. Wan et al. / Energy 198 (2020) 117366 Table 1 Details regarding the physical property of gas and solid phase chosen. Description

Value

Bed material density (kg/m3) Mean diameter of bed material (mm) Particle size distribution Biomass density (kg/m3) Biomass mean diameter (mm) Close pack volume fraction Initial bed height Outlet pressure Steam supply to the gasifier (kg/h) Steam supply to the loop-seal (kg/h) The 1st air supply to the combustor (kg/h) The 2nd air supply to the combustor (kg/h) The 3rd air supply to the combustor (kg/h)

3560 0.488 Lognormal distribution 550 5.7 0.56 2.5 Atmosphere 85.6 27.1 36 260 362

5

implemented for the cyclone outlet and gasifier outlet; the Neumann boundary condition is assigned for other boundaries. For the temperature, the Neumann boundary condition is assigned for the boundaries besides the gasifier inlet and combustor inlet. Moreover, the mass flow rates for the 2nd, 3rd air inlets, and propane inlet of the combustor are 260 kg/h, 362 kg/h and 19.5 kg/h, respectively. The gas temperature of the 2nd and 3rd air inlets of the combustor is 632 K and 648 K, respectively. In the practical operation, the polydispersity of bed material is prevalent, and the presence of wide particle size distribution exerts a critical impact on the flow hydrodynamics [58]. Thus, the lognormal size distribution of heat carrier is selected to represent the wide distribution of bed material. The lognormal distribution of particle size can be described as

2230 mm, respectively. The propane inlet located in the middle of the combustor provides the additional heat to control the temperature of the DFBG.

3.1. Computational setups Via the SnappyHexMesh tool in the OpenFOAM package [57], three grid sets with a total number of 744,138 (fine grid), 244,296 (mediate grid), 72,054 (coarse grid) hexahedral cells are adopted to test the grid independence study. The corresponding average cell length for these grid sets is nearly 4 cm, 6 cm, and 9 cm, respectively. As can be seen from Fig. S1 in the Supporting Information, the time-averaged gas vertical flux in the central line of the gasifier slightly changes when refining from the mediate grid set to fine one, demonstrating the grid independence of the numerical results obtained. Thus, the mediate grid set is selected for the numerical simulation in this work to speed up the simulation, which corresponds to side-lengths of nearly 6 cm  6 cm  6 cm in each control volume. Table 1 lists the detailed information regarding the gas and solid phase chosen in this work. A total number of 549,685 numerical particles are used to describe the solid phase in the DFBG. Bed material has a density of 3560 kg/m3, and a lognormal distribution pattern with a mean particle diameter of 0.488 mm. Before the start of numerical simulation, an initial packed bed of heat carrier with a height of 2.5 m and a packing fraction of 0.56 is generated. The biomass material explored has a diameter and density 5.7 mm of 550 kg/m3, respectively. Details for the proximate and ultimate analyses of the biomass material are presented in Table 2 [49]. Proper boundary conditions should be specified for different boundaries of the DFBG. For the velocity, the steam with a mass flow rate and temperature of 85.6 kg/h and 640 K, respectively, is assigned for the gasifier inlet; the air with a flow rate and temperature of 36 kg/h and 602 K is applied for the combustor inlet; the no-slip boundary condition is selected for the wall; and the Neumann boundary condition is assigned for the cyclone outlet and gasifier outlet. For the pressure, a fixed value of 0.1 MPa are

Fig. 2. Comparison of the mole fraction of dry gas species released from the gasifier outlet of DFBG with a monodisperse sand size of 0.488 mm.

Table 2 Proximate and ultimate analyses of biomass [49]. Proximate analysis

(wt. %)

Ultimate analysis

(wt. %)

Fixed carbon Moisture Ash Volatile

20.20 5.18 2.09 72.53

C H O N S Cl

51.3 5.29 40.9 0.66 0.01 0.04

Fig. 3. Instantaneous distribution of solid particles colored with the velocity magnitude (Us, m/s) at time instant of 100 s (a), and the spatial distribution of concentration (qs) of solid phase (b) in the DFBG with a size distribution width (d/m) of 0.1.

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Z. Wan et al. / Energy 198 (2020) 117366

simulated and the data of the previous 30 s are discarded when evaluating the statistical property. 3.2. Numerical validation In the open-source OpenFOAM platform [57], the cold MP-PIC solver (called MPPICFoam) has been well validated for the gassolid hydrodynamics, such as the bubble dynamics in the dual fluidized bed [38], the gas-solid flow in the cyclone separator [59], the catalyst residence time distribution in the methanol to olefins (MTO) fluidized bed [60], demonstrating the proper implementation of numerical model and the reliability of the model for predicting the gas-solid flow in the fluidizing apparatus. In this work, this cold solver is extended with the consideration of heat transfer, and the complicated heterogeneous and homogeneous reactions. To verify the effectiveness of the developed reactive models, validation has been carried out by comparing the numerical results with the experimental data of bubbling fluidized bed gasifier [61] and spouted bed gasifier [62]. Details regarding the model validation can be found in our previous work [63]. To validate the model setup for the apparatus explored in this work, Fig. 2 presents the Fig. 4. 3-D presentation of the spatial distribution of the gas flux in the DFBG with a PSD width (d/m) of 0.1. (a) horizontal gas flux (Fgx, kg/(m2s)); (b) vertical gas flux (Fgz, kg/(m2s)).

  fm;log normal x ¼


lnðxÞ  mÞ2 1 pffiffiffiffiffiffi exp  2 xd 2p 2d

(33)

where m and d stand for respectively the arithmetic mean diameter and the standard deviation. In this work, the mean diameter of sand material is kept constant to be 0.488 mm but changes the standard deviation when evaluating the effect of PSD. In line with the experimental work of Chew et al. [58], five lognormal distributions with distribution width (defined as d/m) of 0.1, 0.3, 0.5, 0.7, and 0.9 are simulated. To maintain a stable simulation, the time step is set in the range of 5  105 s to 5  104 s. A total number of 100 physical seconds is

Fig. 5. Spatial distribution of solid flux in the DFBG with a PSD width (d/m) of 0.1. (a) horizontal solid flux (Fsx, kg/(m2s)); (b) vertical solid flux (Fsz, kg/(m2s)).

Fig. 6. 3-D illustration of the gas density distribution in the DFBG with a particle size width (d/m) of 0.1.

Z. Wan et al. / Energy 198 (2020) 117366

7

Fig. 7. Effect of size distribution width (d/m) of heat carrier on the distribution of gas density in the DFBG. (a) axial distribution in the combustor; (b) axial distribution in the gasifier; (c) horizontal distribution along the x coordinate.

comparison of mole fraction of dry gas species released from the gasifier outlet of the DFBG. As shown in the figure, the difference between the simulation results and the experiment data [49] is small, demonstrating the reliable prediction of the reactive MP-PIC model. 4. Results and discussion 4.1. General flow behavior of solid phase Fig. 3a shows the 3-D illustration of particles colored by the velocity magnitude in the DFBG with a size distribution width (d/m) of 0.1, t ¼ 100 s. The different flow regimes of combustor, gasifier and loop seal give rise to different flow intensity of solid phase in these components. The combustor operates in the fast fluidization regime, in which the particles are vigorously dragged upward by the gas introduced from the combustor inlet. A small velocity of solid phase in the lower part of combustor can be observed. Along the vertical direction, particle velocity increases due to the acceleration effect. Particles are transferred from the combustor outlet to the cyclone, in which the high-temperature solid particles are separated from the gas phase, followed by the accumulation in the loop seal. The loop seal operates in the nearly packed bed status with an extremely slow solid velocity. Moreover, the particles in the loop seal are fluidized by the introduced gas, and part of them fall

into the gasifier. Relatively slow motion of solid phase in the gasifier can be observed as this component operates under the bubbling fluidizing regime characterized by the presence of multiple bubbles. Apparent large particle velocity appears in the freeboard region, which corresponds to the elutriation of fine sand particles by the gas flow and small biomass particles due to the gasification. The transportation of sand particles from the gasifier to the combustor via the chute results in the return of low-temperature heat carrier to the combustor for the next cycle. Fig. 3b presents the spatial distribution of solid concentration in the DFBG. Dense distribution of solid phase in the gasifier can be observed. Large solid concentration appears in the lower part of three components, owing to the accumulation of large particles. In the combustor, the solid concentration becomes more and more dilute, demonstrating the typical feature of fast fluidization regime, i.e., the heterogeneous distribution of bed material with dense distribution in the bottom but dilute one in the upper. 4.2. Spatial distribution of gas flux The complex bed structure gives rise to the complicated gassolid flow dynamics in the system. Fig. 4 demonstrates the spatial distribution of the horizontal and vertical gas flux in the DFBG operating with a PSD width (d/m) of 0.1. Apparently, large horizontal gas flux appears in the outlet region of combustor, which

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Z. Wan et al. / Energy 198 (2020) 117366

Fig. 8. Impact of size distribution width (d/m) on the viscosity of gas phase in the DFBG. (a) axial distribution in the combustor; (b) axial distribution in the gasifier; (c) horizontal distribution along the x coordinate.

corresponds to the gas flow induced by the horizontal duct. Relatively large horizontal gas flux appears near the fuel inlet in the riser, and the cyclone due to the strong swirling flow in this part. As the combustor operates in the fast transportation regime, apparent large vertical gas flux appears in the component. Vigorous downward gas flux appears in the central region of the cyclone due to the downward swirling flow of gas phase in this component. Moreover, the fishtailing phenomenon of gas phase is evident in the cyclone. Comparatively, small gas vertical flux exists in the bubbling fluidized gasifier.

combustor, respectively. Furthermore, a large horizontal flux of solid phase exists in the region near the surface of the gasifier, attributing to the horizontal transportation of solid material from the loop seal. Chaotic solid motion along horizontal direction can be observed in the gasifier due to the vigorous bubble motion. In the combustor, large vertical flux of solid phase appears along the bed height, while vigorous solid back-mixing results in the large falling flux of solid phase near the combustor wall. A comparatively smaller vertical flux of solid phase is observed in the gasifier.

4.4. Gas density distribution 4.3. Solid flux distribution To explore the flow intensity of solid phase, Fig. 5 gives the spatial variation of the horizontal and vertical solid flux in the DFBG with a PSD width (d/m) of 0.1. Near the combustor inlet, the gas introduced from the combustor inlet, the particle stream transferred from the chute, together with the strong solid back-mixing near the combustor wall perplex the gas-solid motion in this region. Thus, chaotic solid motion with large horizontal flux magnitude appears in this zone. In the upper part of combustor, the horizontal solid flux decreases. As expected, large magnitude of horizontal solid flux appears in the loop seal and the chute as the particles are horizontally transported into the gasifier and

Different roles and regimes of combustor and gasifier lead to the spatial variation of gas temperature, which further affects the density and thermal properties of gas phase. Fig. 6 gives the 3-D illustration of gas density in the full-loop DFBG operating with a size distribution width (d/m) of 0.1. The gas density in the combustor is obviously larger than that in the gasifier, as the gas phase in the combustor is mainly composed of the O2, N2, and CO2. The largest gas density appears in the lowest part of the combustor, due to the smallest gas temperature in this region. Along the combustor height, a continuous decrease of gas density results from the enlarged gas temperature by the combustion of gas fuel in this component. Moreover, small gas density appears in the loop seal

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m). Overall, gas density is the highest in the combustor, and lowest in the gasifier along the horizontal direction, which is mainly due to the difference of gas species. The maximum gas density of nearly 0.9 kg/m3 appears near the biomass inlet due to the introduction of biomass particles. 4.5. Gas viscosity distribution The chemical reactions and complex hydrodynamics give rise to the spatial variation of the thermal properties of gas phase. Fig. 8 provides the spatial distribution of gas viscosity in the DFBG operating with different PSD width. According to Boltzmann’s law, fluid viscosity is independent of density but varies with the temperature. Higher temperature causes a greater thermal movement of gas molecules, which further increases the gas viscosity. In the combustor, small gas viscosity in the region below the fuel inlet can be observed as no chemical reactions are involved in this part. The sharp variation of the gas viscosity near the fuel inlet results from the introduction of low-temperature gas fuel. Above the fuel inlet, the fuel combustion raises the gas temperature and thus increases the viscosity. In the gasifier (Fig. 8b), the sharp increase of gas viscosity near the inlet corresponds to the heat exchange between the low-temperature steam introduced from the gasifier inlet and the bed material in this component. This variable sharply decreases at the axial position of biomass inlet, due to the heat absorption of the drying process of biomass. Above this position, gas viscosity decreases due to the vigorous gasification reactions of biomass particles. In the freeboard region, a slight variation of gas viscosity can be observed. Due to the high-temperature combustion in the combustor, large gas viscosity appears in this component (Fig. 8c), and decreases along the horizontal direction. The lowest gas viscosity can be observed in the gasifier especially the horizontal position with biomass introduction. Enlarging the PSD width increases the gas viscosity in the upper part of the combustor. 4.6. Gas temperature distribution

Fig. 9. 3-D distribution of the gas temperature (Tg, K) in the DFBG with a size distribution width (d/m) of 0.1.

due to the presence of very hot bed material here. Comparatively, large gas density exists in the lower part of the gasifier, as the lowtemperature steam is introduced here. Along the gasifier height, the gas density decreases due to the heat transfer from the hot bed material. The largest gas density appears near the biomass inlet, corresponding to the low-temperature biomass particles injected. Gas density does not change too much in the cyclone and chute as no chemical processes are involved. Fig. 7 presents the impact of size distribution width (d/m) of heat carrier on the lateral and vertical distribution of gas density in the DFBG. The presence of gas fuel inlet in the combustor significantly enlarges the gas density in this region due to the introduction of the low-temperature gas fuel. In the gasifier, biomass introduction results in a visible peak of gas density. The PSD width exerts a negligible impact on the gas density in the lower part of combustor, and the dense region of gasifier. A slight decrease of gas density in the upper part of combustor, and the increase in the freeboard region of gasifier can be observed with enlarging the PSD width (d/

Fig. 9 presents the spatial distribution of gas temperature in the DFBG. A small gas temperature appears in the lower part of the combustor, but significantly increases above the fuel inlet, owing to the heat release of gas combustion in this zone. Moreover, the relatively low temperature of gas phase at three axial positions corresponds to the addition of the 2nd air flow, 3rd air flow, and the propane into the combustor. A low gas temperature exists in the gasifier, especially near the biomass inlet. Low gas temperature near the gasifier inlet attributes to the low-temperature steam introduced. The heat exchange between bed material and steam enlarges the gas temperature in the gasifier. A slight decrease of gas temperature in the freeboard region of gasifier results from the endothermic homogeneous reactions involved in this part. Furthermore, relatively low gas temperature in the lower part of the loop seal is mainly due to the lower temperature of stream injected. Fig. 10 shows the spatial variation of gas temperature in the DFBG operating with different PSD width (d/m) of heat carrier. As shown in Fig. 10a, gas temperature maintains a nearly constant value in the region of z < 2 m of the combustor. The gas temperature rises rapidly as the combustion reaction proceeds. A slight decrease of gas temperature mainly attributes to the heat transfer to solid material. Gas temperature in the lower part of gasifier does not change too much. An apparent decrease of gas temperature at the axial position of biomass inlet results from the endothermic drying and devolatilization of biomass material. In the upper part of freeboard region, a slight decrease of gas temperature appears due to the endothermic homogeneous chemical reactions of gas

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Fig. 10. Effect of PSD width (d/m) on the temperature distribution of gas phase in the DFBG. (a) axial distribution in the combustor; (b) axial distribution in the gasifier.

Fig. 11. Impact of the size distribution width (d/m) of heat carrier on the thermal conductivity of gas phase in the DFBG. (a) axial distribution in the combustor; (b) axial distribution in the gasifier; (c) horizontal distribution along the x coordinate.

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Fig. 12. Impact of size distribution width (d/m) of heat carrier on the specific heat capacity of gas phase in the DFBG. (a) axial distribution in the combustor; (b) axial distribution in the gasifier; (c) horizontal distribution along the x coordinate.

species. Enlarging the PSD width does not obviously change the gas temperature in the lower part of combustor and gasifier, but increases the gas temperature in the upper part of combustor.

4.7. Thermal conductivity distribution Fig. 11 illustrates the spatial distribution of gas conductivity in the DFBG with different PSD width (d/m). A higher temperature enhances the molecular movement, and gives rise to a faster heat transfer. In the combustor, the thermal conductivity of gas phase changes little in the lower part of this component, but decreases slightly in the upper region. In the gasifier, gas thermal conductivity increases in the dense region due to the continuous heating of gas phase by the hot bed material. A slight decrease of gas thermal conductivity in the upper part of the gasifier primarily attributes to the endothermic homogeneous gasification reactions involved. Specifically, the thermal conductivity of gas phase in the combustor is within the range of 0.06e0.08 J/(s*m*K), while that in the gasifier ranges from 0.08 to 0.14 J/(s*m*K). Although the gas temperature in the combustor is obviously larger than that in the gasifier, the larger gas thermal conductivity in the gasifier is mainly due to the species difference of gas mixture. Thus, different from gas viscosity, the gas thermal conductivity continuously increases from the combustor to the gasifier along the horizontal direction (Fig. 11c). The PSD width exerts a negligible effect on the thermal conductivity in the lower

part of combustor and that in the dense region of gasifier. However, the enlargement of the PSD width results in larger thermal conductivity in the upper part of combustor. 4.8. Specific heat capacity distribution Fig. 12 shows the spatial distribution of gas specific heat capacity in the DFBG with different PSD width. As shown in Fig. 12a, the specific heat capacity in the combustor increases along the bed height. The apparent decrease of gas specific heat capacity appears in the gasifier. As shown in Fig. 12c, the gas mixture in the combustor contains various gas species with relatively low specific heat capacity (e.g., CO2), giving rise to a lower specific heat capacity of gas phase in this component. Large specific heat capacity of gas mixture in the gasifier mainly attributes to the large fraction of H2O in this reactor. The increase of PSD width enlarges the specific heat capacity of gas phase in the upper part of combustor. 4.9. Species distribution Fig. 13 presents the spatial variation of gas species in the DFBG with a size distribution width (d/m) of 0.1. Small air content appears in the lower part of the combustor, and obviously increase below the propane inlet, due to the introduction of second air flow. With the injection of propane from fuel inlet, the air concentration in the

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Fig. 13. Distribution of the gas species in the DFBG with PSD width (d/m) of 0.1. (a) Air; (b) C2H4; (c) CO; (d) CO2; (e) H2; (f) H2O.

Fig. 14. Time evolution of gas flow rate released from the outlet of combustor (a) and gasifier (b) of the DFBG with a PSD width (d/m) of 0.1.

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Fig. 15. Effect of PSD width (d/m) on the vertical distribution of gas species in the combustor of the bed. (a) N2; (b) O2.

upper part of combustor is significantly reduced due to the oxygen consumption of fuel combustion. In the gasifier reactor, biomass injected from the right side absorbs a large amount of heat due to the drying and pyrolysis processes, giving rise to large contents of gas species (C2H4, CO, CO2, and H2) around the biomass inlet and the upper part of the container. In general, similar spatial distribution can be observed for the gas species of C2H4, CO and H2, which are released from the biomass and participate in the homogeneous reactions. Moreover, the mass concentration of carbon monoxide is highest, followed by carbon dioxide, and the lowest content of ethylene. The spatial distribution of these gas species is strongly affected by the spatial distribution of biomass introduction. After introducing into the gasifier, the heated biomass particles release the volatile and thus these gas species are large in the region close to the inclined wall. In the freeboard region, the homogeneous reactions involved, gives rise to a slight variation of these gas species. A similar distribution behavior exists for the CO2 in the gasifier. The difference lies in that this gas species also exists in the combustor especially the region near the propane inlet, due to the propane combustion. As the gasifying agent, large steam fraction presents in the lower part of gasifier due to the vertical introduction from gasifier inlet. The devolatilization process of biomass releases the gas species in the right region of the gasifier, which correspondingly reduces the mass fraction of steam. In the upper part, the participation of steam in the homogeneous reactions significantly reduces the mass fraction of steam. Fig. 14 presents the time evolution profile of the flow rate of gas species released from the outlet of combustor and gasifier in the DFBG with a PSD width (d/m) of 0.1. In the combustor, oxygen in the air reacts with gas fuel to form carbon dioxide, while nitrogen does participate in any reaction. The consumption of oxygen gives rise to a small decrease of flow rate in the initial period. Moreover, the combustion gives rise to a decrease of the flow rate of N2 due to the production of other species. After the initial period, the nitrogen in the combustor keeps a high mass flow rate and oxygen and carbon dioxide have a mass flow rate of only about 0.02 kg/s. In the gasifier reactor, the continuous enlargement of gas species attributes to the volatile release after introducing the biomass particles. Moreover, the mass flow rates of H2, CO, and CO2 are more than that of the C2H2 and CH4. Among them, the content of H2 is the highest, followed by CO and CO2 due to the steam gasification. Fig. 15 shows the vertical content distribution of gas species in the combustor of the DFBG with different PSD width (d/m) of heat carrier. Introduced from the combustor inlet, the sharp decrease of

the mass fraction in the axial region of 0.5e2 m mainly results from the steam addition from the chute (Fig. 13f). Above the fuel inlet, the mass fraction of the N2 and O2 slightly increases due to the injection of additional 2nd and 3rd air flow. Along the combustor height, the size distribution of heat carrier exerts a negligible impact on the content of gas species in the combustor. Fig. 16 shows the content distribution of gas species in the gasifier of the DFBG operating with different PSD of heat carrier. Since the gasification reaction generates gaseous products, the mass fraction of steam continuously decreases. The gaseous products (CH4, CO, CO2) steadily increase in the dense region due to the release of volatile in the biomass particles. Above the bed surface, a slight decrease of the gas species can be observed owing to the homogeneous gasification reactions.

5. Conclusions Via the multiphase particle-in-cell approach, numerical simulation of biomass gasification in a dual fluidized bed gasifier with wide size distribution of heat carrier is conducted to study the spatial distribution of bed hydrodynamics in the apparatus. Based on the numerical simulation, the following tips can be drawn as: 1) The different flow regimes in the combustor and gasifier result in the different concentration and flow intensity of solid phase. Large vertical gas flux appears in the combustor. Strongly swirling flow together with fishtailing phenomenon appear in the cyclone. Intensive chaotic solid motion exists near the combustor inlet, while obviously horizontal solid motion appears close to the bed surface of gasifier. 2) Different flow regimes and the reactions result in different distribution of gas thermal properties. Gas density decreases along the bed height in the combustor. Large gas viscosity appears in the upper part of the gasifier. Obviously high gas temperature exists in the upper part of the combustor. The thermal conductivity and specific heat capacity of gas phase are large in the gasifier due to the large content of the steam in the apparatus. 3) Biomass inlet results in large content of gaseous products in the right part and freeboard region of gasifier. PSD width exerts a negligible effect on the gas density, temperature, conductivity, viscosity, in the lower part of combustor and the dense region of gasifier, but the noticeable impact in the upper region of combustor and the freeboard region of gasifier. Enlarging the

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Fig. 16. Effect of PSD width (d/m) of heat carrier on the vertical distribution of gas species in the gasifier of the DFBG. (a) CH4; (b) C2H4; (c) CO; (d) CO2; (e) H2; (f) H2O.

PSD width of heat carrier increases the contents of gas species in the gasification.

Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

CRediT authorship contribution statement Zhanghao Wan: Validation. Shiliang Yang: Writing - review & editing. Yonggang Wei: Resources. Jianhang Hu: Investigation. Hua Wang: Funding acquisition. Acknowledgments The authors thank the financial support from the National

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Natural Science Foundation of China-Yunnan joint fund (Grant No. U1602272) and National Natural Science Foundation (Grant No. 51966007).

Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.energy.2020.117366.

Nomenclature As CD Cp,i Cv ds Dgk Ds Es fD fs Fgs Fgx Fgz Fsx Fsz g hg hg,i DHrg mc mmoisture ms mvolatile Nugs pg Ps q Q_ D

Qradi Qreact Qsg Res Sgs Sgw t

T0 Tb,local Tg Ts ug ui uj us us Yg;i Yg,k

surface area of particle, m2 drag coefficient, dimensionless specific heat capacity of gas species i, J/kg$K specific heat capacity of particle, J/kg$K particle diameter, m turbulent mass diffusion rate, m2/s drag function, dimensionless particle enthalpy, J/kg particle distribution function obtained by collapsing the velocity dependence, dimensionless particle distribution function, dimensionless inter-phase momentum exchanging rate per volume, kg/(m3$s) horizontal flux of gas phase, kg/(m2$s) vertical flux of gas phase, kg/(m2$s) horizontal flux of solid phase, kg/(m2$s) vertical flux of solid phase, kg/(m2$s) gravitational acceleration, m/s2 gas mixture enthalpy, J/kg heat formation of gas species i, J/kg gas reaction heat, W/m3 mass of carbon, kg mass of moisture, kg mass of particle, kg mass of volatile, kg Nusselt number, dimensionless gas pressure, Pa positive constant with the unit of pressure, Pa fluid heat flux, W/m2 enthalpy diffusion, W/m3 radiative heat transfer between the gas and particle, W chemical reaction heat transfer between the gas and particle, W convective heat transfer between the gas and particle, W particle Reynolds number, dimensionless inter-phase energy exchanging rate, W/m3 convective heat transfer between the gas and wall, W/ m3 time, s reference temperature, K local bed-temperature, K gas temperature, K solid temperature, K gas velocity, m/s velocity of gas species i, m/s velocity of gas species j, m/s particle velocity, m/s local mass-averaged particle velocity, m/s mass fraction of gas species i, dimensionless mass fraction of gas species k, dimensionless

15

Greek symbols model parameter, dimensionless inter-phase momentum exchanging coefficient, kg/ (m3$s) g constant parameter, dimensionless Gs particle acceleration, m/s2 dm_ s mass source term of solid phase, kg/(m3$s) dij Kronecker delta, dimensionless dm_ k;react mass transfer between the gas species, kg particle volume fraction, dimensionless εs εg gas voidage, dimensionless qg gas volume fraction, dimensionless qs solid phase volume fraction, dimensionless qcs volume fraction of close-packing solid phase, dimensionless qc volume fraction of carbon, dimensionless d standard deviation of size distribution, m lg gas thermal conductivity, J/s$m$K m arithmetic mean diameter, m ml gas phase laminar viscosity, kg/(m$s) mt sub-grid scale viscosity of gas phase, kg/(m$s) mg shear viscosity of gas phase, kg/(m$s) rg gas density, kg/m3 rs particle density, kg/m3 s Stefan-Boltzmann constant, W/(m2$K4) gas phase stress tensor, Pa tg tD damping time, s ts normal stress of solid phase, Pa

a bgs

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