Three-Dimensional EulerianâEulerian Simulation of Coal Combustion

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Three-dimensional Eulerian-Eulerian Simulation of Coal Combustion under Air Atmosphere in a Circulating Fluidized Bed Combustor Ying Wu, Daoyin Liu, Jiliang Ma, and Xiaoping Chen Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b01084 • Publication Date (Web): 07 Jul 2017 Downloaded from http://pubs.acs.org on July 11, 2017

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Three-dimensional Eulerian-Eulerian Simulation of Coal Combustion under Air Atmosphere in a Circulating Fluidized Bed Combustor Ying Wu, Daoyin Liu*, Jiliang Ma, Xiaoping Chen a

Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education, School of

Energy and Environment, Southeast University, Nanjing, 210096, China.

KEYWORDS: Three-dimensional simulation; Circulating fluidized bed; Coal combustion; Eulerian-Eulerian approach

ABSTRACT: Circulating fluidized bed (CFB), due to its unique advantages like high fuel flexibility and low pollutant emission, has become a competitive technology for coal combustion. With the fast development of the computational fluid dynamics (CFD) technology, the numerical simulation has become a significant method to study the complicated characteristics of the CFB system. Based on a pilot-scale 50kWth CFB combustor, this paper develops a comprehensive three-dimensional CFB coal combustion model under the air atmosphere by the Eulerian-Eulerian approach. Good agreement is observed for the general behaviours of the CFB combustor between simulated results and experimental data. A full-physics picture including flow structure, temperature and gas composition in a three-dimensional space of the pilot-scale CFB combustor is provided. The simulation in this paper helps in a better understanding of the CFB combustion.

1. INTRODUCTION

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The circulating fluidized bed (CFB), as a chemical reactor, has attained great attention due to its unique advantages of high fuel flexibility, excellent heat transfer and low pollutant emission. Nowadays, it is still under rapid development and has been experimentally studied in the chemical, energy and environmental industries for applications1-4 like fluid catalytic cracking (FCC), fuel combustion and gasification, as well as pollutant emission reduction. Besides, with the increasing concerns about global warming, it is also a promising method for CO2 capture when combined with the O2/CO2 combustion technology5-8. However, due to the complicated mechanism of multiphase flow in the CFB, the deep theoretical analysis and experimental study of the CFB system is restricted. The computational fluid dynamics (CFD) provides an economical and effective way to study the CFB system. Besides, it exhibits great potential to replace the conventional empirical or semi-empirical models for the industrial-scale CFB design in future.

Numerical simulations of the fuel combustion or gasification in the CFB system by the CFD technology have been widely reported in the literature. Wang et al.9 investigated the chemical looping combustion processes (CLC) processes in a dual CFB reactor by means of the CFD method, and concluded that the CFD model with the cluster structure-dependent drag model was well suitable to predict the dynamic and reactive characteristics in the CFB reactor. A CFD model was implemented by Zhang et al.10 to simulate the coal gasification processes in a CFB gasifier, results of which predicted a reasonable distribution of temperature and gas composition in the riser, and agreed well with the experimental data. In the former work of our research


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group, Zhou et al.11, 12 developed a comprehensive CFD model of coal combustion in a CFB combustor. In her work, the simulated results of the flow characteristics, temperature, gas composition and reaction rate profile were analyzed in details. Limited by the expensive computational cost required by the multiphase characteristics in the CFB system, most of the excellent studies mentioned above were

conducted

under

the

two-dimensional

assumption.

However,

the

three-dimensional effects should not be overlooked as significant differences between two-dimensional and three-dimensional simulations were found by various scholars13-16. Li et al.17 presented two-dimensional and three-dimensional simulations of three different CFB risers and concluded that the two-dimensional simulation could only be used as a tool for qualitative studies while the three-dimensional simulation was needed to accurately capture the quantitative flow behavior in the CFB riser. Recently, with the continuous development of the computational hardware, three-dimensional simulations of fuel combustion or gasification in the CFB riser have become affordable18-20. Xie et al.21 developed a three-dimensional numerical model to study the gaseous pollutant emissions in a CFB combustor. Results showed the flow characteristics and distributions of the gas compositions in the axial direction, but neglected the temperature distribution validated by the experiment as well as the simulated profile in the radial direction. Besides, Adamczyk et al.22 conducted a three-dimensional simulation of an industrial CFB boiler under air- and oxy-fuel combustion modes. However, only the pressure and temperature profile in the simulated results were validated by the measured data.


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Thus, the comprehensive three-dimensional simulation of the coal combustion in the CFB system is not paid enough attention and should be further studied.

Usually, there exist two main approaches to simulate the multiphase flow in the circulating fluidized bed, namely Two Fluid Model (TFM) within the Eulerian-Eulerian framework and Discrete Element Method (DEM) within the Eulerian-Lagrangian framework23. Compared to the Eulerian-Lagrangian approach, the advantage of the Eulerian-Eulerian approach lies in the realistic computational time as both the fluid and solid phases are treated to be an interpenetrating continuum, which is suitable for the simulation of the large-scale equipment.

Based on a pilot-scale 50kWth CFB combustor in the Southeast University of China, a three-dimensional numerical simulation of coal combustion under the air atmosphere by two different coal types is conducted. The aim of this work is to develop a comprehensive Eulerian-Eulerian CFD model of coal combustion in the CFB combustor, which couples the gas-solid multiphase flow with heat and mass transfer, as well as various homogeneous and heterogeneous chemical reactions. The outcome of the model provides a full-physics picture including flow structure, temperature and gas composition. The detailed simulation of such a pilot-scale CFB combustor is limited in the literature as most studies focus on lab-scale facilities. As a method offering detailed qualitative and quantitative profiles, it successfully predicts critical parameters that are difficult to be obtained by experiment on a pilot scale, which exhibits great potential to improve the conventional empirical or semi-empirical models for the CFB scale-up. Besides, with the help of the


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comprehensive model in this paper, prediction of the oxy-fuel CFB combustion characteristics can be expected in the future research.

2. MATHEMATICAL FORMULATION

2.1 Experimental setup and materials The experiment is conducted in a pilot-scale 50kWth CFB combustor, which is simplified in Figure 1. It consists of a riser fitted with a distributor at the bottom, a high-temperature cyclone and a solid circulating system. The primary air is uniformly fed through the distributor, which adopts the cap type with an estimated distributor-to-bed pressure ratio of 1/3. The secondary air is fed from a secondary air inlet located at a port above the bed. The coal, delivered by the coal distributing air, is fed into the riser from a coal feed inlet. The solid particles escape from the furnace outlet and are separated from the gas by the high-temperature cyclone. The recycled particles are delivered back to the riser through the solid circulating inlet. The whole equipment is surrounded by the electrical heaters to make up for the heat loss and maintain the furnace temperature.

Two different coal types of bituminite and anthracite are selected in this paper. The ultimate and proximate analysis of two different coal types is shown in Table 1.

2.2 Model configuration The simulation is carried out by the commercial software ANSYS FLUENT version 16.0. Geometry of the CFB riser is shown in Figure 2. The riser is divided


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into three different zones, namely, a dense zone (0-800mm), a transition zone (800mm-1000mm) and a dilute zone (1000mm-4200mm). The inner diameters of the dense zone and the dilute zone are 122mm and 150mm. The riser consists of a primary air inlet at the bottom, a coal inlet at the bed height Z=700mm, a secondary air inlet at the bed height Z=900mm, a solid circulating inlet at the bed height Z=200mm and a furnace outlet at the bed height Z=4100mm.

The mesh sensitivity analysis is conducted by adopting four different grid cell number, containing about 54 391, 72 156, 98 277 and 121 813 grid cells. It is tested that the simulated deviation of the latter three is very small. Thus, the simulated results of the 98 277 grid cells are selected in this paper.

To ensure the good convergence during the whole simulation process, some assumptions are adopted as follows:

(1) The coal particle and the inert bed material are considered to be perfectly mixed in a single solid phase. The inert bed material is treated as the pure ash specie of the coal particle. Besides, a monodispersed solid phase is considered without considering the particle size distribution, effects of which on the simulated results will be studied in future. In this paper, the surface to volume mean diameter is used to represent the average diameter of the solid phase, which is calculated by,

d p = 1/ ∑ i

xi di

(1)

Where, d p is the surface to volume mean diameter, xi is the mass fraction of


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particles with diameter of d i . (2) Simulation of the high-temperature cyclone and the solid circulating system is not considered on the assumption that no chemical reactions occur in these parts. The method which maintains the solid inventory in the riser is based on the ash equilibrium,

Ashoutlet = Ashinlet + Ashre

(2)

Where, Ashinlet is the inlet ash from the coal feed inlet, Ashre is the inlet ash from the solid recirculating inlet and Ashoutlet is the outlet ash from the furnace outlet. Besides, the solid species composition of the solid circulating inlet is the same as that of the furnace outlet, and the solid temperature of the solid circulating inlet is determined with consideration of the temperature drop caused by the heat loss in the high-temperature cyclone and the solid circulating system. The assumptions mentioned above are realized by implementing a user defined function (UDF) into FLUENT.

(3) The small temperature difference in the CFB system indicates that the thermal radiation has little influence on the temperature distribution and is ignored in this paper.

2.3 Governing equations

2.3.1 Continuity equations


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r ∂ αg ρg ) +∇⋅ αg ρg ug = Sgs ( ∂t

(3)

r ∂ (αs ρs ) +∇⋅ αs ρs us = Ssg ∂t

(4)

αg + αs = 1

(5)

(

)

(

)

For the heterogeneous reactions, the source term is given by,

  Sgs = −Ssg = ∑ Ri ∑(γ R'' ,i − γ R' ,i )MWi  R  i 

(6)

'' ' Where, Ri is the reaction rate of species i ; γ R,i and γ R,i is the is the

stoichiometric coefficients of the product and the reactant; MWi is the molecular weight of species i .

2.3.2 Momentum equations

r r r r r r ∂ α g ρg u g +∇⋅ αg ρg u g u g = −αg ∇Pg +∇αg ⋅τ g + α g ρg g + β (us − u g ) + Sgs us ∂t

(7)

r r r r r r ∂ α s ρs u s + ∇ ⋅ α s ρs u s u s = −α s ∇Ps + ∇α s ⋅τ s + α s ρs g − β (u s − u g ) + Ssg u s ∂t

(8)

(

)

(

)

(

)

(

)

Where, the gas stress tensor τ g is,

r r T 2 r τ g = µg ∇u g + ∇ug  − αg µg ∇u g 

( ) 

3

( )

(9)

The solid stress tensor is closed by the constitutive relations adopted from the Kinetic Theory of Granular Flow (KTGF) 24,


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r r T 2 r r τ s = µs ∇us + ∇us  − µs ∇⋅ us + λs ⋅∇⋅ us

( ) 



(

3

)

(10)

Here, µs is the solid shear viscosity and λs is the bulk viscosity, 1/2 4  Θs  αs ρsdp Θsπ  2  P sinφ µs = αs ρsdp g0,ss (1+ ess )   + 1+ (1+ ess )(3ess −1)αs g0,ss  + s  5 6(3− ess )  5  2 I2D (11) π 

1/2

4 Θ  λs = αs ρsdp g0,ss (1+ ess )  s  3 π 

(12)

The radial distribution function g0,ss is defined as,

  α g0,ss = 1− ( s )1/3   αs,max 

−1

(13)

The granular temperature is introduced as a measure for the fluctuation energy of the particles, with its equation given as,

r r 3 ∂  α ρ Θ +∇⋅ α ρ u Θ = ( − PI + τ ):( ∇ u s s ) +∇⋅ (κs∇Θs ) − γ s − 3βΘs ( ) s s s s s s  s s 2  ∂t 

(

Where,

)

r (−Ps I + τ s ) : (∇u s ) , ∇⋅ (κ s∇Θs ) and γ s

(14)

is the generation, the

conduction and the dissipation rate of the fluctuation energy, with the granular conductivity κs and the collisional rate of energy dissipation per unit volume γ s expressed as, 2 150ρs d p Θsπ  6 Θ  κs = 1+ αs g0,ss (1+ ess ) + 2αs2 ρsd p g0,ss (1+ ess ) s  384(1+ ess )g0,ss  5 π 

(15)

r 4 Θs −∇⋅ us ) dp π

(16)

γ s = 3(1− ess2 ) g0,ss ρsαs2Θs (


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In this paper, the gas-solid drag coefficient β is the Wen-Yu model25 corrected by the Energy-Minimization Multi-Scale (EMMS) method26-28,

r

3 4

β = hβ CD

r

αsαg ρg us − ug ds

αg −2.65

 24 0.687 α Re 1+ 0.15(αg Res )  Res < 1000 CD =  g s  0.44 Res ≥ 1000  r

Res =

(17)

(18)

r

ρg ds us − ug µg

(19)

Here, hβ refers to the EMMS correction factor provided by Wang28, with detailed expression shown in Table 2.

2.3.3 Energy equations

r ∂ αg ρg Hg ) +∇⋅ αg ρg ug Hg =∇(λg∇Tg ) + Qgs + Sgs Hs ( ∂t

(20)

r ∂ (αs ρs Hs ) +∇⋅ αs ρs us Hs = ∇(λs∇Ts ) + Qsg + Ssg Hs ∂t

(21)

(

(

)

)

Here, Qsg = −Qgs refers to the gas-solid heat transfer term,

Qsg = hsg (Ts −Tg )

hsg =

(22)

6kgαsαg Nu dp2

(23)

The gas-solid heat transfer coefficient adopts the Gunn model29,


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0.333 0.333 Nu = (7 −10αg + 5αg2 )(1+ 0.7Re0.2 ) + (1.33 − 2.4αg +1.2αg2 )Re0.7 s Pr s Pr

(24)

2.3.4 Species equations

r ∂ αg ρgYi,g ) +∇⋅ αg ρg ugYi,g = −∇⋅αg J i,g + Ri,g ( ∂t

(25)

r ∂ αs ρsYi,s ) +∇⋅ αs ρs usYi,s = −∇⋅αs J i,s + Ri,s ( ∂t

(26)

(

(

)

)

Yi, g and Yi,s are the mass fraction of gas species and solid species. J i is the diffusion term. The mass source of the species equation, Ri is the reaction rate related to species i , as analyzed below. 2.4 Kinetic model As the coal particle is fed into the furnace, the moisture is quickly dried at first, and then there exist a release of the volatile called the pyrolysis process. The pyrolysis products undergo the homogeneous oxidation, whereas the remaining char undergoes the heterogeneous oxidation and gasification. Besides, the formation of the pollutants like NOx and SO2 is accompanied. In this paper, the chemical reactions in the coal combustion mainly involve the moisture evaporation, the volatile pyrolysis, the pyrolysis gaseous products oxidation, the char oxidation and gasification, and the pollutant formation. 2.4.1 Moisture evaporation, volatile pyrolysis and pyrolysis gaseous products oxidation Usually, the moisture evaporation occurs quickly upon the entry of the coal to the


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furnace. This process is not the emphasis in the simulation and has little influence on the overall performance. Thus, an Arrhenius expression of the moisture evaporation is applied, which is fast enough to be realistic, but slow enough to prevent pressure solver failure (R1).

( R1 ) : H2O( l ) → H2O( g ) The volatile pyrolysis is a process where various gaseous products are released through the decomposition of fuel. It is a very complicated process, and the distribution of products depends on many factors such as the coal type, temperature or species distribution. In this paper, the pyrolysis process is expressed according to MGAS (METC Gasifier Advanced Simulation) model10, 30. The pyrolysis products contain CH4, H2, CO2, CO, H2O(g), Tar, NH3 and H2S. Based on the mass balance of each component, the molecular formula of volatile (CaHbOcNdSe) and Tar (CxHyOz) and the stoichiometric coefficients of pyrolysis products are determined (R2).

( R2 ) :Volatile →α1Tar +α2CO +α3CO2 +α4CH4 +α5H2 +α6H2O( g ) +α7 NH3 +α8H2S For bituminite,

Volatile: C2.2H8.22O1.036N0.035S0.032 Tar: C10.79H11.43O0.18

α1 = 0.1246,α2 = 0.1419,α3 = 0.0659,α4 = 0.6469

α5 =1.2797,α6 = 0.7398,α7 = 0.0350,α8 = 0.0320


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For anthracite,

Volatile: C1.59H12.27O0.136N0.022S0.034 Tar: C10.79H95.55O0.18

α1 = 0.0239,α2 = 0.0184,α3 = 0.0085,α4 =1.3044 α5 = 2.2224,α6 = 0.0959,α7 = 0.0220,α8 = 0.0340 The pyrolysis products will undergo the homogeneous oxidation after the pyrolysis process (R3- R6).

( R3 ) : CH4 +1.5O2 → CO + 2H2O( g ) ( R4 ) : CO + 0.5O2 → CO2

( R5 ) : H2 + 0.5O2 → H2O( g )

( R6 ) : Cx HyOz + (x +

y z y − )O2 → xCO2 + H2O( g ) 4 2 2

2.4.2 Char oxidation and gasification The char oxidation is a typical heterogeneous reaction, in which the char particle is assumed to be a spherical particle surrounded by a stagnant boundary layer. In this paper, the char oxidation rate is controlled by the mixture of the kinetic resistance and mass transfer resistance. Besides, both CO and CO2 are the primary products (R7).


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 2 2  1 m + + n)O2 → 2 −  CO +  −1 CO2 + mNO + nSO2 φ 2  φ φ 

( R7 ) : CNmSn + (

 2p + 2 dp < 0.05mm  p+2    φ = 2 p + 2 − ( p / 0.095)(100dp − 0.005) 0.05mm ≤ dp ≤1.0mm  p+2    dp >1.0mm 1.0  p = 2500exp(−5.19*104 / RTs ) For bituminite, m=0.008, n=0.0075;

For anthracite, m=0.0077, n=0.0119;

On the other hand, the char gasification with CO2 and H2O(g) is considered (R8-R9).

( R8 ) : CNmSn + (1+ m + 2n)CO2 → (2 + m + 2n)CO + mNO + nSO2

( R9 ) : CNmSn + (1+ m + 2n)H2O( g ) → (1+ m + 2n)H2 + CO + mNO + nSO2 2.4.3 Pollutant formation The complicated mechanism, as well as the large amounts of species in low concentrations, makes the CFD modeling of pollutant formation one of the most challenging tasks. To reduce the computational difficulty and obtain acceptable results, the pollutant formation process is treated as follows: (1) The sulfur and nitrogen is partitioned between the volatile form (CaHbOcNdSe) and the char form (CNmSn), and the mass fraction in the volatile is identical to that in the dry, ash-free


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original coal31; (2) The sulfur and nitrogen is devolatilized into H2S and NH3 in the pyrolysis process (R2), but oxidized into SO2 and NO in the char oxidation (R3); (3) The H2S and NH3 will be further oxidized into SO2 (R10) and NO (R11); (4) It is assumed that NO is the only form of NOx since NO usually accounts for over 90% of NOx32, and reduction reactions between NO and char/CO/NH3 are introduced (R12-R14); (5) From the ash composition analysis of bituminite in Table 3, the calcium oxide (CaO) contained in the ash can absorb a part of SO2 into the calcium sulfate (CaSO4), which is also considered in the simulation (R15).

( R10 ) : H2S +1.5O2 → SO2 + H2O( g )

( R11 ) : NH3 +1.25O2 → NO +1.5H2O( g )

( R12 ) : CNmSn + (1+ 2n)NO →CO + (0.5 +

m + n)N2 + nSO2 2

( R13 ) : CO + NO → CO2 + 0.5N2 ( R14 ) : NH3 + NO + 0.25O2 → N2 +1.5H2O

( R15 ) : CaO + SO2 + 0.5O2 → CaSO4 The reaction rates and kinetic parameters in this paper are shown in Table 430, 33-36.

3. PARAMETER SETTING For the material, the gas phase consists of 12 species (methane CH4, oxygen O2, carbon dioxide CO2, carbon monoxide CO, water vapor H2O(g), hydrogen H2, sulfur dioxide SO2, Tar CxHyOz, ammonia NH3, nitric oxide NO, hydrogen sulfide H2S and


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nitrogen N2) and the solid phase consists of 6 species (volatile CaHbOcNdSe, char CNmSn, moisture H2O(l), calcium oxide CaO, calcium sulfate CaSO4 and ash). It needs to be noted that, CaO is separated from ash as it plays a key role in the SO2 absorption. Thus, ash is composed of all the components in Table 3 except CaO during the whole simulation process. Properties of the volatile, ash and Tar are estimated by the method proposed by Eisermann37.

For the boundary conditions, all the inlets (the primary air inlet, the secondary air inlet, the coal feed inlet and the solid recirculating inlet) adopt the velocity-inlet type, while the furnace outlet adopts the pressure-outlet type. At the inlets, the gas and solid phase is uniformly fed into the furnace with constant homogeneous velocity. The no-slip boundary is used for the gas phase while the partial slip boundary is used for the solid phase. At the initial time, the gas atmosphere is air and the ash particles are stacked up to 400mm with the solid volume fraction set to be 0.55. Temperature of the gas and the solid phase is 1123K. Some main parameters used in this simulation are shown in Table 5. A time-step of 1.0*10-4s with 50 iterations per time step is used. The phase-coupled SIMPLE algorithm and the first-order upwind scheme are used to discretize the governing equations. A residual of less than 1.0*10-3 for all the variables is imposed as the stopping criterion except the temperature, whose stopping criterion is 1.0*10-6 as the solution of the energy equation has great effects on the temperature and gas component distribution.


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4. RESULTS AND DISCUSSION The CFB coal combustion simulation by two different coal types is conducted. As the thermal input remains unchanged, there exists a slight difference in the coal feed rate and superficial gas velocity by two different coal types, as shown in Table 6.

The simulation is run for 50s, and it takes about two months by using twelve CPU cores. Taking bituminite as an example, the simulated O2 and CO2 outlet volume fraction with time is shown in Figure 3. Various reactions of the coal combustion are accompanied by a decrease of O2 and an increase of CO2. Besides, it can be observed that the result reaches a steady value from about 25s, which indicates the stable condition in the furnace. Under the stable condition, the time-averaged solid flux is 8.78 kg/(m2s) with the circulating ratio of approximately 70. The circulating ratio γ is defined as,

γ = mre / min

(27)

Where, mre is the time-averaged solid mass flow of the solid recirculating inlet,

min is the time-averaged solid mass flow of the coal feed inlet. The profile of the hydrodynamics, temperature and gas composition is analyzed as follows. To easily compare the difference of the simulated results, the time-averaged curve and the transient contour are shown in one figure. Besides, for the transient contour, a common colorbar is used.

4.1 Hydrodynamics


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The solid volume fraction profile by two different coal types is compared with the experimental data in Figure 4. The solid volume fraction of two coal types exhibits an exponential reduction along the bed height with the simulated results showing a good agreement with the experimental data, indicating the effectiveness of the EMMS drag model in improving the simulation accuracy. It can be observed that the anthracite curve is a little steeper than the bituminite curve, which is mainly caused by the slight decreased superficial gas velocity (Table 6).

The solid Z-velocity profile by bituminite is shown in Figure 5. The axial profile shows that the solid Z-velocity in the dilute zone is much larger than that in the dense zone. The reason lies in two aspects: (1) on the one hand, the injection of the secondary air increases the superficial gas velocity and promotes the gas-solid momentum transfer; (2) on the other hand, the EMMS model takes the drag reduction caused by the cluster agglomerates into consideration, as a result of which, the high solid volume fraction in the dense zone indicates a poor gas-solid contacting and the gas-solid drag coefficient in the dense zone is much lower than that in the dilute zone. From the radial profile, in the lower part of the dilute zone (Z=1000mm), the typical ‘core-annulus’ flow structure with the solid flowing upward in the center and flowing downward near the wall can be apparently observed, which is consistent with the flow structure of the fast fluidization regime. With the increase of the bed height, the solid Z-velocity near the wall becomes gradually positive due to the decreased solid concentration.

4.2 Temperature


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Figure 6 shows the axial gas temperature profile by two different coal types. In the dense zone, the cold primary air is heated and the temperature increases due to the exothermic reactions of the char oxidation and the pyrolysis gaseous products oxidation. When a large amount of the cold secondary air is injected in the transition zone, the temperature reduces to some extent. With the further oxidation reactions of the gaseous products that released from the volatile pyrolysis process, the temperature in the dilute zone increases with the bed height at first and reaches the maximum at a position where the amount of heat produced is equivalent to the amount being removed, then begins to cool down due to the heat loss through the wall. The simulated temperature curve by two different coal types closely follows the trend observed in the experimental data with the maximum deviation of about 50K. Compared with the bituminite curve, the anthracite curve shows a higher temperature level in the dense zone, but a lower temperature level in the dilute zone. The reason lies in the fact that anthracite has a high fixed carbon but a low volatile content, as the char oxidation mainly occurs in the dense zone whereas the gaseous products released from the volatile pyrolysis are mainly oxidized in the dilute zone.

Figure 7 shows the radial gas temperature profile by bituminite at different bed heights (Z=700mm, Z=800mm, Z=900mm, Z=3500mm). At the bed height of the coal feed inlet Z=700mm, the cold coal distributing air is heated and its temperature rapidly increases. At the bed height Z=800mm, the temperature on the left side is higher than that on the right side, which is due to the fast chemical reactions of the pyrolysis gaseous products oxidation. At the bed height of the secondary air inlet


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Z=900mm, the injection of the cold secondary air on the right side intensifies the radial difference of the gas temperature. At the bed height Z=3500mm, the temperature distribution is radially uniform and it gradually decreases in the radial direction due to the heat loss to the surroundings.

The axial gas and solid temperature profile by bituminite is compared in Figure 8. It can be easily found that the overall temperature difference between the gas and solid phase is rather small, which is attributed to the excellent gas-solid heat transfer. There exists obvious temperature difference in the secondary air injection zone. With the injection of the cold secondary air, the gas temperature fluctuation is more obvious than the solid temperature as the specific heat of gas is lower than that of solid.

4.3 Gas composition The time-averaged outlet gas volume fraction (on the dry basis) by two different coal types is compared with the experimental data in Figure 9. A good agreement between the simulated results and the experimental data is achieved, indicating that the selected chemical reaction models are acceptable.

The axial gas distribution profile (on the wet basis) by two different coal types is shown in Figure 10- Figure 14. The mass fraction of O2, CH4, CO, SO2 and NO is studied in this section.

In the dense zone, O2 in the primary air is consumed and its concentration decreases due to the char oxidation and the pyrolysis gaseous products oxidation.


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Then, the injection of the secondary air increases the O2 concentration in the transition zone to some extent. With the further oxidation reactions, the O2 concentration begins to fall down and finally reach a steady value when the combustibles almost burn out. It can be observed that, although there is little difference for the outlet O2 concentration of two different coal types, the O2 concentration of anthracite exhibits a lower value in the dense zone, which is caused by its higher fixed carbon content as the char oxidation mainly occurs in the dense zone.

CH4, produced by the pyrolysis process, is mainly distributed near the coal feed inlet and then rapidly consumed due to its fast oxidation reaction rate. The same variation trend can be found for other pyrolysis products such as Tar, H2, NH3 and H2S. Unlike CH4, CO is widely distributed at the bottom of the riser since CO is mainly produced by the char oxidation, not only the pyrolysis process. Effects of the secondary air injection and fast oxidation reaction rate on the CO concentration can also be found. For anthracite, its high fixed carbon but low volatile content indicates a low CH4 concentration but a high CO concentration profile compared to bituminite.

As mentioned before, the sulfur and nitrogen in the coal is partitioned between the volatile form and the char form. Thus, the pollutants SO2 and NO are produced by the char oxidation and the H2S & NH3 oxidation. Both SO2 and NO show a similar variation trend. At first, the SO2 (NO) concentration increases due to the fast oxidation rate in the dense zone. Then the injection of the secondary air decreases


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the SO2 (NO) concentration to some extent. As the combustibles gradually burn out, the absorption (reduction) reactions begin to dominate and a gradual decrease of the SO2 (NO) concentration in the dilute zone is observed. Compared to bituminite, the anthracite curve exhibits a high SO2 concentration but a low NO concentration, which corresponds to the S and N content in the ultimate analysis of two different coal types (Table 1).

Figure 15 shows the radial gas distribution profile (taking O2 as an example) by bituminite at different bed heights (Z=700mm, Z=800mm, Z=900mm, Z=3500mm). At the bed height of the coal feed inlet Z=700mm, O2 in the coal distributing air is consumed and its concentration rapidly decreases. At the bed height Z=800mm, the O2 concentration on the left side is obviously lower than that on the right side, which is due to the fast chemical reactions of the pyrolysis gaseous products oxidation. At the bed height of the secondary air inlet Z=900mm, the injection of the secondary air on the right side intensifies the radial difference of the O2 concentration. At the bed height Z=3500mm, O2 is uniformly distributed.

5. CONCLUSION The three-dimensional computational fluid dynamics simulation is conducted for a CFB combustor by two different coal types in an Eulerian-Eulerian framework. The established model couples the gas-solid flow with mass transfer, heat transfer and chemical reactions. The profile of the hydrodynamics, temperature and gas composition is analyzed in the axial and radial direction. Good agreement is


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observed between the simulated results and the experimental data:

(1) Based on the EMMS drag model, the simulated results of the solid volume fraction profile show a good agreement with the experimental data. From the solid Z-velocity profile, the ‘core-annulus’ flow structure with the solid flowing upward in the center and flowing downward near the wall can be observed.

(2) The simulated temperature curve closely follows the trend observed in the experimental data. The temperature difference in the radial direction at different bed heights is also theoretically analyzed.

(3) A good agreement between the simulated results and the experimental data is achieved with respect to the outlet gas volume fraction, indicating the effectiveness of the selected chemical reaction models. The mass fraction of the main gas components in the axial and radial direction is successfully predicted.

In summary, the simulation successfully establishes a comprehensive model of the CFB coal combustion system, and provides a full-physics picture the pilot-scale CFB combustor, which is beneficial for the scale-up design and development of oxy-fuel CFB combustion. AUTHOR INFORMATION Corresponding Author *E-mail for Daoyin Liu: [email protected]. Tel/Fax : +86-25-8379-3453. Notes The authors declare no competing financial interest.


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ACKNOWLEDGEMENTS Financial support of this work by National Key Research and Development Plan (No.2016YFB0600802) is gratefully acknowledged.

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[13] Xie, N.; Battaglia, F.; Pannala, S. Effects of using two- versus three-dimensional


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computational modeling of fluidized beds: Part I, hydrodynamics. Poweder Technology 2008, 182, 1-13.

[14] Xie, N.; Battaglia, F.; Pannala, S. Effects of using two- versus three-dimensional computational modeling of fluidized beds: Part II, budget analysis. Poweder Technology 2008, 182, 14-24.

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[18] Alvarez, L.; Gharebaghi, M.; Jones, J.M.; Pourkashanian, M.; Williams, A.; Riaza, J.; Pevida, C.; Pis, J.J.; Rubiera, F. CFD modeling of oxy-coal combustion: Prediction of burnout, volatile and NO precursors release. Applied Energy 2013, 104, 653-665.

[19] Wang, X.; Jin, B.; Zhang, Y.; Zhang, Y.; Liu, X. Three dimensional modeling of a coal-fired chemical looping combustion process in the circulating fluidized bed fuel reactor. Energy & Fuels 2013, 27, 2173-2184.


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NOMENCLATURE Abbreviation CFB

Circulating Fluidized Bed

CFD

Computational Fluid Dynamics

FCC

Fluid Catalytic Cracking

CLC

Chemical Looping Combustion

TFM

Two Fluid Model

DEM

Discrete Element Method

UDF

User Defined Function

KTGF

Kinetic Theory of Granular Flow

EMMS

Energy Minimization Multi Scale

MGAS

METC Gasifier Advanced Simulation


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SIMPLE

Semi Implicit Method for Pressure Linked Equations

Symbols

dp

Surface to volume mean diameter

α

Volume fraction

ρ

Density

u

Velocity

S

Source term

R

Reaction rate term

P

Pressure

τ

Stress tensor

β

Drag coefficient

µs

Solid shear viscosity

λs

Bulk viscosity

g 0, ss

Radial distribution function

κs

Granular conductivity

γs

Collisional rate of energy dissipation per unit volume

hβ

EMMS correction factor

hsg

Gas solid heat transfer coefficient

T

Temperature

Pr

Prandtl number

Re

Reynolds number

Nu

Nusselt number


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Y

Mass fraction of species

Ji

Diffusion term

C

Species concentration

r

Reaction rate


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5

1

4 6 2

3 1-Riser 2-Coal feeder 3-Primary air 4-Secondary air 5-High-temperature cyclone 6-Solid circulating system

Figure 1. The simplified structure of the pilot-scale 50kWth CFB combustor


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Energy & Fuels

1-Coal inlet 2-Primary air 3-Secondary air 4-Recycle 5-Outlet

Figure 2. Geometry of the CFB combustor from different angles


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Outlet volume fraction (%)

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O2

22 20 18 16 14 12 10 8 6 4 2 0 -2

CO2

0

10

20

30

40

50

Time (s)

Figure 3. The simulated O2 and CO2 outlet volume fraction with time by bituminite


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Solid volume fraction 0.6 0.49 0.38 0.27 0.16 0.05

Bituminite

Anthracite Z

Y

(a)

X

(b)

Figure 4. The solid volume fraction profile by two different coal types (a) time-averaged curve (b) transient contour


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Z=4000mm

Solid Z-velocity (m/s) 4.5 Z=3000mm

3.1 1.7 0.3

Z=2000mm

-1.1 -2.5

Z=1000mm

Y

Y

X Y

Y

Z

Z

Z

X

X

Z

X

Surface Y=0

(a)

(b)

Figure 5. The solid Z-velocity profile by bituminite (a) axial contour (b) radial contour


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Temperature (K) 1400 1200 1000 800 600 400

Bituminite

Anthracite Z

Y

(a)

X

(b)

Figure 6. The axial gas temperature profile by two different coal types (a) time-averaged curve (b) transient contour


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Z=3500mm

Temperature (K) 1400 Z=900mm

1200 1000 800

Z=800mm

600 400

Z=700mm

Y

Z

Y

X Z

(a)

X

(b)

Figure 7. The radial gas temperature profile by bituminite (a) time-averaged curve (b) transient contour


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Temperature (K) 1400 1200 1000 800 600 400

Gas

Solid Z

X Y Surface Y=0

(a)

(b)

Figure 8. Comparison of gas and solid temperature profile by bituminite (a) time-averaged curve (b) transient contour


Energy & Fuels

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Figure 9. The gas emission characteristics by two different coal types


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Mass fraction (%) 24.0 20.0 16.0 12.0 8.0 4.0

Bituminite

Anthracite Z

Y

(a)

X

(b)

Figure 10. The axial distribution of O2 by two different coal types (a) time-averaged curve (b) transient contour


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Mass fraction (%) 9.0 7.3 5.6 3.9 2.2 0.5

Bituminite

Anthracite Z

Y

(a)

X

(b)

Figure 11. The axial distribution of CH4 by two different coal types (a) time-averaged curve (b) transient contour


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Energy & Fuels

Mass fraction (%) 4.2 3.4 2.6 1.8 1.0 0.2

Bituminite

Anthracite Z

Y

(a)

X

(b)

Figure 12. The axial distribution of CO by two different coal types (a) time-averaged curve (b) transient contour


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Mass fraction (%) 0.5 0.4 0.3 0.2 0.1 0.0

Bituminite

Anthracite Z

Y

(a)

X

(b)

Figure 13. The axial distribution of SO2 by two different coal types (a) time-averaged curve (b) transient contour


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Mass fraction (%) 0.15 0.12 0.09 0.06 0.03 0.001

Bituminite

Anthracite Z

Y

(a)

X

(b)

Figure 14. The axial distribution of NO by two different coal types (a) time-averaged curve (b) transient contour


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Z=3500mm

Mass fraction (%) 24.0 Z=900mm

20.0 16.0 12.0

Z=800mm

8.0 4.0

Z=700mm

Y

Z

Y

X X Z

(a)

(b)

Figure 15. The radial distribution of O2 by bituminite (a) time-averaged curve (b) transient contour


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Table 1 The ultimate and proximate analysis of two different coal types Ultimate analysis (wt%)

Bituminite

Anthracite

Cad

58.97%

55.65%

Had

3.65%

1.31%

Oad

7.30%

0.23%

Nad

0.67%

0.52%

Sad

1.76%

2.74%

Char

47.33%

54.43%

Volatile

25.02%

6.02%

Moisture

2.10%

1.32%

Ash

25.55%

38.23%

Low heating value, LHV (MJ/kg)

23.54

24.64

Proximate analysis (wt%)


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Table 2 The fitting formula of the heterogeneity index hβ Fitting Formula ( hβ = a * Reb )

Range of α g

 a = -6848.6908+47360.3831α g +72699.0872 α g2 ln(α g )-41781.9224α g3 +494.7451/α g  2 2 3 b = -1845.2146+4138.4718 α g +410.4788/α g -4630.2697α g -36.4527/α g +2067.2072α g

0.4 < α g ≤ 0.5170

2 2  a = (0.1181-0.3862α g +0.3212α g )/(1-3.7165α g +3.4796α g )  5.4444 b = 0.1913/(1+exp(23.4801-45.0888 α g ))

0.5170 < α g ≤ 0.6297

 a = (-1084.3396+1080.9754α g-0.8837 ) -0.4603  3 b = -0.3246+0.4399α g -0.1985α g +0.00014/log(α g )+0.1769/α g

0.6297 < α g ≤ 0.9853

 a = 1.8069α g433.8562 +0.5966 α g39.9478  2 2 b = (0.0978-0.0981 α g )/(1-1.0006α g )

0.9853 < α g ≤ 0.9997

 a = 1.0  b = 0.0

0.9997 < α g ≤ 1

( ρg =0.3326kg / m3, ug =5.55m/ s , ρp =1610kg / m3 , dp = 0.35mm, µg = 4.3376*10−5 Pa⋅ s )


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Energy & Fuels

Table 3 The ash composition analysis of bituminite SiO2

Al2O3

Fe2O3

CaO

MgO

PbO

SO3

K2O

Na2O

CuO

35.69%

24.47%

6.71%

14.89%

1.44%

0.89%

13.99%

1.08%

0.57%

0.27%


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Table 4 The reaction rates and kinetic parameters for the coal combustion Reaction

Reaction rate

Kinetic parameter

R1

r1 = k1C H 2O ( l )

k1 = 1.1*10 5 exp( − 1.07 *10 4 / Ts )

R2

r2 = k 2CVolatile

k 2 = 1.14 *10 4 exp( − 8.78 *10 3 / Ts )

R3

0.7 r3 = k 3 C CH C O0.82 4

k 3 = 5.01*1011 exp( − 2.42 *10 4 / Tg )

R4

r4 = k 4 C CO C O0.52 C H0.52O

k 4 = 1.3*1011 exp( − 1.51*10 4 / Tg )

R5

r5 = k 5 C H 2 C O2

k 5 = 1.08 *1013 exp( − 1.52 *10 4 / Tg )

R6

r6 = k 6 C Tar C O2

k 6 = 3.8 *10 7 exp( − 6.68 *10 3 / Tg )

R7

r7 =

6ε s ρ sYchar k c C NO d p ρc

k7 = 8.91*103 exp(−1.8*104 / Tg ) kd = φ ShDg wc / d p RTg , Sh = 2ε g + 0.69(Re/ ε g )0.5 Sc0.333

RT / wc kc = (1 / k d ) + (1 / k 7 )

R8

Re = ug − us d p ρ g / µ g , Sc = µ g / ρ g Dg

k8 = 4.4Ts exp(−1.77*104 / Ts )

0.6 r8 = k8C CO

2

r9 = k9 C H0.6O ( g )

k9 = 1.33Ts exp( −1.77 *104 / Ts )

R10

r10 = k10 C H0.22 S C O1.32

k10 = 2.12 *1011 exp( − 2.45 *10 4 / Tg )

R11

r11 = k11C NH 3 C O2

k11 = 2.73 *1017 exp( − 3.82 *10 4 / Tg )

R12

r12 =

R13

r13 = k13 (

R14

0.5 0.5 r14 = k14 C NH CO0.52 C NO 3

R15

r15 =

R9

2

*Unit:

6ε s ρ sYchar k16 C NO d p ρc

k12 = 1.3 *10 5 exp( − 17111 / Tg )

aC NO (bCCO + c ) ) aC NO + bCCO + c

k13 = 1.95 *10 7 exp( −1.9 *10 4 / Tg ) a = 182.6, b = 7.86, c = 0.002531 k14 = 3.38 *1013 exp( − 2.94 *10 4 / Tg )

ε s ρ sYCaO η k15C SO ρ CaO

2

k15 = 1.1*10 6 exp( −7.16 *10 3 / Tg )

η = exp( −5.71 X ), X = CCaSO4 / (CCaO + CCaSO4 )

r : kmol /(m3 ⋅ s),C: kmol / m3,T : K


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Energy & Fuels

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Energy & Fuels

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Table 5 Main parameters used in this simulation Parameter

Value

Parameter

Value

Particle diameter

0.35mm

Coal distributing air ratio

0.05

Primary air temperature

298K

Material returning air ratio

0.05

Secondary air temperature

298K

Restitution coefficient

0.95

Coal distributing air temperature

298K

Solid packing limit

0.63

Material returning air temperature

700K

Wall thickness

300mm

Primary air ratio

0.6

Outer wall temperature

298K

Secondary air ratio

0.3

Furnace outlet pressure

-50Pa


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Energy & Fuels

Table 6 Gas parameters of two different coal types Condition

Bituminite

Anthracite

Thermal input

50kWth

50kWth

Coal feed rate

0.00212 kg/s

0.00203 kg/s

Theoretical gas flow (273K)

6.03 Nm3/kg (coal)

5.38 Nm3/kg (coal)

Excess air coefficient

1.2

1.2

Practical gas flow (273K)

7.24 Nm3/kg (coal)

6.46 Nm3/kg (coal)

Superficial gas velocity (1123K)

3.57 m/s

3.05 m/s


Three-Dimensional EulerianâEulerian Simulation of Coal Combustion

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