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Numerical simulation of semi-dry flue gas desulfurization process in the powder-particle spouted bed
Advanced Powder Technology xxx (xxxx) xxx
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Advanced Powder Technology journal homepage: www.elsevier.com/locate/apt
Original Research Paper
Numerical simulation of semi-dry flue gas desulfurization process in the powder-particle spouted bed Feng Wu a,⇑, Kai Yue a, Weiwei Gao a, Ming Gong a, Xiaoxun Ma a,⇑, Wenjing Zhou b a b
School of Chemical Engineering, Northwest University, Xi’ an 710069, China School of Chemical Engineering and Technology, Xi’an Jiaotong University, Xi’an 710049, China
a r t i c l e
i n f o
Article history: Received 9 January 2019 Received in revised form 17 October 2019 Accepted 24 October 2019 Available online xxxx Keywords: Powder-particle spouted bed Semi-dry flue gas desulfurization Numerical simulation Optimization analysis
a b s t r a c t Computational fluid dynamics (CFD) combined with the two-fluid model (TFM) was used for simulation of water vaporization and semi-dry flue gas desulfurization process in a two dimensional powder-particle spouted bed (PPSB), on the basis of gas-solid two-phase flow, the mathematical and physical models of water vaporization process and flue gas desulfurization reaction process have been established through reasonable hypothesis and simplification of the system. The numerical method was used to simulate the desulfurization reaction process and the heat and mass transfer in the powder-particle spouted bed. Simulation results indicate that water vaporization rate was high in spout and annular regions. The main area where flue gas desulfurization reaction occurs was annular area, as a result, the maximum value of desulfurization product rate appears in the annulus. Under the same condition, the desulfurization efficiency of simulation value is 75.75% when the value of slurry water content equals 40 kg-H2O/kgdry_sorbent, which is close to but greater than the experimental value (75.03%). The desulfurization efficiency of spouted bed increases first and then decreases with the increase of water content of desulfurization slurry, and the optimum slurry water content for desulfurization process in powder-particle spouted bed was obtained by numerical simulation, which was 40 kg-H2O/kg-dry_sorbent. Ó 2019 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.
1. Introduction The increasing use of fossil fuels to meet energy needs has led to higher SO2 and CO2 emissions into the atmosphere, which is dangerous to the environment and human health [1–5], and the emission of SO2 and CO2 from power plants that burn fossil fuels form acid rain in the atmosphere, and then result in serious environmental problems [6–16]. Removal of SO2 and CO2 from gas has been a worldwide concern. Various technologies for flue gas desulfurization (FGD) can be classified into three different types: wet scrubber, semi-dry processes and dry processes [11]. Semidry FGD processes have been developed and adopted commercially since the 1980s. The spray dry scrubber process, as a typical semidry FGD process, is effective in removing SO2 from flue gas and has been developed as an alternative to wet scrubbers [6–10]. For flue gas cleaning, many different processes have been developed for SO2 and CO2 removal. Ma et al. [7–9] experimentally investigated the use of limestone for SO2 removal from flue gas in the semidry FGD process with a powder-particle spouted bed ⇑ Corresponding authors. E-mail addresses: [email protected] (F. Wu), [email protected] (X. Ma).
(PPSB), they also studied the influence of gas components on removal of SO2 from flue gas in the semidry FGD process with a PPSB, and results confirm that PPSB has the advantage of higher SO2 capture efficiency over the conventional existing processes. However, they have not built mathematical model of the desulfurization process and analyzed it in a powder–particle spouted bed. Xu et al. [10] experimentally tested a new semi-dry desulfurization process, and the process uses the so-called powder-particle spouted bed PPSB as the reactor in which coarse medium particles, usually silica sand of several hundred micrometers in size, are fluidized with hot flue gas. Wu et al. [11] studied the effect of the pore-size distribution of lime on the reactivity for the removal of SO2 in the presence of high-concentration CO2 at high temperature. Wang [12] experimentally investigated effects of humidification water parameters (ratio of humidification water to total water, location of humidification water) on semi-dry flue gas desulfurization removal efficiency. Rahimi et al. [13,14] developed a mathematical model for investigation of SO2 and CO2 removal in a powder-particle spouted bed for non-isothermal operating condition. The one-dimensional stream-tube model is used and many assumptions of the model are applied, which cannot provide much information to a comprehensive understanding of the
https://doi.org/10.1016/j.apt.2019.10.024 0921-8831/Ó 2019 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.
Please cite this article as: F. Wu, K. Yue, W. Gao et al., Numerical simulation of semi-dry flue gas desulfurization process in the powder-particle spouted bed, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.10.024
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F. Wu et al. / Advanced Powder Technology xxx (xxxx) xxx
Nomenclature C ds dm s g0 H dm s o Re t T sh U x, y o
concentration [kg/m3] particle diameter [mm] Spalding mass number [—] radial distribution coefficient [—] vessel height [mm] gas mass product from reactions [kg/m2 s] water evaporation rate per unit area [kg/m2 s] Reynolds dimensionless number [—] time [s] thermodynamic temperature [K] Sherwood dimensionless number [—] superficial gas velocity [m/s] Cartesian coordinates [m] gas-liquid interface vapor mass fraction [—]
desulfurization process in powder-particle spouted bed. Haghnegahdar et al. [15] experimentally studied the removal of carbon dioxide in an powder-particle spouted bed reactor, and a laboratory scale PPSB is employed to investigate the effects of operating parameters such as approach to saturation temperature, static bed height, Ca/C molar ratio, inlet CO2 concentration and type of sorbent on CO2 removal efficiency. Recently, Fakhari et al. [16] experimentally investigated simultaneous absorption of CO2 and SO2 by NaOH solution in a powder particle spouted bed (PPSB) reactor. The Taguchi method was used for design of the experiments on the effect of the operating parameters of molar ratio of sorbent to CO2, inlet gas temperature and flow rate, and CO2 and SO2 concentrations in the gas stream on removal of CO2. The above-mentioned review of literature indicates that most of the semidry FGD processes with a powder-particle spouted bed were studied by experimental methods, and very few theoretical analyses have been made on it, especially using the CFD simulation method on the detailed analysis of desulfurization process in powder-particle spouted bed. In the present study, computational fluid dynamics (CFD) combined with two-fluid model (TFM) was used for simulation of water vaporization and semi-dry flue gas desulfurization process in PPSB, on the basis of the gas-solid two phase flow, the mathematical and physical models of water vaporization process and flue gas desulfurization reaction process have been established through reasonable hypothesis and simplification of the system. The numerical method was used to simulate and optimize the desulfurization reaction process and the heat and mass transfer in the powder-particle spouted bed.
Greek symbols bgs fluid-particle friction coefficient [kg/(m3 s)] e volume fraction [—] h granular temperature [m2/s2] q density [kg/m3] l shear viscosity [kg/(m s)] n water content of slurry [kg-H2O/kg] subscripts g gas [—] q phase type (solid or gas) [—] s solids [—]
@ eq qq þ r eq qq ! v q ¼ d ms @t where
!
vq
is the velocity of phase q and eq is the phase volume frac-
tion, d m stands for the gas mass product from reactions. s
The momentum balance of gas and solid phase is given as Gas phase:
@ eg qg mg þ r eg qg mg mg ¼ eg rPg þ eg qg g @t þ bgs ms mg þ r sg
In the PPSB process, the sorbent slurry is fed continuously to a spouted bed in which coarse particles are being spouted with hot gas containing SO2. The reaction between SO2 gas and the sorbent takes place simultaneously with the drying of slurry in the bed. Finally. The unspent sorbent particle and the reaction products are entrained with clean gas in the form of dry powder and collected by a bag filter. The governing equations and the associated constitutive models of the two-fluid model (TFM) that are used in the simulation of PPSB are summarized in this section. The mass conservation equation for gas phase (q = g) and solid phase (q = s) is
ð2Þ
Solid phase:
@ ðes qs ms Þ þ r ðes qs ms ms Þ ¼ es rP rPs þ r ss þ es qs g @t þ bgs mg ms
ð3Þ
where es ¼ 1 - eq . The transport equation for granular temperature, h(kinetic fluctuation energy of particles) is given as
3 @ ðes qs hÞ þ r ðes qs hÞv s I ¼ rps I þ ss 2 @t
: rv s þ r ðks rhÞ cs þ /s þ Dgs ð4Þ
The constitutive models used are as follows: Gas and solid phase stress tensors:
2 3
sg ¼ eg lg rv g þ rv g T rv g I
2. Numerical method 2.1. Hydrodynamic model
ð1Þ
2 3
ð5Þ
ss ¼ es ls rv s þ rv s T þ es ks ls rv s I
ð6Þ
Radial distribution function:
h i1 g 0 ¼ 1 ðes =es;max Þ1=3
ð7Þ
Collisional energy dissipation:
cs ¼
12 1 e2s g 0 pffiffiffiffi qs e2s h3=2 s ds p
ð8Þ
Solid pressure [17]
ps ¼ es qs h þ 2qs ð1 þ es Þe2s g 0 h
ð10Þ
Please cite this article as: F. Wu, K. Yue, W. Gao et al., Numerical simulation of semi-dry flue gas desulfurization process in the powder-particle spouted bed, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.10.024
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F. Wu et al. / Advanced Powder Technology xxx (xxxx) xxx Table 1 The size and parameters of the spouted bed.
Solid shear viscosity [18]
rffiffiffiffi h
4 5
ls;k ¼ es 2 qs dg 0 ð1 þ eÞ
p
pffiffiffiffiffiffi 2 10qs d ph 4 þ 1 þ es g 0 ð1 þ eÞ 96ð1 þ eÞes g 0 5
ð11Þ
Frictional viscosity as given by Schaeffer et al. [19] is
P sin/ 2 I2D
ls;fr ¼ spffiffiffiffiffiffi
ð12Þ
The diffusivity of granular temperature is given by [18]
khs ¼
pffiffiffiffiffiffiffiffi 2 150ds qs hs p 6 1 þ es g 0 ð1 þ es Þ 5 384ð1 þ es Þg 0 rffiffiffiffi hs 2 þ 2qs ds es g 0 ð1 þ es Þ
p
ð13Þ
Solid bulk viscosity is given as [17]
4 ns ¼ es qs dg 0 ð1 þ eÞðh=pÞ2 3
ð14Þ
The drag model as given by Gidaspow [20] is expressed as
8 > <
bgs ¼
as ag qg jug us j 3 C ds 4 D
a2:65 ; ag P 0:8 g
2 > : 150 as l2g þ 1:75 as qg jug us j ; ag < 0:8 ds
ð15Þ
ag ds
where
( CD ¼
24
ag Res
0:687 1 þ 0:15 ag Res ; 0:44;
ðRes < 1000Þ ðRes P 1000Þ
Computer run
Description
Computer run
Diameter of the bed
53.5 mm
0.56 m/s
Inclined angle
60°
Bed height Diameter of the spout gas inlet Static bed depth Particle recovery coefficient
400 mm 14.3 mm
Minimum spouting velocity Maximum solid volume fraction Particle density Particle diameter
107 mm 0.9
Operating pressure Internal friction angle of particles
0.55 2700 kg/m3 460 lm 101325 Pa 28.7°
The energy conservation equation is given as
@ @P aq qq hq þ r aq qq v q hq ¼ - aq q þ s @t @t n X ! ! : rv q - r q q þ F pq þ mpq v pq mqp v pq þ Sq
ð18Þ
p¼1
In the formula, h is the enthalpy (J/kg) of q component. The experimental data of Ma et al. [7–9] for cylindrical spouted bed with conical base are used to validate the numerical model of water vaporization and semi-dry flue gas desulfurization process in a PPSB. The bed and grid structures for the computational domain are illustrated in Fig. 1, and the corresponding parameters selected for the present simulation are listed in Table 1. 2.2. Water evaporation model
ð16Þ
qg ds ug us Res ¼ lg
Description
ð17Þ
The desulfurization slurry is coated on moving particle surfaces and vaporizes in the high temperature gas environment. The water evaporation rate per unit area of desulfurization slurry is expressed as [21]
Fig. 1. The structure size, boundary condition and grids of the spouted bed (unit: mm).
Please cite this article as: F. Wu, K. Yue, W. Gao et al., Numerical simulation of semi-dry flue gas desulfurization process in the powder-particle spouted bed, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.10.024
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ma ¼
Dg Shqtotal BM 2Rd
ð19Þ
where Dg is the gas diffusion rate, Rd is the particle radius, qtotal is the total density of the gas phase for gas-liquid interface (kg/m3). Sh is the Sherwood dimensionless number, which is mathematically expressed as [22]
1 1 Sh ¼ 2:0 þ 0:6Re2 Sc3 ð1 þ BM Þ0:7
Table 3 Boundary conditions for numerical simulation. Initial and boundary conditions
Parameter
Inlet of gas
The turbulent velocity distribution, spouting inlet gas velocity U, (m s1), turbulence kinetic intensity is 2% No particles enter for solid phase, inlet gas temperature T = 520 K, the mass fraction of each component of the inlet gas is: H2O = 0, SO2 = 0.00118, O2 = 0.23264 The velocity is 0.0260 m/s, the direction of velocity is vertical boundary surface and T = 300 K
ð20Þ
where BM is the difference between mass fraction in the gas-liquid interface saturation vapor and mass fraction of water vapor in the gas phase subject value, which can be expressed as
Inlet of desulphurization slurry Outlet
Y sat ð1 uÞ BM ¼ 1 Y sat
Wall
ð21Þ
2.3. Desulfurization model
Symmetrical axis
The reaction of desulfurization process can be divided into the following steps: (a) The SO2 in high temperature flue gas diffuses from the gas phase to the gas-liquid interface. (b) The SO2 diffuses from the gas-liquid interface into the liquid phase, and begins to dissolve in the liquid membrane surface. (c) The SO2 dissolves in the water and generates H2SO3, and then dissociate. (d) The S ion in the liquid film diffuses to liquid phase center. (e) The dissolved ionized process of desulfurizer Ca(OH)2. Above all, the overall reaction equation is expressed as.
SO2 ð g Þ þ CaðOHÞ2 ðsÞ ! CaSO3 1=2H2 OðsÞ þ 1=2H2 OðlÞ
ð22Þ
The definition of desulfurization efficiency is
gSO2
Uniform velocity distribution for fluid phase No particle exits for solid phase No slip for fluid phase, standard Wall Functions for near-wall treatment and adiabatic boundary condition for energy transport equation, zero shear stress for solid phase Axisymmetric boundary
in the calculation domain, and the total number of grid cells is 13475. The component materials and physical properties of inlet gas are listed in Table 2 and the boundary conditions for numerical simulation are listed in Table 3. A comparative investigation on the velocity of particle phase between numerical simulation results (by the standard k-x turbulence model and the standard k-e turbulence model) and experimental results has been made, as shown in Wu et al. [23]. The maximum deviations of particle velocity and voidage of gas phase at different heights in spouted bed are less than 26% and 6.9% between numerical results and experimental data, respectively. Considering the error produced by the measurement, we think that the deviation is acceptable, which can be attributed to the simulation of the desulfurization process in spouted bed. 3. Results and discussion
C in;SO2 C out;SO2 ¼ C in;SO2
ð23Þ
2.4. Simulation procedure The set of governing equations presented in Section 2.1 is solved by CFD code (Fluent 15). The coupled calculation of water vaporization and desulfurization process (Eqs. (19)–(23)) is realized by user-defined function (UDF) and is updated continuously in iteration. The phase-coupled PC-SIMPLE algorithm is used for the pressure-velocity coupling. A second-order upwind discretization scheme is used for momentum, turbulence kinetic energy and turbulence dissipation rate equations. Transient simulations are performed with a constant time step of 0.00001 s with 30 iterations per time step. The convergence criterions for the solution are that all variable residuals such as velocity are less than 1 103. The geometry and grid description of the two dimensional spouted beds are shown in Fig. 1. Structured grid is used
3.1. Flow of gas and particle and water vaporization in spouted bed Fig. 2 shows the instantaneous solid volume fraction in a spouted bed with U = 0.7 m/s. It can be seen that when t = 5 s, a stable particle internal spouting and circulation can be established with the three flow regions, that is, the spout, annulus and fountain being clearly visualized. The contours of water vaporization rate in spouted bed are shown in Fig. 3. It can be seen from Fig. 3(a) that values of water vaporization rate are high in spout and outside of annulus which are the main regions of the water evaporation, which reveals that, high gas temperature in spout region is beneficial for the evaporation of water. Fig. 3(b) reveals that the value of water vaporization rate decreases with the increase of crosssectional height in spouted beds, and on the other hand, Particle movement is slow in the outside area of annulus (Fig. 3(a) and (b)), as a result, slurry retention time in spouted bed is long, which is also beneficial for the evaporation of water. On the contrary, the value of water vaporization rate is low in the interior area of the
Table 2 Component materials and physical properties of inlet gas. Chemical formula of components
H2O
SO2
O2
N2
Density (kg/m3) Heat capacity at constant pressure (J/kg K) Coefficient of thermal conductivity (W/m K) Dynamic viscosity (kg/m s) Molar mass (kg/kmol) Standard enthalpy of formation (J/kmol) Reference temperature (K)
0.5542 2014 0.0261 1.34 105 18.01534 2.418379 108 298
2.77 622.28 0.0104 1.2 105 64.0648 2.968612 108 298
1.2999 919.31 0.0246 1.919 105 31.9988 0 298
1.138 1040.67 0.0242 1.663 105 28.0134 0 298
Please cite this article as: F. Wu, K. Yue, W. Gao et al., Numerical simulation of semi-dry flue gas desulfurization process in the powder-particle spouted bed, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.10.024
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Fig. 2. Instantaneous solids volume fraction in a spouted bed with U = 0.7 m/s.
Fig. 3. The distribution of water vaporization rate in spouted bed (unit: kg/m3 s).
Fig. 4. Distribution of desulfurization results in the spouted bed.
Please cite this article as: F. Wu, K. Yue, W. Gao et al., Numerical simulation of semi-dry flue gas desulfurization process in the powder-particle spouted bed, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.10.024
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Fig. 5. Radial distribution of desulfurization results at different height of the spouted bed.
Fig. 6. Comparison of desulfurization efficiency between simulation and experimental results.
annulus and fountain regions due to the lack of transverse mixing of particle movement, which is disadvantageous for the dispersion of slurry and leads to the reduction of contact area between gas and liquid. Fig. 4(a) displays the contours of volume fraction of SO2 in spouted bed. It can be seen that the maximum value of volume fraction of SO2 appears in the gas inlet of spouted bed and decreases with the increase of height of spouted bed, and the
minimum value of volume fraction of SO2 appears in the annulus, which is in accordance with the law in Fig. 3 and indicates that the annulus is the main region of the desulfurization reaction in the spouted bed. Fig. 4(b) presents the contours of desulfurization (CaSO3) products rate in the spouted bed. It can observed that the maximum value of desulfurization product rate, up to 0.300 (kg m3 s1), appears in the annulus, which is the major region of flue gas desulfurization reaction due to large particle concentration, large gas-solid contact area and large mass transfer area in the annulus. The volume fraction distribution of desulfurization products (CaSO3) is shown in Fig. 4(c). It can be seen that the most abundant desulfurization products are in the annular zone and at the spouted bed outlet. There are more desulfurization products in this area because of the high product rate of desulfurization process in annular zone. The desulfurization reaction takes place on the surface of particles, and the desulfurized products are taken out of the bed by high-speed gas after severe collision and friction between particles. Fig. 5 shows the radial distribution of desulfurization results at different heights of the spouted bed. It is clearly observed from Fig. 5(a) that the volume fraction of SO2 is the largest in the spout zone, with the increase of radial distance, the volume fraction of SO2 decreases gradually, and close to zero near the bed wall (in annulus) due to the high gas velocity in the injection zone, the short contact time between slurry and gas and the insufficient time for desulfurization reaction. On the contrary, the accumulation of desulfurization slurry in annulus makes more time for contact between gas and slurry, the desulfurization reaction is more adequate, and large amount of SO2 is removed. As a results, the value
Please cite this article as: F. Wu, K. Yue, W. Gao et al., Numerical simulation of semi-dry flue gas desulfurization process in the powder-particle spouted bed, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.10.024
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Fig. 7. Distribution of water vaporization rate (kg m3 s1) in the spouted bed under different water contents (kg-H2O/kg-dry_sorbent).
Fig. 8. Distribution of desulfurization product (CaSO3) rate (kg m3 s1) in the spouted bed at different water contents (kg-H2O/kg-dry_sorbent).
of volume fraction of SO2 in annuls becomes very small, which equals almost zero. Fig. 5(b) reveals that the desulfurization reaction in spouted bed mainly occurs in the low bed area (cylindrical cone area), and the desulfurization reaction rate decreases with the increase of bed height. Fig. 5(c) indicates that the volume fraction of CaSO3 increases first and then decreases with the increase of radial distance, which also increases with the increase of crosssectional height in spouted beds. Fig. 6 compares the desulfurization efficiency between simulation and experimental results [7–9]. In general, the agreement with experimental data is quite encouraging, and the simulation deviation is 0.96%, which reveals that the numerical model of desulfurization in spouted bed is reasonable. 3.2. Effect of water content of slurry on desulfurization efficiency Fig. 7 compares the distribution of water vaporization rate (kg m3 s1) in the spouted bed under different water contents, and the physical meaning of slurry water content is the quantity of water contained in one kilogram of dry base absorbent. It is seen that the rate of water vaporization decreases gradually with the increase of water content, which indicates that as the water content increases, the water content in spouted bed increases, while the gas temperature remains unchanged, which leads to the decrease of water temperature and the decrease of water vaporization rate. Fig. 8 shows the distribution of desulfurization product
(CaSO3) rate (kg m3 s1) in the spouted bed at different water contents. It is seen that the distribution trend of desulfurization product formation rate in spouted bed is consistent as a whole at the condition of different water contents, and the desulfurization product formation rate is the largest in annular zone. When the water content changed from 30 kg-H2O/kg-dry_sorbent to 70 kgH2O/kg-dry_sorbent, the rate of product formation first increased to 0.461 kg m3 s1 and then gradually decreased. Particles enter the nozzle through the spout, fountain region and annulus region, and then form a very regular internal circulation of particle movement because of the special flow structure of spouted bed. Particles in spout and fountain region obtain more kinetic energy because of high velocity of gas, and the residence time of particles in the bed becomes shorter, as a result, the reaction time between flue gas and slurry adhering to the surface of particles becomes shorter. On the other hand, according to the distribution of particle volume fraction in Fig. 2, when the spouted bed reaches stable spouting state, a large number of particles accumulate in the annulus region, which increases the contact area between particles and gases, prolongs the contact time and maximizes the product rate of desulfurization products in the annulus region. Fig. 9 shows the radial distribution of the rate of desulfurization reaction product (CaSO3) in the spouted bed at different water contents. It is interesting to note that when the bed height z equals 0.03 m and 0.06 m, the rate of desulfurization reaction product formation increases gradually along the radial direction in spout
Please cite this article as: F. Wu, K. Yue, W. Gao et al., Numerical simulation of semi-dry flue gas desulfurization process in the powder-particle spouted bed, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.10.024
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Fig. 9. Radial distribution of the rate of desulfurization reaction product (CaSO3) in the spouted bed at different water contents.
Fig. 10. Comparison of desulfurization rates under different value of water contents of slurry.
region, and a larger value appears at the interface between the spout and the annulus. With the increase of water content, the rate of product formation first increases and then decreases. The comprehensive analysis reveals that the desulfurization reaction rate in spouted bed first increases and then decreases with the water content, and the desulfurization effect is the best when value of water content equals 40 kg-H2O/kg-dry_sorbent.
Fig. 10 compares the desulfurization efficiency in spouted bed under different values of water contents of slurry. It can be seen that, under the same simulated conditions, the desulfurization rate in spouted bed increases first and then decreases with the increase of water content, which reveals that when the slurry water content increases from 30 kg-H2O/kg-dry_sorbent to 40 kg-H2O/kgdry_sorbent, the reaction rate of desulfurization in spouted bed increases gradually with the increase of the slurry water content, which is due to that the increase of water content causes an increase in the amount of calcium hydroxide dissolved, which in turn increases the amount of calcium ions involved in the ion reaction, as shown in Fig. 11, that is, the increase of the maximum and overall values of volume fraction of absorbent (Ca(OH)2.When the value of slurry water content continues to increase, the concentration of desulfurizer and water phase will be significantly reduced because the total calcium ions in the solution are reduced (Fig. 11). Besides, the lower the temperature, the lower the rate of water vaporization in spouted bed, and the lower the rate of desulfurization reaction. When the water content is 40 kg-H2O/ kg-dry_sorbent, the desulfurization rate reaches the maximum of 75.75%, while the experimental value is 75.03%, which is the closest to the experimental value. 4. Conclusions (1) The values of water vaporization rate are high in spout and outside of annulus which are the main regions of the water evaporation, and high gas temperature in spout region is beneficial for the evaporation of water. The value of water
Please cite this article as: F. Wu, K. Yue, W. Gao et al., Numerical simulation of semi-dry flue gas desulfurization process in the powder-particle spouted bed, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.10.024
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Fig. 11. Volume fraction of absorbent (Ca(OH)2) at different water contents.
vaporization rate is low in the interior area of the annulus and fountain regions due to the lack of transverse mixing of particle movement. (2) The maximum value of desulfurization product rate appears in the annulus, which is the major region of flue gas desulfurization reaction due to large particle concentration, large gas-solid contact area and large mass transfer area in the annulus. The minimum value of volume fraction of SO2 appears in the annulus, which indicates that the annulus is the main area of the desulfurization reaction in spouted bed. (3) The desulfurization efficiency of spouted bed increases first and then decreases with the increase of water content of desulfurization slurry. The desulfurization efficiency is the highest when the water content is 40 kg-H2O/kgdry_sorbent and is close to the experimental value. The optimum slurry water content for desulfurization process in powder-particle spouted bed was obtained by numerical simulation, which was 40 kg-H2O/kg-dry_sorbent.
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. Acknowledgement This work is supported by National Natural Science Foundation of China (Grant No. 21878245), Natural Science Foundation of Shaanxi Province (Grant No. 2019JM-039) and Cyrus Tang Foundation. References [1] F. Garcıa-Labiano, A. Rufas, F. Luis, M. de las Obras-Loscertales, P. Gayan, A. Abad, J. Adanez, Calcium-based sorbents behaviour during sulphation at oxyfuel fluidised bed combustion conditions, Fuel 90 (2011) 3100–3108. [2] L. Jia, Y. Tan, C. Wang, E.J. Anthony, Experimental study of oxy-fuel combustion and sulfur capture in a mini-CFBC, Energy Fuels 21 (6) (2007) 3160–3164. [3] D. Shun, D.H. Bae, I.K. Jang, K.H. Park, S.K. Park, Kinetic study of sulfur dioxide elimination by limestone through the lab scale circulating fluidized bed combustor, Adv. Mater. Phys. Chem. 2 (4) (2012) 189–192. [4] T. Wall, Y. Liu, C. Spero, L. Elliott, S. Khare, R. Rathman, An overview on oxyfuel coal combustion–state of the art research and technology development, Chem. Eng. Res. Des. 87 (2009) 1003–1016.
[5] M. Lupion, R. Diego, L. Loubeau, B. Navarrete, CIUDEN CCS project: dtatus of the CO2 capture technology development plant in power product, Energy Procedia 4 (2011) 5639–5646. [6] Q. Guo, N. Iwata, K. Kato, Process development of effective semidry flue gas desulfurization by a powder-particle spouted bed, Kagaku Kogaku Ronbunshu 22 (6) (1996) 1400–1407. [7] X. Ma, T. Kaneko, Q. Guo, G. Xu, Removal of SO2 from flue gas using a new semidry flue gas desulfurization process with a powder-particle spouted bed, Can. J. Chem. Eng. 77 (2) (1999) 356–362. [8] X. Ma, T. Kaneko, T. Tashimo, T. Yoshida, K. Kato, Use of limestone for SO2 removal from flue gas in the semidry FGD process with a powder-particle spouted bed, Chem. Eng. Sci. 55 (20) (2000) 4643–4652. [9] X. Ma, T. Kaneko, G. Xu, K. Kato, Influence of gas components on removal of SO2 from flue gas in the semidry FGD process with a powder-particle spouted bed, Fuel 80 (5) (2001) 673–680. [10] G. Xu, Q. Guo, T. Kaneko, K. Kato, A new semidry desulfurization process using a powder-particle spouted bed, Adv. Environ. Res. 4 (2000) 9–18. [11] S. Wu, A. Uddin, C. Su, S. Nagamine, E. Sasaoka, Effect of the pore-size distribution of lime on the reactivity for the removal of SO2 in the presence of high-concentration CO2 at high temperature, Ind. Eng. Chem. Res. 41 (2002) 5455–5458. [12] N. Wang, X. Zhang, Effect of humidification water on semi-dry flue gas desulfurization, Procedia Environ. Sci. 11 (Part B) (2011) 1023–1028. [13] A. Rahimi, M.S. Hatamipour, M. Gholami, M.R. Haghnegahdar, Non-isothermal modeling of the flue gas desulphurization process using a semi-dry spouted bed reactor, Chem. Eng. Res. Des. 89 (2011) 777–784. [14] M.A. Fakhari, A. Rahimi, M.S. Hatamipour, A. Fozooni, Non-isothermal modeling of simultaneous CO2 and SO2 removal in a semi-dry spouted bed reactor, Process Saf. Environ. Prot. 98 (2015) 342–353. [15] M.R. Haghnegahdar, M.S. Hatamipour, A. Rahimi, Removal of carbon dioxide in an experimental powder-particle spouted bed reactor, Sep. Purif. Technol. 72 (2010) 288–293. [16] M.A. Fakhari, A. Rahimi, M.S. Hatamipour, A. Fozooni, Experimental study of simultaneous removal of CO2 and SO2 in a spouted bed reactor, Canad. J. Chem. Eng. 95 (2017) 1150–1155. [17] C.K. Lun, S.B. Savage, D.J. Ieffrey, N. Chepurniy, Kinetic theories for granular flow: inelastic particles in Couette flow and slightly inelastic particles in general flow field, J. Fluid Mech. 140 (1984) 223–256. [18] D. Gidaspow, R. Bezburuah, J. Ding, Hydrodynamics of circulating fluidized beds. Kinetic Theory Approach, Fluidization VII, in: Proceedings of the Seventh Engineering Foundation Conference on Fluidization, 1992, pp. 75–82. [19] D.G. Schaeffer, Instability in the evolution equations describing incompressible granular flow, J. Different. Eq. 66 (1987) 19–50. [20] D. Gidaspow, R. Bezburuah, J. Ding, Hydrodynamics of circulating fluidized beds. Kinetic Theory Approach. Fluidization VII, in: Proceedings of the Seventh Engineering Foundation Conference on Fluidization, 1992, pp. 75–82. [21] R.J. Haywood, R. Nafziger, M.A. Renksizbulut, Detailed examination of gas and liquid transient processes in convection and evaporation, ASME J. Heat Transf. 111 (1989) 495–502. [22] M. Mezhericher, A. Levy, I. Borde, Heat and mass transfer of single droplet/wet particle drying, Chem. Eng. Sci. 63 (2008) 12–23. [23] F. Wu, X. Zhang, W. Zhou, X. Ma, Numerical simulation and optimization of hydrodynamics in a novel integral multi-jet spout-fluidized bed, Powder Technol. 336 (2018) 112–121.
Please cite this article as: F. Wu, K. Yue, W. Gao et al., Numerical simulation of semi-dry flue gas desulfurization process in the powder-particle spouted bed, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.10.024
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Advanced Powder Technology journal homepage: www.elsevier.com/locate/apt
Original Research Paper
Numerical simulation of semi-dry flue gas desulfurization process in the powder-particle spouted bed Feng Wu a,⇑, Kai Yue a, Weiwei Gao a, Ming Gong a, Xiaoxun Ma a,⇑, Wenjing Zhou b a b
School of Chemical Engineering, Northwest University, Xi’ an 710069, China School of Chemical Engineering and Technology, Xi’an Jiaotong University, Xi’an 710049, China
a r t i c l e
i n f o
Article history: Received 9 January 2019 Received in revised form 17 October 2019 Accepted 24 October 2019 Available online xxxx Keywords: Powder-particle spouted bed Semi-dry flue gas desulfurization Numerical simulation Optimization analysis
a b s t r a c t Computational fluid dynamics (CFD) combined with the two-fluid model (TFM) was used for simulation of water vaporization and semi-dry flue gas desulfurization process in a two dimensional powder-particle spouted bed (PPSB), on the basis of gas-solid two-phase flow, the mathematical and physical models of water vaporization process and flue gas desulfurization reaction process have been established through reasonable hypothesis and simplification of the system. The numerical method was used to simulate the desulfurization reaction process and the heat and mass transfer in the powder-particle spouted bed. Simulation results indicate that water vaporization rate was high in spout and annular regions. The main area where flue gas desulfurization reaction occurs was annular area, as a result, the maximum value of desulfurization product rate appears in the annulus. Under the same condition, the desulfurization efficiency of simulation value is 75.75% when the value of slurry water content equals 40 kg-H2O/kgdry_sorbent, which is close to but greater than the experimental value (75.03%). The desulfurization efficiency of spouted bed increases first and then decreases with the increase of water content of desulfurization slurry, and the optimum slurry water content for desulfurization process in powder-particle spouted bed was obtained by numerical simulation, which was 40 kg-H2O/kg-dry_sorbent. Ó 2019 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.
1. Introduction The increasing use of fossil fuels to meet energy needs has led to higher SO2 and CO2 emissions into the atmosphere, which is dangerous to the environment and human health [1–5], and the emission of SO2 and CO2 from power plants that burn fossil fuels form acid rain in the atmosphere, and then result in serious environmental problems [6–16]. Removal of SO2 and CO2 from gas has been a worldwide concern. Various technologies for flue gas desulfurization (FGD) can be classified into three different types: wet scrubber, semi-dry processes and dry processes [11]. Semidry FGD processes have been developed and adopted commercially since the 1980s. The spray dry scrubber process, as a typical semidry FGD process, is effective in removing SO2 from flue gas and has been developed as an alternative to wet scrubbers [6–10]. For flue gas cleaning, many different processes have been developed for SO2 and CO2 removal. Ma et al. [7–9] experimentally investigated the use of limestone for SO2 removal from flue gas in the semidry FGD process with a powder-particle spouted bed ⇑ Corresponding authors. E-mail addresses: [email protected] (F. Wu), [email protected] (X. Ma).
(PPSB), they also studied the influence of gas components on removal of SO2 from flue gas in the semidry FGD process with a PPSB, and results confirm that PPSB has the advantage of higher SO2 capture efficiency over the conventional existing processes. However, they have not built mathematical model of the desulfurization process and analyzed it in a powder–particle spouted bed. Xu et al. [10] experimentally tested a new semi-dry desulfurization process, and the process uses the so-called powder-particle spouted bed PPSB as the reactor in which coarse medium particles, usually silica sand of several hundred micrometers in size, are fluidized with hot flue gas. Wu et al. [11] studied the effect of the pore-size distribution of lime on the reactivity for the removal of SO2 in the presence of high-concentration CO2 at high temperature. Wang [12] experimentally investigated effects of humidification water parameters (ratio of humidification water to total water, location of humidification water) on semi-dry flue gas desulfurization removal efficiency. Rahimi et al. [13,14] developed a mathematical model for investigation of SO2 and CO2 removal in a powder-particle spouted bed for non-isothermal operating condition. The one-dimensional stream-tube model is used and many assumptions of the model are applied, which cannot provide much information to a comprehensive understanding of the
https://doi.org/10.1016/j.apt.2019.10.024 0921-8831/Ó 2019 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.
Please cite this article as: F. Wu, K. Yue, W. Gao et al., Numerical simulation of semi-dry flue gas desulfurization process in the powder-particle spouted bed, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.10.024
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F. Wu et al. / Advanced Powder Technology xxx (xxxx) xxx
Nomenclature C ds dm s g0 H dm s o Re t T sh U x, y o
concentration [kg/m3] particle diameter [mm] Spalding mass number [—] radial distribution coefficient [—] vessel height [mm] gas mass product from reactions [kg/m2 s] water evaporation rate per unit area [kg/m2 s] Reynolds dimensionless number [—] time [s] thermodynamic temperature [K] Sherwood dimensionless number [—] superficial gas velocity [m/s] Cartesian coordinates [m] gas-liquid interface vapor mass fraction [—]
desulfurization process in powder-particle spouted bed. Haghnegahdar et al. [15] experimentally studied the removal of carbon dioxide in an powder-particle spouted bed reactor, and a laboratory scale PPSB is employed to investigate the effects of operating parameters such as approach to saturation temperature, static bed height, Ca/C molar ratio, inlet CO2 concentration and type of sorbent on CO2 removal efficiency. Recently, Fakhari et al. [16] experimentally investigated simultaneous absorption of CO2 and SO2 by NaOH solution in a powder particle spouted bed (PPSB) reactor. The Taguchi method was used for design of the experiments on the effect of the operating parameters of molar ratio of sorbent to CO2, inlet gas temperature and flow rate, and CO2 and SO2 concentrations in the gas stream on removal of CO2. The above-mentioned review of literature indicates that most of the semidry FGD processes with a powder-particle spouted bed were studied by experimental methods, and very few theoretical analyses have been made on it, especially using the CFD simulation method on the detailed analysis of desulfurization process in powder-particle spouted bed. In the present study, computational fluid dynamics (CFD) combined with two-fluid model (TFM) was used for simulation of water vaporization and semi-dry flue gas desulfurization process in PPSB, on the basis of the gas-solid two phase flow, the mathematical and physical models of water vaporization process and flue gas desulfurization reaction process have been established through reasonable hypothesis and simplification of the system. The numerical method was used to simulate and optimize the desulfurization reaction process and the heat and mass transfer in the powder-particle spouted bed.
Greek symbols bgs fluid-particle friction coefficient [kg/(m3 s)] e volume fraction [—] h granular temperature [m2/s2] q density [kg/m3] l shear viscosity [kg/(m s)] n water content of slurry [kg-H2O/kg] subscripts g gas [—] q phase type (solid or gas) [—] s solids [—]
@ eq qq þ r eq qq ! v q ¼ d ms @t where
!
vq
is the velocity of phase q and eq is the phase volume frac-
tion, d m stands for the gas mass product from reactions. s
The momentum balance of gas and solid phase is given as Gas phase:
@ eg qg mg þ r eg qg mg mg ¼ eg rPg þ eg qg g @t þ bgs ms mg þ r sg
In the PPSB process, the sorbent slurry is fed continuously to a spouted bed in which coarse particles are being spouted with hot gas containing SO2. The reaction between SO2 gas and the sorbent takes place simultaneously with the drying of slurry in the bed. Finally. The unspent sorbent particle and the reaction products are entrained with clean gas in the form of dry powder and collected by a bag filter. The governing equations and the associated constitutive models of the two-fluid model (TFM) that are used in the simulation of PPSB are summarized in this section. The mass conservation equation for gas phase (q = g) and solid phase (q = s) is
ð2Þ
Solid phase:
@ ðes qs ms Þ þ r ðes qs ms ms Þ ¼ es rP rPs þ r ss þ es qs g @t þ bgs mg ms
ð3Þ
where es ¼ 1 - eq . The transport equation for granular temperature, h(kinetic fluctuation energy of particles) is given as
3 @ ðes qs hÞ þ r ðes qs hÞv s I ¼ rps I þ ss 2 @t
: rv s þ r ðks rhÞ cs þ /s þ Dgs ð4Þ
The constitutive models used are as follows: Gas and solid phase stress tensors:
2 3
sg ¼ eg lg rv g þ rv g T rv g I
2. Numerical method 2.1. Hydrodynamic model
ð1Þ
2 3
ð5Þ
ss ¼ es ls rv s þ rv s T þ es ks ls rv s I
ð6Þ
Radial distribution function:
h i1 g 0 ¼ 1 ðes =es;max Þ1=3
ð7Þ
Collisional energy dissipation:
cs ¼
12 1 e2s g 0 pffiffiffiffi qs e2s h3=2 s ds p
ð8Þ
Solid pressure [17]
ps ¼ es qs h þ 2qs ð1 þ es Þe2s g 0 h
ð10Þ
Please cite this article as: F. Wu, K. Yue, W. Gao et al., Numerical simulation of semi-dry flue gas desulfurization process in the powder-particle spouted bed, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.10.024
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F. Wu et al. / Advanced Powder Technology xxx (xxxx) xxx Table 1 The size and parameters of the spouted bed.
Solid shear viscosity [18]
rffiffiffiffi h
4 5
ls;k ¼ es 2 qs dg 0 ð1 þ eÞ
p
pffiffiffiffiffiffi 2 10qs d ph 4 þ 1 þ es g 0 ð1 þ eÞ 96ð1 þ eÞes g 0 5
ð11Þ
Frictional viscosity as given by Schaeffer et al. [19] is
P sin/ 2 I2D
ls;fr ¼ spffiffiffiffiffiffi
ð12Þ
The diffusivity of granular temperature is given by [18]
khs ¼
pffiffiffiffiffiffiffiffi 2 150ds qs hs p 6 1 þ es g 0 ð1 þ es Þ 5 384ð1 þ es Þg 0 rffiffiffiffi hs 2 þ 2qs ds es g 0 ð1 þ es Þ
p
ð13Þ
Solid bulk viscosity is given as [17]
4 ns ¼ es qs dg 0 ð1 þ eÞðh=pÞ2 3
ð14Þ
The drag model as given by Gidaspow [20] is expressed as
8 > <
bgs ¼
as ag qg jug us j 3 C ds 4 D
a2:65 ; ag P 0:8 g
2 > : 150 as l2g þ 1:75 as qg jug us j ; ag < 0:8 ds
ð15Þ
ag ds
where
( CD ¼
24
ag Res
0:687 1 þ 0:15 ag Res ; 0:44;
ðRes < 1000Þ ðRes P 1000Þ
Computer run
Description
Computer run
Diameter of the bed
53.5 mm
0.56 m/s
Inclined angle
60°
Bed height Diameter of the spout gas inlet Static bed depth Particle recovery coefficient
400 mm 14.3 mm
Minimum spouting velocity Maximum solid volume fraction Particle density Particle diameter
107 mm 0.9
Operating pressure Internal friction angle of particles
0.55 2700 kg/m3 460 lm 101325 Pa 28.7°
The energy conservation equation is given as
@ @P aq qq hq þ r aq qq v q hq ¼ - aq q þ s @t @t n X ! ! : rv q - r q q þ F pq þ mpq v pq mqp v pq þ Sq
ð18Þ
p¼1
In the formula, h is the enthalpy (J/kg) of q component. The experimental data of Ma et al. [7–9] for cylindrical spouted bed with conical base are used to validate the numerical model of water vaporization and semi-dry flue gas desulfurization process in a PPSB. The bed and grid structures for the computational domain are illustrated in Fig. 1, and the corresponding parameters selected for the present simulation are listed in Table 1. 2.2. Water evaporation model
ð16Þ
qg ds ug us Res ¼ lg
Description
ð17Þ
The desulfurization slurry is coated on moving particle surfaces and vaporizes in the high temperature gas environment. The water evaporation rate per unit area of desulfurization slurry is expressed as [21]
Fig. 1. The structure size, boundary condition and grids of the spouted bed (unit: mm).
Please cite this article as: F. Wu, K. Yue, W. Gao et al., Numerical simulation of semi-dry flue gas desulfurization process in the powder-particle spouted bed, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.10.024
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F. Wu et al. / Advanced Powder Technology xxx (xxxx) xxx
ma ¼
Dg Shqtotal BM 2Rd
ð19Þ
where Dg is the gas diffusion rate, Rd is the particle radius, qtotal is the total density of the gas phase for gas-liquid interface (kg/m3). Sh is the Sherwood dimensionless number, which is mathematically expressed as [22]
1 1 Sh ¼ 2:0 þ 0:6Re2 Sc3 ð1 þ BM Þ0:7
Table 3 Boundary conditions for numerical simulation. Initial and boundary conditions
Parameter
Inlet of gas
The turbulent velocity distribution, spouting inlet gas velocity U, (m s1), turbulence kinetic intensity is 2% No particles enter for solid phase, inlet gas temperature T = 520 K, the mass fraction of each component of the inlet gas is: H2O = 0, SO2 = 0.00118, O2 = 0.23264 The velocity is 0.0260 m/s, the direction of velocity is vertical boundary surface and T = 300 K
ð20Þ
where BM is the difference between mass fraction in the gas-liquid interface saturation vapor and mass fraction of water vapor in the gas phase subject value, which can be expressed as
Inlet of desulphurization slurry Outlet
Y sat ð1 uÞ BM ¼ 1 Y sat
Wall
ð21Þ
2.3. Desulfurization model
Symmetrical axis
The reaction of desulfurization process can be divided into the following steps: (a) The SO2 in high temperature flue gas diffuses from the gas phase to the gas-liquid interface. (b) The SO2 diffuses from the gas-liquid interface into the liquid phase, and begins to dissolve in the liquid membrane surface. (c) The SO2 dissolves in the water and generates H2SO3, and then dissociate. (d) The S ion in the liquid film diffuses to liquid phase center. (e) The dissolved ionized process of desulfurizer Ca(OH)2. Above all, the overall reaction equation is expressed as.
SO2 ð g Þ þ CaðOHÞ2 ðsÞ ! CaSO3 1=2H2 OðsÞ þ 1=2H2 OðlÞ
ð22Þ
The definition of desulfurization efficiency is
gSO2
Uniform velocity distribution for fluid phase No particle exits for solid phase No slip for fluid phase, standard Wall Functions for near-wall treatment and adiabatic boundary condition for energy transport equation, zero shear stress for solid phase Axisymmetric boundary
in the calculation domain, and the total number of grid cells is 13475. The component materials and physical properties of inlet gas are listed in Table 2 and the boundary conditions for numerical simulation are listed in Table 3. A comparative investigation on the velocity of particle phase between numerical simulation results (by the standard k-x turbulence model and the standard k-e turbulence model) and experimental results has been made, as shown in Wu et al. [23]. The maximum deviations of particle velocity and voidage of gas phase at different heights in spouted bed are less than 26% and 6.9% between numerical results and experimental data, respectively. Considering the error produced by the measurement, we think that the deviation is acceptable, which can be attributed to the simulation of the desulfurization process in spouted bed. 3. Results and discussion
C in;SO2 C out;SO2 ¼ C in;SO2
ð23Þ
2.4. Simulation procedure The set of governing equations presented in Section 2.1 is solved by CFD code (Fluent 15). The coupled calculation of water vaporization and desulfurization process (Eqs. (19)–(23)) is realized by user-defined function (UDF) and is updated continuously in iteration. The phase-coupled PC-SIMPLE algorithm is used for the pressure-velocity coupling. A second-order upwind discretization scheme is used for momentum, turbulence kinetic energy and turbulence dissipation rate equations. Transient simulations are performed with a constant time step of 0.00001 s with 30 iterations per time step. The convergence criterions for the solution are that all variable residuals such as velocity are less than 1 103. The geometry and grid description of the two dimensional spouted beds are shown in Fig. 1. Structured grid is used
3.1. Flow of gas and particle and water vaporization in spouted bed Fig. 2 shows the instantaneous solid volume fraction in a spouted bed with U = 0.7 m/s. It can be seen that when t = 5 s, a stable particle internal spouting and circulation can be established with the three flow regions, that is, the spout, annulus and fountain being clearly visualized. The contours of water vaporization rate in spouted bed are shown in Fig. 3. It can be seen from Fig. 3(a) that values of water vaporization rate are high in spout and outside of annulus which are the main regions of the water evaporation, which reveals that, high gas temperature in spout region is beneficial for the evaporation of water. Fig. 3(b) reveals that the value of water vaporization rate decreases with the increase of crosssectional height in spouted beds, and on the other hand, Particle movement is slow in the outside area of annulus (Fig. 3(a) and (b)), as a result, slurry retention time in spouted bed is long, which is also beneficial for the evaporation of water. On the contrary, the value of water vaporization rate is low in the interior area of the
Table 2 Component materials and physical properties of inlet gas. Chemical formula of components
H2O
SO2
O2
N2
Density (kg/m3) Heat capacity at constant pressure (J/kg K) Coefficient of thermal conductivity (W/m K) Dynamic viscosity (kg/m s) Molar mass (kg/kmol) Standard enthalpy of formation (J/kmol) Reference temperature (K)
0.5542 2014 0.0261 1.34 105 18.01534 2.418379 108 298
2.77 622.28 0.0104 1.2 105 64.0648 2.968612 108 298
1.2999 919.31 0.0246 1.919 105 31.9988 0 298
1.138 1040.67 0.0242 1.663 105 28.0134 0 298
Please cite this article as: F. Wu, K. Yue, W. Gao et al., Numerical simulation of semi-dry flue gas desulfurization process in the powder-particle spouted bed, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.10.024
F. Wu et al. / Advanced Powder Technology xxx (xxxx) xxx
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Fig. 2. Instantaneous solids volume fraction in a spouted bed with U = 0.7 m/s.
Fig. 3. The distribution of water vaporization rate in spouted bed (unit: kg/m3 s).
Fig. 4. Distribution of desulfurization results in the spouted bed.
Please cite this article as: F. Wu, K. Yue, W. Gao et al., Numerical simulation of semi-dry flue gas desulfurization process in the powder-particle spouted bed, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.10.024
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F. Wu et al. / Advanced Powder Technology xxx (xxxx) xxx
Fig. 5. Radial distribution of desulfurization results at different height of the spouted bed.
Fig. 6. Comparison of desulfurization efficiency between simulation and experimental results.
annulus and fountain regions due to the lack of transverse mixing of particle movement, which is disadvantageous for the dispersion of slurry and leads to the reduction of contact area between gas and liquid. Fig. 4(a) displays the contours of volume fraction of SO2 in spouted bed. It can be seen that the maximum value of volume fraction of SO2 appears in the gas inlet of spouted bed and decreases with the increase of height of spouted bed, and the
minimum value of volume fraction of SO2 appears in the annulus, which is in accordance with the law in Fig. 3 and indicates that the annulus is the main region of the desulfurization reaction in the spouted bed. Fig. 4(b) presents the contours of desulfurization (CaSO3) products rate in the spouted bed. It can observed that the maximum value of desulfurization product rate, up to 0.300 (kg m3 s1), appears in the annulus, which is the major region of flue gas desulfurization reaction due to large particle concentration, large gas-solid contact area and large mass transfer area in the annulus. The volume fraction distribution of desulfurization products (CaSO3) is shown in Fig. 4(c). It can be seen that the most abundant desulfurization products are in the annular zone and at the spouted bed outlet. There are more desulfurization products in this area because of the high product rate of desulfurization process in annular zone. The desulfurization reaction takes place on the surface of particles, and the desulfurized products are taken out of the bed by high-speed gas after severe collision and friction between particles. Fig. 5 shows the radial distribution of desulfurization results at different heights of the spouted bed. It is clearly observed from Fig. 5(a) that the volume fraction of SO2 is the largest in the spout zone, with the increase of radial distance, the volume fraction of SO2 decreases gradually, and close to zero near the bed wall (in annulus) due to the high gas velocity in the injection zone, the short contact time between slurry and gas and the insufficient time for desulfurization reaction. On the contrary, the accumulation of desulfurization slurry in annulus makes more time for contact between gas and slurry, the desulfurization reaction is more adequate, and large amount of SO2 is removed. As a results, the value
Please cite this article as: F. Wu, K. Yue, W. Gao et al., Numerical simulation of semi-dry flue gas desulfurization process in the powder-particle spouted bed, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.10.024
F. Wu et al. / Advanced Powder Technology xxx (xxxx) xxx
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Fig. 7. Distribution of water vaporization rate (kg m3 s1) in the spouted bed under different water contents (kg-H2O/kg-dry_sorbent).
Fig. 8. Distribution of desulfurization product (CaSO3) rate (kg m3 s1) in the spouted bed at different water contents (kg-H2O/kg-dry_sorbent).
of volume fraction of SO2 in annuls becomes very small, which equals almost zero. Fig. 5(b) reveals that the desulfurization reaction in spouted bed mainly occurs in the low bed area (cylindrical cone area), and the desulfurization reaction rate decreases with the increase of bed height. Fig. 5(c) indicates that the volume fraction of CaSO3 increases first and then decreases with the increase of radial distance, which also increases with the increase of crosssectional height in spouted beds. Fig. 6 compares the desulfurization efficiency between simulation and experimental results [7–9]. In general, the agreement with experimental data is quite encouraging, and the simulation deviation is 0.96%, which reveals that the numerical model of desulfurization in spouted bed is reasonable. 3.2. Effect of water content of slurry on desulfurization efficiency Fig. 7 compares the distribution of water vaporization rate (kg m3 s1) in the spouted bed under different water contents, and the physical meaning of slurry water content is the quantity of water contained in one kilogram of dry base absorbent. It is seen that the rate of water vaporization decreases gradually with the increase of water content, which indicates that as the water content increases, the water content in spouted bed increases, while the gas temperature remains unchanged, which leads to the decrease of water temperature and the decrease of water vaporization rate. Fig. 8 shows the distribution of desulfurization product
(CaSO3) rate (kg m3 s1) in the spouted bed at different water contents. It is seen that the distribution trend of desulfurization product formation rate in spouted bed is consistent as a whole at the condition of different water contents, and the desulfurization product formation rate is the largest in annular zone. When the water content changed from 30 kg-H2O/kg-dry_sorbent to 70 kgH2O/kg-dry_sorbent, the rate of product formation first increased to 0.461 kg m3 s1 and then gradually decreased. Particles enter the nozzle through the spout, fountain region and annulus region, and then form a very regular internal circulation of particle movement because of the special flow structure of spouted bed. Particles in spout and fountain region obtain more kinetic energy because of high velocity of gas, and the residence time of particles in the bed becomes shorter, as a result, the reaction time between flue gas and slurry adhering to the surface of particles becomes shorter. On the other hand, according to the distribution of particle volume fraction in Fig. 2, when the spouted bed reaches stable spouting state, a large number of particles accumulate in the annulus region, which increases the contact area between particles and gases, prolongs the contact time and maximizes the product rate of desulfurization products in the annulus region. Fig. 9 shows the radial distribution of the rate of desulfurization reaction product (CaSO3) in the spouted bed at different water contents. It is interesting to note that when the bed height z equals 0.03 m and 0.06 m, the rate of desulfurization reaction product formation increases gradually along the radial direction in spout
Please cite this article as: F. Wu, K. Yue, W. Gao et al., Numerical simulation of semi-dry flue gas desulfurization process in the powder-particle spouted bed, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.10.024
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Fig. 9. Radial distribution of the rate of desulfurization reaction product (CaSO3) in the spouted bed at different water contents.
Fig. 10. Comparison of desulfurization rates under different value of water contents of slurry.
region, and a larger value appears at the interface between the spout and the annulus. With the increase of water content, the rate of product formation first increases and then decreases. The comprehensive analysis reveals that the desulfurization reaction rate in spouted bed first increases and then decreases with the water content, and the desulfurization effect is the best when value of water content equals 40 kg-H2O/kg-dry_sorbent.
Fig. 10 compares the desulfurization efficiency in spouted bed under different values of water contents of slurry. It can be seen that, under the same simulated conditions, the desulfurization rate in spouted bed increases first and then decreases with the increase of water content, which reveals that when the slurry water content increases from 30 kg-H2O/kg-dry_sorbent to 40 kg-H2O/kgdry_sorbent, the reaction rate of desulfurization in spouted bed increases gradually with the increase of the slurry water content, which is due to that the increase of water content causes an increase in the amount of calcium hydroxide dissolved, which in turn increases the amount of calcium ions involved in the ion reaction, as shown in Fig. 11, that is, the increase of the maximum and overall values of volume fraction of absorbent (Ca(OH)2.When the value of slurry water content continues to increase, the concentration of desulfurizer and water phase will be significantly reduced because the total calcium ions in the solution are reduced (Fig. 11). Besides, the lower the temperature, the lower the rate of water vaporization in spouted bed, and the lower the rate of desulfurization reaction. When the water content is 40 kg-H2O/ kg-dry_sorbent, the desulfurization rate reaches the maximum of 75.75%, while the experimental value is 75.03%, which is the closest to the experimental value. 4. Conclusions (1) The values of water vaporization rate are high in spout and outside of annulus which are the main regions of the water evaporation, and high gas temperature in spout region is beneficial for the evaporation of water. The value of water
Please cite this article as: F. Wu, K. Yue, W. Gao et al., Numerical simulation of semi-dry flue gas desulfurization process in the powder-particle spouted bed, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.10.024
F. Wu et al. / Advanced Powder Technology xxx (xxxx) xxx
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Fig. 11. Volume fraction of absorbent (Ca(OH)2) at different water contents.
vaporization rate is low in the interior area of the annulus and fountain regions due to the lack of transverse mixing of particle movement. (2) The maximum value of desulfurization product rate appears in the annulus, which is the major region of flue gas desulfurization reaction due to large particle concentration, large gas-solid contact area and large mass transfer area in the annulus. The minimum value of volume fraction of SO2 appears in the annulus, which indicates that the annulus is the main area of the desulfurization reaction in spouted bed. (3) The desulfurization efficiency of spouted bed increases first and then decreases with the increase of water content of desulfurization slurry. The desulfurization efficiency is the highest when the water content is 40 kg-H2O/kgdry_sorbent and is close to the experimental value. The optimum slurry water content for desulfurization process in powder-particle spouted bed was obtained by numerical simulation, which was 40 kg-H2O/kg-dry_sorbent.
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. Acknowledgement This work is supported by National Natural Science Foundation of China (Grant No. 21878245), Natural Science Foundation of Shaanxi Province (Grant No. 2019JM-039) and Cyrus Tang Foundation. References [1] F. Garcıa-Labiano, A. Rufas, F. Luis, M. de las Obras-Loscertales, P. Gayan, A. Abad, J. Adanez, Calcium-based sorbents behaviour during sulphation at oxyfuel fluidised bed combustion conditions, Fuel 90 (2011) 3100–3108. [2] L. Jia, Y. Tan, C. Wang, E.J. Anthony, Experimental study of oxy-fuel combustion and sulfur capture in a mini-CFBC, Energy Fuels 21 (6) (2007) 3160–3164. [3] D. Shun, D.H. Bae, I.K. Jang, K.H. Park, S.K. Park, Kinetic study of sulfur dioxide elimination by limestone through the lab scale circulating fluidized bed combustor, Adv. Mater. Phys. Chem. 2 (4) (2012) 189–192. [4] T. Wall, Y. Liu, C. Spero, L. Elliott, S. Khare, R. Rathman, An overview on oxyfuel coal combustion–state of the art research and technology development, Chem. Eng. Res. Des. 87 (2009) 1003–1016.
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Please cite this article as: F. Wu, K. Yue, W. Gao et al., Numerical simulation of semi-dry flue gas desulfurization process in the powder-particle spouted bed, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.10.024