Optimization the operation parameters of SDA desulfurization tower by flow coupling chemical reaction model

Abstract Spray Drying Absorber (SDA) has been widely used for large-scale desulfurization. However, it also has some limitations. For example, the liquid absorbent easily causes scaling, which impedes the contact between the serous fluid and the flue gas and reduces the chemical reaction rate and desulfurization efficiency. This paper establishes the mathematical and physical model of gas and liquid two-phase flow and droplet evaporation and heat transfer in rotary spray desulfurization tower. To study the accumulation and distribution of chemical reaction precipitates in the desulfurization tower and analyze the removal efficiency of sulfur dioxide (SO2) in different atomization diameters, this paper establishes a simulation model concerning the coupling of desulfurization reaction and flow field calculation based on the absorption and reaction mechanism of SO2. Baffle in different widths are set to optimize the internal flow field and balance the distribution of flue gas. By setting baffles of different widths to optimize the flow field in the tower and changing the distribution of flue gas, this model reduces the scaling while ensuring the desulfurization efficiency. The results of the simulation experiment have verified that the droplet with a diameter of 50 μm is the optimal option, which can effectively remove the scaling and ensure that the desulfurizing tower runs in high efficiency and stability. When the width of baffles is 2250 mm, the efficiency of desulfurization exceeds 95%, and the amount of scaling on the desulfurization tower main wall is controlled at the minimum level, which is the optimal option for production.


INTRODUCTION
As one of the major air pollutants, sulfur dioxide (SO 2 ) poses a great threat to the ecological environment. By contributing to acid rain, it can have signifi cant impacts on humans, animals and plants. For humans, when SO 2 is breathed in, most of them is retained in the upper respiratory tract. Then sulfurous acid with corrosiveness will form on the wet mucous membrane, with part of them being oxidized to sulfuric acid, which will irritate the respiratory system. If a human inhales SO 2 at the concentration of 100 ppm at one time, there will be obvious irritation sign of bronchial and lungs. More seriously, the lung tissues will be damaged in eight hours without timely treat and cure. As SO 2 can be absorbed into the blood, it can poison the other parts of the body by destroying the enzymatic activities, reducing the human metabolism, and damaging the liver function 1 .
During the sintering process of steel, a large amount of SO 2 is produced and emitted with the concentration between 400 to 2000 mg/m 3 , which may cause huge pollution to the environment. Therefore, to reduce the emission of SO 2 , the desulfurizing techniques should be adopted during this process.
Developed by a Denmark company called GEA Niro in the 1970s, Spray drying absorber (SDA) is a large--scale desulfurization method used by companies all over the world. The specifi c workfl ows of SDA are showed as follows: A high-speed motor drives the atomizing wheel to rotate at a high speed, generating a strong centrifugal force. Then the turbid liquid or emulsion with alkaline substance Ca(OH) 2 is spouted out and atomized into micro-sized droplets through the nozzle on the atomizing wheel and then evenly sprayed into the reaction zone inside the tower 2 . The sintering fl ue gas enters the tower through the fl ue gas distributor and then thoroughly contacts with the atomized droplets of Ca(OH) 2 , of which the acidic substances are neutralized in a short time for desulfurization. Meanwhile, the dry resultants formed in the acid-base neutralization reaction accumulating inside the tower are collected in the bag-type dust collector.
The SDA method possesses many advantages. For example, it can easily adapt to the changes of smoke compositions, fl ow rate, temperature, and concentration of SO 2 , with simple workfl ows and high effi ciency of desulfurization. Moreover, the by-products can be recycled 3 . However, this method also has some limitations. As the absorbent is liquid, scaling can be caused (as shown in Fig. 1), which changes the air fl ow distribution inside the tower. As a result, the contact between the serous fl uid and the fl ue gas will be impeded, which lowers the chemical reaction rate and desulfurization effi ciency 4 .
Polish Journal of Chemical Technology, 22,1,[35][36][37][38][39][40][41][42][43][44][45]10.2478/pjct-2020-0006 With the popularization of rotary spray and drying technology, many scholars and researchers have conducted researches on the acid-base absorption reaction, gas-liquid fl ow and heat-transfer characteristics of the desulfurizing tower, particularly on the solid component content of alkaline particles in the serous fl uid, the concentration of SO 2 in the fl ue gas, the ratio of Ca/S and SO 2 in the serous fl uid as well as the drying time of the serous fl uid. The numerical simulation method has been adopted to study the factors which can improve the desulfurization effi ciency of the fl ue gas, such as fl ue gas temperature, atomization and evaporation characteristics of the serous fl uid, as well as gas fl ow structure.
HT Karlsson 5 suggested that the concentration of lime slurry signifi cantly infl uenced the desulfurization process. By studying the infl uence of the concentration of serous fl uid on the desulfurization effi ciency through experiments, he found that if there were too much solid lime particles in the serous fl uid, the diffusion of SO 2 on the surface of the serous fl uid would be impeded. With SO 2 failing to react with the serous fl uid completely, the desulfurization effi ciency was thus lowered. But if the lime slurry was in moderate concentration , the concentration of SO 2 in the fl ue gas had no obvious impact on the desulfurization effi ciency.
According to the research of FF. Hill 6 , the absorption effi ciency of SO 2 was related to the drying condition when the Ca(OH) 2 content was relatively high. However, under this circumstance, the mass transfer resistance of liquid would be the main factor to lower the absorption effi ciency. Gullett 7 studied the characteristics of the serous fl uid particles in the absorbent and the concentration of SO 2 in the desulfurization reaction. The research results indicated that the overall reaction rate was impacted by the diffusion of gas fi lm when the concentration of SO 2 was lower than 800 ppm. However, when the concentration of SO 2 was higher than 800 ppm, the reaction rate was controlled by the dissolution rate of lime. According to the previous studies analyzing the infl uence of the calcium-sulfur ratio on the desulfurization effi ciency through the laboratory reactor 8 , it was found that if the calcium-sulfur ratio reached 1.2, the removal rate of SO 2 would exceed 95%.
Studying the reaction process of the serous fl uid and fl ue gas, SR Dantuluri 9 , Wentz 10 and Ranz 11 divided the serous fl uid evaporation and desulfurization reaction into three stages, which were the normal-speed reaction stage, the slow-down reaction stage, and the quasi-equilibrium reaction stage. Partridge 12 holds that as the serous fl uid droplets were heated continuously in the slow-down reaction stage, the surface of the large particles would be dry, with small pores on it. That is, when the serous fl uid lost a large amount of water, a shell would be formed on the surface of the solid particle clusters to block the contact between the fl ue gas and the serous fl uid. As a result, the desulfurization reaction would be terminated.
Studying on the spray drying process through experiment, Sang SK 13 found that when the SO 2 molecules were fully exposed to the wet droplets, the total surface area of droplet was the key factor imparting the desulfurization process. If the atomized droplets were too small or the evaporation speed was too fast, the shell forming in a short time on the surface will prevent the SO 2 molecules from suffi ciently transferring to the surface of the droplets, which would lower the desulfurization effi ciency.
By adding hygroscopic agent to prolong the liquid state of serous fl uid, TC Keener 14 improved the utilization of lime and the removal rate of SO 2 . He found that over 90% of SO 2 was removed when additives were used moderately.
A theoretical model of the semi-dry fl ue gas desulfurization method has been built by Chen Minggong 15 . The retention time of the serous fl uid in the desulfurization tower was calculated based on the desulfurizing reaction model and the drying model. He found that when the desulfurizing reaction time was longer than the drying time, the factors deciding the retention time of serous fl uid ware SO 2 content in the fl ue gas, the liquid-phase diffusion coeffi cient of the SO 2 molecules, the reaction rate constant, the concentration of Ca(OH) 2 in the absorption liquid, and the mass transfer coeffi cient of the SO 2 liquid fi lm. However, when the drying time was longer than the desulfurization time, factors that decided the retention time were evaporation amount, the droplet density, the vaporization enthalpy of solution and gas, the heat conductivity coeffi cient, and the temperature of fl ue gas and droplets.
As for the studies on the infl uence of droplet evaporation, H Kim 16 has conducted numerical simulation researches and found that in low temperature, the evaporation time of droplets would become longer with the increase of pressure, while in high temperature, the evaporation time would get shortened with the increase of pressure. A mathematical model concerning the droplet movement and evaporation in the gas fl ue has been established by Kang Meiqiang 17 , to study the impacts of parameters, including fl ue structure, fl ue gas temperature, spray particle diameter and installation position of the nozzle, on evaporation characteristics. Moreover, Han Fangliang 18 simulated the three-dimensional fl ow fi eld inside the fl ue gas desulfurization tower based on the turbulence model and random orbit model, which optimized the operation conditions of the desulfurization tower.
To research the infl uence of various operating parameters on desulfurization effi ciency, Fabrizio Scala 19-21 established a steady-state one-dimensional spray dryer model and a single-molecule rigid droplet model of SO 2 absorption to conduct the numerical simulation on the instantaneous and irreversible chemical reactions occurs in the desulfurization tower. It was found that the Ca/S molar feed ratio, average initial droplet size and lime particle size had a signifi cant infl uence on the desulfurization effi ciency. Chen Jianzhong 22 suggested that temperature drop and temperature difference of adiabatic saturation of the fl ue gas in the desulfurization tower had a great impact on the desulfurization effi ciency. He held that as the reaction time of liquid desulfurization became longer, the evaporation time of droplets would be affected, causing the desulfurization effi ciency drop exponentially with the adiabatic saturation temperature rising. Establishing an integrated model applicable to the semi-dry desulfurization tower and the bag-type dust collector and considering the gas-phase resistance, liquid-phase mass transfer resistance and lime dissolution and the height is 40 m in total (see Fig. 2a). The height of the inlet fl ue is 8.9 m and its width reaches 6.8 m.
With the weir-type fl ue gas distributor being installed at the lower inlet of the fl ue gas (see Fig. 2b), the fl ue gas goes through the upper or lower path after entering the desulfurization tower. Consisting of 120 blades, the blade grid is installed at the upper inlet of the fl ue gas with the inclined angle of each blade is 135 o , which helps to change the fl ow direction of the fl ue gas. The length and width of the fl ue outlet are both 6 m. The alkali liquor is ejected through injection ports on the atomizing wheel. The diameter of the atomizing wheel is 360 mm and the injection port is 1 mm. Each atomizing wheel contains 12 injection ports. resistance, Wang Naihua 23 has conducted researches on calcium-sulfur ratio, adiabatic saturated temperature, the initial particle diameter of limestone serous fl uid, and the fl ue gas temperature at the SO 2 inlet.
The simultaneous removal of SO 2 and polycyclic aromatic hydrocarbons (PAHs) by adding calcium-based additives CaO, Ca(OH) 2 , and CaCO 3 during sewage sludge incineration was investigated in a fl uidized bed incinerator by Qin Linbo and Han Jun 24 . Novel integrated desulfurization (NID) technology applied for sintering fl ue gas SO 2 removal was studied to investigate the effects of approach to saturation temperature, CaO/S and H 2 O/ CaO on SO 2 removal effi ciency by Qin Linbo 25 . Separate gasifi cation of Fenton-oxidized or CaO-treated sludge released H 2 S emission respectively from sulfonic acid/ sulfone/heterocyclic-S and inorganic sulfi de in char was studied by Liu Huan 26 . Qin Linbo 27 studied the effect of sewage sludge-CaO weight ratio, calcination temperature and hydration time on desulfurization effi ciency. He found that SO 2 removal effi ciency was increased from 88.7% to 97.3% after using the lime modifi ed with sewage sludge.
The SDA technology has been widely investigated from the early time, and great achievements has been gained in theory and experimental research. However, the former researches mainly studied the factors which infl uence the desulfurization effi ciency, rather than resolve the problems of dust-adhibiting, wall built-up and scaling in the production process. The existing researches that conducted numerical simulation of the gas-liquid fl ow fi eld inside the desulfurization tower did not involve the process of acid-base absorption reaction, and thus failed to predict the amount of the reaction products and the their distribution in the desulfurization tower.
In order to solve the problems of dust-adhibiting and scaling during the production process in the rotary spray desulfurization tower, this paper establishes a mathematical-physical model to characterize the gas-liquid two-phase fl ow and the heat transfer and evaporation of droplets and a simulation model of the desulfurization reaction and the fl ow fi eld calculation coupling in accordance with the absorption reaction mechanism of SO 2 . To optimize the structure and operation parameters of the desulfurization tower, we take the on-site data as the boundary condition to study the infl uence of atomized particle with different diameters on the desulfurization effi ciency and the amount of scaling on the inner wall of the tower. By setting baffl es of various widths, the distribution of the fl ue gas in the upper and lower fl ue baffl e and the fl ow fi eld distribution of the fl ue gas in the desulfurization tower are optimized. By successfully resolving the problems of scaling and dust-adhibiting, the production can be carried out more smoothly and stably.

Geometric model and grid generation
The external desulfurization tower consists of a cylinder part and a cone part, with the main components such as the inlet and outlet, fl ue, fl ue baffl e, fl ue gas distributor, atomizing wheel and upper fl ue blade grid. The diameter of the desulfurization tower is 20.3 m, The tetrahedral unstructured grids are adopted to build the desulfurization tower (see Fig. 3). The girds are refi ned at the injection pores, the fl ue gas distributors and the blade grids to improve the calculation accuracy. This model includes 8,759,060 grid cells and 1,463,562 nodes in total.

Mathematical-physical model and chemical reaction
The fl ue gas inside the desulfurization tower is in form of incompressible turbulent fl ow. The alkali liquor is atomized into droplets with diameters ranging from 50 to 80 microns, and then ejected into the desulfurization tower. heated by the fl ue gas, the droplets will evaporate inside the tower, and then react with SO 2 in the fl ue gas. Therefore, this gas-liquid two-phase fl ow process involves the phase change and heat transfer.

Equation of fl ow control
(1) Continuity equation 28 (1) In the Equation (1),  refers to the fl uid density, and u, v and w are the components of the velocity vectors at the directions of x, y and z, respectively.
(2) Momentum conservation equation 28 In the Equation (3), k represents the fl uid heat transfer coeffi cient, which equals to 0.0332 W/m 2 · K. T is the fl uid temperature, which is 403 K. cp is the specifi c heat at constant pressure, which is1004.4 J/kg K.  refers to the density, which is 0.8752 kg/m 3 .
(4) RNG k - turbulence model 29 The equation of the kinetic energy k and turbulent energy dissipation rate  can be defi ned as follows: In the Equation (4) and (5), the turbulent viscosity coeffi cient . G k is aterm of the turbulent kinetic energy k generated by the average velocity gradient, and its calculation formula is as follows: (10) In the Equation (6)

Multiphase fl ow model
The Lagrange method was adopts to calculate the movement and evaporation of droplet particles. In the Lagrange coordinate system, the dispersed phase is analyzed in discrete particles, and the motion equation of each discrete particle is solved through integral. The movement trajectory of dispersed phase is obtained by taking the stress of the dispersed phase in the continuous phase and the turbulent diffusion trajectory into account. As for the continuous phase, the Navier-Stokes dynamic equation is solved in the Euler coordinate system. The motion equation of droplets is shown in the equation (11) 6 : In this equation, F d (u -u p ) is the unit mass drag force of the particles, with the unit of N/kg. u represents the fl uid velocity, with the unit of m/s. u p represents the particle velocity.  p is the particle density, and its unit is kg/m 3 . The item has taken the gravity and buoyancy of the particles into account.
When simulating two-phase fl ow, the particle coupling option was fully coupled. In the simulation, the drag force option was adopted as Schiller Naumann 32 .

Chemical reaction of acid-base neutralization
The reaction between SO 2 and lime serous fl uid is a continuous process, which contains the steps including the gas phase diffusion of SO 2 , the absorption of SO 2 in water fi lm, the ionization of SO 2 in water fi lm, the transfer of acidic ions from aqueous layer to the reaction interface, the surface dissolution of absorbent, the transfer of the dissolved absorbent to the reaction interface, and fi nally the chemical reaction between the dissolved absorbent and the dissociated acid gas ions. The alkali liquor contains water and calcium hydroxide. During the chemical reaction, Assuming that the calcium hydroxide molecules are thoroughly dissociated into ions, and the reaction is shown as follows: Ca(OH) 2  Ca 2+ + 2OH -SO 2 fi rstly reacts with the water in the serous fl uid, forming the acid solution, which contains water and sulfuric acid. Assuming that the sulfuric acid molecules are completely dissociated into ions, the following reaction will occur: H 2 SO 4  2H + + SO 4 2-Moreover, Ca 2+ will react with OH -: Ca 2+ + OH -+ SO 4 2-+ H +  CaSO 4 + H 2 O In order to simulate the chemical reaction, it is assumed that the serous fl uid is composed of single-molecule rigid droplet, the chemical reactions mainly takes place on the surface of the droplet instantaneously and irreversibly, and ignore the infl uence of the volume change under thermal evaporation on the chemical reactions.
Reactions and reaction kinetics can be modelled using CFX Expression Language (CEL), together with appropriate settings for Component sources. The reaction and reaction rate were modelled using a basic Eddy Break Up (EBU) formulation for the component and energy sources, so that, the transport equation for mass fraction of acid was (12) where mf is mass fraction, D A is the Kinematic Diffusivity and i is the stoichiometric ratio. The right hand side represents the source term applied to the transport equation for the mass fraction of acid. The left hand side consists of the transient, advection and diffusion terms.
For acid-alkali reactions, the stoichiometric ratio is usually based on volume fractions. The reaction is modelled by introducing source terms for the acid, alkali and product components.

Material parameters
The main components of the fl ue gas are air and SO 2 . The serous fl uid consists of 20% of Ca(OH) 2 and 80% of water, with the mixed density of 1250 kg/m 3 . The fl ue gas is regarded as the incompressible fl uid. The impact of other components in the fl ue gas on the reaction is ignored.

Inlet conditions
The fl ue gas temperature reaches 130°C, and the inlet velocity of the fl ue gas is 6.88 m/s.

Outlet conditions
The outlet pressure is -1000 pa measured when the desulfurization tower is the operating.

Wall conditions
The wall surface, tower surface and bottom surface are heat insulation and designed to use rough material, with the roughness of 0.046 (equal to the roughness of steel). If the droplets are in the liquid when contacting the wall, they will lose their velocity in horizontal and vertical directions. If the droplets are dried before contacting to the smooth wall, the original fl ow fi eld will not be affected.

Droplet spray parameters
The rotation speed of the atomizing wheel is 10000 r/min, ejecting droplets of serous fl uid with a diameter of 30 to 70 μm. The mass fl ow rate of all ejection ports and temperature are 1.2 kg/s and 40 o C respectively. The injection amount of Ca (OH) 2 in this paper is calculated by the amount of SO 2 measured in the literature. When the SO 2 concentration at the system outlet has reached the design requirement, n(Ca)/n(S) is equal to 1.28. The process of rotary atomization of the serous fl uid is simplifi ed and regarded as the serous fl uid particles entering the desulfurization tower in accordance with the tangential direction of the rotation of the atomization wheel. That is, a tangential velocity represents the rotation process.

Initial conditions
In the simulation, the inlet velocity is used as the initial value of the fl ow fi led. The initial static pressure and temperature of the calculation domain are 10 6 Pa and 130 o C respectively. The turbulence intensity is 5%.

Verifi cation of the calculation simulation model
The applicability and feasibility of the numerical calculation model can be verifi ed through the measured data. The above-mentioned calculation model of the fl ow, heat transfer and chemical reaction is applied to the technological parameters of SDA, which is used for the No. 4 sintering machine of Shagang Group provided by Environmental Engineering 30 . The SO 2 concentration is calculated at the outlet, and compared with the measured value, to determine the accuracy and availability of the calculation model. The comparison of calculated value and measured value are shown in Figure 4.
The main operating parameters of the desulfurization tower in Environmental Engineering 30 are as follows: the average treatment capacity of fl ue gas is 2.04 million m 3 /h, and the concentration of fl ue gas ranges from 4000 to 5000 mg/m 3 , with the average fl ue gas temperature of 130 o C and the highest temperature of 180 o C. The rota- Figure 4. Comparison of experimental data and simulation data tion speed of the serous fl uid in the atomizing process is 10000 r/min, with the amount of spray serous fl uid of 10.1876 to 24.3054 t/h. Meanwhile, the Ca/S ratio is 1.48 to 1.71m, which is higher than the theoretical design value of 1.2. The atomization diameter is 50 μm.
By comparison, it is found that the concentration of the fl ue gas at the outlet generated by numerical calculation is higher than the monitoring value, which means that the desulfurization effi ciency is lower than the reality. The main reasons for this phenomenon are concluded as follows: Firstly, the numerical study simplifi es the components of fl ue gas at the inlet by neglecting the moisture and assumes the fl ue gas is composed of pure air and SO 2 . However, there is 10% of water vapor in the sintering fl ue gas in the actual production. The water vapor impedes the evaporation of the serous fl uid during the reaction, making the serous fl uid left in liquid phase, but the desulfurization reacts mainly when the serous fl uid are in liquid. Therefore, the reaction time of the serous fl uid particles and the fl ue gas in the actual desulfurization process is longer than that of the numerical simulation, so that the actual concentration value of SO 2 in the fl ue gas after the desulfurization is lower than the calculated value.
Secondly, there is CO 2 in the fl ue gas in the actual reaction process, and CaCO 3 will be formed when CO 2 reacts with the serous fl uid. Moreover, HSO 4 -, SO 3 2and SO 4 2ionized in the serous fl uid will react with HCO 3 and CO 3 2in CaCO 3, according to the principle of replacing weak acid with strong acid. Therefore, the carbon dioxide will further improve the reaction effi ciency, which makes the actual desulfurization effi ciency higher than the calculated value.
The third reason is that as the utilization effi ciency of the serous fl uid is less than 100% in actual production, it is generally use excessive serous fl uid to desulfurize the fl ue gas. According to the data provided by Gu, He and Jiang 30 , the calcium-sulfur ratio ranges between 1.47 and 1.71 in actual operation. In calculation of this paper, the reaction coeffi cient is set as 1.2, which means that the ratio of the serous fl uid and SO 2 is 1.2:1. To satisfy the requirement of actual production, the amount of spray serous fl uid is relatively higher, so that the SO 2 are absorbed more thoroughly. Therefore, the calculated desulfurization effi ciency is lower than actual value.

Numerical analysis
ANSYS ICEM 31 is employed to generate the 3D structural grids of the models. A high-resolution scheme is used to discretize the turbulence formulations. The high--resolution scheme uses the second order backward Euler scheme wherever and whenever possible and are reverted to the fi rst order backward Euler scheme to maintain a bounded solution. The convection and diffusion terms of equations are discretized with the fi rst-order upwind and central difference scheme, respectively. ANSYS CFX 16.0 32 commercial code is used to solve all governed equations. The under-relaxation factors for velocity, energy, and mass provided damping for the above equation set. In a steady-state calculation, the default value of 0.75 has been found to be suffi ciently small to dampen solutions. The convergence criteria are that the values of root mean square of residuals are below 10 -4 .

RESULTS AND DISCUSSION
The droplet diameter is a key factor which affects the evaporation and chemical reactions of the serous fl uid. If the diameter of the droplet is too small, the droplet will rapidly evaporate once contacting to the fl ue gas, and the dry serous fl uid particles will fail to react with SO 2 , thus reducing the desulfurization effi ciency. But if the diameter of the droplet is too large, the evaporation will be prolonged, and dust-adhibiting and scaling will be generated if the droplets are not dry enough when contacting to the inner wall of the desulfurization tower. Therefore, the movement trajectory of the droplets with different particle diameters under the same fl ue gas temperature and fl ue gas amount were studied.

The impact of droplet diameter on desulfurization effi ciency
The droplets with the diameter of 30 μm, 40 μm, 50 μm, 60 μm and 70 μm are adopted for calculation. The movement trajectories of the droplets under different working conditions are shown in Fig. 5, and the evaporation time of the droplets are calculated by equation (13) and shown in Table 1.
Where τis evaporation time, h refers to the heat transfer coeffi cient of the wet droplets and the air fl ow. T g and T p represent the temperature of the environment and the temperature of wet particles or droplets, respectively. L w is the latent heat of water vaporization and d r is droplet diameter.
Calculate the removal amount of SO 2 based on the concentration of SO 2 at the inlet and outlet, so as to explore the impact of the diameters of droplets on the desulfurization effi ciency. The calculation formula of the desulfurization effi ciency η is shown in the Equation (14): In this equation,  represents the desulfurization efficiency. c in is the concentration of SO 2 at the inlet, while c out is the concentration of SO 2 at the outlet.
According to the comparative analysis in Figure 5a, the serous fl uid will rapidly evaporate if the diameter of droplets is 30 μm, and the droplets will be dried within 0.55 s. According to the research results of Sang SK 33 , if SO 2 fully contact with the wet surface of droplets, the change of the total surface area of droplets will become the key factor that impact the desulfurization. When the evaporation rate of droplets is too fast, the SO 2 fails to fully transfer to the surface of the droplets, so that the Table 1. Evaporation time and desulfurization effi ciency of droplets with different diameters serous fl uid fails to react with SO 2 molecules effectively, thus reducing the desulfurization effi ciency. Therefore, the desulfurization effi ciency is relatively low when the diameter of droplets is 30 μm.
As shown in Figure 5b, when the diameter of particle of the serous fl uid is 40 μm, the time required for the droplet evaporation reaches 0.72 s based on the calculation, which is longer compared with that of 30 μm. As a result, the desulfurization effi ciency has been improved, but does not reach 95%, which fails to meet the requirement of production.
As shown in Figure 5c, when the diameter of droplet is 50 μm, the length of the movement trajectory is signifi cantly increased. When contacting to the inner wall, the droplets are just in dry status, and the evaporation time of the serous fl uid is about 0.9 s by calculation. The desulfurization effi ciency is 95.5%, which fulfi ll the requirement of production. By observing the scaling amount, it can be found that as the diameter of the particles increases, the scaling amount increases signifi cantly. Therefore, the particles with a diameter of 50 μm generate the minimal amount of scaling without reducing the desulfurization effi ciency.
According to Figures 5e, when the droplet diameter is 70 μm, the evaporation of the serous fl uid will be prolonged to 2.19 times than that of. droplet diameter 30 μm. In this case, more evaporation time means that serous fl uid is given more time to absorb SO 2 . Therefore, the desulfurization effi ciency under this circumstance is higher than that of the droplet with other small diameters. However, it was also shown that since the serous fl uid in liquid phase has not evaporated completely when contacting to the inner wall, the serous fl uid would stick to the wall and produce scaling, which poses a negative impact on production. Thus, the particles with a diameter between 60 to70 μm are unsuitable for production, as the desulfurization effi ciency fails to be signifi cantly improved. Instead, the amount of scaling increases markedly By analyzing Figure 5 and Table 1, it can be seen that the diameter of the serous fl uid droplets has a great impact on the desulfurization effi ciency. When the diameter of droplets increases from 30 μm to 50 μm, the desulfurization effi ciency increases 19.9%. When the diameter of droplet increases from 50 μm to 70 μm, the desulfurization effi ciency just increases 2.1%, instead, the amount of scaling increases signifi cantly. According to the national standard of China, the emission concentration of fl ue gas should be lower than 100 mg/m 3 , while in this paper, the initial concentration of fl ue gas is 2000 mg/m 3 . Therefore, the desulfurization effi ciency should be higher than 95% to meet the requirements of sintering exhaust gas emission. In other word, the diameter of the droplets should be larger than 50 μm to meet the requirements of environmental protection for desulfurization. However, if the diameter of droplet is over 60 μm, there would generate 1.23 times scaling more than that case of 50 μm droplet, however the desulfurization effi ciency only enhances 1.6%. The redundant scaling would blocks the paths in the desulfurization tower, and poses a threat to the stable production. Therefore, according to the simulation results, the droplets with a diameter of 50 μm is an optimum option which can ensure not only the high desulfurization effi ciency, but also the stable operation of the desulfurization.

The impact of baffl es of diff erent widths on the amount of scaling inside the tower
The degree of completion of the desulfurization reaction is determined by the distribution of the fl ue gas inside the tower. When a large amount of fl ue gas fl ows through the spray area of the serous fl uid evenly, the chemical reactants can fully contact and react with each other and SO 2 in the fl ue gas can be absorb to the maximum extent. If the fl ue gas is distributed unevenly and some of them fail to pass through the reaction zone, the short-circuited airfl ow will be formed. If the fl ue gas fl ows through the reaction zone too fast, SO 2 will not be absorbed completely, thus reducing the desulfurization effi ciency. Moreover, excess serous fl uid impacting the inner wall of the desulfurization tower will produce scaling through the chemical reaction, which adheres to the inner wall of the tower. The scaling is forms in two ways. Some are formed by unreacted and dry Ca(OH) 2 sticking to the inner wall after heated by the fl ue gas, and the others are salts generated by chemical reaction occurring close to the inner wall also under the infl uence of water mist.
In order to distribute the fl ow fi eld evenly and promote reaction of the chemical reactants suffi ciently, and reduce the scaling on the inner wall, it is proposed to fi x a baffl e at the inlet of the desulfurization tower (as shown in Fig. 6a). By adjusting the width of the baffl e, the distribution ratio of the fl ue gas at the upper and lower sides of the inlet can be optimized. There are fi ve types of baffl es with the width ranging from 0 to 2.65 m in this study. As shown in Figure 6, the fl ue gas is divided into upper and lower two streams after entering the desulfurization tower. Entering the desulfurization tower through the rotary fl ue, the upper stream forms a vortex under the infl uence of the rotating inertia, and contacts and reacts with the alkaline droplets. Impacting the rotary atomizer, the lower stream also contacts with droplets in the middle of the desulfurization tower. Therefore, the fl ow ratio of the upper and lower stream has been regulated by the width of the baffl e. By analyzing the Figure 6, it can be found that when the width of the baffl e is 2.25 m, the stream lines refl ect the fl ow trajectory of the fl ue gas concentrating inside the tower ,thus the fl ow fi eld is quite disorder in the middle of the tower. It is also shown that there are less fl ow trajectory lines near the wall compared with other parts. Therefore, it can be predicted that amount of scaling on the wall will be the least under this circumstance.
As shown in Table 2, with the increase of width of baffl e, the amount of the fl ue gas passing through the upper fl ue decreases, while the amount of the fl ue gas going through the lower fl ue increases. When the width of baffl e increases from 0 to 2.25 m, the desulfurization effi ciency increases 5%, but when the width of baffl e increases from 2.25 to 2.65 m, the desulfurization effi ciency decreases 3.25%. Therefore, at the point of acquiring the maximum desulfurization effi ciency, the width of baffl e should be 2.25 m.
On the other hand, the amount of scaling on the tower wall under the different baffl e widths should be calculated and compared. The distribution and mass fraction of the resultants on the inside wall of the tower has been shown in Fig. 7. Table 2. The fl ue gas distribution ratio, desulfurization effi ciency and the amount of scaling with baffl es of different widths Figure 7. Distribution of scaling formed in the inner wall of tower with baffl es of different width It can be seen from Figure 7 that without any baffl e, totally 36.07 kg scaling sticks to the inner wall of the tower, which blocks the fl ow channel. As a result, the production must be stopped frequently to clean the tower. With the width of the baffl e increasing to 2.25 m, the fl ue gas ratio of upper and lower stream is 0.67, and the amount of scaling is 28.5883 kg which is the least among all study cases. However, when the width of the baffl e is larger than 2.25 m, the balance of fl ow fi eld is broken, redundant fl ue gas from lower smoke pipe is unevaporated and contacts to the inner wall, so 2 kg more scaling generates, which impedes the production. Therefore, the baffl e with a width of 2.25 m is the optimal option for production, with a higher desulfurization effi ciency and the least scaling on the inner wall of the desulfurization tower.

CONCLUSION
This paper studies the fl ow fi eld and desulfurization effi ciency in the rotary spray tower using the method of numerical simulation with the coupling of chemical reactions. By comparing the calculated concentration of SO 2 at the outlet with the monitoring values, this paper verifi es the correctness of stimulating the gas-liquid two--phase fl ow fi eld in the desulfurization tower through the RNG k- turbulence model and the particle trajectory physical model coupling chemical reaction.
It has been found that the diameter of the serous fl uid droplets signifi cantly impact the desulfurization effi ciency. The desulfurization effi ciency is improved with the increase of the droplet diameter. However, if the droplet diameter is too large, as the serous fl uid is still wet when contacting to the tower wall, a large amount of scaling will be produced, which blocks the desulfurization tower and impedes the stable operation of production. The droplet with a diameter of 50 μm is an optimum parameter which can ensure not only the high desulfurization effi ciency, but also the stable operation of the desulfurization.
The distribution of the fl ue gas inside the tower affects the completion level of the desulfurization reaction. Installing plate baffl e can regulate the fl ow ratio of the upper to the lower stream. With the width of the baffl e increasing, the upper stream decreases while the upper stream increases. When the width of baffl e is 2.25 m, the desulfurization effi ciency in the tower reaches the maximum, and the amount of the scaling on the surface of the tower wall is the least, which meets the requirements of production.