Parameter Investigation of Flyash Jet Mill with Superheated Steam

To improve the utilization rate of flyash, refining flyash is an important approach, and steam power grinding with superheated steam is an important equipment to meet the fine particle size requirements of flyash grinding. A 2.4 m height jet mill with four Laval nozzles has been employed here, and FLUENT is adopted to numerically simulate the distribution of fluid velocity and pressure in the flow field within the model. Meanwhile, parameter investigation is also carried out to reveal the effect of nozzle spacing and nozzle inclination angle on fluid filed and average particle velocity distribution. It is found that both spacing and nozzle inclination angle can affect the average particle velocity simultaneously. From the present parameter investigation, it can be concluded that Model 2 (nozzle spacing is 455 mm) with an inclination angle of 4° is the best choice since it can provide largest particle velocity distribution along the vertical direction.


Introduction
With the increasingly strict requirements of national environmental protection policies and regulations, flyash solid waste has become one of the factors restricting the survival and development of thermal power plants.At the same time, the cost of flyash treatment is high, it is difficult, and it occupies a lot of space, which brings significant economic and environmental pressure to the operation of power plants.The traditional sales area of flyash not only meets the needs of surrounding cement factories, but also partially enters the commercial concrete market, and has some applications in road engineering.Flyash is an excellent admixture that can produce morphological effects, activation effects, and micro aggregate effects.To improve the utilization rate of flyash, refining flyash is an important approach.However, the smaller the particle size, the higher the processing cost, which is the main reason restricting the large-scale utilization of flyash.If low-cost and large-scale production of flyash with a specific surface area of 6000-12000 cm 2 /g can be achieved, it will greatly improve the utilization rate of flyash, improve its environmental pollution, and have good market prospects.The particle size of ultrafine flyash is about 5-15 μm, and steam power grinding is an important equipment to meet the fine particle size requirements of flyash grinding.
The work of others has shown that jet grinding, as a low-cost and applicable new technology, can be further developed for pulverizing flyash, delivering food functional factors, and micronizing pharmaceutical ingredients [1][2][3].Moreover, superheated steam jet grinding is a new and effective method for modifying desulfurization gypsum, which has enormous application potential in the cement industry [4].Some scholars have used spiral flow fields to study airflow mills, particles with a balance zone between centrifugal force and radial resistance in a spiral mill can be mainly ground through high-speed collisions between particles circulating near the top and bottom walls of the grinding chamber [5].At present, the discrete element method (DEM) is generally used to study particle collisions [6-9]，but DEM take much more computational time.And Lee et al. used a discrete element model (DEM) and computational fluid dynamics (CFD) simulation to clarify the relationship of the correlation with the actual design variables by predicted the motion of particles with the operating conditions and obtained optimized conditions [10].
Therefore, finite element simulation guidance for the design of airflow mills has a certain application background.In this study, we focused on studying the distribution of fluid velocity and pressure in the flow field within the model.Meanwhile, parameter investigation is carried out to reveal the effect on fluid filed and average particle velocity distribution.And a 2.4 m height jet mill has been established with four Laval nozzles, and the height and spacing of the nozzles have also been preliminarily designed.In finite element simulation, we incorporate particle injection to simulate the motion behavior of particles in an airflow mill.The convergence of the grid used for calculation in the article has also been verified.

Control Equations
The control equations of the fluid in the jet mill can be given below: (1) Continuity equation (2) Momentum equation

Problem Statement
The geometry of the jet mill and the inner part of the overheat vapor is given in Figure 1, and the total height in the y-direction is about 2416 mm, and the distance between the nozzles is about 615 mm, and this is one of the key parameters that affect the velocity and pressure field inside the jet mill.The diameter of the cylinder at the nozzle zone and the top of the mill are 780 mm and 915 mm respectively.FLUENT is emploed here to simuate the fluid field, and Figure 2 shows the mesh of the computational domain, and total 37080 nodes was involved in the meshing.
In the numerical simulation, the standard k-ε model is employed for turbulent flow.Attentions should be paid to setting the preheated steam as an ideal gas and selecting the nasa9 empirical formula for specific heat.The total pressure at the inlet is set to 1 MPa, and the temperature is set to 280 °C (553.15K).The outlet is set as a pressure outlet, with a pressure setting of -1 kPa and set to suppress reflux (for easy convergence).The particle density of flyash is set to 2435.9 kg/m 3 , the specific heat according to industrial experience is given by 975 J/(kg•K).The diameter of the flyash particle is 75 m, and the feeding rate is 500 kg/h.). 5 shows the typical fluid velocity field of the vertical section of the jet mill, and it is cleat the the maximal velocity appears at the nozzle area, and the velocity distribution is not symmetrical about the vertical axis of the structure, and this is caused by the asymmetric outlet.There are four symmetrical inlet at the bottom, while only one outlet at the top of the jet mill.Figures 6-8 give the velocity vectors of the particles in the jet mill of Model 1 with three different inclination angles.It is clearly revealed from the results that nozzle with an inclination angle can generate larger particle velocity as well as impact kinetic energy.It is also noted that, although it is obvious that nozzle angle has a higher upward impact momentum, which is speculated to give particles more velocity, it is necessary to quantitatively evaluate the effect of the inclination angle of the nozzle.
Figure 9 shows a typical particle tracks in the jet mill, it can be seen the whole tracks of all the 20 particles and it seems like a random phenomenon.In order to quantitatively evaluate the effect of the inclination angle of the nozzle, average particle velocities across 9 different horizontal cross sections are extracted and illustrated.As shown in the Figure 9, the tracks of the particles cover the entire inner space of the jet mill.Figure 10 shows the 9 different horizontal cross sections with a thickness of 2 cm.The numerical results are given in dpmrpt files and then processed using Combined Programming Language, which is very suitable for processing hundreds of thousands of rows of data.As mentioned earlier, different nozzle spacing and inclination angles can result in different flow fields and velocity vectors, which lead to different particle velocity distributions.Based on massive numerical simulation, Figures 11-14 show the variations of particle velocity with horizontal section of the jet mill for different incident angles and different nozzle spacing.11-14, the average particle velocity distribution along the vertical direction varies with nozzle spacing and nozzle inclination angle.The maximal average particle velocity can reach to 35 m/s for Model 5 with nozzle inclination angle of 4° near the nozzle zone.While Model 1 gives the minimal average particle velocity distribution compared with the other 4 Models due to the large nozzle spacing which decrease the collision probability of the particles.When the nozzle spacing is 615 mm (Model 1) or 455 mm (Model 2), nozzle inclination angle of 4° can effectively increase the average particle velocity.When the nozzle spacing is 535 mm (Model 4), inclination angle of 7° can provide particles with a larger average velocity.When the nozzle spacing is 455 mm (Model 3) or 295 mm (Model 5), it is observed that different inclination angle exhibit different particle velocity performance, and it is hard to determine the optimal design.

Conclusions
Numerical simulation of the flyash jet mill with superheated steam is conducted with FLUENT in the present investigation and the effect of key parameters such as nozzle spacing and nozzle inclination angle on the average particle velocity are also given.It is found that both spacing and nozzle inclination angle can affect the average particle velocity simultaneously.From the present parameter investigation, it can be concluded that Model 2 (nozzle spacing is 455 mm) with an inclination angle of 4° is the best choice since it can provide largest particle velocity distribution along the vertical direction.Further investigation should focus on the effect of outlet boundary condition and internal vortex generator.

Figure 1 .
Figure 1.The structure of the jet mill.Figure 2. Meshing of the inner part.

Figure 2 .
Figure 1.The structure of the jet mill.Figure 2. Meshing of the inner part.

Figure 3 .
Figure 3. Variations of fluid pressure with horizontal coordinate (x-axis) at different cross sections of the jet mill.

Figure 4 .
Figure 4. Variations of fluid velocity with horizontal coordinate (x-axis) at different sections of the jet mill.It can be seen from Figure3and Figure4that, the average fluid pressure increases with the increment of the y-coordinate, and near the outlet area the pressure is about 600 Pa, it is also noted that in the middle of the nozzle area, the peak pressure is larger than 1 kPa due to the inlet fluid collision.Meanwhile the fluid velocity field is rather uniform, and the peak velocity appears at the middle of the nozzle area, which reaches to about 200 m/s.

Figure 5 .
Figure 5.Typical fluid velocity field of the vertical section of the jet mill.

Figure 8 .
Figure 8. Velocity vectors of the particles in the jet mill (Model 1-inclination angle 7°). 5 shows the typical fluid velocity field of the vertical section of the jet mill, and it is cleat the the maximal velocity appears at the nozzle area, and the velocity distribution is not symmetrical about the vertical axis of the structure, and this is caused by the asymmetric outlet.There are four symmetrical inlet at the bottom, while only one outlet at the top of the jet mill.Figures6-8give the velocity vectors of the particles in the jet mill of Model 1 with three different inclination angles.It is clearly revealed from the results that nozzle with an inclination angle can generate larger particle velocity as well as impact kinetic energy.It is also noted that, although it is obvious that nozzle angle has a higher upward impact momentum, which is speculated to give particles more velocity, it is necessary to quantitatively evaluate the effect of the inclination angle of the nozzle.Figure9shows a typical particle tracks in the jet mill, it can be seen the whole tracks of all the 20 particles and it seems like a random phenomenon.In order to quantitatively evaluate the effect of the inclination angle of the nozzle, average particle velocities across 9 different horizontal cross sections are extracted and illustrated.As shown in the Figure9, the tracks of the particles cover the entire inner space of the jet mill.Figure10shows the 9 different horizontal cross sections with a thickness of 2 cm.The numerical results are given in dpmrpt files and then processed using Combined Programming Language, which is very suitable for processing hundreds of thousands of rows of data.As mentioned earlier, different nozzle spacing and inclination angles can result in different flow fields and velocity vectors, which lead to different particle velocity distributions.Based on massive numerical simulation, Figures 11-14 show the variations of particle velocity with horizontal section of the jet mill for different incident angles and different nozzle spacing.

Figure 9 .
Figure 9. Particle tracks in the jet mill.Figure 10.Illustration of different horizontal section.

Figure
Figure 12.Variations of particle velocity with horizontal section of the jet mill for different incident angles of Model 2.

FigureFigure 14 .
Figure 13.Variations of particle velocity with horizontal section of the jet mill for different incident angles of Model 3.

Figure 15 .
Figure 15.Variations of particle velocity with horizontal section of the jet mill for different incident angles of Model 5.As shown in Figure11-14, the average particle velocity distribution along the vertical direction varies with nozzle spacing and nozzle inclination angle.The maximal average particle velocity can reach to 35 m/s for Model 5 with nozzle inclination angle of 4° near the nozzle zone.While Model 1 gives the minimal average particle velocity distribution compared with the other 4 Models due to the large nozzle spacing which decrease the collision probability of the particles.When the nozzle spacing is 615 mm (Model 1) or 455 mm (Model 2), nozzle inclination angle of 4° can effectively increase the average particle velocity.When the nozzle spacing is 535 mm (Model 4), inclination angle of 7° can provide particles with a larger average velocity.When the nozzle spacing is 455 mm (Model 3) or 295 [1] Duanle Li, Rui Sun, Dongmin Wang, Caifu Ren, Kuizhen Fang 2021 Study on the pozzolanic activity of ultrafine circulating fluidized-bed fly ash prepared by jet mill Fuel 291 120220 [2] Carmine Sabia, Tommaso Casalini, Luca Cornolti, Marco Spaggiari, Giovanni Frigerio, Luca Martinoli, Alberto Martinoli, Antonio Buffo, Daniele L. Marchisio, Maurizio C. Barbato 2022 A novel uncoupled quasi-3D Euler-Euler model to study the spiral jet mill micronization of pharmaceutical substances at process scale: model development and validation Powder Technology 405 117573 [3] Hang Li, Tian-Xiao Yang, Qing-Sheng Zhao, Shou-Bu Hou, Rong-Rong Tian, Bing Zhao 2023 Comparative study of encapsulated cannabidiol ternary solid dispersions prepared by different techniques: The application of a novel technique jet milling Food Research International 168 112783 [4] Zhonghui Xu, Dan Hu, Ran An, Longyuan Lin, Yingling Xiang, Linpei Han, Yunlin Yu, Liping Ning, Jing Wu 2022 Preparation of superfine and semi-hydrated flue gas desulfurization gypsum powder by a superheated steam powdered jet mill and its application to produce cement pastes Case Studies in Construction Materials 17 e01549 [5] Kizuku Kushimoto, Kaya Suzuki, Shingo Ishihara, Rikio Soda, Kimihiro Ozaki, Junya Kano 2023 Analysis of the particle collision behavior in spiral jet milling Advanced Powder Technology 34 103993 [6] S.S. Bhonsale, Bard Stokbroekx, Jan Van Impe 2020 Assessment of the parameter identifiability of population balance models for air jet mills Computers & Chemical Engineering 143 107056 [7] Kizuku Kushimoto, Shingo Ishihara, Junya Kano 2019 Development of ADEM-CFD model for analyzing dynamic and breakage behavior of aggregates in wet ball milling Advanced Powder Technology 30 1131-1140 [8] Kizuku Kushimoto, Shingo Ishihara, Samuel Pinches, Mitchell L. Sesso, Shane P. Usher, George V. Franks, Junya Kano 2020 Development of a method for determining the maximum van der Waals force to analyze dispersion and aggregation of particles in a suspension Advanced Powder Technology 31 2267-2275 [9] Genlian Fan, Qibing Liu, Akira Kondo, Makio Naito, Kizuku Kushimoto, Junya Kano, Zhanqiu Tan, Zhiqiang Li Self-assembly of nanoparticles and flake powders by flake design strategy via dry particle coating Powder Technology 418 118294 [10] Hong Woon Lee, Sinae Song, Hee Taik Kim 2019 Improvement of pulverization efficiency for micro-sized particles grinding by uncooled high-temperature air jet mill using a computational simulation Chemical Engineering Science 207 1140-1147