Improving the electrostatic precipitation removal efficiency by desulfurized wastewater evaporation

Abstract

A novel technique was developed to improve the removal of fine particles by electrostatic precipitation. The performance of a lab-scale ESP spraying desulfurized wastewater was investigated under controlled conditions in a coal-fired thermal system. The fine particles' properties, electric performance, wet flue gas desulfurization efficiency and pH were analyzed. Moreover, the factors that influence the removal efficiency of ESP, including operating voltage, wastewater flow rate, and atomized droplets' diameter, were also analyzed. It was found that the average diameter of particles increases from 0.15 to 0.5 μm due to the evaporation of desulfurized wastewater, which is confirmed by SEM. Fine particles removal efficiency by ESP was greatly improved from 68% to 83%. Moreover, the ESP removal fine particle efficiency could be affected by the wastewater flow rate, operating voltage, and atomized droplet diameter. Finally, the influence of the WFGD system was analyzed, indicating that the desulfurized wastewater evaporation had little impact on the desulfurization system. This novel technique can improve the removal efficiency of ESP with zero discharge by implementing the desulfurization wastewater evaporation.

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1. Introduction

Electrostatic precipitators (ESP) are widely used for removing particles in various industrial processes.¹ Although the total efficiency of ESPs is as high as 99%, there are still many submicron particles that are emitted into the atmosphere, especially particles in the diameter range of 0.1–1 μm.^2,3 This low efficiency is not only a key constraining factor for ESP efficiency but also an important control index for the environmental air quality in China.

The limestone–gypsum wet flue gas desulfurization (WFGD) system, widely applied for the removal of SO₂, has many advantages such as high efficiency and reliability, and adaptability to various types of coals.^4,5 In the desulfurization slurry cycle, chlorine and heavy metals accumulate, resulting in unexpected negative influences, such as reducing the pH of the desulfurization slurry and rendering the equipment prone to corrosion and erosion. To avoid corrosion and erosion as well as meet the quality requirements for the desulfurization slurry, the concentration of chloride ions in the desulfurization slurry must be controlled within the prescribed range. Thus, there will be a lot of desulphurization waste liquid discharge within the desulfurization system to ensure that the concentration of chloride ions falls within the prescribed range. Moreover, the wastewater must be completely treated before discharging. The conventional methods, such as chemical precipitation, filtration, electrodialysis, ion exchange and ultrafiltration, have been applied to many WFGD wastewater treatment systems. However, conventional treatments have disadvantages such as high cost and secondary pollution.^6–9

In recent years, a novel evaporation treatment has been added to wastewater treatment with an advantage of zero-discharge performance. The wastewater discharged from the WFGD was atomized into droplets by dual fluid atomizer nozzles, located at the duct between the air preheater and the ESP.^10,11 The atomized droplets rapidly evaporated at high temperature and this increased the flue gas humidity. As a result, the resistance of the fly ash reduced, whereas the particle-charged ability and the particle removal efficiency increased. The precipitation of the solid matter in the wastewater will be removed by the ESP.^12,13 This novel technology is simple and requires less investment. However, the influence on the electric performance while using DWES technology has not been sufficiently reported.

In this study, a controlled experiment was conducted to further explore the performance of ESP after wastewater evaporation. During the experiment, the FGD wastewater was sprayed into the duct, and it was ensured that it completely evaporated into the ESP. In addition, erosion and corrosion were avoided to ensure that the flue gas temperature did not significantly decrease. The influencing factors that were studied include temperature, the diameter of the spray droplets, the flow rate of wastewater, and the SO₃ concentration.

2. Experimental section

2.1 The properties of coal and wastewater

To investigate the influence of wastewater evaporation on the ESP, anthracite coal was used during the test. The proximate and ultimate analyses of anthracite are given in Table 1.

Table 1 Proximate and ultimate analyses of experimental coal (the subscripts ar = as received and ad = air dry)

Sample	Proximate analysis wad/%				Ultimate analysis wad/%
	Mtol	Aar	Var	Caf	Car	Har	Oar	Nar	Sar
Stone coal	2.38	8.84	10.00	78.78	64.39	3.18	0.72	0.81	1.70

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The desulfurization wastewater was taken from the actual coal-fired power WFGD set-up, which was the supernatant fluid obtained after the precipitation of the desulfurization wastewater for four hours; the pH of the desulfurization wastewater was 5. Table 2 shows the FGD wastewater quality analysis; the wastewater was characterized by an extremely complex composition matrix. The FGD wastewater carried various total dissolved solids (TDS) and contained a lot of F⁻, SO₄²⁻, S²⁻, Cl⁻, etc.

Table 2 Analysis of desulfurization wastewater

Parameter	Range (mg L^−1)	Parameter	Range (mg L^−1)
pH	5	Cu	0.11
Ca^2+	1000	Pb	0.50
Mg^2+	600	Cd	0.07
Cl^−	7000	Ni	0.25
F^−	12.0	Hg	0.12
NH3/NH4	500	Cr	0.046

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2.2 Experimental facility

A test device was designed and set up to simulate a power plant for the flue gas treatment process, as shown in Fig. 1. The instrument was mainly made up of a coal-fired boiler, a buffer vessel, a wastewater evaporation system, an ESP, a WFGD, and an analysis detecting system. Flue gas at approximately 350 m³ h⁻¹ was provided by a coal-fired boiler. A stirrer and an electric heater were installed in the buffer vessel to ensure the constant particle concentration and size distribution and to regulate the temperature of the flue gas. The wastewater was atomized into droplets by dual fluid atomizer nozzles, and the flue gas and wastewater countercurrent were sufficiently contacted in the evaporation chamber (height = 4000 mm and diameter = 400 mm). The mixture gas entered the ESP by a booster fan. An ESP is a barb-plate tube with an operating voltage of 50 kV. When the flue gas passed through the WFGD, it contacted the spraying slurry. During the process of desulfurization, the desulfurization liquid returned to the crystallization tank for oxidation and recycling. This operation was conducted under different conditions in the experiment and the specific parameters are shown in Table 3.

Fig. 1 Schematic diagram of experimental system.

Table 3 Operation parameters of the experiment

Parameter	Range
Flue gas flow rate (m^3 h^−1)	350
Evaporation temperature (°C)	150–230
The EPS voltage (kV)	10–50
Desulfurization waste water flow rate (L h^−1)	10–25

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2.3 Measurement technique

An electrical low pressure impactor was used to monitor the particle size distribution and concentration. The ELPI consisted of 13 stages (12 channels), and the cut off diameter (D₅₀) of each stage and the geometrical mean diameter (D_i) of each channel are shown in Table 4. The ELPI operating principle was based on the inertial classification. When the particles passed through the channels, they were electrically charged; the number was calculated according to the electrical charge by the different channels. A thermocouple was used to measure the temperature of the flue gas. A pitot tube anemometer was used to measure the volume flow velocities and pressures. The particles were collected on the aluminum film of the impact plate of the PM_2.5/10 impactor for morphology and component analysis, which were carried out using Zeiss Ultra Plus scanning electron microscope (SEM) equipped with an energy-dispersive spectroscopy (EDS) detector system.

Table 4 Impactor properties

Impactor stage no.
1	2	3	4	5	6	7	8	9	10	11	12	13
Aerodynamic D50% (μm)
0.023	0.030	0.050	0.098	0.211	0.317	0.576	0.891	1.505	2.243	3.758	6.285	9.314
Aerodynamic Di (μm)
0.026	0.039	0.070	0.144	0.259	0.427	0.716	1.158	1.837	2.903	4.860	7.651

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3. Results and discussion

3.1 The desulfurization wastewater evaporation time

The desulfurization wastewater evaporation process is a complicated heat and mass transfer process. In order to calculate the evaporation time of the droplet, the following assumptions were made: (1) spherical droplets approximation, internal uniform; (2) droplet evaporation in a state of balance; (3) the thermal radiation effects were not considered; and (4) the flue space is infinite.

The droplet evaporation process was divided into two stages: heat absorption and evaporation. When the droplet temperature was lower than the critical evaporation temperature, the droplet absorbed heat from the flue gas to achieve the critical evaporation temperature; however, mass exchange did not occur between the droplets and the smoke flow. The convective heat balance equation for the droplets evaporation is as follows:¹⁴

where T_p is the instantaneous temperature of the droplet (K); m_p is the quality of the droplets (kg); c_p is the heat capacity of the droplet (J kg⁻¹ K⁻¹); T_∞ is the temperature of the flue gas (K); h is the convective heat transfer coefficient (W m⁻² K⁻¹); and A_p is the surface area of the droplet (m²).

When the instantaneous temperature of the droplet was higher than the critical evaporation temperature, the temperature of the droplet remained unchanged during the process of boiling (equal to the boiling point temperature). The boiling evaporation rate equation is as follows:¹⁵

where c_p,∞ is the heat capacity of the flue gas (J kg⁻¹ K⁻¹).

When calculating the parameters selected for the experimental conditions, the natural parameters of flue gas are as shown in Table 5, whereas the natural parameters for the wastewater droplets are shown in Table 6.

Table 5 Natural parameters of flue gas

T∞ (K)	ρ (kg m^−3)	μ (kg m^−1 s^−1)	k∞ (W m^−1 K^−1)	c∞ (J kg^−1 K^−1)
423	0.868	22.7 × 10^−6	3.53 × 10^−2	1084

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Table 6 Natural parameters of wastewater droplets

Tp (K)	cp (J kg^−1 K^−1)	kp (W m^−1 K^−1)	ρp (kg m^−3)	μp (kg m^−1 s^−1)	Tpvap (K)	Tpbp (K)
323	2890	0.570	1228	2.5 × 10^−3	373	380

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Based on the main components of the desulphurization wastewater, as well as the colligative properties of the dilute solution, the droplet evaporation time becomes longer with an increase in the concentration of the solution. Evaporation times for the different diameters of droplets were calculated and are shown in Fig. 2.

Fig. 2 Evaporation times for different diameters of droplet.

On experimenting with double liquid atomization nozzle having an average size of 40 microns, the evaporation time was about 0.215 s, and the retention period of the droplet in the evaporation chamber was about 0.8 s under typical conditions, which means that the droplet can be completely evaporated within the chamber.

3.2 Effect on the properties of particles

3.2.1 Particle size distribution before and after the wastewater evaporation. The concentrations of the fine particles, before and after the evaporation, in the chamber were measured by the ELPI, and the evaporation chamber temperature and wastewater flow rate were set at 150 °C and 15 L h⁻¹, respectively. As shown in Fig. 3, the fine particles size distribution demonstrated two different peaks: a sub-micron peak at 0.114 μm and a coarse peak at 0.956 μm, displaying a bimodal concentration distribution. The peak values are 9.9 × 10⁴ cm⁻³ and 2.3 × 10⁴ cm⁻³, which are similar to the flue gas distribution obtained from the coal-fired power plants.¹⁶ In comparison with the original gas, after injecting the desulfurization wastewater, the size distribution obviously shifted to the bigger value and displayed a unimodal concentration distribution. The peak was at approximately 0.5 μm and number concentration was 7.9 × 10⁴ cm⁻³. This is because the desulfurization wastewater formed a large number of small droplets by the two-fluid atomization nozzle. The particles and droplets collided and agglomerated with each other in the evaporation chamber due to the inherent cohesiveness of the particle surfaces caused by the van der Waals' interactions or the capillary attraction. At high temperature, the liquid bridge force between the particle surfaces changed into a solid bridge force.^17–19

Fig. 3 Change of diameter distribution of particles.

3.2.2 Morphology and elements. Fig. 4 shows the micrographs and the elements of the particles at the inlet and outlet of the evaporation chamber. The evaporation chamber temperature and wastewater flow rate were set at 150 °C and 15 L h⁻¹, respectively. As indicated in Fig. 4(a) without wastewater evaporation, the particles were uniform in size with a spherical appearance and the main elements were Si, Ca, and Al, together with lesser proportions of Na, S, K, and Mg. After the desulfurization wastewater evaporation, the particle elements changed and the morphology became markedly distinct from that observed before the evaporation. Most of the micrometer particles gathered together into bigger particles, presenting an aggregated flocculate. The concentration of the elements Ca, Cl and O distinctly increased. Before the desulfurization, the fine particles were primarily fly ashes from coal combustion. The desulfurization wastewater evaporation and inorganic impurities condensed on the surface of the fly ash. To confirm this phenomenon, the micrographs of the fly ash samples were studied using SEM. It can be observed from Fig. 4(a) that the fly ash sample contained spherical, nearly spherical, and irregularly shaped particles. However, when desulfurization wastewater evaporation, the (Fig. 4(b)) particles adhered by some flocculate was observed.

Fig. 4 SEM micrographs of and the elements in particles of wastewater evaporation.

3.3 Enhancement of the ESP removal efficiency by DWES

Fig. 5 shows the mass and number concentration at the ESP outlet. The addition amount of the desulfurization wastewater was 15 L h⁻¹, and the flue gas temperature at the position of addition was 150 °C. The operating voltage of the ESP was 50 kV. The number concentration of the electric outlet particles was 6.1 × 10⁶ cm⁻³ and the mass concentration was 59 mg cm⁻³. After opening the DWES, the particle number concentration and mass concentration at the electric outlet reduced to 5.4 × 10⁶ cm⁻³ and 38 mg cm⁻³, respectively. This is because the evaporation of the desulfurization wastewater increased the size of the fine particles; therefore, the larger particles picked up the charges at a faster rate than the smaller particles. The electric charged mechanism for the fine particles was divided into two kinds. One is by ions under the action of an electric field force, impact and adhesion on the dust particles, referred to as the electric field charged. Another kind is caused by the thermal motion of chaotic spread of colliding ion and dust particles, and adhesion on dust particles makes the dust charged; this charged way is called diffusion charge.^20–22 The particles with sizes greater than 1 μm mainly charged by electric field. On the particle size less than 0.1 μm microns particles mainly charged by diffusion charged. After injecting desulfurization wastewater, fine particles coalescence, which enhanced the number of charged particles in the electric field, as shown in Fig. 3. Moreover, the inorganic precipitation (such as CaCl₂·2H₂O, MgSO₄·H₂O, NaCl) generated by the evaporation of desulfurization wastewater can reduce the dust ratio resistance. The flue gas temperature was reduced and the humidity was increased, which also had an effect on reducing resistance of fly ash. The flue gas relative humidity increased in the electric field, which can absorb a large number of electronic, the negatively molecules charged into slow negative ions, the number of free electrons was decreased, ionization intensity was abated, the corona current decreased, and breakdown voltage was higher, avoid the phenomenon of back corona premature effectively, thus increasing the surface density current under the optimal working voltage, improved the electric dust removal efficiency of PM_2.5.

Fig. 5 The mass and number concentration at ESP outlet.

The experiment of classification removing efficiency was defined as the level changes in a certain level of particle number concentration and the initial state when the percentage of the particle number concentration ratio can be written as follows:

where η_Ni is the resolved efficiency, N_i0 is the initial state particle number concentration, and N_it is the removal of particle number concentration at the outlet of the ESP. Fig. 6 shows the ESP grade removal efficiency, which can be described as with desulfurization wastewater evaporation or not in the evaporation chamber outlet. The evaporation chamber temperature and wastewater flow rate were set to 150 °C and 15 L h⁻¹, respectively. The removal efficiency increased rapidly with the increase of particle sizes from 0.01 to 0.1 μm. This phenomenon was due to the low 0.1–1 μm particle charge power. Charged particles in the filter mechanism were mainly divided into field charged and diffusion charged; moreover, with the increase of particle size, the field charged proportion increased and the diffusion of charged proportion decreased, but the integrated performance for particle sizes in the 0.1 to 1 μm range of charged particles was weakest.²³ After desulfurization wastewater evaporation, the removal efficiency of the particles between 0.1 and 1 μm increased from 70% to 85%. Each section particle number concentration range was determined by diffusion coagulation, deposition, collision agglomeration and capture. The size range from 0.07 to 0.1 μm was also reduced by Brownian coagulation, which is noticeable at high number concentration. Note that the diffusion of capture efficiency is calculated as follows:¹⁴where u is the air flow velocity (m s⁻¹); D_c is the particle diameter (m); and D is the diffusion coefficient (m² s⁻¹). Using eqn (7), the particle capture efficiency can be obtained with the increase of pe. By eqn (8), pe was proportional to the particle diameter and inversely proportional to diffusion coefficient. Under the same working conditions, the properties of the smaller particle exhibited a large diffusion number and a small pe number, and therefore particles with size under 0.1 μm microns had higher efficiency. Due to their inertia collision effect, for larger particles, the capture efficiency was also higher. For a certain particle size, the change of particle number concentration was mainly decided by the following aspects: small size of the particles due to the collision reunion or growth of the particle size. The diameter of fine particles increased due to the reunion or generation of new particles of large size, whereas the size of particles was arrested because of inertia or diffusion. Two kinds of effect makes the particle number concentration decrease. Therefore, for a certain section, particle number concentration changes mainly depends on the several kinds of effect the result of a joint.

Fig. 6 Grade removal efficiency of fine particles by the ESP.

3.4 Factors influencing the ESP removal efficiency

3.4.1 The operating voltage. Fig. 7 shows the number concentration of particles at the ESP outlet and removal efficiency under different voltages. For the desulfurization wastewater with a flow of 10 L h⁻¹, collection efficiency started at 40.5% at 10 kV, and with increasing voltage the removal efficiency enhanced to 98.5% at 50 kV. The efficiency increased with an increase in the voltage, and the efficiency was closely related to the particle charge and its residence time inside the ESP. On increasing the operating voltages in the ESP, the electric field became stronger. This results in faster charging kinetics as well as higher migration velocities; therefore, the collection efficiency improved. Fig. 7 also suggests that vapor has important effects on the particles; the collection efficiency of particles increased significantly with an increase in the relative humidity. The collection efficiency increases from 40.5% to 50% when the wastewater flow was increased from 10 L h⁻¹ to 20 L h⁻¹ at 10 kV. However, the relative humidity had a little influence on the collection efficiency in high voltage. Because H₂O molecules became charged due to the interaction with the electrons, and the ion mobility decreased, which caused diffusion charging that enhanced especially for diameters smaller than 2.5 μm. Moreover, the increase of relative humidity benefited the agglomeration of particles. Thus, the collection efficiency was greatly improved with the increase of relative humidity.

Fig. 7 The ESP removal efficiency under different voltages by DWES operation.

3.4.2 Wastewater flow rate. The evaporation flow of desulfurization wastewater significantly influenced the particle resistivity and ESP removal efficiency. The evaporation flow of wastewater was adjusted steadily from 5 L h⁻¹ to 20 L h⁻¹, and the remaining experimental parameters were the same as that in Section 2.1. Fig. 8 shows the PM_2.5 removal efficiency under various flow conditions. The removal efficiency increased with the increase of wastewater flow, improving from 75.32% to 94.88%. The major reason was that the increase of the wastewater flow reduced the dust resistivity; therefore, fine particles were more likely to be captured by ESP. Moreover, the increase in the content of water vapor in the flue gas can improve the operation of the electrostatic precipitators' volt–ampere characteristics. The abovementioned two aspects comprehensively promote effects that enhance the electric dust removal efficiency (Fig. 9).

Fig. 8 The ESP removal efficiency under different wastewater flows.

Fig. 9 The dust rate resistance at different temperatures by DWES operation.

To explore the wastewater flow influence on the fly ash resistance, the wastewater flow was adjusted between 10 L h⁻¹ and 20 L h⁻¹. The other operating parameters were the same as those listed in Section 2.1. The flue gas temperature was adjusted from 100 °C to 150 °C. Fig. 8 shows the dust resistivity under different temperature conditions, and the dust resistivity increased from 7.2 × 10⁹ to 1.8 × 10¹¹ Ω cm with an increase in temperature. The greater the wastewater flow, the lower the decrease in the resistance. Generally, fly ash absorbs moisture and forms liquid film on the surface in wet flue gas. The liquid film increased the surface conductivity, and hence the fine particles exhibited lower resistivity. Moreover, water evaporated gradually and the suspended solids condensed on the particles surfaces, which displayed a good effect on the resistivity. Temperature was a parameter for fly ash electrical resistivity. The dust resistivity is decided by the volume and surface resistivity.^24–27 Volume resistivity was inversely proportional to the working temperature, and surface resistivity was mainly proportional to the temperature. Usually, the volume and surface resistivity were near the maximum at 150 °C.

3.4.3 The atomized droplets diameter. The atomized droplet diameter was of great importance for the heat and mass transfer during the desulfurization evaporation process. In the experiments, the evaporation chamber temperature was set at 150 °C, the wastewater flow was set at 15 L h⁻¹, and the spray nozzle was used with an average particle size between 20 μm and 70 μm. As shown in Fig. 10, the ESP outlet number concentration obviously decreased under small droplets evaporation. In addition, the number concentration reduced from 5.2 × 10⁵ cm⁻¹ to 4.8 × 10⁵ cm⁻¹, and the removal efficiency increased from 8.5% to 19.6% under the small droplets spray evaporation. This was because the small particle size droplets effectively enhanced the collision probability between the fine particles and the wastewater droplets. Moreover, the droplets colliding with the particles can build liquid bridges between these particles surfaces. The liquid bridges can be changed into a solid bridge in high temperature flue gas. When the particle size increased, the number of droplets and the collision probability reduced. On the other hand, according to the numerical results of 3.1, the droplets that completely evaporated need to spend some time in the flue gas; large droplets might not completely evaporate, so the smaller droplet enhanced the ESP removal efficiency. Probably, both the collision improvement and the particle agglomeration finally increased the ESP removal efficiency.

Fig. 10 The ESP removal efficiency for different droplet diameters.

3.5 Effect on the WFGD

The effect of the desulfurization wastewater evaporation on the FGD system was studied. The desulfurization efficiency and pH value of the slurry were measured, as shown in Fig. 11. It can be seen that the desulfurization efficiency displayed a small improvement after starting the DWES. The main reason is that the desulfurization is an exothermic reaction; the desulfurization wastewater evaporation absorbs heat in the flue gas, and it contributes to the desulfurization reaction. Moreover, the pH value of the slurry changed because the desulfurization reaction reduced the pH of the desulfurization slurry, which varied from 5.4 to 5.6. Wastewater evaporation had a little effect on the pH of the desulfurization slurry; however, the humidity of the flue gas increased when the DWES was running, which resulted in more water condensing inside the WFGD tower, which could reduce water consumption.

Fig. 11 Desulfurization efficiency and slurry pH value.

4. Concluding remarks

Experimental and theoretical studies were performed to investigate the wastewater evaporation on a laboratory-scale coal-fired thermal state experimental system. The particle properties, ESP removal efficiency and the influence factors were examined at the same time. The results showed that, under the rated operating conditions, the size of the fine particles significantly increased, and the electric outlet particle number concentration and mass concentration reduced to 5.4 × 10⁶ cm⁻³ and 38 mg cm⁻³, respectively, after the wastewater evaporation, ESP removal efficiency of PM_2.5 improved to 92.5%, and ESP grade removal efficiency of the fine particles significantly increased, especially for the particle sizes from 0.1 to 1 μm, which were higher than those in the general ESP. Moreover, with increasing voltage, the removal efficiency increased from 40.5% to 98.5%. When the desulfurization wastewater flow was 10 L h⁻¹ and 20 L h⁻¹, the resistance changed from 1.4 × 10¹¹ to 1.1 × 10¹¹ Ω cm. The condensation of the suspended solids on the particles surfaces had a good effect on reducing the resistivity. An increase in the content of water vapor in the flue gas can improve the ESP volt–ampere characteristics. In addition, the atomized droplet diameter was of great importance during the desulfurization evaporation process. The smaller particle size of the droplets effectively enhanced the collision probability between the fine particles and the wastewater droplets. The increased removal efficiencies of the fine particles improved from 8.5% to 19.6% using the smaller droplets spray nozzle. According to the tests, the wastewater evaporation had a little effect on the pH of the FGD system and the desulfurization efficiency. Importantly, this technology can achieve zero discharge of wastewater and improve the removal efficiency of the ESP.

Acknowledgements

This work was supported by the National Basic Research Program of China (973 Program No. 2013CB228505), the Fundamental Research Funds for the Central Universities, the Ordinary University Graduate Student Scientific Research Innovation Projects of Jiangsu province, China (No. KYLX15-0072) and the Scientific Research Foundation of Graduate School of Southeast University (No. YBJJ1508).

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