Study on the performance of the dust removal efficiency test device

In China, the particulate matter (PM) pollution has caused serious environmental and health concerns, and it is still a huge challenge to control PM pollution. In order to alleviate the hazardous of PM pollution, the dust collector is widely used to separate PM from gas streams. However, after a period of operating, whether the removal efficiency of the dust collector still meet the requirement of the emission limit is unknown. The dust removal efficiency test device was designed and its performance was investigated in this study. The flow field simulation result showed that the test device had a good uniformity, which was consistent with previous measured result. Moreover, it found that the linear relationship between the light scattering concentration and the filtration membrane weighed concentration was y=2.285x, and when the inlet wind speed was about 0.5 m/s, the dust removal efficiency of the dust collector was more than 99.73%, the outlet dust concentration was (0.03-0.04) mg/m3, which was below the emission limit of 15 mg/m3. When the inlet dust concentration was about 200 mg/m3 and the inlet wind speed was (0.25-0.75) m/s, the dust removal efficiency was more than 99.98%. To sum up, the designed test device can be used to estimate the dust removal efficiency of various types of dust collectors.


Introduction
With the rapid development of industrialization and urbanization, air pollution has been causing growing concerns worldwide.Because of the complex structure, diverse sources and intricate evolution mechanism of the PM, it is still a huge challenge to control PM pollution [1].At present, the PM pollution has caused serious environmental and health concerns, such as cardiovascular disease, atherosclerosis.In order to alleviate the hazardous of PM pollution, the emission limit of PM is stipulated in relevant standards in China, such as <<Emission standard of air pollutants for thermal power plants>> (GB13223) execute the the emission limit of 30 mg/m 3 for the coal-fired boiler, and the emission limit is much stricter in the important areas which is 20 mg/m 3 [2].Moreover, <<Emission standard of air pollutants for steelmaking industry>> (GB28664) execute the the emission limit of 20 mg/m 3 for other production equipmentes of new facility, and the emission limit is 15 mg/m 3 for other production equipmentes in the important areas [3].

Materials and methods
In this study, the test device was made up of dust generating device, air blower, air injection nozzle, turbulent zone, test zone, transitional zone, detachable test filter cartridge, back pulse jetting, piezometer, dust concentration meter, removable cylinder, precise filter, flowmeter, draught fan, compress screw, control system, which its operational principle could be seen in the previous literature [5].Moreover, the dust collector was connected with the transitional zone, and fan and air outlet were successively connected with the dust collector.
In order to investigate the feasibility of the test device on evaluating the removal efficiency of the dust collector, the dust dispersion process was simulated firstly.In this study, the dust concentration was measured by the dust concentration meter of light scattering and 5μm of fly ash, and the dust concentration meter was made in Langyi Electromechanical Equipment Co. Ltd., China.The specific test procedure could be found in the previous literature [5].At the end, the dust removal efficiency was calculated by the dust concentration before and after the dust collector.

Flow field simulation of the test device
In this study, SCDM (ANSYS Space Claim Direct Modeler)-Fluent was chose to model the test device, and ICEMCFD (The Integrated Computer Engineering and Manufacturing Code for Computational Fluid Dynamics) was chose to divide the grids.The size of the grid was set as 30 mm, which it was composite grid of tetrahedral and hexahedral, and partial grid was encrypted, and the total number of the grid was 65016 (Figure 1).Meanwhile, the independence verification of grid was implemented to ensure calculation accuracy, and the relative error of the calculated result was 0.3%, which it could ensure that the calculation result is within the allowable range of error.The divided grids were imported into software Fluent for simulating calculation, and the calculated conditions were as follows: the whole calculation was performed in steady state (continuous phase, discrete phase), and the vertical gravity acceleration was considered.SST k-ω model was used to simulate the turbulence motion.Moreover, gas-solid bidirectional coupling calculation was chose, and the discrete phase model (DPM) was selected to simulate the dust diffusion, which the material was ashsolid, the diameter was 10 μm, the flow rate was 0.0000 1kg/s, and the incidence was in the normal direction.The air medium was selected as the gas phase, which adopted the SIMPLE pressure velocity coupling solution method and the second-order wind algorithm, and the residual error, the inlet velocity and the outlet pressure were set at 0.001, 2.0 m/s and 101 KPa respectively.The standard non-slip wall was chose, and the boundary condition for the discrete phase was reflect, and the inlet and outlet were escape.
The axial velocity cloud diagram and the pressure cloud diagram of the test device are shown in Figure 2. It shows that when the inlet velocity is 0.5 m/s, the pressure of the dust increases from 0.024 Pa to 0.050 Pa, which due to the restriction of the compasses tube wall.Meanwhile, the velocity decreases from 0.5 m/s to 0.27 m/s.Because of the space of the cylindrical cavity becomes suddenly larger, the velocity of the downward-moving dust reduce to 0.03 m/s, and the pressure in the calibration area is constant at about 0.030 Pa, which indicates the dust is in the relatively static state.However, the outlet pressure decrease suddenly to 0.019 Pa, and the dust is ejected rapidly at 0.15 m/s.In the view of the axial velocity and the pressure, the distribution of the dust was uniformly in the calibration area.
Similarly, when the inlet velocity is 2.0 m/s, the axial velocity cloud diagram and the pressure cloud diagram of the test device are simulated (Figure 3).It shows that the pressure of the dust increases from -0.84 Pa to 0.47 Pa, and the velocity decreases from 2.0 m/s to 0.44 m/s.At last, the velocity of the downward-moving dust reduce to 0.1 m/s, and the pressure in the calibration area is constant at about 0.37 Pa, and the dust is in the relatively static state.However, the outlet pressure decrease suddenly to 0.068 Pa, and the dust is ejected rapidly at 0.66 m/s.In a word, the distribution of the dust was also uniformly in the calibration area.
The sections of the device can be seen in Figure 4.It can be found that the distance of section A and the calibration area is (0.95-1.50) m.Therefore, different sections were selected to simulate the process of the dust diffusion, which the distance away from section A was 0.  Figure 5 shows that the diffused area of the dust, which blue area represents no diffused area.It indicated that when the distance away from section A was (0.05-0.65) m (Fig. 5 e-j), the diffused area of the dust became gradually larger as they penetrated into the cylinder device.Comparing with the inlet velocity of 2.0 m/s, the diffused speed of the inlet velocity of 0.5m/s was much faster, the reason might be that the bigger the inlet velocity, the faster the dust penetrated into the test device, and resulted in the dust could not remain and diffuse to much farther section.When the dust penetrated into the calibration area of the test device, which the distance away from section A was (0.95-1.35) m, the diffused area of the dust were similar at the inlet velocity of 0.5 m/s and 2.0 m/s, and the dust concentration remained at about 0.013 kg/m 3 (Fig. 5 k,m,o,q,s) and 0.0035 kg/m 3 (Fig. 5 l,n,p,r,t), which proved ulteriorly the diffused dust in the calibration area of the test device was uniform.Some researchers found that the wider the diffusion area and horizontal distribution range of the calibration area, the better the diffused degree [6].In this study, the diffused area at the distance away from section A of (1.05-1.35)m was larger than the diffused area at the distance away from section A of 0.95 m, therefore, the diffusion area in the calibration area of the device was large, indicated that the uniformity of the device was good.Moreover, previous measurement on the uniformity of the test device, which three dust concentrations (110 mg/m 3 , 360 mg/m 3 , 600 mg/m 3 ) at five positions were chose, and the results showed its uniformity was less than 7%, which was consistent with the simulated results [5].(e: 0.5 m/s of inlet velocity and 0.05 m away from section A; f: 2.0 m/s of inlet velocity and 0.05 m away from section A; g: 0.5 m/s of inlet velocity and 0.35 m away from section A; h: 2.0 m/s of inlet velocity and 0.35 m away from section A; i: 0.5 m/s of inlet velocity and 0.65 m away from section A; j: 2.0 m/s of inlet velocity and 0.65 m away from section A; k: 0.5 m/s of inlet velocity and 0.95 m away from section A; l: 2.0 m/s of inlet velocity and 0.95 m away from section A; m: 0.5 m/s of inlet velocity and 1.05 m away from section A; n: 2.0 m/s of inlet velocity and 1.05 m away from section A; o: 0.5 m/s of inlet velocity and 1.15 m away from section A; p: 2.0 m/s of inlet velocity and 1.15 m away from section A; q: 0.5 m/s of inlet velocity and 1.25 m away from section A; r: 2.0 m/s of inlet velocity and 1.25 m away from section A; s: 0.5 m/s of inlet velocity and 1.35 m away from section A; t: 2.0 m/s of inlet velocity and 1.35 m away from section A.)

Evaluation of the dust removal efficiency
Because of the dust concentration was measured by the dust concentration meter of light scattering in this study, but the method of filtration membrane weighing was used widely as the standard traceability method [7], therefore, the congruent relationship between the dust concentration of light scattering and the filtration membrane weighed dust concentration was investigated, which the specific process was as follows: the dust concentration meter of light scattering was placed at the location of 0.2 m away from the wall of the calibration area, which the calibration area was 1.25 m away from section A. The filtration membrane sampling device was located at the same section, and it was 0.1 m away from the light scattering meter.The results could be found in Table 1 and Figure 6, it showed that the linear relationship between the light scattering concentration and the filtration membrane weighed concentration was y=2.285x, and their correlation coefficient was 0.994.Table 2 shows the error between the corrected light scattering concentration and the membrane weighed concentration, and all of their absolute values are not more than 20%, which meet the requirement that the error of the dust concentration meter of <<dust concentration measuring Instrument>> (JJG 846-2015) did not exceed ±20% [8].Therefore, the light scattering concentration could be corrected by this linear relationship, and the corrected light scattering concentration could be traceable to the filtration membrane weighed concentration.In this study, bag filter was used to be as the investigated dust collector, and the dust removal efficiency was selected to be as the performance index of the dust collector, which was calculated by the dust concentration before and after the dust collector.The dust concentration of (10-400) mg/m 3 and various inlet wind speeds were chose to estimate the dust removal efficiency.It could be found that when the inlet wind speed was about 0.5 m/s, the dust removal efficiency was more than 99.70% (Table 3), which met the requirement of the dust removal efficiency of multiple cyclones should be more than 94% in HJ/T 286 [9].Furthermore, the outlet dust concentration of the dust collector was (0.03-0.04) mg/m 3 , which was also below the emission limit of 15 mg/m 3 .Table 4 shows the dust removal efficiencies of different inlet wind speeds.When the inlet dust concentration is about 200 mg/m 3 and the inlet wind speed is (0.25-0.75) m/s, the dust removal efficiency is more than 99.98%.In the future,the dust removal efficiency could be estimated by the test device at various experimental conditions such as the inlet dust concentration, the inlet wind speed and different types of the dust collector.

Conclusion
In this study, the dust removal efficiency test device was designed and its performance was investigated.
The flow field of the test device was simulated by SCDM-Fluent and ICEMCFD, and the result showed that the it had a good uniformity.In addition, the linear relationship between the light scattering concentration and the filtration membrane weighed concentration was obtained and it was y=2.285x.By the measurement of the dust concentration meter of light scattering, it found that when the inlet wind speed was about 0.5 m/s, the dust removal efficiency of the dust collector was more than 99.73%, and the outlet dust concentration was (0.03-0.04) mg/m 3 , which was below the emission limit of 15 mg/m 3 .When the inlet dust concentration was about 200 mg/m 3 and the inlet wind speed was (0.25-0.75) m/s, the dust removal efficiency was more than 99.98%.In a word, the designed test device could be used to estimate the dust removal efficiency of various types of dust collectors.

Figure 1 .
Figure 1.The divided grids of the test device.(a: The inlet of PM; b: Calibration area; c: The outlet of PM) 05 m, 0.35 m, 0.65 m, 0.95 m, 1.05 m, 1.15 m, 1.25 m and 1.35 m respectively.

Figure 2 .Figure 3 .Figure 4 .
Figure 2. The cloud diagram at 0.5 m/s of inlet velocity.(B: The axial velocity cloud diagram; C: the pressure cloud diagram)

Figure 5 .
Figure 5.The cloud diagram of the dust concentration at different inlet velocity and different sections.

Figure 6 .
Figure 6.The linear relationship between the light scattering concentration and the filtration membrane weighed concentration.

3 )
The dust concentration of light scattering meter (mg/m 3 )

Table 1 .
The measured result of the light scattering dust concentration and the filtration membrane weighed dust concentration.

Table 2 .
The error between the corrected light scattering concentration and the membrane weighed concentration.

Table 3 .
The dust removal efficiencies of different dust concentrations.

Table 4 .
The dust removal efficiencies of different inlet wind speeds.