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Electrostatic precipitation under coal pyrolysis gas at high temperatures
Journal Pre-proof Electrostatic precipitation under coal pyrolysis gas at high temperatures
Quanlin Chen, Mengxiang Fang, Jianmeng Cen, Yifei Zhao, Qinhui Wang, Yuwei Wang PII:
S0032-5910(19)31071-X
DOI:
https://doi.org/10.1016/j.powtec.2019.11.108
Reference:
PTEC 14990
To appear in:
Powder Technology
Received date:
18 March 2019
Revised date:
9 November 2019
Accepted date:
26 November 2019
Please cite this article as: Q. Chen, M. Fang, J. Cen, et al., Electrostatic precipitation under coal pyrolysis gas at high temperatures, Powder Technology(2019), https://doi.org/ 10.1016/j.powtec.2019.11.108
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2019 Published by Elsevier.
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Electrostatic precipitation under coal pyrolysis gas at high temperatures Quanlin Chena, Mengxiang Fanga, Jianmeng Cena,1 , Yifei Zhaoa, Qinhui Wanga, Yuwei Wanga a
State Key Lab of Clean Energy Utilization, Zhejiang University, Hangzhou 310027, China
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Abstract
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An experimental-scale electrostatic precipitator (ESP) was built to investigate the influence
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of temperature and gas media on collection efficiency and energy consumption. High temperature was harmful to the performance of the ESP and had a considerable influence on
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large particles. The maximum collection efficiency was lower and energy consumption was
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higher under coal pyrolysis gas media than under air. Two improvement methods, namely, gas
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media conditioning and positive polarity power supply, were studied in this work. Both
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methods can improve the performance of high-temperature ESP under coal pyrolysis gas media. The improvement in collection efficiency under the effect of gas conditioning decreased and that under the effect of a positive power supply increased with the increase in temperature. Gas conditioning and positive power supply increased the maximum collection efficiencies of the ESP at 600 °C under coal pyrolysis gas media by 3.0% and 11.8%, respectively. Key words: high-temperature; ESP; coal pyrolysis gas; gas conditioning; positive power supply. 1 Corresponding author. Tel.: +86-0571-87952205. E-mail address: [email protected]
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1. Introduction High-temperature dust removal technologies are crucial for the development of clean coal utilization technologies, such as pressurized fluidized bed combustion (PFBC) and integrated gasification combined cycle (IGCC) [1-4]. High-temperature dust removal technologies include cyclone separators, ceramic filters, granular filters, and electrostatic precipitators
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(ESPs) [5, 6]. Although cyclone separators have simple structures and are insensitive to high temperatures, they present low removal efficiencies for fine dust particles [7]. Ceramic filters
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have high dust removal efficiencies but have large pressure drops and are fragile. ESPs are
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widely used in conventional coal-fired power plants at operating temperatures of less than
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150 °C because of their high dust removal efficiencies and low pressure drops [7, 8]. Nevertheless, high-temperature electrostatic precipitation requires further exploration.
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Villot et al. [9] established a wire-cylinder ESP for the purification of biomass syngas and
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obtained a collection efficiency of 96% at temperatures above 500 °C. Xiao et al. [10-12]
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found that the dust removal efficiency of a wire-cylinder ESP under air atmosphere can reach 99.6% at the temperature range of 350 °C–700 °C. Xu et al. [13] constructed a wire-plate ESP to study the relationship between temperature and dust removal efficiency and reported that dust removal efficiency decreased from 99.8% to 95% as temperature increased from 300 K to 900 K. Gu et al. [14, 15] developed a new type of ESP based on thermionic emission cathodes and tested the thermionic emission properties of three cathodes. The ESP achieved a dust removal efficiency of more than 90% at temperatures above 673 K. The discharge characteristics of a coal pyrolysis gas atmosphere are different from those of an air atmosphere and a conventional flue gas atmosphere and affect particle charging and
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removal by ESPs [16, 17]. Thus, this study aims to investigate the characteristics of electrostatic precipitation under coal pyrolysis gas media at high temperatures. The results of this work will be beneficial for the design and application of high-temperature ESPs in IGCC, PFBC, and coal polygeneration systems.
2. Experimental setup
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2.1 ESP
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The schematic of the experimental system is shown in Fig. 1. The system comprises five
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parts, namely, an ESP, a high-temperature gas generator, a particle feeding and measuring
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system, a high-voltage DC power supply, and an electrical parameter measurement system. The ESP is mainly composed of a high-voltage electrode, a low-voltage electrode, and an
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insulator. The high-voltage electrode is a stainless steel wire with a length of 1,200 mm and a
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diameter of 2 mm. The low-voltage electrode is a stainless steel tube with a length of 1,200
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mm with an internal diameter of 100 mm. The electrode distance of the device is 49 mm. Electrical insulation remains challenging at high temperatures because the performance of insulating materials deteriorates as temperature increases. In this work, a corundum insulator is used as the insulating material between the high- and low-voltage electrodes.
2.2 Particle feeding and measurement The particle feeding and measuring system consists of an electromagnetic vibrating feeding device and a PM10 impactor. The feeding rate of the electromagnetic vibrating feeding device is 1–5 g/min. Particle concentration in the gas flow is sampled by the PM10 impactor and
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measured by an electronic balance. The particle concentration sampling point is located at the outlet of the ESP. Many particles that pass through the ESP are removed by inertial force. The particle concentration sampling point is set at the outlet of the dust collector to eliminate the influence of inertial force. The particle concentration at the sampling point is called the inlet
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concentration of the ESP under the current voltage.
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concentration of the ESP when the output voltage is 0 kV and is called the outlet
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The particles used in the experiments are fly ash produced by a coal pyrolysis
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polygeneration system. Table 1 provides the chemical composition of the particles. The
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resistivity of ash is measured by a high-temperature particle resistivity meter over the temperature range of 100 °C to 700 °C. As shown in Fig. 2(a), the maximum resistivity of ash
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is 5.33 × 106 Ω × cm at the temperature of 100 °C. Resistivity decreases to 4.86 × 103 Ω ×cm
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as the temperature increases to 700 °C. Particle size distribution is measured by using a
38.017 μm.
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Mastersizer 2000 apparatus. As shown in Fig. 2(b), the median particle size is approximately
2.3 Electric circuit
The high-voltage negative power supply provides an adjustable DC negative voltage of 0– 60 kV and has a maximum output current of 20 mA. In this work, the voltage supplied by a high-voltage power supply is called the output voltage and that between the anode and cathode is called the port voltage, which is measured by a high-voltage probe and an Agilent 34970A data acquisition system. A 0.8 MΩ resistor is used to protect the high-voltage power
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supply in the case of spark breakdown. The discharge current is measured by the Agilent 34970A data acquisition system, which has a measurement accuracy of 1 nA.
2.4 Experimental procedure The coal pyrolysis gas used in this work is a mixture of pure gas. The composition of coal pyrolysis gas is shown in Table 2. The gas components are measured by using an Agilent
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7890A gas chromatography system. Air and N2 are also investigated for comparison with coal
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pyrolysis gas. A mass flow controller is used to adjust the gas flow rate such that the flow rate
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in the ESP is approximately 0.12 m/s. That is, the residence time of the dust-containing gas in the ESP is 10 s.
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The testing temperatures on each gas media are set from 400 °C to 700 °C with a 100 °C
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interval. The inlet particle concentration is controlled to 1 g/Nm3 by adjusting the feeder.
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follows:
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The definitions of the collection efficiency and energy consumption index are expressed as
=
min mout P , min min mout ,
where η is the collection efficiency; min is the inlet mass concentration of particles, g/Nm3 ; mout is the outlet mass concentration of particles, g/Nm3 ; φ is the energy consumption index, W/(g/Nm3 ); and P is the electric power consumed by the ESP, W. Certain tests are repeated to check the repeatability of the experimental data. Results highlight the positive performance of the experimental system.
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3. Results and discussion The influence of temperature and atmosphere on the collection efficiency and energy consumption of ESP was studied to obtain a comprehensive understanding of high-temperature ESP under coal pyrolysis gas media. Discharge is a fundamental process for ESP and has an important impact on subsequent particle charging and removal. Therefore, the
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effect of temperature and atmosphere on the discharge process was briefly described in this work. The abilities of two improvement methods, namely, gas media conditioning and
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positive polarity power supply, to address the low collection efficiency and high energy
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3.1 Effect of temperature
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consumption of high-temperature ESP under coal pyrolysis gas media were tested.
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3.1.1 Effect of temperature on discharge
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The discharge current correspondingly increases with the increase in output voltage, as shown in Fig. 3. The increase in temperature can promote gas discharge. As temperature increases, the corona onset voltage decreases, whereas the discharge current increases. However, the corona onset voltage decreases from 10.1 kV to 8.3 kV as temperature increases from 400 °C to 600 °C. At an output voltage of 12 kV, the discharge current increases from 259 μA at 400 °C to 681 μA at 600 °C. High temperature enlarges the mean free path of electrons and gas molecules and helps electrons gain additional energy from the imposed electric field. Thus, the possibility of inelastic collisions between electrons and gas molecules increases. As a result, additional electron-positive ion pairs are produced. The production of
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excess electron-positive ion pairs, in turn, reduces corona onset and spark breakdown voltages, as well increases the discharge current at the same voltage [18]. Furthermore, the reduction in the work function of the cathode by the high temperature of the cathode surface results in additional thermionic electrons and increases current.
3.1.2 Effect of temperature on dust removal efficiency
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Fig. 4 shows that the dust removal efficiency at each temperature can be roughly divided
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into two stages with the increase in voltage. At the first stage, the voltage is low, the discharge
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current is small, and dust removal efficiency increases with the increase in voltage. At the second stage, the voltage is high, the discharge current is large, and collection efficiency
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remains nearly unchanged with the increase in voltage. To illustrate, dust removal efficiency
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at 500 °C increases from 50.12% to 96.54% when output voltage increases from 6 kV to 10
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kV and from 96.54% to 98.85% as the output voltage increases from 10 kV to 18 kV.
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These results are attributed to the different distributions of space charge and electric field at various voltages. As described in Section 3.1.1, the discharge current correspondingly increases with the increase in voltage. This effect increases the probability that particles are charged. In addition, particle migration velocity increases with the increase in voltage because of electric field intensity and the Coulomb force exerted on charged particles [19]. According to Deutsch’s formula in Eq. (1), collection efficiency η increases as particle migration velocity increases. 𝜂 = 1−𝑒
𝐴 𝑄
− 𝑤
,
(1)
where w is the particle migration velocity, A is the area of the collection tube in the ESP, and
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Q is the gas flow in the ESP. Moreover, as electric field intensity increases, the saturated charge of particles increases. In this work, field charging is dominant. Thus, the saturated charge of a particle can be written as follows [20]: 𝑞=
3𝜀 𝑟 𝜀 𝑟+2
𝜋𝜀0 𝑑2 𝐸,
(2)
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where ε0 is the permittivity of the free space, εr is the relative permittivity of gas media, d is
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the diameter of the particle, and E is the intensity of the electric field.
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Fig. 5 shows the variation in dust removal efficiency with temperature. At the first stage,
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dust removal efficiency increases with the increase in temperature. At a low output voltage,
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the discharge current is small and the particles have yet to reach the saturated state of charge. The current and particle charge increase with the increase in temperature. These effects
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increase dust removal efficiency. For example, when the output voltage is 8.5 kV, dust
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removal efficiency increases from 66.86% to 93.28% when temperature increases from
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400 °C to 700 °C. However, dust removal efficiency decreases with the increase in temperature at the second stage. The discharge current is large and particles have reached the saturated charge state because the output voltage is high at this stage. Thus, a high current at high temperature does not improve particle charge. Furthermore, high current results in low collection efficiency because it leads to the high voltage of protective resistance and the low port voltage of the ESP. To illustrate, when the output voltage is 18 kV, the discharge current increases from 1458 μA to 3217 μA, the port voltage decreases from 16.8 kV to 15.4 kV, and dust removal efficiency decreases from 99.65% to 96.01% as the temperature increases from 400 °C to 600 °C.
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Fig. 6 depicts the particle collection efficiency and migration velocity at different temperatures for various particle size ranges, namely, PMd > 10 (particles with an aerodynamic diameter of more than 10 μm), PM10 > d > 2.5 (particles with an aerodynamic diameter between 2.5 and 10 μm), and PMd < 2.5 (particles with an aerodynamic diameter of less than 2.5 μm). Particle migration velocity w is calculated with Deutsch’s formula as follows: 𝐴
𝑤 = − ln( 1 − 𝜂).
(3)
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𝑄
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The following results were obtained for the three particle size ranges: At 400 °C, the
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collection efficiencies for PMd > 10 , PM10 > d > 2.5 , and PMd < 2.5 are 99.92%, 98%, and 90%,
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respectively. The collection efficiencies for PMd > 10 , PM10 > d > 2.5 , and PMd < 2.5 decrease to
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96.44%, 95%, and 82%, respectively, as the temperature increases to 600 °C. The trends shown by migration velocity are the same as those shown by collection efficiency: They
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decrease from 17.83, 9.78, and 5.76 mm/s to 8.34, 7.49, and 4.29 mm/s, respectively. The
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change in the migration velocity of large particles is more substantially than that in the
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migration velocity of small particles. The increase in temperature enhances the thermal motion of small particles. This effect enables small particles to efficiently agglomerate and reduce their number. The migration velocity of small particles decreases at a slower rate at high temperature than that of large particles because of the agglomeration effect.
3.1.3 Effect of temperature on energy consumption Fig. 7 depicts the variation in energy consumption index with voltage. As the temperature increases from 400 °C to 600 °C, the energy consumption index increases from 3.08 W/(g/Nm3 ) to 8.14 W/(g/Nm3 ) at an output voltage of 12 kV. According to Eq. (4), the energy
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consumption index is proportional to the current. The increase in temperature and voltage can increase the discharge current, which results in an increase in the energy consumption index.
Up I P = min mout min mout ,
(4)
where Up is the port voltage of the ESP. In summary, the increase in voltage will increase dust removal efficiency by strengthening
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particle charge and increasing migration velocity. However, the increase in dust removal efficiency as a result of increasing output voltage is limited. At the first stage, increasing
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voltage can drastically improve dust removal efficiency. At the second stage, however,
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increasing voltage negligibly affects dust removal efficiency but significantly increases
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energy consumption. Dust removal efficiency and energy consumption at this stage should be comprehensively considered.
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Fig. 8 displays the energy consumption index and outlet mass concentration at 500 °C
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when the output voltage ranges from 8.5 kV to 18 kV. China’s environmental protection
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requirements state that outlet mass concentration should not exceed 20 mg/Nm3 . An output voltage of 10.6–18 kV is preferred when the outlet mass concentration is less than 20 mg/Nm3 . The upper limit of the energy consumption index is set as 10 W/(g/Nm3 ) to save energy. Thus, the optimal voltage range that can meet the requirements of outlet mass concentration and energy consumption is 10.6–13 kV.
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3.2 Effect of gas media 3.2.1 Effect of gas media on discharge The V–I characteristics of air, N 2 , and coal pyrolysis gas at temperatures of 400 °C and 600 °C are compared in Fig. 9. A corona is present during air discharge but not during N2 discharge [12, 16, 21]. At 400 °C, N2 discharges at 11.5 kV and quickly breaks down because
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it is a nonelectronegative gas medium and cannot attach electrons. The N2 discharge current is
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higher than the air discharge current because of the difference in discharge types. To illustrate,
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the discharge currents of air and N2 at 400 °C and 12 kV output voltage are 259 and 6,480 μA, respectively. In addition, the air discharge current is composed of negative ions and electrons,
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whereas the N2 discharge current is mainly composed of electrons. These results suggest that
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the dust removal efficiency of N2 is extremely lower than that of air. By contrast, energy
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consumption under N2 is higher than that under air.
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The main components of coal pyrolysis gas, such as CH 4 , H2 , and N2 , are nonelectronegative gas media. Therefore, the discharge characteristics of coal pyrolysis gas are similar to those of N2 . However, the starting discharge voltage under coal pyrolysis gas is lower than that under N2 . At 600 °C, the coal pyrolysis gas discharges at 3 kV, whereas N2 discharges at 9.67 kV.
3.2.2 Effect of gas media on dust removal efficiency The collection efficiencies of air, N2 , and coal pyrolysis gas at different temperatures are compared in Fig. 10. Regardless of temperature and output voltage, dust removal efficiencies
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are ranked as follows: air > coal pyrolysis gas > N 2. These results can be attributed to the differences in discharge characteristics. In contrast to air discharge, a corona is absent during N2 discharge, and the discharge current is mainly composed of electrons. The lower charge efficiency of electrons on particles than that of ions results in low dust removal efficiency under N2 . Coal pyrolysis gas is mainly composed of nonelectronegative gas, such as CH 4 , H2 ,
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and N2 . It also contains electronegative gas, such as CO 2. Therefore, the dust removal
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efficiency of coal pyrolysis gas is between that of air and N2 . For example, the collection
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efficiencies of air, N2 , and coal pyrolysis gas at 400 °C and 12 kV output voltage are 99.12%,
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83.34%, and 90.56% respectively.
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With the increase in temperature, the difference between coal pyrolysis gas and N2 narrows, whereas that between coal pyrolysis gas and air widens. At an output voltage of 18 kV and a
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temperature of 400 °C, the collection efficienc ies of air, N2 , and coal pyrolysis gas are 99.65%,
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83.52% and 90.23%, respectively. Thus, the difference between the dust removal efficiencies
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of coal pyrolysis gas and N2 is 6.71% and that between the dust removal efficiencies of coal pyrolysis gas and air is 9.42%. As the temperature increases to 600 °C and the output voltage is maintained at 18 kV, the difference in dust removal efficiency between coal pyrolysis gas and N2 decreases to 0.55%, whereas that between coal pyrolysis gas and air increases to 19.03%.
3.2.3 Effect of gas media on energy consumption The energy consumption of air, N 2 , and coal pyrolysis gas at various temperatures are compared in Fig. 11. As discussed in Section 3.2.1, the discharge characteristics of coal
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pyrolysis gas are similar to those of N2 . Thus, the energy consumption of coal pyrolysis gas is similar to that of N2 . The energy consumption of N 2 is lower than that of coal pyrolysis gas at output voltages of 8.5 and 10 kV and at a temperature of 400 °C because N2 has yet to be discharged. The energy consumption of coal pyrolysis gas is considerably larger than that of air given
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the difference in discharge type and current composition. For example, the energy
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consumption indexes of coal pyrolysis gas and air at a temperature of 600 °C and output
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voltage of 12 kV are 58.35 and 8.14 W/(g/Nm3 ), respectively.
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In summary, the dust removal efficiency of coal pyrolysis gas is lower than that of air,
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whereas the energy consumption of coal pyrolysis gas is higher than that of air because of differences in discharge type and current composition.
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3.3 Methods for the improvement of dust removal
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The low collection efficiency and high energy consumption of high-temperature ESP under coal pyrolysis gas media are identified as problems on the basis of the results presented above. This work aimed to solve these problems by improving the performance of the ESP through gas conditioning and positive polarity power supply.
3.3.1 Gas media conditioning Table 3 lists the composition of conditioned coal pyrolysis gas. The contents of H2 , CH4 , and other combustible gases in conditioned coal pyrolysis gas have decreased from 82% to 60% relative to those in coal pyrolysis gas. The CO2 content of conditioned coal pyrolysis gas is 12%
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higher than that of coal pyrolysis gas. Gas media conditioning can improve dust removal efficiency to a certain extent. However, the improvement in dust removal efficiency under the effect of gas media conditioning decreases as temperature increases, as shown in Fig. 12. At 400 °C, the maximum collection efficiency of conditioned coal pyrolysis gas is 96.54%, which is 6.02% higher than that of
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coal pyrolysis gas. The difference between the maximum dust removal efficiencies of the
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conditioned coal pyrolysis and coal pyrolysis gases decreases to 3% as the temperature
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increases to 600 °C.
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The energy consumption indexes of coal pyrolysis gas and conditioned coal pyrolysis gas
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are compared in Fig. 13. The energy consumption index of conditioned coal pyrolysis gas is slightly lower than that of coal pyrolysis gas. However, the difference in energy consumption
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between conditioned coal pyrolysis gas and coal pyrolysis gas decreases with the increase in
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temperature. The energy consumption indexes of conditioned coal pyrolysis gas and coal
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pyrolysis gas are 45.27 and 49.35 W/(g/Nm3 ), respectively, at a temperature of 400 °C and an output voltage of 12 kV. The difference between the energy consumption indexes of conditioned coal pyrolysis gas and coal pyrolysis gas decreases from 4.08 W/(g/Nm3 ) to 1.88 W/(g/Nm3 ) as temperature increases from 400 °C to 600 °C and the output voltage is maintained at 12 kV. The V–I characteristics of coal pyrolysis gas and conditioned coal pyrolysis gas are compared in Fig. 14. Conditioned coal pyrolysis gas has stronger electronegativity, higher initial discharge voltage, and lower current under the same voltage than coal pyrolysis gas because the CO2 content of conditioned coal pyrolysis gas is higher than that of coal pyrolysis
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gas. At 400 °C, coal pyrolysis gas begins to discharge at 4.5 kV, whereas conditioned coal pyrolysis gas begins to discharge at 5.07 kV. The discharge currents under coal pyrolysis gas and conditioned coal pyrolysis gas are 6,863 and 6,046 μA, respectively, at a temperature of 400 °C and an output voltage of 12 kV. The difference in discharge characteristics between coal pyrolysis gas and conditioned coal pyrolysis gas decreases with the increase in
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temperature. As the temperature increases from 400 °C to 600 °C and the output voltage is
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maintained at 12 kV, the difference in discharge current between coal pyrolysis gas and
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conditioned coal pyrolysis gas decreases from 817 μA to 8 μA.
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Gas conditioning improves collection efficiency and energy consumption mainly by
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increasing CO2 content. CO2 is an electronegative gas medium that can attach electrons to form negative ions. Negative ions charge particles more efficiently than electrons. Therefore,
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the dust removal efficiency of conditioned coal pyrolysis gas is higher than that of coal
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pyrolysis gas. In addition, negative ions are considerably less mobile than electrons. This
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characteristic accounts for the lower discharge current of the conditioned coal pyrolysis gas compared with that of coal pyrolysis gas. Thus, the energy consumption of conditioned coal pyrolysis gas is lower than that of coal pyrolysis gas. The mean free path of gas molecules and electrons increases and the ability of CO 2 to attach electrons decreases with the increase in temperature. Therefore, the improvement in collection efficiency and energy consumption under the effect of gas media conditioning is weakened.
3.3.2 Positive polarity power supply Nonelectronegative gases, such as CH 4 , H2 , and N2 , account for 82% of the pyrolysis gas
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content. Under nonelectronegative gas media, the current is mainly composed of electrons because they cannot attach electrons during conventional negative discharge [18, 22]. The abnormally large discharge current can be ascribed to the fact that the charge mobility of electrons is approximately 1,000 times higher than that of ions. In addition, inadequate particle charge and low dust removal efficiency can be attributed to the lower charge
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efficiency of electrons on particles than that of ions. That is, the existence of the electronic
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current results in low collection efficiency and high energy consumption [16]. However, the
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charge in the drift region is mainly composed of ions during positive discharge [17]. Inspired
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by the different current compositions during positive and negative discharges, this study aims
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to use a positive polar power supply to improve the dust removal efficiency and energy consumption of high-temperature ESP under coal pyrolysis gas media.
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The positive and negative discharge characteristics under coal pyrolysis gas media are
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compared in Fig. 15. The initial discharge voltage and current of positive discharge are lower
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than those of negative discharge. Negative and positive discharges begin at 4.53 and 8.9 kV, respectively, at 400 °C. The negative and positive discharge currents of coal pyrolysis gas are 6,863 and 2,348 μA, respectively, at a temperature of 400 °C and an output voltage of 12 kV. In addition to these differences, a corona exists during positive discharge, and the discharge current is mainly composed of ions. The collection efficienc ies of negative and positive ESPs are compared in Fig. 16. At a temperature of 400 °C and an output voltage of 8.5 kV, the collection efficiency of positive ESP is 65.34%, which is lower than that of negative ESP. This difference may be attributed to the positive ESP, which has yet to discharge. Except for this condition, the collection
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efficiency of positive ESP is higher than that of negative ESP. At 400 °C, the maximum dust removal efficienc ies of positive and negative ESPs are 96.18% and 90.52%, respectively. When the temperature rises to 600 °C, the maximum dust removal efficiencies of positive and negative ESPs decrease to 88.92% and 77.12%, respectively. Therefore, the difference between the maximum dust removal efficienc ies of positive and negative ESPs increases from
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5.66% to 11.8%. That is, a high temperature will enhance the improvement effect of the
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positive power supply on dust removal efficiency under coal pyrolysis gas media.
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The energy consumption indexes of negative and positive ESPs are compared in Fig. 17.
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Energy consumption by the positive ESP is lower than that by the negative ESP because the
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positive discharge current is lower than the negative discharge current. At a temperature of 600 °C and an output voltage of 10 kV, the energy consumption indexes of the positive and
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negative ESPs are 17.01 and 39.54 W/(g/Nm3 ), respectively.
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A positive polarity power supply improves dust removal efficiency and energy
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consumption mainly because of the differences in discharge type and current composition between positive and negative discharges. During negative discharge under coal pyrolysis gas media, the corona is nearly absent and the current is mainly composed of electrons. However, the corona exists and the current is mainly composed of ions during positive discharge [17]. With the increase in temperature, the improvement in collection efficiency under the effect of gas conditioning decreases, whereas that under the effect of the positive power supply increases. This finding indicates that positive power supply may be the superior method for improving the performance of high-temperature ESP under coal pyrolysis gas media.
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4. Conclusions This work investigates the influence of temperature and gas media on the collection efficiency and energy consumption of ESP. It also explores the improvement in collection efficiency and energy consumption under the effect of gas media conditioning and a positive polar power supply. The following conclusions are drawn: As temperature increases, the maximum dust removal efficiency decreases, whereas
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1.
energy consumption increases. Under air atmosphere, the maximum collection efficiency
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decreases from 99.65% to 96.01%, whereas the energy consumption index increases
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from 24.58 W/(g/Nm3 ) to 51.6 W/(g/Nm3 ) as the temperature increases from 400 °C to
influence on large particles.
The maximum collection efficiency under coal pyrolysis gas media is lower than that
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2.
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600 °C. Temperature has diverse effects on different particle sizes and a considerable
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under air. By contrast, energy consumption under coal pyrolysis gas media is higher than
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that under air. At 400 °C, the maximum collection efficiencies under air and coal pyrolysis gas are 99.65% and 90.52%, respectively, and the corresponding energy consumption indexes are 24.58 and 49.35 W/(g/Nm3 ), respectively. 3.
The improvement in collection efficiency and energy consumption through gas conditioning is mainly attributed to the increase in CO2 content. At 400 °C, gas conditioning increases the maximum collection efficiency of the ESP under coal pyrolysis gas media by 6.02%. However, the improvement in collection efficiency under the effect of gas conditioning weakens as the temperature increases.
4.
The provision of a positive power supply is a good method for improving the
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performance of ESP under coal pyrolysis gas media. The improvement in collection efficiency attributed to the effect of the positive power supply increases with the increase in temperature. At 600 °C, the maximum collection efficiency of ESP under coal pyrolysis gas media increases by 11.8% under the effect of the positive power supply.
Acknowledgments
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The authors acknowledge the support from the National Key R&D Program of China (grant
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number: 2018YFB060500).
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[6] Xiao G, Wang X, Zhang J, Ni M, Gao X, Luo Z, et al. Granular bed filter: A promising technology for hot gas clean-up. POWDER TECHNOL. 2013;244:93-9. [7] Jaworek A, Krupa A, Czech T. Modern electrostatic devices and methods for exhaust gas cleaning: A brief review. J ELECTROSTAT. 2007;65(3):133-55. [8] Noda N, Makino H. Influence of operating temperature on performance of electrostatic
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precipitator for pulverized coal combustion boiler ☆ . ADV POWDER TECHNOL.
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2010;21(4):495-9.
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[9] Villot A, Gonthier Y, Gonze E, Bernis A, Ravel S, Grateau M, et al. Separation of
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particles from syngas at high-temperatures with an electrostatic precipitator. SEP PURIF
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TECHNOL. 2012;92:181-90.
[10] Xiao G, Wang X, Yang G, Ni M, Gao X, Cen K. An experimental investigation of
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electrostatic precipitation in a wire–cylinder configuration at high temperatures. POWDER
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TECHNOL. 2015 2015-01-01;269:166-77.
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[11] Ni M, Wang X, Xiao G, Qiu K, Yang G, Gao X, et al. Development of back corona discharge in a wire-cylinder electrostatic precipitator at high temperatures. POWDER TECHNOL. 2015 2015-12-01;286:789-97. [12] Xiao G, Wang X, Zhang J, Ni M, Gao X, Cen K. Characteristics of DC discharge in a wire-cylinder configuration at high ambient temperatures. J ELECTROSTAT. 2014 2014-02-01;72(1):13-21. [13] Xu X, Gao X, Yan P, Zhu W, Zheng C, Wang Y, et al. Particle migration and collection in
a
high-temperature
2015-03-25;143:184-91.
electrostatic
precipitator.
SEP
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2015
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[14] Gu Z, Xi X, Yang J, Xu J. Properties of RE –W cathode and its application in electrostatic
precipitation
for
high
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clean-up.
FUEL.
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2012-05-01;95:648-54. [15] Xu J, Gu Z, Xi X, Zhang J, Liu J. Loss mechanism and lifetime estimation of Ce–W cathode
in
high
temperature
thermionic
emission
electrostatic
precipitation.
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ELECTROSTAT. 2014 2014-08-01;72(4):336-41.
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[16] Chen Q, Fang M, Cen J, Liu J. Characteristics of negative DC discharge in a
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wire-cylinder configuration under coal pyrolysis gas components at high temperatures. RSC
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ADV. 2018;8(40):22737-47.
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[17] Chen Q, Fang M, Cen J, Lv M, Shao S, Xia Z. Comparison of positive and negative DC discharge under coal pyrolysis gas media at high temperatures. POWDER TECHNOL. 2019
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2019-01-01;345:352-62.
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[18] Xu X, Zhu D. Gas Discharge Physics: Fudan University Press 1996.
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[19] Long Z, Yao Q. Evaluation of various particle charging models for simulating particle dynamics in electrostatic precipitators. J AEROSOL SCI. 2010;41(7):702-18. [20] Skodras G, Kaldis SP, Sofialidis D, Faltsi O, Grammelis P, Sakellaropoulos GP. Particulate removal via electrostatic precipitators — CFD simulation. FUEL PROCESS TECHNOL. 2006;87(7):623-31. [21] Xiao G, Wang X, Zhang J, Ni M, Gao X, Cen K. Current analysis of DC negative corona discharge in a wire-cylinder configuration at high ambient temperatures. J ELECTROSTAT. 2014 2014-04-01;72(2):107-19. [22] Rinard G, Rugg DE. High-Temperature High-Pressure Electrostatic Precipitator
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Electrical Characterization and Collection Efficiency. IEEE T IND APPL. 1987;23(1):114-9.
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Table 1. Chemical composition of particles Table 2. Composition of coal pyrolysis gas Table 3. Composition of conditioned coal pyrolysis gas Fig. 1. Schematic of the experimental setup Fig. 2. Property of particles: (a) resistivity and (b) particle size distribution
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Fig. 3. V–I characteristics under air atmosphere at different temperatures
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Fig. 4. Collection efficiency vs. output voltage under air atmosphere at different temperatures
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Fig. 5. Collection efficiency vs. temperature at different output voltages
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Fig. 6. Performance of the ESP vs. temperature at an output voltage of 18 kV: (a) fractional
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particle collection efficiencies and (b) particle migration velocities Fig. 7. Energy consumption index vs. output voltage at different temperatures
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Fig. 8. Energy consumption index and outlet mass concentration vs. output voltage at 500 °C
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Fig. 9. V–I characteristics under different gas media at (a) 400 °C and (b) 600 °C
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Fig. 10. Variation in collection efficiency with atmosphere at different output voltages Fig. 11. Energy consumption index vs. output voltage under different atmospheres at (a) 400 °C and (b) 600 °C
Fig. 12. Comparison of collection efficiency under coal pyrolysis and conditioned coal pyrolysis gases Fig. 13. Comparison of energy consumption index under coal pyrolysis and conditioned coal pyrolysis gases Fig. 14. Comparison of V–I characteristics under coal pyrolysis and conditioned coal pyrolysis gases
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Fig. 15. Negative and positive discharge characteristics of coal pyrolysis gas at (a) 400 °C and (b) 600 °C Fig. 16. Comparison of the collection efficiency of negative and positive ESPs
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Fig. 17. Comparison of energy consumption by negative and positive ESPs
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Table 1 Chemical composition of particles C
SiO2
Al2 O3
Fe2 O3
CaO
MgO
K2 O
Na2 O
Loss
Content (%)
39.85
27.20
21.63
4.26
2.90
1.50
0.36
0.16
2.14
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Pr
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Composition
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Table 2 Composition of coal pyrolysis gas H2
CH4
CO2
N2
CO
Volume (%)
30
44
8
10
8
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Component
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Table 3 Composition of conditioned coal pyrolysis gas H2
CH4
CO2
N2
CO
Volume (%)
20
30
20
20
10
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Pr
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Component
Journal Pre-proof Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
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Highlights: High temperature was harmful to the performance of the ESP. The performance of ESP in coal pyrolysis gas is inferior to that in air. Gas conditioning and positive power supply can improve the performance of ESP.
Quanlin Chen, Mengxiang Fang, Jianmeng Cen, Yifei Zhao, Qinhui Wang, Yuwei Wang PII:
S0032-5910(19)31071-X
DOI:
https://doi.org/10.1016/j.powtec.2019.11.108
Reference:
PTEC 14990
To appear in:
Powder Technology
Received date:
18 March 2019
Revised date:
9 November 2019
Accepted date:
26 November 2019
Please cite this article as: Q. Chen, M. Fang, J. Cen, et al., Electrostatic precipitation under coal pyrolysis gas at high temperatures, Powder Technology(2019), https://doi.org/ 10.1016/j.powtec.2019.11.108
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© 2019 Published by Elsevier.
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Electrostatic precipitation under coal pyrolysis gas at high temperatures Quanlin Chena, Mengxiang Fanga, Jianmeng Cena,1 , Yifei Zhaoa, Qinhui Wanga, Yuwei Wanga a
State Key Lab of Clean Energy Utilization, Zhejiang University, Hangzhou 310027, China
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Abstract
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An experimental-scale electrostatic precipitator (ESP) was built to investigate the influence
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of temperature and gas media on collection efficiency and energy consumption. High temperature was harmful to the performance of the ESP and had a considerable influence on
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large particles. The maximum collection efficiency was lower and energy consumption was
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higher under coal pyrolysis gas media than under air. Two improvement methods, namely, gas
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media conditioning and positive polarity power supply, were studied in this work. Both
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methods can improve the performance of high-temperature ESP under coal pyrolysis gas media. The improvement in collection efficiency under the effect of gas conditioning decreased and that under the effect of a positive power supply increased with the increase in temperature. Gas conditioning and positive power supply increased the maximum collection efficiencies of the ESP at 600 °C under coal pyrolysis gas media by 3.0% and 11.8%, respectively. Key words: high-temperature; ESP; coal pyrolysis gas; gas conditioning; positive power supply. 1 Corresponding author. Tel.: +86-0571-87952205. E-mail address: [email protected]
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1. Introduction High-temperature dust removal technologies are crucial for the development of clean coal utilization technologies, such as pressurized fluidized bed combustion (PFBC) and integrated gasification combined cycle (IGCC) [1-4]. High-temperature dust removal technologies include cyclone separators, ceramic filters, granular filters, and electrostatic precipitators
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(ESPs) [5, 6]. Although cyclone separators have simple structures and are insensitive to high temperatures, they present low removal efficiencies for fine dust particles [7]. Ceramic filters
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have high dust removal efficiencies but have large pressure drops and are fragile. ESPs are
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widely used in conventional coal-fired power plants at operating temperatures of less than
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150 °C because of their high dust removal efficiencies and low pressure drops [7, 8]. Nevertheless, high-temperature electrostatic precipitation requires further exploration.
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Villot et al. [9] established a wire-cylinder ESP for the purification of biomass syngas and
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obtained a collection efficiency of 96% at temperatures above 500 °C. Xiao et al. [10-12]
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found that the dust removal efficiency of a wire-cylinder ESP under air atmosphere can reach 99.6% at the temperature range of 350 °C–700 °C. Xu et al. [13] constructed a wire-plate ESP to study the relationship between temperature and dust removal efficiency and reported that dust removal efficiency decreased from 99.8% to 95% as temperature increased from 300 K to 900 K. Gu et al. [14, 15] developed a new type of ESP based on thermionic emission cathodes and tested the thermionic emission properties of three cathodes. The ESP achieved a dust removal efficiency of more than 90% at temperatures above 673 K. The discharge characteristics of a coal pyrolysis gas atmosphere are different from those of an air atmosphere and a conventional flue gas atmosphere and affect particle charging and
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removal by ESPs [16, 17]. Thus, this study aims to investigate the characteristics of electrostatic precipitation under coal pyrolysis gas media at high temperatures. The results of this work will be beneficial for the design and application of high-temperature ESPs in IGCC, PFBC, and coal polygeneration systems.
2. Experimental setup
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2.1 ESP
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The schematic of the experimental system is shown in Fig. 1. The system comprises five
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parts, namely, an ESP, a high-temperature gas generator, a particle feeding and measuring
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system, a high-voltage DC power supply, and an electrical parameter measurement system. The ESP is mainly composed of a high-voltage electrode, a low-voltage electrode, and an
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insulator. The high-voltage electrode is a stainless steel wire with a length of 1,200 mm and a
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diameter of 2 mm. The low-voltage electrode is a stainless steel tube with a length of 1,200
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mm with an internal diameter of 100 mm. The electrode distance of the device is 49 mm. Electrical insulation remains challenging at high temperatures because the performance of insulating materials deteriorates as temperature increases. In this work, a corundum insulator is used as the insulating material between the high- and low-voltage electrodes.
2.2 Particle feeding and measurement The particle feeding and measuring system consists of an electromagnetic vibrating feeding device and a PM10 impactor. The feeding rate of the electromagnetic vibrating feeding device is 1–5 g/min. Particle concentration in the gas flow is sampled by the PM10 impactor and
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measured by an electronic balance. The particle concentration sampling point is located at the outlet of the ESP. Many particles that pass through the ESP are removed by inertial force. The particle concentration sampling point is set at the outlet of the dust collector to eliminate the influence of inertial force. The particle concentration at the sampling point is called the inlet
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concentration of the ESP under the current voltage.
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concentration of the ESP when the output voltage is 0 kV and is called the outlet
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The particles used in the experiments are fly ash produced by a coal pyrolysis
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polygeneration system. Table 1 provides the chemical composition of the particles. The
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resistivity of ash is measured by a high-temperature particle resistivity meter over the temperature range of 100 °C to 700 °C. As shown in Fig. 2(a), the maximum resistivity of ash
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is 5.33 × 106 Ω × cm at the temperature of 100 °C. Resistivity decreases to 4.86 × 103 Ω ×cm
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as the temperature increases to 700 °C. Particle size distribution is measured by using a
38.017 μm.
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Mastersizer 2000 apparatus. As shown in Fig. 2(b), the median particle size is approximately
2.3 Electric circuit
The high-voltage negative power supply provides an adjustable DC negative voltage of 0– 60 kV and has a maximum output current of 20 mA. In this work, the voltage supplied by a high-voltage power supply is called the output voltage and that between the anode and cathode is called the port voltage, which is measured by a high-voltage probe and an Agilent 34970A data acquisition system. A 0.8 MΩ resistor is used to protect the high-voltage power
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supply in the case of spark breakdown. The discharge current is measured by the Agilent 34970A data acquisition system, which has a measurement accuracy of 1 nA.
2.4 Experimental procedure The coal pyrolysis gas used in this work is a mixture of pure gas. The composition of coal pyrolysis gas is shown in Table 2. The gas components are measured by using an Agilent
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7890A gas chromatography system. Air and N2 are also investigated for comparison with coal
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pyrolysis gas. A mass flow controller is used to adjust the gas flow rate such that the flow rate
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in the ESP is approximately 0.12 m/s. That is, the residence time of the dust-containing gas in the ESP is 10 s.
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The testing temperatures on each gas media are set from 400 °C to 700 °C with a 100 °C
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interval. The inlet particle concentration is controlled to 1 g/Nm3 by adjusting the feeder.
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follows:
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The definitions of the collection efficiency and energy consumption index are expressed as
=
min mout P , min min mout ,
where η is the collection efficiency; min is the inlet mass concentration of particles, g/Nm3 ; mout is the outlet mass concentration of particles, g/Nm3 ; φ is the energy consumption index, W/(g/Nm3 ); and P is the electric power consumed by the ESP, W. Certain tests are repeated to check the repeatability of the experimental data. Results highlight the positive performance of the experimental system.
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3. Results and discussion The influence of temperature and atmosphere on the collection efficiency and energy consumption of ESP was studied to obtain a comprehensive understanding of high-temperature ESP under coal pyrolysis gas media. Discharge is a fundamental process for ESP and has an important impact on subsequent particle charging and removal. Therefore, the
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effect of temperature and atmosphere on the discharge process was briefly described in this work. The abilities of two improvement methods, namely, gas media conditioning and
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positive polarity power supply, to address the low collection efficiency and high energy
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3.1 Effect of temperature
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consumption of high-temperature ESP under coal pyrolysis gas media were tested.
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3.1.1 Effect of temperature on discharge
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The discharge current correspondingly increases with the increase in output voltage, as shown in Fig. 3. The increase in temperature can promote gas discharge. As temperature increases, the corona onset voltage decreases, whereas the discharge current increases. However, the corona onset voltage decreases from 10.1 kV to 8.3 kV as temperature increases from 400 °C to 600 °C. At an output voltage of 12 kV, the discharge current increases from 259 μA at 400 °C to 681 μA at 600 °C. High temperature enlarges the mean free path of electrons and gas molecules and helps electrons gain additional energy from the imposed electric field. Thus, the possibility of inelastic collisions between electrons and gas molecules increases. As a result, additional electron-positive ion pairs are produced. The production of
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excess electron-positive ion pairs, in turn, reduces corona onset and spark breakdown voltages, as well increases the discharge current at the same voltage [18]. Furthermore, the reduction in the work function of the cathode by the high temperature of the cathode surface results in additional thermionic electrons and increases current.
3.1.2 Effect of temperature on dust removal efficiency
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Fig. 4 shows that the dust removal efficiency at each temperature can be roughly divided
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into two stages with the increase in voltage. At the first stage, the voltage is low, the discharge
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current is small, and dust removal efficiency increases with the increase in voltage. At the second stage, the voltage is high, the discharge current is large, and collection efficiency
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remains nearly unchanged with the increase in voltage. To illustrate, dust removal efficiency
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at 500 °C increases from 50.12% to 96.54% when output voltage increases from 6 kV to 10
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kV and from 96.54% to 98.85% as the output voltage increases from 10 kV to 18 kV.
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These results are attributed to the different distributions of space charge and electric field at various voltages. As described in Section 3.1.1, the discharge current correspondingly increases with the increase in voltage. This effect increases the probability that particles are charged. In addition, particle migration velocity increases with the increase in voltage because of electric field intensity and the Coulomb force exerted on charged particles [19]. According to Deutsch’s formula in Eq. (1), collection efficiency η increases as particle migration velocity increases. 𝜂 = 1−𝑒
𝐴 𝑄
− 𝑤
,
(1)
where w is the particle migration velocity, A is the area of the collection tube in the ESP, and
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Q is the gas flow in the ESP. Moreover, as electric field intensity increases, the saturated charge of particles increases. In this work, field charging is dominant. Thus, the saturated charge of a particle can be written as follows [20]: 𝑞=
3𝜀 𝑟 𝜀 𝑟+2
𝜋𝜀0 𝑑2 𝐸,
(2)
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where ε0 is the permittivity of the free space, εr is the relative permittivity of gas media, d is
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the diameter of the particle, and E is the intensity of the electric field.
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Fig. 5 shows the variation in dust removal efficiency with temperature. At the first stage,
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dust removal efficiency increases with the increase in temperature. At a low output voltage,
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the discharge current is small and the particles have yet to reach the saturated state of charge. The current and particle charge increase with the increase in temperature. These effects
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increase dust removal efficiency. For example, when the output voltage is 8.5 kV, dust
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removal efficiency increases from 66.86% to 93.28% when temperature increases from
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400 °C to 700 °C. However, dust removal efficiency decreases with the increase in temperature at the second stage. The discharge current is large and particles have reached the saturated charge state because the output voltage is high at this stage. Thus, a high current at high temperature does not improve particle charge. Furthermore, high current results in low collection efficiency because it leads to the high voltage of protective resistance and the low port voltage of the ESP. To illustrate, when the output voltage is 18 kV, the discharge current increases from 1458 μA to 3217 μA, the port voltage decreases from 16.8 kV to 15.4 kV, and dust removal efficiency decreases from 99.65% to 96.01% as the temperature increases from 400 °C to 600 °C.
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Fig. 6 depicts the particle collection efficiency and migration velocity at different temperatures for various particle size ranges, namely, PMd > 10 (particles with an aerodynamic diameter of more than 10 μm), PM10 > d > 2.5 (particles with an aerodynamic diameter between 2.5 and 10 μm), and PMd < 2.5 (particles with an aerodynamic diameter of less than 2.5 μm). Particle migration velocity w is calculated with Deutsch’s formula as follows: 𝐴
𝑤 = − ln( 1 − 𝜂).
(3)
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𝑄
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The following results were obtained for the three particle size ranges: At 400 °C, the
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collection efficiencies for PMd > 10 , PM10 > d > 2.5 , and PMd < 2.5 are 99.92%, 98%, and 90%,
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respectively. The collection efficiencies for PMd > 10 , PM10 > d > 2.5 , and PMd < 2.5 decrease to
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96.44%, 95%, and 82%, respectively, as the temperature increases to 600 °C. The trends shown by migration velocity are the same as those shown by collection efficiency: They
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decrease from 17.83, 9.78, and 5.76 mm/s to 8.34, 7.49, and 4.29 mm/s, respectively. The
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change in the migration velocity of large particles is more substantially than that in the
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migration velocity of small particles. The increase in temperature enhances the thermal motion of small particles. This effect enables small particles to efficiently agglomerate and reduce their number. The migration velocity of small particles decreases at a slower rate at high temperature than that of large particles because of the agglomeration effect.
3.1.3 Effect of temperature on energy consumption Fig. 7 depicts the variation in energy consumption index with voltage. As the temperature increases from 400 °C to 600 °C, the energy consumption index increases from 3.08 W/(g/Nm3 ) to 8.14 W/(g/Nm3 ) at an output voltage of 12 kV. According to Eq. (4), the energy
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consumption index is proportional to the current. The increase in temperature and voltage can increase the discharge current, which results in an increase in the energy consumption index.
Up I P = min mout min mout ,
(4)
where Up is the port voltage of the ESP. In summary, the increase in voltage will increase dust removal efficiency by strengthening
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particle charge and increasing migration velocity. However, the increase in dust removal efficiency as a result of increasing output voltage is limited. At the first stage, increasing
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voltage can drastically improve dust removal efficiency. At the second stage, however,
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increasing voltage negligibly affects dust removal efficiency but significantly increases
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energy consumption. Dust removal efficiency and energy consumption at this stage should be comprehensively considered.
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Fig. 8 displays the energy consumption index and outlet mass concentration at 500 °C
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when the output voltage ranges from 8.5 kV to 18 kV. China’s environmental protection
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requirements state that outlet mass concentration should not exceed 20 mg/Nm3 . An output voltage of 10.6–18 kV is preferred when the outlet mass concentration is less than 20 mg/Nm3 . The upper limit of the energy consumption index is set as 10 W/(g/Nm3 ) to save energy. Thus, the optimal voltage range that can meet the requirements of outlet mass concentration and energy consumption is 10.6–13 kV.
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3.2 Effect of gas media 3.2.1 Effect of gas media on discharge The V–I characteristics of air, N 2 , and coal pyrolysis gas at temperatures of 400 °C and 600 °C are compared in Fig. 9. A corona is present during air discharge but not during N2 discharge [12, 16, 21]. At 400 °C, N2 discharges at 11.5 kV and quickly breaks down because
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it is a nonelectronegative gas medium and cannot attach electrons. The N2 discharge current is
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higher than the air discharge current because of the difference in discharge types. To illustrate,
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the discharge currents of air and N2 at 400 °C and 12 kV output voltage are 259 and 6,480 μA, respectively. In addition, the air discharge current is composed of negative ions and electrons,
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whereas the N2 discharge current is mainly composed of electrons. These results suggest that
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the dust removal efficiency of N2 is extremely lower than that of air. By contrast, energy
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consumption under N2 is higher than that under air.
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The main components of coal pyrolysis gas, such as CH 4 , H2 , and N2 , are nonelectronegative gas media. Therefore, the discharge characteristics of coal pyrolysis gas are similar to those of N2 . However, the starting discharge voltage under coal pyrolysis gas is lower than that under N2 . At 600 °C, the coal pyrolysis gas discharges at 3 kV, whereas N2 discharges at 9.67 kV.
3.2.2 Effect of gas media on dust removal efficiency The collection efficiencies of air, N2 , and coal pyrolysis gas at different temperatures are compared in Fig. 10. Regardless of temperature and output voltage, dust removal efficiencies
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are ranked as follows: air > coal pyrolysis gas > N 2. These results can be attributed to the differences in discharge characteristics. In contrast to air discharge, a corona is absent during N2 discharge, and the discharge current is mainly composed of electrons. The lower charge efficiency of electrons on particles than that of ions results in low dust removal efficiency under N2 . Coal pyrolysis gas is mainly composed of nonelectronegative gas, such as CH 4 , H2 ,
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and N2 . It also contains electronegative gas, such as CO 2. Therefore, the dust removal
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efficiency of coal pyrolysis gas is between that of air and N2 . For example, the collection
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efficiencies of air, N2 , and coal pyrolysis gas at 400 °C and 12 kV output voltage are 99.12%,
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83.34%, and 90.56% respectively.
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With the increase in temperature, the difference between coal pyrolysis gas and N2 narrows, whereas that between coal pyrolysis gas and air widens. At an output voltage of 18 kV and a
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temperature of 400 °C, the collection efficienc ies of air, N2 , and coal pyrolysis gas are 99.65%,
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83.52% and 90.23%, respectively. Thus, the difference between the dust removal efficiencies
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of coal pyrolysis gas and N2 is 6.71% and that between the dust removal efficiencies of coal pyrolysis gas and air is 9.42%. As the temperature increases to 600 °C and the output voltage is maintained at 18 kV, the difference in dust removal efficiency between coal pyrolysis gas and N2 decreases to 0.55%, whereas that between coal pyrolysis gas and air increases to 19.03%.
3.2.3 Effect of gas media on energy consumption The energy consumption of air, N 2 , and coal pyrolysis gas at various temperatures are compared in Fig. 11. As discussed in Section 3.2.1, the discharge characteristics of coal
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pyrolysis gas are similar to those of N2 . Thus, the energy consumption of coal pyrolysis gas is similar to that of N2 . The energy consumption of N 2 is lower than that of coal pyrolysis gas at output voltages of 8.5 and 10 kV and at a temperature of 400 °C because N2 has yet to be discharged. The energy consumption of coal pyrolysis gas is considerably larger than that of air given
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the difference in discharge type and current composition. For example, the energy
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consumption indexes of coal pyrolysis gas and air at a temperature of 600 °C and output
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voltage of 12 kV are 58.35 and 8.14 W/(g/Nm3 ), respectively.
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In summary, the dust removal efficiency of coal pyrolysis gas is lower than that of air,
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whereas the energy consumption of coal pyrolysis gas is higher than that of air because of differences in discharge type and current composition.
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3.3 Methods for the improvement of dust removal
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The low collection efficiency and high energy consumption of high-temperature ESP under coal pyrolysis gas media are identified as problems on the basis of the results presented above. This work aimed to solve these problems by improving the performance of the ESP through gas conditioning and positive polarity power supply.
3.3.1 Gas media conditioning Table 3 lists the composition of conditioned coal pyrolysis gas. The contents of H2 , CH4 , and other combustible gases in conditioned coal pyrolysis gas have decreased from 82% to 60% relative to those in coal pyrolysis gas. The CO2 content of conditioned coal pyrolysis gas is 12%
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higher than that of coal pyrolysis gas. Gas media conditioning can improve dust removal efficiency to a certain extent. However, the improvement in dust removal efficiency under the effect of gas media conditioning decreases as temperature increases, as shown in Fig. 12. At 400 °C, the maximum collection efficiency of conditioned coal pyrolysis gas is 96.54%, which is 6.02% higher than that of
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coal pyrolysis gas. The difference between the maximum dust removal efficiencies of the
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conditioned coal pyrolysis and coal pyrolysis gases decreases to 3% as the temperature
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increases to 600 °C.
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The energy consumption indexes of coal pyrolysis gas and conditioned coal pyrolysis gas
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are compared in Fig. 13. The energy consumption index of conditioned coal pyrolysis gas is slightly lower than that of coal pyrolysis gas. However, the difference in energy consumption
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between conditioned coal pyrolysis gas and coal pyrolysis gas decreases with the increase in
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temperature. The energy consumption indexes of conditioned coal pyrolysis gas and coal
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pyrolysis gas are 45.27 and 49.35 W/(g/Nm3 ), respectively, at a temperature of 400 °C and an output voltage of 12 kV. The difference between the energy consumption indexes of conditioned coal pyrolysis gas and coal pyrolysis gas decreases from 4.08 W/(g/Nm3 ) to 1.88 W/(g/Nm3 ) as temperature increases from 400 °C to 600 °C and the output voltage is maintained at 12 kV. The V–I characteristics of coal pyrolysis gas and conditioned coal pyrolysis gas are compared in Fig. 14. Conditioned coal pyrolysis gas has stronger electronegativity, higher initial discharge voltage, and lower current under the same voltage than coal pyrolysis gas because the CO2 content of conditioned coal pyrolysis gas is higher than that of coal pyrolysis
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gas. At 400 °C, coal pyrolysis gas begins to discharge at 4.5 kV, whereas conditioned coal pyrolysis gas begins to discharge at 5.07 kV. The discharge currents under coal pyrolysis gas and conditioned coal pyrolysis gas are 6,863 and 6,046 μA, respectively, at a temperature of 400 °C and an output voltage of 12 kV. The difference in discharge characteristics between coal pyrolysis gas and conditioned coal pyrolysis gas decreases with the increase in
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temperature. As the temperature increases from 400 °C to 600 °C and the output voltage is
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maintained at 12 kV, the difference in discharge current between coal pyrolysis gas and
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conditioned coal pyrolysis gas decreases from 817 μA to 8 μA.
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Gas conditioning improves collection efficiency and energy consumption mainly by
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increasing CO2 content. CO2 is an electronegative gas medium that can attach electrons to form negative ions. Negative ions charge particles more efficiently than electrons. Therefore,
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the dust removal efficiency of conditioned coal pyrolysis gas is higher than that of coal
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pyrolysis gas. In addition, negative ions are considerably less mobile than electrons. This
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characteristic accounts for the lower discharge current of the conditioned coal pyrolysis gas compared with that of coal pyrolysis gas. Thus, the energy consumption of conditioned coal pyrolysis gas is lower than that of coal pyrolysis gas. The mean free path of gas molecules and electrons increases and the ability of CO 2 to attach electrons decreases with the increase in temperature. Therefore, the improvement in collection efficiency and energy consumption under the effect of gas media conditioning is weakened.
3.3.2 Positive polarity power supply Nonelectronegative gases, such as CH 4 , H2 , and N2 , account for 82% of the pyrolysis gas
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content. Under nonelectronegative gas media, the current is mainly composed of electrons because they cannot attach electrons during conventional negative discharge [18, 22]. The abnormally large discharge current can be ascribed to the fact that the charge mobility of electrons is approximately 1,000 times higher than that of ions. In addition, inadequate particle charge and low dust removal efficiency can be attributed to the lower charge
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efficiency of electrons on particles than that of ions. That is, the existence of the electronic
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current results in low collection efficiency and high energy consumption [16]. However, the
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charge in the drift region is mainly composed of ions during positive discharge [17]. Inspired
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by the different current compositions during positive and negative discharges, this study aims
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to use a positive polar power supply to improve the dust removal efficiency and energy consumption of high-temperature ESP under coal pyrolysis gas media.
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The positive and negative discharge characteristics under coal pyrolysis gas media are
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compared in Fig. 15. The initial discharge voltage and current of positive discharge are lower
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than those of negative discharge. Negative and positive discharges begin at 4.53 and 8.9 kV, respectively, at 400 °C. The negative and positive discharge currents of coal pyrolysis gas are 6,863 and 2,348 μA, respectively, at a temperature of 400 °C and an output voltage of 12 kV. In addition to these differences, a corona exists during positive discharge, and the discharge current is mainly composed of ions. The collection efficienc ies of negative and positive ESPs are compared in Fig. 16. At a temperature of 400 °C and an output voltage of 8.5 kV, the collection efficiency of positive ESP is 65.34%, which is lower than that of negative ESP. This difference may be attributed to the positive ESP, which has yet to discharge. Except for this condition, the collection
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efficiency of positive ESP is higher than that of negative ESP. At 400 °C, the maximum dust removal efficienc ies of positive and negative ESPs are 96.18% and 90.52%, respectively. When the temperature rises to 600 °C, the maximum dust removal efficiencies of positive and negative ESPs decrease to 88.92% and 77.12%, respectively. Therefore, the difference between the maximum dust removal efficienc ies of positive and negative ESPs increases from
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5.66% to 11.8%. That is, a high temperature will enhance the improvement effect of the
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positive power supply on dust removal efficiency under coal pyrolysis gas media.
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The energy consumption indexes of negative and positive ESPs are compared in Fig. 17.
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Energy consumption by the positive ESP is lower than that by the negative ESP because the
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positive discharge current is lower than the negative discharge current. At a temperature of 600 °C and an output voltage of 10 kV, the energy consumption indexes of the positive and
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negative ESPs are 17.01 and 39.54 W/(g/Nm3 ), respectively.
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A positive polarity power supply improves dust removal efficiency and energy
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consumption mainly because of the differences in discharge type and current composition between positive and negative discharges. During negative discharge under coal pyrolysis gas media, the corona is nearly absent and the current is mainly composed of electrons. However, the corona exists and the current is mainly composed of ions during positive discharge [17]. With the increase in temperature, the improvement in collection efficiency under the effect of gas conditioning decreases, whereas that under the effect of the positive power supply increases. This finding indicates that positive power supply may be the superior method for improving the performance of high-temperature ESP under coal pyrolysis gas media.
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4. Conclusions This work investigates the influence of temperature and gas media on the collection efficiency and energy consumption of ESP. It also explores the improvement in collection efficiency and energy consumption under the effect of gas media conditioning and a positive polar power supply. The following conclusions are drawn: As temperature increases, the maximum dust removal efficiency decreases, whereas
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1.
energy consumption increases. Under air atmosphere, the maximum collection efficiency
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decreases from 99.65% to 96.01%, whereas the energy consumption index increases
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from 24.58 W/(g/Nm3 ) to 51.6 W/(g/Nm3 ) as the temperature increases from 400 °C to
influence on large particles.
The maximum collection efficiency under coal pyrolysis gas media is lower than that
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2.
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600 °C. Temperature has diverse effects on different particle sizes and a considerable
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under air. By contrast, energy consumption under coal pyrolysis gas media is higher than
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that under air. At 400 °C, the maximum collection efficiencies under air and coal pyrolysis gas are 99.65% and 90.52%, respectively, and the corresponding energy consumption indexes are 24.58 and 49.35 W/(g/Nm3 ), respectively. 3.
The improvement in collection efficiency and energy consumption through gas conditioning is mainly attributed to the increase in CO2 content. At 400 °C, gas conditioning increases the maximum collection efficiency of the ESP under coal pyrolysis gas media by 6.02%. However, the improvement in collection efficiency under the effect of gas conditioning weakens as the temperature increases.
4.
The provision of a positive power supply is a good method for improving the
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performance of ESP under coal pyrolysis gas media. The improvement in collection efficiency attributed to the effect of the positive power supply increases with the increase in temperature. At 600 °C, the maximum collection efficiency of ESP under coal pyrolysis gas media increases by 11.8% under the effect of the positive power supply.
Acknowledgments
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The authors acknowledge the support from the National Key R&D Program of China (grant
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number: 2018YFB060500).
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References
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Table 1. Chemical composition of particles Table 2. Composition of coal pyrolysis gas Table 3. Composition of conditioned coal pyrolysis gas Fig. 1. Schematic of the experimental setup Fig. 2. Property of particles: (a) resistivity and (b) particle size distribution
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Fig. 3. V–I characteristics under air atmosphere at different temperatures
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Fig. 4. Collection efficiency vs. output voltage under air atmosphere at different temperatures
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Fig. 5. Collection efficiency vs. temperature at different output voltages
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Fig. 6. Performance of the ESP vs. temperature at an output voltage of 18 kV: (a) fractional
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particle collection efficiencies and (b) particle migration velocities Fig. 7. Energy consumption index vs. output voltage at different temperatures
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Fig. 8. Energy consumption index and outlet mass concentration vs. output voltage at 500 °C
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Fig. 9. V–I characteristics under different gas media at (a) 400 °C and (b) 600 °C
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Fig. 10. Variation in collection efficiency with atmosphere at different output voltages Fig. 11. Energy consumption index vs. output voltage under different atmospheres at (a) 400 °C and (b) 600 °C
Fig. 12. Comparison of collection efficiency under coal pyrolysis and conditioned coal pyrolysis gases Fig. 13. Comparison of energy consumption index under coal pyrolysis and conditioned coal pyrolysis gases Fig. 14. Comparison of V–I characteristics under coal pyrolysis and conditioned coal pyrolysis gases
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Fig. 15. Negative and positive discharge characteristics of coal pyrolysis gas at (a) 400 °C and (b) 600 °C Fig. 16. Comparison of the collection efficiency of negative and positive ESPs
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Fig. 17. Comparison of energy consumption by negative and positive ESPs
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Table 1 Chemical composition of particles C
SiO2
Al2 O3
Fe2 O3
CaO
MgO
K2 O
Na2 O
Loss
Content (%)
39.85
27.20
21.63
4.26
2.90
1.50
0.36
0.16
2.14
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Composition
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Table 2 Composition of coal pyrolysis gas H2
CH4
CO2
N2
CO
Volume (%)
30
44
8
10
8
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Component
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Table 3 Composition of conditioned coal pyrolysis gas H2
CH4
CO2
N2
CO
Volume (%)
20
30
20
20
10
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Component
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☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
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Highlights: High temperature was harmful to the performance of the ESP. The performance of ESP in coal pyrolysis gas is inferior to that in air. Gas conditioning and positive power supply can improve the performance of ESP.