Sulfuric Acid Aerosol Formation and Collection by Corona Discharge

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Sulfuric Acid Aerosol Formation and Collection by Corona Discharge in Wet Electrostatic Precipitator Zhengda Yang, Chenghang Zheng, Xuefeng Zhang, Cunjie Li, Yi Wang, Weiguo Weng, and Xiang Gao Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b01090 • Publication Date (Web): 29 Jun 2017 Downloaded from http://pubs.acs.org on June 30, 2017

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Manuscript No. ef ef--20172017-01090e

Sulfuric Acid Aerosol Formation and Collection by Corona Discharge in Wet Electrostatic Precipitator

Zhengda Yang, Chenghang Zheng, Xuefeng Zhang, Cunjie Li, Yi Wang, Weiguo Weng and Xiang Gao*

* Corresponding author: Prof. Xiang Gao State Key Lab of Clean Energy Utilization, Institute for Thermal Power Engineering Zhejiang University Hangzhou 310027 P.R. China Tel: +86-571-87951335 Fax: +86-571-87951616 Email:[email protected]

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ABSTRACT Wet electrostatic precipitator (ESP) is considered as an effective technology for the control of dust and acid mist after wet flue gas desulfurization. In this study, a lab-scale wet ESP was designed to investigate dust collection with residual SO2 in the flue gas. Results showed that dust collection efficiency was reduced in the presence of SO2. This finding is attributed to the sulfuric acid aerosol formation during corona discharge process. An exponential increase of aerosol number concentration was observed with the increasing applied voltage, which could be two orders of magnitude higher than that without corona discharge. Fractional aerosol size distribution indicated that aerosol penetration ratio was extremely low in size range of 0.1–1.0 µm. The formation of sulfuric acid aerosol could only be detected when the specific energy density exceeded a threshold value. Dust collection efficiency decreased by 13.5% when the SO2 concentration increased from 0 to 25 ppm as a consequence of aerosol formation. KEYWORDS: wet electrostatic precipitator, aerosol formation, corona discharge, threshold specific energy density, particle collection


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INTRODUCTION Particulate matter (PM) pollution has caused global concerns over the past years

due to its hazardous effects on human health 1. PM emissions from coal combustion sources typically consist of filterable and condensable PM 2. In coal-fired power plants, they can be formed during both combustion and post-combustion processes. Filterable PM is mainly produced by fragmentation and vaporization mechanisms at high temperature, whereas condensable PM is primarily generated by condensable substances such as SO3, NH3, and other soluble components at low temperature

3–5

.

Once discharged into the atmosphere, both will contribute to severe haze problems. Filterable PM can be effectively collected by an ESP featured as durable and cost-effective 6. Inside an ESP, a high voltage is applied to the discharge wire to ionize the gas and generate non-uniform electric field. When particles in the flue gas flow through the ESP, they are charged by field and diffusion charging. The charged particles would migrate to the collection plates by Coulomb force under the electric field, and the flue gas is thereby purified 7. Nevertheless, condensable PM is usually formed downstream of the ESP at low temperature gas conditions. In recent years, the formation of sulfuric acid aerosol during wet flue gas desulfurization (WFGD) process from coal-fired power plants has attracted increasing attention. Several researchers studied the nucleation of sulfuric acid aerosol due to the super-saturation during a quenching process by both experimental and simulation methods 8–10. The removal characteristics of sulfuric acid aerosol in a WFGD system were investigated under different conditions. However, the removal of sulfuric acid aerosol by WFGD showed less efficiency 11–13. Wet ESP was considered as a possible technology for sulfuric acid aerosol removal, and the key parameters for its removal performance were examined

14–16

. Ions and electrons

produced by corona discharge are necessary for aerosol charging in wet ESP, whereas various other reactive species such as O, OH, O3, H2O2, and HO2 radicals

17

are

simultaneously produced, which may induce SO2 oxidation into SO3. Furthermore, the saturation gas condition inside wet ESP is beneficial for homogeneous nucleation and consequently results in sulfuric acid aerosol formation. Anderlohr et al.


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reported that the sulfuric acid aerosol can be generated in the wet ESP in a pilot plant. The formation and removal characteristics of sulfuric acid aerosol in a WFGD system have been described in a number of publications, whereas minimal work has been conducted on the mechanisms of sulfuric acid aerosol formation and threshold formation condition in wet ESP. In this study, a lab-scale wet ESP was designed to investigate sulfuric acid aerosol formation and its effects on dust collection. The correlation between corona discharge and aerosol formation was evaluated by analyzing the correlation of aerosol emission and specific energy density. The characteristics of formed sulfuric acid aerosol were described by fractional aerosol size distribution. Furthermore, the threshold condition for aerosol formation at different gas temperatures within the wet ESP was systematically investigated. Finally, the wet ESP performance for dust collection was evaluated under different SO2 concentrations.

2

EXPERIMENTAL SETUP AND METHODS

2.1

Experimental setup

A schematic of the experimental system is illustrated in Figure 1. The system mainly consists of a gas simulation system, a scrubbing tower, and a wet ESP reactor with high frequency power supply (50 kV, 2mA, 20 kHz, and negative DC), as well as gas analysis instruments.


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Figure 1. Schematic of the experimental system.

An air fan was used to provide a constant flow rate of 120 m3/h for all experiments. The gas was heated using an electric heater which was connected to a programmable temperature controller. The temperature was measured by an S-type thermal couple and fed back to the programmable temperature controller to adjust the heating power of the electric heating elements. After attaining the set temperature, the temperature was kept almost unchanged with a deviation of within ±0.1 °C. To change the gas composition, SO2 and dusts were introduced into the flue gas upstream of the scrubbing tower. The SO2 flow rate was adjusted by a mass flow controller (Seven Star Huachuang Co., Ltd, China) with an accuracy of ±0.1 mL/min. The dusts were added into the gas using a vibration feeder, and the injection amount could be controlled by the vibration frequency. The total gas flow rate was measured by a rotameter and adjusted by a valve between the rotameter and the wet ESP. The calculated gas velocity within the wet ESP was 0.93 m/s, and the corresponding treatment time was 1.29 s. The wet ESP was a horizontal flow reactor with wire-to-plate configuration as shown in Figure 2. The discharge wire was made of stainless steel energized with a negative DC voltage. The collection plates were two grounded steel plates with dimensions of 300 and 1200 mm in height and length, respectively, and the distance between the discharge wire and the collection plates was 60 mm. The wet ESP has 10 discharge wires, and they were placed at 100-mm intervals. All the wires were


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connected as a whole and installed in the middle of gas passage. They were insulated from other parts of the wet ESP using two Teflon insulators with a diameter of 50 mm. To avoid unwanted moisture condensation, an electric heater was placed outside the insulator surface with a temperature of 80 °C.

Figure 2. Arrangement of the discharge wire and the collection plates. Both SO2 and aerosols were sampled downstream of the wet ESP with a sampling probe. The SO2 concentration was measured by a gas analyzer (Testo Instrumental Trading, Co., Ltd, Shanghai). An electrical low pressure impactor (ELPI+, Dekati Ltd., Finland) was used as an online method for sulfuric acid aerosol and dust measuring. Its working principle consists of three continuous processes: aerosol charging, size classification and electrical detection with sensitive electrometers. This measured signal is directly proportional to particle number concentration and size, thus the ELPI+ gives real-time aerosol size distribution and concentration

19

. The

volatile nature of sulfuric acid aerosol makes the size distribution measurement by ex-situ method challenging. To obtain sufficient accuracy, the sampling probe maintained the same temperature with the flue gas and no diluter was used in this work. The results for sulfuric acid aerosol measurement have showed good accuracy 18, 20

. Sulfuric acid aerosol and dust cannot be classified by ELPI+, so the

concentration measured in this study was referred as the total amounts of both sulfuric acid aerosol and dust.


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2.2

Experimental approach

An index of aerosol penetration ratio ( R p ) was defined through a number basis to describe the penetration of formed aerosol from the wet ESP:

Rp =

N out, on N out, off

Eq. (1)

where N out,on and N out,off denote the aerosol number concentration (1/cm3) with and without corona discharge, respectively. R p is the aerosol penetration ratio. R p > 1 means that the aerosol number with corona discharge is larger than that without corona discharge, that is, more aerosol is formed than collected. The dust used in this study was collected from an ESP hopper in a coal-fired power plant. Particle size distribution was measured by ELPI+, and the median diameter was 4.417 µm. The wet ESP performance for dust collection was evaluated by the fractional and total collection efficiencies calculated using the following equations, respectively:

ηi (%) =

N out ,off ( ri )- N out ,on ( ri )

η total (%) =

N out ,off ( ri )

∑m

out , off

× 100%

Eq. (2)

( ri ) − ∑ mout ,on ( ri )

∑m

out , off

( ri )

× 100%

Eq. (3)

where Nout ,on (ri ) and Nout ,off (ri ) denote the fractional particle number concentration (1/cm3) with and without corona discharge, respectively. mout ,on (ri ) and mout,off (ri ) denote the fractional particle mass concentration (mg/cm3) with and without corona


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discharge, respectively. ηi (%) and ηtotal (%) are the fractional and total collection efficiencies, respectively. U-I curve was typically used to describe the corona discharge characteristics of wet ESP. Another index of specific energy density (SED) was defined to reflect the correlation between corona discharge and aerosol formation:

SED=(U × I )/Q

Eq. (4)

where U is the applied voltage (kV), I is the corona current (mA), and Q is the gas flow rate (m3/s).

3

3.1

RESULTS AND DISCUSSION

Electrical characteristics of wet ESP

Figure 3a shows the voltage–current under different SO2 concentrations when the gas flow rate was 120 m3/h. Both applied voltage and corona current were monitored by the DC power supply. Results showed that the corona current was a function of applied voltage affected by SO2 concentration. The corona current of all SO2 concentrations increased monotonically as applied voltage increased, presenting an almost exponential relationship with applied voltage. When the applied voltage was 32 kV, the corona current of pure air and air with 10 ppm SO2 were 0.56 and 0.47 mA, respectively. The corona current is generated by gas ionization, and the reduction of corona current can be attributed to the decreasing ionization rate between electrons and gas molecules because of the increased electronegativity of SO2 molecule 21, 22. In addition, the corona current can also be suppressed by space charge once a large number of fine sulfuric acid aerosols exist in the flue gas 23.


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Figure 3. Corona discharge characteristics in wet ESP. (a) U-I curve under different SO2 concentrations and (b) photographs of corona discharge under different corona currents.

Figure 3b shows the photos of corona discharge taken from the side view of the wet ESP under different corona currents. The photos were captured at night with all lights off, and the exposure time was 3 min. The corona occurred (> 0.001 mA) when the applied voltage exceeded 16 kV, which was considered as the corona onset voltage. One or more bright spots appeared on the discharge wire surface under this condition. As the applied voltage increased, several bright spots appeared along the discharge wire surface with slight hissing sounds. As the applied voltage reached the breakdown voltage, the bright spots spread along the discharge wire while they were still not uniform, and spark-over occurred occasionally on some points. As proven by Mok et al., radicals and active species such as O, OH, O3, and HO2 could be produced during corona discharge

17, 24

. However, the HO2 lifetime is short and its production depends

on the amounts of O and OH. Therefore, O, OH, and O3 are considered significant in


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the conversion of SO2 in the corona discharge reactor. SO2 molecule can be activated and oxidized by these radicals and active species to form SO3 and corresponding acid aerosols by the following reactions:

SO2 + O → SO3

R1

SO2 + O3 → SO3 + O2

R2

SO2 + OH → HSO3

R3

HSO3 + OH → H2SO4

R4

HSO3 + O2 → HO2 + SO3

R5

SO3 + H2O → H2SO4

R6

During these reactions, SO2 was mainly oxidized into SO3 by O, OH, and O3, and SO3 was combined with H2O to produce the H2SO4 molecule. Large amounts of sulfuric acid aerosols were formed by nucleation afterwards under the wet flue gas condition.

3.2

Formation of sulfuric acid aerosol

3.2.1 Correlation between corona discharge and aerosol formation The evolution of aerosol number concentration was continuously measured downstream of the wet ESP as shown in Figure 4. Dusts were added into the flue gas from approximately 40 s, and a relatively stable number concentration approximately 3.5×104 1/cm3 was maintained for more than 60 s. A high voltage of 30 kV was implemented from about 120 s, and the number concentration dropped to around 1.3×104 1/cm3. From approximately 290 s, SO2 was introduced into the flue gas, and the SO2 concentration was changed about every 60 s. Once 5 ppm SO2 was introduced, the number concentration accelerated to more than 2.0×105 1/cm3, and it increased as SO2 concentration rose.


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Figure 4. Aerosol number evolution under different SO2 concentrations.

Figure 5. Aerosol number concentration and specific energy density under different voltages.

The SO2 conversion rates were dominated by the intensities of radical and active species. The intensities of these radicals and species were proven to be closely related to specific energy densities

25

. As shown in Figure 5, both aerosol number

concentration and specific energy density increased exponentially with the applied voltage when it exceeded 20 kV. The fitting relationship of aerosol number concentration and specific energy density was N=2.62 × 10 6 exp(SED/782)-2.77 × 10 6 , in which R2 was 0.99. Therefore, the aerosol number concentration strongly depended


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on a specific energy density.

3.2.2 Size distribution of formed sulfuric acid aerosol Sulfuric acid aerosols are usually formed by homogeneous nucleation and enlarged through coagulation and collision. These lead to a complicated size distribution downstream of the wet ESP. Figure 6 displays the size distribution of sulfuric acid aerosol under different applied voltages. A mono-modal-like size distribution mode was observed, which differed from the findings in the combustion burner and the atmospheric environment

26, 27

. The number concentration decreased

with increasing aerosol diameter under high applied voltages. Besides, the aerosol diameter with discharge tended to be smaller, mainly in the range of < 0.1 µm, than that without discharge. U-shaped aerosol penetration curves are observed in Figure 6b, indicating an extremely low penetration ratio in the size range of 0.1–1.0 µm. For example, for the condition of 22 kV, the penetration ratio was approximately 1 in this size range. The penetration ratio decreased with increasing aerosol diameter for aerosol size < 0.1 µm, whereas it increased as the particle diameter rose for aerosol size >1.0 µm because particle concentration in the background was relatively high in the size range 0.1–1.0 µm (Figure 6a). When the number concentration of sulfuric acid aerosol and foreign nuclei (background particle) was in the same level, the effects of heterogeneous nucleation cannot be ignored. After the interaction with foreign nuclei, the formed aerosols enlarged, which increased the concentration in the size range >1.0 µm.


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Figure 6. Sulfuric acid aerosol after the wet ESP with (a) aerosol number size distribution and (b) penetration ratio under different applied voltages.

3.2.3 Threshold condition for aerosol formation Figure 7 illustrates the penetration ratio under different specific energy densities with different gas temperatures. Evidently, the penetration ratio without SO2 decreased continuously with increasing specific energy density. In contrast, it decreased slightly to a minimum value first and then increased significantly as the specific energy density rose when SO2 was presented in the flue gas. For example, at gas temperature of 28.7 °C, the penetration ratio changed from 0.87 to 0.79 when the specific energy density increased from 0.5 to 11.8 J/m3, whereas the penetration ratio increased from 0.79 to 166.53 when the specific energy density increased from 11.8 to 905.8 J/m3. Sulfuric acid aerosols were generated by corona discharge in wet ESP, but some were subsequently collected by charging and migration. The formation and collection of sulfuric acid aerosol were two simultaneous processes, which were both positively influenced by the corona discharge intensity. The amounts of radical and active species produced by corona discharge were extremely small under lower specific energy density, indicating that only limited aerosols were formed. The amounts of radical and active species produced by corona discharge increased rapidly with increasing specific energy density. Therefore, the collection rate dominated under lower specific energy density, whereas the formation rate increased faster than the


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collection rate under higher specific energy density.

Figure 7. Sulfuric acid aerosol penetration ratio under different specific energy densities.

This study proposed a threshold specific energy density to describe the aerosol formation and collection processes, which referred to the specific energy density when the penetration ratio was equivalent to 1. The aerosol number concentration increased when the specific energy density exceeded the threshold one, and it increased with increasing gas temperature as presented in Table 1. The variation of threshold specific energy density with gas temperature is consistent with the gas temperature effects on aerosol formation and collection processes. Aerosol concentration was continuously monitored for more than 18 min during gas temperature increasing process (as shown in Figure 8) to investigate its effects on aerosol formation. Results showed that the aerosol number concentration generated by corona discharge increased as a function of gas temperature. It increased from 4.0×106 to 1.0×107 1/cm3 when the gas temperature increased from 36.0 to 56.0 °C. Particle collection efficiency has been proven to increase with increasing gas temperature due to the changes in the corona current and ion thermal mobility

28

. The threshold

specific energy density increased, suggesting that the influence of gas temperature on the aerosol collection was more obvious than the aerosol formation.


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Figure 8. Aerosol number concentration when gas temperature increased.

Table 1. Threshold specific energy density at different gas temperatures (fitting results according to the data in Figure 7). Gas temperature (℃)

28.7 33.6 38.3

43.8

49.6

54.6

Threshold specific energy density (J/m3) 16.7 22.3 30.2 100.7 128.4 154.4

The results discussed above are for total aerosols formed in the wet ESP. The fractional aerosol number concentrations under different specific energy densities were measured at gas temperature of 28.7 and 49.6 °C. As shown in Figure 9, the fractional aerosol number concentrations at two temperatures presented similar tendencies. The number concentrations for all sizes first decreased to a minimum value and then increased as the specific energy density rose. The specific energy densities corresponding to this minimum number concentration varied with aerosol diameter. These specific energy densities were much larger for aerosols ranging from 0.1 to 1.0 µm than that of other aerosols. Take gas temperature of 28.7 °C for example; they were 11.8, 326.0, and 31.7 J/m3 for aerosols with diameter of 0.016, 0.430, and 1.217 µm, respectively. This difference can be attributed to the size dependence of aerosol charging. The charge amount of charged aerosol in the size range 0.1–1.0 µm


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is usually less than that of smaller or larger aerosols due to the combined effects of diffusion and field charging mechanisms 6. Once the aerosol is charged, the energy barriers for condensation can be reduced

29, 30

. Two aspects contribute to the

difference of transition energy density for different aerosol sizes.

Figure 9. Aerosol number concentration for different aerosol sizes, (a) 28.7 and (b) 49.6 °C.

3.3

Aerosol formation influence on dust collection

As shown in Figure 10, the wet ESP performance for dust collection was evaluated under different SO2 concentrations. Results showed that the dust collection efficiency of all sizes decreased with SO2, and a negative collection efficiency was observed for particles smaller than 0.1 µm, indicating that the collected particle amount was less than that generated under this applied voltage. This finding can be attributed primarily to the generated aerosol number that happened to be high in this size range according to the data above. In addition, particles were primarily charged by diffusion charging in the size range < 0.1 µm. The suppression of corona current due to the SO2 electronegative effects or space charge of charged aerosols may affect the particle charging process, which could also diminish collection efficiency.


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Figure 10. Particle collection efficiency under different SO2 concentrations, (a) fractional collection efficiency, (b) total collection efficiency (gas temperature = 28.7 °C, applied voltage = 30 kV)

The total collection efficiency in Figure 10b is calculated by mass basis. The mass concentration (mg/m3) obtained by ELPI+ is based on several assumptions according to the following equation 31:

m(ri ) = N (ri ) × ( D(ri ))3 × π (1/ 6) × ρ × 0.001

Eq. (5)

where N ( ri ) is the aerosol number concentration (1/cm3), D ( ri ) is the median aerodynamic diameter (µm), ρ is particle mass density, and 1 g/cm−3 is used in this study. Although the fractional collection efficiency evidently decreased for particles smaller than 0.1 µm, the total collection efficiency was reduced from only 93.0% to 79.5% when the SO2 concentration increased from 0 to 25 ppm, and the corresponding particle concentration at the outlet increased from 1.08 to 3.14 mg/m3. Thus, even though the amount of aerosol generated with corona discharge was two orders of magnitude higher than that without corona discharge, it still accounted for a little part in the weight of particles downstream of the wet ESP. However, it will be a serious problem for the plants burning high sulfur content coal in real application, and


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the specific energy density should not rise too high to avoid the excess formation of sulfuric acid aerosols.

4

CONCLUSIONS

Wet ESP is typically used for sulfuric acid aerosol removal from wet flue gas. However, the formation of sulfuric acid aerosol by wet ESP is observed in the presence of SO2, and the process is influenced by specific energy density. In this study, a lab-scale wet ESP was designed to systematically investigate the sulfuric acid aerosol formation and collection during corona discharge. The aerosol number concentration after the wet ESP could be two orders of magnitude higher than that before the wet ESP. The aerosol number concentration increased exponentially with the applied voltage. The aerosol size distribution measured by ELPI+ indicated that the formed aerosol was mainly in the range of < 0.1 µm. Results showed that a threshold specific energy density for new sulfuric acid aerosol, which was influenced by gas temperature and aerosol diameter, existed. The dust particle removal efficiency decreased from 93.0% to 79.5% when the SO2 concentration increased from 0 to 25 ppm as a consequence of aerosol formation. Therefore, the specific energy density of a wet ESP should not rise too high to avoid the excess formation of sulfuric acid mist in actual applications.

5

ACKNOWLEDGEMENTS

This work was supported by the National Program on Key Basic Research Project (973 Program) (No. 2013CB228504), the National Key Research and Development Program of China (No. 2016YFC0203701), the Environmental Welfare Project of Ministry of Environmental Projection of China (No. 201509012), the Science and Technology Development Project of Shandong Province (No. 2014GJJS0501), and the Hangzhou Key Research & Development Innovation Project (No. 20142011A28).


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Sulfuric Acid Aerosol Formation and Collection by Corona Discharge

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