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Effect of corona discharge on PPS filter material of electrostatic-bag composite precipitators Yin Ye, and Zuwu Wang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b03829 • Publication Date (Web): 04 Jan 2018 Downloaded from http://pubs.acs.org on January 4, 2018

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Industrial & Engineering Chemistry Research

Effect of corona discharge on PPS filter material of electrostatic-bag composite precipitators Yin Ye, Zuwu Wang* School of Resource and Environmental Science, Wuhan University, No. 299，Bayi Road，Wuchang District，Wuhan City, Hubei Province 430079, China Email: [email protected] Email: [email protected] *Corresponding author

Abstract: The polyphenylene sulfide (PPS) filter bags of some electrostatic-bag composite precipitators are prone to being damaged within 4-12 months. In this work, to study the degradation mechanisms of the PPS filter material used in electrostatic-bag precipitators, we simulated the working environment of the precipitator. Additionally, after the aging corrosion, we analyzed the mechanical properties, morphology, crystallinity and chemical elemental composition of the PPS filter material, along with the components of flue gas in the precipitator. The results showed that oxidizing agents (O3, NO2 and SO3) can be produced in the process of corona discharge of the flue gas and that they accelerate the crystallization and the degradation of crystalline and amorphous regions of the PPS, leading to the degradation of PPS in the electrostatic-bag precipitator. With the coexistence of NO2 and SO3, the degradation energy barrier of PPS was reduced, thus resulting in faster degradation. Keywords: Electrostatic-bag composite precipitator; PPS; Degradation; Corona discharge; Oxidizing gas

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1. Introduction Flue gas from coal-fired power plants is a main source of atmospheric particulate pollution. In China, with the development of emission standards, traditional electrostatic precipitators have been unable to meet the existing processing requirements.1-3 Therefore, many precipitators in coal-fired boilers are subject to upgrade and transformation.4 To overcome this shortcoming, electrostatic-bag composite precipitators have been developed.5 The design concept of the electrostatic-bag composite precipitators is to retain the first electric field and remove the second and third electric fields; thus a bag precipitator is realized.6 In an electrostatic-bag composite precipitator, most of the dust can be removed by the electrostatic precipitator, whereas a small proportion of fine dust can be removed by the bag precipitator.6, 7 Because dust particulates carry some charge, they can repulse each other, and form a uniform mass concentration in an orderly arrangement on the filter bags, resulting in low resistance and easy cleaning.8-10 The combination of electrostatic precipitators and filter bags offers the advantages of both precipitators; thus, the composite precipitator is considered to be the “best combination”.11 However, the actual processes involve certain problems that are difficult to resolve. The failure of polyphenylene sulfide (PPS) nonwoven fabric bags, which is the core component of an electrostatic-bag composite precipitator, occurs sporadically during practical use.12-18 For instance, an electrostatic-bag composite precipitator was used in the transformation of 2×135 MW generator units of a coal-fired power plant in Henan Province, China. Its PPS filter bags had to be replaced because of extensive damage after only 8 months of operation. In another coal-fired power plant in China, two electrostatic-bag composite precipitators were used for a 200 MW generator unit. The PPS filter bag began to deteriorate dramatically after 4 months; consequently, all of the bags were replaced after only 1 year.13, 14 PPS nonwoven fabric filter material is a cost-effective filter materials used in processing the flue gas of coal-fired power plants.12, 15, 16, 18-20 In China, approximately 80% of the PPS filter material is used in coal-fired boilers.13 The service life of a PPS filter bag can reach 5 years provided that the bag


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is maintained and used properly. However, the service lives of some PPS filter materials of electrostatic-bag composite precipitators have been substantially shortened.19,

20

Research has

suggested that the O3 produced via corona discharge is the primary reason for the shortened service life of filter materials,21 whereas other researchers could not detect O3 in the electrostatic-bag precipitator using on-site tests,22 thus making the problem difficult to solve. In actual engineering projects, oxidation and acid-base corrosion are the main factors causing damage to the PPS fiber.23-25 O2, SO2, NO and other substances in the flue gas of coal-fired power plants have corrosive effects on PPS.26 Tanthapanichakoon et al. found that the mechanical properties of PPS fabrics are related to two phenomena: the crystallization of the material and the degradation of amorphous regions and some parts of the crystallized regions. 12, 15 These two processes compete with each other.27 Using differential scanning calorimetry (DSC), Park et al. found that the curing and heat treatment of PPS in air can cause oxidation and cross-linking.28 Gies et al. used matrix-assisted laser desorption/ionization–time-of-flight (MALDI-TOF) mass spectrometry, and found that the main chain of PPS can form a sulfoxide structure.29 Mao et al. studied the corrosion and failure processes of PPS filter material in an SO2 environment and proposed that PPS had good corrosion resistance against SO2, although high temperatures could accelerate the corrosion PPS by SO2.30 Tanthapanichakoon et al. reported that NO at high temperatures increase the deterioration rate of crystallized and non-crystallized regions of PPS but does not affect the process of crystallization of PPS.18 Additionally, O2 can simultaneously increase the rate of crystallization and the deterioration rate of PPS.12 However, existing theories cannot fully explain the apparent reduction in the service life of the PPS filter material in electrostatic-bag composite precipitators used in coal-fired power plants. Therefore, it is very important to carry out studies on the aging and failure processes of PPS filter material in electrostatic-bag composite precipitators and to determine the main reason for the significant reduction in the service life of the filter material. Therefore, in the present work, the gas experiment method was used to investigate the oxidation of PPS filter material in a simulated electrostatic-bag filter environment, which was used to reflect the actual operating environment of the



filter material. Moreover, the changes in composition of flue gas in the simulated electrostatic-bag precipitator were also studied to investigate the reasons for the accelerated aging of PPS could be investigated.

2. Experiments 2.1 Methodology The experiments were conducted under normal pressure. The experimental device consisted of two systems: a needle-plate discharge system at the front and a high-temperature 10 L aging vessel at the rear. The needle-plate discharge system was used to simulate the electrostatic precipitator system in the electrostatic-bag composite precipitator. The maximum output voltage of the high-voltage DC power supply was 60 kV, whereas the measurable maximum current on the ammeter was 2000 µA with a precision of 0.5. The high-temperature aging vessel was used to simulate the exposure of the system to a corrosive environment. The aging vessel comprises the heating system and stainless-steel tank in the setup. The setup was run in continuous mode, and its schematic is shown in Figure 1.

Figure 1. Schematic of the experimental setup. During each group of experiments, the PPS filter material was placed in the aging vessel. The simulated flue gas generated by the gas distribution system was dried by a dehumidification device. The dry gas passed through the needle-plate discharge system, then entered the aging vessel. The


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discharge system maintained a stable high-voltage corona discharge at 25 kV during the entire testing process. The aging vessel was maintained at a high temperature (200 °C). The simulated flue gas entered the experimental system at a constant flow rate (2 L/min) and was purged in the absorption bottle as exhaust gas. The PPS filter material was removed from the vessel after aging corrosion and was then cooled in a vacuum dryer for 24 h. After cooling, the material was used for further testing and analysis. The filter material was cut into small pieces with dimensions of 20 cm × 5 cm to analyze the mechanical properties. The mechanical properties were measured by an electronic universal material testing machine (Material Testing System i-strentek 1510, Labthink Instruments Co., Ltd., Jinan, China). In addition, a 200-kV field-emission scanning electron microscope (Sigma 500, Carl Zeiss, Inc., Germany) was used to observe the morphology of the samples. The crystallinity was determined by a differential scanning calorimeter (Diamond DSC LT, Mettler Toledo Co., Ltd., Switzerland). The temperature range, heating rate and flow rate of N2 in DSC measurements were measured from room temperature to 350 °C, 30 °C/min and 100 ml/min, respectively. The change in crystallinity was directly evaluated from the peak area of the fusing heat.28 The elemental analysis was performed by a CHNS analyzer (Elementar Vario MICRO cube, Elementar Co., Ltd., Germany). In these experiments, the mechanical properties, change in crystallinity and elemental change in filter material were the most important evaluation indices. Among these, the mechanical properties of the filter material were expressed through the retention of breaking strength, which is defined as Equation (1). Retention of strength =

strength after corrosion × 100% original strength

(1)

Fiber orientation and fiber strength are the two main parameters that determine the bulk strength of needled nonwoven fabric in various directions, e.g., machine and transverse directions. The machine direction means the direction parallel to the direction of the movement of the fabric. The transverse direction is known as the cross-machine direction. Given that the fiber orientation does not substantially change with time after gas exposure, the tensile tests of the machine and transverse directions are used to investigate the mechanical properties of the PPS fabric, which means that the



retention of strength of machine and transverse directions are two parameters that were used during experimentation. The strength testing method is wildly used in the research of needled nonwoven fabric.13 Meanwhile, after the corona discharge, the composition of the flue gas was characterized to determine the corrosive substances generated in the process of corona discharge that can accelerate the aging and corrosion of PPS.

2.2 Experimental conditions In the electrostatic-bag composite precipitator, the normal operating temperature for the PPS filter bag was defined as 200 °C or less.31 For the PPS, the rate of the crystallization process is the fastest at approximately (Tg + Tm)/2 (approximately 198.5 °C).12 Therefore, the temperature of 200 °C was selected as the experimental temperature. The selected NO and SO2 concentrations were 500 and 1000 ppm, respectively, and there are several reasons why these concentrations were selected. First, for PPS filters, when the maximum surge temperature is 190 °C, the maximum operating concentration of SO2 is 1000 ppm, and the maximum operating concentration of NO is 500 ppm.13, 30 Secondly, many researchers have suggested setting the operating concentrations to these values, as in the following examples: “NO concentration is several times than that in the real incinerating plants so as to reduce the exposure time required to evaluate the degradation, i.e., 500ppm and 1000ppm.”12 “If the volume ratio of SO2 in mixing gas is more than 700 ppm, we should deal with it beforehand, or make its temperature below 170 °C.”30 Finally, many power plants that have installed electrostatic-bag precipitators were investigated, and the average data from them for 12 months are as follows: 710.92 ≤ [SO2] ≤ 1036.37 ppm, 426.77 ≤ [NO] ≤ 509.55 ppm. Therefore, the selected concentrations were slightly higher than the concentrations in the actual operating environment, which can accelerate the aging and corrosion of PPS. The O2 concentration was chosen to be 5%, which ensured that the simulated flue gas environment was close to the actual power plant environment for the PPS. The needle-plate discharge system was a scaled-down electrostatic precipitator. The needle-plate spacing was proportionately reduced to 40 mm. The experimental conditions are presented in Table 1.


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In the design process, the selection of voltage for the electrostatic precipitator was based on the standard 0.36 kV/mm.2 For this reason, the theoretical operating voltage was 14.4 kV. The volt-ampere characteristic test for the needle-plate discharge system showed that the onset voltage and the spark voltage were not the same under various flue gas conditions. The highest onset voltage was 10 kV, whereas the lowest spark voltage was 26 kV. On the basis of these results, a voltage of 25 kV which was closer to the spark voltage was selected as the voltage for aging tests to ensure the maximum effect during a short time period. To ensure the stability of the flue gas concentration in the experimental system, the gas flow rate was maintained at 2 L/min. Table 1. Experimental conditions for the exposure tests. Test materials

PPS

Temperature (°C)

200

Time (h)

100, 200, 300, 400, 500

Moisture content (%)

0

Corona voltage (kV)

25

Concentration of various gas components

(1) 5% O2 (2) 5% O2 + 500 ppm NO (3) 5% O2 + 1000 ppm SO2 (4) 5% O2 + 500 ppm NO + 1000 ppm SO2

2.3 Materials PPS (product model No. 6947PPS/PPS 551 MPS CS30). The gases used were N2 (99.9% purity), O2 (99.9% purity), NO (99.9% purity) and SO2 (99.9% purity).

3. Results and Discussion 3.1 Effect of corona discharge on flue gas To determine what substances were generated under the corona discharge conditions, which deepened the corrosion of PPS, the effect of corona discharge on the composition of different flue gases was studied experimentally. The flue gas flow rate and the humidity were maintained at 2 L/min and 0%, respectively. The contents of different substances in the exhaust gas were then measured under various flue gas conditions and a 25 kV discharge voltage. The experimental results are presented in Table 2.



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The experimental data show that O3 was produced in a 5% O2 atmosphere with corona discharge. Additionally, NO2 was produced in 5% O2 + 500 ppm NO atmosphere. For the 5% O2 + 1000 ppm SO2 atmosphere, SO3 was produced, whereas NO2 and SO3 were simultaneously produced in the atmosphere of 5% O2 + 500 ppm NO + 1000 ppm SO3. The production of oxidizing substances was substantial at 25 kV, which was close to the spark discharge. At the same time, O3 was not observed in the mixed flue gas atmosphere (5% O2 + 500 ppm NO, 5% O2 + 1000 ppm SO2, 5% O2 + 500 ppm NO + 1000 ppm SO2), indicating that O3 was not produced in the mixed flue gas, which is in accordance with the actual experience at the power plant. Table 2. Gas concentrations before and after the corona discharge (for 25 kV and at 200 °C). Number Gas Types

O2 (%) In let

3

5% O2 + 500

5% O2 + 1000

ppm NO

ppm SO2

5

5

5

5

(+0.1; -0.1)

(+0.1; -0.1)

(+0.1; -0.1)

(+0.1; -0.1)

5% O2

500

0

SO2 (ppm)

0

0

5 (+0.1; -0.1)

NO (ppm)

0

SO2 (ppm)

0

utl et

2

NO (ppm)

O2 (%)

O

1

O3 (ppm)

(+23.2; -31.5)

0

SO3 (ppm)

0

NO + 1000 ppm SO2

500 (+35.4; -18.8) 1000

(+11.4; -35.2)

(+22.7; -27.1)

5

5

5

(+0.1; -0.1)

(+0.1; -0.1)

(+0.1; -0.1)

0

(+18.9; -26.4) 0

(+22.1; -25.3)

5% O2 + 500 ppm

1000

188.3

298.3

NO2 (ppm)

0

4

(+35.2; -13.2)

738.4

829.4

(+44.4; -26.9)

(+15.6; -38.6)

0

0

0 311.7

0

(+13.6; -31.2) 0

194.3

305.7 (+11.9; -26.5)

261.2

170.6

(+24.3; -17.7)

(+21.2; -27.4)

The results indicate that some gases such as O3, NO2 and SO3 with stronger oxidizing and corrosive properties were generated during the low-temperature plasma process initiated by the corona


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discharge.32, 33 According to several previous studies, the flue gas can be oxidized during the corona discharge process, and the oxidation process is related to the chemical reaction of free radicals.21 During the low-temperature plasma process initiated by the DC corona discharge, the generation of free radicals and the oxidation of flue gas can be divided into three stages as follows: (1) the discharge stage, where the primary and excited free radicals are generated and are consumed within approximately 10-8 s; (2) the post-discharge stage, where the excited free radicals are eliminated, and free radicals with longer lifetimes are formed, which are consumed in approximately 10-3 s; and (3) reaction between free radicals and flue gas, where the time-span order of this stage is a nanosecond or slightly longer. Figure 2 describes the reaction processes of these three stages.34

Figure 2. Oxidation processes of flue gas under corona discharge. In the 5% O2 atmosphere, the reaction was terminated at the second stage, and a large amount of O3 was generated. In the mixed flue gas atmosphere, O3 was generated in Stage II, where NO and SO2 were oxidized into NO2 and SO3, respectively. The oxidation process consumed O3, thus resulting in an unmeasurable concentration of O3 in the exhaust gas. Meanwhile, in the three-component mixed flue gases, the generation efficiency of SO3 decreased, whereas the generation efficiency of NO2 did



not substantially change. The reason is that these two substances are competitive in the coexistence of NO and SO2. In any event, the flue gas can generate a series of more oxidative substances for PPS material through the corona discharge process. These substances include O3, NO2 and SO3 and confirm the hypothesis that the oxidative substances shorten the service life of PPS filter material in an electrostatic-bag precipitator. The existence of these substances deepens the degree of oxidation and corrosion of the PPS filter material in an electrostatic-bag precipitator, accelerates the aging and degradation of PPS, and shortens the service life of the filter material. Therefore, an electrostatic-bag composite precipitator with numerous damaged filter bags may operate at a voltage approximately equal to the voltage for certain reasons.

3.2 Effects of corona discharge on PPS filter material

3.2.1 Changes in crystallinity Annealing usually results in a change in the crystallinity of a polymeric material; however, the intensity of the effect can be changed by the presence of corrosive gases.35 These experiments investigated the effect of corona discharge on the mechanical properties of PPS filter material under various flue gas conditions and at a voltage of 25 kV. The detailed experimental conditions are listed in Table 1, with 10 iterations for each data point. The crystallinity of PPS over time under different flue gas conditions at 200 °C is shown in Figures 3-6. Irrespective of whether corona discharge occurred or not, the crystallinity initially increased because of the annealing process and then decreased monotonically with the passage of time because of the degradation process in the presence of corrosive gases.36


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Figure 3. Change in crystallinity of PPS with time in a 5% O2 atmosphere.

Figure 4. Change in crystallinity of PPS with time in a 5% O2 + 500 ppm NO atmosphere.



Figure 5. Change in crystallinity of PPS with time in a 5% O2 + 1000 ppm SO2 atmosphere.

Figure 6. Change in crystallinity of PPS with time in a 5% O2 + 500 ppm NO + 1000 ppm SO2 atmosphere. The results show that in the same atmosphere, the presence or absence of corona discharge affects the crystallinity of PPS differently. Figure 3 shows that, in 5% O2 atmosphere and in the absence of corona discharge, the crystallinity increased continuously within 200 h and then began to decrease. However, after reaching a peak value at 100 h, the crystallinity started decreasing because of the effect of corona discharge. In the 5% O2 + 500 ppm NO atmosphere, the peak of crystallinity


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variation occurred at 200 h in the absence of corona discharge, whereas the peak of crystallinity variation occurred at 100 h in the presence of corona discharge, as shown in Figure 4. The same situation occurred in the other two atmospheres of 5% O2 + 1000 ppm SO2 and 5% O2 + 500 ppm NO +1000 ppm SO2, as shown in Figures 5 and 6. Similarly, in the same atmosphere, the peak value of the crystallinity variation of the PPS material was higher in the presence of corona discharge than in its absence. Figures 3-6 represent this trend graphically. After 200 h, the crystallinity began to decrease because of the degradation in the amorphous regions and some parts of the crystalline regions. Although not obvious, changes in crystallinity differed after 200 h with or without corona discharge: the decreasing trend of crystallinity was faster under the corona discharge conditions. The reason for this difference is not obvious: the reaction of PPS and flue gas is pseudo-zeroth-order for a solid-gas reaction, and the duration of the experiment was short. The aforementioned results show that under the same atmosphere, the corona discharge condition significantly changed the pattern of change in the crystallinity of the PPS material. This phenomenon may originate from one or more than one substance generated by the corona discharge.37 Such substances can accelerate the crystallization process of PPS during the annealing process and lead to the peak of crystalline variation, which is larger than without corona discharge. Meanwhile, during the degradation process, more than one substance generated by the corona discharge led to more violent degradation reactions of crystalline and amorphous regions and to changes in the crystal structure. Another reason is that these substances generated by the corona discharge are strongly oxidizing toward PPS. The reconstruction of PPS crystals is accelerated by a strongly oxidizing gas; thus, the crystallinity changes rapidly.37

3.2.2 Effect of chemical composition The contents of carbon, hydrogen, oxygen, and sulfur in the original PPS filter material and PPS filter material after aging under different conditions were measured. The results are shown in Table 3. The PPS filter material contained a small amount of O, which may enter the manufacturing process of the



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filter material. After aging and corrosion, the percentages of C, H and S atoms decreased, whereas the percentage of O atoms increased, indicating that the oxidizing corrosion occurred under the action of flue gas; hence, new oxygen-containing functional groups were introduced into the PPS molecular structure, because some PPS molecules were oxidized to form polyarylene sulfoxide, which could be further oxidized to diphenylsulfone.38 Table 3. CHNS analysis of PPS under various corrosion conditions (after 500 h and at 200 °C).

Name PPS 5% O2 5% O2; corona discharge 5% O2 + 500 ppm NO 5% O2 + 500 ppm NO; corona discharge 5% O2 + 1000 ppm SO2 5% O2 + 1000 ppm SO2; corona discharge 5% O2 + 500 ppm NO + 1000 ppm SO2 5% O2 + 500 ppm NO+1000 ppm SO2; corona discharge

C (%) S (%) H (%) O (%) 65.18 27.62 3.56 3.12 63.59 26.79 3.49 4.71 62.85 26.38 3.34 5.29 62.58 27.23 3.53 4.97 61.51 26.10 3.60 6.66 64.41 27.15 3.37 5.03 59.92 25.69 2.96 7.47 62.77 26.01 3.18 5.68 57.81

25.33

2.77

8.55

Under the same flue gas conditions, with or without the corona discharge, a comparison of elemental compositions of the PPS showed that the content of oxygen was increased under the corona discharge conditions, indicating that the corona discharge intensified the oxidation of PPS. Meanwhile, under the conditions of corona discharge, a comparison of elemental compositions under different flue gases indicated that the degree of oxidation followed the order 5% O2 + 500 ppm NO + 1000 ppm SO2 > 5% O2 + 1000 ppm SO2 > 5% O2 + 500 ppm NO > 5% O2. This order confirms that under the corona discharge conditions, the flue gas components produced one or more oxidizing substances, which intensified the oxidizing corrosion of the PPS material.

3.2.3 Effect on mechanical properties The changes in mechanical properties of the filter material directly show the effect of corona discharge on the PPS filter material, and the retention of strength in the machine and transverse directions was used to investigate the mechanical properties of the PPS filter material during the experiments. The experiments investigated the effect of corona discharge on the mechanical properties


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of PPS filter material under various flue gas conditions and at a voltage of 25 kV. The detailed experimental conditions are listed in Table 1, with 10 iterations for each data point; the experimental results are shown in Figures 7-10.

Figure 7. Change in strength of the PPS material with time in 5% O2 atmosphere.

Figure 8. Change in strength of the PPS material with time in 5% O2 + 500 ppm NO atmosphere.



Figure 9. Change in strength of the PPS material with time in 5% O2 + 1000 ppm SO2 atmosphere.

Figure 10. Change in strength of the PPS material with time in 5% O2 + 500 ppm NO + 1000 ppm SO2 atmosphere. As shown in Figure 7, affected by the flue gas containing 5% O2 and at high temperature (200 °C), the mechanical strength of the PPS filter material first slightly increased, and then decreased smoothly regardless of the fact of whether the corona discharge existed or not. This behavior was


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observed with or without the corona discharge because, under conditions of high-temperature oxygen, the crystallization phenomenon occurred in the fiber and the degradations of the crystalline and the amorphous regions occurred simultaneously. The process of crystallization led an increase in the fiber strength, whereas the degradation decreased the strength of the fibers. Initially, in the exposure experiments, the crystallization process in the fibers was dominant so that its strength increased slightly. Then, the degradations of the crystalline and the amorphous regions within the fiber material became dominant so that the strength decreased smoothly. Similar trends were observed under other flue gas conditions, as shown in Figures 8-10. A comparison of results in the presence and absence of corona discharge indicates that the mechanical properties of the PPS filter material gradually decreased because of long-term exposure to corrosive flue gas. However, the corona discharge exacerbated the degradation of PPS material, in accordance with practical experience. Irrespective of which flue gas condition was applied, the mechanical properties of the PPS filter material deteriorated under a voltage of 25 kV. As shown in Figure 7, after 500 h of reaction time in 5% O2 and at 200 °C with corona discharge, the retention of breaking strength of the PPS filter material along the machine and transverse directions was 92.95% and 112.80%, respectively. By contrast, in the absence of corona discharge, the mechanical properties of the PPS filter material decreased by 2.31% (machine direction) and 4.61% (transverse direction). As shown in Figure 8, after 500 h of reaction time in 5% O2 + 500 ppm NO and at 200 °C, the values of the retention of breaking strength of the PPS filter material decreased by 3.04% (machine direction) and 5.69% (transverse direction) because of the corona discharge conditions. Similarly, as shown in Figures 9 and 10, after 500 h of reaction time in 5% O2 + 1000 ppm SO2, and at 200 °C, the retention of strength of the PPS filter material decreased by 6.60% (machine direction) and 7.85% (transverse direction) because of the corona discharge conditions. Similarly, in 5% O2 + 500 ppm NO + 1000 ppm SO2, the values of retention of breaking strength of the PPS filter material decreased by 16.51% (machine direction) and 13.31% (transverse direction) because of the corona discharge conditions. The decrease in strength was greater in the transverse direction fiber orientation



because the fabric strength is higher in the direction that most fibers are laid. Since the nonwoven fabric being tested has more fibers oriented in the transverse direction, this direction exhibits higher strength than the machine direction. The deterioration of fabric strength is hypothesized to be caused mainly by the change of fiber strength. The fabric has fewer fibers oriented in the transverse direction; thus, with the degradation of fibers, the decrease in strength is greater in the transverse direction. The aforementioned results indicate that corona discharge conditions have different effects on the PPS filter material under different flue gas compositions. Obviously, under the condition of corona discharge, a comparison of the mechanical properties of the PPS filter material showed that the reduction of strength decreased in the following order: 5% O2 + 500 ppm NO + 1000 ppm SO2 > 5% O2 + 1000 ppm SO2 > 5% O2 + 500 ppm NO > 5% O2. A more complicated composition of the flue gas produces more oxidizing substances under corona discharge conditions, thus resulting in a greater effect on the filter material. The order again confirms that under the corona discharge conditions, the flue gas components produced one or more oxidizing substances that intensified the oxidizing corrosion of the PPS material. For an electrostatic-bag composite precipitator, the reason for the shortening of the service life of the PPS filter material may be the oxidizing corrosion. The acceleration of degradation reactions at an elevated temperature leads to molecular re-orientation and structural changes in the fibers, and to some cross-linking and branching reactions of PPS. Even if the decrease of the mechanical properties of the PPS filter materials is detected, no morphological changes are observed in the filter structure as shown in Figure 11.12


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Figure 11. Morphology of the PPS fibers before and after exposure to different conditions: (a) origin; (b) 5% O2; (c) 5% O2 + 500 ppm NO; (d) 5% O2 + 1000 ppm SO2; (e) 5% O2 + 500 ppm NO + 1000 ppm SO2; (f) 5% O2, corona discharge; (g) 5% O2 + 500 ppm NO, corona discharge; (h) 5% O2 + 1000 ppm SO2, corona discharge; (i) 5% O2 + 500 ppm NO + 1000 ppm SO2, corona discharge.

3.3 Accelerated degradation mechanism of PPS by corona discharge Different concentrations of the flue gas have different effects on the mechanical properties of PPS filter materials. However, compared with the results when corona discharge was absent, the presence of corona discharge resulted in a stronger effect on the PPS filter material.



The results show that the mechanical properties of the PPS filter material were related to two phenomena: the crystallization phenomenon and the deterioration of amorphous and semi-crystalline regions. These two phenomena were competitive with each other. In the absence of corona discharge, the increase in crystallinity of PPS was slow as 5% O2, 500 ppm NO, 1000 ppm SO2 and the flue gas consisting of their mixture increased the rate of damage to the amorphous and semi-crystalline regions of PPS. However, their crystallization rates were observed to be only slightly affected. Under the condition of corona discharge, gases with stronger oxidizing abilities were generated, which simultaneously increased the crystallization rate and the breakdown rate of the PPS material. Meanwhile, the more oxidative environment exhibited a stronger deteriorating effect on the amorphous region, which led to the accelerated degradation of the PPS filter material under corona discharge conditions.16, 39 After the mechanical properties of PPS reached their peak values, they began to decrease and showed a near-linear relationship with time. During the reaction, the damage to amorphous and semicrystalline regions was dominant and the gas-solid reaction rate was found to be pseudo-zeroth-order. Obviously, the slope differed for different atmospheres, where a more oxidative atmosphere led to a decreasing slope.35 With respect to the reaction mechanism, in addition to the cross-linking and divergence, oligomerization of the polymer also occurred in the PPS, which was exposed to a high-temperature oxidizing atmosphere.24 Scheme 1 reflects the oxidation and cross-linking reactions of the PPS molecules under high-temperature oxidizing atmospheres.39-42 In the oxidation and cross-linking process, oxidation occurred at the thio-ether bond, whereas the cross-linking occurred on the benzene ring. Additionally, the oxygen element linked to the main chain of the PPS in the form of sulfoxide. In the process, the chain molecules connected to each other and formed a network with a dense molecular arrangement.42 Macroscopic observation showed that the toughness of PPS filter material decreased, whereas the brittleness increased and the chain structure was found to be easily broken.


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Scheme 1. Oxidation and cross-linking reactions of the PPS molecules under high-temperature oxidizing atmospheres: (a) formation of peroxy radicals of relatively high activity, (b) formation of stable phenyl radicals and peroxides, (c) the process of producing hydroxyl radicals and phenoxyl radicals and (d) the cross-linking reaction between the phenoxyl radicals and phenyl radicals. Under the long-term action of a high-temperature oxidative atmosphere, the PPS molecules underwent further oxidation and degradation, resulting in cleavage of the benzene-sulfur bonds of the PPS molecules cleaved and the occurrence of different free-radical reactions. Therefore, the PPS molecules were not only oxidized into sulfoxide and sulfone groups but also into compounds with phenolic and carbonyl groups.38 The oxidative degradation mechanism is shown in Scheme 2. When the temperature exceeded 350 °C, the molecule broke and produced SO2. However, this process



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generally does not occur in an electrostatic-bag precipitator because the temperature in the precipitator is normally less than 200 °C.

Scheme 2. Oxidative degradation mechanism of PPS under high-temperature oxidizing atmospheres. The oxidation pathways of NO2 and SO3 on PPS are shown in Scheme 3. After the radical reactions with different transition states, the final product, diphenyl sulfone, was formed.43,

44

In

previous studies, the structures were optimized into two reaction pathways and their energies were calculated. The calculated energy barriers of all stages in the oxidation of PPS are presented in Table 4.45 The highest energy barrier was 274.88 kJ/mol in the oxidation of S into sulfoxide by NO2, which was much higher than the 210.42 kJ/mol observed for SO3. Additionally, the highest energy barrier to SO3 oxidation occurred at the TS2 stage, indicating that SO3 had a stronger oxidative effect on the PPS material. At the same time, SO3 more easily oxidized the sulfoxide into diphenylsulfone in one step, whereas it was difficult for NO2 to accomplish such an oxidation. When NO2 and SO3 coexisted, the TS1 stage energy barrier decreased to 153.28 kJ/mol, whereas the TS2 stage energy barrier decreased to 111.65 kJ/mol for the overall reaction, indicating that the coexistence of NO2 and SO3 would produce synergistic effects for the oxidation of PPS material.


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Scheme 3. Oxidation pathways of NO2 and SO3 on PPS: (a) oxidation pathways of PPS by NO2 and (b) oxidation pathways of PPS by SO3. Table 4. Potential energies for different reaction pathways of oxidation of PPS by NO2 and SO3 TS1

TS2

TS3

TS4

(kJ/mol)

(kJ/mol)

(kJ/mol)

(kJ/mol)

NO2

274.88

111.65

-20.69

-149.69

SO3

153.28

210.42

204.49

144.67

Oxidizer

The results show that under corona discharge conditions, the mechanical properties of the PPS filter material deteriorate faster in the O2 + SO2 mixed atmosphere than that in the O2 + NO mixed atmosphere. In the three-component mixed atmosphere, the corona discharge produces SO3 and NO2. Under the action of O2, SO2, NO, SO3 and NO2, the degradation of the PPS filter material was very rapid. Simultaneously, this rapid degradation theoretically explains the shortened service life of the PPS filter material in an electrostatic-bag precipitator PPS filter.

4. Conclusions



The present experimental work proposes a novel scheme to study the aging and degradation process of PPS material in an electrostatic-bag precipitator. Compared with the degradation of the mechanical properties of the PPS filter material under the no-discharge condition, that under the presence of corona discharge was faster. The main reason for the accelerated degradation of the PPS filter material was the flue gas, which produced gases with stronger oxidizing abilities under corona discharge conditions. Flue gas consisting only of O2 can produce O3 during the discharge; however the mixed flue gas did not produce O3. The discharge of two-component flue gas can produce SO3 or NO2. Three-component flue gas can simultaneously produce SO3 and NO2. The main reason for the accelerated aging of PPS filter material was the production of NO2 and SO3, which were produced by the corona discharge process in the electrostatic-bag composite precipitator. The phenomenon of PPS polymer deterioration can be divided into two phases. The initial phase consists of crystallization, which is similar to the annealing process, where the molecular structure changes and the molecules reposition themselves. The second phase consists of the degradation process of the amorphous and semicrystalline regions. In the absence of corona discharge, the flue gas exhibited little effect on the crystallization process; consequently, the strength depended on the degradation of amorphous and semicrystalline regions. In the presence of corona discharge, gases with stronger oxidizing abilities were generated, which resulted in a simultaneous increase in the crystallization and degradation rates. In summary, all of the aforementioned processes indicate that the mechanical properties of the PPS filter material will deteriorate faster. The polymer oligomerization reaction occurred among the PPS molecules, which were exposed to a high-temperature oxidizing atmosphere. The cross-linking and the degradation of molecules also occurred. The long-term high-temperature conditions further oxidized and decomposed the PPS to form sulfoxide and sulfone structures. The two oxidizing agents, NO2 and SO3, exhibited different oxidative reactions for the PPS filter material, whereas the oxidation ability of SO3 toward PPS was


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stronger than the oxidation ability of NO2. At the same time, the mixture of NO2 and SO3 produced a synergistic effect for the degradation of the PPS material. In summary, the PPS filter material is the first choice for traditional bag precipitators because of its high property/cost ratio. However, the oxidation resistance of the PPS filter material is relatively poor. Therefore, care must be taken when incorporating PPS filter material into electrostatic-bag precipitators. For example, sparking voltage should be avoided in an electrostatic-bag composite precipitator, and the flue gas must be preprocessed to reduce the concentrations of SO2 and NO.

Author Information Corresponding Author *Tel.: +86 027-68775543. Email: [email protected]

Declaration of Conflicting Interests The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Acknowledgments We gratefully acknowledge financial support received for this study from the National High-tech R＆ D Program (863 Program) of China (2013AA065104).

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