Simultaneous Removal of PH3, H2S, and Dust by Corona Discharge

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Simultaneous removal of PH3, H2S and dust by corona discharge Yi Xing Ma, Xue Qian Wang, Ping Ning, Chen Cheng, Fei Wang, Langlang Wang, Yilong Lin, and Yongtao Yu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b01563 • Publication Date (Web): 12 Oct 2016 Downloaded from http://pubs.acs.org on October 20, 2016

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Simultaneous removal of PH3, H2S and dust by corona discharge Yixing1 Ma, Xueqian1 Wang, Ping∗1 Ning, Chen1 Cheng, Fei2 Wang, Langlang1 Wang, Yilong1 Lin, Yongtao1 Yu 1 Faculty of Environmental Science and Engineering, Kunming University of Science and Technology, Kunming 650500, China 2 Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing, 100000, China

Abstract: Conversion of PH3 and H2S and simultaneous removal of dust by corona discharge were studied in a wire-cylinder corona discharge reactor. Under the corona discharge, the removal efficiencies of PH3, H2S, and dust reached 99.5%, 99.0%, and 99.9%, respectively. The results of this study indicated that high conversion of PH3 and H2S were achieved under high SIE (specific input energy). The required SIE for PH3 was lower than H2S to achieve the same conversion efficiency, partially because the chemical bond energy of PH3 is lower than that of H2S. Low content of O2 was favorable for removing PH3 and H2S due to the energy consumption of O2 in the electric field. PH3 was converted to elemental phosphorus, H2, and H3PO4, and H2S converted was to elemental sulfur, H2, and SO2 or SO42when PH3 only or H2S only gas was treated with corona discharge under very low O2 content condition (0.1 vol. %). The gas mixture of PH3 and H2S under low O2 content condition generated H2, H3PO4, P4S10, elemental sulfur, and SO2 or SO42-. However, H2, elemental phosphorus or elemental sulfur was not generated when the O2 content

∗

Corresponding author E-mail: [email protected]

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was more than 0.1 vol. %. Moreover, this study demonstrated that dust had a positive effect on H2S conversion but inhibited the generation of SO2. The conversion of PH3 and H2S proceeded through the cleavage of chemical bond by electron collision with corona discharge, in which the distribution of products depended on the species of coexisting gases. Keywords: Phosphine, Hydrogen sulfide, Dust, Corona discharge Introduction The presence of Phosphine (PH3) and Hydrogen sulfide (H2S) in industrial gases often leads to debasing of gas quality, excessive corrosion of equipment and deactivation and poisoning of catalysts, which limited the recovery and utilization of industrial gases. Moreover, it will cause serious adverse effects on human health and the environment when industrial gases are released without removal of PH3 and H2S. PH3 is one of the highly toxic gases, which might cause immediate death at a level of 50 ppm exposure according to the National Institute for Occupational Safety and Health (NIOSH) [1]. Nevertheless, PH3 is present in coal syngas, acetylene gas, and yellow phosphorous tail gas, and widely used in various industrial processes and fields, such as phosphine fumigation, semiconductor industry, and electronic industry [2-7]. H2S is a major toxic and malodorous gas emitted from industrial processes, primarily from extraction and refining of oil and natural gas, coal gasification, yellow phosphorous tail gas, and acetylene production [8-10]. PH3 and H2S often coexist in yellow phosphorous tail gas, coal syngas and acetylene production, which usually contain dust. Therefore, it is desirable to remove them simultaneously from industrial


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gases. The methods of adsorption and catalytic oxidation to remove harmful gases are extensively studied. Al2O3, V2O5, active carbon and TiO2 are often used as support and loaded with metals such as Fe, Mo, Cu, Ag, Na for removing PH3 and H2S [1, 7, 11-14]. Although these methods can remove PH3 and H2S efficiently, there are some substantial drawbacks associated with their practical applications, such as the specific temperature range, directionality problem, catalyst poisoning and deactivation caused by coexisting gas or other substances [15-17]. Particularly, the dust in industrial gas, such as yellow phosphorus tail gas and coal gas, makes it impossible or impracticable to remove PH3 and H2S by conventional methods of adsorption and catalytic. Since comprehensive control is a growing trend in pollution treatment and gas purification [18-19], the simultaneous removal of PH3, H2S, and dust is highly desired. In recent years, non-thermal plasma has attracted increasing attention from researchers for their potential applications in pollution treatment, especially on reducing highly toxic pollutants, such as CS2, Hg0, VOCs, NOx, H2S, C2H6S, and C7H8O [19, 20-25]. In these studies, although pollutants were removed with non-thermal plasma generated by dielectric barrier discharge (DBD) or direct current discharge (DC), the dust was not collected in these processes. Corona discharge can produce non-thermal plasma efficiently, which is the basic principle of an electrostatic precipitator to catch and collect dust. However, there is still no report about the conversion of PH3 by corona discharge or simultaneous removal of toxic gases and dust by corona discharge. Non-thermal plasma has been widely investigated to


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dissociate H2S to produce H2 and S, which is highly efficient [26-28]. But the main goal is to produce H2 from H2S. Liang [29] and Huang [30] reported the removal of H2S by DBD and achieved high removal rate. However, this method cannot be used to collect dust simultaneously because of the structure of DBD. In this work, simultaneous conversion of PH3, H2S and removal of dust by corona discharge was investigated. The mechanism was also elucidated on the basis of the experimental results and analysis of conversion products. Our results demonstrated that corona discharge is a convenient and efficient process to purify gases. 2 Experimental The experimental set-up wass similar to the study of COS with modifications [31], and their differences are shown in Fig. 1. The flow rate of feeding gas was 300 ml/min and the balance gas was N2. The high purity N2 99.999% employed in the experiment was made by Kunming Messer Gass Products Co., LTD. PH3 and H2S gas were 6000 and 6660 ppm with N2 as balanced gas and made by Dalian Special Gases Co., LTD. All the experiments were conducted at room temperature and atmospheric pressure. An absorption flask with deionized water was added for absorbing tail gas to collect the content of the gas. The absorption solution was analyzed by ion chromatography (ICS3000). PH3, H2S, and SO2 concentrations were determined by an HC-6 sulfur phosphorus microscale analyzer with an FPD detector. H2 test tubes (made by KOMYO RIKAGAKU KOGYO K.K, Japan) were employed to determine whether H2 was generated or not. X-ray diffractometer (XRD, D/MAX-2200), X-ray Fluorescence (XRF, RIGAKU XRF 3640), and X-ray photoelectric spectroscopy


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(XPS, PHI5000 Versaprobe-II) were employed to detect and analyze the chemical composition and structure of solid deposit in corona discharge reactor. The conversion efficiencies of PH3 and H2S were calculated by using the equations (1) and (2). The yield of SO2 was calculated according to equation (3). The specific input energy (SIE, J/L) was chosen to quantitate the power deposited into 1 L of the reaction gas. SIE is closely related to voltage, current, and flow and can be calculated using equation (4) [20-21, 32]. The discharge power correlates with voltage and current, which can be detected by the Negative DC high-voltage power. All of the data in this paper represent the average of at least three replicates. Conv(PH 3 ) =

[PH3 ]in − [PH3 ]out × 100% [PH3 ]in

(1)

Conv(H 2S) =

[H 2S]in − [H 2S]out × 100% [H 2S]in

(2)

[SO2 ]out × 100% [H 2S]in

(3)

discharge power (W) gas flow rate (l / s)

(4)

Yield(SO2 ) =

SIE =

3 Results and discussion 3.1 Qualitative analyses of gases Qualitative analyses of the inlet and outlet gases were performed by FTIR with gas cell. All spectra were collected in the 4000–400 cm−1 frequency ranges at a resolution of 4 cm−1. A total of 16 scans were averaged for each spectrum, corresponding to a time resolution of 19 s. FTIR results of the inlet and outlet gases


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are shown in Fig. 2-4. Fig. 2-4 are FTIR spectra of PH3, H2S, and a mixture of PH3 and H2S, respectively, under different SIE with the low O2 concentration of 0.1 vol. %. In Fig. 2, the absorption peaks at 2320 and 2325 cm-1 were attributed to symmetric and asymmetric P-H stretching modes of PH3[1, 33-35]. Peaks at 2256 and 2395 cm-l corresponded to the vibration-rotation bands of PH3 (g). In addition, weak but distinct δas bands at 1122 and 991 cm-l were observed, which are also characteristic of PH3 [36]. Moreover, as shown in Fig. 2, the transmittance of PH3 decreased with increasing SIE, which indicated that more PH3 was converted with increasing input energy. In the FTIR spectra of H2S (Fig. 3), peaks corresponded to H2S were not observed because the main absorption peaks of H2S in the considered spectral range occurs at far infrared 97.11 cm−1 which are out of middle infrared scope (4000-400 cm−1)[37-38]. However, the absorption peak appeared at 1373 cm−1 was ascribed to the S=O symmetric stretching of SO2 [39], which implied that SO2 was one of the products of H2S conversion. With increasing SIE, SO2 absorption peaks increased first and then decreased. This observation indicated that the amount of H2S converted to SO2 increased with increasing energy input in the SIE range of 0 to 318 J/L. However, when the SIE was greater than 318 J/L, the amount of SO2 started to decrease. As shown in Fig. 4, the FTIR spectra of PH3 and H2S mixture exhibited similar absorption peaks to those in Fig. 2 and 3. With increasing SIE, the peak intensity of PH3 tended to become weaker, whereas the peak intensity of SO2 increased firstly and then diminished. Compared with Fig. 2 and Fig. 3, the variations


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of PH3 absorption peaks in PH3 and H2S mixture under different SIE are the same as those of PH3 only, whereas the similar variations of SO2 absorption peaks require higher SIE than H2S only. This could be attributed to the fact that PH3 conversion is more favorable than H2S conversion, and PH3 is converted preferentially when they coexist. Therefore, part of the energy is consumed for PH3 conversion at first, and higher SIE is needed for the subsequent H2S conversion. H2 cannot be detected by FTIR because of its nonpolar molecule structure. For this reason, H2 test tubes were used to detect whether H2 was generated or not in this study. It was found that H2 was detected no matter in PH3 only, H2S only, and PH3 and H2S mixture with the low concentration of O2. On the basis of these results, it was concluded that PH3 and H2S could be removed by corona discharge and the gas products included H2 and SO2. These results also demonstrated the chemical bonds between P and H in PH3 and chemical bonds between S and H in H2S were cleaved to form new products. 3.2 Conversions of the PH3 and H2S at different inlet concentrations Having investigated the effects of SIE on the conversion of PH3 and H2S, we next examined the influences of inlet concentration on the conversion of PH3 and H2S. PH3 with different inlet concentrations (100, 510, 1080, and 1580 ppm) and H2S with inlet concentrations (105, 410, 1010, and 1500 ppm) were examined under the same conditions. Fig. 5 (PH3) and Fig. 6 (H2S) show the results of PH3 and H2S conversion under different SIE with different inlet concentrations. Fig. 5 clearly shows that the conversion of PH3 increased with increasing SIE at


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all investigated PH3 concentrations. Higher SIE means higher voltage and current when the flow is steady. Consequently, higher voltage results in higher electric field strength and a higher current represent high current density, which would promote the conversion of PH3 and H2S. When PH3 was 105 ppm and SIE was 102.2 J/L, the conversion of PH3 reached 94.3%. However, the PH3 conversions were only 84.3%, 76.3%, and 73.7%, respectively, when the PH3 concentrations were 510, 1080, and 1580 ppm under the same SIE level. Apparently, the higher concentration of PH3 was, the higher SIE was required to achieve the same conversion rate. The similar trend for H2S was observed in Fig. 6, which is reasonable because more energy is required for converting more PH3 or H2S. Compared Fig. 5 with Fig. 6, it can be seen that more energy should be inputted for H2S to gain the same conversion efficiency as the same inlet concentration of PH3, indicating H2S is harder to convert by corona discharge than PH3. This result originates from the fact that the chemical bond energy of H-S (91.2 kJ/mol)[40] is higher than that of P-H (83.9 kJ/mol )[41]. Moreover, the conversion of PH3 and H2S increased with increasing SIE and reached nearly 100% conversion when SIE was high enough, what were well consistent with FTIR results in Fig. 2-4. 3.3 Effect of O2 content on PH3 and H2S conversion Corona discharge generated from high-enough voltage can dissociate gas molecules and generate radicals. The species of radicals are related to the species of parent gas molecule. Under corona discharge, the presence of O2 plays critical roles on the reactions because it generates O • that has a very strong oxidation


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characteristic. Therefore, the effect of O2 content on conversion of PH3 and H2S was investigated in this study. The conversion of PH3 and H2S with different contents of O2 (0.1 vol. %, 0.7 vol. %, 2.8 vol. % and 7 vol. %) under different SIE was investigated systemically. The results of PH3 and H2S are shown in Fig. 7 and Fig. 8, respectively. As shown in Fig. 7, the order of PH3 conversion with different O2 contents was: 0.1 vol. % > 0.7 vol. % >2.8 vol. % > 7 vol. % under the similar SIE. Apparently, a higher content of O2 caused a lower conversion of PH3, implying the content of O2 has an adverse effect on the conversion of PH3. The result in Fig. 8 indicated that the effect of O2 on H2S conversion was identical to PH3. The higher conversion efficiency was achieved with the lowest O2 content because PH3 and H2S can be converted by corona discharge directly. In the presence of oxygen, a part of energy would be consumed by O2 for the dissociation and conversion of oxygen in the electric field. As a result, the energy used for converting PH3 and H2S were reduced correspondingly. The yields of SO2 from H2S conversion with different O2 contents and under different SIE are shown in Fig. 8. With increasing SIE, the yield of SO2 increased first and then decreased at SIE of around 318 J/L at all investigated O2 content. The SO2 yield initially increased because of the increased H2S conversion and more generated oxidative radicals with increasing SIE. The SO2 yield decreased when SIE further increased , which was caused by the conversion of SO2 to SO3 and SO42- with more energy. Since a higher input energy could produce more oxidative radicals and promote oxidation. The yield of SO2 with higher O2 content was higher than that with


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lower O2 content when O2 from 0.1% to 2.8 vol. %, which could be attributed to a certain level of O2 could promote the combination of S with O and generate more SO2. On the other hand, too much O2 would promote the conversion of SO2 to SO3 and SO42-. As a result, the SO2 yield declined when the O2 content was 7%. It was also found that H2 was detected by H2 test tubes when O2 content was 0.1 vol. % while no H2 was detected when the content of O2 was 0.7 vol. %, 2.8 vol. %, or 7 vol. %. This is because PH3 and H2S mainly decomposed and generated H2 when the O2 content was very low. When the O2 content was high, PH3 and H2S would be oxidized. Indeed, prominent red solid was observed on the electrodes when PH3 gas contained 0.1 vol. % O2. In contrast, a transparent liquid was observed rather than any red solid （marked as solid B in the following）when O2 content was 0.7 vol. %, 2.8 vol. %, or 7 vol. %. In the case of H2S gas with 0.1 vol. % O2, yellow solid （marked as solid A in the following）was observed on electrodes although the deposited solid was not as prominent as PH3. In contrast, no deposited solid was observed when the O2 content in H2S gas was 0.7 vol. %, 2.8 vol. %, or 7 vol. %. These results indicated that very low content of O2 resulted in more reduction substances while high content of O2 yielded more oxidation products. 3.4 Conversion of PH3 and H2S mixture Since PH3 and H2S often coexist in the same gas, the conversion of PH3 and H2S mixture was investigated in this study. The PH3 concentration of 1080 ppm and the H2S concentration of 1100 ppm were employed in this experiment. The content of O2 was controlled as 0.1 vol. % according to the results of previous experiments


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involving O2. The conversion of PH3 and H2S mixture and the yields of SO2 are shown in Fig. 9. Compared with the result of the PH3 only, the conversion of PH3 hardly exhibited significant difference when mixing with H2S. However, the conversion gap between single and mixed gas was evident in the case of H2S. It was found that the conversion of H2S in the mixture was considerably lower than that of single component H2S by over 15%, especially under low SIE. On the other hand, the yield of SO2 increased when PH3 and H2S were mixed. This result originates from their differences in gas properties and energy input. As analyzed above, H2S was harder to convert by corona discharge than PH3. Consequently, PH3 would be converted preferentially when mixed with H2S. The reason for increased yield of SO2 is that less energy will be consumed for the conversion of SO2 to SO3 and SO42- when PH3 and H2S coexist. 3.5 Simultaneous removal of PH3, H2S, and dust Industrial gas consisting of PH3 and H2S usually contains dust, such as yellow phosphorus tail gas and coal gas. However, it is a prerequisite that dust must be removed before the removal of PH3 and H2S by adsorption and catalytic method for gas purification or emission. Therefore, simultaneous removal of PH3, H2S, and dust is an ideal process. In this work, dust was introduced to the system with the dust concentration of 5600 ± 5% mg/m3 to explore the efficacy of corona discharge on the simultaneous conversion and removal of PH3, H2S, and dust. The conversions of PH3 and H2S and the yields of SO2 with or without dust are presented in Fig. 10. The results shown that when the dust was introduced into gas, PH3 conversion


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did not exhibit significant variation except that PH3 conversion had a slight reduction under a low SIE. In contrast, the introduction of dust had a positive effect on H2S conversion but reduced the yield of SO2, as shown in Fig. 10. This result is likely attributed to that some components of dust can react with H2S or its intermediates, or the dust could play a catalytic role to promote the conversion of H2S to other types of sulfur-containing compounds, such as elemental S, SO3, or SO42-. The yield of SO2 was up to 43% after the reaction in the absence of dust while the yields were all below 31% after dust was introduced. It was evident that the presence of dust considerably suppressed the generation of SO2. Furthermore, the highest yield of SO2 was observed at lower SIE compared with the result without dust, which demonstrated that the introduction of dust reduced energy consumption and benefited H2S conversion. With corona discharge, dust was also collected by the ground electrode and PH3 and H2S were converted concurrently with dust removal efficiency above 99.9%. Therefore, corona discharge is an ideal process, in which PH3, H2S, and dust can be removed by corona discharge efficiently and simultaneously. 3.6 Analysis of the products and the reaction mechanism Absorption liquid of tail gas and solid products collected were analyzed to study the mechanism of PH3 and H2S conversion by corona discharge. 3.6.1 Analysis of absorption liquid To further investigate the gas content after corona discharge treatment, we analyzed the absorption liquids of reaction tail gas by using ion chromatography. As shown in Fig. 11, the ion chromatograms of absorption liquids of PH3, H2S, and PH3


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and H2S mixture exhibited patterns a, b, and c, respectively. The results of IC analysis demonstrated the presence of sulfate (7 min) and phosphate (18 min) in the deposit. Sulfate is formed through the absorption of SO2 that generated from H2S conversion, which also indicated the conversion of H2S to SO2, SO3, or SO42-. Phosphate should be ascribed to the absorption of H3PO4 aerosols generated from PH3. 3.6.2 Analysis of solid component In this paper, XRF and XRD were used to determine the primary elements and phases of solid products, and X-ray photoelectron spectroscopy (XPS) was used to evaluate the component of solid further. XRD spectra were obtained with a D/MAX-2200 X-ray diffraction at 20~60 kV and 2~50 mA by using Ni-filtered Cu Kα radiation (λ=0.15406 nm) at a rate of 2 °/min from 2θ=10° to 90°. The crystalline phases were identified by matching the JCPDS files. X-ray photoelectron spectroscopy (XPS) analyses were carried out on a Physical Electronics PHI5600 spectrometer using an Al-Kα anode X-ray source with a photo energy of hv 1486.6 eV. XRF data were collected on Rigaku 3640 operated at 40 kV and 70 mA. Powder samples obtained from precipitation were subjected to washing, drying (

Simultaneous Removal of PH3, H2S, and Dust by Corona Discharge

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