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Effects of arc rotation speed on BTX decomposition by atmospheric pressure plasma
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Effects of arc rotation speed on BTX decomposition by atmospheric pressure plasma Hyun-Woo Park, Dong-Wha Park ∗ Department of Chemistry and Chemical Engineering and RIC-ETTP (Regional Innovation Center for Environmental Technology of Thermal Plasma), Inha University, 100 Inha-ro, Nam-gu, Incheon 402-751, Republic of Korea
a b s t r a c t This study was conducted to investigate the decomposition of volatile organic compounds in air by the rotating arc plasma system. The rotating arc plasma decomposed benzene, toluene, and m-xylene (BTX) gases diluted in air. The effects of the gas flow rate and arc rotation speed on conversion rate and energy efficiency were evaluated by high speed images and Fourier transform infrared spectroscopy. Perfect conversions of toluene and m-xylene into environmentally benign species were achieved at the lowest waste gas flow rate of 20 L/min, while the conversion rate of benzene was 79% under the same operating conditions due to the highest chemical bond strength of benzene among BTX. Although the conversion rate decreased with increasing the gas flow rate, the energy efficiency had an optimal gas flow rate, which influenced arc rotation speed. In addition, the oxidation rates of BTX were examined based on analysis of carbon oxides generated under different operating conditions. © 2013 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. Keywords: BTX; Rotating arc plasma; Conversion rate; Energy efficiency; By-product; Carbon balance
1.
Introduction
Volatile organic compounds (VOCs), which spread easily in air, are environmentally harmful substances because they cause many diseases such as allergic reactions, headache, eye irritation, dry cough, nausea, tiredness, and even cancer (Fan et al., 2009). The regulation of VOCs emission into the atmosphere has been introduced and reinforced continuously as awareness of this health issue has increased. As a result, many industries around the world that produce VOCs are being forced to reduce significantly or to eliminate the release of VOCs. Conventional treatment methods applied to air pollutants include carbon adsorption and incineration. However, the carbon adsorption method has the disadvantages of requiring a large amount of power and a low pollutant removal efficiency of 40–50% (Huang et al., 2002). Although thermal or catalytic incineration provides very high removal efficiency via a relatively simple method (Rubio et al., 2011; Indarto et al., 2007; Oda, 2003), additional fuel must be added to enhance the energy intensity to enable treatment of diluted VOCs. Moreover, the incineration process produces hazardous by-products such as dioxins and furans (Indarto et al., 2007).
Non-thermal plasma processes such as silent discharge (Oda, 2003), ferroelectric pellet-packed reactor (Yamamoto et al., 1992), pulsed corona discharge (Van Durme et al., 2007), dielectric barrier discharge (Syner and Anderson, 1998), and catalyst combined systems (Lu et al., 2006) have been introduced as attractive methods for the decomposition of VOCs during the last decade. The VOCs conversion process based on non-thermal plasma has achieved high treatment efficiency via the high kinetic energy of electrons that can disconnect chemical bonds in pollutants. However, the nonthermal plasma conversion method requires improvement of waste throughput and control of harmful by-products generation (Song et al., 2002). Conversely, an energy efficient treatment of waste gas with the suppression of unwanted by-products can be achieved using thermal plasma because it generates a high temperature for thermal cracking and abundant radicals for chemical reactions. A rotating arc plasma system, which was used in the present study, can decompose waste gas with lower energy and higher waste throughput than typical arc plasma systems because of the rotation motion of the arc column. The arc plasma has a very high temperature in the arc channel region owing to
∗
Corresponding author. Tel.: +82 32 874 3785; fax: +82 32 872 0959. E-mail address: [email protected] (D.-W. Park). Received 20 December 2012; Received in revised form 22 August 2013; Accepted 4 October 2013 0263-8762/$ – see front matter © 2013 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cherd.2013.10.004 Please cite this article in press as: Park, H.-W., Park, D.-W., Effects of arc rotation speed on BTX decomposition by atmospheric pressure plasma. Chem. Eng. Res. Des. (2013), http://dx.doi.org/10.1016/j.cherd.2013.10.004
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Joule heating, while the temperature falls sharply as distance from the arc column increases. Therefore, inefficient gas conversion has been attributed to the relatively low temperature in the peripheral region of the typical arc plasma. Conversely, the rotating arc plasma diffuses localized high heat flux throughout the plasma, resulting in enhancement of the reaction volume. The rotating arc plasma can be easily characterized based on the presence of a diffused burn-like flame between two electrodes due to the arc rotation. In the system used in this study, the swirling injection of plasma forming gas drives the rotation motion of the arc column. The major pathways of VOCs decomposition using conventional non-thermal plasma are molecular dissociation by high-energy electrons and chemical reactions by produced oxide ions and free radicals (Wang et al., 2009). On the other hands, the rotating arc is an advantageous method to decompose toxic and dangerous gases even though they are chemically stable species with strong chemical bond energy. Because the arc column directly contacts with waste gases in the rotating arc plasma producing a large amount of reactive radicals. As a result, the decomposition of waste gas is enhanced as the arc rotation speed increases. However, to increase the arc rotation, the gas flow rate must be increased. Since the plasma temperature decreases as a result of the high flow rate of cold gas, the conversion rate of waste gas decreases with increasing gas flow rate. Therefore, the effects of the gas flow rate and the arc rotation speed on the conversion rate and energy efficiency of waste gas treatment were examined in a rotating arc plasma system in the present study. The VOCs, benzene, toluene, and m-xylene (BTX), were used as model waste gases to investigate the conversion experiment and carbon balance to clarify major chemical reaction pathways in the decomposition of BTX by the rotating arc plasma system.
2.
of BTX diluted by air was used directly as the plasma forming gas. The flow rate of waste gas was controlled by a mass flow controller (SmartTrak, Sierra Instruments Inc., USA). A high voltage alternating current (AC) power supply (IHP-1002, EN Technology, Korea) with a frequency of 40 kHz and average input power of 656 W was employed for arc discharge. The dynamic behavior of arc discharge and the arc rotation speed were analyzed using a high-speed camera (V710, Phantom Inc., USA). The gas velocity, which was used to estimate the residence time of waste gas in the torch, was measured using an anemometer (416, Testo Inc., Germany) 30 mm away from the torch exit. Fig. 2 shows the schematic diagram of the rotating arc plasma torch. The electrodes consisted of a tungsten rod and a copper nozzle. The diameter of the tungsten rod was 12 mm, and the inner diameter of the nozzle was expanded from 15 to 20 mm. The waste gas was introduced into the discharge region through six holes with a 45◦ angle relative to the vertical direction of the torch axis. Therefore, the arc was rotated by the drag force of the swirling waste gas. Since the diameter of the gas swirling holes was fixed at 3 mm, the arc rotating speed increased as the gas flow rate increased. Increases in temperature and flow rate of the cooling water were measured by resistance temperature detectors and a flow meter to evaluate the heat loss of the rotating arc plasma torch. Waveforms of the arc voltage and current were measured using an oscilloscope (TDS3012C, Tektronix Inc., USA), a high voltage probe (P6015A, Tektronix Inc., USA), and a current probe (TPCA300 and TPC303, Tektronix Inc., USA) to examine the characteristics of the AC rotating arc discharge. The thermal efficiency of the torch (TE) and specific input energy (SIE) were calculated using the following equations: TE(%) =
˙ p,cw (To − Ti )] Pin − [mC Pin
Experimental setup SIE(J/L) =
Fig. 1 shows the schematic diagram of the rotating arc plasma system for BTX decomposition (Park et al., 2011). Liquid BTX with a purity of 99.99% was added to air at 600 ppm using a syringe pump (KDS 100, KD Scientific Inc., UK). The waste gas
60Pnet V˙
(1)
(2)
˙ (g/s) is the mass flow rate where Pin (W) is the input power; m of the cooling water; Cp,cw (J/g ◦ C) is the heat capacity of the cooling water; Ti and To (◦ C) are the coolant temperature at
Fig. 1 – Schematic diagram of the rotating arc plasma system for BTX decomposition. Please cite this article in press as: Park, H.-W., Park, D.-W., Effects of arc rotation speed on BTX decomposition by atmospheric pressure plasma. Chem. Eng. Res. Des. (2013), http://dx.doi.org/10.1016/j.cherd.2013.10.004
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Fig. 2 – Schematic diagram of the rotating arc plasma torch. the inlet and outlet points, respectively. The net power of the plasma, Pnet (W), was calculated from the input power and the thermal efficiency of the torch. V˙ (L/min) is the flow rate of the plasma forming gas composed of air and BTX. To evaluate the conversion rate and energy efficiency of waste treatment, the BTX concentration was analyzed by Fourier transform infrared spectroscopy (FT-IR: IG-2000, Otsuka Electronics, Japan). The conversion rate of BTX was estimated as follows: (%) =
Cin − Cout × 100 Cout
(3)
where is the conversion rate (%) of BTX; Cin and Cout are the concentrations (ppm) of BTX at the gas inlet and outlet of the rotating arc plasma system, respectively. The energy efficiency (EE) of BTX treatment was calculated using the conversion values as follows: EE(mol/J) =
Cin V˙ ¯ net 60VP
× 106
(4)
where V˙ and V¯ are the volume flow rate (L/min) and molar volume (L/mol) of the plasma forming gas, respectively.
3.
Results and discussion
Table 1 shows experimentally measured data according to the gas flow rate during the decomposition of BTX using the rotating arc plasma system. The waste gas residence time in plasma and specific input energy decreased from 0.84 to 0.25 s and from 1617 to 371 J/L, respectively, as the gas flow rate
Fig. 3 – Voltage waveform of the rotating arc plasma at the gas flow rate of 40 L/min and at the operating AC frequency of 40 kHz. increased from 20 to 100 L/min. These results indicate that a high gas flow rate will affect the conversion rate negatively because of the reduced plasma temperature and reaction time. Conversely, arc rotation speed increased as the gas flow rate increased, which would have a positive influence on the conversion of BTX via enhancement of the contact area between the high temperature arc channel and waste gas. Fig. 3 presents the measured voltage waveform for the gas flow rate of 40 L/min at the applied frequency of 40 kHz. The root mean square (RMS) voltage was 750 V and the average RMS current was 870 mA. As shown in Fig. 3, the waveform of
Table 1 – Experimentally measured data according to the total flow rate in the decomposition of BTX using the rotating arc plasma system. Plasma gas flow rate (L/min) 20 40 60 80 100
Thermal efficiency of torch (%) 85.6 85.8 85.6 86.4 87.2
Net power (W) 539 551 540 575 619
SIE (J/L)
1617 827 540 431 371
Number of arc rotation (rpm) 900 3120 4680 6300 7140
Gas residence time in torch (s) 0.84 0.5 0.38 0.29 0.25
Average frequency of re-strike (Hz) 27 102 244 333 357
RMS voltage (V) 720 725 730 772 825
Please cite this article in press as: Park, H.-W., Park, D.-W., Effects of arc rotation speed on BTX decomposition by atmospheric pressure plasma. Chem. Eng. Res. Des. (2013), http://dx.doi.org/10.1016/j.cherd.2013.10.004
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Fig. 4 – High speed images for rotating arc channel in a restrike cycle at the gas flow rate of 40 L/min.
arc voltage formed a fish bone shape as shown. The amplitude of arc voltage increased for 10 ms, after which it decreased suddenly. These findings can be explained by the arc restrike that is generally observed in DC arc plasmas (Moreau et al., 2006). The arc voltage is proportional to the arc length between the roots on the electrode surface due to the resistivity of the arc channel. Since the axial position of an arc spot is fixed at the end point of a tungsten rod, the other arc root slides along with the inner surface of the copper nozzle during a cycle, increasing the overall arc voltage due to the gas drag force. As a result, both the restrike frequency and RMS voltage increase as the gas flow rate increases (Table 1). The rotating motion of the arc discharge was observed by a high-speed camera that looked at the plasma region inside the torch in the direction of torch axis. The high speed images presented in Fig. 4 were obtained at a gas flow rate of 40 L/min. The first image was selected immediately after a new restrike of the arc and the rotating of the arc channel could be clearly observed. In this figure, one restrike cycle was terminated at 11.1 ms, and this duration of a restrike cycle was in a good agreement with the measured arc voltage shown in Fig. 3. In addition to the arc restrike phenomenon, the arc rotating speed was evaluated from the high speed images. The average arc rotation speed increased linearly from 900 to 7140 rpm as the total flow rate increased from 20 to 100 L/min. A high arc rotating speed is preferable during decomposition of BTX due to the dispersion of high heat flux into the entire plasma region, while a large gas flow rate hinders the decomposition of BTX by decreasing the plasma temperature and thermal cracking. Therefore, the conversion rate and energy efficiency of BTX treatment were examined according to the gas flow rate. The conversion rates of BTX as a function of total flow rate are presented in Fig. 5. The conversion rates decreased as the gas flow rate increased due to the reduced specific input energy. It should be noted that different conversion rates are found according to each BTX species. This is because the
chemical bond strength and molecular stability are related to the decomposition of VOCs (Wang et al., 2009). The dissociation energy of the carbon carbon bond in a benzene ring is 144 kcal/mol at 298 K, while that of the methyl group in toluene or m-xylene is 100 kcal/mol at the same temperature. The carbon carbon bond in the benzene, the methyl group in toluene and m-xylene and C C bonds are disconnected by thermal cracking of the plasma, after which oxidation generates stable and environmentally benign species. Therefore, benzene requires higher energy than toluene and m-xylene to be decomposed, which causes the conversion efficiencies of toluene and m-xylene to be higher than that of benzene at the same gas flow rate (Fig. 5). The maximum conversion rates of benzene, toluene and m-xylene were 79%, 100%, and 100%, respectively, at the lowest gas flow rate of 20 L/min. Fig. 6 presents the energy efficiency of BTX treatment as a function of the gas flow rate. Although the conversion rates
Fig. 5 – Effect of the gas flow rate on the conversion rate of BTX.
Please cite this article in press as: Park, H.-W., Park, D.-W., Effects of arc rotation speed on BTX decomposition by atmospheric pressure plasma. Chem. Eng. Res. Des. (2013), http://dx.doi.org/10.1016/j.cherd.2013.10.004
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Fig. 6 – Effect of the gas flow rate on the energy efficiency of BTX conversion.
decreased with increasing gas flow rate, the energy efficiencies increased with flow rate until 80 L/min, then decreased at 100 L/min. The increased energy efficiency can be explained by the effects of enhanced arc rotation on the decomposition of BTX. Well dispersed high heat flux and enhanced reaction between waste gas and the arc discharge channel during rapid arc rotating leads to the high energy efficiency of BTX treatment in the rotating arc plasma. However, in the case of 100 L/min, however, dead volume where thermal cracking of BTX is unavailable due to low temperature is expected to be large to decompose BTX. Based on these findings, a gas flow rate of 80 L/min was the optimal gas flow rate for the conversion of BTX. The carbon balances according to the total flow rate in BTX decomposition processes are presented in Fig. 7. The major chemical reactions involved in the decomposition of BTX diluted by air are oxidation, partial oxidation, and thermal cracking. As a result, CO, CO2 , hydrocarbons, and solid carbons were measured as by-products. The major by-product was CO2 , while the ratio of hydrocarbon was relatively low for all BTX species. These results indicate that oxidation is the main pathway involved in the conversion of BTX in the rotating arc plasma system. Moreover, the CO2 ratio in the by-products decreased as the gas flow rate increased because chemical reactions including oxidation processes occur more easily at high temperature when there is a long reaction time. According to a research paper reported, the major by-product in the decomposition of low concentration BTX in air by using the combination of corona discharge and catalyst system, was COx (CO + CO2 ) with small amount of O3 (Fan et al., 2009). In addition, major by-products of CO2 , CO, HCOOH, O3 , and carbon solid with small amount of C2 H2 and HCN were produced during the decomposition of benzene in the corona discharge (Satoh et al., 2008). It is because O2 is efficiently converted to O3 by the high kinetic energy of electrons in the corona discharge. On the other hands, the arc plasma produces different species. Large amount of CO, CO2 and H2 O and small amount of acetylene and C2 H2 in the flue gas were detected in the treatment of BTX by the gliding arc plasma (Indarto et al., 2007). In addition, the formation of CO2 , CO and NO2 were found in the decomposition of toluene by the gliding arc plasma (Bo et al., 2008). In the present work, CO2 , CO and NO2 were also found from the decomposition of BTX by the rotating arc plasma. Consequently, VOCs diluted in air are easily converted to CO and CO2 by plasma process due to
Fig. 7 – Carbon balance in treated BTX gases according to the total flow rate (a) benzene, (b) toluene, and (c) m-xylene.
the oxidation reaction between C atoms in VOCs and oxide radicals. Table 2 presents the comparison of treatment capacity, BTX conversion rate and energy efficiency values between the rotating arc plasma and other non-thermal plasmas. The rotating arc plasma used in this work shows the highest total flow rate of waste gas. Toluene and m-xylene were perfectly converted by using the rotating arc plasma. Especially, in the comparison between the rotating arc plasma and the gliding arc plasma, energy efficiency and throughput of BTX have greatly improved by the arc rotation. As shown in Table 2, the energy efficiency of benzene, toluene and m-xylene in the rotating arc plasma are 1.308, 1.657 and 1.657 mol/J, while these are 0.029, 0.03 and 0.046 mol/J in the gliding arc plasma. Therefore, a large amount of VOCs gases can be efficiently treated by using the rotating arc plasma process compared with conventional non-thermal plasmas.
Please cite this article in press as: Park, H.-W., Park, D.-W., Effects of arc rotation speed on BTX decomposition by atmospheric pressure plasma. Chem. Eng. Res. Des. (2013), http://dx.doi.org/10.1016/j.cherd.2013.10.004
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Table 2 – Treatment capacity, conversion rate and energy efficiency for the decomposition of BTX by conventional non-thermal plasmas and rotating arc plasma. Material
Method
Total flow rate (L/min)
Benzene
Corona discharge Combination of plasma and catalyst Silent discharge by bipolar pulse power Silent discharge by AC power Gliding arc plasma Rotating arc plasma
6 6 2.5 2.5 5 20
20 100 74 56 59 79
0.006 0.175 1.395 1.238 0.029 1.308
Toluene
Corona discharge Combination of plasma and catalyst Gliding arc plasma Rotating arc plasma
6 6 5 20
68 100 63 100
0.039 0.344 0.030 1.657
m-Xylene
Silent discharge by bipolar pulse power Silent discharge by AC power Gliding arc plasma Rotating arc plasma
2.5 2.5 5 20
100 98 65 100
3.110 2.166 0.046 1.657
4.
Conclusions
The rotating arc plasma for the decomposition of diluted BTX in air was evaluated based on the conversion rate and energy efficiency with increasing waste gas flow rate (which simultaneously affects plasma temperature, reaction time, and arc rotating speed). The conversion rates decreased as the flow rate of waste gas increased due to reduced plasma temperature and reaction time. Since benzene has a strong carbon carbon chemical bond, it was the most difficult species to decompose among BTX. Moreover, the energy efficiency increased linearly with increasing the gas flow rate until a certain value, above which it slightly decreased. Overall, the results indicate that the rotating motion of the high temperature arc column helps to increase the energy efficiency during the BTX conversion processes. According to the analysis of by-products, oxidation was the main decomposition pathway of BTX in the rotating arc plasma system.
Acknowledgment This work was supported by an Inha University Research Grant.
References Bo, Z., Yan, J., Li, X., Chi, Y., Cen, K., 2008. Scale-up analysis and development of gliding arc discharge facility for volatile organic compounds decomposition. J. Hazard. Mater. 155, 494–501. Fan, X., Zhu, T.L., Wang, M.Y., Li, X.M., 2009. Removal of low-concentration BTX in air using a combined plasma catalysis system. Chemosphere 75, 1301–1306. Huang, Z.H., Kang, F., Zheng, Y.P., Tang, J.B., Liang, K.M., 2002. Adsorption characteristics of trace volatile organic compounds on activated carbon fibers at room temperature. Adsorpt. Sci. Technol. 20, 495–500.
Conversion rate (%)
Energy efficiency (mol/J)
Reference
Fan et al. (2010) Wang et al. (2009) Indarto et al. (2007) This study Fan et al. (2010) Indarto et al. (2007) This study Wang et al. (2009) Indarto et al. (2007) This study
Indarto, A., Yang, D.R., Azhari, C.H., Wan Mothtar, W.H., Choi, J.W., Lee, H., Song, H.K., 2007. Advanced VOCs decomposition method by gliding arc plasma. Chem. Eng. J. 131, 337–341. Lu, B., Zhang, X., Yu, X., Feng, T., Ya, S., 2006. Catalytic oxidation of benzene using DBD corona discharges. J. Hazard. Mater. 137, 633–637. Moreau, E., Chazelas, C., Mariaux, G., Vardelle, A., 2006. Modeling the restrike mode operation of a DC plasma spray torch. J. Therm. Spray Technol. 15, 524–530. Oda, T., 2003. Non-thermal plasma processing of environmental protection: decomposition of diluted VOCs in air. J. Electrostat. 57, 293–311. Park, H.W., Choi, S., Park, D.W., 2011. Conversion of BTX by rotating arc plasma at atmospheric pressure. Proceedings of 20th International Symposium on Plasma Chemistry EEC04. Rubio, S.J., Quintero, M.C., Rodero, A., 2011. Application of microwave air plasma in the destruction of trichloroethylene and carbon tetrachloride at atmospheric pressure. J. Hazard. Mater. 186, 820–826. Satoh, K., Matsuzawa, T., Itoh, H., 2008. Decomposition of benzene in a corona discharge at atmospheric pressure. Thin Solid Films 516, 4423–4429. Song, Y.H., Kim, S.J., Choi, K.I., Yamamoto, T., 2002. Effect of adsorption and temperature on a nonthermal plasma process for removing VOCs. J. Electrostat. 55, 189–201. Syner, H.R., Anderson, G.K., 1998. Effect of air and oxygen content on the dielectric barrier discharge decomposition of chlorobenzene. IEEE Trans. Plasma Sci. 26, 1695–1699. Van Durme, J., Dewulf, J., Sysmans, W., Leys, C., Van Langenhove, H., 2007. Efficient toluene abatement in indoor air by a plasma catalytic hybrid system. Appl. Catal. B: Environ. 74, 161–169. Wang, H., Li, D., Wu, Y., Li, J., Li, G., 2009. Removal of four kinds of volatile organic compounds mixture in air using silent discharge reactor driven by bipolar pulsed power. J. Electrostat. 67, 547–553. Yamamoto, T., Ramanathan, K., Lawless, P.A., Ensor, D.S., Newsome, J.R., Plank, N., Ramsey, G.H., 1992. Control of volatile organic compounds by an AC energized ferroelectric pellet reactor and pulsed corona reactor. IEEE Trans. Ind. Appl. 28, 528–534.
Please cite this article in press as: Park, H.-W., Park, D.-W., Effects of arc rotation speed on BTX decomposition by atmospheric pressure plasma. Chem. Eng. Res. Des. (2013), http://dx.doi.org/10.1016/j.cherd.2013.10.004
CHERD-1383; No. of Pages 6
chemical engineering research and design x x x ( 2 0 1 3 ) xxx–xxx
Contents lists available at ScienceDirect
Chemical Engineering Research and Design journal homepage: www.elsevier.com/locate/cherd
Effects of arc rotation speed on BTX decomposition by atmospheric pressure plasma Hyun-Woo Park, Dong-Wha Park ∗ Department of Chemistry and Chemical Engineering and RIC-ETTP (Regional Innovation Center for Environmental Technology of Thermal Plasma), Inha University, 100 Inha-ro, Nam-gu, Incheon 402-751, Republic of Korea
a b s t r a c t This study was conducted to investigate the decomposition of volatile organic compounds in air by the rotating arc plasma system. The rotating arc plasma decomposed benzene, toluene, and m-xylene (BTX) gases diluted in air. The effects of the gas flow rate and arc rotation speed on conversion rate and energy efficiency were evaluated by high speed images and Fourier transform infrared spectroscopy. Perfect conversions of toluene and m-xylene into environmentally benign species were achieved at the lowest waste gas flow rate of 20 L/min, while the conversion rate of benzene was 79% under the same operating conditions due to the highest chemical bond strength of benzene among BTX. Although the conversion rate decreased with increasing the gas flow rate, the energy efficiency had an optimal gas flow rate, which influenced arc rotation speed. In addition, the oxidation rates of BTX were examined based on analysis of carbon oxides generated under different operating conditions. © 2013 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. Keywords: BTX; Rotating arc plasma; Conversion rate; Energy efficiency; By-product; Carbon balance
1.
Introduction
Volatile organic compounds (VOCs), which spread easily in air, are environmentally harmful substances because they cause many diseases such as allergic reactions, headache, eye irritation, dry cough, nausea, tiredness, and even cancer (Fan et al., 2009). The regulation of VOCs emission into the atmosphere has been introduced and reinforced continuously as awareness of this health issue has increased. As a result, many industries around the world that produce VOCs are being forced to reduce significantly or to eliminate the release of VOCs. Conventional treatment methods applied to air pollutants include carbon adsorption and incineration. However, the carbon adsorption method has the disadvantages of requiring a large amount of power and a low pollutant removal efficiency of 40–50% (Huang et al., 2002). Although thermal or catalytic incineration provides very high removal efficiency via a relatively simple method (Rubio et al., 2011; Indarto et al., 2007; Oda, 2003), additional fuel must be added to enhance the energy intensity to enable treatment of diluted VOCs. Moreover, the incineration process produces hazardous by-products such as dioxins and furans (Indarto et al., 2007).
Non-thermal plasma processes such as silent discharge (Oda, 2003), ferroelectric pellet-packed reactor (Yamamoto et al., 1992), pulsed corona discharge (Van Durme et al., 2007), dielectric barrier discharge (Syner and Anderson, 1998), and catalyst combined systems (Lu et al., 2006) have been introduced as attractive methods for the decomposition of VOCs during the last decade. The VOCs conversion process based on non-thermal plasma has achieved high treatment efficiency via the high kinetic energy of electrons that can disconnect chemical bonds in pollutants. However, the nonthermal plasma conversion method requires improvement of waste throughput and control of harmful by-products generation (Song et al., 2002). Conversely, an energy efficient treatment of waste gas with the suppression of unwanted by-products can be achieved using thermal plasma because it generates a high temperature for thermal cracking and abundant radicals for chemical reactions. A rotating arc plasma system, which was used in the present study, can decompose waste gas with lower energy and higher waste throughput than typical arc plasma systems because of the rotation motion of the arc column. The arc plasma has a very high temperature in the arc channel region owing to
∗
Corresponding author. Tel.: +82 32 874 3785; fax: +82 32 872 0959. E-mail address: [email protected] (D.-W. Park). Received 20 December 2012; Received in revised form 22 August 2013; Accepted 4 October 2013 0263-8762/$ – see front matter © 2013 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cherd.2013.10.004 Please cite this article in press as: Park, H.-W., Park, D.-W., Effects of arc rotation speed on BTX decomposition by atmospheric pressure plasma. Chem. Eng. Res. Des. (2013), http://dx.doi.org/10.1016/j.cherd.2013.10.004
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ARTICLE IN PRESS chemical engineering research and design x x x ( 2 0 1 3 ) xxx–xxx
Joule heating, while the temperature falls sharply as distance from the arc column increases. Therefore, inefficient gas conversion has been attributed to the relatively low temperature in the peripheral region of the typical arc plasma. Conversely, the rotating arc plasma diffuses localized high heat flux throughout the plasma, resulting in enhancement of the reaction volume. The rotating arc plasma can be easily characterized based on the presence of a diffused burn-like flame between two electrodes due to the arc rotation. In the system used in this study, the swirling injection of plasma forming gas drives the rotation motion of the arc column. The major pathways of VOCs decomposition using conventional non-thermal plasma are molecular dissociation by high-energy electrons and chemical reactions by produced oxide ions and free radicals (Wang et al., 2009). On the other hands, the rotating arc is an advantageous method to decompose toxic and dangerous gases even though they are chemically stable species with strong chemical bond energy. Because the arc column directly contacts with waste gases in the rotating arc plasma producing a large amount of reactive radicals. As a result, the decomposition of waste gas is enhanced as the arc rotation speed increases. However, to increase the arc rotation, the gas flow rate must be increased. Since the plasma temperature decreases as a result of the high flow rate of cold gas, the conversion rate of waste gas decreases with increasing gas flow rate. Therefore, the effects of the gas flow rate and the arc rotation speed on the conversion rate and energy efficiency of waste gas treatment were examined in a rotating arc plasma system in the present study. The VOCs, benzene, toluene, and m-xylene (BTX), were used as model waste gases to investigate the conversion experiment and carbon balance to clarify major chemical reaction pathways in the decomposition of BTX by the rotating arc plasma system.
2.
of BTX diluted by air was used directly as the plasma forming gas. The flow rate of waste gas was controlled by a mass flow controller (SmartTrak, Sierra Instruments Inc., USA). A high voltage alternating current (AC) power supply (IHP-1002, EN Technology, Korea) with a frequency of 40 kHz and average input power of 656 W was employed for arc discharge. The dynamic behavior of arc discharge and the arc rotation speed were analyzed using a high-speed camera (V710, Phantom Inc., USA). The gas velocity, which was used to estimate the residence time of waste gas in the torch, was measured using an anemometer (416, Testo Inc., Germany) 30 mm away from the torch exit. Fig. 2 shows the schematic diagram of the rotating arc plasma torch. The electrodes consisted of a tungsten rod and a copper nozzle. The diameter of the tungsten rod was 12 mm, and the inner diameter of the nozzle was expanded from 15 to 20 mm. The waste gas was introduced into the discharge region through six holes with a 45◦ angle relative to the vertical direction of the torch axis. Therefore, the arc was rotated by the drag force of the swirling waste gas. Since the diameter of the gas swirling holes was fixed at 3 mm, the arc rotating speed increased as the gas flow rate increased. Increases in temperature and flow rate of the cooling water were measured by resistance temperature detectors and a flow meter to evaluate the heat loss of the rotating arc plasma torch. Waveforms of the arc voltage and current were measured using an oscilloscope (TDS3012C, Tektronix Inc., USA), a high voltage probe (P6015A, Tektronix Inc., USA), and a current probe (TPCA300 and TPC303, Tektronix Inc., USA) to examine the characteristics of the AC rotating arc discharge. The thermal efficiency of the torch (TE) and specific input energy (SIE) were calculated using the following equations: TE(%) =
˙ p,cw (To − Ti )] Pin − [mC Pin
Experimental setup SIE(J/L) =
Fig. 1 shows the schematic diagram of the rotating arc plasma system for BTX decomposition (Park et al., 2011). Liquid BTX with a purity of 99.99% was added to air at 600 ppm using a syringe pump (KDS 100, KD Scientific Inc., UK). The waste gas
60Pnet V˙
(1)
(2)
˙ (g/s) is the mass flow rate where Pin (W) is the input power; m of the cooling water; Cp,cw (J/g ◦ C) is the heat capacity of the cooling water; Ti and To (◦ C) are the coolant temperature at
Fig. 1 – Schematic diagram of the rotating arc plasma system for BTX decomposition. Please cite this article in press as: Park, H.-W., Park, D.-W., Effects of arc rotation speed on BTX decomposition by atmospheric pressure plasma. Chem. Eng. Res. Des. (2013), http://dx.doi.org/10.1016/j.cherd.2013.10.004
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Fig. 2 – Schematic diagram of the rotating arc plasma torch. the inlet and outlet points, respectively. The net power of the plasma, Pnet (W), was calculated from the input power and the thermal efficiency of the torch. V˙ (L/min) is the flow rate of the plasma forming gas composed of air and BTX. To evaluate the conversion rate and energy efficiency of waste treatment, the BTX concentration was analyzed by Fourier transform infrared spectroscopy (FT-IR: IG-2000, Otsuka Electronics, Japan). The conversion rate of BTX was estimated as follows: (%) =
Cin − Cout × 100 Cout
(3)
where is the conversion rate (%) of BTX; Cin and Cout are the concentrations (ppm) of BTX at the gas inlet and outlet of the rotating arc plasma system, respectively. The energy efficiency (EE) of BTX treatment was calculated using the conversion values as follows: EE(mol/J) =
Cin V˙ ¯ net 60VP
× 106
(4)
where V˙ and V¯ are the volume flow rate (L/min) and molar volume (L/mol) of the plasma forming gas, respectively.
3.
Results and discussion
Table 1 shows experimentally measured data according to the gas flow rate during the decomposition of BTX using the rotating arc plasma system. The waste gas residence time in plasma and specific input energy decreased from 0.84 to 0.25 s and from 1617 to 371 J/L, respectively, as the gas flow rate
Fig. 3 – Voltage waveform of the rotating arc plasma at the gas flow rate of 40 L/min and at the operating AC frequency of 40 kHz. increased from 20 to 100 L/min. These results indicate that a high gas flow rate will affect the conversion rate negatively because of the reduced plasma temperature and reaction time. Conversely, arc rotation speed increased as the gas flow rate increased, which would have a positive influence on the conversion of BTX via enhancement of the contact area between the high temperature arc channel and waste gas. Fig. 3 presents the measured voltage waveform for the gas flow rate of 40 L/min at the applied frequency of 40 kHz. The root mean square (RMS) voltage was 750 V and the average RMS current was 870 mA. As shown in Fig. 3, the waveform of
Table 1 – Experimentally measured data according to the total flow rate in the decomposition of BTX using the rotating arc plasma system. Plasma gas flow rate (L/min) 20 40 60 80 100
Thermal efficiency of torch (%) 85.6 85.8 85.6 86.4 87.2
Net power (W) 539 551 540 575 619
SIE (J/L)
1617 827 540 431 371
Number of arc rotation (rpm) 900 3120 4680 6300 7140
Gas residence time in torch (s) 0.84 0.5 0.38 0.29 0.25
Average frequency of re-strike (Hz) 27 102 244 333 357
RMS voltage (V) 720 725 730 772 825
Please cite this article in press as: Park, H.-W., Park, D.-W., Effects of arc rotation speed on BTX decomposition by atmospheric pressure plasma. Chem. Eng. Res. Des. (2013), http://dx.doi.org/10.1016/j.cherd.2013.10.004
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Fig. 4 – High speed images for rotating arc channel in a restrike cycle at the gas flow rate of 40 L/min.
arc voltage formed a fish bone shape as shown. The amplitude of arc voltage increased for 10 ms, after which it decreased suddenly. These findings can be explained by the arc restrike that is generally observed in DC arc plasmas (Moreau et al., 2006). The arc voltage is proportional to the arc length between the roots on the electrode surface due to the resistivity of the arc channel. Since the axial position of an arc spot is fixed at the end point of a tungsten rod, the other arc root slides along with the inner surface of the copper nozzle during a cycle, increasing the overall arc voltage due to the gas drag force. As a result, both the restrike frequency and RMS voltage increase as the gas flow rate increases (Table 1). The rotating motion of the arc discharge was observed by a high-speed camera that looked at the plasma region inside the torch in the direction of torch axis. The high speed images presented in Fig. 4 were obtained at a gas flow rate of 40 L/min. The first image was selected immediately after a new restrike of the arc and the rotating of the arc channel could be clearly observed. In this figure, one restrike cycle was terminated at 11.1 ms, and this duration of a restrike cycle was in a good agreement with the measured arc voltage shown in Fig. 3. In addition to the arc restrike phenomenon, the arc rotating speed was evaluated from the high speed images. The average arc rotation speed increased linearly from 900 to 7140 rpm as the total flow rate increased from 20 to 100 L/min. A high arc rotating speed is preferable during decomposition of BTX due to the dispersion of high heat flux into the entire plasma region, while a large gas flow rate hinders the decomposition of BTX by decreasing the plasma temperature and thermal cracking. Therefore, the conversion rate and energy efficiency of BTX treatment were examined according to the gas flow rate. The conversion rates of BTX as a function of total flow rate are presented in Fig. 5. The conversion rates decreased as the gas flow rate increased due to the reduced specific input energy. It should be noted that different conversion rates are found according to each BTX species. This is because the
chemical bond strength and molecular stability are related to the decomposition of VOCs (Wang et al., 2009). The dissociation energy of the carbon carbon bond in a benzene ring is 144 kcal/mol at 298 K, while that of the methyl group in toluene or m-xylene is 100 kcal/mol at the same temperature. The carbon carbon bond in the benzene, the methyl group in toluene and m-xylene and C C bonds are disconnected by thermal cracking of the plasma, after which oxidation generates stable and environmentally benign species. Therefore, benzene requires higher energy than toluene and m-xylene to be decomposed, which causes the conversion efficiencies of toluene and m-xylene to be higher than that of benzene at the same gas flow rate (Fig. 5). The maximum conversion rates of benzene, toluene and m-xylene were 79%, 100%, and 100%, respectively, at the lowest gas flow rate of 20 L/min. Fig. 6 presents the energy efficiency of BTX treatment as a function of the gas flow rate. Although the conversion rates
Fig. 5 – Effect of the gas flow rate on the conversion rate of BTX.
Please cite this article in press as: Park, H.-W., Park, D.-W., Effects of arc rotation speed on BTX decomposition by atmospheric pressure plasma. Chem. Eng. Res. Des. (2013), http://dx.doi.org/10.1016/j.cherd.2013.10.004
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Fig. 6 – Effect of the gas flow rate on the energy efficiency of BTX conversion.
decreased with increasing gas flow rate, the energy efficiencies increased with flow rate until 80 L/min, then decreased at 100 L/min. The increased energy efficiency can be explained by the effects of enhanced arc rotation on the decomposition of BTX. Well dispersed high heat flux and enhanced reaction between waste gas and the arc discharge channel during rapid arc rotating leads to the high energy efficiency of BTX treatment in the rotating arc plasma. However, in the case of 100 L/min, however, dead volume where thermal cracking of BTX is unavailable due to low temperature is expected to be large to decompose BTX. Based on these findings, a gas flow rate of 80 L/min was the optimal gas flow rate for the conversion of BTX. The carbon balances according to the total flow rate in BTX decomposition processes are presented in Fig. 7. The major chemical reactions involved in the decomposition of BTX diluted by air are oxidation, partial oxidation, and thermal cracking. As a result, CO, CO2 , hydrocarbons, and solid carbons were measured as by-products. The major by-product was CO2 , while the ratio of hydrocarbon was relatively low for all BTX species. These results indicate that oxidation is the main pathway involved in the conversion of BTX in the rotating arc plasma system. Moreover, the CO2 ratio in the by-products decreased as the gas flow rate increased because chemical reactions including oxidation processes occur more easily at high temperature when there is a long reaction time. According to a research paper reported, the major by-product in the decomposition of low concentration BTX in air by using the combination of corona discharge and catalyst system, was COx (CO + CO2 ) with small amount of O3 (Fan et al., 2009). In addition, major by-products of CO2 , CO, HCOOH, O3 , and carbon solid with small amount of C2 H2 and HCN were produced during the decomposition of benzene in the corona discharge (Satoh et al., 2008). It is because O2 is efficiently converted to O3 by the high kinetic energy of electrons in the corona discharge. On the other hands, the arc plasma produces different species. Large amount of CO, CO2 and H2 O and small amount of acetylene and C2 H2 in the flue gas were detected in the treatment of BTX by the gliding arc plasma (Indarto et al., 2007). In addition, the formation of CO2 , CO and NO2 were found in the decomposition of toluene by the gliding arc plasma (Bo et al., 2008). In the present work, CO2 , CO and NO2 were also found from the decomposition of BTX by the rotating arc plasma. Consequently, VOCs diluted in air are easily converted to CO and CO2 by plasma process due to
Fig. 7 – Carbon balance in treated BTX gases according to the total flow rate (a) benzene, (b) toluene, and (c) m-xylene.
the oxidation reaction between C atoms in VOCs and oxide radicals. Table 2 presents the comparison of treatment capacity, BTX conversion rate and energy efficiency values between the rotating arc plasma and other non-thermal plasmas. The rotating arc plasma used in this work shows the highest total flow rate of waste gas. Toluene and m-xylene were perfectly converted by using the rotating arc plasma. Especially, in the comparison between the rotating arc plasma and the gliding arc plasma, energy efficiency and throughput of BTX have greatly improved by the arc rotation. As shown in Table 2, the energy efficiency of benzene, toluene and m-xylene in the rotating arc plasma are 1.308, 1.657 and 1.657 mol/J, while these are 0.029, 0.03 and 0.046 mol/J in the gliding arc plasma. Therefore, a large amount of VOCs gases can be efficiently treated by using the rotating arc plasma process compared with conventional non-thermal plasmas.
Please cite this article in press as: Park, H.-W., Park, D.-W., Effects of arc rotation speed on BTX decomposition by atmospheric pressure plasma. Chem. Eng. Res. Des. (2013), http://dx.doi.org/10.1016/j.cherd.2013.10.004
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Table 2 – Treatment capacity, conversion rate and energy efficiency for the decomposition of BTX by conventional non-thermal plasmas and rotating arc plasma. Material
Method
Total flow rate (L/min)
Benzene
Corona discharge Combination of plasma and catalyst Silent discharge by bipolar pulse power Silent discharge by AC power Gliding arc plasma Rotating arc plasma
6 6 2.5 2.5 5 20
20 100 74 56 59 79
0.006 0.175 1.395 1.238 0.029 1.308
Toluene
Corona discharge Combination of plasma and catalyst Gliding arc plasma Rotating arc plasma
6 6 5 20
68 100 63 100
0.039 0.344 0.030 1.657
m-Xylene
Silent discharge by bipolar pulse power Silent discharge by AC power Gliding arc plasma Rotating arc plasma
2.5 2.5 5 20
100 98 65 100
3.110 2.166 0.046 1.657
4.
Conclusions
The rotating arc plasma for the decomposition of diluted BTX in air was evaluated based on the conversion rate and energy efficiency with increasing waste gas flow rate (which simultaneously affects plasma temperature, reaction time, and arc rotating speed). The conversion rates decreased as the flow rate of waste gas increased due to reduced plasma temperature and reaction time. Since benzene has a strong carbon carbon chemical bond, it was the most difficult species to decompose among BTX. Moreover, the energy efficiency increased linearly with increasing the gas flow rate until a certain value, above which it slightly decreased. Overall, the results indicate that the rotating motion of the high temperature arc column helps to increase the energy efficiency during the BTX conversion processes. According to the analysis of by-products, oxidation was the main decomposition pathway of BTX in the rotating arc plasma system.
Acknowledgment This work was supported by an Inha University Research Grant.
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Please cite this article in press as: Park, H.-W., Park, D.-W., Effects of arc rotation speed on BTX decomposition by atmospheric pressure plasma. Chem. Eng. Res. Des. (2013), http://dx.doi.org/10.1016/j.cherd.2013.10.004