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Removal of low-concentration formaldehyde in air by DC corona discharge plasma
Chemical Engineering Journal 171 (2011) 314–319
Contents lists available at ScienceDirect
Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
Removal of low-concentration formaldehyde in air by DC corona discharge plasma Yajuan Wan, Xing Fan, Tianle Zhu ∗ School of Chemistry and Environment, Beihang University, No.37 Xueyuan Road, Haidian District, Beijing 100191, China
a r t i c l e
i n f o
Article history: Received 7 November 2010 Received in revised form 6 April 2011 Accepted 7 April 2011 Keywords: Formaldehyde DC corona discharge plasma MnOx /Al2 O3 catalyst Ozone Indoor air
a b s t r a c t The effects of discharge polarity, discharge electrode configuration, gas composition and downstream catalyst on the removal of low-concentration HCHO in air were systematically investigated in a link tooth wheel-cylinder plasma reactor energized by a DC power. Experimental results show that the positive DC corona discharge is much more effective in removing HCHO as compared to the negative one. The discharge electrode configuration significantly influences the energy input to the plasma reactor. For a given specific energy density (SED), longer reaction zone favors the HCHO conversion; however, the HCHO conversion is almost constant in spite of the different discharge electrode configurations within a fixed reaction zone. The conversion of HCHO increases with the increase of gas humidity, and decreases with increasing coexisting toluene in the gas stream. On the other hand, introduction of MnOx /Al2 O3 catalyst downstream the plasma reactor significantly enhances the HCHO conversion and reduces the O3 emission. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Low-concentration formaldehyde (HCHO) is still a primary pollutant in indoor air and has gathered escalating public concern in recent years due to its adverse health effects, such as eye, nose and throat irritation, allergic asthma, pulmonary function damage and even cancer [1,2]. The major source of indoor HCHO is the pressed wood products made with urea–formaldehyde and phenol–formaldehyde resins, such as plywood, particleboard and medium-density fiberboard, which are often used in home building and remodeling [1–4]. Besides, textiles, cosmetics, combustion appliances and tobacco products also contribute to HCHO pollution [1,2,4]. Conventional technologies, such as adsorption, catalytic and photo-catalytic oxidation, have been widely investigated for the removal of HCHO [5–10]. Complete oxidation of 100-ppm HCHO was achieved over a Pt/TiO2 catalyst even at room temperature [8]. However, the limited removal capacities of adsorbent materials and the expensive cost of noble metals still limit their widespread applications. As an alternative approach, non-thermal plasma (NTP) technology appears to be more appropriate for indoor air purification because it is capable of removing various pollutants such as particulate matters, bacteria and volatile organic compounds (VOCs) simultaneously under ambient conditions. In recent years
∗ Corresponding author. Tel.: +86 10 82314215; fax: +86 10 82314215. E-mail address: [email protected] (T. Zhu). 1385-8947/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.cej.2011.04.011
in fact, various dielectric barrier discharge (DBD) plasma reactors have been developed for removing HCHO from gas streams [11–14], mainly at much higher concentration levels (e.g. hundreds of ppm) than those observed in actual indoor environments (hundreds of ppb to several ppm) [1,2]. As long as the NTP process is operated in air-like mixtures, the formation of O3 is unavoidable. Since O3 also contributes to poor indoor air quality, it should be removed from the treated gases. On the other hand, although O3 itself is a relatively weaker oxidant in NTP as compared to • O and • OH radicals, it is promising if the oxidative capacity of the O3 molecules can be exploited by combining NTP with an O3 decomposition catalyst in series. In fact, our previous study [15] has demonstrated that the BTX (mixture of benzene, toluene and p-xylene) conversion can be significantly promoted by introducing MnOx /Al2 O3 catalyst after the discharge zone, at the same time harmful O3 can be removed from the exit gas stream. It has been proven that O3 decomposition over manganese oxide catalyst produces atomic oxygen and peroxide as the intermediate species [16,17]. These highly active oxygen species may trigger catalytic oxidation of not only BTX [15,18,19], but also HCHO. The purpose of this work is to optimize a link tooth wheelcylinder plasma reactor for the removal of low-concentration (2.2 ± 0.1 ppm) HCHO in air. In this specific reactor, different amounts of discharge teeth wheels can be linked in different forms through a central rod to serve as the discharge electrode. To produce stable and reproducible plasma conditions for quantitative experimental studies, DC high voltage power supply was used. The
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315
Fig. 1. Schematic diagram of (a) the experimental system, and (b) the discharge electrodes.
effects of discharge polarity, discharge electrode configuration and gas composition, including humidity and coexistence of toluene, on the NTP removal of HCHO were systematically investigated. Besides, the MnOx /Al2 O3 catalyst was introduced downstream of the plasma reactor and the effects were evaluated on two aspects: enhancement of HCHO removal and reduction of O3 emission. 2. Experimental 2.1. Experimental set-up A schematic diagram of the experimental system is shown in Fig. 1a. It consists of a link tooth wheel-cylinder plasma reactor with a 25 kV/5 mA positive/negative DC high voltage power supply, a cylinder catalytic reactor, reaction gas supply and analytical instrumentation. A stainless steel cylinder with an inner diameter
of 42 mm and a length of 300 mm was used as the ground electrode of the plasma reactor, while a stainless steel rod (o.d. 6 mm) through which 9 discharge teeth wheels were linked with a space interval of 10 mm as the high voltage electrode, except for the discharge electrode configuration-effect experiments. As shown in Fig. 1b, 9 discharge teeth wheels linked with space intervals of 5, 10 and 20 mm were used as the discharge electrodes during the tooth wheel-interval (i) experiments, resulting in reaction zone lengths of 49, 89 and 169 mm, respectively. For the tooth wheel-number (n) experiments, the reaction zone length was kept constant at around 85 mm by linking 5, 9 or 15 discharge teeth wheels with intervals of 20, 10 and 5 mm, respectively. A stainless steel cylinder with an inner diameter of 27 mm and a length of 300 mm was connected to the plasma reactor in series, where MnOx /Al2 O3 catalyst could be introduced to construct a hybrid plasma-catalysis system (Fig. 1a).
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2.2. Experimental methods Gaseous HCHO, toluene and water vapor were introduced by passing dry air through three temperature-controlled bubble towers, containing 36 wt.% HCHO solution, liquid toluene and deionized water, respectively, and were completely mixed with the dilution air in a buffer flask before the plasma reactor. The flow rates of air were controlled by a set of mass flow controllers. The reaction gas containing (2.2 ± 0.1)-ppm HCHO passed through the plasma reactor and then the catalytic reactor at a rate of 6.0 L/min. Relative humidity (RH) of the reaction gas was controlled at around 30% except for the humidity-effect experiments, during which the RH was changed between 0% and 70% by adjusting the air flow rate through the water bubble tower. All the tests were carried out at room temperature and atmospheric pressure. MnOx /Al2 O3 (5 wt.% Mn, 2.5–3.5 mm in diameter) prepared by the impregnation method was used as the catalyst in this study. Loading amount of the catalyst was 15.0 g, with the residence time in the catalyst bed being around 0.21 s. When the catalyst was introduced, the reaction was started by energizing the plasma reactor with DC only when the HCHO concentration at the exit of the catalytic reactor reached a steady state, meaning that initial adsorption–desorption equilibrium of HCHO over the catalyst surface was achieved. HCHO concentration in gas stream was analyzed by the acetylacetone spectrophotometric method [11] while toluene using an Agilent HP-6890N GC (USA), equipped with an FID and a 30-m HP-5 capillary column. The conversion of HCHO and toluene is calculated based on their inlet and outlet concentrations, respectively. O3 outlet concentration was determined by the indigo disulphonate spectrophotometric method [15] in both the presence and absence of HCHO in the gas stream in this study (results prove that the presence of low-concentration HCHO hardly influences the O3 outlet concentration). Besides, temperature and humidity monitoring was conducted with a SMART SENSOR AR837 device (Hong Kong). All the experimental results were compared on the basis of specific energy density (SED, the ratio of discharge power to the gas flow rate, J/L) since SED directly reflects the energy consumption and can be used for the scale-up of the NTP reactor.
3. Results and discussion 3.1. Effects of discharge polarity Fig. 2 shows the removal of HCHO by applying positive and negative DC as functions of SED. It can be seen that the HCHO conversion increases with the increase of SED for both polarities. For a
Fig. 2. Effects of discharge polarity on HCHO conversion and O3 outlet concentration.
fixed SED, the HCHO conversion by applying positive DC is higher than that by applying negative DC. To achieve the same HCHO conversion, power consumption requirement of negative DC is much larger. Similar observations were obtained by Hensel et al. [20] using a multi-point-to-mesh discharge reactor. One explanation is that the positive corona discharge plasma has more chemical active species towards HCHO conversion as compared to the negative one. In fact, Fig. 2 also presents the O3 outlet concentration by applying positive and negative DC. It is obvious that under the same SED, the O3 concentration in positive corona is much higher than that in negative corona. Considering the energy efficiency for HCHO conversion, positive DC corona discharge was adopted in the following researches. 3.2. Effects of discharge electrode configuration 3.2.1. Effects of tooth wheel-interval The effects of discharge tooth wheel-interval (i) on energy input and HCHO removal in the plasma reactor were investigated. Results presented in Fig. 3a show that under a given applied voltage, more energy can be injected into the reactor with longer interval between discharge teeth wheels. For an applied voltage of 14 kV, the SED was 21.0, 37.8 and 49.0 J/L for space intervals of 5, 10 and 20 mm, respectively. This phenomenon may be attributed to an enhanced
Fig. 3. Effects of discharge tooth wheel-interval (i) on (a) energy input, and (b) HCHO conversion.
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Fig. 4. Effects of discharge tooth wheel-number (n) on (a) energy input, and (b) HCHO conversion.
propagation of the corona between discharge teeth wheels with longer interval. On the other hand, dependence of HCHO conversion on the space interval (i) and SED is presented in Fig. 3b. It is seen that for identical SED, the HCHO conversion slightly increased with increasing the interval from 5 to 10 mm, and then leveled off with further increase of the interval to 20 mm. This result indicates that a space interval of 10 mm is adequate for the best use of plasma reactive species to remove HCHO. In order to optimize the energy input and HCHO conversion as well as to minimize the plasma reactor volume, a discharge tooth wheel-interval of 10 mm (a reaction zone length of 89 mm) is optimum. 3.2.2. Effects of tooth wheel-number Fig. 4 shows the effects of discharge tooth wheel-number (n) on energy input and HCHO conversion in a fixed reaction zone (length of around 85 mm). As seen from Fig. 4a, more discharge teeth wheels promote the energy input to the plasma reactor, probably due to the increasing available discharge channels. For an applied voltage of 14 kV, the SED was 16.8, 37.8 and 89.6 J/L with 5, 9 and 15 discharge teeth wheels, respectively. Despite this, more discharge teeth wheels would increase the cost of the discharge electrode. On the other hand, the test results in Fig. 4b show that for a given SED, the HCHO conversion is almost constant in spite of the different amounts of discharge tooth wheel. This result suggests that the amounts of plasma-generated energetic electrons, active radicals and excited species may only depend on the SED in the reactor. Therefore, the number of discharge tooth wheel within a fixed reaction zone will be mainly determined by the energy input and the cost of the discharge electrode rather than by the HCHO conversion. 3.3. Effects of gas composition 3.3.1. Effects of humidity The effects of humidity on HCHO removal were investigated since indoor environments have variable humidity levels. Fig. 5 shows the HCHO conversion and O3 outlet concentration as functions of SED for dry air, 30% and 70% RH. It is seen that the increase of humidity significantly promotes the HCHO conversion although it inhibits the O3 production. For an SED of 80 J/L, the conversion of HCHO increased from 42% for dry air to 54% for 30% RH and 57% for 70% RH, while the corresponding O3 outlet concentration decreased from 282 to 162 and 157 ppm. O3 formation mechanisms research shows that the reduction of O3 production in humid air could be primarily attributed to the removal of ·O by H2 O [21], lead-
Fig. 5. Effects of humidity on HCHO conversion and O3 outlet concentration. Table 1 Reactions and reaction rate constants for HCHO and oxidative plasma species (O3 , • O, • OH) at 298 K. Reaction
k (cm3 /(molecule s))
HCHO + O3 → products HCHO + • O → CHO + • OH HCHO + • OH → CHO + H2 O
2.1 × 10−24 1.7 × 10−13 1.0 × 10−11
ing to efficient generation of • OH radicals and hence higher HCHO conversion, since the reaction of • OH with HCHO was found to be the primary route in HCHO oxidation by reactive species (Table 1) [22,23]. 3.3.2. Effects of coexisting toluene Also investigated were the effects of coexisting toluene, another representative pollutant in indoor air. Experiments were done under a fixed SED of 60 J/L. Fig. 6 presents the conversion of HCHO, the conversion and the removed amount of toluene as functions of toluene inlet concentration. It can be observed that the HCHO conversion decreased slightly (∼11%) as the inlet concentration of toluene increased from 0 to 13.2 ppm. The coexistence of toluene in the feed gas does not favor the removal of HCHO. On the other hand, the absolute removal of toluene molecules increased although the conversion of toluene decreased with increasing toluene inlet con-
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emission. For an SED of 20 J/L, the conversion of HCHO increased from 36% for plasma alone to 87% for the hybrid plasma-catalysis system; meanwhile, the O3 outlet concentration decreased from 58 to 14 ppm. It is clear that mainly O3 induced by gas discharge was decomposed catalytically as expected. The O3 initiated catalytic oxidation reactions are not only effective for BTX conversion but also for the removal of HCHO. Besides, considering that some other stable or metastable plasma species may also have sufficiently long lifetime to reach the downstream catalyst, further studies of effects of these species on the catalytic removal of HCHO are required to clarify the catalyst activation mechanisms. In addition, it should be noticed that even in the presence of MnOx /Al2 O3 catalyst, the O3 emission (14 ppm for an SED of 20 J/L) is still much higher than the guideline of 0.16 mg/m3 (75 ppb) by “Indoor Air Quality Standard” of China. In future research, it will be tested if the simultaneous catalytic removal of HCHO and O3 can be improved by introducing catalysts that are more reactive towards O3 decomposition. Fig. 6. Dependence of the conversion of HCHO, the conversion and the removed amount of toluene on toluene inlet concentration (SED: 60 J/L).
4. Conclusions Effects of discharge polarity, discharge electrode configuration, gas composition and downstream catalyst on the removal of lowconcentration HCHO in air by DC corona discharge plasma were systematically investigated in this study. The main findings can be summarized as follows. (1) The positive DC corona discharge is much more effective in removing HCHO as compared to the negative one. (2) The discharge electrode configuration significantly influences the energy input to the plasma reactor, but hardly affects the HCHO conversion in a fixed reaction zone for a given SED. (3) Increasing humidity has significantly positive effects while coexisting toluene in the gas stream has slightly adverse effects on the removal of HCHO. (4) The presence of MnOx /Al2 O3 catalyst after the plasma reactor significantly enhances the conversion of HCHO and reduces the emission of O3 . Acknowledgement
Fig. 7. Effects of downstream catalyst on HCHO conversion and O3 outlet concentration.
The authors would like to thank the Hi-Tech Research and Development Programs of China (Nos. 2007AA061404 and 2010AA064904) for the financial support of this work. References
centration. As a result, the competitive consumption of energetic electrons and oxidative radicals by toluene molecules increased, leading to a lower conversion of HCHO. Moreover, Fig. 6 also shows that the conversion of toluene is much higher than that of HCHO under the same conditions. This finding suggests that toluene molecules are more selectively reactive towards plasma species as compared to HCHO. 3.4. Effects of downstream catalyst Fig. 7 indicates the profiles of HCHO conversion and O3 outlet concentration as functions of SED for plasma process with and without downstream MnOx /Al2 O3 catalyst. As seen from the figure, no HCHO conversion occurred without discharge power whether the catalyst was introduced or not, showing that discharge plasma is indispensable for the removal of HCHO. The MnOx /Al2 O3 catalyst cannot be activated for HCHO oxidation under ambient conditions. As compared to plasma alone, the presence of MnOx /Al2 O3 catalyst significantly enhances the HCHO removal and reduces the O3
[1] T. Salthammer, S. Mentese, R. Marutzky, Formaldehyde in the indoor environment, Chem. Rev. 110 (2010) 2536–2572. [2] X.J. Tang, Y. Bai, A. Duong, M.T. Smith, L.Y. Li, L.P. Zhang, Formaldehyde in China: production, consumption, exposure levels, and health effects, Environ. Int. 35 (2009) 1210–1224. [3] D.A. Missia, E. Demetriou, N. Michael, E.I. Tolis, J.G. Bartzis, Indoor exposure from building materials: a field study, Atmos. Environ. 44 (2010) 4388–4395. [4] T.J. Kelly, D.L. Smith, J. Satola, Emission rates of formaldehyde from materials and consumer products found in California homes, Environ. Sci. Technol. 33 (1999) 81–88. [5] R.H. Wang, J.H. Li, OMS-2 catalysts for formaldehyde oxidation: effects of Ce and Pt on structure and performance of the catalysts, Catal. Lett. 131 (2009) 500–505. [6] C.Y. Li, Y.N. Shen, M. Jia, S.S. Sheng, M.O. Adebajo, H.Y. Zhu, Catalytic combustion of formaldehyde on gold/iron-oxide catalysts, Catal. Commun. 9 (2008) 355–361. [7] X.F. Tang, J.L. Chen, Y.G. Li, Y. Li, Y.D. Xu, W.J. Shen, Complete oxidation of formaldehyde over Ag/MnOx –CeO2 catalysts, Chem. Eng. J. 118 (2006) 119–125. [8] C.B. Zhang, H. He, K.I. Tanaka, Perfect catalytic oxidation of formaldehyde over a Pt/TiO2 catalyst at room temperature, Catal. Commun. 6 (2005) 211–214. [9] F. Shiraishi, D. Ohkubo, K. Toyoda, S. Yamaguchi, Decomposition of gaseous formaldehyde in a photocatalytic reactor with a parallel array of light sources – 1. Fundamental experiment for reactor design, Chem. Eng. J. 114 (2005) 153–159.
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Contents lists available at ScienceDirect
Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
Removal of low-concentration formaldehyde in air by DC corona discharge plasma Yajuan Wan, Xing Fan, Tianle Zhu ∗ School of Chemistry and Environment, Beihang University, No.37 Xueyuan Road, Haidian District, Beijing 100191, China
a r t i c l e
i n f o
Article history: Received 7 November 2010 Received in revised form 6 April 2011 Accepted 7 April 2011 Keywords: Formaldehyde DC corona discharge plasma MnOx /Al2 O3 catalyst Ozone Indoor air
a b s t r a c t The effects of discharge polarity, discharge electrode configuration, gas composition and downstream catalyst on the removal of low-concentration HCHO in air were systematically investigated in a link tooth wheel-cylinder plasma reactor energized by a DC power. Experimental results show that the positive DC corona discharge is much more effective in removing HCHO as compared to the negative one. The discharge electrode configuration significantly influences the energy input to the plasma reactor. For a given specific energy density (SED), longer reaction zone favors the HCHO conversion; however, the HCHO conversion is almost constant in spite of the different discharge electrode configurations within a fixed reaction zone. The conversion of HCHO increases with the increase of gas humidity, and decreases with increasing coexisting toluene in the gas stream. On the other hand, introduction of MnOx /Al2 O3 catalyst downstream the plasma reactor significantly enhances the HCHO conversion and reduces the O3 emission. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Low-concentration formaldehyde (HCHO) is still a primary pollutant in indoor air and has gathered escalating public concern in recent years due to its adverse health effects, such as eye, nose and throat irritation, allergic asthma, pulmonary function damage and even cancer [1,2]. The major source of indoor HCHO is the pressed wood products made with urea–formaldehyde and phenol–formaldehyde resins, such as plywood, particleboard and medium-density fiberboard, which are often used in home building and remodeling [1–4]. Besides, textiles, cosmetics, combustion appliances and tobacco products also contribute to HCHO pollution [1,2,4]. Conventional technologies, such as adsorption, catalytic and photo-catalytic oxidation, have been widely investigated for the removal of HCHO [5–10]. Complete oxidation of 100-ppm HCHO was achieved over a Pt/TiO2 catalyst even at room temperature [8]. However, the limited removal capacities of adsorbent materials and the expensive cost of noble metals still limit their widespread applications. As an alternative approach, non-thermal plasma (NTP) technology appears to be more appropriate for indoor air purification because it is capable of removing various pollutants such as particulate matters, bacteria and volatile organic compounds (VOCs) simultaneously under ambient conditions. In recent years
∗ Corresponding author. Tel.: +86 10 82314215; fax: +86 10 82314215. E-mail address: [email protected] (T. Zhu). 1385-8947/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.cej.2011.04.011
in fact, various dielectric barrier discharge (DBD) plasma reactors have been developed for removing HCHO from gas streams [11–14], mainly at much higher concentration levels (e.g. hundreds of ppm) than those observed in actual indoor environments (hundreds of ppb to several ppm) [1,2]. As long as the NTP process is operated in air-like mixtures, the formation of O3 is unavoidable. Since O3 also contributes to poor indoor air quality, it should be removed from the treated gases. On the other hand, although O3 itself is a relatively weaker oxidant in NTP as compared to • O and • OH radicals, it is promising if the oxidative capacity of the O3 molecules can be exploited by combining NTP with an O3 decomposition catalyst in series. In fact, our previous study [15] has demonstrated that the BTX (mixture of benzene, toluene and p-xylene) conversion can be significantly promoted by introducing MnOx /Al2 O3 catalyst after the discharge zone, at the same time harmful O3 can be removed from the exit gas stream. It has been proven that O3 decomposition over manganese oxide catalyst produces atomic oxygen and peroxide as the intermediate species [16,17]. These highly active oxygen species may trigger catalytic oxidation of not only BTX [15,18,19], but also HCHO. The purpose of this work is to optimize a link tooth wheelcylinder plasma reactor for the removal of low-concentration (2.2 ± 0.1 ppm) HCHO in air. In this specific reactor, different amounts of discharge teeth wheels can be linked in different forms through a central rod to serve as the discharge electrode. To produce stable and reproducible plasma conditions for quantitative experimental studies, DC high voltage power supply was used. The
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315
Fig. 1. Schematic diagram of (a) the experimental system, and (b) the discharge electrodes.
effects of discharge polarity, discharge electrode configuration and gas composition, including humidity and coexistence of toluene, on the NTP removal of HCHO were systematically investigated. Besides, the MnOx /Al2 O3 catalyst was introduced downstream of the plasma reactor and the effects were evaluated on two aspects: enhancement of HCHO removal and reduction of O3 emission. 2. Experimental 2.1. Experimental set-up A schematic diagram of the experimental system is shown in Fig. 1a. It consists of a link tooth wheel-cylinder plasma reactor with a 25 kV/5 mA positive/negative DC high voltage power supply, a cylinder catalytic reactor, reaction gas supply and analytical instrumentation. A stainless steel cylinder with an inner diameter
of 42 mm and a length of 300 mm was used as the ground electrode of the plasma reactor, while a stainless steel rod (o.d. 6 mm) through which 9 discharge teeth wheels were linked with a space interval of 10 mm as the high voltage electrode, except for the discharge electrode configuration-effect experiments. As shown in Fig. 1b, 9 discharge teeth wheels linked with space intervals of 5, 10 and 20 mm were used as the discharge electrodes during the tooth wheel-interval (i) experiments, resulting in reaction zone lengths of 49, 89 and 169 mm, respectively. For the tooth wheel-number (n) experiments, the reaction zone length was kept constant at around 85 mm by linking 5, 9 or 15 discharge teeth wheels with intervals of 20, 10 and 5 mm, respectively. A stainless steel cylinder with an inner diameter of 27 mm and a length of 300 mm was connected to the plasma reactor in series, where MnOx /Al2 O3 catalyst could be introduced to construct a hybrid plasma-catalysis system (Fig. 1a).
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2.2. Experimental methods Gaseous HCHO, toluene and water vapor were introduced by passing dry air through three temperature-controlled bubble towers, containing 36 wt.% HCHO solution, liquid toluene and deionized water, respectively, and were completely mixed with the dilution air in a buffer flask before the plasma reactor. The flow rates of air were controlled by a set of mass flow controllers. The reaction gas containing (2.2 ± 0.1)-ppm HCHO passed through the plasma reactor and then the catalytic reactor at a rate of 6.0 L/min. Relative humidity (RH) of the reaction gas was controlled at around 30% except for the humidity-effect experiments, during which the RH was changed between 0% and 70% by adjusting the air flow rate through the water bubble tower. All the tests were carried out at room temperature and atmospheric pressure. MnOx /Al2 O3 (5 wt.% Mn, 2.5–3.5 mm in diameter) prepared by the impregnation method was used as the catalyst in this study. Loading amount of the catalyst was 15.0 g, with the residence time in the catalyst bed being around 0.21 s. When the catalyst was introduced, the reaction was started by energizing the plasma reactor with DC only when the HCHO concentration at the exit of the catalytic reactor reached a steady state, meaning that initial adsorption–desorption equilibrium of HCHO over the catalyst surface was achieved. HCHO concentration in gas stream was analyzed by the acetylacetone spectrophotometric method [11] while toluene using an Agilent HP-6890N GC (USA), equipped with an FID and a 30-m HP-5 capillary column. The conversion of HCHO and toluene is calculated based on their inlet and outlet concentrations, respectively. O3 outlet concentration was determined by the indigo disulphonate spectrophotometric method [15] in both the presence and absence of HCHO in the gas stream in this study (results prove that the presence of low-concentration HCHO hardly influences the O3 outlet concentration). Besides, temperature and humidity monitoring was conducted with a SMART SENSOR AR837 device (Hong Kong). All the experimental results were compared on the basis of specific energy density (SED, the ratio of discharge power to the gas flow rate, J/L) since SED directly reflects the energy consumption and can be used for the scale-up of the NTP reactor.
3. Results and discussion 3.1. Effects of discharge polarity Fig. 2 shows the removal of HCHO by applying positive and negative DC as functions of SED. It can be seen that the HCHO conversion increases with the increase of SED for both polarities. For a
Fig. 2. Effects of discharge polarity on HCHO conversion and O3 outlet concentration.
fixed SED, the HCHO conversion by applying positive DC is higher than that by applying negative DC. To achieve the same HCHO conversion, power consumption requirement of negative DC is much larger. Similar observations were obtained by Hensel et al. [20] using a multi-point-to-mesh discharge reactor. One explanation is that the positive corona discharge plasma has more chemical active species towards HCHO conversion as compared to the negative one. In fact, Fig. 2 also presents the O3 outlet concentration by applying positive and negative DC. It is obvious that under the same SED, the O3 concentration in positive corona is much higher than that in negative corona. Considering the energy efficiency for HCHO conversion, positive DC corona discharge was adopted in the following researches. 3.2. Effects of discharge electrode configuration 3.2.1. Effects of tooth wheel-interval The effects of discharge tooth wheel-interval (i) on energy input and HCHO removal in the plasma reactor were investigated. Results presented in Fig. 3a show that under a given applied voltage, more energy can be injected into the reactor with longer interval between discharge teeth wheels. For an applied voltage of 14 kV, the SED was 21.0, 37.8 and 49.0 J/L for space intervals of 5, 10 and 20 mm, respectively. This phenomenon may be attributed to an enhanced
Fig. 3. Effects of discharge tooth wheel-interval (i) on (a) energy input, and (b) HCHO conversion.
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Fig. 4. Effects of discharge tooth wheel-number (n) on (a) energy input, and (b) HCHO conversion.
propagation of the corona between discharge teeth wheels with longer interval. On the other hand, dependence of HCHO conversion on the space interval (i) and SED is presented in Fig. 3b. It is seen that for identical SED, the HCHO conversion slightly increased with increasing the interval from 5 to 10 mm, and then leveled off with further increase of the interval to 20 mm. This result indicates that a space interval of 10 mm is adequate for the best use of plasma reactive species to remove HCHO. In order to optimize the energy input and HCHO conversion as well as to minimize the plasma reactor volume, a discharge tooth wheel-interval of 10 mm (a reaction zone length of 89 mm) is optimum. 3.2.2. Effects of tooth wheel-number Fig. 4 shows the effects of discharge tooth wheel-number (n) on energy input and HCHO conversion in a fixed reaction zone (length of around 85 mm). As seen from Fig. 4a, more discharge teeth wheels promote the energy input to the plasma reactor, probably due to the increasing available discharge channels. For an applied voltage of 14 kV, the SED was 16.8, 37.8 and 89.6 J/L with 5, 9 and 15 discharge teeth wheels, respectively. Despite this, more discharge teeth wheels would increase the cost of the discharge electrode. On the other hand, the test results in Fig. 4b show that for a given SED, the HCHO conversion is almost constant in spite of the different amounts of discharge tooth wheel. This result suggests that the amounts of plasma-generated energetic electrons, active radicals and excited species may only depend on the SED in the reactor. Therefore, the number of discharge tooth wheel within a fixed reaction zone will be mainly determined by the energy input and the cost of the discharge electrode rather than by the HCHO conversion. 3.3. Effects of gas composition 3.3.1. Effects of humidity The effects of humidity on HCHO removal were investigated since indoor environments have variable humidity levels. Fig. 5 shows the HCHO conversion and O3 outlet concentration as functions of SED for dry air, 30% and 70% RH. It is seen that the increase of humidity significantly promotes the HCHO conversion although it inhibits the O3 production. For an SED of 80 J/L, the conversion of HCHO increased from 42% for dry air to 54% for 30% RH and 57% for 70% RH, while the corresponding O3 outlet concentration decreased from 282 to 162 and 157 ppm. O3 formation mechanisms research shows that the reduction of O3 production in humid air could be primarily attributed to the removal of ·O by H2 O [21], lead-
Fig. 5. Effects of humidity on HCHO conversion and O3 outlet concentration. Table 1 Reactions and reaction rate constants for HCHO and oxidative plasma species (O3 , • O, • OH) at 298 K. Reaction
k (cm3 /(molecule s))
HCHO + O3 → products HCHO + • O → CHO + • OH HCHO + • OH → CHO + H2 O
2.1 × 10−24 1.7 × 10−13 1.0 × 10−11
ing to efficient generation of • OH radicals and hence higher HCHO conversion, since the reaction of • OH with HCHO was found to be the primary route in HCHO oxidation by reactive species (Table 1) [22,23]. 3.3.2. Effects of coexisting toluene Also investigated were the effects of coexisting toluene, another representative pollutant in indoor air. Experiments were done under a fixed SED of 60 J/L. Fig. 6 presents the conversion of HCHO, the conversion and the removed amount of toluene as functions of toluene inlet concentration. It can be observed that the HCHO conversion decreased slightly (∼11%) as the inlet concentration of toluene increased from 0 to 13.2 ppm. The coexistence of toluene in the feed gas does not favor the removal of HCHO. On the other hand, the absolute removal of toluene molecules increased although the conversion of toluene decreased with increasing toluene inlet con-
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emission. For an SED of 20 J/L, the conversion of HCHO increased from 36% for plasma alone to 87% for the hybrid plasma-catalysis system; meanwhile, the O3 outlet concentration decreased from 58 to 14 ppm. It is clear that mainly O3 induced by gas discharge was decomposed catalytically as expected. The O3 initiated catalytic oxidation reactions are not only effective for BTX conversion but also for the removal of HCHO. Besides, considering that some other stable or metastable plasma species may also have sufficiently long lifetime to reach the downstream catalyst, further studies of effects of these species on the catalytic removal of HCHO are required to clarify the catalyst activation mechanisms. In addition, it should be noticed that even in the presence of MnOx /Al2 O3 catalyst, the O3 emission (14 ppm for an SED of 20 J/L) is still much higher than the guideline of 0.16 mg/m3 (75 ppb) by “Indoor Air Quality Standard” of China. In future research, it will be tested if the simultaneous catalytic removal of HCHO and O3 can be improved by introducing catalysts that are more reactive towards O3 decomposition. Fig. 6. Dependence of the conversion of HCHO, the conversion and the removed amount of toluene on toluene inlet concentration (SED: 60 J/L).
4. Conclusions Effects of discharge polarity, discharge electrode configuration, gas composition and downstream catalyst on the removal of lowconcentration HCHO in air by DC corona discharge plasma were systematically investigated in this study. The main findings can be summarized as follows. (1) The positive DC corona discharge is much more effective in removing HCHO as compared to the negative one. (2) The discharge electrode configuration significantly influences the energy input to the plasma reactor, but hardly affects the HCHO conversion in a fixed reaction zone for a given SED. (3) Increasing humidity has significantly positive effects while coexisting toluene in the gas stream has slightly adverse effects on the removal of HCHO. (4) The presence of MnOx /Al2 O3 catalyst after the plasma reactor significantly enhances the conversion of HCHO and reduces the emission of O3 . Acknowledgement
Fig. 7. Effects of downstream catalyst on HCHO conversion and O3 outlet concentration.
The authors would like to thank the Hi-Tech Research and Development Programs of China (Nos. 2007AA061404 and 2010AA064904) for the financial support of this work. References
centration. As a result, the competitive consumption of energetic electrons and oxidative radicals by toluene molecules increased, leading to a lower conversion of HCHO. Moreover, Fig. 6 also shows that the conversion of toluene is much higher than that of HCHO under the same conditions. This finding suggests that toluene molecules are more selectively reactive towards plasma species as compared to HCHO. 3.4. Effects of downstream catalyst Fig. 7 indicates the profiles of HCHO conversion and O3 outlet concentration as functions of SED for plasma process with and without downstream MnOx /Al2 O3 catalyst. As seen from the figure, no HCHO conversion occurred without discharge power whether the catalyst was introduced or not, showing that discharge plasma is indispensable for the removal of HCHO. The MnOx /Al2 O3 catalyst cannot be activated for HCHO oxidation under ambient conditions. As compared to plasma alone, the presence of MnOx /Al2 O3 catalyst significantly enhances the HCHO removal and reduces the O3
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