Hexane decomposition without particle emission using a novel dielectric barrier discharge reactor filled with porous dielectric balls

Accepted Manuscript Hexane decomposition without particle emission using a novel dielectric barrier discharge reactor filled with porous dielectric balls Qi Jin, Boqiong Jiang, Jingyi Han, Shuiliang Yao PII: DOI: Reference:

S1385-8947(15)01481-3 http://dx.doi.org/10.1016/j.cej.2015.10.070 CEJ 14346

To appear in:

Chemical Engineering Journal

Received Date: Revised Date: Accepted Date:

19 July 2015 7 October 2015 8 October 2015

Please cite this article as: Q. Jin, B. Jiang, J. Han, S. Yao, Hexane decomposition without particle emission using a novel dielectric barrier discharge reactor filled with porous dielectric balls, Chemical Engineering Journal (2015), doi: http://dx.doi.org/10.1016/j.cej.2015.10.070

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Hexane decomposition without particle emission using a novel dielectric barrier discharge reactor filled with porous dielectric balls

Qi Jin, Boqiong Jiang, Jingyi Han, Shuiliang Yao* School of Environmental Science and Engineering, Zhejiang Gongshang University, Hangzhou 310018, China

*Corresponding author. Tel.: +86 571 28008240; fax: +86 571 28008240. E-mail address: [email protected] (S. Yao).

Abstract—This study investigated the decomposition of n-hexane in 15% O2 (N2 balance) in dielectric barrier discharge (DBD) reactors filled with three different kinds of dielectric balls, viz. quartz, less porous, and porous alumina balls. Products of n-hexane decomposition were analyzed by using a gas chromatograph (GC), gas chromatography-mass spectrometry (GC-MS), Fouriertransform infrared (FTIR) spectroscopy, and scanning mobility particle sizer (SMPS). The main products of n-hexane decomposition are CO, CO2, and liquid particles. The DBD reactor filled with porous alumina balls can completely adsorb the liquid particles and reduce particle emission. GCMS and FTIR analysis results show that the liquid particles mainly compose 3-hexanone and 2hexanone.

Keywords—DBD; n-hexane decomposition; dielectric ball; particle concentration; particle composition

1. Introduction

Recent years, haze, which is the major atmospheric pollution concern, has made people suffer a lot in major districts, such as Beijing, Nanjing, Singapore, and Southeastern United States [1–4]. Haze is mainly caused by the presence of high-concentration particulate matter (PM) and photochemical substances (e.g., hydrocarbons and NOx) in the atmosphere [4−6]. Hydrocarbons are alkane hydrocarbons, polycyclic aromatic hydrocarbons, and aliphatic acids [7]. Cai et al. [8] found that the concentration of total volatile organic compounds (TVOCs) on haze days is higher than that on clear days, indicating TVOCs contribute significantly to hazy weather. Especially, the alkane concentration in working days is about 35% higher than that at weekends. An et al. [9] reported that alkanes provide the largest percentage of TVOCs to atmosphere. Gasoline vehicles are possibly the main source of atmospheric alkanes [10–12]. Today, several end-of-pipe technologies, such as activated carbon adsorption, condensation, thermal combustion, catalytic oxidation, and nonthermal plasma (NTP), are available for removing VOCs from air [13]. Among them, the NTP is very cost-effective for treating VOCs emission with low concentration and large volume [14–17]. Many studies focused on using dielectric barrier discharges (DBD) with or without catalysts for the decomposition of VOCs, such as benzene [18–22], toluene [23–31], xylene [26], halo-hydrocarbons [32–33], and formaldehyde [34]. Furthermore, select studies use corona discharges with catalysts to decompose alkanes, such as n-heptane and hexane [35]. Although alkanes are the main VOCs in the atmosphere, only a few studies focused on alkane decomposition. Hill et al. [36] applied a dielectric packed-bed plasma reactor for the destruction of propane in air. They found the end-products of the propane decomposition were CO and CO2, with no other hydrocarbon intermediate detected.

Marotta et al. [37] used DC corona discharges to decompose n-hexane and 2,2,4-trimethylpentate (ioctane). They suggested that alkane decomposition is initiated by O and OH to form alkane free radicals, which are then oxidized by O2 to ketone and free radicals of fewer carbons. Pekarek et al. [35] used NTP and TiO2 to decompose n-heptane. The existing NTP technologies normally produce a large number of particles when VOCs are decomposed. Parissi et al. [38] and Borra [39] reported that particles were mainly produced from the nucleation of dehydrogenation products of VOCs, forming poly-aromatic hydrocarbons and hydroxides compounds. Moreover, NTP-induced particles have usually a diameter range from a few nanometers to around 1 µm, and a number concentration ranging from 10 3 to 10 6 #/cm3. Zhang et al. [40] studied the particle formation from styrene decomposition with corona discharges and reported that the total number concentration of the particles is around 1×106 #/cm3 with diameters between 28 and 157 nm. Particle components included about 61% of C–C and/or C–H, 22% of C–O–C and/or C–OH, 15% of C=O, H–C=O and/or aromatic–OH, and 2% of COOH and/or COOR related compounds. Yamamoto et al. [41] also reported that when CFC-113 in air is decomposed by using a packed bed plasma reactor, the aerosol number concentration increased when the discharge power was increased. The particles in the atmosphere are harmful to human health. It was indicated that annually about 800,000 deaths were associated with PM2.5 exposure [42] and several toxicological studies [43–45] highlighted a clear correlation between chronic exposure to PM2.5 and mortality risk. The highest relative mortality risks were related to ischemic heart diseases and lung cancer, with stronger correlations for non-smokers than smokers. It has also been reported that fine particles penetrate deep in the lungs and those smaller than 300 nm can cross cellular membranes [46]. So the regulations concerning submicron particles became more stringent, and searching for new

particle cleaning devices of higher collection efficiency became more important in last decades. Furthermore, technologies based on electrostatic forces are successfully applied to remove fine particles, but their effectiveness in removing particles in the ultrafine range is less known [46]. Therefore, studies on a better understanding of particle formation, composition, particle number concentration, and size distribution inside the NTP system are required. The simultaneous removal of gaseous contaminants and PM has been a new trend in gas cleaning technology development. n-Hexane, as a typical alkane, is widely used as a solvent in industry [47]. This study investigates the decomposition of n-hexane using a planar DBD reactor filled with three kinds of dielectric balls to stimulate n-hexane decomposition as well as to reduce the emission of particle products.

2. Experimental

2.1. Experimental system Experiments for n-hexane decomposition were carried out using the set-up in Fig. 1. Three mass flow controllers (MFC, D07, Sevenstar, Beijing, China) were used to generate a 500 ml/min gas mixture of nitrogen (N2, purity 99.999%) and oxygen (O2, purity 99.999%). The gas flow rate of O2 was fixed at 75 ml/min to simulate the exhaust gas of 15% O2 from a VOCs source. N2 gas flow was used to bubble the liquid n-hexane (purity 99%, Aladdin, Shanghai, China) in a bubbler (held in a water bath at 25 oC) to generate a gas mixture with n-hexane. The initial concentration of the nhexane in the gas mixture from the mixer (held with an electric heater at 50 oC) was controlled in a range of 293–367 ppmv via changing N2 gas flow rate (F1).

As shown in Figs. 1, 2, and S1, the DBD reactor generally consists of a stainless steel frame, two alumina plate blocks, two alumina plates (purity 96%, 115×115×1 mm3), two stainless steel electrodes (95×95×0.3 mm3 with terminals), two organic glass spacers (115×10×2 mm3), and two organic glass holders (115×10×1 mm3). The organic glass spacers were used to keep a space distance of 2 mm between two alumina plates and to allow all gas mixture to pass the space between alumina plates. Three kinds of dielectric balls, viz. quartz (Q) balls (purity 96% SiO2， EKEAR, China), less porous (LP) balls (66.2% SiO2, 11.1% Al2O3, 5.98% CaO, 4.78% K2O, 4.38% ZnO, 2.32% MgO, 1.80% Na2O, 1.57% Fe2O3, 1.08% B2O3, Judong Industrial, China), and porous alumina (PA) balls (purity 95% Al2O3, Zibo Jianlong, China) were used. The diameters of the dielectric balls ranged from 1.79 mm to 2.29 mm. A pulse power supply (DP-12K5-SCR, PECC, Japan) was used to supply pulse voltage to stainless steel electrode terminals in the DBD reactor to generate pulsed DBD within the discharge space (95×95×2 mm3) between two alumina plates covered with two stainless steel electrodes. The waveforms of discharge voltage and current were measured with a voltage probe (VP, P6015A, Tektronix, USA), a current transformer (CT, TCP 0030, Tektronix, USA), and a digital phosphor oscilloscope (DPO 3034, Tektronix, USA). The gas from the outlet of the DBD reactor was analyzed with a GC (GC1690, Hangzhou Kexiao, China) equipped with a Porapak-N column, a SE30 capillary column, and two flame ionization detectors (FID) for each column. The Porapak-N column was used to separate CO, CO2, and hydrocarbons with a carbon number less than 5. The SE-30 capillary column was used to separate hydrocarbons with a carbon number higher than 5. Two Ni catalytic conversion ovens after the Porapak-N and SE-30 capillary columns were used prior to each FID to convert carbon compounds (such as CO and CO2) into alkanes (such as CH4).

The particles in the gas stream from the outlet of the DBD reactor were detected using a scanning mobility particle sizer (SMPS, Grimm, Germany) equipped with a differential mobility analyzer (DMA) in an aerodynamic diameter range from 10.25 nm to 1093.95 nm and a condensation particle counter (CPC) which can measure particles in a number concentration range of 0~107 particles (#)/cm3. Ozone concentration in the gas stream from the outlet of the DBD reactor was analyzed using an ozone meter (UV-100, Eco Sensors, USA). <> <> 2.2. Characterization of dielectric balls The physical properties of the dielectric balls were tested with a N2 adsorption apparatus (Micromeritics JW-BK 132F, Beijing JWGB Sci. Tech., China). The dielectric balls were ground into powders, from which those with diameters less than 100 meshes were selected by using a 100mesh stainless steel wire sieve, and heated at 150 oC for 10 h before Brunauer–Emmertt–Teller (BET) measurement. N2 adsorption isotherms were measured at the liquid N2 temperature (77 K) and N2 pressures ranging from 10 -6 to 1.0 P/P0. Surface areas of the dielectric balls were measured according to BET method and their pore size distributions were obtained according to the Barret– Joyner–Halenda (BJH) method. The properties of the dielectric balls were listed in Table 1. The scanning electron microscope（SEM）photos of them were shown in Fig. 3. <> The infrared spectra of the dielectric balls before and after use in the discharge space were measured using a FTIR spectrometer (Thermo Nicolet NEXUS, ThermoFisher, USA). All the ball

samples were ground with KBr into powders before measurement. 2.3. GC-MS analyses

The n-hexane and its decomposition products in the gas steam from the outlet of the DBD reactors were continuously absorbed using 5-ml decane liquids (purity 99%, Aladdin, Shanghai, China). After 10 h discharges, 16.1 g Q balls, 15.5 g LP balls, and 8.6 g PA balls were severally extracted with 10-ml CH2Cl2 liquids (HPLC purity, TEDIA, Fairfield, USA) for 16 h, and the supernatant liquids were taken for GC-MS analyses. The n-hexane and its decomposition products in 5-ml decane liquids after absorption and in 10-ml CH2Cl2 supernatant liquids after extraction were analyzed using a GC-MS (GC MS-7890A-5975C, Agilent, USA) equipped with a 0.25 µm×0.25 mm×30 m HP-5MS capillary column. The HP-5MS capillary column was set in an oven, and the temperature of the oven after sample injection was kept at 40 oC for 4 min, then raised to 270 oC at a rate of 10 oC/min, and then held at 270 oC for 3 min. The sample inlet temperature was kept at 250 oC during the whole process. Helium carrier gas was used at a flow rate of 0.7 ml/min and the split ratio was 10:1. Mass spectrometer was operated in a standard electronic impact mode with electron energy of 70 eV. The temperatures of interface, MS source, and MS quadrupole were 280, 300, and 180 oC, respectively. Mass spectra were recorded with a scan velocity of 0.33 seconds per scan in the range of 25–500 m/z. n-Hexane and its decomposition products were identified according to the retention time and mass spectrum data obtained from a NIST library. 2.4. Definitions The energy injection P in W from the pulse power supply to the DBD reactor was calculated using Eq. (1). V +V I +I P = f ∑( i i +1 )( i i +1 )(ti +1 -ti ) i

2

2

(1)

Where, Vi and Vi+1 in V are the discharge voltage at discharge time ti and ti+1 in s, respectively. Ii and Ii+1 in A are the discharge current at discharge time ti and ti+1, respectively. The discharge voltage and current were obtained from the datum sequences of the discharge voltage and current waveforms. i and i+1 are step values of the datum sequences, and i values are from 0 to 9999. f is the pulse frequency in Hz. The specific energy density (SED) in J/L is the ratio of the energy injection P to the total gas flow rate F in L/min, as described in Eq. (2). (2) n-Hexane conversion x was calculated using Eq. (3). (3) Where C0 and C in ppmv are the concentrations of the n-hexane in the inlet and outlet gas streams of the DBD reactor, respectively. Energy efficiency of n-hexane decomposition E in mol/kWh is shown in Eq. (4). (4) Where R and T are the gas constant (0.082 J/mol/atm) and gas temperature (298 K), respectively. A unit conversion factor of 0.036 is used to provide E in mol/kWh. The selectivities of CO and CO2 were defined as Eqs. (5) and (6). (5)

(6) Where [CO] and [CO2] in ppmv are the concentrations of CO and CO2 from n-hexane

decomposition. [CT] in ppmv was calculated from the following Eq. (7): (7) The carbon balance was calculated by using Eq. (8): (8) Where ∑SD and ∑SND are the sums of peak areas of all the compounds which were detected by the GC equipped with SE-30 capillary column with or without discharge, respectively. It must be noted that only CO, CO2, and n-hexane were found using GC analysis. All discharge experiments were carried out after the dielectric balls were saturated with nhexane.

3. Results and discussion

3.1. Discharge characterization The typical waveforms of discharge voltage and current of the DBD reactor filled with PA balls were shown in Fig. 4, where n-hexane concentration C0 was 367.3 ppmv. The voltage waveform has a positive pulse and a negative pulse. The peak voltage and rise time of the positive pulse are 7.0 kV and 1.6 µs, respectively. The lowest voltage of the negative pulse is −7.8 kV. Discharge current increases with the increase in pulse voltage and peaks at 1.58 A, decreases to a lowest value of −0.93 A with the decrease in pulse voltage. The energy injection P was calculated with Eq. (1) using the voltage and current data shown in Fig. 4. The energy injection P was 2.3 W, resulting in a SED value of 276 J/L. It is notable that the waveforms of discharge voltage of different DBD rectors are almost the

same if the same peak voltages are applied. However, the waveforms of discharge current of different DBD rectors have the same shapes but different current altitudes when the same peak voltages are applied (Fig. S2). <> Fig. 5 shows the relation of energy injection as a function of peak voltage. The energy injection increases slightly when the peak voltage increases from zero to the breakdown voltage. The energy injection increases obviously when the peak voltage is higher than the breakdown voltage, indicating that the pulsed corona discharges occur within the discharge space. The values of the breakdown voltage are 7.25 kV for the DBD reactors without balls (None), 7.0 kV for the DBD reactor with Q balls, 6.5 kV for the DBD reactor with LP balls, and 6.1 kV for the DBD reactor with PA balls. The DBD reactor filled with PA balls has the lowest breakdown voltage and highest energy injection at a certain peak voltage. As the distance between two alumina plates was fixed at 2 mm, the differences in breakdown voltages are obviously due to the presence of different dielectric balls. Pulse voltage is applied across on the whole DBD reactor, including two alumina plates, dielectric balls, and gases in it. In comparison with the DBD reactor without dielectric balls, the space for gas discharges between alumina plates and dielectric balls is reduced. The pulse voltage across on dielectric balls is lower than that across on gases of the same space without dielectric balls (as the relative permittivities of dielectric balls are higher than that of the gases), resulting in the increase in pulse voltage across on the reduced gas space. That is, the gases in the reduced space in the DBD reactor with dielectric balls can be broken-down at a lower peak voltage than the gases in normal DBD reactor without dielectric balls. The decrease effect in breakdown voltage is proportional to the relative permittivities of the dielectric balls. The DBD reactor with PA

balls has the lowest breakdown voltage since PA balls (Al2O3) have the highest relative permittivity (9.9). The DBD reactors with Q and LP balls have breakdown voltages higher than that with PA balls because the relative permittivities of Q balls (SiO2, 3.9) and LP balls (mixture of SiO2 and some metal oxides, such as Al2O3, within 3.9-9.9) are lower than that of PA balls. <> 3.2. Typical n-hexane decomposition When the pulse voltage of a peak voltage higher than the breakdown voltage is applied to the DBD reactors, pulsed corona discharges occur within the discharge space, which results in the decomposition of n-hexane. Fig. 6(a) presents the conversions of n-hexane using different DBD reactors at various SED. n-Hexane conversions increase with increasing SED. Since the increase of SED can result in the increase of the discharge energy into the discharge space, and the formation of energetic electrons and active species (such as O atoms, superoxide radicals, hydroxyl radicals, and metastable nitrogen atoms) is promoted, n-hexane decomposition can be enhanced [48–50]. nHexane conversion in the DBD reactor with PA balls is slightly higher than that without balls (None), and obviously higher than those with Q and LP balls. This result implies that the presence of PA dielectric balls is conducive to the decomposition of n-hexane, but Q and LP balls are not. The energy efficiency of n-hexane decomposition as a function of SED is illustrated in Fig. 6(b). The energy efficiency in all DBD reactors decreases with increasing SED. This result is similar to VOCs decomposition results using DBD reactors as reported by other scientists [22, 25]. It is mainly because that when SED increased, more energy is converted into heat and used for forming byproducts such as O3 (detected in the latter of this study), instead of decomposing pollutants. At a SED of 83.3 J/L, the maximum energy efficiencies are 0.115, 0.82, 0.113, and 0.15 mol/kWh for the

DBD reactors without balls (None), with Q, LP, and PA balls, respectively. The n-hexane decomposition reactions can be divided into two stages. Firstly, n-hexane is decomposed by energetic electrons and active species to form intermediate products. Secondly, intermediate products are further oxidized into final products. As can be seen from Table 1 and Fig. 3, PA balls have many mesopores and macropores, LP balls have only macropores, while Q balls have neither mesopores nor macropores. So the intermediate products may be adsorbed or absorbed onto the PA balls and the second stage reactions are partly hindered. Therefore, more energetic electrons and active species can react with n-hexane in first stage decomposition reactions, resulting in the highest conversion and energy efficiencies. In the same way, LP balls have higher conversion and energy efficiencies than Q balls. <> In order to compare the energy efficiencies of n-hexane decomposition with other VOCs decomposition, Fig. 7 summaries the energy efficiencies of various VOCs decomposition as a function of initial VOCs concentrations. The energy efficiencies of C2HCl3 [17, 50–51] and CCl4 [50,52] decomposition are in the range of 1.2–2.9 mol/kWh and 0.2–0.35 mol/kWh, respectively. The energy efficiencies of benzene decomposition [20,22] and CH3OH decomposition [53] are 0.27–0.35 mol/kWh and 0.22 mol/kWh, respectively. All above values are higher than that of any other kinds of VOCs decomposition. The energy efficiencies of 2-heptanone [55], toluene [24], and n-hexane decomposition (this study) are in the range of 0.08–0.15 mol/kWh. Energy efficiencies of other VOCs decomposition listed in Fig.7 are lower than this study. <>

The main products from n-hexane decomposition are CO and CO2. Fig. 8 presents the selectivities of CO and CO2 and the ratio of CO2 selectivity to CO selectivity at various SED. Generally, the selectivities of CO and CO2 increase with the increase in SED. As it is expounded in the former of the paper, more active species were produced when SED increased, which can oxidize the intermediate products into final products effectively, resulting in higher CO and CO2 selectivities. The CO selectivity using the DBD reactor filled with PA balls is the lowest (Fig. 8(a)); the CO2 selectivity using the DBD reactor filled with Q balls is the highest at a certain SED (Fig. 8(b)). It may be because the Q balls nearly have no pores, the intermediate products are more likely to be oxidized to final products immediately instead of adsorbing on the Q balls. It was found that there is no obvious difference in the ratios of CO2 selectivity to CO selectivity among the DBD reactors except for that with PA balls. The ratio of CO2 selectivity to CO selectivity using the DBD reactor filled with PA balls is 2 to 3 times higher than other DBD reactors. This finding suggests that the CO formation from n-hexane decomposition in the DBD reactor filled with PA balls differs from those in other DBD reactors. Considering that when the DBD reactor filled with PA balls is used, the CO selectivity is the lowest while the CO2 selectivity is almost the same as those using the DBD reactor without dielectric balls, CO is possibly produced from the oxidation of the intermediate products of h-hexane decomposition, and PA balls can inhibit CO formation reactions via effective adsorption of the intermediate products onto their surfaces. <> The total selectivities of CO and CO2 using the DBD reactor filled with PA balls at different SED is in the range of 32% to 68%, implying that there are other products except for CO and CO2. The carbon balance was then calculated using Eq. (8) (Fig. 9). The carbon balance values using the

DBD reactors without balls (None), with Q and LP balls are at constant levels of 83.4%, 92.8%, and 87.6%, respectively. However the carbon balance using the DBD reactor with PA balls decreases with increasing SED. As shown in Table 1, the specific surface area of PA balls is 384 m2/g, which is much larger than those of Q and LP balls. It is notable that more intermediate products generated at high SED values were effectively adsorbed on the PA balls. The intermediate products on three kinds of dielectric balls were then analyzed by using FTIR. In Fig. 10(a), the FTIR spectra of Q balls before and after n-hexane decomposition are almost the same, indicating that there is no –

product adsorbed on Q balls. In Fig. 10(b), a few products with –CH2– changed angle (1460 cm 1) and C=O (ketone, 1653 cm-1) functional groups were found on the LP balls. In Fig. 10(c), the intensity of the bands at 1362, 1394, 1433, 1485, 1608, 1665, 1727, and 3125 cm-1 increased greatly after n-hexane decomposition due to the presence of R–NO2 (aliphatic), COOH, –OH (carboxylic avid), –OH (alcohol), COOH, C=O (ketone), C=O (saturated fat aldehyde), and –OH (alcohol) groups, respectively; indicating that a large amount of intermediate products from n-hexane decomposition were adsorbed on PA balls. Furthermore, there is an interesting finding that the relative absorbance of the OH stretching vibration at 3480 cm–1 (Si–OH) on Q and LP balls after n-hexane decomposition increased (Fig. 10(a,b)). It is possibly due to the absorption of OH radicals from electric discharges on the surface of SiO2 during the n-hexane decomposition. However, the relative absorbance of the OH stretching –1

vibration at 3497 cm

(Al–OH) decreased after n-hexane decomposition (Fig. 10(c)), indicating

that OH groups may be consumed by the intermediate products of n-hexane decomposition. <>

In order to investigate the composition of the intermediate products of n-hexane decomposition, gas products from the outlet of the DBD reactor was absorbed using the n-decane liquid and analyzed with GC-MS. As shown in Fig. 11(a), 2-hexanone and 3-hexanone were found to be the main products in the decane absorption liquid. Except for the peak of n-hexane, acetic acid, 1nitrate-1,2,3-propanetriol, methyl cyclopentane, nitro ethane, propanoic acid, pentanal, 1,5-dinitrate pentanediol, 1-nitro propane, 3-hexanol, and hexanal were also found. Among those products, methyl cyclopentane is the impurity from the raw n-hexane. 3-Hexanone, 2-hexanone, 3-hexanol, and hexanal are obviously produced from the dehydrogenation and oxidation of n-hexane. Meanwhile, acetic acid, 1-nitrate-1,2,3-propanetriol, nitro ethane, propanoic acid, pentanal, 1,5dinitrate pentanediol, and 1-nitro propane are from the breaking of C–C bonds of n-hexane. The peak areas of 2-hexanone and 3-hexanone from the DBD reactor with PA balls are typically smaller than those with Q and LP balls, indicating that PA balls can adsorb 2-hexanone and 3-hexanone more effectively than Q and LP balls. <> The CH2Cl2 supernatant liquids after extracting the decomposition products adsorbed on Q, LP, and PA balls were then analyzed with GC-MS and shown in Fig. 11(b). 2-Hexanone and 3-hexanone were found as the main products on PA balls. Products including trimethyl oxirane, nitro ethane, 2pentanone, propanoic acid, pentanal, 2,3-dihydro-2,5-dimethyl furan, 3-hexanol, hexanal, 1-hexanol, and butyrolactone were also found on PA balls. Those products have C–OH, C=O, COOH, and R– NO2 functional groups, proving that GC-MS analysis results match with the FTIR results in Fig. 10.

3.3. Particle formation All the intermediate products detected by GC-MS have higher boiling points than n-hexane, so the number concentration of the particles in the gas stream from the outlet of the DBD reactor was also detected using the SMPS. Fig. 12(a) shows the particle number concentrations as a function of particle diameter at a SED range of 288−304 J/L. The curve of particle number concentration (None) has three peaks, viz. 9.85×104, 2.59×105, and 3.17×105 #/cm3 at 22.7, 51.4, and 100.9 nm, respectively. The curve of particle number concentration (Q balls) has two peaks: 6.14×104 #/cm3 at 20.8 nm and 9.73×104 #/cm3 at 56.5 nm. The curve of particle number concentration using the DBD reactor with LP balls also has two peaks: 7.58×10 2 #/cm3 at 68.24 nm and 8.21×102 #/cm3 at 213.77 nm. However, the particle number concentration using the DBD reactor with PA balls is the lowest, which is less than 15 #/cm3. <> The total particle number concentration was defined as the sum of the number concentrations over all particle diameters. Fig. 12(b) is the total particle number concentrations as a function of SED. For the DBD reactors without balls (None) and with Q balls, total particle concentrations increase and saturate to an order within 1×106–6×106 #/cm3 with increasing SED. This saturated order is very close to that of the aerosol from styrene removal with an AC/DC streamer corona plasma system in air, as reported by Zhang et al. [40]. For the DBD reactor with LP balls, the total particle concentration also increases with increasing SED and saturate to 1.78×104 #/cm3. For the DBD reactor with PA balls, the total particle concentration is at a very low level of 33–83 #/cm3. It has the lowest total particle concentration of 33.2 #/cm3 at 526.8 J/L, which are 2 to 5 orders of magnitude lower than that of other DBD reactors. Those facts indicate that using the DBD reactor

filled with PA balls, the emission of the particles in a wide range from 10 nm to 1000 nm can be effectively reduced. As Q balls have a very small pore volume and surface area (Table 1), and considering the fact that the total number concentrations of the particles from the DBD reactors without balls (None) and with Q balls are very close (Fig. 12(b)), it is concluded that Q balls have no reduction effect on particle emission during n-hexane decomposition. The total number concentration of the particles from the DBD reactor with LP balls is lower than that without balls, suggesting that LP balls reduce particle emission. The total number concentration of the particles from the DBD reactor with PA balls is less than 83 #/cm3, implying that PA balls have the strongest effect on reducing particle emission from the DBD reactor. As the sizes of the particles from n-hexane decomposition are all bigger than the mesopores (4.31 nm) in the PA balls, these mesopores have no effect of solid particle collection, but effect on liquid particle adsorption. Furthermore, there were a large number of macropores on the PA balls (Fig. 3) even much more than the LP balls, so these macropores on the PA balls should have an important effect on collecting solid and liquid particles and reducing the emission of particles from the DBD reactor. In order to better understand the role of discharge and adsorption in reducing particle emission, an additional DBD reactor filled with PA balls but without supplying pulse voltage was installed downstream the DBD reactor without balls. The particle number concentration was also detected using the SMPS. The additional reactor can effectively reduce the emission of particles with diameters less than 450 nm (Fig. 13). However, no inhibition effect could be found when the particle diameter is larger than 450 nm. This fact suggests that the PA balls adsorb only the particles of a diameter less than 450 nm. When the DBD reactor was filled with PA balls, the particles were

completely adsorbed over the whole diameter range. This finding implies that besides the adsorption of particles on the PA balls, there must be other effects promoting particle reduction. These effects most possibly include electrostatic precipitation. The similar result was reported by Jaworek et al. [56]. <> As shown in Fig. 11(a), the peak areas of 2-hexanone and 3-hexanone in the gas stream from the DBD reactor with PA balls are typically lower than that from the DBD reactor without balls. The total particle numbers from the DBD reactor with PA balls are also lower than that from the DBD reactor without balls (Fig. 12(b)). This fact implied that the particles mainly compose 2-hexanone and 3-hexanone. A small number of particles was found in unsaturated n-hexane gas steams even without discharges (SED=0) (Fig. 12(b)), indicating that n-hexane with a boiling point of 68.5 to 69.1 oC can condense to form particles even at a low concentration. This suggested that, as the main products of n-hexane decomposition, 2-hexanone (boiling point 127.2 oC, melting point –55.5 oC) and 3-hexanone (boiling point 124 oC, melting point –55.5 oC) can reasonably condense into liquid particles since they have boiling points much higher than n-hexane and melting points lower than room temperature (25 oC). 3.4. O3 concentration It is well known that O3 is easily generated in the DBD reactor. So we also observed the O3 concentration. The O3 concentration as a function of SED using different DBD reactors is shown in Fig. 14. Generally, the O3 concentration of the DBD reactors with balls is lower than that of DBD reactors without balls, indicating that the balls has an inhibition effect on O3 emission. The lowest O3 concentration was obtained using the DBD reactor with PA balls, almost 1/3 to 1/4 of others,

which suggested that the DBD reactor with PA balls has the strongest inhibition on O3 emission. This effect may be due to the largest specific surface area which can adsorb/absorb a great amount of O3. On the other hand, there were a lot of intermediate products generated from n-hexane decomposition on the PA balls, which may react with O atoms, resulting in less O3 production from combination of O atoms with O2. Moreover, O3 can react with the intermediate products adsorbed/absorbed on the PA balls to generate deeply oxidized hydrocarbons, such as acetic acid and propanoic acid, which can be found from GC-MS analysis result (Fig. 11(b)). <>

4. Conclusions

In this study, the decomposition of n-hexane was carried out in four kinds of DBD reactors at atmospheric pressure, the main conclusions are summarized as follows: (1) n-Hexane is decomposed mainly to CO2, CO, and hydrocarbons containing oxygen such as 2hexanone and 3-hexanone. The hydrocarbons containing oxygen can presence as liquid particles since they have boiling points higher than n-hexane and melting points lower than room temperature. (2) n-Hexane conversion using the DBD reactor filled with PA balls is slightly higher than that without balls, but obviously higher than those with Q and LP balls. This is due to PA balls can more effectively adsorb the intermediate products of n-hexane decomposition than Q and LP balls, resulting in more reactive species react with n-hexane.

(3) The selectivity of CO using the DBD reactor filled with PA balls is much lower than other reactors, suggesting PA balls can inhibit CO formation. (4) The DBD reactor filled with PA balls can completely adsorb the liquid particles for a wide range of 10~1000 nm and the total particle number concentration in decomposition products is 2 to 5 orders of magnitude lower than that of other reactors. Adsorption and electrostatic precipitation may contribute to the reduction of particle emission from the DBD reactor.

Acknowledgments

Financial supports are provided by Zhejiang Provincial Natural Science Foundation of China (No. LY13B070004), the Program for Zhejiang Leading Team of S&T Innovation (No. 2013TD07).

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Fig. 1. Schematic of experimental system.

Terminal connected to pulse power supply Electrode (95×95×0.3 mm3)

Dielectric balls (Φ2 mm)

Alumina plates (115×115×1 mm 3)

Organic glass spacer (115×10×2 mm3) Organic glass holder (115×10×1 mm 3)

Electrode (95×95×0.3 mm 3) Terminal connected to pulse power supply

Fig. 2. Basic structure of the DBD reactor.

(a)

(b)

(c)

(d)

Fig. 3. SEM photos of the surfaces of Q ball (a), LP ball (b, c), and PA ball (d).

Voltage (kV)

10 5 0 -5 -10

Current (A)

2.0 1.0 0.0 -1.0 0

20

40

60

80

100

Time (µs)

Fig. 4. Typical waveforms of discharge voltage and current (with PA balls, 367.3 ppmv C0).

6

None

Q balls

LP balls

PA balls

Energy injection (W)

5 4 3

Breakdown voltage 7.25 kV

2

Breakdown voltage 7.0 kV

Breakdown voltage 6.5 kV

Breakdown voltage 6.1 kV

1 0 0

2

4

6

8

10

Peak voltage (kV)

12 0

2

4

6

8

10

Peak voltage (kV)

12 0

2

4

6

8

10

12 0

Peak voltage (kV)

Fig. 5. Energy injection as a function of peak voltage.

2

4

6

8

10 12

Peak voltage (kV)

0.20

Conversion x (%)

(a)

80 60 40

None Q balls LP balls PA balls

20 0 0

100 200 300 400 500 600 SED (J/L)

Energy efficiency (mol/kWh)

100

(b)

0.15

None Q balls LP balls PA balls

0.10 0.05 0.00 0

100 200 300 400 500 600 SED (J/L)

Fig. 6. n-Hexane conversion x (a) and energy efficiencies (b) versus specific energy density SED.

1.0E+01

Evans, C2HCl3 Penetrante, C2HCl3

1.0E+00

Energy efficiency (mol/kWh)

Jiang, C6H6

Oda, C2HCl3

This study, C6H14, PA balls

Penetrante, CCl4 Bromberg, CCl4

1.0E-01

Yao, CH3OH

This study, C6H14, None This study, C6H14, LP balls

Nunes, toluene

This study, C6H14, Q balls

Futamura, C6H6, MnO2

Lu, C 6H6 , MnO2 Lu, C 6H6, TiO2

Ye, C6H6

Ayrault, 2-heptanone Demidouk., toluene, Pt

Lu, C6H6

Demidouk., toluene

Tamon, (CH3)2S Tamon, CH3SH

Liang, toluene, TiO2/BaTiO3 Liang, toluene, TiO2 Liang, toluene, BaTiO3

Futamura, C6H6

1.0E-02

Chun, C6H6, with oscillation Chun, C6H6, without oscillation

Ayrault, 2-heptanone, Pt

Futamura, Cl2C=CCl 2, FB Futamura, Cl 2C=CHCl, FB Futamura, CH2=CH2, FB Futamura, C6H6, FB Futamura, CH3 Br, FB Futamura, CH3Cl, FB Futamura, CH4 , FB

Tamon, SO2

1.0E-03 Tamon, COS

1.0E-04 1.E+01

1.E+02

1.E+03

1.E+04

1.E+05

Initial concentration (ppmv)

Fig. 7. Comparison of energy efficiencies as a function of initial VOCs concentration [17–25, 50–55].

100 None

40

Q balls LP balls

30

PA balls

20 10

10

(b)

80

8

60

6

40

None

CO2/CO

(a)

CO2 selectivity (%)

CO selectivity (%)

50

(c)

None Q balls LP balls PA balls

4

Q balls

20

LP balls

2

PA balls

0

0 0

100 200 300 400 500 600

SED (J/L)

0 0

100 200 300 400 500 600

SED (J/L)

0

100 200 300 400 500 600

SED (J/L)

Fig. 8.

Selectivities of CO (a) and CO2 (b) and the ratio of CO2 selectivity to CO selectivity (c) at various specific energy densities SED.

Carbon balance (%)

100 80 60 40

None Q balls LP balls PA balls

20 0 0

100 200 300 400 500 600

SED (J/L)

Fig. 9. Carbon balance as a function of specific energy density SED.

1.0

(a)

0.8

Q balls (N)

SiO2

0.6

Q balls (Y) OH

0.4 0.2

Relative absorbance

0.0 1.0

0

500

1000

1500

2000

(b)

0.8

3500

4000

3500

4000

3500

4000

LP balls (Y) Si-C

0.4

3000

LP balls (N)

Fe2O3

0.6

2500

CH2 C=O

0.2

R-OH

0.0 1.0

0

500

1000

1500

2000

(c)

0.8

C-OH COOH COOH C-OH R-NO2 C=O C-OH

Al2O3

0.6 0.4

R-NO2

0.2

2500

3000

PA balls (N) PA balls (Y) R-OH

C=O

0.0 0

500

1000

1500

2000

Wave number

2500

3000

(cm-1)

Fig. 10. FTIR spectra of three kinds of dielectric balls before (N) and after (Y) n-hexane decomposition.

Fig. 11. GC-MS analysis results of gas products absorbed in decane (a) and products on dielectric balls extracted with CH2Cl2 (b).

90

1.0E+05

80 70

1.0E+04

60 50

1.0E+03

40 None Q balls LP balls PA balls

1.0E+02 1.0E+01

30 20 10 0

1.0E+00 10

100

Particle diameter (nm)

(10) 1000

Total number concentration (#/cm3)

1.0E+07

100

(a)

Number concentration (#/cm3)

Number concentration (#/cm3)

1.0E+06

(b) 1.0E+06

None Q balls

1.0E+05 1.0E+04

LP balls

1.0E+03 1.0E+02

PA balls

1.0E+01 1.0E+00 0

100

200

300

400

500

600

SED (J/L)

Fig. 12. Particle number concentration versus particle diameter (a) and total particle number concentration as a function of SED (b).

100

None: 1.0E+05

90 80 70

1.0E+04

60

None+adsorption:

1.0E+03

50 40 30

1.0E+02

PA balls:

20 10

1.0E+01

0 1.0E+00 10

100

Number concentration (#/cm3)

Number concentration (#/cm3)

1.0E+06

(10) 1000

Particle diameter (nm)

Fig. 13. Particle number concentration versus particle diameter at an SED range of 288– 304 J/L.

O3 concentration (ppmv)

5000 None

4000

Q balls LP balls

3000

PA balls

2000 1000 0 0

100 200 300 400 500 600

SED (J/L)

Fig. 14. O3 concentration as a function of SED at 293-321 ppmv C0.

Table 1 Properties of dielectric balls. Balls SBET Pore sizes

Pore volumes

Diameters

Weights

Packing densities

(m2/g)

(nm)

(cm3/g)

(mm)

(g)

(g/ml)

Q

0.615

12.6

0.00194

1.91–2.29

23.6

1.31

LP

0.513

14.9

0.00191

1.90–2.27

23.7

1.32

PA

384

4.31

0.413

1.79–2.29

13.0

0.722

n-Hexane decomposition in a DBD reactor filled with dielectric balls is investigated. Products were analyzed using GC, GC-MS, FTIR, and SMPS. Main products are CO, CO2, and particles. Particles mainly compose 3-hexanone and 2-hexanone. The DBD reactor with porous alumina balls can completely reduce particle emission.