Removal of NOx, SO2, and Hg From Simulated Flue Gas ... - IEEE Xplore

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IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 41, NO. 2, FEBRUARY 2013

Removal of NOx, SO2, and Hg From Simulated Flue Gas by Plasma–Absorption Hybrid System Meiyan Wang, Yifei Sun, and Tianle Zhu

Abstract—The simultaneous removal of NOx , SO2 , and Hg from simulated flue gas by a plasma–absorption hybrid system was investigated. In the nonthermal plasma reactor, NO could be effectively oxidized to NO2 . However, Hg0 oxidation was significantly restrained since NO concentration and its reactivity with O3 are much higher than those of Hg0 . In the absorber, SO2 and NO2 were absorbed by (NH4 )2 SO3 solution, in which the − S(IV) ions (SO2− 3 and HSO3 ) were found to be dominant for NO2 absorption. The S(IV) ions were significantly oxidized during the absorption, causing an increase in NO2 concentration with operating time. However, the addition of S2 O2− inhibited the S(IV) 3 oxidation and promoted the removal of NO2 . With a followed electric mist eliminator, the NH3 slipped from the absorber can be captured, and Hg0 was efficiently oxidized, which can be further removed by water absorption. Index Terms—Absorption, Hg, NOx , nonthermal plasma (NTP), S2 O2− 3 , SO2 .

I. I NTRODUCTION

V

ARIOUS pollutants from coal combustion, including SO2 , NOx , and Hg, are emitted into the atmosphere. SO2 and NOx are major causes of acid rain, photochemical smog, and respiratory organ disease, and mercury is considered as a serious toxic heavy metal due to its environmental and neurological health impact. Hence, developing technologies to reduce the NO, SO2 , and mercury emissions has been a problem of paramount concern. For multipollutant reductions from flue gas, the commonly adopted pathway is that these pollutants are respectively treated using different processes, e.g., NOx reductions by ammonia selective catalytic reduction (SCR), SO2 reduction by lime/limestone wet flue gas desulfurization (WFGD), and Hg removal by activated carbon injection. Although these techniques are efficient for flue gas purification, they are involved with high construction and operation cost, as well as large installation space, owing to the complex treatment processes Manuscript received August 21, 2012; revised October 28, 2012; accepted December 6, 2012. Date of publication January 11, 2013; date of current version February 6, 2013. This work was supported by the National Natural Science Foundation of China under Grant 20977003, by the High-Tech Research and Development (863) of China under Grant 2012AA062503, and by the Key Laboratory for Advanced Technology in Environmental Protection of Jiangsu Province under Grant AE201003. M. Wang is with the School of Chemistry and Environment and Key Laboratory of Bio-Inspired Smart Interfacial Science and Technology of Ministry of Education, Beihang University, Beijing 100191, China, and also with the School of Energy and Environmental Engineering, Hebei University of Technology, Tianjin 300401, China. Y. Sun and T. Zhu are with the School of Chemistry and Environment and Key Laboratory of Bio-Inspired Smart Interfacial Science and Technology of Ministry of Education, Beihang University, Beijing 100191, China (e-mail: [email protected]). Digital Object Identifier 10.1109/TPS.2012.2234483

and numerous apparatus. Therefore, novel technology aimed at simultaneous removal of NOx , SO2 , and Hg from flue gas is desperately required. In the flue gas, about 95% of NOx is NO. Absorption process exhibits high removal efficiency for NO2 , whereas it is incapable of NO removal due to the low solubility. On the other hand, mercury compounds in the flue gas may be emitted as elemental mercury (Hg0 ), oxidized mercury (Hg2+ ), and particle-associated mercury (Hgp ). Most Hgp and Hg2+ can be effectively trapped by conventional air pollution control systems such as electrostatic precipitators, fabric filters, and WFGD; however, it is difficult for Hg0 removal with its high vapor pressure and low water solubility [1]. Therefore, preoxidation of NO and Hg0 is a crucial step for their absorption removal. A series of studies were conducted based on gaseous or aqueous oxidation of NO and Hg0 , including nonthermal plasma (NTP) oxidation, injecting ozone or chlorine to the gas flow, and adding oxidative agents such as sodium chlorite/hypochlorite to the absorption solution [2]–[5]. Among them, NTP is considered to be one of the most promising technologies for oxidation of Hg0 and NO. NTP has been studied for several decades and is recognized as a potential process for simultaneous removal of NOx , SO2 , and trace elements [6], [7]. Active species such as O, OH, HO2 , and O3 generated from the gas discharge can induce the oxidation of NO and Hg0 . A lot of studies have been carried out to investigate NOx removal by an electron-beam NTP-based process [8]. Masuda and Nakao first proposed the electrical NTP process for NO oxidation, and encouraging results have been obtained in both experimental and industrial investigations [9]–[13]. Furthermore, Byun et al. reported effective oxidation of Hg0 with a dielectric barrier discharge and a pulsed corona discharge reactor [14]–[16]. A dc wire-cylinder reactor is more promising for practical applications because of the low cost of dc power supply and the ability to remove mist, aerosol, and gaseous pollutants simultaneously. However, the oxidation of Hg0 by dc corona discharge has not been extensively represented in literature. In recent years, an integrated technology of NTP combined with chemical absorption has been put forward for flue gas decontamination, which gives a significant cost saving in replacing the individual processes (SCR + WFGD + carbon injection) [17]–[21]. Mizuno et al. also reported a single-stage wet-type plasma reactor in which the absorbent (water, NaOH, or Na2 SO3 ) film on the ground electrode induced in situ absorption of NOx /SO2 [22]–[24]. During our previous study, the effects of reaction conditions, including discharge polarity, electrode configuration, and gaseous components on the Hg0

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WANG et al.: REMOVAL OF NOx , SO2 , AND Hg FROM FLUE GAS BY PLASMA–ABSORPTION HYBRID SYSTEM

Fig. 1.

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Schematic of the experimental setup.

and NO removal, were respectively investigated by the NTP alone or the NTP–absorption combined process [25], [26]. However, the simultaneous removal of NOx , SO2 , and Hg by the NTP–absorption combined method has rarely been investigated. In this paper, a hybrid system of dc NTP combined with ammonia absorption was proposed for the simultaneous removal of NOx , SO2 , and Hg from flue gas. The (NH4 )2 SO3 solution was used as absorbent for the following two reasons: 1) the ammonia resource widely exists in China; 2) the product ammonium sulfate can be used as fertilizer, realizing recovery of the sulfur resource in the flue gas. The removal strategy is based on the oxidation of NO to NO2 in the NTP reactor, the absorption of NO2 and SO2 by the (NH4 )2 SO3 solution in the absorber, followed by the capture of NH3 slip and oxidation of Hg0 to Hg2+ in the electric mist eliminator, and finally the absorption of Hg2+ along with other soluble pollutants into water. The removal performance of the system and the oxidation and absorption mechanisms were investigated. II. E XPERIMENTAL S ETUP AND M ETHODS A. Experimental Setup A schematic of the experimental setup is shown in Fig. 1. It consists of a gas feeding unit, a plasma–absorption hybrid gas treatment system, and a set of analytical instruments. The plasma–absorption hybrid system was composed of a wirecylinder NTP reactor with positive dc high-voltage power supply (25 kV/5 mA), an absorber with ammonium sulfite

solution, an electric mist eliminator, and an absorber with water, sequentially. For the NTP reactor, a stainless steel cylinder (i.d. 42 mm) was used as the ground electrode, whereas a stainless steel rod (o.d. 6 mm), through which the discharge tooth slices equidistantly linked with a space interval of 10 mm, was used as the high-voltage electrode. The effective discharge length and discharge gap were 160 and 16 mm, respectively. Four discharge points were evenly distributed on each tooth slice. The electric mist eliminator was a modified NTP reactor, in which the stainless steel rod was wrapped with an insulating Teflon layer to avoid the creepage phenomenon caused by mist condensation, and the plexiglass connector was used to link the rod and the cylinder in order to observe the mist condition. In the NTP reactor and electric mist eliminator, the visual aspect of the discharge is a luminous plasma column completely filling the interelectrode space, which is indicative of the streamer corona discharge. Gas absorption was conducted in bubble column absorbers. A 2.5-L (NH4 )2 SO3 solution (mixture of NH4 OH and H2 SO3 solution) with or without S2 O2− 3 addition (0.3 mol/L) was used as absorbent in the first absorber, with pH kept in the range of 6.0–6.5. In the latter absorber, only water was needed as absorbent. B. Experimental Methods All the experiments were performed at atmospheric pressure and room temperature. The simulated flue gas consisted of 6% O2 , 12% CO2 , 3% H2 O, 120-ppm NO, 525-ppm SO2 , and

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110-μg/m3 Hg0 , using N2 as balance gas, with a total flow rate of 6 L/min. The energy density (ED, J/L) was used to evaluate the energy consumption of the NTP reactor, which is defined as the power deposited into 1 L of reaction gas and calculated as follows: ED =

60U I Input electrical power = Gas flow rate Q

(1)

where U is the applied voltage (kV) measured with a highvoltage probe (Nissin EP-50K, Japan); I is the discharge current (mA) calculated by measuring the voltage across a 10-Ω resistor with a multimeter; Q is the flow rate of the reaction gas (L/min). Concentrations of gaseous NOx , SO2 , Hg, NH3 , and O3 at the outlet of each reactor were measured to evaluate the removal performance of the plasma–absorption system. A flue gas analyzer (Testo 335, Germany) and a SO2 analyzer (Kane SGA94, Britain) were used to monitor the concentrations of NOx and SO2 , respectively. O3 , NH3 , and Hg were analyzed by indigo disulphonate spectrophotometry, sodium salicylate–sodium hypochlorite spectrophotometry, and dithizone spectrophotometry, respectively [27]–[29]. To measure the O3 , NH3 , and Hg concentrations, a constant gas flow (O3 : 0.5 L/min; NH3 and Hg: 1 L/min) was pumped from a sample point toward the inlet of an absorption bottle filled with 10-mL solution (O3 : 0.5-g/L indigo disulphonate; NH3 : 0.005-mol/L sulfuric acid; Hg2+ : 0.5-mol/L sulfuric acid; Hg0 : 0.1-mol/L potassium permanganate and 10% v/v sulfuric acid) followed by colorimetry analysis with a spectrophotometer. Small samples of the absorption solution were periodically collected and used to analyze. The concentrations of nitrite and − nitrate ions (NO− 2 and NO3 ) were determined with ion chromatography (Methrohm 792 Basic, Switzerland). Sulfite/S(IV) − 2− ions (SO2− 3 and HSO3 ) and thiosulfate ion (S2 O3 ) concentrations were measured by an iodometric titration method [30]. III. R ESULTS AND D ISCUSSION A. Oxidation of NO, SO2 , and Hg0 by NTP Reactor Fig. 2 indicates the concentration profiles of NOx , SO2 , and Hg at the outlet of the NTP reactor as functions of ED. It is shown that the NO concentration at the outlet of the NTP reactor significantly decreased with increasing the ED from 0 to 80 J/L and then leveled off with a further increase in ED. Accordingly, the NO2 concentration increased. In addition, the SO2 and Hg0 concentrations slightly decreased, and the Hg2+ increased with the increase in ED. This can be ascribed to the increment of active species such as energetic electrons, i.e., O, OH, HO2 radicals, and O3 , due to the increased discharge power [31]. On the other hand, the oxidation efficiencies of Hg0 and SO2 were much lower than that of NO with identical ED. The initial concentrations of NO, SO2 , and Hg0 were 120 ppm, 525 ppm, and 110 μg/m3 , respectively. For an ED of 80 J/L, the NO, SO2 , and Hg0 concentrations decreased to 26 ppm, 420 ppm, and 94 μg/m3 after the NTP reactor, corresponding to the oxidation efficiencies of 78%, 20%, and 15%, with NO2 and

Fig. 2.

NOx , SO2 , and Hg concentrations at the outlet of the NTP reactor.

Hg2+ increased to 101 ppm and 14 μg/m3 , respectively. Previous studies have found that O3 is the major oxidative species for the oxidation of NO and Hg0 [25], [26]. As shown by reactions (2) and (3) [32], [33], the rate constant of reaction (2) is five orders of magnitude higher than that of reaction (3); additionally, the inlet concentration of NO (120 ppm) far exceeded that of Hg0 (110 μg/m3 , i.e., 12 ppb); thus, Hg0 oxidation was significantly restrained by NO coexistence due to its preferential consumption of O3 . Thus NO + O3 → NO2 + O2 k298 = 1.8 × 10−14 cm3 /molecule/s

(2)

Hg + O3 → HgO + O2 k298 = 7.5 × 10−19 cm3 /molecule/s

(3)

Similarly, the oxidation efficiency of SO2 was found to be quite low due to the low rate constant of SO2 −O3 reaction (4). SO2 +O3 → SO3 +O2

k298 = 2.0 × 10−22 cm3 /molecule/s (4)

Considering the reduction in energy consumption as the cobenefit with satisfactory oxidation efficiency, the ED of the NTP reactor was maintained at about 80 J/L during the treatment process in the flowing investigation. The corresponding applied voltage and discharge current were 13.6 kV and 0.59 mA, respectively.


Fig. 3. NOx , SO2 , and Hg concentrations at the outlet of the absorber. (Discharge conditions for the NTP reactor: ED = 80 J/L, U = 13.6 kV, I = 0.59 mA.)

B. Removal of NOx , SO2 , and Hg by Absorption Fig. 3 shows the NOx , SO2 , and Hg concentrations at the outlet of the absorber as functions of operating time. It is shown that the NO concentration at the outlet of the absorber was almost identical with the inlet (i.e., NTP outlet) and did not change with operating time, indicating ineffective removal of NO by absorption. NO2 was effectively absorbed by the fresh (NH4 )2 SO3 solution, with an outlet concentration of 12.8 ppm initially; however, the concentration significantly increased with operating time, being 74 ppm at 70 h. The reason is that, NO2 may react with S(IV) ions in the absorption solution via reactions (5) and (6) [34], which promoted the absorption of NO2 . + − 2− 2NO2 + SO2− 3 + H2 O → 2H + 2NO2 + SO4 − + − 2NO2 + HSO3 + H2 O → 2H + 2NO2 + HSO2− 4

(5) (6)

However, the plasma-induced O3 , O radical, and particularly the dissolved NO2 led to the formation of sulfite radical (SO•− 3 • and HSO•− 3 , noted as R ), which initiated a series of radical chain reactions and caused the oxidation of S(IV) ions to S(VI) ions [35], [36]. Consequently, the NO2 concentration increased as a result of insufficient reaction with S(IV) ions. Fig. 3 also shows that the SO2 can be effectively absorbed in spite of insufficient oxidation. The SO2 outlet concentration decreased with operating time, from 108 ppm at the start to 0 ppm after 30 h. This can be explained by the fact that the SO2 absorption was promoted by the following reactions: − SO2 + SO2− 3 + H2 O ↔ 2HSO3 + − SO2 + H2 O ↔ H + HSO3

(7) (8)

and the decreased S(IV) concentration in the absorption solution may lead to a forward-direction movement of the reaction equilibrium for the reversible reaction (8). On the other hand, Hg0 was hardly dissolved in the absorbent; thus, its concentration kept constant at about 95 μg/m3 after absorption, and the Hg2+ concentration remained below 3 μg/m3 . The aforementioned results show that S(IV) ions were crucial for NO2 absorption; however, they were oxidized during the absorption process. Therefore, as sulfite oxidation inhibitor, a 0.3-mol/L S2 O2− 3 ion was added to the absorbent in order to promote the NO2 absorption.

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Fig. 4. NOx , SO2 , and Hg concentrations at the outlet of the absorber (with the addition of S2 O2− 3 ). (Discharge conditions for the NTP reactor: ED = 80 J/L, U = 13.6 kV, I = 0.59 mA.)

Fig. 4 gives the NOx , SO2 , and Hg concentrations at the outlet of the absorber in the presence of S2 O2− 3 as functions of operating time. It is shown that NO2 absorption was promoted by the addition of S2 O2− 3 , with the outlet concentration slightly increased from 8.3 ppm at the beginning to 18.6 ppm at 70 h. This can be attributed to the fact that the oxidation inhibitor S2 O2− 3 acted as a free radical scavenger, which reduced the sulfite oxidation rate by reacting with free radicals and terminating the chain reactions, as shown in reactions (9) and (10) [34]. •− R• + S2 O2− 3 → R + S2 O3

S2 O•− 3

+

S2 O•− 3

→

S4 O2− 6

(9) (10)

In addition, in the presence of S2 O2− 3 , SO2 was efficiently absorbed despite of high S(IV) concentration, which could be attributed to reactions (11) and (12). Hg0 and Hg2+ concentrations followed a similar pattern to the situation without S2 O2− 3 . − S2 O2− 3 + SO2 + H2 O → 2HSO3 + S

(11)

2− S + SO2− 3 → S2 O 3 .

(12)

C. Analysis of the Absorption Solution Composition In order to further investigate the effects of S(IV) ions on the − absorption of NO2 , the concentrations of S(IV), NO− 2 , NO3 , − − and total nitrogen (TN: NO2 plus NO3 ) ions in the absorption solution were detected with operating time, as shown in Fig. 5, where TN’ is the calculated concentration of TN ion by assum− ing that all dissolved NO2 was converted to NO− 2 or NO3 . It is shown in Fig. 5 that the S(IV) concentration remarkably declined, from 0.99 mol/L in the initial absorbent to 0.03 mol/L after 36 h, implying that S(IV) ions were con− sumed by oxidation. On the other hand, no NO− 2 or NO3 was observed during the initial 24 h, which seems incompatible with reactions (5) and (6). This might be explained by the fact that the dissolve of SO2 into the absorption solution yields HSO− 3 , and consequently, a set of complex − reactions between NO− 2 and HSO3 took place, resulting in the formation of nitrogen–sulfur compounds such as

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− 0.35 mol/L at 70 h. In addition, the NO− 2 and NO3 ions were not observed in the absorption process due to reaction (13).

D. Effect of Electric Mist Eliminator

Fig. 5. Ion concentrations in the absorption solution.

Fig. 6. Ion concentrations in the absorption solution (with the addition of S2 O2− 3 ).

HO3 SNO, H2 N2 O2 , (HO3 S)2 NOH, (HO3 S)NHOH, NH2 OH, (HO3 S)3 N, NH(HSO3 )2 , and (NH2 )(HSO3 ), which would partially decompose to gaseous N2 , NO, or N2 O, as summarized by reaction (13) [37]. − HNO2 + HSO− 3 → products + HSO4

E. Removal Model

(products : nitrogen-sulfur compounds, N2 , NO, N2 O). (13) After 24 h, NO− 2 ion was observed because the effects of reaction (13) were attenuated with a S(IV) concentration below 0.07 mol/L. At 36 h, NO− 3 started to be formed, and both of − and NO ion concentrations increased afterward via the NO− 2 3 reaction (14), with a further declined S(IV) concentration below 0.03 mol/L. On the other hand, the TN concentration was far below TN’, suggesting that the formed NO− 2 ion was significantly consumed to produce the nitrogen–sulfur compounds, N2 , NO, or N2 O. − 2NO2 + H2 O → 2H+ + NO− 2 + NO3

NH3 slip is usually considered as an undesirable byproduct resulted from the ammonia absorption methods. Additionally, SO3 mist and gypsum rain are common in the WFGD system. Therefore, the electric mist elimination is necessary for the trap of acid and NH3 mist. In this paper, we focused on the removal of the NH3 slip and the oxidative effect of the residual NO and Hg0 by the electric mist eliminator. The NH3 concentration at the outlet of the absorber was 9 ppm. After the electric mist eliminator with an ED of 80 J/L (U = 13.6 kV, I = 0.59 mA), it was below 1 ppm due to the capture effect by the strong electric field and the consumption through the reactions with the plasma-induced energetic electrons, i.e., O and OH radicals. It can be further reduced below 0.5 ppm after water absorption. Therefore, the NH3 slip can be effectively controlled in the plasma–absorption system. On the other hand, NO in the electric mist eliminator was almost completely oxidized, with an outlet concentration constant below 3 ppm. Therefore, Hg0 was easily oxidized along with the low-concentration NO, with its concentration decreased to 27 μg/m3 . The Hg2+ and low-concentration NO2 and SO2 can be effectively absorbed by water. After the test, some white solid products were visible on the discharge electrode and the inner wall of the electric mist eliminator. In order to analyze the formed byproduct, the discharge electrode and the inner wall of the electric mist eliminator were washed by deionized water and analyzed by ion chro− − 2− 2− matography. NH+ 4 , NO3 , NO2 , SO3 , and SO4 ions were detected in the washing liquids, implying that the NH3 –NOx and NH3 –SOx reactions occurred in the electric mist eliminator, which caused the deposition of the solid NH4 NO2 , NH4 NO3 , (NH4 )2 SO3 , and (NH4 )2 SO4 .

(14)

Ion concentrations in the absorption solution with the addition of S2 O2− 3 are shown in Fig. 6. It is shown that the decrease in S(IV) concentration became slow in the presence of S2 O2− 3 , from 0.95 mol/L initially to

Based on the aforementioned results, a remove model of NOx , SO2 , and Hg from flue gas by the plasma–absorption system was proposed. Fig. 7 illustrates the possible reaction pathways in the reactors. The removal pathways for NOx , SO2 , and Hg can be described as follows. 1) NO was effectively oxidized to NO2 in the NTP reactor (a small part of NO2 further converted to NO3 ). NO2 was absorbed in the absorber in the presence of S(IV) ions, − which converted to NO− 2 and NO3 ions, nitrogen–sulfur compounds, or gaseous N2 /NO/N2 O. The residual NO was totally oxidized in the electric mist eliminator, which resulted the formation of NO2 and a small quantity of solid byproduct NH4 NOx . Finally, the NO2 was absorbed in water. 2) SO2 was partially oxidized in the NTP reactor and then significantly absorbed in the absorber. Some residual SO2 may form SO3 or NH4 SOx in the electric mist eliminator and then absorbed in water.


Fig. 7.

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Proposed chemical pathways for NOx , SO2 , and Hg removal by the plasma–absorption hybrid system.

3) Hg0 was hardly oxidized in the NTP reactor with coexistence of high-concentration NO (120 ppm) and hardly absorbed in the absorber. However, in the electric mist eliminator, with NO concentration being about 22 ppm, it could be sufficiently oxidized to HgO or HgOH, which was easily dissolved into water. in the In general, with the addition of 0.3-mol/L S2 O2− 3 absorption solution, the total removal efficiencies of NOx , SO2 , and Hg by the plasma–absorption hybrid system reached 93%, 100%, and 75%, respectively. IV. C ONCLUSION The simultaneous removal of NOx , SO2 , and Hg from flue gas by a plasma–absorption hybrid system has been systematically investigated in this paper. The main findings can be summarized as follows: 1) NO was effectively oxidized to NO2 by the NTP reactor, whereas Hg0 oxidation was significantly restrained due to the preferential consumption of O3 by NO. 2) NO2 and SO2 were readily absorbed by (NH4 )2 SO3 − solution, where the S(IV) ions (SO2− 3 and HSO3 ) were mainly responsible for the removal of NO2 . 3) The S(IV) ions were oxidized during the process, causing a decrease in NO2 removal with operating time. The addition of S2 O2− 3 oxidation inhibitor reduced the S(IV) oxidation rate and thus promoted the removal of NO2 . 4) The Hg0 was oxidized, and the NH3 slipped from the absorber can be reduced by using the electric mist eliminator.

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Meiyan Wang was born in Henan, China, on February 15, 1985. She received the B.E. degree in environmental engineering from the University of Science and Technology Beijing, Beijing, China, in 2007 and the Ph.D. degree in combined M.E. and Ph.D. program in environmental engineering from Beihang University, Beijing, China, in 2012. She is involved in research on flue gas treatment using nonthermal plasma.

Yifei Sun was born in Liaoning, China, on October 31, 1973. She received the B.E. degree in environmental engineering from Beijing Institute of Technology, Beijing, China, in 1996 and the M.E. and Ph.D. degrees in municipal environmental engineering from Kyoto University, Kyoto, Japan, in 2003 and 2006, respectively. During 2006–2009, she was a Postdoctoral Researcher with Osaka Institute of Technology. She is currently an Associate Professor with the School of Chemistry and Environment, Beihang University, Beijing. Her research interests include solid waste disposal and utilization and the application of thermochemical technology in pollution control, including nonthermal plasma technology.

Tianle Zhu was born in Jiangxi, China, on June 2, 1963. He received the B.E. degree in chemical engineering from Nanchang Hangkong University, Nanchang, China, in 1983; the M.E. degree in environmental engineering from the Xi’an University of Architecture and Technology, Xi’an, China, in 1989; and the Ph.D. degree in environmental engineering from Tsinghua University, Beijing, China, in 2000. During 2001–2003, he was a Postdoctoral Researcher with Tsinghua University. He is currently a Professor with the School of Chemistry and Environment, Beihang University, Beijing. His research interests include the applications of nonthermal plasma in flue gas treatment and indoor air pollution control.