Chemical Engineering Journal 192 (2012) 21–28

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Chemical Engineering Journal

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Simultaneous removal of SO2, NO and mercury using TiO2-aluminum silicate fiberby photocatalysis

Yuan Yuan a, Junying Zhang a,⇑, Hailong Li a,b, Yang Li a,c, Yongchun Zhao a, Chuguang Zheng a

a State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, Wuhan, Hubei 430074, Chinab School of Energy Science and Engineering, Central South University, Changsha, Hunan 410083, Chinac State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, Dalian, Liaoning 116024, China

a r t i c l e i n f o

Article history:Received 2 November 2011Received in revised form 14 March 2012Accepted 15 March 2012Available online 30 March 2012

Keywords:TiO2-aluminum silicate fiberPhotocatalysisDesulfurizationDenitrificationMercury

1385-8947/$ - see front matter � 2012 Elsevier B.V. Ahttp://dx.doi.org/10.1016/j.cej.2012.03.043

⇑ Corresponding author. Tel.: +86 27 8754 2417; faE-mail address: jyzhang@hust.edu.cn (J. Zhang).

a b s t r a c t

A novel TiO2-aluminum silicate fiber (TAS) nanocomposite, synthesized by a sol–gel method, is proposedto use as a photocatalyst for the removal of multiple pollutants. The photocatalyst has been characterizedby XRD, SEM, EDX, UV–Vis spectra and BET. The TAS calcined at 500 �C exhibited the biggest BET surfacearea and highest photocatalytic activity and was used as the photocatalyst for subsequent experiments.The oxidation and removal efficiencies of SO2, NO and elemental mercury (Hg0) in simulated coal com-bustion flue gas by the TAS catalyst were tested under UV irradiation. Experiments were conducted ina fixed-bed reactor at temperatures ranging from 30 to 120 �C. In simulated flue gas (4% O2, 12% CO2,2% H2O, 400 ppm SO2, 50 ppm NO), the removal efficiencies for SO2, NO and Hg0 at 120 �C and withUV intensity of 3 mW cm�2 can reach 33%, 31% and 80%, respectively. NO inhibited SO2 oxidation dueto its competition for active adsorption sites. SO2 also had a prohibitive effect on NO removal. In contrast,SO2 was found to have a promotional effect on Hg0 oxidation due to the formation of HgSO4. NO inhibitedthe photocatalytic removal of mercury. During the simultaneous removal of SO2, NO and Hg0 on TAS, thephotocatalytic oxidation efficiency decreased from 30 to 120 �C. O2 exhibited a promotional effect on thephotocatalytic oxidation due to the formation of lattice oxygen. However, the addition of water vapor tothe simulated flue gas inhibited the oxidation of SO2, NO and Hg0. The UV intensity was the most impor-tant factor in the photocatalytic oxidation. Our discussion on the possible reaction mechanism providessome useful information for developing effective photocatalysts to oxidize SO2, NO and Hg0 in simulatedcoal combustion flue gas.

� 2012 Elsevier B.V. All rights reserved.

1. Introduction

As the most abundant fossil fuel, coal plays an important role inmaintaining a steady supply of energy and promoting growth ofthe world economy. According to the International Energy Agency(IEA), the greatest demand in the future for fossil fuels will be forcoal, which will remain the primary fuel source until 2030 [1].However, coal combustion is the greatest anthropogenic sourceof toxic air pollution [2]. SOx, NOx and trace metal mercury arethe main toxic pollutants from coal combustion.

The emissions of SOx and NOx have caused serious environmen-tal problems including acid rain, photochemical smog and tropo-spheric ozone destruction [3,4]. Typically, SOx and NOx in flue gasconsist of more than 98% of sulfur dioxide (SO2) and over 90–95%of nitric oxide (NO) [5]. Consequently, considerable attention hasbeen given to the removal of SO2 and NO from flue gas combustion

ll rights reserved.

x: +86 27 8754 5526.

systems [5]. At present, wet flue gas desulfurization (WFGD) tech-nology is still the most effective and widely used method for SO2

removal, and SCR is considered as the most effective method forNOx reduction [6].

Mercury is considered to be a global pollutant. Mercury emis-sion has caused significant concern because of its extreme toxicity,persistence and the bio-accumulation of its compounds [7–10].Coal-fired boilers are the largest source of anthropogenic mercuryemissions, accounting for approximately one-third of the annual150 tons of mercury emission in the United States [11]. By April2010, more than 20 US states had proposed or adopted rules thatwere more stringent than the Clean Air Mercury Rule (CAMR)[12,13]. By December 2011, the US Environmental ProtectionAgency (EPA) announced the Mercury and Toxic Standards to pro-tect American families from power plant emissions of mercury andtoxic air pollution [14,15]. Flue gas from coal-fired boilers containsthree basic forms of mercury: elemental mercury (Hg0), oxidizedmercury (Hg2+) and particle bound mercury (Hgp) [16]. Hg2+ iswater-soluble, so it can be captured by wet flue gas desulfurization(FGD) equipment. Hgp can be captured by particulate matter (PM)

22 Y. Yuan et al. / Chemical Engineering Journal 192 (2012) 21–28

control devices such as electrostatic precipitators (ESP) or fabricfilters (FF). Hg0 is insoluble and extremely volatile at the operatingtemperatures of typical air pollution control devices (APCD). It istherefore very difficult to capture [17–20]. Consequently, moreeffective Hg0 control technologies need to be developed [21]. Thetechnology of sorbent injection, particularly activated carboninjection (ACI), is being investigated intensively at present [22–24].

The current methods for removal of SO2, NO and Hg0 need largeand complex systems, and none of them can integrate the removalof multiple pollutants. This results in large investments and oper-ating costs [25]. Therefore, the development of a new integratedremoval technology has become a major investigation in the fieldof flue gas pollutant control. Among available technologies, cata-lytic oxidation technology has received most attention [26–32].Many catalytic oxidation methods have focused on H2O2 or TiO2

doped with Pt, Si, V and other metals [33–37]. Liu et al. [33] studiedthe simultaneous removal of NO and SO2 from coal-fired flue gasusing a UV/H2O2 advanced oxidation process in an UV-bubble col-umn reactor. Nasonova et al. [34] investigated NO and SO2 removalusing a non-thermal plasma discharge process combined with aTiO2 photocatalyst. Li et al. [35] studied the oxidation of NOon Pt/TiO2 catalysts prepared by wet impregnation and photo-deposition methods. Pitoniak et al. [36] used a high surface areaSiO2–TiO2 composite to remove elemental mercury vapor fromcombustion sources. Li et al. [15] studied oxidation and captureefficiencies of elemental mercury over SiO2–TiO2–V2O5 catalystsfrom simulated low-rank coal combustion flue gases. However,these studies have focussed on the removal of one or two pollu-tants. Little research on the simultaneous removal of SO2, NO andHg0 has been reported.

In our efforts to develop a novel catalyst for simultaneous re-moval of multiple pollutants, TiO2-aluminum silicate fiber (TAS)was synthesized by a sol–gel method. It has been successfully em-ployed to remove SO2, NO and Hg0 simultaneously from simulatedcoal combustion flue gas.

Aluminum silicate fiber is a fire-resistant and thermal insula-tion material and has been widely used in industry and building.Its maximum operating temperature is 1200 �C. The TAS nanocom-posite has excellent mechanical and thermal properties and highcorrosion resistance. In this work, systematic experiments wereconducted to investigate the removal efficiencies for SO2, NO andHg0 over the TAS nanocomposite, the effects of SO2 and NO onthe photocatalytic desulfurization, denitrification and mercury re-moval and also the effects of temperature, O2, H2O and UV inten-sity on the photocatalytic oxidation efficiency. The ultimate goalis to develop a non-toxic catalyst with high oxygen capacity forthe simultaneous removal of multiple pollutants from flue gas.

2. Materials and methods

2.1. Preparation of TiO2-aluminum silicate fiber

The TAS nanocomposite was prepared by the sol–gel methodusing tetrabutyl titanate (TEOT), water, ethanol and acetic acid.First, the aluminum silicate fiber was immersed in a mixed solu-tion of ethanol and TEOT (2:3, by volume). After intensely stirringfor half an hour, the mixed solution was added dropwise to theprepared mixture of ethanol, acetic acid and distilled water(1:2:0.6, by volume) under vigorous stirring. After completion ofthe reaction, the aluminum silicate fiber was removed from thesol and aged at room temperature for 24 h. Then it was placed ina drying oven at 100 �C for 2 h to vaporize liquid from within theTiO2-aluminum silicate fiber. Finally, it was calcined in a mufflefurnace from 300 to 700 �C for 2 h to remove the physically ad-sorbed water and organic components.

2.2. Characterization of TiO2-aluminum silicate fiber

The crystallinity of the catalyst was detected by X-ray diffrac-tion (XRD) (D8-FOCUS, Bruker AXS, Germany) using CuKaradiation, operating at 40 kV and 40 mA and a scanning rate of10� min�1. The morphology of the catalyst was determined usinga scanning electron microscope (SEM) (Sirion 200, FEI, Nether-lands) with an acceleration voltage of 20 kV. Energy dispersingX-ray (EDX) analysis was carried out using an X-ray detector (GEN-ESIS, EDAX Inc., United States). UV–Vis absorption spectra were re-corded on a UV–Vis spectrophotometer (Lambda 35, Perkin–Elmer,United States). The surface area and pore volume analysis wereperformed using the Brunauer–Emmett–Teller (BET) technique(ASAP 2000, Micromeritics, United States) involving nitrogenadsorption–desorption isotherm measurements at �196 �C. Priorto the adsorption measurement, the catalyst was degassed at200 �C for 30 h in nitrogen.

2.3. Experimental methods

The schematic diagram of the experimental system is shown inFig. 1. The system consisted of the simulated flue gas, a fixed bedreactor, a portable infrared gas analyzer and a mercury analyzer.The simulated flue gas was a mixture of 4–8% O2, 12% CO2, 2–4%H2O, 400–1200 ppm SO2, 50–300 ppm NO and balanced with N2,with a total flow rate of 1 L min�1. The simulated flue gas compo-nents, except water vapor, were cylinder gases. Water vapor wasgenerated from a heated water bath. The N2 flow was divided intothree branches. One of the N2 streams converged with the O2, CO2,SO2 and NO to form the main gas flow, in order to adjust the totalgas flow. The second stream of N2 with a flow rate of 300 mL min�1

passed through a heated water bath to introduce water vapor tothe system. When 2% and 4% H2O were introduced to the gas flow,the water bath temperatures were 40 and 52 �C, respectively.The third stream of N2 with a flow rate of 300 mL min�1 passedthrough the Hg0 permeation tube to introduce Hg0 vapor to thesystem. The Hg0 inlet concentration was around 50 lg m�3. Themercury permeation tube was placed in a U-shape glass tubewhich was immersed in a water bath at constant temperature toensure a constant Hg0 permeation rate.

The fixed bed reactor consisted of inner and outer quartz tubes.A 9 W UV lamp (TUV PL-S, Philips, Netherlands) was placed in theinner tube. This UV lamp provided 3 mW cm�2 intensity at a wave-length of 253.7 nm measured by a UV radiometer (UV-B, Photo-electric Instrument Factory of Beijing Normal University, China).Variations of UV intensity could be achieved by covering differentstainless steel meshes on the UV tube. When the widths of meshopening were 160 and 650 lm, UV intensity were 1 and 2 mW cm�2,respectively. A heating tape was wrapped around the reactor sothat the temperature of the gas mixture could be controlled inthe range from 30 to 120 �C by thermocouple. Cold air generatedby an air pump was used to cool the UV lamp. The TAS catalystwas placed on a stainless steel mesh (2 mm opening) and allowedto react with SO2, NO, and Hg0 under UV irradiation. In each test,4 g TAS, previously calcined at 500 �C, was used. The reactor wasplaced inside an opaque box in order to make maximum use ofUV energy and prevent interference from sunlight and fluorescentlight.

An on-line mercury analyzer (VM-3000 Mercury Vapor Monitor,Mercury Instruments, Germany) based on atomic absorption spec-trometry was used to measure the gas phase concentration of Hg0

in the simulated flue gas. The gas flowed through 1 mol L�1 sodiumhydroxide (NaOH) solution and silica gel before entering the mer-cury analyzer in order to capture acidic gases and water vapor andprevent corrosion of the mercury analyzer. Portable infrared gasanalyzers (FT/IR Gas Analyzer GASMET DX4000, Temet, Finland)

Fig. 1. Schematic diagram of the experimental system.

Fig. 2. XRD patterns of aluminum silicate fiber (a) and TAS calcined at 300 �C,400 �C, 500 �C, 600 �C and 700 �C (b–f).

Y. Yuan et al. / Chemical Engineering Journal 192 (2012) 21–28 23

based on Fourier transform infrared principles were used to mea-sure the SO2 and NO concentrations.

The experimental conditions are summarized in Table 1. In Set I,the SO2 removal efficiency over TAS was first investigated at 30 �C.The effect of SO2 and NO concentration on the photocatalyticdesulfurization was also studied. In Set II, the NO removal ratewas studied at 30 �C and the effect of SO2 on the photocatalyticdenitrification was also investigated. Set III experiments were de-signed to study the Hg0 removal efficiency at 30 �C. In addition,the impact of SO2 and NO on mercury removal was investigated. Fi-nally, Set IV was conducted to study the simultaneous removal ofSO2, NO and Hg0 over TAS. In the presence of 12% CO2, 400 ppmSO2, 50 ppm NO and balanced with N2, the impacts of temperature,O2, H2O and UV intensity on the photocatalytic oxidation efficiencywere studied.

In this work, the Hg0 removal efficiency (EH), SO2 removal effi-ciency (ES) and NO removal efficiency (EN) over the TAS catalystcan be defined using Eqs. (1)–(3), respectively.

EH ð%Þ ¼Hg0

in �Hg0out

Hg0in

� 100% ð1Þ

ES ð%Þ ¼SO2in � SO2out

SO2in� 100% ð2Þ

EN ð%Þ ¼NOin � NOout

NOin� 100% ð3Þ

where Hg0in, SO2in and NOin represent Hg0, SO2 and NO at the inlet of

the reactor, respectively and Hg0out, SO2out and NOout represent Hg0,

SO2 and NO at the outlet of the reactor, respectively.

3. Results and discussion

3.1. Characterization of TiO2-aluminum silicate fiber

Fig. 2 shows the XRD patterns of the aluminum silicate fiber (a)and TAS calcined at 300 �C, 400 �C, 500 �C, 600 �C and 700 �C (b–f).

Table 1Experimental conditions.

Gas

Set I Basic gas (BG: N2, 4% O2, 2% H2O, 12% CO2) + SO2 (400–800 ppm)BG + 400 ppm SO2 + NO (50–200 ppm)

Set II BG + NO (50–200 ppm)BG + 50 ppm NO + SO2 (400–800 ppm)

Set III BGBG + SO2 (400–1200 ppm) + NO (50–300 ppm)

Set IV N2 + 12% CO2 + 400 ppm SO2 + 50 ppm NO + individual gas compon

There was no obvious diffraction peak from the aluminum silicatefiber between 15� and 80�, indicating that the aluminum silicate fi-ber was amorphous. When TAS was calcined at 300 �C, the diffrac-tion peak of anatase appeared at 25.3�. With increasing calcinationtemperature, further characteristic peaks of anatase were found,namely the (103), (004), (112), (200), (105), (211), (204),(116), (220) and (215) peaks at 37.0�, 37.8�, 38.6�, 48.0�, 53.9�,55.1�, 62.7�, 68.8�, 70.3� and 75.1�, respectively, indicating thatthe TiO2 with an anatase phase structure had been formed. More-over, the intensity of the peaks gradually increased with the rise ofcalcination temperature due to the heat-induced growth of titaniaparticles. This growth results in the increase of crystallinity andgreater ordering in the structure of titania particles [38]. Sharpand narrow X-ray peaks were observed for the TAS calcined at

UV intensity (mW cm�2) Temperature (�C)

3 30

3 30

3 30

ents (O2, H2O) 1–3 30–120

24 Y. Yuan et al. / Chemical Engineering Journal 192 (2012) 21–28

500 �C. It should be noted that there was no rutile peak at any cal-cination temperature below 500 �C. Nevertheless, anatase gradu-ally turned into the rutile phase as the calcination temperaturewas further increased. When the TAS was calcined at 700 �C, itwas observed that the main rutile diffraction peak (110) at 27.5�was absent. Other rutile diffraction peaks assigned to (101),(111), (211), (220), (301), (112), (320) and (202) also appearedat 36.1�, 41.2�, 54.3�, 56.7�, 69.0�, 69.8�, 74.5� and 76.5�, respec-tively. Thus, the calcination temperature was a key factor in thecrystalline structure.

The SEM and EDX analysis of the aluminum silicate fiber andTAS calcined at 500 �C is shown in Fig. 3. Aluminum silicate fiberis composed of many tiny columnar fibers (Fig. 3A). These tiny fi-bers are arranged in order with an average diameter of 10 lm.The EDX spectrum confirmed that the aluminum silicate fiber con-tained the elements O, Al and Si. Fig. 3B shows the morphology ofthe TAS catalyst. After loading with TiO2, a large number of parti-cles with a diameter of about 20 nm were observed on the surfaceof the aluminum silicate fiber. In the EDX spectrum of TAS, thepeaks corresponding to O, Al, Si and Ti were all clearly observed,indicating that the particles on the fiber surface were TiO2. Thedata for the elemental compositions of the aluminum silicate fiberand TAS are summarized in Table 2. The aluminum silicate fiberconsisted of 60 wt.% O, 20 wt.% Al and 20 wt.% Si and the mass frac-tion of TiO2 loaded on the fiber was about 10 wt.%.

The UV–Vis spectra of the aluminum silicate fiber and TAS cal-cined at different temperatures are displayed in Fig. 4. There wasno absorption peak from the aluminum silicate fiber in the UVand visible spectral range, indicating that the aluminum silicate fi-ber shows no optical activity. When TAS was calcined at 300 �C, atypical absorption of TiO2 at 300–400 nm was observed. It shouldbe noted that the absorption intensity of TAS gradually increasedwith the rise of the calcination temperature. The most intenseabsorption peak in the UV region appeared for the TAS calcined

Fig. 3. SEM and EDX analysis of aluminum silic

at 500 �C along with an extension of the absorption wavelengthrange. Because the absorption intensity increased and the absorp-tion wavelength range was extended, the rate of formation of elec-tron–hole pairs on the photocatalyst surface also increased greatly,resulting in the photocatalyst exhibiting higher photocatalyticactivity [39,40]. Thus the TAS calcined at 500 �C has the highestcatalytic activity. In addition, when the calcination temperaturewas further increased, the absorbance of the TAS was reducedslightly.

Table 3 lists the BET surface area, pore volume and pore size ofTAS calcined at different temperatures. The BET of aluminum sili-cate fiber was 0.0561 m2 g�1. Its total volume of pores less than72.2610 nm diameter and the average pore size were all very smalland negligible, which indicates that the aluminum silicate fiberwas not porous. Nevertheless, after the nano TiO2 was loaded ontothe aluminum silicate fiber, the BET of TAS increased significantly.It was also found that the BET, pore volume and pore size all in-creased with the rise of calcination temperature. The TAS calcinedat 500 �C gave the largest BET, 87 times greater than that of alumi-num silicate fiber. Moreover, the BET decreased gradually as thecalcination temperature was further increased, indicating thatthe calcination temperature also affected the pore structure ofthe nanocomposite.

The TAS calcined at 500 �C most likely had the best SO2, NO andHg0 removal performances for it showed the highest crystallinity,highest photocatalytic activity and greatest BET surface area.Therefore, the TAS calcined at 500 �C was used as the photocatalystfor subsequent experiments.

3.2. Photocatalytic desulfurization

As shown in Fig. 5a, in the presence of 400 ppm SO2, a low valueof ES of 7% was observed under visible light at 30 �C due to simplephysical adsorption. However, the ES rapidly increased to 39%

ate fiber (A) and TAS calcined at 500 �C (B).

Table 2Elemental compositions of aluminum silicate fiber and TAS calcined at 500 �C.

Sample Element Mass percentage(%)

Atomic percentage(%)

Aluminum silicatefiber

O 59.54 71.70Al 19.49 13.92Si 20.97 14.39

TAS calcined at500 �C

O 54.94 69.05Al 19.19 14.30Si 19.52 13.98Ti 6.35 2.67

Fig. 4. The UV–Vis spectra of aluminum silicate fiber (a) and TAS calcined at 300 �C,400 �C, 500 �C, 600 �C and 700 �C (b–f).

Table 3Physical properties of aluminum silicate fiber and TAS.

Sample BET SA(m2 g�1)

Pore volume(cm3 g�1)

Pore size(nm)

Aluminum silicatefiber

0.0561

TAS calcined at300 �C

0.7696 0.000039 0.20329

TAS calcined at400 �C

3.1688 0.005082 6.41500

TAS calcined at500 �C

4.8761 0.005141 4.21734

TAS calcined at600 �C

1.3699 0.002408 7.03237

TAS calcined at700 �C

0.5054 0.001436 11.3653

Y. Yuan et al. / Chemical Engineering Journal 192 (2012) 21–28 25

under UV irradiation, indicating the occurrence of a photocatalyticoxidation reaction under UV irradiation.

As shown in Fig. 6, the ES decreased from 39% to 30% as the SO2

concentration increased from 400 to 800 ppm. It can be provedthat the photocatalytic desulfurization efficiency graduallydecreased with the increase of SO2 concentration. Moreover, aninhibitory effect of NO on SO2 removal was observed, as shownin Fig. 6. In the range of 50–200 ppm NO, ES decreased to approx-imately 33%, which may be a result of the competitive adsorptionof NO with SO2 for the active sites and the strongly oxidizing �OHand O�2 species. This competition reduced the number of free rad-icals which could be able to react with SO2. The oxidation of SO2

was therefore inhibited, resulting in the reduction of SO2 removalefficiency.

3.3. Photocatalytic denitrification

As shown in Fig. 5b, in the presence of 50 ppm NO, the NO re-moval rate was only 8% under visible light at 30 �C. However, theEN rapidly increased to 41% under UV irradiation due to the oxida-tion of NO on the TAS surface. As shown in Fig. 6, the NO removalefficiency decreased only slightly when the NO concentration in-creased from 50 to 200 ppm, indicating that there was no signifi-cant change in photocatalytic denitrification efficiency. The effectof SO2 on NO removal was found to be inhibitory. As shown inFig. 6, the value of EN decreased to 35% when 400 ppm SO2 wasintroduced to the gas flow. With a further increase of SO2 concen-tration to 800 ppm, EN decreased to approximately 30%. The inhib-itory effect of SO2 also contributed to the competitive adsorption ofSO2 with NO for the active sites and the strongly oxidizing �OH andO�2 species. This competition reduced the number of free radicals

which could be able to react with NO, resulting in the reductionof NO removal efficiency. Furthermore, the formation of surfacenitrates was greatly suppressed by SO2, which resulted in a partialnegative effect on NO oxidation [41].

3.4. Photocatalytic mercury removal

As shown in Fig. 5c, an EH value of only 12% was observed undervisible light at 30 �C. Under UV irradiation, EH rapidly increased to85% due to the occurrence of the photocatalytic oxidation reaction.Hg0 was oxidized to Hg2+ to achieve photocatalytic mercuryremoval.

3.4.1. Effect of SO2 on mercury removalThe promotional effect of SO2 on Hg0 oxidation was observed

over the TAS catalyst. As shown in Fig. 7, in the range of 0–1200 ppm SO2, the Hg0 removal rate increased to approximately92%. Furthermore, In the presence of NO, SO2 was also found tohave a promotional effect on Hg0 oxidation. The introduction of1200 ppm SO2 into the gas flow containing 50 ppm NO improvedEH from 83% to over 90%. The promotional effect of SO2 was dueto the formation of HgSO4. Under UV irradiation, abundant activeradicals O�2 and �OH were generated over the TAS catalyst. SO2

was oxidized by active radicals to form SO3, which constitutednew chemisorption sites for Hg0 and could react with Hg0 to pro-duce HgSO4 [7,42]. The reaction process is given in Eqs. (4)–(6).

2SO2 þ O�2 ! 2SO3 ð4ÞSO2 þ 2 �OH! SO3 þH2O2 ð5Þ2Hgþ 2SO3 þ O2 ! 2HgSO4 ð6Þ

3.4.2. Effect of NO on mercury removalNO was found to have an inhibitory effect on Hg0 oxidation over

the TAS catalyst. As shown in Fig. 7, the introduction of 300 ppmNO into the gas flow resulted in a reduction of EH to 80%. The inhib-itory effect of NO was due to the competition of NO with Hg0 forthe active adsorption sites and radicals. Moreover, in the presenceof 1200 ppm SO2, an inhibitory effect of NO on Hg0 oxidation wasalso observed over the TAS catalyst.

3.5. Integration of desulfurization, denitrification and mercuryremoval

In the presence of 12% CO2, 400 ppm SO2, 50 ppm NO and bal-anced with N2, the effects of O2, H2O, temperature and UV intensityon the photocatalytic oxidation were examined.

Fig. 5. (a) SO2 removal over TAS (gas composition: BG + 400 ppm SO2); (b) NOremoval over TAS (gas composition: BG + 50 ppm NO); and (c) Hg0 removal overTAS (gas composition: BG).

Fig. 6. Effects of NO and SO2 on photocatalytic desulfurization and denitrification,respectively.

Fig. 7. Effects of SO2 and NO on Hg0 removal.

Fig. 8. Effects of O2 and H2O on photocatalytic removal of SO2, NO and Hg0 (gascomposition: N2 + 12% CO2 + 400 ppm SO2 + 50 ppm NO + O2 + H2O).

26 Y. Yuan et al. / Chemical Engineering Journal 192 (2012) 21–28

3.5.1. Effect of O2

In the presence of 2% H2O with UV intensity of 3 mW cm�2, O2

was found to have a promotional effect on the photocatalytic re-moval of SO2, NO and Hg0 at 120 �C. As shown in Fig. 8, introduc-tion of 8% O2 into gas flow improved ES, EN and EH from 27%, 25%and 74% to 35%, 34% and 84%, respectively. The promotional effectof O2 was due to the formation of lattice oxygen. Gas phase O2 gen-erated the lattice oxygen under 253.7-nm UV irradiation, whichwas the most abundant reactive intermediate and served as theoxidant to oxidize SO2, NO and Hg0 into SO3, NO2 and HgO [43].

3.5.2. Effect of H2OIn the presence of 4% O2 with UV intensity of 3 mW cm�2, H2O

was found to have an inhibitory effect on SO2, NO and Hg0 removalat 120 �C. As shown in Fig. 8, in the dry condition, ES, EN and EH

were 36%, 37% and 89%, respectively. When 4% H2O was introduced

to the gas flow, ES, EN and EH decreased to 31%, 28% and 76%,respectively. The inhibitory effect of H2O can be explained by com-petitive adsorption [15,44]. The competitive adsorption of H2O onactive sites inhibited the adsorption of reactive species and re-duced the number of oxidized SO2, NO and Hg0, resulting in thereduction of photocatalytic removal efficiency.

3.5.3. Effect of temperatureIn the presence of 4% O2 and 2% H2O with UV intensity of

3 mW cm�2, the photocatalytic oxidation efficiencies decreasedwith an increase of temperature. As shown in Fig. 9, ES, EN and EH

were reduced from 37%, 35% and 84% to 33%, 31% and 80%, respec-tively, as the temperature increased from 30 to 120 �C. Higher tem-peratures would result in a larger number of collisions between thegas-phase reactive species and the quartz wall. The depositionsand adsorptions of SO2, NO and Hg0 on the catalyst surface wouldbe weakened, which resulted in a possible reduction in both the

Fig. 9. Effects of temperature and UV intensity on photocatalytic removal of SO2,NO and Hg0 (gas composition: BG + 400 ppm SO2 + 50 ppm NO).

Y. Yuan et al. / Chemical Engineering Journal 192 (2012) 21–28 27

amount of reactive species and the reaction rate [32]. Thus,the photocatalytic oxidation efficiency was lower at highertemperatures.

3.5.4. Effect of UV intensityUV intensity had a significant impact on SO2, NO and Hg0 re-

moval over TAS at 120 �C. As shown in Fig. 9, when the UV inten-sity was 3 mW cm�2, ES, EN and EH were 33%, 31% and 80%,respectively. However, these values were rapidly reduced to 14%,12% and 38% when the UV intensity was reduced to 1 mW cm�2,indicating that the intensity of UV irradiation had an important ef-fect on the photocatalytic reaction. There is a reduction of photoex-cited active species with the decrease of UV intensity, whichresulted in the reduction of photocatalytic oxidation efficienciesfor SO2, NO and Hg0.

3.6. Mechanism analysis

The mechanism of Hg0 removal over TiO2 is considered to arisefrom the excitation of photoelectrons from the valence band to theconduction band of TiO2 under UV light irradiation, leaving holes inthe valence band. In the presence of O2 and H2O, the photo-gener-ated electrons and holes react with them to generate active oxygenspecies such as O�2 and �OH radicals, which are responsible for Hg0

oxidation [36,45]. The mechanism can be described by Eqs. (7)–(20) [22,45–47].

TiO2 þ hm! hþ þ e� ð7ÞH2O$ Hþ þ OH� ð8Þ

OH� þ hþ ! �OH ð9Þ

H2Oþ hþ ! �OHþHþ ð10ÞO2 þ e� ! O�2 ð11ÞO�2 þHþ ! �HO2 ð12Þ�HO2 þ e� þHþ ! H2O2 ð13ÞH2O2 þ hm! 2 �OH ð14Þ

Hg0 þ O�2 ! HgO ð15Þ

Hg0 þ �OH! HgO ð16Þ

Similarly, it is considered that O�2 and �OH radicals are responsi-ble for the oxidation of SO2 and NO to form SO3 and NO2,respectively.

SO2 þ O�2 ! SO3 ð17ÞSO2 þ �OH! SO3 ð18ÞNOþ O�2 ! NO2 ð19ÞNOþ �OH! NO2 ð20Þ

4. Conclusions

The TAS catalyst prepared by the sol–gel method has been char-acterized by XRD, SEM, EDX, UV–Vis spectra and BET. Anatase wasobserved to be the exclusive phase of TiO2 when the TAS was cal-cined below 500 �C but the anatase turned gradually into the rutilephase when the calcination temperature was further increased. Alarge number of TiO2 particles, with a diameter about 20 nm, wereobserved on aluminum silicate fiber and the TiO2 content wasabout 10 wt.%. TAS calcined at 500 �C exhibited the highest UV–Vis absorption intensity and greatest photocatalytic activity. Inaddition, the calcination temperature affected the pore structureof the nanocomposite and the TAS calcined at 500 �C was foundto have the largest BET surface area.

In our study of the removal of SO2 over TAS in simulated coalcombustion flue gas, SO2 removal efficiency decreased with the in-crease of SO2 concentration in the range from 400 to 800 ppm andNO exhibited an inhibitory effect on SO2 removal. In the removal ofNO over TAS, there was no significant change in photocatalyticdenitrification efficiency with an increase of NO concentration.SO2 was found to have an inhibitory effect on the photocatalyticremoval of NO due to the competitive adsorption of SO2 with NOfor the active sites and the strongly oxidizing �OH and O�2 species.The Hg0 removal efficiency over TAS was stable at around 84% inthe presence of 400 ppm SO2 and 50 ppm NO at 30 �C under UVirradiation. SO2 promoted the photocatalytic mercury removaldue to the formation of HgSO4. However, NO exhibited an inhibi-tory effect on Hg0 oxidation and capture.

In the simultaneous removal of SO2, NO and Hg0 over the TAScatalyst, the photocatalytic oxidation efficiency decreased withthe increase of temperature. O2 was found to have a promotionaleffect on the photocatalytic removal of SO2, NO and Hg0 due tothe formation of lattice oxygen. H2O exhibited an inhibitory effecton the photocatalytic oxidation due to the competitive adsorptionfor active adsorption sites. In addition, UV irradiation was criticalto the successful oxidation of SO2, NO and Hg0. This study providesinformation about the simultaneous removal of multiple pollu-tants, including the promotional or inhibitory effects of variousfactors on the photocatalytic oxidation. Such knowledge is of fun-damental importance in developing effective pollutant controltechnology.

Acknowledgments

The authors gratefully acknowledge the support from the Na-tional Key Basic Research and Development Program (2011CB201500),the National Natural Science Foundation of China (41172140,51176060, 51021065), and the State Key Laboratory of CleanEnergy Utilization (ZJUCEU2011015).

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