Elemental Mercury Removal from Flue Gas over TiO2 Catalyst in an

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Kinetics, Catalysis, and Reaction Engineering

Elemental mercury removal from flue gas over TiO2 catalyst in an internal-illuminated honeycomb photoreactor Jianping Yang, Siming Ma, Yongchun Zhao, Hailong Li, Junying Zhang, and Chuguang Zheng Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b04417 • Publication Date (Web): 06 Dec 2018 Downloaded from http://pubs.acs.org on December 8, 2018

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Industrial & Engineering Chemistry Research

Elemental mercury removal from flue gas over TiO2 catalyst in an internal-illuminated honeycomb photoreactor Jianping Yang1,2, Siming Ma2, Yongchun Zhao2,*, Hailong Li1, Junying Zhang2, Chuguang Zheng2 1

School of Energy Science and Engineering, Central South University, Changsha 410083, China

2

State Key Laboratory of Coal Combustion, School of Energy and Power Engineering, Huazhong University of Science and Technology, Wuhan 430074, China

*Corresponding

author, Email: [email protected], TEL: 86-27-87542417, FAX: 86-27-87545526

1

ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Abstract: TiO2 catalyst in an internal-illuminated honeycomb photoreactor was prepared for Hg0 removal from flue gas. The Hg0 removal efficiency was above 95% under the optimal operation condition. With the increasing TiO2 coating value, the Hg0 removal efficiency significantly increased. The catalyst calcined at 400 ºC presented optimal Hg0 removal performance, while higher calcination temperature weakened the Hg0 photocatalytic removal activity. Similar Hg0 removal performances were obtained under UV irradiation when the reaction temperature was in the range of 25-90 ºC, and 1.5 mW/cm2 of UV light irradiation was competent for Hg0 photocatalytic removal. With the same quantity utilization of TiO2 catalyst, the internal-illuminated honeycomb photoreactor presented better Hg0 removal performance than fixed-bed reactor. Finally, the procedure of Hg removal from flue gas over TiO2 catalyst in internal-illuminated honeycomb photoreactor was proposed, and the product in the Hg0 photocatalytic removal process was analyzed as well. Keywords: mercury, photocatalytic removal, TiO2, photoreactor, coal combustion

2


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1. Introduction Mercury (Hg) is harmful for human health and the environment due to its extreme toxicity, persistence, and bio-accumulation.1 The “Minamata Convention on Mercury”, which aimed to reduce anthropogenic Hg emission from industrial plants, has taken into force for its signatories on 16th August 2017. Coal combustion is one of the most significant anthropogenic Hg emission sources.1 Thus, there is great demand to develop effective and efficient technologies for reducing Hg emission from coal-fired power plants. Recently, a variety of Hg0 removal technologies including sorbent injection2-14 and catalytic oxidation15-22 were studied. Activated carbon injection (ACI) is the maximum achievable technology for controlling Hg removal. However, the operating cost of ACI technology is too high to be applied commercially in power plants.23 Thus, it is urgent to develop lost-cost technologies to replace ACI. To date, most of power plants installed wet flue gas desulfurization (WFGD), and wet electrostatic precipitator (WESP) was also applied in many power plants downstream of the WFGD. It is a promising strategy to convert gaseous Hg0 into water-soluble Hg2+ so as to facilitate the co-benefit Hg removal by the WFGD or WESP. Photocatalysis is considered as an effective way for oxidizing Hg0 to Hg2+.24-34 TiO2 is the most studied photocatalyst due to its remarkable photocatalytic activity, excellent photostability, low cost, and nontoxicity. To improve the photocatalytic Hg0 oxidation efficiency, extensive studies were performed to develop new TiO2 structures.27, 29, 30, 32, 33 The unique structure and large surface area enhanced the synergy between the photocatalytic oxidation and adsorption of Hg0. Additionally, doping foreign atoms into TiO2 is another useful method to promote the photocatalytic activity of TiO2, since it can prevent the recombination between photogenerated electrons and holes.28, 29, 31, 35-37 3



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However, in the real industrial application, the volume of flue gas required to be purified is extremely large. Thus, besides the modification of TiO2 material, the design of photocatalytic reactors remains an important issue.38 Maximizing the photocatalyst surface and the light utilization are required to improve the Hg0 photocatalytic oxidation performance. The internally-illuminated honeycomb photoreactor has been demonstrated to exhibit superior performance due to the following aspects:39-41 (1) the honeycomb which used as the photocatalyst support can increase the photocatalyst loading amount because of its multiple channels, and (2) the optical fibers distributed inside the honeycomb channels could effectively transmit and scatter light to the catalyst on honeycomb internal channels. However, up to date, no research on Hg0 photocatalytic oxidation using internally-illuminated honeycomb photoreactor was conducted. In this work, TiO2 catalyst was synthesized and coated on the honeycomb. The effects of TiO2 coating value, calcination temperature of catalysts, reaction temperature, UV-light intensity, and reactor type on Hg0 photocatalytic oxidation performance were investigated. Finally, the procedure of Hg removal from flue gas by TiO2-coated ceramic honeycomb was proposed, and the product in the Hg0 photocatalytic removal process was tested. 2. Experimental section 2.1. Sample preparation A dip-coating method was used to immobilize the photocatalysts (TiO2) on the honeycomb internal channels wall.42 In a typical process, the TiO2 precursor solution was firstly prepared by thermal hydrolysis method.42 85 ml the titanium (IV) butoxide (TBOT) was added dropwise into 510 ml HNO3 aqueous solution (0.1 M). The solution was stirred vigorously at 80 ºC for 8 h until obtaining clear sol. To increase the viscosity of sol, 10 g of polyethylene glycol (PEG) was slowly 4


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added into the sol. After that, the honeycomb was immersed into the sol for 30 min and then taken out. The excess sol in the channels was blown away by compressed air. Following, the honeycomb was dried at 105 ºC for 12 h and calcined at controlled temperature (300-600 ºC) for 1 h. The TiO2 loading value on the honeycomb was changed by adjusting the concentration of TiO2 precursor solution. The catalysts were denoted as TiXCH, where X represents the calcination temperature and the subscript of CH represents ceramic honeycomb. In this work, the ceramic honeycomb was purchased from Chauger Honeycomb Ceramics Co. Ltd. (Taiwan). The substrate of ceramic honeycomb was cordierite. The dimension of ceramic honeycomb was 40 mm × 50 mm (D × H), which contained 160 channels (2 mm × 2 mm). For comparison, the TiO2 powder catalysts were synthesized using the same procedure, which were denoted as TiXP and the subscript of P represents the TiO2 powder. 2.2. Characterization of samples The BET surface area and pore structure characteristics of samples were determined by N2 adsorption isotherms (ASAP 2020 porosimeter). The crystal structure was studied by powder X-ray diffraction (XRD, X’Pert PRO diffractometer). The UV-vis absorption property was studied using UV-vis spectrophotometer (Lambda 35, Perkin–Elmer). The surface morphology was studied by environmental scanning electron microscope equipped with energy dispersive X-ray spectroscopy (ESEM-EDS, QUANTA 200). 2.3. Experimental apparatus and procedures Figure 1(a) shows the experimental apparatus. The simulated flue gas (SFG) consisted of 4% O2, 12% CO2, 50 ppm SO2, 50 ppm NO, 10 ppm HCl, 6% H2O, 60 μg/m3 Hg0 vapor and N2. The total flow rate of SFG was 1.2 L/min. The gas hourly space velocity (GHSV) was about 2250 h-1. The Hg0 5



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vapor was provided by a Hg0 permeation tube (VICI, Metronics Inc., Santa Clara, CA), which was kept at a constant temperature to continuously produce Hg0 vapor. N2 (0.4 L· min-1) was used to introduce Hg0 vapor into the system. The reactor consisted of a honeycomb and a UV lamp. The optical fibers (d=1 mm) were inserted into each honeycomb channels (Figure 1(b)). The honeycomb inserted with optical fibers was located at a cylindrical Pyrex glass reactor (375 mL). A quartz window on the front of reactor was used for light irradiation. A UV lamp, which can provide 365 nm UV light, was placed at one end of the reactor. The light intensity was controlled by adjusting the distance between the reactor and UV lamp, which was measured in front of the quartz window. It is noteworthy that there is no obvious attenuation of light intensity at the end of reactor because of the transmission of optical fibers distributed inside the honeycomb channels. Thus, the light intensity measured in the front of reactor can represent the actual light intensity from the optical fibers to the surface of TiO2. Additionally, a powder reactor was used to investigate the effect of reactor types on Hg0 removal performance (Figure 1(c)). Table 1 summarizes the experimental conditions. In Set I, blank experiment was performed by using a fiber-illuminated honeycomb which was not coated TiO2 so as to eliminate the influence of honeycomb substrate and UV light on Hg0 removal. In Set II to Set V, the effects of TiO2 coating value, calcium temperature, reaction temperature, light intensity, and reactor type on the Hg0 removal performance were investigated under N2 atmosphere. In Set VI, experiment was performed to evaluate the stability of photocatalytic activity under SFG atmosphere. The product in the Hg0 photocatalytic removal process was also tested in Set VI of experiment. In this set of experiments, the photocatalytic Hg0 oxidation efficiency of catalyst after saturated with mercury was also tested. To short the experimental period, the catalyst was firstly saturated by high-concentration (600 μg/m3) 6


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Hg0 vapor until the fluctuation of Hg0 concentration at the reactor outlet was less than 5 μg/m3 for more than 1 h. The Hg0 concentration was measured evey 10 h instead of online monitor. Temperature programmed desorption (TPD) test was carried out to identify the mercury species on the spent catalyst. The catalyst was heated from 30 to 750 °C with a heating rate of 10 °C·min−1. The decomposed mercury was carried by pure N2 (500 ml·min−1) to the Hg analyzer. The Hg0 removal performance was evaluated by the instantaneous Hg0 removal efficiency (ET-i) and accumulated Hg0 removal efficiency (ET-a), which was defined as eq. (1) and eq. (2), respectively. ET-i 

ET-a

0 Hgin0  Hg out 100% Hgin0

 

t 0

0 Hgin0   0 Hg out

(1)

t



t

Hgin0 0

 100%

(2)

0 where Hgin0 and Hg out represents the instantaneous Hg0 concentration at the reactor inlet and outlet,



t 0

Hgin0 and



t 0

0 Hg out

represents the accumulation Hg0 concentration at the reactor inlet and outlet in

120 min. When the catalyst was saturated with mercury, the Hg0 oxidation efficiency (Eoxi) was equal to ET. The Hg species and reaction products were analyzed during the experimental period. A Hg species conversion system was applied to determine the Hg2+ concentration (Figure 1(a)). The total Hg (HgT) concentration can be determined when the flue gas passed through SnCl2 solution. The difference of HgT and Hg0 is the Hg2+ concentration. The accumulated Hg0 and Hg2+ amount was calculated by integralling the Hg0 and Hg2+ concentration curve, as illustrated by eq. (3). t2

Q   (Cin  Cout )  v  dt

(3)

t1

7



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where Q represents the accumulated Hg0 or Hg2+ amount (µg⋅g-1), v represents the gas flow rate (m3⋅h-1), and t represents the test time (h). The Hg absorbed on the catalyst (HgS) was determined by RA-915M mercury analyzer with PYRO-915+ pyrolysis equipment. The total Hg amount (QT) was the sum of Hg0 ( QHg ), Hg2+ ( QHg ) and HgS ( QHg ) amount. The proportion of each Hg species was 2

0

S

calculated by dividing the QHg , QHg , and QHg to QT, respectively. 0

2

S

3. Results and discussions 3.1. Sample characterization The surface area, pore volume, and pore size of samples are shown in Table 2. The sample of TiX300 possesses the largest BET surface area and pore volume. As the raise of calculation temperature, the BET surface area and pore volume decreased obviously, especially for the TiX500 and TiX600. This should be attributed to the sintering of samples after calculating at high temperature, resulting in the sink of framework structure or blocking of pore channel of samples. Figure 2 shows the XRD patterns of P25 and TiXCH catalysts calcined at different temperatures. The calcination temperature was a key factor for determining the phase of TiO2 coated on honeycomb. It is observed that only few peaks of anatase phase existed on the samples of Ti300CH and Ti400CH. With the increasing calcination temperature, the characteristic diffraction peaks became sharp and narrow, and the intensity of peaks increased as well. This can be attributed to the heat-induced growth of TiO2 particles, which caused the increase of crystallinity and higher structure order of TiO2 particles. When the sample was calcined at 500 ºC, part of anatase TiO2 was converted into rutile phase. However, only rutile TiO2 existed on the catalyst with the further increase of calculation temperature to 600 ºC. Figure 3 shows the surface morphology of bare and TiO2-coated honeycomb. The bare honeycomb 8


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consisted of large amounts of rectangular particles, and the chemical composition of these particles was 2MgO·2Al2O3·5SiO2, as shown in Figure 3(a). Moreover, many pores existed on the honeycomb. After coated with 0.85 g/m2 TiO2, there was no obvious variation of surface morphology between bare and TiO2-coated honeycomb (Figure 3 (b)). However, combined the EDX analysis of Ti element, it is observed that TiO2 particles were dispersed on the honeycomb surface. After coated with 3.40 g/m2 TiO2, obvious agglomeration of TiO2 was observed on the honeycomb surface. Part of TiO2 filled into the pores of honeycomb (Figure 3 (c)) and another part of them coated on the honeycomb surface as a form of film (Figure 3 (d)). Figure 3 (e) shows the SEM image and EDX pattern of the cross-section of honeycomb when coating 3.40 g/m2 TiO2. The EDX line mapping shows that Ti element was distributed in the range of 0-20 μm of deep direction, suggesting that the thickness of TiO2 coating layer was 20 μm. Moreover, it is apparently observed that the first 20 μm thick of TiO2 coating honeycomb had a higher density than other parts of honeycomb support. This also agrees with EDX analysis, suggesting that TiO2 was successfully coated on the honeycomb. Figure 4 shows the UV-Vis spectra of TiO2-coated honeycomb. The spectra of all samples showed that the typical adsorption edge of TiO2 was in the UV region of 300-400 nm. The absorption wavelength threshold (λAbsorp.Edge) can be obtained by intercepting the tangent to absorption, and the λAbsorp.Edge of Ti300CH, Ti400CH, Ti500CH, and Ti600CH was 399.76 nm, 402.23 nm, 410.43 nm, and 413.20 nm, respectively. The band gap energy can be calculated by the following equation.43 Eg = 1240 / λAbsorp.Edge

(4)

The Eg of Ti300CH, Ti400CH, Ti500CH, and Ti600CH was 3.10, 3.08, 3.02, and 3.00 eV, respectively. This suggests that the raise of calcination temperature resulted in a narrower band gap. Since the Eg of anatase and rutile TiO2 was 3.2 and 3.0 eV,43 the UV-Vis results also agreed with the XRD results, 9



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where the conversion of anatase into rutile phase occurred as the raise of calcination temperature. 3.2. Interference of bare honeycomb and UV light irradiation on Hg0 photocatalytic removal In the blank test, the flue gas containing Hg0 vapor firstly passed through the bare honeycomb under dark environment to eliminate the interference of honeycomb on Hg0 photocatalytic removal. As shown in Figure 5, the Hg0 concentration did not change obviously in 30 min. After that, the UV light (4 mW/cm2) was introduced into the reactor. It is observed that the largest interference was within 0.5 μg·m−3, which was less than 1% of the inlet Hg0 concentration. Thus, the interference of bare honeycomb and UV light irradiation on Hg0 photocatalytic removal could be negligible. In other words, the photocatalytic removal of Hg0 was attributed to the TiO2 catalyst with the assistance of UV light irradiation. 3.3. Effect of TiO2 coating value The effects of TiO2 coating value on Hg0 removal are shown in Figure 6. Under dark condition, only less than 20% Hg0 removal efficiency was obtained on all catalysts at 90 ºC. This suggests that the adsorption of Hg0 was not dominant during photocatalytic Hg0 removal. With the increase of TiO2 coating value from 0.85 to 3.40 g/m2, the Hg0 removal efficiency significantly increased from 35.0% to 95.2% under 1.5 mW/cm2 UV light irradiation. Similar photocatalytic activity was obtained when coating 4.25 g/m2 TiO2. It is reported that, under UV light irradiation, the photocatalytic Hg0 removal over TiO2 was attributed to the transition of photoelectrons from valence band to conduction band, hence generating photogenerated electrons.25,

36

Meanwhile, the holes were leaved in the

valence band. Part of photogenerated electrons and holes could react with O2 and H2O, which were adsorbed on the catalyst surface to generate active species like O2- and •OH radicals. These active species can oxidize Hg0 to form HgO. Thus, with the increase of TiO2 coating value, more active 10


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species can be generated, causing the increase of Hg0 removal efficiency. As the further increase of TiO2 loading ratio to 5.10 g/m2, the photocatalytic activity was slightly inhibited, which might be caused by the pore blockage or agglomeration. 3.4. Effect of calcination temperature The effects of calcination temperature on Hg0 removal are shown in Figure 7. The Hg0 removal efficiency of Ti300CH was 86.7%. As the increasing calcination temperature, the photocatalytic Hg0 removal activity increased. The optimal Hg0 removal performance was obtained when the catalyst calcined at 400 ºC. The increase of photocatalytic activity was attributed to the improvement of crystallization of anatase in Ti400CH. With the further increase of calcination temperature to 500 and 600 ºC, the Hg0 removal efficiency decreased seriously. Particularly, the Hg0 removal efficiency of Ti600CH decreased to 41.2% only. This can be due to the following reasons. First, according to the XRD analysis (Figure 2), part of anatase phase converted into rutile phase when the catalyst was calcined at 500 and 600 ºC. The anatase TiO2 has higher photocatalytic activity than the rutile phase.44 Second, the growth of TiO2 particles caused the decrease of BET surface area of TiO2-coated honeycomb (Table 2), which is unbeneficial for Hg0 adsorption. In this way, the excessive high calcination temperature is adverse to the photocatalytic activity of TiO2. 3.5. Effect of reaction temperature and light intensity The effects of reaction temperature and light intensity on Hg0 removal are shown in Figure 8. As shown, the Hg0 removal efficiency decreased significantly as the increasing reaction temperature. This is because physisorption acted as a dominant role in Hg0 removal at low temperature without UV irradiation,45 and the high temperature is unbeneficial for physisorption. Under 1.5 mW/cm2 UV irradiation, above 90% Hg0 removal efficiency was obtained at below 90 ºC. However, the Hg0 11



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removal performance decreased obviously at 120 ºC. This suggests that the photocatalytic Hg0 removal over TiO2 should follow the Eley-Rideal (E-R) mechanism.46 Gaseous Hg0 was firstly adsorbed on the active adsorption sites of photocatalyst to form adsorbed Hg (Hgads). The Hgads was then oxidized by the active species like O2- and •OH radicals to form HgO. The gaseous reactants attained more kinetic energy at high temperature, hence accelerating the reaction rate.8, 47 However, the higher temperature was unfavorable for the adsorption of Hg0 on photocatalyst surface and the amount of Hgads decreased accordingly. As a result, the overall photocatalytic removal rate decreased as the increasing reaction temperature. Additionally, when the reaction temperature was below 90 ºC, similar Hg0 removal performances were obtained under UV irradiation with light intensity of 1.5 and 4 mW/cm2. Thus, 1.5 mW/cm2 UV light irradiation was competent to generate enough active species for Hg0 oxidation. 3.6. Effect of reactor type The effects of reactor type on Hg0 removal are shown in Figure 9. The P25 and Ti400P were placed on a quartz plate in a powder reactor, while Ti400HC was fixed in the same location of the internal-illuminated honeycomb photoreactor. Apparently, the photocatalytic Hg0 removal activity of Ti400P was far higher than that of P25. According to the XRD analysis, anatase and rutile were co-existed in P25, while anatase was the single phase in Ti400P. Thus, it further confirmed that anatase TiO2 was more active than rutile phase in photocatalytic Hg0 removal. When the same quantity (0.24g, i.e. coating 3.40 g/m2 TiO2) of TiO2 was used, the internal-illuminated honeycomb photoreactor was superior compared to the powder reactor in Hg0 photocatalytic removal. This is because, in the powder reactor, only the TiO2 powder in surface layer was exposed to UV light irradiation. In this way, large amount of TiO2 powder was not useful for photocatalytic Hg0 removal. 12


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However, in the internal-illuminated honeycomb photoreactor, TiO2 was coated on each channel of honeycomb to form a thin film, and the optical fibers can transmit and scatter enough light to the catalyst. The TiO2 was uniformly exposed to UV light irradiation, hence significantly increasing the availability of TiO2. Further, less amount of TiO2 was used to demonstrate the advantage of honeycomb photoreactor. As shown, when coating 0.85 g/m2 (i.e. 0.06g) TiO2, higher Hg0 removal efficiency was obtained in the powder reactor. This is because the thickness of TiO2 powder is small, and most of TiO2 powder was exposed to UV light irradiation. Although the gas residence time is much lower in the powder reactor compared to the honeycomb reactor, large amount of TiO2 powder participated in photocatalytic Hg0 removal. This could fully demonstrate the advancement of internal-illuminated honeycomb photoreactor in photocatalytic Hg0 removal. 3.7. Procedure for Hg0 removal over TiO2 catalyst in internal-illuminated honeycomb photoreactor The Ti400CH showed excellent Hg0 removal performance at 40-100 ºC, which was in the temperature range downstream of a FGD system. Recently, many coal-fired power plants installed wet electrostatic precipitator (WESP) downstream of a FGD system. Since water film was applied to collect particles on the dust-collecting plate of WESP, Hg2+ in flue gas could be efficiently removed by the water film because of its water-solublity. Thus, the TiO2-coated honeycomb reactor can be equipped in the duct between the FGD and WESP. In this way, the Hg2+ formed during photocatalytic oxidation can be captured by WESP rather emitted into atmosphere. Moreover, since most of pollutants were removed when the flue gas passed through the APCDs, the interference of flue gas impurities like SO2 and NO on the photocatalytic activity can be minimized. During the practical application, the photocatalyst stability is extremely important. Thus, the life 13



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of Ti400CH was investigated with continuous UV light irradiation under SFG atmosphere. As shown in Figure 10(a), the Ei of Ti400CH decreased slightly as the reaction process, suggesting that the Hg0 adsorption over catalyst was saturated gradually. Since the TiO2 in an honeycomb photoreactor was originally designed to act as a catalyst instead of an adsorbent. For practical use, this reactor should run continuously for months or even years. Thus, the photocatalytic oxidation efficiency (Eoxi) of catalyst after saturated with mercury was tested. As shown, the Eoxi of mercury saturated Ti400CH catalyst was maintained at about 70% in a 50-h test. This suggests that the photocatalyst has high photocatalytic activity and stability for prolonged use. It should be noteworthy that the Hg0 concentration (60 μg/m3) used in this work was much higher than that in real condition at the WFGD outlet (less than 10 μg/m3). Thus, higher Hg0 removal efficiency and longer lifetime of photocatalyst would be obtained in practical application. Meanwhile, it is reasonable to believe that increasing TiO2 coating value in the photoreactor or UV light intensity could also enhance the photocatalytic Hg0 oxidation during prolonged use. The Hg species and reaction products were analyzed during the experimental period. As shown in Figure 10(b), during the first 10-h, 83.5% Hg was absorbed on the photocatalyst (HgS); 4.4% Hg2+ and 12.1% Hg0 emitted into atmosphere. The mercury species on the catalyst was identified by a TPD experiment. Figure 11 shows that the mercury on the catalyst decomposed in temperature range of 400-500 °C with a peak at about 480 °C, which was matched with the decomposition temperature of HgO.48 There was no other mercury species observed over the TPD spectra. Thus, during the photocatalytic removal process, most of Hg0 was oxidized by the O2•— and OH• radicals to form HgO, which was subsequently deposited on the photocatalyst surface. Meanwhile, little amount of gaseous Hg2+ was formed and escaped into atmosphere. After passed an impinger containing WESP 14


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effluent (collected from power plant), there was no Hg2+ detected. This suggests that the Hg2+ including both HgO and water-soluble Hg2+ were not emitted into atmosphere as a co-benefit of WESP. As the reaction process, the mercury adsorption capacity of catalyst was saturated gradually, while most of Hg2+ escaped into flue gas in the following 50-h test. Subsequently, the Hg2+ was captured by the WESP effluent rather emitted into atmosphere, as shown in Figures 10(c) and 10(d). Thus, the photocatalytic Hg0 removal from flue gas over TiO2 catalyst in an internal-illuminated honeycomb photoreactor under UV irradiation was promising. Even though, it should be noteworthy that the gas stream is saturated with humidity after FGD. Generally, H2O competes active oxidation sites with Hg0 on the catalyst surface, causing a decrease in Hg0 oxidation performance.49 Decreasing the superhydrophilicity of TiO2 like introducing carbon-functional groups into catalyst might cause less adsorption competition from H2O for Hg0 adsorption, 49 which will be studied in the future. 4. Conclusions A novel Hg0 removal technology from flue gas by using TiO2 catalyst in an internal-illuminated honeycomb photoreactor was developed. TiO2 coating value and catalyst calcination temperature played important roles in Hg0 removal. The catalyst with a TiO2 coating value of 3.40 g/m2 obtained optimal Hg0 removal performance when calcined at 400 ºC. The catalyst presented excellent Hg0 removal performances in a wide of temperature of 25-90 ºC, and 1.5 mW/cm2 of UV light irradiation was enough for Hg0 photocatalytic removal. With the optimal sample and operating condition, Hg0 removal efficiency of 95% was achieved in internal-illuminated honeycomb reactor, which was much higher than that in a powder reactor. Above 70% photocatalytic Hg0 oxidation efficiency was maintained during prolonged use. In the first 10-h reaction stage, 83.5% Hg was absorbed on the photocatalyst; 4.4% Hg2+ and 12.1% Hg0 emitted into atmosphere. As the reaction process, most of 15



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Hg2+ escaped into flue gas, which could be subsequently captured by WESP. These knowledge provides valuable information for developing efficient Hg0 removal technology from coal combustion flue gas. Acknowledgments This

research

was

supported

by

the

National

Key

Technologies

R&D

Program

(2016YFB0600604), the National Natural Science Foundation of China (No. 51476189, 51776227), and the Natural Science Foundation of Hunan Province, China (2018JJ1039, 2018JJ3675). Author would like to thank anonymous reviewers for their critical comments. References (1) UNEP, 2013. Global Mercury Assessment 2013: Sources, Emissions, Releases and Environmental Transport. UNEP Chemicals Branch, Geneva, Switzerland. (2) Xu, H; Qu, Z; Zong, C; Huang, W; Quan, F; Yan, N. MnOx/Graphene for the Catalytic Oxidation and Adsorption of Elemental Mercury. Environ. Sci. Technol. 2015, 49, 6823. (3) He, C.; Shen, B.; Chen, J.; Cai, J. Adsorption and Oxidation of Elemental Mercury over Ce-MnOx/Ti-PILCs. Environ. Sci. Technol. 2014, 48, 7891. (4) Liao, Y.; Xiong, S.; Dang, H.; Xiao, X.; Yang, S.; Wong, P. K. The Centralized Control of Elemental Mercury Emission from the Flue Gas by a Magnetic Rengenerable Fe-Ti-Mn Spinel. J. Hazard. Mater. 2015, 299, 740. (5) Xie, J.; Qu, Z.; Yan, N.; Yang, S.; Chen, W.; Hu, L.; Huang, W.; Liu, P. Novel Regenerable Sorbent Based on Zr-Mn Binary Metal Oxides for Flue Gas Mercury Retention and Recovery. J. Hazard. Mater. 2013, 261, 206. (6) Chiu, C.; Kuo, T.; Chang, T.; Lin, S.; Lin, H.; Hsi, H. Multipollutant Removal of Hg0/SO2/NO 16


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22


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Table 1 Summary of experimental conditions Experiments Photocatalysts

TiO2 coating

Reaction

Light intensity

value (g/m2)

temperature (ºC)

(mW/m2)

I

CH

0

90

0, 1.5, 4

II

Ti400CH

0.85, 1.70,

90

1.5

3.40

90

1.5

2.55, 3.40, 4.25, 5.10 III

Ti300CH Ti400CH Ti500CH Ti600CH

IV

Ti400CH

3.40

25, 60, 90, 120

0, 1.5, 4

V

Ti400CH

3.40

90

1.5

0.85, 3.40

90

1.5

Ti400P P25 VI

Ti400CH

23



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Table 2 Surface area, pore volume, and pore size of the powder samples BET surface area

Pore volume

Pore size

(m2/g)

(cm3/g)

(nm)

Ti300P

127.44

0.131

4.11

Ti400P

100.42

0.106

4.52

Ti500P

13.59

0.029

8.77

Ti600P

0.194

0.005

104.23

P25

50.26

-

-

Samples

24


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Figure 1 (a) Experimental schematic diagram of Hg0 adsorption system, (b) schematics of the TiO2-coated honeycomb reactor and illumination optical fibers, (c) schematics of the powder reactor

(a)

(b)

(c)

25



Figure 2 XRD patterns of P25 and TiXCH calcined at 300, 400, 500, and 600 ºC

26


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Figure 3 SEM images, elemental mapping and EDX analysis of bare and TiO2-coated honeycomb. (a) bare ceramic honeycomb, (b) Ti400CH (coating 0.85 g/m2 TiO2), (c) Ti400CH (coating 3.40 g/m2 TiO2), (d) Ti400CH (coating 3.40 g/m2 TiO2), (e) cross-section of Ti400CH (coating 3.40 g/m2 TiO2)

(a)

(b)

(c)

27



(d)

(e)

28


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Figure 4 UV-Vis spectra of photocatalysts

29



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Figure 5 Interference of bare honeycomb without TiO2 coating and UV light irradiation on Hg0 photocatalytic removal

30


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Figure 6 Effect of TiO2 loading value on Hg0 removal performance

31



Figure 7 Effect of calcination temperature on Hg0 removal performance

32


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Figure 8 Effect of reaction temperature and light intensity on Hg0 removal performance

33



Figure 9 Effect of reactor type on Hg0 removal performance

34


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Figure 10 (a) Hg0 removal performance of Ti400CH under SFG atmosphere; (b) Hg species and reaction products; (c) Hg0 oxidation performance of mercury-saturated Ti400CH under SFG atmosphere; (d) Hg species and reaction products

35



Figure 11 Temperature-programmed desorption (TPD) spectra of mercury species

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For Table of Contents Only

37


Elemental Mercury Removal from Flue Gas over TiO2 Catalyst in an

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