Preparation of fly ash adsorbents utilizing non-thermal plasma to add S active sites for Hg0 removal from flue gas

Fuel 266 (2020) 116936

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Full Length Article

Preparation of fly ash adsorbents utilizing non-thermal plasma to add S active sites for Hg0 removal from flue gas

T

⁎

Ruize Sun, Hailu Zhu, Mengting Shi, Guangqian Luo , Yang Xu, Xian Li, Hong Yao State Key Laboratory of Coal Combustion, School of Energy and Power Engineering, Huazhong University of Science and Technology, Wuhan, Hubei 430074, China

G R A P H I C A L A B S T R A C T

A R T I C LE I N FO

A B S T R A C T

Keywords: Mercury Non-thermal plasma modification H2S Fly ash

This paper proposed a method of preparing efficient, cost-effective and eco-friendly fly ash (FA) adsorbents utilizing non-thermal plasma technology in H2S atmosphere to remove elemental mercury (Hg0) in the coal-fired flue gas. Characterization of fly ash samples showed plasma treatment had little effect on porous structure, surface morphology and crystal structure of fly ash samples. It was found that elemental sulfur (S) particles, which were active site for Hg0 removal, was incorporated successfully on the adsorbents’ surface. The loaded S particles showed superior Hg0 oxidation ability and low remission possibility. The Hg0 removal experiments were conducted in a bench-scale fixed-bed reactor. After plasma treatment, the initial Hg0 removal efficiency of modified fly ash had increased significantly compared to raw fly ash (2.8–3.6 times). The effects of treatment time (0.5–5 min), adsorption temperature (60–180 °C) and flue gas components on Hg0 removal efficiency were also analyzed in the present study. The Hg0 removal efficiency had a positive correlation with adsorption temperature. HCl, O2 and NO in the flue gas could promote the removal of Hg0, whereas SO2 had an adverse effect. The Hg0 adsorption performance was better with longer treatment time largely due to more S was loaded on the fly ash surface, which was confirmed by XPS and SEM results. Additionally, the temperature-programmed decomposition (TPD) experiments further proved existing species of mercury in used modified fly ash were black HgS and red HgS. Finally, the mechanism of fly ash adsorbents modified by non-thermal plasma technology to remove Hg0 was proposed.

1. Introduction Mercury (Hg) has been considered as global pollutant and attracted global attention, owing to its high toxicity, bioaccumulation and long residence time in atmosphere [1–3]. Coal-fired power plants are considered to be the largest anthropogenic mercury emission source [4,5].

⁎

Therefore, it is necessary to control mercury emission from coal-fired power plants. There are three different forms of mercury in the flue gas: elemental mercury (Hg0), oxidized mercury (Hg2+) and particulate mercury (Hgp). Hg2+ is easily soluble in water which could be effectively removed by existing WFGD equipment. Most of Hgp can be captured by particle control devices effectively. However, Hg0 is

Corresponding author. E-mail address: [email protected] (G. Luo).

https://doi.org/10.1016/j.fuel.2019.116936 Received 29 August 2019; Received in revised form 12 November 2019; Accepted 19 December 2019 Available online 14 January 2020 0016-2361/ © 2019 Published by Elsevier Ltd.

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difficult to synergistic removed by existing air pollutant control devices (APCDs) due to its high volatility and poor solubility in water. Therefore, the purpose of this paper is to enhance removal efficiency of Hg0 [6]. Activated carbon (AC) injection in the upstream of particles control devices is considered one of the most mature mercury removal technologies which has been commercially applied in the United States [7,8]. The modified carbon-based adsorbents have shown great Hg0 removal performance [9,10]. However, the cost of controlling mercury emission by activated carbon is high and carbon-based adsorbents injection will cause problems in the reuse of fly ash [11]. Hence, it is not a promising method to control mercury emission in the long term, then it is imperative to develop an efficient, cost-effective and environmental friendly adsorbent. Copper slag has been proven to be efficient catalyst for Hg0 removal [12]. Owning to its huge reservation, low price, contained unburned carbon and inorganic materials are beneficial to Hg0 removal, fly ash (FA) based adsorbents have become a potential mercury removal adsorbents and received much attention [13–15]. However, the Hg0 removal efficiency of raw fly ash is low which limits its commercial application. The Hg0 adsorption ability is primarily related to the surface morphology, pore structure and chemical composition of adsorbents. As reported, chemical impregnation with halogen species (Cl, Br, I) was an effective way to promote Hg0 removal efficiency [16–18]. However, chemical impregnation process is time-consuming, and the formed HgX (X referred to halogen species) has high potential to remit and cause serious environmental problems. Besides that, elemental sulfur and sulfide species has exhibited great oxidization ability and produced mercuric sulfide (HgS) showed superior stability [19–21]. Hsing-Cheng His et al. analyze Hg0 removal ability of carbonaceous and noncarbonaceous adsorbents impregnated with elemental sulfur [22]. Asasian Neda et al. immobilized elemental sulfur on the activated carbon surface [23]. The results showed the Hg0 removal efficiency had increased a lot and elemental sulfur served as active sites during Hg0 adsorption experiments. However, traditional impregnation method is kind of time-consuming, hence it is crucial to develop a preparation method to synthesize fly ash-based adsorbent featuring high Hg0 removal efficiency without complex and time-consuming procedure. According to our previous research, non-thermal plasma technology has been recognized as an efficient and effective modification method. Zhang et al. used Cl2 plasma technology to modify the activated carbon by producing active chlorine atoms on its surface [24]. Zeng et al. used non-thermal plasma to enhance the sorbents’ regeneration performance [25]. Additionally, non-thermal plasma could accelerate chemical reaction. Reddy E L et al. used non-thermal plasma to accelerate the decomposition of hydrogen sulfide (H2S) and produced elemental sulfur (S) successfully [26]. However, whether non-thermal plasma could immobilize elemental sulfur on fly ash surface have not been investigated yet. In this study, we proposed the method of taking advantage of nonthermal technology to add sulfur active material on fly ash surface to enhance its Hg0 removal efficiency with low mercury remission risk. We also investigated the effects of various treatment time, different adsorption temperature and flue gas constitutes on modified fly ash adsorbent’s performance. Different characterization methods were utilized to compare the physical and chemical properties of raw, fresh and used modified fly ash samples. Combining characterization results with temperature-programmed desorption (TPD) experiments and Thermogravimetric analysis (TA) results, the mechanism of Hg0 removal by non-thermal plasma modified fly ash adsorbents were proposed in this research.

Fig. 1. The schematic diagram of non-thermal plasma treatment system.

2. Material and methods 2.1. Non-thermal plasma treatment with H2S The fly ash samples were collected from the ash hopper of an electrostatic precipitator in a self-circulating fluidized bed boiler and then dried in an oven at 105 °C for 12 h. Afterwards, it was sieved with 80 mesh sifter and fly ash particles smaller than 0.18 mm was obtained. Then, fly ash samples were modified in a bench-scale dielectric barrier discharge non-thermal plasma system in a quartz reactor. The schematic diagram of non-thermal modification system was shown in Fig. 1. It consists of non-thermal plasma power source, frequency controller and quartz reactor. The current supplied by our plasma system is alternate with 10 kHz audio frequency and 3 kV Peak Voltage (DBD-100A, Suman Corp.). A total of 0.5 g fly ash sample was loaded evenly in a quartz reactor and then silicone caulk was used to seal the reactor. Afterwards, 20 vol % H2S balanced by high purity N2 was introduced to the reactor for 1 min to purge the air in the reactor out before every plasma treatment process. Afterwards, the non-thermal plasma power was turned on for 0.5 min. The obtained fly ash sample was abbreviated to FA-T0.5 (0.5 was referred to treatment time). Similarly, we synthesized FA-T1.5, FAT2.5 and FA-T5.0 fly ash samples. Additionally, a special sample, abbreviated to FA-S, was prepared through fly ash sample treated for 10 min to increase sulfur content on fly ash surface only for characterization of sulfur species on fly ash surface by XPS analyzer. 2.2. Sample characterization methods Various methods were used to investigate the characteristics of raw and modified fly ash adsorbents. Elemental analyzer (Elementary Co.) was used to determine the chemical composition of different fly ash samples. The BET surface area and pore size distribution were analyzed via nitrogen adsorption at 77 K (Micromeritics ASAP 2020). SEM (SUPRA 55 SAPPHIRE) was employed to characterize surface morphologies and composition on fly ash samples’ surface. XRD (PANalytical XPert Pro) was applied to determine crystalline structure of fly ash samples. The data of XRD was collected over a 2θ range from 10° to 70° at intervals of 0.013° with a counting time of 1 min per step. XPS (AXIS-ULTRA DLD-600 W) was used to characterize the surface chemistry of fly ash samples. 2.3. Mercury adsorption experiment The Hg0 removal experiments were carried out in a quartz tube reactor as reported in our previous study [27]. The inner diameter of the quartz tube reactor is 10 mm. The adsorption temperature was maintained at 110 °C. A total of 0.5 g raw or modified fly ash adsorbents were placed on the porous quartz plate. 85 ± 1 μg/m3 Hg0, provided by a mercury permeation tube (VICI Metronics Inc.) was carried by 1 L/ 2

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min high purity N2, flowed into reactor inlet. The outlet Hg0 concentration was measured by continuous mercury emission monitor (VM3000, Mercury Instruments GmbH). Furthermore, NO, O2, SO2 and HCl balanced with high purity N2 with the total flow rate of 1 L/min flowed into the reactor to investigate the effects of flue gas constitute on adsorbents’ Hg0 removal performance. The removal efficiency of fly ash adsorbents was evaluated by Hg0 removal efficiency (η), which was calculated by the following equation:

η=

Cinlet − Coutlet × 100% Cinlet

Table 2 BET analysis results of fly ash samples. Sample

BET surface area (m2/ g)

Pore volume (cm3/ g)

Average pore size (nm)

FA FA-T0.5 FA-T1.5 FA-T2.5 FA-T5.0

46.49 46.12 41.59 42.37 46.33

0.050 0.050 0.046 0.047 0.045

4.35 4.32 4.37 4.40 3.87

(1)

where Cinlet and Coutlet represent the instantaneous Hg0 concentration (μg/m3) of the inlet and outlet of the fixed bed reactor.

SiO2 Fe2O3

2.4. Temperature-programmed desorption (TPD) of fly ash samples

Fly ash

The temperature-programmed desorption experiments (TPD) of fresh and used fly ash samples were carried out in the same fixed bed reactor as adsorption experiments to identify thermal stability of different adsorbed mercury species on the fly ash surface. According to difference of thermal stability of various adsorbed mercury species, we could understand mechanisms of mercury adsorption process better by analyzing the occurrence of mercury. Firstly, the TPD experiment of original fly ash sample was carried out to ensure no mercury existence in it. Then 0.3 g fresh or used modified fly ash sample was packed in the fixed reactor, respectively. It was heated from 40 °C to 600 °C in 1 L/ min N2 atmosphere with heating rate of 10 °C/min. The Hg0 concentration released from fly ash was continuously measured by the same continuous mercury monitor (VM3000, Mercury Instruments GmbH).

Modified fly ash

10

3.1. Sample characterization results Table 1 gives the results of the elemental analysis of different fly ash samples. It was evident that more sulfur was loaded in fly ash with longer H2S plasma treatment time. It indicated non-thermal plasma treatment could deposit sulfur species in fly ash successfully. The forms of sulfur species need to be investigated further. Table 2 shows the BET analysis of various fly ash samples. It was obvious that the specific surface area, pore and channel structure of FA, FA-T0.5 and FA-T5.0 remained the same. It suggested that plasma modification process had no obvious effects on the morphology and porous structure of fly ash samples. The XRD results of raw and modified fly ash were shown in Fig. 2. It Table 1 Element analysis results of fly ash samples. H (%)

S (%)

FA FA-T0.5 FA-T1.5 FA-T2.5 FA-T5.0

0.51 0.43 0.48 0.54 0.38

21.67 20.97 21.11 20.87 21.19

1.13 1.09 0.97 0.90 0.45

1.05 1.24 1.69 2.36 3.94

50

60

70

indicated the main crystal structure material in raw fly ash adsorbent was SiO2 and Fe2O3. After the plasma modification, the XRD pattern had no obvious change. There was no evidence of the existence of elemental sulfur in crystal structure which maybe due to the amount of elemental sulfur produced by plasma modification was not enough for XRD analyzer to determine. It inferred non-thermal plasma had ignorable change on fly ash crystal structure. SEM is an auxiliary method to show the product of elemental sulfur by non-thermal plasma modification in our research. The results were shown in Fig. 3. It was clear that the original fly ash had some macroporous structures (> 50 nm) which could provide sites for sulfur deposition. Additionally, the vicinity of the large pores on original fly ash surface was relatively smooth. Compared to the original fly ash, it was obvious that some light gray particles appeared on the surface and inside the pores of modified fly ash after the non-thermal plasma treatment. Even though the comparison of mapping pictures was not significant, we could still find the slight difference of these pictures in the Fig. 4. With longer modification time, the elemental sulfur was more on the FA surface. It suggested small elemental sulfur particles were successfully fabricated on the fly ash surface by non-thermal plasma treatment. The special sample FA-S was utilized in the XPS analysis experiments since the amount of sulfur loaded by few times modification process was not enough for the XPS analyzer to determine. The S2p XPS spectra of raw, fresh and used modified fly ash samples were shown in Fig. 5. The distribution of each sulfur functional groups was shown in Table 3. The binding energy peak at 169.1 eV corresponds to sulfate (SO42−) which dominates sulfur functional groups [28]. The two peaks centered at 164.0 eV, 163.0 eV were observed after modification process, which corresponded to elemental sulfur (S), thiosulfate (S2O32−), respectively [29,30]. It confirmed H2S non-thermal plasma treatment could load elemental sulfur, which has been proven to be vital active adsorption sites for Hg0 oxidization, on fly ash surface successfully [22]. After Hg0 adsorption experiments, a new peak located at 161.4 eV which was referred to sulfide (S2−) appeared on fly ash adsorbents’ surface [31]. This was attributed to the reaction of elemental sulfur and

3. Results and discussion

C (%)

40

Fig. 2. XRD Patterns of raw and modified fly ash samples.

Thermogravimetric analysis in high purity nitrogen atmosphere was also employed to evaluate the thermal properties of raw and modified fly ash samples. The special fly ash sample (FA-S) was tested to magnify the difference. During the experiments, nitrogen was used as carrier gas and its flow rate was 30 mL/min. All fly ash samples were firstly maintained under 120 °C for 2 h then increased temperature to 800 °C with the heating rate of 10 °C/min.

N (%)

30

2θ / (°)

2.5. Thermogravimetric analysis (TGA) of fly ash samples

Sample

20

3

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Fig. 3. SEM images of fly ash samples.

FA (200 nm)

FA-T1.5(200 nm)

FA-T2.5(200 nm)

Hg0 and formed HgS on the fly ash surface.

raw fly ash

SO42-

S2p

3.2. The Hg0 removal performance of fly ash adsorbents

S

S2O3

used modified fly ash 2-

S

160

S2O32- S

162

164

SO42-

166 168 170 Binding energy / eV

172

174

Fig. 5. XPS S2p spectra of different fly ash samples. Table 3 The distribution of sulfur functional groups. Functional groups

3.2.2. Effect of adsorption temperature In order to investigate the influence of adsorption temperature on Hg0 removal, the adsorption experiments of FA-T5.0 at various temperature were also tested. The results were depicted in Fig. 7. In conclusion, the Hg0 removal efficiency enhanced with higher adsorption temperature, indicating chemical adsorption dominated the Hg0 removal. Besides that, the reaction between Hg0 and elemental sulfur is exothermic and irreversible which potentially cause the increase of better Hg0 removal efficiency with higher adsorption temperature. The initial Hg0 removal efficiency increased from 58% to 80%, with the adsorption temperature conducted at from 60 °C to 140 °C. However, when the temperature exceeded 180 °C, Hg0 removal efficiency dropped rapidly which was lower than 20% after 5 min. There are two reasons responsible for this phenomenon. On the one hand, element sulfur loaded on fly ash may melt and block the pores and channels of fly ash adsorbents at this temperature. On the other hand, liquid sulfur could be carried by carrier gas flow and condense on the downstream reactor wall which may cause less contact time of active material (S) and Hg0 [32,33]. The latter reason could be also demonstrated by the appearance of light yellow materials on the downstream tube wall. In the reality, the contact time between adsorbents and flue

FA (200 nm)

fresh modified fly ash SO 24 2-

Intensity

3.2.1. Effect of H2S plasma treatment time In order to investigate the effects of non-thermal treatment time on Hg0 removal efficiency, various fly ash adsorbents were tested in the fixed bed reactor. All the adsorption experiments were conducted at 110 °C and Hg0 vapor was carried by high purity N2 with total flow rate of 1 L/min. As illustrated in Fig. 6, the initial Hg0 removal efficiency of raw fly ash was around 20% which could remarkably rise to 66% after only 0.5 min plasma modification. The superior performance of modified fly ash samples was mainly attributed to formation of elemental sulfur. It also indicated H2S plasma treatment was a promising modification method. The Hg0 removal efficiency increased with longer non-thermal plasma treatment time. Specifically, the initial Hg0 removal efficiency of FA-T1.5, FA-T2.5, FA-T5.0 was 66%, 80% and 90%, respectively. This was because of more elemental sulfur (active adsorption sites) deposited on the fly ash surface with longer treatment time. With the adsorption experiments proceeded, the Hg0 removal efficiency of fly ash samples decreased gradually due to elemental sulfur were consumed during the reaction process proceeded.

SO42− S S2O32− S2−

Electron binding energy (eV)

Relative intensity (%) Raw fly ash

Fresh modified fly ash

Used modified fly ash

169.1 164.0 163.0 161.4

99.73 0.27 0.00 0.00

78.99 8.17 12.84 0.00

70.32 9.68 14.07 5.93

gas was only few seconds [34]. Our modified fly ash adsorbents would still be a promising candidate due to its high initial Hg0 removal efficiency. 3.2.3. Effect of flue gas components It was essential to investigate the influence of flue gas component could have on the mercury form and reactivity. Hence, Hg0 removal activities in simulated gas atmosphere were also tested. The simulated flue gas was composed of 6%O2, 0.3‰NO, 2‰SO2, 0.1‰HCl and balanced with high purity N2. The mercury adsorption experiments of FA-T5.0 were carried out in the same fixed bed reactor as before at 110 °C. The results were shown in Fig. 8. We could find in the first 10 min the Hg0 removal efficiency decreased from 77.5% to 73.4% gradually. After changing the atmosphere from pure nitrogen to simulated flue gas, Hg0 removal efficiency

FA-T1.5(200 nm)

FA-T2.5(200 nm)

Fig. 4. Mapping images of sulfur species on the fly ash samples. 4

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Efficiency of Hg0 removal (%)

100

synergy effect on the Hg0 removal which could be explained by Deacon reaction. Briefly, active Cl atoms could be produced in the presence of O2 [35]. The possible reactions of Hg0 being oxidized are shown in Eqs. (2)–(4):

FA-T5.0 FA-T2.5 FA-T1.5 FA-T0.5 FA

80

60

(2)

Cl + Hg → HgCl

(3)

HgCl + Cl → HgCl2

(4)

0

40

As shown in Fig. S3-4, O2 and NO could promote the removal efficiency of fly ash adsorbents. With higher the concentration of O2 is, the promotion effect of O2 is more obvious. This was largely due to the addition of O2 could supplement the lattice oxygen in fly ash which could oxidize Hg0 [36]. While, NO could react with lattice oxygen on the fly ash surface, producing NO2 which was also beneficial to the Hg0 oxidization. The reaction is shown in Eq. (5) [37]. Whereas SO2 would inhibit the Hg0 removal performance of FA-T5.0. With the addition of 1400 ppm SO2, the Hg0 removal efficiency would decrease significantly from 77% to 34% which was due to SO2 could compete with Hg0 for active sites. These were consistent with others’ research [38–44].

20

0

4HCl + O2 → 4Cl + 2H2O

0

5

10

15

Time (min)

20

25

30

Fig. 6. Effect of treatment time on Hg0 removal efficiency.

4Hg0(ads) + 6NO2(g) → Hg2(NO3)2 + Hg2(NO2)2 + 2NO(g)

(5)

3.3. Mercury removal mechanism 3.3.1. Temperature programmed desorption experiments The Temperature programmed desorption experiments (TPD) results of used modified fly ash sample (FA-S) was shown in Fig. 9. We could find the mercury adsorbed in used modified fly ash mainly remitted from 200 °C to 400 °C. There were two mercury desorption peaks located at 242 °C and 293 °C in used modified fly ash adsorbent which corresponded to black HgS and red HgS, respectively [45]. Combined with the XPS spectra of Hg4f of adsorbent which was shown in Fig. 10. The binding energy of 103.1 eV corresponded to Si2p characteristic peak. There was no evidence of mercury presence in the modified fly ash which indicated the process of plasma modification did not introduce impurities. After the mercury adsorption experiments a new peak located at 100.2 eV appeared which was attributed to Hg2+. The results indicated modified fly ash had ability of adsorbing Hg0 by chemical adsorption. Combined with the results of previous S2p XPS analysis and TPD curves, it was concluded the elemental sulfur could be loaded on the fly ash surface by non-thermal plasma, thereby oxidizing Hg0 and forming HgS.

Fig. 7. Effect of adsorption temperature on Hg0 removal efficiency.

Hg0 removal efficiency / %

100

N2

90

N2+2‰SO2+0.1‰HCl +0.3‰NO+6%O2

80

3.3.2. Thermogravimetric analysis (TGA) of fly ash samples Fig. 11(A) and (B) represented the TG and DTG curves of fresh and used modified fly ash, respectively. When the temperature heated to

70 60

1200 0

5

10 Time / min

15

20

FA-T5.0 black HgS red HgS

1000

Hg0 concentration / g/Nm3

50

Fig. 8. Effect of simulated flue gas condition on Hg0 removal efficiency.

increased rapidly to 89% only in 2 min. It indicated the simulated flue gas had positive effect on modified fly ash performance. Further, we investigated the influence of each flue gas component on adsorbents’ adsorption performance. The experimental results were shown in Fig. S1-5. As shown in Fig. S1, HCl could promote the oxidization of Hg0. When 133 ppm HCl was added, the Hg0 removal efficiency could increase from 73% to 74%. While, when 167 ppm HCl was added, the Hg0 removal efficiency could reach 73% from 70%. Furthermore, we also investigated the effect of HCl in the presence of O2. The results, illustrated in Fig. S2, indicated HCl and O2 could have

800 600

242

293

400 200 0 0

100

200 300 400 Temperature / oC

500

600

Fig. 9. TPD results of used modified fly ash samples. 5

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R. Sun, et al.

Hg4f

raw fly ash

Intensity

fresh modified fly ash

H2S(g) + surface → H2S(ad)

(7)

H2S(ad) + plasma → S(ad) + H2

(8)

Hg0(g) → Hg0(ad)

(9)

S(s) + Hg0(ad) → HgS

(10)

S(ad) + Hg (ad) → HgS 0

used modified fly ash

The elemental sulfur served as active material for Hg removal from our research results. Elemental sulfur was produced by non-thermal plasma treatment under H2S atmosphere successfully. Produced sulfur could be bonded to FA surface or deposited on its surface and reacted with Hg0 further [48].

Hg2+

98

100

102 104 Binding energy / eV

106

108

4. Conclusions

Fig. 10. XPS Hg4f spectra of different fly ash samples.

This paper proposed a method of preparing Hg0 removal adsorbent with superior removal performance utilizing H2S non-thermal plasma technology. The plasma treatment had no effect on morphology, porous and crystal structure of fly ash samples. After modification, fly ash performance enhanced remarkably. And the Hg0 removal efficiency was higher with longer treatment time. The flue gas component HCl, O2, NO could promote the oxidization of Hg0, while SO2 showed a reverse effect. The mechanism of the modified fly ash samples for removing Hg0 was that H2S non-thermal plasma treatment could load element sulfur on fly ash surface successfully which further reacted with Hg0 and formed HgS.

100 Fresh modified fly ash Used modified fly ash

TG / %

90

80

70

60

CRediT authorship contribution statement 0

100 200 300 400 500 600 700 800 900 Temperature / oC

Ruize Sun: Conceptualization. Hailu Zhu: Writing - original draft. Mengting Shi: Writing - review & editing. Guangqian Luo: Supervision. Yang Xu: Writing - review & editing. Xian Li: Data curation. Hong Yao: Supervision.

(A) TG curve

3.5

240 252 310

Declaration of Competing Interest

Fresh modified fly ash Used modified fly ash

3.0

2.0

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

1.5

Acknowledgment

1.0

The authors gratefully acknowledge the financial support provided by the National Key Research and Development Program of China (Grant No. 2016YFB0600603) and the National Natural Science Foundation of China (Grant No. 51776084). The authors also gratefully acknowledge the assistance of the Analytic and Testing Center of Huazhong University of Science and Technology for the experimental measurements.

2.5 DTG (%/min)

(11) 0

0.5 0.0

0

100 200 300 400 500 600 700 800 900 Temperature / oC

(B) DTG curve Fig. 11. TG (A) and DTG (B) curves of fresh and used modified fly ash samples.

Appendix A. Supplementary data 800 °C, 21.7% weight of modified fly ash was lost which was largely attributed to the evaporation of sulfur species in the modified fly ash adsorbents. Additionally, used modified fly ash had an additional weak weight loss peak and the main weight loss peak shifted to the left a little, comparing to fresh fly ash DTG curve. Combined with previous analysis, the weak weight loss peak was contributed to the decomposition of red HgS and the shift of main weight loss peak was because of the weight loss of black HgS. In conclusion, we proposed the possible mechanism of Hg0 removal by non-thermal plasma modified fly ash adsorbents was shown as below [46,47]: H2S(g) + plasma → S(s) + H2(g)

Supplementary data to this article can be found online at https:// doi.org/10.1016/j.fuel.2019.116936. References [1] Qiang Z, Duan Y, Chen M, Meng L, Ping L. Studies on mercury adsorption species and equilibrium on activated carbon surface. Energy Fuels 2017;31(12). acs.energyfuels.7b02699. [2] Radke LF, Friedli HR, Heikes BG. Atmospheric mercury over the NE Pacific during spring 2002: gradients, residence time, upper troposphere lower stratosphere loss, and long-range transport. J Geophys Res Atmos 2007;112(D19). [3] Macdonald RW, Loseto LL. Are Arctic Ocean ecosystems exceptionally vulnerable to global emissions of mercury? A call for emphasised research on methylation and the consequences of climate change. Environ Chem 2010;7(2):133–8.

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Fuel 266 (2020) 116936

R. Sun, et al.

[4] Pacyna EG, Pacyna JM, Sundseth K, Munthe J, Kindbom K, Wilson S, et al. Global emission of mercury to the atmosphere from anthropogenic sources in 2005 and projections to 2020. Atmos Environ 2010;44(20):2487–99. [5] Withum JA, Tseng SC, Locke JE. Evaluation of mercury emissions from coal-fired facilities with SCR and FGD systems. Office of scientific & technical information technical reports. 2004. [6] Cao Y. Coal fired flue gas mercury. Emission Controls 2015. [7] Zeng H, Jin F, Guo J. Removal of elemental mercury from coal combustion flue gas by chloride-impregnated activated carbon. Fuel 2004;83(1):143–6. [8] Zhong L, Zhang Y, Yong J, Norris P, Pan WP. Synthesis of activated carbon from coal pitch for mercury removal in coal-fired power plants. J Therm Anal Calorim 2016;123(1):851–60. [9] Yang W, Chen H, Han X, Ding S, Shan Y, Liu Y. Preparation of magnetic Co-Fe modified porous carbon from agricultural wastes by microwave and steam activation for mercury removal. J Hazard Mater 2020,;381:120981. [10] Yang W, Li Y, Shi S, Chen H, Shan Y, Liu Y. Mercury removal from flue gas by magnetic iron-copper oxide modified porous char derived from biomass materials. Fuel 2019;256:115977. [11] Cetin B, Aydilek AH, Guney Y. Stabilization of recycled base materials with high carbon fly ash. Resour Conserv Recycl 2010;54(11):878–92. [12] Li H, Zhang W, Wang J, Yang, Z, Li L, Shih K. Copper slag as a catalyst for mercury oxidation in coal combustion flue gas. Waste Manage S0956053X1730884X. [13] Maroto-Valer MM, Zhang Y, Granite EJ, Zhong T, Pennline HW. Effect of porous structure and surface functionality on the mercury capacity of a fly ash carbon and its activated sample. Fuel 2005;84(1):105–8. [14] Zhang Y, Duan W, Liu Z, Cao Y. Effects of modified fly ash on mercury adsorption ability in an entrained-flow reactor. Fuel 2014;128:274–80. [15] Hassett DJ, Eylands KE. Mercury capture on coal combustion fly ash. Fuel 1999;78(2):243–8. [16] Gu Y, Zhang Y, Lin L, Xu H, Orndorff W, Pan WP. Evaluation of elemental mercury adsorption by fly ash modified with ammonium bromide. J Therm Anal Calorim 2015;119(3):1663–72. [17] Zhu TY, Kuang JY, Xu WQ, Ye M, Guo YY, Liu W. Study on mercury adsorption performance of modified fly ash. Adv Mater Res 2011;343–344(343–344):246–9. [18] Deng S, Shu Y, Li S, Tian G, Huang J, Zhang F. Chemical forms of the fluorine, chlorine, oxygen and carbon in coal fly ash and their correlations with mercury retention. J Hazard Mater 2016;301(JAN):400–6. [19] López FA, López-Delgado A, Padilla I, Tayibi H, Alguacil FJ. Formation of metacinnabar by milling of liquid mercury and elemental sulfur for long term mercury storage. Sci Total Environ 2010;408(20):4341–5. [20] Liu W, Vidic RD, Brown TD. Optimization of high temperature sulfur impregnation on activated carbon for permanent sequestration of elemental mercury vapors. Environ Sci Technol 2000;34(3):483–8. [21] Li H, Zhu L, Wang J, Li L, Shih K. Development of nano-sulfide sorbent for efficient removal of elemental mercury from coal combustion fuel gas. Environ Sci Technol 2016;50(17):9551. [22] Hsi H, Rood MJ, Rostam-Abadi M, Chen S, Chang R. Mercury adsorption properties of sulfur-impregnated adsorbents. J Environ Eng 2002;128(11):1080–9. [23] Asasian N, Kaghazchi T. Optimization of activated carbon sulfurization to reach adsorbent with the highest capacity for mercury adsorption. Sep Sci Technol 2013;48(13):2059–72. [24] Zhang B, Zeng X, Xu P, Chen J, Xu Y, Luo G, Xu M, Yao H. Using the novel method of non-thermal plasma to add Cl active sites on activated carbon for mercury removal in flue gas. Environ Sci Technol. [25] Zeng X, Yang X, Bi Z, Luo G, Ping S, Zou R, et al. Elemental mercury adsorption and regeneration performance of sorbents FeMnO x enhanced via non-thermal plasma. Chem Eng J 2016;309:503–12. [26] Reddy EL, Biju VM, Subrahmanyam C. Production of hydrogen and sulfur from hydrogen sulfide assisted by nonthermal plasma. Appl Energy 2012;95(2):87–92. [27] Bi Z, Ping X, Yong Q, Qiao Y, Ma J, Hui W, et al. Increasing oxygen functional

[28]

[29]

[30] [31]

[32] [33]

[34] [35]

[36]

[37] [38]

[39]

[40]

[41] [42]

[43]

[44]

[45]

[46]

[47]

[48]

7

groups of activated carbon with non-thermal plasma to enhance mercury removal efficiency for flue gases. Chem Eng J 2015;263:1–8. Sugiyama S, Hayashi H, Tanaka M, Moffat JB, Minami T. Surface and bulk properties of stoichiometric and nonstoichiometric strontium hydroxyapatite and the oxidation of methane. J Solid State Chem 1996;126(2):242–52. Lindberg BJ, Hamrin K, Johansson G, Gelius U, Fahlman A, Nordling C, et al. Molecular spectroscopy by means of ESCA II. Sulfur compounds. Correlation of electron binding energy with structure. Phys Scr 1970;1(5–6):286. Siriwardane RV, Cook JM. Interactions of SO2 with sodium deposited on CaO. J Colloid Interface Sci 1986;114(2):525–35. Agostinelli E, Battistoni C, Fiorani D, Mattogno G, Nogues M. An XPS study of the electronic structure of the ZnxCd1− xCr2 (X= S, Se) spinel system. J Phys Chem Solids 1989;50(3):269–72. Earnshaw A, Greenwood N. Chemistry of the Elements. second ed.; 1998. Mullett M, Pendleton P, Badalyan A. Removal of elemental mercury from Bayer stack gases using sulfur-impregnated activated carbons. Chem Eng J 2012;211212(none):133–42. Yang H, Xu Z, Fan M, Bland AE, Judkins RR. Adsorbents for capturing mercury in coal-fired boiler flue gas. J Hazard Mater 2007;146(1–2):1–11. Zhao L, Li C, Zhang X, Zeng G, Zhang J. A review on oxidation of elemental mercury from coal-fired flue gas with selective catalytic reduction, catalysts. Catal Sci Technol 2015;5(7):3459–72. Zeng X, Yang X, Bi Z, Luo G, Hong Y. Elemental mercury adsorption and regeneration performance of sorbents FeMnOx enhanced via non-thermal plasma. Chem Eng J 2016;309:503–12. Role of NO in Hg~0 oxidation over a commercial selective catalytic reduction catalyst V2O5-WO3/TiO2. J Environ Sci, (12), 126–132. Jampaiah D, Ippolito S, Sabri YM, Tardio J, Periasamy S, Nafady A, Reddy BM, Bhargava SK. Ceria-zirconia modified MnOx catalysts for gaseous elemental mercury oxidation and adsorption. Catal Sci Technol. Yinyin W, Boxiong S, Chuan H, Shiji Y, Fumei W. Simultaneous removal of NO and Hg(0) from Flue Gas over Mn-Ce/Ti-PILCs. Environ Sci Technol 2015;49(15):9355–63. Ruihui, Wenqing, Tong, Tingyu. Role of NO in Hg~0 oxidation over a commercial selective catalytic reduction catalyst V2O5-WO3/TiO2. J Environ Sci 2015;38(12):126–32. Bo Z, Liu X, Zhou Z, Shao H, Xu M. Catalytic oxidation of elemental mercury by Mn–Mo/CNT at low temperature. Chem Eng J 2016;284(2):1233–41. Rumayor M, Diazsomoano M, Lopezanton MA, Martineztarazona MR. Application of thermal desorption for the identification of mercury species in solids derived from coal utilization. Chemosphere 2015;119(119C):459–65. Xu Y, Zeng X, Zhang B, Zhu X, Zhou M, Zou R, Sun P, Luo G, Yao H. Experiment and kinetic study of elemental mercury adsorption over a novel chlorinated sorbent derived from coal and waste polyvinyl chloride. Energy Fuels 2016;30(12). acs.energyfuels.6b01372. Yang X, Zeng X, Luo G, Bi Z, Ping X, Xu M, et al. Chlorine-Char composite synthesized by co-pyrolysis of biomass wastes and polyvinyl chloride for elemental mercury removal. Fuel 2016;183:73–9. Xinbin F, Lu JY, Conrad DG, Yingjie H, Banic CM, Schroeder WH. Analysis of inorganic mercury species associated with airborne particulate matter/aerosols: method development. Anal Bioanal Chem 2004;380(4):683. Feng W, Borguet E, Vidic RD. Sulfurization of carbon surface for vapor phase mercury removal – I: effect of temperature and sulfurization protocol. Carbon 2006;44(14):2990–7. Feng W, Borguet E, Vidic RD. Sulfurization of a carbon surface for vapor phase mercury removal – II: sulfur forms and mercury uptake. Carbon 2006;44(14):2998–3004. Rettner CT, Auerbach DJ, Tully JC, Kleyn AW. Chemical dynamics at the gas surface interface. J Phys Chem 1996;100(31):13021–33.