Regeneration performance of activated coke for elemental mercury removal by microwave and thermal methods

Fuel Processing Technology 199 (2020) 106303

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Research article

Regeneration performance of activated coke for elemental mercury removal by microwave and thermal methods

T

⁎

Donghai Ana, Xiang Wangb, Xingxing Chenga, , Lin Cuia, Xiaoyang Zhanga, Ping Zhoua, ⁎ Yong Donga, a b

National Engineering Laboratory for Coal-fired Pollutants Emission Reduction, Shandong University, Jinan, Shandong 250061, China Key Laboratory for Thermal Science and Power Engineering of Ministry of Education, Department of Thermal Engineering, Tsinghua University, Beijing 100084, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Powdered activated Coke Mercury adsorption Microwave regeneration Thermal regeneration Recovery of mercury

The purpose of this study was to recover mercury and reuse active coke in the field of active coke adsorption of Hg0. The powdered active coke (AC) with high adsorption performance of Hg0 was prepared by a simple way. The regeneration characteristics of AC as a Hg0 sorbent were also investigated using microwave (MG-AC) and thermal methods (TG-AC). Of the two regeneration methods, microwave heating exhibited a faster heating rate to achieve complete regeneration. Following regeneration, adsorption performance of regenerated AC increased due to AC reactivation by microwave. Brunauer-Emmett-Teller (BET) measurements, X-ray photoelectron spectroscopy (XPS), and Laser particle size analyzer were used to analyze surface physical and chemical properties as well as particle size distribution of the samples. The results indicated that regeneration could greatly influence properties of AC, such as specific surface area, pore structure, surface chemical functional groups, and average particle size. Thermal methods had higher desorption content (CO2, CO and NO) than microwave methods. Hence, microwave regeneration had a smaller carbon consumption of 0.12% compared to thermal methods (0.23%). The Hg0 that desorbed from AC was collected using the deep adsorption technique in order to avoid secondary pollution.

1. Introduction The removal of mercury and its derivatives has attracted attention due to their toxic effects on ecological safety and human health [1–3]. There are three forms of mercury in flue gas: elemental mercury (Hg0), oxidation mercury (Hg2+) and particulate mercury (Hgp). Due to its water solubility, Hg2+ can be easily captured by wet flue gas desulfurization, while Hgp can be collected by electrostatic precipitators and fabric filters [4–6]. Hg0 is not easily removed with air pollution control devices [7–10]. Therefore, many sorbent have been researched to reduce Hg0 emission from flue gas and many control methods have been developed, the fly ash and calcium-based were considered as cheap adsorbent, as well as the zeolites (Na-X and NaeP1) [11], zeolite [12], MnOx/ZrO2 [13], Mn/γ-Fe2O3 [14] sorbents and so on, which have a potential application in mercury removal fields. Activated carbon injection (ACI) has been widely used in coal-fired power plant as an effective method to control the Hg0 emissions [15,16]. The high cost of the activated carbon, however, limits application of the ACI method [17–19]. It is therefore necessary to develop a cost-effective adsorbent for Hg0 adsorption. Not only does AC have well-developed pore ⁎

structure and abundant surface functional groups similar to activated carbon, but also it is less expensive cheaper than activated carbon [20–23]. These advantages suggest that more research is needed with regard to AC mercury removal. During the adsorption process, pore and active sites on the surface of AC were occupied by mercury, resulting in a loss of activity for the AC [24–27]. At present, spent AC is usually sent to landfill, incinerated, or regenerated for reuse [28–30], yet landfills and incinerators not only waste resources but produce secondary pollution as well [31]. Regeneration can increase the adsorbent use time, ensuring its economic viability and environmental security [32–34]. Commonly, the regeneration is used for de-NOx, de-SO2, and wastewater treatment, in processes that include thermal regeneration [35], microwave regeneration [36], oxidation regeneration [37] and biological regeneration [38]. Thermal regeneration is widely applied in mercury removal field, α-MnO2 nanotubes [39], CuCl2-MF, Co-MF sample, etc. Although the developed samples presented good mercury removal capacity, the regenerability and reusability of the spent sample is of particular significance for industrial application [40]. Microwave regeneration is extensively used in the regeneration of spent sorbents because of its high heating efficiency, time saving, and selectivity.

Corresponding authors. E-mail addresses: [email protected] (X. Cheng), [email protected] (Y. Dong).

https://doi.org/10.1016/j.fuproc.2019.106303 Received 23 September 2019; Received in revised form 24 November 2019; Accepted 24 November 2019 0378-3820/ © 2019 Published by Elsevier B.V.

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2.3. The regeneration of spent activated coke

However, very few studies have documented the recovery of spent AC from mercury removal processes. Regeneration of deactivated AC is a viable and environmentally friendly option for the mercury control field because of its low cost, recyclability, and capacity to prolong the life of AC. Mercury desorbed from the spent AC should also be recycled, to ensure that it is cannot be released into the atmosphere or groundwater. This research compares the effectiveness of microwave and thermal regeneration methods for restoration of adsorption capacity of AC spent by mercury removal processes. The changes to AC functional groups and pore structure, as well as carbon consumption and particle size distribution. Based on the results of this study, a possible mechanism of mercury desorption on AC was proposed, and powdered sulfur was used to reduce secondary pollution from mercury recovery. This study may accelerate the industrial application of adsorption, desorption, and recovery of mercury in coal-fired power plants.

Fig. 1 illustrates the thermal regeneration process of spent AC. 1 mL of spent AC was placed in the reactor in the center of the temperaturecontrolled tube furnace. The regeneration temperature was performed in the 30–500 °C range with heating rates of 5 °C/min and maintained at 500 °C for 10 min prior to cooling for 30 min while the nitrogen purge continued. As a comparative experiment, a microwave oven (M3L233C, Midea Corp. China) was used for the treatment. The output power of the microwave oven can be adjusted from 0 W to 1200 W. A quartz glass container was selected as the reactor and placed in the center multimode resonant microwave cavity. With the two regeneration methods, N2 was maintained as carrier gas at 500 mL/min. An infrared gas analyzer (GASMET DX4000, Temet Instruments Oy, Helsinki, Finland) was used to check for a gas component in the desorption process.

2. Experiments and methods

2.4. The uncertainty analysis

2.1. Materials preparation

The uncertainty analysis was used to make sure the accuracy of experimental data [41–44]. In this work, the uncertainty analysis of the experimental results (uA), the uncertainty analysis of the experimental apparatus (uB) and the relative synthetic uncertainty (uA+B). The uA, uB and uA+B was determined [45,46] by formula Eqs. (1)–(3), respectively.

Zhundong lignite was used to prepare activated coke under the same conditions of coal-fired hot gas in a drop-tube reactor. Table 1 lists the proximate and ultimate analyses of coke and burn-off of coal. The Zhundong lignite was dried in a drying oven (Shanghai Yiheng, DHP9012/9032) at 100 °C for 8 h, then crushed and screened to an average particle size of 85um for preparation of powder AC. The preparation of AC was as follows: Feed amount 2.91 g/min used N2 6L/min; the preparation temperature was 900 °C; CO2 (10%) 1.91 L/min; O2 (6%) 0.95 L/min; H2O (g) (12%) 1.59 L/min and residence time 6 s and balance N2 is 5.44 L/min. The resulting AC was then further extruded, ground and screened, yielding the final170 um samples. The Burn-off (X) was calculated to be 45.9% by using the equation

(

X= 1−

A coal A char

) × 100%. Where A

coal

uA =

Ra Ca n

(1)

uB =

a k

(2)

uA + B =

uA2 + uB2

(3)

where uA is uncertainty analysis of experimental results,Ra, Ca and n is the range (The difference between the maximum and minimum values in the test results), range coefficient and the number of experiment, respectively, according to type A evaluation of measurement uncertainty in china (NB/T47066-2018), Ca and n was selected 1.13 and 3. Besides a and k is half-width and confidence factor, respectively, according to the type B evaluation of measurement uncertainty in china (NB/T47066-2018), the k was selected 2.576, and a was selected according to the instrument. The relative synthetic uncertainty uA+B consists of uA and uB. Each experiment was repeated 3 times, and the results were listed in the Table 2.

and Achar is ash content in raw

coal and AC, respectively.

2.2. The mercury adsorption test As shown in Fig. 1 (a), the performance of AC adsorption Hg0 was evaluated by the fixed-bed adsorption system, which included a program temperature control system in order to meet the demand of adsorption and desorption, an Hg0 generation system, and a detection system. In each test, 1 mL of prepared AC was put into the reactor and placed in the center of the temperature-controlled tube furnace. An Hg0 permeation tube loaded in a U-shaped glass tube was used to generate Hg0 vapor carried by N2. The concentration of Hg0 was measured using a Thermo Fisher mercury continuous emission monitoring system (Hg0 CEMS). An Hg continuous monitoring system can measure Hg flue gas concentrations in different forms. At each time for regeneration, the AC adsorbed the Hg0 for 60 min under N2, with the adsorption temperature at 60 °C, and the total flow at 1 L/min. The average efficiency and adsorption capacity of AC was 64.5% and 4.71 μg/g, respectively. Following adsorption, spent AC was regenerated through heating and microwaving in N2.

2.5. Characterization The AC pore properties were determined at −196 °C by N2 adsorption, while the micropore volume and average pore diameter were calculated using the Brunauer–Emmett–Teller (BET) method (Quantachrome, Boynto Beach, FL, USA). Surface morphology was characterized using a scanning electron microscope (SEM: DESKV, Denton Vacuum, Cherry Hill, NJ, USA). Surface properties were analyzed with X-ray photoelectron spectroscopy (XPS) using VG Multilab 2000 X-ray photoelectron spectrometer (Thermo Fisher Scientific, Waltham, MA, USA), with Al Kα as the excitation source. IR spectra were recorded on a Shimadzu IR Affinity-1 Fourier transform infrared (FTIR) spectrometer in the 400–4000 cm−1 range. In the process of

Table 1 Proximate and ultimate analyses of raw coal, AC and burn-off of coal. Sample

Raw coal AC

Proximate analysis/%

Elemental analysis/%

Burn-off/%

Mad

Vad

Aad

FCad

Cad

Had

Oad

Nad

Sad

X

11.87 1.78

26.33 2.66

14.17 30.83

52.92 79.73

60.36 81.84

5.45 0.86

16.91 2.12

1.16 0.87

0.96 0.62

45.9

2

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(a)

(b)

Power of sulfur

gas-washing bottle

Silica-gel drier

Fig. 1. Schematic diagram of experimental setup (a); the device of recovery of mercury (b).

than thermal regeneration, indicating a rapid and thorough desorption process. Next, an infrared gas analyzer (GASMET DX4000, Temet Instruments Oy, Helsinki, Finland) was used to measure gas concentration in the outlet of the desorption gas outlet using the two regeneration methods. As shown in Figs. 3 (a) and 4 (a), the concentration of CO was greater than that of NO and CO2 for both regeneration methods. The concentrations of the three gases reached maximum value in a shorter time during the microwave regeneration process than during thermal regeneration. These results agree with the IR spectra analysis. The sources of desorption gas and the desorption mechanism were further analyzed. Because of variation in thermal stability for different functional groups, temperature increase causes functional groups to decompose into CO and/or CO2. Normally, CO2 is derived from carboxyl, anhydride, and lactone; CO is derived from anhydride, the phenol, carbonyl and quinonyl, and NO is derived from pyridinic, pyridonic or pyrrolic, quaternary, nitrogen oxides or nitrates [49]. According to the results, CO was the main product in the desorption process under nitrogen atmosphere.

regeneration under N2 atmosphere, TG (METTLER, TGA/DSC1/ 1600HT) and microwave oven were combined with the IR (TG-IR) and MG (MG-IR) as a convenient method to detect desorbed gas. 3. Results and discussion 3.1. Mercury desorption performance A temperature-programmed desorption (TPD) experiment was conducted to investigate the desorption performance of mercury from the spent AC resulting from thermal and microwave regeneration. The results shown in Fig. 2 (the black curve), showed that two peaks emerged during the desorption process at 130 °C and 390 °C, indicating that there were two kinds of adsorption states on the surface of the AC. At 130 °C, the peaks on the curve represent the physical adsorption of Hg0 on the AC, while the main peaks at 390 °C show decomposition of the peak HgO. The HgO was formed because Hg0 reacts with oxygen groups on the surface of the AC [47]. The red curve shows desorption properties of spent AC by microwave. Where the microwave power was 300 W there was only one peak on the curve, because spent AC can desorb rapidly. Comparing the two desorption methods, in the 5 min, desorption completely of the spent AC by microwave method. It was also observed that the desorption process was faster and more thorough when microwave regeneration, rather than thermal regeneration was used. After desorption, the Hg0 concentration at outlet of the reactor was close to zero, which was measured by Hg0 CEMS. The results showed that the spent AC was desorbed completely.

3.3. Effect of regeneration on structural properties 3.3.1. SEM during regeneration SEM characterization is a convenient approach for investigating the morphology of AC under different regeneration methods. The original AC, obtained by the method described previously, has developed surface pore structure as shown in Fig. 5 (a). Fig. 5 (b) shows how, after a lengthy desorption process, the functional groups of spent AC had decomposed causing pore collapse on the TG-AC. In comparison, as shown in Fig. 5 (c), the microwave regeneration process resulted in new pores being created on the MG-AC, due to the intermolecular vibration with microwaves. The texture, specific surface area, volume, total surface area and volume of the pores in the TG-AC and MG-AC were changed from the original AC; the parameters are listed in Table 3. Carbon consumption was then investigated using the two regeneration methods, and the results were 0.12% (MG-AC) and 0.23% (TG-AC). These mass losses were due to decomposition of functional groups on the surface of AC. Therefore, to depress the decomposition of active functional groups and reduce the carbon consumption, the adsorbents that adsorbed Hg0 must be regenerated at lower temperatures or desorb rapidly, if microwave regeneration is to meet the requirement. The EDS was selected to identify the composition and content on the surface of spent AC, and the results were shown in Fig. 5 (d). Further analysis, Hg

3.2. Desorbed gas with regeneration IR combined with TG and MG (TG-IR and MG-AC) was used to evaluate the AC of the desorbed gas in different regeneration methods. The two desorption conditions showed a number of absorption peaks ranging from 400 cm−1 to 4000 cm−1. Fig. 3 (b) and Fig. 4 (b) show the IR spectra of desorbed gas resulting from thermal regeneration and microwave regeneration, respectively. The IR spectra of the curve showed two main peaks; the peak at 2300 cm−1 shows the C]O vibration peak of CO2, while the peak at 2230 cm−1 shows the absorption peak of CO [48]. Peaks of NO were detected during the entire desorption process as well. The intensity of the adsorption IR spectra increased with regeneration time, that is to say, the concentrations of CO2 and CO increased as the temperature increased. Microwave regeneration showed sharper gas evolution peaks within a shorter time frame

Table 2 The uA, uB and uA+B of mercury desorption, TG-IR, MG-IR, re-adsorption of mercury, the particle size and the results of Tables 3 and 4. Parameter

Mercury desorption (μg/m3)

TG-IR (ppm)

MG-IR (ppm)

Re-adsorption of mercury(μg/m3)

The particle size (μm)

Table 3

Table 4

uA uB uA+B

2.3 × 10−4 1.6 × 10−4 2.8 × 10−4

3.6 × 10−4 2.4 × 10−4 4.3 × 10−4

2.6 × 10−4 2.2 × 10−4 3.4 × 10−4

2.1 × 10−4 1.5 × 10−4 2.6 × 10−4

1.9 × 10−4 1.1 × 10−4 2.2 × 10−4

1.7 × 10−4 1.4 × 10−4 2.2 × 10−4

2.2 × 10−4 1.6 × 10−4 2.7 × 10−4

3

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Fig. 2. Hg-TPD curves on AC by thermal and microwave regeneration.

regeneration for high recovery capacity of Hg0. As shown in Table 3, changes to the porous structure in regenerated samples may be due to the different heating mechanisms used in each regeneration method. Compared to original AC, the specific surface area and pore volume of TG-AC decreased from 348.3 to 292.2 m2/g and 0.1374 to 0.1225 cm3/g, respectively. Further, the average pore size increased from 1.981 to 2.057 nm, indicating that modification by thermal regeneration destroys micropores in spent AC. Nevertheless, the specific surface area of MG-AC increased from 348.3 to 398.2 m2/g, the pore volume increased from 0.1374 to 0.1401 cm3/g, and the average pore size decreased from 1.981 to 1.733 nm, indicating that modification by microwave regeneration resulted in opening of the closed pores in AC. The MG-AC had greater specific surface area and pore volume, with smaller average pore size than TG-AC. All results were consistent with Fig. 6. Consequently, the microwave regeneration preserved the texture to a greater extent than the heating method, and further promoted Hg0 adsorption.

4f spectrum of spent AC was obtained, which contained two peaks. The peak appearing at 101.6 eV could be assigned to Hg2+, generally HgO, while the peak at 102.7 eV was considered to be Si 2p. This is in agreement with the characteristic peak of Hg, which the content of the Hg on the spent AC were listed in the EDS table. 3.3.2. The BET during Regeneration The N2 adsorption isotherms at −196 °C of original AC, TG-AC, and MG-AC are shown in Fig. 6 (a), According to classification of the adsorption isotherm of Brunauer et al. [50], all the samples showed I type adsorption isotherms with a hysteresis loop within a relatively low pressure area (10−6 < P/P0 < 0.05). Microporous adsorption was the main method in this stage; with the increase of relative pressure a hysteresis loop appeared, indicating existence of some mesoporous structures in the sample. The pore size distributions of the original AC, TG-AC, and MG-AC were calculated using the QSDFT method, and the result is shown in Fig. 6 (b). The curve shows pores of original AC to be mainly 0–2 nm micropores, while mesopores of 2–50 nm were rarely distributed. Compared with the pore size distribution after heating regeneration, a large number of micropore structures in the range of 0.75–1.25 nm seemed to have disappeared, due to micropore damage and collapse of the micropore walls, which led to reduced micropore volume and increased mesopore volume. After regeneration by microwave, the micropore structure was more developed, and the pore diameter gradually diminished. A large peak appeared in the range of 0.5–2 nm after regeneration by microwave. The kinetic diameter of mercury was 0.32 nm. According to the adsorption theory, when the pore size is 1.7–3 times than kinetic diameter of mercury, it has the best adsorption effect on mercury than other pore sizes. Micropores with a diameter of approximately 0.75 nm are conducive to the adsorption of Hg0; therefore microwave regeneration may be a better solution than heating 120

3.3.3. The surface chemistry of AC Since surface chemical properties of porous carbon play an important role in the process of adsorbing mercury, the surface functional groups and percentages within the samples of AC, MG-AC, and TG-AC were characterized via XPS analysis. The C1s spectra of original AC, MG-AC, and TG-AC are illustrated in Fig. 7 (a), (c) and (e). The C1speak is composed of three components, appearing at 284.4 eV, 285.4 eV, and 286.5 eV, respectively. The signal at 284.4 eV is considered as C]C bonding [51] and the signals at 285.4 eV and 286.5 eV correspond to sp3 CeC [52] and CeO [53], respectively. The O1speaks of original AC, MG-AC, and TG-AC appeared at about 531.6–531.9 eV and 532.6–533.2 eV as shown in Fig. 7(b) (d) and (f), which were assigned to unsaturated carbon−oxygen double bond (C] O, ester or amides) and C−O single bond (ether-like, ester oxygen or

600 CO2

(b)

(a)

N2

NO

500

CO

80 cut off heating

60

400

300

40

CO2

Temperature (oC)

CO

200

10 20 30 40 50 60 70 80 90 100

NO

in )

Concentration(ppm)

100

e( m

20

Ti

m

100 0 0

20

40

60

80

100

120

500

1000

1500

2000

2500

3000

3500

4000

-1

Time (min)

Wavenumber (cm )

Fig. 3. The thermal regeneration exhausted gas analysis (a). IR spectrum of exhausted gas under thermal regeneration (b). 4

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(a)

(b)

CO

300W

CO2

700

CO2

NO

CO

Concentration(ppm)

600 500 400 NO

300 200 100 10

0 0

2

4

6

10 500

8

1000

1500

Time (min)

2000

2500

3000

3500

9

8

7

6

5

m Ti

4

e(

3

m

2

in

1

)

4000

-1

Wavenumber (cm )

Fig. 4. The microwave regeneration exhausted gas analysis (a). The IR spectra of exhausted gas under microwave regeneration (b).

(b)

(a)

(c)

1 μm

1 μm

1 μm

Intensity (a.u)

(d)

101.6 eV HgO

102.7 eV Si 2p

96 98 100 102 104 106 108 110 Binding Energy (eV)

Hg

10 μm

Fig. 5. SEM images of original AC (a), TG-AC (b), MG-AC (c) and EDS of spent AC.

31.27%, and 41.38%, respectively. The CeO group showed a gradual increase after each regeneration method, and the ratios were 55.69%, 68.73%, and 58.62%. According to the above data, the C=O/C-O ratios calculated and the result were 0.7956, 0.4549 and 0.7059, respectively. In the C]O group, the original AC contained a high proportion of C] O. It has been reported in the literature that ketone, lactones, carbonyl or quinone group (C=O) is conducive to the adsorption of Hg0 [24]. Ratios of C=O/C-O groups decreased from 0.7956 to 0.4549 and 0.7059 after heating regeneration and microwave regeneration. Due to instability of groups of carboxylic, phenolic, hydroxyl and quinone, the acidic functionalities decomposed under the regeneration methods. Meanwhile, high-stability functional groups such as the C − O single bond increased, indicating conversion reactions between different types

Table 3 Pore parameters of original AC, TG-AC, and MG-AC. Samples

BET surface (m2/ g)

Pore volume (cm3/ g)

Average pore diameter (nm)

Original AC TG-AC MG-AC

348.3 292.2 398.2

0.1374 0.1225 0.1401

1.981 2.057 1.733

anhydride hydroxy), respectively [54,55]. The relative contents of each oxygen-containing group were summarized in order to evaluate changes after regeneration, and the sample results are listed in Table 4. The C]O ratios of original AC, MG-AC, and TG-AC were 44.32%,

0.7

(a)

Fresh

TG-AC

MG-AC

120

0.6

100

0.5

(b)

MG-AC

TG-AC

Fresh

-1

-1

dv/dr(cm .g .nm )

3

Adsorption vplume (cm /g)

140

0.5~2 nm >2 nm

0.4

3

80

60

0.3

0.2

40

2.4 nm

0.1

20

0.0

0 0.0

0.2

0.4

0.6

0.8

1

1.0

10

100

Pore width (nm)

Relative pressure, P/P0

Fig. 6. N2 adsorption-desorption isotherm (a) and pore size distribution (b) of original AC, TG-AC, and MG-AC. 5

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C1s

(a)

O1s

(b)

Sp2 C=C

533.1eV

Intensity (a.u)

Intensity(a.u)

C-OH

531.7 eV C=O

Sp3 C-C C-O

279

282

285

288

291

294

525

297

528

531

534

537

540

Binding Energy (eV)

Binding Energy (eV) C1s

(c)

O1s

(d) 532.7 eV C-OH

Intensity(a.u)

Intensity (a.u)

Sp2 C=C

531.6 eV C=O

Sp3 C-C C-O

279

282

285

288

291

294

297

525

528

531

Binding Energy (eV)

534

537

540

Binding Energy (eV)

O1s

C1s

(e)

(f)

Sp2 C=C

532.9 eV

Intensity (a.u)

Intensity(a.u)

C-OH

Sp3 C-C

531.8 eV C=O

C-O

279

282

285

288

291

294

297

Binding Energy (eV)

525

528

531

534

537

540

Binding Energy (eV)

Fig. 7. Chemical structure characterizations of original AC, MG-AC, and TG-AC. C1s spectrum of original AC (a), G-AC (c) and TG-AC (e); O 1 s spectrum of original AC (b), MG-AC (d) and TG-AC (f).

alcohol, phenol, water, and the NeH stretching vibrations of –NH and –NH2; the characteristic peaks at 2300–2400 cm−1 were formed by physical adsorption of CO2.The peaks at 3000–2800 cm−1 can be related to the CeH stretching vibrations of –CH2 (2875 cm−1) and –CH3 (2960 cm−1).The carbonyl(C=O), lactone (O-C=O) and benzene (C=C) groups were also identified through the characteristic stretching vibration at 1700–1395 cm−1.The peaks at 1200–1000 cm−1 were assigned to the CeO stretching vibrations of anhydride and ether [21,56]. After regeneration, when peaks tend to fade, the key peaks of carbonyl, quinone, and lactone, which promote mercury adsorption, decreased. Contents of the groups showed greater reduction after thermal than microwave regeneration. These results agree with the Table 4.

Table 4 the concentration of different type of oxygen (C]O, CeO) in original AC, TG -AC, and MG-AC. Samples

C=O

C-O

C=O/C-O

AC TG-AC MG-AC

44.32% 31.27% 41.38%

55.69% 68.73% 58.62%

0.7956 0.4549 0.7059

of functional groups. The contents of C]O in MG-AC was greater than in TG-AC, because pyrone-like groups were formed by the rearrangement of ether and carbonyl groups at high temperatures [25]. 3.3.4. The IR during Regeneration The Infrared adsorption spectra of original AC, MG-AC, and TG-AC were carried out in the range of 400–4000 cm−1 with a resolution of 2 cm−1, and the results are shown in Fig. 8.The peaks at 3600–3200 cm−1 can be assigned to the OeH stretching vibrations of

3.4. Re-adsorption performance after regeneration and cycle time The adsorption curves and removal efficiency of original AC, MGAC, and TG-AC are shown in Fig. 9. The Hg0 adsorption efficiency of 6

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original AC

TG-AC

0.14 C-C C=O

0.12

Absorbance (a.u.)

O-H

0.10

C-O-C

O-C-O

-NH

-CH 3

-NH2

-CH 2

CO 2

0.08 0.06 0.04 0.02 0.00 4000

3500

3000

2500

2000

1500

1000

500

-1

Wavenumber (cm ) Fig. 8. IR absorption spectra of original AC, MG-AC, and TG-AC. 50

80

Removal efficiency (%)

AC TG-AC

45

MG-AC

3

Concentration( g/Nm )

40 35

MG-AC AC

60

TG-AC

40 20

30

0

25 20 15 10 5 0 0

10

20

30

40

50

60

Time (min) Fig. 9. Mercury adsorption curve and removal efficiency of original AC, MG-AC, TG-AC.

90 MG

TG

70 60 50 40 30

0

Hg adsorption efficiency (%)

80

20 10 0

1

3

2

4

Cycle times Fig. 10. Cycle times of the sorbent for Hg0 removal efficiency.

7

5

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18

MG-AC TG-AC

14 12

Volume (%)

72 %

200 180 160 140 120 100 80 60

Particle size (μm)

Original-AC

16

77 %

81 %

AC TG-AC MG-AC

10 8 6 4 2 0 0

100

200

300

400

Partical size ( m) Fig. 11. The particle size distribution of original AC, MG-AC, TG-AC.

Rapidly Hg0

Hg(ads) MG-AC O

C-O-Hg

CO2+Hg0

Lactone

-O-Hg

CO+Hg 0

Hg

Hg

Hg Hg

Lactone, Quinonyl

-OH

CO

Phenol

-COOH

CO2

Carboxyl N-X

TG-AC

NO

Nitrogen oxide Nitrate

Lenitively Fig. 12. Schematic illustration of desorption mechanism for microwave regeneration and thermal regeneration.

regeneration with MG method, the Hg0 removal efficiency increases at first and then decreased. After the fifth circulation, the Hg0 removal efficiency of MG-AC and TG-AC reaches 65% and 34%, respectively. This suggests that the regeneration method of MG highlights its potential applications in the future.

TG-AC exhibited a general decrease from 68.7% (average efficiency of original AC in1h) in the first-time regeneration to 52.4%. The results indicate that thermal regeneration negatively impacted the Hg0 removal performance of AC, because thermal regeneration damaged both pore structure and chemically active sites. The average Hg0 removal efficiency of MG-AC was 75.3%, higher than that of original AC and TGAC under the same adsorption conditions. Hence, the microwave regeneration promotes the adsorption of Hg0 by recycled coke. The results indicate that despite consumption of the active functional groups by microwave regeneration, the micropores and specific area of the MGAC increased. The literature [57] confirms that micropores promote adsorption of Hg0. It may therefore be concluded that microwave regeneration is an effective way to improve the desorption rate of spent AC and may lead to improved performance of Hg0 removal when compared with heating regeneration. In order to explore the regeneration further, five-cycle experiment was performed in this work. The cycle time of spent AC was carried out by MG and TG methods, respectively. As shown in Fig. 10, with the increasing of cycling time of AC, which was desorbed by TG, the Hg0 removal efficiency decreases sharply. Nevertheless, the AC after

3.5. The recovery of mercury Hg0 is released from spent AC under different regeneration methods. The Hg0 in this experiment was fixed to prevent secondary pollution, and the sulfur used as a curing agent played a key role in the Hg0-fix performance. As shown in Fig. 1 (b), the desorption gas was first passed through the gas-washing bottle to remove impurity, followed by the silica-gel drier to remove water vapor and finally, through a fixed-bed reactor filled with sulfur powder where the S reacted with Hg0 to form HgS at room temperature. The Hg0-fix efficiency of this process was 100%.

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a high-efficiency, economical regeneration method for spent AC that also prevents secondary pollution through mercury adsorption.

3.6. The particle size distribution The particle sizes of original AC, TG-AC, and MG-AC were measured using a Laser particle size analyzer and the results are shown in Fig. 11.The expansion rate (Ɛr) was selected to analyze the change in AC particle size with different desorption methods. When the Ɛr > 1, the particles expand. The Ɛr was calculated using the following formula: D εr = Dchar , where Dchar and Dcac refer to average particle sizes of original cac AC and AC after regeneration, respectively. In the desorption process, the average particle sizes of the three samples were 175 μm (72% of the total), 160 μm (77% of the total) and 168 μm (81% of the total), respectively, and the Ɛr of TG-AC and MG-AC was 0.988 and 0.993 respectively. The results showed that the particle size distribution curve of TG-AC and MG-AC changed to show smaller average particle sizes than original AC, and microwave and thermal regeneration promoted particle reduction. This phenomenon may have been caused by carbon consumption and decomposition of functional groups on the surface of the samples.

CRediT authorship contribution statement Donghai An: Writing - original draft, Data curation. Xiang Wang: Software. Xingxing Cheng: Supervision, Writing - review & editing. Lin Cui: Conceptualization. Xiaoyang Zhang: Formal analysis. Ping Zhou: Methodology. Yong Dong: Project administration, Resources. Declaration of competing interest The authors declared that they have no conflicts of interest to this work. We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted. Acknowledgements This project was supported by The National Natural Science Foundation of China (No.51976108), The National Science Foundation of Xinjiang Uygur Autonomous Region (XJEDU2018Y048, XJEDU20191025), Science and Technology Planning Project of AQSIQ (2017QK178).

3.7. The regeneration mechanism The spent AC desorption reaction paths for thermal and microwave regeneration are summarized in Fig. 12. During the desorption process of spent AC, the phenol, carbonyl, and quinonyl groups connect to the carbon atoms in the benzene ring, forming CO easily. Conversely, CO2 is formed by decomposition of groups not directly connected to the benzene ring. Two main processes cause mercury desorption. In the first, mercury is physically adsorbed but can be easily released after heating. In the second process, Hg0 combines with carbonyl (-C=O-), lactyl, and semi-quinone groups to chemically adsorb to the AC. During the process of thermal or microwave desorption, in this case, the chemical adsorption of –C-OHg decomposes to form Hg0 and CO. Meanwhile, other functional groups that do not react with Hg0, such as hydroxyl group, carboxyl group, nitrogen oxide, and nitrate, decompose to form CO2, CO and NO. Further analysis showed that during the desorption process of spent AC by microwave in the presence of local hot spots, microwave irradiation can create instantaneous high temperature causing rapid desorption of the functional groups as well as Hg0, and causing large amounts of Hg0, CO2, CO and NO to be formed in a short time. By comparison, the decomposition types of functional groups and the content of desorbed gas increased more gradually than in thermal regeneration.

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4. Conclusions This study investigated the regeneration properties of spent AC from both microwave regeneration and thermal regeneration and systematically investigated the regeneration by combining regeneration efficiency. Compared with the traditional thermal regeneration process, the microwave regeneration showed a faster heating rate, requiring only 5 min to achieve complete regeneration. Further analysis of the pore structure of MG-AC showed that new micropores were produced during the microwave process. Because MG-AC had more active functional groups than TG-AC, MG-AC was able to maintain a higher Hg0 removal capacity after Hg−regeneration cycles than TG-AC. Increased porosity development (micropores) and decreased decomposition of functional groups on the AC facilitates mercury re-adsorption, and is therefore a goal of regeneration. In desorption process, at first, the physical adsorption of Hg0 requires desorption from the spent AC. As temperature increased, the chemical bond which was formed by the reaction of Hg0 and active functional groups ruptured to release the Hg0. Besides, CO, CO2 and NO were generated from the functional groups through the regeneration process. Due to carbon consumption, the average particle size decreased after both regeneration processes. In conclusion, this research demonstrates that microwave regeneration is 9

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