Cost-effective Manganese Ore Sorbent for Elemental Mercury

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Energy and the Environment

Cost-effective Manganese Ore Sorbent for Elemental Mercury Removal from Flue Gas Yingju Yang, Sen Miao, Jing Liu, Zhen Wang, and Yingni Yu Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.9b03397 • Publication Date (Web): 01 Aug 2019 Downloaded from pubs.acs.org on August 2, 2019

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Environmental Science & Technology

1

Cost-effective Manganese Ore Sorbent for Elemental Mercury

2

Removal from Flue Gas

3

Yingju Yang, Sen Miao, Jing Liu,* Zhen Wang, and Yingni Yu

4

State Key Laboratory of Coal Combustion, School of Energy and Power Engineering, Huazhong

5

University of Science and Technology, Wuhan 430074, China

6 7

ABSTRACT: Mercury capture from flue gas remains a challenge for environmental protection due

8

to the lack of cost-effective sorbents. Natural manganese ore (NMO) was developed as a

9

cost-effective sorbent for elemental mercury removal from flue gas. NMO sorbent showed excellent

10

Hg0 removal efficiency (>90%) in a wide temperature window (100-250 °C) under the conditions of

11

simulated flue gas. O2, NO, and HCl promoted Hg0 removal due to the surface reactions of Hg0 with

12

these species. SO2 and H2O slightly inhibited Hg0 removal under the conditions of simulated flue gas.

13

O2 addition could also weaken the inhibitory effect of SO2. NMO sorbent exhibited superior

14

regeneration performance for Hg0 removal during ten-cycle experiments. Quantum chemistry

15

calculations were used to identify the active components of NMO sorbent and to understand the

16

atomic-level interaction between Hg0 and sorbent surface. Theoretical results indicated that Mn3O4 is

17

the most active component of NMO sorbent for Hg0 removal. The atomic orbital hybridization and

18

electrons sharing led to the stronger interaction between Hg0 and Mn3O4 surface. Finally, a chemical

19

looping process based on NMO sorbent was proposed for the green recovery of Hg0 from flue gas.

20

The low cost, excellent performance, superior regenerable properties suggest that the natural

21

manganese ore is a promising sorbent for mercury removal from flue gas.

22 1

ACS Paragon Plus Environment


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1. INTRODUCTION

24

Mercury is one of the global pollutants in the world, and has received widespread attention due

25

to its neurological toxicity, bioaccumulation and adverse effects on human health.1 Thermal power

26

plants are considered as the major anthropogenic sources of mercury emission in China.2,3 Minamata

27

Convention on Mercury, an international convention to control and reduce mercury emissions in the

28

world, came into force in August 2017.4,5 Moreover, mercury emission limit of power plants

29

becomes gradually more and more stringent, and the maximum emission concentration is 1 μg/m3 in

30

2030.6 Therefore, mercury removal from flue gas increasingly becomes a global environmental

31

concern.

32

Mercury species in flue gas mainly consist of three forms: elemental mercury (Hg0), oxidized

33

mercury (Hg2+), and particulate-bound mercury (Hgp).7,8 Most of the Hg2+ and Hgp can be removed

34

by using traditional air pollution control devices (APCDs).9 Hg0 is highly volatile and

35

water-insoluble, and is very difficult to remove using the existing APCDs.10 Therefore, one of the

36

major challenges to meet the increasingly strict mercury emission standards of power plants is Hg0

37

removal.

38

Up to now, there are mainly two kinds of Hg0 emission control technologies: catalytic

39

oxidation11,12 and sorbent injection.13-15 Sorbent injection is currently regarded as the most mature

40

technology for mercury removal.5,16 Activated carbon is the commercial sorbent used for mercury

41

removal from flue gas. However, the high operation costs and its side effects on the commercial

42

utilization of fly ash as the raw material of cement production limit its widespread application.17 In

43

addition, mercury capture capability of the raw activated carbon is limited.5 Various chemical

44

modification methods (such as sulfur,18,19 halogen,20-22 and metal oxides23) have been developed to

45

enhance the mercury removal efficiency of raw activated carbon. Nevertheless, this will further 2


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46

increase the operation cost of activated carbon injection technology. Therefore, it is very important to

47

develop low-cost and non-carbon sorbents with high mercury removal efficiency.

48

Natural manganese ore (NMO) mainly consisting of manganese oxides is ubiquitous in soils

49

and sediments, and plays an important role in the environmental protection.24 NMO is very cheap

50

and easily acquirable due to its huge geological reserve. It is reported that NMO is chemically active

51

for a lot of oxidation-reduction and cation-exchange reactions.24,25 Moreover, NMO shows unusually

52

high adsorption capacity and scavenging capability for heavy metals removal from contaminated

53

waters.26,27 Thus, it is speculated that NMO can act as the cost-effective adsorbents for Hg0 removal

54

from flue gas due to its low cost and chemical reactivity. The fundamental studies of mercury

55

removal experiments are the basis of the industrial application of NMO sorbent. However, to date, no

56

attempts have been conducted to investigate the mercury removal by natural manganese ore.

57

In this work, natural manganese ore was developed as sorbent to capture Hg0 from flue gas at

58

different reaction temperatures. The physical and chemical properties of NMO sorbent were

59

characterized by using different analysis methods. The effects of flue gas components on Hg0

60

removal by NMO sorbent were systemically investigated. The atomic-level interaction between Hg0

61

and NMO sorbent was explored using density functional theory calculations. Finally, a novel concept

62

based on chemical looping process was proposed to recover Hg0 from flue gas.

63 64

2. EXPERIMENTAL METHODS

65

2.1. Sorbent Preparation and Characterization

66

The natural manganese ore used in this study was produced from Huangshi mineral field of

67

Hubei province, China. NMO sorbent was prepared by a simple procedure. NMO lump was ground

68

by a planetary ball mill, and then sieved to 200 mesh. After that, NMO particles were dried at 100 °C 3



69

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for 10 h.

70

The Brunauer-Emmett-Teller (BET) surface area and pore structure of NMO sorbent were

71

determined by a nitrogen adsorption apparatus (ASAP2020, Micromeritics). Before the BET

72

measurement, the sample was degassed at 200 °C for 2 h. The chemical composition of NMO

73

sorbent was investigated by X-ray fluorescence (XRF) technique. The percentage of the chemical

74

composition was determined by XRF characterization analysis. Powder X-ray diffraction (XRD)

75

measurement was performed on a diffractometer (X'Pert PRO, PANalytical) with Cu-Kα radiation.

76

XRD pattern was recorded in the 2θ range of 10-90° at a step of 5°/min. X-ray photoelectron

77

spectroscopy (XPS) analysis was used to determine the chemical valence state of different elements,

78

and carried out on a Thermo ESCALAB 250 instrument with Al Kα (hv=1486.6 eV) as the excitation

79

source. The binding energy was calibrated using C1s peak at 284.6 eV. Microcosmic morphology

80

analysis was performed on a scanning electron microscopy (SEM, Sigma300, Carl Zeiss).

81 82

2.2. Mercury Removal Performance Evaluation

83

Mercury removal performance of NMO sorbent was studied in a fixed-bed reactor, as shown in

84

Figure S1. The experimental system mainly includes Hg0 generator, gas feed system, fixed-bed

85

reactor and on-line mercury monitoring system. The stable Hg0 concentration (65 µg/m3) was

86

generated by heating the Hg permeation tube located in the U-shaped glass tube at 45 ºC. The

87

simulated flue gas (SFG) with a total flow rate of 1 L/min consisted of 4% O2, 12% CO2, 5% H2O,

88

500 ppm SO2, 300 ppm NO, 10 ppm HCl, and balance gas N2. The fixed-bed reactor with an internal

89

diameter of 18 mm was placed in a temperature-controlled furnace. In each test, 0.2 g sorbent was

90

mixed with 1.8 g quartz sand to decrease the pressure drop. It has been demonstrated that quartz sand

91

is inert for Hg0 adsorption.28 Therefore, quartz sand has no effects on mercury removal efficiency of 4


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92

NMO sorbent. The corresponding bed height of sorbent-sand mixtures was about 10 mm. Hg0

93

concentration of simulated flue gas was measured by a mercury analyzer (Lumex RA-915M, Russia)

94

based on the differential Zeeman atomic absorption spectrometry (ZAAS). Gaseous Hg2+ was

95

measured by another mercury analyzer (Tekran 2537Xi, USA). To prevent the corrosion of analytical

96

cell, acid gases and water vapor of simulated flue gas was adsorbed by 10% NaOH solution and

97

drying agent before entering the mercury analyzer.

98

Before each experiment, the flue gas bypassed the fixed-bed reactor to attain a stable inlet Hg0

99

concentration (Hg0in ). Flue gas was switched to reactor to measure the outlet Hg0 concentration (Hg0out )

100

when the inlet Hg0 concentration was stabilized at a certain value for 30 min. The reaction time of

101

each experiment was 120 min. Mercury removal efficiency (η) of NMO sorbent was calculated using

102

the following equation:

η=

103

∫

t

0

t

0 Hgin0 dt − ∫ Hg out dt

∫

t

0

0

0 in

Hg dt

× 100%

(1)

104

The detailed experimental conditions are listed in Table 1. In experiment Ⅰ, mercury removal

105

efficiency of NMO sorbent was tested at 50-350 °C under the condition of simulated flue gas. The

106

experiments (Ⅱ-Ⅵ) were conducted to investigate the effects of different flue gas components on

107

mercury removal performance of NMO sorbent. Temperature programmed desorption (TPD)

108

experiments were performed in N2 atmosphere with a flow rate of 1 L/min. The spent sorbents tested

109

in different reaction atmospheres were heated from room temperature to 650 °C at a heating rate of

110

10 °C/min.

111 112 113

3. COMPUTATIONAL DETAILS In this study, density functional theory (DFT) calculations were carried out using the Cambridge 5



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114

Serial Total Energy Package (CASTEP),29 in which plane waves were used to expand the electronic

115

wave functions. Ultrasoft Vanderbilt pseudopotentials were used to describe the electron-ion

116

interactions. Perdew-Burke-Ernzerhoff (PBE)30 functional of generalized gradient approximation

117

(GGA)31 was used to calculate the electronic subsystem. A cutoff energy of 340 eV was employed in

118

the plane wave expansion. Broyden Fletcher Goldfarb Shanno (BFGS) method was used for

119

geometry optimization. DFT calculations involved the following convergence criteria: (1) a SCF

120

tolerance of 2.0×10-6 eV/atom; (2) an energy tolerance of 2.0×10-5 eV/atom; (3) a maximum force

121

tolerance of 5.0×10-2 eV/Å; (4) a maximum displacement tolerance of 2.0×10-3 Å.

122

According to the XRD, XRF and XPS characterization analysis results, NMO sorbent includes

123

different metal oxides which expose different surfaces to provide active sites for Hg0 adsorption.

124

Even though the reactive surfaces can be more effective for Hg0 adsorption than those

125

thermodynamically stable ones, the reactive surfaces cannot stably exist under the reaction

126

conditions. Moreover, some thermodynamically stable surfaces are also effective for Hg0 adsorption.

127

The surface energies of different metal oxides were calculated and compared under the reaction

128

conditions to obtain the thermodynamically stable surfaces. Therefore, these thermodynamically

129

stable surfaces of different metal oxides were constructed to calculate adsorption energy of Hg0 over

130

active components (Figure S2). The vacuum layer thickness of different surfaces is 15 Å.

131 132

The adsorption energy (Eads)32,33 is defined as follows: Eads = E(sorbent–Hg) – (Esorbent + EHg)

(2)

133

where E(sorbent–Hg), Esorbent, and EHg represent the total energy of sorbent-Hg system, the total energy of

134

clean sorbent surface, and the total energy of gaseous Hg0, respectively.

135 136

4. RESULTS AND DISCUSSION 6


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137


4.1. Characterization Analysis

138

The BET surface area of natural manganese ore is 22.85 m2/g. The average pore diameter and

139

total pore volume are 9.62 nm and 0.0549 cm3/g, respectively. XRF analysis results indicated that the

140

elemental compositions of NMO sorbent mainly include Mn, Si and Fe (Table S1). Mn, Si, and Fe

141

accounted for 44.31%, 31.66%, and 15.32%, respectively. Manganese oxides of NMO sorbent

142

existed in the forms of MnO2, Mn2O3 and Mn3O4, as shown in Figure 1a. The crystallinity of

143

manganese oxides was relatively weak. The diffraction peak of iron oxides was not observed in the

144

XRD pattern, which indicated that iron oxides of NMO sorbent exist in an amorphous form. NMO

145

sorbent particles were composed of nanosheets (Figure S3). The average size of nanosheets was

146

about 87.98 nm.

147

The surface chemical state of NMO sorbent can be determined by XPS analysis results. In Mn

148

2p spectra (Figure 1b), the peaks at 642.92 eV, 641.67 eV, and 653.55 eV were attributed to Mn4+ of

149

MnO2,34 Mn3+ of Mn2O3,35 and Mn2+ or Mn3+ of Mn3O4,36 respectively. Therefore, Mn2+, Mn3+ and

150

Mn4+ coexisted in the sorbent. In Fe 2p spectra (Figure 1c), the peaks at 710.57 eV, 711.79 eV,

151

712.76 eV, and 724.29 eV were assigned to Fe3+ of Fe2O3,28 while the peak at 709.82 eV was

152

ascribed as Fe2+ of FeO or Fe3O4.37 In O 1s spectra (Figure 1d), the peaks at 529.67 eV and 531.03

153

eV were attributed to lattice oxygen and adsorbed oxygen, respectively.

154 155

4.2. Hg0 Removal Performance of NMO Sorbent

156

4.2.1. Effects of Reaction Temperature

157

Hg0 removal efficiency of NMO sorbent under the conditions of simulated flue gas and different

158

reaction temperatures (50-350 °C) is shown in Figure 2a. In the temperature range (50-150 °C), Hg0

159

removal efficiency of NMO sorbent increased with increasing reaction temperature. However, Hg0 7



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160

removal efficiency decreased from 98.72% to 71.93% as the reaction temperature increased from

161

150 °C to 350 °C. In the temperature window of 100-250 °C, NMO sorbent showed >90% Hg0

162

removal efficiency. Thus, the optimal reaction temperature window of NMO sorbent is 100-250 °C.

163

It is clear that mercury removal efficiency of NMO sorbent is dependent on the reaction temperature.

164

Moreover, gaseous Hg2+ was measured to distinguish the contributions of adsorption and catalytic

165

oxidation to mercury removal (Figure 2a). At the lower temperatures (≤200 ºC), mercury removal

166

was mainly caused by adsorption. At the higher temperatures (>200 ºC), mercury removal was

167

caused by both adsorption and catalytic oxidation. The sorbent injection technology is usually

168

operated at the lower temperatures (about 150 ºC) in coal-fired power plants, and thus NMO is

169

mainly used as sorbent.

170

Mercury removal by NMO sorbent involved the processes of Hg0 adsorption, surface

171

conversion, and decomposition desorption (mainly occurs at high temperatures). It was reported that

172

the temperature-dependent mercury removal efficiency can be explained by a dimensionless

173

temperature coefficient theory.38 Physically, the temperature coefficient represents the total

174

contribution of promotional reactions and inhibitory reactions to Hg0 removal. Moreover, the

175

temperature coefficient also reflects the temperature-dependent relationship between reaction

176

chemistry and Hg0 removal. Mercury removal by NMO sorbent was controlled by two types of

177

reactions: promotional reactions (such as adsorption, surface conversion) and inhibitory reactions

178

(such as decomposition desorption). In the low temperature range (50-150 °C), the contribution of

179

promotional reactions to Hg0 removal was much larger than that of inhibitory reactions, because the

180

low temperature was beneficial to adsorption reaction. Thus, Hg0 removal efficiency increased with

181

increasing reaction temperature at low temperatures (Figure 2a). At high temperatures (200-350 °C),

182

the inhibitory reactions played a remarkable role in Hg0 removal. The inhibitory reactions dominated 8


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183


Hg0 removal process, leading to the decrease of Hg0 removal efficiency (Figure 2a).

184 185

4.2.2. Effects of Flue Gas Compositions

186

The effects of flue gas compositions (O2, NO, SO2, HCl, H2O) on mercury removal

187

performance of NMO sorbent were also investigated, as shown in Figures 2b-d. O2 addition

188

enhanced Hg0 removal efficiency of NMO sorbent (Figure 2b), because gaseous O2 replenished the

189

consumed surface chemisorbed oxygen which is important for Hg0 removal.39-41 The promotional

190

effect became significant at high temperatures (especially at 250 ºC and 300 ºC). However, the

191

promotional effect was insensitive to O2 concentration variation (Figure S4). This insensitive role

192

was closely associated with the abundant chemisorbed oxygen (Figure 1d), which was enough for

193

ppb-level Hg0 removal. Therefore, the further increase in O2 concentration showed little effects on

194

Hg0 removal.

195

The presence of NO promoted Hg0 adsorption on NMO sorbent surface (Figure 2b). It was

196

reported that NO can be oxidized by surface chemisorbed oxygen into NO2 species via the reaction

197

NO + O* = NO2*.42 NO2 species reacted with adsorbed Hg0 to form Hg(NO3)2 species via the

198

reaction Hg* + 2NO2* + O2* = Hg(NO3)2*,40 leading to the increase of Hg0 removal efficiency.

199

Hg(NO3)2 formation could be confirmed by TPD experiments (Figure S5a). The increase in NO

200

concentration showed little effects on Hg0 removal efficiency of NMO sorbent (Figure S5b).

201

Moreover, Hg0 removal was not significantly affected by the addition of 4% O2 (Figure S5b), which

202

was ascribed to the abundant surface chemisorbed oxygen. The amount of Hg0 adsorption (3.58 μg)

203

and desorption (3.27 μg) was calculated from the adsorption (Figure S5b) and desorption curves

204

(Figure S5a), respectively. It was found that the amount of Hg0 adsorption is similar to that of Hg0

205

desorption. This indicated that the contribution of adsorption to mercury removal is much larger than 9



206

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that of catalytic oxidation. Therefore, NMO is mainly sorbent.

207

SO2 exhibited a slightly inhibitory effect on Hg0 removal (Figure 2c). The inhibitory effect was

208

not sensitive to SO2 concentration. Meanwhile, O2 addition could also weaken the inhibitory effect of

209

SO2. The inhibitory effect was attributed to the competitive adsorption between Hg0 and SO2.

210

SO2-pretreated experiments could be used to demonstrate the competitive adsorption phenomenon.

211

As shown in Figure S6, compared with the fresh NMO sorbent, SO2-pretreated sorbent showed poor

212

Hg0 removal efficiency in N2 atmosphere. This indicated that SO2 molecules occupy the active sites

213

of Hg0 adsorption during SO2 pretreatment. Even though SO2 showed inhibitory effect on Hg0

214

removal, the inhibitory effect could be neglected due to the presence of O2 in flue gas during its

215

practical application at low temperatures (≤150 °C).

216

It is well-known that HCl is the most important species for Hg0 removal. HCl promoted Hg0

217

removal due to the formation of active chlorine species, as shown in Figure 2b. No obvious increase

218

in Hg0 removal efficiency was observed when HCl concentration increased from 10 ppm to 20 ppm

219

(Figure S7). Hg0 removal efficiency decreased from 99% to 90% within 80 min in N2/HCl

220

atmosphere. The promotional effect of HCl declined gradually with increasing reaction time, which

221

was attributed to the consumption of surface chemisorbed oxygen species during HCl activation.

222

However, the addition of 4% O2 to N2/HCl atmosphere could maintain about 100% Hg0 removal

223

efficiency within 120 min. It was reported that O2 molecules can regenerate and restore the

224

consumed chemisorbed oxygen for active chlorine species formation.43 Therefore, O2 played an

225

important role in the promotional effect of HCl on Hg0 removal.

226

The effect of H2O on Hg0 removal is shown in Figure 2d. In the 3% H2O + N2 atmosphere, Hg0

227

removal efficiency slowly decreased from 91.7% to 82.4% with the increase of reaction time. Hg0

228

removal efficiency decreased to 69.6% after 120 min when 5% H2O was added to N2 gas. Compared 10


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229

with pure N2 gas, the inhibitory effect of 5% H2O in simulated flue gas was relatively weaker. NMO

230

sorbent showed a stable Hg0 removal efficiency (> 82%) in the 5% H2O + SFG atmosphere. The

231

inhibitory effect was attributed to the competitive adsorption between Hg0 and H2O on NMO sorbent

232

surface,44 or to the H2O-induced elimination of active chlorine species.7 As mentioned previously, O2,

233

NO and HCl species of simulated flue gas promoted Hg0 removal. Therefore, the promotional effects

234

of O2, NO and HCl species partially compensated the inhibitory effect of H2O.

235

Moreover, the larger time-scale experiments using 0.01 g sorbent were conducted to determine

236

the saturated adsorption capacity (Figure S8). The saturated mercury adsorption capacity of NMO

237

sorbent was about 53.57 mg/g. Meanwhile, TPD experiments were also performed to confirm the

238

amount of adsorbed mercury on NMO sorbent after the reaction. The adsorbed mercury calculated

239

from TPD experiments was approximately 41.36 mg/g, which was close to the experimental value

240

(43.33 mg/g) of the larger time-scale experiments.

241 242

4.3. Regeneration Performance of NMO Sorbent

243

Thermal treatment is widely regarded as the simplest method to regenerate mercury sorbent.45-47

244

Thus, this method was used to investigate the regeneration performance of NMO sorbent. The

245

adsorption experiments were conducted at 150 ºC under the conditions of simulated flue gas.

246

According to the TPD experiments of used sorbent (Figure S8), the maximum desorption peak

247

temperature was 496 ºC. Almost all of mercury species decomposed and desorbed from used sorbent

248

at 500 ºC. Thus, the thermal treatment of used sorbents was performed at 500 ºC in air stream. There

249

were three desorption peaks (Figure S9), which indicated that three different mercury species exist

250

on NMO sorbent surface. It was reported that HgCl2 decomposes in the temperature range of 70-220

251

ºC.48 As a result, the desorption peak at 173 ºC was assigned to the decomposition of HgCl2 species. 11



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252

The desorption peaks at 265 ºC and 496 ºC were attributed to the decompositions of

253

Hg2(NO3)2•2H2O and HgCl2O8•H2O species, respectively.49

254

Hg0 breakthrough curves and removal efficiency of NMO sorbent over 10 regeneration cycles is

255

presented in Figure 3a. NMO sorbent exhibited >90% Hg0 removal efficiency. Mercury removal

256

performance of NMO sorbent did not significantly degrade over 10 regeneration cycles. After 10

257

successive cycles, Hg0 removal efficiency of NMO sorbent remained 93.4%. Moreover, no

258

remarkable changes were observed in the crystalline phases of manganese oxides of used NMO

259

sorbent (Figure 1a). After 10 regeneration cycles, NMO sorbent maintained its nanosheet structure

260

(Figures 3b and 3c). The average size of nanosheets was about 89.63 nm, which was close to that

261

(87.98 nm) of nanosheets of fresh sorbent. Therefore, the regeneration process showed little effects

262

on the microcosmic morphology and average size of NMO sorbent. Based on the above analysis

263

results, NMO sorbent showed excellent regeneration performance for Hg0 removal from flue gas.

264 265

4.4. Active Components of NMO Sorbent

266

XRF analysis results indicated that NMO sorbent consists of different metal oxides (Table S1).

267

It was reported that manganese and iron oxides show reaction activity for Hg0 removal.50-52 XRD and

268

XPS analysis results (Figures 1a-c) suggested that theses metal oxides include different species (such

269

as MnO2, Mn2O3, Mn3O4, FeO, Fe2O3, Fe3O4). However, different species showed different

270

reactivity for Hg0 removal. Therefore, density functional theory calculations were performed to

271

identify the active components which are important for Hg0 removal. Adsorption energy and charge

272

transfer can be used to evaluate the adsorption capacity and oxidation performance of materials,

273

respectively.17

274

The adsorption energy, charge transfer, and structural parameters of the most stable structures of 12


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275

Hg0 adsorption over different metal oxides are presented in Figure 4a. Compared with other metal

276

oxides, Mn3O4 shows the highest adsorption energy (−164.86 kJ/mol) for Hg0 capture. Only 0.10 e

277

charges are transferred from Hg atom to Mn3O4 surface, indicating that Hg0 oxidation occurs

278

difficultly on Mn3O4 surface. As a result, the adsorption role of Mn3O4 is much more important for

279

Hg0 removal than its oxidation role. Mn3O4 is an excellent sorbent material rather than catalyst

280

material. Even though iron oxides present relatively higher adsorption energy, larger charge transfer

281

between Hg0 and iron oxides surface can be observed. Thus, iron oxides mainly serve as catalysts for

282

Hg0 removal rather than sorbents.28,38 MnO2 can be used as excellent catalyst material of Hg0

283

removal due to its significant charge transfer.39,50 Mn2O3 shows relatively lower adsorption energy

284

and charge transfer, which indicates that Mn2O3 shows poor adsorption capacity and oxidation

285

performance for Hg0 removal. Based on the above analysis, Mn3O4 is identified as the most active

286

component of NMO sorbent due to its higher adsorption energy and negligible charge transfer.

287

Moreover, iron oxides and MnO2 can also catalyze partial Hg0 oxidation at certain temperatures. The

288

above DFT calculation results indicate that NMO sorbent shows higher adsorption energy for Hg0

289

capture. Thus, this can further verify the higher mercury removal efficiency of NMO sorbent.

290

The atomic-level interaction between Hg0 and Mn3O4 surface was further understood using the

291

partial density of states (PDOS) and three-dimensional (3D) electron density. As shown in Figure 4b,

292

Hg s-orbital is hybridized with Mn s- and d-orbitals at −3.65 eV and −2.74 eV. Meanwhile, Hg

293

d-orbital also strongly interacts with Mn d-orbital at −6.38 eV. As a matter of fact, the orbital

294

hybridization is the result of electron sharing between Hg and Mn atoms (Figure 4c). Moreover, the

295

electron distribution of surface Mn atom is disturbed by Hg0 adsorption (Figure 4d). Thus, the orbital

296

hybridization and electron sharing between Hg and Mn atoms are closely associated with the

297

stronger interaction between gaseous Hg0 and Mn3O4 surface. 13



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298 299

4.5. Chemical Looping Process for Hg0 Green Recovery

300

Based on the excellent regeneration performance of NMO sorbent, a chemical looping process

301

was proposed for the green recovery of Hg0 from flue gas. The chemical-looping recovery

302

technology is shown in Figure 5. NMO sorbent is used as mercury carrier to concentrate and recover

303

Hg0, and is circulated between flue duct and regeneration reactor. The adsorption and desorption

304

reactions of mercury occur in two separate reactors (flue duct and regeneration reactor). Moreover,

305

NMO sorbent regeneration (mercury desorption) is performed in air atmosphere, avoiding the

306

secondary pollution of other reported regeneration gases (such as HCl, H2S).

307

NMO sorbent is injected into the downstream flue gas of ESP to adsorb Hg0. Subsequently, the

308

used sorbent (Hg-laden sorbent) is efficiently collected by a pulse-jet fabric filter (PJFF) baghouse

309

used by the TOXECONTM technology.53 TOXECONTM is a commercial Hg control technology

310

developed by National Energy Technology Laboratory of USA. This technology does not impact fly

311

ash utilization, because most of fly ashes in flue gas are removed by the ESP. The collected sorbent is

312

regenerated to release all mercury adsorbed on sorbent in a regeneration reactor. The regeneration

313

process of used sorbent produces an ultrahigh-concentration Hg0, which can be easily collected using

314

a simple condensation method to avoid the secondary pollution. Meanwhile, the regenerated sorbent

315

can be reused and injected into flue gas to capture Hg0, closing the mercury adsorption-recovery

316

cycle. The chemical-looping recovery technology based on NMO sorbent injection can realize the

317

ultra-low emission and resource utilization of mercury in power plants.

318 319

■ ASSOCIATED CONTENT

320

Supporting Information 14


Page 15 of 29


321

Schematic diagram of the experimental system, experimental conditions of mercury removal,

322

XRF and SEM analysis results of NMO sorbent, surface models of different active components,

323

effects of flue gas compositions on mercury removal efficiency, larger time-scale experiments, TPD

324

spectra of used sorbent.

325 326

■ AUTHOR INFORMATION

327

Corresponding Author

328

*Tel: +86 27 87545526; fax: +86 27 87545526; e-mail address: [email protected].

329

Notes

330

The authors declare no competing financial interest.

331 332

■ ACKNOWLEDGMENTS

333

This work was supported by National Key Research and Development Program of China

334

(2018YFC1901303), Fundamental Research Funds for the Central Universities (2019kfyRCPY021),

335

National Postdoctoral Program for Innovative Talents (BX20180108), and Program for HUST

336

Academic Frontier Youth Team (2018QYTD05).

337 338

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471 472

21



473

Page 22 of 29

Table 1. Experimental Conditions of Mercury Removal by Natural Manganese Ore Sorbent No.

Flue gas compositions

Temperature (ºC)

Ⅰ

N2, 4% O2, 12% CO2, 500 ppm SO2, 300 ppm NO, 10 ppm HCl

50-350

Ⅱ

N2, 0%, 4%, 12% O2

100-300

Ⅲ

N2, 4% O2, 100-300 ppm NO

100-300

Ⅳ

N2, 4% O2, 10-20 ppm HCl

100-300

Ⅴ

N2, 4% O2, 200-1000 ppm SO2

150

Ⅵ

N2, 4% O2, 12% CO2, 5% H2O, 500 ppm SO2, 300 ppm NO, 10

150

ppm HCl 474 475

22


Page 23 of 29


List of Figures Captions

476 477 478

Figure 1. (a) XRD patterns of fresh sorbent and used sorbent after 10 cycles. (b) Mn 2p XPS spectra.

479

(c) Fe 2p XPS spectra. (d) O 1s XPS spectra. Dot denotes experimental data, blue solid line denotes

480

fitting data.

481

Figure 2. Hg0 removal efficiency of NMO sorbent: (a) Effects of reaction temperature. Flue gas

482

compositions: 4% O2, 12% CO2, 500 ppm SO2, 300 ppm NO, 10 ppm HCl, and balance gas N2. (b)

483

Effects of flue gas compositions (O2, NO, HCl). (c) Effect of SO2 concentration at 150 ºC. (d) Effect

484

of H2O concentration at 150 ºC.

485

Figure 3. (a) Hg0 breakthrough curves and removal efficiency of NMO sorbent over 10 regeneration

486

cycles. Flue gas compositions of adsorption experiments: 4% O2, 12% CO2, 500 ppm SO2, 300 ppm

487

NO, 10 ppm HCl, and balance gas N2. Adsorption temperature: 150 ºC. Regeneration gas: air.

488

Regeneration temperature: 500 ºC. (b) SEM image of fresh NMO sorbent. (c) SEM image of used

489

NMO sorbent after 10 successive regeneration cycles.

490

Figure 4. (a) Adsorption energy, charge transfer, and structural parameters of the most stable

491

structures of Hg0 adsorption on different iron and manganese oxides surfaces. (b) PDOS results of

492

Hg0 adsorption on Mn3O4(001) surface. (c) Three-dimensional (3D) and (d) two-dimensional (2D)

493

electron densities of Hg0 adsorption on Mn3O4(001) surface.

494

Figure

495

ultrahigh-concentration Hg0 can be recovered using a simple condensation method.

5.

Chemical

looping

process

for

Hg0

green

496 497 498 23


recovery

from

flue

gas.

The

(a)

2700

Intensity (a.u.)

2250

♦

1800

SiO2 MnO2 Mn2O3 ♦ Mn3O4 Al2SiO5

(b)

Fresh sorbent

Intensity (a.u.)


900

Used sorbent

20

30

499

50 60 2θ (degree)

80

(d)

711.79

4000 712.76

709.82

3600

500

4000 3500 3000

655

650 645 640 Binding energy (eV)

20000

635

529.67

12000 8000

531.03

4000

3400 740

641.67 642.92

16000

710.57

3800

4500

2000 660

90

4200 724.29

Intensity (a.u.)

70

Intensity (a.u.)

(c)

40

653.55

2500

450 10

5500 5000

1350

Page 24 of 29

735

730


710

705

700

0 540

538

536


528

526

501

Figure 1. (a) XRD patterns of fresh sorbent and used sorbent after 10 cycles. (b) Mn 2p XPS spectra.

502

(c) Fe 2p XPS spectra. (d) O 1s XPS spectra. Dot denotes experimental data, blue solid line denotes

503

fitting data.

504 505

24


Page 25 of 29


80

100

60 40 20 0

50

100

506

150 200 250 Temperature (°C)

300

507

60 40 20

100

150 200 250 Reaction temperature (°C)

300

(d) 100

80 60

N2 N2 + 200 ppm SO2 N2 + 500 ppm SO2 N2 + 800 ppm SO2 N2 + 1000 ppm SO2 N2 + 4% O2 + 500 ppm SO2

40 20

0

20

40

60 80 Time (min)

100

Mercury removal efficiency (%)


(c) 100

0

80

0

350

N2 + 4% O2 N2 + 10 ppm HCl

N2 N2 + 300 ppm NO

(b)

Adsorption Oxidation



(a) 100

80 60 40 20 0

120

SFG + 5% H2O N2 N2 + 5% H2O N2 + 3% H2O SFG

0

20

40

60 80 Time (min)

100

120

508

Figure 2. Hg0 removal efficiency of NMO sorbent: (a) Effects of reaction temperature. Flue gas

509

compositions: 4% O2, 12% CO2, 500 ppm SO2, 300 ppm NO, 10 ppm HCl, and balance gas N2. (b)

510

Effects of flue gas compositions (O2, NO, HCl). (c) Effect of SO2 concentration at 150 ºC. (d) Effect

511

of H2O concentration at 150 ºC.

512

25



1.0

3rd

4th

Regeneration cycles 5th 6th 7th

9th

10th

100 80

0.6

60

0.4

40

0.2

20

0.0

0

200

513

400

600 800 Time (min)

1000

1200

1400

0

(c)

(b)

514

8th

94.6% 91.7% 93.5% 93.6% 93.1% 93.3% 93.4% 91.6% 93.0% 93.4%

0.8

Cout/Cin

2nd


1st

(a)

Page 26 of 29

200 nm

200 nm

515

Figure 3. (a) Hg0 breakthrough curves and removal efficiency of NMO sorbent over 10 regeneration

516

cycles. Flue gas compositions of adsorption experiments: 4% O2, 12% CO2, 500 ppm SO2, 300 ppm

517

NO, 10 ppm HCl, and balance gas N2. Adsorption temperature: 150 ºC. Regeneration gas: air.

518

Regeneration temperature: 500 ºC. (b) SEM image of fresh NMO sorbent. (c) SEM image of used

519

NMO sorbent after 10 successive regeneration cycles.

520 521

26


Page 27 of 29


(a) −320

O

Fe

Hg

Hg

Mn

O QHg=0.10 e

Adsorption energy (kJ/mol)

QHg=0.23 e −240

QHg=0.24 e 2.763 Å

2.912 Å

2.771 Å

Hg

QHg=0.25 e 2.785 Å

−160

QHg=0.34 e 2.583 Å

Eads=−135.35 Eads=−102.36

Eads=−69.50

0

Fe3O4(111)

FeO(111)

2.5

Density of states (electrons/eV)

Fe2O3(1102)

-2.74

2.0 1.5

-6.38

0.5

MnO2(110)

Eads=−49.81

Mn2O3(110)

Mn3O4(001)

(c)

Mn-s orbital Mn-p orbital Mn-d orbital

Mn

1.0

Hg

-3.65

0.0 20

2.845 Å

Eads=−91.73

−80

(b)

Eads=−164.86

QHg=0.13 e

Mn

-20

-10

0

10

-6.38

16

Hg-s orbital Hg-p orbital Hg-d orbital

Hg

12 8

20

(d)

-3.65 -2.74

4

Hg

Mn

0

522

-20

-10

0 Energy (eV)

10

20

523

Figure 4. (a) Adsorption energy, charge transfer, and structural parameters of the most stable

524

structures of Hg0 adsorption on different iron and manganese oxides surfaces. (b) PDOS results of

525

Hg0 adsorption on Mn3O4(001) surface. (c) Three-dimensional (3D) and (d) two-dimensional (2D)

526

electron densities of Hg0 adsorption on Mn3O4(001) surface.

527 528 529 530

27



Page 28 of 29

Flue duct (downstream of ESP) Flue gas (Hg)

Flue gas (No)

Ultrahigh concentration Hg 0-rich air Condensation collection

531

5.

Chemical Looping Process

Used sorbent

Regenerated sorbent

Ultralow concentration

Air Regeneration reactor

Hg0

532

Figure

533

ultrahigh-concentration Hg0 can be recovered using a simple condensation method.

Chemical

looping

process

for

green

534 535

28


recovery

from

flue

gas.

The

Page 29 of 29

536


TOC/Abstract Art Flue duct (downstream of ESP) Flue gas (No)

Chemical Looping Process

Used sorbent

Regenerated sorbent

Flue gas (Hg) Ultralow concentration

Ultrahigh concentration

537

Hg0-rich air Condensation collection

Air Regeneration reactor

538

29


Cost-effective Manganese Ore Sorbent for Elemental Mercury

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