A Diamine-Based Integrated AbsorptionâMineralization Process for

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Energy and the Environment

A diamine-based integrated absorption-mineralisation process for carbon capture and sequestration: energy saving, fast kinetics and high stability Bing Yu, Hai Yu, Kangkang Li, Long Ji, Qi Yang, Xiaolong Wang, Zuliang Chen, and Mallavarapu Megharaj Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b04253 • Publication Date (Web): 22 Oct 2018 Downloaded from http://pubs.acs.org on October 24, 2018

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Environmental Science & Technology

Figure 1. Overall CO2 mass transfer co-efficients, KG, and pH as a function of CO2 loadings (ranging from 0.0 to 1.0 mol/mol) at 25℃ in 2 M DEAPA solutions (a); CO2 loadings, pH and KG for rich DEAPA solution (CO2 loading, 1.0 mol/mol) with various CaO dosages (ranging from 0.0 to 4.0 mol/L) for DEAPA regeneration (b). 50x21mm (300 x 300 DPI)

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Figure 2. The FT-IR spectra of 2 M DEAPA solutions with various CO2 loadings, ranging from 0.0 to 1.0 mol/mol (a); the FT-IR spectra of 2 M rich DEAPA solutions (CO2 loading, 1.0 mol/mol) with various CaO dosages, ranging from 0.0 to 4.0 mol/L: FT-IR (b), and the 13C-NMR analysis of 2 M rich DEAPA solutions (CO2 loading, 1.0 mol/mol) various CaO dosages, ranging from 0.0 to 4.0 mol/L (c).


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Figure 3. Regeneration of DEAPA using B-FA: effect of rotation speed on the CO2 loading of rich DEAPA solution at 20°C and an S/L ratio of 300 g/L (a); effect of temperature on the CO2 loading of CO2 rich DEAPA solution at a rotating speed of 150 rpm and an S/L ratio of 300 g/L (b); effect of S/L ratio on the CO2 loading of CO2 rich DEAPA solution at 40°C and at a rotating speed of 150 rpm (c); predicted versus actual values of response surface model for the reduced CO2 loading of rich DEAPA solution (d); 2D contour (e) and 3D response surface plot of reduced CO2 loading of rich DEAPA solution at a rotating speed of 150 rpm (f).



Figure 4. Four different fly ashes were used with their dosages of 500g/L for DEAPA regeneration (a); the cyclic capacities in 5 CO2 absorption and mineralisation cycles (b); TXRF analysis of the leaching behaviour of heavy metals from fly ash before and after carbonation (c).


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Figure 5. Scanning electron micrographs (SEM) of fresh (a) and carbonated B-FA (b) at 1500x and 3000x, associated with EDX analysis (c). 59x30mm (300 x 300 DPI)



Table of Contents


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1

A diamine-based integrated absorption−mineralisation process

2

for carbon capture and sequestration: energy saving, fast

3

kinetics and high stability

4

Bing Yu1, 2, Kangkang Li2, Long Ji2, Qi Yang3, Xiaolong Wang4, Zuliang Chen1, Hai Yu2*,

5

and Mallavarapu Megharaj1*

6

1. Global Centre for Environmental Remediation, The University of Newcastle, Callaghan,

7

New South Wales 2308, Australia.

8

2. CSIRO Energy, 10 Murray Dwyer Circuit, Mayfield West, NSW 2304, Australia.

9

3. CSIRO Manufacturing Flagship, Clayton, Victoria 3168, Australia

10

4. Huaneng Clean Energy Research Institute, Beijing 102209, China

11

*Corresponding author. Ph: +61-2-49606201.E-mail: [email protected].

12

*Corresponding author. Ph: +61-2-49138734; Email: [email protected].

13

Table of Contents: CaO/CaO-rich fly ash Diamine flow

Clean gas

Carbonation sink

Precipitation sink

Flue gas (10-15% CO2)

Gas flow Filter

Rich solvent

14 15

Energy saving

Fast kinetics

CaCO3 rich products

Lean solvent

High stability

(4.76 cm height x 8.47 cm width)

1



16

Abstract: The high energy requirement of amine regeneration and the uncertainty of safe

17

disposal of the captured CO2 remain big challenges to the large-scale implementation of

18

amine scrubbing process for CO2 capture. Mineral carbonation represents a safe and

19

permanent route to capture and store CO2 with net energy production but typically proceeds

20

at a slow reaction rate. Here we present a new integrated absorption and mineralization (IAM)

21

process that couples a diamine based CO2 absorption with fly ash triggered amine

22

regeneration. The technical feasibility of the IAM process using 3-Diethylaminopropylamine

23

(DEAPA) and CaO containing materials such as CaO and coal fly ashes was verified, and the

24

reaction mechanism involved was investigated. It was found that CaO and CaO rich coal fly

25

ash were effective to regenerate DEAPA by the decomposition of DEAPA carbamate species

26

and the formation of calcium carbonate precipitates. Furthermore, the diamine-based IAM

27

process displayed a fast kinetics and a high stability for CO2 sequestration and can reduce the

28

leachability of some heavy metals in the fly ash. These process properties render this

29

diamine-based IAM process a great potential for carbon capture and sequestration

30

applications.

31

Keywords: chemical regeneration, CO2 mineralization, CaO, diethylaminopropylamine.

32 33 34 35 36 37 38

2


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1. Introduction

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Excessive anthropogenic CO2 emission has been considered to be the major contributor

41

towards abnormal climate changes and global warming.1 In particular, the combustion of coal

42

in power generation sectors is one of the largest CO2 emission sources.2 Even so, coal will

43

continue to play a major role in meeting worldwide energy demands in the coming decades.2,

44

3

45

in the coal fired power stations is essential to reduce global CO2 emissions in the transition to

46

cleaner energy sources.4, 5 To date, the conventional amine-based wet-scrubbing remains the

47

leading technology available for large-scale CO2 capture,6-8 and the captured CO2 is then

48

pressurised to ∼100 bar (or more) and finally transported to a storage site for permanent

49

sequestration.9 Despite the great advancement of amine scrubbing technology in recent

50

decades, the amine processes still suffer from several inherent limitations: 1) the extraction of

51

high-temperature steam for solvent regeneration, which reduces the overall efficiency of the

52

power plant by 20-30% and also causes significant disturbance to power station operation;10 2)

53

solvent degradation and materials corrosion, resulting in high costs for solvent makeup and

54

treatment and plant maintenance;11 3) the storage safety issue, which poses a serious threat to

55

people and the environment in case of CO2 leakage.12 Therefore, the development of energy-

56

saving, low cost, and risk free CCS processes is of great significance.

57

Mineral carbonation is an alternative option for CCS which provides a safe and permanent

58

route for reducing carbon emission from coal-fired power plants.13 Mineral carbonation

59

mimics natural chemical transformations of CO2, such as the weathering of noncarbonated

60

minerals to form geologically and thermodynamically stable mineral carbonates.14 Due to the

61

exothermic nature of CO2 mineralisation, it is believed that mineral carbonation has a high

62

economic viability.13 As a cheap and widely available natural material, alkali-rich natural

63

rocks containing Ca- or Mg-bearing minerals such as wollastonite, olivine, and serpentine

In this context, the deployment of post combustion carbon capture and sequestration (CCS)

3



64

have been identified as promising candidates for CO2 mineralization.15 To improve the

65

viability of the process application at an industrial scale, previous researches have mainly

66

focused on the aqueous batch reaction of pre-treated feedstocks and CO2 at elevated

67

temperatures and/or pressures.16 However, the process (i.e., grinding to increase surface area,

68

acid extraction of oxides) and reaction conditions (i.e., high temperatures and pressures)

69

required for mineral carbonation are energy intensive and costly.17 An alternative source of

70

metal oxides is solid alkaline waste materials with high Ca or Mg content (e.g., steel slag,14

71

coal fly ash,18,

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materials for mineral carbonation. Alkaline industrial solid wastes typically have high surface

73

area without the need for extensive pre-processing, and they have high mass fractions of

74

reactive oxides without requiring energy inputs for crushing reactants or increasing

75

temperature and pressure for reaction.17 Therefore, CO2 sequestration using alkaline wastes

76

can potentially reduce the cost significantly. Among all alkaline industrial wastes, coal fly

77

ash is attracting great interests for CO2 mineral carbonation, and a high CaO content in coal

78

fly ash represents a high potential for this fly ash to be used in mineral carbonation. Montes-

79

Hernandez et al.21 found that 1 kg of fly ash containing 4.1 wt. % of CaO can achieve 0.026

80

kg of CO2 sequestration at moderate conditions. Back et al.22 also indicated that that 1 kg of

81

lignite fly ash with 11.5 wt.% of CaO could sequester 0.23 kg CO2 under optimum conditions

82

(i.e., temperature: 75℃, solid to liquid ratio: 50 g/L and CO2 pressure: 0.01 MPa). The

83

estimated annual worldwide generation of coal fly ash was around 750 million tonnes in

84

2012,23 but the coal fly ash is always regarded as a resource that is not beneficially utilized.

85

In addition, a huge amount of coal fly ash has been accumulated around the world since the

86

industrial revolution, indicating that the fly ash is readily available for large-scale CO2

87

sequestration from coal-fired power plants. The implementation of fly ash based mineral

88

carbonation can not only reduce the CO2 emission of the coal-fired power plants, but also

19

cement kiln dust,17 and waste cement20) have been evaluated as raw

4


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increase the stability of fly ash and thus expanding the utilization of fly ash in construction

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materials24, 25 In addition, the fly ash based mineral carbonation should be a good choice

91

where there are barriers for availability of local geological storage sites or CO2 transportation.

92

Recent reported work related to the coal fly ash based mineral carbonation mainly focused on

93

optimizing the process-related parameters such as temperature, CO2 partial pressure, particle

94

size, and sample water content. Ćwik et al.26 studied the carbonation of high-calcium fly

95

ashes in dry and moist conditions and found that the carbonation can occur both with and

96

without water vapor presence, furthermore, a higher temperature or pressure resulted in a

97

higher carbonation efficiency in the temperature range of 160-290°C and under 1-6 bars of

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CO2. Liu et al.25 investigated the direct gas–solid carbonations of coal fly ash and found that

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the CO2 sequestration efficiency can be improved by increasing the temperature and the

100

contents of CO2 and H2O(g) at atmospheric pressure. However, compared to the traditional

101

amine-based CCS process, the fly ash based mineral carbonation proceeds at a slow reaction

102

rate under natural ambient conditions, which limits its practical applications.

103

Recently, increasing attention has been focused on the development of integrated absorption-

104

mineralisation (IAM) process that combines amine scrubbing and mineral carbonation

105

together, to utilize the advantages of each technology and avoid their unfavourable

106

characteristics for CCS.27, 28 For example, Kang et al.28 tested Ca(OH)2 as a pH swing agent

107

for the regeneration of a sterically hindered amine - 2-Amino-2-methyl-1-propanol (AMP)

108

and a calcium source for CO2 sequestration. It was found that Ca(OH)2 was effective for

109

realizing the decomposition of AMPH+ to fresh AMP together with the CO2 mineralization

110

simultaneously. However, sterically hindered amines are known to exhibit slow kinetics for

111

CO2 absorption. If these amines were used in the IAM process, they will strongly limit the

112

kinetic performance of the capture system. In comparison with sterically hindered amines,

113

primary/secondary amines are advantageous of their fast CO2 absorption kinetics. For this 5



114

reason, the primary and secondary amines based IAM processes were also investigated in our

115

prior work and the results showed that CaO-rich fly ash was able to regenerate the CO2

116

loaded primary/secondary amine solutions quickly at mild conditions (temperature of 40oC

117

and atmospheric pressure).29 However, mono-primary/secondary amines typically suffer low

118

CO2 absorption capacities.30 As a result, a lower CO2 cyclic capacity was obtained in a use of

119

primary/secondary amine in IAM process.

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Evidently, a promising IAM process should meet the following requirements: (i) high CO2

121

absorption capacities, (ii) fast CO2 absorption and mineralisation kinetics, (iii) long-term

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cycling stability in CO2 absorption/amine regeneration, and (iv) amine regeneration at low

123

temperatures to minimize regeneration costs. In this case, we envisioned that

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diamines bearing one primary and one tertiary amine group (1°/3° diamines) could be ideal

125

amine absorbents used in the IAM process. 1°/3° diamines could exert the fast CO2

126

absorption kinetics of primary amine group while retaining the intrinsic high absorption

127

capacity of diamines up to 1.0 mole of CO2 per molecule of amine.31 In addition, 1°/3°

128

diamines should have a good stability since they excludes the secondary amine groups which

129

could promote the formation of undesirable nitrosamines, leading to the amine loss along

130

with environmental issues.32, 33 Furthermore, non-thermally driven chemical regeneration of

131

1°/3° diamines processes based on coal fly ash could dramatically reduce the regeneration

132

energy consumption.

133

3-Diethylaminopropylamine (DEAPA) was selected as a representative 1°/3° diamines in this

134

work, which has been confirmed to be a promising absorbent for CO2 absorption in our prior

135

study.7 Herein, an improved IAM process which uses the DEAPA solution for CO2

136

absorption and the CaO-rich fly ash for amine regeneration was proposed. In this process,

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CO2 is first captured from flue gas by DEAPA solution in the absorber, and the CO2 rich

138

DEAPA solution is then introduced to a carbonation reactor for DEAPA regeneration and 6


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CO2 mineralisation by CaO rich fly ash. The regenerated solvent is sent back to the absorber

140

for continuous CO2 absorption. The objectives of this study are (1) to examine the technical

141

feasibility of the DEAPA based IAM process; (2) to determine the performance of fly ash

142

triggered DEAPA regeneration and reveal its underlying mechanism; (3) to evaluate the

143

kinetics, and stability of this new process alongside the heavy metals’ leachability of fly ash

144

after carbonation. We firstly selected the reactive CaO as a feedstock for DEAPA

145

regeneration, and measured the overall mass transfer coefficient (KG), CO2 loading and pH

146

value changes of CO2 rich DEAPA solutions with various dosages of CaO to assess the

147

comprehensive performance of DEAPA-based IAM in CO2 absorption and regeneration.

148

Meanwhile, the major speciation of the CO2-rich and CO2-lean solutions was identified by

149

Fourier transform infrared (FT-IR) spectroscopy and

150

NMR). Then further experiments were carried out using fly ash instead of CaO, and the

151

characterization of fly ash before and after carbonation was investigated to provide a deep

152

understanding of the reaction mechanisms by the use of scanning electron microscope (SEM),

153

energy-dispersive X-ray spectroscopy (EDX), and X-ray diffraction (XRD). Finally, we

154

performed the kinetic study of DEAPA regeneration by four different fly ashes; the stability

155

of the new process was verified in cycling experiments, and the leachability of heavy metals

156

in fly ash before and after carbonation was examined by total reflection X-ray fluorescence

157

spectrometry (TXRF).

158

2. Experimental section

159

2.1. Chemicals

160

3-(Diethylamino)propylamine (DEAPA, ≥99.0%), and calcium oxide (CaO, reagent grade,

161

≥99.9%) were purchased from Sigma-Aldrich. All chemicals were used without further

162

purification. CO2 gas (99.99%) and N2 gas (99.99%) were purchased from Coregas Australia,

13

C nuclear magnetic resonance (13C-

7



163

and all solutions were prepared with deionized water. Four different fly ashes used in this

164

study were collected from the Vales Point power station and Loy Yang thermal power plant

165

in Australia, and Beijing Huaneng thermal power plant and Yuanping power plant (Shanxi

166

province) in China. Here, they are defined as V-FA, L-FA, B-FA and Y-FA, respectively.

167

2.2. Chemical regeneration of DEAPA using CaO or fly ash

168

A series of 2 M DEAPA samples with various CO2 loadings ranging from 0.0 to 1.0 mol/mol

169

were prepared by introducing a pure CO2 gas stream into DEAPA solutions, and used to

170

perform the DEAPA regeneration experiments. In DEAPA regeneration experiments, 100 ml

171

CO2 rich DEAPA samples (1.0 mol/mol) with various CaO dosages ranging from 0.0 to 4.0

172

mol/L were prepared. The solid CaO sample was mixed with the CO2 loaded DEAPA

173

solution with stirring by a magnetic stirrer at 150 rpm at the temperature of 40ºC and

174

atmospheric pressure. After 40 minutes of reaction time, the solids was filtered via a Büchner

175

funnel and the residual liquid samples were collected to measure their mass transfer

176

coefficients (KG), CO2 loadings and pH values. The parameter KG was measured using a

177

wetted wall column8, 30 and the CO2 loadings of amine solutions were determined by the acid

178

titration.34

179

The experimental procedures used for DEAPA regeneration by fly ash were the same as those

180

described for the CaO based DEAPA regeneration above. 100 ml of CO2 enriched DEAPA

181

samples (1.0 mol/mol) were used for fly ash triggered DEAPA regeneration experiments. The

182

ranges of variable parameters for rotating speed of the magnetic stirrer, liquid temperature,

183

and the ratio of solid to liquid were 150 - 750 rpm, 20 - 60°C, and 100 - 500 g/L, respectively.

184

The exploration of kinetic performance for the DEAPA regeneration based on four different

185

fly ashes (V-FA, L-FA, B-FA and Y-FA) with a constant dosage of 500 g/L, was also carried

186

out in this study. The reaction samples (1 mL) were extracted with a syringe at 0, 2, 5, 10, 15,

8


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20, 25, and 30 min, and the suspension was immediately filtered through a 0.2-μm nylon

188

filter for the measurement of CO2 loadings. For the cycling experiments of CO2 absorption

189

and mineralisation by this fly ash based IAM process, a 500 g/L of Y-FA was used. After 40

190

min reaction time, the suspension was immediately filtered, and the remained CO2 loadings

191

in the obtained liquid samples were measured based on the method described above. Then a

192

CO2 rich DEAPA solution was reprepared for the regeneration cycle study by bubbling a pure

193

CO2 gas stream into the recovered lean DEAPA solution. The CO2-absorption and amine-

194

regeneration experiment was repeated for 5 cycles.

195

2.3. Characterization and analysis

196

A Bruker Ascend 400 MHz NMR instrument was employed to determine the speciation in

197

CO2-loaded DEAPA solutions before and after the addition of CaO or B-FA, and the detailed

198

experimental procedures and data process were described in our previous work.30 FT-IR

199

spectroscopy (VERTEX 70, Bruker Co. Ltd.) was used to analyse the structural changes of

200

speciation in the CaO based DEAPA regeneration system. 2 M rich DEAPA solutions (initial

201

CO2 loading: 1.0 mol/mol) with various dosages of CaO ranging from 0.0 to 4.0 mol/L were

202

prepared for FT-IR analysis. In addition, 2 M rich DEAPA solutions with various CO2

203

loadings ranging from 0.0 to 1.0 mol/mol were also measured using FTIR for comparison.

204

The chemical composition, mineralogy and microstructure of B-FA before and after

205

carbonation were evaluated by XRD (Bruker D8), and SEM (JSM-4500, JEOL) respectively.

206

The distributions of elements on the surface of the fresh and carbonated B-FA were

207

determined using EDX spectroscopy equipped to the SEM. In addition, the leaching

208

behaviour of heavy metals for fresh and carbonated B-FA was tested using a Bruker S2

209

Picofox TXRF with a molybdenum anode X-ray tube. The measurement time was 600 s for

210

each TXRF spectrum with 2 mg/L gallium as the internal standard for quantification. A 100

211

ml of CO2 rich DEAPA sample (1.0 mol/mol) with the dosage of 500 g/L fly ash was 9



212

prepared, and the filtrate was collected after the reaction of 24 hours to determine the

213

concentration of dissolved ions in the liquid phase. Furthermore, the control experiment was

214

also carried out which employed the distilled water to replace the CO2 rich DEAPA solution

215

without changing other experimental conditions, and the liquid phase was measured by

216

TXRF for comparison.

217

3. Results and discussion

218

3.1. CaO triggered DEAPA regeneration and CO2 mineralization

219

Our previous studies29, 35 have confirmed that CaO was the main effective component in B-

220

FA for amine regeneration and CO2 mineralization in the IAM system. Prior to the use of B-

221

FA, the active species of CaO was investigated to test the technical feasibility of DEAPA

222

based IAM system and better understand the underlying mechanism for the DEAPA

223

regeneration by B-FA. As shown in Figure 1a, the KG values in the solution decreased with

224

the increasing CO2 loading, owing to the depletion of free reactive DEAPA. In addition, the

225

pH values also declined as the CO2 loading increased, and this trend is likely due to the

226

formation of pronated DEAPA which increased the proton concentration in the system. Upon

227

the addition of CaO (Figure 1b), a pronounced decline in CO2 loadings was observed along

228

with the rise of CaO dosages. In contrast, KG and pH values experienced a gradual increase as

229

the CaO dosage increased. Specifically, the CO2 loading decreased from 1.03 to 0.58

230

mol/mol whilst the KG value increased from 0 to 0.62 mol/m2·s·kPa, and the pH value

231

increased from 8.96 to 12.29 when CaO dosage increased from 0.0 to 2.8 mol/L. Notably, the

232

CO2 loadings, KG and pH values were almost unchanged as the CaO dosage was over 2.8

233

mol/L. This is likely due to the fact that a larger CaO dosage would promote the

234

agglomeration of CaCO3 on the surface of CaO particles, and thus leading to the reduction of

235

the availability of CaO for DEAPA regeneration.

10


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236

In our prior work, the carbamate species was found to be the main product for the reaction

237

between CO2 and a 1°/3° diamine.31 The carbamate can experience the hydrolysis process to

238

form bicarbonate (HCO3−) and free amine through reaction (1).29 The dissolved CaO provides

239

Ca2+ and OH- via reaction (2), and then HCO3− can form CaCO3 precipitate via reaction (3).

240

The precipitation of CaCO3 reduces concentration of HCO3−, hence benefiting the hydrolysis

241

of carbamate species, and promoting the production of the free amine. In addition, the

242

deprotonation process would also be initiated by the OH − from the dissolved CaO to

243

regenerate free amine molecules via reaction (4).

244

R R NCO O + H O

245

CaO + H O ↔ Ca

246

Ca

247

R R N H + OH → R R NH + H O

248

Therefore, it can be inferred here that the CaO played the key role in both the DEAPA

249

regeneration from its carbamate and protonated species and the mineralization of CO2

250

absorbed.

R R NH + HCO

(R1)

+ 2OH

(R2)

+ OH + HCO → CaCO ↓ + H O

KG pH

b

0.8

1

13 12

0.6

11

0.4

10

0.2

9

0

8

pH

0.8

0.6

0.8 0.4

251

0.2

0.4

0.6

0.8

KG pH CO₂ loading

0.4

0

1

0

CO2 loading (mol/mol)

10.5

9

0.6 0.2

KG 0

12

pH

14

C O 2 lo a d in g (m o l/m o l)

1.2 1

K G ( m o l/m 2 * s* K P a )

(R4)

K G (m o l/m 2 * s* K P a )

a

(R3)

0.4

0.8

1.2

1.6

2

2.4

2.8

CaO dosage (mol/L)

252 11


3.2

3.6

4

7.5

6


253

Figure 1. Overall CO2 mass transfer co-efficients, KG, and pH as a function of CO2 loadings

254

(ranging from 0.0 to 1.0 mol/mol) at 25℃ in 2 M DEAPA solutions (a); CO2 loadings, pH

255

and KG for rich DEAPA solution (CO2 loading, 1.0 mol/mol) with various CaO dosages

256

(ranging from 0.0 to 4.0 mol/L) for DEAPA regeneration (b).

257

To gain insight of the mechanism involved in the reaction of CO2-rich DEAPA with CaO,

258

FT-IR spectroscopy was used to identify the major speciation in the CaO triggered DEAPA

259

regeneration process. Figure 2a displays FT-IR spectra of the DEAPA solution with various

260

CO2 loadings, and the major peaks are highlighted by the dotted arrows. Several peaks

261

appeared as a result of the protonation of the amino group, and the formation of carbamate

262

and bicarbonate. As for fresh DEAPA solution, there are two distinct vibration bands at

263

wavelengths 1475 and 1373 cm−1, which were due to −CH3 asymmetric and symmetric

264

rocking.36 After the DEAPA solutions were loaded with CO2, two peaks at 1564 and 1488

265

cm−1 assigned to COO− asymmetric and symmetric stretching appeared and increased with

266

the increase of the CO2 loading.37 In addition, a new peak at about 1330 cm−1 corresponding

267

to the N-COO- stretching vibration appeared and gradually increased as the CO2 loading

268

increased. The appearance of these new signals confirmed the formation of the carbamate

269

while CO2 was absorbed by the DEAPA solutions. In theory, there should be a typical band at

270

around 1530 cm−1 related to the symmetric NH3+ scissoring appeared after CO2 absorbed,37

271

but it was strongly overlapped by a COO− asymmetric stretch at 1564 cm−1. The bicarbonate

272

species was not able to be confirmed from this FT-IR measurement as the characteristic band

273

of bicarbonate at 1354 cm−1 is strongly overlapped by a stretching vibration of carbamate

274

(C−N) stretch at 1330 cm−1 as already reported elsewhere.38 The FT-IR spectra of the rich

275

DEAPA solutions with various CaO dosages are presented in Figure 2b, in which the

276

characteristic signals of carbamate species at 1564, 1488 and 1330 cm−1 were decreased

277

gradually with the increase of CaO dosage. This suggests that CaO was effective for the

12


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decomposition of the formed carbamate in rich DEAPA solution. The declining of the peak at

279

1564 cm−1, the overlapped signals of the protonated DEAPA and the stretching vibration of

280

COO− asymmetric of carbamate, also supports the reduction of carbamate and protonated

281

amine along with the CaO dosage. In fact, our prior work has already confirmed that CaO

282

could provide the hydroxide ion for the deprotonation of the protonated MEA,35 therefore, the

283

deprotonation process could also be expected to occur in this DEAPA based IAM system.

284

The mechanism of CaO triggered DEAPA regeneration was further examined by the

285

NMR spectroscopy as presented in Figure 2c. Prior to dosing CaO, two carbonyl signals

286

were observed within the range of 160 –168 ppm, the stronger one at 164.5 ppm (marked as f)

287

corresponding to the carbamate, and the weaker one at 161.2 ppm (marked as s) representing

288

the bicarbonate.39 The calculated integral ratio of these two peaks demonstrated that the

289

DEAPA-carbamate was the dominant species alongside the formation of little amount of

290

bicarbonate in CO2-rich DEAPA solution. In Figure 2c, the peaks marked as a, b, c, d, and e

291

represent signals correspond to free DEAPA and its protonated format DEAPAH+, and the

292

peaks marked as a', b', c', d', and e' represent five carbons in DEAPA-carbamate and its

293

pronated DEAPA-carbamate species. The integrals of DEAPA/DEAPAH+ signals were much

294

smaller than that of DEAPA-carbamate and its associated protonated species (in the

295

supporting information Figure S1), and this further confirmed that the carbamate species

296

were the dominant components within CO2 rich DEAPA solution. As the addition of 0.40

297

mol/L CaO, the bicarbonate signal significantly reduced in quantity, suggesting that the CaO

298

could decrease the CO2 loading of rich DEAPA solution via the precipitating the bicarbonate

299

in the early stage. Meanwhile, the peak area of a 13C signal f was gradually decreased along

300

with the increase of CaO dosage from 0.0 to 2.0 mol/L, corresponding to the decomposition

301

of carbamate species. Likewise, the peak area of 13C signals a', b', c', d' and e' representing

302

carbamate species also experienced a decreasing trend as the CaO dosage increased. However,

13


13

C-


303

the peaks representing DEAPA/DEAPAH+ gradually increased, and this correlated to the

304

increase of the concentration of free DEAPA when the CaO dosage increased. Based on the

305

results shown in Figure 1b, when the CaO dosage was greater than 0.40 mol/L, the pH

306

values in CO2 rich DEAPA solutions was over 10.04 which was larger than the pKa value of

307

DEAPA (9.50),40 and this indicated that the amount of free DEAPA was much more than that

308

of protonated DEAPA. Further to the results discussed above, the changes of the

309

concentration of each species along with the CaO dosage increased in the CaO triggered

310

DEAPA regeneration system are provided in the supporting information (Figure S1). The

311

results illustrated that the concentration of free DEAPA increased as the CaO dosage

312

increased, while the concentration of DEAPA-carbamate species in CO2 rich DEAPA

313

solutions was gradually decreased. This further validated that the CaO could achieve the

314

decomposition of DEAPA carbamate species into free DEAPA.

315

14


Page 20 of 35

1373

CaO dosage increase

c

1300

CO₂ loading 0.2 mol/mol CO₂ loading 0.6 mol/mol CO₂ loading 1.0 mol/mol

1450

1600

d

a

f

a′

4.0 mol/L 3.2 mol/L

c

d′

1300

CaO dosage 0.4 mol/L CaO dosage 1.2 mol/L CaO dosage 3.2 mol/L

1450

1600

1750

(cm-1) Wavenumbers e

c′

b b′

e′ a H2N

2.8 mol/L

c

d N

b

e

DEAPA (EDAPAH+)

o

2.0 mol/L 1.2 mol/L 0.8 mol/L 0.4 mol/L 0.0 mol/Lf

1564

1488

CaO dosage 0.0 mol/L CaO dosage 0.8 mol/L CaO dosage 2.0 mol/L CaO dosage 4.0 mol/L

1150

1750

Wavenumbers (cm-1)

1330

1564

1475

CO₂ loading 0.0 mol/mol CO₂ loading 0.4 mol/mol CO₂ loading 0.8 mol/mol

1150

b Absorbance (a.u.)

A b sorb ance (a.u .)

1330

a

1475


1488

Page 21 of 35

-o

a′

f N H

c′ b′

d′ N

e′

DEAPACOO- (H+DEAPACOO-)

o s

a′

a

d

d′

c′

c

b′

b

e

e′

-o

s oH

CO32- (HCO3-)

316 317

Figure 2. The FT-IR spectra of 2 M DEAPA solutions with various CO2 loadings, ranging

318

from 0.0 to 1.0 mol/mol (a); the FT-IR spectra of 2 M rich DEAPA solutions (CO2 loading,

319

1.0 mol/mol) with various CaO dosages, ranging from 0.0 to 4.0 mol/L: FT-IR (b), and the

320

13

321

dosages, ranging from 0.0 to 4.0 mol/L (c).

322

3.2. Optimization of fly ash triggered DEAPA regeneration and CO2 mineralisation

323

Based on above discussions, CaO has been confirmed to be effective for DEAPA

324

regeneration and CO2 mineralisation. To further improve the economic and practical

325

feasibility of this IAM process, the CaO rich coal fly ash was studied in this system to

326

explore whether or not the fly ash could play the same role as CaO in the process. Several

327

operating parameters, such as rotating speed of the magnetic stirrer, liquid temperature and

C-NMR analysis of 2 M rich DEAPA solutions (CO2 loading, 1.0 mol/mol) various CaO

15



328

solid/liquid ratio have direct impacts on the DEAPA regeneration of this IAM process. The

329

rotating speed may affect the mass transfer between liquid and gas phases and thus influence

330

DEAPA regeneration as well as CO2 mineralisation. The maximum reduced CO2 loading at

331

different rotating speeds were investigated, and the results shown in Figure 3a suggested that

332

the rotating speed had little influence on this system under the conditions studied. However,

333

different results have been reported in the direct aqueous carbonation of industrial alkaline

334

wastes,24, 41 where CO2 removal efficiency was significantly affected by the rotating speed.

335

This was because that the direct aqueous carbonation involves the mass transfer among gas,

336

liquid and solid phases, and an increase of rotating speed could enhance mass transfer

337

between liquid and gas phases. However, the IAM process is only related to liquid-solid mass

338

transfer, and the avoidance of mass transfer between liquid and gas phases in this IAM

339

process allows the reduced CO2 loading of rich DEAPA solution to be independent on the

340

rotating speed used here. Therefore, only one rotating speed, 150 rpm, was used in the

341

following work.

342

Temperature was another important factor that may have a significant influence on the

343

DEAPA regeneration because it is related to the dissolution of reactive CaO from coal fly ash

344

into the solution and the growth of CaCO3 precipitates.41 As shown in Figure 3b, a gradual

345

increase of the reduced CO2 loading was observed when the temperature was increased from

346

20 to 40℃, and this was due to the speeded leaching of calcium ion from fly ash with an

347

increase in temperature, which would be beneficial to CO2 carbonation reaction. However, as

348

the temperature increased from 40 to 60℃, the value for the reduced CO2 loading was almost

349

unchanged. This is due to the opposite impact of temperature on calcium ion leaching and

350

carbonation reaction. At higher temperatures, more calcium ion will be obtained in the

351

aqueous solution but the exothermic nature of the carbonation reaction will favour the

352

decomposition of carbonate precipitates thermodynamically.

16


Page 22 of 35

Page 23 of 35


353

To further enhance the DEAPA regeneration and CO2 mineralisation in this IAM process, the

354

solid/liquid ratio should be carefully considered since it was directly related to the amount of

355

reactive CaO from the fly ash involved in the DEAPA regeneration. As shown in Figure 3c,

356

it was found that a remarkable increase of the CO2 loading reduction along with the

357

solid/liquid ratio increase from 0.0 to 500.0 g/L. For example, the maximum CO2 loading

358

reduction reached 0.35 mol/mol when the fly ash dosage was 500.0 g/L. The results revealed

359

that CaO rich fly ash plays the same role as that of CaO in the DEAPA regeneration.

360

To visualize the reduced CO2 loading under various operating factors, a response surface

361

methodology model was employed to analyse the experimental data and identify the optimal

362

condition for DEAPA regeneration (Eq. [1]):2, 42

363

Y = -0.022437 - 0.000040A + 0.006112B + 0.000018C + 1.05134E-20AB + 1.66667E-08AC -

364

6.87500E-07BC + 4.02778E-08A² - 0.000059B² + 9.46875E-07C²

365

where Y was the reduced CO2 loading; and A, B, and C were independent coded variables of

366

rotating speed, temperature and solid/liquid ratio, respectively. Figure 3d shows the

367

comparison of the CO2 loading reduction estimated by the fitting equation with the

368

experimental data, which indicates that the calculated results from the fitting equation were in

369

good agreement with the experimental value. In addition, Figure 3a to c also show the

370

overall trend of experimental results was consistent with that of the fitting equation (blue

371

dash lines). Based on the fitting equation, it was also found that the effect of temperature and

372

solid/liquid ratio on DEAPA regeneration were significant (p < 0.0001). As a result, the

373

calculated CO2 loading reduction based on the given temperature and solid/liquid ratio should

374

be also reliable by this fitting equation (as shown in Fig. 3e and f).

17


(E1)


0.15 0.10 0.05

0.25 0.20 0.15 0.10 0.05

100

200

300

400

500

600

700

10

800

375

20

30

40

50

60

e

0.20

0.10

50

70

Reduced CO2 loading (mol/mol)

S/L ratio (g/L)

Predicted

Actual

0.30

Temperature (℃)

Rotating speed (rpm)

Actual vs. Predicted

S/L ratio model

0.40

0.00

0.00

0.00

0.50


0.20

Temperature model

150

250

350

450

f

S/L ratio (g/L)

Temperature (°C)

Temperature (°C)

376

Figure 3. Regeneration of DEAPA using B-FA: effect of rotation speed on the CO2 loading

377

of rich DEAPA solution at 20°C and an S/L ratio of 300 g/L (a); effect of temperature on the

378

CO2 loading of CO2 rich DEAPA solution at a rotating speed of 150 rpm and an S/L ratio of

379

300 g/L (b); effect of S/L ratio on the CO2 loading of CO2 rich DEAPA solution at 40°C and

380

at a rotating speed of 150 rpm (c); predicted versus actual values of response surface model

381

for the reduced CO2 loading of rich DEAPA solution (d); 2D contour (e) and 3D response

382

surface plot of reduced CO2 loading of rich DEAPA solution at a rotating speed of 150 rpm

383

(f).

384

3.4. The kinetics, stability and leaching behaviour of this IAM process

385

It has been confirmed that diamines which contain at least one primary amino group within

386

their molecules feature in a fast kinetics for CO2 absorption in our prior work. 30 Therefore, to

387

verify whether this DEAPA based IAM process exhibits a fast kinetics for CO2 absorption

388

and sequestration, it is necessary to carry out an investigation of the kinetic performance of

18


550

S/L ratio (g/L) Reduced CO2 loading (mol/mol)

Rotating speed model

0.25

d

c

b 0.30

0.30



a

Page 24 of 35

Page 25 of 35


389

the fly ash based carbonation within this IAM process. As shown in Figure 4a, CO2 loadings

390

of DEAPA solutions in the presence of four different fly ashes were all decreased rapidly in

391

the first 5 min, and this indicated that the mineral carbonation in these systems was a rapid

392

process. For example, the CO2 loading for rich DEAPA solution in the presence of Y-FA

393

reduced from 1.03 to 0.66 mol/mol within 5 min. Notably, compared with the performance of

394

Y-FA, the CO2 loading for rich DEAPA solution in the presence of L-FA or V-FA decreased

395

slightly. It was anticipated that a poor performance for DEAPA regeneration by L-FA and V-

396

FA should be caused by their lean CaO content. In fact, the chemical composition for these

397

four different fly ashes was analysed by XRF (Table S1). It was found that CaO content in

398

V-FA, L-FA, B-FA and Y-FA, was 3.6%, 4.2%, 16.4% and 28.4%, respectively, and these

399

results agreed with the performance of each fly ash in DEAPA regeneration. Therefore,

400

selecting a suitable fly ash with rich CaO content is necessary, and a larger content of CaO in

401

fly ash is beneficial to its utilisation for amine regeneration.

402

Given that the IAM process integrating CO2 absorption and DEAPA regeneration should be

403

continuous and cyclic in application, the assessment of the cyclic stability of this process is

404

required. Figure 4b shows the cyclic capacities for this IAM process remained very stable

405

over 5 CO2 absorption and mineralisation cycles, and this certified the technical viability of

406

using the fly ash to achieve DEAPA regeneration and CO2 sequestration.

407

The IAM process may help to alter the chemical stability of the fly ash, and reduce the

408

leachability of heavy metals. Figure 4c shows the TXRF analysis of the leaching behaviour

409

of heavy metals from fly ash before and after carbonation, and it was found that several

410

heavy metal elements in fly ash can be inhibited from leaching into solution after carbonation.

411

For example, a significant reduction of Zn and Sr ions leaching was achieved after

412

carbonation, e.g., from 42.491 and 4.932 mg/L (in fresh fly ash) to 29.379 and 0.581 mg/L (in

413

carbonated fly ash), respectively. The leaching of elements such as Ti, Mn and Rb with trace 19



414

leaching concentrations of 0.041 and 0.048, and 0.032 mg/L from fresh fly ash, respectively,

415

were all reduced to the detection limit after carbonation. This was likely due to the fact that

416

the precipitate of CaCO3 accumulated on the surface of the carbonated fly ash could act as the

417

protective layer to restrict leaching of heavy metals from the inside of the carbonated fly ash

418

into the solution.41 Another possible reason is that these metal cations can react with

419

hydroxide ion and precipitate as metal hydroxides at a higher pH and thus decrease their

420

leachability. In contrast, the leaching concentrations of other three metals (V, Cr and Cu)

421

were increased slightly after carbonation, and this is because that these metal elements may

422

exist in leachate in the form of amphoteric metals which can readily solve under both acidic

423

and alkaline conditions.43 Since Ca isn’t a heavy metal element, the data for leaching

424

concentrations of Ca is not shown in Figure 4c. However, the TXRF analysis here found that

425

the leaching of Ca ions was reduced from 533.61 to 118.3 mg/L after carbonation, and this

426

indicated that the IAM process is beneficial to the upgrade of the fly ash for its potential

427

utilization as construction materials.

428

20


Page 26 of 35

Page 27 of 35


a

1.1

V-FA

CO 2 loading (mol/mol)

1

L-FA

B-FA

Y-FA

0.9 0.8 0.7 0.6 0.5 0.4 0

Cycling capacity (mol/mol)

b

5

10

15

20

25

Time (min)

0.4

30

0.3

0.2

0.1

0 1

c

2

Elem ent Conc. (m g/L )

3

Cycle number

50

4

5

Fresh fly ash Carbonated fly ash

42.491 40

29.379

30

20

10

0

0.041 0

0.048 0

Ti

Mn

0.032 0

4.932 0.58 0.08 0.21

0.255 0

0.053 0.029

-10

429

Zn

Rb

Sr

Cr

V

Cu

430

Figure 4. Four different fly ashes were used with their dosages of 500g/L for DEAPA

431

regeneration (a); the cyclic capacities in 5 CO2 absorption and mineralisation cycles (b);

432

TXRF analysis of the leaching behaviour of heavy metals from fly ash before and after

433

carbonation (c).

434

3.5. Characterizations and analysis

435

To further understand the fly ash triggered DEAPA regeneration mechanism in this study,

436

SEM analysis was carried out in order to investigate the morphological changes occurring as

437

the fresh fly ash was carbonated. As shown in Figure 5a, the surface of fresh fly ash was

21



438

smooth without any obvious precipitates. After carbonation, the surface of particles became

439

rough (Figure 5b) in comparison with the fresh fly ash (Figure 5a), and a layer of white

440

porous coating was observed on the surface of the carbonated fly ash. The composition in the

441

surface of particles before and after carbonation was further identified by EDX analysis. As

442

shown in Figure 5c, the weight percentage for elements C, O, and Ca were found to be

443

increased after the carbonation reaction, which indicated the produced white porous coating

444

on carbonated fly ash corresponded to the calcium carbonate composition. The examination

445

of the changes on the fly ash surface was also conducted using XRD, and the results in

446

comparison with the fresh fly ash are presented in Figure S2. The fresh fly ash was

447

dominated by calcium-bearing crystalline phases, such as Lime (CaO), Calcite (CaCO3),

448

Calcium Sulfate (CaSO4), Srebrodolskite (Ca2Fe2O5), and Gypsum (CaSO4·2H2O). After the

449

carbonation, the peak representing CaO almost disappears and the intensity of CaCO3 peak

450

significantly increases, suggesting that the component of CaO in fresh fly ash was converted

451

to CaCO3 via carbonation reaction. The mechanism of fly ash triggered DEAPA regeneration

452

was further studied using the 13C-NMR spectroscopy, and the stacked 13C NMR spectra of 5

453

samples with various fly ash dosages are presented in Figure S3. It was found that the

454

concentration of the free DEAPA increased while DEAPA-carbamate species decreased,

455

along with the fly ash dosages increased. In summary, SEM-EDX and XRD analysis of fly

456

ash, combined with the 13C-NMR analysis of speciation in solution, further revealed that the

457

CO2 mineralization could be realized via this fly ash-based DEAPA regeneration process.

458

22


Page 28 of 35

Page 29 of 35


a

b

c

Fresh B-FA

CaCO3 No precipitate

Rough surface

Carbonated B-FA

Smooth surface

459 460

Figure 5. Scanning electron micrographs (SEM) of fresh (a) and carbonated B-FA (b) at

461

1500x and 3000x, associated with EDX analysis (c).

462

Acknowledgements

463

The authors wish to acknowledge financial assistance provided through CSIRO Energy. Bing

464

Yu would like to thank the University of Newcastle for his International Postgraduate

465

Research Scholarship, as well as the University of Newcastle Research Scholarship Central

466

25:75 program for supporting his research.

467

Supporting Information Available

468

The detailed information on determining the concentration of each species in the CaO

469

triggered DEAPA regeneration system; XRD patterns of fresh and carbonated B-FA and 13C-

470

NMR analysis of CO2 rich DEAPA solutions with various fly ash dosages. This information

471

is available free of charge via the Internet at http://pubs.acs.org.

472 473

23



474

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