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Shu-Yuan Pan, Yi-Hung Chen, Chun-Da Chen, Ai-Lin Shen, Environmental Science & Michael Lin, andTechnology Pen-Chiis published Chiang by the American Chemical

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CaCO

Cement

(s)

3 Page Environmental 1 of 26 Ca2+ Science & Technology CO32-(aq)

CaOf C3S C S 2

C3A(s) BOFS C3A·CaCO3·11H

Chemical Effect

CaCO3(s)

Enhance initial strength development

ACS Paragon Plus Environment CaCO3(s)

1. High surface area Physical Effect 2. Nucleation sites 3. As inert filler in voids

Environmental Science & Technology

Page 2 of 26

1

High-gravity Carbonation Process for Enhancing CO2

2

Fixation

3

Steelmaking Industry

4

Shu-Yuan Pan,† Yi-Hung Chen,‡ Chun-Da Chen,§ Ai-Lin Shen,§ Michael Lin,

5

and Pen-Chi Chiang*,†,

and

Utilization

Exemplified

by

the

∥

∥

6 7

†

Graduate Institute of Environmental Engineering, National Taiwan University, 71 Chou-Shan Rd., Da-an Dist., Taipei City, 10673 Taiwan (R.O.C.)

8 9

‡

Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, 1, Sec. 3, Zhongxiao E. Road, Taipei City, Taiwan 10608, Taiwan (R.O.C.)

10 11

§

China Steel Corporation, Kaohsiung, 1 Chung Kang Road, Hsiao Kang, Kaohsiung 81233, Taiwan (R.O.C.)

12

∥

13

Carbon Cycle Research Center, National Taiwan University, 71 Fan-Lan Rd., Da-an Dist., Taipei City, 10672 Taiwan (R.O.C.)

14 15

Author Information

16

Corresponding Author

17

* (P.C.C.) Phone: +886-2-23622510, Fax: +886-2-23661642; E-mail: [email protected]

18

1

ACS Paragon Plus Environment

Page 3 of 26


19

Abstract

20

The high-gravity carbonation process for CO2 mineralization and product utilization as

21

green cement was evaluated using field operation data from the steelmaking industry. The

22

effect of key operating factors including rotation speed, liquid-to-solid ratio, gas flow rate,

23

and slurry flow rate on CO2 removal efficiency was studied. The results indicated that

24

maximal CO2 removal of 97.3% was achieved using basic oxygen furnace slag at a

25

gas-to-liquid ratio of 40, with a capture capacity of 165 kg CO2 per day. In addition, the

26

product with different carbonation conversions (i.e., 0%, 17%, and 48%) was used as

27

supplementary cementitious materials in blended cement at various substitution ratios (i.e.,

28

0%, 10%, and 20%). The performance of the blended cement mortar, including

29

physicochemical properties, morphology, mineralogy, compressive strength and autoclave

30

soundness, was evaluated. The results indicated that the mortar with a high carbonation

31

conversion of slag exhibited a higher mechanical strength in the early stage than pure

32

Portland cement mortar, suggesting its suitability for use as high-early strength cement. It

33

also possessed superior soundness to the mortar using fresh slag. Furthermore, the optimal

34

operating conditions of the high-gravity carbonation were determined by response surface

35

models for maximizing CO2 removal efficiency and minimizing energy consumption.

36

Keywords: rotating packed bed; mineralization; basic oxygen furnace slag; supplementary

37

cementitious materials; response surface methodology; energy consumption.

38

2



39

1. Introduction

40

Intensive CO2 emissions and iron/steelmaking slag utilization are key environmental

41

issues in the steelmaking industry. Basic oxygen furnace slag (BOFS) is accepted as a

42

pozzolanic martial and has been utilized in civil engineering for construction. Fresh BOFS

43

or fly ash is often mandatory in the production of high-strength concrete as supplementary

44

cementitious materials (SCM).1 The high strength is frequently achieved through an

45

increase in the cementitious materials content.2 However, a report from the Portland Cement

46

Association indicates that the use of BOFS as SCM may increase the later-age strength of

47

the concrete but also may reduce the early-age strength, compared with Portland

48

cement-only concrete.2 Therefore, the utilization of fresh BOFS as a concrete product or a

49

road base material is still restricted by several barriers including (1) difficulty in grinding,

50

i.e., energy intensive and not cost effective, (2) low soundness because the contents of

51

free-CaO (CaOf) and -MgO (MgOf) may lead to fatal expansion of hardened cement-BOFS

52

paste, (3) potential environmental impacts of heavy metal leaching and high alkalinity, and

53

(4) low strength of blended cements with fresh BOFS, especially in the early stages (< 3

54

days).3-6 In addition, although BOFS contains large amounts of β-C2S and C3S, which are

55

known as the primary strength-contributing hydraulic phases, the cementitious activity of

56

BOFS was still low due to the large crystal size in BOFS.

57

Application of a rotating packed bed (RPB) for accelerated carbonation of alkaline

58

wastes (high-gravity carbonation, or HiGCarb) should be a promising approach to

59

overcoming BOFS treatment issues, while also mineralizing gaseous CO2 from industrial

60

sources.7-9 A superior CO2 fixation efficiency can be achieved using BOFS via the HiGCarb

61

process within a few minutes under ambient pressure and temperature.10, 11 As reported

62

through a small-scale field operation in our previous study,12 the total energy consumption,

63

including BOFS grinding, was ca. 707 kWh/t-CO2 with a capture capacity of 1.6 kg CO2/d. 3


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64

It is expected that the energy consumption can be further reduced by increasing the

65

operation scale of the process in the future. Since a fine particle size of BOFS (i.e., < 125

66

µm) is required in HiGCarb for achieving high CO2 mineralization efficiency, the

67

disadvantage of the energy-intensive BOFS grinding issue may be compensated if the final

68

carbonated product could be utilized as SCM. Meanwhile, to accelerate the commercial step

69

of the HiGCarb process, an integrated performance evaluation of both CO2 mineralization

70

and product utilization as SCM performed in an industrial plant is needed.

71

The objectives of the present study were (1) to evaluate the effect of key operating

72

factors including rotation speed (ω), liquid-to-solid (L/S) ratio, gas flow rate (QG), and

73

slurry flow rate (QS) on ex-situ CO2 removal efficiency in hot-stove gas from a blast

74

furnace, (2) to characterize improvement in physicochemical properties of BOFS through

75

XRF, SEM, XRD, and toxicity characteristic leaching procedure (TCLP), (3) to assess the

76

performance of carbonated BOFS as SCM in blended cement mortar including workability,

77

mechanical properties and durability, and (4) to determine the optimal operating conditions

78

of the HiGCarb process using nonlinear mathematical programming via response surface

79

methodology (RSM) for maximizing CO2 removal efficiency and minimizing energy

80

consumption.

81

2. Experimental

82

2.1 Materials

83

Both the fresh BOFS and cold rolling wastewater (CRW) were obtained directly from

84

plants at the China Steel Corporation (CSC, Kaohsiung, Taiwan). The specific gravity of the

85

fresh BOFS was 3.14 g/cm2, with a mean particle size of 8.73 µm. The fresh BOFS was rich

86

in CaO (~37.2%) and Fe2O3 (~36.2%), with minor components of SiO2 (~10.7%) and MgO

87

(~8.2%). The Ca(OH)2 and CaOf contents in the fresh BOFS were found to be 7.7% and

4



88

0.8%, respectively. In addition, the CO2 source with an average concentration of 30 ± 2%

89

was supplied directly from a hot-stove furnace at CSC; no capture or concentrated processes

90

were required in advance.

91

2.2 Specification of High-gravity Carbonation (HiGCarb) process

92

The HiGCarb process using the BOFS/CRW slurry was performed for ex-situ CO2

93

fixation and waste treatment at the No.3 Blast Furnace Plant in CSC. Table S1 (see

94

Supporting Information) presents the specifications of RPB used in the previous study13 and

95

that of this study. In this study, the packed bed is in horizontal rotation with a mean diameter

96

and height of 46.5 cm and 19.9 cm, respectively. The reaction volume of the packed bed is

97

about 160 times greater than that used in a previsous lab-scale study.12, 13 In addition, the

98

maximal rotation speed of the packed bed is designed as 900 rpm to provide a centrifugal

99

acceleration of up to 2,065 m/s2 (about 210g).

100

2.3 Experimental Design

101

In this study, the central composite design (CCD) method was used for the CO2

102

fixation experiments. Four factors (X1 is rotation speed; X2 is gas flow rate; X3 is slurry

103

flow rate; X4 is L/S ratio) with five levels were designated for establishing quadratic

104

response surface models for CO2 removal efficiency (Y1) and energy consumption (Y2), as

105

shown in Table S2 (see Supporting Information). A total of 40 runs was performed including

106

3 replicates on center, 2 replicates on axis, and 1 replicate on corner. The independent

107

operating factors including rotation speed (158–541 rpm), gas flow rate (0.38–0.77 m3/min),

108

slurry flow rate (0.33–0.56 m3/h), and L/S ratio (10–20) were coded with minimum and

109

maximum levels in CCD.

110

With a proper design of experiments, the response surface methodology (RSM) can be

111

applied to determine the operating conditions for maximal responses (i.e., CO2 removal 5


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Page 7 of 26


112

efficiency) from a statistical point of view. The least-squares estimation was used to

113

determine the model parameters in an approximating polynomial equation, representing the

114

response surface, with a cubic-order model. The analysis of the fitted response surface is

115

generally equivalent to the analysis of the actual system if the fitted surface is a satisfactory

116

estimation of the true response function.

117

2.4 Determination of CO2 Removal Efficiency and BOFS Carbonation Conversion

118 119

In the gas phase, the CO2 removal efficiency (η, %) of the HiGCarb process was calculated by eq 1:

(%) =

, , , − , , , ×% , , ,

(1)

120

where ρco2,i and ρco2,o (g/L) are the CO2 mass density at the temperature of the inflow and

121

exhaust gas streams, respectively; Qg,i (m3/min) and Qg,o (m3/min) were the volumetric flow

122

rate of the inlet gas and exhaust gas, respectively, and Cg,i (%) and Cg,o (%) were the CO2

123

concentration in the inlet and exhaust gas, respectively, which were measured by a portable

124

gas analyzer (PG-350, HORIBA, Japan).

125

In the solid phase, the carbonation conversion (also referred to as carbonation degree)

126

of BOFS was determined by thermal analysis using eq 2, assuming the CaO-containing

127

compounds are the main reaction species:

δCaO =

Δ ⁄ ! $%&' 1 × × ⁄ 1 − ( Δ ! ) $%'2 &')*)&+

(2)

128

where ∆mCO2 is the weight loss due to the CaCO3 decomposition in BOFS samples; m105oC is

129

the dry weight of the sample measured at 105 oC; MWCO2 is the molecular weight of CO2

130

(i.e., 44 g/mol); MWCaO is the molecular weight of CaO (i.e., 56 g/mol); and CaOtotal is the

131

weight fraction of CaO in the fresh BOFS determined by XRF (i.e., 37.2%). The details

6



132

regarding the methods of thermal analyses for alkaline solid wastes can be found in the

133

literature.14, 15

134

Furthermore, qualitative characterization of BOFS before and after carbonation was

135

carried out using scanning electronic microscope (SEM, TM3000, Hitachi, Germany) and

136

X-ray diffraction (XRD, D8 Advance, Bruker, USA). Loss on ignition (LOI) is determined

137

in accordance with ACI 116

138

ignited at 900–1,000 oC.

139

2.4 Evaluation of Carbonated BOFS Product in Blended Cement

16

as the mass loss in percentage of a constant weight sample

140

In this study, standard-sized 50-mm blended cement cubes were prepared using fresh

141

BOFS (denoted as F-BOFS) or carbonated BOFS (denoted as C-BOFS) to partially replace

142

Portland cement at substitution ratios of 10% and 20% by weight. For the control group, a

143

cube paste with 100% Portland cement type I (i.e., no replacement using BOFS) was

144

prepared. The cubes of blended cement were demolded after 24 hr, and then put into a

145

saturated lime solution for 56 days. The compressive strength of blended cement mortar was

146

measured after curing for 3, 7, 28, and 56 days. In addition, two different carbonation

147

conversions of C-BOFS, i.e., 17% and 49%, were prepared to evaluate the effect of

148

carbonation conversion on the workability and strength development of blended cement.

149

A standard flow of 110 ± 5%, as specified by ASTM C 230,17 was maintained by

150

adjusting the quantity of mixing water to maintain consistent workability in the pastes, and

151

to reduce the effect of compaction during casting of the blended cement into the molds. The

152

Chinese National Standards (CNS) 61-R2001

153

evaluate the feasibility of C-BOFS utilization in the blended cement mortar. The

154

requirements for minimal compressive strength of cement mortar at 3, 7 and 28 days in the

155

CNS standard are 1800, 2800 and 4000 psi, respectively. The CNS specification for

156

Portland cement also specifies a maximum autoclave expansion of 0.80%.18 Moreover,

18

for Portland cement type I was utilized to

7


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157

performance of the toxicity characteristic leaching procedure (TCLP) for both F-BOFS and

158

C-BOFS was carried out in accordance with NIEA R201.14C.

159

3. Results and Discussion

160

3.1 Effect of Key Operating Factors on CO2 Fixation

161

Figure 1(a) shows the effect of the rotation speed, L/S ratio and slurry flow rate on CO2

162

removal efficiency in a hot-stove gas using the HiGCarb process. The CO2 removal was

163

found to be significantly affected by the rotation speed, L/S ratio and slurry flow rate. The

164

rotation speed varied from 150 to 550 rpm, offering a centrifugal acceleration variation

165

from 60 to 770 m/s2. The results indicate that the efficiency of CO2 removal moderately

166

increases as the rotation speed increases up to 300–500 rpm. This was attributed to the

167

reduction of mass transfer resistance (i.e., liquid film thickness) by increasing the rotation

168

speed within this range, which was favorable to carbonation reaction. A CO2 removal of

169

93% can be achieved by the HiGCarb process at a rotation speed of 350 rpm under a

170

gas-to-liquid (G/L) ratio of 160.

171

reduction in CO2 removal efficiency was observed, indicating that the extent of reduction in

172

mass-transfer resistances at higher rotation speed was compensated for by a reduction of the

173

retention time. In addition, a superior CO2 removal performance was achieved at an L/S

174

ratio of 10. In the case of a low L/S ratio, the concentration of reactive species in the slurry

175

(e.g., Ca2+) was expected to be higher than that under a high L/S ratio, leading to a greater

176

driving force of chemical potential for carbonation reaction.

177

CO2 removal was relatively higher at a low L/S ratio.

However, when the rotation speed further increased, a

Therefore, the efficiency of

178

In addition, the CO2 removal efficiency was found to increase significantly with the

179

increase of slurry flow rate (QS) from 0.33 to 0.50 m3/h under a rotation speed of 350 rpm.

180

The increase of slurry flow rate (QS) can improve the liquid-side mass transfer, which

8



181

implies that the overall mass transfer resistance of carbonation reaction in the HiGCarb

182

process might be mainly led on the liquid-phase side according to the two-film theory. In

183

addition, at higher gas flow rates, an increase in gas-liquid contact area and a reduction in

184

gas-phase mass transfer resistance occur. However, the CO2 removal efficiency was

185

observed to decrease at a higher gas flow rate (i.e., a higher G/L ratio), as shown in Figure

186

1(b). In other words, it is confirmed that the carbonation of BOFS/CRW slurry in the

187

HiGCarb process is a liquid-side mass transfer controlled reaction. A maximal CO2 removal

188

of 97.3% was achieved at a G/L ratio of 40, with a capture capacity of 165 kg CO2 per day.

189

This suggests that both the rotation speed and G/L ratio should be the key factors for

190

scale-up design of the HiGCarb process.

191

3.2 Improvement on Physicochemical Properties of Alkaline Wastes

192

Table S3 (see Supporting Information) presents the physicochemical properties of

193

CRW before carbonation, mixed with BOFS, and after carbonation. The fresh CRW

194

exhibited highly alkaline (pH ~11.8) and complex in compositions, where the major ions

195

were found to be Na+, K+, Cl-, and SO42-. Trace amounts of Ca2+, Fe3+, Cu2+, Al3+, Mg+, and

196

TIC could be also observed in the fresh CRW. After the BOFS was introduced into the CRW,

197

the pH of the solution increased to 12.5, and both the conductivity and total dissolved solid

198

concentration also increased. The concentration of calcium ions increased significantly,

199

from 4.5 to 1,009 mg/L, due to the hydration of calcium-containing species such as CaOf

200

and Ca(OH)2 in BOFS. It was noted that the CRW could enhance the hydration of calcium

201

species in BOFS 12.

202

Figs S1. (a) and (b) (see Supporting Information) show the SEM images of F-BOFS

203

and C-BOFS, respectively. It was clearly observed that, before carbonation, the entire

204

F-BOFS was smooth without crystallized precipitates on the surface of slag. After

205

carbonation, the C-BOFS exhibited rhombohedral crystals, with a size of 1 to 3 µm, formed 9


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206

uniformly on the surface of the C-BOFS, which should be calcium carbonate precipitates. In

207

addition, the crystallography of BOFS was examined by XRD analysis, as shown in Fig. S2

208

(see Supporting Information), indicating that the formed product was calcite (i.e., CaCO3).

209

This evidence reveals that the gaseous CO2 is successfully mineralized as calcium carbonate

210

precipitates, thereby attaching onto the surface of the BOFS in the course of high-gravity

211

carbonation.

212

Table S4 (see Supporting Information) presents the physicochemical properties of the

213

fresh and carbonated BOFS used in this study. After carbonation, both the density and

214

particle size distribution of BOFS decreased because the reactive calcium species in the

215

fresh BOFS were gradually leached out during carbonation, resulting in exhibiting a less

216

dense and shrinking matrix. For instance, the mean particle size of BOFS decreased from

217

8.73 µm to 6.99 µm when the fresh BOFS was reacted with CO2 to a carbonation

218

conversion of 17%. With an increase in the carbonation conversion of BOFS to 48%, a finer

219

mean particle size was found to be 5.23 µm. This might also be attributed to the formation

220

of fine CaCO3 precipitates after carbonation, thereby leading to an increase in both fineness

221

and specific surface area. In addition, the content of CaOf was also observed to be

222

eliminated after carbonation, which can be beneficial to mitigate the expansion potential of

223

mortar containing BOFS. With the above observations, it suggests that the carbonation

224

conversion of BOFS should affect the changes in physical properties of BOFS such as

225

particle size and specific surface area.

226

On the other hand, Table 1 presents the TCLP results of BOFS before and after

227

carbonation, which indicate that the accelerated carbonation can effectively prohibit the

228

leaching of heavy metals such as Hg, Cr, Cr(VI), Ag, and Ba from solid matrix. With the

229

increase of carbonation conversion to 48%, the leaching concentrations of Hg, Cr and Cr(VI)

230

from BOFS were not detected by ICP-OES. It was observed that the F-BOFS might be

231

classified as hazardous materials due to its high concentration of total chromium metals 10



Page 12 of 26

232

(e.g., greater than 5 mg/L) according to the TCLP results. However, after the HiGCarb

233

process, the carbonated product can potentially be used as green building materials because

234

both the C-BOFS-1 and C-BOFS-2 were above the related standards. Particularly, the

235

leaching concentrations of total Cr metal were significantly reduced, by 99.3%, after

236

carbonation. It was thus concluded that carbonation using the HiGCarb process can

237

effectively improve the physicochemical properties of BOFS to become environmentally

238

friendly products.

239

3.3 Utilization of Carbonated BOFS as Supplementary Cementitious Materials (SCM)

240

The product of the HiGCarb process, i.e., the carbonated BOFS (C-BOFS), was used

241

as SCM to replace Portland cement type I with different substitution ratios (10% and 20%).

242

The compositions of each BOFS paste were presented in Table S5 (see Supporting

243

Information). Figure 2 shows the autoclave soundness expansion of cement mortar with

244

different substitution ratios using F-BOFS and C-BOFS. The expansion capacity of mortar

245

was found to increase as the substitution ratio of F-BOFS increases due to higher CaOf

246

contents in mortar. The maximal expansion was around 0.3% in the case of 20%

247

substitution using F-BOFS. However, using C-BOFS in blended cement instead, the

248

expansion increment was successfully stabilized at a value of less than 0.15%, even in the

249

case of up to 20% substitution of C-BOFS. This should be attributed to the elimination of

250

reactive CaOf content in C-BOFS with relatively stable compounds (e.g., CaCO3), as

251

indicated previously in Table S4, according to the process chemistry (eqs. 3 to 5) proposed

252

in the literature.12, 19, 20 Similar observations were found in the literature 21 that the durability

253

of cement mixtures was greatly enhanced by employing pure limestone powder in blended

254

cement. CaO (s) + CO2(g) + H2O(l)→ CaCO3(s) + H2O(l), ∆H = –178.3 kJ/mol CO2

11


(3)

Page 13 of 26


CaSiO3(s) + CO2(g) + 2 H2O(l) → CaCO3(s) + H4SiO4(aq), ∆H = –87.9 kJ/mol CO2

(4)

Ca2SiO4(s) + 2 CO2(g) + 2 H2O(l)→ 2 CaCO3(s) + H4SiO4(aq), ∆H = –203.5 kJ/mole (5) CO2 255

Figure 3 shows the effect of carbonation conversion and substitution ratio on the

256

compressive strength of cement mortar. In general, the compressive strength of blended

257

cement mortar decreased as the substitution ratio of C-BOFS in cement increased. The

258

mortar using BOFS with higher carbonation conversion (~48%) exhibited superior 3-day

259

compressive strength to that using pure-Portland cement or fresh BOFS. The results

260

indicated that the relative compressive strength of blended mortar replaced by 10% and

261

20% C-BOFS was 136% and 104%, respectively. Particularly, the 10%-C-BOFS mortar

262

with a carbonation conversion of 48% can attain 3-day compressive strengths of over 4,000

263

psi (~27.6 MPa). This might be due to the fact that the formation of fine CaCO3 precipitates

264

on the surface of BOFS could provide favorable sites for nucleation of hydrate products,

265

increase surface area on bleeding, and behave as nano-fillers in voids between the cement

266

grains, thereby leading to a more compact structure in the early stage. In addition, the

267

hydration of C3A can be enhanced by CaCO3 to form calcium carboaluminate

268

(C3A·CaCO3·11H), as described in eq 6, which helps to develop a higher mechanical

269

strength in the early stage:22 2 C3A + 1.5 CaCO3 + 0.5 Ca(OH)2 + 22.5 H → C3A·CaCO3·11H + C3A·0.5CaCO3·0.5Ca(OH)2·11.5H

(6)

270

This suggests that the C-BOFS should be suitable for use as high-early strength (HES)

271

cement, specified by ASTM C39,23 where rapid strength development is desired. However,

272

partial decomposition of the C3A·CaCO3·11H phase might occur to form denser phases of

273

C3AH6 and CaCO3 on the 7th day, as shown in eq 7. This seems to be one of the key factors

274

that led to a decrease in the compressive strength after the 3rd day. 12



C3A·CaCO3·11H → C3AH6 + CaCO3 +5 H2O

Page 14 of 26

(7)

275

The cement mortar with 10% C-BOFS can meet the requirement of compressive

276

strength for 3, 7 and 28 days in the CNS requirement. The compressive strength of mortar

277

for 7, 28 and 56 days moderately decreases as the carbonation conversion of BOFS

278

increases. However, the compressive strength of mortar with 20% substitution failed to

279

meet the CNS requirement for 7 and 28 days, particularly in the case of high carbonation

280

conversion. This is attributed to the low pozzolanic (cementitious) activity of C-BOFS with

281

a relatively higher carbonation conversion caused by the consumption of Ca(OH)2 during

282

carbonation, leading to low strength development at the late stage. In addition, most of the

283

strength-developing properties of cement are controlled by C3S and C2S, which are also

284

partially consumed during carbonation. It thus suggests that the partial Portland cement

285

replacement by C-BOFS should be up to 20%, as higher contents can have a negative

286

impact on the compressive strength of the cement compositions due to the reduced amount

287

of the main hydration products able to induce bonding properties. With the above

288

observations, it was concluded that the encountered barriers of F-BOFS utilization as SCM

289

in blended cement, such as instable expansion property and low early-age compressive

290

strength, can be overcome by the HiGCarb process.

291

3.4 Balancing the Energy Consumption and Capture Capacity

292

In this study, two major unit operations, including (1) BOFS processing (i.e., grinding)

293

and (2) the HiGCarb process, were employed for estimating the required energy utilized for

294

carbon capture and utilization. For instance, according to Bond’s equation,12, 24 the grinding

295

power of BOFS was estimated to be 136.9 kWh/t-CO2, while the energy consumption of the

296

HiGCarb process including air compressors, stirring machines, blowers, pumps, and RPB

297

reactor, measured directly from the existing equipment, was estimated to be 67.8

13


Page 15 of 26


298

kWh/t-CO2. In this case, the scale of the HiGCarb process was operated at a capture

299

capacity of ~170 kg CO2 per day, producing ~690 kg of C-BOFS per day.

300

Figure 4 shows the effect of CO2 removal efficiency on energy consumption and CO2

301

capture capacity of the HiGCarb process. The results indicate that the overall energy

302

consumption of the HiGCarb process increases with the decrease of CO2 capture efficiency.

303

As suggested by the U.S. Department of Energy (DOE), a cost-effective CO2 capture

304

facility should achieve a removal efficiency (η) of 90%, while maintaining 90% (1→2) was estimated to be 268.6 ± 57.9 kWh/t-CO2

307

captured (with a 95% confidence interval), which was lower than the DOE requirement, i.e.,

308

420 kWh/t-CO2.26 The achievable capture capacity was around 150 kg of CO2 per day (1→3

309

→4). In this case, the COE of the HiGCarb process was estimated to be 22.4 ± 0.1%, which

310

met the goal of maintaining 90%

(8)

Min (energy consumption): ψ= f (X1, X2, X, X4) < 250 kWh per t-CO2

(9)

316

where X1 is rotation speed; X2 is gas flow rate; X3 is slurry flow rate; and X4 is L/S ratio.

317

The effect of key operating factors on both η and ψ values was examined with the analysis

318

of variance table (ANOVA) and visualized by a response surface model from a statistical

319

point of view. The ANOVA results of CO2 removal efficiency and energy consumption

320

presented in Table S6 and Table S7 (see Supporting Information), respectively, indicated 14



Page 16 of 26

321

that the developed response models were significant because the p-value was less than 0.05.

322

Figure 5a and 5b present the 2D contour plots of these established mathematical models, as

323

formulated in terms of the operating factors with coded values in eqs 10 and 11,

324

respectively: η (%) = 39.1 + 10.7*X1 + 14.7*X3 – 15.8*X4 – 20.8*X12 + 136.3*X22

(10)

ψ (kWh) = 432.2 – 32.6*X1 + 270.1*X2 + 21.0*X3 + 15.3*X4 + 38.6*X1X3 – 35.1*X1X4 – 59.1*X3X4 + 110.6*X12

(11)

325

The developed quadratic models were significant, with acceptable R2 values, as shown

326

in Table S8 (see Supporting Information). The prediction/actual values of the developed

327

model for both CO2 removal efficiency and energy consumption are also shown in Figures

328

S3a and S3b, respectively (see Supporting Information). The optimum ranges of the

329

operating factor designs were determined graphically by setting optimization objectives for

330

each targeted response and then creating an overlay contour that highlights an area of

331

operability.

332

As shown in Figure 5 (c), in the case of low L/S ratio (i.e., 10), the optimal rotation

333

speed and gas flow rate should be in the ranges of 259.2–410.2 rpm and 0.34–0.45 m3/min,

334

respectively. For high L/S ratio (i.e., 20), however, both the optimal ranges of rotation speed

335

and gas flow rate were relatively narrowed, e.g., 346.0–496.9 rpm and 0.21–0.29 m3/min,

336

respectively, as shown in Figure 5 (d). According to the above analysis, one of the

337

candidates within the above optimal ranges was estimated to be operated at a rotation speed

338

of 400 rpm with a slurry flow rate of 0.36 m3/h at an L/S ratio of 10, corresponding to a CO2

339

removal efficiency of 98% at energy consumption of 196.6 kWh/t-CO2. It was concluded

340

that the developed HiGCarb process exhibited an efficient CO2 capture performance, which

341

was experimentally determined and mathematically validated, for building up a green

342

waste-to-resource (i.e., slag-to-cement) supply chain.

15


Page 17 of 26


343

Associated Content

344

Supporting Information

345

The Supporting Information is available free of charge on the ACS Publications

346

website at DOI:

347

Acknowledgements

348

Sincere appreciation goes to the Ministry of Science and Technology (MOST) of

349

Taiwan (R.O.C.) under Grant Number MOST 104-3113-E-007-001 and MOST

350

103-2911-I-002-596 for the financial support.

351

References

352

1.

Wilson, M. L.; Kosmatka, S. H., Design and Control of Concrete Mixtures. In

2.

High-Performance Concrete, 15, Ed. Portland Cement Association: Washington, DC, 2011; p 299. Caldarone, M. A.; Taylor, P. C.; Detwiler, R. J.; Bhidé, S. B. Guide Specification for

353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373

3.

High Performance Concrete for Bridges; Portland Cement Association: Canada, 2005; p 62. Zhang, T.; Yu, Q.; Wei, J.; Li, J.; Zhang, P., Preparation of high performance blended cements and reclamation of iron concentrate from basic oxygen furnace steel slag.

4.

Resources, Conservation and Recycling 2011, 56, (1), 48-55. Monkman, S.; Shao, Y.; Shi, C., Carbonated Ladle Slag Fines for Carbon Uptake and

5.

Sand Substitute. J. Mater. Civ. Eng. 2009, 21, 657-665. Wu, H. Z.; Chang, J.; Pan, Z. Z.; Cheng, X., Carbonate Steelmaking Slag to

6.

7.

8.

Manufacture Building Materials. Advanced Materials Research 2009, 79-82, 1943-1946. Pan, S.-Y.; Chang, E. E.; Chiang, P.-C., CO2 Capture by Accelerated Carbonation of Alkaline Wastes: A Review on Its Principles and Applications. Aerosol and Air Quality Research 2012, 12, 770-791. Sanna, A.; Uibu, M.; Caramanna, G.; Kuusik, R.; Maroto-Valer, M. M., A review of mineral carbonation technologies to sequester CO2. Chemical Society reviews 2014, 43, (23), 8049-80. Pan, S.-Y.; Chiang, A.; Chang, E.-E.; Lin, Y.-P.; Kim, H.; Chiang, P.-C., An innovative approach to integrated carbon mineralization and waste utilization: A review. Aerosol 16



374 375

9.

and Air Quality Research 2015, 15, 1072-1091. Pan, S.-Y.; Chiang, P.-C.; Chen, Y.-H.; Tan, C.-S.; Chang, E. E., Kinetics of carbonation

376

reaction of basic oxygen furnace slags in a rotating packed bed using the surface

377 378 379 380

coverage model: Maximization of carbonation conversion. Applied Energy 2014, 113, 267-276. 10. Chang, E. E.; Chen, T.-L.; Pan, S.-Y.; Chen, Y.-H.; Chiang, P.-C., Kinetic modeling on CO2 capture using basic oxygen furnace slag coupled with cold-rolling wastewater in a

381 382

rotating packed bed. J Hazard Mater 2013, 260, 937-946. 11. Chang, E. E.; Pan, S. Y.; Chen, Y. H.; Tan, C. S.; Chiang, P. C., Accelerated carbonation

383 384 385 386 387

of steelmaking slags in a high-gravity rotating packed bed. J Hazard Mater 2012, 227-228, 97-106. 12. Pan, S. Y.; Chiang, P. C.; Chen, Y. H.; Chen, C. D.; Lin, H. Y.; Chang, E. E., Systematic Approach to Determination of Maximum Achievable Capture Capacity via Leaching and Carbonation Processes for Alkaline Steelmaking Wastes in a Rotating Packed Bed.

388 389

Environ Sci Technol 2013, 47, (23), 13677-85. 13. Pan, S. Y.; Chiang, P. C.; Chen, Y. H.; Tan, C. S.; Chang, E. E., Ex Situ CO2 capture by

390

carbonation of steelmaking slag coupled with metalworking wastewater in a rotating

391 392

packed bed. Environ Sci Technol 2013, 47, (7), 3308-15. 14. Chang, E. E.; Wang, Y.-C.; Pan, S.-Y.; Chen, Y.-H.; Chiang, P.-C., CO2 Capture by

393

Using Blended Hydraulic Slag Cement via a Slurry Reactor. Aerosol and Air Quality

394 395

Research 2012, 12, 1433-1443. 15. Chang, E. E.; Chiu, A.-C.; Pan, S.-Y.; Chen, Y.-H.; Tan, C.-S.; Chiang, P.-C.,

396

Carbonation of basic oxygen furnace slag with metalworking wastewater in a slurry

397 398

reactor. International Journal of Greenhouse Gas Control 2013, 12, 382-389. 16. ACI Committee 116, Cement and concrete terminology. In American Concrete

399 400 401 402 403

Institute: 2000; Vol. ACI 116R-00, p 73. 17. ASTM C 230-98, Specification for flow table for use in tests of hydraulic cement. In Annual book of ASTM standards, 2001. 18. CNS 61-R2001, Portland cement. In Bureau of Standards, Metrology and Inspection: Taiwan (ROC), 2011.

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19. Santos, R. M.; Van Bouwel, J.; Vandevelde, E.; Mertens, G.; Elsen, J.; Van Gerven, T., Accelerated mineral carbonation of stainless steel slags for CO2 storage and waste valorization: Effect of process parameters on geochemical properties. International

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Journal of Greenhouse Gas Control 2013, 17, 32-45. 20. Gerdemann, S. J.; O'Connor, W. K.; Dahlin, D. C.; Penner, L. R.; Rush, H., Ex situ

409 410

aqueous mineral carbonation. Environ Sci Technol 2007, 41, (7), 2587-2593. 21. Rashad, A. M.; Seleem, H. E. D. H., A Study on High Strength Concrete with Moderate

411

Cement Content Incorporating Limestone Powder. Building Research Journal 2014, 61, 17


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(1).

413

22. Hawkins, P.; Tennis, P.; Detwiler, R. The Use of Limestone in Portland: A

414 415 416

State-of-the-Art Review; Portland Cement Association: USA, 2003. 23. ASTM C39, Standard Test Method for Compressive Strength of Cylindrical Concrete Specimens. In ASTM American Society for Testing and Materials: West

417

Conshohochen, PA, USA, 2001.

418 419

24. Bond, F. C., Crushing and grinding calculations. Part 1. Br. Chem. Eng. 1961, 6, 378-385.

420

25. Matuszewski, M.; Ciferno, J.; Marano, J. J.; Chen, S. Research and Development Goals for CO2 Capture Technology; U.S. Department of Energy: Washington, DC, 2011.

421 422

26. Datta, S.; Henry, M. P.; Lin, Y. J.; Fracaro, A. T.; Millard, C. S.; Snyder, S. W.; Stiles,

423

R. L.; Shah, J.; Yuan, J.; Wesoloski, L.; Dorner, R. W.; Carlson, W. M., Electrochemical

424 425 426

CO2Capture Using Resin-Wafer Electrodeionization. Ind Eng Chem Res 2013, 52, (43), 15177-15186.

18



427

Figure Captions

428

Figure 1. Influence of (a) rotation speed, L/S ratio and slurry flow rate, and (b)

429

gas-to-slurry (Q/S) ratio on CO2 removal efficiency in hot-stove gas using

430

HiGCarb process

431

Figure 2. Autoclave soundness expansion of cement mortar with different substitution ratios

432

using fresh BOFS (F-BOFS) or carbonated BOFS (C-BOFS) with a carbonation

433

conversion of 17%.

434

Figure 3. Effect of carbonation conversion and substitution ratio on compressive strength.

435

The numbers in terms of percentage represent relative compressive strength to

436

Portland cement type I mortar. Carbonation conversion of F-BOFS = 8.8%,

437

carbonation conversion of C-BOFS-1 = 17.0%, carbonation conversion of

438

C-BOFS-2 = 47.9%.

439

Figure 4. Effect of CO2 removal efficiency on energy consumption and capture capacity of

440

HiGCarb process. Both BOFS grinding and HiGCarb (air compressors, stirring

441

machines, blowers, pumps, and RPB reactor) processes were considered in

442

energy consumption calculation.

443

Figure 5. (a) 2D contour; (b) 3D response surface plot of CO2 removal efficiency

444

(conditions of predicted maximal efficiency: 471 rpm; QG=0.33 m3/min; Qs= 0.36

445

m3/h, and L/S= 10.7). Optimal operating conditions of HiGCarb process in the

446

cases of (c) low and (d) high L/S ratio

447 448

19


Page 20 of 26

Page 21 of 26

449 450


Figure 1. (a) 100

CO2 Removal Efficiency (η , %)

90

Operating conditions CO2 Concn = 30.3 + - 0.2% 3 QG = 0.38 m /min

80 70 60 50 L/S = 13, Qs = 0.33 m3/h L/S = 15, Qs = 0.33 m3/h L/S = 20, Qs = 0.33 m3/h L/S = 20, Qs = 0.40 m3/h L/S = 20, Qs = 0.50 m3/h

40 30 20 10 100

200

400

500

600

Rotation Speed (rpm)

451 452

300

(b) 100

CO2 Removal Efficiency (η , %)

3

QS = 0.33 m /h 3 QS = 0.43 m /h QS = 0.50 m3/h QS = 0.53 m3/h

90 80 70 60 50 40 30

Operating conditions Rotation speed = 400 rpm L/S ratio = 20 mg/L

40

453 454

50

60

70

80

90

100

110

120

Gas-to-Slurry Ratio (-)

455

20


130

140


456

Figure 2. 0.35 F-BOFS (CaOf=0.8%, Ca(OH)2=7.7%, η=6.7%) C-BOFS (CaOf=0%, Ca(OH)2=0%, η=17.0%)

Autoclave Soundness (%)

0.30

10% substitution 20% substitution 30% substitution

0.25 0.20 0.15 0.10 0.05 0.00

F-BOFS

C-BOFS

457 458

21


Page 22 of 26

Page 23 of 26

459


Figure 3. 4500

4500 136%

Compressive Strength (psi)

3 Days

7 Days

100%

4000

4000

3500

3500

98% 90%

87%

10%-sub

84% 100%

3000

104%

91% 83%

3000

68%

CNS 61-R2001

10%-sub

2500

2500 79% 75%

20%-sub

20%-sub

2000

2000

53%

CNS 61-R2001

1500

1500 0

10

20

30

40

0

50

28 Days

20

30

40

50

56 Days

100%

6000

6000

Compressive Strength (psi)

10

6500

6500

100% 91%

5500

5500 88%

5000

5000

78%

79%

4500 73%

74%

73%

4500

75%

10%-sub

10%-sub

4000

4000 CNS 61-R2001

65% 65%

3500

3500 20%-sub

20%-sub

3000

50%

3000 No standards required

2500

2500 0

10

20

30

40

50

0

10

20

30

40

Carbonation Conversion (%)

Carbonation Conversion (%) 10% C-BOFS Substitution

52%

20% C-BOFS Substitution

460 461

22


Limits by Standards in Taiwan

50


Figure 4. 200

Process Energy Consumption (kWh/t-CO2)

550 500

Process scale QG = 0.38-0.80 m3/min

180

450 4 ○

400

140 350

3 ○

120

300

100

2 ○

250 200

80 1 ○

150

60

Energy Consumption Capture Capacity

100

40

50 20

463

160

30

40

50

60

70

80

CO2 Removal Efficiency (%)

464 465

23


90

100

CO2 Capture Capacity (kg/d)

462

Page 24 of 26

Page 25 of 26


466

Figure 5.

467

(a)

(b)

(c)

(d)

468 469

470 471

24



Page 26 of 26

472 Table 1. Toxicity characteristic leaching procedure (TCLP) results of fresh and carbonated BOFS comparable to limits by regulations in Taiwan Values Items

Unit

Carbonation conversion (δCaO)

473 474

F-BOFS

Limits by regulations in Taiwan C-BOFS-1

C-BOFS-2

Utilization Product

Green Building Materials a

Hazardous Materials b

%

8.8

17.0

47.9

-

-

-

Mercury and Mercury compounds

Hg

mg/L

0.0009

0.0005

ND (

High-Gravity Carbonation Process for Enhancing ... - ACS Publications

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