Evaluation of factors affecting mineral carbonation of CO2 using coal fly ash in aqueous solutions under ambient conditions

Chemical Engineering Journal 183 (2012) 77–87

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Evaluation of factors affecting mineral carbonation of CO2 using coal fly ash in aqueous solutions under ambient conditions Ho Young Jo ∗ , Jin Ha Kim, Young Jae Lee, Meehye Lee, Suk-Joo Choh Department of Earth and Environmental Sciences, Korea University, Anam-dong, Seongbuk-gu, Seoul 136-713, Republic of Korea

a r t i c l e

i n f o

Article history: Received 7 October 2011 Received in revised form 5 December 2011 Accepted 7 December 2011 Keywords: CO2 Sequestration Coal fly ash Mineral carbonation Aqueous carbonation

a b s t r a c t This study examined the factors affecting mineral carbonation to sequester CO2 using coal fly ash (CFA) in aqueous solutions under ambient temperature and pressure conditions. Serial extraction and carbonation tests were conducted on CFA obtained from a coal-fired power plant as the following conditions were varied: solid dosage, CaO content, CO2 flow rate, and solvent type. The solid dosage, CaO content, and solvent type affected the Ca extraction efficiency, which was well correlated with the carbonation efficiency. In addition, at a given Ca extraction efficiency, the CO2 flow rate and the solvent type affected the rate and extent of CFA carbonation. Based on the study results, the CO2 sequestration capacity of CFA under ambient temperature and pressure conditions was approximately 0.008 kg of CO2 per 1 kg of CFA at the experimental test conditions (CaO content: 7 wt.%, solid dosage: 100 g/L, CO2 flow rate: 2 mL/min, and solvent: DI water). © 2011 Elsevier B.V. All rights reserved.

1. Introduction Carbon dioxide (CO2 ) emitting from the burning of fossil fuel by humans might be primarily responsible for recent global warming. Among various CO2 sequestration methods to reduce the CO2 concentration in the atmosphere, mineral carbonation, which was first proposed by Seifritz [1], has recently attracted strong research attention because CO2 can be sequestered permanently without potential underground reservoirs [2]. Mineral carbonation involves sequestering CO2 by converting it into a thermodynamically stable form of carbonate minerals by reacting CO2 and alkali rich materials directly or indirectly. Alkali-rich natural rocks containing Ca- or Mg-bearing minerals such as wollastonite, olivine, and serpentine have been extensively evaluated as raw materials to sequester CO2 via the formation of carbonate minerals [2–4]. In addition to natural rocks, industrial solid wastes or byproducts with high Ca or Mg content (e.g., steel slag, fly ash, red mud, cement kiln dust, and waste cement) have been evaluated as raw materials for mineral carbonation [5–14]. Mineral carbonation using industrial solid wastes or by-products has the following three benefits: (1) Ca ions contained in solid wastes are more reactive to CO2 than Mg ions that are thermodynamically stable in the natural rocks, (2) solid wastes generally have high specific surface area, resulting in high reactivity to CO2 , and (3) solid wastes formed

∗ Corresponding author. Tel.: +82 2 3290 3179; fax: +82 2 3290 3189. E-mail address: [email protected] (H.Y. Jo). 1385-8947/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.cej.2011.12.023

in high temperature conditions are thermodynamically unstable, resulting in their high dissolution capacity [6]. Coal fly ash (CFA), which is produced from coal-fired power plants that are a major CO2 emission source, is one of the possible low-cost industrial solid by-products for mineral carbonation. In general, CFA mainly contains silica (SiO2 ), alumina (Al2 O3 ), lime (CaO), and iron oxide (Fe2 O3 ), with a trace amount of unburned carbon. Even though the physical and chemical properties of CFA vary depending on the mineralogy and particle size of the raw material and the type of coal burning process, several studies have shown CFA to be a good raw material for mineral carbonation of CO2 due to its high CaO content [7,11,15,16]. For example, Back et al. [8] conducted kinetic tests on lignite fly ash, which contained 11.5 wt.% CaO, using a CO2 gas flow rate through the reactor of 1 L/min in aqueous solutions under various conditions (e.g., solid to liquid ratio: 25–100 g/L, CO2 concentration: 10–30 vol.%, stirring rate: 300–600 rpm, and temperature: 25–75 ◦ C). Their study showed that the carbonation rate increased with increasing CO2 concentration, stirring rate and temperature, and with decreasing solid to liquid ratio. Regarding the temperature effect, the carbonation rate was maximized at 75 ◦ C, probably due to the elevated extraction capacity of Ca ions from the lignite fly ash. The study indicated that the CO2 sequestration capacity of the lignite fly ash was 0.23 kg of CO2 per 1 kg of lignite with 11.5 wt.% CaO at appropriate conditions (i.e., solid to liquid ratio: 50 g/L, CO2 pressure: 0.01 MPa, temperature: 75 ◦ C). Montes-Hernandez et al. [12] also suggested that 1 kg of CFA with 4.1 wt.% of CaO can sequester 0.026 kg of CO2 at moderate temperature (∼30 ◦ C) and pressure (10–40 bars) conditions. Recently, Reddy et al. [17] showed that

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H.Y. Jo et al. / Chemical Engineering Journal 183 (2012) 77–87

Table 1 Physical properties and chemical composition of coal fly ash (CFA). Material

Specific gravity

CFA Limec

2.41 –

Paste pH

12.4 –

Specific surfacea (m2 /g)

4.73 –

Chemical composition (wt.%) SiO2

Al2 O3

Fe2 O3

CaO

MgO

Na2 O

Others

LOIb

50.3 ND

21.8 ND

7.8 ND

7.2 98.0

1.5 0.5

0.3 0.5

2.5 1.0

8.6 ND

ND: non detectable. a Measured by the BET method. b Loss on ignition. c Lime composition was provided by the manufacturer (Junsei Chemical, Japan).

CO2 , SO2 , Hg, and some heavy metals could be removed from flue gas emitted from a coal-fired power plant using CFA by the mineral carbonation processes. In Korea, more than 4 million tons of coal ash is generated annually from coal-fired power plants. Coal ash is generally mixed with water and transported to ash lagoons for disposal. Although such ash lagoons may create environmental pollution without appropriate pollution control systems [18,19], they can be used for sequestering CO2 from flue gas emitted from a coal-fired power plant by in situ mineral carbonation processes. Thus, in this study a conceptual in situ mineral carbonation method is proposed for sequestrating CO2 in an ash lagoon. The proposed method consists of the following three steps: (1) mixing coal ash obtained from a coal-fired power plant with water, (2) sluicing the mixture into the ash lagoon, and (3) injecting flue gas from a coal-fired power plant through a pipe into the bottom of the ash lagoon at a flow rate favorable for carbonation. The proposed method allows CO2 to be sequestered at a site relatively close to a coal-fired power plant without (1) having to transport large volume of reactants for carbonation over long distances, (2) having to capture CO2 from flue gas, and (3) requiring energy inputs for crushing reactants and for increasing temperature and pressure. Therefore, CO2 sequestration can probably be achieved at a lower cost than in comparable methods. The main study objective was to ensure the viability of the proposed method regarding the CO2 sequestration capacity of CFA under ambient temperature and pressure conditions. The potential of CFA as a raw material was evaluated and the factors affecting the degree of carbonation for sequestering CO2 by indirect and direct aqueous carbonation methods under ambient temperature and pressure conditions were investigated. Because the leaching capacity of alkaline elements and the chemistry of the leachate may greatly affect the carbonation of CO2 , extraction tests were conducted on CFA to evaluate the elemental extraction characteristics as the following conditions are varied: solid dosage, CaO

content, CO2 flow rate, and solvent type. The reagent-graded lime (CaO) was added to CFA to evaluate the influence of CaO content on extraction and carbonation because the CaO content generally varies from 1 wt.% to 40 wt.% depending on the type of coal burned in coal-fired power plants. Carbonation tests were also conducted on supernatants without solid particles (indirect carbonation) and on slurry with solid particles (direct carbonation), which were obtained directly from the extraction tests, by continuously flowing CO2 at various flow rates under ambient temperature and pressure conditions. 2. Materials and methods 2.1. Materials Coal fly ash (CFA) used in this study was obtained from a coalfired power plant in Korea. The physical and chemical properties of CFA are shown in Table 1. The specific gravity of the CFA, which was measured following ASTM C 618 [20], was 2.41. The pH of the paste, which was measured on CFA paste mixed with deionized (DI) water, was 12.4. The specific surface area of 4.73 m2 /g was determined by the Brunauer–Emmett–Teller (BET) method (Model ASAP 2020) [21]. The X-ray fluorescence (XRF) results showed that CFA was mainly composed of SiO2 (50.3 wt.%), Al2 O3 (21.8 wt.%), Fe2 O3 (7.8 wt.%), and CaO (7.2 wt.%) (Table 1). CFA was classified as a class F fly ash in accordance with ASTM C 618-99, as the combined SiO2 , Al2 O3 , and Fe2 O3 content exceeded 70 wt.%. The morphology of CFA was found to be mostly spherical particles with an average size of 20 ␮m by using field emission scanning electron microscopy (FE-SEM, Hitachi S-4300) equipped with energy dispersive X-ray analysis (EDX, Horiba EX-20) in the Engineering Laboratory Center at Korea University. The reagent-graded CaO (Junsei Chemical Co., Ltd., Japan), which mostly consisted CaO (98.0 wt.%) (Table 1), was used to obtain a desired CaO content of CFA.

Table 2 List of experiments. No. of test

Type of solvent

1 2 3 4 5 6 7 8 9 10 11 12 13

DI-water DI-water DI-water DI-water DI-water DI-water Tap-water DI-water DI-water DI-water DI-water DI-water DI-water

Extraction

Carbonation

No. of tests replicated

CaO content (wt.%)

Solid dosage (g/L)

Type of leachate

CO2 flow rate (mL/min)

7 7 7 7 7 7 7 14 27 34 7 7 7

100 140 200 100 140 200 200 200 200 200 200 200 200

Slurry Slurry Slurry Supernatant Supernatant Supernatant Supernatant Supernatant Supernatant Supernatant Supernatant Supernatant Supernatant

4 4 4 4 4 4 4 4 4 4 2 8 10

1 1 1 1 1 3 2 1 2 1 2 2 4

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Table 3 Results of chemical analyses on the leachates from the extraction tests.

pH EC (mS/cm) Al (␮g/L) As (␮g/L) Ca (mg/L) Cd (␮g/L) Cr (mg/L) Cu (␮g/L) K (mg/L) Na (mg/L) Ni (␮g/L) Mg (mg/L) Pb (␮g/L) Se (␮g/L) Sr (mg/L) Zn (␮g/L)

Before extraction

After extraction

Tap water

Solid dosage (CaO content: 7 wt.%, DI water)

7.4 0.5 <0.1a <0.1 200 <0.1 0.1 106 1.8 6.4 21 6.5 3 15 0.1 32

CaO content (solid dosage: 200 g/L, DI water)

100 g/L

140 g/L

200 g/L

14%

27%

34%

11.7 4.7 <0.1 <0.1 503 [5003]b <0.1 0.2 <0.1 2.5 6.7 <0.1 <0.1 <0.1 <0.1 10.1 <0.1

11.6 5.5 238 <0.1 516 [7004] <0.1 0.1 <0.1 5.1 7.6 <0.1 3.0 <0.1 <0.1 23.1 <0.1

12.0 5.7 86 150 688 [10 006] 2 0.3 1 4.3 9.0 16 0.1 1 42 11.4 25

13.0 10.4 101 <0.1 1073 [20 011] 1 0.2 <0.1 7.9 10.6 44 0.1 6 27 19.0 6012

12.7 9.7 53 <0.1 106 [38 594] 2 0.2 2 6.1 9.3 47 0.1 4 27 19.1 22

12.8 9.9 <0.1 <0.1 929 [48 599] 2 0.2 2 6.0 8.0 47 0.5 3 24 14.7 28

Tap water (solid dosage: 200 g/L, CaO content: 7 wt.%)

12.4 5.2 103 1 640 [10 006] 1 0.3 <0.1 5.5 15.5 23 0.6 2 23 3.6 6

All values are average values of the given tests. a < represents below detection limit. b [] represents total Ca concentration at a given condition.

2.2. Extraction tests

Ca extraction efficiency (%)

15

10

5

80

120

140

160

180

200

220

15 DI water Solid dosage: 200 g/L 10

5

(b)

Ca extraction efficiency

5

10

15

20

25

30

35

CaO content (wt.%)

MCa-extraction (g) (1)

where MCa-extraction is the mass of Ca in the extract after the extraction procedure, MT is the total mass of the material used in the extraction test, CaO content is the CaO content in CFA determined by the XRF analysis, MWCa is the molecular weight of Ca (40.087 g/mol), and MWCaO is that of CaO (56.077 g/mol).

Ca extraction efficiency (%)

15

MT (g) × (CaO content (wt.%)/100) × (MWCa /MWCaO )

× 100

100

Solid dosage (g/L)

0

=

DI water CaO content: 7 wt.%

(a) 0

Ca extraction efficiency (%)

Extraction tests were conducted on CFA to evaluate the elemental extraction characteristics under the following various conditions: solid dosage: 100–200 g/L, CaO content: 7–34 wt.%, and extraction solution: DI water and tap water. The specific amount of reagent-graded CaO was added to CFA with approximately 7 wt.% CaO content to obtain the specific CaO content (i.e., 14, 27, and 34 wt.%). Table 2 shows a list of extraction tests. CFA was air-dried and then placed at a specific solid dosage in a 1.5-L Teflon reactor with an extraction solution. The slurry mixture of CFA and extraction solution was then stirred using a motor-driven blender in the Teflon reactor at 45 rpm for 24 h at room temperature. After mixing, the slurry was allowed to settle for 10 min, and the pH and electrical conductivity (EC) were measured immediately using pH and EC probes, respectively. The slurry was filtered through a 0.2␮m filter paper (ADVANTEC® ), stored in a polyethylene bottle, and acidified with a nitric acid solution to pH < 2 for chemical analysis. The Ca extraction efficiency was determined by using the Ca concentrations of the leachates obtained from the extraction tests to investigate how the test conditions affected the Ca extraction capacity. The Ca extraction efficiency is the ratio of the amount of extracted Ca ions to the total amount of Ca ions of the material, according to the following equation:

CaO content: 7 wt.% Solid dosage: 200 g/L 10

5

(c) 0

2.3. Carbonation tests Carbonation tests were conducted on supernatants without solid particles (indirect carbonation) and on slurry with solid

DI water

Tap water

Fig. 1. Ca extraction efficiencies as a function of (a) solid dosage, (b) CaO content, and (c) solvent.

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13

13 140 g/L

12

12 100 g/L 100 g/L pH

11

pH

11 200 g/L 10

10

9

9

140 g/L

Supernatant CaO content: 7 wt.% CO flow rate: 4 ml/min

(a) 20

0

40

60

Slurry CaO content: 7 wt.% CO flow rate: 4 ml/min 2

(b)

2

8

8

80 100 Time (min)

120

140

160

20

0

9

40

60

80 100 Time (min)

120

140

160

9 Supernatant CaO content: 7 wt.% CO flow rate: 4 ml/min

8 7

8 7

2

6 EC (mS/cm)

6 EC (mS/cm)

200 g/L

140 g/L

5 4 100 g/L

3

5

100 g/L

4 200 g/L

3

2

2 200 g/L

Slurry CaO content: 7 wt.% CO flow rate: 4 ml/min

140 g/L

1

1 (c)

2

(d)

0

0 0

20

40

60

80 100 Time (min)

120

140

160

0

20

40

60

80 100 Time (min)

120

140

160

Fig. 2. Temporal pH changes in the carbonation tests on (a) the supernatant without solid particles and (b) the slurry with solid particles, and temporal EC changes in the carbonation tests on (c) the supernatant without solid particles and (d) the slurry with solid particles at various solid dosages.

particles (direct carbonation), which were obtained from the extraction tests by continuously flowing CO2 gas under ambient temperature and pressure conditions. The slurry, which was a mixture of the extraction solution and CFA, was obtained after terminating the extraction tests. The supernatants were obtained by filtering the slurries using a 0.2-␮m membrane filter (ADVANTEC® ) after terminating the extraction tests under various conditions. Table 2 shows a list of carbonation tests. The supernatant or slurry of 0.5–1.0 L was poured into an opened 1-L Erlenmeyer flask at a given solid dosage. The pH and EC probes and a CO2 injection tube were placed in the Erlenmeyer flask. CO2 gas (99.9 vol.% purity) was injected continuously from a gas cylinder into the bottom of the Erlenmeyer flask at various flow rates (4–10 mL/min). The flow rate was controlled by a regulator (KOFLOC – RK1600R). CO2 gas was bubbled through the supernatant or slurry in the bottom of the Erlenmeyer flask. During CO2 injection, the pH and EC were measured automatically using a personal computer with a hyperterminal program. CO2 injection was stopped when the pH of the supernatant or slurry reached approximately 8.3, in order to prevent the dissolution of precipitated calcite

(CaCO3 ) that might be saturated in solution at pH 8.3 at atmospheric CO2 pressure [22]. After stopping CO2 injection, the leachate obtained from the carbonation tests was filtered through a 0.2-␮m filter paper (ADVANTEC® ). The filtered sample was stored in a polyethylene bottle and acidified with a nitric acid solution to pH < 2 for chemical analysis. The precipitate obtained from the filtration was dried in a desiccator and stored in a sealed 50-mL vial for material characterization. The carbonation efficiency was determined using the Ca concentrations in filtered samples obtained from the extraction and carbonation tests and the CaO content of the materials obtained from the XRF analysis. According to the following reactions (2) and (3), the carbonation efficiency can be calculated according to Eq. (4): CaO + H2 O → Ca(OH)2

(2)

Ca(OH)2 + CO2 → CaCO3 (s) + H2 O

(3)

H.Y. Jo et al. / Chemical Engineering Journal 183 (2012) 77–87

13

81

13 34 wt.%

12

12

27 wt.% 11

2 ml/min

14 wt.%

10

4 ml/min

pH

pH

11 7 wt.%

8 ml/min 10 10 ml/min

9 9

8

Supernatant Solid dosage: 200 g/L CO flow rate: 4 ml/min

(a)

Supernatant CaO content: 7 wt.% Solid dosage: 200 g/L

(b)

2

7

8 0

20

40

60

80 100 Time (min)

120

140

0

160

12

20

40

60 Time (min)

80

100

120

6

34 wt.%

10

5 2 ml/min

27 wt.% 4

6 Supernatant Solid dosage: 200 g/L CO flow rate: 4 ml/min

14 wt.%

4

2

EC (mS/cm)

EC (mS/cm)

8

3 4 ml/min 2

8 ml/min

7 wt.% 2

10 ml/min

1 (c)

Supernatant CaO content: 7 wt.% Solid dosage: 200 g/L

(d)

0

0 0

20

40

60

80 100 Time (min)

120

140

160

0

20

40

60 Time (min)

80

100

120

Fig. 3. Temporal pH changes in the carbonation tests on supernatants at (a) various CaO contents and (b) various CO2 flow rates, and temporal EC changes in the carbonation tests on supernatants at (c) various CaO contents and (d) various CO2 flow rates.

Carboantion efficiency =

MCa-extraction − MCa-carboantion MT (g) × (CaO content (wt%)/100) × (MWCa /MWCaO )

× 100

(4)

where MCa-extraction is the mass of Ca in the solution after the extraction test, MCa-carbonation is the mass of Ca in the solution after the carbonation test, MT is the total mass of the material used in the extraction test, CaO content is the CaO content in CFA, MWCa is the molecular weight of Ca (40.087 g/mol), and MWCaO is that of CaO (56.077 g/mol).

Mn, Ni, Pb, Se, and Zn concentrations using an inductively coupled plasma mass spectrometer (ICP-MS) in the Korea Basic Science Institute. The chemical and mineralogical compositions of the raw materials and precipitates were analyzed using an XRF instrument (ZSX Primuss II, Rigaku) and an X-ray diffractometer (XRD; SMART APEXII, Bruker), respectively. The morphology and chemical compositions of the precipitates were analyzed with an FE-SEM instrument (Hitachi S-4300) equipped with EDX (Horiba EX-200). The carbonate content of the precipitates obtained from the carbonation tests was determined by thermogravimetric analysis (TGA 2050). Weight fractions determined by TGA can be distinguished as follows: (1) 25–100 ◦ C: moisture, (2) 105–500 ◦ C: organic carbon, and (3) 500–1000 ◦ C: inorganic carbon (CaCO3 ) [6].

2.4. Chemical analysis and materials characterization 3. Results and discussion The filtered samples from the extraction and carbonation tests were analyzed for their Al, Ca, K, Mg, and Na concentrations using an inductively coupled plasma atomic emission spectrometer (ICPAES, OPTIMA 3000XL, Perkin-Elmer) in the Center for Mineral Resources Research at Korea University, and for As, Cd, Cr, Cu,

3.1. Extraction tests Ca extraction tests were conducted on CFA under various conditions (solid dosage: 100, 140, and 200 g/L, CaO content: 7, 14, 27,

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C

(a)

(b)

C Calcite Supernatant DI water V Vaterite CaO content: 7 wt.% CO flow rate: 4 mi/min

C

2

2

V

Solid dosage V V C

C

V

V

C

C

C V

C

100 g/L V

Slurry C Calcite DI water V Vaterite CaO content: 7 wt.% CO flow rate: 4 ml/min

V

Solid dosage

C V

C

V

C

V

CC

C

VC

V

100 g/L C V

140 g/L 140 g/L

200 g/L 200 g/L 20

30

40

60

50

20

30

40

2 Theta

2 Theta

(c) C

Supernatant DI water CaO content: 7 wt.% Solid dosage: 200 g/L

60

50

o

o

(d)

C Calcite V Vaterite

C

Supernatant C Calcite DI water V Vaterite Solid dosage: 200 g/L CO flow rate: 4 mi/min 2

CO Flow rate 2

VC V V

V

C

C

C V

C C 10 ml/min C V V

CaO content VC V V

V

8 ml/min

C

C

C V

CC V

34 wt.% C V 27 wt.%

4 ml/min 14 wt.% 2 ml/min 20

30

40 o 2 Theta

50

7 wt.% 60

20

30

40 o 2 Theta

50

60

Fig. 4. Results of XRD analysis on the (a) precipitates obtained from the carbonation tests on supernatants at various solid dosages, (b) samples obtained from the carbonation tests on slurries at various solid dosages, and precipitates obtained from the carbonation tests on supernatants at (c) various CO2 flow rates and (d) various CaO contents.

and 34 wt.%, and extraction solution: DI water and tap water) to evaluate their effect on the elemental extraction characteristics of CFA. The results of the chemical analysis on the leachate obtained from the extraction tests are shown in Table 3. The pHs of the leachates after 24-h reaction between CFA and the extraction solution ranged between 11.6 and 13.0, regardless of the test conditions. These pH increases of the leachates were attributed to their CaO content. The EC of the leachate obtained from the test with a solid dosage of 100 g/L was lower (4.7 mS/cm) but the leachates obtained from the tests at solid dosages of 140 g/L (5.5 mS/cm) and 200 g/L (5.7 mS/cm) had similar ECs. The ECs of the leachates obtained from the tests with CaO contents higher than 14 wt.% were similar (9.7–10.4 mS/cm). These results indicate that the mixture of CFA and the extraction solution reached equilibrium at a specific condition, with no further leaching of elements occurring (e.g., a solid dosage of 200 g/L and CaO content of 14 wt.%).

Ca ions were dominant in the leachates, regardless of the test conditions (Table 3). The Ca concentrations in the leachates ranged between 503 mg/L and 1073 mg/L. The Ca concentration increased as the solid dosage was increased from 100 g/L to 200 g/L. The elevated Ca concentrations in the leachates were attributed to the dissolution of Ca-bearing minerals of CFA. The Ca concentration increased from 688 mg/L to 1073 mg/L as the CaO content was increased from 7 wt.% (total Ca concentration: 10 006 mg/L) to 14 wt.% (total Ca concentration: 20 011 mg/L), but was subsequently almost unchanged as the CaO content was increased from 14 wt.% (total Ca concentration: 38 594 mg/L) to 34 wt.% (total Ca concentration: 48 599 mg/L) because the leachate was probably saturated with CaO-bearing minerals at a CaO content above 14 wt.%. These Ca concentration results were comparable to those of the ECs in the leachates, suggesting that the EC of the leachate can be used as an indicator of the elemental concentration (mainly Ca ions).

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Fig. 5. SEM images of precipitates obtained from the carbonation tests on supernatants at solid dosages of (a) 100 g/L, (b) 140 g/L, and (c) 200 g/L under test conditions (CaO content: 7 wt.%, CO2 flow rate: 4 mL/min, and DI water) and at (d) the CaO content of 14 wt.% under test conditions (solid dosage: 200 g/L, CO2 flow rate: 4 mL/min, and DI water).

The other primary elements in the leachates were Na (6.6–12.8 mg/L), K (2.2–7.9 mg/L), and Sr (7.3–36.4 mg/L), regardless of the test conditions, which are comparable to results from Hong et al. [21]. The elevated Na concentration was attributed to the dissolution of Na2 SO4 in CFA [7]. The leachates obtained from the extraction tests had elevated concentrations of some elements such as Cr and Se, regardless of the test conditions (Cr: 0.1–0.3 mg/L and Se: 23–42 ␮g/L) (Table 3). Fig. 1 shows the Ca extraction efficiencies of CFA at various test conditions calculated from Eq. (1). The Ca extraction efficiency generally decreased slightly even though the extracted Ca concentration increased with increasing solid dosage (Fig. 1a). The average Ca extraction efficiencies were 10.0, 7.4, and 6.9 wt.% and the average extracted Ca concentrations were 503, 516, and 688 mg/L at solid dosages of 100 g/L (total Ca concentration: 5003 mg/L), 140 g/L (total Ca concentration: 7004 mg/L), and 200 g/L (total Ca concentration: 10 006 mg/L), respectively. Similarly, the Ca extraction efficiency decreased from 6.9 wt.% to 1.9 wt.% even though the Ca concentration increased from 688 mg/L to 929 mg/L as the CaO content was increased from 7 wt.% (total Ca concentration: 5003 mg/L) to 34 wt.% (total Ca concentration: 48 599 mg/L) at a solid dosage of 200 g/L (Fig. 1b and Table 3). These results indicated that the Ca extraction efficiency could be increased by increasing the amount of extraction solution added to CFA due to the dilution effect [8]. The Ca extraction efficiency for DI water was slightly higher than that for tap water (Fig. 1c), indicating that the pre-existing ions in

tap water affected the Ca extractability from CFA. In general, an increase in the ionic strength of the solution decreases the degree of dissolution of the minerals [22]. 3.2. Carbonation tests 3.2.1. Temporal behaviors of pH and EC during CO2 injection The temporal behaviors of pH and EC during CO2 injection for the supernatants without solids and the slurries with solids, obtained from the extraction tests at various test conditions, are shown in Figs. 2 and 3. The tests were terminated when the pH reached 8.3 to prevent dissolution of the precipitated carbonates. For both the supernatant and the slurry, pH initially decreased slowly from approximately 12.0 to 11.0 and then dropped quickly from about 11.0 to 8.3. On the other hand, EC decreased steadily until pH reached approximately 11.0 and then increased slightly (Fig. 2a and b), indicating that most of the CaCO3 precipitated at pH between 13.0 and 10.0, according to the following carbonation reaction: CO2 + H2 O → CO3 2− + 2H+

(5)

Ca2+ + CO3 2− → CaCO3 (s)

(6)

CO2 gas dissolves in DI water to produce carbonate ions which react with Ca2+ ions that are dissolved from Ca-bearing minerals of CFA, resulting in the precipitation of CaCO3 . Continued

H.Y. Jo et al. / Chemical Engineering Journal 183 (2012) 77–87

3.2.2. Identification of the precipitates After terminating the carbonation tests, white precipitates were observed in both supernatants and slurries. The white precipitates obtained by filtering the leachate after terminating the carbonation tests were analyzed using XRD, TGA, and SEM to identify and quantify their mineralogical phases. The XRD results showed that the precipitates consisted of mainly calcium carbonates (CaCO3 ), regardless of the test conditions (Fig. 4). The TGA results also confirmed that the precipitates obtained from the carbonation tests with the supernatants consisted of more than 90 wt.% carbonates. The SEM results showed that the precipitates obtained from the carbonation tests, regardless of the test conditions, were mainly composed of cubic calcite and spherical vaterite. Fig. 5 shows the SEM images for selected samples. Calcite is a more stable form of calcium carbonates, whereas vaterite, which is a metastable amorphous form of calcium carbonates, generally forms under

10 Supernatant Slurry Carbonation efficiency (%)

dissolution of CO2 gas in water increases the concentration of H+ ions and thus decreases the pH. The decreased Ca concentrations in the filtered samples after the termination of the CO2 injection tests also confirmed the progress of the carbonation reaction during CO2 injection. The carbonation reaction tends to complete when the pH reaches 8.3, at which EC becomes stabilized (Fig. 2c and d). The carbonation reaction can proceed longer in the slurry than in the supernatant because the consumed Ca2+ ions can be continuously replenished by the dissolution of Ca-bearing minerals in the slurry. In this study, however, the longer time for carbonation completion (i.e., at pH = 8.3) occurred in the supernatants than in the slurries at the solid dosages of 100 g/L and 140 g/L except at the solid dosage of 200 g/L, probably because calcium carbonates precipitated on the surface of the Ca-bearing minerals and thereby prevented their further dissolution [6,7,11,13]. In addition, the initial extracted ionic concentration (mainly Ca concentration) affected the time for the carbonation completion. The time for the carbonation completion increased with increasing the initial EC for both the supernatants and the slurries, regardless of the solid dosage. For example, for the supernatant the longer time occurred at the solid dosage in the order of 140, 100, and 200, which had the initial EC of 8.0, 4.6, and 3.8, respectively, probably due to the heterogeneity of CFA with respect to the content of Ca-bearing minerals (Fig. 2c). Thus, in this study, carbonation tests were conducted on the supernatants to evaluate the relation between the Ca extraction capacity and the carbonation capacity. When the reagent-graded CaO was added to CFA, the CaO content affected the time for carbonation completion (Fig. 3a and b). As the CaO content was increased from 7 wt.% to 34 wt.% at a fixed CO2 injection rate, the time for carbonation completion increased from 60 min to 160 min because more Ca2+ ions were released by dissolution of Ca-bearing minerals. However, the rate of carbonation, which can be approximated by the slope of the curve of reaction time vs. EC, remained almost constant at ∼0.1 mS/cm/min and was unaffected by the CaO content due to the fixed CO2 injection rate. In Fig. 3(b), a large disturbance in the 14 wt.% curve between 80 and 100 min occurred probably because of unknown pH measurement errors. The rate of carbonation was affected by the CO2 injection rate at a fixed CaO content (7 wt.%) and solid dosage (200 g/L) (Fig. 3c and d). As the CO2 injection rate was increased from 2 mL/min to 10 mL/min, the rate of carbonation increased approximately from 0.05 mS/cm/min to 0.2 mS/cm/min, indicating that the overall carbonation rate in the supernatants is limited by the rate of CO2 delivery to the solution. For the supernatant with dissolved Ca2+ ions, the CO2 gas dissolved in the supernatant reacted quickly with the dissolved Ca2+ ions, resulting in the precipitation of Ca carbonates until the pH reached about 8.3.

5

DI water CaO content: 7 wt.% CO2 flow rate: 4 ml/min

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1.0 Supernatant Slurry Normalized carbonation efficiency

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DI water CaO content: 7 wt.% CO2 flow rate: 4 ml/min

(b) 0.0 80

100

120

140

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Solid dosage (g/L) Fig. 6. (a) The carbonation efficiencies and (b) the normalized carbonation efficiencies as a function of the solid dosage obtained from the carbonation tests on the supernatants without solids and the slurries with solids under the test conditions (solvent: DI water, CaO content: 7 wt.%, and CO2 flow rate: 4 mL/min).

supersaturation conditions with respect to Ca2+ and CO3 2− ions and is easily transformed to calcite as the supersaturation ratio decreases. 3.2.3. Carbonation efficiencies at various conditions The carbonation tests at various test conditions (i.e., solid dosage, CaO content, CO2 flow rate, and solvent type) were conducted on supernatants without solid particles obtained from the extraction tests to evaluate how the test conditions affected the carbonation efficiency of CFA. The carbonation efficiencies at the various test conditions calculated using Eq. (4) are shown in Figs. 6(a) and 9(a). It is assumed that Ca removal from the solution only occurred by Ca conversion into calcite formed during CO2 injection because precipitates after obtained from the carbonation tests with the supernatants mainly consisted of CaCO3 (>90 wt.%) according to the XRD and TGA results. Thus, the carbonation efficiency represents the percentage of Ca content removed by conversion of CaCO3 per total Ca content of the material. The normalized carbonation efficiency as shown in Figs. 6(b) and 9(b) represents the ratio of the carbonation

H.Y. Jo et al. / Chemical Engineering Journal 183 (2012) 77–87

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10

CaO content: 7 wt.% Solid dosage: 200 g/L Carbonation efficiency (%)

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15 20 25 CaO content (wt.%)

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35

Fig. 7. (a) The carbonation efficiencies and (b) the normalized carbonation efficiencies as a function of the CaO content obtained from the carbonation tests on the supernatants without solids under the test conditions (solvent: DI water, solid dosage: 200 g/L, and CO2 flow rate: 4 mL/min).

efficiency to the Ca extraction efficiency. The normalized carbonation efficiency indicates how the test condition affects the carbonation efficiency at a given Ca extraction efficiency. 3.2.3.1. Effect of the solid dosage. The carbonation efficiency was almost unaffected by the solid dosage at a fixed CaO content of 7 wt.%, for both the supernatant and the slurry, even though the carbonation efficiency for the supernatant at the solid dosage of 100 g/L was exceptionally higher (Fig. 6a), probably due to heterogeneity of CFA with respect to the content of Ca-bearing minerals. The average carbonation efficiency for the supernatant was 8.6, 4.1, and 5.5 wt.% at a solid dosage of 100, 140, and 200 g/L, respectively. The normalized carbonation efficiency was not significantly affected by the solid dosage (Fig. 6b). These results suggest that the Ca extraction is a primary factor affecting the carbonation efficiency of CFA, rather than the solid dosage. In contrast, the amount of CO2 sequestered by CFA increased with increasing solid dosage because of the simultaneous increase in the extracted Ca concentrations. The carbonation efficiency for the supernatant was comparable to that for the slurry at a fixed solid dosage except at 100 g/L for the supernatant, probably because the precipitation on the CFA

0.0 0

2

4 6 8 CO2 flow rate (ml/min)

10

12

Fig. 8. (a) The carbonation efficiencies and (b) the normalized carbonation efficiencies as a function of the CO2 flow rate obtained from the carbonation tests on the supernatants without solids under the test conditions (solvent: DI water, CaO content: 7 wt.%, and solid dosage: 200 g/L).

surface prevented further leaching of Ca ions during CO2 injection, although the carbonation efficiency was determined based on only the change in the Ca concentration during CO2 injection. The temporal variation of pH and EC also showed that the longer times for carbonation reaction completion (i.e., pH = 8.3) for the supernatant generally occurred than for the slurry (Fig. 2), suggesting that insignificant continuous leaching of Ca in the slurry occurred during CO2 injection. In addition, several studies showed that CaCO3 coatings in steel slag by reacting CO2 caused a decrease in the carbonation efficiency of the steel slag with time [6,7,11]. 3.2.3.2. Effect of the CaO content. The increase in the CaO content of the sample with addition of the reagent-graded CaO slightly decreased the carbonation efficiency of the sample at a fixed solid dosage of 200 g/L (Fig. 7a). The average carbonation efficiency decreased from 5.5 wt.% to 1.7 wt.% as the CaO content was increased from 7 wt.% to 34 wt.%, which was attributed to the constant solubility of Ca-bearing minerals, especially Ca(OH)2 , regardless of their mass in the solution.

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Carbonation efficiency (%)

CaO content: 7 wt.% Solid dosage: 200 g/L CO2 flow rate: 4 ml/min

5

that the other conditions were unchanged. These results suggest that the CO2 flow rate is the main factor affecting the carbonation efficiency at a given Ca extraction ratio. The decrease in the carbonation efficiency with increasing CO2 flow rate was attributed to the increase in the CO2 delivery rate into the solution. The fast CO2 delivery rate caused the rapid decrease in pH, as shown in Fig. 3, leading to early carbonation completion. For the supernatant with elevated Ca concentration, CO2 gas dissolved in the supernatant and reacted with the dissolved Ca ions, resulting in precipitation of Ca carbonates at pH > 8.3, but most carbonation occurred at pH > 10.3 [8]. These results suggest that the CO2 delivery into the solution might be the rate determining step of the carbonation reaction. Thus, a slower CO2 flow rate will enhance the carbonation efficiency and the amount of CO2 sequestered when the indirect aqueous carbonation method is used.

(a) 0 DI water

Tap water

Normalized carbonation efficiency

1.0 CaO content: 7 wt.% Solid dosage: 200 g/L CO2 flow rate: 4 ml/min

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3.2.3.4. Effect of the extraction solutions. Tap water was used as a solvent instead of DI water to investigate whether the chemistry of the solvent affected the carbonation efficiency and to evaluate its potential application. Under the same experimental conditions, the carbonation efficiency (5.5 wt.%) for the carbonation test using DI water was slightly higher than that (3.0 wt.%) using tap water (Fig. 9a). These results were attributed to the higher extracted Ca concentration for DI water than for tap water, as shown in Fig. 2, probably due to the common ion effect of tap water [21]. The normalized carbonation efficiency was also higher for DI water than for tap water (Fig. 9b), suggesting that the chemistry of the solvent also affected the carbonation efficiency. The extraction efficiency for DI water (6.9 wt.%) was higher than that for tap water (6.2 wt.%) and the carbonation efficiency for DI water was also higher (4.9 wt.% vs. 3.0 wt.%). Mg ions in tap water may have inhibited the formation of calcite [23,24].

0.2 4. Conclusions

(b) 0.0 DI water

Tap water

Fig. 9. (a) The carbonation efficiencies and (b) the normalized carbonation efficiencies obtained from the carbonation tests on the supernatants without solids using DI water and tap water as solvents under the test conditions (CaO content: 7 wt.%, solid dosage: 200 g/L, and CO2 flow rate: 4 mL/min).

The Ca concentration in the leachates obtained from the extraction tests increased from 688 mg/L to 1073 mg/L as the CaO content was increased from 7 wt.% to 14 wt.%, but was subsequently unaffected as the CaO content was increased from 14 wt.% to 34 wt.%, probably because of the equilibrium attained between the extraction solution and the Ca-bearing minerals as the CaO content exceeded 14 wt.%. The normalized carbonation efficiency also revealed an absence of any change at a CaO content above 14 wt.% (Fig. 7b). Thus, the amount of CO2 sequestered for each test, which is the amount of CO2 converted into CaCO3 at a given CFA, remained constant at a CaO content above 14 wt.%, probably due to the solubility of Ca(OH)2 . 3.2.3.3. Effect of the CO2 flow rate. At a given CFA, the carbonation efficiency decreased with increasing CO2 flow rate (Fig. 8a). The average carbonation efficiency decreased from 5.7 wt.% to 2.0 wt.% as the CO2 flow rate was increased from 2 mL/min to 10 mL/min for the carbonation tests with CFA having a CaO content of 7 wt.% and a solid dosage of 200 g/L. The normalized carbonation also decreased with increasing CO2 flow rate (Fig. 8b). Similarly, the amount of CO2 sequestered decreased with increasing CO2 flow rate, provided

In this study, the two-step indirect aqueous carbonation method with separate extraction and carbonation steps was used to evaluate the carbonation efficiency of CFA. The aqueous mineral carbonation process of the study method probably consisted of (1) leaching of Ca from the Ca-bearing mineral phase into the solution, (2) delivery of gaseous CO2 into the solution, (3) conversion of dissolved CO2 (aq) into bicarbonate and carbonate ions, and (4) precipitation of Ca carbonates mainly at pH > 10. The carbonation efficiency was well correlated with the Ca extraction efficiency, suggesting that the Ca extraction step primarily controls the rate and extent of CFA carbonation for the overall indirect aqueous mineral carbonation process. The experimental factors of solid dosage, CaO content, and solvent type affected the Ca extraction efficiency. In addition, at a given Ca extraction efficiency, the CO2 flow rate and the solvent type affected the rate and extent of CFA carbonation. Based on the study results, the maximum carbonation efficiency of approximately 0.11 g of CO2 per g of CaO was obtained at a solid dosage of 100 g/L, CO2 flow rate of 2 mL/min for CFA, and at 25 ◦ C for CFA with a CaO content of 7 wt.% using DI water as a solvent. The CO2 sequestration capacity of CFA with a CaO content of 7 wt.% under ambient temperature and pressure conditions based on the maximum carbonation efficiency (0.11 g of CO2 per g of CaO) was approximately 0.008 kg of CO2 per 1 kg of CFA, which is approximately 30 and 3 times lower than the results for CFA with CaO contents of 11.5 wt.% and 4.1 wt.% reported by Back et al. [8] and Montes-Hernandez et al. [12], respectively. However, their higher temperature and CO2 pressure indicated higher energy inputs for the carbonation tests than in this study.

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Although the in situ mineral carbonation process under ambient temperature and pressure conditions proposed in this study has several advantages such as lower energy inputs, further research on enhancing the Ca extraction efficiency, using flue gas instead of pure CO2 gas, and using flow-through column reactors should be conducted to increase the potential field applications. In addition, replicate tests would be conducted on CFA having various CaO contents obtained from different sources because of potential fundamental changes of CFA properties by adding reagent-graded CaO and the variability in CFA mineralogy. Acknowledgement This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (grant number: 2009-0076614). References [1] W. Seifritz, CO2 disposal by means of silicates, Nature (1990) 345. [2] K.S. Lackner, C.H. Wendt, D.P. Butt, E.L. Joyce, D.H. Sharp, Carbon dioxide disposal in carbonate minerals, Energy 20 (1995) 153–1170. [3] R. Baciocchi, A. Polettini, R. Pomi, V. Prigiobbe, V.N. Zedwitz, A. Steinfeld, CO2 sequestration by direct gas-solid carbonation of air pollution control (APC) residues, Energy Fuel 20 (2006) 1933–1940. [4] M. Hänchen, V. Prigiobbe, G. Storti, T.M. Seward, M. Mazzotti, Dissolution kinetics of forsteritic olivine at 90–150 ◦ C including effects of the presence of CO2 , Geochim. Cosmochim. Acta 70 (2006) 4403–4416. [5] A. Iizuka, M. Fujii, A. Yamasaki, Y. Yanagisawa, Development of a new CO2 sequestration process utilizing the carbonation of waste cement, Ind. Eng. Chem. Res. 43 (2004) 7880–7887. [6] W.J. Huijgen, G.-J. Witkamp, R.N.J. Comans, Mineral CO2 sequestration by steel slag carbonation, Environ. Sci. Technol. 39 (2005) 9676–9682. [7] W.J. Huijgen, R.N.J. Comans, Carbonation of steel slag for CO2 sequestration: leaching 4 of products and reaction mechanisms, Environ. Sci. Technol. 40 (2006) 2790–2796. [8] M. Back, M. Kuehn, H. Stanjek, S. Peiffer, Reactivity of alkaline lignite fly ashes towards CO2 in water, Environ. Sci. Technol. 42 (2008) 4520–4526.

87

[9] D. Bonenfant, L. Kharoune, S. Sauvé, R. Hausler, P. Niquette, M. Mimeault, M. Kharoune, CO2 sequestration potential of steel slags at ambient pressure and temperature, Ind. Eng. Chem. Res. 47 (2008) 7610–7616. [10] R.M. Dilmore, B. Howard, Y. Soong, C. Griffith, S.W. Hedges, A.D. DeGalbo, B. Morreale, J.P. Baltrus, D.E. Allen, K.F. Jaw, Sequestration of CO2 in mixtures of caustic byproduct and saline waste water, Environ. Sci. Technol. 26 (2009) 1325–1333. [11] D.N. Huntzinger, J.S. Gierke, S.K. Kawatra, T.C. Eisele, L.L. Sutter, Carbon dioxide sequestration in cement kiln dust through mineral carbonation, Environ. Sci. Technol. 43 (2009) 1986–1992. [12] G. Montes-Hernandez, R. Pérez-López, F. Renard, J.M. Nieto, L. Charlet, Mineral sequestration of CO2 by aqueous carbonation of coal combustion fly-ash, J. Hazard. Mater. 161 (2009) 1347–1354. [13] S. Kashef-Haghighi, S. Ghoshal, CO2 sequestration in concrete through accelerated carbonation curing in a flow-through reactor, Ind. Eng. Chem. Res. 49 (2010) 1143–1149. [14] L. Wang, Y. Jin, Y. Nie, Investigation of accelerated and natural carbonation of MSWI fly ash with a high content of Ca, J. Hazard. Mater. 174 (2010) 334–343. [15] Y. Soong, D.L. Fauth, B.H. Howard, J.R. Jones, D.K. Harrison, A.L. Goodman, M.L. Gray, E.A. Frommel, CO2 sequestration with brine solution and fly ashes, Energy Convers. Manage. 47 (2006) 1676–1685. [16] A. Uliasz-Bochenczyk, E. Mokrzycki, M. Mazurkiewicz, Z. Piotrowski, Utilization of carbon dioxide in fly ash and water mixtures, Chem. Eng. Res. Des. 84 (A9) (2006) 843–846. [17] A.J. Reddy, S. John, H. Weber, M.D. Argyle, P. Bhattacharyya, D.T. Yaylor, M. Christensen, T. Foulke, P. Fahlsing, Simultaneous capture and mineralization of coal combustion flue gas carbon dioxide (CO2 ), Energy Procedia 4 (2011) 1574–1583. [18] KEPC, Korea Electric Power Corporation Annual Report, Seoul, Korea, 2001, pp. 7–10 (in Korean). [19] S. Choi, S. Lee, Y. Song, H. Moon, Leaching characteristics of selected Korean fly ashes and its implications for the groundwater composition near the ash disposal mound, Fuel 81 (2002) 1083–1090. [20] ASTM, Annual Book of Standards, American Society of Testing Materials, West Conshohocken, PA, 2004. [21] J. Hong, H. Jo, S. Yun, Coal fly ash and synthetic coal fly ash aggregates as reactive media to remove zinc from aqueous solutions, J. Hazard. Mater. 164 (2009) 235–246. [22] W. Stumm, J.J. Morgan, Aquatic Chemistry, Chemical Equilibria and Rates in Natural Water, 3rd ed., John Wiley & Sons, Inc., New York, 1996, pp. 150–152. [23] R.A. Berner, The role of magnesium in the crystal growth of calcite and aragonite from sea water, Geochim. Cosmochim. Acta 39 (1975) 489–504. [24] M.M. Reddy, Effect of magnesium ion on calcium carbonate nucleation and crystal growth in dilute aqueous solutions at 25 ◦ C, in: F.A. Mumpton (Ed.), Studies in Diageneses, U.S. Geological Survey Bulletin 1578, Denver, Colorado, 1986, pp. 169–182.