Wollastonite Carbonation in Water-Bearing Supercritical CO2: Effects

Subscriber access provided by UNIVERSITY OF THE SUNSHINE COAST

Article

Wollastonite Carbonation in Water-bearing Supercritical CO2: Effects of Particle Size Yujia Min, Qingyun Li, Marco Voltolini, Timothy J Kneafsey, and Young-Shin Jun Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b04475 • Publication Date (Web): 02 Oct 2017 Downloaded from http://pubs.acs.org on October 7, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Environmental Science & Technology is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 33

Environmental Science & Technology

Wollastonite Carbonation in Water-bearing Supercritical CO2: Effects of Particle Size

Yujia Min,† Qingyun Li,† Marco Voltolini,‡ Timothy Kneafsey,‡ Young-Shin Jun*† †

Department of Energy, Environmental and Chemical Engineering, Washington University in St. Louis, MO 63130

‡

Earth and Environmental Science, Lawrence Berkeley National Laboratory, CA 94720

E-mail: [email protected] Phone: (314) 935-4539 Fax: (314) 935-7211 http://encl.engineering.wustl.edu/ Submitted: August 2017


*To Whom Correspondence Should be Addressed

ACS Paragon Plus Environment


1

ABSTRACT

2

The performance of geologic CO2 sequestration (GCS) can be affected by CO2 mineralization and

3

changes in the permeability of geologic formations resulting from interactions between water-

4

bearing supercritical CO2 (scCO2) and silicates in reservoir rocks. However, without understanding

5

size effects, the findings in previous studies using nanometer or micrometer size particles cannot

6

be applied to the bulk rock in field sites. In this study, we report the effects of particle sizes on the

7

carbonation of wollastonite (CaSiO3) at 60 oC and 100 bar in water-bearing scCO2. After

8

normalization by the surface area, the thickness of the reacted wollastonite layer on the surfaces

9

was independent of particle sizes. After 20 hours, the reaction was not controlled by the kinetics

10

of surface reactions, but by the diffusion of water-bearing scCO2 across the product layer on

11

wollastonite surfaces. Among the products of reaction, amorphous silica, rather than calcite,

12

covered the wollastonite surface and acted as a diffusion barrier to water-bearing scCO2. The

13

product layer was not highly porous, with 10 times smaller specific surface area than the altered

14

amorphous silica formed at the wollastonite surface in aqueous solution. These findings can help

15

us evaluate the impacts of mineral carbonation in water-bearing scCO2.

1 ACS Paragon Plus Environment

Page 2 of 33

Page 3 of 33

16


TOC Art

17



18

INTRODUCTION

19

To mitigate the adverse impacts of global climate change, anthropogenic CO2 can be

20

captured from point sources and sequestered in subsurface environments, such as deep saline

21

aquifers or depleted oil and gas reservoirs.1 The safety and efficiency of CO2 trapping in these

22

storage sites are related to interactions among supercritical CO2 (scCO2), water, and rock.2 For

23

example, the carbonation of silicates can trap CO2 in mineral phases and cause changes in the

24

wettability and porosity of reservoir rocks, which can in turn alter their permeability. Furthermore,

25

a change of permeability in geologic formations can influence the transport of CO2 and potentially

26

affect CO2 leakage. For similar reasons, scCO2–water–rock interactions can be important in other

27

subsurface CO2 injection processes, including scCO2-enhanced oil recovery and scCO2-energized

28

hydraulic fracturing.3-7

29

While most previous studies on scCO2–water–rock chemical interactions focused on

30

mineral dissolution and precipitation in the presence of bulk aqueous phase,2 recent studies have

31

revealed the following importance of interactions between scCO2 and minerals in the absence of

32

aqueous solutions.8-10 First, because injected scCO2 will displace brine, most of the contact areas

33

between scCO2 and caprocks and a certain portion of the contact areas between scCO2 and

34

formation rocks, are in the absence of bulk aqueous solution.11 Second, scCO2 has higher

35

diffusivity, lower viscosity, and thus lower capillary entry pressure than aqueous phase fluid (i.e.,

36

CO2-dissolved brine).12 Therefore, scCO2 can enter smaller pore throats which brine cannot enter.

37

Third, recent studies reported that water dissolved in scCO2 has higher reactivity on mineral

38

surfaces than that of water in the aqueous phase.8, 12, 13 Based on these considerations, the reactions

39

of minerals with water-bearing scCO2 are at least equally, if not more, important than reactions of

40

mineral with aqueous solutions.12, 14


Page 4 of 33

Page 5 of 33


41

Our current understanding of water-bearing scCO2–mineral interactions focuses on the

42

carbonation of various Ca-, Mg-, or Fe-bearing minerals, including wollastonite,15-18 forsterite,11,

43

14, 19-22

44

water saturation percentage of CO2,14, 18, 19, 21, 25 temperature,17, 18, 20, 23 and organic ligands,22 on

45

carbonation reactions, we need a better understanding of the dependency of water-bearing scCO2–

46

silicates reactions on particle sizes. There are several knowledge gaps: First, previous studies

47

mostly used micrometer or even nanometer size particles,14-23 a far cry from the bulk rock in field

48

sites. Without understanding the effects of particle sizes, we cannot know whether the outcomes

49

obtained with small particles are applicable to bulk rock in field sites. Second, findings from

50

different studies are not directly comparable due to the different particle sizes used. Thus, results

51

from previous studies on water-bearing scCO2–silicate reactions cannot be combined. Third,

52

investigating the effects of particle sizes can reveal the mechanisms of mineral carbonation

53

reactions. For example, a recent study of metal oxide carbonation (i.e., CaO) in dry CO2 found that

54

the reacted fraction is constant for different particle sizes and concluded that the reaction extent of

55

CaO carbonation is determined by the porosity of CaO particles.26 Similarly, investigating the

56

effects of particle sizes on silicate carbonation in water-bearing scCO2 can inform us about whether

57

the reaction is controlled by the porosity of silicates or other limiting factors, such as the kinetics

58

of silicate dissolution in water films. However, to the best of the authors’ knowledge, there have

59

been no studies on the effects of particle sizes on water-bearing scCO2–silicate interactions.

fayalite,23 dolomite,24 and phlogopite.8 While these studies investigated the effects of the

60

The goal of this study is, therefore, to understand the effects of particle size on the

61

carbonation of wollastonite in water-bearing scCO2. We chose wollastonite because it has been

62

frequently used as a representative of silicates in previous studies on silicate reaction in scCO2 and

63

in aqueous solutions.17, 18, 27, 28 We hypothesize that the reaction is controlled by the kinetics of



64

wollastonite hydrolysis. If anisotropic reactivity of different crystal faces is not significant, the

65

reaction extent of different size particles should be similar after normalization to surface area. To

66

test this working hypothesis, particles with five different size ranges were reacted at 60 oC and 100

67

bar, conditions closely relevant to GCS. The information provided can contribute to better

68

understanding the water-bearing scCO2–silicate reactions and scaling up findings in laboratory

69

studies using small size mineral particles to the larger scale bulk rock formations.

70

EXPERIMENTAL METHODS

71

Minerals

72

Natural wollastonite particles with five different size ranges were purchased from NYCO

73

Company (Willsboro, NY). By using X-ray powder diffraction (XRPD, Bruker D8), its structure

74

was identified to be wollastonite-1A, the most common polymorph in the natural environment.29

75

The size distribution and shape for each size range were determined by laser diffraction and image

76

analysis (Microtrac Inc.) performed by NYCO Company. Most particles were in cylindrical shape

77

(Figure S1). The spherical equivalent volumetric mean diameters were 3.8, 5.2, 11.1, 17.8, and

78

82.0 µm, and the respective typical aspect ratios were 3:1, 3:1, 3:1, 4:1, and 15:1. The detailed size

79

distribution is available in the Supporting Information (Figure S1). These size ranges were chosen

80

because they can provide detectable and significantly different reaction extents. Using N2 as

81

adsorbate and 11 points on the isotherm in the BET method (AX1C-MP-LP, Quantachrome

82

Instruments), the specific surface areas were determined to be 4.46±0.01, 3.61±0.01, 1.95±0.01,

83

1.45±0.01, and 0.54±0.01 m2/g, respectively, for 3.8, 5.2, 11.1, 17.8, and 82.0 µm particles. The

84

error ranges are based on duplicate measurements. X-ray fluorescence (Table S1) showed that the

85

Ca/Si ratio was 0.959. Thermogravimetric analysis (TGA, Q5000IR, TA Instruments) showed


Page 6 of 33

Page 7 of 33


86

0.45 % mass loss between 150–780 oC. As shown by the XRD patterns in Figure S5, the unreacted

87

sample contains aragonite, which is estimated to be 1 wt%, based on 0.45% mass loss in TGA. No

88

further treatment was performed after the wollastonite were obtained from the vendor.

89

Carbonation in water-bearing scCO2 at simulated GCS conditions

90

Carbonation experiments were conducted in a 300 mL high pressure and high temperature

91

reactor (Parr Instruments, Moline, IL) modified from the reactor used in our previous studies.30-34

92

A schematic diagram of the reaction system setup is available in the Supporting Information

93

(Figure S2). The conditions (60 oC and 100 bar CO2) are similar to typical conditions at GCS sites:

94

65 oC and 150 bar in the Frio formation,35 37 oC and 100 bar at the Sleipner site,36 and 63 oC and

95

140 bar at the Weyburn field site.37 To investigate carbonation of wollastonite in the absence of

96

aqueous phase, wollastonite particles were placed in 10 ml Teflon tubes, while ultra-purified

97

deionized water (Barnstead, > 18.2 MΩ·cm) was added outside the tubes. After scCO2 was injected

98

into the vessel, water dissolved in the scCO2 and generated water-bearing scCO2. Five PTFE tubes

99

were placed in the reactor during each test. Considering the volume taken up by tubes and liners

100

in the reactor, the volume of CO2 during the experiment was 201 mL. The tubes were capped, but

101

have small holes allowing contact between the wollastonite and water-bearing scCO2. The

102

solubility of water in CO2 was predicted using Spycher’s model.38 At 35 and 60 oC, the mole

103

fraction of 100% saturated water in 100 bar CO2 is 0.41 and 0.49%, which indicates 234 and 117

104

µL water in the 201 mL scCO2. The solubility of water in scCO2 is affected by the salinity of water.

105

As a starting point, ultra-purified water was used for simplicity. This result can serve as an

106

important underpinning for understanding the effects of salinity on silicate carbonation in the

107

future. Each sample contained 0.3 g particles. To check the possible influence of particle stacking

108

inside the tubes on the reaction, results were compared with samples using 0.05 g particles.



Page 8 of 33

109

Selected samples were analyzed using XRPD. In this study, no attempt of XRPD quantitative

110

analysis was made. Instead, the reaction extent was determined by TGA analysis. A previous study

111

has shown that the results from XRPD quantitative analysis are comparable with the results from

112

TGA analysis.18

113

Determination of the reaction extent

114

The reacted fraction of wollastonite was determined based on TGA results. After the

115

sample was recovered from the reactor, the samples were stored at atmospheric pressure and

116

analyzed within 24 hours. The powder sample was briefly mixed using a plastic spatula, and

117

approximately 10 mg was used for each TGA analysis. At least duplicate samples were analyzed.

118

Samples were heated to 900 oC with a ramp of 20 oC/min under N2 flow (25 ml/min). Wollastonite

119

and amorphous silica were stable below 900 oC, while calcium carbonate completely decomposed

120

into CaO and CO2.18 The amount of CO2 was measured by the mass loss between 150-780 oC

121

during TGA (Figure 1B). Given the amount of CO2, the reacted fraction of the original wollastonite

122

was derived using this equation:

123 124

(1) (2)

125

where mL is the mass loss during TGA, which is the mass of CO2 from CaCO3 decomposition.

126

According to equation (2), the mass loss is equal to the mass of CO2 consumed during reaction

127

with water-bearing scCO2, so the mass of wollastonite consumed as reactant can be calculated

128

using mL and the molecular weight of CO2 and wollastonite; mo is the mass of the sample reacted

129

with water-bearing scCO2, which includes the mass of the original wollastonite (wollastonite

130

before reaction with CO2) and the mass of sorbed CO2 on wollastonite (equal to mL); and M is the


Page 9 of 33


131

molecular weight of different species. Given the 0.45% mass loss in unreacted wollastonite, the

132

detection limit of this method is estimated to be 1%. The reacted fraction of the largest particle

133

size used (82 µm) is 7.8%, so the uncertainty of the reaction extent determined using this method

134

is no more than 15%.

135

Calculation of the reacted thicknesses of wollastonite samples

136

Based on the reacted fraction, we calculated the thicknesses of the reacted layer by using a

137

shrinking core model. The schematic diagram in Figure 2A shows the geometry. Before the

138

reaction, each particle was assumed to be cylindrical in shape, with aspect ratio R and diameter D,

139

calculated using equation (3) based on the spherical equivalent diameter (Deq) measured by laser

140

diffraction by the NYCO Company. During the reaction, a shell with thickness L was consumed

141

as the reactant, and according to the model, the wollastonite particle shrank to a cylindrical shape

142

core with diameter D-2L. The volume fraction of the shell is the reacted fraction of the particle.

143

By integrating all the size intervals shown in the size distribution (Figure S1), the reacted fraction

144

of each size range was calculated. The reacted thickness L was obtained by solving equation (4),

145

which equated the calculated reacted fraction and the reacted fraction determined by TGA.

146

147

(3) ∑

(4)

∑

148

In Equations 3 and 4, L is the thickness of the reacted layer, F is the reacted fraction, Deq is the

149

spherical equivalent diameter of each size range from laser diffraction results provided by NYCO

150

Company, D is the cylindrical equivalent diameter of each size range, and R is the aspect ratio. L

151

was obtained by solving this equation. For small particle size ranges, in which (R × D – 2 × L)

Wollastonite Carbonation in Water-Bearing Supercritical CO2: Effects

Recommend Documents