Coupling Effects of Supercritical CO2 Sequestration in Deep Coal

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Environmental and Carbon Dioxide Issues

Coupling effects of super-critical CO2 sequestration in deep coal seam Beining Zhang, Weiguo Liang, P. G. Ranjith, Zhigang Li, Chang Li, and Dongsheng Hou Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b03151 • Publication Date (Web): 03 Dec 2018 Downloaded from http://pubs.acs.org on December 6, 2018

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Coupling effects of super-critical CO2 sequestration in deep coal seam Beining Zhang†,‡,§, Weiguo Liang†,‡,*, Pathegama G. Ranjith§, Zhigang Li†,‡, Chang Li†,‡, Dongsheng Hou†,‡ †

College of Mining Engineering, Taiyuan University of Technology, Taiyuan 030024, PR China

‡

Key Laboratory of In-situ Property-improving Mining of Ministry of Education, Taiyuan University of

Technology, Taiyuan 030024, PR China §

Deep Earth Energy Laboratory, Department of Civil Engineering, Monash University, Building 60,

Melbourne, Victoria 3800, Australia

ACS Paragon Plus Environment

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Abstract: CO2 sequestration in deep un-minable coal seams is currently identified as a research hotspot to reduce CO2 emissions, due to the potential large-scale storage capacity and complicated physical and chemical reactions, especially for super-critical CO2 (scCO2). Hitherto, the interaction mechanisms between scCO2 and coal mass in situ conditions are still unclear. Therefore, the main objective of this study is to fully address the coupling effects of scCO2 sequestration on coal mass and provide a comprehensive evaluation of the interrelation of these variations. Five cycles of Helium and scCO2 injection were replicated on a sub-bituminous coal sample to investigate the permeability variation with scCO2

saturation

time.

Meanwhile,

gas

chromatography-mass

spectrometry

(GC-MS),

gas

chromatography (GC), fourier transform infrared spectroscopy (FTIR), proximate analysis and low-pressure-temperature nitrogen (N2) isotherm analyses were employed to characterize the transformation in coal mass. The test result shows that: (1) ScCO2 tended to mobilize a higher proportion of aliphatics than aromatics, and the concentration of the yielded hydrocarbons decreased with CO2 saturation time. (2) Carbonate and silicate cemented minerals were partly dissolved due to the formation of an acidic solution containing H2CO3. (3) The hydrocarbon extraction and mineral dissolution resulted in the corresponding FTIR absorbance bands being weakened and the volatile matter content and the ash content decreased by approximately 15% and 26%, respectively. (4) The coal pore volume and the Brunauer−Emmett−Teller (BET) surface area decreased by approximately 24% and 12%, respectively. (5) Due to CO2 adsorption and the reduction of Young’s modulus with saturation time, the volumetric strain increased from 0.23% to 3.26%, which led to coal permeability decreased from 0.042 md to 0.029 md. After analyzing the interrelation of these variations, the interaction mechanisms between scCO2 and coal mass in situ conditions were described and an overall negative effect on coal permeability was found. Keywords: CO2 sequestration; permeability; super-critical CO2 extraction; matrix swelling; pore structure


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1. Introduction

CO2 sequestration has been regarded as an economical and promising solution to solve atmospheric CO2 emission, especially in deep un-minable coal seams due to the potential large-scale storage capacity and enhanced enhance coalbed methane (CBM) recovery. When the target depth is over 800m, the reservoir conditions can reach the supercritical temperature and pressure of CO2 (31.1 °C, 7.38 MPa) and the CO2 density is sufficiently high (~500 to 700 kg/m3) to limit the storage volume required [1-3]. However, previous studies have found some disadvantages of the CO2 sequestration, especially in coal seam due to the associated physical and chemical changes. Coal is a glassy, strained, cross-linked macromolecular systems and is not in its lowest energy state [4]. According to Gibbs’ adsorption theory [5] and Griffith’s failure theorem [6], CO2 adsorption leads to a reduction in surface energy, which lowers the limitation of the intramolecular interaction energy in coal. As a result, the coal mass becomes rubbery and the coal strength decreases [7-9], which is a major problem in terms of long-term safety. Due to 1.5 MPa sub-critical CO2 saturation, a reduction in both the compressive strength (by 13%) and the elastic modulus (by 26%) of lignite samples was observed under uniaxial conditions [10]. Compared with sub-critical CO2, super-critical CO2 has a greater adsorption potential, which caused a greater reduction of compressive strength [11]. Perera et al. [12] conducted a series of UCS tests at 33℃and observed that super-critical CO2 saturation resulted in a 40% higher UCS strength reduction and 100% higher Young’s modulus reduction in bituminous coal compared with sub-critical CO2 saturation. The reduction of coal mass strength, especially under in-situ conditions, changed the porosity and permeability of coal seam and eventually affected coal CO2 sequestration process. In addition, CO2 adsorption into the coal matrix also caused the visco-elastic relaxation of its


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strained, glassy/rubbery and highly cross-linked macromolecular structure [13-15], leading to coal matrix swelling. According to previous research, due to higher chemical potential and sorption capacity, super-critical CO2 creates greater adsorption-induced swelling than sub-critical CO2 [16], and the increase of the injection pressure also contributed to coal matrix swelling [11,17]. Coal matrix swelling caused a decrease of cleat aperture and the shrinkage of the flow path, resulting in a reduction of the overall permeability [18]. Perera et al. [19] investigated the effect of saturation time on coal swelling and observed a gradual reduction of coal permeability after three cycles of 15h super-critical CO2 saturation, mainly due to further coal matrix swelling over time. Similar permeability reductions (by 42.5%, 38.4% and 11%, respectively) occurred in bituminous coal after 20h, 40h and 60h of super-critical CO2 saturation [20]. As coal mass strength and Young’s modulus are closely associated with coal swelling, their interaction needs to be considered in CO2 sequestration. However, to date, there has been only limited research conducted on this issue, and it became fewer when chemical changes were involved. Super-critical CO2 has the potential to extract hydrocarbons and dissolve mineral matters in the coal mass, which causes physical changes and chemical alterations, resulting in coal structure rearrangement and eventually affecting permeability [21,22]. According to previous research [23-25], super-critical CO2 injection leads to the dissolution of some silicate cemented minerals, including quartz, feldspar-bearing minerals and clay, which significantly affects the grain-to-grain contacts, resulting in variations in the pore structure. Jonathan and Robert [26] injected super-critical CO2 into three high volatile bituminous coals and observed that a higher proportion of aliphatics relative to aromatics were mobilized. Hydrocarbon extraction and mineral dissolution changed the pore morphology. Extraction effect and chemical interaction may contribute to the change in coal surface chemistry. ScCO2 treatment degraded the accessibility of micropores with different coal rank, but increased macropore volume without


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confining stress [27,28]. However, these experiments have not considered the effect of in situ stresses, and the variation trend may be different considering the reduction of coal strength and matrix swelling. Therefore, it becomes increasingly necessary to conduct a comprehensive investigation of the interaction mechanisms of scCO2 sequestration on coal seam in situ conditions. To date, previous studies only focused on one or two aspects of these physical and chemical changes, e.g. the permeability, mechanical properties, pore structure, mineral dissolution, and hydrocarbon extraction, while a comprehensive understanding of interaction mechanisms between scCO2 and coal mass in situ conditions are still absent. Therefore, a series of He permeability and scCO2 extractions were investigated at constant stress (20 MPa) and temperature (50 ℃). The interrelations of physical and chemical variations were described, which provides a further understanding of various effects of scCO2 sequestration on coal seam.

2. Methodology

2.1 Sampling

The Datong coalfield is located in the north of Shanxi Province, China (Figure 1), with a total area of 1827 km2 from 39°52′ to 40°10′ north latitude and 112°49′32″ to 113°9′30″ west longitude [29]. It consists of two coal-bearing strata: upper Jurassic coal-bearing strata with 60.8 Gt of coal reserves and lower Carboniferous and Permian coal-bearing strata with 369.1 Gt of the coal reserves. It is a main large-scale coalfield in China, as well as one of the potential CO2 sequestration sites. The sub-bituminous coal samples used for the present study were collected from the fresh surface at a depth of around 400 m, located at the 15th coal seam of the Datong coalfield. These large coal blocks, wrapped in multiple layers of plastic sheet, were then transported from the coalfield to the Key


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Laboratory of In-situ Property-improving Mining of Ministry of Education at Taiyuan University of Technology, China. A diamond wire cutter (Figure 2(a)), with the advantage of little vibration, was used to avoid damage caused by the vibration of the traditional cutter to the sample structure. The processing path of the cutting-wire was controlled with the precision of 0.1mm. The large coal block was processed directly to the required sample size (100×100×200 mm). To simulate CO2 injection into an in-situ coal seam, the direction of the CO2 injection was parallel to the bedding plane. As coal is chemically heterogeneous, coal properties in different regions are variable. To avoid the heterogeneous effect on experimental results, 4 powdered coal samples in adjacent regions (Region 1 and Region 2, highlighted in red in Figure 2(a)) were chosen to check the difference between these regions and the mean test results were used to investigate the variation of the physical and chemical properties of the coal sample before and after scCO2 saturation.


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Figure 1. The geological map of Datong coalfield [29]

Diamond wire

1 before

2 after

after

before

(a) Processing the sample

(b) coal sample

Figure 2. Coal sample


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2.2 Experimental Apparatus

A super-critical CO2 extraction apparatus was used to conduct the scCO2 extraction experiment. This apparatus consists of a triaxial sample holder which can accept a 100×100×200mm cuboid sample and withstand 70 MPa pressure, two independent syringe pumps which provide the axial and confining stress, a heating blanket with a maximum temperature of 70℃, and a pressure regulator and a back-pressure regulator which control the upstream and the downstream pressure, respectively (Figure 3). To isolate the coal sample from the confining fluid and prevent the super-critical CO2 from extracting the hydrocarbons in the rubber membrane, the coal sample was first wrapped with silver paper and then placed inside a rubber jacket. The fluid yielded at downstream was injected into the CO2-extract separator. As dichloromethane (CH2Cl2) has a greater solubility and wider extraction yields than super-critical CO2 [26], it was used in the solvent trap to capture the extracted hydrocarbons. The ice-water bath around the separator was used to chill the solvent trap. In addition, a milli-gas-flow counter was connected to the gas outlet to measure the flow rate.

Silver paper

Rubber jacket

Back-pressure regulator

Flow meter

Pressure regulator

Preheater

CO2-extract separator

Pressure booster Confining pressure pump

Axial pressure pump

CO2 container Ice-water bath

Figure 3. Sketch of experimental apparatus


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2.3 Experimental procedure

In this study, the coal sample was subjected to a definite confining pressure (20 MPa) while the system temperature was maintained at 50℃. The downstream pressure was maintained constant at 8.0 MPa to ensure that the super-critical CO2 condition was achieved in the coal sample. Helium (He) was injected before and after CO2 injection to investigate the coupled effects of super-critical CO2 injection on coal permeability. Table 1 shows the testing conditions for the sample and the permeability test was conducted using the drained testing method. Prior to CO2 injection, He was initially injected to determine the original permeability of the coal sample at injection pressures of 9-13 MPa. Subsequently, super-critical CO2 was injected into the coal sample at constant 9 MPa injection pressure for 24 hours. Meanwhile, the fluid yielded at the outlet was injected into the CO2-extract separator where the organic matters were collected in a chilled CH2Cl2 solvent trap and the remaining gas was collected for analysis. He was then injected again from 9 MPa to 13 MPa to investigate the influence of super-critical CO2 injection on coal permeability. Subsequently, four further similar cycles of 24 hours of supercritical CO2 injection and the following He injection were conducted. The bituminous substances yielded were analyzed by gas chromatography–mass spectrometry (GC–MS) and the yielded gas was analyzed by gas chromatography in each case. The hydraulic oil variation in the syringe pump was monitored and recorded at every one-minute interval to calculate the volumetric strain of the confined coal sample [30,31]. After the permeability test, the coal sample was recovered from the sample holder and two small coal blocks in adjacent regions (highlighted in red in Figure 2(a)) were collected to conduct the proximate analysis (PA), Fourier transform infrared spectroscopy (FTIR) and low-pressure-temperature N2 adsorption to investigate the variation of the physical and chemical properties of the coal sample after


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super-critical CO2 saturation.

2.4 Permeability equation

Darcy’s law was used to calculate the coal permeability. The permeability equation for gas flow can be obtained as follows: K

2QP2 L  P12  P22  A

(1)

where, K is the permeability, 1012 μm2;  is the dynamic viscosity of the injecting fluid, Pa·s; Q is the downstream flow rate, m3·s−1; P1 is the upstream pressure, Pa; P2 is the downstream pressure, Pa; L and A is the mean length and area of the sample, m and m2, respectively.

3. Results and Analysis

3.1 Helium flow behavior

3.1.1 Helium flow rate

Helium (He), an inert gas, was used to investigate the coupled effects of super-critical CO2 saturation on coal permeability because He causes only a minimal swelling effect [32] and guarantees a minimum degree of gas slippage [33]. He flow rates were measured with a milli-gas-flow counter at the downstream. Each He permeability test was completed when the flow rate at downstream was steady. For the first He injection at 9 MPa (Figure 4), the flow rate increased slowly to equilibrium, which took less time to become stable at the following injection pressures (10-13 MPa). At each injection pressure, the stabilized equilibrium flow rate was used to calculate the permeability. After 24 hours of super-critical CO2 saturation, He was reinjected into the coal sample to investigate the permeability


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variation. An obvious fluctuation of the flow rate was observed for the initial He injection at 9 MPa (Figure 5). The initial He flow rate fluctuated consistently from a maximum 63.85 ml/s to a minimum 2.07 ml/s, and it took a very long time (nearly 5-6 hours) to undergo steady-state flow. The yielded gas was consistently analyzed via gas chromatography (GC) and the detection results indicated that CO2 was desorbed during the fluctuation period and no more CO2 was detected when the flow rate became stable. Meanwhile, CO2 desorption caused associated matrix shrinkage (Figure 17), which is discussed in the following section. CO2 desorption and matrix deformation resulted in the flow rate fluctuation at the initial He injection. The similar fluctuation of the flow rate was observed for the following initial He injection in each case (Figure 6). According to previous research [34,35], gas transportation could be Darcy or non-Darcy flow in porous media. Darcy’s law assumes a linear relationship between pressure gradient and gas flow rate [36]. Figure 7 shows the He flow rate versus the injection pressure observed in each case. The results clearly indicated a linear variation of the flow rate with injection pressure. Table 1 shows the regressed equations of the trend lines added to the data set for each He injection. The determination coefficient (R2) ranged from 0.9794 to 0.9959, which showed a good linear fit. Therefore, He flow along the coal sample was in accordance with Darcy's law. Interestingly, the slopes of these regressed equations decreased with saturation time, which implied that continuous CO2 injection led to less increase of the flow rate with injection pressure.


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60 13MPa

Flow rate (ml·s-1)

50 40

12MPa

Slowly increasing to equilibrium

30

11MPa

20 Increasing injection pressure

10MPa

10 9MPa

0 0

5

10

15 Time (h)

20

25

30

Figure 4. He flow rate with the injection time at outlet for the first He injection (injection pressures were marked against each stable stage)

70 60

Flow rate (ml·s-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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50 Fluctuation region, CO2 desorption, Matrix shrinkage

40 30

13MPa 12MPa

20

11MPa 10MPa

10 9MPa

0 0

5

10

15

20

Time (h)

25

30

35

Figure 5. He flow rate with the injection time at outlet for the second He injection (injection pressures were marked against each stable stage)


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50 45 13MPa

40

Flow rate (ml·s-1)

Fluctuation region, CO2 desorption, Matrix shrinkage

35 30

12MPa

25 20

11MPa

15 10MPa

10 5

9MPa

0 0

5

10

15

20

25

30

35

Time (h) Figure 6. He flow rate with the injection time at outlet for the third He injection (injection pressures were marked against each initial stage) 60

1st He flow 2nd He flow 3rd He flow 4th He flow 5th He flow 6th He flow Trend line for 1st flow Trend line for 2nd flow Trend line for 3rd flow Trend line for 4th flow Trend line for 5th flow Trend line for 6th flow

50

Flow rate (ml·s-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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40 30 20 10 0 8

9

10

11

12

Injection pressure (MPa)

13

14

Figure 7. He flow rate versus injection pressure for each He injection Table 1. Regressed equations of trend lines Trend line

Regressed equation

R2

1st He flow

Q = 12.057P - 101.98

0.9959

2nd He flow

Q = 9.642P - 81.34

0.9912

3rd He flow

Q = 9.385P - 79.632

0.9911

4th He flow

Q = 9.244P - 79.977

0.9855

5th He flow

Q = 9.185P - 79.159

0.9900

6th He flow

Q = 9.118P - 79.617

0.9794


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0.050 0.045 Permeability (md)

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0.040 0.035 0.030 1st He flow 3rd He flow 5th He flow

0.025 0.020 8

9

10 11 12 Injection pressure (MPa)

2nd He flow 4th He flow 6th He flow

13

14

Figure 8. He permeability versus injection pressure for each He injection

3.1.2 Permeability evolution with saturation time

Changes in He permeability after each phase of super-critical CO2 injection are shown in Figure 8. It was expected that He permeability would increase with the progressive injection pressure since He adsorption caused a negligible matrix swelling, and furthermore, the increase of pore pressure resulted in the reduction of effective stress and the dilatation of the cleats, facilitating He flow. At the first He injection, He permeability increased by 1.31 times as injection pressure was incremented from 9 MPa to 13 MPa. At subsequent He permeability tests, this rate of permeability increase maintained around 1.18~1.43 times. At the second He injection, according to the detected results by gas chromatography, no more CO2 was desorbed after over 6 hours of He injection, but there was still a 10.84% permeability reduction at 9 MPa injection pressure and about 20% permeability reduction at 10-13 MPa injection pressure. He permeability presented an average reduction of approximately 19.44% after the first super-critical CO2 injection, which was the maximum reduction after each cycle of CO2 saturation. Previous research proved


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that CO2 adsorption-induced swelling narrowed down the gas flow path and caused a predominant reduction in permeability [18]. After that, due to further adsorption-induced swelling, the magnitude of average permeability reduction decreased after each period of saturation, which was calculated as 19.44%, 5.51%, 4.93%, 3.17%, and 2.45%, respectively. In addition to coal swelling, the gradual reduction of permeability is also related to the extraction of organic matters, mineral dissolution and mechanical property alteration, which is an integrated result of coupled effects and will be discussed in the following sections.

3.2 Extraction due to CO2 injection

3.2.1 Extraction analysis

Five successive extractions were collected after five cycles of super-critical CO2 injection, and the extracts were named in order according to the order of CO2 injection (Figure 9). These extractions were successively evaporated using a rotary evaporator and a gentle stream of nitrogen to reduce to a final volume

of

approximately

0.5

ml.

Subsequently,

the

extractions

were

analyzed

via

gas

chromatography-mass spectrometry (GC-MS) using an Agilent 6890 gas chromatograph interfaced with an Agilent 5973 mass selective detector. The potential loss of the extracted organic matters and the solvent (CH2CL2) is acknowledged due to the dynamic injection of super-critical CO2. In order to minimize their evaporation, the CO2 flow rate in the solvent trap was limited through the valves at both sides of the solvent trap. Based on the NIST library, a semi-quantitative analysis of the detected components in the extractions was conducted according to the peak area. Figure 10 shows the chromatograms of the extracts after the second super-critical CO2 injection and the most obvious aliphatic and aromatic fractions are highlighted.


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The aliphatic compounds detected in the extracts are composed of n-alkanes from n-C10 to n-C24 and some cyclic and branched alkanes. The other detected compounds are aromatic hydrocarbons with one, two or more benzene rings. In addition, some oxygen-containing aliphatic and aromatic compounds, such as aldehydes, ketones, alcohols, and carboxylic esters, were detected, but the relative concentrations were very low. The yielded oxygen-containing hydrocarbons were approximately 10 percent of the total extracts. Multiple cycles of super-critical CO2 injection extracted additional aliphatics and aromatics (Figure 11). Similar to the trend of the He permeability variation, the concentrations of alkanes and aromatics generally decreased with each successive CO2 injection. The maximum reduction in the concentrations occurred after the first super-critical CO2 injection, and the aliphatics and aromatics decreased by 33.8% and 12.5%, respectively. After the fifth CO2 injection, approximately 47% of alkane concentrations and 63% of the aromatics concentrations were observed compared to the first extractions. Interestingly, the yielded aromatics was around 15%~22% percent of the alkanes in each cycle, which reveals that super-critical CO2 tends to mobilize a higher proportion of aliphatics compared to aromatics [26].


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Figure 9. Evaporated extracts with successive super-critical CO2 injection

Figure 10. Chromatograms of extracts after second super-critical CO2 injection


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Figure 11. Aliphatics and aromatics yielded after each cycle of scCO2 injection

Coal can be viewed generally as a three-dimensional macromolecular polymer network consisting of condensed aromatic and hydroaromatic compounds, which are cross-linked via short alkyl bridges, ether linkages, and thioether linkages [37,38]. Some small organic molecules, mainly oxy-compounds and hydrocarbons, are trapped in the macro- and micro-pores or bonded with some weak non-covalent bonds, such as hydrogen bonds or other even weaker interactions, which are easily mobilized when super-critical CO2 is injected into the coal [39,40]. Some n-alkanes and PAHs in the coal bitumen are dissolved or physically trapped in the asphaltenes, which are polar compounds composed of nitrogen, sulfur, and oxygen functionalities and are hardly dissolved in super-critical CO2 [41,42]. It is a slow process to mobilize these hydrocarbons in the asphaltenes because super-critical CO2 is unable to significantly disrupt these polar associations present within bitumen [43]. This accounts for why a sharp drop in measured hydrocarbon concentrations occurred after the first CO2 injection, but the declines became gradual in the following successive extractions. Some small-molecule hydrocarbons in the coal sample were also mobilized due to super-critical CO2 injection, and some specific gases, such as CH4, C2H4, and C2H6, in the yielded gas were detected via gas


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chromatography (GC) (Figure 12). Similarly, the concentrations of CH4, C2H4, and C2H6 decreased with successive cycles of CO2 injection and the maximum reduction of the concentrations took place at the second CO2 injection. At the end of the fifth cycle of CO2 injection, the average concentrations of CH4, C2H6, and C2H4 were 0.0050%, 0.0223% and 0.0012%, respectively, which were about 30% of those at the first cycle of CO2 injection.

Figure 12. Concentration of yielded gas with extraction time

3.2.2 Proximate analysis

As mentioned earlier, adjacent coal blocks (Region 1 and Region 2, highlighted in Figure 2(a)) were processed into powdered coal to investigate the variations of the coal sample after super-critical CO2 saturation. These powdered coals were then analyzed via proximate analysis, FTIR (Bruker VERTEX 70) and low-pressure-temperature N2 adsorption (ASAP2020).


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Table 2 shows the variation of moisture, ash, and volatile matter content before and after scCO2 saturation. As discussed previously, the alkanes and PAHs trapped in the macromolecular polymer network were mobilized due to the super-critical CO2 extraction, resulting in the loss of hydrocarbons from the coal matrix. Since the boiling points of these alkanes and PAHs are basically below 600℃ [44], they will become gaseous or be decomposed in the volatile matter content test (around 900℃), which means the extracted alkanes and PAHs partly contribute to the decrease of volatile matter content after super-critical CO2 injection. As scCO2 has the ability to completely dissolve and displace moisture in coal [45], an average 89.68% decline of the moisture content was observed. CO2 can dissolve in water and form an acidic solution, which dissolves minerals in the coal sample. As a result, the mean ash content (air-dried) decreased by 25.98%. Table 2. Variations of moisture, ash, and volatile matter content before and after scCO2 extraction Moisture content (air dried, %)

Ash content (air dried, %)

Volatile matter content (dried ash free, %)

State Region 1

Region 2

Region 1

Region 2

Region 1

Region 2

Before scCO2 saturation

6.23

6.07

7.28

7.31

38.24

36.81

After scCO2 saturation

0.69

0.58

5.29

5.51

32.22

31.43

Decrease percent/%

88.92

90.44

27.34

24.62

15.74

14.62

Average decrease percent/%

89.68

25.98

15.18

3.2.3 Functional groups analysis

Figure 13 shows the FTIR spectra of the coal in Region 1 and Region 2 before and after super-critical CO2 saturation, and reflects the obvious variations of the aliphatic and aromatic functional


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groups. A predominant decrease of the adsorption intensity was observed at the region of 1480-1800 cm-1 which are assigned to the C=O and aromatic C=C stretching bands. Specifically, the absorption at 1600 cm-1 represents the aromatic C=C bonds in a single benzene ring, the absorption at 1570 cm-1 belongs to the aromatic C=C bonds in two or three rings, and the absorption at 1490 cm-1 is assigned to the aromatic C=C bonds in three or more rings [39,46]. The distinct trend of decline of the aromatic groups is consistent with the corresponding PAHs detected in the yielded extractions. In addition, the 1730 cm-1 region was taken as the characteristic absorption of hydrogen-bonded carbonyl or carboxyl [47], which precisely verified the oxygen-containing hydrocarbons in the extracts. The adsorption band of the aliphatic groups can be found in the regions of 2750-3000 cm-1, which mainly consist of methyl (CH3), methylene (CH2) and methine (CH). Small aliphatic molecules are embedded in the macro- and micro- pores or bonded with some weak non-covalent bond [39], which are easily mobilized by super-critical CO2. Therefore, great amounts of alkane were detected in the yielded extractions (Figure 10, Figure 11), and a similar decreasing trend was also observed in these infrared characteristic regions. The absorbance bands of carbonate minerals and clays located in the regions of 600-1000 cm-1, and the adsorption peaks at 876 cm-1, 800 cm-1, 750 cm-1, represented calcite, quartz, and illite, respectively [48,49]. Given that water was present in the coal sample (6.36% moisture content in the raw coal), the water in contact with carbon dioxide became acidic and the measured pH varied from 2.80 to 2.95 in the super-critical CO2 extracted from water-containing samples [50]. Obviously, due to the newly-formed carbonic acid, part of the minerals is dissolved, resulting in a predominant declining trend in the stretched absorption peaks (876 cm-1 and 750 cm-1) and 25.98% decrease of the mean ash content after multiple cycles of super-critical CO2 injection (Table 2). Furthermore, the areas at 700-900 cm-1 were also


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assigned to aromatic CH out-of-plane bands [51,52], which were weakened due to super-critical CO2 extraction. As a result, mineral dissolution and aromatic extraction contributed to weaker overlapping bands in the region 700-900 cm-1.

(a) Region 1


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(b) Region 2 Figure 13 FTIR spectra of coal before and after super-critical CO2 extraction

3.2.4 Pore structure analysis

Low-pressure-temperature (10nm in diameter) are the main gas flow path for CO2 transportation in the coal mass. Furthermore, the specific surface area, pore volume, and average pore size were also calculated to investigate pore morphology variations due to CO2 saturation (Figure 15). The specific surface area was determined by the Brunauer−Emmett−Teller (BET) model [59] and the pore volume was obtained from the desorption branch of the nitrogen isotherm according to the Barrett, Joyner, Halenda (BJH) theory [60]. After super-critical CO2 saturation, the BET surface areas of Region 1 and Region 2 decreased by 24.02% and 23.29%, respectively, and the BJH pore volumes decreased by 12.23% and 11.74%, respectively.


Absorbed

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Silt-shaped pore

H3 type

P/P0

1

(a) Region 1

H3 type Absorbed

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Silt-shaped pore

P/P0

1

(b) Region 2 Figure 14. Low-pressure N2 adsorption isotherms at 77 K of coal samples


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(a) Region 1

(b) Region 2 Figure 15. Variations of pore size distribution before and after ScCO2 saturation, pore classification system by Hodot: micro-pores (< 10 nm in diameter), transition pores (10–100 nm in diameter), meso-pores (100–1000 nm in diameter), and macro-pores (> 1000 nm in diameter)


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For low-rank coal with a Ro. max of

Coupling Effects of Supercritical CO2 Sequestration in Deep Coal

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