Interaction between sequestered supercritical CO2 and minerals in deep coal seams

International Journal of Coal Geology 202 (2019) 1–13

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International Journal of Coal Geology journal homepage: www.elsevier.com/locate/coal

Interaction between sequestered supercritical CO2 and minerals in deep coal seams Renxia Jiang, Hongguan Yu

T

⁎

School of Chemical and Environmental Engineering, Shandong University of Science and Technology, 266590 Qingdao, China

ARTICLE INFO

ABSTRACT

Keywords: Coal Carbon dioxide Minerals Pore structure Interaction Sequestration

This study investigates the interaction between supercritical CO2 (scCO2) and minerals in coal that may occur during CO2 sequestration into deep coal seams. Two low-rank coals with low-ash content, one low-rank coal with high-ash content and one high-rank coal with low-ash content were used to study the interactions. A coal-CO2 geochemical interaction experiment with a typical high-ash coal was conducted to investigate change of the water-soluble elements after scCO2 interaction. X-ray powder diffraction (XRD) was used to determine the effects of scCO2–H2O on the minerals in the four coals. In addition, the influence of scCO2 on pore structure of four coals was studied based on liquid nitrogen adsorption experiment. The results suggest that scCO2 can effectively enhance the solubility of all elements from coal in water because of scCO2 interaction. ScCO2 can significantly improve the solubility of trace elements than that of major elements. Because of the relatively low mineral content in coals and the short experimental period, secondary minerals formed by scCO2-mineral-water interaction were not determined with XRD. However, the content of primary minerals changed; the apparent content of quartz increased and the content of kaolinite, carbonate minerals and pyrite decreased. For the experimental coals in this study, scCO2 can enlarge the pore size of the micropores for low-ash coal and mesopores for lowrank coal, but shrinks that of micropores for high-ash coal and mesopores for high-rank coal.

1. Introduction Geological CO2 storage into deep unminable coal seams is an important topic of interest as it not only potentially enhances the recovery of coalbed methane, but also sequesters CO2 to reduce greenhouse gas CO2 emissions to the atmosphere. The injected CO2 is in a supercritical state (scCO2) with a high-pressure and relatively high temperature in the deep coal seam, in which pressure is higher than 7.38 MPa and the temperature is higher than 304.13 K. Coal is a mixture of inorganic minerals with different amounts and types and organics in a complex and three-dimensional network, which may change during CO2 sequestration process. The coal reservoirs often contain groundwater that includes bound water, free water and adsorbed water. Therefore, a scCO2-H2O-coal geochemical reaction system is formed when CO2 is injected into a deep coal seam. The injected CO2 will form a mixed zone of CO2, water and coal in coal reservoir. The injected CO2 dissolves in water to form carbonic acid, where the acid water can react with the minerals in coal and change the concentration of ions in the water and the composition of minerals in coal. Minerals mainly undergo the following three changes after CO2 is injected into a deep brine reservoir: mineral dissolution (Wdowin et al., ⁎

2014), the transformation of minerals (Wang et al., 2018), and the formation of new minerals (Wdowin et al., 2014). Many scholars focused on the interaction between CO2 and organic matter in coal and its impact on CO2 sequestration (Cao et al., 2011; Gathitu et al., 2009; Larsen, 2004; Li et al., 2017; Mirzaeian and Hall, 2006; Scherf et al., 2011; Wang et al., 2017; Zhang et al., 2017), while few studies have focused on the interaction between CO2 and minerals in coal. Studies of the effects of weak acidic/basic solutions on coal have frequently focused on the demineralization of the coals to produce low-ash coal (Meshram et al., 2015) and the leachability of the toxic heavy metals from coal to investigate environmental and health concerns (Verma et al., 2015). Studies of the CO2-water-mineral and CO2saline aquifers interactions have frequently focused on the CO2-induced changes in aqueous phase pH (Harvey et al., 2013), mobilisation of contaminants (Lu et al., 2010), mineral trapping (Riaz and Cinar, 2014) or salt precipitation (Miri and Hellevang, 2016) during CO2 storage, potential impacts of CO2 leakage (Little and Jackson, 2010). Results gathered from these studies are of questionable applicability to CO2 sequestration in coal. CO2 is dissolved in water to form carbonic acid (H2CO3) solutions, which reduces the pH of the solution. Some minerals in coal react with

Corresponding author. E-mail address: [email protected] (H. Yu).

https://doi.org/10.1016/j.coal.2018.12.001 Received 7 September 2018; Received in revised form 2 December 2018; Accepted 2 December 2018 Available online 05 December 2018 0166-5162/ © 2018 Published by Elsevier B.V.

International Journal of Coal Geology 202 (2019) 1–13

R. Jiang, H. Yu

the acidic water, which will result in the dissolution of the minerals and the changes of elements composition in the acidic water (White et al., 2005). The use of CO2-ECBM/sequestration techniques may affect the composition of the produced waters, CO2 adsorption capacity of coal and surface properties of coal. Alkaline-earth metals are easily removed from some minerals (such as calcite and dolomite) in coal with the acidic water. We have little definitive information on the long-term effects of CO2 on mineral matter in coal. The dissolution and mobilisation of several elements caused by the geochemical reaction of scCO2 with minerals in coal had been studied by several researchers. Ca and Mg were removed from a brown coal and two subbituminous coals by treating with CO2 dissolved in water under a pressure of 600 kPa (Hayashi et al., 1991). The release and migration of the major and trace elements from coal caused by CO2-water-coal interaction were studied at 25–35 °C and pressures of 5–11 MPa (Zhao et al., 2018). Their results showed that the concentration of major elements (K, Na, Ca and Mg) in CO2-H2O solution is lower than that of N2-H2O solution, and the migration capacity of trace elements is in the order of As > Zn > Ni > Ba > Sr > Hg > Cd > Co > Tl > Li > Mo > Ag > Sb > V > Cu. The results of the static batch experiments with scCO2-H2O at 9.5 MPa and 40 °C indicated mobilisation of elements (such as Ca, Mg and Fe) due to dissolution minerals and displacement of ion exchangeable metals (Dawson et al., 2011). The mobilisation of elements from coal was studied using the batch reactor experiments with CO2-water and at 40 °C and 9.5 MPa. The results showed that several elements (Al, Ba, Ca, Fe, Mg and Mn) consistently preferentially mobilised from coal by CO2-H2O, whereas elements (Ag, B, Hg, K, Li, Na and Sb) were much less likely to be preferentially mobilised by CO2-acidified water (Dawson et al., 2015). A few scholars have studied the CO2 adsorption and minerals change caused by the reaction of scCO2 with minerals in coal. Clays in coal can store CO2 (Karacan, 2003; Karacan and Mitchell, 2003). Clays have the highest CO2 adsorption rate (Karacan and Mitchell, 2003), and clay region was compressed in coal and the highest amount of CO2 was stored in clay regions (Karacan, 2003) as a result of imbibition of CO2. The relative carbonate content decreases with time and the relative clay content increases after the reaction with CO2 and water (Guo et al., 2018). There was a difference in mineral dissolution rates, where scCO2-H2O has the strongest effect on calcite, followed by dolomite, aluminum hydroxide minerals, chlorite, and albite, but the effects are not obvious for illite, kaolinite, and quartz (Du et al., 2018). The experiments of CO2-minerals-water interaction under 40 °C and 5 MPa showed that obvious changes have taken place in coal minerals (X-ray diffraction) and minerals in the fracture of coal (especially trace elements) can be dissolved in water (Hedges et al., 2007). To investigate CO2–brine–rock interaction during CO2 sequestration in deep coal seams, batch experiments were conducted for reacting powdered rock samples with CO2 and brine and CO2 at 160 °C and 15 MPa. The results showed that the contents of quartz, plagioclase, illite and chlorite increased considerably, whereas the contents of illite/smectite, biotite and kaolinite decreased more or less for lithic sandstone after reaction with CO2-brine (Wang et al., 2016). The effects of scCO2 on pore structure are governed by coal rank, mineralogy, water content, temperature, and pressure (Liu et al., 2018). (1) Water plays an important role in the reactions between scCO2 and minerals in coal. The reaction of carbonic acid with coal shows that the minerals in coal are dissolved in water and form ions dissolved in water. The ion leaves the coal with the flow of acidic water, which leads to the increase of porosity or permeability of the coal and ultimately enhances CO2 adsorption capacity of the coal (Massarotto et al., 2010). (2) scCO2 can increase the pore volume (PV) of macropores and decrease that of micropores for lignite coal, but scCO2 can decrease the PV of macropores and increase that of micropores for bituminous coal, (Massarotto et al., 2010; Wang et al., 2014). ScCO2 has a minor impact on macropores and mesopores for anthracite but increases the PV of micropores (Liu et al., 2010; Mirzaeian and Hall, 2006). ScCO2 can increase specific

surface area (SSA) of micropores, decrease the SSA of macropores and mesopores for bituminous coal and anthracite (Gathitu et al., 2009). The effects of scCO2 on coal pore structure and connectivity have generally been recognized, but the changes in coal pore structure and connectivity caused by scCO2 are still controversial (Liu et al., 2018). The interaction mechanisms between coal and CO2 are not yet fully understood because of the complex heterogeneous processes of interaction. The processes involve the swelling of coal matrix during coal adsorption of CO2, the extraction of small organic molecules from coal, the reaction of CO2 and minerals, and scCO2 drying coal. The effects of scCO2 on pore structure are complex. The influencing factors include characteristics of coal (such as coal rank, grade, structure and mineralogy), the existing form and content of water in coal, and experimental conditions (temperature and pressure). This study investigated the dissolution of some elements from coal in water under the condition of CO2 pressure up to 10 MPa and a temperature of 40 °C. The possible changes of minerals from coal were studied by comparing XRD measurement data of raw coal and interacted-coal. The low-temperature liquid nitrogen adsorption method is used for analyzing the change of pore size by comparing the measured data of raw coal and scCO2 interacted-coal for a period of 30 days. 2. Materials and methods 2.1. Coal sample characteristics For this study, one low-rank Longkou coal with high-ash content, two low-rank coals (Yinni and Caili) with low-ash content, and one high-rank Shanxi coal with low-ash content were selected to investigate the interaction between scCO2 and minerals in coal. Some of these coals were also used to study scCO2 adsorption on coal (Jiang et al., 2017; Yu et al., 2017a) and scCO2 extraction of small organics from coal (Yu et al., 2017b). The basic information on proximate analysis, total sulfur and calorific value for the four coals are given in Table 1. Chinese national standards were used to obtain the basic information, where proximate analysis was conducted according to GB/T212–2008, the content of total sulfur in coal was tested according to GB/T214–2007 and the calorific value of coal followed GB/T213–2008. The high-ash Longkou coal was used to study the change of soluble elements from the raw-coal and scCO2 interacted-coal. The composition of oxides for coal ash and the major elements in coal are shown in Table 2. The content of X element in a dried-coal can be calculated by the content of oxides (CXaOb) in coal ash, as shown in the following equation:

CX = CXaOb Ad

aMX 100MXaOb

(1)

where, CX and CXaOb is the content of element X in dried-coal and the content of oxide XaOb in coal-ash, respectively (in %), Ad is ash content Table 1 Basic information on proximate analysis and calorific value of the coal sample used in the experiment. Sample

Yinni Caili Longkou Shanxi a

Qgr,dd (MJ/kg)

Proximate analysis and total Sulfur (%) Mada

Adb

Vdafc

FCdafc

St,db

6.89 8.50 9.82 2.87

9.73 14.40 38.23 10.82

50.48 49.74 56.02 9.61

49.52 50.26 43.98 90.39

0.77 0.64 0.81 0.83

27.13 21.23 15.27 31.01

Mad is moisture content on an air-dry basis. Ad and St,d are ash and total‑sulfur content on a dry basis, respectively. c Vdaf and FCdaf are volatile matter and fixed carbon content on a dried, ashfree basis, respectively. d Qgr, d is gross calorific value on a dry basis. b

2

International Journal of Coal Geology 202 (2019) 1–13

R. Jiang, H. Yu

Table 2 Ash compositions and content of major elements in coal for Longkou coal sample (%). Oxides/Elements

SiO2/Si

Al2O3/Al

Fe2O3/Fe

MgO/Mg

CaO/Ca

TiO2/Ti

K2O/K

Na2O/Na

Oxides in ash (XaOb), CXaOb Elements in coal (X), CX

55.26 9.87

17.10 3.46

5.11 1.37

8.22 1.90

4.23 1.16

1.22 0.28

0.89 0.28

0.78 0.22

of the interaction experiment and determination. In order to study the effect of moisture in coal on the interaction between scCO2 and minerals in coal, four samples of Longkou coal were carried out the experiment, which are the dried-coal, air-drying coal, wet coal with a moisture content value 10 wt% and 30 wt%, respectively. The interaction time was 30 days, test temperature was (40 ± 0.5) °C and the pressure of CO2 was (10 ± 0.4) MPa. In order to speed up the test progress, four sample vessels in parallel were used for the interaction test. Fig. 3 is a schematic diagram of the test device. This test does not require accurate pressure monitoring, so the device uses four pointer pressure gauges with a maximum pressure of 40 MPa. Four coal samples of Longkou coal with a mass of about 30 g (m g) were weighed, one was directly added to the sample vessel as the airdried coal sample, the second was dried at 105 °C for 2 h, and the third and the fourth were wet coal with 10 wt% and 30 wt% total-water, respectively. Deionized water was added to the air-dried coal sample to prepare the wet coal, and the amount of water added is determined according to the air-dry moisture in coal (Mad), the coal sample mass (m), and the moisture content of the wet coal sample. The temperature was controlled at 40 °C by a water bath, and CO2 pressure of sample vessel was increased to 10 MPa by a booster pump. If the pressure dropped > 0.4 MPa during the test, the pressure was restored to 10 MPa by the booster pump. The interaction time between coal and scCO2 continued for 30 days. At the end of the 30-days reaction experiment, all the coal samples were removed from the sample vessel and put in the beaker. Deionized water was added to the beaker with the interacted-coal sample to ensure that the volume of deionized water added is 10 mL per gram of drycoal. The volume of deionized water added is determined according to the content of moisture in the interacted-coal and the coal sample mass (m). Thus, the volume of the leachate prepared is VT, which is about 10 × m (mL). The coal-water solution was fully stirred and filtered by filter paper to obtain the leachate. At the same time, similar operations for unreacted raw-coal were carried out for comparison of the interacted-coal and raw-coal. A volume (VSX) of prepared-filtrate was diluted to a volume (V0) with deionized water to meet the needs of AAS measurements. The contents of soluble elements were determined by AAS. The content of elements measured in the leachate (CSX) is converted to the content of soluble elements per unit coal mass (CmSX), and the formula is as follows:

Table 3 Content of trace elements in Longkou coal sample (μg/g). Elements

Zn

Ni

Mn

Cu

Cr

Pb

Content

35

17

13

21

24

38

2500

a.Quartz b.Calcite c.Pyrite d.Kaolinite e.Siderite f.Muscovite

a 2000

1500

b 1000

a d

500

d 0

10

20

b c

ab a a ecb f aba bb a a cb

30

40

50

60

a 70

a aa 80

90

2

Fig. 1. X-ray diffraction profiles of Longkou coal.

in coal on a dry basis (in %), MX and MXaOb is the atomic weight of element X and molecular weight of XaOb (in g/mol), respectively, a is the number of element X and b is the number of oxygens in XaOb. Table 3 shows the contents of some trace elements in Longkou coal, measured by atomic absorption spectrophotometry (AAS). The XRD of Longkou coal is shown in Fig. 1. The main minerals in Longkou sample are quartz (SiO2), calcite (CaCO3), kaolinite (Al2Si2O5(OH)4), siderite (FeCO3), pyrite (FeS2) and muscovite. Quartz (35.6%) and calcite (22.8%) content were the highest, followed by kaolinite (17.9%) and muscovite (15.7%), siderite (4.6%) and pyrite content (1.9%) were the lowest. 2.2. Experimental approach The scheme of scCO2-mineral interaction experiments and the techniques used in this study is shown in Fig. 2. High-ash Longkou coal was used to investigate the change of soluble elements caused by the reaction of scCO2, water and minerals in coal. In addition to Longkou coal, low-rank Yinni and Caili coal and high-rank Shanxi coal with low-ash content were used for the X-ray diffraction (XRD) analysis to investigate the change of minerals caused by scCO2mineral-water interaction, where Longkou coal involve air-dry coal and wet coal with 30% water, and the other three coals involve air-dry coal. The nitrogen adsorption analysis of the raw-coals and interacted-coals were used to study the change of pore structure caused by the interaction between coal and scCO2. Because adding water to coal under high pressure will make coal matrix expand and the four coals used in the N2 adsorption experiment have higher air-dry moisture, only air-dry interacted-coals and raw-coal of the four coals were used the N2 adsorption analysis. The sample was crushed to pass through a 200 mesh sieve, and a magnet was used to remove iron from the sample brought by crushing equipment. The prepared samples were sealed and kept for the purpose

CmSX =

CSX VSX VT 100 mV0 100 Mad

(2)

where CmSX is the content of soluble element X in the coal expressed by a dry basis (in μg/g), CSX is the concentration of element X measured by AAS (in μg/mL), VT is the total volume of leachate prepared (in mL). VSX is the volume of the leachate collected for preparation of the solution determined by AAS (in mL), V0 is the diluted volume of the leachate for the determination of AAS (in mL). In addition, 50 mL of filtrate was dried by natural drying, and the soluble-material mass in the unit mass coal (mSM) was calculated with the following equation:

mSM = 3

mSM50 VT 100 50m 100 Mad

(3)

International Journal of Coal Geology 202 (2019) 1–13

R. Jiang, H. Yu

One high-ash coal

Raw coal

One high-ash coal and three low-ash coals

Raw coal+Water+scCO2 Dried coal

Wet-coal with 10% water

Raw coal+Water+scCO2

Wet-coal with 30% water

Air-dry coal

High-ash coal with 30% water

Raw coal

Air-dry coal

Leaching with water

AAS

XRD

Liquid N2 adsorption

Soluble element

Minerals change

Change of pore size

Interaction between scCO2 and minerals in coal Fig. 2. Scheme of scCO2-mineral interaction experiments and the techniques used in this study. P

Pt

TTr

Sample vessel-1 Needle valve

Manometer

Booster pump

P

Pt

Release valve

TTr

Sample vessel-2

PE tube Needle valve

Air

P Coal-1

Needle valve CO2 storage tank

Compressed air

P Coal-2 Pt

P

Release valve

TTr

Sample vessel-3 Needle valve

Pt

P

Coal-3 P

Pt

Atmosphere Release valve

Vacuum pump

TTr CO2 cylinder

Compressor

Pt: Platinum resistor P: Manometer TTr:Temperature transmitter

Sample vessel-4 Needle valve

Water bath

Coal-4

Release valve

Computer

Fig. 3. Simplified diagram of the experimental apparatus for interaction between mineral matter in coal and ScCO2.

where mSM is the content of soluble material (SM) in coal expressed by a dry basis (in mg/g), mSM50 is the mass of SM in the 50 mL filtrate (in mg/mL). Water has a significant effect on the interaction between scCO2 and minerals in coal. The dried-coal, air-dry coal and wet coals with 10 wt% and 30 wt% water were used to study the difference in the amount of soluble matter (mSM) and the concentration of soluble-element (CmSX) in a dry coal.

software package (MDI Jade6) was used to quantify the minerals in the coal. The pore structure parameters, including pore volume, specific surface area and pore diameter were determined by the low-temperature liquid nitrogen adsorption experiment. The N2 adsorption/desorption experiment was performed on an automated specific surface areas analyzer (SSA-4200) produced by Beijing Piaode Electronic Technology Co., Ltd. Prior to N2 adsorption/desorption, all the coal samples were fully degassed under vacuum conditions to remove residual gas and moisture in coals at 105 °C for 12 h. Then, nitrogen adsorption isotherms were obtained for relative pressure (P/P0) varying from 0.01 to 0.995, where P and P0 are the equilibrium and the saturation pressure of N2 at 77 K. The adsorption/desorption experiment and calculation of pore structure parameters can be performed automatically by the computer software of SSA-4200.

2.3. Analytical techniques Raw-coal and CO2 interacted-coal were ground to passing through 200 mesh sieve before using it for XRD analysis. X-ray diffraction data were obtained using an X-ray diffractometer (Rigaku D/Max2500PC, Japan). Samples were run from 5° to 90° 2θ with a step of increment of 10°/min, at 40 kV and 30 mA. A quantitative X-ray diffraction analysis 4

International Journal of Coal Geology 202 (2019) 1–13

R. Jiang, H. Yu

Table 4 The content of dissolved matter and elements of Longkou raw-coal and CO2-interaction coal. Sample

Raw coal Dried interacted-coal Air-dry interacted-coal interacted-coal with 10 wt% water interacted-coal with 30 wt% water

Soluble matter mSM (mg/g)

0.05 0.15 0.15 0.27 0.33

Soluble elements,CmSX (μg/g) Ca

Mg

Fe

Al

0.02 1.06 1.25 3.09 4.23

1.95 2.37 3.60 4.28 6.25

1.21 1.78 2.12 3.56 10.28

NA NA NA 0.11 0.28 ⁎

K

Na

Zn

Ni

Mn

Cu

Cr

Pb

3.64 5.87 6.35 9.50 13.57

35.35 37.26 40.38 67.35 100.25

0.23 0.75 0.78 0.85 1.02

0.02 0.02 0.02 0.03 0.05

NA NA NA NA NA

0.04 0.08 0.1 0.14 0.28

1.78 2.35 2.89 4.25 6.21

NA NA NA NA NA

NA⁎ indicates that the element was not analyzed.

3. Results and discussion

detected in leachates from raw-coal, interacted dried-coal and interacted air-drying-coal. The amount of Fe in the leachate from scCO2 interacted-coal with 10 wt% water is greater than that of Mg from the interacted-coal with 30 wt% water. The amount of dissolved trace-elements is from large to small: Cr > Zn > Cu > Ni, and their contents increase with the increase of moisture in coal. The relative amount of soluble matter (RmSM) and relative concentration of each soluble-element (RCmSX) were used to express the mSM and CmSX, which were calculated with the following equations:

3.1. Soluble matter and elements The amount of soluble matter (mSM) and content of soluble elements (CmSX) with unit mass coal on a dry basis for Longkou raw-coal and CO2-interaction coal is shown in Table 4. It can be seen from Table 4 that the content of soluble matter is from small to large: raw coal < interacted dried-coal = interacted airdrying-coal < interacted coal with 10 wt% water < interacted coal with 30 wt% water. That is to say, the soluble amount of CO2-interacted coal is significantly greater than that of the non-interacted coal (raw coal), and the amount increases with the increase of water content in coal. Metal ions combined with organic matter in coal (Schatzel and Stewart, 2003; Vassilev and Tascón, 2003; Ward, 2002) can be extracted by scCO2 in the presence of water (Erkey, 2000). CO2 with a dipole-quadrupole structure will show a certain polarity after being induced by polar H2O (Pourmortazavi and Hajimirsadeghi, 2007). Therefore, water can be used as a modifier for scCO2 extraction of metallic organics in coal. Under the action of high-pressure scCO2, the wetting angle of coal decreases (Plug et al., 2008; Sakurovs and Lavrencic, 2011), which makes it easier for water and CO2 to contact and beneficial to the dissolution of some minerals in coal. So, the minerals in the coal are easier to leach out by scCO2 and water, which is caused by coal swelling, pore expansion and modifier of water. The amount of soluble matter increases with the increase of water amount, indicating that water has a significant influence on the formation of soluble matter. Just as shown in Table 4, except that Mn and Pb were not detected in leachates from all coals and no Al was detected in leachates from raw-coal and the dry and air-drying interacted -coal, all other listed elements were detected. Except for Al, Mn and Pb, the concentrations of elements mobilised are from small to large: raw coal < interacted dried-coal < interacted air-drying-coal < interacted coal with 10 wt% water < interacted coal with 30 wt% water. That is, the contents of all the soluble elements from scCO2 interacted-coal are higher than that of raw coal, and the amount of soluble elements increases with the increase of water content in the coal. Elements such as Na, K, Mg, Ca, Fe and Al are often major constituents in coal. On average, the content of the six mobile elements is from large to small: Na > K > Mg > Fe > Ca > Al. No Al was

RmSM =

mSM,IC mSM,RC

RCmSX =

(4)

CmSX,IC CmSX,RC

(5)

where RmSM is the ratio of the amount of soluble matter from interacted coal (mSM,IC) to that from raw-coal (mSM,RC), RCmSX is the ratio of the content of soluble element X from interacted-coal (CmSX,IC) to that from raw-coal (CmSX,RC). The ratio of the soluble-matter amount and solubleelement concentration from interacted-coal to that from raw-coal are shown in Table 5. The RmSM and RCmSX represent the increments in the solubility of soluble matter (SM) and soluble elements (SX) relative to the solubility of raw-coal. As can be seen from Table 5, scCO2 can enhance the solubility of elements in coal, and water can promote the dissolution of some elements. In all soluble elements, the element with maximum incrementsolubility from scCO2 interacted-coals is Ca and that with a minimum is Ni as compared with raw-coal. Compared with raw coal, Ca solubility in CO2 interacted-coal is 53–211.5 times that of raw coal, while that of other elements in the interacted-coal is 1.1–8.5 times that of raw coal. The contents of elements in the coal vary greatly as shown in Tables 2 and 3, it is reasonable that the amount of soluble elements in leachates obtained in the experiment varies greatly. In order to understand the proportion of the dissolved-elements amount to the total amount of the elements in raw coal, the ratio (in percent) of the amount of soluble elements from the leachates (CmSX, Table 4) to total amount of the element in coal (CX, Tables 2 and 3) was calculated and expressed in RRCmSX (%). The formulas used are shown in Eqs (6) and (7), where Eq. (6) applies to the major elements shown in Table 2, and Eq. (7) applies to the trace elements shown in Table 3.

RRCmSX = 10

4

×

CmSX CX

(6)

Table 5 The ratio of the soluble-matter amount and soluble-element concentration from interacted-coal to that from raw-coal. Sample

Dried interacted-coal Air-dry interacted-coal interacted-coal with 10 wt% water interacted-coal with 30 wt% water

Soluble matter RmSM

3.0 3.0 5.4 6.6

Soluble element, RCmSX Ca

Mg

Fe

Al

K

Na

Zn

Ni

Cu

Cr

53.0 62.5 154.5 211.5

1.2 1.9 2.2 3.2

1.5 1.8 2.9 8.5

– – ∞ ∞

1.6 1.7 2.6 3.7

1.1 1.1 1.9 2.8

3.3 3.4 3.7 4.4

1.0 1.0 1.5 2.5

2.0 2.5 3.5 7.0

1.3 1.6 2.4 3.5

5

International Journal of Coal Geology 202 (2019) 1–13

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Table 6 The ratio of the amount of soluble elements obtained by the experiment to total amount of the elements in coal (%). Sample

Soluble element, RRCmSX (%) Ca

Raw-coal Dried interacted-coal Air-dry interacted-coal interacted-coal with 10 wt% water interacted-coal with 30 wt% water

RRCmSX = 100

Mg −6

1.7 × 10 9.1 × 10−5 1.1 × 10−4 2.7 × 10−4 3.7 × 10−4

Fe −4

1.0 × 10 1.3 × 10−4 1.9 × 10−4 2.3 × 10−4 3.3 × 10−4

CmSX CX

Al −5

8.8 × 10 1.3 × 10−4 1.6 × 10−4 2.6 × 10−4 7.5 × 10−4

– – – 3.2 × 10−6 8.1 × 10−6

CO2 (aq) + H2 O + H+ (aq)

H2 CO3

CO2 (gas) + CaCO3 + H2 O

CaCO3 (calcite) + 2H+ (aq)

Ca2+ (aq) + 2HCO3 (aq)

1.6 × 10 1.7 × 10−2 1.8 × 10−2 3.1 × 10−2 4.6 × 10−2

Zn

Ni

Cu

Cr

0.7 2.1 2.2 2.4 2.9

0.1 0.1 0.1 0.2 0.3

0.2 0.4 0.5 0.7 1.3

7.4 9.8 12.0 17.7 25.9

Al2 Si2 O5 (OH)4 + 4SiO2 + 2 K+ (10a)

2NaAlSi3O8 (Albite) + H2 O +

2H+

Al2 Si2 O5 (OH)4 + 4SiO2 +

2Na+ (11a)

CaAl2 Si2 O8 (Anorthite) + H2 O +

2H+

Al2Si2O5 (OH)4 +

Ca2 +

(12a)

Therefore, the chemical equilibrium between feldspar and the CO2 aqueous solution is as follows:

2CO2 (gas) + 2KAlSi3 O8 + 3H2 O

Al2 Si2 O5 (OH)4 + 4SiO2 + 2 K+

+ 2HCO3 (aq) 2CO2 (gas) + 2NaAlSi3 O8 + 3H2 O

2CO2 (gas) + CaAl2Si2O8 + 3H2 O

(10b)

Al2Si2O5 (OH)4 + 4SiO2 + 2Na+

+ 2HCO3 (aq)

(11b)

Al2Si2O5 (OH)4 + Ca2 +

+ 2HCO3 (aq)

(12b)

There is the chemical equilibrium between kaolinite and the CO2 aqueous solution as follows (Harvey et al., 2013):

(8)

Fig. 4 shows the pH (a) and concentration of CO32− (b), HCO3− (c) and CO2 (d) in CO2 solution at pressure up to 30 MPa and 35, 50, 75 and 100 °C calculated with Phreeqc 3.1.4. As can be seen from Fig. 4, the pH value of the aqueous solution decreases rapidly from 7.0 to < 4 with the increase of CO2 pressure. Under supercritical conditions, the pH value of the aqueous solution of CO2 is about 3.0–3.4. The concentrations of CO32− HCO3− and CO2 in CO2 solution increase with the in, crease of pressure. The concentration of HCO3− (5–60 mg/L) in CO2 aqueous solution is significantly greater than that of HCO32− (3.5 × 10−6-7 × 10−6 mg/L). It should be noted that the CO2 solubility in Fig. 4(d) is the amount of CO2 in water, not the amount of CO2 in the gas phase in equilibrium between CO2 and water. When CO2 and Ca2+ coexist in aqueous solution, Eq. (9a) is in equilibrium. When CO2 coexists with water and calcite, there is a chemical equilibrium of Eq. (9b) solution (Marini, 2007; Song and Zhang, 2013). Thus, the mineral calcite (CaCO3) is formed if CO2 coexists with a solution containing Ca2+, and Ca2+ is released from calcite when CO2 coexists with water and calcite.

CO2 (gas) + H2 O + Ca2 + (aq)

1.3 × 10 2.1 × 10−3 2.3 × 10−3 3.4 × 10−3 4.9 × 10−3

−2

2KAlSi3O8 (Sanidine) + H2 O + 2H+

HCO3 (aq)

CO3 2 (aq) + 2H+ (aq)

Na −3

sanidine, albite and anorthite), kaolinite (Al2Si2O5(OH)4) and quartz (SiO2) (Carroll et al., 2013):

(7)

The ratio of the amount of soluble elements (RRCmSX) obtained by the experiment to the total amount of the elements in coal is shown in Table 6. From total content of elements in raw coal, the trace elements in coal can be effectively dissolved in water by scCO2, especially Cr and Zn. Relative to trace elements, the major elements in coal are difficult to be leached out by water, especially Al, Ca, Mg and Fe. Relative to other major elements, the major elements of Na and K are more easily dissolved in water. Trace elements in coal mainly exist in inorganic and organic association (Finkelman et al., 2018; Vejahati et al., 2010). ScCO2 extraction of small organic matter from coal can promote the dissolution of trace elements in the organic association. CO2 is dissolved in water to form carbonic acid (H2CO3) solutions, and the further decomposition of carbonic acid reduces the pH of the solution and causes a large amount of C(4) ions (HCO3− and CO32−) in the solution, just as the following chemical reaction equilibrium:

CO2 (g) + H2 O

K

6CO2 (gas) + Al2 Si2 O5 (OH)4 + 5H2 O

2Al3 + + 2H 4 SiO4 + 6HCO3 (aq) (13)

SiO2 + 2H2 O

H 4 SiO4

(14)

When feldspar forms a chemical equilibrium in CO2 aqueous solution, there are not only the primary minerals (preexisting minerals) but also the formation of secondary minerals (newly formed minerals) and soluble elements. The secondary minerals include kaolinite, quartz and silicic acid, and the soluble elements include HCO3−, Al3+ and K+ (Na+ or Ca2+). Therefore, when scCO2 coexists with wet-coal containing feldspars (including sanidine, albite and anorthite) and kaolinite, the amount of K+, Na+, Ca2+ and Al3+ increases in the leachates. On the other hand, Ca2+ formed can be combined with CO32− in the solution to form calcite, and the amount of Ca2+ will decrease. 3.2. XRD analysis Fig. 5 shows the XRD patterns of raw-coal and scCO2 interacted-coal of four coals. Table 7 shows the minerals composition and their content (%) in the raw-coal and interacted-coal identified by XRD. It should be noted that XRD is mainly used for qualitative analysis of minerals in coal, but the quantitative analysis of minerals in coal is rarely used and has some limitations. These limitations are mainly due to the changes in mineral crystallinity in coal, the complexity of coal, difficulty in locating coal samples, the X-ray absorption interference of other minerals, and the interference of non-qualitative organic matter in coal (Mandile and Hutton, 1995; Ruan and Ward, 2002; Ward et al., 2001; Wertz, 1990). The minerals content is only the approximate amount of the minerals in total mineral quantity, which is basically a semi-quantitative analysis. Each coal sample was repeatedly determined twice by XRD. Table 7 provides the average content of mineral in each sample

(9a) (9b)

Therefore, when the calcite in coal coexist with CO2 and water, Ca2+ in calcite will be released, making the concentration of Ca2+ increase in the leachate. However, the reaction must involve water, so the concentration of Ca2+ in leached solution for dried-coal is basically the same as that of raw coal. Similar to calcite, dolomite and siderite also exist in similar chemical equilibrium with Eqs. (9a) and (9b), resulting in increased concentration of Mg2+ and Fe2+ in the leachates. The hydrogen ion in water of siliciclastic reservoirs can be controlled by equilibrium interactions between feldspars (including 6

International Journal of Coal Geology 202 (2019) 1–13

R. Jiang, H. Yu

5.0

7.0E-6

4.6

35 50 75 100

4.4 4.2

pH

concentration of CO32- (mg/L)

(a) pH

4.8

4.0 3.8 3.6 3.4 3.2 3.0

6.5E-6

5

10

15

20

25

CO32-

6.0E-6 5.5E-6 5.0E-6

35 50 75 100

4.5E-6 4.0E-6 3.5E-6 3.0E-6

0

(b)

30

0

5

10

7E4

(c) HCO3-

60 55

concentration of CO2 (mg/L)

Concentration of HCO3- (mg/L)

65

15

20

50 45 40 35 30

35 50 75 100

25 20 15 10

30

(d) CO2

6E4 5E4 4E4 3E4

35 50 75 100

2E4 1E4

5 0

25

Pressure (MPa)

Pressure (MPa)

0 0

5

10

15

20

25

30

0

5

10

15

20

25

30

Pressure (MPa)

Pressure (MPa)

Fig. 4. The (a) pH, concentration of (b) CO32−, (c) HCO3− and (d) solubility of CO2 gas in CO2 solution.

and its error. The error is expressed in each case as an absolute percentage; thus, a determination of 29.2 (1.1) % refers to 29.2% average mineral-content with an error of 1.1%. It can be seen from Table 7 that quartz (SiO2) and kaolinite (Al2Si2O5(OH)4) were detected from Yinni raw-coal and extracted-coal. In addition to quartz and kaolinite, other four major minerals of calcite (CaCO3), pyrite (FeS2), siderite (FeCO3) and muscovite (KAl2(Si3Al) O10(OH)2) were detected from raw-coal and extracted-coal of Caili and Longkou. The main minerals of quartz, kaolinite, calcite, pyrite, dolomite (CaMg(CO3)2), rutile (TiO2) and strontianite (SrCO3) were detected from Shanxi raw-coal and extracted-coal. There is no difference in mineral species between scCO2 extracted-coal and raw-coal, that is, no secondary minerals were detected in the four scCO2 interacted-coals. Change of the mineral content may not only be due to an increase or decrease in this mineral content as shown in Table 7, but also due to the unknown change of other minerals content that was not detected by XRD. Except for Longkou interacted-coals with 30% water content (interacted-coal 2), the content of quartz in the four interacted-coal is higher than that of raw-coal, while the content of kaolinite is lower than that of raw coal. The contents of four carbonate minerals (calcite, siderite, strontianite and dolomite), muscovite and pyrite in the interacted-coal are lower than that of raw-coal. The content of all detected minerals for Longkou interacted-coal with 30% water content (interacted-coal 2) is lower than that of its raw-coal and the interacted-coal with air-dry water (interacted-coal 1). The content of rutile in Shanxi interacted-coal is higher than that of its raw-coal. Although there is no feldspar detected with XRD in coal samples, the

possibility of feldspar in the coal cannot be ruled out due to the limitations of XRD mentioned above. In coal, feldspar (including sanidine, albite and anorthite) can be converted to quartz and kaolinite in the presence of water and scCO2 (Eqs. (10b), (11b) and (12b)), increasing the content of quartz and kaolinite in the interacted coal. However, when kaolinite coexisted with the CO2 aqueous solution, there is a chemical equilibrium as shown in Eq. (13), which reduced the content of kaolinite in the interacted-coal. The final result depends on the content of feldspar and kaolinite in coal and the equilibrium conditions (CO2 pressure, temperature and moisture in coal). There is a chemical equilibrium between quartz and silicic acid in CO2 aqueous solution, just as shown in Eq. (14). The quartz in Longkou coal with 30% water will convert to silicic acid, but quartz in Longkou and other coals with air-dry water cannot convert to silicic acid. For Longku coal, the quartz content in the interacted-coal with 30% water is lower than that in raw-coal and the interacted-coal with air-dry water. Calcite, dolomite, siderite and strontianite are all carbonates. In the coexistence of these four minerals with scCO2 and water, there is a similar chemical equilibrium of calcite and the CO2 aqueous solution (Eq. (9b)), which leads to the content of calcite, dolomite, siderite and strontianite in raw-coal is higher than that of its interacted-coal. Muscovite (KAl2(Si3Al)O10(OH)2) is similar to kaolinite (Al2Si2O5(OH)4), the chemical equilibrium between muscovite and CO2 aqueous solution is as follows (Dethlefsen et al., 2012):

10CO2 (gas) + KAl2 (Si3 Al)O10 (OH)2) + 10H2 O + 10HCO3 (aq) 7

K+ + 3Al3 + + 3H 4 SiO4 (15)

International Journal of Coal Geology 202 (2019) 1–13

R. Jiang, H. Yu

4000

5000

(a) Yinni coal

(b) Caili coal

a. Quartz b. Kaolinite

4000

3000 Interacted-coal

b a d b c e

3000

a

2000

b

1000

b

b

b

ba a

a

1000

a

f

a

2000

a

a. quartz b. calcite c. pyrite d. kaolinite e. siderite f. muscovite a a a Interacted-coal

a

cb

a

Raw-coal

d b

0

e f

a

a

50 2

60

Raw-coal

a

0 10

20

30

40

50

60

70

80

90

10

20

30

40

2

7000

(c) Longkou 6000 coal

a. quartz b. calcite c. pyrite

5000

Interacted-coal with 30 wt.

d. kaolinite e. siderite f. muscovite

14000 12000

water

(d) Shanxi coal

a. quartz b. calcite c. pyrite d. kaolinite

10000

4000

70

80

90

e.dolomite f. strontianite g. rutile Interacted-coal

8000

a

3000 6000

Interacted coal with air-dry water 2000

a

1000

d

d

a

4000

b b c e

0

a

f

a

2000

Raw-coal

a

a d

0

b d g fc e c ab a

a

a

a

60

70

Raw-coal

-1000 10

20

30

40

50

60

70

80

90

10

20

30

40

50

80

90

2

2

Fig. 5. X-ray diffraction patterns of raw-coals and their interacteded coal with scCO2. Table 7 The content of mineral matter in raw-coals and scCO2 interacted-coal using XRD (%). Samples Yinni Caili Longkou Shanxi

⁎ ⁎⁎

Raw-coal Interacted-coal⁎ Raw-coal Interacted-coal⁎ Raw-coal Interacted-coal 1⁎ Interacted-coal 2⁎⁎ Raw-coal Interacted-coal⁎

Quartz

Calcite

Kaolinite

Pyrite

Siderite

Muscovite

Dolomite

Rutile

Strontianite

Total

29.2(1.1) 30.5(1.3) 35.0(1.2) 45.7(1.4) 35.6(1.2) 45.5(1.4) 32.4(1.0) 57.4(1.1) 58.4(1.3)

– – 23.0(0.9) 20.5(1.1) 22.8(0.7) 17.6(1.0) 13.5(1.2) 10.3(0.9) 7.6(0.8)

34.1(1.2) 32.1(1.4) 17.3(0.9) 9.9(1.1) 17.9(1.1) 13.0(1.1) 13.0(1.3) 10.8(1.2) 9.5(0.9)

– – 1.7(0.4) 0.6(0.2) 1.9(0.3) 1.8(0.3) 0.5(0.2) 6.3(0.8) 5.0(0.8)

– – 4.8(0.3) 1.9(0.1) 4.6(0.4) 4.3(0.4) 4.0(0.3) – –

– – 15.1(0.5) 11.9(0.4) 15.7(0.7) 10.8(0.6) 10.2(0.6) – –

– – – – – – – 4.9(0.3) 2.9(0.1)

– – – – – – – 0.8(0.2) 1.0(0.3)

– – – – – – – 8.1(0.3) 5.7(0.2)

63.3 62.6 96.9 90.5 98.5 92.7 73.9 98.6 90.1

Interacted-coal 1 means scCO2 interacted-coal with air-dry water; Interacted-coal 2 means scCO2 interacted-coal with 30 wt% water.

So, the content of muscovite in the interacted-coal is lower than that in the raw-coal, which will lead to an increase in the content of K and Al elements in the interacted-coal. There is a chemical equilibrium between pyrite and CO2 aqueous solution as shown below:

FeS2 + 2CO2 (gas) + 2H2 O

Fe2 + + 2H2 S(gas) + 2CO3 (aq)

scCO2, which lead to a decrease in the content of pyrite in interactedcoal. Therefore, the content of pyrite in interacted-coal is lower than that in raw-coal. The content of rutile in Shanxi coal is low. The content of rutile in interacted-coal is higher than that in raw coal, which can be attributed to the measurement error and the change of other mineral content. The dissolution of the minerals and the migration of dissolved elements in coal, and the formation of secondary minerals is a long-term

(16)

The pyrite in raw-coal can be dissolved into water in the presence of 8

International Journal of Coal Geology 202 (2019) 1–13

R. Jiang, H. Yu

18 Quantity adsorbed, V(mL/g STP)

16 14 12 10 8 6

3.5

35

(a) Yinni coal

3.0

4

2.0

Adsorption on raw-coal Desorption on raw-coal

1.5

Adsorption on interacted-coal Desorption on interacted-coal

1.0

0.5

0.0

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.50

0.55

0.60

4 2 0 0.0

(b) Caili coal

5

30

2.5

Quantity adsorbed, V(mL/g STP)

20

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

25

15 10

2

1

0.20

0.25

0.30

0.35

0.1

0.2

0.3

16 Quantity adsorbed, V(mL/g STP)

Quantity adsorbed, V(mL/g STP)

18 Adsorption on raw-coal Desorption on raw-coal

(c) Longkou coal

Adsorption on interacted-coal Desorption on interacted-coal 0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.50

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0.8

0.9

1.0

Relative pressure, P/P0

Relative pressure, P/P0

30 28 5.0 26 4.5 24 4.0 22 3.5 20 3.0 18 2.5 16 14 2.0 12 1.5 10 8 6 4 2 0 0.0

0.40

5 0 0.0

1.0

Adsorption on raw-coal Desorption on raw-coal Adsorption on interacted-coal Desorption on interacted-coal

3

20

0.45

(d) Shanxi coal Adsorption on raw-coal Desorption on raw-coal Adsorption on interacted-coal Desorption on interacted-coal

14 12 10 8 6 4 2

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0 0.0

1.0

0.1

0.2

0.3

Relative pressure, P/P0

0.4

0.5

0.6

0.7

Relative pressure, P/P0

Fig. 6. Nitrogen adsorption/desorption isotherms of raw-coal and scCO2 interacted-coal for four coals.

geochemical process under in situ conditions. Although the process is similar to that of injecting CO2 into a deep brine reservoir, the coal with a complex porous structure and coexisting mineral make the injected CO2 more difficult to diffuse to coal matrix than that of brine reservoir. The difficulty of CO2 diffusion is not conducive to the reaction of minerals with scCO2. Therefore, the geochemical interaction between scCO2 and coal in situ is a long-term process. Due to the slow chemical reaction between minerals and CO2, it is difficult to obtain accurate information on the change of minerals in coal by scCO2 using laboratory research. Difficulties in on-line detection, slow reaction rate between CO2 and minerals in coal, and difficulty in quantifying minerals in coal are all difficult to overcome in laboratory research. It is somewhat one-sidedness that the use of soluble elements expresses the change of minerals to characterize the interaction between scCO2 and minerals in coal in the laboratory. The soluble element does not indicate the change of minerals in coal during the interaction, but only the result after the interaction. In addition, only a small amount of metal elements was examined in this paper. Similarly, the conclusions of the formation of secondary minerals characterized by XRD have limitations. One is that the organic matter in coal affects the determination of XRD, and the other is that the interaction time between CO2 and coal is too short compared with a long time in situ CO2 sequestration into coal. The computer software for simulating geochemical reactions can be used to simulate minerals-CO2-water interaction to improve the laboratory experiments, such as PHREEQC.

3.3. Pore-size distribution Fig. 6 depicts N2 adsorption/desorption isotherms of four samples, including their raw-coals and scCO2 interacted-coals. All N2 adsorption isotherms exhibited the same characteristics between raw-coal and scCO2 interacted-coal, which indicates that the raw-coal and interactedcoal have a similar pore structure. For the adsorption branch, the adsorption quantity increases slowly then follows a rapid increase as the relative pressure (P/P0) increases to 0.9. There is not a deflection point for all coals, which is often used to separate the monolayer and multilayer adsorption phase (Liu et al., 2017). Except for Shanxi coal, the hysteresis loop between the adsorption and desorption was observed for the other three samples, indicating the existence of mesopores in the coals. There is no hysteresis loop on Shanxi coal, indicating there is the smallest pore (width = 1 nm) (Zeng et al., 2017) in Shanxi coal. The adsorption isotherms would belong to type III based on the IUPAC (International Union of Pure and Applied Chemistry), which means there is a weak interaction between N2 and coal. Both the N2 adsorption and desorption branches have been used to interpret the pore size distributions of samples. However, the adsorption branches of the isotherms often used to calculate pore size distribution in order to avoid the effect of false peaks happened in desorption branch (Clarkson et al., 2012; Jin et al., 2016). In addition, there is no hysteresis loop on Shanxi coal. So, the specific surface area (SSA) of each sample was calculated using the Brunauer-Emmet-Teller (BET) equation according to the adsorption branch of the isotherm in the P/P0 range of 0.05–0.30. The total pore volume (TPV) was estimated using 9

International Journal of Coal Geology 202 (2019) 1–13

R. Jiang, H. Yu

0.00040

0.035

0.00035

0.025

0.00030

0.020 0.015 0.010 0.005 0.000 10

0.0001

Cumulative pore volume,V (mL/g)

0.0002

0.050 0.045

0.030

dV/dr (mL·g-1·Å-1)

Cumulative pore volume, V (m2·g -1)

dV/dr (mL·g-1·Å-1)

0.0003

100

1000

Pore radius, r (Å)

(a) Yinni coal Raw-coal Interacted-coal

0.0000 10

0.00025 0.00020

0.00015 0.00010 0.00005 0.00000 10

(c) Longkou coal

0.005 100

1000

100

0.035

0.0006

0.040 0.035

0.0005

0.030 0.025 0.020 0.015 0.010 0.005 0.000 10

100

1000

Pore radius, r (Å)

Raw-coal Interacted-coal

0.0004 0.0003

1000

Pore radius r (Å)

0.030 0.025 0.020 0.015 0.010 0.005 0.000 10

100

0.0000 10

1000

Pore radius, r (Å)

0.0002 0.0001

100

1000

Pore radius, r (Å)

Cumulative pore volume,V (mL/g)

0.00020

0.010

0.00000 10

1000

dV/dr (mL·g-1·Å-1)

Cumulative pore volume, V (m2·g -1)

dV/dr (mL·g-1·Å-1)

0.00025

0.015

Raw-coal Interacted-coal

0.045

0.00030

0.020

(b) Caili coal

0.00010

0.0007

0.00035

0.025

Pore radius, r (Å)

0.050

0.00040

0.030

0.000 10

0.00015

Pore radius, r (Å)

0.00045

0.035

0.00005

100

0.00050

0.040

Raw-coal Interacted-coal

(d) Shanxi coal 100

1000

Pore radius, r (Å)

Fig. 7. Pore size distributions (PSDs) curve using N2 adsorption branch for raw-coals and ScCO2 interacted coals.

the Barrett-Joyner-Halenda (BJH) model according to the adsorption branch. The BET equation is suitable for the description of micropore (pore size < 2 nm), and the BJH model is suitable for the description of mesopore (pore size 2–50 nm). So, the specific surface area of micropore is expressed with SSA-BET, and the total pore volume of mesopore is expressed with TPV-BJH in this paper. Fig. 7 is a distribution curve of BJH differential pore volume (dV/dr) with pore radius (r) of four rawcoals and their CO2 interacted-coal, and the small graph shows the BJH cumulative pore volume with pore radius (r). The specific surface area (SSA) can be obtained according to the distribution curve of BJH differential pore volume (dV/dr) with pore radius (r), which is expressed with SSA-BJH. Fig. 8 is the specific surface area distributions (SSADs) curve using the BJH model for raw-coals and interacted-coals. The distribution of coal mesopores can be seen from Fig. 7, and the distribution of SSA for coal mesopores can be seen from Fig. 8. The SSABET, SSA-BJH and TPV-BJH data for raw-coal and interacted coal are shown in Table 8. The pore size of each sample was generated by the TPV and the SSA. There are three different ways to represent pore size. (1) The average pore size (APS) is calculated from the total pore volume (TPV-BJH) and BET specific surface area (SSA-BET), which contains all pores and only has the upper limit of the pore size. (2) The average pore size of BJH (APS-BJH) is calculated from the cumulative total pore volume of BJH (TPV-BJH) and the total surface area of BJH (SSA-BJH), which is the average value of all pore size of mesopore. (3) The most probable pore size of BJH (MPPS-BJH) represents the most probable mesopore size within the samples. MPPS-BJH is obtained by the abscissa value (pore

radius, r) corresponding to the peak points of the ordinate (dV/dr) in pore size distributions (PSDs) curve (Zhang and Cui, 2018). The specific surface area (SSA-BET and SSA-BJH), total pore volume (TPV-BJH) and pore size (APS, APS-BJH and MPPS-BJH) of raw-coal and its interacted-coal for four coal samples are shown in Table 8. For low-rank Yinni and Caili coal with low-ash, there is no significant difference in pore size distributions (PSDs) between the rawcoal and interacted-coal as shown in Fig. 7, and the pore size of mesopore (MPPS-BJH) between raw-coal and interacted-coal differs by only 0.1 nm, just as shown in Table 8. The specific surface area of micropores (SSA-BET) and mesopores (SSA-BJH), the total pore volume of mesopores (TPV-BJH), the average pore size (APS) and pore size of mesopores (APS-BJH) for the interacted-coals are higher than that of raw-coals for two coal samples. This indicates the effect of scCO2 on the pore-size expansion of micropores and mesopores of the two coals. For high-ash Longkou coal, there is a significant difference in PSDs between raw-coal and interacted-coal. The pore-structure parameter values of mesopores (SSA-BJH, TPV-BJH, APS, APS-BJH and MPPS-BJH) calculated with BJH model for interacted-coal are higher than that of its raw-coal. This indicates the effect of scCO2 on the pore-size expansion of mesopores of Longkou coal. The specific surface area of micropores (SSA-BET) for the interacted-coal is lower than that of raw-coal. The N2 adsorption of the interacted-coal is significantly lower than that of the raw-coal as the relative pressure is lower than 0.5, as shown in Fig. 6(c). The change in adsorption capacity for the interacted-coal being lower than that of raw-coal is consistent with the change of SSA-BET, obtained using the BET equation in the P/P0 range of 0.05–0.30. This indicates 10

International Journal of Coal Geology 202 (2019) 1–13

R. Jiang, H. Yu

6

0.6

SSA-BJH (m2·g-1)

7.37 7.15

0.5

5 4 3 2 1 0 10

0.4

100

1000

Pore radius, r (Å)

0.3 0.2

Raw-coal Interacted-coal

0.1 10

100

1000

0.90 (b) 0.85 Caili coal 0.80 0.75 0.70 0.65 0.60 0.55 0.50 0.45 Raw-coal 0.40 Interacted-coal 0.35 0.30 0.25 0.20 0.15 10

Pore radius, r (Å) 1.4

14 13

1.2

11

SSA-BJH (m2·g-1)

0.9 0.8

1.2

10

13.21 11.55

9 8 7

1.0

6 5 4 3 2

0.7

1 0 10

0.6

100

1000

Pore radius, r (Å)

0.5

(c) Longkou coal

0.4 0.3

0.1

8 7 6 5 4 3 2 1 0 10

10

100

1000

Pore radius, r (Å)

100

1000

12

12.62

11

0.9 0.8 0.7 0.6

9 8

7.29

7 6

Raw-coal Interacted-coal

5 4 3 2 1

0.5

0 10

0.4

100

1000

Pore radius, r (Å)

0.3

Raw-coal Interacted-coal

0.2

11.31 10.25

9

10

1.1

SSA-BJH (m2·g-1)

Cumulative AAS (m2·g -1)

1.0

10

13

(d) Shanxi coal

1.3

12

1.1

11

Pore radius r (Å)

Cumulative AAS(m2·g -1)

1.3

12

Cumulative AAS (m2·g -1)

7

SSA-BJH (m2·g -1)

0.7

(a) Yinni coal Cumulative AAS (m2·g -1)

0.8

0.2 0.1

100

1000

0.0

10

100

Pore radius r, (Å)

1000

Pore radius, r (Å)

Fig. 8. Specific surface area distributions (SSADs) curve using BJH model for raw-coals and ScCO2 interacted coals. Table 8 Specific surface area (SSA), total pore volume (TPV) and pore size (PS) of raw-coal and its interacted-coal. Specific surface area (m2/g)

Sample

SSA-BET Yinni Caili Longkou Shanxi

Raw-coal Interacted-coal Raw-coal Interacted-coal Raw-coal Interacted-coal Raw-coal Interacted-coal

4.71 5.13 8.29 9.49 9.85 7.39 4.44 9.58

TPV-BJH (mL/g)

SSA-BJH −2

7.15 7.37 10.25 11.31 11.55 13.21 7.29 12.62

2.94 × 10 3.19 × 10−2 3.21 × 10−2 4.85 × 10−2 4.01 × 10−2 4.73 × 10−2 2.59 × 10−2 2.83 × 10−2

the effect of scCO2 on the pore-size contraction of micropores of Longkou coal. For high-rank Shanxi coal with low-ash content, the adsorption capacity for the interacted-coal is significantly larger than that of rawcoal. The SSA-BET, SSA-BJH and TPV-BJH of interacted-coal are higher than that of raw-coal, but pore size of mesopores (APS, APS-BJH and MPPS-BJH) is lower than that of raw-coal. APS-BJH for the interactedcoal is 1.73 times that of the raw-coal, TPV-BJH for the interacted-coal is 1.09 times that of the raw-coal. The increase of SSA-BJH is greater than the increase of TPV-BJH, so, the pore size of mesopores of

Pore diameter, 2r (nm) APS

APS-BJH

MPPS-BJH

23.4 23.6 15.0 20.1 15.9 24.0 21.8 11.1

16.4 17.3 12.5 17.1 13.9 14.3 14.2 9.0

3.1 3.2 3.6 3.5 3.0 4.1 3.6 2.8

interacted-coal is lower than that of raw-coal. This indicates the effect of scCO2 on the pore-size contraction of mesopores of Shanxi coal. SSABET for the interacted-coal is 2.16 times that of the raw-coal, which indicates the effect of scCO2 on the pore-size expansion of micropores of Shanxi coal. So, scCO2 enlarges the pore size of the micropores and shrinks the pore size of the mesopores for Shanxi coal. The small organic molecules in coal can be extracted by scCO2, and minerals in coal can react with CO2. So, the difference of pore size between CO2 interacted-coal and raw-coal is a combined result of the extraction and reaction. The scCO2 extraction from coal is accompanied 11

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R. Jiang, H. Yu

by the following processes: the diffusion of CO2 and extracts in coal pores, the dissolution of small organic molecules, the swelling of coal matrix and the change of pore structure, which includes increase or decrease in pore size, shrinkage or collapse of pores caused by coal swelling, the formation of new pores, the colloidization of some micropores and disappear of some pores (Qin et al., 2009). However, the effects the process depend on coal rank, coal quality and interacted conditions (such as time, temperature and pressure). The scCO2 extraction yield of small molecular organics from coal decreased with the increase of coal rank (Qin et al., 2009; Yu et al., 2017b). Since scCO2 not only has a geochemical reaction with minerals in coal in the presence of water, but also has an extraction effect on small organic molecules in coal, the low-temperature nitrogen adsorption experiments cannot distinguish whether the pore structure changes are caused by the geochemical reaction or the extraction effect on the molecules. From the perspective of the content of minerals in coal, scCO2 enlarges the pore size of the micropores for low-ash coal (Yinni, Caili and Shanxi coal), but shrinks that of micropores for high-ash coal (Longkou coal). From the perspective of coal-rank, scCO2 enlarges the pore size of the mesopores for low-rank coal (Yinni, Caili and Longkou coal), but shrinks that of mesopores for high-rank coal (Shanxi coal). The dissolution of scCO2 into water produces carbonic acid, which reacts with some minerals in coal to liberate some water-soluble ions. These equilibrium reactions are related to the CO2 partial pressure. CO2 and water will flow to the area of lowest pressure in the coal seam, causing the CO2 concentration and acidity of the water to decrease. This causes the water-soluble ions to precipitate again when the pressure becomes less than the threshold pressure of each mineral. The precipitation will clog the coal matrix pores, thus reducing the permeability of coal and preventing the diffusion and flow of CO2. Eventually, these minerals will be redissolved as the CO2 pressure amasses behind them, and the redissolved ions migrate with water. So, the interaction between scCO2 and minerals in coal not only affects the pH, the solubility of minerals in groundwater and the pore size of coal, but also affects the migration of CO2 in the coal seam, and then affects the CO2 injection process.

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4. Conclusions The amount of soluble matter and soluble elements in scCO2 interacted-coal is significantly greater than that of raw-coal, and their amount increases with the increase of water content in coal. Relative to the elements in raw coal that can be dissolved by water, the solubility increase of scCO2 to Ca and Al elements is significantly higher than that of other major-elements and all trace-elements. Compared with the total content of the elements (including soluble and insoluble elements) in raw-coal, the trace elements in coal can be effectively dissolved in water by scCO2, and the major elements in coal are difficult to be leached out from scCO2 interacted-coal by water. Relative to raw-coal, no new mineral formation was detected by XRD, but the mineral content in interacted-coal changed. When coal is interacted with lower water content by scCO2 and for a long time, the apparent content of quartz increases and the content of carbonates (calcite, siderite, strontianite and dolomite), kaolinite, pyrite and muscovite decreases. ScCO2 can enlarge the pore size of the micropores for low-ash coal, but shrinks that of micropores for high-ash coal. ScCO2 can enlarge the pore size of the mesopores for low-rank coal, but shrinks that of mesopores for high-rank coal. Acknowledgements This research was supported by the National Natural Science Foundation of China (Grant Nos. 51174127 and 21176145) and Shandong Province Natural Science Foundation (No. ZR2011DM005). 12

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