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Abstract

The interactions between supercritical CO₂ and coal and their effects on changes in the coal pore structure and organic groups play a critical role in the CO₂ geological storage-enhanced coalbed methane recovery. To investigate the effects of supercritical CO₂ on organic groups in coals of different ranks and its mechanisms under different temperature and pressure conditions, CO₂ sequestration processes in bituminous coals and high-rank coals were replicated using a high-pressure reactor. Four coal samples of different ranks were exposed to supercritical CO₂ and water under three temperatures and pressures for 240 h. Fourier transform infrared spectroscopy was used to provide semiquantitative ratios and Fourier transform infrared spectra of coal samples before and after the supercritical CO₂–H₂O treatment. The results show that interactions between supercritical CO₂ and coal were controlled by the coal macromolecular structure, and semianthracite is the inflection point of interaction characteristics for coal samples of different ranks. Bituminous coal, including high- and low-volatility bituminous coal, has a low degree of condensation of its aromatic structure, and its aromatic nuclei can facilitate addition reactions. Swellings primarily break cross-links between aromatic nuclei in the same aromatic layer. These characteristics favor the polymerization addition of aliphatic side chains of aromatic nuclei, causing an increase in the degree of condensation of the aromatic structures in bituminous coal. High-rank coals including semianthracite and anthracite have a high degree of condensation of their aromatic structures, and the aromatic nuclei favor substitution reactions. Swellings primarily break cross-links connecting different aromatic layers, and bond dissociation reactions and sulfuration reactions are more significant for high-rank coal. These characteristics cause a decrease in the degree of condensation of the aromatic structure in high-rank coal. Temperature and pressure have a great impact on interactions between supercritical CO₂ and coal and are controlled by the reaction types of the organic groups. With the increase in experimental temperature and pressure, the changes in the organic group content can be classified as the descending type, the rising type, the lower opening parabola type, and the upper opening parabola type. 45.0°C and 10 MPa is the inflection point of the changes in the organic group content. Descending- and rising-type changes favor addition, bond dissociation, and sulfuration reactions, which are endothermic. The reaction rate of supercritical CO₂ and the organic groups increases, and the effects caused by temperature and pressure decrease as the temperature and pressure increase. Lower opening parabola- and upper opening parabola-type changes favor substitution, oxidation, and addition polymerization reactions, which are exothermic. These changes were significantly affected by a variety of reactions and were suppressed by high temperature and pressure. When the temperature is ≤45.0°C and the pressure is ≤10 MPa, supercritical CO₂ has remarkable effects on alkyl and hydroxy groups and has a stronger effect on bituminous coal. When the temperature is >45.0°C and the pressure is >10 MPa, supercritical CO₂ has remarkable effects on oxygen- and sulfur-containing groups and has a greater effect on high-rank coals.

Keywords

aromatic structure cross-link Fourier transform infrared spectra absorbance peak coal rank

Introduction

Due to the competitive adsorption advantage, CO₂ sequestration in a coal seam can effectively displace CH₄, which has become a new method for the improvement of coalbed methane (CBM) development and the reduction of CO₂ emissions (Bergen et al., 2011). CO₂ geological storage-enhanced coalbed methane recovery (CO₂-ECBM) is an attractive technology for reducing greenhouse gas emissions and exploiting new energy sources, and it has attracted much attention from many researchers (Rodosta et al., 2011). At the reservoir temperature and pressure (burial depth >800 m), CO₂ is likely to be present in a supercritical state (i.e. Tc = 31.06°C, Pc = 7.38 MPa) (Massarotto et al., 2010). The effect of supercritical CO₂ (ScCO₂) on the transformation of the organic composition and inorganic minerals of coal may lead to physical and chemical structural changes in coal, which lead in turn to changes in permeability and the adsorption capacity of the coal, which is particularly critical for the effectiveness of CO₂-ECBM (Day et al., 2008; Kiyama et al., 2011; Lin et al., 2008; Siemons and Busch, 2007).

A ScCO₂–H₂O system affects coal in two different ways: (1) CO₂ can dissolve in water and form an acidic solution containing H₂CO₃. The acidic solution can leach out inorganic minerals in coal, e.g. calcite, dolomite, magnesite, etc., which can change the pore–fracture structure and the connectivity of coal (Dawson et al., 2015; Liu et al., 2015, 2010; Massarotto et al., 2010; Wang et al., 2016a, 2016b). Recent studies of interactions between a ScCO₂–H₂O system and coal have focused on that aspect. (2) ScCO₂ has good diffusivity and dissolving capacity that can dissolve some of the organic groups on the inner surface of the pores and fractures of coal during the migration process within them, which changes the physical and chemical structure of the coal (Kolak and Burruss, 2006; Mazumder et al., 2006; Zhang et al., 2017). The interactions between ScCO₂ and coal organic groups are complex, and their effects are relative to the solubility of a compressed gas and affected by temperature, pressure, pore and fracture widths, coal rank, etc. (Jonathan et al., 2015; Kolak and Burruss, 2006; Pasquali et al., 2008; Stahl et al., 1978).

The ScCO₂ extraction rate of coal organic groups slightly increased as the temperature and pressure increased, but the characteristics of the changes are different for coals of different ranks (Kolak and Burruss, 2006; Pasquali et al., 2008; Stahl et al., 1978). In a low-pressure range (7–10 MPa), ScCO₂ can extract fewer polar hydrocarbons and lipids, e.g. alkanes and polycyclic aromatic hydrocarbons of lower molecular weight, esters, ethers, lactones, epoxy compounds, etc. (Stahl et al., 1978). The effects of ScCO₂ on polar functional groups are weak (Massarotto et al., 2010). In a high-pressure range (>10 MPa), ScCO₂ can extract some aliphatic hydrocarbons, hydroxyl groups, and other oxygen-containing groups and cause changes in the coal macromolecular structure and the destruction of the microcrystalline structure (Gathitu et al., 2009; Jonathan et al., 2015; Mirzaeian and Hall, 2006; Okolo et al., 2015). ScCO₂ extraction rates of lignite/brown coals, bituminous coals, and anthracites increased at first but subsequently decreased and peaked when the coal carbon content was approximately 86% (Bertier et al., 2006; Liu and Smirnov, 2009; Mirzaeian et al., 2010; Wang et al., 2010).

Interactions between ScCO₂ and coal proceed by two kinds of mechanisms. (1) ScCO₂ adsorption and diffusion in coal can cause swelling. Swelling decreases interactions among coal macromolecular chains and extends coal macromolecular chains. Low-rank coals swell more readily, and temperature has a weak effect (Day et al., 2008; Gathitu et al., 2009; Siemons and Busch, 2007). (2) ScCO₂ can weaken intermolecular forces in coal, causing the dissociation of low molecular weight compounds from the coal macromolecular structure (Tsotsis et al., 2004). Water can leach these low molecular weight compounds from the coal matrix, which may change the pore–fracture structure and rearrange coal physical structure (Tsotsis et al., 2004). The interactions between ScCO₂ and low molecular weight compounds are affected by temperature and pressure. Other researchers have suggested that ScCO₂ is a nonpolar solvent whose polarity does not change with pressure. Therefore, ScCO₂ has a limited effect on low molecular weight compounds in coal, which are mainly polar compounds (Massarotto et al., 2010).

Scientists generally agree that the interactions between ScCO₂ and coal organic groups can loosen the surface of coal particles and lead to coal macromolecular rearrangements and obvious plastification, which can increase the porosity and permeability of coal (Day et al., 2008; Siemons and Busch, 2007; Tsotsis et al., 2004; Wang et al., 2010). Meanwhile, different specific surface areas (both of the absolute content and the percentage composition) of pores with different widths decrease, which enhances the desorption ability of a coal (André et al., 2007; Dutka et al., 2012, 2013; Hao et al., 2013). Other scientists have suggested that ScCO₂ can only break subagents in coal structure units but are unable to break the space frames of coal formed by chemical cross-links (Kutchko et al., 2013; Mastalerz et al., 2010). Therefore, ScCO₂ can slightly change the pore size distribution in coal. Meanwhile, a consensus also exists that the interactions between ScCO₂ and coal organic groups can effectively improve the adsorption capacity, specific surface area, and porosity of coal (Kutchko et al., 2013; Mastalerz et al., 2010). In conclusion, the effects of the interactions between ScCO₂ and organic groups on coal porosity, permeability, and adsorption capacity and their important roles in CO₂-ECBM have been generally recognized. However, the effects of ScCO₂ on the changes in coal organic groups and their mechanisms are still controversial. A poor understanding of interactions between a ScCO₂–H₂O system and organic groups limits our understanding of CO₂-ECBM and hinders engineering exploration.

In the present study, simulations of CO₂ sequestration processes in coal at different temperatures and pressures combined with Fourier transform infrared spectroscopy (FTIR) were employed to describe the characteristics of the changes in the semiquantitative ratios and the absorbance peaks in FTIR spectra. Interaction mechanisms between a ScCO₂–H₂O system and organic groups are also discussed for bituminous coals and high-rank coals. This study aims to provide a better understanding of the theoretical effectiveness of CO₂-ECBM, which can guide the optimization of CO₂-ECBM engineering explorations.

Materials and methodology

Samples

Four groups of Chinese coal samples of different ranks were systematically collected: high-volatility bituminous coal from the Yangzhuang Mine, low-volatility bituminous coal from the Xinyuan Mine, semianthracite from the Yuwu Mine, and anthracite from the Sihe Mine, and these were, respectively, designated Coal #1 to Coal #4 (Table 1). Coal #1 was collected from coal seam #9 in the Taiyuan formation in the Bohaiwan basin. The Bohaiwan basin is rich in unconventional gas resources, including CBM and shale gas. Coals #2 to #4 were collected from coal seam #3 of the Shanxi formation in the southern Qinshui basin. The southern Qinshui basin is the most active region for CBM exploration in China, and it is the only CO₂-ECBM demonstration study area in China, which presents favorable conditions for research (Liu et al., 2016, 2017, 2015). The collection, retention, and preparation of the coal samples were conducted according to the relevant standard GB/T 19222-2003 in China (Zhong et al., 2003) and the international standard ISO 7404-2:1985. To prevent these coal samples from further oxidizing, coal samples collected from the coal mine working faces were wrapped in absorbent paper and hermetically sealed in plastic bags at 5°C. The key properties of these coals are shown in Table 1.

Table 1.

Properties of the coals used.

Samples	Sampling location	R_o,max (%)	Proximate (wt%)				Ultimate (wt%)
Samples	Sampling location	R_o,max (%)	M_ad	A_ad	V_daf	FC_ad	O_daf	C_daf	H_daf	N_daf
#1	Yangzhuang Mine	0.72	2.29	5.49	38.72	57.91	7.99	83.28	5.27	1.22
#2	Xinyuan Mine	1.81	0.81	5.35	15.26	80.20	9.30	80.32	4.43	1.14
#3	Yuwu Mine	2.19	1.10	11.98	13.44	76.19	2.44	91.73	4.12	2.44
#4	Sihe Mine	3.33	1.48	13.12	6.32	81.39	2.98	93.45	2.15	1.00

A_ad: ash yield, air-drying base; C_ad: content of carbon, dry ash-free basis; FC_ad: fixed carbon content, air-drying base; H_ad: content of hydrogen, dry ash-free basis; M_ad: moisture, air-drying base; N_ad: content of nitrogen, dry ash-free basis; O_daf: content of oxygen, dry ash-free basis; V_daf: volatile matter, dry ash-free basis; wt%: weight percent.

ScCO₂–H₂O treatment

High-pressure reactor

The interactions between ScCO₂–H₂O and coal were carried out in a high-pressure reactor (Figure 1). The reactor can simulate the conditions for CO₂ sequestration in a deep coal seam, which are a maximum of 35 MPa and 200°C. The key component of this apparatus is a high-pressure tubular stainless steel sample cell, which is 100 mm in diameter and 200 mm in height. A coal sample was held in a 60–80 mesh (0.20–0.25 mm) stainless steel tube in the sample cell, which allowed the water and CO₂ to flow through the coal sample.

Figure 1.

Sketch of the high-pressure reactor.

Coal grain size

In principle, it is better to choose coal with a large particle size for an experimental simulation of in situ conditions because it captures the actual conditions more realistically. In contrast, for laboratory experiments, using coal with a large particle size will result in an inconveniently long reaction time. Therefore, the coal samples used for ScCO₂–H₂O treatment were coal particles sieved into 4–8 mm grain sizes. Because the volume of the sample cell was approximately 1500 ml, a 200 g coal sample was used for ScCO₂–H₂O treatment.

Temperatures, pressures, and time in the experiment

The simulation experiments were performed at three burial depths: 1000, 1500, and, 2000 m. Temperatures and pressures of the three burial depths were calculated from the geothermal gradient and the pressure gradient at the sampling location. The ancient sedimentary environment of the Shanxi formation in the Southern Qinshui basin and the Taiyuan formation in the Bohaiwan basin are both believed to be an epicontinental sea setting and belong to the Permo-Carboniferous system of the north China old craton. Their underground constant temperature zones have the same temperature and depth. The temperature and depth of underground constant temperature zones, average geothermal gradient, and average pressure gradient are approximately 9°C, 20 m, 3.54°C/100 m, and 1.0 MPa/100 m (for a burial depth <2500 m), respectively. The temperatures and pressures at the three burial depths according to the average geothermal and pressure gradients are shown in Table 2. These parameters indicate that the CO₂ was in a supercritical state during the simulation.

Table 2.

Temperatures and pressures for the simulation experiments.

Sampling location	Samples	Temperature (°C)			Pressure (MPa)
Sampling location	Samples	1000 m	1500 m	2000 m	1000 m	1500 m	2000 m
Bohaiwan basin	#1	45.0	62.5	80.0	10.0	15.0	20.0
Southern Qinshui basin	#2 to #4	45.0	62.5	80.0	10.0	15.0	20.0

To ensure that an experiment reached reaction equilibrium, an experimental duration of 240 h (10 days) was chosen. Figure 2 shows that the temperatures and pressures in the reactor were very stable over the course of the simulations, giving a pressure of ±0.78 MPa from a predetermined pressure and a temperature of ±0.8°C from a predetermined temperature.

Figure 2.

Experimental conditions of the ScCO₂–H₂O treatment ((a) pressure; (b) temperature).

ScCO₂–H₂O treatment process

The basic ScCO₂–H₂O treatment process is as follows.

After the prepared coal samples were placed in the sample cell (Figure 1), it was sealed and then preheated to a predetermined temperature.

The sample cell was first evacuated three times, and then 300 ml deionized water was pumped into the sample cell at a flow rate of 5 ml min⁻¹, ensuring that deionized water ran over the coal sample.

Gas was pumped into the sample cell to a predetermined pressure. The temperature and pressure were monitored during the experiment to ensure system stability.

After 240 h, the system pressure was reduced to atmospheric pressure, and the coal sample was removed from the sample cell and immediately vacuum dried at 50°C for 24 h.

Determination of organic group types and contents

FTIR spectroscopy

FTIR spectroscopy is a common technique for determining coal chemical structure parameters and accurately displays the type and semiquantitative ratio of coal functional groups (Chen et al., 2012, 2015; Mastalerz et al., 2012; Wang et al., 2016b).

FTIR was carried out with a VERTEX 80v FTIR spectrometer produced by Bruker, Germany. To obtain accurate infrared spectra, the coal samples used in the experiment were no larger than 200 mesh (76 µm) and were vacuum dried at 50°C for 24 h. The coal samples were prepared by the KBr disc technique. Pulverized coal was evenly mixed with KBr in a proportion of 1:180 (the sum of the weights was 0.1 g). A KBr disc (0.1 g) was used as a blank background. The scanned area was 400–4000 cm⁻¹ with a scanning resolution of 4 cm⁻¹. The coal samples were scanned 32 times.

Types and ratios of the major organic groups

The types and ratios of the major organic groups of coal were acquired by a semiquantitative analysis of the infrared spectra of the coal molecules. The primary features of the analysis are as follows.

According to the infrared spectra of the four coals, polynomial fitting could be used in the baseline correction. Then, the absorbance peaks could be preliminarily analyzed via the second derivative using the OMNIC software package. By comparing the absorption frequency attributes of the coal molecules with the coal molecular structure, the major organic groups of the coal could be determined.

Overlapping peaks could be separated by the peak resolution function of the OMNIC software. Then, the ratios of the major organic groups were analyzed semiqualitatively by calculating the peak areas of the various organic groups. The semiquantitative analysis of the infrared spectra was based on the Lambert–Beer law (equation (1))

A (ν) = \lg \frac{1}{T (ν)} = K (ν) b c

(1)

where A(v) is the absorbance, K(v) is the absorbance coefficient, b is the thickness of a coal sample in centimeter, c is the concentration of a coal sample in mol l⁻¹, and T(v) is the percent transmittance.

Results and discussion

Semiquantitative ratios of the organic groups after the ScCO₂–H₂O treatment

There are five coal organic group categories—aromatic groups (aromatic nucleus), alkyl groups, hydroxy groups, oxygen-containing groups, and sulfur-containing groups (Figure 3). The four coals mostly contained hydroxy groups, oxygen-containing groups, and CH_x (2.27–36.20, 8.53–28.28, and 17.41–51.49%, respectively), followed by Ar groups (aromatic groups) (5.52–15.84%), with sulfur-containing groups as the fewest (7.50–32.35%) (Figure 3). The semiquantitative ratios of the organic groups of the four coals are greatly different because of differences in the materials that formed the coal. After the ScCO₂–H₂O treatment, distinct changes in semiquantitative ratios of the organic groups were evident. As the temperature and pressure increased, the changes became significantly regular.

Figure 3.

Semiquantitative ratios of five categories of coal organic groups before and after the ScCO₂–H₂O treatment. Note: Ar, aromatic nucleus; –CH_x, alkyl groups, e.g. methyls, methylenes, and methines; Hyd, hydrogen bonding groups, e.g. free hydroxyls, self-association hydrogen bonds, and hydroxyls; Oxy, oxygen-containing groups, e.g. carboxyls, carbonyls, etc.; Sul, sulfur-containing groups, e.g. mercapto group and disulfide bond.

As the coal rank increased, the Ar semiquantitative ratios of Coals #1 to #3 (10.37, 15.10, and 15.84%, respectively) increased. After the ScCO₂–H₂O treatment, the Ar semiquantitative ratios in Coals #1 and #3 declined slightly compared to raw coal, and it slightly increased in Coals #2 and #4 (Figures 3 and 4).

There are two types of CH_x in coal samples—aromatic CH_x (Ar–CH_x) and aliphatic CH_x (C_nH_2n+1–CH_x). The semiquantitative ratios of Ar–CH_x (13.47–47.49%) in coal samples are much larger than C_nH_2n+1–CH_x (1.05–10.19%) (Figure 4), which means that the changes in the semiquantitative ratios of CH_x are primarily determined by Ar–CH_x. The C_nH_2n+1–CH_x semiquantitative ratios gradually decreased (10.19, 7.66, 4.00, and 1.05%, respectively) as R_o,max increased. As the temperature and pressure increased, the semiquantitative ratios of Ar–CH_x in Coals #2 to #4 showed a decreasing tendency, but in Coal #1, they showed a significant increase compared to raw coal (Figure 4). Differently than for Ar–CH_x, as the temperature and pressure increased, the C_nH_2n+1–CH_x semiquantitative ratio in Coals #1 to #3 decreased, but it slightly increased in Coal #4 (Figure 4).

Figure 4.

Semiquantitative ratios of coal organic groups before and after the ScCO₂–H₂O treatment. Note: Ar, aromatic nucleus; Ar–CH_x, aromatic CH_x; C_nH_2n+1–CH_x, aliphatic CH_x; ·OH, free hydroxyls; OH–O, self-association hydrogen bonds in ethers; –OH, hydroxyls in alcohols, phenols, and ethers; –COOH, carboxyls in fatty alcohols, aldehydes, and ketones; –CO–N, amides in aliphatic compounds; –C–O–, oxygen bonds in aliphatic ethers, phenols, and aromatic oxides; R–C=O, carbonyls in aldehydes, ketones, and esters; Ar–C=O, carbonyls in anhydride; RC=OR′, quinonyls in aromatic ketones; –S–S–, disulfide bonds; –SH, mercapto groups.

Hydrogen bonding plays a vital role in the establishment of the skeletal structure of coal macromolecules (Wang et al., 2016b). The coals contained three types of hydroxy group—OH, an OH–O bond (ether), and –OH groups. The four coals mostly contained –OH, with extremely low semiquantitative ratios of ·OH and OH–O (Figure 4). Therefore, the changes in the hydroxy group semiquantitative ratios are primarily determined by –OH. Coal #1 and #4 had the highest semiquantitative ratios of –OH of 30.76 and 30.26%, respectively, and Coal #3 had the lowest semiquantitative ratio of –OH of 0.20% (Figure 4). Compared to raw coal, the hydroxy group semiquantitative ratios in Coals #2 to #4 significantly increased, but they decreased in Coal #1 (Figure 3). As the temperature and pressure increased, the semiquantitative ratios of ·OH in Coals #2 to #4 decreased, but –OH greatly increased in Coals #2 to #4 (Figure 4). Compared to raw coal, the semiquantitative ratios of ·OH slightly increased in Coal #1, but –OH decreased (Figure 4). OH–O in Coals #2 and #3 was not detected in raw coal, and the semiquantitative ratios of the four coals all increased compared to raw coal.

The four coals mostly contained –C–O– groups at an extremely low semiquantitative ratio (generally <3.00%) of other oxygen-containing groups, e.g. –COOH groups, –CO–N groups, R–C = O groups, Ar–C = O groups, and RC = OR′ groups (Figure 4). The changes in the oxygen-containing group semiquantitative ratios are primarily determined by –C–O–. Some oxygen-containing groups were not detected, e.g. –CO–N in Coals #1 and #3; R–C = O in Coals #1, #3, and #4; and RC = OR′ in Coals #1, #2, and #4 (Figure 4). The semiquantitative ratios of different oxygen-containing groups in Coal #4 are all extremely low, generally <2.00%. As R_o,max increases, the semiquantitative ratios of –C–O– gradually decreased for the four coals (20.90, 24.09, 14.29, and 5.71%, respectively). As the temperature and pressure increased, the oxygen-containing group semiquantitative ratios in Coals #2 to #4 significantly decreased, but they increased in Coal #1 compared to raw coal (Figure 3). The –C–O– semiquantitative ratio in Coals #2 to #4 significantly decreased as the temperature and pressure increased, but –COOH slightly increased (Figure 4). The –C–O– semiquantitative ratio in Coal #1 slightly increased, but –COOH decreased (Figure 4). The semiquantitative ratios of Ar–C = O, –CO–N, R–C = O, and RC = OR′ are low in raw coal, decreased after the ScCO₂–H₂O treatment, and approached or reached zero at temperatures greater than 45.0°C and pressures greater than 10 MPa, except for RC = OR′ in Coal #3 (Figure 4).

The coals primarily contained two types of sulfur-containing groups: –S–S– and –SH. The –S–S– semiquantitative ratios (4.80–19.20%) are slightly higher than –SH (2.33–13.15%) (Figure 4). The –S–S– semiquantitative ratios and –SH in Coal #4 are extremely high, which are next to –OH (30.62%). After the ScCO₂–H₂O treatment, the sulfur-containing group semiquantitative ratios in Coals #1 and #3 slightly increased compared to raw coal, but slightly declined in Coals #2 and #4 (Figure 3). As the temperature and pressure increased, the –S–S– semiquantitative ratio in Coals #1 and #3 slightly increased (Figure 4). On the contrary, the –S–S– semiquantitative ratio in Coals #2 and #4 continuously declined (Figure 4). After the ScCO₂–H₂O treatment, the –SH semiquantitative ratio in Coals #1, #2, and #4 slightly decreased (Figure 4), but slightly increased in Coal #3 (Figure 4).

In addition, the fluctuating amplitudes of semiquantitative ratios of the organic groups compared to raw coal decreased at temperatures greater than 45.0°C and pressures greater than 10 MPa, indicating that the interactions between ScCO₂ and organic groups were attenuated.

FTIR spectra of the organic groups after the ScCO₂–H₂O treatment

Absorbance peak of the aromatic structure

As R_o,max increased, the changes in the absorbance peak intensities of Ar ranged from a significant decrease for Coal #1 (1600.63 cm⁻¹) and a slight decrease for Coal #2 (1596.77 cm⁻¹) to a slight increase for Coal #3 (1598.70 cm⁻¹) and a significant increase for Coal #4 (1598.70 cm⁻¹) (Figure 5). Semianthracite (#3) is the inflection point of the changes in the absorbance peak intensities, indicating that coalification affects the reactions of ScCO₂ with Ar. The changes in the absorbance peak intensities of Ar for Coals #1 and #2 are different for their semiquantitative ratios, which means that the changes in their semiquantitative ratios were caused by the changes in the total organic group content. In addition, the peak positions in the spectrum of Coal #2 moved to a higher frequency after the ScCO₂–H₂O treatment because of inductive effects of oxygen-containing and hydroxy groups.

Figure 5.

Graph of FTIR spectra of coal samples.

The Ar–CH_x of the four coals mostly comprises –CH₂ (747–743 cm⁻¹) and –CH₃ (1379–1373, 1460–1439 cm⁻¹), with weak absorbance peaks of –CH (3060–3032 cm⁻¹) (Figure 5). The absorbance peak intensity of –CH₂ for Coals #2 to #4 was slightly larger than –CH₃, and –CH₃ had the highest absorbance peak intensity compared to –CH₂ and –CH in Coal #1. Ar–CH_x mostly undergoes 1,2-substitution (770–735 cm⁻¹), following by 1,3-substitution (810–750 cm⁻¹) and 1,4-substitution (860–800 cm⁻¹) (Figure 5). Some single substitutions (710–690 cm⁻¹) were also evident in Coal #1 (Figure 5).

After the ScCO₂–H₂O treatment, the absorbance peak intensity of Ar–CH_x in Coals #1 to #3 significantly decreased, indicating that the Ar–CH_x content was lower (Figure 5). The changes in the absorbance peak intensities of Ar–CH_x are weak at temperatures greater than 45.0°C and pressures greater than 10 MPa. Furthermore, Coal #1 had the largest decreasing amplitude for a 1,2-substitution (746.32 cm⁻¹) and a 1,3-substitution (804.17 cm⁻¹) (Figure 5). The Ar–CH_x content decrease indicates that the 4H (750 ± 10 cm⁻¹), 3H (810 ± 10 cm⁻¹), and 1H (875 ± 10 cm⁻¹) content in Ar increased by different amounts, and bond dissociation reactions and 1,2-substitution, 1,3-substitution, and 1,4-substitution addition reactions occurred as a result of the ScCO₂–H₂O treatment.

The absorption band at approximately 3060–3032 cm⁻¹ is the stretching vibration –CH absorption peak, which can be considered as measurement of the adjacent aromatic hydrogens per ring (Wang et al., 2016b). The –CH absorbance peak intensity for Coals #1 to #3 significantly decreased (Figure 5), confirming the increase in adjacent aromatic hydrogens per ring and occurrence of bond dissociation reactions. The absorption band at approximately 747–743 cm⁻¹ is the rocking vibration –CH₂ absorption peak, whose intensity is proportional to the number of continuously linked –CH₂ groups in a molecular chain and can indicate the length of the methylene chains in coal (Wang et al., 2016b). The length of the methylene chains of Coals #1 to #3 decreased (Figure 5), also indicating that bond dissociation reactions were strong during ScCO₂–H₂O treatment and were weakly affected by temperature and pressure (Figure 5). The absorbance peak intensity for single substitutions (692.32 cm⁻¹) and 1,4-substitutions (863.95 cm⁻¹) in Coal #1 greatly increased at temperatures greater than 45.0°C and pressures greater than 10 MPa (Figure 5). Some substituent groups in some 1,2- and 1,3-substitutions were reduced to H. Therefore, an increase in single substitution content also confirmed the occurrence of bond dissociation reactions. The para-position on the benzene ring experiences less steric hindrance, so it is easily substituted. Therefore, the increase in 1,4-substitutions indicates that bond dissociation reactions, addition reactions, and electrophilic substitution reactions were synchronized.

The Ar–CH_x absorbance peak intensity in Coal #4 significantly increased, weakly affected by the temperature and pressure (Figure 5). The number of substituent groups increased after the ScCO₂–H₂O treatment, but the adjacent aromatic hydrogens per ring in Coal #4 were lower. That means that Coal #4 undergoes more substitution reactions during ScCO₂–H₂O treatment. The symmetrical and symmetrical variable-angle absorbance peak of –CH₃ (1430.92 cm⁻¹), the rocking vibration absorbance peak of –CH₂ (746.32 cm⁻¹), and the stretching vibration absorbance peak of –CH₂ (3031.55 cm⁻¹) increased in intensity, which confirmed that substitution reactions had occurred and that branched chains had increased.

Aliphatic hydrocarbon structure absorbance peak

Two absorbance peaks of C_nH_2n+1–CH_x are present in the coal samples, the symmetrical stretching vibration (2858–2847 cm⁻¹) and the asymmetrical stretching vibration (2935–2918 cm⁻¹) of –CH₂ (Figure 5). C_nH_2n+1–CH_x in coal samples is mostly –CH₂, indicating that coal samples mostly contain long linear and alicyclic structures with fewer branched chains. As R_o,max increased, the changes in the absorbance peak intensities of C_nH_2n+1–CH_x varied from a significant decrease for Coal #1 (2917.77 and 2854.13 cm⁻¹) and a slight decrease for Coal #2 (2912.00 and 2858.00 cm⁻¹) to a slight increase for Coal #3 (2913.91 and 2854.13 cm⁻¹) and a significant increase for Coal #4 (2913.91 and 2852.20 cm⁻¹) (Figure 5). Semianthracite (#3) is the inflection point of the changes in the absorbance peak intensities. The semiquantitative ratios behave in a similar fashion—the changes in the absorbance peak intensities of C_nH_2n+1–CH_x are weak at temperatures greater than 45.0°C and pressures greater than 10 MPa. Therefore, the changes in the C_nH_2n+1–CH_x semiquantitative ratios were caused by the changes in total organic group content, and coalification had a critical role in reactions between ScCO₂ and C_nH_2n+1–CH_x.

Hydroxy group absorbance peaks

The ·OH (3684–3625 cm⁻¹) and OH–O (3624–3610 cm⁻¹) absorption bands are not obvious, but superimposed, e.g. in raw Coal #3 and #4 (Figure 5). The changes in the ·OH absorbance peak intensity are consistent with the semiquantitative ratios—Coal #1 was slightly increased, and Coals #2 to #4 were decreased (Figure 5). The absorbance peak intensity of OH–O had a slight increase (Figure 5), indicating that a trace amount of OH–O was produced during the ScCO₂–H₂O treatment.

The –OH groups in the four coals are mostly –OH (alcohol) (3550–3200 cm⁻¹), with a low content of –OH (phenol) (940–900 cm⁻¹) (Figure 5). The –OH absorption bands of Coals #3 and #4 are not obvious but are superimposed over the absorption bands of silicate minerals (1033 or 1010 cm⁻¹) (Figure 5). The changes in the –OH (alcohol) absorbance peak intensities are consistent with the semiquantitative ratios in that absorbance peak intensities showed large variations. The –OH (phenol) absorbance peak intensity of Coal #1 had a clear increase compared to raw coal (Figure 5), indicating that a small quantity of –OH (phenol) was produced during the ScCO₂–H₂O treatment. The –OH (phenol) absorbance peak intensity of Coals #2 to #4 slightly decreased (Figure 5), which shows that the changes in –OH content were caused by changes in alcohol –OH groups.

Oxygen-containing groups absorbance peaks

Coal #1 mostly contains –C–O– (aliphatic ether) (1125–1110 cm⁻¹) groups, with a low –C–O– (phenol) (1300–1150 cm⁻¹) and –C–O– (aromatic oxide) (1351–1260 cm⁻¹) content, and the –C–O– (aliphatic ether) and –C–O– (aromatic oxide) contents are slightly greater than the –C–O– (phenol) content (Figure 5). Coal #3 mostly contains –C–O– (aromatic oxide), and the –C–O– content in Coal #4 is low (Figure 5). The –C–O– (aliphatic ether) content is slightly greater than the –C–O– (phenol) and –C–O– (aromatic oxide) contents, and the–C–O– absorption bands were superimposed over those of carbonate minerals (Figure 5). ScCO₂–H₂O treatment significantly decreased the absorbance peak intensity and the –C–O– content in the coal samples, but temperature and pressure had little effect (Figure 5). The changes in the –C–O– semiquantitative ratio of Coal #1 were caused by the changes in the total organic group content. The –C–O– of Coal #3 is unusual in that at temperatures greater than 45.0°C and pressures greater than 10 MPa, the –C–O– (aliphatic ether) (1153.22 cm⁻¹) and –C–O– (phenol) (1116.58 cm⁻¹) absorbance peak intensities increased as the –C–O– (aromatic oxide) decreased (Figure 5).

The coal samples mostly contain –COOH (fatty alcohol) (2780–2350 cm⁻¹) and no –COOH (aldehyde and ketone) (1720–1690 cm⁻¹) (Figure 5). Only Coals #1 and #3 had obvious –COOH (fatty alcohol) absorption peaks at approximately 2362.37 cm⁻¹, which are not obvious in Coals #2 and #4 (Figure 5). Overall, the ScCO₂–H₂O treatment slightly increased the –COOH (fatty alcohol) absorbance peak intensities in Coals #2 to #4 but decreased them in Coal #1.

The Ar–C = O stretching vibration absorption band is usually split into two absorption bands at 1860–1800 and 1800–1750 cm⁻¹ because of the coupling of stretching vibrations of the two C = O groups. ScCO₂–H₂O treatment significantly decreased the Ar–C = O absorbance peak intensity consistently with changes in the semiquantitative ratios (Figure 5). The Ar–C = O stretching vibration absorption band of Coals #1 to #3 moved to a higher frequency (1868.68, 1884.11, and 1874.47 cm⁻¹, respectively), indicating that Coals #1 to #3 mostly contain open chain aliphatic and cyclic aliphatic anhydrides. In addition, the Ar–C = O absorbance peak of Coal #4 is not obvious because the Ar–C = O content of Coal #4 is low (Figure 5).

The –CO–N (1650–1640 cm⁻¹) absorbance peaks were usually superimposed over the Ar (1635–1595 cm⁻¹) and RC = OR′ (1690–1650 cm⁻¹) peaks and were not obvious (Figure 5) due to the conjugative effect between Ar and carbonyl groups. No –CO–N absorbance peaks were seen in Coals #1 and #3, and they disappeared in Coals #2 and #4 after the ScCO₂–H₂O treatment (Figure 5). The R–C = O absorbance peaks (1736–1722 cm⁻¹) were also not obvious. The only R–C = O absorbance peaks were found in raw Coal #2 and disappeared after the ScCO₂–H₂O treatment (Figure 5). The only RC = OR′ absorbance peaks were found in Coal #3, and their intensity decreased after the ScCO₂–H₂O treatment (Figure 5). The changes in the –CO–N, R–C = O, and RC = OR′ absorbance peak intensities after ScCO₂–H₂O treatment were consistent with their semiquantitative ratios.

Sulfur-containing group absorbance peaks

The changes in the –S–S– content and its absorbance peak intensity (466–478 cm⁻¹) were closely linked to coalification but little affected by temperature and pressure (Figure 5). After ScCO₂–H₂O treatment, absorbance peak intensity in the high- and low-volatility bituminous coals (Coals #1 and #2) decreased, and the decrease in amplitude decreased with the increase in coalification (Coal #1 > Coal #2) (Figure 5). The absorbance peak intensity of the high-rank coals (Coal #3 and #4) was greater, and the increase in amplitude decreased with the increase in coalification (Coal #3 > Coal #4) (Figure 5). The absorbance peak intensity was highest for semianthracite. The changes in the –SH content and absorbance peak intensity (540 cm⁻¹) are identical to –S–S– and are closely associated with coalification (Figure 5).

In addition, the changes in the –S–S– semiquantitative ratios of Coal #1 and the –SH semiquantitative ratio of Coal #4 were caused by a change in the total organic group content.

Chemical reactions between a ScCO₂–H₂O system and organic groups

The effects of swelling on Ar, CH_x, and hydroxy groups

Solvated electrons in ScCO₂ can dissolve some coal and cause the coal to swell (Dawson et al., 2015; Gathitu et al., 2009). Swelling decreased the interactions among the coal macromolecules, causing the extension and reorientation of the coal macromolecules, rupture of the hydrogen bonds, and rearrangement of coal chemical structure (Dawson et al., 2015; Gathitu et al., 2009). Swelling is accompanied by bond dissociation reactions, polyaddition reactions, and hydrogen bond interactions and is a general expression of above-mentioned chemical reactions.

The ratio between the peak areas of CH_x with absorption bands of 700–900 cm⁻¹ and the Ar (Ar–CH_x/Ar) stretching vibration is a measurement of the degree of condensation of Ar (Chen et al., 2012). The degree of condensation of Ar Coals #1 to #4 progressively increased, which is in agreement with our knowledge of coal ranks (Table 3). After the ScCO₂–H₂O treatment, the degree of condensation of Ar Coals #2 to #4 decreased greatly, and the decrease in amplitude increased with the coal rank (Table 3). Therefore, the condensation degree of Ar in coal with a lower coal rank (#3) is higher than that with higher coal rank (#4). The degree of condensation of Ar in Coal #1 increased as the temperature and pressure increased (Table 3), indicating that the decrease in the degree of cross-linking caused by swelling is more pronounced in a high-rank coal.

Table 3.

Degree of condensation of coals before and after ScCO2–H2O treatment.

Samples	#1		#2		#3		#4
Samples	Ar–CH_x/Ar	Variation (%)	Ar–CH_x/Ar	Variation (%)	Ar–CH_x/Ar	Variation (%)	Ar–CH_x/Ar	Variation (%)
Raw coal	0.47		1.15		1.65		1.75
1000 m	0.53	13.36	0.80	−30.18	0.93	−43.76	0.83	−52.62
1500 m	0.65	38.36	0.86	−25.08	0.82	−50.21	0.68	−61.19
2000 m	0.72	52.80	0.75	−34.45	0.99	−39.63	0.80	−54.37

Note: “−” means decrease.

The short-range order of the macromolecular structure of a high-rank coal depends on the directional arrangement of the aromatic layers between the coal macromolecules (Mathews and Chaffee, 2012; Mathews et al., 2011). Macromolecules of a high-rank coal in the same aromatic layer are cross-linked by carbon–carbon bonds (C_Ar–C_Ar), and hydrogen bonds and Van Der Waals forces almost disappear, instead of reactions among unfixed π bonds whose structure and chemical properties are stable and have almost no swelling (Mathews and Chaffee, 2012; Mathews et al., 2011). Macromolecules of high-rank coal in different aromatic layers are cross-linked by methyne, ether, thioether, methylene ether, and methyne thioether bonds, etc. (Mathews and Chaffee, 2012; Mathews et al., 2011). Swelling caused by ScCO₂ primarily breaks cross-links connecting different aromatic layers, decreasing the forces acting among the aromatic layers and the degree of cross-linking of the macromolecules. The effect becomes stronger as the coal rank increases. Cross-link breakage can produce hydrogen free radicals (·H). ·H can be substituted by CH_x, causing Ar–CH_x and C_nH_2n+1–CH_x, to increase or it can be changed into –OH, causing an increase in alcoholic hydroxyl groups. Cross-link breakage among aromatic layers and the formation of –OH increase the asymmetry and polarity of the hydrogen bonds (hydroxyl groups are hydrogen bonding groups) and of C_Ar–C_Ar. The common effects of polar group conjugations, hydrogen bond interactions, and symmetric interactions of coal macromolecules increase the absorbance peak intensity of Ar.

The short-range order of the macromolecular structure of bituminous coal depends on the directional arrangement of the aromatic layers in the coal macromolecules (Castro-Marcano et al., 2012; Grzybek et al., 2002). The aromatic structure of bituminous coal is mainly polycyclic aromatic hydrocarbons with rings arranged in a straight line, and the aromatic layers are weakly cross-linked (Castro-Marcano et al., 2012; Grzybek et al., 2002). Swelling primarily breaks cross-links in the same aromatic layer. Some branched chains and organic groups with low bond energy were broken by swelling, causing the rearrangement of the coal chemical structure, increased directionality in the aromatic layers, and finally the formation of C_Ar–C_Ar cross-links (equation (2)). The formation of C_Ar–C_Ar cross-links is also known as a polyaddition reaction. Polyaddition reactions can further increase the degree of condensation and degree of cross-linking of the aromatic structure, and it becomes weaker as the coal rank increases. The breakage of strong polar cross-links and the increase in the symmetry of C_Ar–C_Ar concomitantly increase the absorbance peak intensity of Ar

C_{6} H_{5} {-CH}_{3} {CH}_{2} \cdot + \cdot {CH}_{2} {CH}_{3} {-C}_{6} H_{5} \to C_{6} H_{5} {-CH}_{3} {CH}_{2} {-CH}_{2} {CH}_{3} {-C}_{6} H_{5}

(2)

The other effects of swelling caused by ScCO₂ on organic groups are effects on the CH_x and hydroxy group contents and absorbance peak intensities. Cross-link breakage, e.g. of methyne, ether bonds, and methylene ether bonds, is an important reason for the decrease in the CH_x, –OH, and –C–O– contents. These cross-links are primarily found in bituminous coal. Therefore, the decrease in their contents is more pronounced in Coals #1 and #2. Cross-link breakage, produces a large number of ·H, which changes the associations between the aliphatic chains and the organic groups. More –OH exist in coal macromolecules in the form of OH–O, which is an important reason for the increase in the OH–O content and absorbance peak intensity. Meanwhile, the increase in the OH–O contents and absorbance peak intensity confirmed the occurrence of swelling. Some ·H also changed to ·OH via the capture of O. ·OH can combine with Ar or CH_x to form –OH. In Coal #1, some ·H combined with Ar to form –OH (phenol). However, the polyaddition reaction inhibited the reaction of ·H and CH_x, causing a decrease in –OH (alcohol) and an increase in ·OH. In Coals #2 to #4, the combination of ·H with Ar is difficult because the polycyclic aromatic structure and the breakage of cross-links due to swelling decrease –OH (phenol) groups. However, bond dissociation and hydrolysis reactions increased the –OH (alcohol) content in Coals #2 to #4.

The effects of endothermic reactions on organic groups

Addition reactions.

There are three types of addition reactions between a ScCO₂–H₂O system and coal organic groups—chlorination, conjugated addition, and nucleophilic addition. Chlorination and conjugated addition meanly affect Ar content and absorbance peak intensity, and nucleophilic addition acts on hydroxy group and oxygen-containing groups.

Cl is a common element in coal (Xu et al., 2016). X-ray photoelectron spectroscopy shows that a large amount of Cl is present in coal, and the Cl content decreases with an increase in coalification (Table 4). The acid environment caused by the dissolution of ScCO₂ in water can dissolve Cl in inorganic chlorides. These Cl can participate in addition reactions with Ar under heating (<100°C) and pressurized conditions, and such reactions are termed chlorination (Limantseva et al., 2008). ScCO₂ and water are both ionizing solvents that can accelerate chlorination (Arash et al., 2016).

Table 4.

Results of X-ray photoelectron spectroscopy (XPS) of coal samples (Source gun type: Al K Alpha).

SamplesElement	#1		#2		#3		#4
SamplesElement	Peak BE	Atomic %	Peak BE	Atomic %	Peak BE	Atomic %	Peak BE	Atomic %
C1s	283.06	86.83	283.10	86.73	283.74	81.17	284.12	58.22
O1s	530.84	7.00	530.99	7.53	531.20	12.78	531.03	25.06
N1s	398.66	2.70	397.84	2.07	–	–	400.66	2.09
Na1s	–	–	–	–	–	–	1071.66	0.78
Ca2p	–	–	–	–	346.62	0.63	–	–
Cl2p	198.29	0.38	199.18	0.63	–	–	–	–
Si2p	101.44	1.44	101.26	1.46	101.33	3.09	101.86	7.52
Al2p	72.92	1.65	72.96	1.57	73.61	2.34	73.76	6.33

Peak BE: peak binding energy (eV).

In the early phase of chlorination, H on Ar groups can be substituted by Cl, releasing HCl (equation (3))

RH \overset{Cl}{\to} RCl+HCl

(3)

In the later phases, after H is significantly decreased, the addition reaction occurs (equation (4))

C_{|}^{|} = C_{|}^{|} \overset{Cl}{\to} Cl - C_{|}^{|} - C_{|}^{|} - Cl

(4)

Chlorination destroys carbon–carbon double bonds (C = C) in Ar, causing the decrease in the Ar content and absorbance peak intensity in Coals #1 and #2.

The coal aromatic structure consists of Ar with a different degree of condensation and contains a small amount of hydroaromatic rings, aziridine, and sulfur heterocyclic rings (Castro-Marcano et al., 2012; Grzybek et al., 2002; Mathews and Chaffee, 2012; Mathews et al., 2011). No clear conclusions are possible about the coal macromolecular structure. Studies of coal macromolecular structure are focused on bituminous coal, and research on anthracite is lacking. The best knowledge at present is that the number of condensed aromatic rings in coal is associated with coalification and the carbon content (Castro-Marcano et al., 2012; Grzybek et al., 2002; Mathews and Chaffee, 2012; Mathews et al., 2011). As coalification increases, the number of condensed rings increases, and the oxygen-containing group content decreases. The number of condensed rings in bituminous coal with a carbon content between 70 and 83% is approximately 2–3, and the aromatic structure is mainly benzene, naphthalene, and phenanthrene. The number of condensed rings of bituminous coal with a carbon content between 83 and 90% increases to 3–5, and the aromatic structure is mainly phenanthrene, anthracene, and pyrene. The number of condensed rings of a high-rank coal with a carbon content greater than 90% is greatly increased because of the graphite structure. For a high-rank coal with a carbon content greater than 93%, the number of condensed rings spikes to more than 30 (Castro-Marcano et al., 2012; Grzybek et al., 2002; Mathews and Chaffee, 2012; Mathews et al., 2011).

Coal #1 is a high-volatility bituminous coal with a carbon content of 83.28% (Table 1). Its aromatic structure mostly contains benzene, naphthalene, and phenanthrene. Naphthalene and phenanthrene are polycyclic aromatic hydrocarbons with rings arranged in a straight line. Their resonance energy decreases as the number of rings increases, and they are more reactive than benzene. Coal #2 is low-volatility bituminous coal, with a carbon content of 80.32% (Table 1). Its aromatic structure mostly contains phenanthrene, anthracene, and pyrene. Pyrene has a condensed multibenzene ring structure such that its chemical stability is similar to benzene. The pyrene in Coal #2 makes chlorination more difficult. Coals #3 and #4 are semianthracite and anthracite with carbon contents of greater than 90% (Table 1). The chemical properties of Coals #3 and #4 are stable, and chlorination is difficult. Therefore, Coal #1 is easily chlorinated, greatly increasing its Ar content. Chlorination can also occur in Coal #2, but the increase in the Ar content is less. Little chlorination occurs in Coals #3 and #4, so the Ar content is stable.

ScCO₂ and water are both polar solvents that can promote the conjugated addition of Ar. Solvated electrons can participate in conjugated addition with 1,3-dienes in Ar and produce free anions. The free anion captures a proton and forms a free radical. The free radical then acquires a solvated electron and becomes an anion. The anion is an alkali that can capture a proton to form a cycloolefin. Conjugated addition can also destroy C = C bonds in Ar. Its response intensity in different rank coals is similar to chlorination.

Furthermore, swelling and addition reactions together drove the changes in the absorbance peak intensities of Ar and had opposite effects on Ar. Coal #1 mostly underwent addition reactions so that the absorbance peak intensity of Ar decreased. The effects of addition reactions are slightly stronger than swelling in Coal #2, so the absorbance peak intensity of Ar slightly decreased. The effects of swelling are slightly stronger than addition reactions in Coal #3, so the absorbance peak intensity of Ar slightly increased. Coal #4 had the most swelling, so the absorbance peak intensity of Ar increased.

Water is a nucleophilic reagent and can participate in nucleophilic addition with C = O in R–C = O, Ar–C = O, and RC = OR′ groups under acidic conditions (equation (5)). Nucleophilic addition can form –OH groups and decrease the R–C = O, Ar–C = O, and RC = OR′ contents and absorbance peak intensities

\begin{array}{l} {R–C=O+H}_{2} O \to R–C– {(OH)}_{2} \\ {HCHO+H}_{2} O \to H_{2} C {(OH)}_{2} \\ {CH}_{3} {CHO+H}_{2} O \to {CH}_{3} CH {(OH)}_{2} \\ {CH}_{3} {COCH}_{3} {+H}_{2} O \to {({CH}_{3})}_{2} C {(OH)}_{2} \end{array}

(5)

2. Bond dissociation reactions.

Bond dissociation reactions are important reasons for the changes of CH_x. Under acidic conditions caused by ScCO₂ and heating conditions, chemical bonds of CH_x can have hemolysis and form free radical (·C), which is known as bond dissociation reaction of CH_x (equation (6)) (Liang et al., 2014; Wang et al., 2016b). According to the bond dissociation energy, the longer the molecular chains of alkane, the smaller the bond dissociation energy of ·C is. Smaller bond dissociation energy makes ·C instability that can easily be broken. Larger macromolecules are more easily broken in the middle of the molecular chain

\begin{array}{l} {CH}_{3} {-CH}_{3} \to 2 {CH}_{3} \\ {CH}_{3} {CH}_{2} {-CH}_{3} \to {CH}_{3} {CH}_{2} \cdot + \cdot {CH}_{3} \\ {CH}_{3} {CH}_{2} {-CH}_{2} {CH}_{3} \to 2 {CH}_{3} {CH}_{2} \\ {({CH}_{3})}_{2} {CH-CH}_{3} \to {({CH}_{3})}_{2} CH \cdot + \cdot {CH}_{3} \\ {({CH}_{3})}_{3} {C-CH}_{3} \to {({CH}_{3})}_{3} C \cdot + \cdot {CH}_{3} \end{array}

(6)

In addition, C = C of Ar also have small bond dissociation energy, particularly polycyclic aromatic hydrocarbons with rings arranged in a straight line, e.g. naphthalene and anthracene, that their aromatic structures can be easily destroyed. That is, one of the reasons for the decrease in Ar contents in Coals #1 and #2.

3. Sulfuration reaction.

Organic sulfur is the main heteroatom in coal and is in the form of a cross-linked structure and heterocyclic ring that are difficult to dissolve and remove (Lei et al., 1994; Sun et al., 2009). Research shows that with the increase in coalification, inorganic sulfur can translate into organic sulfur, making the total sulfur content increase constantly (Lei et al., 1994; Sun et al., 2009). That is, with the increase in coalification, the content of organic sulfur tends to rise, and the sulfur-containing group semiquantitative ratios are anomalously high. Meanwhile, with the increase in coalification, thioether (–S–S–) and mercaptan (–SH) change into thiophene sulfur—the main organic sulfur in high-rank coal.

Under acidic conditions caused by ScCO₂ and heating conditions, sulfide minerals in coal, e.g. pyrite, can be dissolved to form H₂S. H₂S can have direct reactions with organic matter of coal to form organic sulfides, e.g. mercaptan. Mercaptan and thioether can change into each other such that –SH creates –S–S–, and –S–S– breaks into –SH (equation (7)). The cross-linking reaction between S²⁻ and organic matter of coal is known as a sulfuration reaction. A sulfuration reaction can connect one or more S to the coal macromolecules to form cross-links. That is why the sulfur-containing group contents in Coals #3 and #4 increased

2-SH ⇌ H–S–S–H

(7)

In addition, –S–S– and –SH in Coals #1 and #2 are primarily cross-links in coal macromolecules, which can be broken by swelling, making the decrease in sulfur-containing group contents. Moreover, –S–S– and –SH are no longer the main cross-links in Coals #3 and #4 that were weakly affected by swelling and primarily affected by sulfuration reaction.

The effects of exothermic reaction on organic groups

Substitution reaction.

There are two types of substitution reactions between a ScCO₂–H₂O system and coal organic groups—electrophilic substitution and hydrolysis reaction. They act on CH_x and oxygen-containing groups, respectively.

H on Ar can be easily substituted by other organic groups, and the acidic conditions caused by ScCO₂ promote the substitution reaction making the increase in substituent, e.g. CH_x (Liang et al., 2014; Wang et al., 2016b)

C_{6} H_{6} {+–CH}_{3} \to C_{6} H_{5} {-CH}_{3} {+H}^{+}

(8)

According to the effects of swelling and endothermic reactions, the substitution reaction is weak because of the symmetrical cross-linked structure of bituminous coal (Coals #1 and #2), together with swelling and bond dissociation reaction, causing the decrease in contents of Ar–CH_x and C_nH_2n+1–CH_x. The substitution reaction is strong because of the polar cross-linked structure of high-rank coal (Coal #3 and #4), causing the increase in contents of Ar–CH_x and C_nH_2n+1–CH_x.

Moreover, the changes of Ar–CH_x and C_nH_2n+1–CH_x are the results of swelling, bond dissociation reaction, and substitution reaction. Swelling and bond dissociation reaction decrease CH_x contents, while substitution reaction increases CH_x contents.

There were more substituent groups and branched chains of aliphatic hydrocarbons in bituminous coal (Coals #1 and #2), which are easily broken. Therefore, swelling and bond dissociation reaction are stronger than substitution reaction, causing the decrease in CH_x contents. ·C produced by bond dissociation reaction is so unstable that free radical reaction will continue until the free radicals disappear. The free radical reaction can combine two ·C together that shorten the aliphatic hydrocarbons and increase the degree of condensation of the coal. At the high-rank coal stage (Coals #1 and #2), aliphatic series, aliphatic functional groups, and side chains of Ar have decreased, causing the swelling and bond dissociation reaction of CH_x to become weaker. The large number of ·H produced by swelling promoted the substitution reaction, making the substitution reaction stronger than swelling and bond dissociation reaction. The dominance of substitution reaction increased CH_x content. In a word, semianthracite (Coal #3) is the inflection point of interaction types for CH_x. For Coal #3, C_nH_2n+1–CH_x are more stable than in Ar. Bond dissociation reaction mainly continued to Ar–CH_x, while substitution reaction primarily affected C_nH_2n+1–CH_x, causing the decrease in Ar–CH_x and increase in C_nH_2n+1–CH_x. These have been proved by the increase in absorbance peak intensity of –C–O– (aliphatic ether) and the decrease in absorbance peak intensity of –C–O– (aromatic ether) in Coal #3.

Hydrolysis reaction is the important reason for the decrease in contents and absorbance peak intensity of oxygen-containing groups. Hydrolysis reaction is actually nucleophilic substitution reaction. Oxygen bonds in –CO–N and –C–O– are unstable and can be easily hydrolyzed under acid conditions and heating conditions (Wang et al., 2016b). The products of the hydrolysis reactions of different groups have some differences

Hydrolysis reaction of amide: ${R=O–NH}_{2} \overset{H^{+}, H_{2} O}{\to} {R=O-OH+NH}_{4}^{+}$

Hydrolysis reaction of –C–O–: ${R–O–R'+H}_{2} O \to ROH+R'OH$

Under the acidity and heating conditions, the hydrolysis reaction of –CO–N and –C–O– caused the decrease in –CO–N and –C–O– content. Oxygen bonds in –CO–N and –C–O– are inactive. Therefore, the reactivity of –CO–N and –C–O– is weak, and the decreasing percentages of their contents are small.

2. Oxidizing reaction.

R–C = O, Ar–C = O, and RC = OR′ are easily oxidized and form –COOH. The reaction conditions of R–C = O, Ar–C = O, and RC = OR′ oxidization are moderate. These oxidizations maybe caused by oxidation reagents in coal or reactive solvents, e.g. Cr, H₂O₂, and Br. Their essences are nucleophilic additions of –OH and –COOH. These oxidizations may also be caused by oxygen during the experimental process, which is auto-oxidation

Oxidizing reaction: $2RCOH \overset{-OH}{\to} {RCH}_{2} {OH+RCOO}^{-}$ , ${RCOO}^{-} \overset{\cdot H}{\to} RCOOH$

Auto-oxidation reaction: ${2RCOH+O}_{2} \to 2 RCOOH$

The oxidizing reaction of C = O bonds is an important reason for the increase in –COOH content.

The effects of temperatures and pressures on reactions between a ScCO₂–H₂O system and organic groups

Under ScCO₂–H₂O treatment, as the temperature and pressure increased, the changes in the organic group content can be classified into four types: descending type, rising type, lower opening parabola type, and upper opening parabola type.

Descending type

The typical organic groups of descending type include oxygen-containing groups with low contents (e.g. R–C = O, Ar–C = O, and RC = OR′), Ar, C_nH_2n+1–CH_x, –S–S–, and –SH in Coals #1 and #2 and ·OH in Coals #2 to #4. With the increase in temperature and pressure, contents and absorbance peak intensity of descending type decrease, indicating that interactions between ScCO₂ and organic groups enhanced. When temperatures are >45.0°C and pressures are >10 MPa, their decrease in amplitude significantly decreases, which means that the effects of temperature and pressure on reactions between ScCO₂ and organic groups are attenuated.

Under the supercritical state, the dissolving capacity and reaction capacity of ScCO₂ to organic groups slightly change with the changes in temperature and pressure. The increase in the organic group content is primarily affected by the reaction capacity and reaction type of the organic groups. Descending type gives priority to endothermic reaction. With the increase in temperature, the chemical properties are more reactive, and the reaction capacity and reaction rates of the organic groups increase. Furthermore, reaction types of the organic groups can be determined. R–C = O, Ar–C = O, and RC = OR′ mostly contained nucleophilic addition. Ar in Coals #1 and #2 primarily had chlorination or conjugated addition (including C_Ar–C_Ar cross-linking). C_nH_2n+1–CH_x, –S–S–, and –SH in Coals #1 and #2 and ·OH in Coals #2 to #4 mostly contained bond dissociation reactions caused by swelling.

Rising type

The typical organic groups of rising type include ROR′–OH; –OH in Coals #2 to #4; and Ar, –S–S, and –SH in Coals #3 and #4. With the increase in temperature and pressure, contents and absorbance peak intensity of descending type increase, indicating that interactions between ScCO₂ and organic groups enhanced. Additionally, when temperatures are >45.0°C and pressures are >10 MPa, the effects of temperature and pressure on reactions between ScCO₂ and organic groups are attenuated.

Rising type also gives priority to endothermic reaction. Furthermore, ROR′–OH and –OH in Coals #2 to #4 mostly contained nucleophilic addition and bond dissociation reactions caused by swelling. Ar in Coals #3 and #4 primarily had a bond dissociation reaction, and –S–S and –SH in Coals #3 and #4 mostly contained sulfuration reaction.

Lower opening parabola type

The typical organic groups of lower opening parabola type include C_nH_2n+1–CH_x in Coals #3 and #4, Ar–CH_x in Coal #4, and –COOH in Coals #2 to #4. With the increase in temperature and pressure, the contents and absorbance peak intensity of lower opening parabola type increase first and then decrease when temperatures are >45.0°C and pressures are >10 MPa. After the ScCO₂–H₂O treatment, the organic group contents are higher than raw coal. With the increase in temperature and pressure, the reaction capacity of ScCO₂ decreases.

The lower opening parabola type gives priority to exothermic reaction. High temperature has an inhibiting effect on the exothermic reaction. Moreover, C_nH_2n+1–CH_x in Coals #3 and #4 and Ar–CH_x in Coal #4 were primarily formed by substitution reaction of CH_x, and –COOH in Coals #2 to #4 mostly contained oxidation reaction.

Upper opening parabola type

The typical organic groups of upper opening parabola type include –C–O–, Ar–CH_x in Coals #1 to #3, and –OH and –COOH in Coal #1. With the increase in temperature and pressure, contents and absorbance peak intensity of upper opening parabola type decrease first, and then increase when temperatures are >45.0°C and pressures are >10 MPa. After the ScCO₂–H₂O treatment, the organic group contents are above raw coal. With the increase in temperature and pressure, reaction capacity of ScCO₂ decrease.

Upper opening parabola type also gives priority to exothermic reaction and affected by a variety of chemical reactions. Ar–CH_x in Coals #1 to #3 affected by substitution reaction and bond dissociation reaction, and –C–O–, –OH, and –COOH were affected by nucleophilic addition, swelling, bond dissociation reaction caused by swelling, and polyaddition reaction (exothermic reaction). With the increase in temperature and pressure, substitution reaction and polyaddition reaction attenuate, and bond dissociation reaction enhances, causing the organic group contents decrease first and then increase.

In conclusion, 45.0°C and 10 MPa is the inflection point of the changes in the organic group content. When temperatures are >45.0°C and pressures are >10 MPa, the reaction capacity of the organic groups increases, but the effects of temperature and pressure on reactions attenuate, or the reaction capacity decreases. The effects of temperature and pressure on reactions between ScCO₂ and organic groups are controlled by reaction types of the organic groups. When temperatures are ≤45.0°C and pressures are ≤10 MPa, ScCO₂ has remarkable effects on CH_x and hydroxy groups and has a better effect on bituminous coal than high-rank coal. When the temperatures are >45.0°C and pressures are >10 MPa, ScCO₂ has remarkable effects on oxygen-containing groups and sulfur-containing groups and has a greater effect on high-rank coal. In this sense, the burial depth of ScCO₂ to bituminous coal should not be too deep, and the burial depth of ScCO₂ to high-rank coal can be up to 2000 m.

The effects of coalification on reactions between a ScCO₂–H₂O system and organic groups

According to “Chemical reactions between a ScCO₂–H₂O system and organic groups”, coal coalification has great effects on ScCO₂–H₂O treatment. Coal coalification determines coal macromolecular structure, including characteristics of the aromatic structures, categories of main organic groups, and essences of cross-links. With the increase in coalification, coal macromolecules show obvious differences, causing the differences in reaction types and reaction results between ScCO₂–H₂O and different ranks of coals. For instance, the differences in characteristics of the aromatic structures cause Ar in bituminous coal to give priority to addition reaction and to substitution reaction in high-rank coal, resulting in the decrease in Ar contents in bituminous coal and the increase in absorbance peak intensity in high-rank coal. The differences in categories of main organic groups of coal, especially side chains of aliphatic series, cause side chains of aliphatic series in bituminous coal to have an intense polyaddition reaction and those in high-rank coal to have an obvious bond dissociation reaction, resulting in the increase in the degree of condensation of Ar and decrease in CH_x contents in bituminous coal and the decrease in the degree of condensation of Ar and increase in CH_x contents in high-rank coal. The differences in cross-links cause cross-link breakage in the same aromatic layer in bituminous coal and breakage of cross-links connecting different aromatic layers in high-rank coal, resulting in the increase in the degree of condensation of Ar and decrease in contents of oxygen-containing groups and sulfur-containing groups in bituminous coal and the decrease in the degree of condensation of Ar and increase in the contents of oxygen-containing groups and sulfur-containing groups in high-rank coal.

In conclusion, semianthracite is the inflection point of reactions between ScCO₂–H₂O and organic groups in coal samples of different ranks. Semianthracite corresponds to the “third saltation” of coalification. In this phase, oxygen-containing functional groups and side chains in coal almost disappear, and the number of condensed rings rapidly increases. Furthermore, coal macromolecules tend to be ordered. Semianthracite is a demarcation point from a quantitative change to a qualitative change in the coal macromolecules, making it an important transition in ScCO₂–H₂O treatment.

Conclusions

ScCO₂–H₂O treatments of four coal samples of different ranks were carried out at the three temperatures and pressures, which are 45.0°C and 10 MPa, 62.5°C and 15 MPa, and 80.0°C and 20 MPa. FTIR spectroscopy provides semiquantitative ratios and FTIR spectra of coal samples before and after the ScCO₂–H₂O treatment. Then, the changes in the organic groups and effects of temperature and pressure and coalification on reactions between ScCO₂–H₂O and organic groups were discussed in terms of organic chemistry. The following conclusions can be drawn from this study.

The changes in the organic group content are the result of a variety of reactions and are affected by temperature and pressure and coalification. Reaction types between ScCO₂ and coal were controlled by coal macromolecular structure. Semianthracite is the inflection point of reaction types for coal samples of different ranks. The effects of temperature and pressure on interactions between ScCO₂ and coal were controlled by the reaction types of the organic groups. With the increase in temperature and pressure, the changes in the organic group content can be classified into four types, which are descending type, rising type, lower opening parabola type, and upper opening parabola type. 45.0°C and 10 MPa is the inflection point of the changes in the organic group content.

Bituminous coal including high- and low-volatility bituminous coal has low degree of condensation of the aromatic structure, and its Ar can facilitate addition reactions. The swellings primarily break cross-links between Ar in the same aromatic layer. These characteristics promote the polymerization addition of aliphatic side chains of Ar, causing the increase in the degree of condensation of the aromatic structure and the decrease in contents of CH_x, oxygen-containing groups, and sulfur-containing groups in bituminous coal. High-rank coal including semianthracite and anthracite has a high degree of condensation of the aromatic structure, and its Ar gives priority to substitution reactions. The swellings primarily break cross-links connecting different aromatic layers. Bond dissociation reaction and sulfuration reaction are more significant for high-rank coal. These characteristics cause the decrease in the degree of condensation of the aromatic structure and the increase in contents of CH_x and sulfur-containing groups in high-rank coal.

Descending type and rising type favor addition reaction, bond dissociation reaction, and sulfuration reaction, which are endothermic reactions. The reaction rate of ScCO₂ and organic groups increases as the temperature and pressure increase. Lower opening parabola type and upper opening parabola type favor substitution reaction, oxidation reaction, and addition polymerization, which are exothermic reactions. They were significantly affected by a variety of reactions and were suppressed by high temperature and pressure. Thus, when the temperatures are ≤45.0°C and pressures are ≤10 MPa, ScCO₂ has remarkable effects on CH_x and hydroxy groups and has a better effect on bituminous coal. When the temperatures are >45.0°C and pressures are >10 MPa, ScCO₂ has remarkable effects on oxygen-containing groups and sulfur-containing groups and has a greater effect on high-rank coal. These results indicate that the burial depth of ScCO₂ to bituminous coal should not be too deep, and the burial depth of ScCO₂ to high-rank coal can be up to 2000 m.

Footnotes

Acknowledgements

We would like to thank a number of research students from China University of Mining and Technology for their assistance in the coal sampling and some experiments.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This study was supported by the National Natural Science Foundation of China (Nos. 41330638, 41402135, and 41727801), the Natural Science Foundation of Jiangsu Province (No. BK20171188), the Scientific Research Foundation of Key Laboratory of Coal-based CO₂ Capture and Geological Storage, Jiangsu Province (China University of Mining and Technology) (Nos. 2015A04 and 2016B03), the State Scholarship Fund of China Scholarship Council (CSC) (No. 201706425014), Geological survey project of China Geological Survey (No. 12120115008101), and the Science Foundation of Wuhan Institute of Technology (No. K201726).

References

André

Audigane

Azaroual

et al . (2007) Numerical modeling of fluid-rock chemical interactions at the supercritical CO₂-liquid interface during CO₂ injection into a carbonate reservoir, the Dogger aquifer (Paris Basin, France). Energy Conversion and Management 48(6): 17821797.

Arash

Thomas

Zhu

(2016) Activated carbon monoliths with hierarchical pore structure from tar pitch and coal powder for the adsorption of CO₂, CH₄ and N₂. Carbon 103: 115124.

Bergen

Tambach

Pagnier

(2011) The role of CO₂-enhanced coalbed methane production in the global CCS strategy. Energy Procedia 4: 3112–3116.

Bertier

Swennen

Laenen

et al . (2006) Experimental identification of CO₂-water-rock interactions caused by sequestration of CO₂ in Westphalian and Buntsandstein sandstones of the Campine Basin (NE-Belgium). Journal of Geochemical Exploration 89(1–3): 10–14.

Castro-Marcano

Lobodin

Rodger

et al . (2012) A molecular model for Illinois No. 6 Argonne Premium coal: Moving toward capturing the continuum structure. Fuel 95: 35–49.

Chen

Mastalerz

Schimmelmann

(2012) Characterization of chemical functional groups in macerals across different coal ranks via micro-FTIR spectroscopy. International Journal of Coal Geology 104: 22–33.

Chen

Zou

Mastalerz

et al . (2015) Applications of micro-Fourier Transform Infrared Spectroscopy (FTIR) in the geological sciences – A review. International Journal of Molecular Sciences 16(12): 30223–30250.

Dawson

GKW

Golding

Biddle

et al . (2015) Mobilisation of elements from coal due to batch reactor experiments with CO₂ and water at 40°C and 9.5 MPa. International Journal of Coal Geology 140: 63–70.

Day

Duffy

Sakurovs

et al . (2008) Effect of coal properties on CO₂ sorption capacity under supercritical conditions. International Journal of Greenhouse Gas Control 2(3): 342–352.

10.

Dutka

Kudasik

Pokryszka

et al . (2013) Balance of CO₂/CH₄ exchange sorption in a coal briquette. Fuel Processing Technology 106: 95–101.

11.

Dutka

Kudasik

Topolnicki

(2012) Pore pressure changes accompanying exchange sorption of CO₂/CH₄ in a coal briquette. Fuel Processing Technology 100: 30–34.

12.

Gathitu

Chen

Michael

(2009) Effect of coal interaction with supercritical CO₂ physical structure. Industrial & Engineering Chemistry Research 48(10): 5024–5034.

13.

Grzybek

Pietrzak

Wachowska

(2002) X-ray photoelectron spectroscopy study of oxidized coals with different sulphur content. Fuel Processing Technology 77–78: 1–7.

14.

Hao

Kim

Qin

(2013) Enhanced CO₂ gas storage in coal. Industrial & Engineering Chemistry Research 52(51): 18492–18497.

15.

Jonathan

Paul

Leslie

et al . (2015) Using ground and intact coal samples to evaluate hydrocarbon fate during supercritical CO₂ injection into coal beds effects of particle size and coal moisture. Energy & Fuels 29(8): 5187–5203.

16.

Kiyama

Nishimoto

Fujioka

et al . (2011) Coal swelling strain and permeability change with injecting liquid/supercritical CO₂ and N₂ at stress-constrained conditions. International Journal of Coal Geology 85(1): 56–64.

17.

Kolak

Burruss

(2006) Geochemical investigation of the potential for mobilizing non-methane hydrocarbons during carbon dioxide storage in deep coal beds. Energy & Fuels 20(2): 566–574.

18.

Kutchko

Goodman

Rosenbaum

et al . (2013) Characterization of coal before and after supercritical CO₂ exposure via feature relocation using field-emission scanning electron microscopy. Fuel 107(9): 777–786.

19.

Lei

Ren

Tang

et al . (1994) Sulfur-accumulating model of superhigh organosulfur coal from guiding, China. Chinese Science Bulletin 39(21): 1817–1821.

20.

Liang

Wang

Zeng

et al . (2014) Effect of demineralization on lignite structure from Yinmin coalfield by FTIR investigation. Journal of Fuel Chemistry and Technology 42(2): 129–137.

21.

Limantseva

Makhnach

Ryzhenko

et al . (2008) Formation of dawsonite mineralization at the Zaozernyi deposit, Belarus. Geochemistry International 46(1): 62–76.

22.

Lin

Fujii

Takisawa

et al . (2008) Experimental evaluation of interactions in supercritical CO₂/water/rock minerals system under geologic CO₂ sequestration conditions. Journal of Materials Science 43(7): 2307–2315.

23.

Liu

Wang

Sang

et al . (2010) Changes in pore structure of anthracite coal associated with CO₂ sequestration process. Fuel 89(10): 2665–2672.

24.

Liu

Wang

Sang

et al . (2015) Fractal analysis in pore structure of coal under conditions of CO₂ sequestration process. Fuel 139: 125–132.

25.

Liu

Smirnov

(2009) Carbon sequestration in coal-beds with structural deformation effects. Energy Conversion and Management 50(6): 1586–1594.

26.

Liu

Sang

Liu

et al . (2015) Growth characteristics and genetic types of pores and fractures in a high-rank coal reservoir of the southern Qinshui basin. Ore Geology Reviews 64: 140–115.

27.

Liu

Sang

Pan

et al . (2016) Study of characteristics and formation stages of macroscopic natural fractures in coal seam #3 for CBM development in the east Qinnan block, Southern Quishui Basin, China. Journal of Natural Gas Science and Engineering 34: 1321–1332.

28.

Liu

Sang

Wang

et al . (2017) FIB-SEM and X-ray CT characterization of interconnected pores in high-rank coal formed from regional metamorphism. Journal of Petroleum Science and Engineering 148: 21–31.

29.

Massarotto

Golding

Bae

et al . (2010) Changes in reservoir properties from injection of supercritical CO₂ into coal seams – A laboratory study. International Journal of Coal Geology 82(3–4): 269–279.

30.

Mastalerz

Drobniak

Walker

et al . (2010) Coal lithotypes before and after saturation with CO₂; insights from micro- and mesoporosity, fluidity, and functional group distribution. International Journal of Coal Geology 83(4): 467–474.

31.

Mastalerz

Goodman

Chirdon

(2012) Coal lithotypes before, during, and after exposure to CO₂: Insights from direct Fourier transform infrared investigation. Energy & Fuels 26: 3586–3591.

32.

Mathews

Chaffee

(2012) The molecular representations of coal – A review. Fuel 96: 1–14.

33.

Mathews

van Duin

ACT

Chaffee

(2011) The utility of coal molecular models. Fuel Processing Technology 92: 718–728.

34.

Mazumder

van Hemert

Busch

et al . (2006) Flue gas and pure CO₂ sorption properties of coal: A comparative study. International Journal of Coal Geology 67(4): 267–279.

35.

Mirzaeian

Hall

(2006) The interactions of coal with CO₂ and its effects on coal structure. Energy & Fuels 20(5): 2022–2027.

36.

Mirzaeian

Hall

Jirandehi

(2010) Study of structural change in Wyodak coal in high-pressure CO₂ by small angle neutron scattering. Journal of Materials Science 45(19): 5271–5281.

37.

Okolo

Everson

Neomagus

HWJP

et al . (2015) Comparing the porosity and surface areas of coal as measured by gas adsorption, mercury intrusion and SAXS techniques. Fuel 141: 293–304.

38.

Pasquali

Comi

Pucciarelli

et al . (2008) Swelling, melting point reduction and solubility of PEG 1500 in supercritical CO₂. International Journal of Pharmaceutics 356(1–2): 76–81.

39.

Rodosta

Litynski

Plasynski

et al . (2011) Status update and results from the U.S. department of energy’s regional carbon sequestration partnership initiative: Update on validation and development phases. Energy Procedia 4: 3457–3464.

40.

Siemons

Busch

(2007) Measurement and interpretation of supercritical CO₂ sorption on various coals. International Journal of Coal Geology 69(4): 229–242.

41.

Stahl

Schilz

Schütz

(1978) A quick method for the microanalytical evaluation of the dissolving power of supercritical gases. Angewandte Chemie International Edition in English 17(10): 731–738.

42.

Sun

Wei

Liu

et al . (2009) Release of organonitrogen and organosulfur compounds during hydrotreatment of Pocahontas No. 3 coal residue over an activated carbon. Energy & Fuels 23(10): 5284–5286.

43.

Tsotsis

Patel

Najafi

et al . (2004) Overview of laboratory and modeling studies of carbon dioxide sequestration in coal beds. Industrial & Engineering Chemistry Research 43(12): 2887–2901.

44.

Wang

Wei

Wang

et al . (2010) Sorption-induced swelling/shrinkage and permeability of coal under stressed adsorption/desorption conditions. International Journal of Coal Geology 83(1): 46–54.

45.

Wang

et al . (2016a) Experimental study of CO₂-brine-rock interaction during CO₂ sequestration in deep coal seams. International Journal of Coal Geology 154–155: 265–274.

46.

Wang

Yang

et al . (2016b) Chemical composition and structural characteristics of oil shales and their kerogens using Fourier Transform Infrared (FTIR) spectroscopy and Solid-State ¹³C Nuclear Magnetic Resonance (NMR). Energy & Fuels 30: 6271–6280.

47.

Liu

Zhang

et al . (2016) Role of chlorine in ultrafine particulate matter formation during the combustion of a blend of high-Cl coal and low-Cl coal. Fuel 184: 185–191.

48.

Zhang

Cheng

et al . (2017) Influence of supercritical CO₂ on pore structure and functional groups of coal: Implications for CO₂ sequestration. Journal of Natural Gas Science and Engineering 40: 288–298.

49.

Zhong

Chen

Ren

(2003) China National Standards: Sampling of Coal Petrology (GB/T 19222-2003). Beijing: Standards Press of China.

The effects of CO 2 on organic groups in bituminous coal and high-rank coal via Fourier transform infrared spectroscopy

Abstract

Keywords

Introduction

Materials and methodology

Samples

ScCO2–H2O treatment

High-pressure reactor

Coal grain size

Temperatures, pressures, and time in the experiment

ScCO2–H2O treatment process

Determination of organic group types and contents

FTIR spectroscopy

Types and ratios of the major organic groups

Results and discussion

Semiquantitative ratios of the organic groups after the ScCO2–H2O treatment

FTIR spectra of the organic groups after the ScCO2–H2O treatment

Absorbance peak of the aromatic structure

Aliphatic hydrocarbon structure absorbance peak

Hydroxy group absorbance peaks

Oxygen-containing groups absorbance peaks

Sulfur-containing group absorbance peaks

Chemical reactions between a ScCO2–H2O system and organic groups

The effects of swelling on Ar, CHx, and hydroxy groups

The effects of endothermic reactions on organic groups

The effects of exothermic reaction on organic groups

The effects of temperatures and pressures on reactions between a ScCO2–H2O system and organic groups

Descending type

Rising type

Lower opening parabola type

Upper opening parabola type

The effects of coalification on reactions between a ScCO2–H2O system and organic groups

Conclusions

Footnotes

Acknowledgements

Declaration of conflicting interests

Funding

References

ScCO₂–H₂O treatment

ScCO₂–H₂O treatment process

Semiquantitative ratios of the organic groups after the ScCO₂–H₂O treatment

FTIR spectra of the organic groups after the ScCO₂–H₂O treatment

Chemical reactions between a ScCO₂–H₂O system and organic groups

The effects of swelling on Ar, CH_x, and hydroxy groups

The effects of temperatures and pressures on reactions between a ScCO₂–H₂O system and organic groups

The effects of coalification on reactions between a ScCO₂–H₂O system and organic groups