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Article

The Extraction Effect of Supercritical CO2 on Coal Organic Matter Based on CO2 Sequestration in Unmineable Coal Seam

by
Renxia Jiang
and
Hongguan Yu
*
School of Chemical and Biological Engineering, Shandong University of Science and Technology, Qingdao 266590, China
*
Author to whom correspondence should be addressed.
Minerals 2022, 12(10), 1254; https://doi.org/10.3390/min12101254
Submission received: 20 August 2022 / Revised: 23 September 2022 / Accepted: 28 September 2022 / Published: 30 September 2022
(This article belongs to the Special Issue CO2 Geological Storage: Fluid–Rock Interactions and Geochemical Modeling)
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Abstract

:
On the basis of the effect of extraction components of supercritical CO2 (Sc-CO2) from coal on groundwater in the fields of greenhouse gas CO2 sequestration into deep and unmineable coal seams, Sc-CO2 extracts from coals were analyzed using GC/MS to investigate the compositions and their contents of the extracts under different experimental conditions. The results show that Sc-CO2 extracts from coals contain hydrocarbons and organic compounds containing heteroatoms. The main compound in the extract is hydrocarbons which include a large concentration of acyclic alkanes and alkenes and a small concentration of cycloalkanes and aromatic hydrocarbons. Even-numbered n-alkane dominates in the extract, and hexacosene is the main alkene in the extracts from lignite and bituminous coal. The aromatic hydrocarbons are more difficult to extract and their concentration decreases with the increase of coal rank. The main oxygen-containing compounds are esters and carboxylic acids which are more easily extracted from lignite. The concentrations of nitrogen-containing compounds are very small and are more difficult to extract from coal with the rank increase. A small concentration of sulfur-containing compounds is extracted from coal. The results demonstrate that Sc-CO2 has the potential to mobilize organic compounds from coal seams, which affect the transport of CO2 in coal seams and cause groundwater pollution.

1. Introduction

CO2, the most important synthetic raw material in the chemical industry, is a carbon-based chemical and energy industry product [1,2] and is one of the greenhouse gases with a longer lifetime [3]. Although CO2 sequestration into deep coal seams is technically feasible [4,5] to reduce CO2 emission, there are some limitations in measurements of supercritical CO2 adsorption on coals with manometric equipment [6,7]. On the other hand, the environmental problems caused by the technology are still unclear [8].
As an alternative to conventional organic solvent [2], supercritical CO2 (Sc-CO2) can extract some of the organic molecules in coal and shale [9,10] which can be used to obtain valuable information about coal structure and organic matter from coal [11,12]. As a result, some organic matter can be extracted from the coal after CO2 injection in coal seams. Some extracts are toxic substances that enter groundwater and will cause water pollution [13]. Some compounds in the extracts can be used to identify biomarkers in coal.
Solvent extracts of coal include hydrocarbons [14] and heteroatomic compounds [15,16,17]. Among them, hydrocarbons include saturated hydrocarbon, branched-chain hydrocarbon, alkyl benzene, steroid, isoprene, and terpenoids. Heteroatomic compounds can be divided into oxygen-, oxygen-, and sulfur-containing organic matter.
Gas chromatography/mass spectrometry (GC/MS) has been widely used in the qualitative analysis of the components of extracts from coal and the identification of biomarkers in coal [18,19].
In this paper, GC/MS was used to identify the components of Sc-CO2 extracts from coals, including lignite, bituminous coal, and anthracite coal. The distribution characteristics of the components from extracts involved in CO2 injection in coal seams are discussed. The results can lay the foundation to study the effects of CO2 injected into coal seams, such as groundwater pollution, and at the same time explore the effect of Sc-CO2 extracts on CO2 flow in coal seams.

2. Materials and Methods

2.1. Materials

Neimeng, Shanxi, and Yinni coal samples were used to investigate the effect of coal rank on the distribution of compositions in extracts. Neimeng coal sample is from the No. 3-1 seam with a depth of 150 m in the Shangwan coal mine, Ordos coalfield, Inner Mongolia Autonomous Region, China. Shanxi coal is from the No. 3 coal seam with a depth of 470 m in Yechuan mine, southeast of the Qinshui Basin, Shanxi province, China. Yinni sample is from commercial coal imported from Indonesia. The quality indices of the three coals are shown in Table 1.
The volatile matter content on a dry-ash-free basis (Vdaf) and ash content on a dry basis (Ad) of Yinni coal are 50.08 wt.% and 9.73 wt.%, respectively, so the coal is lignite with high volatile content and low ash content. The Vdaf and Ad of Neimeng coal are 36.84 wt.% and 7.81 wt.%, respectively. Therefore, this coal is bituminous coal with high volatile and low ash content. The Vdaf and Ad of Shanxi coal are 9.57 wt.% and 10.82 wt.%, respectively, so the coal is anthracite with low volatile matter and ash content.

2.2. Methods

2.2.1. Experimental Apparatus

Experimental apparatus has been used in our previous studies [20]. The extraction device used in this study is developed based on the classical Sc-CO2 extraction system. A simplified layout of the experimental apparatus is shown in Figure 1.
The experimental device comprises four integrated parts: pressurization for CO2 injection, coal extraction, collection and enrichment of extracts, and extraction yield measurement. (1) The part of pressurization for CO2 injection provides clean Sc-CO2. It consists of a CO2 cylinder, a booster pump for CO2 pressurization, a needle valve for opening/cutting CO2 into the extraction cell, a CO2 storage tank containing activated carbon to remove organic matter from CO2, and an air-compressor for driving the booster pump. (2) The extraction part can be used to extract organic molecules from coal with Sc-CO2. It includes an extraction cell, a manometer for the determination of CO2 pressure in the extraction cell, a needle valve to control CO2 flow, and a water bath to maintain the extraction. (3) The extracts collection part is used to completely capture the extraction in CO2. It mainly includes a needle valve, two collection vessels, a restrictor to keep the flow-rate constant, an anti-volatilization conduit to avoid extracts volatilization with CO2, a solvent for trapping extracts, and an ice bath for keeping extracts at a low temperature. (4) In the extracts measurement part, the extracts are separated from the solvents and their quantity is determined using a nitrogen blowing system to remove the solvent and an analytical balance to obtain the mass of extracts.

2.2.2. Experimental Methods

Single-stage re-extraction of extracted coal was used to assess the components of extracts from raw coal and Sc-CO2-extracted coal. As presented in Figure 2, the raw coal was extracted with Sc-CO2 at 40 °C. The extracted coal from raw coal at 40 °C (EC40) was re-extracted at 60 °C, and the residual coal of EC40 at 60 °C (EC60) was re-extracted with Sc-CO2 at 80 °C, respectively. The extracts were analyzed with GC/MS.
The method of extraction from raw coal by Sc-CO2 at 40 °C (E40) is as follows. Approximately 100 g of air-dried raw coal sample (<0.2 mm) was put into an extraction cell. The temperature of the water bath was set to 40 °C. The extraction tank was pressurized repeatedly with a booster pump above 10 MPa until it was stabilized at 10 MPa within 2 h. Two 5 mL portions of acetone were added to the two collection vessels, respectively. The restrictor was inserted into the first solvent vessel, and the anti-volatilization conduit was inserted into the second vessel. The extraction program consisted of a 2 h static (no CO2 flow) step followed by a dynamic step at a flow rate of approximately 5 mL/min. The extract carried by CO2 was collected with acetone at a flow rate of 5 mL/min. When the pressure of the sample cell dropped to 0 Mpa, the solvent was transferred into a weighing vial. The solvent was evaporated under a gentle stream of nitrogen. The extract in the vial was weighed and the Sc-CO2 extraction yield of coal was calculated by Equation (1).
η daf = 100   m E m C ( 100 − M ad − A ad ) × 10 6
where ηdaf is the extraction yield of coal expressed on a dry, ash-free basis, mg/kg; mE is the mass of the extract, g; mC is the mass of experimental coal samples, g; and Mad and Aad are the moisture and ash content on an air-dry basis in coal, respectively (Table 1).
Except for the extraction temperature, the extraction methods of E60 and E80 are the same as those of E40.
To investigate the effect of coal rank on components in extracts, the extracts from Neimeng bituminous coal, Shanxi anthracite, and Yinni lignite were analyzed with GC/MS, which included raw coal at 40 °C, EC40 at 60 °C, and EC60 at 80 °C.
The extraction yields of raw coal, EC40, and EC60, and their distribution showed obvious differences. The masses of extracts from raw coal at 40 °C and extracted coal at 60 and 80 °C are provided in Table 2.
The Sc-CO2 extracts were firstly dissolved by acetone for GC/MS analysis. The total ion chromatograms (TICs) of extracts were obtained with GC/MS. The optimum condition for GC/MS determination is that the concentration of the extract in acetone ranges from 0.5 mg/mL to 0.7 mg/mL. The concentration of extract in the acetone solution used in this study was 0.6 mg/mL. To ensure the same concentration of extracts in acetone (0.6 mg/mL) for GC/MS analysis, the added volume of the solvent is also given in Table 2.

2.2.3. GC/MS Conditions

The Sc-CO2 extracts were analyzed via GC/MS using an Agilent 6890 Gas Chromatograph interfaced with an Agilent 5973 Mass Selective Detector (Santa Clara, CA, USA).
The amount of 1 μL of extracts was injected in the splitless mode and separated on an HP-5MS column using He as a carrier gas (1.0 mL/min). The following GC oven program was used during the extracts analyses: initial temperature held at 100 °C for 3 min, followed by a 10 °C/min ramp to 300 °C, and then held at the final temperature for 10 min. The solvent delay was at 5 min.
The MS conditions were as follows. The ionization mode was electron ionization (EI), and the electron bombarding energy was 70 eV. The scan range from m/z 40 to 550 at 1 scan/s was used. The GC/MS interface, ion source, and quadrupole analyzer temperatures were 260, 230, and 150 °C, respectively.

2.2.4. Identification and Quantification of the Components

NIST MS SEARCH 2.0 was used for the component identification of TIC of extracts from coal. Some documentation [21,22,23] on GC/MS detection for coal extraction was also used to help identify the components of extracts.
There are more than one hundred components in Sc-CO2 extracts, and quantification of each component using external or internal standard methods is difficult. The internal standard method was used for the quantification of the components in this paper. n-Hexacosane, squalene, hopane, and anthracene were used for the quantification of acyclic alkanes, alkenes, cycloalkanes, and aromatic hydrocarbons. Alcohols, esters, carboxylic acids, and ketones were quantitated using cholesterol, di-(2-ethylhexyl) phthalate (DEHP), n-hexadecanoic acids, and triazolone. N-methylmorpholine and Benzo[b]thiophene were used for the quantification of nitrogen- and sulfur-containing compounds.

3. Results

TICs of extracts for Yinni, Neimeng, and Shanxi coal are shown in Figure 3, Figure 4 and Figure 5, respectively. The top, middle, and bottom of these Figures are TICs of extracts from raw coal at 40 °C, EC40 at 60 °C, and EC60 at 80 °C, respectively. The GC/MS TICs of extracts from raw coal and re-extracted coal have a one-hump shape of unresolved complex mixtures (UCM), and most of the peaks are not eluted [24,25].
All extracts were determined twice by GC/MS. Table 3 shows the average compositions of Sc-CO2 extracts from raw coal, EC40, and EC60. As can be seen from Table 3, the hydrocarbon content was relatively high in the extracts from raw coal, while the oxygen-, nitrogen-, and sulfur-containing compounds were relatively low. This is because the small organic molecules in coal are mainly hydrocarbons, and the content of heteroatomic compounds is low, which is common knowledge among coal researchers [4,8,12,15].

3.1. Hydrocarbons

The hydrocarbons in Sc-CO2 extracts from coal included acyclic alkanes, alkenes, cycloalkanes, and aromatic hydrocarbons. Hydrocarbons were the main components of extracts, except for EC60 of Neimeng coal. It accounted for almost 80% of the total extract from three raw coals.

3.1.1. Acyclic Alkanes

The number of acyclic alkanes in three coals was relatively high. They accounted for one-third of all extracts from three raw coals at 40 °C. The alkanes included a large concentration of n-alkanes and a small concentration of their isomers.
The carbon preference index (CPI) is a ratio of peak heights or peak areas for odd-to-even-numbered n-alkanes in the range nC24–nC34 [26]. Petroleum that originates from terrigenous organic matter has a high CPI that decreases toward 1.0 with increasing maturation. CPI is affected by both source and maturity of crude oils, and its value can be used as an index to reflect the maturity of organic matter or source rock.
The extracts from Yinni raw coal covered a broad range of carbon numbers with nC14–nC34 at 40 °C, and nC18–nC34 dominant in the extracts. Even-numbered n-alkane predominated in the extracts, especially nC26H54 (8.82%). CPI was 0.25 for the raw coal. The carbon number of extracts from EC40 of Yinni lignite at 60 °C was nC21–nC34, CPI was 0.86, and the main n-alkane was nC29H60 (8.36%) in the extract. The carbon number of extracts from EC60 of Yinni coal was nC16–nC25, and the main n-alkane was C20H42 (9.53%), and there were no odd carbon n-alkanes except for C25H52 (7.03%).
The extracts from Neimeng raw coal covered carbon numbers with C16–C30 n-alkanes at 40 °C, and even-numbered n-alkane predominated in the extracts. CPI was 0.03, especially n-C24H50 (6.63%). The carbon number of extract from EC40 of Neimeng coal was nC15-nC34, and it was still dominated by even carbon, mainly eicosane (14.85%). There was a small amount of tetracosane (0.51%) in the extract from EC60 of Neimeng coal.
The carbon numbers of extracts were nC14–nC27, nC14–nC30, and nC16–nC30 for raw coal, EC40, and EC60 of Shanxi coal, respectively. The even-carbon predominance increased with the increase of extraction temperature, and CPI was 0.72, 0.36, and 0.17 for extracts of raw coal, EC40, and EC60, respectively. The main n-alkane was C26H54 in extracts from raw-coal and extracted-coal.
The carbon number of the branched alkane was significantly lower than that of n-alkane. The carbon number of a branched alkane was C21-C22 in the extract from Yinni raw coal, and there was no branched alkane in the extract from EC40. There was a small amount of branched alkane in the extract from Neimeng raw coal. The branched alkanes were present in the extract of Shanxi raw coal and its extracted coal, and their content increased with the increase of temperature.

3.1.2. Alkenes

The number of alkenes in the extract was higher than that of cycloalkanes and aromatics for three coals at all temperatures. The alkenes included a large concentration of n-alkenes and a small concentration of their isomers. The maximum carbon number of the alkenes reached 35.
The relative amount of the alkenes was 34.19% and 27.94% for Yinni raw coal at 40 °C and EC40 at 60 °C, respectively. The amount reached 34.05 % for Neimeng raw coal at 40 °C. The main alkene in extracts from Yinni lignite and Neimeng bituminous coal was Hexacosene (C26H52), which accounted for 13.26% and 19.48% of the two coals, respectively. There was a higher number of alkenes for raw and extracted coal for Shanxi anthracite, and the main alkene was C25H48, which accounted for 11.09%, 12.17%, and 10.31% for raw coal at 40 °C, EC40 at 60 °C, and EC60 at 80 °C, respectively.

3.1.3. Cycloalkanes

The carbon number of cycloalkanes in extracts of the three coals was C10-C32. The number of cycloalkanes in extracts for raw coal at 40 °C was approximately 10%. Compared with bituminous coal and lignite, cycloalkanes were easy to extract from Shanxi anthracite. The main cycloalkane compound in the extract from Yinni raw coal was C29H50, which accounted for 3.27% of the total account, and its structure is shown in Figure 6a. The main cycloalkane for Neimeng raw coal was C20H40 (6.27%), whose structure is shown in Figure 6b. The main cycloalkane for Shanxi raw coal was C14H28 (2.92%), as shown in Figure 6c.

3.1.4. Aromatic Hydrocarbons

Compared with other hydrocarbons, the Polycyclic aromatic hydrocarbons (PAHs) in Sc-CO2 extracts from coal were low. With the increase of coal metamorphism, the content of PAHs in the extract increased. The PAHs in extracts were detected only from raw coal of Yinni and Neimeng coal at 40 °C, with no PAHs found in extracts from EC40 and EC60 of two coals. PAHs were detected in Shanxi coal at all temperatures, and their content increased with the increase in temperature.
The main PAHs in the extract from Yinni raw coal was C16H30, which accounted for 1.26% of the total account, and its structure is shown in Figure 7a. The main one for Neimeng raw coal was C20H22 (1.42%), as shown in Figure 7b. The main PAHs for Shanxi coal were C16H30, which accounted for 0.92%, 0.92%, and 4.67% of the total account for raw coal at 40 °C, EC40 at 60 °C, and EC60 at 80 °C, respectively. C14H16 (Figure 7c) was detected from extracts for Shanxi coal, which accounted for 0.46% and 0.95% of the total account for raw coal at 40 °C and EC40 at 60 °C, respectively.

3.2. Oxygen-Containing Compounds

The oxygen-containing compounds in Sc-CO2 extracts from coal included alcohols, esters, carboxylic acids, and ketones, among others. Except for EC40 and EC60 of Yinni coal, the content of the compounds in the extract did not exceed 20%. The content of oxygen-containing compounds from Yinni lignite was the highest in the extract, followed by Shanxi anthracite and Neimeng bituminous coal. Of all the extracts from coal, esters were the most abundant oxygen-containing compounds.

3.2.1. Alcohols

Compared with other oxygen-containing compounds, the content of alcohols in extracts from coal was low. The carbon number of alcohols in the extracts from all raw coal samples was C10–C28 at 40 °C, and hexadecyl alcohol (C16H34O) was detected in the extracts from raw coal.
Only cholesterol (C27H46O) was detected from the extract from EC40 of Yinni coal at 60 °C, and no alcohols were detected from EC60 of Yinni coal at 80 °C. Only 1-Octacosanol (C28H58O) was detected from the extract of EC40 of Neimeng coal at 60 °C, and no alcohols were detected from EC60 of Neimeng coal at 80 °C. Although the content of alcohols in the extract from Shanxi coal was low, they were detected at all temperatures.

3.2.2. Esters

The content of ester compounds in extracts from coal was higher than that of other oxygen-containing compounds. The carbon number of esters in the extract from coal samples ranged from 8 to 34. C24H38O4 (Figure 8a) was detected in all extracts, and its content was 3.27%, 14.57%, and 16.16% from raw coal, EC40, and EC60 of Yinni coal, respectively.
Esters were extracted in extracts from raw coal, EC40, and EC60, and their contents increased with the increase of temperature, indicating that the main controlling factor of esters in Sc-CO2 extract from coal is extraction temperature.
The extracts from Yinni coal had the highest content of esters, and there was no difference in the content of esters in extracts of Neimeng bituminous coal and Shanxi anthracite.

3.2.3. Carboxylic Acids

The carbon number of carboxylic acid compounds detected in extracts from coals was 6-23. The content of carboxylic acid in extracts from Yinni and Neimeng coal increased with the increase of temperature, but its types decreased. The content of the acid in extracts from Yinni coal was higher than those of Neimeng and Shanxi coal, and the content in the extract from Neimeng-coal extract was lower than that of the other two coals.
The main carboxylic acids in the extract from Yinni and Shanxi raw coal was C16H26O3, as shown in Figure 8b, which accounted for 2.77% and 1.64% of the total account for Yinni and Shanxi coal. The main acid in the extract from Neimeng coal was terephthalic acid (C8H6O4), which accounted for 1.19% of all extracts.

3.2.4. Ketones

No ketones were detected in Sc-CO2 extracts from EC60 of Yinni coal and Neimeng raw coal and its extracted coal (EC40 and EC60). The carbon number of ketones detected in extracts from coals was 14–30. The main ketone is C30H50O, as shown in Figure 8c, which accounted for 2.48% and 2.82% of the total account from Yinni and Shanxi raw coal, respectively.

3.2.5. Other Oxygen-Containing Compounds

Only a small amount of ether was detected in the extract from Yinni and Neimeng raw coal at 40 °C, but the extract from Yinni coal contained aldehyde compounds. Ether and aldehyde compounds were detected from extracts from Shanxi raw coal and its extracted coal.

3.3. Nitrogen-Containing Compounds

Only C20H24N2O4 (Figure 9a) and C16H13N (Figure 9b) were detected in extract from Yinni raw coal at 40 °C; its content was 4.18%, and 0.41%, respectively. Oleic acid amide (Figure 9c) was detected in the extract from Neimeng coal, and its content was 1.40%, 24.71%, and 75.08% from raw coal, EC40, and EC60, respectively. The main nitrogen-containing compound was C22H43NO (Figure 9d) in the extract from Neimeng raw coal, which accounted for 5.07%. The extracts from Shanxi raw coal and its extracted coal had a low content of nitrogen-containing compounds.

3.4. Sulfur-Containing Compounds

The main sulfur-containing compound was C7H5NO2S2 (Figure 9e) in the extract from Yinni raw coal, which accounted for 0.23%. No sulfur-containing compounds were detected in the extract from Yinni extracted-coal and Neimeng coal. The content of sulfur-containing compounds from Shanxi coal was higher than that of Yinni coal, and their content decreased with the increase in temperature. The main sulfur-containing compound was C14H8N2S2 (Figure 9f) in the extract from Shanxi raw coal and EC40, and only C7H5NOS (Figure 9g) was detected from the extract of Shanxi EC60 at 80 °C.

4. Discussion

4.1. Effect of Supercritical CO2 Extracts on CO2 Flow in Coal Seam

To understand the effect of Sc-CO2 extract on fluid transport in coal seams, it is necessary to understand (1) the type of pores where small organic molecules reside and (2) the role of different types of pores in fluid transport. In addition, it is necessary to dynamically understand the changes in pressure during fluid migration and the resulting changes in coal pores.
The coal matrix is heterogeneous and is characterized by three different porosity systems: micropore (<2 nm), mesopore (2–50 nm), and macropore (>50 nm in diameter) [27]. According to the two-phase model of coal [28], the small organic molecules of coal exist mainly in the macropores and mesopores of middle- and low-rank coal, and micropores of high-rank coal. It has been reported that only micropores will play a significant role in gas adsorption, and macro- and mesopores most likely act primarily as transport conduits [29]. The interaction with Sc-CO2 drastically changes the mesopore and micropore surface area and volume distribution in both bituminous coal and lignite [30]. The mesopores become the dominant roles for low-rank coal, and the micropores are the dominant roles for high-rank coal. The mesopore size diminishes and the number of micropores ascends with the coalification effect increases for the higher rank coals [31].
Section 3 shows that with the deepening of coal metamorphism, Sc-CO2 extraction of coal at 40 °C increases and the extraction becomes increasingly difficult. It indicates that the small molecule exists mainly in the macropores and mesopores of lignite and bituminous coal but exists in the micropores of anthracite. Therefore, Sc-CO2 has a great impact on the migration of fluid in bituminous coal and lignite and has a great impact on the CO2 sequestration amount of anthracite. Hydrocarbons are the main components in the extracts, and the proportion of hydrocarbons in the raw coal extract is higher than 75%. Therefore, hydrocarbons, especially acyclic alkanes and alkenes, should be considered first when considering the transport of CO2 in coal seams. The CO2 sequestration into coal appears to have the potential to increase significantly coal microporosity which is very advantageous for CO2 storage.
Sc-CO2 extraction of small molecules from coal is affected by CO2 pressure, and the extraction amount increases with the increase of CO2 pressure [4,14,20]. The influence of the dynamic change of CO2 pressure on the solubility of small organic molecules will cause the change in coal pore structure and affect fluid flow.
CO2 pressure decreases from the point of injection to the production well for enhanced coalbed methane (ECBM) and CO2 sequestration. The injected CO2 flows from the high-pressure areas near the injection well toward the low-pressure areas near the production well along the path of least resistance. The organic compounds dissolved in Sc-CO2 pass through the high permeability area of coal seams and flow with CO2 to the low-pressure area. The solubility of small molecules in Sc-CO2 decreases with the decrease of CO2 pressure [4,14,20]. The organic compounds will be redeposited in the low-pressure area due to the decrease of their solubility in CO2. The deposited organic matter blocks will begin to clog or plug the coal’s pores, causing areas of high permeability to transform to low permeability. This will lead to an increase in exhaust pressure, thereby increasing the CO2 dissolution capacity. The precipitated components then begin to redissolve with an increase in CO2 pressure, and the fluid begins to flow again [4,8,32,33,34].
Eventually, the flowing CO2 finds its way to the area of lowest pressure at the production well. But in some areas, the precipitated organic molecules may no longer be dissolved to block coal pores and affect CO2 injection conditions and may even cause excessive local pressure. The dissolution and precipitation of small organic molecules in coal will change the flow path of CO2. It is important to note that this is only a hypothesis and needs to be confirmed in laboratory or field studies.
It should be noted that although Sc-CO2 does not dissolve the macromolecular of coal, Sc-CO2 will cause plasticization of coal [35]. The plasticization of Sc-CO2 on coal can cause swelling of the coal matrix. Coal matrix swelling can compress the cleat space of the coal seams thereby reducing the permeability of the coal seams. Thus, coal matrix swelling adversely affects the migration of CO2 in coal seams [36,37]. On the other hand, coal matrix swelling induced by Sc-CO2 reduces the pore surface area and volume of coals [38] and is associated with the fluid type and coal rank [39]. A change in the pore structure of low-rank coals is more pronounced than that of high-rank coals [40,41].
The following key points need to be noted here. (1) The effect of extracts on fluid flow is only a hypothesis and needs to be confirmed in laboratory or field studies. (2) The extraction and migration behaviors of these compounds depend not only on the coal rank but also on other parameters, including, but not limited to, the temperature of the coal sample, CO2 pressure, interaction time between CO2 and coal, and water content of the coal sample [4,8,20,42,43]. (3) It is an important issue for future research on how organic compounds dissolve in CO2 affect field-scale CO2 sequestration or ECBM projects. Although experimental confirmation is lacking, it is hypothesized that compounds dissolved in CO2 can affect CO2 migration in coal seams. (4) Coal matrix swelling adversely affects the migration of CO2 within coal seams.

4.2. Environmental Ramifications for CO2 Sequestration

This experiment demonstrated that aromatic hydrocarbon and nitrogen-containing compounds in the coal matrix can be dissolved and migrated out by injecting supercritical CO2 into coal samples under laboratory conditions. Polycyclic aromatic hydrocarbons (PAHs) were detected in Sc-CO2 extracts of the coal sample, although the concentration was low.
Toxic organic compounds dissolved in Sc-CO2, especially PAHs and nitrogen-containing compounds, can flow with CO2 and be dissolved in water, resulting in groundwater contamination. Because oxygen-containing compounds can form hydrogen bonds with water, the solubility of organic matter in water is increased, so that the solubility of PAHs in water increases. The toxic organic PAHs mobilized by Sc-CO2 and CO2–water mixtures may persist in the fluid phase even when CO2 concentrations have diminished significantly.

5. Conclusions

Sc-CO2 can extract hydrocarbons and heteroatom-containing organic compounds from coal. (1) The main compound in extracts is hydrocarbons, which include a large concentration of acyclic alkanes and alkenes and a small concentration of cycloalkanes and aromatic hydrocarbons. Even-numbered n-alkane dominates in the extract. Hexacosene is the main alkene in the extracts from lignite and bituminous coal, and the carbon number of alkenes in the extract from anthracite is 25–26. The aromatic hydrocarbons are more difficult to extract by Sc-CO2 and their concentration decreases with the coal-rank increase. Aromatic hydrocarbons can be extracted completely by Sc-CO2 from lignite and bituminous at 40 °C, but aromatic hydrocarbon in anthracite is difficult to extract from coal by Sc-CO2. (2) Oxygen-containing compounds in coal are most easily extracted by Sc-CO2 from lignite. The compounds in the extract are mainly ester and carboxylic acid. (3) The concentration of nitrogen-containing compounds in the extract is very small and they are more difficult to be extracted from coal with the rank increase. (4) No sulfur-containing compounds are extracted from bituminous coal. The sulfur-containing compound is not only extracted from anthracite raw coal at 40 °C, but also extracted from extracted coal (EC40 and EC60) at 60 °C and 80 °C. (5) Sc-CO2 has a great impact on the migration of fluid in the coal seams of bituminous coal and lignite and on the CO2 sequestration amount in the coal seams of anthracite.
Small organic compounds in coal dissolved in supercritical CO2 can affect the flow of injected CO2 in coal seams, and hydrocarbons are the main compounds affecting the CO2 flow. Organic materials dissolved in CO2 can cause groundwater pollution, especially PAHs and nitrogenous compounds.

Author Contributions

R.J. collected the samples, conducted all of the GC/MS analysis, wrote the initial draft, and prepared all of the illustrations; H.Y. designed the project, helped interpret the data, and assisted with the writing. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China (Grant No. 51174127) and Shandong Province Natural Science Foundation (No. ZR2011DM005).

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

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Minerals 12 01254 g001 550
Figure 1. A simplified layout of the experimental apparatus.
Figure 1. A simplified layout of the experimental apparatus.
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Minerals 12 01254 g002 550
Figure 2. Simplified diagram of the experimental procedure.
Figure 2. Simplified diagram of the experimental procedure.
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Minerals 12 01254 g003 550
Figure 3. Total ion chromatograms of extracts from Yinni coal.
Figure 3. Total ion chromatograms of extracts from Yinni coal.
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Minerals 12 01254 g004 550
Figure 4. Total ion chromatograms of extracts from Neimeng coal.
Figure 4. Total ion chromatograms of extracts from Neimeng coal.
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Minerals 12 01254 g005 550
Figure 5. Total ion chromatograms of extracts from Shanxi coal.
Figure 5. Total ion chromatograms of extracts from Shanxi coal.
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Minerals 12 01254 g006 550
Figure 6. The structure of (a) 28-Nor-17 alpha (H)-hopane; (b) Cembrane; and (c) 1,2,4,5-Tetraethylcyclohexane.
Figure 6. The structure of (a) 28-Nor-17 alpha (H)-hopane; (b) Cembrane; and (c) 1,2,4,5-Tetraethylcyclohexane.
Minerals 12 01254 g006
Minerals 12 01254 g007 550
Figure 7. The structure of (a) 8a-Ethyl- 1,1,4a,6-tetramethyldecahydronaphthalene; (b) 1,2,3,6,7,8,9, 10,11,12-Decahydrobenzo[e]pyrene; and (c) 2,3,6,7-Tetramethylnaphthalene.
Figure 7. The structure of (a) 8a-Ethyl- 1,1,4a,6-tetramethyldecahydronaphthalene; (b) 1,2,3,6,7,8,9, 10,11,12-Decahydrobenzo[e]pyrene; and (c) 2,3,6,7-Tetramethylnaphthalene.
Minerals 12 01254 g007
Minerals 12 01254 g008 550
Figure 8. The structure of (a) Bis(2-ethylhexyl) phthalate; (b) Dodecenyl succinic anhydride; and (c) Urs-20-en-16-ol.
Figure 8. The structure of (a) Bis(2-ethylhexyl) phthalate; (b) Dodecenyl succinic anhydride; and (c) Urs-20-en-16-ol.
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Minerals 12 01254 g009 550
Figure 9. The structure of (a) Methyl 20-hydroxy-19-oxo-2,16-didehydrocuran-17-oate; (b) α-Naphthylphenylamine; (c) Oleamide; (d) Erucylamide; (e) 1,2-Benzisothiazoline-3-thione, 1,1-dioxide; (f) 2,2’-Bibenzothiazole; and (g) 2(3H)-Benzothiazolone.
Figure 9. The structure of (a) Methyl 20-hydroxy-19-oxo-2,16-didehydrocuran-17-oate; (b) α-Naphthylphenylamine; (c) Oleamide; (d) Erucylamide; (e) 1,2-Benzisothiazoline-3-thione, 1,1-dioxide; (f) 2,2’-Bibenzothiazole; and (g) 2(3H)-Benzothiazolone.
Minerals 12 01254 g009
Table
Table 1. Proximate analysis (wt.%) of coal samples.
Table 1. Proximate analysis (wt.%) of coal samples.
Coal SampleMoisture, MadAshVolatile Matter
Vdaf		Fixed Carbon
FCdaf
AadAd
Yinni6.899.069.7350.0849.92
Neimeng3.877.517.8136.8463.16
Shanxi2.8710.5110.829.5790.43
Table
Table 2. The extracts mass and the added volume of acetone for GC/MS analysis.
Table 2. The extracts mass and the added volume of acetone for GC/MS analysis.
Coal SamplesExtracts Mass (g)Volume of Acetone (mL)
YinniNeimengShanxiYinniNeimengShanxi
Raw coal0.02100.00650.0064351111
EC400.01810.00570.005530910
EC600.01480.00460.00482588
Table
Table 3. Compositions of Sc-CO2 extracts from experimental coal samples (%).
Table 3. Compositions of Sc-CO2 extracts from experimental coal samples (%).
CompositionsYinni CoalNeimeng CoalShanxi Coal
Raw CoalEC40EC60Raw CoalEC40EC60Raw CoalEC40EC60
HydrocarbonsAcyclic alkanes29.8425.3232.2833.0242.980.5133.1636.7134.73
Alkenes34.1927.946.9734.0517.197.5534.9029.0334.21
Cycloalkanes9.953.843.649.181.34-10.488.498.35
Aromatics1.53--2.96--1.682.194.67
Sum75.5157.142.8979.2161.518.0680.2276.4281.96
Oxygen-containing compoundsAlcohols1.303.08-3.200.51-0.350.980.94
Esters7.1529.0449.075.848.7312.795.78.4912.7
Carboxylic acids6.306.478.041.782.764.073.603.771.26
Ketones4.484.31----4.335.030.26
Other0.34--3.50--1.080.762.40
Sum19.5742.957.1114.321216.8615.0619.0317.56
Nitrogen-containing compounds4.59--6.4726.4975.081.361.470.37
Sulfur-containing compounds0.33-----3.363.080.11
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Jiang, R.; Yu, H. The Extraction Effect of Supercritical CO2 on Coal Organic Matter Based on CO2 Sequestration in Unmineable Coal Seam. Minerals 2022, 12, 1254. https://doi.org/10.3390/min12101254

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Jiang R, Yu H. The Extraction Effect of Supercritical CO2 on Coal Organic Matter Based on CO2 Sequestration in Unmineable Coal Seam. Minerals. 2022; 12(10):1254. https://doi.org/10.3390/min12101254

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Jiang, Renxia, and Hongguan Yu. 2022. "The Extraction Effect of Supercritical CO2 on Coal Organic Matter Based on CO2 Sequestration in Unmineable Coal Seam" Minerals 12, no. 10: 1254. https://doi.org/10.3390/min12101254

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