Research on high-pressure adsorption of supercritical CO2 and the characterization of coal structure change in anthracite

CO2-Enhanced Coal Bed Methane ( CO2-ECBM) Technology has been widely used in deep coalbed methane (DCBM) extraction and CO2 sequestration. CO2 is in the supercritical state ( ScC - O2 ) in deep coal seam reservoir, and has mechanical, physical and chemical effects on the coal body, making understanding ScCO2 adsorption crucial. Based on the self-developed supercritical isothermal adsorption device, ScCO2 high-pressure adsorption experiments were carried out in anthracite. Besides, using multiple characterisation methods to analyse the changes in microstructures of coal before and after ScCO2 adsorption and reveal the mechanism of processing. The results indicate that an obvious mutation zone in adsorption during high-pressure conditions is carried out, with the minimal adsorption capacity in the mutation zone; 2) The effect of ScCO2 on the microstructure of the coal mainly focuses on the micropores and mesopores (below 50 nm ), resulting in an improved coal adsorption capacity; 3)Providing three assessment indexes of potential of DCBM resource exploitation and CO2 sequestration includes the buried depth, porosity and fracture permeability. The above research outcomes provide important theoretical foundation for the development of DCBM resources, assessment of CO2 sequestration potential and engineering application.


Introduction
CO ଶ is the primary component of greenhouse gases [1].According to a pervious report [2], 90% of total CO ଶ emissions come from human life and 80% of China's CO ଶ emissions is from coal fuels consumption.Presently, through injecting CO ଶ into deep unrecoverable coal seams, which improves the recovery rate of CBM while achieving geological sequestration and emission reduction of CO ଶ , it is an important way to realize the high-quality and low-carbon development of China's coal industry, and evaluating the injectability and storage capacity of CO ଶ is a bottleneck problem to be solved urgently [3,4].The features of China's DCBM resources are rich, high gas saturation and high free gas abundance.However, the overall degree of development is relatively low.In general, the temperature and pressure of the reservoir will be greater than 31.1 ‫ל‬ C and 7.38MPa when the depth exceeds 800 m, at which point the CO ଶ will be transformed from the 1335 (2024) 012047 IOP Publishing doi:10.1088/1755-1315/1335/1/012047 2 gaseous to the supercritical state.Compared with the gaseous, supercritical CO ଶ (ScCO ଶ ) is a state with physical properties such as density, viscosity and diffusion coefficient between the liquid phase and the gas phase, which significantly affects the permeability and adsorption capacity of coal [5].Based on the similarity of the mechanism of waterrock action of root fluids on rocks, ScCO ଶ has 'liquid-like' physical properties in the coal microstructure, so the actions on the coal body mainly consists of mechanical, physical and chemical action.At present, most of the studies focus on the effects of mechanical action mainly to change the ScCO ଶ on the permeability characteristics of coal, while the physical and chemical action on the ScCO ଶ produced by the coal body adsorption mechanism is not clear enough.Studies have showed that ScCO ଶ interacts with organic functional groups in the coal matrix and changes the chemical structure.The extracted organic matters include a large number of acyclic alkanes and olefins, as well as a small number of cycloalkanes and aromatic hydrocarbons [6].Due to the mineral dissolution and coal matrix adsorption and swelling, the findings demonstrated that the pore volume and specific surface area of BET increase of CO ଶ injection pressure in coal, while different types of pores showed different trends [7][8][9][10].In addition, some scholars researched the changes of mineral composition, pore structure and organic functional groups of coal after ScCO ଶ under different temperature conditions, as well as the temperature variation of coal bodies caused by ScCO ଶ adsorption and desorption [11][12][13].However, all ScCO ଶ isothermal adsorption experiments carried out from the gaseous state and analyzed the ScCO ଶ properties of coal, which neglected the effects of CO ଶ phase transition on adsorption in process.As CO ଶ is supercritical state, its physical properties, especially density, are very sensitive to temperature and pressure changes, which significantly affects the chemical and structural changes of the coal and then affects the analysis of ScCO ଶ adsorption capacity.
In this paper, ScCO ଶ whole course high-pressure adsorption experiment of anthracite was carried out, which was directly from supercritical state to avoid the effect of CO ଶ phase transition on adsorption.It combined Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) and specific surface area (BET), analyzing the response characteristics of chemical structure and pore structure caused by ScCO ଶ high-pressure adsorption.Considering the adsorption and solvation deformation to modify the adsorption equation, then reveal the characteristics and mechanism of ScCO ଶ high-pressure adsorption in.It is meaningful for solving the critical technological bottleneck of CO ଶ -ECBM in deep coal seam.

Experimental methodology
The samples were taken from No. 3 coal seam in the geological 5309 working face of the Permian Shanxi Formation in Chengzhuang Mining, Jincheng, Shanxi province.In order to avoid oxidation of the samples, the coal samples were vacuum-packed and sealed for preservation after sampling at the working face.In the laboratory, the raw coal was ground into 60-80 mesh powder particles for ScCO ଶ .Supercritical carbon dioxide isothermal adsorption experimental equipment high pressure adsorption experiment by using mortar and sieves of different particle sizes (Figure 1).Table 1 shows the industrial analysis and microscopic composition analysis results.Check the airtightness of the test system by using He and then vacuum the system.2) Turn on the constant temperature system, until the temperature stabilize, calibrate the volume of free space of the reference and experimental kettle with He. 3) Inject high-pressure ‫ܱܥ‬ ଶ into the reference kettle through the booster pump, and wait for the temperature and pressure of acquisition module to stabilize before high pressure isothermal adsorption test.4) Control valve ܸ ଶ to inject about ‫ܱܥܽܲܯ5.7‬ଶ from the reference kettle to the experimental kettle firstly to ensure that ‫ܱܥ‬ ଶ in the experimental kettle is in the supercritical state.5) Dynamic adsorption equilibrium is reached when the fluctuation range of gas pressure in the reference kettle and experimental kettle in the acquisition module is ±0.1%.Record the value of pressure, temperature and equilibrium time.6 ) Control ܸ ଶ , quickly inject about ‫ܱܥܽܲܯ5.0‬ଶ into the experimental kettle and observe the variation of pressure value.After 48h, the display remains stable, repeat the injection operation at ‫ܽܲܯ5.0‬ interval until the pressure in the experimental kettle is from7.5 MPa to 9.0 MPa.7) Control V2, rapidly inject about 1.0 MPa CO2 into the experimental kettle, close the connected valve, observe the pressure display and keep stable after 48 ݄.Repeat the injection operation with 1.0 MPa pressure interval until the pressure in the experimental kettle from ‫ܽܲܯ0.9‬ to 13.0MPa.The whole test lasted approximately 384h (Figure 3).According to static volume method [14], the measured CO ଶ adsorption capacity, i.e., the excess adsorption capacity, is calculated using [12]: where m ୟ is the adsorption capacity of CO ଶ ( g), ߩ is the free CO ଶ density in the reference kettle at pressure is P and temperature is T ( g/cm ଷ ), ߩ and ߩ ௦ are the free CO ଶ density in the reference and experimental kettles at adsorption equilibrium at each set pressure value, respectively (g/cm ଷ ), ܸ and ܸ ௦ are the remaining volume of the reference and experimental kettles, respectively (cm ଷ ), m ୡ is the mass of coal sample (g), and n ୣ୶ is the excess adsorption capacity of CO ଶ ( m ଷ /t).
At specific temperature and pressure conditions, the free CO ଶ density can be calculated using: where M is the mole mass of CO ଶ (44 g/mol), P is CO ଶ pressure (MPa), T is the equilibrium temperature (K), Z (,) is the gas compression coefficient at specific T and P, R is the gas constant (8.314J/(mol ‫ڄ‬ K).
Since Equation ( 2) ignores the thickness of adsorption layer, the excess adsorption capacity n ୣ୶ is lower than the absolute adsorption capacity (n ୟୠ ).Thus, according to the Gibbs model and GibbsHelmholtz equation, the absolute adsorption capacity n ୟୠ of ScCO ଶ can be derived [15,16]: where ߩ ௗ is the adsorption phase density of ScCO ଶ ( kg/m ଷ ).In this study, ߩ ௗ = 1180 kg/ m ଷ [17].

Characterization test of microstructure of coal
Three test methods were used to characterize the pore and chemical structure of anthracite before and after high pressure adsorption of ScCO ଶ include Low temperature liquid nitrogen adsorption (LPN ଶ GA ), Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS), as shown in .

Characterization of the pore structure
After ScCO ଶ adsorption, the changes in microstructure will lead to the differences in coal adsorption capacity.In order to quantitatively analyze the effect of ScCO ଶ adsorption on coal pore volume (PV) and specific surface area (SSA), using LP-N ଶ GA to investigate the changes in the pore structure of the coal body.The N ଶ adsorption/desorption isotherms obtained before and after ScCO ଶ adsorption by anthracite coal are shown in Figure 5, where the N ଶ adsorption/desorption isotherms are tested at temperature of 77 K and relative pressure (P/P ) in the range of 0.01 to 0.995.According to the IUPAC classification of adsorption isotherms [18], N ଶ adsorption is the multimolecular layer adsorption and there are significant changes in the mesopores distribution of samples before and after ScCO ଶ adsorption.When P/P င 0.8, the adsorption capacity increases slowly, with the P/P > 0.8, it rapidly increases, and the adsorption isotherm shows a concave shape.When approaching the saturated vapor pressure, the adsorption capacity increases sharply and the adsorption isotherm become steep.During the desorption processing, with the desorption pressure decreasing, N ଶ is gradually desorbed.The desorption curve is located above the adsorption curve and the adsorption/desorption curve shows significant hysteresis characteristics.The PV and pore size distribution of anthracite before and after ScCO ଶ adsorption are quantitatively calculated using the BJH െ N ଶ model, and the specific surface area is obtained by using the BET theory (Figure 5,Table 2 ).The results show that the PV, average pore diameter (APD) and SSA of anthracite increase after ScCO ଶ adsorption.Before and after adsorption, the PV is 0.0022ml/g and 0.0129ml/g, and the SSA is 0.4676 m ଶ /g and 0.5405 m ଶ /g , which are increased by 486.36% and 15.59% , respectively.Under the effects of ScCO ଶ adsorption swelling/shrinkage and dissolution, macropore and a series of mesoporous with diameters longer than 5 nm are produced in anthracite coal, resulting in an in-crease in pore volume and specific surface area.The contribution of mesoporous with pore size of 2 െ 5 nm to the specific surface area is much larger than that of 10 െ 50 nm.

Fractal characterization of pore
Based on the fractal theory, the 3D fractal dimension and characteristics of anthracite before and after ScCO ଶ adsorption are studied by using Frenkel-Halsey-Hill (FHH) Equation ( 5) [19,20]: where V is the gas adsorption capacity at equilibrium pressure P(ml/g), P is the gas saturation vapor pressure (MPa), P is the equilibrium pressure of gas adsorption (MPa), A is the slope of the fitted straight line, related to the fractal dimension D, B is a constant.According to the studies [16], when the gas adsorption pressure is low, the force between gas molecules and coal particles is mainly Van der Waals force, and the adsorption layer is less.The fractal dimension ‫ܦ‬ ଵ is calculated by Equation ( 6): ‫ܦ‬ ଵ = ‫ܣ3‬ ଵ + 3 (6) At the high adsorption pressure, the gas molecule condense on the solid surface and pores, with the force between them is mainly capillary cohesion.The relationship between A ଶ and fractal dimension D ଶ is Equation ( 7): ‫ܦ‬ ଶ = ‫ܣ‬ ଶ + 3 (7) As shown by the desorption curves in Figure 5, the curves decrease significantly near the relative pressures of 0.5 , indicating that the coal samples have different pore characteristics at different relative pressure stages.According to the Equation ( 5), the low-temperature liquid nitrogen adsorption data are calculated and section fitting (Figure 7) and the fractal dimension D. D includes D ଵ high pressure section (relative pressure greater than 0.5 ) and D ଶ low pressure section (relative pressure less than 0.5 ).The fractal results of pore structure are shown in .According to the classical fractal theory in lowpressure section, it can be seen that the value of fractal dimension after adsorption is 2.4234 , which is larger than before adsorption value of 2.3418 indicating that the surface roughness of the anthracite increases by ScCO ଶ .In the high-pressure section, the fractal dimension value of 2.7865 after adsorption is greater than that of 2.7561 before adsorption, which demonstrates that the microstructure of the coal samples is much more complex and the micropore structure is more irregular after ScCO ଶ adsorption.The fractal dimension of coal can reveal the CO ଶ adsorption capacity to a certain extent.The higher the fractal dimension, the more irregular the surface of coal and the more complex the pore structure, i. e. coal has a larger surface area and stronger adsorption capacity.

Functional groups change induced by ‫ܱܥܿܵ‬ ଶ high adsorption
The FTIR spectra of coal samples before and after ScCO ଶ high adsorption were plotted by Peakfit, as shown in Figure 8, and the spectra were analyzed semi-quantitatively by curve fitting to reveal the changes in the chemical structure of the coal body.The FTIR spectroscopy of coal samples can be divided into -OH hydroxyl (3600 െ 3000 cm ିଵ ) , aliphatic C െ H ୶ (3000 െ 2800 cm ିଵ ) , aromatic C-H (900700 cm ିଵ ) and oxygen-containing functional groups (1800 െ 1000 cm ିଵ ) [21][22][23].Before and after ScCO ଶ adsorption, the positions and intensities of the characteristic peaks have changed, indicating that the proportions of the functional groups changes.ScCO ଶ high adsorption does not change the type of functional groups of anthracite, but affects the account of functional groups, and different functional groups are affected in different degrees.Analysis of the spectra of different functional groups such as aromatic structure, oxygenated functional group and aliphatic structure before and after ScCO ଶ adsorption.Figure 9 shows that the structure of anthracite aromatic hydrocarbons (900 െ 700 cm ିଵ ) is mainly change the benzene ring with 3 substituents has the highest content before and after ScCO ଶ adsorption (30.52% and 50.72%, respectively), and the content increases, while the content of other forms of aromatic hydrocarbon structures decreases after ScCO ଶ adsorption, and the benzene ring with 5 substituents decreases the most from 29.19% before to 14.01% after adsorption.Anthracite (1800 െ 1000 cm ିଵ ) this wavelength range is mainly the stretching vibration of oxygen-containing functional groups, the number of oxygen-containing functional groups after adsorption is larger than that before, in which the largest change is attributed to the ether bond stretching vibration of phenol, alcohol and ether in the range of 1350 െ 1033 cm ିଵ .The ether-oxygen bond of the main oxygen-containing functional groups in anthracite, decreasing from 84.27% before adsorption to 45.91% after, which indicates that ScCO ଶ makes anthracite macromolecules shed carbon and oxygen elements.
Aliphatic hydrocarbons (3000 െ 2800 cm ିଵ ) include methyl antisymmetric stretching vibration, methylene symmetric and antisymmetric stretching vibration, and methylidyne stretching vibration.Before ScCO ଶ adsorption, the contents of methyl antisymmetric stretching vibration, methylene symmetric and antisymmetric stretching vibration, and methylidyne stretching vibration of anthracite samples are relatively average (30.11%,10.89%, 28.75%, and 30.25%, respectively).After adsorption, the content of methylidyne stretching vibration changed the most ( 30.25% decreased to 2.31% ), and the content of methyl antisymmetric stretching vibration, methylene symmetric and antisymmetric stretching vibration all increased (to 48.53%, 14.6% and 34.56%, respectively).The more െCH ଶ and CH ଷ side chain in coal, the stronger adsorption capacity of coal surface.It can be seen that after ScCO ଶ adsorption, the content of െCH ଶ and െCH ଷ increases, and the adsorption capacity of coal surface is enhanced.The hydroxyl groups in anthracite are mainly hydrogen-bonded functional groups, after ScCO ଶ adsorption, OH െ OH and cyclic hydroxyl content increases, while both OH െ N and OH െ ߨ content decreases.The reason is when the coal samples adsorb ScCO ଶ , the dissolution produces a weak acid, which provides H + to the system.Some existing hydrogen bonds are destroyed by H ା , and OH െ N and OH െ ߨ are combined with the most easily broken hydrogen bonds, which are converted into OH െ OH and cyclic hydroxyl groups.

Mechanism analysis and modification model of ‫ܱܥܿܵ‬ ଶ high adsorption
The adsorption capacity of coal depends on the dual effects of pore and chemical structure.ScCO ଶ has a transformation effect on the microstructure such as SSA, APZ and PV of anthracite mesopores, including the closed and semi-closed pores and pore throat positions filled with some organic matter in coal.After ScCO ଶ solubilization reaction with microstructures, the new pores are formed or closed pores connected, providing more adsorption space.In addition, the adsorption capacity is related to the surface roughness and pore structure complexity of coal samples based on pore size fractal characteristics.The fractal dimension of coal samples increases after ScCO ଶ adsorption, indicating that the surface roughness of coal pores increases, and the microscopic pore structure of coal is more complex and irregular.
The evolution of chemical structure of coal is another important factor affecting the adsorption capacity.It provides potential energy for gas adsorption and controls the main sites of gas adsorption.Based on the FTIR analysis, comparing the changes of oxygen-containing functional groups in coal samples before and after ScCO ଶ adsorption, the largest was in the range of 1350 െ 1033 cm ିଵ , which was attributed to the ether bond stretching vibration of the phenol, alcohol and ether.The ether bond is one of the bridge bonds connecting the condensed aromatic ring structural units in the macromolecular structure of coal.The ether-oxygen bond is the main oxygencontaining functional groups in anthracite, which is decreased from 84.27% before adsorption to 45.91% after.It demonstrated that ScCO ଶ made some carbon and oxygen elements in anthracite macromolecules fall off, which could provide more adsorption sites for CO ଶ and enhance the adsorption capacity of coal.Gibbsian surface excess model Furthermore, ScCO ଶ is a substance with physical properties such as density, viscosity and diffusion coefficient between liquid and gas phase.Particularly, the density, which changes significantly near the critical point, is very sensitive to temperature and pressure changes, which in turn affects the analysis of the ScCO ଶ adsorption properties of coal.In solid-gas adsorption, there exists an adsorption layer with a certain thickness at the adsorption interface, and the density of the adsorption phase is different from the density of the free phase(Figure 10) [24].Considering the elastic-plasticity of coal, the solid skeleton deforms after gas adsorption.Therefore, the volume change (ȟV ) caused by the ScCO ଶ adsorption on anthracite in this experiment mainly includes the volume change caused by the swelling/shrinkage of coal adsorption and the volume change due to the dissolution of ScCO ଶ .The equation of absolute adsorption capacity of ScCO ଶ was modified [25]: The relative volume change (ȟV ୡ ) of anthracite adsorbing ScCO ଶ is related to the pressure change under isothermal conditions, which can be calculated by the following equation: where ߛ is compression coefficient (MPa ିଵ ), V ୡ is the volume of adsorbent at pressure P(cm ଷ /g).
The initial volume of adsorbent ܸ at the adsorption initial pressure ܲ .The volume ܸ under pressure ܲ can be expressed as: The initial volume of the adsorbent measured with helium, the difference between the final and initial volume of the adsorbent (ȟV ୡ ) is: ȟܸ = ܸ ൫݁ ିఊ(ି బ ) െ 1൯ (11) Substituting Equation (11) into Equation ( 8) obtains the corrected equation of absolute adsorption capacity of coal considering ScCO ଶ adsorption swelling/shrinkage and dissolution as follow: According to the Equation ( 12), the ScCO ଶ isothermal adsorption curve was obtained (Figure 11).It can be seen that with the increase of adsorption equilibrium pressure, the ScCO2 adsorption capacity has a sudden change near the pressure of 8MPa.When the pressure is below 8MPa, the adsorption capacity decreases rapidly, while the pressure is above 8MPa, the adsorption capacity increases gradually with the increase of pressure, and finally tends to saturation.It can be seen that ScCO ଶ adsorption belongs to high-pressure adsorption, the adsorption capacity has a complex functional relationship with pressure, and the smallest capacity at the mutation point.Currently, the commonly used IUPAC isothermal adsorption line classification only applies to normal pressure adsorption, which does not conform to the high-pressure adsorption characteristics.Near the critical value, these isotherms change sharply non-monotonously and there are obvious extreme values.
The experimental results in this paper are in good consistence with existing studies [13,16,19].

Conclusion
This paper conducted ScCO ଶ high-pressure adsorption experiments on anthracite, combining various characterization methods such as FTIR, XPS and BET tests to study the characteristics of microstructure changes caused by ScCO ଶ and modified adsorption equation by considering swelling deformation, so as to reveal the characteristics and mechanism of ScCO ଶ high pressure adsorption on coal.The primary conclusions are as follows.
(1)The ScCO ଶ adsorption on anthracite has a transformation effect on the pore structure of coal.The pore volume before and after adsorption is 0.0022ml/g and 0.0129ml/g, and the specific surface area is 0.4676 m ଶ /g and 0.5405 m ଶ /g , which increased by 486.36% and 15.59% , respectively, leading to the mesopore volume, average pore size, specific surface area and fractal dimension increased, new pores are generated and connected, and the pore structure is more complex and irregular, resulting in enhanced adsorption of coal.
(2)The chemical structure of anthracite is changed after ScCO ଶ adsorption.The chemical bonds in the organic macromolecular structure of coal are destroyed, and the interaction with the organic functional groups in the pore structure, providing more adsorption sites, and the surface adsorption capacity of coal is enhanced.
(3) ScCO ଶ adsorption is high-pressure adsorption.The calculation of absolute adsorption capacity is modified and establish the equation with considering the swelling/shrinking and dissolution effects of coal body caused by ScCO ଶ .There is a mutation point in the ScCO ଶ isothermal adsorption line, which occurs in the near-critical region (near the CO ଶ critical temperature and critical pressure), and the adsorption capacity is the minimum at the point.

Figure 1 .
Figure 1.The preparation of sample

Figure 3 .Figure 4 .
Figure 3.A diagram of the high-pressure adsorption process

Figure 6 .
Figure 6.Pore size distribution of anthracite coal before and after adsorbs ScCO ଶ

Table 1 .
Proximate and microscopic composition analysis of test coal samples The high-pressure isothermal adsorption test at 35 ‫ל‬ ‫ܥ‬ were conducted based on the Chinese GB/T 19560-2008 standard.The sample coal was crushed to 60 െ 80 mesh with a mass of 40 ݃ and then was dried at 105 ‫ל‬ ‫ܥ‬ for 24 ݄ in a vacuum drying oven.The gas purity of ‫ܱܥ‬ ଶ and ‫݁ܪ‬ was 99.999% and 99.99%, respectively.The detailed test step are as follows.1)

Table 2 .
Pore volume and specific surface area changes of anthracite coal before and after adsorbs ScCO2

Table 3 .
Fractal dimensions before and after anthracite coal adsorbs supercritical CO ଶ