Flue gas separation at organic-inorganic interface under geological conditions
Graphical abstract
Introduction
Problems of global warming caused by a large amount of flue gas emitted by high energy consumption industries are becoming more and more serious. Flue gas, consists of CO2 and N2, is an important part of greenhouse gas [1]. To prevent the greenhouse effect, CO2 capture and separation techniques have been regard as promising methods [2]. Among which, an innovative approach to inject CO2 into shale reservoirs was proposed and researched [3], [4], [5]. On the one hand, this technique can remove CO2 from flue gas using the depleted gas reservoirs. On the other hand, CO2 can replace methane in shale slits and generate some accessional benefits (shale gas). Therefore, it is meaningful to explore the adsorption behavior of flue gas in shale reservoirs.
Shale reservoirs are the complicated sedimentary rocks composed of organic and inorganic substances, which has a lot of nanoporous channels [6,7]. Due to the confused reservoir conditions, it is difficult to study the adsorption behaviors by experimental methods [8]. Molecular simulation can reveal the microscopic mechanism of gas-solid interface interaction [9]. Currently, most researchers focus on the adsorption behavior of gas molecules on the single type of slit-pore of shale matrix (organic or inorganic).
For organic reservoirs, two disconnected and independent graphite slabs are typically regarded as slit pores to represent the complicated nanopore [3,7,[10], [11], [12]]. Liu et al. [3] simulated the adsorption selectivity of CO2 from CO2/N2 mixture in a perfect graphite basal nanopore at 298 K. The results indicated that the interaction energy between CO2 and the graphite surface is bigger than that between N2 and the graphite surface. Besides, Wu et al. [11] reported the mechanism about displacement of methane in carbon nano-channel by molecular dynamics (MD) simulation. Their findings demonstrated that the adsorption ability of CO2 is powerful than that of methane, and CO2 can displace the adsorbed methane. Furthermore, Yuan et al. [7] revealed the evolution processes for enhanced recovery of methane with CO2 via MD simulation. The results proved that desorption of methane by CO2 injection depends on the displacement angle.
For inorganic reservoirs, quartz, clay and calcite are the general constituent substances [6]. Sun et al. [13] described the adsorption property of methane and CO2 in quartz nanopore by MD simulation. They found the apparent competitive adsorption behavior of CO2 over methane on nanopore surface in the temperature range of 313-353 K, because of interactions between CO2 and quartz surface. In addition, Zhou et al. [14] investigated the mechanism of adsorption of CO2 and methane in kaolinite clay via Monte Carlo simulation. They observed two adsorption layers on kaolinite surface, and the selectivity parameter over 7 has been found due to the bigger adsorption ability of CO2 than methane. Furthermore, Franco [15] and Santos [16] used MD simulation to predict the adsorption behaviors of CO2 and methane as well as their diffusion behaviors on calcite surface. On this basis, Tao et al. [17] described the kinetic information of CO2 adsorption on calcite surface by an experiment-simulation collaborative method [18,19].
However, the above mentioned studies have not actually represented geological conditions due to a single type of shale matrix. To further understand shale reservoirs, organic-inorganic models should be considered, this is because many organic and inorganic matters coexist in actual reservoirs [20]. For this reason, Chen et at. [21] constructed the graphite-montmorillonite slit pore to study the adsorption behavior of CH4 and CO2 by MD simulation. However, under geological conditions, graphite is idealized as organic reservoirs, which cannot represent the real environment, because the metal ion impurities will be doped into organic matters. To the best of the authors’ knowledge, the study of flue gas adsorption behavior on heterogeneous interfaces containing metal ion impurities is blank.
In this study, multiscale simulations were performed to compare the adsorption characteristics of flue gas at organic-inorganic interface with and without metal ion impurity at different temperatures. First of all, the adsorption behaviors of flue gas at heterogeneous interface were shown in detail to intuitively understand the adsorption habits of gas molecules, and the relative concentrations of flue gas at the interface were quantified. Furthermore, the diffusion coefficients were calculated by Einstein diffusion law, which illustrates the effect of heterogeneous nanopore on confined fluids. Next, the electronic properties of gas on organic and inorganic surfaces were calculated by density functional theory (DFT) to acquire the physicochemical heterogeneity (e.g. electron density distribution, adsorption energy and adsorption distance). Finally, in order to quantitatively evaluate the separation performance of flue gas at heterogeneous interface, the selectivity parameter was calculated. This study provides new insights into flue gas adsorption in more realistic shale reservoirs.
Section snippets
Organic-inorganic interface model
Given that slit-shaped pore of shale reservoir was proved by scanning electron microscopy [22,23], the organic-inorganic nanopore as well as its relevant parameters were defined and two gas molecules were illustrated as shown in Fig. 1. Calcite is one of the primary components in shale reservoirs and the (104) cleave surface for calcite is the most stable crystallographic plane [17,24], so three layers of (104) cleave surface were used to represent inorganic matter. Besides, kerogen is
Adsorption behavior of flue gas at heterogeneous interfaces
In this section, to explore the differences in the adsorption flue gas at GC and Ca-GC interfaces, snapshots of flue gas adsorbed on heterogeneous surfaces were first shown at different temperatures. Then, the adsorption configurations were described by preferential orientation of CO2 and N2. Next, the relative concentrations were calculated to quantitatively characterize the adsorption of two gases at the organic-inorganic interface. Finally, the diffusion coefficients were used to analyze the
Conclusion
Multiscale simulations are used to study the difference in the adsorption behavior of flue gas at GC and Ca-GC interfaces. MD simulation results show that CO2 is preferentially adsorbed on GC surface, while N2 is squeezed into the central region of the interface. At Ca-GC interface, the adsorption of CO2 on graphite surface is enhanced due to the presence of Ca ion. Further, according to the adsorption orientation of gas molecules, CO2 molecules present the angle of 45° erect adsorption
Author statement
Lin Tao: Conceptualization, Methodology, Formal analysis, Writing - original draft. Junchen Huang: Formal analysis, Writing - review & editing. Davoud Dastan: Formal analysis, Writing - review & editing. Jing Li: Resources, Visualization. Xitao Yin: Conceptualization, Supervision, Resources. Qi Wang: Methodology, Writing - review & editing, Software, Resources.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
We are very grateful for the support of the National Natural Science Foundation of China (Fund number: 51874169, 51634004, and 51974157).
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