Article pubs.acs.org/EF
Interactions of Supercritical CO2 with Coal Dengfeng Zhang,*,† Lili Gu,† Songgeng Li,‡ Peichao Lian,† and Jun Tao† †
Faculty of Chemical Engineering, Kunming University of Science and Technology, Kunming 650500, People’s Republic of China State Key Laboratory of Multi-Phase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China
‡
ABSTRACT: Carbon dioxide sequestration on coal with enhanced coalbed methane recovery (CO2-ECBM) is acknowledged as a promising way to mitigate CO2 emissions. For successfully understanding and implementing CO2-ECBM process, the potential interactions of CO2 with coal during CO2 sequestration in coal seams were investigated. Research methods consisting of lowtemperature nitrogen adsorption−desorption and chromatographic analysis were used to address the transformation of coal pore morphology and the capability of supercritical CO2 extraction when coal contacts with high pressure CO2. According to the test results, interaction of coal with high pressure CO2 does not create a significant influence on pore shape and mesoporous volume distribution of any rank of coal. However, this causes the coal surface fractal dimension and specific surface area to be changed, which implies that the coal’s pore morphology change due to CO2 sorption is irreversible. The results also indicate that the injection of high-pressure CO2 does not only change the pore morphology of coal but also has the ability to extract the hydrocarbons present in the coal matrix. The extracted hydrocarbons are of biological toxicity and can be mobilized with gas or water to other geologic structures and aquifers. Thus, the potential environmental safety and health issues (ES&H issues) related to CO2 sequestration in deep coal seams require thorough assessment. tion with an intra- and intermolecular (secondary) model,12 has been widely accepted to describe the organic matrix. Gases are mainly adsorbed in the coal matrix and thus the morphology and surface chemistry of matrix are crucial for gas sorption.13,14 According to Larsen et al.,15 this cross-linked macromolecular system is glassy, strained, and does not hold a lower free energy state. Hence, this structure is brittle. When treated with organic solvent and CO2 sorption,16−19 swelling phenomenon will occur. For methane recovery or CO2 sequestration, swelling is detrimental and will lead to profound changes in the cleat porosity and permeability of the reservoir system,20 as the swelling induced by methane or CO2 dissolution will cause matrix volumetric expansion and also cause the cleats to close.19 The permeability of matrix and cleats decreases due to swelling and negatively influence the ability of gas and fluid to move through coal.18 Moreover, the distance between coal atoms increases due to swelling and the mechanical strength decreases.21 Additionally, the debate on coal pores has lasted a lone time. Bond proposed that coal pore morphology consisted of an extensive network of cavities interconnected.22 However, Larsen et al. confirm that most of the coal pores are not connected to the external surface.15 Recently, He et al. adopted neutron scattering and further demonstrate that most of the pores in low-porosity coals are inaccessible to fluids but even highly porous coals have a significant proportion of closed porosity.23 CO2 and methane might penetrate the coal matrix and cause coals to expand to some extent.24 Both swelling and diffusion induced by high-pressure CO2 probably cause the coal pore morphology to change. Results show that the permeability
1. INTRODUCTION Carbon dioxide (CO2) mainly emitted from fossil fuel combustion causes global warming. In response to this challenge, CO2 capture and sequestration (CCS) is emerging as a promising method to mitigate the atmospheric CO2 concentration.1 With respect to options for CO2 sequestration, sequestration of CO2 in coal seams with enhanced coalbed methane recovery (CO2-ECBM) has received considerable attention.2,3 The injected high pressure CO2 first flows in naturally occurring fractures and then adsorbs in the coal matrix. When adsorption of CO2 takes place, the adsorbed CO2 molecules can replace coalbed methane due to stronger interaction between CO2 and coal.4,5 It is estimated that the CO2 storage capacity of unmineable coal seams in the worldwide is between 140 and 3000 Gt,6 which can simultaneously recover 1.45 × 1013 m3 of methane resource.3 Currently, in order to acquire the information on the CO2 storage amount and methane recoverable of the target coal seams, most literature published are focused on the investigation on high pressure methane and CO2 adsorption on coal;7−9 however, in consideration of the characteristics of coal and high pressure CO2, other potential interactions still exist between coal and CO2 besides the adsorption effect. It is known that the pore system of coal seams comprises organic matrix porosity and nature or induced network of fractures, i.e., cleat system.3 The fluid transport in these two sources of porosity also incorporates two hydrodynamic mechanisms: diffusion in matrix described by Fick’s Law and laminar flow in cleats modeled using Darcy’s Law.10 Firouzi et al. adopted a molecular pore network model to represent the coal pore system and indicate that the morphological characteristics and energetic effects play a dominant role in the flow and transport properties of fluids.11 A covalently crosslinked, three-dimensional macromolecular model, in connec© 2012 American Chemical Society
Received: July 17, 2012 Revised: December 14, 2012 Published: December 15, 2012 387
dx.doi.org/10.1021/ef301191p | Energy Fuels 2013, 27, 387−393
Energy & Fuels
Article
coal mine, drilled at a depth about 200 m), Zhangji coal (Huainan coal mine, drilled at a depth between 560 and 580 m), Liulin coal (Hedong coal mine, drilled at a depth about 300 m), and Qinshui coal (Jincheng coal mine, drilled at a depth between 200 and 300 m), respectively. In order to prevent oxidation related transformation of coal structure, all the coal samples were preserved in a sealed plastic bag with helium (He). In order to make a preliminary investigation on the interactions of supercritical CO2 with coal and accelerate the interaction process, all the samples were crushed and sieved to obtain particles with diameters between 180 and 250 μm. Before interaction tests, samples were dried at 105 °C for 24 h under vacuum conditions. The proximate analysis, ultimate analysis, and petrographic composition of each coal are presented in Table 1.9 The Qinshui
of naturally fractured black coal is significantly reduced due to matrix swelling, and the matrix swelling also plays an important part in fluid adsorption capacity estimation.25,26 Although numerous studies have been conducted to study the coal swelling induced by CO2, the knowledge about how CO2 molecules influence the coal pore morphology including pore shape, fractal dimension, porous volume distribution profile, and surface area is still limited. For CO2 sequestration in coal seams, the temperature and pressure of coal seams suitable for sequestration are far above the critical points of CO2 (Tc = 31.05 °C, Pc = 7.3 MPa).27 Supercritical fluid (SCF) solvents are intermediates between liquid and gas and considered important in the separation processes based on the physicochemical characteristics including density, viscosity, diffusivity, and dielectric constant, which are easily manipulated by pressure and temperature.28 Thus, the supercritical fluid extraction (SFE) process has been widely used to separate high value components of the extract, such as value organic component extraction from natural products and oil extraction from the subjected material. As a solvent which is low cost, easily attainable, nontoxic, inexpensive, nonflammable, and nonpolluting, supercritical CO2 (SC-CO2) is always used as a nonpolar solvent for the supercritical extraction process. As can be seen in Figure 1,29
Table 1. Proximate Analysis, Ultimate Analysis, and Petrographic Composition of Coal Samples analyses moisture ash volatile matter fixed carbon carbon hydrogen nitrogen sulfur oxygen vitrinite liptinite inertinite Ro max(%) a
Bulianta
Zhangji
Liulin
proximate analysis (wt %, ada) 7.00 1.96 0.66 4.55 8.76 11.32 31.66 32.64 20.64 56.79 56.64 67.38 ultimate analysis (wt %, ada) 72.09 74.55 78.46 4.85 4.97 4.28 0.59 0.97 0.84 0.19 0.40 0.41 17.73 10.35 4.69 maceral composition (vol %, mmfb) 82.30 74.3 60.20 0.00 5.50 0.00 17.70 20.20 39.80 0.47 0.87 1.35
Qinshui 1.82 5.77 4.70 87.71 88.17 2.86 0.56 0.32 2.32 93.80 0.00 6.20 4.06
Air-dried basis. bMineral matter free basis.
anthracite has low volatile matter and Ro max is 4.06%. In contrast, Bulianta coal, Zhangji coal, and Liulin coal contain high volatile matter, together with Ro max ranging between 0.47% and 1.35%, which can all be categorized into bituminous coals. 2.2. Apparatus and Operation Procedure. The study of interactions of CO2 with coal was carried out based on the main apparatus which is shown in Figure 2. The heart of the experimental apparatus consists of a set of reference and sample cells. All the cells, connected tubing, and valves were placed in a temperature regulated air bath, which can maintain the temperature within the 0.1 °C
Figure 1. Two phase model of coal structure.
the three-dimensional network of coal is composed of condensed aromatic and hydroaromatic compounds, which are connected via short alkyl bridges, ether linkages, and thioether linkages. Low-molecular weight organic compounds (molecular weight Liulin coal (20.64%) > Qinshui coal (4.70%). Additionally, it is also noticed that more aliphatic hydrocarbons are found in macromolecular system of low rank coal than that of high rank coal.52 From the above two aspects, the extracted hydrocarbon type of Bulianta coal (Ro max = 0.47%) is plentiful as shown in Table 4. It may also suggest that environmental issues derived from CO2 sequestration in low rank coal seams will be even more serious.
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AUTHOR INFORMATION
Corresponding Author
*Phone: +86(871)5920242. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work is conducted with funding from the project supported by the National Natural Science Foundation of China (Grant No. 40672100) and the Science Foundation of the Education Department of Yunnan Province (Grant No. 2012Y538). The authors acknowledge Mrs. X. Zhou and Mr. Z. M. Lun of Petroleum Exploration & Production Research Institute, SINOPEC, for their assistance on installment of experimental apparatus and chromatographic analysis.
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4. CONCLUSION Interactions of supercritical CO2 with coal including pore morphology change and supercritical CO2 extraction were studied in this work. The major conclusions can be summarized as follows. 392
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