Abstract
Coal has complex pore structures including micro-, meso-, and macropores and cracks, and contains organic micromolecules. To in-depth study the effects of organic micromolecules in coal on the pore structure and gas diffusion characteristics, organic micromolecules were extracted by tetrahydrofuran using microwave-assisted method from anthracite and bituminous coal samples to obtain the residual coal samples at 50 \(^{\circ }\)C and atmospheric pressure. Changes in raw and residual coal samples were explored using a series of methane desorption and low-temperature nitrogen adsorption experiments and their pore structure parameters were compared using samples with same granule size (0.180–0.250 mm). The results showed that (1) although the micropores of both raw and residual coal granules are characterized by fractal, residual coal has lower fractal dimension than raw coal; (2) residual coal samples have higher gas emission amount and rate than the raw coal samples and lower gas diffusion resistance, indicating that gas in residual coal is easier to flow from micropores through mesopores into macropores and cracks. Based on the fractal theory and the diffusion model, extraction of organic micromolecules increased the number and diameter of pores and cracks and dredged the rawly clogged pores and cracks, all of which decreased the resistance of gas diffusion in the coal matrix. In addition, extraction of organic micromolecules has the most obvious effect on diffusion pores. After extraction, the proportion of meso-/macropores increased, while that of micropores reduced, thereby reducing the resistance of gas flowing from micropores through mesopores into macropores and cracks, and subsequently increasing the amount and rate of gas emission. The study is of great significance for pushing forward the boundary of our recognition of the influences exerted by micromolecules on gas diffusion.
References
Busch, A., Gensterblum, Y.: CBM and CO2-ECBM related sorption processes in coal: A review. Int. J. Coal Geol. 87(2), 49–71 (2011)
Cai, Y., Liu, D., Yao, Y., Li, J., Liu, J.: Fractal characteristics of coal pores based on classic geometry and thermodynamics models. Acta Geol. Sin. Engl. Ed. 85(5), 1150–1162 (2011)
Cai, Y.D., Liu, D.M., Pan, Z.J., Yao, Y.B., Li, J.Q., Qiu, Y.K.: Pore structure and its impact on CH4 adsorption capacity and flow capability of bituminous and subbituminous coals from Northeast China. Fuel 103, 258–268 (2013)
Chen, H., Ge, L.M., Li, J.W.: Optimization of microwave-assisted extraction coal and analysis of soluble fraction and residue. J. China Coal Soc. 4, 022 (2009)
Chen, X., Cheng, Y., He, T., Li, X.: Water injection impact on gas diffusion characteristic of coal. J. Min. Saf. Eng. 30(3), 443–448 (2013)
Cheng, Y.P.: Theories and engineering applications on coal mine gas control. China University of Mining and Technology Press, Xuzhou (2010)
Clarkson, C., Bustin, R.: The effect of pore structure and gas pressure upon the transport properties of coal: a laboratory and modeling study. 1. Isotherms and pore volume distributions. Fuel 78(11), 1333–1344 (1999a)
Clarkson, C.R., Bustin, R.M.: The effect of pore structure and gas pressure upon the transport properties of coal: a laboratory and modeling study. 2. Adsorption rate modeling. Fuel 78(11), 1345–1362 (1999b)
Clarkson, C.R., Marc Bustin, R.: Variation in micropore capacity and size distribution with composition in bituminous coal of the Western Canadian Sedimentary Basin: Implications for coalbed methane potential. Fuel 75(13), 1483–1498 (1996)
Day, S., Sakurovs, R., Weir, S.: Supercritical gas sorption on moist coals. Int. J. Coal Geol. 74(3), 203–214 (2008)
Fang, P.H., Wong, R.: Evidence for fullerene in a coal of Yunnan, Southwestern China. Mater. Res. Innov. 1(2), 130–132 (1997)
Harpalani, S., Chen, G.: Influence of gas production induced volumetric strain on permeability of coal. Geotech. Geol. Eng. 15(4), 303–325 (1997)
He, X.: The effect of alternative electromagnetic field on adsorption of gas by coal. J. China Coal Soc. 21(1), 63–67 (1996)
He, X., Nie, B.: Diffusion mechanism of porous gases in coal seams. J. China Univ. Min. Technol. 30(1), 1–4 (2001)
Hu, S., Li, M., Xiang, J., Sun, L., Li, P., Su, S., Sun, X.: Fractal characteristic of three Chinese coals. Fuel 83(10), 1307–1313 (2004)
Iino, M., Takanohashi, T., Ohsuga, H., Toda, K.: Extract ion of Coals with \(\text{ CS }_{2}\) -N-methyl-2-pyrrolidinone mixed solvent at room temperature: effect of coal rank and synergism of the mixed solvent. Fuel 67, 1639–1647 (1988)
Ji, H.J., Li, Z.H., Peng, Y.J., Yang, Y.L., Tang, Y.B., Liu, Z.: Pore structures and methane sorption characteristics of coal after extraction with tetrahydrofuran. J. Nat. Gas Sci. Eng. 19, 287–294 (2014)
Jiang, W.: Study on features comparison and mechanism of methane adsorbed by coal before and after solvent extraction. Coal Sci. Technol. 41(3), 114–119 (2013)
Krooss, B., Van Bergen, F., Gensterblum, Y., Siemons, N., Pagnier, H., David, P.: High-pressure methane and carbon dioxide adsorption on dry and moisture-equilibrated Pennsylvanian coals. Int. J. Coal Geol. 51(2), 69–92 (2002)
Laxminarayana, C., Crosdale, P.J.: Role of coal type and rank on methane sorption characteristics of Bowen Basin, Australia coals. Int. J. Coal Geol. 40(4), 309–325 (1999)
Li, Y.H., Lu, G.Q., Rudolph, V.: Compressibility and fractal dimension of fine coal particles in relation to pore structure characterisation using mercury porosimetry. Part. Part. Syst. Charact. 16(1), 25–31 (1999)
Liu, B., Xiong, D., Xian, X.: Adsorption and seepage characteristics of coal to methane under electric field. J. Chongqing Univ. (Nat. Sci. Ed.) 29(2), 83–85 (2006)
Mandelbrot, B.B.: The fractal geometry of nature. Macmillan (1983)
Marzec, A.: Towards an understanding of the coal structure: a review. Fuel Process. Technol. 77–78, 25–32 (2002)
Mathews, J.P., Chaffee, A.L.: The molecular representations of coal—a review. Fuel 96(1), 1–14 (2012)
Moore, T.A.: Coalbed methane: a review. Int. J. Coal Geol. 101, 36–81 (2012)
Nie, B., Guo, Y., Wu, S., Zhang, L.: Theoretical model of gas diffusion through coal particles and its analytical solution. J. China Univ. Min. Technol. 30(1), 19–22 (2001)
Qin, Y., Fu, G.: Study on fractal characteristic of pore in coal and moisture absorbing property of coal. J. China Coal Soc. 25(1), 55–59 (2000)
Qin, Z., Gong, T., Li, X., Li, H., Zhang, D., Sun, h: TEM analysis of coal extract ion and coal inbuilt state structural model. J. China Univ. Min. Technol. 37(4), 443–449 (2008)
Qin, Z., Li, H., Zhang, D., Ding, S., Sun, h, Jiang, C.: Characteristics of micropore-inbuilt form of micromolecules in coal and their solubilization rules. J. China Univ. Min. Technol. 36(5), 586–591 (2007)
Ruckenstein, E., Vaidyanathan, A.S., Youngquist, G.R.: Sorption by solids with bidisperse pore structures. Chem. Eng. Sci. 26(9), 1305–1318 (1971)
Rus’ ianova, N., Maksimova, N., Jdanov, V., Butakova, V.: Structure and reactivity of coals. Fuel 69(11), 1448–1453 (1990)
Shi, J.Q., Durucan, S.: A bidisperse pore diffusion model for methane displacement desorption in coal by CO2 injection. Fuel 82(10), 1219–1229 (2003)
Takanohashi, T., Terao, Y., Iino, M.: Sorption behaviors of methanol vapor by coal extracts and residues. Fuel 79(3), 349–353 (2000)
Wang, E., He, X.: Fractal descript ion of coal seam porosity and crack system and its application. J. Fuxin Min. Inst.(Nat. Sci.) 15(4), 19–23 (1995)
Wang, F., Zhang, D.J., Li, X.P., Yang, Ml: Adsorption behaviors for nitrogen by coal, its extractions and residues. J. Fuel Chem. Technol. 31(5), 395–399 (2003)
Xie, K.: Coal structure and its reactivity. Science Press, Beijing (2002)
Yang, Q., Wang, Y.: Theory of methane diffusion from coal cuttings and its application. J. China Coal Soc. 8(3), 87–93 (1986)
Yang, Q., Wang, Y.: Mathematical simulation of the radial methane flow in spherical coal grains. J. China Univ. Min. Technol. 4, 44–48 (1988)
Yang, R.T.: Gas separation by adsorption processes. Butterworth Publishers, Buffalo (1987)
Yuan, W., Pan, Z., Li, X., Yang, Y., Zhao, C., Connell, L.D., Li, S., He, J.: Experimental study and modelling of methane adsorption and diffusion in shale. Fuel 117, 509–519 (2014)
Zeng, F.: The fractal surface of coal. Coal Convers. 18(2), 7–13 (1995)
Zhang, D.J., Wang, F., Li, X.P., Xian, X.F.: Effect of solvent extraction on pore character and granularity of bituminous coal. J. Fuel Chem. Technol. 32(1), 18–22 (2004)
Zhu, Z., Gao, J.: Coal chemistry. In., pp. 194–198. Shanghai science and Technology Press, Shanghai (1984)
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This study is supported by the National Basic Research Program of China (973 Program) (No. 2011CB201200) and the National Natural Science Foundation of China (No. 51304189).
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Ji, H., Li, Z., Yang, Y. et al. Effects of Organic Micromolecules in coal on its Pore Structure and Gas Diffusion Characteristics. Transp Porous Med 107, 419–433 (2015). https://doi.org/10.1007/s11242-014-0446-9
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DOI: https://doi.org/10.1007/s11242-014-0446-9