Experimental Research on Pore Structure and Gas Adsorption Characteristic of Deformed Coal

China Petroleum Processing and Petrochemical Technology2014,Vol.16,No.4,pp55-64December 31. 2014Experimental Research on Pore Structure and gasAdsorption Characteristic of Deformed coalGuo Deyong; Guo Li; Miao Xinhui(School of resource and safety Engineering, China University of Mining and Technology( beijing),Beijing 100083)Abstract: The pore structure and gas adsorption property of deformed coal with different degrees of metamorphism weretested by low-temperature nitrogen adsorption and isothermal adsorption experiments. The fractal theory and the Langmuiradsorption theory were used to analyze the experimental data. The test results showed that the deformed coal had more heterogeneous pore structures and open pores, and its specific surface area(SSA)and fractal dimension(D)were higher. Thereis a polynomial relationship between D and specific surface area as well as gas adsorption capacity(Vi). The gas adsorptioncapacity of deformed coal is infuenced by pore structure, coal rank, deformation and stress together, among which the porestructure is the main influencing factor for the adsorption capacity of deformed coal. The test pressure could affect the aduracy of the adsorption constants a and b, so the highest experiment pressure should be greater than the actual pressure ofcoal seam in order to reduce the deviation of adsorption constantKey words: deformed coal; pore structure; nitrogen adsorption; isothermal adsorption1 Introductiontemperature, pressure, etc. The development of deformedAs a porous medium, coal has various pores of differ-coal reduces the gas permeability of coal seams and hasent sizes that are formed by organic matters and mineral an important impact on coal seam gas extraction.Thematters on the surface and body of coal, which ranges research on the pore structure and gas adsorption of defrom 10 0 m to the micron levelformed coalh low tture liquid nitrogare generally studied in the adsorption experiments. The adsorption experiments and isothermal adsorption experiinternal surface of coal pores is the main space available ments is of great guiding significance for the prediction offor methane adsorption The size, shape and develop- coal and gas outburst and coal seam gas extractionment of coal pores directly affect the coal property for 2 Experimentaladsorption and desorption of methane. The research oncoal pore characteristics is an important approach for in- 2.1 Coal sampling and coal quality analysisterpreting the gas adsorption of coal and ascertaining the Coal samples were collected from Jiaozuo(JZ)Jiulishanmechanism of migration and flow of gas in coal seamsCoalmine, Jiaozuo(JZ)Guhanshan Coalmine, PingdingDeformed coal is the coal whose primary structure Is shan(PDS)No8 Coalmine, Pingdingshan(PDS)No10damaged by tectonism b-I. Because of suffering from theCoalmine and Zhengzhou(Zz) Chaohua Coalmine. Fivection of tectonic stress the deformed coal is character-sets of deformed coal samples and corresponding unized by low strength, high energy storage, quick desorpdeformed coal samples were selected for experimentaltion, low permeability, etc, and its gas adsorption capacity is also changed. Coal and gas outburst occurs mainly study, with the coal quality analyzed (Table 1)in deformed coal development zones. Coal adsorptivityis affected comprehensively by multiple factors such as Received date: 2014-10-10; Accepted date: 2014-11-14pore structure, coal rock composition, coal metamor- Corresponding Author: Prof. Guo Deyong, Telephone: +86phism degree, coal mass deformation, geologic structure, 10-62331517; E-mH中国煤化工CNMHGChina Petroleum Processing and Petrochemical Technology2014,16(4):55-64Table 1 Quality analysis parameters of coal samples used in experimentsolatile contentlo, SampleAsh content Fixed carbon Maximum reflectance of№12(Va),%Coal(Aa),%(FC), ovitrinite(Roma), %oJ-G2.7958.29014.03577.6753.07AnthraciteJJ-Y3.09011.13580.4902.63Anthracite6.0308.02085950Anthracite345678JG-Y2.2557.54082.4402.45AnthraciteP8-G26.21562.025Fat coaP8-Y0.78525.88513.08561.030Fat coalP10-G1.25524.2752123554.485101Fat coalP10-Y1.11031.3859.68058935Fat coal9 ZCH-GI0.61511.86075.3101.88Lean coal10ZCH-G213.12577.220Lean coalNote: JJ-G, JJ-Y, JG-G, JG-Y, P8-G, P8-Y, P10-G, P10-Y, ZCH-GI and ZCH-G2 respectively represent the deformed coal of Jz Jiulishan, the un-deformed coal of Jz Jiulishan the deformed coal of JZ Guhanshan the un-deformed coal of JZ Guhanshan. the de formed coal of PDS No 8 Coalminethe un-deformed coal of pdsCoalmine. the deformed coal of pDs No 10 Coalmine. the un-deformed coal of pds no 10 Coalmine. zz deormed coal 1 and zz deforme2.2 Low temperature liquid nitrogen adsorption ex- the value of a is a saturated adsorption quantity, reflectingperimentthat coal has the maximum adsorptivity; the value of bThe instrument used in the experiment is an ASAP2000 is the ratio of adsorption rate to desorption rate, reflect-type specific surface area and pore size distribution tester ing the velocity and difficulty of gas adsorption by themanufactured by the Micromeritics Instrument Corpora- internal surface of coal. a and b are adsorption constanttion,USA. The coal samples before testing in the experi- and are common parameters for calculating coal seam gasments were dried. The theoretical basis for this method is content in actual coalmine workthe law of gas adsorption on solid surface and actually the3 Results and Discussionclassical volumetric method 9. The measured pore sizerange is from 1. 43 nm to 160 nm. a classification fromHodot for coal pore size is used in this work, which 3.1 Low temperature liquid N2 adsorptiondefines micropores(<10 nm), mesopores(10--100 nm), 3.1.1 Low temperature liquid N, adsorption experimentsand macropores(100 nm)The test results obtained from the low temperature liquid2.3 Isothermal adsorption experimentnitrogen adsorption experiments are shown in Table 2The isothermal adsorption experiment of coal is an impor-According to Table 2, the bet specific surface area oftant method for coal seam gas testing. A WY-98B adsorpdeformed coal is 3. 5--35 times that of un-deformed coaltion constant tester was used in the experiments to simu- and the percentage of micropores of deformed coal islate the adsorption of methane by coal when the specified 1.1-5.0 times that of un-deformed coal. It can be knowntemperature is 30 C Coal adsorptivity is a function of the that the specific surface area of deformed coal is muchcoal's physical property and external temperature, pres- larger than that of un-deformed coal, and the proportionsure and adsorption medium. In case of a given tempera- of micropores of deformed coal is also much higher thanture and adsorption medium, the law of gas adsorption that of un-deformed coal. This is because coal structureby coal can be expressed in Langmuir equation: V=V/ is damaged after being affected by the action of tectonic(PL+P). Let a- Vi and b=1/PL; then this equation is con-ess. and thereverted to P/V=Pla+1/(ab). As known from references-I3, formed coal weYH中国煤化工 In the de-of microporesCNMHGGuo Deyong, et al. Experimental Research on Pore Structure and Gas Adsorption Characteristic of Deformed CoalTable 2 Specific surface area and pore size distribution in coal measured by low temperatureliquid nitrogen adsorption methodPercentage of different types of pores,Roman,% Specific surface area, mlg Total pore volume,cmM3.0704940.0084438.8143.2817910.29750.00857638325.560.93100.0051649.15JG-Y2.450.1910.007947,1464.2928.57P8-G1.050.35070.0023116.0024.00P8-Y0.02790.0075112.5062.5025.00P10-G0.35570.0027014.89P10-Y0.01010.0073813.3362.22CH-G0.30200.0086012.1230.30CH-G22.100.30040.0085912.20634224.38could increase the specific surface area of coal. The total nels of larger porespore volume of un-deformed coal in both JZ coal sample The pore size distribution of both JZ un-deformed coaland PDS coal sample is slightly larger than that of de-( Figure 2(a) and PDS un-deformed coal( Figure 2( b))isformed coal because of the high proportion of mesopores uniform, where the specific surface area is provided jointin the un-deformed coally by micropores and mesopores, and the development3.1.2 Analysis of the relationship between pore size degree of mesopores is higher than that of micropores. Theand specific surface areaproportion of mesopores in the un-deformed coal is largerThe experimental results indicate that the specific surfaceas compared to the greater ratio of micropores in the de-and mesopores, but the specific surface area of the coal changes the size, quantity and size distribution of poep ectarea of deformed coal is mainly composed of microporesformed coal, indicating that tectonically destructive effwith different metamorphism degrees and structure types 3.1.3 Adsorption-desorption curves of coal and poreis slightly different. The pore size distribution(Figure 1type analysis(a)) of jz deformed coal shows a single-peak shape, in Figure 3 shows the coals adsorption-desorption isothermwhich micropores less than 5 nm in diameter provides curves obtained by the low temperature liquid nitrogenthe main specific surface area, and pores with a diameter experiments, which conform to the adsorption-condensaof around 3.0 nm are quite developed. The main con- tion theory+->. The pore shape structure of coal samplestributor to the specific surface area of Zz Chaohua coal can be analyzed according to the adsorption loop in the(Figure 1(b)comprises micropores with diameter 10 research on coal's pore structurell6-19). In general, opennm, while mesopores also provide certain specific surface pores can generate an adsorption loop, and the pores witharea. The pore size distribution(Figure 1(c))of pDs de- one end closed would not generate an adsorption loop. Informed coal is uniform, with micropores and mesopores addition, although one end is closed, "ink bottle"poresjointly providing the main specific surface area, and the can generate an adsorption loop due to their particularity,development degree of micropores is higher than that of and an inflection point will appear on the adsorption loopmesopores. This indicates that the biggest contributorThe adsorption-desorption curves of jz deformed coal aregas adsorption in the deformed coal is micropores Micro- shown in Figure 3(a), and the adsorption loop is very olpores are not only the main space for adsorbing gas but vious. The first half section of the adsorption curve showsalso function as gas motion media and connection chan- an upcony中国煤化工sure is lessCNMHGChina Petroleum Processing and Petrochemical Technology2014,16(4):55-6415≌s寓Average pore diameter, nr(a)Jz deformed coal(a)Jz un-deformed coal1.00.lb)Zz Chaohua coal(b)PDs un-deformed coalFigure 2 Relationship between specific surface area andpore size distribution of un-deformed coal10would gradually increase accordingly; moreover, in caseof reaching the critical relativeto a pore size, capillary condensation also occurs. Smallradius pores will be firstly full of condensed liquid. As therelative pressure of gas increases, the pores with slightlylarger radius will also be filled with condensed liquid successively, and thereby the wall adsorption layer thicknessc)PDS deformed coalof the pores with larger radius continues to increase. ThisFigure I relationship between specific surface area and is manifested by the fact that the second half section ofpore size distribution of deformed coalthe curve rises sharply at the position with a relative pressure being in the range of 0.9-1.0, indicating that capil-than 0.2 and then slowly rises; when the relative prelary condensation occurs in the relatively large pores,sure reaches 0.9, it rises sharply, indicating to a sharp resulting in sharp increase in adsorption quantityincrease in adsorption quantity. This can be interpreted The adsorption loop(b)of Zz Chaohua deformed coalas follows. When the relative pressure is less than 0.2, is obvious, indicating that open pores and fine flaskdu- type pores are developed. The adsorption and desorptionally reaches a saturated state of single-layer adsorption. curves( c)of other coal samples are separated at a relativeAfter condensation, nitrogen can humidify the pore wall. pressure in the range of 0.60--0.98. The pore radius corTherefore, as the relative pressure of gas increases gradi中国煤化工lated as perally, the gas adsorption layer thickness on each pore wall Kelvin equationthe relativelyCNMHGGuo Deyong, et al. Experimental Research on Pore Structure and Gas Adsorption Characteristic of Deformed Coal10small pores are gas-proof pores with one end closed. Theadsorption loop has small separation scope and short dis-tance, indicating that most of the pores with a radius ex-月8ceeding 1.86 nm are the semi-closed pores which cannotgenerate an adsorption loop, and a part of them are theopen pores which can generate an adsorption loop3.1. 4 Fractal characteristics of poresfractal dimension D(hereinafter abbreviated as D)is aRelative pressure(p/Po)tool for analyzing the geometric and structural character(a)JZ deformed coalistics of surface and pore structure> D is often used toteristics: 2