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Coal-like material for coal and gas outburst simulation tests
International Journal of Rock Mechanics & Mining Sciences 74 (2015) 151–156
Contents lists available at ScienceDirect
International Journal of Rock Mechanics & Mining Sciences journal homepage: www.elsevier.com/locate/ijrmms
Technical Note
Coal-like material for coal and gas outburst simulation tests Qianting Hu a,b, Shutong Zhang a,b,c,n, Guangcai Wen a,b, Linchao Dai a,b, Bo Wang a,b a b c
State Key Laboratory of Gas Disaster Detecting, Preventing and Emergency Controlling, Chongqing 400037, PR China China Coal Technology Engineering Group, Chongqing Research Institute, Sha Pingba District, Chongqing 400037, PR China School of Energy and Safety, Anhui University of Science and Technology, Huainan 232001, PR China
art ic l e i nf o Article history: Received 21 August 2014 Received in revised form 18 December 2014 Accepted 9 January 2015 Available online 3 February 2015
1. Introduction Coal and gas outbursts are extremely complicated dynamical phenomena that occur in coal mines [1]. So far, researcher's views on the mechanisms of coal and gas outbursts have agreed that coal and gas outbursts are caused by the combined action of in-situ stress, gas pressure and the mechanical properties of coal [2–4]. In view of the uncertainty and danger, it is difficult to observe the occurrence and process of coal and gas outbursts directly. Consequently, the coal and gas outburst analog simulation test becomes an effective way to further study the mechanical mechanism in the process of coal and gas outburst. An analog material and model are the requirement for an analog stimulation test. So, in order to improve the reliability of the test results, the analog material must be guaranteed to satisfy the demands of simulation [5]. In 1951, one-dimensional outburst simulation tests were conducted using briquettes which was cold pressure formed by culm from bursting proneness of Mazurk coal seam [6]. In 1960, Kuroiwa and Tashiro pressure formed cylindrical similar models of volume 120 cm3 using fine coal [7], and Bodziony et al. formed cylindrical similar models in size of ∅97.4 mm 281 mm, using fine coal of less than 0.2 mm size [8]. In 1994, Kuroiwa and Tashiro formed 96.4 mm diameter, length of 300 mm cylindrical similar models using fine coal of less than 0.2 mm size [9]. In 1984 and 1985, Zhi et al. formed pore putty similar material using the mixture of fine coal, cement and air entraining agent [8,10,11]. In 1959, Fami made 10 cm 10 cm 30 cm and 25 cm 200 cm 300 cm briquettes. In 2012, Skoczylas formed 48 mm diameter, length of 110 mm briquettes using fine coal [12], and in 2014, Skoczylas et al. prepared coal briquettes of various porosity
n Corresponding author at: China Coal Technology Engineering Group, Chongqing Research Institute, Sha Pingba District, Chongqing 400037, PR China. E-mail address: [email protected] (S. Zhang).
http://dx.doi.org/10.1016/j.ijrmms.2015.01.005 1365-1609/& 2015 Elsevier Ltd. All rights reserved.
from 13.5% to 33%, and the coal briquettes have the similar mechanical and gaseous properties with normal and altered coal [13]. Meanwhile, researchers in China also prepared many similar materials to march the coal and gas outburst similar simulation tests. In 1989, Deng formed briquettes of IV, V type strength using fine coal in cold pressure method [4]. From 1990 to 2013, Zhou et al. cold pressure formed briquette using less than 0.1 mm, 0.5 mm, or 1 mm, and 0.1–0.2 mm, 10–80 mesh fine coal with or without water [2,3,14–17]. In 1999 and 2002, Jiang et al. pressure formed similar material using fine coal and diesel oil [18,19], and in 2012, Ou et al. obtained similar material using less than 1 mm fine coal and coal oil [20]. These studies on coal and gas outburst similar materials and tests put forward to new views, and improved the development of coal and gas outburst mechanism. But, due to the low similarity in physical and mechanical parameters of these similar materials with original coal, there exist much qualitative research results but little quantitative research results in coal and gas outburst mechanisms. This paper put forward a coal-like material that has high similarity in physical and mechanical properties with the original coal, using uniform design experiment.
2. Choice of the raw materials in outburst similar simulation test According to previous researches of analog materials, the selection of raw materials should comply the principles as follows: (1) satisfy the requirements of the coal's characteristics; (2) the ratios of raw materials have large influence on the physical and mechanical parameters of similar materials; (3) keep environmental security, resources abundant and low-cost; (4) ensure that the fabrication process is simple and fast molding [5]. Nowadays, about the coal and gas outburst simulation tests, researchers usually chose the raw materials and preparation technologies of analog materials as follows: (1) cold briquetting of a
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Table 1 Raw materials of coal-like material for coal and gas outburst similar simulation test. Category
Material
Size fraction
Remarks
Skeletal material Grouting agent Auxiliary material
Crushed coal Cement Sand Activated carbon Water
80–40 mesh/40–20 mesh 425# ordinary Portland cement 40–20 mesh ∅5.6 5.3 mm2 –
Outburst coal seam – River sand Granule Ordinary tap water
certain sizes of crushed coal; (2) cold briquetting of a certain sizes of crushed coal with water; (3) cold briquetting of a certain sizes of crushed coal with grouting agent of cement or gypsum; (4) hot briquetting of a certain sizes of crushed coal and grouting agent of asphalt, zetar or rosin. The coal size in raw materials usually is less than 1 mm, and the pressing formed pressure is about 20–100 MPa [2,3,8,9,13,14,15,17,21,22]. Based on the results of similar materials preparation technology in coal and gas outburst simulation tests, in order to meet the needs of the large-size of coal and gas outburst simulation models, this experimental study on the coal-like material chooses crushed coal, cement, sand, activated carbon and water as raw materials (Table 1). Coal material is acquired from N2808 workface, coal seam 8# in the Yuyang Mine (Songzao Mining Area, China). There have occurred several coal and gas outbursts in this coal seam 8#, which contains anthracite coal. The approximate analysis of the coal seam 8# is as follows: volatile matter content Vdaf ¼ 9.87– 10.97%, ash content Ad ¼11.53–19.13%, moisture content Mad ¼ 0.56–2.55%, true density TRD¼ 1.5–1.53 g/cm3, apparent density ARD¼1.34–1.38 g/cm3, porosity F¼ 7.95–10.7%, gas adsorption constant a ¼32.8564–34.1459, b¼1.0598–1.4098, index of initial velocity of diffusion of coal gas Δp ¼22–29, hardiness coefficient f ¼0.21–0.38, uniaxial compressive strength Rt r1 MPa, III–V type destructed coal body which has low strength and can be easily crumbled.
3. Proportioning test of coal-like material sample
Table 2 Coal-like material uniform experimental design (mass ratio, %). Level
1 2 3 4 5 6 7 8 9 10 11 12
Factor Cement
Sand
Water
Activated carbon
Crushed coal
2 3 4 5 6 7 8 9 10 11 12 13
3.5 6.5 3 6 2.5 5.5 2 5 1.5 4.5 1 4
8.25 7 9 7.75 6.5 8.5 7.25 9.25 8 6.75 8.75 7.5
0.88 0.82 0.76 0.7 0.9 0.84 0.78 0.72 0.92 0.86 0.8 0.74
85.37 82.68 83.24 80.55 84.1 78.16 81.97 76.03 79.58 76.89 77.45 74.76
reducing test times in large quantities, shortening experiment time, and saving labor and cost [24,25]. The raw materials ranges in this paper are cement 2–13%, sand 1–6.5%, water 6.5–9.25% and activated carbon 0.7–0.92%. It will select the fine coal which grain size is 80–40 mesh and 40–20 mesh, and their mass ratio is 1:1. The ratio of cement has important influence on uniaxial compressive strength of coal-like material. So, the table of uniform experimental is designed according to gradient of cement ratio of 1% (Table 2). There is good uniformity of this designing scheme (Fig. 1).
3.1. Similarity index of the coal-like material The coal and gas outburst similar simulation test should obey the similarity theory. According to the mechanism of mechanical effects on coal and gas outburst [23], the coal and gas outburst is consisted of three continuous or alternate stages. In the three stages, the static deformation and destruction of coal body occur during the outburst preparation stage, the fracturing of gassy coal body, and the movement of crushed coal and gas in the mining space during the outburst forming and developing stage. So, in order to obtain the similar rulers between the outburst models and the prototype, it is required that the coal-like materials must have the similar physical and mechanics characteristics with the outburst coal seam. Judging criteria of similarity between coal-like material and outburst coal seam is according to the parameters such as uniaxial compressive strength, hardiness coefficient, elastic modulus, porosity, density, adsorption constants (a,b), index (Δp) of initial velocity of diffusion of coal gas. 3.2. Designing scheme This paper uses the uniform experimental design method for make the proportion plan of coal-like materials. The uniform experimental design is a good method to handle the experimental design with multi-factors and multi-levels. It has the advantages of
3.3. Experiment method and result According to Table 2, each level contains two samples, so 24 cylindrical coal-like material samples were formed by means of two-sided compression of 25 MPa in heavy-walled steel tube with the inner diameter 50 mm; the wall of the tube is 25 mm, its length is 210 mm. The manufactured coal-like material samples were removed from the briquetting machine, and cured seven days. Then the height of coal-like material samples were set to about 100 mm. The 24 cylindrical standard coal-like material samples were weighed and uniaxial compression tested, so such physical and mechanical indexes as density, uniaxial compressive strength, elasticity modulus of the 24 cylindrical standard coal-like material samples were acquired, (Table 3). Comprehensive analysis on physical and mechanical indexes of 24 coal-like materials samples in Table 3, it can be found that two coal-like material samples of each level have similar physical and mechanics indexes, the results have good uniformity. The density is among 1.34–1.40 g/cm3, uniaxial compressive strength is 0.543– 1.803 MPa, and elasticity modulus is 24.8–237.3 MPa. The physical and mechanical indexes of these coal-like material varies in similar range to the outburst dangerous coal (III–V type destructed coal), they can meet the requirements of coal coal-like material for coal and gas outburst simulation test.
Q. Hu et al. / International Journal of Rock Mechanics & Mining Sciences 74 (2015) 151–156
4. Analysis of effect factors of coal-like material 4.1. Relationship between mass ratio of grouting agent and the mechanical indices of coal-like material 4.1.1. Compressive strength Grouting agent has a large influence to the physical and mechanical properties of coal-like material. And mass ratio of grouting agent determines the strength of coal-like material. According to Tables 2 and 3, it is found that the relationship between mass ratio of cement and uniaxial compressive strength of coal-like material is fit to linear relation by regression analysis, which is shown in Fig. 2: Rt ¼ 0:1194M C þ 0:2145
ð1Þ
where Rt is uniaxial compressive strength of coal-like material, MPa; Mc is cement mass ratio, %. 4.1.2. Hardiness coefficient The hardiness coefficient indicates the ability to resist the damage of coal. According to the reference of GB/T 23561.122010 [26], the procedure of determining coal hardiness coefficients is as follows: (1) Coal cinder of diameter of 20–30 mm is obtained
Fig. 1. Factors distribution map of uniform experimental design.
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through sieving. Then it is divided into 15 groups by quality of 50 g. (2) Each group coal is put into the mashed cylinder of inner diameter 74.5 mm and length 680 mm, then crushed with cylindrical hammer of 2.4 kg from a height of 600 mm for three times. Then, the crushed coal is collected with container. (3) Each five groups of crushed coal are mixed and sieved with sieve of aperture 0.5 mm. The pulverized coal of size less than 0.5 mm is put into measuring tube of inner diameter 23 mm to measure the height L. The hardiness coefficient of this five groups coal cinder is calculated with the formulation f ¼20n/L, in which n is crushed times of each group coal. Then the average hardiness coefficient of three hardiness coefficients is the coal hardiness coefficient. The coal-like material samples in Table 3 were divided into four groups: level 1–3, level 4–6, level 7–9 and level 10–12. Then the hardiness coefficient of each group coal-like material samples was measured. Because of the same quality of each level in the same group, the cement mass ratio can be equivalent to 3%, 6%, 9% and 12%. The relationship between cement mass ratio and hardiness coefficients of coal-like material samples is obtained (Fig. 3): f ¼ 0:009M C þ 0:095
ð2Þ
where f is hardiness coefficient of the coal-like material.
Fig. 2. Relationship between cement mass ratio and uniaxial compressive strength of coal-like material.
Table 3 Physical and mechanical indexes of 24 coal-like materials samples. No. 1 2 3 4 5 6 7 8 9 10 11 12
Sample no.
Quality (kg)
Diameter (mm)
Height (mm)
Density (g/cm3)
Compressive strength (MPa)
Elastic modulus (MPa)
1-1 1-2 2-1 2-2 3-1 3-2 4-1 4-2 5-1 5-2 6-1 6-2 7-1 7-2 8-1 8-2 9-1 9-2 10-1 10-2 11-1 11-2 12-1 12-2
0.266 0.265 0.268 0.264 0.266 0.265 0.271 0.267 0.260 0.254 0.268 0.270 0.261 0.260 0.267 0.271 0.264 0.266 0.265 0.263 0.268 0.266 0.269 0.266
49.5 49.48 49.52 49.51 49.51 49.51 49.5 49.49 49.57 49.5 49.51 49.53 49.51 49.49 49.51 49.52 49.54 49.54 49.52 49.51 49.59 49.52 49.46 49.58
101.88 102.14 102.21 101.18 100.89 100.84 102.42 100.78 100.09 98.65 99.48 100.46 99.95 99.53 98.74 100.26 99.55 100.12 99.8 98.94 99.78 99.26 99.69 99.08
1.36 1.35 1.36 1.36 1.37 1.37 1.37 1.38 1.35 1.34 1.40 1.39 1.36 1.36 1.40 1.40 1.38 1.38 1.38 1.38 1.39 1.39 1.40 1.39
0.543 0.543 0.586 0.56 0.578 0.594 0.818 0.751 0.85 0.793 1.162 1.033 0.965 1.095 1.55 1.507 1.626 1.471 1.576 1.353 1.803 1.679 1.674 1.521
24.8 25.2 31.3 29.2 30.8 32.0 54.9 45.2 63.1 52.7 113.0 62.1 58.6 86.8 142.5 141.7 146.0 125.3 178.0 100.2 237.3 210.0 160.5 142.4
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Fig. 3. Relationship between cement mass ratio and hardiness coefficients of coallike material samples.
Fig. 5. Relationship between crushed coal ratio and density of coal-like material.
Table 4 The No. 5, 6 and 7 level coal-like material samples' adsorption and desorption indexes. No. Cement mass Porosity ratio (%) F (%)
Gas adsorption constant a
5 6 7
Fig. 4. Relationship between elasticity modulus and uniaxial compressive strength of coal-like material.
4.1.3. Elasticity modulus Elasticity modulus is the important physical index of coal-like material. By the regression analysis, the test data in Table 3 is found that the relationship between elasticity modulus and uniaxial compressive strength of coal-like material (Fig. 4): E ¼ 0:0127e1:6121Rt
ð3Þ
where E is Young's modulus of elasticity of the coal-like material, GPa. The elasticity modulus will increase along with the increase of uniaxial compressive strength of the coal-like material, which is in line with the previous results [27].
4.2. Relationship between mass ratio of skeletal material and physical indexes of coal-like material 4.2.1. Density When the forming condition is the same, the mass ratio of crushed coal has large influence on the density of coal-like material. Using the regression analysis of Table 3, can obtain the relationship between crushed coal mass ratio and density of coallike material, it is shown in Fig. 5. With the increasing of crushed coal mass ratio, the density of coal-like material will decrease linearly because of crushed coal's small density among raw materials:
ρ ¼ 0:0047Ma þ 1:752
ð4Þ
where ρ is the density of the coal-like material, g/cm³; Ma is crushed coal mass ratio, %.
6 7 8
5.92 5.36 7.06
Index (Δp) of initial velocity of diffusion of coal gas
b
31.7329 1.4762 23 33.9787 1.6270 23 31.6595 1.6533 20
4.2.2. Adsorption and desorption indexes Gas adsorption constant (a) indicates the unit mass coal's gas adsorption quality under the condition of infinite gas pressure. Gas adsorption constants (a, b) are used to calculate gas content of coal seam with Langmuir Equation, and measured with methane of purity 99.9% and 30 1C test temperature. Index (Δp) of initial velocity of diffusion of coal gas indicates the gas desorption velocity of gassy coal. It is a pressure difference Δp (mm Hg) caused by the free methane in a certain volume vacuum space and 20 1C test temperature. The free methane is diffused by the coal of 3.5 g and size 0.2–0.25 mm which has adsorbed methane of purity 99.9% in 0.1 MPa methane pressure in the time 10–60 s. According to the test on III, IV type crushed coal, the uniaxial compressive strength is about 1 MPa [19]. So, the No. 5, 6 and 7 level coal-like material samples which have about 1 MPa uniaxial compressive strength are chose to measure the adsorption and desorption indexes (Table 4). Shown from Table 4, No. 5–7 level coal-like material samples have similar porosity, gas adsorption constant, index (Δp) of initial velocity of diffusion of coal gas with coal seam 8# in the Yuyang Mine. Shown from Table 2, the coal's mass ratio (78.16–84.1%) of No. 5–7 level coal-like material samples covers nearly the mass ratio of No. 1–12 (74.76–85.37%). Because the mass ratio of crushed coal has large influence to coal-like material samples' gas adsorption and desorption property, the gas adsorption and desorption property of No. 1–12 satisfy the similarity with coal seam 8# in the Yuyang Mine.
5. Research on the relationship between forming size and strength of coal-like material The size of coal and gas outburst similar models is larger than coal-like material sample of diameter 50 mm. Although there is the
Q. Hu et al. / International Journal of Rock Mechanics & Mining Sciences 74 (2015) 151–156
Table 5 The uniaxial compressive strength of 5 cuboid standard coal-like materials. Level
Cement mass ratio (%)
Uniaxial compressive strength (MPa)
2 4 6 8 10
3 5 7 9 11
0.44 0.50 0.89 1.01 1.29
Fig. 6. Uniaxial compressive strength relationship between diameter 50 mm cylindrical and cuboid coal-like material samples.
same forming condition, strength difference between them because of the influence of steel tube's boundary conditions. According to the No. 2, 4, 6, 8 and 10 level in Table 2, five coal-like material samples were formed by means of two-sided compression of 25 MPa in heavy-walled steel tube with the inner diameter 100 mm; the wall of the tube is 25 mm, its length is 210 mm. The manufactured coal-like material samples were also removed from the briquetting machine, and cured more than 20 days. Then the coal-like material samples were set to cuboid of 50 mm 50 mm 100 mm. The uniaxial compressive strengths of the five cuboid standard coal-like materials were measured (Table 5). Compared with Tables 3 and 5, the uniaxial compressive strength relationship between diameter 50 mm cylindrical and cuboid coal-like material samples is shown in Fig. 6. The uniaxial compressive strength of diameter 50 mm cylindrical coal-like material samples is larger than that of the cuboid coal-like material samples, and their ratio is 1.15–1.49. For the same size coal-like material samples, the uniaxial compressive strength is raised with the increase of cement mass ratio. The difference of uniaxial compressive strength between them is small because of the small uniaxial compressive strength of coal-like material samples. So, the research results which obtained with diameter 50 mm cylindrical coal-like material samples can be used to guide the proportioning of the coal and gas outburst coal-like materials.
6. Conclusions (1) A method of selecting the cement, crushed coal, water, sand and activated carbon as raw materials for coal and gas outburst coallike material was proposed. The five factors and 12 variable levels table of raw materials' ratio was designed using uniform design method. And the physical and mechanical indexes as density, uniaxial compressive strength, elasticity modulus of the coal-like material samples were acquired through weigh, uniaxial compression and hardiness coefficient test. (2) Quantitative relation between effect factors and physical and mechanical indexes of coal-like material is analyzed with regression analysis method. And it is found that the relationship
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between mass ratio of cement (Mc) and uniaxial compressive strength of coal-like material (Rt) is fit to linear relation Rt ¼ 0.1194Mc þ0.2145, the relationship between cement mass ratio (Mc) and hardiness coefficients of coal-like material samples (f) is fit to linear relation f¼0.009Mc þ0.095, the relationship between elasticity modulus (E) and uniaxial compressive strength of coal-like material (Rt) is fit to exponential function E¼ 0.0127e1.612Rt, the relationship between coal mass ratio (Ma) and density of coal-like material (ρ) is fit to linear relation ρ ¼ 0.0047Ma þ 1.752. (3) The coal-like material which coal from original mold coal seam have the similar gas adsorption and desorption property with the original mold coal seam. Through experimental study on physical and mechanical indexes of coal-like material samples, the coal and gas coal-like material preparation method is acquired which has 0.54–1.80 MPa uniaxial compressive strength and similar gas adsorption and desorption property with the original mold coal seam. (4) Compared with uniaxial compressive strength of different sized coal-like material samples, the difference of uniaxial compressive strength between small and large samples is small, and the research results which obtained with diameter 50 mm cylindrical coal-like material samples can be used to guide the proportioning of the coal and gas outburst coal-like materials.
Acknowledgments This work was financially supported by the National Basic Research Program of China (Project no. 2012CB724206) and National Natural Science Foundation of China (Project no. 51104087). References [1] Yu BF. Coal and gas outburst mechanism. Beijing: China Coal Industry Publishing House; 1985 (in Chinese). [2] Meng XY, Ding YS, Chen L, Bai RS, Tan QM. 2D simulation test of coal and gas outburst. J Chin Coal Soc 1996;21(1):57–62 (in Chinese). [3] Cai CG. Experimental study on 3-D simulation of coal and gas outbursts. J Chin Coal Soc 2004;29(1):66–9 (in Chinese). [4] Deng QF, Luan YX, Wang YA. Coal and gas outbursts similar simulation test. Saf Coal Min 1989;20(11):5–10 (in Chinese). [5] Gu DZ. Equivalent material and simlitude models. Xuzhou: China University of Mining and Technology Press; 1995 (in Chinese). [6] Sichuan In Min. Summarizing of coal and gas outburst mechanism in abroad. Min Saf Environ Prot 1976;S1:1–19 (in Chinese). [7] Kuroiwa T, Tashiro T. Experimental study on coal pulverization and gas emission in a moment of outbursts of gas and coal. J Jpn Min 1960;76 (862):227–30. [8] Zhang CH, Liu ZG. Review and prospect on coal and gas outburst simulation experiment in laboratory. Chin Saf Sci J 2011;21(3):48–53 (in Chinese). [9] Nelieki A, Topolnicki J. Experimental stand for the investigation of outbursts of porous materials saturated with gas. Arch Min Sci 1994;39(3):301–12. [10] Ujihira M, Isobe T, Higuehi K. On the flaking destruetive phenomena of porous material induced by involved high pressure gas.Study on coal and gas outbursts (1st. Report). J Min Met Inst Jpn 1984;100(3):225–57. [11] Ujihira M, Isobe T, Higuehi K. On the proeess of destruetion of porous material in which high pressure gasis involved. Study on coal and gas outbursts (2nd Report). J Min Met Inst Jpn 1984;100(5):397–403. [12] Skoczylas N. Laboratory study of the phenomenon of methane and coal outburst. Int J Rock Mech Min Sci 2012;55:102–7. [13] Skoczylas N, Dutka B, Sobczyk J. Mechanical and gaseous properties of coal briquettes in terms of outburst risk. Fuel 2014;134:45–52. [14] He XQ, Zhou SN. Rheological hypothesis of coal and gas outburst mechanism. J Chin Univ Min Technol 1994;4(1):15–23 (in Chinese). [15] Jiang CL, Yu QX. Spherical shell lossing stability hypothesis and control techniques of coal and gas outburst mechanism. Xuzhou: China University of Mining and Technology Press; 1998 (in Chinese). [16] Tao YQ. Study on the gassy coal THM coupling model and coal and gas outburst simulation. (Ph.D. thesis). Chongqing: Chongqing University; 2009 (in Chinese).
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Contents lists available at ScienceDirect
International Journal of Rock Mechanics & Mining Sciences journal homepage: www.elsevier.com/locate/ijrmms
Technical Note
Coal-like material for coal and gas outburst simulation tests Qianting Hu a,b, Shutong Zhang a,b,c,n, Guangcai Wen a,b, Linchao Dai a,b, Bo Wang a,b a b c
State Key Laboratory of Gas Disaster Detecting, Preventing and Emergency Controlling, Chongqing 400037, PR China China Coal Technology Engineering Group, Chongqing Research Institute, Sha Pingba District, Chongqing 400037, PR China School of Energy and Safety, Anhui University of Science and Technology, Huainan 232001, PR China
art ic l e i nf o Article history: Received 21 August 2014 Received in revised form 18 December 2014 Accepted 9 January 2015 Available online 3 February 2015
1. Introduction Coal and gas outbursts are extremely complicated dynamical phenomena that occur in coal mines [1]. So far, researcher's views on the mechanisms of coal and gas outbursts have agreed that coal and gas outbursts are caused by the combined action of in-situ stress, gas pressure and the mechanical properties of coal [2–4]. In view of the uncertainty and danger, it is difficult to observe the occurrence and process of coal and gas outbursts directly. Consequently, the coal and gas outburst analog simulation test becomes an effective way to further study the mechanical mechanism in the process of coal and gas outburst. An analog material and model are the requirement for an analog stimulation test. So, in order to improve the reliability of the test results, the analog material must be guaranteed to satisfy the demands of simulation [5]. In 1951, one-dimensional outburst simulation tests were conducted using briquettes which was cold pressure formed by culm from bursting proneness of Mazurk coal seam [6]. In 1960, Kuroiwa and Tashiro pressure formed cylindrical similar models of volume 120 cm3 using fine coal [7], and Bodziony et al. formed cylindrical similar models in size of ∅97.4 mm 281 mm, using fine coal of less than 0.2 mm size [8]. In 1994, Kuroiwa and Tashiro formed 96.4 mm diameter, length of 300 mm cylindrical similar models using fine coal of less than 0.2 mm size [9]. In 1984 and 1985, Zhi et al. formed pore putty similar material using the mixture of fine coal, cement and air entraining agent [8,10,11]. In 1959, Fami made 10 cm 10 cm 30 cm and 25 cm 200 cm 300 cm briquettes. In 2012, Skoczylas formed 48 mm diameter, length of 110 mm briquettes using fine coal [12], and in 2014, Skoczylas et al. prepared coal briquettes of various porosity
n Corresponding author at: China Coal Technology Engineering Group, Chongqing Research Institute, Sha Pingba District, Chongqing 400037, PR China. E-mail address: [email protected] (S. Zhang).
http://dx.doi.org/10.1016/j.ijrmms.2015.01.005 1365-1609/& 2015 Elsevier Ltd. All rights reserved.
from 13.5% to 33%, and the coal briquettes have the similar mechanical and gaseous properties with normal and altered coal [13]. Meanwhile, researchers in China also prepared many similar materials to march the coal and gas outburst similar simulation tests. In 1989, Deng formed briquettes of IV, V type strength using fine coal in cold pressure method [4]. From 1990 to 2013, Zhou et al. cold pressure formed briquette using less than 0.1 mm, 0.5 mm, or 1 mm, and 0.1–0.2 mm, 10–80 mesh fine coal with or without water [2,3,14–17]. In 1999 and 2002, Jiang et al. pressure formed similar material using fine coal and diesel oil [18,19], and in 2012, Ou et al. obtained similar material using less than 1 mm fine coal and coal oil [20]. These studies on coal and gas outburst similar materials and tests put forward to new views, and improved the development of coal and gas outburst mechanism. But, due to the low similarity in physical and mechanical parameters of these similar materials with original coal, there exist much qualitative research results but little quantitative research results in coal and gas outburst mechanisms. This paper put forward a coal-like material that has high similarity in physical and mechanical properties with the original coal, using uniform design experiment.
2. Choice of the raw materials in outburst similar simulation test According to previous researches of analog materials, the selection of raw materials should comply the principles as follows: (1) satisfy the requirements of the coal's characteristics; (2) the ratios of raw materials have large influence on the physical and mechanical parameters of similar materials; (3) keep environmental security, resources abundant and low-cost; (4) ensure that the fabrication process is simple and fast molding [5]. Nowadays, about the coal and gas outburst simulation tests, researchers usually chose the raw materials and preparation technologies of analog materials as follows: (1) cold briquetting of a
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Table 1 Raw materials of coal-like material for coal and gas outburst similar simulation test. Category
Material
Size fraction
Remarks
Skeletal material Grouting agent Auxiliary material
Crushed coal Cement Sand Activated carbon Water
80–40 mesh/40–20 mesh 425# ordinary Portland cement 40–20 mesh ∅5.6 5.3 mm2 –
Outburst coal seam – River sand Granule Ordinary tap water
certain sizes of crushed coal; (2) cold briquetting of a certain sizes of crushed coal with water; (3) cold briquetting of a certain sizes of crushed coal with grouting agent of cement or gypsum; (4) hot briquetting of a certain sizes of crushed coal and grouting agent of asphalt, zetar or rosin. The coal size in raw materials usually is less than 1 mm, and the pressing formed pressure is about 20–100 MPa [2,3,8,9,13,14,15,17,21,22]. Based on the results of similar materials preparation technology in coal and gas outburst simulation tests, in order to meet the needs of the large-size of coal and gas outburst simulation models, this experimental study on the coal-like material chooses crushed coal, cement, sand, activated carbon and water as raw materials (Table 1). Coal material is acquired from N2808 workface, coal seam 8# in the Yuyang Mine (Songzao Mining Area, China). There have occurred several coal and gas outbursts in this coal seam 8#, which contains anthracite coal. The approximate analysis of the coal seam 8# is as follows: volatile matter content Vdaf ¼ 9.87– 10.97%, ash content Ad ¼11.53–19.13%, moisture content Mad ¼ 0.56–2.55%, true density TRD¼ 1.5–1.53 g/cm3, apparent density ARD¼1.34–1.38 g/cm3, porosity F¼ 7.95–10.7%, gas adsorption constant a ¼32.8564–34.1459, b¼1.0598–1.4098, index of initial velocity of diffusion of coal gas Δp ¼22–29, hardiness coefficient f ¼0.21–0.38, uniaxial compressive strength Rt r1 MPa, III–V type destructed coal body which has low strength and can be easily crumbled.
3. Proportioning test of coal-like material sample
Table 2 Coal-like material uniform experimental design (mass ratio, %). Level
1 2 3 4 5 6 7 8 9 10 11 12
Factor Cement
Sand
Water
Activated carbon
Crushed coal
2 3 4 5 6 7 8 9 10 11 12 13
3.5 6.5 3 6 2.5 5.5 2 5 1.5 4.5 1 4
8.25 7 9 7.75 6.5 8.5 7.25 9.25 8 6.75 8.75 7.5
0.88 0.82 0.76 0.7 0.9 0.84 0.78 0.72 0.92 0.86 0.8 0.74
85.37 82.68 83.24 80.55 84.1 78.16 81.97 76.03 79.58 76.89 77.45 74.76
reducing test times in large quantities, shortening experiment time, and saving labor and cost [24,25]. The raw materials ranges in this paper are cement 2–13%, sand 1–6.5%, water 6.5–9.25% and activated carbon 0.7–0.92%. It will select the fine coal which grain size is 80–40 mesh and 40–20 mesh, and their mass ratio is 1:1. The ratio of cement has important influence on uniaxial compressive strength of coal-like material. So, the table of uniform experimental is designed according to gradient of cement ratio of 1% (Table 2). There is good uniformity of this designing scheme (Fig. 1).
3.1. Similarity index of the coal-like material The coal and gas outburst similar simulation test should obey the similarity theory. According to the mechanism of mechanical effects on coal and gas outburst [23], the coal and gas outburst is consisted of three continuous or alternate stages. In the three stages, the static deformation and destruction of coal body occur during the outburst preparation stage, the fracturing of gassy coal body, and the movement of crushed coal and gas in the mining space during the outburst forming and developing stage. So, in order to obtain the similar rulers between the outburst models and the prototype, it is required that the coal-like materials must have the similar physical and mechanics characteristics with the outburst coal seam. Judging criteria of similarity between coal-like material and outburst coal seam is according to the parameters such as uniaxial compressive strength, hardiness coefficient, elastic modulus, porosity, density, adsorption constants (a,b), index (Δp) of initial velocity of diffusion of coal gas. 3.2. Designing scheme This paper uses the uniform experimental design method for make the proportion plan of coal-like materials. The uniform experimental design is a good method to handle the experimental design with multi-factors and multi-levels. It has the advantages of
3.3. Experiment method and result According to Table 2, each level contains two samples, so 24 cylindrical coal-like material samples were formed by means of two-sided compression of 25 MPa in heavy-walled steel tube with the inner diameter 50 mm; the wall of the tube is 25 mm, its length is 210 mm. The manufactured coal-like material samples were removed from the briquetting machine, and cured seven days. Then the height of coal-like material samples were set to about 100 mm. The 24 cylindrical standard coal-like material samples were weighed and uniaxial compression tested, so such physical and mechanical indexes as density, uniaxial compressive strength, elasticity modulus of the 24 cylindrical standard coal-like material samples were acquired, (Table 3). Comprehensive analysis on physical and mechanical indexes of 24 coal-like materials samples in Table 3, it can be found that two coal-like material samples of each level have similar physical and mechanics indexes, the results have good uniformity. The density is among 1.34–1.40 g/cm3, uniaxial compressive strength is 0.543– 1.803 MPa, and elasticity modulus is 24.8–237.3 MPa. The physical and mechanical indexes of these coal-like material varies in similar range to the outburst dangerous coal (III–V type destructed coal), they can meet the requirements of coal coal-like material for coal and gas outburst simulation test.
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4. Analysis of effect factors of coal-like material 4.1. Relationship between mass ratio of grouting agent and the mechanical indices of coal-like material 4.1.1. Compressive strength Grouting agent has a large influence to the physical and mechanical properties of coal-like material. And mass ratio of grouting agent determines the strength of coal-like material. According to Tables 2 and 3, it is found that the relationship between mass ratio of cement and uniaxial compressive strength of coal-like material is fit to linear relation by regression analysis, which is shown in Fig. 2: Rt ¼ 0:1194M C þ 0:2145
ð1Þ
where Rt is uniaxial compressive strength of coal-like material, MPa; Mc is cement mass ratio, %. 4.1.2. Hardiness coefficient The hardiness coefficient indicates the ability to resist the damage of coal. According to the reference of GB/T 23561.122010 [26], the procedure of determining coal hardiness coefficients is as follows: (1) Coal cinder of diameter of 20–30 mm is obtained
Fig. 1. Factors distribution map of uniform experimental design.
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through sieving. Then it is divided into 15 groups by quality of 50 g. (2) Each group coal is put into the mashed cylinder of inner diameter 74.5 mm and length 680 mm, then crushed with cylindrical hammer of 2.4 kg from a height of 600 mm for three times. Then, the crushed coal is collected with container. (3) Each five groups of crushed coal are mixed and sieved with sieve of aperture 0.5 mm. The pulverized coal of size less than 0.5 mm is put into measuring tube of inner diameter 23 mm to measure the height L. The hardiness coefficient of this five groups coal cinder is calculated with the formulation f ¼20n/L, in which n is crushed times of each group coal. Then the average hardiness coefficient of three hardiness coefficients is the coal hardiness coefficient. The coal-like material samples in Table 3 were divided into four groups: level 1–3, level 4–6, level 7–9 and level 10–12. Then the hardiness coefficient of each group coal-like material samples was measured. Because of the same quality of each level in the same group, the cement mass ratio can be equivalent to 3%, 6%, 9% and 12%. The relationship between cement mass ratio and hardiness coefficients of coal-like material samples is obtained (Fig. 3): f ¼ 0:009M C þ 0:095
ð2Þ
where f is hardiness coefficient of the coal-like material.
Fig. 2. Relationship between cement mass ratio and uniaxial compressive strength of coal-like material.
Table 3 Physical and mechanical indexes of 24 coal-like materials samples. No. 1 2 3 4 5 6 7 8 9 10 11 12
Sample no.
Quality (kg)
Diameter (mm)
Height (mm)
Density (g/cm3)
Compressive strength (MPa)
Elastic modulus (MPa)
1-1 1-2 2-1 2-2 3-1 3-2 4-1 4-2 5-1 5-2 6-1 6-2 7-1 7-2 8-1 8-2 9-1 9-2 10-1 10-2 11-1 11-2 12-1 12-2
0.266 0.265 0.268 0.264 0.266 0.265 0.271 0.267 0.260 0.254 0.268 0.270 0.261 0.260 0.267 0.271 0.264 0.266 0.265 0.263 0.268 0.266 0.269 0.266
49.5 49.48 49.52 49.51 49.51 49.51 49.5 49.49 49.57 49.5 49.51 49.53 49.51 49.49 49.51 49.52 49.54 49.54 49.52 49.51 49.59 49.52 49.46 49.58
101.88 102.14 102.21 101.18 100.89 100.84 102.42 100.78 100.09 98.65 99.48 100.46 99.95 99.53 98.74 100.26 99.55 100.12 99.8 98.94 99.78 99.26 99.69 99.08
1.36 1.35 1.36 1.36 1.37 1.37 1.37 1.38 1.35 1.34 1.40 1.39 1.36 1.36 1.40 1.40 1.38 1.38 1.38 1.38 1.39 1.39 1.40 1.39
0.543 0.543 0.586 0.56 0.578 0.594 0.818 0.751 0.85 0.793 1.162 1.033 0.965 1.095 1.55 1.507 1.626 1.471 1.576 1.353 1.803 1.679 1.674 1.521
24.8 25.2 31.3 29.2 30.8 32.0 54.9 45.2 63.1 52.7 113.0 62.1 58.6 86.8 142.5 141.7 146.0 125.3 178.0 100.2 237.3 210.0 160.5 142.4
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Fig. 3. Relationship between cement mass ratio and hardiness coefficients of coallike material samples.
Fig. 5. Relationship between crushed coal ratio and density of coal-like material.
Table 4 The No. 5, 6 and 7 level coal-like material samples' adsorption and desorption indexes. No. Cement mass Porosity ratio (%) F (%)
Gas adsorption constant a
5 6 7
Fig. 4. Relationship between elasticity modulus and uniaxial compressive strength of coal-like material.
4.1.3. Elasticity modulus Elasticity modulus is the important physical index of coal-like material. By the regression analysis, the test data in Table 3 is found that the relationship between elasticity modulus and uniaxial compressive strength of coal-like material (Fig. 4): E ¼ 0:0127e1:6121Rt
ð3Þ
where E is Young's modulus of elasticity of the coal-like material, GPa. The elasticity modulus will increase along with the increase of uniaxial compressive strength of the coal-like material, which is in line with the previous results [27].
4.2. Relationship between mass ratio of skeletal material and physical indexes of coal-like material 4.2.1. Density When the forming condition is the same, the mass ratio of crushed coal has large influence on the density of coal-like material. Using the regression analysis of Table 3, can obtain the relationship between crushed coal mass ratio and density of coallike material, it is shown in Fig. 5. With the increasing of crushed coal mass ratio, the density of coal-like material will decrease linearly because of crushed coal's small density among raw materials:
ρ ¼ 0:0047Ma þ 1:752
ð4Þ
where ρ is the density of the coal-like material, g/cm³; Ma is crushed coal mass ratio, %.
6 7 8
5.92 5.36 7.06
Index (Δp) of initial velocity of diffusion of coal gas
b
31.7329 1.4762 23 33.9787 1.6270 23 31.6595 1.6533 20
4.2.2. Adsorption and desorption indexes Gas adsorption constant (a) indicates the unit mass coal's gas adsorption quality under the condition of infinite gas pressure. Gas adsorption constants (a, b) are used to calculate gas content of coal seam with Langmuir Equation, and measured with methane of purity 99.9% and 30 1C test temperature. Index (Δp) of initial velocity of diffusion of coal gas indicates the gas desorption velocity of gassy coal. It is a pressure difference Δp (mm Hg) caused by the free methane in a certain volume vacuum space and 20 1C test temperature. The free methane is diffused by the coal of 3.5 g and size 0.2–0.25 mm which has adsorbed methane of purity 99.9% in 0.1 MPa methane pressure in the time 10–60 s. According to the test on III, IV type crushed coal, the uniaxial compressive strength is about 1 MPa [19]. So, the No. 5, 6 and 7 level coal-like material samples which have about 1 MPa uniaxial compressive strength are chose to measure the adsorption and desorption indexes (Table 4). Shown from Table 4, No. 5–7 level coal-like material samples have similar porosity, gas adsorption constant, index (Δp) of initial velocity of diffusion of coal gas with coal seam 8# in the Yuyang Mine. Shown from Table 2, the coal's mass ratio (78.16–84.1%) of No. 5–7 level coal-like material samples covers nearly the mass ratio of No. 1–12 (74.76–85.37%). Because the mass ratio of crushed coal has large influence to coal-like material samples' gas adsorption and desorption property, the gas adsorption and desorption property of No. 1–12 satisfy the similarity with coal seam 8# in the Yuyang Mine.
5. Research on the relationship between forming size and strength of coal-like material The size of coal and gas outburst similar models is larger than coal-like material sample of diameter 50 mm. Although there is the
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Table 5 The uniaxial compressive strength of 5 cuboid standard coal-like materials. Level
Cement mass ratio (%)
Uniaxial compressive strength (MPa)
2 4 6 8 10
3 5 7 9 11
0.44 0.50 0.89 1.01 1.29
Fig. 6. Uniaxial compressive strength relationship between diameter 50 mm cylindrical and cuboid coal-like material samples.
same forming condition, strength difference between them because of the influence of steel tube's boundary conditions. According to the No. 2, 4, 6, 8 and 10 level in Table 2, five coal-like material samples were formed by means of two-sided compression of 25 MPa in heavy-walled steel tube with the inner diameter 100 mm; the wall of the tube is 25 mm, its length is 210 mm. The manufactured coal-like material samples were also removed from the briquetting machine, and cured more than 20 days. Then the coal-like material samples were set to cuboid of 50 mm 50 mm 100 mm. The uniaxial compressive strengths of the five cuboid standard coal-like materials were measured (Table 5). Compared with Tables 3 and 5, the uniaxial compressive strength relationship between diameter 50 mm cylindrical and cuboid coal-like material samples is shown in Fig. 6. The uniaxial compressive strength of diameter 50 mm cylindrical coal-like material samples is larger than that of the cuboid coal-like material samples, and their ratio is 1.15–1.49. For the same size coal-like material samples, the uniaxial compressive strength is raised with the increase of cement mass ratio. The difference of uniaxial compressive strength between them is small because of the small uniaxial compressive strength of coal-like material samples. So, the research results which obtained with diameter 50 mm cylindrical coal-like material samples can be used to guide the proportioning of the coal and gas outburst coal-like materials.
6. Conclusions (1) A method of selecting the cement, crushed coal, water, sand and activated carbon as raw materials for coal and gas outburst coallike material was proposed. The five factors and 12 variable levels table of raw materials' ratio was designed using uniform design method. And the physical and mechanical indexes as density, uniaxial compressive strength, elasticity modulus of the coal-like material samples were acquired through weigh, uniaxial compression and hardiness coefficient test. (2) Quantitative relation between effect factors and physical and mechanical indexes of coal-like material is analyzed with regression analysis method. And it is found that the relationship
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between mass ratio of cement (Mc) and uniaxial compressive strength of coal-like material (Rt) is fit to linear relation Rt ¼ 0.1194Mc þ0.2145, the relationship between cement mass ratio (Mc) and hardiness coefficients of coal-like material samples (f) is fit to linear relation f¼0.009Mc þ0.095, the relationship between elasticity modulus (E) and uniaxial compressive strength of coal-like material (Rt) is fit to exponential function E¼ 0.0127e1.612Rt, the relationship between coal mass ratio (Ma) and density of coal-like material (ρ) is fit to linear relation ρ ¼ 0.0047Ma þ 1.752. (3) The coal-like material which coal from original mold coal seam have the similar gas adsorption and desorption property with the original mold coal seam. Through experimental study on physical and mechanical indexes of coal-like material samples, the coal and gas coal-like material preparation method is acquired which has 0.54–1.80 MPa uniaxial compressive strength and similar gas adsorption and desorption property with the original mold coal seam. (4) Compared with uniaxial compressive strength of different sized coal-like material samples, the difference of uniaxial compressive strength between small and large samples is small, and the research results which obtained with diameter 50 mm cylindrical coal-like material samples can be used to guide the proportioning of the coal and gas outburst coal-like materials.
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