- Email: [email protected]
Control of combustion area using electrical resistivity method for underground coal gasification
International Journal of Mining Science and Technology 22 (2012) 351–355
Contents lists available at SciVerse ScienceDirect
International Journal of Mining Science and Technology journal homepage: www.elsevier.com/locate/ijmst
Control of combustion area using electrical resistivity method for underground coal gasification Selivanova Tatiana ⇑, Grebenyuk Igor, Belov Alexey Engineering School, Far Eastern Federal University, Vladivostok, Russian Federation
a r t i c l e
i n f o
Article history: Received 28 October 2011 Received in revised form 15 November 2011 Accepted 5 December 2011 Available online 15 May 2012 Keywords: Underground coal gasification Monitoring Electro conductivity Laboratory experiment Coal specimen
a b s t r a c t Underground coal gasification (UCG) is one of the clean technologies to collect heat energy and gases (hydrogen, methane, etc.) in an underground coal seam. It is necessary to further developing environmentally friendly UCG system construction. One of the most important UCG’s problems is underground control of combustion area for efficient gas production, estimation of subsidence and gas leakage to the surface. For this objective, laboratory experiments were conducted according to the UCG model to identify the process of combustion cavity development by monitoring the electrical resistivity activity on the coal samples to setup fundamental data for the technology engineering to evaluate combustion area. While burning coal specimens, that had been sampled from various coal deposits, electrical resistivity was monitored. Symmetric four electrodes system (ABMN) of direct and low-frequency current electric resistance method was used for laboratory resistivity measurement of rock samples. Made research and the results suggest that front-end of electro conductivity activity during heating and combusting of coal specimen depended on heating temperature. Combusting coal electro conductivity has complicated multistage type of change. Electrical resistivity method is expected to be a useful geophysical tool to for evaluation of combustion volume and its migration in the coal seam. Ó 2012 Published by Elsevier B.V. on behalf of China University of Mining & Technology.
1. Introduction At the moment, Underground coal gasification (UCG) is one of the new clean energy technologies with zero emission [1,2]. Underground coal gasification provides pollution control, especially regarding to emissions of nitrous oxides, mercury and sulfur, eliminates ash disposal after coal [1,3]. UCG is an alternative for surface gasifier, since underground gasification process is conducted underground in the virgin coal seam, thus largely reducing the cost for coal mining and transportation [4,5]. UCG can recover energy from coal seams that are difficult to be mined with traditional mining technology. In fact, UCG is regarded as the most promising way to increase the coal resources available for utilization by gasifying un-mineable thin or deep coal seams under difficult geological and mining condition [6]. Although modern sensing and control techniques reduce UCG’s impact on environment it is necessary for further developing environmentally friendly UCG system construction [5,7]. When the combustion reactor is operating, the gasification area is moved along the linking hole. The gasification induces many cracks resulting from stress changes around gasification area [8,9]. These fracture activities can cause gas leakage to the surface, water
⇑ Corresponding author. Tel.: +7 4232 316294. E-mail address: [email protected] (T. Selivanova).
pollution and well collapse. So, one of the most important UCG’s problems is evaluation of the combustion area in underground coal seams and rock masses. The monitoring of the ground combustion area is necessary not only for efficient gas production but also for estimation of subsidence and gas leakage to the surface [1]. Control of the combustion area around gasification cavity is necessary for UCG safeguarding. Some coal deposits located in the Russian Far East territory can be used for UCG [10]. Coal of far-eastern deposits of Russia contains some mineral admixture like lens, interbeds and dispersions. Mineral substance of coal generally are represented by clay minerals (kaolin, illite), iron disulphate (pyrite), carbonates (siderite, ankerit, calcite), oxides (quartz), illites (limonite), sulfides, phosphates, sulfates, silicates, different salts, rare-earth elements [11–13]. Morphological varieties of kaolin are fine round grains spread in coal filling cracks and plant tissue vesicle. Calcite and siderite are found as small masses located at the coal fractures. Pyrite is found very rarely. Usually pyrite is occurred small-grained, regularshaped masses, lens located at the coal seam roof. Often the same coal fracture may be mineralized by kaolin, calcite, siderite, quartz. Terrigenous substance of coal consists of fine clay, debris of feldspar and quartz. These inclusions randomly distributed in coal volume and have not coal type correlation. Based on ash volume far eastern coal may be divided on the following groups: low-ash of 7–20% and specific weight of 1.2–1.3 g/cm3; variable-ash of 7–40%; high-ash of 20–45% and specific weight of 1–1.7 g/cm3.
2095-2686/$ - see front matter Ó 2012 Published by Elsevier B.V. on behalf of China University of Mining & Technology. http://dx.doi.org/10.1016/j.ijmst.2012.04.012
352
T. Selivanova et al. / International Journal of Mining Science and Technology 22 (2012) 351–355
Coal samples used in this study were obtained from brown coal deposit (Pavlovskoe deposit, Primorskiy region, Russian Far East) and coal deposit (Urgalskoe deposit, Khabarovskiy region, Russian Far East).
The coal specimen N1 (approximately 18 cm 36 cm 22 cm, rotational form) was sampled from the top commercial seam of Pavlovskoe brown coal deposit (Fig. 1). Original color of the sample was brown. The sample had horizontal stratification. Organically and mineral inclusions visually were not determined. The sample had laminated structure. Based on structure-texture specific the sample may be related to large striated lithologic type. The sample had exogenous and hypergenic fractures crossing the samples in two orthogonally related planes. Dominant forms of cleavages were scaly, fission fragment, irregular ones. Fracture forms were varied widely changed from even flat fractures to irregular hatched fractures. Fracture surface was heterogenic knobby one. Fracture forms were V-shaped and had parallel envelops. Coal substance was transfixed by macro pores and transitions pores different direction and depth. Numerous fractures were exogenous; hypergenic fractures practically were not visible. Exogenous fractures visually made up three systems. Fracture’s width was 0.05–0.5 cm. The coal was crumpled and it fell to pieces under weak mechanical pressure. Factor of general fracturing of the coal specimen N1 was 4. The coal specimen N2 (approximately 18 cm 36 cm 22 cm, rotational form) was sampled from the top commercial seam of Pavlovskoe brown coal deposit (as shown in Fig. 2). This specimen differs from the specimen N2 only greater specific weight and smaller degree of fracturing. Factor of general fracturing of the coal specimen N2 was 2. Proximate and ultimate chemical analyses of the coal samples N1, 2 (Pavlovskoe brown coal deposit) are shown in Table 1. The coal specimen N3 was selected from the top commercial seam of Urgalskoe coal deposit. The specimen has rotational form and sample size was 25 cm 35 cm 25 cm (as shown in Fig. 3). Original color of the specimen was bright black with metallic luster. The specimen had exogenous and hypergenic fractures crossing the specimen in two orthogonally related planes. Dominant forms of cleavages were stepped. Fracture surface was homogeneous, smooth, unruffled and parallel hatched. Coal substance was transfixed by macro pores and transitions pores different direction and different depth. Numerous fractures were exogenous; hypergenic fractures practically were not visible. The coal was crumpled and it fell to pieces under weak mechanical pressure. Factor of general fracturing of coal specimen was 2. The coal samples were covered with concrete to 5–10 cm thickness for persistence of specimens during burning and for minimization current lines transformation on ‘‘coal sample-air’’ boundary. Samples were not keeping in water previously.
Fig. 1. Coal specimen N1 sampled from Pavlovskoe brown coal deposit.
Fig. 2. Coal specimen N2 sampled from Pavlovskoe brown coal deposit.
Coal seams must be seen as heterogeneous system composed of solid mineral substance, organic mass high porosity, gas and liquid phases. Thermo-chemical conversion mechanism of different metamorphic phases and petro-graphical composition coal is based on identical principals. Coal thermo-chemical conversion is complicated multistage process composed of consecutive reactions [5,14,15]. Porosity, normal water contents, pore channel convolution are changed under heating and combusting of rock. Variation of these parameters has an effect on physical characteristics of rock (resistivity, polarization, acoustic, magnetic and so on) [6,16]. So, geophysical methods may be used for control UCG combustion area. Electrical resistivity method, one of widely applicable geophysical method, is expected to be a useful geophysical tool to control UCG combustion area. Coal combusting makes for water content expulsion and vapor condensation outlying combustion zone forming conducting channels. It is possible to assume that electro conductivity of combusting coal will be changed under heating [9,17,18]. To applicability check of electro conductivity method for monitoring of UCG combustion area location laboratory experiments have been made as an activity of the Ministry of Education and Science of the Russian Federation and Far Eastern Federal University. Our investigation based on a theoretical analysis and laboratory simulation tests. Typical results of the laboratory experiments are presented below.
2. Experimental UCG simulation tests were made in the UCG laboratory of the Far Eastern Federal University in Vladivostok (the Russian Federation). This experiment was undertaken to indentify the process of combustion cavity development by monitoring the electrical resistivity activity on the coal samples to set up fundamental information for the technology engineering to evaluate the location of the combustion area.
2.1. Coal specimens
353
T. Selivanova et al. / International Journal of Mining Science and Technology 22 (2012) 351–355 Table 1 Integrated chemical composition of far eastern coal. Chemical composition (%)
SiO2
Al2O3
Fe2O3
TiO2
CaO
MgO
Na2O
K2O
Variation range Mean value
31.6–75.7 40.8
10.9–37.1 16.1
0.5–6.4 1.4
1.4–.5.5 2.1
0.7–8.4 2.7
0.5–5.0 1.5
0.2–1.7 0.6
0.5–2.7 0.9
Fig. 3. Coal specimen N3 selected from Urgalskoe coal deposit.
2.2. Simulation test of UCG A scheme of the UCG model gasifier used for these tests is shown in Fig. 4. After cementing of the coal samples, a gasification tunnel with a diameter of 20 mm was bored along the lateral direction. These boreholes included an initial injection hole and a production hole, which can be used for moving the oxygen supply and gas production. 2.3. Experimental setup Laboratory resistivity measurement methodology of rock samples is widely known. Usually symmetric four electrodes system (ABMN) of direct and low-frequency current electric resistance method is used for laboratory resistivity measurement of rock samples. Parameters of using electrical system were according to the following factors: the sample’s size; possibility of potential difference and current measurement; observance of equivalence principle. Vertical electrical sounding was made to determine the optimal size of the electrical system. Size of AB line was changed
Fig. 5. Experimental setup for coal specimen.
from 3 to 12 cm, MN line-from 1 to 2 cm. Cuprous electrodes to 0.9 cm diameter and 15 cm long were used. Fig. 5 presents the experimental setup. For 2–3 h during coal combustion, potential difference and current measurements were made. The following widely known formula was used for resistivity extend
R ¼ KU=J where K is the electrode system coefficient; U the potential difference at the receiving circuit, MN; J the current intensity at the circuit feeder, AB. 2.4. Gas supply and monitoring system An electrical source was switched on to ignite the coal. After ignition, air was injected and the coal on the side of injection borehole began to burn and a high temperature profile gradually was formed. With the gasification of the coal seam, the fire front moved forward and along the coal seam. The gasifying agents, air and oxygen, were supplied by an air compressor. The gas produced by UCG was pumped away. All the data of measured potential (voltage), current were transferred to the control computer for real-time monitoring. 3. Results and discussion 3.1. Results of experiments
Fig. 4. UCG model gasifier used for tests.
The specimen N1 was heated until 800 degree centigrade. After stable gasification controlled steam/oxygen ratio was adjusted, resistivity activity during heating and combustion coal sample was studied. Fig. 6 presents resistivity change during heating and
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T. Selivanova et al. / International Journal of Mining Science and Technology 22 (2012) 351–355
the specimen N2 was slight. As a result, resistivity of the specimen slow decreased during heating (the resistivity gradient was about 0.03 for 1 min). Heating time was not enough for water content reduction, thermal expansion of mineral spots and, as consequence, resistivity was not sharply changed. The specimen N3 was heated for 4 h until 600–800°. Stable process of UCG was not exacted. Fig. 8. presents resistivity activity during heating and combustion test N3. The result shows that resistivity of this specimen changed chaotically.
0.4
ρ (Ω ⋅ m)
0.3 0.2 0.1
3.2. Discussion remarks
0
20
40
60 80 t (min)
100
120
Fig. 6. Resistivity activity during heating and combustion test (the brown coal specimen N1).
0.030
ρ (10 4 Ω ⋅ m)
0.025 0.020 0.015 0.010
Obtained experimental results let us assume that electro conductivity activity during heating and combusting test of coal specimen depends on normal water content value. Coal combusting makes for the following water transformation process: (a) Water content expulsion; (b) Water vapor relocation to outlying combustion zone; (c) Water vapor condensation; (d) Conducting channel (surface) forming. It the beginning of coal combusting (to 300–500 °C) abrupt decreasing of resistivity is observed. Resistivity gradient depends on coal normal water content. Some later, resistivity is increasing under the influence of thermal expansion of mineral spots. The thermal expansion of mineral spots changes the following coal parameters: (a) Porosity factor; (b) Electrical channel convolution; (c) Double electrical layer conductivity and solution conductivity relation.
0.005 4. Conclusions
0
20
40
60
80
100
120
140
160
t (min) Fig. 7. Resistivity activity during heating and combustion test (the coal specimen N2).
0.035
ρ (10 4 Ω ⋅m)
0.030 0.025 0.020 0.015 0.010 0.005 0
40
80
120 160 t (min)
200
240
Fig. 8. Resistivity activity during heating and combustion test (the coal specimen N3).
combustion test of the brown coal specimen N1. From this graph, the results show that for the first 15 min from the start of heating and combusting test the resistivity deeply decreased (the resistivity gradient was 0.3 for 1 min). Furthermore heating for 60 min, resistivity was relativity stable (the resistivity gradient was about 0.01 for 1 min) and then resistivity began to increase with gradient 0.3 for 1 min). The specimen was heated until 600–800°. Fig. 7 shows resistivity activity during heating and combustion test of the coal specimen N2. At opposed to the specimen N1 the specimen N2 had bigger specific weight and smaller degree of fracturing. Open porosity of
Carried out research and these results suggest that front-end of electro conductivity activity of coal specimen depend on heating and combusting. Combusting coal electro conductivity has complicated multistage type of change. Resistivity decreasing of brown coal samples at the start time of heating depended on water content expulsion and it relocation to outlying combustion zone. Later resistivity of brown coal decreased under water vapor condensation and conducting channel (surface) forming. Farther heating temperature growth made for electrical resistivity increased under water content reduction, thermal expansion of mineral spots and electrical channel convolution change. Unfortunately, the ccorrelation between heating temperature and electrical resistivity activity of coal specimen was not intercommunicated. Electrical resistivity method is expected to be a useful geophysical tool to for evaluation of combustion volume and its migration in the coal seam. In the near future, we are planning to develop electrical resistivity monitoring technology for evaluation of the ground combustion area. Acknowledgements This investigation was provided by the Ministry of Education and Science of Russian Federation (No. P1679), Far Eastern Federal University. The authors gratefully acknowledge their support. References [1] Bian Z, Inyang HI, Daniels JL, Otto F, Struthers S. Environmental issues from coal mining and their solutions. Min Sci Technol 2010;20(2):215–23. [2] Liu H, Feng C, Xia P, Yang K, Liu S. Method of oxygen-enriched two-stage underground coal gasification. Min Sci Technol 2011;21(2):191–6. [3] Liu S, Li J, Mei M, Dong D. Groundwater pollution from underground coal gasification. J Chin Univ Min Technol 2007;17(4):467–72. [4] Kondyrev BI, Belov AV, Grebenyk IV. Underground coal gasification experience in People’s Republic of China. Min Inf Anal Bull: Sci Tech J 2005;10:286–9. [5] Kreynin EV. Non-conventional terminal technologies of hard quarry fuel digging. Moscow: Gasprom Press; 2004.
T. Selivanova et al. / International Journal of Mining Science and Technology 22 (2012) 351–355 [6] Gao Y, Gao F, Zhang Z, Zhang T. Visco-elastic-plastic model of deep underground rock affected by temperature and humidity. Min Sci Technol 2010;20(2):183–7. [7] Zhu L, Zhuang Z. Framework system and research flow of uncertainty in 3D geological structure models. Min Sci Technol 2010;20(2). 30611-311. [8] Itakura H. Fundamental experiments on UCG. Muroran: Muroran Institute of Technology Press; 2009. [9] Wang H, Cheng Y, Yuan L, Wang L. Similarity model tests of movement and deformation of coal–rock mass below steps. Min Sci Technol 2010;20(2):188–92. [10] Kondyrev BI, Belov AV, Grebenyk IV. Underground coal gasification perspective on Far-eastern coal deposits. Coal 2008;10:36–7. [11] Liu S, Wang Y, Zhao K, Yang K. Enhanced-hydrogen gas production through underground gasification of lignite. Min Sci Technol 2009;19(3):389–94. [12] Podolyan VI. Brief geological and tectonic information of regional coal structure. In: Coal basins and deposits of Sabiria, Kazachstan and FAR east of USSR. Moscow: Nedra Press; 1979. p. 82–128.
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[13] Selivanova TV. Delimitation of coal seam geometrics using geophysical well survey (by Primorskiy region’s coal deposits example). Vladivostok: Fareastern National Technical University Press; 2008. [14] Kondyrev BI, Zhukov AB, Belov AV, Grebenyk IV. Study of physical and chemical processes of boring and reaming technology of coal production. Vladivostok: Far-eastern National Technical University Press; 2008. [15] Liu SQ, Liang J. Characteristics of underground gasification of different kinds of coal. J Chin Univ Min Technol 2003;32(6):624–8. [16] Nie B, He X, Liu F, Zhu C, Wang P. Chaotic characteristics of electromagnetic emission signals during deformation and fracture of coal. Min Sci Technol 2009;19(2):189–93. [17] Liang J, Zhang Y, Wei C, Feng Y. Experiment research on underground coal gasification of Xiyang anthracite. J Chin Univ Min Technol 2006;35(1). 25-28, 34. [18] Yu H, Yang L, Feng W, Liu S, Song. Reaction kinetic equation for char combustion of underground coal gasification. J Chin Univ Min Technol 2006;16(2):141–6.
Contents lists available at SciVerse ScienceDirect
International Journal of Mining Science and Technology journal homepage: www.elsevier.com/locate/ijmst
Control of combustion area using electrical resistivity method for underground coal gasification Selivanova Tatiana ⇑, Grebenyuk Igor, Belov Alexey Engineering School, Far Eastern Federal University, Vladivostok, Russian Federation
a r t i c l e
i n f o
Article history: Received 28 October 2011 Received in revised form 15 November 2011 Accepted 5 December 2011 Available online 15 May 2012 Keywords: Underground coal gasification Monitoring Electro conductivity Laboratory experiment Coal specimen
a b s t r a c t Underground coal gasification (UCG) is one of the clean technologies to collect heat energy and gases (hydrogen, methane, etc.) in an underground coal seam. It is necessary to further developing environmentally friendly UCG system construction. One of the most important UCG’s problems is underground control of combustion area for efficient gas production, estimation of subsidence and gas leakage to the surface. For this objective, laboratory experiments were conducted according to the UCG model to identify the process of combustion cavity development by monitoring the electrical resistivity activity on the coal samples to setup fundamental data for the technology engineering to evaluate combustion area. While burning coal specimens, that had been sampled from various coal deposits, electrical resistivity was monitored. Symmetric four electrodes system (ABMN) of direct and low-frequency current electric resistance method was used for laboratory resistivity measurement of rock samples. Made research and the results suggest that front-end of electro conductivity activity during heating and combusting of coal specimen depended on heating temperature. Combusting coal electro conductivity has complicated multistage type of change. Electrical resistivity method is expected to be a useful geophysical tool to for evaluation of combustion volume and its migration in the coal seam. Ó 2012 Published by Elsevier B.V. on behalf of China University of Mining & Technology.
1. Introduction At the moment, Underground coal gasification (UCG) is one of the new clean energy technologies with zero emission [1,2]. Underground coal gasification provides pollution control, especially regarding to emissions of nitrous oxides, mercury and sulfur, eliminates ash disposal after coal [1,3]. UCG is an alternative for surface gasifier, since underground gasification process is conducted underground in the virgin coal seam, thus largely reducing the cost for coal mining and transportation [4,5]. UCG can recover energy from coal seams that are difficult to be mined with traditional mining technology. In fact, UCG is regarded as the most promising way to increase the coal resources available for utilization by gasifying un-mineable thin or deep coal seams under difficult geological and mining condition [6]. Although modern sensing and control techniques reduce UCG’s impact on environment it is necessary for further developing environmentally friendly UCG system construction [5,7]. When the combustion reactor is operating, the gasification area is moved along the linking hole. The gasification induces many cracks resulting from stress changes around gasification area [8,9]. These fracture activities can cause gas leakage to the surface, water
⇑ Corresponding author. Tel.: +7 4232 316294. E-mail address: [email protected] (T. Selivanova).
pollution and well collapse. So, one of the most important UCG’s problems is evaluation of the combustion area in underground coal seams and rock masses. The monitoring of the ground combustion area is necessary not only for efficient gas production but also for estimation of subsidence and gas leakage to the surface [1]. Control of the combustion area around gasification cavity is necessary for UCG safeguarding. Some coal deposits located in the Russian Far East territory can be used for UCG [10]. Coal of far-eastern deposits of Russia contains some mineral admixture like lens, interbeds and dispersions. Mineral substance of coal generally are represented by clay minerals (kaolin, illite), iron disulphate (pyrite), carbonates (siderite, ankerit, calcite), oxides (quartz), illites (limonite), sulfides, phosphates, sulfates, silicates, different salts, rare-earth elements [11–13]. Morphological varieties of kaolin are fine round grains spread in coal filling cracks and plant tissue vesicle. Calcite and siderite are found as small masses located at the coal fractures. Pyrite is found very rarely. Usually pyrite is occurred small-grained, regularshaped masses, lens located at the coal seam roof. Often the same coal fracture may be mineralized by kaolin, calcite, siderite, quartz. Terrigenous substance of coal consists of fine clay, debris of feldspar and quartz. These inclusions randomly distributed in coal volume and have not coal type correlation. Based on ash volume far eastern coal may be divided on the following groups: low-ash of 7–20% and specific weight of 1.2–1.3 g/cm3; variable-ash of 7–40%; high-ash of 20–45% and specific weight of 1–1.7 g/cm3.
2095-2686/$ - see front matter Ó 2012 Published by Elsevier B.V. on behalf of China University of Mining & Technology. http://dx.doi.org/10.1016/j.ijmst.2012.04.012
352
T. Selivanova et al. / International Journal of Mining Science and Technology 22 (2012) 351–355
Coal samples used in this study were obtained from brown coal deposit (Pavlovskoe deposit, Primorskiy region, Russian Far East) and coal deposit (Urgalskoe deposit, Khabarovskiy region, Russian Far East).
The coal specimen N1 (approximately 18 cm 36 cm 22 cm, rotational form) was sampled from the top commercial seam of Pavlovskoe brown coal deposit (Fig. 1). Original color of the sample was brown. The sample had horizontal stratification. Organically and mineral inclusions visually were not determined. The sample had laminated structure. Based on structure-texture specific the sample may be related to large striated lithologic type. The sample had exogenous and hypergenic fractures crossing the samples in two orthogonally related planes. Dominant forms of cleavages were scaly, fission fragment, irregular ones. Fracture forms were varied widely changed from even flat fractures to irregular hatched fractures. Fracture surface was heterogenic knobby one. Fracture forms were V-shaped and had parallel envelops. Coal substance was transfixed by macro pores and transitions pores different direction and depth. Numerous fractures were exogenous; hypergenic fractures practically were not visible. Exogenous fractures visually made up three systems. Fracture’s width was 0.05–0.5 cm. The coal was crumpled and it fell to pieces under weak mechanical pressure. Factor of general fracturing of the coal specimen N1 was 4. The coal specimen N2 (approximately 18 cm 36 cm 22 cm, rotational form) was sampled from the top commercial seam of Pavlovskoe brown coal deposit (as shown in Fig. 2). This specimen differs from the specimen N2 only greater specific weight and smaller degree of fracturing. Factor of general fracturing of the coal specimen N2 was 2. Proximate and ultimate chemical analyses of the coal samples N1, 2 (Pavlovskoe brown coal deposit) are shown in Table 1. The coal specimen N3 was selected from the top commercial seam of Urgalskoe coal deposit. The specimen has rotational form and sample size was 25 cm 35 cm 25 cm (as shown in Fig. 3). Original color of the specimen was bright black with metallic luster. The specimen had exogenous and hypergenic fractures crossing the specimen in two orthogonally related planes. Dominant forms of cleavages were stepped. Fracture surface was homogeneous, smooth, unruffled and parallel hatched. Coal substance was transfixed by macro pores and transitions pores different direction and different depth. Numerous fractures were exogenous; hypergenic fractures practically were not visible. The coal was crumpled and it fell to pieces under weak mechanical pressure. Factor of general fracturing of coal specimen was 2. The coal samples were covered with concrete to 5–10 cm thickness for persistence of specimens during burning and for minimization current lines transformation on ‘‘coal sample-air’’ boundary. Samples were not keeping in water previously.
Fig. 1. Coal specimen N1 sampled from Pavlovskoe brown coal deposit.
Fig. 2. Coal specimen N2 sampled from Pavlovskoe brown coal deposit.
Coal seams must be seen as heterogeneous system composed of solid mineral substance, organic mass high porosity, gas and liquid phases. Thermo-chemical conversion mechanism of different metamorphic phases and petro-graphical composition coal is based on identical principals. Coal thermo-chemical conversion is complicated multistage process composed of consecutive reactions [5,14,15]. Porosity, normal water contents, pore channel convolution are changed under heating and combusting of rock. Variation of these parameters has an effect on physical characteristics of rock (resistivity, polarization, acoustic, magnetic and so on) [6,16]. So, geophysical methods may be used for control UCG combustion area. Electrical resistivity method, one of widely applicable geophysical method, is expected to be a useful geophysical tool to control UCG combustion area. Coal combusting makes for water content expulsion and vapor condensation outlying combustion zone forming conducting channels. It is possible to assume that electro conductivity of combusting coal will be changed under heating [9,17,18]. To applicability check of electro conductivity method for monitoring of UCG combustion area location laboratory experiments have been made as an activity of the Ministry of Education and Science of the Russian Federation and Far Eastern Federal University. Our investigation based on a theoretical analysis and laboratory simulation tests. Typical results of the laboratory experiments are presented below.
2. Experimental UCG simulation tests were made in the UCG laboratory of the Far Eastern Federal University in Vladivostok (the Russian Federation). This experiment was undertaken to indentify the process of combustion cavity development by monitoring the electrical resistivity activity on the coal samples to set up fundamental information for the technology engineering to evaluate the location of the combustion area.
2.1. Coal specimens
353
T. Selivanova et al. / International Journal of Mining Science and Technology 22 (2012) 351–355 Table 1 Integrated chemical composition of far eastern coal. Chemical composition (%)
SiO2
Al2O3
Fe2O3
TiO2
CaO
MgO
Na2O
K2O
Variation range Mean value
31.6–75.7 40.8
10.9–37.1 16.1
0.5–6.4 1.4
1.4–.5.5 2.1
0.7–8.4 2.7
0.5–5.0 1.5
0.2–1.7 0.6
0.5–2.7 0.9
Fig. 3. Coal specimen N3 selected from Urgalskoe coal deposit.
2.2. Simulation test of UCG A scheme of the UCG model gasifier used for these tests is shown in Fig. 4. After cementing of the coal samples, a gasification tunnel with a diameter of 20 mm was bored along the lateral direction. These boreholes included an initial injection hole and a production hole, which can be used for moving the oxygen supply and gas production. 2.3. Experimental setup Laboratory resistivity measurement methodology of rock samples is widely known. Usually symmetric four electrodes system (ABMN) of direct and low-frequency current electric resistance method is used for laboratory resistivity measurement of rock samples. Parameters of using electrical system were according to the following factors: the sample’s size; possibility of potential difference and current measurement; observance of equivalence principle. Vertical electrical sounding was made to determine the optimal size of the electrical system. Size of AB line was changed
Fig. 5. Experimental setup for coal specimen.
from 3 to 12 cm, MN line-from 1 to 2 cm. Cuprous electrodes to 0.9 cm diameter and 15 cm long were used. Fig. 5 presents the experimental setup. For 2–3 h during coal combustion, potential difference and current measurements were made. The following widely known formula was used for resistivity extend
R ¼ KU=J where K is the electrode system coefficient; U the potential difference at the receiving circuit, MN; J the current intensity at the circuit feeder, AB. 2.4. Gas supply and monitoring system An electrical source was switched on to ignite the coal. After ignition, air was injected and the coal on the side of injection borehole began to burn and a high temperature profile gradually was formed. With the gasification of the coal seam, the fire front moved forward and along the coal seam. The gasifying agents, air and oxygen, were supplied by an air compressor. The gas produced by UCG was pumped away. All the data of measured potential (voltage), current were transferred to the control computer for real-time monitoring. 3. Results and discussion 3.1. Results of experiments
Fig. 4. UCG model gasifier used for tests.
The specimen N1 was heated until 800 degree centigrade. After stable gasification controlled steam/oxygen ratio was adjusted, resistivity activity during heating and combustion coal sample was studied. Fig. 6 presents resistivity change during heating and
354
T. Selivanova et al. / International Journal of Mining Science and Technology 22 (2012) 351–355
the specimen N2 was slight. As a result, resistivity of the specimen slow decreased during heating (the resistivity gradient was about 0.03 for 1 min). Heating time was not enough for water content reduction, thermal expansion of mineral spots and, as consequence, resistivity was not sharply changed. The specimen N3 was heated for 4 h until 600–800°. Stable process of UCG was not exacted. Fig. 8. presents resistivity activity during heating and combustion test N3. The result shows that resistivity of this specimen changed chaotically.
0.4
ρ (Ω ⋅ m)
0.3 0.2 0.1
3.2. Discussion remarks
0
20
40
60 80 t (min)
100
120
Fig. 6. Resistivity activity during heating and combustion test (the brown coal specimen N1).
0.030
ρ (10 4 Ω ⋅ m)
0.025 0.020 0.015 0.010
Obtained experimental results let us assume that electro conductivity activity during heating and combusting test of coal specimen depends on normal water content value. Coal combusting makes for the following water transformation process: (a) Water content expulsion; (b) Water vapor relocation to outlying combustion zone; (c) Water vapor condensation; (d) Conducting channel (surface) forming. It the beginning of coal combusting (to 300–500 °C) abrupt decreasing of resistivity is observed. Resistivity gradient depends on coal normal water content. Some later, resistivity is increasing under the influence of thermal expansion of mineral spots. The thermal expansion of mineral spots changes the following coal parameters: (a) Porosity factor; (b) Electrical channel convolution; (c) Double electrical layer conductivity and solution conductivity relation.
0.005 4. Conclusions
0
20
40
60
80
100
120
140
160
t (min) Fig. 7. Resistivity activity during heating and combustion test (the coal specimen N2).
0.035
ρ (10 4 Ω ⋅m)
0.030 0.025 0.020 0.015 0.010 0.005 0
40
80
120 160 t (min)
200
240
Fig. 8. Resistivity activity during heating and combustion test (the coal specimen N3).
combustion test of the brown coal specimen N1. From this graph, the results show that for the first 15 min from the start of heating and combusting test the resistivity deeply decreased (the resistivity gradient was 0.3 for 1 min). Furthermore heating for 60 min, resistivity was relativity stable (the resistivity gradient was about 0.01 for 1 min) and then resistivity began to increase with gradient 0.3 for 1 min). The specimen was heated until 600–800°. Fig. 7 shows resistivity activity during heating and combustion test of the coal specimen N2. At opposed to the specimen N1 the specimen N2 had bigger specific weight and smaller degree of fracturing. Open porosity of
Carried out research and these results suggest that front-end of electro conductivity activity of coal specimen depend on heating and combusting. Combusting coal electro conductivity has complicated multistage type of change. Resistivity decreasing of brown coal samples at the start time of heating depended on water content expulsion and it relocation to outlying combustion zone. Later resistivity of brown coal decreased under water vapor condensation and conducting channel (surface) forming. Farther heating temperature growth made for electrical resistivity increased under water content reduction, thermal expansion of mineral spots and electrical channel convolution change. Unfortunately, the ccorrelation between heating temperature and electrical resistivity activity of coal specimen was not intercommunicated. Electrical resistivity method is expected to be a useful geophysical tool to for evaluation of combustion volume and its migration in the coal seam. In the near future, we are planning to develop electrical resistivity monitoring technology for evaluation of the ground combustion area. Acknowledgements This investigation was provided by the Ministry of Education and Science of Russian Federation (No. P1679), Far Eastern Federal University. The authors gratefully acknowledge their support. References [1] Bian Z, Inyang HI, Daniels JL, Otto F, Struthers S. Environmental issues from coal mining and their solutions. Min Sci Technol 2010;20(2):215–23. [2] Liu H, Feng C, Xia P, Yang K, Liu S. Method of oxygen-enriched two-stage underground coal gasification. Min Sci Technol 2011;21(2):191–6. [3] Liu S, Li J, Mei M, Dong D. Groundwater pollution from underground coal gasification. J Chin Univ Min Technol 2007;17(4):467–72. [4] Kondyrev BI, Belov AV, Grebenyk IV. Underground coal gasification experience in People’s Republic of China. Min Inf Anal Bull: Sci Tech J 2005;10:286–9. [5] Kreynin EV. Non-conventional terminal technologies of hard quarry fuel digging. Moscow: Gasprom Press; 2004.
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