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Effects of electrical resistance on the spontaneous combustion tendency of coal and the interaction matrix concept
Mineral
Journal of University of Science and TechnologyBeijing Volume 13, Number 1, February 2006, Page 1
Effects of electrical resistance on the spontaneous combustion tendency of coal and the interaction matrix concept O.S. Yildirim'),C. Sensogu?), and M.K. Gokay3) 1) Ministry of Energy, Ankara, Turkey 2) Mining Engineering Department,Dumlupinar University, Kutahya, Turkey
3) Mining Engineering Department, Selcuk University, Konya, Turkey (Received 2005-02-22)
Abstract: There have been several developments in determining the spontaneous combustion liability of coal. Most of the methods of concern have purely been based on the internal properties of the coal itself. The relation between the crossing-point method and the electrical resistance of coal was examined here to outline the spontaneous combustion tendency of coal. The electrical resistance property of coal was looked into as a decision-making parameter of the interaction matrix concept for the final decision on the spontaneous combustion tendency. Key words: spontaneous combustion; electrical resistance; interaction matrix
1. Introduction Coal itself as a material is a semiconductor and its electrical conductivity is changed in the range of 1x10-2-1x10'2 O m . The electrical conductivity of coal differs with respect to its direction of stratification of meaning that changes with the anisotropic properties of coal. Consequently, the specific resistance of coal to electricity is found to be dependent on many factors such as rank, temperature, voltage, water content, petrography, carbon content and the friability behavior of coal [l-lo]. The specific resistance of coal decreases when the voltage changes in the range of 0300 V. The increase in the water content of coal causes a decrease in its specific resistance. It is presented that the specific resistance to electricity decreases sharply when the moisture content of coal increases. This can be explained by the alkaline mineral content of coal. Alkaline mineral ions are not active in dry coal samples. When the moisture content of coal increases, ionic activities in coal increase causing an increase in electrical conductivity. Laboratory tests performed to measure the electrical conductivity of a coal sample showed significant differences according to conductivity measuring direction. This is possible due to coal stratification direction influencing the inorganic mineral content of coal [2]. In general, clay minerals constitute the main parts of the coal seams' inert mineral content. Clay minerals Corresponding author: C. Sensogut,E-mail: [email protected]
easily absorb water to their mineral composition. This affects the specific resistance of coal significantly [11. Specific resistance changes with temperature as well and it decreases with increasing temperature. The rate of decrease in specific resistance is significant for the temperature range of 0-80°C [2]. Macroscopic lithotopes, which are the petrographic elements of the texture of coal, are named as vitrain, clarain, durain, and fusain [ll]. Researchers reported that fusain has low specific resistance (1.0-lx104 Sz-m-') while vitrain, clarain, and durain have higher values of specific resistance to electricity. It is also shown that the specific resistance of coal increases when vitrain content increases and fusain content decreases for the temperature range of 20-400°C [ll. In addition to these factors, the inorganic content of coal consists of minerals and trace elements; they have three kinds of source in general [121, (1) original minerals (originated from vegetation), (2) primary minerals (crystallized during coalification), and (3) secondary minerals (exterior or re-crystallized minerals in coal seam). Minerals, carbonates, sulphates, chlorites, silicates, salts, oxides, and hydroxides can be the main inorganic content of any coal seam.
2. Experimental 2.1. Electrical resistivity It is a well-known fact that the resistance to elec-
J. Univ. Sci. Technol. Beijing, Vol13,No.Z, Feb 2006
2
tricity is directly related to the length of the wire or conductor. However, it is also indirectly related to the conductor’s cross-sectional area. In addition to these facts, the resistance depends also on the material properties of the metal that the conductor is made up of [13-141. The electrical resistance value of solid materials can be sorted into three main groups [151, (1) low-level resistance, 0-1 Q, (2) mid-level resistance, l-lx105Q, and (3) high-level resistance, >lx105Q. The “high-level” resistance is quite high, therefore, the materials that have high electrical resistance are also called “electrical isolators” [15]. 2.2. Measurement of electrical resistivity To determine the relation between the electrical re-
sistance and the spontaneous combustion tendency of coal, several laboratory tests were performed. The results of the electrical resistance measurements, chemical analyses, and spontaneous combustion index values of coal are given in Table 1. The spontaneous combustion tendency indices in Table 1 were determined by using ignitability techniques and crossingpoint approach [16-171. Test samples were first taken, from the coal mine, in hermetic boxes, and transported safely to the laboratory. They were, then, cut into prismatic blocks (1.5 cmxl cm, 5 cmx2.0 cm) by a diamond saw. The electrical resistance of the samples was then measured by using an ohmmeter connected to the prepared test set as seen in Fig. 1.
Table 1. Laboratory test results of the coal samples taken from different collieries of Turkish Coal Board
Test Samples
1
2
4 5
6
8
Colliery Aegean Lignites Aegean Lignites Park Lignites Western Lignites Western Lignites Western Lignites Western Lignites Western Lignites Western Lignites
Mine
Moisme/%
Ash/ wt%
Volatile matter 1 wt%
Fixed carbod
Total sulphurl
Liability index I
wt%
wt%
&-1
Risk groups
Electrical resistance of coal I Ma
Electrical resistance of solution/
31.638
333
14.331
1250
n
Darkale
20.7
34.71
45.34
19.4.5
4.94
9.2
Merkez
17.8
17.95
43.04
39.01
1.99
9.7
Mediumhigh Mediumhigh
Cayirhan
28.3
18.8
46.49
35.43
5.84
8.8
Medium
1.779
454
Tuncbilek-A
9.3
29.6
32.86
28.18
1.12
12.2
High
36.398
1526
Tuncbilek-B
12.3
15.52
33.58
38.60
1.02
11.6
High
36.398
555
Tuncbilek-C
10.3
30.5
30.79
28.87
1.49
14.0
High
47.275
1739
Omerler-A
14.8
18.01
35.74
31.40
1.22
15.2
High
21.994
1639
Omerler-B
16.2
16.76
28.36
38.68
4.27
8.5
Medium
2.438
344
Omerler-C
12.1
32.35
27.77
27.79
1.65
9.6
21.994
169
Medium-
hi
potential difference of 41.3 V was attributed to the sample by using an energy supplier (port 2). Electrical current passing through the coal specimen was measured by an ampere meter (port 1). As the electrical resistance of the test samples was very high at the pretest measurements, specially selected resistances (RS 200-1 MQ) were used (port 3). The electrical resistance value of each sample was then determined according to the electrical current passing through the samples. The measured values are given in Table 1. Fig. 1. Measurement of electrical resistance. 1- ampere meter; 2- energy supplier; kesistance; 6test port.
The resistance values obtained here were perpendicular to the coal stratification direction. Prismatic test samples were connected to the test port (port 4).A
2.3. Statistical analysis of the test results To interpret the laboratory test results, regression analysis was used to determine the relation between the electrical resistance values and the spontaneous combustion tendency index. The result of this analysis
O.S. Yildirim et al., Effects of electrical resistance on the spontaneous combustion tendency
...
3
led to the following equation, l/y = 0.1 14-(8.359~10")~
(1)
where y gives the tendency index and x is the electrical resistance value in this equation. The regression coefficient of their relation was found to be -0.70. The relation between these variables is presented in Fig. 2. As is seen from Eq. (l), an increase in the electrical resistance of coal causes an increase in tendency index. Fig. 3. Test apparatus to obtain the electrical resistivity of dissolved coal: l-energy source; 2-ampere meter; Wolution; 4- electrodes.
X
..-b 10.5 'T: c(
8.5 0
10 20 30 40 Electrical resistance of coal / MQ
50
Fig. 2. Relation between electrical resistance and tendency index.
2.4. Electrical resistance of dissolved coal
As seen in Fig. 3, two electrodes made up of copper plates with 3.5 cmx3.5 cm were put into the baker. The surface of the electrodes facing each other was prepared to be conductive. However, the other sides of the electrodes were covered with a special isolation material. After the test set was prepared, 20 V was passed to the test set, which was controlled by means of an energy supplier. Electrical current passing through each test solution was measured. Electrical resistance of each test sample was then calculated using Ohm law.
Electrical resistance differs due to coal stratification. The inorganic minerals in the stratified layers, fissures, and cracks affect electrical resistivity. As most of the alkaline inorganic minerals in coal seams are of secondary origin, that is, re-crystallized minerals, statistical analysis of the electrical resistance measurements of coal is impossible. Therefore, it was decided to measure the electrical resistance of dissolved coal samples. In this procedure, coal was first dissolved in water and then the resistance of the solution obtained was measured. The measured resistance values here were not affected by fissures and inorganic minerals as the test sample was a solution. When the results obtained from the solution test are compared with the resistance obtained from solid coal samples, the effects of discontinuities can be determined.
y=8.4393x10118.4565x
The test procedure followed here complied with the Turkish Standard 9106 [18]. The standard consists of the standardized test steps to obtain conductivity of coal seams and salt content in the coal seams. As a first step, coal samples were crushed and then they were ground up to particle sizes of -0.5 mm. To prepare a test solution, 5.0 g of ground coal was put into a test baker which contains 90 mL pure water. When all the test samples were prepared according to the standard procedure, they were shaken for 60 min by using an electrical shaker. After shaking, they were filtrated to separate the solid particle content. The filtrated specimens were then put into bakers of 100 mL for measurement. The electrical resistance of these solutions were measured by using the test apparatus seen in Fig.
In this equation, y represents the tendency index and x is the electrical resistance of the dissolved coal sample. The linear regression coefficient of this relation was found to be 0.83, which is high enough to define the relation. Graphical representation of this relation can be seen in Fig. 4.As observed from the graph, it can be concluded that the electrical resistance of the solution, which depends on the dissolved alkaline content of coal, has a correlation with the spontaneous combustion tendency of coal. When the electrical resistance of the sample is determined to be high, the spontaneous combustion tendency of the coal seam, from where the sample was taken, is expected to be high as well. The results obtained from the solution tests have similarities with the results shown in Fig. 3.
2.5. Statistical analysis of test values Just as a statistical analysis was performed for the solid coal test results, the test results obtained from the dissolved coal samples were also analyzed using statistical regression analysis. The relation that was tried to be set is between electrical resistance and spontaneous combustion tendency index. The values obtained from the tests are also listed in Table 1. The result deducted from the regression analysis is expressed as follow, (2)
J. Univ. Sci. Technol.Beijing, Vo1.13, No.1, Feb 2006
4
To consider other parameters that are effective on spontaneous combustion tendency, its mechanism was analyzed to list the effective parameters. When the control parameters such as electrical conductivity and resistance of the coal seam are determined, it becomes easier to interpret the relations between them.
cording to this procedure, the row and column members of the rated matrix were added to each other. The resultant added values were attributed as a co-ordinate of the decision parameters. Consequently, the decision parameters have the following type of co-ordinate values, Pi (total weighting of the ith row, total weighting of the ith column).
A
Electrical resistance of solution / kQ
Fig. 4. Relation between spontaneous combustion tendency index and electrical resistance of the dissolved coal samples.
/4p2i-a3 \ I p4
I
2.6. Interaction matrix application Hudson [19] presented the interaction matrix concept to analyze factors affecting the decisions on a defined engineering system. The parameters that are listed for a system can also be sorted according to their causes and effects on the system. The interaction matrix concept [20] is based on the definition of the parameters' interrelations two by two. These relations are also called pair-wise relations. The main point here is the definition of the relation according to the experience of the domain experts. However, the logical relations that are observed in the field should be interpreted here with numerical values, weighting values, to analyze the defined decision conditions by using mathematical procedures. To standardize the procedure, Hudson presented a weighting scale to weight logical relation [21-221. Fig. 5 shows the interaction matrix mapped for spontaneous combustion tendency of a typical coal seam. To obtain this weighted matrix, first, decision parameters effective on spontaneous combustion were determined. They were selected to confine the decision environment and they were assigned to the diagonal members of the matrix. Off-diagonal members of the matrix were then assigned to the weighting values representing the cause or the effect relations between the diagonal members. The interactions of the parameters were defined as critical, strong, medium, poor, and nonrelative to each other. The weighting values of 4, 3, 2, 1, and 0 were assigned to them, respectively. The interaction matrix was then presented as a procedure to be used in engineering projects. Ac-
I
4
12
'
I
20 Cause
28
36
Fig. 5. Interaction matrix evaluation, decision map for defined parameters.
After determining the co-ordinate values for each decision parameter, they were plotted as an x-y graph (Fig. 6). Interpretation of this graph is detailed in literatures [19,21-231. If the parameter has the coordinate value that is plotted as a point located around the upper-right corner of the graph, it is interpreted as critically interactive decision parameters. Similarly, if the co-ordinate gives a plotting point located around the lower-right corner, it shows the parameters dominant on the defined decision environment. Fig. 6 presents the cause-effect graph of the parameters that were selected to analyze spontaneous combustion tendency. According to this graph some parameters (shown in Table 2) were selected according to their level of interaction. These results show the criticality and dominance level of the parameters according to the interaction matrix rating during this study (Fig. 5). According to the experiences gained, the rank of coal is found to be very important and critical for the spontaneous combustion tendency. Electrical resistance occupies the second level on the criticality list. Therefore, it is interpreted as a highly interactive decision parameter on spontaneous combustion. Electrical resistance is deduced to be a dominated parameter in this decision environment.
O.S. Yildirim et al., Effects of electrical resistance on the spontaneous combustion tendency
13
12
11
8
13
12
17
19
9
22
...
5
35
Rank
13 15
Temperature Moisture
10 15
Sulphur Other Minerals
11 11
Porosity Thermal Conductivity
8 2 16
Friability Bacteria Electrical Resistance
P=
13.6
Fig. 6. Cause-effect graph of the given decision mapping in Fig. 5. Table 2. Parameters selected according to their level of interaction Sorted dominance (max. to min.) 1. Rank 6. Porosity 2. Moisture 7. Electrical resistance 3. Other minerals 8. Thermal conductivity 4. Sulphur content 9. Bacteria 5. Temperature 10. Friability
Sorted criticality (max. to min.) 6. Friability 2. Electrical resistance 7. Temperature 3. Other minerals 8. Porosity 4. Thermal conductivity 9. Sulphur content 5. Moisture 10. Bacteria 1. Rank
3. Conclusions
References
The test results showed that the relation between electrical resistance and spontaneous combustion tendency can be determined using statistical analysis. The results showed that when the electrical resistance of a coal seam sample is determined as high resistive, the spontaneous combustion tendency index and its risk level for that coal seam are determined to be higher as well. It was also resolved that a solid coal sample’s electrical resistance is affected by inorganic mineral content. Their total effects on the resistivity of coal should be defined to interpret the relation between the electrical resistivity of coal and its combustion tendency. In this study, tests were performed using solid coal samples and samples obtained by their dissolution in water. The test results showed that coal and its inorganic and alkaline content dissolved in water with a relatively low ratio because of a higher electrical resistance.
[l] A.M. Elliott, Chemistry of Coal Utilisation, Wiley, New York, 1981. [2] N.S. Tiwory and A. Mukhdeo, Measurement of electrical resistivity of coal samples, Fuel, 1993, No.72, p.1099. [3] V. Mahadevan and A.M. Ramlu, Fire risk rating of coal mines due to spontaneous heating, J. Mines Met. Fuels, 1985, p.357. [4] M. Guney, Certain factors affecting the oxidation and spontaneous combustion of coal, Min. SOC. Mag., 1968, p.71. [5] N. Eroglu and J.M. Gouws, Theories related to spontaneous combustion of coal, Min. J. Chamber Min. Eng. Turkey (in Turkish), 1993, No.32, p.13. [6] V. Didari, Spontaneous combustion of coal in underground collieries and risk iIndices, Min. J. Chamber Min. Eng. Turkey (in Turkish), 1986, No.15, p.29. [7] R.Morris and T. Atkinson, Seam factor and spontaneous heating of coal, Min. Sci. Technol., 1988, No.”, p.149. [8] R.N. Sing, S. Demirbilek, and M. Turney, Application of spontaneous combustion risk index to mine planning, safe storage and shipment of coal, J. Mines Met. Fuels, 1984, No.7, p.347. [9] N. Berkowitz, An Introduction to Coal Technology, Academic Press, New York, 1979, p.345. [lo] C. Sensogut and I. Cinar, A research on the tendency of Ermenek district coals to spontaneous combustion, Miner. Resour. Eng., 9(2000), No.4, p.421. [ 113 E. Nakoman, Coal (in Turkish), Technical Notes, University of Dokuz Eylul, MM-IED-85-081, 1985, p.348. [ 121 0. Kural, Technology and Chemistry of CoaZ (in Turkish), Ozgun Printing, Istanbul, 1989, p.657. [ 131 E. Hughes, Electrical Technology, Prentice-Hall, New Jersey, 1988.
Electrical resistance is considered to be one of the main decision parameters on spontaneous combustion tendency and it is considered with nine other parameters to form interaction matrix evaluation. According to the field experience, electrical resistance is interpreted as the second critical parameter among the selected ten decision parameters. Consequently, it was decided to continue the research on electrical resistance until uncertain information became clear. This will help mine managers when they set out to explain the liability of coal seam to spontaneous combustion.
6 [14] N. Tirben, Electricity - Laboratory Manual (in Turkish), Publication of Ministry of Education, 1979. [151 K. Anasiz, Electricity Measurement Apparatus and Electrical Measurements (in Turkish), Publication of Ministry of Education, 1977. [16] S. Sarac, Spontaneous combustion tendency of turkish lignites, Geosound, 1993, No.6, p.39. [17] C. Sensogut and I. Cinar, Spontaneous combustion liability of turkish coals - a case study, [in] Int. Symp. on Mine Planning and Equipment Selection, Ukraine, 1999, p.421. [18] TS 9106, Coal-Value of Electrical Conductivity and Determination of Salt Content,TSE Press, 1991, p.4. [19] J.A. Hudson, Atlas of rock engineering mechanisms: un-
J. Univ. Sci. Technol. Beijing, vol.13, No.1, Feb 2006 derground excavations, Int. J. Rock Mech. Sci. Geomech. Abstr., 1991, p.523. [20] M.Q. Phan, R.W. Longman, S.C. Lee, et al., System identification from multiple-trial corrupted by non-repeating periodic disturbances, Int. J. Appl. Math. Comput. Sci., 13(2003),N0.2, p.185. [21] J.A. Hudson, Rock Engineering Systems, Theory and Practice, Ellis Honvood, Chichester, 1992. [22] J.A. Hudson and Y . Jiao, Analysis of Rock Engineering Projects, Elsevier, 2000, p.300. [23] M.K. Gokay, Evaluation of project parameters and interaction matrix method, J. Turk. SOC.Rock Mech., 1993, No.9, p.19.
Journal of University of Science and TechnologyBeijing Volume 13, Number 1, February 2006, Page 1
Effects of electrical resistance on the spontaneous combustion tendency of coal and the interaction matrix concept O.S. Yildirim'),C. Sensogu?), and M.K. Gokay3) 1) Ministry of Energy, Ankara, Turkey 2) Mining Engineering Department,Dumlupinar University, Kutahya, Turkey
3) Mining Engineering Department, Selcuk University, Konya, Turkey (Received 2005-02-22)
Abstract: There have been several developments in determining the spontaneous combustion liability of coal. Most of the methods of concern have purely been based on the internal properties of the coal itself. The relation between the crossing-point method and the electrical resistance of coal was examined here to outline the spontaneous combustion tendency of coal. The electrical resistance property of coal was looked into as a decision-making parameter of the interaction matrix concept for the final decision on the spontaneous combustion tendency. Key words: spontaneous combustion; electrical resistance; interaction matrix
1. Introduction Coal itself as a material is a semiconductor and its electrical conductivity is changed in the range of 1x10-2-1x10'2 O m . The electrical conductivity of coal differs with respect to its direction of stratification of meaning that changes with the anisotropic properties of coal. Consequently, the specific resistance of coal to electricity is found to be dependent on many factors such as rank, temperature, voltage, water content, petrography, carbon content and the friability behavior of coal [l-lo]. The specific resistance of coal decreases when the voltage changes in the range of 0300 V. The increase in the water content of coal causes a decrease in its specific resistance. It is presented that the specific resistance to electricity decreases sharply when the moisture content of coal increases. This can be explained by the alkaline mineral content of coal. Alkaline mineral ions are not active in dry coal samples. When the moisture content of coal increases, ionic activities in coal increase causing an increase in electrical conductivity. Laboratory tests performed to measure the electrical conductivity of a coal sample showed significant differences according to conductivity measuring direction. This is possible due to coal stratification direction influencing the inorganic mineral content of coal [2]. In general, clay minerals constitute the main parts of the coal seams' inert mineral content. Clay minerals Corresponding author: C. Sensogut,E-mail: [email protected]
easily absorb water to their mineral composition. This affects the specific resistance of coal significantly [11. Specific resistance changes with temperature as well and it decreases with increasing temperature. The rate of decrease in specific resistance is significant for the temperature range of 0-80°C [2]. Macroscopic lithotopes, which are the petrographic elements of the texture of coal, are named as vitrain, clarain, durain, and fusain [ll]. Researchers reported that fusain has low specific resistance (1.0-lx104 Sz-m-') while vitrain, clarain, and durain have higher values of specific resistance to electricity. It is also shown that the specific resistance of coal increases when vitrain content increases and fusain content decreases for the temperature range of 20-400°C [ll. In addition to these factors, the inorganic content of coal consists of minerals and trace elements; they have three kinds of source in general [121, (1) original minerals (originated from vegetation), (2) primary minerals (crystallized during coalification), and (3) secondary minerals (exterior or re-crystallized minerals in coal seam). Minerals, carbonates, sulphates, chlorites, silicates, salts, oxides, and hydroxides can be the main inorganic content of any coal seam.
2. Experimental 2.1. Electrical resistivity It is a well-known fact that the resistance to elec-
J. Univ. Sci. Technol. Beijing, Vol13,No.Z, Feb 2006
2
tricity is directly related to the length of the wire or conductor. However, it is also indirectly related to the conductor’s cross-sectional area. In addition to these facts, the resistance depends also on the material properties of the metal that the conductor is made up of [13-141. The electrical resistance value of solid materials can be sorted into three main groups [151, (1) low-level resistance, 0-1 Q, (2) mid-level resistance, l-lx105Q, and (3) high-level resistance, >lx105Q. The “high-level” resistance is quite high, therefore, the materials that have high electrical resistance are also called “electrical isolators” [15]. 2.2. Measurement of electrical resistivity To determine the relation between the electrical re-
sistance and the spontaneous combustion tendency of coal, several laboratory tests were performed. The results of the electrical resistance measurements, chemical analyses, and spontaneous combustion index values of coal are given in Table 1. The spontaneous combustion tendency indices in Table 1 were determined by using ignitability techniques and crossingpoint approach [16-171. Test samples were first taken, from the coal mine, in hermetic boxes, and transported safely to the laboratory. They were, then, cut into prismatic blocks (1.5 cmxl cm, 5 cmx2.0 cm) by a diamond saw. The electrical resistance of the samples was then measured by using an ohmmeter connected to the prepared test set as seen in Fig. 1.
Table 1. Laboratory test results of the coal samples taken from different collieries of Turkish Coal Board
Test Samples
1
2
4 5
6
8
Colliery Aegean Lignites Aegean Lignites Park Lignites Western Lignites Western Lignites Western Lignites Western Lignites Western Lignites Western Lignites
Mine
Moisme/%
Ash/ wt%
Volatile matter 1 wt%
Fixed carbod
Total sulphurl
Liability index I
wt%
wt%
&-1
Risk groups
Electrical resistance of coal I Ma
Electrical resistance of solution/
31.638
333
14.331
1250
n
Darkale
20.7
34.71
45.34
19.4.5
4.94
9.2
Merkez
17.8
17.95
43.04
39.01
1.99
9.7
Mediumhigh Mediumhigh
Cayirhan
28.3
18.8
46.49
35.43
5.84
8.8
Medium
1.779
454
Tuncbilek-A
9.3
29.6
32.86
28.18
1.12
12.2
High
36.398
1526
Tuncbilek-B
12.3
15.52
33.58
38.60
1.02
11.6
High
36.398
555
Tuncbilek-C
10.3
30.5
30.79
28.87
1.49
14.0
High
47.275
1739
Omerler-A
14.8
18.01
35.74
31.40
1.22
15.2
High
21.994
1639
Omerler-B
16.2
16.76
28.36
38.68
4.27
8.5
Medium
2.438
344
Omerler-C
12.1
32.35
27.77
27.79
1.65
9.6
21.994
169
Medium-
hi
potential difference of 41.3 V was attributed to the sample by using an energy supplier (port 2). Electrical current passing through the coal specimen was measured by an ampere meter (port 1). As the electrical resistance of the test samples was very high at the pretest measurements, specially selected resistances (RS 200-1 MQ) were used (port 3). The electrical resistance value of each sample was then determined according to the electrical current passing through the samples. The measured values are given in Table 1. Fig. 1. Measurement of electrical resistance. 1- ampere meter; 2- energy supplier; kesistance; 6test port.
The resistance values obtained here were perpendicular to the coal stratification direction. Prismatic test samples were connected to the test port (port 4).A
2.3. Statistical analysis of the test results To interpret the laboratory test results, regression analysis was used to determine the relation between the electrical resistance values and the spontaneous combustion tendency index. The result of this analysis
O.S. Yildirim et al., Effects of electrical resistance on the spontaneous combustion tendency
...
3
led to the following equation, l/y = 0.1 14-(8.359~10")~
(1)
where y gives the tendency index and x is the electrical resistance value in this equation. The regression coefficient of their relation was found to be -0.70. The relation between these variables is presented in Fig. 2. As is seen from Eq. (l), an increase in the electrical resistance of coal causes an increase in tendency index. Fig. 3. Test apparatus to obtain the electrical resistivity of dissolved coal: l-energy source; 2-ampere meter; Wolution; 4- electrodes.
X
..-b 10.5 'T: c(
8.5 0
10 20 30 40 Electrical resistance of coal / MQ
50
Fig. 2. Relation between electrical resistance and tendency index.
2.4. Electrical resistance of dissolved coal
As seen in Fig. 3, two electrodes made up of copper plates with 3.5 cmx3.5 cm were put into the baker. The surface of the electrodes facing each other was prepared to be conductive. However, the other sides of the electrodes were covered with a special isolation material. After the test set was prepared, 20 V was passed to the test set, which was controlled by means of an energy supplier. Electrical current passing through each test solution was measured. Electrical resistance of each test sample was then calculated using Ohm law.
Electrical resistance differs due to coal stratification. The inorganic minerals in the stratified layers, fissures, and cracks affect electrical resistivity. As most of the alkaline inorganic minerals in coal seams are of secondary origin, that is, re-crystallized minerals, statistical analysis of the electrical resistance measurements of coal is impossible. Therefore, it was decided to measure the electrical resistance of dissolved coal samples. In this procedure, coal was first dissolved in water and then the resistance of the solution obtained was measured. The measured resistance values here were not affected by fissures and inorganic minerals as the test sample was a solution. When the results obtained from the solution test are compared with the resistance obtained from solid coal samples, the effects of discontinuities can be determined.
y=8.4393x10118.4565x
The test procedure followed here complied with the Turkish Standard 9106 [18]. The standard consists of the standardized test steps to obtain conductivity of coal seams and salt content in the coal seams. As a first step, coal samples were crushed and then they were ground up to particle sizes of -0.5 mm. To prepare a test solution, 5.0 g of ground coal was put into a test baker which contains 90 mL pure water. When all the test samples were prepared according to the standard procedure, they were shaken for 60 min by using an electrical shaker. After shaking, they were filtrated to separate the solid particle content. The filtrated specimens were then put into bakers of 100 mL for measurement. The electrical resistance of these solutions were measured by using the test apparatus seen in Fig.
In this equation, y represents the tendency index and x is the electrical resistance of the dissolved coal sample. The linear regression coefficient of this relation was found to be 0.83, which is high enough to define the relation. Graphical representation of this relation can be seen in Fig. 4.As observed from the graph, it can be concluded that the electrical resistance of the solution, which depends on the dissolved alkaline content of coal, has a correlation with the spontaneous combustion tendency of coal. When the electrical resistance of the sample is determined to be high, the spontaneous combustion tendency of the coal seam, from where the sample was taken, is expected to be high as well. The results obtained from the solution tests have similarities with the results shown in Fig. 3.
2.5. Statistical analysis of test values Just as a statistical analysis was performed for the solid coal test results, the test results obtained from the dissolved coal samples were also analyzed using statistical regression analysis. The relation that was tried to be set is between electrical resistance and spontaneous combustion tendency index. The values obtained from the tests are also listed in Table 1. The result deducted from the regression analysis is expressed as follow, (2)
J. Univ. Sci. Technol.Beijing, Vo1.13, No.1, Feb 2006
4
To consider other parameters that are effective on spontaneous combustion tendency, its mechanism was analyzed to list the effective parameters. When the control parameters such as electrical conductivity and resistance of the coal seam are determined, it becomes easier to interpret the relations between them.
cording to this procedure, the row and column members of the rated matrix were added to each other. The resultant added values were attributed as a co-ordinate of the decision parameters. Consequently, the decision parameters have the following type of co-ordinate values, Pi (total weighting of the ith row, total weighting of the ith column).
A
Electrical resistance of solution / kQ
Fig. 4. Relation between spontaneous combustion tendency index and electrical resistance of the dissolved coal samples.
/4p2i-a3 \ I p4
I
2.6. Interaction matrix application Hudson [19] presented the interaction matrix concept to analyze factors affecting the decisions on a defined engineering system. The parameters that are listed for a system can also be sorted according to their causes and effects on the system. The interaction matrix concept [20] is based on the definition of the parameters' interrelations two by two. These relations are also called pair-wise relations. The main point here is the definition of the relation according to the experience of the domain experts. However, the logical relations that are observed in the field should be interpreted here with numerical values, weighting values, to analyze the defined decision conditions by using mathematical procedures. To standardize the procedure, Hudson presented a weighting scale to weight logical relation [21-221. Fig. 5 shows the interaction matrix mapped for spontaneous combustion tendency of a typical coal seam. To obtain this weighted matrix, first, decision parameters effective on spontaneous combustion were determined. They were selected to confine the decision environment and they were assigned to the diagonal members of the matrix. Off-diagonal members of the matrix were then assigned to the weighting values representing the cause or the effect relations between the diagonal members. The interactions of the parameters were defined as critical, strong, medium, poor, and nonrelative to each other. The weighting values of 4, 3, 2, 1, and 0 were assigned to them, respectively. The interaction matrix was then presented as a procedure to be used in engineering projects. Ac-
I
4
12
'
I
20 Cause
28
36
Fig. 5. Interaction matrix evaluation, decision map for defined parameters.
After determining the co-ordinate values for each decision parameter, they were plotted as an x-y graph (Fig. 6). Interpretation of this graph is detailed in literatures [19,21-231. If the parameter has the coordinate value that is plotted as a point located around the upper-right corner of the graph, it is interpreted as critically interactive decision parameters. Similarly, if the co-ordinate gives a plotting point located around the lower-right corner, it shows the parameters dominant on the defined decision environment. Fig. 6 presents the cause-effect graph of the parameters that were selected to analyze spontaneous combustion tendency. According to this graph some parameters (shown in Table 2) were selected according to their level of interaction. These results show the criticality and dominance level of the parameters according to the interaction matrix rating during this study (Fig. 5). According to the experiences gained, the rank of coal is found to be very important and critical for the spontaneous combustion tendency. Electrical resistance occupies the second level on the criticality list. Therefore, it is interpreted as a highly interactive decision parameter on spontaneous combustion. Electrical resistance is deduced to be a dominated parameter in this decision environment.
O.S. Yildirim et al., Effects of electrical resistance on the spontaneous combustion tendency
13
12
11
8
13
12
17
19
9
22
...
5
35
Rank
13 15
Temperature Moisture
10 15
Sulphur Other Minerals
11 11
Porosity Thermal Conductivity
8 2 16
Friability Bacteria Electrical Resistance
P=
13.6
Fig. 6. Cause-effect graph of the given decision mapping in Fig. 5. Table 2. Parameters selected according to their level of interaction Sorted dominance (max. to min.) 1. Rank 6. Porosity 2. Moisture 7. Electrical resistance 3. Other minerals 8. Thermal conductivity 4. Sulphur content 9. Bacteria 5. Temperature 10. Friability
Sorted criticality (max. to min.) 6. Friability 2. Electrical resistance 7. Temperature 3. Other minerals 8. Porosity 4. Thermal conductivity 9. Sulphur content 5. Moisture 10. Bacteria 1. Rank
3. Conclusions
References
The test results showed that the relation between electrical resistance and spontaneous combustion tendency can be determined using statistical analysis. The results showed that when the electrical resistance of a coal seam sample is determined as high resistive, the spontaneous combustion tendency index and its risk level for that coal seam are determined to be higher as well. It was also resolved that a solid coal sample’s electrical resistance is affected by inorganic mineral content. Their total effects on the resistivity of coal should be defined to interpret the relation between the electrical resistivity of coal and its combustion tendency. In this study, tests were performed using solid coal samples and samples obtained by their dissolution in water. The test results showed that coal and its inorganic and alkaline content dissolved in water with a relatively low ratio because of a higher electrical resistance.
[l] A.M. Elliott, Chemistry of Coal Utilisation, Wiley, New York, 1981. [2] N.S. Tiwory and A. Mukhdeo, Measurement of electrical resistivity of coal samples, Fuel, 1993, No.72, p.1099. [3] V. Mahadevan and A.M. Ramlu, Fire risk rating of coal mines due to spontaneous heating, J. Mines Met. Fuels, 1985, p.357. [4] M. Guney, Certain factors affecting the oxidation and spontaneous combustion of coal, Min. SOC. Mag., 1968, p.71. [5] N. Eroglu and J.M. Gouws, Theories related to spontaneous combustion of coal, Min. J. Chamber Min. Eng. Turkey (in Turkish), 1993, No.32, p.13. [6] V. Didari, Spontaneous combustion of coal in underground collieries and risk iIndices, Min. J. Chamber Min. Eng. Turkey (in Turkish), 1986, No.15, p.29. [7] R.Morris and T. Atkinson, Seam factor and spontaneous heating of coal, Min. Sci. Technol., 1988, No.”, p.149. [8] R.N. Sing, S. Demirbilek, and M. Turney, Application of spontaneous combustion risk index to mine planning, safe storage and shipment of coal, J. Mines Met. Fuels, 1984, No.7, p.347. [9] N. Berkowitz, An Introduction to Coal Technology, Academic Press, New York, 1979, p.345. [lo] C. Sensogut and I. Cinar, A research on the tendency of Ermenek district coals to spontaneous combustion, Miner. Resour. Eng., 9(2000), No.4, p.421. [ 113 E. Nakoman, Coal (in Turkish), Technical Notes, University of Dokuz Eylul, MM-IED-85-081, 1985, p.348. [ 121 0. Kural, Technology and Chemistry of CoaZ (in Turkish), Ozgun Printing, Istanbul, 1989, p.657. [ 131 E. Hughes, Electrical Technology, Prentice-Hall, New Jersey, 1988.
Electrical resistance is considered to be one of the main decision parameters on spontaneous combustion tendency and it is considered with nine other parameters to form interaction matrix evaluation. According to the field experience, electrical resistance is interpreted as the second critical parameter among the selected ten decision parameters. Consequently, it was decided to continue the research on electrical resistance until uncertain information became clear. This will help mine managers when they set out to explain the liability of coal seam to spontaneous combustion.
6 [14] N. Tirben, Electricity - Laboratory Manual (in Turkish), Publication of Ministry of Education, 1979. [151 K. Anasiz, Electricity Measurement Apparatus and Electrical Measurements (in Turkish), Publication of Ministry of Education, 1977. [16] S. Sarac, Spontaneous combustion tendency of turkish lignites, Geosound, 1993, No.6, p.39. [17] C. Sensogut and I. Cinar, Spontaneous combustion liability of turkish coals - a case study, [in] Int. Symp. on Mine Planning and Equipment Selection, Ukraine, 1999, p.421. [18] TS 9106, Coal-Value of Electrical Conductivity and Determination of Salt Content,TSE Press, 1991, p.4. [19] J.A. Hudson, Atlas of rock engineering mechanisms: un-
J. Univ. Sci. Technol. Beijing, vol.13, No.1, Feb 2006 derground excavations, Int. J. Rock Mech. Sci. Geomech. Abstr., 1991, p.523. [20] M.Q. Phan, R.W. Longman, S.C. Lee, et al., System identification from multiple-trial corrupted by non-repeating periodic disturbances, Int. J. Appl. Math. Comput. Sci., 13(2003),N0.2, p.185. [21] J.A. Hudson, Rock Engineering Systems, Theory and Practice, Ellis Honvood, Chichester, 1992. [22] J.A. Hudson and Y . Jiao, Analysis of Rock Engineering Projects, Elsevier, 2000, p.300. [23] M.K. Gokay, Evaluation of project parameters and interaction matrix method, J. Turk. SOC.Rock Mech., 1993, No.9, p.19.