Effects of electrical resistance on the spontaneous combustion tendency of coal and the interaction

Journal of University of Science and Technology BeijingVolume 13, Number 1, February 2006, Page IMineralEffects of electrical resistance on the spontaneous combustion tendencyof coal and the interaction matrix conceptO.S. Yildirim", C. Sensogut, and M.K. Gokay1)Ministry of Energy, Ankara, Turkey2)Mining Engineering Department, Dumlupinar University, Kutahya, Turkey3)Mining Engineering Department, Selcuk University, Konya, Turkey( Received 2005-02Abstract: There have been several developments in determining the spontaneous combustion liability of coal. Most of the methodsof concern have purely been based on the internal properties of the coal itself. The relation between the crossing-point method andthe electrical resistance of coal was examined here to outline the spontaneous combustion tendency of coal. The electrical resistanceproperty of coal was looked into as a decision-making parameter of the interaction matrix concept for the final decision on the spon-taneous combustion tendencyKey words: spontaneous combustion; electrical resistance; interaction matrix1. Introductioneasily absorb water to their mineral composition. Thisaffects the specific resistance of coal significantly [1]Coal itself as a material is a semiconductor and its Specific resistance changes with temperature as wellelectrical conductivity is changed in the range of and it decreases with increasing temperature. The rate1x10-2-1x102Q2m. The electrical conductivity of of decrease in specific resistance is significant for thecoal differs with respect to its direction of stratificatemperature range of 0-80%C [2]tion of meaning that changes with the anisotropicproperties of coal. Consequently, the specific resis-pic lithotopes, which are the petrographictance of coal to electricity is found to be dependent onelements of the texture of coal. are named as vitrainmany factors such as rank, temperature, voltage, waterclarain, durain, and fusain [11]. Researchers reportedcontent, petrography, carbon content and the friabilitythat fusain has low specific resistance (1.0-1x10behavior of coal [1-10. The specific resistance of coalQ m)while vitrain, clarain, and durain have higherdecreases when the voltage changes in the range of 0- values of specific resistance to electricity. It is also300 V. The increase in the water content of coal shown that the specific resistance of coal increasescauses a decrease in its specific resistance. It is pre- when vitrain content increases and fusain content de-sented that the specific resistance to electricity decreases for the temperature range of 20-400C [1]. Increases sharply when the moisture content of coal in-addition to these factors, the inorganic content of coalcreases. This can be explained by the alkaline mineral consists of minerals and trace elements; they havecontent of coal. Alkaline mineral ions are not active inthree kinds of source in general [12],(1)original min-dry coal samples. When the moisture content of coal erals(originated from vegetation ),( 2)primary miner-increases, ionic activities in coal increase causing anals(crystallized during coalification), and (3)seconncrease in electrical conductivity. Laboratory testsdary minerals(exterior or re-crystallized minerals inperformed to measure the electrical conductivity of acoal seam)coal sample showed significant differences accordinMinerals, carbonates, sulphates, chlorites, silicates,to conductivity measuring direction. This is possible salts, oxides, and hydroxides can be the main inor-due to coal stratification direction influencing the ganic content of any coal seaminorganic mineral content of coal [2]2. Experime中国煤化工In general, clay minerals constitute the main parts 2.1. ElectricalCNMHGof the coal seams' inert mineral content. Clay mineralsIt is a welllat uc resistance to elec-Corresponding author: C, Sensogut, E-mail: sensogut@ mail dumlupinar.edu, tr2J. Univ. Sci. Technol. Beijing, VoL 13, No 1, Feb 2006tricity is directly related to the length of the wire or sistance and the spontaneous combustion tendency ofconductor. However, it is also indirectly related to the coal, several laboratory tests were performed. The reconductor's cross-sectional area. In addition to these sults of the electrical resistance measurements chemipends also on the material cal analyses, and spontaneous combustion index val-properties of the metal that the conductor is made up ues of coal are given in Table 1. The spontaneousof [13-14]. The electrical resistance value of solid combustion tendency indices in Table I were determaterials can be sorted into three main groups [15], (1) mined by using ignitability techniques and crossing-low-level resistance, 0-1 Q2,(2)mid-level resistance, point approach [16-17]. Test samples were first taken,1-1x10Q2, and (3)high-level resistance, >1x10Q2from the coal mine, in hermetic boxes, and transportedThe"high-level"resistance is quite high, therefore, safely to the laboratory. They were, then, cut intothe materials that have high electrical resistance are prismatic blocks(1.5 cmx1 cm, 5 cmx2.0 cm) by aalso called"electrical isolators'151diamond saw. The electrical resistance of the samples2.2. Measurement of electrical resistivitywas then measured by using an ohmmeter connectedTo determine the relation between the electrical reto the prepared test set as seen in Fig. ITable 1. Laboratory test results of the coal samples taken from different collieries of Turkish Coal boardVolatileFixed Totallatter/ carhElectrical Electricalresistance resistanceMineSamplesture/% wt%bon/ phuof coal/ of solution/wto wt%oMedium-0.734.7145.3419454.949.231.638LignitesAegeanMerkez17.8179543.0439011.999.714.3311250ParkCayirhan28.318.8464935.435848.8 Medium1.779454WesternTuncbilek-A9329.6328628.181.12122HiLiesternTuncbilek-B 12.35.5233.5838.601.0211.6High36.398555Lignites Tuncbilek-C 10.3 30.5 30.79 28.87 1.49 14.0 High47.275Omerler-A14.8180135.7431.401.2215.221,99416398 Lignites Omerler-B16.216.76283638.684.278.5 Mediun2.438344WesternMedium-Omerler-C12.132.3527.7727.791.6:9.621994potential difference of 41.3 v was attributed to thesample by using an energy supplier(port 2). Electriccurrent passing through the coal specimen was meaured by an ampere meter(port 1). As the electrical resistance of the test samples was very high at the pretest measurements, specially selected resistances (RS200-1 MQ2) were used (port 3). The electrical resistance value of each sample was then determined ac-cording to the electrical current passing through thesamples. The measured values are given in TableFig. 1. Measurement of electrical resistance. 1-ampere 2.3. Statistical analysis of the test resultsmeter; 2energy supplier; 3--resistance; 4test port.To interpretlaboratory test results, regressionThe resistance values obtained here were perpen- analysis was u中国煤化工 tion betweendicular to the coal stratification direction. PrismaticCNMH SPontaneoutest samples were connected to the test port (port 4). a combustion tendency index ' Ihe result of this analysisO.S. Yildirim et al, Effects of electrical resistance on the spontaneous combustion tendencyled to the following equation,l/y=0.114-(8.359×104)xwhere y gives the tendency index and x is the electrical resistance value in this equation. The regressioncoefficient of their relation was found to be -0.70The relation between these variables is presented inFig. 2. As is seen from Eq. (1), an increase in theelectrical resistance of coal causes an increase in ten3dency indexFig 3. Test apparatus to obtain the electrical resistivity ofdissolved coal: 1-eney3--solution: electrodes.As seen in Fig. 3, two electrodes made up of coppplates with 3.5 cmx3. 5 cm were put into the bakerElectrical resistance of coal/MQThe surface of the electrodes facing each other wasFig. 2. Relation between electrical resistance and tendency prepared to be conductive. However, the other sides ofthe electrodes were covered with a special isolationmaterial. After the test set was prepared, 20 V was2. 4. Electrical resistance of dissolved coalpassed to the test set, which was controlled by meansElectrical resistance differs due to coal stratificationupplier. Electthrough each test solution was measured. ElectricalThe inorganic minerals in the stratified layers, fissures, resistance of each test sample was then calculated usand cracks affect electrical resistivity. As most of theing ohm lawalkaline inorganic minerals in coal seams are of secondary origin, that is, re-crystallized minerals, statisti- 2.5. Statistical analysis of test valuescal analysis of the electrical resistance measurementsJust as a statistical analysis was performed for theof coal is impossible. Therefore, it was decided to solid coal test results. the test results obtained from themeasure the electrical resistance of dissolved coal dissolved coal samples were also analyzed using staamples. In this procedure, coal was first dissolved in tistical regression analysis. The relation that was triedwater and then the resistance of the solution obtainedto be set is between electrical resistance and spontaneous combustion tendency index. The values obtainedwere not affected by fissures and inorganic minerals from the tests are also listed in Table 1. The result deas the test sample was a solution. When the results ducted from the regression analysis is expressed asobtained from the solution test are compared with the followresistance obtained from solid coal samples, the ef-fects of discontinuities can be determinedy=8.4393×10184565The test procedure followed here complied with theIn this equation, y represents the tendency indexTurkish Standard 9106 [18]. The standard consists of and x is the electrical resistance of the dissolved coalhe standardized test steps to obtain conductivity of sample. The linear regression coefficient of this relacoal seams and salt content in the coal seams. Astion was found to be 0.83, which is high enough to defirst step, coal samples were crushed and then they fine the relation. Graphical representation of this relawere ground up to particle sizes of -0.5 mm. To pre- tion can be seen in Fig 4. As observed from the graphpare a test solution, 5.0 g of ground coal was put into a it can be concluded that the electrical resistance of thetest baker which contains 90 mL pure water. When all solution, which depends on the dissolved alkalinehe test samples were prepared according to the stan- content of coal, has a correlation with the spontaneousdard procedure, they were shaken for 60 min by using combustion tendency of coal. When the electrical rean electrical shaker. After shaking, they were filtrated sistance of the sample is determined to be high, theto separate the solid particle content. The filtrated spe- spontaneous combustion tendency of the coal seam,cimens were then put into bakers of 100 mL for meas- from where中国煤化工 pected to beurement.The electrical resistance of these solutions highCNMHGthe solutionwere measured by using the test apparatus seen in Fig. tests have similarities with the results shown in Fig 3J. Univ. Sci. Technol. Beijing, VoL 13, No 1, Feb 2006To consider other parameters that are effective on cording to this procedure, the row and column mem-spontaneous combustion tendency, its mechanismbers of the rated matrix were added to each other Theanalyzed to list the effective parameters. Whenresultant added values were attributed as a co-ordinatecontrol parameters such as electrical conductivity and of the decision parameters. Consequently, the decisionresistance of the coal seam are determined, it becomes parameters have the following type of co-ordinateeasier to interpret the relations between them(total weighting of the ith row, totalweighting of the ith column)14(l12Electrical resistance of solution /kQ2Fig 4. Relation between spontaneous combustion tendency index and electrical resistance of the dissolved coal sam-ples.ParaM(Dominance2.6. Interaction matrix applicationHudson [19 presented the interaction matrix con4Causccept to analyze factors affecting the decisions on a defined engineering system. The parameters that are listig. 5. Interaction matrix evaluation, decision map fored for a system can also be sorted according to theidefined parameters.causes and effects on the system. The interaction matrix concept [20] is based on the definition of the pa-After determining the co-ordinate values for eachrameters' interrelations two by two. These relations decision parameter, they were plotted as an x-y graphare also called pair-wise relations. The main poi(Fig. 6). Interpretation of this graph is detailed in lithere is the definition of the relation according to theratures [19, 21-23]. If the parameter has the co-experience of the domain experts. However, the logiordinate value that is plotted as a point locatedcal relations that are observed in the field should be the upper-right corner of the graph, it is interpreted asnterpreted here with numerical values, weighting valcritically interactive decision parameters. Similarly, ifues, to analyze the defined decision conditions by uthe co-ordinate gives a plotting point located aroundthe lower-right corner, it shows the parameters domicedure, Hudson presented a weighting scale to weight nant on the defined decision environment. Fig. 6 prelogical relation [21-22sents the cause-effect graph of the parameters thatFig. 5 shows the interaction matrix mapped fowere selected to analyze spontaspontaneous combustion tendency of a typical coaldency. According to thseam. To obtain this weighted matrix, first, decision (shown in Table 2)were selected according to theirarameters effective on spontaneous combustion werelevel of interactiondetermined. They were selected to confine the deciThese results show the criticality and dominancesion environment and they were assigned to the di- level of the parameters according to the interactionagonal members of the matrix. Off-diagonal members matrix rating during this study(Fig. 5). According toof the matrix were then assigned to the weighting val- the experiences gained, the rank of coal is found to beues representing the cause or the effect relations be- very important and critical for the spontaneous com-the diagonal members. The interactions of the bustion tendency. Electrical resistance occupies theparameters were defined as critical, strong, medium, second level on the criticality list. Therefore, it is inpoor, and nonrelative to each other. The weighting terpreted as a highly interactive decision parameter orvalues of 4, 3, 2, 1, and 0 were assigned to them, re- spontaneous combustion. Electrical resistance is despectively. The interaction matrix was then presented duced to be a dominated parameter in this decisions a procedure to be used in engineering projects. Ac- environmentHCNMHGO.S. Yildirim et al, Effects of electrical resistance on the spontaneous combustion tendency4|4444|4|330|100332P30112Mo200P430Sulphur3002P。23203Other mineralsPorosityThermal ConductivityP80 300000|P90Bacteria0 PIElectrical Resistance13121181312171992213.6Fig. 6. Cause-effect graph of the given decision mapping in Fig. 5.Table 2. Parameters selected according to their level of interactionorted dominance(max to min)Sorted criticality(max. to minRank6. Porosity1. Rank6. Friability2. Moisture7. Electrical resistance2. Electrical resistance7, Temperature3. Other mineral8. Thermal conductivity3. Other minerals4. Sulphur content9. Bacteria4. Thermal conductivity9. Sulphur content5. Temperature10. Friability5. Moisture10. Bacteria3. ConclusionsReferencesThe test results showed that the relation between [11 A M. Elliott, Chemistry of Coal Utilisation, Wiley, Newelectrical resistance and spontaneous combustion ten-York. 1981dency can be determined using statistical analysis. The [2] N.S. Tiwory and A Mukhdeo, Measurement of electricalesults showed that when the electrical resistance of aresistivity of coal samples, Fuel, 1993, No 72, p 1099coal seam sample is determined as high resistive, the 3V. Mahadevan and A.M. Ramlu, Fire risk rating of coalmines due to spontaneous heating, J. Mines Met. Fuelsspontaneous combustion tendency index and its risk985,p.35level for that coal seam are determined to be higher as [4 M. 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