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A study of spontaneous combustion characteristics of Nigerian coals
Short Communications 17
18 19 20
21 22 23 24
Given, P. H. and Derbyshire, F. J. Quarterly Progress Reports, DOE-PC6081 l-3,4,5, Pennsylvania State Univ., US DOE, 1985 Derbyshire, F. J., Davis, A., Epstein, M. and Stansberry, P. G. Fuel 1986,65,1233 Derbyshire, F. J., Terrer, M.-T., Davis,A.and Lin,R. Fuel 1988,67,1029 Davis, A., Derbyshire, F. J., Finseth, D. H.. Lin. R.. Stansberrv. P. G. and Terrer; M.-T. Fuel 1986, 65; 500 Planchon, D. PhD Thesis Univ. Libre de Bruxelles, Belgium, 1987 Ghodsi, M. and Neumann-Tihe, C. Thermochim. Acta 1983, 62, 1 Cypres, R. and Furfari, S. Fuel 1981, 60, 768 Ratnasamy, P. and Fripiat, J. J. Trans. Faraday Sot. 1970, 66, 2897
A study
Olayinka
of spontaneous
I. Ogunsola*
and
25
26 27 28
29
30
Ratnasamy, P., Rodrique, L. and Leonard, A. J. J. Phys. Chem. 1973, 77, 2242 Chung, K. S. and Massoth, F. E. J. Catal. 1980, 64, 332 Sermon, P. A. and Bond, G. C. Cutal. Res. Sci. Eng. 1973, 8, 211 Hodnett. B. K. and Delmon. B. in ‘Catalytic Hydrogenation, Studies in Surface Science and Catalysis 27’ (Ed. L. Cerveny), Elsevier, Amsterdam, 1986, p. 53 Bond, G. C. in ‘Spillover of Adsorbed Species’ (Ed. G. M. Pajonk, S. J. Teichner and J. E. Germain), Elsevier, Amsterdam, 1983, p. 1 Ledoux, M. J., Agostini, G., Benazonz, R. and Michaux, 0. Bull. Sot. Chim. Be(g. 1984, 93(8-9), 635
combustion
Randy
characteristics
31 32 33 34 35
36 37 38
39
Ratnasamy, P. and Sivashanker, S. Catal. Res.-Sci. Eng. 1980, 22, 401 Valyon, J. and Hell, W. K. J. Catal. 1983, 84, 216 Moseley, F. and Paterson, D. J. Inst. Fuel 1967, 40, 523 Anthony, D. B. and Howard, I. B. AIChE J. 1976,22,625 British Coal Research Establishment, Final Report, ECSC project, 1220EC/827, 1987 Lowenthal, G., Wanzl, W. and Van Heek, K. H. Fuel 1986, 65, 346 Chow, C. K. Fuel 1983,62, 317 Teckaly, P., Bacaud, R., Charcosset, H., Delpuech, J.-J., Kister, J., Nicole, D. and Oberson, M. Fuel 1988, 67, 932 Utz, 8. R., Appell, H. R. and Blaustein, B. D. Fuel 1986, 65, 1085
of Nigerian
coals
J. Mikula
CANMET, Energy, Mines and Resources Alberta, Canada TOC 1EO (Received 25 September 7990)
Canada,
Coal Research
Laboratory,
PO Bag
1280,
Devon,
The results of a study evaluating the spontaneous combustion characteristics of four Nigerian coals of varying rank are reported in this paper. Crossing point temperature (CPT) and liability index for the four coals were determined. The CPT was found to decrease with increase in coal rank. The liability index, which gives a better evaluation of susceptibility of coal to spontaneous heating, was also found to decrease with increase in rank and with decrease in oxygen content and moisture holding capacity of the coal. Of the four coals studied, the high volatile B bituminous coal had the lowest susceptibility to spontaneous combustion while the subbituminous was the most susceptible. (Keywords: coal; combustion; rank)
Fires which occur during the mining, transportation and storage or stockpiling of coal are largely caused by autogenous heating of coal which can result in spontaneous combustion when other favourable conditions exist. The history of spontaneous combustion is as old as that of coal mining itself and there has been a considerable effort to study and understand the mechanisms and factors responsible for spontaneous combustion. When coals are exposed to an oxidizing atmosphere even under ambient conditions, exothermic oxidative reactions, which release large amounts of heat, are known to occur’. Such heat release, if not appropriately removed may lead to a temperature rise which in turn can result in spontaneous combustion and subsequent tires.
* Permanent address: Faculty University of Port Harcourt, Nigeria 00162361/91/020258%04 0 1991 Butterworth-Heinemann
258
FUEL,
1991,
of Engineering, Port Harcourt,
Ltd.
Vol 70, February
Coals prone to oxidation such as lignite and subbituminous are often more susceptible to spontaneous combustion than coals of higher rank. However, rank dependency of susceptibility to spontaneous combustion is not detinitive’. Rather, liability to spontaneous combustion differs with different coals and stockpiling or handling conditions3. While some coal properties have been found to influence spontaneous combustion characteristics of some coals, they do not affect that of some others. For example, oxidation of pyrite in pyrite-rich coal to pyrophoric Fe’+ sulphide, Fe’+ and Fe3+ sulphide as reported by Mapstone4, may generate a lot of heat and form pyrite/coal interfaces for increased oxidation’. However, spontaneous combustion of some sulphur-free coals has been reported’. Also, while liability has been found to increase with increased moisture content, some high moisture-containing coals have demonstrated a relatively low liability to
spontaneous combustion3. Other coal properties that influence coal oxidation autogenous heating include and/or maceral composition, oxygen content, friability, thermal conductivity, heat of wetting, porosity and surface area. Susceptibility to autogenous heating is enhanced by high oxygen and exinite heat of wetting6-” and contents5, friability, and by decrease in thermal conductivity of coal. In addition to the variation in the effect of the various intrinsic coal properties on liability to spontaneous combustion, other factors such as methods of mining, transportation and storage conditions also play a major role. It is therefore essential to determine the spontaneous combustion characteristics of coal from different mines and even different seams. Safe mining, transportation and storage of Nigerian coals, proven reserves of which areestimated at 639 million tons’*, have not been reported. Nigerian coals, which are mainly of low rank (lignite and
Short
fvww-
Communications
Thermocoupls
Thermocoupbr
c N2 Outlot - Air Outlot
.. . . . . . * . . .
Reactor I Programmabto Furnace
t
. . . . . . . . *
. . . . . . . . .
. . . . . . . . .
I . . . . . . . . .
. . . . . . . . .
1
,
-Sampb
0 0 Figure 2
-Y
I
100 zw
JM 400 500 600 x.2 TIME b”inned
Thermogram for Coal A
25or 225
rl-
ma 115
V
Air tnbt
N2w
Flowmeter
flowmeter Figure 1
Schematic
diagram
of the spontaneous
combustion
test system
00 Table 1
Some properties
of coal sample
100 200 300 4w 5w 600 100 TIME (m,““,crJ
used Figure 3
Property
Coal A
Coal B
Coal C
Coal D
Residual moisture (wt%) Ash (wt%) Ash (et%, d.b.) Vol. matter (wt%, daf) Fixed carbon (wt%, daf) Equilibrium moisture (wt%) Heating value (MJ kg-’ daf.)
36.96 4.24 6.13 59.18 40.82 31.11 27.91
15.98 10.08 12.00 48.61 51.39 18.89 31.16
11.28 7.33 8.26 46.42 53.58 16.40 31.88
7.35 10.52 11.35 46.71 53.29 9.72 33.10
Carbon (wt%, daf.) Hydrogen (wt%, daf.) Nitrogen (wt%, daf.) Sulphur (wt%, daf.) Oxygen (wt%, daf.)
68.61 5.38 1.10 1.64 23.21
76.58 5.44 1.85 0.74 15.39
77.82 5.38 1.84 0.41 14.55
80.18 5.82 2.18 0.60 11.24
Thermogram
for Coal B
250225200 175 ii .- 150a
0
subbituminous), have been reported to have good potential for gasification and liquefaction processes” and for making form coke’j. While the spontaneous combustion characteristics of US coals14-16, British coals1s~i9, Australian coals*‘, Indian coals*.“, Russian coals** and Canadian coals3*23-26 have been studied, those of Nigerian coals have not been reported. The results of the first study aimed at evaluating the spontaneous combustion characteristics of Nigerian coals are reported in this paper.
EXPERIMENTAL Samples and their preparation Four Nigerian coals (a lignite a subbituminous (coal B), and volatile bituminous (coals C supplied by the Nigerian Coal ation (NCC) were used for this
(coal A), two high and D)) Corporstudy.
The bulk raw coal samples collected from the various mines were packed and shipped in air-tight metal containers filled with nitrogen to prevent oxidation. One representative split of each sample was then stage-crushed and screened in a nitrogen-flushed glove box. The -60+200 mesh size fraction of each sample was taken and stored in nitrogen until ready for use. Table I shows some of the properties of the four coals used for this study. Experimental system The experimental set up developed by Chakravorty and co-workers3 at the Surface Mining section of CANMETs Coal Research Laboratory in Devon, Alberta for studying the spontaneous combustion characteristics of Canadian coals was used. The system consists mainly of a 37 mm internal diameter by 105 mm long stainless steel reactor fitted with three Iron-Constantan (Type J)
J 100 200 3w 400 500 6W IM) TlME (minurcs)
0
Figure 4
Thermogram
for Coal C
Thermogram
for Coal D
250225200 1150 i.lY) L
Figure 5
thermocouples (one on the outside of the reactor to measure the oven temperature, one on the inside wall of the reactor to determine the reactor inside wall temperature, and one in the middle of the reactor to monitor the coal sample bed
FUEL,
1991,
Vol 70, February
259
Short Communications temperature), a gas flow line, a flowmeter and a water bubbler; a programmable furnace; and a multi-channel chart recorder for continuous monitoring of the thermocouple outputs. The system was linked with a personal computer for continuous data acquisition and analysis. Figure 1 shows the schematic diagram of the system. Details of the system design can be found elsewhere3. Experimental procedure
The procedure used was similar to that used by Chakravorty and co-workers3*26. For each sample in each run, 50 g of the -60 + 200 mesh fraction was weighed into two reactors and placed in a programmable furnace. Air at the flow rate of 80 cm3 min- ’ was passed through a water bubbler into one of the reactors while nitrogen at the same flow rate was passed into the other reactor. The furnace was set at a heating rate of 0S”C min- ‘. The temperature rise in the furnace and coal bed of the two reactors was continuously monitored and the temperature-time history of the oven and in the two reactors was obtained. Two indices of liability to spontaneous combustion and the crossing point temperature were determined from the thermograms obtained. These measurements are the first to be reported for Nigerian coals. RESULTS AND DISCUSSIONS Figures 2-5 illustrate
the thermograms for coals A, B, C and D respectively. Each Figure contains the temperature-time profile for the oven, the nitrogen test and the air test. Three common observations can be made from the Figures for the four coal samples: (a) the oven temperature increases linearly with increase in time; (b) the temperature in the coal bed when either nitrogen or air is passed also increases but not linearly with increase in time and at a lower rate than that of the oven; and (c) the nitrogen curve does not cross the oven curve at any point while the air curve crosses the oven curve at some point. The temperature at which the air curve crosses the oven curve is generally referred to as the crossing point temperature (CPT). The reason for the coal bed temperatures exceeding the oven temperature above the crossing point is due to the exothermic oxidation reactions which heats the coal beyond the oven temperature3. Coals highly susceptible to autogenous heating would therefore be expected to have a lower CPT value than those that are less susceptible. The CPT values for the four coals studied are shown in Table2. The crossing point temperature differs for each coal. The data indicate that the lower rank coals have higher CPT value. This trend is opposite to that for western Canadian coals studied3 using the same experimental system and procedure. Also, the
260
FUEL,
1991,
Vol 70, February
CPT value for the Nigerian coals are lower than those of Western Canada of similar rank. This implies that susceptibility of coal to spontaneous combustion is not simply rank dependent. Rather, it is more or less coal specific. Two liability indices I, and I, which have been used by other researchers3sz6 have been used to overcome some of the limitations ofthe CPT. Liability index (I,) was derived from the ratio of CPT and the coal bed heating rate between 10 min before and 10 min after the crossing point. I, was calculated, as given by Mansour and co-workersz6 from the averaged incremental area between the air and nitrogen curves obtained from the thermograms shown in Figures 2-5. The area between the air and nitrogen thermograms can be related to the heat produced as a result of oxidation. A plot of change in the incremental area versus coal bed temperature for the four coals is illustrated in Figure 6. At lower temperature, z58O”C, the incremental area for the four coals appears similar with little or no change. However, the change in incremental area (which is a measure of amount of heat produced during oxidation), increases linearly with temperature >8O”C and is different for each coal. It can be concluded from the slope of these curves and the data presented in Table 2 that coal D is the least susceptible to spontaneous combustion of the four Nigerian coals studied. This was followed by coals A, C and B respectively. Values of the two liability indices for the four coals studied are shown in Table 2. Index I, is an improvement on I, since it measures the heating effects at a temperature more likely to be found before combustion startsz6. These indices coupled with the crossing point temperature provide better evaluation of coal’s susceptibility to spontaneous combustion. Coals highly susceptible to spontaneous combustion are expected to have high liability index as well as low CPT value. The liability indices have been reported to correlate better with intrinsic coal properties3. An increase with decrease in coal rank is evident in Table 2. However, an exceptional coal A (a lignite) with a relatively high moisture content (see Table 1) which exhibits a lower liability than the two subbituminous coals. This is in agreement with the results of Chakravorty et aL3. The reason for the unexpectedly lower susceptibility to spontaneous combustion shown by same
Table 2
Figure 6 Incremental area between the air and nitrogen curves as a function of coal bed temperature
0
0
Figure 7 Variation of liability index with the carbon, oxygen, and equilibrium moisture contents
Summary of results
Crossing point temperature (“C) Liability index (I,) Liability index (I,) heating rate (“C min- ‘)
Coal A
Coal B
Coal C
Coal D
163.5 10.3 1.1 1.7
146.5 15.9 2.1 2.3
145.0 13.8 1.7 2.0
140.0 8.5 1.2 1.2
Short Communications high moisture lignite (coal A) is probably a result of the cooling effect due to evaporation of the large amount of moisture in the coal. This is a limitation of the liability defined in this way. Chakravorty et al.3 found that the inverse relations between liability and rank did not hold for coals containing > 12% moisture. Figure
liability oxygen, contents.
7 shows a composite index as a function equilibrium and
ACKNOWLEDGEMENT The coal samples used for this work were supplied by the Nigerian Coal Corporation. The authors are very grateful to Kanti Kar, Drs Chakravorty and Mansour of the Surface Mining Laboratory, Energy, Mines and Resources, CRL Devon for their help.
plot of the of carbon, moisture
A decrease in the liability index with an increase in carbon content and with a decrease in oxygen and equilibrium moisture contents could be observed with the exception of the lignite. Coals with high oxygen content have greater tendency to chemically bind the moisture thereby rendering the surface more hydrophilic and reactive.
CONCLUSIONS Spontaneous combustion characteristics of four Nigerian coals of varying rank have been studied. The susceptibility to spontaneous combustion decreases with increase in coal rank and with decrease in both the moisture holding capacity and oxygen content of the coal except for the high moisture-containing lignite. This trend compares with that of Western Canadian coals although the Nigerian coals tend to be more susceptible to spontaneous combustion than Western Canadian coals of similar rank. The Nigerian coals have a lower crossing point temperature and a higher liability index than the western Canadian coals of same rank.
9
Bhattacharyya, K. K., Hodges, B. J. and Hinsley, F. B. Min. Eng. (London) 1969.
10
Guney, M. Can. Inst. Min. BUN. 1971.
101,274
11 12 13
64(707), 138 Shea, F. L. and Hsu, H. L. Ind. Eng. Chem. Prod. Res. Dev. 1972, 11, 184 Ogunsola, 0. I. and Azhekesan, M. Fuel 1988,67, 1008 Afonja, A. A. Fuel Proc. Technol. 1983, 7, 293
14 15
Davis, J. and Bvrone, J. F. J. Am. Ceram. Sot. 1924,7, 8b9 L. D., Elder. J. D.. Schmiddt. Steiner, W. A. and Davis, J. D. Am. Gas Assoc. Mon. 1945, 27
16 17
REFERENCES Berkowitz, N. Atmospheric oxidation of Coal, in ‘Sample Selection, Aging, and Reactivity of Coal’ (Ed. Klein),- John Wilev and Sons Inc.. New York. 1989., pp. 217-281 Tanafdar, M. N. and Guha, D. Fuel 1989, 68,315 Chakravorty, R. N. and Kar, K. ‘Characteristics of Western Canadian Coals with Respect to their Susceptibility to Spontaneous Combustion’, CANMET Report No. ERPjCRL 86-151 (TR), Energy, Mines & Resources, Coal Research Lab. 1986 Mapstone, G. E. Chem. Ind. (London)
18
Engr. 1966, 126(74), 121
19 20 21
Inst. Min. Eng. 1966, 126, 121
Guney, M., Hodges, D. G. and Himsley, F. B. Min. Engr. 1969,129( 1lo), 67 Sevenster, P. G. Fuel 1961,40, 7 Banerjee, S. C., Nandy, D. K., Banerjee, D. D. and Chakravorty, R. N. Trans. Min. Geol. & Met. Inst. of India 1972, 69(2), 15
22
Handa, T., Nishimato, T., Morita, M. and Kodama, J. Fire Sci. Technol. Tokyo
23
Kaji, R., Hishinuma, Y. and Nakamura, Y. Fuel 1985,64, 297 Feng, K. K., Chakravorty, R. N. and Cochrane. T. S. CIM Bull. 1973.66(738), 75 Feng, K. K. CIM Bull. 1985,78,71 Mansour, N. A., Chakravorty, R. N., Stewart, D. B. and Kar. K. ‘Interoretation of Thermograms to Determine Spontaneous Combustion Susceptibility of Coals’. Pacer for oresentation at the Fifty ‘International Pittsburgh Coal Conference, Pittsburgh, PA, Sept. 1988
1985,5,
24
1954, 658
Chamberlain, E. A. C. and Ball, D. A. Colliery Guardian 1973, Feb. 65 Berkowitz, N. and Schein, H. G. Fuel 1951,30,94 Hodges, D. J. and Hinsley, F. B. Trans. Inst. Min. Eng. 1963, 123, 211 Hodges, D. J. and Acherjee, B. Trans.
Kuchta, J. M., Rowe, V. R. and Burgess, D. S. US Dept. of Int., Bur. Mines Rep. 1980, No. 8474 Smith, A. C. and Lazarra, C. P. US Eur. of Mines Invesst. Rep. 1987, No. R9079 Hodges, D. G. and Acherjee, B. Min.
25 26
21
FUEL, 1991, Vol 70, February
261
18 19 20
21 22 23 24
Given, P. H. and Derbyshire, F. J. Quarterly Progress Reports, DOE-PC6081 l-3,4,5, Pennsylvania State Univ., US DOE, 1985 Derbyshire, F. J., Davis, A., Epstein, M. and Stansberry, P. G. Fuel 1986,65,1233 Derbyshire, F. J., Terrer, M.-T., Davis,A.and Lin,R. Fuel 1988,67,1029 Davis, A., Derbyshire, F. J., Finseth, D. H.. Lin. R.. Stansberrv. P. G. and Terrer; M.-T. Fuel 1986, 65; 500 Planchon, D. PhD Thesis Univ. Libre de Bruxelles, Belgium, 1987 Ghodsi, M. and Neumann-Tihe, C. Thermochim. Acta 1983, 62, 1 Cypres, R. and Furfari, S. Fuel 1981, 60, 768 Ratnasamy, P. and Fripiat, J. J. Trans. Faraday Sot. 1970, 66, 2897
A study
Olayinka
of spontaneous
I. Ogunsola*
and
25
26 27 28
29
30
Ratnasamy, P., Rodrique, L. and Leonard, A. J. J. Phys. Chem. 1973, 77, 2242 Chung, K. S. and Massoth, F. E. J. Catal. 1980, 64, 332 Sermon, P. A. and Bond, G. C. Cutal. Res. Sci. Eng. 1973, 8, 211 Hodnett. B. K. and Delmon. B. in ‘Catalytic Hydrogenation, Studies in Surface Science and Catalysis 27’ (Ed. L. Cerveny), Elsevier, Amsterdam, 1986, p. 53 Bond, G. C. in ‘Spillover of Adsorbed Species’ (Ed. G. M. Pajonk, S. J. Teichner and J. E. Germain), Elsevier, Amsterdam, 1983, p. 1 Ledoux, M. J., Agostini, G., Benazonz, R. and Michaux, 0. Bull. Sot. Chim. Be(g. 1984, 93(8-9), 635
combustion
Randy
characteristics
31 32 33 34 35
36 37 38
39
Ratnasamy, P. and Sivashanker, S. Catal. Res.-Sci. Eng. 1980, 22, 401 Valyon, J. and Hell, W. K. J. Catal. 1983, 84, 216 Moseley, F. and Paterson, D. J. Inst. Fuel 1967, 40, 523 Anthony, D. B. and Howard, I. B. AIChE J. 1976,22,625 British Coal Research Establishment, Final Report, ECSC project, 1220EC/827, 1987 Lowenthal, G., Wanzl, W. and Van Heek, K. H. Fuel 1986, 65, 346 Chow, C. K. Fuel 1983,62, 317 Teckaly, P., Bacaud, R., Charcosset, H., Delpuech, J.-J., Kister, J., Nicole, D. and Oberson, M. Fuel 1988, 67, 932 Utz, 8. R., Appell, H. R. and Blaustein, B. D. Fuel 1986, 65, 1085
of Nigerian
coals
J. Mikula
CANMET, Energy, Mines and Resources Alberta, Canada TOC 1EO (Received 25 September 7990)
Canada,
Coal Research
Laboratory,
PO Bag
1280,
Devon,
The results of a study evaluating the spontaneous combustion characteristics of four Nigerian coals of varying rank are reported in this paper. Crossing point temperature (CPT) and liability index for the four coals were determined. The CPT was found to decrease with increase in coal rank. The liability index, which gives a better evaluation of susceptibility of coal to spontaneous heating, was also found to decrease with increase in rank and with decrease in oxygen content and moisture holding capacity of the coal. Of the four coals studied, the high volatile B bituminous coal had the lowest susceptibility to spontaneous combustion while the subbituminous was the most susceptible. (Keywords: coal; combustion; rank)
Fires which occur during the mining, transportation and storage or stockpiling of coal are largely caused by autogenous heating of coal which can result in spontaneous combustion when other favourable conditions exist. The history of spontaneous combustion is as old as that of coal mining itself and there has been a considerable effort to study and understand the mechanisms and factors responsible for spontaneous combustion. When coals are exposed to an oxidizing atmosphere even under ambient conditions, exothermic oxidative reactions, which release large amounts of heat, are known to occur’. Such heat release, if not appropriately removed may lead to a temperature rise which in turn can result in spontaneous combustion and subsequent tires.
* Permanent address: Faculty University of Port Harcourt, Nigeria 00162361/91/020258%04 0 1991 Butterworth-Heinemann
258
FUEL,
1991,
of Engineering, Port Harcourt,
Ltd.
Vol 70, February
Coals prone to oxidation such as lignite and subbituminous are often more susceptible to spontaneous combustion than coals of higher rank. However, rank dependency of susceptibility to spontaneous combustion is not detinitive’. Rather, liability to spontaneous combustion differs with different coals and stockpiling or handling conditions3. While some coal properties have been found to influence spontaneous combustion characteristics of some coals, they do not affect that of some others. For example, oxidation of pyrite in pyrite-rich coal to pyrophoric Fe’+ sulphide, Fe’+ and Fe3+ sulphide as reported by Mapstone4, may generate a lot of heat and form pyrite/coal interfaces for increased oxidation’. However, spontaneous combustion of some sulphur-free coals has been reported’. Also, while liability has been found to increase with increased moisture content, some high moisture-containing coals have demonstrated a relatively low liability to
spontaneous combustion3. Other coal properties that influence coal oxidation autogenous heating include and/or maceral composition, oxygen content, friability, thermal conductivity, heat of wetting, porosity and surface area. Susceptibility to autogenous heating is enhanced by high oxygen and exinite heat of wetting6-” and contents5, friability, and by decrease in thermal conductivity of coal. In addition to the variation in the effect of the various intrinsic coal properties on liability to spontaneous combustion, other factors such as methods of mining, transportation and storage conditions also play a major role. It is therefore essential to determine the spontaneous combustion characteristics of coal from different mines and even different seams. Safe mining, transportation and storage of Nigerian coals, proven reserves of which areestimated at 639 million tons’*, have not been reported. Nigerian coals, which are mainly of low rank (lignite and
Short
fvww-
Communications
Thermocoupls
Thermocoupbr
c N2 Outlot - Air Outlot
.. . . . . . * . . .
Reactor I Programmabto Furnace
t
. . . . . . . . *
. . . . . . . . .
. . . . . . . . .
I . . . . . . . . .
. . . . . . . . .
1
,
-Sampb
0 0 Figure 2
-Y
I
100 zw
JM 400 500 600 x.2 TIME b”inned
Thermogram for Coal A
25or 225
rl-
ma 115
V
Air tnbt
N2w
Flowmeter
flowmeter Figure 1
Schematic
diagram
of the spontaneous
combustion
test system
00 Table 1
Some properties
of coal sample
100 200 300 4w 5w 600 100 TIME (m,““,crJ
used Figure 3
Property
Coal A
Coal B
Coal C
Coal D
Residual moisture (wt%) Ash (wt%) Ash (et%, d.b.) Vol. matter (wt%, daf) Fixed carbon (wt%, daf) Equilibrium moisture (wt%) Heating value (MJ kg-’ daf.)
36.96 4.24 6.13 59.18 40.82 31.11 27.91
15.98 10.08 12.00 48.61 51.39 18.89 31.16
11.28 7.33 8.26 46.42 53.58 16.40 31.88
7.35 10.52 11.35 46.71 53.29 9.72 33.10
Carbon (wt%, daf.) Hydrogen (wt%, daf.) Nitrogen (wt%, daf.) Sulphur (wt%, daf.) Oxygen (wt%, daf.)
68.61 5.38 1.10 1.64 23.21
76.58 5.44 1.85 0.74 15.39
77.82 5.38 1.84 0.41 14.55
80.18 5.82 2.18 0.60 11.24
Thermogram
for Coal B
250225200 175 ii .- 150a
0
subbituminous), have been reported to have good potential for gasification and liquefaction processes” and for making form coke’j. While the spontaneous combustion characteristics of US coals14-16, British coals1s~i9, Australian coals*‘, Indian coals*.“, Russian coals** and Canadian coals3*23-26 have been studied, those of Nigerian coals have not been reported. The results of the first study aimed at evaluating the spontaneous combustion characteristics of Nigerian coals are reported in this paper.
EXPERIMENTAL Samples and their preparation Four Nigerian coals (a lignite a subbituminous (coal B), and volatile bituminous (coals C supplied by the Nigerian Coal ation (NCC) were used for this
(coal A), two high and D)) Corporstudy.
The bulk raw coal samples collected from the various mines were packed and shipped in air-tight metal containers filled with nitrogen to prevent oxidation. One representative split of each sample was then stage-crushed and screened in a nitrogen-flushed glove box. The -60+200 mesh size fraction of each sample was taken and stored in nitrogen until ready for use. Table I shows some of the properties of the four coals used for this study. Experimental system The experimental set up developed by Chakravorty and co-workers3 at the Surface Mining section of CANMETs Coal Research Laboratory in Devon, Alberta for studying the spontaneous combustion characteristics of Canadian coals was used. The system consists mainly of a 37 mm internal diameter by 105 mm long stainless steel reactor fitted with three Iron-Constantan (Type J)
J 100 200 3w 400 500 6W IM) TlME (minurcs)
0
Figure 4
Thermogram
for Coal C
Thermogram
for Coal D
250225200 1150 i.lY) L
Figure 5
thermocouples (one on the outside of the reactor to measure the oven temperature, one on the inside wall of the reactor to determine the reactor inside wall temperature, and one in the middle of the reactor to monitor the coal sample bed
FUEL,
1991,
Vol 70, February
259
Short Communications temperature), a gas flow line, a flowmeter and a water bubbler; a programmable furnace; and a multi-channel chart recorder for continuous monitoring of the thermocouple outputs. The system was linked with a personal computer for continuous data acquisition and analysis. Figure 1 shows the schematic diagram of the system. Details of the system design can be found elsewhere3. Experimental procedure
The procedure used was similar to that used by Chakravorty and co-workers3*26. For each sample in each run, 50 g of the -60 + 200 mesh fraction was weighed into two reactors and placed in a programmable furnace. Air at the flow rate of 80 cm3 min- ’ was passed through a water bubbler into one of the reactors while nitrogen at the same flow rate was passed into the other reactor. The furnace was set at a heating rate of 0S”C min- ‘. The temperature rise in the furnace and coal bed of the two reactors was continuously monitored and the temperature-time history of the oven and in the two reactors was obtained. Two indices of liability to spontaneous combustion and the crossing point temperature were determined from the thermograms obtained. These measurements are the first to be reported for Nigerian coals. RESULTS AND DISCUSSIONS Figures 2-5 illustrate
the thermograms for coals A, B, C and D respectively. Each Figure contains the temperature-time profile for the oven, the nitrogen test and the air test. Three common observations can be made from the Figures for the four coal samples: (a) the oven temperature increases linearly with increase in time; (b) the temperature in the coal bed when either nitrogen or air is passed also increases but not linearly with increase in time and at a lower rate than that of the oven; and (c) the nitrogen curve does not cross the oven curve at any point while the air curve crosses the oven curve at some point. The temperature at which the air curve crosses the oven curve is generally referred to as the crossing point temperature (CPT). The reason for the coal bed temperatures exceeding the oven temperature above the crossing point is due to the exothermic oxidation reactions which heats the coal beyond the oven temperature3. Coals highly susceptible to autogenous heating would therefore be expected to have a lower CPT value than those that are less susceptible. The CPT values for the four coals studied are shown in Table2. The crossing point temperature differs for each coal. The data indicate that the lower rank coals have higher CPT value. This trend is opposite to that for western Canadian coals studied3 using the same experimental system and procedure. Also, the
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CPT value for the Nigerian coals are lower than those of Western Canada of similar rank. This implies that susceptibility of coal to spontaneous combustion is not simply rank dependent. Rather, it is more or less coal specific. Two liability indices I, and I, which have been used by other researchers3sz6 have been used to overcome some of the limitations ofthe CPT. Liability index (I,) was derived from the ratio of CPT and the coal bed heating rate between 10 min before and 10 min after the crossing point. I, was calculated, as given by Mansour and co-workersz6 from the averaged incremental area between the air and nitrogen curves obtained from the thermograms shown in Figures 2-5. The area between the air and nitrogen thermograms can be related to the heat produced as a result of oxidation. A plot of change in the incremental area versus coal bed temperature for the four coals is illustrated in Figure 6. At lower temperature, z58O”C, the incremental area for the four coals appears similar with little or no change. However, the change in incremental area (which is a measure of amount of heat produced during oxidation), increases linearly with temperature >8O”C and is different for each coal. It can be concluded from the slope of these curves and the data presented in Table 2 that coal D is the least susceptible to spontaneous combustion of the four Nigerian coals studied. This was followed by coals A, C and B respectively. Values of the two liability indices for the four coals studied are shown in Table 2. Index I, is an improvement on I, since it measures the heating effects at a temperature more likely to be found before combustion startsz6. These indices coupled with the crossing point temperature provide better evaluation of coal’s susceptibility to spontaneous combustion. Coals highly susceptible to spontaneous combustion are expected to have high liability index as well as low CPT value. The liability indices have been reported to correlate better with intrinsic coal properties3. An increase with decrease in coal rank is evident in Table 2. However, an exceptional coal A (a lignite) with a relatively high moisture content (see Table 1) which exhibits a lower liability than the two subbituminous coals. This is in agreement with the results of Chakravorty et aL3. The reason for the unexpectedly lower susceptibility to spontaneous combustion shown by same
Table 2
Figure 6 Incremental area between the air and nitrogen curves as a function of coal bed temperature
0
0
Figure 7 Variation of liability index with the carbon, oxygen, and equilibrium moisture contents
Summary of results
Crossing point temperature (“C) Liability index (I,) Liability index (I,) heating rate (“C min- ‘)
Coal A
Coal B
Coal C
Coal D
163.5 10.3 1.1 1.7
146.5 15.9 2.1 2.3
145.0 13.8 1.7 2.0
140.0 8.5 1.2 1.2
Short Communications high moisture lignite (coal A) is probably a result of the cooling effect due to evaporation of the large amount of moisture in the coal. This is a limitation of the liability defined in this way. Chakravorty et al.3 found that the inverse relations between liability and rank did not hold for coals containing > 12% moisture. Figure
liability oxygen, contents.
7 shows a composite index as a function equilibrium and
ACKNOWLEDGEMENT The coal samples used for this work were supplied by the Nigerian Coal Corporation. The authors are very grateful to Kanti Kar, Drs Chakravorty and Mansour of the Surface Mining Laboratory, Energy, Mines and Resources, CRL Devon for their help.
plot of the of carbon, moisture
A decrease in the liability index with an increase in carbon content and with a decrease in oxygen and equilibrium moisture contents could be observed with the exception of the lignite. Coals with high oxygen content have greater tendency to chemically bind the moisture thereby rendering the surface more hydrophilic and reactive.
CONCLUSIONS Spontaneous combustion characteristics of four Nigerian coals of varying rank have been studied. The susceptibility to spontaneous combustion decreases with increase in coal rank and with decrease in both the moisture holding capacity and oxygen content of the coal except for the high moisture-containing lignite. This trend compares with that of Western Canadian coals although the Nigerian coals tend to be more susceptible to spontaneous combustion than Western Canadian coals of similar rank. The Nigerian coals have a lower crossing point temperature and a higher liability index than the western Canadian coals of same rank.
9
Bhattacharyya, K. K., Hodges, B. J. and Hinsley, F. B. Min. Eng. (London) 1969.
10
Guney, M. Can. Inst. Min. BUN. 1971.
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