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The role of desorption of moisture from coal in its spontaneous heating
The role of desorption of moisture from coal in its spontaneous heating K. K. Bhattacharyya” Department of Mining Engineering, University of Nottingham, Nottingham, (Received 19 March 1971)
UK
As a part of investigations on the influence of humidity on the initial stage of the spontaneous heating of coal, the effect of desorption of moisture from the coal was studied. Laboratory experiments with different coals were carried out under conditions where desorption of water from the coals by air was certain to take place. Thermal changes were measured by a specially designed calorimeter in isothermal conditions, mostly at 3O’C. The results showed that in each case there occurred a cooling of the coal, indicating that the rate of heat loss from a coal due to this process was greater than the rate of lieat release due to oxidation of the coal. The data suggest that moisture desorption acts as an inhibitor to the spontaneous heating of coal. For a particular coal, the rate of heat loss increases with the increase in the equilibrium humidity deficiency of the air. The effects of rank, particle size and weathering on the process are also discussed.
on the influence of moisture on the spontaneous heating of coal have been carried out both in field and laboratory over a long period and in several countries. The common conclusion is that damp conditions favour the self-ignition of coal in mines and storage. It has also been found that the moisture content is the most variable of all parameters of the mine atmosphere’. Both air and coal are capable of holding certain amounts of moisture under given conditions of pressure, temperature and availability of water vapour. In an atmosphere of constant humidity, the water vapour pressures in the coal and air remain in equilibrium states. When the humidity of the atmosphere rises, the coal begins to sorb moisture from the surroundings until a new equilibrium is attained. Conversely, if the surroundings get drier, then the air picks up moisture from the coal. Studies on the thermal changes in coals during the first two conditions have been reported earlier2*3. A laboratory study of the thermal effect of desorption of water from coals by the surrounding atmosphere is described here. Although previous investigators4-6 have studied this particular aspect of desorption of water vapour from the coal, the experimental conditions used appear to be rather extreme. The purpose of the present investigation was, therefore, to measure the heat changes during desorption of moisture from different coals in various humid atmospheres. The effects of rank, particle size and weathering on the process were also examined. Investigations
EXPERIMENTAL The apparatus and its operational method are described in detail in an earlier publication’. In brief, a specially designed calorimeter8 capable of measuring heat loss from the material in its cell was used. All experiments were carried out in an isothermal condition at 30°C with the exception of one test during which the temperature was 35*C. The equilibrium humidities of the moisture carrier * Present address: Faculty of Engineering Science, The University of Western Ontario, London, Ontario, Canada
214
FUEL, 1972, Vol 51, July
and the coals were chosen from 0, 43, 75, 80 and 100% relative humidities at the experimental temperature. The origins and analyses of the coals used are shown in Table 1. About 5 g of coal (-72 mesh BS) was placed in the calorimeter cell and an air (or nitrogen) flow of 2.5 ml/mm g of dry coal was passed through it during each test. The coal sample in the cell was brought to stable thermal and hygrometric conditions by passing through it the desired flow of nitrogen equilibrated at the chosen humidity. The flow of nitrogen was then replaced by air of lower humidity. During two experiments with moist coal and dry air the calorimeter outlet was connected for definite periods to two U-tubes in series containing a known amount of phosphorus pentoxide to absorb the moisture desorbed from the coal sample. The average hourly rate of loss of water from the coal was subsequently calculated from the gain in weight of the phosphorus pentoxide tube and the time of determination. The influence of particle size on the thermal change during desorption of water was studied by using four different sized samplesof -25 t 36, -36 + 52, -52 + 72 and -72 t 200 mesh BS made from coal F. To investigate the effect of weathering on the process, three oxidized samples from the low-rank coal II were tested. A bulk sample of this coal (-72 mesh BS) was oxidized continuously at room temperature in a dry condition by pure oxygen, and the sub-samples were taken out after 30, 50 and 70 days of oxidation.
RESULTS AND DISCUSSION The progress of the experiments was noted by continuously recording the calorimeter output, and the rates of heat change were subsequently calculated from the therms grams. The- results show that in each case there occurred a cooling of the coal. The heat loss data presented here are expressed on a dry coal basis. To estimate the rate of heat loss during desorption of water from the small amount of glass wool, which was mixed with the coal sample during each experiment, tests were done by passing dry air
K. K. Bhattacharyya: Table 1
Analyses of coals used (as supplied
by the NCEl Coal Survey
Moisture desorption from coal and spontaneous heating
Laboratories) Analysis
Sample details
(%) dmmf*
air-dried
Coal
Colliery
Seam
Moisture
Ash
VM
Fixed carbon
Total S
VM
C
H
A 0 C D E F G H
Cynhaidre t Easington Thoresby Bentinck Whitewell Denby Drury Measham
Pumpquart t High Main Top Hard Yard High Hazles Belpar-Lawn Stockings
1.6 1.9 1.6 7.5 5-8 8.4 4-6 5.3
1.3 5-o 5-l 4-9 7.4 2.7 7.5 6.0
4.8 29.7 32.1 30-2 31.6 33.3 34.2 36.7
92.3 634 61.2 57-4 55.2 55.6 53.7 52.0
1.2 o-7 1.6 0.8 l-4 1-3 1-9 1.6
4.8 30.6 33.9 33.5 354 36-9 39.9 42.1
94.2 88.5 86.6 83.7 83.9 82-8 81-5 80.7
2.8 5.2 5-4 4-9 5.6 5.4 5.8 5.2
* t
Dry, mineral-matter Supplied by Cardiff
Lowe
free (Parr’s basis) Coal Survey Laboratory,
Sample no.CSL
2708
through a sample of the wool saturated with water vapour at 30°C. The rate of heat loss from the glass wool was found to be insignificant. The variations of the rate of heat loss with time for all the tests made are shown in Figures 1-3. While coals A, C, E and F were used for a particular humidity condition only, the others were tested under several conditions. A few experiments were performed using nitrogen instead of air during a part or the whole of a test. It appears from these results and those reported earlier’ that under the present test conditions the effect of heat release due to oxidation of the coal is negligible. The loss of heat from the coals indicates that the rate of heat change in a coal during the tests is governed mainly by the process of desorption of water from the coal. Similar observations have been made by Stott4 and by Hodges and Acherjee6. Tests with moist coals and dry air All the coals were used in this series of experiments, and the results are plotted in Figures 1 and 2. The relation between the rate of heat loss and time of desorption of moisture is seen to be similar for the anthracite (coal A, Figure 1) and the high-rank coal B (Figure 2) tested at several humidity conditions. The plot of the results from the test with coal C (Figure I), carried out under conditions similar to those of coal A, shows two peaks prior to any consistent decrease in the rate of heat loss. The tests with the medium-rank coal D (Figure 2) initially equilibrated at 43, 75 and 80% RH. at 30°C show that in each case the rate of heat loss gradually increases to an apparent maximum and does not show any appreciable decrease during the experimental period. However, when the initial equilibration humidity of the coal is raised to 100% R.H., the rate increases with the time of drying after a slight decrease at the outset. The results of the tests with coals E and F (Figure I), both saturated with water vapour at 3O”C, show different rate-time relations. While for the former the rate of heat loss becomes more or less constant after a steady initial rise, for coal F it increases during the first hour of drying and then decreases slightly for some time before increasing again. The irregularity of this latter type becomes more pronounced during the tests with the low-rank coals G and H (Figure 2). When coal G equilibrated at 43% R.H. at 30°C is subjected to dry air, the variation of the rate of heat loss with time is somewhat similar to that observed with coal C. However, the results from the tests with coal
G containing higher initial moisture and with coal H are more or less similar to those obtained with coal F. The difference in the nature of variation of the rate of heat loss with the time of drying of different moist coals may be explained when the type of coal surface, the state of sorbed water, the gravimetric rate of water loss, and the heat of desorption of unit weight of water are taken into consideration. Owing to experimental limitations it was only possible to estimate the rate of loss of water from the coal during two experiments. According to some investigators’34, when dry air is passed through a moist coal, the rate of loss of water reaches its maximum within a few minutes from the start and then decreases continuously throughout the process. The maximum rate of water loss depends on the initial moisture content of the coal and the rate of air flow. The general shape of such a gravimetric rate curve is similar to that of the rate-of-heat-loss curves presently obtained with the anthracite coal A and the highrank coal B. This is possibly because the water molecules
L.0
0 0
l
,&-
L
B
12 Time(h)
16
20
L 2L
Figure 1 Variation in the rate of heat loss from coal with time during oxidation and/or desorption of water vapour by dry air. Coal saturated at 100% R.H. at 3O”C, and dry air *coal A, n coal C,Acoal E,ncoal F
FUEL, 1972, Vol 51, July
215
Moisture desorp tion from coal and spontaneous heating: K. K. Bha ttacharyya I
I
I
I
Coai
I
6
05 0
L
8
I
I
L
0
Time (h)
I
I
I
I
Coal
I
12 Time ( hl
16
20
I
I
16
20
L.0
0
3.0
2.0
I.0
I
I 01
0
Time
(h 1
*
1 Cal/g = 4.187
216
J/g
FUEL, 1972, Vol51,
July
12 Time
Variation in the rate of heat loss from coal with time during oxidation figure 2 OCoal equilibrated @Coal equilibrated at 43% R.H. at 30°C and dry air/N2 q Coal equilibrated n Coal equilibrated at 80% R.H. at 30°C and dry air
are held rather loosely in these coals of hydrophobic nature. The similarity also suggests that the heat of drying for these coals probably remains more or less constant throughout the desorption process. In contrast to the above, the rate-of-heat-loss curves obtained from the medium- and low-rank coals are disThe results similar from the rate-of-water-loss curve. apparently indicate that the heat loss per unit weight of water desorbed from these coals may increase with the During the experiment with coal D extent of drying. equilibrated at 80% R.H. at 3O”C, it was estimated that while the average heat of drying for the first six hours was 614 Cal/g* of water desorbed, that for the next six hours was 812 Cal/g. For the test with coal H equilibrated at 75% R.H. at 3O”C, the average heat loss owing to desorption of water was found to increase from 845 Cal/g during the first four hours to 906 Cal/g for the next eighteen hours. Observations in agreement with these results have also been made by other investigators4y6. Owing to their increasingly hydrophilic nature, the
I
2L
(h)
and/or desorption of water vapour by dry air at 75% R.H. at 30°C and dry air at 100% R.H. at 30°C and dry air
medium- and low-rank coals sorb water vapour in a manner different from the high-rank coal and anthracite. The forces of attraction between the coal surface and the water molecules are fairly strong in the former types. The occurrence of multilayer sorption of water in these coals suggests that the layers below the top one are under compression, partly owing to the forces of attraction and partly because each layer is compressed by other layers above it. So the compression of the layer decreases from the innermost to the outermost layers. Moreover, the water molecules inside the pores and capillaries of the coal are attracted on all sides. Therefore it is to be expected that the heat of desorption in such cases would increase as the coal gets drier. Tests with
moist
coals and moist air
This series of tests was carried out with coals B, D, G and H; the results are shown in Figure 3. The nature of variation in the rate of heat loss with time appears to be similar for coals B, D and G. During any particular test the rate increased with time to a maximum value and then
K. K. Bhattacharyya: I
I
I
I
Coal
Time
Moisture desorption from coal and spontaneous heating
I 0
Coal G
I
I
I
I
I
1,
8
12 Time (hl
16
20
[h)
2-o------------(Coal
0
Figure
0
L
3 Variation aCoal equilibrated OCoal equilibrated n Coal equilibrated nCoal equilibrated ACoal equilibrated
8
12 Time I h)
16
I 20
OO-'-'
Effects of some variables on the rate of heat loss The influence of some factors on the rate of heat loss due to desorption of water from the coal has been studied by a common basis of comparison. The basis chosen is QtZZo, the total heat loss from a coal during twenty hours
16
2l
Time (h)
in the rate of heat loss from coal with time during oxidation at 100% R.H. and air equilibrated at 43% R.H. at 30°C at 100% R.H. and air equilibrated at 80% R.H. at 30°C at 80% R.H. and N2/air equilibrated at 43% R.H. at 35’C at 80% R.H. and air/N2 equilibrated at 43% R.H. at 30°C at 80% R.H. and air equilibrated at 75% R.H. at 30°C
decreased progressively as the coal approached the new equilibrium. The maximum rate of heat loss increases with an increase in the initial equilibrium deficiency of the air. When this deficiency is small, the curves become flatter. Contrary to this, the rate curves for coal H show not only some irregularity, but there is also a tendency for the rate to increase with time of desorption of water. This tendency is more pronounced when the initial equilibrium deficiency of the air is larger. The increase in the rate of heat loss with the period of desorption of water from coal H clearly suggests that the water molecules in this low-rank coal are held by fairly strong forces.
H
and/or
desorption
of water vapour by moist air
of an experiment. This is termed the characteristic rate of heat loss, and in effect represents twenty times the average hourly heat loss during the early stage of desorption. In certain cases where the tests have been discontinued before the twentieth hour, extrapolated values are taken from the respective graphs.
Effect of the deficiency in the equilibrium humidity of air The variation of the characteristic rates of heat loss from coals B, D, G and H with the initial equilibrium deficiency of the moisture carrier, denoted by -e, is shown in Figure 4. The results of the tests with dry air are plotted in Figure 4a and those done with moist air in Figure 4b. value at zero e for any particular coal in While the Q,,u Figure 4u is the characteristic rate of heat release during its dry oxidation, that in Figure 4b is represented by the average of the QtZZo values calculated from the results of
FUEL, 1972, Vol 51, July
217
Moisture desorption from coal and spontaneous heating: K. K. Bhattacharyya 60
explanation, necessary.
I
I
I
a
I 25
-10’ 0
I
I
50
15
I
100
however, further definitive investigation is
Effect of coal rank The plots of the characteristic rate of heat loss values and the equilibrium moisture contents of the coals saturated at 30°C against the parameters of coal rank are shown in Figure 5. The QtZz,-,value for each coal is taken from the result of the test with the coal saturated at 10% R.H. at 30” and dry air (or nitrogen). Despite the abnormality of coal G (C = 81.5% and VM = 399%), the general relation between the characteristic rates of heat loss and carbon contents or volatile matter of the coals is similar to that between the equilibrium moisture contents of the coals and the parameters of rank. It seems, therefore, that other conditions being similar the characteristic rate of heat loss from a coal due to desorption of moisture is in general dependent on its hygroscopicity.
R.H., -e (%I 30
I
I
b i 60
3%Mo
C I% d.m.m.f1
Figure 5
Variations in the characteristic equilibrium moisture content with coal’rank lQp2B, Oequilibrium moisture
I
I
20
LO R.H., -e
Figure rate of a tests Ocoal
,
I%1
4 Effect of equilibrium deficiency heat loss from coals with dry air, b tests with moist air B, Dcoal D, x coal G, + coal H
of air on the characteristic
oxidation of the coal carried out under several moist conditions. These data are taken from the results reported in an earlier publication’. As the oxidation of a coal is an exothermic reaction in contrast to the currently discussed endothermic process of desorption of water from the coal, the QfZ2u values at zero e are plotted on a negative scale. It is evident from F&me 4 that the characteristic rate of heat loss from each coal due to desorption of moisture increases with the increase in the dryness of the air, and the relation is not rectilinear. This may partly be explained by taking into consideration the apparent inconsistency in the heat of desorption with the advancement of the process, and the usual sigmoid nature of the coal-water desorption isotherms obtained with these coalsg. For a complete
218
FUEL, 1972, Vol 51, July
V.M. (% d.m.mf) rate
of heat
loss and
Effect of coal particle size Four different-sized fractions of coal F were tested under similar conditions, dry air passing through a sample saturated at 10% R.H. at 30°C. The results plotted in Figure 6 show a more or less similar pattern of change in the rate of heat loss with the time of drying. However, there appears to be no clear correlation between the average particle diameter of the coal and its rate of cooling. For example, the rate of heat loss from the finest sized fraction used, -72 + 200 mesh BS, is the highest of the four at the very early stage of the process, but it becomes the lowest after about ten hours and continues to be so until the end of the twenty-hour test. The characteristic rates of heat loss from these sized fractions and their equilibrium moisture contents at 10% R.H. at 30°C are given in Table 2. Whilethe former tends to show a slight decrease with decrease in the average particle size below -36 + 52 mesh, the latter remains constant. According to R. L. Bond et al”, reduction of the particle size below -14 t 25 mesh has very little effect on the moisture content of a coal. It is also shown in a previous Section that for a particular coal the rate of heat loss is mainly dependent on its initial moisture content. Therefore, it is expected that no significant difference in the rate of heat loss should occur with the sized fractions tested, if the heat of desorption per unit weight of water remains the same in each case. The results, however, show
K. K. Bhattacharyya:
Moisture desorption from coal and spontaneous heating
that for much of the time slightly higher rates of heat loss are observed with the coarser particles.
I
I
I
I
I
0
L
u
lb
IL
‘u
LU
Time (h) Figure 7 Effect of weathering of coal on the variation in the rate of heat loss from coal H with time during oxidation and desorption of water vapour by dry air. Coal saturated at 100% R.H. at 30°C,
I
I
L
0
I
12 Time
and dry air l unoxidized, 0 oxidized 0 oxidized 70 days
I
I
16
20
-72
+200
?.A
mesh BS
Tab/e 3 Effect of weathering content and the characteristic moisture Days of oxidation Equilibrium moisture (%, w/w) OF20 (Cal/g, dry coal)
Table 2 Effect of coal particle size on the equilibrium moisture content and the characteristic rate of heat loss due to desorption of moisture Sized fraction (mesh BS) Equilibrium moisture (%, w/w) 0~20 (Cal/g, dry coal)
-25
+ 36
-36
50 days,
[h 1
figure 6 Effect of coal particle size on the variation in the rate of heat loss from coal F with time during oxidation and/or desorption of water vapour by dry air. Coal saturated at 100% R.H. at 3O’C. and dry air 0 -25 + 36 mesh BS, 0 -36 + 52 mesh BS, n -52 + 72 mesh BS, q
30 days, n oxidized
+ 52
-52
+ 72
-72
15.4
15-4
15.4
15.4
67.3
68.9
64-5
64.0
+ 200
Effect of weathering of coal The rate of heat loss from each oxidized sample of the low-rank coal H saturated at 100% R.H. at 30°C was measured during drying with dry air. The results in Figure 7, together with those obtained during a similar test with raw coal H, show that the rate of heat loss in each case gradually increases with the time of drying, and that the curves for the weathered samples are more regular than that for the unoxidized one. The change in the nature of the rate curves for the weathered samples may be an indication of a change in the state of water in the oxidized coal. It appears from Figure 7 that the rates of heat loss from the oxidized samples are generally higher than that from the fresh coal, but there seems to be no proportionality between the rate and the extent of preoxidation. This is also evident from Table 3, where the characteristic values of rate of heat loss are shown with the equilibrium moisture contents of these samples at the saturationvapour pressure
0 19.2 56.5
of coal on the equilibrium moisture rate of heat loss due to desorption of
30
50
70
20-B 58.8
21.8 70.0
23.4 68.5
at 3O’C. The increased moisture-holding capacity of weathered coal is an established fact, and it is also shown in an earlier Section that under similar test conditions the rate of heat loss from a particular coal is dependent on its initial moisture content.
CONCLUSIONS The experimental technique used appears to be particularly suitable for this kind of study of measuring thermal changes in coals during desorption of moisture from them at low temperatures. In the temperature range of 30-35°C the exothermic reaction of coal oxidation is overshadowed by the endothermic process of desorption of water from the coal; the resultant effect is cooling of the coal. The observed relations between the characteristic rate of heat loss and factors such as rank, particle size and weathering of coal indicate that the process is in part governed by the basic physico-chemical structure of the coal. The investigation shows too that at low temperatures the desorption of water from a moist coal by its surroundings has an inhibiting effect on its self-ignition. As long as the partial pressure of water vapour in the air is less than that on the coal surface, the chance of self-heating of the coal is lower. The rate of heat loss from a particular coal due to desorption of its moisture increases with increase in the equilibrium deficiency of the air. In practice, by keeping the ventilation air in a mine dry the danger of spontaneous heating of the coal can be reduced. The
FUEL, 1972, Vol 51, July
219
Moisture desorp tion from coal and spontaneous hea ring: K. K. Bhattacharyya
absence of any artificial source of water in mines will allow the dry incoming air to desorb moisture from the coal seam and its surrounding strata, and thus to keep the coal at a lower temperature. ACKNOWLEDGEMENTS The author wishes to thank Dr D. J. Hodges and Emeritus Professor F. B. Hinsley of the Department of Mining Engineering, University of Nottingham, for their supervision of this work. Thanks are also due to Professor H. J. King, Head of the Department, for his help and permission to publish this paper. The provision of financial aid and the supply of the coal samples by the National Coal Board, UK, are gratefully acknowledged.
220
FUEL, 1972, Vol 51, July
REFERENCES 2 3 4 5 6 I
Migdalski, H. Bergbuutechnik 1957,7, (I), 3 Bhattacharyya, K. K. J. MinesMetals Fuels 1970, 18, (l), 5 Bhattacharyya, K. K. Fuel, Lond. 1971,50,367 Stott, J. B. Coal Mining Research Ann. Rep., University of Otago, N.Z., 1958, 1 Hodges, D. J. and Hinsley, F. B. Trans. Instn Min. Engrs, Lond. 1963-64, 123, 211 Hodges, D. J. and Achejee, B. Trans. Instn Min. Engrs, Lond. 1966-67. 126, 121 Bhattacharyy~, K. K., Hodges, D. J. and Hinsley, F. B. Min. Engr, Lond. 1969,77,274 Hodges, D. J. and Acherjee, B. Lab. Pmt. 1965,40, (7), 842 Bhattacharyya, K. K. PhD Thesis, University of Nottingham, 1968 Bond, R. L., Griffith, M. and Maggs, F. A. P. Fuel, Lond. 1950,29, (4), 83
98 10
UK
As a part of investigations on the influence of humidity on the initial stage of the spontaneous heating of coal, the effect of desorption of moisture from the coal was studied. Laboratory experiments with different coals were carried out under conditions where desorption of water from the coals by air was certain to take place. Thermal changes were measured by a specially designed calorimeter in isothermal conditions, mostly at 3O’C. The results showed that in each case there occurred a cooling of the coal, indicating that the rate of heat loss from a coal due to this process was greater than the rate of lieat release due to oxidation of the coal. The data suggest that moisture desorption acts as an inhibitor to the spontaneous heating of coal. For a particular coal, the rate of heat loss increases with the increase in the equilibrium humidity deficiency of the air. The effects of rank, particle size and weathering on the process are also discussed.
on the influence of moisture on the spontaneous heating of coal have been carried out both in field and laboratory over a long period and in several countries. The common conclusion is that damp conditions favour the self-ignition of coal in mines and storage. It has also been found that the moisture content is the most variable of all parameters of the mine atmosphere’. Both air and coal are capable of holding certain amounts of moisture under given conditions of pressure, temperature and availability of water vapour. In an atmosphere of constant humidity, the water vapour pressures in the coal and air remain in equilibrium states. When the humidity of the atmosphere rises, the coal begins to sorb moisture from the surroundings until a new equilibrium is attained. Conversely, if the surroundings get drier, then the air picks up moisture from the coal. Studies on the thermal changes in coals during the first two conditions have been reported earlier2*3. A laboratory study of the thermal effect of desorption of water from coals by the surrounding atmosphere is described here. Although previous investigators4-6 have studied this particular aspect of desorption of water vapour from the coal, the experimental conditions used appear to be rather extreme. The purpose of the present investigation was, therefore, to measure the heat changes during desorption of moisture from different coals in various humid atmospheres. The effects of rank, particle size and weathering on the process were also examined. Investigations
EXPERIMENTAL The apparatus and its operational method are described in detail in an earlier publication’. In brief, a specially designed calorimeter8 capable of measuring heat loss from the material in its cell was used. All experiments were carried out in an isothermal condition at 30°C with the exception of one test during which the temperature was 35*C. The equilibrium humidities of the moisture carrier * Present address: Faculty of Engineering Science, The University of Western Ontario, London, Ontario, Canada
214
FUEL, 1972, Vol 51, July
and the coals were chosen from 0, 43, 75, 80 and 100% relative humidities at the experimental temperature. The origins and analyses of the coals used are shown in Table 1. About 5 g of coal (-72 mesh BS) was placed in the calorimeter cell and an air (or nitrogen) flow of 2.5 ml/mm g of dry coal was passed through it during each test. The coal sample in the cell was brought to stable thermal and hygrometric conditions by passing through it the desired flow of nitrogen equilibrated at the chosen humidity. The flow of nitrogen was then replaced by air of lower humidity. During two experiments with moist coal and dry air the calorimeter outlet was connected for definite periods to two U-tubes in series containing a known amount of phosphorus pentoxide to absorb the moisture desorbed from the coal sample. The average hourly rate of loss of water from the coal was subsequently calculated from the gain in weight of the phosphorus pentoxide tube and the time of determination. The influence of particle size on the thermal change during desorption of water was studied by using four different sized samplesof -25 t 36, -36 + 52, -52 + 72 and -72 t 200 mesh BS made from coal F. To investigate the effect of weathering on the process, three oxidized samples from the low-rank coal II were tested. A bulk sample of this coal (-72 mesh BS) was oxidized continuously at room temperature in a dry condition by pure oxygen, and the sub-samples were taken out after 30, 50 and 70 days of oxidation.
RESULTS AND DISCUSSION The progress of the experiments was noted by continuously recording the calorimeter output, and the rates of heat change were subsequently calculated from the therms grams. The- results show that in each case there occurred a cooling of the coal. The heat loss data presented here are expressed on a dry coal basis. To estimate the rate of heat loss during desorption of water from the small amount of glass wool, which was mixed with the coal sample during each experiment, tests were done by passing dry air
K. K. Bhattacharyya: Table 1
Analyses of coals used (as supplied
by the NCEl Coal Survey
Moisture desorption from coal and spontaneous heating
Laboratories) Analysis
Sample details
(%) dmmf*
air-dried
Coal
Colliery
Seam
Moisture
Ash
VM
Fixed carbon
Total S
VM
C
H
A 0 C D E F G H
Cynhaidre t Easington Thoresby Bentinck Whitewell Denby Drury Measham
Pumpquart t High Main Top Hard Yard High Hazles Belpar-Lawn Stockings
1.6 1.9 1.6 7.5 5-8 8.4 4-6 5.3
1.3 5-o 5-l 4-9 7.4 2.7 7.5 6.0
4.8 29.7 32.1 30-2 31.6 33.3 34.2 36.7
92.3 634 61.2 57-4 55.2 55.6 53.7 52.0
1.2 o-7 1.6 0.8 l-4 1-3 1-9 1.6
4.8 30.6 33.9 33.5 354 36-9 39.9 42.1
94.2 88.5 86.6 83.7 83.9 82-8 81-5 80.7
2.8 5.2 5-4 4-9 5.6 5.4 5.8 5.2
* t
Dry, mineral-matter Supplied by Cardiff
Lowe
free (Parr’s basis) Coal Survey Laboratory,
Sample no.CSL
2708
through a sample of the wool saturated with water vapour at 30°C. The rate of heat loss from the glass wool was found to be insignificant. The variations of the rate of heat loss with time for all the tests made are shown in Figures 1-3. While coals A, C, E and F were used for a particular humidity condition only, the others were tested under several conditions. A few experiments were performed using nitrogen instead of air during a part or the whole of a test. It appears from these results and those reported earlier’ that under the present test conditions the effect of heat release due to oxidation of the coal is negligible. The loss of heat from the coals indicates that the rate of heat change in a coal during the tests is governed mainly by the process of desorption of water from the coal. Similar observations have been made by Stott4 and by Hodges and Acherjee6. Tests with moist coals and dry air All the coals were used in this series of experiments, and the results are plotted in Figures 1 and 2. The relation between the rate of heat loss and time of desorption of moisture is seen to be similar for the anthracite (coal A, Figure 1) and the high-rank coal B (Figure 2) tested at several humidity conditions. The plot of the results from the test with coal C (Figure I), carried out under conditions similar to those of coal A, shows two peaks prior to any consistent decrease in the rate of heat loss. The tests with the medium-rank coal D (Figure 2) initially equilibrated at 43, 75 and 80% RH. at 30°C show that in each case the rate of heat loss gradually increases to an apparent maximum and does not show any appreciable decrease during the experimental period. However, when the initial equilibration humidity of the coal is raised to 100% R.H., the rate increases with the time of drying after a slight decrease at the outset. The results of the tests with coals E and F (Figure I), both saturated with water vapour at 3O”C, show different rate-time relations. While for the former the rate of heat loss becomes more or less constant after a steady initial rise, for coal F it increases during the first hour of drying and then decreases slightly for some time before increasing again. The irregularity of this latter type becomes more pronounced during the tests with the low-rank coals G and H (Figure 2). When coal G equilibrated at 43% R.H. at 30°C is subjected to dry air, the variation of the rate of heat loss with time is somewhat similar to that observed with coal C. However, the results from the tests with coal
G containing higher initial moisture and with coal H are more or less similar to those obtained with coal F. The difference in the nature of variation of the rate of heat loss with the time of drying of different moist coals may be explained when the type of coal surface, the state of sorbed water, the gravimetric rate of water loss, and the heat of desorption of unit weight of water are taken into consideration. Owing to experimental limitations it was only possible to estimate the rate of loss of water from the coal during two experiments. According to some investigators’34, when dry air is passed through a moist coal, the rate of loss of water reaches its maximum within a few minutes from the start and then decreases continuously throughout the process. The maximum rate of water loss depends on the initial moisture content of the coal and the rate of air flow. The general shape of such a gravimetric rate curve is similar to that of the rate-of-heat-loss curves presently obtained with the anthracite coal A and the highrank coal B. This is possibly because the water molecules
L.0
0 0
l
,&-
L
B
12 Time(h)
16
20
L 2L
Figure 1 Variation in the rate of heat loss from coal with time during oxidation and/or desorption of water vapour by dry air. Coal saturated at 100% R.H. at 3O”C, and dry air *coal A, n coal C,Acoal E,ncoal F
FUEL, 1972, Vol 51, July
215
Moisture desorp tion from coal and spontaneous heating: K. K. Bha ttacharyya I
I
I
I
Coai
I
6
05 0
L
8
I
I
L
0
Time (h)
I
I
I
I
Coal
I
12 Time ( hl
16
20
I
I
16
20
L.0
0
3.0
2.0
I.0
I
I 01
0
Time
(h 1
*
1 Cal/g = 4.187
216
J/g
FUEL, 1972, Vol51,
July
12 Time
Variation in the rate of heat loss from coal with time during oxidation figure 2 OCoal equilibrated @Coal equilibrated at 43% R.H. at 30°C and dry air/N2 q Coal equilibrated n Coal equilibrated at 80% R.H. at 30°C and dry air
are held rather loosely in these coals of hydrophobic nature. The similarity also suggests that the heat of drying for these coals probably remains more or less constant throughout the desorption process. In contrast to the above, the rate-of-heat-loss curves obtained from the medium- and low-rank coals are disThe results similar from the rate-of-water-loss curve. apparently indicate that the heat loss per unit weight of water desorbed from these coals may increase with the During the experiment with coal D extent of drying. equilibrated at 80% R.H. at 3O”C, it was estimated that while the average heat of drying for the first six hours was 614 Cal/g* of water desorbed, that for the next six hours was 812 Cal/g. For the test with coal H equilibrated at 75% R.H. at 3O”C, the average heat loss owing to desorption of water was found to increase from 845 Cal/g during the first four hours to 906 Cal/g for the next eighteen hours. Observations in agreement with these results have also been made by other investigators4y6. Owing to their increasingly hydrophilic nature, the
I
2L
(h)
and/or desorption of water vapour by dry air at 75% R.H. at 30°C and dry air at 100% R.H. at 30°C and dry air
medium- and low-rank coals sorb water vapour in a manner different from the high-rank coal and anthracite. The forces of attraction between the coal surface and the water molecules are fairly strong in the former types. The occurrence of multilayer sorption of water in these coals suggests that the layers below the top one are under compression, partly owing to the forces of attraction and partly because each layer is compressed by other layers above it. So the compression of the layer decreases from the innermost to the outermost layers. Moreover, the water molecules inside the pores and capillaries of the coal are attracted on all sides. Therefore it is to be expected that the heat of desorption in such cases would increase as the coal gets drier. Tests with
moist
coals and moist air
This series of tests was carried out with coals B, D, G and H; the results are shown in Figure 3. The nature of variation in the rate of heat loss with time appears to be similar for coals B, D and G. During any particular test the rate increased with time to a maximum value and then
K. K. Bhattacharyya: I
I
I
I
Coal
Time
Moisture desorption from coal and spontaneous heating
I 0
Coal G
I
I
I
I
I
1,
8
12 Time (hl
16
20
[h)
2-o------------(Coal
0
Figure
0
L
3 Variation aCoal equilibrated OCoal equilibrated n Coal equilibrated nCoal equilibrated ACoal equilibrated
8
12 Time I h)
16
I 20
OO-'-'
Effects of some variables on the rate of heat loss The influence of some factors on the rate of heat loss due to desorption of water from the coal has been studied by a common basis of comparison. The basis chosen is QtZZo, the total heat loss from a coal during twenty hours
16
2l
Time (h)
in the rate of heat loss from coal with time during oxidation at 100% R.H. and air equilibrated at 43% R.H. at 30°C at 100% R.H. and air equilibrated at 80% R.H. at 30°C at 80% R.H. and N2/air equilibrated at 43% R.H. at 35’C at 80% R.H. and air/N2 equilibrated at 43% R.H. at 30°C at 80% R.H. and air equilibrated at 75% R.H. at 30°C
decreased progressively as the coal approached the new equilibrium. The maximum rate of heat loss increases with an increase in the initial equilibrium deficiency of the air. When this deficiency is small, the curves become flatter. Contrary to this, the rate curves for coal H show not only some irregularity, but there is also a tendency for the rate to increase with time of desorption of water. This tendency is more pronounced when the initial equilibrium deficiency of the air is larger. The increase in the rate of heat loss with the period of desorption of water from coal H clearly suggests that the water molecules in this low-rank coal are held by fairly strong forces.
H
and/or
desorption
of water vapour by moist air
of an experiment. This is termed the characteristic rate of heat loss, and in effect represents twenty times the average hourly heat loss during the early stage of desorption. In certain cases where the tests have been discontinued before the twentieth hour, extrapolated values are taken from the respective graphs.
Effect of the deficiency in the equilibrium humidity of air The variation of the characteristic rates of heat loss from coals B, D, G and H with the initial equilibrium deficiency of the moisture carrier, denoted by -e, is shown in Figure 4. The results of the tests with dry air are plotted in Figure 4a and those done with moist air in Figure 4b. value at zero e for any particular coal in While the Q,,u Figure 4u is the characteristic rate of heat release during its dry oxidation, that in Figure 4b is represented by the average of the QtZZo values calculated from the results of
FUEL, 1972, Vol 51, July
217
Moisture desorption from coal and spontaneous heating: K. K. Bhattacharyya 60
explanation, necessary.
I
I
I
a
I 25
-10’ 0
I
I
50
15
I
100
however, further definitive investigation is
Effect of coal rank The plots of the characteristic rate of heat loss values and the equilibrium moisture contents of the coals saturated at 30°C against the parameters of coal rank are shown in Figure 5. The QtZz,-,value for each coal is taken from the result of the test with the coal saturated at 10% R.H. at 30” and dry air (or nitrogen). Despite the abnormality of coal G (C = 81.5% and VM = 399%), the general relation between the characteristic rates of heat loss and carbon contents or volatile matter of the coals is similar to that between the equilibrium moisture contents of the coals and the parameters of rank. It seems, therefore, that other conditions being similar the characteristic rate of heat loss from a coal due to desorption of moisture is in general dependent on its hygroscopicity.
R.H., -e (%I 30
I
I
b i 60
3%Mo
C I% d.m.m.f1
Figure 5
Variations in the characteristic equilibrium moisture content with coal’rank lQp2B, Oequilibrium moisture
I
I
20
LO R.H., -e
Figure rate of a tests Ocoal
,
I%1
4 Effect of equilibrium deficiency heat loss from coals with dry air, b tests with moist air B, Dcoal D, x coal G, + coal H
of air on the characteristic
oxidation of the coal carried out under several moist conditions. These data are taken from the results reported in an earlier publication’. As the oxidation of a coal is an exothermic reaction in contrast to the currently discussed endothermic process of desorption of water from the coal, the QfZ2u values at zero e are plotted on a negative scale. It is evident from F&me 4 that the characteristic rate of heat loss from each coal due to desorption of moisture increases with the increase in the dryness of the air, and the relation is not rectilinear. This may partly be explained by taking into consideration the apparent inconsistency in the heat of desorption with the advancement of the process, and the usual sigmoid nature of the coal-water desorption isotherms obtained with these coalsg. For a complete
218
FUEL, 1972, Vol 51, July
V.M. (% d.m.mf) rate
of heat
loss and
Effect of coal particle size Four different-sized fractions of coal F were tested under similar conditions, dry air passing through a sample saturated at 10% R.H. at 30°C. The results plotted in Figure 6 show a more or less similar pattern of change in the rate of heat loss with the time of drying. However, there appears to be no clear correlation between the average particle diameter of the coal and its rate of cooling. For example, the rate of heat loss from the finest sized fraction used, -72 + 200 mesh BS, is the highest of the four at the very early stage of the process, but it becomes the lowest after about ten hours and continues to be so until the end of the twenty-hour test. The characteristic rates of heat loss from these sized fractions and their equilibrium moisture contents at 10% R.H. at 30°C are given in Table 2. Whilethe former tends to show a slight decrease with decrease in the average particle size below -36 + 52 mesh, the latter remains constant. According to R. L. Bond et al”, reduction of the particle size below -14 t 25 mesh has very little effect on the moisture content of a coal. It is also shown in a previous Section that for a particular coal the rate of heat loss is mainly dependent on its initial moisture content. Therefore, it is expected that no significant difference in the rate of heat loss should occur with the sized fractions tested, if the heat of desorption per unit weight of water remains the same in each case. The results, however, show
K. K. Bhattacharyya:
Moisture desorption from coal and spontaneous heating
that for much of the time slightly higher rates of heat loss are observed with the coarser particles.
I
I
I
I
I
0
L
u
lb
IL
‘u
LU
Time (h) Figure 7 Effect of weathering of coal on the variation in the rate of heat loss from coal H with time during oxidation and desorption of water vapour by dry air. Coal saturated at 100% R.H. at 30°C,
I
I
L
0
I
12 Time
and dry air l unoxidized, 0 oxidized 0 oxidized 70 days
I
I
16
20
-72
+200
?.A
mesh BS
Tab/e 3 Effect of weathering content and the characteristic moisture Days of oxidation Equilibrium moisture (%, w/w) OF20 (Cal/g, dry coal)
Table 2 Effect of coal particle size on the equilibrium moisture content and the characteristic rate of heat loss due to desorption of moisture Sized fraction (mesh BS) Equilibrium moisture (%, w/w) 0~20 (Cal/g, dry coal)
-25
+ 36
-36
50 days,
[h 1
figure 6 Effect of coal particle size on the variation in the rate of heat loss from coal F with time during oxidation and/or desorption of water vapour by dry air. Coal saturated at 100% R.H. at 3O’C. and dry air 0 -25 + 36 mesh BS, 0 -36 + 52 mesh BS, n -52 + 72 mesh BS, q
30 days, n oxidized
+ 52
-52
+ 72
-72
15.4
15-4
15.4
15.4
67.3
68.9
64-5
64.0
+ 200
Effect of weathering of coal The rate of heat loss from each oxidized sample of the low-rank coal H saturated at 100% R.H. at 30°C was measured during drying with dry air. The results in Figure 7, together with those obtained during a similar test with raw coal H, show that the rate of heat loss in each case gradually increases with the time of drying, and that the curves for the weathered samples are more regular than that for the unoxidized one. The change in the nature of the rate curves for the weathered samples may be an indication of a change in the state of water in the oxidized coal. It appears from Figure 7 that the rates of heat loss from the oxidized samples are generally higher than that from the fresh coal, but there seems to be no proportionality between the rate and the extent of preoxidation. This is also evident from Table 3, where the characteristic values of rate of heat loss are shown with the equilibrium moisture contents of these samples at the saturationvapour pressure
0 19.2 56.5
of coal on the equilibrium moisture rate of heat loss due to desorption of
30
50
70
20-B 58.8
21.8 70.0
23.4 68.5
at 3O’C. The increased moisture-holding capacity of weathered coal is an established fact, and it is also shown in an earlier Section that under similar test conditions the rate of heat loss from a particular coal is dependent on its initial moisture content.
CONCLUSIONS The experimental technique used appears to be particularly suitable for this kind of study of measuring thermal changes in coals during desorption of moisture from them at low temperatures. In the temperature range of 30-35°C the exothermic reaction of coal oxidation is overshadowed by the endothermic process of desorption of water from the coal; the resultant effect is cooling of the coal. The observed relations between the characteristic rate of heat loss and factors such as rank, particle size and weathering of coal indicate that the process is in part governed by the basic physico-chemical structure of the coal. The investigation shows too that at low temperatures the desorption of water from a moist coal by its surroundings has an inhibiting effect on its self-ignition. As long as the partial pressure of water vapour in the air is less than that on the coal surface, the chance of self-heating of the coal is lower. The rate of heat loss from a particular coal due to desorption of its moisture increases with increase in the equilibrium deficiency of the air. In practice, by keeping the ventilation air in a mine dry the danger of spontaneous heating of the coal can be reduced. The
FUEL, 1972, Vol 51, July
219
Moisture desorp tion from coal and spontaneous hea ring: K. K. Bhattacharyya
absence of any artificial source of water in mines will allow the dry incoming air to desorb moisture from the coal seam and its surrounding strata, and thus to keep the coal at a lower temperature. ACKNOWLEDGEMENTS The author wishes to thank Dr D. J. Hodges and Emeritus Professor F. B. Hinsley of the Department of Mining Engineering, University of Nottingham, for their supervision of this work. Thanks are also due to Professor H. J. King, Head of the Department, for his help and permission to publish this paper. The provision of financial aid and the supply of the coal samples by the National Coal Board, UK, are gratefully acknowledged.
220
FUEL, 1972, Vol 51, July
REFERENCES 2 3 4 5 6 I
Migdalski, H. Bergbuutechnik 1957,7, (I), 3 Bhattacharyya, K. K. J. MinesMetals Fuels 1970, 18, (l), 5 Bhattacharyya, K. K. Fuel, Lond. 1971,50,367 Stott, J. B. Coal Mining Research Ann. Rep., University of Otago, N.Z., 1958, 1 Hodges, D. J. and Hinsley, F. B. Trans. Instn Min. Engrs, Lond. 1963-64, 123, 211 Hodges, D. J. and Achejee, B. Trans. Instn Min. Engrs, Lond. 1966-67. 126, 121 Bhattacharyy~, K. K., Hodges, D. J. and Hinsley, F. B. Min. Engr, Lond. 1969,77,274 Hodges, D. J. and Acherjee, B. Lab. Pmt. 1965,40, (7), 842 Bhattacharyya, K. K. PhD Thesis, University of Nottingham, 1968 Bond, R. L., Griffith, M. and Maggs, F. A. P. Fuel, Lond. 1950,29, (4), 83
98 10