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The role of moisture in the self-heating of low-rank coals
ELSEVIER
Fuel Vol. 75, No. 7, pp. 891-895, 1996 Copyright © 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0016-2361/96 $15.00+0.00
S0016-2361(96)00010-5
The role of moisture in the self-heating of low-rank coals Anthony H. Clemens and Trevor W. Matheson Coal Research Limited, P.O. Box 31-244, Lower Hutt, New Zealand (Received 10 March 1995; revised 2 January 1996)
The self-heating of coal mainly involves exothermic reactions of oxygen at reactive radical sites within the coal and the enhancing or moderating effect that water has on these reactions. The thermal response of samples of low-rank coals, dried by heating under nitrogen flow at 105°C and exposed to dry oxygen, is similar to or slightly less than that observed when they are flow-dried at 30°C and tightly bound moisture remains. The most likely reason is that moisture affects the nature of the radical sites where oxidation occurs. By hindering the formation of stabilized radicals, it encourages faster oxidation which may lead to enhanced thermal response, although some of the extra heat may be taken up by the residual moisture. When loosely bound moisture is allowed to remain in the coal, the thermal response on exposure to dry oxygen decreases very quickly, due mainly to hindered access to reactive sites and dissipation of heat generated by any oxidation that does occur. The effect of desorption is comparatively minor and the course of the oxidation reaction responsible for generating heat does not appear to be changed by the presence of small quantities of loosely bound moisture. Copyright © 1996 Elsevier Science Ltd. (Keywords: low-rank coals; serf-heating; moisture)
The self-heating of coal is influenced by many factors, but mainly it involves exothermic reactions between reactive sites in the coal and oxygen from the air and the enhancing or moderating effects that water has on these reactions. Between them, these processes are largely responsible for initiating self-heating and determining the extent to which it develops. The reaction between coal and oxygen at ambient temperatures has been widely investigated and the general consensus 1'2 is that it proceeds through attack of oxygen at reactive radical sites within the coal to generate peroxy compounds in a manner analogous to the oxidation of hydrocarbons. The subsequent breakdown of these peroxy compounds releases heat into the system which, if not sufficiently dissipated, leads to localized temperature increases, thus helping to promote the self-heating process. The influence of water on the self-heating process is complex. If present in the coal in sufficient quantities, it suppresses self-heating by reducing the number of available reactive sites, blocking access of oxygen to these sites--the rate of diffusion of oxygen in air is reportedly 3 four orders of magnitude greater than in w a t e r - - a n d by taking up the heat released by oxidation as it occurs. Further suppression can result from the endothermic process of desorption of water from the coal 4. If the moisture content of the coal is lowered, it may be expected that these moderating influences will become less effective and a significantly greater level of self-heating will occur on exposure of the partially dried coal to oxygen. It has been reported 3 that brown coals undergo oxidation most rapidly when their moisture
content is reduced to between 5 and 10wt%; a New Zealand subbituminous coal was found to undergo oxidation most rapidly when it contained between 7 and 17 wt% moisture 5. It appears that the presence of a small amount of moisture enhances oxidation, and this is sometimes ascribed to a catalytic effect of the moisture and its known ability to encourage formation of the oxidation accelerant sulfuric acid through oxidation of pyritic sulfur in the coal 6. The removal of water brings about the collapse of the colloidal gel-like structure 7-9 found in low-rank coals, although if this were to significantly inhibit self-heating there would be no danger of the process occurring in dried coals. The extent and method of drying influences the concentration and nature of the reactive sites within a coal 1°-12. It is reported that the presence of water may also change the oxidation reaction products 1334. In addition, the water may be bound in several ways within a coal. These include loosely bound water in pores, capillary condensed water and water that is hydrogen-bonded to polar groups 15-17. The self-heating of three low-rank coals has been the subject of an ongoing investigation in the authors' laboratory. In the first stages 1839 the thermal and chemical responses of dried samples of the coals on exposure to dry oxygen at different temperatures including nearly atmospheric (30°C) were investigated. These studies confirmed that the major reaction contributing to heat generation at this temperature involved the breakdown of peroxy functional groups to form carboxylic acid products. It was also found that reactions of this sort continued to be major contributors over the
Fuel 1996 Volume 75 Number 7
891
The role of moisture in the self-heating of low-rank coals." A. 14. Clemens and T. W. Matheson Table
1 Compositionof freshcoals (wt%)~
Moisture Ash C H N Total S Organic S PyriticS SulfateS O
New Vale
Kai Point
Kopako
39.4 5.4 64.9 5.4 0.7 0.4 0.39 0.00 0.01 23.2
28.7 5.4 69.1 5.5 0.9 4.1 4.00 0.08 0.02 15.0
26.0 3.2 71.8 5.7 1.3 0.1 0.10 0.00 0.00 17.9
a Dry basis (exceptmoisture);oxygenby difference temperature range from atmospheric to near the ignition point. Later studies showed l° that the removal of water from these coals increased the concentration and changed the types of radical sites present within them. The outcome depended on a balance between the decarboxylation reactions producing new radicals and the removal of water molecules from heteroatomic sites within the coal macrostructure, allowing greater interaction between unpaired electrons and the heteroatoms. The balance was affected by the drying method used (vacuum or flow drying). This paper examines the tendency of the three lowrank coals to undergo self-heating as their moisture content is changed from dry to ,,~50% of the original level. The tendency to self-heat was measured by the thermal responses that accompanied exposure to oxygen of the dry and partially dry samples of the coals. The experiments were devised to highlight as much as possible each of the several roles played by water, to determine the moderating or enhancing effect of each and their relative importance to the initiation of the selfheating process in coals. A future paper will examine the effect of exposing dry samples of coal to reactant gases containing different amounts of moisture.
EXPERIMENTAL The samples investigated were the two lignites, New Vale and Kai Point, and the subbituminous Kopako coal used in the previous studies 1°'~8'19. Their properties are listed in Table 1. They were obtained fresh from the mine and stored as lumps under water until needed. Before use, the coals were reduced in size while still under water and transferred to a nitrogen-filled glovebag where all subsequent grinding, sieving and storage operations were carried out. Initial grinding was done with a micro hammer mill followed by further grinding (to <76/zm) with a mortar and pestle. All the thermal experiments involved measuring the thermal response when an accurately weighed sample of coal (10-20mg) placed in the sample chamber of a differential scanning calorimeter (d.s.c.) was exposed to a 20 ml min -1 flow of reactant gas. In all cases the thermal response was monitored for 30min and calculated in MJkg -1 of dry coal from the area under the thermogram. The first experiments were run to ensure that no artificial thermal responses were being generated by the design of the experimental apparatus. Each coal was placed in the sample chamber of the d.s.c., which was
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Fuel 1996 Volume 75 Number 7
preset to 30°C and flushed with dry nitrogen (20 ml min-1). After 20 min, by which time only tightly bound moisture remained and a constant reading was being maintained, the coal was exposed to the gas flowing through the reactant gas line, which in this case was simply more dry nitrogen. No thermal response was observed during 30 min monitoring. A second set of experiments was run to determine the tendency toward self-heating of coals retaining tightly bound moisture. This involved repeating the above experiments except that dry oxygen instead of nitrogen was introduced from the reactant line. In a third series of experiments the coals were introduced to the sample chamber of the d.s.c, preset to 30°C and flushed with moist nitrogen generated by bubbling through a saturated potassium carbonate solution (44% r.h. at 18.5°C). After 20rain, by which time the coal contained both tightly and loosely bound moisture and the system had equilibrated, the gas flow was switched to dry oxygen and the thermal response was monitored. To determine the contribution due to desorption, the experiments were repeated with dry nitrogen introduced from the reactant gas line. The third series of experiments was repeated with the d.s.c, chamber in which the coals were placed swept with moist nitrogen generated by passing through a saturated solution of potassium acetate (20% r.h. at 20.0°C). This gave the coal samples a lower moisture content than that obtained by equilibration over saturated potassium carbonate. In a final series of experiments the coals were dried by placing in the sample chamber of the d.s.c, and heating to 105°C under a flow of nitrogen, held at this temperature for 10min, then cooled to 30°C, held a further 10min and then exposed to the gas (dry nitrogen or oxygen) in the reactant gas line. The blank run with dry nitrogen again gave no response. To check whether the presence of moisture was changing the oxidation reaction products, the second and third series of experiments were duplicated in, and monitored by, an FT-i.r. spectrometer. Samples of the coals were placed in a Harrick HVC-DR2 vacuum reaction chamber FT-i.r. accessory which was set at 30°C and flushed with either dry nitrogen or moist nitrogen generated by passing through one or other of the saturated solutions. After 20 min, the gas was switched to dry oxygen and the infrared spectra of the samples were monitored until reaction products were observed. The moisture contents of the coals used in each of the above experiments were determined by exposing them to a dry or moist nitrogen flow until a constant moisture content was obtained. It was assumed that the same equilibrium moisture content had been reached when coal in the d.s.c, was exposed to moist nitrogen flows and the d.s.c, response was steady. RESULTS AND DISCUSSION When New Vale lignite was placed under a flow of dry nitrogen at 30°C, the moisture content of the lignite quickly fell from 39.4 to 6.7 wt% and then remained at that level (Table 2). Under a nitrogen flow at 105°C this residual moisture was removed. When equilibrated under an atmosphere generated from a saturated solution of potassium acetate, the moisture level fell to 9.9wt% and from saturated potassium carbonate to
The role of moisture in the serf-heating of low-rank coals: A. H, Clemens and T. W. Matheson Table 2 Thermal response on exposure of dried and partially dried coals to dry gases
Drying methoda
Moisture (wt%)
Reactant gas
Thermal response (MJ kg -1 dry coal)
New Vale
A A A B B C C D D
0 0 0 6.7 6.7 9.9 9.9 12.9 12.9
O2 N2 Air 02 N2 02 N2 02 N2
+0.026 0.000 +0.006 +0.026 0.000 +0.016 -0.002 +0.003 -0.004
Kai Point
A A B B C C D D
0 0 6.1 6.1 8.0 8.0 12.6 12.6
02 N2 02 N2 02 N2 O2 N2
+0.026 0.000 +0.029 0.000 +0.015 -0.002 +0.002 -0.005
Kopako
A A B B C C D D
0 0 6.O 6.0 8.8 8.8 13.4 13.4
02 N2 O N2 02 N2 02 N2
+0.024 0.000 +O.024 0.000 +0.013 -0.003 +0.003 -0.005
Coal
aA, N 2 flow at 105°C; B, N 2 flow at 30°C; C, N 2 equilibrated over saturated KOAc; D, N2 equilibrated over saturated K2CO3
3.0
2.25
E v
~= =~
1.5
r
0,75
I
I
I
~
0
5
10
15
l
20
I
25
30
Time ( min )
Figure 1 Thermal response when dry New Vale lignite is exposed to dry oxygen
12.9 wt%. Therefore for New Vale the moisture contents ranged from zero to ,,~30% of the levels found in the asreceived sample. For Kai Point and Kopako the range was from zero to near 50% (Tables 1, 2). When the New Vale sample dried under nitrogen flow at I05°C was exposed to dry oxygen, an immediate exothermic response was observed. It reached a maximum value within 1 min and tailed off slowly thereafter (Figure 1). The heat released during 30min exposure (0.026MJkg -1 dried coal) was 4.5 times greater than that (0.006MJkg -1) found when the dried coal was exposed to dry air. The heat differential reflects the relative oxygen contents of the two reactant gases and suggests that reactive sites within the coal are not saturated when exposed to oxygen alone. The observation of initial rapid heat release followed
by a gradual tailing-off is as expected for the addition of oxygen to reactive radical sites within the dried coal. Such processes are exothermic and proceed rapidly during the initial stages while radical sites are readily available, then slow down as the concentration of reactive sites is diminished. The values obtained in this study indicate an uptake of,-~8 × 10-5 mol of oxygen per gram of dry coal, based on the heat release data for carbonaceous materials and other low-rank coals 2°'2l on exposure to oxygen. The apparatus used in these experiments did not allow accurate measurement of oxygen uptake, but previous studies 22 have shown that exposure of dried samples of these coals to a 30:70 oxygen-argon atmosphere at 30°C for 5 h leads to the consumption of ,,~1 z 10-4tool of oxygen per gram of dry coal. When the New Vale sample dried by nitrogen gas flow at 30°C and containing tightly bound moisture (6.7 wt%) was exposed to dry oxygen, the heat released (0.026MJkg -1) was identical to that found when the sample was dried at 105°C (Table 2), and the thermogram was the same as that shown in Figure 1. Similar results were obtained for the Kopako sample, whilst for Kai Point the heat released from the sample containing tightly bound moisture was slightly greater than that obtained from its totally dry counterpart. These results are in agreement with observations of previous workers 3'5 and could possibly be ascribed to moisture catalysing the oxidation reaction. However, to do so directly, the water molecule would be required to be a hydrogen atom d o n o r - - a n extremely unlikely event J6 at 30°C. It should also be noted that these coals contain little if any pyritic or sulfate sulfur (Table 1). It is therefore unlikely that the moisture acts as an intermediate in the oxidation of pyrite that could lead to sulfuric acid formation and enhanced oxidation of the coal. Rather than catalysing the oxidation reaction, it may be that the presence of tightly bound moisture generates radical sites in the coal (where oxidation occurs) that are more reactive than those derived from the fully dried coal. Support for this comes from previous observations ~° that as the coals are dried, the water molecules associated with heteroatoms are removed, allowing increased interaction between the heteroatoms and the unpaired electron density. This favours formation of more persistent, less reactive radical sites. Tightly bound moisture appears to be preferentially associated with heteroatoms, thereby blocking their interaction with the unpaired electrons and resulting in less persistent, more reactive sites and faster oxidation reactions. Offsetting this is the ability of the remaining water to take up some of the heat generated. In the particular cases of New Vale lignite and Kopako coal the two opposing effects are seen to cancel each other, whilst for Kai Point lignite the sample containing tightly bound moisture is slightly more prone to self-heating than the completely dried sample. The infrared studies suggest that the oxidation reaction mechanism is not changed by the presence of tightly bound moisture. The products (carboxylic acids absorbing at 1690cm-l)lare the same as those observed previously with dry coal 8,19. When New Vale pre-equilibrated with 20% r.h. nitrogen and containing 9.9 wt% moisture was exposed to dry oxygen, an exothermic response was seen immediately. It tailed off during the next several minutes
Fuel 1996 Volume 75 Number 7
893
The role of moisture in the self-heating of low-rank coals: A. H. Clemens and T. W. Matheson 3.0
3.0
225 -
2.25
E
.
~
.
.
.
E
.
~ i1=
1,5
1.5-
Heat released = 3 x 10 "3 MJ/kg
"r
-r
0.75
Heat released = 16 x 10 "3 MJ/kg
075
0
'
5
1'0
;5
2'0
2'5
I 0
30
I 5
/ 10
Figure 2 Thermal response when New Vale with 9.9 wt% moisture is exposed to dry oxygen
I 25
30
Figure 4 Thermal response when New Vale with 12.9 wt% moisture is exposed to dry oxygen
30
3.0
2.25
i
E
E 15
o
I 20
Time ( m i n )
Time ( rain )
2.25
I 15
~ o
J _ 1.5
Heat absorbed = - 2 x 10-3MJ/kg
Heat absorbed = - 4
-1-
x 10"3MJ/kg
"T"
0.75
0175
i
0[
5I
110
15
210
25
I
30
0
110
115
210
I
25
30
Time ( min )
Time ( mir~ )
Figure 3 Thermal response when New Vale with 9.9 wt% moisture is
[
5
Figure 5 Thermal response when New Vale with 12.9 wt% moisture is
exposed to dry nitrogen
exposed to dry nitrogen
(Figure 2).
In this case the increased amount of loosely bound moisture almost completely eliminates the thermal response. Again, only a small part of the decrease is due to moisture desorption; most probably relates to blocked access, fewer radical sites and heat dissipation. The situation is not so clear-cut as in the previous experiments because the infrared studies now show a broad peak centred at l l 0 0 c m -l in addition to the carboxylic acid signal at 1690 cm -1 . The possibility that a different (less exothermic) oxidation reaction is occurring cannot now be ruled out.
Overall the heat released was 0.016 MJ kg -1 . Exposure to dry nitrogen produced an endothermic response of 0.002MJkg -l. The response was not immediate, taking between 3 and 8min to reach a maximum before tailing off (Figure 3). Similar trends were found for Kopako and Kai Point (Table 2). This shows that once a small amount of loosely bound moisture is left in the coal, the thermal response on exposure to dry oxygen begins to decrease rapidly. Very little of the decrease is observed to be due to moisture desorption, and the FT-i.r. studies suggest that the small amount of loosely bound moisture does not change the oxidation reaction mechanism, as the carboxylic acid signal at 1690 cm -l is still evident. The major reason for the decreased thermal response is therefore probably related to moisture inhibiting access of oxygen to reactive radical sites 3'17'23 and dissipating heat released by oxidation as it occurs. The result is perhaps not too surprising, given the very slow diffusion rate of oxygen in water relative to that in air 3. When New Vale containing 12.9wt% moisture was exposed to dry oxygen, an immediate sharp exothermic response occurred, but within 10 min this had subsided to the baseline value (Figure 4). The overall exothermic response was only 0.003 MJ kg -1 . Exposure of the moist New Vale sample to dry nitrogen generated an endothermic response, equivalent to 0.004 MJ kg -l coal (Figure 5). Again the Kai Point and Kopako samples behaved similarly (Table 2) to New Vale.
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Fuel 1996 Volume 75 Number 7
CONCLUSIONS W h e n samples of low-rank coals containing varying amounts of moisture (from dry up to ~50°/s of their asreceived moisture contents) are exposed to dry oxygen, the corresponding heat release is the result of a complex set of competing factors. For all the coals the heat released during 30 rain exposure by the dry sample and the sample containing tightly bound moisture is the same or nearly so. The most likely cause is that the tightly bound moisture favours the formation of radical sites (where oxidation occurs) that are more reactive than when water is removed completely. This is the reason that low-rank coals with 5 to I0 wt°/e moisture are found to undergo oxidation more rapidly than completely dried samples. The increased rate of oxidation may not always translate into more rapid self-heating, because the
The role of moisture in the self-heating of low-rank coals. A. H. Clemens and T. W. Matheson residual m o i s t u r e is able to dissipate s o m e o f the heat g e n e r a t e d f r o m the o x i d a t i o n reactions. W h e n small a m o u n t s o f loosely b o u n d w a t e r are left in the coals, the t h e r m a l response on e x p o s u r e to o x y g e n quickly decreases. T h e d e c r e a s e d response is n o t due to d e s o r p t i o n effects or, in m o s t cases, to a c h a n g e in o x i d a t i o n reaction. R a t h e r it is due p r i m a r i l y to m o i s t u r e b l o c k i n g access to reactive sites a n d further dissipating heat f r o m a n y o x i d a t i o n t h a t m a y t a k e place. A c o n s e q u e n c e o f this finding is that well over 50% o f the original m o i s t u r e c o n t e n t can safely be r e m o v e d f r o m these l o w - r a n k coals w i t h o u t the risk o f self-heating.
ACKNOWLEDGEMENTS T h e a u t h o r s t h a n k the F o u n d a t i o n for Research, Science a n d T e c h n o l o g y for funding s u p p o r t .
4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
REFERENCES 19 1 2 3
Kim, A. G. 'Laboratory Studies on Spontaneous Heating of Coal. A Summary of Information in the Literature', Information Circular 8756, US Bureau of Mines, 1977 Davidson, R. M. 'Natural Oxidation of Coal', IEACR/29, IEA Coal Research, London, 1990 Panaseiko, N. P. Solid Fuel Chem. 1974, (8), 21 (trans. from Khim. Tverd. Topl. 1974, (8), 26)
20 21 22 23
Bhattacharyya, K. K. Fuel 1972, 51,214 Chen, X. D. and Stott, J. B. Fuel 1993, 72, 787 Schmal, R. In 'Chemistry of Coal Weathering', Coal Science and Technology 14 (Ed. C.R. Nelson), Elsevier, Amsterdam, 1989, Ch 6 Gorbaty, M. L. Fuel 1978, 57, 166 Deevi, S. C. and Suuberg, E. M. Fuel 1987, 66, 454 Suuberg, E. M., Otake, Y., Yun, Y. and Deevi, S. C. Energy Fuels 1993, 7, 384 Carr, R. M., Kumagai, H., Peake, B. M., Robinson, B. H., Clemens, A. H. and Matheson, T. W. Fuel 1995, 74, 389 Dack, S. W., Hobday, M. D., Smith, T. D. and Pilbrow, J. R. Fuel 1983, 62, 1510 Buckmaster, H. A. and Kudynska, J. Fuel 1992, 71, 1147 Huggins, F. E., Huffman, G. P., Kosmack, D. A. and Lowenhaupt, D. E. Int. J. CoalGeoL 1983, 3, 157 Gethner, J. S. Appl. Spectrosc. 1987, 41, 50 Ingram, G. R. and Rimstidt, J. D. Fuel 1984, 63, 292 Nelson, C. R. In 'Chemistry of Coal Weathering', Coal Science and Technology 14, (Ed. C. R. Nelson), Elsevier, Amsterdam, 1989, Ch. 1 Vorres, K. S., Wertz, D. L., Malhotra, V., Dang, Y., Joseph, J. T. and Fisher, R. Fuel 1992, 71, 1047 Clemens, A. H., Matheson, T. W. and Rogers, D. E. Fuel 1990, 69, 255 Clemens, A. H., Matheson, T. W. and Rogers, D. E. Fuel 1991, 70, 215 Kaji, R., Hishinuma, Y. and Nakamura, Y. Fuel 1987, 66, 154 O'Neill, M. and Philips, J. J. Phys. Chem. 1987, 91, 2867 Matheson, T. W. and Rogers, D. E. 'Low Temperature Oxidation of Dried New Zealand Lignite', Publication P135, New Zealand Energy Research and Development Committee, 1988 Isaacs, J. J. and Liotta, R. Energy Fuels 1987, 1,349
Fuel 1996 Volume 75 Number 7
895
Fuel Vol. 75, No. 7, pp. 891-895, 1996 Copyright © 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0016-2361/96 $15.00+0.00
S0016-2361(96)00010-5
The role of moisture in the self-heating of low-rank coals Anthony H. Clemens and Trevor W. Matheson Coal Research Limited, P.O. Box 31-244, Lower Hutt, New Zealand (Received 10 March 1995; revised 2 January 1996)
The self-heating of coal mainly involves exothermic reactions of oxygen at reactive radical sites within the coal and the enhancing or moderating effect that water has on these reactions. The thermal response of samples of low-rank coals, dried by heating under nitrogen flow at 105°C and exposed to dry oxygen, is similar to or slightly less than that observed when they are flow-dried at 30°C and tightly bound moisture remains. The most likely reason is that moisture affects the nature of the radical sites where oxidation occurs. By hindering the formation of stabilized radicals, it encourages faster oxidation which may lead to enhanced thermal response, although some of the extra heat may be taken up by the residual moisture. When loosely bound moisture is allowed to remain in the coal, the thermal response on exposure to dry oxygen decreases very quickly, due mainly to hindered access to reactive sites and dissipation of heat generated by any oxidation that does occur. The effect of desorption is comparatively minor and the course of the oxidation reaction responsible for generating heat does not appear to be changed by the presence of small quantities of loosely bound moisture. Copyright © 1996 Elsevier Science Ltd. (Keywords: low-rank coals; serf-heating; moisture)
The self-heating of coal is influenced by many factors, but mainly it involves exothermic reactions between reactive sites in the coal and oxygen from the air and the enhancing or moderating effects that water has on these reactions. Between them, these processes are largely responsible for initiating self-heating and determining the extent to which it develops. The reaction between coal and oxygen at ambient temperatures has been widely investigated and the general consensus 1'2 is that it proceeds through attack of oxygen at reactive radical sites within the coal to generate peroxy compounds in a manner analogous to the oxidation of hydrocarbons. The subsequent breakdown of these peroxy compounds releases heat into the system which, if not sufficiently dissipated, leads to localized temperature increases, thus helping to promote the self-heating process. The influence of water on the self-heating process is complex. If present in the coal in sufficient quantities, it suppresses self-heating by reducing the number of available reactive sites, blocking access of oxygen to these sites--the rate of diffusion of oxygen in air is reportedly 3 four orders of magnitude greater than in w a t e r - - a n d by taking up the heat released by oxidation as it occurs. Further suppression can result from the endothermic process of desorption of water from the coal 4. If the moisture content of the coal is lowered, it may be expected that these moderating influences will become less effective and a significantly greater level of self-heating will occur on exposure of the partially dried coal to oxygen. It has been reported 3 that brown coals undergo oxidation most rapidly when their moisture
content is reduced to between 5 and 10wt%; a New Zealand subbituminous coal was found to undergo oxidation most rapidly when it contained between 7 and 17 wt% moisture 5. It appears that the presence of a small amount of moisture enhances oxidation, and this is sometimes ascribed to a catalytic effect of the moisture and its known ability to encourage formation of the oxidation accelerant sulfuric acid through oxidation of pyritic sulfur in the coal 6. The removal of water brings about the collapse of the colloidal gel-like structure 7-9 found in low-rank coals, although if this were to significantly inhibit self-heating there would be no danger of the process occurring in dried coals. The extent and method of drying influences the concentration and nature of the reactive sites within a coal 1°-12. It is reported that the presence of water may also change the oxidation reaction products 1334. In addition, the water may be bound in several ways within a coal. These include loosely bound water in pores, capillary condensed water and water that is hydrogen-bonded to polar groups 15-17. The self-heating of three low-rank coals has been the subject of an ongoing investigation in the authors' laboratory. In the first stages 1839 the thermal and chemical responses of dried samples of the coals on exposure to dry oxygen at different temperatures including nearly atmospheric (30°C) were investigated. These studies confirmed that the major reaction contributing to heat generation at this temperature involved the breakdown of peroxy functional groups to form carboxylic acid products. It was also found that reactions of this sort continued to be major contributors over the
Fuel 1996 Volume 75 Number 7
891
The role of moisture in the self-heating of low-rank coals." A. 14. Clemens and T. W. Matheson Table
1 Compositionof freshcoals (wt%)~
Moisture Ash C H N Total S Organic S PyriticS SulfateS O
New Vale
Kai Point
Kopako
39.4 5.4 64.9 5.4 0.7 0.4 0.39 0.00 0.01 23.2
28.7 5.4 69.1 5.5 0.9 4.1 4.00 0.08 0.02 15.0
26.0 3.2 71.8 5.7 1.3 0.1 0.10 0.00 0.00 17.9
a Dry basis (exceptmoisture);oxygenby difference temperature range from atmospheric to near the ignition point. Later studies showed l° that the removal of water from these coals increased the concentration and changed the types of radical sites present within them. The outcome depended on a balance between the decarboxylation reactions producing new radicals and the removal of water molecules from heteroatomic sites within the coal macrostructure, allowing greater interaction between unpaired electrons and the heteroatoms. The balance was affected by the drying method used (vacuum or flow drying). This paper examines the tendency of the three lowrank coals to undergo self-heating as their moisture content is changed from dry to ,,~50% of the original level. The tendency to self-heat was measured by the thermal responses that accompanied exposure to oxygen of the dry and partially dry samples of the coals. The experiments were devised to highlight as much as possible each of the several roles played by water, to determine the moderating or enhancing effect of each and their relative importance to the initiation of the selfheating process in coals. A future paper will examine the effect of exposing dry samples of coal to reactant gases containing different amounts of moisture.
EXPERIMENTAL The samples investigated were the two lignites, New Vale and Kai Point, and the subbituminous Kopako coal used in the previous studies 1°'~8'19. Their properties are listed in Table 1. They were obtained fresh from the mine and stored as lumps under water until needed. Before use, the coals were reduced in size while still under water and transferred to a nitrogen-filled glovebag where all subsequent grinding, sieving and storage operations were carried out. Initial grinding was done with a micro hammer mill followed by further grinding (to <76/zm) with a mortar and pestle. All the thermal experiments involved measuring the thermal response when an accurately weighed sample of coal (10-20mg) placed in the sample chamber of a differential scanning calorimeter (d.s.c.) was exposed to a 20 ml min -1 flow of reactant gas. In all cases the thermal response was monitored for 30min and calculated in MJkg -1 of dry coal from the area under the thermogram. The first experiments were run to ensure that no artificial thermal responses were being generated by the design of the experimental apparatus. Each coal was placed in the sample chamber of the d.s.c., which was
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Fuel 1996 Volume 75 Number 7
preset to 30°C and flushed with dry nitrogen (20 ml min-1). After 20 min, by which time only tightly bound moisture remained and a constant reading was being maintained, the coal was exposed to the gas flowing through the reactant gas line, which in this case was simply more dry nitrogen. No thermal response was observed during 30 min monitoring. A second set of experiments was run to determine the tendency toward self-heating of coals retaining tightly bound moisture. This involved repeating the above experiments except that dry oxygen instead of nitrogen was introduced from the reactant line. In a third series of experiments the coals were introduced to the sample chamber of the d.s.c, preset to 30°C and flushed with moist nitrogen generated by bubbling through a saturated potassium carbonate solution (44% r.h. at 18.5°C). After 20rain, by which time the coal contained both tightly and loosely bound moisture and the system had equilibrated, the gas flow was switched to dry oxygen and the thermal response was monitored. To determine the contribution due to desorption, the experiments were repeated with dry nitrogen introduced from the reactant gas line. The third series of experiments was repeated with the d.s.c, chamber in which the coals were placed swept with moist nitrogen generated by passing through a saturated solution of potassium acetate (20% r.h. at 20.0°C). This gave the coal samples a lower moisture content than that obtained by equilibration over saturated potassium carbonate. In a final series of experiments the coals were dried by placing in the sample chamber of the d.s.c, and heating to 105°C under a flow of nitrogen, held at this temperature for 10min, then cooled to 30°C, held a further 10min and then exposed to the gas (dry nitrogen or oxygen) in the reactant gas line. The blank run with dry nitrogen again gave no response. To check whether the presence of moisture was changing the oxidation reaction products, the second and third series of experiments were duplicated in, and monitored by, an FT-i.r. spectrometer. Samples of the coals were placed in a Harrick HVC-DR2 vacuum reaction chamber FT-i.r. accessory which was set at 30°C and flushed with either dry nitrogen or moist nitrogen generated by passing through one or other of the saturated solutions. After 20 min, the gas was switched to dry oxygen and the infrared spectra of the samples were monitored until reaction products were observed. The moisture contents of the coals used in each of the above experiments were determined by exposing them to a dry or moist nitrogen flow until a constant moisture content was obtained. It was assumed that the same equilibrium moisture content had been reached when coal in the d.s.c, was exposed to moist nitrogen flows and the d.s.c, response was steady. RESULTS AND DISCUSSION When New Vale lignite was placed under a flow of dry nitrogen at 30°C, the moisture content of the lignite quickly fell from 39.4 to 6.7 wt% and then remained at that level (Table 2). Under a nitrogen flow at 105°C this residual moisture was removed. When equilibrated under an atmosphere generated from a saturated solution of potassium acetate, the moisture level fell to 9.9wt% and from saturated potassium carbonate to
The role of moisture in the serf-heating of low-rank coals: A. H, Clemens and T. W. Matheson Table 2 Thermal response on exposure of dried and partially dried coals to dry gases
Drying methoda
Moisture (wt%)
Reactant gas
Thermal response (MJ kg -1 dry coal)
New Vale
A A A B B C C D D
0 0 0 6.7 6.7 9.9 9.9 12.9 12.9
O2 N2 Air 02 N2 02 N2 02 N2
+0.026 0.000 +0.006 +0.026 0.000 +0.016 -0.002 +0.003 -0.004
Kai Point
A A B B C C D D
0 0 6.1 6.1 8.0 8.0 12.6 12.6
02 N2 02 N2 02 N2 O2 N2
+0.026 0.000 +0.029 0.000 +0.015 -0.002 +0.002 -0.005
Kopako
A A B B C C D D
0 0 6.O 6.0 8.8 8.8 13.4 13.4
02 N2 O N2 02 N2 02 N2
+0.024 0.000 +O.024 0.000 +0.013 -0.003 +0.003 -0.005
Coal
aA, N 2 flow at 105°C; B, N 2 flow at 30°C; C, N 2 equilibrated over saturated KOAc; D, N2 equilibrated over saturated K2CO3
3.0
2.25
E v
~= =~
1.5
r
0,75
I
I
I
~
0
5
10
15
l
20
I
25
30
Time ( min )
Figure 1 Thermal response when dry New Vale lignite is exposed to dry oxygen
12.9 wt%. Therefore for New Vale the moisture contents ranged from zero to ,,~30% of the levels found in the asreceived sample. For Kai Point and Kopako the range was from zero to near 50% (Tables 1, 2). When the New Vale sample dried under nitrogen flow at I05°C was exposed to dry oxygen, an immediate exothermic response was observed. It reached a maximum value within 1 min and tailed off slowly thereafter (Figure 1). The heat released during 30min exposure (0.026MJkg -1 dried coal) was 4.5 times greater than that (0.006MJkg -1) found when the dried coal was exposed to dry air. The heat differential reflects the relative oxygen contents of the two reactant gases and suggests that reactive sites within the coal are not saturated when exposed to oxygen alone. The observation of initial rapid heat release followed
by a gradual tailing-off is as expected for the addition of oxygen to reactive radical sites within the dried coal. Such processes are exothermic and proceed rapidly during the initial stages while radical sites are readily available, then slow down as the concentration of reactive sites is diminished. The values obtained in this study indicate an uptake of,-~8 × 10-5 mol of oxygen per gram of dry coal, based on the heat release data for carbonaceous materials and other low-rank coals 2°'2l on exposure to oxygen. The apparatus used in these experiments did not allow accurate measurement of oxygen uptake, but previous studies 22 have shown that exposure of dried samples of these coals to a 30:70 oxygen-argon atmosphere at 30°C for 5 h leads to the consumption of ,,~1 z 10-4tool of oxygen per gram of dry coal. When the New Vale sample dried by nitrogen gas flow at 30°C and containing tightly bound moisture (6.7 wt%) was exposed to dry oxygen, the heat released (0.026MJkg -1) was identical to that found when the sample was dried at 105°C (Table 2), and the thermogram was the same as that shown in Figure 1. Similar results were obtained for the Kopako sample, whilst for Kai Point the heat released from the sample containing tightly bound moisture was slightly greater than that obtained from its totally dry counterpart. These results are in agreement with observations of previous workers 3'5 and could possibly be ascribed to moisture catalysing the oxidation reaction. However, to do so directly, the water molecule would be required to be a hydrogen atom d o n o r - - a n extremely unlikely event J6 at 30°C. It should also be noted that these coals contain little if any pyritic or sulfate sulfur (Table 1). It is therefore unlikely that the moisture acts as an intermediate in the oxidation of pyrite that could lead to sulfuric acid formation and enhanced oxidation of the coal. Rather than catalysing the oxidation reaction, it may be that the presence of tightly bound moisture generates radical sites in the coal (where oxidation occurs) that are more reactive than those derived from the fully dried coal. Support for this comes from previous observations ~° that as the coals are dried, the water molecules associated with heteroatoms are removed, allowing increased interaction between the heteroatoms and the unpaired electron density. This favours formation of more persistent, less reactive radical sites. Tightly bound moisture appears to be preferentially associated with heteroatoms, thereby blocking their interaction with the unpaired electrons and resulting in less persistent, more reactive sites and faster oxidation reactions. Offsetting this is the ability of the remaining water to take up some of the heat generated. In the particular cases of New Vale lignite and Kopako coal the two opposing effects are seen to cancel each other, whilst for Kai Point lignite the sample containing tightly bound moisture is slightly more prone to self-heating than the completely dried sample. The infrared studies suggest that the oxidation reaction mechanism is not changed by the presence of tightly bound moisture. The products (carboxylic acids absorbing at 1690cm-l)lare the same as those observed previously with dry coal 8,19. When New Vale pre-equilibrated with 20% r.h. nitrogen and containing 9.9 wt% moisture was exposed to dry oxygen, an exothermic response was seen immediately. It tailed off during the next several minutes
Fuel 1996 Volume 75 Number 7
893
The role of moisture in the self-heating of low-rank coals: A. H. Clemens and T. W. Matheson 3.0
3.0
225 -
2.25
E
.
~
.
.
.
E
.
~ i1=
1,5
1.5-
Heat released = 3 x 10 "3 MJ/kg
"r
-r
0.75
Heat released = 16 x 10 "3 MJ/kg
075
0
'
5
1'0
;5
2'0
2'5
I 0
30
I 5
/ 10
Figure 2 Thermal response when New Vale with 9.9 wt% moisture is exposed to dry oxygen
I 25
30
Figure 4 Thermal response when New Vale with 12.9 wt% moisture is exposed to dry oxygen
30
3.0
2.25
i
E
E 15
o
I 20
Time ( m i n )
Time ( rain )
2.25
I 15
~ o
J _ 1.5
Heat absorbed = - 2 x 10-3MJ/kg
Heat absorbed = - 4
-1-
x 10"3MJ/kg
"T"
0.75
0175
i
0[
5I
110
15
210
25
I
30
0
110
115
210
I
25
30
Time ( min )
Time ( mir~ )
Figure 3 Thermal response when New Vale with 9.9 wt% moisture is
[
5
Figure 5 Thermal response when New Vale with 12.9 wt% moisture is
exposed to dry nitrogen
exposed to dry nitrogen
(Figure 2).
In this case the increased amount of loosely bound moisture almost completely eliminates the thermal response. Again, only a small part of the decrease is due to moisture desorption; most probably relates to blocked access, fewer radical sites and heat dissipation. The situation is not so clear-cut as in the previous experiments because the infrared studies now show a broad peak centred at l l 0 0 c m -l in addition to the carboxylic acid signal at 1690 cm -1 . The possibility that a different (less exothermic) oxidation reaction is occurring cannot now be ruled out.
Overall the heat released was 0.016 MJ kg -1 . Exposure to dry nitrogen produced an endothermic response of 0.002MJkg -l. The response was not immediate, taking between 3 and 8min to reach a maximum before tailing off (Figure 3). Similar trends were found for Kopako and Kai Point (Table 2). This shows that once a small amount of loosely bound moisture is left in the coal, the thermal response on exposure to dry oxygen begins to decrease rapidly. Very little of the decrease is observed to be due to moisture desorption, and the FT-i.r. studies suggest that the small amount of loosely bound moisture does not change the oxidation reaction mechanism, as the carboxylic acid signal at 1690 cm -l is still evident. The major reason for the decreased thermal response is therefore probably related to moisture inhibiting access of oxygen to reactive radical sites 3'17'23 and dissipating heat released by oxidation as it occurs. The result is perhaps not too surprising, given the very slow diffusion rate of oxygen in water relative to that in air 3. When New Vale containing 12.9wt% moisture was exposed to dry oxygen, an immediate sharp exothermic response occurred, but within 10 min this had subsided to the baseline value (Figure 4). The overall exothermic response was only 0.003 MJ kg -1 . Exposure of the moist New Vale sample to dry nitrogen generated an endothermic response, equivalent to 0.004 MJ kg -l coal (Figure 5). Again the Kai Point and Kopako samples behaved similarly (Table 2) to New Vale.
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Fuel 1996 Volume 75 Number 7
CONCLUSIONS W h e n samples of low-rank coals containing varying amounts of moisture (from dry up to ~50°/s of their asreceived moisture contents) are exposed to dry oxygen, the corresponding heat release is the result of a complex set of competing factors. For all the coals the heat released during 30 rain exposure by the dry sample and the sample containing tightly bound moisture is the same or nearly so. The most likely cause is that the tightly bound moisture favours the formation of radical sites (where oxidation occurs) that are more reactive than when water is removed completely. This is the reason that low-rank coals with 5 to I0 wt°/e moisture are found to undergo oxidation more rapidly than completely dried samples. The increased rate of oxidation may not always translate into more rapid self-heating, because the
The role of moisture in the self-heating of low-rank coals. A. H. Clemens and T. W. Matheson residual m o i s t u r e is able to dissipate s o m e o f the heat g e n e r a t e d f r o m the o x i d a t i o n reactions. W h e n small a m o u n t s o f loosely b o u n d w a t e r are left in the coals, the t h e r m a l response on e x p o s u r e to o x y g e n quickly decreases. T h e d e c r e a s e d response is n o t due to d e s o r p t i o n effects or, in m o s t cases, to a c h a n g e in o x i d a t i o n reaction. R a t h e r it is due p r i m a r i l y to m o i s t u r e b l o c k i n g access to reactive sites a n d further dissipating heat f r o m a n y o x i d a t i o n t h a t m a y t a k e place. A c o n s e q u e n c e o f this finding is that well over 50% o f the original m o i s t u r e c o n t e n t can safely be r e m o v e d f r o m these l o w - r a n k coals w i t h o u t the risk o f self-heating.
ACKNOWLEDGEMENTS T h e a u t h o r s t h a n k the F o u n d a t i o n for Research, Science a n d T e c h n o l o g y for funding s u p p o r t .
4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
REFERENCES 19 1 2 3
Kim, A. G. 'Laboratory Studies on Spontaneous Heating of Coal. A Summary of Information in the Literature', Information Circular 8756, US Bureau of Mines, 1977 Davidson, R. M. 'Natural Oxidation of Coal', IEACR/29, IEA Coal Research, London, 1990 Panaseiko, N. P. Solid Fuel Chem. 1974, (8), 21 (trans. from Khim. Tverd. Topl. 1974, (8), 26)
20 21 22 23
Bhattacharyya, K. K. Fuel 1972, 51,214 Chen, X. D. and Stott, J. B. Fuel 1993, 72, 787 Schmal, R. In 'Chemistry of Coal Weathering', Coal Science and Technology 14 (Ed. C.R. Nelson), Elsevier, Amsterdam, 1989, Ch 6 Gorbaty, M. L. Fuel 1978, 57, 166 Deevi, S. C. and Suuberg, E. M. Fuel 1987, 66, 454 Suuberg, E. M., Otake, Y., Yun, Y. and Deevi, S. C. Energy Fuels 1993, 7, 384 Carr, R. M., Kumagai, H., Peake, B. M., Robinson, B. H., Clemens, A. H. and Matheson, T. W. Fuel 1995, 74, 389 Dack, S. W., Hobday, M. D., Smith, T. D. and Pilbrow, J. R. Fuel 1983, 62, 1510 Buckmaster, H. A. and Kudynska, J. Fuel 1992, 71, 1147 Huggins, F. E., Huffman, G. P., Kosmack, D. A. and Lowenhaupt, D. E. Int. J. CoalGeoL 1983, 3, 157 Gethner, J. S. Appl. Spectrosc. 1987, 41, 50 Ingram, G. R. and Rimstidt, J. D. Fuel 1984, 63, 292 Nelson, C. R. In 'Chemistry of Coal Weathering', Coal Science and Technology 14, (Ed. C. R. Nelson), Elsevier, Amsterdam, 1989, Ch. 1 Vorres, K. S., Wertz, D. L., Malhotra, V., Dang, Y., Joseph, J. T. and Fisher, R. Fuel 1992, 71, 1047 Clemens, A. H., Matheson, T. W. and Rogers, D. E. Fuel 1990, 69, 255 Clemens, A. H., Matheson, T. W. and Rogers, D. E. Fuel 1991, 70, 215 Kaji, R., Hishinuma, Y. and Nakamura, Y. Fuel 1987, 66, 154 O'Neill, M. and Philips, J. J. Phys. Chem. 1987, 91, 2867 Matheson, T. W. and Rogers, D. E. 'Low Temperature Oxidation of Dried New Zealand Lignite', Publication P135, New Zealand Energy Research and Development Committee, 1988 Isaacs, J. J. and Liotta, R. Energy Fuels 1987, 1,349
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