Energy & Fuels 1999, 13, 77-81
77
Effect of Coal Interaction with Oxygen on Its Ignition Temperature Jirˇ´ı Medek* and Zuzana Weishauptova´ Institute of Rock Structure and Mechanics, Academy of Sciences of the Czech Republic, 182 09 Prague 8, Czech Republic Received April 24, 1998. Revised Manuscript Received October 19, 1998
Samples of lignite and bituminous coal were subjected to interaction with dry and moist oxygen. It has been found that within the low-temperature range used, the decisive source of the exothermic effect was the adsorption heat of water vapor, and the temperature of coal did not exceed 100 °C regardless of the mode of its previous contact with air oxygen. At temperatures above 100 °C, the only observed result of the coal interaction with oxygen was the formation of a layer of oxidation products on the coal surface. It was established that the oxidation reaction induced close to but below the ignition temperature was unable to overcome, by its heat effect, the small temperature difference required for the ignition of coal; on the contrary, the surface oxides formed increased the ignition point. The amount of oxygen bonded by both the physical adsorption and chemisorption was determined by direct measurements. It has been assumed that the interaction of coal matter with oxygen below the ignition temperature leads to the inactivation of the coal surface without the thermal effects necessary for coal ignition.
Introduction The self-ignition of coal with a possible transition into endogenous fires represents a direct hazard to the safety of the working conditions in both the opencast and underground mines and adversely affects the environment in their surroundings. The essence of the selfignition phenomenon can be defined as a spontaneous temperature increase which must occur without addition of the necessary amount of heat from external sources. In principle, the possible mechanisms of spontaneous temperature increase are explained, up to this time, in two ways. The first mechanism is by heat development in the course of specific physicochemical processes, which directly cause or take a considerable part in the temperature increase of coal until the ignition point. Most frequently the following processes are considered: Chemical reactions or catalytic action of mineral admixtures,1-5 reactions of selective chemical groups or free radicals,6-11 effect of pyrophoric compounds,12,13 * To whom correspondence should be addressed. E-mail:
[email protected]. (1) Cudmore, J. F.; Sanders, R. H. Australian Coal Ind. Res. Lab., ACIRL-PR.84-10, North Ryde, N. S. W., 1984. (2) Bardocz, V. 21st Conference on Safety in Mines; Research Institute of Sydney: Sydney, 1985; p 475. (3) Norton, P. Proc.Int. Conf. Coal Sci. 1985, p 467. (4) Ghosh, R. Fuel 1986, 65, 1042. (5) Backes, C.; Pulford, I. D.; Duncan, H. J. J. Reclam. Revegetat. Res. 1987, 6, 279. (6) Davydova, Z. A.; Sukhov, V. A.; Lukovnikov, A. F. Khim. Tverd. Topl. 1983, 6, 38. (7) Evans, J. C.; Rowlands, Ch. C.; Cross, R. M.; Rigbly, N. Fuel 1984, 63, 1471. (8) Dack, S.; Hobday, M. D.; Smith, T. D.; Pilbrow, J. R. Fuel 1984, 63, 39. (9) Khan, M. R. Energy Fuels 1987, 1, 366. (10) Neuman, H. J. Erdoel, Kohle, Erdgas, Petrochem. 1987, 103, 131.
heat developed by electrochemical processes,14-17 activity of bacteria18,19, and the adsorption heat of water vapor.20-24 The second mechanism is by self-reliant interaction of oxygen with coal, either in the form of a single continuous process or as a system of consecutive oxidation reactions. In this case, attention was paid to the study of partial oxidation reactions taking place in various temperature intervals, mostly within the lowtemperature region.25-32 (11) Wiese, R. G., Jr.; Powell, M. A.; Fyfe, W. S. Chem. Geol. 1987, 63, 29. (12) Mapstone, G. E. Chem. Ind. 1954, 23, 658. (13) Stadnikov, G. L. Spontaneous Combustion of Coals and Rocks; Ugletechizdat: Moscow, 1956. (14) Obukhov, N. K.; Burkov, P. A.; Alexandrov, I. V. Khim. Tverd. Topl. 1976, 4, 73. (15) Kossov, I. I.; Alexandrov, I. V.; Kamneva, A.I. Khim. Tverd. Topl. 1984, 4, 41. (16) Alexandrov, I. V. Khim. Tverd. Topl. 1987, 5, 29. (17) Fuerstenau, D. W.; Rosenbaum, J. M.: You, Y. S. Energy Fuels 1988, 2, 241. (18) Ichiro, I. J. Min. Met. Inst. Jpn. 1960, 76, 524. (19) Lifanov, J.V. Khim. Tverd. Topl. 1970, (3), 150. (20) Bhattacharyya, K. K. Fuel 1971, 50, 367. (21) Panaseiko, S. P. Khim. Tverd. Topl. 1974, 1, 26. (22) Saranchuk, V. I.; Gabishko, L. Y.; Pashchenko, L. V.; Lukyanenko, L. V. Khim. Tverd. Topl. 1978, 1, 9. (23) Nordon, P.; Bainbridge, N. W. Fuel 1983, 62, 619. (24) Singh, R. N.; Demirbilek, S. Min. Sci. Technol. 1987, 4, 155. (25) de Vries, H. A. W.; Bokhoven, C.; Dormnas, H. N. M. Brennst.Chem. 1969, 50, 289. (26) Fredericks, P. M.; Warbrooke, P.; Wilson, M. A. Org. Geochem. 1983, 5, 89. (27) Beafore, F. J.; Cawiezel, K. F.; Montgomery, C. T. J. Coal Qual. 1984, 3, 17. (28) Kaji, R.; Hishimma, Y.; Makamura, Y. Fuel 1990, 64, 297. (29) Banerjee, B.; Bchattacharyya, N. C. Fuel Sci. Technol. 1988, 7, 11. (30) Taraba, B.; Dobal, V.; C ˇ a´p, K.; Harasˇta, N. Fuel 1988, 67, 758. (31) Clemens, A. H.; Matheson, T. W.; Rogers, D. E. Fuel 1990, 69, 255. (32) Kelemen, S. R.; Freund, H. Energy Fuels 1990, 4, 165.
10.1021/ef9800967 CCC: $18.00 © 1999 American Chemical Society Published on Web 12/05/1998
78 Energy & Fuels, Vol. 13, No. 1, 1999
Medek and Weishauptova´
Table 1. Effect of Preoxidation on Low-Temperature Interaction of Lignite with Moist Oxygen pretreatment conditions
results of coal interaction with O2
mode
T [°C]
vacuum
40
time
Ti [°C]
Ts [°C]
Tmax(iso) [°C]
Tmax(adia) [°C]
127.3
25 50 25 50 25 50 25 50 25 50 25 50 25 50
53.0 76.0 53.8 77.0 52.9 77.0 54.0 76.5 54.0 77.3 54.7 77.6 55.0 78.1
96.5 96.2 97.1 96.9 97.0 96.8 96.1 96.3 97.1 97.4 96.8 97.0 97.3 97.1
drier
70
4h
128.5
110
12 h
153.6
110
24 h
154.0
110
14 days
154.2
110
30 days
156.6
110
60 days
156.8
20
14 days
In Contact with Atmospherea 131.1 25 50 134.0 25 50 135.2 25 50
53.9 77.2 53.8 77.1 53.9 77.1
96.4 97.2 96.8 96.8 97.1 97.0
20
30 days
20
60 days
a Samples were heat-insulated in a box from polystyrene. T ) ignition temperature, T ) starting temperature, T i s max(iso) ) maximum temperature attained by isothermic process, Tmax(adia) ) maximum temperature attained by quasi-adiabatic process.
It has been already shown by Oreshko33 that, in principle, four temperature zones may be distinguished, where the particular reactions of oxygen with coal attain an equilibrium state. The whole oxidation process can be divided into the following reaction steps with different activation energies E: Low-temperature oxidation [R1] with formation of peroxidic complexes of the CxOy type with E ) 13-17 kJ/mol, two further mediumtemperature reactions from which the first one [R2] with E ) 25 kJ/mol induces the decomposition of these complexes and the following oxidation reaction [R3] with E ) 67 kJ/mol leads to the formation of oxidized coal, and the final reaction of ignition [R4] with E ) 105145 kJ/mol. It follows, from this scheme linking the individual oxidation reactions, that the amount of heat generated in the system by a certain reaction may be able to directly initiate the subsequent reaction. Therefore, a decisive criterion for the confirmation of the effect of oxidation reactions on attainment of the ignition temperature could consist of direct evocation of reaction R4. However, the starting temperature of reaction R4 should always be lower than the ignition point, so that the reaction itself would increase the coal temperature to the ignition point by its inherently produced heat. The aim of this work is to contribute to the knowledge of (a) what temperature increase can be attained at low temperatures by the contact of moist air with the coal samples previously exposed to air oxidation, (b) how far it is possible to increase the coal temperature up to the ignition point by the action of oxidation reactions only, and (c) whether a system in which the final exothermic reaction R4 has been induced at a temperature near to the ignition point is able to attain the ignition point merely by the heat production of this reaction. (33) Oreshko, V. F. Dokl. Akad. Nauk SSSR, Otd. Technol. Nauk. 1952, 82, 135.
Experimental Section Coal. For the oxidation studies, a lignite L from an openpit mine in the North Bohemian district (moisture ) 19.9 wt %, ash ) 25.8 wt %, V ) 51.8 wt % (daf), sulfur ) 3.42 wt %) and a bituminous coal B from a coal mine in the Ostrava district (moisture ) 2.34 wt %, ash ) 9.48 wt %, V ) 38.1 wt % (daf), sulfur ) 1.48 wt %) were used. The coals were chosen with respect to their high liability to oxidation both in the seam and in the coal stockpile. After extraction, the coal samples were immediately stored in closed containers. Equipment. For the determination of the low-temperature interaction of coal with air, the surface of the lignite L, grain size 2-4 mm, was pretreated by long-term contact with air with natural humidity in two different ways: at elevated temperatures in a drier air stream and at room temperature in a polystyrene box with a perforated cover and bottom making a spontaneous air draught possible. The check sample was prepared in a vacuum box. The temperature of the samples was recorded at all times. The experimental conditions are shown in Table 1. For the successive measurement of the effect of moist air on the temperature increase under dynamic conditions at temperatures below 100 °C, 100 g of coal was spread out on a fritted disk in a glass reactor inserted into a liquid thermostat. The air passing through the reactor at a flow rate 5 cm3/min was saturated with water vapor, for isothermic measurements at 20 °C and for other measurements at the increasing temperature of the bath. The bath temperature controlled to a precision of (0.01 °C was synchronously increased with the coal temperature so as to always remain 2 °C lower, and this difference was equalized by the heat produced in the reactor. This experimental arrangement imitated a well-insulated system corresponding to real conditions in situ. For measurements at temperatures exceeding 100 °C under dynamic conditions, a pair of quartz reactors (Figure 1) synchronously electrically heated (with a precision of ( 0.1 °C) was used. The coal samples of the grain size below 0.2 mm, with identical weights of approximately 1 g, were placed on glass frits in each reactor. Oxygen was passed through the first reactor and nitrogen through the second one, both at the same flow rate of 5 cm3/min. Both gases were saturated with water vapor at 20 °C. The presence of gaseous products in
Effect of Coal Interaction with Oxygen
Figure 1. Apparatus for measurement of coal interaction with oxygen at temperatures above 100 °C. 1, furnace; 2, synchronously heated reactors; 3, saturators of water vapor; 4, heatconductivity detectors; 5, compensations bridge; 6, recorder. carrier gas was recorded by means of a heat-conductivity detector. The purpose of such a parallel arrangement of the reactors was to eliminate the effect of volatile matter adsorbed in the porous system of coal. This experimental arrangement was also used to establish the temperature of the ignition point Ti in the oxygen atmosphere at an increasing rate of 200 °C/ min. In all cases the temperature was measured with a set of miniature thermocouples Fe-constantan connected in a series and located directly in the sample bed. The temperature changes were continuously recorded. The kinetics of oxygen consumption by the coal substance was measured under static conditions by means of SORPTOMATIC 1800 (Carlo Erba). A sample of 1.5 g with a grain size below 0.2 mm was desorbed in a vacuum of 3 × 10-4 Pa for 8 h, the lignite at 40 °C, and bituminous coal at 80 °C. The measurements were performed in two ways. According to the experimental scheme denoted as A, first the oxygen was admitted to the sample at a temperature of 25 °C and pressure of 100 kPa and then the temperature was gradually increased at a rate of 10 °C/min to the final value. According to the scheme denoted as B, the sample was first brought to the required experimental temperature in a vacuum and then the oxygen was admitted at a 100 kPa pressure. The subsequent desorption of the physically adsorbed oxygen was carried out at the same temperature as the sorption. In both cases, the pressure change in the system was recorded at 15 min intervals and the amount of consumed oxygen was calculated from the pressure differences. The SORPTOMATIC device was also used for the determination of the specific surface area of coals from the CO2 isotherm at 25 °C.34
Results Low-Temperature Interaction. During the pretreatment of lignite, no temperature increase regardless of the experimental arrangement has been observed. The results of the subsequent low-temperature interaction of moist air with preoxidized lignite are presented in Table 1. The first column shows the arrangement, temperature, and time of coal pretreatment with air; the second one shows the ignition temperature Ti of lignite samples after pretreatment. The third column indicates the initial temperature of the sample before interaction Ts, chosen at 25 and 50 °C, which under normal conditions represents the approximate limits of the temperature interval in a real environment. The fourth column shows the maximum final temperatures (34) Medek, J. Fuel 1977, 56, 131.
Energy & Fuels, Vol. 13, No. 1, 1999 79
Figure 2. Interaction of lignite with dry oxygen under dynamic conditions. Top band: gaseous products generation. Bottom band: temperature changes. (a) Determination of ignition temperature Ti. (b) Temperature dependence on time.
Figure 3. Interaction of lignite with moist oxygen under dynamic conditions. Top band: gaseous products generation. Bottom band: temperature changes. (a) Oxidation process with two temperature dwells. (b) Oxidation process with two temperature drops and three dwells.
achieved at the isothermic experimental arrangement, and the fifth one indicates the final temperatures by temperature equalization of the surrounding bath with the instantaneous temperature of the sample (quasiadiabatic system). Ignition Temperatures. Ignition temperatures Ti determined in a dry oxygen and oxygen saturated with water vapor atmosphere at 20 °C were found for the original lignite to be 127.3 and 128 °C, respectively, and for the original bituminous coal to be 140 and 140.1 °C, respectively. The presence of water vapor in oxygen does not affect Ti. Interaction at Temperatures above 100 °C under Dynamic Conditions. Figures 2 and 3 show the results for the lignite in dry and moist oxygen under dynamic conditions. The top band represents the relative amount of liberated gaseous reaction products; the bottom one represents the temperature. Figure 2a illustrates the course of both variables during the determination of ignition points. Figure 2b shows the temperature of the sample during the oxidation in an atmosphere of dry oxygen. After a rapid temperature increase, the sample was left at the temperature of the heating jacket, 125 °C, for 6 h without recording any temperature change. After this time interval, the temperature was slowly increased 5 °C/min and the ignition was recorded at a temperature T ) 156.6 °C. Continuous escaping of volatile matter showed a slightly increasing trend during the whole temperature interval.
80 Energy & Fuels, Vol. 13, No. 1, 1999
Figure 4. Time dependence of oxygen sorption on lignite.
The oxidation course of moist oxygen with the ignition temperature 153.6 °C was similar. Figure 3a illustrates the interaction with moist oxygen modified by two isothermic dwells, at 125 °C for 6 h and subsequently at P1 at 148 °C for 1 h. After increasing the reactor temperature at P2, the ignition occurs at 154 °C. Figure 3b shows the recorded course with two drops modeling the alternating temperature decrease and increase. The first occurs after 10 h at P1 and the second after 6 h at P2, in both cases from 125 to 60 °C with an immediate temperature increase to 148 °C following a dwell of 2 h. After this time, at P3 the reactor temperature was directly increased and the sample inflamed at 164.2 °C. Interaction Kinetics of Oxygen with Coal at Temperatures above 100 °C under Static Conditions. Figure 4 shows the time dependence of the amount of sorbed oxygen obtained on lignite at 80, 100, 131, and 154 °C according to experimental scheme A. Figure 5 shows a similar dependence for the samples of bituminous coal at 100, 134, and 153 °C. From both diagrams it is evident that the amount of consumed oxygen and its adsorption rate rapidly rises with increasing temperature during the initial 60 min. However, during the 6 h time interval, no ignition of either lignite or bituminous coal both at temperatures lower or higher than the ignition temperature Ti could be observed. For comparison, Figure 5 illustrates the oxygen sorption on charcoal determined at the same conditions. Although the charcoal exhibited, according to the CO2 isotherm, a considerably higher surface area (1100 m2/g) than the bituminous coal sample (270 m2/ g) and a much more developed system of meso- and macropores, the consumption of sorbed oxygen was at 153 °C, considerably lower, and 100 °C, practically negligible. Figure 6 illustrates the oxygen sorption on lignite at 120, 135, and 153 °C by the shock oxidation according to experimental scheme B. It is found, from the diagram, that the quantity of adsorbed oxygen is lower than in the previous case but the initial sorption rate on the
Medek and Weishauptova´
Figure 5. Time dependence of oxygen sorption on bituminous coal. AC - active coal.
Figure 6. Shock oxidation of lignite.
warm sample surface is much higher than on the gradually heated one, so that the sorption equilibrium begins to be established already after 2 h. After oxidation is finished, the sample was desorbed and the portion of the physically adsorbed gas was determined from the volume of the released gas, the residual gas amount related to oxygen bonded by chemisorption. The parameters of desorption and the resulting values are given in Table 2. The surface area of lignite heated to 153 °C was found as 242 m2/g. Compared with the value of 250 m2/g for the original sample, it can be assumed that the oxidation did not change the surface area. Discussion The samples of lignite mentioned in Table 1 did not exhibit a temperature increase during preoxidation under various treatment conditions (column 1), even if
Effect of Coal Interaction with Oxygen
Energy & Fuels, Vol. 13, No. 1, 1999 81
Table 2. Desorption of Oxygen from Surface of Oxidized Lignite oxygen volume (STP) temp °C 120 135 153
desorption time, min
total, cm3/g
0 15 210a 0 15 210a 0 15 210a
5.202
adsorbed
chemisorbed
cm3/g
%
cm3/g
%
1.052 1.218
20.2 23.4
4.150 3.984
79.8 76.6
0.795 0.933
12.8 15.0
5.420 5.282
87.2 85.0
0.685 0.836
10.3 12.6
5.948 5.797
89.7 87.4
6.215 6.633
a After this time, the desorption attained the macroscopic equilibrium.
the temperature was 110 °C and the pretreatment continued for 60 days. It results, from values quoted in columns 4 and 5, that maximum temperatures attained at the contact of coal with air saturated with water vapor are practically identical regardless of the coal pretreatment. At the isothermic arrangement imitating a thermally uninsulated system with initial temperatures 25 and 50 °C, the temperature increase was within the interval of 2729 °C and the rough equilibrium was established after 18-20 min. In a system operating with equalization of the temperature between the coal and the bath, the limiting value of about 97 °C has been attained after 30 min in all cases. If the jacket temperature was raised above this value, a further temperature increase of coal did not follow even after 6 h owing to the counter-action of the evaporation heat of water condensed in the porous system of coal. The results obtained by this experimental arrangement confirm that a process with sufficiently intensive inherent energy is able to overcome the temperature difference of 2 °C between the coal sample and the bath and to increase the temperature by 30 °C without thermal insulation, both of the original coal and coal oxidized by the air oxygen. The decisive heat source was the exothermic process of adsorption and condensation of water vapor, which should also be considered the single entirely objective cause of the initial increase of the coal temperature in a natural environment. The same temperature increase recorded in a vacuumprepared sample with clean reactive surface offers evidence that for the temperature increasing due to the heat of water sorption, the original coal contains a sufficient amount of oxygen groups on the surface forming its hydrophilic character. Introducing the coal into the state with a temperature of 125 °C, lower than Ti, has been created to achieve the effect of final reaction R4, which had to be overcome, by its inherent energy, the temperature difference of 2.7 or 3 °C, respectively. The evidence of the coarse of oxygen reaction with coal is given by the record of increasing of its gaseous products (Figures 2 and 3).
From the shifting of the ignition temperature it may be assumed, in accordance with results published formerly,35,36 that the oxidized surface layer grows at the same time. Neither of the artificial temperature drops during the oxidation, which had to create conditions for continuation of the reaction on the temperature-stabilized oxidized surface, resulted in any other effect than an increase of the ignition temperature. It can be deduced that in the course of the so-called low-temperature oxidation under natural reaction conditions, the temperature of coal, both in the seam and after extraction, will not exceed 100 °C. At temperatures close to this value, the generated heat is consumed for the temperature holding and phase transformation of water contained in coal. The surface layer of oxides is an equilibrium product of oxidation reactions, and provided that it is not thermally or chemically destroyed, each subsequent interaction of oxygen with the coal surface is realized in this layer. However, the attained temperature is not sufficient to change the composition of the coal surface, so that at temperatures up to 100 °C, the coal attains, after a certain time, the equilibrium conditions of the surrounding atmosphere, from which it can be shifted only by addition of a suitable higher energy from another source. It results, from the course of the kinetic of oxidation, both at the increasing temperature (Figures 4 and 5) following the scheme A and at the shock oxidation (Figure 6) according to scheme B, that at the immediate interaction of oxygen with the coal surface purified by thermal desorption, the ignition did not take place, not even at a temperature exceeding the ignition point. This casts doubt on the concept that the oxidation reaction alone is responsible for the self-ignition of coal. Conclusions The interaction of coal with oxygen leads to gradual saturation of the reaction surface with oxidation products shifting the ignition point toward higher temperatures. It has been proved that with increasing temperature, the amount of oxygen bonded by chemisorption increases and its physically adsorbed amount decreases. The interaction of oxygen with the coal matter takes place evidently at a considerably high rate, and its result is the inactivation of the reaction surface, reducing the ability of coal to ignite even at temperatures considerably exceeding the ignition temperature of the original coal. Therefore, it appears to be necessary to perform the determination of the ignition temperature at a high rate of temperature increase, when the time interval is very short, to create an effective oxide layer. The presence of water vapor affects the temperature increase only in the region up to 100 °C, in the form of the heat of adsorption. EF9800967 (35) Erdmann, E. Brennst.-Chem. 1922, 3, 293. (36) Schroder, H. Brennst.-Chem. 1954, 35, 14.
