March, 1926
INDUSTRIAL A N D ENGI4ITEERINGCHEMISTRY
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Influence of Moisture on the Spontaneous Heating of C oal’22 By Joseph D. Davis and John F. Byrne PITTSBURGII EXPERIMEXT STATION, BUREAU O F MINES,PITTSBURGH, PA.
T
HE role played by moisture in the spontaneous heating in the experiment. Briefly, the apparatus used consisted of coal has long been a favored subject for discussion of a vacuum-jacketed tube holding about 30 grams of coal among investigators of this phenomenon. By some to which oxygen or inert gas a t the temperature of the tube the presence of moisture has been considered as absolutely could be admitted in regulated amounts. Surrounding required before spontaneous combustion will take place; this calorimeter tube was an oil bath, the temperature of others hold that moisture merely facilitates or hastens the which was automatically controlled thermoelectrically so oxidation that would take place without it; while still others as to keep it a t all times a t the same temperature as that claim that even a small amount of moisture will prevent of the coal sample. A recording millivoltmeter gave a spontaneous combustion. Some advise the wetting down of continuous record of the temperature of thc coal. For making a test the coal was coal at the time of storing, while others warn against normally first dried in an e v e n s t o r i n g in a moist atmosphere of nitrogen or Pittsburgh and Sharon coals were oxidized in an adiplace or on damp ground. natural gas a t 140” C. for abatic calorimeter in the “as received” state and after 2 hours, and then placed in Almost any opinion on different degrees of drying treatment, both dry and t h i s generally recognized the calorimeter, where it saturated oxygen being used. Dry inert gas (natural was supplied with an excess factor in spontaneous heatgas) was used to check the operation of the apparatus ing can be substantiated by of air or oxygen previously and to eliminate the heat factor involved in vaporizav o l u m i n o u s data. It IS dried by sulfuric acid and tion of moisture. probable that in some casea heated to the temperature The results show that coal in the “as received” state the influence of other facof the apparatus by passing will not heat in dry oxygen; when oxygen saturated at tors, notably size of coal and through a coil placed in the room temperature is used the coal will heat or cool demethod of piling, has been oil bath. The rate a t which pending on the rate of circulation of oxygen and the rea t t r i b u t e d to moisture. the coal heated was given sultant rate of moisture vaporization. The heat There is no doubt that moisb y t h e time-temperature (latent) abstracted may be of such magnitude as to curve drawn by the temture has a purely mechanical overbalance the oxidation heat even at 70’ C. It is also effect in making for tighter perature recorder. Under shown that spontaneous heating of thoroughly dried packing of the fines and in the conditions of the testcoal proceeds slowly from 70’ C. and tends to stop d i s i n t e g r a t i n g the lump. dry air and dry coal over around 97 C. This indicates that the presence of This is particularly evident the whole temperature range some moisture is required by the spontaneous heating where there is a l t e r n a t e of the test-moisture was reactions. wetting and drying of the neither taken up nor given pile and where there is an off by the coal. The latent opportunity for frost action. heat factor was therefore Whether or not a moist coal will absorb more oxygen than eliminated except as regards water formed by oxidation of a dry one is a point of theoretical interest only. The question the coal which was insignificant in amount. of vital importance to those storing coal is whether or not Tests for Moisture Effects the heat resulting from the increased absorption, if any, is outbalanced by the cooling due to vaporization, which For the moisture effects, a Sharon and a Pittsburgh coal must take place as the temperature of a moist pile rises. ( S o s . 1 and 8, respectively, according to the Ohio classiDetermination of the individual influence of moisture in fication) were used. The samples were sealed at the mine the pile is a matter fraught with difficulties, as the problem and later ground to minus 100 mesh with a minimum exis so complicated by variations in the nature (of the coal, posure to air. The analyses follow: the size, the method of piling, etc., that it practically defies solution. Analyses of Coals Tested for Moisture Effects (Per cent) In the laboratory, however, it is possible to test a coal, --PITTSBURGH COAL-. r SHARON COAL eliminating all variables save the one under considerationMoisture Moisture “As Moisture and “As Moisture and in this case, moisture-and i t is with such tests that this CONSTITUENT received” free ash free received” free ash free paper deals. The studies were made in an adiabatic calorim- Moisture 2.2 ... ... 11.8 ... ... matter 32.1 38.8 39.7 41.9 36.4 37.5 eter, in which all the heat generated by the coal is retained Volatile Fixed carbon 60.6 62.5 53.4 53.8 55.0 58.1 by it and goes to elevating its temperature, there being no Ash 5.3 3.0 5.2 ... 2.7 ... 5.1 5.0 5.2 5.7 5.1 5.4 loss by radiation or conduction. The apparatus and its Hydrogen 77.7 76.2 68.5 Carbon 76.9 81.2 80.1 1.4 1.6 1.7 1.7 1.6 1.6 possibilities have been described in a previous paper.3 There- Nitrogen 21.3 12.2 12.6 Oxygen 10.4 8.6 9.1 in are given curves illustrating the spontaneous heating Sulfur 0.5 0.5 2.5 2.6 0.4 2.4 characteristics of several typical coals, after drying in an Heating value . . . 7 t k 6 8189 ... ... 7656 ... 7894 ... 7583 6750 inert atmosphere a t a temperature higher than that attained Calories 13,780 14,210 B. t. U. 13,650 13,960 14,740 12,150 ~~
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Presented before the Section of Gas and Fuel Chemistry a t the 69th Meeting of the American Chemical Society, Baltimore, M d . , April 6 t o 10, 1925. Published by permission of the Director, U. S. Bureau of Mines. a Davis and Byrne, J. .4m.Ceram. SOL.,7, 809 (1924).
Sharon coal is reputed t o be a n excellent storage cod, spontaneous heating thereof being rare. Operators attribute this both to its low sulfur content and to its high moisture.
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ISDUSTRI.4L A S D E-VGISEERI-YG CHEMISTRY
Pittsburgh coal is much more extensively mined, and a much larger amount is stored. Generally speaking, it is a satisfactory storage coal, although spontaneous heating and even fires in piles of Pittsburgh coal are by no means unknown. In preparing the samples for experiments on influence of moisture, two methods of drying were employed: (1) drying in an evacuated desiccator a t room tempera t u r e f o r extended periods, and ( 2 ) drying a t elevated temperatures in a stream of n a t u r a l gas or nitrogen. Neither of these treatments alters the condition of the coal with regard to subsequent oxidation, the only effect being the removal of m o i s t u r e . Oxygen, both dry and saturated at room temperature, w a s u s e d , and the exhaust gases were analyzed. I n all cases there was a t least 65 per cent excess oxygen in these gases. TIME, HOURS Figure I-Heating Characteristics of
Sharon and Pittsburgh Coals w h e n Most of Moisture Is Removed a n d Oxygen I s
Dry Coal i n Oxygen
A large number of tests were run with both Pittsburgh and Sharon coals dried under various conditions. Using oxygen, both dry and saturated a t room temperature, the coals were allowed to heat from 40" C. and from 70" C. initial temperatures. Most of the tests were begun a t the latter temperature, as such tests furnished more conclusive data for purposes of comparison. Figure 1 illustrates the general characteristics of these coals when most of the moisture has been removed and the oxygen is dry. All curves except S-6 lie close together; this order of variation in moisture content had no significant effect. Practically no moisture was vaporized during the experiments. I n Curve 8-6 the moisture content is higher, which may account for the initial slow rate-i. e., enough moisture was vaporized by the dry oxygen to retard heating. It is possible that the residual moisture acts as a catalyst, so that once the reaction starts the coal heats rapidly. This theory finds some support in the curves of Figure 2 . Sample X-1 was dried for 2 'hours in dry natural gas a t 140" C., while Sample X - 3 of the same coal was treated similarly for 22 hours. When treated with dry oxygen in the calorimeter the former heated spontaneously in the usual manner. The latter, however, heated much more slowly up to about 97" C., when the temperature rise practically stopped, even though the oxygen flow was continued a t the same rate. After 25 hours dry natural gas was substituted for oxygen, and the temperature remained constant for 14 hours, indicating that there was no error in control of of the apparatus. Similar curves were obtained from other tests. Other coal samples dried for extended periods gave heating curves similar to that found above. It was found that 22, 40, and 24 hours' drying were sufficient to reduce the samples to constant weight; that is, there was no further Dry
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loss of moisture on further drying. The inference was that such complete drying removed the last traces of moisture required to catalyze oxidation and thus affected the spontaneous heating rates. However, it was suggested by Professor White4 of the University of Michigan that possible excessive heating during drying would alter the colloidal state of the coal so as to render it less active regardless of its moisture content. With this suggestion in mind some work on the effect of drying on the adsorptive capacity of coal was carried out. Influence of Prolonged Heating in Dry Gas on Colloidal State of Coal
It has long been known that peat when freshly taken from the bog is almost completely dispersable by caustic alkali. The peat, which is made up practically entirely of humic compounds whose chemical structure is unknown, is usually said to dissolve in the alkali; but it can be shown that the solution is colloidal. If, however, the peat has been dried before treatment with alkali, the amount of it which goes into solution with alkali is greatly reduced. The degree of dispersity of the colloidal peat is diminished by drying, or there has been reversion of its colloidal state; one would also expect its capacity for adsorption of vapors and gases such as oxygen would be diminished by drying. Although bituminous coal marks a late stage in the coalification process of which peat is the first, it was thought possible that prolonged drying, particularly when heat is being applied, might lower its degree of dispersity and hence its capacity for taking up oxygen. The diminished heating rate of bituminous coal caused by prolonged drying, as shown in the preceding paragraph, might therefore be due to reversion of colloidal state rather than to a lack of sufficient moisture to catalyze the reaction. In order to obtain evidence on this point two samples of the same coal, one dried a t 140" C. for 2 hours and the other dried a t the same 140
130
120
k'
~'110 P
c 3
f2 100
c
I
E
90
80
70 0
4
8
12
16
20
24
TIME. HOURS Figure 2-Oxidation
28
32
36
40
Rates of T w o Coals, O n e Slightly Moist, t h e Other Dry
temperature for 24 hours, were tested for their comparative capacity for absorption of oxygen. The samples were of very nearly the same weight originally and they were treated in a closed apparatus with oxygen which was circulated a t the same rate in both cases. Provision was made for absorption of products of oxidation during circulation of 4 Suggestion t o the writer during a general discussion of the spontaneous heating rate curves for a previous paper.
ISDC'STRI-IL A S D ESGISEERISG CHEMISTRY
March, 1926
the oxygen and the coal u-as maintained a t 50" C. by an oil bath controlled kn. a thermostat. The volume of oxygen d+ , of time could be read off from a absorbed Over any & buret, xvhich served as a gas reserroir. The results of the test are as follom: Preliminary drying a t 140' C. Test 1 2
Hours 2 24
OXYGEN .%BSORBED. C c . PER GRAM COALPER HOUR 1st 2 2nd 2 hcurs
0.233 0 265
hours 0 144
0.193
=1 further test of the effect of drying on t h e adsorptive capacity of this coal consisted in drying two samples a t 140" C., one for 2 hours and the other for 24 hours, and then exposing them in the same inclosure to a current of inert gas n-hich had first passed through a saturated solution of sodium chloride maintained a t constant temperature. The amount of moisture taken up by the samples was determined el-ery 4 hours by weighing. The rates a t which the samples gained in neight are shoxn in Figure 3.
EXPOSURE. HOURS
Figure 3-Adsorption of Moisture by Coal after 2 Hours' Drying and after 24 Hours' Drying a t 140' C.
335
Moist Coal i n Dry Gas and in Dry Oxygen
Xhen both of these coals in the 'Lasreceived" state were treated in the calorimeter with dry oxygen a t 70" C., the temperature actually fell. The characteristics of the apDaratus Dermit a dror, in temwrature only when heat is being extracted from t h e coal itself. S o doubt some oxidation was taking place in the moist coal, with resultant evolution of heat; however, the dry oxygen took up moisture from the moist coal with absorption of heat. The heat absorbed in vaporization was greater than the heat evolved by oxidation, the net result being negative heating or cooling. This action is illustrated in Figure 4. I n these tests the temperature remained constant when the coal was sealed in a still atmosphere of natural gas. When dry gas was circulated the temperature began to drop, more rapidly, of course, than when dry oxygen was circulated a t the same rate. The difference between the two cooling rates would then indicate the heating effect of the oxygen, since the heat lost by vaporization of moisture ~ o u l dbe approximately the same in both cases. Curl-e M-6 is the cooling rate of Pittsburgh coal (2.2 per cent water) in dry oxygen, and M-5 is the same coal in dry natural gas circulated a t the same rate. The total latent heat of vaporization of 2.2 per cent xater is 12.25 calories. per gram of coal dried. Since the specific heat of this coal is 0.273, these 12.25 calories represent a drop of 45" C. (12.25/0.273) in the coal temperature. However, in 2% hours the coal temperature dropped but 30.5" C. (70.0-39.5). Hence, 30.5/45.0 of 2.2 per cent, or 1.49 per cent of the water, was removed. I n oxygen the coal temperature fell 13" C. (70.0-57.0) in 22 hours. Assuminn that the same amount of moisture was removed both with the gas and with oxygen treatment, since the rate of circulation was the same in both cases, 17.5" C. (30.5- 13.0) represents the temperature rise through oxidation of the coal. This equals 4.78 calories per gram per 22 hours, or 0.217 calorie per gram per hour, average net heat of oxidation of moist coal between 70" and 57" C.,
The results of the above two adsorption tests are not in agreement, although the difference shown in either case should perhaps not be considered great enough t o be significant. However, the result of the oxidation test is opposed t o that of the preceding paragraph wherein a much slower rate of heating followed long-time drying. For 70 none of the preliminary drying periods was an especial effort made to maintain the temperature closely constant 60 a t 140" C. During the 24-hour drying periods it was realized that there would be more opportunity for the temperature to rise above 140" C. than during the 2- $ m hour periods and that excessive temperature might exert 2 a stronger influence than heretofore suspected. I n order @ 4o to test this point another oxygen long-time adsorption test was run wherein the drying temperature was kept 30 above 140" C. (140" to 160" C., to magnify any temperature effect). The rate of oxygen absorption for TIME, HOURS the so dried Was found to be lower than for Figure 4-Temperature Drop of "As Received" Coals Treated with Dry Oxygen a t 70° C. either of the two previous tests. For the second 2 hours, for example, it was 0.09 cc. per gram of coal per hour as compared with 0.144 and 0 183, respectively, for the previous with a reduction in moisture content from 2.2 to 0.71 per cent. The average heat of oxidation of dry coal of this 2 and 24-hour tests. -4lthough these results are someTvhat meager, they do rank between 57" and 70" C. has been shown to be 0.353 indicate that excessive heating teiids to diminish the capacity calorie per gram per h0ur.j This would indicate that the of this coal to take up oxygen. Temperature in excess heat of oxidation for dry coal is 0.136 calorie (0 353-0 217) of 140" C. apparently is the important factor rather than per hour greater than that of moist coal. the time of heating. Furthermore, one would conclude In the case of dried coal, however, neither the temperfrom these results taken in connection XTTith those set forth ature of heating nor the time of drying was excessive (2 in Figure 1 that the degree of drying, and henre catalysis, hours a t 140" C.) and the result is not therefore in conis not an important factor. The heating curve for the flict with that shown by Curie X-3, Figure 2, where the sample dried a t 140" C, (Figure 1 ) is not very different from time of drying was 22 hours. I t merely indicates that those which were d r i d in vacuum and Jvithout :iny heating if the time of drying or temperature of heating has not a t all. 6 Davis and Byrne, THIS J O U R X A L , 17, 125 (1925). 9
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been excessive, dry coal will heat more rapidly than moist coal. Curve 8-7 represents a Sharon coal with 10.3 per cent water, in dry gas, in oxygen, and again in dry gas. The cooling rate is slower in oxygen than in natural gas, owing to the heat evolved by oxidation of the coal. From the foregoing data it seems safe to conclude that in the coal pile that is losing moisture there is little danger of spontaneous heating. The latent heat of water vaporized more than counterbalances the exothermic oxidation until the vapor pressures of water in the air and in the coal are in equijibrium. Moist Coal in Saturated Gas and Saturated Oxygen
The amount of vaporization, and consequently the heat lost, depends upon the difference in vapor pressures of the moisture in the coal and that in the oxygen with which the coal is being treated. As previously shown, ‘las received” coal will not heat when treated with dry oxygen. With oxygen saturated a t room temperature, however, the result is different. Figure 5 illustrates the heating rate of “as received” Pittsburgh coal in saturated oxygen. Following Curve M-7, when saturated oxygen was circulated, the coal temperature fell for 6 hours to 64.5’ C. The rate of oxygen circulation was then decreased, and the temperature-began
Figure 5-Pittsburgh
TIME, HOURS Coal, “As Received,” i n Saturated Oxygen
to rise slowly, indicating that the negative heat of vaporization had been outbalanced by the positive heat of oxidation. When the temperature reached 79.5” C., the rate of flow was increased, and during the interval between 22 and 27 hours the temperature remained practically constant. Then the rate of circulation was retarded somewhat, and the heating proceeded a t the usual accelerated rate. Curve M-8 is also a Pittsburgh coal under similar conditions, with slight variations in the oxygen rate. Between the temperatures of 80” and 110” C. the rate of circulation was slightly greater than in M-7, which accounts for the slower heating rate in the former. The cooling of the coal or the retarding of the heating rate is not due to abstraction of heat by the intake air, as other tests showed that with rates three times as rapid as the maximum here used the intake air is preheated to the bath temperature before reaching the coal. Furthermore, with dry coal and dry oxygen greater rates of gas flow produced no cooling. From Figure 5 it is concluded that the rate of spontaneous heating of moist coal in moist oxygen is dependent to a large extent on the rate of flow of oxygen. With no oxygen, of course, there is no heating; with a very slow flow-that is, with only sufficient oxygen to satisfy the coal-there is heating a t a rate somewhat slower than with dry coal; a t more rapid rates the coal cools because of moisture vapori-
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zation. In brief, the coal temperature will rise unless the heat loss of vaporization counterbalances the heat of oxidation. Even though the oxygen is saturated when it reaches the coal, this is a t an elevated temperature, and consequently moisture will vaporize from it, with a resultant heat loss. I n a coal pile the hazard of spontaneous heating will be greatly reduced if the vapor pressure of the moisture in the coal is greater than that of the air with which it is in contact. Conclusions
Pittsburgh and Sharon coals, “as received,” will not heat in dry oxygen, but the temperature drops instead, owing to moisture vaporization. When treated with oxygen saturated a t room temperature, “as received” coal will heat or cool, depending upon the rate of circulation of oxygen and the resultant rate of moisture vaporization. I n brief, with the same amount of air circulation, the rate of spontaneous heating is a function of the relative vapor pressures of the moisture in the coal and in the air with which it is in contact. Bituminous coal dried in a n inert atmosphere or in vacuum will invariably heat spontaneously from 70” C., provided the temperature of drying has not exceeded 140” C. or the time of drying has not been excessive. Failure to heat when the temperature of drying has been excessive or the time long may be due partly to complete removal of the moisture; it is thought more probable that it is due to a change brought about thereby in the colloidal state of the coal. The fact that peat suffers colloidal reversion on prolonged drying, and considering that coal is also a colloid of similar character, would point t o a colloidal reversion effect. I n applying results of these laboratory tests to actual storage conditions one must use caution, because storage conditions involve indeterminate factors purposely eliminated in the laboratory. It may be inferred, however, that since prolonged drying to as high a temperature as 140” C. is not likely to occur in storage,.reversal of colloidal state is not an important factor in diminlshing heating rates in storage. If the temperature of the pile gets that high experience shows it will go on up and spontaneous combustion will usually occur. Under laboratory conditions a coal that has been moderately dried heats somewhat faster than a moist coal. This indicates that it is better to store the coal moist than dry and such a conclusion is in harmony with theory since pores in the coal which are filled with moisture have no space for the adsorption of oxygen. Furthermore, if the coal is moist enough to be out of equilibrium with the atmosphere, moisture will slowly evaporate and there will be cooling due to latent heat of vaporization. A very dry coal, on the other hand, will take up moisture from the atmosphere and the heat of condensation will raise its temperature.
Poland Closes Wood Distillation Plant Poland’s important wood distillation plant, “Chajnowka,” has been closed down as a result of business depression, according to a report received from Consul General T. Jaeekel, Warsaw, by the Chemical Division of the Department of Commerce. This plant was established by the German authorities in order to utilize the vast timber resources of the Bialowieza forest which were cut very intensively during the time that part of Poland was under German control. The plant was subsequently taken over by the Polish Government and operated by a corporation especially formed for the purpose. It is adapted to the production of charcoal, wood tar, calcium acetate, and methanol. Acetic acid, acetone, refined methanol, formalin, and other derivatives are produced by a group of plants working in close cooperation with the “Chajnowka” establishment.
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