INDUSTRIAL A N D ENGINEERING CHEMISTRY
August, 1924
775
Loss of Heating - Value of Bituminous Coal on Exposure to.Air’,’ By J. F. Byrne and J. D. Davis PITTSBURGH EXPERIMENT STATION, BUREAU OF MINES,PITTSBURGH, PA.
T HAS long been known
sampling, and determining A method was deoised f o r the determination of loss in heating the heating value after havthat bituminous coal value of coal on exposure to air such that a precision of the order of ing exposed it to weathering suffers a loss of heating 0.03 per cent was realized calorimetrically. T h i s was applied to conditions, the difference in value on exposure to the fine coal. which gives the most trouble f r o m spontaneous heating in a heating value being ascribed a t m o s p h e r e . This point coal pile, with the following results: to the effects of weatherhas been repeatedly investi( I ) T h e heating oalue loss per hour f o r thefirst half-hour exposure ing or atmospheric oxidagated in connection with its was nearly twice that f o r the whole hour and approximately eight tion. This experimental bearing on the deterioration times thatf o u n d for 65 hours’ exposure, the temperature being mainmethod, while it has the adof coal in storage. Thus tained constant. vantage that it can be apParr and Wheeler3 found ( 2 ) Rate of heating oalue loss f o r Freeport coal showed a marked plied directly to coal under that Illinois coal, exposed increase with temperature of exposure above 125” C., indicating that practical storage conditions, to air under storage condithis would be a dangerous temperature range in the storage of this coal. is subject to considerable tions, suffered a loss of 5 to (3) Comparatioe experiments with Pittsburgh, Upper Freeport, error. This is mainly be29 B. t.u. per pound per day and Pocahontas coals showed that the Frecporf suffered the greatest cause the second sample, during the first 7 days, loss in heating oalue and the Pocahontas least except for short time through increase in weight from 3.9 to 4.6 B. t. u. per exposure, where the loss f o r Pocahontas was comparatively high. day during 60 days, and due to oxygen absorption, T h i s initial high sensitioity of Pocahontas coal, which is a repredoes not represent the same from 0.3 to1.3B.t.u.perday sentative semibituminous coal, m a y haoe a n important beating o n amount of coal substance as during a year. More restorage. I t is suggested that a pile of this coal should be carefully an equal weight of the cently, Porter and Ovitz4 watched f o r a f e w days after storage; if spontaneous heating does not original sample. Furtherfound the heating value loss deoelop rapidly, the pile m a y doubtless be considered comparatively more, although both heatof Pittsburgh coal to be 4 safe. B. t. u. per pound, that of ing values may be calcuAlthough the method used here was applied to short time exposure Pocahontas 59 B. t. u., and lated to the same basis with tests at relatively high temperatures, it will oboiously apply for a n y that of Sheridan, Wyoming, regard to moisture and ash temperature and for a n y exposure time, prooided only that the coal coal 500 B. t. u.-all for 6 content, errors in sampling tested is so fine that a gram of it will constitute a representatioesample. m o n t h s ’ exposure. Such and the moisture and ash exDeriments are sufficient to determinations mav total. iniicate roughly the order of heating value loss of coal under say, 0.5 per cent, which is quite a large percentage of t i e heat: practical storage conditions, and in this respect they have ing value difference found. served their intended purpose. The experimental methods The experimental method which follows was designed to followed, however, consisted in determining the heating value eliminate these sources of error by exposing to oxidation
I
I
n
I
FIG. I-CONBTANTTEXPERATURE FURNACE a-Thwing controller d-Pyrex tube e-Thermometer in tube b-A. C . electromagnetic relay c-Furnace f-Control couples
on samples reduced from fresh coal in
bulk,
again
Published by permission of the Director, Bureau of Mines, Department of the Interior. * Presented under the title, “Loss of Heating Value of Bituminous Coal during Storage,” before the Section of Gas and Fuel Chemistry a t the 66th Meetinr of the American Chemical Societv. Milwaukee. w i s . ,. Seotember 1
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--.
-~
I
a University of Illinois, Eng. Expt. Sta., Bull. 38 (1909). 4 Bur. Mines, Bull. 136 (1917).
only such weights of coal as could be entirely burned in the calorimeter subsequently. Both the fresh coal Sample and that to be oxidized were weighed from the same representative sample of freshly mined coal. The only err& effecting the diffeEnce in heating value found, therefore, Was that involved in the calorimetric determination. which was less than 0.1 per cent.
INDUSTRIAL A N D ENGINEERING CHEMISTRY
776
An objection which may be raised concerning this method is that it involves the use of fine coal such that, say, a gram of it constitutes a representative sample; hence the method cannot be applied directly to lump coal in storage. It has been shown, however, that it is the fine coal of a storage pile that gives the most trouble from spontaneous heating, and to
w
A
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Vol. 16, No. 8
their placing and removal. Sufficient air was circulated through the tube to insure an excess of oxygen for the coal. The calorimetric determinations were made in a waterjacketed calorimeter of Bureau of Mines design and a Parr bomb. (Fig. 2) Using a platinum resistance thermometer and a bridge and galvanometer (Fig. 3), duplicate samples could be checked to 1 or 2 calories. In view of the fact that in many cases the loss on oxidation was very small, it was necessary to have this degree of accuracy over several determinations. OF THE APPARATUS ACCURACY
For the purpose of this work it was not essential that the water equivalent of the calorimeter be determined with a high degree of precision, but it was necessary that the apparatus give very closely reproducible results on the comparative calorific value of coals. Two determinations on a standard sample of naphthalene from the Bureau of Standards were considered sufficient to establish the water equivalent of the apparatus. The results of these determinations were, respectively, 2766 and 2769 grams as water; 2767 was therefore taken as the water equivalent factor of the calorimeter. The following results will serve to show the comparative accuracy of the apparatus in duplicating results on the same sample of coal: SAMPLE
Run
Heating Value Calories per Gram
Fresh Pittsburgh coal
Pittsburgh coal after heating 3 hours a t 125O C .
7807 7803 7805 A ~ .~ . .7x05 7792 7788 7799 Av. 7790 . . . ~ 8346 8349 8349 Av. 8348 7921 7921 Av.. 7921 8005 7998 8000 Av. 8001 ~~
New River, fresh coal
FIG.2-BUREAU
MINESCALORIMETER &-Drive pulley and jacket stirre1 a-Jacket filler and overflow 1-Sliding cover for bucket opening b-Tap for emptying jacket n-Toluene expansion coif for therc-Heater €or bucket d-Thermostat heating circuit mostat o-Bomb e-Auxiliary heater for jacket +-Thermostat for jacket tempera$-Coal tray ture r-Leads t o firing circuit h-Bucket stirrer
.
OF
this the method is directly applicable. The tests of coals given herein show loss in heating value of coals under accelerated heating conditions such as might be expected in fine coal undergoing spontaneous heating.
~~~
Lower bench, thick Freeport, fresh coal
i i
~~
Upper bench, thick Freeport, fresh coal
From this table it will be seen that by making three runs on a sample of coal and averaging the results it was possible to obtain comparative heating values to within 1 or 2 calories. Results varying more than this were obtained, but in such cases it was evident that variation in the coal sample itself
AND METHOD APPAX~ATUS
The samples to be tested were first weighed in the combustion calorimeter trays. These were put into the oxidizing chamber for an allotted time; the trays were then removed, and the heat units were determined in the calorimeter. I n this way the heating value of the oxidized coal was directly comparable with that of the fresh coal, regardless of any change in weight during the oxidation, in moisture, or in ash content. The oxidizing chamber was an alundum tube furnace, wound with nichrome wire and insulated with Vitrex cement and asbestos. (Fig. 1) Any predetermined temperature was maintained by means of a 5-element copper-constantan thermocouple connected to a Thwing controller, which made contact a t half-minute intervals. This was in series with an electromagnetic relay, which operated a switch across a 110-volt line. Temperatures were read on a mercury thermometer. The oven temperadre was held within *lo C. I n the furnace was a glass tube on the bottom of which was placed a strip of sheet iron. The trays were placed on this strip, which, by reason of its high conductivity, assured an identical temperature for all the samples, as well as facilitating
1
k I
110 volts a. C.
I
F I G . 3-cAI.ORIMETER
a-Calorimeter jacket b-Thermostat c-Bridge d-Mercury thermometer in jacket e-Resistance thermometer in bucket f-Galvanometer
SET-UP
h-Auxiliary jacket heater k-Thermostat circuit 1-Bucket water h ater n-Firing circuit for bomb a-Battery p-Relay
INDUSTRIAL A N D ENGINRERING CHEMISTRY
August, 1924
was the cause of the trouble. Two calories variation in 8000 is less than 0.03 per cent, and it is very doubtful if the ordinary analytical sample is representative to that extent. I n other words, the gram samples weighed out from the usual 4-ounce sample bottle would hardly be expected to be so nearly identical-for example, one might easily introduce 0.03 per cent error by loss of moisture in weighing.
777
E- 2.
Samples of this same coal were treated in the same manner for 3 hours at different temperatures ranging from 50" to 220" C., as given in Table 11. This table is plotted in Fig. 5. Undoubtedly, there is some loss a t 50" C., but in a period as short as the duration of this test the loss is too insignificant to be determined in the calorimeter. Up to 125" C., the loss is small, but above this temperature it increases very rapidly. Attempts were made to test samples a t 250" C., but the coal began to decompose, as indicated by its temperature increase above that of the oven. A crust formed over the sample, rendering i t unfit for the calorimetric test. Fig. 5 indicates that a coal pile may be permitted to attain a temperature of 125" C. (257" F.) without serious loss, but should be watched carefully for further rise. I n an attempt to compare the relative heating rates of several bituminous coals, samples were run a t 125" and 55" C. for periods of 3 and 5 hours. The results appear in Table 111.
3
TABLE 111-COMPARATIVE HEATLOSSES OF PITTSBURGH, UPPERFREEPORT,
4
3 3.
c
3
2.
cz
W
2 2. c7
POCAHONTAS COALS Temperature Calories of Oxidation per Gram -----Lossc. Fresh Coal Calories Per cent 55 7805 2 0.025 55 8022 1 0.012 55 8555 5 0.058 125 7805 15 0.192 125 8022 38 0.474 125 8555 23 0.269 55 7805 11 0.141 55 8022 32 0.399 55 8555 10 0.117 125 7805 39 0.498 125 8022 47 0.586 125 8555 24 0.280 125 8496 30 0.353
AND
B 2 1. W X
3
91. 4
0.
0.
0
8
FIG.4---I,OSS
32 40 48 56 64 72 TIME, HOURS HEATUNITS ON OXIDATION O F UPPER FREEPORT COAL 16
IN
24
DISCUSSIOK OF RESULTS Fresh samples of coal were used in all the experiments. They were sealed in airtight containers a t the mine, and when ready for testing the coal was crushed as quickly as possible and again sealed in bottles, so that there would be a minimum of oxidation. Using coal from the upper bench, Upper Freeport seam, a series of tests was run in air for different time intervals a t a constant temperature of 125" C. The calorimetric values given represent the average of a t least three determinations, all of which checked within + 2 calories. The results are given in Table I. TABLE I-LOS.5 Heating Period Hours Fresh roal 0.5 1 2 3 5 12 16 20 65
IN HEATUNITS AT 125' c . Calories B . t . u. per per Gram Pound Per cent
Calories 8022 8004
ii
7997 7984 7975 7963 7949 7937 7713
0:22 0.27 0.31 0.47 0.58 0.73 0.91 1.06 3.85
32 40 46 68 84 106 131 153 556
22 25 38 47 59 73 85 309
8000
TABLE 11-LOSS
50
85 125 178 185 220
IN
HEATUNITS IN A Calories per Gram
Calories 8022 8021 8008
7984 7735 7558 6297
.. .. 14
.
O:i4 0.27 0.15 0.16 0.12 0.061 0.057 0.053 0.059
38 287 464 1725
INTERVAL B. t . u. per Pound Per cent
3-HOUR
,
.. .. 25
68 516 835 3105
Of the coals tested, the upper bench of the Freeport seam loses most in heating value, and Pittsburgh coal follows. Pocahontas KO. 6 loses more than the others during the 3-hour period a t 55" C. This relatively large loss is significant in that the heat evolved,will assist in raising the temperature of the coal pile to a more dangerous point. Furthermore, Pocahontas is a more friable coal, readily forming fines, so undesirable in a coal pile. Therefore, the relative safety factor for Pocahontas No. 6 is considerably less than that indicated by the smaller heat loss in 5 hours a t 125" C.; and, as experience has indicated, this is not a better storing coal than either Pittsburgh or Upper Freeport.
Per cent per Hour
This table is graphically represented in Fig. 4. At 125" C. the loss per hour is more rapid during the first 5 hours, after which it decreases gradually until the coal becomes saturatedi. e., until equilibrium between the coal and the oxygen is reached. This graph illustrates the great loss during the first few hours-the most vital in the spontaneous combustion history of the coal. Temgerature C. Fresh coal
COALSEAM Pittsburgh TJpper Freeport Pocahontas No. Pittsburgh Upper Freeport Pocahontas No. Pittsburgh Upper Freeport Pocahontas No. Pittsburgh Upper Freeport Pocahontas No. New River
Time of Oxidation Hours 3 3 6 3 3 3 6 3 5 5 6 5 5 5 6 5 5
..
0:i7 0.47 3.57 5.78 21.51
FIG.5-LOSS
IN
HEATING VALUE
OF
UPPER FREEPORT COAL
I n all tests the depreciation in heating value is less a t 55" C., even for 5 hours, than a t 125" C. for 3 hours. This shows that the high temperature is to be particularly avoided in coal storage. The loss during the second 3-hour or second 5-hour period will be considerably less than during the first periods, as given in Table 111. As stated above, the loss is greater immediately after the coal is mined, and decreases a t a gradually diminishing rate, until, at the end of a year, the daily loss is practically nil.
I N D U S T R I A L A N D ENGINEERING CHEMISTRY
778
It is impossible to apply t h e figures given to coal in storage. The samples all passed a 200-mesh sieve, and are far more readily attacked by oxygen than a pile of lump coal would be. I n fact, coal sized a t minus 100 and plus 200-mesh suffers smaller loss than minus 200-mesh size, as shown in the comparative figures given in Table IV. TABLEIV-COMPARISONOF THE HEAT Loss IN MINUS 200-MESH AND MINUS100 AND PLUSZOO-MESHCOAL,OXIDIZED AT 125' C. FOR 5 HOURS -200-Mesh Loss 100f200-Mesh Loss Per cent Per cent Pittsburgh 0.49 0.38 Upper Freeport 0.58 0.56 Pocahontas No. 6 0.28 0.27 New River 0.35 0.35
Vol. 16, No. 8
A coal of minus 100 and plus 200-mesh loses, on the average, 7.8 per cent less in heat than a minus 200-mesh coal, whose surface area is about twice as great. Since a minus 200-mesh coal has nearly five hundred times as much surface area as an equal weight of 0.5-inch lumps, it is evident that the figures shown in the tables are an enormous magnification of the loss in the average coal pile.
ACKNOWLEDGMENT
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The writers wish to acknowledge their indebtedness for the helpful suggestions of A. C. Fieldner, superintendent and supervising chemist of the Pittsburgh Station.
T h e Melting Point of Acetyl Salicylic Acid' By Mark E. Putnam THE Dow CHEMICAL Co., MIDLAND, MICH.
N AN article by Dahm2 there is a careful description of a melting point apparatus especially designed for this work. I n some particulars, however, this author has not emphasized several very essential points necessary for securing a melting point covering a very short range. The range given by Dahm is 133" to 135" C., which is unusually large. The method given herein will serve to shorten this range and a t the same time give great uniformity in the checking of all samples of the pure product. I n connection with the Dahm apparatus, it has been found advisable to hold the melting point tube containing the sample as far as possible above the surface of the heated sulfuric acid bath, for its initial position, or a t least somewhere near the top of the neck of the flask. The bottom of this same melting point tube should extend an inch or more beneath this supporting rod, just as indicated in the Dahm apparatus. Needless to say, the walls of the melting point tube should be very thin (such tubes are best made by drawing out thin test tubes) and the samples of acetyl salicylic acid employed should be very finely powdered and carefully dried a t 85" to 90" C. for 15 to 20 minutes. With such precautions carefully observed, the determinations are made in the following manner :
I
*
The temperature of the sulfuric acid bath is raised to 120' C. and observations are immediately begun on the rate of further heating. The most satisfactory rate found in this laboratory has been taken as 3 " C. per minute, and after some few experiments this rate can readily be attained and carried from 120 O to 140 O C. without much variation. When the rate of heating is found to accord with 3 O C. per minute, and the temperature of the bath is exactly 130" C., the melting point tube is introduced and the observation taken. After the product has melted, the rate of heating of 3' C. per minute is again checked. All determinations are rejected where the rate of 3 " C. per minute is found to be greater or less by 5 seconds in a 60-second interval, within the range 120" to 140" C.
It will be found that the sample of acetyl salicylic acid in melting passes through three well-defined stages: (a) moist stage, ( b ) formation of first globule, (c) complete liquefaction with definite meniscus. Naturally, the appearance of the second stage is that usually considered as more closely representing the true melting point. The following observations were made on three samples of acetyl salicylic acid put out by three separate manufacturers. The thermometers used were glass spindles 31.8 cm. (12.5 inches) in length, reading from 100" to 140" C. and graduated 1 2
Received April 11, 1924. THISJOURNAL, 11, 29 (1919).
in tenths of a degree. They were calibrated by the writer for every one-half degree Centigrade between 124" and 137" C., as against a Reichsanstalt thermometer reading from 95 " to 150°, graduated in two-tenths of a degree, and without any appreciable correction in the range here concerned. Sapple 1 C.
Sample 2 O
133.5 133.8 133.4 Average 133.6
Sayple 3
c.
C. 133.4 133.5 133.3 133.4
133.5 133.4 133.8 133.6
From these averages, therefore, the melting point of pure acetyl salicylic acid may be given as 133.5" C. I n order to give a more severe test to this method of determination of the melting point, two separate observers were given these same samples of acetyl salicylic acid and asked to construct a melting point apparatus after Dahm's description and proceed with the determinations as described above. Their results are as follows: Sample
c.
6 3 6 2
Observer A
B B A B
A
Here the range may be given as approximately 133.3" to 133.6" C. I n other words, the melting point of the purest acetyl salicylic acid that could be obtained does not vary more than between these limits (133.3' to 133.6' C.), and accordingly this is recommended as the correct range for the melting point when taken under conditions described; the value 133.5 " C. is therefore representative of the mean for this range. It is interesting to note the many variations in the melting point of this product as recorded in the literature. The British Pharmacopeia gives the melting point of 133" to 135" C., the Japanese Pharmacopeia that of 135" C., the German, 135" C., and the New and Nonofficial Remedies, U. S. A., 128" to 133" C. The determination given by Dahm, 133" to 135" C., covers too wide a range, but when interpreted as meaning 133' C., referring to incipient fusion, and 135" C., for complete transparency, the figures are sufficiently clear. Cappelli3 has described the conditions emphasized in this paper and has given as a final value 132" C. for incipient fusion, which corresponds to the moist stage described herein. ' These observations were made in the spring of 1920 in collaboration with W. A. Van Winkle, for whose careful work the writer expresses his gratitude and appreciation. 8
Giorn. chim. i n d . applicata, 2, 291 (1920).
