Courtesy, Bureau of N i n e s
BY-PRODUCT COKEOVEN PL.IST
T'ER a period
Efl 0
Electrical
f o u n d . The fiber cylinder was of y e a r s t h e made to fit tightly in the hollow electrical consteel cylinder and was pressed into ductivity of coke has been studied it before t h e c e n t e r h o l e w a s by different investigators (6, 7 ) . drilled and r e a m e d to size. It Only recently have attempts been was necessary to replace this fiber made (3, 5 ) to correlate the cono c c a si0 n a l l y (about every fifty ductirity of coke with its other determinations) because it beU properties and with those of the came pitted with use. The drill coal from which it was produced. rod selected fitted the reamed hole In view of this increasing interest, snugly without binding. the V.S. Bureau of Mines has determined the reactivity, igniFactors Affecting the Election characteristics, and electritrical Resistance of cal C o n d u c t i v i t y of a group of Crushed Coke cokes made under carefully controlled carbonizing conditibns in XOISTURE. Moisture absorbed the Bureau of Mines-American from the air may d e c r e a s e the JOSEPH D. DAVIS AnD H. STUART AUVIL Gas Association retort (9). Ten r e s i s t a n c e of high-temperature of the available thirty-tTvo series U. S. Bureau of Alines Experiment Station, cokes as much as 3 per cent. It of cokes made during the survey seems reasonable to assume that Pittsburgh, Pa. of gas- and coke-making properthe resistance of low-temperature ties of American coals from differcokes would be decreased to an ent coals or blends a t temperatures ranging from 500" to even greater extent since they are more hygroscopic (4). 1100" C. were selected for study. The present report shows Therefore, all cokeswere dried before testing, as recommended how the conductivity of coke is affected by the conditions by Koppers and Jenkner (3). under which the measurements are made, by the plastic Williams and Shuck ( 7 ) and Koppers SIZEOF PARTICLES. and Jenkner ( 3 ) found a gradual increase in resistance as the properties and carbonizing temperatures of the coal, and by the composition of the coke. size of the coke particles was decreased. Their findings have been verified in this study for five successive sizes ranging Apparatus from minus 50- to minus 250-mesh. The resistance of 150to 200-mesh coke was in close agreement with a weighted The apparatus was essentially the same as that used by average of all sizes; since this size gave the most easily reIioppers and Jenkner ( 3 ) . It consisted of a Carver press, a producible results, it was used in all determinations. dial-type Wheatstone bridge with a stated accuracy of 0.1 LESGTH OF SAMPLE. Pressure applied to one side of a per cent, a galvanometer of the closed lamp and scale type, mass of solid particles is not distributed evenly through the and an insulated mold. The insulated mold (Figure 1) was mass as it is in a fluid. Owing to the bridging and binding the only ppecial piece of apparatus needed, and its construeof the particles, there is a pressure gradation a\vay from the tioii wa- -imple after a suitable insulating material had been
Conductivity of Coke
1196
OCTOBER, 1935
INDUSTRIAL AND ENGINEERING CHEMISTRY
point of application. It follows that as the length of sample is increased the average pressure on the individual particles will decrease, and in consequence the contact resistance will increase. Tests showed that the resistance per unit length increased with length of sample. These differences are not variations in the resistance of the coke substance itself but of the contact resistance induced by varying pressures within the sample. The obvious way of minimizing variations in contact resistance is to use a very thin sample. However, the range and sensitivity of the measuring instruments available and the range of resistance encountered in the cokes precluded this possibility. Consequently, it was necessary to use a sample somewhat longer than its diameter; 1.4 times the diameter was adopted, since tests showed that this size gave the desired accuracy in reading and since small variations in length did not cause excessive variations in resistance. For example, with a sample length of 1.4 times the diameter, increases in length of 20, 50, and 100 per cent increased the specific resistance 6, 18, and 49 per cent, respectively. Decreases in length of 20 and 50 per cent decreased the resistances 4 and 11 per cent, respectively. Koppers ( 3 ) seems to have recognized this factor (although no definite dimensions are given in his report), for he states that the material to be tested is compressed to the required size. APPLIED PRESSURE.Figure 2 shows that the resistance and the rate of change of resistance vary inversely with the pressure. -4small variation a t low pressure changes the resirtance markedly. Since it was felt that most of these variations were in contact resistance and would make reproduction of results more difficult, the higher-pressure ranges TI ere used in this study. The lowest pressure on the comparatively flat portion of the curves is 60,000 pounds per square inch (4200 kg. per sq. cm.). Since errors due to slight changes in pressure a t this point are small and since higher pressures deform the fiber retainer, this pressure n-as used in all subequent tests. TIMEOF PRESSURE APPLICATIOX. When samples were placed under a constant pressure of 60,000 pounds per square inch, the specific resistance decreased for about 25 minute% and then remained practically constant, the initial T alue averaging about 8 per cent higher than the final one. If, however, the pressure was released momentarily and the coke allowed to expand sornmhat and redistribute itself, the same constant value was attained in 10 minutes. The results giren in this paper are the constant values reached after momentary relesqe of the pressure.
The determined resistance of a sample of crushed coke is the resistance of the coke substance plus the contact resistance of the particles. The amount of contact resistance depends upon the test conditions. Relative values can be obtained for conductivity of the coke substance if test conditions are duplicated carefully. The conductivity of cokes made from the same coal depends upon and increases with the carbonizing temperature. The average conductivity of cokes produced at 1100' C. is 260,000 times that of cokes produced at
600' C.
119;
In general, results were e a s i l y reproducible; 85 per cent of the duplicate 'determinations agreed within 2 per cent. In the other 15 per cent there was usually binding of t h e p l u n g e r with an accompanying shriek. The shriek and a third determination invariably pointed out the one in error. After the foregoing preliminary tests were made, all coke samples were tested as follows: F r o m a half-kilogram sample, representative of the coke from a single test (about 30 kg.), a 150200 mesh sample was p r e p a r e d , dried at 115" C. for 2 hours, cooled in a desiccator, FIGURE1. APPAR.4TUS FOR hIE.4Sa n d placed i n a u n I w THE CONDUCTIVITY OF COKE tightly stoppered bottle. B 0.454.55 gram sample of this coke (its length under preswre was 1.25 to 1.5 times the diameter) was placed in the mold (Figure 1) and jarred down; then the plunger WRS insert'ed. A load of approximately 3000 pounds (60,000 pounds per square inch of sample) lyas applied, released, again applied, and then held for 10 minutes, a t which time the resistance and the length of the sample were determined. The resistance was calculated as ohms per meter per square millimeter. All determinations were made a t room temperature and in duplicate, the average value being reported. The maximum variation allowed between duplicate tests was 2 per cent. Cokes Tested
The dry, mineral-matter-free, fixed carbon, the proximate analysis, and the plastic properties of the ten coals selected for this study are given in Table I, together with the volatile-
The quantitj and quality of the polatile matter contained i n the coke affect its conductivity to a marked extent. The logarithm of the resistance is a straightline function of the volatile matter, the slope of the line depending on the carbonizing temperature of the coals. Other factors being equal, the conductivity of low-volatile or devolatilized cokes varies with the length of the plastic range as shown by the Layng-Hathorne test. Coke conductivity decreases with decrease i n coal rank. The relation is only general, however, and does not hold for individual cokes.
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TABLE I.
VOL. 27, NO. 10
PROPERTIES 3~ COALS AND VOLATILE-MATTER CONTENT OF DRY800" (ARRANGED IN ORDEROF DECREASING FIXEDCARBON IN COAL)
ANSLYSES AND PLASTIC
Coal No.
Bed
State
23 26 27 8 7 9 28 10 19 21
Pocahontas No. 4 Sewell Sewell Mary Lee (washed) Mary Lee (washed) Pittsburgh Pittsburgh No. 6 Lower Sunnyside Green River
W. Va. W. Va. W. Va. Ala. Ala. Pa. Pa. Ill. Utah
KY.
County McDowell Raleigh Fayette Jefferson Jefferson Fayette Fayette Franklin Carbon Muhlenberg
Modified Layng-Hathorne Plastic Max. range pressure
Fi 84.1 78.6 72.5 69.1 67.7 63.5 62.6 60.6 57.1 57.0
matter content of the cokes produced a t 800" C. The coals range from 15.4 to 38.8 per cent volatile matter (as carbonized) and from 57.0 to 84.1 per cent of dry, mineral-matterfree, fixed carbon. Since these coals cover virtually the entire range in rank of coking coals in America and since the cokes made a t any given temperature were produced under strictly comparable conditions, the results should be free from criticism either as to the range of the coals selected or the comparability of the cokes. Tests were made on dry cokes produced a t 800" C. from all coals listed and on those pro-
C. COKES
Per cent15.4 20.8 26.5 27.6 27.9 33.6 35.1 32.1 38.8 36.2
0.8 1.3 1.9 4.2 1.2 1.9 1.8 7.9 4.6 10.1
C.
M m . HzO
8000
c.
Coke!, Volatile matter Per cent
The logarithms (to the base 10) of the resistances are plotted against carbonizing temperature in Figure 3; the numerical values are given in Table 11. Cokes made from coal 27 have the highest conductivity (or lowest resistivity), and thobe from coals 26, 28, 21, and 10 follow in order. These coals, when arranged in order of decreasing Layng-Hathorne plastic range, fall in the same order. This trend is discussed later. As the carbonizing temperature is increased, the resistance decreases; however, each successive increase in carbonizing temperature is accompanied by a smaller decrease in resistance. The change in resistance between 600" and 700" C. is the greatest; that betaeen 700" and 800" C. is also large compared with those between succeeding 100' C. temperature intervals. The curves shown (Figure 3) are for dry cokes with the exception of the "devolatilized" curve for coal 10, which shows the resistance of samples after being used in the A. S. T. M. volatile-matter determination. The wide difference between the various pairs of values, especially a t the lower carbonizing temperatures (154,000,000 as compared with 331 ohms a t 600" C.), suggested that the volatile content was one of the determining factors in coke TABLE11.
RESISTANCE OF DRY COKESIN ORDER OF DECREAGIXG FIXED CARBON IN COAL
--Resistance 600' C.
Coal NO.
7
26 1,095,000 712,000 27 28 I4,800,000 10 154,000,000 33 1 105 21 19.500.000 a Cokes subjected t o
a t Carbonizing Temperatures of:-700° C. 800' C. 900' C. 1000° C l l O O o C Ohms/meter/sq. mna. 92 75 148 3,590 461 94 68 316 132 2,700 128 86 192 7,660 471 191 162 2,060 361 93,800 162 136 301 245 322 158 122 831 245 5.390 standard .1.S . T. A f . volatile-matter determination.
TABLE111. RESISTANCE OF CARBONACEOUS SUBSTANCES (Ohms per meter per sq. mm.) -1rtificiai graphite 4cheson grade BB5 Natural flake graihite Mexican, containing 7 % ash Commercial by-produit coke (coal 9) Sugar char, heated to 550" C. for 1 hour and devolatiliaed
FIGURE 2. RELATION OF RESISTANCE OF COKE TO APPIJED PRESSURE duced a t 600", i O O " , 900", 1000", and 1100" C. froin coals 26,27,28,10, and 21. Tests were made on the 800" C. cokes and the cokes from coal 10 after they had been subjected to the standard A. S. T. M. volatile-matter determination (1).
OF DRY4 N D DEVOLATILIZED 800" c. TABLE ILr. RESISTANCE COKES, IN ORDER O F DECREaSING FIXEDCARBON I N C O A L
Coal No.
Results Tables 11, 111, and IV and Figures 3, 4, and 5 show the results of these tests. The electrical resistances given are in ohms per meter per square millimeter; they can be converted easily into resistivity or conductivity by the formula: Resistivity (ohms/cm. cube) = 1 conductivity ohms/meter/sq. mm. 10,000
27 34 92 228
4
Coal Rank
23 26
Low-volatile
27 8
Medium-volatile
7 9 28 10 19
-ResistanceDry Devolatilinedo Ohms/meter/sp. mm. 607 150 149 461 316 798
146 163
High-volatile A
1040 566 471
181 181 168
High-volatile B
2060 880
301 274
83 1 291 21 High-volatile C Cokes subjected t o standard A. 5. T. M. volatile-matter determination.
OCTOBER, 1935
INDUSTRIAL AND ENGINEERING CHEMISTRY
1199
The volatile matter and the log resistance curves are roughly parallel. The resistance, however, appears somewhat high for the given volatile content for cokes from coals 7 and 10. This high value can be explained by the fact that these two cokes are high in ash compared with the others shown: The one from coal 7 contains 19.4 per cent; and from coal 10, 18.9 per cent; and the remaining cokes, less than 12 per cent, This is in accord with the findings of Williams and Shuck (7) who concluded that ash contained within the particle increased the resistivity somewhat but not nearly so much as volatile matter. Koppers and Jenkner (5) found that an ash content of less than 10 per cent had little effect on conductivity. Calculations made from a curve included in their report show that 20 per cent ash decreases the conductivity approximately 30 per cent. This variation is of the same order as that mentioned above. The cokes (Figure 5 ) have been arranged in order of decreasing plastic range of the coal. Attention has already been called to the facts that the resistance of the dry cokes from five different coals varied ingj __--* ,- - __ + -~ v ersely with t h e 8s --- length of the plastic E range, as shown by the modified LayngHathorne test, and t h a t t h e r e is a marked dependency of r e s i s t a n c e o n volatile c o n t e n t . This r e l a t i o n s h i p d o e s n o t h o l d so well for ten coals as it did for five. If these cokes are subjected to the standard A. S. T. M. FIGURE 4. RELATION OF RESISTANCE volatile-matter deO F COKE TO VOLATILE-MATTER t e r m i n a t i o n and CONTEYT
~~;;[~i~-~~ ---TAT---
FIGURE 3. RELATION OF RESISTANCE OF COKE TO CARBONIZIKG TEMPERATURE
resistance. Figure 3: show-b the relation found. It can best be expressed by two curves, one for the 700" and 600" C. cokes and the other for the cokes produced a t and above 800" C. If the resistance of the coke depended solely on the quantity of the contained volatile matter, the curve would be continuous. Since it is not, it may be assumed that the quality as well as the quantity of the volatile matter present affects the resistance, especially since it is a known fact that there is a difference in the.character of the volatile products evolved during low- and high-temperature carbonizing processes. It has been noted in the Bureau of Mines-American Gas Association tests ( 2 ) that this change in composition usually occurs between 700" and 800" C. The approximate relation of resistance to volatile matter can be expressed by the following formulas: For coals carbonized at 600" and 700" C.: R = 2.2 X 12.5" For coals carbonized at SOO", goo", lOOO", and llOOo C.: R = 83 X 2.2u where R = resistance, ohrns/meter/sq. mm. v = volatile content, per cent These formulas indicate that the volatile matter in the individual cokes of either group is of similar quality. The assumption is that there are two major groups differentiated by the quality of the volatile matter of the cokes, the critical point being between 700" and 800" C. This dependency on volatile matter largely explains the variation in resistance with temperature. The change in resistance between 600" and 700" C. is due mainly to a change in the quantity of volatile matter present; that between 700' and 800' C. is due to change in quality as well as quantity. Succeeding changes are largely a matter of quantity. The above, however, does not explain the changes in resistance shown for the devolatilized cokes from coal 10. These changes are discussed later. Figure 5 illustrates the relation of resistance to volatile matter for cokes produced from ten different coals a t 800" C.
FIGURE5. RELATIONOF RESISTANCE OF COKE TO THE PIASTICPROPERTIES OF COAL AND VOLATILEMATTER IN COKE
