T H E S I G S I F I C A S C E O F T H E HI’DROGES C O S T E S T O F CHARCOALS BY H. H. LOWRY
Most studies of the thermal decomposition of hydrocarbons are confined to an examination of the composition of the liquid and gaseous products. Among exceptions to this generalization may be mentioned the interest in coke, carbon black, and charcoal. Even in these cases the physical properties rather than the chemical composition are regarded as the factors which drtermine their suitability for specific uses. However, in an earlier papers it was pointed out that certain physical properties of a group of charcoals were rather simply related to the percent hydrogen which was contained in them as determined by ultimate analysis. This group of charcoals was prepared in a gas-fired furnace from a single, specially-selected lot of anthracite coal. As stated in this earlier paper, careful consideration of the commercial records taken a t the time of preparation indicated that the hydrogen content was probably determined by the maximum temperature t o which the samples were heated during their preparation. The hydrogen contents ranged from 0 . 2 1 to 0 . 5 3 % ~ while the probable range of maximum temperature was goo0 to 1zoo0. The presence of hydrogen in these charcoals was shown to be consistent with a point of view that so-called “amorphous” carbons are hydrocarbons of low hydrogen content2 built up of polymerized residues from the thermal decomposition of hydrocarbons of greater hydrogen content. Since the significance of the hydrogen content of charcoals has been generally overlooked, the present study was undertaken in order to evaluate the factors xvhich may ordinarily be varied in the preparation of charcoals for various purposes. The factors which were independently varied in this study were the maximum temperature, the time of heating, the atmosphere surrounding the sample during heating and the raw material. To a limited extent the effect of previous heat treatment was also determined. A later paper will give the results of the study of the correlation of hydrogen content and some adsorptive properties of charcoals prepared under carefully controlled conditions. Preparation of Samples and Method of Analysis Since the samples which are considered in this report were not all prepared specifically for this investigation, the method of preparation of certain series of samples differed from the method used in other series. In general, however, the samples were all.originally ground and sifted to 60-80 mesh and heated in a cont’rolled atmosphere for a known length of time. In all cases, J. A.lm.Chem. Sor., 46, 824 (19241. * F o r instance. see Bancroft: J. Phvs. Chem., 24, 1 2 7 (1920);Lewis and Randall: “Thermodynamics and the Free Energy Chemical Substances,” page 569 (1923).
of
S I G S I F I C h S C E O F H T D R O G E S C O S T E S T O F CO.1Ld
I333
the furnace was an electric furnace of large heat capacity, heated by a platinum rhodium resistor Tyhich was so wound that for a length of about j" there were no temperature differences of more than 1 2 ' . The temperature was niaintained constant a t the desired temperature by means of a Wheatstone bridge controller' specially designed with a recorder by Leeds and Sorthrup ('ompany. With this controller the temperature fluctuations were reduced to less than s o for periods extending to a week when the iliaximum teniperature did not exceed about 1300°, above which temperature the volatilization of the platinum caused a gradual drift in temperature downwards requiring frequent manual adjustment. Except when the effect of the surrounding atmosphere on the hydrogen content was being studied, a pure dry hydrogen atmosphere was maintained. In general, the sample ranging in size from 5 grams to j o grams was placed in a small graphite crucible which in turn was placed in the zone of maximum temperature. Temperature readings taken every j minutes showed that the furnace had recovered its temperature equilibrium within the first j minutes. At the end of the period of heating the crucible was withdrawn to a cool part of the furnace and left in the current of hydrogen until sufficiently cold to handle freely. The hydrogen content was ascertained by means of a simple organic combustion, by which carbon was determined as carbon dioxide by absorption by soda-lime and hydrogen as water by absorption by phosphorw pentoxide. Since our earlier data indicated that the hydrogen mas directly associated with the carbon and not with impurities contained therein, all analyses were calculated to an ash-free basis. In order to avoid errors in the analyses due t o absorbed gas and moisture, the samples mere evacuated a t zoo' for 6 hours and burned in a current of pure oxygen without transfer from the vessel in which they were evacuated. I n this case it was unnecessary t o weigh the original samples since our only interest was in the carbon and hydrogen present in the sample and these were directly determined as oxides in the analyses. Duplicate analyses on the same sample checked in the great ~ if not, further analyses were majority of cases to within o . 0 ~ 7hydrogen; made until the probable error of the average was less than 0.027~ hydrogen Xn alternative method developed by Mr. W.B. Warren of these Laboratories was used for certain samples having hydrogen contents less than o . z o 5 . This method consisted in carrying out the combustion in a closed system in which the sample could first be outgassed and which was provided with suitable fractionating traps so that the carbon dioxide could be measured volumetrically and the water weighed by absorption by phosphorus pentoxide. The weighing procedure was simply an observation by means of a cathetometer of the extension of a quartz-fiber spiral balance2 from which was suspended a small tray of phosphorus pentoxide. The quartz-fiber balance was contained in the closed system in which the combustion took place. This 1 See H. 8.Roberts: J. Wash. Acad Sci., 11,401(1921); E. S . Bunting: J. .Im.Ceramic Soc., 6, 1209 (1923). J. W. McBain and A. hi. Bakr: J. Am. Chem. SOC., 48,690 (1925).
H.H.LOWRY
I334
TABLE I Calculated and Observed Values of Hydrogen Content for Coal 61 Time
Temp "C 800
900 900
Obs.
Cal.(I)
8 16
1.30
1.08
1.25
1.03
-0.24
-0.02
24
.03 I .OI 0.97
1.22
I
.oo
-0.2j
-0.03
24
0.63
0.63
0.61
0.5
0.96
0.82
f0.13
.8I
.76
0.83 .77
+o.14
.o
+0.05
50.04
.73 .63
.TO
.j2
+0.03
+O.OI
.66 .60
-0.02
-0.03
-o.oj
-0.05
.49 .4j
.65 .60 .54 .49
.j5
-0.0;
-0.06
.48
-0.04
-0.03
0.5
0.65
0.60
+O.
.o
.56
2
.o
.51
4.0 8.0
,42
.j4 .49 .43 .39 .34
0.62 .j 6
+o,oj
I
I
2 .O
4.0 8 .o 16 . o 32 . o
15.5
48.0 141. j IIO0
.55
.39 .37 .29
.28
.27 .PI
Cal.(I)
.j 2
Obs.-Cal.(r)
Obs. -Cal.
-0.27
-0.05
0 .oo
+0.02
+o
,02
03
0.00
+0.02
-0.01
-0.01
-0.04
.46 .4I .36
+0.03
0.00
.29
+0.02
.23
+0.07
-0.01
-0.02
+o.or 0.00
+0.05
.o
0.39 .32
0.38 .33
2 .O
.29
29
0.38 .34 .30
4.0 16 . o
.28
.2j
.2j
+0.03
.20
. I8
.I9
+0.02
40.8
.I 4
'I4
.I4
24.0
0.10
0.15
0.Ij
-0.oj
-0.0j
0.25 .I5 . I3
0.24
+O.OI
$0.02
-0.06
-0.04
-0.04
-0.02
0.5 I
1110
5
Hours
I
1000
Hydrogen Content
in
0.01
0.00
0.00
+O.OI -0.02
-0.01
0.03 +O.OI 0.00
.I 2
'17 '14
.IO
.I 2
0.23 'I9 .I5 ' '3 .I1
.09 .06
.09 .06
.08
0.00
48.0
.06
0.00
I220
24.0
0.08
0.07
0.06
+O.OI
$0.02
1400
0.5
0.09
0.10
0.06
-0.01
+0.03
I 500
I
0.046
0.048
0.021
-0.002
+0.02j
I200
0.5
I .o 2.25
4.0
8.0 16.0
.o
.21
-0.02
-0.01
-0.02
-0.01
+O.OI 0
.oo
(2)
S I G N I F I C A S C E O F H T D R O G E S C O S T E N T O F COALS
I335
method permitted the use of small samples while maintaining the same degree of accuracy as ivas obtained by the usual combustion method for samples having much greater hydrogen contents.
Experimental Results In order to determine the effect of the maximum temperature and the time of heating, a large sample of a specially selected anthracite coal was obtained and smaller samples of this heated a t temperatures ranging from 800" to 1 5 0 0 ~ for periods of time ranging from 0.5 to 141.5 hours. The data so obtained are given in Table I. In the fourth and fifth columns are given values calculated on the basis of the following considerations. Anthracite coal is commonly supposed to have been formed by the thermal decomposition under pressure of organic matter. Preliminary examination of the data indicated that the hydrogen content was much more sensitive to a change in temperature than to a change in the length of time of heating a t constant temperature, and that a t constant temperature the decrease in hydrogen content with time was exponential. I n addition to these facts, it was observed experimentally that no measurable thermal decomposition took place below some fairly high temperature, Le., about 480' for this particular anthracite coal. To determine this initial temperature a sample of coal was placed in a quartz tube and connected to a Topler pump. After a preliminary evacuation a t room temperature, a furnace was raised into position surrounding the sample. The temperature was raised in 50' steps and maintained at each temperature until the rate of gas evolution was less than 0.1 C.C. per hour. The gas was collected and analyzed separately for each temperature interval. In Fig. I are plotted the total cubic centimeters of hydrogen evolved to the temperature indicated. The method of obtaining To from these data is also illustrated.' The figures in brackets a t each point represent the number of hours the sample was heated a t each temperature. Several equations were set up which qualitatively were in agreement with the facts outlined above.2 Of these equations, one of the form H = H, e-a(T- Tdt" (1)
was chosen as most satisfactory. In this equation H represents the hydrogen content after heating a t temperature T for time t for a material having an original hydrogen content H, and initial decomposition temperature To, a and n being empirical constants and e the base of natural logarithms. I n calculating the values given in column 4 in Table I the value of T owas determined empirically from the data, while the value of H, was determined by analysis of the raw coal. The equation as used, replacing the constants by their numerical values, becomes
H
=
2.10
10-000210(T-720)t~~~~
(14
The method of measuring this temperature is essentiallv that of 0. A. Nelson and G. A. Hulett: J. Ind. Eng. Chern., 12, 40 (1920). Also see R . Holroyd and R. V. Wheeler: J. Chern. Soc., 1928, 3197. I am indebted to 3Ir. G. G. 3luller for developing these various equations.
I336
n. H .
LOWRY
Since the value of To so obtained, Le., 7 2 0 ° , did not agree with the experimental value of 48oU,the following alternative form of equation was tried:
H = H,
e-b(T
- T,jit" (2)
FIG.I Thermal Decomposition of Anthracite Coal ? I , showing the total cubic centimeters of hydrogen evolved per gram of coal to and including the temperature indicated. The figures in brackets represent the number of hours the sample was maintained at each temperature.
where b and c are constants while the other symbols have the same significance as in equation ( I ) , To being the experimentally determined value. Replacing the constant with their numerical values, equation ( 2 ) becomes 10-0.000004i7(T-480)L."t~.l" H = Io (28)
S I G S I F I C A S C E O F HYDROGEN CONTEST O F COALS
I337
The values obtained by use of ( 2 8 ) are given in column 5 of Table I. I t is to be observed that either equation fits the data remarkably well considering the wide range of temperatures and times covered by t'he data. Equation ( 2 8 ) appears slightly superior in that both the maximum positive and negative deviations obtained with it are less than those from equation (Ia) and since it gives a much closer fit to the data a t the lowest temperature used. Approximately half of the deviations are equal to or less than the probable error of the observed values, considering only analytical errors and not the errors due to possible variations in the method used for preparing the samples. I n order to test the equations further, a few samples were prepared froni two other coals, one an anthracite coal having H, = 2.80% and To = 460", and the other, a coal sold as "Domestic Fuel," having H, = 4 . 0 3 7 ~and TP= 360'. The observed and calculated values are given in Tables I1 and I11 respectively. The equations used for the calculations were, for Anthracite 42
.\gain in the case of these two coals either equation appears to fit the data equally well. This might well be expected since both equations are threeconstant equations, since in the first form To does not appear to have the significance assigned to it, and another constant "c" is necessary when the experimentally determined value is used. There is, however, a possibility
TABLE I1 Calculated and Observed Values of Hydrogen Content of Anthracite Tzmp. C
Time in Hours
IO00
0.;
$2
c;
Obs. -Cal.(I) 0bs.-Cal.(zl
2 . 5
Obs. 0.j7 .54 .45
4 .O
8 .o 16 . o 2 4 . 0
.32
0.j
0.22
.o
.18 '15
. I'i
-0.01
.o 4.0
"7 . I8 '13
.14
fo.03
+o.o~
.I2
.I1
so.01
+0.02
24.0
.IO
.06
.06
+o.o4
+o.o~
I .o
I200
Hydrogen Content
I 2
Cal.(I)
0.61 '
54
Cal.
(2)
0.60 .j4
.46
.46
.42
.42
.4I
.33
.36
.31
.31 .28
.36 .31 .28
0.22
0.21
-0.04 0.00
-0.01 0.00
-0.03 0.00
+0.04 0.00
-0.03 0.00 -0.01
to.01
-0.03 0.00
+0.04 +O.OI 0.00
n. H.
I338
LOWRY
that the value of Todetermined from the hydrogen-content data does represent the minimum temperature a t which hydrogen is liberated by “primary decomposition”’ and that the lower temperature found experimentally is the result of a secondary decomposition of hydrocarbons liberated from the coal by heat.
TABLE I11 Calculated and Observed Values of Hydrogen Content of “Domestic Fuel” Teap.
C
1000
Time in Hours
5
Cal.(rj
C a l . ( a ) Ohs. -Cal.(I)
0.;
0.42
.o
.40 .3i
0.44 .40
.33 .32
0.43 .40 .37 .34 .3’
.37 ,34 .3I
-0.01
-0.01
$0
+O.OI
I
2 .O
4 .O
8 .o 16 .o
I200
Hydrogen Content
Obs.
-0.01
Obs.-Cd.(2j -0.02
0 .oo
0.00
0 .oo
0.00
01
.2j
.28
.28
-0.03
-0.03
3 2 .o
.23
.2j
.2j
-0.01
-0.01
0.j
0.22
0 . 2 0
0.20
.o
.I i
,
I8
-0.01
2 .O
.16
,16
.18 .16
4.0
.I3
.I4
-0.01
I
0.02
0.00
+0.02
-0.01 0.00
8 .o
.I1
,I3
‘I4 , ‘3
24.0
.IO
.IO
.I1
0.00
-0.01
48 . o
.I1
.09
.09
0.02
so.02
-0.02
-0.01
-0.02
It is to be noted that the same value of the time exponent, 0.106, is obtained for both anthracite coals, while a much smaller value, 0.052, is obtained for the Domestic Fuel. This means that time is a less important factor in the case of the Domestic Fuel, the temperature being an even more predominant factor than in the case of the two other coals. It seems likely that this is to be somehow associated with the fact that the Domestic Fuel passes through a molten or plastic state, while both anthracite coals remain rigid solids throughout the temperature interval studied. This difference in the values of the time exponents for the anthracite coals and the Domestic Fuel is in accord with the hypothesis proposed in the earlier paper2 to account for the presence of hydrogen chemically combined to carbon after exposure to high temperatures. I t was suggested then ( I ) that the hydrogen was held by carbon atoms which were PO situated with respect to neighboring atoms that they were unable to take their place in the normal graphite lattice and ( 2 ) that as the temperature was raised the mobility of the carbon atoms was increased and that a certain number would thereby be able to orient themSee R.Holroyd and R.T. Wheeler: loc. cit. J. Am. Chem. Soc., 46, 824 (1924).
S I G N I F I C A N C E OF HYDROGEK COSTENT O F COALS
I339
selves in a graphite lattice and in so doing transfer the force which previously had been sufficient to retain a hydrogen atom to the neighboring carbon atoms. During the molten stage, the carbon atoms of the Domestic Fuel are very mobile and the hydrogen is freely liberated, which is also reflected in the low hydrogen contents. 1180 SAMPLE
*
1140
7. HZ IN RAW COAL 2.13 2 33 2 44 7 54
9
1120.
4
2 57 258 2 62 2 66 2 68
D
1100-
+
1080-
*
1060
e
A
+
2en 3 70 4 03
t
4 46 4 73
x
.
T ‘C. 10401
1020 R..P
1000.
980.
960. 940920900880.
860e
840-
+
n
+ *
820.
L
I?
.
01
.
02
.
03
.
04
.
.
.
.
.
05 06 0.7 08 09 PER CENT HYDROGEN
.
10
.
11
.
I2
,
1.3
FIG. 2
-1plot of hydrogen content data of fourteen different coals heated in a n atmosphere of hydrogen for thirty minutes at the temperatures indicated. In addition there are included point “R,”activated cocoanut charcoal; point “P,” sugar charcoal, and two points “T’,” wood charcoals.
The few experiments conducted t o determine the influence of previous heat treatment indicated that the treatments may be considered additive. For instance, a sample of Anthracite #I was heated to IZOO’ for 4 hours reducing the hydrogen content to 0 . 1 2 7 ~ . Subsequent heating at’ 800’ for
4 hours left the hydrogen content unchanged. This would be expected from an examination of the equations. To reduce further this hydrogen content of 0 . 1 2 7 ~by an amount equal to the probable error of thedetermination would require, according to equation (za), a t least 800 years a t 800’. Similarly, judging from the hydrogen content data alone, preheating a t 800” is equivalent to a much shorter time a t 1200’ if the sample is later to be heated at, I2OO0.
In Fig. z are shown graphically the results obtained on 14 different sample3 of coal, all previously ground and sifted to 60-80 mesh and heated for I z hour in an atmosphere of hydrogen a t the temperature indicated. The hydrogen contents of the raw coals are given in t’he legend. All analyses are reported on an ash-free basis. I t is evident from the figure that the hydrogen contents of the various samples fall within a comparatively small range for a given maximum temperature of treatment, the range becoming less a t the higher temperatures. I n general, the “softer” coals, Le., those having the higher hydrogen contents in the raw state, yield products having less hydrogen than do the “harder“ coals, which is in accord with the previous suggestion that in these cases the fusion of the coal allows the hydrogen to escape more readily than is the case with a non-fusible coal. Four points taken from the literature are also indicated on this figure. The point marked “R”‘is for an activated cocoanut charcoal, that marked “P”2 is for a sugar charcoal and the two marked “77”3are for wood charcoals. The fact that the hydrogen contents are so little dependent on the raw material suggests that the mechanism of thermal decomposition of carbonaceous materials a t high temperatures is relatively simple and probably determined largely by the properties of the carbon atom itself. The mechanism proposed in a preceding paragraph considers a balance between the rigidity of the carbons and their energy contents. I n the preceding figure there was shown a point representing the hydrogen content of a charcoal activated in an oxidizing atmosphere a t 1000’. This charcoal had the same hydrogen content as the other samples prepared in a hydrogen atmosphere. This would indicate that the hydrogen content was not of primary importance from the viewpoint of the activation process, but is determined solely by the temperature. Many experiments have been performed which fully substantiate this conclusion. I n Table IV are given hydrogen analyses of samples prepared in different atmospheres. The only variable in the preparation was the gas passed over the carbon during the period of heating. The carbon was exposed in a thin layer to the gas in a rotating tube so t,hat the granules would be uniformly affected by the atmosphere. With oxidizing atmospheres it was necessary to reset the temperature under the actual conditions of the test since the oxidation was sufficient to raise the temperature of a thermocouple in the furnace as much as z jo. The oxidizing gas was passed over the surface of the granules, the amount used l.4. B. Kay: Chem. and Met. Eng., 2 8 , 977 (1923)
A. R. Powell: J. .Im. Chem. Soc., 45, I (1923). Violette: Ann. Chim. Phys., 32, 322 ~1853).
S I G N I F I C A S C E OF HYDROGES COSTENT OF COALS
1341
ranging from 80 to 800 C.C. (0.16 to 1.6 grams) per gram of sample. The charcoals changed regularly in appearance as the amount of oxidation increased, changing from solids with metallic luster t o ones with the dull matte finish characteristic of adsorbent charcoals. It may be of interest to mention briefly a t this time that the amount of carbon dioxide adsorbed a t oo and j 6 0 mm pressure by these samples was increased by the oxidation from 8 C . C . per gram to 33 C . C . per gram for those prepared a t 1000' and from z C . C . per gram to 2 4 C.C.per gram for those prepared a t 1100'.
TABLE IV The Hydrogen Content of Samples of Anthracite Coal Yo. I prepared a t Different Temperatures in Hydrogen, Air, and Carbon Dioxide Temp. "C Hz Air CO2 Gal.**" 900 0 . 7 6 (4)* 0 . 6 7 (9)** 0 . 6 7 (6)** 0.78 .4Y (4) . 3 3 (1)
.48 ( 5 )
rojo I100
.2Y
(2)
.28 (3)
IIjO
.22
(i)
.22
I200
. 2 0 (2)
. I 8 (3)
I 2 50
. I 3 (1)
1300
. I O (2)
.I3 .09
IO00
.36 (3) (I)
(2) (1)
. 4 i (3) . 3 6 (3)
,49
.28 (6) . 2 2 (8)
.28
.Ii .I4 .09
(15)
. I j
(1)
.I1
(2)
.os
.3i .21
* The figures in the brackets are the number of samples prepared at the conditinns indicated. The hydrogen content given is an average of this number. * * That these 900" values are too low, as may be judged from plotting the data, is t o be attributed to the fact that they were prepared before it y a s appreciated that the use of an oxidizing gas raised the temperature of the furnace above that established with the furnace idle. * * * These samples were calculated using the following modified form of equation za: H = 2.10 x 10-000000~2i(T--16011di assuming t"ID6 = constant value 5 . 2 7 X 1 0 - 6 .
=
1.105 which is combined with the constant ''a" giving the
summary Data have been given which show that at, a constant high temperature, exceeding some temperature characteristic of the material, carbonaceous materials decrease in hydrogen content in a regular manner with increasing time of treatment. The temperatures ranged from 800" to 1500' and thr times of heating ranged from 0 . j t o 141, j hours. Two equations of similar form are given, either of which reproduces the data within the esprrimental error for three different coals. It is pointed out that the effect of t x o separatr heat treatments is additive. I n all cases the hydrogen content n-as much more sensitive to a change in temperature than to a change in the length of time of heating at constant temperature. From a study of the thermal decomposition of 14different coals, ranging in hydrogen content from 2.13 to 4.73%. it was determined that the hydrogen contents, after heating for 0.5 hour at temperatures from about 900' to about
IZOO', fell within a very narrow range. I n general, the greater the hydrogen content of the raw coal the smaller the hydrogen content of the charcoal for a given treatment. This is particularly true where the coal passes through a plastic state. A hypothesis is presented t o account for the phenomena observed. It is shown that the atmosphere in which the sample is heated does not influence the hydrogen content of the resulting charcoal. Analyses of 93 samples prepared a t temperatures ranging from 900 t o 1300' in hydrogen, air and carbon dioxide are given to support this statement. These data indicate t h a t the hydrogen content is not of primary importance from the viewpoint of the activation process as has previously been common belief.
Bell Telephone Laboratories, A-ew York. N . Y.
