August, 1930
INDUSTRIAL AND ENGINEERI-VG CHEMISTRY
danger lies is unknown. The cooperation of oil chemist, entomologist, and plant physiologist is needed in the study of these complex problems with the strong assurance that continued progress in research will open equally large or larger fields than have now been developed.
839
Literature Cited Gray and de IND. ENG.CxEM., 181 1 7 j ( l g 2 6 ) . (2) On& de, Knight, and Chamberlain, H i l s a r d i a (Calif. Agr. Expt. S t n I . 2, 351 (1927). ( 3 ) Sligh, Proc. Am. Soc. Testing M u f e r i a l s , 24, Pt. 11, 1 (1924).
Heat Transfer in the Low-Temperature Carbonization of Coal' V. C. Allison 36 W. BLACKWELL Sr., DOVER, N. J.
In the carbonization of coal, part of the entering EMI-COKE is much results obtained. For the heat raises the coal charge to a suitable reaction temmore easily penetrated sake of simplicity and to perature and the remainder promotes the carbonizaby heat than coal and avoid complicating the probtion, chemical and physical reactions requiring a the transformation of coal to lem to an undue degree, the definite time to proceed to completion. Heat can semi-c o k e therefore overFourier theorem and the use penetrate coal layers less than 0.52 inch thick more comes one of the greatest diffiof Bessel's functions will be rapidly than the carbonization reactions can proceed. culties in the thermal procdisregarded. Therefore, in carbonizing stationary coal layers the essing of coal. The transIt is customary to conthroughput in pounds per square foot of heated surformation occurs a t 390" C. sider that the time, M , reface per hour decreases when the coal layer thickness (735" F.) f o r o n e of t h e quired for carbonization is reis less than 0.52 inch. best known American coking lated to the thickness, R, of Heat enters the coal charge largely by radiation, coals, Pittsburgh seam, and the coal layer which must be but the heat inflow is rhythmically interrupted by the the continued addition of penetrated by the heat and a periodic formation of the plastic layer which, before heat above this point, serves term, K , which is a constant it changes to semi-coke through the completion of the to cure or mature the semia t a n y fixed temperature, carbonizing reactions requiring time, is relatively coke into coke. through a formula of the type impervious to heat penetration either by radiation or This early stage of the carM = KR2 conduction. Restriction of the plastic layer formation bonizing of coal, concerned to one performance will speed up carbonization. with the change of the coal There are certain fundamento semi-coke. has come to be tal objections to a formula generally known as low-temperature carbonization. Low- of this type when applied to the carbonization of coal, and temperature carbonization may be considered as a method one of these objections is that as R is made infinitely small of producing a smokeless solid fuel together with gaseous zero carbonization time is approached and as the carbonizaand tarry by-products, or as a preliminary stage on the ther- tion of coal involves chemical reactions possessing a finite mal process involved in the changing of coal into metallurgical time of reaction zero carbonization time is not to be expected, coke or into gas. Lowtemperature carbonization, as such even with a coal layer of very small thickness. This formula a preliminary stage, is already more or less differentiated does become applicable, however, when a small constant in gas manufacture; but it is customarily submerged or is introduced as disregarded entirely, as a separate stage, in the manufacture M = K ( R + C)z of metallurgical coke. The appearance of coal is deceptive even to those long This formula was developed to fit the experimental results accustomed to its complexity, for close inspection of the seemingly uniform, black material reveals a mixture highly obtained in a series of nine tests where Pittsburgh seam complex both chemically and physically. This complexity coal was carbonized in vertical, cylindrical metal retorts renders very difficult the examination of coal on a small, varying in radius from 1.03 to 6.28 inches and a t retort refined scale. Larger scale apparatus, using sufficient temperatures varying from 590" C. (llOOo F.) to 780" C. coal to overcome the characteristic heterogeneity of the coal, (1440' F.). Low-temperature carbonization was judged butdnot so large as to prevent easy manipulation, is indicated. complete when the center of the middle of the charge attained a temperature of 390" C. (735" F.), for this is the only Development of Formula definite "break-point" on the time-temperature curve above the 100" C. (212' F.) water-evaporation flat section Thermodynamics offers an almost ideal method of attacking reactions which proceed reversibly and under equilibrium and is immediately followed by increased heat penetrability conditions. The carbonization of coal, however, is not of the charge, as is shown by the increased steepness of the generally carried out under equilibrium conditions and the time-temperature curve. Kinety-five per cent of the total carbonization is also an irreversible reaction. Thermo- loss of weight, obtainable under low-temperature carbonizadynamics also gives only the tendency for a reaction to pro- tion conditions, has occurred when this 390" C. (735" F.) ceed in a given direction and has little to say about the point is reached with this particular coal. By means of this formula the carbonizing time, M , in velocity of the reaction. The problem in hand is intimately bound up with reaction velocity, and an attempt will be made minutes, can be calculated for the nine tests mentioned to attack the problem from a purely empirical viewpoint above with the following errors: and then develop a tentative theory from the empirical Average error = *4.7%
S
1
Received February 12, 1930.
Maximum error = *13.4%
INDUSTRIAL AND ENGINEERING CHEMISTRY
840 Probable error = *2/3
4"
= 3=4.7%
n-1
4".
Maximum probable error = '3
The average error is equal to the probable error, and the greatest actual error is less than the maximum probable error, which indicates the validity of the experimental procedure (24, 25). This material is assembled in Table I and representative curves are shown in Figure 1. Table I-Pittsburgh
S e a m Coal, t h r o u g h l/z-Inch. drical Retorts
hf = K ( R + 0.5212; T F. Abs. 1808 1764 1819 1762 1766 1809 1879 1557 1774
K
=
(i:3y ~
3
Vertical Cylin~
~
L
~
4
M
2
R
F. 600 None None 370 495 None None None None
Inches 6.28 5.23 5.23 5.23 5.23 3.59 2.80 1.73 1.03
M ACTUAL CALCD. DISCREPANCY Minutes Minutes Minutes Per cent 335 352 +17 5.07 406 450 +42 10.59 369 397 +28 7.54 325 333 +8 2.46 299 296 -3 1.00 239 207 -32 13.38 115 116 $1 0.87 114 111 +3 1.70 34 34 0 0
There are probably innumerable equations which fit this set of data, but they appear in general to belong to the class and not the class M = KR2 C. The of M = K ( R C)2, constant C is equal to 0.52 for the coal, Pittsburgh seam, carbonized in this series of tests.
+
22, No. 8
+
(24)
= *20.9%
n - 1
VOl.
+
ture to the coal. The empirical formula, M = K ( R C)2, was developed on the basis of cylindrical-retort carbonization experiments, and the constant A is therefore unity for cylindrical retorts. I n this case i t indicates the heat-penetration aid offered by the radial section of the charge; this is often termed the "wedge" or "piece-of-pie" effect. For flat-plate and annulus carbonization the constant A is not unity and the formula becomes M
=
AK(R
+ C)2
The values of A , experimentally determined, for several different shaped coal layers, are given in Table 11. Table 11-Value of Positional C o n s t a n t A (T = F. absolute) VERTICALCYLINDRICAL RETORTS,A Vertical cylindrical retort Vertical annulus retort HORIZONTAL FLAT-LAYER RETORTS Single side heating: No top plate:
1 1/3
7900
No charge preheat 450' F. charge preheat
7540
No charge preheat
+
+,
O
El=
+
(2)
60
H = hours required to carbonize a coal layer R inches in 1 -
thickness or K ( R 60 o.52)z layers of coal of R inches thickness can +
be carbonized in 1 hour.
(3)
A coal layer R inches in thickness is R/12 feet thick. A total thickness of R 12
60
K(R
+ 0.52)2feet
of coal in layers R inches in thickness will be carbonized in 1 hour. One cubic foot of bituminous slack weighs about 50 pounds. For each square foot of carbonizing surface R
50X-X 12
60
K(R
+ 0.52)2
=
250R
K ( R + 0.52)2Pounds
of coal will be carbonized. Pounds per square foot per hour =
250R K ( R 0.E~2)~ (4)
+
Application of F o r m u l a to Various Shaped Retorts
When flat-layer carbonization (both with and without top plate) or annulus carbonization is under consideration, it is necessary to introduce another constant which involves the positional effect. This constant, A , indicates the way the retort shape affects the application of the retort tempera-
-
3T
T
8370^- 3T 1'
500' F. charge preheat Both sides heating: No top plate:
No charge preheat
d
No charge preheat
M = K(R 0.52)2 (1) R = radius of vertical cylindrical retort, in inches 1'316 K = where T is external retort temperature ( T +00.382t)4' in F. absolute and t is preheat temperature in ' F. where T is external retort temperature (2". 0.365t)4' in C. absolute and t is preheat temperature in C. M = minutes required for the middle of charge center to attain 390" C. (735" F.) M K(R 0.52)2 =
3T
T
Top plate:
Top plate:
Discussion of F o r m u l a
r
8840*- 3 T
4690
450" F. charge preheat
-
2T
T
1
No charge preheat, top plate preheated to furnace temperature 2/4970
Significance of F o r m u l a M = K ( R
-
r
21'
+ 0.52)2
K may be termed the "heat-transfer coefficient." K represents the resultant of two effects; it comprises the heatdriving or impelling potential of the thermal gradient existing between the retort wall and the charge of coal or coke and the resistance offered to the transfer of heat from a solid through gas to solid. The fact that the heat-driving part of K is best expressed as the fourth power of the absolute temperature of the retort wall indicates that the heat enters the charge from the inside wall of the retort largely by radiaton (16, 42). This point is also emphasized by the consideration that the heat conductivity of coal is too low to permit the passage of sufficient heat to carbonize the coal in the observed time of carbonization. The conductivity of coal, as given a t the 1928 International Conference on Bituminous Coal, is as follows (41) : C = heat conducotivityin grain calories per square centimeter per centimeter per C. Coking coal, giving a coke of high density: C = 0.003 0.0016 X 10-8t 0.0016 X 10-59 Gas coals, giving a coke of average density: C = 0.003 0.0013 X 10-at 0.0015 X 10-6tZ Gas coals, giving a spongy coke : C = 0.0003 0.0013 X 10-8t 0.001 X 10-6t2
+
+
+
+ +
+
+
The expression ( R 0.52)2is a measure of the quantity of heat that enters the coal. R may be described as that portion of the heat used up in saturating the heat capacity of the coal and raising its temperature to a point where chemical reactions and physical changes of state are initiated. It is therefore based largely upon the specific heat of the charge and the ease with which heat can enter the charge. The temperature of the charge will rise as rapidly as the entering heat can satisfy the heat capacity of the charge.
7,VDUSTRIAL A N D ENGINEERING CHEMISTRY
August, 1930
R is therefore proportional to the distance the heat must travel, and this means the thickness of the coal layer undergoing carbonization. The constant 0.52, however, represents a portion of the heat entering the charge which is utilized in a very different way. It comprises the heat used up in the chemical reactions taking place and the physical changes of state occurring. All such changes of state and all chemical reactions require a definite time to progress to conclusion. The portion of the heat entering the charge expressed by the constant 0.52 is then proportional to the time and more or less independent of the distance the heat must travel. As soon as the R portion of the heat has raised the charge to the reaction temperature and continues to arrive in sufficient quantity to supply the necessary reaction heat, the coal will start using the 0.52 portion of the incoming heat to initiate and conduct chemical reactions and physical changes of state which will in time be completed.
841
stirring or agitation. I n the extreme form of this principle a very thin layer of coal is exposed to a very hot (hightemperature heat is expensive heat) surface for a very short time and then removed to a less active zone to permit the “time-element” factor to be satisfied through the “stewing period.” Fresh coal, of course, is applied to the very hot surface immediately upon the removal of the coal which has been exposed to the heat. A definite carbonization capacity effect can be assigned to each, and all these expedients and devices and these capacities are additive. It is thereby possible to anticipate the carbonizing capacity of a low-temperature carbonization process by a consideration of the sum total of the carbonizing expedients or aids employed. Some of these carbonizing expedients or aids cost more in power and maintenance than their added carbonizing capacity justifies. However, it should be realized that apparently the ultimate limit a p proached by these low-temperature carbonization processes, except for those utilizing exothermicity, seems to be a throughput capacity of about 14 to 17 pounds per square foot of heated surface per hour. The commonly accepted upper temperature limit for lowtemperature carbonization is 600’ C. (1112’ F.). Preheating, except in exothermic carbonization, is merely a method of utilizing low-temperature heat, and there is nothing to be gained in preheating above the temperature a t which the coal changes over into semi-coke. This is 390’ C. (735” F.) for the Pittsburgh seam coal. The optimum thermal conditions for ordinary low-temperature carbonization are, then, a retort temperature of 1112’ F. (1572’ F. absolute) and a preheat temperature of 735” F. From the relation
K = 1.316 X 1014 (T
Corbon/zirq Tlme in Minutes Figure 1
This “time-element” effect is the origin of the “stewing period” which automatically imposes an upper capacity limit in “thin layer” carbonization. When the thickness of the layer of carbonizing coal is less than 0.52 inch, in straight low-temperature carbonization the time required to distil the coal becomes greater than the time required for the heat t o penetrate the charge. The “time-element” factor, 0.52, is more or less independent of the distance the heat must travel, while the heat-penetration factor, R, is dependent upon the distance the heat must travel. Therefore, the carbonizing capacity of a stationary or non-stirring retort, in pounds per square foot of heated surface per hour, decreases when the coal-layer thickness becomes less than 0.52 inch. Heat penetrates coal layers of .less than 0.52 inch thickness more rapidly than the chemical and physical reactions involved in coal carbonization can take place. (Figure 2 ) Limit of Carbonizing Capacity
+ 0.382t)4
we get K = =
1.316 X 1014 (1572 0.382 X 73Z1)~ 11.14
+
Verti’col Cylindrical Re f o r f s
Y
Throuqhpuf
250R K [ R t. .52jZ
li 10
+I
= Retort fadusin
9
a 7 6 5 d
3 2
The majority of externally heated low-temperature carbonization methods being developed today depend upon various expedients and devices designed to reduce the distance the heat must travel in penetrating the coal. Therefore almost all these cases approach, more or less closely, “thin-layer” carbonization as a goal. These expedients are generally of the nature of mechanical stirring, auxiliary heating elements or fins, hot gases through the charge, heat storage, relatively thin coal layers, etc. The highest externally heated throughput capacity is probably obtained in those processes including some form of
I
L a y e r Thickness R i n Inches
Figure 2
The minimum K value (maximum carbonizing rate) under ordinary low-temperature carbonization conditions is then 11.14. Now if R is reduced to a value equal to the radius of a coal-dust particle in a coal-dust cloud, the R term can be neglected in comparison with 0.52 and the formula may be written M
=
K(0.52)2
INDUSTRIAL AND ENGINEERING CHEMISTRY
8-12
and with K
=
11.14,
M
= = =
11.14 X 0.522 11.14 X 0.27 3.02 minutes
No matter, then, how many or what kind of auxiliary carbonizing aids are employed, the minimum carbonizing time for coal, under the maximum temperature conditions prevailing in ordinary low-temperature carbonization, is 3 minutes. This result can be checked. One of the near-successful commercial processes, where coal is carbonized by the passage of hot inert gases through a coal-dust cloud, operates according to the following procedure: Hot inert gas enters the bottom of the vertical shaft carbonizer a t 1500" F. and passes upward through a down-falling coal-dust cloud. The gases leave the top of the carbonizer a t 600" F. The average carbonizing temperature is then 1050" F., or 1510"F. absolute. A drier is superimposed upon the carbonizer and the hot gases enter the bottom of the drier a t 600' F. and leave the top a t 500" F. The rate of hot-gas passage is probably adjusted to secure complete drying, so the assumption may be made that coal dust leaving the drier and entering the carbonizer is preheated to 212' F. From the relation K =
1.316 X l O I 4 (2" 0.382t)4
+
=
1.316 X 1014 (1510 0.382 X 212)4
+
20.4
R can again be neglected and M = 20.4 X 0.522 = 20.4 X 0.27 = 5.51 minutes
The actual time employed is 35 seconds for passage through the drier, 35 seconds for the down fall through the carbonizer, and the dust particle is then left for an additional 290 seconds, the "stewing period," a t the bottom of the carbonizer. The coal-dust particle is thus in the carbonizer for 325 seconds, or 5.42 minutes. The calculated time of 5.51 minutes is seen t o differ from the actual time by only 1.66 per cent. This agreement enhances the validity of the calculation of the minimum coal-carbonizing time, under ordinary low-temperature carbonization conditions, as 3.02 minutes. This 3.02 minutes is represented by that portion of the utilized heat designated as 0.52 in the formula M
=
K(R
+ C)z
where C = 0.52 for Pittsburgh seam coal. This explains why extremely high throughput capacity has not yet been obtained through the use of extremely thin layers of coal. It is the thing which automatically limits the throughput capacity of a low-temperature carbonization process. Mechanism of Heat Penetration of a Charge of Coal
The thermal gradient between the retort wall and the charge of coal appears in the heat-transfer coefficient as the fourth power of the absolute temperature of the retort wall as, O
c.
F.
1.316 X 1014 K = (T+0.382t)4
1.24 x 1013 = (T 0.365Q4
+
This fact, together with the known low heat conductivity of coal, indicates that the major part of the heat utilized in
Vol. 22, No. 8
carbonization penetrates the charge by radiation. But the slowness of the heat penetration suggests that this penetration by radiation is not continuous. When the coal is first placed in the retort, the radiant heat from the retort walls penetrates the charge for a short distance. The coal substance itself is not very penetrable by radiant heat, but the charge voids are easily penetrated by the radiant heat. The coal particles in a certain zone are thus surrounded by and bathed in radiant heat and after a certain time those in this particular heated zone coalesce or melt and form a gas-tight molten coal layer (36, 3 7 ) . This molten layer of coal is even less easily penetrated by radiant heat than the original coal substance and the penetration of radiant heat beyond this plastic zone practically ceases. The only heat then penetrating the plastic zone does so by conduction and the heat conductivity of coal is very low. After due course of time the "time element" or "stewing period" effects the change of the molten coal layer into semi-coke. This semi-coke is honeycombed with gas-filled pores and voids and thus about five times as penetrable by radiant heat as the coal substance. The radiant heat therefore penetrates this new thin layer of semicoke and then penetrates a short distance into the coal charge again. After a brief time this new heated coal zone melts and again restricts the heat passage into the charge to the very slow heat conduction through the plastic layer. Again the molten layer in time changes into semi-coke, easily penetrable by radiant heat, and another ingress of heat occurs. The heat therefore seems to penetrate the charge by the periodic advance of a series of plastic layers. The picture of heat penetration here exhibited is seen to be an alternation of periods of fairly rapid radiant heat penetration of the charge voids or the semi-coke pores, interspersed with periods of slow heat conduction through the gas-tight molten coal layer. It is a rhythmic process. Even in the case of non-coking coals the same general mechanism is in operation, except that the zone or layer of coal undergoing transformation from coal substance (not easily penetrated by radiant heat) to semi-coke (more easily penetrated by radiant heat) is discontinuous and not molten. Exothermic Coal Carbonization
Apparently it is an accepted view that the carbonization of coal is endothermic. This view originates with investigators who have conducted experiments over the complete carbonization range. It is based upon the fact that the sum of the heats of formation of the carbonization products is greater than the heat of combustion of the coal which is carbonized (40). The carbonization products have therefore abstracted heat from the process a t some time during the progress of the carbonization. The reaction heat is therefore endothermic when considered over the complete carbonization range. A careful survey of the literature, however, reveals the fact that the decomposition of most coals is alternately endothermic and exothermic over the temperature range utilized in low-temperature carbonization (up to 600" C. or 1112"F.). This is true, not only of most coals, but even in greater degree of their generic progenitors such as peat and wood. Whether the decomposition of coal is endothermic or exothermic a t a given temperature depends upon whether the algebraic sum of the preceding heats of reaction is positive or negative a t that particular point. A temperature can be so chosen as to yield either an endothermic or exothermic resultant heat of decomposition (23, 13, 4,21, 17, 18, 19, 30, 39, 26, 6,10, 11, 7 , 5',,12,~.8,~Q, 2, 28, 29, 14, 31, 32, 22, 20, 33,3, 15, 27, 2 ) .
INDUSTRIAL AND ENGINEERING CHEMISTRY
August, 1930
Applied Exothermic Carbonization (31, 38, 33, 35, 88, 29, 21)
The extremely slow heat penetration of the coal charge is due to the low radiant-heat penetrability of the series of gastight molten coal layers repeatedly established and represented by the advancing plastic zone. The plastic zone is thus the handicap which retards coal carbonization. If the plastic zone can be restricted to just one performance or occurrence, then this one “stewing period” will suffice for the carbonization of the charge of coal. This restriction of the plastic-zone dominance has been successfully carried out in a t least one low-temperature coal-carbonization process. A charge of coal is preheated (the rate of preheating is important) to 35-50 degrees below the point where a strongly exothermic reaction occurs; this exothermic reaction point is generally the point where the coal becomes plastic. The preheated coal is then suddenly transferred to a much hotter retort. Owing to the large temperature gradient thus established, the radiant heat from these new retort walls quickly penetrates the entire charge of warm, dry coal in sufficient quantity to heat the charge up the few remaining degrees necessary to “trip-off” the desired exothermic reaction. This new hot retort can be maintained a t a temperature from 930” to 1110’ F. (500’ to 600” C.) or, if the new retort is initially heated, to 1380’ F. (750” C.) before the preheated coal is added, the exothermic reactions supply the remaining quantity required to carbonize the charge. When the exothermic reaction takes place, it releases some 65 B. t. u. per pound (36 calories per gram) which raises the temperature of the entire charge about 70” C. (126’ F.); this is for Illinois coal. The entire charge then melts and changes into semi-coke in one “stewing period.” It has been reported that the rate of heat penetration so obtained is 7 inches in 10 minutesthat is to say, the coal in the center of the 7-inch radius retort reaches a certain predetermined temperature, above the transformation point, a t the end of 10 minutes; this transformation temperature is 735” F. (390’ C.) for Pittsburgh seam coal. This is a t the rate of 42 inches per hour, which is about eighty times that obtained by the usual non-stirring methods. Such rapid heat penetration can only be secured in semi-coke, so the rate of low-temperature carbonization must be about the same as the rate of heat penetration, or 42 inches per hour. The dimensions of the retort are given and these indicate a carbonizing capacity of 87 pounds per square foot of heated surface per hour. This is ten times that of most of the other low-temperature carbonization processes. This carbonizing capacity refers, however, only to the formation of semi-coke. The retort is subjected to additional heat for a supplementary period of about 4 hours. During this time the radiant heat from the retort walls is penetrating the easily penetrable semi-coke, and thus “curing” or “sweating” or “maturing” the semi-coke into coke. The metallurgical coke so obtained is stated to be satisfactory.
Substituting these values in the carbonization equation M = K ( R 0.52)2and again neglecting R because the radius of a coal-dust particle is negligible in comparison with 0.52, the following minimum time required for carbonization is obtained.
+
At 1800” F., M = 5 (0.52)2 = 1.35 minutes At 1000” F., M = 29.1 (0.52)2 = 7.87 minutes
The coal can carbonize 7.87/1.35 or 5.8 times as rapidly a t 1800’ F. as a t 1000’ F. This suggests the advisability of so designing a low-temperature carbonization process that the coal is exposed to a very hot surface for a short time and then removed to a somewhat cooler and less expensive zone and there permitted to transform to semicoke during its “stewing period.” If cheap low-temperature heat is available it is advisable to preheat. This means resistant high-temperature alloys capable of withstanding mechanical strain a t high temperatures in the case of the agitator or stirrer type processes. This same consideration applies to the exothermic schemes where continuously intermittent charging and discharging expose the hot retort walls to frequent action of air and gases. Research along the line of high-temperature resistant alloys as a part of the low-temperature carbonization program should not be neglected. A preliminary very brief exposure of the coal to a very high temperature followed by a “stewing period” in less highly heated, and therefore less expensive, sections, applied to low-temperature carbonization in general will mean large carbonization capacity for a small unit, and this indicates low first cost and low operating charges. Literature Cited
Application of High Temperature to Low-Temperature Carbonization
The effect of the retort-wall temperature in driving heat into the coal charge is shown to be the function of the fourth power of this temperature in degrees absolute, as K =
At 1800° F. (980’
1.316 X 1014 ( T 0.382t)4
+
e.),K
1316
(1800
x 1014 +
460)i = 5
x 1014 = 29.1 At 1000° F. (540” e.),K = (;Ooo 460)i 1316
843
(38) (39) (40) (41)
+
(42)
Audibert, Fuel Science Pracfice, 7, 226 (1928). Chapman, I b i d . , 5, 355 (1926). Coffman and Layng, IND. ENG.CHEM.,20,165 (1928). Constam and Cobb, J. Gas Lighting, 107, 697 (1909). Davis, Fuel Science Practice, 4, 927 (1925). Davis and Byme, Carnegie Inst. Tech., Cooperative Mining, Bull. 8 (1922). Davis and Byrne, J . Am. Ceram. Soc., 7 , 809 (1924). Davis and Byme, IND. END.CHEX.,17, 125 (1925). Davis and Byme, Ibid., 16, 233 (1926). Davis and Place, Zbid., 16,589 (1924). Davis and Place, Fuel Science Practice, 3,434 (1924). Davis, Place, and Edeburn, Ibid., 4, 286 (1925). Euchene, J . Gas Lighting, 76, 1080 (1900). Gentry, Proc. Intern. Conference Bifuminous Coal, 1926,436. Gentry, “Technology of Low Temperrture Carbonization,’’ (1928). Haslam and Russell, ”Fuels and Their Combustion” (1926). Hollings and Cobb, Gas J . , 126, 917 (1914). Hollings and Cobb, J . Chem. SOL.,107T, 1106 (1915). Hulett and Capps, J. IND. END.CHBM.,9, 927 (1917). Keppeler, Slahl Eisen, 46, 631 (1927). Klason, Z.ongew. Chem., 22, 1025 (1909). Layng and Coffman, IND. ENG.CHEM.,19, 924 (1927). Mahler, Compt. rend., 113, 862 (1891). Mellor, “Higher Mathematics for Students of Chemistry and Physics,” p. 522 (1922). Palmer, “Theory of Measurements,” p. 55 (1912). Parr, Gas Age-Record, 50,531 (1922). Parr, Proc. Intern. Conference Bituminous Coal, 1926, Vol I, 56. Parr, Ibid., 1926,646. Parr, Ibid., 1926,635. Parr and Layng, J. IND. ENG.CHEM.,13, 14 (1921). Parr and Layng, British Patent 249,886 (March 27, 1925). Parr and Layng, British Patent 256,192 (July 30, 1925). Parr and Layng, BritishPatent 261,799 (July 30, 1925). Parr and Layng, British Patent 277,955 (September 1, 1926). Parr and Layng, British Patent 263,783 (January 2 , 1926). Rose, Proc. Am. Gas Assocn., Chapt. 111, p. 26 (1926). 1577F (Tulv. Rose. Am. Inst. Mining Met. Ene.. - Paaer . - _ .1926). Strache and Fromm, Brennstofchem., 3, 340 (1922). Strache and Grau, Zbid., 2, 97 (1921). Taylor, “Treatise on Physical Chemistry,” p. 208 (1925). Terres, Proc. Intern. Conference Bituminous Coal, 1928, Vol. 11, 666, 679. Thwaites, Gas J., 173, 851 (1924).
