STRUCTURE in COIC
A series of thin sectionb of coke are studied, with and without the admixture of breeze, both before and after burning. It is found that the distribution and structure of ash in coke in relation to the Combustible is dependent on the structure of the original coke. It is indicaled that the combustible material surrounds and envelops the ash residue. The addition of 5 per cent breeze to the charged coal has little effect upon the structure of the resulting coke or itsashresidue. The direction i n which a coking coal expands i n respect to its banded slructure is indicated in a preliminary manner.
LOU1 S SHNIDMAN Rochester Gas and Electric Corporation, Rochester, N. Y.
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OKE ( I ) has been defined as “the solid, cellular, coherent residue from destructive distillation of bituminous coal. said residue having such physical structure as results from the hardening of a fused or semiliquid mass”. Pflulce (4, 6) reported that coke breeze could be added to 100 per cent high-volatile coal prior to carbonization without detrimcntal effects on coke strength, provided the following limits were 01,served : 1. Up to 5 per cent of breeze, no larger than minus 1/32-inrli ~
square-mesh. 2. Up to 3 per cent of breeze no larger than minus 1/16-inrh square mesh. 3. 1 per cent of any other size up to minus l/a-inch squarc mcdi. ~
Pfluke further showed that, provided these limits are observed both with 100 per cent high-volatile coal and blended coal, thc percentage of usable coke from the initial charge remains almost unchanged when breeze is added compared to the percentage of usable coke from the initial charge with no breeze added. The term “ash structure in cokc” is used here to indicate the probable distribution of the ash or mineral matter in relation to the combustible material-that is, the position of each particle of mineral mat>terwith respect to the combustible. A survey of the literature indicated that little information was available relating to ash structure in coke. A number of investigations h a w been made on the distribution of ash in solid fuels (2, 3 ) , but no study has been directed toward the manner in which the ash structure is built or distributed in the solid fuel sample in relation to the combustible. As a result an attempt was made to study this subject. STRUCTURE A N D DISTRIBUTION O F ASH
Figure 1.
Photographs of Original Cokes and Residues
after Combustion
Since coke is an abrasive material, a Carborundum wheel was used to secure thin slices. Sections a / l ~inch thick \yere cut from the lumps of coke in different directions. They were carefully placed on a lime asbestos board, phot’ographed, inserted in a cold electric muffle furnace, and slo~vlyraised to 1300’ F. After remaining in the furnace for 16 hours, the lime asbestos board containing the ash from the coke was carefully removed, allowed to cool, and rephotographed in the same position as before burning. The coke samples were comp1etel.v burned during this treatment, and only the ash or noncombustible matter remained. The coke was made from Pittsburgh high-volatile gas coal crushed to -11/2 inch size, charged into by-product coke ovens 1262
INDUSTRIAL A N D ENGINEERING CHEMISTRY
December, 1943
1263
(Koppers-Becker type), and carbonized at 2450' F. for 12 hours net, The approximate analysis of the coke follows: Volatile matter. yo Ash % Fixdd carbon, % Sulfur % ' Fusio; point O F . Heating value, B. t. u./lb.
0 7
9.4 89.9
0.85
2600 A
12,810
PHOTOGRAPHIC RECORD
COKEASH. Figure 1 shows how the coke behaved on burning and the structure and distribution of its ash. The photographs are typical of the results obtained from a large number and variety of samples examined in the course of this study. The samples of coke and ash shown are pieces of approximately half-oven width; when the sections are described as longitudinal, it means parallel to a line across the oven perpendicular to the walls; the opposite is meant for the cross section. Figure 1A shows two samples of coke with the addition of 5 per cent breeze (fine coke) in the charged coal cut longitudinally from the coke sample. The characteristic fissures and porous structure of the coke is clearly seen. B presents the same samples after the combustible material has been burned away and only the ash remains; the shape and height of the ash samples are very similar to the unburned coke. Furthermore, the major fissures and minor cracks in the coke sample are clearly visible ahd maintained in the residue. The porous nature of the ash residue is also revealed. The light and dark areas in the residue give further indication as to how the component parts of the ash are distributed. Near the lower right-hand corner of the specimen a t the left of Figure 1A is a piece of foreign matter (slate) which is somewhat darker than the remaining sample. The left-hand specimen in B indicates clearly the presence of this impurity in the ash in the same location as before burning. Figure 1C shows two samples of coke with the addition of 5 per cent breeze in the charged coal, cut in cross section from the coke sample. The characteristic porous structure, major and minor fissures, as well as impurities are apparent. D represents the same samples after ignition. The ash structure is very similar t o the coke structure. The photographs show that the distribution of ash within & sample of coke is dependent upon the presence of foreign matter and impurities as well as upon the coke structure. Further, the distribution of the ash can be ascertained rather closely from a knowledge of the coke structure. It further indicates that the combustible matter surrounds and envelops the ash residue. The volume of the ash residue was almost the same as the original coke sample. Figure 1E presents cross and longitudinal sections of the same sample of coke t o which no breeze had been added in the coal charge. F shows the same samples after burning away the combustible material. As indicated previously, the ash structure follows closely the structure of the coke. The color, porosity, ancl general appearance of the ash structure of the cokes with and without breeze was the same. This study or ash structure corroborates the findings of Pfluke (5) who showed that the shatter, tumbler, and screen tests of coke, with and without the addition of breeze were the same. In each case the photographs show that the combustible matter is built around the ash structure. The light and dark areas represent the distribution of the component parts of the ash. COAL ASH. Figure 2A presents sections of bituminous gas coal from the Pittsburgh field. At the left is a section cut perpendicular to the bond structure; at the right, a section cut parallel to the bonds and comprising several bonds. The approximate analysis of the coal was: Volatile matter, yo
Ash %
Fixe'd carbon, % Sulfur % Fusiod point, O F.
34.3 6.8 59.4
1.0
2600 -!-
Figure 2.
Photographs of Original Coal and Residues after Combustion
These samples were somewhat difficult to prepare, since the coal was soft and oily. The left-hand section shows the characteristic band structure of the coal. Figure 2B represents the same samples after slow burning by the method indicated for coke. Combustion of this sample was difficult because it was a coking coal and passed through a plastic stage before coking and final burning. This behavior obscured the true picture as to how ash is present in coal. However, B gives a preliminary indication as to the direction of expansion on heating. The lefthand section expanded considerably, but principally in the direction perpendicular to the plane of the bands in the unburned coal. This was further evidenced by the nature of the ash structure of the sample on the right. This ash residue was approximately the same size as the unburned coal b u t was considerably higher, an indication that expansion had occurred in the direction perpendicular to the plane of the bands. During softening and swelling of the coal there was no exuding of an ash-free pitch or tar, but an expansion of the coal as a whole. Figure 2A and B show individual lumps which do not correspond t o the coal charge in a by-product oven. From this preliminary study it appears that this type of bituminous coal expands in a direction perpendicular t o the plane of its banded structure. It appears, then, that as the coal passes through the plastic state during the coking process, there is no segregation of ash; that is, the mineral matter does not separate from the combustible but remains thoroughly distributed. The method employed may be applied to record the visible changes that take place in the ash and fuel a t different periods during combustion. The behavior of the Pittsburgh gas coal indicates the difficulty of predicting how and where such coal ash would be distributed during the coking process or during combustion. ACKNOWLEDGMENT
The author wishes to acknowledge the cooperation of Ralph M. Bishop of the laboratory staff in taking the photographs. LITERATURE CITED
(1) Am. SOC.for Testing Materials, Standards, Part 111, D-121-26T. p. 584 (1939). (2) Gauger, A. W., Penna. State Coll., Tech. Paper 28 (1936). ( 3 ) Marsden, Arthur, Gas J.,202, 54-8 (April 5, 1933). (4) Pfluke, F. J., Am. Gas Assoc. PTOC., 1936, 771-92. ( 5 ) Ibzd., 1937, 619-48.
PRESENTED before the Division of Gas and Fuel Chemistrv a t the 105th Meeting of t h e AMERICAN CHEMICAL SOCIETY, Detroit, Mich.
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