Softening of Coal by Heat Is It a Distinctive and Measurable Characteristic? HORACE C. PORTER 1833 Chestnut Street, Philadelphia, Pa.
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This paper discusses the probable nature and mechanism of the softening and cokeforming process in coal; the difficulties inherent in such a heterogeneous and changing material, in making measurements of softening characteristics; some results of efforts to adapt an extrusion method to such measurements, with only partial success; and the promising features of certain other proposed methods which allow separate measurement of a number of contributing factors in the softening phenomenon.
RACTICALLY all bituminous coals, except those of low rank bordering upon the lignites, become soft in s o m e m e a s u r e when heated. The better coking coals, in general, become the softer under equal conditions. The problem, therefore, is whether this property can be measured satisfactorily and used to characterize and classify different coals in and bordering upon the bituminous coking group. It has much to do with the old and mooted question of what constitutes the coking property of coal and with the mechanism of coke formation. Difficulties attend the study of the phenomenon and measurement of the property, largely because of the fact that it is a transient condition, the viscosity or fluidity changing continuously as a result of progressive decomposition that starts before softening has begun; this decomposition affects the course of the softening greatly, even acting, perhaps, as its primary cause. Unlike lubricating oils and greases, and most of the solid or semi-liquid materials (pitches, waxes, etc.) whose viscosity is measured a t elevated temperatures, softened coal is not constant in composition or physical character as its temperature is raised, or in fact while it is maintained a t one temperature. It is, chemically as well as physically, in a state of flux. The low thermal conductivity of coal introduces a further difficulty-namely, that of securing a uniform temperature and consequently a uniform condition throughout the test portion. The rate of heating, differing in portions close to or remote from the heat source, has a great effect on the degree of softening. Different zones also are affected unequally in physical character by the travel, from one t o the other, of the products of decomposition.
Significant Elements in the Softening Property In view of these difficulties, from lack of constancy or uniformity, it may well be questioned whether such a material admits of a serviceable and practically significant test of its softening characteristics. Probably the answer lies in combining and suitably weighting a number of significant phases or elements of the softening phenomenon, and using such an aggregate as the index to the practical behavior of the coal in this respect. The following are among these significant factors:
*+. 1. Degree of maximum fluidity.
Temperature at which any d e f i n i t e degree of fluidity is reached. 3. Range of temperature over which a c e r t a i n minimum of fluidity exists (often called t h e “plastic” range). 4. R a t e s of rise a n d fall of fluidity within the plastic range, shown by p l o t t i n g temperature against flow resistance. 5. R e l a t i v e ease of decomosition, indicated by rate of gas o r m a t i o n with rise of temperature. 6. Swelling, as controlled by factor 5 and by the fluidity. 2.
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It is hardly possible, with our present knowledge, to weight these factors- and derive a formila or index for softening kehavior or “cokability,” but it is proposed that, by an extended study of the data to be obtained from such measurements on a large number of coals and by correlation with their known properties manifested in use, a proper weighting may in all probability be found. A factor not included above, since i t is hardly measurable, probably influences the softening process-namely, the miscibility or wettability of the changing solid in the liquid being formed. Although certain methods developed recently by other investigators offer better means of measurement of most of these elements in the softening behavior, some results of extrusion experiments will first be given to show, in simple fashion, the manifestation of factors 1 and 6 (i. e., fluidity and swelling) and to show how this combination varies among different varieties of coals and is related to their behavior in use. Extrusion lMethod of Test When a column of sized coal particles, packed to a certain bulk density, is confined in a vertical, cylindrical space on the top of which is a weighted piston, and the coal is heated a t a definite rate, it softens and swells, extruding a portion through any aperture in the bottom or walls of the container. The amount extruded depends on the fluidity and on the pressure applied to the piston. If the latter does not move freely or a t requisite velocity, the internal pressure in the mass, because of swelling (gas bubble expansion), may exceed the piston load and have an effect on the extrusion. In 1931 the author gave an account (9) of some experiments showing the widely varying behavior of eight coking coals, heated a t a rate of 4” C. per minute from 320” to 520” C. in a vertical, cylindrical chamber, under a weighted piston and with a bottom orifice 9 mm. in diameter t o permit ex962
AUGUST, 1935
INDUSTRIAL AND ENGINEERING CHEMISTRY
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ably heavier load for a much longer time, extrudes only 3.5 per cent. Coal 4 (Orient mine), although commercially it has made coke in ovens, exhibits no plasticity under the conditions here used. It has the highest volatile-matter content of the coals mentioned here and decomposes easily under heat, but it does not swell in this test. The extruded portion from coal 1 had 5.7 per cent ash content, as against 7.6 per cent in the original. Coal 1, here shown to be of high fluidity, has, in another test using a piston load of 8 to 10 pounds continuously applied, been made to extrude 95 t o 97 per cent, thus demonstrating its essentially complete liquidity. Coals 1 and 2, behaving so differently in this extrusion test, are both from the Pittsburgh seam in western Pennsylvania, have coking properties, and decompose easily; but coal 2 contains somewhat more oxygen and is mined to the west of the Fayette anticline, while coal 1 is to the east in a different basin. Coals similar to these in source have been found by the Bureau of Mines (6)(their coals 9 and 28) t o vary sharply in "agglutinating index" and in physical quality of the cokes made in the 90-pound laboratory coking test. -PETAIL OF P15TONHEAO-
hIodified Extrusion Test in a Closed Capsule
As a further development of this extrusion method and with the object of making the fluidity itself, as far as possible, the chief factor in the observed result, the closed capsule shown in Figure 2 was used for determining penetration of / I the softened coal into an adjoining layer of sized coke particles. No external pressure was applied, I 1 the flow of the coal being actuated only by gravity iI and by its own swelling or intumescence. Since i l the latter is due entirely, as Audibert and Delmas (2) have pointed out, to the expansion of imprisoned gas bubbles, the placing of a layer of infusible coke particles next to the soft swelling coal s h o u l d h a v e t h e effect, in some measure, of breaking these bubbles and facilitating the gas exit through the coke. This method still measures the resultant of two factors, since t,he driving force causing penetration into the coke layer is the bubble expansion or intumescence, but possibly the greater of the two in this arrangement might be expected to be the fluidity of the coal. The test method is evident, in the main, from Figure 2. The coke layer is below the coal, and both are of sized particles, 10 to 40 mesh, and EXTRUSIOX O F SOFTENED C0.4L FIGTSRE 1. APPARATUS F O R DETERMINING are Dacked to a definite bulk densitv. The heating is a t a definite rate, about 3" C. per mintrusion (Figure 1). The comparisons were not strictly quanute, between 400" and 520" C., and the capsule is suspended titative, since it was found necessary in nearly all cases to in an inert atmosphere (either nitrogen or city gas). The add to the piston load in varying amount and for different lengths of time in order to retain the charge in the heated zone. The extrusion was thus, in these cases, influenced TABLEI. RESULTS OF EXTRUSION TESTS(9) during a fraction of the plastic period by the added pressure Per Cent Vol. Plastic Exdue to swelling. M a t t e r Rangea Added Loadb truded Coal Source 70 C. L b . Yo of period plastic Nevertheless, as may be seen from Table I, the results, after due allowance for this irregularity of pressure, indicate 1- 2 1. Westmoreland Co., Pa. 32.5 62 10 65 1- 2 35.5 unquestionably a wide variation in fluidity and, roughly, its 2. Allegheny Co., Pa. 60 7 31 34.5 3. Dominion, Nova Scotia 1- 2 80 27 2.5 trend. It is quite evident that, notwithstanding the irregu35.6 4. Franklin Co.. Ill. .. ... None 0 1-10 29.0 a. Indiana Co., P a . 86 30 75 larity (in some cases) of the piston pressure applied, a wide 1- 5 25.5 114 7 65 6. Cambria Co., P a . ( D seam) divergence exists among the coals in degree of fluidity. 7. New River W. Va. 20.2 60 None 0 1-15 105 55 22.0 8. Cambria Cb. Pa. (B seam) 3.5 Coals 1 and 2, for example, under equal load conditions, ex1- 2 30 9. Huntingdon bo., Pa. 16.0 40 0 trude 65 and 31 per cent, respectively; coal 3, with the same Plastic range is taken as t h a t between t h e complete softening or ooalescence point a n d t h e setting point. load but continued somewhat longer, extrudes only 2.5 per b Load added during a part of t h e plastic period t o c o u n t e r a d swelling cent; and coal 9, also under the same load for about the same pressure in the coal. time as coal 3, extrudes nothing. Coal 8, under a consider0
depth oi pmetration iiit,o the coke lager is noted, :is well as any extrusion taking place thr
