EFFECT OF SECONDARY HEATING TEMPERATURE ON THE ELECTRICAL RESISTANCE OF COKE

A. V. Vikharev, N. M. Smolyaninova

(Presented by the scientific-methodological seminar of the Faculty of Chemical Technology)

The importance of the problem of studying the effect of secondary heating on the behavior of coke has already been discussed [1]. This problem can be successfully solved only through comprehensive consideration. In this respect it is important to study not only changes in the properties of coke, but also—and to an even greater extent—the structural transformations that ultimately determine all the properties of coke.

One of the widespread, quite reliable, and sensitive methods for studying the structure of coke is the method of measuring electrical resistance, which was used in the present work.

The investigation was carried out on a laboratory apparatus designed by us for measuring electrical resistance over a wide temperature range (Fig. 1).

The apparatus consists of two parts: a measuring part and a heating part. The measuring part includes platinum electrodes 10 with platinum lead-in conductors 5 and a UMB-type measuring bridge.

Heating is effected by the passage of current through a layer of crushed graphite 14 placed in the space between two porcelain tubes 8, 9. The temperature was measured with a platinum—platinum-rhodium thermocouple 13.

For supplying electric current, large-cross-section copper wires 16 and two graphite electrodes 7 were used. Power supply to the heater and regulation of the heating regime were carried out through an RNO-type autotransformer.

The coke powders under investigation were poured into the inner porcelain tube 8, and by means of a lever, through a porcelain punch 6, a pressure of 10 atmospheres was created. The powders had a grinding size of 0—0.25 mm. A single charge amounted to 1.5 g.

The heating rate was maintained within the range of 4—5 deg/min.

Coke samples taken from different places along the length of the piece—“cabbage,” “seam,” “center” [1]—were subjected to investigation.

The results obtained are presented in Fig. 2 in the form of the dependence of the relative change in the electrical resistance of coke on the temperature of secondary heating. Numerical values of the electrical resistance were recorded every 25°C.

As can be seen from the graphs, all samples exhibit a general tendency toward a decrease in electrical resistance (dotted line).

However, in certain temperature regions significant deviations from this basic dependence are observed.

The sharpest fluctuations in electrical resistance are observed in the temperature intervals 350–450°C, 750–850°C, and at 1000–1100°C (depending on the sample).

The greatest decrease in electrical resistance, in comparison with that at room temperature, was found for the “seam,” the smallest for the “cabbage,” while coke samples taken from the center of the lump occupy an intermediate position.

Fig. 2. Relative change in the electrical resistance of coke as a function of temperature.
1—“seam,” 2—“center,” 3—“cabbage”; \(R_t\)—electrical resistance at the given temperature; \(R_0\)—electrical resistance at 0°C.

It can be asserted with a high degree of reliability that the indicated difference in the behavior of the coke samples studied was a consequence of the temperature conditions under which the samples were located in the chamber of the coke oven. Thus, the “cabbage,” which showed the smallest drop in resistance, was exposed for a longer time to higher temperatures than the “center” and “seam” samples. The same is true for the “center” in comparison with the “seam.”

The sharp drop in electrical resistance toward lower values for the “seam” at 1000°C, for the “center” at 1050°C, and for the “cabbage” at 1100°C is also due to the difference in the temperature conditions under which these samples were obtained. In all probability, these drops are observed at temperatures that were, respectively, the final coking temperatures. Heating above this temperature leads to a further increase in electrical conductivity as a result of polycondensation processes.

Of greatest interest are the sharp changes in electrical resistance in the temperature range 350–500°C. It was precisely in this region that an increase in the structural strength of coke had previously been observed [1]. We associate this phenomenon with the relief of internal stresses. The basis for such a

Fig. 1. Schematic of the apparatus for determining the electrical resistance of coke in the hot state:
1—corundum base; 2—thermal insulation; 3—casing; 4—weight, producing pressure on the specimen; 5—platinum conductors; 6—porcelain punch; 7—graphite electrodes; 8—inner porcelain tube; 9—outer porcelain tube; 10—platinum electrodes; 11—corundum gasket; 12—supporting porcelain tube; 13—platinum–platinum-rhodium thermocouple; 14—heater made of crushed graphite; 15—test specimen; 16—supply conductors.

Confirmation of this assertion may be found in the works of Academician V. D. Kuznetsov [2], who indicated that, when internal stresses in solids are removed or arise, a change in the intensity of the lines is observed on X-ray diffraction patterns. Consequently, this process is associated with certain internal molecular rearrangements of the substance, which also affected the electrical conductivity.

Taking into account the commonality of the structure of coke with the structure of graphite, one may suppose that, in this temperature region, a phenomenon is observed analogous to that discovered by V. N. Krylov in work with graphite [3]. He noted that “with increasing temperature the parameter \(c\) of the graphite crystal increases noticeably, reaching 6.91 Å at \(700^\circ\mathrm{C}\), with a noticeable inflection in the curve of the growth of the parameter \(c\) with temperature at \(500^\circ\mathrm{C}\), where its decrease to 6.86 Å is observed. This causes a densification of the graphite crystal and, possibly, explains the fact that mechanical strength increases with increasing temperature.”

The change in electrical resistance at a temperature of \(725^\circ\mathrm{C}\) in the “cap,” at \(750^\circ\mathrm{C}\) in the “center,” and at \(850^\circ\mathrm{C}\) in the “seam” is difficult to explain. Most probably it is connected with the removal of residual volatile substances, the nature of which has not yet been definitively established. In this connection, determination of their exact elemental composition is of interest.

Conclusions

1. The effect of secondary heating to a temperature of \(1400^\circ\mathrm{C}\) on the electrical resistance of coke has been studied.

2. All coke samples showed a tendency toward a decrease in electrical resistance with increasing temperature.

3. Temperature regions have been identified in which the electrical resistance changes sharply, and an attempt has been made to explain the causes of these fluctuations.

References

1. A. V. Vikharev, N. M. Smolyaninova, V. D. Chetvergov. On the question of the effect of secondary heating on the strength of coke matter. Izvestiya TPI, vol. 175, preceding article.

2. V. D. Kuznetsov. Physics of the Solid State, Tomsk, “Krasnoe znamya,” p. 592, 1941.

3. V. N. Krylov, N. N. Vil’k. Carbon-Graphite Materials and Their Use in the Chemical Industry. Leningrad, Khimiya, p. 7, 1965.