ON THE MECHANISM OF OXIDATION OF BITUMINOUS COAL AND THE CAUSES OF ITS SPONTANEOUS COMBUSTION

Article 12.

M. Yu. Grigor’ev.

On the Influence of Carbon Tetrachloride on the Oxidizability of Coals

Impurities whose influence on the rate of oxidation of bituminous coal has been studied by various authors may be divided into three groups: 1) moisture, 2) the ash constituent of the coal itself and various mineral substances artificially mixed with it, and 3) gaseous or vaporous substances with which the coal was treated before determining its oxidizability, or which were introduced into the atmosphere at the moment of determining the oxidizability of the coal.

Moisture apparently has no appreciable chemical influence on the oxidizability of coal. There are indications (Gray) that the moisture contained in coal prevents its spontaneous combustion, since until it has all evaporated the temperature cannot rise above 100°. Erdmann²), Davis and Birns³) found that moist coals do not heat up in dry oxygen. V. M. Pal’velov⁴) notes that, when moist oxygen was passed through coal in Erdmann’s apparatus, the critical temperatures for a number of Kuzbass coals showed a noticeable increase (from 120 to 135°; from 159 to 172°; from 154 to 168°, etc.). However, the oxidation curves at a constant bath temperature (125°, 150°, 175°) gave almost complete agreement for dry oxygen and for oxygen saturated with water vapor at 50°. The author likewise explains this by the fact that water acts only mechanically, removing heat.

As regards the influence of ash, the data cited by various authors are somewhat divergent. In V. M. Pal’velov⁴), after treatment of coals with hydrochloric acid (whereby part of the mineral substances is extracted), the critical temperatures of some Prokop’evsk and Kemerovo coals decreased strongly (in some cases by 30°), but for a number of other coals remained almost unchanged. At the same time, Züstmann and Lenert⁵) found that coals de-ashed with hydrochloric acid oxidized noticeably with greater difficulty. Thus, for a number of coals they observed an increase in the critical temperature from 176° to 187°, from 185° to 206°, from 186° to 198°, and from 168.5° to 197°.

Perhaps this contradiction is explained by differences in the composition of the ash substances. Some of them may facilitate

the oxidation process, while others, on the contrary, hinder it. In addition, the possibility is not excluded of chemical interaction between hydrochloric acid and the organic mass of the coal, which also may affect oxidizability. N. F. Denisova\(^{6}\), wetting coal with solutions of \(\mathrm{Na_2CO_3}\), \(\mathrm{KMnO_4}\), \(\mathrm{Al_2(SO_4)_3}\), \((\mathrm{NH_4})_2\mathrm{SO_4}\) and, after drying in a vacuum, determining the adsorption capacity of the coal with respect to oxygen, found as a result of the treatment some decrease in absorptive capacity.

S. M. Petrov and L. G. Maidanovskaya\(^{7}\), on the contrary, found that aluminum and iron sulfates at \(50^\circ\) and \(100^\circ\) somewhat accelerate the oxidation of coal. After drying at the given temperatures of coals wetted with the indicated sulfates, the subsequent absorption of oxygen by the coal proceeded more slowly.

Kreilen\(^{8}\) determined the initial temperature of coals to which stearic and oleic acids had been added. Both of them, especially the latter, accelerated oxidation. Preliminary chlorination, bromination, and iodination of coal acted in the same direction. Partial hydrogenation of coal increased its resistance to oxygen. Kreilen also studied the influence of gases and vapors \(\mathrm{CO_2}\), \(\mathrm{CO}\), \(\mathrm{H_2S}\), \(\mathrm{SO_2}\), \(\mathrm{NO_2}\), \(\mathrm{CCl_4}\).

It turned out that hydrogen sulfide and carbon tetrachloride somewhat lower the initial temperature.

We were interested in carbon tetrachloride as a substance already used in extinguishing fires. It was of interest to determine whether \(\mathrm{CCl_4}\) is an anticatalyst that retards oxidation, or whether it acts simply mechanically, as a volatile liquid entirely incapable of combustion, the vapors of which cut off access of air to the burning body.

The series of experiments set up for this purpose consisted in the following: oxygen from a gasometer was passed through the Erdmann apparatus, equipped in the usual way, but between the Drechsel wash bottle drying it and the apparatus there was placed a wash bottle with carbon tetrachloride. By means of a water bath of a definite temperature we regulated the consumption of \(\mathrm{CCl_4}\) and, consequently, the content of its vapors in the oxygen. Before the experiment and after its completion the wash bottle was weighed, which made it possible to determine the content of \(\mathrm{CCl_4}\) in 1 liter of gas. The data obtained are given below.

As is seen from the table, in the presence of \(\mathrm{CCl_4}\) for some coals (Zhurninskii seam and seam IV, Internal Kiselevka) an increase was observed in the temperature at which vigorous oxidation begins and in the temperature of spontaneous ignition. For the two other coals this was not the case. In almost all cases, instead of the usual flash, the appearance of smoke was observed, with a sharp jump in temperature over 2–3 seconds. When the Erdmann apparatus was unloaded, the odor of chlorine was noticed.

Evidently, in our experiments \(\mathrm{CCl_4}\) played the role of an inert addition to oxygen, only weakly affecting the rate of oxidation of coal at temperatures up to \(150\)—\(160^\circ\), and somewhat changing the character of the process

Content of CCl₄ in 1 liter of gas

at higher temperatures. In any case, carbon tetrachloride cannot be recognized as an anticatalyst, a small amount of which would be sufficient to prevent the spontaneous combustion of bituminous coal.

The present work was carried out on assignment from the Kuznetsk Scientific Research Coal Institute.

LITERATURE

1. V. S. Krym, Chemistry of Solid Fuel, 2nd ed., p. 277.
2. E. Erdmann, Brennst.—Chem. III, pp. 257, 278, 293 [1922].
3. I. D. Davis, I. F. Byrne—Br. Ch. IV, 166 [1925].
4. V. T. Gal’velov.—Kh. T. T. VIII, p. 791 [1937].
5. Sustmann, R. Lechuert—Br. Ch. 19, 21—27 [1938].
6. Denisova N. F.—Establishment of the conditions for bringing coal into a stable state with respect to atmospheric oxygen (unpublished).
7. Petrov S. M. and Maidanovskaya L. G.—Proceedings of the Siberian Physico-Technical Institute 4, No. 3, pp. 102—111 [1936].
8. I. J. W. Kreilen.—Chem. Weekbl. 29, 534—8, 559—61, 566—7.