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

Article 9.

B. V. Tronov and M. Yu. Grigoriev.

Iodine Numbers of Hard Coals

Ferd. Fischer[^1], treating hard coals with bromine water, and also with a solution of KBr and KBrO₃ in the presence of hydrochloric acid, observed that coals absorb bromine, sometimes in amounts up to 30% and more. From this he drew the following conclusions:

1) In hard coals there are unsaturated compounds which react first of all both with halides and with oxidizing agents.

2) From the amount of bromine absorbed one may judge the tendency of coals toward spontaneous combustion.

Fischer’s opinion was supported by Habermann[^2] and Dering[^3], who likewise treated coals with bromine. Later investigators began to use iodine[^4],[^5].

When the theory of coal oxidation proposed by F. Fischer was called into question[^6], the question arose whether iodine or bromine numbers could be used to characterize the capacity of coals for oxidation. Since the portions of the complex molecules constituting the organic mass of hard coal most accessible to the action of oxidizing agents are, apparently, phenolic groups, it was important to establish whether phenols are capable of reacting with iodine.

Experiments that were carried out showed[^7] that phenols react very readily with iodine and sometimes give very considerable iodine numbers. Moreover, the more readily a given phenol is oxidized by permanganate, the greater is the rate of its reaction with iodine.

From this it could be expected that, despite a different interpretation of the mechanism of coal oxidation, determination of the iodine number would nevertheless remain a method for estimating the reducing activity of coals, equivalent to the permanganate method.

In addition, permanganate and certain other oxidizing reagents can be used only in aqueous solutions, and water wets coals poorly. It is recommended[^8], for better wetting, to add to the solution a special patented agent (“perminal”), but Francis[^9] found that this reagent itself decomposes permanganate. Iodine can be used in the most diverse organi-

organic solvents that readily wet coals. Therefore it may be thought that, in the study of coals, it will prove not equivalent to permanganate, as in the case of phenols, but better than permanganate.

The data available in the literature already spoke in favor of the iodine number as a fairly characteristic constant, making it possible to evaluate the capacity of coals for oxidation. E. V. Voitova^4), for example, compared the iodine numbers of various coals with their spontaneous-ignition points in a current of oxygen, determined by the most generally accepted Erdmann method. The most varied coals were taken, from the very readily oxidized Chelyabinsk and Moscow-region brown coals (spontaneous-ignition temperature 116°–125° and 130°–136°) to the low-activity Tkvarcheli brown coals (spontaneous-ignition temperature 210°–220°). On the whole, good parallelism with the iodine numbers was found.

In the work of G. A. Zilberg, N. N. Saikovich, and N. F. Denisova^5), a comparison was made of different methods for determining the oxidizability of coals. The authors arrived at the rather discouraging conclusion that “the methods applied, failing to capture the peculiarities of the behavior of each coal during oxidation, cannot give a correct characterization of it and therefore compel one to treat the results obtained with great caution.”

However, this conclusion was drawn mainly on the basis of the fact that Kuzbass coals behave differently under natural conditions than in the laboratory; concerning the methods themselves the author notes that they give, for the most part, mutually consistent results.

Of course, no laboratory method of studying coal can by itself give an exact indication of whether coal will catch fire in a given mine. In the laboratory we can obtain only a characterization of the properties of the coal substance itself, whereas the chemical characterization of coal is only one of the factors on which the outbreak of a fire depends. To solve the practical question of the possibility of spontaneous combustion of coal in a particular mine, it is also necessary to take into account the local conditions of occurrence of the seams and their working. If, however, we speak of the chemical side of the question, then the work of Zilberg, Saikovich, and Denisova also indicates that the iodine number generally characterizes well the activity of coals with respect to oxidizing agents.

Coal from the Moshchnyi seam in Khakassia, with a spontaneous-ignition temperature of 135°, gave iodine numbers: after 2 hours—55.1; 4 h.—67.82; 24 h.—91.56. For coal from the Volkovskii seam of the Kemerovo deposit, for the same time intervals the numbers obtained were 9.39; 13.41; 30.19. The spontaneous-ignition point was 160°.

We, while studying the oxidation process of certain Kuzbass coals, also decided to include in the work program the determination of iodine numbers.

Ten samples of Kuzbass coals were taken, for which, together with a number of other tests (see the other papers), the iodine number was determined by the following method (after Margoshes):

A 1 g weighed portion of coal, ground so as to pass through a sieve with 2,500 openings per sq. cm, is treated with 25 ml of a decinormal solution of iodine in alcohol; a further 200 ml of an aqueous solution of alcohol (1:1) is added, and the mixture is shaken vigorously for 5 minutes, after which it is left at rest for 5 minutes. After the indicated time has elapsed, the coal is filtered off on a Büchner funnel. The residue on the filter is thoroughly washed with an aqueous solution of alcohol (1:1) until the yellow color disappears. The filtrate is titrated with a decinormal solution of hyposulfite. A blank experiment is set up in parallel.

In the table below, the iodine numbers of coals are compared with their spontaneous-ignition temperatures according to Erdmann. The iodine numbers are given in grams of iodine reacted per 100 g of coal. In Erdmann’s apparatus there were determined both the usually determined point of spontaneous ignition (i.e., the point of intersection of the temperature curves of the coal and the bath) and the temperature at which a noticeable flash of the coal was observed.

Table 1

Iodine numbers and spontaneous-ignition points of coals.

We see that all coals (5 samples) with a spontaneous-ignition temperature above 160° gave iodine numbers below 10. The coals with the highest and the lowest spontaneous-ignition temperature according to Erdmann gave, respectively, the smallest and the largest iodine numbers: 175°—iodine number 6.5; 135°—17.7. Among coals of intermediate oxidizability, and in general when comparing coals close to one another, small deviations are observed, but these deviations lie almost within the limits of measurement errors. It is possible that with a different duration of the experiments or with changes in other conditions (iodine concentration, amount of solution for the given weighed portion of coal), the agreement could be made still more complete.

In general, of all the reagents we tested, iodine gave results most closely corresponding to what is obtained in pri—

Erdmann apparatus. Meanwhile, determination of the iodine number, especially by Margoshes’ method, is much simpler and requires less time than testing by Erdmann’s method.

Therefore, determination of the iodine number of coals may be recommended as a general method for detecting their reducing capacity, for the time being as a parallel method to Erdmann’s method. If the iodine number proves itself not on tens but on hundreds of coals, then it is possible that this method may in the future replace the Erdmann test, which is cumbersome in its apparatus and requires considerable time.

The work was carried out with funds from KNIUI.

REFERENCES

1. Ferd. Fischer, Zeit. angew. Chem. 1899, pp. 764–7, 787–90.
2. Journ. f. Gasbel 1906, 422.
3. Fischers Jaresber 39, 13 (1908).
4. E. I. Voitova, “Chemistry of Solid Fuels,” VII, No. 3, pp. 249–262 (1906).
5. G. A. Zilberg, N. N. Saikovich, N. F. Devisova. Transactions of the Scientific-Research Institute “Kuzbassugol”, 1935, “Hard Coals of Kuzbass,” pp. 157–179.
6. B. V. Tronov. “On the Mechanism of Oxidation of Hard Coal by Atmospheric Oxygen.”
7. B. V. Tronov and V. Z. Grigorieva. Work submitted for publication.
8. N. Heathevat. Fuel XII, 1, p. 4 (1933).
9. W. Francis, Fuel XVII, No. 12, pp. 363–372 (1938).