I. V. Gebler and G. A. Zilberg

On the Question

of the Change of Certain

Kuznetsk Coals

During Their Oxidation

Tomsk

1929

I. V. Gebler and G. A. Zilbergh.

On the Changes in Some Kuznetsk Coals During Their Oxidation

Weathering and oxidation of hard coals during their more or less prolonged storage can, to a considerable degree, change their properties. These processes, generally lowering the quality of coals, when proceeding at a more vigorous rate lead to spontaneous combustion.

Coal is capable of absorbing a considerable quantity of oxygen; at first, at ordinary temperature, this is a physical process: oxygen is adsorbed by the coal; subsequently part of the adsorbed oxygen, being in close contact with the coal, enters into chemical combination with the organic mass of the coal, which leads to the formation of oxidized organic compounds with the liberation of water and carbon dioxide. Such a process is accompanied by the evolution of heat, which intensifies the oxidation reaction. In this connection there is usually observed a lowering of the calorific capacity of coals and, for caking coals, a diminution of their caking capacity; likewise there changes the yield and character of the volatile products liberated during the dry distillation of coal.

In this connection, the question of oxidation and of the corresponding change in coking coals during their storage in air is of particular interest, especially in the event that, for some reason, the coals cannot be sent for coking immediately after extraction, but are subject, for example, to transport to coke plants remote from the mines and to more or less prolonged storage, which is intended to compensate for irregularity of delivery.

In view of the absence of definite conceptions concerning the chemical composition of the organic part of hard coal—which nevertheless must be regarded as extremely complex—at the present time, despite a number of works in Western Europe and America devoted to questions of the oxidation and spontaneous combustion of coals, the essence of these processes remains little clarified, and the views of different authors are often diametrically opposed, as may be seen, for example, from the article by Eng. N. M. Karavaev*), where a review of individual works is given and the conclusions following from them are summarized. On the basis of these materials one may conclude that, despite a rather considerable amount of factual material relating to various coals, there are no such generalizations that would make it possible to form definite judgments regarding the changeability of coals during their storage in air on the basis of their elementary composition and of those properties which are determined by ordinary technical analysis. In this connection, for the characterization of coals with respect to their capacity for oxidation, it is necessary to study each coal separately, which may be done either by observing changes in the properties of coal under natural conditions of storage in piles in the open air—which, generally speaking, requires much time—or else by, so to speak,

*) Izvestiya Teplotekhnicheskogo Instituta, 1925, No. 8, p. 57.

controlled oxidation in the laboratory, subjecting the coal, under appropriate conditions, to the action of atmospheric oxygen at elevated temperature; in this way the time required for investigation is considerably shortened, while the data obtained may provide interesting material and give quite comparable numerical characteristics for different coals.

It was precisely by this method that certain American coals were investigated at temperatures from 25 to 110° in an apparatus proposed for this purpose by Prof. White*).

The present work is an attempt to elucidate, in this respect, certain coals of the Kuznetsk Basin.

For observations on the change of coals during storage in air, we took coals of three types: the Volkov seam of the Kemerovo deposit, and the Inner Third and Fourth seams of the Prokopyevsk deposit, from batches of coal intended for experimental coking at the Kemerovo Chemical Plant. The time of mining of the coals was about a week before the beginning of the observations.

The Volkov coal was taken in two grades: the residue when screened on a 10 mm sieve, and everything that passed through the sieve. The two other coals were in their natural size. Observations were conducted over a period of 4 summer months; the coals were placed in the plant yard in piles, each with an area of \(1 \tfrac{1}{2}\) square meters and a height of 0.7 m.

From Table 1 one can see the composition of the coals taken for investigation; ash, volatile matter, coke yield, and sulfur in this and in the following tables are calculated in percent of the dry coal substance; the data of ultimate analysis and calorific value are for the organic mass. The analyses were carried out by ordinary methods.

Table 1.

) Industrial and Engineering Chemistry*, February 1925—cited from the article by A. M. Gladshein: “Coal and Iron,” 1926, No. 14.

Under the above-indicated conditions, the coals were left to lie from 18/VI to 28/X; during this period, determinations of the volatile matter and calorific value in average coal samples were made several times; the data of these determinations are given in the following tables and are expressed graphically in diagrams 1–8.

Table 2.

Volkov seam on a 10 mm sieve.

As is evident, with respect to volatile substances, the coal did not change. The drop in calorific value amounts to 2.4% (diag. No. 1); as for caking capacity, it turned out that after 40 days the coal caked weakly, and after 68 days the ability to cake had been completely lost.

Diagram No. 1. Drop in calorific value of Volkov coal on a 10 sieve.

Table 3.

Volkov seam below a 10 mm sieve.

Here too it may be considered that the volatile substances did not change. The calorific value fell by 2.3% (diag. No. 2). After 30 days the coal caked weakly, and after 54 days the coke obtained was powdery.

Diagram No. 2. Decline in the calorific value of Volkov coal through a 10 mm sieve.

Table 4.

Inner 3rd Seam.

Table 5.

Inner 4th Seam.

The caking capacity of the coals of the inner 3rd and inner 4th seams remained unchanged; thus these coals are stable and do not undergo noticeable changes when stored in air for 4 months.

Experiments on the study of coals by means of intensive oxidation were carried out under the following conditions.

The apparatus used for this work consisted of a tin box of hexagonal cross-section, through which passed a glass tube connected with a series of absorption devices for CO₂ and H₂O released as products of coal oxidation. Inside the box were placed carbon electric lamps. The air entering from the gasometer, before entering the tube, passed through ordinary absorbers for purification from impurities. After establishing the required tempera-

temperature in the apparatus—110° C, a weighed portion of coal was introduced into the tube in an aluminum boat. The coal was previously dried in an atmosphere of nitrogen. Every two or three days the boat with the coal was weighed in order to determine the gain in the coal and in the absorbers. The rate of the air passed through was the same in all experiments. During the experiment the temperature fluctuated within 2° in either direction. In such an apparatus it is easy to maintain a constant temperature for a long period of time. (See the appended photograph.)

For the experiments in this apparatus, coals were taken from the Volkov and Kemerovo seams of the Kemerovo mine and the Inner 4th seam of the Prokopievsk deposit. The samples of these coals were taken from seam workings; the sampling technique was as follows: across the entire thickness of the seam a groove 4–5 centimeters wide was cut; the coal broken out of the groove fell into an iron scoop that followed directly behind the pick. A sample weighing 3–4 kilograms was carefully mixed and crushed; the average sample selected from this mass was used for the work.

Table No. 6 characterizes the composition of the coals taken.

Table 6.

Coals.

Tables Nos. 7, 8, and 9 give the results of weighings during the heating of coal at a temperature of 110°.

In column 1 is the gain in coal, in percent; in columns 2 and 3, the loss by the coal of the evolved water and carbonic acid, in percent; in columns 4 and 5, the loss of H and C, recalculated from H₂O and CO₂; in columns 6 and 7, the loss, in percent, of H and C from those originally contained in the coal.

From diagrams Nos. 3, 4, and 5, illustrating the figures of this table, it is seen that over the course of 45 days, from coal subjected to the action of air at a temperature of 110°, 3.06% carbon dioxide gas and 6.18% water were evolved, calculated on absolutely dry coal. This constitutes a loss of 0.69% H and 0.81% C. At the same time the weight of the coal increased by 4.53%. The loss from the hydrogen originally contained in the coal amounts to 15.7%, and from the carbon, 1.00%.

*) Coal grades taken from a memorandum of the Central Laboratory of the Kemerovo Combine of the Sibugol Trust.

Volkov coal at a temperature of 110°.
Table 7.

Diagram labels: “Weight gain”; “Loss of CO₂”; “Loss of H₂O”; horizontal axis: “days.”

Diag. No. 3. Volkov coal at temp. 110.

Diagram No. 4. Volkov coal at \(t = 110\). Loss by the coal of hydrogen and carbon.

Diagram No. 5. Volkov coal at \(t = 110^\circ\). Loss, in %, of hydrogen and carbon from the initial quantity contained in the coal.

Kemerovo coal at a temperature of \(110^\circ\). Table 8.

For Kemerovo coal, at the same temperature, over 45 days there was evolved 4.32% H₂O and 1.53% CO₂, which amounts to 0.48% hydrogen and 0.42% carbon, calculated on the coal. During the same time the coal showed a gain in weight of 2.55%. The loss from the H₂ originally contained in the coal amounts to 9.89%, and C—0.53% (Diagr. Nos. 6, 7 and 8).

Diagr. No. 6. Kemerovo coal at t = 110°.

Diagr. No. 7. Kemerovo coal at t = 110°. Loss by the coal of hydrogen and carbon.

Diagr. No. 8. Kemerovo coal at t = 110°. Loss in % of C and H₂ from those originally contained in the coal.

Internal No. 4 at a temperature of 110°.

Table 9.

From Diagrams Nos. 9, 10, and 11, constructed on the basis of the figures given for Internal No. 4, it is seen that at the same temperature, over the course of 44 days, 1.85% H₂O and 0.89% CO₂ were evolved, which

Diagram No. 9. Internal No. 4 at \(t = 110^\circ\).

amounts to a loss of hydrogen of 0.21% and of carbon of 0.27%. During the same time the coal showed a gain in weight of 2.07%. The loss of the hydrogen originally present in the coal amounts to 4.85% and of the carbon to 0.30%.

Diagram No. 10. Internal 4th. Loss by the coal of hydrogen and carbon.

Diagram No. 11. Internal 4th at \(t = 110^\circ\). Loss, in %, of hydrogen and carbon from that originally contained in the coals.

Comparing the diagrams of the three coals, Nos. 5, 8, 11, which illustrate the gain in the coals and the gain in the absorbers, we see that the gain curves for the Volkovsky and Kemerovo coals have the same character. In the first week the oxidation process proceeds more intensively than on the remaining days. For Internal 4th (diagram No. 9) the gains have the form of straight lines. The process proceeds rather uniformly over the entire interval of time. For all three coals the loss of \(H_2O\) proceeds faster than that of \(CO_2\).

In the Volkovsky coal the loss of H and C proceeds much more intensively than in the Kemerovo and Internal 4th coals.

In the Kemerovo coal the loss of H and C proceeds equally, and the curves, in their character, approach the curves for Internal 4th. Diagrams Nos. 5, 8, 11 express the losses of H and C as a percentage of those originally contained in the coals. The curves have the same character. Table No. 10 gives the results of analyses of the oxidized coals.

Table 10.

Comparing this table with Table No. 6, where the analyses of fresh coals are given, we see that after oxidation the coals yielded a greater amount of volatile matter and, from caking coals, became non-caking; the calorific value decreased. The decrease in calorific value is seen in Diagram No. 12. The difference in calorific value between fresh and oxidized coals is, for Kemerovo, 528 cal. or 6.4%; for Volkovsk, 655 cal. or 7.94%; and for Vnutrennii 4, 291 cal. or 3.4%.

Diagram No. 12. Decrease in calorific value at \(t = 110^\circ\) for Volkovsk, Kemerovo, and Vnutrennii 4 coals.

Diagram No. 13 presents a summary diagram for the weighed portions of all the coals and the oxidation products released.

Diagram No. 13. Summary diagram for Volkovsk, Kemerovo, and Vnutrennii 4 coals.

The investigated coals were subjected to coking with collection of the by-products by the Bayer method; the coals of the Kemerovo and Volkovsk seams were also investigated by this method after oxidation. The principle of the method, as is known, consists in the following: the coal is heated in a glass tube; the products released during decomposi-

volatile substances, pass successively through a series of absorbers, in which they are adsorbed. After the experiment, the absorbed substances are determined quantitatively by the increase in weight of the corresponding absorption apparatus.*)

Into a dry tube of refractory glass, sealed at one end, 15–20 grams of finely ground dry coal are poured. An asbestos plug is inserted upon the layer of coal. A second inserted plug stands away from the first at a distance of 3 cm, forming an air space. Holes are pierced in both plugs. On top of the second plug a layer of fireclay grains is poured, and on top of them a third plug, also with a hole, is inserted. The weighed tube is connected by its open end with a weighed cotton filter placed in a steam bath. By its narrow end the cotton filter is connected with a bent tube. The latter is connected with a Geissler potash apparatus into which 8 cu. cm. of \(H_2SO_4\) \(1/1N\) has been introduced. By its other end the potash apparatus is connected with a U-shaped calcium chloride tube. Before the experiment the total weight of the potash apparatus and the tube is determined. Next, 2 potash apparatuses with a KOH solution and a calcium chloride tube are attached. The total weight is also taken. To the latter are attached 2 apparatuses with paraffin oil and 2 calcium chloride tubes, connected with a gas receiver. The tube with coal is

heated to \(1000^\circ\), and throughout the whole system a vacuum of 45 mm of mercury is produced. The vacuum is produced by a siphon that removes water from the gasometer into the weighed receiver.

At the conclusion of the experiment the absorbers are weighed in the same order. Equality in the weight of the tube before and after the experiment gives the yield of volatiles and coke; the increase in the cotton filter with the bent tube gives the amount of tar; the increase in the potash apparatus with \(H_2SO_4\) and in the calcium chloride tube gives the amount of crude ammonia water. By titrating the sulfuric acid with caustic soda

*) F. Schreiber. — Die Industrie der Steinkohlenveredelung, 1923; also Litinsky. — Kokerei und Gaswerksöfen, 1928.

the amount of free ammonia is obtained. The gain in weight of the potash apparatus with KOH and of the calcium-chloride tube gives the total amount of hydrogen sulfide and CO₂. From the gain in weight of the absorbers with paraffin oil, the amount of crude benzol (light oil) is determined. The amount of gas is measured by the weight of water flowing out of the gas receiver.

The gas analysis is carried out in the usual manner.

Table 11 gives the results of the investigation of the coals taken according to Bayer; Table 12 illustrates those changes which the coals of the Volkov and Kemerovo seams show when investigated by this method after intensive oxidation; the difference in the yield of products is given in percent (+ increase, − decrease), taking the yields for fresh coals as 100.

For both coals, after oxidation, the coke obtained was powdery; there is practically no decrease in the yield of coke from the Kemerovo coal, while the yield of volatile products changed sharply, with a decrease in the total amount of gas, tar, and crude benzol; the yield of ammoniacal water for the Kemerovo coal, however, increased considerably.

On the basis of the experiments and observations carried out, we can draw the following conclusions:

Table 11.

Products by the Bayer method.

1) The coal of the Volkov seam, being unstable, cannot be used for coking after four months’ storage in air during the summer period, owing to the complete loss of its caking capacity.

2) The coals of the Inner 3rd and 4th seams are examples of stable coals and do not change under storage in the same conditions over the same period.

3) After intensive oxidation (110°), the Volkov, Kemerovo, and Inner 4th coals lose the property of caking; their calorific capacity decreases and the amount of volatiles increases.

Table 12.

Increase (+) or decrease (−), in %% of fresh coal (= 100)

4) After intensive oxidation of the same coals, the yield of coke (for the Kemerovo coal, only insignificantly), tar, and gas decreases; the yield of ammoniacal water increases.

To the article by I. V. Gebler and G. A. Zilberg,
“On the Changes in Some Kuznetsk Coals During Their Oxidation.”

Apparatus for testing coals for oxidizability.