On the Mechanism of Oxidation of Hard Coal and the Causes of Its Spontaneous Combustion: Article 10
Abstract Generated abstract
This article examines gaseous products formed during the air oxidation of hard coals and accompanying changes in their bituminous fractions, with attention to implications for spontaneous combustion. Four Kuznetsk-region coals of different maturity were oxidized in a retort at 100, 150, 200, and 250 degrees, while water, carbon dioxide, and carbon monoxide were measured gravimetrically or by conversion to carbon dioxide, and alcohol-benzene extracts were analyzed for bitumen yield, acid number, and saponification number. The experiments indicate that younger coals produced larger quantities of oxidation products, whereas more mature coals produced less and in some cases continued to gain weight at higher temperature. Oxidation generally increased bitumen yield and raised acid and saponification numbers, although the highest-temperature treatment and one coal sample showed exceptions to this pattern.
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ON THE MECHANISM OF OXIDATION OF HARD COAL AND THE CAUSES OF ITS SPONTANEOUS COMBUSTION
Article 10.
M. Yu. Grigor'ev and K. K. Semenova.
On the gaseous products of coal oxidation and on changes in the bituminous part during the oxidation of coals by air.
A considerable number of works, both in our literature and in foreign literature, have been devoted to the study of the oxidation products of fossil coals, as well as to the chemical changes in coal in the process of its oxidation. Interest in such study is explained by the fact that, from the products of oxidation, one may judge the chemical nature of hard coals. Thus, Fr. Fischer and his collaborators, by studying the products of oxidation of coal by the oxygen of air under pressure, gave weighty proof of the lignin theory of the origin of hard coals and of the aromatic structure of the principal mass of coal (Krem—Chemistry of Solid Fuel).
At the assignment of the Kuznetsk Scientific Research Institute, we carried out work to determine the quantity of gaseous products of oxidation released at different temperatures (H₂O, CO₂, and CO), and to study the changes, as a result of oxidation, in the bituminous part of coal. Four coals were taken for this work. The data of the elemental analysis are given below.
Table 1
| Coal | C | H | S + O + N |
|---|---|---|---|
| 1. Pl. Zhurnskii, Zhurinka mine, Leninsk . . . . . . . | 83.16 | 5.84 | 11.0 |
| 2. Pl. IV Inner, mine No. 3, Kiselevka | 85.41 | 5.26 | 9.33 |
| 3. Pl. Gorelyi, mines 5–6, Prokop'evsk | 88.42 | 4.05 | 7.53 |
| 4. Pl. IV Inner, Koksovaya mine, Stalinugol . . . . . . . | 86.94 | 4.37 | 8.69 |
100 g of coal, ground to pass through a sieve with 900 openings per 1 cm², was loaded into a metal retort. The latter was placed in a wide crucible furnace \((d = 12.5\ \text{cm})\). The space between the walls of the crucible furnace and the retort was filled with asbestos wool, into which a thermometer was inserted. The temperature of the furnace was regu-
regulated by a thermoregulator. Air, previously freed from $\mathrm{H_2O}$ and $\mathrm{CO_2}$ by passing through sulfuric acid and soda lime, was fed from a gasometer through the bottom of the retort. For uniform distribution of the air throughout the entire mass of coal, a layer of somewhat compacted asbestos wool was placed on the bottom of the retort. The spent air was led off through the lid of the retort and subsequently passed through 2 calcium-chloride tubes, a potash apparatus, and a calcium-chloride tube, and entered the gasometer. The determination of $\mathrm{H_2O}$ and $\mathrm{CO_2}$ was carried out by the usual gravimetric method. The gas collected in the gasometer for determining carbon monoxide was passed, after preliminary removal of water vapor, through an incandescent quartz tube filled with copper oxide, and the carbon dioxide formed was trapped by caustic alkali in the potash apparatus. From the gain in weight of the potash apparatus the amount of carbon monoxide released in the process of coal oxidation was calculated. The oxidation of the coal was carried out at temperatures of 100°, 150°, 200°, and 250°. At a temperature of 100°, the coal was oxidized for 12 hours. In the remaining cases—for 6 hours. The percentages of $\mathrm{H_2O}$, $\mathrm{CO_2}$, and $\mathrm{CO}$ were calculated relative to the coal. The data obtained are summarized in the table.
Table 2
| Coal | Temperature | Change in weight | $\mathrm{H_2O}$ | $\mathrm{CO_2}$ | $\mathrm{CO}$ | Notes |
|---|---|---|---|---|---|---|
| Pl. Zhurinskii, Zhurinka mine, Leninsk | 100° | 0,25% | 0,17% | 0,009% | 1. Coal at $T^\circ$ of 150°, 200°, 250° was oxidized for 6 hours. At 100° it was oxidized for 12 hours. | |
| Pl. Zhurinskii, Zhurinka mine, Leninsk | 150° | + 1,32 g | 0,31% | 0,22% | 0,013% | 1. Coal at $T^\circ$ of 150°, 200°, 250° was oxidized for 6 hours. At 100° it was oxidized for 12 hours. |
| Pl. Zhurinskii, Zhurinka mine, Leninsk | 200° | 0,99% | 0,49% | 0,06% | 1. Coal at $T^\circ$ of 150°, 200°, 250° was oxidized for 6 hours. At 100° it was oxidized for 12 hours. | |
| Pl. IV Vnutrennii, mine No. 3, Kiselevka | 100° | 0,21% | 0,15% | 0,007% | 1. Coal at $T^\circ$ of 150°, 200°, 250° was oxidized for 6 hours. At 100° it was oxidized for 12 hours. | |
| Pl. IV Vnutrennii, mine No. 3, Kiselevka | 150° | 0,30% | 0,18% | 0,01% | 1. Coal at $T^\circ$ of 150°, 200°, 250° was oxidized for 6 hours. At 100° it was oxidized for 12 hours. | |
| Pl. IV Vnutrennii, mine No. 3, Kiselevka | 200° | + 0,86 | 0,80% | 0,21% | 0,05% | 1. Coal at $T^\circ$ of 150°, 200°, 250° was oxidized for 6 hours. At 100° it was oxidized for 12 hours. |
| Pl. IV Vnutrennii, mine No. 3, Kiselevka | 250° | + 0,28 | 1,29% | 0,58% | 0,07% | 1. Coal at $T^\circ$ of 150°, 200°, 250° was oxidized for 6 hours. At 100° it was oxidized for 12 hours. |
| Pl. Gorelyi, mine 5—6, Prokopyevsk | 100° | + 0,94 | 0,18% | 0,09% | traces | 2. Percentages of $\mathrm{H_2O}$, $\mathrm{CO_2}$, and $\mathrm{CO}$ are given relative to the coal. |
| Pl. Gorelyi, mine 5—6, Prokopyevsk | 150° | 0,41% | 0,12% | 0,005% | 2. Percentages of $\mathrm{H_2O}$, $\mathrm{CO_2}$, and $\mathrm{CO}$ are given relative to the coal. | |
| Pl. Gorelyi, mine 5—6, Prokopyevsk | 200° | 0,77% | 0,23% | 0,05% | 2. Percentages of $\mathrm{H_2O}$, $\mathrm{CO_2}$, and $\mathrm{CO}$ are given relative to the coal. | |
| Pl. Gorelyi, mine 5—6, Prokopyevsk | 250° | + 0,37 | 1,15% | 0,52% | 0,1% | 2. Percentages of $\mathrm{H_2O}$, $\mathrm{CO_2}$, and $\mathrm{CO}$ are given relative to the coal. |
| Pl. IV Vnutrennii, Koksovaya mine, Stalinugol | 100° | 0,21% | 0,10% | 0,005% | 3. Rate of passage of air through the coal: 2 l/hour. | |
| Pl. IV Vnutrennii, Koksovaya mine, Stalinugol | 150° | + 1,02 | 0,92% | 0,12% | 0,006% | 3. Rate of passage of air through the coal: 2 l/hour. |
| Pl. IV Vnutrennii, Koksovaya mine, Stalinugol | 200° | + 0,37 | 1,13% | 0,28% | 0,04% | 3. Rate of passage of air through the coal: 2 l/hour. |
| Pl. IV Vnutrennii, Koksovaya mine, Stalinugol | 250° | − 0,56 | 1,70% | 0,59% | 0,09% | 3. Rate of passage of air through the coal: 2 l/hour. |
As can be seen from the results of the experiments, the greatest quantity of oxidation products is given by the young coals—Leninsk and Kiselevsk. They also show appreciable curves upon oxidation at 150°–200°, which are replaced by a loss in weight in the Leninsk coal at a temperature of 250°. The more mature coals—Prokopyevsk—gave a smaller quantity of oxidation products and a gain in weight even after 6 hours of oxidation at a temperature of 250°.
Both the original and the oxidized coals were subjected to extraction with alcohol-benzene in a Soxhlet apparatus. In the extracted
the acid and saponification numbers of the bitumens were determined. The data obtained are given in the table.
Table 3
| Coal | Oxidation of coal | Bitumen yield, % | Acid number, mg KOH | Saponification number, mg KOH |
|---|---|---|---|---|
| 1. Pl. Gorely | Initial | 0.51 | 57.6 | 117.9 |
| 1. Pl. Gorely | Ox. at 100° | 0.88 | 63.96 | 261.26 |
| 1. Pl. Gorely | Ox. at 250° | 0.41 | 123.9 | 253.9 |
| 2. Pl. IV Vnutrennii, mine No. 3, Kiselevsk | Initial | 1.35 | 80 | 101.7 |
| 2. Pl. IV Vnutrennii, mine No. 3, Kiselevsk | Ox. 150° | 1.41 | 81.2 | 116.5 |
| 2. Pl. IV Vnutrennii, mine No. 3, Kiselevsk | Ox. 250° | 1.24 | 85.3 | 128.3 |
| 3. Pl. Zhurinskii, Zhurinka mine, Leninsk | Initial | 3.35 | 132.6 | 154.3 |
| 3. Pl. Zhurinskii, Zhurinka mine, Leninsk | Ox. 150° | 5.08 | 129.1 | 199.3 |
| 3. Pl. Zhurinskii, Zhurinka mine, Leninsk | Ox. 250° | 3.83 | 247.2 | 288.7 |
| 4. Pl. IV Vnutrennii, Koksovaya mine, Stalinskugol | Initial | 0.51 | 91.9 | 264.5 |
| 4. Pl. IV Vnutrennii, Koksovaya mine, Stalinskugol | Ox. 200° | 0.47 | 50.3 | 105.2 |
| 4. Pl. IV Vnutrennii, Koksovaya mine, Stalinskugol | Ox. 250° | 0.79 | 71.7 | 158.0 |
As is evident from the data presented, in all cases an increase in the bitumen yield is observed in oxidized coals. In three cases (Pl. Gorely, Pl. IV Vnutrennii Kiselevsk, Pl. Zhurinskii) the initial increase in bitumen yield was replaced by a decrease at a temperature of 250°. The exception is the coal from the Koksovaya mine. The acid and saponification numbers, in general, with the exception of the same coal from the Koksovaya mine, showed a considerable increase in our experiments.