ON THE STUDY OF LOW-TEMPERATURE OXIDATION OF BROWN COAL

K. G. KUZNETSOV

(Presented by Honored Worker of Science and Technology, Professor Doctor I. N. Butakov)

In studying the process of low-temperature oxidation of coals, it had previously been established that the rate of oxygen addition decreases monotonically [1, 2], while the rate of evolution of gaseous oxidation products increases as the concentration of oxygen-containing atomic groups increases [3, 4]. As a result, an equilibrium is established. It was assumed that, on the surface freed after removal of the decomposition products, secondary formation of oxygen-containing groups occurs, identical in thermal stability to those that had decomposed earlier (primary groups). To describe the kinetics of the oxidation process, mathematical expressions were proposed [3, 4], the basis of which is the well-known Langmuir equation. However, most studies were carried out with coals containing, before the experiment, only a small amount of oxygen (electrode coal, coke, etc.).

Brown coals are always oxidized to a considerable extent and contain 20–25% oxygen before being placed in a stockpile. Obviously, the “initial” active oxygen-containing atomic groups must influence the process. In the present work the question of the kinetics of oxidation of brown coal is considered. The investigations were carried out with coal from the Nazarovo deposit, the proximate and ultimate analyses of which are presented in Table 1. The experiments were conducted in an apparatus whose schematic is given in Fig. 1. Air, purified of moisture and carbon dioxide by passing it through U-shaped tubes with ascarite and a coil with concentrated acid, entered a preheater, where it was heated to the required temperature, and then was fed into the vessel with coal.

Table 1

Coal crushed to 0.25–1 mm was first brought to constant weight in a stream of dry nitrogen at a temperature of 105°, and then thermostated at the same temperature. The weight of the sample was 50–60 g. The gaseous oxidation products passed through a coil with ser-

sulfuric acid and U-shaped tubes with Ascarite; from the change in the weight of the latter, the amount of water and carbon dioxide released during the experiment was determined. The amount of oxygen taken up was determined

Fig. 1. Apparatus for coal oxidation: 1 — thermostat; 2 — heater; 3 — air preheater; 4 — sample; 5 — stirrer; 6 — contact thermometer; 7 — mercury thermometer; 8 — Ascarite; 9 — sulfuric acid.

from the difference between the change in the weight of the sample and the amount of gaseous oxidation products. The oxidation time was 1100–1200 hours. The experiments were carried out both in air and in a nitrogen atmosphere. The results of the observations showed (Fig. 2, curve 2) that the character of the change in the curve for the yield of gaseous products differs from the character

Fig. 2. Oxygen uptake 1, yield of gaseous oxidation products 2, change in the weight of the sample 3 during oxidation.
(All quantities are expressed as % of the initial weight of the sample.)

established earlier. Indeed, the rate of evolution is maximal at the initial moment, then decreases to a certain

values and, upon further oxidation, remains relatively constant.

The total yield of oxidation products over the time until equilibrium was established amounted to 20–21%. In parallel, an experiment was carried out with a coal sample in a stream of nitrogen. In this case, the evolution of gaseous products, H₂O and CO₂, was also observed; their total amount over 4–5 days was 1.7–1.9, after which evolution practically ceased. Thus, the possibility that thermal decomposition of unstable initial groups has an influence is not excluded, but in the main the process of evolution of oxidation products is due to the decomposition of at least two types of oxygen-containing groups having different stabilities.

The initial rate of the process is apparently due to the oxidation and decomposition of “initial” atomic groups, which are not identical in their chemical activity to the oxygen-containing atomic groups newly formed on the liberated surface. In the final period, after the “initial” groups have been consumed, the principal role is played by the decomposition of the newly formed oxygen-containing atomic groups.

The oxygen uptake curve in Fig. 2 (curve 1) lies practically below the curve for the yield of oxidation products; consequently, at the given temperature, the dominant process is the decomposition of oxygen-containing atomic groups. The kinetic curve of weight change in Fig. 2, curve 3, can be divided into two sections. The first is characterized by a weight increase of 0–0.2%; the second, by a decrease in weight at a variable rate.

The character of the change in this curve is also due to the influence of the decomposition of two types of thermally nonuniform oxygen-containing atomic groups.

Conclusions

1. Isothermal oxidation of brown coal from the Nazarovo deposit was carried out at a temperature of 105°C.

2. It was established that the yield of gaseous oxidation products decreases with time and is due to the decomposition of at least two types of oxygen-containing atomic groups having different stabilities.

References

1. S. M. Petrov, L. G. Maidanovskaya. Proceedings of the Siberian Physico-Technical Institute, vol. 4, no. 3, 1936.

2. V. S. Veselovskii et al. Spontaneous Ignition of Industrial Materials. Nauka Publishing House, 1964.

3. V. V. Pomerantsev, V. A. Lombakh, Yu. A. Rundygin. Engineering Physics Journal, No. 3, 1962.

4. I. A. Yavorskii. Problems in the Theory of Combustion of Fossil Coals and Intensification of Their Ignition, Publishing House of the Siberian Branch of the Academy of Sciences of the USSR, 1961.