PROCEEDINGS
OF THE TOMSK ORDER OF THE RED BANNER OF LABOR POLYTECHNIC
INSTITUTE named after S. M. KIROV

Vol. III 1961

EFFECT OF TEMPERATURE AND HEAT-CARRIER FLOW VELOCITY ON THE ARTIFICIAL DRYING OF MOLDED PEAT

N. M. SMOLYANINOVA, Ya. L. KOTLIKOV

(Presented by Professor, Doctor I. V. Gebler)

The use of peat in metallurgy may be carried out in two ways: either in the form of peat coke, or directly in the form of air-dry molded peat. In our opinion, the latter route is more promising, especially in connection with the future industrial development of the West Siberian iron-ore basin [1].

Among the many factors that influence the mechanical properties of peat and coke made from it (type of peat, degree of its decomposition and processing, molding pressure, etc.), moisture is of great importance.

In addition, the increased moisture content of peat has a negative effect on the operation and heat balance of the blast furnace [2, 3].

At present, almost the only method of dewatering peat is its natural drying. However, it is not possible in this way to dry large quantities of peat to a moisture content below 25%, which is explained by the special properties of peat and by the nature of its interaction with water.

In peat, several types of moisture are distinguished, depending on the nature of its bond with the material [4].

Water of the noncapillaries, capillaries, osmotic water, and part of the water of ultraporous spaces are removed comparatively easily, for example, during natural drying. The remaining water, under pressure and bound into one whole with colloidal particles, can be removed by evaporation only under artificial conditions.

All proposed methods of artificial peat drying have so far been unprofitable.

The simplest method is thermal drying of peat. However, because of the high water content in raw peat, ordinary thermal drying methods are unacceptable, since the amount of thermal energy contained in the dry substance of raw peat is less than the amount required to evaporate the moisture contained in the peat.

At the present stage of development of drying technology, it is considered economically expedient to dry peat only from a moisture content of 60–65% [5].

We carried out work with the aim of finding the optimal conditions for drying with flue gases, ensuring the production of molded peat with the best mechanical properties.

The method of peat processing, preparation of samples, and evaluation of their quality was the same for all investigations and has been described earlier [6].

Two series of experiments were carried out. The first series was conducted at temperatures of 80–180° at a constant heat-carrier velocity. The second series of experiments was conducted at heat-carrier velocities equal to 0.4; 1.15; and 1.85 m/sec.

The results of the experiments are given in Fig. 1 and in Tables 1–4.

Fig. 1. Dependence of peat moisture content on drying duration at different temperatures and heat-carrier velocities. 1—temperature 80°, heat-carrier velocity—0.4 m/sec; 2—80° and 1.15 m/sec; 3—80° and 1.85 m/sec; 4—100° and 0.04 m/sec; 5—100° and 1.15 m/sec; 6—100° and 1.85 m/sec; 7—120° and 1.15 m/sec; 8—150° and 1.15 m/sec; 9—180° and 1.15 m/sec.

The curves shown in Fig. 1 indicate the dependence of the drying duration, to a specified given moisture content in the peat, on temperature. As is evident from the graph, with increasing temperature the drying duration to one and the same moisture content decreases, and to a greater extent the higher the temperature.

Table 1

Heat-carrier temperature, °C

However, from 150° onward the drying duration depends on temperature to a lesser degree, since it is evidently limited only by the rate of moisture diffusion from the internal parts of the material to its surface.

The crushing resistance and strength index of the peat (Table 2) fall sharply in the temperature interval 80–120°; at higher temperatures they decrease considerably more slowly, while the strength index even shows a tendency to increase.

The abrasion of peat at low temperatures (80–100°) is insignificant, but then increases sharply.

The reason for the deterioration of the mechanical properties of peat with increasing drying temperature is the large and nonuniform shrinkage of the material due to the presence of high gradients of moisture content and temperature.

Table 2

Table 3

Table 4

...in the field of peat molding, caused by high drying rates.

Thus, the best results in terms of mechanical strength were obtained for molded peat dried at 80 and 100°, which ensure a relatively low drying rate. In this case the briquettes, although they were cracked, nevertheless retained their integrity, in contrast to drying at higher temperatures, when the molded pieces disintegrated into small fragments.

On this basis, only briquettes dried at 80 and 100° were subjected to thermal treatment.

As can be seen from Table 3, all the mechanical properties of briquettes subjected to thermal treatment are higher for peat dried at 80°.

The strength index and resistance to crushing drop sharply with increasing temperature, reach a minimum in the temperature interval 450—500°, and then slowly increase.

The abrasion of peat increases strongly up to 500°, and then less significantly.

The velocity of the heat carrier (Fig. 1) has a substantial effect on the duration of drying.

The strength index and crushing resistance of peat (Table 4) depend very little on this factor, showing a tendency to decrease as the heat-carrier velocity increases above 1.15 m/sec, which is explained by the more severe drying conditions. At relatively low velocities the abrasion depends almost not at all on the heat-carrier velocity.

Consequently, up to a certain limit, it is advantageous to increase the heat-carrier velocity, since this makes it possible to shorten the drying time considerably without impairing the mechanical properties of the peat.

The indices of the mechanical properties of thermally treated molded peat, presented in Table 5, deteriorate as the heat-carrier velocity increases.

It is interesting to note that I. L. Lyuboshits [7] denies the influence of air-motion velocity within the range 0.5—2.0 m/sec on the drying rate in the moisture interval 50—80%. It is difficult to agree with this, although the contradiction may be only apparent and may be explained by the difference in the hydrodynamic regimes in our experiments and in the experiments of I. L. Lyuboshits.

Conclusions

1. It has been established that increasing the temperature and velocity of the heat carrier substantially reduces the duration of drying, but has a negative effect on the mechanical properties of peat.

2. The best results in terms of the mechanical strength of air-dry peat were obtained at a drying temperature of 80° and a heat-carrier velocity of no more than 1.15 m/sec.

Literature

1. Gebler I. V. and Smolyaninova N. M. The Problem of Metallurgical Fuel for the Use of Iron Ores of Tomsk Oblast. Publishing House of Tomsk University, 1959.

2. Myagkov N. A. Peat Coke. ONTI Publishing House, M.–L., 1936.

3. Kondakov V. V. “Oxygen,” No. 1, p. 35, 1945.

4. Sinyakin S. P. Drying and Harvesting of Sod Peat. Gosenergoizdat, Moscow, 1948.

5. Prokhorov N. I. Artificial Dewatering of Peat. Leningrad Book Publishing House, 1944.

6. Gebler I. V., Smolyaninov S. I., Martynov A. F., and Seve...in B. M. Influence of pressure and moisture on the properties of peat as metallurgical fuel. “Peat Industry,” No. 8, p. 16, 1959.

7. Lyuboshits I. L. Collection Peat in the National Economy of the BSSR. Publishing House of the Academy of Sciences of the USSR, Minsk, 1948.