THE INFLUENCE OF TECHNOLOGICAL FACTORS ON THE STRENGTH OF PEAT THERMAL BRIQUETTES

S. G. Maslov

(Presented by the scientific-methodological seminar of the organic departments of the Faculty of Chemical Technology)

In the literature on thermal briquetting of peat [1–9] there is no clear answer to questions concerning the role of various factors in the properties of thermal briquettes. In the present work, on the basis of the mechanism of peat thermal briquetting set forth in the preceding article [10], an attempt is made to fill this gap to some extent.

The temperature of heating the peat before pressure is applied. Heating in thermal briquetting is dictated by the need to reach a definite level of its thermal decomposition. The maximum strength of thermal briquettes is attained at a temperature that ensures the optimal quantitative combination of free radicals and liquid and plastic products. Above the optimal temperature, the number of saturated bonds decreases and, more importantly, secondary reactions lead the intermediate plastic products to such changes that the latter can no longer ensure the binding of peat particles into a strong briquette.

The increase in the optimal temperature in the case of high-rate heating [5, 7] of peat is caused by a shift of the thermal decomposition reactions of solid fuels toward higher temperatures. The sharp decrease in it when fuel-smelting materials are obtained by the thermal-briquetting method [6] is probably due to the catalytic action of iron ore and fluxes on the reactions occurring during the heating of peat.

Thus, the choice of the optimal temperature in solving technological problems must be linked with the kinetics of the processes of thermal decomposition of the organic mass of peat, which determines the decisive influence of such technological factors as the heating rate, the presence of catalyzing components, and also the time factor, especially in the interval between the moment at which the final heating temperature is reached and the moment at which pressure is applied. The time factor is of such great importance that it must be considered separately.

Holding time before pressure is applied. Under conditions in which the rate of heating of the fuel before briquetting is less than or equal to the rate of the processes of thermal decomposition of the organic mass, posing the question of the influence of the factor under consideration has no meaning, since the time interval between the end of heating and the application of pressure is reduced to a minimum. Since

In practice, this interval will always have a finite value; therefore, this circumstance must be taken into account when choosing the technological regime, depending on one or another hardware design of the process. Ultimately, this may be expressed in an adjustment of the optimum temperature of thermal briquetting. In the case of high-speed heating, the holding time of the peat heated to the final temperature before the application of pressure acquires the significance of a most important technological factor and, in turn, is connected with other parameters that determine the kinetics of the process (heating rate, temperature, catalytic effect of additives).

The holding time before the application of pressure is due to the need to attain a definite level of development of thermal-decomposition reactions. Under conditions of high-speed heating, the rate of heating of the particles is much higher than the rate of pyrolysis reactions; as a result, a time interval is needed for them to proceed sufficiently fully. In briquetting a peat–phosphorite mixture, owing to the catalytic action of mineral additives, this interval is shorter than in high-speed heating of pure peat.

When high-speed heating is used, the heating temperature of the peat may serve as a corrective parameter for the holding time.

Briquetting pressure. The application of pressure is necessary chiefly to achieve sufficient contact between particles of the solid residue and the liquid phase. When the pressure is increased to the optimum value, the increase in the number of contacts occurs mainly through the packing of grains and the realization of brittle and plastic deformations. With further compression of the briquetted mass, elastic deformations begin to predominate in it, while the realization of brittle and plastic deformations requires ever-increasing forces; the growth in the number of contacts, and with it the strength of the thermal briquettes, slows down. The decrease in strength observed in a number of cases when the pressing pressure is increased above a certain value can be explained by the physical squeezing-out of the liquid part of plastic products from the points and surfaces of contact of individual grains, as well as by an increase in the temperature of the charge particles above the optimum level.

The sharp reduction in the pressure required for thermal briquetting (200–300 kgf/cm²), as compared with the usual method of peat briquetting (1000 kgf/cm²), is due to the lubricating action of the liquid products of thermal decomposition, the plasticity of the solid residue, and also to a different mechanism of formation of the strong briquette structure. Zverev and Pushkarev [8] experimentally established that the specific friction force in briquetting peat heated to the thermal-briquetting temperature is 2.4 times less than in briquetting cold dried peat.

The reason for the higher value of the optimum pressure in thermal briquetting of peat of a low degree of decomposition is, in our opinion, the anisometric (fibrous) structure of its particles, which increases the magnitude of friction as they are brought together.

The necessity of applying higher pressures in producing peat–phosphorite thermal briquettes is caused by the increased amount of the nonfusible part, which intensifies the process of “self-attenuation” in solid fuels, i.e., reduces the fluidity of the liquid phase and makes it more viscous.

Holding time under pressure. The necessity of holding under pressure follows from the mechanism of pressure action in thermal briquetting set forth above. Strength has a time dependence; therefore, with increasing holding time, the number of transitions of elastic deformations of particles of the solid residue increases.

into brittle and plastic ones, i.e., stress relaxation occurs and the number of contacts increases. On the other hand, the time factor must play an essential role in the completeness of the chemical reactions that realize the plastic state.

Obviously, the greater the briquetting pressure, the faster the increase in the number of contacts due to deformation, and the higher the rate of the reactions realizing the plastic state; that is, the less time is required to obtain a specified strength of the thermal briquette. Hence the characteristic interrelation between the holding time under pressure and the magnitude of the specific pressing pressure.

Apparently, the greater value of the holding time under pressure for peat of a low degree of decomposition is explained by the same reasons as the need for a higher pressure when briquetting these peats.

Degree of peat comminution before heating. One of the basic propositions of the physicochemical mechanics of porous and fibrous disperse structures states: “Before obtaining a strong body, it is necessary to destroy the initial material and then join it again.” Destruction occurs mainly along existing defects. The greater their number, the more intensive the comminution must be.

In thermal briquetting of peat, the creation of a strong structure occurs mainly as a result of realization of the plastic state. But the finer the peat particles, the greater their surface area, and the more significant the amount of the liquid part of the plastic mass must be in order to bind them. The necessary degree of comminution in the case of thermal briquetting of peat is determined by the optimal combination of both of the above factors. This conclusion is confirmed by its dependence on the type of peat.

REFERENCES

1. Kh. I. Rivkina, Torfyanoe delo, 1934, No. 9, p. 28.

2. K. N. Witt. Briquetting of Wood Waste. Sverdlovsk—Moscow, Goslestekhizdat, 1935.

3. G. L. Petrovskii. Information Bulletin of the All-Union Scientific Research Geological Institute (VSEGEI), 1960, issue 36, p. 30.

4. V. I. Chistyakov, N. A. Basov. Scientific and Technical Information Bulletin of the All-Union Scientific Research Institute of the Peat Industry, 1961, issue 12, p. 30.

5. S. I. Smolyaninov, V. E. Voronin. Torfyanaya promyshlennost’, 1962, No. 4, p. 26.

6. S. I. Smolyaninov, S. G. Maslov. Izvestiya SO AN SSSR, Technical Sciences Series, 1964, No. 6, issue 2, p. 112.

7. S. M. Reprintseva. Thermal Decomposition of Disperse Solid Fuels. Minsk, “Nauka,” 1965.

8. D. P. Zverev, Yu. V. Pushkarev. Khimiya tverdogo topliva, 1967, No. 4, p. 56.

9. S. I. Smolyaninov, G. G. Krinitsyn, S. G. Maslov. Izvestiya TPI, vol. 196, Tomsk, TTU Publishing House, 1969, p. 143.

10. S. I. Smolyaninov, S. G. Maslov. Khimiya tverdogo topliva, 1971, No. 4, p. 76.