STUDY OF NATURAL DRYING OF PEAT FUEL-MELTING MATERIALS

S. I. Smolyaninov, G. G. Krinitsyn

(Presented by the Scientific-Methodological Seminar of KhTF)

The process of drying peat is an unavoidable operation in any method of its production. In obtaining lump fuel from raw peat mass, this stage of the technology is assigned an additional target load—the formation of a strong peat structure. The applicability of the theoretical foundations of drying developed by A. V. Lykov [1] has been demonstrated for peat systems by the works of M. V. Volarovich, G. N. Kuzman, N. V. Churaev, and others [2, 3].

In connection with research in the field of obtaining fuel-melting materials based on machine-formed peat, we conducted experiments on the natural drying of peat and its mixture with iron ore (TPM), whose purpose was, above all, to reflect the features of peat drying in the presence of iron oxides. During the drying process, from the moment of molding until a moisture content of about 20% was reached, the following were determined: the weight, size, and volume of the samples, their moisture content and moisture gradient, temperature at different points of the sample, the dynamics of crack development, the amount of bound water, and the ultimate shear stress.

Peat from the Tagan deposit of Tomsk Oblast was taken for the work. The peat was hypnum-sedge lowland peat, degree of decomposition about 25%, working moisture content—84.8%, ash content on dry matter—9.4%, yield of volatile matter on combustible mass—62.0%. As the iron-ore component, magnetite concentrate from the Abagur concentrating plant was used, ground to pass through a 0.075 mm sieve. The initial peat was mixed with iron ore in a ratio of 100:6 and passed three times through a meat grinder. Cylindrical specimens, 40 by 60 mm, were dried on polystyrene boards. Moisture content was determined by the accelerated method [4], temperature—with thermocouples using a potentiometer at 1 mV. Volume was determined by measuring linear dimensions and volumetrically by the method of Wüst and Ott [5], the dynamics of crack development by the method used for coke quality control [6], bound water—dilatometrically and by the method of negative adsorption of sugar [7], and ultimate shear stress—by a conical plastometer [8] and by the Rehbinder method.

When considering drying kinetics, questions of structure-forming processes may be clarified and determined, since the change in local moisture content and local temperature over time depends on the interrelated mechanism of moisture and heat transfer within the moist material and on mass and heat exchange at the surface—

of the body with the surrounding medium, which is determined by the nature of the bond of water with wet bodies. Therefore, the kinetics of the drying process, in turn, is determined by the physicochemical properties of the drying material itself.

For the analysis of drying processes, of great importance is the construction of drying-rate curves according to A. V. Lykov, which have a characteristic form with four clearly distinguishable drying periods and critical points determined by different mechanisms of mass and heat transfer.

The curves are plotted in coordinates of material moisture content—drying rate. The latter is determined from the curve of change in moisture content by graphical differentiation and is expressed in units of mass of water per unit surface per unit time.

As can be seen from Fig. 1, the drying-rate curves have a characteristic form. It should be noted, however, that the period of constant drying rate is shown by us only conditionally, since the experimental data do not provide sufficient grounds for isolating this period.

The periods of increasing and decreasing drying rates are clearly demarcated; moreover, the latter can also be divided into two stages. Quite unexpectedly, an initial period of decreasing drying rate was revealed, amounting in time to one third of the entire duration of the experiment. Taking into account that the peat and TFM samples were placed for drying immediately after molding, it may be assumed that this section must be characteristic of all disperse systems capable of forming coagulation structures with immobilization of moisture. As our experiments on the kinetics of development of coagulation structures in raw peat mass (without drying) showed, it is precisely during this time that they reach their maximum development.

According to A. V. Lykov, an increasing drying rate is observed at the initial moment and corresponds to warming of the material. In our case, the stage of increasing drying rate obviously cannot be explained from these positions.

On the basis of the properties of liquid in capillaries, N. A. Nasedkin [5] supposes that evaporation of free water will occur in the greater amount the higher the dispersity of the peat and the more it is compressed. Evaporation of water from the surface leads, according to N. A. Nasedkin, to an increase in the curvature of the meniscus, which increases the evaporation surface and, consequently, at the initial stage a section of increasing drying rate is observed.

The explanation of the growth of the drying rate of peat at this stage by an increase in the evaporation surface most likely corresponds to reality. However, in our opinion, this phenomenon must be explained not by an increase in the curvature of the menisci, but by a decrease in their radius with a simultaneous increase in their number. The latter conclusion follows directly from the works of M. P. Volarovich and N. V. Churaev [2], who showed that, upon compression of peat, its active porosity and kinetic specific surface increase, the average sizes of water-conducting pores decrease, but their number increases; this should cause an increase in the specific evaporation surface.

What has been said will be valid in the case when the rate of shrinkage corresponds to the rate of moisture loss; and if this condition is met, the drying stage under consideration will apparently be typical for all capillary-porous colloidal bodies.

In this case, the growth of the drying rate must cease at the moment when, on the surface of the material, a moisture content is reached that corresponds to the complete loss of capillary moisture. In the work cited,

According to M. P. Volarovich and I. V. Churasev, on the basis of a generalization of material from the work of other researchers, the value of the moisture content corresponding to the noted point is taken as equal to \(0.5\ \mathrm{g/g}\), or about \(33.3\%\) relative.

Analysis of our experimental data on the distribution of moisture in the concentric layers of cylindrical specimens fully confirms this proposition.

Finally, a sharp decrease in the drying rate begins after the capillary moisture has been lost by the entire mass of the specimen (Fig. 1).

It is also evident from the same figure that the increase in the drying rate ceases for peat when a moisture content of about \(55\%\) is reached, and for FMM, \(50\%\). From the standpoint of the mechanism considered, this difference is readily explained. Figure 2 presents the results of calculating the mean moisture gradient along the radius of peat and FMM specimens at different drying times. The mean moisture gradient was calculated as the ratio, at each given moment in time, of the difference between the moisture content of the material at the center and at the surface (the latter was found by graphical extrapolation) to the radius of the specimen. Hence it is clear that the noted difference in the times at which the period of increasing drying rate ends is due to the difference in the moisture gradient of the peat and FMM specimens.

Fig. 1. Dependence of the drying rate of peat specimens (curve 1) and fuel-melting materials (curve 2) on moisture content

As A. V. Lykov [1] indicates, the drying periods can be distinguished more accurately by the change in the temperature of the materials as a function of moisture content. Our experiments showed that, under conditions of natural drying in the open air, this method cannot be used because of temperature changes that cannot be strictly accounted for.

Thus, analysis of the experimental material makes it possible to draw the following conclusion regarding the features of the process of natural drying of fuel-melting materials in comparison with pure peat. Drying of FMM proceeds more intensively than drying of specimens of pure peat over the greater part of the complete cycle. At the maximum value, the drying rate of FMM specimens \((\mathrm{mg}/\mathrm{cm}^{2}\cdot\mathrm{h})\) is 1.5 times higher than that of pure peat specimens; however, at a moisture content of less than \(30\%\), the drying rates of both materials are equal. The distinctive feature of FMM drying, equal

Fig. 2. Dependence of the moisture gradient along the radius of cylindrical samples of peat (curve 1) and fuel-smelting materials (curve 2) on moisture content. The moisture content of the FSM is recalculated to peat.

...which, like the characteristic drying periods in both cases, can be explained from the standpoint of the views being developed on the mechanism of structure-forming processes during the drying of disperse systems.

During the drying of FSM, throughout the entire period smaller moisture gradients are observed in comparison with peat, which leads to less cracking of the fuel-smelting materials.

Judging from the change in the content of bound water and the limiting shear stress, the introduction of finely ground iron ore changes the nature of the course of structure-forming processes toward an acceleration of the processes of compact coagulation and a decrease in the average radius of the capillaries in the peat system; moreover, shrinkage phenomena not only are not inhibited by the presence of the mineral filler, but, on the contrary, the rate of shrinkage increases noticeably.

All this, taken together, has a positive effect on the quality of the final product.

The amount of hydration-bound water in peat, determined from the curves of the dependence of the thermogradient coefficient on moisture content, is 40% for peat [2]. This same value of moisture content, corresponding to the second critical point on the drying-rate curves (Fig. 1), is 33% for peat and 22% for FSM. The difference is about 11%. Similar values were obtained when determining bound water by the dilatometric method (14%) and by the method of negative adsorption of sugar (11%).

Conclusions

1. Drying of peat samples with the addition of iron ore proceeds more intensively over the greater part of the full cycle.

2. Characteristic periods of increasing drying rate and the initial period of decreasing rate have been shown; these are explained by the occurrence of structure-forming processes in the raw peat mass and during the drying period corresponding to the removal of capillary-bound moisture.

Literature

1. A. V. Lykov. Theory of Drying. Energiya Publishing House, Moscow, 1968.

2. M. P. Volarovich, N. V. Churaev. Study of Peat Using Radioactive Isotopes. Publishing House of the Academy of Sciences of the USSR, Moscow, 1960.

3. G. I. Kuzhman. Theoretical Foundations and the Process of Obtaining Fine-Lump Peat Fuel for Energo-Gas-Chemical Use. Gosenergoizdat, Moscow–Leningrad, 1961.

4. S. I. Smolyaninov, V. N. Ponomarev, V. I. Makarenko. Accelerated method for determining the moisture content of solid fuels. Published by TPI, vol. 167, pp. 114–117, 1967.

5. N. N. Kulakov. Introduction to the Physics of Peat. Gosenergoizdat, Moscow–Leningrad, 1947.

6. L. D. Gluzman, I. N. Edelman. Laboratory Control of Coke-Chemical Production. Metallurgizdat, Moscow, 1957.

7. F. D. Zhitenkaya. Methods for determining bound water in peat. In: New Physical Methods for the Study of Peat. Gosenergoizdat, pp. 138–148, Moscow–Leningrad, 1960.

8. M. P. Volarovich, S. I. Markov. Determination of ultimate shear stress by the conical plastometer method. In: New Physical Methods for the Study of Peat. Gosenergoizdat, pp. 95–101, Moscow–Leningrad, 1960.