ON THE QUESTION OF “FRACTIONAL” MELTING OF COAL ASH FROM THE BEREZOVSKY DEPOSIT OF THE KANSK–ACHINSK BASIN

I. K. LEBEDEV, A. S. ZAVORIN, S. K. KARYAKIN, G. K. PRIVALIKHIN

(Presented by the scientific seminar of the Department of Boiler Engineering and Boiler Installations)

Taking into account the enormous national-economic importance of developing the coals of the Berezovsky deposit as an energy fuel, a number of research groups (the All-Union Thermal Engineering Institute, Tomsk Polytechnic Institute, and others) began studies of the Berezovsky coals of sector No. 1. The results of the work carried out yielded a sufficient body of material for obtaining calculated data on the thermotechnical characteristics of the coal and the physicochemical properties of the ash. At the same time, a number of important features of the coals of the first sector were revealed: more than 40% of the coals have an ash content of no more than $A^c = 5\%$ [1]; the content of calcium oxide and magnesium oxide in the ash of these low-ash coals reaches 60%, and in individual cases is even higher, which accounts for the very high temperature characteristics of the ash [2]. In addition, in the course of determining the temperature characteristics, while observing the general regularity of their variation with chemical composition, along with difficulties in determining signs of deformation, other interesting phenomena were noted [2], which we also observed for low-ash coal from a trial batch of mine production.

Thus, heating coal ash with $A^c = 5$–$7\%$ in the temperature region before the state $t_1$ is accompanied by the outflow of an easily flowing melt from the lower part of the specimen. With a further increase in temperature, the specimen melts; the main melt formed in this process does not possess flowing properties and has the shape of a slightly flattened hemisphere. The external appearance of the cooled melt also indicates that the outflow of the easily flowing melt takes place before the onset of softening of the specimen (Fig. 1).

In some coal-ash specimens with an ash content of $A^c = 4$–$5\%$, before the temperature $t_1$ is recorded, swelling of the lower part of the specimen begins, while the upper part remains sharp and is preserved unmelted up to the temperature of the liquid-melting state. With a further rise in temperature above $t_1$, the solid upper part gradually sinks into the swollen base. After $t_3$ is reached, the melt does not spread over the plate.

Heating the ash over the entire ash-content interval $A^c = 4$–$7\%$ is accompanied by intense gas evolution, as may be judged from the noticeable decrease in the volume of the specimens and the escape of gas bubbles.

Attempts to explain the peculiarities of melting of the ash of low-ash Berezovsky coal and the phenomena accompanying it, on the basis of systematization of only external features, indicated a stage-by-stage formation of readily fusible eutectics. The different fluidity of the melts was explained by the presence in them of suspended crystalline inclusions carried out of the ash mass during formation of the liquid phase. Taking into account the staged character of liquid-phase formation during ash melting, which indicates selective melting of individual constituents, these phenomena were classified as “fractional” melting. However, the nature of these phenomena still remains unclear.

Meanwhile, “fractional” melting may have a decisive influence on resolving the question of the most rational organization of the furnace process when burning coals from the Berezovsky deposit. Here attention is drawn to the fact that certain signs of “fractional” melting, although manifested more weakly, were observed for some samples from the Nazarovo deposit at an ash content close to the minimum. Along with this, comparisons of Berezovsky coal and the more thoroughly studied coals of the Nazarovo and Irsha-Borodino deposits of the Kansk-Achinsk basin revealed a great similarity in the change of ash chemical composition with ash content [1]. It is known that the practice of burning Kansk-Achinsk coals at power stations has encountered difficulties in the operation of boiler units; therefore there are grounds for concern in this respect also regarding Berezovsky coal. An effective means of combating operating difficulties, verified by practice, has been the combustion of Kansk-Achinsk coals in furnaces with liquid slag removal, whose designs ensure, in the course of combustion, high-temperature treatment of the entire mass of ash.

Fig. 1. View of the cooled melt: 1 — readily flowing melt; 2 — main melt

However, Berezovsky coals cannot be assigned with complete confidence to those that can be burned without particular difficulty in such furnace devices. The high temperatures of the liquid-molten state of the ash will require, for satisfactory slag removal, very high temperatures at the slag tap, which is hardly feasible in existing designs of steam generators with furnaces of highly forced combustion. Therefore, at first glance, it seems expedient when burning low-ash Berezovsky coals to return to the use of furnaces with dry slag removal. But in this case the presence in the ash of a noticeable amount of readily fusible constituents will lead to slagging of the radiant heating surfaces. This may also be caused by an increase in the ash content of the fuel being burned, since a considerable part of section No. 1 is represented by coals with ash content above \(A^{c}=7\%\), corresponding to lower temperature characteristics. For these reasons, the successful use of dry slag removal when burning Berezovsky coals appears doubtful not only because of the formation of cohesive deposits on convective heating surfaces, but also because of slagging of radiant surfaces.

From all that has been set forth it follows that there is as yet no final, unequivocal decision on the applicability of existing furnace devices for burning low-ash coal, while obtaining such a decision is a primary task requiring the quickest possible resolution at the present stage of development of the Berezovsky deposit for energy use.

tion. Prerequisites for a positive solution of the question may appear only with a thorough study of the mineral part and of the melting of coal ash.

For this purpose, we propose to carry out studies establishing the composition and properties of the liquid phase that forms and of the unmelted solid residues; for this it is necessary first of all to solve the question of separating the melts from the main mass of ash. The first stage of this work was as follows: ash samples were heated in a cryptol furnace at the same heating rate as in determining the temperature characteristics, to temperatures from 1150 to 1400°C (at intervals of 50°C), and were divided into two parts—the upper and the lower. On the basis of the described picture of ash melting, one should have expected a distribution of the components of the chemical composition over the height. The change in the temperature characteristics for the upper and lower parts of the samples confirmed this and showed that selective melting begins to affect the distribution of the low-melting components between the “top” and the “bottom” from 1250°C. However, chemical analysis did not answer the question of the degree of participation of individual components of the chemical composition in “fractional” melting, and made it possible to establish only a certain increase in CaO for the “top” and a corresponding increase in SiO₂ for the “bottom,” beginning at 1300°C. No change in the content of other oxides is observed. Apparently, the distribution of the chemical composition is not manifested because of a possible increase, in the liquid phase, of refractory crystalline formations. This circumstance indicates, on the one hand, the imperfection of this method of separating the aggregate of fractions according to the criterion of melting and, on the other hand, the difficulty of carrying out such separation.

Fig. 2. X-ray diffraction pattern of the low-fluidity melt

The separation of the low-fluidity melt that melts out first did not present any difficulty in this respect, but its very small amount does not make it possible to carry out an analysis of the chemical composition. X-ray phase analysis (Fig. 2) made it possible to establish that iron compounds participate in the formation of this eutectic (the lines \(d/n = 1.483;\ 1.61;\ 2.10;\ 2.53\) belong to Fe₃O₄).

X-ray pattern 1 was obtained from a cooled melt that was present as a thin film on a platinum plate. X-ray pattern 2 shows the lines corresponding to platinum.

Thus, the first results of the study of the melting of ash from low-ash Berezovsky coal made it possible to conclude the following:

1. The presence of “fractional” melting of the ash in the temperature range before the onset of the state of initial deformation has been confirmed.

2. Iron compounds participate in the formation of the low-fluidity melt.

LITERATURE

1. N. V. Trikashnyi. Thermal-engineering characteristics of coals from certain deposits of the Kansk-Achinsk basin. Author’s abstract of dissertation, Tomsk, 1968.

2. N. V. Trikashnyi. Temperature characteristics of the ash of coals from the Berezovsky deposit of the Kansk-Achinsk basin and the possibilities of reducing them. In the collection: 1st Scientific and Technical Conference on Heat Exchange and Combustion. Krasnoyarsk, 1970.