QUANTITATIVE ASSESSMENT OF MELT YIELD DURING HEATING OF BEREZOVSKY COAL ASH

I. K. Lebedev, A. S. Zavorin, G. K. Privalikhin,
S. K. Karyakin

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

The identification of the laws and mechanism of selective melting of fractions during the fusion of ash from low-ash Berezovsky coal depends on the success of separating the fluid melt from the solid phase. The searches we conducted in this direction led to the adoption, as the basis for solving the problem posed, of a method for melting ash samples on porous supports. Similar methods for studying partially melting mixtures were used by F. Chedik-Eisenberg, as well as by Ya. I. Olshanskii [1]. The physical essence of the method we used consists in the capillary suction of the low-melting eutectic mixture formed during heating of the sample and flowing to its base.

According to this method, the sample is placed on a porous support, which, on a platinum plate, is put into a heating cryptol furnace. From the change in the weight of the sample and the support during heating to a specified temperature, one can make a quantitative assessment of the migration of the fluid melt into the capillaries. Separate study of the solid unmelted residues and of the removed melt, obtained at various temperatures, in conjunction with data on the amount of meltings, makes it possible to determine the composition of the liquid and solid phases, as well as the sequence of melting of the mineral constituents by ordinary methods of investigation (optical, chemical, spectral, X-ray structural analyses).

The results presented here for separation of the liquid phase were obtained during the melting of Berezovsky coal ash samples with ash content \(A^c = 5.50\%\) on porous supports made of chemically pure magnesium oxide and calcium oxide. It should be noted that the intense gas evolution occurring during heating of the samples is accompanied by a change in the intrinsic weight of the supports, which has no definite regularity; this, apparently, is explained by the instability of the composition of the reducing atmosphere of the furnace and the impossibility of maintaining a constant treatment regime. Therefore, in order to avoid superposition of the irregular change in support weight on the gain in weight due to absorption of the liquid phase, the supports were preliminarily calcined at the same temperatures as those used in heating the samples. In addition, heating of each sample was carried out together with a “blank” support, from the change in weight of which a correction for gas evolution of the sample was introduced. Heating

was carried out up to a temperature, beginning from 1000 to 1400°C at intervals of 50°C and then up to 1550°C at intervals of 25°C. The absence of results obtained at temperatures above 1550°C is explained by the difficulty of separating the very small mass of unmelted residue from the support. For greater reliability, the yield of phases at each temperature was determined from 8–10 samples. Graphical dependences of the change in phase yield during heating of the ash on temperature are presented, as percentages of the sample weight without taking the gas phase into account, in Fig. 1. The results were obtained at the moment the indicated temperatures were reached, i.e., without holding over time, which is related to the resolving power of the heating apparatus.

Fig. 1. Phase yield as a function of temperature: 1 — solid phase; 2 — liquid phase; — support of MgO; — — — support of CaO

Fig. 2. Phase yield during heating of ash of weight fractions: — liquid phase; — — — solid phase; 1 — ash of the original fuel; 2 — ash of the fraction with specific gravity 1.40 g/cm³; 3 — ash of fractions with specific gravity 1.40–1.60 g/cm³; 4 — ash of fractions with specific gravity 2.28–2.86 g/cm³

Samples calcined on magnesium oxide are easily separated from the support, which indicates a sufficiently complete absorption of the melts. The high capillary properties of MgO supports are also confirmed by comparison of the curves shown. Therefore the separation carried out on these supports should be considered the closest to reality.

The analysis we carried out of the temperature characteristics of the ash of different weight fractions of coal dust showed that the fusibility properties of the ash of these fractions differ significantly from one another. Consequently, it may be expected that the behavior of the ash of the original fuel will not reflect the essence of the processes occurring during coal combustion.

To assess the possibility of participation of individual groups of mineral constituents in “fractional” melting, separation of the melt-forming and refractory parts during melting of the ash of coal-dust fractions with specific gravity 1.40; 1.40–1.60 and 2.28–2.86 g/cm³ was carried out by the method of capillary suction during heating on magnesium-oxide supports (Fig. 2).

The results of phase separation during melting of the ash of the density fractions are presented recalculated to the solid and liquid phases. The yield of the gaseous phase is of independent interest and requires more detailed study.

Here attention is drawn to the fact that melting of the ash of fractions with a specific gravity of 2.28–2.86 g/cm³ differs sharply from the melting of the ash of other fractions and of the original fuel, and is characterized by the absence of melts up to 1425°C. After this, rapid melting begins. The course of the curves fully agrees with the melting interval of the ash of this fraction observed in determining the temperature characteristics, which cannot be said of the ash of fractions with a specific gravity of less than 1.40 and 1.40–1.60 g/cm³, or of the ash of the original fuel.

The arrangement of curves 2 and 3, analogous to the distribution of phases during heating of the ash of the original fuel (curve 1), with shifts corresponding to their temperature characteristics, indicates selective melting.

Fig. 3. Change in sample height during heating: 1—on a porous support; 2—on a platinum support.

Analysis of these results makes it possible to conclude that the phenomenon of “fractional” melting of the ash of Berezovsky coal is caused by the mineral matter associated with the organic mass of the fuel, while introduced minerals play no significant role. This is also confirmed by the fact that “fractional” melting is readily detected in the ash of low-ash Berezovsky coals and appears in some samples of Nazarovsky coal at an ash content close to the minimum for this deposit.

It should be noted that samples from the ash of the original dust, as well as fractions with specific gravity of 1.40 and 1.40–1.60 g/cm³, when heated on porous supports, retain their initial shape and sharp edges up to temperatures corresponding to state $t_2$, determined on a platinum support according to GOST 2057—60. At the same time, only in exceptionally rare cases are signs of deformation observed that correspond to $t_1$.

The melting process is characterized by the change in sample height shown in Fig. 3.

Apparently, the melting mineral constituents, when interacting with the refractory part, form eutectic mixtures that distort the melting picture (curve 2), while the amount of melt in contact with the remaining mass of the sample affects the change in fusibility.

Conclusions

1. Introduced minerals do not affect the “fractional” melting of ash. This phenomenon is determined by the mineral part associated with the organic mass of the fuel (primary and secondary ash).

2. The observed features of ash melting in low-ash Berezovsky coal make it difficult to apply the existing method for determining temperature characteristics according to GOST 2057—60 to ash that exhibits signs of “fractional” melting.

References

1. V. Eitel. Physical Chemistry of Silicates. Translated from English. Moscow, Foreign Literature Publishing House, 1962.