RESULTS OF A STUDY OF THE INITIAL MINERAL MATTER OF BEREZOVSKY COAL

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

(Submitted by the scientific seminar of the Department of Boiler Construction
and Boiler Installations)

The study of the mineral matter of Berezovsky coal, as well as the features of its distribution, is important from the standpoint of predicting slag behavior and the formation of deposits during the combustion of this coal in the furnaces of large steam generators. For this purpose, we investigated the distribution of mineral matter throughout the entire mass of pulverized Berezovsky coal by means of separation in liquids of different specific gravity.

Coal dust, prepared by grinding coal with an ash content of 5.65% in an unventilated rod laboratory mill, was subjected to separation in liquids of different specific gravity. The size characteristic of the initial dust is shown in Fig. 1. Separation was carried out in a Hungarian-made centrifuge. The number of revolutions of the centrifuge was 2500–3000, depending on the amount of sample loaded. The centrifugation time was 10 minutes. Separation of the main mass of coal substance was carried out in a liquid with a specific gravity of 1.40 g/cm³ (a mixture of carbon tetrachloride with benzene). To improve the separation of heavy mineral impurities, a liquid with a specific gravity of 1.60 g/cm³ (carbon tetrachloride) was used. Separation of heavy mineral impurities was carried out in two liquids with specific gravities of 2.28 and 2.86 g/cm³ (a mixture of bromoform with alcohol). To determine the effect of dust particle size on the separation process, the fraction with a specific gravity greater than 1.40 g/cm³ was sieved into four groups of size fractions (greater than 88, 88–55, 55–46 μm, and less than 46 μm). The size characteristic of the fraction with a specific gravity greater than 1.40 g/cm³ is also given in Fig. 1 (curve 2). Further separation was carried out for each group of size fractions, with determination of the amount of the separated weight fraction. Thus, five groups of weight fractions were obtained (less than 1.40; 1.40–1.60; 1.60–2.28; 2.28–2.86 g/cm³ and greater

Fig. 1. Size characteristics of dust: 1 — initial sample; 2 — fraction heavier than 1.40 g/cm³

2.86 g/cm³) of four sizes (less than 46, 46–55, 55–88 µm and more than 88 µm).

From the distribution curves of the weight fractions (Fig. 2) it follows that coal dust of this ash content consists mainly of light fractions (the total amount of fractions with a specific gravity up to 1.60 g/cm³ is about 99%). Among the weight fractions with a specific gravity greater than 1.60 g/cm³, whose amount was about 1%, the greatest yield falls on the fraction with a specific gravity of 2.28–2.86 g/cm³. Visual and microscopic observations make it possible to conclude that the named weight fractions consist of quartz and clay minerals; coal matter is completely absent. The color of the fractions is grayish-yellow, and upon

Fig. 2. Distribution of dust by weight fractions: 1—particles greater than 88 µm; 2—88–85 µm; 3—55–46 µm; 4—less than 46 µm; 5—total amount

heating to 800°C it changes to pinkish-red. The ash content is 96–98%. These fractions are the principal impurity minerals that are not associated with the organic mass of the coal. The amount of weight fractions with a specific gravity greater than 2.86 g/cm³ is insignificant. They consist exclusively of pyrite and marcasite. Fractions with a specific gravity of 1.60–2.28 g/cm³ consist of intergrowths of coal matter with mineral impurities. The ash content of these fractions is 45.50–82.10%. The light fractions with specific gravity less than 1.40 and 1.40–1.60 g/cm³ consist mainly of coal matter having an ash content of 4.44–6.42%.

Figure 3 presents the distribution of ash in the weight fractions, recalculated per 1 kg of fuel. The recalculation was carried out according to the formula

\[ A=\frac{A^{c}\cdot G}{100},\ \%, \]

where \(A\) is the amount of ash in the fraction of the given specific gravity, %;
\(A^{c}\) is the ash content of the given fraction, %;
\(G\) is the amount of the given fraction in the coal, %.

From the curve in Fig. 3 it follows that the amount of ash present in the light fractions of the coal constitutes the predominant part (about 4.0%). Consequently, it may be assumed that the value 4.0% is the minimum ash content for coals of the Berezovsky deposit.

Any increase in the ash content of coal can occur only through the introduction into the coal of various rocks during mining, transportation, and storage.

Analysis of the influence of the size of coal particles on separation shows that the separation process depends to some extent on the size of the separated particles.

Fig. 3. Distribution of ash by weight fractions

Fig. 4. Temperature characteristics of ash

of the separated particles. From a comparison of the course of curves 1 and 2 in Fig. 1 it follows that, with a decrease in the size of the particles of the coal dust being separated, as a result of cohesion forces, particles of one specific gravity become entrained by particles of another, depending on the quantitative ratio between them. However, at sizes of 46–65 \(\mu\)m the curves run parallel. Conse-

Consequently, in the range of these sizes the most complete separation occurs. This is also confirmed by the close arrangement of the curves in Fig. 2.

The study of the melting of size and density fractions makes it possible to conclude that the temperature characteristics of the ash are variable and depend both on particle size and on their specific gravity. Figure 4 presents the temperature characteristics of the ash of the initial sample (solid lines) and of the density fraction with a specific gravity greater than \(1.40 \, \text{g/cm}^3\) (dashed lines) as a function of particle size. It is evident that the temperature characteristics of the ash of the fraction heavier than \(1.40 \, \text{g/cm}^3\), although they increase with increasing particle size, are nevertheless significantly lower than those of the initial sample. Consequently, removal of part of the light fractions from the coal leads to a decrease in the ash-fusion temperatures.

During pulverized combustion of coal in the furnace chamber, a process of natural separation of coal particles occurs both by specific gravity and by size. It is most probable that the heavier fractions will enter the slag, while the lighter fractions will be carried into the gas ducts. Consequently, the composition and properties of the slags will be determined by the interaction of only part of the mineral impurities of the coal. At the same time, as shown above, the slag-fusion temperatures will decrease in comparison with the initial ash. This circumstance may create favorable conditions for liquid slag removal during the combustion of Berezovsky coal.

From all that has been set forth, the following conclusions may be drawn:

1. Separation of Berezovsky coal dust occurs in all ranges of the liquids used.

2. Density fractions of coal with a specific gravity of \(2.28\text{--}2.86 \, \text{g/cm}^3\) have been separated; they constitute the main part of the admixed minerals.

3. The quantitative distribution of Berezovsky coal dust by specific gravity has been established, which will subsequently make it possible to determine the distribution of the mineral portion throughout the entire mass of the coal.

4. The ash-fusion intervals of size and density fractions of coal have been established, from which it follows that the temperature characteristics of the ash depend both on particle size and on specific gravity. Consequently, presenting the temperature characteristics in the form of average values for the initial laboratory ash cannot reliably enough reflect the behavior of the ash and slag during combustion of coals from the Berezovsky deposit.