PRELIMINARY RESULTS OF AN EXPERIMENTAL STUDY OF THE PROCESS OF PNEUMATIC SORTING OF RUN-OF-MINE COAL ON A CHAIN GRATE

Engineer I. A. Yavorsky and S. I. Shabanov

The burning of run-of-mine coals on chain grates is associated with a number of difficulties caused by the high content of fines in such coals.

It is known, for example, that the aerodynamic resistance of a bed of run-of-mine coal is much higher, and its permeability to air considerably worse, than that of any sorted coal.

The ability of coal to sinter or coke further increases the resistance of a bed containing fines, and makes the supply of oxygen to the coal particles still more difficult, owing to the partial or complete elimination of pores between the finest particles of coal.

Attempts to intensify combustion by increasing the blast pressure, as a rule, do not lead to positive results, since even with slight nonuniformity in the thickness or density of the bed of run-of-mine coal, the air destroys the bed at its weakest points, exposes the grate there, and beyond the limits of the craters piles up whole heaps of fine coal.

The same phenomenon may also arise as a result of nonuniformity in the sizes of the flow cross sections between the grate bars.

The formation of craters entails a large and useless breakthrough of blast air into the furnace and an excessive increase in the excess-air coefficient in it, and sometimes leads to a drop in pressure in the zones if the forced-draft fan does not have a sufficient reserve of capacity. At the same time, the already poor blowing of air through coal accumulated in heaps sharply deteriorates, which may lead to complete disruption of combustion.

All these phenomena are expressed the more strongly, the larger the open area of the grate and the smaller its aerodynamic resistance, since the air is redistributed inversely proportional to the resistances of the parallel sections. Thus, with a grate open area of 10%, the velocity of the air passing through the exposed parts of the grate is 6–10 times higher than the velocity of its passage through sections covered by a coal bed 165 mm thick.

Analysis of the causes of crater combustion also indicates the way to prevent both the phenomenon itself and its harmful consequences.

First of all, it is completely inadmissible to have any significant leaks in the seals or differences in the size of the gaps between the grate bars, which requires very careful manufacture and assembly of the grate surface.

It is also necessary to strive for the maximum equalization of the aerodynamic resistances of the idle and loaded grate, which can be achieved by increasing the resistance of the grate itself and reducing the resistance of the bed of run-of-mine coal.

As will be seen below, it is most expedient to increase the total resistance of the grate not by raising its resistance coefficient, but by increasing the velocity head. For example, in moder-

for the modernized grate design the resistance coefficient of the grate bars, in comparison with the BTsR-1 grate bars, even decreases, but, owing to the reduction of the free area to 6.2%, the share of the velocity head became \(^{2}/_{3}\) of the total absorbed by the grate.

With such a design, if the grate is exposed, its total resistance rapidly increases at the expense of the velocity head, limiting the breakthrough of air into the furnace.

The resistance of a layer of run-of-mine coal can be reduced by loosening the layer, i.e., by increasing its porosity coefficient¹, for example, by sorting the run-of-mine coal by size in such a way that the largest lumps lie directly on the grate, while the smaller ones are arranged above them in layers in descending order of lump size. In addition to a considerable reduction in the resistance of the layer and the prevention of crater formation, such sorting also ensures good washing of the coal lumps by air and, consequently, a high intensity of combustion.

To carry out such sorting, I. A. Yavorskii proposed a method providing for the use of forced-draft air at elevated pressure for this purpose.

The device for pneumatic sorting of run-of-mine coal directly on the grate is as follows.

Before the first zone of the grate a special sorting zone is arranged, the windbox of which is connected to a separate high-pressure fan. The width of the openings of this zone for standard BTsR grates is 100–120 mm.

The pressure in the zone is maintained such that, on one package of grate bars above the zone, sufficiently intense and uniform boiling of the fuel layer is ensured, that is, the movement of large lumps and the bouncing of smaller ones.

As the grate roller passes over the opening of the sorting zone, the air pressure under this package of grate bars gradually decreases and the boiling of the layer ceases. Naturally, as boiling ceases, the larger and heavier lumps settle onto the grate first, followed by increasingly smaller ones. The very finest coal particles, during boiling, are carried out of the layer into the separation chamber located above the sorting zone and separated from the furnace by a low overhanging threshold.

When the aerosuspension leaves the separation chamber for the furnace, the largest particles of suspended coal fall onto the layer of sorted fuel, and only particles with a diameter smaller than the mesh opening of No. 70 sieve are carried away with the air and burn in the flame above the layer.

It should be noted that pneumatic sorting is most easily carried out when using the aforementioned modernized grate, since its small free area and the execution of the inter-grate-bar channels in the form of converging nozzles promote the creation of large dynamic air heads that facilitate boiling of the layer.

To study the physical aspect of the phenomenon of pneumatic sorting of coal and to obtain design data for it, a whole series of experimental investigations of a layer of cold fuel on a chain grate was carried out at the Transport-Energy Institute of the West Siberian Branch of the Academy of Sciences.

The tasks of these investigations were reduced to the following:

1. Determination of the aerodynamic resistance of a layer of sorted coal as a function of the size of the coal particles, the porosity of the layer and its thickness, as well as the filtration velocity.

1. The same for a uniform mixture of fractions of several sizes at different weight ratios of the components. Experiments were also carried out with a layered arrangement of the components.

2. The same for run-of-mine coal of a definite fractional composition.

All experiments were carried out with coal from the Anzhero-Sudzhensk deposit; the preliminary separation of this coal into fractions of different size was performed by means of screens (in what follows, to designate a fraction, the diameter of the aperture of the screen on which the fraction is retained is indicated first, and then the diameter of the aperture of the screen through which the fraction passes completely).

As a rule, in each experiment the bed was brought to boiling, and after it ceased all the necessary characteristics of the bed were again recorded.

The experimental installation on which the investigations were carried out consisted of a stationary section of a natural chain grate of modernized design, beneath which there was a blast zone supplied by a fan.

Along the length of the section, two packages (12 pieces) of grate bars were laid, and across its width—one and a half grate-bar lengths. Above the grate bars, on three sides, the section was enclosed by vertical metal walls, and on the side where the grate bars had been cut in half, glass was installed tight against the cuts for observing the behavior of the coal in the bed.

To prevent air leakage, all gaps in the side seals and the spaces between grate bars around the perimeter of the test section were carefully sealed and coated with clay.

To exhaust the dust-laden air that had passed through the bed, the entire segment of the grate on which the test section was set apart was covered with a metal box connected to the flue of a nearby boiler.

The flow rate of the blast air was measured with a pneumometric tube installed in a straight section of the air duct from the fan and connected to a “Ksidin” microdraft gauge.

The pressure difference in the zone and above the bed was measured with an ordinary U-shaped liquid differential manometer.

The aerodynamic resistances of the bed were determined from the readings of this differential manometer, from which the intrinsic resistance of the grate was subtracted; the latter was periodically determined on the basis of special experiments with no coal on the grate.

The first series of experiments was carried out for a bed composed of any one fraction of screened fuel. Fractions of sizes 15–20 mm, 10–15 mm, 2–5 mm, and 1–2 mm were investigated.

The graph in Fig. 1 presents the dependence of the required pressure under the grate on the air-filtration velocity ^1) through a coal bed 150 mm thick for fractions of different coarseness, while the curves of Fig. 2 illustrate the dependence of the resistance of the bed on the mean size of the coal particles for different filtration velocities.

As is evident from these curves, the resistance of the bed increases as the size of the coal particles decreases, and the more rapidly, the higher the filtration velocity. At practically used filtration velocities, the resistance of the bed increases especially rapidly as the particle size decreases below 5–6 mm. Consequently, this size is the lower limit of coarseness at which coal can be classified as screened, especially since a bed composed of finer particles loses stability and boils at filtration velocities lower than those required for intensive combustion (Fig. 1).

^1) The filtration velocity is determined by the air flow rate in m³/sec falling on 1 m² of the grate surface area.

Fig. 1

Layer resistance as a function of the average lump size
(Layer thickness 150 mm).

Fig. 2

The influence of the thickness of the bed on its resistance is illustrated by the curves in Figs. 3 and 4. For the coarse fractions this dependence may be taken as linear; however, for the fine fractions, especially at increased filtration velocities, a deviation from the linear law is clearly seen. Apparently, this

Fig. 3

is explained as follows. At a filtration velocity of more than \(0.5\ \mathrm{m^3/m^2\,sec}\), movement of the finest particles begins within the bed. In this process the particles turn in such a way that the open cross-section of the bed increases.

Fig. 4

Evidence of this may be the fact that, as experiment has shown, for fine fractions the coefficient of resistance of the bed, beginning at a certain filtration velocity, decreases as this velocity increases.

The thicker the bed of coal, the greater its specific pressure on the underlying coal particles, caused by the weight of the bed. Therefore, as the thickness of the bed increases, movement of the coal particles and the aerodynamic

the resistance of the bed increases, and its dependence on the velocity approaches more and more closely a quadratic one.

The task of the second group of experiments included the study of the aerodynamic characteristics of a bed composed of a uniform mixture of several fractions taken in definite weight ratios.

In Figs. 5, 6, and 7, the solid lines show the dependences of the bed resistance on the filtration velocity for mixtures composed of equal weight quantities, respectively, of two fractions (1–2 mm and 2–5 mm), three fractions (2–5 mm, 5–10 mm, and 15–20 mm), and four fractions (1–2 mm, 2–5 mm, 5–10 mm, and 15–20 mm).

In the first and third of these experiments, the pressure under the grate was raised until the onset of stable fluidization of the bed (points on the curve marked with double circles), and after some time had elapsed it decreased to a minimum, then rose again and, finally, fell to zero. In the second experiment (Fig. 6), the pressure under the grate was continued to be raised even after the onset of fluidization, and then the same cycle was repeated as in the other two experiments.

Fig. 5

Observations and graphs show approximately the following picture of the phenomenon.

At the first increase in filtration velocity, stirring of the fine particles gradually begins in the thickness of the bed. At a filtration velocity quite definite for each mixture, this stirring passes over into stable fluidization of the bed.

Moreover, the greater the number of components in the mixture, i.e., the wider the range of particle sizes, the more violently fluidization of the bed begins and the more sharply the break in the curve in Figs. 5–7 is expressed.

After the onset of fluidization, over the course of 30–40 sec there is an increase in filtration velocity practically without an increase in pressure under the grate, which indicates a decrease in the resistance of the bed. This increase in velocity continues until the increasing intrinsic resistance of the grate and the reduced resistance of the fluidized bed come into equilibrium with the available pressure under the grate.

It should be noted that the modernized chain grate on which the experiments were carried out has the very valuable property of automatically preventing an excessive increase in air consumption during fluidization of the bed, since, owing to its small free cross-section, the total resistance of the grate increases rapidly with increasing air flow due to the velocity head. At the same time, the high velocities of the air emerging from the inter-grate gaps contribute to the destruction and intense fluidization of the bed.

Observations show that even before fluidization begins, but mainly after it, sorting of the bed by size begins, i.e., the pushing out onto the surface of the bed of the finer particles and the accumulation of coarse ones in the lower layers.

Fig. 6

When the air flow is reduced, fluidization of the bed gradually decreases. Stable fluidization ceases at almost the same filtration velocity at which it began, but at a considerably lower pressure beneath the grate (see the points on the corresponding curves marked by double circles).

In general, the pressure curve beneath the grate, with further reduction of the air velocity, lies much lower than the corresponding curve during the initial increase in velocity.

A repeated process of increasing the filtration velocity gives a certain increase in the resistance of the bed, which is apparently explained by some shaking and compaction of the bed during the process of reducing the velocity. However, repeated...

...the reduction in velocity gives almost the same curve as the first reduction.

The sorting of fuel during the process of boiling causes a large increase in the porosity coefficient and a corresponding decrease in the resistance of the bed. Thus, in the experiment with two fractions the porosity coefficient increased from 0.350 to 0.525, i.e., by 50%, and the pressure under the grate decreased by a factor of 1.51. In the experiment with four fractions the porosity coefficient increased from 0.342 to 0.468, i.e., by 41%, and the pressure under the grate decreased by a factor of 2.11. The decrease in pressure under the grate by a factor of 2.25 in the experiment with three frac-

Fig. 7

tions is not characteristic, since during the vigorous boiling of the bed a considerable quantity of fine coal particles was carried out beyond the limits of the installation.

In parallel with the experiments described, experiments were also carried out on blowing through a bed of fuel composed of the same fractions, but with each fraction arranged in layers on the grate (the coarser fraction at the bottom, and above it progressively finer ones). The results of these experiments are shown in the same Figs. 5–7 by dashed curves.

As can be seen from a comparison of the solid and dashed curves, during the first increase in filtration velocity the resistance for the layered arrangement of the fractions is less than for a uniform mixture, but greater than for the sorted bed. Apparently, this indicates that, during pneumatic sorting, the most complete separation of coal by size takes place, since this separation also occurs within each of the constituent fractions.

After the bubbling of the residual layer, its behavior differs almost not at all from the behavior of a layer made up of a uniform mixture of fractions.

In the third group of experiments, the behavior of a layer of run-of-mine coal was investigated. Figs. 8–10 give the results of experiments for run-of-mine coal of one and the same fractional composition, with layer thicknesses of 98, 165, and 205 mm, respectively.

The nature of the course of the processes in the layer of run-of-mine coal, as is seen from the curves in Figs. 8–10, in essence differs in no way from that for a mixture of fractions; only here all the phenomena are expressed more vividly. For example, the boiling of a layer of run-of-mine coal begins very violently and almost instantaneously

Fig. 8

spreads throughout the entire layer. The physical picture of this phenomenon, resembling an explosion, is approximately as follows. At the moment when the static pressure beneath the layer approximately balances the weight of the latter, the frictional forces between the coal particles are disrupted, and the finest of them are torn out onto the surface of the layer. The sharply reduced resistance of the layer resulting from this, and the rapid increase in air flow, further increase the intensity of boiling.

As for the mixture of fractions, it was found that even before the onset of boiling, within the thickness of the layer of run-of-mine coal there occurs stirring of the coal particles and movement of the finest of them from below upward. These coal particles are not able to pass through the whole layer through the small interpiece channels and become stuck in the upper part of the layer, which causes a gradual increase in the static pressures inside the layer. Ultimately this may lead to boiling of the upper part of the layer at a pressure beneath the grate lower than that necessary for normal boiling.

A similar phenomenon occurred in the last of the experiments mentioned in this group (Fig. 10), during which the rise of pressure beneath the grate was artificially stopped upon reaching 157 mm water column.

After 4 minutes, weak boiling began in one place of the layer, which then gradually spread over its entire surface.

However, as a result of such partial and weak boiling, the porosity coefficient of the layer increased by only 8%, whereas with sufficient

Run-of-mine coal of the Anzhero-Sudzhensk deposit
\(H_{\text{sl}} = 165\ \mathrm{mm};\ \gamma = 968\ \mathrm{kg/m^3}\)

Plot annotations: “fluidization”; \( \mathrm{m^3/m^2\,sec} \)

Fractional composition:

• \(>20\ \mathrm{mm}\) — 97%
• \(15\text{–}20\ \mathrm{mm}\) — 6.2%
• \(10\text{–}15\ \mathrm{mm}\) — 8.1%
• \(5\text{–}10\ \mathrm{mm}\) — 20.5%
• \(2\text{–}5\ \mathrm{mm}\) — 14.0%
• \(1\text{–}2\ \mathrm{mm}\) — 19.0%
• \(<1\ \mathrm{mm}\) — 20.5%

Fig. 9

Run-of-mine coal of the Anzhero-Sudzhensk deposit
\(H_{0} = 205\ \mathrm{mm};\ \gamma = 909\ \mathrm{kg/m^3}\)

Vertical-axis annotation: mm water column
Plot annotations: “fluidization”; \( \mathrm{m^3/m^2\,sec} \)

Fractional composition:

• \(>20\ \mathrm{mm}\) — 97%
• \(15\text{–}20\ \mathrm{mm}\) — 6.2%
• \(10\text{–}15\ \mathrm{mm}\) — 8.1%
• \(5\text{–}10\ \mathrm{mm}\) — 20.5%
• \(2\text{–}5\ \mathrm{mm}\) — 14.0%
• \(1\text{–}2\ \mathrm{mm}\) — 19.0%
• \(<1\ \mathrm{mm}\) — 24.5%

Fig. 10

in the case of vigorous fluidization, in the experiment with a bed thickness of 165 mm, the porosity coefficient increased from 0.281 to 0.397, i.e., by 41%. This phenomenon can be explained only by the fact that, under weak fluidization, sorting occurs only among the fine fractions, while the coarse fractions are sorted only partially. Consequently, increasing the intensity of fluidization of the bed improves the quality of its sorting. It is also quite obvious that good sorting of the bed requires a slow reduction of the pressure under the grate after fluidization.

It should be noted that in all these experiments, during fluidization of the bed there was a comparatively large carryover of fines beyond the limits of the installation, which is easily detected when the charge is weighed again.

Thus, it has been proved experimentally that pneumatic sorting can significantly improve the aerodynamic characteristics of a bed of run-of-mine coal.

Conclusions

1. As the experiments have shown, only fractions with particle sizes greater than 5–7 mm should be assigned to the category of sorted coals, since a bed of smaller particles has a high aerodynamic resistance and, even at low blasting velocities, loses its stability, i.e., begins to fluidize.

2. For coarse fractions, the aerodynamic resistance of the bed increases in direct proportion to its thickness. For a bed composed of fine particles, however, beginning at a certain filtration velocity, the resistance increases more rapidly than the bed thickness.

3. The experiment showed that, in the process of fluidization of a coal bed, it is sorted by size. The largest lumps are located at the bottom, while the finest coal particles are on the surface of the bed.

4. To achieve high sorting quality, it is necessary to ensure sufficiently vigorous fluidization of the bed and a slow cessation of it.

5. As a result of pneumatic sorting, the porosity of a bed of run-of-mine coal increases significantly, and its aerodynamic resistance decreases by a factor of 2–3.

As a result, prerequisites are created for the almost complete elimination of the difficulties encountered when burning run-of-mine coals in a bed (crater-like character and low intensity of combustion).

1. The coefficient of volumetric porosity is the ratio of the volume of voids to the total volume of the charge. ↩