SELECTIVE GRANULATION OF COAL BLENDS FOR COKING

V. M. Vityugin, G. D. Speshchi

(Presented by the scientific-methodological seminar of the Faculty of Chemical Technology)

Granulation as a method of compacting a coal blend was first proposed by staff members of the Institute of Fossil Fuels and the Institute of Metallurgy of the USSR Academy of Sciences [1].

Compaction of the blend makes it possible to increase the charge and, consequently, the productivity of coke ovens; moreover, this is achieved not only by increasing the bulk density, but also by reducing the yield of coke fines.

Granulation has a number of advantages over other methods of compaction, some of which give only a small effect, increasing the bulk density by only 2–5% (the introduction of microadditives, coarse crushing), while others (tamping, briquetting, vibration), which provide greater compaction, are distinguished by high cost and by the complexity of the equipment and its operation. It is for this reason that, in recent times, a number of research groups have carried out studies on the granulation of coal charges under various conditions [2, 3]. In addition, it has been shown that a stronger coke can be obtained from a compacted blend. The compaction of particles into granules leads to an increase in caking ability, similar to the addition of well-caking components, and reduces shrinkage during the coking process.

Compaction during granulation is achieved comparatively simply. We obtained granules with an apparent specific gravity of 1.1 g/cm³ (the bulk density of the coal blend was 0.75 g/cm³). In briquetting, for example, such a density is achieved under a pressure of several hundred atmospheres.

When granules are mixed with an ungranulated blend in a proportion that provides the greatest compaction, the bulk density of the charge increases on average by 10–15%. The use of hydrocarbon liquids or emulsions as a binder, instead of water, additionally increases caking ability and increases the yield of chemical products [3].

Stronger caking of compacted particles is most clearly manifested for weakly caking coals, which, during coking, yield little fusible material. This small amount acts more strongly, since it is sufficient to fill the reduced volumes of the intergrain voids. This leads to a decrease in the gas permeability of the mass, an increase in expansion pressure, and an acceleration of the development of the swelling process. There occurs a denser closing and bonding of the surface grains of the layers of the plastic mass; by the end of the period of the plastic state,

a denser and stronger semicoke is obtained, with smaller pores, the walls of which are then compacted still further by the products of high-temperature decomposition of the primary tar. The strong and rigid skeleton thus formed more strongly resists the change in volume during shrinkage. The coke obtained is more fissured, and the yield of fines increases.

When coking charges with a high content of weakly caking coals, the positive factor of increased caking capacity may prevail over the negative factor of increased fissuring, and the coke from such charges, after preliminary compaction, may be better in mechanical properties [4]. But it may also be the same as without compaction, or it may be worse. Precisely such results were obtained by the workers of the IGI at the Novo-Tula Metallurgical Plant, the Moscow Coke and Gas Plant, and the Nizhny Tagil Metallurgical Combine.

According to data from the Nizhny Tagil plant, where the work was carried out with a charge of production grinding, and 60–80% of the charge was granulated, granulation does not worsen, and in some cases even slightly improves, the quality of the coke. For example, the yield of coke of the +40 class for the ungranulated charge was 42.4%, while for the charge granulated by 57–80% it was from 42.1 to 45.4%.

According to data from experimental coking at the Chelyabinsk Metallurgical Plant and the Magnitogorsk Metallurgical Combine (granulation with fuel oil), with a content of 40 to 50% granules in the charge, the results for the determination of quantities characterizing coke strength are close to those obtained without granulation. The abrasion resistance of the coke was noticeably reduced, and its structural strength increased [3].

The purpose of the present work is to test the possibility of improving coke quality by selective granulation.

Since preliminary compaction has an unequal effect on the coking properties of coals of different grades—the caking capacity of weakly caking coals increases, while well-caking coals in this case give coke of reduced quality because of increased fissuring—it appears advisable to carry out granulation selectively, compacting only those coals, or that coal, on whose behavior during coking this has a favorable effect.

To determine the nature of the influence on coke quality of the granulation of individual components, experiments were set up with coals of production grinding used for preparing the charge at the Kemerovo Coke-Chemical Plant: OS—mine 9-15, G-sh. named after Kirov, K₂—“Fizkulturnik” mine, Zh—Chertinskaya Central Preparation Plant.

The first series of experiments was carried out on a plant charge, or one close to it in composition, according to the following procedure.

For granulation, a disk granulator was used with a pan diameter of 1 m, set at an angle of 47°, at a speed of 19 rpm. Either a mixture of all the constituents, or certain ones of them, were fed to the granulator. In the latter case the remaining constituents were mixed with the granulation product (were added to the charge) without additional moistening (at plant moisture content—7.5%).

The coals subject to granulation were moistened to approximately 14%. In addition to water, kerosene in an amount of about 1% was used for wetting.

The material from the granulator was screened into classes. The yield of the classes was determined. A sample was taken from the finished charge to determine mois-

ness. Then the charge was dropped in an amount of 20 kg from a height of 1.8 m, and its bulk density was measured.

For coking the charge, a welded metal retort of rectangular shape was used. The internal dimensions of the retort were: width—70 mm, length—148 mm, height—220 mm. The coking charge was taken in an amount of about 1.5 kg and, during charging, was compacted to the same degree as during dropping. The retort was inserted into an electric furnace preheated to 600°C. Coking lasted 5 hours.

The following determinations were carried out for the coke: yield by size classes, porosity, shatter strength by the Syskov method, and abrasion strength—according to the procedure developed by staff members of the Fuel Technology Laboratory of the Tomsk Polytechnic Institute.

The measurement results are given in Tables 1 and 2.

Table 1

Characteristics of the charges

The bulk density of the experimental charges in all cases proved to be greater than that of the plant charge. The greatest bulk density was obtained in experiment 4 (846 kg/m³). To a considerable degree this is explained by the reduced moisture content due to the largest amount of undercharge. The coke porosity for this experiment is the lowest. In general, with an increase in the charging density, the density of the coke increases.

In the experiments with selective granulation (2, 3, 4, 5), the increase in the bulk density of the charge is greater.

For the first test the bulk density is higher than for 6, despite the greater moisture content, evidently because the content of granules is closer to the optimum.

In the granulation process it was found that coal of grade OS granulates better. Accordingly, charges containing a larger amount of OS granules have a better granulometric composition.

Table 2

Characteristics of the coke

For all tests with selective granulation, the yield of coke of the +40 class is higher than for the 1st, and the greatest is in test 4.

The indices of mechanical strength of the coke are better for tests 3 and 5, when the largest amount of grade G coal, 38%, is granulated. The most important for metallurgical coke is the index characterizing abrasion. For all tests, except the last, with a decrease in abrasion the drop strength increases. The lowest abrasion index was obtained for the last test (without selective granulation). Incidentally, the yield of metallurgical coke for this test is also the highest, and the density of the charge is the lowest, despite the comparatively low moisture content. The latter is apparently explained by the smaller (in comparison with test 1) content of granules and of well-caking components.

The data of the proposed tests indicate the possibility of improving the mechanical properties of coke by means of selective granulation of certain constituents and in certain amounts.

Conclusions:

1. Work has been carried out to evaluate the quality of coke from selectively granulated coals for the Kemerovo Coke-Chemical Plant.
2. It has been shown that selective granulation provides greater compaction of the charge than ordinary granulation.
3. The yield of metallurgical coke is not thereby reduced.
4. The possibility of improving the mechanical properties of coke through selective granulation of coals has been shown.

LITERATURE

1. P. I. Kanavets, P. N. Melent’ev, A. E. Sporius et al. Trudy IGI, vol. XXII, 154, 1963.
2. N. S. Gryaznov, I. M. Lazovskii, M. G. Feldbrin. Collection of articles “Preparation of Coals for Coking,” Issue 2, 1960.
3. I. P. Bardin, P. I. Kanavets et al. Reports to the conference on problems of the fuel base of ferrous metallurgy of the USSR. Publishing House of the USSR Academy of Sciences, 1959.
4. G. I. Enik. Trudy IGI, vol. XII, 76, 1961.