PROCEEDINGS
OF THE TOMSK ORDER OF THE RED BANNER OF LABOR POLYTECHNIC
INSTITUTE named after S. M. KIROV

Volume 136
1965

INFLUENCE OF HEATING RATE AND CHARGE COMPOSITION ON COKE STRENGTH

N. M. SMOLYANINOVA, Yu. D. YUKHNOVETS

(Presented by the Scientific-Methodological Seminar of the Chemical-Technological Faculty)

The quality of coke depends both on the composition of the charge and the conditions of preparing the coal for coking, and on the technological conditions of the coking process, among which the rate of heating of the coal in the coke oven chamber probably plays a decisive role.

The influence of chamber width on coke quality is determined precisely by the different rate of temperature rise at different points across the width of the coked charge. With an increase in the heating rate, the caking capacity of coals increases, and the dynamics of gas evolution also changes, which affects both fissuring and the structural strength, or the strength of the porous body of the coke.

In most works aimed at determining the influence of the coal-heating rate on the mechanical properties of coke, either the average heating rate was taken into account—which has nothing in common with the actual rates of temperature rise at each given point of the coked charge—or the investigations were carried out under heating regimes that do not occur at all under production conditions with the existing coking technology.

All of the above is of special importance when introducing wide-chamber ovens into production practice, as well as charges with an increased content of gas coals.

In this connection, we have carried out an investigation of the influence of heating rate and charge composition on coke quality.

Four charges were adopted for the tests: two of them were proposed by the Kuznetsk branch of VUKhIN as promising ones; one was the current coke-chemical production charge of KMK; and one charge was composed by us in order to more clearly reveal the influence of additions of gas coals.

The composition and characteristics of the charges are given in Tables 1 and 2.

Table 1

It is known that the temperature regime of heating of the coked charge at different points across the chamber width differs because of the low thermal conductivity of coal, the presence of moisture, and the change in properties during the coking process. This causes coke with different properties to be obtained depending on the position of the piece in the charge with respect to the height and width of the coke chamber. At identical mean coking rates, this difference may reach considerable values—from 17 to 0.2 deg/min.

Table 2

In order to bring the heating regime as close as possible to that existing in an industrial coke oven, and also to determine the influence of the true rates of temperature rise in the coal charge on the strength of coke across the chamber width, we carried out experimental cokings according to regimes corresponding to the layer adjacent to the chamber wall, to layers located at distances of 60 and 120 mm from the wall, and to the center of the charge. The indicated regimes, calculated from curves of the dynamics of temperature change across the width of the coal charge of a coking chamber¹, are given in Table 3.

Table 3

I — at the wall;
II — 50–60 mm from the wall;
III — 130–140 mm from the wall;
IV — at the center of the charge

Coking was carried out in a special steel cassette. To ensure the greatest possible uniformity of heating of the entire charge through the layer thickness, the width of the cassette was selected to be as small as possible (20 mm), sufficient only for obtaining pieces of coke of the dimensions needed to determine structural strength by the adopted method. The remaining dimensions may be arbitrary, depending on the dimensions of the oven and the required quantity of coke.

The production-ground charge was always loaded into the cassette with a constant bulk density of 750 kg/m\(^3\). The heating regime was monitored by means of a thermocouple placed at the center of the coal charge.

To prevent oxidation, which was observed during coking under conditions III and IV as a result of the prolonged residence of the coal at a temperature of 373°K, the indicated experiments were carried out in a nitrogen atmosphere, and the charge cassette was covered from above with pulverized charcoal.

Upon reaching the final temperature, the cassette was removed from the furnace, and the coke was quenched with water.

For all the coke samples obtained, the strength in the hot state at temperatures of 293, 673, and 1073°K was determined by the method developed at the Department of Chemical Technology of Fuel, TPI¹, and the porosity was also determined.

The data obtained are presented in Figs. 1 and 2 in the form of graphs of the dependence of coke strength on the temperature of secondary heating for different charges and coking conditions, and also in the form of the dependence of coke strength at a given temperature on the position of the piece across the width of the chamber (Fig. 3). Fig. 4 shows the change in coke porosity across the width of the chamber.

As can be seen from the graphs (Figs. 1, 2), the dependence of the structural strength of coke on the temperature of secondary heating is expressed, for all conditions and charges, by curves with a characteristic minimum of abrasion resistance in the temperature interval 673—873°K.

The reason for the increase in structural strength is, as indicated earlier, the reduction of internal thermal stresses arising in the coke during its rapid cooling in the process of wet quenching.

The dependence of coke strength on the composition of the charge is more complex. Thus, the abrasion resistance of coke obtained from charges 1, 2, and 3 under coking condition I decreases in the temperature interval 293—723°K with an increase in their content of gas coals. However, at temperatures above 873°K, the coke from charge 1 (18% gas coal) has the lowest abrasion resistance, the coke from charge 3 has the highest, while charges 2 and 4 occupy an intermediate position. This fact once again emphasizes the fundamental shortcoming of methods for testing coke strength only in the cold state.

Samples obtained from charges 1, 2, and 3 under condition II have practically the same structural strength throughout the entire tested temperature interval. A similar picture is also observed when these charges are coked under the fourth (IV) condition. However, the abrasion resistance of the coke here is much higher than under coking according to the second (II) condition.

As for charge 4, under condition II it yielded coke that was much more readily abraded in comparison with all the other conditions.

Under the fourth condition, from this same charge, in the temperature interval 293—723°K, the strongest coke was obtained. However, at higher testing temperatures its structural strength drops sharply (by 10—13%) in comparison with coke samples obtained from the other charges.

Under the third (III) condition, the abrasion resistance of all samples upon heating to 600° changes only slightly (on average by 2—3%), but with a further increase in temperature the structural strength decreases sharply (by a factor of 2) in comparison with the strength at 293°K.

Fig. 3 shows a typical change in the structural strength of coke obtained from charge No. 1 across the width of the chamber at different

Fig. 1. Dependence of coke strength in the hot state on the secondary-heating temperature for charge No. 2 (solid lines) and charge No. 3 (dashed lines) under different coking regimes (Roman numerals by the curves).

Fig. 2. Dependence of coke strength in the hot state on the secondary-heating temperature for charge No. 1 (solid line) and charge No. 4 (dashed line) under different coking regimes (Roman numerals by the curves).

Fig. 3. Dependence of coke strength (from charge No. 1) in the hot state on the position of the piece in the coal charge (distance from the chamber wall) at different temperatures of secondary coke heating: 1—293°K, 2—673°K, 3—1000°K.

Fig. 4. Dependence of coke porosity from different charges (numbers at the curves) on the coking regime (distance from the chamber wall).

temperature of secondary heating of the coke. Similar results were obtained also for charges of another composition.

Comparing the curves presented, one can see that the heating rate corresponding to the second regime (2–3 deg/min), or to a distance of 60 mm from the chamber wall, ensures the production of the strongest coke, regardless of the composition of the charge. The indicated heating regime is apparently optimal, since it provides sufficient caking and not too high a rate of gas evolution, as a result of which the strongest porous coke body is formed.

As is known, structural strength depends both on the strength of the coke substance itself and on the thickness of the pore walls. The higher the porosity, the thinner the pore walls obviously must be, and the higher their brittleness, which is manifested in the yield of fine classes and which characterizes, as a whole, the abrasion strength.

The influence of coke porosity on abrasion resistance is very clearly confirmed by the graph shown in Fig. 4: the maximum strength of coke located at a distance of 60 mm from the heating wall corresponds to minimum porosity. At the same time, the cause of the high abrasion resistance of coke obtained from charge No. 4 (with a predominance of gas coals) under regimes I and II is apparently its higher porosity. For this charge there is also characteristic (Fig. 8) a considerable decrease, over the entire width of the chamber, in the structural strength of the coke at a test temperature of 1073°K.

The graphs in Figs. 1 and 2 show that the abrasion resistance of almost all the tested samples, regardless of the charge-heating regime, increases rather sharply during secondary heating, beginning at a temperature of 723–873°K.

The decrease in the structural strength of the samples under the first heating regime occurs as a result of the formation, with intensive gas evolution, of highly porous coke, since the heating rate of the charge here is high and amounts to values of the order of 5.6–17 deg/min (Table 3). Under the third and fourth regimes the strength of the coke falls because of a decrease in caking capacity, occurring probably as a result of some oxidation, which cannot be completely avoided during prolonged holding of the charge at low temperatures (for 4 and 7.5 hours at a temperature of about 373°K).

The coke from charge No. 4 proved to be the strongest, in comparison with the other samples, under coking regime IV in the temperature interval 293–723°K. Here, apparently, the favorable effect on the strength of coke from charges with a predominance of gas coals of preliminary heat treatment of them, or else of slight oxidation, was manifested. However, at higher temperatures of secondary heating the abrasion resistance of the coke from this charge drops sharply and at 1073°K becomes much greater in comparison with the other samples.

A comparison of the abrasion-resistance curves of coke samples obtained from charges Nos. 1 and 2 with coke from charge No. 3 (KMK) indicates a small difference in their structural strength. At the same time, the strength of lump coke from these charges may differ considerably owing to differences in fissuring.

Conclusions

1. The influence of the heating rate and of the composition of a charge with an increased content of gas coals on the strength of coke in the hot state has been studied.

2. In the investigation, heating regimes at different points of the coal charge across the width of the coking chamber were modeled.

1. It has been established that the best coke strength, regardless of the composition of the charge, is ensured by a heating regime corresponding to a distance of 60 mm from the chamber wall.

2. The change in coke strength as a function of the heating regime is due to differences in the physicochemical conditions under which the porous structure of the coke is formed.

1. N. M. Smolyaninova, A. A. Kaplin, and L. M. Vasil’eva. Investigation of coke strength in the hot state. Coke and Chemistry, No. 5, 1961. ↩↩