Influence of Temperature and Briquetting Pressure on the Mechanical Properties of Peat Thermal Briquettes

S. I. Smolyaninov, A. M. Denisov

(Presented by Prof., Doctor of Technical Sciences I. V. Gebler)

Serious attention is currently being given to the question of using peat as a metallurgical fuel [1]. This is due not only to the well-known favorable properties of peat (low ash and sulfur content), but also to the shortage of coking coals, as well as, in a number of cases, the absence of coal reserves in regions of large iron-ore deposits. The latter situation applies fully to Tomsk Oblast, where only the use of peat can solve the fuel problem in exploiting the recently discovered, very rich reserves of iron ores [2].

The modern blast-furnace process is possible only with a lump fuel of a definite granulometric composition and definite physicochemical and physicomechanical properties. Therefore, solving the problem of using peat in metallurgy is connected above all with obtaining from it a lump material. Semicoking of peat fines and the use of fine semicoke to replace lean coals in the coal charge and for ore agglomeration, while essentially acceptable, solves the indicated problem only partially.

The Department of Chemical Technology of Fuels of the Tomsk Polytechnic Institute investigated the possibility of obtaining fuel-smelting materials on the basis of machine-formed peat.

Such materials obtained under laboratory conditions, in the air-dry state, possess mechanical strength exceeding the strength of the best samples of hard-coal coke. The coke obtained from these materials also has high mechanical strength. At the moment of forming, such a quantity of ore and flux can be added to raw peat as will ensure the normal ratios among these components in the blast-furnace process.

At present, owing to a number of advantages, the milled method of peat extraction has become widespread, in which peat is obtained in the form of fine crumbs. Thus, in 1965 the share of milled peat extraction is planned to be brought to 80–90% of the total volume of extraction [3]. Compared with the extraction of lump peat, the production of milled peat is at present more mechanized and economical. The cost of extracting milled peat is on average two times

less than that of lump peat, while output per worker in the former case is two to three times greater. At present the cost price of a ton of conventional fuel in the form of milled peat has been brought down, at a number of advanced enterprises, to 30–35 rubles [3].

Milled peat can be lumped by the following methods:

a) by thermal briquetting, i.e., by pressing crushed peat preheated to 280–300° under a comparatively low pressure (50–100 atm). The thermal briquettes obtained under such conditions possess high mechanical strength and, after coking, yield satisfactory metallurgical coke. Thermal briquettes could apparently be used as blast-furnace fuel even without preliminary coking. This question, however, has not yet been studied at all;

b) by briquetting dried milled peat under high pressure (1000 atmospheres and more). Coke from such briquettes has comparatively low mechanical characteristics;

c) to the same category may be assigned the production of blast-furnace coke from milled peat through intermediate semicoking, followed by briquetting of the semicoke with a binder and coking of the briquettes.

The first of the indicated methods makes it possible to obtain a strong metallurgical fuel by a comparatively simple technology. A large economic effect may apparently be expected from combining, in the blast furnace, the coking process and the blast-furnace process proper, i.e., from the use of uncoked thermal briquettes in blast-furnace smelting (the gas-blast-furnace process). The advantage of thermal briquettes in the gas-blast-furnace process, in comparison with other types of peat fuel, is the absence of moisture in them, due to the technology of their manufacture.

The prospects of thermal briquettes as blast-furnace fuel were confirmed at a number of conferences [1, 3].

A small number of works have been published on this question. The most serious is the work of Kh. I. Rivkina [4]. This article also reports on the first experiments on thermal briquetting carried out in 1903 in Germany and in 1931 by A. A. Semechkin and F. I. Glotov. The latter briquetted peat heated to 215–280° under a pressure of from 10 to 100 atmospheres. Some of the briquettes obtained had a crushing resistance of up to 88 kg/cm². Coke from these thermal briquettes proved to be weak. In the opinion of the authors, briquetting of the carbonized peat particles proceeded exclusively through “molecular cohesion of carbon that is reaction-active at the moment of its liberation in the exothermic process.”

Kh. I. Rivkina briquetted peat in a heated die. The influence of heating temperature, briquetting pressure, degree of peat comminution, and holding time of the briquette under pressure was investigated. In addition, various heating regimes were studied. These experiments established: the optimum briquetting temperature (270–300°), the necessity of holding the briquette under pressure (for about 2 minutes), the positive influence of comminuting peat particles (0.5–0.2 mm), the required pressure (30–40 atm), and the favorable effect of a variable heating regime with a forced rise in temperature from 250° to the final heating temperature during briquetting. The briquettes obtained were tested for crushing resistance both directly and after coking. This value was 200–400 kg/cm². As to the mechanism of formation of a strong briquette, Kh. I. Rivkina, while acknowledging the arguments of A. A. Semechkin and F. I. Glotov, also attached great importance to the resinous decomposition products as a binder.

We carried out experiments on obtaining thermal briquettes from peat of the Tagan deposit in Tomsk Oblast. The peat used was a lowland hypnum–sedge peat of medium degree of decomposition. The moisture content of the air-dry peat was 13.0%, ash content on a dry basis was 7.6%, and the volatile-matter content on the combustible mass was 70.0%.

Peat ground to pass through a 0.25 mm sieve was loaded into a heated steel die 31 mm in diameter and 70 mm high. One of the die inserts had an opening for a thermocouple placed in the center of the charge. After heating to the specified temperature and holding for a definite time at the required pressure, the briquette was unloaded and cooled in isolation from the surrounding medium. The briquettes obtained were tested both immediately and after heating them, without access of air, to 400 and 800° at a rate of 3–4° per minute. The crushing resistance, the strength index according to K. I. Syskov, and the abrasion coefficient were determined by the methods described earlier [6].

In the experiments carried out, the influence of the final temperature of peat heating before briquetting (at a constant heating rate of 6.5% per minute) and of the briquetting pressure was investigated. The holding time of the briquettes under pressure was 2 minutes.

The principal experimental data are presented in Figs. 1 and 2. The existence of a temperature optimum is explained, on the one hand,

Fig. 1. Dependence of abrasion and crushing resistance of thermal briquettes on briquetting temperature (briquetting pressure—60 atm). 1—thermal briquette; 2—the same after heating to 400°; 3—the same after heating to 800°.

Fig. 2. Dependence of abrasion and crushing resistance of thermal briquettes on briquetting pressure (briquetting temperature—260°). 1—thermal briquette; 2—the same after heating to 400°; 3—the same after heating to 800°.

by the need for sufficient development of decomposition reactions leading to an increase in the amount of reaction-active carbon (according to the views of A. A. Semechkin and F. I. Glotov), and to the accumulation of liquid or plastic decomposition products. On the other hand, with a further increase in temperature the decomposition products undergo secondary transformations that reduce their “binding” properties, and are partly distilled off in the form of resin vapors.

In general, it should be assumed that the optimum temperature determines the most advantageous rate of decomposition of the organic matter of peat, which must be commensurate with the time required for the peat particles to come together under the action of an external load.

The optimum temperature for heating peat before briquetting was also established in the work of Kh. I. Rivkina, but at higher values of this parameter. The latter is explained primarily by the difference in the nature of the peats studied. In addition, the heating rate, as well as other experimental conditions, for example pressure, may play a significant role. When pressure is applied to a heated charge of peat, the latter is heated additionally as a result of compression. This heating may apparently be considerable. In a number of cases we observed a sharp acceleration of decomposition reactions under pressure, leading to an explosion-like release of volatiles. The briquette was destroyed in the process, and its center was carbonized much more strongly than the periphery. In the experiments of Kh. I. Rivkina the optimum temperature of 300° was established at a briquetting pressure of 33 atm; in ours—at 60 atm, the optimum temperature was 260°.

The dependence of the mechanical strength of thermal briquettes on briquetting pressure shows the presence of a maximum strength at a pressure of 150 atm.

In the experiments of Kh. I. Rivkina, a continuous increase in strength was observed from 16 to 55 atm—the upper limit investigated by her. The decrease in strength observed in our experiments after 150 atm can apparently be explained by the strong rise in peat temperature during compression and the consequent exceeding of the optimum briquetting temperature, unless one assumes a possible change in the direction of the decomposition reactions of the organic matter, leading to a deterioration of the “binding” properties.

From the standpoint of the possibility of using thermal briquettes in the blast-furnace process without preliminary coking, it is of interest to trace the change in their strength at different stages of pyrolysis. As can be seen from Figs. 1 and 2, during the coking of thermal briquettes there is a uniform decrease in their strength. In this behavior during coking, thermal briquettes differ from machine-formed peat under corresponding conditions, which has minimum strength at approximately 400° [6]. This indicates that the structure of the coke is formed mainly at the moment of briquetting, and further shrinkage of the organic matter occurs with difficulty, which leads to a decrease in strength. In most of the experiments of Kh. I. Rivkina [3], the crushing strength of the coke was lower than the strength of the initial thermal briquettes, although the opposite was also observed. The latter is probably explained by the test conditions.

As for the determination of the strength index according to K. I. Syskov, this parameter proved to be less characteristic, although the general pattern of its change is analogous to the change in abrasion resistance and crushing strength.

For comparison, we determined under identical conditions the strength of cokes from the Kuznetsk Metallurgical Combine and the Kemerovo

coke-chemical plant. These data, together with the characteristics of the best samples of thermal briquettes, are given in Table 1.

Table 1

As can be seen from Table 1, at all stages of pyrolysis the thermal briquettes exceed good metallurgical coke in crushing resistance, and in abrasion they approach it.

Thus, thermal briquettes, apparently, can be used in large-volume blast furnaces both in the form of coke and directly, without preliminary coking.

Conclusions

1. The influence of temperature and briquetting pressure on the crushing resistance, abrasion, and strength index of peat thermal briquettes has been studied.

2. It has been shown that the maximum mechanical strength of thermal briquettes and of the coke made from them corresponds to comparatively narrow temperature ranges during briquetting. An optimum pressure is also observed. These phenomena are apparently explained by the need for decomposition reactions to proceed at a definite rate.

3. The behavior of thermal briquettes at different stages of pyrolysis provides grounds for a positive assessment of the possibility of using them in a blast furnace without preliminary coking.

Literature

1. Borts A. G. Meeting on the use of peat in the metallurgical industry. “Coke and Chemistry,” No. 2, pp. 56–58, 1959.

2. Gebler I. V., Smolyaninova N. M. The problem of metallurgical fuel for the utilization of iron ores of Tomsk Oblast. Publishing House of Tomsk University, Tomsk, 1959.

3. Tsuprov S. A. On the state of technology and the direction of its development in 1959–1965 in the peat industry of the USSR. In: Prospects for the comprehensive utilization of peat resources of the Novosibirsk economic region. Publishing House of the Novosibirsk Council of National Economy, 1959.

4. Rivkina Kh. I. Obtaining metallurgical coke from milled peat by briquetting during the coking period. “Peat Business,” No. 9, pp. 28–32, 1934.

5. Klimov B. K. New methods of thermal processing of peat. GONTI, 1939.

6. Gebler I. V., Smolyaninov S. I., Martynov A. F., and Severin B. M. Influence of pressure and moisture on the properties of peat as a metallurgical fuel. “Peat Industry,” No. 8, pp. 16–20, 1959.