ON THE STUDY OF THE MECHANISM OF THERMAL BRIQUETTING OF PEAT

S. G. Maslov, S. I. Smolyaninov, N. M. Smolyaninova, I. A. Oshchepkov

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

Several attempts are known to solve the question of the formation of a strong structure in thermal briquettes. A. A. Semechkin and R. I. Glotov [1] believe that briquetting of carbonized peat particles proceeds exclusively through the molecular cohesion of carbon, which is reactive at the moment of its liberation during the exothermic process. Kh. I. Rivkina [1] assumes that thermal briquetting proceeds by means of bitumens. These views do not correctly reflect the essence of the mechanism of thermal briquetting.

More substantiated explanations were given by V. E. Rakovskii [2]. Together with E. A. Novichkova, he experimentally established that peat, upon heating, passes through a stage of the plastic state. The presence of a plastic state in peat is also reported by G. D. Petrovskii et al. [3]. The ideas of V. E. Rakovskii were further developed in work [4], where questions of the mechanism of thermal briquetting under high-rate heating are considered.

Heating peat before thermal briquetting can be explained by the need for sufficient development of pyrolysis reactions leading to the accumulation of plastic products; moreover, the formation of a strong structure in a thermal briquette is determined not only by the plasticity and “adhesive capacity” of these products, but also by their increased chemical activity. In order to understand correctly the question of the accumulation of plastic products, it is necessary to determine the role of the constituent parts of peat in the formation of liquid products of thermal decomposition when peat is heated to the temperature of thermal briquetting (250–300°C). The experimental part of the work was devoted to solving this question.

The experiments were carried out on an installation (Fig. 1) based on the design described by L. A. Prilepskaya [5], with minor changes. Separate trapping was introduced: resins—by a cotton filter, water—by calcium chloride. In this form the installation has a smaller volume of “dead space” and gives better reproducibility of results; however, the resin yield is obtained somewhat too high, and the water yield too low.

For the study, peat was taken from the Vasyugan deposit of Tomsk Oblast (degree of decomposition—20%; \(W^{a}=12.29\%\); \(A^{c}=2.41\%\); \(C^{r}=55.61\%\); \(H^{r}=6.12\%\); \(S^{c}=0.12\%\)) and its constituent parts, isolated by methods of group analysis of peat. The group composition of the peat:

Figure 1. Apparatus for the thermal decomposition of solid fuels in small weighed portions.
1 — stand; 2 — electric furnace, 600°C; 3 — test tube—reactor; 4 — tar-trapping tube; 5 — tar receiver with cotton wool; 6 — electric furnace, 105°C; 7 — U-shaped tube with calcium chloride; 8 — thermometer; 9 — gas meter; 10 — clamps; 11 — gas inlet tube; 12 — thermometer; 13 — gas outlet tube; 14 — manometer; 15 — drain branch pipe; 16 — measuring vessel.

Table 1

Yield of products of thermal decomposition from peat and its constituent parts
(% of combustible mass)

the following: bitumens—10.55%, water-soluble and readily hydrolyzable substances—23.27%, humic acids—24.85%, fulvic acids—22.32%, lignin and cellulose—19.01%.

The initial preparations were subjected to thermal decomposition at temperatures of 200, 250, 300, 350, and 400°C. The sample weight was 1–2 grams. Gas analysis was carried out on a VTI-2. The results of the experiments are given in Table 1.

Let us consider the data obtained from the point of view of the yield of liquid products of thermal decomposition (tar) in the temperature range 250–300°C. The yield of tar during thermal decomposition of the constituent parts of peat in the indicated temperature interval is: from bitumens—3.06%, from humic acids—1.78%, from the residue—1.71%, from water-soluble and readily hydrolyzable substances—0.71%. Taking the total amount of tar released in this temperature interval as 100% and, taking into account the group composition of the peat, we find the yield of tar from the peat components during thermal decomposition of peat: bitumens—16.90%, humic and fulvic acids—57.50%, water-soluble substances—9.50%, residue—16.25%.

From the results presented it is evident that all components of peat take part in the formation of tar. The greatest share of liquid products released during the thermal decomposition of peat in the temperature range 250–300°C is given by humic acids and fulvic acids. The total amount of tar released in the temperature interval of interest to us is about 7% (on the combustible mass). From the literature data and from practice we know that the amount of binders added in the briquetting of solid fuels varies within these limits. Apparently, the liquid products of thermal decomposition that are released play the role of a binder in the thermal briquetting of peat and are one of the causes of the formation of the strong structure of the thermobriquette.

LITERATURE

1. Kh. I. Rivkina. Production of metallurgical coke from milled peat by briquetting during the coking period. Torfyanoe delo, 9, 28, 1934.

2. V. E. Rakovskii, F. L. Kaganovich, E. A. Novichkova. Chemistry of Pyrogenic Processes, Minsk, 1959.

3. G. D. Petrovskii et al. A new method of briquetting and coking brown and hard coal. Information Bulletin of VSEGEI, 36, 145, 1960.

4. S. I. Smol’yaninov, V. E. Voronin. Production of peat thermobriquettes under conditions of high-speed heating. Peat Industry, 4, 1962.

5. L. A. Prilepskaya. Determination of the yield of semi-coking products in small charges. Factory Laboratory, 6, 1953.