Toward the Study of the Mechanism of Peat Thermal Briquetting. Changes in the Group Composition of Peat During Thermal Briquetting

S. G. Maslov, S. I. Smolyaninov

(Presented by the scientific-methodological seminar of KhTF)

One of the reasons for the formation of a strong structure in a thermal briquette is, apparently, chemical interaction between peat particles, which is realized in the form of a condensation structure.

The process of thermal briquetting may be conventionally divided into the stage of heating to the thermal-briquetting temperature and the stage of pressing the heated charge. Accordingly, we shall also distinguish two groups of reactions, the final results of which can provide a definite idea of the change in the group composition of peat at the indicated stages of the thermal-briquetting process.

The work was carried out with peat from the Tagan deposit in Tomsk Oblast \((W^a = 11.11,\ A^c = 8.70,\ V^r = 68.42,\ C^r = 59.27,\ H^r = 6.10,\ N^r = 2.74,\ O^r = 31.89)\).

The initial peat, ground to pass through a 0.5 mm sieve, was heated in a quartz test tube to \(250^\circ\mathrm{C}\) and cooled in liquid nitrogen. Thermal briquettes were obtained at a temperature of \(250^\circ\mathrm{C}\), a pressure of \(300\ \mathrm{kg/cm^2}\), and a holding time under pressure of 2 min.

The group composition of the above samples was determined by the method of the Kalinin Peat Institute [20]. The results are given in Table 1.

The data presented indicate that, when peat is heated to the thermal-briquetting temperature, its group composition changes significantly. The yield of bitumens (from 6.4 to 17%) and humic acids (from 31.0 to 43.2%) increases, while the content of the remaining components decreases, which is consistent with the literature data [1—9].

The presumed causes of the formation of the group of substances extracted by organic solvents were reported earlier [10]. They amount to various processes of destruction of the constituent parts of peat and to the synthesis of new compounds from the fragments formed in the process.

The fact that the amount of humic acids increases appears to contradict their thermal instability, but the increased yield of solid residue in the pyrolysis of these compounds [11—19] indicates that their thermal decomposition proceeds predominantly in the direction of the cleavage of side functional groups while preserving and even enlarging the main nucleus of the molecule. By analogy with the process of peat formation, in the present case, apparently, humic acids are synthesized from the products of thermal decomposition of the group constituents of peat.

Table 1

Change in the group composition of peat during thermal briquetting

According to V. E. Rakovskii [3], the possibility is not excluded that molecules of humic acids are formed from fulvic acids during the heating process.

When pressure is applied to the heated charge (Table 1), the amount of alcohol–benzene extract decreases somewhat (from 17.0 to 16.0%) and that of humic acids decreases (from 43.2 to 41%), while the proportion of residue increases (from 14.8 to 17.0%). This, in our opinion, indicates the occurrence, during the formation of the strong structure of the thermal briquette, of chemical condensation reactions and is direct proof of V. E. Rakovskii’s supposition that such reactions are present at the moment when the plastic state of peat is realized.

Conclusions

1. The change in the group composition of peat during thermal briquetting has been determined.

2. It has been shown that, in the formation of the strong structure of the thermal briquette, a definite role is played by bonds formed as a result of chemical reactions between individual group fractions.

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