SOME FEATURES OF THE THERMAL DECOMPOSITION OF PEAT

V. I. LOZBIN, S. I. SMOLYANINOV

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

One of the most characteristic indicators of the process of thermal decomposition of fuels is the dynamics of the change in the substance under the action of temperature. Usually, in such studies, the process is characterized by the rate of gas evolution or by the change in the weight of the substance by means of continuous weighing of the sample. But in this case it is difficult to present a complete picture of the thermal decomposition of the fuel, since we know nothing about the thermophysical changes [1] occurring in the substance during heating.

On the other hand, in a thermographic study of the processes of thermal decomposition of solid fossil fuels, it becomes possible to investigate the heats of the reactions occurring during pyrolysis of the fuel [2–5], but it is difficult to determine the thermogravimetric characteristic in the study of solid fuels. Difficulties of this kind were overcome with the development of a new method for analyzing solid substances—derivatography.

We carried out derivatographic studies of peats from the Vasyugan deposit in order to study the influence of botanical and group composition on the character of the thermal decomposition of peats.

The studies were carried out on a derivatograph of the system of F. Paulik, I. Paulik, and L. Erdey. Calcined aluminum oxide at \(1200^\circ\mathrm{C}\) was used as the standard; the heating rate was \(5^\circ/\mathrm{min}\); sensitivity: DTA — \(1/5\), DTG — \(1/3\), TG — 500 mg, \(T\) — \(900^\circ\mathrm{C}\). The studies were conducted in an atmosphere of nitrogen purified of oxygen.

To study the influence of the botanical composition of peat on the character of thermal decomposition, the available peat samples were classified by us into 4 main types: sedge, Sphagnum balticum, Sphagnum fuscum, and Sphagnum magellanicum.

The thermal decomposition of peats upon heating is characterized by the presence on the DTG curve of three maxima of the rates of volatile evolution in the temperature intervals \(100\text{–}180^\circ\), \(200\text{–}300^\circ\mathrm{C}\), and \(310\text{–}400^\circ\mathrm{C}\). The complex character of the DTG curve is explained by the stepwise pyrolysis of the organic mass of peat.

A comparison of the extreme temperature values of thermal decomposition on the DTA curves shows that lowland peats have the greatest thermal stability, and raised peats the least. At the same time, a connection is evident between the botanical composition and the character of thermal decomposition:

a) for peats of the Sphagnum magellanicum and sedge types, with an increase in the amount of the principal component, thermal stability decreases;

b) for peats of the Sphagnum balticum and Sphagnum fuscum types, with an increase in the content of the principal component, thermal stability and coke yield increase;

c) the thermal stability of peat of the Sphagnum fuscum type is higher than that of Sphagnum magellanicum and Sphagnum balticum; with an increase in Sphagnum fuscum in the peat, the thermal stability of the peat increases.

Let us consider the influence of the group composition of peat (bitumens, humic acids, water-soluble and readily hydrolyzable substances, and nonhydrolyzable substances) on the course of the DTA curves. The components listed above were extracted according to the method for determining the group composition of peats and plants. The character of thermal decomposition was studied by derivatographic investigations both of individual components of the group composition and by recording derivatograms of the residue after removal of one or another group of substances.

Fig. 1. Thermograms of components of the group composition of peat:
a — original peat, sample 6B; b — peat without humic acids; v — peat without bitumens and humic acids; g — bitumens from peat 6B; d — humic acids from bituminous peat; e — humic acids from end-bituminous peat.
The numbers by the curves are the temperatures of thermal effects on the DTA curves.

Sequential extraction of the components of the group composition and derivatographic investigations were carried out for peat 6B. The experimental data obtained (Fig. 1) will be presented in the order in which the components of the group composition were extracted.

The thermal decomposition of bitumens begins at a temperature of 340°C; the maximum of thermal decomposition is observed at a temperature of 500°C (an endothermic effect on the DTA curve) (Fig. 1, g). The negative thermal effect of the bitumen pyrolysis reaction is apparently due to the occurrence of decarboxylation reactions [7].

Humic acids decompose in the temperature interval (Fig. 1, d, e) 300–400°C with an overall positive thermal effect. This positive thermal effect, in our view, may be explained by the presence in humic acids of bonds of a heterocyclic character; when these bonds are broken under the action of high temperature, a considerable amount of heat is released.

Water-soluble and readily hydrolyzable compounds are represented in peat by carbohydrates, chiefly mono-, di-, and polyoses. The thermal decomposition of these compounds begins at a temperature of 240–310°C, and the positive thermal effect in this temperature interval (Fig. 1, v) can be explained by reactions involving the rupture of heterocycles and isomerization of heterocyclic compounds into linear ones [7], which proceed with the release of a considerable amount of heat.

The last component of the group composition of peat—the nonhydrolyzable residue—decomposes at a temperature of 470–490°C with a positive thermal effect. The exothermic effect of the thermal decomposition of the nonhydrolyzable residue can be explained by the presence of oxygen bonds in the macromolecules of the residue, which, during dehydration at the expense of hydroxyls, form bonds with closure of the carbohydrate into a heterocycle. Further thermal decomposition of heterocyclic compounds (isomerization into linear ones) occurs with a positive thermal effect (Fig. 1, v).

When comparing the derivatograms of the components of the group composition of peat considered by us with the derivatograms of the original peat, a correspondence is observed between the thermal effects on the DTA curves for the individual components both in the original peat and in the isolated groups of substances. Consequently, during the thermal decomposition of the organic mass of peat, the thermal effects are characteristic both of the individual components and of the original peat consisting of these components.

Conclusions

1. A relationship has been established between the botanical composition and the character of thermal decomposition.

2. The relationship between the group composition and the character of thermal decomposition is confirmed. Bitumens decompose in the temperature interval 340–500°C; humic acids, 300–400°C; water-soluble and readily hydrolyzable compounds, 200–310°C; the nonhydrolyzable residue, 460–500°C.

LITERATURE

1. G. O. Piloyan, Introduction to the Theory of Thermal Analysis. Moscow, 1964.

2. P. I. Kanavets, V. I. Chernykh, K. I. Chibisova. Comprehensive thermographic study of the process of thermal decomposition of hard coals and fluxed ore-fuel granules. Proceedings of the Second Conference on Thermography, Kazan, Publishing House of the Academy of Sciences of the USSR, 1961.

3. V. A. Filimonov, V. E. Rakovskii. Exo- and endothermic reactions during the destruction of peat. IFZh, vol. 4, no. 3, 1961.

4. Studies in the Field of Peat and Sapropel. Publishing House of the Academy of Sciences of the Latvian SSR, 1958.

5. Kh. A. Iskhakov. Application of thermography in the study of solid fuels. Author’s abstract, Alma-Ata, 1961.

6. F. Paulik, I. Paulik, L. Erdey. Derivatography. Budapest, 1964.

7. V. A. Filimonov, V. E. Rakovskii. Thermographic study of the processes of thermal decomposition of peat. In: Chemistry and Genesis of Peat and Sapropels, Minsk, 1962.