Proceedings

of the Tomsk Polytechnic Institute named after S. M. Kirov, holder of the Order of the October Revolution and the Order of the Red Banner of Labor

Vol. 257
1973

Thermal Decomposition of Peat Humic Acids

S. I. Smolyaninov, Ya. A. Belikmaer, V. M. Ikryan

(Presented by the scientific-methodological seminar of KhTF)

The study of the kinetics and mechanism of pyrolysis of solid fuels is a task of primary importance at the present stage in the development of pyrogenic processes. Peat may be the most suitable object of investigation in this case. The complex, yet knowable, composition of the organic matter of peat is a rich source of information on the behavior of individual classes of substances during pyrolysis. The study of kinetic regularities in the thermal decomposition of peat will make it possible to determine the role of individual classes of substances in the formation of decomposition products, and at the same time to gain deeper insight into the mechanism of thermolysis, since the technological use of solid fuels requires a thorough study of the changes that occur in the fuel during its processing. The thermal decomposition of peat matter is associated with the release of gaseous, liquid, and solid pyrolysis products; all classes of compounds present in peat take part in their formation. Therefore, it is of interest to study the thermal decomposition not only of peat, but also of individual groups of substances that are components of peat and are extracted from it by the corresponding extractants.

The aim of the present study was to investigate the thermal decomposition of humic acids extracted from two different peats, the characteristics of which are presented in Table 1.

Table 1

Technical and elemental analysis of peats and humic acids (% by weight)

From previously known works [1–7], we have an understanding of the pyrolysis of peat humic acids only within a narrow temperature range, up to \(500^\circ\mathrm{C}\). Thus, according to the data of these authors, the decomposition of humic acids begins already at \(100^\circ\mathrm{C}\), with the release of water and carbon dioxide. The most intense gas evolution, according to Klimov [7], was observed at \(225\text{–}275^\circ\mathrm{C}\), and up to \(300^\circ\mathrm{C}\) carbon oxides and water vapor are released in the main. As the pyrolysis temperature increases, the nature of the decomposition changes: the proportion of carbon dioxide and water of decomposition in the gaseous products decreases, and hydrogen, carbon monoxide, and methane appear. Such a nonuniform character of the decomposition process, in the opinion of a number of researchers [4,5,7], indicates that humic acids include several substances decomposing at different temperatures. The formation of tar, which according to Ivanov [1] begins at \(225^\circ\mathrm{C}\), occurs in insignificant amounts. Other researchers [2,3,6] arrive at the same conclusion. The main product of pyrolysis of humic acids is the solid residue.

Experimental part.

Thermal analysis of humic acids was carried out on a combined setup combining derivatographic and chromatographic methods of analysis under conditions of linear heating at a rate of 5 deg/min. The essence of the analysis method is as follows. Above the crucible with the test sample in the reaction zone of the furnace, a probe is placed, by means of which the pyrolysis products are removed. To create an inert atmosphere and prevent suction of air from outside,

Nitrogen is fed into the reaction space of the furnace. The probe is then connected, through absorbers for the liquid products, to the measuring units of two chromatographs. The rate of formation of \(\mathrm{CO_2}\) is recorded continuously; for this purpose, a differential switching scheme was used for a chromatograph with flow-type detectors. The gas was analyzed for its content of \(\mathrm{H_2}\), CO, and \(\mathrm{CH_4}\) on an KhT-2M chromatograph periodically, every 4 minutes.

As can be seen from the data of thermogravimetric and chromatographic analysis of the products of thermal decomposition, presented in Figs. 1 and 2, the most intense decomposition occurs in the temperature range \(150–500^\circ\mathrm{C}\).

In the figure:
\(\omega\) \((\mathrm{mg}/\mathrm{min}\,\mathrm{g})\); \(T^\circ\mathrm{C}\).
1 — Total volatile products, lowland peat
2 — \(\mathrm{CO_2}\), lowland peat
3 — Total volatile products, raised-bog peat
4 — \(\mathrm{CO_2}\), raised-bog peat

Fig. 1. Dynamics of changes in the rates of decomposition of humic acids and formation of carbon dioxide during pyrolysis.

The principal decomposition products in this period are carbon oxides, water, and tar; moreover, the carbon oxides predominate over the liquid products, and only in the temperature region \(340–500^\circ\mathrm{C}\)

the amount of liquid products increases, while CO₂ and CO decrease. Hydrogen and methane appear above 300°C, with maxima in their rates of formation occurring at 500 and 720°C, respectively. Assuming that, under conditions of an inert medium and rapid removal of decomposition products, secondary processes of decomposition of volatile products are unlikely, one can form an idea of the ratios of the different functional groups whose products are CO₂, CO, H₂, and CH₄. It is especially characteristic that humic acids of completely different peats behave identically during pyrolysis, having identical decomposition products and temperatures of the maxima in the rates of formation of the corresponding gas components. This may be confirmation of an identical pyrolysis mechanism. The next feature

Fig. 2. Dynamics of changes in the rates of formation of gas components during pyrolysis of humic acids.

Legend in the figure:
1 — CO; 2 — CH₄; 3 — H₂ — lowland peat;
4 — CO; 5 — CH₄; 6 — H₂ — raised-bog peat.
Axis labels: \(\omega\) (ml/min); \(T,^\circ\mathrm{C}\).

of the thermal decomposition of humic acids of different peats is the greater rate of decomposition and rate of CO₂ formation for the humic acids of lowland peat, whereas the rate of decomposition of the peat itself is greater for raised-bog peat. This is probably due to the fact that the humic acids of lowland peat contain a more reactive type of acids, whose degree of aromatization is somewhat lower, or else the qualitative composition is the same,

and the quantitative ratio is different. A consequence of this is the unequal amount of decomposition products, as is evident from the data in Table 2.

Table 2

Yield of pyrolysis products of humic acids

Numerator—the yield of pyrolysis products up to 550°C; denominator—the yield of pyrolysis products up to 850°C.

By the method proposed by Nelubin and Alaev [8], we carried out a kinetic assessment of the process of decomposition of humic acids. As a result of the kinetic analysis it was established that the processes of formation of the individual components of the gas are complex parallel–consecutive reactions involving the splitting off of functional groups. As is known, each bond has a definite strength; consequently, the detachment of functional groups must proceed in a definite sequence as the temperature rises. The possible course of parallel–consecutive gas-formation reactions is probably due to the presence of a large number of groups whose bond energies are identical. In reality, as has become clear, these individual gas-formation processes appear on the overall decomposition curve. They can be isolated and evaluated by the method of combining linear heating with isothermal holding upon reaching a specified temperature. Understanding the mechanism of the process is aided by the assumption [8] of a narrow distribution of the energy imparted to bonds during heating. On this basis it can be said that the position of the group and the nature of the influence of neighboring groups do not strongly affect the bond strength in such a complex macromolecule as this is mani-

proceeds in smaller molecules.

The determined activation energies of the gas-formation process lie within the following ranges: for CO$_2$, 20–30 kcal/mol; CO, 20–30 kcal/mol; H$_2$, 50–60 kcal/mol; CH$_4$, 30–60 kcal/mol. It is characteristic that the activation energies of the process of formation of one and the same constituent of the decomposition products differ little from one another.

Conclusions

1. The dynamics of formation of the products of thermal decomposition of humic acids from lowland and raised bog peats have been studied.

2. The rates of formation of the gas components have maximum values at the following temperatures: CO$_2$—200–250°C; CO—250, 400–450, 700°C; H$_2$—700–750°C; CH$_4$—500°C.

3. The total decomposition rate of humic acids from lowland peat exceeds that for raised bog peat in the initial period up to 250°C by a factor of 1.5; in the subsequent period they become equal.

4. The activation energies found for the processes of formation of the individual gas components are in satisfactory agreement with the literature data for the thermal decomposition of model substances.

Literature

1. B. I. Ivanov. KhTT, vol. 5, nos. 9–10, “Nauka,” 1934, p. 754.

2. M. M. Zhuravleva. KhTT, vol. 7, no. 4, “Nauka,” 1936, p. 328.

3. E. E. Kazakov. Izv. AN SSSR, O.T.N., no. 8, Moscow, 1949, p. 152.

4. P. I. Belkevich, K. A. Gaiduk. Transactions of the Institute of Peat, vol. 7, Moscow, Publishing House of the Academy of Sciences of the BSSR, 1959, p. 82.

5. P. I. Belkevich, K. A. Gaiduk. In: Chemistry and Genesis of Peat and Sapropels. Moscow, Publishing House of the Academy of Sciences of the BSSR, 1962, p. 160.

6. V. E. Rakovskii. General Chemical Technology of Peat. Moscow–Leningrad, GEI, 1949.

7. B. K. Klimov, V. A. Lagin, A. V. Lozovoi, M. S. Gorokhlinskaya. Izv. AN SSSR, O.T.N., nos. 11–12, issue 3, Moscow, 1941, p. 112.

8. B. V. Nebol’sin, G. P. Alaev. KhTT, no. 6, “Nauka,” 1969.