THERMAL DECOMPOSITION OF THE LIGNIN–CELLULOSE COMPLEX OF PEAT

S. I. Smolyaninov, Ya. A. Belikhmaer, V. M. Ikrin

(Presented by the scientific-methodological seminar of the KhTF)

This study is subordinated to the general aim of studying the kinetics of the thermal decomposition of peat. Pyrolysis of the individual groups of substances present in the organic mass of peat makes it possible to determine their role in the formation of the products of thermal decomposition and thereby to penetrate more deeply into the mechanism of peat destruction. The lignin–cellulose complex, which represents the residue of peat after extraction from it of bitumens and water-soluble and readily hydrolyzable humic acids, was subjected to decomposition. In a number of studies carried out on plant residue, a general picture is given of the behavior of this complex up to 550°C.

The first period of decomposition, characterized by the formation of a large quantity of carbon oxides and water of decomposition, is attributed by the authors of [1–3] to the decomposition of cellulose. Above 250°C gaseous hydrocarbons and tar appear. The appearance of tar is noted somewhat later than in the other constituent parts of peat. After 300°C the intensity of decomposition decreases markedly, but at this temperature hydrogen appears. The maximum of tar formation occurs at 400°C. With a further rise in the pyrolysis temperature, only the yield of hydrogen increases; the intensity of formation of the remaining gas components decreases.

Work has also been carried out on the pyrolysis of separate preparations of cellulose and lignin. Thus, in the carbonization of cotton cellulose [4] up to 500°C, stages of pyrolysis were distinguished. Up to 150°C adsorbed moisture is released; up to 240°C, bound water is split off. The subsequent process is accompanied by thermal cleavage of certain bonds, –C–O–, –C–C–, with the formation of a large quantity of carbon oxides and groups containing up to 4 carbon atoms, and then

a process of aromatization begins, with the formation of carbonaceous polymers having a structure resembling graphite.

The decomposition of natural lignin [5] leads to its activation and depolymerization, and upon subsequent thermal treatment it undergoes recondensation. In the distillation of lignin obtained by saccharifying Douglas fir with 0.5% sulfuric acid, 55.8% charcoal, 8.4% tar, 18–19% water-soluble acids, and 10.3% gas were obtained.

The thermal analysis carried out by us made it possible to study the dynamics of the thermal decomposition of the lignin–cellulose complex and the formation of gas components over a wide temperature interval. In Fig. 1 the curves of the total rate of decomposition and of carbon dioxide formation are presented for samples prepared from lowland and raised-bog peats. The characteristics of the study samples are represented by the following data: for lowland peat: \(W^{a} - 8.6\%\), \(A^{c} - 8.3\%\), \(C^{\mathrm{T}} - 46.9\%\), \(H^{\mathrm{T}} - 68\%\), \(O^{\mathrm{T}} - 46.3\%\); for raised-bog peat: \(W^{a} - 9.0\%\), \(A^{c} - 5.7\%\), \(C^{\mathrm{T}} - 43.4\%\), \(H^{\mathrm{T}} - 4.3\%\), \(O^{\mathrm{T}} - 52.4\%\). Comparing the curves of the total rate of decomposition for both samples, we observe the presence of similar destruction reactions, the temperatures of whose maxima are identical. In the case of raised-bog peat they are clearly separated, whereas in the case of lowland peat they are not. If one takes into account the quantitative ratio

Fig. 1. Dynamics of changes in the total rates of decomposition and carbon dioxide formation.

Legend in the figure:

1 — total volatile products, lowland peat
2 — \(CO_{2}\), lowland peat
3 — total volatile products, raised-bog peat
4 — \(CO_{2}\), raised-bog peat

Vertical axis: \(W\), \(\left(\dfrac{\mathrm{mg}}{\mathrm{min}}\right)\)
Horizontal axis: \(T^\circ C\)

gaseous products with the total loss of weight, it may be noted that the principal share of the products in the temperature interval \(140–300^\circ\mathrm{C}\) is the water of decomposition, and a small amount is carbon oxides. It is characteristic that the intensity of decomposition of the lowland-peat sample in this interval is lower than that of the raised-bog peat sample. At higher temperatures we observe the opposite picture, probably owing to the presence of a larger amount of lignin in the lowland-peat sample, whose thermal stability is higher. A similar phenomenon is also observed in the quantitative ratio of the gaseous decomposition products (Fig. 2). The maxima of the rates of formation of the gas components appear at the following temperatures: \(\mathrm{CO_2}\)—\(280^\circ\mathrm{C}\), \(\mathrm{CO}\)—\(280\), \(720\), \(760^\circ\mathrm{C}\), \(\mathrm{H_2}\)—\(695^\circ\mathrm{C}\), \(\mathrm{CH_4}\)—\(460^\circ\mathrm{C}\). Such a correspondence of the maxima of the formation rates for the samples of both peats indicates similarity in chemical structure and, consequently, the same mechanism of thermal destruction. Table I gives an idea of the yields of pyrolysis products up to \(550^\circ\mathrm{C}\) and \(850^\circ\mathrm{C}\).

Table I

Yield of pyrolysis products of the lignin–cellulose
complex

*) numerator—yield of products up to \(550^\circ\mathrm{C}\),
denominator—yield of products up to \(850^\circ\mathrm{C}\).

It is interesting to note that the yields of pyrolysis products for both samples are almost indistinguishable, although the different ratios of lignin and cellulose would suggest a greater difference in the products.

Over the entire temperature interval we carried out a kinetic

Fig. 2. Dynamics of changes in the rates of formation of gas components.

Legend:
1 — CO; 2 — CH₄; 3 — H₂ — lowland peat;
4 — CO; 5 — CH₄; 6 — H₂ — raised-bog peat.

assessment of the processes of formation of the gas components. The calculated activation energies for the corresponding gas-formation processes have the following values: CO₂ — 20–30 kcal/mol, CO — 20–50 kcal/mol, H₂ — 50–100 kcal/mol, CH₄ — 50–70 kcal/mol.

The determined activation energies for identical temperature intervals have similar values for identical gas components, but for different samples. Thus, the correspondence between the temperature intervals of formation of the individual gas components and the kinetic parameters of the decomposition process indicates the presence of similar decomposition stages for samples of different peats, and consequently of a similar mechanism of destruction.

References

1. B. I. Ivanov. KhTT, vol. 5, issue 9–10, “Nauka,” 1934, p. 754.
2. M. M. Zhuravlev. KhTT, vol. 7, issue 4, “Nauka,” 1936, p. 328.
3. E. I. Kazakov. Izv. AN SSSR, o.t.n., No. 8, 1949, p. 152.
4. M. M. Tang, R. Bakon. “Carbon,” 1964, 2, No. 3, pp. 211–220. Cited in RZh, 1965, 24P11.
5. F. E. Brauns, D. A. Brauns. Chemistry of Lignin. Moscow, “Lesnaya promyshlennost,” 1964.