FEATURES OF THE THERMAL DECOMPOSITION OF PEAT ACCORDING TO DERIVATOGRAPHIC DATA

S. I. SMOLYANINOV, V. I. LOZBIN, V. M. IKRIN, Ya. A. BELIKHMAER

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

The study of the kinetics and mechanism of the thermal destruction of fuel is most effective when a set of experimental data obtained by different methods is used.

Using a combined derivatographic apparatus [1], we studied the pyrolysis process of various peats. At a constant heating rate (5 degrees/min), the following were determined as functions of temperature: total weight loss (TG), rate of weight loss (DTG), differential thermographic curve (DTA), rates of evolution of individual gas components, as well as a number of kinetic parameters of the process in accordance with methods developed by us [2]. Thirty different peats from deposits of Western Siberia were investigated. The samples used are characterized by the data in Table 1.

Table 1

Characteristics of peats

Continuation of Table 1

Several stages of the thermal decomposition of peat are clearly distinguished on the derivatogram of peat; these are characterized by separate peaks on the DTG curve and by the corresponding extrema on the DTA curve. The first maximum on the DTG curve corresponds to the release of water; the subsequent ones correspond to decomposition of the organic part of the peat.

All peat studies yielded analogous derivatograms, differing, however, in the absolute values of the characteristic points. Figs. 1 and 2 present experimental data and some calculated kinetic parameters as functions of the degree of decomposition of peat.

Fig. 1. Dependence of the characteristic points of the derivatograms on the degree of decomposition of peat. Temperatures of the maxima of the weight-loss rates and the corresponding extrema of DTA: a) moisture release, DTG; b) the same, DTA; c) 1st decomposition peak, DTG; d) the same, DTA; e) 2nd decomposition peak, DTG; f) the same, DTA; g) rate of weight loss in the 1st peak; h) the same, in the 2nd peak.

Fig. 2. Change in the rate of gas evolution as a function of peat pyrolysis temperature: a) total weight loss; b) carbon dioxide evolution; c) carbon monoxide evolution; d) hydrogen evolution; e) methane evolution. 1 — peat, sample 29; 2 — sample 30 (Table 1).

...peat, analogous dependences are also found for the yield of volatile substances and other characteristics of peat. The temperature intervals in which the yield of volatile substances is determined for the first decomposition peak lie within 160–210—260–340°C, for the second, 260–340—315–400°C, and the limits of the third period are 315–400—480–550°C. The data presented in the figures clearly demonstrate both the limits of variation of certain parameters and the existence of their connection with the chemical composition of peat and, apparently, may be used to establish a more rigorous correlation dependence.

It is interesting to note that the process of moisture evolution for all the peats studied formally, but quite accurately, follows a second-order equation (up to the maximum, half of the moisture contained in the peat is evolved), whereas the order of the thermal decomposition reactions of the organic matter of peat, approximately calculated from the isolated first peak, is close to unity.

It should be noted that, in accordance with the works of V. E. Rakovskii and V. A. Filimonov [3, 4], in the first peak, reactions of decomposition of organic matter also proceed with heat absorption. This point will be considered in more detail somewhat below.

In Fig. 3, for two peat samples, curves of the rates of evolution of individual gas components are presented. The present data most...

Fig. 3. Yield of volatile substances in different temperature intervals (a—in the interval of the 1st peak, b—of the 2nd peak, v—in the third stage (up to 410–550° C)) and kinetic parameters of the process, g—moisture release, d, e—1st peak of peat pyrolysis decomposition, as a function of its degree of decomposition.

clearly show the stepwise nature of the decomposition of the organic matter of peat, while their specific, for each gas representative, form and the rather strict and narrow temperature intervals of the emission maxima, independent of the type of peat, most likely indicate that reactions of the same type proceed in these intervals.

Determination of the kinetic parameters from gas-evolution curves for several peats, including the data presented in Fig. 3, showed that for the reaction of formation of CO₂ (calculated from the first peak) the reaction order is close to first, the activation energy lies within 16–25 kcal/mol, and \(K_0\) is of the order of \(10^5\)—\(10^9\ \mathrm{min}^{-1}\); moreover, there is an undoubted connection with the composition of the peat, corresponding to the regularities revealed from Figs. 1 and 2.

The order of the reactions leading to the formation of CO (from the first peak) is close to unity or less, the activation energy is 15–20 kcal/mol, and the rate constant is \(10^5\)—\(10^7\ \mathrm{min}^{-1}\). Hydrogen release in the first peak formally corresponds to second-order reaction, and in the second peak it is closer to first order. The activation energies are, respectively, 36–40 and 30–47 kcal/mol, and \(K_0 = 10^8\)—\(10^9\) and \(10^5\)—\(10^{10}\).

For the reaction of methane formation \(n \approx 2\), \(E = 29\)—\(41\ \mathrm{kcal/mol}\), \(K_0 = 10^6\)—\(10^9\ \mathrm{min}^{-1}\).

Thus, taking into account the data on the dynamics of gas evolution, and considering the mentioned works of V. E. Rakovskii and V. A. Filimonov, it may be assumed that the first maximum on the DTG curve corresponds to the sum of the processes of desorption of hygroscopic moisture and the occurrence of dehydration reactions of organic compounds. In addition, in this interval the decomposition of water-soluble and readily hydrolyzable substances proceeds with significant release of carbon dioxide. In some experiments it can be noted that the corresponding DTA peak is complex.

By the time of the second maximum (the first decomposition peak), decarboxylation reactions develop to the greatest extent; these initially proceed with a negative thermal effect, and then the overall effect of peat pyrolysis becomes positive.

Beginning at approximately 400°C, cracking reactions develop, accompanied by strong heat absorption, which accounts for the maximum methane evolution and the first hydrogen peak at 500°C. By this point the evolution of CO₂ and the first stage of CO formation are essentially completed. The second hydrogen peak (650–700°C) apparently corresponds to condensation reactions proceeding with heat release and leading to the formation of the semicoke structure; a consequence of these reactions is also the evolution of carbon monoxide. Finally, at temperatures above 800°C, condensation reactions develop in the solid phase with the evolution of hydrogen and carbon monoxide; in this process some amount of CO₂ is formed and methane is entirely absent.

REFERENCES

1. Smol’yaninov S. I., Lozbin V. I., Belikhmaer Ya. A., Ikrin V. M. A comprehensive installation for studying the pyrolysis of solid fuels. Izv. TPI, vol. 233, 1974, pp. 37–40.

2. Smol’yaninov S. I., Lozbin V. I., Ikrin V. M., Belikhmaer Ya. A. Determination of the kinetic parameters of the process of thermal decomposition of fuel from derivatographic data. Present volume.

3. Filimonov V. A., Rakovskii V. E. Thermal effects and the mechanism of reactions of thermal decomposition of peat according to data from differential thermal intervals. In: Chemistry and Chemical Technology. (Proceedings of KPI, vol. III), Moscow, Nedra, 1967, pp. 301–302.

4. Rakovskii V. E., Filimonov V. A. Mechanism of reactions of thermal destruction of oxygen-containing components of fuels. Ibid., pp. 302–311.