EFFECT OF THE COMPONENTS OF GROUP COMPOSITION ON THE THERMAL DECOMPOSITION OF PEAT

V. I. Lozbin, S. I. Smolyaninov, V. P. Kudinov

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

The theory of the thermal decomposition of solids has now been developed quite extensively. Determination of the kinetic parameters of the decomposition of various inorganic and individual organic compounds presents no particular difficulties. However, the weakest point in this respect is the process of pyrolysis of solid fossil fuels, despite the fact that thermal processing of fuels has been carried out for more than one hundred years. This is explained by the complexity of the structure of the elementary structural units of solid fuels and by the occurrence of a large number of consecutive-parallel reactions within a narrow temperature interval during thermal decomposition [1].

Whereas the kinetics of the thermal decomposition of solid fuels under rapid heating has been clarified in its main features in the works of Z. F. Chukhanov and co-workers [2], the study of the kinetics of fuel pyrolysis using slow heating became possible only with the appearance of thermobalances with programmed heating of the sample and recording of the rate of weight change (Stanton thermobalances, derivatograph, etc.).

The purpose of the present work was to elucidate the kinetics of the thermal decomposition of peat and to determine the influence of group composition on the pyrolysis process of the initial peat. In numerous studies on the thermal decomposition of peat, the authors give temperature intervals for the decomposition of individual components of the group composition [3—4]. V. E. Rakovskii [4], using the thermographic method, identified maxima characteristic of the decomposition of bitumens, water-soluble and readily hydrolyzable substances, humic acids, and the nonhydrolyzable residue, which appear on the thermogram of the initial peat. However, no works have been published on the rate of decomposition of individual components of the group composition of peat and on the influence of these components on the rate of peat decomposition.

The study was carried out on a derivatograph of the F. Paulik, J. Paulik, L. Erdey system, in a nitrogen atmosphere. The heating rate was 5 degrees/min; sensitivity: T—600°C, TG—200 mg, DTG—1/3, DTA—1/5; sample weight—200 mg.

The characteristics of the peat samples studied are presented in Table 1. For one of the samples (Table 1, No. 6), the components of the group composition were extracted and their derivatographic investigations were carried out. Curves of the rate of volatile release from different components of the group composition are presented in Fig. 1. Then 6 peats were selected that differed in a definite ratio of the components of the group composition (Table 1); their derivatographic studies were carried out, and the effective activation energy of thermal decomposition was determined by the method proposed by Ya. A. Belikhmaer [5].

Discussion of the Results

The curve of the rate of volatile release for the original peat is characterized by a broad temperature range of peat decomposition—150–500°C—with one distinct maximum at a temperature of 265°C and a certain increase in the rate of gas evolution in the range 400–500°C (Fig. 1, a). On the derivatogram of bitumens, the rate of volatile release (Fig. 1, b) gradually increases with increasing temperature from 150 to 500°C, reaching a maximum only at 475°C. After removal of the bitumens, no increase in the rate is observed on the decomposition-rate curve of the debituminized peat (Fig. 1, v) in the range 400–500°C.

Fig. 1. Dependence of the decomposition rate of the components of the group composition on temperature: a) original peat; b) bitumens; v) debituminized peat; g) water-soluble and readily hydrolyzable; d) residue after removal of w/s and r/h; e) humic acids; zh) nonhydrolyzable residue.

The decomposition of the water-soluble and readily hydrolyzable substances proceeds intensively in the range 100–250°C, with a maximum at 160° (Fig. 1, g). After the water-soluble and readily hydrolyzable substances are removed from the peat, the decomposition-rate curve of the part of the peat containing fulvic acids, humic acids, and the nonhydrolyzable residue is characterized by a sharp increase in the decomposition rate in the range 200–350°C, with a maximum at 280°C (Fig. 1, d). Such a sharp increase in the decomposition rate in this temperature range is probably characteristic of the thermal decomposition of fulvic acids, since the thermal decomposition of humic acids (Fig. 1, e) and of the nonhydrolyzable residue (Fig. 1, zh) proceeds comparatively calmly over a broad temperature range.

Apparently, the water-soluble substances, readily hydrolyzable substances, and fulvic acids decompose at a high rate; consequently, during pyrolysis of the original peat, bitumens, humic acids, and the nonhydrolyzable residue limit the rate of volatile release (Fig. 2, a, b).

Fig. 2. Dependence of the decomposition rate on temperature for peats with different contents of the sum of water-soluble, readily hydrolyzable, and fulvic acids.
a) 40% water-soluble, readily hydrolyzable, fulvic acids;
b) 64% water-soluble, readily hydrolyzable, fulvic acids.

The effective activation energy determined for the pyrolysis of peats with different contents of bitumens, humic acids, and nonhydrolyzable residue decreases as the sum of the latter increases (Table 2).

Conclusions

1. The group composition affects the rate of thermal decomposition of peat.

2. As the content of bitumens, humic acids, and nonhydrolyzable residue in peat increases, the decomposition rate decreases.

3. The effective activation energy decreases as the sum of bitumens, humic acids, and nonhydrolyzable residue increases.

LITERATURE

1. V. E. Rakovskii, F. L. Kaganovich, E. A. Novichkova. Chemistry of Pyrogenic Processes. Academy of Sciences of the BSSR, Minsk, 1959.
2. Z. F. Chukhanov. Doklady Akademii Nauk SSSR, Vol. XXII, 4, 1950.
3. O. Abel, H. Luther. Erdöl u. Kohle, No. 2, 15, 1962.
4. V. E. Rakovskii, V. A. Filimonov. In: Chemistry and Chemical Technology, “Nedra,” 1967.
5. Ya. A. Belikhmaer, K. K. Stramkovskaya. Izv. TPI (in press).