STUDY OF THE MECHANISM OF PEAT THERMAL BRIQUETTING BY THE EPR METHOD

S. I. Smolyaninov, S. T. Maslov, G. F. Morozova

(Presented by the scientific-methodological seminar of KhTF)

In obtaining lump fuel from peat by the method of thermal briquetting, preliminary heating of the starting material to temperatures on the order of 260–280°C is necessary. These temperatures are relatively low; however, changes in a number of indices of group composition [1], the yield of pyrobi­tumens [2], functional composition, and plasticity [3] indicate that the initial stages of thermal destruction may proceed rather deeply.

Since the role of chemical processes in the formation of the strong structure of a thermal briquette may prove appreciable, we have attempted to assess it using the method of electron paramagnetic resonance.

At present there is no unified opinion on the nature of paramagnetic centers in solid combustible minerals and solid residues, and on their heat treatment. The first and numerous investigators [4–8] explained the paramagnetism of these materials by the presence of free radicals.

A. A. Berlin [9], as well as N. N. Tikhomirova and co-workers [10], see the cause not in broken chemical bonds, but in a system of conjugated bonds of aromatic condensed networks. A. A. Sidorov and V. F. Anufrienko [6, 7] explain the presence of a broad signal in EPR spectra by the formation of free radicals, and of a narrow one by condensed aromatic systems. N. K. Larina with co-authors [11] attribute paramagnetism in solid fuels to oxygen bound to aromatic carbon in groups of the araxyl type.

Yu. B. Tyutyunnikov and co-workers [12] found on the curve of the dependence of the number of paramagnetic centers in coals on the temperature of their heating two characteristic maxima at 250–300°C and 550–650°C. They associate the first with the presence of conjugated bonds, and the second with the formation of free radicals as a result of the appearance of microcracks during the solidification of the plastic mass.

Without entering into a discussion of this question, we considered it possible, in explaining the change in the paramagnetism of peat at low heating temperatures, to proceed from the standpoint of the existence of free radicals.

In any case, the change in the concentration of paramagnetic centers is connected by us with certain chemical transformations.

organic matter of peat, the result of which is an increase in the reactivity of the solid residue.

Peat samples (Table 1) were heated to various temperatures and were also held for different times at specified temperatures, followed by “freezing.” Measurements were carried out on an RE-1301 radiospectrometer with a side standard of CuCl·2H₂O. The results obtained are shown graphically in Figs. 1, 2 and are given in Table 2.

Table 1

Characteristics of peat samples

From Fig. 1 it is evident that, as the heating temperature increases, the number of PC in peat increases, reaches a maximum at 275–280°C, and begins to decrease up to 300–310°C, after which it again rises sharply. The same regularity was established by Yu. B. Tyutyunnikov [12] for various coals.

Table 2

Changes in the number of paramagnetic centers in peat during thermal briquetting

It is interesting to note that the temperature of the maxima of PC accumulation coincides (allowing for the correction for self-heating) with the optimal temperature of peat thermal briquetting.

When peat is heated to the temperature of thermal briquetting, thermal decomposition of its constituent parts occurs and free radicals are formed in the liquid, gaseous, and solid pyrolysis products.

The first two types of radicals, owing to their high mobility, are rapidly consumed. In the solid residue, however, because of spatial hindrances, the rate of formation of free radicals in individual temperature regions exceeds the rate of their consumption, and active particles accumulate (Fig. 1, Table 2). During pressing of the heated charge, spatial hindrances are eliminated, the number of contacts increases, free radicals react with one another, forming chemical bonds, and their number decreases significantly (Table 2). This fact may serve as direct evidence of the role of chemical-bond formation in creating the strong structure of the thermobriquette.

The results of studying the kinetics of the change in the number of PC in heated peat (Fig. 2) are curves with a maximum, which is characteristic of consecutive reactions and agrees well with the curve of the change in the strength of thermobriquettes as a function of the holding time of the heated peat before pressure is applied [13].

Fig. 1. Change in the number of paramagnetic centers in peat during heating.
1 — ○ — peat 1; 2 — △ — peat 2; 3 — ● — peat 3

Fig. 2. Effect of the holding time of heated peat on the content of paramagnetic centers in it. Heating temperature: 1 — ● — 220°C; 2 — ○ — 250°C; 3 — △ — 280°C

The activation energies of PC formation calculated from the modified Arrhenius equation [14] (Table 3) range from 6 to 20 kcal/mole. The activation energy calculated from the kinetic curves of gas evolution lies within the same limits.

Both of the latter facts speak in favor of the radical hypothesis for the occurrence of PC in solid fuels during heating, and of the intermediate character of the free radicals formed, which, apparently, arise upon cleavage of side functional groups, as well as as a result of the disaggregation of the central molecules of certain constituent parts of peat.

Table 3

Activation energy of formation of paramagnetic centers in peat

In conclusion, we would like to thank Associate Professor Yu. A. Zakharov, who kindly provided the opportunity to record EPR spectra in his laboratory, and engineer A. A. Vasiliev, who carried out the work of recording the spectra.

Conclusions

1. It has been shown that heated peat has one of the maxima in PC content at 270–280°C.
2. The activation energies of PC formation in peat have been calculated.
3. A conclusion is given in favor of the presence of chemical interaction in the formation of the strong structure of a thermal briquette.

References

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