PROCEEDINGS
OF THE TOMSK POLYTECHNIC INSTITUTE
of the Order of the October Revolution and the Order of the Red Banner of Labor
named after S. M. Kirov

Volume 250
1975

ON DETERMINING THE TEMPERATURE OF THE ONSET OF THERMAL DECOMPOSITION OF PEAT FROM THE RESULTS OF DERIVATOGRAPHIC ANALYSIS

S. I. SMOLYANINOV, V. I. LOZBIN

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

From the theory of thermography and derivatography it is known that, with an increase in the heating rate, adjacent peaks on DTA and DTG curves, which characterize the occurrence of certain reactions in a substance, often merge, forming one common maximum. In this case it is impossible to clearly delimit the temperature intervals over which these reactions proceed [1, 2]. A characteristic example of such superposition of rates is the process of removal of adsorption moisture and the initial stage of thermal decomposition during heating of peat at a constant rate of temperature rise.

A decrease in the heating rate leads to partial separation of the peaks; however, it is generally known that in these cases the sensitivity of the apparatus is significantly reduced. Thus, the heating rate can be reduced only to certain limits, at which the rates of evolution of volatiles for the two reactions \(A\) and \(B\) still overlap significantly (Fig. 1).

When air-dry peat is heated under conditions of a linear temperature rise, the process of moisture removal and the beginning of thermal decomposition of the organic mass of peat occur simultaneously in the interval \(100—150^\circ\mathrm{C}\), imposing a definite imprint on the course of the integral (TG) and differential (DTG) curves of weight change. Consequently, in this region the experimental data obtained reflect the total process

\[ \left(\frac{dx}{dT}\right)_{\mathrm{sum.}} = \frac{du}{dT}(E_1;\ B_1;\ u) + \frac{dv}{dT}(E_2;\ B_2;\ v), \tag{1} \]

where \(\dfrac{du}{dT}\) is the rate of water removal,

\(\dfrac{dv}{dT}\) is the rate of peat decomposition.

Having the total curve of the rate of evolution of volatiles, the beginning of decomposition of the organic mass of peat can be determined only in the case when the kinetic constants of the first process are known, that is, \(E_1\) and \(B_1\).

In previous works [3—4] a method was given for determining the activation energy of reactions of this type, and it was shown that the process of removal of adsorbed moisture from peat is described sufficiently well by equations

formal kinetics, and the experimentally determined order of the process is close to two;

\[ \frac{du}{dT}=\frac{K}{b}(a-u)^2, \tag{2} \]

i.e., up to the temperature of maximum rate, half of the adsorbed water of the peat is removed.

Separating the variables and integrating, we obtain

\[ K=\frac{u}{a-u}\cdot\frac{b}{aT}, \tag{3} \]

and for the point of maximum rate

\[ K_m=\frac{b}{aT_m}, \tag{4} \]

Fig. 1. Toward the calculation of the rates of reactions partially superimposed in a certain temperature interval

Fig. 2. Calculated curves of the rate of water removal and thermal decomposition of peat

since \(u_m=a-u=\dfrac{a}{2}\) (for a second-order reaction).

The pre-exponential factor for the first reaction is found from the equation for the dependence of the rate constant on temperature

\[ B=K e^{\frac{E}{RT}}, \tag{5} \]

where

\[ E=3.52\,\frac{T_m T_i}{\Delta T}\quad \text{when}\quad \frac{du}{dT_i}=\frac{1}{2}\frac{du}{dT_m}, \]

\[ T=T_m,\quad K=K_m. \]

The simplicity of the subsequent operations is obvious: for a number of points at temperatures above \(T_m\), the rate constants, current concentrations, and rates for these temperatures are determined according to expressions 5, 3, and 2. By subtracting from the total

\[ \left(\frac{dx}{dT}\right)_{\mathrm{sum}} \]

the obtained values

\[ \left(\frac{du}{dT}\right)_i \]

determines a series of values of the rate of removal of volatiles. By extrapolating this curve to the zero line, the value of the temperature at which the thermal decomposition of peat begins is determined (of course, within the sensitivity limits of the apparatus used).

Table 1

Characteristics of the samples studied

For the investigations, peat samples with degrees of decomposition of 5, 20, and 50% were taken. Derivatographic analyses were carried out under the following conditions: \(T = 300^\circ\mathrm{C}\), TG — 100 mg, DTG — 1/3, heating rate — about 4 deg/min. The necessary characteristics of the peat are shown in Table 1. By the method described above, the course of the curves of the rate of removal of adsorbed moisture from peat at temperatures above \(T_m\) was calculated. The results are presented in Fig. 2. The temperature at which peat pyrolysis begins is approximately the same for all the samples studied and lies within the range 125–135°C. The sample weight has no noticeable effect on the course of the moisture-removal rate curves, on the temperature of the rate maximum, or on the temperature at which pyrolysis begins.

REFERENCES

1. G. O. Piloyan, Introduction to the Theory of Thermal Analysis. Moscow, 1964.
2. H. Jüntgen and K. H. van Heek. Fuel, 2, 103, 1968.
3. S. I. Smol’yaninov, V. I. Lozbin, Ya. A. Belikhmaer. Izv. TPI, vol. 233.
4. S. I. Smol’yaninov, V. I. Lozbin. “Izv. TPI,” vol. 233.