DERIVATOGRAPHIC METHOD FOR DETERMINING THE ANALYTICAL MOISTURE CONTENT OF SOLID FUELS

S. I. Smolyaninov, Ya. A. Belikhmaer, V. I. Lozbin

(Submitted by the scientific-methodological seminar of the CTF)

Methods of integral and differential thermogravimetry in recent years have found wide application in studies of processes occurring during the pyrolysis of solid fossil fuels. The capabilities of these methods are revealed especially fully when they are used jointly in the so-called derivatographic method.

However, in processing experimental data for a number of fuels, in particular for peat and brown coal, a significant overlapping of the processes of release of adsorption moisture and thermal decomposition is observed; therefore, for each sample it is necessary to carry out a preliminary determination of moisture content. We have attempted to develop a method for determining the analytical moisture content of solid fuels from derivatographic-analysis data.

Derivatography was carried out under the following conditions: TG — 200 mg, DTG — 1/3, T — 300°C, heating rate — about 5 deg/min.

Results and Discussion

For calculations of analytical moisture content from derivatographic-analysis data, characteristic points on the TG and DTG curves were taken, respectively: the inflection point on the weight-change curve and the maximum rate of weight change (Fig. 1).

Let us denote by \(x_m\) the amount of water released by the time the temperature corresponding to the maximum rate of weight change is reached. We shall denote the total amount of adsorbed water in the given charge by \(a\) (Table 1). The ratio of the amount of water released up to the inflection point recorded by the TG curve to the total amount may be called the degree of transformation:

\[ \alpha = \frac{x_m}{a}. \]

It is evident from Table 1 that the degree of transformation depends neither on the type of fuel nor on the size of the charge—in all samples, up to the maximum point on the DTG curve (or up to the inflection point on the TG curve), one half of the adsorbed water present in the sample is released (Tables 1, 2). Fol-

Fig. 1. Derivatogram of water removal from peat

Additionally, the analytical moisture content of the samples under study can be determined by the formula:

\[ W^a=\frac{2x_m}{G}\cdot 100\%, \]

where
\(x_m\) is the amount of water released up to the inflection point on the TG curve or up to the maximum on the DTG curve, mg;
\(G\) is the initial sample weight, mg.

A comparison of the results for analytical moisture content calculated from derivatographic analysis data with the moisture content determined by the standard method is shown in Table 2. The relative error, determined for the calculated values of \(w^a\) having the maximum deviation from the values determined by the standard method, does not exceed 2%.

Table 1

Characteristics of the samples studied

Values determined by the standard and derivatographic methods

Table 2

Conclusions

1. A derivatographic method is proposed for determining the analytical moisture content of peat, making it possible to carry out quantitative calculations without the use of preliminary technical analysis.

2. The relative error in determining analytical moisture content by the derivatographic method does not exceed 2%.