ACCELERATED METHOD FOR DETERMINING THE MOISTURE CONTENT OF SOLID FOSSIL FUELS

S. I. Smolyaninov, V. N. Ponomarev, V. I. Makarenko

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

To the present time there is no fully reliable single analytical method that makes it possible to determine the content of one or another type of moisture in solid fossil fuels. Being, for example, satisfactory for determining moisture in hard coals, the same method is not reliable in the analysis of brown coal or peat.

In direct methods, moisture is removed by drying in a stream of inert gas or in a boiling liquid that does not mix with water, and is determined by the gain in weight of the absorbent or by direct measurement of the volume in a graduated receiver.

In indirect methods, moisture is likewise removed from the substances being analyzed, but its content is determined not by direct measurement of the quantity of water evolved, but by the decrease in weight of the dehydrated residue, i.e., by the difference in weight between the initial and the dried substance.

Dehydration of a weighed portion of the substance being analyzed may be carried out in various ways: by heating in a drying oven at a temperature of 102–105°, by keeping it over concentrated sulfuric acid in a desiccator at room temperature, by drying in an electric field of high-frequency currents, or by infrared rays.

Indirect methods based on other principles are also known, for example on changes in dielectric properties, on the use of the heat of dehydration of water of crystallization, etc.

The standard method for determining moisture has become widespread because of its sufficient simplicity and accuracy. But its disadvantage is the duration of the determination.

At present there exists a whole series of accelerated methods for determining moisture. But all of them suffer from a drawback—considerable inaccuracy, and some of them require complex apparatus for carrying out the analysis. In addition, many of the accelerated methods for determining moisture are carried out at high temperatures (about 150°). In this case it is necessary to take into account also that the organic mass of some solid fossil fuels undergoes thermal destruction with the formation of CO₂ and H₂O. This, naturally, distorts the results of the analyses, which is especially noticeable when drying peat and brown coals.

We propose using the SESH-3 drying oven for determining the moisture content of solid fossil fuels.

The drying oven is a small drying chamber protected by a layer of thermal insulation. The drying chamber is equipped with

with a rotating table, driven by a geared motor with a worm reduction unit, placed in the heater casing opposite the exhaust branch pipe of the fan. The operating temperature in the drying chamber is provided by an electric heater located in the lower part of the cabinet and is maintained at the specified level by means of a thermoregulator. Intensive air exchange in the cabinet is effected by a fan. A special cooler is supplied with the drying cabinet for cooling the dried samples.

This apparatus is produced commercially, and therefore there is no particular need to develop special, no less complex instruments, as has been proposed by a number of authors.

In carrying out experiments to determine the moisture content of solid combustible minerals, various samples of hard coals (gas coal, coking coal, anthracite), brown coals, and peat were taken for analysis.

From these samples analytical specimens were prepared, whose moisture content was determined by standard methods and by drying in an SESH-3 drying cabinet. Several aluminum cups with weighed portions of the material under study were placed in the SESH-3 drying cabinet. The weight of a portion was 1–2 g. Every 5 min one of the cups was removed, cooled in the cooler, and the moisture content of the sample was determined from the loss in weight of the portion. All weighings were performed with an accuracy of 0.0002 g. The results of experiments on determining moisture in analytical samples of hard coals are given in Fig. 1. From the graphs it is clear that, for all coals, removal of moisture proceeds very rapidly. Approximately 10 min after the start of heating, the moisture content of the portion reaches a value close to the moisture content determined by the standard method. The discrepancy between the moisture values of the sample under investigation by the two methods does not exceed 0.2 percent, which is within the permissible discrepancies for parallel determinations. Similar results were obtained in determining the moisture content of brown coals, as shown in Fig. 2.

Fig. 1. Dependence of weight loss (in percent) on drying time for hard coals: 1—curve—coking coal; moisture determined by the standard method—1.13%; 2—curve—anthracite; moisture determined by the standard method—1.13%; 3—curve—gas coal; moisture determined by the standard method—1.99%.

Fig. 2. Dependence of weight loss (in percent) on drying time for drying samples of brown coals: 1—curve—brown coal 1; moisture determined by the standard method—4.84%; 2—curve—brown coal 2; moisture determined by the standard method—5.78%.

According to the standard method, drying of an analytical peat sample with a maximum particle size of 0.28 mm lasts 2 hours; each control drying lasts 0.5 hour.

When drying in the SESH-3 drying oven, the drying time of an analytical peat sample is reduced to 15–20 min (Fig. 3). The weighed portions were dried until the difference in weight between two successive weighings became less than 0.001 g. When determining moisture in very wet peat and in hydromass, the duration of the initial drying by standard methods is 4–6 hours, whereas when drying in the SESH-3 drying oven this time is reduced to 30–35 min. The difference between the values obtained by the standard method and by the method we propose is no more than 0.3–0.5 percent (Fig. 4).

Fig. 3. Dependence of weight loss (in percent) on drying time of peat samples: curve 1—Bakcharskii peat; moisture determined by the standard method—8.7%; curve 2—Taganskii peat; moisture determined by the standard method—9.7%.

Fig. 4. Dependence of weight loss (in percent) on drying time of samples of very wet peat: curve 1—Bakcharskii peat; moisture determined by the standard method—81.5%; curve 2—Taganskii peat; moisture determined by the standard method—87.5%.

On the basis of the studies carried out, the following procedure for determining moisture may be proposed.

A weighed portion of an analytical sample of the substance under study, 1–2 g, is placed in an aluminum weighing bottle. The weighing bottles with the portions, open, are placed in a SESH-3 drying oven preheated to a temperature of 105–110°, which is maintained throughout the entire drying period. When determining moisture in coals, the duration of the initial drying is 8–12 min. After drying, the weighing bottles are closed with lids and placed in a cooler. The cooling time is 2–3 min. The check drying lasts 3–4 min. When determining the moisture of an analytical peat sample, the initial drying time is 20 min, and the check drying time is 5 min. All weighings are carried out on an analytical balance with an accuracy of up to 0.0002 g.

When analyzing peat ground to a maximum particle size of 2–3 mm, portions of 2–3 g are taken. The portion is dried for 30–35 min. After cooling, the weighing bottles are weighed and placed in the SESH-3 drying oven for check drying for 10 min, after which the weighing bottles are cooled and weighed again. All weighings are carried out with an accuracy of up to 0.01 g.

The limits of permissible discrepancies in the analysis of the same analytical sample between parallel determinations are 0.3–0.5 percent when the peat is ground to 2–3 mm and 0.2 percent when the peat is ground to a fineness of 0.28 mm.

It follows from the above that the accelerated method for determining moisture in the SESH-3 drying oven, in terms of analytical accuracy, is not inferior to the ty-

according to GOST and significantly reduces the analysis time. The possibility of applying the described method, which has increased accuracy in comparison with other accelerated methods, may be quite broad.

Conclusions

1. It is proposed that determinations of the moisture content of solid fuels be carried out in the SESH-3 drying oven.

2. A procedure has been developed for the accelerated determination of the moisture content of hard coals, brown coals, and peat, reducing the determination time severalfold compared with the standard method, while maintaining the same accuracy.