SETUP FOR STUDYING THE PYROLYSIS PROCESS OF SOLID FUELS

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

(Presented by the Scientific-Methodological Seminar of the CTF)

One of the main tasks in the chemical technology of solid fuel is the study of the mechanism and kinetics of the highly complex process of the thermal destruction of solid combustible minerals. Many investigations, carried out both in our country and abroad, have been devoted to this question. In most cases, the process of pyrolysis of solid fuels was studied either on volumetric or on thermogravimetric installations. Volumetric and thermogravimetric analyses complement each other in revealing the mechanism of the pyrolysis process; however, the different conditions under which the investigations are carried out do not allow the experimental data obtained by different authors to be generalized, even for one and the same object of study. Therefore, because of the complexity of the process and the dependence of the mechanism and kinetics of its course on a large number of factors, from the methodological standpoint it would be more correct to combine different methods of investigation in a single experiment.

The aim of the present work was to create a complex installation combining thermogravimetry and thermography with the simultaneous study of the kinetics of formation of the principal gaseous components during the thermal decomposition of a sample. The scheme of the installation is shown in Fig. 1.

Analysis of the sample (a charge of 300–1000 mg) is carried out in the platinum crucible of a derivatograph. The latter is supplemented with a special quartz cap for continuous withdrawal of volatile products. Tar is trapped in a filter filled with cotton wool. Water vapor is retained in a tube with anhydrone. Gas is drawn off by means of a gasometer at a constant rate, which is controlled by a rheometer. The withdrawal rate is selected experimentally so as to eliminate losses of gas evolved during decomposition of the sample and to ensure minimal dilution of the gaseous products with nitrogen.

The rate of formation of carbon dioxide during thermal decomposition of the sample is automatically recorded by the potentiometer of the chromatograph (in our case a GACH-21 chromatograph was used) with flow-type detectors. Analysis of the gas for the content of H₂, CH₄, CO is carried out on an KhT-2M chromatograph periodically, every 4 minutes.

To create an inert medium, nitrogen is supplied under the quartz beaker of the derivatograph at a rate of 15 liters per hour. Removal of oxygen impurity is accomplished by passing the nitrogen through a layer of granulated copper at a temperature of 200°C.

Fig. 1. Schematic of the setup. 1—nitrogen cylinder, 2—electric furnace, 3—rheometer, 4—tube with CaCl₂, 5—derivatograph furnace, 6—quartz bell jar, 7—quartz beaker, 8—crucible with sample, 9—filter for trapping tar, 10—tube with anhydrone, 11—tube with ascarite, 12—detector, 13—gasometer, 14—KhG-2M chromatograph, 15—GAKh-21 chromatograph.

Figure labels:
Y-axis: % content in the gas
X-axis: Temperature, °C
Legend: experiment 1; experiment 2
Curve labels: CO; H₂; CH₄

Fig. 2. Dynamics of the formation of gas components during the thermal decomposition of the nonhydrolyzable residue of peat

The combination in a single experiment of thermography, thermogravimetry, and volumetry in this setup makes it possible to obtain extensive information on the process of thermal decomposition of a sample (weight loss, rate of weight loss, thermal effects, total rate of gas formation, and the rate of formation of $\mathrm{CO_2}$, $\mathrm{CO}$, $\mathrm{CH_4}$, $\mathrm{H_2}$). The good reproducibility of the curves for the rates of formation of gas components during thermal decomposition of a sample (the nonhydrolyzable residue of peat) is illustrated by Fig. 2. It should be noted that, when constructing curves for the dependence of the rate of formation of gas components on temperature, it is necessary to introduce a correction that depends on the suction rate and on the location at which the gas sample is taken for analysis.

Conclusions

1. A setup has been developed that combines, in a single experiment, thermography and thermogravimetry with the study of the kinetics of formation of the principal gas components during the thermal decomposition of solid fuels in small samples. The setup can be constructed on the basis of a derivatograph supplemented with a special attachment that makes it possible to continuously withdraw the gaseous pyrolysis products of the sample and carry out their chromatographic analysis.