Catalytic properties of some mineral salts in relation to the process of decomposition of pyrolysis tar

Abstract

The paper discusses the results of an experimental study of the thermal decomposition of pyrolytic wood tar in a homogeneous process, as well as in the presence of a number of mineral substances: potassium sulfate, magnesium sulfate, monopotassium phosphate and the mineral residue of the production of potassium salts. The experiments were carried out on a laboratory setup under isothermal conditions at temperatures of 300, 350, and 400 °C by two methods providing two-phase and three-phase contact with the catalyst, respectively. It has been established that the rate of decomposition of pyrolysis tar under the studied conditions obeys the laws described by the Avrami–Erofeev equation with a variable exponent n. The area of change of this indicator in various experiments includes values from 0.415 to 1.238. The mean n value calculated for all study options was 0.694 (95 % CI 0.605 to 0.783), and the median value was 0.639. It has been found that the decomposition rate of the pyrolysis tar increases when MOX particles, potassium sulfate, and magnesium sulfate are introduced into the reaction zone. No effect of monopotassium phosphate on the rate of decomposition of the pyrolysis tar was not found. It is shown that in the case of tailings of potassium salts production an increase in the rate of decomposition of pyrolysis tar is due to the catalytic effect of this substance. The most likely reason for the positive effect of potassium sulfate, as well as magnesium sulfate on the rate of decomposition of pyrolysis tar, is apparently also the catalytic effect of these substances. The Arrhenius parameters of the homogeneous process of thermal decomposition of the pyrolysis tar and the decomposition process in the presence of tailings of potassium salts production, as well as potassium and magnesium sulfates, were determined. According to the data obtained, the activation energy of the homogeneous process was 29.6 kJ/mol, and the pre-exponential factor was 3.15·101 min–1, respectively. The Arrhenius parameters of the decomposition of pyrolysis tar in the presence of tailings of potassium salts production, according to the data of the performed studies, were 23.0 kJ/mol and 1.82·101 min–1. The paper also determined the Arrhenius parameters of the decomposition of pyrolysis tar in the presence of K2SO4 and MgSO4. According to estimates, the activation energy in the presence of these substances was about 50–60 kJ/mol. The research results presented in the article can be used in the design of heat generating equipment, which includes a system for cleaning products of thermochemical conversion of vegetable raw materials.