Kinetic and thermodynamic studies on the thermal behaviour of fly ash from lignite coals

The kinetic and the thermodynamic characteristics of the physical and the physicochemical processes, taking place under thermal treatment of fly ash (FA) from lignite coals, were investigated. The obtained results are important for the thermal stability of this material, as well as for the determination of appropriate regimes for its zeolitization, vitrification and thermal activation. The thermal treatment is essential for the development of advanced technologies for utilization of ash residues. FA was subjected to differential thermal analysis (DTA) as the heating was performed at different rates: 2.5, 5.5, 11.0, 15.0, 25.0 and 35.0 °С min−1. The obtained experimental thermograms were divided conditionally in three thermal zones: up to 200 °C, 200–675 °C and 675–950 °C, where the following thermal effects were registered: first zone – an endothermic effect, attributed to the water desorption and the liberation of adsorbed gases; second zone – two exothermic effects, related to the chemical oxidation of the magnetite in the FA composition; third zone – a small endothermic effect in the interval 760–861 °C related to the glass transition of the amorphous constituents of the FA. The activation energy Ea of the surface and the bulk oxidation of magnetite to hematite, corresponding to the exothermic peaks in the second zone, was calculated by the derived kinetic equations. The process thermodynamic parameters, such as enthalpy ΔH and entropy ΔS, were computed comparing the areas of the characteristic thermal effects of the investigated FA and an appropriate etalon. The values of Ea, ΔH and ΔS of the low- and the high-temperature oxidation were found to be E ¯ a L = 29.3 kJ mol−1, ΔHL = 1.01 kJ mol−1, ΔSL = 1.67 J mol−1 K−1, and E ¯ a H = 51.7 kJ mol−1, ΔHH = 3.97 kJ mol−1 and, ΔSH = 5.05 J mol−1 K−1, correspondingly.