Urea and CaCl2 as inhibitors of coal low-temperature oxidation

Abstract

Thermal analysis was used to study the influence of CaCl2 and urea as possible chemical additives inhibiting coal oxidation process at temperatures 100–300 °C. Weight increase due to oxygen chemisorption and corresponding amount of evolved heat were evaluated as main indicative parameters. TA experiments with different heating rates enabled determination of effective activation energy Ea as a dependence of conversion. In the studied range of temperatures, the interaction of oxygen with (untreated) coal was confirmed rather as a complex process giving effective activation energies changing continuously from 70 kJ mol−1 (at about 100 °C) to ca. 180 kJ mol−1 at temperatures about 250 °C. The similar trend in Ea was found when chemical agents were added to the coal. However, while the presence of CaCl2 leads to higher values of the effective activation energies during the whole temperature range, urea causes increase in Ea only at temperatures below 200 °C. Exceeding the temperature 200 °C, the presence of urea in the coal induces decrease in activation energy of the oxidation process indicating rather catalysing than inhibiting action on coal oxidation. Thus, CaCl2 can only be recommended as a ‘real’ inhibitor affecting interaction of coal with oxygen at temperatures up to 300 °C.