Effect of experimental conditions on parameters derived from TG-DSC measurements of low-temperature oxidation of coal

Abstract

The process of oxygen chemisorption on coal in the temperature range ≈150–300 °C was studied under different experimental conditions using TG-DSC apparatus. As changing experimental conditions, oxygen flow (20 or 200 cm3 min−1), material of crucible (α-Al2O3 or Pt–Rh alloy), and initial sample mass (2–13 mg) were examined with respect to reliability and reproducibility of the parameters derived from TA curves. As parameters quantifying coal oxidation, temperatures of minimal Tmin and maximal Tmax sample mass, mass changes (mass loss WH below Tmin and mass increase WO above Tmin), heat evolution during oxygen chemisorption QO (related to the coal mass increase), and kinetic parameters (activation energy E and frequency factor A) were evaluated. Values of Tmax, E, and A were found to lie in very close intervals independently on experimental conditions (95% confidence intervals were Tmax = 270.2 ± 0.7 °C, E = 81 ± 3 kJ mol−1, log10A = 5.9 ± 0.3 s−1). Thus, these parameters can be used as actual characteristics of oxygen chemisorption stage of coal oxidation irrespective on conditions of TA measurements. Opposite, parameter QO was confirmed to depend clearly on initial sample mass. The dependence is different for crucible materials used; however, it tends to the same value (≈50 kJ g−1) with increasing sample mass. Further, precision of values WH, WO, and Tmin determined from TG was found to be poor. This fact complicates evaluation of the effect of experimental conditions. Finally, the effect of oxygen flow on all above parameters was found to be negligible. Its influence (if any) was hidden by common experimental errors.