The thermal decomposition behavior of hard coal fly ash (HCA2), obtained from the combustion of an Australian hard coal in
thermoelectric power plants, in different atmospheres (air, N2 and N2-H2 mixture), was studied using thermogravimetry (TG), infrared-evolved gas analysis (IR-EGA), differential scanning calorimetry
(DSC) and thermodilatometry (DIL) techniques. It was found that changing of the applied atmosphere affects the carbon content
of the ash which results in different thermal decomposition behaviors. In air, the carbon content was oxidized to carbon dioxide
before the decomposition of carbonate. In N2 or in N2-H2 atmospheres, the carbon content acts as a spacer causing a fewer points of contact between calcium carbonate particles, thus
increasing the interface area which results in a decrease of the carbonate decomposition temperature. Following the carbonate
decomposition, the iron oxide content of the ash undergoes a reductive decomposition reaction with the unburned carbon. This
oxidation-reduction reaction was found to be fast and go to completion in presence of the N2-H2 mixture than in the pure nitrogen atmosphere due to the reducing effect of the hydrogen.
The kinetics of the carbonate decomposition step, in air and N2-H2 mixture was performed under non-isothermal conditions using different integral methods of analysis. The dynamic TG curves
obeyed the Avrami-Erofeev equation (A2) in air, and phase boundary controlled reaction equation (R2) in N2-H2 mixture. The change in the reaction mechanism and the difference in the calculated values of activation parameters with the
change of the atmosphere were discussed in view of effect of the atmosphere on the carbon content of the ash.