Abstract

Non-isothermal oxidation of brown coal with 5 wt% of Cu(NO₃)₂, 5 wt% of Ce(NO₃)₃ and {2.5 wt% Cu(NO₃)₂ + 2.5 wt% Ce(NO₃)₃} additives was studied. The introduction of additives was carried out by an incipient wet impregnation method to ensure uniform distribution of cerium and copper nitrates within the structure of coal powdery samples (according to SEM and EDX mapping). The samples reactivity was studied in an isothermal oxidation regime at 200 °C (1 h) and by DSC/TGA at 2.5 °C/min heating rate. The additives implementation was found to reduce significantly the oxidation onset temperature (ΔT_i = 20–55 °C), the samples oxidation delay time (Δt_i = 2–22 min) and overall duration of the oxidation process (Δt_c = 8–16 min). The additives efficiency could be graded in accordance with the activation on the coal oxidation in the following row: Cu(NO₃)₂ > {Cu(NO₃)₂ + Ce(NO₃)₃} > Ce(NO₃)₃. According to the mass spectroscopy, the obtained row of activation correlates well with the initial temperature of the studied nitrate’s decomposition (from 190 to 223 °C). A presence of nitrates was found to change significantly the trend of heat release taking place during the oxidation of coal samples (according to DSC/TGA data). The influence of coal morphology and volatiles content in initial sample on the parameters of the oxidation process was studied as well. Activation energy (E_a) of the coal oxidation was calculated using Coats–Redfern method. Maximum decrease in E_a from 69 to 58 kJ/mol was observed for the samples with Cu(NO₃)₂.

Graphical abstract