The effect of densification with NaOH on brown coal thermal oxidation behaviour and structure

The thermal behaviour and chemical structure of densified products of two Victorian brown coals prepared with NaOH addition were analysed by differential gravimetric analysis/differential thermal analysis (DGA/DTA) and Fourier transform infrared spectroscopy (FTIR). Acid washing of coal led to lower tendency to spontaneous combustion through a decrease in the total enthalpy change and a shift of the main mass loss stages to higher temperatures due to strengthening coal hydrogen bonding network. The addition of a small amount of NaOH (such that pH < 7.5) shifted the main stages of the DTG/DTA curves to lower temperatures and had a catalytic effect on spontaneous combustion by disrupting the hydrogen bonds network and suppressing cross-linking reactions. The products densified with higher amount of NaOH (such that pH >= 7.5) showed lower proportions of mass loss at lower temperatures stages of the DGA/DTA curves during oxidation with air. The ion exchange of Na ions for hydrogen in acid functional groups, increases the polarity and strength of bonds leading to a more rigid coal structure, reduces the surface area and decreases the coal's tendency to react with oxygen and consequently delays the onset of combustion. FTIR results showed that the order of thermal vulnerability of brown coal's functional groups toward decomposition/oxidation was as follows: aliphatic groups > carboxylic groups > carboxylate and aromatic groups > substituted aromatic groups approximate to polymerized ethers and ketones.