Changes to the organic functional groups of an inertinite rich medium rank bituminous coal during acid treatment processes

A South African inertinite rich medium rank bituminous coal was subjected to a series of acid treatment steps using concentrations of HF and HNO3 ranging between 1.5 and 4 M, temperatures between 65 and 80 °C and reaction time between 1.5 and 3 h for each acid. A HF and HCl procedure was used to reduce mineral matter to less than 2.5%. Analyses indicate that oxygen and nitrogen containing species are incorporated in the coal structure during the HF/HNO3 leaching processes, but not during the HCl/HF/HCl procedure. The HF/HNO3 methods lead to increased amounts of = N–OH groups formed on the remaining coal structure, whereas the HCl/HF/HCl procedure did not. The –COOH content upon HCl/HF/HCl treatment increased slightly possibly due to protonation of COO− groups by HCl. The results show that the CO2 reactivity doubled after the HF/HNO3 procedures and increased by a factor of 2.5 for the HCl/HF/HCl treatment. Surface areas of the treated coal samples increased with the highest increase for the HCl/HF/HCl method. The increased surface areas may cause the increased CO2 reactivity. The results indicate that the described acid treatment procedures with mixtures containing nitric acid have slight, but measurable, effects on the coal structure.