The influence of nitric acid oxidation of low rank coal and its impact on coal structure

FT-i.r. was used to examine the behaviour of a Spanish lignite during its oxidative treatment with nitric acid with a view to assessing the different forms of sulphur reduction and the changes produced in the coal structure, as a result of the action of the reagent. Inorganic sulphur decreases rapidly and practically disappears, even under mild attack conditions (50 degreesC, 20% acid concentration). At first, organic sulphur undergoes a rapid decrease but more energetic conditions are required to maintain the reduction. Reduction may even reach 53% under such conditions. Unfortunately, energetic attacks (90 degreesC) lead to a high-level of coal organic matter solubilization and an increase in oxygen content. Basically, the oxygen appears as carbonyl group within the desulphurized coal. The nitric acid causes effective nitration of the coal, the nitrogen being incorporated especially as aromatic nitrogen. The substitution is easily produced (50 degreesC) when there are two adjacent aromatic hydrogens per ring. The isolated hydrogens in aryl- or polycyclic aromatic structures, are more resistant to attack under mild conditions but not so at 90 degreesC. As nitration progresses, more electrophilic molecules appear and aliphatic hydrogen tends to increase after initially decreasing under mild attack and increases more so under more energetic condition. This aliphatic hydrogen compensates for the decrease in aromatic hydrogen. (C) 2003 Elsevier Ltd. All rights reserved.