The influence of oxidation with HNO3 on the surface composition of high-sulphur coals: XPS study

The study reported has been undertaken to characterise the changes taking place on the surface of the pyrite-free coal oxidised with nitric acid. The method applied for this purpose was XPS and the samples studied were high-sulphur brown coal Mequinenza from Spain and Labin from Croatia. It was found that the oxidative method of pyrite removal by using HNO3 enables to efficiently desulphurise coal surfaces. The changes on the coal surface as a result of the oxidation have been proved to depend on the individual characteristic of the coal studied. In general, the exposure to nitric acid leads to an increase in the content of oxidised organic sulphur species (sulphones and sulphoxides) on the coal surface. According to the susceptibility of a given coal surface to oxidation it is either enriched in the functional nitrogen groups as a result of HNO3 oxidation or the nitrogen species formed as a result of the oxidation are removed from the coal surface in gas form (NO or/and NO2).