Chemical desulphurisation of Thailand low-rank coals with mixture of hydrogen peroxide and formic acid - a mild oxidising treatment / Rafedah Rakal @ Zakaria

Chemical desulphurisation of Thailand coals namely Mae Moh 1 (MM1), Mae Moh (MM II) and Ban Pu (BP) with mixture of hydrogen peroxide and formic acid, a mild oxidising treatment was successfully investigated. The effect of some process parameters, such as reagent mixed ratio, peroxide concentration, leaching temperature and leaching time, was investigated. The removal of total sulphur, pyrite, sulphate and organic sulphur forms were done via chemical leaching using mixture of hydrogen peroxide and formic acid at leaching temperatures of 30, 50 and 70°C at ambient pressure with various leaching times. The optimum conditions for MMI were achieved using reagents volume mixed ratio of 30ml 6% (O2) hydrogen peroxide: 70ml formic acid with leaching temperature and time of 50°C and 24 hours, respectively. The results obtained indicated that pyrite, sulphate and organic sulphur removal increased with increasing leaching temperature at various leaching times. The reagent mixture was able to remove approximately 49% of the total sulphur and 40% of ash. Almost all of the sulphate and 25% of the organic sulphur have been removed with 72% of coal yield achieved at ambient pressure. There was a slight decreased in calorific value performance of the leached coal, which is within a tolerable limit. Further, the pyrite-free coal of MM1 was treated at optimum conditions and proved that the reagent mixture was efficient to remove inorganic and organic sulphur. A kinetic analysis on pyrite removal reaction of MMI was found to correlate well with a second-order rate equation that gave activation energy of 45.0 x 106 J kmol-1. Desulphurisation of various coals involving MMII and BP mixed with the mild oxidising agent and single reagent treatment were done under optimal conditions. Preliminary study on MMII and BP coals at different pressures (ambient and elevated pressure) was also carried out under optimal conditions. These coals were selected according to the sulphur distributions that have high organic sulphur in comparison to MMI and suitable to be applied at elevated pressure. The mild oxidising treatment used in this study was found to slightly affect the coal microstructure as revealed by the Scanning Electron Microscope – Energy Dispersive X-ray.