Desulphurization of South African coal using low power microwave energy

The effect of microwave irradiation has been experimented with using South African bituminous coals and various alkali solutions. Optimization of the treatment process was carried out on both low and high sulphur coals at varying power levels and retention times. Particle size and alkali concentration change effects, in addition to the above were studied. Changes in structure and combustion characteristics after desulphurization using a strong caustic solution were investigated. The optimum sulphur removal time and power for the high sulphur coal were difficult to detect as a result of the pyrite particle unevenly distributed within the coal structure. Coal particles of size 212 μm and 74 μm showed optimum sulphur removal after exposure time of 10 min at a power of 650 W. The larger particle size optimum was unpredictable due to the nature and distribution of the coal within the sample. Decreased sulphur contents were noted with a decrease in particle size. Structural as well as combustion characteristics were largely unaltered. Low sulphur coal exhibited very low desulphurization rates possibly due to pyrite occurrence in the coal. Pyrite occurred predominantly in the organic phase and in small quantities. Structural and combustion characteristics were largely unaltered after microwave treatment. Economic viability of low sulphur microwave treatment is thus questionable. Treatment with strong KOH solutions as well as mixtures of NaCl and NaOH yielded improved desulphurization values than that of the NaOH solution alone. Conventional leaching with NaOH was found to show enhanced sulphur removal results than that of coals treated using the microwave energy.