Biodesulfurization of a high-sulphur and medium-ash coal product was carried out in a 5-m tall packed bed bioreactor using a mixed culture of iron- and sulfur- oxidizing microorganisms. The feed coal (4-ton) was obtained from a heavy media cyclone product from Takht-Zeitoon coal mine (Tabas, Iran) with a density of -1.4 g/cm 3 and a grain size of 0.5-25 mm, having the ash and sulfur contents of 14.92% and 2.46%, respectively. The pilot scale process was performed in a semi-continuous mode with a solution injection rate of 2.5-4.5 lit/min/m 2 , aeration rate of 4-12 m 3 /h/m 2 and solution pH of 1.7-2, over a period of 190 days. Iron and sulfate ions were controlled during the process in two 1500-L stirred tank reactors by goethite and gypsum precipitation processes, respectively. The coulumn bioleaching in a period of 170 days was followed by a washing step with HCl to remove sulfate precipitates. Results showed that the total sulfur and ash contents reached to 1.81% and 11%, respectively, which were corresponded to 26.12% sulfur removal and 25.80% ash removal. It was indicated that biodesulfurization has a good industrial potential to treat coarse-grained high sulfur coals.
