The microbial removal process of pyritic sulfur from coal by the acidophilic, iron-oxidizing bacterium Thiobacillus ferrooxidans has been studied for clean coal production. The effectiveness of Thiobacillus ferrooxidans for the removal of sulfur compound from coal has been determined and the influence of some important process variables such as mineral salt concentrations, sulfur content of coal, surfactant, and pulp density and particle size of coal has been investigated.
The oxidation of aqueous acidic ferrous sulfate by Thiobacillus ferrooxidans has been studied over the initial ferrous sulfate concentration of 4∼16 g $Fe^(2+}$/L in the shake flask cultures. The oxidation rate of ferrous sulfate increased linerly with increasing initial substrate concentrations. Also, the effect of concentrated mineral salts (so call 18K medium) on the oxidation of ferrous sulfate was found to be negligible. The ferrous iron oxidation by Thiobacillus ferrooxidans was depressed by formaldehyde condensate of sodium naphthalene sulfonate (CWM 1002). This surfactant was proposed as an effective inhibitor for prevents the formation of acidic drainage from the coal refuse.
The effect of pulp density on microbial desulfurization of coal has been studied over a range of pulp density from 5% to 20% in shake-flask cultures. Approximately 82% of pyritic sulfur was removed from a 5% slurry of 0.2∼0.5mm coal particles having an initial pyritic sulfur content of 2.69% 30 days. More than 50% pyritic sulfur was removed from a 20% slurry. At low pulp densities, the rate of pyritic iron removal based on unit coal mass was decreased only slightly, therefore the rate of pyritic iron removal based on unit volume increased almost linearly with the pulp density when it is lower than 15%. However, at 20% pulp density, the rate of pyritic iron removal based on unit coal mass decreased significantly, resulting in a slight increase in the rate of pyritic iron removal based on unit volume. The use of preadapted cells as an inoculum reduced the lag time and improved the sulfur removal rate by 8% and 2% for 10% and 20% pulp density, respectively.
The effect of particle size (0.2 - 0.5mm∼2.0 - 4.0mm) on microbial desulfurization of coal has been investigated at a pulp density of 10%. The rate of sulfur removal decreased significantly with increasing particle size.
Also, the microbial desulfurization of coal has been performed in an internal loop airlift reactor. Approximately 67% of pyritic sulfur was removed from a 20% slurry of 0.2∼0.5mm coal particles having an initial pyritic sulfur content of 2.69% for 20 days. The pyritic sulfur removal was higher than that obtained in shake-flask although the experiment was run for a shorter period. The use of an external $CO_2$ supply could not improved the desulfurization rate.