Microbial desulfurization of a domestic anthracite coal by using an acidophilic, sulfur-oxidizing bacterium, Thiobacillus ferrooxidans has been studied in an airlift slurry reactor of 12 L volume. Effects of coal slurry density and CO2 supplement on microbial pyrite removal have been evaluated. High sulfur removal rates have been obtained even for very high coal slurry densities (up to 70% w/v). About 90 similar to 95 % of the sulfur in the coal could be removed in 15 similar to 20 days. The efficiency of microbial desulfurization was significantly improved with C0(2) enriched air supply for high coal slurry densities.