Pyrite microbial desulfurization

Rudimentary investigations of microbial desulfurization have received little attention in the literature at this time (2). One successful example of desulfurization is the removal of pyrite from coal by Thiobacillus sp. and Ferrobaccus sp. (3). While studies of the complex hydrocarbon-sulfur systems are of great value, being closer to in situ reality, investigation of a defined system should form the foundation of these more detailed studies. 

Beyer et al. (49) found that, during microbial desulfurization, pyritic sulfur decreases and elemental sulfur increases with time, whereas the organic sulfur remains unchanged. They suggested that microbial oxidation of pyrite produces ferric sulfate [Fe2(S04)3] and that the simultaneous inorganic reaction of ferric iron with pyrite produces elemental sulfur and ferrous iron, as follows ... 

The full potential for removing pyritic sulfur from various coals by physical coal cleaning is significant but difficult to achieve. However, SO2 control by precombustion removal of pyrite could be an important S02-emissions reduction strategy. The cleaned coal produced could be used in coal-fired utilities, constructed both pre-and post-NSPS, as well as in industrial boilers. To realize the potential for coal cleaning in actual practice, however, new techniques must be demonstrated in the laboratory and then at the "proof-of-concept" scale (approximately one ton of coal per hour). These new coal beneficiation techniques could be advanced physical-coal-cleaning (PCC) processes, or they could employ microbial desulfurization or chemical desulfurization to remove organic sulfur. These latter processes could be used by themselves or in concert with PCC processes. 

Mechanism of Microbial Desulfurization. The microbial dissolution of pyritic sulfur in coal by acidophilic bacteria has been thoroughly investigated (17,18,29). The pyrite is readily oxidized by oxygen or ferric ion, resulting in the ferrous state as follows ... 

Most microbial desulfurization studies have been conducted in the laboratory shake-flask type experiments, and the major drawback cited against such a process has been that the rates of pyritic sulfur removal were not high enough to reduce the reactor size to a reasonable capacity (2,6). In this study an attempt has been made to determine the effectiveness of T. ferrooxidans under simulated pipeline conditions for pyritic sulfur removal. Since the microbial desulfurization process is conducted under acidic environment, an attempt has been made to determine the corrosion rates under dynamic conditions using Illinois //6 and Indiana 3 bituminous coals and to investigate the effectiveness of a commercial corrosion inhibitor for controlling the corrosivity. 

About 80% pyritic sulfur removal has been achieved by microbial desulfurization of Illinois 6 and Indiana 3 coals using T. ferrooxidans in laboratory shake-flask experiments and in a two-inch pipeline loop. The 10 to 25 wt% coal/water slurry was recirculated at 6-7 ft/sec for 7 to 12 days at 70-90°F. Results also show that the rates of bacterial desulfurization are higher in the pipeline loop under turbulent flow conditions for particle sizes, 43 to 200/m as compared to the shake-flask experiments. It is visualized that the proposed coal slurry pipelines could be used as biological plug flow reactors under aerobic conditions. The laboratory corrosion studies show that use of a corrosion inhibitor will limit the pipeline corrosion rates to acceptable levels. 

The chemical methods are not effective in removing major amounts of organic sulfur. Thus, removal of total sulfur to produce a clean-burning fuel is very difficult. It is envisioned that the optimum desulfurization of coal will include physical, chemical, and microbial treatment. The raw coal will be ground fine to liberate most of the pyritic sulfur and then processed by using the column... 

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