Bacterial removal of sulphur from three different coals
Introduction
In the coal dependent developing countries, including China and India, there is concern over potential sulphur emissions in the future. Some micro-organisms are capable of metabolising both sulphur and pyrite in coal and to utilise the energy, released to support their growth. The idea of exploiting acidophilic pyrite oxidizing bacteria as a means of cleaning coal is quite old [1]. Zarubina studied the oxidation of pyrite in Soviet coals in the presence of Thiobacillus ferrooxidans. The principle underlying microbial coal desulphurisation, with special emphasis on inorganic sulphur removal, is based on a complex combination of spontaneous (non-biological) and microbiologically catalysed oxidations of inorganic sulphide minerals present in coal. The microorganisms convert the inorganic sulpur to sulphate which is soluble in water [2]. After the completion of the cleaning process, the coal is separated from the liquid phase containing the sulphate and washed with water to obtain fuel with a lower sulphur content.
In the present investigation of microbial desulphurisation of coal, Thiobacillus cultures isolated from three coals were used for desulphurisation. The effect of various parameters including pH, particle size, pulp density, media composition etc. on microbial desulphurisation were studied in all the three cases using shake flasks. In addition, the effect of contact time on microbial desulphurisation of lignitic coal was studied with 2% (w/v) pulp density.
Section snippets
Coal samples
Three coal samples, an Assam coal, a Polish bituminous coal and Rajasthan lignite, were used for the microbial desulphurisation experiments. The Assam coal sample was obtained from the North Eastern coal fields, Tinsukia district, Assam. It contained 5–6 wt% of sulphur and 0.92 wt% of ash. Much of the sulphur was in organic form. The Polish coal sample was from the Janina coal mine at Libiaz. This contained 2–3 wt% of sulphur and 14.6 wt% of ash. The lignite sample was collected from Giral lignite
Results and discussion
The three coal samples were desulphurised using their respective native Thiobacillus isolates Tf-R, Tf-A and Tf-P.
Conclusions
The lignite was found to be the most suitable coal for microbial desulphurisation experiments because:
(a) It is a comparatively young coal. The pyrite in such coals are weakly attached to the coal making the bond easy to break under microbial influence [13].
(b) It has high content of pyritic sulphur for which the adaptability of the bacterial culture, Tf-R, is good [21].
(c) The photomicrographic studies suggested that the pyrite in lignites was present in framboidal form or as isolated euhedral
Acknowledgements
The authors wish to thank Dr R.P. Das, Director and Head, Hydro and Electrometallurgy, Regional Research Laboratory, Bhubaneswar, for his permission to publish this paper. Also they wish to thank Dr Irena Twardoska, Scientist, Institute of Environmental Engineering, Zabrze, Poland, for providing the Polish coal sample. The first author wishes to thank CSIR for granting her Senior Research Fellowship.
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