BIODESULFURIZATION OF ORGANIC SULFUR IN COAL BY MULTI-STAGE BIOPROCESS METHODS

• Main Author: Paisal, Yustin
• Format: Dissertations
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/54698
• Institution: Institut Teknologi Bandung

Biodesulfurization of coal, in particular organic sulfur, has received a great deal of attention in recent years, since the combustion of coal containing high sulfur causes severe environmental pollution such as acid rain because of sulfur dioxide emission. Sulfur is present in coal mainly in three forms–pyritic, organic and sulfate sulfur, and organic sulfur is the most recalcitrant form of sulfur due to its nature finely dispersed in coal matrix, which is thus more resistant to conventional chemical desulfurization and high investment and operation costs are imposed to desulfurize the most recalcitrant molecules. Therefore, the application of biodesulfurization processes using microorganisms has attracted attention as a new eco-technology or a clean alternative method to remove sulfur (particularly organic sulfur) from coals in order to achieve more efficient desulfurization as well as to minimize the environmental problems associated with combustion of high sulfur coals. In this study, biodesulfurization of organic sulfur in Tondongkura coal was carried out using a mixotrophic bacterium Pseudoclavibacter sp. strain SKC/XLW-1 capable of producing biosurfactants and oxidizing iron-sulfur by multi-stage bioprocess methods (designated BMT), consisting of biooxidation for 15 days or 40 days, followed by biomodification for 30 minutes and ended up with column flotation. In addition to the two main treatments, two control treatments were also conducted, that were column flotation only and a combination of biomodification for 30 minutes and column flotation. To facilitate description, each treatment was coded: column flotation only (method A), a combination of biomodification for 30 minutes and column flotation (method B), a combination of biooxidation for 15 days, biomodification for 30 minutes and column flotation (method C), and a combination of biooxidation for 40 days, biomodification for 30 minutes and column flotation (method D). The characteristics of coal and bacterial cells and their metabolites as well as the sulfur removal (total sulfur, sulfate, organic sulfur, pyritic sulfur) were investigated in conjunction with HPLC (high-performance liquid chromatography), FTIR (Fourier transform infrared spectroscopy), XRD (X-ray diffraction), SEM-EDX (scanning electron microscopy with energy dispersive X-ray spectroscopy) and TEM (transmission electron microscopy). The bacterium Pseudoclavibacter sp. strain SKC/XWL-1 was successfully employed to remove organic sulfur from Tondongkura coal by BMT methods by which a significant organic sulfur removal was observed for both main treatments (methods C and D), which had levels of 26.44% and 25.86%, respectively. These removals were greater than those for the two control treatments, only reaching levels of 8.85% (for method A) and 16.64% (for method B). The extent of organic sulfur removal for each treatment was in the following sequence of method C > method D > method B > method A under parameter conditions of 15 ml collector (diesel oil + coconut oil)/130 g coal/10 L tap water and 2.5 ml frother (pine oil)/ 130 g coal/10 L tap water. This observation was also supported by FTIR analyses, showing that there was a reduction of peaks at 620–690 cm-1 (ascribed to aliphatic thiol: C-S bond) and 2530–2580 cm-1 (assigned to aliphatic thiol: S-H bond). Likewise, the same observation was also evident for the removal of total sulfur, which was achieved with the extent in the following sequence: method C (48.29%) > method D (40.09%) > method A (26.21%) > method B (25.51%). Over the course of the biooxidation experiments, the bacterium Pseudoclavibacter sp. strain SKC/XWL-1 produced eight types of organic acids, including oxalic acid, citric acid, succinic acid, lactic acid, formic acid, acetic acid, propionic acid and butyric acid. Of these organic acids, the oxalic and formic acids formed complexes such as iron(II) oxalate dihydrate [Fe(C2O4).2H2O] and sodium formate [NaHCOO], which subsequently covered the surfaces of the coal particles in the sink fraction of the column flotation. The formation of both complexes was assumed to have an important role as natural depressants, resulting in the removal of sulfide and oxide minerals from coal. Moreover, the bacterium Pseudoclavibacter sp. strain SKC/XLW-1 and its metabolic products such as extracellular polymeric substances (EPS), biofilms, and organic acids were suggested to play a strategic role in creating a favorable micro-environment for the establishment of coal-trace element natural separation (cobalt iron molybdenum oxide [(Co0.7Fe0.3)MoO4] and copper vanadium sulfide [Cu3S4V]). Correspondingly, whether or not the bacterium Pseudoclavibacter sp. strain SKC/XLW-1 and its metabolic products could potentially be utilized in industrial desulfurization to be a more environmentally friendly flotation reagent, the effects of flotation reagents used in this study (i.e., diesel oil, coconut oil and pine oil) on four treatments established were statistically analyzed by estimating the significance of differences between the treatments. The main treatment of method C was observed to be statistically significant (F0.05 (1.8) = 3.46) compared with other treatments (methods A, B and D), indicating that the utilization of the bacterium Pseudoclavibacter sp. strain SKC/XLW-1 and its metabolic products (EPS, biofilms, and organic acids) is highly potential to be a more eco-friendly alternative flotation reagent in biodesulfurization processes of coal. Thereby, the findings of this study may provide an encouraging insight into the improvement of biodesulfurization processes in recalcitrant organic sulfur-containing coals by employing bacteria and their metabolites in order to create an economical and more eco-friendly desulfurization process.