| Sulfur dioxide released from the combustion of fuel oils results in the serious environmental problems such as acid rain and air pollution. As concerns about environment increases, more and more stringent regulations on sulfur content in oils are being implemented to mitigate those problems. Biodesulfurization (BDS) is expected to be a complement or a promising alternative process to hydrodesulfurization (HDS) for efficient removal of heterocyclic sulfur compounds such as dibenzothiophene (DBT) at moderate operating conditions. In order to develop the new technology for desulfurization, efforts have been taken to isolate new desulfurizing bacteria native from China and study the mechanism of microbial desulfurization and its application for desulfurization of fuel oils.The first task was to screen bacterial strains with high activity to remove sulfur from organic compounds via a sulfur-specific pathway (also called 4s pathway). After isolation and incubation, five strains (YS-S-4, NCC-1, NCC-2, LB-H et at), which can specifically remove sulfur from the model compound of DBT, were selected from samples of Beijing, Shanghai, Shandong and Tianjin. Gas chromatography with mass spectrometry (GC-MS) and high performance liquid chromatography (HPLC) analysis confirmed that all the isolated strains can convert DBT to 2-hydroxybiphenyl (2-HBP) via the 4S pathway. The strain NCC-1 was identified as Rhodococcus erythropolis NCC-1 by Institute of Microbiology, Chinese Academy of Sciences and was deposited at the China General Microbiological Culture Collection Center (No. CGMCC 1.3783).The second task was to evaluate the growth and the ability of the isolated strains of biodesulfurization. DBT was used as a model compound to evaluate the isolated strains for their desulfurization ability and to assess the effects of incubation conditions. In aqueous phase, the conversion efficiency of DBT to 2-HBP exceeded 95% by the strains of YS-S-4, NCC-1 and LB-H. And the desulfurization ability was linked with the growth of strains in BSM medium containing DBT as sole source of sulfur. The optimal initial pH growth varies from 6 to 8 and the optimal temperature for growth was 30℃ for strain of Rhodococcus erythropolis NCC-1. Glycerol was the optimal carbon source and theoptimal concentration was 10 g/L. Ammonium chloride was the appropriate source of nitrogen at the concentration of 1 g/L. Strain NCC-1 could be cultivated with inexpensive inorganic sulfur and a few of DBT as a mix source of sulfur. Rhodococcus erythropolis NCC-1 able to convert dibenzothiophene sulfone (DBTO2) and 4,6-dimethyldibenzothiophene (4,6-DMDBT) to 2-HBP and 2-hydroxy-3,3'dimethyl biphenyl (2-HDMBP) respectively. The isolates could also desulfurize Benzo[b]thiophene (BT) and Phenyl Sulfide (PS). Rhodococcus erythropolis NCC-1 showed better activity and more efficient desulfurization of DBT and other forms of organic sulfur in the water phase, so it should be useful for the practical microbial desulfurization of fossil fuels.Thirdly, different type of oils, kerosene, a model oil (n-hexadecane(n-C16) containing DBT) and a hydrodesulfurized diesel oil were treated with the isolated strains. When biodesulfurized with Rhodococcus erythropolis NCC-1 cells, the total sulfur content significantly decreased, from 150mg/L to 20 mg/L for n-C16 and from 554 mg/L to 328 mg/L for diesel oil.Finally, a novel bioreactor of desulfurization from fuel oils were designed and manufactured to suit the features of bacterial growth, desulfurization activity and reaction conditions. And a new tentative plan was proposed to obtain low sulfur diesel fuel via ultrasound assisted electrochemical oxidative desulfurization. |
