Study On New Methods For Fuels Desulfurization Based On Reduction With Sodium Borohydride

The consumption of coal and oil accounts for the major part of the total energy consumption in China. Both coal and oil contain a larger amount of sulfides which will be transferred into large amounts of sulfur oxides after combustion. So many sulfur oxides emitted into the atmosphere will bring about a lot of environmental pollution problems such as acid rain, haze. Furthermore, the sulfides in fuel oils can poison the catalyst in automotive exhaust treatment system. In order to mitigate the increasingly serious environmental pollution problems, it is very necessary to desulfurize coal and oil. On the other hand, most countries around the world have established more stringent regulation to restrict the sulfur content in fuel oil. China also is accelerating the upgrading of the quality of fuel oil. The standard of the sulfur content in fuel oil is gradually approaching the level of developed countries. Both the hydrodesulfurization for the desulfurization of fuel oil and the traditional technologies for the desulfurization of coal prior to combustion have their drawbacks. Therefore, recently all countries around the world are devoted to the development of new technologies for the desulfurization of coal and fuel oil. In this study, based on the reduction desulfurization method with NaBH4, two new methods were established for the desulfurization of coal and fuel oil. One is the extraction-reduction method which is a combination of reduction desulfurization method with NaBH4 and extraction method with ionic liquid(IL). The other one is NaBH4 reduction desulfurization-electrochemical regeneration method which is a combination of reduction desulfurization method with NaBH4 and regenation method of NaBH4 by electrochemical reduction. The conditions of desulfurization reactions and desulfurization mechanism were investigated. The aim of this study is to establish new desulfurization methods of fuel oil and coal and provide new options for the desulfurization industries of fuel oil and coal. The main research contents and results are as follows:(1) Model and real gasoline were desulfurized by extraction-reduction method in which the ionic liquid 1-butyl-1-methylpyrrolidinium trifluoromethanesulfonate([C4mpyr][OTf]) was used as the extractant and NaBH4 and nickel salts was used as the reductant. The factors that influenced desulfurization efficiency and the regeneration performance of IL were investigated. Results indicated that desulfurization efficiency reached more than 97% for model gasoline and more than 93% for real gasoline under the conditions of B/S molar ratio = 9, Ni/S molar ratio = 3, oil/IL volume ratio = 3, water content in IL = 5%, reaction time = 50 min. The IL [C4mpyr][OTf] has negligible solubility in real gasoline whereas the solubility of real gasoline in [C4mpyr][OTf] was 4.5 wt%. The 1H, 13 C and 19 F NMR spectra of the recycled ILs were nearly similar to that of the original ILs, which indicated that the ILs maintained their original structures after desulfurization and regeneration. On the other hand, the desulfurization performance of the recycled ILs was also investigated and results showed that the desulfurization efficiency suffered from a slight decrease after every recycle.(2) Desulfurization mechanism and reaction kinetics of extraction-reduction method were investigated. The desulfurization reactivity of organosulfur compounds followed the order of BT(DBT) > 3-MBT > 4, 6-DMDBT, which indicated that the desulfurization reactivity is significantly affected by steric hindrance. The products of different model organosulfur compounds after desulfurization were analyzed by Gas Chromatography-Mass Spectrometer(GC-MS) and their corresponding reaction routes were proposed. The effectiveness of different nickel salts followed the order of NiCl2(Ni(OAc)2) > NiSO4 > Ni(NO3)2. Elements contents of various aqueous solutions from desulfurization process were analyzed. Results showed that most of the removed sulfur(S) was transformed into S2- and almost all the Ni from NiCl2·6H2O was precipitated as nickel boride. A large part of NaBH4 hydrolyzed to produce active hydrogen and a small part of NaBH4 reacted with NiCl2 to give nickel boride. Finally, desulfurization kinetics was also probed and results showed that desulfurization reactions of both model sulfur compounds and real gasoline could be treated as pseudo-first-order reactions.(3) Model and real diesel fuel were desulfurized by NaBH4 reductive desulfurization-electrochemical regeneration method in which the working electrode, counter electrode and reference electrode were the boron-doped diamond(BDD) thin film electrode, graphite electrode and saturated calomel electrode, respectively. Pulse voltage was supplied for the electrolysis. The voltage range for electroreduction of NaBO2 into NaBH4 and the factors that influenced desulfurization efficiency were investigated. The result of cyclic voltammetry analysis showed that the voltage range for electroreduction of NaBO2 into NaBH4 should be from-1.2 V to-1.8 V. Desulfurization efficiency reached more than 93% for model diesel fuel and more than 86% for real diesel fuel under the conditions of-1.5 V forward pulse voltage, 0.3 V reverse pulse voltage, 1.5 s forward pulse duration, 0.5 s reverse pulse duration, 0.2 mol/L NaBO2 concentration, 1.2 mmol/L NiCl2 concentration, 1/3 volume ratio of oil to electrolyte, 1.5 h electrolytic time.(4) Desulfurization mechanism and reaction kinetics of NaBH4 reductive desulfurization-electrochemical regeneration method were investigated. The result of 11 B Nuclear Magnetic Resonance(NMR) confirmed that NaBO2 was converted into NaBH4 by electroreduction. The products of model diesel fuel after desulfurization were analyzed by GC-MS and their corresponding reaction routes were proposed. Elements contents of various aqueous solutions from desulfurization process were analyzed. Results showed that most of the removed S was transferred to the electrolyte and almost all the Ni from NiCl2·6H2O was precipitated as nickel boride. Most of the boron(B) still remained in the electrolyte, i.e., most of the B can be recycled. Desulfurization mechanism was proposed based on the above analysis. Finally, desulfurization kinetics was also probed and results showed that desulfurization reactions of both model organosulfur compounds and real diesel fuel could be treated as pseudo-first-order reactions.(5) Coal was desulfurized by NaBH4 reductive desulfurizationelectrochemical regeneration method in which the working electrode, counter electrode and reference electrode were the boron-doped diamond thin film electrode, graphite electrode and saturated calomel electrode, respectively. Pulse voltage was supplied for the electrolysis. The factors that influenced desulfurization efficiency and the change of characteristics of coal after desulfurization were investigated. Results indicated that the optimal reaction conditions were as follows.-1.5 V forward pulse voltage, 0.5 V reverse pulse voltage, 2 s forward pulse duration, 1 s reverse pulse duration, 0.2 mol/L NaBO2 concentration, 0.8 mmol/L NiCl2 concentration, 50 g/L coal concentration and 2.5 h electrolytic time. Under the optimal reaction conditions, the removal efficiency of total sulfur reached 64.1%. Thereinto, the removal efficiency of iron sulfide(81.9%) was highest, the removal efficiency of sulfate(78.9%) took second place and the removal efficiency of organic sulfur(52.5%) was lowest. The analytical results of physicochemical and combustion characteristics of coal before and after desulfurization indicated that the calorific value of coal increased by 1.8% and the ignition temperature of coal decreased 10 ℃ after desulfurization. All these results indicated that high desulfurization efficiency of coal could be obtained by NaBH4 reductive desulfurization-electrochemical regeneration method. Furthermore, the quality of coal could not be destroyed after desulfurization.