Multi-step Upgrading Of Bio-oil In Supercritical Fluids

Current energy and environmental issues facing humanity are gradually becoming the two crucial factors that retard social and economic development severely. The finite reserves and the declining supplies of fossil fuels make developing new effective energy alternatives a matter of great urgency. Biomass is currently the most cost-effective route to fuels, and is expected to be "the source of great practical value to produce renewable liquid fuels". One efficient way for biomass application is to fast pyrolysis to get the pyrolysis oil (also known as "bio-oil"). Its physico-chemical properties are different from those of petroleum-derived oil, such as high moisture and oxygen content, large distillation residue amount, low pH value, wide boiling and viscosity range, and fast aging. Thermal instability, corrosiveness, low calorific value and noninfiammability, have severe adverse effects on the combustion behavior of bio-oil. So bio-oil cannot be directly applied in energy production in standard equipment unless being modified and upgraded.Unlike the traditional upgrading method which is based on the improvement of the calorific value of bio-oil via catalytic cracking or hydrodeoxygenation process, in this work, the aim is to produce stable and flammable oxygenated organic compounds (such as alcohols, ethers, ketones, and esters) through the process of coupling or merging the multiple reactions, so as to greatly reduce the hydrogen and energy consumption, significantly improve the atomic utilization of Crude bio-oil and the yield of the refined oil, and improve the fuel performance of the oil. In this work, the bio-oil was upgraded mainly through merging multiple reactions such as catalytic esterification and hydrocracking. The feasibility of application of multi-step refining method to the bio-oil upgrading process was focused on, and finally the multi-step refining system was successfully established. Moreover, the scale-up experiment and fuel performance test of bio-oil were preliminarily probed into.The feasibility of catalytic esterification of Crude bio-oil in supercritical ethanol conditions was first investigated. It was concluded that this process can be well conducted under this condition; the acids and aldehydes in the resultant oils significantly decreased, and the esters greatly increased. Furthermore, in hydrogen atmosphere, the effect of the supported metal catalysts such as Pt/Al2(SiO3)3, Pd/Al2(SiO3)3, Rh/Al2(SiO3)3, Pt/C, Pd/C, Pt/MgO, Pd/MgO and Pt/Hydrotalcite on hydrocracking of Crude bio-oil was also investigated. The obtained oil products were characterized through GC-MS and 31P-NMR analysis. The results showed that the basic supports supported catalysts exhibited the better upgrading performance; after upgrading reactions, the distillation residues were less, and the target product compounds such as alcohols/ethers, ketones and esters in light fraction were high in content.The multi-step refining method was adopted to optimize the bio-oil upgrading process. Crude bio-oil was first separated by vacuum distillation into two fractions, high water content (-75 wt.%) of low-boiling fraction (LBF,40 wt.%), and high-boiling fraction (HBF,60 wt.%) almost without water. As for LBF, supercritical methanol condition can greatly facilitate the esterification of acid constituents. By studying the effect of three types of supports Al2(SiO3)3, activated carbon and MgO supported Pt catalysts on LBF upgrading reactions, it was demonstrated that in the presence of hydrogen, the total amount of the three groups acids, aldehydes and phenols decreased from 53.8% to less than 3% on Pt/C and Pt/MgO, meanwhile the amount of esters increased from 2.1% to more than 70%. By investigating the model reactions of furfural and acetol combined with the existing experiment results, the conversion scheme of the typical components (acids, furfural and acetol) in LBF was proposed. In addition to esterification of acids, the esters can also be transformed from furfural and acetol.As for HBF, the influence of different supercritical media (n-hexane, THF, ethanol and methanol) on HBF upgrading reactions was first investigated, and the results showed that methanol was proved to be a promising supercritical medium; supercritical methanol can significantly promote the esterification reactions, and the alcoholysis ability of methanol facilitated HBF upgrading reactions to some extent. During these reactions, esterification and cracking (alcoholysis and hydrocracking) were the two dominant processes. After investigating HBF upgrading reactions in supercritical methanol conditions over a series of supported mono-metallic Pt, Ni catalysts and bi-metallic PtNi, PdNi catalysts, PtNi/MgO exhibited good performance, and gave a high yield (72.4 wt.%) of refined oil. The acid-base properties of the supports had an important effect on the coke deposition on the catalyst surface. The acidic catalysts gave the somewhat lower product yields, but tended to inhibit coking reaction. For the refined oil obtained over PtNi/MgO, the density and kinematic viscosity significantly decreased, and its pH value was near neutral; moreover, as the distillate product, this oil had no inorganic residue, and after removing the solvent and deducting the influence of water, the heating value significantly increased. The refined oil has comparable density and kinematic viscosity with bio-diesel, and comparable heating value with ethanol, which implies it could be a potential substitute, or at least partial substitute for the fossil transportation fuel. From the application of view, the multi-step refining method was a better solution. Starting from Crude bio-oil, the light fraction of finally obtained refined oil accounted for about 40 wt.% of Crude bio-oil, coke about 9 wt.%, and the heavy residue about 11 wt.%.By optimizing the experimental conditions, the more suitable condition for HBF upgrading reactions in supercritical methanol was 40 mL of methanol,10 mL of HBF ethanol dispersion,0.5 g of PtNi/MgO,0.5 MPa of initial hydrogen pressure,563 K of reaction temperature,700 rpm of stirring speed,5 h of reaction time. Under these conditions, the yield of refined oil was 63.6 wt.%, the carbon selectivity was 24.3%, the volume of gas product was 561.0 mL, O/C molar ratio was 0.62. This optimized condition was used in the scale-up process, and the results showed that the yield of refined oil greatly decreased (19.8 wt.%), the coke selectivity increased significantly (36%). After dehydration and decoloration, the product oil was used in the engine bench test to test its fuel performance; to further enhance the fuel performance of the refined oil, deep dehydration of the oil was needed.