Production And Upgrading Of Bio-oil In Sub-and Supercritical Fluids

Being different from solar, wind and other renewable energy resources, biomass is the only renewable energy resource that contains carbon. Biofuels derived from biomass have the potential to replace gasoline, diesel and natural gas as the alternate transportation fuels in the future. Supercritical fluids present unique heat and mass transfer abilities and controllable properties, and have a wide range of applications in material synthesis, chemical reaction and other fields. Supercritical fluid technology has been proven to be able to improve certain processes in biomass conversion and biofuels production. As biomass feedstocks are highly distributed with low energy densities, thus are difficult to collect and transport, it is more appropriate to first locally convert biomass feedstocks to intermediate products such as bio-oil or bio-crude to increase energy density and decrease transportation cost. Then the intermediate products are transported to a central biorefinery and upgraded to high quality biofuels. This paper was based on the above biomass utilization and biofuels production route. Fast pyrolysis and hydrothermal liquefaction (HTL) processes were applied to locally convert lignocellulosic biomass and low lipid content microalgae to produce pyrolysis bio-oil and HTL bio-crude, respectively. Supercritical fluid technology was introduced to improve the HTL process. Upgrading of bio-oil and HTL bio-crude in supercritical fluids were also investigated.In this paper bio-oil from fast pyrolysis of Pinus sylvestris L. through a fluidized bed reactor was upgraded over bifunctional catalysts (Pt and Pd on HZSM-5and SO42-/ZrO2/SBA-15) in supercritical monoalcohols (methanol and ethanol). Upgrading of the bio-oil distillation residue performed effectively as the removal of acids and aldehydes and the decrease of ketones, phenols, sugars, and PAHs were achieved. Esters became dominant in the upgraded oil, and the heating value of bio-oil was elevated. The effects of solvents and catalysts on the upgrading process were investigated. Ethanl and Pd were favorable to lower the coke formation while mesoporous catalyst support SO42-/ZrO2/SBA-15was conductive to the removal of aldehydes.Although the pretreatment achieved over98wt.%water removal, but the upgraded oil still contained significant amount of water. To solve this problem and to effectively use the bio-oil distillate, upgrading of the bio-oil without pretreatment was proposed. In this paper, upgrading of the bio-oil was conducted under the same reaction condition, and the upgraded oil was compared to that from upgrading of the bio-oil distillation residue. The upgraded oil from upgrading of bio-oil was higher in heating value and water content. The feasibility and advantages of this path was verified based on the analysis of the upgrading products and the used catalysts.On the other hand, this paper demonstrated the feasibility of producing bio-crude from low lipid content microalgae Chlorella pyrenoidosa through HTL using ethanol as the solvent. The effects of temperature, processing gas and catalyst on bio-crude yield and quality under sub-and supercritical ethanol conditions were investigated. Supercritical ethanol condition was essential for the conversion of microalgae. The bio-crude yields decreased and the higher heating value (HHV) increased as the temperature further increased. Hydrogen as the processing gas increased the bio-crude yields and HHV, and inhibited the formation of char. Catalysts had no significant effect. A potential reaction pathway for the presented HTL process was proposed. As a conclusion, the presented HTL process produced bio-crude with HHV of36MJ.kg and H/C and O/C ratio of1.53and0.11, respectively. Ethanol consumption and volatile components ultilization were the major concerns to improve the energy efficiency according to mass and energy balance analysis. Initial attempts of upgrading of the HTL bio-crude in supercritical ethanol was conducted with supported Pt, Pd catalysts. The upgrading process had no significant effect on bio-crude yield or elemental composition, however, the high boiling point components in bio-crude decreased due to the promoted cracking reactions by metal catalysts. This paper further compared the HTL of microalgae under subcritical water and supercritical ethanol conditions. The effects of operating parameters including solvent, feedstock load ratio, solid load ratio and initial nitrogen pressure on products yields and system pressures were studied. The results showed that HTL under subcritical water conditions was insensitive to the operating parameters within the measuring range. While HTL under supercritical ethanol conditions produced higher bio-crude yields, however the solid residue yields increased dramatically under high feedstock load ratio conditions. This paper originally proposed the use of ethanol-water solution as the solvent to improve the presented HTL process, and the products yields were succesfully optimized under high feedstock load ratio condition.