(38a) Hydrothermal Upgrading of Algal Bio-Oil By Supercritical Water

Global warming is a significant problem the world is currently facing as result of greenhouse gasses being emitted from the continual usage of fossil fuels. One particular solution that has been examined to reduce the rate of global warming is using bio-crude as a fuel replacement. Bio-crude is a more desired renewable fuel because liquid fuels are easier to transport and have higher energy densities than solid and gaseous fuels. Bio-crude is produced by the thermochemical or biochemical conversion of biomass through methods such as liquefaction, pyrolysis, solvent extraction, etc. Just like crude oil, bio-crude needs to be refined before it can be used; this is to remove oxygen and other heteroatoms that lower its fuel quality. The most common method of upgrading crude oil is hydro-deoxygenation which involves treating crude with hydrogen at high temperatures. Hydrogen is predominantly produced by natural gas reforming, thus it would be counter intuitive to use a fossil fuel in order to improve a renewable energy technology.

Supercritical water (SCW) treatment is examined as an alternative method to upgrade crude in a more sustainable manner. Water at supercritical conditions becomes a single phase and exhibits compressibility and density similar to that of gasses and liquids, respectively. Furthermore, its dielectric constant significantly decreases with increasing temperature, causing it to behave like a nonpolar solvent. SCW is also considered an environmentally-friendly solvent as it turns to regular water at ambient temperatures. These ideal solvent properties are proposed to be useful in crude upgrading as it is expected that SCW breaks down longer chain hydrocarbons while promoting oxidation reactions to remove heteroatoms from the crude.

This goal of this study was to upgrade bio-crudes produced from hydrothermal liquefaction (HTL) of microalgae (N. salina) with SCW in order to remove impurities, decrease oxygen content, reduce viscosity, and produce desired low carbon (C₆-C₁₄) alkane products. The bio-crude was treated with and without Pd/C catalyst at temperatures of 380, 400, and 420 °C at a fixed SCW density of 278 kg m^-3_, thus allowing pressure to be monitored than controlled. The upgraded bio-crude quality was assessed by evaluating energy content, ultimate analysis, GCMS, FTIR, NMR, viscosity, and density. The energy content of the treated bio-oil increased with reaction temperature and catalyst loading as a result of the decrease in oxygen and sulfur content. Viscosity and density decreased due to the decomposition of large molecules, creating shorter straight chain hydrocarbons.