Catalytic Hydrothermal Liquefaction of Eucalyptus: Effect of Reaction Conditions on Bio-oils Properties.

Abstract
Hydrothermal liquefaction (HTL) is a more promising technique for the direct conversion of a variety of wet biomass feedstocks into bio-oils with high conversion rates and improved fuel properties. As a wet processing technology, HTL is generally carried out in aqueous media of at least 50%–60% water, around 280–400°C temperature, and pressures between 10 and 30 MPa, making it very suitable for most natural biomasses and organic residues. The applicability of lignocellulosic biomass as a promising feedstock for next generation transport fuels has been demonstrated by the successful integration of first generation bioethanol and biodiesel into the existing infrastructure. In comparison to FeSO4, and ZnSO4 catalysts, alkaline catalysts such as Na2CO3, NaOH, K2CO3 and KOH have shown a better effect on the yield and quality of bio-oil obtained during the HTL of lignocellulosic biomass feedstocks. Previously, liquefaction of sweet sorghum bagasse was carried out using K2CO3 and KOH as catalysts and a high biocrude yield of 61.8% and 42.3% was obtained with a low yield of solid biochar as side product. Eucalyptus as a main source of fiber for pulp is widely used for paper production, furniture and construction material, which generates large amounts of biomass residues, can be important renewable raw materials for the production of biofuels and high-value chemicals. In the present study, catalytic HTL of eucalyptus biomass was carried out with K2CO3 at 350 and 400 °C temperature and the results thus obtained were compared with the biocrudes obtained without catalyst under similar reaction conditions. All the samples including eucalyptus biomass (dry basis), biocrudes, solid products and aqueous phases were characterized using different characterization techniques such as CHN, GC-MS, TGA, ICP, HHV, TOC (total organic carbon) and potassium concentration analyzer. Furthermore, the effect of reaction conditions on the biocrude yield, biocrude properties and amount of solid products (biochar) were also analysed. The results showed that the change in reaction temperature from 350 to 400 °C have influenced the product distribution, allowing to decrease both the biocrude yield (35.79 to 29.86%) and energy recovery (70.19 to 58.90%) for each phase. However, the solid biochar yield was increased from 9.46 to 13.64% under similar reaction conditions. Moreover, the aqueous phase obtained from the first HTL run was also used for the recirculation experiments by keeping the catalyst amount constant to see the influence of water-soluble organics on the energy balance, yields and properties of HTL biocrude. Therefore, the present study provides a unique method to develop a feasible process under controlled operating conditions for the valorization of lignocellulosic biomass feedstocks to refinery intermediates.