Production of Biofuels from Various Lignocellulosic Streams by Liquefaction

Abstract

This PhD research project aimed to study the application of lignocellulosic biomass and waste lignin in a conventional hydrothermal liquefaction process aiming production of bio-liquids as a value-added by-product to achieve middle-range distillates used in the biorefinery streams delivering transportation fuels. Pine wood, wheat straw and LignoBoost lignin were characterised and selected for hydrothermal liquefaction and liquefaction in molten salts. To fully understand the thermal degradation of the studied feedstock, biochemical components cellulose, xylan (hemicellulose) and Organosolv lignin were also introduced and characterised by analytical pyrolysis (Py-GC-MS). Analytical pyrolysis allowed the prediction of feedstocks’ thermal degradation reaction network and assessing potential constituents of bio-liquids (bio-crudes). Metal analysis was conducted to give a better insight into the inorganic components of biomass and their potential interaction with molten salts during liquefaction. Kinetic assessments, using Kissinger, Kissinger-Akahira-Sunose (KAS) and Flynn-Wall-Ozawa (FWO) models, on the studied biomass and biochemical components were calculated using the results acquired from TGA. The activation energies from the kinetic models (KAS and FWO) were used to obtain the thermodynamic parameters, including enthalpy, Gibbs free energy and entropy. Enthalpy and Gibbs free energy results have shown that additional heat is required during the thermal decomposition. The entropy results revealed that the reaction was moving from a disorderly state to get closer to a thermodynamic equilibrium. Hydrothermal liquefaction (HTL) experiments were conducted in a 300 ml batch reactor. The impact of the temperature (275, 300 and 325 °C) during HTL processing was investigated using LignoBoost lignin and pine wood. LignoBoost lignin was found with the highest bio-crude yield of 9.31 wt. % at the highest processing temperature of 325 °C. Whilst pine wood favoured a lower processing HTL temperature (300 °C) and obtained a bio-crude yield of 8.21 wt. %. Bio-products obtained from HTL processing (gas, liquids and solids) were characterised using various analytical equipment. The aqueous phase was analysed with the specifically developed methodology using high-performance liquid chromatography (HPLC). The HPLC revealed methanol and carboxylic acids (formic, acetic and propanoic acid) were found in HTL-derived aqueous phases of LignoBoost lignin and pine wood. The molten salt mixture of ZnCl2:KCl:NaCl was used in biomass liquefaction at a molar ratio of 44.3:41.9:13.8 %, respectively. The pure molten salt was characterised using x-ray diffraction (XRD) and differential scanning calorimetry (DSC) to understand the crystallinity and the thermal behaviours when used in the liquefaction processing. The DSC was used as a benchmark analysis for the assessment of thermal behaviour and quality from the molten salts, recovered after liquefaction experiments, prior to their re-application. From the DSC analysis, the recycled molten salts maintained the desirable melting point at the temperature region of 200 °C, which was in line with the theoretical and actual melting point of the unprocessed ‘pure’ molten salt.