Thesis title: Hydrothermal liquefaction of lignocellulosic biomass to produce high quality biocrude
Hydrothermal liquefaction (HTL) is a promising technology for converting lignocellulosic biomass into high-quality biocrude, known as a sustainable alternative to fossil fuels. Hydrogen producers like Fe and Ni as hydrogenation catalyst are added to the HTL reactor to maximize the bio-crude yields and quality. To establish the best operating process conditions, the influence of variation of temperature and reaction time on the product yields was also examined.
Red mud, a main waste of aluminum industry containing high amount of Fe2O3 (20–30 %), was used for the first time as iron source in HTL; aiming to maximize biocrude yield and quality, exploiting the Fe oxidation with water to produce in-situ H2. The red mud capacity to produce H2 was investigated by reducing it with the hydrochar produced through HTL. HTL tests were performed at 330 °C for 10 min, adding an amount of red mud containing 6 wt% of Fe with respect to the biomass. The reduced red mud (RRM) demonstrated the highest activity in the conversion of biomass into high quality biocrude (yield of 49 wt% while a blank test has a yield of 33%, HHV = 30.81 MJ/kg), acting both as H2 producer and as a catalyst. Furthermore, to minimize the process waste, the recycle of water phase (WP) and the RRM were performed for 5 consecutive runs demonstrating the feasibility of the proposed process with a considerable increase in bio-crude yield (60 wt%) and quality (HHV = 30.89 MJ/kg).
Furthermore, thermal and acid treatment of red mud was conducted for HTL of lignocellulosic biomass along with its impact on biocrude yield, composition, and stability. A challenge found in red mud and Fe-catalyzed tests was that using red mud leads to metal leaching, particularly Fe and Al, which can contaminate the biocrude and hinder downstream upgrading. As a positive effect of acid washing significantly reduces metal leaching while has still the positive impact as same as non-acid washed on biocrude yields (47% yield) and improved deoxygenation reactions (HHV= 31.0 MJ/kg). In Addition, the results of HTL experiments using single red mud components (Fe203, AL2O3, SiO2, TiO2, CaO, CaCO3) are provided to see their effect on the HTL separately and make a comprehensive reaction pathway. Among them CaO has shown the maximum catalytic activity with producing approximately 43% yield.
The produced Fe assisted biocrudes still require further upgrading steps to improve its quality by decreasing heteroatoms such as oxygen, making it a suitable precursor for biofuel production. Firstly, the study focused on the heterogenous catalytic hydrothermal liquefaction (HTL) of Brachychiton populneus biomass seed (a Moroccan lignocellulosic biomass), using Ni as hydrogenation catalyst and Fe as active hydrogen producer. The activity of Ni metal and of Ni/Al2O3 in the HTL of seed (BS) and of a mixture of seed and shell (BM) was studied. The highest biocrude yields of 57.18% and 48.23% for BS and BM, respectively, in the presence of Ni/Al2O3 as catalyst and Fe as hydrogen donor. Elemental analysis results showed that in these operative conditions, an increase of the higher heating value (HHV) from 25.14 MJ/kg to 38.04 MJ/kg and from 17.71 MJ/kg to 31.72 MJ/kg was obtained for BS and BM biomass, respectively, when the combination of Fe and Ni/Al2O3 was used.
Another upgrading process has been conducted on the Fe-catalyzed biocrude by using a synthesized NiMo/Al2O3 sulfided catalyst and 80 bar pressurized H2 in Aalborg University, Advanced Biofuel Research Group. The primary objective was to evaluate the impact of Fe contamination on catalyst performance and bio-crude upgrading efficiency. Three types of bio-crudes (Blank oil (Fe-free), Fe-oil (Fe-containing), and Treated oil (Fe-reduced via citric acid washing)) were hydrotreated to assess differences in yield distribution, elemental composition, and properties. Screening tests are conducted on key parameters such as temperature, reaction time, and Catalyst/biocrude ratio to find optimal condition of upgrading process. After reaction, treated biocrude produce the highest yield and HHV with value 66% and 36 MJ/kg. Additionally, spent catalysts were characterized using TGA, SEM-EDS, XRD, BET, and FTIR to investigate catalyst deactivation mechanisms, including coke formation, metal deposition, and active site loss.
Finally, techno-economic analyses (TEA) and life cycle assessment (LCA) were conducted on an iron-assisted hydrothermal liquefaction (HTL) process for converting lignocellulosic biomass into gasoline to see process commercialize capability and environmental impact, comparing two approaches for minimizing by-product streams. The primary difference between the two approaches lies in their hydrogen source for upgrading bio-crude to bio-gasoline. Scheme 1 utilizes residual water-soluble and gaseous compounds from the process to generate the H2 needed for upgrading. Scheme 2, on the other hand, converts these waste streams into heat to supply part of the required energy, while external H2 from steam methane reforming (with or without CO2 capture) or water electrolysis (green hydrogen) is used for upgrading. Red mud, after the reduction of Fe2O3 to metallic iron, is employed in the HTL reactor as a hydrogen producer, enhancing both the yield and quality of the bio-crude while minimizing the H2 consumption in the upgrading unit. The HTL reactor was modeled based on optimal operating conditions experimentally determined while sensitivity analyses were performed on the other scheme’s units to determine their optimal conditions. Both schemes produce 459 tonnes of gasoline equivalent per day, consuming 33 tonnes of H2. Scheme 2 achieves a minimum fuel selling price (MFSP) of $0.94 per liter of gasoline equivalent (LGE), with methane reforming and CO2 capture providing the lowest emissions (1.13 kg CO2-Eq per kg of LGE). Scheme 1 has a slightly higher MFSP of $0.96 per LGE but is more environmentally sustainable, with an LCA showing 1.11 kg CO2-Eq per kg of LGE.
This research provides valuable insights into waste valorization for sustainable biofuel production, offering practical solutions to improve HTL biocrude in terms of both quantity and quality, reduce waste, and promote circular economy principles.