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Given its global accessibility and high organic content, biogenic waste such as sewage sludge currently represents a valuable renewable resource for energy production. Hydrothermal liquefaction (HTL) stands out as one of the most suitable technologies to convert these feedstocks into biocrude, a valuable biofuel precursor. This process operates at moderate temperatures and high pressure in the presence of water, eliminating the need for energy-intensive preliminary dewatering steps when treating high-moisture feedstocks. This study focuses on investigating the effects of various reaction temperatures and holding times on biocrude yield and quality obtained from batch HTL conversion of digested secondary sludge derived from paper mill facilities. Experiments were conducted at temperatures of 280, 300 and 330 °C, with holding times ranging from 0 to 35 minutes. Optimal conditions were identified at 300 °C and holding times between 10 and 35 minutes, resulting in biocrude yields of 20-21%, higher heating values (HHV) of 35 MJ kg-1 and energy recovery of 54-59%. The findings underscore the potential application of HTL in waste biomass disposal cycles, contributing to waste minimisation and enhancing the bioenergy recovery.

Syngas tars are responsible for clogging and corrosion of pipelines and equipments due to their high condensation temperatures. At the same time, considering their energy content they cause a reduction in the energy efficiency of the conversion process. To overcome these drawbacks, the addition of a downstream reactor to perform hot catalytic gas cleaning has been proposed. However, the occurrence of significant carbon deposition and sulphide formation on the catalytically active surfaces can easily lead to early deactivation of the catalyst. Chemical looping tar reforming is based on a solid material, known as oxygen carrier, that undergoes two reactions steps: (1) reduction by interaction with the gas and tar streams; (2) regeneration by oxidation with ambient air, which also involves the combustion of any deposits on the particle surface. In this work, the first reaction step of the process is investigated in an integrated setup involving the pyrolysis or the gasification of hazelnut shells and the reaction with the oxygen carrier for tar abatement. Two reactor configurations have been considered: (1) single reactor, where the biomass and the oxygen carrier beds are loaded in series into the same reactor; (2) two reactors, where the two beds are loaded into different reactors in series. Blank tests for pyrolysis and gasification are also carried out for comparison. The results indicate that the two beds configuration enables higher tar conversion (89% wt for pyrolysis and 75% wt for steam gasification), though the presence of the oxygen carrier causes a reduction in the energy content of the syngas, especially in terms of H2 concentration, which is reduced from around 34% to 21% mol for pyrolysis and from 28% to 21% mol for steam gasification.

Hydrothermal liquefaction is a promising technology for liquid biofuel production from a wide range of organic wastes. Waste biogenic feedstocks with a high moisture content are particularly suitable for this purpose due to the possibility to feed wet materials and to obtain high liquid yields in hydrothermal liquefaction (HTL). Although, yields and quality of the obtained liquid products are usually strongly dependent on the composition of the feedstocks, and due to their variability, it is often difficult to have reliable predictions. However, biogenic waste can be easily schematize based on their content of organic macro-components, mainly polysaccharides, proteins, and lipids. This work tries to summarize the effect of the variation of feedstock’s composition on yields and the quality of HTL products, with a particular focus on binary interactions between the macro-components. Cellulose, egg albumin and sunflower oil are used as model compounds to represent polysaccharides, proteins, and lipids, respectively. HTL tests are carried out in micro autoclaves of 10 mL using these model compounds alone and in binary and ternary mixtures as feedstocks, at 330°C and 10 minutes of retention time. Results showed that the biocrude yields did not follow the behaviour predicted by the linear combination of the three compounds but an increase of biocrude production and a reduction of solid residue is obtained for the mixtures. GC-MS results showed the presence of compounds related to Maillard reactions and amides formation. Some general reaction pathways were summarized to explain these results. The comprehension of these interactions can guide the future research to obtain a prediction model for biofuel production through a HTL process.

Hydrothermal liquefaction (HTL) is one of the advanced biomass conversion technologies to produce bio-crude from wet lignocellulosic feedstocks. Hydrochar and water phase containing organics are always generated as by-products and their efficient re-use could be fundamental to decrease the whole energy consumption. Hydrogen producers like Fe are often added to the HTL reactor to maximize the bio-crude yields and quality. Furthermore, Fe can be easily recovered from the biochar at the end of the reaction and re-used, after a reduction treatment. In this work, the feasibility to produce bio-crude through HTL of pinewood in a continuous zero-waste process scheme is evaluated in an Aspen Plus® simulation. The zero- waste configuration was carried out using the water phase containing organics instead of distillate water in the HTL reactor and the hydrochar as renewable reductant of iron oxides. Experimental data were used to model the HTL (Ryield) while the iron oxide reduction and the combustion of the side streams were simulated in the Gibbs reactor (Rgibbs). Red mud, a waste stream of the Bayern process for aluminium production, containing 50 wt % of hematite (Fe2O3), was selected as low-cost iron source. The iron oxides reduction with hydrochar was performed at different temperature (400-1200 °C) to determine the optimal value (complete conversion to Fe). Based on the simulation results, hydrochar allows the complete red mud reduction at 780 °C but the separation of the carbon excess before Fe recirculation must be considered. The proposed continuous zero-waste HTL plant consumed a total energy of 5080 J s-1, mostly of it related to the HTL (4945 J s-1) and iron oxide reduction (640.2 J s-1). However, the combustion of both off gases and hydrochar excess can approximately provide the heat needed to plant (-5012 J s-1). Finally, the results demonstrated that the proposed HTL scheme might be a zero-waste process in terms of both mass and energy flows.

Chemical recycling is considered one of the most sustainable solutions to limit the environmental issues related to plastic waste pollution, whereby plastic is converted into more valuable compounds when mechanical recycling is not feasible. Among the most critical fast-growing components of municipal solid waste, polystyrene represents 1/3 of the filling materials in landfills. In this work, the chemical recycling of polystyrene via two main thermochemical processes is investigated: pyrolysis and hydrothermal liquefaction (HTL). The influence of temperature (HTL: 300–360 °C and pyrolysis: 400–600 °C) and reaction time (HTL: 1–4 h; pyrolysis: 30 min) on the products obtained was studied. The obtained liquid and solid products were analyzed by using gas chromatography-mass spectrometry (GC-MS), an elemental analysis (EA), Fourier-transform infrared spectroscopy (FT-IR) and a thermogravimetric analysis (TGA). During HTL, a temperature of 360 °C and reaction time of 4 h were needed to completely decompose the polystyrene into mainly oil (83%) and water-soluble compounds (10%). The former was mainly composed of aromatics while the water phase was mainly composed of aromatics and oxygenated compounds (benzaldehyde and acetophenone). The pyrolysis led to the formation of 45% gas and 55% oil at 500 °C, and the oil was 40% styrene. Pyrolysis was thus more selective towards the recovery of the styrene monomer while the HTL can be an effective process to produce renewable aromatics.

Hydrothermal liquefaction (HTL) is one of the most promising technologies to produce valuable compounds from biomass and waste. The use of water as solvent makes this process extremely convenient for high moisture feedstock and also environmentally sustainable. However, the obtained product, the bio-crude, is not ready to the end use, its oxygen content is quite high making the oil physically and chemically instable and thus difficult to handle and store. The use of heterogeneous catalysts and hydrogen producers can improve the product quality during the hydrothermal process being also easy to be separated and recirculated. In this work the use of reduced red mud acting as hydrogen producer is tested in the hydrothermal process of oak wood. Red mud is composed mainly by Fe2O3 that was reduced with the char produced by HTL and by a simulated syngas which is obtained from char gasification. The reduced red mud was mixed with the biomass and fed into the HTL batch reactor with variable red mud biomass ratio. The reduction temperature was optimized in order to obtain zero valent Fe able to produce hydrogen reacting with water in HTL conditions. The tests were conducted at 330 °C with a reaction time of 10 min. The obtained bio-crude was characterized with elemental analysis. The results in terms of oil yield and quality were compared with those obtained with pure iron powder showing that red mud can be used successfully as hydrogen producer in HTL process and recycled after its reduction with char or syngas. The use of red mud leads to an increase of the oil yield of 20% with respect to the blank test and looking at the oil composition the hydrogenation effect is evident, the amount of hydrogen increases while the amount of oxygen decreases.

The use of H2 as fuel of the future is closely linked to the development of Fuel Cells, among them Proton Exchange Membrane Fuel Cells (PEMFCs) are the most attractive. To avoid the irreversible poisoning of the platinum-based catalyst placed on the PEMFC electrodes, pure H2 (CO < 10 ppm) is required. Steam iron process (SIP) is a cyclical process which allows, at high temperature and low pressure, the direct production of pure H2 by redox cycles of iron. Syngas is generally used as reducing agent while steam water is used to oxidize iron and to produce pure H2. However, iron oxides powders suffer from deactivation in few redox cycles due to their low thermal stability. The aim of this study is to improve iron oxides resistance adding Al2O3 as high thermal stability material. Bioethanol is used as renewable sources of syngas to makes the process totally sustainable. To evaluate the effect of Al2O3 addition, different Fe2O3 / Al2O3 ratios were tested (40 wt%, 10 wt%, 5 and 2 wt%). The stability of the synthetized particles was evaluated with 10 redox cycles comparing the results with that of commercial Fe2O3 powders. Al2O3 does not behave as inert material in the process but it actively participates in the reduction step, catalysing coke formation due its acidity. With the sample 98 wt% Fe2O3- 2 wt% Al2O3 the best performances in terms of particles stability and hydrogen purity were obtained.