How Do Hydrothermal Liquefaction Conditions and Feedstock Type Influence Product Distribution and Elemental Composition?

Effect of temperature on hydrothermal liquefaction of high lipids and carbohydrates content municipal primary sludge

The study assessed the valorisation of primary sludge through HTL and the influence of temperature on the product distribution. The experiments were conducted at different temperatures, 30 min reaction time, and 100 rpm stirring rate. The maximum yield of biocrude produced was 39.47% at 270 °C. The best yield of oils was 23.96% at 300 °C. The lowest yield of asphaltenes was 12.50% at 240 °C. HHV for biocrude were always between 39 and 41 MJ/kg, close to petroleum. Best energy recovery for biocrude was 82% at 270 °C.

Waste animal fat with hydrothermal liquefaction as a potential route to marine biofuels

Unused animal waste rendered fat is a potential feedstock for marine biofuels. In this work, bio-oil was generated using hydrothermal liquefaction (HTL) of nitrogen-free and low sulfur rendered bovine fat. Maximum bio-oil yield of 28 ± 1.5% and high heating value of 38.5 ± 0.16 MJ·kg‒1 was obtained at 330 °C at 50% animal fat solid load and 20 min retention time. The nitrogen and sulfur content were negligible, making the produced bio-oil useful marine biofuel, taking into account current stringent regulations on NOx and SOx emissions. The economic analysis of the process, where part of the bovine fat waste is converted to the bio-oil and the semi-solid residues can be used to supply the heat demand of the HTL process and alternately generate electricity, showed that our process is likely to generate a positive profit margin on a large scale. We also showed the growing economic importance of electricity in the revenues as commercial production becomes more energy efficient.

Conversion of cud and paper waste to biochar using slow pyrolysis process and effects of parameters

A series of laboratory studies were undertaken in Gondar to explore the effects of temperature, air mass flow rate, heating rate, and residence duration on cud and waste paper char yields in slow pyrolysis. Cud and waste paper were burned at a low pyrolysis temperature to generate biochar (167 °C). The rate of decomposition depends on the feedstock and the process conditions. The biochar yield is mostly governed by the applied regulated temperature and airflow rate, according to the data. During the experiment, the main airflow rate delays the pyrolysis process. The temperature rises when both the primary and secondary air inlets open at the same time, resulting in lesser biochar output. The experiment was carried out at a slow pyrolysis temperature of 167 °C, with 15% biomass moisture, 60% humidity, and a 0.35-1.5 kg/s air mass flow rate. At this temperature, 30 kg of feedstock, cup, and paper in the reactor generate 10 kg-23kg and 10-20 kg of biochar, respectively, at a 0.35 m/s airflow rate. As the airflow rate increases within the restricted values, a temperature gradient appears and tends to increase. However, as the pyrolysis temperature and airflow rate rise, the biochar yield decreases.

Hydrothermal liquefaction of sewage sludge anaerobic digestate for bio-oil production: Screening the effects of temperature, residence time and KOH catalyst

Due to sewage sludge being an abundant biobased resource, and with the number of biogas plants utilizing sewage sludge increasing, digested sewage sludge (DSS) is a promising feedstock for producing bio-oil. This study uses DSS from a biogas plant to produce bio-oil in a hydrothermal liquefaction process adjusting time from 2 to 6 hours, temperature from 280 to 380°C and the presence of a base as a depolymerization agent and potential catalyst. High conversion yields are obtained, with the maximum of 58 wt% on a dry, ash free basis and an energy recovery of up to 94%. The oils contain compounds with a potential for utilization as biofuels and building blocks, especially fatty acids as biodiesel feedstock and biobased phenols, glycols and aliphatic alcohols.

Direct conversion of almond waste into value-added liquids using carbon-neutral catalysts: Hydrothermal hydrogenation of almond hulls over a Ru/CNF catalyst

The almond industry leaves behind substantial amounts of by-products, with almond hulls being the primary residue generated. Given that one way to improve food security is by decreasing waste to reduce environmental impacts, developing sustainable processes to manage this by-product is necessary. Herein, we report on the hydrothermal hydrogenation of almond hulls over a carbon-neutral Ru supported on carbon nanofibres (Ru/CNF) catalyst, addressing the temperature, H₂ pressure, time and catalyst loading. These variables controlled the distribution of the reaction products: gas (0-5%), liquid (49-82%) and solid (13-51%), and ruled the composition of the liquid effluent. This aqueous fraction comprised oligomers (46-81 wt%), saccharides (2-7 wt%), sugar alcohols (2-15 wt%), polyhydric alcohols (1-8 wt%) and carboxylic acids (7-31 wt%). The temperature and reaction time influenced the extension of hydrolysis, depolymerisation, deamination, hydrolysis, hydrogenation and dehydration reactions. Additionally, the initial H₂ pressure and catalyst loading kinetically promoted these transformations, whose extensions were ruled by the amount of H₂ effectively dissolved in the reaction medium and the prevalence of hydrogenations over dehydration/decarboxylation reactions or vice versa depending on the catalyst loading. Process optimisation revealed that it is feasible to convert up to 67% of almond hulls into merchantable oligomers at 230 °C, 35 bar initial H₂, using 1 g catalyst/g biomass (0.4 g Ru/g biomass) for 360 min. Additionally, decreasing the temperature to 187 °C without modifying the other parameters could convert this material into oligomers (31 wt%) and small oxygenates (17 wt% carboxylic acids, 11 wt% sugar alcohols and 6 wt% polyhydric alcohols) concurrently. The theoretical energy assessment revealed that the total and partial combustion of the spent solid material could provide the required energy for the process and allow catalyst recovery and reutilisation. This environmental friendliness and holistic features exemplify a landmark step-change to valorising unavoidable food waste.

Inorganic Salt Catalysed Hydrothermal Carbonisation (HTC) of Cellulose

The presence of inorganic salts either as part of the substrate or added to the reaction medium are known to significantly affect the reaction pathways during hydrothermal carbonisation (HTC) of biomass. This work aims to understand the influence of salts on hydrothermal carbonisation by processing cellulose in the presence of one or more inorganic salts with different valency. Batch experiments and Differential Scanning Calorimetry were used to investigate the change in reaction pathways during hydrothermal conversion. The effect of salts on the rate of HTC of cellulose can be correlated with the Lewis acidity of the cation and the basicity of the anion. The effect of the anion was more pH-dependent than the cation because it can protonate during the HTC process as organic acids are produced. The introduction of salts with Lewis acidity increases the concentration of low molecular weight compounds in the process water. The addition of a second salt can influence the catalytic effect of the first salt resulting in greater levulinic acid yields at the expense of hydrochar formation. Salts also play an important role in cellulose dissolution and can be used to modify the yield and composition of the hydrochars.

Co-Hydrothermal Liquefaction of algal and lignocellulosic biomass: Status and perspectives

Hydrothermal liquefaction (HTL) effectively converts biomass to biofuels, thereby limiting the endless reliance on petroleum products derived from fossil fuels. However, the conversion is based on individual feedstock in the HTL process. In order to, further boost the conversion, HTL can be done by blending various feedstock, mainly algal and lignocellulosic biomass. Bibliometric analysis was carried out, and it was observed that there have been very few studies on Co-Hydrothermal Liquefaction (Co-HTL). There still exist several crucial gaps in process optimization when co-reactants are used due to their synergistic effects. The reaction kinetics and mechanism, catalyst screening and by-products application require further studies. Therefore, R&D is necessary to optimize the process to completely utilize the complementarity of the feedstocks under study resulting in better quality of products which require minor/ no upgradation steps.

Biocrude Production from Hydrothermal Liquefaction of Chlorella: Thermodynamic Modelling and Reactor Design

Hydrothermal liquefaction can directly and efficiently convert wet biomass into biocrude with a high heating value. We developed a continuous hydrothermal liquefaction model via Aspen Plus to explore the effects of moisture content of Chlorella, reaction pressure and temperature on thermodynamic equilibrium yields, and energy recoveries of biocrude. We also compared the simulated biocrude yield and energy recoveries with experiment values in literature. Furthermore, vertical and horizontal transportation characteristics of insoluble solids in Chlorella were analyzed to determine the critical diameters that could avoid the plugging of the reactor at different flow rates. The results showed that the optimum moisture content, reaction pressure, and reaction temperature were 70–90 wt%, 20 MPa, and 250–350 °C, respectively. At a thermodynamic equilibrium state, the yield and the energy recovery of biocrude could be higher than 56 wt% and 96%, respectively. When the capacity of the hydrothermal liquefaction system changed from 100 to 1000 kg·h−1, the critical diameter of the reactor increased from 9 to 25 mm.

Hydrothermal Liquefaction of Food Waste: Effect of Process Parameters on Product Yields and Chemistry

Increasing food waste generation (1.6 billion tons per year globally) due to urban and industrial development has prompted researchers to pursue alternative waste management methods. Energy valorization of food waste is a method that can reduce the environmental impacts of landfills and the global reliance on crude oil for liquid fuels. In this study, food waste was converted to bio-crude oil via hydrothermal liquefaction (HTL) in a batch reactor at moderate temperatures (240–295°C), reaction times (0–60 min), and 15 wt.% solids loading. The maximum HTL bio-crude oil yield (27.5 wt.%), and energy recovery (49%) were obtained at 240°C and 30 min, while the highest bio-crude oil energy content (40.2 MJ/kg) was observed at 295°C. The properties of the bio-crude oil were determined using thermogravimetric analysis, fatty acid methyl ester (FAME) analysis by gas chromatography with flame ionization detection, CHNS elemental analysis, and ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectroscopy (FT-ICR MS). FT-ICR MS results indicated that the majority of the detected compounds in the bio-crude oil were oxygen-containing species. The O4 class was the most abundant class of heteroatom-containing compounds in all HTL bio-crude oil samples produced at 240°C; the O2 class was the most abundant class obtained at 265 and 295°C. The total FAME content of the bio-crude oil was 15–37 wt.%, of which the most abundant were palmitic acid (C16:0), palmitoleic acid (C16:1), stearic acid (C18:0), and polyunsaturated fatty acids (C18:3N:3, C18:3N:6).

Hydrothermal liquefaction of sewage sludge; energy considerations and fate of micropollutants during pilot scale processing

The beneficial use of sewage sludge for valorization of carbon and nutrients is of increasing interest while micropollutants in sludge are of concern to the environment and human health. This study investigates the hydrothermal liquefaction (HTL) of sewage sludge in a continuous flow pilot scale reactor at conditions expected to reflect future industrial installations. The processing is evaluated in terms of energy efficiency, bio-crude yields and quality. The raw sludge and post-HTL process water and solid residues were analyzed extensively for micropollutants via HPLC-MS/MS for target pharmaceuticals including antibiotics, blood pressure medicine, antidepressants, analgesics, x-ray contrast media, angiotensin II receptor blockers, immunosuppressant drugs and biocides including triazines, triazoles, carbamates, a carboxamide, an organophosphate and a cationic surfactant. The results show that a positive energy return on investment was achieved for all three HTL processing temperatures of 300, 325 and 350 °C with the most beneficial temperature identified as 325 °C. The analysis of the HTL by-products, process water and solids, indicates that HTL is indeed a suitable technology for the destruction of micropollutants. However, due to the large matrix effect of the HTL process water it can only be stated with certainty that 9 out of 30 pharmaceuticals and 5 out of 7 biocides products were destroyed successfully (over 98% removal). One compound, the antidepressant citalopram, was shown to be moderately recalcitrant at 300 °C with 87% removal and was only destroyed at temperatures ≥325 °C (>99% removal). Overall, the results suggest that HTL is a suitable technology for energy efficient and value added sewage sludge treatment enabling destruction of micropollutants.

Feedstock-Dependent Phosphate Recovery in a Pilot-Scale Hydrothermal Liquefaction Bio-Crude Production

Microalgae (Spirulina) and primary sewage sludge are considerable feedstocks for future fuel-producing biorefinery. These feedstocks have either a high fuel production potential (algae) or a particularly high appearance as waste (sludge). Both feedstocks bring high loads of nutrients (P, N) that must be addressed in sound biorefinery concepts that primarily target specific hydrocarbons, such as liquid fuels. Hydrothermal liquefaction (HTL), which produces bio-crude oil that is ready for catalytic upgrading (e.g., for jet fuel), is a useful starting point for such an approach. As technology advances from small-scale batches to pilot-scale continuous operations, the aspect of nutrient recovery must be reconsidered. This research presents a full analysis of relevant nutrient flows between the product phases of HTL for the two aforementioned feedstocks on the basis of pilot-scale data. From a partial experimentally derived mass balance, initial strategies for recovering the most relevant nutrients (P, N) were developed and proofed in laboratory-scale. The experimental and theoretical data from the pilot and laboratory scales are combined to present the proof of concept and provide the first mass balances of an HTL-based biorefinery modular operation for producing fertilizer (struvite) as a value-added product.