• Energy Research

The technological development of preparing bio-oil from low-temperature hydrothermal conversion of agricultural and forestry waste has positive significance for alleviating the shortage of oil energy supply and reducing environmental pollution. This paper selects typical oxides (Al2O3, CeO2, MgO, SiO2, TiO2, and ZnO) as catalysts to set up a low-temperature (220 °C) hydrothermal conversion process of cotton stalk containing pretreatment processes including chopping. For moderate amplification estimation, lab-scale experimental data is used as a benchmark for calculation, and the functional unit for this study is set to be a 1 kg bio-oil product. The results suggest that the cerium dioxide-involved process with the highest bio-oil yield and highest synthetic consumption, and the silica-involved process with the lowest bio-oil yield, caused the highest environmental impact, resulting in greenhouse gas (GHG) emissions of 67.729 kg CO2e/kg and 60.001 kg CO2e/kg, respectively. It indicates that catalysts need to consider the balance between synthetic consumption and catalytic performance. Magnifying lab-scale data to an industrial scale using scale-up frameworks introduces a low model uncertainty, as the practical value had little effect on the overall evaluation results. However, existing equipment data should be used to reduce the uncertainty of the model itself. The environmental sustainability of bio-oil production by low-temperature hydrothermal liquefaction still needs to be improved, especially by catalyst recovery and bio-oil yield improvement.

The technological development of preparing bio-oil from low-temperature hydrothermal conversion of agricultural and forestry waste has positive significance for alleviating the shortage of oil energy supply and reducing environmental pollution. This paper selects typical oxides (Al2O3, CeO2, MgO, SiO2, TiO2, and ZnO) as catalysts to set up a low-temperature (220 °C) hydrothermal conversion process of cotton stalk containing pretreatment processes including chopping. For moderate amplification estimation, lab-scale experimental data is used as a benchmark for calculation, and the functional unit for this study is set to be a 1 kg bio-oil product. The results suggest that the cerium dioxide-involved process with the highest bio-oil yield and highest synthetic consumption, and the silica-involved process with the lowest bio-oil yield, caused the highest environmental impact, resulting in greenhouse gas (GHG) emissions of 67.729 kg CO2e/kg and 60.001 kg CO2e/kg, respectively. It indicates that catalysts need to consider the balance between synthetic consumption and catalytic performance. Magnifying lab-scale data to an industrial scale using scale-up frameworks introduces a low model uncertainty, as the practical value had little effect on the overall evaluation results. However, existing equipment data should be used to reduce the uncertainty of the model itself. The environmental sustainability of bio-oil production by low-temperature hydrothermal liquefaction still needs to be improved, especially by catalyst recovery and bio-oil yield improvement.

Hydrothermal liquefaction technology (HTL) is a promising thermochemical method to convert biomass into novel liquid fuels. The introduction of oxides and inorganic acids/bases during the hydrothermal process significantly impacts the yield and composition of bio-oil. However, systematic research on their effects, especially at lower temperatures, remains limited. In this paper, we examine the effects of acidity and alkalinity on cotton stalk hydrothermal bio-oil by introducing homogeneous acids and bases. Given the operational challenges associated with product separation using homogeneous acids and bases, this paper further delves into the influence of heterogeneous oxide catalysts (possessing varying degrees of acidity and alkalinity, as well as distinct microstructures and pore architectures) on the production of cotton stalk hydrothermal bio-oil. The effects of nanoscale oxides (CeO2, TiO2, ZnO, Al2O3, MgO and SiO2) and homogeneous acid–base catalysts (NaOH, K2CO3, Na2CO3, KOH, HCl, H2SO4, HNO3) on the quality of cotton stalk bio-oil under moderate hydrothermal conditions (220 °C, 4 h) were investigated. Characterization techniques including infrared spectroscopy, thermogravimetric analysis, elemental analysis, and GC-MS were employed. The results revealed that CeO2 and NaOH achieved the highest bio-oil yield due to Ce3+/Ce4+ redox reactions, OH-LCC disruption, and ionic swelling effects. Nano-oxides enhanced the formation of compounds like N-ethyl formamide and aliphatic aldehydes while suppressing nitrogen-containing aromatics. The total pore volume and average pore width of oxides negatively correlated with their catalytic efficiency. CeO2 with low pore volume and width exhibited the highest energy recovery. The energy recovery of cotton stalk bio-oil was influenced by both acid and base sites on the oxide surface, with a higher weak base content favoring higher yields and a higher weak acid content inhibiting them. The findings of this research are expected to provide valuable insights into the energy utilization of agricultural solid waste, such as cotton stalks, as well as to inform the design and development of highly efficient catalysts.

Hydrothermal liquefaction technology (HTL) is a promising thermochemical method to convert biomass into novel liquid fuels. The introduction of oxides and inorganic acids/bases during the hydrothermal process significantly impacts the yield and composition of bio-oil. However, systematic research on their effects, especially at lower temperatures, remains limited. In this paper, we examine the effects of acidity and alkalinity on cotton stalk hydrothermal bio-oil by introducing homogeneous acids and bases. Given the operational challenges associated with product separation using homogeneous acids and bases, this paper further delves into the influence of heterogeneous oxide catalysts (possessing varying degrees of acidity and alkalinity, as well as distinct microstructures and pore architectures) on the production of cotton stalk hydrothermal bio-oil. The effects of nanoscale oxides (CeO2, TiO2, ZnO, Al2O3, MgO and SiO2) and homogeneous acid–base catalysts (NaOH, K2CO3, Na2CO3, KOH, HCl, H2SO4, HNO3) on the quality of cotton stalk bio-oil under moderate hydrothermal conditions (220 °C, 4 h) were investigated. Characterization techniques including infrared spectroscopy, thermogravimetric analysis, elemental analysis, and GC-MS were employed. The results revealed that CeO2 and NaOH achieved the highest bio-oil yield due to Ce3+/Ce4+ redox reactions, OH-LCC disruption, and ionic swelling effects. Nano-oxides enhanced the formation of compounds like N-ethyl formamide and aliphatic aldehydes while suppressing nitrogen-containing aromatics. The total pore volume and average pore width of oxides negatively correlated with their catalytic efficiency. CeO2 with low pore volume and width exhibited the highest energy recovery. The energy recovery of cotton stalk bio-oil was influenced by both acid and base sites on the oxide surface, with a higher weak base content favoring higher yields and a higher weak acid content inhibiting them. The findings of this research are expected to provide valuable insights into the energy utilization of agricultural solid waste, such as cotton stalks, as well as to inform the design and development of highly efficient catalysts.

Liquefaction conversion technology has become one of the hottest biomass conversion methods due to its flexible material selection and extensive product applications. Exploring biomass liquefaction conversion focuses on catalysts, biomass/water ratio, and reaction temperature. However, it is found that solvents are crucial in the biomass liquefaction process and significantly impact the type of liquefied products and bio-oil yield. Given the current rapid development trend, timely sorting and summary of the solvent effect in the biomass liquefaction process can promote the subsequent development and industrialization of more efficient and cleaner biomass liquefaction technology. Therefore, this review first introduces the characteristics of water as the liquefaction solvent, then summarizes the effects of organic solvents on liquefaction, and finally elaborates on the synergistic effect of co-solvents, which provides a more systematic overview of solvent effects in the liquefaction process. Meanwhile, prospects are put forward for the future development of biomass liquefaction conversion.

Liquefaction conversion technology has become one of the hottest biomass conversion methods due to its flexible material selection and extensive product applications. Exploring biomass liquefaction conversion focuses on catalysts, biomass/water ratio, and reaction temperature. However, it is found that solvents are crucial in the biomass liquefaction process and significantly impact the type of liquefied products and bio-oil yield. Given the current rapid development trend, timely sorting and summary of the solvent effect in the biomass liquefaction process can promote the subsequent development and industrialization of more efficient and cleaner biomass liquefaction technology. Therefore, this review first introduces the characteristics of water as the liquefaction solvent, then summarizes the effects of organic solvents on liquefaction, and finally elaborates on the synergistic effect of co-solvents, which provides a more systematic overview of solvent effects in the liquefaction process. Meanwhile, prospects are put forward for the future development of biomass liquefaction conversion.