The fractionation of components of lignocellulosic biomass is important to be able to take advantage of biomass resources. The hydrothermal–ethanol method has significant advantages for fraction separation. The first step of hydrothermal treatment can separate hemicellulose efficiently, but hydrothermal treatment affects the efficiency of ethanol treatment to delignify lignin. In this study, the efficiency of lignin removal was improved by an ultrasonic-assisted second-step ethanol treatment. The effects of ultrasonic time, ultrasonic temperature, and ultrasonic power on the ultrasonic ethanol treatment of hydrothermal straw were investigated. The separated lignin was characterized by solid product composition analysis, FT-IR, and XRD. The hydrolysate was characterized by GC-MS to investigate the advantage on the products obtained by ethanol treatment. The results showed that an appropriate sonication time (15 min) could improve the delignification efficiency. A proper sonication temperature (180 °C) can improve the lignin removal efficiency with a better retention of cellulose. However, a high sonication power 70% (840 W) favored the retention of cellulose and lignin removal.

For hydrothermal liquefaction of dry biomass to produce liquid fuels, water needs to be added or the aqueous phase products can be recycled. This paper focuses on understanding the relationship between hydrothermal degradation of the dry biomass and oil formation under the condition of accumulative recycling of the aqueous phase. Completely dried corn stalk and deionized water were used for the hydrothermal liquefaction (HTL) experiment. The aqueous products for subsequent recycling were not diluted. It was demonstrated that the recycling of the aqueous can promote the enrichment of organic acids and the conversion of ketones and phenols in the aqueous, improving the yield and quality of Bio-crude oil. After recycling, the yield of Bio-crude oil increased from 20.42% to 24.31% continuously, and the oxygen content decreased from 13.34% to 9.90%. Although the process was accompanied by solid deposition and had a negative impact on the hydrothermal degradation efficiency, the formation of carbon microspheres during the deposition enhanced the utilization of nondegradable solids, while the formation of metal salt particles had a positive impact on oil production. After three rounds of recycling, the solid deposition effect was weakened. At this time, oil production and solids degradation can be promoted simultaneously.

It is meaningful to understand the conversion pathways of nitrogen during the hydrothermal liquefaction process of microalgae to reveal the related reaction mechanisms and develop effective methods to prevent N from ending in biocrude, which eventually increases the quality of biocrude. Extending from our previous works that mainly focused on two high-protein (>50 wt%) microalgae (Chlorella sp. and Spirulina sp.), Nannochloropsis sp., which has a high lipid content (>70 wt%), was used as the feedstock for this project using the same methodology. The high lipid content in Na. induced less nitrogen during the oil phase and as a result, reduced the heteroatom content while also improving the quality of biocrude. It is worth noting that another investigation was conducted on the model compounds with different types of amino acids to specify the effects of the types of amino acids in the proteins in microalgae on the N pathway and their distribution in the products (aqueous phase, oil, solid, and gas). It was found that the basic amino acid in microalgae caused the formation of more N-heterocyclic compounds in the biocrude. The mass flow based on the mass balance was demonstrated to further refine the map showing the predicted reaction pathway of nitrogen from the previous version.