Investigation of the formation and interaction of solids and renewable crude during hydrothermal liquefaction

Abstract

The hydrothermal liquefaction (HTL) is a highly promising method for converting biomass into renewable crude (RNC) oil using subcritical water. Near the critical point, water exhibits unique properties such as lower viscosity and higher solubility of organic materials, making it an excellent medium for efficient and homogeneous reactions. However, during HTL, the co-products of aqueous phase, hydrochar, and gases are formed alongside the renewable crude. The solid and renewable crude mixture is challenging to separate, even with solvent extraction methods. As a result, the actual yield of renewable crude is lower than expected. The reasons for oil trapping and how experimental parameters affect product yields and oil trapping in hydrochar were not well understood. This study aims to re-evaluate the HTL process of biomass, with a focus on the formation and interaction between solid and renewable crude. Published data were analysed to gather additional information that could shed light on the solid-renewable crude interaction. Understanding the influence of experimental parameters on product yields and the trapping of renewable crude in solids during HTL reactions is crucial. In this investigation, pure carbohydrate was chosen as the raw material. The parameters examined included temperature (260-350°C), residence time (10-25 minutes), and biomass-to-water ratio (0.25-1), which were analysed to determine their effect on the trapping of renewable crude into solids and the overall product yields. Dichloromethane (DCM) was utilised as a solvent to identify the recovered renewable crude from the solid phase. The Source Rock Analyser (SRA) was employed to identify the light and heavy oils trapped in the solids before and after solvent extraction. The optimal conditions for higher renewable crude yields were found to be 350°C, 10 minutes residence time, and a 0.5 biomass-to-solvent ratio. However, increasing the residence time and biomass-to-water ratio resulted in decreased yields of renewable crude by 37% and 7%, respectively. Maximum biocrude trapping in solids was observed at 320°C, and solvent extraction was able to extract up to 58% of the crude oil, with a higher extraction efficiency for light oil compared to heavy oil. Furthermore, the study investigated the fundamental reasons behind the interaction between solid and renewable crude using a mixture of model compounds (cellulose, lignin, protein, and lipid) and real biomass (pine wood, sludge, and microalgae). Mixtures such as 50% cellulose + 50% protein, 50% cellulose + 50% lipid, 50% lignin + 50% protein, and 50% lignin + 50% lipid were prepared to ensure sufficient renewable crude production, as cellulose and lignin primarily produced solid, while lipid and protein mainly produced crude. HTL of binary mixtures of model compounds generally resulted in higher yields of renewable crude at higher temperatures, except for cases where secondary cracking occurred, leading to decreased yields. For the cellulose (50%) and lipid (50%) mixture, 57% to 71% of RNC was trapped in the hydrochar, with maximum and minimum yields recorded at 350°C and 260°C, respectively. Similarly, the cellulose (50%) and protein (50%) mixture trapped 64% to 77% of RNC, with maximum and minimum yields observed at 290°C and 350°C. The lignin (50%) and lipid (50%) mixture trapped 64% to 77% of RNC, with maximum and minimum yields at 320°C and 260°C. Lastly, the lignin (50%) and protein (50%) mixtures trapped 60% to 66% of RNC, with maximum and minimum yields at 350°C and 260°C. In terms of real biomass, pine wood resulted in maximum and minimum RNC yields of 14% and 8% at 320°C and 350°C, respectively, due to secondary cracking occurring at 350°C. Sludge exhibited maximum and minimum RNC yields of 20% and 14% at 350°C and 260°C, while microalgae showed maximum and minimum RNC yields of 21% and 12% at 260°C and 350°C, indicating that the secondary cracking of oil started from the beginning and resulted in decreased RNC yields at higher temperatures for microalgae. The analysis with source rock analyser (SRA) suggested that a significant amount of RNC was trapped in the hydrochar, with 42% to 61% for pine wood, 50% to 67% for sludge, and 48% to 71% for microalgae. During the investigation of the fundamental reasons for solid-oil interaction, it was found that the functional groups of hydrochar, acid value, wettability capability, and viscosity of the renewable crude were responsible for the interaction between solid and renewable crude.