Enhancing biomass properties for biofuels: A comparative analysis of pyrolysis and hydrothermal carbonization technologies

Abstract

This study analyzes and compares the effects of pyrolysis and hydrothermal carbonization (HTC) on biomass sourced from southern Spain, focusing on their potential for biofuel production. Five biomass types including orange, olive and pine-tree prunings, and almond and pistachio shells were selected due to their widespread availability and renewable energy potential. Pyrolysis experiments were conducted in a fixed-bed reactor at 450, 500, and 550 ºC for 30 and 60 min, while HTC was performed in an autoclave reactor at 180, 220, and 260 ºC for 60, 120 and 240 min. The resulting biochar and hydrochar were characterized through elemental analysis, ash content, higher heating value (HHV), energy yield (EY), and global energy efficiency (GEE). The results indicate that, when temperature increases, both processes reduce solid yield while promoting gas formation and pyrolysis generally enhances the carbon content and HHV of the biomass, particularly in pine and pistachio, which reached maximum HHV values of 27.32 MJ/kg and 25.39 MJ/kg, respectively. However, pyrolysis also leads to increased ash content. Conversely, HTC significantly reduces ash content offering a cleaner solid product and retains a higher EY, with pine hydrochar achieving an EY of 89.26 % at 180◦C for 240 min. The global energy efficiency (GEE) analysis further supports HTC as a more energy-efficient process, with values reaching 91.80 % for pine hydrochar. Additionally, structural and combustion analyses of the best samples (obtained from pine biomass) revealed that while pyrolysis produces a more thermally stable and porous carbon matrix, HTC-treated samples retain more functional groups, exhibit a two-stage combustion profile, and display significantly higher initial combustion rates, underscoring their potential for rapid energy release in biofuel applications. These findings highlight the trade-offs between both methods.