Torrefaction of woody biomass (Acacia nilotica) in fluidized bed reactor: study of solid fuel properties

Abstract

Torrefaction is a mild pyrolysis process that converts waste biomass into a high-quality solid biofuel which can be effectively blended with coal for use in thermal power plants. In this research, Acacia nilotica was subjected to torrefaction in a fluidized bed reactor at temperatures of 210 °C, 240 °C, and 270 °C, utilizing nitrogen flow at atmospheric pressure with a heating rate of 10 °C min−1 and a constant retention time of 60 min. The samples produced were designated as dried Acacia nilotica (DAN) and torrefied Acacia nilotica (TAN210, TAN240, TAN270) according to the respective torrefaction temperatures. The solid yield exhibited a decline from 62.72 to 30.23% as the temperature increased, attributed to the substantial degradation of hemicellulose. Correspondingly, the energy yield also decreased from 67.20% at 210 °C to 50.06% at 270 °C. However, the higher heating value (HHV) significantly increased from 16.08 to 26.63 MJ kg−1. The fuel property such as the fuel ratio (FR) increases from 0.066 (DAN) to 0.27 (TAN270) indicating its suitability for blending with coal. Additionally, the torrefied biomass exhibited enhanced flow characteristics and hydrophobicity. After 5-day moisture reabsorption assessment at 66% relative humidity, DAN absorbed 35.72% moisture whereas TAN270 absorbed only 11.85%. The hydrophobic nature resulted from reduced hydroxyl groups and alterations in surface structure. Characterization techniques including TGA, FTIR, and SEM–EDX validated the decomposition of hemicellulose. Overall, torrefied Acacia nilotica shows enhanced fuel properties, hydrophobicity, and stability positioning it as a promising solid biofuel for energy applications. © The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature 2025.