A comprehensive study on torrefaction of empty fruit bunches: Characterization of solid, liquid and gas products

Empty fruit bunches (EFB), after fruits-bunches separation, are highly potential as an energy source. However, due to some inherited inferior fuel properties such as bulk density, hydrophobicity, moisture content and calorific value (CV), pretreatment is required before EFB can be utilized as a solid fuel. Torrefaction is promising to upgrade EFB fuel properties, and thorough products characterization is essential to reveal process mechanism and potential application. In this study, the effects of temperature (225–300 °C) and retention time (20–60 min) on EFB torrefaction were investigated in a fixed bed reactor. While an increased temperature and retention time had led to a lower energy and mass of solid yields, the liquid and gaseous yields showed otherwise. The oppositely reduced O (37%) and increased C (45%) contents of torrefied EFB with increasing temperature, and the resulting decrease in O/C ratio, and H and O contents, had greatly improved the CV of the torrefied solid up to 46% compared to its raw form. In addition, the chlorine content was significantly reduced (21–54%) after torrefaction. Both the raw and torrefied solid EFB showed a strong thermal degradation of hemicellulose with lower decomposition rate of the latter due to mass depletion during torrefaction. Oxygen removal from EFB during torrefaction migrated to the liquid and gaseous products, most dominant as water and CO2, followed by CO and oxygen-containing organic compounds. The liquid product consisted of condensable components; water (main) and functionalised compounds, among which were acids (as relatively higher concentration), furans, ketones, alcohols and phenols. The gaseous product was dominated by CO2 and CO in all torrefaction conditions. The findings contribute partly as biomass resource database for future bioenergy upscaling and deployment. •Torrefied EFB has greater energy yield, carbon content, calorific value, atomic H/C and O/C.•Greater morphological changes in torrefaction leads to reduced crystallinity for bioconversion.•Condensable liquid consists of mainly oxygenated compounds - acids, phenols and ketones.•Non-condensable gases found in the order of CO2 > CO > CH4.•Oxygen migration upgrades EFB into an energy-dense solid biofuel.