Catalytic effects of aliphatic acids on wet torrefaction mechanisms and fuel potential of empty fruit bunches

Wet torrefaction (WT) enables the treatment of oil palm biomass to synthesise coal-like solids known as hydrochars, and the addition of catalysts can further enhance their physicochemical properties. In this study, acetic, formic, levulinic, and citric acids at two different concentrations (0.1 and 1.0 M) were added as catalysts in the WT of empty fruit bunches (EFB) at 180 and 220 °C for 12, 24, and 48 h. The effects of different catalysts on the physicochemical properties of the hydrochars and the reaction mechanisms for WT of EFB were examined and revealed. The fuel application of the hydrochars was also explored, and their commercial counterparts, such as coals and biomass pellets, were compared. Among the aliphatic acid catalysts employed, acetic acid had a marginal effect on both the physicochemical properties of the hydrochars and the reaction mechanisms of the synthesis process. On the other hand, formic and citric acids showed better catalytic effects in synthesising sugar, as relatively higher sugar concentrations were observed. Among the acids, 0.1 M of levulinic acid at 220 °C and 48 h (LA-0.1-220-48) enhanced the hydrochars with the highest HHV (29.43 MJ kg −1 ). This could be attributed to the polymerisation of 5-hydroxymethylfurfural (HMF) onto the hydrochars as secondary deposits. The HHV and ash content of LA-0.1-220-48 hydrochars (29.43 MJ kg −1 , 3.90 wt%), in particular, were superior to the two locally sourced coals (13.86 and 17.08 MJ kg −1 , 32.13 and 22.51 wt%), thus demonstrating greater potential as a solid fuel. • Aliphatic acid-catalysed wet torrefaction reveals conversion potentials for EFB. • Reaction mechanisms in EFB wet torrefaction with various aliphatic acids. • Hydrochars at 0.1 M levulinic acid, 220 °C, and 48 h show best fuel properties. • Formic & citric acids effectively enhance sugar generation.