Torrefaction of low-value agro-industrial wastes using macro-TGA with GC-TCD/FID analysis: Physicochemical characterization, kinetic investigation, and evolution of non-condensable gases

Abstract

Two low-value agro-industrial residues, passion fruit peel waste (PF) and pineapple peel waste (PA), were considered in this study as feedstocks to produce solid biofuels by torrefaction. The experiments were conducted in a macro-TGA with GC-TCD/FID analysis to investigate their torrefaction characteristics. The GC-TCD/FID analysis allowed quantifying the non-condensable gases, including CO, CO2, CH4 and H2. The impacts of pretreatment with torrefaction on the physicochemical properties were investigated by using proximate analysis, ultimate analysis, and calorific values. The torrefaction of PF and PA was investigated at three different temperatures (200, 250, and 300 °C) and two residence times (15 and 60 min). The torrefaction pre-treatment led to higher levels of elemental carbon, fixed carbon, HHV and energy density than in the raw biomass. Experimental results showed that the most favorable torrefaction condition for producing a solid biofuel with a high energy density is a moderate temperature of 250 °C and 15 min of residence time. Two models were applied to the torrefaction kinetics of the two agro-industrial wastes: one with the one-step method and the other with the two-step method. The two-step reaction method was the most appropriate for describing the torrefaction behavior of the two agro-industrial wastes in all conditions studied. As determined by the two-step reaction method, the torrefaction activation energies ranged from 17.9 to 199.9 kJ mol−1 for PF and 5.3–84.0 kJ mol−1 for PA. Findings from this study are potentially valuable for the design of large-scale equipment for the torrefaction of PF and PA residues.