Upgrading of banana leaf waste to produce solid biofuel by torrefaction: physicochemical properties, combustion behaviors, and potential emissions

Abstract

This study is the first report that focuses on investigating the effects of torrefaction on the bioenergy-related properties, combustion behavior, and potential emissions of banana leaf waste (BLW). Experiments were first conducted in a bench-scale fixed-bed reactor operating at light (220 °C), mild (250 °C), and severe (280 °C) torrefaction conditions to torrefy the raw BLW. Torrefaction pretreatments reduced the weight of the raw BLW by about 60%, but the resulting solid biofuel can preserve up to 77% of the energy content of the raw biomass. It was found that torrefied BLW contains more concentrated fixed carbon than the raw BLW, volatile matter content of up to 59.8 wt.%, and a higher HHV of up to 20.7 MJ kg^-1 with higher concentrations of carbon, nitrogen, and ash. Bulk density increased 13.0% over the raw BLW, and the torrefied BLW became a solid biofuel with 51.5% greater energy density under the severe torrefaction condition. The upgrading of BLW by torrefaction enhanced its combustion performance in terms of comprehensive combustion, ignition, burnout, and flammability indices. Compared with commercial hard coal, BLW torrefied at the mild condition (250 °C) had lower potential emissions per unit of energy, 25.3% less CO₂ emission, 3.1% less CO emission, 96.4% less SO₂ emission, and 18.4% less dust emission, except for NO_X emission. This study conclusively indicates that BLW after torrefaction has enhanced bioenergy-related properties, improved combustion performance, and reduced emissions potential, proving to be a promising method for its valorization.

Keywords: Bioenergy; Emission indices; Fuel characteristics; Lignocellulosic residue; Performance indexes; Thermochemical pretreatment; Torrefied biomass.