Bio-Coal Production from Torrefaction of Coconut Husk with Food Waste As Binder: Effect of Pellet Size and Binder Ratio

Coconut husk, a by-product of the coconut industry, holds promise as a sustainable feedstock for bio-coal production. However, pelletizing coconut husk poses challenges due to its low moisture content, low bulk density, and high ash content. To overcome these challenges, this study explored the use of food waste as a binder and investigated the effect of pellet size and binder ratio on bio-coal production from torrefaction of coconut husk. The study conducted experiments with three different pellet sizes (16 mm, 20 mm, 28 mm) and varying binder ratios (25%, 50%, 75%). Torrefaction was performed at 290°C for 60 minutes. The results demonstrated a clear relationship between pellet size and the properties of the resulting bio-coal. Smaller pellet sizes exhibit higher energy density and heating value, with the 16 mm pellet achieving the highest higher heating value (HHV) of 20.26 MJ/kg and an energy density of 1.25. Conversely, increasing the pellet size from 16 mm to 28 mm leads to a decrease in energy density and heating value. These results were corroborated by a Van Krevelen plot which revealed 16 mm pellet to have the lowest O/C and H/C ratio and was in vicinity with moderate quality natural coal.

The quality of the bio-coal produced was assessed through various testing and analysis techniques, including thermogravimetric analysis (TGA), proximate analysis, lignocellulose analysis, Fourier transform infrared (FTIR) spectroscopy, and field-emission scanning electron microscopy (FESEM). FESEM analysis clearly showed the presence of large pores in coal from 16 mm pellets, a very desirable property for any coal and FTIR showed the absence of OH, CO, and COOH groups that led to hydrophobicity of the bio-coal, another desirable property. In conclusion, use of two different biomasses in different roles, one as main feedstock, and other one as binder, makes this work very novel and provides alternative to effective waste management. The findings in this work provide insights for developing efficient and sustainable waste management processes, contributing to the utilization of vast waste biomass for renewable energy generation.