Combustion Performance and Water Absorption of Mixed Shrub and Waste Bio-Pellets at Various Molding Temperatures and Durations

Abstract

Bio-pellets represent an environmentally sustainable alternative fuel with the potential to supply fossil fuels. Raw materials considered potential for making bio-pellets should have low economic value and be abundantly available. Materials that meet these criteria include shrubs and plantation waste. This study aimed to identify the optimal material composition and molding condition (temperature and duration) in the production of shrubs and waste mixed bio-pellet based on its combustion characteristics (calorific value and burning rate) and water absorption. Bio-pellets produced form mixture of Gamal shrub (Gliricidia sepium), Lamtoro shrub (Leucaena leucocephala), and coffee husk waste (Coffea arabica), utilizing 6 distinct raw material weight ratios of 1:1:2, 1:2:1, 2:1:1, 0:1:1, 1:0:1, and 1:1:0. The bio-pellets were molded at 2 different temperatures, 180 and 225 °C, for durations of 3 and 5 min. The parameters analyzed included the combustion rate, calorific value, water uptake, and dimensional swelling. The findings of this study reveal that molding temperature exerts the most significant influence on the combustion characteristics of bio-pellets. Bio-pellets molded at 225 °C demonstrated the highest calorific value, reaching 4,767 kcal/kg, alongside the lowest combustion rate at 0.22 g/min. In contrast, the bio-pellets molded at 180 °C exhibited lower calorific values and higher combustion rates. The raw material composition and molding duration did not exhibit a significant effect on combustion rate and calorific value, although the combustion rate tended to decrease with increasing molding duration. Moisture response was strongly temperature-dependent. Increasing temperature from 180 to 225 °C reduced longitudinal swelling (43.68% to 2.95%), transversal swelling (3.73% to 0.77%), volume swelling (55.90% to 4.57%), and water uptake (73.50% to 3.95%), with strong uptake-swelling correlations. These findings suggest that the regulation of molding temperature is a critical factor in optimizing bio-pellet quality, particularly in enhancing energy efficiency, reducing combustion rate, and increasing moisture resistance. HIGHLIGHTS Molding temperature has the most significant influence on combustion characteristics of bio-pellets compared to molding duration and raw material composition. Bio-pellets molded at 225 °C exhibited the highest calorific value of 4,767 kcal/kg with the lowest combustion rate of 0.22 g/min Raw material composition and molding duration did not significantly affect combustion parameters. Regulating molding temperature is critical for optimizing bio-pellet quality and energy efficiency. Optimizing molding temperature can enhance energy efficiency and reduce combustion rate of bio-pellets. GRAPHICAL ABSTRACT

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