Drying Kinetics and Convective Heat Transfer of Coconut-Shell Biobriquettes in a Dual-Blower Tray Dryer

Abstract

Biobriquette quality is strongly determined by the drying process, yet the relationship between drying conditions, product quality, and the underlying convective heat transfer mechanism is rarely quantified together. This study evaluates the drying kinetics, product quality, and convective heat transfer characteristics of coconut-shell biobriquettes dried using a dual-blower tray dryer with IoT-based temperature control. Biobriquettes were dried at temperatures of 75-105 °C and durations of 60-150 minutes, and the results were compared with sun drying and with the Indonesian National Standard (SNI 01-6235-2000). The lowest moisture content, 3.81 ± 0.37%, was obtained at 105 °C for 120 minutes, meeting the SNI requirement (maximum 8%), while extending the drying time to 150 minutes caused surface cracking from excessive internal stress. The tray dryer reduced moisture content roughly eight times more than sun drying (3.81% vs. 30.29%) and achieved a drying rate about 4.5 times higher (0.000780 vs. 0.000171 g.cm-2·min-1); these differences, together with the effect of drying temperature and time, were statistically significant (p < 0.05; independent-samples t-test for the tray-dryer-vs-sun-drying comparison and one-way ANOVA with Tukey’s HSD post-hoc test for the temperature and time comparisons). The mean ash content (8.66 ± 0.09%, n = 2) remained slightly above the SNI threshold, indicating room for improvement in carbonization or binder formulation. Convective heat transfer analysis showed a turbulent airflow regime (Re = 660,158.76) with a heat transfer coefficient of 41.01 W.m-2·K-1, higher than a comparable tray-dryer study (25.47 W.m-2·K-1), consistent with forced convection being an important contributor to the observed moisture removal and product quality; as no direct comparison with a single-blower configuration was performed within this study, this interpretation is based on comparison with values reported elsewhere in the literature rather than a controlled experimental comparison.