The Effect of Torrefaction Holding Time on the Characteristics of Rice Husk as a Raw Material for Biopellet Production

Abstract

Rice husk is an abundant agricultural residue in Indonesia with potential for renewable solid fuel production. However, its use as fuel is constrained by relatively high moisture and volatile matter contents, low calorific value, poor grindability, and moisture absorption. Torrefaction followed by pelletization offers a route to improve these properties and combustion performance. This study investigated how torrefaction holding time affects rice husk characteristics and the combustion performance of biopellets. Rice husk samples were torrefied at 300 °C using holding times of 0, 30, and 60 min. Untreated rice husk served as a reference. Characterisation included proximate analysis, higher heating value measurement, grindability, hydrophilicity, and pellet density. Combustion performance was evaluated using a water boiling test.  Torrefaction reduced moisture content from 10.31% to 3.38–4.86% and volatile matter from 55.64% to 21.75–26.82%, while fixed carbon increased from 12.93% to 36.96–42.10%. The higher heating value increased from 3,174.50 cal/g to a maximum of 4,529.56 cal/g at 60 min. Moisture uptake decreased from 9.04% for untreated rice husk to 3.06% after 60 min of torrefaction. The proportion of particles passing the 75 μm sieve increased from 4.28% to 28.40–31.24%, indicating improved grindability. Biopellet density increased from 0.605 to 0.776 g/cm³. In the water boiling test, the 60-min torrefied biopellets required 14 g of fuel and 3.11 min to boil 200 mL of water, compared with 22.5 g and 4.25 min for untreated biopellets, and produced a maximum flame temperature of 894 °C. These findings indicate that torrefaction at 300 °C, particularly for 60 min among the conditions examined, improves the fuel properties and combustion performance of rice husk biopellets for practical solid biofuel energy applications.