Optimized Torrefaction of Corn Straw in a Screw Reactor: Energy Balance Analysis and Biochar Production Enhancement

Torrefaction is a promising pretreatment method to enhance the physical and chemical properties of corn straw for bioenergy applications. In this study, torrefaction experiments were conducted in a continuous screw reactor under varying temperatures and feed rates. The quality of the resulting biochar was assessed using color difference analysis, with a defined threshold to determine product qualification (i.e., compliance rate). Results showed that the compliance rate dropped from 78% to 61% as the feed rate increased from 0.5 kg/h to 1.5 kg/h. To address this, process parameters were optimized. Increasing the flow of the hot carrier gas significantly improved product quality: at a carrier gas temperature of 550 °C, a flow rate of 9.4 kg/h, and a feed rate of 1 kg/h, the compliance rate reached 81%. An energy balance was established through proximate and ultimate analyses and measurements of the higher heating value (HHV). Under optimized conditions, the mass yield (MY) and energy yield (EY) were 58.84% and 66.48%, respectively. Maintaining the carrier gas temperature above 550 °C ensured a stable and self-sustaining torrefaction process. These findings provide practical insights for the design and operation of energy-efficient, continuous biomass torrefaction systems, contributing to the advancement of sustainable biochar production at industrial scales.