Development of surface torrefaction process to utilize agro-byproducts as an energy source

Highlights: • Developed as surface torrefaction that improved disadvantage of the torrefaction. • Validation for surface torrefaction of pepper stem pellet as a fuel was conducted. • The optimum process conditions of surface torrefaction were derived. • Surface torrefaction improved calorific value, energy yield and hydrophobicity. In this study, to improve the fuel characteristics of unused agro-byproducts and utilize these as an energy source after pelletizing, a thermochemical process termed as surface torrefaction was developed. After torrefaction, the amount of available energy and energy yield decreased due to mass reduction. Thus, surface torrefaction minimized the amount of input energy and energy yield. During this process, the carbonized surface of pellets and any moisture was removed from inside the pellet which is advantageous for storage and transportation. Changes in color and physiochemical characteristics were also observed. The calorific value and moisture content of the wood pellet (WP) and pepper stem pellet (PP) before surface torrefaction were 20.1 MJ kg⁻¹ and 8% and 18.1 MJ kg⁻¹ and 6.5%, respectively. After surface torrefaction, the WP and PP calorific values and energy yields were: 20.3–20.7 MJ kg⁻¹ and 87.3–92.5%, and 18.5–20.1 MJ kg⁻¹ and 86.8–95.9%, respectively. After torrefaction and surface torrefaction, the change in mass yield and fuel characteristics were compared. The optimal conditions for WP and PP treatments, considering hygroscopicity, calorific value, and energy yield, were determined as 300 °C for 4.5 min and 300 °C for 5 min, respectively at surface torrefaction.