A comparative study of thermal and mechanical properties of binder-free castor stalk pellets produced via pre- and post-torrefaction routes

Abstract

This study investigates the thermal, mechanical, moisture-absorption, and energy balance characteristics of torrefied castor stalk (TCS) pellets made through two different processing methods. In route-1, biomass was first torrefied at elevated temperatures (250 ◦C, 275 ◦C, and 300 ◦C) and then pelletized (pre-TCS). In route-2, pellets were prepared from raw biomass and then torrefied (post-TCS) at various temperatures (250 ◦C, 275 ◦C, and 300 ◦C). At 300 ◦C, the post-TCS pellets showed a higher heating value (23.80 ±0.08 MJ/kg) compared to pre- TCS (21.96 ±0.165 MJ/kg) and CS-raw pellets (18.80 ±0.22 MJ/kg). Multi-linear regression analysis using SPSS version 28 revealed that impact of moisture, lignin, and extractive contents on the mechanical properties of TCS pellets. The post-TCS pellets absorbed less moisture than the pre-TCS pellets, resulting in only a slight decrease in HHV of 1.65 MJ/kg, even after 144 h of exposure to open laboratory conditions. The thermal stability was evaluated by STA/DSC, and FE-SEM confirmed a fused surface morphology, which increased the mechanical strength of post-TCS-300 pellets. The post-TCS-300 pellets demonstrated a higher energy balance of 60.97 MJ/kg compared to pre-TCS-300 pellets (55.51 MJ/kg). Overall, route-2 showed improved fuel qualities, including higher heating value, energy yield, mass yield, energy density, moisture-absorption resistance, thermal stability, durability, and O/C & H/C ratios. These results suggest that the post-torrefied pelletization process in route-2 is a more valuable option, suitable for both domestic and industrial uses