Comparative advantages of biomass-derived biochars via torrefaction under flue gas and nitrogen atmosphere

Torrefaction is a widely recognized thermochemical process for converting biomass into energy-dense biochar with improved combustion properties and enhanced grindability. This research performed a comparative evaluation of biochars obtained under traditional nitrogen (N₂) torrefaction and flue gas FG torrefaction to explore the potential applications of biochars. The experimental findings indicated that FG torrefaction achieved a higher reduction in solid yield and HHV improvement, thus leading to a higher energy density. The biochar yield from N₂ torrefaction ranged from 98.73 and 68.13 %, whereas FG torrefaction resulted in the yield between 97.35 % and 54.73 %. Additionally, compared to N₂ torrefaction, a larger surface area, a higher degree of carbonization, and more stable pyrolysis properties were achieved. The carbonization index ranged from 1.00 to 1.23 under N₂ conditions and from 1.01 to 1.32 under FG conditions. The higher HGI value and strong correlation analysis suggested that biochar grindability was strongly linked to torrefaction temperature and carbonaceous properties, irrespective of the torrefaction method. The HGI index ranged from 23 to 48 under FG torrefaction conditions and from 23 to 45 under N₂ torrefaction conditions. This study highlights the unique potential of FG torrefaction for biochar production, presenting a sustainable approach to waste management and carbon sequestration.