A review of comparative life cycle assessment of dry, wet, and microwave torrefaction pathways for sustainable bioenergy systems

Understanding the lifecycle carbon impacts of different torrefaction methods is critical to advancing sustainable bioenergy solutions. While prior research has explored individual torrefaction pathways, comprehensive comparisons of carbon flows and global warming potential (GWP) across dry (conventional), wet (hydrothermal carbonization), and microwave-assisted torrefaction remain limited. This review compares the life cycle assessment (LCA) results of these torrefaction technologies, focusing on their carbon footprints, GWP per experimental configuration, and net carbon benefits (NCB). Conventional torrefaction exhibits GWP values ranging from 0.1 to 0.4 kg CO2 eq per kg biomass, while electricity generation from torrefied biochar generally ranges between 0.1 and 0.3 kg CO2 eq·kWh−1. Microwave-assisted torrefaction demonstrates the lowest GWP (∼0.110 kg CO2 eq·kWh−1), making it a promising route for producing coal-like solid fuels. Conversely, hydrochar-based electricity from hydrothermal carbonization tends to yield higher GWP. Feedstock type, energy source, process temperature, and LCA boundary conditions influence environmental impacts across torrefaction methods. Among the pathways evaluated, microwave torrefaction offers higher NCB, lower emissions, and greater process efficiency for energy recovery. This review also outlines methodological challenges in LCA harmonization and highlights opportunities for carbon credit validation and policy support toward net-zero biomass energy systems.