A review on advances of torrefaction technologies for biomass processing

Torrefaction is a thermochemical pretreatment process at 200–300 °C in an inert condition which transforms biomass into a relatively superior handling, milling, co-firing and clean renewable energy into solid biofuel. This increases the energy density, water resistance and grindability of biomass and makes it safe from biological degradation which ultimately makes easy and economical on transportation and storing of the torrefied products. Torrefied biomass is considered as improved version than the current wood pellet products and an environmentally friendly future alternative for coal. Torrefaction carries devolatilisation, depolymerization and carbonization of lignocellulose components and generates a brown to black solid biomass as a productive output with water, organics, lipids, alkalis, SiO2, CO2, CO and CH4. During this process, 70 % of the mass is retained as a solid product, and retains 90 % of the initial energy content. The torrefied product is then shaped into pellets or briquettes that pack much more energy density than regular wood pellets. These properties minimize on the difference in combustion characteristics between biomass and coal that bring a huge possibility of direct firing of biomass in an existing coal-fired plant. Researchers are trying to find a solution to fire/co-fire torrefied biomass instead of coal in an existing coal-fired based boiler with minimum modifications and expenditures. Currently available torrefied technologies are basically designed and tested for woody biomass so further research is required to address on utilization of the agricultural biomass with technically and economically viable. This review covers the torrefaction technologies, its’ applications, current status and future recommendations for further study.