Upgrading biomass fuels via wet torrefaction: A review and comparison with dry torrefaction

Biomass pretreatment is an essential step prior to several thermochemical conversion processes. Wet torrefaction, a biomass pretreatment method in hydrothermal media or hot compressed water at temperatures within 180–260°C, has been receiving a lot of attention because it possesses some advantages over other pretreatment methods. Apart from the undoubted benefits of upgrading biomass fuels to closer to coal properties, wet torrefaction has the capacity to work with wet or even extremely wet biomasses and enhance the ash removal from the biomass. The technology has recently attracted great interest from both academic groups and industrial companies. This review aims at providing a comprehensive overview of recent research and development activities in the field with focus on improvements in the chemical, physical and fuel properties of the solid product after wet torrefaction. Moreover, a brief introduction to dry torrefaction, a more conventional thermal pretreatment of biomass in the absence of oxygen under atmospheric pressure and in a temperature range of 200–300°C, is also given and compared with wet torrefaction. Main differences in the properties of the solid products from the two torrefaction methods are also discussed.