Torrefaction of ionic-liquid impregnated lignocellulosic biomass and its comparison to dry torrefaction

Torrefaction of woody biomass as a thermal pretreatment to increase biomass energy density is prerequisite to many thermochemical conversions. An ionic-liquid (IL) biomass torrefaction route was proposed and investigated as an alternative to conventional dry and wet torrefaction for enhancing the rate of biomass torrefaction and for potentially improving the quality of torrefied solid products. Ionic liquid-impregnated aspen, birch and sawdust were torrefied in the 240–280 °C temperature range using [Emim][OTf], [Emim][BF4] and [Hmim][NTf2] ILs. To assess the benefits of IL impregnated biomass, dry and IL torrefaction were compared in terms of mass yield, energy density, energy yield, hydrophobicity and ultimate moisture uptake of torrefied solid products using thermogravimetry and fixed-bed setups. At equal reaction time and treatment temperature, the decomposition rates of birch, aspen and sawdust in ILs were considerably enhanced compared to their dry torrefaction counterparts. IL torrefaction led to increased energy density of treated aspen, birch and sawdust solids alike, whereas improved hydrophobicity of IL-torrefied solids translated in nearly 40–45% reduction of their ultimate moisture uptake. Finally, char and ash leftovers building up in spent ILs after repeated IL torrefaction cycles were specifically addressed as potential factors responsible for IL decomposition.