Pretreatment methods to obtain pumpable high solid loading wood–water slurries for continuous hydrothermal liquefaction systems

Feedstock pretreatment is a prerequisite step for continuous processing of lignocellulosic biomass through HTL, in order to facilitate the pumpability of biomass aqueous slurries. Until now, HTL feedstock pumpability could only be achieved at solid mass content below 15%. In this work, two pretreatment methods to obtain wood-based slurries with more than 20% solid mass content, for continuous processing in HTL systems, are proposed. The effect of biomass particle size and pretreatment method on the feedstock pumpability is analyzed. The experimental results show that pumpable wood-based slurries containing 20% solids can be prepared using recycle HTL biocrude as carrier fluid, if particles smaller than 0.125 mm are used. The recycle biocrude concentration used for slurry make-up is strongly affected by the sawdust size distribution. A second pretreatment option is feedstock thermal treatment with alkalis. This method is less sensitive to particle size or wood type. 1 mm particles of either softwood or hardwood could be converted into pumpable liquid feedstock by thermal treatment with NaOH at 180 °C. Wood-water-biocrude slurries viscosity is reduced from 100 to 1000 Pa s to about 1 Pa s, when thermal treatment is applied.