Advanced Biomass-to-value Chains by Integrating Hydrothermal Carbonization Into Complex Conversion Process Schemes

Abstract

This study explores the potential of biomass hydrothermal carbonization (HTC) for developing innovative, integrated waste-to-value conversion chains. Experiments with two lignocellulosic residuals, silver fir sawdust (FIR) and wheat straw (STRAW), give valuable hints for foreshadowing novel process schemes. The batch carbonization reactions lasted from 0 to 120 min, at constant water/biomass ratios (FIR, 7/1; STRAW, 5/1), at 180, 200, and 230 °C, and autogenous pressure. Within the first 60 min of reaction, both biomasses release green platform chemicals in the liquid phase at increasing concentrations without resorting to catalysts or additives. Longer reaction times cause the depletion of liquid-phase chemicals due to secondary hydrochar production. The highest concentrations of 5-hydroxymethylfurfural (80 mM) and furfural (400 mM) detected with FIR suggest exploiting this biomass in a short-time HTC, renouncing the improvement of the solid fuel. Waste-to-energy is unanimously considered the route of choice for STRAW. In this case, the conventional use of HTC would be pre-treating the biomass for feeding a gasification stage properly. Evidence of this study suggests that a two-step HTC would combine the requirements of fuel production with those of recovery of value-added chemicals from the liquid phase. During the reactor warm-up to 230 °C, furfural concentration builds up to 300 mM, then halves within the first 15 min of the HTC process and drops to zero at 120 min, a reaction time required for a satisfactory STRAW energy densification (raw biomass, 15 MJ/kg; 120 min hydrochar, 21 MJ/kg). The results of this study pave the way for further investigation aiming to set the optimal operational conditions.