Understanding the influence of biomass particle size and reaction medium on the formation pathways of hydrochar

The chemical-physical processes controlling hydrothermal carbonization (HTC) are still not completely understood. This paper focuses on two aspects: the influence on the hydrochar formation of the particle size of the feedstock and the presence of solved compounds in the feedwater. To address these, brewer’s spent grains were crushed to < 1 mm and separated in three fractions. In addition, residual process water (rPW) from 5-hydroxymethylfurfural (HMF) production instead of bi-distilled H2O was added in a series of experiments for recycling. The results show a transfer limitation of hydrolysis products through pores for the particle size fractions > 250 μm proved by HPLC analysis of liquid byproducts, particularly when rPW, containing readily condensable/polymerizable intermediates, is added. This has a positive effect on the yield and carbon content of the hydrochars caused mainly by an increase in its secondary char fraction. The reaction pathways involved are discussed in detail.