Hydrothermal conversion of different lignocellulosic biomass feedstocks – Effect of the process conditions on hydrochar structures

Abstract

Five biomass feedstocks (Coffee residues, Rice waste, Whitewood, Zilkha black, and Lignin) were hydrothermally processed in a semi-continuous flow rig using 9 different processing conditions (75, 150, 250 °C, and 1, 50, 240 bar). Solid residues produced at low temperature (<150 °C) did not show significant structural changes. At more severe conditions, structural changes could be linked to the lignocellulosic composition and divided into three categories: (i) biomass with higher hemicellulose-cellulose and lower cellulose-lignin structures, (ii) lower hemicellulose-cellulose and higher cellulose-lignin structures, and (iii) only cellulose-lignin structures. Both hemicellulose and cellulose structures in category (i) and (ii) were successfully degraded under subcritical conditions (250 °C and 50 bar) to produce hydrochar with higher lignin content. Biomasses with higher levels of lignin did not show the same degree of transformation. Category (i) produced a low hydrochar yield (39 wt%) due to the degradation of higher hemicellulose-cellulose structures. Category (ii) had higher hydrochar yields (58–62 wt%) due to the lower amount of cellulose and hemicellulose. Category (iii) had the highest hydrochar yields (73–90 wt%) thanks to the lack of hemicellulose and lower cellulosic structures. A novel concept called “displacement”, based on a thermogravimetric profiling method, was used to quantify changes in the pyrolysis behaviour of the hydrochar compared to the original feedstock. The degree of “displacement” correlated with hydrochar yield and reactivity, the highest level of displacement was observed with category (i- higher hemicellulose-cellulose biomasses) while the lowest displacement was observed with category (iii- higher lignin biomasses). This novel technique could be used to quantify the effects of hydrothermal treatment on any given biomass.