Physico-chemical characterization of carbons produced from technical lignin by sub-critical hydrothermal carbonization

Morphology, physical and chemical properties of carbonaceous materials (CMs) synthesized by hydrothermal carbonization (HTC) of a technical lignin, HP-L™, derived from a bio-refinery waste-stream is reported. The technical lignin, HP-L™, was hydrothermally carbonized at four processing temperatures; 200, 250, 300 and 350 °C. The yield of derived CMs was highest at the lowest processing temperature (200 °C), and decreased with increasing temperature. L350, obtained at 350 °C was further chemically activated with KOH. SEM analysis showed that lignin exhibited agglomerated structures with some vesicles at elevated temperatures. Overall, major structural changes in the HP-L™ lignin only occurred after 300 °C. Vibrational spectroscopy showed bands due to lignin functionalities disappeared at 350 °C. This was supported by deconvolution and peak analysis using NMR, which revealed the evolution of a more condensed structure characterized by aromatic C when temperature reached 350 °C. Unlike carbohydrates and other biomass types which can easily be hydrothermally carbonized at temperatures (160–220 °C), HP-L™ lignin remains relatively unchanged up until 350 °C where significant aromatization occurs. 350 °C is therefore the desired temperature for the hydrothermal treatment of HP-L™ lignin.