Influence of feedwater pH on the CO2 reactivity of hydrochars. Co-carbonisation with a bituminous coal

Abstract

Hydrothermal carbonisation (HTC) is recognised as an environmentally advantageous method to pre-treat biomass feedstocks. This study examined the hydrothermal carbonisation of two lignocellulosic biomasses at different pH levels. Lower pH promoted hydrolysis, dehydration and decarboxylation reactions, which led to more condensed and aromatic structures of hydrochars. These observations were confirmed by means of elemental analysis and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The initial pH also had a strong influence on the ash chemistry of the hydrochars. Lower pH promoted the removal of alkali and alkaline earth metals from the biomass. This was reflected in a decrease in the reactivity of bio-cokes produced from hydrochars compared with those obtained from untreated biomass. Based on these findings, the possibility of adding 20 wt% of hydrochar samples to a low volatile bituminous coal was studied. A significant decrease in the reactivity of the resulting bio-cokes compared to the values obtained for the same blends with raw biomass was observed (37% and 20% for PS and RH hydrochars, respectively).