Hydrothermal carbonization of pretreated pine needles: The impacts of temperature and atmosphere in pretreatment on structural evolution of hydrochar

Abstract

Hydrothermal carbonization (HTC) is a process for enhancing energy content of biomass through deoxygenation reactions, effectiveness of which depends on composition of biomass. Thermal pretreatment could induce structural change of biomass and thus affects the chemical inertness/reactivity in HTC. This was investigated herein by HTC of the pine needles, thermally treated at 250 or 350 °C with/without presence of 11.5% O2, at 250 °C for 10 h. The results showed that pretreatment at 250 °C could increase overall yield of hydrochar in subsequent HTC, due to the enhanced aromatization and the resistance towards fragmentation in HTC. Nonetheless, the HTC worked on biochar formed at 250 °C, facilitating deoxygenation to remarkably decrease O/C ratio. In comparison, the pretreatment at 350 °C removed substantial amount of volatile fractions, significantly increasing chemical inertness of resulting char-350 during the followed HTC. Oxygen presence induced oxidation reactions in pretreatment, forming more bio-oil/gases and making resulting hydrochar more hydrophilic. Especially, oxidation at 250 °C formed more aliphatic structures that could be easier removed during followed HTC, enhancing thermal stability of resulting hydrochar. Although a portion of cellulose retained in the biochar obtained from pretreatment at 250 °C, the elimination of aliphatics suppressed formation of spheric structures of hydrochar, while retained original fibrous structures from pine needles.