Hydrothermal carbonization of cellulose in aqueous phase of bio-oil: The significant impacts on properties of hydrochar

Abstract

Aqueous phase of bio-oil (APBO) is a major fraction of bio-oil deriving from pyrolysis of biomass. APBO mostly contains water, carboxylic acids and different kinds of organics, which can afford a potential reactive medium for hydrothermal carbonization (HTC) of biomass. In this study, the HTC of cellulose in APBO was conducted, aiming to probe the effects of the APBO organics on the properties of the hydrochar. The results showed that the organics such as sugars, sugar-derivatives and phenolics in APBO could adsorb on the surface of cellulose and suppress the hydrolysis at lower temperature (i.e. 180 °C), while they could cross-polymerize with cellulose derivatives at higher temperatures (i.e. 220 or 260 °C). Compared with the hydrochar obtained in water, the APBO remarkably enhanced hydrochar yield by 11.3% and resulted in the abundant formation of aliphatic structures, making the hydrochar carbon and hydrogen-rich but with a lower thermal stability. In addition, the involvement of the APBO organics within the cross-polymerization interfered with the nucleation of the sugars/furans and inhibited the formation of the spheres with a hydrophilic shell and a hydrophobic core, resulting in the formation of an irregularly shaped hydrochar. Such a process also made the hydrochar rather hydrophobic, while the combustion performance was highly improved, exhibiting a lower ignition temperature (170 °C in APBO vs 306 °C in water).