Rheological, thermophysical, and morphological features of original and hydrogenated bio-oils

Abstract

Bio-oils are multicomponent heterogeneous systems with different origins and prehistory, which implies that they have a complex rheological and thermophysical behavior. The high viscosity of bio-oils is the reason for their special treatment, including hydrogenation, which affects the structure, properties, and, accordingly, the possibility of using the resulting products as liquid fuels. This makes it important to understand the relationship between the structural features of bio-oils and their rheology. In this study, the rheological behavior of four samples of original and hydrogenated bio-oils was studied in a wide temperature range from −30 °C to 70 °C. Bio-oil samples exhibited viscoelasticity and were characterized by the presence of a yield stress, the value of which depended on the content of the dispersed phase in the bio-oil and the type of continuous medium. From a morphological point of view, the original bio-oils represented water-in-oil emulsions, while the hydrogenated samples were oil-in-water ones. According to differential scanning calorimetry data, bio-oils tended to undergo significant supercooling and did not exhibit phase transitions when cooled to at least −50 °C. Arrhenius activation energies can be used to express the tendency of bio-oils to undergo viscosity increase and structure formation (leading to the appearance of a yield stress) with varying the temperature. It was shown that hydrogenated bio-oils have a lower yield stress and apparent viscosity measured at high shear stresses. The complex rheological behavior of bio-oils makes it incorrect to characterize them by a single viscosity value, since it depends not only on the phase composition of the bio-oil, but also on the measuring conditions, which must be tailored individually, taking into account future bio-oil applications.