Viscosity of Biomass Pyrolysis Oils from Various Feedstocks

Bio-oil is a renewable energy source that is produced from the pyrolysis of lignocellulosic biomass. The pyrolysis oils are emulsion-like fluids, containing aqueous and phenolic phases, and can be more than 400 times more viscous than water at 25 degrees C. A series of rheological tests were performed on a set of bio-oils from different feedstocks and pyrolysis conditions. In general, the viscosity of the oils was independent of the shear rate (i.e., Newtonian). However, some of the hardwood samples shear thin at lower temperatures (-5 degrees C) and high shears (> 100 s(-1)). Oscillatory frequency sweeps were also performed. All of the oil samples were found to be viscous liquids, and the loss modulus (G) was orders of magnitude greater than the storage modulus (G'). A strong dependence of viscosity upon the temperature showed that the viscosity of poplar and oak 500 degrees C oils increased over 220-fold between 55 and 5 degrees C. Water content and acidity were also measured and compared to viscosity. The water content was found to have a stronger effect on viscosity than acidity. Generally, the oils that had higher water contents had lower viscosities. Viscosity does not correlate with the acid number or pH. While the acid number and pH are independent measurements of the acidity of the bio-oils, no correlation between the acid number and pH was observed. The microstructure of the oils was investigated using optical microscopy and small-angle neutron scattering. Optical microscopy did not show discrete boundaries between the aqueous and organic phases. The neutron-scattering profiles showed that a fractal structure is present in two of the three oils studied.