4.2. Physiochemical properties of bio-oil
The physicochemical properties of the bio-oil from three experiments under the same reaction conditions are shown in Table 3. The water content is one of the key properties to rate quality of bio-oil, which affects the heating values, pH, viscosity and the storage characteristics of bio-oil. The water contents of bio-oil 1 and 3 were almost the same, 36.03 wt% and 38.06 wt%, respectively, and higher than bio-oil 2 which had a water content of 32.95 wt%. The water contents of bio-oil 1, 2 and 3 were all higher than that of the Grade G bio-oil in ASTM D7544-12. The reason why pyrolysis of biomass produces high amounts of water is the intrinsic high oxygen content of biomass structure consisting mainly of cellulose, hemicellulose and lignin [19]. Generally, a higher content of water yields a low value of viscosity. In the bio-oil 3, the water content and viscosity were 38.06 wt% and 4.368 mm2 s−1, respectively. Higher viscosity indicates poor flow characteristics and stability. In addition, the high viscosity will cause high-pressure drops in pipelines leading to higher pumping costs [9]. The viscosity of the bio-oil 1, 2, 3 were all much lower than the Grade G bio-oil in ASTM D7544-12, which may be because of the higher contents of water.