The differences in processing conditions result in significant differences in the product yield and product composition of bio-oils. Recently, Lu et al. (2009) intensively reviewed the fuel properties fast pyrolysis oils and discusses how these properties affect the utilization of bio-oils. In general, bio-oils are complex mixtures of volatile hydrocarbons, alcohols, organic acids, aldehydes, ketones, ethers, furans, phenols and other non-volatile compounds. The unstable fragments in bio-oil could rearrange through condensation, cyclization, and polymerization to form new compounds, such as aromatics. Table 3 describes selected properties of bio-oils produced from hydrothermal liquefaction of swine manure and pyrolysis of wood. For comparison purposes, the characteristics of heavy petroleum fuel oil were also presented in Table 3.

As shown in Table 3, liquefied oils have much lower oxygen and moisture contents, and consequently much higher energy value, as compared to oils from fast pyrolysis. The corresponding HHV of liquefied oil from swine manure is 36.05 MJ/kg, which about 90% of that of heavy fuel oil (40 MJ/kg). The properties of bio-oil from both processes are significantly different from heavy petroleum fuel oil. Compared with heavy petroleum fuel oil, the bio-oils have the following undesired properties for fuel applications: high viscosity, high water and ash contents, high oxygen content and low heating value.

Pyrolysis oil is acidic in nature, polar and not miscible with conventional crude oil. In addition, it is unstable, as some (re)polymerization of organic matter in the oil causes an increase in viscosity over time. Overall, bio-oils can not be directly used as transportation fuels due to their high viscosity, high water and ash contents, low heating value, instability and high corrosiveness. Therefore, bio-oil has to be upgraded before it can be used as an engine fuel.