Fast pyrolysis of biomass to produce bio-oils is still in its early stage of development. This technology has strong adaptability to biomass resources, and it offers a convenient way to produce liquid fuels with high bio-oil yields and low production costs.
Bio-oils are low-grade liquid fuels when compared with petroleum fuels. The poor fuel properties include the complex multiphase structures, high contents of oxygen, water, solids and ash, low heating values, high viscosity and surface tension, chemical and thermal instability, low pH values, and poor ignition and combustion properties. In spite of these poor fuel properties, bio-oils also have some promising properties. They usually possess some lubricity and they are less toxic and more biodegradable than petroleum fuels.
For the commercialization of bio-oils as liquid fuels, fuel specifications need to be established as in the case of petroleum fuels. Currently, there are no nationally or internationally recognized fuel specifications for bio-oils, but preliminary specifications have been proposed [8]. These specifications cover a range of applications in different thermal devices which have different requirements on the fuel properties of bio-oils. At present, many bio-oils cannot meet the fuel specifications, and therefore, they cannot be accepted commercially. Further work is required to be done in the future to improve the properties of bio-oils.