Fast pyrolysis of biomass in the absence of air at atmospheric pressure and 450-550°C will produce a mixture of gas, liquid and char (Fig. 4.7). The liquid is known as bio­oil, pyrolysis oil or bio-crude with yields as high as 80% depending on conditions. Bio-oil has a low calorific value at 16-18 MJ/kg but has the advantage of being a renewable fuel with low levels of sulfur and low net emissions of CO2 (section ‘Pyrolysis’, Chapter 4). However, bio-oil is acidic, has a high viscosity, and is thermally unstable, and therefore requires processing before it can be used as a fuel. As a conse­quence bio-oil is regarded as a second-generation biofuel as it is not produced com­mercially at present. Bio-oil properties are compared with heavy fuel oil and diesel in Table 7.3, where the differences in viscosity and energy content are clear.

Bio-oil is a complex mixture containing some 300 compounds including acids, alcohols, aldehydes, esters, ketones, sugars, phenols, guaiacols, syringols, furans and lignin-derived compounds. Most of the compounds identified are phenols with alde­hydes and ketones attached, which gives a high oxygen and highly hydrated content. The oxygen content needs to be reduced before the bio-oil can be used and the fol­lowing methods have been used.