Esterification. The carboxylic acids and aldehydes of the bio-oil can be converted into esters by removing oxygen in the form of H2O. The reaction takes place at temperatures of 50-80°C in presence of acid catalysts. Alcohols such as methanol and ethanol, obtained from fermentation of cellulosic materials, are used for the esterification process. The esterification process drastically reduces the aging rate of bio-oil. Methanol was found to reduce the aging rate by a factor of 20 (Diebold and Czernik 1997). The esterification reactions can be represented by the following equations (Bulushev and Ross 2011).

RjCOOH + R-OH^R1COOR + H2O (3)

R1CHO + 2R-OH^R1CH(OR)2 + H2O (4)

Where R and R1 are alkyl groups.

Aqueous Reforming. Recently Dumesic and others have proposed aqueous reforming techniques to selectively produce alkane products, with reduced or no need of external hydrogen. The process takes place through aldol condensation and hydrogenation of carbohydrate-derived compounds, to make large water soluble intermediates which are then converted into alkanes (Barrett et al. 2006; Chheda and Dumesic 2007; West et al. 2008).

Steam Reforming. Reforming of the bio-oil using steam produces H2-rich syngas which can be used as a source of hydrogen for hydrogenation reactions in bio-oil upgrading and conversions.