Hydrothermal conversion of lignin: A review

Lignin is a carbon-rich renewable source owning aromatic structure units, which is an important constituent in biomass. Hydrothermal conversion of lignin is widely studied as a promising method to produce not only bioenergy but also value-added useful chemicals. Fuel gas, aromatic aldehydes and phenolic products can be obtained from lignin hydrothermal gasification, wet oxidation and hydrothermal liquefaction, respectively. This article discusses and compares the three methods of lignin hydrothermal conversion, including their process parameters, possible conversion routes, catalysts, application of products. Effects of hot-compressed organic solvent–water mixture solution on conversion of lignin and effects of lignin in biomass hydrothermal conversion are commented. Wet oxidation of lignin is an efficient mean of recovering value-added aromatic aldehydes, especially vanillin. Hydrothermal liquefaction of lignin is a promising way of recovering phenolics-rich bio-oils. Both aromatic aldehyde and phenolic compound are important chemical intermediates. There are strict requirements of process conditions and relative high costs to get fuel gas from direct hydrothermal gasification of lignin. However, further studies on improving gasification of lignin seem necessary in order to get fuel gas from hydrothermal gasification of the whole biomass.