Catalytic conversion of LignoBoost Kraft lignin into liquid products in near-critical water
Lignin, one of the three main components of lignocellulosic biomass, is the second most abundant organic polymer found on Earth. Due to its aromatic nature, lignin is recognized as being a potential feedstock for producing transportation fuel and high value-added chemicals. Nowadays, most of the lignin (almost 99%) produced in the Kraft pulping process is used as internal fuel. A modern Kraft mill has an energy surplus and, therefore, the potential of being a large scale biorefinery: one option is to extract lignin from black liquor, make it a new source of specialty chemicals and fuel. Furthermore, a new process, called “LignoBoost”, has recently been developed to extract a high quantity of pure lignin and has gained commercial status. Therefore, in years to come, a huge amount of LignoBoost Kraft lignin is expected to be available for valorisation.
In this work, the catalytic conversion of LignoBoost Kraft lignin into liquid products at near-critical condition in water, using ZrO2/K2CO3 as the catalytic system and phenol as the co-solvent, was carried out in the small pilot unit, developed by, and located at, Chalmers University of Technology in Gothenburg, Sweden. The plant, operated in continuous mode, was fed with lignin slurry at a flow rate of 1 kg/h. The analytical procedure for the reaction products has been developed in order to determine the composition of the liquid products. In addition, the influence of K2CO3 concentration and reaction temperature was investigated in order to optimise the yields of the liquid products obtained.
The results show that the K2CO3 concentration and reaction temperature exert different effects in terms of the composition and yields of the resulting products. The reaction products obtained from this process consist of water-soluble organics (5–11% on a dry lignin basis), lignin-oil (69–88%) and char (16–22%). The main 1-ring aromatic compounds (found in water-soluble organics and diethyl ether-soluble lignin-oil) are anisoles, alkylphenols, guaiacols and catechols, showing different trends with K2CO3 concentration and reaction temperature. In addition, the reaction temperature has a relatively large effect on alkylphenols, whereas K2CO3 has a relatively large effect on anisoles. The lignin-oil, being partially deoxygenated, has higher carbon content and heat value, but lower content of sulphur, than lignin in the feed.