Hydrothermal Liquefaction of Lignin into Bio-Oil

Abstract: Lignin, one of the three main components of lignocellulosic biomass, is the second most abundant organic polymer found on Earth. Nowadays, most of the lignin (almost 99%) produced in the Kraft pulping process is used as internal fuel. However, modern Kraft mills have an energy surplus, which provides an opportunity for extracting lignin that can be used as a new source of specialty chemicals as well as transportation fuel. Furthermore, a new process, called “LignoBoostTM”, has been developed recently to extract a large quantity of pure lignin and has gained commercial status.

In this work, hydrothermal liquefaction (HTL) was used to produce bio-oil from LignoBoostTM Kraft lignin in subcritical water, using ZrO2/K2CO3/KOH as the catalytic system and phenol as the capping agent, in a small pilot unit (in continuous mode) developed and located at Chalmers University of Technology in Gothenburg, Sweden. An analytical procedure for the reaction products was developed in order to analyse the liquid products. In addition, the influence of the concentration of K2CO3 and the reaction temperature was investigated to optimise the yields and quality of the resulting liquid products. The stability of bio-oil is a significant factor to study since it influences the further upgrading of bio-oil into fuel to be used in industry: high stability makes it more versatile and thus suitable for wider range of applications. The stability of bio-oil was, therefore, studied under natural (room temperature, 2 years) and accelerated aging (up to 80°C, up to 1 month); the accelerated aging of bio-oil fractions was also studied to obtain a deeper understanding of the aging mechanism.

The results show that these two variables, i.e. the concentration of K2CO3 and the reaction temperature, affect the products obtained differently: these products consist of bio-oil (69–88%), water-soluble organics (5–11%) and char (16–22%). The main monomers are anisoles, alkyl phenols, guaiacols and catechols, the relative amounts of which varied with the reaction conditions. Being partially deoxygenated, lignin HTL bio-oil contains less water and ash than lignin, which is beneficial for achieving bio-oil of high quality. This bio-oil was found to be remarkably stable at both room temperature and elevated temperature. Furthermore, its stability was found to be enhanced by the removal of insoluble high Mw molecules.