LIGNIN HYDROTHERMAL LIQUEFACTION IN SUBCRITICAL WATER TO PRODUCE BIOFUEL AND CHEMICALS
Lignin is one of the most abundant amorphous macromolecules found in nature. Along with cellulose and hemicellulose, it forms a main component of biomass, and is mainly responsible for providing strength, rigidity and resistance to degradation. From a chemistry perspective, lignin is an important source of aromatics. In the kraft process, lignin is recovered in the form of “black liquor”, which is considered a low-value by-product. Nowadays, energy efficiency measures in the pulp and paper industry have improved to a level of having an energy surplus in the mill, making it is possible to add value to the black liquor, which represents the surplus energy, in various ways. One scenario is to extract lignin from the black liquor, which has become technically feasible by developing LignoBoost technology, and then converting the lignin into high-value products, such as specialty chemicals or bio-fuel.
In this work, hydrothermal liquefaction (HTL) of LignoBoost kraft lignin has been carried out in a small pilot plant, using sub-critical water as the medium, ZrO2, K2CO3/Na2CO3 and/or KOH/NaOH as the catalytic system, methanol as the co-solvent and phenol as the capping agent to suppress repolymerisation (e.g. formation of char). With the aim of developing the HTL process, different investigations were carried out to study the influence of methanol, the pH (8.9-10.4) through the use of different concentrations of potassium hydroxide and the use of phenol as the capping agent (2-10%); different fractions of sodium in alkali (Na/(Na+K) from 0.0-1.0) were also investigated. The reactions were performed in a fixed-bed reactor (500 cm3) at 350°C with the exception of the methanol investigation, where it was varied between 280 and 350°C, and a pressure of 25 MPa. The reactor outlet was comprised mainly of two liquid phases: one aqueous and one oil.
The pH and methanol investigations showed different results in terms of operability and yields. The yields of bio-oil, WSO and char were affected by different levels of pH and concentrations of methanol. In addition, the use of methanol led to operational difficulties due to the extensive formation of solids. For the phenol and sodium/potassium investigations, the overall yield was not affected considerably either by different phenol concentrations (2-10%) or sodium fractions in the alkali. It was possible to lower the phenol concentration in the feed to 2% and maintain fairly constant overall yields. In the case of the sodium series, it was shown that it was also possible to replace the potassium ion in the feed with the sodium ion without it having a strong effect on the product yield. This HTL process gave the same major individual compounds such as guaiacol, anisole, catechol and alkylphenols, in all of the investigations undertaken, with different trends and influences being observed depending on the parameters studied.