LIGNIN DEPOLYMERISATION IN NEAR- CRITICAL WATER TO PRODUCE BIOFUEL AND CHEMICALS: EFFECT OF CO-SOLVENT AND pH

Licentiatavhandling, 2015

Lignin is a complex three-dimensional amorphous polymer, which, along with cellulose and hemicellulose, forms a main component of biomass. Its main role is in giving 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 “black liquor” and used as a fuel to meet the heating needs of the pulping process. Nowadays, energy efficiency measures in the pulp industry have improved to a level of having an energy surplus in the mill. Thus, it is possible to add value to the black liquor, which represents the surplus energy, in different possible ways. One scenario is to extract lignin from black liquor, which has become technically feasible by developing LignoBoost technology, and then converting lignin to high value products like specialty chemicals or bio-fuel. In this work, depolymerisation of LignoBoost Kraft lignin has been carried out in a small pilot plant (1kg/h) using methanol as a co-solvent and hydrogen donor in a first investigation, with phenol as a capping agent to suppress repolymerisation (e.g. formation of char), ZrO2 as a heterogeneous catalyst and potassium carbonate as co-catalysts. In a second investigation the pH varied by using different potassium hydroxide concentration, with the same reaction mixture as in the first investigation except methanol. The reaction was carried out in a continuous flow fixed-bed reactor (500 cm3) at 280-350°C and a pressure of 25 MPa. The reactor outlet consisted mainly of two liquid phases, one aqueous and one oil phase. The pH and methanol investigations showed different results in terms of operability and yields. The char yields for the methanol series varied between 14% and 26%, while for the pH series it varied between 12.6% and 18%. In addition, using methanol led to operational difficulties due to the formation of solids in oil, which were soluble in THF (Tetrahydrofuran). On the other hand, the formation of suspended solids for the pH series showed a clear dependence on the pH, with a shifting point (pH product= 8.1) to a higher suspended solid yield. For the phenolic compounds in aqueous phase, both investigations showed the formation of the same class of compounds with different yields. A clear influence of the pH on the water soluble organics (WSO) yield was observed, as reflected in an increase in the yield from about 23% to 40%. For methanol, WSO varied from 9% to 24%.