De-polymerization and Purification of Kraft Lignin Utilizing the Liquid-Lignin Phase

Abstract: The effects of global warming are apparent and the work towards a sustainable future is underway. In order to reduce the environmental impact caused by the transportation sector, a transition from fossil fuels to those based on renewable resources are important. Using agricultural land to produce raw materials, e.g. wheat or corn, for biofuel is controversial as it competes with food production. Furthermore, a single process would not be able to keep up with the demand for biofuels. Using previously unused residual products or upgrading their use to increase efficiency and value is a great way forward. Black liquor is exactly that, a residual product in pulp mills that can be used more efficiently. A large portion of black liquor is lignin, a large and complex aromatic structure that when broken down into smaller units can function as fuel. However, black liquor contains all the cooking chemicals used during the cooking process and needs to be extracted. There are already methods for extraction and purification of lignin from black liquor but more cost efficient methods are needed in order for a large-scale production of lignin-based biofuel. A new method developed by SunCarbon consists of four parts; membrane filtration of black liquor, base catalyzed de-polymerization of the complex structure, CO2 induced precipitation of a heavy lignin-rich liquid-phase and purification with the Aqueous Lignin Purification with Hot Acids (ALPHA) process. The ALPHA process purifies lignin with mixtures of acetic acid and water which acts as an anti-solvent and phase separation between a heavy lignin-rich liquid-phase and a light phase occurs, were the metals prefer the light phase and the lignin is thus purified. The red line of this method is that the product and the intermediary products are kept liquid and can thus be pumped. This eliminates problems with lignin in powder form, which otherwise is common, and the final liquid product can be introduced to existing refineries without altering their process, i.e. a drop-in fuel. In this work, all but membrane filtration has been investigated experimentally and weight has been put on precipitation and purification. It was found that base catalysis successfully de-polymerizes lignin at 220-240 °C and that an increase in temperature yields a higher degree of de-polymerization. 250 °C was also tested but the formation of solid material in the reactor made it unpumpable. The largest lignin molecules (~100 kDa) are completely removed and instead lignin dimers and monomers are formed. It was also shown that a heavy lignin-rich liquid-phase forms at 110-150 °C and when pH was ~9 or lower. The lignin yield in the heavy liquid-phase was high; 76-85% of the initial lignin had precipitated and a lower temperature improved yield. The decanted light liquid-phase removed a large portion, 72%, of the initial ash content and again a lower temperature was preferable. Initial attempts of purification with the ALPHA process on the precipitated lignin failed. A model substance, LignoBoost powder, was thus used to test the method. Purification with the model substance successfully separated in two liquid phases that could easily be separated. The viscosity of the heavy phase was too high to be pumped with, e.g. a centrifugal pump, but a screw pump could be used. A two-step purification of the model substance showed that a high yield can be expected, >92%, from the ALPHA process. Two-step purification of the precipitated lignin showed that ash content decreases but the formation of a heavy liquid phase is crucial for achieving a high yield.