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

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.Global warming is a hot topic and its effects, e.g. melting glaciers and increased intensity of hurricanes, are often discussed in media and terms like the greenhouse effect and greenhouse gases often comes up. Greenhouse gases in the atmosphere such as carbon dioxide, methane and nitrous oxide, are to a large extent responsible for the greenhouse effect [1]. Sunlight passes these gases on its way down to earth, but as they hit earth, the light energy transforms to heat energy, infrared radiation. Heat is constantly radiated from Earth’s surface out into space but greenhouse gases hinder this by blocking the infrared radiation. This is the so-called greenhouse effect and because not all energy supplied by the sun is radiated back into space, it leads to an unbalance. More energy comes in than what comes out, which in turn leads to an increase in mean temperature around the globe, this is what’s called global warming. The fight against global warming takes many shapes but maybe the best known is the transition from fossil fuel to a renewable alternative. Fossil fuels have been used heavily since the industrial revolution (1750) and the use, burning, of fossil fuels creates carbon dioxide which is the foremost greenhouse gas [1]. According to the U.S. Environmental Protection Agency (EPA) the emissions of carbon dioxide represents 81% of the total greenhouse gas emissions from the U.S. in the years 1990-2014 [2]. Reduction of carbon dioxide emission are a necessity if the fight against global warming is to be won. Reduction of carbon dioxide emissions require that fossil fuel is replaced with a sustainable and renewable alternative, e.g. electricity or biofuel. Electric cars may very well have an important role in this transition but heavier transportation such as trucks, ships and aircrafts will be needing liquid fuel, biofuels, in the foreseeable future. A new process developed by SunCarbon aims to produce a biofuel which originates in the Swedish forest. 69% of Sweden’s total land area is covered with forest and this has led to a strong and globally competitive pulp and paper industry [3-5]. During pulping a residual product called black liquor is produced, it contains the chemicals used for producing pulp from wood chips but also extracted wood substances [6]. The most interesting substance found in black liquor is lignin, one of the main building blocks of all trees, grasses and other plants [7]. Lignin is a large and complex molecule but when it’s cut up in smaller fragments, it can function as a raw material for the production of biofuel [8]. However, the pulping chemicals found in black liquor needs to be removed in order to produce a biofuel. This is where the SunCarbon process comes in. A portion of the lignin in black liquor will be removed with filtration and then broken down into smaller fragments by heating it in a reactor together with catalysts. The smaller lignin fragments will be purified in two steps, first by using carbon dioxide that will purify the lignin by separating it from excess water and chemicals. In the next stage, hot acetic acid and water mixtures will be used to further purify the lignin. The result after the second purification stage is a heavy lignin-rich bio-oil which can be sent to existing refineries for final treatment. The SunCarbon process was tested in this master thesis and the results seems promising. Lignin was successfully broken down into smaller fragments and purification with carbon dioxide removed 72% of the unwanted pulping chemicals while still recovering up to 85% of the lignin fragments. The second purification stage proved difficult to master but some successful experiments were preformed and showed that a high recovery of lignin can be expected