This project deals with key issues in the development of second-generation (2 G) biofuels via gasification and Fischer-Tropsch synthesis (FTS), more specifically the importance of contaminants in the gas phase from high-temperature gasification. The synthetic fuel produced this way is well suited as diesel and jet fuel, and the feedstock can be all forms of biomass, such as waste and residues from forestry and agriculture. The process consists of many steps, including gasification, purification of the syngas, synthesis of hydrocarbons (e.g. FTS), and refining and upgrading of products. Ash compounds, sulfur and other unwanted elements in the biomass can form volatile compounds because of the high temperatures in the gasifying reactor, and these components can be transported with the gas downstream and poison the catalysts used in synthesis of the fuel components. We have studied several aspects of this process. Solid sorbents for gas cleaning at high temperature have been studied in collaboration with SINTEF in a separate subproject. If we are successful in developing sorbents that are efficient at high temperatures, the process can be redesigned with higher thermal efficiency (reduced energy consumption) and improved profitability as a result. We have chosen to study manganese-based sorbents for removal of sulfur components, important catalyst poisons. The influence of the chemical composition (choice of support) on the capacity and stability has been investigated experimentally, in capacity and cycli stability studies. The results show that the manganese-on-carrier systems are promising, and even if there is some loss of capacity by repeated cycles (sorption-regeneration), it is possible to design a cyclic process with low reactor volume. This work is continued in a new project, Chemical Looping Desulfurization, starting in 2017, where the goal is to develop this into a unique process.
The second main activity dealt with the effect of the inorganic components (ash) on the catalysts for synthesis of fuels from the syngas. The chemical effect has been demonstrated earlier through model studies, in this project we have focused on gas phase transport and realistic deposition routes. The main ash component, potassium (K) in the form of various potassium salts, were deposited as aerosol particles on the cobalt catalyst used for the synthesis of 2G fuels. This work was done in close cooperation with the Linneaus University in Växjø in Sweden, where they have extended experience with this type of deposition. The method was chosen to simulate how potassium salts behave in high-temperature gasification of biomass. We have shown that even if potassium salts are deposited as solid nanoparticles on the external surface of the catalysts, and not in direct contact with the cobalt active sites, they still have a dramatic effect on the catalyst activity. It is however, still unclear how K deposited on the external surface of the catalyst particles actually has an effect on the catalyst activity, which essentially is related to the cobalt particles on the inner surface of the catalyst. We speculate that this is because the salts are mobile at reaction conditions, and are transported rapidly to the active sites.
We have also studied how potassium affects the cobalt catalyst used in the Fischer-Tropsch synthesis. This work is mainly done through theoretical calculations of the interactions between cobalt surfaces and potassium and reactants in the FTS. For this we have used Density Functional Theory (DFT), a simplified quantum chemical method that enables calculations of electron structures and chemical bond for complicated systems. This work has shown us how potassium occupies key sites on the surface, thus offering an explanation for the severe effect on the activity. Also here the mobility of potassium is a key point, we show that there are very small barriers towards potassium diffusion on the surface. Potassium will thus mobile and occupy the most favorable points on the surface, these points are then probably also important for activation of the reactants and for the synthesis of hydrocarbons on the surface. The results confirm that the ash components must be cleaned from the syngas down to very low levels before the gas reaches the synthesis step of the process. Based on the price link between potassium concentration and the loss of activity, we can now formulate justified requirements for purification of the syngas, where the cost of cleaning can be weighted against the loss of activity over time with different level of potassium on the catalyst.
One PhD candidate is educated and one postdoctoral candidate has been working in the project. In addition, three master students prepared master theses with projects related to the project.
We propose a research project addressing key steps in a syngas-based thermochemical conversion process for making second generation biofuels. The steps we propose to study are steps we believe are key to the success of such a process: Central catalytic is sues related to converting the synthesis gas to fuel products, in particular the action of catalyst poisons in the biomass, and development and studies of adsorption and adsorbents for the high-temperature removal of pollutants carried in the syngas. The purpose is to improve catalyst lifetime, and
reducing losses in efficiency linked to conventional cleaning processes. The project addresses fundamental issues, with important practical applications and involves theoretical studies, development of new met hods for poisoning studies of Fischer-Tropsch catalysts, and development of new sorbents for high-temperature gas cleaning. 2 researcher candidates will be trained through the project, and a wide national and international network will be maintained.