Entwicklung von porösen Silica-Membranen zur CO$_{2}$-Abtrennung aus dem Rauchgas fossil befeuerter Kraftwerke

From a global perspective the share of coal in energy production currently grows faster than the share of renewable energy. Carbon dioxide capture and storage from energy-related sources therefore is the key technology to bridge the gap until we have succeeded in establishing a sustainable energy supply. Membrane-based systems to capture CO$_{2}$ from flue gas streams are considered a promising alternative to conventional absorption technology due to potentially lower efficiency penalty. Apart from showing adequate selectivity and permeability, suitable membranes have to be thermally stable and chemically resistant under application conditions. Direct contactwith flue gas irreversibly damages polymer membranes. Ceramic membranes in contrast may have an advantage over polymer membranes because of an assumed superior resistance. Against this background this thesis focuses on the development of a silica-based membrane as well as on the study of its behavior under real flue gas conditions. From a simple point of view the separation task in the flue gas can be reduced to a CO$_{2}$/N$_{2}$ separation. Due to little differences regarding their molecular mass and kinetic diameter, separation of CO$_{2}$ and N$_{2}$ cannot be achieved using molecular sieving or Knudsen diffusion. This research investigates the possibility of CO$_{2}$ selective transport through a silica membrane whose affinity towards CO$_{2}$ is enhanced by amine modification. For this a two-step approach is followed: First the development of a silica layer with suitable pore size and second post-synthesis grafting of aminofunctional silanes on the pore walls. Pore size tailoring is aspired by blending silica sol with a cationic tenside acting as surfactant template molecule. The blended sol is applied on ceramic support disks using a dip-coating process. Thermal treatment leads to the decomposition of template molecules, leaving small mesopores. After thermal treatment the layer thickness is below100 nanometers. Liquid- and gas-phase deposition techniques are employed in post-synthesis grafting of aminofunctional alkoxysilanes on the pore walls of such membranes. Using a binary CO$_{2}$/N$_{2}$ gas mixture to probe the gas permeation characteristics, individual membranes exhibit a relative enrichment of CO$_{2}$ in the permeate stream with respect to the feed from 15 up to 79 vol% at 30°C and 67 vol% at 90°C. CO$_{2}$ permeance is approximately 1·10$^{-8}$ mol·m$^{-2}$·s$^{-1}$·Pa$^{-1}$ which is high in comparison with literature values from similar membranes. However, reproducibility of the postsynthesis grafting process is still limited. In parallel to the membrane development, the degradation behavior of ceramic membranes in direct contact with flue gas is systematically investigated for the first time. For this purpose a test rig is built up in the lignite-fired RWE power station “Kraftwerk Niederaußem” as well as in the hard-coal fired EnBW station “Rheinhafen-Dampfkraftwerk”, in each case after the flue gas desulfurization plant. Membrane alterations caused by the flue gas exposition such as theformation of a filter cake, pore blocking, corrosion and phase transformations are correlated with the particular flue gas conditions and underlying mechanisms are explained. These findings are an important contribution for ongoing, target-oriented membrane development since they allow the development and implementation of specific procedures to prevent membrane degradation