Researchers in the membrane research group (MEMFO) at The Norwegian University of Science and Technology (NTNU) in Trondheim have developed and are patenting a new nano-structured plastic membrane for the low-cost separation of CO2 from flue gases from power plants.
The polyvinylamine membranes use a fixed-site-carrier (facilitated transport) mechanism. CO2 is transported through the membrane by both solution-diffusion and fast “hops” between reactive fixed-site-carriers. The effectiveness of the membrane increases proportionally to the concentration of CO2 in feed gas mixture.
The novelty is that instead of using a filter that separates directly between CO2 and other molecules, we use a so-called agent. It is a fixed carrier in the membrane that helps to convert the gas we want to remove.—Professor May-Britt Hägg, MEMFO
The agent helps the CO2 molecules combine with moisture to form HCO3 (bicarbonate) which is quickly transported through the membrane. Conventional membranes without carrier sites may lose their separation ability when swollen by water vapor in the system. By contrast, this polyvinylamine membrane shows even more enhanced CO2 transport in the presence of water molecules by the reversible formation of bicarbonates.
In recent testing under various conditions (pressure of 2-15 bar and temperature of 25-55°C and relative humidity of 30-95%), the team found that both CO2 permeance and CO2 selectivity over N2 were dependent on humidity change in the system.
The permeance increased continuously with humidity and even exceeded more than 0.3m3(STP)/m2•bar•hr, while the selectivity (CO2/N2) showed convex curves with a maximum of approximately 120-250 depending on temperature and pressure.
The CO2 capture performance (high permeance, excellent selectivity and durability) under the presence of water vapor in the system indicates a potential for omitting water removal units from the CO2 capture process when applying this membrane for real power plant process.
According to the team, the excellent CO2 selectivity over N2 (150–250) even at relatively high pressure (10-20 bar) has strong potential for replacing the conventional CO2 capture process using alkanolamine solutions.
MEMFO recently joined a consortium of 26 European businesses and institutions within a project named NanoGloWa—Nanostructured Membranes against Global Warming. The consortium has received €13 million (US$18 million) to develop such membranes.
Within a five-year period, the plan is to test the membrane technology in four large power plants in Europe. We believe this will result in an international breakthrough for energy-efficient CO2 membranes.—May-Britt Hägg
The new technology is very suited for coal-powered plants. In gas-fired plants, the concentration of CO2 is so low that the pressure in the waste gas must be increased before the gas can be cleaned with this method. Statoil is currently developing a method for pressurized exhaust that could be combined with this membrane technology, and that would make it suitable for purification in gas-powered plants as well, according to Professor Hägg.