Pre-combustion Membranes for Carbon Capture
Carbon capture and storage from fossil-based power generation is a critical component of realistic strategies for arresting the rise in atmospheric CO2 concentrations, but capturing substantial amounts of CO2 using current technology would result in a prohibitive rise in the cost of producing energy. The National Energy Technologies Laboratory-Regional University Alliance (NETL-RUA) is pursuing a multifaceted approach, which leverages cutting-edge research facilities, world-class scientists and engineers, and strategic collaborations to foster the discovery, development, and demonstration of efficient and economical approaches to carbon capture.
A typical coal gasification process produces H2, CO2, and steam at about 260°C and 25 bar after completion of the water-gas shift reaction. Separating CO2 from H2 at this point, before H2 is sent on for combustion in a turbine, is known as pre-combustion carbon capture. While liquid solvents are the traditional choice for separating CO2 in this application, membranes have shown the potential to further reduce energy requirements for pre-combustion capture. Membranes separate gases according to differences in their rate of permeation through the membrane material. The driving force for gas permeation in a membrane is the partial pressure difference of the gas species across the film. Since there is a relatively high concentration of CO2 (roughly 30 percent) and it is at high pressure in a coal gasification scheme, membranes are well suited for this application.
The purpose of this research is to develop CO2-selective supported ionic liquid membranes (SILMs) that make use of advanced ionic liquid and polymer materials that are capable of functioning at pre-combustion capture conditions. SILMs are composite membranes comprised of an ionic liquid (IL) suspended within the pores of a glassy polymer support. Ionic liquids are salt solutions that are molten at room temperature, have very low vapor pressures, and tend toward thermal stability and high CO2 solubility. Integrating ILs into polymer membranes increases their selectivity and permeability because the gas permeability of the IL is typically one to three orders of magnitude greater than the polymer support structure. Currently, several different classes of ionic liquids that designed for high temperature separation of CO2 from H2 are being studied. In nature, 5-, 6-, and 7-membered cyclic molecules are naturally quite stable. Ionic liquids which form ring complexes upon interaction with CO2, known as ring-forming ILs, are being designed to create facilitated transport membranes which have good CO2 selectivity at elevated temperatures. Also, ILs with carboxyl based anions (such as acetate) are known to have high selectivity for CO2 over H2 but are incompatible with water. Modified carboxyl ILs that are designed to be hydrophobic are also being synthesized as candidate materials.
SILMs are being studied in hollow fiber configurations due to their high surface area per unit volume, which makes them useful for industrial applications. The materials and morphology of the fibers are being optimized in order to produce the best combination of gas transport properties and mechanical strength. High porosity fibers lead to high CO2/H2 permeance and selectivity, since an increased pore volume also maximizes the IL content of the fibers. By contrast, low porosity fibers have the best mechanical strength because higher polymer content leads to greater structural rigidity.
Several dozen new ring-forming IL compositions have been designed and synthesized. Preliminary results show promise that these materials will be CO2/H2 selective at elevated temperature, although finding the optimum level of interaction between the IL and CO2 remains as a challenge. Ring-forming ILs that have two primary amine groups will capture CO2 at elevated temperature but do not easily release it. The solubility of these ILs can be high, but the diffusivity tends to be low. Ionic liquids that are designed to have a combination of primary and secondary or tertiary amines have shown improved performance for membrane applications.
Hollow fiber supports made from rigid Matrimid and Torlon polymers were fabricated and have been shown to have high strength and temperature stability. It was shown that hollow fibers made from these materials have gas permeances that exceed flat sheet membranes when tested using the same baseline ionic liquid, 1-hexyl-3-methylimidalzolium bis(trifluoromethylsulfonyl)imide ([hmim][Tf2N]). X-ray computed tomography (CT) scanning has been used to produce the first truly non-destructive images of an SILM. Using this technique, a complete three dimensional mapping of a hollow fiber SILM was created, which has the unique ability to show the location of the IL within the fiber without disturbing it through conventional microscopy preparation techniques.
Membrane technologies will be developed and evaluated under realistic testing conditions at successively larger scales with eventual bench scale testing in the presence of real fuel gas at the National Carbon Capture Center. The technologies will then be transferred to industrial partners for further scale up and commercialization.
The research will accelerate the development (ranging from the discovery of innovative materials through evaluation in real systems) of efficient, cost-effective fossil fuel conversion systems that meet the programmatic goal of capturing 90 percent of the CO2 produced by an integrated gasification combined cycle power plant at a cost of less than $40/tonne CO2.
Figure 1. Schematic of a typical hollow fiber membrane with an ionic liquid filled selective layer.
Figure 2. Scanning electron microscope image of a hollow fiber membrane cross-section.