Transformational Technologies 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 portion of the capture effort is devoted to the study of “transformational” technologies. This portion of the program is designed to examine novel concepts in solvents, sorbents, membranes, and other less conventional technologies.
Figure 1: Film made from a phase-segregating poly(ionic liquid).
Transformational technologies are short, high-risk, high-reward projects. Transformational technology research evaluates emerging ideas and highly novel concepts for CO2 capture. The work encompasses both development of advanced materials and the evaluation of new process innovations. Concepts are evaluated against state of the art commercial technologies using highly efficient molecular modeling, synthesis, fabrication, and systems analysis techniques. A project typically lasts one year or less, and in that time sufficient information is obtained to adequately assess the concept’s potential. Based on the results obtained, projects are either terminated or moved into other parts of the capture portfolio for further development. Among the materials currently being examined are advanced polymers based on inorganic phosphazines and cyclic ethers, solvents made up of nanoparticles, core-shell MOFs, and hybrid organic-inorganic hydroxides. Process concepts include redox-swing capture materials and capture of carbon dioxide concurrent to coal gasification.
Figure 2: Ionic liquid solvents.
Since the NETL-RUA began to examine transformational technologies, about twenty concepts have been studied. Five of those concepts showed sufficient promise to be selected for development as carbon capture technologies: eutectic solvents, flue gas dehumidification, layered-double hydroxides, hybrid solvent-membrane capture systems, and phase-segregating poly(ionic liquids).
Ionic liquids (ILs) have relatively high CO2 solubility and selectivity over N2 and H2 but suffer from high viscosity limiting the overall usefulness of otherwise quite interesting materials. Eutectic ionic liquid solvents grew from a recognition that crystalline solids tend to melt to form low viscosity liquids. By mixing highly crystalline ionic solids, NETL-RUA researchers were able to create ionic liquids with excellent CO2 capture capacity and viscosities as low as 25 cP.
Figure 3: Hybrid solvent-membrane process.
The presence of large amounts of moisture in flue gas severely limits the options for separation of CO2, but no sufficiently inexpensive process for removing water prior to capture exists. A process was developed which can economically remove the moisture from flue gas, use a physical sorbents to capture CO2, and regenerate the moisture sorbent using the CO2 as a sweep gas. The result is an efficient and highly novel post-combustion capture process.
The limiting factor in the efficiency of current solid sorption processes is the temperature stability of the sorbent. It is desirable to create materials which can be regenerated at higher temperatures in order to allow production of higher pressure CO2. Double-layered hydroxide sorbents are novel hydrotalcite analogues with excellent stability and potentially high CO2 capture capacity.
Phase-segregating poly(ionic liquids) (PILs) are block copolymers which phase separate at the nanoscale to form an ionic transport phase and a neutral structural phase, which are covalently bonded to one another. When the materials are cast as membranes, the result is a film with excellent mechanical and transport properties.
Excellent synergies arise when an amine solvent capture process is combined with a CO2-selective membrane. By using a sweep gas in the regeneration column, the solvent is made much more efficient, resulting in a pressurized and concentrated stream containing the process CO2. The membrane stage can then be used to conduct the final purification step.
Figure 4: Combined moisture/CO2 removal process.
The transformational research portfolio allows new concepts and materials to be evaluated and integrated into the larger capture effort. Once integrated in the larger program, the new technologies will be evaluated at successively larger scales with eventual bench scale testing in the presence of real fuel and flue gases 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 at a cost of less than $40/tonne CO2.