Post-combustion Solvents 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.
Liquid solvents are the most mature technology available for CO2 capture from post-combustion flue gas, with monoethanolamine (MEA) preferred. Although MEA is a good performer for CO2 gas separation, NETL’s Cost and Performance Baseline for Fossil Energy Plants estimated that the use of MEA to capture 90 percent of CO2 in a pulverized coal power plant would impose a 30 percent energy penalty and ultimately result in an 85 percent increase in COE. Most of this energy loss is due to solvent regeneration. Additionally, MEA is corrosive and environmentally toxic.
Current NETL-RUA research in post-combustion CO2-capture solvents is examining several solvent classes for improved performance: phase change amino acids (AA), aprotic heterocyclic anion (AHA) ionic liquids (ILs), and low-viscosity eutectic solvents. Phase change amino acid solvents alleviate the harmful decomposition products produced in amine scrubbing and reduce regeneration energy. The formation of a solid upon CO2 sorption gives rise to economic benefits in terms of capacity. Because the precipitate can be separated from the bulk solvent and only that portion requires regeneration, the required regeneration energy should be lower. Some ILs that have a strong affinity for CO2 also undergo a large increase in viscosity after absorbing CO2. Aprotic heterocyclic anion (AHA) ILs do not suffer from a large viscosity increase and are therefore more attractive for CO2 capture solvent applications. Certain unusual liquid materials including eutectic solvents have unique properties which may lead to substantial increases in CO2 capture performance. Eutectic solvents take advantage of the fact that highly crystalline solids tend to undergo a sharp solid-liquid transition, going from a crystalline solid to a low viscosity liquid. Unfortunately, these materials also tend to freeze at relatively high temperatures, meaning they are solids under post-combustion capture conditions. Eutectic melting point depression in organic salts leads to low-viscosity liquids with desirable properties for CO2 capture.
Research in post-combustion solvents is focused on the development of amino acid solutions, ILs, and low-viscosity eutectic solvents. Of particular interest in these studies is the characterization and optimization of numerous performance parameters, including capacity, sensible heat, and heat of vaporization, which can be used for techno-economic assessment. Each of these properties is a parameter which may be tuned in solvent development, so their effect on economics will serve to guide materials development, through collaborative efforts with NETL system studies. Solvents are evaluated using simulated flue gas with and without gas contaminants (SO2 and NOx). In additional to smaller lab-scale thermal analysis instruments, including thermogravimetric analyzers (TGAs) and differential scanning calorimeters (DSCs), solvents can be evaluated in larger-scale systems including continuously stirred tank reactors (CSTRs) and a Hiden IGA microbalance system. AHA ILs with properties tailored for post-combustion CO2 capture conditions are being examined through a combined modeling and experimental approach to probe their tolerance to contaminants. Calorimetry is being performed performed on the amino acid formulations to confirm heats of reaction and heat capacity, and process engineering will help formulate a conceptualized CO2 removal process. Eutectic mixtures of ionic solids have been created, and these mixtures were observed to have low molar volume as well as low viscosity, a combination of properties which is important for capture performance but difficult to achieve in pure substances. Capacity testing and full characterization is now underway.
Figure 1: Flow diagram for the CSTR apparatus.
The phase change amino acids (AA) project has made significant progress in proving out the novel concept of incurring precipitate formation while capturing CO2 from simulated flue gas. Various families of amino acids have been screened down to a few viable candidates, and the chemistry of the intermediate and final products formed has been identified. Current efforts include identifying and quantifying the base/buffer effect versus the amino acid effect on measured CO2 removal levels. Regeneration of the recovered solids will be studied through either a) conventional temperature swing, or b) pH swing. Future effort will focus on implementing a conceptual process while handling solids formation. Additional calorimetry is also warranted to prove out the lessened parasitic energy of regeneration that is projected. The effect of flue gas contaminants will be also studied.
The AHA IL solvent effort has enjoyed considerable success to date. The chemical informatics effort has been highly instrumental in the screening of over 10 million ILs for CO2 solubility, as well as the discovery of a universal relationship between the CO2 solubility and inverse density for those ionic liquids. Researchers at NETL have utilized click chemistry to validate the initial results obtained from this work. The effect of branching was evaluated in ionic liquids, and it was found that branching results in low solubility and selectivity. Disubstituted triazolide AHA ILs were synthesized using a new method to mitigate undesirable side reactions. Several AHA ILs have been successfully evaluated in a CSTR. Similar to the amino acid effort, the effect of flue gas contaminants on AHA ILs will be studied in a CSTR during FY-14.
Figure 2: Schematic of the reactor for the CSTR.
A detailed study on the formation of eutectic mixtures was completed. More than 15 organic salt mixtures were developed and characterized for thermal transitions and viscosity. A sharp decrease in viscosity at the eutectic point has been observed for several of these mixtures. Viscosities as low as 22 cP were observed, which are among the lowest viscosities ever seen for ionic liquids with good CO2 solubility at room temperature. CO2 solubility testing of the most promising eutectic combinations was completed. The results indicate that increasing the proportion of the methyl pyridinium compounds improves solubility. However, even the best result is still below the desired CO2 solubility (that demonstrated by the [hmim][Tf2N]). The molar volume of the eutectic salts is extremely low which results in low CO2 uptake. Synthesis of high molar volume crystalline ILs is currently being performed to improve CO2 uptake while lowering the viscosity. The eutectic solvents appear quite promising, but stability concerns make it more likely that the materials will be useful as post-combustion solvents. In FY14, eutectic solvents will be examined for CO2 capture performance under ideal conditions. Future solvent work will include the optimization of these solutions for CO2 capture and the integration of the technology into a flue gas system.
Solvent technologies will be developed and evaluated under realistic testing conditions at successively larger scales with eventual bench scale testing in the presence of real flue 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 a pulverized coal power plant at a cost of less than $40/tonne CO2.
Figure 3. Photograph of the CSTR.