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October 1, 2022 | By Scott Jenkins, Chemical Engineering magazine
An expanding array of CO2-capture approaches are quickly developing for hard-to-abate industrial sectors. The key objective is to significantly lower the costs of traditional CO2-capture techniques, which are based on aqueous alkanolamine absorption and desorption
Achieving climate mitigation objectives, such as carbon neutrality by 2050, requires aggressive large-scale deployment of CO2 capture, utilization and storage (CCUS) technologies. A critical piece of the broad CCUS universe involves capturing CO2 emitted from industrial plants where full and immediate decarbonization is technologically unfeasible or cost-prohibitive in the near- to mid-term. Such facilities include natural-gas power plants, cement- and steel-making plants, among other heavy industry sectors. The current decade is increasingly viewed as critical for reducing CO2 emissions, placing further importance on developing and applying cost-competitive CO2 capture.
Because the largest fraction (estimated at up to 70–80%) of the costs associated with CCUS solutions involve capturing the CO2 and separating it from the capture medium, research and development efforts on a large number of different carbon-capture approaches are currently focused on reducing costs. Specifically, researchers, engineers and technology developers from the public and private sectors are teaming up to test various technologies to minimize or offset the parasitic energy penalty associated with CO2 capture, while also improving process performance.
These efforts have been boosted recently by private investment, government funding programs and legislation, such as the recent Inflation Reduction Act in the U.S, which broadened tax credits for carbon sequestration, among other policy initiatives. In August, the U.S. Department of Energy’s (DOE) Office of Fossil Energy and Carbon Management (FECM) announced more than $31 million in funding for 10 projects to develop carbon capture technologies capable of capturing at least 95% of CO2 emissions generated from natural-gas power plants, waste-to-energy power plants, and industrial applications, including cement and steel.
The diversity of industries to which CO2 capture could be applied, combined with differences in fluegas compositions and process configurations, will likely mean that many different post-combustion carbon-capture technologies will be required throughout heavy industrial sectors to meet variable CO2-mitigation needs that may fit best with one technology approach more than others.
Traditional technology for post-combustion carbon capture is largely based on using alkanolamines, such as monoethanolamine (MEA), or similar solvents. The process works by introducing CO2-containing fluegas into the bottom of an absorber tower, where it flows upward against a down-flowing aqueous amine stream, which removes CO2 from the fluegas. The solvent is then regenerated and the CO2 captured in a desorber at differing temperature or pressure conditions. One approach to reducing the energy requirements, and with it, the costs, of CO2 capture involves phase-separation schemes.
Researchers from the Prairie Research Institute (PRI; prairie.illinois.edu) at the University of Illinois at Urbana-Champaign have developed a novel biphasic solvent-based CO2 absorption process (known as BiCAP) that has the potential to lower energy and equipment costs for regenerating the CO2 from the solvent, once it is absorbed. The technology is based on a blended solvent that separates according to hydrophilicity and density into a CO2-rich phase and CO2-lean phase after contacting the fluegas. Most (~95%) of the CO2 absorbed ends up in the rich phase, which is about half (or less) of the volume of the original solvent, notes principal investigator Yongqi Lu, a chemical and environmental engineer at PRI. Because almost all the CO2 is contained in half the volume of solvent, less energy is required for thermal desorption, and the stripping capacity is increased, due to the higher CO2 loading in the “rich” solvent phase. In addition, less energy and compression work are required to compress the CO2, which is desorbed at an elevated pressure, according to the research team.
Figure 1. University of Illinois researchers built this pilot unit to test and develop a biphasic amine solvent that lowers energy requirements for CO2 capture
The group tested the process at a 0.7-ton/day integrated small pilot-scale capture unit at Abbott Power Plant in early 2022 (Figure 1) and validated the improved energy performance over the conventional absorption process. The team is planning is to scale up the technology at the 2–20 ton/day scale at industry sites.
Another angle on the phase-separation idea, known as the DMX process, comes from IFP Energies Nouvelles (www.ifpenergiesnouvelles.fr) and Axens (both Rueil-Malmaison, France; www.axens.net). DMX, a patented process stemming from research at IFP and being marketed by Axens, uses a solvent that reduces the energy intensity for carbon capture by nearly 30% compared to the MEA reference process. DMX is based on absorption by a de-mixing solvent (the DMX solvent), which is specifically designed to separate, under specific temperature and CO2 partial-pressure conditions, into a water-rich phase with high CO2 loading, and an amine-rich phase that is low in CO2, IFP says. The separation reduces the flow to the stripper, so less regeneration energy is required.
The DMX process was developed for CO2 capture on coal-power-plant fluegas and steel-making offgas, as well as waste incinerators, cement plants and biomass energy plants.
In May of this year, CarbonOrO Products B.V. (Naardem-Vesting, the Netherlands; www.carbonoro.com) received support from a cement industry consortium (Global Cement and Concrete Association) to develop its technology, which uses a proprietary CO2-capture amine solvent that is designed to undergo a phase shift. Due to the unique properties of the biphasic solvent, desorption of CO 2 is not just driven by chemistry, but also by a physical process (phase-shift), the company says. This allows for desorption of CO2 at much lower temperatures (starting at ~70°C) than prevailing amines solvents (starting at ~130°C). “Also, CO2-evasion is more or less instantaneous instead of gradual and correlated to temperature,” the company says. Consequently, energy use by the CarbonOrO process is lower than traditional technology.
For industrial applications, CarbonOrO targets energy use of 2.4 GJ/ton CO 2, significantly below the current industry (MEA) benchmark of 3.6 GJ/ton CO 2. In addition, the CarbonOrO solvent shows (in laboratory testing), less thermal and oxidative degradation than commercial solvents.
Another of the projects that recently was awarded funding from DOE is the University of Kentucky dual-solvent system. In June of this year, researchers at the University of Kentucky (Lexington; www.uky.edu) received $2.4 million to develop and demonstrate a cost-effective system to capture CO2 and produce hydrogen at natural-gas-combined-cycle (NGCC) power plants. The team has devised a dual-loop, two-solvent CO2-capture system that reduces CO2 concentrations in fluegas from NGCC plants to lower than that found in the atmosphere generally, making it a net-negative-CO2 emissions technology, co-principal investigator Kunlei Liu says (Figure 2).
Figure 2. University of Kentucky researchers are testing a dual-loop CO2-capture system that generates hydrogen to offset the cost of the capture
The system has a primary loop using an amine as the solvent. Depending on the external electricity demand, the main loop removes 80 to 95% of CO2 from the NGCC fluegas (which contains 4 vol.% CO2) and regenerates the solvent thermally. Then, the system uses a potassium hydroxide (KOH) solution to remove more CO2 in a second polishing loop that brings the overall CO2 capture to greater than 99%, Liu says. Because KOH cannot be regenerated thermally, the researchers developed an electrochemical system that generates pure H2 while regenerating a portion of the capture solvent. H2 production partially offsets the cost of the process.
The researchers are retrofitting the dual-solvent system on an existing 0.1-MW bench-scale facility using natural-gas-derived fluegas.
Other companies are advancing CO2 capture solutions that evolve from the traditional approaches by exploring water lean or non-aqueous amines. ION Clean Energy (Boulder, Colo.) is commercializing proprietary liquid absorbent and process technologies which are more effective and cost-efficient than current commercial solutions.
While ION’s solvent technologies are applicable to several applications, ICE-31 (its newest), is especially well suited for natural gas combined cycles (NGCC) and natural gas boilers because it is extremely stable in oxygen-rich environments and can adapt and ramp up with commercial dispatch of power stations, the company says. In an environment where every ton of CO2 captured is critical for capture economics, these key performance characteristics will drive CO2 emissions even lower for load-following facilities without impacting their dispatch rate.
ION has tested and proven its solvents at the National Carbon Capture Center in Alabama and the CO2 Technology Centre Mongstad in Norway, the world’s largest and most advanced CO2-capture test facility.
Most recently, ION has been the technology of choice for DOE-funded commercial front-end engineering design (FEED) studies for CO2 capture systems at Tampa Electric Company’s Polk Power Station and Calpine’s Delta Energy Center. Additionally, ION’s commercial pilot at Calpine’s Los Medanos Energy Center will be operational in early 2023. There, ICE-31’s long-term performance will be empirically demonstrated on a commercially dispatched NGCC, sponsored by DOE-NETL and Calpine.
ION has also explored improved gas-liquid contacting devices with its patent-pending Modular Adaptive Packing (MAP) technology. ION has leveraged the power of 3D printing to create optimal contacting devices that have been successfully benchmark-tested against the best commercially available optimized packing designs. 3D printing gives ION’s engineers the ability to completely reimagine the structure and design of G/L contacting devices to optimize cooling, mass transfer, liquid hold-up, and pressure drop, ION says.
Non-aqueous CO2-selective solvents are also the focus of another project recently garnering DOE support. The project stems from research by RTI International (Research Triangle Park, N.C.; www.rti.org), and involves a host of commercial and academic partners.
RTI’s non-aqueous solvents (NAS) are optimized mixtures of hydrophobic amines, which capture CO2 as amine carbamates, and hydrophobic diluents. NAS, such as the ones developed by RTI, offer several significant advantages that lower the cost of CO2 capture. The key advantage is the lower parasitic energy penalty, explains Vijay Gupta, a chemical engineer at RTI. “NAS does not rely on the need to generate water vapor in the solvent for CO2 stripping in the regenerator,” Gupta says. “This de-coupling of the solvent regeneration from the boiling point of water allows the solvent to be regenerated at lower temperatures, allowing for use of lower-quality steam, leading to an increase in plant efficiency.” The specific reboiler duty for NAS is 35-40% lower than that of 30 wt.% MEA, a leading aqueous solvent used in CO2 capture, Gupta adds.
In addition, NAS allows regeneration of CO2 at elevated pressures (4.2 bars) with minimal increase in regeneration energy. This lowers cost and power consumption for CO2 compression, Gupta says.
Figure 3. Engineers tested non-aqueous solvents for carbon capture at Technology Centre Mongstad in Norway
RTI is engaged in several projects testing different variations on its solvents. A large-scale testing and demonstration project for the use of NAS in a 12-MW CO2-capture plant is currently wrapping up at Technology Centre Mongstad. Data from the testing will be used to finalize the process setup and conduct a technoeconomic assessment of the technology. Two other ongoing projects are looking at process intensification of NAS for cement and NGCC plants.
Meanwhile, the company C-Capture Ltd. (Leeds, U.K.; www.c-capture.co.uk) is exploring solvent-based CO2 capture that doesn’t involve amines. The company was spun out of research by Chris Rayner at the University of Leeds to capture CO2 from biogas and landfill gas. The company has developed an organic solvent for CO2 capture that Rayner calls “amine-free.” Although the company won’t divulge the structure of the proprietary solvent, it is based on fundamentally different chemistry than that found in amine solvent systems, C-Capture says. It is capable of significantly lowering the energy requirements for releasing the CO2 and regenerating the solvent compared to amine-based solvents.
The C-Capture process reduces the energy required for CO2 removal by up to 90% when treating biogas, according to Rayner. Along with the solvent, Rayner and the C-Capture team have subsequently re-engineered the process equipment around it to maintain mild temperatures and pressures as a way to avoid high costs for equipment.
The company is moving the technology forward on several fronts. It has built a small pilot plant and a larger (1 ton of CO2 per day) demonstration plant to develop the technology. C-Capture has worked with the Drax Power generation facility in the U.K. on CO2 capture, and in July 2022, announced a partnership with the Bioenergy Infrastructure Group to provide carbon capture for the Ince Bioenergy Power facility, a biomass waste gasification facility in the U.K.
Linde, BASF and Mitsubishi Heavy Industries are among the others exploring improved solvents for CO2 capture. But while efforts to lower the costs of solvent-based CO2 capture will continue to be a main focus for CCUS, other approaches are also gaining momentum and support.
Another in the recent raft of funding awards from the DOE went to conduct a FEED study to retrofit an iron-making plant with CryoCap FG, an Air Liquide CO2-capture technology based on cryogenic separation. Cryocap combines pressure-swing adsorption capabilities with cryogenic refrigeration technologies to achieve high CO2-capture rates with high CO2 purity, according to project leaders. Cryocap FG has been piloted at facilities in France and Denmark.
Earlier this year, Air Liquide signed a memorandum of understanding with Lhoist to apply a CryoCap system to Lhoist’s lime production facility in Rety, France.
Several companies are developing CO2 capture with membranes rather than with sorbent materials. An example of this comes from Membrane Technology and Research Inc. (Newark, Calif.; www.mtrinc.com), which has developed polymeric CO2-capture membranes, known as the Polaris class. The company says its membranes offer the highest combination of CO2 permeance and CO2/N2 selectivity of any commercial polymeric membrane. The membranes are combined with a novel two-step process design (developed by MTR) that uses incoming combustion air to sweep membranes and recycle CO2 to the combustion process. The benefits of this two-step membrane design include an increased CO2 concentration to the membrane capture step and a reduction in the fraction of CO2 removal required by the capture step, the company says. MTR design calculations estimate that this membrane process can capture CO2 at a cost of below $40/ton under partial capture conditions.
The membranes are being tested in large-scale pilot tests designed to assess the cost-effectiveness of membrane-based CO2 capture compared to solvent-based capture. One project, at the Wyoming Integrated Test Center in Gillette, Wyo. will capture 150 tons of CO2 per day.
While the solvent-based CO2-capture systems are overall the most developed, several other CO2-capture technologies have gained significant interest and investment. The following describe some examples.
Molten salts. Mantel (Boston, Mass.; www.mantelcapture.com) recently received seed funding for its CO2-capture technology, which is based on molten borate salts. The borate salts are in the liquid phase at high temperatures (~600ºC), such as those found in industrial heating, cement, steel and hydrogen production applications. In the capture phase, CO2 is absorbed into the liquid salts, generating heat that can be recovered as high-quality heat. This heat can drive steam production. The approach can reduce energy losses by more than 60%, and overall costs by half, says Cameron Halliday, co-founder and CEO of Mantel. “Operating carbon capture at high temperatures offers thermodynamic advantages for energy efficiency, and since the borates are liquids at these elevated temperatures, we can also realize the advantages of liquid handling over solids,” he says.
Mantel is using its new funding to continue work on building a large prototype and to accelerate design of a 50-ton/d demonstration plant to evaluate a host of questions on operating the system. Mantel envisions its solutions for hard-to-abate industries such as industrial heat, cement, steel, and hydrogen, as well as CO2 removal from the atmosphere through pairing with biogenic sources of emissions, such as bioenergy, waste-to-energy and pulp and paper.
Nanoporous networks. Jeff Reimer, a researcher at the University of California at Berkeley and Lawrence Berkeley National Laboratory, recently published some work exploring melamine nanoporous networks (MNNs) for CO2 capture. At the kilogram scale, Reimer and colleagues demonstrated a solid-state, polyamine-appended, cyanuric acid–stabilized MNNs that is effective, scalable, recyclable, and capable of high-capacity CO2 capture, the team says. According to the researchers, MNNs (synthesized from commercial melamine and paraformaldehyde) are promising for reversible CO2 capture “owing to the intriguing advantages of their robust flake-like structures, high surface areas, tunable surface chemistries and industrial-scale capture capabilities.” The project is probing the nature of the chemisorption mechanism at the atomic level to aid further the design of MNNs with high CO2 adsorption capacity for CCUS.
Electro-swing adsorption. In April of this year, Verdox Inc. (Boston, Mass.; www.verdox.com) was awarded a $1 million milestone award from XPrize Foundation and Musk Foundation for carbon removal. Verdox has developed a system for capturing CO2 based on electrical charge that requires much less energy than approaches using temperature or pressure differentials. The electroswing adsorption (ESA) platform is similar in principle to pressure-swing or temperature-swing adsorption, but uses 70% less energy, the company says. The ESA platform is a modular system consisting of stacks of flat electrodes that are functionalized with specially designed quinones. The capturing electrode has high affinity for CO2, but only when charged. When a cell is charged, it can bind CO2 in a chemically irreversible step,” explains Sahag Voskian, Verdox chief technology officer. “When the cell is discharged, using a counter electrode to balance the charge and complete the electrochemistry, the affinity for CO2 falls to zero and the CO2 is released in a pure form.” The company is now conducting field trials and plans to begin construction of a pilot plant.
Metal organic frameworks (MOFs). MOFs are viewed as possible sorbent material for CO2. An interesting example is Nuada, a technology developed by MOF Technologies (Belfast, U.K.; www.moftechnologies.com), that operates via vacuum-swing adsorption (VSA). Nuada is a combination of a proven, mature technology (the VSA) and a novel, high-capacity MOF sorbent material, applied to point-source carbon capture. By using pressure instead of heat to release the captured CO2, Nuada cuts energy consumption by up to 80% versus state-of-the-art amine scrubbing solutions, the company says.
Nuada is designed as a compact and modular carbon-capture plant.
Ionic liquids. Ionic liquids (ILs) are another class of materials being explored for use as a sorbent material for CO2 capture. The company IoLiTec GmbH (Heilbronn, Germany; www.iolitec.de) is exploring various combinations of IL anions and cations for capturing CO2 from different fluegas compositions. In general, some potential advantages of ILs for CO2 capture include faster reaction kinetics than CO2 absorption with MEA, higher CO2 loading and less corrosion impact. The company is constructing a pilot plant in Greece to capture CO2 from a coal-fired power plant.
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