Researchers at the Department of Energy’s Pacific Northwest National Laboratory (PNNL) have created a new system—the least costly to date—that efficiently captures CO₂ and converts it into methanol. The new PNNL carbon capture and conversion system brings the cost to capture CO₂ down to about $39 per metric ton.

The process takes flue gas from power plants, uses a PNNL-patented solvent to strip out CO₂, then converts the CO₂ into methanol. PNNL chemist David Heldebrant, who leads the research team behind the new technology, compares the system to recycling. Just as one can choose between single-use and recyclable materials, so too can one recycle carbon.

As described in an open-access paper in the journal Advanced Energy Materials, the new system is designed to fit into coal-, gas-, or biomass-fired power plants, as well as cement kilns and steel plants.

An efficient and selective heterogeneous catalyst is identified for the condensed-phase hydrogenation of captured CO₂ in the presence of an advanced water-lean post-combustion capture solvent, (N-(2-EthoxyEthyl)-3-MorpholinoPropan-1-Amine), 2-EEMPA. The catalysts commonly used for gas-phase CO₂ hydrogenation (e.g., Cu/Zn/Al₂O₃) cause deactivation of amine promoters via N-methylation by C—O cleavage of formamide intermediates.

A heterogeneous catalyst system that suppresses N-methylation of amine solvents is identified, demonstrating how Pt, supported by reducible metal oxides CeO₂ or TiO₂, can be selective for C—N cleavage to produce methanol.

This is the first known demonstration of integrated low-temperature thermocatalytic capture and conversion of CO₂ to methanol in an economically viable CO₂ capture solvent. Technoeconomic analyses performed on the state-of-technology suggest that methanol can be produced with a minimum selling price of $4.4/gallon ($1,460/metric ton) when using CO₂ captured from a 650 MW natural gas combined cycle plant. Ultimately, a road map of how realistic and achievable improvements to space velocity and methanol selectivity of this integrated process can enable near cost parity to fossil-derived methanol, with a selling price of ≈$1.4/gal ($470/metric ton), is presented.

—Kothandaraman et al.

By using renewably sourced hydrogen in the conversion, the team can produce methanol with a lower carbon footprint than conventional methods that use natural gas as a feedstock. Methanol produced via CO₂ conversion could qualify for policy and market incentives intended to drive adoption of carbon reduction technologies.

A significant amount of work remains to optimize and scale this process, and it may be several years before it is ready for commercial deployment. But, said Casie Davidson, manager for PNNL’s Carbon Management and Fossil Energy market sector, displacing conventional chemical commodities is only the beginning; the team’s integrated approach opens up a world of new CO₂ conversion chemistry.

Commercial systems soak up carbon from flue gas at roughly $46 per metric ton of CO₂, according to a DOE analysis. The PNNL team’s goal is to continually chip away at costs by making the capture process more efficient and economically competitive.

The team brought the cost of capture down to $47.10 per metric ton of CO₂ in 2021. A new study described in the Journal of Cleaner Production explores the cost of running the methanol system using different PNNL-developed capture solvents, and that figure has now dropped to just below $39 per metric ton of CO₂.

We looked at three CO₂-binding solvents in this new study. We found that they capture over 90 percent of the carbon that passes through them, and they do so for roughly 75 percent of the cost of traditional capture technology.

—Yuan Jiang, lead author of the Journal of Cleaner Production paper

Different systems can be used depending on the nature of the plant or kiln. But, no matter the setup, solvents are central. In these systems, solvents wash over CO₂-rich flue gas before it’s emitted, leaving behind CO₂ molecules now bound within that liquid.

Creating methanol from CO2 is not new. But the ability to both capture carbon and then convert it into methanol in one continuously flowing system is. Capture and conversion has traditionally occurred as two distinct steps, separated by each process’s unique, often non-complementary chemistry.

Traditional conversion technology typically requires highly purified CO₂ notes Heldebrant; the new system is the first to create methanol from “dirty” CO₂.

This technology is available for licensing. This work was supported by the Department of Energy’s Technology Commercialization Fund, the Office of Fossil Energy and Carbon Management, and Southern California Gas. Part of the work was performed at EMSL, the Environmental Molecular Sciences Laboratory, a DOE Office of Science user facility at PNNL.

Resources

• Kothandaraman, J., Lopez, J. S., Jiang, Y., Walter, E. D., Burton, S. D., Dagle, R. A., Heldebrant, D. J. (2022) “Integrated Capture and Conversion of CO2 to Methanol in a Post-Combustion Capture Solvent: Heterogeneous Catalysts for Selective C—N Bond Cleavage.” Adv. Energy Mater. doi: 10.1002/aenm.202202369

• Yuan Jiang, Paul M. Mathias, Richard F. Zheng, Charlies J. Freeman, Dushyant Barpaga, Deepika Malhotra, Phillip K. Koech, Andy Zwoster, David J. Heldebrant (2023) “Energy-effective and low-cost carbon capture from point-sources enabled by water-lean solvents,” Journal of Cleaner Production, doi: 10.1016/j.jclepro.2022.135696