A case study conducted by the U.S. Department of Energy’s (DOE) NETL examined the performance and cost of solvent-based direct air capture (DAC) system configurations that remove carbon dioxide (CO₂) from the atmosphere.

DAC is an emerging carbon dioxide removal (CRD) technology that concentrates CO₂ found in the ambient air rather than a power plant or industrial facility flue gas, thereby addressing both current and legacy emissions. Atmospheric concentrations of CO₂ (~415 parts per million) are much lower than those found in effluent streams from power plants or industrial facilities, presenting a greater technical and cost challenge for technologies to concentrate the CO₂ to the degree necessary for storage or utilization.

“There are many tools at our disposal for achieving a carbon emission-free electricity sector by 2035 and economy-wide net-zero emissions by 2050. One DAC tool in our box is solvent-based carbon capture, which uses liquid solvents to capture atmospheric CO₂ through chemical reaction directly from the atmosphere,” said Tim Fout, a member of NETL’s Strategic Systems Analysis and Engineering (SSAE) directorate. “The Lab is examining a variety of DAC technology options to prioritize research and development – the ultimate goal is to facilitate commercialization by the private sector.”

The solvent-based DAC system evaluated in this case study, which has been developed by Carbon Engineering,  comprises air contactors that remove 74.5% of the CO₂ from the inlet air using a potassium hydroxide solvent, pellet reactors that convert potassium carbonate to calcium carbonate pellets and potassium hydroxide using calcium hydroxide, steam slakers that dry the calcium carbonate pellets and also regenerate calcium hydroxide through the reaction of calcium oxide with water, an oxy-fired calciner in which the calcium carbonate pellets are converted to calcium oxide and CO₂, a low-pressure air separation unit to provide oxygen to the calciner, and a CO₂ compressor for the CO₂ product.

Continuing research, development and demonstration is expected to result in more advanced DAC solvents, process equipment, and system configurations, leading to lower costs that will make widespread application practical.

In recent years, there has been a significant increase in research focused on DAC. A number of DAC developers have advanced proprietary technologies to small pilot-scale testing, and published performance and cost estimates; however, these technologies require further research, development, and demonstration to reduce costs. Estimates of the cost to remove CO₂ from the atmosphere disclosed by various sorbent- and solvent-based DAC technology developers roughly span the range of $95–600 per ton, but varying study assumptions and inadequate detail make assessments and direct comparisons of the reported costs difficult.

The NETL solvent-based DAC study has been published and can be viewed here.

DAC is a priority for the DOE and figures prominently in the Bipartisan Infrastructure Law (BIL), in which $3.5 billion has been dedicated to development of regional DAC Hubs that will each capture at least 1,000,000 metric tons of CO₂ from the atmosphere. In addition, the DOE established the Carbon Negative Shot, which seeks to reduce the cost of CRD technologies to $100/net metric ton CO₂ removed from the atmosphere within ten years.

NETL is a U.S. Department of Energy national laboratory that drives innovation and delivers technological solutions for an environmentally sustainable and prosperous energy future. By leveraging its world-class talent and research facilities, NETL is ensuring affordable, abundant and reliable energy that drives a robust economy and national security, while developing technologies to manage carbon across the full life cycle, enabling environmental sustainability for all Americans.