Carbon dioxide (CO2) scrubbers are devices or processes designed to remove CO2 from a gaseous mixture, such as air or industrial exhaust. They are used to purify gas streams for industrial purposes or to reduce CO2 concentration for environmental reasons. As an integral part of carbon capture technologies, scrubbers extract the targeted molecule. The captured CO2 is then handled for storage or utilization, supporting localized environment control and global decarbonization efforts.
The Basic Science of Carbon Capture
CO2 scrubbers operate using two principal scientific mechanisms: chemical absorption and physical adsorption. Chemical absorption involves a liquid solvent, frequently an aqueous solution of organic compounds called amines, which reacts directly with the CO2 molecules. The gas flows through a contactor where the solvent chemically binds the carbon dioxide. The CO2-rich solvent is then transferred to a separate stripping unit where heat is applied, reversing the chemical reaction to release a concentrated stream of CO2 and regenerate the solvent for reuse.
Physical adsorption relies on porous solid materials like activated carbon, zeolites, or Metal-Organic Frameworks (MOFs) to trap the CO2 molecules. This process, known as physisorption, is a surface phenomenon where gas molecules are held to the solid material by weak forces. The CO2 is typically released from the solid sorbent for collection by manipulating pressure or temperature in processes like Pressure Swing Adsorption (PSA) or Temperature Swing Adsorption (TSA). The choice between methods is often determined by the CO2 concentration; chemical methods are favored for lower concentrations, while physical methods can be more cost-effective for very high-concentration streams.
Industrial and Life Support Applications
CO2 scrubbers are used in specialized, confined environments where CO2 buildup poses an immediate threat. In spacecraft and rebreather systems, for instance, the scrubber maintains breathable air by removing exhaled carbon dioxide. Early NASA missions, including the Apollo program, relied on expendable canisters containing solid lithium hydroxide (LiOH). This compound chemically reacts with CO2 to form lithium carbonate, a process valued for its high absorption capacity and small size.
For larger, longer-duration confined spaces like nuclear submarines, regenerative liquid systems using monoethanolamine (MEA) are often employed. On a much larger scale, industrial point-source scrubbing targets exhaust gas streams, such as those from power generation or cement manufacturing. The CO2 concentration in these flue gases can range from 3% to 20%, making the capture process significantly less challenging than removing CO2 from ambient air. These systems utilize mature technologies to capture emissions directly at the source.
Direct Air Capture Technology
Direct Air Capture (DAC) technology is a distinct application of CO2 scrubbing designed to remove carbon dioxide dispersed in the atmosphere. The fundamental challenge for DAC systems stems from the extremely low concentration of CO2 in ambient air, which is only about 420 parts per million (ppm). This concentration is orders of magnitude lower than the CO2 levels found in industrial flue gas, demanding a highly selective and energy-intensive capture process.
To compensate for the low concentration, DAC facilities must process massive volumes of air, often using large arrays of fans and contactors. The technology typically utilizes either liquid solvents or solid sorbents exposed to the air to capture the CO2. Because of the vast amount of air that must be moved and the energy required to regenerate the sorbent or solvent, DAC is a highly energy-intensive process. Some applications require thousands of kilowatt-hours of energy per ton of CO2 captured, making DAC a complex and costly technology.
Handling the Captured Carbon
Once CO2 is scrubbed and concentrated, it must be managed to achieve a net environmental benefit. The two main outcomes are permanent geological sequestration and utilization in commercial applications. Carbon Capture and Storage (CCS) involves compressing the captured CO2 and injecting it deep underground into stable geological formations. These formations include deep saline aquifers or depleted oil and natural gas reservoirs.
Alternatively, the captured CO2 can be directed toward Carbon Capture and Utilization (CCU) pathways, where it is used as a feedstock for new products. A long-standing utilization method is Enhanced Oil Recovery (EOR), where CO2 is injected into oil fields to increase extraction, though its net climate benefit is often debated. Other applications include incorporating the captured CO2 into building materials like concrete, or converting it into synthetic fuels or chemicals. However, using it for fuels or chemicals may result in the eventual re-emission of the CO2 when the product is consumed.