Most researchers today are concerned with reducing the emission of carbon dioxide by designing more efficient engines and switching to renewable sources of energy. Researchers at Georgia Institute of Technology have chosen another route; they are studying the direct extraction of carbon dioxide from air using newly developed adsorbent materials.
The technology was initially used to supply CO2 for industrial applications such as biofuel production from algae and enhanced oil recovery. However, the technology was later expanded to supplement the air filtration of power plant flue gas by extracting the emitted CO2.
However, CO2 from flue gas accounts for only a portion of the greenhouse gas emitted each year. David Sholl, professor in Georgia Tech’s School of Chemical & Biomolecular Engineering, says that to make significant cuts in emission the only viable option is direct extraction from air.
Christopher Jones, another professor at Georgia Tech, adds that direct air capture is necessary since much of the emissions come from mobile sources such as buses, cars, planes and ships. However, he acknowledges that this would be more costly.
In a detailed economic feasibility study, the researchers projected that a CO2 removal unit the size of an ocean shipping container could extract about a thousand tons of the gas each year. The operation costs of the extraction would be around $100 per ton.
Global Thermostat, a startup company, is collaborating with the researchers to establish a pilot plant to demonstrate the direct air capture technique. The method is similar to the extraction technique for removing the gas from smokestack emissions, although the researchers have to optimize the extraction efficiency. Optimization is important since flue gas typically contains 15 percent CO2 while the atmosphere contains less than 400 parts per million of the gas.
Sholl notes that the extraction efficiency could be improved by building the capture equipment in the sequestration sites; this would eliminate losses from transport. Jones further adds that new process improvements could bring down operational costs from the previous estimates.
The technique modeled by the Georgia Tech team is a batch extraction process that involves blowing air through a ceramic honeycomb structure coated with adsorbent. The adsorbent is a dry amino-modified silica material that has a high affinity for CO2. The adsorbent can be recycled by passing steam through the structure to remove the adsorbed gas. The technique could produce carbon dioxide that is 90 percent pure.
Jones says the technical challenges of the research are demonstration at scale, demonstration of long-term adsorbent stability and demonstration of process feasibility. He further asserts that increased funding for air capture research is needed.
Sholl and Jones have been collaborating on a number of researches on air capture and developing adsorbent materials, including metal-organic framework (MOF) materials. Their past works included a study published in the Journal of the American Chemical Society that describes the role of Zirconium in enhancing the efficiency of amine-based adsorbents.
Another study published in ChemSusChem describes the adsorptive capacity of primary, secondary and tertiary amines for carbon dioxide. The results show primary amines have the greatest affinity for CO2 and can selectively capture the gas from air.
The study that describes a detailed cost estimates for the air capture process was published in Industrial & Engineering Chemistry Research.
Jones believed that air capture should be among the options explored to address global warming. The US Department of Energy funded the Georgia Tech research into air capture techniques.