Researchers believe they can boost the economic feasibility of carbon capture and storage by turning captured carbon into a useable product.
Scientists at the US Department of Energy’s Idaho National Laboratory (INL) have developed a process for turning captured carbon dioxide into syngas, a mixture made up primarily of hydrogen and carbon monoxide that can be used to make fuels and chemicals. The team has published its results in Green Chemistry, a publication of the Royal Society of Chemistry.
According to INL, traditional approaches for reusing the carbon from CO2 involve a reduction step that requires high temperatures and pressures. At lower temperatures, the CO2 doesn’t stay dissolved in water long enough to be useful.
The process developed at INL is said to address this challenge by using specialised liquid materials that make the CO2 more soluble and allow the carbon capture medium to be directly introduced into a cell for electrochemical conversion to syngas.
“For the first time it was demonstrated that syngas can be directly produced from captured CO2 – eliminating the requirement of downstream separations,” the researchers said in the paper.
The newly described process uses switchable polarity solvents (SPS), liquid materials that can shift polarity when exposed to a chemical agent. This property makes it possible to control what molecules will dissolve in the solvent.
In an electrochemical cell, water oxidation occurs on the anode side, releasing oxygen and hydrogen ions that migrate through a membrane to the cathode.
The hydrogen ions then react with bicarbonate (HCO3-, the form in which CO2 is captured in the SPS), allowing the release of CO2 for electrochemical reduction and formation of syngas.
On the release of CO2, the SPS switches polarity back to a water-insoluble form, allowing for the recovery and reutilisation of the carbon capture media.
In early experiments, too much hydrogen and not enough syngas was being produced. The results improved when the team introduced a supporting electrolyte to increase the ionic conductivity. Adding potassium sulphate increased electrolyte conductivity by 47 per cent, which allowed the efficient production of syngas.
INL add that when syngas can be produced from captured CO2 at significant current densities, it boosts the process chances for industrial application. Unlike other processes that require high temperatures and high pressures, the SPS-based process is said to have shown best results at 25 degrees C and 40psi.
The team has filed a provisional patent and is discussing the approach with a company involved in electrochemical technology research and development, Lister said.
“It integrates two areas that have been on parallel tracks: carbon capture and sequestration (CCS) and CO2 utilisation,” said Diaz Aldana, principal investigator on the experiment. “The problem with CCS has been its economic feasibility. If you can get some extra value out of the CO2 you are capturing, it’s a different story.”