Scientists from the Netherlands Organisation for Applied Scientific Research (TNO) have proposed an effective method for purifying carbon dioxide intended for underground injection or industrial use. They have developed a technology that makes it possible to remove carbon monoxide (CO) from CO₂ via so-called chemical cycling, a method that not only improves the purity of carbon dioxide but also helps extract additional energy from impurities that would normally have to be removed.
The biggest challenge of carbon capture and storage is that CO₂ is rarely completely pure. Depending on the source and capture technology, CO2 may contain impurities that can damage pipelines, underground storage facilities or catalysts during subsequent use. Carbon monoxide content is subject to especially strict requirements: its concentration must not exceed 100–300 ppm (parts per million). The existing purification methods either require significant energy inputs, complex equipment or can introduce additional contaminants into the gas stream.
Seeking to address this problem, the researchers have proposed the use of chemical cycling technology. This two-stage process uses particles of metal oxide, a so-called oxygen carrier.
During the first stage, a stream of CO₂ mixed with CO is passed through a layer of metal oxide. The oxide releases oxygen to the CO molecules, causing the carbon monoxide to oxidize and convert into CO₂. At the same time, the metal oxide is partially reduced. During the second stage, it gets reoxidized with air, which brings it back to its original state and prepares it for a new cycle. This way, the system acts as a chemical filter, purifying the gas without introducing foreign substances.
The researchers conducted a thermodynamic analysis first to select the most suitable materials. They looked at nickel, copper, manganese and iron oxides under relatively mild conditions, with temperatures below 700°C and low CO concentrations. Copper and iron compounds proved to be the most promising, while nickel and manganese demonstrated less stable results. Both stages of the process for the selected materials were exothermic, which means that they were accompanied by the release of heat. This opens up the possibility of using the released energy to, among other things, generate steam or electricity.
Particular attention in the experiments was paid to iron, as it is readily available, non-toxic and relatively inexpensive. The scientists tested several types of iron oxide-based materials with the addition of aluminum oxide as a carrier. The best results were demonstrated by the sample obtained via the impregnation method: it withstood about 200 cycles at temperatures ranging from 400°C to 550°C, stably oxidizing carbon monoxide to CO₂. Experiments also showed that the technology works effectively even at low CO concentrations of some 2,000–4,000 ppm, which corresponds to conditions in a real industrial flow.
The researchers also found that, while raising the temperature to about 450°C significantly extends the time required for complete carbon monoxide conversion, further heating yields progressively less additional benefit. This is why the optimal temperature regime will depend on the specific process flow diagram and the opportunities for utilizing the heat generated.
While iron-containing materials showed good results, the scientists suggest that there may exist compounds with higher reaction rates or greater durability. In the future, the researchers plan to test other materials and conduct a feasibility study of the technology. This will allow them to find out how this purification method could become more cost-effective than the existing methods, such as cryogenic separation or catalytic oxidation.
