MIT researchers have developed a new system that could potentially be used for converting power plant emissions of carbon dioxide into useful fuels for cars, trucks and planes, as well as into chemical feedstocks for a wide variety of products.
This new membrane-based system was developed by MIT postdoc Xiao-Yu Wu and Ahmed Ghoniem, the Ronald C. Crane professor of mechanical engineering. The membrane is made of a compound of lanthanum, calcium and iron oxide. This allows oxygen from a stream of carbon dioxide to migrate through to the other side, leaving carbon monoxide behind. Other compounds, known as mixed ionic-electronic conductors, are also under consideration in their lab for use in multiple applications including oxygen and hydrogen production.
Carbon monoxide produced during this process can be used as a fuel by itself, or combined with hydrogen and/or water to make many other liquid hydrocarbon fuels, as well as chemical including methanol, syngas and so on. Ghoniem’s lab is exploring some of these options. This process could become part of the suite of technologies known as carbon capture, utilization and storage, or CCUS, which, if applied to electricity production, could reduce the impact of fossil fuel use on global warming.
The membrane has a structure known as perovskite, which is 100 percent selective for oxygen and allows only those atoms to pass. The separation is driven by temperatures of up to 990 degrees Celsius. The key to making this process work is to keep the oxygen that separates from carbon dioxide flowing through the membrane until it reaches the other side. This could be done by creating a vacuum on the side of the membrane opposite the carbon dioxide stream. This would require a lot of energy to maintain.
Instead of the vacuum, the researchers used a stream of fuel, like hydrogen or methane. These materials are so oxidized that they will actually draw the oxygen atoms through the membrane without requiring a pressure difference. The membrane also prevents the oxygen from migrating back and recombining with the carbon monoxide, to form carbon dioxide all over again. Ultimately, and depending on the application, a combination of some vacuum and some fuel can be used to reduce the energy required to drive the process and produce a useful product.
The energy input needed to keep the process going is heat, which could be provided by solar energy or by the waste heat. Some of this heat could come from the power plant itself and some other sources. Essentially, the process makes it possible to store that head in chemical form, for use whenever it’s needed. Chemical energy storage has very high energy density — the number of energy stores for a given weight of material — as compared to many other storage forms.
At this point, the team has demonstrated that the process works. Ongoing research is examining how to increase the oxygen flow rates across the membrane, perhaps by changing the material used to build the membrane, changing the geometry of the surfaces or adding catalyst materials on the surfaces. The researchers are also working on integrating the membrane into working reactors and coupling the reactor with the fuel production system. They are examining how this method could be scaled up and how it compares to other approaches to capturing and converting carbon dioxide emissions in terms of costs and effects on overall power plant operations.
In a natural gas power plant Ghoniem’s group has working on previously, Wu says the incoming natural gas could be split into two streams. One stream would be burned to generate electricity while producing a pure stream of carbon dioxide and the other stream would go to the fuel side of the new membrane system. This provides the oxygen-reacting fuel source. That stream would produce a second output from the plant — a mix of hydrogen and carbon monoxide known as syngas — which is widely used as an industrial fuel and feedstock. The syngas can be added to the existing natural gas distribution network.
This method may not only cut greenhouse emissions, it could also produce another potential revenue stream to help defray the costs.
The process can work with any level of carbon dioxide concentration, but the higher the concentration, the more efficient the process is. So it was well-suited to the concentrated output stream from conventional fossil fuel-burning power plants or those designed for carbon capture, like oxy-combustion plants.
The research on this study was published in the journal ChemSusChem.