With myriad materials to choose from, simulations help speed up the process of selecting the right materials for the job.
With most of the world still relying on fossil fuel-driven power plants for their energy, carbon dioxide emissions remain a global concern. Reducing greenhouse gas emissions, particularly carbon dioxide, is paramount in combating climate change.
One way to do this is to capture the carbon dioxide (CO₂) emissions directly from the smokestacks of the power plants before they enter the atmosphere.
Dr. Lourdes Vega, Director of the Khalifa University Research and Innovation Center on CO₂ and Hydrogen (RICH) and Professor of Chemical Engineering, is leading a collaborative research team that is analyzing different types of materials to determine their potential for post-combustion carbon dioxide capture and separation.
The team includes Dr. Daniel Bahamon, Research Scientist, and Dr. Maryam Khaleel, Assistant Professor of Chemical Engineering, both from Khalifa University, along with Dr. Santiago Builes from EAFIT University, Colombia, and Wei Anlu from China University of Petroleum, a student who spent six months at the RICH Center at Khalifa University for performing part of this study. They published their work in January in the journal Frontiers in Chemistry.
“Mitigation strategies such as carbon capture, utilization and storage (CCUS) play an import role in limiting the contribution of CO₂ emissions to global climate change. One key approach to this is capturing post-combustion CO₂ from flue gas at power stations and chemical manufacturing plants,” explained Dr. Vega.
Flue gas is the by-product gas that leaves a fossil fuel power station or plant via a chimney known as a flue. While its composition depends on what is being burned, it mostly comprises nitrogen, carbon dioxide, water vapor and a number of pollutants such as particulate matter, carbon monoxide, nitrogen oxides and sulfur oxides. The ‘smoke’ seen pouring from these flues is not smoke at all, but the water vapor in the gas forming a cloud as it meets cooler air.
Carbon dioxide is the second largest component of flue gas at around 4 to 25 percent, depending on the source. Although there are technologies available now to capture this carbon dioxide before it can wreak havoc on the atmosphere, they have several disadvantages.
Currently, aqueous amine solutions, which are solutions containing water and amines, organic compounds derived from ammonia and containing a nitrogen atom attached to hydrogen and carbon atoms, are used to capture CO₂ in large-scale applications. Amine solutions are excellent at trapping the CO₂, making them the most popular and developed carbon capture technology. However, the disadvantage to this technology is that in order to separate the trapped CO₂ from the amine solution, it has to be heated, requiring additional energy, and some of the amines are lost in this high energy process.
To overcome the shortcomings of amine solutions, solid sorbent materials are a viable alternative. Solid sorbents can selectively adsorb CO₂, however some solid sorbent materials perform better than others, and finding the most optimal carbon capturing material was the focus of Dr. Vega’s investigation.
A good adsorbent is a highly porous material with a large internal surface, full of holes to collect the CO₂. Metal-organic frameworks (MOF) materials possess enhanced stability, greater CO₂ cycling capacities and lower regeneration energies, making MOFs a material of choice for solid sorbent-based carbon capture.
However, MOFs alone are not enough to adsorb the CO₂ from flue gas at low pressures, especially since water vapor in the gas can compete with the carbon dioxide for adsorption. To counter this, attention has turned to amine-functionalized MOFs, where amines are grafted onto the open metal sites to increase CO₂ adsorption selectivity and capacity. These materials combine the benefits of both the MOFs and the amines, avoiding the disadvantages of the need for heating the solvent for removing the CO2 or the evaporation of the amines.
There are multiple types of amines, each of which has different characteristics relevant to CO₂ capture. Finding the optimal amine for each real-world application can be a time-consuming endeavor.
“Molecular simulations can allow the systematic and precise study of the various relevant variables of a system,” explained Dr. Vega. “We can isolate and quantify the effect of each functionalized MOF on the performance of the system, making simulation an excellent tool for the rational design of materials.”
The research team used molecular simulations to explore the relationship between the structure of different kinds of amine-grafted MOFs and their CO₂ adsorption performance.
A series of amine-grafted MOFs were screened, establishing the most promising materials for adsorbing low-concentration CO₂, while considering their regeneration performance, or how many cycles the MOFs could operate before degrading.
“Our work offers a molecular understanding of how functionalization takes place on MOFs and how it affects their final performance, providing guidance on the design of the best material/amine combination for optimal post-combustion CO₂ capture,” said Dr. Vega. Once the best material is found with this procedure, it will be synthesized and tested in a reactor at the conditions required for CO2 capture from different sources.
Dr. Vega’s team’s work is a significant contribution to the development of efficient and sustainable carbon capture utilization and storage solutions, as part of the RICH Center effort to find optimal materials to produce clean energy.
15 April 2021