U.K. RESEARCHERS EXAMINE CALCIUM-BASED SORBENTS IN SITU AT NANOSCALE

Tamar Hallerman
GHG Monitor
08/03/12

Developers looking to improve calcium-based sorbent materials for carbon capture need to find materials that are less susceptible to degradation so that systems can run more reliably, according to a group of United Kingdom-based researchers. In a study recently published in the journal Energy & Environmental Science, researchers at the University of Leeds examined calcium oxide (CaO)-based sorbents on the nanoscale level using the Diamond Light Source, the United Kingdom’s national synchrotron. By conducting a series of smaller capture tests on the machine’s high resolution powder diffraction beamline, the researchers were able to study the capture and hydration processes associated with CaO-based materials in situ in a much more detailed way than in previous examinations.

While other researchers have been able to conduct laboratory X-ray tests of the sorbents, none have been as detailed as the Diamond test, and few have been able to view the capture process in situ, said Tim Comyn, a head author on the paper and professor of engineering at the University of Leeds. Instead, researchers have had to move forward with the process at a particular temperature, halt operations and then examine the conditions, leading to a less accurate readout of the process. Being able to examine the CO2 capture process while it was occurring, at temperature gave the Leeds team the ability to determine the true size and strain the materials experience during the process, Comyn said in an interview.

Calcium Sorbents a Cheaper Option for Post-Combustion Capture

Used primarily for post-combustion capture, calcium sorbent-based materials are considered among the most mature CO2 capture methods for existing power plants. The materials are abundant, relatively low cost and have large sorption capacity and fast reaction rates during the chemical process. The materials can help capture CO2 at a range of 400°C to 800°C via the formation of calcium carbonate (CaCO3), which can be regenerated and used over again. However, after multiple regeneration cycles, the material sinters and becomes more dense, losing surface area and subsequently decreasing capture capacity. System operators can temporarily restore that surface area by hydrating the material with steam. However, the sorbent is subsequently reduced of its mechanical strength and eventually “falls to pieces,” according to Comyn, needing to be replaced if CO2 capture is to be continued.

By comparing their results with those gained from more conventional laboratory X-ray tests, the Leeds team was able to determine that the stresses in the calcium hydroxide phase when bound to CaO were more than 20 times higher than its strength, leading to the degradation of the materials once exposed to steam, according Comyn, a professor of engineering at the University of Leeds. “What we’ve done is look at calcium oxide mixed with calcium hydroxide, and we’ve been able to see a mechanism that explains why, when there’s water around, these things fall to bits and disintegrate,” Comyn said. “The analysis that we did allows us to gauge the fact that there’s a huge stress that develops in these materials when there’s water around, and that stress is about many times higher than what you need to break the thing apart.”

Next Step: Develop Better Materials

The team’s observations could bring insights that could help improve the efficiency of the capture method as it is further developed, according to Comyn. He said that developers of new materials for post-combustion capture will need to develop materials with good surface area and solid mechanical friability for the capture systems to continue to be reliable over a longer period of time. “This is a first step along the way,” Comyn said. “If we were to do much more experimentation, we might be able to tell the CCS community ‘look, you need this particular amount of steam,’ but I think there’s another side to this as well, and that’s perhaps that we need to be thinking of modifying these materials a little bit so that we get the best of both worlds. We want something which is highly reactive and captures carbon very easily, but it’s no good if the thing is going to fall to bits and not work under certain cycles. There’s materials science that needs to go on to improve the materials and there’s also some experimentation that needs to go on in situ to find the best way of using these materials.” He added that he is hoping to do a larger-scale experiment using the Diamond facility in the near future.