A Crash Course in Carbon Capture

Carbon dioxide, or CO₂, is essential for the survival of most living organisms – including us. For example, plants convert it to the oxygen we need to breathe. But, too much of it in the atmosphere warms the planet, causing climate change. 

Human activities have caused levels of CO₂ to skyrocket - for the past two centuries in particular - and scientists and policy makers have been working to reverse emissions to preserve Earth and ensure society’s future prosperity. 

Katherine Hornbostel, assistant professor of mechanical engineering materials science at the University of Pittsburgh Swanson School of Engineering, has devoted much of her career to developing carbon capture methods, the process where CO₂ is removed from power plant exhaust or the environment then reused or stored permanently underground.

She recently led a discussion with curious Pittsburghers about this process at the Carnegie Science Center’s Café Scientifique.

“At the end of the day, we have to do everything we can to fight this,” Hornbostel said. 

Why Do We Need to Catch Carbon?  

The Biden-Harris Administration set ambitious reduction goals in 2021, which would put the US on course to reach net-zero emissions by 2050. But how can carbon capture and storage help us get there? 

First, we need to understand where emissions are coming from and the technology being developed to mitigate it, Hornbostel explained. 

CO₂ comes from several places and activities – some more obvious than others. There are ways we can reduce some of our carbon footprint on our own, like choosing public transportation rather than driving a car on our daily commute. Individual buildings, like Pitt’s Mascaro Center for Sustainable Innovation (MCSI) LEED-certified building at the Swanson School, are becoming more proactive in mitigating their emissions. However, industrial processes like the production of steel and cement are much more carbon intensive because of their dependency on fossil fuels. This is where carbon capture comes in. 

“We should do carbon capture with industrial sources,” Hornbostel said. “Maybe even more so than natural gas. You can’t really decarbonize these processes, so it makes a lot of sense to go with carbon capture.” 

What Does Carbon Capture Look Like? 

Carbon capture technology can attach to a building; however, as Hornbostel noted, it can be a logistical nightmare. 

“The cool thing here is that we’re already moving a lot of air when we have buildings,” Hornbostel said, “We have built-in fans and blowers, and we’re exhaling in buildings, so the carbon dioxide levels are much higher in the building than in the surrounding atmosphere.” 

On the industrial and power generation side, however, it’s a different story – which is where the bulk of carbon emissions need to be reduced anyway. Hornbostel and like-minded researchers have been working to design attachments to these plants that are both practical and cost-effective in a relatively small complex. 

The first is through chemical reactions: liquid chemicals (solvents) and solid chemicals (sorbents). Gas can be moved from the industrial source through a bed of either solvents or sorbents. They’ll start to react with the CO₂ selectively, then the CO₂ will be removed from the material once it’s full so it can be recirculated and used again. 

“You’re trying to remove the carbon dioxide from that material once it’s full,” Hornbostel explained. “So, you might send high temperatures, like steam, to pull the carbon dioxide out of this material and then reuse it. These are big systems.” 

Another method for power and industrial carbon capture uses membranes, which are similar to air filters. CO₂ selectively gets pulled across the membrane, separating it from the rest of the gas. One membrane design that Hornbostel has worked with are hollow-fiber membranes, which look like tiny straws packed into a bundle. The beauty of this design is that it has a huge surface area in a small volume, which means that the membrane system can be much more compact than a solvent or sorbent system.

Doing carbon capture from fossil power production and industrial processes is a great start, but it’s not going to be enough to mitigate climate change long-term. In addition to capturing CO₂ from the sources emitting it, we will have to clean up the CO₂  that’s already been emitted into the environment. This approach is called “negative emissions,” which encompasses capturing CO₂ from the air (“direct air capture”) and capturing CO₂ from the ocean (“direct ocean capture”). One of Dr. Hornbostel’s direct air capture research projects involves using metal-organic frameworks, or MOFs, that can trap CO₂ in their cores, while blocking other gasses like water. On the direct ocean capture side, Hornbostel was recently awarded research funding from the National Oceanic and Atmospheric Administration and Office of Naval Research to modify existing desalination membranes to simultaneously perform carbon capture. 

Ok, Now Where Does All That Carbon Go? 

 Regardless of the method, these are gigatons of CO₂ that must be captured each year. 

“The scale is mind-boggling about how much CO₂ we’re talking about,” Hornbostel said. 

Before anything, it’ll need to be transported with CO₂ pipelines, (that don’t exist yet). Then, it can either be used or stored. There are plenty of use strategies for CO₂- ranging from carbonated beverages to concrete- but the vast majority of the CO₂ will have to go underground due to the sheer volume being emitted. Most of the CO₂ will be placed underground, but not to worry, according to Hornbostel. As it starts to be pushed underground, it can either become more like a rock or liquid formation, depending on the surrounding environment. 

“The good news is that the research on carbon storage is mature,” said Hornbostel, “and there are plenty of underground rock and saltwater formations where we can safely store CO₂ long-term.”

Café Scientifique, presented by PPG, is a program held at the Carnegie Science Center for anyone looking to learn and discuss today’s most probing scientific questions with local experts. Hornbostel’s talk,”Carbon Capture 101,” was the latest segment featured in the series.