Carbon capture at the source

Scientists from Rice University have created a simple and inexpensive way to separate carbon dioxide from natural gas at wellheads. Their research has been published in the journal Nature Communications.

Natural gas is considered the cleanest and most abundant fossil fuel, but when it comes out of the ground, this gas is contaminated by 10-20% (sometimes even as high as 70%) CO₂. As noted by Professor James Tour, CO₂ is currently separated from natural gas through aqueous amine towers - a process which is energy, heat and space intensive - and once the CO₂ has been removed, it is typically vented into the air.

“Amines are corrosive and hard on equipment,” Professor Tour added. “They do capture CO₂, but they need to be heated to about 140°C to release it for permanent storage. That’s a terrible waste of energy.”

Researchers in the Tour lab, led by graduate student Chih-Chau Hwang, invented a nanoporous material of carbon with nitrogen or sulfur, made from inexpensive polymers, to perform this process in a more environmentally friendly way. Tour explained that the polymers are heated to 600°C, with a base like potassium hydroxide, and this creates black powder.

“This black powder will trap the CO₂ as it comes out with the natural gas, and … we can do this at the wellhead,” Professor Tour said.

Graduate student Chih-Chau Hwang and Professor James Tour show a vial of material that has the ability to capture carbon from gas flowing out of a well and hold it until it can be put back underground or otherwise used. Credit: Jeff Fitlow/Rice University.

The powder, which has a massive surface area, pulls CO₂ molecules from flowing natural gas and polymerises them while under pressure naturally provided by the well. When the pressure is released, the CO₂ spontaneously depolymerises and frees the sorbent material to collect more. All of this occurs at ambient temperatures.

“Nobody’s ever seen a mechanism like this,” Tour said. “You’ve got to have that nucleophile (the sulfur or nitrogen atoms) to start the polymerisation reaction. This would never work on simple activated carbon; the key is that the polymer forms and provides continuous selectivity for CO₂.”

The sulfur-infused powder was found to absorb 82% of its weight in CO₂, and the nitrogen-infused powder was not far behind. In comparison, the aqueous amine tower only traps about 10% of CO₂ by weight.

Particles of nitrogen-containing porous carbon are able to capture carbon dioxide from natural gas under pressure at a wellhead by polymerising it. When the pressure is released, the carbon dioxide returns to gaseous form. Credit: Tour Group/Rice University.

Additionally, said Professor Tour, “Our technique allows one to specifically remove CO₂ at the source. It doesn’t have to be transported to a collection station to do the separation. This will be especially effective offshore, where the footprint of traditional methods that involve scrubbing towers or membranes are too cumbersome.

“This will enable companies to pump carbon dioxide directly back downhole, where it’s been for millions of years, or use it for enhanced oil recovery to further the release of oil and natural gas. Or they can package and sell it for other industrial applications.”

The technology has been licensed by Apache, a Houston-based oil and gas exploration and production company, which also funded the research.

Source