13th August 2014

CO2 "sponge" could soak up pollution

A new polymer that could help to absorb man-made emissions from power plants has been announced by the American Chemical Society.

A sponge-like plastic that soaks up the greenhouse gas carbon dioxide (CO2) might ease our transition away from polluting fossil fuels and toward new energy sources, such as hydrogen. The material — a relative of the plastics used in food containers — could play a role in the U.S. government's plan to cut CO2 emissions 30 percent by 2030, and could also be integrated into power plant smokestacks in the future. A report on the new material is one of nearly 12,000 presentations at the 248th National Meeting & Exposition of the American Chemical Society (ACS), the world’s largest scientific society, taking place in San Francisco this week.

“The key point is that this polymer is stable, it’s cheap, and it adsorbs CO2 extremely well,” says Andrew Cooper, Ph.D. “It’s geared toward function in a real-world environment. In a future landscape where fuel-cell technology is used, this adsorbent could work toward zero-emission technology.”

Adsorbents are most commonly used to remove greenhouse gas pollutants from smokestacks at power plants where fossil fuels are burned. However, Cooper and his team intend this adsorbent — a microporous organic polymer — for a different application. The new material would become part of an emerging technology called integrated gasification combined cycle (IGCC), which can convert fossil fuel into hydrogen gas. Hydrogen holds great promise for use in fuel-cell cars and electricity generation, because it produces almost no pollution. IGCC is a bridging technology that is intended to jump-start the hydrogen economy, or the transition to hydrogen fuel, while still using the existing fossil-fuel infrastructure. But the IGCC process yields a mixture of hydrogen and CO2 gas, which must be separated.

Cooper, who is from the University of Liverpool, claims that the sponge works best under the high pressures intrinsic to the IGCC process. Just like a kitchen sponge swells when it takes on water, the adsorbent swells slightly when it soaks up CO2 in the tiny spaces between its molecules. When the pressure drops, the adsorbent deflates and releases the CO2, which can then be collected for storage or conversion into useful compounds.

The material — a brown, sand-like powder — is made by linking together many small carbon-based molecules into a network. Cooper explains that the idea to use this structure was inspired by polystyrene, a plastic used in styrofoam and other packaging material. Polystyrene can adsorb small amounts of CO2 by the same swelling action.

One advantage of using polymers is that they tend to be very stable. The material can even withstand being boiled in acid, proving it should tolerate the harsh conditions in power plants where CO2 adsorbents are needed. Other CO2 scrubbers — whether created from plastics or metals or in liquid form — do not always hold up well, he says. Another benefit of this new adsorbent is its ability to adsorb CO2 without also taking on water vapour, which can clog up other materials and make them less effective. Its low cost, reusability, and long lifetime also makes the sponge polymer attractive. In his report, Cooper also describes how it is relatively simple to embed the spongy polymers in the kinds of membranes already being evaluated to remove CO2 from power plant exhaust. Combining two types of scrubbers could make even better adsorbents, by harnessing the strengths of each.

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