N-doped biochar from chicken poo captures CO2

ENGINEERS in South Korea have found that nitrogen-doped biochar beads made from chicken manure could be an effective, sustainable way to capture CO₂ from waste gas streams.

The most common capture method for CO₂ uses amine liquids, but these have a high regeneration cost and can react with other gases in flue gas. Other solid adsorption methods require high pressures or temperatures or are expensive. Minh-Viet Nguyen and Byeong-Kyu Lee at the University of Ulsan investigated the use of biochar, which has a very high internal surface area, but usually will only achieve good adsorption capacities at a high CO₂ partial pressure. The researchers used nitrogen-doping, also known as amine modification, to improve the adsorption capacity. NH₃ groups have been shown to interact with the CO₂, increasing selectivity towards it.

Nguyen and Lee used chicken manure, a cheap and commonly-available waste product, as a source of carbon. They first dried the manure then pyrolysed it at 450?C for one hour to produce biochar.  The biochar was mixed with a solution of HNO₃, then NaOH, before being placed in a reactor at 450?C with ammonia for one hour, producing amine-modified biochar (AMBC). The researchers then mixed this with a natural polymer called α-L-gulopyranuronate, and dropped the mixture into a calcium chloride solution to produce the beads.

The researchers tested the beads using a lab-scale fixed bed column at ambient pressure. The AMBC beads adsorbed 10.15 mmol/g of CO₂ at 20?C, far higher than the beads made from undoped biochar, which adsorbed 3.72 mmol/g, and higher than conventional activated carbon, which adsorbed 8.91 mmol/g. Even after ten adsorption and desorption cycles, the AMBC beads retained 88% of their adsorptive capacity, compared to just 62% for the activated carbon beads. Tests with a mixture of CO₂ and nitrogen showed that AMBC beads showed a high selectivity towards CO₂ of 79.1%. The researchers point out that in use, the bead format allows easy recovery.

“Based on these findings, the AMBC beads can be used as an alternative adsorbent for CO₂ removal in ambient conditions,” the researchers say.

Process Safety and Environmental Protection DOI: 10.1016/j.psep.2016.04.007