Ammonia-based post-combustion capture

Ammonia-based carbon capture is an emerging technology using a chilled ammonia process to remove CO2 from flue gas. Trialled in the power industry, its use in the cement industry is now the subject of R&D efforts.

Ammonia-based post combustion capture (PCC) used a chilled ammonia process (CAP) to separate carbon dioxide (CO2) from flue gas. The process is relatively new, only patented in 2006, and provides a number of advantages over the more mature amine-based PCC technology, including lower costs and global availability of aqueous ammonia, higher stability in the presence of flue gas impurities, such as NOx and SOx, and lower energy requirements.

The process is similar to amine-based PCC – with the addition of a cooling step for the flue gases. After the cooler, the flue gases are sent through an absorber, where the CO2 is removed by the ammonia solution. The now CO2-rich solution then moves to the desorber, where it is heated, stripping out the CO2, which is captured and processed for transported to geological storage or onward use. 

The ammonia solution can then be re-used within the system, after controlling for ammonia slip, without the need for the purging and topping-up required with amine-based solvents, which degrade over time.

The technology has been demonstrated at a pilot scale by Alstom for CO2 concentrations of between 3% and 16%, which are lower than those found in cement plant flue gas. The higher CO2 concentration of cement plant flue gas would require some changes to the process, such as higher liquid-to-gas flow rate ratio, higher ammonia concentration in the solvent, higher CO2 loading of the solvent, or a combination of these.

There is also the known possibility of clogging due to the formation of solids within the process equipment. Current CAP operations avoid this by limiting the ammonia and CO2 concentration in the absorber; however, creates challenges when it comes to system optimisation and could be a particular obstacle to implementation in the higher CO2 concentrations found in cement plant flue gases. As a result, second-generation CAP technology is designed to take advantage of and control solid formation as part of the carbon capture process.  Ammonia-based PCC of CO2 does offer some competitive advantages over conventional amine-based PCC. It is however yet to be proved at commercial scale or within the flue gas conditions found in the cement industry. It also adds to health and safety challenges, due to the need for a refrigeration unit to cool the flue gas and the risk of explosion associated with the dry CO2-ammonia reaction.