I enjoyed Jeff Johnson’s article on the CO2 capture trial to be run at a Wisconsin coal-fired power plant (C&EN, March 3, page 7). As I understand it, the system uses chilled ammonia to convert CO2 to ammonium bicarbonate. The aqueous ammonium bicarbonate is precipitated by concentration and transported to another facility where it is heated to release ammonium carbonate and CO2.
The fate of the evolved CO2 was not disclosed, but it seems to me to represent an inefficiency in the process. I imagine the chemistry is something like this:
CO2 + NH4OH → NH4HCO3
How about adding lime to the process:
NH4HCO3 + CaO → CaCO3 + NH4OH
No heating or cooling is required. The precipitated limestone can be used in paper coating, concrete, or several other applications, and the regenerated ammonia is recycled. No CO2 is evolved for treatment elsewhere.
This seems so simple to me that there must be something I’m missing. Any ideas?
Michael H. Auerbach