Carbon dioxide transfer capacities of aqueous ammonia solution and monoethanolamine (MEA) solution were compared. The ammonia process for CO2 capture was simulated using a semibatch reactor, where the flow of gas is continuous. The CO2 carrying capacity in g CO2 per g of NH3 solution (8 wt.%) circulated is 0.07 as compared with 0.036 g CO2 per g MEA solution (20 wt.%). The energy requirement for liquid mass circulation of ammonia solution is approximately 50% of MEA solution for equal weight of CO2 carried. In another comparison, the thermal energy required to regenerate CO2 from the rich solution is substantially less as compared to the MEA process. A 3-cycle absorption–regeneration test was conducted in the semibatch reactor to simulate an approach-to-steady state in a flow system. pH values of the absorbent solution was found to oscillate between 9.6 (CO2-lean) and 8.8 (CO2-rich) under the test conditions. Thermodynamics study shows that ammonium bicarbonate required the least thermal energy among the ammonium compounds for CO2 regeneration.