Carbon dioxide capture from gasification syngas via cyclic carbonation/calcination

Significant research has been carried out to investigate the carbonation of CaO as a potential method for CO2 capture and sequestration. Up to date, the majority of this work has been related with CO2 removal from combustion flue gases with little attention focused on the carbonation reaction kinetics under gasification syngas conditions. The intrinsic rate constants of the CaO-CO2 reaction was studied via a grain model for two naturally occurring calcium oxide based sorbents using a thermogravimetric analyzer. An apparent kinetic model was used to cover both the chemical reaction and diffusion rate control regimes to enable the development of a single phase, plug flow, moving bed carbonator reactor model. Over temperatures ranging from 580-700°C, it was observed that the presence of CO and H2 during carbonation caused a significant increase in the initial rate of carbonation which has been attributed to the CaO surface sites catalyzing the water-gas shift reaction increasing the local CO2 concentration. The water-gas shift reaction was assumed to be responsible for the increase the activation energy from 29.7 to 60.3 kJ/mol for limestone and 17.4 to 21.6 kJ/mol for dolomite based on the formation of intermediate complexes. Structural differences between the two sorbents are believed to be accountable for the difference in activation energies. Changes in microporosity due to particle sintering during calcination have been credited with the rapid initial decrease in cyclic CaO maximum conversion for limestone particles whereas the presence of steam during carbonation has been shown to improve the long term maximum conversion in comparison to previous studies without steam present. The dolomite showed a minimal decrease in CaO maximum conversion over repeated cycles as the majority of the carbonation takes place in the larger pores/voids. The effects of major operating parameters such as inlet CaO and feed gas superficial velocities, inlet temperature, and number of calcination/carbonation cycles on the behaviour of the carbonation reaction in a moving bed reactor have been determined. It was revealed that the process is most sensitive to conditions where convective removal of the heat from carbonation is insufficient, limiting the rate of reaction.