The impact of variable carbonation and decarbonation conditions on the CO2 sorption capacity of CaO-based sorbents

Abstract

The study focuses on the utilisation of natural limestones for repeatable high-temperature chemisorption of carbon dioxide from flue gas. Two methods are compared to improve the problem of the capacity decrease with repeated uses: the steam regeneration of limestone and the conversion of the same limestone to a CaO-based sorbent via an organic intermediate. Moreover, the study extends the knowledge of the sorption behaviour of the limestones by assessing the impact of the composition of the calcination and carbonation atmosphere measured independently using a self-designed laboratory apparatus with a fluidised-bed reactor. After ten cycles, performed without any regeneration procedure, the capacity achieved by the natural limestones decreased from ca 290 to 370 to only 80 and 130 mg g−1. The application of steam regeneration showed good results in capacity stabilisation. From the fifth to the tenth cycles, the limestones had an average capacity of 220 mg g−1. Steam regeneration was not, however, the only method to improve the capacity effectively. The CaO sample prepared by the chemical modification of the natural limestone was resistant to the drop in capacity and achieved the value of 210 mg g−1 after ten cycles. Since the pilot-scale units used worldwide utilise oxy-fuel combustion flue gas as the decarbonation medium, the influence of pure CO2 on sorption capacity has been investigated as well. This atmosphere supports a decrease in the capacity of limestones, which drops to 30–50 mg g−1 after ten cycles. The CaO sample exhibited higher resistance and maintained the capacity of ca 110 mg g−1. In addition, the influence of the residual concentrations of sulphur dioxide present in the simulated flue gas has been tested. It has been confirmed that SO2 seriously deteriorates sorption properties; therefore, its presence in the gas feed must be avoided.