Development of the Al₂O₃-supported NaNO₃-Na₂Mg(CO₃)₂ sorbent for CO₂ capture with facilitated sorption kinetics at intermediate temperatures

Abstract

For the development of a dry solid sorbent having quite fast CO₂ sorption kinetics in an intermediate temperature range of 245–300 °C to be applicable to a riser-type fluidized bed carbonator, samples of Al₂O₃-supported MgCO₃ (1.2 mmol/g) promoted with different molar amounts of Na₂CO₃ (1.2, 1.8 mmol/g) and/or NaNO₃ (0.6 mmol/g) were prepared by incipient wetness pore volume impregnation. For a reference, an unsupported bulk phase sorbent of NaNO₃-Na₂Mg(CO₃)₂ was also prepared. From the sorption reaction using a gas mixture containing CO₂ by 2.5–10% at 1 bar for the sorbents after their activation to MgO, Al₂O₃-supported sorbents were featured by their rapid carbonation kinetics in contrast to the unsupported sorbent showing a quite slow carbonation behavior. The addition of Na₂CO₃ to the MgCO₃/Al₂O₃ sorbent made MgO species more reactive for the carbonation, bringing about a markedly enhanced kinetic rate and conversion, as compared with the unpromoted MgCO₃/Al₂O₃ sorbent having a small negligible reactivity. The addition of NaNO₃ to MgCO₃/Al₂O₃ or to Na₂CO₃-MgCO₃/Al₂O₃ induced the same promotional effects, but to a lesser magnitude, as observed for the Na₂CO₃ addition. It was also characteristic for all these MgCO₃-based sorbents that initial carbonation conversions with time appeared as sigmoid curves. For the Al₂O₃-supported sorbent comprised of NaNO₃, Na₂CO₃, and MgCO₃ by 0.6, 1.8, and 1.2 mmols, respectively, per gram sorbent, showing the best kinetic performance, a kinetic equation capable of reflecting such sigmoid conversion behavior was established, and its applicability to a riser carbonator was examined throughout a simple model calculation based on the kinetics obtained.