Improvement of Limestone-Based CO2 Sorbents for Ca Looping by HBr and Other Mineral Acids

The effects of mineral-acid doping on the long-term reactivity of limestone-based sorbents for CO2 capture was investigated in this work. Havelock (Canada), Longcliffe (U.K.), and Purbeck (U.K.) limestones were doped with a range of mineral acids (HCl, HBr, HI, and HNO3), and the effects of concentration were also studied. Doped samples were subjected to repeated cycles of carbonation and calcination in a fluidized-bed reactor. The experimental results showed that HBr and HCl as dopants with a 0.167 mol % doping concentration significantly improved the long-term reactivity of Havelock and Longcliffe limestones (doping with HI marginally improved the reactivity); however, doping Havelock limestone with a similar concentration of HNO3 reduced its CO2 uptake. Purbeck limestone was not significantly improved in reactivity by any dopant. Gas adsorption analyses showed that sorbents have a very small surface area: less than 4 m(2)/g. The pore size distribution appears to change significantly upon doping for those sorbents that are improved by doping, and it is likely that optimizing the pore size distribution upon cycling is one reason for the enhanced reactivity observed. The pore-size distributions of the initially calcined limestones and the changes thereof with cycling and doping explain the differences in the behaviors of the limestones.