The Effects of Repeated Cycles of Calcination and Carbonation on a Variety of Different Limestones, as Measured in a Hot Fluidized Bed of Sand

Abstract

The capacity of calcined limestone to react repeatedly with CO 2, according to CaO (cr) + CO 2(g) = CaCO 3(cr) (eq I), and also its regeneration in the reverse reaction have been studied in a small, electrically heated fluidized bed of sand, for five different limestones. The forward step of eq I is a promising way of removing CO 2 from the exhaust of, for example, a coal-fired power station, ready for sequestration or as part of a scheme to generate H 2 using an enhanced water−gas shift reaction. The reverse step regenerates the sorbent. The uptake of CO 2 by CaO, produced by calcining limestone, was measured using a bed of sand fluidized by N 2 at ∼1023 K. For each experiment, a small quantity of limestone particles was added to the hot sand, whereupon the limestone calcined to produce CaO. Calcination was completed in ∼500 s for particles of a mean diameter of ∼600 μm. Next, CO 2 was added to the fluidizing nitrogen to carbonate the CaO for ∼500 s. Measurements of [CO 2 ] in the off-gases enabled the rates of calcination and the subsequent carbonation to be measured as functions of time. Many successive cycles of calcination and carbonation were studied. The forward step of reaction I is shown to exhibit an apparent final conversion, which decreases with the number of cycles of reaction; the final conversion fits well to a correlation from the literature. The reverse (calcination) reaction always proceeded to completion. Particles of limestone, removed from the reactor after several cycles, in either their partially carbonated or fully calcined state, were studied using X-ray diffraction, gas adsorption analysis, mercury porosimetry, and scanning electron microscopy. It was found that the carrying capacity of CaO for CO 2 on the n th cycle of carbonation was roughly proportional to the voidage inside pores narrower than ∼150 nm in the calcined CaO before carbonation began. Thus, morphological changes, including reduction in the volume of pores narrower than 150 nm within a calcined limestone, were found to be responsible for much of the fall in conversion of reaction I with increasing numbers of cycles. The rate of attrition of the particles of limestone in a fluidized bed, while cycling between the calcined and carbonated states, was also studied. It was found that most limestones lost less than 10% of their mass due to attrition over the course of a typical experiment, lasting ∼8 h.