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 CO2, according to CaO(cr) + CO2(g) = CaCO3(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 CO2 from the exhaust of, for example, a coal-fired power station, ready for sequestration or as part of a scheme to generate H2 using an enhanced water−gas shift reaction. The reverse step regenerates the sorbent. The uptake of CO2 by CaO, produced by calcining limestone, was measured using a bed of sand fluidized by N2 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, CO2 was added to the fluidizing nitrogen to carbonate the CaO for ∼500 s. Measurements of [CO2] 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 CO2 on the nth 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.

Additive effects of steam addition and HBr doping for CaO-based sorbents for CO 2 capture

“…The carrying capacity here is considered as the uptake of CO2 per mole of the original sorbent; therefore, any attrition of particles that results in losses of material from the bed will reduce the observed carrying capacity from the actual fractional conversion of the particles in the bed. Attrition has been shown to vary across different limestones and treatment methods and to be problematic in FBRs of a similar scale as the one used for this paper (Blamey et al, 2010b, Fennell et al, 2007b as well as on a pilot scale (González et al, 2010). Table 3 shows the mass loss data for all the limestones, after 13 cycles, doped and undoped, with or without steam present.…”

“…The biggest issue for kinetic studies is that the temperature swing from 900 °C to 650 °C is not instantaneous, so that a substantial proportion of the reaction occurs whilst the temperature is varying. (Donat et al, 2012) This is in contrast to experiments where the concentration of gas into the bed is varied, which can be done in seconds, (Fennell et al, 2007b) but unfortunately does not lead to a realistic decay in carrying capacities, owing to the mild calcination environment. For these reasons, reaction kinetics will only be briefly discussed here qualitatively.…”

“…There are inherent thermodynamic advantages to this process, compared to other post-combustion CCS technologies, and a number of researchers have determined that this process imposes an efficiency penalty on a power station significantly lower than that imposed by either oxyfuel combustion or MEA scrubbing, at 6-8% points, as opposed to 10 -12 points for the latter technologies. (Romeo et al, 2010, Lin et al, 2011, Daval et al, 2011, Martinez et al, 2012 One issue which has received significant attention during previous research is that the ability of CaO produced from natural limestone reduces significantly (from ~ 0.7 mol CO2/mol CaO) when it is first used to capture CO2 to a significantly lower level (~0.1 mol CO2/mol CaO) after 30 cycles of calcination and carbonation (Fennell et al, 2007a, Blamey et al, 2010a) (hereafter, the number of moles of CO2 reacted per mole of CaO will be referred to as the carrying capacity of the limestone) .…”

Estudo do aproveitamento de resíduo da mineração rico em calcário para captura de dióxido de carbono

“…Uma forma atrativa de realizar esta separação é utilizando o óxido de cálcio, através da realização de ciclos consecutivos das reações carbonatação-calcinação (Nikulshina et al, 2009), sendo a primeira a reação de formação do carbonato de cálcio a partir da reação do óxido de cálcio com o CO 2 e a segunda a reação reversa (Abanades, 2002;Al-Jeboori et al, 2012;Wang et al, 2007). A vantagem deste processo é a utilização de uma matéria-prima abundante e barata (Abanades et al, 2004), já a desvantagem é o fato da capacidade de captura diminuir aolongo dos ciclos de reações (Rhida et al, 2012;Fennell et al, 2007), devido principalmente à sinterização decorrente da etapa de calcinação. A fim de amenizar este problema, pode-se incluir uma etapa intermediária de hidratação do CaO (Phalak et al, 2012), a qual contribui para aumentar o volume dos poros.…”

Carbon capture in molten salts

“…High-temperature post-combustion removal (HT-CCS) by carbonate looping has been identified by The European Technology Platform for Zero-Emission Power Generation (ZEP) as one of the most promising methods in the developing stage [2][3][4][5][6][7][8][9][10][11][12][13][14][15]. The concept relies on absorption of CO 2 by CaO with formation of CaCO 3 in the solid state.…”