Statistical and continuum models of fluid-solid reactions in porous media

Field emission scanning electron microscopy (FESEM), two-dimensional X-ray diffraction (2D-XRD), and transmission electron microscopy coupled with selected area electron diffraction (TEM-SAED) analyses of the reactant/product textural relationship show that the thermal decomposition of Iceland spar single crystals according to the reaction CaCO3(s) → CaO(s) + CO2(g) is pseudomorphic and topotactic. This reaction begins with the formation of a mesoporous structure made up of up to four sets of oriented rod-shaped CaO nanocrystals on each rhombohedral cleavage face of the calcite pseudomorph. The four sets formed on (1014)calcite display the following topotactic relationships: (1) (1210)calcite//(110)CaO; (2) (1104)calcite⊥ (110)CaO; (3) (1018)calcite//(110)CaO; and (4) (0114)calcite⊥(110)CaO; with [841]calcite//[110]CaO in all four cases. At this stage, the reaction mechanism is independent of P CO2 (i.e., air or high vacuum). Strain accumulation leads to the collapse of the mesoporous structure, resulting in the oriented aggregation of metastable CaO nanocrystals (~5 nm in thickness) that form crystal bundles up to ~1 μm in cross-section. Finally, sintering progresses up to the maximum T reached (1150 °C). Oriented aggregation and sintering (plus associated shrinking) reduce surface area and porosity (from 79.2 to 0.6 m2/g and from 53 to 47%, respectively) by loss of mesopores and growth of micrometer-sized pores. An isoconversional kinetic analysis of non-isothermal thermogravimetric data of the decomposition of calcite in air yields an overall effective activation energy E α = 176 ± 9 kJ/mol (for α > 0.2), a value which approaches the equilibrium enthalpy for calcite thermal decomposition (177.8 kJ/mol). The overall good kinetic fit with the F1 model (chemical reaction, first order) is in agreement with a homogeneous transformation. These analytical and kinetic results enable us to propose a novel model for the thermal decomposition of calcite that explains how decarbonation occurs at the atomic scale via a topotactic mechanism, which is independent of the experimental conditions. This new mechanistic model may help reinterpret previous results on the calcite/CaO transformation, having important geological and technological implications.

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Thermal decomposition of calcite: Mechanisms of formation and textural evolution of CaO nanocrystals

Sintering of nascent calcium oxide

The importance of calcium-based sorbents, especially natural limestones, for CO{sub 2} removal necessitates an investigation into the sotbent decay mechanism. This study starts from pore size distributions for samples from tests under various calcination/carbonation cycling conditions. A sintering model is formulated to describe the cyclic behavior of sorbents during cyclic calcination and carbonation. It explains the similar reversibility shown by sorbents under different test conditions. A balance between shorter cumulative sintering time and higher calcination rates appears to be responsible for the similar degrees of sintering and sorbent reversibility.

The effect of CaO sintering on cyclic CO2 capture in energy systems

The enormous anthropogenic emission of the greenhouse gas CO2 is most likely the main reason for climate change. Considering the continuing and indeed growing utilisation of fossil fuels for electricity generation and transportation purposes, development and implementation of processes that avoid the associated emissions of CO2 are urgently needed. CO2 capture and storage, commonly termed CCS, would be a possible mid-term solution to reduce the emissions of CO2 into the atmosphere. However, the costs associated with the currently available CO2 capture technology, that is, amine scrubbing, are prohibitively high, thus making the development of new CO2 sorbents a highly important research challenge. Indeed, CaO, readily obtained through the calcination of naturally occurring limestone, has been proposed as an alternative CO2 sorbent that could substantially reduce the costs of CO2 capture. However, one of the major drawbacks of using CaO derived from natural sources is its rapidly decreasing CO2 uptake capacity with repeated carbonation-calcination reactions. Here, we review the current understanding of fundamental aspects of the cyclic carbonation-calcination reactions of CaO such as its reversibility and kinetics. Subsequently, recent attempts to develop synthetic, CaO-based sorbents that possess high and cyclically stable CO2 uptakes are presented.

CaO-based CO2 sorbents: from fundamentals to the development of new, highly effective materials.

The rates of calcination of two types of limestones, ranging in particle size from 1 to 90 μm, were measured over the temperature range 516 to 1,000°C. A kinetic model based on the B.E.T. (Brunauer-Emmett-Teller method) surface area of the CaCO3 correlates the results over 5 orders of magnitude in reaction rate. The B.E.T. surface area of CaO formed by rapid calcination in dispersed-particle systems is 50 to 90 m2/g.

Calcination kinetics and surface area of dispersed limestone particles

The paper discusses results of measuring the rate of reaction of calcined limestone (CaCO/sub 3/) with sulfur dioxide (SO/sub 2/) and oxygen (O/sub 2/) at conditions that eliminate all resistances not associated with the lime (CaO) grain surface. Reactivity increased with the square of the Brunauer, Emmett, and Teller (B.E.T.) surface area when grain size was varied as an experimental parameter. The observed effects of surface area and temperature account for the SO/sub 2/ capture reported for boiler tests of the limestone-injection process.

R. H. Borgwardt

Papers

Calcination kinetics and surface area of dispersed limestone particles

Effect of specific surface area on the reactivity of CaO with SO2