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CO2 capture with solid sorbents: materials characterization and reaction kinetics
Full text linkThe enormous anthropogenic emission of carbon dioxide is most likely one of the main reasons for the global warming and the climate change problems ([1], [2], [3]). Considering the continuing and progressively growing utilization of fossil fuels, mainly in the power generation sector where fossil fuel-based combustion and gasification power plants are predominant, the development and implementation of processes that avoid the associated CO2 emissions must be urgently identified. Carbon dioxide capture and storage, commonly termed CCS, represents a range of technologies oriented to affordably and efficiently sequester carbon dioxide from these sources and would be a possible mid-term solution to mitigate the emissions of CO2 into the atmosphere ([4]). However, the costs (especially in terms of penalties in the power plants efficiency) associated with the current industrially available CO2 capture techniques, such as amine-based scrubbing, are prohibitively high, thus making the development of new CO2 sorbents an highly important research challenge.
Among several strategies currently under investigation, calcium oxide (CaO), readily obtained through a calcination stage of naturally occurring calcium carbonate (CaCO3), has been proposed as an alternative CO2 solid sorbent that could significantly reduce the costs of carbon dioxide capture systems. The technique, widely discussed in the literature and recently reviewed by several authors ([5], [6], [7] and [8]), is based on the reversible reaction CaO (s) + CO2 (g) ↔ CaCO3 (s) and is applied through cyclic stages of carbonation and of calcination, offering a number of advantages. However, a few issues, including especially the decline of sorbent capacity when they are cycled through multiple CO2 capture-and release stages, still call into question its widespread deployment on industrial applications. The improvement of this technology and the development of new calcium-based solid sorbents are currently a matter of study and, despite the apparent simplicity of the chemistry involved, several aspects of the carbonation reaction and its kinetics are still not clearly understood.
The determination of the surface reaction kinetic parameters is one of the open disputes. Several contributions investigating the carbonation reaction and its kinetics have reported thus far activation energies varying in a range of about 20 ÷ 30 kJ/mol ([9], [10]) and 70 ÷ 80 kJ/mol ([11], [12], [13], [14]); a few authors otherwise asserted that the carbonation reaction has a zero-activation energy ([15], [16]). These values were estimated from CaO conversion versus time profiles obtained from CO2 absorption analysis carried out in a wide range of operating conditions in terms of carbonation temperatures and CO2 partial pressures and hence, the observed uncertainty in the mentioned activation energies is reasonably related to the quality of the experimental data. The accuracy of the experimental data is a questionable matter especially when the data are obtained through the thermo-gravimetric approach because, as well known, TGA experiments are typically affected by mass transfer limitations. The external diffusion is particularly important because it weighs on the gas (CO2) diffusion towards the solid sorbent (CaO) surface that is essential to support the carbon dioxide mole consumption due to the chemical reaction. Even though several strategies can be applied to reduce the external mass diffusion during the CaO carbonation studied in a TGA system (typically increasing the gas flow rates), evidences of a complete removal of such resistance cannot be easily provided. In fact, the typical circumstance that at high gas flow rates the conversion versus time curves can show no changes when increasing the gas flow rate does not imply that the external mass diffusion resistance is eliminated, but only that such resistance cannot be further reduced in the TGA geometry and operating conditions used. Indeed, the local velocities reached around/inside a TGA crucible (especially above the sorbent particles contained in a common sample holder) could be low even when the average velocity in the furnace is increased by increasing the gas flow rate, so that the local velocities around/inside a TGA crucible cannot be increased enough to compensate the very high consumption rate of CO2 due to the fast carbonation surface reaction. Therefore, alternative method has to be studied in order to measure CaO conversion versus time profile actually not limited by the external mass diffusion, and to check the validity of the thermo-gravimetric data currently available.
A second aspect concerns the structural properties characterizing the solid sorbent particles and how such properties can affect the CO2 absorption performances of CaO. Since, CaO-CO2 is a typical gas-solid reaction, it is most likely that specific surface and pore volume distribution can affect the reaction kinetics of CaO sorbent particles, as well as their absorption capacity. Several studies have been carried out to comprehend the carbonation reaction and kinetics in terms of these structural properties (porosity, specific surface, structural parameter, or the whole pore size distribution) through the development and application of random pore/grain models ([15], [17], [10], [13], [18], [19]). Most of these contributions related the transition from the fast regime to the slow product-layer diffusion controlled regime, characterizing the CaO carbonation, to the filling of small pores and/or to the development of a critical carbonate layer, and focused the attention on the impact of the pore size distribution on the critical CaCO3 product layer thickness, for which an unambiguous value or a direct measure has not been anyway proposed. Additionally, even though CaO and CaCO3 are crystalline species and their crystalline structures could reasonably affect both the carbonation reaction kinetics and mechanism, very few contributions have been focused thus far to study their impact on the carbonation reaction, insomuch as the influence of CaO/CaCO3 crystalline domain sizes on the carbonation reaction with CaO-based solid sorbents has never been investigated.
The research project summarized in this work of thesis has been focused on the investigation of the CaO carbonation reaction with the goal of clarifying these unresolved aspects.
Sorbent samples were first characterized by thermo-gravimetric analysis (TGA). CaO particles, directly produced in the TGA apparatus through stages of thermal decomposition in N2 atmosphere (temperature range from 650°C and 900°C), were tested to investigate their reactivity in the CO2 capture process, aiming at identifying the absorption specific rates, and confirming as common TGA analysis are reasonably affected by physical limitations, mainly mass transfer resistances. The TGA unit was fed with gas consisting of pure carbon dioxide or of a N2/CO2 mixture so that different CO2 partial pressures were used within a range of 0.05 and 1 bar while carbonation temperatures were varied from 450°C up to 650°C.
Some CaO sorbent samples were also preliminary prepared through a stage of calcination realized in a separate muffle furnace. Different operating conditions in terms of calcination temperatures (especially 900°C) and residence times at high temperature (from few minutes up to some hours) were used in order to produce CaO sorbent samples with different structural properties, mainly in terms of porosity and specific surface area. In fact, these factors, which are closely related to the sorbent modifications due to high temperature treatments, reasonably affect the carbonation reaction. Specific surface area measurements by N2 adsorption were performed to complete the characterization of the samples by means of BET analysis. The samples were afterwards tested during CO2 absorption processes carried out in the TGA unit under a gas flow of pure carbon dioxide (total pressure of 1 bar). Based on CaO conversions and the corresponding reaction rates measured, a simple reaction mechanism was applied to determine the kinetic parameters. An activation energy of about 45 kJ/mol was estimated, but it was reasonably associated to apparent kinetic rates. Moreover, the relationship between variation of the specific surface and porosity due to sintering and their effect on the carbonation reaction were not clearly quantify because of the uncertainty of the experimental data obtained, caused by the mass-transfer limitations that affected the TGA experiments.
The X-ray powder diffraction technique was therefore applied since it can provide an alternative method to the thermo-gravimetric analysis for studying the CaO-CO2 reaction. X-ray diffraction experiments were carried out (in collaboration with the Department of Geosciences at the University of Padova) to determine the structural changes of the sorbent samples (namely phase evolution and crystallite size modifications) as a function of temperature and CO2 partial pressure. Several tests were performed using a high temperature reaction chamber, with a controlled gas inlet composition, both during the thermal decomposition (calcination/regeneration) and during the absorption processes. Calcination experiments, carried out in a N2 atmosphere (total pressure = 1 bar) and a temperature range varying between 650 and 950°C, allowed to observe that, after the complete decomposition of calcium carbonate precursor, the average crystallite size of CaO domains formed (approximately of 40 nm) considerably changes, when kept for long residence times at high temperatures. We also verified that even a low concentration of CO2 in the calcination atmosphere promotes CaO crystal size growth during the CaCO3 thermal decomposition and significantly increases the size of the nascent CaO crystalline domains. After the preparation stage of thermal decomposition, carbonation experiments using fresh calcines directly produced within the reaction chamber were performed. It was observed that differences in the crystallite size of the CaO samples apparently influence the solid sorbent reactivity in the following CO2 capture process. At the same carbonation isotherm (temperatures applied were in the range of 400-650°C), with a CO2 partial pressure of 1 bar, samples with a larger CaO crystal size (at the beginning of carbonation) showed a lower overall carbon dioxide absorption capacity, suggesting that the carbonation reaction (kinetics) could be affected by initial CaO sorbent particle crystallite size. Unfortunately, the low time resolution provided by the available standard laboratory instrumentation was not sufficient to obtain detailed information about the transformations occurring in the sample particles, especially during the initial very fast stage of the carbonation reaction, whereas the surface chemical reaction should reasonably occur with negligible effects of the product layer diffusion.
Therefore, in-situ synchrotron radiation X-ray powder diffraction (SR-XRPD), performed at the Advanced Photon Source (APS) facilities of the Argonne National Laboratory, was finally applied to investigate the CaO carbonation reaction more in detail. A set of CO2 absorption experiments were conducted in a high temperature reaction capillary with a controlled atmosphere (CO2 partial pressure of 1 bar), in the temperature range between 450°C and 750°C using CaO based sorbents obtained by calcination of commercial calcium carbonate. The evolution of the crystalline phases during CO2 uptake by the CaO solid sorbents was monitored for a carbonation time of 20 min as a function of the carbonation temperature and of the calcination conditions. The Rietveld refinement method was applied to estimate the calcium oxide conversion during the reaction progress and the average size of the initial (at the beginning of carbonation) calcium oxide crystallites. The measured average initial carbonation rate (in terms of conversion time derivative) of 0.280 s-1 (± 13.2% standard deviation) is significantly higher than the values obtained by thermo-gravimetric analysis and reported thus far in the scientific literature. Additionally, a dependence of the conversion versus time curves on the initial calcium oxide crystallite size was observed and a linear relationship between the initial CaO crystallite size and the calcium oxide final conversion was identified. The evolution of the CaCO3 crystalline phase during the CaO carbonation was also investigated by means of the same technique. Maximum sizes of the calcium carbonate crystalline domains were observed in the CaCO3 crystallite size versus time curves, (specifically during the first rapid stage of the carbonation) and were identified as the average values of the critical CaCO3 product layer thickness. A relationship between this parameter and the corresponding calcium oxide conversion (at which the transition to the second slow reaction stage occurs), as well as a dependence of the carbonate product layer thickness with the initial CaO particle porosity, were found. Finally, CaCO3 critical product layer thicknesses were used to estimate the initial specific surface areas of the CaO sorbent particles afterwards utilized to calculate the kinetic parameters of the intrinsic surface carbonation reaction. A reaction rate constant of 1.89 × 10-3 mol/m2 s, with zero-activation energy, has been obtained
CaCO3 Crystallite Evolution during CaO Carbonation: Critical Crystallite Size and Rate Constant Measurement by In-Situ Synchrotron Radiation X-ray Powder Diffraction
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