**Gas Capture Processes**

Special Issue Editors

**Tohid N. Borhani Zhien Zhang Muftah H. El-Naas Salman Masoudi Soltani Yunfei Yan**

MDPI • Basel • Beijing • Wuhan • Barcelona • Belgrade • Manchester • Tokyo • Cluj • Tianjin


Muftah H. El-Naas Qatar University Qatar

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This is a reprint of articles from the Special Issue published online in the open access journal *Processes* (ISSN 2227-9717) (available at: https://www.mdpi.com/journal/processes/special issues/ gas capture).

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## **Contents**




## **About the Special Issue Editors**

**Tohid N. Borhani** is currently an Assistant Professor at Heriot-Watt University teaching different subjects in chemical engineering. Before that, he was a Research Fellow at Cranfield University, Research Associate at The University of Sheffield, Research Associate at Imperial College London, and Postdoctoral Researcher at UTM. His main fields of study include separation process, process modelling and simulations, and chemometric and quantitative structure–property relationships (QSPRs). He has published several papers and book chapters in high level journals and publishers. He is an Associate Fellow of Higher Education Academy, U.K., and an Academic Member of the U.K. Carbon Capture and Storage Research Centre (UKCCSRC).

**Zhien Zhang** is currently a Research Fellow in the William G. Lowrie Department of Chemical and Biomolecular Engineering at Ohio State University, USA. His research interests include advanced processes and materials for gas capture, gas separation, and carbon capture, utilization and storage (CCUS). Dr. Zhang has published more than 80 peer-reviewed journal papers and 16 editorials in high impact journals such as Renewable & Sustainable Energy Reviews and Applied Energy. He is the Editorial Supervisor of the Journal of Natural Gas Science and Engineering and Editor of some international journals, e.g., Membranes, Environmental Chemistry Letters, etc.

**Muftah El-Naas** is the Director of the Gas Processing Center at Qatar University, where he also served as the QAFCO Industrial Chair Professor. His areas of expertise include CO<sup>2</sup> capture and sequestration, water treatment and purification, membrane separation, and plasma technology. Most of his recent research focuses on the development of new and environmental-friendly technologies for the oil and gas industry. Dr. El-Naas has authored more than 170 papers in international journals and conferences, in addition to several book chapters and patent applications. He has recently developed and patented a new process and a reactor system for the management of CO<sup>2</sup> emissions and desalination reject brine.

**Salman Masoudi Soltani** (Ph.D) is a lecturer in chemical engineering at Brunel University London, U.K., and a founding member of the Chemical Engineering Department at the university. His research has mainly centred on separation processes. Prior to this position, he worked as a postdoctoral research associate with the Clean Fossil & Bioenergy Research Group at Imperial College London, U.K., and as a postdoctoral Knowledge Transfer Partnership (KTP) research associate (industrial postdoc) at the University of Nottingham, UK. He is a Chartered Engineer (CEng/MIChemE), a Fellow of Higher Education Academy, U.K., and an Academic Member of the U.K. Carbon Capture and Storage Research Centre (UKCCSRC).

**Yunfei Yan** is a professor in the college of energy and power engineering at Chongqing University, and a professor in the Key Laboratory of Low Grade Energy Utilization Technology and System, Ministry of Education (Chongqing University) and Evaluation Center of Industrial Energy Conservation and Green Development, Ministry of Industry and Information Technology, China. Dr. Yan worked as a visiting scholar at Boston University, USA, from 2010 to 2011. His areas of expertise include catalytic combustion, micro energy and power system, heat and mass transfer, gas separation and carbon capture, multiphase flow and environmental protection, new energy, and renewable energy use and conversion. Dr. Yan has authored more than 60 papers in international journals and conferences, in addition to several patent applications.

### *Editorial* **Gas Capture Processes**

#### **Zhien Zhang 1,\* , Tohid N. Borhani 2, Muftah H. El-Naas 3, Salman Masoudi Soltani <sup>4</sup> and Yunfei Yan <sup>5</sup>**


Received: 31 December 2019; Accepted: 2 January 2020; Published: 4 January 2020

**Abstract:** The increasing trends in gas emissions have had direct adverse impacts on human health and ecological habitats in the world. A variety of technologies have been deployed to mitigate the release of such gases, including CO2, CO, SO2, H2S, NOx and H2. This special issue on gas-capture processes collects 25 review and research papers on the applications of novel techniques, processes, and theories in gas capture and removal.

**Keywords:** global warming; gas emission; capture; CO2

#### **1. Introduction**

The increasing trends in environmental gas emissions have had direct adverse impacts on human health and ecological habitats. Various technologies have been deployed to mitigate the release of such gases, including CO2, CO, SO2, H2S, NOx and H2 [1–3]. Nevertheless, many of these technologies have demonstrated poor performance or are yet to become economically feasible for commercial deployment. Therefore, devising efficient capture processes and associated technologies has become significantly more urgent in the past few years.

In the current special issue of *Processes*, 25 review and research papers on the applications of novel techniques, processes, and theories in gas capture and removal are presented. This special issue is available online at: https://www.mdpi.com/journal/processes/special\_issues/gas\_capture. A concise summary of the presented works in this special issue is outlined hereafter.

#### **2. Overview of Papers in This Special Issue**

In the work of Mendoza et al. [4], iron ore was studied as a CO2 absorbent. Carbonation was carried out via mechanochemical and high temperature–high pressure (HTHP) reactions. Kinetics of the carbonation reactions were studied for the two methods. In the mechanochemical process, the kinetics were analyzed as a function of the CO2 pressure and the rotation speed of the planetary ball mill, while in the HTHP process, the kinetics were studied as a function of pressure and temperature. The highest CO2-capture capacities achieved were 3.7341 mmol of CO2/g of sorbent in ball-milling (30 bar of CO2 pressure, 400 rpm, and 20 h) and 5.4392 mmol of CO2/g of absorbent in HTHP (50 bar of CO2 pressure, 100 ◦C, and 4 h). To overcome the kinetic limitations, water was introduced to all carbonation experiments. The calcination reactions were carried out in an argon atmosphere using

thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) analysis. Siderite can be decomposed in the same temperature range (100 ◦C to 420 ◦C) as the samples produced by both methods. This range reached higher temperatures compared with pure iron oxides due to the decomposition temperature increase with decreasing purity. Calcination reactions yield magnetite and carbon. A comparison of recyclability (use of the same material in several cycles of carbonation–calcination), kinetics, spent energy, and the amounts of initial material needed to capture 1 ton of CO2 revealed the advantages of the mechanochemical process compared with HTHP.

Osagie et al. [5] conducted steady-state simulation and exergy analysis of a 2-amino-2-methyl-1 -propanol (AMP)-based post-combustion capture (PCC) plant. Exergy analysis was able to identify the location, sources of thermodynamic inefficiencies, and magnitude in the thermal system. Furthermore, thermodynamic analysis of different configurations of the process helped to identify opportunities to reduce the steam requirements for each of the configurations. Exergy analysis performed for the AMP-based plant and the different configurations revealed that a rich split with intercooling configuration gave the highest exergy efficiency of 73.6%, while those of the intercooling and the reference AMP-based plant were 57.3% and 55.8%, respectively. Thus, exergy analysis of flowsheeting configurations could lead to significant improvements in plant performance and to cost reduction for amine-based CO2-capture technologies.

Mahmud et al. [6] investigated the reaction kinetics of carbon dioxide with blends of N-methyldiethanolamine and L-arginine using the stopped-flow technique. The experiments were performed over a temperature range of 293 to 313 K, and at solution concentrations up to 1 mol/L of different amino acid/amine ratios. The overall reaction rate constant (kov) was found to increase with increasing temperature and amine concentration, as well as with increased proportion of L-arginine concentration in the mixture. The experimental data were fitted to the zwitterion and termolecular mechanisms using a nonlinear regression technique with average absolute deviations (AAD) of 7.6% and 8.0%, respectively. A comparative study of the promoting effect of L-arginine with that of the effect of glycine and diethanolamine (DEA) in N-methyldiethanolamine (MDEA) blends showed that the MDEA–arginine blend exhibited faster reaction rate with CO2 with respect to the MDEA–DEA blend, while the case was reversed when compared to the MDEA–glycine blend.

In the study of Zhang et al. [7], four kinds of pore-forming material were screened and utilized to prepare sorbent pellets via the extrusion–spheronization process. In addition, the impacts of additional content of pore-forming material and their particle sizes were also investigated comprehensively. It was found that the addition of 5 wt. % polyethylene resulted in the highest CO2-capture capacity (0.155 g CO2/g sorbent in the 25th cycle) and a mechanical performance of 4.0 N after high-temperature calcination, results approximately 14% higher and 25% improved, respectively, compared to pure calcium hydrate pellets. Smaller particle sizes of the pore-forming material were observed to lead to a better performance in CO2 sorption, while for mechanical performance, there was an optimal size for the pore-former used.

Gutierrez et al. [8] reported the conceptual design of an amine-based carbon dioxide (CO2) separation process for enhanced oil recovery (EOR). A systematic approach was applied to predict the economic profitability of the system while reducing the environmental impacts. Firstly, they modeled the process with UniSim and determined the governing degrees of freedom (DoF) through a sensitivity analysis. They then proceeded with the formulation of the economic problem, where the employment of econometric models allowed them to predict the highest dynamic economic potential (DEP). In the second part of the study, the waste reduction (WAR) algorithm was applied to quantify the environmental risks of the studied process. This method was based on the minimization of the potential environmental indicator (PEI) by using the generalization of the waste reduction algorithm. Results showed that the CO2 separation plant was promising in terms of economic revenue. However, the PEI value indicated that the higher the profitability, the larger the environmental risk. The optimal value of the DEP corresponded to 0.0274 kmol/h and 60 ◦C, with a plant capacity according to the molar flow

rate of the produced acid gas. In addition, the highest environmental risk was observed at the upper bounds of the DoF.

Baena-Moreno et al. [9] presented a method for regeneration of a sodium hydroxide (NaOH) solution as a valuable byproduct from a biogas-upgrading unit through calcium carbonate (CaCO3) precipitation, as an alternative to the elevated energy consumption required by the physical regeneration process. The purpose of this work was to study the main parameters that might affect NaOH regeneration using an aqueous sodium carbonate (Na2CO3) solution and calcium hydroxide (Ca(OH)2) as a reactive agent for regeneration and carbonate slurry production, in order to outperform the regeneration efficiencies reported in earlier works. Moreover, Raman spectroscopy and scanning electron microscopy (SEM) were employed to characterize the solid obtained. The studied parameters were reaction time, reaction temperature, and molar ratio between Ca(OH)2 and Na2CO3. In addition, the influence of small quantities of NaOH at the beginning of the precipitation process was studied. The results indicated that regeneration efficiencies between 53% and 97% could be obtained by varying the main parameters mentioned above, and both Raman spectroscopy and SEM images revealed the formation of a carbonate phase in the obtained solid. These results confirmed the technical feasibility of this biogas-upgrading process through CaCO3 production.

Liu et al. [10] studied the effect of physical and mechanical activation on the physicochemical structure of coal-based activated carbons (ACs). In the stage of CO2 activation, a rapid decrease of the defective structure and the growth of aromatic layers accompanied by the dehydrogenation of aromatic rings resulted in the ordered conversion of the microstructure and severe carbon losses on the surfaces of Char-PA, while the oxygen content of Char-PA, including C=O (39.6%), C–O (27.3%), O–C=O (18.4%), and chemisorbed O (or H2O) (14.7%), increased to 4.03%. Char-PA presented a relatively low SBET value (414.78 m2/g) owing to the high value of non-*V*mic (58.33%). In the subsequent mechanical activation from 12 to 48 h under N2 and dry ice, the strong mechanical collision caused by ball-milling destroyed the closely arranged crystalline layers and caused the collapse of mesopores and macropores, resulting in disordered conversion of the microstructure and the formation of a defective structure; a sustained increase in the SBET value from 715.89 to 1259.74 m2/g was found with prolonged ball-milling time. There was a gradual increase in the oxygen content from 6.79% to 9.48% for Char-PA–CO2-12/48 obtained by ball-milling under CO2. Remarkably, the variations of physicochemical parameters of Char-PA-CO2-12/48 were more obvious than those of Char-PA-N2-12/48 under the same ball-milling time, which was related to the stronger solid–gas reactions caused by the mechanical collision under dry ice. Finally, the results of the SO2 adsorption test of typical samples indicated that Char-PA–CO2-48 with a desirable physicochemical structure can maintain 100% efficiency within 30 min and its SO2 adsorption capacity could reach 138.5 mg/g at the end of the experiment. After the 10th cycle of thermal regeneration, Char-PA–CO2-48 still had a strong adsorptive capacity (81.2 mg/g).

Liu et al. [11] studied the catalytic effect of NaCl (1 and 3 wt. %) in the presence of oxygen functional groups on the improvement of the physicochemical structure of coal-based activated carbons. A large quantity of Na can be retained in 1NaJXO and 3NaJXO with the presence of oxygen functional groups to promote further its catalytic characteristics during pyrolysis, resulting in disordered transformation of the carbon structure. In addition, the development of micropores was mainly affected by the distribution and movement of Na catalyst, whereas the growth of mesopores was mainly influenced by the evolution of oxygen functional groups. The active sites of 3NaJXO-800 were no longer preferentially consumed in the presence of Na catalyst during subsequent CO2 activation to facilitate the sustained disordered conversion of the microstructure and the rapid development of the micropores, resulting in an obvious high SBET value as activation proceeded. A high SBET/burn-off ratio value (41.48 m2/g/%) of 3NaJXO-800 with a high value of SBET (1995.35 m2/g) at a low burn-off value (48.1%) was obtained, associated with a high efficiency of pore formation. Finally, the SO2 adsorption efficiency of 3NaJXO-800-48.1 was maintained at 100% for 90 min. After 180 min, 3NaJXO-800-48.1 still presented a high adsorptive capacity (140.2 mg/g). It was observed that a large micropore volume in the case of hierarchical pore structure necessarily ensured optimal adsorption of SO2.

Karamian et al. [12] investigated the effect of different nanofluids, such as water/Al2O3, water/Fe2O3, or water/SiO2, on absorption rate. The results showed that the absorption of CO2 and SO2 in nanofluids significantly increased by up to 77% in comparison with the base fluid. It was also observed that the type of gas molecules and nanoparticles determined the mechanism of mass transfer enhancement by nanofluids. Additionally, the results indicated that the values of mass transfer coefficient of SO2 in water/Al2O3, water/Fe2O3, and water/SiO2 nanofluids were, respectively, 50%, 42%, and 71% higher than those of SO2 in pure water (kLSO2−water <sup>=</sup> 1.45 <sup>×</sup> <sup>10</sup>−<sup>4</sup> <sup>m</sup>/s). Moreover, the values for CO2 in the above nanofluids were, respectively, 117%, 103%, and 88% higher than those of CO2 in water alone (kLCO2−water <sup>=</sup> 1.03 <sup>×</sup> <sup>10</sup>−<sup>4</sup> <sup>m</sup>/s). Finally, this study tried to offer a new, comprehensive correlation for mass transfer coefficient and absorption rate prediction.

Petrovic and Soltani [13] optimized a post-combustion carbon-capture unit using monoethanolamine (MEA), based on a Taguchi experimental design, to understand the impacts of the operational parameters on the energy consumption of the capture unit. An equilibrium-based approach was employed in Aspen Plus to simulate 90% capture of the CO2 emitted from a 600 MW combined-cycle gas turbine power plant. The effects of inlet flue gas temperature, absorber column operating pressure, exhaust gas recycle ratio, and amine concentration on the energy demand were evaluated using signal-to-noise ratios and analysis of variance. The optimum parameters were found to be: flue gas temperature = 50 ◦C, absorber pressure = 1 bar, exhaust gas recirculation = 20%, and amine concentration = 35 wt. %, with a relative importance of amine concentration > absorber column pressure > exhaust gas recirculation > flue gas temperature. This configuration gave a total capture unit energy requirement of 5.05 GJ/tCO2, with a reboiler energy requirement of 3.94 GJ/tCO2. All the studied factors, except for the flue gas temperature, demonstrated a statistically significant association to the response (i.e., energy demand).

In the study of Ge et al. [14], low-cost activated carbons were prepared from waste polyurethane foam by physical activation with CO2 and chemical activation with Ca(OH)2, NaOH, or KOH. The activation conditions were optimized to produce microporous carbons with high CO2 adsorption capacity and CO2/N2 selectivity. The sample prepared by physical activation showed CO2/N2 selectivity of up to 24, much higher than that of the sample prepared by chemical activation. This was mainly due to the narrower microporosity and the rich N content produced during the physical activation process. However, physical activation samples showed inferior textural properties compared to chemical activation samples, which led to a lower CO2 uptake of 3.37 mmol/g at 273 K. Porous carbons obtained by chemical activation showed a high CO2 uptake of 5.85 mmol/g at 273 K, comparable to the optimum activated carbon materials prepared from other wastes. This was mainly attributed to large volumes of ultra-micropores (<1 nm) up to 0.212 cm3/g and a high surface area of 1360 m2/g. Furthermore, in consideration of the presence of fewer contaminants, lower weight losses of physical activation samples, and the excellent recyclability of both physically and chemically activated samples, the waste polyurethane-foam-based carbon materials have potential application prospects in CO2 capture.

Ghasem [15] studied the simultaneous absorption of CO2 and NO2 from a mixture of gases (5% CO2, 300 ppm NO2, balance N2) by aqueous sodium hydroxide solution in a membrane contactor. For the first time, a mathematical model was established for the simultaneous removal of the two undesired gas solutes (CO2, NO2) from flue gas using a membrane contactor. The proposed model considers the reaction rate and radial and axial diffusion of both compounds. The model was verified and validated with experimental data and found to be in good agreement. The model was used to examine the effect of the flow rate of liquid, gas, and inlet solute molar fraction on the percent removal and molar flux of both impurity species. The results revealed that an increased liquid flow rate improved the percent removal of both compounds. A high inlet gas flow rate decreased the percent removal. It was possible to obtain the complete removal of both undesired compounds. The model was confirmed to be a dependable tool for the optimization of such process, and for similar systems.

Wang et al. [16] investigated the structures and electronic properties of monolayer arsenene doped with Al, B, S, and Si, based on first-principles calculations. The dopants exerted great influence on the properties of the arsenene monolayer. The electronic properties of the substrate were effectively tuned by substitutional doping. After doping, NO adsorption onto four kinds of substrate was investigated. The results demonstrated that NO exhibited a chemisorption character on Al-, B-, and Si-doped arsenene, and a physisorption character on S-doped arsenene with moderate adsorption energy. Due to the adsorption of NO, the band structures of the four systems had great changes; the adsorption reduced the energy gap of Al- and B-doped arsenene and opened the energy gap of S- and Si-doped arsenene. The large charge depletion between the NO molecule and the dopant demonstrated that there was a strong hybridization of orbitals at the surface of the doped substrate because of the formation of a covalent bond, except for S-doped arsenene. The charge depletion also indicated that Al-, B-, and Si-doped arsenene might be good candidate gas sensors to detect NO gas molecules, owing to their high sensitivity.

Shoukat et al. [17] studied various novel amine solutions both in aqueous and non-aqueous (monoethylene glycol (MEG)/triethylene glycol (TEG)) forms for hydrogen sulfide (H2S) absorption. The study was conducted in a custom-built experimental setup at temperatures relevant to subsea operation conditions and atmospheric pressure. Liquid-phase-absorbed H2S and amine concentrations were measured analytically to calculate H2S loading (mol of H2S/mol of amine). The maximum achieved H2S loadings as the function of pKa, gas partial pressure, temperature, and amine concentration were presented. The effects of solvent type on absorbed H2S were also discussed. Several new solvents showed higher H2S loading compared to aqueous N-methyldiethanolamine (MDEA) solution, which is the current industrial benchmark compound for selective H2S removal in natural gas sweetening processes.

Wang et al. [18] investigated the carbonaceous deposits on the surface of a coking chamber. Scanning electron microscopy (SEM), X-ray fluorescence spectrum (XRF), Fourier-transform infrared spectrometer (FTIR), Raman spectroscopy, X-ray diffraction spectrometry (XRD), and X-ray photoelectron spectroscopy (XPS) were applied to investigate the carbonaceous deposits. FTIR revealed the existence of carboxyl, hydroxyl, and carbonyl groups in the carbonaceous deposits. SEM showed that different carbonaceous deposit layers presented significant differences in morphology. XRF and XPS revealed that the carbonaceous deposits mainly contained C, O, and N elements, with smaller amounts of Al, Si, and Ca elements. It was found that the C content gradually increased in the formation of carbonaceous deposits, while the O and N elements gradually decreased. It was also found that the absorbed O2 and H2O took part in the oxidation process of the carbon skeleton to form =O and –O– structures. The oxidation and elimination reaction resulted in a change in the bonding state of the O element, and finally formed compact carbonaceous deposits on the surface of the coking chamber. Based on these analyses, the formation and evolution mechanisms of carbonaceous deposits were discussed.

Zhang et al. [19] proposed an oxygen recovery (OR) process for a supercritical water oxidation (SCWO) system based on the solubility difference between oxygen and CO2 in high-pressure water. A two-stage gas–liquid separation process was established, using Aspen Plus software to obtain the optimized separation parameters. Accordingly, energy consumption and economic analyses were conducted for the SCWO process with and without OR. Electricity, depreciation, and oxygen costs were the major contributions to the cost of the SCWO system without OR, accounting for 46.18, 30.24, and 18.01 \$/t, respectively. When OR was introduced, the total treatment cost decreased from 56.80 \$/t to 46.17 \$/t—a reduction of 18.82%. Operating costs were significantly reduced at higher values of the stoichiometric oxygen excess for the SCWO system with OR. Moreover, the treatment cost for the SCWO system with OR decreased with increasing feed concentration for increased reaction heat and oxygen recovery.

Szpalerski and Smoli ´nski [20] presented a strategy for maximizing recovery of flare gases in industrial plants processing hydrocarbons. The functioning of a flare stack and the depressurization systems in a typical refinery plant was described, and the architecture of the depressurization systems and construction of the flares were shown in a simplified way. A proposal to recover the flare gases together with their output outside the industrial plant, in order to minimize impact on the environment (reduction of emissions) and to limit consumption of fossil fuels was presented. Contaminants that might be found in these depressurization systems were indicated. The proposal presented in the article assumed the injection of an excess stream of gases into an existing natural gas pipeline system. A method of monitoring was proposed, aiming to eliminate the introduction of undesirable harmful components into the systems.

Jin et al. [21] experimentally and theoretically studied the mechanism for gas transportation in emerging 2D-material-based membranes. They measured the gas permeances of hydrogen and nitrogen from their mixture through the supported MXene lamellar membrane. Knudsen diffusion and molecular sieving through straight and tortuous nanochannels were proposed to elucidate the gas transport mechanism. An average pore diameter of 5.05 Å in straight nanochannels was calculated by linear regression in the Knudsen diffusion model. The activation energy for H2 transport in the molecular sieving model was calculated to be 20.54 kJ mol−1. The model indicated that the gas permeance of hydrogen (with a smaller kinetic diameter) is contributed to by both Knudsen diffusion and the molecular sieving mechanism, but the permeance of larger molecular gases like nitrogen is from Knudsen diffusion. The effects of critical conditions such as temperature, the diffusion pore diameter of structural defects, and the thickness of the prepared MXene lamellar membrane on hydrogen and nitrogen permeance were also investigated to better understand the different contributions to hydrogen permeation of Knudsen diffusion and molecular sieving. At room temperature, the total hydrogen permeance was 18% due to Knudsen diffusion and 82% due to molecular sieving. The modeling results indicated that molecular sieving plays a dominant role in controlling gas selectivity.

Zhao et al. [22] studied the influence of gas molar fraction and activity in aqueous phase while predicting phase equilibrium conditions. In pure gas systems, such as CH4, CO2, N2 and O2, the gas molar fraction in the aqueous phase was proposed as one of phase equilibrium conditions, and a simplified correlation of the gas molar fraction was established. The gas molar fraction threshold maintaining three-phase equilibrium was obtained by phase equilibrium data regression. The UNIFAC model, the predictive Soave–Redlich–Kwong equation, and the Chen–Guo model were used to calculate aqueous phase activity and the fugacity of gas and hydrate phases, respectively. The calculations showed that the predicted phase equilibrium pressures were in good agreement with published phase equilibrium experiment data, and the percentages of absolute average deviation pressures were given. The water activity, gas molar fraction in the aqueous phase, and the fugacity coefficient in vapor phase were discussed.

Wang et al. [23] established a numerical calculation model for the rapid estimation of coal seam gas content based on the characteristic values of gas desorption at specific exposure times. Combined with technical verification, a new method which avoids the calculation of gas loss for the rapid estimation of gas content in the coal seam was investigated. Study results showed that the balanced adsorption gas pressure and coal gas desorption characteristic coefficient (*K*t) satisfied the exponential equation, and the gas content and *K*t were linear equations. The correlation coefficient of the fitting equation gradually decreased as the exposure time of the coal sample increased. Using the new method to measure and calculate the gas content of coal samples from two different working faces of the Lubanshan North mine (LBS), the deviation of the calculated coal sample gas content ranged from 0.32% to 8.84%, with an average of only 4.49%. Therefore, the new method meets the needs of field engineering technology.

In the work of Yang et al. [24], four samples with different coal ranks were collected and diffusion experiments were conducted under different pressures through the adsorption and desorption processes. Three widely used models, i.e., the unipore diffusion (UD) model, the bidisperse diffusion (BD) model, and the dispersive diffusion (DD) model, were adopted to compare their applicability and to calculate the diffusion coefficients. Results showed that for all coal ranks, the BD model and DD model could match the experimental results better than the UD model. Concerning the fast diffusion coefficient *Dae* of the BD model, three samples displayed a decreasing trend with increasing gas pressure, while the other sample showed a V-type trend. The slow diffusion coefficient *Die* of the BD model increased with

gas pressure for all samples, while the ratio β is an intrinsic characteristic of coal and remained constant. For the DD model, the characteristic rate parameter *k*<sup>Φ</sup> did not change sharply and the stretching parameter α increased with gas pressure. Both *Dae* and *Die* were in proportion to *k*Φ, which reflected the diffusion rate of gas in the coal. The impacts of pore characteristic on gas diffusion were also analyzed. Although pore size distributions and specific surface areas were different between the four coal samples, correlations were not apparent between pore characteristic and diffusion coefficients.

Wang et al. [25] presented a mechanism analysis of air reactor (AR) coupling in a high-flux, coal-direct chemical looping combustion (CDCLC) system and provided a theoretical methodology for optimal system design with favorable operation stability and low gas leakages. First, they presented dipleg flow diagrams of the CDCLC system and concluded the feasible gas–solid flow states for solid circulation and gas leakage control. On this basis, semi-theoretical formulas of gas leakage were proposed to predict the optimal regions of the pressure gradients of the AR. Meanwhile, an empirical formula of critical sealing was also developed to identify the advent of circulation collapse, so as to ensure the operational stability of the whole system. Furthermore, the theoretical methodology was applied in condition design of a cold system. The resulting favorable gas–solid flow behaviors, together with the good control of gas leakages, demonstrated the feasibility of the theoretical methodology. Finally, the theoretical methodology was applied to carry out a capability assessment of a high-flux CDCLC system under a hot state in terms of the restraint of gas leakages and the stability of solid circulation.

Wang et al. [26] investigated the fundamental effects of air reactor (AR) coupling on oxygen carrier (OC) circulation and gas leakages with a cold-state experimental device of the proposed in situ gasification chemical looping combustion (iG-CLC) system. The system exhibited favorable pressure distribution characteristics and good adaptability of solid circulation flux, demonstrating the positive role of the direct coupling AR method in the stabilization and controllability of the whole system. The OC circulation and the gas leakages were mainly determined by the upper and lower pressure gradients of the AR. With an increase in the upper pressure gradient, the OC circulation flux increased initially and later decreased until the circulation collapsed. Additionally, the upper pressure gradient exerted a positive effect on the restraint of gas leakage from the FR to the AR, but a negative effect on the suppression of gas leakage from the AR to the FR. Moreover, gas leakage of the J-valve to the AR, which is directly related to the solid circulation stability, was exacerbated with an increase of the lower pressure gradient of the AR. In real iG-CLC applications, the pressure gradients should be adjusted flexibly and optimally to guarantee balanced OC circulation together with an ideal balance of all gas leakages.

Krištof et al. [27] presented preliminary results on the spray characteristics of a spiral nozzle used for gas absorption processes. First, they experimentally measured the pressure impact footprint of the spray generated. Effective spray angles were then evaluated from the photographs of the spray and via Archimedean spiral equation using the pressure impact footprint records. Using classical photography, areas of primary and secondary atomization were determined together with the droplet size distribution, which were further approximated using selected distribution functions. Radial and tangential spray velocities of droplets were assessed using laser Doppler anemometry. The results showed atypical behavior related to different types of nozzles. In the investigated measurement range, the droplet size distribution showed higher droplet diameters (about 1 mm) compared to those from, for example, air-assisted atomizers. The results were similar for the radial velocity, which was lower (a maximum velocity of about 8 m/s) compared to, for example, effervescent atomizers, which can produce droplets with a velocity of tens to hundreds of m/s. In contrast, the spray angle ranged from 58◦ and 111◦ for the inner small and large cones, respectively, to 152◦ for the upper cone and, in the measured range, was independent of the inlet pressure of liquid at the nozzle orifice.

Ibrahim et al. [28] reviewed the main research work carried out over the last few years on direct mineral-carbonation process utilizing steel-making waste, with emphasis on recent research achievements and potential for future research.

At the end of this editorial, the editors would like to express their sincere gratitude to the authors for their valuable contributions to this special issue and thank the editorial staff of *Processes* for their help and support during the review process.

#### **References**


© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

## *Article* **Siderite Formation by Mechanochemical and High Pressure–High Temperature Processes for CO2 Capture Using Iron Ore as the Initial Sorbent**

#### **Eduin Yesid Mora Mendoza 1,2,\*, Armando Sarmiento Santos 1, Enrique Vera López 1, Vadym Drozd 2, Andriy Durygin <sup>2</sup> , Jiuhua Chen <sup>2</sup> and Surendra K. Saxena <sup>2</sup>**


Received: 6 September 2019; Accepted: 5 October 2019; Published: 14 October 2019

**Abstract:** Iron ore was studied as a CO2 absorbent. Carbonation was carried out by mechanochemical and high temperature–high pressure (HTHP) reactions. Kinetics of the carbonation reactions was studied for the two methods. In the mechanochemical process, it was analyzed as a function of the CO2 pressure and the rotation speed of the planetary ball mill, while in the HTHP process, the kinetics was studied as a function of pressure and temperature. The highest CO2 capture capacities achieved were 3.7341 mmol of CO2/g of sorbent in ball milling (30 bar of CO2 pressure, 400 rpm, 20 h) and 5.4392 mmol of CO2/g of absorbent in HTHP (50 bar of CO2 pressure, 100 ◦C and 4 h). To overcome the kinetics limitations, water was introduced to all carbonation experiments. The calcination reactions were studied in Argon atmosphere using thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) analysis. Siderite can be decomposed at the same temperature range (100 ◦C to 420 ◦C) for the samples produced by both methods. This range reaches higher temperatures compared with pure iron oxides due to decomposition temperature increase with decreasing purity. Calcination reactions yield magnetite and carbon. A comparison of recyclability (use of the same material in several cycles of carbonation–calcination), kinetics, spent energy, and the amounts of initial material needed to capture 1 ton of CO2, revealed the advantages of the mechanochemical process compared with HTHP.

**Keywords:** CO2 capture; iron ore; carbonation; calcination; recyclability; mechanochemical reactions; carbonation kinetics

#### **1. Introduction**

The planet temperature has been monitored since the 19th century, however only since the 1980s has the world realized that global warming was occurring, according to Goddard Institute for Space Studies (GISS) report [1]. The global temperature has increased in the last 50 years by about 1 ◦C. CO2 atmospheric concentration rose to 401 ppm in 2015, which is an increase of 110 ppm since the start of the industrial age [1–3]. Even though some countries have defined policies using international deals, such as Rio de Janeiro (1992), Kyoto (1997), and Copenhagen (2009), the global warming problem remains. A premise in the Paris agreement (2016) is to invest more than 100 billion dollars a year to achieve a solution to the green-house effect of gasses emission. Indeed, these agreements have helped to develop and improve different strategies and technologies to reduce the amount of CO2 emitted to the atmosphere, as it is the main cause of the green-house effect [3–5].

Renewable energy and energy-efficient use as preventive approaches to reduce CO2 emissions are still far from suitable solutions. Several techniques can be used for CO2 separation from flue gases and its subsequent sequestration. These are classified as either physical and chemical methods. The chemical absorption reactions use chemical absorbents, such as amines, and are the most mature technology. Although absorption by amine is the method mainly used in industry, the required energy for the absorbents' regeneration is considerably high [6–8]. Membranes are not considered as applicable solutions due to low mass transfer [9,10]. However, some recent studies have shown remarkable improvements in the CO2/N2 selectivity of the membranes [10]. Cryogenics and micro algals are laboratory-scale technologies [11].

As a result of an exothermic reaction, carbonates generated from metallic oxides, such as MgO, FeO, or CaO, and carbon dioxide, which can be taken from flue gases, are a suitable means of capturing CO2. These carbonates can be heated, and after that, using an endothermic reaction, pure CO2 is released. This pure gas can be used in valuable industrial applications, such as oil recovery oil in the food industry, etc., allowing the recovery of the metal oxides, which, in turn, can be used in a new cycle of carbonation–calcination until the active sites disappear [5,12]. Iron oxides have good thermodynamic properties to be a CO2 sorbent. Kumar et al. [13] demonstrated that a mixture of magnetite and iron can be carbonated with 57% conversion efficiency by adding water as a catalyst. Graphite and iron are suitable reducing agents for carbonation. In addition, they are highly available in steel making industries. Recent research found a novel way to carbonate iron oxides and iron using a ball milling process, reaching almost pure siderite without the use of water [14].

In addition, wustite, hematite, magnetite, and even siderite, iron ore can contain different compounds, such as CaO, MnO, Al2O3, K2O, S, SiO2, P2O5 [15]. Siderite is used in industrial processes. Hence, its thermal decomposition has generated interest. Depending on the atmosphere, different calcination products are formed. Hematite (Fe2O3) is common in an oxidizing atmosphere. Magnetite is obtained in a CO2 atmosphere, while magnetite and wustite (FeO) are found in an inert atmosphere or in vacuum [14,16,17].

In this work, the CO2 capture capacity of iron ore from the El Uvo mine, Colombia, was studied. Thermodynamic simulations supported the results of carbonation–calcination reactions. The CO2 capture capacity was studied for the HTHP process as a function of pressure, temperature, and reaction time, while in the ball milling process, it was studied as a function of pressure, revolution speed, and reaction time. The kinetics of mechanochemical reactions was studied, and finally, energy spend in both carbonation methods was compared.

#### **2. Experimental**

The iron ore samples were prepared from a mineral rock, which was crushed and ground using a mortar and pestle, and, finally, processed by ball milling in an inert atmosphere for two hours. The chemical composition of the ore was studied by X-ray fluorescence analysis (XRF), as shown in Table 1. The major component was iron, but there are some impurities, such as SiO2, CaO, Al2O3, MgO, MnO, P2O5, Na2O, K2O, S, and Zn.

The reactor for carbonation in the HTHP process was a closed cylindrical vessel of length 31.75 mm and an internal diameter of 8.89 mm. The reactor was loaded with 0.25 g of iron ore and iron (Good Fellow, 99% purity, <60 μm) mixture. The molar ratio of iron ore/Fe was defined according to the stoichiometry of reactions (1) and (2).

$$\rm{Fe\_2O\_3(s) + Fe + 3CO\_2(g) \to 3FeCO\_3(s)}\tag{1}$$

$$\text{2FeOOH}(\text{s}) + \text{Fe} + \text{3CO}\_2(\text{g}) \rightarrow \text{3FeO}\_3(\text{s}) + \text{H}\_2\text{O} \tag{2}$$

Zero-point one five milliliters of water was added to the vessel to improve the kinetics of the carbonation reaction. High purity CO2 gas (99.99% purity, Airgas) was introduced into the system at desired pressures from 30 to 50 bar. Before the experiments, the reactor was flushed with CO2 gas three times to ensure an inert atmosphere in the system.


**Table 1.** Chemical composition of iron ore in wt.% according to X-ray fluorescence (XRF) analysis.

Mechanochemical reactions between iron ore and carbon dioxide were performed at room temperature and elevated CO2 pressure (10–30 bar). Planetary ball mill Retsch PM100 was operated at 200 to 400 revolutions per minute. The vessel for the ball milling was a stainless-steel jar of 50 mL volume capable of holding up to 100 bar gas pressure. High purity CO2 gas (Airgas, 99.999%) was loaded into the reactor at different pressures together with 3.00 g of iron ore and 0.5 mL of water. The average temperature in the reactor during the ball milling process was 32 ◦C. The mechanochemical reaction was run for different periods, and each 1 h milling interval was followed by half an hour cooling interval to avoid the overheating of the sample. The powder to balls (stainless steel) weight ratio was 2:27. The reactor was flushed several times with CO2 gas to ensure a pure CO2 atmosphere inside the reactor. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were conducted in a temperature range of 25 to 1000 ◦C using a TA Instruments SDT Q600 instrument. Experiments were performed in air and Ar atmospheres with a heating rate of 10 ◦C /min. Powder X-ray diffraction patterns were collected using Bruker GADDS/D8 diffractometer equipped with Apex Smart CCD Detector and molybdenum rotating anode. Collected 2D diffraction patterns were integrated using Fit2D software [18]. Quantitative phase analysis of the samples was performed using the Rietveld method and GSAS package [19,20]. The CO2 sorption capacity was calculated using the results generated by the Rietveld refinement of XRD patterns. Raman spectroscopy characterization was used to identify carbon in siderite decomposition products. A continuous-wave (CW) argon ion (Ar+) laser (model 177G02, Spectra Physics) of 514.4 nm in wavelength was used as a source of monochromatic radiation. Backscattered Raman spectra were collected by a high-throughput holographic imaging spectrograph (model HoloSpec f/1.8i, Kaiser Optical Systems) with volume transmission gratings, a holographic notch filter, and thermoelectrically cooled charge-coupled device (CCD) detector (Andor Technology). The spectra were usually collected with 10 min exposure.

#### **3. Results**

#### *3.1. Iron Ore Characterization*

The iron ore sample was analyzed by XRD technique. As shown in Figure 1, Fe can be found in the form of hematite Fe2O3 (JCPDS card number #00-072-0469), goethite FeOOH (JCPDS card number#00-081-0462), and siderite FeCO3 (JCPDS card number #00-029-0696).

**Figure 1.** XRD pattern of iron ore sample.

Rietveld refinement shows that iron ore is composed of Fe2O3 (48.02%), FeCO3 (21.15%), and FeOOH (30.83%). These results defined chemical reactions (1) and (2).

Reactions (1) and (2) use Fe iron as a reducing agent. Moreover, Sulfur (S) included in the iron ore in the form of sulfate or sulfide can act as a reducing agent. In García et al. [21], siderite is obtained from hematite, carbon dioxide, sulfur dioxide, and water at suitable conditions.

The regeneration reaction studied in this work is shown below:

$$2\text{6FeCO}\_3(\text{s}) \rightarrow 2\text{Fe}\_3\text{O}\_4(\text{s}) + \text{C}(\text{s}) + 5\text{CO}\_2(\text{g})\tag{3}$$

After regeneration, products can be further recycled back and used in a new carbonation reaction to complete the cycle.

#### *3.2. Thermodynamics Simulation of Iron Ore Carbonation*

FACTSAGE software and the databases therein, FACT- F\*A\*C\*T 5.0, SGPS- SGTE, and SGSL were used to verify the thermodynamic feasibility of the carbonation process at equilibrium for the system iron ore–CO2. According to XRD analysis and chemical composition of the ore, carbonation simulations were performed considering iron ore as a mixture of hematite and goethite. Carbonation in ball milling was simulated at a temperature of 32 ◦C (the average temperature measured in the reactor) and CO2 pressures between 1 and 50 bar. As can be seen in Figure 2, siderite is stable at those conditions.

Figure 3 shows simulations at HTHP conditions for iron ore carbonation in the temperature range of 25 to 325 ◦C and CO2 pressures of 10 and 50 bar. Figure 3a,b do not include Fe in the reaction, while Figure 3c,d include this metallic element. The presence of siderite is evident in all cases. The stability of siderite increased with pressure. This behavior was observed in both systems. It is clear that when Fe is included in the system, the decomposition temperature of siderite increased for the same CO2 pressure. For example, the siderite decomposition temperature at 50 bar began at 150 and 225 ◦C for systems without and with Fe, respectively, revealing the advantage of including Fe due to higher temperatures favoring the thermodynamic conditions for carbonation.

**Figure 2.** FACTSAGE simulation of siderite stability at 32 ◦C and various pressures for the system, iron ore–CO2.

**Figure 3.** FACTSAGE simulations at various temperatures for the system iron ore–CO2, (**a**) 10 bar, (**b**) 50 bar, and for the system iron ore–CO2–Fe (**c**) 10 bar, (**d**) 50 bar.

#### *3.3. Iron Ore Carbonation in Mechanochemical Process*

Siderite yield in the carbonation reaction increased (JCPDS card number # 029-0696) as a result of carbonation by the ball milling method. Figure 4 shows the increases in iron carbonate formation at 30 bar, 400 rpm, 32 ◦C, and 20 h of time reaction.

**Figure 4.** Carbonation of iron ore in ball milling at 30 bar, 400 rpm, 32 ◦C, and 20 h of reaction time.

Initially, the experiments were performed without water, but the siderite amount did not change. That can be related to kinetics limitations. It is clear that water acts as a catalyst in the carbonation process of metal oxides [13,22]. Figure 4 reveals that a considerable amount of siderite can be obtained by 20 h of milling. For those conditions, the CO2 capture capacity of hematite and goethite is 0.1643 g CO2/g sorbent or 3.7341 mmol CO2/g sorbent calculated from the Rietveld refinement of the XRD pattern. This value translates to a 26.82% conversion rate. The calculation was performed, taking into account the initial amount of FeCO3 contained in the ore and that the amount of initial absorbent is the sum of the weights of goetite and hematite.

Table 2 shows the calculations of CO2 capture capacity at different conditions of pressure, revolution speed, and duration of the reaction. As can be seen in the table, the CO2 capture at the same temperature by iron ore increased at higher pressures and longer reaction times.


**Table 2.** CO2 capture capacity of iron ore at different conditions of pressure, revolution speed, and time reaction in the mechanochemical process.

Additionally, values in Table 2 reveal that the ball milling process is affected by the revolution speed of the ball mill. At faster speeds, the siderite yield increased due to the transfer of higher kinetic energy, promoting the appearance of defects, which generate more active sites in goethite and hematite, facilitating the reaction with CO2. According to [23] in the ball milling process, there is a critical revolution speed above which the balls will be pinned to the inner walls of the vial and do not fall to exert any impact force. In these experiments, the speeds of revolution were kept below the critical speed.

#### *3.4. Iron Ore Carbonation in the HTHP Process*

Using this method, neither using graphite as a reducing agent and water as a catalyst nor iron as a reducing agent and no water, the siderite yield was increased. Figure 5 confirms an increase of siderite yield (JCPDS card number #00-029-0696) as a result of carbonation by the HTHP method at 50 bar, 100 ◦C, and 4 h with the addition of metallic iron and water to iron ore. For these conditions, the CO2 capture capacity was 0.2393 g CO2/g sorbent or 5.4392 mmol CO2/g sorbent calculated from the Rietveld refinement of the XRD pattern. This value translates to a 39.08% conversion rate.

**Figure 5.** Carbonation of iron ore in the thermo pressure process at 50 bar, 100 ◦C, and 4 h of time reaction.

Table 3 presents the CO2 capture capacity at different conditions of pressure, temperature, and reaction time. Longer reaction times increased the CO2 capture for all cases. Siderite stability at higher temperatures decreased. For example, at 200 ◦C, 50 bar, and 4 h, a decrease of 53.2% in siderite formation was evidenced compared to 150 ◦C. These considerations can be confirmed with FACTSAGE simulations, which showed that at 50 bar, siderite began to decompose at temperatures around 200 ◦C. In this case, it can be supposed that the decomposition temperature is lower due to the presence of water. The CO2 capture capacity of iron ore increased at higher pressures while keeping the temperature and reaction time constant, confirming the same effect observed in the mechanochemical process.


**Table 3.** CO2 capture capacity of iron ore at different conditions of pressure, temperature, and reaction time in the high temperature–high pressure (HTHP) process.

#### *3.5. Thermal Decomposition of Siderite Studied by Thermogravimetric Analysis*

The siderite decomposition reaction was studied on two samples. The first one was obtained by the mechanochemical process at 30 bar of CO2 pressure, 400 rpm, and 20 h of reaction time, and the second one was obtained at 50 bar of CO2 pressure, 100 ◦C, and 4 h of reaction time by the HTHP process. The decomposition temperature of siderite was experimentally identified using thermogravimetric analysis. Figure 6a,b show the TG–DSC plots for siderite obtained in mechanochemical and HTHP processes, respectively, in an argon atmosphere.

**Figure 6.** Thermogravimetric (TG)-heat flow plot of thermal decomposition of the product formed at 30 bar CO2 pressure, 400 rpm, and 20 h (**a**), and at 50 bar, 100 ◦C, 4 h (**b**), heating rate 10 ◦C/min in an argon atmosphere.

The reaction mechanism for decomposition of siderite samples synthesized from pure iron oxides without water in an argon atmosphere was studied in [14]. Here, according to the thermal gravimetry (TG) plots, mass losses around 100 ◦C occurred for both plots. They were equivalent to 18 wt% and 6 wt%, respectively, and corresponded to the release of adsorbed water by iron ore. The release of CO2 started from 100 ◦C. Some research works have reported that in the temperature interval between 250 and 375 ◦C, there are losses of weight corresponding to the dehydration of the goethite phase and iron hydroxides [24,25].

The thermal decomposition of siderite was significant at the temperature range of 100 to 420 ◦C. According to [14], this range reaches higher temperatures compared to the decomposition temperature of siderite synthesized from pure iron oxides, due to siderite decomposition temperature increasing with decreasing purity [15,26]. Patterson et al. [27] found that magnesium, manganese, or calcium increases the siderite decomposition temperature.

To identify the products after calcination, the siderite sample obtained by the mechanochemical reaction at 30 bar of CO2 pressure, 400 rpm, and 20 h of reaction time was decomposed. The X-ray diffraction pattern in Figure 7 evidences the presence of magnetite (JCPDS # 001-1111), hematite (JCPDS # 00-001-1053), and graphite (JCPDS # 00-026-1079) after decomposition in a vacuum at 300 ◦C for 1 h. As can be seen, at this temperature, siderite was completely decomposed. The same products were identified for decomposed siderite, obtained by the HTHP process, and under the same decomposition conditions.

**Figure 7.** XRD pattern after siderite decomposition at 300 ◦C in a vacuum. Magnetite, hematite, and graphite were identified.

Raman spectroscopy is a suitable technique for graphite identification, due to its high sensitivity to highly symmetric covalent bonds with little or no natural dipole moment. The carbon–carbon bonds that make up these materials fit this criterion perfectly, and as a result, Raman spectroscopy is highly sensitive to these materials and able to provide a wealth of information about their structure. Every band in the Raman spectrum corresponds directly to a specific vibrational frequency of a bond within the molecule. The 1582 cm−<sup>1</sup> band of graphite is known as the G band, and at 1370 cm−1, a characteristic line appears, which is named D mode for a disorder-induced mode of graphite [28–30].

According to Figure 8, decomposed siderite at 300 ◦C in vacuum evidences the presence of graphite, by mean of peaks at 1582 cm−<sup>1</sup> and 1370 cm−1. Figure 8a,b show the Raman patterns of decomposed siderite produced by mechanochemical and HTHP processes, respectively. These results confirm that reaction (3) occurs during siderite decomposition obtained by both ball milling and HTHP.

**Figure 8.** Raman pattern of decomposed siderite from processes, (**a**) mechanochemical (**b**), HTHP.

#### *3.6. Carbonation–Calcination Cycles*

Samples of decomposed siderite were studied in various cycles in CO2 absorption/release reactions to confirm if the materials can be reused. Initially, using the two calcinated samples, there was no CO2 capture, neither in mechanochemical nor in HTHP

For the mechanochemical method at 30 bar CO2 pressure, 400 rpm, 20 h, adding water to the samples, siderite yield was accomplished. After recarbonation, samples were decomposed at 300 ◦C in a vacuum. Table 4 shows the CO2 capture capacity of the transformed material for several cycles in the mechanochemical process.


**Table 4.** CO2 capture capacity for several cycles of carbonation–calcination using themechanochemical process.

The addition of magnetite as a new chemical to the absorbent mixture and carbon as a new reducing agent improved the CO2 capture capacity in subsequent cycles. Here, one additional carbonation reaction is:

$$2\text{Fe}\_2\text{O}\_4(\text{s}) + \text{C}(\text{s}) + 5\text{CO}\_2(\text{g}) \rightarrow 6\text{FeCO}\_2(\text{s}).\tag{4}$$

Hence, iron ore can be used for multiple cycles according to the combination of (1), (2), (3), and (4) reactions.

For HTHP, the recarbonation was studied at 50 bar, 100 ◦C, and 4 h. Graphite was used as a reducing agent in the first place due to its availability and cost. There was no siderite formation after adding extra-graphite to the mixture. It was necessary to add iron and water to achieve new carbonation. The iron addition is not propitious in terms of cost. After recarbonation, samples were decomposed at 300 ◦C in a vacuum. Table 5 shows the CO2 capture capacity of material in the second and third cycles. As can be seen, it was necessary to include extra iron in both cycles. Here, only three cycles were studied because CO2 capture capacity decreases dramatically during cycles.


**Table 5.** CO2 capture capacity for several cycles of carbonation–calcination using the HTHP process.

#### *3.7. Discussion*

It is clear that the addition of water to the mixtures facilitates CO2 sorption and thus, affects the reactivity and capacity of the materials. The presence of moisture increases the mobility of alkaline ions and thus, accelerates the reactions [13,31–33]. Here, in the two methods of carbonation studied, reactions without water did not allow the siderite formation due to kinetics limitations. According to [13], water on the sorbent surface before and after calcinations facilitates the reaction with CO2, which results in the formation of CO3 <sup>2</sup><sup>−</sup> and H<sup>+</sup> ions. Free Fe+<sup>2</sup> ions can further react with CO3 <sup>2</sup><sup>−</sup> to form FeCO3. The presence of water has a dual effect. It not only helps CO2 uptake of sorbent but also affects the siderite stability [13,14].

With increasing siderite formation over time, its layer thickens, which inhibits the contact between Fe+<sup>2</sup> and CO3 <sup>2</sup><sup>−</sup> harming the formation of new siderite. The mechanochemical process provides a way to remove the outer layer of FeCO3; this layer is generally nonporous. This fluidization regime allows the carbonation reaction to remain more active [14,34,35]. This is likely the reason the carbonation process did not need an extra-reducing agent, such as iron, to obtain siderite in all of the cycles, which is an advantage compared with the HTHP process, which needed metallic iron. Initially, the presence of iron allowed high levels of CO2 capture; however, with cycles, the actives sites vanished, and the CO2 capture was practically negligible.

The advantage of the fluidization regime used in the mechanochemical process can be explained through kinetics. Figure 9 shows the siderite yield vs. time for two samples at 20 bar CO2 pressure and 32 ◦C. The first one was treated at 400 rpm and the other one at 200 rpm. As was explained above, the carbonation depends strongly on the revolution speed for the same conditions of pressure and temperature, if the revolution speed is lower than the critical speed. The product yield at 3 h of reaction time and 400 rpm is about three times higher than the mass gained at 200 rpm speed.

**Figure 9.** Siderite formation as a function of rotation speed at 20 bar CO2 pressure and 32 ◦C in the ball milling process.

According to Alkaç and Atalay [36], using the mass fractional conversion *x* with respect to time, it is possible to calculate f(*x*), the reaction model, which comprises the particular fractional conversion and related mechanism in terms of mathematical equations [15,36]. If f(*x*) vs. t has high lineality, it indicates a suitable fitting for a given model and the slope gives the value of the rate constant, k, at a fixed temperature. Constant k is directly proportional to the reaction rate. For example, Figure 10 depicts f(*x*) as a function of time, taking the Jander three dimensional diffusion model which presented high lineality. This model expresses f(*x*) as

$$\mathbf{f}(\mathbf{x}) = \left[1 - (1 - \mathbf{x})^{\frac{\mathbf{f}}{\mathbf{f}}}\right]^2. \tag{5}$$

**Figure 10.** The Jander three-dimensional diffusion model evaluated for fractional mass conversion values (*x*) as a function of time at 20 bar and 32 ◦C in the ball milling process.

The slopes represent k according to the expression above. Hence, if the machine operates to 400 rpm, the rate constant increases 4.14 times compared to k at 200 rpm. Hence, if the kinetics energy transferred to the absorbent is bigger, carbonation conditions improve [37,38].

Another important consequence of the mechanochemical treatment is the improvement in calcination conditions. Some researches [39,40] have reported siderite, which has been treated in the ball milling process, as having lower decomposition temperatures.

According to carbonation reactions, it is possible to establish a projection of the amount of raw material that would be used in large-scale CO2 emissions. Table 6 shows the needed material amount to capture 1 ton of CO2 in carbonation reactions, assuming 100% conversion. The amount of formed siderite is 2.63 tons in both reactions.

**Table 6.** Amount of raw material needed to capture 1 ton of CO2 in several carbonation reactions, assuming 100% of conversion yield.


In terms of steel production in a blast furnace, to produce one ton of steel, around 1.8 tons of CO2 emissions are generated. Here, for carbonation by mechanochemical interaction, 3.43 tons of iron ore are needed operating at 30 bar, 400 rpm for 36 h for the almost total transformation of iron ore. Otherwise, using HTHP, 2.26 tons of iron ore and 0.75 tons of metallic iron are needed as initial materials. However, the capture is reduced to a maximum of 43.15% in HTHP due to kinetics limitations and surface area conditions. Since carbonation by the mechanochemical method has remarkable advantages, the amount of the recovered material is calculated from the calcination reaction (3). In this case, 4.37 tons of siderite will be transformed into 2.91 tons of magnetite, 0.073 tons of carbon, and 1.38 tons of CO2. Magnetite and carbon will be used in the next cycle as sorbent and reducing agent, respectively, and pure CO2 can be used in industrial applications.

Another important point to consider is the energy needed in each process. According to [41], the total spent energy in ball milling can be calculated as a function of the filling factor of the reactor, ball mass, ball diameter, number of balls, reaction time, rotation speeds of plate and reactor, the radius of plate and reactor, and sample mass. A sample processed at 30 bar, 400 rpm, and 3 h consumes 14.062 W-h per gram of absorbent having a CO2 capture capacity of 2.7816 mmol CO2/g sorbent. In HTHP, the energy can be calculated by multiplying the values of electrical current, electrical voltage, a factor of heat losses, and time reaction. The power factor was taken as one, due to the total impedance in the electrical circuit that warms the reactor being completely resistive. To compare the expenses of energy demands from the mechanochemical and HTHP processes, similar values of CO2 capture capacities were taken. Hence, the spent energy to process a sample at 50 bar, 100 ◦C, and 1 h, which produced a CO2 capture capacity of 2.9118 mmol CO2/g sorbent was calculated. This process needed 41.58 W-h per gram. This energy is almost three times larger than the energy in the mechanochemical method. In addition, the CO2 pressure is higher which is an extra penalty.

#### **4. Conclusions**

Iron ore was studied for CO2 capture. The CO2 capture capacity was evaluated for two methods, ball milling and HTHP. Water was always added to accomplish carbonation. Higher levels of capture were achieved at higher pressures and reaction times. Faster revolution speed allowed an increase in the siderite formation in ball milling. In HTHP, carbonation reactions were favored by temperatures between 100 and 150 ◦C, but at 200 ◦C, an inverse reaction was observed. The range of regeneration of iron oxides was identified between 100 ◦C and 420 ◦C in both methods, reaching higher temperatures than the siderite decomposition temperatures formed from synthetic iron oxides, due to decomposition temperature increases with decreasing purity. Magnetite and carbon were identified as decomposition products. It was necessary to add water to accomplish re-carbonation in the mechanochemical process while in the HTHP process, metallic iron and water were needed. Carbonation by the mechanochemical process was studied for four carbonation–calcination cycles presenting suitable conditions, while HTHP was only studied for three cycles due to its CO2 capture capacity decreasing. The amounts of hematite, goethite, and metallic iron needed to capture 1 ton of CO2 in carbonation reactions were calculated. A projection of the material needed to carbonate, as well as the material recovered in the calcination, for a real application was carried out. Kinetics and energy requirements confirmed the advantages of carbonation by mean of the mechanochemical process compared to the HTHP method.

**Author Contributions:** All authors contributed to the manuscript. E.Y.M.M. and V.D. proposed the initial idea; A.D. designed and managed the technological equipment; E.Y.M.M. and V.D. performed the simulations; A.D., E.Y.M.M. and V.D. performed the experimentation. V.D. and E.Y.M.M. wrote the manuscript; J.C., A.S.S., E.V.L. and S.K.S. improved the manuscript.

**Funding:** This research received no external funding.

**Acknowledgments:** The authors acknowledge the financial support by Departamento Administrativo de Ciencia, Tecnología e Innovación, Colciencias in Colombia. The work in part is supported by the NSF Geophysics Program (EAR 1723185).

**Conflicts of Interest:** The authors declare no conflict of interests.

#### **References**


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