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CO2 capture performance and mechanical properties of Ca(OH)2-based sorbent modified with MgO and (NH4)2HPO4 for Calcium Looping cycle
Fuel 256 (2019) 115924
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Full Length Article
CO2 capture performance and mechanical properties of Ca(OH)2-based sorbent modified with MgO and (NH4)2HPO4 for Calcium Looping cycle
T
Plínio César de Carvalho Pintoa, Geison Voga Pereirab, Leonardo Schiavo de Rezendeb, ⁎ Flávia C.C. Mourab, Jadson Cláudio Belchiorb, a b
Instituto Federal do Espírito Santo, Campus Vila Velha, Av. Ministro Salgado Filho 1000, Vila Velha 29106-010, ES, Brazil Departamento de Química, Universidade Federal de Minas Gerais, Av. Antônio Carlos 6627, Belo Horizonte 31270-901, MG, Brazil
ARTICLE INFO
ABSTRACT
Keywords: Sorbent Calcium Looping Calcium hydroxide Struvite CO2 capture
The present work deals with an efficient synthesis of calcium hydroxide-based materials modified with magnesium oxide and ammonium phosphate for Calcium Looping technology in Post-combustion CO2 capture. This modification produced 3 %wt of struvite, MgNH4PO4·6H2O, and hydroxyapatite Ca10(PO4)6(OH)2, in the material. These minerals are very hard, which contributes to increase the mechanical strength of the materials. The modified materials are four times more resistant to impact and abrasion than materials containing pure calcium hydroxide freshly prepared, and therefore, can generate less fractures and dusts. In addition, struvite and hydroxyapatite can create open pores and channels into the sorbent structure when they release water or ammonia by thermal decomposition above 200 °C identified by X-ray microtomography. Due to this porous structure the materials are more resistant to sintering during absorption/regeneration cycles. The new developed absorbent are harder, less brittle, generate less frictional dust, and are more resistant to high temperature sintering (400–700 °C) compared to pure Ca(OH)2 based materials. The results showed that calcium hydroxide modified materials can be considered highly efficiency to CO2 absorption, with particular achievement of greater than 50%wt.
1. Introduction The scientific community believes that the increase in the concentration of atmospheric CO2 has been producing the global warming through the intensification of the greenhouse effect. As a result, climate changes can lead to many environmental disasters such as drought, desertification, hurricanes and tornadoes as well as floods, with many impacts on flora, fauna and human society [1]. Various human activities that use fossil fuels for energy generation contribute to these problems. For example, industries, transportation, and electric power generation sectors are potential source of emissions of greenhouse gases, mainly CO2. Thus, mitigation of climate change occurs through the adoption of clean technologies that can use renewable energy sources or can control the emission of greenhouse gases. Among other possibilities, in order to reduce CO2 emissions one can considerer, for example, the technology known as Calcium Looping. Generally, Calcium Looping applied in industries is associated with technologies called “Carbon Capture and Storage” (CCS). The latter is an approach that can capture (separation) the CO2 from other substances emitted by a source of pollution. Following this stage one can ⁎
provide its compression and transportation to a storage site where the gas is injected into appropriate geological formations. This procedure is well established and there are already technologies applied in power plants, cement plants, refineries and other industrial sectors [2–4]. Calcium Looping technology is based on the absorption of CO2 in calcium oxide particles for the production of calcium carbonate in a fluidized bed reactor (carbonator) operating at atmospheric pressure and temperature of about 650 °C. Lime stone, which is the precursor material for calcium oxide, is abundant in nature and has been considered as a very low-cost input. Partially carbonated solid particles after short residence periods (a few minutes) are circulated in a second gas–solid reactor (calciner), where CaO is regenerated by calcination between 900 °C and 1000 °C at atmospheric pressure. The gas stream of the highly concentrated calciner in CO2 is then compressed for geological storage (Fig. 1) [5–7]. The efficiency of Calcium Looping technology in the CO2 capture is strongly influenced by the characteristics of the absorbent material such as: chemical absorption of CO2, resistance to embrittlement due to thermal fatigue, sintering resistance and mechanical resistance to friction and impact [7,8].
Corresponding author. E-mail address: [email protected] (J. Cláudio Belchior).
https://doi.org/10.1016/j.fuel.2019.115924 Received 7 May 2019; Received in revised form 26 July 2019; Accepted 29 July 2019 0016-2361/ © 2019 Published by Elsevier Ltd.
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strong turbulence, which favours the solid-gas reactions, but damages the absorbent material [7]. Therefore, some strategies are adopted to improve the performance of absorbent materials. Additives such as kaolin, bentonite and some other clays act as binders and aid in the production of pellets. Unfortunately, these substances minimize the absorption capacity of CO2 and do not offer an increase in the mechanical resistance of the pellets [16,17]. Kaolin and bentonite have covalent chemical bonds with low affinity for the calcium oxide ionic bonds. Calcium aluminate cements increase the hardness of sorbents [17], aluminosilicates can generate greater resistance to friction [18]. In particular, calcium and magnesium phosphates have also recently been used as binders promoting higher resistance to friction, higher hardness and resistance to sintering, but do not contribute to absorb CO2 [19]. On the other hand, oxides of magnesium, iron oxide, and ceramics of alkali metals have also been tested as materials with high level of CO2 absorption. In addition, they contribute to the increase in mechanical strength and resistance to sintering, with an advantage of absorption of CO2 at temperatures between 200 °C and 500 °C [18,19]. Calcium and lithium ceramics are two common CO2 sorbents. Lithium ceramics offer thermal stability, present small volume change during CO2 sorption/desorption process, and low regeneration energy compared to CaO. However, lithium raw materials are less abundant and more expensive. Furthermore, lithium materials show a CO2 absorption kinetics and CO2 capture efficiency lower compared to CaO-based sorbents [20,21]. Other additives and different strategies are applied to minimize the effects of the loss of CO2 absorption of the absorbent material. Reagents such as organic acids, surfactants, chelating agents, coals, biomass, polymers are used to create pores. These substances are added and subsequently, with appropriate treatment, usually thermal treatment, are removed from the pellets creating additional pores [22–24]. The pores tend to increase the surface area and allow greater diffusion and interaction of CO2 with the active sites of the absorbent material [16]. The most commonly material used is calcium oxide for the synthesis processes that involves calcination of calcium hydroxide and calcination of calcium carbonate in high temperature furnaces [18]. However, other methods of efficient synthesis in laboratory scale have been used such as sol–gel precipitation, wet impregnation, chemical vapor deposition, spray-dryer, pyrolysis by ultrasonic spray, the use of chemical compounds as spacers, and control of synthesis for exposition of more active crystalline basic (O2–) sites [8,25–27]. The geometry and size of the absorbers are also important parameters that influence CO2 capture. The larger the size, the lower the absorption capacity, due to a limitation of the diffusional process. And spherical bodies are more stable and suffer less mechanical wear, especially in fluidized bed reactors [19,25,28]. This work presents a new calcium rich material with semi-sphere geometry applied in CO2 capture between 400 and 700 °C. The proposed material has higher mechanical strength, higher resistance to sintering and high level of absorption of CO2 compared to pure calcium oxide. The approach with these characteristics is built by incorporating small amounts of phosphate, magnesium and ammonium ions in the materials. A new method of sorbents synthesis based on Ca(OH)2 is also proposed to form hydroxyapatite, Ca10(PO4)6(OH)2, which provides greater hardness to the sorbent and struvite, MgNH4PO4·6H2O, which form more pores during its decomposition and release of ammonia and water with the Calcium Looping reaction temperature. Therefore, there is a significant change in the chemical structure of the materials, which favours the CO2 capture process with high level of absorption.
Fig. 1. Cyclic process of CO2 absorption and regeneration of CaO-based absorbent.
The chemical absorption is related to the molar absorptivity of CO2, which is an intrinsic property of each material, and is expressed by the ratio CO2 absorbed mass/absorbent material mass. Pure calcium oxide has a theoretical capture efficiency of 0.786 gCO2/g through the absorbent material used. Materials considered highly efficient of CO2 absorption have demonstrated to produce greater than 50%wt. This means that such materials have to absorb more than 0.5 gCO2/g of absorbent material [8,9]. Thermal fatigue and sintering are processes that involve the transfer of heat in materials. Thermal fatigue occurs when the material is subjected to several cycles of heating and cooling, suffering successive dilations and volumetric contractions that weaken the structure, due to the generation of several cycles of cracks and fractures. Ceramic materials based on calcium oxide do not support successive dimensional variations because they do not present enough elastic behaviour. Sintering is a process of grain agglutination at temperatures below melting temperature and changes the microstructure of a solid material. In Calcium Looping, such a process occurs during the regeneration step, which reaches the temperature of 900 °C–1000 °C. Sintering impairs the CO2 capture process because it leads to the reduction of pore numbers and their sizes, diffusion channels and surface area of the absorbent material. The Sintering process is also more evident in pulverized material and can be minimized through pelletizing [10,11]. The Pelletizing procedure is performed by three main methods well decribed in the litetarure namely rotation, extrusion and extrusion-spheronization. The rotation is characterized by a vessel that, in a controlled conditions, injects a spray of water while stirring the material by a pair of rotor blades and a chopper, forming pellets by the agglutination of the powder particles with water [12]. Extrusion promotes the mixing and compaction of the raw materials through helical pistons that rotate and force the passage of the material through cylinders with holes at the end of the path that define the shape of the pellets. Finally, these wires are cut to define the final pellet size [13]. Extrusion and spheronization is the combination of the extrusion technique that forms yarns of the material with the spheronization technique to cut the yarns and give the rounded finish to pellet formation in the form of spheres [14]. The graphite casting granulation is a new pelletization technique based on injecting a slurry of the precursor material into a spherical graphite powder mold, followed by drying the material at 70° C and calcination at 600 °C in air to remove the graphite dust. This technique requires less mechanical force in pellet production compared to previous techniques. This tends to reduce the material compaction and favor the CO2 capture process [15]. More general industrial applications are the use of pellets in fluidized bed reactors but usually one can observe drawbacks. This is basically due to the dynamics imposed on the absorbent bodies. The latter promote interactions that are determinant to wear them, causing them to lose mass, generating dust or suffering fractures. The gas flow in the reactors is intense enough that the absorber bodies enter in a regime of
2. Materials and methods 2.1. Reagents All the chemical reactions were performed in deionized water (type III). The CaO was purchased from Belocal-Lhoist Brazil (99% purity). 2
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Magnesium oxide was purchased from Neon commercial (99% purity). Di-ammonium hydrogen phosphate was purchased from Vetec-SigmaAldrich Company and used as received (100% purity). The CO2 and N2 gases were purchased from Linde (analytical grade).
lid under rotation of 25 rpm for 5 min in a Friability Tester Ethik Technology. 2.4. CO2 absorption performance of the Ca(OH)2-based sorbent modified with MgO and (NH4)2HPO4
2.2. Synthesis of materials
The CO2 capture was realized by using a thermogravimetric analysis (TG) performed on PERSEUS® STA 449 F3 JUPITER® NETZSCH equipment with temperature programme: 25 °C to 600 °C with a heating rate of 50 °C min−1, 600 °C–650 °C with a heating ratio of 5 °C min−1, and then an isotherm at 650 °C for 30 min using a flow of 71% CO2 (20 mL min−1 N2 + 50 mL min−1 CO2).
Calcium oxide powder was hydrated in a ratio of 1:1.5 (CaO:H2O) to form a mass of calcium hydroxide. The calcium hydroxide mass was added manually to the 10 mm diameter semi-spheres silicone templates. The calcium hydroxide semi-spheres (10 mm diameter) were dried in an oven at 80 °C for 1 h and they were packed in sealed plastic bags and the sample was named 100%Ca(OH)2. The production of calcium oxide semi-spheres involves a thermal treatment step (600 °C for 1 h under air atmosphere) of the calcium hydroxide semi-spheres prior to packaging generating the sample 100%CaO. Different materials were obtained by partially replacing the calcium oxide to magnesium oxide (3, 6 and 10 %wt) resulting in three new materials: 97%Ca(OH)2 3%Mg(OH)2, 94%Ca(OH)2 6%Mg(OH)2, 90%Ca(OH)2 10%Mg(OH)2. In addition, the water used during the hydrating step was partially replaced by a solution of di-ammonium hydrogen phosphate to react stoichiometrically with MgO and produce struvite (NH4MgPO4·6H2O), generating, finally, other three different materials named: 97%Ca(OH)2 3%MgNH4PO4, 94%Ca(OH)2 6% MgNH4PO4, and 90%Ca(OH)2 10%MgNH4PO4.
2.5. Reuse of the material in several cycles of CO2 absorption and thermal regeneration The regeneration capacity of the materials was studied by thermogravimetric analysis using ten cycles of CO2 absorption and regeneration of the materials by thermal decomposition. In each cycle an absorption of CO2 at 650 °C (50 mL min−1 CO2 + 20 mL min−1 N2) was carried out for 30 min and regeneration in N2 (70 mL min−1) at 900 °C for 5 min. The heating and cooling ramp between 650 °C and 900 °C was performed at a rate of 20 °C min−1 in N2. 3. Results and discussion
2.3. Characterization of materials
Two materials were initially prepared in a semi-sphere morphology. Calcium hydroxide was synthesized by the reaction of calcium oxide with water, 100%Ca(OH)2, (Eq. (1)), and calcium oxide obtained by thermal decomposition of the calcium hydroxide semi-sphere at 600 °C for 1 h, 100%CaO, (Eq. (2)). Furthermore, a modification in the chemical composition of these semi-spheres was proposed through the partial substitution of CaO by MgO in different amounts (3, 6, and 10 % wt) since magnesium contributes to increase the resistance to sintering of the material considering 97%Ca(OH)2 3%Mg(OH)2, 94%Ca(OH)2 6% Mg(OH)2, 90%Ca(OH)2 10%Mg(OH)2, (Eqs. (3) and (4)) [25,28–31]. Other modifications of the chemical composition were established by the incorporation of MgO and di-ammonium hydrogen phosphate ((NH4)2HPO4) (in 3, 6, and 10 %wt), also considering 97%Ca(OH)2 3% MgNH4PO4, 94%Ca(OH)2 6%MgNH4PO4, and 90%Ca(OH)2 10% MgNH4PO4, to reach different properties: i) increase the mechanical resistance of the material and avoid material particles losses in the reactor by friction and impact; ii) increase the resistance to sintering to enable a greater reutilisation of the material in several cycles of CO2 capture. As expected, it increases the mechanical strength that comes from the formation of hydroxyapatite (Ca10(PO4)6(OH)2) (Eq. (5)). The latter is the main mineral constituent of animal bones [32], and MgO and di-ammonium hydrogen phosphate contributes to the formation of struvite (NH4MgPO4·6H2O) (Eq. (6)), which is the basis of magnesium phosphate cements. Phosphate-bonded refractories are also in widespread use, exploiting the property of cold setting to form products that are stable at high temperatures, i. e. 600–1000 °C [33–35].
The materials were characterized by different techniques. XRD patterns were collected in a Siemens D5000 instrument using a Ni-filtered CuKα radiation (k = 1.5418A°) and a graphite monochromator in the diffracted beam. A scan rate of 1° min−1 was applied to record a pattern in the 2θ range of 20–80°. The XRD lines were encountered in JCPDS (Joint Committee on Powder Diffraction Standard). Infrared spectra were obtained from a Perkin-Elmer Spectrum RX FT-IR spectrometer, using KBr disks in the 4000–400 cm−1 region with 64 scans and 4 cm−1 of spectral resolution. Thermogravimetric analysis (TG) and differential scanning calorimetry (DSC) were carried out by a PERSEUS® STA 449 F3 JUPITER NETZSCH at temperatures ranging from 25 °C to 900 °C with a heating rate of 20 °C min−1 in nitrogen flux of 70 mL min−1. The morphology of the materials was studied by N2 adsorption (−196 °C) in a Quantachrome IQ2 system. The surface area was calculated using the BET equation. The total pore volume was estimated from the amount of nitrogen adsorbed at P/P0 = 0.95. Scanning electron microscopies (FEG QUANTA 200 FEI) were performed using an accelerating voltage of 20 kV. The samples were coated with Au by the sputtering method to make them conductive. Images were acquired using magnification of 5000× for the evaluation of semi-spheres microstructures. The X-ray microtomographies were obtained by a microCT scanner (SkyScan 1174, Bruker) using a 50 kV voltage source, 800 mA source current and 8.05 mm pixel size. The light image was projected with an objective magnifying lens onto a 1304 × 1024 charge coupled device (CCD) detector array. No filters were used. The samples were attached to a stage that rotated 180° with images acquired every 0.7°. The acquired shadow projections were further reconstructed into 2D slices using the NRecon software interface, and CT Analyser software (Skyscan, Bruker micro-CT) was used for 3D analysis and 3D surface rendering. CT Vol 2.3.1.0 version software (Skyscan, Bruker micro-CT) was used for 3D volumetric visualization. The mechanical properties were analysed by a compressive strength that is the force required to fracture a compressed specimen, and It was determined by the arithmetic mean of ten 10 mm diameter samples submitted to compression in a portable digital Hardness Tester Ethik Technology. Friability is the percentage of dust mass generated by friction and the impact of the pellets in an acrylic cylindrical plate with
Ca(OH) 2 (s)
(1)
CaO(s) + H2 O(g)
(2)
CaO(s) + H2 O(l) Ca(OH) 2 (s)
Mg(OH) 2 (s)
(3)
MgO(s) + H2 O(g)
(4)
MgO(s) + H2 O(l) Mg(OH) 2 (s)
10CaO(s) + 6(NH 4 )2 HPO4 (aq)
Ca10 (PO4 )6 (OH) 2 (s) + 8H2 O(l) + 6NH3 (aq)
(5) MgO(s) + 5 H2 O(l) + (NH 4 ) 2 HPO4 (aq)
NH 4 MgPO4 ·6H2 O(s) + NH3 (aq)
(6)
The struvite (NH4MgPO4·6H2O) can contribute to the formation of a structure with higher mechanical and thermal resistance. In addition, it generates a more porous structure, because of the release of ammonia 3
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Fig. 3. Infrared spectra of pure CaO and Ca(OH)2 semi-spheres and semispheres modified with 10% Mg(OH)2 and 10% MgNH4PO4. Fig. 2. X-ray diffraction patterns of pure CaO and Ca(OH)2 semi-spheres and semi-spheres modified with MgO and (NH4)2HPO4.
As can be seen in Fig. 4(a), the CaO and Ca(OH)2 semi-spheres present two events of weight loss in the TG curve, in N2 atmosphere. The decomposition of Ca(OH)2 between 300 °C and 500 °C, forming CaO and H2O (Eq. (2)), and the decomposition of CaCO3 between 600 °C and 800 °C (Eq. (8)) [39].
gas and steam of water by thermal decomposition at 100 °C (Eq. (7)) [36].
NH 4 MgPO4 ·6H2 O(s)
MgHPO4 (s) + 6 H2 O(g) + NH3 (g)
(7)
CaCO3 (s)
CaO(s) + CO2 (g)
(8)
The thermal behaviour of the semi-spheres produced with CaO e MgO (Ca(OH)2/Mg(OH)2) are similar to the CaO and Ca(OH)2 semispheres, since the Mg(OH)2 decomposes together with Ca(OH)2 (Eq. (4)). The semi-spheres produced with CaO, MgO and (NH4)2HPO4 (Ca (OH)2/MgNH4PO4) show the weight losses of the Ca(OH)2 and CaCO3 decomposition, but also a weight loss below 400 °C from the water and ammonia release (Eq. (7)). Assuming that all MgO added in the synthesis was converted to Mg(OH)2 or NH4MgPO4·6H2O and based on the decomposition of Ca(OH)2 and CaCO3, one can calculate the chemical composition of the semi-spheres, and this is shown according to Table 1. It is also observed (Fig. 4) that the semi-spheres with higher concentration of CaO or Ca(OH)2 are those that absorb the greater amount of CO2 when heated in CO2 atmosphere (Fig. 4b), according to Eqs. (9) and (10). However, calcium hydroxide only absorbs CO2 above 300 °C when it begins to decompose thermally and releases water (Eq. (2)). The total CO2 absorption of the calcium hydroxide should take into account the mass change in CO2 atmosphere with the addition of the amount of water released in N2 atmosphere (32%wt + 22%wt = 54% wt).
3.1. Characterization of materials Fig. 2 shows the X-ray diffractograms of pure CaO and Ca(OH)2 semi-spheres and semi-spheres modified with 3% or 6% of Mg(OH)2 and 3% or 6% of NH4MgPO4. The semi-sphere of calcium hydroxide was identified portlandite as crystalline phase by X-ray diffraction analysis. The semi-sphere of calcium oxide consists mainly of lime, CaO, but with a small amount of portlandite that can be formed in the slow cooling in the muffle furnace by the absorption of water. The semi-sphere with 3% NH4MgPO4 showed the presence of portlandite, but also calcite, CaCO3. Also, hydroxyapatite, Ca10(PO4)6(OH)2, signals have been identified in this sample. The reaction of MgO and CaO with (NH4)2HPO4 in solution (Eq. (5) and (6)) occurs with the formation of ammonia, which can promote the elevation of pH and the formation of hydroxyapatite and calcite by the absorption of CO2 from the air. The semi-spheres that were synthesized with 3 to 10% MgO formed portlandite as main phase. No signs of brucite, Mg(OH)2, were identified due to the low concentration of MgO in the material. Struvite (NH4MgPO4·6H2O) was not identified by the X-ray diffraction analysis of the semi-spheres synthesized with 3 to 10% MgNH4PO4, due to the low concentration in the material. Amorphous impurities in the materials are not possible to be observed in the diffractograms. Fig. 3 shows the spectra obtained in the infrared region of pure CaO and Ca(OH)2 semi-spheres and semi-spheres modified with 10% Mg (OH)2 and 10% NH4MgPO4. The infrared spectra of pure CaO and Ca(OH)2 semi-spheres show that both materials present calcium hydroxide (portlandite), including the calcium oxide semi-sphere that can absorb water during the cooling after calcination step. The portlandite shows a characteristic sharp band at 3640 cm−1 related to O–H stretching bond to Ca. In the semi-sphere modified with MgO the presence of brucite was suggested through the thin and intense band in 3698 cm−1 due to O–H stretching bond to Mg [37]. In the semi-sphere of 97%Ca(OH)2 3%MgNH4PO4, 94%Ca(OH)2 6%MgNH4PO4, and 90%Ca(OH)2 10%MgNH4PO4, no signal is observed that suggests the presence of brucite. There is a strong indication that the magnesium has been converted to ammonium and magnesium phosphate, which is a more thermodynamically stable phase. However, due to the low concentration of the Mg in the material, it was not detected by infrared spectroscopy [38].
CaO(s) + CO2 (g)
CaCO3 (s)
Ca(OH) 2 (s) + CO2 (g)
CaCO3 (s) + H2 O(g)
(9) (10)
According to the stoichiometry of the reaction, (Eq. (9)), the total absorption of CO2 by calcium oxide can reach a maximum of 78 %wt. However, this value is not experimentally obtained under normal conditions, because CaO is very reactive and absorbs water and atmospheric CO2 quickly. In order to reach CO2 absorption values above 60 %wt, it is necessary to carry out the treatment of the reagent and to promote the synthesis of the material in an inert atmosphere of N2 or Ar. Under normal conditions the maximum CO2 absorption varies from 55 %wt to 63 %wt [31]. It has been experimentally found that CaObased materials do not absorb more than 35 %wt of CO2 under the following conditions: i) an impure lime, CaO; and (ii) inadequate storage of the reagent or synthesis of the product (in contact with atmospheric air). The partial substitution of CaO by MgO without any special treatment in Ca(OH)2-based absorbers contributes to the reduction in the CO2 amount captured, according to Fig. 4b. The higher the CaO substitution per MgO the greater the reduction of CO2 capture efficiency because the conversion of MgO to MgCO3. This conversion is 4
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Fig. 4. TG curves of Ca(OH)2-based semi-spheres modified with MgO and (NH4)2HPO4 performed in different atmospheres: (a) N2, and (b) CO2 (25 °C to 600 °C with a heating rate of 50 °C min−1, 600 °C to 650 °C with a heating ratio of 5 °C min−1, and then an isotherm at 650 °C for 30 min).
thermodynamically favoured below 300 °C. Above 500 °C the inverse decomposition reaction of magnesite is favoured. In addition, the kinetics of CO2 absorption by MgO is slow and needs to be catalysed by water vapour and alkali metals such as sodium and potassium [40–43]. 3.2. CO2 capture studies Fig. 5 shows the CO2 absorption efficiency of the proposed synthetized materials in several reuse cycles after thermal decomposition at 900 °C through thermogravimetry. It is observed (Fig. 5) that the semi-spheres with 100 %wt CaO, 100 %wt Ca(OH)2 and 97 %wt Ca(OH)2 have an initial absorption of CO2 above 45 %wt. This value (52%wt) for Ca(OH)2 also includes the loss of water above 300 °C in the 1st cycle that corresponds to approximately 22 %wt (Fig. 4). The carbonation mechanism of Ca(OH)2 at high temperatures appears to be complex, but it has already been shown that water catalyses the carbonation reaction and promotes a higher sorption of CO2 by the materials based on CaO [44,45]. The CO2 absorption efficiency of pure calcium materials greatly reduces when absorption and regeneration are repeated several times [28–30,46]. The semispheres with the mixture of Ca(OH)2 and Mg(OH)2 present lower initial CO2 uptake than the pure Ca(OH)2 semi-spheres. The increase in magnesium hydroxide concentration in the semi-sphere reduces the initial absorption of CO2. However, the presence of the magnesium ion in the semi-sphere structure reduces the loss of efficiency in the absorption of CO2 from the material in several cycles of reuse as also previously verified [47]. Magnesium oxide does not react at high temperature with CaO, and then it contributes to inhibit the sintering of the material during the regeneration cycles of Calcium Looping [48].
Fig. 5. Efficiency of CO2 absorption of materials in several cycles of absorption (650 °C) and regeneration (900 °C).
We observed that the inhibition of sintering during regeneration by thermal decomposition at 900 °C is even more pronounced when the magnesium ion, ammonium ion and the phosphate ion are combined in the material structure. The material with the best CO2 absorption efficiency in several reuse cycles consists of 97 %wt Ca(OH)2 and 3 %wt NH4MgPO4. According to the literature, the struvite, NH4MgPO4·6H2O, will be thermally decomposed to release ammonia, water and increase the porosity of the material (Eq. (7)) [33]. This hypothesis was studied by N2 adsorption analysis, scanning electron microscopy and X-ray
Table 1 Calculated chemical composition of CaO and Ca(OH)2 semi-spheres modified with MgO and (NH4)2HPO4. Expected chemical composition
100% CaO 100% Ca(OH)2 97% Ca(OH)2 3% Mg(OH)2 94% Ca(OH)2 6% Mg(OH)2 90% Ca(OH)2 10% Mg(OH)2 97% Ca(OH)2 3% NH4MgPO4·6H2O 94% Ca(OH)2 6% NH4MgPO4·6H2O 90% Ca(OH)210% NH4MgPO4·6H2O
Calculated chemical composition (wt%) CaO
Ca(OH)2
CaCO3
Mg(OH)2
NH4MgPO4·6H2O
73.0 4.6 – – – 2.3 4.9 8.7
21.2 87.0 87.3 87.3 83.9 85.4 80.7 72.4
5.8 8.4 9.7 6.7 6.1 9.3 8.4 8.9
– – 3.0 6.0 10.0 – – –
– – – – – 3.0 6.0 10.0
5
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The material containing 100% Ca(OH)2 is characterized by nanoparticles (similar to sheet forms) forming micrometric aggregates [49]. The other materials have smaller particles with the appearance of irregular flakes (Fig. 7). Fig. 8 shows the distribution of the chemical elements in the calcined absorbent material containing 97% Ca(OH)2 and 3% MgNH4PO4 obtained by EDS probe. According to Fig. 8, one can observe that the calcium and oxygen are homogeneously distributed in the sample, since it consists mainly of calcium oxide. Magnesium is well dispersed in the material but has some agglomeration points. This may be attributed to a failure in the homogenization of magnesium oxide during the synthesis or by the crystallization of struvite. Nitrogen was eliminated in the thermal decomposition and phosphorus are not identified because its signal was overlaped by the Au peak (from the sample coating) at 2.010 eV.
Table 2 Surface area of some absorbent materials synthesized in this work. Material
Surface área
100% Ca(OH)2 100% Ca(OH)2 (calcined) 97% Ca(OH)2 + 3% NH4MgPO4 97% Ca(OH)2 + 3% NH4MgPO4 (calcined)
34 m2 g−1 8 m2 g−1 28 m2 g−1 20 m2 g−1
microtomography. 3.3. Physical characterization of materials According to Table 2, nitrogen adsorption analysis of the calcined materials at 900 °C/1h revealed that the material containing only calcium hydroxide is the one with the largest specific surface area. After calcination there is a drastic reduction of the specific surface area of the material. However, this reduction of the area is not so marked in the material synthesized with 3% of phosphate, magnesium and ammonium ions. This can be justified based on a higher resistance to sintering and the lower loss of efficiency in the CO2 absorption from the material during the regeneration cycles. Fig. 6 shows the internal structure of the material seen by a vertical section of some semi-spheres synthesized in this work that were obtained by X-ray microtomography (non-destructive technique). Table 3 shows the porosity of CaO based-semi-spheres obtained by X-ray microtomography. The internal 3D architecture analysis of the semi-spheres allows to obtain information about its porosity. One can verified that the calcined material based on calcium hydroxide modified with ammonium, phosphate and magnesium ions has a higher porosity when compared to the material synthesized with pure calcium hydroxide. In addition, the pores are opened and this allows a greater diffusion of the CO2 through the material. These pore conditions can therefore produce a greater gas–solid interaction and consequently greater absorption of CO2 after the thermal regeneration. The greater amount of open pores in the materials structure tends to connect the micropores and create more macropores (> 100 nm). This contributes to a more efficient sorption of CO2 as also observed in the work of Soleimanisalim et al. [46]. The analysis of Fig. 6 and Table 3 clearly shows the formation of these micropores. In addition, the 100% Ca (OH)2 calcined material (Fig. 6) shows cracks that tend to contribute to its mechanical embrittlement. This evidence is clearly observed by Xray microtomography (Table 3). However, the incorporation of small amount (approximately 3 %wt) of magnesium oxide and ammonium phosphate contributes significantly to the increase in mechanical resistance to impact and abrasion of the materials. In the context of Calcium Looping it is more interesting to have a mechanically resistant material with a large amount of macropores that contribute to the greater absorption of CO2. Fig. 7 shows typical scanning electron microscopy images of cross sections of the absorbent materials.
3.4. Mechanical properties of materials Table 4 shows the mechanical properties, compressive strength and friability, of the semi-spheres freshly prepared. The semi-spheres of Ca(OH)2 are those that most produce dust by friction (high friability). The pure CaO and Ca(OH)2 semi-spheres present low compressive strength. The crushing or compression force required to fracture the material is 10 N which is the lowest value measured. This means that the material can fracture manually if it is compressed. This does not happen if this compressive strength is greater than 25 N for a semi-sphere of 9 mm diameter. As one observes, the addition of magnesium oxide and di-ammonium hydrogen phosphate promotes an increase of about four times in the fracture resistance of the material. The semi-spheres with NH4MgPO4 present the best physical properties for CO2 absorption: low friability and high compressive strength. The physical properties depend on the geometry and size of the absorbent material. Therefore, we cannot directly compare our results with other studies that used materials with different geometries and sizes. In general, it is believed that 1 N of crushing force is sufficient for a material (pellets of 1–3 mm diameter) to be applied in large-scale industrial processes as previously reported [40]. The common size of the pellets used in fluidized bed reactors is 300–500 μm. However, the pellets synthesized in this work are much larger (9 mm diameter) because they are being investigated in CO2 capture with possible application in automobiles. This type of application is not allowed to generate dust and the gas pressure is close to the environment. It is also important to evaluate the mechanical properties of the material after the CO2 absorption and regeneration cycles. The thermogravimetry (TG) analysis was made with a small fraction (about 20 mg) of several crushed hemispheres. Therefore, it was not possible to perform the mechanical analysis after the tenth cycles in the TG analysis. However, CO2 absorption experiments were also performed in a tubular furnace, using the whole hemispheres. It has been demonstrated that the mechanical fragility of the absorbents is higher in the new material. The calcination produces a small reduction in the mechanical strength of the material due to the decomposition of the material, as can
Fig. 6. Images obtained by X-ray microtomography of semi-spheres of (a) 100% Ca(OH)2 calcined, (b) 97% Ca(OH)2 3% Mg(OH)2 calcined, (c) 97% Ca(OH)2 3% MgNH4PO4 calcined. 6
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Table 3 Porosity of CaO based-semi-spheres obtained by X-ray microtomography. Sample
Closed porosity (%)
Open porosity (%)
100% Ca(OH)2 calcined 97% Ca(OH)2 and 3% Mg(OH)2 calcined 97% Ca(OH)2 and 3% MgNH4PO4 calcined
0.53 0.39 0.02
43.71 46.46 69.76
(a)
(b)
(c)
5 µm
5 µm
5 µm
Fig. 7. Images obtained by scanning electron microscopy of the absorbents in natura with: (a) 100% Ca(OH)2, (b) 97% Ca(OH)2 and 3% Mg(OH)2, (c) 97% Ca(OH)2 and 3% MgNH4PO4.
Fig. 8. Distribution of the chemical elements in the calcined absorbent material containing 97% Ca(OH)2 and 3% MgNH4PO4 obtained by EDS probe.
conclusion can be made about the behaviour of the materials after the CO2 sorption cycles in fluidized bed regeneration but this deeper study was not performed and is under future investigations. The greater amount of open pores in the modified materials contribute to inhibit sintering and explain the low decrease of efficiency in CO2 sorption after several cycles when compared to CaO or Ca(OH)2 materials.
Table 4 Compressive strength and friability of the semi-spheres. Sample
Compressive strength (N)
Friability (wt.%)
100% CaO 100% Ca(OH)2 97% Ca(OH)2 + 3% Mg(OH)2 94% Ca(OH)2 + 6% Mg(OH)2 90% Ca(OH)2 + 10% Mg(OH)2 97% Ca(OH)2 + 3% NH4MgPO4 94% Ca(OH)2 + 6% NH4MgPO4 90% Ca(OH)2 + 10% NH4MgPO4
6.5 10.0 29.5 42.0 49.0 46.5 56.0 47.0
0.8 6.5 1.6 2.3 3.7 1.5 0.8 0.4
4. Conclusions Sorbent materials based on Ca(OH)2 exhibit high initial absorption of CO2 (greater than 50 %wt). The synthesis of the material was very efficient and can be controlled by the purity of the chemical inputs. The incorporation of small amount (approximately 3 %wt) of magnesium oxide and ammonium phosphate contributes significantly to the increase in mechanical resistance to impact and abrasion of the materials. In addition, this chemical modification produced a material with very developed porosity identified by X-ray microtomography with a higher specific surface area after thermal treatment at high temperature. In this way, the material becomes resistant to sintering and more efficient in long CO2 capture cycles and thermal regeneration. Finally, one can point out that these new proposed materials are of great applications for contributing to reduce the CO2 emissions and, therefore, minimizes, in principle, the green house effects.
be observed in Table 4. The 100% CaO sample shows the same behaviour of 100% calcined Ca(OH)2 sample. However, the increase in the mechanical strength after carbonation is much more pronounced than the decrease during calcination, and this can be attributed to the formation of crystalline CaCO3 as similar achievement found in ref. [48]. In addition, Tables 2 and 3 show that materials with NH4MgPO4 have a higher amount of open pores, which allows a greater permeation of the gas through the material. This material suffer less changes in the surface area after calcination, which tends to reduce embrittlement of it by large volume changes (compaction in calcination and expansion in carbonation). The better mechanical strength of the materials modified with MgO, NH3 and H3PO4 is evidenced in the freshly materials. However, no 7
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Acknowledgements
[23] Sun J, Liang C, Wang W, Liu W. Screening of naturally Al/Si-based mineral binders to modify CaO-based pellets for CO2 capture. Energy Fuels 2017. https://doi.org/ 10.1021/acs.energyfuels.7b03252. [24] Sun J, Liang C, Tong X, Guo Y, Li W, Zhao C, et al. Evaluation of high-temperature CO 2 capture performance of cellulose-templated CaO-based pellets. Fuel 2019. https://doi.org/10.1016/j.fuel.2018.11.123. [25] Sayyah M, Ito BR, Rostam-Abadi M, Lu Y, Suslick KS. CaO-based sorbents for CO2 capture prepared by ultrasonic spray pyrolysis. RSC Adv 2013. https://doi.org/10. 1039/c3ra44566f. [26] Zhao M, Shi J, Zhong X, Tian S, Blamey J, Jiang J, et al. A novel calcium looping absorbent incorporated with polymorphic spacers for hydrogen production and CO2 capture. Energy Environ Sci 2014. https://doi.org/10.1039/c4ee01281j. [27] Mutch GA, Shulda S, McCue AJ, Menart MJ, Ciobanu CV, Ngo C, et al. Carbon capture by metal oxides: unleashing the potential of the (111) facet. J Am Chem Soc 2018. https://doi.org/10.1021/jacs.8b01845. [28] Wang T, Liu J, Lackner KS, Shi X, Fang M, Luo Z. Characterization of kinetic limitations to atmospheric CO2 capture by solid sorbent. Greenh Gases Sci Technol 2016. https://doi.org/10.1002/ghg.1535. [29] Sanna A, Maroto-Valer MM. CO2 capture at high temperature using fly ash-derived sodium silicates. Ind Eng Chem Res 2016. https://doi.org/10.1021/acs.iecr. 5b04780. [30] Salaudeen SA, Acharya B, Dutta A. CaO-based CO2 sorbents: a review on screening, enhancement, cyclic stability, regeneration and kinetics modelling. J CO2 Util 2018. https://doi.org/10.1016/j.jcou.2017.11.012. [31] Valverde JM. Ca-based synthetic materials with enhanced CO2 capture efficiency. J Mater Chem A 2013. https://doi.org/10.1039/c2ta00096b. [32] Rey C, Combes C, Drouet C, Glimcher MJ. Erratum to: bone mineral: update on chemical composition and structure. Osteoporos Int 2009. https://doi.org/10. 1007/s00198-009-1063-2. [33] Walling SA, Provis JL. Magnesia-based cements: a journey of 150 years, and cements for the future? Chem Rev 2016. https://doi.org/10.1021/acs.chemrev. 5b00463. [34] Gao X, Zhang A, Li S, Sun B, Zhang L. The resistance to high temperature of magnesia phosphate cement paste containing wollastonite. Mater Struct Constr 2016. https://doi.org/10.1617/s11527-015-0729-9. [35] Fang Y, Cui P, Ding Z, Zhu JX. Properties of a magnesium phosphate cement-based fire-retardant coating containing glass fiber or glass fiber powder. Constr Build Mater 2018. https://doi.org/10.1016/j.conbuildmat.2017.12.059. [36] Frost RL, Weier ML, Erickson KL. Thermal decomposition of struvite. J Therm Anal Calorim 2004. https://doi.org/10.1023/b:jtan.0000032287.08535.b3. [37] Ugliengo P, Zicovich-Wilson CM, Tosoni S, Civalleri B. Role of dispersive interactions in layered materials: a periodic B3LYP and B3LYP-D* study of Mg(OH)2, Ca (OH)2 and kaolinite. J Mater Chem 2009. https://doi.org/10.1039/b819020h. [38] Korchef A, Saidou H, Ben Amor M. Phosphate recovery through struvite precipitation by CO2 removal: effect of magnesium, phosphate and ammonium concentrations. J Hazard Mater 2011. https://doi.org/10.1016/j.jhazmat.2010.11.045. [39] Menéndez E, Vega L, Andrade C. Use of decomposition of portlandite in concrete fire as indicator of temperature progression into the material: application to fireaffected builds. J. Therm. Anal. Calorim. 2012. https://doi.org/10.1007/s10973011-2159-4. [40] Hu Y, Liu X, Zhou Z, Liu W, Xu M. Pelletization of MgO-based sorbents for intermediate temperature CO2 capture. Fuel 2017. https://doi.org/10.1016/j.fuel.2016. 09.066. [41] Jin S, Ko KJ, Lee CH. Direct formation of hierarchically porous MgO-based sorbent bead for enhanced CO2 capture at intermediate temperatures. Chem Eng J 2019. https://doi.org/10.1016/j.cej.2019.04.020. [42] Vu AT, Park Y, Jeon PR, Lee CH. Mesoporous MgO sorbent promoted with KNO3 for CO2 capture at intermediate temperatures. Chem Eng J 2014. https://doi.org/10. 1016/j.cej.2014.07.088. [43] Dong W, Chen X, Yu F, Wu Y. Na2CO3/MgO/Al2O3 solid sorbents for low-temperature CO2 capture. Energy Fuels 2015. https://doi.org/10.1021/ef502400s. [44] Materic V, Smedley SI. High temperature carbonation of Ca(OH)2. Ind Eng Chem Res 2011. https://doi.org/10.1021/ie200367w. [45] Mutch GA, Anderson JA, Vega-Maza D. Surface and bulk carbonate formation in calcium oxide during CO2 capture. Appl Energy 2017. https://doi.org/10.1016/j. apenergy.2017.05.130. [46] Soleimanisalim AH, Sedghkerdar MH, Karami D, Mahinpey N. Pelletizing and coating of synthetic zirconia stabilized calcium-based sorbents for application in calcium looping CO2 capture. Ind Eng Chem Res 2017. https://doi.org/10.1021/ acs.iecr.6b04771. [47] Erans M, Manovic V, Anthony EJ. Calcium looping sorbents for CO2 capture. Appl Energy 2016. https://doi.org/10.1016/j.apenergy.2016.07.074. [48] Yan X, Li Y, Ma X, Zhao J, Wang Z, Liu H. CO 2 capture by a novel CaO/MgO sorbent fabricated from industrial waste and dolomite under calcium looping conditions. New J Chem 2019. https://doi.org/10.1039/c8nj06257a. [49] Montes-Hernandez G, Pommerol A, Renard F, Beck P, Quirico E, Brissaud O. In situ kinetic measurements of gas-solid carbonation of Ca(OH)2 by using an infrared microscope coupled to a reaction cell. Chem Eng J 2010. https://doi.org/10.1016/j. cej.2010.04.041.
The authors would like to acknowledge to Petrobras S.A., Fiat Chrysler Automobiles (FCA), and FAPEMIG for financial support and also the Center of Microscopy at the Universidade Federal de Minas Gerais (http://www.microscopia.ufmg.br) for providing the equipment and technical support for experiments involving electron microscopy, and Marivalda Magalhães Pereira for the X-ray microtomography images. References [1] Intergovernmental Panel on Climate Change. Climate Change 2014 Synthesis Report – IPCC; 2014. doi: 10.1017/CBO9781107415324. [2] Bui M, Adjiman CS, Bardow A, Anthony EJ, Boston A, Brown S, et al. Carbon capture and storage (CCS): the way forward. Energy Environ Sci 2018. https://doi. org/10.1039/c7ee02342a. [3] Hornberger M, Spörl R, Scheffknecht G. Calcium looping for CO2 capture in cement plants – pilot scale test. Energy Procedia 2017. https://doi.org/10.1016/j.egypro. 2017.03.1754. [4] Theeyattuparampil VV, Zarzour OA, Koukouzas N, Vidican G, Al-Saleh Y, Katsimpardi I. Carbon capture and storage: state of play, challenges and opportunities for the GCC countries. Int J Energy Sect Manag 2013. https://doi.org/10. 1108/IJESM-04-2013-0010. [5] Ortiz C, Chacartegui R, Valverde JM, Becerra JA, Perez-Maqueda LA. A new model of the carbonator reactor in the calcium looping technology for post-combustion CO2 capture. Fuel 2015. https://doi.org/10.1016/j.fuel.2015.07.095. [6] Ortiz C, Chacartegui R, Valverde JM, Becerra JA. A new integration model of the calcium looping technology into coal fired power plants for CO2 capture. Appl Energy 2016. https://doi.org/10.1016/j.apenergy.2016.02.050. [7] Valverde JM, Sanchez-Jimenez PE, Perez-Maqueda LA. Calcium-looping for postcombustion CO2 capture. On the adverse effect of sorbent regeneration under CO2. Appl Energy 2014. https://doi.org/10.1016/j.apenergy.2014.03.081. [8] Radfarnia HR, Sayari A. A highly efficient CaO-based CO2 sorbent prepared by a citrate-assisted sol-gel technique. Chem Eng J 2015. https://doi.org/10.1016/j.cej. 2014.09.074. [9] Hu Y, Liu W, Peng Y, Yang Y, Sun J, Chen H, et al. One-step synthesis of highly efficient CaO-based CO2 sorbent pellets via gel-casting technique. Fuel Process Technol 2017. https://doi.org/10.1016/j.fuproc.2017.02.016. [10] Zhu D, Choi SR, Miller RA. Development and thermal fatigue testing of ceramic thermal barrier coatings. Surf Coatings Technol 2004. https://doi.org/10.1016/j. surfcoat.2004.08.017. [11] Kierzkowska AM, Pacciani R, Müller CR. CaO-based CO2 sorbents: from fundamentals to the development of new, highly effective materials. ChemSusChem 2013. https://doi.org/10.1002/cssc.201300178. [12] Manovic V, Wu Y, He I, Anthony EJ. Spray water reactivation/pelletization of spent CaO-based sorbent from calcium looping cycles. Environ Sci Technol 2012. https:// doi.org/10.1021/es303252j. [13] Qin C, Yin J, An H, Liu W, Feng B. Performance of extruded particles from calcium hydroxide and cement for CO 2 capture. Energy Fuels 2012. https://doi.org/10. 1021/ef201141z. [14] Sun J, Liu W, Hu Y, Wu J, Li M, Yang X, et al. Enhanced performance of extrudedspheronized carbide slag pellets for high temperature CO2 capture. Chem Eng J 2016. https://doi.org/10.1016/j.cej.2015.10.026. [15] Li H, Qu M, Yang Y, Hu Y, Liu W. One-step synthesis of spherical CaO pellets via novel graphite-casting method for cyclic CO2 capture. Chem Eng J 2019. https:// doi.org/10.1016/j.cej.2019.05.214. [16] Ridha FN, Manovic V, Anthony EJ, MacChi A. The morphology of limestone-based pellets prepared with kaolin-based binders. Mater Chem Phys 2013. https://doi. org/10.1016/j.matchemphys.2012.11.007. [17] Kierzkowska AM, Poulikakos LV, Broda M, Müller CR. Synthesis of calcium-based, Al 2 O 3 -stabilized sorbents for CO 2 capture using a co-precipitation technique. Int J Greenh Gas Control 2013. https://doi.org/10.1016/j.ijggc.2013.01.046. [18] Dong W, Chen X, Wu Y. TiO2-doped K2CO3/Al2O 3 sorbents for CO2 capture. Energy Fuels 2014. https://doi.org/10.1021/ef500133e. [19] Kumar S, Saxena SK. A comparative study of CO2 sorption properties for different oxides. Mater Renew Sustain Energy 2014. https://doi.org/10.1007/s40243-0140030-9. [20] Zhang Y, Gao Y, Pfeiffer H, Louis B, Sun L, O’Hare D, et al. Recent advances in lithium containing ceramic based sorbents for high-temperature CO2 capture. J Mater Chem A 2019. https://doi.org/10.1039/c8ta08932a. [21] Hu Y, Liu W, Yang Y, Qu M, Li H. CO2 capture by Li4SiO4 sorbents and their applications: Current developments and new trends. Chem Eng J 2019. https://doi. org/10.1016/j.cej.2018.11.128. [22] Broda M, Müller CR. Synthesis of highly efficient, Ca-based, Al2O3-stabilized, carbon gel-templated CO 2 sorbents. Adv Mater 2012. https://doi.org/10.1002/ adma.201104787.
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Full Length Article
CO2 capture performance and mechanical properties of Ca(OH)2-based sorbent modified with MgO and (NH4)2HPO4 for Calcium Looping cycle
T
Plínio César de Carvalho Pintoa, Geison Voga Pereirab, Leonardo Schiavo de Rezendeb, ⁎ Flávia C.C. Mourab, Jadson Cláudio Belchiorb, a b
Instituto Federal do Espírito Santo, Campus Vila Velha, Av. Ministro Salgado Filho 1000, Vila Velha 29106-010, ES, Brazil Departamento de Química, Universidade Federal de Minas Gerais, Av. Antônio Carlos 6627, Belo Horizonte 31270-901, MG, Brazil
ARTICLE INFO
ABSTRACT
Keywords: Sorbent Calcium Looping Calcium hydroxide Struvite CO2 capture
The present work deals with an efficient synthesis of calcium hydroxide-based materials modified with magnesium oxide and ammonium phosphate for Calcium Looping technology in Post-combustion CO2 capture. This modification produced 3 %wt of struvite, MgNH4PO4·6H2O, and hydroxyapatite Ca10(PO4)6(OH)2, in the material. These minerals are very hard, which contributes to increase the mechanical strength of the materials. The modified materials are four times more resistant to impact and abrasion than materials containing pure calcium hydroxide freshly prepared, and therefore, can generate less fractures and dusts. In addition, struvite and hydroxyapatite can create open pores and channels into the sorbent structure when they release water or ammonia by thermal decomposition above 200 °C identified by X-ray microtomography. Due to this porous structure the materials are more resistant to sintering during absorption/regeneration cycles. The new developed absorbent are harder, less brittle, generate less frictional dust, and are more resistant to high temperature sintering (400–700 °C) compared to pure Ca(OH)2 based materials. The results showed that calcium hydroxide modified materials can be considered highly efficiency to CO2 absorption, with particular achievement of greater than 50%wt.
1. Introduction The scientific community believes that the increase in the concentration of atmospheric CO2 has been producing the global warming through the intensification of the greenhouse effect. As a result, climate changes can lead to many environmental disasters such as drought, desertification, hurricanes and tornadoes as well as floods, with many impacts on flora, fauna and human society [1]. Various human activities that use fossil fuels for energy generation contribute to these problems. For example, industries, transportation, and electric power generation sectors are potential source of emissions of greenhouse gases, mainly CO2. Thus, mitigation of climate change occurs through the adoption of clean technologies that can use renewable energy sources or can control the emission of greenhouse gases. Among other possibilities, in order to reduce CO2 emissions one can considerer, for example, the technology known as Calcium Looping. Generally, Calcium Looping applied in industries is associated with technologies called “Carbon Capture and Storage” (CCS). The latter is an approach that can capture (separation) the CO2 from other substances emitted by a source of pollution. Following this stage one can ⁎
provide its compression and transportation to a storage site where the gas is injected into appropriate geological formations. This procedure is well established and there are already technologies applied in power plants, cement plants, refineries and other industrial sectors [2–4]. Calcium Looping technology is based on the absorption of CO2 in calcium oxide particles for the production of calcium carbonate in a fluidized bed reactor (carbonator) operating at atmospheric pressure and temperature of about 650 °C. Lime stone, which is the precursor material for calcium oxide, is abundant in nature and has been considered as a very low-cost input. Partially carbonated solid particles after short residence periods (a few minutes) are circulated in a second gas–solid reactor (calciner), where CaO is regenerated by calcination between 900 °C and 1000 °C at atmospheric pressure. The gas stream of the highly concentrated calciner in CO2 is then compressed for geological storage (Fig. 1) [5–7]. The efficiency of Calcium Looping technology in the CO2 capture is strongly influenced by the characteristics of the absorbent material such as: chemical absorption of CO2, resistance to embrittlement due to thermal fatigue, sintering resistance and mechanical resistance to friction and impact [7,8].
Corresponding author. E-mail address: [email protected] (J. Cláudio Belchior).
https://doi.org/10.1016/j.fuel.2019.115924 Received 7 May 2019; Received in revised form 26 July 2019; Accepted 29 July 2019 0016-2361/ © 2019 Published by Elsevier Ltd.
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strong turbulence, which favours the solid-gas reactions, but damages the absorbent material [7]. Therefore, some strategies are adopted to improve the performance of absorbent materials. Additives such as kaolin, bentonite and some other clays act as binders and aid in the production of pellets. Unfortunately, these substances minimize the absorption capacity of CO2 and do not offer an increase in the mechanical resistance of the pellets [16,17]. Kaolin and bentonite have covalent chemical bonds with low affinity for the calcium oxide ionic bonds. Calcium aluminate cements increase the hardness of sorbents [17], aluminosilicates can generate greater resistance to friction [18]. In particular, calcium and magnesium phosphates have also recently been used as binders promoting higher resistance to friction, higher hardness and resistance to sintering, but do not contribute to absorb CO2 [19]. On the other hand, oxides of magnesium, iron oxide, and ceramics of alkali metals have also been tested as materials with high level of CO2 absorption. In addition, they contribute to the increase in mechanical strength and resistance to sintering, with an advantage of absorption of CO2 at temperatures between 200 °C and 500 °C [18,19]. Calcium and lithium ceramics are two common CO2 sorbents. Lithium ceramics offer thermal stability, present small volume change during CO2 sorption/desorption process, and low regeneration energy compared to CaO. However, lithium raw materials are less abundant and more expensive. Furthermore, lithium materials show a CO2 absorption kinetics and CO2 capture efficiency lower compared to CaO-based sorbents [20,21]. Other additives and different strategies are applied to minimize the effects of the loss of CO2 absorption of the absorbent material. Reagents such as organic acids, surfactants, chelating agents, coals, biomass, polymers are used to create pores. These substances are added and subsequently, with appropriate treatment, usually thermal treatment, are removed from the pellets creating additional pores [22–24]. The pores tend to increase the surface area and allow greater diffusion and interaction of CO2 with the active sites of the absorbent material [16]. The most commonly material used is calcium oxide for the synthesis processes that involves calcination of calcium hydroxide and calcination of calcium carbonate in high temperature furnaces [18]. However, other methods of efficient synthesis in laboratory scale have been used such as sol–gel precipitation, wet impregnation, chemical vapor deposition, spray-dryer, pyrolysis by ultrasonic spray, the use of chemical compounds as spacers, and control of synthesis for exposition of more active crystalline basic (O2–) sites [8,25–27]. The geometry and size of the absorbers are also important parameters that influence CO2 capture. The larger the size, the lower the absorption capacity, due to a limitation of the diffusional process. And spherical bodies are more stable and suffer less mechanical wear, especially in fluidized bed reactors [19,25,28]. This work presents a new calcium rich material with semi-sphere geometry applied in CO2 capture between 400 and 700 °C. The proposed material has higher mechanical strength, higher resistance to sintering and high level of absorption of CO2 compared to pure calcium oxide. The approach with these characteristics is built by incorporating small amounts of phosphate, magnesium and ammonium ions in the materials. A new method of sorbents synthesis based on Ca(OH)2 is also proposed to form hydroxyapatite, Ca10(PO4)6(OH)2, which provides greater hardness to the sorbent and struvite, MgNH4PO4·6H2O, which form more pores during its decomposition and release of ammonia and water with the Calcium Looping reaction temperature. Therefore, there is a significant change in the chemical structure of the materials, which favours the CO2 capture process with high level of absorption.
Fig. 1. Cyclic process of CO2 absorption and regeneration of CaO-based absorbent.
The chemical absorption is related to the molar absorptivity of CO2, which is an intrinsic property of each material, and is expressed by the ratio CO2 absorbed mass/absorbent material mass. Pure calcium oxide has a theoretical capture efficiency of 0.786 gCO2/g through the absorbent material used. Materials considered highly efficient of CO2 absorption have demonstrated to produce greater than 50%wt. This means that such materials have to absorb more than 0.5 gCO2/g of absorbent material [8,9]. Thermal fatigue and sintering are processes that involve the transfer of heat in materials. Thermal fatigue occurs when the material is subjected to several cycles of heating and cooling, suffering successive dilations and volumetric contractions that weaken the structure, due to the generation of several cycles of cracks and fractures. Ceramic materials based on calcium oxide do not support successive dimensional variations because they do not present enough elastic behaviour. Sintering is a process of grain agglutination at temperatures below melting temperature and changes the microstructure of a solid material. In Calcium Looping, such a process occurs during the regeneration step, which reaches the temperature of 900 °C–1000 °C. Sintering impairs the CO2 capture process because it leads to the reduction of pore numbers and their sizes, diffusion channels and surface area of the absorbent material. The Sintering process is also more evident in pulverized material and can be minimized through pelletizing [10,11]. The Pelletizing procedure is performed by three main methods well decribed in the litetarure namely rotation, extrusion and extrusion-spheronization. The rotation is characterized by a vessel that, in a controlled conditions, injects a spray of water while stirring the material by a pair of rotor blades and a chopper, forming pellets by the agglutination of the powder particles with water [12]. Extrusion promotes the mixing and compaction of the raw materials through helical pistons that rotate and force the passage of the material through cylinders with holes at the end of the path that define the shape of the pellets. Finally, these wires are cut to define the final pellet size [13]. Extrusion and spheronization is the combination of the extrusion technique that forms yarns of the material with the spheronization technique to cut the yarns and give the rounded finish to pellet formation in the form of spheres [14]. The graphite casting granulation is a new pelletization technique based on injecting a slurry of the precursor material into a spherical graphite powder mold, followed by drying the material at 70° C and calcination at 600 °C in air to remove the graphite dust. This technique requires less mechanical force in pellet production compared to previous techniques. This tends to reduce the material compaction and favor the CO2 capture process [15]. More general industrial applications are the use of pellets in fluidized bed reactors but usually one can observe drawbacks. This is basically due to the dynamics imposed on the absorbent bodies. The latter promote interactions that are determinant to wear them, causing them to lose mass, generating dust or suffering fractures. The gas flow in the reactors is intense enough that the absorber bodies enter in a regime of
2. Materials and methods 2.1. Reagents All the chemical reactions were performed in deionized water (type III). The CaO was purchased from Belocal-Lhoist Brazil (99% purity). 2
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Magnesium oxide was purchased from Neon commercial (99% purity). Di-ammonium hydrogen phosphate was purchased from Vetec-SigmaAldrich Company and used as received (100% purity). The CO2 and N2 gases were purchased from Linde (analytical grade).
lid under rotation of 25 rpm for 5 min in a Friability Tester Ethik Technology. 2.4. CO2 absorption performance of the Ca(OH)2-based sorbent modified with MgO and (NH4)2HPO4
2.2. Synthesis of materials
The CO2 capture was realized by using a thermogravimetric analysis (TG) performed on PERSEUS® STA 449 F3 JUPITER® NETZSCH equipment with temperature programme: 25 °C to 600 °C with a heating rate of 50 °C min−1, 600 °C–650 °C with a heating ratio of 5 °C min−1, and then an isotherm at 650 °C for 30 min using a flow of 71% CO2 (20 mL min−1 N2 + 50 mL min−1 CO2).
Calcium oxide powder was hydrated in a ratio of 1:1.5 (CaO:H2O) to form a mass of calcium hydroxide. The calcium hydroxide mass was added manually to the 10 mm diameter semi-spheres silicone templates. The calcium hydroxide semi-spheres (10 mm diameter) were dried in an oven at 80 °C for 1 h and they were packed in sealed plastic bags and the sample was named 100%Ca(OH)2. The production of calcium oxide semi-spheres involves a thermal treatment step (600 °C for 1 h under air atmosphere) of the calcium hydroxide semi-spheres prior to packaging generating the sample 100%CaO. Different materials were obtained by partially replacing the calcium oxide to magnesium oxide (3, 6 and 10 %wt) resulting in three new materials: 97%Ca(OH)2 3%Mg(OH)2, 94%Ca(OH)2 6%Mg(OH)2, 90%Ca(OH)2 10%Mg(OH)2. In addition, the water used during the hydrating step was partially replaced by a solution of di-ammonium hydrogen phosphate to react stoichiometrically with MgO and produce struvite (NH4MgPO4·6H2O), generating, finally, other three different materials named: 97%Ca(OH)2 3%MgNH4PO4, 94%Ca(OH)2 6% MgNH4PO4, and 90%Ca(OH)2 10%MgNH4PO4.
2.5. Reuse of the material in several cycles of CO2 absorption and thermal regeneration The regeneration capacity of the materials was studied by thermogravimetric analysis using ten cycles of CO2 absorption and regeneration of the materials by thermal decomposition. In each cycle an absorption of CO2 at 650 °C (50 mL min−1 CO2 + 20 mL min−1 N2) was carried out for 30 min and regeneration in N2 (70 mL min−1) at 900 °C for 5 min. The heating and cooling ramp between 650 °C and 900 °C was performed at a rate of 20 °C min−1 in N2. 3. Results and discussion
2.3. Characterization of materials
Two materials were initially prepared in a semi-sphere morphology. Calcium hydroxide was synthesized by the reaction of calcium oxide with water, 100%Ca(OH)2, (Eq. (1)), and calcium oxide obtained by thermal decomposition of the calcium hydroxide semi-sphere at 600 °C for 1 h, 100%CaO, (Eq. (2)). Furthermore, a modification in the chemical composition of these semi-spheres was proposed through the partial substitution of CaO by MgO in different amounts (3, 6, and 10 % wt) since magnesium contributes to increase the resistance to sintering of the material considering 97%Ca(OH)2 3%Mg(OH)2, 94%Ca(OH)2 6% Mg(OH)2, 90%Ca(OH)2 10%Mg(OH)2, (Eqs. (3) and (4)) [25,28–31]. Other modifications of the chemical composition were established by the incorporation of MgO and di-ammonium hydrogen phosphate ((NH4)2HPO4) (in 3, 6, and 10 %wt), also considering 97%Ca(OH)2 3% MgNH4PO4, 94%Ca(OH)2 6%MgNH4PO4, and 90%Ca(OH)2 10% MgNH4PO4, to reach different properties: i) increase the mechanical resistance of the material and avoid material particles losses in the reactor by friction and impact; ii) increase the resistance to sintering to enable a greater reutilisation of the material in several cycles of CO2 capture. As expected, it increases the mechanical strength that comes from the formation of hydroxyapatite (Ca10(PO4)6(OH)2) (Eq. (5)). The latter is the main mineral constituent of animal bones [32], and MgO and di-ammonium hydrogen phosphate contributes to the formation of struvite (NH4MgPO4·6H2O) (Eq. (6)), which is the basis of magnesium phosphate cements. Phosphate-bonded refractories are also in widespread use, exploiting the property of cold setting to form products that are stable at high temperatures, i. e. 600–1000 °C [33–35].
The materials were characterized by different techniques. XRD patterns were collected in a Siemens D5000 instrument using a Ni-filtered CuKα radiation (k = 1.5418A°) and a graphite monochromator in the diffracted beam. A scan rate of 1° min−1 was applied to record a pattern in the 2θ range of 20–80°. The XRD lines were encountered in JCPDS (Joint Committee on Powder Diffraction Standard). Infrared spectra were obtained from a Perkin-Elmer Spectrum RX FT-IR spectrometer, using KBr disks in the 4000–400 cm−1 region with 64 scans and 4 cm−1 of spectral resolution. Thermogravimetric analysis (TG) and differential scanning calorimetry (DSC) were carried out by a PERSEUS® STA 449 F3 JUPITER NETZSCH at temperatures ranging from 25 °C to 900 °C with a heating rate of 20 °C min−1 in nitrogen flux of 70 mL min−1. The morphology of the materials was studied by N2 adsorption (−196 °C) in a Quantachrome IQ2 system. The surface area was calculated using the BET equation. The total pore volume was estimated from the amount of nitrogen adsorbed at P/P0 = 0.95. Scanning electron microscopies (FEG QUANTA 200 FEI) were performed using an accelerating voltage of 20 kV. The samples were coated with Au by the sputtering method to make them conductive. Images were acquired using magnification of 5000× for the evaluation of semi-spheres microstructures. The X-ray microtomographies were obtained by a microCT scanner (SkyScan 1174, Bruker) using a 50 kV voltage source, 800 mA source current and 8.05 mm pixel size. The light image was projected with an objective magnifying lens onto a 1304 × 1024 charge coupled device (CCD) detector array. No filters were used. The samples were attached to a stage that rotated 180° with images acquired every 0.7°. The acquired shadow projections were further reconstructed into 2D slices using the NRecon software interface, and CT Analyser software (Skyscan, Bruker micro-CT) was used for 3D analysis and 3D surface rendering. CT Vol 2.3.1.0 version software (Skyscan, Bruker micro-CT) was used for 3D volumetric visualization. The mechanical properties were analysed by a compressive strength that is the force required to fracture a compressed specimen, and It was determined by the arithmetic mean of ten 10 mm diameter samples submitted to compression in a portable digital Hardness Tester Ethik Technology. Friability is the percentage of dust mass generated by friction and the impact of the pellets in an acrylic cylindrical plate with
Ca(OH) 2 (s)
(1)
CaO(s) + H2 O(g)
(2)
CaO(s) + H2 O(l) Ca(OH) 2 (s)
Mg(OH) 2 (s)
(3)
MgO(s) + H2 O(g)
(4)
MgO(s) + H2 O(l) Mg(OH) 2 (s)
10CaO(s) + 6(NH 4 )2 HPO4 (aq)
Ca10 (PO4 )6 (OH) 2 (s) + 8H2 O(l) + 6NH3 (aq)
(5) MgO(s) + 5 H2 O(l) + (NH 4 ) 2 HPO4 (aq)
NH 4 MgPO4 ·6H2 O(s) + NH3 (aq)
(6)
The struvite (NH4MgPO4·6H2O) can contribute to the formation of a structure with higher mechanical and thermal resistance. In addition, it generates a more porous structure, because of the release of ammonia 3
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Fig. 3. Infrared spectra of pure CaO and Ca(OH)2 semi-spheres and semispheres modified with 10% Mg(OH)2 and 10% MgNH4PO4. Fig. 2. X-ray diffraction patterns of pure CaO and Ca(OH)2 semi-spheres and semi-spheres modified with MgO and (NH4)2HPO4.
As can be seen in Fig. 4(a), the CaO and Ca(OH)2 semi-spheres present two events of weight loss in the TG curve, in N2 atmosphere. The decomposition of Ca(OH)2 between 300 °C and 500 °C, forming CaO and H2O (Eq. (2)), and the decomposition of CaCO3 between 600 °C and 800 °C (Eq. (8)) [39].
gas and steam of water by thermal decomposition at 100 °C (Eq. (7)) [36].
NH 4 MgPO4 ·6H2 O(s)
MgHPO4 (s) + 6 H2 O(g) + NH3 (g)
(7)
CaCO3 (s)
CaO(s) + CO2 (g)
(8)
The thermal behaviour of the semi-spheres produced with CaO e MgO (Ca(OH)2/Mg(OH)2) are similar to the CaO and Ca(OH)2 semispheres, since the Mg(OH)2 decomposes together with Ca(OH)2 (Eq. (4)). The semi-spheres produced with CaO, MgO and (NH4)2HPO4 (Ca (OH)2/MgNH4PO4) show the weight losses of the Ca(OH)2 and CaCO3 decomposition, but also a weight loss below 400 °C from the water and ammonia release (Eq. (7)). Assuming that all MgO added in the synthesis was converted to Mg(OH)2 or NH4MgPO4·6H2O and based on the decomposition of Ca(OH)2 and CaCO3, one can calculate the chemical composition of the semi-spheres, and this is shown according to Table 1. It is also observed (Fig. 4) that the semi-spheres with higher concentration of CaO or Ca(OH)2 are those that absorb the greater amount of CO2 when heated in CO2 atmosphere (Fig. 4b), according to Eqs. (9) and (10). However, calcium hydroxide only absorbs CO2 above 300 °C when it begins to decompose thermally and releases water (Eq. (2)). The total CO2 absorption of the calcium hydroxide should take into account the mass change in CO2 atmosphere with the addition of the amount of water released in N2 atmosphere (32%wt + 22%wt = 54% wt).
3.1. Characterization of materials Fig. 2 shows the X-ray diffractograms of pure CaO and Ca(OH)2 semi-spheres and semi-spheres modified with 3% or 6% of Mg(OH)2 and 3% or 6% of NH4MgPO4. The semi-sphere of calcium hydroxide was identified portlandite as crystalline phase by X-ray diffraction analysis. The semi-sphere of calcium oxide consists mainly of lime, CaO, but with a small amount of portlandite that can be formed in the slow cooling in the muffle furnace by the absorption of water. The semi-sphere with 3% NH4MgPO4 showed the presence of portlandite, but also calcite, CaCO3. Also, hydroxyapatite, Ca10(PO4)6(OH)2, signals have been identified in this sample. The reaction of MgO and CaO with (NH4)2HPO4 in solution (Eq. (5) and (6)) occurs with the formation of ammonia, which can promote the elevation of pH and the formation of hydroxyapatite and calcite by the absorption of CO2 from the air. The semi-spheres that were synthesized with 3 to 10% MgO formed portlandite as main phase. No signs of brucite, Mg(OH)2, were identified due to the low concentration of MgO in the material. Struvite (NH4MgPO4·6H2O) was not identified by the X-ray diffraction analysis of the semi-spheres synthesized with 3 to 10% MgNH4PO4, due to the low concentration in the material. Amorphous impurities in the materials are not possible to be observed in the diffractograms. Fig. 3 shows the spectra obtained in the infrared region of pure CaO and Ca(OH)2 semi-spheres and semi-spheres modified with 10% Mg (OH)2 and 10% NH4MgPO4. The infrared spectra of pure CaO and Ca(OH)2 semi-spheres show that both materials present calcium hydroxide (portlandite), including the calcium oxide semi-sphere that can absorb water during the cooling after calcination step. The portlandite shows a characteristic sharp band at 3640 cm−1 related to O–H stretching bond to Ca. In the semi-sphere modified with MgO the presence of brucite was suggested through the thin and intense band in 3698 cm−1 due to O–H stretching bond to Mg [37]. In the semi-sphere of 97%Ca(OH)2 3%MgNH4PO4, 94%Ca(OH)2 6%MgNH4PO4, and 90%Ca(OH)2 10%MgNH4PO4, no signal is observed that suggests the presence of brucite. There is a strong indication that the magnesium has been converted to ammonium and magnesium phosphate, which is a more thermodynamically stable phase. However, due to the low concentration of the Mg in the material, it was not detected by infrared spectroscopy [38].
CaO(s) + CO2 (g)
CaCO3 (s)
Ca(OH) 2 (s) + CO2 (g)
CaCO3 (s) + H2 O(g)
(9) (10)
According to the stoichiometry of the reaction, (Eq. (9)), the total absorption of CO2 by calcium oxide can reach a maximum of 78 %wt. However, this value is not experimentally obtained under normal conditions, because CaO is very reactive and absorbs water and atmospheric CO2 quickly. In order to reach CO2 absorption values above 60 %wt, it is necessary to carry out the treatment of the reagent and to promote the synthesis of the material in an inert atmosphere of N2 or Ar. Under normal conditions the maximum CO2 absorption varies from 55 %wt to 63 %wt [31]. It has been experimentally found that CaObased materials do not absorb more than 35 %wt of CO2 under the following conditions: i) an impure lime, CaO; and (ii) inadequate storage of the reagent or synthesis of the product (in contact with atmospheric air). The partial substitution of CaO by MgO without any special treatment in Ca(OH)2-based absorbers contributes to the reduction in the CO2 amount captured, according to Fig. 4b. The higher the CaO substitution per MgO the greater the reduction of CO2 capture efficiency because the conversion of MgO to MgCO3. This conversion is 4
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Fig. 4. TG curves of Ca(OH)2-based semi-spheres modified with MgO and (NH4)2HPO4 performed in different atmospheres: (a) N2, and (b) CO2 (25 °C to 600 °C with a heating rate of 50 °C min−1, 600 °C to 650 °C with a heating ratio of 5 °C min−1, and then an isotherm at 650 °C for 30 min).
thermodynamically favoured below 300 °C. Above 500 °C the inverse decomposition reaction of magnesite is favoured. In addition, the kinetics of CO2 absorption by MgO is slow and needs to be catalysed by water vapour and alkali metals such as sodium and potassium [40–43]. 3.2. CO2 capture studies Fig. 5 shows the CO2 absorption efficiency of the proposed synthetized materials in several reuse cycles after thermal decomposition at 900 °C through thermogravimetry. It is observed (Fig. 5) that the semi-spheres with 100 %wt CaO, 100 %wt Ca(OH)2 and 97 %wt Ca(OH)2 have an initial absorption of CO2 above 45 %wt. This value (52%wt) for Ca(OH)2 also includes the loss of water above 300 °C in the 1st cycle that corresponds to approximately 22 %wt (Fig. 4). The carbonation mechanism of Ca(OH)2 at high temperatures appears to be complex, but it has already been shown that water catalyses the carbonation reaction and promotes a higher sorption of CO2 by the materials based on CaO [44,45]. The CO2 absorption efficiency of pure calcium materials greatly reduces when absorption and regeneration are repeated several times [28–30,46]. The semispheres with the mixture of Ca(OH)2 and Mg(OH)2 present lower initial CO2 uptake than the pure Ca(OH)2 semi-spheres. The increase in magnesium hydroxide concentration in the semi-sphere reduces the initial absorption of CO2. However, the presence of the magnesium ion in the semi-sphere structure reduces the loss of efficiency in the absorption of CO2 from the material in several cycles of reuse as also previously verified [47]. Magnesium oxide does not react at high temperature with CaO, and then it contributes to inhibit the sintering of the material during the regeneration cycles of Calcium Looping [48].
Fig. 5. Efficiency of CO2 absorption of materials in several cycles of absorption (650 °C) and regeneration (900 °C).
We observed that the inhibition of sintering during regeneration by thermal decomposition at 900 °C is even more pronounced when the magnesium ion, ammonium ion and the phosphate ion are combined in the material structure. The material with the best CO2 absorption efficiency in several reuse cycles consists of 97 %wt Ca(OH)2 and 3 %wt NH4MgPO4. According to the literature, the struvite, NH4MgPO4·6H2O, will be thermally decomposed to release ammonia, water and increase the porosity of the material (Eq. (7)) [33]. This hypothesis was studied by N2 adsorption analysis, scanning electron microscopy and X-ray
Table 1 Calculated chemical composition of CaO and Ca(OH)2 semi-spheres modified with MgO and (NH4)2HPO4. Expected chemical composition
100% CaO 100% Ca(OH)2 97% Ca(OH)2 3% Mg(OH)2 94% Ca(OH)2 6% Mg(OH)2 90% Ca(OH)2 10% Mg(OH)2 97% Ca(OH)2 3% NH4MgPO4·6H2O 94% Ca(OH)2 6% NH4MgPO4·6H2O 90% Ca(OH)210% NH4MgPO4·6H2O
Calculated chemical composition (wt%) CaO
Ca(OH)2
CaCO3
Mg(OH)2
NH4MgPO4·6H2O
73.0 4.6 – – – 2.3 4.9 8.7
21.2 87.0 87.3 87.3 83.9 85.4 80.7 72.4
5.8 8.4 9.7 6.7 6.1 9.3 8.4 8.9
– – 3.0 6.0 10.0 – – –
– – – – – 3.0 6.0 10.0
5
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The material containing 100% Ca(OH)2 is characterized by nanoparticles (similar to sheet forms) forming micrometric aggregates [49]. The other materials have smaller particles with the appearance of irregular flakes (Fig. 7). Fig. 8 shows the distribution of the chemical elements in the calcined absorbent material containing 97% Ca(OH)2 and 3% MgNH4PO4 obtained by EDS probe. According to Fig. 8, one can observe that the calcium and oxygen are homogeneously distributed in the sample, since it consists mainly of calcium oxide. Magnesium is well dispersed in the material but has some agglomeration points. This may be attributed to a failure in the homogenization of magnesium oxide during the synthesis or by the crystallization of struvite. Nitrogen was eliminated in the thermal decomposition and phosphorus are not identified because its signal was overlaped by the Au peak (from the sample coating) at 2.010 eV.
Table 2 Surface area of some absorbent materials synthesized in this work. Material
Surface área
100% Ca(OH)2 100% Ca(OH)2 (calcined) 97% Ca(OH)2 + 3% NH4MgPO4 97% Ca(OH)2 + 3% NH4MgPO4 (calcined)
34 m2 g−1 8 m2 g−1 28 m2 g−1 20 m2 g−1
microtomography. 3.3. Physical characterization of materials According to Table 2, nitrogen adsorption analysis of the calcined materials at 900 °C/1h revealed that the material containing only calcium hydroxide is the one with the largest specific surface area. After calcination there is a drastic reduction of the specific surface area of the material. However, this reduction of the area is not so marked in the material synthesized with 3% of phosphate, magnesium and ammonium ions. This can be justified based on a higher resistance to sintering and the lower loss of efficiency in the CO2 absorption from the material during the regeneration cycles. Fig. 6 shows the internal structure of the material seen by a vertical section of some semi-spheres synthesized in this work that were obtained by X-ray microtomography (non-destructive technique). Table 3 shows the porosity of CaO based-semi-spheres obtained by X-ray microtomography. The internal 3D architecture analysis of the semi-spheres allows to obtain information about its porosity. One can verified that the calcined material based on calcium hydroxide modified with ammonium, phosphate and magnesium ions has a higher porosity when compared to the material synthesized with pure calcium hydroxide. In addition, the pores are opened and this allows a greater diffusion of the CO2 through the material. These pore conditions can therefore produce a greater gas–solid interaction and consequently greater absorption of CO2 after the thermal regeneration. The greater amount of open pores in the materials structure tends to connect the micropores and create more macropores (> 100 nm). This contributes to a more efficient sorption of CO2 as also observed in the work of Soleimanisalim et al. [46]. The analysis of Fig. 6 and Table 3 clearly shows the formation of these micropores. In addition, the 100% Ca (OH)2 calcined material (Fig. 6) shows cracks that tend to contribute to its mechanical embrittlement. This evidence is clearly observed by Xray microtomography (Table 3). However, the incorporation of small amount (approximately 3 %wt) of magnesium oxide and ammonium phosphate contributes significantly to the increase in mechanical resistance to impact and abrasion of the materials. In the context of Calcium Looping it is more interesting to have a mechanically resistant material with a large amount of macropores that contribute to the greater absorption of CO2. Fig. 7 shows typical scanning electron microscopy images of cross sections of the absorbent materials.
3.4. Mechanical properties of materials Table 4 shows the mechanical properties, compressive strength and friability, of the semi-spheres freshly prepared. The semi-spheres of Ca(OH)2 are those that most produce dust by friction (high friability). The pure CaO and Ca(OH)2 semi-spheres present low compressive strength. The crushing or compression force required to fracture the material is 10 N which is the lowest value measured. This means that the material can fracture manually if it is compressed. This does not happen if this compressive strength is greater than 25 N for a semi-sphere of 9 mm diameter. As one observes, the addition of magnesium oxide and di-ammonium hydrogen phosphate promotes an increase of about four times in the fracture resistance of the material. The semi-spheres with NH4MgPO4 present the best physical properties for CO2 absorption: low friability and high compressive strength. The physical properties depend on the geometry and size of the absorbent material. Therefore, we cannot directly compare our results with other studies that used materials with different geometries and sizes. In general, it is believed that 1 N of crushing force is sufficient for a material (pellets of 1–3 mm diameter) to be applied in large-scale industrial processes as previously reported [40]. The common size of the pellets used in fluidized bed reactors is 300–500 μm. However, the pellets synthesized in this work are much larger (9 mm diameter) because they are being investigated in CO2 capture with possible application in automobiles. This type of application is not allowed to generate dust and the gas pressure is close to the environment. It is also important to evaluate the mechanical properties of the material after the CO2 absorption and regeneration cycles. The thermogravimetry (TG) analysis was made with a small fraction (about 20 mg) of several crushed hemispheres. Therefore, it was not possible to perform the mechanical analysis after the tenth cycles in the TG analysis. However, CO2 absorption experiments were also performed in a tubular furnace, using the whole hemispheres. It has been demonstrated that the mechanical fragility of the absorbents is higher in the new material. The calcination produces a small reduction in the mechanical strength of the material due to the decomposition of the material, as can
Fig. 6. Images obtained by X-ray microtomography of semi-spheres of (a) 100% Ca(OH)2 calcined, (b) 97% Ca(OH)2 3% Mg(OH)2 calcined, (c) 97% Ca(OH)2 3% MgNH4PO4 calcined. 6
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Table 3 Porosity of CaO based-semi-spheres obtained by X-ray microtomography. Sample
Closed porosity (%)
Open porosity (%)
100% Ca(OH)2 calcined 97% Ca(OH)2 and 3% Mg(OH)2 calcined 97% Ca(OH)2 and 3% MgNH4PO4 calcined
0.53 0.39 0.02
43.71 46.46 69.76
(a)
(b)
(c)
5 µm
5 µm
5 µm
Fig. 7. Images obtained by scanning electron microscopy of the absorbents in natura with: (a) 100% Ca(OH)2, (b) 97% Ca(OH)2 and 3% Mg(OH)2, (c) 97% Ca(OH)2 and 3% MgNH4PO4.
Fig. 8. Distribution of the chemical elements in the calcined absorbent material containing 97% Ca(OH)2 and 3% MgNH4PO4 obtained by EDS probe.
conclusion can be made about the behaviour of the materials after the CO2 sorption cycles in fluidized bed regeneration but this deeper study was not performed and is under future investigations. The greater amount of open pores in the modified materials contribute to inhibit sintering and explain the low decrease of efficiency in CO2 sorption after several cycles when compared to CaO or Ca(OH)2 materials.
Table 4 Compressive strength and friability of the semi-spheres. Sample
Compressive strength (N)
Friability (wt.%)
100% CaO 100% Ca(OH)2 97% Ca(OH)2 + 3% Mg(OH)2 94% Ca(OH)2 + 6% Mg(OH)2 90% Ca(OH)2 + 10% Mg(OH)2 97% Ca(OH)2 + 3% NH4MgPO4 94% Ca(OH)2 + 6% NH4MgPO4 90% Ca(OH)2 + 10% NH4MgPO4
6.5 10.0 29.5 42.0 49.0 46.5 56.0 47.0
0.8 6.5 1.6 2.3 3.7 1.5 0.8 0.4
4. Conclusions Sorbent materials based on Ca(OH)2 exhibit high initial absorption of CO2 (greater than 50 %wt). The synthesis of the material was very efficient and can be controlled by the purity of the chemical inputs. The incorporation of small amount (approximately 3 %wt) of magnesium oxide and ammonium phosphate contributes significantly to the increase in mechanical resistance to impact and abrasion of the materials. In addition, this chemical modification produced a material with very developed porosity identified by X-ray microtomography with a higher specific surface area after thermal treatment at high temperature. In this way, the material becomes resistant to sintering and more efficient in long CO2 capture cycles and thermal regeneration. Finally, one can point out that these new proposed materials are of great applications for contributing to reduce the CO2 emissions and, therefore, minimizes, in principle, the green house effects.
be observed in Table 4. The 100% CaO sample shows the same behaviour of 100% calcined Ca(OH)2 sample. However, the increase in the mechanical strength after carbonation is much more pronounced than the decrease during calcination, and this can be attributed to the formation of crystalline CaCO3 as similar achievement found in ref. [48]. In addition, Tables 2 and 3 show that materials with NH4MgPO4 have a higher amount of open pores, which allows a greater permeation of the gas through the material. This material suffer less changes in the surface area after calcination, which tends to reduce embrittlement of it by large volume changes (compaction in calcination and expansion in carbonation). The better mechanical strength of the materials modified with MgO, NH3 and H3PO4 is evidenced in the freshly materials. However, no 7
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Acknowledgements
[23] Sun J, Liang C, Wang W, Liu W. Screening of naturally Al/Si-based mineral binders to modify CaO-based pellets for CO2 capture. Energy Fuels 2017. https://doi.org/ 10.1021/acs.energyfuels.7b03252. [24] Sun J, Liang C, Tong X, Guo Y, Li W, Zhao C, et al. Evaluation of high-temperature CO 2 capture performance of cellulose-templated CaO-based pellets. Fuel 2019. https://doi.org/10.1016/j.fuel.2018.11.123. [25] Sayyah M, Ito BR, Rostam-Abadi M, Lu Y, Suslick KS. CaO-based sorbents for CO2 capture prepared by ultrasonic spray pyrolysis. RSC Adv 2013. https://doi.org/10. 1039/c3ra44566f. [26] Zhao M, Shi J, Zhong X, Tian S, Blamey J, Jiang J, et al. A novel calcium looping absorbent incorporated with polymorphic spacers for hydrogen production and CO2 capture. Energy Environ Sci 2014. https://doi.org/10.1039/c4ee01281j. [27] Mutch GA, Shulda S, McCue AJ, Menart MJ, Ciobanu CV, Ngo C, et al. Carbon capture by metal oxides: unleashing the potential of the (111) facet. J Am Chem Soc 2018. https://doi.org/10.1021/jacs.8b01845. [28] Wang T, Liu J, Lackner KS, Shi X, Fang M, Luo Z. Characterization of kinetic limitations to atmospheric CO2 capture by solid sorbent. Greenh Gases Sci Technol 2016. https://doi.org/10.1002/ghg.1535. [29] Sanna A, Maroto-Valer MM. CO2 capture at high temperature using fly ash-derived sodium silicates. Ind Eng Chem Res 2016. https://doi.org/10.1021/acs.iecr. 5b04780. [30] Salaudeen SA, Acharya B, Dutta A. CaO-based CO2 sorbents: a review on screening, enhancement, cyclic stability, regeneration and kinetics modelling. J CO2 Util 2018. https://doi.org/10.1016/j.jcou.2017.11.012. [31] Valverde JM. Ca-based synthetic materials with enhanced CO2 capture efficiency. J Mater Chem A 2013. https://doi.org/10.1039/c2ta00096b. [32] Rey C, Combes C, Drouet C, Glimcher MJ. Erratum to: bone mineral: update on chemical composition and structure. Osteoporos Int 2009. https://doi.org/10. 1007/s00198-009-1063-2. [33] Walling SA, Provis JL. Magnesia-based cements: a journey of 150 years, and cements for the future? Chem Rev 2016. https://doi.org/10.1021/acs.chemrev. 5b00463. [34] Gao X, Zhang A, Li S, Sun B, Zhang L. The resistance to high temperature of magnesia phosphate cement paste containing wollastonite. Mater Struct Constr 2016. https://doi.org/10.1617/s11527-015-0729-9. [35] Fang Y, Cui P, Ding Z, Zhu JX. Properties of a magnesium phosphate cement-based fire-retardant coating containing glass fiber or glass fiber powder. Constr Build Mater 2018. https://doi.org/10.1016/j.conbuildmat.2017.12.059. [36] Frost RL, Weier ML, Erickson KL. Thermal decomposition of struvite. J Therm Anal Calorim 2004. https://doi.org/10.1023/b:jtan.0000032287.08535.b3. [37] Ugliengo P, Zicovich-Wilson CM, Tosoni S, Civalleri B. Role of dispersive interactions in layered materials: a periodic B3LYP and B3LYP-D* study of Mg(OH)2, Ca (OH)2 and kaolinite. J Mater Chem 2009. https://doi.org/10.1039/b819020h. [38] Korchef A, Saidou H, Ben Amor M. Phosphate recovery through struvite precipitation by CO2 removal: effect of magnesium, phosphate and ammonium concentrations. J Hazard Mater 2011. https://doi.org/10.1016/j.jhazmat.2010.11.045. [39] Menéndez E, Vega L, Andrade C. Use of decomposition of portlandite in concrete fire as indicator of temperature progression into the material: application to fireaffected builds. J. Therm. Anal. Calorim. 2012. https://doi.org/10.1007/s10973011-2159-4. [40] Hu Y, Liu X, Zhou Z, Liu W, Xu M. Pelletization of MgO-based sorbents for intermediate temperature CO2 capture. Fuel 2017. https://doi.org/10.1016/j.fuel.2016. 09.066. [41] Jin S, Ko KJ, Lee CH. Direct formation of hierarchically porous MgO-based sorbent bead for enhanced CO2 capture at intermediate temperatures. Chem Eng J 2019. https://doi.org/10.1016/j.cej.2019.04.020. [42] Vu AT, Park Y, Jeon PR, Lee CH. Mesoporous MgO sorbent promoted with KNO3 for CO2 capture at intermediate temperatures. Chem Eng J 2014. https://doi.org/10. 1016/j.cej.2014.07.088. [43] Dong W, Chen X, Yu F, Wu Y. Na2CO3/MgO/Al2O3 solid sorbents for low-temperature CO2 capture. Energy Fuels 2015. https://doi.org/10.1021/ef502400s. [44] Materic V, Smedley SI. High temperature carbonation of Ca(OH)2. Ind Eng Chem Res 2011. https://doi.org/10.1021/ie200367w. [45] Mutch GA, Anderson JA, Vega-Maza D. Surface and bulk carbonate formation in calcium oxide during CO2 capture. Appl Energy 2017. https://doi.org/10.1016/j. apenergy.2017.05.130. [46] Soleimanisalim AH, Sedghkerdar MH, Karami D, Mahinpey N. Pelletizing and coating of synthetic zirconia stabilized calcium-based sorbents for application in calcium looping CO2 capture. Ind Eng Chem Res 2017. https://doi.org/10.1021/ acs.iecr.6b04771. [47] Erans M, Manovic V, Anthony EJ. Calcium looping sorbents for CO2 capture. Appl Energy 2016. https://doi.org/10.1016/j.apenergy.2016.07.074. [48] Yan X, Li Y, Ma X, Zhao J, Wang Z, Liu H. CO 2 capture by a novel CaO/MgO sorbent fabricated from industrial waste and dolomite under calcium looping conditions. New J Chem 2019. https://doi.org/10.1039/c8nj06257a. [49] Montes-Hernandez G, Pommerol A, Renard F, Beck P, Quirico E, Brissaud O. In situ kinetic measurements of gas-solid carbonation of Ca(OH)2 by using an infrared microscope coupled to a reaction cell. Chem Eng J 2010. https://doi.org/10.1016/j. cej.2010.04.041.
The authors would like to acknowledge to Petrobras S.A., Fiat Chrysler Automobiles (FCA), and FAPEMIG for financial support and also the Center of Microscopy at the Universidade Federal de Minas Gerais (http://www.microscopia.ufmg.br) for providing the equipment and technical support for experiments involving electron microscopy, and Marivalda Magalhães Pereira for the X-ray microtomography images. References [1] Intergovernmental Panel on Climate Change. Climate Change 2014 Synthesis Report – IPCC; 2014. doi: 10.1017/CBO9781107415324. [2] Bui M, Adjiman CS, Bardow A, Anthony EJ, Boston A, Brown S, et al. Carbon capture and storage (CCS): the way forward. Energy Environ Sci 2018. https://doi. org/10.1039/c7ee02342a. [3] Hornberger M, Spörl R, Scheffknecht G. Calcium looping for CO2 capture in cement plants – pilot scale test. Energy Procedia 2017. https://doi.org/10.1016/j.egypro. 2017.03.1754. [4] Theeyattuparampil VV, Zarzour OA, Koukouzas N, Vidican G, Al-Saleh Y, Katsimpardi I. Carbon capture and storage: state of play, challenges and opportunities for the GCC countries. Int J Energy Sect Manag 2013. https://doi.org/10. 1108/IJESM-04-2013-0010. [5] Ortiz C, Chacartegui R, Valverde JM, Becerra JA, Perez-Maqueda LA. A new model of the carbonator reactor in the calcium looping technology for post-combustion CO2 capture. Fuel 2015. https://doi.org/10.1016/j.fuel.2015.07.095. [6] Ortiz C, Chacartegui R, Valverde JM, Becerra JA. A new integration model of the calcium looping technology into coal fired power plants for CO2 capture. Appl Energy 2016. https://doi.org/10.1016/j.apenergy.2016.02.050. [7] Valverde JM, Sanchez-Jimenez PE, Perez-Maqueda LA. Calcium-looping for postcombustion CO2 capture. On the adverse effect of sorbent regeneration under CO2. Appl Energy 2014. https://doi.org/10.1016/j.apenergy.2014.03.081. [8] Radfarnia HR, Sayari A. A highly efficient CaO-based CO2 sorbent prepared by a citrate-assisted sol-gel technique. Chem Eng J 2015. https://doi.org/10.1016/j.cej. 2014.09.074. [9] Hu Y, Liu W, Peng Y, Yang Y, Sun J, Chen H, et al. One-step synthesis of highly efficient CaO-based CO2 sorbent pellets via gel-casting technique. Fuel Process Technol 2017. https://doi.org/10.1016/j.fuproc.2017.02.016. [10] Zhu D, Choi SR, Miller RA. Development and thermal fatigue testing of ceramic thermal barrier coatings. Surf Coatings Technol 2004. https://doi.org/10.1016/j. surfcoat.2004.08.017. [11] Kierzkowska AM, Pacciani R, Müller CR. CaO-based CO2 sorbents: from fundamentals to the development of new, highly effective materials. ChemSusChem 2013. https://doi.org/10.1002/cssc.201300178. [12] Manovic V, Wu Y, He I, Anthony EJ. Spray water reactivation/pelletization of spent CaO-based sorbent from calcium looping cycles. Environ Sci Technol 2012. https:// doi.org/10.1021/es303252j. [13] Qin C, Yin J, An H, Liu W, Feng B. Performance of extruded particles from calcium hydroxide and cement for CO 2 capture. Energy Fuels 2012. https://doi.org/10. 1021/ef201141z. [14] Sun J, Liu W, Hu Y, Wu J, Li M, Yang X, et al. Enhanced performance of extrudedspheronized carbide slag pellets for high temperature CO2 capture. Chem Eng J 2016. https://doi.org/10.1016/j.cej.2015.10.026. [15] Li H, Qu M, Yang Y, Hu Y, Liu W. One-step synthesis of spherical CaO pellets via novel graphite-casting method for cyclic CO2 capture. Chem Eng J 2019. https:// doi.org/10.1016/j.cej.2019.05.214. [16] Ridha FN, Manovic V, Anthony EJ, MacChi A. The morphology of limestone-based pellets prepared with kaolin-based binders. Mater Chem Phys 2013. https://doi. org/10.1016/j.matchemphys.2012.11.007. [17] Kierzkowska AM, Poulikakos LV, Broda M, Müller CR. Synthesis of calcium-based, Al 2 O 3 -stabilized sorbents for CO 2 capture using a co-precipitation technique. Int J Greenh Gas Control 2013. https://doi.org/10.1016/j.ijggc.2013.01.046. [18] Dong W, Chen X, Wu Y. TiO2-doped K2CO3/Al2O 3 sorbents for CO2 capture. Energy Fuels 2014. https://doi.org/10.1021/ef500133e. [19] Kumar S, Saxena SK. A comparative study of CO2 sorption properties for different oxides. Mater Renew Sustain Energy 2014. https://doi.org/10.1007/s40243-0140030-9. [20] Zhang Y, Gao Y, Pfeiffer H, Louis B, Sun L, O’Hare D, et al. Recent advances in lithium containing ceramic based sorbents for high-temperature CO2 capture. J Mater Chem A 2019. https://doi.org/10.1039/c8ta08932a. [21] Hu Y, Liu W, Yang Y, Qu M, Li H. CO2 capture by Li4SiO4 sorbents and their applications: Current developments and new trends. Chem Eng J 2019. https://doi. org/10.1016/j.cej.2018.11.128. [22] Broda M, Müller CR. Synthesis of highly efficient, Ca-based, Al2O3-stabilized, carbon gel-templated CO 2 sorbents. Adv Mater 2012. https://doi.org/10.1002/ adma.201104787.
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