Crystallographic and Morphological Transformation of Natrite and

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Cite This: Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystallographic and Morphological Transformation of Natrite and Nahcolite in the Dry Carbonate Process for CO2 Capture Santiago Medina-Carrasco*,† and Jose Manuel Valverde*,‡ †

X-Ray Laboratory (CITIUS), Universidad de Sevilla, Avenida Reina Mercedes, 4B, 41012 Sevilla, Spain Facultad de Física, Universidad de Sevilla, Avenida Reina Mercedes s/n, 41012 Sevilla, Spain

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‡

ABSTRACT: The reversible reaction between natrite (Na2CO3) and nahcolite (NaHCO3) is at the root of the dry carbonate process (DCP) recently proposed for the capture of CO2. In this manuscript, the crystallographic transformations that take place during carbonation of natrite in the presence of CO2 and H2O to produce nahcolite and the reversible decarbonation of nahcolite under realistic conditions have been studied by in situ powder X-ray diffraction (PXRD) analysis. Carbonation shows a great diversity of emerging phases with the presence of trona (Na2CO3·NaHCO3·2H2O), wegscheiderite (Na2CO3· 3NaHCO3), and thermonatrite (Na2CO3·H2O) in addition to natrite and nahcolite. On the other hand, decarbonation occurs through the decomposition of nahcolite only in natrite. An analysis of the crystallite size of the intervening phases by means of the Le Bail method reveals that natrite generally has a larger crystallite size (∼300 nm) than the rest of phases during carbonation. In decarbonation, the crystallite size of natrite remains stable from its formation around 20−30 nm, whereas nahcolite exhibits a crystallite size notably greater than natrite of about 50 nm. The evolution of weight percentages for all the phases that appear in each of the reactions has been studied using the Rietveld method. In carbonation, a complete transformation of natrite into nahcolite is achieved at 70 °C under CO2 and high humidity. The monoclinic nahcolite phase observed in that case has a space group P21/n instead of the P21/c observed for the rest of the cases. Trona is an intermediate phase during carbonation of natrite and appears before transforming into wegscheiderite, which is seen at 80 °C. Thermonatrite is formed under conditions of high humidity at 60 and 70 °C and only under a N2 atmosphere. Generally, it is found that carbonation requires long residence times in the ranges of tested temperatures (between 60 and 80 °C as proposed in the DCP), whereas decarbonation occurs fast at temperatures above 150 °C under CO2 as required in practice. We hope that our crystallographic study serves to shed further light on the reaction mechanisms that take place in the DCP, which are particularly complex in the carbonation process. sodium carbonate.6,9 The Solvay process, named after its inventor Ernest Solvay (1838−1922), has been employed for many years to produce Na2CO3.9 In the Solvay process (‘“ammonia-soda process”’), Na2CO3 is produced from NaCl and limestone (CaCO3), with the addition of ammonia (NH3).9 Ammonia recycling is an essential feature of the Solvay process since it has a greater value than sodium bicarbonate. The net reaction once ammonia has been recovered is

1. INTRODUCTION The reversible chemical reaction between natrite (sodium carbonate, Na2CO3, or soda ash)1−3 and nahcolite (sodium bicarbonate, NaHCO3, or baking soda)4,5 is well-known from its use in a variety of industrial applications:6 Na 2CO3(s) + CO2(g) + H 2O(g) V 2NaHCO3(S) Δr H 0 = −129.1 kJ mol−1

(1)

Soda ash is a main product for many industrial uses such as the production of glass6 and detergents.6 Sodium bicarbonate is used in medicine (frequently as an antacid),7 as a leavening agent in baking (“baking soda”),8 and in the manufacture of © XXXX American Chemical Society

Received: April 14, 2018 Revised: June 9, 2018 Published: June 22, 2018 A

DOI: 10.1021/acs.cgd.8b00563 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

Article

2NaCl + CaCO3 → Na 2CO3 + CaCl 2

Some salts that can be formed from the following reactions (eqs 8−11):27,28

(2)

NaHCO3 can be produced by the reaction 1 using the Na2CO3 obtained by means of this process. An exploration of natural sodium carbonate resources in the United States revealed that Na2CO3 may be produced in a cheap way from available natural sodium carbonaceous minerals (e.g., trona, Na2CO3·NaHCO3·2H2O,10,11 or nahcolite) or naturally occurring sodium carbonate-bearing brines.9,12,13 Of these minerals, trona or sodium sesquicarbonate is one of the most widely used. As is shown in Table 1, natural trona consists of

wt % 46.53 34.82 0.568 2.98 14.92 0.182

solid phases nahcolite−ice−decahydrate nahcolite−decahydrate−trona trona−heptahydrate− decahydrate trona−monohydrate− heptahydrate nahcolite−trona− wegscheiderite monohydrate−trona− anhydrate trona−wegscheiderite− anhydrate

(4)

HCO3− + H 2O → H3O+ + CO32 −

(5)

Na 2CO3 → 2Na + CO3

Na + + HCO3− → NaHCO3

Na2CO3 molality

NaHCO3 molality

T (°C)

0.46 2.11 4.37

0.58 0.60 0.087

−3.37 20.5 32.3

4.84

0.064

35.6

2.08

1.55

73.7

4.14

0.87

102.4

3.91

1.43

112.5

that the trona phase cannot exist above the trona− wegscheiderite−anhydrate invariant point at about 112.5 °C. Similar arguments can be given for anhydrate at the triple point, always appearing at temperatures above 112.5 °C. The reactions summarized above give an idea of the considerable complexity of the process, where a wide diversity of combinations of the different compounds can appear in different mixtures of Na2CO3 and NaHCO3 with CO2 and H2O, depending on the reaction conditions. In this work, we have carried out a novel in situ PXRD analysis for further understanding the crystallographic transformations that take place under conditions of high humidity, high partial pressure of CO2, and temperatures of practical interest for the dry carbonate process (DCP) that relies on reaction 1 to capture CO2 in fuel-fired power plants.12,27 1.1. Dry Carbonate Process for CO2 Capture. The DCP for CO2 capture is based on reaction 1.12,27,34 CO2 present in the flue gas is captured through the chemical binding of CO2 to Na2CO3 in a carbonator reactor at operating temperatures below 100 °C. In the carbonator reactor, Na2CO3 is converted to NaHCO3 by the chemical reaction with CO2 in the presence of H2O. Na2CO3 is regenerated in a separate reactor by heating NaHCO3 to temperatures above 100 °C under a high CO2 concentration atmosphere, thus releasing almost pure CO2 after water condensation. In the process, there is a continuous

(3)

CO2 + 2H 2O → H3O+ + HCO3−

2−

(11)

Table 2. Calculated Invariant Points28

Trona is stable at temperatures up to 57 °C under dry conditions. Intermediate salts such as Wegscheider’s salt, also called wegscheiderite or “decimite” (Na2CO3·3NaHCO3)22,23 and sodium carbonate monohydrate or thermonatrite (Na2CO3·H2O)24 are produced between 57 and 160 °C.25 Above 160 °C, trona decomposes into Na2CO3 and NaHCO3. NaHCO3 decomposes to Na2CO3, H2O, and CO2 in the temperature range of 100−200 °C (eq 1). Reaction kinetics is very fast at 200 °C.26 In the decomposition process of trona, Na2CO3, water, and CO2 are obtained as final products. A proposed mechanism to describe the reaction between Na2CO3 and CO2 is eqs 4−7:27,28

+

(10)

In this way, in addition to NaHCO3, other salts can be formed from the reaction between CO2, H2O, and sodium carbonate anhydrate (Na2CO3) such as sodium carbonate decahydrate or natron (Na2CO3·10H2O),29,30 sodium carbonate heptahydrate (Na2CO3·7H2O),31 thermonatrite, wegscheiderite, and trona.32,33 Haynes showed the equilibrium diagrams in the low and high-temperature domains for the aforementioned phases based on the molality of Na2CO3 and NaHCO3.28 Invariant points are given in Table 2. A conclusion from his work was

2(NaCO3 ·NaHCO3 ·2H 2O)(s) → 3Na 2CO3(s) + CO2(g) Δr H 0 = 133.39 kJ mol−1

(9)

→ 2(Na 2CO3 ·NaHCO3 ·2H 2O)

approximately 46% Na2CO3 and 35% NaHCO3 it being relatively abundant and widely distributed around the world.14 The main places where trona can be found are United States (Wyoming),15 Turkey,16,17 Tanzania,18 Kenya,18 Uganda,18 Sudan,18 and China.19 The mineral trona appears usually in a mixture with nahcolite.17,20 Calcination and dehydration of trona are mainly used for the production of Na2CO3. In general, it is also possible to carry out these processes to remove the carbonate, thus decreasing the consumption of acid if an acid treatment is applied, or to reduce the weight of the material for minimizing transport costs, which can be high if the material is hydrated. In the production of Na2CO3 from trona, the monohydrate process is the most developed method.13 The first step of this method is the decomposition of the mineral at temperatures in a range between 200 and 600 °C, which leads to the production of Na2CO3 by means of the following reaction:13,21

+ 5H 2O(g)

2Na + + CO32 − + 7H 2O → Na 2CO3 ·7H 2O

3Na + + CO32 − + HCO3− + 2H 2O

Table 1. Natural Trona composition component

(8)

2Na + + CO32 − + H 2O → Na 2CO3 ·H 2O

13

Na2CO3 NaHCO3 Na2SO4 insolubles hydration water others

2Na + + CO32 − + 10H 2O → Na 2CO3 ·10H 2O

(6) (7) B



Article

Figure 1. General scheme of the dry carbonate process. Adapted from refs 27 and 35.

wear and loss of the sorbent material, due to interparticle collisions and the nonreversible reaction with SO2 and HCl that may be present in the coal combustion gas. In order to solve it, a certain amount of sorbent is periodically replenished into the system. Figure 1 shows a schematic flow diagram of the DCP. The DCP is particularly suitable for retrofitting coal-fired power plants (with a previous desulfurization of wet combustion gases) and natural gas power plants. It is estimated that commercial scale natural gas (500 MWe) with DCP would require an initial sorbent charge of approximately 387 t and a fresh sorbent replenishment rate of approximately 0.2 t/h.35 After integration of the DCP, the net efficiency of the plant would suffer a drop from 40.5% to 33.4% (penalty of 7.1%). For power plants fed only with coal, there is a higher concentration of CO2 in the combustion gas, and a greater amount of sorbent is needed for the capture of CO2, while the loss of efficiency is expected to be similar. The reactions involved in the capture of CO2 using Na2CO3 result in the reversible formation of NaHCO 3 and wegscheideirite according to eqs 1 and 12:35

Na 2CO3(s) + SO2(g) +

(14)

The formation of NaCl and Na2SO4 leads to a depletion of Na, thus the capacity of Na2CO3 to capture CO2 is reduced in each cycle. However, if a desulfurization step is introduced before CO2 capture, HCl and SO2 are present at relative concentrations about an order of magnitude below the concentration of CO2 in the combustion gas, which mitigates the irreversible loss of conversion caused by this issue. Another dry capture process widely investigated in the last years is the calcium-looping process (CaL), which shows a high potential to mitigate CO2 emissions.36,37 The CaL process is based on the reversible carbonation/calcination of CaO/ CaCO3 at high temperatures: CaO(s) + CO2(g) V CaCO3(s) Δr H 0 = −177.8 kJ mol−1

(1)

Na 2CO3(s) + 0.6CO2(g) + 0.6H 2O(g) V 0.4[Na 2CO3 · 3NaHCO3(s)] Δr H 0 = −135.98 kJ mol−1

(15)

An important advantage of the DCP over the CaL process is that, in the former, carbonation can be carried out at relatively low temperatures (60−70 °C), whereas the latter requires temperatures around 625−680 °C. Moreover, regeneration of Na2CO3 is possible at moderate temperatures (120−200 °C), which allows the use of medium heat sources and even renewables such as solar thermal or biomass.12 In contrast, in the CaL process, it is necessary to reach temperatures in the calciner in the range of 900−950 °C for the regeneration of the sorbent CaO, which requires coal oxy-combustion, thus increasing the energy penalty of the technology. Na2CO3 is also abundant and relatively cheap (∼100 €/ton).12 In this manuscript, an in situ PXRD study has been carried out for the first time to our knowledge on both the carbonation and decarbonation reactions involved in the DCP at conditions for CO2 capture. A quantitative analysis of the emergent phases and their crystallite sizes is performed.

Na 2CO3(s) + CO2(g) + H 2O(g) V 2NaHCO3(S) Δr H 0 = −129.1 kJ mol−1

1 O2(g) V Na 2SO4(s) + CO2(g) 2

(12)

The production of other possible reaction byproducts, such as trona and sodium bicarbonate hydrate (NaHCO3·2H2O), is negligible at the conditions of interest. Both direct reactions are exothermic. Therefore, the integration of heat is important for an efficient implementation of the process in a commercial system.27 Potential contaminants present in the flue gas, such as SO2 and HCl, react with Na2CO3 according to eqs 13 and 14:

2. EXPERIMENTAL AND ANALYSIS METHODS The raw materials used in our work were powdered reagent-grade Na2CO3 (CAS no. 497-19-8) and NaHCO3 (CAS no. 144-55-8) both from Sigma-Aldrich.

Na 2CO3(s) + 2HCl(g) V 2NaCl(s) + CO2(g) + H 2O(g) (13) C



Article

Figure 2. Diffractograms recorded by in situ PXRD analysis during carbonation for 60 °C (a), 70 °C (b), and 80 °C (c) for the experiments of the first type at 90% RH in CO2. Phases of natrite (Na2CO3, a and c, main Bragg peaks marked with orange squares), nahcolite (NaHCO3, a, b and c, main Bragg peaks marked with blue triangles), trona (Na2CO3·NaHCO3·2H2O, a, b and c, main Bragg peaks marked with solid dark green circles), and wegscheiderite (Na2CO3·3NaHCO3, c, main Bragg peaks marked with red diamonds) were obtained. instrumental contribution for structural adjustments in a wide range of angles. Measurements were taken within a 2θ range from 10° to 60°, a step of 0.02°, time per step of 0.2 s, and tube conditions of 40 kV and 40 mA, with a total time duration of approximately 9 min per scan. Carbonation took place under isothermal conditions at diverse temperatures in the range proposed for CO2 capture (60, 70, and 80 °C) under CO2.12 Two different types of carbonation tests were carried out. In the first type, the sample was exposed to CO2 from the beginning of the test. The system then took about 45 min for stabilization once the desired temperature of the isotherm was reached. During this time, the initially dry sample was exposed to humidity that was increased from 5% to 90% RH, after which it was kept constant at 90% RH in the isotherm. In the second type of carbonation tests, the transitory heating process was carried out in the

Carbonation reaction tests were carried out in a Bruker D8 Discover A25 diffractometer equipped with a Lynxeye-XE positionsensitive detector and an Anton Paar CHCplus humidity chamber. This camera allows precise control of the relative humidity (% RH) and temperature avoiding condensation during the test. The decarbonation experiments were performed in a Bruker D8 Advance diffractometer equipped with a Bruker Vantec 1 position-sensitive detector and an Anton Paar XRK 900 temperature chamber. Both Bruker diffractometers had radial Soller slits and 60 mm Göbel mirrors for CuKα radiation (λ = 0.15405 nm) with parallel geometry in the incident beam. NiCr/NiAl thermocouples placed near the sample holder provided a reliable measurement and control of temperature. The diffractometers were calibrated mechanically according to the manufacturer specifications. Corundum, LaB6, and silicon standards were used to check the resolution and to obtain the D



Article

Figure 3. Time evolution of crystallite size (a) and phases wt % (b) for the carbonation experiments of the first type at 90% RH in CO2, for nahcolite (NaHCO3), natrite (Na2CO3), trona (Na2CO3·NaHCO3·2H2O), and wegscheiderite (Na2CO3·3NaHCO3) at 60, 70, and 80 °C. presence of N2 at 5% RH to avoid carbonation in this period. A first PXRD scan was recorded at 30 °C and a second one at the target temperature of the isotherm. Then the desired humidity level was introduced into the system while the environment was kept in N2. Another PXRD scan was recorded, and then CO2 was introduced at the desired isothermal conditions. In the decarbonation tests, CO2 was passed at atmospheric pressure across the powder at a small flow rate (100 cm3 min−1), and the temperature was linearly increased at 12 °C min−1 from room temperature to reach isothermal conditions at 135, 150, and 165 °C. The target temperature was held constant for 2 h in carbonation and for 1 h in decarbonation, while PXRD scans were continuously recorded. Quantification of the weight percentage (wt %) of the present phases during the carbonation and decarbonation tests was performed by Rietveld refinement.38 Le Bail fits of the patterns were used to calculate coherent crystal lengths (crystallite size, L).39 The software TOPAS 6 (Bruker) was employed for the adjustments.40 Rietveld and Le Bail refinements were performed using the fundamental parameter method. Zero error (2θ), sample displacement, and absorption and lattice parameters of the phases were allowed to vary in order to search for the best fittings. The background was fitted by a fourthorder Chebychev polynomial. Lorentz and polarization geometric factors for the measurement configuration were used. Crystallite sizes were calculated using the best combinations of Gaussian and Lorentzian functions. The values of the goodness of fit (GOF) of the adjustments were checked to obtain values close to unity. At the same time, values of residual factors (Rwp and RBragg) were obtained. In the reported results, these values were generally small, indicating

coherent data.38 It was observed that there was practically no presence of amorphous material in all the tests.

3. RESULTS AND DISCUSSION 3.1. Carbonation Tests Results. 3.1.1. Carbonation Tests Results of First Type. Figure 2 illustrates a waterfall representation of PXRD scans recorded for the carbonation of Na2CO3 under isothermal conditions at T = 60 °C (Figure 2a), 70 °C (Figure 2b), and 80 °C (Figure 2c). Measurements in these tests were taken at 90% RH with an initial time of 45 min for stabilization from 5% RH to 90% RH under CO2. Main Bragg peaks of the detected phases are marked. For a temperature of 60 °C (Figure 2a), a mixture of the phases of natrite (Na2CO3, monoclinic structure, space group C2/m, 12),1−3 nahcolite (NaHCO3, monoclinic structure, space group P21/c, 14),4,5 and trona (Na2CO3·NaHCO3·2H2O, monoclinic structure, space group C2/c, 15)10,11 was observed. For the temperature of 70 °C (Figure 2b), only a mixture of nahcolite (NaHCO3, monoclinic structure, space group P21/n, 14)41 and trona (same structure and space group as the one obtained at 60 °C) was found. At 80 °C (Figure 2c), the crystalline phases present were natrite, nahcolite, and trona (all three with the same structure and space group as those obtained at 60 °C), in addition to wegscheiderite (Na2CO3· 3NaHCO3, triclinic structure, space group P1̅, 2).22,23 E



Article

Figure 4. Diffractograms recorded by in situ PXRD analysis during carbonation at T = 60 °C (a, 90% RH), 70 °C (b, 90% RH), and 80 °C (c, 37.5% RH) for the experiments of the second type under CO2 starting at t = 42 min. Previous scans were recorded under N2 at 5% RH and 30 °C in t = 0 min (a, b, and c) and at 60 °C (a), 70 °C (b), and 80 °C (c) in t = 9 min and 90% RH at t = 27 min. Phases of natrite (Na2CO3, a, b and c, main Bragg peaks marked with orange squares), thermonatrite (Na2CO3·H2O, a, and b, main Bragg peaks marked with blue squares), trona (Na2CO3·NaHCO3·2H2O, a and b, main Bragg peaks marked with solid dark green circles), and wegscheiderite (Na2CO3·3NaHCO3, c, main Bragg peaks marked with red diamonds) were obtained.

order of 25 nm. The trona crystals increased in size from about 10 nm until reaching a maximum of about 120 nm at 80 min and then decreased to stabilize at a size of about 70 nm. For the temperature of 70 °C, the size of the nahcolite crystals (space group P21/n) is stable at a size of 100 nm. At this temperature, the calculation of the crystal sizes of trona presents a very high error, and coherent values cannot be obtained. As can be seen in Figure 3b, the amount of trona at 70 °C is very low practically from the beginning, which leads to significant errors in the adjustment. For the temperature of 80 °C, the natrite presents a high error that prevents deriving coherent values. The size of nahcolite is kept around 40 nm

Figure 3 shows data of the crystallite size (L) and wt % for the phases present in the first type of experiments presented in Figure 2. In the study of crystallite size (Figure 3a), for the temperature of 60 °C, it can be seen that Na2CO3 has a size considerably greater than NaHCO3 and trona, and even larger than the rest of the phases obtained to 70 and 80 °C. For 60 °C the errors obtained in the adjustment of the sizes for natrite are large, and from the sixth scan (approximately 90 min), it is not possible to obtain coherent values, although the size must be kept in the same order. Regarding the size of the crystals of natrite, coherent values were obtained on the order of 400 nm, while the nahcolite crystals are maintained in a size on the F



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Figure 5. Time evolution of crystallite size (a) and phases wt % (b) for the carbonation experiments of the second type for natrite (Na2CO3), thermonatrite (Na2CO3·H2O), trona (Na2CO3·NaHCO3·2H2O), and wegscheiderite (Na2CO3·3NaHCO3) at 60 °C, 70 °C (90% RH), and 80 °C (37.5% RH) in CO2 from t = 42 min to the finish. The sample was initially subjected to N2 at 5% RH at 30 °C, while the temperature was increased up to 60, 70, and 80 °C, respectively, in t = 9 min, and the humidity increased to 90% RH at t = 27 min.

3.1.2. Carbonation Tests Results of Second Type. Figure 4 shows the PXRD scans recorded for the carbonation of Na2CO3 under isothermal conditions at T = 60 °C (Figure 4a), 70 °C (Figure 4b), and 80 °C (Figure 4c) for the experiments of the second type. Measurements of the isotherm were taken at 90% RH for 60 °C (Figure 4a) and 70 °C (Figure 4b) and at 37.5% RH for 80 °C (Figure 4c). To make a complete followup of the phase changes that occurred and to avoid the formation of phases in the process of stabilization of the system, measurements were initially made in N2 at 30 °C and at the temperature of the isotherm at 5% RH. Then, humidity was introduced, a new PXRD scan was recorded, and the N2 atmosphere was replaced by CO2 maintaining the RH constant while scans were continuously recorded. Main Bragg peaks of the present phases are marked in Figure 5. For the temperatures of 60 °C (Figure 4a) and 70 °C (Figure 4b), a mixture of the phases of natrite (Na2CO3, monoclinic structure, space group C2/m, 12), 1−3 thermonatrite (Na2CO3·H2O, orthorhombic structure, space group PCa21, 29),24 and trona (Na2CO3·NaHCO3·2H2O, monoclinic structure, space group C2/c, 15)10,11 was observed. At 80 °C (Figure 4c), the crystalline phases present were natrite (with the same structure and space group as those obtained at 60 and 70 °C) and wegscheiderite (Na2CO3·3NaHCO3, triclinic structure, space group P1̅, 2).22,23 Results of the time evolution of crystallite size (L) and wt % for the phases present in the second type of experiments are

from the beginning, and the size of trona grows from an initial value of 40 nm until reaching a stable value of 100 nm. The wegscheiderite crystal size is maintained throughout the experiment at around 55 nm. Figure 3b shows the time evolution of the wt % of the phases present during the carbonation reaction. As can be seen, at T = 60 °C, natrite is first detected at a 66 wt %, after which it decreases to about a 45 wt %. At the same time, the wt % of nahcolite, which initially is 25 wt %, decreases to an amount of 7 wt %. The decrease of natrite and nahcolite wt % leads to the formation of trona, whose wt % starts at 8 wt % and ends at 50 wt %. At T = 70 °C, we have from the beginning nahcolite as a dominant component, growing from an initial 90 wt % to 97%, while trona decreases until disappearing. At T = 80 °C, there is a mixture of four phases, natrite, which goes down from an initial 40 wt % until it stabilizes at 25 wt %, nahcolite, which remains constant at 10 wt %, trona, which remains constant at around 25 wt %, and wegscheiderite, which grows from an initial 30 wt % until stabilizing at around 42 wt %. The process of stabilization of the system in this type of experiment involved a time of 45 min, and during that time no X-ray diffraction measurements were carried out, so it is not possible to describe the phase changes that took place in this transitory period. For this reason the second type of experiments were carried out in which carbonation was avoided during the system stabilization. G



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Figure 6. Diffractograms recorded by in situ PXRD analysis during decarbonation of NaHCO3 at T = 120 °C (a), 135 °C (b), 150 °C (c), and 165 °C (d) under CO2. Phases of nahcolite (NaHCO3, main Bragg peaks marked with blue triangles) and natrite (Na2CO3, main Bragg peaks marked with orange squares) were obtained.

in the crystallite size at T = 60 and 80 °C, reaching a maximum value of about 300 nm for both cases, to stabilize with time around 275 nm for the case of 60 °C and at about 250 nm for T = 80 °C. The case of T = 70 °C shows a different behavior with a very slow growth of the crystallite size until stabilizing at about 220 nm. For the cases of T = 60 and 70 °C, when introducing humidity under N2, thermonatrite (Na2CO3·H2O) appears (Figure 5b), which does not happen for T = 80 °C. The presence of thermonatrite favors the growth of the crystallite size of natrite. For T = 60 °C the crystallite size of

shown in Figure 5. Regarding crystallite size (Figure 5a), the size of Na2CO3 has an initial value of about 110 nm at 30 °C in the presence of N2, which is kept constant as the temperature is increased to the value of the isotherm, either T = 60, 70, or 80 °C. Once humidity is introduced, an increase of L occurs for T = 60 °C until reaching 190 nm and for T = 70 °C until it reaches a value of 180 nm. In the case of T = 80 °C, the crystallite size remains constant even when humidity is introduced. After the introduction of CO2, while keeping the temperature of each isotherm fixed, there is a constant increase H



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Figure 7. Time evolution of crystallite size (a) and phases wt % (b) for the experiments of decarbonation in CO2, for nahcolite (NaHCO3), and natrite (Na2CO3) at 120, 135, 150, and 165 °C.

wt % and grows slowly until it stabilizes at 20 wt % after introducing CO2 at 60 and 70 °C. In the presence of CO2 trona is also formed at these temperatures, starting from an initial percentage of 5 wt % for 60 °C and 10 wt % at 70 °C, and growing to a value of 45 wt % to 60 °C and 40 wt % to 70 °C. The presence of CO2 at 80 °C will result in the formation of wegscheiderite, which grows from a very low initial value to a wt % that stabilizes at just over 20 wt %. In this case, unlike in the others, 37.5% RH has been used, which may have conditioned the presence only of wegsheiderite and natrite, as well as the low concentration of wegscheiderite compared to natrite. 3.2. Decarbonation Tests Results. Figure 6 illustrates the PXRD scans recorded for decarbonation of NaHCO3 at T = 120 °C (Figure 6a), 135 °C (Figure 6b), 150 °C (Figure 6c), and 165 °C (Figure 6d) under CO2. In all cases, the initial NaHCO3 decomposes into Na2CO3 (eq 1) with different kinetics depending on the temperature of the isotherm.

thermonatrite starts at 100 nm under N2 and decreases slowly until it stabilizes around 80 nm when introducing CO2, while in the case T = 70 °C the initial crystallite size is much smaller, about 30 nm, and grows slowly until it stabilizes around 80 nm. For the temperatures of 60 and 70 °C, when introducing CO2, trona appears (Figure 5b) with an initial crystallite size of 45 nm for the case of 60 °C and 75 nm for the case of 70 °C. Crystallite size grows slowly for both temperatures until it stabilizes at 120 nm. At T = 80 °C, after the introduction of CO2, wegscheiderite appears, with an initial value of the crystallite size that cannot be calculated given the initial low percentage of the phase (Figure 5b). In the next scan, it can be calculated with a value that will stabilize around 120 nm remaining practically constant from the beginning. From the time evolution of the wt % of the present phases (Figure 5b), it may be seen that in the presence of dry N2 the only existing phase is natrite. Thermonatrite appears when introducing humidity at the temperatures of 60 and 70 °C. The initial wt % of thermonatrite in N2 is very low, approximately 5 I



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Figure 8. Expanded regions of the diffractogram shown in Figure 2c on the PXRD pattern recorded 135 min after the beginning of the carbonation test at T = 80 °C illustrating the contribution of the main reflection peaks in the angle range between 28.5 and 38.5° for nahcolite (NaHCO3, thick blue line), natrite (Na2CO3, thick black line), trona (Na2CO3·NaHCO3·2H2O, thick green line), and wegscheiderite (Na2CO3·3NaHCO3, thick turquoise line) phases from the best Rietveld fit (thin red line) to the experimental pattern (thin blue line). The bottom gray line shows the deviation between the best fit and the experimental diffractogram.

information on the crystallite sizes for each type of experiment, although in situ PXRD provides us with a direct measure as the reaction evolves at the relevant conditions for CO2 capture. Figure 8 shows an example of the Rietveld analysis results on the PXRD pattern that was recorded 135 min after the beginning of the experiment at T = 80 °C (Figure 2c), from which the wt % of the four phases involved are obtained. Expanded regions of the angle range between 28.5 and 38.5° for nahcolite, natrite, trona, and wegscheiderite phases can be seen, showing the main reflections for the different structures. The presence of several crystalline phases implies a high complexity, which requires a meticulous analysis of the results in order to make these adjustments. 3.3.2. Carbonation Tests of Second Type. At T = 60 and 70 °C, once humidity is introduced under N2, thermonatrite is formed, which coexists with the initial phase of natrite and with trona that appears once CO2 is introduced later. The presence of thermonatrite favors the growth of the crystallite size of natrite (3.1.2). The formation of thermonatrite is well-known from the decomposition of natron (Na2CO3·10H2O) at moderate to high temperatures in arid climates, either as a primary evaporate or by dehydration.30 Another mechanism of formation is the one that has been observed in this work, from the exposure of natrite to humidity.46 Dehydration of thermonatrite has been previously studied,47 it being one of the industrial production methods of natrite.46,48 The presence of trona at the same temperatures and of natrite at 60 °C has already been observed in our first type of carbonation experiments (3.1.1). At T = 80 °C the appearance of wegscheiderite is observed, which, as explained for the experiment of the first type at the same temperature, usually appears as an intermediate phase from the decomposition of trona,21,25,43 although in the procedure followed in these experiments it appears directly. Liang claims that for reaction 1 wegscheiderite appears at temperatures above 70 °C in most of the partial pressures studied of H2O and CO2.49 For the three temperatures of the second type of experiments (3.1.2) in our work natrite also appears which is a remnant of the starting product. For the carbonation tests of the second type (3.1.2), as with the phases obtained in the experiments of the first type (3.1.1), to our knowledge no calculations of coherent crystal length sizes have been reported previously by PXRD. In addition to the SEM micrographs of the works discussed for the experiments of the first type (3.1.1) reported in the literature, additional works have also been reported for thermonatrite under N2 at constant water-vapor pressure,47 or in ethylene

Data obtained from the calculations of crystallite size (L) and wt % are plotted in Figure 7. In the study of crystallite sizes (Figure 7a) for natrite, it can be seen that at T = 120 °C, this phase appears from minute 6 with a crystallite size of 40 nm, which gradually decreases until it stabilizes around 20 nm. For the rest of temperatures, the natrite crystallite size remains stable from its appearance around 30 nm, with a slight decrease at the end of the process to 25 nm at T = 160 °C. Regarding the crystallite size of nahcolite (Figure 7b), it is much less stable, although it must be taken into account that at T = 135, 150, and 165 °C the wt % of this phase goes down rather quickly (Figure 7b), which makes it difficult to get a reliable trend. In general, it can be established that the crystallite sizes are greater for nahcolite than for natrite with a size around 50 nm. Figure 7b clearly shows that the kinetics of decarbonation is accelerated with the increase of temperature. For the temperature of 120 °C, decomposition takes approximately 80 min for all NaHCO3 to be transformed into Na2CO3, while it takes 30 min for 135 °C, 12 min for 150 °C, and only 6 min are needed at 165 °C for the reaction to be completed. 3.3. Carbonation Tests Discussion. 3.3.1. Carbonation Tests of First Type. It is interesting to note that the structure obtained for nahcolite at 70 °C is from space group P21/n, different from the P21/c obtained in all the other cases studied. Both structures are very similar, with slight differences in the way the atoms are arranged. In the first type of experiments (3.1.1), trona (Na2CO3· NaHCO3·2H2O) is present in all cases for the carbonation temperatures T = 60, 70, and 80 °C. A number of studies have been reported on the decomposition of trona to obtain natrite in the presence of CO2 and humidity,21,32,42 but the mechanisms of formation of trona from natrite in the range of temperatures used in this work have not been previously analyzed. In the same way, the nahcolite phase is also present at all three temperatures coexisting with trona. Trona is decomposed in wegscheiderite at 80 °C while coexisting with nahcolite according to the description made in previous works.21,25,43 In the cases of 60 and 80 °C, there is still a remnant of natrite that has not reacted completely. Prior to this work, no calculations of coherent crystal length sizes have been reported to our knowledge by in situ PXRD for the phases involved in the carbonation/decarbonation process summarized by eq 1. Images have been reported by scanning electron microscopy (SEM) of nahcolite, natrite, trona, and wegscheiderite phases formed or grown in N2 and CO2,42,44,45 or in an organic or mixed organic-aqueous solvent using ethylene glycol.21,25,43 From these images, it is possible to infer J



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Figure 9. Expanded regions of the diffractogram shown in Figure 5b on the PXRD pattern that started 105 min after the beginning of the experiment at T = 70 °C, illustrating the contribution of the main reflection peaks from the angle range between 27 and 38.5° for natrite (Na2CO3, thick black line), thermonatrite (Na2CO3·H2O, thick violet line), and trona (Na2CO3·NaHCO3·2H2O, thick green line) phases from the best Rietveld fit (thin red line) to the experimental pattern (thin blue line). The bottom gray line shows the deviation between the best fit and experimental diffractogram.

Table 3. Phases, Structure, and Space Group Observed in the Processes of Carbonation and Environmental Conditions of Appearance (Temperature, Gas, and % RH) phase

chemical formula

structure

Natrite Nahcolite

Na2CO3 NaHCO3

monoclinic monoclinic

Trona Thermonatrite Wegscheiderite

Na2CO3·NaHCO3·2H2O Na2CO3·H2O Na2CO3·3NaHCO3

monoclinic orthorhombic triclinic

space group

T (°C) at which appears

atmosphere

% RH at which appears

C2/m, 12 P21/c, 14 P21/n, 14 C2/c, 15 PCa21, 29 P1̅, 2

60, 70, and 80 60 and 80 70 60, 70, and 80 60 and 70 80

CO2 CO2 CO2 CO2 N2 CO2

90 90 90 90 90 90 37.5

80 °C, while at T = 70 °C (where natrite does not appear) it has a P21/n space group. Trona appears in all cases of the first (3.1.1) and second type (3.1.2) of carbonation tests, except for the second type (3.1.2) at 80 °C where natrite and wegscheiderite appear. Having reached the temperature of 80 °C in the presence of N2 and then introducing moisture and CO2, the system has not been allowed to evolve for the trona phase to appear. Thermonatrite only emerges in the carbonation experiments of the second type (3.1.2) at 60 and 70 °C under N2 when moisture is introduced. Once it appears, it is maintained after the introduction of CO2. The phase of wegscheiderite appears at 80 °C in CO2 with humidity in the two types of experiments (3.1.1 and 3.1.2), mixed with natrite, nahcolite, and trona in the first type (3.1.1) and mixed with natrite in the second type (3.1.2), taking into account that in this second case only a 37.5% RH is introduced. In the first type (3.1.1), in the 45 min of stabilization under CO2 and humidity, the system has been allowed to evolve with the four mentioned phases appearing. The only temperature where the complete transformation of natrite into nahcolite has been achieved is 70 °C for the first type (3.1.1) of carbonation tests. In Figure 10 we show a graphical representation of the crystal structures obtained in this work with their lattice parameters refined by the Rietveld method for the different temperatures. The represented phases are natrite (monoclinic structure, space group C2/m (12))1−3 at 70 °C, thermonatrite (orthorhombic structure, space group PCa21, 29)24 at 70 °C, nahcolite (monoclinic structure, space group P21/c, 14)4,5 at 60 °C, nahcolite (monoclinic structure, space group P21/n, 14)41 at 70 °C, trona (monoclinic structure, space group C2/c, 15)10,11 at 70 °C, and wegscheiderite (triclinic structure, space group P1̅, 2)22,23 at 80 °C. Sizes of the structures is on the same scale for them to be directly comparable.

glycol solution.46,48 In the paper by Oosterhof, a Coulter multisizer was used to measure the particle size distribution of the thermonatrite obtained.46 The particle sizes obtained for the temperatures of 50, 60, and 70 °C were compared, observing that, for 70 °C, slightly higher particle sizes were obtained than in the other cases. Even though particle size and crystallite size cannot be directly compared both, properties are usually correlated. Regarding the crystallite sizes obtained in our work, for the temperature of 70 °C, the crystallite size is always slightly higher than in the case of 60 °C, which is consistent with the results reported by Oosterhof.46 Figure 9 illustrates an example of the Rietveld analysis of the PXRD pattern that was started 105 min after the beginning of the experiment at T = 70 °C (Figure 4b), which yields the wt % of the three phases involved. Expanded regions of the angle range between 27 and 38.5° for natrite, thermonatrite, and trona phases can be seen, showing the main reflections for the different structures. The complexity of the adjustment shown in the example is also high, as in the case of the carbonation experiments of the first type (3.1.1).38 In previous published works on the phases present in the carbonation process of Na2CO3, no quantification of the present phases has been carried out by means of the Rietveld method.46,48,50 3.3.3. Crystallographic Transformations. Table 3 summarizes the phases observed during carbonation and the environmental conditions at which they appear (temperature, type of gas, and %RH) including their chemical formula, structure, and spatial group. Natrite is the starting phase, which appears at 60, 70, and 80 °C for all the carbonation experiments, except for first type tests (3.1.1) at 70 °C in which it does not appear. This is because the system takes 45 min to stabilize at T = 70 °C and 90% RH. Thus, the whole natrite transforms into nahcolite and into trona, which is a mixture of both phases. Nahcolite only appears in the first type of experiments (3.1.1), with a P21/c space group at T = 60 and K



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Figure 10. Crystallographic representation of natrite (Na2CO3, monoclinic structure, space group C2/m, 12)1−3 at 70 °C, thermonatrite (Na2CO3· H2O, orthorhombic structure, space group PCa21, 29)24 at 70 °C, nahcolite (NaHCO3, monoclinic structure, space group P21/c, 14)4,5 at 60 °C, nahcolite (NaHCO3, monoclinic structure, space group P21/n, 14)41 at 70 °C, trona (Na2CO3·NaHCO3·2H2O, monoclinic structure, space group C2/c, 15)10,11 at 70 °C, and wegscheiderite (Na2CO3·3NaHCO3, triclinic structure, space group P1̅, 2)22,23 at 80 °C including lattice parameters.

3.4. Decarbonation Tests Discussion. In the case of 120 °C where kinetics is the slowest, high intensity peaks are observed. This is explainable by texturization of the crystallites formed, which has been taken into account when calculating crystallite sizes (L) and wt %.40,51 Heda has studied the thermal decomposition of NaHCO3 under different atmospheres according to eq 1.26 Various nonisothermal methods of kinetic analysis were employed for estimating the Arrhenius kinetic parameters, activation energy, and frequency factor. The results showed that the most probable reaction mechanism under dry CO2 is the AvramiErofeev equation, with n = 1.5.26 They concluded that, in

general, the activation energy decreases with an increase in the heating rate and the onset and final decomposition temperatures increase with an increase in the heating rate. In the work of Liang, NaHCO3 was heated from ambient to 100 °C at a rate of 5 °C/min and then to 120 °C at 1 °C/min under a He gas flow. Decomposition of NaHCO3 to Na2CO3 was observed to begin at about 100 °C and was complete soon after the temperature reached 120 °C.49 A previous study on the calcination of limestone (CaCO3) showed that the presence of helium in the calciner environment leads also to a significant acceleration of the reaction, which is arguably due to the enhancement of heat transfer and CO2 diffusivity in He.52 In L



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Figure 11. Expanded regions of the diffractogram shown in Figure 9b on the PXRD pattern that started 45 min after the beginning of the decarbonation experiment T = 135 °C, illustrating the contribution of the main reflection peaks from the angle range between 28.5 and 35.5° for natrite (Na2CO3, thick black line) and nahcolite (NaHCO3, thick blue line) phases from the best Rietveld fit (thin red line) to the experimental pattern (thin blue line). The bottom gray line shows the deviation between the best fit and experimental diffractogram.

and a mixture of natrite, nahcolite, trona, and wegscheiderite at 80 °C. In general, crystallite sizes remain stable during these experiments. Once the system stabilizes, trona and natrite become the dominant phases at 60 °C, nahcolite remains as the only phase at 70 °C, and wegscheiderite, trona, and natrite coexist at 80 °C. In the second type of carbonation test, the sample was subjected to a N2 flow until target humidity and temperature conditions for the carbonation reaction were reached, after which CO2 was introduced for carbonation to evolve. In these tests, thermonatrite and trona have been obtained in addition to natrite at 60 and 70 °C and a mixture of natrite and wegscheiderite at 80 °C. In general, crystallite sizes remain stable except for natrite, whose crystallite size grows until it stabilizes. Once the system reaches a stable state, natrite and trona are the predominant phases at T = 60 and 70 °C, while natrite is prevailing at 80 °C. The crystallite sizes obtained for natrite are clearly greater than for the rest of the phases involved in both carbonation test types. The only temperature at which a complete transformation of natrite into nahcolite has been achieved is 70 °C under CO2 atmosphere with humidity introduced since the beginning of the experiment. The monoclinic nahcolite phase obtained in that case has a space group P21/n instead of the P21/c obtained for the rest of tests. In any case, the carbonation kinetics is rather slow with residence times for the reaction to be significantly advanced on the order of 60 min. In the decarbonation process under CO2, as required in practice, the initial NaHCO3 decomposes into Na2CO3 at neatly different rates depending on temperature. At T = 120 °C it takes 80 min for all NaHCO3 to be transformed into Na2CO3. This time is reduced to 30 min at T = 135 °C, 12 min at 150 °C, and just to 6 min at 165 °C. The crystallite size of natrite remains stable from its formation around 20−30 nm, whereas the crystallite size of nahcolite is greater than for natrite, with a size around 50 nm. In decarbonation, short residence times for the reaction to be fully achieved, as it is normally desirable in the practical application, are attained at temperatures above 150 °C.

the present work, the decomposition reaction has been carried out under pure CO2, which significantly slows down decarbonation. It must be remarked that realistic calcination conditions for CO2 capture necessarily involve calcination under a high CO2 concentration environment since CO2 has to be extracted in the most pure form possible from the calciner to be compressed and stored. To our knowledge, the present study analyzes for the first time both the kinetics and structural transformation of NaHCO3 decarbonation under these realistic conditions. As can be seen in 3.2., a useful, practical result from our tests is that sorbent regeneration in short residence times under a high CO2 concentration environment as needed in practice would require a temperature around 165 °C or above. In the study of Ball, it was reported, as observed in the present work, that wegscheiderite does not appear in the decarbonation process of NaHCO3.44 However, no quantification of the wt % of the present phases has been carried out to our knowledge using the Rietveld method for this reaction. Figure 11 shows an example of the Rietveld analysis results on the PXRD pattern that was recorded starting 45 min after the beginning of the experiment at T = 135 °C (Figure 9b), which yields the wt % of the two phases involved in the reaction. Expanded regions of the angle range between 28.5 and 35.5° for natrite and nahcolite phases can be seen, showing the main reflections for both structures. In this case only two phases appear in contrast with the carbonation tests, albeit deconvolution of the peaks has a similar complexity.38

4. CONCLUSIONS The DCP to capture CO2 relies on the use of natrite (Na2CO3) as a dry sorbent at relatively low to medium temperatures for both carbonation and sorbent regeneration, which would help reduce significantly the energy penalty for retrofitting fuel fired power plants with CO2 capture. In the present manuscript, the crystallographic transformation of the Na2CO3/NaHCO3 carbonation/decarbonation reactions has been analyzed in detail by means of in situ PXRD analysis at industrially relevant conditions for the first time to our knowledge. Carbonation of natrite in the presence of CO2 and H2O has shown a great diversity of emerging phases whose crystalline structures have been characterized. On the other hand, decarbonation of nahcolite under CO2 is far less complex with the only presence of the two mentioned phases. Two types of carbonation tests have been carried out. In the carbonation tests performed under CO2 and high humidity from the beginning of the test until reaching the carbonation temperature isotherm, nahcolite and trona have been observed in addition to natrite at 60 °C, nahcolite and trona at 70 °C

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AUTHOR INFORMATION

Corresponding Authors

*(S.M.-C.) E-mail: [email protected]. *(J.M.V.) E-mail: [email protected]. ORCID

Santiago Medina-Carrasco: 0000-0003-1353-9331 Jose Manuel Valverde: 0000-0002-2345-8888 Notes

The authors declare no competing financial interest. M



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ACKNOWLEDGMENTS This work was supported by the Spanish Government Agency Ministerio de Economiá y Competitividad (Contracts CTQ2014- 52763-C2 and CTQ2017-83602-C2, FEDER funds). We gratefully acknowledge the X-ray service of the Innovation, Technology and Research Center of the University of Seville (CITIUS).

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Crystallographic and Morphological Transformation of Natrite and

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