Understanding the Impacts of Impurities and Water Vapor on

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Understanding the Impacts of Impurities and Water Vapor on Limestone Calcination in a Lab-scale Fluidized Bed Shuai Guo, Hui Wang, Dunyu Liu, Li Yang, Xing Wei, and Shaohua Wu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.5b01218 • Publication Date (Web): 13 Oct 2015 Downloaded from http://pubs.acs.org on October 17, 2015

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Understanding the Impacts of Impurities and Water Vapor on Limestone Calcination in a Lab-scale Fluidized Bed Shuai Guo a, Hui Wang a,*, Dunyu Liu b, Li Yang a, Xing Wei a, Shaohua Wu a a

b

School of Energy Science and Engineering, Harbin Institute of Technology, Harbin 150001, China;

School of Energy and Power Engineering, University of Shanghai for Science and Technology, Shanghai 200093, China

Abstract: In-furnace desulfurization has been widely used in circulating fluidized bed (CFB) boiler. SO2 is removed by reacting with limestone during the process of sulfation after calcination in air combustion. Although CaCO3 is the main component of limestone，there are also other impurities such as CaMg(CO3)2 and SiO2 which can influence the desulfurization. The porous CaO produced by calcination plays an important role in sulfation, and water vapor in the furnace influences the calcination. This paper aims to understand the impacts of impurities and water vapor on limestone calcination. Two kinds of China limestone were used to investigate the issues in a rotatable Fluidized-Bed

Reactor

(FBR).

Mercury

injection

apparatus

(MIP),

scanning

electron

microscope-Energy Dispersive Spectrometer (SEM-EDS), and X-ray diffraction (XRD) techniques were employed to analyze the pore structure, micro-morphology and crystal structure of the CaO calcined respectively. The results show that the water vapor improves calcination rate and shortens the reaction time and those influences are stronger for higher impurity limestone possibly because of more defects in the crystal structure. Water vapor can directly influence the chemical reaction of calcination without affecting the diffusion property of CO2. Higher water vapor content results in slightly lower ultimate degree of conversion of limestone, but for different kinds of limestone the difference is not obvious. The results of SEM and MIP also mean that the existence of water vapor improves sintering and growth of grains. The results of XRD give further evidence to the previous conclusion. These tests and analysis give rise to the mechanisms behind the impacts of water vapor

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on limestone calcination: the binding ability of H2O to active site O* in Ca-O is stronger than that of CO2. H2O tends to replace CO2 on the active site to increase the release of CO2 in calcination. Water vapor also accelerates sintering, most possibly in the initial stage when sintering neck is formed. There exist two possibilities: H2O molecules are absorbed on the active site of Ca-O* to promote the formation of the sintering neck of CaO by interaction between H2O molecules (such as hydrogen bond). Water vapor can also act as a solvent to improve the solid state diffusion from surface to sintering neck which also benefits the fusion and growth of minicrystals. Key words: Impurities; Water vapor; Limestone calcination; Mechanism; Fluidized bed

1. INTRODUCTION In-furnace desulfurization has been widely used in CFB boiler. Limestone or dolomite with certain particle sizes are normally added as sorbents into furnace where coal and limestone particles are suspended and mixed both horizontally and vertically. SO2 emitted by burning coals is removed by reacting with sorbents in the furnace

[1,2]

.The combustion temperature of CFB (800-950 °C) is

considered as the best temperature for the desulfurization by limestone [3]. When the molar ratio of Ca/S is above 2, more than 90% of SO2 is removed while only 20-60% of sorbents are effectively used [4]. Compared with wet flue gas desulfurization, this dry method of capturing SO2 in situ has the advantages of low cost, high flexibility and low water consumption. That is why it is widely acknowledged and applied. There have been various studies about the technology of desulfurization at high temperatures in the boiler where coal is fired with biomass [5,6], the effect of CO2 in oxy-fuel combustion [2,7] and modification of sorbents [4,8]. However, the problem on utilization of sorbents is still unsolved.


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The major mineral components of limestone are CaCO3 and CaMg(CO3)2, and there are also minor impurities in natural limestone such as SiO2, Al2O3 and Fe2O3. The impurities are mostly oxides which have good thermo-stability as well as high melting points. So under the temperature of CFB boiler, only the major components of limestone are subject to decomposition and the calcination products are then reactive to SO2. The reactions involved are (1) - (4), in which (2) is called calcination of CaCO3, (3) and (4) are called sulfation of CaO and MgO, respectively. But (4) is restrained beacuse MgSO4 decomposes at higher temperatures above 900 °C. CaMg(CO3)2 (s) →CaCO3 (s)+MgO (s) +CO2 (g) CaCO3 (s) →CaO (s) +CO2 (g) CaO (s) +SO2 (g) +1/2O2 (g) →CaSO4 (s) MgO (s) +SO2 (g) +1/2O2 (g) →MgSO4 (s)

（1）（2）（3）（4）

As soon as limestone particles are added into furnace, decomposition occurs at high-temperature, and produces porous CaO that has large specific surface area which is of great importance to the following sulfurization. The results from Borgwardt [9,10] showed that the specific surface area of CaO calcined was more than 104 m2/g and sulfurization reactivity increased with the BET surface area of CaO following a power law. With the progress of calcination, the porous CaO which is under high temperature will sinter [11]. As a result pores are closed, specific surface area reduces and desulfurization ability reduces. The decomposition of limestone which contains more CaMg(CO3)2 could show some different characteristics [12,13]. The research about the influences of water vapor on CaMg(CO3)2 decomposition has been scarce[14]and not in-depth enough. Meanwhile, the components of impurities contained in limestone also impact the calcination reaction in many aspects. The research results of Sulaiman [15] and Anbalagan [16] show that the presence of impurities enhances the


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rate of calcination and lower the decomposition temperature of limestone, and the activation energy for decomposition of limestone also decreases compared with pure calcium carbonate

[10,17]

, the

reason of which may be that impurities could function as catalysis owing to their influence in the crystalline structure and increase the concentration of defects in the crystal lattice Borgwardt et al.

[10]

[16]

. Besides,

found that the sintering of CaO has been enhanced by the presence of impurities

ions in the crystal structure of CaO so that the impurities also affect the micro-morphology and pore structure of the CaO calcined. The results of Beruto

[18]

et al. indicate that impurities play a major

role in changing the kinetics of the decomposition and the subsequent sintering of CaO caicined to form different micro-morphology patterns. Manovic

[19]

et al. also indicate that in the presence of

impurities, melting and change in particle morphology can be seen even under those relatively mild conditions. Souza

[20]

et al. point out that the presence of magnesium and other impurities favors the

opening of porosity prior to CaCO3 calcination. For coal-fired CFB combustors, the flue gas usually consists of about 15% CO2, 3-5% O2, 5-15% H2O, small amounts of SOx, NOx and N2 balance

[21,22]

. For example, water vapor accounts for 8.4%

after burning East Kentucky coal; under the same condition, there are 12.4% water vapor for Sack lignite and 10.6% for Highvale sub-bituminous coal. CFB boiler is normally used to burn low quality fuels such as high moisture lignite, which produces as high as 20% water vapor. Its high concentration will influence desulfurization of limestone to some extent[21- 23 ] but previous researchers seldom considered water vapor which was also neglected by some mathematical models related to desulfurization[24,25]. The influence of water vapor is both on the calcination of CaCO3 and the sulfation of CaO. This paper focuses on the former. Previous studies mainly focused on the influence of water vapor on the decomposition rate and


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temperature of calcium carbonate. Senum

[26]

(1976) gave a review about the influence of water

vapor in a US government report. Controlling factors of decomposition reaction of limestone are heat transfer, mass transfer and chemical reactions. Water vapor can improve the reaction as a catalyst, the effect of which is weaker with the increase of temperature. Boynton [27] and Burnham [28] (1980) have also drawn similar conclusions. They conducted the experiments on impacts of H2O (g), CO2 and N2 on the calcination at temperatures from 400 to 900 °C, and a heating rate of 4 °C/min. In experiments, the formation of CaO calcined was monitored by XRD. Results show H2O (g) has the largest impact, followed by CO2 and then N2. The decomposition rate of limestone increases and decomposition temperature decreases with the introduction of water vapor. Kim [29] et al. (1998) also point out that the degree of conversion of limestone calcination had been improved with 8% H2O (g) addition. Recent experiments from Shang [30] and Song [31] (2010) confirmed the previous conclusions. These experiments were preceded in TG with 5, 10 and 15% water vapor provided respectively under temperatures from 700 to 850 °C and at the heating rate of 20 °C/min. The fact is that as the concentration of water vapor increases, the decomposition rate increases. Besides, Wang [32] et al. (2008) studied the influence of water vapor on limestone calcination in a FB reactor at high CO2 concentrations. CO2 was diluted with 60% water vapor or N2 respectively. Results show that both of them improve the decomposition rate but to different extents. Since thermal conductivity of water vapor is larger than N2, it is deduced that the influence results from different heat transfer abilities of reactant particles at different atmospheres. Water vapor can improve the sintering in calcination by influencing the pore structure. Borgwart [33]

et al. (1989) conducted calcination experiments on a 10 µm particle size and CaO obtained was

further sintered at CO2 and water vapor atmospheres. These tests of pore structure by N2 absorption


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method showed that the existence of CO2 and water vapor promoted sintering and decreased the specific surface area and porosity of CaO calcined. Khraisha

[34]

et al. (1991) also carried out

experiments on limestone decomposition in a suspension reactor at temperatures from 801 to 853 °C in presence of water vapor. The concentration of water vapor ranged from 2.22 to 6.09%. It was shown that the degree of conversion was improved when the concentration was 2.22% and reduced when the concentration was 6.09%. Therefore, different concentrations of water vapor play different roles in decomposition. A low concentration has a positive role as a catalyst while a high concentration has a negative role by sintering. Thus, an optimum concentration of water vapor exists in improving calcination. Soares

[35]

et al. (2008) indicate that water vapor can not only catalyze

reaction but also promote sintering. Recently, Huang [36] (2013) and Wang [37] (2014) used a tube furnace to study the concentration of water vapor on calcination. The results showed that with the increase of the concentration of water vapor, calcination time, the specific surface area, and the specific pore volume of CaO calcined were reduced. This indicates that pore distribution moves towards macropore. Furthermore, water vapor enhances the degree of conversion of CaCO3 and a higher concentration is usually more effective. The study above is mainly focused on the impacts of water vapor on the reaction rate, degree of conversion, decomposition temperature and pore structure characteristics, but without discussing the mechanisms behind. This kind of study is still scarce. Maclntire

[14]

et al. (1953) pointed out that

water vapor was a catalyst in calcination of dolomite or limestone. McIntosh [38] et al. (1990) also conducted an experiment with TGA which showed that the decomposition temperature under wet N2 atmosphere was lower than that under dry N2 atmosphere. The reason is possibly that H2O molecules replace -CO2 in CaCO3 (CaO-CO2) and therefore the reaction rate is higher. Wang [39] et al. (1995)


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studied the catalytic mechanism of water vapor for limestone calcination on the basis of chemical reaction kinetics by TGA and DXRD. The comparison on the absorption characteristics between H2O and CO2 suggests that under 300 °C, H2O and CO2 are absorbed on the surface of limestone; the former owns a stronger force to replace CO2, which leads to the fact that the absorption of H2O can weaken the connection between CaO- and -CO2 so that CaCO3 decomposed faster. This compensates the negative effect for CO2 diffusion because of water vapor to some extent. Overall, there have been many studies about this issue but mostly in fixed bed reactor or TGA but rarely in a fluidized bed. Khraisha [34] and Wang [32] conducted experiments in a suspended bed. This is also different from CFB, and there was too much CO2 in Wang’s experiments. The particle size, temperature and reaction time involved were also different from those in actual CFB boiler. Based on the above two points, it is necessary to simulate the temperature and particle size of actual CFB boiler for studying the influence of water vapor on calcination. Besides, the combined impacts of impurities and water vapor have not been mentioned in published papers. Therefore, the influence of CaMg(CO3)2 and impurities on limestone calcination in presence of water vapor is another focus of this study . This paper focuses on the calcination reaction rate and degree of conversion of two kinds of limestone which contains different mineral compositions. The experiments were conducted under different concentrations of water vapor from 0 to 15% in a FB reactor under 850 °C. SEM-EDS was used to observe the micro-morphology of the CaO calcined. The pore and crystal structure were tested with MIP and XRD to understand the impact mechanism.


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2. EXPERIMENTAL SECTION 2.1. Samples. In the experiment, two kinds of limestone from different geographical origins in China were used as the reactant. The components are listed in Table 1 according to the China Standard GB/t 3286-2012. The particle size distribution was measured by a Malvern Mastersizer 2000 as shown in Figure 1 and the specific surface area of the two kinds of limestone are also listed in Table 1. Table 1 Figure 1 2.2. FBR Apparatus: Description and Operating Conditions. Figure 2 The experiment was conducted in a rotatable Fluidized-Bed Reactor (FBR), as shown in Figure 2, which consisted of a preheating section, a furnace section, a water-vapor generator, the measuring section, the gas-supply section as well as the heating and insulation section. The mixed gas (including water vapor) was fed into furnace after being heated by the preheating section and the released gas was then extracted for testing its components quantitatively. The main devices included: (1) a preheating section: the temperature was controlled by 5 kW electric heaters; (2) a furnace section: the rotatable design was useful for replacing the reactants with new ones quickly without wasting time on the cooling of the furnace. The inner diameter of the furnace was 50 mm and the height was 950 mm. It was made of 310S stainless steel and circled by 8 kW electric heating tube furnace. The heating process was controlled by a PID controller. A thermocouple of 1 mm diameter was inserted into the furnace for monitoring the actual temperature with an inspecting instrument (Omega-DaqPRO 5300, Israel); (3) the water-vapor generator was made of the source, peristaltic


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pump, heater and vaporization pipe. The source was provided by ultra-pure water (Ulupure, 18.25 MΩ•cm) excluding the effects of Na+, Ca2+ and Mg2+ in tap water; The flow rate of water was controlled by a peristaltic pump (LEAD LIUID-BT50S) at 0.00016-190ml/min and its rotating speed was 0.1-50 rounds/min at the precision of 0.5%; The electric power of the heater was 2.5 kW and the working temperature was 500 °C; a vaporization pipe in the heater was spiral to improve the degree of vaporization and gas mixing, which also made the structure of heater more compact shown in Figure 2. It was made of 316L stainless steel with the pipe diameter of 16 mm, the working temperature being 500 °C. The working process of the water-vapor generator can be given as follows. Water was firstly fed into the vaporization pipe through a peristaltic pump at a controllable speed through a silica gel tube. The amount of water was limited by the rotational frequency of the peristaltic pump. The liquid water became high temperature steam after the vaporization pipe. The temperature of the heater was kept at 500 °C to ensure the complete vaporization of liquid water. After uniformly mixing water vapor with stimulated flue gas in the vaporization pipe, they passed from the outlet of vaporization pipe into the preheating section. To prevent water vapor from condensing, heating tapes (200 °C) which were covered with thermal insulation materials and high temperature resistance tinfoil were wrapped around the pipe between the water-vapor generator and the preheating section; (4) the gas-supply section: the flow rate of gases were controlled by mass flow meters (Sevenstar, China). The detailed description of limestone calcination was as follows: 20g limestone was introduced into the furnace when the inner temperature reached 900 °C. Limestone was dried beforehand in a vacuum drying oven for 2 hours at 110 °C to eliminate the liquid water. The mixture of N2 and water vapor which was being preheated passed through a gas distributor below the furnace at a rate of 25


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L/min so that limestone was completely fluidized. The amount of water vapor was determined by the rotating speed of peristaltic pump with its volume fraction from 0 to 15%. The concentrations of gases were monitored by the FTIR (GASMET-DX4000, Finland), the average values of which were determined as the actual concentrations in the experiment. Figure 3 shows that the concentration of water vapor and the mass of the liquid water follow linear relationships against the frequency of peristaltic pump. FTIR testing results agree reasonable well with those from theoretical calculations. The reaction rate and degree of conversion were calculated by the amount of CO2 measured by an infrared CO2 analyzer (measuring range 0-20%, precision ±1% FS). After the reaction was completed, the CaO caicined was taken from the rotating furnace after same residence time in the furnace so that the sintering condition would be the same. The residence time was 1 min in the experiment. In the whole process, there was a thermocouple inserted into the furnace to monitor the temperature inside. Figure 4 shows that temperature firstly decreases then increases and finally stabilizes. The initial drop in the temperature is because the calcination is an endothermic reaction. The absorption of heat causes the temperature to decrease and then return after the reaction is complete. When the samples are fed into the furnace at 900±2 °C, its average temperature of the reaction is 850±5 °C which is the actual temperature of the calcination by experience. The conditions of experiments are shown in Table 2. Figure 3 Figure 4 Table 2 The CaO calcined at different concentrations of water vapor were taken from the furnace and tested for micro-morphology, element distribution, pore structure, mineral compositions and crystal


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structure after cooling. The micro-morphology was observed by a SEM of Quanta 200EFG with the magnifying power of 12- >1,000,000x and the resolution power of 1.5 nm. EDS uses SDD to get the energy resolution power of 128eV at the counting rate of 60,000 CPS for qualitative or quantitative analyzing the elements of micro-area (elements from B-U). The porosity, pore size distribution, specific surface area and specific pore volune were measured by an AutoPore 9500 Mercury Injection Apparatus made by Micromeritics from US. The low pressure of the apparatus was 227.527 kPa with the range of pore size from 3.6 to 360 µm and the high pressure was 227.527 kPa with the pore size at 5.5 nm. Mineral compositions and crystal structures were studied by XRD (Empyrean, Finland) using Cu target as X-ray source in the 2θ range of 0-120°, with a step size of 0.01313° 2θ and a counting time of 78.795 s per step. The X-ray tube was operated at 40 kV and 40 mA. The samples were powdered by hand using an agate mortar. The mineral compositions and crystal structure of the samples were interpreted and modeled using the Whole Pattern Fitting and Rietveld (WPFR) Refinement method by jade 6.5.

3. RESULTS AND DISSCUSSION 3.1. The Influence of Impurities and Water Vapor on Calcination. 3.1.1. The influence of Degree of Conversion. The limestone calcination follows the principle of the shrinking core model, that is, the reaction proceeds from the outside surface of the reactant to inside. As the reaction goes on, product layer gradually gets thicker, but the reaction happens invariably on the boundary of CaCO3 and CaO, and the CO2 spread from CaO calcined layer. So the limestone calcination is controlled by three factors [40]

: ① heat transfer, namely the process where heat is transferred from the CaO calcined layer to

the interface where CaCO3 is decomposed; ② chemical reaction, namely the decomposition of


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CaCO3 to CO2 and CaO; ③ mass transfer, namely the process where the released CO2 is transferred from internal surface of CaO calcined layer to the external surface then to the environment. Wang [39] pointed out that when the particle size of CaCO3 was large heat transfer and mass transfer were the main controlling factors. Only when the size was small (according to other researchers, no more than 38 µm), the chemical reaction itself was the main controlling factor. That is why the small particle size is chosen in the study of CaCO3 chemical reaction kinetics. Yu

[41]

made use of MFBRA to

study the decomposition kinetics of CaCO3 particles in the temperature range of 700-900 °C. The size was from 45 to 75 µm to minimum the effect of heat transfer and mass transfer. The apparent activation energy was calculated as 142.73 kJ/mol. When the temperature is high (>800 °C), the influence of heat transfer on the calcination is small enough to be ignored and the process becomes limited by the production and diffusion rate of CO2. The former is controlled by the chemical reaction itself and the latter by the internal and superficial pore structure of CaO calcined layer. Since the particle size chosen in this paper is relatively large (d50>290µm), it is speculated that the process is limited by CO2 diffusion property at the temperature of this paper (T≈850 °C). In other words, the process becomes limited by the diffusion, since chemical reaction rate is high. Therefore the decomposition process of CaCO3 particles can be characterized according to the CO2 released, and the degree of conversion can be calculated by the measuring results of CO2 releasing [41]. Figure 5 Figure 5 shows how degree of conversion changes in the limestone calcination in presence of water vapor. Three stages can be observed [41]: ① the first stage is very short. In this stage, limestone is heated and the temperature increases quickly; ② in the second stage, degree of conversion of limestone increases quickly as reaction progresses; ③ the third stage is the one at which the degree


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of conversion increases slowly and finally becomes steady. As is shown in Figure 5(1), when the concentration of water vapor increases, the reaction rate in the second stage also increases and the reaction time is shortened. In other words, the turning point from the second stage to the third one appears earlier but no changes happen to the third stage. If moving the curve describing how degree of conversion changes in 15% H2O along the X-axis, we can find that after the turning point, a nearly complete overlap exists for different concentrations of water vapor, meaning that water vapor cannot affect the third stage of limestone calcination. The calcination of limestone is accompanied by the production and diffusion of CO2. Considering that the chemical reaction is strong in the second stage, water vapor may affect the chemical reaction itself directly but it cannot exclude the possibility that water vapor could influence the diffusion property of CO2. In order to ensure the influence that water vapor has on calcination, the diffusivities of CO2 at different atmospheres are calculated according to the equation presented elsewhere by Poling [42]. The calculative results show that the diffusivity of CO2-N2 (1.76 cm2/s) is higher than that of CO2-H2O (1.69 cm2/s), so the existence of water vapor does not favor the diffusion of CO2. Thus, the influence of water vapor is on the chemical reaction itself, namely water vapor can affect the process of producing CO2 instead of the diffusion property. Figure 5(1) also shows that as the concentration of water vapor increases, the ultimate degree of conversion becomes lower slightly. Konist

[43]

have

also drawn the similar conclusions in his research. He pointed that the degree of conversion of limestone calcination decreased at water vapor atmosphere and further decreased with the increasing of water vapor. Khraisha

[34]

explained that the negative influence of water vapor on limestone

calcination was because the existence of water vapor decreased the diffusion rate of CO2. Figure 5(2) are the curves showing how degree of conversion changes for different limestone in calcination with


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the same concentration of water vapor. Figure 5(2a) shows that different limestone at the same concentration of water vapor share nearly the same reaction rate in the second stage but the durations are different, and Hebei limestone which contains more impurities have a longer duration, in other words, the turning point for the second and the third stage of Hebei limestone occurs later. If moving the curve of Liaoning limestone along the X-axis for about 40 s, the turning points of different limestone are nearly on the same vertical line, which means that the second stage of calcination reaction for Hebei limestone in 5% H2O lasts 40 s longer than that for the other one, but the total reaction time for Heibei limestone is shorter, so the Hebei limestone releases CO2 faster. Borgwardt [44]

explained that when the reactants contained more impurities, the defects of crystal as a result

accelerated the solid state diffusion during the reaction. It may be the reason why the releasing rate of Hebei limestone is faster than that of Liaoning one. Figure 5(2b) shows that the results above have nothing to do with water vapor. So it can be speculated that the reason may be related to the characteristic of limestone itself. Specifically more impurities in Heibei limestone mean more defects in crystal structure favorable for chemical reaction. Figure 5(2) also shows that the changing characteristics of degree of conversion curves for various kinds of limestone are different in the third stage, mainly because of the difference in pore structures. However, the ultimate degree of conversion for each limestone is all above 90% without obvious differences. 3.1.2. The influence of micro-morphological. Figure 6 Figure 6 shows the micro-morphology of Liaoning limestone calcination products at different concentrations of water vapor. Figure 6(a) shows the calcination products without water vapor under the magnification of 10000 and 40000 times. It can be seen that the CaO particles are composed of


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many small grains which are also composed of many smaller mini-crystals. Figure 6(a)-(c) show that the calcination products without water vapor is full of obvious stripe gaps which divide the surface of CaO particles into many grains, and the grains are composed of many smaller mini-crystals. Besides, the boundaries between grains and mini-crystals are all quite clear. When there is 5% H2O, gaps on the surface of calcination products decrease obviously and are replaced by round tiny pores. It can be clearly seen that the edges on either side of the gaps have been connected and gradually disappeared. Under the magnification of 40000 times, it can be seen that two grains have been fused but there is an unobvious boundary between them. The same thing has happened to mini-crystals. Above all it can be proved that the existing of water vapor could improve the fusion of mini-crystals and make the gaps between them disappear. Figure 6(c) shows that when there is 15% H2O, although several gaps again appear on the surface of calcination products, fusion phenomenon between grains or mini-crystals is more obvious. For example, in Figure 6(b), many round tiny pores are replaced by fairly smooth surface. The surface on the whole particle is so smooth that it is impossible to distinguish different grains or mini-crystals. Chen [45] pointed out that under oxy-fuel combustion, the high concentration of CO2 will improve the growth of grains in CaO calcined. Thus, it can be speculated that water vapor can also increase the grain sizes of calcination products and the higher the concentration of water vapor, the larger the grain size will become. Figure 7 Figure 7 shows the micro-morphology of Hebei limestone calcination products at different concentrations of water vapor. To have a more comprehensive understanding of the micro-morphology of calcination products, the magnification times are increased from 5000 to 40000 which are much wider than the previous one. Figure 7 shows that there are obvious stripe gaps


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on the surface of calcination products without water vapor and two kinds of crystal morphology exist on the surface. One kind contains independent grains but the grains of the other kind have already fused each other. It is different from the limestone from Liaoning at the same concentration of water vapor, as shown in Figure 6, which only contains one kind of crystal morphology on the surface. The reason may be that the Hebei limestone contains more CaMg(CO3)2 which can decompose at the temperature of 250-300 °C, as is shown in Reaction 1. Figure 8 Table 3 In the experiments of 850 °C, the MgO calcined will experience serious sintering which changes the crystal morphology. As CaCO3 has a high decomposition temperature, the CaO calcined still maintains the initial crystal morphology. It may be the reason why the calcination products of Hebei limestone contain two kinds of different crystal morphology. EDS was used to measure the different crystal morphology on the surface, the results are shown in Figure 8. The mass and atomic number percentage of each element is given in Table 3. It can be seen that different kinds of crystal morphology on the surface are corresponding to different element distribution, which can be specifically reflected in the different content of Ca and Mg. So the hypothesis above that different kinds of crystal morphology are mainly caused by the sintering of MgO can be justified. It should be noticed that much Si and Fe are contained in the results. The reason can be inferred that during the formation of limestone in the nature, SiO2 and Fe2O3 which have higher hardness in limestone is exposed to the outside while CaCO3 which has lower hardness is wrapped inside [46]. When CaCO3 experiences decomposition at the high temperature of 850 °C, SiO2 and Fe2O3 are nearly insusceptible due to the high melting point, leading to the enrichment of Si and Fe outside.


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Furthermore, only the radial depth of 0.5-5 µm on the surface can be precisely measured by EDS, which may cause the high contents of Si and Fe in the results. After magnifying the products calcined for 40000 times in Figure 7(a), it can be seen that although some of the grains on the surface have fused, most of them still maintain independent with each other and have very obvious gaps on the surface. The gaps divide the surface of calcination products into smaller grains of different sizes. Figure 7(b) shows that the gaps on the surface of calcination products have obviously decreased when there is 5% H2O. This is probably due to the fact that water vapor can accelerate the fusion of grains, which could make the gaps close. This conclusion is further justified by the magnification time of 40000. It is very obvious that different grains are fused on the surface and the boundary between them blurs. Some of the grains even have completely fused and formed a very smooth surface only with few unobvious boundaries between them. Figure 7(c) is the micro-morphology of products in 15% H2O. It can be seen that very obvious gaps again appear on the surface of calcination products just like Figure 7(a) but with fewer number, and it is obvious that the two sides of the gaps was connective previously. Thus it can be inferred that the gaps are caused by force which is probably stemming from the internal force caused by the releasing of accumulated CO2 during calcination. It can be seen from Figure 7(c) of which the magnification time is 20000 that the grains on the surface of calcination products have been obviously fused, some of which have been connected to very smooth surface. In addition, when there is no water vapor, boundary between the two kinds of crystal morphology blurs. When the magnification time is 40000, it can be seen that the smooth surface due to the fusion of grains is more than that in Figure 7 (a) and (b), which means that the fusion of grains is serious. The results above show that water vapor has obvious influence on the limestone calcination. The


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limestone calcination is a typical shrinking core reaction, which means the reaction starts layer by layer from the outside to the inside of the reactant. As the reaction goes, the product layer becomes thicker. The resistance against the diffusion of CO2 becomes larger. Gradually the reaction is limited by the diffusion of CO2 [47]. When there is no water vapor, the gaps on the surface of the calcination products are caused by the diffusion of CO2 through the inside pores to the external surface of particles then to the environment. The testing results prove that when there is 5% H2O, the existence of water vapor will promote the fusion and growth of grains or mini-crystals which make the pores close and also cause the stripe gaps to disappear. When there is 15% H2O, the sintering phenomenon on the surface becomes more obvious, leading to the formation of the smooth areas on the surface. 3.2. Mechanisms of Impurities and Water Vapor on Limestone Calcination. 3.2.1. Pore Structure. Figure 9 Figure 9 shows how pore size distribution of different limestone changes with different concentrations of water vapor. Pore sizes of 10-100 nm are more obviously influenced by water vapor. As the increasing of water vapor, the average pore size will decrease and the pore size distribution will be more concentrated. This proves that water vapor will decrease the number of pores because of sintering, which is achieved by accelerating the fusion between grains. The sintering can normally be divided into three stages: the initial, the second and the final one. In the initial stage, the boundary between minicrystals forms sintering neck by mass transfer such as evaporation-condensation or diffusion; in the second stage, atoms move to the boundary between minicrystals to enlarge the sintering neck and minicrystals will fuse and grow; in the final stage, minicrystals continue to grow and densification becomes stronger. Water vapor is most likely to


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affect the mass transfer by diffusion in the initial stage of sintering. There are three ways of diffusion: surface-sintering neck, lattice-sintering neck and crystal boundary-sintering neck, of which the first one is likely influenced by water vapor

[48]

. Specifically, H2O molecules possibly improve the

solid-state diffusion from surface to sintering neck as a solvent. The improvement of mass transfer by diffusion accelerates the fusion of minicrystals as well as sintering. Figure 9 also shows that for Hebei limestone there is a small peak within the pore sizes of 20-30 nm whether with or without water vapor. It is speculated that the pores may be generated by CO2 released from CaMg(CO3)2 or MgCO3 calcination. Comparing with Liaoning limestone, Hebei limestone will release CO2 twice. The CO2 released in the first time will generate pores with very small sizes on the surface of calcination products due to large resistance for CO2 releasing. While the CO2 released in the second time will enlarge the pore size to be 60-100 nm through the previous sizes of 20-30 nm. This may be the reason why there are pores of two ranges on the surface of Hebei limestone calcination products. What’s more, the small peak within the range of 20-30 nm reflects, to some degree, the sintering degree of MgO. The peak height gradually decreases with the increasing of water vapor, proving that the sintering of MgO becomes serious and the number of pores becomes less. 3.2.2. Mineral Compositions and Crystal Structure. The above results prove that water vapor can improve the sintering. Since the inner mechanism of sintering is the fusion and growth of minicrystals, this part will focus on the influence of water vapor on crystal structure. Figure 10 Figure 10 shows the XRD patterns and mineral compositions of different limestone calcination products at different concentrations of water vapor, of which the mass percent of each mineral


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composition is calculated by semi-quantitative analysis with the K value method[49]. Figure 10(a) shows that the XRD pattern of Liaoning limestone calcinaton products mainly contain three phases, the main phase CaO crystal (2θ=32.1°,37.2°,53.8°,64.1°,67.4°,79.6°,88.5°), the sub-phase CaCO3 crystal (2θ=29.3°) and the impurity phase SiO2 crystal (2θ=26.5°). The main phase is the product of calcination and the sub-phase is the reactant. From Figure 10(b), it can be seen that with the increase of water vapor, the content of sub-phase increases gradually, meaning that the degree of conversion of limestone reactant goes down to some degree. For Hebei limestone, besides the above-mentioned three phases, there is still a certain number of MgO crystal (2θ=42.9°, 62.2°) and CaMg(CO3)2 crystal (2θ=30.9°, 51.1°). This is similar with the components of Hebei Limestone shown in Table 1, meaning that the reactants of the limestone contain not only CaCO3 but also CaMg(CO3)2 and MgCO3. In addition, with the concentration of water vapor increasing, the content of undecomposed reactants increases gradually, this is in accordance with the previous conclusion (Figure 5). That is to say, the existence of water vapor will play a positive role on the degree of conversion of limestone calcination. Borgwardt

[9, 10]

pointed that sintering changed pore structure of CaO calcined by influencing the

size of minicrystals; since the study shows that water vapor could improve sintering, it is speculated that it will affect the minicrystals size of CaO calcined. When the micro strain does not exist, the size of minicrystals can be calculated by Scherrer equation below. L=

kλ β cos θ

（5）

Here L is the minicrystal size of CaO calcined, Å；β is the main peak broadening in the XRD pattern; λ is X-ray wavelength, 1.5406Å; θ is Bragg angle, degree; k is Scherrer constant, generally equals to 1.


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Figure 10(a) and 10(c) show that with the participation of water vapor, the main peak of CaO is higher and narrower but without any change in position. The results allow the calculation of minicrystal size by the Scherrer equation since the microstrain does not exist and speculates that as the concentration of water vapor increases, the size of minicrystals increase. The minicrystal size firstly increases but then decreases with the concentration of water vapor increasing shown in Figure 11. Figure 11 Figure 12 Figure 12 reflects the calcination mechanism of limestone. The irregular crystal structure of CaCO3 makes it easy to decompose. At the decomposition temperature, it firstly breaks the chemical bond of C-O/O-O in crystal structure of CaCO3 like in positions 1, 2 and 3, where the active sites of O are formed (that is O* ). The isolated O-C-O (CO2) is then absorbed by O* instead of being released as a gas as Equation (6) shows. After that, O-C-O absorbed on the active site of Ca-O* is desorbed and released as CO2 (shown in Figure 12(c)) and leaves the crystal structure of CaO with the shape of regular hexahedron. Ca is located at the center of each face while O is at the body-center (shown in Figure 12(b)). Its shape of crystal structure makes it relatively stable with a high melting point and difficult to decompose. The mechanism of CaCO3 calcination can be described in Equation (7) and (8) [39], in which nLCO2 indicates that a CO2 molecule absorbs in n active sites L by chemical interaction. The range of n is between 0-2. Ca—O*—O=C=O—O*—Ca

（6）

CaCO3 nL →CaO+ nL(CO2)

（7）

nL(CO2) →nL + CO2


（8）

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Wang [39] explained the mechanism on how H2O improved the calcination that the binding ability of H2O with the active site O* was stronger than that of CO2. So CO2 was replaced by H2O and released as gas phase. The replacement of the active site by H2O and CO2 follows the Equations (9) and (10). That is, a CO2 molecule needs two active sites while an H2O molecule needs only one, which explains why CaO absorbs more H2O than CO2. H2O+* →H—O—H

（9）

* CO2+2* →O=C=O

（10）

*

*

It has been considered that CaO consists of hexahedron unit cells shown in Figure 12(b). The above study points out that water vapor influences the formation of sintering neck. When all the active sites of a unit cell are occupied by H2O, the adsorbed H2O molecule interacts with another one that is also adsorbed in another unit cell by intermolecular force such as hydrogen bond, which in turn accelerates the sintering as well as the growth of minicrystals. The process is shown in Figure 13. As mentioned before, H2O molecules can also act as a solvent to accelerate the mass transfer between the surface and sintering neck. A higher concentration of water vapor brings stronger influence. The high speed of minicrystals fusion leads to the closure of pores as a result of sintering before the second stage of calcination finishes. The accumulation of CO2 gives enough internal stress to break the CaO calcined layer. Therefore, the fused minicrystals are broken and the grain size slightly reduces but still higher than that without water vapor shown in Figure 11, and leaving the obvious gaps seen in Figure 6(c) and 7(c). Due to the fact that the temperature in calcination is very high, the crystal structure which is formed by the active site of O* in Ca-O and H2O is unsteady. It may explain why there is no Ca (OH)2 in the CaO calcined.


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Figure 13

4. CONCLUSION Limestone calcination experiments were conducted in a rotatable FBR to study the influence of impurities and water vapor concentration on calcination reaction rate and degree of conversion. Besides, MIP, SEM-EDS and XRD were also used to study the mechanism. The conclusions are as follows: (1) The existence of water vapor will improve the limestone calcination reaction and shorten the reaction time and the influences are stronger when containing higher content of impurities are contained possibly because of more defects in the crystal structure of Hebei limestone; water vapor can directly influence the reaction itself instead of the diffusion property of CO2. The ultimate degree of conversion goes slightly lower as the concentration of water vapor increases. (2) The results of SEM-EDS show that water vapor can improve the sintering of crystal structure as well as the fusion and growth of minicrystals, and two kinds of crystal morphology exist on the surface of Hebei limestone calcination products. The results from EDS also prove that different kinds of crystal morphology correspond very well to different distribution of elements of Ca and Mg; MIP shows that water vapor can reduce the number of pores whose sizes are 10-100 nm, enlarging the average pore size and concentrating pore size distribution of CaO calcined; the results of XRD also prove that above. (3) The above analysis gives the conclusion about the mechanisms that the binding ability of O* in Ca-O with H2O is stronger than that with CO2. The replacement of CO2 by H2O speeds up the rate of CO2 released in calcination. The influence of water vapor on the sintering is most possibly on the formation stage of sintering neck. H2O absorbed on the active site of Ca-O* accelerates the formation


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of sintering neck of CaO calcined by the interaction between H2O molecules (such as hydrogen bond). Meanwhile, H2O molecules, as a kind of solvent of mass transfer by diffusion from the surface to sintering neck, may also facilitate the solid state diffusion as well as the fusion and growth of minicrystals.

■ AUTHOR INFORMATION Corresponding Author *Telephone: +86-451-86413231.Fax:+86-451-86412528. Email: [email protected].

Notes The authors declare no competing financial interest.

■ ACKNOWLEDGMENTS We gratefully acknowledge the financial support provided by the Key Technologies Research and Development Program of China (Grant No: 2012BAA02B01-04), the support by the Foundation for Innovative Research Groups of the National Natural Science Foundation of China (Grant No. 51421063) and the Collaborative Innovation Center of Clean Coal Power Plant with Poly-generation.

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of sulfur capture by limestone in selected conditions of air-fired and oxy-fuel circulating fluidized-bed boilers. Energy Fuels 2011, 25, 2968-2979. [3] Leckner, B.; Åmand, L.E. Emissions from a circulating and a stationary fluidized bed boiler: a comparison. In Mustonen JP, editor. Proceedings of the Ninth International Conferenceon FBC; ASME: Boston MA: 1987, 891-897. [4] Park, Y.G.; Kim, S.H.; Jo, Y.M. Desulfurization with a modified limestone formulation in an industrial CFBC boiler. Energy Fuels 2006, 20, 138-141. [5] Lasek, J. A.; Kazalski, K. Sulfur self-retention during cocombustion of fossil fuels with biomass. Energy Fuels 2014, 28(4): 2780-2785. [6] Husmann, M.; Hochenauer, C.; Meng, X.; de Jong, W.; Kienberger, T. Evaluation of sorbents for high temperature in situ desulfurization of biomass-derived syngas. Energy Fuels 2014, 28(4), 2523-2534. [7] Rahiala, S.; Hyppänen, T.; Pikkarainen, T. Bench-scale and modeling study of sulfur capture by limestone in typical CO2 concentrations and temperatures of fluidized-bed air and oxy-fuel combustion. Energy Fuels 2013, 27(12), 7664-7672. [8] Naktiyok, J.; Bayrakçeken, H.; Özer, A. K.; Gülaboğlu, M. Ş. Flue gas desulfurization by calcined phosphate rock and reaction kinetics. Energy Fuels 2013, 21(3), 1466-1472. [9] Borgwardt, R. H.; Bruce, K. R. Effect of specific surface area on the reactivity of CaO with SO2. AIChE J. 1986. 32, 239-246. [10] Borgwardt, R. H. Sintering of nascent calcium oxide. Chem. Eng. Sci. 1989, 44, 53-60. [11] Li, Z. S.; Cai, N. S.; Huang, Y. Y. Effect of preparation temperature on cyclic CO2 capture and multiple carbonation-calcination cycles for a new Ca-based CO2 sorbent. Ind. Eng. Chem. Res. 2006, 45, 1911-1917.


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[12] de Souza, F.; Bragança, S.R. Thermogravimetric analysis of limestones with different contents of MgO and microstructural characterization in oxy-combustion. Thermochim Acta 2013, 561:19-25. [13] Fang, F.; Li, Z. S.; Tang, X. Y.; Cai, N. S.; Yang, H. T. AFM Investigation of solid product layer of MgSO4 generated on MgO surfaces in the reaction of MgO with SO2 and O2. Chem. Eng. Sci. 2011, 66(6), 1142-1149. [14] Maclntire, W. H.; Stansel, T. B. Steam catalysis in calcinations of dolomite and limestone fines. Ind. Eng. Chem. 1953, 45(7), 1548-1555. [15] Haji-Sulaiman, M. Z.; Scaroni, A. W. The rate limiting step in the sulfation of natural limestones during fluidized bed coal combustion. Fuel Process. Technol. 1992, 31(3), 193-208. [16] Anbalagan, G.; Rajakumar, P. R.; Gunasekaran, S. Non-isothermal decomposition of Indian limestone of marine origin. J Therm Anal Calorin 2009, 97, 917-921. [17] Calvo, E. G.; Arranz, M. A.; Leton, P. Effect of impurities in the kinetics of calcite decomposition. Thermochim Acta 1990, 170, 7-11. [18] Beruto, D.T.; Botter, R.; Cabella, R.; Lagazzo, A. A consecutive decomposition sintering dilatometer method to study the effect of limestone impurities on lime microstructure and its water reactivity. J Eur Ceram Soc 2010, 30(6), 277-286. [19] Manovic, V.; Charland, J. P.; Blamey, J.; Fennell, P. S.; Lu, D. Y. Influence of calcination conditions on carrying capacity of CaO-based sorbent in CO2 looping cycles. Fuel 2009, 88, 1893-1900. [20] de Souza, F.; Braganca, S. R. Thermogravimetric analysis of limestones with different contents of MgO and microstructural characterization in oxy-combustion. Thermochim Acta 2013,561,19-25. [21] Stewart, M. C.; Manovic, V.; Anthony, E. J.; Macchi, A. Enhancement of indirect sulphation of limestone by steam addition. Environ. Sci. Technol. 2010, 44 (22), 8781-8786.


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Captions Table 1 Composition of Limestone Samples (Mass Fraction/%) Table 2

Operating Conditions for Fluidized Bed Reactor Experiment

Table 3

Element distribution of Hebei limestone calcination products without water vapor

Figure 1

Particle size distribution of limestone samples

Figure 2

Schematic of the rotatable fluidized bed reactor

Figure 3

Relationship between frequency and water vapor content or liquid water mass

Figure 4

Temperature change during the limestone calcination

Figure 5

Degree of conversion of limestone changes against time (a) From the same geographical origin (b) From various geographical origins

Figure 6

Micro-morphological change at different concentrations of water vapor for Liaoning Limestone

Figure 7

Micro-morphological change at different concentrations of water vapor for Hebei Limestone

Figure 8

Testing zone of Hebei limestone calcination product by EDS

Figure 9

Pore size distributions at different concentrations of water vapor

Figure 10

X-ray diffraction patterns and mineral compositions change at different concentrations of water vapor

Figure 11

Minicrystal size changes at different concentrations of water vapor

Figure 12

Schematic of CaCO3 calcination reaction mechanism

Figure 13

The process of Ca-O absorbs H2O


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Table 1. Composition(Mass Fraction/%) and specific surface area(m2/g) of samples Sample

CaO

Liaoning Limestone 52.61 Hebei Limestone

44.84

MgO Al2O3 Fe2O3 SiO2 LOI Others specific surface area 0.5

0.69

0.53

2.58 42.7

0.39

0.124

4.1

1.86

1.12

6.12 40.5

1.46

0.0783


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Table 2. Operating Conditions for Fluidized Bed Reactor Experiment Items

Details

Geographical Origins(China)

Liaoning, Hebei

Samples Weight(g)

20

The Initial Reaction Temperature(°C)

900

The Average Reaction Temperature(°C)

850

Gas Flow（L/min）

25

Sintering Time（min）

1

H2O（%）

0/5/15

N2（%）

Balance


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Table 3. Element distribution of Hebei limestone calcination products without water vapor 1

2

Wt%

At%

Wt%

At%

O

22.07

36.55

19.68

33.68

Mg

8.95

9.75

2.15

2.43

Al

10

9.82

12.18

12.36

Si

27.44

25.88

30.17

29.41

K

7.19

4.87

8.99

6.29

Ca

9.83

6.5

15.39

10.51

Fe

10.85

5.15

7.44

3.65

Zn

3.66

1.48

3.98

1.67

Note:Wt%—mass percent；At%—Atom number percent


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Figure 1. Particle size distribution of limestone samples


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Figure 2. Schematic of the rotatable fluidized bed reactor


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Figure 3. Relationship between frequency and water vapor content or liquid water mass


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Figure 4. Temperature change during the limestone calcination


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(2)

(1)

Figure 5. Degree of conversion of limestone changes against time (a) From the same geographical origin (b) From various geographical origins


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a) 0% H2O

b) 5% H2O

c) 15% H2O

Figure 6. Micro-morphological change at different concentrations of water vapor for Liaoning Limestone


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a) 0% H2O

b) 5% H2O

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c) 15% H2O

Figure 7. Micro-morphological change at different concentrations of water vapor for Hebei Limestone


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Figure 8. Testing zone of Hebei limestone calcination product by EDS


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Figure 9. Pore size distributions at different concentrations of water vapor


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（a）

（b）

（c）

（d）

1-CaO 2-CaCO3 3-SiO2 4-MgO 5-CaMg(CO3)2 Figure 10. X-ray diffraction patterns and mineral compositions change at different concentrations of water vapor


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Figure 11. Minicrystal size changes at different concentrations of water vapor


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(b) CaO(s) (c) CO2 (g) (a) CaCO3(s) Figure 12. Schematic of CaCO3 calcination reaction mechanism


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Figure 13. The process of Ca-O absorbs H2O


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Understanding the Impacts of Impurities and Water Vapor on

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