Influence of Water Vapor on Surface Morphology and Pore Structure

Apr 13, 2016 - during Limestone Calcination in a Laboratory-Scale Fluidized Bed ... College of International Education, Shenyang Institute of Engineer...
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Influence of Water Vapor on Surface Morphology and Pore Structure during Limestone Calcination in a Laboratory-Scale Fluidized Bed Hui Wang,*,† Shuai Guo,† Dunyu Liu,‡ Li Yang,†,§ Xing Wei,† and Shaohua Wu† †

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 § College of International Education, Shenyang Institute of Engineering, Shenyang 110136, China ‡

ABSTRACT: Capturing SO2 using natural limestone in situ is an important way of desulfuration in a circulating fluidized bed (CFB) boiler. The limestone desulfuration mainly consists of two processes in the condition of standard pressure and air atmosphere: calcination and sulfuration. Water vapor in the flue gas influences the calcination, and the physical characteristics such as the pore structure of calcined CaO play an important role in the following sulfation. The impacts of water vapor during calcination may be reflected on the surface micromorphology and pore structure of calcined CaO. This work aims to understand the influence of water vapor on surface morphology and pore structure of calcined CaO. One kind of Chinese limestone was used to study the issues in a rotatable fluidized bed reactor. Scanning electron microscope (SEM), confocal scanning laser microscope (CSLM), mercury injection apparatus (MIP), and N2 adsorption instrument were employed to test the micromorphology and pore structure of calcined CaO. Results show that the existence of water vapor accelerates calcination and shortens the reaction time, but higher water vapor content results in slightly lower ultimate degree of conversion. Testing results of SEM and CSLM show that water vapor improves sintering and growth of grains of calcined CaO. The change of surface roughness also proves the above conclusion; besides, the results of MIP and N2 adsorption instrument show that there are pores with a size range of 15−80 nm inside calcined CaO without H2O(g), and after sintering under the condition of H2O(g), they will combine together to form relatively bigger pores with a size range of 40−100 nm. As a result, the average pore size increases and the specific surface area decreases but the specific pore volume is less influenced. The fractal dimensions of pore structure for calcined CaO under different concentrations of water vapor were calculated using the data of N2 adsorption. Results show that as the concentration of water vapor increases, the fractal dimension first decreases and then slightly increases but is still lower than that without H2O(g). It means that the pore structure of calcined CaO becomes simpler with the impact of water vapor, which is beneficial for the reaction between calcined CaO and gases such as SO2. There is as high as about 15% water vapor in the flue gas after coal combustion under air atmosphere in the CFB boiler.6 In addition, CFB boiler is usually used to burn low quality fuels such as high moisture lignite and industrial and urban waste; thus the water vapor content may be higher, reaching almost 20%, and for oxy-fuel combustion, 30% may exist. The existence of water vapor in the furnace will influence limestone desulfuration,7,8 especially for calcination.9 Therefore, it is important to study the influence of water vapor on limestone calcination under fluidized bed combustion. There have been several studies about the influences of water vapor on the decomposition temperature, reaction rate, and degree of conversion during limestone calcination. The earliest study is a review of Senum9 who pointed out that water vapor was a catalyst to improve the limestone decomposition rate. Boynton10 and Burnham et al.11 studied the influence of H2O(g) on limestone calcination by monitoring the decomposition of CaCO3 and the formation of calcined CaO by XRD. Results show that the calcination reaction rate increases and decomposition temperature decreases in the presence of water vapor. Khraisha and Dugwell12 carried out experiments on

1. INTRODUCTION The desulfuration technology in situ for circulating fluidized bed (CFB) boilers is widely acknowledged due to the advantages of low investment cost, high flexibility, and low consumption of water.1 The most common sorbent used for desulfuration in CFB boiler is limestone. The limestone particles are injected into the furnace by a nozzle at near the dense phase zone. SO2 emitted by the burning coals is removed by reacting with sorbents to form insoluble sulfate.2 The main component of limestone is CaCO3. The desulfuration consists of two processes: the production of CaO and CO2 by calcination (1) and the following sulfation of CaO (2): CaCO3(s) → CaO(s) + CO2 (g)

(1)

CaO(s) + SO2 (g) + (1/2)O2 (g) → CaSO4 (s)

(2)

Limestone decomposes as soon as being introduced into the furnace, and then porous CaO is produced. This kind of pore structure plays an important role in the following sulfation. Studies indicate that the large specific surface area of porous calcined CaO favors the sulfation reaction.3,4 But the pore structure of calcined CaO is affected by the physicochemical properties of limestone and reaction conditions.5 © 2016 American Chemical Society

Received: January 11, 2016 Revised: March 23, 2016 Published: April 13, 2016 3821

DOI: 10.1021/acs.energyfuels.6b00067 Energy Fuels 2016, 30, 3821−3830

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Energy & Fuels Table 1. Composition of Limestone Sample (Mass Fraction (%)) sample

CaO

MgO

Al2O3

Fe2O3

SiO2

LOI

others

HLJ limestone

53.83

0.4

0.54

0.25

1.91

42.48

0.59

1983 when Avnir et al.19 introduced the fractal theory to the porous materials, a rapid development has been gained. It has been widely applied for describing pore surface roughness and the complexity of pores. Liu and Zhao,20 Shang et al.,21 and Wang et al.22 have already used the MIP method to study fractal features of the pore structure during limestone calcination. They mainly investigate the influences of limestone composition, calcination temperature, time, and additive proportion on the fractal dimension of the pore structure. Presently, the influences of water vapor on the fractal feature of the pore structure for calcined CaO have not been published. Therefore, it is also necessary to study this issue. Above all, water vapor has great influences on the reaction rate and degree of conversion during limestone calcination, and these impacts are mainly reflected on the changes of surface micromorphology and the pore structure of calcined CaO. So far there are no published works about the effects of water vapor on surface micromorphology especially on the 3D morphology of calcined CaO. Besides, there is no related research about the application of the fractal theory on limestone calcination in the presence of water vapor. Relevant studies on the issues are necessary. This work aims to study the calcination reaction rate and degree of conversion of a Chinese limestone at different concentrations of water vapor from 0 to 15% in a laboratoryscale fluidized bed reactor under the temperature of 850 °C. SEM and CSLM techniques were used to observe the surface micromorphology of calcined CaO. Besides, the pore structure was tested by MIP and N2 adsorption instruction to understand the impacts of water vapor on pore structure during calcination.

limestone decomposition under water vapor in a suspension reactor. The results prove that a low concentration of water vapor can improve the limestone decomposition rate while a high one inhibits it. Shang13 and Song14 also studied the same issue as above. The results show that the decomposition rate of limestone has been improved when the concentration of water vapor increases from 5% to 15%. The impacts of water vapor on limestone calcination are directly reflected on the changes of surface micromorphology of calcined CaO, and the surface condition including the roughness of calcined CaO is closely related to its reactivity with SO2. Some studies have been carried out to understand the influences of water vapor on the surface micromorphology of CaSO4 products during limestone desulfuration. Jiang et al.15 tested the surface micromorphology of CaSO4 products during limestone desulfuration without or with 40% H2O(g) by the SEM technique. Results indicate that CaSO4 crystal formed in the presence of water vapor has a smooth particle surface and exists in the form of agglomerate. Manovic et al.8 used SEMEDS to analyze the agglomerate formed during limestone desulfuration in the presence of water vapor. The existence of water vapor greatly affects CaO sulfation and solid state diffusion. This results in enhanced agglomeration and following sulfate fouling, and the connections between particles due to sulfate fouling are often stronger. Stewart et al.6 also studied the same issue. An obvious increase in the CaSO4 crystal grain size was observed in the presence of water vapor. Microstructural changes and observed increase in reaction rate are attributed to the enhanced solid state diffusion. Although the influences of water vapor on the micromorphology of CaSO4 products during limestone desulfuration have been studied extensively, the studies on micromorphology of calcined CaO during limestone calcination are scarce; so is the research on the 3D morphology of calcined CaO. Therefore, it is necessary to do some research on the surface micromorphology especially 3D morphology of calcined CaO in different concentrations of water vapor. Water vapor causes sintering of limestone calcination products and also influences pore structure. Borgwardt16 tested the pore structure of calcined CaO in the presence of water vapor by N2 absorption method. The results show that the existence of water vapor can obviously decrease the specific surface area and porosity of calcined CaO. Soares et al.17 conducted experiments in TGA, with results showing that water vapor can not only catalyze the reaction but also accelerate sintering so as to influence the pore structure. Huang18 (2013) and Wang et al.7 (2014) also made use of an online weighing tube furnace to study the issue. As the concentration of water vapor increases, the specific surface area and specific pore volume of calcined CaO reduce. The average pore size moves toward the macropore. Sintering of calcined CaO is improved. The macroscopic characteristics of pore structure were obtained previously, but the roughness of the pore surface and the complexity of pores have not been quantitatively described. The surface reactivity as well as diffusion ability of gas molecules inside the pores can be influenced by them. This will in turn influence the calcination itself and the gas−solid reaction characteristics between calcined CaO and SO2. Since

2. EXPERIMENTAL SECTION 2.1. Limestone Samples. In the experiments, one natural limestone from Heilongjiang (HLJ) province in China was used as a reactant. Titration was used to test the components according to the China Standard GB/t 3286-2012. The results are shown in Table 1. The particle size distribution was tested by a Malvern Mastersizer 2000 as shown in Figure 1. 2.2. Experimental Setup and Operating Conditions. The experiments were conducted in a rotatable fluidized bed reactor which consisted of five main parts: a water vapor generator, a preheating furnace, a reaction furnace, the sampling and measuring system, and

Figure 1. Particle size distribution of HLJ limestone sample. 3822

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Energy & Fuels the gas-supply system. The details of the experimental setup and operating procedure as well as the calibrated results of the water vapor generator have been discussed elsewhere.23 The conditions of calcination experiments in the laboratory-scale fluidized bed are shown in Table 2.

trations of water vapor. CaCO3 as the major composition of limestone decomposes and releases CO2. As the reaction proceeds, the amount of CO2 increases and reaches its maximum and then decreases until the end of the reaction. CO2 released with time follows the Gaussian distribution. As the concentration of water vapor increases, the released CO2 in calcination reaches the maximum value earlier with a higher volume fraction. The existence of water vapor accelerates the reaction rate and shortens the reaction time. Figure 3 shows the degree of conversion changes against time in the presence of water vapor. The process proceeds in

Table 2. Experimental Conditions for Laboratory-Scale Fluidized Bed Reactor experimental conditions

details

limestone weight (g) reaction temperature (°C) gas flow (L/min) H2O (%) N2 (%)

20 850 25 0, 5, or 15 balance

The calcination products obtained at different concentrations of water vapor were examined for micromorphology and pore structure. SEM (FEI-Quanta 200FEG, Hillboro, OR, USA) and CSLM (OLYMPUS OLS3000, Tokyo, Japan) were both used to test the micromorphology of the calcined CaO. SEM can realize the precise measurement for surface micromorphology of samples, while the surface 3D morphology and roughness can be obtained by CSLM. The magnifying ratios of SEM and CSLM were 12−1,000,000× and 120− 14400×, respectively. The resolution for SEM was 1.5 nm and for CSLM was 0.12 μm at the x-, y-axis directions and 0.01 μm at the zaxis direction. Besides, the pore structure parameters including porosity, pore size distribution, specific surface area, and specific pore volume were measured by MIP (Micrometritics AutoPore 9500, Norcross, GA, USA) and surface area analyzer (Micromeritics ASAP 2420) based on the N2 adsorption−desorption method. The macropore/mesopore structure was measured by MIP, with a high pressure of 228 MPa which corresponds to a measurement range of pore size from 5 nm to 360 μm. While the mesopore/micropore structure especially for micropore was measured by the surface area analyzer. The samples were degassed at 200 °C in a vacuum pressure of 1.33 Pa (10 μm Hg) for 12 h. Then, N2 adsorption at 77.2 K was measured at a relative pressure of 0.01−0.998. The specific surface area of the samples was calculated by the Brunauer−Emmett−Teller (BET) equation using the linear part (0.06 < P/P0 < 0.27) of the adsorption curve by assuming a closely packed BET monolayer, and the pore size distribution of the samples was calculated by the Barrett− Joyner−Halenda (BJH) equation.

Figure 3. Degree of conversion changes against time during HLJ limestone calcination.

three stages.24 The first stage is very fast, only lasting for 20 s when limestone particles absorb heat to reach a high temperature, but the degree of conversion hardly changes. In the second stage, the degree of conversion increases very fast, but it lasts only for 30−40 s. The third stage is the longest one, lasting nearly for 600 s. The calcination of limestone is influenced by three factors, namely, heat transfer, chemical reaction, and CO2 diffusion.25 Wang and Thomson26 pointed out that when the particle size of CaCO3 is big, heat transfer and mass transfer are the main limiting factors. Only when the size is small enough (d50 < 38 μm) is the chemical reaction itself the main limiting factor. Since the particle size of limestone involved is relatively large (d50 > 290 μm), this reaction is mainly restrained by CO2 diffusion at 850 °C. In the second stage, the strong chemical reaction as a result of adequate reactants can counteract some proportion of the resistance caused by CO2 diffusion, so the degree of conversion increases quickly in this stage. However, as the reaction proceeds, not much CaCO3 is left. The diffusion resistance of CO2 increases; the reaction rate becomes slower until it stops. This explains why the second stage is short while the third stage is long during limestone calcination. Figure 3 shows that the ultimate degree of conversion becomes lower slightly as the concentration of water vapor increases. The reason for the negative influence of water vapor on the degree of conversion may be that the existence of water vapor decreases the diffusion rate of CO2.12 3.2. Influence of Water Vapor on Micromorphology of Calcined CaO during Calcination. 3.2.1. Micromorphological Measurement Using SEM. Figure 4 is the different magnifications of micromorphology for limestone calcination products at different concentrations of water vapor. Figure 4a shows that the calcined CaO without water vapor is full of

3. RESULTS AND DISSCUSSION 3.1. Influence of Water Vapor on CO2 Releasing and Degree of Conversion during Calcination. Figure 2 describes CO2 release during calcination at different concen-

Figure 2. CO2 releasing process against time during HLJ limestone calcination. 3823

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Figure 4. Micromorphological change in different concentrations of water vapor: (a) 0%; (b) 5%; (c) 15%.

stripe gaps. Figure 4b shows the surface micromorphology of calcined CaO in 5% H2O(g). Compared to that without water vapor, there are round tiny pores instead of stripe gaps on the surface and the number is much less. In the molten state it is relatively obvious that some part of the surface does not have pores after magnifying 5000 times; Figure 4c is the surface of calcined CaO in 15% H2O(g). Compared with the previous two cases, the particle surface is covered with only a few round tiny pores with a large part of the area almost without any pores on the surface and there are instead some obvious stripe gaps similar to that without water vapor (Figure 4a). Results show that the existence of water vapor has an influence on the limestone calcination. A previous study found that25 the calcination is primarily influenced by the production and diffusivity of CO2. The former is limited by the chemical reaction itself and the latter by the internal and superficial pore structure of limestone. Figure 4a shows the gaps on the surface of particles emerge when CO2 releases and diffuses outside. Water vapor can influence the calcination; in other words, it can affect the release of CO2.23 The CO2 released will spread from the reaction interface to the outside surface through internal pores of the calcined CaO layer and then to the outside environment through superficial pores. Therefore, the pore structure on the surface of particles reflects the calcination reaction characteristics. The results from the experiment with 5% H2O(g) show that water vapor could improve the fusion and growth of grains or minicrystals so that pores of calcined CaO are closed which results in the disappearance of stripe gaps. Therefore, water vapor can accelerate sintering during calcination. When there is 15% water vapor, sintering on the surface is more serious. As pores are closed and CO2 cannot be released, the internal stress is accumulated by CO2, which leads to the gaps on the surface. After magnifying the gaps 5000 times, the trace of snapping because of force between integrated grains can be seen clearly in Figure 4c. In the third stage of calcination, as the diffusion resistance of CO2 increases and not much CaCO3 is left, CO2 fails to overcome the resistance of the calcined CaO layer, the reaction rate becomes slower until it stops. The higher concentration of water vapor means more closed pores, so the ultimate degree of conversion will be lower and the time needed for the reaction will be shorter (see Figure 3). 3.2.2. Measurement of 3D Morphology and Roughness Using CSLM. The surface 3D morphology of calcined CaO in different concentrations of water vapor is measured with CSLM, and the roughness parameters are also calculated. Figure 5 shows the aerial and 3D views of calcined CaO in different concentrations of water vapor. For comparison, the 3D views of calcined CaO in different concentrations of water vapor are set at the same height. However, the coordinate scales of x- and y-axes are different, leading to the fact that different samples obtain different scanned areas. The reason is that natural limestone owns various shapes, in order to show the surface morphology of different samples as much as possible,

Figure 5. Aerial (a−c) and 3D views (d−f) of calcined CaO in different concentrations of water vapor: (a, d) 0%; (b, e) 5%; (c, f) 15%.

the testing range of x- and y-axes needs to be adjusted, which make the testing range different for different samples. It is necessary to point out that the gray scale of the images shown in Figure 5 is 256, namely, different gray scales stand for the area higher or lower than the average height, which also shows the surface pore distribution of different samples. It can be seen from Figure 5 that the surfaces of different samples all have a certain roughness, which is difficult to observe by SEM technique because of its testing principle. Specifically, CSLM makes use of laser as the source; the scanning on the sample surface is proceeding layer by layer along the radial direction at the set height range and then translates the information scanned into images, whose different gray scales stand for different heights, while SEM scans point by point or line by line along the sample surface using its electronic gun. Thus, samples will send various physical signals and then translate them into the surface characteristics of samples which will also be shown on images. It can be easily seen that SEM fails to show the information along the height direction especially the roughness. Besides, in order to make the samples measured by SEM possess electrical conductivity, the sample surface should be metal sprayed before the testing of SEM, which will also influence the testing results. Panels a and d of Figure 5 show the aerial and 3D views of calcined CaO without H2O(g). The 3D view of Figure 5d shows that there are amounts of micrometer doming on the surface of calcined CaO with the height of 20−30 μm which are scattering around the particle surface but not intensively, and the aerial view of Figure 5a also indicates that the gray scales shown on the surface are also different, meaning that the doming mentioned above is of different heights, leading to the rough surface. However, it can be seen from the aerial view of Figure 5b which shows the surface of calcined CaO with 5% H2O(g), there are amounts of intensive micrometer doming, obviously more than that without H2O(g), and the distribution is relatively even and the color of the surface is uniform, showing that the doming is of a similar height. That is why the 3D view of Figure 5e shows a relatively flat surface. Panels c and f of Figure 5 are respectively the aerial and 3D views of the surface of calcined CaO with 15% H2O(g). The aerial view shows that the doming on the surface is almost as intensive as that of Figure 5b, but not as even as it. Instead, the doming is divided by some holes and crevices, forming some grain clusters. Besides, the color is also not as uniform as that of Figure 5b, so there are different heights of doming on the surface. Therefore, the 3D view of Figure 5f is a relatively rough 3824

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Energy & Fuels surface, but still not as rough as that of Figure 5d when there is no water vapor added during calcination. The above studies are only some qualitative observations and comparisons of the roughness between calcined CaO in different concentrations of water vapor. In order to make the conclusions more convincing, the quantitative analysis is also necessary by calculating the roughness parameters. The roughness of the solid surface can greatly influence the properties such as gas absorption which will also influence its chemical reaction. Recently, there are some studies on the 3D morphology of materials with attention especially paid to the surface roughness. Three-dimensional roughness is the average result of roughness through testing the different areas on the sample surface. It can provide major information, and the results are representative. CSLM is a powerful tool to measure the 3D morphology. Its own analysis software can give a precise calculation of 3D roughness parameters. Range is one of the major parameters to describe the surface morphology, which can be used to represent the vertical deviation of the surface.27 Ra and Rq are the two most commonly used parameters, and their mathematical expressions are eq 3 and eq 4, respectively. Ra =

Rq =

1 n

Figure 6. Roughness change in different concentrations of water vapor.

analysis shows that Rq is more sensitive to the height change than Ra. That is why Rq is higher than Ra at the same condition shown in Figure 6. But the bound on the error of Rq is also larger than that of Ra due to its sensitivity. Above all, there are plenty of micrometer domings of different heights on the surface of calcined CaO without H2O(g), leading to the rough surface, and the domings are not very close to each other; they are randomly scattered on the surface with some gaps between them; however, the doming is nearly the same height with 5% H2O(g) compared with the condition without H2O(g), resulting in a relatively smooth surface of calcined CaO. Besides, the domings are closely connected to each other and the number is much more than that without H2O(g). For the condition of 15% H2O(g), the closely connected domings still exist and are also in great amount, but they are divided by some holes and crevices. As a result, the surface is rougher than that with 5% H2O(g) but still smoother than that without H2O(g). It is speculated that the reason for the difference of surface morphology and roughness of CaO as the concentration of water vapor calcined changes is that water vapor promotes the sintering between grains on the surface. Specifically, for the condition of 5% H2O(g), the sintering phenomenon results in fusion between grains of calcined CaO so that the surface is relatively smoother, but for the condition of 15% H2O(g) the sintering is more serious, causing the complete close parts without pores on the surface, which also leads to the difficulty of CO2 diffusion. The internal force produced by accumulated CO2 maybe break the product layer, leaving some holes and crevices on the surface. The result shown in Figure 4c has also confirmed the preceding conclusion. 3.3. Influence of Water Vapor on Pore Structure of Calcined CaO during Calcination. Although SEM and CSLM can show directly how the surface morphology of calcined CaO changes, they are still subject to the limitation of the testing principle and resolution. It is hard to know how the pore structure changes inside the surface of calcined CaO through the SEM or CSLM technique. Therefore, we try to study how the water vapor influences the pore structure of calcined CaO by the method combining MIP and N 2 absorption. 3.3.1. Pore Structure and Pore Size Distribution Change during Calcination. The pore structure parameters of calcined CaO in different concentrations of water vapor were measured by MIP and N2 absorption method with the parameters including specific surface area, specific pore volume, mean pore

n

∑ |yi | i=1

1 n

(3)

n

∑ yi 2 i=1

(4)

Here, Ra is the mean roughness, μm; Rq is the root-mean-square (RMS) roughness; yi is the radial height of the i position on the sample surface, i = 1 − n; and n is the number of chosen positions on the surface in the process of calculation. Ra is one of the parameters which was first proposed to describe the microgeometrical shape of the surface. To some degree, it reflects the degree of discreteness between the profile height and the datum middle line. But its description on the microgeometrical shape of the surface is not good enough mostly because of the discreteness of the results in different positions which causes the calculating values to be unstable, and it is also not suitable for a too rough or too smooth surface to be evaluated by Ra. In order to solve the problem of oversimplified average and being insensitive to the changes of height for Ra, Rq is used to improve the results. Rq is the standard deviation commonly used in mathematical statistics to show how much the profile line deviates from the datum middle line. It should be noted that Ra and Rq both average the profile offset distances within the sampling length but in different ways for averaging; Ra is the average results of absolute values while Rq is the results of RMS. Therefore, Rq is much more accurate as it is in accordance with statistics and it can also connect the statistical method and random process.28 The aforementioned two parameters are both calculated in this work for mutual complementation. Ra and Rq calculated in this work are both the average results of roughness for three single-particle samples which are chosen randomly, and the roughness of each single-particle sample is the average value for five different positions on the surface so as to make the results more representative. The calculating results of Ra and Rq are shown in Figure 6. It can be seen from the figure that as the concentration of water vapor increases, the surface roughness of calcined CaO first decreases and then increases, which is in accordance with the results shown in Figure 5. The preceding 3825

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Figure 7. Pore structure parameters change in different concentrations of water vapor: (a) specific surface area and pore volume; (b) pore diameter and porosity.

size, and porosity. As the small pores could collapse when the high pressure of mercury was injected into the pores during MIP testing, the results obtained by MIP are divided into two parts. As the testing range of MIP is from 5 to 360000 nm and N2 absorption instrument is from 2 to 100 nm, the specific surface area (Smercury) and specific pore volume (Vmercury) of calcined CaO are calculated based on the pore size larger than 5 and 100 nm, respectively. Smercury(5nm) and Vmercury(5nm) stand for the specific surface area and specific pore volume whose pore size is larger than 5 nm. Similarly, Smercury(100nm) and Vmercury(100nm) stand for the specific surface area and specific pore volume respectively whose pore size is larger than 100 nm. Besides, SBET and VN2 are the BET specific surface area and specific pore volume measured by N2 absorption method. Figure 7 shows how the pore structure of calcined CaO changes in different concentrations of water vapor. It can be seen from Figure 7a that the SBET first increases and then decreases as the concentration of water vapor increases, and Smercury(5nm) follows the same trend with SBET, while Smercury(100nm) hardly changes with the concentration of water vapor. It shows that water vapor has less influence on the pores with the size larger than 100 nm during limestone calcination. Similarly, it can also be seen from the figure that VN2, Vmercury(5nm) and Vmercury(100nm) hardly change with the concentration of water vapor. The surface area of the porous structure is mainly determined by micropores, while specific pore volume is decided by macropores. As the calculating results show in Table 3, the specific surface area of calcined CaO without water vapor accounts for nearly 97.8% of the total specific surface area when pore size is smaller than 100 nm, and the specific pore volume of calcined CaO without water vapor accounts for only 24.6% of the total specific pore volume when pore size is smaller than 100 nm. Because of the weak effect of water vapor on pores with big size, the specific pore volume of

calcined CaO in different concentrations of water vapor fails to change obviously. Under the condition of 5% H2O(g), the reduction of pores with small size leads to the decrease of the specific surface area of calcined CaO, while for the condition of 15% H2O(g), the sintering phenomenon of pores has been further strengthened which also leads to the difficulty in the diffusion of CO2 produced during the reaction. Therefore, the strong internal force caused by the accumulation of CO2 finally breaks the product layer, leaving crevices on the surface. As is shown previously, water vapor mainly influences the pores of 5−100 nm size. Table 3 shows the specific surface area, specific pore volume for pores with the size range of 5−100 nm, and their proportions with respect to the total values of surface area or pore volume. The specific surface area for pore size from 5 to 100 nm decreases obviously with water vapor, and this is because the number of small pores decreases. The changing process of the proportion of the specific surface area in different concentrations of water vapor also proves the above conclusion. Besides, comparing the proportion of specific surface area and specific pore volume, the pores of this range mainly determine the specific surface area but have little impact on the specific pore volume. Table 3 shows the proportion of specific pore volume in different concentrations of water vapor remain nearly unchanged, which further proves the above conclusions. The Dmercury and DN2 in Figure 7b stand for the mean pore size which is measured by MIP and N2 absorption methods, respectively. It can be seen from the figure that they both increase first and then decrease. As concluded previously, water vapor can decrease the number of pores with small size, with the average pore diameter increasing obviously under the condition of 5% H2O(g). However, for the condition of 15% H2O(g), the internal force caused by the accumulated CO2 produces some pores with the shape of the crevice on the surface (see Figure 4c), decreasing the average pore size again. Besides, the comparison between the changing range of Dmercury and DN2 shows that although water vapor can directly influence the pores with small sizes, there is a much more obvious change on the average pore size for the pores with big sizes. Therefore, it can be concluded that the influence mechanism of water vapor on pores with small size is to combine them together to form relatively bigger pore sizes. It can also be seen from Figure 7b that the porosity measured by MIP does not change much as the concentration of water vapor increases which also confirms

Table 3. Specific Surface Area and Pore Volume for Pore Size of 5−100 nm specific surface area vol fraction of water vapor (%)

value (m2/g)

0 5 15

23.42 11.38 12.39

specific pore volume

proportion (%)

value (mL/g)

proportion (%)

97.8 92.8 94.4

0.17 0.17 0.16

24.6 24 23.2 3826

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vice versa. All of the preceding discussions also explain that the porosity remains unchanged under different concentrations of water vapor. In conclusion, the influence mechanism of water vapor on limestone calcination is the sintering of pores for calcined CaO within the size range from 15 to 80 nm so that they combine together to form pores with sizes of 40−100 nm, leading to the increase of average pore size while porosity is still unchanged. Besides, the specific surface area of calcined CaO decreases as pores with small sizes reduce, but the specific pore volume is less influenced. 3.3.2. Pore Type and Shape Change. The aforementioned studies show that water vapor mainly influences the pores with sizes of 40−100 nm, so the N2 absorption method is more suitable than the MIP method for this study. The comparison between N2 adsorption−desorption isotherms and the hysteresis loop of calcined CaO in different concentrations of water vapor are discussed as follows to study how the water vapor influences the pore type and shape of calcined CaO. Figure 9 illustrates N2 adsorption−desorption isotherms of calcined CaO under different concentrations of water vapor. It can be seen from the adsorption isotherm curve, in the low pressure zone (0 < P/P0 < 0.2), adsorption capacity does not change much as relative pressure P/P0 increases, which shows that the samples do not contain any micropores(D < 2 nm), while in the medium pressure zone (0.2 < P/P0 < 0.8), the adsorption capacity also does not change at all when relative pressure P/P0 is low under the condition of 0% and 15% H2O(g); only when the relative pressure P/P0 increases up to 0.65 or 0.75 does the adsorption capacity show an obvious increasing. But under the condition of 5% H2O(g) the adsorption capacity does not change much within the medium pressure zone. The results above indicate that there are a few mesopores (2 nm < D < 50 nm) within calcined CaO under the condition of 0% and 15% H2O(g), while for the condition of 5% H2O(g) there are no mesopores in the calcined CaO. Finally, in the high pressure zone (P/P0 > 0.8), adsorption capacity increases quickly with relative pressure P/P0, not arriving at the saturation state even though P/P0 ≈ 1, which indicates that there must be some macropores (D > 50 nm) in the samples. On the basis of the preceding results, it can be known that the adsorption and desorption isotherms illustrated in Figure 9 do not belong to the six categories of isotherms defined by the International Union of Pure and Applied Chemistry (IUPAC). However, they are similar to category V when the relative pressure P/P0 is low. As P/P0 increases multilayer absorption and subsequent capillary condensation will happen. This curve is different from category V of isotherms in that the adsorption

the aforementioned conclusions. Specifically, as the pores with small sizes combine with each other instead of closing and disappearing under the condition of water vapor, the porosity does not change in the case of a fixed volume. However, this still needs to be confirmed. The average pore size and porosity of different samples are both closely related to the pore size distribution. In order to further clarify the influence mechanism of water vapor on pore structure, the MIP method is used to measure the pore size distribution of calcined CaO under different concentrations of water vapor and the results are shown in Figure 8. The MIP method is used to get a much wider range of pore size than the N2 absorption method in order to have a more comprehensive understanding of the pore structure.

Figure 8. Pore size distribution in different concentrations of water vapor.

It can be seen from Figure 8 that the pore size of calcined CaO is mainly in two ranges, and the calcined CaO under different concentrations of water vapor all have pores with sizes from 5000 to 10000 nm. Besides, the calcined CaO without H2O(g) has pores with the size of 15−80 nm, while the size of pores changes to the range from 40 to 100 nm with H2O(g). Therefore, the pores with small sizes are more influenced by water vapor. In the presence of water vapor, the pore size increases. Since pores in this range primarily affect the specific surface area, it can contribute to the decline of specific surface area, while the pores ranging from 5000 to 10000 nm primarily influence the pore volume, the little influence of water vapor on these pores also results in minor changes of pore volume. Figure 8 shows that the range of pore size does not change much under different conditions. If the pores with small sizes have more contribution for the total pore volume, then pores with big sizes have less contribution for the total volume and

Figure 9. N2 adsorption−desorption isotherms for different samples. 3827

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Figure 10. Fitting line of ln(ln(P0/P)) and ln(Q/Qmono) in different concentrations of water vapor.

capacity does not show any limitation when P/P0 is high. It further proves that there are not only mesopores which could cause capillary condensation but also macropores within the samples. It should be clarified that multilayer adsorption occurs without experiencing the monolayer adsorption stage for this study because the acting force between pore surface and adsorbate gas molecules (N2 in this work) is weaker than that between adsorbate gas molecules themselves. As a result, monolayer adsorption has not been accomplished; multilayer adsorption has already begun. In other words, the acting force between absorbent and absorbate is very weak. The preceding discussions show that at a relatively high pressure, “capillary condensation” will happen to mesopores of each sample, leading to the formation of a “hysteresis loop”, as is shown in Figure 9. Adsorption and desorption isotherms do not enclose. The phenomenon of hysteresis loop is primarily related to factors such as pore size, pore shape, and the interaction strength between adsorbent and adsorbate. The different types of hysteresis loop for calcined CaO in different concentrations of water vapor stand for the differences in pore size and pore shape. According to the classification of hysteresis loop by IUPAC, the hysteresis loop shown in Figure 9a belongs to the H3 type. The typical characteristic is that there are pores of crevice or wedge shape in the sample, and this is consistent with the results shown in Figure 4a. The hysteresis loop shown in Figure 9b is similar to H1 type as the adsorption isotherm is almost parallel to the desorption isotherm and an almost vertical increase or decrease at a certain P/P0 can be observed. This type of hysteresis loop reflects the typical pore structure of similar size and round shape, and this is also consistent with the result shown in Figure 4b. However, the hysteresis loop shown in Figure 9c is different from the above two curves. It can be treated as a combination of H1 and H3 types, and this curve possesses characteristics of both types. It can be speculated that there are pores of both crevice and round shape in the sample, which is also consistent with the result in Figure 4c. It can also be seen from Figure 9 that as the concentration of water vapor increases during limestone calcination, the area of hysteresis loop first decreases and then increases slightly, and the P/P0 at the initial separation point between adsorption and desorption isotherms also follows the same trend. The area of hysteresis loop shows the amount of condensate produced by capillary condensation which usually happens in micropores and mesopores. The discussions earlier show that there are no micropores in the samples, so the capillary condensation phenomenon probably happens to mesopores. That is to say, the area of hysteresis loop shows the number of mesopores. Therefore, the number of mesopores first decreases and then increases as the concentration of water vapor increases. The change of initial separation point under different concentrations

of water vapor also confirms the earlier conclusion, as capillary condensation first happens in the pores of small size; after filling them all, it will subsequently happen in the pores of large size. So the increase of P/P0 at the initial separation point also confirms the decline of pores with small size. 3.3.3. Fractal Dimension Change. Pore structure parameters of calcined CaO under different concentrations of water vapor have already been acquired by the MIP and N2 adsorption methods, and macroscopic characteristics of pore structure are determined but the roughness of the pore surface or the complexity of the pores is still not quantitatively described. Aforementioned results show that water vapor mainly influences pores with a size range of 40−100 nm, and the fractal theory is employed to analyze the influence of water vapor on roughness and complexity on pore structure surface during limestone calcination based on the N2 adsorption data. At present, researchers have already conducted lots of studies based on the gas adsorption approach to calculate fractal dimension and proposed many calculation models. Among them the FHH (Frenkel−Halsey−Hill) equation is an effective approach for calculating fractal dimensions. The FHH equation is presented as follows:29 ⎛ Q ⎞ ⎡ ⎛ ⎛ P ⎞⎞⎤ ⎟⎟ = A⎢ln⎜ln⎜ 0 ⎟⎟⎥ + constant ln⎜⎜ ⎣ ⎝ ⎝ P ⎠⎠⎦ ⎝ Q mono ⎠

(5)

Here, Q is the gas adsorption amount (mL/g(STP)) when the equilibrium pressure is P; Qmono is the gas adsorption amount (mL/g(STP)) under the condition of monolayer adsorption; A is the power factor relevant to the fractal dimension D and the adsorption mechanism, P0 is the saturation pressure (Pa) of gas adsorption, and P is the equilibrium pressure (Pa) of gas adsorption. On the basis of eq 5, the slope of the fitting line A can be obtained, with ln[ln(P0/P)] and ln(Q/Qmono) as the horizontal and vertical coordinates. Fractal dimension D can be related to A based on eq 6, and based on this, D can be calculated. A=D−3

(6)

Figure 10 shows the relationship between ln[ln(P0/P)] and ln(Q/Qmono) obtained by FHH equation with the data provided by N2 adsorption isotherms of each sample. It can be seen from the figure that the curves all have more than two linear sections and each section has a relatively good linear fitting. Therefore, choosing the appropriate section for linear fitting is crucial to precisely calculate the fractal dimension. The preceding results show that the adsorption and desorption isotherms are relatively similar to category V when the P/P0 is low; that is to say, multilayer adsorption will directly happen without experiencing the monolayer 3828

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as there is strong reactivity between calcined CaO with H2O(g) addition and SO2.6,7

adsorption stage and capillary condensation will happen in the case of high P/P0. Therefore, the fractal dimension calculated by the linear fitting with the data provided by multilayer absorption and capillary condensation section can illustrate the interaction between absorbed gas molecules as well as their agglomeration characteristics. Other research shows that the fractal dimension obtained by linear fitting with the data from capillary condensation is related to the pore structure of samples. A higher fractal dimension indicates more complex pore structure and results in the stronger gas absorption resistance on the pore surface.30 That is also the reason why the acting force between the pore surface and adsorbate gas molecules is weaker than that between adsorbate gas molecules themselves. Yao et al.31 also worked out fractal dimension by linear fitting used the data provided by the capillary condensation section at high P/P0 and found that it could represent the fractal features of pore structure. Liu et al.32 also used the data from the capillary condensation section to calculate the fractal dimension of ultrafine pulverized coal particles, and the results also show that the fractal dimension could represent the fractal features of pore structure. Therefore, the data from the capillary condensation section is used to calculate the fractal dimension D in order to represent the complexity of the pore structure. The calculation process and reliability analysis of different samples are shown in Figure 10. The results of fractal dimensions are shown in Figure 11. It

4. CONCLUSION One kind of Chinese limestone was used for calcination experiments in a rotatable FBR. SEM, CSLM, MIP, and N2 adsorption instrument were also used to study the influences of the water vapor on surface morphology and pore structure of calcined CaO. The conclusions are as follows: (1) The existence of water vapor could accelerate the reaction rate and shorten the reaction time during limestone calcination, but the ultimate degree of conversion decreases slightly as the concentration of water vapor increases. (2) Water vapor facilitates the fusion of grains, resulting in the acceleration of sintering, and these are proved by SEM and CSLM. Besides, as the water vapor concentration increases, the decrease in the surface roughness of calcined CaO is also a proof of sintering. (3) The testing results of MIP and N2 absorption show that water vapor can greatly influence the pores of 15−80 nm size within calcined CaO while its influences on pores of more than 100 nm size are weak; The influence mechanism of water vapor is to promote sintering of the pores with small size and combine them together to form pores of 40−100 nm size so that the mean pore diameter increases and specific surface area decreases while specific pore volume hardly changes. (4) Water vapor influences pores of small size. The fractal dimensions of pore structure for calcined CaO under different concentrations of water vapor are calculated by using the data provided by N2 absorption. As the concentration of water vapor increases, the fractal dimension first decreases and then increases, but it is still much lower than that without H2O(g) even though there is 15% H2O(g). So under the influences of water vapor, the pore structure of calcined CaO becomes simpler, which will benefit the reaction between calcined CaO and SO2.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-451-86413231. Fax:+86-451-86412528. E-mail: [email protected]. Notes

Figure 11. Fractal dimensions in different concentrations of water vapor.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the financial support provided by the Key Technologies Research and Development Program of China (Grant No. 2015BAA04B03), the support by the Fundamental Research Funds for the Central Universities (Grant No. HIT.NSRIF.2017048), and the support by the Foundation for Innovative Research Groups of the National Natural Science Foundation of China (Grant No. 51421063).

can be seen from the figure that the fractal dimensions of the calcined CaO are between 2 and 3,19 and as the concentration of water vapor increases, the fractal dimension first decreases and then increases. But the fractal dimension obtained in 15% H2O(g) is still lower than that without H2O(g), showing that the pore structure of calcined CaO becomes simpler in the presence of water vapor. This is probably because the sintering caused by water vapor during limestone calcination combines pores with small size and complex structure into pores with large size and simple structure. Besides, the fractal dimension under 15% H2O(g) is slightly higher than that under 5% H2O(g). The reason is probably that the number of pores with small size decreases as a result of sintering, which could hold CO2 inside; the release of accumulated CO2 regenerates pores of small size on the surface. Therefore, water vapor is able to modify the pore structure of calcined CaO so that the the gas diffusion property in the pores is promoted; so is the calcination itself. Meanwhile, the desulfuration is also enhanced

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REFERENCES

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