ie300596x[1]

Physical and Chemical Mechanism for Increased Surface Area and Pore Volume of CaO in Water Hydration Zhenchao Sun, Hao Chi, and Liang-Shih Fan* William G. Lowrie Department of Chemical and Biomolecular Engineering, Ohio State University, Columbus, Ohio 43210, United States ABSTRACT: The present work explores the fundamental mechanism behind the increased surface area and pore volume of CaO after hydration. First, a widely believed mechanism, the “physical attrition theory”, is experimentally examined and is found to have limitations in explaining this phenomenon. Next, to explain the improvement of morphological properties by hydration, a typical water hydration process is examined by dividing the process into four independent chemical and physical substeps. The morphological changes of Ca(OH)2 and its derived CaO by each substep are measured by Brunauer−Emmett−Teller (BET) analysis. During the first step, the intrinsic chemical conversion from CaO to Ca(OH)2, the formed Ca(OH)2 product layer disintegrates because of its low tensile strength and weak crack resistance, which explains the increases in surface area and pore volume by steam/moisture hydration as well as the rapid heat release during hydration. The physical interaction with water (the second step) slightly decreases the surface area and pore volume, possibly by lodging microparticles into the porous structure of bigger particles and inducing stronger particle agglomeration. The Ca(OH)2 solid can further chemically bond water molecules (the third step), which significantly enlarges the solid volume during water-bonding and consequently generates a more porous structure during dehydration. The final precipitation of the dissolved Ca(OH)2 (the fourth step) decreases the solid’s surface area and pore volume. This decrease is attributed to the formed microparticles from solution, which can plug some surface pores on the larger particles during the drying process. ■ INTRODUCTION The increase of surface area and pore volume of CaO by hydration was revealed during the early 20th century or even earlier, when the hydrated lime became a commercial product for modern society.1,2 Although the apparent reactions involved in this process are very simple, as shown in eqs 1 and 2, the surface area and pore volume of the solid CaO can be significantly increased after a hydration step and a subsequent dehydration step.3−8 + →hydration:CaO H O Ca(OH)(s) 2 (g or l) 2(s) (1) → +dehydration:Ca(OH) CaO H O2(s) (s) 2 (g) (2) Because of this unique property of CaO, both water- hydration and steam-hydration are widely applied as an activation/reactivation strategy of CaO-based sorbents in the capture of SO2, CO2, and other acidic gases from syngas and coal-combustion gas.3−11 Application of hydration to CaO- based sorbents for SO2 capture in fluidized bed combustion (FBC) and from coal-combustion flue gas started around the 1980s,11 while it received wide attention for CO2 capture during the past decade with the arising global warming concern. In the past decades, this CaO sorbent activation/reactivation approach has been successfully demonstrated in many experimental setups and reaction schemes, which shows promising prospects for large-scale commercialization.3−11 To study the mechanism of this phenomenon, a number of experimental and theoretical works were performed, and several theories were proposed, which were summarized in various works.1,2,5,12 However, most of these theories tried to explain this phenomenon by varying one single reaction parameter in the overall hydration process, for example, water-to-CaO ratio, temperature, pressure, additives, and other individual parame- ters. However, a hydration process, especially a water-hydration process, is a process of multiphase interaction, which involves multiple physical and chemical substeps. The change of one reaction parameter can generally affect more than one substep in the hydration process. For example, the temperature of water-hydration can affect the hydration reaction rate, the Ca(OH)2 solubility, the physical liquid−solid interaction by vibration of water molecules, the formation of Ca(OH)2·nH2O, etc., which could all possibly affect the resulting solid properties and complicate the interpretation of this mechanism. Hence, conclusions based on such methodology and experimental designs are often disconnected and sometimes inconsistent.1,12 In this study, a widely believed “single-factor” mechanism, the “physical attrition theory”,1,2 is first tested to demonstrate its limitation in explaining the hydration mechanism. A typical water-hydration is divided into several chemical and physical substeps, which are then independently probed. The effect of each substep on the solid morphological properties (surface area and pore volume) is individually studied with the Brunauer−Emmett−Teller (BET) analysis. ■ EXPERIMENTAL SECTION Materials. CaO powder (99.95% pure, <50 μm, named “raw CaO powder”, used to make CaO pellet in this study), Received: March 5, 2012 Revised: July 2, 2012 Accepted: July 30, 2012 Published: July 30, 2012 Article pubs.acs.org/IECR © 2012 American Chemical Society 10793 dx.doi.org/10.1021/ie300596x | Ind. Eng. Chem. Res. 2012, 51, 10793−10799 Ca(OH)2 powder (99% pure, <50 μm, named “raw Ca(OH)2 powder”), and naturally occurring limestone (∼97% pure CaCO3, <50 μm) were obtained from Aldrich Chemicals, MP Biomedicals, and Graymont, respectively. Ethanol (100% pure) was from Decon laboratories. 99.99% pure N2 was used for calcining Ca(OH)2 and limestone to CaO for the BET tests. Morphological Property Test. The morphological prop- erties of Ca(OH)2 and CaO solids were tested in a NOVA 4200e analyzer (Quantachrome Co.). The BET surface area and pore volume were measured at −193 °C using liquid N2 as adsorbent. For each Ca(OH)2 sample, its derived CaO sample was prepared on a Perkin-Elmer (Pyris 1) Thermogravimetric Analyzer (TGA) apparatus by calcining Ca(OH)2 at 700 °C for 5 min. This fixed calcination condition is to exclude the effect of calcination condition on the resulting CaO sorbents. To ensure accuracy, the surface area and pore volume presented in this study are averaged values. In addition, key experiments were repeated using other CaO precursors, which show consistent results but are not presented here to avoid data redundancy. Examination of the “Physical Attrition” Theory (Hydration with Controlled Heat Release). A batch of limestone was calcined at 700 °C for 30 min as the unsintered CaO, which is referred to as the “limestone-CaO”. A batch of sintered CaO sample was prepared by calcining limestone at 900 °C for 2 h (named “sintered-CaO”). The regular hydration was carried out by gradually adding ∼15 mL of deionized water directly onto the “sintered-CaO” sample (∼4.5 g). This solid sample was dried and calcined to a CaO sample (named “water- hydration-CaO”) for the BET test. The hydration with controlled heat release, as shown in Figure 1, was performed with the same amount of “sintered-CaO” sample put into container 1. The bottom of this container was submerged into liquid N2 (∼−193 °C). After stabilization, deionized water was slowly added into container 1. As soon as the water droplet contacted the CaO solid and container, it immediately froze into supercold ice (∼−193 °C) and covered the CaO solid. After the same amount of water was added, container 1 was slowly moved out of the liquid N2 and placed into the ambient condition. After the ice fully melted into liquid and the liquid− solid mixture stabilized for a while, the solid sample was dried and calcined into a CaO sample (named “LN-hydration-CaO”) for the BET test. Effect of the Chemical Conversion of CaO to Ca(OH)2 (Moisture Hydration of a CaO Pellet). “Raw CaO powder” was pressed into a cylindrical pellet (d = 6 mm, h = 3 mm) at 5 MPa pressure. The pressed CaO pellet was sintered at 1200 °C for 3000 min to simulate a CaO particle. This sintered CaO pellet is named “sintered CaO pellet” in the following sections. As shown in Figure 2, this well-sintered CaO pellet was put on a support stage and placed into a sealed container with some water at the bottom. The CaO pellet reacted with the moisture, while it is isolated from the ambient CO2 by the sealed container. The morphological change of this CaO pellet during the moisture-hydration was photographed by a digital optical camera (Nikon D5000) on the first three days. The surface area and pore volume of the “raw CaO powder”, the “sintered CaO pellet”, and the CaO derived from moisture-hydration of the CaO pellet (named “moisture-hydration-CaO”) were meas- ured. A similar moisture hydration experiment was repeated by using calcined lime as the starting material. Effects of the Physical and Chemical Interactions between Ca(OH)2 and Water. The physical interaction of Ca(OH)2 and water was studied by putting 2 g of “raw Ca(OH)2 powder” in 40 and 140 mL of ethanol, respectively. The liquid−solid mixtures were heated at about 100 °C to vaporize the ethanol, producing “Ca(OH)2-40 mL-ethanol” and “Ca(OH)2-140 mL-ethanol” samples, respectively. Their calcined CaO samples are named “Ca(OH)2-40 mL-ethanol- CaO” and “Ca(OH)2-140 mL-ethanol-CaO”, respectively. To study the effect of chemical interaction between Ca(OH)2 and water, 2 g of “raw Ca(OH)2 powder” was first mixed with 40 mL of deionized water. From this mixture, the undissolved solid was filtered out and dried at ∼100 °C, which is named “Ca(OH)2-40 mL-undissolved”. Its calcined CaO sample is named “Ca(OH)2-40 mL-undissolved-CaO”. The surface area and pore volume of these Ca(OH)2 samples and their derived CaO samples were measured by BET. Effect of the Precipitation of the Dissolved Ca(OH)2. The effect of precipitation of dissolved Ca(OH)2 was studied by drying whole mixtures of 2 g of “raw Ca(OH)2 powder” with 40 and 140 mL of deionized water, respectively. Surface area and pore volume of the dried Ca(OH)2 samples (named “Ca(OH)2-40 mL-water” and “Ca(OH)2-140 mL-water”, respectively) and their calcined CaO samples (named “Ca- (OH)2-40 mL-water-CaO” and “Ca(OH)2-140 mL-water- CaO”, respectively) were tested by BET. ■ RESULTS AND DISCUSSION Examination of the “Physical Attrition Effect” Theory (Hydration with Controlled Heat Release). It is common knowledge that the CaO hydration process is usually coupled with rapid/strong heat release. Thus, many previous researchers believed that this rapid/strong heat release induces the inner pore expansion and interparticle attrition, which break the bigger particles into smaller ones.1,2 Other “evidence” used to support this theory was that a smaller average particle size was found after hydration.13 Hence, this theory is widely used to explain the mechanism of the increases of surface area and pore Figure 1. Experimental setup of hydration of sintered CaO at a controlled heat release rate. Figure 2. Experimental setup of moisture-hydration of a well-sintered CaO pellet, which simulates a CaO particle. Industrial & Engineering Chemistry Research Article dx.doi.org/10.1021/ie300596x | Ind. Eng. Chem. Res. 2012, 51, 10793−1079910794 volume of CaO solids after hydration, and many experimental results about hydration were explained on the basis of this theory, such as the “drowning of the quicklime”.1,2 In this study, this theory was first examined by comparing a regular water-hydrated sample (“water-hydration-CaO”) with a sample hydrated at a controlled heat release (“LN-hydration- CaO”). As shown in Figure 1, at about −193 °C, the hydration reaction rate would be close to zero. This means that there was no quick heat release by reaction at the moment the water (or ice) contacts the CaO solid. After the CaO solid was frozen into a big ice shell and slowly moved out of the liquid nitrogen, the hydration reaction gradually started, from which the reaction heat was slowly released and quickly absorbed by the surrounding water (or ice). In this manner, any effect due to quick heat release was minimized. During this process, no bubble or hissing noise that often occurs during a regular water hydration was observed, implying a minimal level of violent reaction. While in the regular hydration, hissing noise was heard as the CaO contacted the water, indicating a regular level of violent reaction. As shown in Table 1, CaO samples derived from both hydration processes demonstrate noticeable increases in surface area and pore volume. If such improvement is caused by the physical attrition or pore cracking due to the violent reaction- heat release, as many researchers would believe, there should be an obvious difference between these two samples. However, by comparing the “LN-hydration-CaO” and “water-hydration- CaO”, the surface area and pore volume of both samples are at the same levels, which indicates that preventing quick heat release does not impede the overall increases in surface area and pore volume. This experimental result suggests that the widely believed “physical attrition theory” cannot explain the increase of surface area and pore volume of CaO after hydration. Substeps in a Typical Water Hydration. A typical water hydration process can be illustrated in Figure 3, regardless of whether it is in a water-to-CaO mode or a CaO-to-water mode. Upon contact between solid CaO and water, the chemical conversion from CaO to Ca(OH)2 occurs instantly. This chemical conversion induces solid volume expansion due to the higher molar volume of Ca(OH)2 and a consequently increase in internal stress. Whether this formed product layer can hold this increased stress is mainly dependent on its mechanical strength and other intrinsic material properties. In case of CaO hydration in moisture/steam, this intrinsic conversion denotes the entire process, which involves no further interaction with liquid water after the initial reaction. After step 1, the formed Ca(OH)2 further interacts with water both physically (step 2) and chemically (step 3). The weakly bonded particles physically disperse in water and reintegrate after drying. Meanwhile, the Ca(OH)2 may further chemically react with water. In the fourth step, a small portion of solid dissolves into water and precipitates out during drying. Effect of the Chemical Conversion from CaO to Ca(OH)2 (Moisture Hydration of a CaO Pellet). To observe the morphological change during the chemical conversion from CaO to Ca(OH)2, a well-sintered CaO pellet (“sintered CaO pellet”, d = 6 mm) was used to simulate a calcined CaO particle (d < 100 μm). Using a sintered CaO pellet can facilitate visual observation of the entire moisture-hydration process by avoiding complicated preparation, transportation, isolation from the ambient CO2, and particle locating in multiple microscopic analyses. Similar approaches have been widely applied in the studies of solid-phase product layer growth in gas−solid reactions.10,14−17 The CaO pellet was then put into a moist environment. Images of the changing pellet were photographed on the first, second, and third days. As shown in Figure 4, the CaO pellet gradually swelled and disintegrated into smaller fragments and fine powder. These particles were collected and calcined to CaO (“moisture-hydration-CaO”) for the BET tests. In the previous studies about cement18 and dental materials,19 Ca(OH)2 has been reported to have extremely Table 1. Morphological Information of CaO Samples in the Study of “Physical Attrition Theory” sample I.D. surface area (m2/g) pore volume (cc/g) brief notation limestone- CaO 19.600 0.099 CaO calcined from limestone at 700 °C for 30 min sintered- CaO 5.194 0.038 CaO calcined from limestone at 900 °C for 2 h LN- hydration- CaO 37.736 0.227 CaO calcined from “sintered-CaO” hydration without rapid heat release by LN freezing water- hydration- CaO 37.001 0.225 CaO calcined from regular hydration of “sintered-CaO” with rapid heat release Figure 3. Illustration of a typical water-hydration process of CaO: (step 1) chemical conversion from CaO to Ca(OH)2; (step 2) physical interaction of Ca(OH)2 with water; (step 3) chemical reaction of Ca(OH)2 and water; and (step 4) precipitation of dissolved Ca(OH)2. Figure 4. Images of the morphological change of a sintered CaO pellet during moisture-hydration: (a) sintered CaO pellet on the first day, (b) the second day, (c) the third day, and (d) after a gentle touch on the third day. Industrial & Engineering Chemistry Research Article dx.doi.org/10.1021/ie300596x | Ind. Eng. Chem. Res. 2012, 51, 10793−1079910795 low crack resistance and weak tensile strength. Hence, the weak Ca(OH)2 product layer cannot hold the increased stress due to the volume expansion, therefore breaking into smaller frag- ments. It is noted that solid volume expansion does not necessarily lead to powder formation and pellet breaking. It has been reported in the literature that CaO pellets prepared by the identical method can still maintain the pellet integrity after chloridation, sulfation, or sulfidation, although these three reactions also induce pellet volume expansion.10,14,15 Hence, the extremely low mechanical strength of Ca(OH)2 is the key factor for the particle breaking. In addition, the unique Ca(OH)2 product layer breaking process can also explain the violent heat release in CaO hydration. For most volume-increasing gas−solid or liquid− solid reactions, the overall reaction has two steps: (1) the surface chemical reaction step, and (2) the solid-phase ionic diffusion step. Given the low ionic diffusivity in solids, the solid- phase ionic diffusion step is usually the rate-controlling step. Yet in the hydration of CaO, its weak mechanical strength causes cracks in the swelling Ca(OH)2 product layer as the reaction proceeds, which circumvents the slow solid-phase ionic diffusion and opens pathways for direct water (or steam) penetration onto the interior unreacted CaO. Without the diffusion resistance of the product layer, the CaO can react with water (or steam) at a very high rate, resulting in the concentrated reaction-heat release within a short time. This property can also explain why the CaO hydration is a violent exothermic reaction, even though its reaction heat (ΔH = −109 kJ/mol) is not significantly high as compared to most gas−solid and liquid−solid reactions. Therefore, we propose that the rapid heat release is the “effect” rather than the “cause” of Ca(OH)2 particle breaking, which is misinterpreted by many researchers.1,2 It should be noted that the concentrated heat release and product layer breaking are not necessary results of volume expansion from CaO (16.7 cm3/mol) to Ca(OH)2 (33.5 cm 3/ mol).2 There are many gas−solid reactions that have higher molar volume expansion and higher reaction heat, such as the CaO carbonation (ΔH = −182 kJ/mol; V(CaCO3) = 36.9 cm3/mol) and CaO sulfation (ΔH = −481.4 kJ/mol; V(CaSO4) = 45 cm 3/mol).3,6 Yet these reactions are not reported to proceed quickly at room temperature and generate higher surface area and pore volume like the CaO hydration reaction. Hence, we can conclude with reasonable confidence that the low mechanical strength of Ca(OH)2 is the most important reason for the particle breaking and the resulting increase of surface area and pore volume in the steam/moisture hydration. Even though steam/moisture hydration at different reaction conditions (such as temperature and pressure) can produce Ca(OH)2 solids with different morphological properties, the major increase can all be attributed to this particle breaking. Different reaction conditions would affect the mechanical cracking process, thereby resulting in the difference in morphological properties. Future work can focus on the mechanical change during the product layer expansion, which is a much researched topic in the field of materials science and engineering.20−22 To quantitatively confirm the increases of surface area and pore volume after this moisture-hydra