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Effect of Temperature on the Carbonation Reaction of CaO with CO2 Zhen-shan Li,*,† Fan Fang,‡ Xiao-yu Tang,† and Ning-sheng Cai† †

Key Laboratory for Thermal Science and Power Engineering of the Ministry of Education, Beijing Municipal Key Laboratory for CO2 Utilization and Reduction, Department of Thermal Engineering, Tsinghua University, 100084 Beijing, China ‡ Xi′an Thermal Power Research Institute Co., Ltd (TPRI), 710032 Xi′an, China ABSTRACT: It is well-known that carbonation is characterized by a rapid initial rate followed by an abrupt transition to a very slow reaction rate. The slow period is believed to be controlled by the diffusion of reacting species throughout the product layer of CaCO3. The thickness of the carbonate layer formed on the free surfaces of CaO is a critical parameter to mark the end of the fast reaction period. This study addresses the question of how temperature affects the reaction process. For example, when the carbonation reaction enters the product layer diffusion-controlled stage at a low temperature such as 500 °C, how does an increase to 600 °C affect the conversion as a function of time and what changes occur in the CaCO3 product layer morphology? This work discusses the interesting finding that the fast reaction stage is recovered again when the temperature is increased. To understand and explain this phenomenon, it is necessary to investigate the mechanism of the temperature effect on the carbonation reaction. Many phenomena are not well explained by the theory of a critical product layer thickness, which is now used almost exclusively to explain the “maximum” conversion during carbonation reaction cycles. Therefore, we provide a new insight into this issue from a nanoscale point of view by combining thermogravimetric analysis (TGA) with the trapping mode (TM) of an atomic force microscope (AFM) to explain the mechanism of the reaction temperature’s effect on the reaction rate and solid conversion characteristics.

1. INTRODUCTION The carbonation reaction of CaO with CO2 is very important in many industrial processes such as CO2 adsorption gasification,1 hydrogen production from integrated coal gasification,2 sorption enhanced steam methane reforming,3 sorption enhanced water−gas shift,4 and CO2 separation from flue gas5 or syngas.6 The reaction of CaO with CO2 is a gas−solid noncatalytic reaction and has been studied by many researchers.7−16 It has been observed that the temperature has a significant effect on CaO conversion; that is, increasing the temperature will increase the carbonation conversion.15,16 Another observed phenomenon is that, in the initial stage, the reaction of CaO with CO2 is fast and is controlled by chemical kinetics.12−14 After the fast initial reaction stage and a transitional stage, a slower reaction stage controlled by diffusion in the pores or in the solid product layer takes place, resulting in slower reaction rates.12−14 The formation of the product layer has two major effects on the subsequent gas−solid reaction. The formation and growth of solid product occurs at the interface between the solid reactant and solid product, protecting the reactant surface from contact with gas and forcing the gas or ions to diffuse through the solid product layer. Furthermore, the formation and growth of solid product plugs the porous structure by filling the pores, inhibiting the diffusion of gas through the porous structure. The thickness of the carbonate layer formed on the free surfaces of CaO is a critical parameter to mark the end of the fast reaction period; the average value for the critical CaCO3 product layer thickness has been measured as 49 nm.17 The theory of a critical product layer thickness is now used almost exclusively to explain the “maximum” conversion during carbonation reaction cycles; however, many phenomena are not well explained by this theory. For example, once the carbonation reaction of CaO © 2012 American Chemical Society

with CO2 enters into the product layer diffusion stage, it is impossible for this reaction to enter the kinetics controlled stage again, because the solid product has already covered the reactant surface. However, a very interesting phenomenon has been observed both in thermogravimetric analysis (TGA) studies (Figure 1) and for fluidized bed experiments (Figure 4 in ref 8); that is, the fast reaction stage appears to reoccur again as the temperature is increased. The understanding and explanation of this phenomenon involves the intrinsic mechanism of the effect of reaction temperature on the carbonation reaction of CaO with CO2. From a macroscopic point of view, the reaction temperature is well-known to have an important effect on the reaction kinetics and solid conversion for gas−solid reactions in the chemical kinetics controlled stage. With increasing temperature, the apparent carbonation rate and conversion of CaO reacting with CO2 increases.11 The effect of reaction temperature on the gas−solid reaction rate can be understood according to the collision theory or activated complex theory. However, the mechanisms of the effects of temperature on the solid conversion are complicated, and less is known about them. For gas−solid reactions such as CaO carbonation, the solid product is generated at the solid reactant surface, and the nucleation, growth, and arrangement of the solid product on the reaction surface is crucial to the progress of the gas−solid reaction. Although experiments clearly reflect that the temperature is the most important controlling variable, the reaction mechanism and how temperature affects the reaction are still unclear. Therefore, we provide a new insight into the reaction by Received: October 10, 2011 Revised: February 25, 2012 Published: March 1, 2012 2473

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Figure 1. Effect of temperature on the carbonation reaction for limestone I with different particle sizes. thermogravimetric analyzer and pure N 2 was fed into the thermogravimetric analyzer for 10 min at a flow rate of 100 mL/ min. The temperature was increased at a rate of 20 °C/min to a final temperature of 100 °C, which was maintained for 10 min. Then, the temperature was increased again at a rate of 50 °C/min to the reaction temperature, which was maintained for 5 min. When the temperature was stable, a gas mixture of CO2 and N2 was introduced into the TGA to induce the carbonation reaction. After a certain period of reaction time, the crystal was cooled to room temperature with 100 mL/min dry pure N2. The crystal was then kept in a dry N2 atmosphere to protect the reacted crystals from any change due to atmospheric humidity until it was tested by AFM. The flow rates of N2 and CO2 from high purity cylinders were controlled by mass flow controllers. The carbonation rate for the reaction between CaO and CO2 is fast. The temperature and CO2 concentration must be at specific levels in order to observe the morphology of the product on the CaO single crystal’s surface during the first fast reaction stage; therefore, 0.5 vol % of CO2 and 99.5 vol % N2 were used for 200 °C carbonation, 10 vol % of CO2 and 90 vol % N2 were used for 500 °C carbonation, and 0.03 vol % of CO2 and 99.97 vol % were used for 600 °C carbonation. For the CaCO3 single crystal experiment, a high concentration of CO2 was necessary in order to prevent CaCO3 decomposition at high temperature. For the CaCO3 single crystal experiment, the procedures were similar to those of the CaO single crystal experiment; however, the main difference was that a gas mixture containing 90 vol % CO2 and 10 vol % N2 was introduced into the thermogravimetric analyzer for the whole process. 2.3. Exploration of the Reacted Single Crystal. The surface morphology of the single crystals before and after reaction was observed by TM-AFM (SPA-300HV, Seiko). The AFM probes were ultrasharp silicon tips (OMCL-AC160TS-C2, Olympus) manufactured in sharpened tetrahedral silicon single crystal with a nominal tip radius of 7 nm, a spring constant of 42 N/m, and a resonance frequency of ∼300 kHz.

imaging the CaO and CaCO3 surfaces reacting with CO2 using the trapping mode (TM) of an atomic force microscope (AFM) to conduct a nanoscale investigation of the mechanism behind the reaction temperature’s effect on the reaction rate and solid conversion.

2. EXPERIMENTAL SECTION 2.1. CaO and CaCO3 Single Crystal. Two types of limestone with different particle sizes were used to study the effect of temperature on the carbonation reaction. The composition of limestone after calcination is shown in Table 1.

Table 1. Composition of Limestone after Calcination (wt %) limestone I limestone II

CaO

MgO

SiO2

Al2O3

Fe2O3

other

98.8 96.6

0.1 1.3

0.6 1.1

0.2 0.6

0.2 0.2

0.1 0.3

CaO and CaCO3 single crystals were used also as experimental samples to investigate the morphological change caused only by the product layer. The uniform and identical surface condition of the samples provided good consistency among all the experiments; that is, the initial conditions of each experiment were the same. CaO and CaCO3 single crystals (10 mm ×5 mm ×0.5 mm) were used for reactions with CO2. An atomic force microscope (AFM) with trapping mode (TM) was used to measure the smoothness and uniformity of single crystals before and after reaction. Before reactions, CaO and CaCO3 single crystals have smooth surfaces; therefore, using these single crystals for reaction with CO2 eliminates the effect of pores and distinguishes the solid product from the solid reactant. 2.2. Thermogravimetric Analysis Experiments. A thermogravimetric analyzer (Q500) was used to carry out the reaction of limestone particles and single crystals of CaO and CaCO3 with CO2. For the CaO single crystal experiment, CaO single crystal was loaded in the 2474

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3. RESULTS 3.1. Effect of Temperature on the Carbonation Reaction of CaO with CO2. Figure 1 shows the reaction process of CaO with CO2. The CaCO3 particle was heated at 850 °C in pure N2 atmosphere. When all CaCO3 had been decomposed to CaO, the temperature was decreased to 500 °C and kept at this temperature for about 5 min. Then 15 vol % CO2 and 85 vol % N2 was introduced into the TGA instrument. The results show that the CaO reacted quickly with CO2 in the initial stage. After about 2−3 minutes, the reaction rate decreased abruptly, indicating the existence of a product layer and the transition to a diffusion limited regime. After about 20 to 30 min for the reaction at 500 °C, the temperature was increased to 600 °C at a rate of 50 °C/min. The reaction rate increased quickly again, as shown in Figure 1. The carbonation reaction of CaO with four different particle sizes ( 0 ⎝ ∂T ⎠

(3)

4 3 πr ρΔGv 3

and

⎛ ∂ΔG* ⎞ ⎜ ⎟ > 0 ⎝ ∂T ⎠

(6)

It should be noted that ΔG* has a strong influence on the density (N*) of stable nuclei, expressed as28

If Δγ ≤ 0, then ϕ = 0 ; this case corresponds to the layer growth, as shown in Figure 11 (1). If Δγ > 0 (ϕ > 0), island growth will occur, as shown in Figure 11 (2). For the islandlayer growth, Δγ ≤ 0 for the first layers and Δγ > 0 for the islands, as shown in Figure 11 (3). As reported by Wang,26 for a chemical reaction, Δγ > 0 will hold; therefore, the CaCO3 product will grow in accordance with the island’s shape. It can be concluded from the results and discussions that the solid product occurs as three-dimensional islands and these islands nucleate and grow heterogeneously on the solid surface. This is because the solid product nucleates at the position with less free energy. Terraces, steps, kinks, and vacancies can exist on the solid surface with lower free energy, and they are the perfect position for nucleation. As the temperature is increased, small islands grow and coalesce, forming large islands. The nucleation of solid products is a key step for a gas solid reaction. From a thermodynamic point of view, the overall free energy change for the formation of a single product nucleus can be expressed as27 ΔG = 4πr 2 σ +

(5)

Variation of solid surface energy, σ, with temperature is usually small. Therefore, the size of a nucleus is mainly dependent on the free energy change of the reaction. The reaction of CaO with CO2 is exothermic and the values of −ΔGv increase with decreasing temperature. Thus, r* is smaller at lower temperatures than at higher temperatures under otherwise identical conditions:

where ϕ is the angle between the CaO−CaCO3 interface and the CaCO3−vacuum interface. We define Δγ as follows: Δγ = γO + γS/O − γS = γO(1 − cos ϕ)

2σ ρΔGv

⎛ ΔG* ⎞ N * = ns exp⎜ − ⎟ ⎝ kBT ⎠

(7)

where ns is the total nucleation site density. In the initial reaction stage, the growth of product islands is governed by both thermodynamics and kinetics. The product island growth involves the diffusion of product molecules or ions along the surface and the boundary between the solid reactant and product. There are three main types of diffusion: surface diffusion, grain boundary (GB) diffusion, and lattice diffusion, as shown in Figure 13. Surface diffusion plays an important role in product island nucleation and growth steps because of the large surface-to-bulk ratio of the product islands. Surface diffusion will lead to product molecules or ions escaping from small islands and joining a larger one, and this will result in more fresh CaO surface coming into contact with gas phase CO2. For product layer diffusion, the boundary between grains is the most important diffusing path. Less tightly bound ions at the nonlattice grain boundary and dislocation are expected to be more mobile than lattice ions. The treatment of GB diffusion can be found in the paper published by Fisher.29 The effect of GB diffusion on the CaO carbonation with CO2 during the product layer diffusion stage has also been studied in detail by Mess et al.13 The diffusion coefficient can be written as

(4)

where ΔGv is the free energy change due to the chemical aspects of the reaction; ρ the molar density; r the radius of the nucleus; and σ is the surface energy per unit area. There exists a critical radius of nuclei, r*, at which the value of ΔG reaches the maximum, and r* can be determined by letting27

⎛ −E ⎞ Di = D0i exp⎜ i ⎟ ⎝ kT ⎠

∂ΔG /∂r = 0 2478

(8)

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Figure 10. AFM images of the CaCO3 surface at 700 °C for different times.

Figure 11. Three different growth models of solid product on the CaO surface. (With kind permission from Springer Science+Business Media: Principles of Surface Physics, Chapter 2 Thermodynamics, 2003, page 62, F. Bechstedt, Figures 2.12, and any original (first) copyright notice displayed with material.)

where i represents surface (S), grain boundary (GB), and lattice (L) diffusion; Ei is the activation energy for diffusion. The activation energy for surface diffusion is typically smaller than those of the other diffusion mechanisms; therefore, the initial product island’s growth is dominated by surface diffusion.

During the product layer diffusion controlled stage, both GB and lattice diffusion control the growth. The effective product layer diffusion coefficient can be expressed as30 Deff = fDGB + (1 − f )DL 2479

(9)

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molecular volume. The free energy per molecule μi is defined as28 μi =

4 d πri3/Ω 3

(

)

=

2Ωσ ri (11)

If μi is large, the effective molecular concentration inside a product island is larger, forcing them to escape to a location with a lower μi value. Thus, the smaller nuclei will have a great amount of free energy and will diffuse along the surface from smaller islands to larger islands, which grow at the expense of the former. Based on these findings, a new mechanism for product layer formation and growth is proposed here. The carbonation reaction might consist of the following steps: (1) reactant gas molecules are adsorbed on the solid surface; (2) adsorbed molecules react with solid molecules to form solid product; (3) product molecules or ions diffuse on the reaction surface; (4) product molecules or ions come into contact to create the possibility of nucleating a new island or finding and joining an existing island; or (5) product molecules or ions escape from small islands and join larger ones. Among these steps, the surface diffusion step is more important. The increase in temperature enhances surface diffusion and causes the formation of larger islands, making more fresh CaO surface available to the CO2. Based on this mechanism for product layer formation and growth, the experimental phenomena shown in Figures 1−3 can be explained as follows. Step I: at low temperature such as 500 °C, the initial carbonation reaction is fast, because some fresh CaO surface is available for direct contact between CaO and CO2, as shown in Figure 14. The product molecules or ions diffuse slowly at

Figure 12. Equilibrium of forces at CaO surface, product island, and vacuum. (With kind permission from Springer Science+Business Media: Principles of Surface Physics, Chapter 2 Thermodynamics, 2003, page 62, F. Bechstedt, Figures 2.13, and any original (first) copyright notice displayed with material.)

Figure 13. Diffusion behavior involvement for the reaction of CaO with CO2.

where f is the volume fraction of the grain boundary. In eq 9, the effective product layer diffusion coefficient, Deff, is related to the lattice diffusion and grain boundary diffusion coefficients. It should be noted that GB changes with the increase of reaction time,13,31 and therefore, f also changes. When the grain has a small diameter, the boundary between grains becomes a dominating factor, having a grain boundary diffusion coefficient as the effective diffusion coefficient. With the increase of reaction time, grains become larger, and the grain boundary decreases,13,31 leading to the decrease of f and Deff. In this case, the importance of lattice diffusion through the product layer crystals increases with time relative to the diffusion through the grain boundaries. In the initial reaction stage, nucleation and growth are competing processes32 and the outcome of this competition is determined by both the chemical reaction rate and surface diffusion, and the number of nuclei can be expressed as the ratio of chemical reaction to surface diffusion, R Ṅ ∼ DS

d(4πri2 σ)

(10) Figure 14. Mechanism of temperature effect on the carbonation reaction of CaO with CO2.

where R is the chemical reaction rate and Ds is the surface diffusion coefficient. At lower temperatures, the ion diffusion distance (Ds) on the surface is small, and a high density of stable nuclei (Ṅ ) with small r* is formed. When the temperature is increased, some nuclei with larger sizes are formed because the ion diffusion distance (Ds) on the surface increases, and the larger nuclei grow at the expense of the smaller nuclei. The driving force for this growth is the minimization of surface free energy of all product islands. For a spherical product island with radii ri, the surface free energy is Gsur = 4πri2σ. The number of molecules contained by the island is given by 4/3πri3/Ω where Ω is the

lower temperature compared higher temperature, resulting in a high density of small islands. Step (II): with the reaction proceeding, the product islands gradually cover the CaO surface. When the surface is fully covered by the product, the reaction enters into the product layer diffusion stage, and the CO2 or ions have to diffuse through the CaCO3 product layer. This stage is controlled by both GB diffusion and lattice diffusion. The relative importance of lattice diffusion to GB diffusion increases with time due to the decrease of the volume fraction of GB, as shown in eq 9. Step (III): as the temperature is increased to 600 °C, simultaneous CaCO3 decomposition 2480

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and recarbonation accelerates, and the Ca2+ and O2− on the surface diffuse faster than they did at the lower temperature (500 °C). More importantly, the island density is reduced with the formation of large islands at higher temperatures, and surface morphology is rearranged. Step (IV): thus, these two factors result in the exposure of fresh CaO surface to the reactant gas again, and the dominant restrictive factor is changed to chemical kinetics from product layer diffusion. Step (V): with the reaction proceeding, the product gradually covers the CaO surface again, and then the reaction enters into the product layer diffusion stage. Steam may also enhance the surface diffusion and GB diffusion similar to temperature. This possibility will be explored in a later publication.

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5. CONCLUSIONS To study the effect of temperature on the carbonation reaction of CaO with CO2, in this study, we provided a new insight into the gas−solid reaction by imaging the CaO and CaCO3 surfaces reacting with CO2, using TM-AFM, from a nanoscale point of view. Based on the experimental results and discussion, the CaCO3 product grows with an island morphology on the CaO surface, and most of the islands appear at the position with terraces, steps, and kinks. With the increase of reaction temperature, CaCO3 product island sizes and heights increase while the island density decreases. These two phenomena lead to the increase of solid conversion in the fast reaction stage. Surface diffusion is important for the fast reaction stage, which competes with the nucleation rate. The surface diffusion distance at lower temperatures is shorter than that at high temperatures. High density groups of islands with smaller size at low temperature are formed while low density, larger sizes of islands are formed at high temperatures. For the product layer controlled stage, both grain boundary diffusion and lattice diffusion dominate the process; the boundary between grains is initially a dominating factor. With reaction time increasing, grains grow with larger sizes, and the grain boundary decreases. The importance of lattice diffusion through the product layer crystals increases with time relative to the diffusion through the grain boundaries. The decomposition−carbonation mechanism is proposed in this study to characterize the net CO2 exchange and surface rearrangement and to explain the mechanism of the effect of temperature on the carbonation reaction of CaO with CO2.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Fund of China (No. 50806038 and No. 51061130535) and the National Basic Research Program of China (2011CB707301).

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