Influence of Water Vapor on Carbonation of CaO in the Temperature

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Influence of Water Vapor on Carbonation of CaO in the Temperature Range 400
550 °C Isak Linden,*,† Peter Backman,† Anders Brink,† and Mikko Hupa† †

Åbo Akademi Process Chemistry Centre, Biskopsgatan 8, FI-20500 Åbo, Finland ABSTRACT: The effect of water vapor on the kinetics of CO2 capture using CaO in the form of calcined limestone was determined by means of a thermogravimetric analyzer. The effect was studied at three different temperatures, 400, 450, and 550 °C; three different partial pressures of CO2, 0.05, 0.15, and 0.5 atm; and four different partial pressures of H2O, 0, 0.02, 0.1, and 0.3 atm. Calcined limestone with a particle size of 100
200 μm was used. For each condition the conversion as a function of time was determined. At 400 and 450 °C, water vapor had an accelerating effect on the conversion of CaO to CaCO3. At 500 °C, water vapor had no accelerating effect, instead it had a slightly retarding effect on carbonation. Regardless of the partial pressure of water vapor, higher temperature resulted in higher conversion. To study possible mass transfer limitations the effect of diluting the CaO sample with inert SiO2 was also studied.

1. INTRODUCTION Burnt lime, CaO, is widely used in many industrial processes. CaO readily reacts with flue gas components such as H2O, SO2, and CO2 and may be used to capture these. The reaction between CaO and CO2, carbonation, can either be intentional, such as in the capture CO2 from flue gases, which can then be released separately in a concentrated stream, or unintentional or even harmful, if lime is to be used for some other purpose, and needs to be in the form of CaO. Harmful carbonation occurs for example in limestone calcination processes and during oxyfuel combustion. Carbonation of lime is covered in a review article by Stanmore and Gilot.1 CaO reacts with H2O and CO2 in the following ways CaO ðsÞ þ CO2 ðgÞ f CaCO3 ðsÞ

ð1Þ

CaO ðsÞ þ H2 O ðgÞ f CaðOHÞ2 ðsÞ

ð2Þ

CaðOHÞ2 ðsÞ þ CO2 ðgÞ f CaCO3 ðsÞ þ H2 O ðgÞ

ð3Þ

2

According to Boynton, the rate of reaction 1 is negligible at temperatures below 290 °C in the absence of H2O. In the presence of water vapor reaction 2 occurs, and since reaction 3 is faster than reaction 1, H2O is believed to have a strong accelerating effect on carbonation when calcium hydroxide is thermodynamically stable. For example Zeman3 studied the difference between capture of CO2 by CaO and Ca(OH)2 and found that Ca(OH)2 captured CO2 much faster than CaO. The reversibility of the reaction between CaO and CO2 was studied by Barker4 who found that after calcination, carbonation only proceeded to 40
80% of the maximum theoretical conversion. Bhatia and Perlmutter5 concluded that carbonation first proceeds by a rapid surface reaction-controlled stage, followed by a slower internal diffusion-controlled process. As carbonation proceeds, pores are closed and a layer of calcium carbonate is formed on the surface of the calcium oxide particles, slowing down the reaction significantly. They noted that after 60% conversion, the reaction almost slows down so much that it r 2011 American Chemical Society

can be considered to be completed. Carbonation will thus be faster in the beginning of the reaction and then slow down. The same phenomenon was also described by Mess et al.6 Bhatia and Perlmutter5 studied the effect of several parameters on carbonation. They found that particle size and partial pressure of CO2 had only little impact on carbonation. The reaction was strongly dependent on temperature however. Both the rate of the reaction and the final conversion increased as the temperature rose. Mess et al.,6 as well as Boynton,2 reports the same findings regarding the effect of temperature. The effect of water vapor on carbonation of CaO has been studied by Nikulshina et al.7 and Wang et al.8 Nikulshina et al.7 studied the carbonation of CaO and Ca(OH)2 at 500 ppm CO2 in air at 200
450 °C. They also studied the effect of a water vapor addition of 50 volume-% to the air during carbonation and found that it significantly enhances the reaction kinetics. In their study, calcium oxide pro analysi in the form of fine powder and of high purity was used. Wang et al.8 studied the effect of water vapor on carbonation of CaOcontaining fly ash during oxy-fuel combustion, where there is a CO2 partial pressure of 80
85%. They found that the presence of H2O resulted in higher conversion of CaO to CaCO3, especially at temperatures below 400 °C. At higher temperatures, the effect was much smaller. Wang et al.8 used fly ash with a CaO content of about 50%. Other studies related to the system CaO-H2O-CO2 include Borgwardt9 and Zeman.3 According to Bogwardt,9 the presence of H2O and CO2 causes sintering of CaO, which reduces its reactivity, although mainly at high temperatures, around 800 °C. None of the studies found in the literature dealt with carbonation at typical partial pressures of CO2 found in flue gases. Wang et al.8 focuses on very high partial pressures of CO2, while Nikulshina et al.7 focuses on very low partial pressures. Also, none of the studies previously mentioned used actual limestone as samples. Received: May 6, 2011 Accepted: October 28, 2011 Revised: October 23, 2011 Published: October 28, 2011 14115

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Table 1. Experimental Conditions partial

partial

temperature (°C)

pressures of H2O (atm)

pressures of CO2 (atm)

400

0, 0.03, 0.1, 0.3

-

450

0, 0.03, 0.1, 0.3

0,005, 0,15, 0,5

-

550

0, 0.03, 0.1, 0.3

-

1/3 CaO, 2/3 CaO

dilution -

Figure 2. The stability curve of CaO and Ca(OH)2, with the conditions of the experiments marked on the graph.

Figure 1. A drawing of the sample holder containing the lime sample and the thermogravimetric analyzer (TGA) setup.

In this study, the effect of water vapor on the kinetics of CO2 capture using calcined limestone was determined by means of a thermogravimetric analyzer (TGA). The effect was studied at three different temperatures, 400, 450, and 550 °C; three different partial pressures of CO2, 0.05, 0.15, and 0.5 atm; and four different partial pressures of H2O, 0, 0.02, 0.1, and 0.3 atm. The temperatures were chosen so that they would be around the stability curve of calcium hydroxide. The effect of diluting CaO in an inert material was also studied. For each condition the conversion as a function of time was determined. It was also investigated which reaction takes precedence, hydroxide or carbonate formation, and if calcium hydroxide can be considered to be an intermediate product during carbonation. Lime samples carbonated with different partial pressures of water vapor were also studied with SEM.

2. EXPERIMENTAL SECTION The experiments were performed in a thermogravimetric analyzer (TGA), which is shown in Figure 1. In the TGA, the sample holder was hanging in the reactor by a chain, which was attached to a balance above the reactor. The gas flow entered the reactor from the bottom. The flow of water vapor was controlled by a pump which fed water to an electrically heated steam generator, while the flow of CO2 and N2 was regulated by mass flow controllers. The temperature was measured with a thermocouple placed immediately under the sample holder. The sample holder was cylindrical, and the sample was placed between an inner core and an outer net (Figure 1). Thus, the thickness of the sample was uniform, and a large ratio of outer surface to the mass of the sample was obtained. In these experiments, the thickness of the sample layer was approximately 1 mm. The sample holder

and the net were made of platinum. A limestone from Verdal, Norway, with a calcium carbonate content of 99 weight-%, was used for the experiments. The limestone was crushed and sieved, and the fraction used had an average particle size of 100
200 μm. For the experiments with diluted limestone, the limestone was diluted to one-third by weight with SiO2 pro analysi, sieved to a fraction of 100
200 μm. During the experiments, a gas mixture of CO2, H2O, and N2 was flowing through the reactor. The total pressure in the reactor was 1 atm. Two series of experiments were performed, one with a set partial of pressure of water vapor and one with a set partial pressure of CO2. Additionally, the effect of diluting CaO was determined. The experimental conditions can be found in Table 1. During all of these conditions, calcium carbonate was stable, meaning that carbonation was thermodynamically possible. However, calcium hydroxide was not stable during most of the runs. The stability curve of calcium hydroxide, plotted along with the conditions of the experiments, can be seen in Figure 2. The thermogravimetric experimental procedure is displayed in Figure 3. 250 mg of limestone was used for each measurement. The limestone was first calcined in the TGA in an N2 atmosphere at 800 °C, so that the CaCO3 in the sample was fully converted to CaO. The temperature was then adjusted to the desired temperature of the carbonation experiment. The flow of H2O and CO2 was then switched on, and the sample was allowed to react with the gas flow while the weight of the sample was registered. After the reaction, the H2O and CO2 flows were switched off, so that only N2 was flowing through the reactor. The temperature was then adjusted to 450 °C, where calcium hydroxide is no longer stable and decomposes into CaO and H2O. Then, the temperature was adjusted to 800 °C, where calcium carbonate decomposes into CaO and CO2. In this way it was possible to determine if the weight gain was due to carbonate or hydroxide formation.

4. RESULTS AND DISCUSSION As the decomposition of calcium hydroxide and calcium carbonate was determined after each experiment, only decomposition of 14116

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Figure 3. A graphical description of the experimental thermogravimetric procedure. The picture shows the thermogravimetric curves during during one example run, with descriptions of the different stages of the experiment.

Figure 4. A close-up of the initial reaction stage, during which the reaction is much faster than in the later stage, during all experiments with a constant partial pressure of carbon dioxide (0.15 atm).

calcium carbonate was detected. There was no weight loss at 450 °C for any of the experiments, only at last step with the final temperature of 800 °C. The entire mass gain which had occurred during the reaction was thus considered to be solely due to formation of CaCO3. The conversion of CaO to CaCO3 was calculated from this mass gain. According to Figure 2, calcium hydroxide should be a stable product in three of the experiments. If it was actually formed, it was immediately turned into calcium carbonate. The first 40 s of each carbonation experiment can be seen in Figure 4. Regardless of the conditions, carbonation proceeds as a fast reaction during this time interval, relative to the following reaction stage. After the first stage, a much slower reaction takes place, which continues until the experiment is ended. The reaction rate of the first stage is slightly accelerated by the addition of water vapor at 400 °C but is largely unaffected by the partial pressure of water vapor at higher temperatures, although a slight acceleration can be seen. The reaction is slightly faster at higher temperatures. Figure 5 shows the effect of partial pressure of water vapor on the conversion of CaO to CaCO3 at different temperatures, with a constant partial pressure of CO2. It can be seen that the partial

pressure of water vapor influences the conversion of CaO to CaCO3 reached before the reaction enters its slow stage, while it has little effect on the initial rate of the reaction. At 400 °C, experiments with water vapor produced higher conversion than the experiment without water vapor. However, the highest conversion was achieved at a partial pressure of 0.1 atm and not at the highest partial pressure of water vapor, 0.3 atm. At 450 °C, the experiments without water vapor still gave the lowest conversion, but 0.02 atm water vapor gave the highest conversion, higher than for 0.1 and 0.3 atm of water vapor. At 500 °C, water vapor only slowed down the carbonation reaction. At this temperature the experiment with no water vapor gave the highest conversion. Figure 6 shows the effect of temperature on conversion of CaO to CaCO3 at certain partial pressures of water vapor and a constant partial pressure of CO2. It can be seen that the effect of temperature on carbonation is more pronounced at low partial pressure of water vapor. At higher partial pressures, the effect of temperature is smaller. It is also shown that a small addition of only 0.02 atm of water vapor can have a significant effect on conversion. It is difficult to explain why higher water vapor contents cause lower conversion to CaCO3. One explanation could be that the higher water content caused more sintering, which in turn lowered the reactivity of the lime. At 500 °C, the effect of water vapor should in any case be small, as reported by Wang et al.8 The results of this study still partly contradicts those of Wang et al.8 since they noticed a small accelerating effect of water vapor at 550 °C. However, Wang et al.8 used a much higher partial pressure of CO2, 0.85 atm, and a different source of CaO (fly ash). Whether or not calcium hydroxide was stable during the experiments does not seem to have any significant impact on the results either. The effect of the partial pressure of CO2 can be seen in Figure 7. CO2 pressure had a slight accelerating effect on the initial rate of the carbonation reaction, but it did not to any greater extent influence the conversion which was reached before the reaction entered its slow stage. This finding is consistent with that of Bhatia and Perlmutter,5 who also found CO2 to influence the initial reaction rate. The effect of diluting the sample with 2/3 SiO2, under a gas flow of 0.15 atm CO2 and 0.85 atm N2, can be seen in Figure 8. Dilution also affected the initial stage of the reaction by slowing it down but did not notably influence what conversion was reached before the reaction slowed down and 14117

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Figure 5. The effect of water vapor partial pressure at temperatures of 400 °C, 450 °C, and 550 °C and a CO2 partial pressure of 0.15 atm.

Figure 6. The effect of temperature at a H2O partial pressure of 0, 0.02, 0.1, and 0.3 atm and CO2 partial pressure of 0.15 atm.

entered the slow stage. The effect of CO2 pressure on the initial, fast stage of the reaction does not in itself tell if the reaction is limited by kinetics or mass transfer, but as both the pressure of CO2 and dilution of the sample only affected the reaction rate of

the initial, fast stage of the reaction, it can be concluded that this stage is limited by mass transfer. Samples which were studied in the SEM were first carbonated for 200 s at a temperature of 400 °C and at a CO2 partial pressure 14118

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repeatability of the setup. The conversion versus time curves were very close to those of the longer experiments at the same temperature, showing that the experiments were repeatable. The SEM pictures, which are shown in Figure 9, revealed that the surfaces in general were very similar. There were minor differences, for example the sample treated in 0.3 atm of water vapor show large cracks. Cross sections of the samples were also studied in the SEM, in order determine if there was a reaction front in the particles. No difference in the carbonate content in the particles could be detected, however. The cross-section images did not reveal any differences between samples carbonated at different partial pressures of water vapor either. Figure 7. The effect of CO2 partial pressure at a temperature of 450 °C and a H2O partial pressure of 0.10 atm.

Figure 8. The effect of dilution of the sample with SiO2 at a CO2 partial pressure of 0.15 atm and no water vapor.

5. CONCLUSIONS The partial pressure of water vapor has a clear effect on the conversion of CaO to CaCO3 in the temperature range of 400
550 °C. Water vapor influences how far the reaction proceeds before entering its slow stage. The partial pressure of carbon dioxide had an effect on the rate of the early initial reaction, but no clear effect on how far the reaction goes before slowing down. Dilution of the sample also slowed down the initial stage of the reaction but did not clearly affect the conversion before the reaction entered into the slow stage. At 400 and 450 °C, the conversion of CaO to CaCO3 was higher when water vapor was added to the atmosphere. However, the highest partial pressure of water vapor added, 0.3 atm, gave a lower conversion compared to 0.02 and 0.1 atm. At 550 °C, water vapor only had a retarding effect on the reaction. SEM images did not reveal any causes for this phenomenon. Even though water vapor has an effect on the reaction, it cannot be said that water vapor unambiguously accelerates or retards carbonation. Calcium hydroxide could not be identified as an intermediate product, and if it was formed, it was immediately turned into calcium carbonate. Water vapor had an accelerating effect on carbonation regardless of whether calcium hydroxide was stable or not during the experimental conditions. ’ AUTHOR INFORMATION Corresponding Author

*Phone: +358503758210. E-mail: isak.linden@abo.fi.

’ ACKNOWLEDGMENT This work was part of the activities at the Åbo Akademi University Process Chemistry Centre within the Finnish centre of Excellence Programme (2000-2011) by the Academy of Finland. Metso Oyj is acknowledged for their funding. ’ REFERENCES

Figure 9. SEM-images of calcium oxide treated in 0, 0.02, 0.1, and 0.3 atm of H2O and 0.15 atm CO2 at 400 °C for 200 s.

of 0.15 atm, in four different partial pressures of H2O; 0, 0.02, 0.1, and 0.3 atm. The time 200 s was chosen since it is in the beginning of the slower conversion stage, after the initial fast reaction stage. The experiments were also used to check the

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(6) Mess, D.; Sarofim, A. F.; Longwell, J. P. Product Layer Diffusion during the Reaction of Calcium Oxide with Carbon Dioxide. Energy Fuel 1999, 13, 999. (7) Nikulshina, V.; Galvez, M. E.; Steinfeld, A. Kinetic analysis of the carbonation reactions for the capture of CO2 from air via the Ca(OH)2
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