ARTICLE pubs.acs.org/IECR
Kinetic Study of High-Pressure Carbonation Reaction of Calcium-Based Sorbents in the Calcium Looping Process (CLP) Fu-Chen Yu and Liang-Shih Fan* William G. Lowrie Department of Chemical and Biomolecular Engineering, The Ohio State University, 140 West 19th Avenue, Columbus, Ohio 43210, United States ABSTRACT: In this study, the high-pressure carbonation kinetics of calcium oxide (CaO) derived from three calcium-based sorbents, namely, limestone (CaCO3), calcium hydroxide [Ca(OH)2], and precipitated calcium carbonate (PCC), used in the calcium looping process (CLP) system were studied using a magnetic suspension balance (MSB) analyzer. Different total pressures (100015000 torr) and concentrations of CO2 (1030%) were tested to determine their effects on the carbonation reaction rate at a specific operating temperature of the CLP system, namely, 700 °C. The carbonation reaction rate was found to increase with increasing concentration of CO2 (1030%) at a constant total pressure of 5000 torr and to exhibit first-order kinetics. However, the total pressure has an effect on the carbonation reaction rate only at lower total pressures. With a 20% CO2 stream, the reaction rate was observed to increase until the total pressure reached 4000 torr, beyond which a further increase in total pressure had a negative effect on the rate of the carbonation reaction of CaO derived from all three precursors. Further, the carbonation reaction had a different reaction order with respect to the partial pressure of CO2. It was found that the reaction was first-order at lower total pressures but changed to zeroth-order when the total pressure exceeded 4000 torr. The different reaction order under elevated pressures can be explained by the Langmuir mechanism. In addition, the reaction rate of carbonation conducted at high total pressure was greater than that at atmospheric pressure, under cyclic testing. The results also showed that there was no significant difference in the behavior of the carbonation reaction of CaO at elevated pressures, regardless of the different precursors used to generate the CaO.
’ INTRODUCTION With rising energy demands and concerns over global warming, it is desirable to develop clean and efficient energy conversion systems. Hydrogen, a clean and carbon-free fuel, is regarded as an ideal energy carrier. Thus, hydrogen generation using highly efficient processes will be crucial. Large-scale hydrogen production depends on the conversion of carbonaceous fuel processes, such as steam methane reforming, coal gasification, catalytic cracking of natural gas, and partial oxidation of heavy oils.1,2 The gases obtained from these reactions are then sent to downstream watergas-shift (WGS) reactors to enhance the hydrogen production. An effective technique to improve the hydrogen productivity through the WGS reaction is to remove CO2 in situ from the reaction mixtures. The continuous removal of the CO2 product from the WGS reactor incessantly drives the equilibrium-limited WGS reaction in the forward direction. Thus, the removal of CO2 will allow this exothermic reaction to be carried out at high temperatures and high pressures, leading to faster kinetics in the forward direction. The efficiency of a hydrogen production process of this nature will be significantly higher compared than that of current processes, especially when CO2 sequestration is integrated. In recent years, a number of novel processes have been proposed in the literature that aim at high-efficiency hydrogen production with CO2 capture. Among these processes are calcium-based looping processes include the Zero Emission Coal Alliance (ZECA) process,3 the hydrogen production by reaction integrated novel gasification (HyPr-RING) process, the CO2 acceptor process, the ALSTOM hybrid combustion r 2011 American Chemical Society
gasification process, and the GE fuel-flexible process.47 Despite the varied schemes in the reactions of these processes, in essence, they all involve the use of calcium-based sorbents to remove CO2 in producing high-purity hydrogen. These systems usually operate at very high pressures and require excess steam to produce high purity hydrogen.8 The calcium looping process (CLP) system,912 which is being developed at The Ohio State University, integrates several reactions into one single reactor, including the WGS reaction, CO2 capture, and sulfur and halide removal at high temperatures, resulting in process intensification. This system is composed of two main reactors, a carbonator and a calciner. The carbonator can be operated at 550700 °C and different pressures depending on the purity of hydrogen required. The calciner is operated at 8001000 °C at atmospheric pressure for spent sorbent regeneration while producing sequestration-ready CO2.13 The major reactions occurring in the CLP are as follows Carbonator CO þ H2 O f H2 þ CO2 CaO þ CO f CaCO3 CaO þ H2 S f CaS þ H2 O CaO þ COS f CaS þ CO2 CaO þ 2HCl f CaCl2 þ H2 O
ð1Þ ð2Þ ð3Þ ð4Þ ð5Þ
Received: January 13, 2011 Accepted: September 2, 2011 Revised: August 20, 2011 Published: September 25, 2011 11528
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Calciner CaCO3 f CaO þ CO2
ð6Þ
With growing interest in CO2 capture, calcium-based sorbents have been gaining special attention. Extensive studies have been conducted on the calcination reaction. Fuertes et al. showed that calcium carbonate decomposes at a definite boundary between CaO and CaCO3.14 Dennis and Hayhurst found lower calcination reaction rates at higher system pressures even without CO2 in the bulk gases.15 The effects of CO2 concentration and total pressure on the calcination reaction were also investigated by Garcia-Labiano et al.16 They found that increasing the CO2 partial pressure or the total pressure decreased the calcination reaction rate. They developed a model to predict the calcination reaction behavior over a wide range of CO2 concentrations, total pressures, particle sizes, and temperatures. Kinetic studies on the carbonation reaction have been extensive; little research, however, has been carried out at elevated pressures. Von Nitsch found that the initial carbonation reaction has zero activation energy at 800850 °C.17 Bhatia and Perlmutter employed the random-pore model to illustrate their experimental results and obtained zero activation energy using an atmospheric thermogravimetric analyzer.18 Sun et al. and Chen et al. found that an increase in the partial pressure of CO2 can enhance the carbonation reaction rate.19,20Further investigation of the carbonation reaction under high total pressures and high partial pressures of CO2 over wider ranges is necessary. Bench-scale testing of the CLP has shown that a pure hydrogen stream (>99.9%) can be obtained at a moderate operating pressure of 15000 torr.911 In addition, the CLP system yields an efficiency of 63% [based on the high heating value (HHV)] similar to that of existing state-of-art processes, which generate hydrogen at much higher pressures but without CO2 capture.21 Thus, the study of the carbonation kinetics of calcium-based sorbents for the CLP at elevated pressures is important. The objective of this work was to investigate the effects of the partial pressure of reacting gases and the total pressure on the carbonation reaction rate of CaO obtained from different precursors, namely, limestone (CaCO3), calcium hydroxide [Ca(OH)2], and precipitated calcium carbonate (PCC). The effect of pressure on the carbonation reaction rate with number of cycles was also investigated.
Figure 1. MSB apparatus.
Powdered samples and pelletized samples were examined in this study. Before each experiment, 0.30.5 g of the sample was loaded into a quartz crucible. Next, the MSB was purged with nitrogen to introduce an inert atmosphere. The crucible was then heated to the desired calcination temperature, 700 °C, for 30 min, which was sufficient to fully calcine the samples to CaO. In addition, to ensure the proper calcination rate, the calcination reaction was conducted at 1000 torr under a nitrogen environment. To compare the reaction rates of the various calcium-based sorbents, a flow of about 120 mL/min of reactant gas, comprising 1030% CO2 in nitrogen, was introduced into the MSB at 700 °C and different total pressures after the completion of calcination. The weight change of the sample was continuously recorded as a function of time for further analysis. The initial rate of carbonation is mainly related to the concentration of CO2 and the total pressure. Hence, these two parameters were used to characterize the effects of pressure on the carbonation reaction. The sorbent reactivity (weight capture, %) and carbonation reaction rate are defined as follows
’ EXPERIMENTAL METHODS
weight capture ð%Þ ¼ C ¼
Particle and Pellet Preparation. Calcium-based sorbents
were prepared in the form of powders and pellets. Naturally occurring limestone and calcium hydroxide were obtained from Graymont Inc. PCC was prepared according to the procedure described by Agnihotri et al.22 In addition, raw powders were manually pressed into cylindrical pellets (with a diameter of 5 mm and a height of 2 mm). Particle Reactivity and Recyclability. A magnetic suspension balance (MSB) analyzer from VTI Corporation was used in this study to test the performance of calcium-based sorbents at elevated pressures. Figure 1 shows the MSB apparatus, which consists of a set of mass flow controllers conveying the reaction gas mixture into a high-pressure reactor enclosed in a hightemperature furnace.The measuring accuracy, pressure tolerance, and maximum temperature of the system can be up to 1 μg, 22000 torr, and 1000 °C, respectively.
ðWt W0 Þ 100 W0
ð7Þ
dC ð8Þ dt It is known that the carbonation reaction can be considered as comprising two regimes, with the first regime involving a rapid chemical reaction and the second consisting of a slow reaction characterized by CaCO3 layer formation that impedes diffusion of the reactant gas. In this study, the carbonation reaction rates for various sorbents were compared for the first regime of the reaction and were calculated based on the initial rate. Moreover, to study multiple calcinationcarbonation reaction cycles, the gases introduced into the MSB were switched between inert gases and reacting gases every 30 min for five cycles. Both carbonation and calcination reaction were conducted at 700 °C with a 120 mL/min gas flow rate. The calcination reaction was carbonation reaction rate of CaO ð%=minÞ ¼
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Figure 2. Effect of pressure on the weight measured in the MSB with inert gas.
Figure 3. Variation in the weight of the quartz crucible with pressure.
conducted at 1000 torr, but the carbonation reaction was conducted at 5000 torr with a 10% CO2 stream (in N2). The change in weight was then monitored across cycles to evaluate the recyclability. Morphological properties such as surface area, pore volume, and pore size distribution were determined by the Brunauer EmmettTeller (BET) method based on the adsorption and/or desorption curve, using a NOVA 4200e analyzer (Quantachrome Company). The BET analysis is based on the nondestructive measurement of pore properties and requires only small amounts (∼0.2 g) of well-mixed samples. The BET surface area, pore volume, and pore size distribution were measured at 196 °C using N2 as the adsorbent. All samples were vacuum-degassed at 300 °C for 4 h prior to BET analysis.
’ RESULTS AND DISCUSSION Buoyancy Effect of MSB. The high-pressure MSB analyzer can be used to test the gas sorption capacity of solid sorbents and to study industrially relevant temperature and pressure reaction conditions. Unlike an atmospheric thermogravimetric analyzer, the MSB analyzer is operated at elevated pressures, and thus, the measurement of the system weight is highly sensitive to the buoyancy effect. To offset this sensitivity, the weight buoyed
Figure 4. Effect of CO2 fraction on the carbonation reaction rate at a fixed total pressure of 5000 torr: (a) CaCO3_CaO, (b) PCC_CaO, (c) Ca(OH)2_CaO. Conditions: carbonation temperature, 700 °C; total flow rate, 120 mL/min; sorbents in the form of powders.
needs to be taken into account. Therefore, an empty quartz crucible was used to test the buoyancy effect. Next, the MSB was run at several pressures ranging from 1000 to 15000 torr using helium as the inert gas. The results are given in Figure 2. The buoyancy effect is indicated by the decreasing weight of the empty quartz crucible with increasing operating pressure. Using the gas law equation, the weight buoyed by the flowing gas due to the increasing operating pressures can be obtained. Hence, the weight of the empty quartz crucible was adjusted with the equation PVM ð9Þ RTZ It can be seen from Figure 3 that the weight of the empty quartz crucible was stable after this correction. Once the buoyancy effect had been considered, the calcium-based sorbents were tested at different total pressures with a constant CO2 actual weight ¼ weight measured þ
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the carbonation reaction is a first-order reaction for all three calcium-based sorbents (n = 1). In addition, based on the grain model,23 the reaction rate for the CaOCO2 system in the first regime can be expressed as a specific rate given by R0 ¼
dX ¼ 3rð1 XÞ1=3 dtð1 XÞ
¼ 56ks ðPCO2 PCO2 , eq Þn S
ð11Þ
At the initial time, t = 0, the surface area is S0, and the reaction rate is r0, so eq 11 becomes R0 0 ¼ Figure 5. Carbonation reaction rate at different partial pressures of CO2 for PCC_CaO, CaCO3_CaO, and Ca(OH)_CaO. Conditions: carbonation temperature, 700 °C; carbonation pressure, 5000 torr; sorbents in the form of powders.
concentration and at different CO2 molar fractions with a fixed total pressure. Carbonation Reaction Rate with Different Fractions of CO2at a Fixed Total Pressure. In the CLP, desirable calciumbased sorbents should have the properties of high reactivity and good recyclability, to reduce the reactor size and optimize the reactor design. In addition, a high reaction rate between CO2 and the calcium-based sorbent is also necessary to achieve a reactor with a reasonable size. The syngas composition is highly dependent on the type of gasifier, and the carbonation reaction rate is highly dependent on the CO2 partial pressure. The typical partial pressure of CO2 in gasifiers is 3003200 torr.11 Therefore, it is important to study the effect of different CO2 molar fractions on the carbonation reaction rate at elevated pressures. As the residence time of the sorbents in the CLP system is on the order of seconds, the first regime of the carbonation reaction is of primary interest. Thus, in this study, the effect of CO2 partial pressure on the rate of the carbonation reaction was first investigated with the total pressure fixed. Three calcium-based sorbents were tested in different fractions of CO2 with a fixed total pressure of 5000 torr. The four representative fractions used were 10%, 15%, 20%, and 30%. As expected, the initial rate of carbonation was found to increase with increasing CO2 fraction because of the higher driving force for the carbonation reaction. Indeed, Figure 4 shows the weight capture versus time curves for the three calcium-based sorbents in the form of powders at different CO2 fractions. An increase in the partial pressure of CO2 produced a clear increase in the carbonation rate. That is, increasing the CO2 fraction in the system caused a shift in equilibrium and drove the reaction forward faster. As can be seen from Figure 4, for each sorbent, the highest carbonation reaction rate occurred at a CO2 fraction of 30% at a fixed total pressure. The effect of CO2 partial pressure at a constant system pressure on the reaction rate of the three calcium-based sorbents in the form of powders is plotted in Figure 5. The carbonation reaction rate can be expressed simply as reaction rate ¼ kðPCO2 Þn
ð10Þ
Therfore, the reaction order can be obtained by linearly fitting the experimental data with eq 10. As can be seen from Figure 5,
dX ¼ 3r0 ¼ 56ks ðPCO2 PCO2 , eq Þn S0 dt
lnðr0 Þ ¼ n lnðPCO2 PCO2 , eq Þ þ lnðS0 Þ þ ln
ð12Þ 56ks 3 ð13Þ
At a given temperature, ks and S0 are constant. The equilibrium partial pressure of CO2 can thus be calculated as24 PCO2 , eq ¼ 7:6 10ð 8308=T þ 9:079Þ
ð14Þ
Therefore, the order of the CaOCO 2 reaction can be determined by the slope of a plot of ln(r 0 ) versus ln[(PCO2 PCO2,eq)/100]. Such plots are shown in Figure 6. It can be clearly seen from the figure that the CaOCO2 reaction is a first-order reaction for all three calcium-based sorbents in the form of powders as well. The carbonation reaction can be considered as two elementary steps, namely, adsorption/desorption and reaction, based on the Langmuir mechanism CaO þ CO2 T CaO 3 CO2 f CaCO3
ð15Þ
Let the rate constants for adsorption, desorption, and reaction be k1, k1, and k2, respectively, and let CaO, CO2, and CaO 3 CO2 be denoted as S, A, and AS, respectively. The reaction rate can then be expressed as r ¼
dCA k1 k2 CA CS ¼ dt k1 CA þ k1 þ k2
ð16Þ
At a total pressure of 5000 torr, the adsorption step is most likely to be the rate-limiting step. Hence, k2 . k1CA, and k2 . k1, so that the expression for the reaction rate becomes r ¼ k1 CA CS
ð17Þ
Based on eq 17, the reaction is of first order, as shown in Figure 6. It can also be seen from Figure 5 that, at any given CO2 partial pressure, the CaO obtained from PCC (PCC_CaO) has the highest carbonation reaction rate, followed by the Ca(OH)2derived CaO [Ca(OH)2_CaO] and then the limestone-derived CaO (CaCO3_CaO). For example, the carbonation reaction rate of PCC_CaO is almost twice that of CaCO3_CaO at a CO2 partial pressure of 1500 torr, corresponding to 30% CO2 at a total pressure of 5000 torr. The reasons for the different performances of sorbents are based on the different morphological properties of the precursors and the CaO samples derived from them. The surface areas and pore volumes of the three calcium-based sorbents and their precursors are listed in Table 1. CaCO3_CaO, 11531
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Figure 6. Reaction order plot with varying CO2 partial pressures at a constant total pressure of 5000 torr: (a) CaCO3_CaO, (b) PCC_CaO, (c) Ca(OH)2_CaO. Conditions: carbonation temperature, 700 °C; total flow rate, 120 mL/min; sorbents in the form of powders.
Table 1. Surface Areas and Pore Volumes of the Precursors and the Derived CaO surface area (m2/g)
pore volume (cm3/g)
limestone
0.90
0.003
Ca(OH)2
10.16
0.047
PCC
32.14
0.263
limestone_CaOa Ca(OH)2_CaOa
20.21 25.9
0.152 0.159
material
PCC_CaOa a
5.951
0.037
Obtained from the corresponding precursor by calcination at 700 °C.
Ca(OH)2_CaO, and PCC_CaO were obtained by calcining their respective precursors at 700 °C for a sufficient time. From the table, it can been seen that PCC has a higher surface area and a higher pore volume than limestone and Ca(OH)2. In addition, PCC has a unique pore size distribution, with its maximum pore volume at 30 nm, as shown in Figure 7. The unique specific mesoporous structure and higher surface area and pore volume in this precursor enable favorable kinetics of carbonation reaction for PCC_CaO, even though the surface area and pore volume are lower after calcination.13 Unlike the PCC precursor, CaCO3 and Ca(OH)2 are composed of microporous structures, as indicated in Figure 7. Most of the pores of CaCO3 and Ca(OH)2 lie in the diameter range of 5 nm. In the absence of a unique structure from the precursor, the performance of the derived sorbents is instead determined by their surface properties such as surface area and pore volume. As can be seen from the Table 1, Ca(OH)2_CaO has a higher surface area and pore volume than does CaCO3_CaO. Hence, Ca(OH)2_CaO exhibits a better CO2 capture capacity and initial reaction rate than CaCO3_CaO. The use of fine powder sorbents over multiple calcination and carbonation cycles can be hampered by the difficulty of separating the sorbents from other fine powders.25 Hence, limestone
Figure 7. Pore size distributions for different calcium-based sorbents before and after calcination.
was tested in the form of pellets as well. These experiments were conducted at 10%, 15%, 20%, and 25% CO2for a total pressure of 3800 torr. A notable increase in the carbonation reaction rate with increasing CO2 partial pressure can be observed in Figure 8a, where the highest reaction rate is shown for 25% CO2. Moreover, the carbonation of CaCO3_CaO in pellet form is also a first-order reaction, as shown in Figure 8b. Even though the particles are in different physical forms, they display similar performances with respect to the partial pressure of CO2 at a fixed total pressure. Hence, the differences in the morphological structures of the different calcium-based sorbents are believed to be responsible for their differences in performance, rather than their physical forms (i.e., powder or pellet). Carbonation Reaction Rate with Different Total Pressures at a Constant CO2 Molar Fraction. To examine the effect of total pressure on CaO, the three precursors were calcined at 1000 torr and 700 °C under a nitrogen flow of 120 mL/min. The CaO generated was then exposed to the reactant gas mixture comprising of CO2 and N2 at the desired carbonation pressure in the 11532
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Figure 8. (a) Effect of CO2 fraction on the carbonation reaction rate of pelletized CaCO3_CaO. (b) Reaction order plot with varying CO2 partial pressures at a constant total pressure. Conditions: carbonation temperature, 700 °C; carbonation pressure, 3800 torr; total flow rate, 120 mL/min.
range of 100015000 torr at 700 °C under an overall gas flow of 120 mL/min. All experiments were conducted at a constant molar fraction (20%) of reactant gas CO2 in N2. Because an increase in the operating pressure also represents an increase in the reactant gas partial pressure, an increase in the operating pressure would be expected to yield an increase in the reaction rate. The curves of weight capture versus time at selected total pressures are plotted in Figure 9. As shown in the figure, however, the carbonation reaction rate was found to increase with an increase in the total pressure from 1000 to 4000 torr but then began to decrease when the total pressure exceeded 4000 torr for all three calcium-based sorbents. When the total pressure reached 15000 torr, the initial carbonation reaction rate was slightly lower than the reaction rate for a total pressure of 4000 torr. The effects of different total pressures on the carbonation reaction rate of CaO obtained from the three precursors are plotted in Figure 10. The decrease of the carbonation reaction rate at total pressures higher than 4000 torr is similar to the decrease of the sulfidation reaction rate for calcium-based sorbents.26,27 Several factors contribute to this behavior: total pressure, partial pressure, and gas dispersion.28 At the carbonation temperature of 700 °C, CaO is not expected to sinter to an extent that can affect the immediate sorbent reactivity within the first 5 min. However, increasing the total pressure of the system led to lower volumetric and linear velocities of the gas, because the inlet gas flow rate was fixed at 120 mL/min. Lower gas velocities resulted in increased external masstransfer resistance, which impeded the reaction between CO2 and CaO. In other words, the decrease in reaction rate
Figure 9. Effect of total pressure on the carbonation reaction rates with a 20% CO2 stream: (a) CaCO3_CaO, (b) PCC_CaO, (c) Ca(OH)2_CaO. Conditions: carbonation temperature, 700 °C; total flow rate, 120 mL/min.
at higher pressure is likely a result of the increased masstransfer resistance. A plot of ln(r0) versus ln[(PCO2 PCO2,eq)/100] for the three investigated calcium-based sorbents at different total pressures is shown in Figure 11. As can be seen, the carbonation reaction is first-order at lower partial pressures of CO2 but becomes zeroth-order at higher pressures. The transition point occurs at corresponding partial pressures of CO2 between 600 and 800 torr. This is in good agreement with the results shown in Figure 10. The total pressure of 4000 torr was taken as an approximation for the transition. The first-order reaction was found only for partial pressure of CO2 (PCO2) less than 800 torr, above which the zeroth-order reaction prevailed. Such a 11533
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Figure 10. Effects of different total pressures on the carbonation reaction rates of PCC_CaO, CaCO3_CaO, and Ca(OH)2_CaO. Conditions: carbonation temperature, 700 °C; CO2 percentage, 20%.
change in the carbonation reaction order in the first regime was also found in the study of Sun et al.;19 the transition point found in this study, however, differs significantly from that identified in their work. They observed that the carbonation reaction rate was not further enhanced when PCO2 was over 75 torr.19 Bhatia and Perlmutter conducted the carbonation reaction at CO2 partial pressures of less than 75 torr at 615 °C and found the reaction to be first-order.18 However, Kyaw et al. performed the carbonation reaction at higher partial pressures of CO2 (>150 torr) and a total pressure of 750 torr and observed zeroth-order reactions for both limestone and dolomite.29 The difference in terms of transition pressure could be governed by the operating conditions such as carbonation temperature and total pressure, as well as the precursors of the calcium-based sorbents. The change in the reaction order can also be explained by eq 16. Once the reaction step becomes the rate-limiting step, so that k2 , k1CA and k2 , k1, the reaction rate is given by r ¼
dCA k1 k2 CA CS K 1 k 2 CA C S ¼ ¼ dt k1 CA þ k1 K 1 CA þ 1
ð18Þ
where k1 K1 ¼ k1
ð19Þ
At low total pressures, this expression yields r ¼ k1 k2 CA CS
ð20Þ
At high total pressures, it yields r ¼ k2 CS
ð21Þ
Thus, eq 20 reflects a first-order reaction with respect to species A at low total pressures, whereas eq 21 reflects a zeroth-order reaction with respect to species A at high total pressures, given that CS, K1, and k2 are constant at a given temperature. Multiple-Cycle Testing. The CaO sorbents undergo carbonation to form CaCO3, which is regenerated by calcination to form CaO, releasing a pure CO2 stream. For commercial viability, the spent calcium-based sorbents must be able to be regenerated for multiple cycle uses and to successfully maintain consistent reactivity. As can be seen in Figure 12, even though the reaction rates for different calcium-based sorbents decreased
Figure 11. Reaction order plot with varying CO2 partial pressures corresponding to different total pressures with a 20% CO2 stream: (a) CaCO3_CaO, (b) PCC_CaO, (c) Ca(OH)2_CaO Carbonation temperature, 700 °C.
with increasing number of cycles, the carbonation reaction rate at higher pressures was greater than that at atmospheric pressure in each cycle. In addition, it can be seen in Figure 13 that the reactivity decayed with increasing number of cycles for the three calciumbased sorbents at higher carbonation pressures as well. Even though the kinetics of the carbonation reaction can be improved by conducting the reaction at higher pressures, the spent sorbents still need to be regenerated at elevated temperatures. Higher calcination temperatures will induce thermal sintering, and this sintering would cause a deterioration of the porosity and affect the CO2 sorption. The size of the reactor can be reduced by conducting the reaction under pressure. Information on CaO carbonation reaction kinetics at high pressures is not available in the literature. The results obtained in this work can thus be helpful 11534
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Figure 12. Reaction rate in multiple-cycle testing of PCC_CaO, CaCO3_CaO, and Ca(OH)2_CaO. Conditions: carbonation temperature, 700 °C; carbonation pressure, 5000 torr; CO2 fraction, 10%; carbonation time, 30 min. Notation: p, high pressure; a, atmospheric pressure.
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three calcium-based sorbents, namely, CaCO3, Ca(OH)2, and PCC. The carbonation reaction can be considered as comprising two regimes, with the first regime involving a rapid chemical reaction and the second consisting of a slow reaction characterized by CaCO3 layer formation that impedes diffusion of the reactant gas. The effects of the total pressure and the CO2 partial pressure on the carbonation reaction in the first regime were investigated. The results indicate that the rate of carbonation increases linearly with CO2 partial pressure and exhibits a first-order dependence at a constant total pressure. However, the rate of the carbonation reaction initially increased with increasing total pressures up to 4000 torr, beyond which the total pressure did not further enhance the reaction rate for all of the calcium-based sorbents investigated. The difference in the reaction order of the carbonation reaction can be explained by the Langmuir mechanism. In addition, the carbonation reaction was found to be of first order at lower total pressures but to change to zeroth-order at higher total pressures. Further, the carbonation reaction rate under pressurized conditions was found to decay more slowly than that under atmospheric conditions in multiple-cycle tests. The high-pressure kinetics data obtained in this work will be useful in understanding the behavior of calcium-based sorbents in the CLP at elevated pressures.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT This work was supported by the Ohio Coal Development Office of the Ohio Air Quality Development Authority (Project CDO/D-05-8/9), the U.S. Department of Energy (Project DEFC26-07NT43059), The Ohio State University, and an industrial consortium. The assistance of Dr. Fanxing Li on the experiments and the helpful discussions with Mr. Nihar Phalak and Mr. Zhenchao Sun are gratefully acknowledged. Figure 13. Weight capture in multiple-cycle testing of PCC_CaO, CaCO3_CaO, and Ca(OH)2_CaO. Conditions: carbonation temperature, 700 °C; carbonation pressure, 5000 torr; CO2 fraction, 10%; carbonation time, 30 min.
for the analysis and design of pressurized calcium-based looping systems.
’ CONCLUDING REMARKS The calcium looping process (CLP) is a novel chemical looping process that efficiently converts syngas into hydrogen. The production of high-purity hydrogen from fuel gas obtained from coal gasification is limited by the equilibrium relationship of WGS reaction. However, with the assistance of calciumbased sorbents, for the in situ removal of CO2 and other pollutants, the extent of the hydrogen production in the WGS reactor can be significantly enhanced while the overall scheme for the syngas conversion to produce hydrogen is appreciably simplified. The CLP is operated at elevated pressures to ensure the production of pure hydrogen while simultaneously enhancing the thermal efficiencies of the CLP. In this study, experimental data for high-pressure carbonation were obtained at a temperature of 700 °C using an MSB analyzer with
’ NOTATION CA = concentration of CO2 CA = concentration of intermediate complex CaO 3 CO2 CS = total number of sites of CaO k = rate constant [1/(min 3 torrn)] k1 = rate constant of adsorption k1 = rate constant of desorption k2 = rate constant of reaction ks = rate constant {[mol/(m2 3 s)] 3 torrn} M = molecular weight (mol/g) n = reaction order P = pressure (torr) PCO2 = partial pressure of CO2 (torr) PCO2,eq = equilibrium partial pressure of CO2 at a given temperature (torr) r = reaction rate in the grain model (1/s), slope of [1 (1 X)1/3] versus t r0 = reaction rate at t = 0 (1/s) R = gas constant [62.36 L 3 torr/(K 3 mol)] R0 = specific reaction rate (1/s) R0 0 = specific reaction rate at t = 0 (1/s) S = specific surface area (m2/g) 11535
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Industrial & Engineering Chemistry Research S0 = specific surface area at t = 0 (m2/g) t = time (min) T = temperature (K) V = volume (L) W0 = weight of the sample after completion of calcination (g) Wt = weight of the sample at any given time t (g) X = conversion of CaO Z = compression factor
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