Activation Strategies for Calcium-Based Sorbents for CO2

ARTICLE pubs.acs.org/IECR

Activation Strategies for Calcium-Based Sorbents for CO2 Capture: A Perspective Fu-Chen Yu, Nihar Phalak, Zhenchao Sun, 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: The chemical looping process (CLP) using calcium-based sorbents to capture CO2 through cyclic carbonation calcination reaction (CCR) before, during, or after the conversion of carbonaceous fuel occurs, is a viable CO2 control technology. With extensive past and current research efforts, the basic process concept has been found to be attractive at larger scales. Additionally, process simulations based on experimental results indicate that the parasitic energy consumption for this high temperature process is relatively low compared to low temperature processes such as the amine-based process. The ability of the calcium-based sorbents to maintain stable reactivity and physical integrity in cyclic reaction under severe operating conditions is one of the most important criteria for the success of the calcium looping technology. Despite being abundant and cheap, calcium-based sorbents derived from naturally occurring precursors, such as limestone and dolomite, suffer from rapid reactivity deterioration after high-temperature calcination and/or several CCR cycles. This deactivation is attributed to the morphological change at both macroscopic and microscopic levels, including pore pluggage, surface area reduction, and alteration of crystallographic plane distribution on the CaO surface. Much attention has recently been placed on sustaining and/or retaining the sorbent reactivity through sorbent modifications and/or reactivation. This paper provides an overview of the optimization and reactivation strategies of calcium-based sorbents with focus on three methods—modification of precursors, addition of dopants and/or supports, and reactivation through steam/water hydration.

’ INTRODUCTION With growing world energy demand and increasing concerns over the anthropological carbon dioxide (CO2) emission from the fossil fuel conversion systems, considerable attention in recent years has been placed on improving fossil energy conversion efficiency as well as integrating with the carbon capture and sequestration (CCS) techniques.1,2 For fossil fuel conversion systems, the CO2 capture can be carried out through oxy-fuel combustion, precombustion, or postcombustion.2,3 Oxy-fuel combustion refers to direct combustion of fossil fuels using high-purity O2 instead of air. Such a combustion system produces a N2-free stream where CO2 can be readily separated. However, in order to provide a vast quantity of O2, an energy and cost intensive air separation unit (ASU) is required. Additionally, the direct combustion with high-purity O2 is a highly exothermic process and requires stringent combustor temperature control through flue gas recycle. Most wet processes aim at CO2 scrubbing from postcombustion flue gas using liquid alkaline solutions, e.g., MEA. To date, wet processes are the most common processes used for CO2 separation; however, their intrinsic disadvantages include low energy and cost efficiency, solvent instability at high temperature, and undesired solvent reactions with certain compounds in flue gas.4,5 Unlike solvent-based processes and oxy-fuel combustion, solid sorbents/oxygen-carriers can perform with wider operating conditions and a higher efficiency. The high temperature solid sorbents/oxygen-carriers reaction schemes can be represented by the cyclic CarbonationCalcination Reaction (CCR) for solid sorbents and the reductionoxidation (REDOX) reactions for oxygen-carriers particles.2,3,6,7 Calcium-based looping processes are attractive because they can achieve in situ combustion or postcombustion CO2 capture r 2011 American Chemical Society

in the carbonation step and release the CO2 for sequestration in the calcination step. A number of such processes have been developed at various scales up to pilot scale, for example, The Ohio State University (OSU) Calcium Looping Process (CLP),7 Hydrogen Production by Reaction Integrated Novel Gasification (HyPr-RING) Process,8,9 ALSTOM’s Hybrid Combustion Gasification Process,10 the GE Fuel-Flexible Process11 and the CO2 Acceptor Process.12 Much work has been conducted to examine the process design and reactant conversion of these processes. The challenge, however, remains with respect to the sorbent conversion and recyclability. As an example, the results of a test conducted on a naturally occurring limestone obtained from Graymont (from quarry at Pleasant Gap, Pennsylvania) that undergoes calcination and carbonation reactions are given in Figure 1. It can be seen that the CO2 sorption capacity decreases over multiple cycles. The CO2 sorption capacity can be defined as follows: weight capture ð%Þ ¼

ðWt W0 Þ 100 W0

ð1Þ

where t is the time (min), W0 is the weight of the sorbent after complete calcination (g), and Wt is the weight of sorbent at a given time t (g). The decrease of the CO2 capture capacity of calcium-based sorbents over multiple carbonationcalcination cycles has been Special Issue: Nigam Issue Received: April 15, 2011 Accepted: June 22, 2011 Revised: June 21, 2011 Published: June 22, 2011 2133

dx.doi.org/10.1021/ie200802y | Ind. Eng. Chem. Res. 2012, 51, 2133–2142

Industrial & Engineering Chemistry Research

ARTICLE

Figure 1. Loss of reactivity during multiple CCR cycles testing of Graymont limestone in a thermogravimetric analyzer (TGA) at 700 °C; calcination in N2 for 30 min, carbonation under 10% CO2 (balanced with N2) for 30 min.

widely reported.13,14 It is known that high calcination temperatures used for the sorbent regeneration have a deteriorating effect on the reactivity and recyclability of these sorbents. Such effect has been attributed to the sintering of the sorbents.1321 The carbonation reaction involves several elementary reaction steps, including CO2 absorption and calcium carbonate (CaCO3) formation. Therefore, both macrostructural and microstructural properties, such as surface area, pore volume and structure, as well as orientation of the calcium oxide (CaO) surface affect its reactivity to a great extent. The arrangement of atoms determines the orientation of the surface, and this gives rise to a specific local configuration. Different surfaces have different polarity which has a direct bearing on the physical adsorption on the surface.22 Although the initial properties of sorbents are important, maintaining or recovering such properties after the cyclic regeneration step plays an equally important role. To improve initial sorbent properties and/or rejuvenate deactivated sorbents, several approaches have been attempted, including synthesis/modification of sorbents, doped and supported sorbents, and reactivation through steam/water hydration, some of which show promising results while others require further work to substantiate their viability for commercial applications. This article discusses the underlying mechanism, effectiveness, and long-term viability of these approaches along with relevant experimental findings from literature. The challenges for commercial implementation are also discussed.

1. SYNTHESIS OF CALCIUM-BASED SORBENTS FROM DIFFERENT PRECURSORS The rapid decay of the CO2 sorption capacity of CaO derived from naturally occurring sources prompted the examination of CaO precursors that result in sorbents with different physical and chemical properties. Different calcium precursors can yield distinct characteristics of CaO for CO2 capture. The reactivity of CaO obtained from different precursors studied in the literature has mainly been associated with their physical properties such as surface area and porosity. A high surface area and a large pore

volume of the sorbents favor the gas molecule diffusion and the formation of CaCO3. Precipitated calcium carbonate (PCC) with a high surface area can be synthesized by bubbling CO2 through a Ca(OH)2 slurry.6,20 By using an anionic dispersant (N40 V), morphology of PCC can be tailored to achieve higher reactivity. The synthesized PCC was found to have a unique mesoporous structure (530 nm) and thus the derived CaO was able to maintain high reactivity over several cycles. This was further explored by obtaining CaO from commercially available inorganic and organometallic precursors.2325 These precursors include CaO, calcium hydroxide (Ca(OH)2), CaCO3, calcium nitrate tetrahydrate (Ca(NO3)2 3 4H2O), calcium acetate (Ca(CH3COO)2), calcium propionate (Ca(C2H5COO)2), calcium acetylacetonate (Ca(CH3COCHCOCH3)2), calcium oxalate (Ca(COO)2), calcium 2-ethylhexanoate (Ca(C7H15COO)2), and calcium D-gluconate monohydrate. As can be seen from Table 1, CaO derived from calcium acetate, calcium propionate, and calcium D-gluconate monohydrate exhibited a higher CO2 capture capacity due to its mesoporous and macroporous structure which provided larger surface area and pore volume. On the other hand, calcium oxalate-derived CaO was microporous and thus its CO2 capture capacity is less. All these studies indicate the importance of sorbent morphology in relation to CO2 capture capacity. Recent studies with more exotic calcium precursors have also confirmed the importance of these morphological properties.25 While some studies continue toward finding better precursors, others have applied techniques to modify natural precursors by chemical treatments. Among them, CaO obtained from naturally occurring limestone was modified with ethanol/water solutions.26 No major difference in the carbonation conversion of sorbents was found in the first cycle, but CaO sorbents modified with ethanol solution showed a higher conversion over subsequent cycles. Furthermore, a higher concentration of ethanol in the solution resulted in a bextter pore structure (higher surface area and pore volume). It is hypothesized that the presence of ethanol improves H2O molecule affinity and penetrability to CaO, yielding a change in pore structure and 2134

dx.doi.org/10.1021/ie200802y |Ind. Eng. Chem. Res. 2012, 51, 2133–2142


ARTICLE

Table 1. CO2 Capture Capacity and Physical Properties of CaO Obtained from Various Precursors carbonation

calcination CO2 capture capacity

surface area

pore volume

(wt %)

(m2/g)

(cm3/g)

0 (He)

51.9

12.8

0.027

0 (He)

49.5

5.3

0.08

30

0 (He)

19.6

13.9

0.15

750

30

0 (He)

2.0

4.2

0.02

0.3

750

30

0 (He)

75.0

15.0

0.18

300

0.3

750

30

0 (He)

70.7

20.0

0.22

700 700

300 300

0.3 0.3

750 750

30 30

0 (He) 0 (He)

60.5 39.3

12.0 5.9

0.09 0.02

Ca(C7H15COO)2CaO24

700

300

0.3

750

30

0 (He)

19.6

9.3

0.015

Ca D-gluconate monohydrateCaO25

650

30

0.15

900

10

0 (N2)

69.6

17.0

0.27

T (°C)

t (min)

PCO2 (atm)

T (°C)

t (min)

CaCO3CaO23

600

300

0.3

750

30

Ca(OH)2CaO23

600

300

0.3

750

30

CaOCaO23

600

300

0.3

750

Ca(NO3)2CaO23

600

300

0.3

Ca(C2H5COO)2CaO24

700

300

Ca(CH3COO)2CaO24

700

Ca(CH3COCHCOCH3)2CaO24 Ca(COO)2CaO24

PCO2 (atm)

Figure 2. BET surface area and pore volume for sorbents calcined at 920 °C with and without modification.2628

particle size of CaO. Limestone exhibited better carbonation conversion over 100 cycles compared to that of sorbents derived from the original limestone, when modified with acetic acid solution.27,28 Similar results were obtained with dolomite.29 As

can be seen from Figure 2, modification by acetic acid or ethanol/ water solution can delay or prevent the sintering to preserve the pore structure over cycles, giving rise to better durability of sorbents. 2135



ARTICLE

Table 2. BET Surface Area and Pore Volume of Various Sorbents by FSP6,39

Figure 3. Comparisons of CO2 capture capacity for Havelock limestone with different type of additions.3638

Further, CaCO3 particles formed in different geometric structures have been investigated to tailor the properties of the sorbents, including platelets, hollow spheres, hexagonal plates, rodlike particles, and multibranched hierarchical structures with the presence of organic additives or templates.3034 However, these particles have not been tested for their application as a high temperature CO2 adsorbent. Recently, Yang synthesized a novel CaO-based sorbent with a hollow structure, denoted as CaO Nanopads.35 This sorbent was prepared by bubbling CO2 through Ca(OH)2 slurry in the presence of the triblock copolymer surfactant, P123 (PEO20PPO70PEO20). The presence of P123 was found to stabilize CO2 bubbles, causing the tailored CaCO3 to appear as a hollow pod containing numerous nanosized CaCO3 platelets. This unique cocoon-like hollow structure was shown to yield a superior CO2 sorption capacity of 39.3% after 50 cycles at temperatures of 600700 °C due to the retardation of sintering. Even though sorbents with superior morphology can be obtained using above methods, the effects of high regeneration temperatures that are encountered in typical calciners have not been tested. Further, most studies have been only conducted in a TGA for limited cycles, and thus the applicability of such novel sorbents in a practical process is not yet clearly known.

2. DOPED OR SUPPORTED CALCIUM OXIDES Researchers have also attempted to maintain the CO2 removal capacity by tuning the morphology of sorbents through doping or the addition of supports instead of changing CaO precursors.3646 The strategy is to use dopants or supports to prevent or delay sintering and thereby improve sorbent recyclability for extended use. However, some dopants have been found to enhance the sintering of the sorbents. The effect of doping CaO with sodium chloride (NaCl) and sodium carbonate (Na2CO3) was investigated for applications in fluidized bed combustion (FBC) systems.36 It was shown that the addition of NaCl retained the CO2 captuzre capacity of the sorbents at 0.4 over 10 cycles due to favorable changes in the pore structure and surface area of the sorbent, while the addition of Na2CO3 did not appear to have significant effects on the extent of carbonation as shown in Figure 3. Previously conducted studies, for enhancing sulfur dioxide (SO2) capture using calcium sorbents modified with sodium salts, hypothesize that the presence of salt induces structural rearrangement which results in optimum pore-size distribution.37 However, the performance of these calcium oxides with NaCl or Na2CO3 differs significantly in FBC conditions as

sorbents

surface area (m2/g)

pore volume (cm3/g)

FSP Si/Ca (1:10)

78

0.34

FSP Ti/Ca (1:10) FSP Cr/Ca (1:10)

61 74

0.38 0.39

FSP Co/Ca (1:10)

80

0.42

FSP Ce/Ca (1:10)

89

0.42

FSP Zr/Ca (1:10)

74

0.23

FSP Zr/Ca (1:5)

67

0.23

FSP Zr/Ca (3:10)

71

0.24

PCC

38

0.11

compared to TGA conditions. When the doped sorbents were tested in the fluidized bed, both NaCl and Na2CO3 caused a decrease in the CO2 sorption capacity of the sorbents which could be attributed to a coating formed over the surface of the sorbents in the calcination stage, leading to pore pluggage. These findings indicate the complexity in interpreting the results for large-scale systems. Encouraging results from sodium-doped CaO have led to further studies using other group 1A elements, including lithium (Li), potassium (K), rubidium (Rb) and cesium (Cs). One investigation has linked the CO2 capture performance of an alkali metal-doped CaO to the electropositivity of the metal.38 It is observed that a Cs-doped CaO sorbent with 20% Cs/CaO has the highest sorption capacity at 50 wt % in the first cycle compared to CaO doped with other alkali metals. The improvement of the CO2 capture capacity resulted from an increase in basic sites on the surface. An enhancement in the rate of CO2 adsorption due to doping with Cs was also reported. Since a higher surface area of the sorbent has been associated with a better CO2 capture capacity, a new method of synthesizing calcium-based nanosorbents with a wide range of refractory dopantssilicon (Si), titanium (Ti), chromium (Cr), cobalt (Co), zirconium (Zr), and cerium (Ce)by flame spray pyrolysis (FSP) has also been explored.39 Though all the doped sorbents showed a lower CaO conversion compared to nascent CaO in the first cycle due to less CaO content, the durability of the doped sorbents was found to be better. Among the resulting CaO, the Zr-doped CaO exhibited the best CO2 sorption capacity under similar operational conditions. With a Zr/Ca molar ratio of 0.3, the sorbents were capable of resisting the sintering agglomeration, leading to 50 wt % reactivity after 100 cycles. As shown in Table 2, the surface area and pore volume of all the FSP-made sorbents was at least two times that of PCC, leading to a faster and higher CO2 uptake capacity.6,39 Recently, supports like aluminum oxide (Al2O3), cement (contains CaO and Al2O3), magnesium oxide (MgO), and silica (SiO2) have also been shown to improve the durability of CaO sorbents. A new regenerable calcium-based sorbent, CaO/Ca12Al14O33, was tested at mild (850 °C, 100% N2) and severe (980 °C, 100% CO2) calcination conditions.40,41 It was observed that this new sorbent can attain the reactivity of 41 wt % after 50 cycles under mild conditions and 22 wt % after 56 cycles under severe conditions.41 The Al2O3 reacted with CaO to form Ca12Al14O33 in the calcination step, yielding a stable structure among CaO micrograins, and thus retarding the sintering of CaO. Additionally, the CaO/Ca12Al14O33 obtained from different prescursors with Al2O3, such as Ca(OH)2 and Ca(CH3COO)2, yields similar 2136



ARTICLE

Table 3. CO2 Capture Capacity of Various CaO Sorbents with Ca12Al14O33 carbonation

calcination

T (°C)

t (min)

PCO2 (atm)

T (°C)

t (min)

PCO2 (atm)

40

700

30

0.2

850

5

0 (N2)

50

CaO/Ca12Al14O3340

650

30

0.2

980

5

1

56

22

Ca(OH)2/Ca12Al14O3341

690

30

0.15

850

10

0 (N2)

45

27.3

Ca(CH3COO)2/Ca12Al14O3341

690

30

0.15

850

10

0 (N2)

45

32.6

Ca(OH)2/calcium aluminate cement42

800

10

0.5

800

10

0 (N2)

1000

CaO/Ca12Al14O33

Table 4. Morphological Properties of Various Samples54 surface area

pore volume

average pore size

(m2/g)

(cm3/g)

(nm)

SBA-15

659

0.83

5.27

CaO/SBA-15a

367

0.667

5.05

CaO/SBA-15b

155

0.432

4.75

samples

a

Solvent was removed by filtration. b Solvent was removed by evaporation.

results from other studies.4042 Alternatively, the CaO/ Ca12Al14O33 sorbents can be made in the form of pellets. CaObased pellets prepared from hydrated lime (Ca(OH)2) and calcium aluminate cements were tested for calcination/carbonation cycles.43 It was found the pellets exhibited a similar trend as powders, maintaining a high residual capacity of 22% after 1000 cycles. The durability of various CaO sorbents with Ca12Al14O33 is summarized in Table 3. A mixture of 26 wt % MgO with CaO was observed to maintain 45 wt % CO2 removal capacities over 100 calcination carbonation reaction cycles.44 It was also reported that CaO with 20 wt % MgO can maintain 24 wt % CO2 capacity after 1250 cycles.45 The method of introduction of MgO in CaO was indicated to play an important role in determining its effectiveness. It is well-known that dolomite-derived CaO performs better than limestone-derived CaO over multiple cycles because of the presence of MgO which limits sintering.44 However, it is reported that the performance of synthetically prepared MgO-doped CaO is superior than CaO derived from dolomite.46 Nano-CaCO3, which has been widely used in plastics, has better morphological properties compared to micro-CaCO3.47 The CO2 capture capacity of CaO from nanosized CaCO3 was first studied by Barker in the application of energy storage.48 Further, nanoCaCO3 precursor was mixed with Al2O3, yielding a stable adsorption ratio of 68.3% after 15 cycles.49 Basically, this approach combines the concepts of nanosorbents and aluminum dopants. It was found that nano-CaCO3 can react with Al2O3 at a lower temperature (800 °C) than micro-CaCO3 to form Ca12Al14O33. Besides Al2O3, nano-CaO sorbent has been coated with TiO2 which is able to transform to CaTiO3/nano-CaO.50 The CaTiO3/ nano-CaO showed a higher CO2 capture capacity after 40 cycles compared to that of nano-CaO. It is likely that CaTiO3 has a high thermal stability and thus provided a stable structure at elevated temperatures. More recently, the addition of potassium permanganate (KMnO4) using wet impregnation methods was shown to improve CO2 sorption capacity of calcium-based sorbents for long-term calcination/carbonation cycles.51 It was indicated that KMnO4 helps to maintain particle stability by controlling the surface area and pores to a specific range. Novel CaO/La2O3 sorbents have also been developed by different

cycles

CO2 capture capacity (%) 41

22

methods, including dry physical mixing, wet chemistry, and sol gel combustion synthesis.52 The well-dispersed lanthanum (III) oxide (La2O3) can prevent the agglomeration of CaO, leading to a carbonation conversion of 65% after 50 cycles. Unlike MgO, which acts as an inert material, La2O3 reacts with CO2 and forms La2O2CO3 simultaneously in the carbonation reaction and decomposes back to La2O3 during the calcination reaction. Mesoporous silicates (MCM-41 and SBA-15) have been recognized as a new class of inorganic materials.53 The MCM-41 represents a hexagonal ordering of cylindrical channels with amorphous silica having pore diameters in the range of 1.510 nm. SBA15 contains larger pores (>30 nm) compared to MCM-14, yielding better stability. Because of high surface areas and well-defined pore structures, these materials have been incorporated with CaO. The calcium-based sorbents on SBA-15 for CO2 adsorption have been studied in the literature.54 The SBA-15 mesoporous sieve provides a stable framework for supporting CaO and is composed of a very high surface area and pore volume as given in Table 4. The stable structure of CaO sorbents with SBA-15 was found to prohibit the sintering of CaO in cyclic testing. The results indicated that the sorbents possessed 63% of the capture capacity after 40 cycles. The CO2 capture capacity of the sorbent with various supports in long series testing is given in Table 5. The dopants or supports are capable of providing more stable structure under cyclic testing. Though they perform better over many cycles, the overall CO2 capture capacity may, however, suffer due to low CaO content. The impact on the process material and energy balances also needs to be considered due to the fact that the inert material will circulate in the loop. The cost of these sorbents is also higher and thus their performance may not be sustainable for an economic process to be developed.

3. STEAM/WATER HYDRATION An intermediate step, reactivation through hydration, has been suggested as a path to overcome the problem of decay in reactivity of calcium-based sorbents and seems to be the most promising among the various alternatives that are available. The reactivation of CaO/CaSO4 sorbents by hydration was attempted for improving their utilization in SO2 capture in fluidized bed combustion applications.55 In this reactivation process, Ca(OH)2, which has a higher molar volume than CaO, formed from the hydration reaction yields an expanded volume of the calcium core in the particle, thereby cracking the sulfated calcium particle layer and allowing continued effective diffusion of SO2. The hydration of CaO for CO2 capture, on the other hand, has a different reactivation mechanism in which hydrated calcium sorbents serve as a precursor for CaO generation. It was observed that Ca(OH)2derived CaO performed better in SO2 capture than the one derived from CaCO3.56 Aside from hydration, the “melting 2137



ARTICLE

Table 5. Long Series Testing of CO2 Capacity of CaO with Different Supports carbonation

calcination T (°C)

t (min)

PCO2 (atm)

1

758

30

0.25

750

30

40

0.33

800

600

10

0.2

600

10

0.2

KMnO4/CaO51

700

20

La2O3/CaO52

650

La2O3/CaO52 SBA-15/CaO54

650 700

T (°C)

t (min)

44

MgO/CaO

758

30

MgO/CaO45

750

20

Al2O3/nano-CaO49

600

CaTiO3/nano-CaO50 TiO2/nano-CaO50

PCO2 (atm)

cycles

CO2 capture capacity (%)

0 (He)

100

45.0

0 (N2)

1250

24.0

5

0 (N2)

50

53.7

750

10

0 (N2)

40

33.0

750

10

0 (N2)

40

23.0

0.15

850

15

0 (Air)

100

27.5

15

0.15

850

10

0 (N2)

50

51.1

15 60

0.15 1

950 910

10 30

1 0 (N2)

50 40

22.0 62.9

technique” was also employed to improve acceptor properties in the development of the CO2 Acceptor Process. In this technique, the concentration of Ca(OH)2 in the melts was correlated to the sorbent activity.12 Further, the activities of the melts were independent of the precursor used in the preparation of the melts— fresh or deactivated limestone. The conversion of inactive CaO to Ca(OH)2 was indicated to be the key to success of this technique. These findings have contributed to the current application of CaO hydration for improving long-term CO2 capture, and it has been experimentally verified to be a viable technique. The addition of steam during the carbonation reaction has also been examined.57 In this case the steam does not actually react with CaO but it enhances the solid state diffusion in the product layer and as a result, improves the carbonation conversion, especially for lower carbonation temperatures and more sintered sorbents. Similarly, the use of steam during calcination has also been investigated and it is reported that the presence of steam reduces the extent of sintering of CaO resulting in higher reactivity toward CO2.58 However, these results seem contradictory to earlier findings which showed that steam enhances sintering.59 These approaches will not be as effective as independent and direct hydration of CaO sorbents. Since the reactor design in calcium-based CO2 capture systems will be based on the fast reaction regime for carbonation, the enhancement of carbonation conversion in the slow reaction regime, limited by solid state diffusion, is of little significance. Hydration of CaO is a fast and exothermic reaction: CaO þ H2 O T CaðOHÞ2 ΔH° ¼ 109 kJ=mol

ð2Þ

Two options exist for hydrating CaO: using water or using steam. Both have been studied by several researchers. The high temperature and pressure steam hydration of CaO was investigated for application in the HyPr-RING process developed in Japan.60 It was reported that the sorbent was more durable in repetitive CO2 sorption with the intermediate hydration step. Since the calcium sorbent and steam are directly added to the gasifier in the HyPr-RING process, the hydration of CaO is inevitable at the operating conditions. It is important to note that the operating conditions are such that both hydration and carbonation of CaO can occur. Ultimately, the hydrated CaO reacts with CO2 to form CaCO3. The advantage of such in situ reactions is that the heat of hydration can balance the endothermic gasification. However, direct interaction of coal minerals and ash with the calcium

sorbent in the gasifier can negatively affect CO2 capture capacity. Controlling the extent of hydration is also a challenge. It has been suggested that hydration causes cracks to be formed in CaO particles, creating channels extending to the interior of the particles and hence improving CO2 capture.61 While most studies have investigated the use of hydration treatment after the calcination reaction and prior to the carbonation reaction, steam reactivation of spent sorbents in the form of CaCO3, in the same manner as that for CaSO4 reactivation for SO2 capture discussed earlier, has also been investigated.62,63 It was found that hydration of the sorbents derived from the calciner in the form of CaO would be better. Zeman has suggested performing hydration at 300 °C and an atmospheric pressure to avoid the extra costs associated with high pressure steam reactors.64 On the other hand, Fennell et al. have studied regeneration of spent CaO sorbents by reaction with humid air.65 Another study shows that steam hydration is more effective than water hydration for six cycles;66 however, single cycle tests conducted at OSU indicate otherwise.58 The mechanism of hydration of CaO was extensively studied in the period 19501960. The major results can be summarized as follows: (1) The mechanism can differ depending on the calcination time and temperature used to obtain CaO. Water hydration predominantly occurs “through solution”.67,68 In this mechanism, supersaturated solutions with respect to crystalline Ca(OH)2 are formed, which facilitates the formation and growth of crystals in the pores and spaces previously occupied by CaO. This leads to a decrease in the pore volume and pore size. (2) The “advancing interface mechanism” had also been proposed for hydration by both water and water vapor. In this case, the hydration proceeds inward from outside of each CaO crystallite.69,70 The product is formed at the original sites of the reactant and is accompanied by overall expansion because the specific volume of the product is greater (1.98) than that of CaO. The surface area of CaO derived from Ca(OH)2 is usually higher as compared to CaO obtained from CaCO3. However, the surface area of CaO is also a function of the decomposition temperature and pressure of Ca(OH)2, and thus there exists an optimum temperature where the surface area is maximum.71 There are two opposing effects—recrystallization and sintering. While recrystallization leads to an increase in the surface area, sintering creates particles with a smaller surface area and a larger grain size. These effects have been considered in the experimental and modeling studies of ultrafast calcination and sintering of Ca(OH)2.72 Besides the effect of temperature and pressure on the calcination reaction, residence time is also one of the important 2138



ARTICLE

Figure 4. Effect of calcination time on the hydrated sorbent at 900 °C (red square, original sorbents; blue diamond, sintered sorbents; green triangle, hydrated sorbents).

Figure 5. Effect of the hydration on the decay rate of CO2 sorption capacity of CaO sorbent (both calcination and carbonation at 700 °C for 30 min in each cycle; hydration with water was performed after the 15th cycle).

variables. It is reported that recrystallization dominates over sintering in a shorter residence time. The importance of hydration in calcium-based CO2 capture systems can be underscored by the number of recent studies that highlight different aspects of hydration. While there is a general agreement on improvement in CO2 capture capacity over multiple cycles, the attrition of hydrated particles can pose a challenge for operating the hydrator as a fluidized bed reactor.7377 Approaches like using fixed-bed reactors and pelletization have been used to deal with particle attrition.73,74,77 Since hydration of CaO is a highly exothermic reaction, operation of fixed-bed reactors require additional heat transfer considerations. Pressure drop is also a potential issue in fixed bed operation. Though pelletization should help reduce attrition effects, hydration over multiple cycles might ultimately induce friability and break the strength of the pellets. A scheme where only a fraction of the sorbent is

hydrated during a continuous operation has also been suggested to minimize steam consumption.78 This can be accomplished by either splitting the calcined sorbent into two streams and hydrating only one or hydrating all the sorbent periodically. While this scheme is likely to reduce steam consumption, a higher solids circulation will be required to achieve the same percentage of the CO2 removal since the full potential of hydration is not being exploited. For CO2 removal from the flue gas stream produced from coal combustion, the first test using hydrated lime was carried out on a 120 kWth subpilot coal combustor in the CCR process with intermediate hydration, developed at OSU.79,80 In this subpilot demonstration, utilizing coal at 20 pounds per hour along with natural gas, over 90% CO2 capture and near 100% SO2 capture was achieved on a once through basis at a Ca/C mole ratio of 1.3. At this molar ratio, the sorbent reactivity could be maintained 2139


Industrial & Engineering Chemistry Research over multiple carbonationcalcinationhydration cycles. In the cyclic loop operation, CaCO3 is calcined to CaO and then hydrated by water to Ca(OH)2. Over the six cycles tested, the sorbent reactivity remained constant. Further studies are being undertaken to examine parameters like steam to CaO ratio, steam residence time, types of reactors, modes of operation, etc., with the aim of designing an actual commercial-scale steam hydrator. A detailed mechanism study of hydration has also been performed, and the results have been interpreted using density function theory (DFT) calculations.22 As can be seen from Figure 4, original sorbents calcined at 900 °C for 22 h can only maintain ∼10 wt % CO2 capture capacity; however, the reactivity can be restored to ∼60 wt % after hydration. Further, the effect of hydration on the extent of sintering is also shown in Figure 4. All the sintered sorbents, denoted as blue dots in Figure 4, are obtained from calcining the original sorbent after hydration at 900 °C for different time periods. In addition, the green dots are merely used to denote that the reactivity of the sorbents can be restored by hydration regardless of the level of the sintering. The reactivity of hydrated sorbents decays slower than that of the original sorbent; that is, the time required to reduce the reactivity of hydrated sorbents to ∼10 wt % is 15 times longer (∼336 h) than that of the original sorbent (∼22 h) at a given temperature. Thus, the unique morphological properties of CaO after hydration can retard sintering and effectively recover the CO2 sorption capacity. The durability of the sorbent is more important than its single cycle reactivity. It was found that hydration decreases the decay rate of the CO2 sorption capacity. As can be seen from Figure 5, the weight capture of original limestone reduced to ∼32% after 15 cycles; however, the hydrated sorbents can retain ∼53 wt % CO2 capture capacity under 15 calcination and carbonation reaction cycles, which is also in agreement with other studies.75 It was found that the rate of carbonation in the diffusion controlled region is accelerated due to the hydration, hence reducing the decay rate of sorbent reactivity. Water hydration can result in a superior performance on the CO2 capture capacity of CaO sorbents.81 However, the operation of the hydrator at high temperatures (400600 °C) can provide high quality heat for use in heat integration in the process. Hydration using high pressure steam at high temperatures does not require significant reheating and cooling of the sorbent, thereby significantly reducing the parasitic energy consumption of the process. However, the temperatures above 500 °C will require pressurized vessels. The specific hydrator operating conditions will ultimately be dictated by the economics.

’ CONCLUDING REMARKS The calcium-based processes involving postcombustion or precombustion CO2 capture confront a common challenge, that is, decreasing CO2 capture capacity of sorbents over multiple cycles of operation. Considerable recent works have been geared toward identifying methods that can overcome this challenge so that the sorbent can be used in a more sustainable manner. The morphology of sorbents has been identified as a key factor that affects the sorbent performance. The initial morphology of fresh sorbents can be improved through modified precursors, addition of dopants and supports, and/or water/steam hydration. Under high calcination temperature and multiple calcinationcarbonation reaction cycles, however, the morphological properties of fresh sorbents tend to undergo undesired, irreversible changes

ARTICLE

that lead to deteriorated sorbent performance. Therefore, reactivation strategies that can effectively restore the desired morphology can potentially extend the sorbent lifetime and improve the process performance. Among the various potential options, water/steam hydration is likely to be the most promising approach for restoring the aforementioned properties at both macrostructural and microstructural levels. Both experimental findings and quantum calculations suggest that hydration can effectively recover the sorbent properties, leading to improved CO2 capture capacity. From a practical standpoint, the hydration step can also potentially be integrated in the overall energy conversion process in an efficient and economical manner. For instance, the heat released from the highly exothermic hydration reaction can be readily integrated in the overall process. Additionally, the operation of the hydrator can also be flexible, that is, hydration can be performed after each carbonationcalcination reaction cycle or once in a few cycles.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT The work conducted at OSU was supported by the U.S. Department of Energy, Ohio Coal Development Office, and an industrial consortium. Their support is gratefully acknowledged. ’ REFERENCES (1) Plaza, M. G.; Pevida, C.; Arias, B.; Casal, M. D.; Martín, C. F.; Fermoso, J.; Rubiera, F.; Pis, J. J. Different Approaches for the Development of Low-Cost CO2 Adsorbents. J. Environ. Eng. 2009, 135, 426–432. (2) Fan, L.-S. Chemical Looping Systems for Fossil Energy Conversions; Wiley: New York, 2010. (3) Li, F.; Fan, L.-S. Clean Coal Conversion Processes—Progress and Challenges. Energy Environ. Sci. 2008, 1, 248–267. (4) Aaron, D.; Tsouris, C. Separation of CO2 from Flue Gas: A Review. Sep. Sci. Technol. 2005, 40, 321–348. (5) Goff, G. S.; Rochelle, G. T. Monoethanolamine Degradation: O2 Mass Transfer Effects under CO2 Capture Conditions. Ind. Eng. Chem. Res. 2004, 43, 6400–6408. (6) Gupta, H.; Fan, L.-S. CarbonationCalcination Cycle Using High Reactivity Calcium Oxide for Carbon Dioxide Separation from Flue Gas. Ind. Eng. Chem. Res. 2002, 41, 4035–4042. (7) Ramkumar, S.; Fan, L. -S. Calcium Looping Process (CLP) for Enhanced Noncatalytic Hydrogen Production with Integrated Carbon Dioxide Capture. Energy Fuels 2010, 24, 4408–4418. (8) Lin, S.-Y.; Suzuki, Y.; Hatano, H.; Harada, M. Developing an Innovative Method, HyPr-Ring, to Produce Hydrogen from Hydrocarbons. Energy Convers. Manage. 2002, 43, 1283–1290. (9) Lin, S.-Y.; Harada, M.; Suzuki, Y.; Hatano, H. Process Analysis for Hydrogen Production by Reaction Integrated Novel Gasification (HyPr-Ring). Energy Convers. Manage. 2005, 46, 869–880. (10) Andrus, H. E.; Burns, G.; Chiu, J. H.; Liljedahl, G. N.; Stromberg, P. T.; Thibeault, P. R. Hybrid Combustion-Gasification Chemical Looping Power Technology Development. ALSTOM Technical Report DE-FC2603NT41866, 2006. (11) Rizeq, R. G.; West, J.; Frydman, A.; Subia, R.; Kumar, R.; Zamansky, V. Fuel-Flexible Gasification-Combustion Technology for Production of H2 and Sequestration-Ready CO2. Annual DOE Technical Progress Report, 2002. (12) Curran, G. P.; Fink, C. E.; Gorin, E. Carbon Dioxide-Acceptor Gasification Process. Studies of Acceptor Properties. Adv. Chem. Ser. 1967, 69, 141–165. 2140


Industrial & Engineering Chemistry Research (13) Barker, R. The Reversibility of the Reaction CaCO3 = CaO + CO2. J. Appl. Chem. Biotechnol. 1973, 23, 733–742. (14) Oates, J. Lime and Limestone: Chemistry and Technology, Production and Uses; Wiley-VCH: Weinheim/New York, 1998. (15) Abanades, J. C. The Maximum Capture Efficiency of CO2 Using a Carbonation/Calcination Cycle of CaO/CaCO3. Chem. Eng. J. 2002, 90, 303–306. (16) Grasa, G. S.; Abanades, J. C. CO2 Capture Capacity of CaO in Long Series of Carbonation/Calcination Cycles. Ind. Eng. Chem. Res. 2006, 45, 8846–8851. (17) Laursen, K.; Duo, W.; Grace, J. R.; Lim, C. J. Cyclic Steam Reactivation of Spent Limestone. Ind. Eng. Chem. Res. 2004, 43, 5715– 5720. (18) Bhatia, S. K.; Perlmutter, D. D. Effect of the Product Layer on the Kinetics of the CO2-Lime Reaction. AIChE J. 1983, 29, 79–86. (19) Abanades, J. C.; Alvarez, D. Conversion Limits in the Reaction of CO2 with Lime. Energy Fuels 2003, 17, 308–315. (20) Iyer, M. V.; Gupta, H.; Sakadjian, B. S.; Fan, L.-S. Multicyclic Study on the Simultaneous Carbonation and Sulfation of High-Reactivity CaO. Ind. Eng. Chem. Res. 2004, 43, 3939–3947. (21) Sun, P.; Grace, J. R.; Lim, C. J.; Anthony, E. J. The Effect of CaO Sintering on Cyclic CO2 Capture in Energy Systems. AIChE J. 2007, 53, 2432–2442. (22) Yu, F. C.; Luo, S.; Fan, L.-S. Interpretation of the Effects of Hydration on the CO2 Adsorption Capacity in CaO Using the Density Functional Theory. Surf. Sci., in review. (23) Lu, H.; Reddy, E. P.; Smirniotis, P. G. Calcium Oxide Based Sorbents for Capture of Carbon Dioxide at High Temperatures. Ind. Eng. Chem. Res. 2006, 45, 3944–3949. (24) Lu, H.; Khan, A.; Smirniotis, P. G. Relationship between Structural Properties and CO2 Capture Performance of CaO-Based Sorbents Obtained from Different Organometallic Precursors. Ind. Eng. Chem. Res. 2008, 47, 6216–6220. (25) Liu, W.; Low, N. W.; Feng, B.; Diniz da Costa, J. C.; Wang, G. X. Calcium Precursors for the Production of CaO Sorbents for Multicycle CO2 Capture. Environ. Sci. Technol. 2010, 44, 841–847. (26) Li, Y.; Zhao, C.; Qu, C.; Duan, L.; Li, Q.; Liang, C. CO2 Capture Using CaO Modified with Ethanol/Water Solution during Cyclic Calcination/Carbonation. Chem. Eng. Technol. 2008, 31, 237–244. (27) Li., Y.; Zhao, C.; Chen, H.; Liu, Y. Enhancement of Ca-Based Sorbent Multicyclic Behavior in Ca Looping Process for CO2 Separation. Chem. Eng. Technol. 2008, 32, 548–555. (28) Li., Y.; Zhao, C.; Chen, H.; Liang, C.; Duan, L.; Zhou, W. Modified CaO-Based Sorbent Looping Cycle for CO2 Mitigation. Fuel 2009, 88, 697–704. (29) Li, Y.; Zhao, C.; Duan, L.; Liang, C.; Li, Q.; Zhou, W.; Chen, H. Cyclic Calcination/Carbonation Looping of Dolomite Modified with Acetic Acid for CO2 Capture. Fuel Process Technolo. 2008, 89, 1461–1469. (30) Falini, G.; Albeck, S.; Weiner, S.; Addadi, L. Control of Aragonite or Calcite Polymorphism by Mollusk Shell Macromolecules. Science 1996, 271, 67–69. (31) DeOliveira, D. B.; Lausen, R. A. Control of Calcite Crystal Morphology by a Peptide Designed to Bind to a Specific Surface. J. Am. Chem. Soc. 1997, 119, 10627–10631. (32) Guo, X. H.; Yu, S. H.; Cai, G. B. Crystallization in a Mixture of Solvents by Using a Crystal Modifier: Morphology Control in the Synthesis of Highly Monodisperse CaCO3 Microspheres. Angew. Chem., Int. Ed. 2006, 45, 3977–3981. (33) K€uther, J.; Seshadri, R.; Knoll, W.; Tremel, W. Templated Growth of Calcite, Vaterite and Aragonite Crystals on Self-Assemble Monolayers of Substituted Alkylthiols on Gold. J. Mater. Chem. 1998, 8, 641–650. (34) Chen, B. D.; Cilliers, J. J.; Davey, R. J.; Garside, J.; Woodburn, E. T. Templated Nucleation in a Dynamic Environment: Crystallization in Foam Lamellae. J. Am. Chem. Soc. 1998, 120, 1625–1626. (35) Yang, Z.; Zhao, M.; Florin, N. H.; Harris, A. T. Synthesis and Characterization of CaO Nanopods for High Temperature CO2 Capture. Ind. Eng. Chem. Res. 2009, 48, 10765–10770.

ARTICLE

(36) Salvador, C.; Lu, D.; Anthony, E. J.; Abanades, J. C. Enhancement of CaO for CO2 Capture in an FBC Environment. Chem. Eng. J. 2003, 96, 187–195. (37) Shearer, J. A.; Johnson, I.; Turner, C. B. Effects of Sodium Chloride on Limestone Calcination and Sulfation in Fluidized-Bed Combustion. Environ. Sci. Technol. 1979, 13, 1113–1118. (38) Reddy, E. P.; Smirniotis, P. G. High-Temperature Sorbents for CO2 Made of Alkali Metals Doped on CaO Supports. J. Phy. Chem. B 2004, 108, 7794–7800. (39) Lu., H.; Khan, A.; Pratsinis, S. E.; Smirniotis, P. G. Flame-Made Durable Doped-CaO Nanosorbents for CO2 Capture. Energy Fuels 2009, 23, 1093–1100. (40) Li, Z. S.; Cai, N. S.; Huang, Y. Y.; Han, H. J. Synthesis, Experimental Studies, and Analysis of a New Calcium-Based Carbon Dioxide Absorbent. Energy Fuels 2005, 19, 1447–1452. (41) Li, Z. S.; Cai, N. S.; Huang, Y. Y. Effect of Preparation Temperature on Cyclic CO2 Capture and Multiple CarbonationCalcination Cycles for a New Ca-Based CO2 Sorbent. Ind. Eng. Chem. Res. 2006, 45, 1911–1917. (42) Martavaltzi, C. S.; Lemonidou, A. A. Development of new CaO Based Sorbent Materials for CO2 Removal at High Temperature. Microporous Mesoporous Mater. 2008, 110, 119–127. (43) Manovic, V.; Anthony, E. J. Long-Term Behavior of CaO-Based Pellets Supported by Calcium Aluminate Cements in a Long Series of CO2 Capture Cycles. Ind. Eng. Chem. Res. 2009, 48, 8906–8912. (44) Chen, Z.; Song, H. S.; Portillo, M.; Lim, C. J.; Grace, J. R.; Anthony, E. J. Long-Term Calcination/Carbonation Cycling and Thermal Pretreatment for CO2 Capture by Limestone and Dolomite. Energy Fuels 2009, 23, 1437–1444. (45) Albrecht, K. O.; Wagenbach, K. S.; Satrio, J. A.; Shanks, B. H.; Wheellock, T. D. Development of a CaO-Based CO2 Sorbent with Improved Cyclic Stability. Ind. Eng. Chem. Res. 2008, 47, 7841–7848. (46) Li, L.; King, D. L.; Nie, Z.; Howard, C. Magnesia-Stabilized Calcium Oxide Absorbents with Improved Durability for High Temperature CO2 Capture. Ind. Eng. Chem. Res. 2009, 48, 10604–10613. (47) Florin, N. H.; Harris, A. T. Reactivity of CaO Derived from Nano-Sized CaCO3 Particles through Multiple CO2 Capture-andRelease Cycles. Chem. Eng. Sci. 2009, 64, 187–191. (48) Barker, R. The Reactivity of Calcium Oxide towards Carbon Dioxide and Its Use for Energy Storage. J. Appl. Chem. Biotechnol. 1974, 24, 221–227. (49) Wu, S. F.; Li, Q. H.; Kim, J. N.; Yi, K. B. Properties of a Nano CaO/Al2O 3 CO2 Sorbent. Ind. Eng. Chem. Res. 2008, 47, 180–184. (50) Wu, S. F.; Zhu, Y. Q. Behavior of CaTiO3/Nano-CaO as a CO2 Reactive Adsorbent. Ind. Eng. Chem. Res. 2010, 49, 2701–2706. (51) Li., Y.; Zhao, C.; Chen, H.; Duan, L.; Chen, X. Cyclic CO2 Capture Behavior of KMnO4-Doped CaO-Based Sorbent. Fuel 2010, 89, 642–649. (52) Luo, C.; Zheng, Y.; Ding, N.; Wu, Q.; Bian, G.; Zheng, C. Development and Performance of CaO/La2O3 Sorbents during Calcium Looping Cycles for CO2 Capture. Ind. Eng. Chem. Res. 2010, 49, 11778–11784. (53) Zhao, D. Y.; Feng, J. L.; Huo, Q. S.; Melosh, N.; Fredrickson, G. H.; Chmelka, B. F.; Stucky, G. D. Triblock Copolymers Syntheses of Mesoporous Silica with Periodic 50300 Angstrom Pores. Science 1998, 279, 548–552. (54) Huang, C. H.; Chang, K. P.; Yu, C. T.; Chiang, P. C.; Wang, C. F. Development of High-Temperature CO2 Sorbents Made of CaOBased Mesoporous Silica. Chem. Eng. J. 2010, 161, 129–135. (55) Shearer, J. A.; Smith, G. W.; Moulton, D. S.; Smyk, E. B.; Myles, K. M.; Swift, W. M.; Johnson, I. Hydration Process for Reactivating Spent Limestone and Dolomite Sorbent for Reuse in Fluidized-Bed Coal Combustion. Proceedings of the Sixth International Conference on Fluidized Bed Combustion, Atlanta, GA, April 911, 1980. (56) Bruce, K. R.; Gullet, B. K.; Beach, L. O. Comparative SO2 Reactivity of CaO Derived from CaCO3 and Ca(OH)2. AIChE J. 1989, 35, 37–41. 2141


Industrial & Engineering Chemistry Research (57) Manovic, V.; Anthony, E. J. Carbonation of CaO-Based Sorbents Enhanced by Steam Addition. Ind. Eng. Chem. Res. 2010, 49, 9105–9110. (58) Ramkumar, S.; Fan, L.-S. Thermodynamic and Experimental Analyses of the Three-Stage Calcium Looping Process. Ind. Eng. Chem. Res. 2010, 49, 7563–7573. (59) Borgwardt, R. H. Calcium Oxide Sintering in Atmospheres Containing Water and Carbon Dioxide. Ind. Eng. Chem. Res. 1989, 28, 493–500. (60) Kuramoto, K.; Fujimoto, S.; Morita, A.; Shibano, S.; Suzuki, Y.; Hatano, H.; Lin, S. Y.; Harada, M.; Takarada, T. Repetitive CarbonationCalcination Reactions of Ca-Based Sorbents for Efficient CO2 Sorption at Elevated Temperatures and Pressures. Ind. Eng. Chem. Res. 2003, 42, 975–981. (61) Hughes, R. W.; Lu, D.; Anthony, E. J.; Wu, Y. Improved LongTerm Conversion of Limestone-Derived Sorbents for in Situ Capture of CO2 in a Fluidized Bed Combustor. Ind. Eng. Chem. Res. 2004, 43, 5529–5539. (62) Manovic, V.; Anthony, E. J. Sequential SO2/CO2 Capture Enhanced by Steam Reactivation of a CaO-Based Sorbent. Fuel 2008, 87, 1564–1573. (63) Manovic, V.; Lu, D.; Anthony, E. J. Steam Hydration of Sorbents from a Dual Fluidized Bed CO2 Looping Cycle Reactor. Fuel 2008, 87, 3344–3352. (64) Zeman, F. Effect of Steam Hydration on Performance of Lime Sorbent for CO2 Capture. Int. J. Greenhouse Gas Control 2008, 2, 203–209. (65) Fennell, P. S.; Davidson, J. F.; Dennis, J. S.; Hayhurst, A. N. Regeneration of Sintered Limestone Sorbents for the Sequestration of CO2 from Combustion and other Systems. J. Inst. Energy 2007, 80, 116–119. (66) Han, L.; Wang, Q.; Ma, Q.; Guan, J.; Luo, Z.; Cen, F. Hydration Reactivation of CaO-Based Sorbent for Cyclic CalcinationCarbonation Reactions. Proceedings of the 20th International Conference on Fluidized Bed Combustion, Xi’an, China, May 1821, 2009. (67) Birss, F. W.; Thorvaldson, T. The Mechanism of the Hydration of Calcium Oxide. Can. J. Chem. 1955, 33, 881–886. (68) Ramachandran, V. S.; Sereda, P. J.; Feldman, R. F. Mechanism of Hydration of Calcium Oxide. Nature 1964, 201, 288–289. (69) Glasson, D. R. Reactivity of Lime and Related Oxides. II. Sorption of Water Vapor on Calcium Oxide. J. Appl. Chem. 1958, 8, 798–803. (70) Glasson, D. R. Reactivity of Lime and Related Oxides. III. Sorption of Liquid Water on Calcium Oxide (Wet Hydration). J. Appl. Chem. 1960, 10, 38–42. (71) Mikhail, R. Sh Activation and Sintering Behavior of Calcium Oxide—The Effect of Hydration on the Surface Area of the Oxide Produced by Thermal Decomposition. J. Phy. Chem. 1963, 67, 2050–2054. (72) Ghosh-Dastidar, A; Mahuli, S.; Agnihotri, R.; Fan, L.-S. Ultrafast Calcination and Sintering of Ca(OH)2 Powder: Experimental and Modeling. Chem. Eng. Sci. 1995, 50, 2029–2040. (73) Blamey, J.; Paterson, N. P. M.; Dugwell, D. R.; Fennell, P. S. Mechanism of Particle Breakage during Reactivation of CaO-Based Sorbents for CO2 Capture. Energy Fuels 2010, 24, 4605–4616. (74) Wu, Y.; Blamey, J.; Anthony, E. J.; Fennell, P. S. Morphological Changes of Limestone Sorbent Particles during Carbonation/Calcination Loping Cycles in a Thermogravimetric Analyzer (TGA) and Reactivation with Steam. Energy Fuels 2010, 24, 2768–2776. (75) Materic, V.; Sheppard, C.; Smedley, S. I. Effect of Repeated Steam Hydration Reactivation on CaO-Based Sorbents for CO2 Capture. Environ. Sci. Technol. 2010, 44, 9496–9501. (76) Manovic, V.; Anthony, E. J. Steam Reactivation of Spent CaOBased Sorbent for Multiple CO2 Capture Cycles. Environ. Sci. Technol. 2007, 41, 1420–1425. (77) Lu, D. Y.; Hughes, R. W.; Reid, T.; Anthony, E. J. Hydration and Pelletization of CaCO3-Derived Sorbents for in-Situ CO2 Capture. Proceedings of the 20th International Conference on Fluidized Bed Combustion, Xi’an, China, May 1821, 2009. (78) Arias, B.; Grasa, G. S.; Abanades, J. C. Effect of sorbent hydration on the average activity of CaO in a Ca-Looping system. Chem. Eng. J. 2010, 163, 324–330.

ARTICLE

(79) Fan, L.-S.; Ramkumar, S.; Wang, W.; Statnick, R. Separation of Carbon Dioxide from Gas Mixtures by Calcium Based Reaction Separation Process. U.S. Provisional Patent Application 61/116172, 2008; PCT Application No. PCT/US2009/065224, 2009. (80) Wang, W.; Ramkumar, S.; Li, S.; Wong, D.; Iyer, M.; Sakadjian, B. B.; Statnick, R. M.; Fan, L.-S. Subpilot Demonstration of the CarbonationCalcination Reaction (CCR) Process: High-Temperature CO2 and Sulfur Capture from Coal-Fired Power Plants. Ind. Eng. Chem. Res. 2010, 49, 5094–5101. (81) Yu, F. C.; Fan, L. -S. Reactivation of Calcium-Based Sorbents through Hydration with Water. Fuel, in review.

2142


Activation Strategies for Calcium-Based Sorbents for CO2

Recommend Documents