Review pubs.acs.org/EF
Performance Enhancement of Calcium Oxide Sorbents for Cyclic CO2 CaptureA Review Wenqiang Liu,†,‡ Hui An,‡ Changlei Qin,‡ Junjun Yin,‡ Guoxiong Wang,§ Bo Feng,*,‡ and Minghou Xu† †
State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, Wuhan 430074, China School of Mechanical & Mining Engineering, §School of Chemical Engineering, The University of Queensland, St Lucia, Qld 4072, Australia
‡
ABSTRACT: While calcium oxide has been identified to be the best candidate for capturing CO2 at high temperature, it suffers a well-known problem of loss-in-capacity; that is, its capacity for carbon capture decays dramatically during cyclic carbonation/ calcination processes. Recently, extensive research work has been conducted on the enhancement of the cyclic performance of calcium oxide through either improving the performance of natural minerals, such as water/steam hydration and pretreatment or modification of calcium oxide sorbents by some techniques such as doping and synthesis. This work summarizes the state-of-theart research in the literature aiming to identify potential solutions to the loss-in-capacity problem. It is found that hydration during or after calcination is effective in recovering the capacity of natural minerals and mixing can produce highly effective synthetic sorbents. Periodic hydration of synthetic sorbents could be a good strategy to overcome the technical issues associated with loss-in-capacity while meeting the requirements of the physical properties of sorbents in many potential applications.
1. INTRODUCTION Global warming due to emissions of CO2 into the atmosphere has been a public concern, and one effective solution that has been identified is CO2 capture, storage, and utilization (CCSU).1 In the process of CCSU, CO2 is captured from coal power plants, transported, and stored or utilized (for example converted to methanol2,3) rather than emitted to the atmosphere. The adoption of CCSU is currently limited by the prohibitively high cost of CO2 capture, which accounts for about 75% of the total cost.1 Therefore, current efforts in the world focus on the development of low-cost CO2 capture technologies. Among the promising technologies, calcium looping process (CLP) is one of the cheapest ones that have good commercialization potential, therefore attracting a great level of interest in recent years. The basic concept of CLP for CO2 separation, which is based on the reversible reaction CaO + CO2 ⇔ CaCO3, is illustrated in Figure 1. The CO2-containing gas passes through a carbonator, and CO2 is captured by the sorbent (CaO) in the reactor. As a result, CO2-free gas leaves the carbonator and CaO becomes CaCO3. The carbonated sorbent (CaCO3) is
then transported to a regenerator for decomposition and the regenerated CaO is sent back to the carbonator for reuse. The produced CO2 in the regenerator can be compressed and sequestered. In this process, the sorbent (CaO) is repeatedly used for multiple cycles, and therefore, the process is called calcium looping. The concept of calcium looping is not new. As early as 1867, DuMotay and Marechal used limes (main component is CaO, and therefore, the major reactions are the same calcium looping reaction) to capture CO2 in carbon gasification with steam.4 The renewed interest in the CLP is because of the abundant reserves of potential sorbents and high efficiency or low cost associated with the process and its various potential applications such as postcombustion CO2 capture, precombustion CO2 capture, and energy storage. (a). Postcombustion CO2 Capture. The process of utilizing CLP for postcombustion CO2 capture from coal combustion flue gas is schematically shown in Figure 2a, originally proposed by Shimizu et al.5 CO2 in the flue gas is captured by calcium based sorbent derived from limestone (main component is CaCO3) in a carbonator at a temperature between 500 and 650 °C. The carbonated sorbent is transferred to a calciner (or regenerator) at a temperature between 900 and 950 °C to regenerate the sorbent, utilizing the heat from burning coal with pure O2 (i.e., oxyfuel combustion), which was produced by an external air separation unit. The produced CO2 is compressed and sequestered subsequently. Some of the sorbent will be deactivated due to the high temperature in the calciner (which will be discussed further later) and thus removed from the calciner and fresh limestone sorbent needs to be supplied. The main advantage of the process is that the Received: February 7, 2012 Revised: April 12, 2012 Published: April 13, 2012
Figure 1. General scheme of the cyclic CO2 capture process using the calcium based sorbent, that is, calcium looping process (CLP). © 2012 American Chemical Society
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Figure 2. Schematics of the applications of CLP in CO2 capture; (a) postcombustion process (Adapted with permission from Blamey et al.11 Copyright 2010 Elsevier.); (b) combined CLP and water gas shift (Adapted with permission from Harrison.19 Copyright 2008 American Chemical Society.); (c) the concept of HyPr-RING process (Adapted with permission from Feng et al.13 Copyright 2007 American Chemical Society.); (d) combined CLP and steam methane reforming (Adapted with permission from Li and Fan.12 Copyright 2008 Royal Society of Chemistry).
water−gas shift reaction. The process of CLP applied in water− gas shift reaction is schematically shown in Figure 2b. The syngas from the oxyfuel gasifier enters the reformer/carbonator, in which CO2 capture and water−gas shift reaction occur simultaneously at a temperature of 550−650 °C and pressure of 2−3 MPa, generating hydrogen of high purity.14 The CaCO3 is sent to the calciner/oxyfuel combustor for decomposition at atmospheric pressure and 800−1000 °C.14 The produced CO2 is compressed and sequestered subsequently. The simulated results using ASPEN exhibited that the process had a high efficiency of 63% (higher heating value, HHV), in comparison to 57% (HHV) without CLP.14 The HyPr-RING process, first proposed by Lin et al.15 in 2001 and then extensively studied by his group,16−18 is another example of the application of CLP for precombustion CO2 capture. As shown in Figure 2c, the process integrates steam coal gasification reaction, water−gas shift reaction, and capturing CO2 in a single reactor (i.e., coal gasifier). CaCO3 is regenerated in a second reactor (regenerator). The claimed system efficiency is over 53%.13 The operating temperature of the coal gasifier could vary from 500 to 1000 °C at a pressure of 1−110 bar.13
overall cost is significantly lower than that of the competing technologies. This is mainly because the heat from the exothermic carbonation reaction in the carbonator could be used to generate extra power. The estimated cost for capturing CO2 in this CLP was 15 USD/t, in comparison to 23.8 USD/t for a standard oxyfuel circulating fluidized bed combustion (CFBC) process (2007 estimate).6 In addition, there is no need to modify the existing power plant, which is another advantage of the capturing process. The other advantage is that large-scale reactors for carbonation and calcination are technically proven, and oxyfuel combustion of coal (for generating heat for sorbent regeneration) is also at the stage of pilot scale demonstration. Currently, a few postcombustion capture pilot plants based on CLP are being demonstrated in Canada,7 Spain,8 U.K.,9 and Germany,10 showing commercialization potential of the technology. (b). Precombustion CO2 Capture. In precombustion CO2 capture, CO2 is captured before the combustion process. Normally, CO2 is captured during or after a gasification process, and hydrogen is the resulting product gas. Potential applications of CLP in precombustion capture include CO2 capture in coal gasification, steam−methane reforming, or 2752
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stable performance over multiple cycles, good mechanical strength, and low attrition rate.13 However, it has been discovered by many researchers that all tested CaO-based sorbents suffer a problem of loss-in-capacity; that is, the capacity of CO2 capture (i.e., the utilization of CaO or conversion of CaO) decreases sharply after cycles of carbonation/calcination. This phenomenon had been reported by many researchers and some typical results from five groups22,25,29−31 are shown in Figure 4. It was found that the calcium conversion decreased from over 90% to a residual value 7−8% after prolonged cycles of carbonation/calcination (up to 500 cycles).30 The loss-in-capacity problem of CaO is currently one of the major problems limiting the widespread application of the calcium looping process. This problem adds extra variability, costs, and difficulties to the control of carbon capture processes. If natural limestone is used as the sorbent in a postcombustion capture process shown in Figure 2a in a power plant, it is estimated that the total amount of fresh makeup limestone may exceed that of coal. This will bring significant operating challenges to the plant. From the ́ simulation results, Martinez et al.32 reported that the lower fresh limestone makeup required, the higher the net thermal efficiency of the system, although high CO2 capture efficiencies could be sustained with higher solid recirculation rates between reactors.32,33 Therefore, it is critically important to solve this problem so that the amount of makeup sorbent can be reduced. Currently, a large portion of the effort in the research area of calcium looping has focused on this problem, trying to find a good solution. The capacity decay of CO2 capture of CaO over cycles has been mainly attributed to the sintering of the CaCO3 during the regeneration process. The mechanism is believed to be as follows. The CaO particle is comprised of microporous grains of various sizes, which, in turn, is comprised of face-centeredcubic crystals. During carbonation of CaO, CO2 is transported through the spaces between the grains and then through the spaces between the crystals before it reaches the crystals and converts CaO crystals into CaCO3 crystals. When the CaCO3 crystals is regenerated subsequently at high temperature, which is normally higher than their sintering temperature (or Tammann temperature, ∼527 °C),20,22,34 sintering occurs severely. The sintering leads to the aggregation of calcium crystals/grains, which results in reduced surface area of the produced calcium oxide (i.e., the spaces between the crystals) available for the carbonation reaction in the next cycle. Thus, the rate, as well as the extent of the gas−solid reaction, decays significantly after each cycle of carbonation/calcination. In the past few years, a great portion of the work on calcium looping has focused on overcoming the loss-in-capacity problem of CaO. According to the above mechanism, there are possibly two strategies: (i) breaking the sintered crystals again back into smaller crystals and therefore recovering the surface area (and performance) of natural minerals after sintering; and (ii) synthesizing sintering-resistant sorbents by separating calcium crystals/particles with inert particles to prevent sintering. Literature results show that both strategies would work; however, the first strategy would not be able to meet some other requirements on sorbent physical properties, while the second strategy would be able to meet all the requirements for practical applications. The objective of this work is to identify potential solutions to the problem by systematically evaluating the latest results in the literature. The
The application of CLP in steam methane reforming is quite similar to the HyPr-RING system for coal gasification, except that the fuel used is now natural gas (consisting primarily of methane). As shown in Figure 2d, the process integrates steam methane reforming, water−gas shift reaction, and CO2 capture in a single reactor (reformer) and the sorbent is regenerated in a regenerator. It has been shown that high purity H2 of 95− 98% (dry basis) can be produced in the process.19 The efficiency of the process is also expected to be higher than the conventional reforming process. (c). Thermal Energy Storage. The thermal energy storage systems have come under close scrutiny, and the reversible chemical reactions have attracted much interest, owing to the higher energy storage density in comparison to latent or sensible heat and less strict insulation required through storing it as chemical potential energy. The reversible reactions should include a forward endothermic reaction to absorb heat from the heat source and a reverse exothermic reaction to release the heat. Many chemical cycles as heat storage systems have been proposed on the basis of this concept, and the reversible reaction of CaO/CO2, which can store thermal energy at a high temperature level of around 700−950 °C from high temperature heat sources such as concentrated solar energy and nuclear power plant,24 has attracted attention.20−23 The feasibility of a chemical heat pump using heat storage and heat output operations of the CaO/CO2 reaction has been evaluated.25−28 A chemical pump system using CaO/PbO/CO2 is shown in Figure 3. In heat storage mode, CaCO3 is
Figure 3. A schematic description of process of CaO/PbO/CO2 energy storage system. (Adapted with permission from Kyaw.24 Copyright 1997 Elsevier.)
decomposed and the reaction energy is supplied by low temperature and low pressure environment (or low temperature thermal energy is stored in CaO). In the meantime, CO2 is stored in PbO (forming PbCO3). Subsequently, in heat output mode, CO2 is released from PbCO3 and reacts with CaO to release reaction heat to a high temperature environment. In this way energy is “pumped” from low temperature to high temperature. Apparently, the properties of CaO and PbO are important for the performance of the system. In the above CLP-based processes, CO2 sorbent is a key component, and the sorbent is expected to be capable of being utilized for multiple carbonation/calcination cycles under the conditions relevant to the applications. To be practically usable, a CO2 sorbent must possess the properties of fast kinetics, 2753
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Figure 4. Problem of loss-in-capacity of CaO sorbent with the number of cycle of carbonation/calcination (typical results in the literature, operating at atmospheric pressure except otherwise stated).
chambers and the gas after/during gasification all naturally contain steam. Therefore, the understanding of steam effect on CaO carbonation will be helpful for the design of the systems. In the calcination stage, steam can be introduced into the calciner to mix with original CO2 stream as a carrier gas. Water/ steam could be also introduced as separate hydration treatment before subjecting to carbonation/calcination cycles, after carbonation, or after calcination, as an additional step. The effectiveness of different hydration methods is discussed below. 2.1.1. Hydration during Carbonation. Steam is one of the factors that can affect calcium conversion during the carbonation reaction of calcium oxide. Although in many practical systems the gas atmosphere naturally contains steam, many studies on CaO−CO2 reaction did not consider the presence of steam mainly because of experimental limitation. The experimental results showed that hydration had positive effects on carbonation, with lime/limestone,15−17,36−42 dolomite,35 or synthetic sorbents36 as the raw materials, when being tested in CO2 (N2/air balance),17,35−40,42 simulated syngas,38,39 simulated flue gas38−40 or gas produced by coal gasification.15−17 The major features of steam effect on CaO carbonation reaction are presented in this section. First, the effect is temporary. Although a sudden increase of the conversion rate was observed when 50% N2 was switched to 50% steam to dilute CO2, after the conversion had reached the diffusion control stage, the carbonation rate almost dropped back to its original value when the gas environment was changed back to N2.35 These temporary effects were also found in air diluted CO2 and simulated syngas.38 Second, the steam effect was more apparent at lower temperatures. The conversions were generally reported to be increased after steam addition, particularly at lower temperature. The high carbonation conversions (up to 40%) were reported in the temperature range 250 °C ≤ T ≤ 400 °C, while no carbonation would occur without the presence of H2O at these temperatures.40 However, these increases were less apparent at higher temperatures. For example, at the carbonation temperature of 800 °C, the conversions of sorbents calcined in N2 with or without the presence of steam were very close.36 Interestingly, at the temperature of interest for carbonation (600 °C), the conversion was reported to be more than doubled when 8% or 15% steam was added.40
results are summarized in two sections corresponding to the two strategies, that is, enhancing the performance of natural minerals and synthesizing sintering resistant sorbents. To put our focus on the enhancement of CO2 capture of sorbents, this review ignores the effect of SO2 presence or sorbent sulfation.
2. ENHANCING PERFORMANCE OF NATURAL MINERALS Many groups have investigated the possibility of improving the performance of natural calcium-based minerals. Currently, two methods have been reported: hydration using liquid water or steam and thermal pretreatment of natural minerals. 2.1. Hydration Treatment. Depending on the stage at which water/steam is introduced, the hydration method can be further divided into different approaches, as shown in Figure 5: during carbonation;15−17,35−42 during calcination;37,41,43−48 and separate hydration treatment on CaO/CaCO3.37,49−54 It is worth noting that, in many practical systems, during the carbonation process, steam is an existing component of the gas to be treated. For example, the flue gas from combustion
Figure 5. Options of water/steam addition at different stages in a typical carbonation and calcination cycle. 2754
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reduction of CaO carbonation conversion is known to be CaO/ CaCO3 sintering during the regeneration process of CaO by calcination at high temperature. It is generally accepted that decreasing the calcination temperature will diminish the effect of sintering,55,56 while the lower the partial pressure of CO2 in the sweep gas, the lower the decomposition temperature of CaCO3.56,57 One effective way to decrease the partial pressure of CO2 is diluting CO2 with steam.39 Recent studies41,46,47 have provided evidence in support of the positive effect of steam dilution. The decomposition temperature becomes lower when steam dilution percentage in the sweep gas increases, shown in Figure 6a. For example, in order to achieve nearly complete
Third, steam has positive effect on carbonation with or without hydration of CaO. The steam partial pressure can be controlled to ensure the forward reaction dominates in the equilibrium system of CaO + H2O ⇔ Ca(OH)2. Therefore, Ca(OH)2 exists in the process for reacting with CO2, the sorption capacity of which is superior to CaO. However, significant improvements were also reported with the addition of steam, while ensuing steam partial pressure is lower than the equilibrium partial pressure to eliminate the possibility of Ca(OH)2 production in the tests.42 The explanations for the positive effect of steam on carbonation are controversial. Basically, there are two theories: catalytic effect of H2O by forming Ca(OH)2 during the kinetic controlled stage and CaO product layer break-up mainly during the diffusion controlled stage. It is known that the CaO carbonation involves two stages: (i) a chemical reaction control stage and (ii) a diffusion control stage. Steam may be expected to enhance the carbonation during these two stages by different mechanisms. During the chemical reaction control stage, the reaction was dominated by the chemical reaction of CaO and CO2. It was proposed that the carbonation improvements in this stage was a result of the formation of transient Ca(OH) 2 as an intermediate, the carbonation of which is thermodynamically favored as compared to that of CaO.38,39,41 When temperature is sufficiently high, a stable Ca(OH)2 compound does not exist, according to equilibria of Ca(OH)2 over water H2O, in which case a part of Ca(OH)2 was still assumed to exist for a short period to react with CO2 to form CaCO3 before decomposing back to CaO.40 On the other hand, the effect of steam addition during carbonation is not pronounced when considering the fast stage only.37 In addition, the steam can still greatly improve the carbonation even when the steam partial pressures at a certain temperature were lower than the corresponding thermodynamic equilibrium partial pressures of steam thus the formation of Ca(OH)2 was not favored.40,42 In this circumstance, the hydration conversion was much less than CaO carbonation conversion, and therefore, the improvement should not be mainly contributed by the formed Ca(OH)2. The steam catalysis hypothesis was also questionable for two reasons.36 First, the formation of Ca(OH)2 contributes to the fast step of the sorbent carbonation reaction while the overall reaction is limited by the diffusion control step. Second, the activation energy increases in the present of H2O, which was found by Symonds et al.,39 while the activation energy was supposed to be reduced to speed up the reaction if H2O is a catalyst. Therefore, it is postulated that the enhancement is mainly a result of product layer break-up during the diffusion control stage.36 2.1.2. Hydration during Calcination. During calcination reaction, CO2 is released and transferred by the carrier gas. Pure CO2 can be used as a carrier gas in the calcination process in order to obtain a purified stream of CO2. Steam can also be used to dilute the CO2 stream because the mixture can be readily separated through steam condensation after the decomposition process. Nearly pure CO2 can be obtained ready for subsequent compression and storage. On the other hand, steam also has positive effect on the decomposition rate, hence enhancing the subsequent carbonation conversion of CaO. First, the introduction of steam in the decomposition stage enhances the rate of decomposition. The major cause for the
Figure 6. (a) Effect of temperature on the decomposition conversion of limestone in various atmospheres; (b) effect of dilution gas percentage on the decomposition conversion of limestone.41.
conversion of CaCO3 to CaO within 40 min, the decomposition temperature has to be as high as ∼1027 °C in a pure CO2 atmosphere, while at a constant steam dilution of 60%, this temperature can be lowered to 920 °C. When the sweep gas is pure steam, the decomposition temperature is further reduced to only 850 °C. When the temperature and calcination time is similar (for example 920 °C for 40 min), steam dilution has the ability to achieve higher conversion of CaCO3 to CaO, shown in Figure 6b. The figure shows that higher decomposition conversion is obtained when CO2 partial pressure is reduced by either steam or N2. However, steam dilution leads to a better conversion than N2 dilution at all percentages investigated. The difference between the two profiles was the largest at a percentage of 20%, 2755
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Although decomposition of CaCO3 in a steam dilution atmosphere can regenerate CaO with better carbonation reactivity than that produced in pure CO2, and steam and CO2 can also be readily separated through steam condensation, it has to be noted that the decomposition process may become less energy effective as a result of the need of steam.43 It is thus recommended that an optimum ratio of CO2 to steam in the sweep gas be identified in future studies. 2.1.3. Separate Hydration Treatment on CaO/CaCO3. Hydration treatment of CaO before subjecting to carbonation/ calcination cycles can improve its performance, owing to higher pore area and pore volume from the cracks formed during steam prehydration of CaO.56 The pretreated CaO sorbent had a high conversion of 52% at the 20th carbonation and calcination cycle.56 The effectiveness of prehydration can be further enhanced by mixing other solvent such as ethanol to enhance H2O molecule affinity to penetrate CaO more easily, resulting larger pores after calcination, and hence better cyclic performance in capturing CO2 was achieved.58 In addition, the steam reactivation step could be separately applied on CaO/ CaCO3 after carbonation reaction,37,51,52,59,61 or after calcination reaction.36,49,50,53,54,56,59−61 The effectiveness of steam reactivation after carbonation reaction is limited. The carbonation reaction of CaO occurs from outer surface to inner core, and a calcium product layer can be formed during the reaction. Therefore, the CaCO3 product layer after carbonation makes the hydration difficult, which in turn limits the positive effect of hydration on the multicycle performance of reactivated sorbent.37 However, if the CaCO3 product layer is thin (because of low level of carbonation), Manovic et al.61 claimed that the product layer is not the major reason to hinder the hydration. While the positive effects of hydration on sorbent after carbonation are controversial, the successful reactivation of sorbent is generally agreed when steam is applied on calcined sorbent.37,53,54,59,62 Both low-temperature steam and liquid water were able to reactivate the calcined and/or extremely sintered CaO sorbents, and 10 min liquid water worked the best, which reactivated the calcined sorbents after 15 cycles back to its original carbonation conversion with even better reactivity.37 In contrast, Han et al.49 found that steam hydration was slightly better than liquid hydration. The improvement may be due to regeneration of the small pores in the sorbent structure because decrease of CO2 capture ability is mainly due to the loss of these small pores.19 The possible explanation for the increase of pore spaces and surface area was the lower density of hydrated lime in comparison to lime and limestone.54 The steam also had positive effect on reactivation of the spent sorbent collected from calciner in a 75 kWth pilot-scale
and the conversions became closer gradually when CO2 partial pressure was further reduced, which was probably because steam dilution and N2 dilution perform similarly in heat transfer as a result of the combined effect of lower density and higher thermal conductivity of steam.41 Second, steam dilution during calcination improves subsequent carbonation reactivity of the produced CaO. As discussed above, steam dilution reduces the partial pressure of CO2 and hence enlarges the difference between CO2 equilibrium pressure and CO2 partial pressure, resulting in faster decomposition rate and thus shortening the residence time of CaCO3 in the calcinator.41,47 While lower decomposition temperature will effectively reduce the effect of sintering, shorter residence time of CaCO3 in the calcinator will also inhibit sintering. As a result, the CaO regenerated with steam dilution exhibits better carbonation reactivity due to the lessening of the sintering effect.41,46,47 This has been supported by Wang and his group,47 who conducted carbonation reactivity experiments of the CaO produced in various steam dilution atmospheres (850 and 920 °C) and in 100% CO2 atmosphere (1020 °C) in a TGA with a carbonation temperature of 650 °C and a CO2 partial pressure of 0.1 MPa. The results are shown in Figure 7. The figure shows that
Figure 7. Comparison of carbonation reactivity of the CaO produced by limestone decomposition with various steam dilution percentages.47
higher carbonation conversion is achieved for the CaO regenerated in an atmosphere with higher steam dilution. For example, the CaO produced in the absence of steam reaches a final conversion of ∼35%, while the conversion is above 60% for the CaO produced in pure steam.
Table 1. Summary of the Experimental Conditions for the Representative Results of Hydration Reactivation of Sorbent after Each Calcination during the Cyclic Test ref Kuramoto et Kuramoto et Kuramoto et Kuramoto et Kuramoto et Zeman54 Han et al.49 Han et al.49
sample al.50 al.50 al.50 al.50 al.50
calcined calcined calcined calcined calcined calcined calcined calcined
Ca(OH)2 CaCO3 CaCO3 CaCO3 CaCO3 CaCO3 CaCO3 CaCO3
hydration condition
carbonation condition
liquid distilled water, 0.1 MPa 500 °C, 10 min, steam in N2, 6 MPa 500 °C,10 min, steam in N2, 6 MPa liquid distilled water 500 °C, 10 min, steam in N2, 9 MPa 300 °C, 5 min, 1 bar CO2, excess steam sufficient distilled water, 0.1 MPa 300 °C, 10 min, steam in N2, 2 MPa 2756
500 600 700 700 700 780 750 750
°C, 25 min, 20% CO2 in N2, 0.1 MPa °C, 25 min, 20% CO2 in N2, 6 MPa °C, 25 min, 20% CO2 in N2, 6 MPa °C, 25 min, 20% CO2 in N2, 6 MPa °C, 25 min, 20% CO2 in N2, 9 MPa °C, 100% CO2, 0.1 MPa °C, 20 min, 25% CO2 in N2, 2 MPa °C, 20 min, 25% CO2 in N2, 2 MPa
calcination condition 900 900 900 900 900 960 950 950
°C, °C, °C, °C, °C, °C, °C, °C,
10 min, N2, 0.1 MPa 10 min, N2, 0.1 MPa 10 min, N2, 0.1 MPa 10 min, N2, 0.1 MPa 10 min, N2, 0.1 MPa 100% CO2, 0.1 MPa 15% CO2 in N2, 2 MPa 15% CO2 in N2, 2 MPa
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atmospheric dual fluidized bed combustion system. The recovered surface area and the increased pore volumes of 50 nm, as well as the large cracks also indicated the promise of using in situ steam for the reactivation of sorbents in an atmospheric FBC.59 Arias et al.63 proposed a process with partial hydration (20 and 60% of calcined sorbents) and reported an improved average activity. The intermediate hydration can be integrated with the hydration step after each calcination during the cycles to improve the reactivity.49,50,54 The higher frequency of hydration after calcination can achieve better performance;49 however, higher hydration frequency means higher energy requirement. Therefore, periodic hydration after calcination is one option to save energy requirement, and Manovic and Anthony105 have proved that periodic hydration after calcination (after 30, 90, 120, 240, and 300 cycles) reactivated the samples significantly and the conversion was maintained as high as 42% at 300 cycles. Materić et al.62 also confirmed the effectiveness of repeated hydration every 18 cycles. The results of steam reactivation of sorbent after each calcination step from different researchers49,50,54 are summarized in Table 1 and Figure 8. The figure shows that the performance varies largely with experimental conditions indicating the need of condition optimization.
capacities, higher capacities can be observed after many cycles.64 Thus, the greater average capacity of the sorbent over many cycles is expected. In contrast, Manovic et al.65 reported that thermal pretreatment did not work for highpurity limestone (La Blanca) even after trying different pretreatment conditions; thus, it was believed that thermal pretreatment only has beneficial effect on some natural sorbents. In addition, the self-reactivation behavior is very susceptible to preheating conditions when investigating the thermal pretreatment on nano-sized CaCO3.67 2.3. Summary of Natural Minerals Performance Enhancement. There have been sufficient evidence showing that the presence of steam/water has positive effect on the performance of natural minerals. Steam can enhance the carbonation reaction in particular in the diffusion control stage, possibly due to the break-up of product layer by steam or the catalytic effect of steam on carbonation. Steam can increase the decomposition rate of CaCO3 and enhance subsequent carbonation conversion of calcined CaCO3, probably due to the combined effects of reduced CO2 partial pressure, reduced decomposition temperature and thus less sintering, and higher heat conductivity of steam. Addition of steam during or after calcination appears to be the most effective in recovering the capacity of natural minerals. Thermal pretreatment may increase the overall conversion of calcined limestone, but its effectiveness also depends on the original limestone.
3. SYNTHESIS OF SINTERING-RESISTANT SORBENTS Synthesis of sintering-resistant calcium based sorbent is another strategy to overcome the loss-in-capacity problem. A range of synthesis methods have been tested with varying degrees of success so far, such as (i) the utilization of potentially sinteringresistant calcium precursors, (ii) doping with metal salts, and (iii) the dispersion of CaO particles into inert matrix. These are discussed separately as below. 3.1. Utilization of Sintering-Resistant Calcium Precursors. The screening of potential calcium precursors is very important, not only because it is possible to produce sinteringresistant sorbents directly from some calcium precursors but also because it can provide criteria for selecting the precursors for synthesis because the reactivity of CaO produced from these precursors directly affects the cyclic performance of synthesized sorbents. Limestone (CaCO3), lime (CaO), or hydrated lime (Ca(OH)2) have been the preferred precursors to be employed for capturing CO2 because of low costs and abundant availability. Although some precursors such as calcium nitrate were not considered to be suitable for producing CaO because of zero CO2 uptake, as the precursor melted upon calcination,34 there are also a number of other potential calcium precursors, and three most promising groups are nano-sized CaO/CaCO3, organometallic precursors, and precipitated CaCO3 (PCC). The performances of calcium oxide sorbents produced from these precursors are discussed below. 3.1.1. Nano-sized CaO/CaCO3. The group of calcium precursors that attracted a lot of attention is nano-sized CaO/CaCO3. As discussed, the conversion−time curves usually show a two-stage behavior. It is suggested that if the particle size is sufficiently small, no diffusion control stage exists because the reaction completes within chemical control stage.23,68 It is necessary to define the boundary conditions of nano-sized material first. Assuming spherical particles and calculating the thickness of formed product layer under
Figure 8. Steam reactivation of sorbent after each calcination step from different researchers.
Despite of the potential advantage of steam addition in the calcium looping system, it is worth noting that the additional processes for generating steam may even raise the energy requirement and cost for regeneration, as indicated by simulations.43 It should also be noted that the large cracks and pores due to hydration also result in fragile particles, which may hinder the use of the method in real systems.62 2.2. Thermal Pretreatment. Thermal pretreatment of the sorbent at high temperatures showed beneficial effects on the cyclic conversion of CaO,53,64−66 and the conversion could even increase with the number of cycles; this phenomenon has been called self-reactivation.53,67 A pore-skeleton model can explain the phenomena associated with the results.53 Using the model, it was proposed that preheating stabilizes the structure and form hard skeleton as a result of continuous ion diffusion during calcination after the complete decomposition of CaCO3. With pretreatment at 1000 °C, the conversion could achieve up to 50% after 30 cycles.53 Thermal pretreated sorbents also had better mechanical properties.53 Although the pretreated sorbents under severe conditions exhibit lower initial sorption 2757
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Table 2. Summary of the Precursors and Test Conditions for CaO from Different Precursors ref
precursors
reactor
carbonation conditions
calcination conditions
Barker22
conventional microsized CaCO3
TGA
Barker23 Martavaltzi and Lemonidou76 Martavaltzi and Lemonidou76 Liu et al.73 Liu et al.73 Liu et al.73 Liu et al.73 Liu et al.73 Liu et al.73 Liu et al.73 Liu et al.73 Liu et al.73 Lu et al.34
CaCO3
