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Steam Hydration Reactivation of CaO-Based Sorbent in Cyclic CarbonationCalcination for CO2 Capture Nai Rong, Qinhui Wang, Mengxiang Fang, Leming Cheng, Zhongyang Luo, and Kefa Cen Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/ef4007214 • Publication Date (Web): 19 Aug 2013 Downloaded from http://pubs.acs.org on August 20, 2013
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Steam Hydration Reactivation of CaO-Based Sorbent in Cyclic Carbonation/Calcination for CO2Capture Nai Rong, Qinhui Wang*, Mengxiang Fang, Leming Cheng, Zhongyang Luo and Kefa Cen State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou 310027, People’s Republic of China KEYWORDS: Calcium looping; CO2 capture; Ca-based sorbent; Steam hydration ABSTRACT: The calcium looping process is one of the most promising approaches for CO2 capture, which is based on the cyclic carbonation/calcination reactions of Ca-based sorbent. However, the sorbent suffers from an unavoidable deactivation of CO2 capture capacity and durability during cyclic CO2 capture process. Separate steam hydration after calcination is valid to regenerate the reactivity of spent sorbent. In this study, the effects of hydration temperature, steam concentration and hydration frequency on the sorbent reactivity during 10 carbonation/calcination cycles were investigated using a pressurized thermogravimetric analyzer with reagent grade CaCO3 used as a precursor under atmospheric pressure. The morphology changes of spent sorbent after calcination under various conditions were observed by a scanning electron microscope. The results revealed that the reactivity and durability of spent sorbent is significantly recovered by separate hydration after calcination. In addition, the enhancement was more pronounced at lower hydration temperature and higher steam concentration. Separate hydration after every calcination performed far better than the low frequency hydration with hydrated just once or every 3 cycles. Comparing with other steam reactivation strategies, such as the steam addition during carbonation and calcination process, separate steam hydration after calcination has shown excellent reactivation performance. 1
INTRODUCTION
CO2 emission from conventional fossil fuel utilization process accounts for the major source of anthropogenic CO2. With growing fossil fuel energy demand, increasing atmospheric concentration of CO2 and tightening CO2 mitigation policy, considerable attention has been focused on the combination of high-efficiency energy conversion and promising CO2 capture and storage (CCS) techniques in recent decades1. However, the existing commercialized and demonstrated CCS technologies are high energy-consumption, investment and construction cost. Consequently, developing relative low-cost CO2 capture techniques is imperative for large scale deployments of CCS2. Comparing with other CCS techniques, such as oxy-fuel combustion and MEA scrubbing, promising calcium looping process (CLP) is more attractive due to its wide operating temperature range, high capture efficiency and low sorbent cost3-5. The CLP concept, which 1 ACS Paragon Plus Environment
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combines capture and concentration of CO2, is conducted via the reversible carbonationcalcination reaction CaO(s) + CO2(g) = CaCO3(s) The CO2 contained in combustion flue gas or gasification syngas is in-situ captured by CaO in the forward carbonation step. The carbonation product CaCO3 is calcined to release CO2 for sequestration in the backward calcination process. Several novel energy utilization systems based on CLP concept have been proposed, such as CO2 acceptor gasification6, GE fuel-flexible process7, NEDO HyPr-RING8, Ohio State Univ. CLP9 and Near Zero Emission Coal Utilization System in our previous work10. Nevertheless, dramatically inevitable decay of sorbent CO2 capture capacity and durability generally occurs during the cyclic carbonation-calcination process, which poses a technical challenge to the development of these systems. The most thoroughly investigated problem is the loss of the sorbent reactivity with increasing number of CO2 capture cycles, which is typically caused by sintering accompanied with the decrease of the sorbent surface area and the loss of small pores necessary for the storage of the produced CaCO311-14. To regenerate the sorbent capacity for cyclic CO2 capture, various reactivation approaches have been investigated. These approaches include steam/water hydration or addition, thermal pretreatment, using innovative sorbent precursors, doping and incorporating with inert matrix. Aforementioned reactivation schemes have been well reviewed by Yu15 and Liu16. Due to the superior availability of steam and convenience of hydration or steam addition, steam reactivation of spent Ca-based sorbent has been extensively investigated. Generally, steam is applied in three different approaches: steam addition during carbonation12, 17-23 ， steam addition during calcination24-28 and separate hydration after calcination13, 19 , 29-31. Steam addition during carbonation enhances the solid state diffusion in the slow diffusion-control reaction regime17,18, thus it improves the carbonation conversion. The presence of steam during carbonation has remarkable effects on the reactivation of severe sintered sorbent. However, for the calcium sorbent which is not seriously sintered, the fast kinetic control stage of the sorbent generally finished at a conversion close to the “conversion maximum” ( namely, carbonation almost completes during the kinetic control stage). Then the presence of steam during carbonation shows negligible enhancement18. Steam addition during calcination has also been proved capable of enhancing the subsequent carbonation conversion24, 25, 27, 28. After calcination, steam could be easily separated by condensation and a concentrated CO2 stream ready for sequestration can be obtained. On the contrary, previous work also indicates that steam might promote the sintering of sorbent during calcination process26. Due to the conflicting roles of steam plays no clear consensus has been reached on its effect on calcium sorbent reactivation during calcination so far. Separate hydration after calcination under low temperature, with excellent reactivation performance, has already been confirmed as a promising approach to regenerate the capture capacity and pore structure of spent CaO-based sorbent24, 29, 30. Surface area and porosity of sorbent was remarkably recovered by hydration-dehydration process, i.e. CaO→Ca(OH)2→CaO. A key point of separate hydration is that the precursor of sorbent changes from CaCO3 to Ca(OH)2. Enhanced cyclic CO2 capture capacity of hydroxide-derived CaO, comparing with carbonate-derived CaO, has been reported12. Wu et al.13 hydrated (steam hydration, 130 °C, for 5min，atmospheric pressure) four various kinds of spent natural limestone sorbent undergoing 10 cycles (carbonation at 650 °C for 5 min, calcination at 900 °C for 5 min, reaction atmosphere: 15vol% CO2, N2 balance). The reactivity of spent sorbents was recovered from 20% to 65-80% 2 ACS Paragon Plus Environment
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after hydration, then the sorbent rapidly deactivated in subsequent cycles. Similar hydration performance could also be found in the study by Manovic and Anthony29. The conversion of synthesis sorbent was enhanced by around 50% via hydration (hydrated after 30, 120, 143 cycles, 100 °C saturated steam, 5 min). Materić et al.30 reported that the sorbent reactivity is thoroughly regenerated to ~100% by repeated steam hydration ( hydrate every 18 cycles, 110 °C, 1 hr). Even humid air was also proved capable of regenerating the CO2 capture capacity of spent sorbent31. Nevertheless, the effect of separate steam hydration was just temporary for multiple CO2 capture cycles. The sorbent reactivity would still decrease rapidly with increasing number of cycles during the subsequent cycles after hydration13, 29. From the practical point of view, durability of sorbent is much more important than the single cycle reactivity. Therefore, more explorations should be dedicated to maintain the durability of CaO-based sorbent for cyclic carbonation/calcination. The primary objective of this work is to investigate the influence of separate steam hydration after calcination on the CaO-based sorbent cyclic carbonation/calcination performance and durability, by focusing on the effects of hydration temperature, steam concentration and hydration frequency. The sorbent carbonation conversion of each cycle was measured by a modified thermogravimetric analyzer (TGA) and the typical surface morphology of spent sorbent was observed. Moreover, the reactivation performances of different steam reactivation strategies were also compared to examine the feasibility and practical operability. 2
EXPERIMENTAL 2.1
Samples
Reagent grade CaCO3 (≥99% purity from manufacturer’s specs) was used as precursor for this study. The XRF elemental analysis of the sorbent is given in Table.1. The particles size of uncalcined CaCO3 was about 20-60 µm according to the analysis report from MALVERN MASTERSIZER2000 particle size analyzer. The CaO sorbent used in the cyclic experiments was prepared by calcining the reagent grade CaCO3 at 900 °C for 10 min in N2 atmosphere. To prevent the freshly-calcined CaO exposing in the air and absorbing the moisture, the cyclic carbonation/calcination was immediately performed after the calcination of precursor CaCO3 without moving out the sorbent from TGA furnace. Table1. Sorbent XRF elemental analysis (wt%)
Sorbent Reagent Grade CaCO3
2.2
CaO 55.49
MgO 0.27
Na2O 0.11
SiO2 0.03
Al2O3 0.04
SrO 0.01
Loss on fusion 43.96
Others 0.09
Apparatus
Cyclic carbonation/calcination CO2 capture experiments were conducted using a ThermoCahn TherMax 500 pressurized thermogravimetric analyzer (PTGA) manufactured by ThermoFisher. The PTGA comprises gas metering system, sensitive balance, electrically heated furnace, pressure control system and data acquisition system. The maximum weighting capacity is 100 grams with the sensitivity of 1 µg. The maximum pressure is up to 7 MPa at temperatures below 1000 °C. The sample zone can be heated up to 1100 °C at the heating rates of 0-25 °C/min under 3 ACS Paragon Plus Environment
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atmospheric pressure. Sample is loaded in a quartz crucible with 15mm diameter and 4mm height. The reaction temperature, pressure and sample weight signals are monitored and recorded online continuously by a data acquisition software. Fig 1 shows a schematic diagram of the experimental apparatus and the gas flow routes. Purge gas, furnace gas, and two reaction gases are controlled by mass flow controllers and the system pressure is controlled by a back pressure regulator (BPR). Steam is generated using a steam generator and the deionized water flow rates are regulated and measured by a high pressure liquid chromatography (HPLC) pump. The entire steam transfer line is heat-traced to about 200 °C to avoid steam condensation in the pipe. Likewise, the reaction gas is also preheated to about 200 °C before being mixed with steam.
Figure 1. Schematic diagram of the thermogravimetric analyzer
2.3
Procedure
Four various patterns of cyclic CO2 capture processes, namely, cycles without hydration, separate hydration after calcination, steam addition during carbonation, and steam dilution during calcination were carried out. All samples were prepared by calcining reagent grade CaCO3 at 900 °C for 10 min under N2 atmosphere. For the cyclic carbonation/calcination without hydration, the temperature program started with the preparation of CaO through calcination at 900 °C under N2 for 10 min. Then, the furnace was naturally cooled down to 650 °C and kept constant for 20 min for the carbonation process. The average cooling rate of the furnace is about 10 °C /min. Gas stream was switched to 15vol% CO2 (N2 balance) during carbonation,. Then the furnace was reheated to 900 °C at the heating rate of 25 °C/min. The CaO sorbent was calcined for 10 min at 900 °C in N2 atmosphere . And then the furnace was cooled down to 650 °C again, this carbonation/calcination process was repeated up to 10 cycles. As for CO2 capture cycles with the presence of steam during carbonation or calcination, the only difference compared with those without hydration was the steam addition during every carbonation or calcination reaction stage. For cycles with steam addition during carbonation, carbonation atmosphere was a mixture of 20vol% or 40vol% steam, 15vol% CO2 4 ACS Paragon Plus Environment
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and balanced with N2, while calcination atmosphere was still N2. Similarly, for cycles with steam addition during calcination, carbonation was conducted in 15vol% CO2 (N2 balance), and calcination was carried out in an atmosphere of 20vol% or 40vol% steam with N2 balance. For cycles with separate hydration after calcination, hydration was performed for 20 min using a mixture of steam and N2 at different temperatures of 200, 300, 350, 400 and 500 °C. After separate hydration, sample was heated to 650 °C under N2 at 25 °C/min for carbonation (15vol% CO2, N2 balance, 25min), then reheated to 900 °C for 10 min calcination under N2. Afterwards, sample was naturally cooled to specific temperature under N2 for hydration once again. It is defined that the first separate hydration was the hydration before the first carbonation of CaO sample in 10 repeated CO2 capture cycles. To investigate the influence of separate hydration frequency on the sorbent cyclic performance, three different frequencies were adopted, i.e. hydration during each cycle, hydration every three cycles and hydration only once before the first carbonation. According to preliminary tests, about 10 mg CaO and total gas flow rate of 1.5 SLM were chosen to avoid gas external diffusion effect. CaO-based sorbent carbonation conversion was defined as below: %
100
where XN is the carbonation conversion of Nth cycle, mN is the mass of the sample after Nth carbonation, m0 is the original calcined CaO-based sample mass, P is the CaO content in the original sorbent and M is mole mass. 2.4
Morphology
The sorbent morphology was observed using a FEI SIRON200 field-emission scanning electron microscope (SEM) with 25KV accelerating voltage. Before each SEM examination, the sample was gold-coated in order to improve the electronic signal and obtain better image quality. 3
RESULTS & DISCUSSION 3.1
Effect of separate steam hydration temperature
Cyclic carbonation/calcination performances of cycles with separate hydration for every cycle at different temperatures are plotted in Fig 2. The conversion curve of cycles without any hydration is also presented for comparison. The carbonation conversion drops rapidly from 73.8% in the 1st cycle to 26.7% in the 10th cycle. It can be seen that the conversion decay trend of cyclic carbonation/calcination without hydration almost overlaps with the reference curve derived from the empirical equation proposed by Grasa and Abanades14: (k=0.52,Xr=0.11)
The result also shows consistency with those of previous works11-14: with increasing number of cycles, the CO2 capture capacity of CaO-based sorbent deactivates significantly due to the decrease of surface area and the loss of porosity caused by sorbent sintering.
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Figure 2. Effect of hydration temperature on cyclic CO2 capture performance. Experimental conditions: carbonation at 650 °C for 25 min in 15vol% CO2 (N2 balance); calcination at 900 °C for 10 min in N2; hydration in 20vol% steam concentration (N2 balance) for 20 min at various temperatures.
It is evident that the sorbent reactivity was regenerated by the separate hydration after every calcination, which restrains the decay of the sorbent carbonation conversion. When hydrating sorbent at 300 °C, the decline of carbonation conversion stops after the 3rd cycle and stabilizes at around 51%. The conversion even increases slightly to 54% in 10th cycle, about 27% higher than that of the cycles without hydration. The similar trend can also be found in the cases of separate hydration at 350 °C and 400 °C, where conversion decreases remarkably in the initial 4 cycles, and then stabilizes at 40-50%. However, the conversions for the case at 500 °C are even lower than the cycles without hydration. The reason could be that hydration reaction is not favored at 500 °C under a steam partial pressure (PH O) of 20 kPa when the equilibrium temperature of hydration is 437 °C32. It is also interesting to find that the conversion of cycles conducted at 200 °C is slightly lower than that at 300 °C. This might be contributed to that the sorbent underwent longer residence time when the furnace cooled down from 900 °C to 200 °C than the case of cooling down from 900 °C to 300 °C. Longer residence time probably results in slightly severer sintering of the spent sorbent. In the separate hydration reactivation process, Ca(OH)2 was formed as the product of the hydration reaction between CaO sorbent and steam. The hydration conversion of CaO was about 98% calculated by the weight change, indicating that the majority of the spent sorbent was regenerated to hydroxide. The steam reactivation can be explained by the mechanism proposed by Ramachandran36 and Glasson34, 35. Glasson proposed the “advancing interface mechanism” by systematically investigating the CaO hydration with steam34 and liquid water35. The mechanism stated that hydration proceeds from the surface into the interior of the unhydrated CaO core. Crystalline Ca(OH)2 is formed at the sites previously occupied by CaO. The mole volume of Ca(OH)2 (~33 cm3/mol) is close to that of CaCO3 (~37 cm3/mol) and much larger than the mole volume of CaO (~17 cm3/mol)36, 37. When the hydration completes, the size of sorbent crystalline will be 1.98 times as large as the original one due to the larger mole volume of Ca(OH)234. 2
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Therefore, cracks will be formed on the hydrated sorbent surface. During the process of recrystallizing CaO to Ca(OH)2, the specific surface area of sorbent increases simultaneously34, which generates new pore structures and reaction surfaces. In the subsequent dehydration process, Ca(OH)2 decomposes to CaO again, with the reopening of micro pores and the exposure of CaO sintered during calcination. Ramachandran33 developed an analogous mechanism of steam hydration, by which the hydration takes place on the solid-vapor interface as a solid-state reaction. According to this mechanism, the expanding of sorbent particles is accompanied by crystalline texture recrystallization and pore structures reorganization during hydration process, which restrains the decrease of sorbent surface area and the loss of porosity38 and thus maintains the cyclic carbonation conversion of spent sorbent at a relatively high level. Fig 2 also reveals that steam reactivation performance is more pronounced at lower hydration temperature in the range of 200 °C - 500 °C. Undoubtedly, the sintering during the hydration stage was more severe at higher hydration temperature due to very low sintering temperature (as a reference, Tammann temperature is 280 °C under 1000 bars pressure39, 40, but in the case of atmospheric pressure, unfortunately, there are no literatures can be used for references) of Ca(OH)2. It’s very likely that the sintering of Ca(OH)2 takes place during the hydration treatment. As Ca(OH)2 is the precursor of CaO, its sintering should have some critical influence on the Ca(OH)2-derived CaO. Sintering of precursor can cause the loss of surface area and porosity of the hydroxide-derived CaO. As a result, CaO sorbent hydrated at high temperature could have lower carbonation conversion during cyclic process. It is interesting to observe that, after the initial decline, the carbonation conversion in the cases with separate hydration slightly increases with growing number of cycles. Kuramoto et al.40 also found the similar phenomenon in their repetitive calcination-hydration-carbonation (CHC) tests, where the carbonation conversion slightly increases by about 2~3% from the 3rd cycle to the 6th cycle (with high pressure liquid-phase hydration after every calcination). However, they didn’t give explanation to this phenomenon in the literature. Similarly, for sorbent pretreated by high temperature thermal reactivation, carbonation conversion also increased with growing number of cycles44, which was named as “self-reactivation”. Manovic and Anthony41 have proposed a poreskeleton model to explain the so-called self-reactivation phenomenon. During the cyclic carbonation/calcination, two kinds of mass transfer occur in parallel, i.e. bulk diffusion correlated to CaCO3 and ion diffusion of CaO crystal structure. As a result of the competition between bulk and ion diffusion, sorbent matrix structure evolves to two types of skeleton, i.e. inward hard skeleton and outward soft skeleton. The hard skeleton could stabilize and maintain the pore structure of the sorbent, while the outward soft skeleton could participate in the carbonation reaction. The outward soft skeleton of the sorbent that undergoes thermal pretreatment may grow with increasing number of cycles, which results in the enhanced cyclic performance of spent sorbent. Therefore, it is speculated that CaO sorbent experiences analogous skeleton transformation during hydration, similar to the thermal pretreated CaO. The growth of outward skeleton of spent CaO during hydration-dehydration process leads to the tiny enhancement of carbonation conversion with increasing number of cycles. Whether or not there is a maximum value for the tiny increase of carbonation conversion with separate hydration needs further exploration. 3.2
Effect of steam concentration
Fig 3 presents the influences of hydration steam concentration on the cyclic carbonation/calcination conversion with hydration at 300 °C. The results show clearly that CO2 7 ACS Paragon Plus Environment
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capture capacity increases with hydration steam concentration. Cycles with hydration conducted under higher steam concentration had better CO2 capture performance. After the initial rapid decay, the carbonation conversion of cycles with 80vol% steam separate hydration stabilizes at around 60%, which is about 10% higher than those with 20vol% steam hydration. Comparing with the case without steam hydration, separate hydration after every calcination shows positive effect on the reactivity and durability of CaO-based sorbent during CO2 capture cycles. The conversion of the 10th cycle with 80vol% steam hydration is about 37% higher than that of the cycles without hydration.
Figure 3. Effect of hydration steam concentration on cyclic CO2 capture performance. Experimental conditions: carbonation at 650 °C for 25 min in 15vol% CO2 (N2 balance); calcination at 900 °C for 10 min in N2; hydration for 20 min at 300 °C under various steam concentrations (N2 balance).
To further illuminate the reason for the different CO2 capture performances among the cases of various steam concentrations, carbonation conversion profiles of the 5th cycle with steam hydration under various steam concentrations and without steam hydration are displayed in Fig 4. For the sake of brevity, conversion profiles of other cycles are not shown here. As can be seen clearly, the carbonation consists of two typical reaction regime: the initial fast kinetic control stage where the process is controlled by the reaction between CaO and CO2, and the subsequent diffusion control stage where carbonation proceeds much slower and is governed by the diffusion of CO2 through the built-up compact product layer of calcium carbonate. The conversion curves show the typical shape that agrees with the previous works11, 17, 18, 42.
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Figure 4. Carbonation profile of the 5th cycle. Experimental conditions are the same to those in Fig 3.
Fig 4 also shows that the CO2 capture capacity of sorbent is remarkably regenerated after separate steam hydration. For the sorbent reactivated with 80vol% steam, the carbonation conversion at the end of kinetic-control regime is about 55%, higher than the conversion of 48% for the sorbent with 20vol% steam hydration. However, the conversion of fast kinetic-control reaction stage for the sorbent without hydration is only ~30%. The final conversion of the kinetic-control stage is influenced by the fraction of relatively small pores (
