A Novel Method for CO2 Sequestration via Indirect Carbonation of

Article pubs.acs.org/IECR

A Novel Method for CO2 Sequestration via Indirect Carbonation of Coal Fly Ash Lanlan He, Dunxi Yu,* Weizhi Lv, Jianqun Wu, and Minghou Xu* State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, 1037 Luoyu Road, Wuhan 430074, People’s Republic of China ABSTRACT: Coal fly ash is a potential candidate for CO2 mineral sequestration. If calcium is extracted selectively from coal fly ash prior to carbonation (namely indirect carbonation), a high-purity and marketable precipitated calcium carbonate (PCC) can be obtained. In the extraction process, recyclable ammonium salt (i.e., NH4Cl/NH4NO3/CH3COONH4) solution was used as a calcium extraction agent in this study. The influence of time, temperature, agent concentration, and solid-to-liquid ratio on calcium extraction efficiency was explored. NH4Cl/NH4NO3/CH3COONH4 are confirmed to be effective calcium extraction agents for the high-calcium coal fly ash investigated, and about 35−40% of the calcium is extracted into the solution within an hour. The calcium extraction performance is best for CH4COONH4, followed by NH4NO3 and NH4Cl. Increasing temperature from 25 to 90 °C and agent concentration from 0.5 to 3 mol/L only subtly increases calcium extraction efficiency for NH4Cl and NH4NO3, while the positive effect of increasing temperature and agent concentration is more obvious for CH3COONH4. In the carbonation process, carbonation efficiency, namely conversion of Ca2+ into precipitated calcium carbonate(PCC), is only 41− 47% when the leachate is carbonated by CO2. A newly proposed method of substituting CO2 with NH4HCO3 as the source of CO32− yields much higher carbonation efficiency (90−93%). Furthermore, the carbonation reaction rate is also largely improved when carbonating the leachate by NH4HCO3. In addition to these benefits, CO2 capture and storage can be simultaneously realized on-site if integrating the leachate carbonation process with an ammonia−water CO2 capture process using NH4HCO3 as a connector. In this way, the costs associated with CO2 compression and transportation can be eliminated. PCC with a purity up to 97−98% is obtained, which meets the purity requirement (≥97%) of industrially used PCC. It is estimated based on the experimental results that 0.17 tons of PCC can be produced from 1 ton of coal fly ash by this method, bounding 0.075 tons of CO2 at the same time, and 0.036 tons more CO2 can be avoided if the obtained PCC is substituted for the PCC manufactured by the conventional energy-intensive method.

1. INTRODUCTION CO2 mineral sequestration, namely bounding CO2 as stable Ca/Mg carbonates, is a leakage-free and environmentally friendly CO2 storage method compared to geological CO2 storage methods.1 This method is highlighted by its huge storage capacity and considered to be a long-term option for CO2 storage.2−4 The raw materials used for mineral sequestration can be natural Ca/Mg silicate ores or industrial alkaline residues.5 Nevertheless, the carbonation of Ca/Mg silicate ores under natural conditions is very slow and proceeds on a long geological time scale. In addition, measures to enhance the reaction kinetics are very energy-intensive and expensive.6 As an alternative option to Ca/Mg silicate ores, industrial alkaline residues have received increasing attention in recent years. The advantages of industrial alkaline residues include low price, availability near large points of CO2 emission, and higher reactivity toward CO2 due to their chemical instability.2−5 Furthermore, they themselves are waste flows that need to be handled or reused. Carbonation of alkaline residues can not only reduce CO2 emissions but also improve their properties for further utilization.7−11 Various types of alkaline residues have been investigated,3 including steelmaking slag,4,9,10,12−20 coal fly ash,21−29 red mud,30−32 waste concretes,33−36 municipal solid waste incineration residues,7,37,38 ultramafic mining waste,39 and so on. © 2013 American Chemical Society

There are three basic routes for alkaline residue carbonation, namely the direct dry/semidry route, direct aqueous route, and indirect route.3,40 For both direct dry/semidry and direct aqueous routes, the carbonated product is a valueless complex mixture that can only be disposed of in a landfill, or in better situations, can be used in construction sectors.9,40 The indirect carbonation route, on the other hand, is highlighted by its capability to produce value-added high-purity precipitated calcium carbonate (PCC) that has wide applications in various areas.40 For the indirect carbonation route, a suitable extraction agent is the most crucial factor determining if the process is economically viable.40 Among the various extraction agents investigated (hydrochloric acid,41 acetic acid,22,40 water,42 carbonic acid, 35 mixing organic acid, 18 ammonium salts,12−14,17,40 etc.), ammonium salts strikingly stand out for a balance between calcium extraction efficiency and selectivity.40 More importantly, they can be easily recovered by spontaneous reactions for reutilization.14,15 However, as the most promising calcium extraction agents, ammonium salts have only been investigated in the extraction of calcium from Received: Revised: Accepted: Published: 15138

July 23, 2013 September 25, 2013 September 27, 2013 September 27, 2013 dx.doi.org/10.1021/ie4023644 | Ind. Eng. Chem. Res. 2013, 52, 15138−15145

Industrial & Engineering Chemistry Research

Article

Table 1. Chemical Composition of Fresh Coal Fly Ash by XRF Analysis (wt %) CaO

SiO2

Al2O3

SO3

Fe2O3

MgO

Na2O

K2O

P2O5

30.47

20.64

12.65

21.29

7.1

5.0

1.04

1.03

0.78

steelmaking slag.12−14,17,40 Whether they are also effective for other alkaline residues, e.g., coal fly ash, needs further investigation. In this work, ammonium salts were used to extract calcium from a high-calcium coal fly ash for subsequent carbonation. Three ammonium salts were evaluated, i.e., NH4Cl, NH4NO3, and CH3COONH4. The influence of extraction time, temperature, extraction agent concentration, and solid-to-liquid ratio on calcium extraction efficiency was investigated. In the carbonation process, two methods for introducing CO32− into the leachate were explored: (1) the conventional one, namely introducing CO2 gas into the leachate,12−14,17,22,40 and (2) a newly proposed one, by adding NH4HCO3 into the leachate. The preliminary results show that adding NH4HCO3 largely enhances both the carbonation rate and efficiency. If the NH4HCO3 needed for calcium carbonation is provided by the ammonia−water CO2 capture process, CO2 capture and storage processes can be intergraded using NH4HCO3 as a connector. The proposed process is considered to be a prospective option for on-site CO2 mineral sequestration.

leachate was determined using complexmetric titration with standard disodium EDTA solution. To evaluate the calcium extraction ability of ammonium salt solution in a given time period, the calcium extraction efficiency is calculated with eq 1, where WCa‑ex (g) is the amount of calcium that can be extracted for time t (min) and WCa‑total (g) is the total amount of calcium contained in the ash sample used for calcium extraction. χe =

WCa ‐ ex × 100 (%) WCa ‐ total

(1)

CCa2+

WCa‑ex in eq 1 is expressed as eq 2, where (mg/L) is the concentration of Ca2+ in the leachate at time t (min), and v (mL) is the total volume of the leachate in the beginning. WCa ‐ ex =

CCa 2+ v × (g) 1000 1000

(2)

WCa‑total in eq 1 is expressed as eq 3, where CCa (wt %) is the calcium concentration in the fly ash sample, determined by ICP-AES (Optima 4300DV). m (g) is the weight of the ash sample used for calcium extraction.

2. MATERIALS AND METHODS The fly ash sample investigated was collected from the electrostatic precipitator of a typical lignite-fired power plant (nominal load: 100 MW). Its properties were characterized by X-ray fluorescence spectroscopy (XRF) and X-ray diffraction (XRD). The composition data are presented in Table 1. It is evident that CaO is the major constituent (30.47%) of the lignite ash, followed by SO3, SiO2, and Al2O3. The high content of SO3 indicates that part of CaO occurs as sulfates, which is confirmed by XRD analysis that detected a strong peak of anhydrite. To determine the calcium concentration in fly ash, a certain dose of the fly ash was totally digested and measured by ICP-AES (Optima 4300DV). The results show that the calcium concentration is 20.95 wt %. The pH value of the ash was determined to be 12.33. Measurements with a Master Min laser particle size analyzer (Malvern Instruments Ltd.) show that more than 90% of the ash particles (on a volume basis) are smaller than 100 μm. For calcium extraction, three ammonium salts, i.e., NH4Cl, NH4NO3, and CH3COONH4, were tested. Deionized water was used to prepare the solution. The prepared ammonium salt solution was put into a sealed round-bottom flask that was surrounded by a water bath. When the solution was heated up to the temperature required, a certain dose of fly ash was added and well mixed with the solution by a temperature and stirring rate controllable DF-101S magnetic stirrer. The stirring rate was fixed at 500 rpm. The influence of extraction time (0−120 min), temperature (10−90 °C), ammonium salt concentration (0−3 mol/L), and solid-to-liquid ratio (20g/L-100g/L) on the calcium extraction efficiency was investigated. The variation of Ca2+ concentration in the leachate with time was monitored by interval sampling and measurements. A total of 5 mL of liquid was sampled with a precleaned syringe at given time intervals (i.e., 2, 5, 10, 20, 30, 60, 90, 120 min) and immediately filtered using a 0.45 μm syringe filter unit. Since the solution was drastically stirred, the sampled liquid can be treated as a uniform part of the solution. The concentration of Ca2+ in the

WCa ‐ total =

CCa ×m 100

(3)

In the carbonation process, two methods for introducing CO32− into the Ca-rich leachate were tested to determine whether the carbonation efficiency could be further improved. One method was to carbonate the leachate by introducing pure CO2, as conventionally used in the literature,12,13,17,22,40 and the other was by adding NH4HCO3. Since room temperature is usually adopted in the production of industrial PCC, the carbonation temperature for the experiments was maintained at 25 °C. The carbonation efficiency χp is calculated with eq 4, where CCa2+−0 (mg/L) is the Ca2+ concentration of the fresh leachate before carbonation. CCa2+−t (mg/L) is the Ca2+ concentration of the leachate carbonated for time t (min). χp =

CCa 2+−0 − CCa 2+− t × 100(%) CCa 2+−0

(4)

3. RESULTS AND DISCUSSION 3.1. Calcium Extraction. 3.1.1. Influence of Extraction Time. For all three calcium extraction solutions, calcium extraction proceeds very fast in the first few minutes and slows down afterward (Figure 1). The features of calcium extraction for coal fly ash are similar to those found for steelmaking slag,14,40 which consists of two different stages, namely, an initial chemical reaction controlled fast extraction stage and a following diffusion controlled slow extraction stage. The calcium extraction efficiency increases rapidly in the beginning and nearly levels off in an hour. The final value falls between 35% and 41%. Comparatively speaking, CH 3 COONH 4 demonstrates apparently a higher calcium extraction efficiency than NH4Cl and NH4NO3. In the fast calcium extraction stage, the extracted calcium may be primarily from calcium-containing species on particle surfaces, which are in good contact with the liquid phase and 15139

dx.doi.org/10.1021/ie4023644 | Ind. Eng. Chem. Res. 2013, 52, 15138−15145


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Figure 1. Variation of calcium extraction efficiency with time (25 °C, ammonium salt concentration 1 mol/L, solid-to-liquid ratio 50g/L, 500 rpm, 120 min).

Figure 2. Influence of extraction temperature on Ca extraction efficiency (ammonium salt concentration 1 mol/L, solid-to-liquid ratio 50g/L, 500 rpm, 60 min).

readily react with the extraction agent. When the extraction proceeds, calcium in the surface layer decreases rapidly, resulting in an inert surface layer that is enriched with insoluble SiO2/Al2O3 and hinders the contact of reactive calcium in the particle core with the reaction solution.4,14 Consequently, the calcium extraction rate declines significantly, indicating the commencement of the diffusion controlled stage. In addition, the increased pH value caused by calcium extraction also contributes to the slow rate of calcium extraction in the later stage. Both NH4Cl and NH4NO3 are strong acid−weak base salts while CH3COONH4 is a weak acid−weak base salt. It is expected that NH4Cl and NH4NO3 are more favorable for calcium extraction than CH3COONH4, because they provide weak acidic circumstances in the solution. However, Figure 1 shows that CH3COONH4 has a better calcium extraction performance than NH4Cl and NH4NO3. The result suggests that a more acidic condition does not necessarily enhance calcium extraction. In fact, all three of the ammonium salts can react with fly ash to form corresponding calcium salts with very high solubility in water, which was suggested to be the major calcium extraction mechanism for ammonium salt solutions.40 Under the conditions investigated, the concentration of Ca2+ in the leachate is far from saturation. Therefore, the differences in solubility between CaCl2, Ca(NO3)2, and Ca(CH3COO)2 cannot account for the better calcium extraction performance for CH3COONH4. A possible explanation is that Ca2+ and CH3COO− can form a Ca(CH3COO)+ complex43 that reduces Ca2+ in the solution and is expected to promote the dissolution of the calcium from the solid matrix. 3.1.2. Influence of Extraction Temperature. The influence of temperature on calcium extraction efficiency was investigated by extraction experiments in a temperature range of 10−90 °C. According to Figure 1, the calcium extraction efficiency almost levels off after 1 h. Therefore, a total reaction time of 60 min was selected for all the tests. For comparison, extraction experiments with deionized water were also carried out. Apparently, the calcium extraction efficiency for ammonium salt solutions is more than 3 times that for deionized water, as shown in Figure 2. In calcium extraction with deionized water, Ca2+ in the leachate primarily comes from lime in the fly ash. Calcium in complex silicate form can hardly dissolve in water, explaining the poor calcium extraction performance. For

NH4Cl and NH4NO3 solutions, increasing temperature from 25 to 90 °C only has a subtle effect on calcium extraction efficiency. But for CH3COONH4 solution, an apparent increase in calcium extraction efficiency is observed at high temperatures of 70−90 °C. As to the deionized water, the calcium extraction efficiency decreases slightly with increasing temperature, because the solubility of Ca(OH)2 is lower at higher temperatures. 3.1.3. Influence of Agent Concentration. To investigate the influence of agent concentration, 10 g of fly ash was dissolved in 200 mL of solution with different agent concentrations (0−3 mol/L) at 25 °C. A great increase in calcium extraction efficiency is observed when the agent concentration increases from 0 to 0.5 mol/L (Figure 3). Further increasing the agent

Figure 3. Influence of extract agent concentration on Ca extraction efficiency (25 °C, solid-to-liquid ratio 50g/L, 500 rpm, 60 min).

concentration from 0.5 to 3 mol/L only has little effect on calcium extraction efficiency for NH4Cl and NH4NO3 but continuously promotes the calcium extraction efficiency for CH3COONH4. The extraction mechanism of calcium by NH4Cl/NH4NO3 is the formation of highly soluble CaCl2/ Ca(NO3)2.40 A total of 10 g of fly ash contains 0.05 mol of CaO that needs 0.1 mol NH4Cl/NH4NO3 for stoichiometric 15140



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3.1.5. Mineralogy of Coal Fly Ash and Its Extraction Residues. XRD patterns of the fresh fly ash and its corresponding extraction residues are analyzed and compared in Figure 5. According to the extraction agent used, the

reactions. Therefore, 200 mL of NH4Cl/NH4NO3 solution with a concentration of 0.5 mol/L is stoichiometrically sufficient even if all the calcium is dissolved. Consequently, when the NH4Cl or NH4NO3 concentration increases from 0.5 to 3 mol/L, only subtle changes in calcium extraction efficiency are observed in Figure 3. As discussed in subsection 3.1.1, calcium extraction mechanisms in CH3COONH4 are more complex. In addition to the formation of highly soluble Ca(CH3COO)2, the formation of the Ca(CH3COO)+ complex may promote calcium dissolution as well.43 The complex reactions are expected to be enhanced by higher concentrations of CH3COO− and can account for the persistent increase in the calcium extraction efficiency when the agent concentration increases from 0.5 to 3 mol/L. 3.1.4. Influence of Solid-to-Liquid Ratio. Figure 4 shows the influence of solid-to-liquid ratio on calcium extraction efficiency

Figure 5. XRD patterns of the fresh fly ash and residues. 1, fresh fly ash; 2, NH4Cl-residue; 3, NH4NO3-residue; 4, CH3COONH4-residue. A, Anhydrite; Q, Quartz; C, Calcite; E, Ettringite; H, Hematite; L, Lime; G, Gypsum.

extraction residues are denoted as NH4Cl-residue, NH4NO3residue, and CH3COONH4-residue, respectively. The data show that the major Ca-containing phases in the fresh fly ash include lime (CaO), ettringite (3CaO·Al2O3·3CaSO4·32H2O), and anhydrite (CaSO4). Lime and ettringite are reactive Cacontaining phases. The calcium in them can be readily extracted by ammonium salt solutions. In contrast, anhydrite is a relatively inert phase, contributing insignificantly to the Ca2+ in the leachate. In all the extraction residues, lime and ettringite are not detected, confirming that calcium as lime and ettringite largely dissolved in ammonium salt solutions under the conditions investigated. The strong signal of anhydrite persists in the residues, and gypsum resulting from hydration of anhydrite is detected. 3.2. Carbonation of Ca-Rich Leachate. In the literature, the Ca-rich leachate was usually carbonated by introducing CO2.12−14,17,22,40 In this work, direct introduction of CO2 for calcium carbonation was tested. Moreover, a new method was proposed to introduce CO32− by adding NH4HCO3 for the purpose of improving the carbonation efficiency. For both carbonation methods, the leachates used were prepared under the same calcium extraction conditions, namely by dissolving 25 g of ash in 500 mL of 1 mol/L solution of NH4Cl/ NH4NO3/CH3COONH4 at 25 °C and removing the solid residues by filtration after extraction for 60 min. The stirring rate was fixed at 500 rpm throughout all of the carbonation experiments. The carbonation results are reported as follows. 3.2.1. Carbonation by CO2. The leachates were carbonated by bubbling pure CO2 (50 mL/min) through a glass tube immersed in the solution under atmospheric pressure and at 25 °C. The carbonation time was 1 h. According to the extraction agent, the leachates are denoted as NH4Cl-leachate, NH4NO3leachate, and CH3COONH4-leachate, respectively. The carbonation efficiency and the solution pH with respect to time are reported in Figures 6−8. Apparently, for all the leachates, calcium carbonation proceeds very fast in the first 10−20 minutes. The reactions almost cease after 30 min, and the final carbonation efficiency

Figure 4. Influence of solid-to-liquid ratio on Ca extraction efficiency (a) and Ca2+concentration of leachate (b) (25 °C, ammonium salt concentration 1 mol/L, 500 rpm, 60 min).

(Figure 4a) and calcium concentration (Figure 4b) in the leachate. As expected, higher calcium extraction efficiency is obtained at a lower solid-to-liquid ratio, which favors calcium dissolution. However, the concentration of Ca2+ in the solution decreases rapidly with a decreasing solid-to-liquid ratio, which means an increase in reactor size is required to precipitate a certain amount of PCC in a given time.15,40 In this regard, for practical applications, an optimal solid-to-liquid ratio has to be determined. 15141



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product of CaCO3. In the beginning, the solution contains dissolved NH3 resulted from the calcium extraction process. When bubbling CO2, dissolved CO2 reacts with NH3 to form (NH4)2CO314 (eq 5). (NH4)2CO3 can totally ionize to provide sufficient CO32− for Ca2+ precipitation (eq 6, X represents Cl/ NO3/CH3COO). Consequently, calcium carbonation efficiency increases rapidly in the first 10−20 minutes. Simultaneously, ammonium salts can be easily recovered by spontaneous reaction between (NH4)2CO3 and CaCl2/Ca(NO3)2/Ca(CH3COO)2. Therefore, they can be reused in the calcium extraction process. With the proceeding of CO2 dissolution and calcium precipitation, NH3 is gradually consumed. As a result, H+ generated from H2CO3 dissociation cannot be neutralized in the later stage, causing a continuous decrease in solution pH value until an acidic medium is reached, which further hinders the dissociation of H2CO3. Therefore, the CO32− concentration in the leachate decreases and calcium precipitation almost ceases after 30 min.

Figure 6. Carbonation of NH4Cl-leachate by CO2.

2NH3 + CO2 + H 2O → (NH4)2 CO3

(5)

(NH4)2 CO3 + CaX 2 → CaCO3 + 2NH4X

(6)

3.2.2. Carbonation by NH4HCO3. Figures 6−8 show that the carbonation efficiency when using CO2 is only 41−47%. In the literature, attempts to improve the carbonation efficiency have been made, including base addition40 and CO2 pressurization.12,22 The enhancement mechanism of both methods is to increase CO32− concentration. In this work, a new method to increase CO32− concentration is investigated. NH4HCO3, rather than CO2, is selected as the source of CO32−. The reason is that, theoretically, NH4HCO3 is more favorable than CO2 for increasing the carbonation efficiency. On one hand, 1 mol of NH4HCO3 reacts with 1 mol of NH3 to form 1 mol of (NH4)2CO3 (eq 7), providing 1 mol of CO32−. However, 1 mol of CO2 needs 2 mol of NH3 to provide 1 mol of CO32− (eq 5). Therefore, more (NH4)2CO3 (i.e., CO32−) will be formed before NH3 is depleted when NH4HCO3 is used for carbonation. On the other hand, even when NH3 is depleted, the amount of CO32− provided by NH4HCO3 hydrolysis is several magnitudes larger than that by H2CO3 dissociation, because NH4HCO3 dissociates in only one step to produce CO32− while H2CO3 has to undergo a two-step dissociation.44

Figure 7. Carbonation of NH4NO3-leachate by CO2.

NH3 + NH4HCO3 → (NH4)2 CO3

(7)

In the NH4HCO3 carbonation experiments, the leachates used were prepared under the same conditions as those used in carbonation by CO2. Solid NH4HCO3 (analytically pure) was added into the leachate. The quantity of NH4HCO3 needed for carbonation was determined according to a stochiometric reaction between Ca2+ and CO32− (i.e., Ca2+/NH4HCO3 = 1:1). The carbonation time was half an hour. The carbonation efficiency with respect to time is depicted in Figure 9. Strikingly, the leachates achieve a high carbonation efficiency of >82% in the first 2 min when NH4HCO3 is added. The carbonation reaction nearly finishes in 10 min (Figures 6−8). The final carbonation efficiency by adding NH4HCO3 is up to 90−93%, nearly doubling that by bubbling CO2. CH3COONH4-leachate has a lower carbonation efficiency than the other two leachates, similar to bubbling CO2. Apparently, NH4HCO3 is much more effective in improving carbonation efficiency compared to CO2. In addition to the

Figure 8. Carbonation of CH3COONH4-leachate by CO2.

falls between 41 and 47%. CO2 dissolution and calcium precipitation are accompanied by a continuous decrease in solution pH value. The NH4Cl-leachate has the highest carbonation efficiency, followed by the NH4NO3-leachate and the CH3COONH4-leachate. In the carbonation process, the basic principle is to introduce CO32− into the leachate. Calcium carbonate precipitates when the ion product of Ca2+ and CO32− exceeds the solubility 15142



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Figure 9. Carbonation of leachate by adding NH4HCO3.

Figure 10. XRD patterns of the PCC samples. 1, commercial PCC sample; 2, PCC derived from NH4Cl-leachate; 3, PCC derived from NH4NO3-leachate; 4, PCC derived from CH3COONH4-leachate.

improved carbonation efficiency, there is another potential advantage when using NH4HCO3 for carbonation. That is onsite CO2 capture and storage can be realized if the carbonation process is integrated with the ammonia−water CO2 capture process, which has been widely investigated and considered to be a promising candidate to alternate the amine-based CO2 capture technology.45 In the ammonia−water CO2 capture process, NH4HCO3 is produced as an intermediate product. Pure CO2 is obtained, and NH3 is recovered during the thermal decomposition of NH4HCO3. For a coal-fired power plant equipped with ammonia−water CO2 capture system, if one stream of NH4HCO3 formed in the CO2 capture process is extracted and used to carbonate the Ca-rich leachate derived from coal fly ash, CO2 can be sequestered on site with PCC as a value-added byproduct. In this way, both CO2 capture and storage can be realized at the same emission point. Costs associated with CO2 compression and transportation in conventional CO2 storage methods like geological storage can be eliminated. Therefore, the competitiveness of ammonia-based CO2 capture technology against other CO2 capture technology can be further increased. 3.2.3. Properties of the PCC. The PCC samples obtained from the carbonation process were washed by deionized water three times and dried at 105 °C overnight. The properties of the PCC were analyzed by chemical titration methods, TGA, XRF, and XRD. The purity and the polymorphs of the PCC were investigated. The purity of the obtained PCC is determined by two methods, i.e., TGA and the method as described in the Chinese standard of precipitated calcium carbonate for industrial use. A detailed procedure can be found elsewhere.46 The purity of PCC measured by the two methods is in good agreement, being about 97−98%. This value meets the purity requirements (≥97%) of PCC for industrial use. There exist three basic calcium carbonate polymorphs in nature, namely calcite, aragonite, and vaterite. Calcite is the most stable phase, followed by aragonite and vaterite. In a spontaneous precipitation process, in the absence of additives and at ambient temperature, both vaterite and calcite can precipitate.47 In contrast, the formation of aragonite usually requires higher temperature or additives.47 Since carbonation experiments in this work were carried out at room temperature and without the introduction of additives, no aragonite is observed in the PCC samples, as shown in Figure 10. PCC derived from the NH4Cl-leachate contains a pure calcite phase, while the PCC samples derived from the NH4NO3-leachate and

CH3COONH4-leachate consist of calcite and vaterite. For comparison, a commercial PCC sample was analyzed by XRD. The data are also shown in Figure 10. It is apparent that the composition of the PCC derived from NH4Cl-leachate is the closest to that of the commercial PCC sample. 3.3. CO2 Uptake Capacity. Taking NH4Cl as an example, the CO2 uptake capacity of the coal fly ash investigated is estimated when NH4HCO3 is used for calcium carbonation. The results show that approximately 0.17 tons of PCC can be produced from 1 ton of lignite fly ash, and 0.075 tons of CO2 can be bounded in calcium carbonates (calcium extraction conditions: 1 mol/L NH4Cl solution, 25 °C, solid-to-liquid ratio of 50g/L, 1 h, 500 rpm; carbonation conditions: Ca2+/ NH4HCO3 = 1:1, 25 °C, 0.5 h). If the obtained PCC is used to substitute commercial PCC produced from limestone, CO2 emissions can be further eliminated. Currently, PCC for industrial use is mainly produced from lime obtained by calcining limestone. The lime is hydrated with water to form Ca(OH)2 slurry, which is then carbonated by CO2 to produce PCC. Although CO2 produced in the limestone calcination process is rebounded in the carbonation step, CO2 emissions due to the enormous energy input to support the limestone calcination reaction cannot be avoided. The net CO2 emission from the current PCC production method is estimated to be 0.21 kg of CO2/kg of PCC.48 When taking this into consideration, approximately 0.036 tons more CO2 can be avoided if 0.17 tons of commercial PCC is substituted by the PCC produced from lignite fly ash. Therefore, the total CO2 uptake capacity for the method described in this work is about 0.111 ton for 1 ton of lignite fly ash. The CO2 uptake capacity of coal fly ash in this work is compared with those adopting different carbonation routes, as shown in Table 2. Apparently, the value in this work is more than 14 times that of carbonating a Victorian lignite fly ash containing high CaO (39.8 wt %) through the direct semidry carbonation route.27 The results show that the proposed indirect carbonation method not only manufactures a highpurity PCC end product but also has a higher CO2 uptake capacity. The feasibility of the carbonation process in this study can be further strengthened if it is integrated into the ammonia−water CO2 capture process to store CO2 on-site, as discussed in subsection 3.2.2. In addition, the carbonation process of coal fly ash in this study can serve as a reference for carbonation of other alkaline residues. 15143



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Table 2. Comparison of Experimental Conditions and Results in the Literature with This Study reference 27

material detail

28

39.8% CaO 5% CaO

this work

30.5% CaO

CO2 capacity (kg/kg)

route

conditions

direct semidry route direct aqueous route indirect route

40 °C, 3−6 MPa, L/S ratio = 0−0.7, 12 h 20−60 °C, 1−4 MPa, L/S ratio = 6.7− 20, 2 h 25 °C, L/S ratio = 20, 1 mol/L NH4Cl solution, 2 h

0.026 0.111

remarks optimized L/S ratio = 0.2, CO2 pressure does not influence capacity temperature, CO2 pressure, and L/S ratio do not influence capacity extraction and carbonation can proceed very fast and efficiently under ambient conditions

*Tel.: 86-27-87545526. Fax: 86-27-87545526. E-mail: mhxu@ hust.edu.cn.

4. CONCLUSIONS Indirect carbonation of a high calcium coal fly ash is investigated, with the aim of sequestering CO 2 and simultaneously producing valuable high-purity PCC. The highlights of this process involve using ammonium salt solution as a calcium extraction agent and using NH4HCO3, rather than CO2, to carbonate the leachate. On the basis of the experimental results, the following conclusions can be made. Ammonium salts are effective calcium extraction agents for the coal fly ash investigated. Approximately 35 to 40% of the calcium in lignite fly ash dissolves in ammonium salt solution. The extraction performance is best for CH4COONH4, followed by NH4NO3 and NH4Cl. Calcium extraction proceeds fast in the first few minutes and nearly ceases in an hour. Increasing the temperature from 25 to 90 °C and increasing agent concentration from 0.5 to 3 mol/L only subtly increase the calcium extraction efficiency for NH4Cl and NH4NO3, while the positive effect of increasing temperature and agent concentration is more obvious for CH3COONH4. A decreasing solid-to-liquid ratio is beneficial for extracting calcium into solution but accompanied by more water consumption and a larger reactor size, which means an optimal solid-to-liquid ratio has to be determined. In the leachate carbonation stage, carbonating by bubbling CO2 yields poor carbonation efficiency (41−47%). Substituting CO2 with NH4HCO3 as the provider of CO32− is confirmed to have a much higher carbonation efficiency and rate than carbonating by CO2, and about 90−93% of the calcium is precipitated. Carbonating the Ca-rich leachate with NH4HCO3 not only improves the carbonation efficiency and rate but also shows the possibility to capture and store CO2 on site. That is, if the NH4HCO3 used for calcium carbonation is provided by NH4HCO3 formed in the ammonia−water CO2 capture process, CO2 capture and storage processes can be intergraded using NH4HCO3 as a connector. It is considered to be a prospective option for on-site CO2 mineral sequestration, eliminating the costs associated with CO2 compression and transportation. The purity of obtained PCC is up to 97−98%, meeting the purity requirements (≥97%) of PCC for industrial use. PCC derived from the NH4Cl-leachate contains a pure calcite phase that is closest to the composition of commercial PCC, while the PCC samples derived from the NH4NO3-leachate and CH3COONH4-leachate are mixtures of calcite and vaterite. The total CO2 uptake capacity of coal fly ash used in this study is calculated to be 0.111 tons of CO2 per ton of coal fly ash, competitive with the direct carbonation routes reported in the literature.

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0.007 66

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by National Key Basic Research and Development Program of China (No. 2013CB228501), National Natural Science Foundation of China (Nos. U1261204, 51076051), and Program for New Century Excellent Talents in University (No. NCET-11-0192).

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A Novel Method for CO2 Sequestration via Indirect Carbonation of

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