A review on carbon dioxide capture and storage technology using coal fly ash

Applied Energy 106 (2013) 143–151

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A review on carbon dioxide capture and storage technology using coal fly ash Jung-Ho Wee ⇑ Department of Environmental Engineering, The Catholic University of Korea, 43 Jibong-ro, Wonmi-gu, Bucheon-si, Gyeonggi-do 420-743, Republic of Korea

h i g h l i g h t s " The potential of the CCS technology using coal fly ash (FA) is reviewed. " Alkali species in FA are dissolved and consumed to sequestrate CO2 in the wet process. " FA can be used as a support or as a raw material of dry sorbents to capture CO2. " The technology can stabilize the harmful components in FA during the process. " Therefore, the technology may be another option of CCS to a limited extent.

a r t i c l e

i n f o

Article history: Received 1 October 2012 Received in revised form 3 January 2013 Accepted 23 January 2013 Available online 16 February 2013 Keywords: Carbon dioxide capture and storage Coal fly ash Mineral carbonation Leaching Dry sorbents

a b s t r a c t This work reviews the availability and the potential of the carbon capture and storage (CCS) technology using coal fly ash (FA). Because the technology can be effectively applied on-site to coal fired power plants and as FA contains sufficient alkali components, the technology may be another option of CCS technology to a limited extent. The technology can be divided into wet and dry processes. In the former, the available components for CCS in FA are leached into solution by the solvent where they are subsequently consumed for carbonation to store CO2. Particularly, the CO2 storage capacity of CaO-enriched FA solution mixed with brine under high pressure may be equal to or greater than the true CO2 emission reduction achieved by applying FA as a cement additive. In the dry process, FA can be used as a direct support or as the raw material of the sorbent supports for CO2 capture. The dry process is effectively applied for CO2 capture rather than storage because the sorbents should be regenerated. Another advantage of the technology is the stabilization of the harmful components present in FA, which are mostly co-precipitated with carbonated FA during the process. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction The amount of solid waste residue (SWR) generated from the large-scale industrial processes such as coal fired power plant (CFPP), cement plant, steel, paper, oil shale industry and solid waste incinerator is increasing every year and some SWRs are substantially harmful to the environment. Therefore, the SWR disposal situation has been intensively aggravated to become an important issue [1–5]. Considering that most of the processes that generate SWR emit a great amount of CO2, SWR can be directly or indirectly used as a material for on-site CO2 capture and storage (CCS), which is another option of CCS technology [6–13]. This potential is based on the following reasons. Firstly, because industrial SWRs contain substantial alkali and alkali earth metals ⇑ Tel.: +82 2 2164 4866; fax: +82 2 2164 4765. E-mail addresses: [email protected], [email protected] 0306-2619/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apenergy.2013.01.062

[14–17], the mineral carbonation with SWR is readily achieved to store CO2 and which may be a permanent solution compared to geological and ocean storage. In addition, CO2 can be partially recovered from the instable carbonated (or bi-carbonated) SWR products. Secondly, they are geochemically instable and reactive because they are generally formed at very high temperature (over 1200 °C) and subsequently produced by rapid cooling [7]. Finally, they are porous materials with relatively high surface area. The first study on CCS using SWR was conducted by Reddy et al. group with alkaline byproduct of oil shale combustion process in the mid-1980s [18] and they proposed the technology to increase the reaction rate of former process in the early 1990s [19]. Both works had inspired a number of subsequent literatures dealt with mineral carbonation with various SWRs [8,20–22]. Among industrial SWRs, interest has focused on coal fly ash (FA) generated from the pulverized CFPP because the CO2 emissions from CFPP and the FA portion are the largest [23–28]. Although

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the characteristics of FA differ substantially according to the coal and combustion conditions [29], currently generated FA is mostly used as cement and concrete additives [30,31]. However, its utilization ratio remains less than 30% of the total generated amount [32] in terms of the worldwide production, which is estimated at 700 million tons per year. In US, at least 150 million tons FA is generated annually and 27% is reused, while the remaining is landfilled or surface-impounded [30]. Although CO2 emission reduction using FA can be achieved via some methods, the primary technology is based on mineral carbonation because FA contains a high content of alkali components, which are essential materials for mineral carbonation [33–35]. For example, the CaO and MgO contents are more than 20% in Class-C FA (FA generated from younger lignite or sub-bituminous coal burning). Furthermore, the CaO concentration is very high in FA collected at flue gas desulfurization (FGD) process [36]. Therefore, several attempts have been made to use FA as the caustic material to increase the pH of the acidic brine solution generated from oil and gas production. One of the advantages of FA use for CO2 emission reduction is its on-site application in CFPP. This is very effective to minimize the CCS cost while saving or reducing the current transfer, treatment and disposal costs of FA. In addition, the final carbonated FA products have the potential to be effectively reused as construction materials or additives because their physical structure might be changed, and their mechanical strength and leaching resistance are improved. Therefore, these can substantially enhance the economy of the carbonation process and decrease the environmental impact of FA. Many papers have reported the FA carbonation effect by leaching process, irrespective of CCS. However, the research focusing on the application of FA to one of the CCS technologies began to be reported since the late 1990s. The present paper reviews the CCS technologies using FA based on previously reported studies. Firstly, the technology using FA is divided into two categories: wet and dry processes. Secondly, the features and performance of each technology are investigated in detail. Finally, the availability and CO2 emission reduction potential of the technology are discussed. 2. Status of coal fly ash (FA) in Korea

produced, which substantially increase the chemical resistance and the long-term strength of the cement [42]. Therefore, this Portland FA cement (PFAC) is generally used as the concrete in dam construction. Another effect of FA application as a cement additive is to reduce CO2 emission by reducing the cement consumption. In Korea Standards, PFAC is classified into three types according to the FA content: A (10% of FA), B (10–20%) and C (20–30%). Currently, type B is most widely used. Therefore, the average portion of FA in PFAC is assumed to be 15%. When 1 ton of Portland cement is produced, 0.83 ton of CO2 is known to be emitted, of which 0.46 ton is generated from sintering of CaCO3 and the balance is due to energy consumption [43]. Therefore, assuming that the energy consumption is unchanged and considering the portion of FA substitution for CaCO3, the CO2 emission reduction achieved by using FA as a cement additive is roughly estimated to be 0.069 ton per ton of cement production. However, the true CO2 emission reduction by FA replacement with cement is less because of the additional CO2 emissions generated from FA treatment as a cement additive such as fractionation, mixing and transportation processes. Due to these penalties, the CO2 emission reduction contribution of FA as a cement additive may be substantially less than this value. Therefore, when FA is used as a direct or indirect material for CCS in CFPP on-site, its detailed effects should be analyzed compared to FA application in cement plant. In this case, various conditions should be considered such as additional energy consumption to treat FA for wet or dry based application. The beneficial effects should be also considered which include reuse of carbonated FA products as construction materials and stabilization of harmful components in FA precipitated during carbonation. Although, this feasibility study based on all these conditions would very meaningful, the present review does not include this study. The paper focuses on the CCS capacity of FA or its potential when it is used as a material for CCS. Except as a cement additive, FA application to other fields has been extensively investigated, including ceramic balls for water purification, zeolite synthesis, polish materials of steel plate, anti-flaming, building and fill materials [44]. However, these are minor applications in terms of CO2 emission reduction.

2.1. Amount of FA generated 3. Study on CCS technology using fly ash (FA) Total amount of coal ash, including FA, generated per year since 2001 in Korea is listed in Table 1. The nation’s total electricity production in 2010 was 433,604 GW h [37], of which 45%, 195,000 GW h, was generated from CFPP via 5 major electric utilities. For this electricity generation, 78.89 million tons of coal was consumed to generate 8.41 million tons of coal ash [38–40]. The amount of generated FA was estimated to be 5.29 million tons [41], of which more than 90% was utilized as additives in cement, concrete and brick. 2.2. Application as a cement additive As aforementioned, most FA is currently used as a cement additive. When FA is mixed with the Portland cement, SiO2 and Al2O3 in FA react with slaked lime and water according to the Pozzolan reaction. Therefore, calcium silicate and calcium aluminate hydrates are

Recently, many papers have investigated the development of CCS technology using FA. These studies can be divided into three classifications according to the applied technology. The first category is wet and dry processes. In wet technology [32,45–51] CCS components in FA such as Ca, Na, Mg and K are leached by aqueous (acidic) solution and then consumed on carbonation reaction to store CO2. In the dry process [52–55], FA and some additives are used as direct CCS materials. In addition, FA is used to support the catalytic sorbent for CO2 capture [56–58] and as a raw material of zeolite [59–63]. The second classification is based on the level of pressure during the carbonation. The reaction can be carried out in atmospheric or high pressurized condition. Generally, carbonation with high pressure is carried out in an autoclave with the object of CO2 storage. Final classification depends on the number of processes involved. In a one-step process, FA treatment and carbon-

Table 1 Annual coal ash and coal fly ash (FA) generation in Korea. Year

2001

2002

2003

2004

2005

2006

2007

2008

2009

2010

Coal ash (million tons) Coal fly ash (million tons)

4.92 3.09

5.14 3.23

5.19 3.26

5.38 3.38

5.95 3.74

5.84 3.67

6.02 3.79

7.61 4.79

8.35 5.25

8.41 5.29

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Table 2 Process features and carbon dioxide storage capacity of wet-based FA application to CCS, as reported in the literature. Paper

Carbonated system

Experimental conditions Reactor type Temperature Solid (FA)/liquid (solvent) CO2 partial pressure Stirring speed Time Others

[45]

Aqueous solution mixed with Ca(OH)2 and FA leachates

N.A.

[46]

Moderately wetted FA (Humidity portion in FA; 5–50%)

[47]

Class-C FA (10%) and NaOH added brine filtrates and slurry

[48]

Lignite FA-water slurry

[32]

FA-water slurry

[49]

Aqueous mixed slurry of Class-C FA and saline

[50]

Lignite FA- water slurry

[51]

Aqueous mixed slurry of fractionated FA and brine

Batch 25–50 °C – 50–125 psi – 24–72 h – Autoclave 20 °C – 1.36 MPa – 2h – Batch (3.7 L) 25–75 °C 50 g/L 0.01–0.3 MPa 600 rpm 4.5 h – Autoclave 30–60 °C 100 g/1 L 40 bar 450 rpm 2h – Closed MFR Ambient – 9.62 psi g 1500 rpm 2h Gas feeding rate; 300 mL/ min Autoclave 35 °C 1.5:1 (by weight) 1 MPa Shaken 24 h – Autoclave (6 L) 90 °C 1 4 MPa 600 rpm 2h –

Storage capacity

Remarks

– S in leachates enhanced CaCO3 production rate – Mg in leachates hindered CaCO3 formation – Storage capacity of solution was independent of CO2 flow rate and concentration – CaO addition to FA enhanced carbonation rate – Humidity, CO2 concentration and partial pressure were strongly dependent on carbonation

–

– 2.77 g CO2 and 29.63 g ppt/L filtrates – 22.88 g CO2 and 21.9 g ppt/L slurry

– Mineral carbonation was effectively available with components in brine and in FA – FA addition to brine increased pH of brine from 3.8 to 8.3 within 1.5 h – NaOH addition to FA brine slurry increased carbonation rate

– 0.23 g CO2/g FA

– Storage capacity was proportional to CO2 partial pressure, stirring speed, temperature and solvent/FA ratio

– 0.026 g CO2/g FA

– Carbonation efficiency was independent on the initial pressure of CO2 – Carbonation was not significantly affected by reaction temperature (20–60 °C) and fly ash dose (50–150 g)

– 33.4 g CO2 at 23.6% solids in slurry by weight (FGD) – 1.54 g CO2 at 23.3% solids in slurry by weight (Class-C)

– Synergistic effect of saline cations and dissolved CCS components of FA in slurry was observed 2 h later from the start

– 0.055 g CO2/g FA (average)

– Carbonated FA (CO2 bonding FA) used as construction materials was estimated 1.3 million tons per year in Poland

– 0.072 g CO2/g 20–150 lm sized FA – 0.036 g CO2/g > 150 lm sized FA

– CaO composition of FA with the particle size of 20–150 lm was the highest (9.3%) and >150 lm was the least (5.89%) – Concentration of significant trace elements in FA was reciprocal to FA particle size

ation are almost simultaneously carried out. On the other hand, in multi-unit involved process, components in FA are firstly extracted or FA is modified before they are subsequently used for carbonation reaction. Generally, the multi process is not desirable because it involves solvent regeneration and side reactions. In the present paper, many studies are classified according to their wet and dry basis and they are reviewed in the next section. In addition, their

critical information such as carbonated system, experimental condition and storage capacity of FA is summarized in Tables 2 and 3. 3.1. Wet process In 1992, Schramke [45] carried out the mineral carbonation feeding CO2 into Ca(OH)2-added FA leachates manufactured by

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Table 3 Process features and carbon dioxide capture capacity of dry-based FA application to CCS, as reported in the literature.

a b

Paper

Carbonated system

Experimental conditions Reactor type Temperature CO2 partial pressure

[52]

KOH added and CPAHCL impregnated FA

DRIFTSa/TPDb 120 °C 10.1 kPa

[53]

PEI (50%) impregnated MCM41 generated from FA (K2CO3 added, 40 mol%)

[54]

Li4SiO4 generated from FA (K2CO3 added, 40 mol%)

TGA 75 °C 10.1 kPa TGA 600 °C 101.3 kPa

[55]

FA

Fluidized bed 43–54 °C 15 kPa

Capture capacity

– 0.008 g CO2/g fresh sorbent – 0.006 g CO2/g regenerated sorbent – 0.112 g CO2/g sorbent – 0.003 g CO2/g PEI free sorbent – 0.107 g CO2/g sorbent

– 0.207 g CO2/g FA

Remarks

– KOH was not essential component to increase capture performance of the sorbent

– Sorption and desorption rates was rapid

– Sorbents was maintained its original capacity during 10 regeneration cycles – Maximum capture capacity reached within reaction time of 15 min – SO2 and Hg was co-precipitated in carbonated FA

Diffuse reflectance infrared transform spectroscopy. Temperature programmed desorption.

extraction of FA using water as the solvent. Originally, the work did not aim to store CO2 but to control the pH of the FA leachates absorbing CO2. The author measured the neutralizing effect of CO2 injection and Ca(OH)2 addition to FA leachates. As a result, the author claimed CO2 was substantially converted and stored in the form of precipitated CaCO3 in the leachates. Many Ca components inherent in FA were also substantially included in the carbonates. Therefore, the paper demonstrated that FA could be used as a CCS material via mineral carbonation. In 1995, Tawfic et al. [46] reported the effect of high pressurized CO2 injection to moderately wetted FA. This study was not focused on CO2 storage. The authors intended to reduce the pH of wetted FA and simultaneously to eliminate the trace metals presented in

FA such as Cd, Pb, Cr, As and Se co-precipitating with CaCO3. Finally, however, the result of the paper led to interest in the FA application to CO2 storage. Therefore, these two papers [45,46] may offer insight into many subsequent works dealing with the CCS technology using FA. In 2006, Soong et al. [47] investigated the carbonation effect of FA in mixed slurry and filtrates of FA, NaOH and brine solution. To utilize Ca, Mg, and Fe in the brine as the CCS components, the initial pH of brine, which is approximately 3–5, should be firstly increased to over 7.8. Therefore, the authors used FA to increase the pH of the brine. Among five kinds of FA, Class-C FA was selected and FA mixed-brine slurry and filtrates were prepared. Thereafter, mineral carbonation with the slurry was carried out according to

Fig. 1. Conceptual process for CO2 sequestration via brine and fly ash (FA) [47].

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147

Fig. 3. Schematic experimental system for mineral sequestration of CO2 by aqueous carbonation of fly-ash in a stirred reactor [32]. Fig. 2. Reaction progress in fly ash (FA) experiment with water and CO2 (s/lratio = 75 g/L; initial pCO2 = 0.02 MPa; stirring rate=300 rpm). A: pH, pCO2 and EC. B: total uptake of CO2, precipitated amount of carbonate, total dissolved inorganic carbon (TDIC) and dissolved contents of Ca and Mg per gram FA [48].

the process shown in Fig. 1, which includes FA recycling, separation and causticization. As a result, the authors claimed that the CO2 was substantially captured in the solution and stored as the precipitated material (ppt). However, the carbonation was conducted at high pressure in an autoclave and the reported Ca composition of 17–29% in FA was very high compared to traditional FA. In 2008, Back et al. [48] reported the mechanism of the mineral carbonation conducted with the mixed slurry of lignite FA and water. In addition, they investigated the effect of various conditions such as CO2 concentration and temperature on the carbonation. After analyzing the variation of pH, electrical conductivity (EC) and CO2 conversion rate during the reaction, they claimed that the carbonation in the slurry was carried out via the three consecutive stages shown in Fig. 2. The first step with a slurry pH over 12 proceeded during the initial 30 min where CO2 and Ca began to be dissolved, which initiated CaCO3 formation. They claimed that the CO2 dissolution was maximized in this stage and became the rate controlling step. The second step was the period between 10 and to 60 min from the start, during which the pH was under 10.5. The carbonation was dominantly carried out to CaCO3 in this stage. The slight dissolution of MgO into water was significantly involved in the final stage, in which soluble MgHCO3 was generated. Based on this mechanism, the authors calculated the CO2 storage capacity of FA to be 90% of the acid neutralizing capacity value of FA. In addition, they highlighted that the Ca conversion ratio into CaCO3 was over 75% of the dissolved Ca from FA. In 2009, Montes-Hernadez et al. [32] investigated the CO2 storage capacity of FA (4.1% of CaO)-water slurry conducting the highly pressurized carbonation in an autoclave, as shown in Fig. 3. They reported that the carbonation was carried out according to two reactions. Firstly, CaO in FA was dissolved by water, which rapidly increased the pH of the slurry. Thereafter, CaCO3 was pro-

Fig. 4. XRD spectra of starting fly-ash and solid products after carbonation during 2 h. C: calcite, M: mullite, Q: quartz, L: lime. These spectra demonstrate the total consumption of lime and the production of calcite [32].

duced by the rapid carbonation in the slurry, as confirmed by the XRD analysis shown in Fig. 4. The authors also estimated the conversion ratio of CaO to CaCO3 as 82%. However, the glass-phase Ca, which was not converted CaCO3, was substantially remained with a concentration of 800 mg/L [64] of slurry after carbonation. In addition, they highlighted the heavy metals in FA such as Se, which was co-precipitated with CaCO3, as well as Mg and S, which hindered the formation of CaCO3. Furthermore, they claimed that the CaCO3 co-precipitated with SO2 4 and Mg components was more soluble than pure CaCO3. In 2009, Dirmore et al. [49] reported the carbonation performance of a mixed slurry of FA collected from the FGD unit and saline waste water. The Ca composition in this FA was substantially high because the lime had previously been used in the FGD process

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to eliminate SOx in the flue gas, so that substantially more Ca remained than in traditional FA. The authors claimed that the amount of CO2 stored by FGD-based FA was even larger than that of Class-C FA, as listed in Table 2, and it was proportional to the FA feeding amount. Moreover, they investigated whether the hazardous elements were extracted from the carbonated FA by measuring their composition in the carbonated FA leachates. As a result, Ba was solely traced in leachates, leading them to claim that most hazardous materials were stabilized during the FA carbonation. In 2009, Uliasz-Bochen´czyk et al. [50] carried out the carbonation of the mixed slurry of water and CaO-enriched lignite FA, with a weight ratio of 1.5. The carbonation was conducted by shaking an autoclave filled with slurry and highly pressurized CO2 for a very long time. As a result, the authors claimed that the CO2 storage capacity was very strongly dependent on the reaction time, being 35 and 79 g of CO2 per kg of FA after the initial 15 min and

25 days later, respectively. However, the water content in the slurry was very high compared to those of other works. In 2011, Nyambura et al. [51] investigated the carbonation of aqueous slurry mixed with fractionated Class-F FA (FA generated from harder and older anthracite or bituminous coal burning) and brine. As a result, CaCO3, plagioclase (NaAlSi3O8–CaAl2Si2O8) and anhydrite (CaSO4) were confirmed to be produced, as shown in Figs. 5 and 6. The FA used in the study was previously fractionated according to particle size and the authors claimed that the CO2 storage capacity of the relatively smaller FA with a size of 20–150 lm was larger than that of the larger FA. They ascribed this result to the fact that the smaller FA contained more CaO (9.3%). Furthermore, they reported that the composition of trace elements such as S, Sr, and Ba in smaller FA were higher than that of larger FA. In addition, they noted that the brine substantially enhanced the degree of carbonation.

Fig. 5. Phase identification and quantification of fresh FA by XRD; M (mullite), Q (quartz), Mt (magnetite), L (lime), H (hematite) [51].

Fig. 6. Phase identification and quantification for carbonated FA by XRD; (90 °C, 4 MPa, bulk ash at an S/L ratio of 1); M (mullite), Q (quartz), C (calcite), A (anhydrite), P (plagioclase), B (bassanite), Mt (magnetite) [51].

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149

Fig. 7. XRD profiles of samples FA-1 (Pure FA) and FA-1⁄ (Li4SiO4 synthesized with FA) [54].

3.2. Dry process Recently, research on dry sorbents for CO2 capture has been intensively conducted due to its relatively more advantageous features compared to the wet process. As aforementioned, FA can be directly used as a support for the sorbent in CO2 capture and as the raw material of supports such as zeolite. Dry sorbents can be effectively used for CO2 capture rather than mineral carbonation because the sorbents are generally regenerated. The works on dry sorbents using FA are reviewed below. In 2004, Graya et al. [52] analyzed the CO2 capture performance of the amine-enriched dry sorbents manufactured by using Pittsburgh-seam coal FA as the sorbent supports. FA was treated before use by solvent via the authors’ own agglomeration process. The primary CO2 absorption component in the sorbent was 3-chloropropylamine-hydrochloride, which was impregnated onto FA with KOH in the aqueous solution. CO2 capture with the sorbents was carried out in their manufactured reactor under a CO2 composition of 10% with the balance of He mixed humid gases. The reported CO2 capture capacity of the fresh and regenerated sorbents is listed in Table 3. In the paper, the authors also claimed although the performance of the sorbent was only 9% of the commercialized one, the performance based on the unit surface of the sorbent was higher. In 2009, Majchrzak-Kuceba et al. [53] investigated the performance as a sorbent support of MCM-41, which is produced by FA collected in a circulating fluidized bed. This meso-porous substance was manufactured via a hydrothermal process where the supernatants in FA were used as its main raw materials and surfactants were added as the structure-directing agents. Thereafter, polyethylenimine (PEI), which is a CO2 capture component, was impregnated onto MCM-41 to produce the sorbent. The reported CO2 capture capacity of their sorbents is listed in Table 3. The absorption was conducted with a gas mixture in which the CO2, O2 and N2 compositions were 10%, 10% and 80%, respectively. In addition, the authors analyzed the desorption rate of the sorbent. Therefore, they concluded that micro-sized FA can be used as the raw material for meso-porous substance such as MCM-41.

In 2010, Olivares-Marín et al. [54] manufactured Li4SiO4 via solid state method using SiO2 in FA as the raw materials and reported its CO2 capture capacity. Li4SiO4 was reacted with CO2 to produce Li2SiO3, according to Eq. (1).

Li4 SiO4 ðsÞ þ CO2 ðgÞ ! Li2 SiO3 ðsÞ þ Li2 CO3 ðsÞ

ð1Þ

The sorbent was synthesized by sintering the mixture of FA and Li2CO3 at 950 °C and it was confirmed to be Li4SiO4 via XRD analysis, as shown in Fig. 7. Thereafter, CO2 capture and desorption tests were carried out many times and their performance was compared to the referenced original Li4SiO4. In addition, they analyzed the effect of K2CO3 addition to the sorbent. The authors claimed although the capture performance of their sorbent was less than that of the reference sorbent, as shown in Fig. 8, their sorbent was better in terms of sorbent regeneration. Furthermore, the authors explained that K2CO3 addition to the sorbent substantially increased the capture capacity of the sorbent in a temperature dependent manner up to 600 °C.

Fig. 8. CO2 sorption profiles of P-Li4SiO4 (Pure Li4SiO4) and FA-1⁄ (Li4SiO4 synthesized with FA) in 100 vol% CO2 at 500 °C [54].

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Fig. 9. Preliminary experiment set-up for CO2 capture and mineralization (Inset shows the testing at the plant) [55].

In 2011, Reddy et al. [55] carried out a pilot-scale mineral carbonation experiment with an FA content of 100–300 kg, as shown in Fig. 9. In the pre-experiment, the carbonation effect of FA was investigated by directly subjecting the FA to contact with the flue gas of the CFPP in a fluidized reactor. They claimed that the concentration of CO2 and SO2 in the flue gas was decreased from 13% to 9.6% and from 107.8 ppmv to 15.1 ppmv within 2 min, respectively. In addition, the concentration of Hg in the flue gas was substantially reduced. The authors claimed that if FA is exposed to flue gas, CO2 is preferably reacted with Ca and other oxides in FA to generate CaCO3 and various other carbonates such as thaumasite (Ca3Si(SO4)CO3(OH)612H2O) dawsonite (NaAl(CO3)(OH)2), and alumohydrocalcite (CaAl2(CO3)2(OH)43H2O). In addition, the authors highlighted the small amount of acid presented in the flue gas such as H2SO4 and HNO3. Such remnant components are inevitable because the flue gas had previously been passed through the scrubbing process to eliminate SOx and NOx, so that various components in FA such as Al, Fe, Ca, Mg, Na, K, and S were slightly dissolved and then co-precipitated in the carbonated FA. In fact, the amorphous silicate in FA is known to be substantially soluble and reactive in acidic condition [65]. From their results, the authors claimed that when 90% of the CO2 emitted from a 532 MW CFPP is stored, the estimated cost is $11/t CO2, which equates to a CO2 storage capacity of 207 kg CO2/t of FA. However, they did not describe the treatment of carbonated FA, which is a critical issue. 4. Conclusions The present paper has discussed the availability and the potential of CCS technology using FA by analyzing the results from previous studies in this field. From the reviews, the following conclusions were obtained.  The technology is applicable to CFPP on-site and FA contains sufficient alkali components such as CaO. Therefore, FA may be effectively used as a material for CCS.  The technology can be classified into wet and dry processes. In the former, the available components for CCS in FA such as Ca, Na, Mg and K are dissolved into solution by leaching and subsequently consumed for carbonation to store CO2.

 Currently, most FA is used as a cement additive. The CO2 emission reduction achieved by FA substitution to cement material was roughly calculated to be less than 0.069 ton per ton of cement.  Class-C FA (particularly lignite coal based), FA collected from the FGD unit in CFPP, and FA with a size of 20–150 lm were the most effective materials for CO2 storage in the wet process due to their higher CaO composition. In addition, the CO2 storage capacity of brine solution mixed with CaO-enriched FA under high pressure can be significantly increased to a level that might be sufficiently competitive to the case of FA application to cement.  In the dry process, FA is directly used to support the sorbent on which CO2 capture components such as amine can be impregnated. In addition, FA can be the raw material of meso-porous substances, zeolite and SiO2, which can be the sorbent supports for CO2 capture. Therefore, the dry process can be effectively used for CO2 capture rather than storage because the sorbents should be regenerated. However, FA may be used as a CCS component for direct contact with the flue gas of the CFPP in the fluidized reactor. The storage capacity in the process was reported to be 0.207 g CO2/g FA.  If the technology with the highest storage capacity reported by dry process were applied to Korean situation, the annual CCS potential is estimated to be 1.10 million tons. In the same way, the global CCS potential is calculated to be 144.9 million tons per year and which equates the total CO2 emissions of a medium-sized European country like Belgium.  Another advantage of CCS technology using FA is the stabilization of the harmful components present in FA such as Cd, Pb, Cr, As, Se, Al, and S. These toxins are mostly co-precipitated with carbonated FA during the process.

Acknowledgements This research was supported by Basic Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (20100009938) as well as supported by the Catholic University of Korea, Research Fund, 2012.

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