Current status and challenges of the ammonia-based CO2 capture technologies toward commercialization

International Journal of Greenhouse Gas Control 14 (2013) 270–281

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International Journal of Greenhouse Gas Control journal homepage: www.elsevier.com/locate/ijggc

Review

Current status and challenges of the ammonia-based CO2 capture technologies toward commercialization Kunwoo Han ∗ , Chi Kyu Ahn, Man Su Lee, Chang Houn Rhee, Je Young Kim 1 , Hee Dong Chun CO2 Project, Research Institute of Industrial Science & Technology, San 32 Hyoja-dong, Pohang 790-330, Republic of Korea

a r t i c l e

i n f o

Article history: Received 26 June 2012 Received in revised form 24 December 2012 Accepted 4 January 2013 Keywords: CO2 capture Aqueous ammonia

a b s t r a c t The present work reviews the current status and prospects of ammonia-based CO2 capture technology, an alternative to conventional amine-based CO2 capture technology. The absorption chemistry and engineering issues for the process development and commercialization are dealt with. Representative developers at pilot-scale testing are Alstom, Powerspan, Commonwealth Scientific and Industrial Research Organization, and Research Institute of Industrial Science & Technology, while lab- and benchscale studies have at Korea Institute of Energy Research, Tsinghua University and Norwegian University of Science & Technology, etc. Published works on CO2 capture using aqueous ammonia state the removal efficiency of CO2 can be 90% and the product purity exceeds 98%, implying that the technical feasibility has been proven. Although being said that it is in the pre-stage of commercialization, some technical issues including the ammonia slip should be resolved to secure economic plausibility. Suggestions are made for the successful development of ammonia-based CO2 capture process for commercialization: suppression of ammonia vaporization, heat integration, minimization of absorbent flow rate, and bicarbonate-prevalent operation. © 2013 Elsevier Ltd. All rights reserved.

Contents 1. 2. 3.

4.

5.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Absorption chemistry of ammonia-based CO2 capture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ammonia-based CO2 capture technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Alstom’s chilled ammonia process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Powerspan’s ECO2 process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. CSIRO process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4. KIER process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5. RIST process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Issues and resolutions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Common technical issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Technology-specific issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3. Process economics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction Over the past decades, the drastic reduction of carbon dioxide (CO2 ) emissions has gained the attention of both the lay public and

∗ Corresponding author at: AMT Pacific Co., Ltd., Daejeon, Republic of Korea Tel.: +82 54 279 6291; fax: +82 54 279 6729. E-mail address: [email protected] (K. Han). 1 Je Young Kim is currently employed at AMT Pacific Co. Ltd., Daejeon, Republic of Korea. 1750-5836/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijggc.2013.01.007

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researchers. It is gaining importance as the growth of atmospheric CO2 concentrations accelerates. A bridge technology for achieving massive reductions of growing emission levels of CO2 from large point sources and carbon dioxide capture and storage, a.k.a. CCS, is garnering a lot of attention these days. Commercial plants for natural gas processing and by-product CO2 removal are under operation at various industries (Table 1 of Bandyopadhyay, 2010; MIT, 2012a). Carbon dioxide is being captured at a rate of a few tens to several hundred tons of CO2 per day, and therefore CO2 capture technology has reached the commercial

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Table 1 Qualitative comparison between amines and ammonia as CO2 capture absorbent. Amines

Ammonia

Remark

CO2 capture capacity Regeneration energy

0.5 mol-CO2 /mol-MEA 4.0 GJ/t-CO2 (MEA) 2.7 GJ/t-CO2 (NSC pilot)

1.0 mol-CO2 /mol-NH3 Lower than 2.0 GJ/t-CO2 a

Theoretical value Total thermal energy requirement is not only associated with the regeneration of absorbent, but also absorbent recovery

Absorption/regeneration rate Volatility

Faster Low

Fast High

Thermal degradation Corrosiveness Chemical stability Absorbent cost

Severe Vulnerable Forms heat stable salts Expensive

Negligible Resistant Stable Cheap

a

Volatility is closely related to the chemical loss and absorbent make-up cost

Based on the claim that the regeneration energy can be lower than over 60% compared to that of MEA with 14 wt% aqua ammonia (Resnik et al., 2004).

testing stage. Captured CO2 has been used for urea ((NH2 )2 CO) production, EOR (enhanced oil recovery), food/beverage industry, etc. However, it is noteworthy that the CO2 capture capacities at running facilities are relatively smaller compared with the emission amount of 3 Mt-CO2 /yr from 500 MW of a typical coal power station. It is obvious that although chemical or other industries are running CO2 capture facilities for various purposes, massive capture of CO2 at a scale of ∼Mt-CO2 /yr at large stationary sources such as power stations, iron and steel industry or cement industry has not been fully verified or commercialized. Among the various approaches to separate CO2 from flue gas or chemically transformed gas stream, the absorption-based CO2 capture technology is known to be the most practical method mainly due to its technical maturity and large capacity of gas treating volume. The general procedure to develop an absorbentbased CO2 capture technology is as follows: selection of absorbent, design of columns and process, calculation of hydraulics, etc. Criteria for absorbent selection are: (a) absorption or loading capacity, (b) regeneration energy, (c) absorption/regeneration rate, (d) volatility, (e) thermal and chemical stability, (f) corrosiveness, (g) absorbent cost, etc. Amines and mixed amines are regarded as the most mature solution to the post-combustion CO2 capture. Absorbent development to overcome the drawbacks such as higher regeneration energy, degradation problem, and high corrosive nature has been conducted using advanced amines (Tanaka et al., 2009; Goto et al., 2011; Iijima et al., 2011; Kim et al., 2011a). A seemingly outdated concept of capturing CO2 using aqueous ammonia is based on extensive experience of sweetening sour gases in the gas industry. Recently, the technology has regained the attention of researchers, giving them a viable option for massive CO2 capture, since the aqueous ammonia-based CO2 capture technology conveys several technical and economical advantages over conventional amine-based CO2 capture technology including high stability, low corrosiveness, high CO2 loading capacity, cheap chemical cost, low regeneration energy duty, etc. Simulation studies insisting that the aqueous ammonia-based CO2 capture technologies are much more economical from the energyconsumption standpoint have been published (Mathias et al., 2009; Valenti et al., 2012; Zhuang et al., 2011). This study examines the CO2 capture technologies based on aqueous ammonia at different organizations. Technical barriers toward the commercialization of the aqueous ammonia-based CO2 capture process are to be addressed and current efforts to overcome the difficulties are to be discussed. A brief discussion on the process economics focused on the regeneration energy requirement will follow.

2. Absorption chemistry of ammonia-based CO2 capture Following the previous discussion on the amine-based CO2 capture technology, a comparison between amines and ammonia for CO2 capture was made in Table 1. Slower absorption kinetics than amine solution (Qin et al., 2010) and high volatility are the two main demerits of ammonia-based CO2 capture. However, ammoniabased CO2 capture technology is superior to amine-based CO2 capture technology in such characteristics as low regeneration temperature (accordingly, lower regeneration energy requirements), high loading/capture capacity, low corrosiveness, and less degradability. The chemical reactions associated with the absorption of CO2 into aqueous ammonia can be found elsewhere (Bai and Yeh, 1997; Darde et al., 2010a; Rhee et al., 2011; Versteeg and Rubin, 2011). Capturing CO2 using aqueous ammonia occurs via the acid-base reaction between the acidic gas components in the target gas such as CO2 , H2 S, SOx and the ammonia liquor. Absorption and desorption of CO2 into ammonia solution can be described as follows (Eqs. (1)–(13)). Ions are formed as CO2 and/or NH3 react with water molecules. Thus the formed ammonium ions (NH4 + ) and bicarbonate ions (HCO3 − ), and other species in the solution tend to react to form ammonium salts such as ammonium carbamate (NH4 COONH2 ), ammonium carbonate ((NH4 )2 CO3 ) and ammonium bicarbonate (NH4 HCO3 ) in the solution. In general, CO2 absorption reactions in ammonia solution can be represented as Eqs. (1)–(8) for the vapor-liquid-solid reactions (Versteeg and Rubin, 2011). Additionally, ammonium sesquicarbonate ((NH4 )2 CO3 ·2NH4 HCO3 ) formation reaction was suggested by Darde et al. (2010a) (Eq. (9)): 2H2 O ↔ H3 O+ + OH−

(1)

+

CO2 + 2H2 O ↔ H3 O + HCO3 +

−

HCO3 + H2 O ↔ H3 O + CO3

−

(2)

2−

(3)

−

(4)

−

NH3 + HCO3 ↔ H2 NCOO + H2 O

(5)

NH4 + + HCO3 − ↔ NH4 HCO3(s)

(6)

NH4 + + NH2 COO− ↔ NH2 COONH4(s)

(7)

2NH4 + + CO3 2− + H2 O ↔ (NH4 )2 CO3 ·H2 O(s)

(8)

4NH4 + + CO3 2− + 2HCO3 − ↔ (NH4 )2 CO3 ·2NH4 HCO3(s)

(9)

+

NH3 + H2 O ↔ NH4 + OH −

A meaningful approach to understand the chemistry associated with the absorption mechanism has been recently raised by

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Kim et al. (2011b). The computational study has revealed that ammonia molecules are involved with the absorption reaction as the catalyst and reactant, simultaneously. Moreover, termolecular reaction among H2 O–CO2 –NH3 prevails in the formation of ammonium salts rather than the zwitterions mechanism, which is a two-step reaction mechanism. The regeneration heat is mainly composed of desorption reaction heat, evaporation energy, and sensible heat of the solution. Desorption reaction equations with their reaction heats are presented in Eqs. (10)–(13). The reaction equations reveal that the desorption (or the regeneration of absorbent solution) heat largely depends on the species, i.e., ammonium carbamate, ammonium carbonate, and ammonium bicarbonate. From the comparison of the reaction heats, it is recommended to operate the process at bicarbonate-prevalent condition, since the ammonium bicarbonate has the lowest reaction heat among the ammonium salts in the solution (Versteeg and Rubin, 2011). Therefore, one should know the concentrations of ionic species to better monitor and control the process for the operation with lowest energy consumption: NH4 HCO3(aq) ↔ NH3(aq) + H2 O(l) + CO2(g)

H = 64.3 kJ/mol (10)

(NH4 )2 CO3(aq) ↔ 2NH3(aq) + H2 O(l) + CO2(g)

H = 101.2 kJ/mol (11)

2NH4 HCO3(aq) ↔ (NH4 )2 CO3(aq) + H2 O(l) + CO2(g) H = 26.9 kJ/mol NH2 COONH4(aq) ↔ 2NH3(g) + CO2(g)

(12) H = 72.3 kJ/mol

(13)

3. Ammonia-based CO2 capture technologies Absorption using ammonia solutions has been recognized as a mature technology in the gas industry for treating acid gases such as CO2 , SOx, etc. One of the major remaining issues for the commercialization of aqueous ammonia-based CO2 capture technology is the ammonia slip due to its intrinsic chemical nature of high volatility. Table 2 is a summary of current worldwide development status on CO2 capture technologies using aqueous ammonia. Several research groups are developing the technology to replace amine-based CO2 capture technology; Alstom, Powerspan, CSIRO (Commonwealth Scientific and Industrial Research Organization), KIER (Korea Institute of Energy Research) and RIST (Research Institute of Industrial Science & Technology). Ammonia-based CO2 capture process is also being developed in the academy, for example, Tsinghua University in China and Norwegian University of Science and Technology (NTNU) in Norway (Qin et al., 2010, 2011). Detailed descriptions of each process will be given together with the technical characteristics in the following sections.

drawn in Fig. 1. The current development status of Alstom’s CAP can be found in Telikapalli et al. (2011). The process consists of feed gas cooling, CO2 separation and capture, absorbent regeneration and production of high purity CO2 . Feed gas is cooled to ∼0 ◦ C by direct contact coolers (two steps). Absorbent solution, ammonium carbonate (AC) or CO2 -lean solution, absorbs CO2 . Then, the solution is converted into ammonium bicarbonate (ABC) or CO2 -rich solution in a slurry form, which can be regenerated by heating. The study contends that the process shows high CO2 capture capacity, production of highly pure CO2 , process robustness to the impurities, stability to thermal degradation, no emission of pollutants, and low chemical cost. Based on the operation result of Pleasant Prairie Carbon Capture Pilot Plant (P4, 1.7 MW slipstream from the plant, We Energies Pleasant Prairie, WI, USA), the CO2 capture ratio has reached over 90% (Telikapalli et al., 2011). A public announcement of the progress of a chilled-ammonia CCS validation project (American Electric Power (AEP)’s Mountaineer Plant in New Haven, WV, USA) has been released (Alstom, 2011; GCCSI, 2012). The project is the first CCS facility from a coal-fired power station, which treats flue gas at a 20 MW scale (from 1300 MW). Key results from the operation are: ∼100,000 t-CO2 /yr (injection for storage ∼7000 t-CO2 /yr), CO2 capture ratio 75–90%, CO2 purity >99%, and stable operation during load changes. Alstom is accelerating the technology development and plans to extend its deployment worldwide with large-scale projects (>200 MWe) such as Pioneer with Transalta in Canada and the Mountaineer plant with AEP, both planned to start operations in 2015. However, due to the financial reasoning the Pioneer project was canceled on April 27, 2012 (MIT, 2012b). Recently, Alstom, together with Technical Center Mongstad (TCM), set a plan to operate CAP for 12-18 months according to a public release of Alstom (2012). From a simulation study, Zhuang et al. (2011) insisted that the process consumes about 60% of energy compared to the MEAbased CO2 capture or 2.5 GJ/t-CO2 . They claimed that the optimum ammonia concentration of the process is 20–30 wt%. Recent work based on process simulation from the University of West Virginia (2012) reports that the electricity cost will increase by 45% when a virtual process, Unit 100(CAP), is employed (from $0.06/kWh to $0.1045/kWh (550 MW power station)). According to the newest process simulation, for 90% CO2 capture the cost of electricity generation was estimated at $US 105/MWh (Versteeg and Rubin, 2011). Even Darde et al. (2010b) insisted that the energy requirement be lowered to 2 GJ/t-CO2 in the regeneration column. A recent simulation study with two electrolyte models (e-NRTL model and Extended UNIQUAC model) has been reported (Darde et al., 2012a). It was insisted that the Extended UNIQUAC model appear to be more satisfactory for the description of experimental data, however, e-NRTL model simulate the energy requirement better resulting in the stripping energy of 2.4 GJ/t-CO2 . Different process configuration has been simulated with the Extended UNIQUAC model and the heat requirement was claimed to be in the same range as that with advanced amine process (Darde et al., 2012b). However, it should be noted that the quantitative analysis on the regeneration energy obtained from the pilot facility operation data are still insufficient.

3.1. Alstom’s chilled ammonia process 3.2. Powerspan’s ECO2 process One of the leading groups in the early development of ammoniabased CO2 capture technology is the power company Alstom. They have been developing the CO2 capture process called chilled ammonia process (CAP), which cools the flue gas down to very low temperatures, preferably below 10 ◦ C. It treats the exhaust gas from the power plant using a high concentration (∼28 wt%, a typical initial mass fraction) ammonia solution, and forms the slurry of ammonium salts (Darde et al., 2010b). A representative figure is

Powerspan has been developing a CO2 capture process using aqueous ammonia, the ECO2 process. The main feature of the process is that it could be combined with a de-SOx process, ECO-SO2 , by utilizing the reaction of ammonia with SOx, which produces the ammonium sulfates. Powerspan had acquired a proprietary license from NETL (National Energy Technology Laboratories) in 2007 (McLarnon and Duncan, 2009). Pilot testing was carried out at

Table 2 Summary of ammonia-based CO2 capture processes: from test results (as of October 2012). ALSTOMa (chilled ammonia)

Working partners Key features

American Electric Power Feed gas cooling High pressure product stream

Technology Center Monstad Exhaust gases from a Residue Catalytic Cracker and Natural Gas Combined Heat and Power plant

Project location

20 MW, Validation-scale testing at AEP’s Mountaineer Plant

Monstad, Norway

Operation Conditions

Feed gas NH3 conc. Absorber Regenerator

Performance Test results

Status and prospect

a b c d e f

Recovery [%] CO2 purity [%] Production [t/yr] Operation

CO2 ∼ 10% T: 0–10 ◦ C ∼28 wt% T: 0–10 ◦ C T > 100 ◦ C P: 20–40 bar 75–90 >99 ∼100,000 September 2009–June 2011 Commercialization expected in 2015

Bandyopadhyay, 2010; Alstom, 2011; Alstom, 2012. Powerspan, 2011. Yu et al., 2011. Yi and Kim, 2008. MIT, 2012c. Nm3 /hr means cubic meter per hour at 0 ◦ C and 1 atm

Powerspanb (ECO2 )

CSIROc

KIERd

RIST

FirstEnergy Combined with de-SOx process (ESO2 ) Ammonium sulfate (SOx removal) production 1 MW, R.E. Burger Plant, Shadyside, OH, USA

Delta Electricity Two absorbers Flue gas cooler

Pressurized regeneration

POSCO Combined with waste heat recovery system

CO2 : 11–12% Coal-fired

∼6000 December 2008–December 2010

CO2 : 9–12% Coal-fired ∼6 wt% T: 10–30 ◦ C T: 90–150 ◦ C P: 3–8.5 atm ∼85 >99 ∼3000 February 2009–

CCS demonstration project for EOR on holde

Pilot project completed in 2010

>90

2012 start up Scheduled to run 12-18 months

Munmorah Power Station, Australia

Bench-scale apparatus at KIER, Daejeon, Republic of Korea 40-100 Nm3 /hf Coal-fired ∼13 wt% T: 20–25 ◦ C T: 80 ◦ C P: 6.5 bar 90 99.9

Pohang Steel Works, POSCO, Republic of Korea CO2 ∼ 23% (BFG) <10 wt% T: ∼40 ◦ C T: ∼80 ◦ C P: 1 atm 90 >95 ∼3,000 May 2011– Design of commercial plant by 2014

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Developer

273

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Fig. 1. Process schematic of Alstom’s chilled ammonia process (CAP) (Kozak et al., 2009).

First Energy’s R.E. Burger Plant (Shadyside, Ohio, 1 MW-slipstream from coal-fired power station) from 2008 to 2010. The ECO2 process has been developed to recover and reuse the highly volatile ammonia. The basic working principle of the process is as follows (Fig. 2); (a) ammonia vapor is collected and used for SOx removal (ECO-SO2 ), (b) ammonia deficient scrubbing liquid is circulated and captures ammonia, (c) the ammonia-enriched liquid is sent to ECO-SO2 ; ammonium sulfate production for cost compensation. The technology has been developed to capture CO2 , SOx, NOx simultaneously (Resnik et al., 2004). A CO2 capture study has been conducted using model gas (15% CO2 , N2 balance) at temperatures in the range of 60–100 ◦ F with ammonia concentrations of 7, 14, and 21%. It was shown that the energy consumption using 14% ammonia solution could be lowered much less than by 64% compared with MEA-based CO2 capture process due to the higher absorption capacity, lower heat for reaction and evaporation (absorption temperature of 80◦ F (27 ◦ C) and regeneration temperature of 180 ◦ F (82 ◦ C)). Pilot-scale research has been presented at GHGT-9 (International Conference on Greenhouse Gas Technologies) (McLarnon and Duncan, 2009). Using the 1-MW scale pilot facility for CO2 capture, CO2 could be captured at ∼25 t/d (Powerspan, 2011); inlet CO2 concentration 11–12%. An independent evaluation report has been released by Worley Parsons that it was expected to capture and compress CO2 at a cost lower than $40/t-CO2 at 220 MWe using ECO2 (Powerspan, 2010) (CO2 recovery 90%, 1000 Btu/lb-CO2 or 2.3 GJ/t-CO2 ). The ECO2 process had been selected by Basin Electric (Antelope Valley Station, North Dakota, 1 Mt-CO2 /yr, CO2 for EOR) as 120 MW commercial demonstration process scheduled to run in 2012. As of December 2010, however, the initial plan was canceled according to the MIT web site (MIT, 2012a). A recent publication reveals that the absorbent solution was changed to mixed ammonium/alkali solution for CO2 capture (Duncan et al., 2010). Another report states that the process has

been reformulated to use a mixture of aqueous amine (GCCSI, 2012). 3.3. CSIRO process An ammonia-based CO2 capture process has been developed at CSIRO independently using a very low concentration ammonia liquor. Fig. 3 shows the schematic process configuration of CSIRO’s process. The pilot facility has a flexible configuration of absorbers to test various absorption conditions. A flue gas cooler has been installed to pretreat the exhaust gas. The operating condition for absorption: ammonia concentration <5 wt%, gas temperature ∼10 ◦ C, absorption inlet temperature (absorber) ∼45 ◦ C, CO2 concentration of the feed gas ∼13% (at 105 kPa), regeneration at 500 kPa. CSIRO has been developing a CO2 capture pilot facility at Munmorah power station with Delta Electricity at a rate of 3000 tCO2 /yr. Recent published work on the field pilot at Munmorah power station states that the overall regeneration energy requirement is 4–4.2 GJ/t-CO2 , comparable to that of MEA process (Yu et al., 2011). It was found that the CO2 removal efficiency could be up to 85% even with a low ammonia content of 6 wt%. The product CO2 concentration is generally over 99%. The pre-treatment column removed up to 95% of SOx from the flue gas. It was insisted that the ammonia concentration could be lower than 200 ppm when the regeneration takes place under pressurized conditions, at 850 kPa, and at low temperatures of 20–25 ◦ C. 3.4. KIER process KIER has been developing an ammonia-based CO2 capture technology jointly with Korea Advanced Institute of Science & Technology (KAIST) since mid-2000 (You et al., 2007; Kim, 2008; Kim et al., 2010). The process flow is a typical solution-based absorption process at an absorption temperature of 25 ◦ C, as

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Fig. 2. Process schematic of aqueous ammonia multi-pollutant control system (Figure 6, Cerfino et al., 2005).

shown in Fig. 4. Regeneration temperatures are in the range of 80–120 ◦ C depending on the operation conditions. A semi-batch lab apparatus and a continuous absorptionregeneration apparatus have been installed and experiments have been carried out using a model gas (CO2 10%) and aqueous ammonia (ammonia concentration 11–17%). The optimum temperature for absorption was found to be 20–30 ◦ C and the optimum ammonia concentration was 13 wt% at regeneration temperature of 80 ◦ C when the absorption temperature was 25 ◦ C. According to You et al. (2007), the ammonia-based CO2 capture process can be operational

without any precipitation of solid salts when the ammonia concentration is over 8 wt% (5 M/kg-H2 O), since the typical CO2 loading at absorption is less than 0.5 mol-CO2 /mol-NH3 , and the inlet gas temperature is ∼50 ◦ C. Kim et al. (2010) have reported the experimental results on the regeneration temperature and pressure depending on the ammonia concentration. Pressurized CO2 product stream of 10 bar can be obtained at regeneration temperatures above 140 ◦ C and at ammonia concentrations of 20–32%. Ascertaining the technical feasibility of the process, KIER has constructed and operated a bench-scale pilot facility (100 Nm3 /h).

Fig. 3. Process schematic of CSIRO process (Yu et al., 2011).

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Fig. 4. Process schematic of KIER process (Yi and Kim, 2008).

Using the flue gas from a coal-fired power plant, the facility has been operated and the performances were; CO2 recovery ratio of 90%, CO2 concentration >99.9%, transfer capacity ∼0.08 kg-CO2 /kgsolution (Kim, 2009). 3.5. RIST process A unique CO2 capture process using a low concentration ammonia solution (<10 wt%) targeted for the iron and steel industry has been developed (Kim et al., 2009, 2011c; Rhee et al., 2011). The process consists of three columns as shown in Fig. 5; namely absorber, stripper (regenerator), and concentrator. The concentrator was adopted for the recovery and reuse of ammonia. BFG (blast furnace gas) is one of the by-products at iron industry and used for combustion. In the process BFG, at temperatures of 35–70 ◦ C, is introduced at the bottom of the absorber. The key characteristics of the RIST process are as follows: (a) integrated with waste heat recovery system for the generation of steam and its supply to reboilers, (b) application of soft-sensing technology for process monitoring, control and optimization, and (c) maximized heat integration within the process. Experiments of absorption-regeneration using lab scale apparatus (2 Nm3 /h) with model gas to examine the technical performances such as the capture ratio (Kim et al., 2009) have been conducted. Succeeding the previous technical feasibility with lab apparatus, the 1st stage pilot plant was constructed in late 2008. The facility could treat BFG at the rate of 50 Nm3 /h. Field tests with a total running time of over 1000 h had been conducted using the facility. From the 1st stage pilot plant operation, it was found that the CO2 recovery ratio was over 90%, and the purity of CO2 product was over 95%. Construction of the 2nd stage pilot plant (1000 Nm3 /h), a 20fold-scale-up facility of the 1st stage pilot plant, was completed and followed by field test runs in 2011. Detailed descriptions on the

test results with discussion, specifically with the steam generated from the waste heat recovery system at low and mid-temperature in the iron and steel industry, can be found in the literature (Kim et al., 2011c). With the current test facility, typical CO2 removal efficiency of over 90% at a purity of over 98% was obtained. From a qualitative point of view, a waste heat recovery system for the steam generation and supply was successfully operated. Although it seems to be an energy-intensive process, it is noteworthy that all the steam required for running the process can be satisfactorily supplied by the low- and mid-temperature waste heat in the ironand steel-making process. The salient feature of the ammonia-based CO2 capture process is the relatively low regeneration temperature, ca. 80 ◦ C compared to that of MEA or ∼120 ◦ C, which makes the process quite economical if the regeneration energy may be obtained from recovering waste heat at low temperature, which might otherwise be abandoned. Recall that the waste heat recovered from a boiler stack at 140–150 ◦ C could be successfully supplied to the current CO2 capture facility. As insisted in the previous publication (Rhee et al., 2011), it is highly possible to run the process more economically if there exists available waste heat at low temperature or less than 150 ◦ C. Unlike the post-CO2 capture process at power plants, the CO2 -free BFG leaving the absorber will be used as a fuel for combustion in the RIST process, which accordingly saves the cost and energy consumption for the power generation via reduction of gas volume and increase of heating value; an increased heating value of BFG by 20% is expected. 4. Issues and resolutions 4.1. Common technical issues It can be generally accepted that the technical feasibility of CO2 capture using aqueous ammonia is proven, i.e., the CO2 capture

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277

Fig. 5. Process schematic of RIST process (Kim et al., 2011c).

ratio can be maintained at 90% with CO2 purity of over 95%. In this section, we address the common issues involving process development and the difficulties each ammonia-based CO2 capture process faces. It is worth noting that the questions regarding the successful development and commercialization of ammonia-based CO2 capture technology narrow down to the question of increasing the process economics. Since the technology is not fully optimized and running at commercial scales anywhere in the world, it might be quite premature to critically evaluate the process economics at present. However, we believe that a rough evaluation of process economics is meaningful in a sense that we can investigate the process details and propose ways to improve the economics for full commercialization. First, the reaction mechanisms associated with the capturing mechanism should be understood in-depth. Although the chemistry associated with the absorption and desorption of CO2 with absorbent solution has been covered in the literature (Bai and Yeh, 1997; Rhee et al., 2011), it is not fully understood as pointed out by Kim et al. (2011a). Specifically the system associated with the electrolytes is hardly handled with accuracy in most of the simulators, though in reality, electrolytes are abundant in the carbonated aqueous ammonia solution or slurry. Second, the ammonia vaporization issue should be resolved. High volatility is an intrinsic burden that the ammonia-based CO2 capture technology bears, which leads to the ammonia slip issue. The suppression of ammonia slip is of primary importance for the successful development of the process technically and economically. In general, ammonia vapor can be easily captured using water, which is employed in the Alstom, KIER, and RIST processes. CAP tries to avoid the problem by cooling the feed gas lower than room temperature. The ammonia concentration can be maintained below 200 ppm under pressurized condition in the stripper of the CSIRO process. According to the Korean government regulation of

exhaust gas from industry, the ammonia level should be controlled to less than 50 ppm. Although the reported NH3 concentrations in the literature are not higher than 50 ppm in many processes, the issue remains critical during the technology development since the method to control the ammonia slip is inconvenient and costly. Two approaches are available to extricate the ammonia slip issue either by utilizing or suppressing it. The former can be realized as manufacturing salts such as fertilizers (ammonium sulfate, ammonium nitrate, etc.) using the excess ammonia vapor as in the ECO2 process. The exact materials balance should be maintained for this case. The latter option, suppression of ammonia slip, can be achieved by adding chemicals or changing the operation conditions. To directly minimize the ammonia slip, additives or washing water should be applied. The selection criteria of additives to resolve the issue might be (a) suppression ability of ammonia vaporization, (b) sustainability of CO2 absorption efficiency, and (c) reusability of additives. Researchers suggested candidate additives such as AMP (2-amino-2-methyl-1-propanol), AMPD (2-amino-2-methyl-1-propandiol), AEPD (2-amino-2-ethyl1-propandiol), THAM (tri-methoxy-amine) and glycerol for the prevention of ammonia vaporization (Yi and Kim, 2008; Seo et al., 2011). Using the additives for the suppression of ammonia slip naturally poses the following secondary issues; operating conditions change such as the regeneration temperature and pressure due to the chemistry change, make-up cost of additives due to the chemical loss, and the formation of heat stable salts requiring extra treatment. Using large amounts of washing water can lead to the increased heating duty for ammonia recovery. Optimization of process conditions can be performed by changes in operation variables and process configuration or process improvement by heat integration. Third, the quantification technique of gaseous ammonia concentration both in the CO2 -free stream and product CO2 stream has to

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be developed. Accurate measurement of ammonia vapor concentration is very important to process monitoring and control. If not specifically mentioned for gaseous ammonia concentration measurement method, it is assumed that most of the ammonia-based CO2 capture processes use gas detection tube, IR (infrared) method or wet analysis. An on-line IR method (Fuji Electronic) was applied to RIST process; however, it was hard to maintain the measurement system stably, and sometimes the display values were quite different from the detection tube measurement due to the condensation of water vapor (moisture). The typical methods for quantifying the gaseous concentration of ammonia are indophenol method and ammonia ion-selective electrode method (American Public Health Association, 1998). However, it is highly recommended to develop and use an easier and facile method due to the fast data acquisition requirement. Finally, gas cooling is another issue in many processes that use ammonia as an absorbent possess. In general, the reaction rate can be increased as the temperature of the reactants increases, while the increased absorbent temperature stands in the way of accelerating the absorption. Therefore, there exists a tradeoff or optimum value for gas cooling degree. The gas cooling process can be realized by adopting gas cooling apparatus, however, excessive gas cooling results in an unfavorable cost increase. Therefore, the economic benefit can be extremely manifested when there is “free or cheap” coolant such as ocean water. Conclusively, if the CO2 capture facility is located in a coastal region, the process economics can be greatly improved. 4.2. Technology-specific issues According to the public release report by Alstom (2011), the technology seems to be technically mature for commercialization. However, as pointed out by NETL’s report (2007), the energy consumption associated with gas cooling should be minimized for Alstom’s CAP. The process can be economically infeasible if there is no cheap coolant or cooling water. Due to the high concentration of ammonia (∼28 wt%, Darde et al., 2010b), ammonia slip is another big issue. Countermeasures to resolve the ammonia slip together with its quantification have to be developed. And it should be noted that the cooling duty for cooling the flue gas is of great concern, which makes the process economically feasible only if abundant cold water such as ocean water exists. Slurry handling may be an issue for engineering and commercialization due to the high content of ammonia and low temperature. Some of the unresolved issues associated with the commercialization of the ECO2 process are the energy requirement for cooling the target gas (typical exhaust gas temperature from a power plant is around 50 ◦ C), materials imbalance issue for capturing SOx considering the inlet concentration and flow rate of SOx and those of ammonia liquid should be considered. For the stable production of ammonium sulfate by utilizing the ammonia vapor, the material balance for SOx and ammonia should be easily maintained. Typically, the concentration of SOx after desulfurization facility is ∼10 ppm, which might be too small compared to the amount of ammonia vapor leaving the absorber. The CSIRO process casts some questions about the technical performances from the reported works; high regeneration energy requirement, high NH3 vapor concentration, etc. (Yu et al., 2011). Results on controlling the ammonia slip and the energy requirement show the need for further process improvement. Due to the low concentration of ammonia in the absorbent solution, it can be assumed that the ammonia slip is not as severe as with other processes. However, like the head and tail of a coin, the regeneration energy of the solution will be presumably higher than those of other ammonia-based processes, and the electricity cost for circulating the liquid is probably much higher than those of others because of

its low ammonia concentration. It should be noted that the regeneration energy was obtained under the conditions of pressurized regeneration and low temperature absorption (under 30 ◦ C). Like CAP, the CSIRO process requires heavy cooling duty since it adopts the gas cooling system, too. Although ammonia vaporization is seemingly not critical due to the low concentration of ammonia, post-treatment of ammonia either by recovery or utilization has not been clearly described. The CSIRO process seems to suffer from ammonia loss and salt precipitation depending on the operating conditions and ammonia concentration. It was insisted that the high pressure regeneration process provide comparable energy savings compared to that of ambient regeneration condition for KIER process (Kim, 2009; Yi and Kim, 2008). The pressurized regeneration does not reduce the total compression energy drastically but does reduce energy consumption to some degree by positioning the compression function within the capture facility. However, high pressure regeneration can be costeffective only if the ammonia slip can be suppressed by slightly increasing the energy requirement. The energy consumption in the “recovery column” (equivalent to concentration column in the RIST process) has not been discussed in detail, nor has it been clearly described on the total energy requirement. To the authors’ knowledge, there has been no published work on energy consumption comparison of the KIER process: energy requirement for the comparison between the regeneration at atmospheric and under high pressure. There are several issues with the process being developed at RIST. By lowering the ammonia concentration and supplying washing water, the ammonia slip can be controlled to less than 10 ppm in the product stream. To substantially reduce the ammonia level as the gas leaves the absorber, additive search and process improvements are being implemented. Steam can be provided by recovering the waste heat from boiler stack gases. However, to extend the applicability of the process to other industries, it is recommended to reduce the energy requirement in the regeneration and concentration columns. 4.3. Process economics Detailed analysis on the process economics for CO2 capture plants can be found elsewhere (IEA, 2005, 2007; Valenti et al., 2012; Versteeg and Rubin, 2011). The core factor in the analysis of process economics is the energy requirement for regeneration. Typical amine-based CO2 capture processes employ reclaimers for the recovery of absorbent solution. However, in the ammonia-based technologies, recovery unit is required for the ammonia recycle (except ECO-SO2 process of Powerspan). Therefore, the energy requirement should be viewed as a whole, since a decrease in certain categories of energy requirement can cause an unexpected increase in other components of energy requirements due to the interactions of materials and energy flow in the process. Selected process performances with regeneration energy requirements have been tabulated in Table 3. Note that most of the numbers in the energy requirement column are not obtained from the operation results of large scale facilities, i.e., they are produced by process simulators or obtained from a small-scale pilot facility (less than 10 MW). Moreover, some are not clearly described to identify each component of the economics evaluation, which makes it more difficult to analyze. An early report on steam consumption between amine- and ammonia-based CO2 capture stated that the latter uses less than 1/3 of the former for unit CO2 recovery (Ciferno et al., 2005). Moreover, it is worth mentioning that estimated total energy consumption of ammonia-based CO2 capture can be lowered to the level of 27% of MEA-based CO2 capture process (McLarnon and Duncan, 2009). Although most of the figures are obtained

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Table 3 Performance summary of current ammonia-based CO2 capture technologies. Developer

Study method

Alstom

Test Simulation

Aqueous ammonia [wt%]

CO2 removal [%] 75–90

CSIRO

Test

6

85

Simulation

5

86.5

Test Test Simulation

a b c d

99

5b

2.3

200c

4–4.2

90

9 9

Regeneration energy [GJ/t-CO2 ]

2.4a

Test

RIST

NH3 conc. [ppm]

∼20

Powerspan

KIER

CO2 purity [%]

99–100

2.94

90

>99

90

>98

d

5

Study basis of pilot facility or simulation

References

20 MW, AEP’s Mountaineer Power Plant Flue gas flow rate of 782 kg/s

Alstom (2011) Darde et al. (2012a)

1 MW, FirstEnergy

Powerspan (2010)

∼0.5 MW at Munmorah Power Station 500 MW

Yu et al. (2011)

3

Dave et al. (2009)

3.4

100 Nm /hr

Kim (2009)

3.1

1000 Nm3 -BFG/h, POSCO 1000 Nm3 -BFG/h

Kim et al. (2011c) Unpublished work

Value obtained using e-NRTL model (cf. 2.4 GJ/t-CO2 with Extended UNIQUAC model). From McLarnon and Duncan (2009). At the stripper (under pressureized condition of 850 kPa). Energy consumption (based on 800 kcal/kg-CO2 ).

from simulations, from the energy requirement viewpoint, the ammonia-based CO2 capture process looks promising; 2.0 GJ/tCO2 for CAP (Darde et al., 2010b), 2.3 GJ/t-CO2 for Powerspan (1000 Btu/lb-CO2 , Powerspan, 2010). The regeneration energy requirement looks high for the CSIRO process (Dave et al., 2009), while that of KIER has been reported as 3.4 GJ/t-CO2 (Kim, 2009). RIST has operated the 2nd stage CO2 capture pilot facility utilizing waste heat as regeneration energy. Simulator for process modeling is under development and a recent result shows the regeneration energy requirement of 3.1 GJ/t-CO2 with ammonia concentration of 9 wt% (unpublished work). Using KS-1 solvent, Mitsubishi Heavy Industry (MHI) insisted that the KM-CDR process is operational at 3.1 GJ/t-CO2 (750 kcal/kgCO2 , Tanaka et al., 2009) as specific heat consumption. A more recent publication by Iijima et al. (2011) states that the newly developed absorbent consumes 2.44 GJ/t-CO2 (583 kcal/t-CO2 , recovery ratio of 77.8%, without cooling of CO2 absorption section). As an example of CO2 capture technology development in the iron and steel industry, a pilot field test result of an amine-based CO2 capture process has been reported and the regeneration energy requirement was 2.7 GJ/t-CO2 by NSC-Kimitsu Works (Mimura et al., 2011). Comparing the heat consumption, possibly the regeneration energy requirement, of ammonia-based technology with those of amines, the energy consumption of ammonia-based process is comparable to the reported values of amine-based processes; however, advancement on the process is needed to be more competitive. It is recommended to compare the overall cost analysis including the capital expenditure and operating and management cost for economics evaluation. Caution has to be taken since most of the literature has dealt with the simulation only (Darde et al., 2010b; Dave et al., 2009). Although accepting that it is premature to accurately provide the total cost for CO2 capture, it might be informative to conduct a rough estimation for the successful commercialization of the technology. In other words, the operating cost comparison with larger facilities might be more instructive. And the total energy consumption comparison should not be performed by merely comparing the regeneration energy requirement. Rather, it is recommended to evaluate the process economics on the total energy consumption together with the regeneration energy requirement. A simulation study on the chilled ammonia absorption process has revealed that the electric energy for chilling is comparable to that of stripping of both CO2 and NH3 , though the combined heat duty is around 3.0 GJ/t-CO2 (Darde et al., 2011). A study on the comparison of CO2 capture cost at different industries when MEA is used as absorbent material has been reported (Ho et al., 2011). According to the study,

the capture cost ranges from 60 to 100 AUD/t-CO2 depending on the industry. It is worth mentioning that the economics should be evaluated on the same or at least similar basis such as the CO2 capture ratio at 90% and CO2 purity of the product stream at 99%. Most researchers have focused on the regeneration energy requirement, for it is a major factor of the total thermal energy requirement. However, head to head comparison on each component of energy requirement is needed for the precise evaluation and comparison for process development. Since there are not many published works on the testing results, process simulation might serve the purpose for the time being. A rough estimation of operating cost from the pilot facility operations of the RIST process indicated that the majority was accounted for by the electricity consumption (unpublished work). The operating cost for CO2 capture can be significantly reduced when the thermal energy is provided by the waste heat recovery system, i.e., the cost of thermal energy consumption is very low. One of the most critical issues associated with the process economics is the heat integration. Note the CO2 absorption reaction is highly exothermic and the regeneration energy requirement is very high. Analysis using a process simulator is very helpful to configure the process equipment and improve the heat integration within the process. Well-designed heat integration approach will also be beneficial to reduce the ammonia slip by lowering the temperature and flatten the temperature profile wherever it is necessary. Based on previous knowledge and experience, to improve the process economics we suggest the following: (a) Suppress the ammonia slip. Gas cooling might be an option, but it is an energy-intensive process. Ammonia slip can be minimized either by additives or process improvement. One way of achieving the mission is to operate pressurized absorption and/or pressurized regeneration process. Pressurized absorption and/or regeneration may lead to the cost reduction. However, it has to be thoroughly verified via field test with large-scale facility, since the operation cost increases if the high pressure blower is installed. (b) Maximize waste heat recovery and utilize process heat when available. Electricity for circulating the cooling water imposes a heavy duty on the operation cost. Therefore, heat integration within the process can greatly improve the process economics. Examples are the selection of optimum type and size of the heat exchangers and method development for recycling and reuse of process water. (c) Minimize the circulating flow rate of absorbent solution.

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Large portion of thermal energy consumption comes from the sensible heat of absorbent solution for regeneration. Since the process is not at equilibrium in reality, the flow rate demand of absorbent solution is higher than theoretical value. Minimizing it can be achievable through the proper selection of packing materials, heat integration, acceleration of absorption rate, and so on. (d) Operate the absorption process as bicarbonate-prevalent status. Depending on the CO2 loading (per ammonia concentration), the ionic species profiles vary. Therefore, it is not always recommended to fully desorb all the CO2 molecules in the regenerator. Instead, running the process with carbonated ammonia liquor may lead to a reduced thermal energy requirement. Prior to find the optimum operating condition, one has to precisely monitor the process, which can be performed by the extension and application of the previous work of Ahn et al. (2011). 5. Conclusions From the investigation of the current status of ammonia-based CO2 capture technology, we found that the technology has been proven to be technically feasible and the technology readiness level can be said to be appropriate for pilot testing. Some variants mainly due to the target application process with different ammonia concentrations and operating conditions are found; Alstom’s Chilled Ammonia Process, Powerspan’s ECO2 Process, CSIRO’s Process, KIER’s Process and RIST’s Process. Most of the ammonia-based CO2 capture processes have announced they achieved CO2 recovery ratio over 90%, and the purity of product stream exceeds 98%, which meet the CO2 gas requirement for geological storage. Accepting that it is technically quite feasible, economic plausibility should be investigated in-depth before attempting commercialization. The prevention of ammonia vaporization is the most critical issue among the technical difficulties the process faces. Additive search for the suppression of ammonia vaporization, utilization of ammonia vapor for the production of salts, or pressurized regeneration are being investigated to resolve the issue with moderate success. More comprehensive study on the process simulation to tackle the problem should be initiated with the deep understanding on the chemical reactions associated with the absorption/desorption chemistry or electrolytes in aqueous absorbent solutions. Since the ammonia-based CO2 capture technology features the relatively low regeneration energy and low regeneration temperature at under 100 ◦ C, it can be suitably established where there is available waste heat for generating low grade steam to provide regeneration energy. To become more competitive option, it is recommended that the regeneration energy requirement of the ammonia-based CO2 capture technology, steam consumption for stripping, be lower than 2.5 GJ/t-CO2 considering the reported values of the amine-based CO2 capture technology. Collaboration is mandatory to resolve the common issues such as rapid and facile quantification of ammonia vapor concentration and ammonia slip. Acknowledgement This research was supported by the Energy Efficiency & Resources of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Knowledge Economy (No. 20122010200040). References Ahn, C.K., Lee, H.W., Chang, Y.S., Han, K., Kim, J.Y., Rhee, C.H., Chun, H.D., Lee, M.W., Park, J.-M., 2011. Characterization of ammonia-based CO2 capture process using ion speciation. International Journal of Greenhouse Gas Control 5, 1606–1613.

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