Current status and challenges of the ammonia escape inhibition technologies in ammonia-based CO2 capture process

Applied Energy 230 (2018) 734–749

Contents lists available at ScienceDirect

Applied Energy journal homepage: www.elsevier.com/locate/apenergy

Current status and challenges of the ammonia escape inhibition technologies in ammonia-based CO2 capture process ⁎

Fu Wanga,b, Jun Zhaoc, He Miaoa, Jiapei Zhaoa, Houcheng Zhangd, Jinliang Yuana, , Jinyue Yane,

T

⁎

a

Faculty of Maritime and Transportation, Ningbo University, Ningbo 315211, China Ningbo RK Solar Tech. Ltd., 315200 Ningbo, China c Key Laboratory of Efficient Utilization of Low and Medium Grade Energy (Tianjin University), Ministry of Education of China, Tianjin 300072, China d Department of Microelectronic Science and Engineering, Ningbo University, Ningbo 315211, China e Department of Chemical Engineering, Royal Institute of Technology, SE 100 44 Stockholm, Sweden b

H I GH L IG H T S

escape mass transfer is presented. • Ammonia state of developments in ammonia escape abatement technologies are reviewed. • Current gaps in ammonia escape abatement technologies are developed. • Research • Challenges and future prospects of ammonia escape abatement are suggested.

A R T I C LE I N FO

A B S T R A C T

Keywords: CO2 capture Ammonia escape Ammonia solvent Mechanism Chilled ammonia process Additives

CO2 capture using ammonia solvent is an alternative to the conventional amine-based CO2 capture technology. While ammonia escape is one of the main barrier limiting its implementation. The present work reviews the current status of ammonia escape mechanisms and its inhibition technologies. The chemistry of ammonia-based absorption and desorption are analyzed, and the mass transfer of the ammonia escape are presented and discussed. Most suppression approaches for ammonia slip are in lab- and bench-scale studies. The representative development of the pilot-scale tests involves NH3 abatement and recycling process and chilled ammonia process (CAP). Some other novel processes have been reported the potential to reduce ammonia slip significantly and relatively lower energy penalty, but some technical issues including the process modification and parameters optimization should be resolved to secure economic feasibility. Integration of different ammonia inhibition approaches is suggested for the future development of ammonia slip suppression process.

1. Introduction Coal accounts for roughly 25% of the world energy supply and 40% of the carbon emissions [1]. Coal-fired power plants, iron and steel manufacturing industries, cement industries, as well as other chemical industries are the energy-intensive and high-CO2 generating emission sources [2–4]. Over the past decades, the drastic reduction of CO2 emissions has gained much attentions of the public, government and researchers. To limit the increase in temperature resulting from global warming under a threshold of 2 °C [5]. it is necessary to take proactive measures to reduce the CO2 emission. Carbon capture and storage (CCS) is receiving great attentions in recent decades, which is considered as a transitional technology for achieving massive reductions of CO2 from

⁎

large emission sources [6–8]. CCS technologies may be implemented with less risk and uncertainty as compared to other mitigation options, e.g., application of renewable energy or renovation of existing production process technologies. Among the various approaches to separate CO2 from the flue gas or chemically transformed gas stream, the chemical absorption-based CO2 capture technology, e.g., monoethanolamine (MEA), ammonia, K2CO3 and alkaline metal solution (NaOH, KOH), is known to be the most practical method mainly due to its technical maturity and advantage of treatment of large gas volume [4,9,10]. Table 1 provides a qualitative comparison among these technologies. As can be seen, the ammoniabased CO2 capture has several advantages including low cost, no degradation in the presence of O2 and SO2, high CO2 capture capacity, low

Corresponding authors. E-mail addresses: [email protected] (J. Yuan), [email protected] (J. Yan).

https://doi.org/10.1016/j.apenergy.2018.08.116 Received 12 April 2018; Received in revised form 29 July 2018; Accepted 21 August 2018 0306-2619/ © 2018 Elsevier Ltd. All rights reserved.

Applied Energy 230 (2018) 734–749

F. Wang et al.

Table 1 Qualitative comparison of solution-based CO2 capture absorbent. Characteristics

Amines

Ammonia

CO2 capture capacity

0.5 (molCO2/molMEA)

1.0 (molCO2/molNH3)

K2CO3

Alkaline metal solution (NaOH, KOH)

Comments

1.0 (molCO2/mol

Theoretical value

Depending on the loading and absorbent concentration

< 2.5d

2.2–2.78e

0.99 (molCO2/ molNaOH) –

∼115

160f

Fast Low

Thermal degradation

Fast Low Easy degradation by SO2 and O2 Severe

∼80 (KIER, RIST) > 100 (ALSTOM) 90–150 (CSIRO) Slow High SOx reacts reversibly with ammonia Negligible

Negligible

Fast Low SOx reacts with solution Negligible

Corrosiveness Absorbent cost

Severe Expensive

Mild Cheap

Mild Expensive

Severe Expensive

K2CO3)

Regeneration energy (GJ/ tCO2)

Regeneration temperature (°C) Absorption rate Volatility Effect of impurities

a b c d e f

4.2 (MEA)a ∼3.0 (KS-Solvents)b 3.1 (RITE-B)c 2.7 (NSC pilot) ∼120

Total thermal energy requirement (not only associated with the regeneration of absorbent, but also absorbent recovery)

Closely related to chemical stability/loss issue-heat stable salt formation and side reactions

From [17]. From [18]. From [19]. Simulation results [20]. Absorption characteristics of CO2 in aqueous K2CO3/piperazine solution is reported in [21]. From[22]

grade energy consumption, low efficiency penalty, as well as the potential of capturing multiple acid gases (NOx, SOx and CO2) in the flue gas and of producing value-added products [11,12]. This technology is often considered as a suitable alternative to the conventional amine solvents such as MEA, and aqueous carbonate salt, particularly potassium carbonate (K2CO3) [13–16]. However, the issue of ammonia slip is becoming one of the most critical technical and economic challenges for the commercial application of the ammonia-based CO2 capture technology [9]. Due to its intrinsically high volatility of ammonia, ammonia vapor leaves from the aqueous solution and then enters into the gas phase in both the absorber and stripper columns during the CO2 capture process. The simple schematic of the ammonia escape in the absorption and desorption processes are shown in Fig. 1. The escaped NH3 concentration in the vent gas is usually over 10,000 ppmv [15,20,23,24], which is far more than the emission standard of 50 ppm [25,26]. The high NH3

escape rate will reduce the NH3 concentration leading to a decrease in the performance of the capture process. For example, reduction in the concentration of ammonia in the absorbent results in a decline in absorption capacity and hence an increase in the amount of ammonia required for absorption of a given amount of CO2. In addition, the evaporated ammonia from the stripper is likely to react with CO2 in the gas phase to form ammonia salt crystals, thus block pipelines and valves which leads to increased pressure and ultimately shutdown of the operation [27]. What is more, if handled improperly, the ammonia will leak into the air, consequently cause serious secondary pollution. Therefore, the development of effective approaches is imperative to suppress ammonia slip or to recover the escaped ammonia. Various ammonia inhibition approaches have been developed based on both physical and chemical measures, including installing water (acid) washing device, membrane absorption, adding additives, process flowsheet modification and parameters optimization, as outlined in

NH3(g)

Gas phase

NH3(aq)

dissociation

+

NH4

CO2(g)

NH3(aq) diffusion

diffusion

hydrolysis

reaction

NH2COONH4

NH3·H2O

ionization

desorption

diffusion

NH3(aq)

Liquid phase

NH3(g)

desorption

desorption

Gas-liquid interface

CO2(g)

reaction

NH4HCO3/ (NH4)2CO3

decomposition CO2(g)

bubble

Fig. 1. Schematic of ammonia escape in the absorption and desorption process. 735

NH3(aq)

Applied Energy 230 (2018) 734–749

F. Wang et al.

Ammonia escape control technologies

Scrubbing

Water

Acid

washing

Membrane

Inorganic

New process flowsheets

Additives

Organic

CAP

Ionic

ECO2 Mixed-salt

liquid

washing

Parameters optimization

Ammonia concentration pH Gas flowrate Liquid flowrate CO2 loading CO2 partial pressure Reaction temperature Stripper pressure

Fig. 2. Ammonia escape control technologies.

4NH4

Fig. 2. These methods have the potential of inhibiting the ammonia escape to some extent with specific advantages and disadvantages. In the review paper, we aimed to provide: (1) the chemistry of the CO2 capture process and desorption process based on ammonia solvent, (2) the mass transfer mechanism of the ammonia escape, the parameters affecting the ammonia escape and the theoretical solutions in reducing ammonia escape, (3) outline the current status of the major approaches of ammonia inhibition technologies in the existing studies, (4) some guidelines to researchers in selecting the better technology for technically and economically solving the ammonia escape issue, and (5) some ideas about mechanism, cost, energy, and efficiency of each method.

The absorption of CO2 using aqueous ammonia occurs via the acidbase reaction between the acid gas components in the target gas such as CO2, H2S, SOx and the ammonia solution. The chemical reactions can be found in the previous studies [13,28-30]. Main chemical reactions relating to the absorption and desorption of CO2 into ammonia solution can be described in Eqs. (1)–(13). In general, CO2 absorption reactions in the ammonia solution can be presented as Eqs. (1)–(9) for the vaporliquid-solid reactions [29,30]. Kim et al. [31] investigated the CO2 conversion mechanism in aqueous ammonia to understand the chemistry. The ab initio calculations and kinetic simulations have revealed that ammonia molecules perform multiple roles as catalyst and reactant, base, and product controller simultaneously. Meanwhile, termolecular reaction among CO2-NH3-H2O prevails in the formation of ammonium salts rather than the zwitterions mechanism, which is a two-step reaction mechanism.

HCO3

−

+ H2 O ↔ H3 O + +

NH3 + H2 O ↔ NH4 NH3 + HCO3 NH4

+

NH4

+

2NH4

−

+ HCO3

+

CO32 −

+ OH− −

↔ H2 NCOO + H2 O −

↔ NH4 HCO3 (s)

+ H2 NCOO

+

+

−

CO32 −

−

↔ H2 NCOONH4 (s)

+ H2 O ↔ (NH4)2 CO3 ·H2 O(s)

−

+ 2HCO3− ↔ (NH4)2 CO3 ·2NH4 HCO3 (s)

(9)

NH4 HCO3 (aq) ↔ NH3 (aq) + H2 O(l) + CO2 (g) mol

ΔH = 64.3 kJ/ (10)

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

ΔH = 101.2 kJ/ (11)

2NH4 HCO3 (aq) ↔ (NH4)2 CO3 (aq) + H2 O(l) + CO2 (g) 26.9 kJ/mol H2 NCOONH4 (s) ↔ 2NH3 (g) + CO2 (g)

ΔH = 72.3 kJ/mol

ΔH = (12) (13)

3. Mass transfer in ammonia escape process The mechanism of the ammonia escape in the process of CO2 absorption was explored in [32]. Ammonia is a volatile with low density, low boiling point, and high vapor pressure. In the CO2 capture process, a dynamic balance with free ammonia exists in the ammonia solution, as shown in Eq. (14): NH4

(1)

CO2 + 2H2 O ↔ H3 O+ + HCO3

+ CO32

The possible desorption reactions and their reaction related heat are presented in Eqs. (10)–(13). From the comparison of the reaction heat, it is obvious that the desorption heat largely depends on the species of the CO2-rich solvent, i.e., ammonium carbamate, ammonium carbonate, and ammonium bicarbonate. Since the ammonium bicarbonate has the lowest reaction heat among the ammonium salts in the solution, it is recommended to operate the process at bicarbonate-prevalent condition [29]. Therefore, in the desorption process, one should know the concentrations of ionic species to better monitor and control the process for operating with lowest energy consumption:

2. Chemistry of ammonia-based CO2 capture process

2H2 O ↔ H3 O+ + OH−

+

(2)

+

(aq) + OH− (aq) ↔ NH3 ·H2 O(aq) ↔ NH3 (aq) + H2 O(aq) (14)

The mass transfer in the ammonia escape process from the liquid phase to the gas phase can be explained by Lewis-Whitman’s two-film theory as shown in Fig. 3. In the liquid phase, the mass transfer of ammonia is a single-phase diffusion process. This process of non-ionized ammonia gradually approaches to the liquid film surface through molecular diffusion or free flow with the solvent updated. When the dissolved ammonia diffuses in the liquid and approaches the gas-liquid interface, ammonia molecules permeate the equilibrious liquid-gas film in sequence through the diffusion. This can also be described by Henry’s law as shown in Eqs. (15)–(17). Finally, the single-phase diffusion

(3) (4) (5) (6) (7) (8) 736

Applied Energy 230 (2018) 734–749

F. Wang et al.

Interface Liquid bulk phase

4. Ammonia escape inhibition technologies

Gas bulk phase

4.1. Scrubbing

CL liquid

Ci

Scrubbing is the most used approaches in chemical process for both separating gas components or inhibit harmful gas to the atmosphere. The ammonia escape inhibition technologies can be divided into water washing and acid washing according to the scrubbing solution used.

Pi gas PG

NH3(L)

4.1.1. Water washing Currently, water washing is one of the most common methods used for controlling ammonia loss due to its high NH3 absorption capacity. The water scrubber can be a multi-stage section affiliated to the upper of the column, or a separate ammonia abatement system consisting of a NH3 absorber and NH3 stripper, to capture and recycle the vaporized NH3 using large amounts of washing water. This approach has been widely employed in Alstom’s chilled ammonia process (CAP) and Korea Institute of Energy Research (KIER) process [37]. Due to its high solubility in water, the evaporated ammonia can be removed efficiently [38]. The capture efficiency of escaping ammonia in the vent gas can reach 95–99.5% [39]. However, some previous studies also indicated that using water washing is still difficult to fully recover the escaped ammonia. Under the ordinary operating conditions, the ammonia concentration in the vent gas after passing the water column is still higher than 50 ppm, which is higher than the emission standard [40]. To further reduce the escaped ammonia concentration, a two-step absorption tower or a multi-stage absorption tower is an effective approach. A two-step absorption process is shown in Fig. 4, as introduced in a Chinese invention patent [41]. The two-step absorption process in a composite tower involves the concentrated ammonia solvent being sprayed into the lower part of the column to absorb CO2. In this process, the ammonia will evaporate and mix with the flue gas, CO2 and NH3 are simultaneously absorbed by water in the upper part of the column. Meanwhile, Ministry of Science and Technology of China (MOST) and the U.S. Department of Energy (DOE) established a multi-tower (the main absorption tower, vice absorption tower, washing tower) system in series, as shown in Fig. 5. Compared with the conventional oncethrough absorption tower, the ammonia escape could be maintained below 5 ppm, and the CO2 removal rate could be increased by 2–5%. However, this process needs a large area which increases the capital costs [32]. Water washing is a simple and effective way applied in the most of ammonia treatment processes, but it also suffers from several inherent drawbacks. Large amounts of water are consumed and the waste water needs to further processed. If the produced dilute ammonia solution is injected into the ammonia solvent, this will result in a system water imbalance, due to the fact that the washing water used is more than the

NH3(g)

Fig. 3. Two-film approach for describing the ammonia escape mass transfer.

process of ammonia in the gas phase is similar to that of the liquid phase. During this process, the ammonia molecular diffuses from the surface of the gas film to the gas phase through the molecular diffusion or gas flow updated [33]. CO2 (g) ↔ CO2 (aq)

PCO2 = HCO2 ·CCO2

(15)

NH3 (g) ↔ NH3 (aq)

PNH3 = HNH3 ·CNH3

(16)

NH3 (aq) ↔ NH3 (g)

ΔH = 29.69 kJ/mol

(17)

where P is partial pressure, CCO2 or CNH3 is liquid-phase molar concentration, HCO2 or HNH3 is the Henry’s law constant, and ΔH is the enthalpy change. According to Fick’s first law, the flux of NH3 is proportional to the NH3 concentration gradient in the diffusion direction opposite to CO2 absorption. The proportionality factor is the diffusion coefficient of NH3 in the medium. This theory is appropriate for absorption and desorption. The flux can be illustrated as the function of a mass transfer coefficient and the corresponding driving force, which is reflected as a pressure difference. According the derivation of the two-film model, the relationship between the mass transfer coefficient and overall ammonia escape molar flux can be expressed as [34]:

NNH3 = kGNH3 (PL−PG ) = kGNH3 (CL HNH3−PG )

(18)

1 1 1 = + kG kG′ kGNH3

(19)

kL0 ENH3 HNH3

(20)

kG′ =

where NNH3 is the overall ammonia escape molar flux, mol/(m2 s), kGNH3 is the overall mass transfer coefficient of ammonia escape, with mol/ (m2 s Pa), kG is the gas side mass transfer coefficient, mol/(m2 s Pa), PL and PG indicate the partial pressure of NH3 in liquid bulk and gas phase. kL0 is the physical mass transfer coefficient in the liquid phase, m/s, which shows the transport of free ammonia from liquid bulk to the gasliquid interface without a liquid chemical reaction. ENH3 is the enhancement factor for NH3. The mass transfer of the gas phase depends on partial pressure difference of ammonia. Thus, changing the NH3 partial pressure in the gas phase can vary the evaporation and mass transfer rate of ammonia [35]. Ma et al. [34] measured the volumetric mass transfer coefficients of ammonia escaping in a bubbling reactor, and found that the temperature and inlet CO2 molar fraction are the important factors to the volumetric mass transfer coefficients. Chu et al. [36] pointed out that the increase of orifice size of the bubble column can enhance CO2 mass transfer and inhibit ammonia escape.

Fig. 4. Two-step absorption tower. 737

Applied Energy 230 (2018) 734–749

F. Wang et al.

Fig. 5. Multi-stage CO2 capture process.

absorb the escaped ammonia. In the acid washing, the flue gas is bubbled through a vessel called saturator [49]. The generated products can be used as compound fertilizer after processing. Khakharia et al. [40] installed an acid wash scrubber downstream of the water washing, and sulfuric acid was used to treat the flue gas. Additional ammonia (up to 150 mg/Nm3) was added into the treated flue gas to test the acid wash scrubber. The experimental results indicated that it was possible to reduce ammonia emissions below 5 mg/Nm3. Meanwhile, Bonalumi [50] and Gal et al. [51] proposed the integration of the wet flue gas desulfurization process with ammonia recovery system. The SOx and H2S derived from the flue gas enters a wet sulfuric acid process, and the resulting acid is used to control ammonia slip in CO2 capture process based on ammonia scrubbing.

water make up which then affects its continuous operation. Thus, regenerating and recycling ammonia scrubbing is necessary. This process is called NH3 abatement and recycling process, which is similar to the main ammonia-based CO2 capture process. Steam is used to regenerate the ammonia water. Thus, the NH3 abatement and recycling process also places an extra energy consumption burden and facilities on the entire CO2 capture system, which leads to a substantial increase in operating costs. Specifically, the thermodynamic analysis of Alstom’s chilled ammonia process by Mani et al. [42] showed that the duty of NH3 abatement regenerator reached 2377 kJ/kg CO2, while the CO2 stripper duty was only 2291 kJ/kg CO2 under the conditions of 26 wt% NH3 and 10 °C absorption temperature. Niu et al. [43] carried out a rigorous simulation of the CO2 capture process by aqueous ammonia at the room temperature and the normal pressure. Their results also showed that the energy consumption for the NH3 abatement system (1703 kJ/kg) would be much higher than that for CO2 regeneration (1285 kJ/kg CO2). The same or similar findings are presented in both studies, i.e., the energy penalty for recovering slipped ammonia might be equivalent to or more than the energy penalty for CO2 capture alone. To regenerate ammonia efficiently and to reduce the energy penalty, several effective methods were also investigated [44–46]. A weakly ion-exchange resin containing amine functional groups used to regenerate ammonia through absorbing carbonic acid was studies by Huang et al. [44]. Li and Yu [26,45] proposed a simple but effective process for ammonia abatement and recycling by using the waste heat in flue gas to regenerate NH3 as shown in Fig. 6. This process can solve the problems of ammonia slip, ammonia make up and flue gas cooling in the ammonia-based CO2 capture process. Fang et al. [46] presented a vacuum membrane distillation (VMD) system to regenerate ammonia from ammonia scrubbing water to replace the conventional heat regeneration of ammonia. The results showed that the ammonia removal efficiency can be up to 95.6% with a 120 min continuous circulation time, which validated that the VMD system has the potential to recover ammonia and solve the problem of ammonia loss.

4.2. Membrane Ammonia losses due to its high volatility being certainly worsened by a direct contact between gas and liquid, the indirect gas/liquid contact is allowed by membrane contactors, which could lead to the ammonia slip mitigation. Various studies reported that the membrane contactor offers promising perspectives due to its large interfacial area and the potential for limiting ammonia slip [52–54]. As shown in Fig. 7, in the CO2 absorption process, involving the liquid- and gas- phase countercurrent flows, the mass transfer among the two phases takes place without dispersing one phase into the other. Ammonia vaporization can be limited via ensuring the operating conditions close to the CO2 saturation in the liquid membrane. This saturation will minimize free NH3 content and thus NH3 volatility will also be minimized [47]. It is also because the membrane is highly permeable to CO2 but less permeable to NH3, thus could drastically lower ammonia slip [55]. In the CO2 absorption process using the membrane contactors, the loss of ammonia in the contactor is related to several factors: the nature of the membrane, the degree of filling of the module, the amount of NH3 present in the solution, and the absorption temperature [56]. In term of membrane materials, microporous hollow fibers with impressive intensification factors have been commonly applied [57–59]. Mehdipour et al. [60] developed a mathematical model based on the coupled partial differential equations for CO2 separation from a CO2/N2 gas mixture with the NH3 solution in a hollow fiber membrane contactor. The effect of the ammonia evaporation, ammonia solvent

4.1.2. Acid washing In order to improve the ammonia absorption efficiency, acid washing is another alternative option which has a faster reaction rate and higher removal efficiency than the water washing [47,48]. Sulfuric acid, hydrochloric acid, hydrogen sulfide and other acid can be used to 738

Applied Energy 230 (2018) 734–749

F. Wang et al.

Fig. 6. Schematic diagram of ammonia-based CO2 capture process with novel NH3 abatement and recycle system.

acting as a physical barrier facing to the liquid phase is coated to a microporous support [66,67]. Further the composite fibers with the dense layer have been successfully used and tested [68,69]. It is found that the wetting is suppressed and the mass transport performances are comparable to those observed in microporous membrane contactors in the peculiar case if the dense skin is properly chosen [55,70]. Karami et al. [42] found that the dense skin membranes showed stable and attractive performances, in terms of the reduced ammonia slip and intensified CO2 mass transfer, compared to the packed column. Makhloufi et al. [55] performed CO2 absorption experiments in the ammonia with porous polypropylene membranes (Oxyphan) and with two different dense skin composite hollow fibers: tailor made (Teflon AF2400) and commercial (TPX). The experimental results confirmed that the intensification potential of the membrane contactors for the ammonia absorption process compared to the packed columns was high, and a significant reduction of ammonia slip was observed. Moreover, the dense skin composite membrane modules showed the stable performances, with the ammonia concentration and process

temperature and ammonia concentration was investigated. The results indicated that the ammonia loss can be significantly reduced, only 0.03% of the inlet absorbent with a solution containing 5 wt% of NH3 evaporated at 298 K. The mass transfer resistance of the liquid phase was negligible, based on analyzing the effects of gas and liquid velocities. Nevertheless, membrane wetting usually takes place after a long period use, which results in a significant resistance to the mass transfer [61]. The use of a hydrophobic membrane could prevent flooding problems in the membrane contactor. The hydrophobic polymers are often used for example, polypropylene (PP), polyethylene (PE), polyvinylidene fluoride (PVDF), and polytetrafluoroethylene (PTFE) membranes [56]. However, in practice, the aqueous solutions with organic absorbents can penetrate the pores and wet the hydrophobic membrane, then affect the mass transfer performances [62–65]. Meanwhile, the hydrophobic microporous membranes cannot offer stable performances, due to the salt precipitation and pore blocking [55]. To keep the membrane contactors in a non-wetted condition, a thin dense layer

Fig. 7. System description of the membrane absorption process. 739

Applied Energy 230 (2018) 734–749

F. Wang et al.

significantly reduce the ammonia loss during the stripping process and Cu2+ can maintain a high NH3 concentration, thereby enhance the absorption capacity in the CO2 absorption process. The results in [74] indicated that the ammonia loss in the regeneration process was reduced by about 42% due to the complexation of copper and ammonium ions. Ma et al. [73] comprehensively investigated the inhibitory effect of Ni(II) ions on the ammonia escape during the absorption and desorption processes. The concentration of Ni(II) was varied from 0.01 mol/ L to 0.09 mol/L. They found that the optimum concentration of the additive is 0.06 mol/L. The amount of ammonia escape was reduced by more than 30% at this concentration. Li et al. [75] theoretically and experimentally investigated the potential of Ni(II), Cu(II) and Zn(II) as the additives to reduce the ammonia volatilization in the ammonia-based CO2 capture power plant. A thermodynamic equilibrium model of M(II)-NH3-CO2-H2O system was established to provide theoretical guidance for the study and analysis of experimental results. The effects of the ammonia concentration, absorption time, CO2 loading and regeneration on ammonia loss and CO2 absorption rate were studied. Among the experimental metal species, Cu(II) had the best adaptability to the variation of pH and CO2 loading, and Ni(II) suppressed the ammonia volatilization most effectively. The addition of the M(II) ions significantly reduced the ammonia loss in both the absorption and regeneration processes, and only slightly decreased the rate of CO2 absorption. Furthermore, the metal additives can accelerate the CO2 desorption rate. However, the investigations also suggested that the complexation of ammonia with the metal ions effectively decline the concentration of the free ammonia, resulting in a reduced evaporative loss but also reduced reactivity towards CO2 [78]. Ma et al. [77] further pointed out that Co(II) had a higher coordination number and thus the formed complex with ammonia had a higher ionization constant than other complex compounds, such as Cu (II) and Zn(II). The experimental results showed that the addition of Co (II) could reduce the ammonia escape by 60% in the absorption process and could also improve CO2 desorption efficiency by 2–5% in the desorption process.

temperature having a significant effect on the mass transfer performances. Molina et al. [71] investigated the effects of several membrane contactor parameters, such as the phase compositions, phase velocities and packing fraction, on the CO2 removal efficiency and ammonia loss. The results illustrated that the CO2 removal efficiency can reach more than 99% with a low gas velocity, and the ammonia loss can be limited to 1% of the solvent. Several hollow fiber composite membrane contactors with a dense skin layer coated in the polypropylene (PP) support were tested and evaluated by Molina et al. [56]. The potentialities of dense skin membrane contactors are discussed with regard to both the increased CO2 mass transfer performances and the mitigation of the ammonia volatilization. These results demonstrated that the membrane limited the ammonia losses but did not eliminate it. Karami et al. [72] proposed that, to avoid NH3 slip completely, NH3 vapor in gas phase can be effectively absorbed with water into another hollow fiber membrane contactor and returned to the CO2 capture cycle. 4.3. Additives 4.3.1. Inorganic additives Metal ions are commonly used as inorganic additives recently. The inhibition mechanism is that the metal ions and free ammonia in the absorbents form a complex [M(NH3)n]2+ (M stands for metal element) as shown in Eq. (21). The central ion M2+ and the body NH3 is coordinated by a coordination bond to form a sub-stable structure. The formed complexes can reduce the free ammonia concentration and thus decrease the ammonia escape. When the concentration of NH3 is reduced, the locked NH3 could be released again [73]. Several studies showed that ammonia can form the complexes with most of the transition metal ions such as copper, nickel, zinc, cobalt. Therefore, the current review mainly focuses on the impacts of those metal ion additives on the ammonia escape inhibition performance in the absorption and desorption processes. θ

Kd

M2 + + NH3 ⇄ [M(NH3)n]2 +

(n = 1−4, 6)

(21)

M2 + + 2OH−

Table 2 summaried the relevant studies of metal ion additives on ammonia escape. Mani et al. [42] initially studied the effects of the zinc salts (chlorides, nitrates or sulphates) on the ammonia absorption and desorption. They found that the addition of zinc(II) salts to the NH3 solution increased the overall capacity of CO2 absorption without appreciably affecting the removal efficiency and stripping of pure CO2 from HCO3− solutions. Meanwhile, this led to a rapid release of about 30–35% of the initially captured CO2. Kim et al. [74] conducted continuous operations in the packed column by using Cu2+ as the additive to the ammonia solution for CO2 capture. The addition of Cu2+ can

M2 + + CO32 −

K sp,OH

⇄

M(OH)2 ↓

(22)

K sp,CO2 −

⇄

3

MCO3 ↓

(23)

However, the main obstacle of using inorganic additives is the decrease impact on CO2 absorption efficiency, which will result in increased equipment size and increased equipment cost. The formed complexes between Zn2+, Cu2+ and ammonia have relatively high stability. The ionization constant of the complex is low (see Table 3), this will affect the CO2 absorption rate. Meanwhile, as the ammonia

Table 2 Summary of relevant studies of metal ion additives on ammonia escape. Additives

Reactor

Concentration/Ratio

Ammonia escape inhibitiona

Conditions

Reference

NiCl2

Bubbling Reactor

0.06 mol/L

30%

NH3 = 8 wt%, T = 15 °C

Ma et al. [73]

Cu(OH)2

Packed column

0.2 g/L

∼42%

NH3 = 9 wt%, CO2 = 20 vol%

Kim et al. [74]

NiCl2·6H2O CuCl2·6H2O ZnCl2

Wetted wall column

0.2 mol/L

26.0–39.7% 18.9–24.9% 23.7–28.6%

NH3 = 3–6 mol/L, CO2 = 0–5 kPa, T = 25 °C 18.9–24.9% 23.7–28.6%

Li et al. [75]

NiCl2·6H2O CuCl2·6H2O ZnCl2

Wetted wall column

0.2 mol/L

42.55% 36.17% 31.91%

CO2 loading = 0.5 molCO2/molNH3, T = 80 °C 36.17% 31.91%

Li et al. [76]

PZ + NiCl2

Bubbling Reactor

0.025 mol/L + 0.05 mol/L

36.45%

NH3 = 2 wt%, CO2 = 15%, T = 20 °C

Ma et al. [77]

CoCl2

Bubbling Reactor

0.005 mol/L

∼60%

NH3 = 8 wt%, CO2 = 15%, T = 15 °C

Ma et al. [78]

NiCl2··6H2O CuCl2·2H2O ZnCl2

Wetted wall column

0.086 mol/L 0.122 mol/L 0.122 mol/L

– 0.122 M 0.122 M

NH3 = 3 M, CO2 = 1–5 kPa

Li et al. [79]

a

Ammonia escape decrease compared to the case of no additive. 740

Applied Energy 230 (2018) 734–749

F. Wang et al.

hydroxyl groups, e.g., 2-amino-2-methyl-1-propanol (AMP), 2-amino-2methyl-1,3-propandiol (AMPD), 2-amino-2-ethyl-1,3-propandiol (AEPD), tri(hydroxymethyl) aminomethane (THAM), amino ethyl ethanol amine (AEEA) and triethylenetetramine (TETA), were studied [85,96,97]. The molecular structures of some typical chemical additives are shown in Fig. 8. The loss of ammonia by vaporization was reduced by the additives, whereas the removal efficiency of CO2 was slightly improved. You et al. [85] further optimized the molecular structures and binding energies from the geometries of (ammonia + additives) and (ammonia + additives + CO2) through density functional theory (DFT) calculations at the standard state [98,99]. The geometry and binding energies derived by DFT revealed that the reduction of the loss of ammonia and the enhancement of CO2 capture were related to the intermolecular interactions between the additives and ammonia/or CO2. In particular, the molecular structures of ammonia with the additives and blended ammonia absorbents with CO2 at an optimized state, such as bond angles, bond distances, and dihedral angles, were strongly influenced by the hydrogen bonding. Asif et al. [89] demonstrated that AMP not only reduced ammonia escape, but also enhanced CO2 capture efficiency and reduced the ammonia regeneration energy. However, it also causes AMP loss. The results showed that the AMP loss rate due to evaporation was 0.042 kg/day. Kang et al. [84] analyzed the inhibitory effects of the alcohols and amine additives on ammonia escape. It was found that triethanolamine (TEA) additive had the best effect on the ammonia escape inhibition, meanwhile the ternary blended NH3 + AMP + TEA solvent enhanced the effect on the ammonia escape inhibition. However, Li et al. [91] pointed out that the piperazine (PZ) promoted aqueous ammonia solution would increase NH3 loss though the mass transfer coefficients of CO2 in aqueous NH3 could be significantly increased. Physical absorption processes include Fluor process using propylene carbonate, Rectisol process using methanol, Selexol process using dimethyl ether of polyethylene glycol, and Sulfinol process using the mixture of Sulfolane and alkanolamine solution, which have been widely used in the chemical industry [100-102]. The introduction of these physical absorbents as the additives to the ammonia solution will increase solubility of CO2 in the aqueous ammonia solution hence increase the CO2 concentration in the solution and facilitate the reaction of ammonia with CO2. This, in return, could lower the concentration of free ammonia in the solution and lead to less ammonia loss. Based on this fact, Yu et al. [23] investigated the performance of two physical absorbents, Sulfolane (TMS) and Propylene carbonate (PC), as the additives to reduce ammonia loss in the aqueous-ammonia-based CO2 capture process. Under the conditions employing of 3 mol/L aqueous ammonia, 0.3 mol/L PC and CO2 loading of 0.4, the ammonia loss can be reduced by 38%, meanwhile the overall gas mass transfer coefficient can be increased by 10% compared to the case of no additive. Table 5 summaried current studies of organic additives on ammonia escape. Yang et al. [103] pointed out that aqueous ammonia-fulvic acid solution is a feasible approach to inhibit ammonia escape. The fluvic acid (FA) is heterogeneous, and consists of numerous oxygen-containing functional groups, such as methoxyls, carboxyls, hydroxyls, carbonyls, and so on. The two weakly acidic groups (–COOH and –OH) are capable of interacting with the aqueous ammonia to form the soluble ammonium fulvate by the ammoniation. Compared with the aqueous ammonia, the formed ammonium fulvates are relatively stable, which could inhibit the ammonia escape to some extent. In summary, the combination of these organic additives with NH3 is mainly based on the weak physical or physicochemical intermolecular interactions. The formed weak bonds are sensitive to the external variables (temperature, pressure) and will break again to release the ammonia molecular to improve the CO2 capture capacity.

Table 3 Ionization constant of the representative ammonia complex [79]. Ammonia complex

Ionization constant/Kdθ

[Cu(NH3)4]2+ [Zn(NH3)4]2+ [Ni(NH3)6]2+ [Co(NH3)6]2+

4.78 × 10−14 3.47 × 10−10 1.82 × 10−9 7.75 × 10−6

Table 4 Solubility constant of the hydroxide and carbonate [78]. Hydroxide Cu(OH)2 Zn(OH)2 Ni(OH)2 Co(OH)2

Solubility constant −20

2.2 × 10 3.0 × 10−17 5.48 × 10−16 5.92 × 10−15

Carbonate

Solubility constant

CuCO3 ZnCO3 NiCO3 CoCO3

1.4 × 10−10 1.46 × 10−10 1.42 × 10−7 1.4 × 10−15

absorbent is alkaline, metal ions can easily form hydroxide or carbonate precipitation according to Eqs. (22) and (23). The formation of the precipitation has low solubility (see Table 4) and will cause the packing and pipeline blockage, increasing the pressure drop of the packing layer and affecting the system operation.

4.3.2. Organic additives Organic additives are usually used in the development of blend solvents, in which the additives play a crucial role in improving the performance of a pristine absorbent, as the physicochemical properties of blended absorbents are strongly influenced by the intermolecular interactions between the pristine absorbents and additives. The formulation of the ammonia absorbent with the functional additives could possibly reduce the loss of NH3 from the absorbent solution. The inhibition mechanism can be explained in two aspects. Firstly, the addition of the additive increases the surface tension of the absorbing liquid and intergranular surface energy, which increases the attraction force between molecules. Secondly, some of the additives contain the functional groups that can combine with the free ammonia, thereby inhibiting the evaporation of ammonia [88]. It has been found that some of the amines or alcohols with functional groups (e.g., –NH2 or –OH), are able to bind with NH3 in the solution by hydrogen bonding to avoid NH3 escape [83,84,91]. The hydrogen bonding generated during the CO2 capture process was verified by FT-IR spectra and computational calculation [85]. Pellegrini et al., and Gao et al. [86,87] proposed the idea of using the ethanol additives to reduce ammonia loss, but did not carry out further research on the inhibitory effect. Ma et al. [88] studied the effects of the ethylene glycol, glycerol and isoamyl alcohol additives on the ammonia escape. After adding 1% of those additives, the volatilization of ammonia is decreased by 35–44%. Seo et al. [81] carried out the CO2 absorption experiments using the bubble reactor, and the impacts of the organic additives including glycine, ethylene glycol and glycerol on ammonia volatilization were investigated. Their results indicated that under the same experimental conditions, the best performance on the ammonia suppression is achieved by the glycerol additive. Additives such as primary, secondary, tertiary and sterically hindered amines, as well as piperazine, have been extensively used to modify the properties of the absorbents to inhibit NH3 escape [92–95]. Meanwhile, the introduction of certain steric hindrance in the organic amine molecule can significantly improve the selectivity of the absorbent and reduce the amount of solvent circulation and energy consumption, then decline the operating costs. Thus, the sterically hindered amines are often used as activators in the conventional absorbent, and their contained hydroxyl groups also gains inhibitory effect on ammonia escape. Several kinds of the additives including amine and

4.3.3. Ionic liquid Ionic liquids (ILs) are becoming the promising absorbents because of their special properties such as negligible vapor pressure, wide liquid 741

Applied Energy 230 (2018) 734–749

F. Wang et al.

H2 N

OH H N

N

OH

HO

HO

HO

OH

diethanolamine

methyldiethanolamine

OH

2-amino-2-methyl-1-propanol

HN

NH2 HO

N

NH OH

HO

triethanolamine

piperazine

2-amino-2-methyl-1,3-propandiol

OH

NH2

NH2 HO

HO OH

OH

2-amino-2-ethyl-1,3-propandiol

tri(hydroxymethyl)aminomethane

H N

H N HO

NH2

NH2

H2N

N H

amino ethyl ethanol amine

triethylenetetramine

O O

S O

O

O

propylene carbonate

sulfolane

Fig. 8. Molecular structure of typical chemical additives.

hydrogen bonding could decrease the ammonia concentration and then inhibit NH3 escape. On the other hand, the ILs could meet different requirements by tuning various structures of the cations and anions to enhance CO2 absorption or NH3 absorption. For instance, a higher NH3 absorption capacity was achieved by introducing the hydroxyl group on the imidazolium cation. Xu et al. [123] experimentally demonstrated the influence of six kinds of the ILs additives on the ammonia-based CO2 capture. The results indicated that all the additives were capable of suppressing the ammonia escape. [Choline]+ and Cl− were more effective in reducing the ammonia loss among the tested cation and anion groups. Meanwhile, the promotion on the inhibition of the ammonia escape is also helpful for the CO2 absorption capacity. The further quantum calculation demonstrated the mechanism, in which the inhibition of ammonia slip was attributed to the strong hydrogen bonding in the NH3-cations complexes, whereas van der Vaals (VDW) interactions between CO2 and anions should be responsible for the promotion of CO2 removal.

temperature range, high thermal stability [104] and adjustability [105]. Similar to NH3, ILs also have the potential of capturing CO2 [106–109], SO2 [110–113] and H2S [114–116], and have gained great attentions recently. ILs is usually blended with amine to promote CO2 removal efficiency and to reduce amine loss [117]. Using ILs as the additive is also a feasible attempt to enhance CO2 absorption and suppress the ammonia slip. On one hand, ILs have great potential for NH3 absorption. Yokozeki et al. [118,119] demonstrated that the conventional ionic liquids such as [Bmim][PF6], [Hmim][Cl] had high NH3 absorption performance and the anion had little effect on NH3 solubility. Li et al. [120] pointed out that NH3 solubility increased when the length of cation's alkyl increased. Li et al. [121] investigated the effects of the hydroxyl cation, anionic structures, pressure and temperature on the NH3 absorption performance. Shi et al. [122] testified that the cation played the leading role in determining NH3 solubility and a strong hydrogen bonding could be formed between NH3 molecule and the ring H atom of the [Emim] cation by molecular simulation, the formed 742

Applied Energy 230 (2018) 734–749

F. Wang et al.

Table 5 Summary of relative studies of organic additives on ammonia escape. Additives

Reactor

Concentration/ratio

Ammonia escape inhibitiona

Conditions

Researcher

Glycerol

Bubbling Reactor

1%

43.81%

NH3 = 8 wt%, T = 15 °C, CO2 = 15%

Ma et al. [80]

Glycine Ethylene glycol Glycerol

Container

1 wt%

5.1% 38.48% 59.91%

NH3 = 9 wt%, T = 40 °C

Seo et al. [81]

Taurine

Wetted wall column

2.0 M

∼31.3–71.9%

NH3 = 3 M, T = 15 °C, CO2 loading = 0–0.3 molCO2/molalkalinity

Yang et al. [82]

AMP

Wetted wall column

30 wt%

27.4%–44.1%

NH3 = 1 wt%, CO2 loading = 0–0.5 mol CO2/ molNH3, T = 40 °C, CO2/SO2 = 15 vol%

Jeon et al. [83]

AMP Ethylene glycol Glycerol PZ MDEA DEA TEA AMP + TEA

Agitated vessel

20 wt% 3 wt%

NH3 = 5 wt%, T = 40 °C NH3 = 10 wt%, T = 40 °C

Kang et al. [84]

20 wt% + 1–7 wt%

1.38%b 11.9% 14.8% 4.2% 6.1% 7.4% 24.5% 0.02–0.4%c

AMP AMPD AEPD THAM

Bubbling Reactor

1 wt%

THAM > AMP > AEPD > AMPD

NH3 = 10 wt%, T = 40 °C, P = 1 atm

You et al. [85]

Ethanol

Bubbling Reactor

–

–

–

Pellegrini et al. [86], Gao et al. [87]

Ethylene glycol Glycerin Isoamyl alcohol

Bubbling Reactor

1%

35.86% 46.38% 42.87%

NH3 = 8 wt%, T = 15 °C, CO2 = 12 vol%

Ma et al. [88]

AEEA AEPD AMP TETA THAM

Bubbling Reactor

1%

3.3%d 33.47% 33.06% 28.1% 27.69%

NH3 = 13%, CO2 = 12%, T = 30 °C

Lv [97]

AMP

Packed column

30 wt%

64%

NH3 = 3 wt%, T = 15 °C, CO2 = 15 vol%, CO2 loading = 0.07 molCO2/molNH3

Asif et al. [89]

NHD

Bubbling Reactor

5%

24.86%

NH3 = 1.2 wt%, T = 20 °C

Ma et al. [90]

Sulfolane (TMS) Propylene Carbonate (PC)

Wetted wall column

0.3 M

13–17% 18.5–38%

NH3 = 3 M, T = 15 °C, CO2 loading = 0–0.4 molCO2/molNH3

Yu et al. [23]

a b c d

NH3 = 5 wt%, T = 40 °C

Ammonia escape decrease compared to the case of no additive. Ammonia loss of the solvent. Ammonia concentration in the outlet flue gas. Increase in ammonia loss.

1%, and at its level the absorber column without the washing column can be employed [126]. However, an extra cooling system is needed to cool the flue gas and ammonia solvent to the required temperature. Due to the high refrigeration load, the energy consumption of the chilled ammonia process was found to be equivalent to that of the amine-based process, which probably results in lowering the competitiveness of the chilled ammonia process [127]. Formation of the solids in the absorber is expected in the CAP process. The precipitation is problematic for a packed column design because of the risk of plugging. Sutter et al. [128] analyzed the thermodynamics of the CO2-NH3-H2O system with a focus on solid formation via phase diagrams and the critical streams for solid formation were identified to make a thorough understanding of the relevant solidsolid-liquid-vapor phase equilibria. As an alternative, a combined spray/flooded tray column was proposed [129]. This column setup has the advantage of reducing the level of ammonia slip by allowing the use of higher CO2 loadings, and enhancing the precipitation of solids. However, the potential for reduced heat requirement in the case of high CO2 loadings is counterbalanced by the heat required to dissolve the solids before entering into the stripper column [130]. Another absorber

4.4. Flowsheet modifications In order to overcome the problem of a large amount of the escaped ammonia in the conventional ammonia-based CO2 capture, many flowsheet modifications have been incorporated and established to identify potential methodologies for improvements in the energy performance and the ammonia escape inhibition, such as Chilled Ammonia Process (CAP) developed by Alstom, ECO2 developed by Powerspan and other novel flowsheets. The use of the chilled ammonia to capture CO2 was first patented in 2006 by Eli Gal [124], as shown in Fig. 9. The description of this process can be referred in [125]. The mass fraction of ammonia in the solvent is typically up to 28%. The pressure in the absorber should be close to the atmospheric pressure, while the temperature should be in the range 0–20 °C, and preferably 0–10 °C. The advantage of operating CO2 absorption in the lower temperature range is to prevent ammonia evaporation and reduce the flue gas volume. The experimental results indicated that the CO2 removal efficiency was only about 80% when the ammonia solution was at 2 °C, which was lower than the commercialized standard efficiency (85%). The ammonia loss rate was lower than

743

Applied Energy 230 (2018) 734–749

F. Wang et al.

Fig. 9. Process description of chilled ammonia process.

Fig. 10. Flow scheme of the modified chilled ammonia process.

NH3 slip. Meanwhile, a rich stream was bypassed before the rich/lean heat exchanger (RL-HEX) and fed to the top stage of the desorber. A rigorous performance assessment was conducted to compare the overall energy penalty between the new proposed process and the standard conventional ammonia-based capture process without solid formation. The results indicated that the new process with controlled solid formation achieved a Specific Primary Energy Consumption for CO2 avoided (SPECCA) of 2.43 MJ/kg CO2 compared to the process without solid formation with a SPECCA of 2.93 MJ/kg CO2. In addition, heat integration between precipitation and dissolution, of which include the enthalpy of crystallization and low grade heat, enables the performance improvement. Hu et al. [134] proposed the idea of using the gas-liquid-solid (GLS) three-phase circulating fluidized bed to capture CO2 by aqueous ammonia. Compared with the conventional reactors, the three-phase circulating fluidized bed can provide larger interface area, higher efficiency of mass transfer, and longer residence time [135]. With the assistance of Aspen Plus, the three-phase circulating fluidized bed was

design option to reduce the ammonia slip in packed columns is to use staged absorption with intermediate cooling [131]. The staging can be obtained either with a two-absorber setup or with a pump around system [132]. Intercooling temperature varied between 18 and 25 °C can reduce the NH3 emission concentration of the absorber below 1.2% [12]. Rhee et al. [28] found that employing a side stream cooler in the absorber can significantly reduce the NH3 slip. When increasing the ratio of the side stream to the total circulating flow rate at a fixed amount of washing water over 50%, the temperature of absorption column can be decreased more than ∼5 °C and the NH3 slip concentration can be reduced to less than ∼100 ppm. Based on their previous work [128], Sutter et al. [133] proposed a chilled ammonia process to avoid the precipitation of the solids and to inhibit the ammonia escape, as shown in Fig. 10. A pump around is considered for both the CO2 absorber and ammonia absorber, by which a friction of the rich stream leaving from the column was cooled and recycled to the absorber directly. In terms of the CO2 absorber, the pump around was recycled to the top of the column to control the temperature and the 744

Applied Energy 230 (2018) 734–749

F. Wang et al.

Fig. 11. Flow scheme of the new process of CO2 capture with the reinforced crystallization.

process. The lower bed in the absorption column has ammonia-rich solvent at the bottom (with ammonia concentration decreasing upwards), and the upper bed has ammonia-lean solvent. This enables the ammonia to be captured in the upper bed, which decreases the ammonia loss. In order to maintain this differential concentration in the absorption column, two solvent streams (one rich in ammonia, and the other rich in potassium) are taken from the regeneration column. Due to the application of heating in the regeneration column at two different temperatures, two reboilers can be used to adapt the temperature requirement [137]. This technology has the potential of reducing more than 50% of parasitic power demand compared to the conventional MEA technology, and a 50–60% reduction in water usage compared to the chilled ammonia process as well as a significant reduction in the capital, operating and maintenance costs. In the mixed-salt process, ammonia emissions are greatly reduced due to the use of K2CO3 and the two-stage absorber approach. For an absorber operating at 30 °C with the mixed-salt process, the ammonia vapor pressure is more than an order of the magnitude smaller than that of other state-of-the-art ammonia-based processes. A successful demonstration of this mixed-salt technology with a capacity of 0.25 tonne/day has been carried out in the USA recently [138]. The heat duty for SRI’s mixed-salt process was calculated to be 2.0 MJ/kg CO2, which offers a much lower energy penalty than Fluor Econamine FG Plus technology and conventional MEA-based technology. Kang et al. [137] further optimized the performance of this novel process and compared with the conventional

designed and simulated. When the mole fraction of aqueous ammonia is about 5.16%, more than 90% of the CO2 can be absorbed and the ammonia escaping ratio is less than 0.07%. Gao et al. [87] put forward a new process of CO2 capture by ammonia with the reinforced crystallization. This process is displayed in Fig. 11. Crystal appears in the solution once the ratio of carbon to nitrogen reaches to 0.57–0.63. After the solution with crystal discharged from the bottom of absorption-crystallization tower, crystal product is obtained via liquid-solid separation device. The separated crystal is sent to the heating and regenerating device. The mixed gas produced by decomposition goes through the separation device of ammonia-carbon. NH3 in the mixture will be separated and sent back to the absorption tower for recycling with the liquid obtained by liquid-solid separation device together. Then CO2 in the flue gas is finally enriched. To reinforce crystallization in the absorption process, alcohols is added into the part with low concentration of the ammonia solution. Based on the preliminary experiments and calculation, it was found that the absorbent capacity could reach 1.64 kgCO2/kgNH3, and the ammonia escape at the outlet could be neglected. Meanwhile, the thermal energy consumption for the regeneration was only about 1/3 as much as that of the conventional rich solution regeneration. SRI International (SRI) is developing a solvent-based and mixed-salt technology [136]. The mixed-salt process involves the use of ammonia to absorb CO2 from the flue gas, and potassium carbonate to control the ammonia slip. Fig. 12 shows a conceptual design of the mixed-salt

Fig. 12. Conceptual diagram of the mixed-salt process. 745

Applied Energy 230 (2018) 734–749

F. Wang et al.

MEA process. A five-case sensitivity study of the mixed-salt process indicated that it is competitive with the piperazine process and consistently outperforms the MEA process. Compared to the absorption process, the ammonia escape in the stripper cannot be neglected. To control the ammonia concentration in the regeneration process and reduce the energy consumption, Jiang et al. [139] proposed an advanced flash stripping process. The majority of the rich solvents was sent to a steam heater to reach the required temperature, then the absorbent was fed to a flash stripper where vapor (including CO2, NH3 and H2O) and liquid were separated. The vapor mixture from the flash tank entered into the bottom of the stripper. A small portion of the cold rich absorbent was fed to the top to cool the vapor mixture, thereby to liquefy the water vapor and absorb the NH3 vapor. A regeneration duty of 1.86 MJ/kg CO2 was achieved for a stripper pressure of 12 bar and an NH3 concentration of 10.2 wt%.

concentration in the CO2 stream, the optimum regeneration conditions were suggested to be at roughly 50% of CO2 release [38]. 5. Issues and resolutions The ammonia-based post-combustion CO2 capture has attracted great attention in the recent years and exciting results have been reported. However, it is worth noting that the ammonia escape is of primary importance for the successful development of the ammoniabased CO2 capture process technically and economically. Since most of the researches are still at the laboratorial or pilot- scale exploration stage. Therefore, a lot of efforts will be required to obtain deeper understanding the mechanisms of the NH3 suppression, to investigate the ammonia escape inhibition process details and to propose ways to improve the technical performance and economics for full commercialization. In general, Water washing is still the most common way to suppress ammonia escape, which is employed in the Alstom, KIER and RIST processes [15]. This method is usually used at the end of the CO2 capture process to finally control the ammonia emission levels. Although the reported NH3 concentrations in the reported literature are lower than 50 ppm in many processes when water washing is employed, the water washing still remains energy and costly issues. Recovery unit is required for the ammonia recycle which leads to the increased heat duty. Therefore, more attentions should be paid on the ammonia abatement and recycling system to reduces water usage and energy consumption. Since both the CO2 stripping and ammonia regenerating feature the relatively low regeneration energy and low regeneration temperature under 100 ◦C, it can be suitably established where there is available waste heat for generating low grade steam to provide regeneration energy. Meanwhile, the acid washing is a more efficient solution to reduce water usage and energy consumption, this process can be integrated with the fertilizer production to achieve low waste emission and reduced costs if there is a demand for fertilizer. The reactor configurations and their corresponding processes play an important role in reducing the ammonia escape. Membrane reactor can reduce the ammonia loss less than 1% of the solvent due to the indirect gas/liquid contact. The ammonia escape volumetric mass transfer coefficient show different trend between the bubble column and the packed column [36,145]. The CAP aims to avoid the ammonia slip by cooling the flue gas and feed solvent lower than room temperature. In addition, the operating parameters of the ammonia-based CO2 capture system have significant impact on the ammonia escape rate. The escaped ammonia concentration can be maintained below 200 ppm under pressurized condition in the stripper of the CSIRO process. Additives have the potential of inhibiting the ammonia escape. researchers suggested candidate inorganic additives, organic additives and ionic liquid for the prevention of ammonia slip with different reaction mechanisms such as complex, hydrogen bond. The exact molecular interactions between the additives and ammonia associated with the absorption/desorption chemistry or electrolytes in the aqueous absorbent solutions should be conducted and confirmed. For the effective utilization of additives, the selection criteria of additives should consider the capability, sustainability and stability. For example, the inorganic additives pose the problems of the formation of heat stable salts. Thus, the additives in the larger temperature range having good chemical stability is to ensure the premise of recycling of absorbent. Meanwhile, the binding properties between the additives and absorbent need to be studied further, especially the effect of the additives on the CO2 absorption-desorption characteristics during system operation, such as changes in regeneration temperature and pressure caused by changes in operating conditions. Most of the ammonia inhibition technologies are focused on the absorption process. In fact, the ammonia escape in the desorption is more complicated. The escaped ammonia may react with CO2 and H2O

4.5. Parameters optimization In terms of parameters analysis and optimization, the research findings suggest that the key process parameters affecting the amount of ammonia escape are the flue gas temperature, pH value of the solvent, ammonia solvent concentration, liquid to gas ratio (L/G), stripping temperature and pressure, etc. [140]. Ma et al. [141] analyzed the mass transfer coefficient of the ammonia escape in the absorption process based on two-film theory. The experimental study showed that the mass transfer coefficient of the ammonia escape was strongly influenced by temperature, CO2 partial pressure, gas flow rate and L/G. Budzianowski et al. [47] pointed out that the flue gas flow rate was the main factor affecting the ammonia escape in the absorption process. Mclarnon [142] testified that the escaped ammonia vapor partial pressure increases with the pH and temperature of solution. Zhang et al. [25] simulated and optimized the CO2 absorption and regeneration processes of a research scale pilot plant involving the effects of the lean solvent flow rate, NH3 concentration, CO2 loading, and temperature on the absorption process. the optimization analysis for the CO2 regeneration process was carried out including temperature of the rich solvent, and condenser, the inner diameter and packed height of the stripper. The results showed that the rate of the ammonia escape increases with the increase of the lean liquid flow rate, NH3 concentration and lean liquid temperature, and decreases with the increase of CO2 loading. Ma et al. [141] suggested that low temperature, high CO2 partial pressure, low flue gas flow rate and high L/G (> 0.7) could reduce the mass transfer coefficient of ammonia escape. For different types of reactor configurations, the NH3 vaporization varies significantly. The bench-scale experiments realized in the counter-current packed bed reactor revealed that the NH3 vaporization can be minimized under the conditions of low temperature, pH, and flow rate of flue gas, as well as high pressure and flow rate of aqueous ammonia. A detailed 2D modeling of aqueous ammonia process in the falling film reactor was found that the NH3 vaporization could be mitigated by making use of the mechanisms of negative enhancement of mass transfer and of migrative mass transport [47]. Regeneration pressure is also one of the key parameters affecting the desorption performance. High pressure regeneration is preferable for obtaining higher CO2 stripping rate and retaining NH3 in the absorbent [38]. When the regeneration is carried out at 20 atm, the NH3 concentration is only half of the slip under the ambient pressure. Duan and Qi [143,144] both revealed that the increase in desorption pressure could effectively reduce the ammonia escape. However, a high regeneration pressure is corresponding to a high regeneration temperature. At the higher regeneration temperatures, although more CO2 could be regenerated, the NH3 vapor concentration would increase more quickly. In addition, the rich absorbent should not be deeply regenerated to aqueous ammonia as deep regeneration will cause the surge of the ammonia vapor into the gas phase, and also consume more energy. In terms of the regeneration efficiency and NH3 vapor 746

Applied Energy 230 (2018) 734–749

F. Wang et al.

and Development Program of China (Grant No. 2016YFE0102400). The author also thank for K.C. Wong Magna Fund in Ningbo University. One of the authors, Fu Wang, acknowledges the financial support from China Scholarship Council (CSC).

to form carbonate, and result in pipeline blocking. Thus, the desorption process poses the following issues; desorption properties such as the ammonia escape concentration, mechanism of the precipitation reaction and the separation of NH3 from CO2 stream in the CO2 purification process. The escaped NH3 into the CO2 stream could possibly be removed together with moisture in the subsequent compression steps. It has been found that other impurities in the CO2 may be removed in this way [146]. The single-step ammonia suppression technology is difficult to achieve the emission standard. Integration of two or more different ammonia inhibition methods is the solution. For example, a water washing device is still installed in the CAP plant. Different ammonia control designs based on the combinations of ammonia escape inhibition technologies are evaluated [49]. But the combination may also face the energy and cost issues. Thus, it is essential to develop an economic analysis and evaluation for the performance and cost in order to obtain technical and economic optimization of these collaboration.

References [1] Chu S. Carbon capture and sequestration. Science 2009;325(5948). 1599–1599. [2] Gielen D. CO2 removal in the iron and steel industry. Energy Convers Manage 2003;44(7):1027–37. [3] Bosoaga A, Masek O, Oakey JE. CO2 capture technologies for cement industry. Energy Procedia 2009;1(1):133–40. [4] Rochelle GT. Amine scrubbing for CO2 capture. Science 2009;325(5948):1652–4. [5] Metz B, Davidson O, De Coninck H, Loos M, Meyer L. IPCC special report on carbon dioxide capture and storage. Geneva (Switzerland): Intergovernmental Panel on Climate Change; 2005. Working Group III. [6] Haszeldine RS. Carbon capture and storage: how green can black be? Science 2009;325(5948):1647–52. [7] Wang F, Zhao J, Li H, Deng S, Yan J. Preliminary experimental study of postcombustion carbon capture integrated with solar thermal collectors. Appl Energy 2017;185:1471–80. [8] Lee ZH, Lee KT, Bhatia S, Mohamed AR. Post-combustion carbon dioxide capture: evolution towards utilization of nanomaterials. Renew Sustain Energy Rev 2012;16(5):2599–609. [9] Zhao B, Su Y, Tao W, Li L, Peng Y. Post-combustion CO2 capture by aqueous ammonia: a state-of-the-art review. Int J Greenhouse Gas Control 2012;9:355–71. [10] Cullinane JT, Rochelle GT. Carbon dioxide absorption with aqueous potassium carbonate promoted by piperazine. Chem Eng Sci 2004;59(17):3619–30. [11] Hanak DP, Biliyok C, Manovic V. Efficiency improvements for the coal-fired power plant retrofit with CO2 capture plant using chilled ammonia process. Appl Energy 2015;151:258–72. [12] Li K, Yu H, Qi G, Feron P, Tade M, Yu J, et al. Rate-based modelling of combined SO2 removal and NH3 recycling integrated with an aqueous NH3-based CO2 capture process. Appl Energy 2015;148:66–77. [13] Bai H, Yeh AC. Removal of CO2 greenhouse gas by ammonia scrubbing. Ind Eng Chem Res 1997;36:2490–7. [14] Kozak F, Petig A, Morris E, Rhudy R, Thimsen D. Chilled ammonia process for CO2 capture. Energy Procedia 2009;1:1419–26. [15] Han K, Ahn CK, Lee MS, Rhee CH, Kim JY, Chun HD. Current status and challenges of the ammonia-based CO2 capture technologies toward commercialization. Int J Greenhouse Gas Control 2013;14:270–81. [16] Han K, Ahn CK, Lee MS. Performance of an ammonia-based CO2 capture pilot facility in iron and steel industry. Int J Greenhouse Gas Control 2014;27:239–46. [17] Sander MT, Mariz CL. The fluor daniel econamine FG process: past experience and present day focus. Energy Convers Manage 1992;33(5–8):341–8. [18] Mimura T, Simayoshi H, Suda T, Iijima M, Mitsuoka S. Developments of energy saving technology for flue gas carbon dioxide recovery in power plant by chemical absorption method and steam system. Energy Convers Manage 1997;38:S57–62. [19] Goto K, Okabe H, Chowdhury FA, Shimizu S, Fujioka Y, Onoda M. Development of novel absorbents for CO2 capture from blast furnace gas. Int J Greenhouse Gas Control 2011;5:1214–9. [20] Darde V, Thomsen K, van Well WJM, Banalumi D, Valenti G, Macchi E. Comparison of two electrolyte models for the carbon capture with aqueous ammonia. Int J Greenhouse Gas Control 2012;8:61–72. [21] Kim YE, Choi JH, Nam S, Yoon YI. CO2 absorption characteristics in aqueous K2CO3/piperazine solution by NMR spectroscopy. Ind Eng Chem Res 2011;50:9306–13. [22] Yoo M, Han SJ, Wee JH. Carbon dioxide capture capacity of sodium hydroxide aqueous solution. J Environ Manage 2013;114:512–9. [23] Yu J, Wang S, Yu H. Experimental studies on suppression of ammonia vaporization by additives. Greenhouse Gases Sci Technol 2013;3(5):415–22. [24] Zhang M, Guo Y. Rate based modeling of absorption and regeneration for CO2 capture by aqueous ammonia solution. Appl Energy 2013;111:142–52. [25] Zhang M, Guo Y. Process simulations of NH3 abatement system for large-scale CO2 capture using aqueous ammonia solution. Int J Greenhouse Gas Control 2013;18:114–27. [26] Li K, Yu H, Tade M, Feron P, Yu J, Wang S. Process modeling of an advanced NH3 abatement and recycling technology in the ammonia-based CO2 capture process. Environ Sci Technol 2014;48(12):7179–86. [27] Yu H, Li L, Morgan S, Allport A, Cottrell A, McGregor J, Wardhaugh L, Feron P. Results from trialling aqueous NH3 based post combustion capture in a pilot plant at Munmorah power station: solvent regeneration energy. Chemeca 2012: Quality of life through chemical engineering: 23–26 September 2012, Wellington, New Zealand. 2012. p. 1097–107. [28] Rhee CH, Kim JY, Han K, Ahn CK, Chun HD. Process analysis for ammonia-based CO2 capture in ironmaking industry. Energy Procedia 2011;4:1486–93. [29] Versteeg P, Rubin ES. A technical and economic assessment of ammonia based post-combustion CO2 capture at coal-fired power plants. Int J Greenhouse Gas Control 2011;5:1596–605. [30] Darde V, van Well WJM, Stenby EH, Thomsen K. Modeling of carbon dioxide absorption by aqueous ammonia solutions using the extended UNIQUAC model. Ind Eng Chem Res 2010;49:12663–74. [31] Kim DY, Lee HM, Cho Y, Hwang I-C, Han K, Kim JY, et al. CO2 capturing mechanism in aqueous ammonia: NH3-driven decomposition- recombination

6. Conclusions Ammonia-based CO2 capture is one of the main sorption methods. While the prevention of the ammonia escape is the most critical issue among the technical problems of this process. The main approaches to control the ammonia slip are summarized in the present paper, and the current status and prospects of these technologies were reviewed. Using water washing is a relative efficient and mature technology applied in the most of ammonia-based recovery plants. The escaped ammonia concentration can be controlled to a certain level in the most cases. Whereas, the NH3 abatement and recycling process places an extra energy consumption burden and facilities. Several new processes such as using multi-stage absorption column, integrating the flue gas pretreatment and desulfurization provide a useful way of reducing water usage and energy consumption. These feasible processes need further investigations. The chilled ammonia process might be an option of reducing the ammonia escape via the gas chilling. Pilot plant operation has proved that this technology is feasible, but it is an energy-intensive process. The refrigeration and circulating the chilling water imposes more than 12.6% of total energy duty on the overall process. the heat requirement has been reported with wide ranges from 0.93 MJ/kg CO2 to more than 3.0 MJ/kg CO2. This makes the overall energy consumption and economic feasibility should be evaluated to address its competitiveness. NH3 escape can be remarkably improved by the physical and chemical enhancement methods, such as the membrane reactor and additives. The membrane contactor offers promising perspectives for reducing the ammonia slip limited to 1% of the solvent from its indirect gas/liquid contact. Membrane wetting can be solved by employing the hydrophobic microporous membrane and composite fibers with a thin dense layer. Additive materials for the suppression of the ammonia vaporization are being investigated to resolve the issue with a moderate success, up to 60% escaped ammonia can be inhibited for inorganic and organic additives. Some of the additives can also enhance the CO2 absorption performance. In general, the ammonia escape rate is affected by the ammonia concentration, gas and liquid flowrates, reaction temperature and desorption pressure. The CO2 partial pressure, CO2 loading and desorption pressure are in favor of decreasing of ammonia escape, while the ammonia concentration, liquid flowrate and reaction temperature have adverse influence. For the complete circulation process, the effect of the above parameters on the ammonia evaporation is interrelated. The ammonia escape rate can be optimized for the specific design system. Acknowledgement This work was supported by National Natural Science Foundation of China (Grant No. 51706112 and 51506149) and National Key Research 747

Applied Energy 230 (2018) 734–749

F. Wang et al.

pathway. J Phys Chem Lett 2011;2:689–94. [32] Ma S, Song H, Wang M, Yang J, Zang B. Research on mechanism of ammonia escaping and control in the process of CO2 capture using ammonia solution. Chem Eng Res Des 2013;91(7):1327–34. [33] Shakerian F, Kim KH, Szulejko JE, Park JW. A comparative review between amines and ammonia as sorptive media for post-combustion CO2 capture. Appl Energy 2015;148:10–22. [34] Ma S, Chen G, Zhu S, Han T, Yu W. Mass transfer of ammonia escape and CO2 absorption in CO2 capture using ammonia solution in bubbling reactor. Appl Energy 2016;162:354–62. [35] Wang WD. Effect of surface tension on mass transfer process. Chem Eng (China) 2004;32:14–8. [36] Chu F, Yang L, Du X, Yang Y. Mass transfer and energy consumption for CO2 absorption by ammonia solution in bubble column. Appl Energy 2017;190:1068–80. [37] Yu H, Qi G, Wang S, Morgan S, Allport A, Cottrell A, et al. Results from trialling aqueous ammonia-based post-combustion capture in a pilot plant at Munmorah Power Station: gas purity and solid precipitation in the stripper. Int J Greenhouse Gas Control 2012;10:15–25. [38] Zhuang Q, Pomalis R, Zheng L, Clements B. Ammonia-based carbon dioxide capture technology: issues and solutions. Energy Procedia 2011;4:1459–70. [39] Sun L. Research on ammonia escape in CO2 capture process by ammonia method Master thesis Harbin Institute of Technology; 2012 China. [40] Khakharia P, Huizinga A, Jurado Lopez C, Sanchez Sanchez C, de Miguel Mercader F, Vlugt TJ, et al. Acid wash scrubbing as a countermeasure for ammonia emissions from a post-combustion CO2 capture plant. Ind Eng Chem Res 2014;53(33):13195–204. [41] Wen LX, Wang C, Chen JF. Methods for CO2 capture of flue gas from power plant. CN Patent 102225311A; 2011. [42] Mani F, Peruzzini M, Barzagli F. The role of zinc (II) in the absorption-desorption of CO2 by aqueous NH3, a potentially cost-effective method for CO2 capture and recycling. ChemSusChem 2008;1(3):228–35. [43] Niu Z, Guo Y, Zeng Q, Lin W. Experimental studies and rate-based process simulations of CO2 absorption with aqueous ammonia solutions. Ind Eng Chem Res 2012;51(14):5309–19. [44] Huang H, Chang SG, Dorchak T. Method to regenerate ammonia for the capture of carbon dioxide. Energy Fuels 2002;16(4):904–10. [45] Yu J, Wang S. Development of a novel process for aqueous ammonia based CO2 capture. Int J Greenhouse Gas Control 2015;39:129–38. [46] Fang M, Ma Q, Wang Z, Xiang Q, Jiang W, Xia Z. A novel method to recover ammonia loss in ammonia-based CO2 capture system: ammonia regeneration by vacuum membrane distillation. Greenhouse Gases Sci Technol 2015;5(4):487–98. [47] Budzianowski WM. Mitigating NH3 vaporization from an aqueous ammonia process for CO2 capture. Int J Chem Reactor Eng 2011;9(1). [48] Valenti G, Bonalumi D, Macchi E. A parametric investigation of the Chilled Ammonia Process from energy and economic perspectives. Fuel 2012;101:74–83. [49] Jilvero H, Normann F, Andersson K, Johnsson F. Ammonia-based post combustionThe techno-economics of controlling ammonia emissions. Int J Greenhouse Gas Control 2015;37:441–50. [50] Bonalumi D, Giuffrida A. Investigations of an air-blown integrated gasification combined cycle fired with high-sulphur coal with post-combustion carbon capture by aqueous ammonia. Energy 2016;117:439–49. [51] Gal E. Use of SO2 from flue gas for acid wash of ammonia: U.S. Patent 7867322; 2011. [52] Lu JG, Lu CT, Chen Y, Gao L, Zhao X, Zhang H, et al. CO2 capture by membrane absorption coupling process: application of ionic liquids. Appl Energy 2014;115:573–81. [53] Li X, Remias JE, Neathery JK, Liu K. NF/RO faujasite zeolite membrane-ammonia absorption solvent hybrid system for potential post-combustion CO2 capture application. J Membr Sci 2011;366(1):220–8. [54] Dai Z, Noble RD, Gin DL, Zhang X, Deng L. Combination of ionic liquids with membrane technology: a new approach for CO2 separation. J Membr Sci 2016;497:1–20. [55] Makhloufi C, Lasseuguette E, Remigy JC, Belaissaoui B, Roizard D, Favre E. Ammonia based CO2 capture process using hollow fiber membrane contactors. J Membr Sci 2014;455:236–46. [56] Molina CT, Bouallou C. Carbon dioxide absorption by ammonia intensified with membrane contactors. Clean Technol Environ Policy 2016;18(7):2133–46. [57] Mansourizadeh A, Ismail AF. Hollow fiber gas-liquid membrane contactors for acid gas capture: a review. J Hazard Mater 2009;171(1):38–53. [58] Golkhar A, Keshavarz P, Mowla D. Investigation of CO2 removal by silica and CNT nanofluids in microporous hollow fiber membrane contactors. J Membr Sci 2013;433:17–24. [59] Amaral RA, Habert AC, Borges CP. Performance evaluation of composite and microporous gas-liquid membrane contactors for CO2 removal from a gas mixture. Chem Eng Process Process Intensif 2016;102:202–9. [60] Mehdipour M, Keshavarz P, Seraji A, Masoumi S. Performance analysis of ammonia solution for CO2 capture using microporous membrane contactors. Int J Greenhouse Gas Control 2014;31:16–24. [61] Chabanon E, Roizard D, Favre E. Membrane contactors for postcombustion carbon dioxide capture: a comparative study of wetting resistance on long time scales. Ind Eng Chem Res 2011;50(13):8237–44. [62] Atchariyawut S, Feng C, Wang R, Jiraratananon R, Liang DT. Effect of membrane structure on mass-transfer in the membrane gas-liquid contacting process using microporous PVDF hollow fibers. J Membr Sci 2006;285(1–2):272–81. [63] Wang R, Zhang H, Feron P, Liang D. Influence of membrane wetting on CO2

[64]

[65]

[66] [67]

[68]

[69]

[70]

[71] [72]

[73]

[74]

[75]

[76]

[77]

[78]

[79] [80] [81]

[82]

[83]

[84]

[85]

[86] [87]

[88]

[89]

[90] [91]

[92] [93]

748

capture in microporous hollow fiber membrane contactors. Sep Purif Technol 2005;46(1–2):33–40. Zhang HY, Wang R, Liang DT, Tay JH. Theoretical and experimental studies of membrane wetting in the membrane gas-liquid contacting process for CO2 absorption. J Membr Sci 2008;308(1–2):162–70. Lv Y, Yu X, Tu ST, Yan J, Dahlquist E. Experimental studies on simultaneous removal of CO2 and SO2 in a polypropylene hollow fiber membrane contactor. Appl Energy 2012;97:283–8. Simons K, Nijmeijer K, Wessling M. Gas-liquid membrane contactors for CO2 removal. J Membr Sci 2009;340(1–2):214–20. Favre E, Svendsen HF. Membrane contactors for intensified post-combustion carbon dioxide capture by gas-liquid absorption processes. J Membr Sci 2012;407–408:1–7. Chabanon E, Bouallou C, Remigy J, Lassaeuguette E, Medina Y, Favre E, et al. Study of an innovative gas-liquid contactor for CO2 absorption. Energy Procedia 2011;4:1769–76. Chabanon E, Roizard D, Favre E. Membrane contactors for post-combustion carbon dioxide capture: a comparative study of wetting resistance on long time scales. Ind Eng Chem Res 2011;50(13):8237–44. Nguyen PT, Lasseuguette E, Medina-Gonzalez Y, Remigy JC, Roizard D, Favre E. A dense membrane contactor for intensified CO2 gas/liquid absorption in postcombustion capture. J Membr Sci 2011;377(1–2):261–72. Molina CT, Bouallou C. Assessment of different methods of CO2 capture in postcombustion using ammonia as solvent. J Cleaner Prod 2015;103:463–8. Karami MR, Keshavarz P, Khorram M, Mehdipour M. Analysis of ammonia separation from purge gases in microporous hollow fiber membrane contactors. J Hazard Mater 2013;260:576–84. Ma S, Chen G, Han T, Zhu S, Yang J. Experimental study on the effect of Ni(II) additive on ammonia escape in CO2 capture using ammonia solution. Int J Greenhouse Gas Control 2015;37:249–55. Kim Y, Lim SR, Park JM. The effects of Cu(II) ion as an additive on NH3 loss and CO2 absorption in ammonia-based CO2 capture processes. Chem Eng J 2012;211:327–35. Li K, Yu H, Tade M, Feron P. Theoretical and experimental study of NH3 suppression by addition of Me(II) ions (Ni, Cu and Zn) in an ammonia-based CO2 capture process. Int J Greenhouse Gas Control 2014;24:54–63. Ma S, Chen G, Zhu S, Wen J, Gao R, Ma L, et al. Experimental study of mixed additive of Ni (II) and piperazine on ammonia escape in CO2 capture using ammonia solution. Appl Energy 2016;169:597–606. Ma S, Song H, Zang B, Chen G, Zhu S. Experimental study of Co (II) additive on ammonia escape in carbon capture using renewable ammonia. Chem Eng J 2013;234:430–6. Li L, Conway W, Burns R, Maeder M, Puxty G, Clifford S, et al. Investigation of metal ion additives on the suppression of ammonia loss and CO2 absorption kinetics of aqueous ammonia-based CO2 capture. Int J Greenhouse Gas Control 2017;56:165–72. Yang HX, Ling Z, Yan XR. Inorganic chemistry. Beijing: Higher Education Press; 2002. Ma SC, Chen GD, Ma XY. Ammonia escape control in carbon dioxide capture using ammonia method. CIESC J 2014;65(10):4086–93. Seo JB, Jeon SB, Kim JY, Lee GW, Jung JH, Oh KJ. Vaporization reduction characteristics of aqueous ammonia solutions by the addition of ethylene glycol, glycerol and glycine to the CO2 absorption process. J Environ Sci 2012;24(3):494–8. Yang N, Yu H, Xu D, Conway W, Maeder M, Feron P. Amino acids/NH3 mixtures for CO2 capture: effect of neutralization methods on CO2 mass transfer and NH3 vapour loss. Energy Procedia 2014;63:773–80. Jeon SB, Lee S, Kang MK, Kang DJ, Oh KJ. Characteristics of absorbent loss and CO2-selective absorption of CO2/SO2 gas into aqueous 2-amino-2-methyl-1-propanol/ammonia solution. Ind Eng Chem Res 2013;52(13):4881–7. Kang MK, Jeon SB, Lee MH, Oh KJ. Improvement in CO2 absorption and reduction of absorbent loss in aqueous NH3/triethanolamine/2-amino-2-methyl-1-propanol blends. Korean J Chem Eng 2013;30(6):1171–80. You JK, Park HS, Yang SH, Hong WH, Shin W, Kang JK, et al. Influence of additives including amine and hydroxyl groups on aqueous ammonia absorbent for CO2 capture. J Phys Chem B 2008;112(14):4323–8. Pellegrini G, Strube R, Manfrida G. Comparative study of chemical absorbents in postcombustion CO2 capture. Energy 2010;35(2):851–7. Gao J, Zhang Y, Feng D, Du Q, Yu M, Xie M, et al. A new technique of carbon capture by ammonia with the reinforced crystallization at low carbonized ratio and initial experimental research. Fuel Process Technol 2015;135:207–11. Ma S, Song H, Zang B, Chen G. Experimental study on additives inhibiting ammonia escape in carbon capture process using ammonia method. Chem Eng Res Des 2013;91(12):2775–81. Asif M, Bak C, Kim WS. Energy minimization and ammonia abatement for CO2 capture using a blend of ammonia and 2-amino-2-methyl-1-propanol solution. Sep Sci Technol 2015;50(10):1565–76. Ma SC, Sun YX, Cui JW, Zhao Y. Experiment and analysis of ammonia escape from decarburization absorbent inhibited by NHD. CIESC J 2011;62(5):1408–13. Li L, Han W, Yu H, Tang H. CO2 absorption by piperazine promoted aqueous ammonia solution: absorption kinetics and ammonia loss. Greenhouse Gases Sci Technol 2013;3(3):231–45. Ali SH. Kinetics of the reaction of carbon dioxide with blends of amines in aqueous media using the stopped-flow technique. Int J Chem Kinet 2005;37(7):391–405. Ali SH, Merchant SQ, Fahim MA. Reaction kinetics of some secondary alkanolamines with carbon dioxide in aqueous solutions by stopped flow technique. Sep Purif Technol 2002;27(2):121–36.

Applied Energy 230 (2018) 734–749

F. Wang et al.

[94] Paul S, Ghoshal AK, Mandal B. Removal of CO2 by single and blended aqueous alkanolamine solvents in hollow-fiber membrane contactor: modeling and simulation. Ind Eng Chem Res 2007;46(8):2576–88. [95] Seo DJ, Hong WH. Effect of piperazine on the kinetics of carbon dioxide with aqueous solutions of 2-amino-2-methyl-1-propanol. Ind Eng Chem Res 2000;39(6):2062–7. [96] Choi WJ, Min BM, Shon BH, Seo JB, Oh KJ. Characteristics of absorption/regeneration of CO2-SO2 binary systems into aqueous AMP+ammonia solutions. J Ind Eng Chem 2009;15(5):635–40. [97] Lv Z. Experimental study of new absorbents for chemical absorption CO2 capture process Master thesis China: Zhejiang University; 2011 [98] Becke AD. Becke’s three parameter hybrid method using the LYP correlation functional. J Chem Phys 1993;98:5648–52. [99] Lee C, Yang W, Parr RG. Development of the Colle-Salvetti correlation-energy formula into a functional of the electron density. Phys Rev B 1988;37(2):785. [100] Yu CH, Huang CH, Tan CS. A review of CO2 capture by absorption and adsorption. Aerosol Air Qual Res 2012;12(5):745–69. [101] Mathias PM, Jasperson LV, VonNiederhausern D, et al. Assessing anhydrous tertiary alkanolamines for high-pressure gas purifications. Ind Eng Chem Res 2013;52(49):17562–72. [102] Huang K, Chen YL, Zhang XM, et al. SO2 absorption in acid salt ionic liquids/ sulfolane binary mixtures: experimental study and thermodynamic analysis. Chem Eng J 2014;237:478–86. [103] Yang J, Gao H, Hu G, Wang S, Zhang Y. Novel process of removal of sulfur dioxide by aqueous ammonia-fulvic acid solution with ammonia escape inhibition. Energy Fuels 2016;30(4):3205–18. [104] Maton C, De Vos N, Stevens CV. Ionic liquid thermal stabilities: decomposition mechanisms and analysis tools. Chem Soc Rev 2013;42(13):5963–77. [105] Fredlake CP, Crosthwaite JM, Hert DG, Aki SNVK, Brennecke JF. Thermophysical properties of imidazolium-based ionic liquids. J Chem Eng Data 2004;49(4):954–64. [106] Dai C, Wei W, Lei Z, Li C, Chen B. Absorption of CO2 with methanol and ionic liquid mixture at low temperatures. Fluid Phase Equilib 2015;391:9–17. [107] Tomé LC, Isik M, Freire CSR, Mecerreyes D, Marrucho IM. Novel pyrrolidiniumbased polymeric ionic liquids with cyano counter-anions: high performance membrane materials for post-combustion CO2 separation. J Membr Sci 2015;483:155–65. [108] Ramdin M, Balaji SP, Vicent-Luna JM, Gutiérrez-Sevillano JJ, Calero S, de Loos TW, et al. Solubility of the precombustion gases CO2, CH4, CO, H2, N2, and H2S in the ionic liquid [bmim][Tf2N] from monte carlo simulations. J Phys Chem C 2014;118(41):23599–604. [109] Nonthanasin T, Henni A, Saiwan C. Densities and low pressure solubilities of carbon dioxide in five promising ionic liquids. RSC Adv 2014;4(15):7566–78. [110] Cui G, Lin W, Ding F, Luo X, He X, Li H, et al. Highly efficient SO2 capture by phenyl-containing azole-based ionic liquids through multiple-site interactions. Green Chem 2014;16(3):1211–6. [111] Zeng S, Zhang X, Gao H, He H, Zhang X, Zhang S. SO2-induced variations in the viscosity of ionic liquids investigated by in situ fourier transform infrared spectroscopy and simulation calculations. Ind Eng Chem Res 2015;54(43):10854–62. [112] Huang K, Chen YL, Zhang XM, Xia S, Wu YT, Hu XB. SO2 absorption in acid salt ionic liquids/sulfolane binary mixtures: experimental study and thermodynamic analysis. Chem Eng J 2014;237:478–86. [113] Cui G, Wang C, Zheng J, Guo Y, Luo X, Li H. Highly efficient SO2 capture by dual functionalized ionic liquids through a combination of chemical and physical absorption. Chem Commun 2012;48(20):2633–5. [114] Ahmadi MA, Haghbakhsh R, Soleimani R, et al. Estimation of H2S solubility in ionic liquids using a rigorous method. J Supercrit Fluids 2014;92:60–9. [115] Faúndez CA, Díaz-Valdés JF, Valderrama JO. Testing solubility data of H2S and SO2 in ionic liquids for sulfur-removal processes. Fluid Phase Equilib 2014;375:152–60. [116] Ma YQ, Wang R. H2S absorption capacity of ionic liquid-MDEA-H2O combined desulfurizers. Chem J Chin Univ 2014;35:1515–22. [117] Yang J, Yu X, Yan J, Tu ST, Dahlquist E. Effects of SO2 on CO2 capture using a hollow fiber membrane contactor. Appl Energy 2013;112:755–64. [118] Yokozeki A, Shiflett MB. Ammonia solubilities in room-temperature ionic liquids. Ind Eng Chem Res 2007;46(5):1605–10. [119] Yokozeki A, Shiflett MB. Vapor-liquid equilibria of ammonia+ionic liquid mixtures. Appl Energy 2007;84(12):1258–73. [120] Li G, Zhou Q, Zhang X, Zhang S, Li J. Solubilities of ammonia in basic imidazolium ionic liquids. Fluid Phase Equilib 2010;297(1):34–9. [121] Li Z, Zhang X, Dong H, Zhang X, Gao H, Zhang S, et al. Efficient absorption of

[122]

[123]

[124] [125] [126] [127]

[128]

[129] [130]

[131]

[132]

[133]

[134]

[135] [136]

[137]

[138]

[139]

[140]

[141]

[142] [143]

[144]

[145]

[146]

749

ammonia with hydroxyl-functionalized ionic liquids. RSC Adv 2015;5(99):81362–70. Shi W, Maginn EJ. Molecular simulation of ammonia absorption in the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide ([emim][Tf2N]). AIChE J 2009;55(9):2414–21. Xu Y, Zhao J, Wu W, Jin B. Experimental and theoretical studies on the influence of ionic liquids as additives on ammonia-based CO2 capture. Int J Greenhouse Gas Control 2015;42:454–60. Gal E. Ultra cleaning combustion gas including the removal of CO2, world intellectual property. Patent WO 2006022885; 2006. Darde V, Thomsen K, Van Well WJM, Stenby EH. Chilled ammonia process for CO2 capture. Energy Procedia 2009;1(1):1035–42. Bak C, Asif M, Kim WS. Experimental study on CO2 capture by chilled ammonia process. Chem Eng J 2015;265:1–8. Mathias PM, Reddy S, O’Connell JP. Quantitative evaluation of the chilled-ammonia process for CO2 capture using thermodynamic analysis and process simulation. Int J Greenhouse Gas Control 2010;4(2):174–9. Sutter D, Gazzani M, Mazzotti M. Formation of solids in ammonia-based CO2 capture processes-Identification of criticalities through thermodynamic analysis of the CO2-NH3-H2O system. Chem Eng Sci 2015;138:170–80. Chilled ammonia post combustion CO2 capture system – laboratory and economic evaluation results. Palo Alto (CA): EPRI; 2006. Jilvero H, Normann F, Andersson K, Johnsson F. Heat requirement for regeneration of aqueous ammonia in post-combustion carbon dioxide capture. Int J Greenhouse Gas Control 2012;11:181–7. Darde V, Maribo-Mogensen B, van Well WJM, Stenby EH, Thomsen K. Process simulation of CO2 capture with aqueous ammonia using the extended UNIQUAC model. Int J Greenhouse Gas Control 2012;10:74–87. Jilvero H, Normann F, Andersson K, Johnsson F. The rate of CO2 absorption in ammonia implications on absorber design. Ind Eng Chem Res 2014;53(16):6750–8. Sutter D, Gazzani M, Pérez-Calvo JF, et al. Solid formation in ammonia-based processes for CO2 capture–turning a challenge into an opportunity. Energy Procedia 2017;114:866–72. Hu K, Liu J, Yang H, Zhang H, Liu Q. New CO2 capture process with gas-liquidsolid three-phase circulating fluidized bed. Ind Eng Chem Res 2013;52(45):15905–11. Jin Y. Fluidization engineering principles. Beijing: Tsinghua University Press; 2001. Jayaweera I, Jayaweera P, Elmore R, Bao J, Bhamidi S. Update on mixed-salt technology development for CO2 capture from post-combustion power stations. Energy Procedia 2014;63:640–50. Kang CA, Brandt AR, Durlofsky LJ, Jayaweera I. Assessment of advanced solventbased post-combustion CO2 capture processes using a bi-objective optimization technique. Appl Energy 2016;179:1209–19. Jayaweera I, Jayaweera P, Kundu P, et al. Results from process modeling of the mixed-salt technology for CO2 capture from post-combustion-related applications. Energy Procedia 2017;114:771–80. Jiang K, Li K, Yu H, Chen Z, Wardhaugh L, Feron P. Advancement of ammonia based post-combustion CO2 capture using the advanced flash stripper process. Appl Energy 2017;202:496–506. Yu J, Wang S, Yu H. Experimental studies and rate-based simulations of CO2 absorption with aqueous ammonia and piperazine blended solutions. Int J Greenhouse Gas Control 2016;50:135–46. Ma S, Chen G, Gao R, Wen J, Ma L. Experimental research of ammonia escape in CO2 absorption using ammonia solution in wetted-wall column. Int J Greenhouse Gas Control 2016;51:254–9. McLarnon CR, Duncan JL. Testing of ammonia based CO2 capture with multipollutant control technology. Energy Procedia 2009;1(1):1027–34. Duan LQ, Yang Y, Zhang SH, Yang YP. Energy consumption analysis and parameters optimization of ammonia-based CO2 removing system from flue gas of coal fired power plant. J North China Electr Power Univ 2012;39(1):6–12. Qi G, Wang S, Yu H, Wardhaugh L, Feron P, Chen C. Development of a rate-based model for CO2 absorption using aqueous NH3 in a packed column. Int J Greenhouse Gas Control 2013;17:450–61. Chu F, Liu Y, Yang L, Du X, Yang Y. Ammonia escape mass transfer and heat transfer characteristics of CO2 absorption in packed absorbing column. Appl Energy 2017;205:1596–604. Koepke D, Eggers R, Mieske K, Kather A. 2nd oxyfuel young researchers forum, Gothenburg, Germany, September 18, 2008.