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The Rate of CO2 Absorption in AmmoniaImplications on Absorber Design Henrik Jilvero,* Fredrik Normann, Klas Andersson, and Filip Johnsson Department of Energy and Environment, Chalmers University of Technology Hörsalsvägen 7B, Göteborg, 41296 Sweden ABSTRACT: We propose design specifications and operating conditions for an absorber in an ammonia-based, post-combustion carbon capture process. We used a rate-based multistage column model to simulate a baseline-case scenario involving an absorber installed at a large-scale CO2 emission source. The goal was to exploit the chemistry of the NH3−CO2−H2O system to evaluate the performance of the absorber with respect to carbon capture, ammonia slip, and the height of the absorber. Thus, we identified as the two important prerequisites for absorber design: (1) that staged absorption is used and (2) that the rich CO2 loading does not exceed 0.5.

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INTRODUCTION Post-combustion carbon capture prevents emissions of carbon dioxide (CO2) from carbon-based combustion processes. The flue gas from the combustion process is fed into an absorption column (unit 2; Figure 1), where it comes in contact with a

Amines (e.g., monoethanolamine; MEA) diluted in water are commonly used as absorbents. The choice of absorbent is an important area of research. Ideally, the absorbent should exhibit high reactivity, low heat requirement for regeneration, low volatility, low toxicity, high loading capacity, and resistance to degradation. Ammonia has several of these characteristics. The heat requirement for regeneration of ammonia is lower than that for MEA, as shown in a previous study conducted by Jilvero et al.1 Ammonia is not susceptible to degradation and it has a high loading capacity.2 The two major drawbacks associated with the use of ammonia are that (1) it is highly volatile and (2) it reacts slowly with CO2. The high volatility of ammonia necessitates the inclusion of ammonia control before discharge of the flue gas (units 5 and 6; Figure 1). As the cost of the ammonia slip cannot be ignored, it is important to strive for an operation that minimizes the ammonia slip. The rate of the reaction between ammonia and CO2 is important for the design of the equipment used in the capture process, particularly with respect to the absorber. The reaction rate defines the residence time of the flue gas in the absorber that is required to achieve the desired capture efficiency, which is usually in the range of 80%−90% of the CO2 in the flue gas. The required residence time of the flue gases is proportional to the packed height of the absorber. As the investment cost for the absorber represents a significant proportion of the overall cost of the capture facility,3 there is a strong incentive to evaluate how the reaction rate of ammonia affects the design of the absorber. Most of the work to date on the design of ammonia-based post-combustion plants has been conducted by Alstom. The most recent demonstration of the “chilled ammonia process” (CAP) comprises the 40-MWth carbon capture plant at Mongstad.4 Nevertheless, experimental data regarding absorber performance is limited to the results from the Munmorah pilot plant.5 This pilot plant has provided experimental data on CO2 capture efficiency

Figure 1. Schematic of an ammonia-based, post-combustion, carbon capture process. The main units are 1, pretreatment; 2, absorber; 3, rich/lean heat exchanger; 4, stripper; 5, water-wash; 6, ammonia stripper; 7, water-wash; 8, CO2 compression.

chemical absorbent, which binds the CO2 in an aqueous solution. The absorption column contains a packing material over which the liquid absorbent is distributed, so as to maximize the gas−liquid contact area. The liquid stream that enters the absorber column is termed the “CO2-lean stream”, while the CO2-containing liquid stream that exits the absorber column is termed the “CO2-rich stream”. The CO2-rich stream is heated (unit 3; Figure 1) and fed into a stripper column (unit 4; Figure 1). In the stripper column, heat is applied to reverse the absorption reactions, thus releasing the carbon dioxide. The heat required to release a specified amount of CO2 is defined as “the heat requirement for regeneration”. After cleaning (unit 7; Figure 1) and compression (unit 8; Figure 1), the CO2 stream is transportable. © 2014 American Chemical Society

Received: Revised: Accepted: Published: 6750

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either with a two-absorber setup (Figure 2a) or with a pumparound system (Figure 2b). The main goal of this setup is to reduce ammonia slip. Absorption Chemistry. The reaction characteristics of absorbents in post-combustion capture are liquid-film-controlled.11 The reactions R1−R4 represent the global exothermal reactions for CO2 absorption in the liquid phase.12,13 Gaseous CO2 is absorbed in reactions R1−R3 and can be bound as the ionic species of carbamate (NH2COO−), bicarbonate (HCO3−), and carbonate (CO32−). The global reactions indicate the energy required to release CO2 from the liquid, and the reaction enthalpy (ΔH in kJ/mol CO2) is given for each reaction. From a enthalpy of reaction perspective it is preferable to absorb carbon dioxide through reactions R1 and R3 rather than reaction R2. Solid precipitation (reaction R4) should also be avoided since this will increase the enthalpy of reaction without increasing the CO2 absorption.

along the height of the absorber, which can be used to validate modeling of the capture process. Rate-based process simulations of ammonia-based CO2 absorption have previously been presented.6−8 The studies performed by Niu et al.6 and Qi et al.7 focused on validating a rate-based absorber model through comparisons with experimental data. In the work of Zhang and Guo,8 the dimensions of a large-scale absorber were considered for a coal-fired power plant. Despite these previous findings, evaluations of different absorber designs based on the absorption chemistry of the NH3−CO2−H2O system are urgently needed. The aim of the present study is to define an absorber design for post-combustion capture with ammonia as the absorbent. This work is based on an evaluation of the absorption chemistry, which determines the general design targets and optimal operating conditions. The governing absorption reaction rates are reviewed and evaluated by comparisons with data in the literature for pilot plants. The absorber configurations and dimensions are assessed with respect to capture efficiency, ammonia slip, and absorber column height.

NH3(aq) + CO2 (g ) + H 2O ⇌ NH+4 + HCO−3 ΔH = 64.26 kJ/mol CO2

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THEORETICAL BACKGROUND TO THE ABSORBER Absorption columns come in various designs, including traytype and packed-type columns. A packed column with either a random or structured packing material is the first choice for an absorber in the post-combustion setting. However, the construction of the absorber column is dependent upon the type of absorbent to be used. For example, in the initial process configuration of the CAP,9 the formation of solids in the absorber was expected. The formation of solids is problematic for a packed column design because of the risk of plugging. As an alternative, a combined spray/flooded tray column was proposed.2 This column setup has the advantage that the level of ammonia slip is low, as it allows the use of higher CO2 loadings, thereby enabling 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 entry into the stripper column.1 Another absorber design option to reduce ammonia slip in packed columns is to use staged absorption with intermediate cooling,10 as illustrated in Figure 2. The staging can be obtained

(R1)

2NH3(aq) + CO2 (g ) + H 2O ⇌ 2NH+4 + CO32 − ΔH = 101.22 kJ/mol CO2

(R2)

2NH3(aq) + CO2 (g ) ⇌ NH+4 + NH 2COO− ΔH = 72.32 kJ/mol CO2

(R3)

NH+4 + HCO−3 ⇌ NH4HCO3(s) ΔH = 26.3 kJ/mol CO2

(R4)

H 2O ↔ H+ + OH−

(R5)

NH3 + H 2O ↔ NH+4 + OH−

(R6)

CO2 + H 2O ↔ HCO−3 + H+

(R7)

HCO−3 ↔ CO32 − + H+

(R8)

NH3 +

HCO−3

−

↔ NH 2COO + H 2O

(R9)

Figure 2. Design options for staged absorption in post-combustion carbon capture. Staged absorption may be achieved through either a two-absorber setup (a) or a pump-around system (b). 6751

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In equilibrium modeling of the NH3−CO2−H2O system the reaction system is usually represented by five ionic reactions, R5−R9. In rate-based modeling the bicarbonate and carbamate formation reactions (R1 and R3) are considered ratedetermining.14 According to equilibrium speciation data by Ahn et al.,15 CO2 is predominantly bound as carbamate at low CO2 loadings. Formation of bicarbonate is the main reaction for increasing the load above 0.5. The carbamate formation is therefore considered to be the main reaction route for CO2 absorption at CO2 loadings below 0.5. The carbamate reaction may be represented by either a two-step zwitterion mechanism (R7 and R8)11,16,17 or an Arrhenius expression.18,19 In R10 and R11, the letter ‘B’ denotes any base that is present in the liquid solution. In the Aspen Plus process simulation software, the forward and backward reaction rates for bicarbonate R12 and R13 and carbamate (R14 and R15) occur in the film.7 Thus, reactions R12 and R13 replace reaction R7 and reactions R14 and R15 replace reaction R9 in the modeling of the liquid film in the absorber simulations. The default rates in Aspen Plus for all reactions are taken from the studies of Pinsent et al.18,20 CO2 (aq) + NH3(aq) ⇌ NH+3 COO−

(R10)

NH+3 COO− + B ⇌ NH 2COO− + BH+

(R11)

−

HCO−3

(R12)

HCO−3 → CO2 (aq) + OH−

(R13)

CO2 (aq) + OH →

absorber operational parameters for post-combustion capture. Table 1 also lists the typical operating conditions and design aspects of these parameters. The nomenclatures used for flows into the absorber in postcombustion capture are as follows: the L/G ratio; the lean stream CO2 loading; the flue gas CO2 concentration; and the lean stream ammonia concentration. Each of these parameters determines the levels of ammonia, CO2, water, and inert gases that enter the absorber. Although these parameters determine the flow of each species, they are not suitable for analyses of the reactions that occur in the absorber. In the present work, the ratio between the reactive species in the column is introduced as a complement to the L/G ratio. This parameter is defined as the mole-basis ratio of the ammonia in the lean stream to the CO2 in the flue gases (absorber loading). Assuming that all the previously mentioned flow parameters are defined on a mole basis, this ratio can be calculated using eq 1. The absorber loading was used in our previous work,1 which was concerned with equilibrium-based process modeling. In that study, the absorber loading was set at 0.5. The absorber loading is a good measure of reactivity or the ability to absorb CO2 even at low CO2 concentrations. However, as an equilibrium-based study represents the maximum absorption efficiency, the reactivity may need to be increased when the rates of the absorption reactions are considered. The absorber loading forms the basis for the evaluation of different absorber designs in the present work.

NH3(aq) + CO2 (aq) + H 2O → NH 2COO− + H3O+ (R14) −

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+

NH 2COO + H3O → NH3(aq) + CO2 (aq) + H 2O

⎞ yCO ⎛ 1 CO2 = L 2 ⎜⎜ + CO2 loadlean⎟⎟ (mole‐basis) NH3 ⎝ x NH3 ⎠ G

(1)

METHODOLOGY A rate-based process model was used to evaluate the parameters of absorption rate; absorber height; and absorber operating conditions. The Aspen Plus version 8.0 process simulation

(R15)

Absorber Dimensions and Operating Conditions. The size specification and operation of the absorber are dependent upon several parameters. Table 1 summarizes the most important

Table 1. Parameters Important for Absorber Design and Their Respective Design Aspects absorber parameter

design aspects

height

In post-combustion CO2 capture, a capture efficiency of 80% to 90% is commonly reported. The absorption rate of the absorbent used is directly correlated to the packed column height. Operating conditions, such as temperature and CO2 loading, also influence the reaction rate.

diameter

The diameter of the absorber is defined by either a desired pressure drop or the superficial gas velocity when flooding occurs in the column. A common design target is to adjust the liquid flow rate (L) and the gas flow rate (G) so that the superficial gas velocity is 80% of the flooding limit. Absorbers are usually operated at a pressure drop that is within the range of 0.25−0.4 in. H2O/ft column (2.0−3.2 mbar/m column).

L/G ratio

A prerequisite for reaching the desired capture efficiency is that the stoichiometric conditions of the reactants in the absorber are sufficient. This is usually achieved by adjusting the ratio of the liquid flow rate (L) to the gas flow rate (G).

CO2 loading

The CO2 loading (CO2/NH3 on mole-basis) reflects the capacity of the lean solution to absorb CO2. However, reaching a low CO2 loading requires an increase in the level of heat for regeneration of the ammonia in the stripper column. In our previous work,1 we showed that a lean CO2 loading of 0.25 is preferable. Although a higher CO2 loading results in a slightly lower heat requirement, the poor capture efficiency and the lack of absorption capacity make this unfeasible.

ammonia concentration

The concentration of ammonia in the liquid stream (xNH3) is an important design parameter. With a low concentration of ammonia, the volume of extra water that needs to be heated, cooled, and recirculated makes the process energy-intensive. Meanwhile, a high concentration of ammonia can lead to precipitation of the solid in the CO2 rich stream. Operating at conditions close to the point at which solid precipitation occurs is desirable,1 with a typical design range being 5−15 wt %.

flue gas CO2 concentration

The driving force to absorb CO2 is determined by the difference between the actual gas phase CO2 concentration (yCO2) and the equilibrium partial pressure of CO2 for the corresponding conditions.

absorber temperature

The operating temperature of the absorber is lower when ammonia is used than when MEA is used as the absorbent. This occurs because the partial pressure of ammonia is far higher than that of MEA. Thus, the lower temperature prevents loss of the absorbent during operation. However, absorber cooling can be energy-intensive. Process performance is greatly enhanced by having access to low-temperature cooling water.21 The operating temperature should be as low as the available cooling media allow. 6752

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software was used for all the simulations. A model based on the conditions of the Munmorah pilot plant was used to compare the experimental data from the pilot plant with the model prediction and to evaluate the carbamate reaction. The second part of the results is devoted to evaluations of the operating conditions and design of the absorber. The simulations of the chemistry in the absorber lay the foundations for the suggested absorber design. The most important parameter valuated is the absorber loading. The present work is based on analyses of liquid and gas compositions. The flue gas in both cases is assumed to be cooled and condensed, and all inert gases are assumed to be nitrogen. The design targets are based on common post-combustion performance indicators, such as the CO2 capture efficiency, the ammonia slip, and the packed section height in the absorber. For this analysis, a baseline-case scenario of a large-scale absorber was used. The thermodynamic model for the liquid bulk requires a more complex framework. When CO2 is absorbed in aqueous ammonia an electrolyte solution is formed. This solution has both ion−ion and ion−molecule interactions and is far from ideal. In the present work, the electrolyte NRTL-based model for the NH3−CO2−H2O system22 was used. Table 2 lists the auxiliary

Table 3. Experimental Settings in the Simulations of the Absorber at the Munmorah Pilot Plant

Munmorah pilot plant

large-scale absorber

mass transfer interfacial area heat transfer liquid holdup

Onda et al.23 Onda et al.23 Chilton and Colburn analogy Stichlmair et al.25

Bravo et al.24 Bravo et al.24 Chilton and Colburn analogy Bravo et al.26

value 0.6 m 7.8 m Pall rings, 25 mm (plastic) 207 m2/m3 0.94 20 rate-based stages 134 L/min 4.5 wt % 0.22 660 kg/h 10 vol %

Table 4. Experimental Settings in the Simulations of a Generic Large-Scale Absorber

Table 2. Models Included in the Column Simulations of the Present Work transport model

setting absorber diameter packed height packing material surface area void fraction stages lean streamflow rate ammonia concentration lean stream CO2 loading flue gas mass flow flue gas CO2 concentration

setting

value

absorber diameter packing material surface area void fraction stages ammonia concentration lean-stream CO2 loading CO2 mass flow flue gas CO2 concentration intercooling temperature

12 m Mellapak 250Y (metal) 256 m2/m3 0.987 30 rate-based stages 10% (mole basis, CO2-free) 0.25 150 t/h 10.5 vol % 10 °C

case was calculated by the process simulations in Aspen Plus. The diameter of the absorber was kept constant at 12 m, which gave the desired pressure drop over the column. The absorber diameter was highly dependent upon the gas flow rate (G), which was kept constant, and this is why also the diameter was kept constant. The liquid to gas flow ratio (L/G) was varied between 2 and 4. For the L/G ratios presented in the results, the pressure drop ranged from 1.5 to 3.2 mbar/m (0.2−0.4 in. H2O/ft); that is, the liquid flow rate did not have a significant impact on the diameter of the absorber. The pressure drop of the Mellapak Y250 packing material was calculated with the built-in correlation for this packing in Aspen Plus. The height of the absorber is also dependent upon the gas-phase partial pressure of CO2. When a two-absorber setup with intercooling is simulated, the two stages are represented by two multistage column models that are connected in series. The absorbers are hereinafter referred to as “Absorber 1″, and “Absorber 2”. Because of the existing modeling framework, the intercooler is placed at the bottom stage of Absorber 1. The heat rejected from the bottom stage is varied until the liquid stream has obtained the designated intercooling temperature (10 °C). A common flash unit could not be employed, as this would imply that the solution reaches equilibrium, that is, a cooler with infinite residence time. On the top stage of the second absorber, the “on-stage liquid” regime was used, since the “above-stage” regime would imply that the stream is flashed upon entering the second column.

models used in these simulations. The property models used in this work are based on the property methods package ENRTL-RK, with some modifications.14 Because of model consistency, the basis of the reaction rate was recalculated on an activity basis rather than on the default molarity basis. The conversion procedure has been described previously.7 The equilibrium constants for R12 and R13 and R14 and R15 were calculated from the reference-state Gibbs energies according to the work of Que and Chen.22 Absorber Simulations Using Data from the Munmorah Pilot Plant. To establish a reliable modeling framework, the results obtained from the Munmorah pilot plant described in the work of Yu et al.5 were compared with the outcomes of the column simulations. The absorption conditions used for the tests at the Munmorah pilot plant are characterized by a large excess of free ammonia, which favors the formation of carbamate. The results from the absorber modeling were compared with what is referred to as the “Measurement Series 3” in the work of Yu et al.5 For this series, the CO2 capture efficiency along the height of the absorber was determined. Table 3 lists the settings for the simulations of the absorber at the Munmorah pilot plant. In the real pilot plant, there are two absorbers connected in series, giving a total packed height of 7.8 m. In these simulations, the two absorbers are simulated as a single absorber of the same total height. Simulations of a Large-Scale Absorber. For the baselinecase (Table 4), a flow rate of CO2 of 150 tons/h was applied (1.3 Mt/year) to represent a large-scale emission source. Absorber packed heights in the range of 0−40 m were evaluated, and the CO2 capture efficiency for each simulation

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EVALUATION OF THE CARBAMATE REACTION The first part of the results describes the reaction rates of the CO2 absorption in an aqueous ammonia solution. Figure 3 summarizes the apparent reaction rates for the carbamate reaction R11 at an ammonia concentration of 1 kmol/m3.11,16−19 6753

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Figure 4. Capture efficiency as a function of the height in the absorber. The experimental data from the Munmorah pilot plant are compared with the reaction rates derived from laboratory-scale experiments. The three lines represent simulations in which the reaction rates reported by Pinsent et al.18 (continuous line) and Puxty et al.19 (dotted line), and the “Log. mean” (dashed line) are used to represent the carbamate formation reaction (on an activity basis).

Figure 3. Apparent kinetic reaction rates for carbamate formation R10, R11, and R15 as a function of temperature (molarity basis). The conditions correspond to a CO2-free solution with an ammonia concentration of 1 kmol/m3. The reaction rates described by Puxty et al.19 and Pinsent et al.18 and the “Log. mean” use an Arrhenius expression, while the remaining rates11,16,17 are expressed as two-step zwitterion mechanisms. The apparent kinetic reaction rate for the MEA carbamate reaction is taken from Versteeg and van Swaaij.27

measurements made in the Munmorah pilot plant.5 The triangle symbols in Figure 4 indicate the experimentally determined CO2 capture efficiencies along the height of the absorber. The lines represent the absorber simulations that used the rate expressions for the carbamate formation reaction proposed by Pinsent et al.18 and Puxty et al.,19 and the “Log. mean”. The reaction rates derived by Puxty et al.19 and Pinsent et al.18 yield, respectively, a too-fast and a too-slow representation of the CO2 capture efficiency, as compared with the reaction rate reported by Pinsent et al.18 The average reaction rate, Log. mean, provides the most accurate prediction of the experimental data. It should be noted that when the total absorption column is modeled, the differences between the rates are not as pronounced, as indicated in Figure 3. For the listed operational conditions at the pilot plant, there is a large excess of ammonia. The level of free ammonia in the lean stream is several times higher than the level of CO2 in the flue gases. Thus, even though almost all the CO2 is captured in the absorber, the CO2 loading of the liquid only increases from 0.22 to 0.33. The operating conditions at the Munmorah pilot plant result in an absorber loading of 0.11. At such low CO2 loadings, only the carbamate reaction is of importance. The reason for this can be deduced from the stoichiometric difference between R1 and R3, in which ammonia and CO2 react at stoichiometric ratios of 1:1 and 1:2, respectively. The general trend is that for CO2 loadings 0.5, the bicarbonate reaction will

The reaction rate for the corresponding carbamate reaction in MEA absorption27 is also included in Figure 3 for reference, together with the equivalent MEA concentration. The temperature range in Figure 3 (0−40 °C), corresponds to the absorber operating temperature. It is evident that there is a large discrepancy between the measured reaction rates (the y-axis is logarithmic), which raises uncertainties regarding the design of the absorber. In addition, Figure 3 confirms the previous indication that the absorption rate of ammonia is lower than that of MEA. In the present work, the slowest18 and the fastest19 of the reaction rates were evaluated. To represent the remaining reaction rates, a medium-fast reaction rate is also proposed and evaluated in this work. The medium reaction rate (Log. mean) are taken as the logarithmic average of the reaction rates described by Pinsent et al.18 and Puxty et al.,19 respectively. Table 5 shows the preexponential factor (k) and the activation energy (E) of R12−R15 used in this work. The reaction rates are presented both on a molarity basis (eq 2) and activity basis (eq 3). i=1 n (−E /(RT ))

r = kT e

∏ Cia

i

(2)

N i=1

r = kT n e(−E /(RT )) ∏ (xiγi)ai

(3)

N

Figure 4 shows the NH3−CO2 chemistry resulting from comparing the reaction rates presented in Table 5 with the

Table 5. Reaction Rates of R12−R15 Used in the Present Work. The Reaction Rates Are Presented on Both a Molarity Basis and Activity Basis k (kmol/m3·s) reaction R12 R13 R14 R15 R14 R15 R14 R15

source

n

(molarity)

Pinsent et al.20

0 0 0 0 0 0 0 0

4.32 2.80 1.35 1.03 6.51 4.97 1.66 1.27

Pinsent et al.18 Log. mean (This work) Puxty et al.19

6754

× × × × × × × ×

1013 1013 1011 1019 1013 1021 1014 1022

(activity)

E (cal/mol)

× × × × × × × ×

13 249 25 818 11 585 16 180 14 362 18 957 14 577 19 172

1.33 8.63 4.16 1.76 2.01 8.51 5.12 2.17

1017 1016 1014 1024 1017 1026 1017 1027

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bicarbonate formation. Figure 5 also indicates the preferred operating conditions. Operating at an absorber loading at 0.33 is not desirable, as even with an absorber height of 40 m, the CO2 capture efficiency does not exceed 60%. Thus, if a CO2 capture efficiency of 85% is to be achieved, an absorber loading of less than 0.25 is required. In summary, the ideal operating conditions for most applications would be an absorber loading that generates a CO2 capture efficiency of 80%−90%. The packed height of the absorber needs to be designed to have sufficient residence time for the fast carbamate reaction to end. The closer the operating conditions of the absorber are to the equilibrium capture efficiency, the lower is the heat requirement for regeneration. At operating conditions close to the equilibrium line, more of the CO2 is bound as bicarbonate than as carbamate or carbonate. As the dissociation of carbamate or carbonate requires more heat R2 and R3 than the dissociation of bicarbonate R1, the total heat requirement would be lower closer to the equilibrium line. One way to take advantage of the fast absorption through carbamate formation while retaining a low level of ammonia slip is to use staged absorption. Staged absorption involves the use of two absorbers (Absorber 1 and Absorber 2) with different operating conditions and objectives. The objective of Absorber 1 is to absorb CO2, while the objective of Absorber 2 is to reduce the gas-phase ammonia concentration. This is illustrated in Figure 6, which compares the one-absorber and two-absorber setups. The total height of both absorber setups is set at 20 m, whereby the two-absorber setup is divided into a 15-m absorber and a 5-m absorber, which are connected in series. Figure 6a presents the gas-phase CO2 concentrations along the height of the absorber. In Absorber 1 of the twoabsorber setup, most of the CO2 is absorbed within 10 m, whereby the absorption is governed by the fast carbamate reaction. In the one-absorber setup, the reactions within the height interval of 8−20 m are characterized by the same fast reaction. The extent to which the packed height is characterized by the carbamate reaction is determined by the absorber loading. In the simulations shown in Figure 6, the same (0.225) is used. However, since more of the packed height in the oneabsorber setup is characterized by the fast reaction, the capture efficiency is higher. The advantage of using staged absorption is evident in Figure 6b, which shows the ammonia concentrations along the height of the absorber for the same absorber setups. The ammonia slip is determined by the conditions at the top of the last absorber. If the liquid stream is preloaded with CO2 upon entering the last absorber, the vapor pressure of ammonia will be suppressed, and as a consequence, the level of ammonia slip is also reduced. If a one-absorber setup is used, the ammonia slip may be as high as 5% (50 000 ppm). After the gas stream enters the bottom of Absorber 2, the ammonia concentration decreases rapidly. Thus, the height of Absorber 2 can be less than that of Absorber 1. The main purpose of Absorber 2 is the removal of ammonia from the flue gases, as almost no CO2 is absorbed in Absorber 2. Using staged absorption, the ammonia slip can be reduced from 50 000 ppm to 3 000 ppm. The gas phase NH3 concentration has a peak at 10 m above the bottom of the Absorber 1. This is due to the temperature profile of the absorber, which has a similar shape as the NH3 concentration with a peak at 5 m. The peak in NH3 concentration is at 10 m of Absorber 1, because the CO2loading is much lower at the top of the absorber compared to the bottom (see Figure 7b). In Figure 6, panels c and d show

absorb CO2. As the equilibrium carbamate concentration is much higher than bicarbonate concentration at CO2 loadings