J. Environ. Eng. Manage., 20(1), 1-7 (2010)
1
REMOVAL OF CO2 FROM FLUE GAS WITH AMMONIA SOLUTION
IN A PACKED TOWER
Hsin-Ta Hsueh, Chi-Liang Hsiao and Hsin Chu*
Department of Environmental Engineering and Sustainable Environment Research Center,
National Cheng Kung University
Tainan 701, Taiwan
Key Words: Carbon dioxide, ammonia, packed tower, absorbent utilization
ABSTRACT
Ammonia was used for the major absorbent while sodium hydroxide was used as a reference in
either a bubble column (semi-batch reaction) or a packed tower (continuous reaction) to absorb CO2
in this study. In the bubble column, bubbling time as a function of temperature, pH, and dissolved
inorganic carbon concentration in the solution was measured to see the characteristics of the reaction
and the capacity of ammonia with regard to CO2 absorption. Its CO2 removal efficiency (RE) and/or
absorbent utilization (AU) in the packed tower were quantified as a function of packing type,
concentration/pH of absorbent, concentration of simulated flue gas (CO2, O2, SO2, NO), gas flow
rate, and liquid/gas ratio. Additionally, an empirical formula obtained, via the analysis of a multiple
regression, was used to relate CO2 RE and the major operation parameters.
In the bubble column, the capacity of ammonia on CO2 absorption is 1.4 kg CO2 kg-1 NH3 in an
exothermic reaction. In the packed tower, the optimum number of packing layers is 21 layers in an
orderly arrangement with a reaction length of 220 mm. In addition, the optimum CO2 RE and AU are
93 and 19%, respectively, at a given condition. After a multiple regression, the empirical formula
indicates four major parameters responsible for CO2 removal, namely ammonia concentration, CO2
concentration, gas flow rate, and initial pH of ammonia solution. The results of this work are feasible
for practical application with regard to CO2 absorption with ammonia in a packed tower.
INTRODUCTION
The atmospheric concentration of CO2 has increased from 280 ppm for the years 1000-1750 to 368
ppm in the year 2000, while the global mean surface
temperature increased by 0.6 ± 0.2 ºC over the 20th
century, as described in the IPCC third assessment report [1]. It is thus necessary to reduce greenhouse gas
emissions, and a number of technologies to mitigate of
CO2 emission have been developed [2]. For major industrial CO2 emission sources, such as fossil fuel
power plant stacks, these include absorption by liquids,
adsorption by solids, extraction by refrigeration, advanced power cycle and the use of alternative energy
sources.
The removal of CO2 from a gas mixture by
washing with alkaline solutions is one of the most
widely practiced industrial gas-absorption processes
[3-5], and the performance and capacity of ammonia
have been reported to be better than those of monoethanolamine (MEA) in a batch study [3]. Several
*Corresponding author
Email: [email protected]
possible pathways of CO2 absorption with ammonia
have also been proposed as follows [6]:
CO2(g) + 2NH3(g) ↔ NH2COONH4(s)
(1)
NH2COONH4(s) + H2O(g) ↔ (NH4)2CO3(s)
(2)
CO2(g) + 2NH3(aq) ↔ CO(NH2)2(s) + H2O(g)
(3)
CO2(g) + 2NH3(aq) → NH4+(aq) + NH2COO-(aq)
(4)
CO2(g) + 2NH3(aq) + H2O(g) ↔ (NH4)2CO3(s)
(5)
CO2(g) + NH3(aq) + H2O(g) ↔ NH4HCO3(s)
(6)
CO2(g) + 2NH3(aq) + H2O(l) ↔ (NH4)2CO3(s)(aq)
(7)
(8)
CO2(g) + NH3(aq) + H2O(l) ↔ NH4HCO3(s)(aq)
According to the above reactions, the complex
mechanisms of CO2 absorption with ammonia are due
to several reactants in the three-phase process. Therefore, it is difficult to analyze the recovery of carbon
and nitrogen due to the multiple reactions, including
volatilization (NH3(g)), dissolution, and condensation
(NH2COONH4(s), (NH4)2CO3(s), CO(NH2)2(s), and
J. Environ. Eng. Manage., 20(1), 1-7 (2010)
2
carbonate) were determined with an ion chromatograph (IC, DX-100) about every minute, as were the
pH and temperature.
NH4HCO3(s)). However, the reactants in the threephase process can be easily reused. For example,
gaseous ammonia was captured in the acidic solution
and reheated for recovery and the condensed product
was used as a nitrogen source for a fertilizer [7]. In
addition, methods for the regeneration of ammonia for
the capture of CO2 have also been proposed [8].
Research on CO2 absorption with an ammonia
solution in a batch or semi-continuous reactor has
been conducted [3,6]. However, this process should be
undertaken in a continuous reactor, to enhance its
practical application. In addition, a packed tower, a
kind of high performance scrubber, has been applied
in CO2 absorption with several amines and NaOH [911]. Consequently, CO2 absorption with ammonia in a
packed tower was carried out under various operating
conditions in this study. Several parameters, including
ammonia concentration, CO2 concentration, gas flow
rate (or retention time), initial pH of ammonia solution,
temperature, and liquid/gas (L/G) ratio, were used to
analyze the CO2 removal efficiency (RE) and absorbent utilization (AU). In addition, we also combined
NO and/or SO2 with CO2 to simulate the compositions
of flue gas and examine their effects on CO2 removal.
2. Reaction between CO2/NOX/SO2 and Ammonia in
a Packed Tower
The removal of CO2/NOX/SO2 (simulated flue
gas) by the ammonia solution was studied in a packed
tower. The system includes a simulated flue gas system, a packed tower, and a sampling and analysis system (Fig. 1). The flue gas simulation system was
composed of five cylinders (air, CO2, NO, SO2, and
N2 (balanced gas)), five mass flow controllers, a two
stage mixer filled with glass beads, and an electrical
temperature controlled heater. The reactor, a custommade Lucite spraying absorber, was packed with Raschig rings. The characteristics of each ring packing
are outside diameter of 7.65 mm, inside diameter of 5
mm, height of 8.3 mm, packing density of 1.97 × 106
m-3, total surface area per unit packed volume of 1,720
m2 m-3, and void fraction of 0.43. The length of the
reaction zone from the gas inlet to the spraying nozzle
was 22 cm, and the internal diameter of the absorber
was 4.9 cm. The spray nozzle was made by System
Spraying Co. (Unijet 1/4TT-SS+TG-SS0.4+W6051
SS-100). The liquid recirculation pump (K33MYFY233, Micropump Co.) had a maximum capacity of 250
mL min-1. A rotameter (AALBORG T54/1-102-5S, <
150 mL min-1) was used to control the flow rate of the
spraying solution.
The concentrations of CO2/O2 were measured
with a CO2/CO/O2 analyzer (Model 300, California
Analytical Instruments, Inc., fuel cell type) and the
concentrations of NOX/SO2 were determined with a
NOX/SO2 analyzer (Model ZRF2-FGG11G2, Califor-
EXPERIMENTAL METHODS AND
MATERIALS
1. Reaction between CO2 and Ammonia in a Bubble
Column
This pre-experiment was designed to check the
feasibility of this study. 99.9% CO2 at 450 mL min-1
was bubbled into 200 mL 0.53 M ammonia solution in
a 250 mL bottle. The concentrations of dissolved inorganic carbon (DIC; all bicarbonate was converted to
BYPASS 2
BYPASS 1
GAS
OUTLET
HOOD
LIQUID INLET
COOLER
MIXER
PACKING
GAS
INLET
PUMP
DILUTED
SULFURIC
ACID
(1N)
BYPASS 3
CO2/CO/O 2
ANALYZER
LIQUID
OUTLET
N2
CO 2
AIR
NOx
SO 2
BAG
BYPASS 4
NO X / SO 2
ANALYZER
FILTER
Fig. 1. The experimental scheme of the packed tower.
Hsueh et al.: CO2 Removal in a Packed Tower
nia Analytical Instruments, Inc., NDIR type). Before
these two analyzers, a diluted sulfuric acid solution
was used to prevent NH3 affecting the analysis.
3
(a)
3. Operating Parameters and Statistical Analysis
26
10
AU =
CO2 RE
Stoichiometric ratio
X m2 / m
X n2 / n
(11)
RESULTS AND DISCUSSION
1. Reaction in a Bubble Column
The reaction of CO2 continuously bubbled into a
given volume of ammonia solution is actually a semibatch one. Therefore, the pH of the ammonia solution
decreases from 11.4 to 7.3 and the temperature increases from 23.8 to 26.9 ºC, due to the bicarbonate/carbonate forming and exothermic reaction, reTable 1. Operating parameters and their ranges in the
packed tower
1
2
3
4
5
6
7
8
Parameters, unit
CO2 concentration, % (v/v)
Ammonia concentration, M
Temperature, ºC
Retention time, s
L/G ratio, mL L-1
Oxygen concentration, % (v/v)
Initial pH
NO and SO2 concentration, ppm
24
Ammonia
NaOH
6
23
(10)
In Eq. 11, m and n are the degrees of freedom (df)
belonging to the Xm and Xn distributions, respectively.
The remaining parameters were input in the final step,
and the deletion step was repeated until the values of
all the parameters were larger than four.
System
pH
(9)
Ammonium bicarbonate was the dominate product in this study, with its reaction expressed as Eq. 8
[6], and the AU was determined on the basis of this.
Stepwise regression was carried out by a backward elimination method using SYSTAT software
[12]. The procedure was initiated by inputting all the
operating parameters and then deleting the one with
the smallest F value, defined as follows:
Fm, m =
25
8
Range
5-20
0-2.7
5-75
5.5-16.4
15-60
0-13.7
10-13
0-4,185
Dissolved inorganic carbon concentration (mg L-1 as C)
CO2 RE
[CO2 ]inlet − [CO2 ]outlet
=
[CO2 ]inlet
Temperature
Ammonia
NaOH
pH
The operating parameters and their ranges are
listed in Table 1, and their effects with regarding to
CO2 RE and AU which are defined in the following
Eq. 9 and 10:
Temperature (°C)
27
12
3500
(b)
3000
2500
2000
1500
Ammonia
NaOH
1000
500
0
0
4
8
12
Time (min)
16
20
Fig. 2. The variations of (a) pH and temperature; and (b)
dissolved inorganic carbon concentration under
bubbling CO2 into the ammonia or NaOH
solutions.
spectively (Fig. 2a). This result is similar to the reaction of 8% (v/v) CO2 and 7% (w/w) ammonia [3]. As
for the NaOH solution at the same bubbling rate, its
pH decreases from 11.6 to 5.6 and the temperature increases from 25.7 to 26.3 ºC. The initial concentration
of ammonia, 0.53 M (pH = 11.4), is much higher than
that of NaOH, 0.0038 M (pH = 11.6), even though the
initial pH values are similar. In addition, the Gibbs
free energies of ammonia and NaOH on the absorption of CO2 are -5.3 and -13.4 kcal mol-1, respectively.
This means that NaOH has more potential to absorb
CO2 than ammonia at the same concentration. As for
the DIC concentration in the solution, it increases to
3,750 mg L-1, but is only 270 mg L-1 in the case of
NaOH solution (Fig. 2b). Consequently, the absorption capacity of ammonia with regard to CO2 is 1.4 kg
CO2 kg-1 NH3. The DIC concentration in the case of
NaOH solution (270 mg L-1) can be taken as the contribution of OH- in the ammonia solution of 0.53 M.
J. Environ. Eng. Manage., 20(1), 1-7 (2010)
4
2. Reaction in a Packed Tower
100
(a)
CO2RE
AU2
60
8
40
6
20
4
0
2
0.0
60
50
CO2 RE (%)
1.0
1.5
2.0
2.5
Ammonia concentration (M)
(b)
1.60 M NH4OH
0.53 M NH4OH
NaOH only
80
3.0
60
40
20
0
10.5
11.0
11.5
12.0
12.5
13.0
Initial pH of absorbent solution
Fig. 4. The variations of CO2 RE and AU at various (a)
ammonia concentrations, and (b) initial pHs.
(Retention time: 6.5 s; CO2: 15%; O2: 6%; L/G
ratio: 50 mL L-1; temperature: 50 ºC)
ammonia concentration increases from 0 to 1.1 M (Fig.
4a). This may be due to the increased collision probability between ammonia and CO2 and/or the increase
in overall mass transfer coefficient as the ammonia
concentration increases [9,13]. At the ammonia concentration 2.7 M, the CO2 RE increases to 85%. As for
ammonia utilization, it decreases from 13.6 to 3.6% as
the ammonia concentration increases from 0.3 to 2.7
M. According to Le Chatlier’s principle, ammonia
concentration in the solution depends on the pH in the
following reaction:
40
30
20
10
0
0.5
100
10.0
2.2. Effects of Ammonia Concentration and Initial pH
The CO2 RE rises from 2 to 67% quickly as
10
AU (%)
CO2 RE (%)
2.1. Determination of the Number of Packing Layers
To determine the optimal packing type in the
packed tower, an experiment was carried out at room
temperature with a gas flow rate of 4 L min-1, 15%
CO2, a liquid flow rate of 200 mL min-1, ammonia
concentration of 0.53 M, and 6% O2. As shown in
Fig. 3, the CO2 RE increases from 15 to 35% as the
length of the reaction zone without packing increases
from 104 to 220 mm. In the presence of packing, the
CO2 RE increases from 35 to 43% when the 220 mm
reaction zone is packed with 14 layers of packing. As
the number of packing layer increases from 14 to 21,
the CO2 RE further increases from 43 to 52%. However, with over 21 layers of packing, the CO2 RE in
fact decreases, probably resulting from the inappropriate distance (too close) between the spraying nozzle
and the top of the packing. Hence, the optimum packing is determined as 21 layers in an orderly arrangement along 220 mm reaction length (a total of 648 Raschig rings).
12
CO2 RE (%)
The optimal packing type in the packed tower is
determined on the basis of the number of packing layers.
14
80
NH3(g) ↔ NH3(aq) + H2O ↔ NH4+(aq) + OH-(aq)
A
B
C
D
Packing type
Fig. 3. The variations of CO2 RE under various packing
types in the packed tower. A: without packing,
104 mm of reaction length; B: without packing,
220 mm of reaction length; C: 14 layers packing,
220 mm of reaction length; D: 21 layers packing,
220 mm of reaction length. (Retention time: 4.9 s;
CO2: 15%; O2: 6%; ammonia concentration: 0.53
M; L/G ratio: 50 mL L-1; temperature: 50 ºC).
(12)
The initial pH of the ammonia solution was set by
adding HCl or NaOH at various ammonia concentrations. As shown in Fig. 4b, the increase in CO2 RE is
observed at the lower ammonia concentrations (0.53
M NH4OH and the case without ammonia) as the pH
increases from 12.5 to 13. In contrast, there are no
variations of CO2 RE for the high ammonia concentration at various pHs (Fig. 4b). Even though adding hydroxyl ions would enhance the production of ammonia
in the gas phase (Eq. 12), no critical effect is observed
in this system.
Hsueh et al.: CO2 Removal in a Packed Tower
100
30
(a)
[NH4OH]
80
25
80
1.6 M
0.53 M
0M
70
20
CO2RE (%)
5
60
2.66 M
1.60 M
1.07 M
0.53 M
40
15
10
20
5
0
0
5
10
15
CO2 inlet concentration (%)
20
CO2 RE (%)
[NH4OH]
AU (%)
60
50
40
30
20
10
0
A B C D E F G A B C D A B C D
80
(b)
Conditions
75
Fig. 6. The variations of CO2 RE at 15% CO2 combined
with various concentrations of NO and/or SO2. A:
15% CO2 only; B: combined with 500 ppm NO;
C: combined with 1,000 ppm SO2; D: combined
with 500 ppm NO and 1,000 ppm SO2; E:
combined with 3,537 ppm NO; F: combined with
4,185 ppm SO2; G: combined with 3,537 ppm
NO and 4,185 ppm SO2 (Retention time: 6.5 s;
O2: 6%; L/G ratio: 50 mL L-1; temperature: 50
ºC).
65
2
CO2RE (%)
70
60
[NH4OH]
1.60 M
0.53 M
55
50
45
0
5
10
O2 inlet concentration (%)
15
Fig. 5. The variations of CO2 RE and AU at various (a)
CO2 concentrations at 6% O2, and (b) oxygen
concentrations at 15% CO2. (Retention time: 6.5
s; L/G ratio: 50 mL L-1; temperature: 50 ºC).
2.3. Effects of CO2, O2, NO and/or SO2 Inlet
Concentrations
The CO2 RE decreases as the CO2 inlet concentration increases at each ammonia concentration (0.5,
1.1, 1.6, and 2.7 M; Fig. 5a), which reflects the fact
that the mass transfer coefficient is inversely proportional to CO2 inlet concentration [9,13,15]. As the
CO2 inlet concentration increases, the stoichiometric
ratio decreases, but the AU rises. For example, the
stoichiometric ratio decreases from 69.6 to 17.6 and
the AU increases from 1.3 to 4.8% as the CO2 concentration increases from 5 to 20% at 2.7 M ammonia
concentration. As for the case of the lowest ammonia
concentration (0.5 M), the stoichiometric ratio decreases from 14.1 to 3.5 and AU increases from 4.5 to
11.1% as the CO2 concentration increases from 5 to
20%. This demonstrates that the increase in CO2 inlet
concentration has a similar effect on the CO2 RE and
AU at each ammonia concentration. As for the effect
of O2 concentration on CO2 RE, a small decrease in
CO2 RE is observed as the O2 concentration increases
from 0 to 13.7% at both the 0.5 and 2.7 M ammonia
concentrations (Fig. 5b). In addition, several hundreds
to thousands of ppm NO and SO2 coexist with CO2
and O2 in the flue gas. As shown in Fig. 6, no dominant variations of CO2 RE are observed when adding
NO and SO2 singly or simultaneously at each given
ammonia concentration. Although NO and SO2 can
react with ammonia to form ammonia nitrate and ammonia sulfate, their concentration levels are much
lower than that of CO2 in this study, and thus these reactions did not occur.
2.4. Effect of Operating Temperature
The CO2 RE increases from 39 to 48% as operation temperature rises from 5 to 50 ºC at 0.5 M ammonia concentration, and from 66 to 76% at 1.6 M
ammonia concentration (Fig. 7). Although the increase in temperature decreases the gas solubility, it
may enhance the diffusion of CO2 and the kinetic collision of CO2 and ammonia. As the temperature rises
further, from 50 to 75 ºC, the CO2 RE decreases, and
this might be due to increased volatilization of gaseous ammonia at such a high temperature. This result is
also observed in the CO2 removal study with MEA
[14].
J. Environ. Eng. Manage., 20(1), 1-7 (2010)
90
30
80
25
[NH4OH]
1.60 M
0.53 M
60
50
30
AU (%)
15
CO2 RE [NH4OH]
1.60 M
0.53 M
75
20
40
12
80
CO2 RE (%)
70
70
65
8
AU [NH4OH]
1.60 M
0.53 M
60
10
55
20
10
6
50
5
[NH4OH]
1.60 M
0.53 M
10
0
0
10
20
30
45
4
4
6
8
0
40
50
60
70
10
12
14
Retention time (s)
16
18
80
O
Operating temperature ( C)
90
Fig. 7. The variations of CO2 RE and AU at various
operating temperatures. (Retention time: 6.5 s;
CO2: 15%; O2: 6%; L/G ratio: 50 mL L-1).
80
2.6. Carbon Recovery
Carbon recovery is defined as the ratio of the
amount of CO2 reduction in the gas phase measured
by NDIR analysis and the increase in DIC in the liquid phase with IC analysis. The carbon recovery
ranged between 66 and 118% under various conditions (Table 2). In the present study, we only measured the total concentrations of carbonate and bicarbonate formed based on Eqs. 2 and 5-8, resulting in a
carbon recovery of less than 100% in most of the experiments. In addition, we have also found that an unknown peak overlapped with the peak of carbonate/bicarbonate, resulting in the carbon recovery of
above 100%.
20
CO2 RE [NH4OH]
0.53 M
1.60 M
70
CO2 RE (%)
2.5. Effects of Retention Time and L/G Ratio
The retention time is defined as the reaction region volume divided by the total gas flow rate. As the
retention time increases, the gas and liquid flow rate
will decrease at the same time on the basis of a given
L/G ratio of 50 mL L-1. The results show that the CO2
RE increases from 68 to 88% as the retention time increases from 5.5 to 16.4 s at a 1.60 M ammonia concentration (Fig. 8a).
The CO2 RE increases from 27 to 56% at 0.5 M
ammonia concentration and from 39 to 88% at a 1.6
M ammonia concentration, as the L/G ratio increases
from 15 to 60 mL L-1 (Fig. 8b). This shows that there
are two possible pathways to increase CO2 RE. One is
a higher liquid flow rate, which will extend the reaction area of packing, due to the extension of the spray
angle. The other pathway is the increase of overall
mass transfer coefficient. Although the CO2 RE increases along with the L/G ratio, AU decreases from
9.1 to 5.2% at a 1.6 M ammonia concentration as the
L/G ratio increases from 15 to 60 mL L-1.
(b)
60
15
AU [NH4OH]
0.53 M
1.60 M
AU (%)
CO2 RE (%)
(a)
85
AU (%)
6
50
10
40
30
5
20
10
20
30
40
50
L/G ratio (mL L-1)
60
Fig. 8. The variation of CO2 RE and AU at various (a)
retention times at 50 mL L-1 of L/G ratio, and (b)
L/G ratios at 6.5 sec retention time. (CO2: 15%;
O2: 6%; temperature: 50 ºC)
2.7. Regression Expression of Operating Parameters
The stepwise regression was used to relate CO2
RE and the operating parameters (Table 3). The procedure was initiated by inputting all the operating parameters and then deleting the one with the smallest F
value. The remaining parameters were then input in
the final step, and the deletion step was repeated until
the values of all parameters were larger than four.
The “excluded” parameters based on an F value
smaller than four are temperature, L/G ratio, and oxygen concentration, and the “included” parameters (F
value > 4) are ammonia concentration, CO2 concentration, total gas flow rate, and pH in the third step of the
stepwise regression (Table 3). Consequently, we can
find an empirical formula for a packed tower with 21
layers of packing in an orderly arrangement along 220
mm reaction length (with a total of 648 Raschig rings)
as follows.
CO2 RE (%) = 7.1 + 23.3 × ammonia concentration
(M) – 1.2 × CO2 concentration (%) – 7.6 ×
total gas flow rate (L min-1) + 5.6 × pH (13)
Hsueh et al.: CO2 Removal in a Packed Tower
Table 2. Carbon recoveries under various conditions
(Retention time: 6.5 s; O2: 6%; L/G ratio: 50
mL L-1)
Run
Variations
1 CO2 concentration (%)
2
3
4
5 Ammonia concentration (M)
6
7
8
9
10 Temperature (ºC)
11
12
13
14
5
10
15
20
0.3
0.5
1.1
1.6
2.7
5
30
40
50
75
Carbon recovery (%)
80
66
76
114
118
76
96
65
80
99
109
107
76
93
Table 3. Results of stepwise regression in the third step
System
Included
2
1
4
7
Parameter
Constant
Ammonia concentration
CO2 concentration
Gas flow rate
pH
Excluded
3
Temperature
5
L/G ratio
6
O2 concentration
Coefficient Std. Error df
23.3
-1.2
-7.6
5.6
2.012
0.435
3.465
1.825
Part. Corr.
0.1
0.3
-0.0
-
F
1 134.3
1 7.0
1 4.9
1 9.4
1
1
1
0.8
3.3
0.1
CONCLUSIONS
Ammonia has potential for use in CO2 absorption. Its capacity is 1.4 kg CO2 kg-1 NH3 in an exothermic reaction. In the packed tower, the optimum
type of packing is 21 layers in an orderly arrangement
along 220 mm of its reaction length. In addition, at a
given condition, the optimum CO2 RE and AU are 93
and 19%, respectively. After a multiple regression, the
empirical formula indicates four major parameters responsible for CO2 removal, namely ammonia concentration, CO2 concentration, gas flow rate, and initial
pH of ammonia solution. Consequently, those results
may be applied to practical application with regard to
CO2 absorption with ammonia in a packed tower.
ACKNOWLEDGMENT
The authors gratefully acknowledge the National
Science Council, Republic of China, for their financial
support (NSC94-2211-E-006-086)
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Discussions of this paper may appear in the discussion section of a future issue. All discussions should
be submitted to the Editor-in-Chief within six months
of publication.
Manuscript Received: February 12, 2009
Revision Received: June 7, 2009
and Accepted: June 20, 2009