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Simultaneous removal of NO and SO2 with hexamminecobalt(II) solution coupled with the hexamminecobalt(II) regeneration catalyzed by activated carbon
Applied Catalysis B: Environmental 54 (2004) 25–32
Simultaneous removal of NO and SO2 with hexamminecobalt(II) solution coupled with the hexamminecobalt(II) regeneration catalyzed by activated carbon Xiang-Li Long∗ , Zhi-Ling Xin, Hong-Xin Wang, Wen-De Xiao, Wei-Kang Yuan UNILAB, State Key Laboratory of Chemical Reaction Engineering, East China University of Science and Technology, Shanghai 200237, PR China Received 31 December 2003; received in revised form 25 April 2004; accepted 31 May 2004 Available online 22 July 2004
Abstract The wet ammonia desulfurization process can be retrofitted for combined removal of SO2 and NO from the flue gases by adding soluble cobalt(II) salt into the aqueous ammonia solution. Activated carbon is used to catalyze the reduction of hexamminecobalt(III) to hexamminecobalt(II) to maintain the capability of removing NO of the hexamminecobalt solution. The effects of temperature, pH, activated carbon particle size, and superficial liquid flow velocity on hexamminecobalt(III) conversion have been investigated. An apparent activation energy is obtained. According to the experimental results, the catalytic reduction reaction rate increases with temperature. The batch reactor experiments show that the best pH range lies in between 3.5 and 6.5. In a fixed-bed reactor, superficial liquid flow velocity obviously affects the reaction and a high yield of cobalt(II) is obtained at a pH value lower than 9.0. The experiments manifest that the hexamminecobalt solution coupled with catalytic regeneration of hexamminecobalt(II) can maintain a high nitric oxide removal efficiency during a period of time. © 2004 Elsevier B.V. All rights reserved. Keywords: Nitric oxide; Catalytic reduction; Activated carbon; Hexamminecobalt; Absorption
1. Introduction Power plant flue gases frequently contain NOx and SO2 that cause acid rain and urban smog. Removal of these contaminants to comply with the environmental emission standards is necessary and various wet and dry processes have been put forward. Among the existing treatment processes for removing NOx from the flue gases, selective catalytic reduction (SCR) using NH3 at 300–500 ◦ C is supposed to be the best available NOx control technology. However, application of the SCR is limited because of its high capital and operating costs. There is still an urgent need for a more economical method for controlling NOx emission. It has been agreed that a flue gas desulfurization (FGD) process that promotes NO removal merely by using chemical additives to an aqueous scrubbing solution may have ∗ Corresponding author. Tel.: +86 21 64253267; fax: +86 21 64252814. E-mail address: [email protected] (X.-L. Long).
0926-3373/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2004.05.020
a significant impact on the control strategies. However, the treatment cost using a strong oxidant, such as OCl2 − [1,2] and H2 O2 [3–6], has been too high to make it a practical NOx control process. Other approaches treating NO in aqueous solution scrubbers include addition of heavy metal chelators to sequester nitric oxide for the subsequent removal [7–10], and even addition of yellow phosphorous emulsions and O2 to oxidize nitric oxide to form a combination of nitrite and nitrate salts [11,12], have not yet been commercialized. The process using an ammonia scrubber to recover sulfur dioxide from the flue gas has been developed and put into commercial application. It is of great significance to capitalize on the ammonia process to abate nitric oxide in the flue gas. The authors put forward a novel technique for simultaneous removal of NO and SO2 from the flue gas by adding soluble cobalt(II) salt into the aqueous ammonia solution. The hexamminecobalt(II) formed by cobalt(II) binding with ammonia can not only coordinate nitric oxide but also activate oxygen molecules in the aqueous ammonia solution. The oxidant is the oxygen coexisting in the flue gas. Therefore,
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X.-L. Long et al. / Applied Catalysis B: Environmental 54 (2004) 25–32
NO can be absorbed and oxidized simultaneously in the hexamminecobalt(II) solution. In order to regenerate the active constituent hexamminecobalt(II) to maintain the capability of removing NO from the gas streams, water is used to reduce cobalt(III) to cobalt(II) with activated carbon as the catalyst. The proposed process can be easily integrated into the existing wet ammonia scrubbers for the simultaneous removal of SO2 and NO.
hexamminecobalt solution will reduce as the reaction proceeds. The hexamminecobalt(III) ion has to be reduced to hexamminecobalt(II) ion once again in order to maintain the removing NO capability. Electrochemical half-cell reduction potential of Co(NH3 )6 3+ /Co(NH3 )6 2+ (Eq. (6)) shows that hexamminecobalt(III) ions cannot be easily reduced to hexamminecobalt(II). A catalyst needs to be chosen to catalyze the reduction of hexamminecobalt(III).
2. Theoretical
[Co(NH3 )6 ]3+ + e− ↔ [Co(NH3 )6 ]2+ ,
It is known that NO is insoluble in water. However, nitric oxide may react with hexamminecobalt(II) to form a nitrosyl complex as follow: NO(g) ↔ NO(aq)
(1)
Co(NH3 )6 2+ (aq) + NO(aq) → [Co(NH3 )5 NO]2+ (aq) + NH3
(2)
Reaction (2) enhances the nitric oxide solubility in the aqueous solution. The hexamminecobalt(II) ion, [Co(NH3 )6 ]2+ , is oxygenated in the aqueous ammonia, the dioxygen displacing one of the ammonia ligands. A brown binuclear complex with bridging dioxygen is then formed [13,14]: 2Co(NH3 )6 2+ + O2 ↔ [(NH3 )5 Co–O–O–Co(NH3 )5 ]4+ +2NH3
(3)
The bridging O–O group is considered a peroxide ion with an O–O distance of 1.47 Å, and a torsion angle of 146◦ about the Co–O–O bond. Hence it is assumed that the oxidizability of [(NH3 )5 Co–O–O–Co(NH3 )5 ]4+ is similar to that of H2 O2 . Such a process is actually activation of the oxygen molecule which makes some processes not possible with free gaseous dioxygen carry out. Therefore, nitric oxide may be oxidized to soluble NO2 in the aqueous solution by [(NH3 )5 Co–O–O–Co(NH3 )5 ]4+ : [(NH3 )5 Co–O–O–Co(NH3 )5 ]4+ + H2 O + 2NH3 +[Co(NH3 )5 NO]2+ → 2Co(NH3 )6 3+ + 2OH− +[Co(NH3 )5 NO2 ]2+
(4)
The following reaction will take place as the produced nitric dioxide is dissolved in the aqueous ammonia:
ECo(NH3 )6 3+ /Co(NH3 )6 2+ = 0.1 V
(6)
It is well known that activated carbon has been successfully applied in industry as a catalyst due to its enormous surface area, porous structure and characteristic flexibility. Since activated carbon has been proven capable of transferring electrons for reduction reactions, it has been reported that a number of chemical species can be reduced in the presence of granular activated carbon. For example, Razvigorova et al. [15] found that nitrate was reduced to nitrite after contacting with an apricot stone-based carbon. Voudrias et al. [16] presented that chlorite was reduced to chloride by granular activated carbon. In addition, Suidan et al. [17] modeled the reduction of free chlorine to chloride in the presence of granular activated carbon. Pérez-Candela et al. [18] found that chromium(VI) could be reduced to chromium(III) by the carbon made from olive stone, and Mary et al. [19] investigated the reduction of bromate to bromide. Carbon atoms at edge of the graphene layers of the activated carbon are reactive and form surface functional groups by bonding with heteroatoms such as oxygen, etc. Surface functional groups are mainly divided into acidic groups and basic groups. Typical acidic groups include surface oxides such as carbonyl, carboxyl, phenolic hydroxyl, lactone and quinone. Usually the structures corresponding to chromeneand pyrone-like ones belong to the basic groups. Because of the peculiar characteristics of activated carbon, coexisting of both acidic groups and basic groups on its surface makes the activated carbon suitable to catalyze hexamminecobalt(III) reduction. It is reported [20] that NH3 molecules may react with the acidic part of carbonyl groups and phenolic hydroxyl groups to form CO− (NH4 )+ complexes on the carbon surface. Formation of these complexes may accelerate disintegration of Co(NH3 )6 3+ ions to cobalt(III) (Eq. (7)): AC
2[Co(NH3 )5 NO2 ]
2+
6AC-OH + Co(NH3 )6 3+ ↔Co3+ + 6AC-CO− (NH4 )+
+ H2 O + 4NH3 → NH4 NO2
+NH4 NO3 + 2Co(NH3 )6 2+
(5)
Hexamminecobalt(II) can be oxidized to hexamminecobalt(III) as the nitric oxide is oxidized. The hexamminecobalt(III) ion, [Co(NH3 )6 ]3+ , cannot be oxygenated in the aqueous ammonia to form dioxygen complex. In other words, the hexamminecobalt(III) ion is unable to activate oxygen molecules. The oxidizing ability of the
(7) Electrochemical half-cell reduction potential of Co3+ /Co2+ (Eq. (8)) shows that cobalt(III) ions are strong oxidants and can be reduced to cobalt(II). 2Co3+ + 2e− ↔ 2Co2+ ,
ECo3+ /Co2+ = 1.82 V
(8)
On the other hand, the basicity of an activated carbon is mainly due to the presence of basic oxygen-containing
X.-L. Long et al. / Applied Catalysis B: Environmental 54 (2004) 25–32
functional groups (e.g. pyrones or chromenes) and/or graphene layers acting as Lewis bases and forming electron donor–acceptor (EDA) complexes with H2 O molecules. Montes-Morán et al. [21] studied the causes of the basic nature of carbonaceous materials and concluded that an unconventional bond might be established between the H3 O+ ions and the cloud of electrons of the aromatic rings of the activated carbon. These basic sites are located at the electron-rich regions within the basal planes of carbon crystallites away from the crystallite edges [22]. This delocalized electron system can act as a Lewis base in the aqueous solution [22] (Eq. (9)): −C + 2H2 O ↔ C − H3 O+ + OH−
(9)
This delocalized electron system (−C) can act as a cobalt(III) reduction center. It should also be noticed that the electrochemical half-cell reduction potentials of O2 /OH− (Eq. (10)) and O2 /H2 O (Eq. (11)) show that hydroxyl ions are more liable to be oxidized to O2 than H2 O molecules by cobalt(III) ions (Eq. (12)). In other words, activated carbon may catalyze the oxidation of water. − 1 2 O2 (g) + H2 O + 2e
↔ 2OH− (aq),
EO2 /OH− = 0.401 (10)
O2 (g) + 4H+ (aq) + 4e− ↔ 2H2 O,
EO2 /H2 O = 1.229 (11)
2Co3+ + 2OH− → 2Co2+ + H2 O + 21 O2
(12)
The global reaction for the regeneration of cobalt(II) ions can be written as follows: AC
2[Co(NH3 )6 ]3+ + H2 O + 10H+ ↔12NH4 + + 2Co2+ + 21 O2
(13)
Cobalt(II) ions may combine with ammonia in the solution to form hexamminecobalt(II) ions repeatedly (Eq. (14)). Thus, the regeneration of hexamminecobalt(II) ions is completed. As a result, the NO removal efficiency can be maintained and lasts long at a high level. Co2+ + 6NH3 → Co(NH3 )6 2+
(14)
In the process discussed above, oxidation and absorption of nitric oxide are performed simultaneously. The oxidant is the oxygen coexisting in the flue gas. The hexamminecobalt(II) ions can be used repeatedly catalyzed by the activated carbon. The reductant of hexamminecobalt(III) is H2 O in the solution. Nitric oxide is converted into nitrite and nitrate (Eq. (15)). 2NO + O2 + 2OH− → NO2 − + NO3 − + H2 O
(15)
27
3. Experimental 3.1. Methods Batch experiments were performed in a stirred glass flask of 500 mL to investigate the hexamminecobalt(III) reduction catalyzed by the activated carbon. 150 mL hexamminecobalt(III) solution and appropriate dose of activated carbon were introduced into the glass flask. The initial hexamminecobalt(III) concentration was 0.01 mol/L for most of the test runs. A commercial coconut activated carbon was used. The pH values of the solutions were adjusted before addition of activated carbon by adding appropriate amounts of H2 SO4 or NaOH. The hexamminecobalt(III) catalyzed reduction was carried out under a stirring speed of 200 rpm. The liquid samples were withdrawn every 3 min to determine the changes of hexamminecobalt(II) concentration during the experiments. Continuous experiments were also carried out to investigate the hexamminecobalt(III) reduction in the regeneration reactor shown in Fig. 1. The hexamminecobalt(III) solution flew through the regeneration reactor (shown in Fig. 1) upward with various velocities. The outlet liquid samples were analyzed periodically to determine the hexamminecobalt(III) conversion. Experiments for the simultaneous removal of SO2 and NO were performed in a packed column (18 mm i.d., 1000 mm long) absorber. The schematic diagram of the experimental apparatus is shown in Fig. 1. The absorber temperature was controlled using a jacket through which water from a thermostatic bath was circulated. Two percent of NO in nitrogen was supplied from a cylinder, and was diluted with N2 to the desired concentration before feeding into the absorber. SO2 was supplied in a similar manner. Ten percent of ammonia aqueous solution, employed as the absorption (scrubbing) solution, together with a measured amount of CoAc2 were added into the 500 mL glass circulation tank. The absorber was operated with a continuous influent gas feeding at 0.2 L/min from the bottom and a continuous scrubbing solution feeding, at a superficial flow rate of 5 m3 /m2 h (25 mL/min) at the top. The absorbent effusing from the packed column was fed into the circulation tank. When the regeneration of hexamminecobalt(II) started, the absorbent in circulation tank flew into the regeneration reactor upwardly and directly into the packed column to scrub NO and SO2 . The regeneration reactor was a fixed-bed with 800 mm in length and 20 mm in diameter. The experimental runs were carried out under atmospheric pressure. 3.2. Analytical methods Cobalt(II) was determined spectrophotometrically at 25 ◦ C from the absorbance at 690 nm of a 9 mol/L solution prepared by diluting an aliquot of sample solution with concentrated HCl. This determination was made using a 5-cm cell. The cobalt(II) calibration curve was obtained
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X.-L. Long et al. / Applied Catalysis B: Environmental 54 (2004) 25–32
Fig. 1. Flowchart of experimental setup: (1) packed column; (2) pump; (3) circulation tanker; (4) regeneration reactor; (5) cylinder; (6) massmeter.
using standard cobalt acetate solutions ranged from 0.001 to 0.02 mol/L. Least-squares fits to the data yield Eq. (16) with a correlation coefficient (r2 ) 0.9999. An eight-sample relative standard deviation of cobalt(II) measurement was determined to be 0.9%. C = −0.0027 + 6.13366X
(16)
where X stands for absorbency and C for cobalt(II) concentration (mol/L). During the experiments, the quantitative analysis of gas compositions was achieved by an on-line Fourier transform infrared spectrometer (Nicolet E.S.P. 460 FT-IR) equipped with a gas cell and a quantitative package, named Quant Pad. The inlet and outlet gases were directly introduced into the gas cell of the FTIR, with pipes insulated through regulated electric coils to obtain the transient NO, NO2 , SO2 , and H2 O concentrations in both inlet and outlet gases, as well as the transient NO conversion. This set-up was conveniently operated to monitor the absorption effect of NO and SO2 . The compositions of the spent scrubbing solution were determined using a Dionex 500 ion chromatograph with a conductivity detector.
must be used to accelerate the process. It is possible to utilize activated carbon to catalyze the hexamminecobalt(III) reduction commercially. 4.2. Effect of granular activated carbon size on hexamminecobalt(III) reduction Fig. 3 shows the experimental results obtained in the fixed-bed reactor with different activated carbon particle sizes. The results express that the effect of activated carbon particle size on hexamminecobalt(III) reduction is negligible: at a flow rate of 10 mL/min, the hexamminecobalt(III) conversions are 97.82, 99.44, and 99.77% with 4–8 mesh, 8–20 mesh and 20–40 mesh activated carbon applied, respectively. Therefore, it can be concluded that the activated carbon size has an insignificant effect on hexamminecobalt(III) reduction in the fixed-bed reactor. The reason may be that the amount of activated carbon in the fixed-bed is so large that the influence of activated carbon size is reduced. Hence, large granular activated carbon pellet is suggested to catalyze
4. Results and discussion
3+
4.1. Effect of activated carbon on hexamminecobalt(III) reduction
Co(NH3)6 conversion(%)
100
Fig. 2 shows the experimental results obtained in the stirred cell at 80 ◦ C and pH 4.1. It can be observed that the activated carbon greatly speeds up the reduction of hexamminecobalt(III). After 15 min, the hexamminecobalt(III) conversion reaches up to 96.60% with 26.7 g/L activated carbon in the solution, while only 8.18% hexamminecobalt(III) is reduced without activated carbon existing in the solution. It can be concluded that the reduction of hexamminecobalt(III) proceeds so slowly that a catalyst
80 60 40 20 0 2
4
6
8
10
12
14
16
t(min) Fig. 2. Effect of activated carbon on reduction of hexamminecobalt(III) (pH 4.1, 80 ◦ C): (䉱) without activated carbon; () 26.7 g/L activated carbon.
X.-L. Long et al. / Applied Catalysis B: Environmental 54 (2004) 25–32
3+
[Co(NH3)6] conversion(%)
100 90 80 70 60 50 0
10
20
30
40
50
Time(min) Fig. 3. Effect of activated carbon size on hexamminecobalt(III) reduction in the fixed-bed (pH 4.5, 60 ◦ C, liquid flow rate = 10 mL/min): (䊏) 4–8 mesh; (䉬) 8–20 mesh; (䉱) 20–40 mesh.
the hexamminecobalt(III) reduction in practical application so that the resistance in the fixed-bed can be minimized. In terms of such a conclusion, 8–20 mesh activated carbon was filled in the fixed column during the next experimentation. 4.3. Effect of pH on hexamminecobalt(III) reduction Knowledge of the optimum pH is very important since pH affects not only the surface charge of the activated carbon, but also the degree of ionization and speciation of adsorbate during reaction. To examine the effect of pH on hexamminecobalt(III) reduction, experiments were carried out in the stirred cell at 80 ◦ C to investigate the effect of pH on hexamminecobalt(III)reduction. The hexamminecobalt(III) conversions for a reaction of 15 min are shown in Fig. 4. It can be found that hexamminecobalt(III) conversion remains at about 95.5% as the pH value of the solutions changes from 3.5 to 6.5. But the conversion decreases as pH is lower or higher.
29
The adsorption of metallic ions on activated carbons is greatly influenced by the pH of the solution, because the mean surface charge density of the carbon is determined by the pH. At some pH (the isoelectric point, pHPZC ) the net overall surface charge of the carbon particle is zero. Each carbon shows a positive charge in solutions with pH below its pHPZC and a negative charge in solutions with a pH above its pHPZC . Therefore, at pH > pHPZC the carbon surface, covered by deprotonated carboxyl groups, attracts cations from solution, while at pH < pHPZC it attracts anions [23,24]. Thus it can be concluded that the higher the pH, the greater negative charge density on the surface of activated carbon, the easier the cationic hexamminecobalt(III) ions’ absorption. It is very difficult for hexamminecobalt(III) ions to be adsorbed on the activated carbon surface when the pH of the solution is lower than its pHPZC . Hence, the reduction of hexamminecobalt(III) will be affected detrimentally while pH is lowered. Furthermore, the reductant of hexamminecobalt(III) is the water of the aqueous solution. It can also be concluded from the electrochemical half-cell reduction potentials of Eqs. (10) and (11) that water is more easily oxidized in high pH solutions. Therefore, the hexamminecobalt(III) conversion decreases as the pH becomes lower than 3.5. On the other hand, as previously discussed, the disintegration of the hexamminecobalt(III) is prerequisite to the regeneration of cobalt(II). In acidic solutions hexamminecobalt(III) is liable to disintegrate for the formation of NH4 + ions. According to the equilibrium between NH3 and NH4 + , NH4 + ions become fewer with increasing pH. Disintegration of hexamminecobalt(III) is therefore inhibited as the basicity of the solution becomes stronger. As a result, the hexamminecobalt(III) conversion decreases as the pH increases above 6.5. So, there is an optimal pH range for hexamminecobalt(III) reduction catalyzed by activated carbon in aqueous solutions. Fig. 5 depicts the experimental results obtained in the fixed-bed. The hexamminecobalt(III) conversions are as high as 99.77% at the pH range between 1.8 and 8.4. These results are different from those obtained from the stirred cell.
90 100
[Co(NH3)6] conversion(%)
80 70 60
3+
Co(NH3)6 conversion(%)
100
3+
50 40 30 1
2
3
4
5
6
7
8
9
10
pH Fig. 4. Effect of pH on hexamminecobalt(III) reduction in the stirred cell (activated carbon = 26.7 g/L, 80 ◦ C).
80 60 40 20 0 1
2
3
4
5
6
7
8
9
10
pH
Fig. 5. Effect of pH on hexamminecobalt(III) reduction in the fixed-bed (60 ◦ C, liquid velocity = 10 mL/min).
X.-L. Long et al. / Applied Catalysis B: Environmental 54 (2004) 25–32
4.4. Contact time on hexamminecobalt(III) reduction Experiments were performed in the fixed reactor to investigate the effect of contact time on hexamminecobalt(III) reduction at pH 5.5 and 60 ◦ C. The experimental results depicted in Fig. 6 show that hexamminecobalt(III) conversion increases with the contact time. The hexamminecobalt(III) conversion increases from 73.43 to 99.77% when the contact time increases from 5 to 12.5 min. 4.5. Effect of temperature on hexamminecobalt(III) reduction Experiments were carried out in the fixed-bed reactor with a liquid flow rate of 14 mL/min and pH 5.6 to investigate the effect of temperature on hexamminecobalt(III) reduction. A conclusion can be drawn from the experimental results shown in Fig. 7 that the hexamminecobalt(III) conversion increases with temperature. The NO removal efficiency increases from 74.08 to 99.72% when the temperature rises from 50 to 80 ◦ C. The explanation is given as follows. First, the diffusivity of solute through the external
100
80
70 50
60
70
80
o
Temperature( C)
Fig. 7. Effect of temperature on hexamminecobalt(III) reduction in a fixed-bed (pH 5.6, liquid flow rate = 14 mL/min).
laminar layer into the micropores of the activated carbon increases with temperature. Second, high temperature is liable to make hexamminecobalt(III) disintegrate easily, and conducible to produce more cobalt(III) ions. Third, dynamically, the reaction rate increases with temperature. The last point is, oxygen solubility decreases with temperature, causing the oxygen produced by reaction (Eq. (12)) stripe much more quickly from the activated carbon. All these factors are beneficial to the hexamminecobalt(III) reduction. The authors have found that the reaction is pseudo-firstorder with respect to hexamminecobalt(III). The global rate expression for hexamminecobalt(III) reduction can be written as E −rCo(NH3 )6 3+ = k0 exp − (17) CCo(NH3 )6 3+ RT where rCo(NH3 )6 3+ is the global rate of hexamminecobalt(III) reduction, k0 the frequency factor, E the apparent activation energy of the reaction, and CCo(NH3 )6 3+ the concentration of hexamminecobalt(III). Fig. 8 shows an Arrhenius-type plot of ln k versus 1/T, providing an estimation of the reaction activation energy. The apparent activation energy E has a value of 45.53 ± 4.51 kJ/mol.
100
2.0 1.8 1.6 1.4
90
-lnk
1.2
3+
[Co(NH3)6] conversion(%)
90
3+
It can be concluded that the influence of pH on hexamminecobalt(III) reduction in the fixed-bed is slighter than that in the stirred cell. The reason may be that the amount of activated carbon packed in the fixed column is so large that the basic and acidic groups on the activated carbons counterbalance the influence of the pH. The results shown in Fig. 5 demonstrate that the hexamminecobalt(III) reduction can be finished in basic solutions which is the suitable condition for oxidation of nitric oxide in the aqueous solution. Therefore, it is possible to integrate the removal of NO and SO2 with the regeneration of [Co(NH3 )6 ]2+ into one system to realize a continual operation propitiously. It may also be noticed that the hexamminecobalt(III)conversion decreases dramatically to 22.70% as the pH increases to 9.60. The reason may be that almost no NH4 + ions formed under such a high pH condition.
[Co(NH3)6] conversion(%)
30
1.0 0.8
80
0.6 0.4
6
8
10
12
contact time (min)
0.2 2.80
2.85
2.90
2.95
3.00 -1
1000/T(K ) Fig. 6. Contact time on hexamminecobalt(III) reduction in the fixed-bed reactor (pH 5.5, 60 ◦ C).
Fig. 8. Plot of −ln k vs. 1000/T.
3.05
3.10
X.-L. Long et al. / Applied Catalysis B: Environmental 54 (2004) 25–32 100 90
80 ˚C
90˚ C
NO Removal (%)
80 70 60 50 40 30 20
Start regeneration
10 0 0
10
20
30
40
50
60
70
80
90
100
Time (hour)
Fig. 9. NO removal efficiency vs. time, showing the effect of regeneration on removal efficiency (50 ◦ C, NO = 748 ppm, SO2 = 1812 ppm, O2 = 5.2%).
4.6. Simultaneous removal of NO and SO2 Fig. 9 illustrates the variation of NO removal efficiency with time coupled with the regeneration of cobalt(II) catalyzed by activated carbon. For the first 11 h the operation is conducted without the regeneration of the solution, and then the regeneration is started which continues during the rest of the run. The experiment is carried out at 50 ◦ C with 5.2% oxygen, an initial hexamminecobalt(II) concentration of 0.02 mol/L, and inlet NO and SO2 concentrations of 748 and 1812 ppm, respectively. The flow rate of scrubbing solution fed into the regeneration column was 25 mL/min, and the initial regeneration temperature was 80 ◦ C. The system has an initial NO removal efficiency of 100%, which declines to about 29% after 11 h. After activating the regeneration system, the NO removal efficiency increases and stabilizes at a level between 70 and 81%. After 82 h, the regeneration temperature increases to 90 ◦ C, and the NO removal efficiency increases to 82%. The removal efficiency depends on the operating conditions employed in the regeneration system. This test shows that the regeneration method works effectively. The authors believe that further improvement of the NO removal efficiency can achieve by increasing the regeneration time and temperature, and modifying the activated carbon. During the whole operation, no SO2 is detected in the outlet gas stream by FTIR. Ion chromatographic analyses of the spent scrubbing solution show that the NO absorbed is converted to nitrate (NO3 − ). The nitrite produced has been completely oxidized to nitrate. Most of the sulfite produced by SO2 dissolving in the ammonia solution is oxidized to sulfate with activated carbon as the catalyst. Such a system also realizes the absorption and oxidation of SO2 at the same time. The ammonium nitrate and ammonium sulfate can be manufactured as byproducts. 5. Conclusions A novel process is put forward to scrub NO and SO2 from flue gas streams by adding soluble cobalt salt into the aque-
31
ous ammonia solution. NO is absorbed and oxidized simultaneously by the aqueous ammonia solution. Activated carbon is utilized to catalyze reduction of hexamminecobalt(III) to hexamminecobalt(II) by H2 O in order to maintain the capability of absorbing NO with such an absorbent. This process could be used to retrofit the existing ammonia desulfurization scrubbers for simultaneous removal of SO2 and NO. SO2 and NO are converted into the useful sulfate and nitrate, respectively. The experiments demonstrate that activated carbon significantly speeds up reduction of hexamminecobalt(III). The activated carbon size has insignificant effect on hexamminecobalt(III) reduction. The conversion of hexamminecobalt(III) increases with temperature. The reduction of hexamminecobalt(III) is also affected by the pH of the aqueous solution. The regeneration of hexamminecobalt(III) carries on efficiently in a fixed-bed at the pH range between 1.8 and 8.4. The experimental results also depict that NO removal efficiency can last long at a high level after the regeneration of hexamminecobalt(II) starts. Though the study on hexamminecobalt(III) reduction has been performed systematically and some useful results have been obtained, it is imperative to make further studies into the process.
Acknowledgements This work was supported by the NSFC (No. 29633030), the Ministry of Science and Technology of China (No. 2001CB 711203), and the Development Project of Shanghai Priority Academic Discipline.
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[17] M.T. Suidan, V.L. Snoeyink, R.A. Schmitz, J. Environmental Engineering Division 1977. 103 (EE4), 677-691, ASCE Proceedings Paper 13138. [18] M. Pérez-Candela, J.M. Mart´ın-Mart´ınez, R. Torregrosa-Maciá, Water Res. 29 (1995) 2174. [19] M.Jo Kirisits, V.L. Snoeyink, J.C. Kruithof, Water Res. 34 (2000) 4250. [20] H. Teng, Y.T. Tu, Y.C. Lai, C.-C. Lin, Carbon 39 (2001) 573.
[21] M.A. Montes-Morán, J. Angel Menéndez, E. Fuente, D. Suárez, J. Phys. Chem. B 102 (1998) 5595. [22] L.R. Radovic, C. Moreno-Castilla, J. Rivera-Utrilla, Chem. Phys. Carbon 27 (2001) 227. [23] F. Rodridguez-Reinoso, Carbon 36 (1998) 159. [24] C.A. Leon y Leon, L.R. Radovic, in: P.A. Thrower, (Ed.), Chemistry and Physics of Carbon, vol. 24, Marcel Dekker, New York, 1994, p. 213.
Simultaneous removal of NO and SO2 with hexamminecobalt(II) solution coupled with the hexamminecobalt(II) regeneration catalyzed by activated carbon Xiang-Li Long∗ , Zhi-Ling Xin, Hong-Xin Wang, Wen-De Xiao, Wei-Kang Yuan UNILAB, State Key Laboratory of Chemical Reaction Engineering, East China University of Science and Technology, Shanghai 200237, PR China Received 31 December 2003; received in revised form 25 April 2004; accepted 31 May 2004 Available online 22 July 2004
Abstract The wet ammonia desulfurization process can be retrofitted for combined removal of SO2 and NO from the flue gases by adding soluble cobalt(II) salt into the aqueous ammonia solution. Activated carbon is used to catalyze the reduction of hexamminecobalt(III) to hexamminecobalt(II) to maintain the capability of removing NO of the hexamminecobalt solution. The effects of temperature, pH, activated carbon particle size, and superficial liquid flow velocity on hexamminecobalt(III) conversion have been investigated. An apparent activation energy is obtained. According to the experimental results, the catalytic reduction reaction rate increases with temperature. The batch reactor experiments show that the best pH range lies in between 3.5 and 6.5. In a fixed-bed reactor, superficial liquid flow velocity obviously affects the reaction and a high yield of cobalt(II) is obtained at a pH value lower than 9.0. The experiments manifest that the hexamminecobalt solution coupled with catalytic regeneration of hexamminecobalt(II) can maintain a high nitric oxide removal efficiency during a period of time. © 2004 Elsevier B.V. All rights reserved. Keywords: Nitric oxide; Catalytic reduction; Activated carbon; Hexamminecobalt; Absorption
1. Introduction Power plant flue gases frequently contain NOx and SO2 that cause acid rain and urban smog. Removal of these contaminants to comply with the environmental emission standards is necessary and various wet and dry processes have been put forward. Among the existing treatment processes for removing NOx from the flue gases, selective catalytic reduction (SCR) using NH3 at 300–500 ◦ C is supposed to be the best available NOx control technology. However, application of the SCR is limited because of its high capital and operating costs. There is still an urgent need for a more economical method for controlling NOx emission. It has been agreed that a flue gas desulfurization (FGD) process that promotes NO removal merely by using chemical additives to an aqueous scrubbing solution may have ∗ Corresponding author. Tel.: +86 21 64253267; fax: +86 21 64252814. E-mail address: [email protected] (X.-L. Long).
0926-3373/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2004.05.020
a significant impact on the control strategies. However, the treatment cost using a strong oxidant, such as OCl2 − [1,2] and H2 O2 [3–6], has been too high to make it a practical NOx control process. Other approaches treating NO in aqueous solution scrubbers include addition of heavy metal chelators to sequester nitric oxide for the subsequent removal [7–10], and even addition of yellow phosphorous emulsions and O2 to oxidize nitric oxide to form a combination of nitrite and nitrate salts [11,12], have not yet been commercialized. The process using an ammonia scrubber to recover sulfur dioxide from the flue gas has been developed and put into commercial application. It is of great significance to capitalize on the ammonia process to abate nitric oxide in the flue gas. The authors put forward a novel technique for simultaneous removal of NO and SO2 from the flue gas by adding soluble cobalt(II) salt into the aqueous ammonia solution. The hexamminecobalt(II) formed by cobalt(II) binding with ammonia can not only coordinate nitric oxide but also activate oxygen molecules in the aqueous ammonia solution. The oxidant is the oxygen coexisting in the flue gas. Therefore,
26
X.-L. Long et al. / Applied Catalysis B: Environmental 54 (2004) 25–32
NO can be absorbed and oxidized simultaneously in the hexamminecobalt(II) solution. In order to regenerate the active constituent hexamminecobalt(II) to maintain the capability of removing NO from the gas streams, water is used to reduce cobalt(III) to cobalt(II) with activated carbon as the catalyst. The proposed process can be easily integrated into the existing wet ammonia scrubbers for the simultaneous removal of SO2 and NO.
hexamminecobalt solution will reduce as the reaction proceeds. The hexamminecobalt(III) ion has to be reduced to hexamminecobalt(II) ion once again in order to maintain the removing NO capability. Electrochemical half-cell reduction potential of Co(NH3 )6 3+ /Co(NH3 )6 2+ (Eq. (6)) shows that hexamminecobalt(III) ions cannot be easily reduced to hexamminecobalt(II). A catalyst needs to be chosen to catalyze the reduction of hexamminecobalt(III).
2. Theoretical
[Co(NH3 )6 ]3+ + e− ↔ [Co(NH3 )6 ]2+ ,
It is known that NO is insoluble in water. However, nitric oxide may react with hexamminecobalt(II) to form a nitrosyl complex as follow: NO(g) ↔ NO(aq)
(1)
Co(NH3 )6 2+ (aq) + NO(aq) → [Co(NH3 )5 NO]2+ (aq) + NH3
(2)
Reaction (2) enhances the nitric oxide solubility in the aqueous solution. The hexamminecobalt(II) ion, [Co(NH3 )6 ]2+ , is oxygenated in the aqueous ammonia, the dioxygen displacing one of the ammonia ligands. A brown binuclear complex with bridging dioxygen is then formed [13,14]: 2Co(NH3 )6 2+ + O2 ↔ [(NH3 )5 Co–O–O–Co(NH3 )5 ]4+ +2NH3
(3)
The bridging O–O group is considered a peroxide ion with an O–O distance of 1.47 Å, and a torsion angle of 146◦ about the Co–O–O bond. Hence it is assumed that the oxidizability of [(NH3 )5 Co–O–O–Co(NH3 )5 ]4+ is similar to that of H2 O2 . Such a process is actually activation of the oxygen molecule which makes some processes not possible with free gaseous dioxygen carry out. Therefore, nitric oxide may be oxidized to soluble NO2 in the aqueous solution by [(NH3 )5 Co–O–O–Co(NH3 )5 ]4+ : [(NH3 )5 Co–O–O–Co(NH3 )5 ]4+ + H2 O + 2NH3 +[Co(NH3 )5 NO]2+ → 2Co(NH3 )6 3+ + 2OH− +[Co(NH3 )5 NO2 ]2+
(4)
The following reaction will take place as the produced nitric dioxide is dissolved in the aqueous ammonia:
ECo(NH3 )6 3+ /Co(NH3 )6 2+ = 0.1 V
(6)
It is well known that activated carbon has been successfully applied in industry as a catalyst due to its enormous surface area, porous structure and characteristic flexibility. Since activated carbon has been proven capable of transferring electrons for reduction reactions, it has been reported that a number of chemical species can be reduced in the presence of granular activated carbon. For example, Razvigorova et al. [15] found that nitrate was reduced to nitrite after contacting with an apricot stone-based carbon. Voudrias et al. [16] presented that chlorite was reduced to chloride by granular activated carbon. In addition, Suidan et al. [17] modeled the reduction of free chlorine to chloride in the presence of granular activated carbon. Pérez-Candela et al. [18] found that chromium(VI) could be reduced to chromium(III) by the carbon made from olive stone, and Mary et al. [19] investigated the reduction of bromate to bromide. Carbon atoms at edge of the graphene layers of the activated carbon are reactive and form surface functional groups by bonding with heteroatoms such as oxygen, etc. Surface functional groups are mainly divided into acidic groups and basic groups. Typical acidic groups include surface oxides such as carbonyl, carboxyl, phenolic hydroxyl, lactone and quinone. Usually the structures corresponding to chromeneand pyrone-like ones belong to the basic groups. Because of the peculiar characteristics of activated carbon, coexisting of both acidic groups and basic groups on its surface makes the activated carbon suitable to catalyze hexamminecobalt(III) reduction. It is reported [20] that NH3 molecules may react with the acidic part of carbonyl groups and phenolic hydroxyl groups to form CO− (NH4 )+ complexes on the carbon surface. Formation of these complexes may accelerate disintegration of Co(NH3 )6 3+ ions to cobalt(III) (Eq. (7)): AC
2[Co(NH3 )5 NO2 ]
2+
6AC-OH + Co(NH3 )6 3+ ↔Co3+ + 6AC-CO− (NH4 )+
+ H2 O + 4NH3 → NH4 NO2
+NH4 NO3 + 2Co(NH3 )6 2+
(5)
Hexamminecobalt(II) can be oxidized to hexamminecobalt(III) as the nitric oxide is oxidized. The hexamminecobalt(III) ion, [Co(NH3 )6 ]3+ , cannot be oxygenated in the aqueous ammonia to form dioxygen complex. In other words, the hexamminecobalt(III) ion is unable to activate oxygen molecules. The oxidizing ability of the
(7) Electrochemical half-cell reduction potential of Co3+ /Co2+ (Eq. (8)) shows that cobalt(III) ions are strong oxidants and can be reduced to cobalt(II). 2Co3+ + 2e− ↔ 2Co2+ ,
ECo3+ /Co2+ = 1.82 V
(8)
On the other hand, the basicity of an activated carbon is mainly due to the presence of basic oxygen-containing
X.-L. Long et al. / Applied Catalysis B: Environmental 54 (2004) 25–32
functional groups (e.g. pyrones or chromenes) and/or graphene layers acting as Lewis bases and forming electron donor–acceptor (EDA) complexes with H2 O molecules. Montes-Morán et al. [21] studied the causes of the basic nature of carbonaceous materials and concluded that an unconventional bond might be established between the H3 O+ ions and the cloud of electrons of the aromatic rings of the activated carbon. These basic sites are located at the electron-rich regions within the basal planes of carbon crystallites away from the crystallite edges [22]. This delocalized electron system can act as a Lewis base in the aqueous solution [22] (Eq. (9)): −C + 2H2 O ↔ C − H3 O+ + OH−
(9)
This delocalized electron system (−C) can act as a cobalt(III) reduction center. It should also be noticed that the electrochemical half-cell reduction potentials of O2 /OH− (Eq. (10)) and O2 /H2 O (Eq. (11)) show that hydroxyl ions are more liable to be oxidized to O2 than H2 O molecules by cobalt(III) ions (Eq. (12)). In other words, activated carbon may catalyze the oxidation of water. − 1 2 O2 (g) + H2 O + 2e
↔ 2OH− (aq),
EO2 /OH− = 0.401 (10)
O2 (g) + 4H+ (aq) + 4e− ↔ 2H2 O,
EO2 /H2 O = 1.229 (11)
2Co3+ + 2OH− → 2Co2+ + H2 O + 21 O2
(12)
The global reaction for the regeneration of cobalt(II) ions can be written as follows: AC
2[Co(NH3 )6 ]3+ + H2 O + 10H+ ↔12NH4 + + 2Co2+ + 21 O2
(13)
Cobalt(II) ions may combine with ammonia in the solution to form hexamminecobalt(II) ions repeatedly (Eq. (14)). Thus, the regeneration of hexamminecobalt(II) ions is completed. As a result, the NO removal efficiency can be maintained and lasts long at a high level. Co2+ + 6NH3 → Co(NH3 )6 2+
(14)
In the process discussed above, oxidation and absorption of nitric oxide are performed simultaneously. The oxidant is the oxygen coexisting in the flue gas. The hexamminecobalt(II) ions can be used repeatedly catalyzed by the activated carbon. The reductant of hexamminecobalt(III) is H2 O in the solution. Nitric oxide is converted into nitrite and nitrate (Eq. (15)). 2NO + O2 + 2OH− → NO2 − + NO3 − + H2 O
(15)
27
3. Experimental 3.1. Methods Batch experiments were performed in a stirred glass flask of 500 mL to investigate the hexamminecobalt(III) reduction catalyzed by the activated carbon. 150 mL hexamminecobalt(III) solution and appropriate dose of activated carbon were introduced into the glass flask. The initial hexamminecobalt(III) concentration was 0.01 mol/L for most of the test runs. A commercial coconut activated carbon was used. The pH values of the solutions were adjusted before addition of activated carbon by adding appropriate amounts of H2 SO4 or NaOH. The hexamminecobalt(III) catalyzed reduction was carried out under a stirring speed of 200 rpm. The liquid samples were withdrawn every 3 min to determine the changes of hexamminecobalt(II) concentration during the experiments. Continuous experiments were also carried out to investigate the hexamminecobalt(III) reduction in the regeneration reactor shown in Fig. 1. The hexamminecobalt(III) solution flew through the regeneration reactor (shown in Fig. 1) upward with various velocities. The outlet liquid samples were analyzed periodically to determine the hexamminecobalt(III) conversion. Experiments for the simultaneous removal of SO2 and NO were performed in a packed column (18 mm i.d., 1000 mm long) absorber. The schematic diagram of the experimental apparatus is shown in Fig. 1. The absorber temperature was controlled using a jacket through which water from a thermostatic bath was circulated. Two percent of NO in nitrogen was supplied from a cylinder, and was diluted with N2 to the desired concentration before feeding into the absorber. SO2 was supplied in a similar manner. Ten percent of ammonia aqueous solution, employed as the absorption (scrubbing) solution, together with a measured amount of CoAc2 were added into the 500 mL glass circulation tank. The absorber was operated with a continuous influent gas feeding at 0.2 L/min from the bottom and a continuous scrubbing solution feeding, at a superficial flow rate of 5 m3 /m2 h (25 mL/min) at the top. The absorbent effusing from the packed column was fed into the circulation tank. When the regeneration of hexamminecobalt(II) started, the absorbent in circulation tank flew into the regeneration reactor upwardly and directly into the packed column to scrub NO and SO2 . The regeneration reactor was a fixed-bed with 800 mm in length and 20 mm in diameter. The experimental runs were carried out under atmospheric pressure. 3.2. Analytical methods Cobalt(II) was determined spectrophotometrically at 25 ◦ C from the absorbance at 690 nm of a 9 mol/L solution prepared by diluting an aliquot of sample solution with concentrated HCl. This determination was made using a 5-cm cell. The cobalt(II) calibration curve was obtained
28
X.-L. Long et al. / Applied Catalysis B: Environmental 54 (2004) 25–32
Fig. 1. Flowchart of experimental setup: (1) packed column; (2) pump; (3) circulation tanker; (4) regeneration reactor; (5) cylinder; (6) massmeter.
using standard cobalt acetate solutions ranged from 0.001 to 0.02 mol/L. Least-squares fits to the data yield Eq. (16) with a correlation coefficient (r2 ) 0.9999. An eight-sample relative standard deviation of cobalt(II) measurement was determined to be 0.9%. C = −0.0027 + 6.13366X
(16)
where X stands for absorbency and C for cobalt(II) concentration (mol/L). During the experiments, the quantitative analysis of gas compositions was achieved by an on-line Fourier transform infrared spectrometer (Nicolet E.S.P. 460 FT-IR) equipped with a gas cell and a quantitative package, named Quant Pad. The inlet and outlet gases were directly introduced into the gas cell of the FTIR, with pipes insulated through regulated electric coils to obtain the transient NO, NO2 , SO2 , and H2 O concentrations in both inlet and outlet gases, as well as the transient NO conversion. This set-up was conveniently operated to monitor the absorption effect of NO and SO2 . The compositions of the spent scrubbing solution were determined using a Dionex 500 ion chromatograph with a conductivity detector.
must be used to accelerate the process. It is possible to utilize activated carbon to catalyze the hexamminecobalt(III) reduction commercially. 4.2. Effect of granular activated carbon size on hexamminecobalt(III) reduction Fig. 3 shows the experimental results obtained in the fixed-bed reactor with different activated carbon particle sizes. The results express that the effect of activated carbon particle size on hexamminecobalt(III) reduction is negligible: at a flow rate of 10 mL/min, the hexamminecobalt(III) conversions are 97.82, 99.44, and 99.77% with 4–8 mesh, 8–20 mesh and 20–40 mesh activated carbon applied, respectively. Therefore, it can be concluded that the activated carbon size has an insignificant effect on hexamminecobalt(III) reduction in the fixed-bed reactor. The reason may be that the amount of activated carbon in the fixed-bed is so large that the influence of activated carbon size is reduced. Hence, large granular activated carbon pellet is suggested to catalyze
4. Results and discussion
3+
4.1. Effect of activated carbon on hexamminecobalt(III) reduction
Co(NH3)6 conversion(%)
100
Fig. 2 shows the experimental results obtained in the stirred cell at 80 ◦ C and pH 4.1. It can be observed that the activated carbon greatly speeds up the reduction of hexamminecobalt(III). After 15 min, the hexamminecobalt(III) conversion reaches up to 96.60% with 26.7 g/L activated carbon in the solution, while only 8.18% hexamminecobalt(III) is reduced without activated carbon existing in the solution. It can be concluded that the reduction of hexamminecobalt(III) proceeds so slowly that a catalyst
80 60 40 20 0 2
4
6
8
10
12
14
16
t(min) Fig. 2. Effect of activated carbon on reduction of hexamminecobalt(III) (pH 4.1, 80 ◦ C): (䉱) without activated carbon; () 26.7 g/L activated carbon.
X.-L. Long et al. / Applied Catalysis B: Environmental 54 (2004) 25–32
3+
[Co(NH3)6] conversion(%)
100 90 80 70 60 50 0
10
20
30
40
50
Time(min) Fig. 3. Effect of activated carbon size on hexamminecobalt(III) reduction in the fixed-bed (pH 4.5, 60 ◦ C, liquid flow rate = 10 mL/min): (䊏) 4–8 mesh; (䉬) 8–20 mesh; (䉱) 20–40 mesh.
the hexamminecobalt(III) reduction in practical application so that the resistance in the fixed-bed can be minimized. In terms of such a conclusion, 8–20 mesh activated carbon was filled in the fixed column during the next experimentation. 4.3. Effect of pH on hexamminecobalt(III) reduction Knowledge of the optimum pH is very important since pH affects not only the surface charge of the activated carbon, but also the degree of ionization and speciation of adsorbate during reaction. To examine the effect of pH on hexamminecobalt(III) reduction, experiments were carried out in the stirred cell at 80 ◦ C to investigate the effect of pH on hexamminecobalt(III)reduction. The hexamminecobalt(III) conversions for a reaction of 15 min are shown in Fig. 4. It can be found that hexamminecobalt(III) conversion remains at about 95.5% as the pH value of the solutions changes from 3.5 to 6.5. But the conversion decreases as pH is lower or higher.
29
The adsorption of metallic ions on activated carbons is greatly influenced by the pH of the solution, because the mean surface charge density of the carbon is determined by the pH. At some pH (the isoelectric point, pHPZC ) the net overall surface charge of the carbon particle is zero. Each carbon shows a positive charge in solutions with pH below its pHPZC and a negative charge in solutions with a pH above its pHPZC . Therefore, at pH > pHPZC the carbon surface, covered by deprotonated carboxyl groups, attracts cations from solution, while at pH < pHPZC it attracts anions [23,24]. Thus it can be concluded that the higher the pH, the greater negative charge density on the surface of activated carbon, the easier the cationic hexamminecobalt(III) ions’ absorption. It is very difficult for hexamminecobalt(III) ions to be adsorbed on the activated carbon surface when the pH of the solution is lower than its pHPZC . Hence, the reduction of hexamminecobalt(III) will be affected detrimentally while pH is lowered. Furthermore, the reductant of hexamminecobalt(III) is the water of the aqueous solution. It can also be concluded from the electrochemical half-cell reduction potentials of Eqs. (10) and (11) that water is more easily oxidized in high pH solutions. Therefore, the hexamminecobalt(III) conversion decreases as the pH becomes lower than 3.5. On the other hand, as previously discussed, the disintegration of the hexamminecobalt(III) is prerequisite to the regeneration of cobalt(II). In acidic solutions hexamminecobalt(III) is liable to disintegrate for the formation of NH4 + ions. According to the equilibrium between NH3 and NH4 + , NH4 + ions become fewer with increasing pH. Disintegration of hexamminecobalt(III) is therefore inhibited as the basicity of the solution becomes stronger. As a result, the hexamminecobalt(III) conversion decreases as the pH increases above 6.5. So, there is an optimal pH range for hexamminecobalt(III) reduction catalyzed by activated carbon in aqueous solutions. Fig. 5 depicts the experimental results obtained in the fixed-bed. The hexamminecobalt(III) conversions are as high as 99.77% at the pH range between 1.8 and 8.4. These results are different from those obtained from the stirred cell.
90 100
[Co(NH3)6] conversion(%)
80 70 60
3+
Co(NH3)6 conversion(%)
100
3+
50 40 30 1
2
3
4
5
6
7
8
9
10
pH Fig. 4. Effect of pH on hexamminecobalt(III) reduction in the stirred cell (activated carbon = 26.7 g/L, 80 ◦ C).
80 60 40 20 0 1
2
3
4
5
6
7
8
9
10
pH
Fig. 5. Effect of pH on hexamminecobalt(III) reduction in the fixed-bed (60 ◦ C, liquid velocity = 10 mL/min).
X.-L. Long et al. / Applied Catalysis B: Environmental 54 (2004) 25–32
4.4. Contact time on hexamminecobalt(III) reduction Experiments were performed in the fixed reactor to investigate the effect of contact time on hexamminecobalt(III) reduction at pH 5.5 and 60 ◦ C. The experimental results depicted in Fig. 6 show that hexamminecobalt(III) conversion increases with the contact time. The hexamminecobalt(III) conversion increases from 73.43 to 99.77% when the contact time increases from 5 to 12.5 min. 4.5. Effect of temperature on hexamminecobalt(III) reduction Experiments were carried out in the fixed-bed reactor with a liquid flow rate of 14 mL/min and pH 5.6 to investigate the effect of temperature on hexamminecobalt(III) reduction. A conclusion can be drawn from the experimental results shown in Fig. 7 that the hexamminecobalt(III) conversion increases with temperature. The NO removal efficiency increases from 74.08 to 99.72% when the temperature rises from 50 to 80 ◦ C. The explanation is given as follows. First, the diffusivity of solute through the external
100
80
70 50
60
70
80
o
Temperature( C)
Fig. 7. Effect of temperature on hexamminecobalt(III) reduction in a fixed-bed (pH 5.6, liquid flow rate = 14 mL/min).
laminar layer into the micropores of the activated carbon increases with temperature. Second, high temperature is liable to make hexamminecobalt(III) disintegrate easily, and conducible to produce more cobalt(III) ions. Third, dynamically, the reaction rate increases with temperature. The last point is, oxygen solubility decreases with temperature, causing the oxygen produced by reaction (Eq. (12)) stripe much more quickly from the activated carbon. All these factors are beneficial to the hexamminecobalt(III) reduction. The authors have found that the reaction is pseudo-firstorder with respect to hexamminecobalt(III). The global rate expression for hexamminecobalt(III) reduction can be written as E −rCo(NH3 )6 3+ = k0 exp − (17) CCo(NH3 )6 3+ RT where rCo(NH3 )6 3+ is the global rate of hexamminecobalt(III) reduction, k0 the frequency factor, E the apparent activation energy of the reaction, and CCo(NH3 )6 3+ the concentration of hexamminecobalt(III). Fig. 8 shows an Arrhenius-type plot of ln k versus 1/T, providing an estimation of the reaction activation energy. The apparent activation energy E has a value of 45.53 ± 4.51 kJ/mol.
100
2.0 1.8 1.6 1.4
90
-lnk
1.2
3+
[Co(NH3)6] conversion(%)
90
3+
It can be concluded that the influence of pH on hexamminecobalt(III) reduction in the fixed-bed is slighter than that in the stirred cell. The reason may be that the amount of activated carbon packed in the fixed column is so large that the basic and acidic groups on the activated carbons counterbalance the influence of the pH. The results shown in Fig. 5 demonstrate that the hexamminecobalt(III) reduction can be finished in basic solutions which is the suitable condition for oxidation of nitric oxide in the aqueous solution. Therefore, it is possible to integrate the removal of NO and SO2 with the regeneration of [Co(NH3 )6 ]2+ into one system to realize a continual operation propitiously. It may also be noticed that the hexamminecobalt(III)conversion decreases dramatically to 22.70% as the pH increases to 9.60. The reason may be that almost no NH4 + ions formed under such a high pH condition.
[Co(NH3)6] conversion(%)
30
1.0 0.8
80
0.6 0.4
6
8
10
12
contact time (min)
0.2 2.80
2.85
2.90
2.95
3.00 -1
1000/T(K ) Fig. 6. Contact time on hexamminecobalt(III) reduction in the fixed-bed reactor (pH 5.5, 60 ◦ C).
Fig. 8. Plot of −ln k vs. 1000/T.
3.05
3.10
X.-L. Long et al. / Applied Catalysis B: Environmental 54 (2004) 25–32 100 90
80 ˚C
90˚ C
NO Removal (%)
80 70 60 50 40 30 20
Start regeneration
10 0 0
10
20
30
40
50
60
70
80
90
100
Time (hour)
Fig. 9. NO removal efficiency vs. time, showing the effect of regeneration on removal efficiency (50 ◦ C, NO = 748 ppm, SO2 = 1812 ppm, O2 = 5.2%).
4.6. Simultaneous removal of NO and SO2 Fig. 9 illustrates the variation of NO removal efficiency with time coupled with the regeneration of cobalt(II) catalyzed by activated carbon. For the first 11 h the operation is conducted without the regeneration of the solution, and then the regeneration is started which continues during the rest of the run. The experiment is carried out at 50 ◦ C with 5.2% oxygen, an initial hexamminecobalt(II) concentration of 0.02 mol/L, and inlet NO and SO2 concentrations of 748 and 1812 ppm, respectively. The flow rate of scrubbing solution fed into the regeneration column was 25 mL/min, and the initial regeneration temperature was 80 ◦ C. The system has an initial NO removal efficiency of 100%, which declines to about 29% after 11 h. After activating the regeneration system, the NO removal efficiency increases and stabilizes at a level between 70 and 81%. After 82 h, the regeneration temperature increases to 90 ◦ C, and the NO removal efficiency increases to 82%. The removal efficiency depends on the operating conditions employed in the regeneration system. This test shows that the regeneration method works effectively. The authors believe that further improvement of the NO removal efficiency can achieve by increasing the regeneration time and temperature, and modifying the activated carbon. During the whole operation, no SO2 is detected in the outlet gas stream by FTIR. Ion chromatographic analyses of the spent scrubbing solution show that the NO absorbed is converted to nitrate (NO3 − ). The nitrite produced has been completely oxidized to nitrate. Most of the sulfite produced by SO2 dissolving in the ammonia solution is oxidized to sulfate with activated carbon as the catalyst. Such a system also realizes the absorption and oxidation of SO2 at the same time. The ammonium nitrate and ammonium sulfate can be manufactured as byproducts. 5. Conclusions A novel process is put forward to scrub NO and SO2 from flue gas streams by adding soluble cobalt salt into the aque-
31
ous ammonia solution. NO is absorbed and oxidized simultaneously by the aqueous ammonia solution. Activated carbon is utilized to catalyze reduction of hexamminecobalt(III) to hexamminecobalt(II) by H2 O in order to maintain the capability of absorbing NO with such an absorbent. This process could be used to retrofit the existing ammonia desulfurization scrubbers for simultaneous removal of SO2 and NO. SO2 and NO are converted into the useful sulfate and nitrate, respectively. The experiments demonstrate that activated carbon significantly speeds up reduction of hexamminecobalt(III). The activated carbon size has insignificant effect on hexamminecobalt(III) reduction. The conversion of hexamminecobalt(III) increases with temperature. The reduction of hexamminecobalt(III) is also affected by the pH of the aqueous solution. The regeneration of hexamminecobalt(III) carries on efficiently in a fixed-bed at the pH range between 1.8 and 8.4. The experimental results also depict that NO removal efficiency can last long at a high level after the regeneration of hexamminecobalt(II) starts. Though the study on hexamminecobalt(III) reduction has been performed systematically and some useful results have been obtained, it is imperative to make further studies into the process.
Acknowledgements This work was supported by the NSFC (No. 29633030), the Ministry of Science and Technology of China (No. 2001CB 711203), and the Development Project of Shanghai Priority Academic Discipline.
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