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Nitric oxide absorption into cobalt ethylenediamine solution
Separation and Purification Technology 55 (2007) 226–231
Nitric oxide absorption into cobalt ethylenediamine solution Xiang-li Long ∗ , Zhi-ling Xin, Mao-bing Chen, Wen-de Xiao, Wei-kang Yuan ∗ State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, People’s Republic of China Received 2 November 2006; received in revised form 19 December 2006; accepted 20 December 2006
Abstract NO pollutant causes acid rain and urban smog. The removal of NO from exhausted gas streams is necessary to meet stringent effluent discharge limits. NO can be removed from exhausted gas streams by putting soluble cobalt salts and ethylenediamine (H2 NCH2 CH2 NH2 ) into basic solutions. The Co(en)3 3+ (en stands for ethylenediamine) ion produced by ethylenediamine binding cobalt acts as a homogeneous catalyst to oxidize NO into soluble NO2 and realize the oxidation and absorption of nitric oxide in the same reactor. The dissolved oxygen in equilibrium with the residual oxygen in the exhausted gas stream acts as the oxidant. The experiments manifest that this process is superior to the methods using Fe(II)–ethylenediaminetetraacetate (EDTA) solution and H2 O2 solution as absorbents in removing NO from the exhausted gas stream. NO removal efficiency decreases with the increase of the gas flow rate. NO removal efficiency increases with the Co(en)3 3+ concentration. pH of the solution affects the NO removal efficiencies obviously. Under anaerobic condition, the NO removal efficiency decreases with pH when pH is over 7.73. Under aerobic condition, there is an optimal pH for NO absorption into the Co(en)3 3+ concentration. More than 90% of NO in the feed gas can be removed by the Co(en)3 3+ solution. © 2007 Elsevier B.V. All rights reserved. Keywords: Absorption; Catalytic oxidation; Nitric oxide; Ethylenediamine; Cobalt
1. Introduction NOX is blamed for the formation of ozone in the troposphere, the production of acid deposition and photochemical smog and respiratory problems to mankind. In recent years, NOX emissions have become the focus of air pollution control. The NOX treatment techniques are broadly classified as dry and wet processes. The dry processes are further classified as selective catalytic reduction (SCR) [1,2], selective non-catalytic reduction (SNCR), adsorption and nonthermal plasmas [3,4]. SCR of NO by NH3 at 300–400 ◦ C to form water and nitrogen is the best developed and worldwide used technology for the control of NOX emissions in fuel combustion from stationary sources [5]. Major concerns about SCR are the deleterious effects that SO2 , particulates and water vapor present in the flue gas have on catalyst life, which is an important component of the technology’s cost [6]. The difficulties with catalyst poisoning, ammonia slippage and high overall operating expenses have prompted a search for alternative methods for the control of NOX emissions.
∗
Corresponding author. Tel.: +86 21 6425 2884; fax: +86 21 6425 3528. E-mail addresses: [email protected] (X.-l. Long), [email protected] (W.-k. Yuan). 1383-5866/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.seppur.2006.12.018
The development of efficient processes for simultaneous SO2 and NOX control for power-plant flue gas is particularly important because fossil-fuel-fired steam boilers represent a major source of sulphur and nitrogen oxide emissions. Nitric oxide is 90–95% of the NOX present in typical flue gas streams. The existing wet flue-gas-desulphurization (FGD) scrubbers in power plants are incapable of eliminating NO from flue-gases because of its low solubility in water. Several methods have been developed to enhance NO absorption, including the use of oxidants to oxidize NO to the more soluble NO2 [7–20] and the addition of various iron(II) chelates to bind and activate NO [21–27]. So far, none of these methods have been put into commercial application. The authors [28,29] have put forward to remove NO from the exhausted gas stream with cobalt ethylenediamine solution by dissolving soluble cobalt salts and ethylenediamine into aqueous solution. The Co(en)3 3+ ion produced by ethylenediamine binding cobalt acts as a homogeneous catalyst to oxidize NO into soluble nitrogen dioxide and realize the oxidation and absorption of nitric oxide in the same reactor. The dissolved oxygen in equilibrium with the residual oxygen in the exhausted gas stream acts as the oxidant. This paper reports the results of tests performed in a packed column and discusses the factors affecting the NO removal efficiency. The experiments
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227
in bench scale scrubbers indicate that this approach displays very well.
The net reaction is
2. Theoretical
As discussed above, Co(en)3 3+ does not take part in the net reaction and acts as a catalyst to accelerate the oxidation of nitric oxide to soluble nitrogen dioxide in the aqueous solution. The oxygen coexisting in the exhaust gas acts as the oxidant of nitric oxide. The absorption of NO into aqueous solutions is realized favorably with the Co(en)3 3+ system.
The NO oxidation by O2 in gas phase can be expressed as follow: 2NO (g) + O2 (g) → 2NO2 (g)
(1)
rate = kair [NO]2 [O2 ]
(2)
The reaction between NO and O2 in gas phase becomes very slow as nitric oxide levels drop into the low ppm range. Catalysts are needed to speed up the nitric oxide oxidation reaction in typical exhausted gas streams because of its low concentration. However, the catalyst may be easily poisoned because of the presence of water, particulates or SO2 in the exhausted gases. Nitric oxide can also be oxidized by oxygen in aqueous solutions. It can be found that the rate law for liquid phase is identical to the one for gas phase. 4NO (aq) + O2 (aq) + 2H2 O (aq) → 4NO2 − (aq) + 4H+ (aq) d[NO2 − ] = kaq [NO]2 [O2 ] dt
(3) (4)
It has been found that kaq is greater than kair by a factor of 400; however, the small Henry’s constants for NO and O2 render the observed nitric oxide loss rates approximately equal [30]. The nitric oxide concentration in the exhausted gas stream is only as low as ppm range and oxygen concentration may be as high as 15%. Therefore, the rate of NO oxidation by oxygen in aqueous solutions may be accelerated with the increase of the NO solubility in aqueous solution. Co(en)3 3+ can bind nitric oxide in basic solution to form nitrosyl complex (Eq. (5)).
2NO (g) + O2 (g) + 2OH− → NO2 − + NO3 − + H2 O
(9)
3. Experimental Experiments were performed in a packed column (18 mm in diameter, 1000 mm in length) absorber and a bubble column (18 mm in diameter, 820 mm in length) absorber. Glass ring pack was packed in the columns. The schematic diagram of the experimental apparatus of the packed column is shown in Fig. 1. The absorber temperature was maintained at 50 ◦ C with the use of a jacket through which water from a thermostatic bath was circulated because the optimal temperature for NO absorption into the Co(en)3 3+ solution is 50 ◦ C with 5.2% O2 present in the gas phase [28]. Two percent of NO in nitrogen was supplied from a cylinder, and it was diluted with N2 to the desired concentration before being fed into the absorber. NO concentration in the feed gas stream ranged from 250 to 900 ppm (by volume). There was no NO2 in the feed gas stream. Measured amounts of cobalt acetate and H2 NCH2 CH2 NH2 were dissolved in 500 ml distilled water. Co(en)3 2+ cation was formed after CoAc2 was dissolved in aqueous solution. It was oxidized, by simple aeration, to form a more stable complex ion Co(en)3 3+ . A pH-electrode was immersed into the liquid to check the pH-value, which is adjusted by adding NaOH or H2 SO4 . The absorber was operated with a continuous feed of influent gas, at 0.2 l min−1 , at the bottom and a recycled scrubbing solution, at a superficial flow rate of 5 m3 m−2 h−1
Co(en)3 3+ + OH− + NO (g) → Co(en)2 (NO)OH2+ + en (5) The reaction between Co(en)3 3+ and NO can improve NO solubility in the aqueous solution. Subsequently, the NO oxidation reaction performs in the aqueous solution as follow: Co(en)2 (NO)OH2+ + 1/2O2 (aq) → Co(en)2 (NO2 )OH2+ (6) In terms of the symbiosis rule, OH− and en can combine with central atoms to form steady complexes. Therefore, OH− may substitute NO2 from Co(en)2 (NO2 )OH2+ to form complex ions in strong basic solutions (Eq. (7)). Nitrite and nitrate can be produced by NO2 dissolving in the basic solutions. 2Co(en)2 (NO2 )OH2+ + 4OH− → 2Co(en)2 (OH)2 + + NO2 − + NO3 − + H2 O
(7)
The Co(en)3 3+ cations are regenerated by following reaction (Eq. (8)) Co(en)2 (OH)2 + + en → Co(en)3 3+ + 2OH−
(8)
Fig. 1. Experimental apparatus for removing NO and SO2 : (1) gas cylinder, (2) flow meter, (3) packed column, (4) circulation tank and (5) pump.
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(25 ml min−1 ), over the top. Experimental runs were carried out at atmospheric pressure. Quantitative analysis of gas compositions was made by an on-line Fourier transform infrared spectrometer (FTIR) (Nicolet E.S.P. 460 FT-IR) equipped with a gas cell and a quantitative software package Quant Pad. The length of the gas cell in the FTIR is 2 m. The bands in the region 2850–2935 cm−1 , 2150–2225 cm−1 and 1875–1960 cm−1 were used to identify NO2 , N2 O and NO, respectively. The influent and effluent gas samples were directly introduced into the gas cell of the FTIR, with pipes insulated through the regulated electric coils to obtain the transient N2 O, NO and NO2 concentrations of the gaseous samples. This set-up was employed to monitor the overall removal of NO from the feed gas stream. The compositions of the spent scrubbing liquor were determined using a Dionex 500 ion chromatograph equipped with a conductivity detector. A Dionex AS11-HC anion separation column was used for the liquor composition analysis. The eluant was 0.025 mol l−1 NaOH.
run. H2 O2 solution can also remove 100% NO from the flue gas because of a high NO oxidation rate obtained at the start of run. But the NO removal efficiencies decrease quickly as the reactions proceed and begin to be lower than that of Co(NH3 )6 2+ after the reactions have carried out 20 min. The NO removal of Fe(II)–EDTA is 63% and that of H2 O2 is 61% after 1 h. But the NO removal efficiency of Co(en)3 3+ solution can still be sustained at 80%. The main reason is that ferrous chelates are readily oxidized by the oxygen coexisting in the flue gas to ferric chelates [31–34], which are incapable of binding NO. The NO oxidation rate declines as hydrogen peroxide is reduced to H2 O when it oxidizes NO to NO2 . 4.2. Effect of cobalt ethylenediamine concentration on NO removal
The aqueous solutions of Fe(II)–EDTA and H2 O2 are the two NO absorbents that have been extensively investigated. The addition of iron(II)–EDTA (EDTA, ethylenediaminetetraacetate) into the scrubbing liquor to promote the solubility of NO by the formation of iron(II)(EDTA)NO. Hydrogen peroxide in the scrubbing solution turns NO into the valuable nitric acid without generating any other polluting byproduct. A series of experiments are carried out in the bubble column to compare the NO removal efficiencies of Fe(II)–EDTA and H2 O2 solution with that of Co(en)3 3+ . The concentrations of Fe(II)–EDTA and Co(en)3 3+ are 0.01 mol l−1 and that of H2 O2 is 10% (weight). A conclusion can be drawn from the experimental results depicted in Fig. 2 that Co(en)3 3+ solution is superior to H2 O2 and Fe(II)–EDTA solution in removing NO. The rate of Fe(II)–EDTA binding NO is so fast that 100% nitric oxide removal efficiency can be obtained at the start of
Experiments have been performed to investigate the effect of Co(en)3 3+ concentration on NO removal at 50 ◦ C and gas flow rate of 200 ml min−1 in the packed column. The experimental results are shown in Fig. 3. It can be seen from Fig. 3 that NO removal efficiency increases with the Co(en)3 3+ concentration. It can be known from the discussion in Section 2 that there are four different kinds of cobalt ethylenediamine complexes (such as Co(en)3 3+ , Co(en)2 (NO)OH2+ , Co(en)2 (NO2 )OH2+ and Co(en)2 (OH)2 + ) in the solution after the reaction has performed for some time. Therefore, with the progression of the reaction, the practical concentration of the active constituent Co(en)3 3+ becomes smaller than that at the start of the reaction. As a result, the NO removal efficiency decreases as the reaction proceeds. For example, NO removal efficiency of 100% can be obtained at the start of the operation with an initial Co(en)3 3+ concentration of 0.01 mol l−1 . The NO removal efficiency begins to decrease after 2 h operation and is sustained at about 78% after 7 h. The explanation may be that the Co(en)3 3+ concentration in the solution is maintained constant after a dynamic equilibrium is realized between the regeneration and consumption of Co(en)3 3+ . Similar changes of the NO removal efficiencies are also seen in Fig. 3 when the Co(en)3 3+ concentrations are 0.02 and 0.04 mol l−1 , respectively. But the NO removal efficiencies are maintained at
Fig. 2. NO removal with different absorbents in a bubble reactor (50 ◦ C, NO = 570 ppm, O2 = 5.2%, gas flow rate = 200 ml l−1 , pH 12.86).
Fig. 3. Effect of Co(en)3 3+ concentration on NO removal in a packed column (50 ◦ C, NO = 780 ppm, O2 = 5.2%, gas flow = 200 ml l−1 , pH 12.86).
4. Results and discussions 4.1. Effect of different absorbents on NO removal
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Fig. 4. Ion chromatograms of the diluted (100 times) spent scrubber solution from the NO removal experiment with cobalt ethylenediamine solution. The Dionex ion chromatograph was equipped with a conductivity detector and a Dionex AS11-HC anion separation column for the analysis. The eluant was 0.025 mol/l NaOH. 3+
higher levels because the initial Co(en)3 concentrations are higher. Ion chromatographic analyses of the spent scrubbing liquor demonstrated in Fig. 4 demonstrate that the NO absorbed is converted to a mixture of nitrite (NO2 − ) and nitrate (NO3 − ). More nitrate than nitrite is found in the spent scrubbing liquor.
229
Fig. 6. Effect of pH on the NO removal under anaerobic condition (without oxygen, Co(en)3 3+ = 0.02 mol l−1 , NO = 500 ppm, gas flow rate = 300 ml l−1 , 50 ◦ C).
is shortened with the increase of the gas flow rate. Therefore, the NO removal efficiency decreases because NO catalytic conversion decreases. Secondly, the amount of the exhausted gases to be treated in unit time increases as the gas flow rate increases. The quick consumption of active principle Co(en)3 3+ gives rise to the decrease of the NO removal efficiency. 4.4. Effect of pH on NO removal
4.3. Effect of gas flow rate on NO removal Experiments have been carried out to investigate the effect of gas flow rate on NO removal at 50 ◦ C in the packed column. The experimental results are depicted in Fig. 5. It can be concluded from Fig. 5 that the NO removal efficiency decreases with the increase of the gas flow rate. For example, the NO removal efficiencies decrease from 100% at the start of the operation to 76.8% after 4 h operation when the gas flow rate is 400 ml min−1 . But the NO removal efficiency is still sustained at 97.2% after 9 h operation when the gas flow rate is 100 ml min−1 . The effect of gas flow rate on NO removal can be explained as follows: first, the residence time of the gas stream in the packed column
Fig. 5. Effect of gas flow rate on NO removal in a packed column (50 ◦ C, 0.01mol l−1 Co(en)3 3+ , NO = 780 ppm, O2 = 5.2%, pH 12.86).
Experiments have been made to test the effect of pH on NO removal in the packed column at 50 ◦ C under anaerobic conditions. The experimental results are shown in Fig. 6. It can be concluded from Fig. 6 that the NO removal efficiencies decrease with the increase of pH. For example, after 60 min operation, the NO removal efficiencies decrease to 94.7, 87.0 and 76.2% with initial pH of 9.86, 11.75 and 13.3, respectively. However, the NO removal efficiency is still maintained at 100% after 1 h operation with an initial pH of 7.73. Under anaerobic conditions, the NO removal is only due to the coordination between NO and Co(en)3 3+ . The Co(en)3 3+ concentration in the aqueous solution may decrease because of the formation of Co(en)2 (OH)2 − . The greater the pH value, the more the Co(en)2 (OH)2 − produced. The Co(en)2 (OH)2 − is unable to bind NO. Therefore, the NO removal efficiencies decrease with pH values because of the increase of Co(en)2 (OH)2 − in the aqueous solution. Experiments have also been carried out to investigate the effect of pH on NO removal with 4.6% oxygen present in the gas phase. The experimental results are depicted in Fig. 7. A conclusion contrary to the one drawn from Fig. 6 can be obtained from Fig. 7 that NO removal efficiencies increase as the initial pH values increase from 7.82 to 12.86. After 60 min continuous treatment, the NO removal declines from 97.34 to 70.26% with an initial pH of 7.82 while the NO removal efficiencies are still sustained at 98.11 and 90.33% with initial pH of 12.86 and 11.76, respectively. However, it can also be seen from Fig. 7 that the NO removal efficiency may decrease as the pH of the solution increases further above 12.86. After 120 min operation, the NO
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Fig. 7. Effect of pH on the NO removal under aerobic condition (O2 = 4.6%, Co(en)3 3+ = 0.02 mol l−1 , NO = 500 ppm, gas flow rate = 300 ml l−1 , 50 ◦ C).
removal efficiencies decline to 91.93 and 88.68% with initial pH of 13.12 and 13.26, respectively; but the NO removal efficiency is still sustained at 95.22% with an initial pH of 12.86. The electrochemical half-cell reduction potentials of 0 ENO/NO − under basic condition and acidic condition are as 2 follows: Basic conditions 2NO + 4OH− → 2NO2 − + 2H2 O + 2e− , 0 ENO/NO
2
−
= +0.46 V
Acidic conditions 2NO + 2H2 O → 2HNO2 + 2H+ + 2e− , 0 ENO/NO
2
−
= −0.98 V
The Nernst equation shows that the electrochemical half-cell 0 reduction potentials of ENO/NO − increase with the pH of the 2 solution. Therefore, the rate of NO oxidation in aqueous solutions increases with pH. In fact, nitrous acid is a strong oxidant and it can be reduced to nitric oxide again in acid solutions. However, on the other hand, the Co(en)3 3+ concentration decreases because the formation of Co(en)2 (OH)2 + , Co(en)(OH)4 − and even Co(OH)3 deposit increases with the pH value in the aqueous solution, which gives rise to the decrease of NO removal efficiency. In terms of the discussion above, there is an optimal pH value for NO absorption into the Co(en)3 3+ solution under aerobic condition. From the experimental results, the best NO removal efficiency is obtained at about pH 12.86 at 50 ◦ C and 4.6% O2 with 0.02 mol l−1 Co(en)3 3+ solution. 5. Conclusions The following specific conclusions for the catalyst system can be made from the experimental results.
1. NO can be removed from exhausted gas streams by putting soluble cobalt salts and ethylenediamine (H2 NCH2 CH2 NH2 ) into basic solutions. The cobalt ethylenediamine can realize the oxidation and absorption of nitric oxide in the same reactor. The dissolved oxygen in equilibrium with the residual oxygen in the exhausted gas stream acts as the oxidant. 2. This process is superior to the methods using Fe(II)–EDTA solution and H2 O2 solution as absorbents in removing NO from the flue gas stream. 3. The NO removal efficiency decreases with the increase of the gas flow rate. NO removal efficiency increases with the Co(en)3 3+ concentration 4. pH of the solution affects the NO removal efficiencies obviously. Under anaerobic condition, the NO removal efficiency decreases with pH when pH is over 7.73. Under aerobic condition, there is an optimal pH for NO absorption into the Co(en)3 3+ concentration. Acknowledgements The present work is supported by the Ministry of Science and Technology of China (No. 2006AA05Z307) and the Development Project of Shanghai Priority Academic Discipline. References [1] S.M. Cho, Properly apply selective catalytic reduction for NOx removal, Chem. Eng. Prog. (January) (1994) 39–45. [2] A.J. Sweeney, Y.A. Liu, Use of simulation to optimize NOx abatement by absorption and selective catalytic reduction, Ind. Eng. Chem. Res. 40 (2001) 2618–2627. [3] M.B. Chang, H.M. Lee, F. Wu, Simultaneous removal of nitrogen oxide/nitrogen dioxide/sulfur dioxide from gas streams by combined plasma scrubbing technology, J. Air Waste Manage. Assoc. 54 (2004) 941–949. [4] T. Yamamoto, M. Okubo, K. Hayakawa, K. Kitaura, Towards ideal NOx control technology using a plasma-chemical hybrid process, IEEE Trans. Ind. Appl. 37 (2001) 1492–1498. [5] F. Nakahjima, I. Hamada, The state-of-the-art technology of NOx control, Catal. Today 29 (1996) 109–115. [6] H.M. Heck, R.J. Farrauto, Catalytic Air Pollution Control: Commercial Technology, Van Nostrand Reinhold, New York, 1995. [7] S. Uchida, T. Kobayashi, S. Kageyama, Absorption of nitrogen monoxide into aqueous KMnO4 /NaOH and Na2 SO3 /Fe2 SO4 solutions, Ind. Eng. Chem. Process Des. Dev. 22 (1983) 323–329. [8] C. Brogren, H.T. Karlsson, I. Bjerle, Absorption of NO in an alkaline solution of KMnO4 , Chem. Eng. Technol. 20 (1997) 396– 402. [9] H. Chu, S.Y. Li, T.W. Chien, The absorption kinetics of NO from flue gas in a stirred tank reactor with KMnO4 /NaOH solutions, J. Environ. Sci. Health, Part A A33 (1998) 801–827. [10] H. Chu, T.W. Chien, S.Y. Li, Simultaneous absorption of SO2 and NO from flue gas with KMnO4 /NaOH solutions, Sci. Total Environ. 275 (2001) 127–135. [11] C.L. Yang, H. Shaw, Aqueous absorption of nitric oxide induced by sodium chlorite oxidation in the presence of sulfur dioxide, Environ. Prog. 17 (1998) 80–85. [12] C. Brogren, H.T. Karlsson, I. Bjerle, Absorption of NO in an aqueous solution of NaClO2 , Chem. Eng. Technol. 21 (1998) 61–70. [13] T.W. Chien, H. Chu, Removal of SO2 and NO from flue gas by wet scrubbing using an aqueous NaClO2 solution, J. Hazard. Mater. B 80 (2000) 43–57.
X.-l. Long et al. / Separation and Purification Technology 55 (2007) 226–231 [14] T.W. Chien, H. Chu, H.T. Hsueh, Spray scrubbing of the nitrogen oxides into NaClO2 solution under acidic conditions, J. Environ. Sci. Health, Part A 36 (2001) 403–414. [15] H. Chu, T.W. Chien, B.W. Twu, The absorption kinetics of NO in NaClO2 /NaOH solutions, J. Hazard. Mater. B 84 (2001) 241–252. [16] T.W. Chien, H. Chu, H.T. Hsueh, Kinetic study on absorption of SO2 and NOX with acidic NaClO2 solutions using the spraying column, J. Environ. Eng. 129 (2003) 967–974. [17] J.L. Paiva, G.C. Kachan, Modeling and simulation of a packed column for NOx absorption with hydrogen peroxide, Ind. Eng. Chem. 37 (1998) 609–614. [18] J.M. Kasper, C.A. Clausen III, C.D. Cooper, Control of nitrogen oxide emissions by hydrogen peroxide-enhanced gas-phase oxidation of nitric oxide, J. Air Waste Manage. Assoc. 46 (1996) 127–133. [19] J.M. Haywood, C.D. Cooper, The economic feasibility of using hydrogen peroxide for the enhanced oxidation and removal of nitrogen oxides from coal-fired power plant flue gases, J. Air Waste Manage. Assoc. 48 (1998) 238–246. [20] E.B. Myers, T.J. Overcamp, Hydrogen peroxide scrubber for the control of nitrogen oxides, Environ. Eng. Sci. 19 (2002) 321–327. [21] P. Van der Maas, T. Van de Sandt, B. Klapwijk, P. Lens, Biological reduction of nitric oxide in aqueous Fe(II)–EDTA solutions, Biotechnol. Prog. 19 (2003) 1323–1328. [22] P. Van der Maas, S. Weelink, L. Harmsen, B. Klapwijk, P. Lens, Denitrification in aqueous FeEDTA solutions, J. Chem. Technol. Biotechnol. 79 (2004) 835–841. [23] P. van der Maas, P. van den Bosch, B. Klapwijk, P. Lens, NOX removal from flue gas by an integrated physicochemical absorption and biological denitrification process, Biotechnol. Bioeng. 90 (2005) 433–441. [24] P. Van der Maas, S. Peng, B. Klapwijk, P. Lens, Enzymatic versus nonenzymatic conversions during the reduction EDTA-chelated
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Nitric oxide absorption into cobalt ethylenediamine solution Xiang-li Long ∗ , Zhi-ling Xin, Mao-bing Chen, Wen-de Xiao, Wei-kang Yuan ∗ State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, People’s Republic of China Received 2 November 2006; received in revised form 19 December 2006; accepted 20 December 2006
Abstract NO pollutant causes acid rain and urban smog. The removal of NO from exhausted gas streams is necessary to meet stringent effluent discharge limits. NO can be removed from exhausted gas streams by putting soluble cobalt salts and ethylenediamine (H2 NCH2 CH2 NH2 ) into basic solutions. The Co(en)3 3+ (en stands for ethylenediamine) ion produced by ethylenediamine binding cobalt acts as a homogeneous catalyst to oxidize NO into soluble NO2 and realize the oxidation and absorption of nitric oxide in the same reactor. The dissolved oxygen in equilibrium with the residual oxygen in the exhausted gas stream acts as the oxidant. The experiments manifest that this process is superior to the methods using Fe(II)–ethylenediaminetetraacetate (EDTA) solution and H2 O2 solution as absorbents in removing NO from the exhausted gas stream. NO removal efficiency decreases with the increase of the gas flow rate. NO removal efficiency increases with the Co(en)3 3+ concentration. pH of the solution affects the NO removal efficiencies obviously. Under anaerobic condition, the NO removal efficiency decreases with pH when pH is over 7.73. Under aerobic condition, there is an optimal pH for NO absorption into the Co(en)3 3+ concentration. More than 90% of NO in the feed gas can be removed by the Co(en)3 3+ solution. © 2007 Elsevier B.V. All rights reserved. Keywords: Absorption; Catalytic oxidation; Nitric oxide; Ethylenediamine; Cobalt
1. Introduction NOX is blamed for the formation of ozone in the troposphere, the production of acid deposition and photochemical smog and respiratory problems to mankind. In recent years, NOX emissions have become the focus of air pollution control. The NOX treatment techniques are broadly classified as dry and wet processes. The dry processes are further classified as selective catalytic reduction (SCR) [1,2], selective non-catalytic reduction (SNCR), adsorption and nonthermal plasmas [3,4]. SCR of NO by NH3 at 300–400 ◦ C to form water and nitrogen is the best developed and worldwide used technology for the control of NOX emissions in fuel combustion from stationary sources [5]. Major concerns about SCR are the deleterious effects that SO2 , particulates and water vapor present in the flue gas have on catalyst life, which is an important component of the technology’s cost [6]. The difficulties with catalyst poisoning, ammonia slippage and high overall operating expenses have prompted a search for alternative methods for the control of NOX emissions.
∗
Corresponding author. Tel.: +86 21 6425 2884; fax: +86 21 6425 3528. E-mail addresses: [email protected] (X.-l. Long), [email protected] (W.-k. Yuan). 1383-5866/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.seppur.2006.12.018
The development of efficient processes for simultaneous SO2 and NOX control for power-plant flue gas is particularly important because fossil-fuel-fired steam boilers represent a major source of sulphur and nitrogen oxide emissions. Nitric oxide is 90–95% of the NOX present in typical flue gas streams. The existing wet flue-gas-desulphurization (FGD) scrubbers in power plants are incapable of eliminating NO from flue-gases because of its low solubility in water. Several methods have been developed to enhance NO absorption, including the use of oxidants to oxidize NO to the more soluble NO2 [7–20] and the addition of various iron(II) chelates to bind and activate NO [21–27]. So far, none of these methods have been put into commercial application. The authors [28,29] have put forward to remove NO from the exhausted gas stream with cobalt ethylenediamine solution by dissolving soluble cobalt salts and ethylenediamine into aqueous solution. The Co(en)3 3+ ion produced by ethylenediamine binding cobalt acts as a homogeneous catalyst to oxidize NO into soluble nitrogen dioxide and realize the oxidation and absorption of nitric oxide in the same reactor. The dissolved oxygen in equilibrium with the residual oxygen in the exhausted gas stream acts as the oxidant. This paper reports the results of tests performed in a packed column and discusses the factors affecting the NO removal efficiency. The experiments
X.-l. Long et al. / Separation and Purification Technology 55 (2007) 226–231
227
in bench scale scrubbers indicate that this approach displays very well.
The net reaction is
2. Theoretical
As discussed above, Co(en)3 3+ does not take part in the net reaction and acts as a catalyst to accelerate the oxidation of nitric oxide to soluble nitrogen dioxide in the aqueous solution. The oxygen coexisting in the exhaust gas acts as the oxidant of nitric oxide. The absorption of NO into aqueous solutions is realized favorably with the Co(en)3 3+ system.
The NO oxidation by O2 in gas phase can be expressed as follow: 2NO (g) + O2 (g) → 2NO2 (g)
(1)
rate = kair [NO]2 [O2 ]
(2)
The reaction between NO and O2 in gas phase becomes very slow as nitric oxide levels drop into the low ppm range. Catalysts are needed to speed up the nitric oxide oxidation reaction in typical exhausted gas streams because of its low concentration. However, the catalyst may be easily poisoned because of the presence of water, particulates or SO2 in the exhausted gases. Nitric oxide can also be oxidized by oxygen in aqueous solutions. It can be found that the rate law for liquid phase is identical to the one for gas phase. 4NO (aq) + O2 (aq) + 2H2 O (aq) → 4NO2 − (aq) + 4H+ (aq) d[NO2 − ] = kaq [NO]2 [O2 ] dt
(3) (4)
It has been found that kaq is greater than kair by a factor of 400; however, the small Henry’s constants for NO and O2 render the observed nitric oxide loss rates approximately equal [30]. The nitric oxide concentration in the exhausted gas stream is only as low as ppm range and oxygen concentration may be as high as 15%. Therefore, the rate of NO oxidation by oxygen in aqueous solutions may be accelerated with the increase of the NO solubility in aqueous solution. Co(en)3 3+ can bind nitric oxide in basic solution to form nitrosyl complex (Eq. (5)).
2NO (g) + O2 (g) + 2OH− → NO2 − + NO3 − + H2 O
(9)
3. Experimental Experiments were performed in a packed column (18 mm in diameter, 1000 mm in length) absorber and a bubble column (18 mm in diameter, 820 mm in length) absorber. Glass ring pack was packed in the columns. The schematic diagram of the experimental apparatus of the packed column is shown in Fig. 1. The absorber temperature was maintained at 50 ◦ C with the use of a jacket through which water from a thermostatic bath was circulated because the optimal temperature for NO absorption into the Co(en)3 3+ solution is 50 ◦ C with 5.2% O2 present in the gas phase [28]. Two percent of NO in nitrogen was supplied from a cylinder, and it was diluted with N2 to the desired concentration before being fed into the absorber. NO concentration in the feed gas stream ranged from 250 to 900 ppm (by volume). There was no NO2 in the feed gas stream. Measured amounts of cobalt acetate and H2 NCH2 CH2 NH2 were dissolved in 500 ml distilled water. Co(en)3 2+ cation was formed after CoAc2 was dissolved in aqueous solution. It was oxidized, by simple aeration, to form a more stable complex ion Co(en)3 3+ . A pH-electrode was immersed into the liquid to check the pH-value, which is adjusted by adding NaOH or H2 SO4 . The absorber was operated with a continuous feed of influent gas, at 0.2 l min−1 , at the bottom and a recycled scrubbing solution, at a superficial flow rate of 5 m3 m−2 h−1
Co(en)3 3+ + OH− + NO (g) → Co(en)2 (NO)OH2+ + en (5) The reaction between Co(en)3 3+ and NO can improve NO solubility in the aqueous solution. Subsequently, the NO oxidation reaction performs in the aqueous solution as follow: Co(en)2 (NO)OH2+ + 1/2O2 (aq) → Co(en)2 (NO2 )OH2+ (6) In terms of the symbiosis rule, OH− and en can combine with central atoms to form steady complexes. Therefore, OH− may substitute NO2 from Co(en)2 (NO2 )OH2+ to form complex ions in strong basic solutions (Eq. (7)). Nitrite and nitrate can be produced by NO2 dissolving in the basic solutions. 2Co(en)2 (NO2 )OH2+ + 4OH− → 2Co(en)2 (OH)2 + + NO2 − + NO3 − + H2 O
(7)
The Co(en)3 3+ cations are regenerated by following reaction (Eq. (8)) Co(en)2 (OH)2 + + en → Co(en)3 3+ + 2OH−
(8)
Fig. 1. Experimental apparatus for removing NO and SO2 : (1) gas cylinder, (2) flow meter, (3) packed column, (4) circulation tank and (5) pump.
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(25 ml min−1 ), over the top. Experimental runs were carried out at atmospheric pressure. Quantitative analysis of gas compositions was made by an on-line Fourier transform infrared spectrometer (FTIR) (Nicolet E.S.P. 460 FT-IR) equipped with a gas cell and a quantitative software package Quant Pad. The length of the gas cell in the FTIR is 2 m. The bands in the region 2850–2935 cm−1 , 2150–2225 cm−1 and 1875–1960 cm−1 were used to identify NO2 , N2 O and NO, respectively. The influent and effluent gas samples were directly introduced into the gas cell of the FTIR, with pipes insulated through the regulated electric coils to obtain the transient N2 O, NO and NO2 concentrations of the gaseous samples. This set-up was employed to monitor the overall removal of NO from the feed gas stream. The compositions of the spent scrubbing liquor were determined using a Dionex 500 ion chromatograph equipped with a conductivity detector. A Dionex AS11-HC anion separation column was used for the liquor composition analysis. The eluant was 0.025 mol l−1 NaOH.
run. H2 O2 solution can also remove 100% NO from the flue gas because of a high NO oxidation rate obtained at the start of run. But the NO removal efficiencies decrease quickly as the reactions proceed and begin to be lower than that of Co(NH3 )6 2+ after the reactions have carried out 20 min. The NO removal of Fe(II)–EDTA is 63% and that of H2 O2 is 61% after 1 h. But the NO removal efficiency of Co(en)3 3+ solution can still be sustained at 80%. The main reason is that ferrous chelates are readily oxidized by the oxygen coexisting in the flue gas to ferric chelates [31–34], which are incapable of binding NO. The NO oxidation rate declines as hydrogen peroxide is reduced to H2 O when it oxidizes NO to NO2 . 4.2. Effect of cobalt ethylenediamine concentration on NO removal
The aqueous solutions of Fe(II)–EDTA and H2 O2 are the two NO absorbents that have been extensively investigated. The addition of iron(II)–EDTA (EDTA, ethylenediaminetetraacetate) into the scrubbing liquor to promote the solubility of NO by the formation of iron(II)(EDTA)NO. Hydrogen peroxide in the scrubbing solution turns NO into the valuable nitric acid without generating any other polluting byproduct. A series of experiments are carried out in the bubble column to compare the NO removal efficiencies of Fe(II)–EDTA and H2 O2 solution with that of Co(en)3 3+ . The concentrations of Fe(II)–EDTA and Co(en)3 3+ are 0.01 mol l−1 and that of H2 O2 is 10% (weight). A conclusion can be drawn from the experimental results depicted in Fig. 2 that Co(en)3 3+ solution is superior to H2 O2 and Fe(II)–EDTA solution in removing NO. The rate of Fe(II)–EDTA binding NO is so fast that 100% nitric oxide removal efficiency can be obtained at the start of
Experiments have been performed to investigate the effect of Co(en)3 3+ concentration on NO removal at 50 ◦ C and gas flow rate of 200 ml min−1 in the packed column. The experimental results are shown in Fig. 3. It can be seen from Fig. 3 that NO removal efficiency increases with the Co(en)3 3+ concentration. It can be known from the discussion in Section 2 that there are four different kinds of cobalt ethylenediamine complexes (such as Co(en)3 3+ , Co(en)2 (NO)OH2+ , Co(en)2 (NO2 )OH2+ and Co(en)2 (OH)2 + ) in the solution after the reaction has performed for some time. Therefore, with the progression of the reaction, the practical concentration of the active constituent Co(en)3 3+ becomes smaller than that at the start of the reaction. As a result, the NO removal efficiency decreases as the reaction proceeds. For example, NO removal efficiency of 100% can be obtained at the start of the operation with an initial Co(en)3 3+ concentration of 0.01 mol l−1 . The NO removal efficiency begins to decrease after 2 h operation and is sustained at about 78% after 7 h. The explanation may be that the Co(en)3 3+ concentration in the solution is maintained constant after a dynamic equilibrium is realized between the regeneration and consumption of Co(en)3 3+ . Similar changes of the NO removal efficiencies are also seen in Fig. 3 when the Co(en)3 3+ concentrations are 0.02 and 0.04 mol l−1 , respectively. But the NO removal efficiencies are maintained at
Fig. 2. NO removal with different absorbents in a bubble reactor (50 ◦ C, NO = 570 ppm, O2 = 5.2%, gas flow rate = 200 ml l−1 , pH 12.86).
Fig. 3. Effect of Co(en)3 3+ concentration on NO removal in a packed column (50 ◦ C, NO = 780 ppm, O2 = 5.2%, gas flow = 200 ml l−1 , pH 12.86).
4. Results and discussions 4.1. Effect of different absorbents on NO removal
X.-l. Long et al. / Separation and Purification Technology 55 (2007) 226–231
Fig. 4. Ion chromatograms of the diluted (100 times) spent scrubber solution from the NO removal experiment with cobalt ethylenediamine solution. The Dionex ion chromatograph was equipped with a conductivity detector and a Dionex AS11-HC anion separation column for the analysis. The eluant was 0.025 mol/l NaOH. 3+
higher levels because the initial Co(en)3 concentrations are higher. Ion chromatographic analyses of the spent scrubbing liquor demonstrated in Fig. 4 demonstrate that the NO absorbed is converted to a mixture of nitrite (NO2 − ) and nitrate (NO3 − ). More nitrate than nitrite is found in the spent scrubbing liquor.
229
Fig. 6. Effect of pH on the NO removal under anaerobic condition (without oxygen, Co(en)3 3+ = 0.02 mol l−1 , NO = 500 ppm, gas flow rate = 300 ml l−1 , 50 ◦ C).
is shortened with the increase of the gas flow rate. Therefore, the NO removal efficiency decreases because NO catalytic conversion decreases. Secondly, the amount of the exhausted gases to be treated in unit time increases as the gas flow rate increases. The quick consumption of active principle Co(en)3 3+ gives rise to the decrease of the NO removal efficiency. 4.4. Effect of pH on NO removal
4.3. Effect of gas flow rate on NO removal Experiments have been carried out to investigate the effect of gas flow rate on NO removal at 50 ◦ C in the packed column. The experimental results are depicted in Fig. 5. It can be concluded from Fig. 5 that the NO removal efficiency decreases with the increase of the gas flow rate. For example, the NO removal efficiencies decrease from 100% at the start of the operation to 76.8% after 4 h operation when the gas flow rate is 400 ml min−1 . But the NO removal efficiency is still sustained at 97.2% after 9 h operation when the gas flow rate is 100 ml min−1 . The effect of gas flow rate on NO removal can be explained as follows: first, the residence time of the gas stream in the packed column
Fig. 5. Effect of gas flow rate on NO removal in a packed column (50 ◦ C, 0.01mol l−1 Co(en)3 3+ , NO = 780 ppm, O2 = 5.2%, pH 12.86).
Experiments have been made to test the effect of pH on NO removal in the packed column at 50 ◦ C under anaerobic conditions. The experimental results are shown in Fig. 6. It can be concluded from Fig. 6 that the NO removal efficiencies decrease with the increase of pH. For example, after 60 min operation, the NO removal efficiencies decrease to 94.7, 87.0 and 76.2% with initial pH of 9.86, 11.75 and 13.3, respectively. However, the NO removal efficiency is still maintained at 100% after 1 h operation with an initial pH of 7.73. Under anaerobic conditions, the NO removal is only due to the coordination between NO and Co(en)3 3+ . The Co(en)3 3+ concentration in the aqueous solution may decrease because of the formation of Co(en)2 (OH)2 − . The greater the pH value, the more the Co(en)2 (OH)2 − produced. The Co(en)2 (OH)2 − is unable to bind NO. Therefore, the NO removal efficiencies decrease with pH values because of the increase of Co(en)2 (OH)2 − in the aqueous solution. Experiments have also been carried out to investigate the effect of pH on NO removal with 4.6% oxygen present in the gas phase. The experimental results are depicted in Fig. 7. A conclusion contrary to the one drawn from Fig. 6 can be obtained from Fig. 7 that NO removal efficiencies increase as the initial pH values increase from 7.82 to 12.86. After 60 min continuous treatment, the NO removal declines from 97.34 to 70.26% with an initial pH of 7.82 while the NO removal efficiencies are still sustained at 98.11 and 90.33% with initial pH of 12.86 and 11.76, respectively. However, it can also be seen from Fig. 7 that the NO removal efficiency may decrease as the pH of the solution increases further above 12.86. After 120 min operation, the NO
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Fig. 7. Effect of pH on the NO removal under aerobic condition (O2 = 4.6%, Co(en)3 3+ = 0.02 mol l−1 , NO = 500 ppm, gas flow rate = 300 ml l−1 , 50 ◦ C).
removal efficiencies decline to 91.93 and 88.68% with initial pH of 13.12 and 13.26, respectively; but the NO removal efficiency is still sustained at 95.22% with an initial pH of 12.86. The electrochemical half-cell reduction potentials of 0 ENO/NO − under basic condition and acidic condition are as 2 follows: Basic conditions 2NO + 4OH− → 2NO2 − + 2H2 O + 2e− , 0 ENO/NO
2
−
= +0.46 V
Acidic conditions 2NO + 2H2 O → 2HNO2 + 2H+ + 2e− , 0 ENO/NO
2
−
= −0.98 V
The Nernst equation shows that the electrochemical half-cell 0 reduction potentials of ENO/NO − increase with the pH of the 2 solution. Therefore, the rate of NO oxidation in aqueous solutions increases with pH. In fact, nitrous acid is a strong oxidant and it can be reduced to nitric oxide again in acid solutions. However, on the other hand, the Co(en)3 3+ concentration decreases because the formation of Co(en)2 (OH)2 + , Co(en)(OH)4 − and even Co(OH)3 deposit increases with the pH value in the aqueous solution, which gives rise to the decrease of NO removal efficiency. In terms of the discussion above, there is an optimal pH value for NO absorption into the Co(en)3 3+ solution under aerobic condition. From the experimental results, the best NO removal efficiency is obtained at about pH 12.86 at 50 ◦ C and 4.6% O2 with 0.02 mol l−1 Co(en)3 3+ solution. 5. Conclusions The following specific conclusions for the catalyst system can be made from the experimental results.
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