Removal of Sulfur Dioxide and Nitric Oxide Using Cobalt

UNILAB, State Key Laboratory of Chemical Reaction Engineering, East China University of Science and ... Lime desulfurization scrubbers can be retrofit...
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Ind. Eng. Chem. Res. 2005, 44, 686-691

Removal of Sulfur Dioxide and Nitric Oxide Using Cobalt Ethylenediamine Solution Xiang-li Long, Wen-de Xiao, and Wei-kang Yuan* UNILAB, State Key Laboratory of Chemical Reaction Engineering, East China University of Science and Technology, Shanghai, 200237, People’s Republic of China

NO can be removed from exhaust gas streams by placing soluble cobalt salts and ethylenediamine (H2NCH2CH2NH2) into basic solutions. The cobalt ethylenediamine acts as a homogeneous catalyst to oxidize NO into soluble nitric dioxide and realize the oxidation and absorption of nitric oxide simultaneously. The dissolved oxygen in equilibrium with the residual oxygen in the exhaust gas stream acts as the oxidant. Lime desulfurization scrubbers can be retrofitted for combined removal of SO2 and NOx from the flue gas by adding cobalt ethylenediamine to lime slurries. The experimental results in a bench-scale scrubber indicate that such a catalyst system can maintain a high NO removal for a long time. NO removal rate increases with a feed oxygen concentration up to 7.8% or higher. According to the experiments performed, a minimum of 0.02 M of CaO is necessary to ensure a high NO removal rate when 1500 ppm of SO2 is present in the feed. The NO removal rate remains the same with an excess of CaO, due to solubility limitations. The optimal temperature is about 50 °C. More than 90% of NO and nearly 100% of SO2 in the feed gas are removed by the Co(en)33+ in a CaO slurry scrubbing solution. Introduction The role of NOx and SO2 pollutants in acid-rain formation and the destruction of lakes and forest ecosystems has been established.1,2 The removal of these contaminants to comply with environmental emission standards is imperative. Wet processes have certain economic advantages in combined NOx/SO2 elimination. The development of efficient processes for simultaneous SO2 and NOx control in power-plant flue gas is particularly important because fossil-fuel-fired steam boilers represent a major source of sulfur and nitrogen oxide emissions. Nitric oxide is 90-95% of the NOx present in typical flue gas streams.3 However, existing wet fluegas-desulfurization (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 4-6 and the addition of various iron(II) chelates to bind and activate NO.7-9 So far, none of these methods have been put into commercial application. We have developed a homogeneous catalyst system that is made up by dissolving soluble cobalt salts and ethylenediamine into aqueous solution to remove NO from exhaust gas streams. Such a catalyst system may be introduced to the existing wet desulfurization process using a lime slurry to accomplish sequential absorption and catalytic oxidation of both NO and SO2. This paper reports the results of tests performed in a packed column and discusses the factors affecting the NO removal efficiency. The experiments in bench-scale scrubbers indicate that this approach works very well. Experimental Section Experiments were performed in a packed column (18 mm i.d., 1000 mm length) absorber. The schematic * To whom correspondence should be addressed. E-mail: [email protected].

Figure 1. Experimental apparatus for removing NO and SO2 (1) gas cylinder; (2) flowmeter; (3) packed column; (4) circulation tank; (5) pump.

diagram of the experimental apparatus is shown in Figure 1. The absorber temperature was controlled with the use of a jacket, through which water from a thermostatic bath was circulated. NO (2% in nitrogen) was supplied from a cylinder,10 and it was diluted with N2 to the desired concentration before being fed into the absorber. SO2 was supplied in a similar manner. NO concentration in the feed gas stream ranged from 250 to 900 ppm (by volume), and SO2 concentration ranged from 800 to 2500 ppm. Measured amounts of cobalt acetate and H2NCH2CH2NH2 (ethylenediamine, abbreviated as en) were dissolved in 500 mL of distilled water or CaO slurry. The Co(en)32+ cation is formed after CoAc2 is dissolved in aqueous solution. It is oxidized, by simple aeration, to form the more stable complex ion Co(en)33+. The absorber was operated with a continuous feed of influent gas, at 0.2 L/min, at the bottom and a continuous feed of scrubbing solution, at a superficial flow rate

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of 5 m3/(m2 h) (25 mL/min), over the top. Experimental runs were carried out under atmospheric pressure. Periodic addition of dilute NaOH solution was necessary to maintain an optimum basic pH of 10.0, since acids were produced from oxidation of NO and SO2. Quantitative analysis of gas compositions was made by an on-line Fourier transform infrared spectrometer (FTIR) (Nicolet ESP 460 FT-IR) equipped with a gas cell and the quantitative software package Quant Pad. The length of the gas cell in the FTIR is 2 m. The bands in the regions 1150-1200, 2850-2935, 2150-2225, and 1875-1960 cm-1 were used to identify SO2, NO2, N2O, 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 N2O, NO, NO2, and SO2 concentrations of the gaseous samples. This setup was employed to monitor the overall removal of NO and SO2 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 M NaOH. Theoretical Section The oxidation of nitric oxide by O2 in the gas phase has been investigated extensively, and different reaction mechanisms have been put forward. It has been proved that the order of the reaction in oxygen is 1 and that in nitric oxide is 2:

2NO(g) + O2(g) f 2NO2(g)

(1)

rate ) kair[NO]2[O2]

(2)

Though the reaction is fast at high NO concentration, it becomes slow as nitric oxide levels drop into the lowppm range. Catalysts are needed to accelerate the nitric oxide oxidation reaction in typical exhaust gas because of its low concentration. 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 that for the gas phase.

4NO(aq) + O2(aq) + 2H2O(aq) f 4NO2-(aq) + 4H+(aq) (3) d[NO2-] ) kaq[NO]2[O2] dt

(4)

It has been found that kaq is greater than kair by a factor of 400; however, the small Henry constants for NO and O2 render the observed nitric oxide loss rates approximately equal.11 The concentration of nitric oxide in the exhaust stream is usually as low as the ppm range, and that of oxygen may be as high as 15%. It can be assumed that an absorbent able to bind NO should be selected to increase the solubility of NO in aqueous solution, in order to speed up the rate of NO oxidation by oxygen under aqueous conditions. Therefore, NO can be absorbed and oxidized at the same time in aqueous solutions. Co(en)33+ can coordinate nitric oxide in basic solution to form a nitrosyl complex:

Co(en)33+ + OH- + NO(g) f Co(en)2(NO)OH2+ + en (5) The coordination reaction above can improve NO solubility in aqueous solution. Subsequently, the NO oxidation reaction occurs in aqueous solution as follows:

Co(en)2(NO)OH2+ + 1/2O2(aq) f Co(en)2(NO2)OH2+ (6) According to the symbiosis rule, OH- and en can combine with central atoms to form steady-state complexes. Therefore, OH- may displace NO2 from Co(en)2(NO2)OH2+ to form complex ions in strongly basic solutions:

2Co(en)2(NO2)OH2+ + 4OH- f 2Co(en)2(OH)2+ + NO2- + NO3- + H2O (7) NO2- and NO3- can be produced by dissolving NO2 in the basic solutions. The Co(en)33+ cations are regenerated by the reaction

Co(en)2(OH)2+ + en f Co(en)33+ + 2OH-

(8)

The overall equation for NO removal is

2NO(g) + O2(g) + 2OH- f NO2- + NO3- + H2O (9) As discussed above, Co(en)33+ does not take part in the net reaction and acts as a catalyst to accelerate the oxidation of nitric oxide in the aqueous solution. The oxygen coexisting in the exhaust gas is the oxidant of nitric oxide. The oxidation and absorption are performed simultaneously by such homogeneous catalytic processes. Lime (CaO) slurry scrubbing is state-of-the-art technology for flue gas desulfurization. There is, for instance, a class of novel technologies for removing SO2 from industrial waste gases by absorption into aqueous slurries of calcium hydroxide and the in situ catalytic autoxidation of sulfite species to harmless and usable gypsum.12 The mechanism of sulfur dioxide absorption into aqueous slurries of calcium hydroxide can be expressed as

CaO + H2O f Ca(OH)2(s)

(10)

Ca(OH)2(s) T Ca(OH)2(aq)

(11)

Ca(OH)2(aq) T Ca2+ + 2OH-

(12)

H+ + OH- T H2O

(13)

Ca2+ + SO32- T CaSO3(aq)

(14)

CaSO3(aq) + 1/2H2O T CaSO3‚1/2H2O(s)

(15)

It is therefore possible to remove both NO and SO2 by simply adding appropriate amounts of soluble cobalt

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Figure 2. Effect of oxygen on NO removal (0.04 M Co(en)33+, 50 °C, 780 ppm of NO, 5.2% O2).

Figure 3. Effect of temperature on NO removal (0.04 M Co(en)33+, 780 ppm of NO, 5.2% O2).

salt and ethylenediamine to the lime slurry scrubbing solution employed in the desulfurization process. Results and Discussion Effect of Oxygen Partial Pressure. Figure 2 shows the effect of oxygen partial pressure on NO removal efficiency. It can be seen that, whether there is oxygen in the gas stream or not, 100% of the nitric oxide is eliminated from the gas phase within 6 h. However, the nitric oxide elimination efficiency starts to decrease quickly when there is no oxygen in the gas phase and only 69.26% of the nitric oxide is eliminated after 12 h. However, the NO removal efficiency decreases only slightly when there is oxygen in the gas streams. The NO removal efficiency is still maintained at 95.85% with an oxygen concentration of 5.2% and 97.66% with an oxygen concentration of 7.6% after 12 h, respectively. The NO removal efficiency increases with the oxygen concentration in the gas phase. The effect of oxygen on NO absorption efficiency can be explained by the mechanism discussed previously. Under anaerobic conditions, the coordination between Co(en)33+ and nitric oxide is the only reaction that occurs to eliminate NO from the gas phase. The concentration of Co(en)33+ cations decreases gradually as the reaction proceeds. Therefore, the nitric oxide absorption rate may decrease continually because of the depletion of Co(en)33+ cations. On the other hand, oxidation of nitric oxide in aqueous solution may take place under aerobic conditions. The Co(en)33+ cations may be regenerated by reactions 7 and 8. Therefore, the concentration of Co(en)33+ cations may be held almost constant and the nitric oxide removal efficiency can be kept at a high level. Effect of Temperature. Figure 3 shows that 50 °C is the best temperature for NO absorption into Co(en)33+ solutions. At 30 °C, the NO removal efficiency is at 93% after 3 h. At 50 °C, after 7 h, the NO removal efficiency starts to decline from 100% to about 95% and is maintained at this level. At 65 °C NO removal efficiency starts to decrease from 99% after 3 h to 69.78% after 9 h. It is reported13,14 that the physical solubility of NO increases (the Henry constant, HNO, decreases) with temperature. Dynamically, the NO oxidation rate also increases with temperature. Hence, NO removal efficiency may increase with temperature. On the other hand, two negative influences of raising the temperature on NO absorption performance should be mentioned. First, the disintegration of cobalt ethylenediamine complexes may be enhanced because ethylenediamine

Figure 4. Plot of NO removal vs time (0.04 M Co(en)33+, 50 °C, 780 ppm of NO, 5.2% O2).

becomes more volatilizable with temperature. Second, the solubility of oxygen in aqueous solutions decreases as the temperature is increased. When the temperature is above 50 °C, the advantages of temperature cannot counterbalance the effects that lower NO absorption rate. Study on Nitric Oxide Removal Efficiency for a Long Period of Time. Figure 4 shows the nitric oxide removal with Co(en)33+ solution studied in a packed column for 200 h without intermission. It demonstrates that the nitric oxide removal efficiency is reduced from 100% to 74.18% after the absorption has been carried out for 100 h. Detection of the pH value of the solution shows that it has dropped from 11 at the start of the reaction to 9 after 100 h. Then sodium hydroxide is added into the solution to bring the pH value back to 11 once again. The nitric oxide absorption is improved, and the removal efficiency is immediately restored to 95.42%. The nitric oxide removal efficiency fluctuates between 89% and 95% in the second 100 h of reaction. The change of pH and the loss of ethylenediamine bring about a fluctuation of the nitric oxide removal efficiency. Sodium hydroxide is dissolved into the solution now and again to keep the pH value greater than 10, and ethylenediamine is put into the solution to redeem the losses caused by its volatilization. Ion chromatographic analyses of the spent scrubbing liquor depicted in Figure 5 (data of the ion chromatograms are given in Table 1) 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. It might be possible to make use of nitric oxide in exhaust gas streams to produce nitrate salts.

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Figure 6. Effect of SO2 on NO removal (0.01 M Co(en)33+, 50 °C, 780 ppm of NO, 5.2% O2). Figure 5. Ion chromatograms of the diluted (100×) spent scrubber solution from the NO removal experiment with cobalt ethylenediamine solution. We used a Dionex ion chromatograph equipped with a conductivity detector and a Dionex AS11-HC anion separation column for the analysis. The eluant was 0.025 M NaOH. Table 1. Data of the Ion Chromatograms no. 1 2 3 4 5 total

ret time (min)

peak name

2.93 5.08 6.10 8.49 11.73

Ac na NO2 na NO3

oeak height (µS)

rel area (%)

type

0.099 0.102 0.506 0.218 28.92 29.847

0.12 0.14 0.82 0.64 98.28 100

BMB BMB BMB BMB BMB BMB

Figure 4 supports the fact that Co(en)33+ solution can hold high nitric oxide removal efficiency for a very long time because of its homogeneous catalysis. The experiment also shows that the pH value of the solution plays an important role in abating NO by Co(en)33+ solution. The relation between pH and nitric oxide removal efficiency can be analyzed in terms of the impacts of pH on electrochemical half-cell reduction potentials, the stability of nitrous acid, the rate of nitric oxide oxidation, and the regeneration of Co(en)33+ cations. First, the nitric oxide oxidation under basic condition and acidic condition can be contrasted on the basis of its electrochemical half-cell reduction potentials. Under basic conditions

2NO + 4OH- f 2NO2- + 2H2O + 2e- E° ) 0.46 V and under acidic conditions

2NO + 2H2O f 2HNO2 + 2H+ + 2e- E° ) 0.98 V Electrochemical half-cell reduction potentials show that oxidation of nitric oxide is thermodynamically more favorable under basic conditions than under acidic conditions. In fact, nitrous acid is a strong oxidant and it can be reduced to nitric oxide again in acid solutions. Second, in acidic solution, nitrous acid may be formed by NO2- combining with H+. Nitrous acid is unstable in aqueous solution and decomposes into NO and NO3-:

3HNO2 T NO3- + 2NO + H3O+

(16)

Third, though oxygen is a better oxidant, its oxidizing rate is very low under acidic conditions. However, its oxidizing rate is much higher in basic solutions. There-

fore, the fast oxidation of nitric oxide by oxygen can be realized in basic solutions. Above all, the Co(en)33+ cations should be reproduced under basic conditions. As is shown in a previous part of this paper, the oxidation reaction (6) is followed by the reaction (7), where NO2 is substituted by OH- in the complexes. However, reaction (7) must take place under basic conditions. The displacenent of NO2 by OHis a crucial step for the regeneration of Co(en)33+ cations. If the reproduction of Co(en)33+ cations cannot be realized propitiously, the homogeneous catalytic oxidation of nitric oxide will not take place successfully. In a word, the pH value of the aqueous solution is a vital factor affecting the nitric oxide absorption efficiency. It can be concluded that the oxidation and absorption of nitric oxide will not take place in acidic Co(en)33+ solution. Effect of SO2 without Lime. When lime is absent in the Co(en)33+ solution, the presence of SO2 in the feed gas has an adverse effect on NO removal rate, as shown in Figure 6. After 5.5 h, the NO removal efficiency with 1500 ppm of SO2 in the gas stream was 7% lower than that without SO2 in the gas stream. There is no SO2 detected in the outlet streams over the duration of the experiment. In other words, the SO2 removal efficiency can be almost kept at 100% because SO2 may easily dissolve into the aqueous solution with a pH value greater than 10 to produce sulfite. The reason SO2 decreases the absorption of NO into Co(en)33+ solution may be that Co(en)33+ concentration will be reduced gradually by the formation of Co2(SO3)3 deposit. As a result, the reaction rate between NO and Co(en)33+ may decrease and finally results in a decrease of NO removal. Effects of CaO on NO Removal. When there is a sufficient amount of CaO in the Co(en)33+ scrubbing solution, SO2 in the feed gas has virtually no adverse effect on the overall NO removal, as is demonstrated by the data shown in Figure 7. The reaction mechanism can be described as eqs 10-15. The calcium sulfite deposit (pKsp ) 6.5) produced may restrain the sulfite concentration from increasing. High SO2 and NO removal efficiencies can be obtained by adding Co(en)33+ into calcium oxide slurries. Effect of the Amount of CaO on NO Removal. According to Figure 8, a minimum amount of CaO in the Co(en)33+ scrubbing slurry is necessary to overcome the adverse effect of SO2 in the feed gas and the figure also shows that too much CaO results in no further improvement in NO removal. To effectively remove NO from a feed gas stream containing 1500 ppm of SO2,

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Figure 7. Effect of calcium lime on NO removal (0.04 M Co(en)33+, 50 °C, 780 ppm of NO, 5.2% O2).

Figure 10. Effect of scrubbing solution composition on NO removal (0.04 M Co(en)33+, 50 °C, 780 ppm of NO, 1500 ppm of SO2, 5.2% O2).

CaO slurry. Nearly 100% removal of SO2 is observed in all experimental runs. Magnesium hydroxide is formed when MgO dissolves into aqueous solution:

MgO + H2O f Mg(OH)2

(17)

It has been reported in the literature15 that Mg(OH)2 slurry could effect a high NO2 removal when NO2 and SO2 are scrubbed simultaneously, because NO2 is reduced to N2: Figure 8. Effect of CaO concentration on NO removal (0.04 M Co(en)33+, 50 °C, 780 ppm of NO, 1500 ppm of SO2, 5.2% O2).

2NO2 + 4HSO3- + 2Mg(OH)2(s) f 2MgSO4 + 2SO42- + 4H2O + N2 (18) However, our experimental results indicate that Mg(OH)2 has the lowest NO removal efficiency, because MgSO3 has a solubility product much greater than that of Mg(OH)2 (pKsp ) 10.7). In other words, Mg(OH)2 cannot counteract the adverse effect of SO2 on NO removal. On the other hand, ammonia also cannot produce a deposit with sulfite. That is, adding ammonia into Co(en)33+ solution is not able to restrain the formation of cobalt sulfite deposit. The reason CaCO3 cannot gain a NO removal efficiency as high as CaO is their different desulfurization mechanisms. The mechanism of CaCO3 desulfurization is

Figure 9. Effect of SO2 concentration on NO removal (0.04 M Co(en)33+, 0.065 M CaO, 50 °C, 780 ppm of NO, 5.2% O2). 3+

more than 0.015 M of CaO in 0.04 M Co(en)3 solution is necessary, while 0.065 M appears to be more than sufficient. The behavior displayed in Figure 8 suggests that about 0.025 M of CaO may be the right concentration for operation with 1500 ppm of SO2. Effect of SO2 Concentration. Given a sufficient amount of CaO in the Co(en)33+ scrubbing solution, a stable, high NO removal is maintained when much more SO2 is present in the feed. Figure 9 shows that, 10 h after the treatment begins, the NO removal efficiency is still more than 95% for cases of both 1500 and 3000 ppm of SO2 in the gas feed streams. Effect of Scrubbing Solution Composition. It is known that CaCO3, Mg(OH)2, and aqueous ammonia are desulfurizing agents. It is necessary to test their effects on the combined removal of NO and SO2 with Co(en)33+ solution. The experimental results of different desulfurizing agents are shown in Figure 10. This figure demonstrates that the catalyst performs the best in the

SO2 + H2O f H2SO3

(19)

H2SO3 f H+ + HSO3-

(20)

H+ + CaCO3 f Ca2+ + HCO3-

(21)

Ca2+ + HSO3- + 2H2O f CaSO3‚2H2O + H+ (22) H+ + HCO3- f H2CO3

(23)

H2CO3 f CO2 + H2O

(24)

According to the mechanism above, in the CaCO3 system, the generation of Ca2+ depends on the concentration of H+ and the existence of CaCO3. It has been found that the optimal pH values for CaCO3 desulfurization are 5.8∼6.2. The pH value for NO absorption into Co(en)33+ solution is greater than 10. Such a pH value is unfavorable for the production of Ca2+ cations in the CaCO3 system. Therefore, the Ca2+ provided is not enough to counteract the adverse effect of SO2 on

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NO elimination. In terms of the mechanism of CaO desulfurization discussed previously, the generation of Ca2+ cations is only dependent on the existence of CaO. The basic solution is also favorable to the production of CaSO3 deposits in calcium oxide slurries. Conclusions A homogeneous catalyst system has been invented to remove NO from exhaust gas streams. Such system can also be used to realize the combined removal of NO and SO2 from the flue gas. The following specific conclusions for such a system can be drawn from the experimental results. 1. It is prepared by dissolving cobalt acetate and ethylenediamine in aqueous basic solution, followed by a short period of aeration to form stable Co(en)33+ ions. Co(en)33+ ions act as the catalyst, and dissolved oxygen, in equilibrium with residual oxygen in the scrubber feed gas, acts as the oxidant. 2. To employ this catalyst system for simultaneous removal of NO and SO2 from flue gas streams, soluble cobalt salts and ethylenediamine may be dissolved in the CaO slurry. The existing lime desulfurization process can be retrofitted to accomplish the absorption of NO and SO2 in the same reactor. 3. Without CaO in the scrubbing solution, SO2 in the feed gas adversely affects the NO removal by reducing the availability of dissolved Co(en)33+ required for the oxidation of NO in aqueous solutions to form nitrite and nitrate. 4. Depending on the SO2 concentration in the feed, a minimum amount of CaO is required in the scrubbing solution to overcome its adverse effect on NO removal. To ensure an effective NO removal, more than 0.015 M of CaO should be present in the scrubbing solution when there is 1500 ppm of SO2 in the feed, while 0.065 M of CaO is sufficient for 3000 ppm of SO2. 5. The NO removal efficiency increases with residual oxygen concentration in the flue gas feed. The best temperature for NO removal is 50 °C. 6. Though a study on NO and SO2 absorption into Co(en)33+ solutions under conditions of industrial importance has been performed systematically and some useful results have been obtained, it is imperative to carry out further studies into such a process. Acknowledgment The present 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. Many thanks are given to Professor Wei-chi Ying for his valuable assistance in helping us finish this paper. Literature Cited (1) Cosby, B. J.; Hornberger, G. M.; Galloway, J. N.; Wright, R. F. Time scales of catchment acidification. Environ. Sci. Technol. 1985, 19, 1144. (2) Hileman, B. Forest decline from air pollution. Environ. Sci. Technol. 1984, 18, 8A. (3) Pereira, C. J.; Amiridis, M. D. In NOx Control from Stationary Sources; Pereira, C. J., Amiridis, M. D., Eds.; ACS Symp. Ser. 552; American Chemical Society: Washington, DC, 1995; Vol. 552, p 1. (4) Chang, S. G.; Liu, D. K. Removal of nitrogen and sulphur oxides from waste gas using a phosphorus/alkali emulsion. Nature 1990, 343, 151. (5) Downey, G. D. Process of removing nitrogen oxides from gaseous mixtures. Eur. Patent 0 008 488, 1980. (6) Cooper, H. B. H. Removal and recovery of nitrogen oxides and sulfur dioxide from gaseous mixtures containing them. U.S. Patent 4 426 364, 1984. (7) Chang, S. G.; Littlejohn, D.; Liu, D. K. Use of Ferrous Chelates of SH-Containing Amino Acid and Peptides for the Removal of NOx and SO2 from Flue Gas. Ind. Eng. Chem. Res. 1988, 27, 2156. (8) Pham, E. K.; Chang, S. G. Removal of NO from Flue Gases by Absorption to an Iron(II) Thiochelate Complex and Subsequent Reduction to Ammonia. Nature 1994, 369, 139. (9) Yao, S.; Littlejohn, D.; Chang, S. G. Integrated Tests for Removal of Nitric Oxide with Iron Thiochelate in Wet Flue Gas Desulfurization System. Environ. Sci. Technol. 1996, 30, 3371. (10) Long, X. L. Simultaneous Removal of Sulfur Dioxide and Nitric Oxide. Ph.D. Dissertation, East China University of Science and Technology, Shanghai, People’s Republic of China, 2002. (11) Pires, M.; Rossi, M. J.; Ross, D. S. Kinetic and Mechanistic Aspects of the NO Oxidation by O2 in Aqueous Phase. Int. J. Chem. Kinet. 1994, 26, 1207. (12) Karlsson, H. T.; Bengtsson, S.; Bjerle, J.; Klingspor, J.; Nilsson, L.-J.; Stro¨mberg, A.-M. 1985, Oxidation of sulfite to sulfate in flue gas desulfurization system. In Processing and Utilization of High Sulfur Coals; Elsevier: Amsterdam, 1985; Coal Science and Technology Series 9. (13) Thomas, D.; Vanderschuren, J. Effect of Temperature on NOx Absorption into Nitric Acid Solutions Containing Hydrogen Peroxide. Ind. Eng. Chem. Res. 1998, 37, 4418. (14) Joshi, J. B.; Mahajani, V. V.; Juvekar, V. A. Absorption of NOx gases. Chem. Eng. Commun. 1985, 33, 1. (15) Eizo, S.; Hidehiro, K.; Muhammad, A. B. Single and Simultaneous Absorption of Lean SO2 and NO2 Into Aqueous Slurries of Ca(OH)2 or Mg(OH)2 Particles, J. Chem. Eng. Jpn. 1979, 12, 111.

Received for review June 6, 2004 Revised manuscript received November 24, 2004 Accepted December 1, 2004 IE049513V