Evaluation of Nitric Oxide Removal from Simulated Flue Gas by Fe(II

(12, 26) Additionally, the citrate buffer system maintains a pH of 5–7, which is desirable for the chemical ... Fe(II)EDTA complex stock solutions wer...
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Evaluation of Nitric Oxide Removal from Simulated Flue Gas by Fe(II)EDTA/Fe(II)citrate Mixed Absorbents Nan Liu, Bi-Hong Lu, Shi-Han Zhang, Jin-Lin Jiang, Ling-Lin Cai, Wei Li,* and Yi He* Institute of Industrial Ecology and Environment, Zhejiang University (Yuquan Campus), Hangzhou, 310027, China ABSTRACT: Fe(II)EDTA is an effective absorbent for the integrated chemical absorption−biological reduction system in removing nitric oxide (NO) from flue gas. However, this absorbent is subject to some defects, such as oxidation by oxygen. In order to overcome this drawback, instead of using Fe(II)EDTA solely, a mixed absorbent containing both Fe(II)EDTA and Fe(II)Cit (Cit = citrate) is employed to absorb NO from simulated flue gas. The mixed absorbent not only shows a high NO absorption capacity similar to a Fe(II)EDTA absorbent, but also exhibits high NO absorption rate and good resistance to oxidation in simulated flue gas. This mixed absorbent can maintain 80% NO removal efficiency at 323 K for more than two hours at an inlet NO concentration of 670 mg·m−3, which is almost as effective as the Fe(II)EDTA absorbent at the same Fe(II) concentration. The oxidation rate constant of Fe(II) in the mixed absorbent is also slower than that in the Fe(II)EDTA absorbent. The optimal molar ratio of Fe(II)Cit to Fe(II)EDTA in the mixed absorbent was found to be 3:1. The effects of several key factors for NO removal, such as the inlet concentrations of NO (200−670 mg·m−3), O2 (1−6.5%), and SO32− (0−43 mg·L−1) and the pH of the mixed absorbent, have been studied. Interestingly, results seem to suggest that SO32− is beneficial for the removal of NO in the system. These findings provide fundamental data for the design of NO removal system for industrial applications with mixed Fe(II)EDTA/Fe(II)Cit absorbent.

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The first two reactions belong to chemical absorption step. One is the formation of Fe(II)L-NO by ferrous complexes in scrubbing liquid and NO in flue gas (reaction 1). This reaction greatly enhances the absorption rate of NO. The other is the oxidation of ferrous complexes, caused by oxygen in flue gas (reaction 2). This oxidation reduces the amount of ferrous complexes for the NO removal.17,18 The biological recovery step for the approach also includes two reactions, as well. Both reactions required the carbon source such as C6H12O6. The recovery of Fe(II)L-NO and Fe(III)L is completed by denitrifying bacteria and iron-reducing bacteria, respectively, as shown in reactions 3 and 4. The NO removal efficiency strongly depends on the ferrous complexes in the scrubbing liquid.12,19 The commonly used ferrous complexes include mercapto compounds (-SH) such as ferrous cysteine (Fe(II)(CyS)2) and amino carboxylic complexes (EDTA, NTA, citrate). Fe(II)(CyS)2 is a good absorbent for NO removal because of its high absorption rate of NO and high resistance to oxidation caused by oxygen. However, the possibility of forming secondary compounds hinders its applications in NO removal.15,19−21 Ferrous EDTA (Fe(II)EDTA) is another absorbent for NO removal that has a relative high NO absorption rate and capacity. However, it is costly and easy to be oxidized into Fe(III)EDTA by oxygen in flue gas.20−23 Moreover, the slow biodegradability of EDTA can be harmful to the environment because it could chelate toxic heavy metals and increase their mobility.10,24 Compared with Fe(II)EDTA, Fe(II)Cit (C6H5Na3O7) is more environmentally friendly, more resistant to oxidation, and less expensive.25−28

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Received: April 9, 2012 Revised: July 20, 2012 Published: July 23, 2012

1. INTRODUCTION Nitrogen oxides (NOx) are usually released from the combustion of fossil fuels, which causes serious environmental issues such as the depletion of the ozone layer and the formation of secondary compounds including photochemical smog and nitric acid.1−3 The major constituent of NOx in the flue gas is nitric oxide (NO). It has a very low solubility in aqueous solutions, which makes it hard to remove. To date, several technologies, including selective catalytic reduction (SCR), selective noncatalytic reduction (SNCR), absorption, and adsorption,4−6 have been used for NO removal. However, these conventional methods suffer from high cost and the risk of secondary pollution.7,8 An integrated chemical absorption−biological reduction approach has been found to be effective in removing NO from flue gas and cost-effective.9−11 This approach uses ferrous chelate complexes (Fe(II)L, L = chelate agent) to enhance the absorption of NO into the scrubbing liquid.12−14 The reaction mechanism of this approach is as follows:15,16 Fe(II)L2 − + NO(aq) ↔ Fe(II)L‐NO2 −

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4Fe(II)L2 − + O2 (aq) + 2H 2O → 4Fe(III)L− + 4OH− (2)

12Fe(II)L‐NO2 − + C6H12O6 microorganism

⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ 12Fe(II)L2 − + 6H 2O + 6CO2 (g) + 6N2(g)

24Fe(III)L− + C6H12O6 + 24OH− microorganism

2−

⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ 24Fe(II)L

+ 18H 2O + 6CO2 (g)

© 2012 American Chemical Society

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Figure 1. Configuration of lab-scale scrubber reactor for measurement of NO absorption: 1, gas cylinders; 2, mass flow controllers; 3, gas mixing chamber; 4, scrubber reactor; 5, thermostatic water bath; 6, holding tank; 7, 8, pump; 9, liquid flow meter; 10, cooler; 11,: NO−NO2−NOx analyzer.

Citrate can be used not only for wet flue gas desulfurization29,30 but also for NO removal.12,26 Additionally, the citrate buffer system maintains a pH of 5−7, which is desirable for the chemical absorption−biological reduction approach.10,31 The only pitfall of Fe(II)Cit is that its NO absorption rate is much slower than that of Fe(II)EDTA.32,33 Currently, there is no single absorbent that is perfect for the integrated chemical absorption−biological reduction approach. Therefore, using a mixed absorbent seems to be a potential way to achieve both high NO absorption rate and good resistance to oxidation, which are two of the most important criteria in selecting absorbents for the approach. In this work, a mixed absorbent containing both Fe(II)Cit and Fe(II)EDTA was examined for NO removal. The objective was to mitigate the deficiencies of Fe(II)EDTA absorber, particularly its tendency to oxidation, while keeping high NO removal efficiency. The molar ratios of Fe(II) to citrate and EDTA to citrate have been varied respectively to search for the optimal composition for the mixed absorbent. The effects of several key factors on NO removal, such as the inlet concentrations of NO, O2, and SO32− and the pH of the mixed absorbent, were also examined to provide desired operating conditions for the integrated chemical absorption−biological reduction NO removal system. The results of these investigations provide insights into the configuration and operation of NO removal system.

2.2. Preparation of Fe(II)Cit and Fe(II)EDTA. Fe(II)Cit complex stock solutions were prepared by mixing 20 mmol·L−1 FeSO4·7H2O with 10, 20, 40, 60, 80, 120 mmol·L−1 trisodium citrate, respectively, under anoxic conditions. The pH was adjusted to 7 with 0.1 mmol·L−1 NaOH or HCl. Fe(II)EDTA complex stock solutions were prepared by mixing 20 mmol·L−1 EDTA tetrasodium with 20 mmol·L−1 FeSO4·7H2O under anoxic conditions. The pH was adjusted to 7 with 0.1 mmol·L−1 NaOH or HCl. The prepared solutions were kept in glass serum vials (250 mL) under N2 positive pressure to prevent oxidation of Fe(II)Cit and Fe(II)EDTA.12 2.3. Apparatus. The schematic diagram of the apparatus is shown in Figure 1. A cylindrical reactor (total volume, height: 0.79 L, 0.6 m) with an effective working volume of 0.52 L and a holding tank (5 L) for the absorbent solution reserved was used for NO removal. 4.5 L aqueous solution was run countercurrent to the gas recycled into the reactor. The liquid sampling points were obtain from the holding tank. The packings, which are commercial products with 10 cm in length, 8 mm in diameter, and about 1200 m2·m−3 in total specific surface area, were randomly packed in the reactor. The reactor was operated in a countercurrent mode. The scrubbing liquid was continuously withdrawn from the holding tank and cycled into the top of the reactor by the electromagnetic pump. A mixing chamber was provided before the inlet gas entered the absorber. The flow rates of the simulated flue gas and scrubbing liquid in the experiments were maintained at 800−2000 mL·min−1 and 40 L·h−1, respectively. The reactor was immersed in water at a constant temperature of 50 ± 0.5 °C to simulate the typical flue gas temperature (45−55 °C) after the flue gas desulphurization (FGD) process. 2.4. NO Absorption Experiments. NO absorption experiments were conducted in the scrubber reactor. The reactor was scrubbed with water before each experiment. The ratios of Fe(II)Cit and Fe(II)EDTA studied were 1:1, 3:1, 4:1, and 7:1. The concentration of Fe(II) in the reactor was 20 mmol·L−1. The simulated flue gas contained 15% (v/v) CO2,

2. MATERIALS AND METHODS 2.1. Chemicals. Trisodium citrate (C6H5Na3O7·2H2O, 99.95%, Sigma), ethylenediaminetetraacetic acid tetrasodium (EDTANa4·4H2O, 99.5%, Sigma), FeSO4·7H2O (99.5%), and D-glucose (99.5%) were obtained from Shanghai Chemical Reagent Co. Ltd. (Shanghai, China). NO (5% in N2, v/v), N2 (99.999%), O2 (99.999%), and SO2 (0.5% in N2, v/v) were obtained from Zhejiang Jin-gong Gas Co. (Hangzhou, China). All other chemicals were of analytical grade, commercially available, and used without further purification. 4911

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0−6.5% (v/v) O2, 50−300 mg·m−3 (v/v) SO2, and 0−670 mg·m−3 NO. During the absorption process, samples were taken at regular intervals for the measurement of Fe(II), pH value, and the outlet concentration of NO. The NO removal efficiency (η) of the absorption system, measured at various ratios of Fe(II)Cit and Fe(II)EDTA, was defined as follows: η=

C in − Cout C in

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Here, Cin and Cout denote inlet and outlet NO concentrations (mg·m−3) in the gas phase. 2.5. Analytical Methods. The concentration of ferrous ions in solution was determined by 1,10-phenanthroline colorimetry at 510 nm with a spectrophotometer.12 The concentration of NO was measured with a chemiluminescent NO−NO2−NOx analyzer (Thermo, U.S.). The data shown in Tables 1 and 2 are the mean values of duplicate experiments, and the data shown in Figures 2−7 are the mean values of triplicate experiments.12

Figure 2. NO absorption capacity with different proportions of Fe(II) and citrate: gas flow rate = 650 mL·min−1; NO = 670 mg·m−3; initial Fe(II)Cit = 20 mmol·L−1; T = 298 K; pH = 6.9.

Table 1. Effects of Inlet O2 Concentration on the Oxidation of Fe(II)a this work O2 % 1 3 5 6.5

ref 38

K′, h−1

O2 %

± ± ± ±

1.2 2.8 4.8 6.8

0.0948 0.2310 0.4062 0.4782

of NO absorption capacity at high Fe(II)/citrate ratio, on the other hand, can be explained not only by the stereohindrance effect of increasing concentration of citrate, but also by the excess unshared-pair electrons of citrate. These electrons occupied the unfilled orbit of Fe(II) in Fe(II)/citrate and make it inaccessible for NO. To maximize the NO absorption capacity, therefore, the optimal ratio of Fe(II) to citrate was kept to 1:2 in subsequent experiments. 3.2. Molar ratios of EDTA to Citrate in the Mixed Absorbent System. For a mixed absorbent system, it is also necessary to determine the optimal ratio of citrate to EDTA for NO removal. A comparative test of several mixed absorption systems was performed. The results in Figure 3 show that the mixed absorbent could maintain 80% NO removal efficiency for more than 2 h at an inlet NO concentration of 670 mg·m−3, when the molar ratio of citrate to EDTA is 2:1 or 6:1. They are almost as effective as Fe(II)EDTA absorbent at the same Fe(II) concentration. The average reduction rate of Fe(II)L by

0.0011 0.0015 0.0017 0.0077

K′, h

−1

0.1302 0.2916 0.5256 0.7470

a G = 1000 mL·min−1; NO = 670 mg·m−3; initial Fe(II)L = 20 mmol·L−1; T = 323 K; pH = 6.80.

Table 2. Effects of SO32− Concentrations on NO Absorption Concentrationa SO32− concn, mg·L−1 0 1.5 7.4 14.8 29.6 42.9

NO concn, mmol·L−1 2.19 3.48 4.06 4.75 4.85 4.91

± ± ± ± ± ±

0.11 0.1 0.27 0.35 0.25 0.06

Gas flow rate = 1000 mL·min−1; NO = 670 mg·m−3; O2 = 3% (v/v); initial Fe(II)L = 20 mmol·L−1; T = 323 K; pH = 6.80.

a

3. RESULTS AND DISCUSSIONS 3.1. Molar Ratios of Fe(II) to Citrate. While some previous studies22,34 have shown that the optimal molar ratio of Fe(II) to EDTA for NO removal is 1:1 when the pH of scrubbing liquid is above 3, there are few reports that show the optimal ratios of Fe(II) to citrate for the purpose of NO absorption. Therefore, it is necessary to determine the optimal ratio of Fe(II) to citrate in this study. We measured the NO absorption capacity at a series of ratios of Fe(II) to citrate. The results in Figure 2 show that there is a optimal ratio of Fe(II) to citrate (1:2) where the NO absorption capacity reaches the maximum (nNO/nFe(II) = 0.47) at a pH of 6.9. The decrease of NO absorption capacity at high Fe(II)/citrate can be attributed to the low stability constant of Fe(II)Cit, 1.26 × 103,31 which determines that only a small portion of the Fe(II) can chelate with citrate and be available for NO adsorption. The decrease

Figure 3. NO absorption efficiency in mixed absorbent systems: gas flow rate = 1000 mL·min−1, NO = 670 mg·m−3, O2 = 3% (v/v), initial Fe(II)L = 20 mmol·L−1, T = 323K, pH = 6.80. (■) Fe(II)EDTA; (●) citrate, EDTA = 2:1; (▲) citrate: EDTA = 6:1; (▼) citrate, EDTA = 8:1; (◀) citrate, EDTA = 14:1; (▶) Fe(II)Cit; (★) Fe(II)EDTA = 5 mmol·L−1. 4912

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pseudomonas sp. strain DN-2 (4.17 mmol·gDCW−1·h−1)14 can be satisfied with 1.07 mmol·h−1 of NO absorption rate easily, which means that this kind of mixed absorbent can be quickly regenerated by microorganisms in the chemical absorption− biological reduction integrated system.13,14 When the molar ratios of citrate to EDTA were set at 8:1 and 14:1, the η decreased to 70%. So the citrate to EDTA molar ratio was selected to be 6:1 for NO removal, which is equivalent to a 3:1 molar ratio of Fe(II)Cit to Fe(II)EDTA. This ratio was used in the subsequent experiments. 3.3. Characteristics of Nitric Oxide Removal in the Mixed Absorption System. 3.3.1. Effects of Inlet Oxygen Concentration on NO Absorption. Flue gas from power plants usually contains a small amount of oxygen (3−8%) depending on combustion conditions. Oxygen has undesirable effects on NO absorption. Specifically, dissolved oxygen (DO) can promote the chemical oxidation of Fe(II)L. For this reason, it is of vital importance to explore the effects of oxygen on NO removal. As shown in Figure 4, the higher the concentration of O2 in flue gas, the faster Fe(II)L reacts with O2. NO removal efficiency declined with the decrease of the concentration of Fe(II)L in solution. The NO removal efficiency was maintained

for 240 min of operation. The concentration of Fe(II) fell below 5.49 mmol·L−1 when the simulated flue gas contained O2 at concentrations of 5% and 6.5%. NO removal efficiency was mainly influenced by the concentration of Fe(II)L in this mixed absorbent system. However, oxygen had limited influence on NO removal efficiency when concentration of Fe(II)L was over 5.5 mmol·L−1. It has been reported that the oxidation of Fe(II)EDTA is a first order reaction when the pH is above 3. The apparent oxidation rate constant has a liner relationship with O2 concentration.35 The reaction between Fe(II)EDTA and oxygen is as follows:17,18,36 k1

Fe(II)EDTA2 − + O2 (aq) → Fe(III)EDTA− + O2−

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First, Fe(II)EDTA reacted with O2. Then, Fe(II)EDTA changed to Fe(III)EDTA through electron transfer. The oxidation rate can be expressed as follows: −d[Fe(II)]/dt = k1[Fe(II)][O2 ] = K ′[Fe(II)]

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DO (dissolved oxygen) is here assumed to be constant. k1[O2] was described as K′: [Fe(II)] = Fe(II)EDTA, mmol ·L−1 [O2 ] = DO, mg·L−1 K′ = k1[O2 ], (mmol · L−1)−1 ·min−1 −d[Fe(II)]/dt = K ′[Fe(II)]

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The integral equation was then obtained as follows: ln([Fe(II)]/[Fe(II)]0 ) = −K ′t K ′ = apparent oxidized rate constant, min−1

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[Fe(II)] is here assumed to be equal to [Fe(II)L]. It can be calculated that ln ([Fe(II)] /[Fe(II)]0) increases linearly with increasing inlet O2 concentrations (data not shown). Table 1 shows that the apparent reaction rate constants K′ are obtained from the oxidation rate of Fe(II)L in Figure 5b accorded with pseudo-first-order conditions.36 The oxidation rate constant k1 of [Fe(II)L] solution was found to be 0.0892 ± 0.0048 kPa−1·h−1, which is faster than that with 0.016 kPa−1·h−1 of [Fe(II)Cit] solution reported by Xu.28 Moreover, this conclusion is consistent with previous studies37,38 on the oxidation reaction of Fe(II)EDTA. It was also reported that K′ increased linearly with increasing inlet O2 concentrations of 1.2%, 2.8%, 4.8%, and 6.8%, respectively. The oxidation rate constant of [Fe(II)EDTA] solution was found to be 0.1093 ± 0.0019 kPa−1·h−1.35,37 The oxidation rates of Fe(II) in this mixed absorbent system shows slower than that of the Fe(II)EDTA system. In the chemical absorption−biological reduction integrated process, the NOx removal efficiency strongly relies on the concentration of Fe(II)L in the scrubbing liquid.13 So, the low oxidation rate of Fe(II) is one of the approaches to keeping Fe(II)L on an appropriate level for NOx removal. Compared with the three mixed absorbent systems, the existence of citrate protects Fe(II) from oxidation. As a result, the oxidation rate of Fe(II) was decreased in Fe(II)Cit and Fe(II)EDTA mixed absorbent, while the NO removal efficiencies were almost the same as that in Fe(II) EDTA. 3.3.2. Effects of Inlet NO Concentration on NO Absorption. The concentration of NO is present in the range 400−1000 mg·m−3 in actual flue gas from power plants; so, it is

Figure 4. Effects of inlet O2 concentration on NO absorption and concentration of Fe(II): gas flow rate = 1000 mL·min−1; NO = 670 mg·m−3; O2 = 0−6.5% (v/v); initial Fe(II)L = 20 mmol·L−1; T = 323 K; pH = 6.80. (a) NO removal efficiency; (b) concentration of Fe(II). (■) 0%; (●) 1%; (▲) 3%; (▼) 5%; (◆) 6.5%. 4913

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2NO2 (aq) + H 2O → 2H+ + NO2− + NO3−

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2Fe(II)L2 − + NO2− + 2H+ ↔ Fe(II)L‐NO2 − + Fe(III)L− + H 2O

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2Fe(II)L‐NO2 − + 2Fe(II)L2 − + 4H+ → 4Fe(III)L− + N2(g) + 2H 2O

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3.3.3. Effects of Gas Flow Rate on NO Absorption. The effects of gas flow rate on NO absorption were also studied in this work. Figure 6 shows the results of experiments performed

Figure 5. NO absorbance capacities and concentrations of Fe(II) at different inlet NO concentrations: gas flow rate = 1000 mL·min−1; NO = 200−670 mg·m−3; O2 = 3% (v/v); initial Fe(II)L = 20 mmol·L−1; T = 323 K; pH = 6.80. (a) NO removal efficiency; (b) concentrations of Fe(II). (■) 200 mg·m3; (●) 335 mg·m3; (▲) 470 mg·m3; (▼) 670 mg·m−3.

vital to investigate NO removal efficiency and Fe(II) concentration impacted by the fluctuation of NO inlet concentrations. The results show, in Figure 5, the NO absorption experiments performed under different inlet NO concentrations with 3% oxygen in gas phase. The NO removal efficiency was maintained as high as 80% for 90 min and showed no significant change when inlet NO concentration is varied from 200 to 670 mg·m−3. A decrease in η of NO removal became apparent after 120 min of operation. The oxidation rate of Fe(II)L with high concentrations of NO was faster than that with low concentrations of NO. It seems that the oxidation rate of Fe(II)L could be accelerated by the NO present in flue gas, which can be explained as follows. First, Fe(II)L could chelate with NO to form Fe(II)L-NO, and this compound oxidizes more readily than Fe(II).38 Second, NO could be oxidized to NO2 by oxygen in flue gas. Then, the NO2 dissolved in the form of HNO2 or HNO3, which could oxidize Fe(II)L. In this way, the higher the concentration of NO, the faster the oxidation rate of Fe(II)L. The reactions are shown below:39 2NO(g) + O2 (g) → 2NO2 (g)

Figure 6. NO absorption capacity at different gas flow rates: gas flow rate = 800−2000 mL·min−1; NO = 670 mg·m−3; O2 = 3% (v/v); initial Fe(II)L = 20 mmol·L−1; T = 323 K; pH = 6.80. (a) NO removal efficiency; (b) concentration of Fe(II). (■) 800 mL·min−1; (●) 1000 mL·min−1; (◆) 1500 mL·min−1; (▼) 2000 mL·min−1.

at a gas flow rate of 800−2000 mL·min−1. The faster the gas flow rate, the lower the NO removal efficiency. NO removal efficiency decreased sharply to 20% after 240 min of operation when the simulated gas flow rate was 2000 mL·min−1. An efficiency of 40% was maintained after 270 min operation when the simulated gas flow rate was 800 mL·min−1. Possible reasons for these phenomena are as follows. First, the total amount of inlet NO and dissolved oxygen increased with the gas flow rate, meaning that any absorbent in solution would be consumed

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faster. Second, both the vapor/liquid ratio and the gas velocity increased as the gas flow rate increased, which made it more difficult for absorbent to contact with flue gas efficiency. 3.3.4. Effects of pH on NO Absorption. Previous studies22,34,40 have shown that the pH value of scrubbing liquid has strong impact on the NO removal efficiency. The NO removal efficiency and Fe(II) concentration at three pH values (3.47, 6.52, and 8.54) are shown in Figure 7. NO

be 2.1 × 105 M−1 and 1.19 × 105 M−1 at temperatures of 25 and 55 °C. At pH 8.0, the equilibrium constants of Fe(II)EDTA is reported to be 6.3 × 106 M−1, at which Fe(II)EDTA chelates with NO more easily.26,31 Fe(II)Cit can exist in several forms. Fe(II)Cit mainly exists over a pH range of 4.0−6.5 and Fe(II)Cit(OH) dominates at pH values above 6.5. When pH value increases past 7.5, NO absorption capacity decreases. This is because Fe(II)Cit(OH)2 forms mainly at high pH values, and NO was prevented from chelating with Fe(II)Cit hydroxylization of Fe(II)Cit(OH)2. 3.3.5. Effects of SO32− on NO Absorption. SO2 appears in actual flue gas within the range of 1500−9000 mg·m−3, which drops to 50−300 mg·m −3 after wet flue gas desulphurization (FGD), producing sulfite/bisulfate ions. These can cause undesirable side reactions in mixed absorption systems. For this reason, it is necessary to evaluate the effects of SO2 on this process. SO2 mainly has effects on NOx removal from flue gases in the form of SO32−. Therefore, the evaluation of the effect of SO2 on NO removal was made through the variation of the concentration of SO32−. Results in Table 2 show the effects of different initial concentrations (0−43 mg·L−1) of SO32− on NO absorption. After 8 h operation, the final nNO/nFe(II)L ratios were found to be 0.53, 0.82, 0.93, 1.14, 1.15, and 1.16, under the concentrations of SO32− 0, 1.5, 7.4, 14.8, 29.6, and 42.9 mg·L−1, respectively. NO absorption capacity was proportional to the concentration of SO32− at the low molar ratio of SO32−/Fe(II)L. There was a optimal ratio of nNO/nFe(II) (1.10), where the SO32−/Fe(II)L ratio reached 5. This is 2.3 times that observed without SO32−. When SO32−/ Fe(II)L was over 5, the NO absorption capacity remained stable. Previous studies10,41 have reported that the sulfite ions have an adverse effect on the denitrifying bacteria in pure Fe(II)EDTA systems, but SO32− showed a positive effect on this Fe(II)L system. One potential reason for this phenomenon is as follows. Citrate was originally used in the desulfurization of flue gas, and SO32− could reduce Fe(III) to Fe(II). In this way, it was possible to maintain certain concentrations of unoxidized Fe(II) for NO absorption. Therefore, it is possible that NOx and SO2 may be removed simultaneously in this mixed absorption system.

Figure 7. Effects of pH on NO absorption capacity: gas flow rate = 1000 mL·min−1; NO = 670 mg·m−3; O2 = 3% (v/v); initial Fe(II)L = 20 mmol·L−1; T = 323 K; pH = 3.47, 6.52, 8.54. (a) NO removal efficiency; (b) concentration of Fe(II). (■) 3.47; (●) 6.52; (▲) 8.54.

4. CONCLUSION In this work, we demonstrate a mixed Fe(II)Cit/Fe(II)EDTA absorbent in removing NO from simulated flue gas. The mixed absorbent shows a high NO absorption capacity and absorption rate similar to a Fe(II)EDTA absorbent. 80% NO removal efficiency can be maintained for more than 2 h with this mixed absorbent at an inlet NO concentration of 670 mg·m−3, which is almost as effective as the Fe(II)EDTA absorbent at the same Fe(II) concentration. The oxidation rate constant of Fe(II) in the mixed absorbent is found to be 0.0892 ± 0.0048 kPa−1·h−1, which is lower than that of Fe(II) EDTA when the concentration of inlet O2 is below 6.5%. The optimal molar ratio of Fe(II)Cit to Fe(II)EDTA is 3:1. The consumption of EDTA can be reduced by 75% relative to other systems. This system could reduce the secondary pollution caused by EDTA and the operational costs. In addition, we observe that SO32− shows positive effects in this mixed absorbent system. The optimal pH value for the scrubbing liquid is found to be about 7.0.

removal efficiency decreases slowly for pH 6.52 and pH 8.54. At a pH of 3.47, NO removal efficiency decreased rapidly. It can be concluded that Fe(II)L is suitable for absorbing NO under alkaline conditions. However, the oxidation rate of Fe(II)L was decreased apparently at pH values more than 6.52. So, it is better to keep the pH value below 7.5 (pH = 6.8−7.0), which is also suitable for microbial growth and beneficial to absorption capacity. The effects of pH and temperature on NO absorption efficiency are mainly determined by the equilibrium constants of Fe(II)Cit-NO and Fe(II)EDTA-NO. It is reported that, when pH remains between 4.0 and 6.5, the equilibrium constants of Fe(II)Cit-NO are 4.9 × 104 M−1 at a temperature of 25 °C and 1.3 × 104 M−1 at 55 °C.26,31 At a pH value of 8.5, the equilibrium constants of Fe(II)EDTA-NO are reported to 4915

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Corresponding Author

*Tel.: +86 571 8898 2038. Fax: 86-571-87952771. E-mail: w_ [email protected]. Tel.:+86-153-1469-2819. E-mail: [email protected]. cn. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was sponsored by the National High Technology Research and Development Program of China (No. 2006AA06Z345), National Natural Science Foundation of China (No. 20676120).

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dx.doi.org/10.1021/ef300538x | Energy Fuels 2012, 26, 4910−4916