Spray Absorption and Electrochemical Reduction of Nitrogen Oxides

Article pubs.acs.org/est

Spray Absorption and Electrochemical Reduction of Nitrogen Oxides from Flue Gas Qingbin Guo,† Tonghua Sun,† Yalin Wang,† Yi He,‡ and Jinping Jia†,* †

School of Environmental Science and Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, P. R. China ‡ Department of Sciences, John Jay College and the Graduate Center, The City University of New York, New York 10019, United States S Supporting Information *

ABSTRACT: This work developed an electrochemical reduction system which can effectively scrub NO× from flue gas by using aqueous solution of Fe(II)(EDTA) (ethylenediaminetetraacetate) as absorbent and electrolyte. This new system features (a) complete decomposition of NOX to harmless N2; and (b) fast regeneration of Fe(II)(EDTA) through electrochemical reaction. The Fe(II)(EDTA) solution was recycled and reused continuously during entire process, and no harmful waste was generated. The reaction mechanism was thoroughly investigated by using voltammetric, chromatographic and spectroscopic approaches. The operating conditions of the system were optimized based on NOX removal efficiency. Approximately 98% NO removal was obtained at the optimal condition. The interference of SO2 in flue gas and the system operating stability was also evaluated.

1. INTRODUCTION Nitric oxide (NO) and nitrogen dioxide (NO2), jointly referred to as NOX,1 are important pollutants in lower atmosphere. It has been known for decades that NOX, together with sulfur dioxide (SO2), are the major contributors for acid rains that cause harmful damage to buildings, forests, crops and aquatic life.2,3 Combined with volatile organic compounds (VOCs) via photochemical reactions, NOx are responsible for formation of ground-level ozone and urban smog, which may potentially trigger serious respiratory diseases.4,5 With the increasing knowledge of pollution and public health problems caused by NOX, stringent regulations on NOX emission have been passed in many countries.6,7 Several abatement techniques are currently available to treat NOX emission from both stationary and mobile sources.8−15 The most widely used technique is selective catalytic reduction (SCR) process, by using which high treatment efficiency (above 85%) can be obtained and the reaction product is harmless N2.16,17 This approach, however, bears shortcomings such as use of high temperature, catalysts deactivation, ammonia escape and expensive operation costs.18−20 Another approach that is of research and application interest is wet process by adding Fe(II)(EDTA) (EDTA, ethylenediaminetetraacetate) into the scrubbing solution to improve the solubility of NO via formation of Fe(II)(EDTA)(NO).21−25 High NO removal efficiency could be obtained, however, Fe(II)(EDTA) was easily oxidized to Fe(III)(EDTA) and lost its binding capability to NO.26−28 The concentration of active Fe(II)(EDTA) in the scrubbing solution decreased quickly as the absorption process, and so did the consequent NO removal efficiency. Regeneration of Fe(II)(EDTA) in scrubbing solution thus has become © 2013 American Chemical Society

a fundamental issue that has to be addressed when using this technology. Several reducing reagents such as hydrazine,29 Na2S2O430 and polyphenolic compounds31 have been used to reduce Fe (III) (EDTA). Although with certain success in laboratory test, low regeneration rate of Fe(II)(EDTA) and consumption of large amount of reducing reagents are two hurdles impeding industrialization of these methods.32−36 An additional issue along with the use of conventional wet denitrification process is that the typical reaction product is nitrous oxide (N2O),37−39 which has long duration effect in depletion of the ozone layer40 when discharged into atmosphere and raises another environmental concern. To address these two issues, this work specifically designed, optimized and evaluated a new electrochemical reactor to remove NOX from flue gas. Aqueous Fe(II)(EDTA) was used as absorbent and electrolyte. This new system features: (a) complete decomposition of NOX to harmless N2; and (b) fast regeneration of Fe(II)(EDTA), which shortens the recycle time of Fe(II)(EDTA) solution and minimizes the use of other chemicals. Figure 1 illustrates the reactor design and the procedure of operation. The whole system can be divided into four parts as shown in Figure1 from left to right: (1) simulated flue gas synthesis unit; (2) Fe(II)(EDTA) spray and NOXabsorption region; (3) NOX reduction and Fe(II)(EDTA) regenerating region; and (4) chemical analysis unit. The heart Received: Revised: Accepted: Published: 9514

March 6, 2013 June 10, 2013 July 22, 2013 July 22, 2013 dx.doi.org/10.1021/es401013f | Environ. Sci. Technol. 2013, 47, 9514−9522

Environmental Science & Technology

Article

Figure 1. Schematic design of the electrochemical reactor used for removing NOX from flue gas. This figure provides the top view of the Fe(II)(EDTA) spray and NOX absorption region and NOX reduction and Fe(II)(EDTA) regenerating region. The front view of these two regions and cross-section of F−F′ are shown in SI (S1 and S2). (M: countercurrent variable speed DC motor, A: cathode, B: particle filter, C: ice box impingement condenser, D: anode, E: proton exchange membrane).

at low temperature. It also has such advantages as high NOX removal efficiency, generating harmless reaction products, ease of installation and operation, and minimum use of absorbant through chemical recycling.

of the reactor was two corrugated impellers installed in NOX reduction and Fe(II)(EDTA) regenerating region. The first impeller rotated at high speed and the second one kept still during operation. These two impellers served as working electrodes where NOX was decomposed to N2 and Fe(III) (EDTA) was converted to Fe(II)(EDTA). The high rotating speed enhanced turbulence of liquid and gas phase, improved the contact of liquid and gas and facilitated forming liquid film on the surface of impellers and accelerating the decomposition of NOX and regeneration of Fe(II)(EDTA). The nonrotating impeller was a demister to settle down fine droplets in the gas stream for recovering the aqueous absorbant and electrolyte. During operation, Fe(II)(EDTA) solution was first mixed and sprayed with simulated flue gas into the Fe(II)(EDTA) spray and NOX absorption region, where fine Fe(II)(EDTA) mist was formed and NOX was complexed with Fe(II)(EDTA) and absorbed by the mist. When the fine mist drifted to the NOXreduction and Fe(II)(EDTA) regenerating region and contacted with the surface of the corrugated impellers, Fe(II)(EDTA)(NO) was electrochemically converted to N2 and Fe(II)(EDTA) (see below for reaction mechanism). The corrugated surfaces helped convert the fine mist to solution again. The regenerated Fe(II)(EDTA) solution was collected in a storage tank and ready to be pumped back and reused. Because this was a close system, there was no liquid waste discharge and therefore potential secondary pollution was avoided. At last, the treated flue gas was monitored before it was discharged into the air. The working mechanisms of the electrochemical process of NOX reduction and Fe(II)(EDTA) regeneration were investigated. Critical operating parameters such as Fe(II)(EDTA) concentrations, pH values, voltages, liquid−gas ratio, rotating speed of corrugated impellers and concentrations of NO and SO2 in flue gas were optimized and the treatment performance was evaluated. Compared with existing NO X removal techniques, this method does not need catalysts and operates

2. EXPERIMENTAL SECTION 2.1. Reagents. 99.999% NO, 99.9% SO2, 99.9% N2, and 99.999% Ar were provided by Dalian Date Gas Co. Ltd., China. Na2EDTA and FeSO4·7H2O were purchased from Sinopharm Chemical Reagent Co. Ltd., China. Na2SO4 was from Shanghai Lingfeng Chemical Reagent Co. Ltd., China. All chemicals were analytical reagent grade and used without any further purification. 2.2. Denitrification Procedure. The setup of the electrochemical reactor was shown in Figure. 1. The main body of the reactor was made of plexiglass cuboids, which was 125 cm wide, 715 cm long, and 165 cm high and had an effective volume of 11.35 L. Simulated flue gas was synthesized by mixing proper amount of N2, NO, SO2 and air. NO was used in the simulated flue gas since it is the major component of NOX (90−95%) and can reasonably present NOX. To mimic the actual situation in industrial process, the simulated flue gas was heated to 50 °C by an electrical heating tape before entering into Fe(II)(EDTA) spray and NOX absorption region. The flow rate of each individual gas was controlled by a mass flow controller (CS200, Sevenstar Electronics Co., Ltd. Beijing). The total flow rate of simulated flue gas was 20 L·min−1. All gas flow meters and rotameters were calibrated with a bubble meter (Humonic digital flow meter 650) or a dry gas meter (Shimagawa DKSCF-T) before use. A Venturi tube linking simulated flue gas, absorption solution and the reactor was used to form aerosol. When passing the Venturi tube, aqueous Fe(II)(EDTA) solution was carried by the heated flue gas and sprayed into the reactor chamber (Fe(II)(EDTA) spray and NOX absorption region), where 9515

dx.doi.org/10.1021/es401013f | Environ. Sci. Technol. 2013, 47, 9514−9522


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Fe(II)(EDTA) aerosol was fully mixed with the flue gas and NOX was complexed and absorbed by the fine mist. Two identical sets of corrugated impellers were used as cathode. The impeller was composed of 10 titanium sheets with approximate surface area of 0.628 m2 for each. The titanium sheets were arranged in parallel. Each impeller was driven by a countercurrent variable speed DC motor and the rotating speed was calibrated with a digital tachometer (JDTong DT-2234C +). The anode for this reactor was a rectangle graphite sheet with surface area of 0.01 m2. It was immersed in saturated sodium sulfate solution. The anode region was separated from the cathode region by a Nepem-117 proton exchange membrane (BEST Industrial and Trading Co., Ltd.). Voltage was provided using a DC regulated power supply (Shanghai Liyou Electrification CO. Ltd., 0−50 V) between the cathode and anode. In order to simulate the industrial operation, Fe(II)(EDTA) solution was prepared by adding equimolar amount of FeSO4·7H2O and Na2EDTA into tap water. Sodium sulfate solution was prepared by dissolving sodium sulfate anhydrous in deionized water. The Fe(II)(EDTA) solution was continuously cycled by a pump from storage tank to Fe(II)(EDTA) spray and NOX absorption region. pH value of the Fe(II)(EDTA) solution was monitored using a pH meter (Shanghai Precision & Scientific Instrument Co. Ltd.) and maintained at pH 5.5 by adding either concentrated sulfuric acid or 1 M sodium hydroxide. 2.3. Experimental Setup for Investigation of the Reduction Mechanism. A specific electrochemical chamber (Figure 2) was set up and used to investigate the mechanism of

between the anode and the cathode was 6 cm. The electrolytes in the anode region and the cathode region were saturated Na2SO4 solution and Fe(II)(EDTA)solution/Fe(II)(EDTA)(NO), respectively. Fe(II)(EDTA)(NO) solution was obtained from the Fe(II)(EDTA) spray and NOXabsorption region. The potential between the two electrodes was controlled by a DC power supply. The solution was agitated using a magnetic stirring plate during experiment. The electrolyte was sampled through two sampling ports located in cathode and anode region. The same setup, but with some modification, was used for investigation of the gaseous reduction products as well. The system was sealed gastight. The anode region was filled with saturated Na2SO4 solution, about 2/3 amount. The sampling hole of the cathode region was plugged by a silicon stopper, through which an inlet tubing, an outlet tubing and the electrode were fitted. The cathode region was first fully filled with Fe(II)(EDTA)(NO) solution, and then argon gas was pumped in through the inlet tubing and the solution was driven out from the outlet tubing. The argon gas flow was stopped when the solution level decreased to approximately 2/3 amount and the inlet and outlet was sealed. The headspace of the cathode Fe(II)(EDTA)(NO) solution was argon gas. Then the reduction reaction started and the gaseous reduction products of Fe(II)(EDTA)(NO) in the cathode area were sampled at 0, 1, and 2 h with a 1 mL gastight syringe. The gaseous sample (1 mL) was injected into a gas chromatography (GC-14B, SHIMADZU, Japan) immediately after sampling. The chromatographic conditions were injection port temperature 100 °C, using 5A molecular sieve column (Quan Dao company, Shanghai) operated isothermally at 60 °C, and TCD detector temperature at 120 °C. The carrier gas was 99.999% argon. 2.4. Chemical Analysis. The contents of NO, O2 and SO2 in inlet and outlet gas were measured using a flue gas analyzer (KM900, Kane International Co.). This device could measure O2, SO2 and NO in the ranges of 0−21.0 vol. %, 0−5000 ppm and 0−5000 ppm, respectively, with correspondent resolution of 0.1 vol. % and 1 ppm. N2O and NH3 were quantitatively determined using an online Fourier transform infrared spectrometer (FTIR) (Nicolet E. S. P. 460, ThermoFisher). The outlet flue gas was monitored every 5 min when the system was stable. An ice box impingement condenser (Pyrex) and a particle filter (Balston 95S6, Parker) were installed between the sampling port and the gas analyzer. A trap equipped with desiccant (silica gel) was used to remove moisture before the gas was analyzed. The concentrations of NO3−, NO2− and NH4+ in the regenerated Fe(II)(EDTA) solution were determined using ion chromatography (Metrohm 882, Switzerland, column: METROSEP A SUPP; size: 4.0 × 250 mm; mobile phase: 3.2 mM Na2CO3/1.0 mM NaHCO3; flow rate: 0.70 mL·min−1; temperature: 35 °C). Cyclic voltammetry was conducted using CHI382b electrochemical analyzer (Shanghai Chenhua Instruments), which used titanium as the working electrode, a graphite sheet as the auxiliary electrode, and a saturated calomel electrode (SCE) as the reference electrode. The scan range was from −1.8 to 5.1 V and the scan rate was 50 mV s−1. There were 200 mL of Fe(II)(EDTA)(NO) in the cathode region and 200 mL of saturated Na2SO4 in the anode region. All measurements were repeated at least twice, and the average data was used in the figures and tables.

Figure 2. The electrochemical chamber used for studying the reaction mechanism: (1) DC power supply, (2) sampling port, (3) the cathode Ti, (4) proton exchange membrane, (5) the anode graphite, (6) electrolytic cell, (7) magnetic stirring plate, (8) cover, (9) magnetic stirrer, and (10) perspex divide.

NOX reduction and Fe(II)(EDTA) regeneration. An plexiglass cell with total volume of 600 mL was equally divided by a proton exchange membrane to two compartments. The cathode was a semicircular titanium foil with surface area of 40 cm2 and the anode is a graphite sheet with equal surface area. Before it was used for the first time, the titanium foil was polished with 600-mesh sandpaper, cleaned by soaking in an ultrasonic water bath for 5 min, and then rinsed with ultrapure water. The anode was rinsed with ultrapure water. The distance 9516



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catholic peaks were −1.0 V and −0.5 V. Fe (II) (EDTA) (NO) was reduced at −1.0 V (peak C). Peaks A and B were a pair of redox peak, where Fe (II) (EDTA)/Fe(III)(EDTA) was oxidized or reduced to Fe(III)(EDTA)/Fe (II) (EDTA), and detailed investigation was listed in part: Reduction of Fe (III) (EDTA) solution. Hydrogen was produced slowly at −1.5 V (peak D). Figure 4 clearly showed that the gaseous reduction

3. RESULT AND DISCUSSION 3.1. Electrochemical Reduction of Fe (II) (EDTA) (NO). NOX removal process was affected by several parameters such as the concentration and pH of Fe(II)(EDTA) solution, potential applied between the anode and the cathode, rotating speed of the corrugated impellers, liquid−gas ratio (L/G), NO and SO2 concentrations in the flue gas. At optimal operation conditions using 60 mM pH 5.5 Fe(II)(EDTA) solution, voltage of 6 V, L/G ratio of 4L·m−3, rotating speed at 157 rpm, NO concentration at 850 mg·m−3 and SO2 concentration at 660 ppm, the reaction mechanism was investigated and described here. The process optimization was summarized in Section 3.2. When flue gas was in contact with Fe(II)(EDTA) mist, NO was complexed and absorbed by Fe(II)(EDTA):41 Fe(II)(EDTA) + NO → Fe(II)(EDTA)(NO)

(1)

After that Fe (II) (EDTA) (NO) was reduced by electrons provided by DC power supply and N2 and free Fe (II) (EDTA) were formed. Fe (II) (EDTA) (NO) may also be reduced by H2, which was generated from electrolysis of water. 2Fe(II)(EDTA)(NO) + 4H+ + 4e → N2 + Fe(II)(EDTA) + 2H 2O

(2) Figure 4. Gas chromatogram of the reduction products of Fe (II) (EDTA) (NO).

Fe(II)(EDTA)(NO) + H 2 → N2 + Fe(II)(EDTA) + H 2O

(3)

product of the Fe (II) (EDTA) (NO) solution N2 and H2 was formed during the process as well. Before the reaction started (0 h), there was no N2 and H2 in the headspace. However, the amount of these two gases continuously increased in the following two hours. Quantity of the Nitrous Compounds. The possible nitrous compounds generated in the process were NO2−, NO3−, NH4+, N2O, N2, and NH3, among which NO2− and NO3− were oxidation products of NO, and others were reduction products. By measuring the amount of outlet NO, NO2−, NO3−, NH4+ and N2O, the amount of N2 in outlet was calculated. For a typical inlet gas with NO concentration of 850 mg·m−3 and flow rate of 20 L·min−1, the amount of NO in 1 min flow was 0.57 mmol, the outlet NO amount was measured as 0.01 mmol, and the amount of other individual compounds were illustrated in Figure 5. NO2− concentration in the electrolyte was 0.04 mmol through ion chromatography. NO3− in the electrolyte, N2O and NH3 in outlet gas were not detected through IC and FTIR. The conventional Fe(II)(EDTA)(NO) reduction process may form N2O, however, we did not observe N2O in outlet by FTIR measurement, which suggested a complete reduction of Fe(II)EDTA(NO). (The ion chromatogram and FTIR spectrum were attached in the Supporting Information (SI) Figure S3 and S4.) The above phenomenon can be explained by reactions 2 and 3 and they follow these equations:42−47

At the same time, the reaction of the anode was 2H 2O − 4e → 4H+ + O2

(4)

H+ would move to the cathode region through the Nepem-117 proton exchange membrane and participate in the reduction and regeneration of Fe(II)(EDTA) (NO), as shown in eq 2. The reaction mechanism was verified below by cyclic voltammetry and gas chromatography observation of the gaseous products. The quantity of the potential nitrogencontaining products was determined using various analytical techniques and the mass balance calculation. As shown in Figure 3, the anodic peak was around potential of 0.5 V and the

2NO3− + 10e + 12H+ → N2 + 6H 2O

(5)

NO3− + 2e + 2H+ → NO2− + H 2O

(6)

−

+

2NO2 + 6e + 8H → NO2 + 4H 2O Figure 3. Cyclic voltammogram of Fe(II)(EDTA) (NO) with a scan rate of 50 mV s−1 (A: oxidation peak of Fe (II) (EDTA), B: reduction peak of Fe(III)(EDTA) C: reduction peak of Fe (II) (EDTA) (NO) and D: reduction peak of hydrogen).

−

2NO2 + 3H 2 → N2 + 6OH

−

2NO2− + 2H 2 → N2 + 4OH− 9517

(7) (8) (9)



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Figure 5. Estimation of the N2 amount through mass balance calculation.

Fe(II)(EDTA)(NO) + SO32 − → Fe(II)(EDTA) + N2O + SO4 2 − +

N2O + 2H + e → N2 + H 2O

RE = (10) (11)

4Fe(II)(EDTA) + 2O2 + 4H+ (12)

2Fe(II)(EDTA) + 2NO + 2H+ → 2Fe(III)(EDTA) + N2O + H 2O

(13)

2Fe(II)(EDTA)(NO) + 2Fe(II)(EDTA) + 4H+ → Fe(III)(EDTA) + N2 + H 2O

(14)

According to Figure 3, the anodic peak and the cathodic peak were around 0.5 V and −0.5 V respectively. For a typical voltammetric curve, if the ratio between i pa and i pc approximately equals to 1, it suggests a reversible process associated with the anodic peak and the cathodic peak.33 In this study, both ipa and ipc were ∼2 mA, and it gave |i pa| |i pc|

=

2mA =1 2mA

(15)

Peak B in Figure 3 suggested Fe(III)(EDTA) was reduced to Fe(II)(EDTA): Fe(III)(EDTA) + e → Fe(II)(EDTA)

(17)

where Cin and Cout with unit of mg·m−3 were the NO concentration in the inlet and outlet gas flow, respectively. 3.2.1. Effect of the Fe(II)(EDTA) Concentration on NO Removal. The NO absorption capability was affected by the Fe(II)(EDTA) concentration (SI Figure S8). For simulated flue gas containing 850 mg·m−3 of NO, 5% oxygen and operating at 323 K, the NO removal efficiency increased quickly from 15.7 to 52% with the increase of Fe(II)(EDTA) concentration form 20 to 60 mM. However, the improvement significantly slowed down with further increase of Fe(II)(EDTA) concentration to 160 mM. Judged by the consideration on operating cost, 60 mM Fe(II)(EDTA) solution was used in following experiments. 3.2.2. Effect of pH Value of Fe(II)(EDTA) Solution on NO Removal. In the denitrification process using aqueous Fe (II) (EDTA), pH of aqueous Fe (II) (EDTA) directly affects the NO removal efficiency and the operating stability of the process. For treating gas stream with 850 mg·m−3 of NO and using 60 mM Fe (II) (EDTA) solution, SI Figure S9 showed that NO removal efficiency improved gradually with the increase of pH. The NO removal efficiency reached the maximum, 62.8%, at pH 5.5, and then decreased from pH 5.5 to 8. The changes of NO removal efficiency with pH value of aqueous Fe (II) (EDTA) were related to the complex forms of aqueous EDTA and FeSO4. EDTA was a weak acid with four carboxylic groups. In strong acidic solution, the main complex that reacts with FeSO4 was Fe (HEDTA). Unfortunately, Fe(HEDTA) had no complexation with NO and resulted in the decreased NO removal efficiency. When pH value increased gradually, the main complex turned to be Fe(II)(EDTA) which had strong complexation with NO. The changes of the complex formed with pH are following eq 18.50

Reduction of Fe (III) (EDTA) Solution. Since the flue gases generally contain oxygen and NO, Fe(II)(EDTA) is likely to be partially oxidized to Fe(III)(EDTA).48

→ 4Fe(III)(EDTA) + H 2O

c in − cout × 100% c in

(16)

Experimentally, we observed that Fe(II)(EDTA) solution was light yellow originally, but was oxidized to brown color Fe (III) (EDTA). However, after the brown solution was reduced and regenerated, the color turned to light yellow again. This phenomenon also proved the regeneration of Fe(II)(EDTA).49 The information for current rate, current density and electrolysis temperature of the electrochemical reaction was provided in SI S5−S7. 3.2. Process Optimization. Operating parameters significantly affects the treatment performance. Evaluated by NO removal efficiency, critical parameters were investigated and the optimum condition was selected. The denitrification process was evaluated by NO removal efficiency (RE) using following equation:

+H +

Fe2 + + EDTA ⎯⎯⎯⎯→ Fe(HEDTA)Fe(II)(EDTA)

(18)

2+

With further increase of solution pH, Fe might react with OH− in alkaline solution (eq 19) and resulted in a decreased NO removal efficiency. Through this experiment, pH 5.5 was selected as the optimum value for Fe(II)(EDTA) solution. Fe2 + + 2OH− → Fe(OH)2

(19)

3.2.3. Effect of Voltage on NO Removal. DC voltage ranging from 2 to 12 V was applied between the anode and the cathode to investigate the effect of voltage on treatment 9518



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which are common SO2 concentration found in real flue gas,53−55 were introduced to the simulated flue gas. Figure S14 in SI shows that NO removal efficiency decreased when SO2 concentration was low, but it increased with further increase of SO2 concentration. SO32‑ ions produced by dissolving SO2 in aqueous solution (eq 20) competes with NO in reacting with Fe(II)(EDTA) (eq 21).

efficiency. Compared with no voltage, the removal efficiency significantly improved from about from 63% to 85%. With the presence of electrons, fresh Fe(II)(EDTA) was generated continuously through reduction of Fe(III)(EDTA) (eq 16) and decomposition of Fe(II)(EDTA) (NO) (eqs 2 and 3). No big difference on NO removal was observed when various voltages were applied to the electrodes. However, for electrochemical reduction of nitrogen-containing compounds, such as Fe(II)(EDTA) (NO), NO3−, and NO2−, the electrolysis time was decreased significantly when voltages increased from 2 V to 12 V. Due to the time and cost consideration, 6 V was used in this study. The interferences of ions in tap water to the electrochemical reactions were also investigated by comparing results obtained using solution prepared by deionized water. Similar results were obtained (SI Figure S10), which suggested tap water can be used for preparing Fe (II) (EDTA) solution. This feature provided convenience for future industrialization of this technique. 3.2.4. Effect of Liquid−Gas Ratio on NO Removal. Liquid− gas (L/G) ratio, defined as the ratio of absorption solution flow and flue gas flow, has been shown as an important parameter in adjusting absorption property of wet desorption device.51 As shown in Figure S11 of SI, with L/G ratio increased from 0.67 to 4.5, the NO removal efficiency improved from 26.6% to 58.2%. When using higher L/G ratio, relatively large amount of Fe(II)(EDTA) was presented in the spray and thus enhanced the overall complexing and absorption capability. Although the NO removal efficiency increased with the increase of L/G ratio, however, a ratio of 4 was used to balance the operating cost and treatment efficiency. 3.2.5. Effect of the Rotating Speed of Corrugated Plates on NO Removal. Improving mass transfer is critical to remove gas from liquid matrix. The NO removal is a liquid film controlled process due to the low solubility of NO in water,52 and therefore decreasing the liquid film thickness and refreshing contact surface between gas and liquid phase frequently can improve NO transfer. Various rotating speed of the corrugated plates were evaluated to investigate its effect on NO removal (SI Figure S12). NO removal efficiency increased almost linearly with the increase of rotating speed. At rotating speed of 157 rpm, NO removal efficiency (83%) was about 1.4 times compared with no rotating was used. 3.2.6. Effect of NO Concentration in the Inlet Gas. The effect of NO concentration in the inlet gas on NO removal was investigated using 60 mM Fe(II)(EDTA) aqueous solution, pH of 5.5, L/G of 4, rotating speed of 157 rpm, and voltage of 6 V. To mimic the actual flue gas, O2 concentration was adjusted to 5% and the gas temperature was maintained at 323K. The NO removal efficiency dropped from 96% to 92% when NO concentration in the inlet gas increased from 200 to 1500 mg·m −3 (SI Figure S13). With the increase of NO concentration, the available Fe(II)(EDTA) absorbing capacity decreased until eventually saturated, which resulted in a decreased treatment efficiency. In actual operation, higher Fe(II)(EDTA) concentration should be considered when relative high NO content is found in inlet gas. 3.2.7. Effect of SO2 Concentration in Inlet Gas on NO Removal. SO2 commonly coexists with NO in flue gas. It is important to study the effect of SO2 (sulfur dioxide) on NO removal because it reacts with Fe(II)(EDTA) as well, and sulfite/bisulfite formed by dissolving SO2 in the aqueous solution may interfere complexing and absorption of NO by Fe(II)(EDTA) solution. In this study, 100−800 ppm SO2,

SO2 + H 2O → H 2SO3 → HSO3− + H+ → SO32 − + 2H+

Fe(II)(EDTA) + SO32 − → Fe(II)(EDTA)(SO32 −)

(20) (21)

However, with the presence of NO, Fe(II)(EDTA) does not react with SO32‑ to a great extent because of marked reactivity of NO with Fe(II)(EDTA). As a result, the NO removal efficiency decreased slightly at the beginning of the curve. When SO2 concentration in the system was higher, the formed complex Fe(II)(EDTA)(SO32‑) in turn facilitated the removal of NO through driving following reaction (eq 22) to the right, leading to an increased NO removal efficiency. Similar result was obtained by other researchers as well.56 Fe(II)(EDTA)(SO32 −) + NO → Fe(II)(EDTA)(SO32 −)(NO)

(22)

Fe(II)(EDTA)(SO32‑)(NO)

When was converted to Fe(II)(EDTA)(NO) (eq 23), higher SO32‑ concentration in solution drove eq 10 to right and resulted in improved recovery of Fe(II)(EDTA). 57 Fe(II)(EDTA)(SO32 −)(NO) → Fe(II)(EDTA)(NO) + SO32 −

(23)

In addition, SO2 would also directly react with NO before forming Fe(II)(EDTA)(NO) (eq 24),58 suggesting high SO2 content in flue gas played a positive role in NO removal. SO2 + 2NO → SO3 + N2O

(24)

SO3 + H 2O → H 2SO4

(25)

The electrochemical process further facilitated the reduction of N2O to N2 (eq 11, 26), and Fe(II)(EDTA)(SO32‑)(NO) to Fe(II)(EDTA) and N2 (eq 26). N2O + H 2 → N2 + H 2O

(26)

2Fe(II)(EDTA)(SO32 −)(NO) + 2e → 2Fe(II)(EDTA) + N2 + 2SO4 2 −

(27)

3.3. System Stability. The operating stability of the system was investigated by continuously treating simulated flue gas for 10 h at optimal condition. Operating conditions were NO concentration 850 mg·m−3, SO2 concentration 660 ppm, Fe(II)(EDTA) concentration 100 mM, pH of Fe(II)(EDTA) solution 5.5, L/G of 4, rotating speed of 157 rpm, voltage of 6 V, 5% O2 and temperature at 323 K. NO concentration in outlet gas was monitored on very one hour. As shown in Figure 6, the NO removal efficiency (about 97.5%) was very stable over the 10 h experiment. The result was very promising for future application of this technique in industry. To keep pH stable at 5.5, pH of absorption solution was monitored by an 9519



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AUTHOR INFORMATION

Corresponding Author

*Phone: 086-21-54742817; fax: 086-21-54742817; e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The present work is supported by the Natural Science Foundation of China (no.20937003 and 50878126) and Ph.D. Program Foundation of Ministry of Education of China (no.20090073110033).

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Figure 6. Investigation of system stability of NO removal in simulated flue gas (NO=850 mg·m−3, SO2 = 660 ppm, Fe(II)(EDTA) = 100 mM, pH 5.5, L/G = 4, rotating speed = 157 rpm, voltage = 6 V, O2 = 5%, T = 323 K) Small vertical scale was used to enlarge the view of fluctuation.

online pH meter and adjusted by adding concentrated sulfuric acid when pH reached 6.5. Because of the entrainment of absorption solution by flue gas, small amount (0.005% of original volume) of Fe(II)(EDTA) solution was added during operation to maintain the amount of absorption solution. 3.4. Comparison with Other NOX Abatement Techniques. This study was compared with other Fe(II)(EDTA) based NOX abatement techniques reported in literature. As shown in Table 1, the NO removal efficiency obtained by this Table 1. Comparison with Other Fe(II)(EDTA) Based NOX Abatement Techniques reactor type packed scrubber turbulent contact absorber scrubber circulating fluidized bed spray/ electrochemical reduction

NO concentration 500−900 mg·m−3 267 mg·m−3

removal efficiency

reference

5.8

70%

52

2.76−6.44

50−70%

59

65%

31

65.5%

60

97.5%

this study

L/G (L·m−3)

up to 1300 mg·m−3 850 mg·m−3

4

work was significantly higher than other methods. The additional advantages offered by this setup including simple reactor design, easy to use, generating harmless final products, minimizing the use of chemical reagents and avoiding secondary pollution.

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REFERENCES

ASSOCIATED CONTENT

S Supporting Information *

The front view and F−F′ section draw of the chamber; ion chromatogram of the electrolyte; FTIR spectrum of the outlet flue gas; Effects of Fe(II)(EDTA) concentration, pH, voltages, liquid−gas ratio, rotating speed, NO concentration, SO2 concentration and electrolysis temperature on NO removal efficiency; Current density and current rate. This material is available free of charge via the Internet at http://pubs.acs.org. 9520



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Spray Absorption and Electrochemical Reduction of Nitrogen Oxides

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