Study on Simultaneous Removal of NOx and SO2 with NaClO2 in a

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Study on Simultaneous Removal of NOx and SO2 with NaClO2 in a Novel Swirl Wet System Amin Pourmohammadbagher, Esmaeel Jamshidi, Habib Ale-Ebrahim,* and Sassan Dabir Department of Chemical Engineering, Amirkabir University of Technology (Tehran Polytechnic), Petrochemical Center of Excellency, Tehran 15875-4413, Iran ABSTRACT: This work attempts to utilize sodium chlorite to clean up NOx and SO2 gases simultaneously from flue gas in a novel swirl scrubber system. Experiments were carried out to evaluate the effects of various operating parameters such as initial SO2 concentration, scrubbing medium pH, sodium chlorite concentration, and liquid and gas volumetric flow rates at 35 °C. In addition, reaction mechanisms of simultaneous denitrification and desulfurization using sodium chlorite in acidic and basic solutions are proposed. Complete oxidation of NO into NO2 occurred using 0.2 M sodium chlorite solution as the scrubbing medium. Complete (100%) SO2 and 81% NOx removal efficiencies were achieved under optimized conditions. The NOx removal increased with decreasing pH. Input SO2 enhanced the NOx absorption. The effect of the SO2 concentration on NOx removal was more intense at higher pH values. Using sodium chlorite as the scrubbing medium in the novel swirl scrubber was found to be quite promising for the simultaneous removal of NOx and SO2.

1. INTRODUCTION Nitrogen oxides (NOx) and sulfur dioxide are greenhouse gases that accumulate in the atmosphere with other greenhouse gases, causing a gradual rise in Earth’s temperature. This will lead to increased risks to human health, a rise in sea level, and other adverse changes to plant and animal habitats. The formation of acid rain and the resultant acidification caused by SO2 and NOx is one of the crucial air pollution problems. Typically, 9095% of nitrogen oxides by volume are emitted as NO and 510% as NO2. Under normal conditions of 25 °C and atmospheric pressure, NO is an odorless gas, whereas NO2 is a pungent reddish brown gas. They are both noxious and contribute to the formation of photochemical smog and resultant atmospheric pollution. SO2 gas is also an important air pollutant with destructive effects on forests, agriculture, and river ecology. Emissions of SO2 to the atmosphere form acid rain that can result in critical environmental problems. As a result, for controlling air pollution by these gases, the simultaneous removal of NOx and SO2 is an ongoing process. Engineers mostly design separate air pollution control devices for individual gas emission treatments.1,2 NOx treatment processes can generally be characterized as either dry or wet techniques. The dry techniques are further classified as selective catalytic reduction and non selective catalytic reduction. The scarcity and expensive nature of catalysts have hindered the acceptance of these methods. In wet techniques, wet scrubbing absorption in which the flue gas is subjected to a liquid wash to remove NOx pollutants is economically the most competitive treatment method.3 Likewise, for SO2, wet desulfuration using alkaline solutions as absorbents is the most widely used process.2 Wet scrubbing is one of the best techniques for the simultaneous removal of NOx and SO2. Conventional wet processes cannot remove NOx efficiently because of the low solubility of NO in aqueous solutions. As NOx gases are mostly NO, the oxidation of NO to NO2, which is soluble in alkaline solutions, is a crucial process. In general, oxidizing agents, such as sodium r 2011 American Chemical Society

chlorite,49 potassium permanganate,10 chlorine dioxide,11 or hydrogen peroxide,12 are added to the scrubbing medium to convert insoluble NO to soluble NO2. Among these oxidants, sodium chlorite has proved to be the most efficient. In wet scrubbing, the polluted flue gas is continuously scrubbed in the column, and the scrubber medium is circulated batchwise. This process involves absorption of the gaseous pollutants into the scrubber medium. The gas absorption in the scrubbing solution is further improved by providing a large gasliquid interfacial area for larger mass transfer. There have been many reports on the enhancement of wet scrubbers’ performances.1316 Jung et al.17 used a solution of 0.03 M iron(II) ethylenediaminetetraacetic acid [Fe(II)-EDTA] þ 0.024 M ascorbic acid þ 0.024 M adipic acid þ 0.09 M sodium sulfite in a wet scrubber to remove SO2 from flue gas. The liquid/gas flow rate of their work was 58.98 L/m3, and 96% removal efficiency was achieved. For removing NOx gases Chandresekara et al.18 used a HNO3 and Ag(I) mixture in an integrated wet scrubberelectrochemical system. NOx was removed from the gas stream with 75% efficiency when the liquid/ gas flow rate was 400 L/m3. Chien and Chu19 used a sieve tray wet scrubber for removing NO gas. They used NaClO2 as the scrubber medium to convert insoluble NO to soluble NO2, which could then be removed by alkaline solution. They could remove NOx from the flue gas stream with 51.5% efficiency using a liquid/gas flow rate of 7 L/m3. Deshwal et al. used aqueous chlorine dioxide scrubbing solution in a bubbling reactor to remove NOx from flue gas. Using a liquid/gas flow rate of 44 L/m3, they removed NOx with 60% efficiency.20 Because wet scrubbers use mostly high liquid/gas flow rates, it is very common to have water pollution problems. High Received: November 9, 2010 Accepted: May 3, 2011 Revised: February 16, 2011 Published: May 24, 2011 8278

dx.doi.org/10.1021/ie102272x | Ind. Eng. Chem. Res. 2011, 50, 8278–8284

Industrial & Engineering Chemistry Research

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Figure 1. Schematic diagram of the novel swirl scrubber system.

concentrations of absorbents are also used to improve removal efficiency in wet scrubbers, which not only causes environmental problems but also leads to significant corrosion problems inside and outside the nozzles, tubes, and walls of scrubbers. Swirl scrubbers usually have internal fans to move the polluted gas through them. A scrubbing medium is also sprayed into the fan inlet, where the rotor shears the liquid into dispersed droplets. The fan also acts as a turbulence producer. The turbulence from the fan increases the liquidgas contacting surface area. Despite the shorter liquidgas contact time in swirl scrubbers, their efficiency for gas removal is good. Swirl scrubbers are used in industrial applications, but reports related to them are scarce in the literature. In this work, a novel swirl scrubber has been developed to remove hazardous gases using a low liquid/gas flow ratio and a low absorbent concentration to overcome many wet scrubber drawbacks,21 and some process parameters that have a direct effect on simultaneous NOx and SO2 removal were studied. These include SO2 concentration, volumetric gas flow rate (FG), volumetric liquid flow rate (FL), oxidant concentration, scrubbing medium pH, contact time, and FG/FL ratio. An aqueous solution of sodium chlorite was used as the scrubbing medium. The results of this study show the effects of each parameter in achieving high simultaneous removal of NOx and SO2 pollutants from flue gas.

2. EXPERIMENTAL SECTION

Figure 2. Novel swirl scrubber system dimensions.

2.1. Experimental Setup. Figure 1 shows a schematic diagram of the novel swirl scrubber system that we developed for the simultaneous removal of hazardous gaseous pollutants. The novel swirl scrubber consists of a cylindrical chamber with an interior axial plate and an 1800 rpm electromotor. Figure 2 shows the main dimensions of the scrubber system. Polluted gas has seven chances to contact the devices and the scrubbing medium in the novel swirl scrubber system. First, the gas stream containing gaseous pollutants enters the scrubber through the vane attached to the top. The gas stream comes into contact with the

scrubber wall. Second, the gas stream meets the spray of the scrubbing medium. The spray depends on the volumetric injection rate of the scrubbing medium. Third, a mist and fog zone exists in the upper part of the scrubber zone, which makes an additional contribution to removal efficiency. Fourth, the gas stream comes into contact with the wall of the scrubber by centrifugal forces. The scrubber medium flows down and produces a water coating on the scrubber body. The circulating gas is sent downward and comes into contact with the absorbent 8279

dx.doi.org/10.1021/ie102272x |Ind. Eng. Chem. Res. 2011, 50, 8278–8284


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Table 1. Operating Conditions in Experiments NaClO2 runs

inlet

liquid flow

concentration(s) concentration(s) rate(s) gas flow rate(s) (M) (m3/min) pH of SO2 (ppm) (L/min)

14

0.2

0, 100, 300, 500 3.3

5

6

58

0.2

0, 100, 300, 500 3.3

5

4.5

912

0.2

0, 100, 300, 500 3.3

5

8.5

1315 0.2

100

3.3

6.1, 7.2, 8.3

6

1618 0.2

100

0.8, 1.7, 5

5

6

1922 0, 0.1, 0.35, 0.5

100

3.3

5

6

coating of the scrubber body. Fifth, the gas stream passing the swirl plate zone comes into contact with the swirl plate by inertial impaction, direct interception, and Brownian diffusion. Sixth, the scrubbing medium is circulated with considerable swirl on the swirl plate zone, forming significant fine droplets of absorbent with a fineness that depends on the volumetric flow rate of the medium fed into the scrubber. The swirl plate zone and the fine absorbent droplets produced on it are the most important parts of the scrubber system to improve the removal efficiency of gaseous pollutants. To build a proper liquid layer circulating on the swirl plate zone and also to produce very fine droplets of the scrubbing medium, it is necessary to adjust the volumetric flow rate of the scrubbing medium. Seventh, the scrubbing medium passes through the swirl plate and flows downward, resulting in an absorbent coating on the scrubber wall. The absorbent coating improves the removal efficiency of gaseous pollutants inside the scrubber. The scrubber medium is then collected in an absorbent tank with a capacity of 15 L and circulated to the top of the system. Clean gas after passing a demister exits from the system to the atmosphere with an average residence time of 35 s. The swirl plate in this system causes the scrubbing medium and polluted gas to circulate with a significant swirl and provides a large gasliquid interfacial area. Therefore, higher mass transfer and very good removal efficiencies can be achieved with this system. The enhancement in the gasliquid interfacial area leads to a low consumption of absorbents and produces less secondary waste. 2.2. Experimental Procedure. The operating time of each experiment was 4 min, which was determined by several experimental equilibrium conditions. Table 1 lists the operating conditions and their ranges in the experiments. The solution pH was adjusted using a buffer of phosphoric acid, acetic acid, boric acid, and sodium hydroxide, and NaClO2 was added to the buffer solution with the required pH. Therefore, the pH of the reaction solution remained almost constant.22 The effects of different parameters, namely, the concentration of SO2 gas, the volumetric flow rate of flue gas (FG), the volumetric flow rate of absorbent (FL), the concentration of sodium chlorite, the solution pH, the gasliquid contact time, and the FG/FL ratio, on the simultaneous removal of NOx and SO2 were studied. 2.3. Analysis Method. The SO2 and NOx concentrations of samples from the input and output gas streams were analyzed with a KANE_940 toxic gas analyzer at atmospheric pressure and an operating temperature of 35 °C. Gas samples with known NOx and SO2 contents were used for calibration. The removal efficiency was calculated based on the inlet and outlet concentrations of polluted gases as removal efficiency ð%Þ ¼

Figure 3. Simultaneous removal of NOx and SO2 with time using 0.2 M NaClO2 at 35 °C, pH 6, FG = 5 m3/min, FL = 3.3 L/min, input NO concentration = 280 ppm, and input SO2 concentration = 100 ppm.

where Ci and Co are the inlet and outlet polluted gas concentrations, respectively, in ppm.

3. RESULTS AND DISCUSSION 3.1. Simultaneous Removal of Nitrogen Oxide and Sulfur Dioxide. The simultaneous removal of NOx and SO2 was studied

at 35 °C and pH 6, using sodium chlorite solution with a concentration of 0.2 M as the scrubber medium with input NO and SO2 concentrations of 280 and 100 ppm, respectively. The volumetric gas and liquid flow rates were fixed at 5 m3/min and 3.3 L/min, respectively. The NOx and SO2 removal efficiencies and residual concentrations with the passage of time are presented in Figure 3. It can be seen that the novel swirl scrubber can remove both NOx and SO2 quite efficiently. In acidic media, NaClO2 decomposes as follows2225 5ClO2 þ 4Hþ f 4ClO2 þ Cl þ 2H2 O

ð1Þ

ClO2 þ Cl þ 4Hþ f Cl2 þ 2H2 O

ð2Þ

Also, SO2 and NO reactions in the scrubbing medium can be considered to follow the mechanisms in eqs 37 and 917, respectively.2530 SO2 reactions SO2 þ H2 O f HSO3 þ Hþ

ð3Þ

ClO2 þ HSO3 f SO4 2 þ Hþ þ ClO

ð4Þ

ClO þ HSO3 f SO4 2 þ Hþ þ Cl

ð5Þ

2Cl þ H2 O f HClO þ Hþ þ Cl

ð6Þ

HClO þ HSO3 f SO4 2 þ 2Hþ þ Cl

ð7Þ

The overall reaction for SO2 removal can be considered as 5SO2 þ 2ClO2 þ 6H2 O f 5H2 SO4 þ 2HCl

ð8Þ

NOx reactions

ðCi Co Þ 100 Ci 8280

ClO2 þ NO f NO2 þ ClO

ð9Þ

ClO þ NO f NO2 þ Cl

ð10Þ



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Figure 4. Effect of input NO concentration on NOx removal at various solution pH values. Experimental conditions: FG = 5 m3/min, FL = 3.3 L/ min, inlet SO2 concentration = 100 ppm, NaClO2 concentration = 0.2 M.

2NO2 þ H2 O f NO2 þ NO3 þ 2Hþ

ClO2 þ NO2 f NO3 þ ClO

ð11Þ

ð13Þ

2Cl þ H2 O f HClO þ HCl

ð14Þ

HClO þ NO2 f NO3 þ HCl

NO2 þ NO þ H2 O f 2NO2 þ 2H

considered as6,28,29,3335

ð12Þ

ClO þ NO2 f NO3 þ Cl

Figure 5. Effect of input SO2 concentration on NOx removal at various solution pH values. Experimental conditions: FG = 5 m3/min, FL = 3.3 L/ min, inlet NO concentration = 250 ppm, NaClO2 concentration = 0.2 M.

ð15Þ þ

SO2 þ H2 O f HSO3 þ Hþ

ð19Þ

ClO2 þ HSO3 f SO4 2 þ HClO

ð20Þ

HClO þ HSO3 f SO4 2 þ 2Hþ þ Cl

ð21Þ

ClO2 þ Hþ þ NO f NO2 þ HClO

ð22Þ

HClO þ NO f NO2 þ Hþ þ Cl

ð23Þ

ð16Þ

4NO2 þ ClO2 þ 4OH f 4NO3 þ Cl þ 2H2 O ð24Þ

ð17Þ

4NO þ 3ClO2 þ 4OH f NO3 þ 3Cl þ 2H2 O ð25Þ

The overall reaction for NOx removal can be considered as26,27

When polluted gas is introduced into the scrubbing medium without any SO2, the reaction for removing NO is eq 25. In the presence of SO2, reactions 1921 also take place in the medium. The products of these reactions act as reactants for eqs 22 and 23. These reactions improve the removal of NO from the gas stream. Therefore, the effect of the SO2 concentration on NOx removal in basic solution is more intense. 3.4. Effect of Gas Flow Rate. To evaluate the effect of the gas flow rate on the simultaneous removal of NOx and SO2, some experiments were performed at different gas flow rates. The solution pH was constant at 6, and 0.2 M sodium chlorite at a constant flow rate of 3.3 L/min was used as the scrubbing medium. The NO and SO2 inlet concentrations were 250 and 100 ppm, respectively. Figure 6a shows the changes in the removal efficiencies of NO, NOx, and SO2 with increasing gas velocity. SO2 was completely (100%) removed in all runs, but the removals of both NO and NOx decreased with increasing gas flow rate. This was quite expected because, at higher gas flow rates, the gas phase would be allowed to contact the scrubbing medium for a shorter time. For better understanding, Figure 6b shows the effect of the gas residence time on the simultaneous removal efficiency. 3.5. Effect of Liquid Flow Rate. To investigate the effect of the liquid flow rate on the simultaneous removal of NO, NOx, and SO2, a series of measurements were made at a constant sodium chlorite concentration of 0.2 M, a pH of 6, and a gas flow rate of 5 m3/min with NO and SO2 concentrations of 250 and 100 ppm, respectively. The results are shown in Figure 7a. It can be seen that

NO2 þ HClO f Cl þ NO2 þ Hþ þ NO3 5NO þ 3ClO2 þ 4H2 O f 5HNO3 þ 3HCl

ð18Þ

3.2. Effect of Input NO Concentration. Experiments were carried out at different NO concentrations to investigate the effect of the input NO concentration on the removal efficiency of NOx using 0.2 M sodium chlorite solution at an input SO2 concentration of 100 ppm and volumetric gas and liquid flow rates of 5 m3/min and 3.3 L/min, respectively. The NOx removal efficiencies at different solution pH values are shown in Figure 4. It is clear that, with increasing inlet NO concentration, the removal efficiency decreases. This effect is more obvious at high pH. In an acidic scrubbing medium, the feed NO concentration has no obvious effect on NOx removal efficiency. This behavior is further discussed in section 3.7. 3.3. Effect of Input SO2 Concentration. Some experiments were also performed to investigate the effect of the SO2 concentration at different solution pH values on the removal efficiency of NOx using 0.2 M sodium chlorite solution when the concentration of NO was maintained at 250 ppm and the volumetric gas and liquid flow rates were fixed at 5 m3/min and 3.3 L/min, respectively. As can be seen from Figure 5, the removal efficiency of NOx increases with increasing SO2 concentration. Enhancement of the NOx removal is more obvious when the solution pH is high. This behavior might be resulting from the catalytic effect of products of SO2 hydrolysis on NOx absorption.31,32 In basic solution, the reactions of NO and SO2 in the scrubbing medium can be

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Figure 6. (a) Effect of gas flow rate on NO, NOx, and SO2 removal efficiencies. (b) NO, NOx, and SO2 removal efficiencies as functions of residence time. Experimental conditions: NaClO2 concentration = 0.2 M, pH = 6, FL = 3.3 L/min, inlet NO concentration = 250 ppm, and inlet SO2 concentration = 100 ppm.

the higher the liquid flow rate, the better the removals of NO and NOx. Note that an increase in the flow rate of the scrubbing medium could provide more scrubbing solution to the same amount of polluted gas, and therefore, higher removal can be expected. For SO2, its total removal (100%) from the flue gas remained unaffected at different scrubbing medium flow rates. As Figures 6a and 7a show, a correct matching of the liquid and gas flow rates should exist for high removals of gaseous pollutants. Figure 7b shows the effect of the liquid/gas flow rate ratio (FL/FG) on the simultaneous removal of NO, NOx, and SO2. It can be seen that higher removals of NO and NOx can be achieved at higher liquid/gas flow rate ratios. This result can be attributed to the higher contacting surface area and the resulting higher mass transfer. Comparing the results of this study with previous works shows that, even though high removal efficiencies are achieved, a very low liquid/gas flow rate ratio is used in the novel swirl scrubber. This could lead to a very low consumption of the scrubbing medium despite the high removal efficiency. 3.6. Effect of NaClO2 Concentration. Some experiments were also performed to evaluate the effect of the sodium chlorite concentration on the simultaneous removal of NO, NOx, and SO2 at pH 6 and NO and SO2 concentrations of 250 and 100 ppm, respectively. The liquid and gas flow rates were fixed at 3.3 L/min and 5 m3/min, respectively. It can be seen from Figure 8 that the removal efficiencies increased as the sodium chlorite concentration in the scrubbing solution increased and attained constant values. Sodium chlorite as the scrubber medium oxidized NO into NO2 completely, and a constant NOx absorption efficiency of about

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Figure 7. (a) Effect of liquid flow rate on NO, NOx, and SO2 removal efficiencies. (b) NO, NOx, and SO2 removal efficiencies as functions of the FL/FG ratio. Experimental conditions: NaClO2 concentration = 0.2 M, pH = 6, FG = 5 m3/min, inlet NO concentrations = 250 ppm, and inlet SO2 concentration = 100 ppm.

Figure 8. Effect of NaClO2 concentration on simultaneous NOx and SO2 removal at pH 6, FG = 5 m3/min, FL = 3.3 L/min, input NO concentration = 250 ppm, and input SO2 concentration = 100 ppm. 8282



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removal systems. Using sodium chlorite as the scrubbing medium in the novel swirl scrubber system proved quite promising for the simultaneous removal of NOx and SO2 with very high efficiencies. Theoretical analyses of the swirl scrubber system will be reported in future.

’ AUTHOR INFORMATION Corresponding Author

*Fax: þ98-021-66405847. E-mail: [email protected].

’ REFERENCES Figure 9. Effect of scrubbing medium pH on NO and NOx removal using 0.2 M NaClO2 at FG = 5 m3/min, FL = 3.3 L/min, input NO concentration = 250 ppm, and input SO2 concentration = 100 ppm.

81% was observed. Total SO2 removal (100%) was observed at an optimum sodium chlorite concentration of 0.2 M. 3.7. Effect of pH. A crucial parameter in the removal of NOx using sodium chlorite solution is the scrubbing medium pH. The effect of solution pH on NO and NOx removals at constant NO and SO2 concentrations of 250 and 100 ppm, respectively, is reported in Figure 9. The gas flow rate was maintained at 5 m3/min, and the scrubber medium flow rate was 3.3 L/min. Figure 9 shows that the NO removal increased as the solution pH decreased. The removal efficiency of NO, however, was found to be significantly affected by solution pH. It was higher and remained almost stable when the solution pH was varied from 4.5 to 6; thereafter, it decreased with increasing solution pH. This behavior can be attributed to the production of chlorine dioxide from the sodium chlorite in acidic solution (eqs 1 and 2). Each molecule of ClO2 contains 19 valence electrons that can transit between the oxygen and chlorine atoms; therefore, these molecules act like free radicals36 and can provide a high removal efficiency of NO by their strong oxidative ability. They oxidize NO into NO2, which is subsequently absorbed in the form of nitrate.25 Also, chlorine gas produced along with chlorine dioxide is again a strong oxidant and is capable of oxidizing NO into NO2 and nitrate.30 However, the effect of pH on NOx removal is less intense. This is attributed to the fact that sodium chlorite has a good oxidizing ability for NO at lower pH but a good absorbing ability for NO2 at higher pH.20 Therefore, pH is a crucial parameter for oxidizing NO into NO2 and absorbing NO2 thereafter.

4. CONCLUSIONS In this work, 100% SO2 removal, 100% NO oxidation, and 81% NOx removal efficiency were attained in a novel swirl scrubber using NaClO2 as the scrubber medium. The NOx removal efficiency increased mainly with decreasing solution pH value. Increasing the initial SO2 concentration enhanced the NOx removal efficiency, and this enhancement was more intense at higher pH values. A higher removal efficiency was obtained with increasing sodium chlorite concentration, but a supply of sodium chlorite above 0.2 M did not improve the removal efficiency considerably. The novel swirl scrubber of this study produced very large gasliquid interfacial areas, and therefore, high removal efficiencies were observed using a very low concentration of scrubbing medium and a low FL/FG ratio. As a result, this system can overcome many drawbacks of conventional gaseous pollutant

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Study on Simultaneous Removal of NOx and SO2 with NaClO2 in a

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