Simultaneous Removal of SO2 and NO from Flue Gas Using

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Simultaneous Removal of SO2 and NO from Flue Gas Using Multicomposite Active Absorbent Yi Zhao,* Yinghui Han, and Cheng Chen School of Environmental Science and Engineering, North China Electric Power University, Baoding City, Hebei Province 071003, P. R. China ABSTRACT: A multicomposite active absorbent was prepared using the liquid-phase complex of NaClO2 and NaClO as well as solid-phase slake lime to simultaneous desulfurization and denitrification at a flue gas circulating fluidized bed (CFB). The effects of influencing factors on the removal efficiencies of SO2 and NO were investigated. Removal efficiencies of 96.5% for SO2 and 73.5% for NO were obtained, respectively, under the optimal experimental conditions. The characterization of the spent absorbent was carried out by using energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), and chemical analysis methods, from which the simultaneous removal mechanism of SO2 and NO based on this absorbent was proposed.

1. INTRODUCTION Coal combustion is a major anthropogenic emission source of SO2 and NOx, which will lead to worsening local air quality as well as regional acid rain pollution. China is one of the few countries in the world whose coal consumption constitutes more than 70% of the country’s total energy sources, and over half of the coal burned by coal-fired power plants. By now, the most successful and commercialized processes for SO2 and NOx reduction are limestone
gypsum flue gas desulfurization (FGD) and selective catalytic reduction (SCR), respectively. However, the combined system of these two processes will occupy much area and is expensive both in capital and operation cost. Therefore, many researchers have carried out experiments to develop simultaneous desulfurization and denitrification processes.1
9 Nevertheless, most of the experiments were conducted in a small-scale reactor and were not developed into practical technology; especially, those performed in a flue gas CFB have not been further developed. In this paper, a new process that uses a multicomposite active absorbent to remove SO2 and NO simultaneously is reported. This process is a promising, better method for flue gas simultaneous desulfurization and denitrification from coal-fired boliers equipped with low NOx burner, because the removal efficiencies for SO2 and NO can meet the discharge standard of air pollutants for thermal power plants in China. Furthermore, the presumed reaction mechanism of simultaneous desulfurization and denitrification was proposed. As a new effective SO2 and NOx emission control strategy for flue gas cleaning, this work has great academic significance and application value.

pressure by using an induced draft fan, and the pressure drop is 800 Pa in the reactor. For a typical coal-fired power plant in China, actual flue gas consists chiefly of CO2, N2, O2, SO2, NOx, water vapor, fly ash, etc., in which the concentration ranges of CO2, N2, O2, and water are generally 10
15%, 78
80%, 5
6%, and 6
8%, respectively, and those of SO2, NOx and fly ash will change with coal species and combustion conditions of the boiler. The temperature range of flue gas is 120
150 °C, and the pressure is about 1 atm. In general, the concentrations of CO2, N2, O2, and fly ash have less effect on the desulfurization and denitrification in the semidry flue gas CFB system. For water in flue gas, it directly affects two flue gas CFB operating parameters: one is the inlet flue gas temperature and another is flue gas humidity. Hence, the simulating gas mixture in our experiments consists of air, SO2, NO, and water. As shown in Figure.1, SO2 and NO were supplied from each compressed gas steel cylinder, air could be from an air compressor, and this gas mixture was heated by an electric heater. The equipment to supply the humidified water was placed at the bottom of the flue gas CFB reactor. The multicomposite active absorbent was produced by the liquid-phase NaClO2 and NaClO complex and the solid-phase slaked lime. The liquid-phase complex absorbent was made from sodium chlorite and sodium hypochlorite, in which according to the required concentration and the ratio of liquid-phase complex absorbent, a given mass of sodium chlorite and sodium hypochlorite was added into the water tank, stirred gently to produce homogeneous solution, and then sprayed into the flue gas CFB reactor. At the same time, the solid slaked lime powders were fed into the reactor by a screw feeder, from which the multicomposite active absorbent was formed in the flue gas CFB reactor. Commercial slaked lime and sodium chlorite used were technical-pure grade (Tianjin Tanggu Chemical Reagent Factory, Tianjin, China). Sodium

2. EXPERIMENTAL SECTION 2.1. Experimental Equipment and Reagents. The experiments on simultaneous desulfurization and denitrification were performed on a flue gas circulating fluidized bed (CFB) system, as shown in Figure 1. The key part of the flue gas CFB reactor is a vertical cylinder with a length of 4500 mm and an inner diameter of 250 mm, on which several temperature station points are located. The experimental system operates under negative r 2011 American Chemical Society

Received: August 31, 2011 Accepted: December 1, 2011 Published: December 01, 2011 480

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Figure 1. Experimental apparatus of flue gas CFB system: 1, air inlet; 2, steel bottle of SO2; 3, steel bottle of NO; 4, relief valve; 5, glass-rotor flow meter; 6, buffer bottle; 7, total flow control valve; 8, summation flowmeter indicator; 9, electric heater; 10, screw material-fed machine; 11, water tank; 12, high-pressure pump; 13, Y-type spray nozzle; 14, fluidized bed reactor; 15, cyclone dust collector; 16, material-circling leg; 17, gas analysis instrument; 18, induced-draft fan.

Figure 2. Effect of the concentration of NaClO2/NaClO complex absorbent on the efficiencies of simultaneous desulfurization and denitrification. The inlet temperature of flue gas is 120 °C, the flue gas humidity is 6% (v), the molar ratio of NaClO2 to NaClO is 0.5, Ca/ (S + N) ratio is 1.2, the flow rate is 250 m3/h, the residence time is 2.8 s, and the inlet concentrations of SO2 and NO are 3258 mg/m3 and 1000 mg/m3, respectively.

hypochlorite was analytical pure reagent (Tianjin Chemical Factory, Tianjin, China). Hydrochloric acid was analytical pure reagent (Zhenxing Chemical Factory, Tianjin, China). Sodium hydroxide was analytical pure reagent (Huadong Chemical Reagent Factory, Tianjin, China). 2.2. Experimental Methods. During the experiments, the atomized droplets containing NaClO2/NaClO complex absorbent resulting from a high-pressure pump were sprayed into the flue gas CFB system at a spray rate of 10
33.4 kg/h from a Y-type nozzle situated at the bottom of the flue gas CFB reactor, and slaked lime powder was fed to the reactor by a screw feeder, meanwhile passing the simulated flue gas containing SO2, NO, H2O, and air. The reacted and unreacted solid particles of absorbent were collected by a cyclone dust collector and recirculated into the reactor. Here, the circulating flow rate is 3.8 kg/(m2 s), and the circulation multiple is 85.10 In flue gas desulfurization technology, an important operational factor, Ca/S, is usually used to evaluate the utilization of the absorbent and serves as a critical technological parameter to determine the added amount of the absorbent. For the simultaneous desulfurization and denitrification from flue gas mentioned in this article, Ca/(S + N) is used instead of Ca/S, and is expressed as the following:10 Ca=ðS þ NÞ ¼ n1 =n2

(XRD, D8 ADVANCE type, BRUKER-AXS in Germany) were used to determine the components of the spent absorbent. The contents of sulfate, sulfite, nitrate, and nitrite in the spent absorbent were analyzed by the methods of chemical analysis, those of sulfate and sulfite were determined by barium chromate photometry, the content of nitrite was determined by N-(1-naphthyl)-ethylenediamine photometry, and that of nitrate was determined by process of the reduction of zinc powder.

3. RESULTS AND DISCUSSION 3.1. Effects of the Running Parameters on Desulfurization and Denitrification. Eight running parameters affecting the

removal efficiency, such as the concentration of NaClO2/NaClO complex absorbent, molar ratio of NaClO2 to NaClO, residence time, Ca/(S + N) ratio, inlet flue gas temperature, flue gas humidity, content of O2, and concentrations of SO2 and NO were investigated experimentally. Figure 2 shows the effect of concentration of NaClO2/NaClO complex absorbent on desulfurization and denitrification. Although the concentration of NaClO2/NaClO complex absorbent has a weak effect on desulfurization, it has a significant effect on denitrification. The removal efficiency is enhanced rapidly when the concentration of NaClO2/NaClO complex absorbent is between 0.002 and 0.008 mol/L. However, the turning point occurs at 0.008 mol/L; thereafter, the removal efficiencies almost stay constant, which might be due to saturation of the “oxidizing points” on the surface of calcium-based absorbent with the increase in the concentration of NaClO2/NaClO complex absorbent. Various molar ratio of NaClO2 to NaClO from 0 to 4 were examined. It can be seen from Figure 3 that the molar ratio has obvious effect on NO removal, but desulfurization has less effect by increasing the molar ratio, especially at the higher molar ratio. The efficiencies of denitrification enhance rapidly when the molar ratio of NaClO2/NaClO complex absorbent is between 0 and 0.7, which shows that the concentration of NaClO2 is significantly correlated with NO removal, when the concentration of NaClO remains constant. Nevertheless, because the price

ð1Þ

Here, n1 and n2 stand for the molar weights of slaked lime in absorbents and the gas mixture (SO2 + 1/2NO) in flue gas, respectively. The definition of stoichiometric relationship is based on the feed rate of slaked lime, SO2, and NO to the reactor and the expected removal products mainly include CaSO4/ CaSO3 and Ca(NO3)2/Ca (NO2)2 in the spent absorbent.1 2.3. Analysis Methods. The concentrations of SO2 and NOx at inlet (virtual flue gas temperature, °C, 1 atm) and outlet (virtual flue gas temperature, °C, 1 atm) were measured by a portable flue gas analyzer (MRU95/3 CD, MRU Company, Heilbronn, Germany), from which the removal efficiencies from the measured concentrations of SO2 and NO were calculated. An energy dispersive X-ray spectrometer (EDS, Vantage DIS type, Thermo NORAN Company) and X-ray diffraction 481

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Figure 3. Effect of the molar ratio of NaClO2 to NaClO on the efficiencies of simultaneous desulfurization and denitrification. The inlet temperature of flue gas is 120 °C, the flue gas humidity is 6% (v), Ca/ (S + N) ratio is 1.2, the flow rate is 250 m3/h, the residence time is 2.8 s, and the inlet concentrations of SO2 and NO are 3000 mg/m3 and 1100 mg/m3, respectively.

Figure 4. Effect of flue gas humidity on the efficiencies of simultaneous desulfurization and denitrification. The inlet temperature of flue gas is 120 °C, the flow rate is 250 m3/h, the residence time is 2.8 s, the concentration of NaClO2/NaClO complex absorbent is 0.008 mol/L, the molar ratio of NaClO2 to NaClO is 0.5, Ca/(S + N) ratio is 1.2, and the inlet concentrations of SO2 and NO are 3146 and 1020 mg/m3, respectively.

of NaClO2 is much higher than that of NaClO,11 in order to reduce the sorbent cost, the optimal molar ratio of NaClO2 to NaClO was selected as 0.5. The experiments on the effect of the residence time on the efficiencies for SO2 and NO removal were carried out. The results showed that the removal efficiencies increased rapidly especially for denitrification when the residence time was between 0 and 2.8 s. This result could be due to the fact that the contact of flue gas with absorbents is more sufficient with an increase in the residence time, which leads to the rise of removal efficiencies. For NO removal, because of the low water solubility, it can hardly be absorbed by the absorbent.12 Therefore, the oxidation of NO to NO2 is necessary, which can be carried out by an increase in the residence time. However, when the residence time was between 2.8 and 3.1 s, the removal efficiencies basically remained steady, which might be because the reaction between the absorbent and NO or SO2 became the control step of removal reaction at 2.8 s. As is well-known, excessive residence time would increase the volume capacity of reactor and operating cost. The optimal residence time was determined to be 2.8 s in the experiments. The effects of Ca/(S + N) on desulfurization and denitrification were investigated. When the Ca/(S + N) value was between 0.8 and 1.2, the removal efficiencies increased rapidly. Thereafter, the removal efficiencies remained almost constant, which may be due to the sedimentation of reaction products, such as CaSO4, CaSO3 , Ca(NO3 )2 , and Ca(NO2 )2, etc. on the surface of calcium-based absorbent. Thus, the optimal Ca/(S + N) ratio was determined to be 1.2. The effect of the inlet flue gas temperature on the removal efficiencies of SO2 and NO was studied experimentally. The results showed that the removal efficiencies enhanced rapidly when the flue gas temperature was between 90 and 110 °C. Thereafter, the removal efficiencies remained constant. However, when the temperature was higher than 120 °C, the removal efficiencies decreased slowly as temperature increased. Hence, the optimal inlet flue gas temperature was selected at 120 °C accordingly. For this experiment, removal of SO2 and NO is a semidry reaction process between porous solids and gases, in which the adsorption, absorption, and diffusion of SO2 and NO on the surface of absorbent are affected greatly by the

Table 1. Effect of Inlet Concentrations of SO2 and NOx on the Simultaneous Removal Efficiencies SO2 inlet

NO

outlet

efficiency

items (mg/m3) (mg/m3)

inlet

outlet

efficiency

(mg/m3) (mg/m3)

(%)

(%)

1

1000

36

96.4

200

54

73.1

2

1467

48

96.7

398

105

73.6

3

2012

78

96.1

605

162

73.2

4

2496

87

96.5

764

200

73.5

5 av

3000

96

96.8 96.5

1000

253

74.1 73.5

Table 2. Contents of Sulfur and Nitrogen Species in the Fresh Absorbent and the Spent Absorbent (mmol/g) contents of sulfur and [SO32‑]

[NO3
]

[NO2
]

nitrogen species

[SO42‑]

fresh absorbent

0.001

0

0

0

spent absorbent

1.277

0.325

0.564

0.286

flue gas temperature. When the temperature was lower than 120 °C, the diffusion and absorption were predominant and the efficiencies increased with the temperature.13 However, when the flue gas temperature was higher than 120 °C, desorption of gas molecules on the surface of the absorbent was enhanced, causing a decrease of equilibrium absorptive capacity of the gas. Therefore, the removal efficiencies evidently decreased with the temperature. In addition, the oxidizing power of the NaClO2/ NaClO complex would be reduced because of its decomposition at higher temperature, resulting in a decrease in the removal efficiency. Obviously, 120 °C is a turning point for the diffusion, absorption, and desorption of SO2 and NO, and for the decomposition of NaClO2/NaClO complex. Commonly, the flue gas temperatures in the tail of power boiler 482

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Figure 5. Energy dispersive spectrum of the spent absorbent.

are about 800
900 °C; after passing economizer, air preheater, and electric precipitator or bag dust collector, it drops to about 120
150 °C, which is close to the optimum temperature (120 οC) obtained in this experiment. In order to further explore the reaction mechanism and to take measures to control the flue gas temperature around 120 °C in the flue gas CFB reactor, the effects of temperature on NO oxidation with injection of NaClO2/NaClO in the absence of slake lime were investigated experimentally in liquid phase system. The results showed that the variations of removal efficiencies for SO2 and NO as a function of the inlet flue gas temperature were similar to those of our previous works,11 in which the inlet flue gas temperature range was 110
120 °C and the corresponding reaction temperature range was 40
50 °C. Flue gas humidity is one of the important factors affecting removal efficiencies. The experimental results of desulfurization and denitrification are shown in Figure 4, when the flue gas humidity ranges from 0 to 8% (v). The results indicate that removal efficiencies increase as the flue gas humidity increases, especially at the lower flue gas humidity range, which may give consideration to the removal reactions being mainly chemical adsorption and slow gas
solid reaction at low flue gas humidity. The water membrane containing NaClO2/NaClO complex absorbent would be formed gradually on the surface of calcium-based absorbent as an increase in the flue gas humidity, especially for the formation of the monomolecular layer. Hence, the SO2 and NO could be easily oxidized to higher valence sulfur and nitrogen oxides by strong oxidizing liquid membrane, and then dissolved in the surface of absorbent. Meanwhile, the rapid ionizing reactions between Ca(OH)2 and higher valence sulfur and nitrogen oxides occur; thus, the removal efficiencies are obviously enhanced. Figure 4 also shows that the removal efficiencies appear constantly with the flue gas humidity when the flue gas humidity exceeds 6% (v), where adsorbed liquid on the surface of the absorbent exceeds monomolecular layer, and the diffusion of SO2 and NO to the surface of the absorbent would be delayed. Therefore, the

Figure 6. XRD patterns of the spent absorbent (I) and commercial slaked lime (II).

velocity of ionizing reactions slows down, a fact consistent with that shown by Yoon14 and Stouffer.15 In order to elucidate the role of O2 in desulfurization and denitrification, experiments about variation of removal efficiencies at the concentration of O2 ranging from 2% to 10% were done, and the results showed that the efficiency of desulfurization maintained at 100% in the whole experimental range; however, that of denitrification was accelerated when the content of O2 increased from 2% to 8%, and thereafter almost kept constant. This suggested that O2 contributed to the conversion of NO to NO2 and that the chemical reaction of O2 with NO on the surface of CaSO3 may occur.16 The experiments on simultaneous desulfurization and denitrification were carried out under the optimal experimental conditions, as follows: The concentration of NaClO2/NaClO complex absorbent is 0.008 mol/L, the molar ratio of NaClO2 to NaClO is 0.5, the inlet flue gas temperature is 120 °C, the flue gas humidity is 6% (v), the residence time is 2.8 s, and the Ca/ (S + N) ratio is 1.2. As shown in Table1, the removal efficiencies vary slightly in SO2 concentration range 1000
3000 mg/m3 and in NO concentration range 200
1000 mg/m3, respectively, indicating that the proposed process is suitable for to the different types of coal and combustion conditions. At the same time, 483

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Figure 7. Effect of the solid to liquid ratio on the efficiencies of simultaneous desulfurization and denitrification. The inlet temperature of flue gas is 120 °C, the flue gas humidity is 6% (v), the concentration of NaClO2/NaClO complex absorbent is 0.008 mol/L, the molar ratio of NaClO2 to NaClO is 0.5, Ca/(S + N) ratio is 1.2, the flow rate is 250 m3/h, the residence time is 2.8 s, and the inlet concentrations of SO2 and NO are 2500 and 500 mg/m3, respectively.

Figure 8. Effect of inlet concentration of SO2 on NO removal. The inlet temperature of flue gas is 120 °C, the flue gas humidity is 6% (v), the concentration of NaClO2/NaClO complex absorbent is 0.008 mol/L, the molar ratio of NaClO2 to NaClO is 0.5, Ca/(S + N) ratio is 1.2, the flow rate is 250 m3/h, the residence time is 2.8 s, and the inlet concentration of NO is 700 mg/m3.

average removal efficiencies of 96.5% for SO2 and 73.5% for NO were achieved, respectively. Taking a 300 MW coal-fired power plant (Huaneng Dandong power plant in Liaoning province of China) as an illustration, SO2 and NOx in raw boiler gas are about 2300 and 400 mg/m3, respectively; if the flue gases are treated by this process, the residual amount of SO2 and NOx will be below 80.5 and 106 mg/m3, respectively. According to the latest emission standard of air pollutants for thermal power plants (GB 13223-2011), the limitation of SO2 and NO pollutants is 400 and 200 mg/m3, respectively. Obviously, it is fully meeting the environment discharge standard in China. 3.2. Analysis of Removal Products. The contents of sulfur and nitrogen species resulting from chemical analysis are listed in Table 2 for the fresh absorbent and the spent absorbent. From Table 2, it is clear that the complicated reactions of SO2, NOx, and calcium species had been carried out. It was seen that the molar ratio of sulfate and sulfite in the spent absorbent was 3.93, and that of nitrate and nitrite was 1.97. With regard to fresh absorbent, a small quantity of sulfate was detected, which might come from slaked lime. Moreover, sulfite, nitrate, and nitrite were not detected, which indicates that sulfate was the main desulfurization product, and nitrate was the main denitrification one. Figure 5 shows the energy dispersive spectra on the surface of the spent multicomposite active absorbent. It is found that peaks of nitrogen and sulfur appear obviously, which confirms the absorptions of nitrogen and sulfur species. In addition, there are peaks of sodium and chlorine elements, which may be resulting from NaClO2/NaClO complex absorbent. The powder X-ray diffraction patterns of the spent multicomposite active absorbent (top) and commercial slaked lime (bottom) are shown in Figure 6. By contrast, many new diffraction peaks appear in the XRD pattern of the spent absorbent, indicating that the chemical reactions occur among SO2, NO2, slaked lime, and the multicomposite active absorbent. It also can be found that the characteristic peaks of CaSO4 and Ca(NO3)2 are stronger than those of CaSO3 and Ca(NO2)2, demonstrating that the major removal products

are CaSO4 and Ca(NO3)2, and minor ones are CaSO3 and Ca(NO2)2. 3.3. Removal Reaction Mechanism. Because of the interaction between NaClO2 and NaClO and the decomposition of NaClO2 in the presence of NaClO, a mass of active chlorine dioxide exists in the humidified water containing NaClO2/ NaClO complex absorbent.17 The reaction equations may include as follows: 2ClO2
þ ClO
þ 2Hþ f 2ClO2 þ Cl
þ H2 O ð2Þ 5ClO2
þ 4Hþ f 4ClO2 þ Cl
þ 2H2 O

ð3Þ

It can be seen from Figures 2 and 3 that the addition of NaClO2/NaClO complex has less effect on SO2 removal, which was verified by the experiments on the effect of solid (slaked lime) to liquid (NaClO2/NaClO complex) ratio on the efficiencies of simultaneous desulfurization and denitrification. As shown in Figure 7, when slaked lime is absent, the removal efficiency of SO2 reaches about 69%. The removal efficiency enhances rapidly when the ratio of solid to liquid is between 0 and 20. Thereafter, the removal efficiencies increase slowly, which indicates that the desulfurization may be the comprehensive results of the absorption reaction between SO2 and slaked lime as well as the oxidation reaction between SO2 and NaClO2/NaClO complex, and the former is primary. From the downward tendency of removal efficiency of NO as the ratio of solid to liquid decreases, it is also found that denitrification is strongly dependent on the concentration of NaClO2/NaClO complex; in this case, NO is oxidized into NO2, and then absorbed by Ca(OH)2 to form Ca(NO3)2. The effects of the presence of SO2 on NO removal were examined, and the results are shown in Figure.8. When the concentration of SO2 increases from 500 to 2500 mg/m3, the removal efficiency of NO increases gradually and reaches a maximum value; thereafter, the removal efficiency of NO remains basically constant, which might be due to the presence of NaClO2/NaClO complex, as the synergy effect of SO2 to promote NO oxidation reaction is not obvious. According to the 484

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crucial turning point of the denitrification efficiency, the desired ratio of SO2/NO in flue gas can be identified as 3.57. According to Figures 7and 8 as well as the experimental results of EDS, XRD, and the chemical analysis, and referencing refs 18 and 19, the possible reaction mechanism in flue gas CFB can be inferred as follows: SO2 ðgÞ f SO2 ðaqÞ

metal pipeline and secondary environmental problems. Moreover, according to the technological characterizations of flue gas CFB, the absorbent and the removal products are all in dry form during the desulfurization and denitrification process; thus, the chloride corrosion of equipment can also be avoided. Although there are small amounts of sulfite and nitrite in the removal products, they cannot be decomposed unless they are in acidic conditions or at high temperature. Because the byproducts containing unreacted Ca(OH)2 are alkaline, the sulfite and nitrite can easily become sulfate and nitrate when in contact with air or wet gas. From the utilization point of view, those byproducts can be used for landfill linears, panel wall, and steam brick as the main material, or paving road and reconstructing highway embankment as the concrete additive instead of part cement.22

ð4Þ

SO2 ðaqÞ þ H2 OðlÞ f H2 SO3 ðlÞ f HSO3
ðaqÞ þ Hþ ðaqÞ f SO3 2
ðaqÞ þ 2Hþ ðaqÞ

ð5Þ

5SO2 ðaqÞ þ 2ClO2 ðaqÞ þ 6H2 OðlÞ f 5SO4 2
ðaqÞ þ 2Cl
ðaqÞ þ 12Hþ ðaqÞ

ð6Þ

4. CONCLUSIONS The following conclusions can be drawn. (1) On the basis of the multicomposite active absorbent, a new process is developed for simultaneous desulfurization and denitrification at a flue gas CFB. Under the optimal conditions, the efficiencies of desulfurization and denitrification are 96.5% and 73.5%, respectively. (2) There are eight operating parameters affecting the removal efficiencies, among them, the concentration of NaClO2/NaClO complex absorbent, the molar ratio of NaClO2 to NaClO, the flue gas temperature, the flue gas humidity, and the residence time are considered to be most important. (3) The desulfurization may are be the comprehensive result of the absorption reaction between SO2 and slaked lime as well as the oxidation reaction between SO2 and NaClO2/ NaClO complex, and the former is primary. The denitrification is strongly dependent on the concentration of NaClO2/NaClO complex; in this case, NO is oxidized to into NO2, and then absorbed by Ca(OH)2.

2SO2 ðaqÞ þ ClO2
ðaqÞ þ 2H2 OðlÞ f 2SO4 2
ðaqÞ þ Cl
ðaqÞ þ 4Hþ ðaqÞ

ð7Þ

SO3 2
ðaqÞ þ O2 ðaqÞ þ NOðgÞ f active complex compound f SO4 2
ðaqÞ þ NO2 ðaqÞ ð8Þ CaðOHÞ2 ðsÞ f CaðOHÞ2 ðaqÞ f Ca2þ þ 2OH


ð9Þ

Ca2þ þ SO3 2
f CaSO3 ðsÞ

ð10Þ

Ca2þ þ SO2
4 f CaSO4 ðsÞ

ð11Þ

2NOðgÞ þ ClO2 ðaqÞ f 2NO2 ðaqÞ þ ClðaqÞ

ð12Þ

2NOðgÞ þ ClO2
ðaqÞ f 2NO2 ðaqÞ þ Cl
ðaqÞ

ð13Þ

2NO2 ðaqÞ þ H2 OðlÞ f NO2
ðaqÞ þ NO3
ðaqÞ þ 2Hþ ðaqÞ

ð14Þ

’ AUTHOR INFORMATION Corresponding Author

NO2 ðgÞ þ NOðgÞ þ H2 OðlÞ f 2NO2
ðaqÞ þ 2Hþ ðaqÞ

*E-mail: [email protected]. Tel.: +86-0312-7522343. Fax: +86-0312-7522192.

ð15Þ 2NO2
ðaqÞ þ ClO2 ðaqÞ f 2NO3
ðaqÞ þ ClðaqÞ ð16Þ 2NO2
ðaqÞ þ ClO2
ðaqÞ f 2NO3
ðaqÞ þ Cl
ðaqÞ ð17Þ


þ 2NO2 fCaðNO2 Þ2 ðsÞ

ð18Þ

Ca2þ þ 2NO3
fCaðNO3 Þ2 ðsÞ

ð19Þ

Ca

2þ

’ ACKNOWLEDGMENT The authors appreciate the financial support provided by “The Major State Basic Research Development Program of China (973 Program, No. 2006CB200300-G)”. ’ REFERENCES (1) Nelli, C. H.; Rochelle, G. T. Simultaneous sulfur dioxide and nitrogen dioxide removal by calcium hydroxide and calcium silicate solids. J. Air Waste Manage. Assoc. 1998, 48, 819. (2) Xu, G. W.; Wang, B.; Suzuki, H.; Gao, S. Q.; Ma, X. X.; Nakagawa, N.; Kato, K. Removal efficiency of the combined desulfurization/denitration process using powder-particle fluidized bed. J. Chem. Eng. Jpn. 1999, 32, 89. (3) Xu, G. W.; Luo, G. H.; Akamatsu, H.; Kato, K. An adaptive sorbent for the combined desulfurization/denitration process using a power-particle fluidized bed. Ind. Eng. Chem. Res. 2000, 39, 2190. (4) Chu, H.; Chien, T. W.; Li, S. Y. Simultaneous absorption of SO2 and NO from flue gas with KMnO4/NaOH solutions. Sci. Total Environ. 2001, 275, 127.

It can be seen from removal reactions that chloride ion (Cl
) appears in the removal products. According to the optimal absorbent (0.008 mol/L) in this process, 0.011 mmol/L of total remaining free Cl
may be contained in the humidified water; this concentration is far lower than the critical chloride concentration of stress corrosion cracking for stainless steel20,21 and the Chinese standard of urban sewage treatment factory (