from Flue Gas by Ozone Oxidation and NaOH Absorption - American

Article pubs.acs.org/IECR

Simultaneous Removal of NO and SO2 from Flue Gas by Ozone Oxidation and NaOH Absorption Jia Zhang,† Rui Zhang,† Xin Chen,‡ Ming Tong,‡ Wanzhong Kang,‡ Shaopeng Guo,† Yanbo Zhou,† and Jun Lu*,† †

Key Laboratory of Coal Gasification and Energy Chemical Engineering of Ministry of Education, East China University of Science & Technology, No. 130 Meilong Road, Shanghai 200237, China ‡ SINOPEC Ningbo Engineering Co., Ltd., Ningbo 315103, Zhejiang, China ABSTRACT: Exhaust flue gas from fossil fuel combustion usually contains a large quantity of SO2 and NO. In this paper, a process of simultaneous removal of NO and SO2 by ozone oxidation combined with NaOH absorption was chosen. The main investigations involved O3 decomposition, factors affecting NO oxidation (O3 dosage, reaction temperature, NO initial concentration, and presence of SO2), and NaOH absorption. The results indicated O3 decomposition rate increased as temperature rose and was less affected by initial concentration of O3. The optimal temperature for NO oxidation was 150 °C. NO oxidation efficiency increased with the increase of O3 dosage at a fixed temperature. NO initial concentration and the presence of SO2 had a slight effect on NO oxidation. The NO oxidation efficiency remained above 90% when nO3/nNO was 1. Absorption by NaOH solution resulted in the final removal of above 99% NO, 90% NO2, and nearly 100% SO2 at pH above 11.

1. INTRODUCTION Various kinds of air pollutants are formed during coal combustion, among which sulfur dioxide (SO2) and nitrogen oxides (NOx) are abundant in the flue gas. These pollutants will cause serious acid precipitation and air pollution; hence, they are a grave environmental problem. Conventionally, wet flue gas desulfurization (WDFG) is a widely applied technology for removing SO2 in most power stations, and SO2 removal can reach above 95%.1,2 As for controlling the emission of NOx, selective catalytic reduction (SCR) technology and selective noncatalytic reduction (SNCR) technology are preferably applied commercially in the removal systems, of which SCR is more efficient in removal of NOx (>90%).3−5 From the consideration of process economy, removing SO2 and NOx separately requires more investment and operation cost; therefore, technologies for simultaneous removal of SO2 and NOx have been developed.6,7 Usually the technologies can be classified into catalytic reduction and oxidation absorption according to the method of denitrification. Catalytic reduction processes such as SNOX,8−11 SNRB,12 DESONOX,13 or activated carbon process14−16 remove NO in the presence of catalysts or reductants. In oxidation absorption processes, NO is converted to NO2 by oxidants or free radicals. NOXSO, nonthermal plasma technology, and electron beam irradiation technology are of this process category.17−19 The most widely applied technology today is WFGD (with CaCO3) combined with SCR, which can remove SO2 above 90% and NO above 80%. However, it shows some drawbacks mainly related to the oxidation of SO2 in the SCR systems. About 0.2 to 2% SO2 in the system will be oxidized to SO3. The formed SO3 will react then with CaO and ammonia to produce CaSO4 and (NH4)2SO3. These undesired products can easily form scale so that NO removal efficiency decreases because of catalyst deactivation, clogging, and corrosion sped up in the heat exchangers and other equipment.20 Ozone oxidation process is © 2014 American Chemical Society

a potential technology with high removal efficiency, fast reaction without presence of catalysts, and hence low equipment investment. In the ozone oxidation process for simultaneous desulfurization and denitrification, NO is first oxidized to oxides having higher valence nitrogen (such as NO2 and N2O5), followed by absorption of NOx and SO2 simultaneously by alkali reagents in the wet scrubber. LoTOx technology is a selective, lowtemperature oxidation technology developed by Belco Technologies Corporation (BOC, U.S.). The final removal efficiency of NO is higher than 90%.21,22 Mok et al.23 realized the simultaneous removal of NOx and SO2 by ozone injection in a bench scale test facility and identified the strong oxidizing property of O3. Wang et al.4,24 confirmed that it was possible to achieve the goal of removing about 97% of NO and nearly 100% of SO2 through ozone oxidation and wet scrubbing at 100 °C. Moreover, they also concluded that the presence of SO2 had little effect on the NO oxidation process. Ma et al.25 applied O3 oxidation and absorption with alkaline solution to study the removal of NO and SO2 in the flue gas within the temperature range of 20−60 °C. The results showed that NO removal efficiency could reach 93% when nO3/nNO was 0.8 and removal of SO2 was nearly 100%. The main byproducts of the ozone oxidation process for removing NO and SO2 simultaneously are nitrite, nitrate, sulfite, and sulfate. In most industries, the products are first converted to nitrate and sulfate to reduce the chemical oxygen demand and then discharged into wastewater pools or discharged directly.26,27 Meanwhile, many scholars are studying Received: Revised: Accepted: Published: 6450

October 11, 2013 March 19, 2014 March 26, 2014 March 26, 2014 dx.doi.org/10.1021/ie403423p | Ind. Eng. Chem. Res. 2014, 53, 6450−6456

Industrial & Engineering Chemistry Research

Article

cooling pipe to be cooled and then entered the bubbling reactor where NOx and SO2 were removed simultaneously. The concentrations of NO, NO2, and SO2 were monitored by a flue gas hand-held analyzer Optima 7 (MRU, Germany) from the sample connections labeled b and c in Figure 1. The concentration of O3 was measured according to the iodometric method (CJ/T 3028.2-1994) from sample connections a and b, calculated by

methods of recycling them because of their extensive applications. For example, nitrate sodium can be used as fertilizer and sulfate sodium can be used for paper production.28,29 Sinopec is one of the main petroleum corporations in China. With the strict air pollution regulations, Sinopec is making an effort to develop technologies aiming at decreasing the content of NO and SO2 in flue gas. A SNCR process has been adopted in the Sinopec Zhenhai Refining & Chemical Company, but because of its poor removal efficiency, the outlet NO concentration of SNCR of still nearly 250 mg/Nm3 demands further treatment (“Nm3” specifies the volume at normal temperature and pressure). Therefore, ozone oxidation process is an option. The purpose of this work is to design a flue gas treatment process utilizing ozone as an oxidant and sodium hydroxide as an absorbent to realize the efficient simultaneous removal of NO and SO2 from flue gas. According to the available literature, almost all the reported investigations were carried out at low temperature below 100 °C. In this paper, the research was carried out within a wide range of temperature from 50 to 250 °C, closer to the actual flue gas temperature.

CO3 = 2.4 × 104ANa B /V0

(1)

where ANa is consumption of Na2S2O3 (mL), B standard concentration of Na2S2O3 (mol/L), and V0 sampling volume of O3 (mL). The concentration of O3 could be adjusted by changing the electric current of the ozone generator. When the inflow of O2 was 1 L/min, the higher the electric current applied to the DBD, the more ozone was produced while electric current varied from 0 mA to 300 mA (peak value). The relationship between O3 concentration and electric current is indicated in Figure 2. According to the requirement of the following experiments, 80 mA was chosen to be the optimal electric current.

2. EXPERIMENTAL SETUP AND METHOD The experimental setup consists of an ozone generator, a gasphase reactor with temperature control, a cooling pipe, and a glass bubbling reactor as shown in Figure 1. Ozone was

Figure 2. Concentration of O3 by varying electric current.

In all experiments, the total flow rate and oxygen content of the simulated flue gas were fixed at 1.05 Nm3/h and 12%, respectively. Decomposition of ozone was first tested to see how it behaved before oxidation of NO. The parameters in this paper included reaction temperature from 50 to 250 °C, residence time from 1.2 to 5 s, and O3 initial concentration from 190 to 480 mg/Nm3. Oxidation of NO alone was conducted with variation of O3 dosage (nO3/nNO) from 0.3 to 1.3, reaction temperature from 50 to 250 °C, and NO initial concentration from 150 to 1000 mg/Nm3 to investigate the effects of O3 dosage and NO initial concentration on oxidation and determine the optimal reaction temperature. SO 2 concentration of 2000 mg/Nm3 was added into the flue gas, and the NO oxidation efficiency in the presence of SO2 was compared with that without SO2. Finally, through NaOH absorption, the final removal efficiencies of NO, NO2, and SO2 were all calculated and the pH values of NaOH solution were recorded. The relationship between pH value and removal efficiency was discussed.

Figure 1. Schematic diagram of the experimental apparatus. 1, air pump; 2, gas volumetric flow meter; 3, gas-phase reactor; 4, cooling pipe; 5, ozone generator; 6, bubbling reactor; 7, thermostatic water bath; 8, rubber stopple; 9, aerator; 10, rotor; 11, pH meter; a, b, c, d, sample connections.

produced by dielectric barrier discharge method (DBD) through the ozone generator manufactured by Qingdao Guolin Industry Co., Ltd. (model CF-G-3-10). The ozone generator required O2 to be a gas source while some kinds of ozone generators need air instead of O2. The gas-phase reactor was made of stainless steel, with an inner diameter of 32 mm and length of 700 mm. The reactor was heated by an electric jacket. The bubbling reactor was a cylindrical container with a total volume of 1.4 L. The simulated flue gas was prepared by air, N2, NO, and SO2. O2 was fed at 1 L/min into the ozone generator as the gas source to produce O3. The flow rates of all the gas feeds were controlled by gas volumetric flow meters. After oxidation with O3 in the stainless steel reactor, NO2 was produced and the flue gas containing NO, NO2, and SO2 first went through the

3. RESULTS AND DISCUSSION 3.1. Decomposition of Ozone. Ozone’s life cycle plays an important role in the success of flue gas oxidation technologies. If ozone decomposes into O2 far more quickly than NO is 6451

dx.doi.org/10.1021/ie403423p | Ind. Eng. Chem. Res. 2014, 53, 6450−6456


Article

oxidized at a certain inlet temperature, there will be no sense in applying this kind of ozone oxidation technology. Therefore, according to the actual temperature of SNCR in Sinopec Zhenhai Refining & Chemical Company, which is around 135 °C under normal conditions or more than 200 °C in abnormal situations, this set of experiments was first done to study ozone decomposition rates within the temperature range of 50 to 250 °C and the residence time of 1.2 s (minimum value) to 5 s. The gas mixture of air and ozone generated by the generator was adjusted to 275 mg/Nm3 as the initial ozone concentration, and the results are shown in Figure 3, where [O3]/[O3]0 is the ratio Figure 4. Concentration of O3 by varying O3 initial concentration at different residence times.

oxidation of NO by O3 is the first step in this process before its removal. The reaction is shown as reaction 3. In addition, many other reactions may take place at the same time, mainly including reactions 4−7.

Figure 3. Concentration of O3 by varying reaction temperature at different residence times.

(3)

NO2 + O3 → NO3 + O2

(4)

NO2 + NO3 → N2O5

(5)

N2O5 → NO2 + NO3

(6)

NO + NO3 → 2NO2

(7)

In this portion of the experiments, oxidation of NO alone was studied, and the simulated flue gas without SO2 was prepared. 3.2.1. Effect of Temperature. The effect of temperature on the oxidation of NO was tested, and the NO initial concentration was set at around 200 mg/Nm3. The results are shown in Figure 5, where nO3/nNO symbolizes molar ratio of O3 dosage and initial content of NO.

of O3 concentration before and after decomposition. The relationship between [O3]/[O3]0 and O3 decomposition rate can be described by eq 2. α = (1 − [O3]/[O3]0 ) × 100%

NO + O3 → NO2 + O2

(2)

where [O3] is O3 concentration after decomposition, sampling from the sample connection b, mg/Nm 3 ; [O 3 ] 0 is O 3 concentration before decomposition, sampling from the sample connection a, mg/Nm3. As Figure 3 shows, the outlet concentration of O3 decreases with the increase of temperature and residence time. When the temperature increases up to 200 °C, the decomposition rate obviously becomes much greater. When the temperature reaches 250 °C, it takes only 4 s to decrease the ozone concentration to 10 mg/Nm3 , and the corresponding decomposition rate is up to 96%. After 5 s, almost all the ozone decomposes and the outlet concentration of ozone is too low to be detected. The ozone decomposition rates at 150 °C were studied by varying O3 initial concentration from 190 to 480 mg/Nm3. The results are shown in Figure 4. As Figure 4 shows, the change trends of ozone decomposition rates are almost the same with the increase of O3 initial concentration at 150 °C. When the O3 initial concentration is 190 mg/Nm3, at 2 and 4 s, the decomposition rates are 16.7 and 27.8%, respectively, while at the O3 initial concentration of 480 mg/Nm3, the decomposition rates are 14.0% and 26.7%. Therefore, it can be concluded that O3 decomposition is mainly influenced by flue gas temperature and less effected by its initial concentration. 3.2. Oxidation of NO Alone by O3. In most practical flue gas, because of the low solubility, removing NO is more difficult than removing any other NOx species with high solubility, such as NO2 and N2O5 which can be easily captured in the downstream SO2 removal equipment. Therefore,

Figure 5. NO oxidation efficiencies under different temperatures and O3 dosages.

As Figure 5 shows, when the NO initial concentration is set at around 200 mg/Nm3, the NO oxidation efficiencies within the temperature range of 50 to 150 °C have no significant difference; the average oxidation efficiencies are around 85% when nO3/nNO is 0.9. The NO oxidation efficiencies begin decreasing when the reaction temperature reaches 200 °C; significant decrease appears after 250 °C when the NO oxidation efficiencies decrease to 75.9%, which is undoubtedly dependent on O3 decomposition rates as shown in Figure 3. 6452



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The NO oxidation efficiency increases with the increase of O3 dosage when reaction temperature is fixed. For example, at 150 °C, when nO3/nNO is 0.3, 0.6, and 0.9, the NO oxidation efficiencies are 34.3, 66.9, and 87.6%, respectively; when nO3/ nNO reaches 1 and 1.3, the NO oxidation efficiencies are more than 90 and 99%, respectively. Moreover, CHEMKIN dynamics simulation showed that it took only 0.01 s to finish the reaction between O3 and NO at 150 °C.24 The test results are coincident with this simulation. In other words, there is little effect of O3 decomposition on the NO oxidation efficiency at 150 °C. Therefore, considering the temperature of outlet flue gas from SNCR in Sinopec Zhenhai Refining & Chemical Company, 150 °C was chosen as the most suitable temperature in the following experiments. 3.2.2. Effect of NO Initial Concentration. Figure 6 shows the effect of NO initial concentration varying from 150 to 1000

Figure 7. Mass ratios of NO2 and NO under different NO initial concentrations and O3 dosages.

efficiency in the presence of SO2 and thus determine the optimal dosage of O3. During the experiments, NO initial concentration was around 200 mg/Nm3 and SO2 initial concentration was around 2000 mg/Nm3. The results are shown in Figure 8.

Figure 6. NO oxidation efficiency by varying NO initial concentration under different O3 dosages.

mg/Nm3 on the NO oxidation efficiencies. Under the same conditions, the oxidation efficiencies of NO at low concentrations (from 150 to 300 mg/Nm3) are almost the same, whereas the NO oxidation efficiencies at high concentrations (from 500 to 1000 mg/Nm3) are only a little higher. As compared with the effect of nO3/nNO ratio and reaction temperature, the initial concentration of NO has the least effect on the oxidation efficiency of NO. According to reaction 3, 1 mol of NO can be oxidized to 1 mol of NO2 by 1 mol of O3, so the mass ratio calculated by NO2 produced and NO consumed (mNO2/mNO) should be 1.53 before nO3/nNO becomes more than 1. As Figure 7 shows, the mass ratios in the low-concentration range are nearly equal to 1.53 when nO3/nNO is lower than 1, though deviation exists mainly caused by the accuracy of the flue gas hand-held analyzer; after nO3/nNO reaches 1, the mass ratios begin decreasing. However, the mass ratios in the high-concentration range begin decreasing when nO3/nNO is 0.6, which is mainly caused by reaction 4. As reaction 3 continues, NO concentration in the system becomes lower so that reaction 3 slows down. On the other hand, NO2 concentration becomes higher, promoting reaction 4. As observed, the higher the NO initial concentration, the more significant the change trend of mass ratios. 3.3. Simultaneous Removal of NO and SO2. 3.3.1. Simultaneous Oxidation of NO and SO2 by O3. In this set of experiments, SO2 was added to study the NO oxidation

Figure 8. Comparison between NO oxidation efficiencies affected by SO2.

The NO oxidation efficiency becomes lower when NO is in the presence of SO2 in the system. In other words, the existence of SO2 in the flue gas will inhibit the oxidation of NO. But this negative effect is limited, and the NO oxidation efficiency still can reach above 90% when nO3/nNO is 1. At the same time, with reaction 8, SO2 is oxidized by O3 in the gas reactor and the SO2 oxidation efficiencies with different dosages of O3 are only about 5%, which is the main reason for the decrease of the NO oxidation efficiency. The big difference between NO and SO2 oxidation efficiencies is due to the activation energies of reactions 3 and 8 which are 3.176 and 58.17 kJ/mol, respectively.23,30 The higher activation energy of reaction 8 makes the reaction between SO2 and O3 more difficult. In addition, SO2 also can react with NO2 as reaction 9 whose activation energy is as high as 113 kJ/mol.30 Therefore, it is concluded that the presence of SO2 has little impact on the oxidation of NO and the optimal molar ratio (nO3/nNO) is 1; the low oxidation efficiency of SO2 will not cause serious corrosion of equipment, which is good for application of the ozone oxidation process.

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SO2 + O3 → O2 + SO3

(8)

SO2 + NO2 → NO + SO3

(9)



Article

3.3.2. Simultaneous Removal of NO and SO2 after Absorption by NaOH. After the oxidation of NO and SO2 by O3, the flue gas containing NO, NO2 and SO2 passed through the cooling pipe to the bubbling reactor containing 1.1 L of 0.04 mol/L NaOH solution. The reaction temperature was set at 150 °C; the initial concentrations of NO and SO2 were 200 and 2000 mg/Nm3, respectively; nO3/nNO was fixed at 1. The whole absorption process lasted 1.5 h. Removal efficiencies of NO, NO2, and SO2 were calculated and saved automatically in the computer at intervals of 3 to 5 min; pH values of the solution were recorded manually at intervals of 1 to 5 min. The results are shown in Figures 9 and 10.

than 3 have been proven unfavorable for the removal of SO2. The main reactions are as follows: SO2 + 2NaOH → Na 2SO3 + H 2O

(10)

SO2 + NaOH → NaHSO3

(11)

NO2 in the flue gas is produced by oxidation of NO, and nearly all of the oxidation products of NO are NO2 when the NO initial concentration is 200 mg/Nm3 and nO3/nNO is equal to 1. As observed, the removal efficiency of NO2 varies with the variation of pH, which can be classified into 3 stages. In the first 55 min when the pH value of NaOH solution is above 11.13, the removal efficiency of NO2 decreases slowly and can be about 90%; in the second stage, from 55 to 65 min when the pH value drops sharply, removal efficiency also drops quickly from 90% to 60%; after 65 min when the pH value is below 4, removal efficiency begins increasing instead to 67% at the end of the experiment. This phenomenon is coincident with the conclusions made by Zhang who found that the absorption rate of NO2 in the dilute nitric acid was faster than that in the water and the rate increased proportionately with solution acidity.31 The main reactions here are as follows: NO + NO2 + 2NaOH → 2NaNO2 + H 2O

(12)

2NO2 + 2NaOH → NaNO2 + NaNO3 + H 2O

(13)

4. ECONOMIC STUDY Table 1 shows the basic data of the flue gas from Zhenhai Refining & Chemical Company. According to the experimental

Figure 9. pH values of NaOH solution with variation of time.

Table 1. Basic Flue Gas Data

actual flue gas emission standard

flue gas volume (Nm3/h)

NOx (mg/Nm3)

SO2 (mg/Nm3)

dust (mg/Nm3)

400 000

250 ≤100

1200 ≤200

50 ≤30

results, a simple economic study is performed (Table 2) which excludes infrastructure cost and fixed cost such as maintenance, depreciation, and so on. Annual working hours are 8000 h. As Table 2 shows, the total cost for the process is about 7242 yuan/ton and a large amount of the cost results from ozone generation, so controling the ozone consumption will be a key point for further reducing cost.

Figure 10. Removal efficiencies of NO, NO2, and SO2 after absorption.

5. CONCLUSIONS The simultaneous removal of NO and SO2 from flue gas was investigated. We applied ozone oxidation technology to oxidize NO to NO2 first, and then used NaOH as an absorbent to capture NO, NO2, and SO2 simultaneously. Decomposition of O3 was analyzed, and the results showed that temperature was a key factor for ozone decomposition rather than initial concentration of O3. When the temperature was higher than 200 °C, O3 decomposition rate increased much more quickly; almost all ozone decomposed in 5 s at 250 °C. NO oxidation efficiency was mainly in proportion to O3 dosage and in inverse proportion to flue gas temperature. The NO oxidation efficiency began decreasing at 200 °C, so 150 °C was chosen to be the suitable temperature for study according to the actual flue gas temperature of SNCR. NO initial concentration within a narrow range basically had no effect on oxidation of NO, but NO with very high initial concentration could obtain a slight

Figure 9 shows the solution’s pH value during the absorption decreases with time. At the first stage, the pH value decreases gradually from 12.34 to 11.13 in 55 min. After 55 min, the pH value suddenly drops from 11.13 to 3.6 in 10 min before the decrease becomes so slow that the final pH value is below 3 at 90 min. Figure 10 shows the removal efficiencies of NO, NO2, and SO2 at different times during absorption. As observed, the removal efficiency of NO remains stable at nearly 99% owing to the strong oxidizing ability of O3. From the previous studies, the NO oxidation efficiency is nearly 93% at 150 °C when nO3/ nNO is 1; it is reactions 12 and 13 lead to further removal of NO. SO2 can be removed efficiently in the first 70 min when the SO2 removal efficiency is above 90%, while after 70 min, when the pH value is below 3, the removal efficiency decreases quickly from 90% to about 40%. Therefore, pH values lower 6454



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Table 2. Economic Analysis no.

item

unit

unit price (yuan)

yearly consumption

unit cost (yuan/ton)

total cost (yuan)

1 2 3 4

oxygen power consumption circulated cooling water 30% NaOH

m3 kwh t t

0.6 0.6 2.7 450

1 240 000 1 872 000 160 992 20 000

1550 2340 906 2446

744 000 1 123 200 434 678 9 000 000

total

7242

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increase in its oxidation efficiency under the same conditions. The presence of SO2 slightly influenced the oxidation of NO, and the SO2 oxidation efficiency by O3 was only about 5%. At 150 °C, above 90% NO could be oxidized to NO2 when nO3/ nNO was 1. After oxidation by O3 in the gas-phase reactor, the flue gas containing NO, NO2, and SO2 finally entered the bubbling reactor containing NaOH solution. The absorption resulted in the final removal of above 99% NO, 90% NO2, and nearly 100% SO2 at pH above 11. A simple economic study showed the cost for ozone oxidation process was about 7242 yuan/ton.

■

AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-021-64252058. Fax: +86-021-64252737. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■ ■

ACKNOWLEDGMENTS This work was supported by Sinopec Ningbo Engineering Co. Ltd. REFERENCES

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(30) Su, M. The Experimental Research on Simultaneous Removal of SO2 and NOx by Advanced Oxidation Process with O3. Diploma Thesis, North China Electric Power University, Beijing, 2010. (31) Zhang, F.; Wang, X.; Yuan, G.; et al. Absorption of Nitrogen Dioxide in Water and Dilute Nitric Acid Solution with ConstantVolume Absorption System. Chin. J. Inorg. Chem. 2013, 29, 95−102.

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