Experimental study on simultaneous desulfurization and denitrification from flue gas with composite absorbent

Authors

  • Yi Zhao,

    1. School of Environmental Science and Engineering, North China Electric Power University, Baoding 071003, People's Republic of China
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  • Tianxiang Guo,

    1. School of Environmental Science and Engineering, North China Electric Power University, Baoding 071003, People's Republic of China
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  • Zhouyan Chen

    Corresponding author
    1. School of Environmental Science and Engineering, North China Electric Power University, Baoding 071003, People's Republic of China
    • School of Environmental Science and Engineering, North China Electric Power University, Baoding 071003, People's Republic of China
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Abstract

The characteristics of desulfurization and denitrification with composite absorbent were researched through aqueous absorption experiments. The removal efficiencies of SO2 and NO were up to 100 and 95%, respectively. The composite absorbent included NaClO2 and component M. Existence of component M in the solution could reduce the absorbent cost compared with using sodium chlorite alone. Chlorine dioxide, as main reaction intermediate product, participated in oxidation reaction. The optimal experimental conditions involved NaClO2 concentration of1.13 mmol/L, solution pH of 5.5, molar ratio (M/NaClO2) of 4.1, reaction temperature of 323 K. The optimal solution pH and reaction temperature were both in the required ranges of limestone-gypsum wet flue gas desulfurization (FGD) process. The research could offer primary data and important references for simultaneous removal of SO2 and NOx in limestone-gypsum wet FGD process. © 2010 American Institute of Chemical Engineers Environ Prog, 2011

INTRODUCTION

China has become one of those countries in which air pollution problem caused by sulfur oxides and nitrogen oxides is most severe [1, 2]. Sulfur oxides and nitrogen oxides are both important precursors of acid rain. Moreover, nitrogen oxides are also important precursors of photochemical smog. Although China has got very significant performance in flue gas treatment, the status is still severe. At present, traditional wet flue gas desulfurization (FGD) process and selective catalytic reduction denitrification process (SCR) are usually applied for joint removal of SO2 and NOx. But the application of individual treatment mode is restricted due to high initial invest, high running cost, and big floor space. So the exploitation of new simultaneous removal technologies of SO2 and NOx has become hot spot. Wet FGD process has a big proportion in present FGD market. Improving wet FGD process to achieve simultaneous removal of SO2 and NOx will have a wide application prospect. The absorbents for simultaneous removal of SO2 and NOx in aqueous have widely been investigated [3–9], including NaClO2/NaOH, FeSO4/H2SO4, KMnO4/NaOH, Fe(II)EDTA, Na2SO3, FeSO4/Na2SO3, H2O2, CO(NH2)2, Molybdenum blue, and so forth. Among these, NaClO2 is one of the most effective reagents [10–17]. But for simultaneous removal of SO2 and NOx from flue gas, most of the absorbent are consumed by SO2. In addition, it is estimated that about 1.38 pounds of NaClO2 are needed to remove a pound of NOx; therefore, the absorbent cost is higher in large scale because of high price of NaClO2. To reduce the cost, the experimental studies on simultaneous removal of SO2 and NOx with composite absorbent based on NaClO2 were carried out in this article. The results could offer primary data and important references for simultaneous removal of SO2 and NOx in limestone-gypsum wet FGD process.

EXPERIMENTAL

Experimental Apparatus

Figure 1 was the experimental apparatus that was made of flue gas simulation system, reaction system, gas sampling, and analysis system. The reactor was a small bubbling reactor with height of 15 cm and volume of 1 L. The gas flow rates of SO2, NO, N2, and O2 were controlled by the rotameters, respectively. SO2 and NOx were diluted by N2 rapidly in primary surge flask in order to avoid the production of NO2. Simulated flue gas formed when SO2 and NO were diluted by N2 and O2 into acquired concentrations in secondary surge flask. The oxidization and absorption reactions occurred when simulated flue gas entered into bubbling reactor, the exhaust gas was discharged into the atmosphere by passing the exhaust gas processing unit.

Figure 1.

Experimental schematic diagram of simultaneous removal of SO2 and NOx from flue gas with aqueous solution. 1–O2 cylinder, 2–NO cylinder, 3–SO2 cylinder, 4–N2 cylinder, 5–pressure relief valve, 6–valve, 7–rotameter, 8–primary surge flask, 9–secondary surge flask, 10–bubbling reactor, 11–constant temperature water bath shaker, 12–desiccator, 13–flue gas analyzer, 14–exhaust gas processing unit.

Reagents

Sodium chlorite (NaClO2) was technical grade reagent and from Tianjin Tanggu Chemical Reagent Factory. M(NaClO, sodium hypochlorite), hydrochloric acid (HCl), and sodium hydroxide (NaOH) were analytical pure reagents and from Tianjin Chemical Factory, Tianjin Zhenxing Chemical Reagent Acid Factory, Tianjin Huadong Chemical Reagent Factory, respectively.

Experimental Method

During the experiments, SO2 and NO were oxidized and absorbed by NaClO2 and M solutions when simulated flue gas entered into the reactor, the reactions are as follows.

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The inlet and outlet simulated flue gas were analyzed by the analyzer (MRU95/3 CD analyzer, made in Germany, with SO2 measurement range of 0–4000 ppm; NO range of 0–4000 ppm; NO2 range of 0–1000 ppm). Removal efficiencies were calculated according to difference of inlet and outlet concentrations of SO2 and NO, separately. The reaction temperature was regulated by constant temperature water bath shaker (THZ-82 type). The solution pH was regulated by adding HCl or NaOH, and monitored by PH-meter (PHS-3C type). Flow rate of flue gas was controlled by the rotameter.

RESULTS AND DISCUSSION

Effect of Solution pH on Removal Efficiencies

Because of the close relationship between oxidation–reduction potential of the solution and pH, experimental research of the solution pH on removal efficiencies of SO2 and NO was performed. The results are shown in Figure 2.

Figure 2.

Effect of solution pH on removal efficiencies of simultaneous desulfurization and denitrification. The absorption temperature was 298 K; flue gas flow rate was 0.12 m3/h; NaClO2 concentration was 1.0 × 10−4 mol/L; component M concentration was 4.1 × 10−4 mol/L; NO concentration was 600 mg/m3; SO2 concentration was 2000 mg/m3.

As shown in Figure 2, removal efficiency of SO2 almost retained 100% and did not change with variation of the solution pH. It can be seen from Figure 2 that the removal efficiency decreased slowly when the solution pH increased from 2.5 to 5.5, and then decreased quickly when the solution pH increased above 5.5, which may be resulting from the formation of a stronger oxidizing species, ClO2, in acid solution, and this reaction rate was slower in higher solution pH than that in lower solution pH, hence, the removal efficiency of NO decreased in higher solution pH. According to the previous work [18] and our experimental results, the removal efficiency of NO could be raised at pH lower than 2, but considering the application to limestone-gypsum wet FGD process, pH of composite absorbent solution was selected at 5.5.

Effect of Concentrations of M and NaClO2 on Removal Efficiencies

The effect of concentrations of M and NaClO2 on removal efficiencies of SO2 and NO was shown in Table 1. As shown in Table 1, the removal efficiency of SO2 was weakly affected by the concentrations of M and NaClO2, which had significant effect on the removal efficiency of NO. The removal efficiency of NO was very low using M alone as an absorbent and that was also not high using NaClO2 alone as an absorbent. But it was obviously improved by adding M into NaClO2 solution and increased with an increase of the concentration of NaClO2, indicating that M was favorable for simultaneous removal of SO2 and NO when composite absorbent of M and NaClO2 was used as an absorbent.

Table 1. Experimental results with composite absorbent.
Item no.Solution pHM concentration (mol/L)NaClO2 concentration (mol/L)Removal efficiency of SO2 (%)Removal efficiency of NOx (%)
160.000280.0005310063
260.000280.0005310060
360.000280.0009610074
460.000280.0009610076
5600.0009610035
6600.0009610035
760.00028010010
860.00028010012

Determination of Economical Molar Ratio (M/NaClO2)

Because of obvious price advantage of M compared with NaClO2, the absorbent cost would be decreased when M was added into NaClO2 solution in the required removal efficiency of NO. To look for the economical molar ratio (M/NaClO2), the costs of absorbent (NaClO2 price of 16,000 ¥/t, M price of 700¥/t) in different experimental conditions were compared, as shown in Figure 3, on the whole, the cost of absorbent increased gradually with an increase of the required removal efficiency of NO, for the required higher removal efficiency of NO such as, efficiency of 90% and efficiency of 80%, the cost of absorbent decreased markedly, and that decreased gradually in the required lower removal efficiency of NO such as, efficiency of 70% and efficiency of 60% when the molar ratio (M/NaClO2) was between 2 and 4.1. However, the change occurred at 4.1. Thereafter, the cost of absorbent increased. Hence, the molar ratio (M/NaClO2) was selected as 4.1.

Figure 3.

Cost curve with different molar ratios (M/NaClO2) varying. [Color figure can be viewed in the online issue which is available at wileyonlinelibrary.com.]

Effect of Reaction Temperature on Removal Efficiencies

To investigate the effect of the reaction temperature on removal efficiencies, removal experiments in the different reaction temperature were carried out. The results are shown in Figure 4.

Figure 4.

Effect of reaction temperature on removal efficiencies of simultaneous desulfurization and denitrification. The flue gas flow rate was 0.12 m3/h; NaClO2 concentration was 1.0 × 10−4 mol/L; component M concentration was 4.1 × 10−4 mol/L; solution pH was 5.5; NO concentration was 600 mg/m3; SO2 concentration was 2000 mg/m3.

As shown in Figure 4, removal efficiency of SO2 still retained 100% and removal efficiency of NO had a variation trend of increasing at first and then dropping with the reaction temperature rising. There were two reasons for this. On one hand, diffusion rates of correlated molecules and ions accelerated so that reaction rate and absorption rate of NO increased, which was helpful to increase the removal efficiency. But at the same time, solubility of NO would be reduced with the reaction temperature rising. It had adverse effect on the removal efficiency. So, the optimal reaction temperature was selected as at 323 K according to experimental results.

Parallel Experiments

According to experimental results and present wet FGD process conditions, the optimal experimental conditions for simultaneous removing SO2 and NO by composite absorbent were obtained, in which molar ratio (M/NaClO2) was 4.1, reaction temperature was 323 K, and solution pH was 5.5. Parallel experiments were performed under the optimum conditions mentioned above. Results are shown in Table 2.

Table 2. Results of parallel tests.
No12345AverageStandard deviation (%)
Removal efficiency of SO2 (%)1001001001001001000
Removal efficiency of NOx (%)95.59594.295.994.995.10.576

As shown in Table 2, the average removal efficiency of SO2 was 100%, and that of NO was 95.1%. The sample standard deviations were 0 for SO2 and 0.576% for NO, respectively. The better reproducibility of removal efficiencies of SO2 and NO indicates that this method has the stable performance.

REACTION MECHANISM

It can be seen from Table 1 that the removal efficiency of NO was improved when the composite absorbent was used, indicating that the absorption reaction of NO was promoted in the presence of M. When the solution pH was 5.5, the electrode potentials of HClO/Cl and ClO2/ClO2 were 1.3311 and 1.0706 V, respectively, which were calculated according to the Nernst equation, indicating that ClO2 can be oxidized into ClO2 by HClO in slightly acid solution. Therefore, ClO2 as the important intermediate was produced. In addition, a green solution resulting from the dissolution of ClO2 [18, 19] was observed in experiments, and the solution color became deeper with the M concentration increasing, it was confirmed that ClO2 existed in this reaction system. For the oxidation of SO2 and NO, the electrode potentials of SO42−/HSO3 (0.0569 V) and NO3/NO (0.5172 V) were slower than those of ClO2/Cl (1.244 V), HClO/Cl (1.3311 V), and ClO2/Cl (0.999 V); hence, SO2 and NO could be oxidized to SO42− and NO3, respectively, by ClO2, ClO2, and HClO. The reactions of simultaneous desulfurization and denitrification were shown in “Experimental Method” section.

CONCLUSIONS

  • 1With simultaneous removal of SO2 and NOx, the consumption of NaClO2 can be decreased by using the composite absorbent at the same removal efficiencies, therefore, the absorbent cost decreased.
  • 2Removal efficiency of NO decreased with the solution pH increasing, increased first and then decreased with the reaction temperature rising. The optimal experimental conditions involved NaClO2 concentration of 1.13 mmol/L, solution pH of 5.5, molar ratio (M/NaClO2) of 4.1:1, reaction temperature of 323 K. The composite absorbent has better performance of simultaneous desulfurization and denitrification when the solution pH and reaction temperature were both consistent with the required process conditions of present limestone-gypsum wet FGD process.
  • 3Under the optimal experimental conditions, the removal efficiencies of SO2 and NO reached 100 and 95%, respectively. These results can provide the primary data and reference for industrial research of simultaneous removal of SO2 and NOx in limestone-gypsum wet FGD process.
  • 4Chlorine dioxide, as important intermediate reaction product, participated in oxidation reactions.

Acknowledgements

The authors appreciate the financial support provided by “Project of Technology Research and Development of Hebei province.”

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