Removal of SO2 from Flue Gas by Sodium Humate Solution

Jan 20, 2010 - Experiments were carried out to examine the effect of various operating ... efficiency and desulfurization time in a lab-scale bubbling...
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Energy Fuels 2010, 24, 1013–1019 Published on Web 01/20/2010

: DOI:10.1021/ef901052r

Removal of SO2 from Flue Gas by Sodium Humate Solution Zhiguo Sun,† Yu Zhao,† Hanyang Gao,‡ and Guoxin Hu*,† †

School of Mechanical and Power Engineering and ‡School of Environmental Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China Received September 17, 2009. Revised Manuscript Received January 4, 2010

A novel process of flue gas desulfurization (FGD) by sodium humate (HA-Na) solution was developed. Experiments were carried out to examine the effect of various operating parameters, such as the HA-Na concentration, pH, temperature, gas flow rate, O2 concentration, SO2 inlet concentration, and NO2, on the SO2 absorption efficiency and desulfurization time in a lab-scale bubbling reactor. The experimental results indicate that the HA-Na concentration significantly influences the desulfurization time but has little effect on the SO2 absorption efficiency. The desulfurization time increases with the HA-Na concentration reaching 0.06 g/mL, and after this value, it begins to decrease. The SO2 absorption efficiency maintains 99% when pH is above 4.5. A low gas flow rate and low temperature are favorable to SO2 absorption. The increase of the SO2 inlet concentration improves the mass transfer of SO2 and accelerates the SO2 consumption rate. NO2 coexisting with SO2 can promote SO2 absorption because it may speed up oxidation of sulfite to sulfate. HA-Na solution shows great performance in SO2 absorption, and the SO2 absorption efficiency can be maintained above 98% for 1.5 h with 100 mL of HA-Na solution (0.06 g/mL), at the condition of 25 °C, gas flow rate of 0.14 m3/h, and inlet SO2 concentration of 2000 ppm. Moreover, the desulfurization products can be made into the humic acid (HA) compound fertilizer, and recycling water can be obtained in this desulfurization process.

HAs are transformed into modified products suitable for agronomic and industrial applications after they are subject to modifications, such as oximation, sulphonation, and nitration.5-9 HA-Na derived from low-rank coal is a cheap absorbent, and the use of HA-Na in SO2 abatement is of increasing interest in the past few years. Green et al.10,11 investigated the absorption of SO2 by HA-fly ash mixtures. However, neither the removal of NOx nor the desulfurization byproduction was mentioned. Zhao et al.12 made use of HA as a special additive to modify Ca-based adsorbents for flue gas desulfurization. Recently, Hu et al.13,14 used humate for the

1. Introduction The removal of combustion gases, such as SO2 and nitrogen oxides (NOx), mainly produced in power plants by the use of fossil fuels, has been the subject of many studies in recent years because of the environmental problems that these pollutants generate. Although SO2 emitted in the combustion of fossil fuels is one of the key precursors to acid rain and a major air pollutant, it is also an important and useful resource of sulfur fertilizer if it is reasonably adsorbed and transformed. At present, flue gas desulfurization (FGD) is one of the most effective techniques to control the emission of SO2 from the combustion of fossil fuels. Although the once-through nonregenerative wet FGD processes mainly based on limestone scrubbing are frequently used, they have many disadvantages, such as high capital and operating costs, a larger water requirement, poor quality of byproduct, and even causing secondary pollution.1-3 Thus, cost-effective technologies of removing SO2 have become the focus of investigation. Humic acids (HAs) are commonly believed to consist of high-molecular-weight and highly polydisperse heterogeneous molecules. They are widely distributed in nature and extracted from low-rank coal and other natural materials. Currently, HAs are used as additives in fertilizers.4 Moreover,

(5) Peng, C. Study of sodium humate on the property of fixing nitrogen in the ammonium phosphate production. Master’s Thesis, Sichuan University, Chengdu, Sichuan, China, 2005. (6) Stevenson, F. J. Humus Chemistry, 2nd ed.; John Wiley and Sons: New York, 1994. (7) Gondar, D.; Lopez, R.; Fiol, S.; Antelo, J. M.; Arce, F. Characterization and acid-base properties of fulvic and humic acids isolated from two horizons of an ombrotrophic peat bog. Geoderma 2005, 126, 367–374.  zı´ kova, J.; Tokarova, V.; (8) Novak, J.; Kozler, J.; Janos, P.; Ce Madronova, L. Humic acids from coals of the North-Bohemian coal field. I. Preparation and characterization. React. Funct. Polym. 2001, 47, 101–109. (9) Almerndros, G.; Martin, F.; Gonzalez-Vila, F. J.; del Rı´ o, J. C. The effect of various chemical treatments on the pyrolytic pattern of peat humic acid. J. Anal. Appl. Pyrolysis 1993, 25, 137–147. (10) Green, J. B.; Manahan, S. E. Sulphur dioxide sorption by humic acid-fly ash mixtures. Fuel 1981, 60, 330–334. (11) Green, J. B.; Manahan, S. E. Adsorption of sulphur dioxide by sodium humates. Fuel 1981, 60, 488–494. (12) Zhao, R.; Liu, H.; Ye, S.; Xie, Y.; Chen, Y. Ca-Based adsorbents modified with humic acid for flue gas desulfurization. Ind. Eng. Chem. Res. 2006, 45, 7120–7125. (13) Hu, G. X. Using humate to removal of sulphur dioxide (SO2) and nitrogen oxides (NOx) in flue gas and by-produce organic fertilizer. CN Patent 200710045443.2, 2008. (14) Sun, Z. G.; Gao, H. Y.; Hu, G. X. Preparation of sodium humate/R-aluminum oxide adsorbents for flue gas desulfurization. Environ. Eng. Sci. 2009, 26 (7), 1249–1255.

*To whom correspondence should be addressed. Telephone/Fax: þ86-21-34206569. E-mail: [email protected]. (1) Srivastava, R. K.; Jozewicz, W. Flue gas desulfurization: The state of the art. J. Air Waste Manage. Assoc. 2001, 51, 1676–1688. (2) Werther, J. Gaseous emissions from waste combustion. J. Hazard. Mater. 2007, 144 (3), 604. (3) Xu, X. C.; Chen, C. H.; Qi, H. Y.; He, R.; You, C.; Xiang, G. Development of coal combustion pollution control for SO2 and NOx in China. Fuel Process. Technol. 2000, 62, 153–160. (4) Pena-Mendez, E. M.; Havel, J.; Patocka, J. Humic substances: Compounds of still unknown structure: Applications in agriculture, industry, environment, and biomedicine. J. Appl. Biomed. 2005, 3, 13–24. r 2010 American Chemical Society

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Energy Fuels 2010, 24, 1013–1019

: DOI:10.1021/ef901052r

Sun et al.

Figure 1. Schematic diagram of the desulfurization and denitrification process by HA-Na solution.

removal of SO2 and NOx in flue gas and the production of organic fertilizer. On the basis of previous literature, this paper proposed a new process for simultaneous removal of SO2 and NOx from flue gas by HA-Na solution and the production of HA fertilizer. The new process (shown in Figure 1) includes the following stages: (1) HA-Na solution is first prepared by HA-Na powder and water in a dissolving tank and sprayed into an absorber. (2) Then, HA-Na solution simultaneously reacts with SO2 and NOx in the absorber. The desulfurization liquid (mainly containing HA, H2SO3, H2SO4, and HNO3) flows into a reaction tank from the absorber. (3) In the reaction tank, SO32- is oxidized to SO42- through diffused aeration and HA is subject to oxygenolysis by H2SO4 and HNO3. The activity of HA can be improved by oxygenolysis.15 Meanwhile, It is possible that HA is nitrified by the mixed acid of HNO3 and H2SO4;16,17 therefore, some HA is converted into nitro-HA, which is a better HA fertilizer. (4) Afterward, the reaction liquid from the reaction tank flows into a sedimentation tank and stands for 12 h. Because of its poor solubility, HA and nitro-HA may be separated as sediment from acidic solution. (5) The separated HA and nitro-HA can be used as a kind of material for compound fertilizer after drying. (6) The acidic solution from the sedimentation tank flows into a neutralization tank and is neutralized to pH 7 by lime. The neutralized solution is finally sent back to the dissolving tank and meets the requirement of recycling water. From the above process, it can be concluded that the simultaneous removal of SO2 and NOx by HA-Na solution is a resourceful type of environmental protection technology for FGD and has many advantages including: (a) lower costs and energy requirements, (b) almost no waste sludge, (c) the recovery of sulfur and nitrogen as a useful sulfur-containing nitrogen fertilizer,

and (d) the simultaneous removal of SO2 and NOx. Therefore, it is hopeful to be used in a large scale. In this paper, we focus on the effects of the HA-Na concentration, pH, temperature, gas flow rate, O2 concentration, SO2 inlet concentration, and NO2 on the SO2 absorption efficiency and desulfurization time in a lab-scale bubbling reactor, optimizing the operating conditions and analyzing the byproduction of desulfurization, which is the basis for large-scale use. 2. Desulfurization Mechanism The major mechanism of SO2 absorption into HA-Na solution is the acid-base reaction, and the acid-base theory predicts that HA-Na should react with SO2 by the following neutralization reaction:10,11 HA-NaðaqÞ þ SO2 ðgÞ þ H2 O T HAðsÞ þ HSO3 - ðaqÞ þ Naþ ðaqÞ

ð1Þ

At the same time, the following reactions accompanying the desulfurization process should be considered: HSO3 - ðaqÞ T Hþ ðaqÞ þ SO3 2- ðaqÞ

ð2Þ

2SO3 2- ðaqÞ þ O2 ðgÞ f 2SO4 2- ðaqÞ

ð3Þ

H2 O T Hþ ðaqÞ þ OH- ðaqÞ

ð4Þ

SO2 from flue gas is first dissolved into water solution. Then, the dissolved SO2 produces Hþ, HSO3-, and SO32- by ionization. The acidic groups of HA-Na, such as carboxyl (COO-) and hydroxyl (OH-) can react rapidly with Hþ, and HA-Na is transferred to HA sediment. According to eqs 1 and 2, this reaction may move the reaction equilibrium to the right, which results in more SO2 dissolved into solution. When all HA-Na is consumed, the desulfurization reaction is terminated.

(15) Zheng, P. Production and Application of Humic Acid from Coal; Chemical Industry Press: Beijing, China, 1991; pp 107-112. (16) Nandurkar, N. S.; Bhanushali, M. J.; Jagtap, S. R.; Bhanage, B. M. Ultrasound promoted regioselective nitration of phenols using dilute nitric acid in the presence of phase transfer catalyst. Ultrason. Sonochem. 2007, 14, 41–45. (17) Xing, Q. Y. Organic Chemistry; Peoples Education Press: Beijing, China, 1961; pp 389-483.

3. Experimental Section 3.1. Sample Preparation. Powdered HA-Na (g50 wt %) was provided by Shanghai Tongwei Biological and Technology Co., 1014

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: DOI:10.1021/ef901052r

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Figure 3. Comparison of HA-Na solution with water and NaOH. SO2, 2000 ppm; gas flow, 0.14 m3/h; absorption solution, 100 mL; 0.04 g/mL; without O2; 25 °C. Figure 2. Schematic diagram of the experimental apparatus.

HA-Na solution shows much better performance than water in both the SO2 absorption efficiency and desulfurization time. Absorption of SO2 by water is slow because it is only a physical absorption process controlled by molecular diffusion. However, the hydroxyl radicals (OH-) in HA-Na solution can react rapidly with the dissolved SO2, which may reduce the SO2 concentration in the gas-liquid interface; therefore, the SO2 diffusion can be promoted. Furthermore, according to eq 1, HA-Na can react with Hþ and is transformed into HA, which may move the reaction equilibrium to the right. Then, more SO2 is dissolved into water. Therefore, it can be concluded that the desulfurization capability of HA-Na solution mainly dependents upon the consumption of not only OH- but also HA-Na. It is also found that the desulfurization capability of HA-Na solution is better than NaOH solution at the same volume and pH. It can be explained as follows: As for the same volume and pH of NaOH and HA-Na solution, Naþ of NaOH is less than that of HA-Na; that is, OH- of NaOH is less than the acidic groups of HA-Na, such as COO- and OH-. The reason is that NaOH is a strong base but HA-Na is a salt of a weak acid. 4.2. Effect of the Inlet SO2 Concentration. Figure 4 shows the effect of the SO2 inlet concentration on absorption efficiency. It can be seen that the SO2 inlet concentration has not significant effect on the SO2 absorption efficiency, but the desulfurization time decreases greatly as the SO2 inlet concentration increases. The reason is as follows: with the SO2 concentration increasing, the SO2 partial pressure in flue gas is increased and the mass-transfer driving force is enhanced. Hence, the consumption of SO2 is faster, and the breakthrough time is decreased. This is favorable to the absorption of SO2. In addition, the increasing rate of mass transfer is less than that of SO2 in flue gas, which leads to a slight decrease of the SO2 absorption efficiency. 4.3. Effect of the HA-Na Concentration. It can be seen from Figure 5 that the SO2 absorption efficiency is always maintained about 98% with the increase of the HA-Na concentration from 0.01 to 0.12 g/mL, which demonstrates that the HA-Na concentration has little effect on the SO2 absorption efficiency. However, Figure 6 shows that the HA-Na concentration influences the desulfurization time obviously. The horizontal section increases from 10 to

Ltd., in Shanghai, China. Sodium hydroxide (NaOH, AR) was from Sinoharm Chemical Reagent Co. Ltd. (SCRC). Deionized water was applied to prepare the solution. 3.2. Desulfurization Test. A schematic diagram of the experimental apparatus is shown in Figure 2. All of the experiments for SO2 absorption were carried out in a bubbling reactor (diameter of 55 mm) at ambient pressure. A simulated flue gas consisted of 2000 ppm SO2, 340 ppm NO2, 0-10% O2, and the balance N2. The SO2, NO2, O2, and N2 gases were supplied from cylinders. The total flow rate of the simulated flue gas was controlled with a rotameter. The absorption temperature was adjusted with a water bath. The SO2 absorption efficiency can be obtained from the following formula: ðCin -Cout Þ  100% η¼ ð5Þ Cin where η is the SO2 absorption efficiency and Cin and Cout are the inlet and outlet SO2 concentrations, respectively. 3.3. Characterization Methods. The changes of the SO2 concentration at the inlet and outlet of the reactor were monitored by a flue gas analyzer (Testo-350XL, Germany). The functional groups of desulfurization products were identified using a Fourier transform infrared spectrometer (FTIR, EQUINOX 55, Germany BRUKER). The FTIR analysis was performed after drying, grinding the desulfurization products, and mixing them with KBr power to prepare sample KBr pellets.

4. Results and Discussion 4.1. Comparison of HA-Na Solution with Water and NaOH Solution. The change of the SO2 absorption efficiency of HA-Na solution with time is shown in Figure 3. It can be observed that the SO2 absorption curve of HA-Na solution is divided into two sections: a horizontal section and a descending section. In the horizontal section (in the beginning of 55 min), the SO2 absorption efficiency is maintained almost constant (99%); in the descending section (55-91 min), the SO2 absorption efficiency decreases rapidly until SO2 absorption saturation. The breakthrough time of SO2 for 100 mL of HA-Na solution (0.04 g/mL) is about 90 min. The contrast experiments of removing SO2 with water at the same volume and NaOH solution at the same pH 10 were also carried out, respectively. The results illustrate that 1015

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: DOI:10.1021/ef901052r

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Figure 4. Effect of the inlet SO2 concentration. Gas flow, 0.14 m3/h; absorption solution, 100 mL; 0.04 g/mL; without O2; 25 °C.

Figure 7. Effect of the temperature. SO2, 2000 ppm; gas flow, 0.14 m3/h; absorption solution, 100 mL; 0.04 g/mL; without O2.

after the HA-Na concentration reaches 0.06 g/mL. It can be deduced from eq 1 that the increase of the HA-Na concentration (from 0.01 to 0.06 g/mL) may provide more HA and remove more SO2. However, too high of a HA-Na concentration (g0.06 g/mL) makes it difficult for HA transformed from HA-Na to deposit from solution, because of the relative decreases of water with the HA-Na concentration increasing. It hinders eq 1 and decreases the desulfurization time. Hence, the optimum concentration of HA-Na solution is 0.06 g/mL, at which point the SO2 absorption efficiency is above 98% and the horizontal section is 90 min. 4.4. Effect of the Temperature. The effect of the temperature on the SO2 absorption efficiency was also explored. The experimental results (Figure 7) show the absorption temperature has little effect on the SO2 absorption efficiency but influences the desulfurization time. The horizontal section decreases from 64 to 31 min as the temperature ascends from 25 to 85 °C. It is obvious that low temperature is favorable to remove SO2. This can be explained as follows: High temperature decreases SO2 solubility in aqueous solution. According to the dissolution equilibrium of SO2 in water, the equilibrium partial pressure of SO2 is increasing with the temperature rising, which results in the escape of some dissolved SO2 from solution. On the other hand, a high temperature may speed up the relative velocity of gas molecules, and the contact time of gas and liquid is reduced. Although a low temperature can enhance the desulfurization performance of HA-Na solution, taking into account the desulfurization cost, room temperature should be suitable for removing SO2 with HA-Na solution. 4.5. Effect of the O2 Concentration. The effect of the O2 concentration on the SO2 absorption efficiency can be seen in Figure 8. It shows that the O2 concentration has an influence on the SO2 absorption result to some extent. The SO2 absorption efficiency has a slight increase with the O2 concentration rising from 0 to 10 vol %, while the desulfurization efficiency has little decrease. The reason is as follows: The O2 partial pressure in the gas phase rises with the increase of the O2 concentration, and then the O2 equilibrium concentration in the liquid phase is increased, which results in more O2 dissolved into solution. It can be deduced from eq 3 that the dissolved O2 may speed up the oxidation of

Figure 5. Effect of the HA-Na concentration. SO2, 2000 ppm; gas flow, 0.14 m3/h; absorption solution, 100 mL; without O2; 25 °C.

Figure 6. Effect of the HA-Na concentration on the desulfurization time (the horizontal section). SO2, 2000 ppm; gas flow, 0.14 m3/ h; absorption solution, 100 mL; without O2; 25 °C.

90 min when the HA-Na concentration increases from 0.01 to 0.06 g/mL, while the horizontal section begins to decrease 1016

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: DOI:10.1021/ef901052r

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Figure 10. Effect of the gas flow rate. SO2, 2000 ppm; absorption solution, 100 mL; 0.04 g/mL; without O2; 25 °C.

Figure 8. Effect of the O2 concentration. SO2, 2000 ppm; gas flow, 0.14 m3/h; absorption solution, 100 mL; 0.04 g/mL; 25 °C.

absorption efficiency. However, in the third section (5585 min), SO2 absorption efficiency decreases sharply from 99 to 0% with the lowering of pH (from 4.5 to 3.6). The reason is that the mass-transfer resistance of the liquid phase is increased gradually and the alkalinity of the liquid phase has an obvious effect on the SO2 absorption efficiency. In the last section (after 85 min), a constant pH (3.6) is maintained. In this section, HA-Na solution loses the desulfurization capability because most HA-Na has converted to HA sediment. It can be concluded from the above analyses that pH has little effect on SO2 absorption efficiency (above 98%) when pH g4.5. However, the SO2 absorption efficiency decreases sharply with the lowering of pH after the point of pH 4.5. When pH drops to 3.6, the HA-Na solution will lose the desulfurization capability. Hence, pH of HA-Na solution should be above 4.5, to keep the high SO2 absorption efficiency. 4.7. Effect of the Gas Flow Rate. Some experiments were also made to detect the effect of the gas flow rate on the SO2 absorption efficiency. It is found from the experimental results (Figure 10) that both the SO2 absorption efficiency and desulfurization time are decreased as the flow rate increases. The reason is that increasing the flow rate may decrease the contact time of SO2 with HA-Na solution. A part of SO2 is unreacted and released to the environment. Moreover, the increase of the flow rate may accelerate the SO2 desorption and make some SO2 desorb from HA-Na solution. 4.8. Effect of NOx. In the actual condition of the plant, there is 100-800 ppm NOx coexisting with SO2 in flue gas; therefore, the effect of NOx should also be taken into account. It can be seen from Figure 11 that the SO2 absorption efficiency with the presence of 340 ppm NOx is slightly higher than that with the absence of NOx, which indicates that NOx can facilitate SO2 absorption. It can be explained as follows: NO2 is soluble in water solution. The dissolved NO2 in solution can react with HSO3- and SO32-, both of which are oxidized to SO42- by NO2; therefore, the concentration of HSO3- and SO32- are decreased. According to eqs 1 and 2, decreasing the amount of HSO3- and SO32- may move the reaction equilibrium to the right, which will promote SO2 to dissolve into HA-Na solution. Owing to NO2, more SO2 is

Figure 9. Effect of pH. SO2, 2000 ppm; gas flow, 0.14 m3/h; absorption solution, 100 mL; 0.04 g/mL; without O2; 25 °C.

sulfite to sulfate. This decreases the HSO3- concentration in the liquid phase and promotes eq 2 to move to the right. The mass-transfer resistance of the liquid phase is lessened. Hence, the SO2 mass transfer is improved, and the consumption of SO2 is faster, which shortens the desulfurization time. 4.6. Effect of pH. The pH of HA-Na solution is an important influence factor on desulfurization; therefore, a series of experiments were performed to investigate the effect of pH on the SO2 absorption efficiency. Figure 9 clearly illustrates the effect of pH on the SO2 absorption efficiency. The changes of pH and the SO2 absorption efficiency can be divided into four sections. In the first section (at the beginning of 10 min), pH decreases rapidly from 10 to 7.4, because of the faster consumption of OH-. In the second section (10-55 min), pH decreases slowly from 7.4 to 4.5. The reason is that HA-Na solution is a sort of pH buffer solution, which may lower the rate of pH decrease. In the earlier two sections, the SO2 absorption efficiency is not decreased with pH and is maintained above 98%. It can be explained as follows: Because of high alkalinity in this section, the mass-transfer resistance of the liquid phase is small and relies on the gas phase. Hence, the decrease of pH cannot lower the SO2 1017

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: DOI:10.1021/ef901052r

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Figure 11. Effect of NOx. SO2, 2000 ppm; NO2, 340 ppm; gas flow, 0.12 m3/h; O2, 5 vol %; absorption solution, 100 mL; 0.04 g/mL; 25 °C.

Figure 12. FTIR spectra of sample (A) HA-Na and (B) desulfurization products.

absorbed by HA-Na solution. The related reactions are as follows:18 2NO2 ðgÞ þ HSO3 - ðaqÞ þ H2 O f 3Hþ ðaqÞ þ 2NO2 - ðaqÞ þ SO4 2- ðaqÞ

ð6Þ

2NO2 ðgÞ þ SO3 - ðaqÞ þ H2 O f 2Hþ ðaqÞ þ 2NO2 - ðaqÞ þ SO4 2- ðaqÞ

ð7Þ

4.9. Treatment of Desulfurization Products. FTIR spectroscopy is a powerful tool for investigating the desulfurization mechanism. Figure 12 shows the infrared comparison spectra of the sample before and after desulfurization. The characteristic adsorption bands of HA-Na are observed at the wavenumbers of 3689 cm-1 (stretching vibrations of -NH2), 3394 cm-1 (hydrogen-bonded OH stretching of carboxyl, alcohol, and phenol), 1594 and 1374 cm-1 (antisymmetric and symmetric COO- stretching vibrations of carboxylic salt), 1034 cm-1 (C-N stretching vibrations), 1011 cm-1 (C-O stretching vibrations in polysaccharides or polysaccharide-like substances), 912 and 756 cm-1 (outof-plane bending vibrations of aromatic CH groups), and 540 and 504 cm-1 (stretching vibrations of C-C) in the spectra of sample A.19 In comparison to spectra A, it is obvious that three new bands (1243, 1114, and 617 cm-1) appear in the 1400-500 cm-1 spectra of sample B. These three new bands can be assigned to different sulfate (SO42-)

Figure 13. Photo of (A) HA-Na solution and (B) desulfurization liquid.

species.20-22 Moreover, the adsorption band of 1700 cm-1 is ascribed to CdO of COOH,19 which indicates that some HA-Na has converted to HA sediment. This is in agreement with eq 1 and verifies the above desulfurization mechanism. After the desulfurization process, the desulfurization liquid (Figure 13) contains not only HA sediment but also sulfate, all of which are fertilizer components. HA is subject to oxygenolysis by H2SO4, and the oxygenolysis can add the content of an oxygen-containing functional group in HA and improve the activity of HA.15 Because of its poor solubility, HA may be separated as a sediment from acidic aqueous solution and made into a HA compound fertilizer. The acidic solution is neutralized at pH 7 by a little lime (0.3 g of CaO/ 100 mL of acidic solution), and recycling water can be achieved. According to the above results, the operating cost and efficiency of the desulfurization process with HA-Na solution, contrasted with the lime-gypsum process, are shown in Table 1. Obviously, the desulfurization process by HA-Na has a higher efficiency and lower operating cost compared to the lime-gypsum process. Therefore, the desulfurization process of HA-Na solution holds great promise for large commercial application.

(18) Littlejohn, D.; Wang, Y.; Chang, S. G. Oxidation of aqueous sulfite ion by nitrogen dioxide. Environ. Sci. Technol. 1993, 27, 2162– 2167. (19) Tatzber, M.; Stemmer, M.; Spiegel, H.; Katzlberger, C.; Haberhauer, G.; Mentler, A.; Gerzabek, M. H. FTIR-spectroscopic characterization of humic acids and humin fractions obtained by advanced NaOH, Na4P2O7, and Na2CO3 extraction procedures. J. Plant Nutr. Soil Sci. 2007, 170 (4), 522–529. (20) Abdulhamid, H.; Fridell, E.; Dawody, J.; Skoglundh, M. In situ FTIR study of SO2 interaction with Pt/BaCO3/Al2O3 NOx storage catalysts under lean and rich conditions. J. Catal. 2006, 241 (1), 200–210. (21) Mitchell, M. B.; Sheinker, V. N.; White, M. G. Adsorption and reaction of sulfur dioxide on alumina and sodium-impregnated alumina. J. Phys. Chem. 1996, 100 (18), 7550–7557. (22) Wang, Y.; Mohammed Saad, A. B.; Saur, O. FTIR study of adsorption and reaction of SO2 and H2S on Na/SiO2. Appl. Catal., B 1998, 16 (3), 279–290.

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Table 1. Comparative Efficiency and Operating Cost Per Ton of SO2 of FGD Technology23 a byproduct FGD process HA-Na solution lime-gypsum

sorbent consumption (ton)

water consumption (ton)

neutralizer consumption (ton)

gypsum (ton)

fertilizer (ton)

desulfurization efficiency (%)

operating cost (U.S. $)

5.5 1.2

circulating 12

0.3 (lime) 0

0.9 3.7

5.5 0

98 95

15 63

a Operating cost = (sorbent cost þ water cost þ neutralizer cost) - (byproduction cost); lime, 58.65 U.S. dollars/ton; water, 0.29 U.S. dollar/ton; HA/ fertilizer, 146.63 U.S. dollars/ton; gypsum, 2.93 U.S. dollars/ton.

should be above 4.5. A low flow rate of flue gas and low temperature are favorable to the absorption of SO2. Increasing the SO2 inlet concentration may improve the mass transfer of SO2, but the SO2 absorption efficiency has a slight decrease. The increase of the O2 concentration may improve SO2 absorption. NO2 coexisting with SO2 may speed up the oxidation of SO2 to sulfate and promote SO2 absorption. The desulfurization products can be made into compound fertilizer, and recycling water is easy to achieve at low cost. It is possible for the desulfurization process of HA-Na solution to be used for a large scale because of its high efficiency and low operating cost.

5. Conclusion The characteristics of SO2 absorption into HA-Na solution have been investigated in a bubbling reactor. The effects of the HA-Na concentration, pH, temperature, gas flow rate, O2 concentration, SO2 inlet concentration, and NO2 on the SO2 absorption efficiency and desulfurization time have been studied. The experimental results show that HA-Na solution compared to water (at the same volume) and NaOH solution (at the same pH) shows greater performance in SO2 absorption. The HA-Na concentration has little effect on the SO2 absorption efficiency, which can be maintained above 98%, but significantly influences the desulfurization time. Although the desulfurization time increases with the HA-Na solution concentration until 0.06 g/mL, there is a contrary result after the value. To achieve the high SO2 absorption efficiency, pH

Acknowledgment. The authors gratefully acknowledge financial support by the National Science Foundation of China (50876062) and the Ministry of Science and Technology of China (2007AA05Z313). The authors thank the Instrumental Analysis Center of Shanghai Jiao Tong University (SJTU) for FTIR measurements.

(23) Jiang, W. J.; Zhao, J. K.; Yin, H. Q.; et al. The Technical Handbook of Flue Gas Desulfurization and Denitrification; Chemical Industry Press: Beijing, China, 1995; pp 439-453.

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