Energy Fuels 2010, 24, 4931–4936 Published on Web 08/31/2010
: DOI:10.1021/ef100698k
Wet Removal of Sulfur Dioxide and Nitric Oxide from Simulated Coal-Fired Flue Gas by UV/H2O2 Advanced Oxidation Process Yangxian Liu, Jun Zhang,* Changdong Sheng, Yongchun Zhang, and Liang Zhao School of Energy & Environment, Southeast University, Nanjing 210096, China Received April 8, 2010. Revised Manuscript Received August 18, 2010
Wet removal of NO and SO2 from simulated coal-fired flue gas with a UV/H2O2 advanced oxidation process was studied in an ultraviolet (UV)-bubble column reactor. The effects of several factors, including the NO initial concentration, SO2 initial concentration, UV radiation intensity, liquid-gas ratio, H2O2 initial concentration, and O2 content, on NO and SO2 removal efficiencies were studied. The results showed that under all conditions, SO2 could achieve 100% removal. The NO removal efficiency decreased with the increase of NO initial concentration or SO2 initial concentration. With the increase of UV irradiation intensity, liquid-gas ratio, or H2O2 initial concentration, the NO removal efficiency increased. The O2 content only had a little impact on NO removal.
problems, none of them can completely substitute for the combination of WFGD-Ca and NH3-SCR technologies yet.11 Therefore, developing more effective integrated flue gas treatment technologies is still one of the major development trends in the flue gas pollution control field.11 An advanced oxidation process can produce free radicals with strong oxidation ability, such as •OH free radicals, to remove pollutants by oxidation.11 Some advanced oxidation processes, such as plasma removal,7 photocatalytic oxidation,11 and sonochemistry oxidation,12,13 have been studied and developed to achieve integrated removal of multiple pollutants from gas. However, so far, none of them can successfully replace the combination of WFGD-Ca and NH3-SCR technologies in the near term due to challenges in costs or technical implementation.14 Because of its advantages in extremely strong oxidation ability, environmentally friendly character, and a simple and secure process, the UV/H2O2 advanced oxidation process15-17 has been widely studied and applied to degrade and discolor organic pollutants in the water treatment field. In the gas purification field, Cooper et al.18 used UV to excite the photolysis of H2O2 to produce •OH free radicals to oxidize and remove NO from simulated flue gas. The results showed that in the temperature range of 423-723 K, a NO removal efficiency of 40-70% was achieved. Jeong19 used plasma combined with a UV lamp to remove Hg0 and NO in
1. Introduction During the coal burning process, a large number of pollutants, including SO2, NOx, trace elements, and volatile organic compounds (VOCs), are released. These pollutants have brought great harm to human health and the environment.1,2 Although calcium-based wet flue gas desulfurization (WFGD-Ca) and ammonia-based selective catalytic reduction (NH3-SCR) processes have achieved industrial application on a large scale for flue gas treatment in coal-fired power plants, neither of them can achieve the integrated removal of multiple pollutants.3,4 The combination of WFGD-Ca and NH3-SCR technologies can simultaneously remove SO2 and NOx, but the large and complex systems and the high capital and operating costs limit its utilization in the developing world.3,4 Because of great advantages in low capital and operating costs and simultaneous removal of multiple pollutants, integrated removal technology has good prospects for development and application.5,6 Several integrated flue gas treatment technologies, such as plasma removal,7 adsorbent adsorption removal,8 oxidant oxidation removal,9 and complex adsorption removal,10 have been developed and applied in the past several decades. However, due to high costs or technical *To whom correspondence should be addressed. Tel.: þ86 025 83 79 36 12. Fax: þ86 025 83 79 58 24. E-mail:
[email protected]. (1) Bueno-L opez, A.; Garcı´ a-Garcı´ a, A. Energy Fuel 2005, 19, 94– 100. (2) Hutson, N. D.; Krzyzynska, R.; Srivastava, R. K. Ind. Eng. Chem. Res. 2008, 47, 5825–5831. (3) Long, X. L.; Xin, Z. L.; Wang, H. X.; Xiao, W. D.; Yuan, W. K. Appl. Catal. B: Environ. 2004, 54, 25–32. (4) Wang, Z. H.; Zhou, J. H.; Zhu, Y. Q. Fuel Process. Technol. 2007, 88, 817–823. (5) Long, X. L.; Xiao, W. D.; Yuan, W. K. Chemosphere 2005, 59, 811–817. (6) Chu, H.; Chien, T. W.; Wu, B. W. J. Hazard. Mater. 2001, B84, 241–252. (7) Kim, H.; Han, J.; Kawaguchi, I.; Minami, W. Energy Fuel 2007, 21, 141–144. (8) Yang, J.; Zhuang, T. T.; Wei, F.; Zhou, Y. J. Hazard. Mater. 2009, 162, 866–873. (9) Fan, Z. L.; Zhang, J.; Sheng, C. D.; Lin, X. F.; Xu, Y. Q. Energy Fuel 2006, 20, 579–582. (10) Sumathi, S.; Bhatia, S.; Lee, K. T.; Mohamed, A. R. Energy Fuel 2010, 24, 427–431. r 2010 American Chemical Society
(11) Wu, Z. B.; Wang, H. Q.; Liu, Y.; Jiang, B. Q.; Sheng, Z. Y. Chem. Eng. J. 2008, 144, 221–226. (12) Owusu, S. O.; Adewuyi, Y. G. Ind. Eng. Chem. Res. 2006, 45, 4475–4485. (13) Adewuyi, Y. G.; Owusu, S. O. J. Phys. Chem. A 2006, 110, 11098– 11107. (14) Deshwal, B. R.; Jin, D. S.; Lee, S. H. J. Hazard. Mater. 2008, 150, 649–655. (15) Muruganandham, M.; Swaminathan, M. Dyes Pigm. 2007, 62, 269–275. (16) Modirshahla, N. M.; Behnajady, A. Dyes Pigm. 2007, 70, 54–59. (17) Hua, Q.; Zhang, C.; Wang, Z. J. Hazard. Mater. 2008, 154, 795– 803. (18) Cooper, C. D.; Clausen, C. A.; Pettey, L. Environ. Eng. 2002, 128, 68–72. (19) Jeong, J. Annual International Conference on Incineration and Thermal Technologies; Curran Associates, Inc.: Red Hook, NY, 2006; Vol 5, pp 151-159.
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simulated flue gas. The results showed that UV irradiation could effectively improve the oxidation efficiency of Hg0 and NO by producing more •OH free radicals. However, this kind of semidry UV/H 2O2 process18,19 has little potential of further development due to very low utilization of H2O 2 and a lack of effective cooling measures for the ultraviolet lamp in hightemperature flue gas. Compared with the semidry process,18,19 a wet UV/H2O2 process, consisting of a UV lamp and a bubble column reactor, has great advantages in effective utilization of the H2O2 and practical cooling of the UV lamp due to the mild reaction conditions of the wet removal process. Alibegic et al.20 obtained satisfactory results by using a UV/H2O2 process to degrade tetrachlrethene (PCE) from simulated flue gas in a UV-bubble column reactor. On the basis of known results,18-20 wet UV/H2O2 advanced oxidation process has a good prospect of development and application for integrated removal of multiple pollutants (SO2, NOx, trace elements, and VOCs) from coal-fired flue gas by wet scrubbing. However, so far, the combined wet removal of two major pollutants, SO2 and NO, from coal-fired flue gas using the UV/H2O2 advanced oxidation process has not been studied. In this paper, a preliminary study on effects of several factors, including NO initial concentration, SO2 initial concentration, UV irradiation intensity, liquid-gas ratio, H2O2 initial concentration, and O2 content, on NO and SO2 removal efficiencies was carried out in a semicontinuous and small-scale UV-bubble column reactor. The results may be beneficial for increasing the diversities in the integrated removal of pollutants from the flue gas of coal-fired power plants.
Figure 1. Schematic diagram of experimental system. (1-4) N2, NO, SO2, O2 gas gylinders; (5-9) rotameters; (10) buffer tank; (11-12) valves; (13) constant temperature water bath; (14) bubble column reactor; (15) jacket heat exchanger; (16) mercury thermometer; (17) rubber plug; (18) guartz casing; (19) UV lamp; (20) gas distributor; (21) cooling circulating pump; (22) gas dryer; (23) gas analyzer; (24) tail gas absorber; (A) primary gas path; (B) gas bypass; (a) gas inlet; (b) gas outlet; (c) cooling water inlet; (d) cooling water outlet; (e) sampling outlet.
The gas dryer (22), containing anhydrous calcium chloride (granular, Shanghai Chemical Reagent Co., AR), is used to dry the wet simulated flue gas to ensure the measuring accuracy and the safety of electrochemical probes in the gas analyzer (23). The gas analyzer (23) (MRU-VARIO PLUS, Germany) with the ability for simultaneous and continuous measurements of NO, NO2, SO2, O2, CO, and CO2 from gas is used to measure the inlet and outlet concentrations of gaseous pollutants. The residual pollutants in the simulated flue gas are further scrubbed by a tail gas absorber (24), containing 800 mL of a mixed solution of (0.05 mol/L) KMnO4 and (0.1 mol/L) NaOH (both of them are from Shanghai Chemical Reagent Co., AR), to avoid environmental pollution. 2.2. Experimental Procedures. First, 600 mL H2O2 solutions with different concentrations were prepared with a 30% H2O2 solution (Shanghai Chemical Reagent Co., AR) and deionized water according to the required conditions. The initial pH values of the H2O2 solutions were measured with an acidimeter (PHB-3, Shanghai leici instrument Co.) and were naturally kept at 3.2 (in fact, the initial pH value of 2.0 mol/L H2O2 solution itself is just 3.2 owing to hydrolysis of H2O2 to produce hydrogen ions), and the others were adjusted to 3.2 through the addition of HCl (0.5 mol/L) and NaOH (0.5 mol/L) solutions (Shanghai Chemical Reagent Co., AR). The H2O2 solution prepared was added into the bubble column reactor (14) by opening the rubber plug (17). After closing the rubber plug (17), the solution temperatures were adjusted and kept at 298 K for all experiments by regulating the constant temperature water bath (13). Second, four kinds of gases, N2, O2, SO2, and NO (Nanjing Specialty Gas Production Plants, high-purity gases), were used to make the simulated flue gas. The compositions, concentrations, and gas flows of pollutants from the simulated flue gas were regulated by the rotameters (5-9). After opening the valve (12) and closing the valve (11), the inlet concentrations of pollutants from the simulated flue gas were first measured using the gas analyzer (23) through the gas bypass (B). Third, when the solution temperature was steady, the experiment was started. After closing the valve (12) and opening the valve (11), the simulated flue gas first flowed into the bubble column reactor (14) through the primary gas path (A) to clear away the remaining air in the bubble column reactor (14). When the UV lamp (19) was turned on, the pollutants from the simulated flue gas were scrubbed and removed in the bubble column reactor (14), and the outlet concentrations of pollutants with time were also recorded simultaneously (each experimental run was 20 min, and a value was recorded per minute) through the
2. Experimental Section 2.1. Experimental Apparatus. The experimental system is shown in Figure 1. The gases are supplied by cylinders (1-4) filled with different gases. There are some rotameters (5-9) in the lines connecting the cylinders (1-4) for the adjustment of gas compositions and flows. The gases from the cylinders (1-4) are mixed in a buffer tank (10). The flow direction of the simulated flue gas is controlled through switching two valves (11-12). The inlet concentrations of the simulated flue gas are measured by the gas bypass (B), and the experiments are conducted by the primary gas path (A). The body of the photochemical reactor is constructed of a PMMA bubble column reactor (14) (height, 40.0 cm; inside diameter, 8.0 cm), and the top is covered with a rubber plug (17). A sand chip gas distributor (20) (it was made of sand core chip and borosilicate glass by welding, inside diameter =7.0 cm and height =3.0 cm; average pore size =15.0-40.0 μm) located at the bottom of bubble column reactor (14) is used to obtain a uniform distribution of the simulated flue gas. The UV lamp (19) is set inside a quartz casing (18) (outside diameter, 4.0 cm) which is fixed to the rubber plug (17). In order to keep a suitable reaction temperature, a jacket heat exchanger (15) is set outside the bubble column reactor (14), and the temperature is adjusted by a constant temperature water bath (13) (DCW-1015, ( 0.1 °C, Ningbo Jiangnan Instrument Factory) with a cooling circulating pump (21). Solution is added into or moved out of the bubble column reactor (14) by opening the rubber plug (17), and its temperature during the experiment is measured by a mercury thermometer (16). There are five small inlets and outlets, including the inlet (a) and outlet (b) of the simulated flue gas, inlet (c) and outlet (d) of the cooling water, and sampling outlet (e) of the solution, on the body of the photochemical reactor. (20) Alibegic, D.; Tsuneda, S.; Hirata, A. Chem. Eng. Sci. 2001, 56, 6195–6203.
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Table 1. Constant Conditions for Removal Process simulated flue gas compositions N2, SO2, NO, O2
solution 600 mL H2O2 solution
solution temperature
solution initial pH
298 K
3.2
Figure 5. Removal efficiencies of NO and SO2 under different H2O2 initial concentrations. Conditions: liquid-gas ratio, 1.2; NO, 419 ppm; SO2, 948 ppm; O2, 6.28%; UV radiation intensity, 36 W.
Figure 2. Removal efficiencies of NO and SO2 under different NO initial concentrations. Conditions: liquid-gas ratio, 1.2; H2O2, 2.0 mol/L; SO2, 1041 ppm,; O2, 6.25%; UV irradiation intensity, 36 W.
Figure 6. Removal efficiencies of NO and SO2 under different O2 contents. Conditions: liquid-gas ratio, 1.2; NO, 405 ppm; SO2, 1018 ppm; H2O2, 2.0 mol/L; UV radiation intensity, 36 W.
Figure 3. Removal efficiencies of NO and SO2 under different UV radiation intensities. Conditions: liquid-gas ratio, 1.2, H2O2, 2.0 mol/L; SO2, 1013 ppm; NO, 414 ppm; O2, 5.93%.
Figure 7. Removal efficiencies of NO and SO2 under different SO2 initial concentrations. Conditions: liquid-gas ratio, 1.2; H2O2, 2.0 mol/L; NO, 424 ppm; O2, 6.08%; UV radiation intensity, 36 W.
was 20 min, and a concentration value per minute was recorded. The average concentration within 20 min was used as the outlet concentration Cout, and the removal efficiency η was calculated by the following eq 1:
Figure 4. Removal efficiencies of NO and SO2 under different liquid-gas ratios. Conditions: H2O2, 2.0 mol/L; SO2, 1023 ppm; NO, 421 ppm; O2, 5.67%; UV radiation intensity, 36 W.
η ¼
gas analyzer (23). The UV lamp power was changed by replacing and using three sets of UV lamps with different powers. Here, three sets of UV lamps (PL-L18W and PL-L36W, produced by Philips; HL-L72W, produced by Haining Light Factory) were employed, and all of them had the same model (L-L) and wavelength (253.7 nm). Finally, after being scrubbed, the wet simulated flue gas was dried by the gas dryer (22). The remaining pollutants from the simulated flue gas were further scrubbed by the tail gas absorber (24). This experiment was finished by turning off the UV lamp and closing the cylinder values. The gas and solution compositions and constant conditions are summarized in Table 1. The other conditions are listed in Figures 2-7. 2.3. Data Processing. Owing to having a relatively stable concentration change within 20 min, each experimental run
Cin - Cout 100% Cin
ð1Þ
where η is the removal efficiency, Cin is the inlet concentration, and Cout is the outlet concentration.
3. Results and Discussions 3.1. Effect of NO Initial Concentration. Experiments with different NO initial concentrations were carried out, and the results are shown in Figure 2. It could be seen that under all NO concentrations, SO2 achieved 100% removal. With the increase in NO initial concentration from 238 ppm to 1032 ppm, the NO removal efficiency decreased from 82.9% to 60.3%. In the study of degradation of reactive azo 4933
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: DOI:10.1021/ef100698k
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dyes using UV/H2O2, Muruganandham and Swaminathan found that the degradation efficiency of reactive azo dyes significantly decreased with the increase of their initial concentrations. Xu et al.21 also obtained identical results when they used UV and UV/H2O2 to degrade diethyl phthalate (DEP). The effects of NO initial concentration on NO removal efficiency are usually attributed to two reasons. On the one hand, an increase in NO initial concentration will increase the amount of NO through the reactor per unit time, which causes a decrease in the ratio of •OH free radicals to NO and finally leads to a decrease in NO removal efficiency; on the other hand, the Beer-Lambert law equation can be shown by following eq 2:15,16,21 I1 ¼ I0 expð - klcB Þ
density of solution per volume to produce more effective photons and finally generate more •OH free radicals to oxidize and remove NO.15,16,12,13 Therefore, the NO removal efficiency would increase with the increase of UV irradiation intensity. However, when the UV irradiation intensity exceeds a certain value, the further increase of UV irradiation intensity may cause several side reactions in the solution:15,16 H2 O2 þ • OH f HO2 • þ H2 O •
where I1 is the transmitted light intensity, I0 is the incident light intensity, k is the light absorption coefficient, l is the dielectric layer thickness, and cB is the concentration of the light absorption medium. From eq 2, we can see that an increase in NO initial concentration will bring an exponential decrease in the transmitted light intensity, leading to a great decrease in effective utilization of UV energy, and finally inhibiting the removal of NO. 3.2. Effect of UV Irradiation Intensity. As seen in Figure 3, the SO2 removal efficiency was still 100%. But NO removal efficiencies changed with different UV irradiation identities. The NO removal efficiency was only 10.8% when the UV irradiation intensity was zero. After UV radiation was added, the NO removal efficiency had a great increase, increasing from 10.8% to 72.5% when the UV irradiation intensity increased from 0 to 36 W. However, with a doubling in UV irradiation intensity from 36 to 72 W, the growth rate of NO removal efficiency gradually became stable. These are consistent with the results of Li et al.18 obtained from the degradation of 17R-ethynylestradiol with UV/H2O2. The effect of UV irradiation intensity on NO removal efficiency can be explained by the following reasons. On the one hand, when the UV irradiation intensity is zero, NO is only removed by absorption of H2O2. As the oxidation ability of H2O2 for NO is much weaker than that of •OH free radicals,22,23 an absence of UV would result in a low NO removal efficiency; on the other hand, when the UV was added, the photolysis of H2O2 can produce •OH free radicals with strong oxidation ability to effectively oxidize and remove NO according to eqs 3 and 4:15,16,12,13 ð3Þ
NO þ • OH f HNO2
ð4Þ
ð6Þ
Hence, H2O2 is also a scavenger of •OH free radicals besides a releasing agent. The oxidation abilities of HO2• free radicals (1.60 eV) and H2O2 (1.77 eV) produced by side reactions 5 and 6 are much smaller than that of •OH free radicals (2.80 eV).15,16 Therefore, the further increase of UV irradiation intensity would only have a little impact on NO removal efficiency at this time. 3.3. Effect of Liquid-Gas Ratio. The change of the liquidgas ratio had little effect on SO2 removal, but a great effect on NO removal, as shown in Figure 4. The effect of the liquidgas ratio on NO removal can be summarized in two ways. On the one hand, increasing gas flow (decreasing liquid-gas ratio) can enhance the gas-liquid mass transfer process and, finally, increase the gas-liquid absorption rate;5,6 on the other hand, increasing gas flow will make the residence time of NO in the reactor become shorter,5,6 which means that the reaction time of NO in the reactor will decrease. In this reaction system, the latter may play a leading role. Therefore, the NO removal efficiency would increase with the increase in the liquid-gas ratio or the decrease of the gas flow. 3.4. Effect of H2O2 Initial Concentration. Many research results15-17showed that the H2O2 initial concentration had a significant impact on the degradation of pollutants using UV/H2O2. So some experiments were conducted with different H2O2 initial concentrations, and the results are shown in Figure 5. It could be seen that SO2 achieved 100% removal. However, little NO was removed when the H2O2 initial concentration was zero, which showed that the UV/H2O system has almost no removal ability for NO. When the H2O2 initial concentration increased from 0 to 1.5 mol/L, the NO removal efficiency increased from 0 to 69.6%. Then, the further increase from 1.5 mol/L to 2.5 mol/L of H2O2 concentration only caused a slight increase in NO removal efficiency. It is known that H2O2 plays a key role in the photochemical reaction process as a releasing agent of •OH free radicals. The effect of the H2O2 initial concentration on NO removal is generally explained by the following mechanisms. On the one hand, when the H2O2 initial concentration keeps a low level, it may cause a reaction in eq 3 in solution and, finally, increase the NO removal efficiency;15,16 on the other hand, the further increase of the H2O2 initial concentration also may cause some side reactions such as those of eqs 5 and 6 in solution,15,16 leading to a great self-loss in •OH free radicals. Therefore, the further increase of the H2O2 initial concentration would only have a small impact on NO removal. 3.5. Effect of O2 Content. The experiments with different O2 contents were carried out, and the results are shown in Figure 6. The SO2 removal efficiency still reached 100% under different O2 contents. The NO removal efficiency only had a very slight increase when the O2 content increased from
ð2Þ
H2 O2 þ hv f 2• OH
OH þ • OH f H2 O2
ð5Þ
Hence, compared with the reaction system with no UV, the addition of UV can greatly enhance NO removal. Furthermore, the Beer-Lambert law holds that the photochemical reaction yield (or the photochemical reaction rate) is proportional to the UV irradiation intensity, which means that the increase of UV irradiation intensity can increase the energy (21) Xu, X. H.; Ye, Q. F.; Tang, T. M.; Wang, D. H. J. Hazard. Mater. 2008, 158, 410–416. (22) Thomas, D.; Vanderschuren, J. Ind. Eng. Chem. Res. 1997, 36, 3315–3322. (23) Paiva, J. L.; Kachan, G. C. Ind. Eng. Chem. Res. 1998, 37, 609– 614.
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0% to 6.02% and a very slight decrease with a further increase from 6.02% to 12.45% of O2 content. In the study on the degradation of bisphenol A with the UV/H2O2/ microaeration process, Li et al.24 found that the degradation efficiency of bisphenol A could be improved by injecting O2 into a simulated solution. The effects of O2 content on NO removal can be explained by the following reasons. On the one hand, it has been known that O2 usually plays a role of capture intermediate for •OH free radicals, which can effectively restrain the combination of •OH free radicals to each other according to the following reaction (eq 7):25,26 •
•
•
O2 þ OH f HO2 þ O
ð7Þ
2NO þ 3H2 O2 f 2HNO3 þ 2H2 O
ð9Þ
SO2 þ H2 O T HSO3- þ Hþ
ð10Þ
HSO3- T SO23 þ þ Hþ
ð11Þ
ð12Þ
SO2 þ H2 O2 f H2 SO4
ð13Þ
HSO3- þ • OH f • SO3- þ H2 O
ð14Þ
SO23 - þ • OH f • SO3- þ OH -
ð15Þ
HSO3- þ H2 O2 f SO24 - þ Hþ þ H2 O
ð16Þ
SO23 - þ H2 O2 f SO24 - þ H2 O
ð17Þ
From eqs 12-17, we can see that SO2 can consume a lot of OH free radicals and H2O2, reducing the amount of •OH free radicals and H2O2 which react with NO.30,31 So with the increase of the initial SO2 concentration, NO removal efficiency would decrease. Owusu and Adewuyi12,13 also found that the high concentrations of SO2 showed an obvious inhibitory effect on the removal of NO when they used • OH free radicals produced with the ultrasonic cavitation effect to simultaneously remove NO and SO2 from gas. In addition, as shown in Figures 2-7, SO2, especially the SO2 of high concentration, could also achieve 100% removal under all conditions. There may be two main reasons to explain the results. On the one hand, the solubility of SO2 in the H2O2 solution is far larger than that of NO.28 According to the two-film theory, SO2 of high solubility will have a competitive advantage over NO of low solubility in the gas-liquid mass-transfer process,5,9 so compared with NO, SO2 more quickly enters into the liquid phase reaction zone to react with •OH free radicals and H2O2; on the other hand, from eqs 10 and 11, we can see that compared with that of NO, the absorption rate of SO2 also can be increased by the hydrolysis reactions of SO2, such as eqs 10 and 11. Furthermore, according to eqs 14-17, the hydrolysis products of SO2, including HSO3- and SO32-, can also be further oxidized and removed by •OH free radicals and H2O2, leading to a further increase in the SO2 absorption rate. Therefore, under all conditions, especially the high concentrations of SO2, it is possible that SO2 has a higher removal or even a complete removal. Jin et al.32 studied simultaneous oxidation absorption of SO2 and NO by wet scrubbing using an aqueous chlorine dioxide solution. The results showed that SO2 also easily achieved 100% removal. Hutson et al.2 also achieved a complete removal of SO2 by simultaneous removal of SO2, NOx, and Hg from coal flue gas using a NaClO2-enhanced wet scrubber. In addition, in order to avoid secondary pollution or achieve recovery and utilization of ion products in solution, some existing post-treatment methods that are used at full scale,33-35 such as multistage flash distillation, multipleeffect boiling, reverse osmosis, biological denitrification, and the liquid-phase catalytic method have been developed and applied for the removal of soluble ions in solution.
where the O (2.05 eV) free radical is also a kind of oxidant with strong oxidation ability and can effectively remove NO by oxidation. Thus, injecting a small quantity of O2 is beneficial for the removal of NO. On the other hand, the solubility of O2 in H2O2 solution is much higher than that of NO.23 The two-film theory holds that when two gases with different solubilities simultaneously undergo a gas-liquid mass transfer process, the gas of high solubility would have a competitive advantage over the gas of low solubility.5,9 Therefore, when O2 is in excess, the competition effect between O2 and NO in the mass transfer process may be one of the main reasons for the decrease in NO removal efficiency. Furthermore, it should also be seen that the O2 content only had a little impact on NO removal efficiency as a whole. In fact, during our experiments, we saw that the O2 self-yield of the UV/H2O2 system itself could reach 0.5-3.5%, which might provide adequate O2 to participate in the removal reaction. 3.6. Effect of SO2 Initial Concentration. As shown in Figure 7, under different SO2 initial concentrations, SO2 was still removed completely. But NO removal efficiencies changed with different SO2 initial concentrations. When the SO2 initial concentration increased from 0 ppm to 2584 ppm, NO removal efficiency decreased from 72.7% to 50.6%. The effect of the initial SO2 concentration on NO removal efficiency can be explained by two main reasons. On the one hand, the two-film theory holds that when a mixed gas, containing two gases with different solubilities, undergoes a gas-liquid mass transfer process, both of the mass transfer rates of the two gases in the mixed gas will decrease due to the competition effect; on the other hand, on the basis of known results,11-13,22,23,27-29 the oxidation removal reactions of NO and SO2 by •OH free radicals and H2O2 can be listed as follows: ð8Þ
SO2 þ OH f SO3 þ H
•
•
NO þ • OH f NO2 þ • H
•
(24) Li, R. Y.; Gao, N. Y.; Xu, B; Cao, J. China Environ. Sci. 2007, 27, 160–164. (25) Lei, L.; Gao, N. Y.; Hu, L. China Environ. Sci. 2008, 28, 233–236. (26) Bobu, M.; Yediler, A.; Siminiceanu, I. Appl. Catal. B: Environ. 2008, 83, 15–23. (27) Bokotko, R. P.; Hupka, J.; Miller, J. D. Environ. Sci. Technol. 2005, 39, 1184–1189. (28) Colle, S.; Vanderschuren, J.; Thomas, D. Chem. Eng. Sci. 2005, 60, 6472–6479. (29) Colle, S.; Vanderschuren, J.; Thomas, D. Chem. Eng. Prog. 2004, 43, 1397–1402.
(30) Liu, Y. X.; Zhang, J.; Sheng, C. D.; Zhang, Y. C.; Zhao, Li. Sci. China Tech. Sci. 2010, 53, 1839–1846. (31) Liu, Y. X.; Zhang, J.; Sheng, C. D.; Zhang, Y. C.; Zhao, L. Chem. Eng. J. 2010, DOI: 10.1016/j.cej.2010.07.009. (32) Jin, D. S.; Deshwal, B, R.; Park, Y. S.; Lee, H. K. J. Hazard. Mater. 2006, B135, 412–417. (33) Khawaji, A. D.; Kutubkhanah, I. K.; Wie, J. M. Desalination 2008, 221, 47–69. (34) Kapoor, A.; Viraraghavan, T. J. Environ. Eng. 1997, 123, 371–378. (35) Pintar, A.; Bercic, G.; Levec, J. AIChE J. 1998, 44, 2280–2286.
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These existing post-treatment methods may be further improved and potentially used for disposing of the nitric acid solution produced by the removal process of NO and SO2 by wet scrubbing using UV/H2O2.
NO removal efficiency decreased with the increase in initial NO concentration or initial SO2 concentration, and NO removal efficiency increased with the increase in the liquidgas ratio. NO removal efficiency increased at first, and then decreased with the increase of O2 content, but the effect was very little as a whole. More research on the reaction mechanism and mass transfer-reaction kinetics remains to be done due to the high complexity in the simultaneous removal of NO and SO2 from coal-fired flue gas usinig the UV/H2O2 advanced oxidation process.
4. Conclusions The effects of several factors, including the initial NO concentration, initial SO2 concentration, UV radiation intensity, liquid-gas ratio, initial H2O2 concentration, and O2 content, on NO and SO2 removal efficiencies were studied in a semicontinuous and small-scale UV-bubble column reactor. Under all conditions, SO2 could achieve 100% removal. With the increase in initial H2O2 concentration or UV radiation intensity, NO removal efficiency increased at first, and then the growth rate of NO removal efficiency gradually stabilized.
Acknowledgment. This study was supported by the International Cooperation Projects of the National Natural Science Foundation of China (No. 50721140649). The authors are grateful for the test support of the Analysis and Testing Center of SEU.
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