Simultaneous Removal of SO2 and NO x by Calcium Hydroxide at

Nov 1, 2010 - Ca(OH)2 under low-temperature and humid circumstance. The step experiments and pretreatment experiments were carried out to investigate ...
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Simultaneous Removal of SO2 and NOx by Calcium Hydroxide at Low Temperature: Effect of SO2 Absorption on NO2 Removal Guoqing Chen,† Jihui Gao,* Jianmin Gao, Qian Du, Xiaolin Fu, Yijun Yin, and Yukun Qin School of Energy Science and Engineering, Harbin Institute of Technology, 92 West Dazhi Street, Harbin 150001, People’s Republic of China

A fixed reactor was employed to investigate the influence of SO2 absorption process on NO2 removal by Ca(OH)2 under low-temperature and humid circumstance. The step experiments and pretreatment experiments were carried out to investigate the effect of the desulfurization product CaSO3 on NO2 removal. The roles of gas components (O2, H2O) and reaction temperature in SO2 enhancing NO2 removal were discussed by comparing their influences on NO2 removal in and without the presence of SO2. It is interesting to observe that NOx removal first increases and then decreases with the increase of SO2 concentration. The presence of SO2 in the flue gas was found to restrain NO release and enhance NO2 removal. SO2 removal product CaSO3 did provide an additional reaction path for NO2 removal, but the reaction rate of NO2 and CaSO3 was lower than that of Ca(OH)2 due to the lower solubility of CaSO3. The enhancement effect of SO2 on NO2 removal can be explained by the presence of SO32- and HSO3- ions derived from the hydrolysis of SO2 in the water layer. HSO3- and SO32- reacted with NO2 to form sulfate, which was detected in the spent absorbent by X-ray diffraction. The higher relative humidity can accelerate the hydrolysis of SO2 and the dissolution of Ca(OH)2, which are in favor of NO2 removal. However, the oxidization of SO32- to SO42- by O2 lowers NO2 removal. Reaction temperature has negligible influence on the enhancement of NOx removal by the presence of SO2. 1. Introduction The emissions of SO2 and NOx from the combustion of fossil fuel have caused serious environmental problems.1 Many technologies have been developed to control the emission of the precursors to acid deposition.2-4 Among these technologies, the combination of low NOx burner technology, wet flue gas desulfurization (WFGD), and selective catalytic reduction (SCR) was proved to be the most effective method for removing SO2 and NOx. However, high cost of investment and operating cost makes it unsuitable for the developing countries such as China. Presently, attempts have been made to find an effective and economical method to remove SO2 and NOx simultaneously in one reaction cell. Considering the high proportion of the SO2 removal process with Ca-based sorbents in the old existing power plants, developing economical and effective technology for simultaneous removal of SO2 and NOx in the conventional FGD reaction cell has attracted many researchers’ attention.5,6 Oxidizing NO to more soluble NO2 has been proposed as a promising method for enhancing NO removal in conventional FGD processes. NO is first oxidized to NO2 by addition of oxidant such as KMnO4,6,7 O3,8,10 and methanol9 into the flue gas duct at an optimum temperature and then reacts with the conventional alkaline absorbent. A fair amount of work has been conducted concerning NO2 removal by Ca-based material under the conventional dry/ semidry FGD reaction conditions. Zhang et al.11 investigated the NO2 absorption on the surface of a Ca(OH)2 particle and set up the absorption model. Nelli and Rochelle12 analyzed NO2 absorption activity of various alkaline solids and found that NO2 absorption activity was very sensitive to the alkalinity of the absorbent surface. Practically, NO2 removal occurs on the absorbent surface simultaneously with the sulfation reaction of * To whom correspondence should be addressed. Tel.: +86-45186413231, ext 816. Fax: +86-451-86412528. E-mail: [email protected]. † E-mail: [email protected].

absorbent. The coexistence of SOx (mainly SO2; the oxidation of SO2 to SO3 by the oxidant added into the flue gas system is assumed to be negligible) and NO2 in the flue gas is bound to affect the absorption of each. Some researchers have reported that the presence of NO2 in the flue gas could improve the SO2 removal capacity of Ca-based sorbents. Liu et al.14 maintained that the deliquescence of NO2 removal products Ca(NO2)2 and Ca(NO3)2 could collect a great amount of water on the sample surface and enhanced the reaction of SO2 and Ca(OH)2. Lee et al.15 postulated O2 and NO as an oxidizing agent to oxidize SO2 to SO3, which has a higher reaction activity with alkaline solids than SO2. However, Li et al.16 pointed out that the reaction products Ca(NO3)2 could react with SO2 to form gypsum and NO, which was deemed as the reason for explaining NOx enhancement. On the other hand, the effect of SO2 on NO2 removal was also studied by some researchers. O’Dowd13 found that slightly higher NOx removal occurred in both s spray drier and baghouse system with the increase of SO2 concentration. Nelli and Rochelle5 also pointed out that the presence of SO2 in the flue gas could enhance NO2 removal by Ca-based absorbent at the bag-filter conditions. However, little work has been done to investigate the detailed reaction path between NO2 and calcium hydroxide particle in the presence of SO2 under low-temperature and humid conditions. The roles of SO2 removal product in NO2 removal was rarely discussed in previous reports. Most experimental results observed under dry/ semidry FGD reaction conditions were explained by using the conclusions observed from NO2 absorption in the aqueous solution. Moreover, there was little report about the effect of reaction conditions such as flue gas components and reaction temperature on SO2 enhancing NO2 removal. Therefore, it is worthwhile to perform the further investigation on this subject. In this paper, the influence of SO2 absorption process on NO2 removal by calcium hydroxide particle under low-temperature and humid circumstance was studied in detail. The step experiments and pretreatment experiments were carried out to

10.1021/ie101594x  2010 American Chemical Society Published on Web 11/01/2010

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investigate the effect of the desulfurization product CaSO3 on NO2 removal. By comparing the effect of gas components (O2, H2O) and reaction temperature on NO2 removal in and without the presence of SO2, their roles in SO2 enhancing NO2 removal were discussed. Finally, the possible effect mechanism of SO2 on NO2 removal on the surface of the calcium hydroxide particle under low-temperature and humid circumstance was discussed. 2. Experimental Section The raw material used to prepare the absorbent is calcium oxide, which is of reagent grade. The raw calcium oxide was first milled and sieved to ensure that almost all calcium oxide particles were in the size range of 60-80 µm. The powdertype calcium oxide was then added into deionized water at a H2O/CaO weight ratio of 5 at a temperature of 70 °C. The slurry was heated to 90 °C and maintained for 3 h with continuous stirring. After stirring, the resulting slurry was dried at 110 °C, until no weight change, to produce the dry absorbent. The dry absorbent was then crushed and sieved into the required particle size range of 250-500 µm with a specific Brunaer-EmmettTeller (BET) surface area of 7.39 m2/g. The experiments for the reaction between Ca(OH)2 and the simulated flue gas were carried out by a fixed bed reactor system. The details of the experimental setup and procedure were described in our previous report.17 In this study about 1.2 g of absorbent was used for each run, except for the runs especially pointed out. To prevent channeling and absorbent agglomeration in the reactor, about 14 g of silicon dioxide with particle size range of 125-250 µm mixed with absorbent was packed in the reactor. The reactor was heated by a tube furnace. The simulated flue gas containing SO2, NO2, H2O, O2, and N2 entered the reactor and reacted with the absorbent. The concentrations of the flue gas components (SO2, NO2, and O2) were controlled by adjusting their flow rate. The water vapor was supplied by the N2 and O2 flow passing through a water humidifier consisting of a conical flask submerged in the water bath. The desired H2O fraction could be achieved by controlling the flow rate of N2 and the temperature of the water bath. When 143 mL/min of N2 and 90 mL/min of O2 were passed through the conical flask submerged in the water bath at 90 °C, it was about 18% H2O that could be gotten in the simulated flue gas. In the progress of an experiment, the total flow rate of the gas stream was controlled at 1500 mL/min (STP). The concentrations of SO2, NO, NO2, and H2O were measured by an online Fourier transform infrared (FTIR) gas analyzer (GASMET-DX4000), with the measurement error of (2%. A Testo-335 flue gas analyzer was used to measure O2 concentration, with the measurement errors of (0.8 vol %. The X-ray diffraction (XRD; Riguku D/Max-γB) was employed to identify the presence of specific crystalline compounds in the spent absorbents. The removal efficiencies of SO2, NO2, and NOx were defined as follows: SO2 removal:

DeSO2 ) (CSO2,in - CSO2,out)/CSO2,in

(1)

NO2 removal: DeNO2 ) (CNO2,in - CNO2,out)/CNO2,in

(2)

NOx removal:

DeNOx ) (CNO2,in + CNO,in - CNO,out - CNO2,out)/(CNO2,in + CNO,in)

(3) where CSO2,in, CSO2,out, CNO,in, CNO,out, CNO2,in, and CNO2,out represent the concentrations of SO2, NO, and NO2 in the flue gas at the inlet and outlet of the reactor, respectively.

Figure 1. Effect of SO2 concentration on NO2, NOx, and SO2 removal and NO release.

Figure 2. Effect of SO2 concentration on NOx removal.

3. Results and Discussion 3.1. Effect of SO2 Concentration on the Reaction between NO2 and Ca(OH)2. The evolutions of the amount of NOx, NO2, and SO2 removal and NO release, which were calculated by the integration of the difference between inlet and outlet concentrations for 65 min reaction time, are shown in Figure 1 with the variation of SO2 concentration from 0 to 2888 ppm. As can be seen, the amount of NOx and NO2 removal markedly increase with an increase in SO2 concentration from 0 to 844 ppm. Nevertheless, when SO2 concentration was raised from 844 to 2888 ppm, there is no significant change in the NO2 removal amount. Unlike the change trend of the NO2 removal amount, the NOx removal amount decreases with SO2 concentration, increasing from 844 to 2888 ppm. It is wellknown that the reaction between NO2 and Ca(OH)2 not only can form nitrite and nitrate species but also releases NO into the flue gas from the decomposition of HNO2 derived from the hydrolysis of NO2. From Figure 1, it can be found that the amount of NO release first decreases, and then increases with an increase of SO2 concentration. The increase in the NO release amount should be responsible for the reduction of the NOx removal amount. Moreover, it can also be found that the addition of SO2 in the flue gas can restrain the release of NO in the NO2 removal process, but the inhibiting effect gradually weakens with SO2 concentration above 500 ppm. Figure 2 shows the evolution of NOx removal with time for different SO2 concentrations. As can be seen, although SO2 concentration > 844 ppm in the flue gas could relaxedly lower the NOx removal amount

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after 65 min of reaction, the NOx removal rate in the beginning 15 min still increases with an increase of the SO2 concentration. Thus, it can be concluded that the presence of SO2 in the flue gas can enhance NO2 removal on the surface of calcium hydroxide particle under low-temperature and humid conditions. There are many possible reactions between SO2 and NO2, because they are simultaneously fed into the flue gas to react with the absorbent. The interaction of SO2 and NO2 may occur in the gas phase or on the surface of the absorbent. The main possible reaction occurring in the gas phase can be expressed as follow: NO2 + SO2 f SO3 + NO

(4)

However, according to the kinetic data of reaction 4, the reaction rate is negligible at temperature lower than 250 °C.16 In addition, the conversion of NO2 to NO has negative effect on NO2 removal. Therefore, the interaction of SO2 and NO2 must occur on the surface of a Ca(OH)2 particle. As we all know, H2O plays an important role in SO2 and NO2 removal on the surface of a Ca(OH)2 particle under lowtemperature and humid conditions. Without the presence of H2O, SO2 and NO2 are hardly removed by calcium hydroxide. Water molecules are absorbed on the surface of a Ca(OH)2 particle and then form a thin water layer. SO2 and NO2 removal reactions occur in this water layer. Therefore, the interaction of SO2 and NO2 may occur in the water layer on the surface of Ca(OH)2. Concerning the effect of SO2 on NO2 absorption in the aqueous solution, Kameoka and Pigford,18 Takeuchi and Yamanaka,19 and Chen and Rochelle20 performed some relevant works. Takeuchi and Yamanaka19 pointed out that NO2 was absorbed by the NaOH solution in the presence of SO2 in the following reaction manner: SO2 + 2NaOH f Na2SO3 + H2O

(5)

2Na2SO3 + NO2 f 2Na2SO4 + 0.5N2

(6)

Littlejohn et al.21 maintained that the reaction between NO2 and sulfite occurred via a free radical mechanism as follows: NO2 + SO32- f SO3•- + NO2-

(7)

2NO2 + HSO3- + H2O f 3H+ + SO42- + 2NO2-

(8) However, due to the discrepancy of the H2O content between the aqueous reaction and the surface reaction of the Ca(OH)2 particle under low-temperature and humid conditions, the interaction of SO2 and NO2 in aqueous solution may be different from that on the surface of Ca(OH)2. Prior to this study, concerning the effect of the SO2 absorption process on NO2 removal on the surface of absorbent under similar conditions, only Nelli and Rochelle5 performed some relevant works. Nelli and Rochelle maintained that SO2 removal product reacted with NO2 on the exterior of an alkaline solid, which provided the additional route for NO2 removal. Therefore, systematic and rounded research was done to analyze the effects of desulfurization product, the SO2 removal process on NO2 removal. 3.2. NO2 Removal by SO2 Removal Product (CaSO3). To elucidate the role of desulfurization product CaSO3 in NO2 removal by Ca(OH)2, the experiments for the reaction between CaSO3 and NO2 diluted by various gas mixtures were carried out at 70 °C and 60% relative humidity for 45 min. The calcium sulfite used was reagent-grade (purity > 90%, Aladdin). Figure 3 shows NO2 removal by CaSO3 particle under different gas

Figure 3. NO2 removal by CaSO3 under various gas conditions.

conditions. As can be seen, NO2 removal was very low as NO2 alone reacted with CaSO3. The higher NO2 removal in the beginning may be due to the high water adsorption capability of CaSO3. When 18% H2O was added into the flue gas, it was about 25% NO2 absorbed by CaSO3. This suggests that the reaction between CaSO3 and NO2 requires the presence of water vapor. Without the presence of H2O the reaction is a gas-solid reaction. When a certain amount of H2O is fed into flue gas, water molecules can accumulate on the surface and interior pore of absorbent and form a thin water layer. Some CaSO3 will dissolve into the thin water layer, which changes the gas-solid reaction to an aqueous ionic reaction. The presence of O2 in the flue gas slightly lowered NO2 removal in and without the presence of H2O. The reason is that SO32- derived from the dissolution of CaSO3 was oxidized to sulfate by the oxygen. However, the presence of SO2 in gas stream enhances NO2 removal in the beginning. The rapid decrease in the following reaction time was due to the reaction between of SO2 and SO32-. It is well-known that calcium sulfite is an alkalescent material. The hydrolysis of SO2 in the water layer not only forms some SO32- and HSO3-, which are in favor of NO2 removal, but also lowers the pH value of the water layer by forming some H+. The pH value of the water layer is a very important factor in determining the amount of S existing as sulfite. The lower the pH value, the more sulfite is converted to bisulfite. According to Chen and Rochelle’s20 research on NO2 removal characteristics by bisulfite and sulfite, the most effective reagent for reacting with NO2 was sulfite. Hence, because the reaction time > 10 min, SO2 shows a negative influence on NO2 removal. 3.3. Comparison of NO2 Removal by Ca(OH)2 and CaSO3. From Figure 3, it can be concluded that SO2 removal product CaSO3 can react with NO2 and does provide an additional reaction path for NO2 removal under low-temperature and humid conditions. For the purpose of confirming whether the presence of CaSO3 on the surface of Ca(OH)2 can be regarded as a reason for explaining the enhancement of NO2 removal by adding SO2 in the flue gas, the comparison of NO2 removal by Ca(OH)2 and CaSO3 respectively were performed under different gas component conditions. As can be seen from Figure 4, NO2 removal by Ca(OH)2 is higher than by CaSO3 under the same gas component conditions. This indicates the enhancement of NO2 removal by adding SO2 in flue gas shown in Figure 1 cannot be explained by the presence of SO2 removal product CaSO3. A fair amount of works have been done by other researchers on the subject of NO2 absorption in the sulfite solution. Chen

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Figure 4. Comparison of NOx removal by Ca(OH)2 and CaSO3.

and Rochelle’s experimental results indicated that the NO2 absorption rate in water and NaOH solution was lower than that in bisulfite and sulfite solution.20 Kameoka and Pigford18 also found that sulfite solution was the most effective as an absorbent for removing NO2, compared to H2O, sulfuric acid, and NaOH. The discrepancy between our findings and previous conclusions is thought to be due to the difference in the sulfite used in experiment. The sulfite used by Chen and Rochelle20 and Kameoka and Pigford18 are Na2SO3 and NaHSO3, which are soluble salts. Because CaSO3 used in our experiment is sparingly soluble in water, NO2 removal by CaSO3 is very low. Moreover, the solubility product constant of Ca(OH)2 is about 80 times higher than that of CaSO3 at 25 °C. Although the reaction rate of SO32- and NO2 is higher than that of H2O and NaOH, the rate-limiting step of the reaction between SO2 and the CaSO3 particle is the dissolution of CaSO3. Therefore, the absorption rate of NO2 on the Ca(OH)2 particle surface is higher than that on a CaSO3 particle surface. 3.4. Step Experiment for the Effect of SO2 on NO2 Removal by Ca(OH)2. To further elucidate the effect of SO2 on NO2 removal by Ca(OH)2, the step experiment was carried out. During the experiment process, SO2 and NO2 were fed into the reactor separately. A 2.5 g amount of absorbent was first treated with a gas stream involving 470 ppm NO2 but without

Figure 5. Results of step experiment.

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the presence of SO2 for 15 min. Then 1165 ppm SO2 was fed into the gas stream to replace NO2 to react with absorbent for 15 min. The step experiment results were shown in Figure 5. Si (i ) 1, 2, ..., 11) listed in Figure 5 stands for the ith (i ) 1, 2, ..., 11) step experiment. The results of the S2, S6, and S8 in which SO2 was removed alone were not depicted in Figure 5. As can be seen from Figure 5, NO2 concentration detected at the reactor outlet in S1 is slightly higher than that in S3, S5, and S7. This suggests that Ca(OH)2 subjected to SO2 has higher NO2 removal activity. However, by comparing NO2 removal in and without the presence of SO2, it can be seen that NO2 removal in S4 and S9 are higher than that in S3, S5, and S7. When SO2 was excluded from the flue gas in S11, NO2 concentration increased rapidly. That is, only when SO2 and NO2 coexist on the absorbent surface, may SO2 enhance NO2 removal. According to the above-mentioned analysis, the reaction between SO2 and NO2 in gas phase cannot occur under lowtemperature and humid conditions. Although SO2 removal product CaSO3 can react with NO2, its presence on the surface of absorbent cannot account for the enhancement of NO2 removal by the presence of SO2. In addition, it is interesting to observe from Figure 1 that the evolutions of the SO2 removal amount with SO2 concentration are similar to the NO2 removal amount. The more SO2 was absorbed on the surface of the absorbent, the more NO2 removal. This suggests the SO2 removal amount may have correlation with NO2 removal. It seems reasonable to deduce that there must be some intermediate species formed from the reaction between SO2 and Ca(OH)2. This species has higher activity for NO2 removal than Ca(OH)2 or plays a catalytic role in the reaction between NO2 and Ca(OH)2. If the intermediate species can be formed in SO2 removal process, the more SO2 removal will form more intermediate products, and that will promote higher NO2 removal. To confirm whether some intermediate species was formed in the SO2 removal process, SO2 pretreatment experiments were performed. In the experimental process, Ca(OH)2 was first pretreated by 1200 ppm SO2 at 70 °C and 60% relative humidity for different times to obtain the pretreated absorbent containing different amounts of CaSO3 and intermediate species, and then the pretreated absorbent was exposed to NO2 under the same temperature and relative humidity condition. The evolutions of

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Figure 7. Reaction paths for simultaneous absorption of NO2 and SO2 on the surface of a Ca(OH)2 particle.

SO2 · H2O + 2OH- f SO32- + 2H2O

Figure 6. Effect of SO2 pretreatment on NOx removal.

the NOx removal by the absorbent pretreated by SO2 for different times are plotted against time in Figure 6. From Figure 6, it is seen that NOx removal by the pretreated absorbent is higher than by the fresh absorbent. With increasing pretreatment time from 0 to 50 min, NO2 removal markedly increases, which can confirm the presumption that some intermediate species does form in SO2 removal process. However, a declining trend is obtained as the pretreatment time above 50 min. Moreover, it is also found that the optimum NO2 removal by pretreated absorbent is still lower than that by Ca(OH)2 in the presence of SO2. This suggests that the intermediate species is unsteady and can be converted to another product in the reaction process. Hence the activity of the pretreated absorbent decrease as the pretreatment time is above 50 min. The main reaction steps involved in the SO2 removal process on the surface of absorbent are the hydrolysis of SO2, the dissolution of Ca(OH)2, and the formation of product layers. The hydrolysis of SO2 first takes place in the water layer and forms SO32- and H2O under alkaline condition, which subsequently react with Ca2+ ion derived from the dissolution of Ca(OH)2 to form the sparingly soluble CaSO3. In this process, on the one hand, SO32- can be formed from the hydrolysis of SO2; on the other hand, SO32- was also consumed through reacting with Ca2+. Therefore, SO32- can be regarded as an intermediate product of the sulfation of Ca(OH)2. According to Chen and Rochelle’s investigation, the NO2 absorption rate in sulfite solution is higher than that in H2O and NaOH solution. Hence it can also be concluded that SO32- derived from the hydrolysis of SO2 in the water film is the intermediate species that enhances NO2 removal on the surface of the calcium hydroxide particle. 3.5. Mechanism of SO2 Enhancing NO2 Removal under Low-Temperature and Humid Conditions. On the basis of the mechanisms of SO2 removal by Ca(OH)2 and NO2 absorption in H2O and sulfite solution, the mechanism of SO2 enhancing NO2 removal under low-temperature and humid circumstance is summarized as follows. The possible reaction paths are shown in Figure 7. Under humid condition, water molecules are adsorbed on the surface and the inner pore of the absorbent to form a water layer.22 The hydrolysis of SO2 takes place in the water layer and forms SO32- under alkaline condition in the following manner. SO2 + H2O f SO2 · H2O

(9)

(10)

When NO2 is added into the gas stream, the hydrolysis of NO2 also occurs and forms HNO2 and HNO3, which subsequently react with Ca(OH)2 to form Ca(NO3)2 and Ca(NO2)2. Meanwhile, NO2 also reacts with SO32- ion derived from the hydrolysis of SO2. The reactions of NO2 removal in the presence of SO2 can be represented in the following manner: 2NO2 + H2O f HNO2 + HNO3 f HNO2 + H+ + NO3(11) NO + NO2 + H2O a 2HNO2

(12)

SO32- + 2NO2 + H2O f SO42- + 2NO2- + 2H+

(13) As seen in the preceding equations, NO2 not only reacts with H2O but also is absorbed by SO32- ion in the presence of SO2. Takeuchi and Yamanaka19 maintained that NaOH does not directly react with dissolved N2O4 or NO2, but HNO3 and HNO2 formed from the hydrolysis of NO2. The NO2 absorption rate in sulfite solution is higher than in H2O and NaOH. Therefore, when SO2 is excluded from the flue gas, NO2 can only be removed by the reactions 11 and 12. The hydrolysis of SO2 on the absorbent surface can enhance NO2 removal. The more SO2 absorbed by Ca(OH)2 can promote the higher NO2 removal. However, the presence of desulfurization product on the surface of the absorbent lowers the dissolution rate of Ca(OH)2. H+ ions formed from the hydrolysis of SO2 and NO2 cannot be neutralized, and the pH value of the absorbent surface and the hydrolysis rate of SO2 and NO2 decrease with reaction time. Once the water layer is converted to be acidic, the hydrolysis product of SO2 will be HSO3-, and some SO32- ions are also converted to HSO3- in the following manner. SO2 + H2O + SO32- f 2HSO3-

(14)

According to Chen and Rochelle’s research,20 the NO2 absorption rate in HSO3- solution is lower than in SO32solution. Therefore, NOx removal decreases with pretreatment time increasing in Figure 6. In addition, HNO2 formed from the hydrolysis of NO2 is also decomposed to NO under acid condition, which can account for the increase of NO produce with SO2 concentration in Figure 1. To further approve the above mechanism, the spent absorbents subjected to different flue gas components were analyzed by XRD. The analysis results are shown in Figure 8. It can be found that the main component of the fresh absorbent is calcium hydroxide. After being treated with the simulated flue gas containing SO2 and NO2, the spectrum of the spent absorbent has obvious changes. Apart from calcium hydroxide, sulfate (SO42-) was detected as the product of desulfurization reaction.

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Figure 10. Effect of H2O on NO2 removal without the presence of SO2.

Figure 8. XRD spectra of the fresh and spent absorbent.

Figure 11. Effect of O2 on SO2 enhancing NO2 removal.

Figure 9. Effect of H2O on NO2 removal in the presence of SO2.

When NO2 was excluded from flue gas, the desulfurization product was mainly composed of CaSO3. Therefore, sulfite could be converted to sulfate in the presence of NO2, which could confirm the existence of reaction 13 on the absorbent surface. However, no distinct XRD patterns were attributed to nitrite species and nitrate species. This might have been due to the quantity of those crystal phases is small in the spent absorbent or their crystal quality is not good enough for XRD detection. 3.6. Effect of Gas Components on SO2 Enhancing NO2 Removal. 3.6.1. Effect of Relative Humidity on SO2 Enhancing NO2 Removal. The evolution of NOx removal with time exposure at 70 °C for different H2O fractions is depicted in Figure 9. As can be seen, with the relative humidity increase, NOx removal increases markedly. To elucidate the effect of H2O on SO2 enhancing NO2 removal, NOx removal by Ca(OH)2 without the presence of SO2 was shown in Figure 10. NOx removal increases first and then decreases with the relative humidity rise from 0 to 80%. Comparing Figures 9 and 10, one can see that the effect of H2O on NOx removal is more obvious in the presence of SO2. Liu and Shih14 found that the reaction between SO2 and Ca(OH)2 requires the presence of water vapor; the higher the relative humidity, the greater the extent of reaction of Ca(OH)2. The same conclusion was also attainted in our previous research.17 H2O molecules are adsorbed on the exterior surface and the inner pore of the absorbent and accumulate to form a water layer.22,23 The thickness of the water layer is

determined by the relative humidity; the higher the relative humidity, the more H2O molecules are adsorbed on absorbent surface. Therefore, relative humidity has a great effect on the SO32- concentration in the water layer. The enhancement effect of H2O on NOx removal in the presence of SO2 is mainly enhancing the hydrolysis of SO2. This conclusion can further confirm that the hydrolysis of SO2 on an absorbent surface enhances NO2 removal. 3.6.2. Effect of O2 on SO2 Enhancing NO2 Removal. The evolution of the NOx, NO2, and SO2 removal amount for 45 min with O2 concentration is shown in Figure 11. It can be found that NOx and NO2 removal amounts decrease with O2 increasing from 0 to 5%, and then no significant changes in NOx and NO2 removals were observed as O2 concentration varies in the range of 5-15%. This result was in agreement with Bausach’s experiment.24 Chen and Rochelle20 investigated the effect of O2 on NO2 absorption in the solution without the presence of SO2 and found that the addition of 21% O2 into the flue gas had negligible influence on NO2 removal. Takeuchi and Yamanaka19 also found that O2 did not affect the absorption rate of NO2 into NaOH in the absence of SO2. However, the presence of O2 has a negative impact on NO2 removal as gas stream involving SO2. Littlejohn et al.21 maintained that O2 present in the flue gas reacted with SO32- at the liquid boundary of the water layer and thus lowered their respective interfacial concentration, which induced a NO2 absorption decrease. The negative effect of O2 on SO2 enhancing NO2 removal further confirms the sulfite formed from the hydrolysis of SO2 in the water layer on the surface of absorbent is the reason for SO2 enhancing NO2 removal.

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N2O4 and N2O3 in the gas phase at higher temperature. Therefore, increasing reaction temperature decreases the hydrolysis of NO2 in the water layer on the absorbent surface. Moreover, Takeuchi and Yamanaka19 found that increasing the reaction temperature lowered the absorption rate of NO2 in the low-concentration sulfite solution. They also found that the higher the sulfite concentration, the smaller the negative effect of reaction temperature. Therefore, the results in Figures 12 and 13 can be explained by the following reasons. On the one hand, increasing temperature reaction lowered the hydrolysis rate of NO2 and the reaction rate of SO32- with NO2. On the other hand, the amount of SO32- in the water layer increases with reaction temperature increasing. The positive and negative effects of the reaction temperature result in no significant changes in NOx removal. Figure 12. Effect of reaction temperature on NO2 removal in the presence of SO2.

Figure 13. Effect of reaction temperature on NO2 removal without the presence of SO2.

3.7. Effect of Reaction Temperature on SO2 Enhancing NO2 Removal. Figure 12 shows the effect of reaction temperature on the extent of NOx removal in the presence of SO2 at 50% relative humidity. As can be seen, the NO2 removal amount slightly decreased when the reaction temperature was raised from 50 to 80 °C, while NOx removal shows no significant changes. Unlike the change trend of NOx and NO2 removal amounts, the SO2 removal amount increases obviously with the increase of reaction temperature. Lee et al.25 also found that the desulfurization activity of the sorbent synthesized by calcium material increased with increasing reaction temperature at a constant relative humidity. As already mentioned in the preceding part, the reactions of NO2 removal on the surface of the absorbent involve the hydrolysis of NO2 and the reaction between NO2 and SO32- in the presence of SO2. The absorption rate of SO2 has a great impact on NO2 removal; the more SO2 is absorbed, the higher the NO2 removal efficiency. However, the evolution of SO2 removal with reaction temperature is in opposition to that of NOx and NO2 in Figure 12. This result seems contradictory to the mechanism proposed in this paper. To explain this observation, the effect of reaction temperature on NO2 removal by Ca(OH)2 without the presence of SO2 was investigated, and the results are shown in Figure 13. As can be seen in Figure 13, NOx removal decreases with an increase of reaction temperature, which is different from that in the presence of SO2. The explanation for this observation is due to the lower solubility of NO2 and the lower equilibrium concentration of

4. Conclusions The presence of SO2 in the flue gas can enhances NO2 removal on the surface of Ca(OH)2 particle under lowtemperature and humid circumstance. SO2 can restrain the release of NO, but the inhibiting effect gradually weakens as SO2 concentration increases above 500 ppm. SO2 removal product CaSO3 can react with NO2 and does provide an additional reaction path for NO2 removal. However, the reaction rate of NO2 and CaSO3 was lower than that of Ca(OH)2 due to the low solubility of CaSO3 in the water layer adsorbed on the absorbent surface. SO2 enhancing NO2 removal was due to the presence of SO32- derived from the hydrolysis of SO2 in the water layer. SO32- could react with NO2 to form sulfate, which was detected in the spent absorbent by XRD. The conversion of sulfite to bisulfite under the acid condition lowered the enhancement effect of SO2 on NO2 removal. Hence, it was proposed that NO2 removal in sulfite solution could be enhanced by adding some alkaline material into sulfite solution to react with H+ formed from the hydrolysis of NO2 and SO2 and to inhibit sulfite converted to bisulfite. The effects of flue gas components and reaction temperature on SO2 enhancing NO2 removal were also studied. The positive effect of H2O on SO2 enhancing NOx removal was due to H2O enhancing the hydrolysis of SO2. The negative effect of O2 on NO2 removal in the presence of SO2 was due to the oxidization of SO32- to SO42-, which further confirmed SO32- and HSO3formed from the hydrolysis of SO2 in the water layer on the absorbent surface is the reason for SO2 enhancing NO2 removal. The positive and negative effects of the reaction temperature result in no significant changes of NOx removal with reaction temperature increase. Acknowledgment We gratefully acknowledge the financial support from the National Natural Science Foundation of China (Grant No. 50806019) and National High Technology Research and Development program of China (Grant No. 2007AA05Z307). Literature Cited (1) Zhao, Y.; Duan, L.; Xing, J.; Larssen, T.; Nielsen, C. P.; Hao, J. M. Soil Acidification in China: Is Controlling SO2 Emission Enough. EnViron. Sci. Technol. 2009, 43, 8021. (2) Ren, F.; Li, Z.; Chen, Z.; Fan, S.; Liu, G. Influence of the Overfire Air Ratio on the NOx Emission and Combustion Characteristics of a downFired 300-MWe Utility Boiler. EnViron. Sci. Technol. 2010, 44, 6510. (3) Zhou, Y.; Zhang, M.; Wang, D.; Wang, L. Study on a Novel Semidry Flue Gas Desulfurization with Multifluid Alkaline Spray Generator. Ind. Eng. Chem. Res. 2005, 44, 8830.

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ReceiVed for reView April 22, 2010 ReVised manuscript receiVed October 14, 2010 Accepted October 18, 2010 IE101594X