Enhancement Effects of Gas Components on NO Removal by Calcium

Jan 5, 2010 - ... atmospheric NOx sampling. Raffaele Cucciniello , Antonio Proto , Federico Rossi , Oriana Motta. Atmospheric Environment 2013 79, 666...
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Enhancement Effects of Gas Components on NO Removal by Calcium Hydroxide Guoqing Chen,† Jihui Gao,* Shuai Wang, Xiaolin Fu, Lili Xu, and Yukun Qin School of Energy Science and Technology, Harbin Institute of Technology, 92 West Dazhi Street, Harbin 150001, P. R. China

The NO absorption characteristic of calcium hydroxide was investigated in a fixed bed reactor system in the presence of SO2. The effect of O2 and H2O on NO removal was discussed in or without the presence of SO2. When SO2 was excluded from the flue gas, NO removal was slight. The presence of SO2 in the flue gas could enhance NO removal only when both of O2 and H2O were also present. The essence of enhancement of SO2 on NO removal was accelerating the conversion of NO to NO2. The desulfurization reaction product could not oxidize NO to NO2. The bed height experiment revealed that the multimolecular intermediate species, which could decomposed in to NO2, was not formed from the adsorption of NO, H2O, and O2 on the surface of absorbent. The possible reaction path proposed in paper is that there may be some unstable intermediate species formed from the desulfurization reaction, which can enhance the conversion of NO to NO2. The presence of H2O can enhance the presence of unstable intermediate. According to analysis, the possible unstable intermediate is the OH radical. 1. Introduction SO2 and NOx emissions from the combustion of fossil fuels, such as coal and heavy oils, have caused serious environmental problems. Protection of global environment requires abatement of not only SO2 but also NOx. The technologies developed for control of SO2 and NO, still use two separate systems to remove SO2 and NO, respectively. Those technologies require a very high capital cost, though they have a relatively high reliability and removal efficiency. Attempts have been made to find an effective method to remove SO2 and NOx simultaneously in one reaction cell. Due to NO inert chemistry, removing NO effectively has been a key problem for simultaneous removal of NOx and SO2. At present, oxidation of NO to NO2 has been proposed as a promising method for enhancing NO removal in conventional flue gas desulfurization (FGD) processes.1-3 The method where NO is first oxidized to NO2 and then absorbed with alkaline absorbent, such as calcium-based absorbent, has attracted many researches’ attention.4-7 Removal of SO2 and NO from the flue gas with calciumbased absorbent has been widely investigated in earlier work.7-9 The results suggested that the coexistence of NO and SO2 in the flue gas could enhance the absorption of each other. Concerning the effect of NO on the desulfurization, many studies maintained that the presence of NO and O2 in the flue gas enhanced the reaction of SO2 removal. The enhancement mechanism was considered to be due to the formation of NO2 which could oxidize SO2 to SO3.10,11 Concerning the effect of SO2 on NO removal with calciumbased absorbent, there also have been several reports. O’Dowd’s13 experimental results revealed that there was a significant decrease in the Ca utilization for the nitrogen species with increasing SO2 concentration. However, other researchers pointed out that the presence of SO2 together with O2 in the simulated flue gas might improve the denitration ability of calcium-based absorbent. Chu and Rochelle1 reported that the calcium-based absorbent could capture a certain NOx in the presence of SO2 and O2 in the traditional dry and semidry FGD process. Livengood14 suggested that * To whom correspondence should be addressed. Tel.: +86-45186413231ext 816. Fax: +86-451-86412528. E-mail: [email protected]. † E-mail: [email protected].

the formation of the compounds containing nitrogen and sulfur based upon the strong dependence of NO removal on SO2 concentration. Tsuchiai15 found that NO removal activity of calcium-based absorbent increased with an increase in SO2 concentration up to 2000 ppm at 130 °C. Sakai’s experimental resultsshowed that SO2 was indispensable to the removal of NO.8 When SO2 was excluded from the feed gases, the removal of NO could not occur. Although the influence of SO2 in the flue gas on NO removal has been investigated, the conclusions and influence mechanisms were not in agreement. Moreover, there was little report about the role of H2O and O2 in the enhancement effect of SO2 on NO removal by calcium hydroxide. Therefore, it was necessary to perform further investigation and to understand the effect of the flue gas components on NO removal by Ca(OH)2, which was of importance for developing simultaneous removal of NOx and SO2 basing on the traditional FGD processes. The aim of the present paper is to provide new experimental data obtained from the breakthrough curve analyses concerning the effect of SO2 on the reaction of NO and Ca(OH)2. Therefore, the NO absorption characteristic of calcium hydroxide at a low temperature was studied in a fixed bed reactor system. The effect of O2 and H2O on NO removal was discussed in or without the presence of SO2. The role of dusulfurization product in NO removal was also investigated by means of step experiment and bed height experiment. Solid product analyses were also conducted to identify the presence of specific functional groups in the spent absorbents and to gain insight into the effect mechanism of SO2 on NO removal. The possible effect mechanism of SO2 on NO removal by calcium hydroxide was proposed. 2. Experimental Section 2.1. Preparation of Absorbent. The raw materials used to prepare absorbent are calcium oxide and silicon dioxide, which are of reagent grade. In order to prevent channeling and absorbent agglomeration in the reactor, the silicon dioxide was added into the aqueous slurry comprised of calcium oxide and water during the hydration process. For the preparation of absorbent (410.9 g), the raw calcium oxide (90 g) was

10.1021/ie900838f  2010 American Chemical Society Published on Web 01/05/2010

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Figure 1. Schematic diagram of the experimental system.

first milled and sieved to ensure that almost all calcium oxide particles were in the size range 60-80 µm. The powdertype calcium oxide was then added into the deionized water at a weight radio of 5:1 (H2O: CaO) at a temperature of 70 °C. The slurry was heated up to 90 °C and maintained for 3 h with continuous stirring. Until the slurry temperature down to ambient temperature, the SiO2 (350.0 g, size: 60-80um) was then added and continuously stirred for 30 min. 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 100-120 µm. The resulting absorbent (410.9 g) contained 60.9 g of calcium hydroxide and 350 g of silicon with a specific BET surface area of 7.26 m2/g. 2.2. Experimental Apparatus and Procedure. The fixed bed reactor was employed for the experiment. This allows us to focus on the reaction mechanism at well-defined flow conditions but requires nonsteady operation. A schematic diagram of the experimental apparatus is shown in Figure 1. The reactor is a 25.0 mm I.D., 300 mm high quartz column fixed in the tube furnace for isothermal operation, which was heated electrically to the required temperature before every experiment. The temperature of tube furnace was regulated by a proportional-integral-derivative (PID) controller. In the experiment, the absorbent was dispersed in the center of the reactor and was supported by the sand core. The mass of absorbent in the bed was 30 g (containing 4.5 g calcium

hydroxide), giving a static bed height of 40 mm. The simulated flue gas containing 0-1000 ppmv of SO2, 0-500 ppmv of NO, 5% O2, 12% CO2, and balance N2 supplied by the high pressure cylinder was passed through the absorbent. The water vapor was supplied by the N2 and O2 flow passing through a water humidifier consisting of a 250 mL conical flask submerged in the water bath at constant temperature. The desired H2O fraction could be achieved by controlling the flow rate of N2 and the temperature of water bath. The total flow rate of the gas stream was controlled at 2750 mL/min. The tubes water vapor transported in were trapped with the heating tapes at 110 °C to prevent condensation. The mixed gas flowed first via the bypass line in order to measure the compositions of reactant gas. The concentration of SO2, NO, NO2, and H2O were measured by an online Fourier transform infrared (FTIR) gas analyzer (GASMET-DX4000, Finland), with the measurement errors of (2%. A Testo-335 gas analyzer was used to measure the concentrations of O2, with the measurement errors of (0.8 vol %. The X-ray diffraction (XRD) (Riguku D/Max-γB, Japan) and Fourier transform infrared (FTIR) spectroscopy (Thermo Nicolet 5700, USA) were employed to identify the presence of specific functional groups in the spent absorbents and to gain insight into the effect mechanism of SO2 on NO removal. The fractional removal of NO and SO2 were defined as follows:

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SO2 removal

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

(1)

NOx removal DeNO ) (CNO,in - CNO,out - CNO2,out)/CNO,in

(2)

NO conversion CoNO ) (CNO,in - CNO,out)/CNO,in

(3)

where CSO2,in, CSO2,out, CNO,in, CNO,out, and CNO2,out represent the SO2, NO, and NO2 concentration in the flue gas at the inlet and outlet of the reactor, respectively. In the initial stage of the experiment, the reactant gas flowed through the bypass line to measure the gas compositions. After the concentrations of the reactant gas were stable, the experiment was started by quickly switching the reactant gas into the reactor. Because the reactor was purged with air, the outlet NO concentration exhibited very low values at first and in turn rapidly increased. In about 1 min after switching the reactant gas into the reactor, the outlet gas concentration tended to be stabilized, and the gas concentration after 1 min was regarded as the steady state outlet gas concentration. To ensure the accuracy of the experiments, for some cases, two or three replicate measurements were made, and the relative errors was less than 3%. In the paper, only the average values are shown. In addition, the analyses of flue gas and spent absorbent respectively by FTIR and IC are shown in Table 1. From the comparison of N and S from gas analysis and solid analysis, it is observed that good mass balance made the experiments credible.

Figure 2. Effect of H2O and O2 on NO removal in the absence of SO2.

3. Results and Discussion 3.1. NO Removal Using Calcium Hydroxide. 3.1.1. NO Removal Using Calcium Hydroxide in the Absence of SO2. The evolution of NO removal by Ca(OH)2 with time in the absence of SO2 is depicted in Figure 2. In order to elucidate the roles of O2 and H2O in NO removal reaction respectively, four experimental runs were performed. As can be observed in Figure 2, due to NO inert chemical characteristics, it is difficult to get high efficiency for NO removal by calcium hydroxide, although NO is an acid gas. Ca(OH)2 can hardly absorb NO in the absence of SO2. O2 has a slight enhancement on NO absorption. It was about 5% of NO absorbed by Ca(OH)2, when 5% of O2 was added into flue gas. When O2 was excluded from the feed gas, the removal of NO could not occur. The previous reports also pointed out that O2 can improve NO removal, because O2 can oxidize NO to NO2 which reacts with Ca(OH)2 readily.11 Despres16 reported that about 6-10% of NO (500 ppm) was oxidized to NO2 for 5% of oxygen concentration at the temperature between 100 and 200 °C. Therefore, it can be concluded that the precondition of NO removal by calcium hydroxide is that NO must be oxidized to NO2. The presence of H2O together with O2 markedly increases the NO removal. Ma et al. maintained that H2O accumulating on the surface and interior pore of absorbent forms a liquid film, which can make the gas-solid reaction of NO2 and Ca(OH)2

Figure 3. Effect of SO2 concentration on NO conversion and SO2/NOx removal by calcium hydroxide at a reaction time of 45 min.

into an ionic reactionsit is very fast and most effective.17 Nelli and Rochelle’s experiments showed that NO2 reaction rate decreased with relative humidity rise from 0% to 80% at 25 °C.4 However, a different result at 70 °C was gained in Zhang’s work.9 Zhang et al. found that the NO2 absorption rate decreased first with relative humidity being raised from “very low” to 30% and then increased with relative humidity from 30%.9 The further analysis about the role of H2O in NO absorption by calcium hydrate is given in the following part. 3.1.2. NO Removal by Calcium Hydroxide in the Presence of SO2. Figure 3 shows the effect of SO2 concentration on NO removal by calcium hydroxide. The amounts of SO2 removal and NO conversion were expressed here by the moles of SO2 and NO removed from the flue gas at a reaction time of 45 min. The moles of SO2 and NO removed were measured by integration of the difference between inlet and outlet concentrations. The amount of NOx removal was also measured by integration of the difference between inlet an outlet NOx

Table 1. Amount of S and N of Spent Absorbent by Gas and IC Analysisa FTIR analysis of flue gas runs SO2 ) 248 ppm SO2 ) 528 ppm SO2 ) 810 ppm a

IC analysis of spent absorbent

gas N-removal (× 10-1 mmol) gas S-removal (× 10-1 mmol) absorbent N-fixed (× 10-1 mmol) absorbent S-fixed (× 10-1 mmol) 4.88 6.65 8.50

13.55 25.64 38.9

4.68 6.52 8.37

13.33 25.24 38.57

Reaction conditions: 4.5 g of Ca(OH)2, 20% H2O, 360 ppm NO, 5% O2, 2750 mL/min, temperature 110 °C, reaction time 45 min.

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Figure 4. Dependence of NO2 variation at outlet with time on SO2 concentration.

concentration (NOx ) NO + NO2). As can be seen in Figure 3, Without the presence of SO2, the amount of NO conversion is not appreciable. The amount of NO conversion markedly increases with an increase in SO2 concentration up to 810 ppm and then relaxedly increases above 800 ppm. From the analysis of gas components at the outlet of reactor, it is found that NO can be oxidized to NO2 in the presence of SO2 in the flue gas. Figure 4 shows the variation of NO2 concentration at the outlet. The variation of NO2 concentration is strongly dependent on the SO2 concentration. The higher SO2 concentration, the more NO2 was detected at the outlet. Therefore, it is suggested that the essence of the SO2 enhancement effect on NO removal by calcium hydroxide is accelerating the conversion of NO to NO2. As also can be observed in Figure 3, both the NO conversion amount and SO2 removal amount increase with SO2 concentration increased. In other words, the more SO2 reacting with Ca(OH)2, the more NO was oxidized to NO2. Therefore, it can be deduced that NO conversion is strongly dependent on the reaction between Ca(OH)2 and SO2. It is also clear in Figure 3 that the amount of NOx removal increases from 0.064 to 0.85 mmol, with SO2 concentration increasing from 0 to 810 ppm, and then slightly decreases with the SO2 concentration increasing from 810 to 2000 ppm. This result indicates that the enhancement effect of SO2 is significant at low SO2 concentrations but slight at high concentrations. In the previous research, similar conclusions were also attained. Sakai8 reported that the presence of SO2 in the flue gas was indispensable for NO removal by slaked lime. But when the concentration of SO2 increased from 800 to 1600 ppm, it did not show any obvious influence on the removal of NO. O’Dowd13 also found that NO removal activity was not appreciable, with an increase in the SO2 concentration from 1000 to 3000 ppm in a spry drier. Tsuchiai’s15 results revealed that NO removal activity was not appreciable without the presence of SO2. The NO removal increased with an increase in the SO2 concentration up to 2000 ppm at the reaction time of 5 and 10 h and then gradually decreased as the SO2 concentration increased further. Nelli and Rochelle4 found that the NO2 rate of removal increased with increasing SO2 concentration from 274 to 970 ppm. Therefore, the enhancement effect of SO2 is slight at high SO2 concentrations, which can explain the discrepancy among researchers’ conclusions. The reactivity of SO2 reacting with Ca(OH)2 is higher than NO2. The amount of Ca(OH)2 molecules on the surface of the absorbent is constant. If the SO2 concentration increases, more Ca(OH)2 molecules are held by SO2 and the amount of Ca(OH)2 molecules left for

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Figure 5. Ca utilization for NO and SO2.

NOx removal will decrease. Meanwhile, the greater SO2 removal caused more NO to be oxidized to NO2. The competition between both SO2 and NO2 species for the absorption on the solid surface occurs on the surface of absorbent. In an attempt to further identify the influence of SO2 on the NO removal, the spent absorbents were analyzed by IC and Ca utilization for NO and SO2 was calculated and shown in Figure 5. The reactivity of the absorbent toward SO2 and NO was expressed as Ca utilization for SO2 and NO, which was defined as the fraction of the number of moles of Ca in the absorbent reacted with NO or SO2. In Figure 5, the Ca utilization for NO and SO2 was plotted with the concentration of SO2. It is observed that the Ca utilization for NO increases with an increase in the SO2 concentration from 0 to 810 ppm. Concerning the enhancement mechanism of SO2 on NO removal by calcium based absorbent, prior to this study, only Sakai8 and Zhang7 performed some relevant works. Zhang7 assumed that SO2 played a catalytic role in the oxidation of NO to NO2 but did not point out the reaction path. Unlike Zhang’s mechanism, Sakai8 supposed that SO32- formed from the absorption of SO2 played an important role for the absorption of NO by the slaked lime. SO2 first reacted with water, then was absorbed as SO32- on the surface of absorbent, and finally reacted with NO and O2. The reaction approach can be shown as CaSO3 + O2 + NO f active complex f CaSO4 + NO2 Yoshinari18 and Li19 investigated the catalytic oxidation of NO over Al2O3 in the presence of SO2. It was found that the oxidation activity of NO to NO2 was enhanced temporarily by the adsorption of SO2 over Al2O3. The multimolecular intermediate formed from the combination of SO2, NO, O2, and H2O on the absorbent surface could be decomposed into NO2 and sulfate salt.20 In order to illustrate the roles of SO2 and desulfurization reaction product in the oxidation of NO and verify whether multimolecular intermediate was formed on the absorbent surface, the absorption curve of NO in a fixed bed reactor was obtained by means of step experiment and bed height experiment. 3.2. Step Experiment for Removal of NO by Calcium Hydroxide. To investigate the roles of SO2 and desulfurization product in the conversion of NO, SO2 and NO were fed into reactor separately. The absorbent was first treated with a flow of 450 ppm NO, 20% H2O, 5% O2, and 12% CO2 (without SO2) for 10 min at a temperature of 110 °C. The total flow rate was 2750 mL/min. In order to form the desulfurization product over the surface of absorbent, the gas stream containing 700 ppm

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Figure 6. Result of step experiment: (step 1) without SO2, 450 ppm NO; (step 2) without NO, 700 ppm SO2; (step 3) without SO2, 450 ppm NO; (step 4) coexistence of 700 ppm SO2 and 450 ppm NO.

SO2, 20% H2O, 5% O2, and 12% CO2 (without NO) was then fed into reactor to react with absorbent for 10 min (during this time DeSO2 ) 100%). After some desulfurization product was formed over the absorbent, the reactor was blown by N2 and SO2 was replaced by NO. The reaction time was also 10 min. When the NO concentration was constant, SO2 was fed into the gases. The step experiment contained four steps, the roles of which can be found in Figure 6. As can be seen from Figure 6, the absorption and conversion efficiencies of NO in the absence of SO2 are about 6%. The absorption activity of the absorbent treated with SO2 is similar to the fresh absorbent. When SO2 was fed into flue gas, the DeNO and CoNO increased obviously. CoNO is higher than DeNO. These results mean that desulfurization product has negligible influence on NO conversion. The presence of SO2 with NO simultaneously in the flue gas enhances the removal of NO. That is, only when Ca(OH)2 reacts with SO2 in the flue gas may NO be oxidized to NO2. The experimental results shown in Figure 6 further confirm that NO conversion is strongly dependent on the reaction between Ca(OH)2 and SO2. 3.3. Bed Height Experiment for the Conversion of NO. In order to further investigate the effect of the desulfurization product formed from the dry-FGD on NO conversion, the bed height experiments were performed. The step experiment was employed to produce desulfurization product. In order to produce different amounts of desulfurization product on the surface of absorbent, the absorbent was first pretreated with the flue gas without the presence of NO for 4, 8, 12, 16, 20, and 30 min. During the pretreatment process, the removal of SO2 was maintained 100%, except for the case for 30 min. Then, the pretreated absorbents containing different amount of desulfurization product were used to absorb NO in the absence of SO2. Figure 7 shows the amount of NO captured by the pretreated absorbent for 1 h of reaction time with respect to the absorbent pretreated time for the bed heights of 40 and 80 mm. It can also be found that the amount of desulfurization product on the absorbent surface has negligible influence on the NO conversion. As the absorbent pretreated time was increased from 4 to 20 min, the NO captured showed no obvious change for 40 and 80 mm bed heights. This indicates that the desulfurization product cannot oxidize NO to NO2. On the other hand, it is deduced that the multimolecular intermediate is also not formed on the surface of Ca(OH)2. As we know, with increasing the height of absorbent, the active site on the absorbent and total surface area of the absorbent would increase. The higher the

Figure 7. Effect of desulfurization reaction products on NO conversion.

Figure 8. Effect of H2O and O2 on the removal of SO2.

total surface area, the more SO2, NO, and H2O would be adsorbed on the surface. If the multimolecular intermediate can be formed, the NO conversion will increase with the height of absorbent. From Figure 7, it cannot be found that the bed height has positive influence on NO conversion. 3.4. Effect of H2O and O2 on the Oxidation of NO. Figures 8 and 9 show the effect of H2O and O2 on SO2 removal and NO conversion. When the absorbent was subjected to the flue gas without the presence of H2O and O2, SO2 removal was very low, and the CoNO and DeNO were only about 3%. The high absorption activity of SO2 in the initial reaction time can be explained by the high water adsorptive activity of Ca(OH)2. As the sulfate/sulfite salts formed from the desulfurization and nitrite/nitrate salt formed from the reaction of Ca(OH)2 and NO2 covered the surface of the absorbent, the absorbent had no desulfurization activity. Due to low SO2 removal in the absence of H2O, NO removal was also slight in both presence of SO2 and O2.This result suggests that NO can not be oxidized to NO2, if SO2 cannot react with Ca(OH)2. As H2O was injected into the flue gas, the desulfurization activity increased markedly. The absorbent exhibited a 100% removal of SO2 for about 10 min, but the DeNO and CoNO were similar to the case for the absence of both O2 and H2O. This indicates that NO cannot be oxidized to NO2 in the absence of O2, even though the desulfurization reaction occurs over the absorbent surface. When the absorbent was subjected to the flue gas with the presence of O2 and without

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Figure 9. Effect of H2O and O2 on the conversion of NO.

Figure 10. XRD spectra of the fresh absorbent and spent absorbent: (a) calcium hydroxide Ca(OH)2; (b) silicon dioxide SiO2; (c) gypsum CaSO4 · 2H2O.

the presence of H2O, the absorbent can not only react with SO2 but also absorb NO2 formed from the oxidation of NO in the initial reaction time. The CoNO increases with the increase in SO2 removal efficiency and decreases with the decrease of SO2 removal efficiency. From the experimental results shown in Figures 8 and 9, O2 and the reaction between SO2 and Ca(OH)2 are found to play an important role in the conversion of NO to NO2. Only when both O2 and the desulfurization reaction coexist in reactor can NO be oxidized to NO2. The presence of H2O in the flue gas can accelerate desulfurization reaction and NO conversion. As we all know, O2 can oxidize NO into NO2, but with low activity under such experiment conditions. The desulfurization product cannot oxidize NO directly (in Figure 7). Therefore, we assume that there maybe have a kind of unstable intermediate species formed from the desulfurization reaction, which can enhance the conversion of NO to NO2. 3.5. Solid Products Analysis and Mechanism of NO Oxidation. 3.5.1. Solid Products Analysis. The chemical composition of the absorbent before and after being treated with the simulated flue gas was investigated through XRD analysis. The XRD spectra are shown in Figure 10. It can be found that the main components of fresh absorbent are calcium hydroxide and silicon dioxide. After being treating with the simulated flue gas, the spectrum of the spent absorbent has a little change. Apart from calcium hydroxide and silicon dioxide, sulfate

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Figure 11. FTIR spectrum of the spent absorbents. Flue gas component of cases (a) NO-H2O-O2; (b) SO2-NO-H2O; (c) SO2-NO-O2; (d) SO2-NO-H2O-O2.

(SO42-) was detected as the product of desulfurization reaction. No distinct XRD patterns were attributed to nitrite species and nitrate species. This might have been due to the small quantity of those crystal phases in the spent absorbent or their crystal quality is not good enough for XRD detection. In order to detect the specific functional group of the absorbent, spectroscopic analysis tests were carried out using the FTIR. The corresponding FTIR spectra are shown in Figure 11. It is observed that the sharp vibration band approximately at 3640 cm-1 can be attributed to the stretching of CaO-H.21 The brand around 3450 cm-1 is typically caused by hydroxyl groups.21,23 A single peak close to 1638 cm-1 related to hydroxyl groups (O-H bending) was also observed.24 The bands at 1450 and 874.3 cm-1 have been assigned to carbonate (CO32-). The products of desulfurization reaction are mainly sulfate (SO42-) species, which is characterized by a broad brad due to S-O stretching near 1104 cm-1 and S-O bending at 650 cm-1. The intensities of sulfate band for cases c and d are higher than the others. The reason for this is that NO is oxidized into NO2 in those cases, which can oxidize sulfite species to sulfate species. Additionally, a slight and weak band at 1345 cm-1 is associated with nitrate (NO3-) species.24 3.5.2. Mechanism of Desulfurization Processes Enhancing NO Removal with a Calcium Based Absorbent. According to above experimental results, it can be observed that O2 is indispensable for NO removal by calcium hydroxide. The presence of SO2 in the flue gas can enhance the removal of NO only when both H2O and O2 are also present in the flue gas simultaneously. H2O can accelerate the removal of SO2 and NO. The desulfurization product could not oxidize NO to NO2. On the surface of the absorbent, the adsorption of H2O, O2, and NO could not form the multimolecular intermediate which could be decomposed into NO2. As we all know, the removal of SO2 and NO is not observed without the absorbent.8 The more SO2 reacted with absorbent, the more NO is conversed to NO2. Therefore, there must be some unstable intermediate species formed from the desulfurization reaction which can enhance the NO oxidation. There are some reports on the NO oxidation in the flue gas. Li19 reported that NO, O2, and SO2 could react with a surface OH radical formed from solid surface absorbed water and formed an unstable intermediate which could be decomposed into NO2 under a certain condition. Wilde20 also found that NO and O2 absorption on solid surfaces required the simultaneous presence of SO2, NO, and O2. Song25 summarized the mechanism of NO homogeneous oxidation in the presence

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of SO2 and O2 and reported that NO could be oxidized to NO2 if some OH or HO2 radicals were present. The reaction steps are as follows: OH + SO2 + M f HOSO2 + M

(4)

HOSO2 + O2 f SO3 + HO2

(5)

HO2 + NO f OH + NO2

(6)

In the experiment, we also found that NO could be oxidized into NO2, under the condition that O2, H2O, and SO2 must be present in the flue gas simultaneously. Thus, we speculate that the unstable intermediate species formed from the desulfurization reaction may be the OH radical. The reaction path of NO removal by calcium hydroxide can be surmized as SO2, O2, NO, and H2O first adsorbing on Ca(OH)2, and then, a certain SO2 and H2O reacting with Ca(OH)2 and forming some OH. OH formed from desulfurization can react with NO, O2 and SO2 to form NO2 and SO3, which then react with Ca(OH)2. In this process, OH can not directly oxidize NO to NO2. It must be oxidized to HO2 under the action of O2. The latter can react with NO to form NO2. NO cannot be oxidized to NO2 without the presence of O2, even though a large number of OH radicals are generated from SO2 removal by calcium hydroxide. This reaction path can also be used to explain the enhancement of NO on desulfurization. 4. Conclusion O2 is indispensable for NO removal by calcium hydroxide. The presence of SO2 in the flue gas can enhance the removal of NO only when both O2 and H2O are also present in the flue. H2O plays an important role in desulfurization reaction which has a significant influence on NO conversion. Due to low SO2 removal in the absence of H2O, NO conversion is also slight in the presence of both O2 and SO2. The essence of the enhancement effect is accelerating the conversion of NO to NO2. According to the step experiment result, NO can not be oxidized to NO2 under the action of the desulfurization reaction product. The bed height experiment revealed that the multimolecular intermediate species, which can be decomposed into NO2, was not formed from the adsorption of NO, H2O, and O2 on the surface of absorbent. It is deduced that there may be some unstable intermediate species formed from the desulfurization reaction, which can accelerate the conversion of NO to NO2. According to analysis, the unstable intermediate species formed from the desulfurization reaction may be the OH radical. The reaction path of NO removal by calcium hydroxide can be surmized as SO2, O2, NO, and H2O first adsorbing on Ca(OH)2, and then, a certain SO2 and H2O reacting with Ca(OH)2 and forming some OH radical. The OH radical formed from desulfurization could react with NO, O2, and SO2 to form NO2 and SO3, which then react with Ca(OH)2. Acknowledgment The authors gratefully acknowledge 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. 2007AA0 5Z307). Literature Cited (1) Chu, P.; Rochelle, G. T. Removal of SO2 and NOx from Stack Gas by Reaction with Calcium Hydroxide Solids. JAPCA 1989, 39, 175.

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ReceiVed for reView May 21, 2009 ReVised manuscript receiVed December 21, 2009 Accepted December 22, 2009 IE900838F