Effect of Additive Agents on the Simultaneous Absorption of NO

Effect of Additive Agents on the Simultaneous Absorption of NO...
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Effect of Additive Agents on the Simultaneous Absorption of NO2 and SO2 in the Calcium Sulfite Slurry Zhihua Wang,† Xiang Zhang,† Zhijun Zhou,*,† Wei-Yin Chen,‡ Junhu Zhou,† and Kefa Cen† †

State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou 310027, People’s Republic of China Department of Chemical Engineering, University of Mississippi, University, Mississippi 38677, United States

‡

ABSTRACT: It is known that NO in the flue gas can be effectively converted to NO2 by O3. The objective of this work is to investigate the feasibility of simultaneously abating NO2 and SO2 from flue gas by liquid-phase conversion. A suite of costeffective additives for enhancing NO2 absorption through enriching the concentration of sulfite ion, SIV, in the liquid phase has been evaluated under pH similar to traditional flue-gas desulfurization (FGD). Experiments were conducted in a lab-scale washing tower with CaSO3 slurry, including metal and non-metal additives: FeSO4, FeCl2, Fe2(SO4)3, MnSO4, MnCl2, MgSO4, MgCl2, (NH4)2SO4, and NH4Cl. All of these additives enhance the absorption efficiency of NO2. Ferrous sulfate, FeSO4, is the most effective additive, with absorption efficiency reaching 95%, but the loss of additive is high because of the oxidation of FeII into FeIII. Ammonium sulfate, (NH4)2SO4, has similar absorption efficiency but shows lower loss during absorption. Its absorption efficiency improves with an increasing temperature and decreases with a decreasing pH, although NO2 absorption is still noticeable when pH varies between 5 and 7, which is within the operating range of a typical wet FGD. The presence of O2 causes a 4% decrease in NO2 removal when MnSO4 and (NH4)2SO4 are used as additives.

1. INTRODUCTION Nitrogen oxides (NOx) and sulfur dioxide (SO2), emitted from combustion processes, such as fossil-fuel power plants, petroleum refineries, cement rotary kilns, metallurgy furnaces, etc., are major contributors to acid rain and photochemical smog.1,2 Current NOx and SO2 emission control technologies include low NOx burners, air staging, reburning, selective noncatalytic reduction (SNCR), selective catalytic reduction (SCR) for NOx and wet flue gas desulfurization (WFGD) for SO2.3−5 Most of these single-function unit operations suffer high operating costs. Emerging cost-effective technologies for simultaneous NOx and SO2 control include non-thermal plasma (NTP),6−10 electric beam irradiation,11 and advanced wet scrubber process.12−14 NO accounts for more than 95% of the total NOx in the flue gas from coal-fired boilers. It is practically insoluble in water; therefore, it cannot be captured in the common calcium-based WFGD systems. However, the oxidation products of NO, including NO2, NO3, and N2O5, are soluble in water. Thus, it is possible to simultaneously remove both nitrogen oxides and SO2 in the existing calcium-based WFGD system if NO is converted into NO2, NO3, N2O5, etc. Low-temperature ozone oxidation technology has been proposed as an alternative to NTP.15 NO can be easily oxidized into NO2, NO3, and N2O5 by O3, OH, and O radicals at relatively low temperature. These oxidation products can then be absorbed in a typical WFGD system,16,17 along with SO2 and oxidized mercury Hg2+. Indeed, Mok et al. has demonstrated the simultaneous oxidation of NOx and SO2 by ozone injection in a bench-scale test facility.18 In our previous studies, the simultaneous oxidation of NO, SO2, and Hg0 by ozone in nitrogen flow was investigated.19,20 The results of these studies suggest that nearly 90% of NO is oxidized and NO2 is the major oxidation product in the temperature range of © 2012 American Chemical Society

373−473 K. Nevertheless, the absorption efficiency of NO2 is not satisfactory in mixtures containing alkaline ions. Extra ozone is needed to convert NO2 further into NO3. Although around 97% NO and nearly 100% SO2 can be absorbed in the washing tower, an extra dosage of ozone becomes an important cost factor for this technology. Therefore, there is an incentive to improve the technology by reducing ozone consumption as much as possible. Improvement in the absorption efficiency of NO2 in alkaline solutions is one of the viable choices. It has been reported that sulfuric acid, sodium hydroxide, sodium sulfite, and bisulfite can improve the absorption of NO2 in wet scrubber.21−24 Among these species, sodium sulfite, Na2SO3, seems to be the most effective agent because of the presence of dissolved SIV ions in its aqueous solution.25 In a typical calcium-based WFGD process, with pH value 5−7, SIV appears mainly as HSO3− and SO32−, with the former being the dominating species.26 The dissolution of NO2 is governed by its interactions with SIV ions through the following chemical reactions:27 2NO2 + H 2O ↔ NO2− + NO3− + 2H+

(1)

H+ + SO32 − ↔ HSO3−

(2)

2NO2 + SO32 − + H 2O ↔ 2NO2− + SO4 2 − + 2H+

(3)

2NO2 + HSO3− + H 2O ↔ 2NO2− + SO4 2 − + 3H+ (4)

However, as seen from the above-mentioned reactions, Na2SO3 will be consumed by both NO2 and the appearance of oxygen Received: August 31, 2011 Revised: July 26, 2012 Published: July 27, 2012 5583

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in flue gas.13,21 Considering the cost of sodium sulfite, it may not be a good choice for practical use. Therefore, seeking a cost-effective absorbent and additive for NO2 absorption is necessary. It is known that calcium sulfite, CaSO3, is an abundant reaction intermediate in the typical calcium-based WFGD process. Thus, CaSO3 may be a good choice for enhancing the NO2 absorption. First, CaSO3 is a source of SIV ion providing SO32− in the solution. Second, as a precursor of gypsum in typical calcium-based WFGD slurries, the amount of CaSO3 is considerable. Specifically, SO2 first reacts with limestone that generates calcium sulfite: CaCO3 + SO2 ↔ CaSO3 + CO2. It then follows forced oxidation by air that generates CaSO4: CaSO3 + 1/2O2 + H2O ↔ CaSO4 + H2O. Gypsum, CaSO4·2H2O, is formed in this process after separation. CaSO3 can also be oxidized by NO2 through ion reactions 2−4 rather than O2, which can further reduce the operating cost of the WFGD system. Therefore, CaSO3 can be an attractive absorbent for multi-pollution emission control in the low-temperature ozone and calcium-based WFGD combination system. Unfortunately, the solubility of CaSO3 is so low that it limits the production of SO32− through hydrolysis of CaSO3. To enhance the dissolution of calcium sulfite, additives, including manganese(II), iron(II), magnesium sulfate, and chloride, have been selected to improve the performance of desulfurization in calcium-based WFGD.28−31 Tang et al. investigated the absorption of NO2 by the CaSO3 slurry with the addition of MgSO4, MgCl2, and Na2SO4.27 MgSO4 was found to be the most effective additive because it is highly soluble in water, and the dissolved SO42− ion pushes the following two reactions toward the right: Ca 2 + + SO4 2 − ↔ CaSO4

Figure 1. Schematic diagram of the experimental apparatus. Industrial Co., Ltd.) and deionized water, which is pumped into the tower from a 2.5 L glass tank. The 80% filled slurry tank is equipped with a magnetic stirrer (MS300, Bante Instrument). The slurry, containing 0.15 mol/L CaSO3, runs cyclically between the contact tower and the tank at a flow rate of 3 L/h by a peristaltic circulating pump (BT01-100, Baoding Longer Precision Pump Co., Ltd.). The pH value of the slurry is monitored by a pH meter (PHS-25, Cany Precision Instruments Co., Ltd.) and adjusted by a 0.1 mol/L HCl aqueous solution. The simulated flue gas mixture was made of gases from four cylinders that contain 1.2 vol % NO2 balanced by N2, 1.2 vol % SO2 balanced by N2, 99.999 vol % O2, and 99.999 vol % N2, all supplied by New Century Mixed Gas Co., Ltd. The flow rate of each gas stream was controlled by an individual mass flow controller (MFC, Qixinghuachuang Co., China). A teflon tube of 6 mm inside diameter was used to transport the gas mixture to avoid oxidation and absorption of NO2 on the tube surface. During the tests, the concentrations of NO2 and SO2 in the gas stream were maintained at 300 ppm. The total gas flow rate was kept around 5 L/min. The absorption efficiency was calculated on the basis of the following formula:

(5)

CaSO3 ↔ Ca 2 + + SO32 −

(6)

NO2 absorption efficiency =

SO32−

As a result, the formation of is promoted. Moreover, the presence of a Mg2+ ion also enhances the formation of another soluble sulfite species, neutral MgSO30 ion pair,28,32 which, in turn, also helps the NO2 absorption.27 However, the additive loss and performance of absorption at practical low pH values have not been investigated. Huss et al. found that MnII and FeII have catalytic effects on the oxidation of aqueous sulfur dioxide solutions,30,31 which may also benefit the contribution of SO32− formation in solutions. In ammonia-based WFGD, (NH4)2SO3 and NH4HSO3 were found to be the key components for SO2 absorption.33 Therefore, in the present work, the absorption of NO2 and SO2 in the CaSO3 slurry with different additives was investigated in the pH range of 5−8. Additives, including FeSO4, FeCl2, Fe2(SO4)3, MgSO4, MgCl2, MnSO4, MnCl2, (NH4)2SO4, and NH4Cl, were evaluated in detail. Additive loss and the effect of the temperature were also investigated.

[NO2 ] in− [NO2 ]out × 100% [NO2 ]in

(7)

where [NO2]in and [NO2]out are the concentrations of NO2 at the inlet and outlet of the washing tower. Equation 7 was also used to estimate the SO2 absorption efficiency similarly. The MFC and flue gas analyzer are the two major sources of experimental errors. Collectively, they generate an error of ±3%. An online continuous emission monitoring system (CEMS, MGA5, Germany MRU Co., Ltd.) was used to monitor the concentrations of NO, NO2, SO2, and O2 of the gas streams entering and exiting the washing tower. The results were recorded by a computer every 5 s. At 2 min after the experiment started, the readings stabilized and timeaveraged results were chosen for estimating the NO2 absorption efficiency. Solid CaSO4 formed in the slurry after absorption of NO2 was carried to the slurry tank. The slurry in the tank was filtered by filter paper every 30 min, dried at 378 K for 4 h in N2, and analyzed by X-ray diffraction (XRD) spectroscopy (X’Pert PRO). The additive metal ions in the sampling filtrate were measured by an atomic absorption spectrophotometer (Thermo 969), and the ammonium additive was quantified by the ion-selective electrode (ISE, 410P-12, Thermo Orion). Additives tested in this study include FeSO4, FeCl2, Fe2(SO4)3, MnSO4, MnCl2, MgSO4, MgCl2, (NH4)2SO4, and NH4Cl. All of these chemicals were supplied by the China National Pharmaceutical Group Industry Corporation, Ltd. at CP grade. They were introduced to the slurry with a concentration ranging from 0 to 0.5 mol/L. These additives are all soluble in water. After each test, the scrubber system, including washing tower, pump, and glass tank, were cleaned with deionized water and dried to avoid cross-interference. The influence of the slurry temperature on the NO2 absorption was experimentally

2. EXPERIMENTAL SECTION Figure 1 illustrates the experimental apparatus for simultaneous absorption of NO2 and SO2. The system consists of a gas manifold, CaSO3 slurry washing tower, and an online flue gas analyzer. The counter-current washing tower is made of glass with a 640 mm height and 40 mm inside diameter. A single spray nozzle (LJ-1/8-1-SS, Guangzhou Aoopo Spraying Equipments Co., Ltd.) was installed on the top of the tower with a liquid−gas interaction distance of around 450 mm. The gas mixture and slurry droplets entered the tower from the bottom and top of the tower, respectively. The slurry used in this study contains CaSO3·2H2O (98% purity, Shanghai Yansheng 5584

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examined specially from 293 to 343 K, while the other tests were running under room temperature to about 293 K. The slurry temperature was controlled by the thermostatic function of a combined magnetic stirrer/heater (MS300, Bante Instrument).

3. RESULTS AND DISCUSSION 3.1. Performance of Baseline Absorbents. The baseline performance of CaSO3 slurry for NO2 absorption was first investigated and compared to water and Na2SO3 with a pH value ranging from 5 to 8 (Figure 2). The concentrations of

Figure 3. Effects of metal additives on NO2 absorption by the CaSO3 slurry at pH 5.5.

chloride salts have essentially the same effects. It is possible that both FeII and MnII catalyze the oxidation of aqueous sulfur dioxide, and FeII may be a stronger oxidation catalyst than MnII.30 In addition to reaction 2, conversion between SO32− and HSO3−, sulfur may also undergo the following reactions in an aqueous solution:30 Figure 2. Performance of different absorbents for NO2 absorption.

SO2 ·H 2O ↔ H+ + HSO3−

(8)

2HSO3−

(9)

II

CaSO3 and Na2SO3 solution were 0.15 and 0.02 mol/L, respectively. The initial gas stream contained 300 ppm NO2 balanced by N2. The pH value of the absorbents were adjusted by HCl and Na(OH)2. The absorption of NO2 in water is low, only 17%, and is independent of the pH. More than 92 and 73% of NO2 can be absorbed by Na2SO3 and CaSO3, respectively, which agrees with the findings by Tang et al. at pH 8.27 With the pH value increasing from 5 to 8, the NO2 absorption efficiency of Na2SO3 increases from 62 to 92%. The NO2 absorption efficiency of CaSO3 increases only slightly when the pH increases. 3.2. Effect of Additives. 3.2.1. Metal Additives. Tang et al. reported the effects of MgSO4 additive in CaSO3 slurry for NO2 absorption at pH 8.27 Nevertheless, pH is usually set around 5− 6 to guarantee the dissolution of CaCO3 in a typical calciumbased WFGD unit.34 Therefore, the effects of additives, including FeSO4, FeCl2, MnSO4, MnCl2, MgSO4, and MgCl2 in 0.15 mol/L CaSO3 aqueous slurry, were investigated at pH 5.5. The concentration of additives varied from 0 to 0.5 mol/L. The feed gas stream contained 300 ppm NO2 balanced with N2. The NO2 absorption performance is illustrated in Figure 3. About 67% of NO2 is absorbed by the CaSO3 slurry without additives. All metal salts show benefits of NO2 absorption but at distinct levels. NO2 absorption efficiency increases with an increasing additive concentration. The system involving FeSO4 and CaSO3 shows the best performance; NO2 absorption efficiency reaches 95% when 0.5 mol/L FeSO4 is added. For systems involving MnSO 4 and MgSO 4 , the maximum absorption efficiency reaches around 85%. When results of MgSO4 and MgCl2 additives are compared, it is clear that SO42− generates a higher benefit than Cl− on the enhancement of SO32− through reactions 5 and 6. The same trend can be found for MnSO4 and MnCl2. Nevertheless, FeII sulfate and

↔ S2 O5

2−

+ H 2O

II

If Fe and Mn catalyze one of the aforementioned reactions, it shifts reaction 8 to the right, and the SIV ions in the solution are enriched. 3.2.2. FeII and FeIII. To further validate the catalytic roles of ferrous salts, FeIII or Fe2(SO4)3 specifically was added to inhibit the catalytic effect of FeII. The temporal profiles of exiting NO2 and SO2 concentrations during the test are shown in Figure 4. Fe2(SO4)3 was introduced at 300 s following stabilization of the NO2 removal by the CaSO3 slurry. There was an obvious further decrease of the NO2 concentration after introducing 0.1 mol/L FeIII. This benefit diminished after the introduction of 0.2 mol/L FeIII. When the FeIII ion was further increased to 0.3 mol/L at 1400 s, the NO2 concentration increased slightly

Figure 4. Profiles of NO2 and SO2 during absorption of NO2 in the CaSO3 slurry with Fe2(SO4)3. 5585

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interface, where SO2 and NO2 must to be transported from the gas phase into the liquid film before they are chemically removed. When NH4+ appears in the CaSO3 solution, ammonium sulfite forms through the following reaction:

rather than decreased. Moreover, the exiting NO2 concentration was about 100 ppm when 0.3 mol/L Fe2(SO4)3 was added, which corresponded to about 67% NO2 removal. The FeII efficiency, as discussed in the last section, was about 92%. It was also noted that the exiting SO2 increased sharply when 0.3 mol/L Fe2(SO4)3 was introduced, which, in turn, significantly reduced the SIV concentration and inhibited the removal of NO2. This phenomenon is likely attributed to the weak acidity of ferric sulfate. When 0.3 mol/L FeIII was added, the pH value of the solution was lower than 4. Extra H+ pushes eq 8 to the left-hand side, and SIV ions are converted to SO2·H2O. Thus, FeII seems to play an important catalytic role during the conversion of NO2 in the CaSO3 slurry. 3.2.3. Non-metal Additives. In addition to the NO2 removal efficiency, the types and properties of the byproduct are also of concern for environmentally benign and cost-effective operations. Byproducts, such as Fe(OH)3, Mn(OH)2, Mg(OH)2, etc., are likely to influence the gypsum quality if gypsum contains these metal salts. Therefore, non-metal additives, including (NH4)2SO4 and NH4Cl, were also tested for the NO2 removal efficiency. Figure 5 shows the performance of (NH4)2SO4 and NH4Cl in the CaSO3 slurry; results are also compared to the pure

NH4 + + SO32 − ↔ (NH4)2 SO3

(10)

The ammonia sulfite can improve the SO2 removal rate in liquid film, which has been demonstrated by Gao et al.,33 through the following reactions in the liquid film:35

SO2 (g) ↔ SO2 (aq)

(11)

SO2 (aq) + H 2O ↔ H 2SO3

(12)

(NH4)2 SO3 + H 2SO3 ↔ 2NH4HSO3

(13)

The appearance of both (NH4)2SO3 and NH4HSO3 in the solution results in a higher SIV concentration that is beneficial to the removal of NO2. The distribution of nitrogen species within the slurry is thus important to the understanding of the functions of additives during NO2 absorption. In this work, the NO2− and NO3− concentrations of the slurry after NO2 absorption were analyzed by an ion chromatograph (792 Basic IC, Metrohm, Ltd.). Table 1 shows the results of tests with different additives. Table 1. Distribution of Nitrogen Species in the Slurries additive

NO2− (g/L)

NO3− (g/L)

NO2−/NO3−

FeSO4 MnSO4 MgSO4 (NH4)2SO4

3.323 2.477 2.408 3.158

0.826 1.224 1.294 0.716

4.02 2.02 1.86 4.41

The NO2− ions are generated through both NO2 hydrolysis in water and reactions of SIV, as depicted by reactions 1−4. NO3− is mainly generated from NO2 hydrolysis, as depicted by reaction 1. With the introduction of additives, NO2− was found to be the main nitrogen species. The ratio of NO2− and NO3− concentrations shown in Table 1 is also an index of the performance of the removal of NO2, which agrees well with the findings shown in Figures 3 and 5. In the last two sections, we demonstrated that metal and non-metal additives can be effectively used to enhance removal of NO2 in the CaSO3 slurry. The performance of the tested additives has the following trend: Cations: FeII, NH4+ > MnII, FeIII > MgII. Anions: SO32− > SO42− > Cl−. 3.3. Loss of Additives. In addition to catalytic effectiveness, the loss of additives is one of the important factors affecting the techno-economic performance. In the current study, the losses of four additives in the slurry were investigated: MnSO4, FeSO4, MgSO4, and (NH4)2SO4. Each of the additives was maintained at 0.5 mol/L in the entering slurry. Figure 6 shows the additive loss during the process. As expected, about 77.7% of iron ions are lost. It is likely that the ferrous ion is oxidized by NO2 into ferric ion and then converts to ferric hydroxide in the presence of a hydroxyl radical at pH 5−7. The losses of MnSO4 and MgSO4 are less than 20%. Reactions associated with direct NO2 removal do not contribute to these losses. Rather, the precipitation of Mn(OH)2 and Mg(OH)2 along with CaSO4 is the main source

Figure 5. Effects of non-metal additives on the absorption of NO2 by the CaSO3 slurry.

solutions of NH4Cl, (NH4)2SO4, and CaSO4. The ammonium salts in calcium sulfite solutions significantly enhance the NO2 removal efficiency. Specially, the NO2 removal increases from 67 to 90% when 0.5 mol/L (NH4)2SO4 is added to the CaSO3 slurry. It is interesting to note that the pure aqueous solutions of NH4Cl, (NH4)2SO4, and CaSO4 achieve a NO2 removal efficiency of about 21%, which is essentially the performance of pure water. These results clearly demonstrate the positive role of SIV ions in NO2 conversion. Moreover, these results seem to suggest that NO2 hardly reacts with CaSO4, (NH4)2SO4, or NH4Cl. Thus, the roles of SO42− ions (or SVI) and NH4+ ions in NO2 removal deserve additional examination and discussion. Like their metal counterparts, both NH4Cl and (NH4)2SO4 have positive effects on the NO2 removal. SO42− ions push reactions 5 and 6 to the right-hand side and enrich the desired SIV ions in the solution. Because the additives of NH4Cl also have considerable benefit on the absorption of NO2, the appearance of NH4+ must have some positive impact. Both mass transfer and absorption happen at the gas−liquid 5586

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Figure 6. Loss ratio of additives during the absorption process.

Figure 8. Effects of the temperature on the removal of NO2.

of losses. For (NH4)2SO4, a small amount of loss (4.4%) is observed. As discussed above, metal additives can accelerate the oxidation of calcium sulfite while improving the removal of NO2. However, the catalytic oxidation is accomplished with additive loss that has a negative effect on the gypsum properties in the case of metal additives. Figure 7 shows the XRD patterns

used as an additive, NO2 removal increases from 89 to 93% with an increasing temperature. These results indicate that, in the temperature range of the tests, NO2 removal by the CaSO3 slurry with (NH4)2SO4 as an additive improves slightly when the temperature rises. Although the solubility of NO2 in liquid phase decreases with an increasing temperature, chemical reaction rates and mass transfer of NO2 in the liquid phase improve. Therefore, the overall absorption improves with an increasing temperature. 3.5. Effect of the pH. Acidity influences the concentration of H+ ions and the concentration of SIV in the solutions, which, in turn, influences the NO2 removal. Figures 9 and 10 illustrate

Figure 7. XRD patterns of CaSO3 with four different additives. Peak assignments: (◆) CaSO3, (□) CaSO4, and (▽) Fe(OH)3.

of four dried slurry samples after tests with 0.5 mol/L additives. Calcium sulfate peaks are found in all four patterns, implying the oxidation of sulfite to sulfate because of the presence of additives. However, the presence of Fe(OH)3 peaks in the ferrous sample suggests the undesirable loss of additives. Thus, (NH4)2SO4 may be the best additive in terms of NO2 removal and additive loss. 3.4. Effects of the Temperature. The average temperature of typical calcium-based WFGD systems is around 333− 343 K. Thus, the effects of the temperature are studied in the temperature range of 293−343 K in the current study (Figure 8). It suggests that the temperature has a negative impact on the NO2 removal by the calcium sulfite slurry without additives. NO2 removal decreases from 67 to 60% when the temperature increases from 293 to 343 K. It should be mentioned that the solubilities of both CaSO3 and NO2 decrease with an increasing temperature. On the other hand, when ammonium sulfate is

Figure 9. Effects of the pH on removal of NO2 and SO2 by the CaSO3 slurry with different metal additives.

the influence of pH in the range of 5−8 on the simultaneous removals of NO2 and SO2 for the tests with 0.5 mol/L ferrous sulfate, manganese sulfate, and ammonium sulfate as additives. Figure 9 shows the simultaneous removal of NO2 and SO2 as a function of the pH by the CaSO3 slurry with FeSO4 and MnSO4 additives. It is observed that adjustment of pH can slightly influence the performance of the CaSO3 slurry for NO2 removal, while the removal of SO2 is always nearly 100% in these tests. NO2 removal is obviously increased with an increasing pH. Even in the range of pH 5−7, the NO2 removal can still reach around 95% with the additives selected. These results indicate that it is possible for the simultaneous removal 5587

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Figure 10. Effects of the pH on the removal of NO2 and SO2 by the CaSO3 slurry with ammonium sulfate additive.

Figure 11. Effects of O2 on the removal of NO2 and SO2 with different additives.

of SO2 and NO2 in a typical calcium-based WFGD system with appropriate additives. The effects of pH on the performance of the non-metal additive, (NH4)2SO4, are shown in Figure 10. NO2 removal increases from 89 to 98% when the pH increases from 5.1 to 7.3. The impact of non-metal additives is approximately the same as the metal additives shown in Figure 9. It is interesting to notice that some SO2 is released from the sulfite when the slurry pH is lower than 6. About 24 ppm SO2 was found at pH 5, which is not in accordance with the SIV concentration estimated using the Davies equation.26 Because 600 ppm SO2 is in the feed gas and the pH was adjusted by the addition of hydrochloric acid in the experiment, the escape of SO2 is likely attributed to the conversion of CaSO3, as shown in reaction 10, and not to the low absorption efficiency. CaSO3 + 2HCl ↔ CaCl 2 + SO2 ↑ + H 2O IV

4. CONCLUSION Experiments were conducted to demonstrate the ability of the simultaneous removal of NO2 and SO2 by the CaSO3 slurry with additives in one washing tower. Metal and non-metal additives tested in this study include FeSO4, FeCl2, Fe2(SO4)3, MnSO4, MnCl2, MgSO4, MgCl2, (NH4)2SO4, and NH4Cl. The cations of these additives show the following order of NO2 removal efficiency: FeII, NH4+ > MnII, FeIII > MgII The activity of anions shows the following order of NO2 removal efficiency: SO32− > SO42− > Cl−. It is found that losses of metal additives are notable; the loss of FeII ions is nearly 70%. Ammonium sulfate has the best performance in terms of both NO2 removal and additive loss. With 0.5 mol/L ammonium sulfate added in the CaSO3 slurry, the NO2 removal efficiency increases significantly from 67 to 90%. Moreover, nearly 100% of SO2 is simultaneously removed in the same washing tower. The removal efficiency of NO2 by CaSO3 and ammonium sulfate additives can be improved by increasing the temperature from 293 to 343 K. However, both decreasing pH and increasing O2 have a small negative impact on NO2 removal. The technology of combining low-temperature ozone oxidation with the existing calcium-based WFGD system with effective additives can be a potentially viable choice for the simultaneous removal of SO2 and NOx from the flue gas.

(14)

HSO3−

and SO32− when the 5−8. HSO3− has a higher

S appears mainly as pH of the slurry varies in the range of fraction of the total SIV at lower pH because the solution contains more H+ ions. On the other hand, the SO32− fraction increases with the increasing pH. According to the report by Takeuchi et al.,21 the reaction rate of SO32− is much faster than that of HSO3− for NO2 absorption. Therefore, higher pH favors higher NO2 removal. 3.6. Effects of O2. Calcium sulfite can be oxidized to calcium sulfate in the presence of an oxidant. In comparison to NO2, oxygen is likely a stronger oxidizing agent for the oxidation of calcium sulfite. Therefore, the effects of O2 on the performance of NO2 removal were investigated in the current work. When the O2 concentration was varied from 0 to 10%, the absorption performances of SO2 and NO2 in the CaSO3 slurry with additives at different O2 concentrations are shown in Figure 11. The NO2 removal efficiencies are all reduced with an increase of oxygen. With CaSO3 and FeSO4 as additives, the NO2 removal maintains essentially at around 94% and little O2 effect was observed. This is attributed to the extra ferrous ions that are involved in the oxidation reaction. For the MnSO4 and (NH4)2SO4 systems, the NO2 removal efficiencies reduce nearly 4% when the O2 concentration increased from 0 to 10%. Figure 11 also shows that nearly 100% of SO2 can be absorbed by the CaSO3 slurry with additives. As a summary, although oxygen has a weak negative effect on the removal of NO2 by the CaSO3 slurry with additives, SO2 removal is essentially independent of the oxygen concentration.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: (86) 571-87952889. Fax: (86) 571-87951616. Email: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was conducted under the financial support from the National Basic Research Program of China (2012CB214906) and the Program for Introducing Talents of Discipline to University (B08026).

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REFERENCES

(1) Wang, W.; Zhong, Q.; Ye, Z.; Bjerle, I. Fuel 1995, 74, 267−272. (2) Patsias, A. A.; Nimmo, W.; Gibbs, B. M.; Williams, P. T. Fuel 2005, 84, 1864−1873.

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dx.doi.org/10.1021/ef3007504 | Energy Fuels 2012, 26, 5583−5589