ARTICLE pubs.acs.org/EF
Simultaneous Removal of SO2 and NO2 on r-Al2O3 Absorbents Loaded with Sodium Humate and Ammonia Water Yu Zhao, Guoxin Hu,* Zhiguo Sun, and Jitao Yang School of Mechanical and Power Engineering, Shanghai Jiaotong University, Shanghai 200240, People’s Republic of China ABSTRACT: Simultaneous removal of sulfur dioxide (SO2) and nitrogen dioxide (NO2) from simulated flue gas has been investigated with absorbents, which are made of R-Al2O3 impregnated with sodium humate and ammonia water solution. The results of the experiment showed that 100% of SO2 and about 80% of NO2 in simulated flue gas could be removed in a fixed-bed reactor, which is packed with these absorbents. The property of preferential absorption of SO2 is obvious when we used these absorbents to remove SO2 and NO2 at the same time from simulated flue gas. The products of this process were characterized by Fourier transform infrared spectroscopy and X-ray diffraction. It can be concluded that the reaction products are mainly ammonium sulfate, ammonium sulfite [(NH4)2SO3], ammonium nitrate (NH4NO3), ammonium nitrite (NH4NO2), and humic acid in this process. The content of NH4NO3 takes a little part of the products as well as NH4NO2 and (NH4)2SO3. This product can be used for compound fertilizer.
1. INTRODUCTION Many flue gas emissions, which mainly come from power plants by the use of fossil fuels, have been causing serious air pollution. The main reason for serious air pollution is that flue gas contains large amounts of sulfur dioxide (SO2) and nitrogen oxide (NOx) and other pollutants.13 Many researchers have been actively exploring effective flue gas purification technologies. Flue gas desulfurization and denitrification processes are the most effective method to control the emission of SO2 and NOx now. Many processes46 have been suggested for the removal of SO2 from flue gas. These processes may be grouped generally as the wet process, semi-dry process, and dry process. The wet process employs an absorbent solution, usually aqueous, for the removal of SO2 from flue gas. The dry process is an attractive alternative with a simple technology compared to the wet process in terms of low cost because it does not require water or reheating energy. Nonetheless, these processes have not been widely used because of the high absorbent cost, low SO2 removal efficiency, poor absorbent use, and secondary pollution. Therefore, making an economical and effective removal of SO2 from flue gas has become a research hotspot. Various semi-dry processes have been developed to avoid the disadvantages of wet and dry flue gas desulfurization techniques.79 The flue gas denitrification processes may be grouped into the selective catalytic reduction (SCR) and selective noncatalytic reduction (SNCR).10 SCR or SNCR are often installed on the back of desulfurization equipment. In recent years, simultaneous removal of SO2 and NOx from flue gas has been a tendency for flue gas purification in one reaction cell.11,12 Moreover, sodium humate (HANa) made from low-rank coal is a cheap and excellent performance absorbent, and it has been used to remove SO2 by a few researchers in the past few decades. Green et al. used the mixtures of humic acid (HA) and fly ash to absorb SO2 from flue gas in the 1980s.13,14 HA has been used to modify Ca-based adsorbents for flue gas desulfurization by Zhao et al.15 Sun et al. also carried out studies about HANa desulfurization.1618 If humic substances r 2011 American Chemical Society
have been used for simultaneous removal of SO2 and NOx from flue gas in semi-dry processes, power plant consumption would be cut down and there would be no secondary pollution. On the other hand, the byproduct of this process may be made into compound fertilizer. In this paper, we focus on the simultaneous removal of SO2 and nitrogen dioxide (NO2) from stimulated flue gas using absorbents loaded with HANa and ammonia water solution. The product compositions are characterized by Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD).
2. EXPERIMENTAL SECTION 2.1. Experimental Apparatus. A schematic diagram of the experimental setup used in the study is shown in Figure 1. The SO2 and NO2 gases supplied from cylinders were diluted with O2 and N2 and passed through a gas-mixing chamber. Then, this gaseous mixture flowed through the fixed-bed reactor. The fixed-bed reactor device was a quartz tube (inner diameter of 15 mm and 500 mm long) filled with absorbents supported with glass fiber. Rotameters and valves were used to monitor the gas flow. The concentrations of SO2, NO2, and O2 were monitored by a flue gas analyzer. 2.2. Preparation of Absorbents. The preparing method for absorbents referred to the work by Sun et al.17 First, R-alumina fibers (Zhejiang Osmun Crystal Fiber Co., Ltd.) were ground to powders. Sodium silicate (37.8 wt %), water (54 wt %), and R-alumina powders (8.2 wt %) were mixed. This mixture was extruded into the specialized mold shape and dehydrated into cylindrical particles (diameter of 4 mm and height of about 5 mm). These particles were sintered at 800 °C for 2 h. Sample A was obtained after these particles cooled with air at room temperature. Second, sample A was impregnated with HANa solution (6 wt %) for 24 h. Sample B was obtained after the particles were dried at 80 °C for 12 h. HANa was purchased from Shanghai Tongwei Received: May 24, 2010 Revised: May 26, 2011 Published: May 31, 2011 2927
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Figure 1. Schematic of the experimental apparatus: (1) SO2 gas cylinder, (2) O2 gas cylinder, (3) NO2 gas cylinder, (4) N2 gas cylinder, (5) rotameter, (6) gas-mixing chamber, (7) bypass, (8) quartz tube reactor, (9) heat insulator, (10) valve, (11) thermocouple, (12) computer, (13) flue gas analyzer, (14) data acquisition unit, (15) calcium chloride, and (16) sodium hydroxide solution.
Figure 2. Breakthrough curves of absorbents at simultaneous removal of SO2 and NO2 (A, B, C, D, and E, kind of absorbents; SO2, desulfurizatiom; and NO2, denitrification).
Table 1. Definitions of Samples samples
definitions
A B
R-Al2O3 HANa/R-Al2O3
C
HANa/R-Al2O3 of impregnated
D
HANa/R-Al2O3 of impregnated
E
R-Al2O3 of impregnated
2 mL of ammonia water solution (12.5 wt %) 3 mL of ammonia water solution (12.5 wt %) 3 mL of ammonia water solution (12.5 wt %)
Biological and Technology Co., Ltd. Third, samples C and D were prepared by adding 2 and 3 mL of ammonia water solution (12.5 wt %) into 5 g of sample B, respectively. Finally, sample E was made by adding 3 mL of ammonia water solution (12.5 wt %) into 5 g of sample A. The definitions of samples are listed in Table 1. 2.3. Experimental Methods. Experiments were carried out at room temperature (25 °C) and atmospheric pressure. In the process of experiments, the entrance concentration of simulated flue gas was about 2000 ppm for SO2, about 200 ppm for NO2, and about 5% for O2. The flow rate of simulated flue gas was controlled at 0.15 m3/h with a rotameter. The collected products were extracted by washing these absorbents and the reactor walls with deionized water. FTIR spectra were recorded in the 4000500 cm1 range using an EQUINOX 55 FTIR spectrometer (Germany BRUKER company), continuously purged with dry air. Spectra were obtained from pressed KBr pellets. Pellets of 1 cm diameter were prepared by mixing 1 or 2 mg of samples with 200 mg of KBr. A scanning electron microscope (SEM) was used to observe the morphology of absorbents. The used absorbents were characterized by XRD with a D/max-2200/PC type-ray diffraction instrument (Rigaku Corporation) to study the composition of the products. Diffraction patterns were recorded with CuKR (λ = 0.1542 nm) radiation. The X-ray tube was operated at 40 kV and 20 mA. The step scans were taken over the range of 2θ from 10° to 60° at a scanning speed of 5°/min. Ion chromatography (IC) analyzed the ion concentration of product in the washing absorbent solution. Flue gas composition analysis is monitored by a flue gas analyzer (Testo 350XL, Germany Testo Company). The removal efficiency is defined as η = ((Cin Cout) 100%)/(Cin), where Cin and Cout are the inlet and outlet concentrations of SO2 and NO2, respectively, and η is the absorption efficiency of them.
Figure 3. SEM photographs of absorbents.
3. RESULTS AND DISCUSSION 3.1. Breakthrough Curves of Absorbents. The breakthrough curves of absorbents in the simultaneous removal of SO2 and NO2 process show the changes of the outlet concentration with time in Figure 2. It can be seen that samples A and B can be broken through easily by simulated flue gas. Because samples A and B are ammonia- and water-free, no reactions can occur in these processes. However, because of the larger specific surface area of R-alumina,19 SO2 and NO2 in simulated flue gas can be partly adsorbed by sample A. Figure 3 shows the SEM photographs of samples A and B. It can be seen from Figure 3 that HANa covered the surface of alumina fibers in sample B. This HANa lessens the specific surface area of sample B. Therefore, sample B is penetrated more easily. In Figure 2, the breakthrough curves of samples C, D, and E in the simultaneous removal of SO2 and NO2 processes are similar. Desulfurization breakthrough curves are divided into two sections: one is the horizontal section, in which the outlet concentration of SO2 is maintained almost constant, and the other is the ascending section, in which the outlet concentration of SO2 increases rapidly. Denitrification breakthrough curves are also divided into two sections: one is the fluctuant section, in which the outlet concentration of NO2 falls and rises rapidly, and the other is the horizontal section, in which the outlet concentration of NO2 is maintained almost constant. It is the common characteristic that the breakthrough times of samples lengthen in this process when using samples C, D, and E. It can be arrived that the absorption performance of samples A and B was 2928
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Table 2. Desulfurization Breakthrough Time at Different Conditionsa desulfurization efficiency (%) breakthrough time (s) removal of SO2 simultaneous removal of SO2 and NO2
100
80
sample D sample E
937 576
1500 792
sample D
2000
2400
sample E
1000
1200
Gas flow, 0.15 m3/h; O2, 5 vol %; ambient pressure; 25 °C; entrance concentration for SO2, 2000 ppm; and entrance concentration for NO2, 0 or 200 ppm. a
improved after impregnated with different amounts of ammonia water solution. When the same amount of ammonia is loaded, the breakthrough time of sample D is longer than that of sample E. The results of desulfurization breakthrough time of samples D and E at different conditions are summarized in Table 2. The breakthrough time of sample D is almost 2 times longer than that of sample E at the simultaneous removal of SO2 and NO2 process. It can be deduced that the combination of HANa with ammonia water at the same time improves the absorbability of these absorbents. 3.2. Removal Efficiency of Absorbents. Figure 4 shows the simultaneous removal of SO2 and NO2 efficiency curve of absorbents. The NO2 removal efficiency changes from about 60% to nearly 90%, while the desulfurization efficiency of absorbents can reach 100%. A high SO2 absorption efficiency (100%) is maintained at first, and then the SO2 absorption efficiency begins to drop greatly until the SO2 absorption saturated. While the NO2 removal efficiency first decreases and then increases, at last, it keeps stable (80%) for a long time. Thus, it indicates that the sample has the property of preferential absorption of SO2 in the presence of both SO2 and NO2. 3.3. Amounts of Sulfur and Nitrogen Absorbed. It is important to investigate the products of removing SO2 and NO2. The used samples D and E were washed with deionized water by ultrasonic cleaning about 0.5 h. The tested results of the ion concentration are summarized in Table 3. The absorbed SO2 is converted into SO42 and SO32. Meanwhile, the absorbed NO2 is converted into NO3 and NO2. The ion concentration of SO42 in washing absorbent solution of sample D is 2-fold that of sample E. The ion concentration of other ions in it has also increased. It can be deduced that HANa has excellent performance to avoid the loss of ammonia solution in the simultaneous removal of SO2 and NO2 process. 3.4. Analysis of Products. 3.4.1. FTIR Analysis. FTIR spectroscopy is a powerful tool for investigating the mechanism of interaction between gases and solid catalyst surfaces.20,21 Figure 5 shows the infrared spectra of used absorbents and products. The used absorbents were taken after removal of SO2 and NO2 test. Products were flushed from the used absorbents. The characteristic adsorption bands of HANa are observed at the wave numbers of 3420 cm1 (hydrogen-bonded OH stretching of carboxyl, alcohol, and phenol), 1580 and 1386 cm1 (antisymmetric and symmetric COO stretching vibrations of carboxylic salt), and 1113 and 1036 cm1 (CO stretching vibrations in polysaccharides or polysaccharide-like substances)
Figure 4. Absorption efficiency of absorbents at the simultaneous removal of SO2 and NO2 (C, D, and E, kind of absorbents; SO2, desulfurizatiom; and NO2, denitrification).
in the infrared spectra of the used sample B.22 This means that almost no reaction occurred between HANa and flue gas when ammonia water was not added. In comparison to spectra of the used sample B, it is obvious that new bands appear at 3128, 1398, and 620 cm1 in infrared spectra of other samples. The band at 3128 cm1 is the characteristic band of ammonium salt, and the band at 1398 cm1 is the ammonium sulfate [(NH4)2SO4] in the HNH bending vibration peaks.23 Moreover, the band at 620 cm1 can likely be attributed to sulfate. Therefore, it is probable that the product of the reaction was (NH4)2SO4. The adsorption band was 1700 cm1 in the infrared spectra of the used sample D, and the product of sample D is ascribed to CdO of COOH,23 which indicates that some HANa has converted to HA. The band at 1631 cm1 is due to the bending mode of molecularly adsorbed water in samples.24 3.4.2. XRD Analysis. In the above study, FTIR spectroscopy shows that (NH4)2SO4 may exist on the used absorbents and products. Figure 6 is XRD patterns of the used sample D. As shown in Figure 6, the diffraction peaks at 16.48°, 20.26°, 22.90°, 28.49°, and 29.80° are assigned to (NH4)2SO4. This is in agreement with the results of FTIR analyses. Meanwhile, the characteristic peaks at 20.52° and 41.72° are attributed to ammonium sulfite [(NH4)2SO3], and the characteristic peaks at 18.32°, 22.70°, 29.14°, 40.16°, and 33.18° belong to ammonium nitrate (NH4NO3). Therefore, the products of this process contain (NH4)2SO4, (NH4)2SO3, and NH4NO3. 3.5. Analysis of the Mechanism. In the simultaneous removal of SO2 and NO2 process, the main reactions are that SO2 and NO2 from simulated flue gas react with ammonia water. Other reactions may be that SO2 and NO2 from simulated flue gas dissolve into water from ammonia water solution and then react with the acidic groups of HANa, such as carboxyl (COO). The chemical reactions involved in this process may be the following reactions:25,26
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SO2 þ 2NH4 OH f ðNH4 Þ2 SO3 þ H2 O
ð1Þ
ðNH4 Þ2 SO3 þ SO2 þ H2 O f 2ðNH4 HSO3 Þ
ð2Þ
2ðNH4 HSO3 Þ þ NH3 f ðNH4 Þ2 SO3
ð3Þ
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Table 3. Ion Concentration in Solution of Washing Absorbents reactant gas inlet concentration (ppm)
HANa and ammonia water in absorbent
ion concentration (mg/L)
sample
SO2
NO2
HANa (mg)
ammonia water (12.5 wt %) (mL)
SO42
SO32
NO3
D
2000
200
332.3
3.0
2390
112
458.5
36
662
E
2000
200
0
3.0
1050
57
359.4
22.5
444
2ðNH4 Þ2 SO3 þ O2 f 2ðNH4 Þ2 SO4
ð4Þ
2HNO2 þ 2NH4 OH f NH4 NO2 þ H2 O
ð5Þ
2NH4 NO2 þ O2 f 2NH4 NO3
ð6Þ
SO2 þ H2 O f H2 SO3
ð7Þ
2H2 SO3 þ O2 f 2H2 SO4
ð8Þ
H2 SO4 þ 2HANa f 2HA þ Na2 SO4
ð9Þ
According to the literature,27,28 the presence of NO2 in the flue gas enhances SO2 removals. Therefore, the interaction between SO2 and NO2 has also taken place. The reaction can be concluded as follows: NO2 þ HSO3 þ H2 O f 3H2 SO4 þ HNO2 NO2 þ SO3 þ H2 O f 3H2 SO4 þ HNO2 H2 SO4 þ 2NH4 OH f ðNH4 Þ2 SO4 þ 2H2 O
NH4þ
Figure 6. XRD patterns of used sample D.
Figure 5. FTIR spectra of used absorbents and products.
NO2
ð10Þ ð11Þ ð12Þ
Because of the strong absorption capability of HANa, the amount of lost ammonia blown by flue gas decreased. The more ammonia water absorbed in absorbents, the longer the breakthrough time was maintained. Therefore, HANa loaded on absorbents plays an important role in this simultaneous removal of SO2 and NO2 process.
4. CONCLUSION On the basis of the experimental investigations and mechanism discussion presented in this paper, it was revealed that the absorbability of these R-Al2O3 absorbents can be improved through loaded HANa and ammonia water solution on them in the simultaneous removal of SO2 and NO2 process. The presence of NO2 in the simulated flue gas enhances SO2 removals on these absorbents loaded with HANa and ammonia water solution in this process. These absorbents have the property of preferential absorption of SO2 in the presence of both SO2 and NO2 when they are impregnated with HANa and ammonia water solution. The products of this process are (NH4)2SO4, (NH4)2SO3, NH4NO3, NH4NO2, and HA. The content of NH4NO3, NH4NO2, and (NH4)2SO3 takes a little part of the products. Products of this process can be made into compound fertilizer. ’ AUTHOR INFORMATION Corresponding Author
*Telephone/Fax: þ86-21-34206569. E-mail:
[email protected].
’ ACKNOWLEDGMENT The authors gratefully acknowledge financial support from the National Science Foundation of China (50876062) and the Ministry of Science and Technology of China (2007AA05Z313). The authors thank Instrumental Analysis Center of Shanghai Jiaotong University (SJTU) for FTIR measurements. 2930
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