Environ. Sci. Technol. 2010, 44, 6712–6717
Novel Process of Simultaneous Removal of SO2 and NO2 by Sodium Humate Solution G U O X I N H U , * ,† Z H I G U O S U N , † A N D HANYANG GAO‡ School of Mechanical & Power Engineering, Shanghai Jiaotong University, Shanghai 200240, China, and, School of Environmental Science & Engineering, Shanghai Jiaotong University, Shanghai 200240, China
Received June 2, 2010. Revised manuscript received July 29, 2010. Accepted July 30, 2010.
A novel simultaneous flue gas desulfurization and denitrification (FGDD) process using sodium humate (HA-Na) solution was proposed. This study relates to the SO2/NO2 absorption efficiency and products of simultaneous removing SO2 and NO2 in a bubbling reactor, especially the effect of recycled water on the SO2/NO2 absorption. Under alkaline conditions, the sulfate content in S-containing compound decreases with the increase of NO2 concentration, whereas there is a contrary result under acidic conditions. Whether the absorption liquid is alkaline or acidic, the presence of NO2 improves the SO2 absorption into HA-Na solution, because NO2 may promote the oxidation of sulfite to sulfate. It seems that the presence of SO2 is unfavorable for the NO2 absorption, but the NO2 absorption efficiency can be improved with the cycle number rising due to the increasing amount of sulfite. Although all the ion 2concentrations of Na+,SO24 ,SO3 , and NO3 have a gradual increase as the cycle number rises, the ion concentrations of + SO24 and Na are far more than that of the other ions, which results in a slight decrease of the SO2 absorption efficiency. However, the initial pH of HA-Na solution prepared by recycled water decreases from 10 to 8.1 with the cycle number increasing from 1 to 10, whereas the final pH (the pH after absorption reaction is finished) remains almost constant (3.3). The SO2 absorption efficiency is above 98% and the NO2 absorption efficiency may reach above 95% in the optimal condition in this process. The chief byproduct is a compound fertilizer consisting of humic acid (HA), sulfate, and nitrate.
desulfurization (FGD) is wet scrubbing technique, in which the lime/limestone process is a dominating FGD technology (1-3). Among the flue gas denitrification technologies, the selective catalytic reduction (SCR) and selective noncatalytic reduction (SNCR) are frequently used (4-6). In recent years, the interests in combined desulfurization and denitrification from flue gas increased rapidly. Many technologies have been proposed, among which the stage treatment technology is considered to be a mature one. In this traditional technology, a separate NOx control system, for example, SCR or SNCR, should be installed to the back of the desulfurization equipment. Although it succeeded in combined removal of the SO2 and NOx, it is not easy to achieve wide industrial application because of the large occupying area and high running cost. To reduce the cost of flue gas purification, development of new technologies and equipments of simultaneous FGDD has become the leading research direction in the air pollution control field. There are many investigations in the world, but most of them have technical and economic defects, and cannot develop to practicable technologies. One promising approach to simultaneously remove SO2 and NOx from flue gas is the combination of NO oxidation to the more soluble NO2 and conventional wet scrubbing (7). The possible NO oxidants include ozone (O3), hydrogen peroxide (H2O2), sodium chlorite (NaClO2), potassium permanganate (KMnO4), HNO3, etc. (8-12). The NO oxidation product, NO2, can be absorbed easily by several chemical reagents, such as HA-Na solution. HA-Na derived from low-rank coal is a cheap absorbent, and the use of HA-Na in SO2 and NOx abatement is of increasing interest in the past few years. Green et al. (13, 14) investigated the absorption of SO2 by HA-fly ash mixtures. However, neither the removal of NOx nor the desulfurization byproduct was mentioned. Zhao et al. (15) made use of HA as a special additive to modify Ca-based adsorbents for flue gas desulfurization. Recently, Hu et al. (16-18) used humate for the removal of SO2 and NOx in flue gas and the production of organic fertilizer. On the basis of previous literature, this paper proposes a process for simultaneous removal of SO2 and NOx from flue gas by HA-Na solution and the production of HA fertilizer. This process (shown in Figure 1) includes the following stages (18): (a) HA-Na solution is first prepared by HA-Na powder and water in a dissolving tank, and sprayed into an absorber. (b)Then, HA-Na solution simultaneously reacts with SO2 and NOx in the absorber. The desulfurization liquid
1. Introduction The removal of combustion gases, such as SO2 and nitrogen oxides (NOx), mainly produced in power plants by the use of fossil fuels, has been the subject of many studies in recent years because of the environmental problems that these pollutants generate. Many technologies have been proposed to control the emission of SO2 and NOx. At present, FGDD are the most effective techniques to control the emission of SO2 and NOx from the combustion of fossil fuels. The most common method for flue gas * Corresponding author phone/fax: +86-21-34206569; e-mail:
[email protected]. † School of Mechanical & Power Engineering, Shanghai Jiaotong University. ‡ School of Environmental Science & Engineering, Shanghai Jiaotong University. 6712
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FIGURE 1. Schematic diagram of the desulfurization and denitrification process by HA-Na solution. 10.1021/es101892r
2010 American Chemical Society
Published on Web 08/12/2010
(mainly containing HA, H2SO3, H2SO4, and HNO3) flows into a reaction tank from the absorber. (c) In the reaction tank, SO32- is oxidized to SO42- through diffused aeration and HA is subject to oxygenolysis by H2SO4 and HNO3. The activity of HA can be improved by oxygenolysis (19). Meanwhile, it is possible that HA is nitrified by the mixed acid of HNO3 and H2SO4 (20, 21); therefore, some HA is converted into nitroHA, which is a better HA fertilizer. (d) Afterward, the reaction liquid from the reaction tank flows into a sedimentation tank and stands for 12 h. Due to its poor solubility, HA and nitroHA may be separated as sediment from acidic solution. (e) The separated HA and nitro-HA can be used as a kind of material for compound fertilizer after drying. (f) The acidic aqueous solution from the sedimentation tank flows into a neutralization tank and is neutralized to pH 7 by HA-Na. The neutralized aqueous solution is finally sent back to the dissolving tank and meets the requirement of recycling water. From the above process, it can be concluded that the simultaneous removal of SO2 and NOx by HA-Na solution is a resourceful type of environmental protection technology for FGD and has many advantages including (a) lower costs and energy requirements, (b) almost no waste sludge, (c) the recovery of sulfur and nitrogen as a useful sulfur-containing nitrogen fertilizer, and (d) the simultaneous removal of SO2 and NOx. Therefore, it is hopeful to be used in a large scale. In this paper, we focus on the absorption efficiency and the content analysis of products of simultaneous removal of SO2 and NO2 by HA-Na solution, the effect of cycle number, and the changes of ion concentration of recycled water in a lab-scale bubbling reactor.
2. Desulfurization and Denitrification Mechanism The major mechanism of SO2 and NO2 absorption into HANa solution is acid-base reaction, and the acid-base theory predicts that HA-Na should react with SO2 and NO2 by the following reactions (13, 14, 22): HA - Na(aq) + SO2(g) + H2O f HA(s) + HSO3 (aq) + Na+(aq)
(1)
+ 2HSO3 (aq) T H (aq) + SO3 (aq)
(2)
22SO23 (aq) + O2(g) f 2SO4 (aq)
(3)
2NO(g) + O2(g) f 2NO2(g)
(4)
2NO2(aq) + H2O f HNO3(aq) + HNO2(aq)
(5)
2HNO2(aq) f HNO3(aq) + 2NO(aq) + H2O
(6)
HA-Na(aq) + 2H+(aq) + NO3 (aq) + NO2 (aq) f HA(s) + Na+(aq) + NO3 (aq) + NO2 (aq)
(7)
SO2 and NO2 from flue gas are first dissolved into aqueous solution. Dissolved SO2 and NO2 are quickly converted into HSO3-, SO32-, NO3-, NO2-, and H+ by ionization. The acidic groups of HA-Na, such as carboxyl (COO-) and hydroxyl (OH-) can react rapidly with H+, and HA-Na is transferred to HA sediment. According to eqs 1, 2, and 7, this reaction may move the reaction equilibrium to the right, which results in that more SO2 and NO2 are dissolved into solution. When all of HA-Na is consumed, the desulfurization and denitrification reaction is terminated. At the same time, the interaction between the SO2 and NO2 should be considered, and it can be concluded by the following reactions (23-25):
+ 2NO2(g) + HSO3 (aq) + H2O f 3H (aq) + 2NO2 (aq) +
SO24 (aq)
(8)
+ 2NO2(g) + SO3 (aq) + H2O f 2H (aq) + 2NO2 (aq) +
SO24 (aq)
(9)
3. Experimental Section A schematic diagram of the experimental apparatus is shown in Supporting Information (SI) Figure S1. All of the experiments for SO2 and NO2 absorption were carried out in a bubbling reactor (diameter 55 mm) at ambient pressure. A simulated flue gas consisted of 2000 ppm of SO2, 340-740 ppm of NO2, 0-10% of O2, and the balance N2. The SO2, NO2, O2, and N2 gases were supplied from cylinders. The total flow rate of the simulated flue gas was controlled with a rotameter. The absorption temperature was adjusted with a water bath. The SO2/NO2 absorption efficiency can be obtained from the following formula: η)
(Cin - Cout) × 100% Cin
(10)
Where η is the SO2/NO2 absorption efficiency, Cin and Cout is the inlet and outlet of SO2/NO2 concentration. To investigate the effect of recycled water on the absorption efficiency of SO2 and NO2, 10 cycles of regeneration and water recycling were performed. The experimental process is as follows: (a) a 1200 mL HA-Na solution (0.04 g/L) was prepared by powdered HA-Na (g50 wt %, Shanghai Tongwei Biological & technology Co.) and deionized water. Although there was some water loss, due to vacuum pumping and simulated flue gas blowing, the 1200 mL HA-Na solution is enough to complete the water recycling experiment 10 times. (b) The 1200 mL HA-Na solution was divided into two parts, solution A (100 mL) and solution B (1100 mL). First, the experiment of simultaneous removal of SO2 and NO2 was performed by the solution A. When SO2 absorption reached saturation, the experiment was finished, although at this time the NO2 absorption was not saturated. The reason is, at this point, the absorption liquid had lost the ability of removing SO2 (since the SO2 absorption was saturated), and it did not have engineering value. Then, HA sediments were separated from the acidic aqueous solution by vacuum filtration. The content of the separated HA sediments were analyzed after they are dried at 80 °C for 12 h, and the ion concentration of absorption liquid was also measured. (c) In this way, simultaneous absorption of SO2 and NO2 was done with use of the solution B, and HA sediment was separated from aqueous solution. The first recycling experiment (cycle 1) was finished. (d) The separated aqueous solution coming from solution A and solution B was combined and neutralized to pH 7 by adding powdered HA-Na. The neutralized aqueous solution achieved by vacuum filtration was recycled to prepare the new HA-Na solution (0.04 g/L). According to the approach of the first recycling experiment, the new HA-Na solution was divided into two parts, one part of which is 100 mL. The second cycle (cycle 2) experiment began. In this way, the water recycling experiments were completed 10 cycles in all. The changes of SO2 concentration at the inlet and outlet of the reactor were monitored by a flue gas analyzer (Testo350XL, Germany). The functional groups of desulfurization products were identified using a Fourier-transform infrared spectrometer (FTIR, EQUINOX 55, Germany BRUKER). The FTIR analysis was performed after drying, grinding the desulfurization products, and mixing them with KBr power to prepare sample KBr pellets. The ion concentration was tested by an ion chromatograph (MIC, Switzerland Metrohm). VOL. 44, NO. 17, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Absorption Efficiency and Products of Removing SO2 and NO2a reactant gas inlet concentration/ppm sample
SO2
NO2
1 2 3 4 5
2000 2000 2000 2000 0
0 340 540 740 740
6 7 8 9 10
2000 2000 2000 2000 0
0 340 540 740 740
a
S-containing ion concentration in absorbent/(mg · L-1) SO32-
SO42-
N-containing ion concentration in absorbent/(mg · L-1) SO2 absorption efficiency
Alkaline (Reaction Time Is 5 Min) 0 902.4 96.4 41.7 899.6 97.7 156.7 694.8 97.1 209.9 641.65 96.8 Acidic (Reaction Time Is 30 Min) 1148.3 4023.2 99 832.8 4139.6 99.4 734.7 4322 98.4 693.8 4550.4 97.9
NO3-
NO2-
NO2 absorption efficiency
50.7 24.8 21.6 12.32
60.25 74.6 76.4 85.9
95.5 90 88.4 87
151.8 179.8 237.6 400.3
310.8 498.4 740.4 604.5
88.5 87.3 83.1 89.5
Gas flow, 0.14 m3/h; absorption solution, 100 mL, 0.04 g/mL; O2, 5 vol %; 25 °C.
4. Results and Discussion 4.1. Absorption Efficiency and Products of Removing SO2 and NO2. It is important to investigate the absorption efficiency and products of removing SO2 and NO2. The initial pH of HA-Na (0.04 g/mL) is usually 10. After 5 min of reaction, the absorption liquid is still alkaline (pH 8-9.5), whereas the absorption liquid is acidic (pH 4-6) when the reaction time is 30 min. The samples of the absorption liquid, which were obtained, respectively, when the reaction time was 5 and 30 min, were tested by the ion chromatograph. The tested results are summarized in Table 1. When the absorption liquid shows alkaline, the SO2 absorption efficiency in the presence of 340 ppm NO2 is slight higher than that of in the absence NO2, which indicates that NO2 can facilitate SO2 absorption. It can be explained as follows: The dissolved NO2 in solution can react with HSO3- and SO32- via eq 8 and eq 9, both of which are oxidized to SO42- by NO2. Thus the concentrations of HSO3- and SO32- are decreased. According to eqs 1 and 2, decreasing the amount of HSO3- and SO32-may move the reaction equilibrium to the right, which will promote SO2 dissolution into HA-Na solution. However, the SO2 absorption efficiency has a slight decrease with the continuous increase of the inlet NO2 concentration from 340 to 740 ppm. The predominant desulfurization product is SO42-. The SO42content in S-containing products decreases with the increase of NO2 concentration. On the other hand, the NO2 absorption efficiency is higher in the presence of SO2 than that of without SO2. With the increasing of NO2 concentration, the NO2 absorption efficiency is decreased. The denitrification products are mainly NO2- and NO3-, and the concentration ratio of NO2 to NO3 is increased with the NO2 concentration rising. The absorption liquid has showed acidic when the reaction time is 30 min. Under acidic conditions, The SO2 absorption efficiency in the presence of 340 ppm NO2 is higher than that of without NO2. However, the SO2 absorption efficiency has a slight decrease (from 99.4 to 97.9%) with the increase of NO2 concentration (from 340 ppm to 740 ppm). The amount of SO42- is increased with the NO2 concentration increasing from 0 ppm to 740 ppm, which indicates that the NO2 may promote the oxidation of SO32- to SO42-. Moreover, the NO2 absorption efficiency decreases from 89.5% in the absence of SO2 to 83.1% in the presence of SO2. The concentration ratio of NO-2 to NO-3 still increases with the NO2 concentration rising under acidic conditions. Figure 2 shows the changes of the SO2 and NO2 absorption efficiency with the time. It is found that a high SO2 absorption efficiency (98%) is maintained in the first 65 min. Then the 6714
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FIGURE 2. The SO2/NO2 absorption efficiency with time. (SO2, 2000 ppm; NO2, 340 ppm; gas flow, 0.12 m3/h; absorption solution, 100 mL; 0.04 g/mL; O2, 5 vol %; 25 °C). SO2 absorption efficiency begins to drop greatly until the SO2 absorption saturation. The NO2 absorption efficiency is approximately 95% in the first 23 min, and begins to drop after this point. However, the NO2 absorption efficiency begins to increases and may recover to 95% when the SO2 absorption reaches to saturation. Thus it indicates that the HA-Na solution has the property of preferential absorption of SO2 in the presence of both SO2 and NO2. It is also possible that SO23 and HSO3 resulting from dissolved SO2 can maintain the high NO2 absorption efficiency via eq 8 and eq 9. With the absorption carrying on, the pH of HA-Na gradually decreases from 10 (the initial pH) to approximately 3.3 (the final pH). 4.2. Effect of Cycle Number on the SO2/NO2 Absorption Efficiency and Time. It is very important to investigate the effect of cycle number on the desulfurization and denitrification result for this new process of simultaneous removal of SO2 and NO2 by HA-Na solution. It can be seen from Figure 3 that the SO2 absorption efficiency has a slight decrease with the increase of cycle number. However, the SO2 absorption efficiency is still above 98% before the cycle 7, and the SO2 absorption efficiency of the last three cycles is above 95%. Due to the increase of the SO2 absorption efficiency, the desulfurization time has a slightly corresponding increase. It can be explained as follows: With increasing number of cycle, the SO42- concentration is
FIGURE 3. Effect of cycle number on the SO2 absorption efficiency and desulfurization time. (SO2, 2000 ppm; NO2, 340 ppm; gas flow,0.12 m3/h; absorption solution,100 mL; 0.04 g/mL; O2, 5 vol %; 25 °C).
FIGURE 4. Effect of cycle number on the NO2 absorption efficiency. (SO2, 2000 ppm; NO2, 340 ppm; gas flow, 0.12 m3/h; absorption solution, 100 mL; 0.04 g/mL; O2, 5 vol %; 25 °C). increased. From the eq 1 and eq 2 it is deduced that the increasing SO42- concentration hinders the SO2 absorption into HA-Na solution and reduces the SO2 absorption efficiency. The cycle number has a significant effect on the NO2 absorption efficiency (Figure 4). By increasing the number of cycles from 1 to 10, the NO2 absorption efficiency is increased from 77 to 99%. The reason is the SO32- ion concentration is increased with increasing cycle number. It can be deduced from eq 8 and eq 9 the adding SO32- may promote the NO2 absorption efficiency. For each experimental run, the NO2 absorption does not reach saturation, so the denitrification time is not present. The effect of cycle number on the ion concentration is also investigated. The experimental results were depicted in Figure 5. On the whole, all ion concentrations of SO42-, SO32-, NO3-, and Na+ are gradually increased with increasing cycle number. The ion concentrations of SO42- and Na+ are far 2greater than that of the other ions. Compared withSO24 , SO3 , NO3-, and Na+, the amount of NO2- is negligible. The pH of HA-Na solution is a very important influencing factor on SO2/NO2 absorption. Figure 6 describes the changes of initial pH of HA-Na solution and pH after desulfurization and denitrification (the final pH) of each recycle experiment run with the cycle number. As can be seen, the initial pH decreases from 10 to 8.1 with the cycle number varying from 1 to 6, whereas the initial pH is almost invariable after the cycle 6. There is no obvious change for the final pH, which
FIGURE 5. Effect of cycle number on ion concentration of cycle solution. (SO2, 2000 ppm; NO2, 340 ppm; gas flow, 0.12 m3/h; absorption solution, 100 mL; 0.04 g/mL; O2, 5 vol %; 25 °C).
FIGURE 6. Effect of cycle number on the initial pH and final pH. (SO2, 2000 ppm; NO2, 340 ppm; gas flow, 0.12 m3/h; absorption solution, 100 mL; 0.04 g/mL; O2, 5 vol %; 25 °C). is approximately 3.3. The HA-Na solution is a pH buffer since it is a salt of strong base and weak acid. Hence, the ionization equilibrium and hydrolytic equilibrium in the solution must be considered. With increasing cycle number, the Na+ ion concentration is increasing. According to the eq 11 and eq 12, the increasing Na+ ion concentration may hinder the ionization of HA-Na and reduce the amount of HAs, which will decrease the hydrolysis of HA-Na. Consequently, the H+ ion concentration is increased and the initial pH is lowered.
Where
is the structural formula of HA-Na;
is the strucis the structural of HA; tural of HA-. 4.3. Treatment and Analysis of Desulfurization and Denitrification Products. Figure 7 shows the infrared comparison spectra of the sample before and after desulfurization and denitrification. The characteristic adsorption bands of HA-Na are observed at the wavenumbers of 3689 VOL. 44, NO. 17, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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simultaneous removing SO2 and NO2 by HA-Na solution has a higher efficiency and lower capital cost, contrasted with the lime-gypsum process. Therefore, this process holds great promise for large commercial application.
Acknowledgments We gratefully acknowledge financial support by the National Science Foundation of China (No.50876062) and the Ministry of Science and Technology of China (No. 2007AA05Z313). We thank the Instrumental Analysis Center of SJTU for FTIR measurements.
Supporting Information Available
FIGURE 7. FTIR spectra of HA-Na; 2. product of cycle 2; 4. product of cycle 4; 6. product of cycle 6; 8. product of cycle 8; 10. product of cycle 10. cm-1 (stretching vibrations of -NH2), 3394 cm-1 (H-bonded OH stretching of carboxyl, alcohol, and phenol), 1594 cm-1 and 1374 cm-1 (antisymmetric and symmetric COO- stretching vibrations of carboxylic salt), 1034 cm-1 (C-N stretching vibrations), 1011 cm-1 (C-O stretching vibrations in polysaccharides or polysaccharide-like substances), 912 cm-1 and 756 cm-1 (out-of-plane bending vibrations of aromatic CH groups), 540 cm-1 and 504 cm-1 (stretching vibrations of C-C) in the spectra of sample A (26). Compared with spectra of HA-Na, it is obvious that three new bands (1243, 1114, and 617 cm-1) appear in the 1400-500 cm-1 spectra of all the five products of desulfurization and denitrification. These three new bands can be assigned to different sulfate (SO24 ) species (27-30). Moreover, the adsorption band of 1700 cm-1 is ascribed to CdO of COOH (26), which indicates that some HA-Na has converted to HA sediment. There are no obvious bands of nitrate or nitrite in the spectra of the products of desulfurization and denitrification since the content of N-containing species in the product is too little to be detected. The FTIR analysis results are in agreement with eq 1 and eq 7, and verify the desulfurization mechanism discussed above. The XRD analysis (SI Figure S2) also proves the results of FTIR analysis. Comparing the XRD diagrams of the samples before and after desulfurization and denitrification, it is found that there is no obvious peak of sulfite and nitrate in the products of desulfurization and denitrification, but the peaks corresponding to Na2SO4 have been detected obviously in the crystal obtained by drying the supernatant layer. After the desulfurization and denitrification process, the absorption liquid (shown in SI Figure S3) contains not only HA sediment, but also sulfate and nitrate, all of which are fertilizer components. Due to its poor solubility, HA may be separated as sediment from acidic aqueous solution and converted into HA compound fertilizer. The chief product of drying the supernatant layer is Na2SO4. SI Table S1 shows the content analyses of HA compound fertilizer. Compared with the content of HA-Na, both the sulfur and nitrogen content of desulfurization and denitrification products are increased, whereas they are not increased obviously as the cycle number rises. However, the oxygen content has an increase with increasing cycle number. These content changes of HA fertilizer illuminate that it is possible that HA is subject to oxygenolysis by H2SO4 and HNO3. The oxygenolysis can increase the content of oxygen-containing functional group in HA and improve the activity of HA (19). The acidic solution is neutralized pH 7 by a little HA-Na (3.2 g HA-Na per 100 mL acidic solution), and recycled water can be achieved easily. According to the results discussed above and literature (18), it is obvious that the process of 6716
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Schematic diagram of the experimental apparatus (Figure S1), XRD diagrams of HA-Na and products of desulfurization and denitrification (Figure S2), photo of HA-Na solution and products of desulfurization and denitrification (Figure S3), content of HA compound fertilizer (Table S1). This material is available free of charge via the Internet at http:// pubs.acs.org.
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