Song Shuab,
Yiqi Huangc,
Longhua Zoud,
Xinyi Zhangc and
Jianjun Li*ab
aCollege of Carbon Neutrality Future Technology, Sichuan University, Chengdu 610065, China. E-mail: jjli@scu.edu.cn
bNational Engineering Research Center for Flue Gas Desulfurization, Chengdu 610065, China
cCollege of Architecture and Environment, Sichuan University, Chengdu 610065, China
dCollege of Food and Biological Engineering, Chengdu University, Chengdu 610106, China
First published on 6th November 2023
Sodium bicarbonate (NaHCO3) is considered to be an effective alkaline adsorbent for SO2 removal and surprisingly, the concentration of NO is significantly reduced along with the generation of NO2 during its desulfurization. Unfortunately, the mechanism of NO interaction with NaHCO3, SO2 and O2 is ambiguous. In this work, the effects of absorption gas and absorber composition on SO2/NO absorption performance were explored, the absorption products were characterized using XPS and SEM, and the Gibbs free energy of the inferred reaction path was calculated based on density functional theory (DFT). The results showed that SO2 and O2 synergistically promoted the absorption and removal of NO by NaHCO3, which could completely remove SO2 and absorb 90% of NO at 160 °C. Sodium metabisulfite (Na2S2O5) and sodium dithionate (Na2S2O6) were identified as the active substances responsible for efficient NO absorption, and the oxidation of Na2S2O5 to Na2S2O6 is the controlling step of the NO removal reaction. Specifically, Na2S2O5 is an intermediate produced by the reaction of NaHCO3 with SO2, and subsequently reacts with O2 to produce Na2S2O6, which releases reactive oxygen species to oxidize NO to NO2. In addition, when the S/N ratio is greater than 1 and the O2 content is greater than 5%, both SO2 and NO can maintain removal efficiency higher than 90%, indicating that the absorption reaction of SO2 and NO by NaHCO3 is highly adaptable to the flue gas composition.
Surprisingly, the concentration of NOx was simultaneously reduced during the desulfurization with NaHCO3,16 providing the possibility to realize integrated removal of multiple pollutants in a single system. Barbara Walawska et al. stated that the efficiency of NaHCO3 for NOx removal was about 21% at 300 °C,17 but failed in the absence of SO2. It was presumed that the intermediate of Na/SOx generated by the desulfurization reaction plays a key role in NO removal, especially sodium metabisulfite (Na2S2O5). Unfortunately, the efficient Na/SOx intermediates are still ambiguous and the synergistic removal mechanisms of SO2 and NOx are currently lacking in research.
In this work, the effect of gas composition on the absorption of SO2 and NO by NaHCO3 was investigated using a fixed-bed reactor. The Na/SOx intermediates generated by the FGD process were characterized by scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS), and then the adsorption properties of a series of possible Na/SOx on SO2 and NO were examined. Furthermore, the effects of O2 in NO oxidation and Na/SOx formation was evaluated. Most interestingly, the reaction paths were inferred from density functional theory (DFT) calculations of Gibbs freedom to elucidate the mechanism of NO and SO2 removal by Na/SOx. Overall, we pave the way for the development of NaHCO3 for simultaneous desulfurization and denitrification.
MnO2 + 2SO2 = MnS2O6 | (1) |
MnS2O6 + Na2CO3 = Na2S2O6 + MnCO3 | (2) |
To investigate the synergy effects on the adsorption of SO2 and NO by NaHCO3, individual and simultaneous sorption process were designed. The uniform reaction conditions set for SO2 and NO adsorption by NaHCO3 included T = 160 °C, GHSV = 10000 h−1, flow rate = 1 L min−1, 5 vol% O2 and adsorption residence time of 60 min. The composition of the adsorption gas was adjusted as a variable and its specific names are listed in Table 1.
Sample | Gas compositions | |
---|---|---|
0–30 min | 31–60 min | |
S550N0 | 550 ppm SO2 | |
N500S0 | 500 ppm NO | |
S550/N500 | 550 ppm SO2 | 500 ppm NO |
S550-N500 | 550 ppm SO2 | 550 ppm SO2 + 500 ppm NO |
N500-S550 | 500 ppm NO | 500 ppm NO + 550 ppm SO2 |
S550N500 | 550 ppm SO2 + 500 ppm NO |
The desulfurization and denitration properties of the possibility Na/SOx included Na2CO3, NaHSO3, Na2SO3, Na2S2O5, and Na2S2O6 were than examined, respectively. Since NaHSO3 and Na2S2O5 would release SO2 at 160 °C according to the results of pre-experiments, NaHSO3 and Na2S2O5 as additives were set in the ratios of 1:
4 with NaHCO3 to measure their effects on NO removal. In addition, the role of O2 in the oxidation of NO was investigated by coexisting/not coexisting O2 in the sorption gas.
ΔG = Gproduct − Greactant | (3) |
Gg(T,P) = EDFT(K0,P0)+ΔHg(K0 → T,P0) − TSg(T,P0) | (4) |
The sorption performance of the S550-N500, N500-S550 and S550N500 samples with SO2 and NO coexisting was shown in Fig. 3A2, B2 and C2. It is easily found that the coexistence of SO2 significantly promotes the absorption and conversion of NO and increase the production of NO2, inferring that the Na/SOx intermediates produced by the reaction of SO2 with NaHCO3 as active substances stimulate the absorption of NO. When NO is exposed to 550 ppm of SO2 (Fig. 3A2), the NO concentrations immediately decrease to 0 ppm, while a maximum of about 35 ppm of NO2 is detected in 30 min. The date in Fig. 3B2 show that when SO2 is introduced into the NO being absorbed, the concentration of SO2 rapidly drops to 0 ppm and the concentration of NO followed down to a minimum of 100 ppm accompanied by an increase in the concentration of NO2 from 20 to 40 ppm. Similarly, as shown in Fig. 3C2, the concentration of SO2 decreases faster than that of NO when SO2 and NO are in contact with NaHCO3 at the same time, and the absorption of SO2 is significantly higher than that of NO, which indicates that NaHCO3 preferentially absorbs SO2 relative to NO.
Another finding is that the amount of NO2 present is much smaller than the amount of NO absorbed, implying that a large number of nitrogenous species are stored in the absorber. The distribution of nitrogen species given in Fig. 4A for other nitrogenous species exceeds 60% of the NO feeding, indicating that the S550N500 absorber removes NO mainly through the formation of other nitrogenous species. However, it is uncertain whether NO2 is an active intermediate species for NO removal, so experiments of mixed NO2 and SO2 removal were conducted. The results in Fig. 4b show that the NO2 concentration is reduced from 360 to 140 ppm with 60% removal when SO2 is coexisted and this implies that NaHCO3 has good absorption properties for NO2. Therefore, NO2 is an important intermediate for NO removal, and it is the unabsorbed NO2 that is detected in the gas.
In addition, the changes in absorber morphology and pH were also recorded, and the results are shown in Fig. 4C. The unreacted NaHCO3 is a loose, weakly basic salt with a pH of 7.51. When the reaction proceeds for 30 min, a large amount of water mist appears in the tube due to the release of water molecules from the thermal decomposition of NaHCO3, at which time the pH is 9.88. It is noteworthy that the best NO removal efficiency is achieved at this time. As the reaction is prolonged, the absorber gradually condenses into lumps as the pH increases to 11.0, leading to a decrease in NO absorption performance. The SEM image of S550N500 after reaction with SO2 and NO is shown in Fig. 4D. Unlike the morphology of NaHCO3 after thermal decomposition, crystalline products cover the surface of S550N500, confirming the occurrence of the chemisorption reaction.
The XRD data of the absorption products are shown in Fig. 4E, and not surprisingly, Na2CO3, the thermal decomposition product of NaHCO3, is detected at 37.8°. The characteristic peaks of Na2SO3 (JCPD 37-1488) appear at 18.7°, 23.5°, 35.2°, 40.0°, 46.4°, 58.3° and 59.1°, which are easily oxidized to Na2SO4 with 2θ = 31.8°, 33.9°, 41.4°, and 52.3° (JCPD 37-1475). In addition, diffraction peaks located at 22.6°, 30.1° and 48.2° are attributed to NaNO3 according to JCPD 36-1474, confirming the chemical reaction between NO and the absorbent.
Fig. 4F shows the FTIR spectra of the samples after the absorption reaction. In addition to the peaks of Na2CO3 at 863 and 1450 cm−1, the characteristic peaks corresponding to Na2SO4 appear at 618 and 1140 cm−1, while that of NaNO3 is at 1381 cm−1, implying that the reaction products of NaHCO3 with SO2 and NO mainly include Na2CO3, Na2SO4 and NaNO3.25–27
The reaction products were then characterized by XPS, and the high-resolution spectra of S 2p and N 1s were illustrated in Fig. 5. For S 2p, the peaks with binding energy located at 166.7 ± 0.2 eV can be attributed to Na2SO3 and NaHSO3, while the peaks at 168.8 eV correspond to the mixed Na2S2O5 and Na2S2O6,28–30 which are generated by the following eqn (6)–(13).31 In addition, two characteristic peaks corresponding to the product Na2SO4 (Na2SO4 (1) and Na2SO4 (2)) are found at 168.4 eV and 169.6 eV, respectively.32 This is consistent with Tim C. Keener that NaHCO3 can react directly with SO2 or with SO2 after thermal decomposition, which gives rise to a multi-path reaction scheme between NaHCO3 and SO2 with temperature-sensitive properties.24,33 When the reaction temperature reaches 413 K, the formation of Na2SO4 (1) by SO2 uptake occurs mainly in the Na2CO3 micropores produced by the thermal decomposition of NaHCO3,8 and the amount of this Na2SO4 production is huge, accounting for 61% of the total S of S550N0. Moreover, Na2SO4 (2) located at 169.6 eV can be reasonably attributed to the direct reaction generation at the S550N0 surface with a percentage of about 4.4%.
For S550-N500, N500-S550 and S550N500, four forms of sulfur species existed after simultaneously absorption of SO2 and NO, including two types of Na2SO4, Na2S2O5 and Na2S2O6, with Na2S2O6 and Na2SO4 (2) predominating. However, the introduction of NO affects the distribution of sulfur species. Notably, when NO is present in the absorbed gas, Na2SO3/NaHSO3 disappears while Na2SO4 (2) is enriched, indicating that the conversion of SO32− to SO42− is promoted.
The N 1s XPS spectra given in Fig. 5B can distinguish two typical components with peaks located at 407.3 eV and 403.9 eV, which belong to sodium nitrate (NaNO3) and sodium nitrite (NaNO2), respectively.34,35 Combined with the absorption performance of NO in the coexistence of SO2 in Fig. 3, it is confirmed that the main products of NO conversion are nitrate nitrogen (NO2− and NO3−).
Overall reaction:
NaHCO3 + NO + SO2 + O2 → Na2SO4 + NO2 + NaNO3 + H2O + CO2 | (5) |
Step A:
NaHCO3 + SO2 = NaHSO3 + CO2 | (6) |
2NaHSO3 = Na2S2O5 + H2O | (7) |
Step B:
2NaHCO3 ≜ Na2CO3 + H2O + CO2 | (8) |
NaCO3 +SO2 = Na2SO3 + CO2 | (9) |
2Na2CO3 + 3SO2 = Na2SO3 + Na2S2O5 + 2CO2 | (10) |
Step C:
2Na2S2O5 + O2 ![]() | (11) |
Na2S2O6 + NO = Na2S2O5 + NO2 | (12) |
Na2S2O5 = Na2SO3 + SO2 | (13) |
Step D:
2NaHCO3 + 2NO2 = NaNO2 + NaNO3 + 2CO2 + H2O | (14) |
The generation of Na2S2O5 from NaHCO3 during desulfurization is thermodynamically favorable, and steps A and B are considered as two possible reactions with ΔG of −3276 and −5105 kJ mol−1, respectively. Step B shows that the generation of Na2S2O5 from Na2CO3 is also thermodynamically feasible, which ΔG is about −1281 kJ mol−1. This indicates that NaHCO3 can be directly or thermally decomposed into Na2CO3 to react with SO2 to form Na2S2O5. Steps C and D, as key steps in NO removal, describe the oxidation of NO to NO2 by O2 and Na2S2O5 and further conversion of NO2 to NOX− with ΔG of 675 kJ mol−1 (11), −158 kJ mol−1 (12), −858 kJ mol−1 (13) and −1721 kJ mol−1 (14), respectively. This means that the oxidation of Na2S2O5 to Na2S2O6 is the controlling step of the whole reaction (Table 2).
Structure | ΔfG° kJ mol−1 | Structure | EDFT eV | ΔHg(0 K → 500 K,P0) kJ mol−1 | Sg(T,P0) J mol−1 K−1 | Gg(500 K,P0) kJ mol−1 |
---|---|---|---|---|---|---|
NaHCO3 | −852 | O2 | −9.8 | 4.34 | 172.20 | −1031.2 |
Na2CO3 | −1048 | SO2 | −11.0 | 8.76 | 270.49 | −1200 |
Na2SO3 | −1012 | NO | −12.2 | 6.06 | 226.26 | −1290.4 |
Na2SO4 | −1270 | NO2 | −15.5 | 8.1 | 260.64 | −1626.4 |
NaNO2 | −285 | H2O | −13.6 | 6.92 | 206.53 | −1415.8 |
NaNO3 | −367 | CO2 | −22.7 | 8.30 | 234.90 | −2304.9 |
Na2S2O5 | −1354 | N2 | −16.4 | 5.91 | 206.74 | −1683.7 |
Na2S2O6 | −1532 |
In summary, pathways A and B describe the chemical interaction between NaHCO3 and SO2 to produce the intermediate Na2S2O5. According to path C, the reaction properties of NaHCO3 with NO depend on the formation of Na2S2O6 from Na2S2O5. The NO molecule acts as a Lewis acid and reacts on the surface of Na2S2O6 to form NO2.37 Gradually, the generated NO2 achieves a dynamic equilibrium of adsorption–desorption, and then reacts with NaHCO3 to form NOx−. The removal mechanism and reaction pathway are shown in Fig. 8.
Considering that SO2 and O2 are the key factors that synergistically promote NO absorption, it is necessary to optimize the content of coexisting SO2 and O2 for improving the NO absorption performance. The ration of SO2 and NO (S/N) of the absorbed gases are set to 0.5, 1, 2, 3, and 4, respectively, where the NO concentration is fixed at 500 ppm and the SO2 concentration corresponds to 250, 500, 1000, 1500, and 2000 ppm. The results in Fig. 9A show that S/N has a small effect on the SO2 removal, which is maintained above 95%. However, the efficiency of NO removal is only 34% when S/N is 0.5, which is considered that the oxidative absorption of NO is limited by the small number of reactive intermediates generated on the absorber surface. With S/N higher than 1, the absorption efficiency of NO is stabilized at about 90%. As the S/N increases to 1, the NO removal stabilizes to about 90%, emphasizing the necessity of SO2 for NO absorption. As shown in Fig. 9B, the O2 concentration does not affect the SO2 absorption efficiency, which is always maintained at about 100%. However, when the O2 content is increased from 0 to 5%, the NO removal efficiency dramatically increases from 11% to about 91%, and increasing the content of O2 to 20% has no significant effect on the NO removal. Therefore, the NO removal by NaHCO3 has the advantage of being highly adaptable to SO2 and O2 content of flue gas.
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Fig. 9 Effect of (A) the ration of SO2 and NO and (B) the content of O2 on the adsorption of SO2 and NO. |
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