Article pubs.acs.org/IECR
Nitrogen Oxide Removal from Simulated Flue Gas by UV-Irradiated Sodium Chlorite Solution in a Bench-Scale Scrubbing Reactor Shaolong Yang, Xinxiang Pan,* Zhitao Han, Dekang Zheng, Jingqi Yu, Pengfei Xia, Bojun Liu, and Zhijun Yan Marine Engineering College, Dalian Maritime University, Dalian 116026, Liaoning China ABSTRACT: Ultraviolet irradiated sodium chlorite (UV/NaClO2) solution was introduced to remove nitrogen oxide (NOx) from simulated flue gas in a bench-scale scrubbing reactor. Effects of UV irradiation time, NaClO2 concentration, NO inlet concentration, pH value, and O2 concentration were investigated separately. Results showed that NaClO2 solution with UV pretreatment achieved a remarkable promotion in NOx removal efficiency. The ClO2 produced from photodecomposition of NaClO2 in aqueous solution substantially improved the NO absorption process. The NOx removal efficiency by UV/NaClO2 solution increased from 28 to 77% as the UV irradiation time increased from 0 to 600 s. When the NaClO2 concentration increased, the NOx removal efficiency by UV/NaClO2 solution increased, whereas the enhancement factor decreased. The NO absorption rate by UV/NaClO2 solution increased with NO inlet concentration. The NOx removal efficiency by UV/NaClO2 solution was higher than that by NaClO2 solution without UV pretreatment at a pH range of 3−12. The reaction mechanisms of the NO removal process were suggested to be different in acid and alkaline media. The UV/NaClO2 process was demonstrated to significantly improve NOx removal efficiency, which might be developed to be a potentially costeffective method for removing NOx from flue gas.
1. INTRODUCTION Air pollutants emitted from fossil fuel combustion has become increasingly serious over the past few decades. Nitrogen oxide (NOx), as one of the main gaseous pollutants, has caused considerable environmental and health problems (e.g., acid rain, photochemical smog, toxic haze) on a regional scale.1 Consequently, more stringent regulations have been implemented in order to control NOx emissions from vehicles, coalfired boilers, kilns, and incinerators in recent years. Various NOx emission control technologies have been researched and applied to both stationary and mobile NOx emission systems. Among them, the most commonly practiced technologies are exhaust gas recirculation (EGR), selective catalytic reduction (SCR), and selective noncatalytic reduction (SNCR), which are commercially available at present. EGR can reduce the amount of NOx generated during combustion by lowering the peak temperature and oxygen content in the combustion chamber.2 However, more particulates are produced when EGR is working. Thus, a trade-off decision needs to be made between NOx and particulate emission. SCR is an efficient aftertreatment technology currently, which has been used for removing NOx from large-scale NOx emission sources, such as coal-fired power plants and large diesel engines. However, the commercial urea-SCR catalysts have a short lifetime, due to the relatively low exhaust gas temperature as well as the poisoning effects from high concentrations of SO2 and particulate emission.3 This drawback causes frequent replacement of the expensive catalysts in application. Compared with SCR, the SNCR method injects molecular © XXXX American Chemical Society
ammonia into the combustion chamber without using a catalyst. However, this process also requires being operated in very limited high temperature ranges (850−1100 °C) for effective NOx removal from flue gas.4 Therefore, there has been an explosion of interest in alternative wet scrubbing techniques which can be operated even at ambient temperature. A number of absorbents have been investigated to determine their effectiveness in NOx removal, including NaClO2, NaClO, H2O2, KMnO4, Na2S2O8, and O3. According to the literature,5,6 among these absorbents, NaClO2 has a relatively strong oxidizing ability and fast reaction kinetics with NO under normal pressure and temperature. As a promising oxidant for NOx removal, the NaClO2 wet scrubbing process for flue gas cleaning has been studied widely. Sada et al.7,8 performed early experiments in a stirred tank reactor. They measured reaction kinetics, and then proposed reaction mechanisms of NOx absorption by NaClO2 solution in alkaline condition. Brogren et al.9 and Krzyzynska et al.10 found that the pH value of NaClO2 solution played a critical role in NOx removal efficiency and reaction pathways. Under acid medium, NaClO2 mainly acted as an oxidant to convert NO to NO2. As the price of NaClO2 (100% by mass) was relatively high (2360 USD per ton in China market), more studies attempted Received: November 16, 2016 Revised: February 7, 2017 Accepted: February 12, 2017
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DOI: 10.1021/acs.iecr.6b04463 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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installed above the gas inlet tube of the scrubber. A peristaltic pump (YZ1115, Longer Precision Pump, China) was used to transfer the liquid to the scrubber. 2.2. Experimental Procedures. All chemicals used in this study were reagent grade and were purchased from Sinopharm Chemical Reagent Co., Ltd. (China). NaClO2 solution (2.3 L) was prepared using NaClO2 powder and deionized water (15.0 MΩ, ELGA PURELAB Option-S, England). The pH value of NaClO2 solution was adjusted to the desired value by adding H2SO4 solution or NaOH solution for the investigation of the pH factor. Before the scrubbing process, NaClO2 solution was pretreated with UV irradiation in the photochemical reactor. During the pretreatment process, the NaClO2 solution was irradiated by a constant power UV lamp at an intensity of 4.78 × 10−3 W/cm3. The liquid in the photochemical reactor was mixed by a magnetic stirrer (900 rpm). The pH value of NaClO2 solution was monitored using an acid meter (S210, Mettler-Toledo, Switzerland). The UV irradiation time is defined as the time spent from the UV lamp power on to power off. During each run, a sample (5 mL) was withdrawn for determining UV−visible absorptions of both ClO2− and ClO2 species in UV/NaClO2 solution. After UV irradiation pretreatment, the NaClO2 solution was immediately delivered into the scrubber using the peristaltic pump. The flow rate of the scrubbing liquid was kept at 300 mL/min. The simulated flue gas entered the scrubber through a gas inlet tube with a flow rate set to 1.26 L/min in all experiments. The gas−liquid reaction occurred at room temperature (about 25 °C) in the bench-scale scrubber. The gas−liquid contact time (about 19 s) was calculated by the gas residence time in the scrubber column. The scrubbing solution was continuously supplied without recirculation. Meanwhile, the effluent was collected in the bottom tank. During the pH factor experiments, when the outlet gas concentrations were stable (about 4 min after scrubbing), a sample (5 mL) was withdrawn from the liquid outlet tube for ion chromatographic (Dionex ICS-1500) analysis purpose. The outlet concentrations of NO, NO2, NOx, and O2 were continuously measured by an infrared multigas analyzer (MGA 5, MRU, Germany) at an interval of 10 s. 2.3. Data Processing. When the simulated flue gas reacted with NaClO2 solution, NOx was absorbed in the counterflow reactor. The removal efficiency (ηi, %) was calculated by
to use a composite absorbent (NaClO + NaClO2) to simultaneously mitigate NOx and SOx.11,12 However, recent studies show that a process of ultraviolet (UV) irradiation of NaClO2 has emerged as an innovative technology for ClO2 generation.13 As ClO2 is also an outstanding oxidant, NaClO2 solution with UV treatment may be more effective in the NOx absorption process. Furthermore, NOx removal using UV-induced oxidants has been the subject of much research in recent years.14−16 A significant synergistic effect in NOx removal could be achieved by the combination of UV radiation (254 nm) and existing oxidizing absorbents. To the best of our knowledge, NOx removal using NaClO2 with UV treatment has not been reported yet. In the present work, we studied the NOx removal performance by UVirradiated NaClO2 (UV/NaClO2) solution in a bench-scale spraying column. Effects of the UV irradiation time, NaClO2 concentration, NO inlet concentration, O2 concentration, and pH value were investigated experimentally. Results showed that the UV/NaClO2 process was demonstrated to significantly improve the NOx removal efficiency over a wide pH range. The possible reaction mechanisms of NOx absorption using UV/ NaClO2 solution were also discussed.
2. EXPERIMENTAL SECTION 2.1. Experiment Apparatus. All experiments were performed in a lab-scale scrubbing reactor. A schematic diagram of the experimental system is described in Figure 1. The experimental system consists of a simulated flue gas unit, a photochemical reactor, and a bench-scale scrubber.
ηi =
Ci ,inlet − Ci ,outlet Ci ,inlet
× 100% (1)
where i stands for NO or NOx; Ci,inlet is the inlet concentration of gas i (ppm); Ci,outlet is the outlet concentration of gas i (ppm). The NOx is the sum of the NO and NO2. The outlet concentrations of NO and NO2 were obtained by taking averages after 2 min. The enhancement factor (εi) of the removal efficiency of NO or NOx was defined as ηi ,UV − ηi ,non‐UV εi = ηi ,UV (2)
Figure 1. Schematic diagram of experimental system.
The simulated flue gas was prepared by quantitative blending of NO (9.8% NO with 90.2% N2 as balance gas), O2 (99.9%), and N2 (99.9%). The gas flow rate of each gas species was controlled by a mass flow controller (D07-19B, Sevenstar Electronics, China). The photochemical reactor included an LP-UV lamp (UV-C, 253.7 nm, 11 W, Philips, Holland), a column (height 40 cm; effective volume 2.3 L), and a magnetic stirrer. The outside of the photochemical reactor was covered with aluminum foil to reduce the disturbance from sunlight. The bench-scale scrubber (effective height 20 cm; internal diameter 5 cm) was a custom-made Lucite spray column. The diameter of the spray nozzle (B1/4TT-SS+TG-SS0.4, Spraying System Co., USA) was 0.04 cm. A perforated screen plate was
where ηi,UV is the removal efficiency by NaClO2 solution with UV pretreatment; ηi,non‑UV is the removal efficiency by NaClO2 solution without UV pretreatment. For the kinetic calculation, the NO absorption rate per unit volume of the reactor (RNO, mol/(m3·s)) was calculated by17 B
DOI: 10.1021/acs.iecr.6b04463 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Industrial & Engineering Chemistry Research RNO =
PNO,outlet ⎞ Q GPT ⎛ PNO,inlet ⎜⎜ ⎟⎟ − RTVL ⎝ PI ,inlet PI ,outlet ⎠
(3)
where QG is the gas flow rate (L/min); PT is the total pressure of the simulated flue gas (Pa); PNO,inlet and PNO,outlet are the inlet and outlet partial pressures of NO gas in the bulk gas (Pa); PI,inlet and PI,outlet are the inlet and outlet partial pressures of inertia gas I in the bulk gas (Pa); R is the gas constant (R = 8.314 J/(mol·K)); T is the absolute temperature (K); VL is the volume of the scrubbing reactor (VL = 2.3 L).
3. RESULTS AND DISCUSSION 3.1. Effect of UV Irradiation Time. The effect of UV irradiation time in the photochemical reactor on NO and NOx removal by NaClO2 solution was investigated in detail. The UV irradiation time is varied from 0 to 600 s at NaClO2 concentration = 0.002 mol/L and NO inlet concentration = 1000 ppm, while the pH value was uncontrolled. Results are shown in Figures 2, 3, and 4 and Table 1.
Figure 4. Effect of UV irradiation time on outlet concentrations of NO and NO2, and pH value of NaClO2 solution. Conditions: NaClO2 concentration 0.002 mol/L; uncontrolled pH value; NO inlet concentration 1000 ppm; O2 0%.
As shown in Figure 2, when UV irradiation time increased from 0 to 300 s, removal efficiencies of NO and NOx substantially increased from 32 and 28% to 95 and 69%, respectively. However, beyond 300 s, increasing UV irradiation time was observed to result in a less significant improvement in the extent of removal efficiencies of NO and NOx. According to the literature,18 ClO2 was generated through the photodecomposition of NaClO2 in aqueous solution by UV irradiation (eq 4). Results showed that the ClO2 concentration increased as UV irradiation time increased in the photochemical reactor (shown in Table 1). The UV−visible absorptions of the ClO2− and ClO2 species in NaClO2 solution were measured under different UV irradiation times (Figure 3). The concentrations of ClO2− and ClO2 were determined at wavelengths of 260 and 360 nm, respectively (shown in Table 1). The results indicated that NaClO2 was converted to ClO2 by UV irradiation under mild alkaline condition. The ClO2 yield ascended from 1.6 × 10−6 to 1.9 × 10−4 mol/L with UV irradiation time varying from 0 to 600 s. As the redox potential of ClO2/Cl− (1.511 V) was higher than that of ClO2−/Cl− (0.76 V), this strong oxidant contributed to the oxidation absorption of NO from simulated flue gas through eq 5.19−21 It was worth noting that when UV irradiation time reached 300 s, the enhancement factors of NO and NOx increased up to 2.0 and 1.5, respectively. These results implied that removal efficiencies of NO and NOx by NaClO2 solution were significantly improved by UV pretreatment. The ClO 2 generated from photodecomposition of NaClO2 was suggested to make a remarkable contribution to the NOx removal process.
Figure 2. Effect of UV irradiation time on removal efficiencies by NaClO2 solution. Conditions: NaClO2 concentration 0.002 mol/L; uncontrolled pH value; NO inlet concentration 1000 ppm; O2 0%.
3ClO2− + H 2O + hv → 2ClO2 + Cl− + 2OH− + 0.5O2 (4)
5NO + 2ClO2 + H 2O → 5NO2 + 2HCl
(5)
According to the photolysis reaction of NaClO2 (eq 4), OH− was also produced by photodecomposition of NaClO2 in aqueous solution.18 As a result, the pH value of NaClO2 solution was observed to increase from 9 to 10.4 with UV irradiation time increasing up to 600 s (Figure 4). As decomposition of the ClO2 speeded up at a solution pH value of above 10, more ClO2− and ClO3− were suggested to be produced when the irradiation time exceeded 300 s (eq 6).22,23
Figure 3. UV−visible absorption spectra of ClO2− and ClO2 species in NaClO2 solution under different UV irradiation times. Conditions: NaClO2 concentration 0.002 mol/L; uncontrolled pH value.
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DOI: 10.1021/acs.iecr.6b04463 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Industrial & Engineering Chemistry Research Table 1. Removal Efficiencies and Enhancement Factors in Terms of UV Irradiation Timea UV irradiation time (s) NaClO2 conversion percentage ClO2 yield (10−5 mol/L) NO removal efficiency (enhancement factor) NOx removal efficiency (enhancement factor) a
0
15
30
60
150
300
450
600
0% 0.16 32% (0)
0.95% 0.85 37% (0.18)
1.6% 1.5 44% (0.40)
3.2% 2.9 59% (0.85)
6.3% 7.6 73% (1.3)
11% 12 95% (2.0)
16% 17 94% (2.0)
22% 19 97% (2.1)
28% (0)
30% (0.06)
34% (0.23)
44% (0.59)
54% (0.92)
69% (1.5)
70% (1.5)
77% (1.8)
Conditions: NaClO2 concentration 0.002 mol/L; uncontrolled pH value; NO inlet concentration 1000 ppm; O2 0%.
and NOx by UV/NaClO2 solution were higher than those by NaClO2 solution without UV pretreatment under all NaClO2 concentrations. In the absence of UV pretreatment, NO was mainly absorbed by ClO2− through eqs 11−14 in weak alkaline condition.9,26 Accordingly, when the NaClO2 concentration increased from 0.002 to 0.01 mol/L, the removal efficiencies of NO and NOx increased from 36 and 31% to 62 and 52%, respectively. However, in the presence of UV pretreatment, part of NaClO2 in aqueous solution was converted to ClO2 during the UV pretreatment process, which favored NO absorption in the scrubber. Thus, as the NaClO2 concentration increased from 0.002 to 0.01 mol/L, the removal efficiencies of NO and NOx by UV/NaClO2 solution significantly increased from 59 and 46% to 81 and 68%, respectively. In addition, it can be seen from Figure 5 that, to achieve the same NOx removal efficiency of 46%, about 0.006 mol/L NaClO2 without UV pretreatment was needed, whereas 0.002 mol/L NaClO2 with UV pretreatment sufficed. This confirmed that NaClO2 demand for NOx removal was considerably higher under the condition of nonUV than under the condition of UV pretreatment. Therefore, from the perspective of cutting down absorbent dosage, the UV/NaClO2 method might achieve a relatively high removal efficiency with less absorbent cost in a wet scrubbing process.
Consequently, NO removal efficiency did not significantly increase as the UV irradiation time increased from 300 to 600 s. 2ClO2 + 2OH− → ClO2− + ClO3− + H 2O
(6)
Nevertheless, Figure 2 also shows that NOx removal efficiency increased from 69 to 77% with UV irradiation time varying from 300 to 600 s. This result could be due to NO2 absorption under alkaline condition (Figure 4). According to the literature,9,24,25 most of the absorbed NO2 in NaClO2/ NaOH solution was due to hydrolysis (eqs 7−10) instead of oxidation. Thus, NO2 absorption efficiency enhanced as OH− concentration increased. As shown in Figure 4, when UV irradiation time was above 300 s, the increase in pH value strengthened the NO2 absorption, resulting in a drop in the outlet NO2 concentration. As a consequence, NOx removal efficiency increased as the NO2 absorption efficiency increased. 2NO2 + H 2O ↔ HNO2 + H+ + NO3−
(7)
2NO2 ↔ N2O4
(8)
2NO2 + H 2O → HNO2 + HNO3
(9)
HNO2 ↔ H+ + NO2−
(10)
2NO + ClO2− → 2NO2 + Cl−
3.2. Effect of NaClO2 Concentration. A set of experiments was conducted to investigate the variation in NO and NOx removal efficiencies as a function of NaClO2 concentration, where the UV irradiation, NO inlet concentration, and pH value were fixed at 60 s, 1000 ppm, and uncontrolled, respectively. As shown in Figure 5, removal efficiencies of NO
(11)
4NO + ClO2− + 4OH− → 4NO2− + Cl− + 2H 2O (12)
4NO + 3ClO2− + 4OH− → 4NO3− + 3Cl− + 2H 2O (13)
4NO2 + ClO2− + 4OH− → 4NO3− + Cl− + 2H 2O (14)
As shown in Figure 6, the enhancement factor of the NO removal efficiency at a NaClO2 concentration of 0.002 mol/L was more than double those under a NaClO2 concentration of above 0.002 mol/L. This phenomenon might be attributable to the following two points: On the one hand, the ratio of ClO2 to ClO2− in aqueous solution had an impact on the NO absorption efficiency. As the amount of photons emitted by the UV lamp in 60 s was limited, the ratio of ClO2 to ClO2− in aqueous solution decreased as NaClO2 concentration increased. On the other hand, the pH value of UV/NaClO2 solution increased to above 10 when the NaClO2 concentration was beyond 0.002 mol/L. Thus, the decomposition rate of ClO2 accelerated as the NaClO2 concentration increased,22 which in turn led to a reduction of the ratio of ClO2 to ClO2− in aqueous solution. 3.3. Effect of NO Inlet Concentration. The effect of NO inlet concentration on removal efficiencies and absorption rates was studied under conditions of NaClO2 concentration = 0.002
Figure 5. Effect of NaClO2 concentration on removal efficiencies by NaClO2 solution. Conditions: UV 60 s; uncontrolled pH value; NO inlet concentration 1000 ppm; O2 0%. D
DOI: 10.1021/acs.iecr.6b04463 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Figure 6. Effect of NaClO2 concentration on enhancement factor of NO removal efficiency by UV/NaClO2 solution. Conditions: UV 60 s; uncontrolled pH value; NO inlet concentration 1000 ppm; O2 0%.
Figure 8. Effect of NO inlet concentration on NO absorption rate by NaClO2 solution. Conditions: NaClO2 concentration 0.002 mol/L; UV 60 s; uncontrolled pH value; O2 0%.
mol/L, UV irradiation time = 60 s, and uncontrolled pH value of the NaClO2 solution. The results are shown in Figures 7 and 8.
RNO = k Ga(PNO − PNO, i)
(15)
where kG is the gas-phase mass transfer coefficient of NO (mol/ (s·m2·Pa), a is the interfacial area between gas and liquid per unit volume of reactor (m2/m3), PNO is the partial pressure of NO gas in the bulk gas (Pa),and PNO,i is the partial pressure of NO gas at the interface (Pa). In the absence of UV pretreatment, the latter positive factor might play a major role. Therefore, the higher the NO inlet concentration was, the higher the removal efficiencies of NO and NOx were. These results suggested that NO removal by NaClO2 solution was kinetically controlled in the NO inlet concentration range of 200−1000 ppm, which was consistent with previous results.12,28,29 However, in the presence of UV pretreatment, the former negative factor might be a dominant role, which resulted in a decrease of removal efficiencies of NO and NOx as the NO inlet concentration increased. 3.4. Effect of pH Value. Considering that the pH value of NaClO2 solution has a substantial influence on NOx removal efficiency and reaction mechanisms,9,10 the effect of the pH value on the removal efficiencies of NO and NOx by NaClO2 solution with and without UV pretreatment was studied. Experiments were performed with the pH value in the range 3− 12 at NaClO2 concentration = 0.002 mol/L, UV = 60 s, and NO inlet concentration = 1000 ppm. The results are shown in Figure 9. As is clearly seen, removal efficiencies of NO and NOx by NaClO2 solution with UV pretreatment were higher than those by NaClO2 solution without UV pretreatment at all pH values. These results were believed to be related to the ClO2 produced by photodecomposition of NaClO2 in aqueous solution.
Figure 7. Effect of NO inlet concentration on removal efficiencies by NaClO2 solution. Conditions: NaClO2 concentration 0.002 mol/L; UV 60 s; uncontrolled pH value; O2 0%.
As shown in Figure 7, removal efficiencies of NO and NOx by NaClO2 solution without UV pretreatment increased gradually as the NO inlet concentration increased. However, when the NO inlet concentration changed from 200 to 1000 ppm, the removal efficiencies of NO and NOx by UV/NaClO2 solution decreased from 92 and 66% to 64 and 56%, respectively. These results can be explained by the following two aspects: On the one hand, the amount of NO molecules through the scrubbing reactor per unit time increased as the NO inlet concentration increased. This increase diluted the relative molar ratio of absorbent to NO, which might adversely impact the overall removal efficiencies of NO and NOx. On the other hand, according to the two-film theory proposed by Danckwerts P. V.,27 the NO absorption rate can be expressed in eq 15. Therefore, an increase of the NO inlet concentration (i.e., NO partial pressure) promotes the mass transfer driving force of NO gas, which can improve the NO absorption rate (Figure 8).
5ClO2− + 4H+ → 4ClO2 + Cl− + 2H 2O
(16)
4ClO2− + 2H+ → 2ClO2 + ClO3− + Cl− + H 2O
(17)
When the pH value was below 5, removal efficiencies of NO and NOx by NaClO2 without UV pretreatment slowly increased as the H+ concentration increased, which was due to the absorption enhancement by the ClO2 generated from acid activation of NaClO2 (eqs 16 and 17).20,30 When the pH value of NaClO2 solution increased from 5 to 9, removal efficiencies of NO and NOx by NaClO2 without UV pretreatment were almost kept at 28 and 24%, respectively. After that, when the E
DOI: 10.1021/acs.iecr.6b04463 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Figure 10. Effect of pH value on ClO2 yield of NaClO2 solution. Conditions: NaClO2 concentration 0.002 mol/L; UV 60 s.
Figure 9. Effect of pH value on removal efficiencies by NaClO2 solution. Conditions: NaClO2 concentration 0.002 mol/L; UV 60 s; NO inlet concentration 1000 ppm; O2 0%.
pH value of NaClO2 solution increased to above 9, the redox potential of ClO2/Cl− decreased from 1.511 to 0.76 V. In this case, removal efficiencies of NO and NOx by NaClO2 without UV pretreatment declined sharply. Likewise, removal efficiencies of NO and NOx by NaClO2 with UV pretreatment showed a similar trend in terms of pH value. When the pH value changed from 3 to 12, removal efficiencies of NO and NOx by UV/NaClO2 solution decreased from 76 and 57% to 36 and 35%, respectively. In addition, it was also worth noting that the removal efficiency difference between NO and NOx by UV/NaClO2 solution decreased with increasing pH value in Figure 9. In acid condition, the NO absorption rate by UV/NaClO2 was fast, but a large amount of NO2 was desorbed from the liquid. The rapid formation rate of NO2 implied that on the one hand, the oxidation of NO to NO2 by UV/NaClO2 (eqs 5, 11) was favored by a low pH value;9 On the other hand, NO2 absorption was not favored by oxidants in UV/NaClO2 solution directly.5 When pH value of UV/NaClO2 solution increased to above 9, the reaction rate between NO and UV/ NaClO2 solution decreased, resulting in a drop in NO removal efficiency. However, the major part of NO reacted to form NO2− or NO3− instead of NO2 in alkaline condition. For example, nearly complete absorption of NO2 was achieved at pH 12. This might contribute to reducing steps of possible absorption process for exhaust gas cleaning system. To understand removal mechanisms of NO removal by UV/ NaClO2 solution, further experiments were performed to measure the UV−visible absorptions of the scrubbing liquid, as well as analyze reaction products of the spent absorption solution by ion chromatographic analysis. The results are shown in Figures 10 and 11. As shown in Figure 10, the ClO2 yield at pH 3 was almost 2 times than those at a pH value of above 3. This result was attributable to the extra ClO2 generated by decomposition of NaClO2 through eqs 16 and 17 under strong acid condition. It can be seen from Figure 11 that the measured ClO3− at pH 3 was believed to be produced from eq 17, which caused extra generation of the ClO2 in this acid condition. Moreover, the formation rate of ClO2 was further accelerated by the catalytic effect of chloride ions.31 This effect resulted in a high conversion rate of NaClO2 at pH 3, which was consistent
Figure 11. Ion chromatographic peaks for spent reaction solution under different pH values. Conditions: NaClO2 concentration 0.002 mol/L; UV 60 s.
with the peak analysis results of ClO2− in Figure 11. The increase of the ClO2 concentration at a pH value of below 5 enhanced the NO removal efficiency by UV/NaClO2 solution, which was in agreement with the results in Figure 9. When the pH value further increased from 3 to 8, removal efficiencies of NO and NOx by UV/NaClO2 solution decreased from 76 and 57% to 60 and 47%, respectively. Thus, the peak area of NO3− at pH 8 was smaller than that at pH 3. In addition, NO2− was not detected under different pH values, and NO3− was the major reaction product in solution in Figure 11. This result indicated that NO could be oxidized to NO3− by UV/NaClO2 solution under both acid and alkaline conditions. When the pH value of UV/NaClO2 solution reached 11, ClO2 was consumed through the disproportionation reaction (eq 6) in alkaline condition.32,33 This drop in ClO2 yield caused the decrease of removal efficiencies of NO and NOx by UV/NaClO2 solution. Therefore, the results suggested that the oxidative ability of NaClO2 solution with UV pretreatment increased as the pH value decreased. In view of that industrial scrubbers generally operated at a pH range of 4−9, the UV/NaClO2 absorption process was more effective than NaClO2 absorption process in this pH range, and did not require any tight pH control in application. F
DOI: 10.1021/acs.iecr.6b04463 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Industrial & Engineering Chemistry Research 3.5. Effect of O2 Concentration. The effect of O2 concentration on the removal efficiencies of NO and NOx was investigated under conditions of NaClO2 concentration = 0.002 mol/L, UV irradiation time = 60 s, NO inlet concentration = 1000 ppm, and uncontrolled pH value of NaClO2 solution. The O2 concentration was regulated to range from 0 to 25%, along with the overall NOx concentration set to 1000 ppm. Figure 12 displays the removal efficiencies of NO and NOx under different O2 concentrations. The removal efficiencies of
impacting removal efficiencies of NO and NOx by NaClO2 with UV pretreatment.
4. CONCLUSIONS A novel UV/NaClO2 process for NOx removal was proposed and systematically investigated in this paper. The results demonstrate that NOx removal by NaClO2 with UV pretreatment can achieve a significant promotion effect. When the UV irradiation time increased from 0 to 600 s, removal efficiencies of NO and NOx by UV/NaClO2 solution increased from 32 and 28% to 97 and 77%, respectively. The enhancement factors for the removal efficiencies of NO and NOx reached 2.1 and 1.8 as the UV irradiation time was 600 s, which was due to the ClO2 generated from the photodecomposition of NaClO2 in aqueous solution. The removal efficiencies of NO and NOx by UV/NaClO2 solution increased as the NaClO2 concentration increased, whereas the enhancement factor decreased. These results might be attributable to the reduction of the ratio of ClO2 to ClO2− in aqueous solution. When the NO inlet concentration changed from 200 to 1000 ppm, the NO removal efficiency decreased but the NO absorption rate increased. O2 in simulated flue gas diluted the volume ratio of NO to NO2, which might cause a drop in the removal efficiencies of NO and NOx by UV/NaClO2 solution. The removal efficiencies of NO and NOx by NaClO2 solution with UV pretreatment were higher than those by NaClO2 solution without UV pretreatment at the pH range of 3−12. These behaviors resulted from the decomposition of ClO2 and ClO2− under different pH values. In acid medium, UV/NaClO2 showed a strong oxidizing ability, which removed 76% NO at pH 3, NaClO 2 concentration of 0.002 mol/L, and UV irradiation time of 60 s. In alkaline solution, UV/NaClO2 was an effective NOx absorption agent, which absorbed NO without NO2 outgassing at pH 12. The current work facilitates the investigation of the NOx removal performance by UV/NaClO2 solution and primary reaction mechanisms. In a follow-up experiment, the reaction kinetics and material balance of this process will be further investigated.
Figure 12. Effect of O2 inlet concentration on removal efficiencies by NaClO2 solution. Conditions: NaClO2 concentration 0.002 mol/L; UV 60 s; NO inlet concentration 1000 ppm; uncontrolled pH value.
NO and NOx by NaClO2 without UV pretreatment were almost unchanged under all O2 concentrations. However, when the O2 concentration increased from 0 to 25%, the removal efficiencies of NO and NOx by NaClO2 with UV pretreatment decreased from 63 and 55% to 50 and 42%, respectively. These results might be due to the reduction of the volume ratio of NO to NO2 as the O2 concentration increased (Figure 13). As part of NO gas was oxidized by O2 before the scrubbing process, the NO inlet concentration decreased as the O2 concentration increased, thereby decreasing the NO absorption driving force. This might cause a drop in the NO absorption rate, adversely
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Shaolong Yang: 0000-0003-2566-9803 Xinxiang Pan: 0000-0002-0251-5679 Zhitao Han: 0000-0001-5501-6067 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Science and Technology Plan Project of China’s Ministry of Transport (Grant 2015328225150), the National Natural Science Foundation of China (Grants 51402033 and 51479020), the Scientific Research Fund of Liaoning Provincial Education Department of China (Grant L2014198), and the Fundamental Research Funds for the Central Universities (Grant 3132016018).
Figure 13. Effect of O2 concentration on volume ratio of NO to NO2 in simulated flue gas. G
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DOI: 10.1021/acs.iecr.6b04463 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
