Article pubs.acs.org/EF
Kinetics of Nitric Oxide Absorption from Simulated Flue Gas by a Wet UV/Chlorine Advanced Oxidation Process Shaolong Yang, Xinxiang Pan,* Zhitao Han, Dongsheng Zhao, Bojun Liu, Dekang Zheng, and Zhijun Yan Marine Engineering College, Dalian Maritime University, Dalian 116026, P. R. China ABSTRACT: The mass transfer reaction kinetics of NO absorption by UV/chlorine advanced oxidation process were investigated in a lab-scale photochemical bubble reactor. Effects of several parameters on NO absorption rate were studied, including UV power, NO inlet concentration, SO2 concentration, active chlorine concentration of electrolyzed seawater, and reaction temperature. Results showed that NO absorption rate increased gradually with the increase of UV power, NO inlet concentration, and active chlorine concentration of electrolyzed seawater, but was almost independent of SO2 concentration and reaction temperature (below 313 K). The absorption process is a pseudo-0.2-order with respect to NO, as well as a pseudo-0.6order with respect to active chlorine. The mass transfer process is the main rate-determining step for the NO absorption by UV/electrolyzed seawater process. The established NO absorption model is in good agreement with the experimental values.
1. INTRODUCTION Over the past few decades, many cities in southeastern China coastal region have experienced a rapid development in economy and industry. A lot of coal-fired plants and deepwater ports have been built up in these coastal regions. The emissions from coalfired plants and ocean-going vessels have caused tremendous harmful effects to human health and ecosystems,1 which caused serious local and regional air pollution. Public concerns on atmospheric impacts from these large stationary and mobile nitrogen oxide (NOx) sources have increased in recent years. Therefore, there are eager demands on developing cost-efficient technologies and improving the currently used methods for NOx removal from large NOx sources, especially in coastal port cities. NOx emission control technologies mainly include combustion modification methods and post-treatment methods.2 Nowadays, dry post-treatment methods, such as selective catalytic reduction (SCR) and selective noncatalytic reduction (SNCR), have been widely used in many applications, such as coal-fired plants, incinerators, heavy trucks, and ships. However, the versatility of these techniques does not sufficiently consider the emission characteristics of different NOx sources. Consequently, high concentrations of sulfur oxides (SOx) and particles in the flue gas from large coal-fired plants and oceangoing vessels deteriorate the NOx removal performance of these dry methods. In such cases, wet scrubbing techniques promise to be more attractive than those competing dry methods and more efficient than current combustion modification methods.3 Moreover, modification of existing desulphurization tower with newly developed wet scrubbing techniques will be an economically attractive solution to control multipollutants from the flue gas. As nitric oxide (NO), the main component of NOx in flue gas, is sparingly soluble in aqueous solution, various oxidants4 have been investigated for their effectiveness in NOx removal, including KMnO4, H2O2, NaOCl, NaClO2, O3, and Na2S2O8, and Fenton et al. Among these oxidants, NaClO is one of the most attractive absorbents for NOx emission control from large power © XXXX American Chemical Society
plants in coastal port cities because of the following advantages: (a) in situ generation by seawater electrolysis; (b) safe to handle and easy storage with a good shelf life; (c) inexpensive and environmentally friendly; (d) chemicals are stable in solution containing various complex ions (e.g., Cl−, Br−, HCO3−, and OH−). In previous works, much research5−10 has been done to investigate the NO removal performances and reaction pathways by NaClO wet scrubbing process. Some of them11−13 use electrogenerated chlorine from seawater electrolysis to absorb NO from flue gas, which obtained a satisfactory removal efficiency. As NO removal by electrogenerated chlorine mainly uses seawater and electricity, this type of technology is especially suitable for large onshore coal-fired plants and ocean-going vessels in coastal port cities. In recent years, a novel advanced oxidation process of active chlorine solution associated with ultraviolet (UV/Chlorine)14−18 has been reported for disinfection and degradation of microorganisms and organic pollutants in wastewater treatment field. During UV/Chlorine treatment, effective components of active chlorine, HOCl and OCl−, produce several free radicals through photolysis reactions, such as hydroxyl (OH•) and chlorine (Cl•) radicals. Because of the strong and non−selective oxidation characteristics of these free radicals, UV/chlorine method presents significant enhancement in removal efficiencies of multi− pollutants. These developments prompted us to investigate the possibility of using UV/chlorine to oxidize NO from flue gas by a wet scrubbing process. In our previous work,19 NO removal performance by NaClO solution pretreated with UV 254 nm was investigated. Effects of several parameters on NO removal efficiencies were discussed, including UV irradiation time, pH value, and the active chlorine concentration (ACC) of NaClO solution. In this work, the feasibility of utilizing UV/electrolyzed seawater to absorb and oxidize NO was determined using a Received: February 15, 2017 Revised: April 6, 2017
A
DOI: 10.1021/acs.energyfuels.7b00458 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels
Figure 1. Schematic diagram of experimental system: (1−3) NO, SO2, and N2 gas cylinders; (4−6) reduced valves; (7−9) mass flow controllers; (10) gas mixer; (11) constant temperature water bath; (12) gas distributor; (13) UV lamp; (14) bubble column; (15) rubber plug; (16−17) block valves; (18) thermometer; (19) electric condenser; (20) gas analyzer; (21) tail gas absorber; (A) gas primary road; (B) gas bypass. 3000 mg/L [Cl2]) with artificial seawater. The ACC of the bubbling liquid was determined by N,N-diethyl-p-phenylenediamine (DPD) colorimetric method (ASTM 4500 G), using a spectrophotometer (Shimadzu UV1800, Japan) at wavelength of 515 nm. The initial pH value of the bubbling liquid was adjusted to 6 by adding HCl solution (1 mol/L). The pH value of the solution was measured by a pH meter (Mettler-Toledo S210, Switzerland). After that, the bubbling liquid was added into the bubble column (14) by opening the rubber plug (15). The temperature of bubbling liquid was adjusted to designed values by the constant temperature water bath (11). When the inlet concentrations of gas pollutants and solution temperature reached the required values and kept stable, the simulated flue gas (1.26 L/min) was introduced into the bubble column reactor (14) through the gas primary road A. Meanwhile, the UV lamp (13) (0.047 W/cm3) was turn on, and gas pollutants reacted with the absorbent under atmospheric pressure. The outlet concentrations of gas pollutants were simultaneously monitored using the gas analyzer (20) at an interval of 10 s. Each test cycle was kept for 15 min. All experiments were repeated for three times and the mean values of these data were used for further calculation. 2.3. NO Removal Efficiency. The inlet concentration of NO is measured through the gas bypass B. The outlet concentration of NO was determined by taking average of outlet concentration of NO after bubbling for 5 min. Then the NO removal efficiency (η, %) is defined as follows:
bubble column reactor. The mass transfer reaction kinetics of NO absorption by UV/electrolyzed seawater were investigated. Effects of the ACC, UV power, NO inlet concentration, and SO2 concentration on NO absorption rate were studied. The mass transfer reaction kinetic parameters were calculated. A simple NO absorption equation was established and verified. This study could provide a theoretical basis for the numerical simulation of NO absorption by UV/electrolyzed seawater process, as well as the industrial application of this technology.
2. EXPERIMENTAL SECTION 2.1. Experimental Apparatus. Figure 1 describes the schematic diagram of the experimental system. The system is composed of simulated flue gas blending unit, photochemical bubble reactor, and flue gas analysis system. The photochemical bubble reactor includes constant temperature water bath (11), gas distributor (12), UV lamp (13) (UV−C 254 nm, 16 W, Philips, Holland), bubble column (14), and thermometer (18). The bubble column (14) (height 600 mm, inner diameter 50 mm) is custom-made by perspex. The gas distributor (12) (ISO 4793 P4, pore size 10−15 μm, disc diameter 38 mm) is installed at the bottom of the bubble column (14), which is used to distribute the simulated flue gas. The bubble column (14) was placed in a constant temperature water bath (11) (Julabo F34-ED Refrigerated/ Heating Circulator, Germany), which is used to control the absorbent temperature during experiments. The absorbent is poured into the bubble column (14) by opening the rubber plug (15), along with its temperature measured by the thermometer (18) during the experiments. To mitigate the acid corrosion to the gas analyzer (20), the electric condenser (19) is used to remove the water moisture. The gas analyzer (20) (MRU MGA5, Germany) is used to continuously measure the concentrations of multigas pollutants, including NO, NO2, NOx, and SO2. 2.2. Experimental Procedures. The simulated flue gas was a blend of three kinds of gases: NO (10.04% NO with N2 as the balance gas), SO2 (10.1% SO2 with N2 as the balance gas), and N2 (99.999%). The flow rate of each gas from the separate air bottle (1−3) were metered by the mass flow controller (7−9) (MFC, Beijing Sevenstar Electronics Co., Ltd.). The inlet concentrations of gas pollutants were measured by the gas analyzer (20) through gas bypass line B. The absorbent in our experiments was produced by artificial seawater electrolysis in an undivided cell as described previously.13 The artificial seawater was prepared according to the ASTM D1141 standard. To reduce the deterioration of electrogenerated chlorine (less than 2% in 12 h), all the electrolyte was stored in a water bath (293 K) shielded from light. For each run, bubbling liquid (500 mL) was prepared by freshly mixing the electrogenerated chlorine (initial ACC of about
η=
C NO,inlet − C NO,outlet C NO,inlet
× 100% (1) 3
where CNO,inlet is the inlet concentration of NO (mg/m ) and CNO,outlet is the outlet concentration of NO (mg/m3). 2.4. NO Absorption Rate. According to the material balance of NO, NO absorption rate (NNO, mol/(m2·s) can be calculated using the following eq 2:
NNO =
Q GηC NO,inlet
1 VLaNOMNO 60 000
(2)
where QG is the volumetric flue gas flow rate (L/min); VL is the volume of absorbent (VL = 0.5 L); aNO is the interfacial area between gas and liquid per unit volume of the system (m2/m3); MNO is the molecular weight of NO (MNO = 30 g/mol). 2.5. Physical Parameters. The seawater viscosity (μsw, Pa·s) is calculated by the empirical equation proposed by Fabuss et al. at a given viscosity of pure water and temperature.20 The solubility coefficient of NO in liquid phase (HNO,L, mol/(L·Pa)) is calculated by the Van Krevelen and Hoftizer empirical equation with a known solubility coefficient of NO in water (HNO,w, mol/(L·Pa)). The diffusion coefficient of NO in liquid phase (DNO,L, m2/s) is calculated using the B
DOI: 10.1021/acs.energyfuels.7b00458 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels Table 1. Physical and Mass Transfer Parameters of NO Absorption in Seawater System HNO,L (×10−8 mol/(Pa·L))
DNO,L (×10−9 m2/s)
kNO,L (×10−3 m/s)
kNO,G (×10−7 mol/(m2·s·Pa))
aNO (m2/m3)
1.55
1.97
1.99
5.53
40.31
2NO2 (aq) + H 2O ↔ HNO2 + H+ + NO3−
Wilke−Chang empirical equation at a given diffusion coefficient of NO in water (DNO,w, m2/s). HNO,w and DNO,w can be obtained by consulting refs 3 and 21. The diffusion coefficient of NO in gas phase (DNO,G, m2/s) is estimated by the Chapman−Enskog semiempirical equation.22 2.6. Mass Transfer Parameters. The interfacial area and mass transfer coefficients in our gas−liquid bubbling reactor were determined by classical chemical methods. Two absorption systems, CO2 in Na2CO3/NaHCO3−NaClO solution and CO2 in NaOH solution, were used to determine the gas−liquid mass transfer parameters of CO2 absorption process. Then mass transfer parameters of NO absorption can be obtained by the following eqs 3−5:
kNO,L = k CO2,L(D NO,L /DCO2,L)
(3)
kNO,G = k CO2,G(DNO,G /DCO2,G)
(4)
aNO = aCO2
(5)
NO(aq) + HOCl ↔ NO2 (aq) + H+ + Cl−
(7)
3NO2 (aq) + H 2O ↔ 2H+ + 2NO3− + NO(aq)
(8)
(10)
NO2− + HOCl → NO3− + H+ + Cl−
(11)
HOCl ↔ H+ + OCl−
pK a = 7.54 (298 K)
•
•
OCl− + hv → O•− + Cl• •−
O
•
+ H 2O ↔ HO + OH
(12) (13)
HOCl + hv → HO + Cl
(14) −
(15)
It is known that the redox potentials of these free radicals (e.g., HO•, O•−, Cl•) were much higher than those of HOCl or OCl−. Therefore, the NO absorption rate of electrolyzed seawater could be substantially improved under UV irradiation. Furthermore, previous works25,26 showed that high concentrations of Cl− and HCO3− would act as scavengers of free radicals through eqs 16−22. They reacted with active free radicals (e.g., HO• and Cl•) to form CO3•− and Cl2•− radicals, which were often more selective and less reactive than HO• or Cl•.16
3. RESULTS AND DISCUSSION 3.1. Effects of Process Parameters on NO Absorption Rate. 3.1.1. Control Study in Different Systems. As NO removal by UV/chlorine was preliminarily investigated, contrast experiments of different absorption systems were performed in a lab-scale photochemical bubble reactor. The ACC was kept at 250 mg/L[Cl2] in various reaction systems, including UV/NaClO freshwater, UV/NaClO seawater, UV/electrolyzed seawater, and electrolyzed seawater. The results are shown in Figure 2. The NO absorption rate was achieved by different systems in following yield order UV/NaClO freshwater > UV/NaClO seawater ≈ UV/electrolyzed seawater > electrolyzed seawater. As can be seen, the NO absorption by UV without oxidants did not obtain significant NO absorption rate. As the photon energy of UV at wavelength of 254 nm was only 471 kJ/mol, the NO molecule (bond energy 678 kJ/mol) could not be decomposed directly, resulting in a low NO absorption rate. As shown in Figure 2, it is worth noting that the NO absorption rate by UV/electrolyzed seawater was eight times more than that by electrolyzed seawater. This result suggested that a pronounced synergistic effect on the NO absorption was achieved by combining UV and electrolyzed seawater. According to our previous experiments,13 the main oxidant component in electrolyzed seawater was active chlorine (HOCl/OCl−). Thus, NO was mainly removed by the HOCl in electrolyzed seawater at pH 6 (eqs 6−11). (6)
HNO2 ↔ H+ + NO2−
Then, a comparison experiment between electrogenerated chlorine and NaClO reagent under UV irradiation was carried out. As expected, it appears that the NO absorption rate of UV/electrolyzed seawater was almost equal to that of UV/ NaClO seawater. This confirmed that a large amount of strong oxidants were produced during the photodecomposition of active chlorine in seawater, which played an important role in NO removal from simulated flue gas. According to the literature,23,24 several active species (e.g., HO•, O•−, and Cl•) can be produced from UV photolysis of HOCl and OCl− according to the following eqs 12−15:
where kNO,L and kCO2,L are the liquid phase mass transfer coefficients of NO and CO2, respectively (m/s); DNO,L and DCO2,L are the liquid phase diffusion coefficients of NO and CO2, respectively (m2/s); kNO,G and kCO2,G are the gas phase mass transfer coefficients of NO and CO2, respectively (mol/(m2·s·Pa)); DNO,G and DCO2,G are the gas phase diffusion coefficients of NO and CO2, respectively (m2/s); aNO and aCO2 are the specific interfacial areas of NO and CO2, respectively (m2/m3). Table 1 summarizes the related physical and mass transfer parameters for NO absorption under conditions of solution temperature of 293 K and gas flow rate of 1.26 L/min.
NO(g) ↔ NO(aq)
(9)
HCO3− + HO• → H2O + CO3•−
(16)
HCO3− + Cl• → H+ + Cl− + CO3•−
(17)
HCO3− + Cl 2•− → H+ + 2Cl− + CO3•−
(18)
CO32 − + ClO• → ClO− + CO3•−
(19)
−
•
Cl + Cl ↔ Cl 2 −
•
•−
(20)
Cl + HO ↔ ClOH
•−
Cl− + ClOH•− ↔ Cl 2•− + OH− −
(21) (22)
HCO3−
As a result, Cl and ions in the artificial seawater consumed part of active free radicals. In this case, the NO absorption rate of UV/NaClO seawater was expected to be lower than that of UV/NaClO freshwater, which was in agreement with the results shown in Figure 2. 3.1.2. Effect of Active Chlorine Concentration. Figure 3 shows the effect of ACC in electrolyzed seawater on NO absorption rate. The target ACC of the absorbent was prepared by freshly diluting the electrolyzed seawater with artificial seawater. It can be seen that the NO absorption rate gradually increased from 2.6 × 10−5 to 3.3 × 10−5 mol/(m2·s) with increasing ACC from 250 to 1500 mg/L [Cl2]. As UV power was kept constant C
DOI: 10.1021/acs.energyfuels.7b00458 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels during the experiments, the same amount of photons were suggested to be absorbed by electrolyzed seawater. In such case, it can be implied that mainly the HOCl instead of free radicals in electrolyzed seawater increased with the increase of ACC. As a result, the NO absorption rate did not increase substantially with the increase of ACC. According to the literature,26 when the ACC was beyond a certain value, the excessive amount of active chlorine would react with active free radicals (eqs 23−26), which might, on the contrary, reduce the concentrations of free radicals. Furthermore, a high concentration of active chlorine would increase the yield of possible harmful by−products during decomposition process.14 Based on the fact that NO could be absorbed readily by free radicals,27,28 the results in Figure 3 implied that ACC of
Figure 4. Effect of UV power on NO absorption rate. Conditions: NO 1000 ppm; ACC 250 mg/L [Cl2]; temperature 293 K.
Figure 3. Effect of ACC on NO absorption rate. Conditions: NO 1000 ppm; UV 16 W; temperature 293 K.
250 mg/L [Cl2] was sufficient to absorb NO from simulated flue gas under UV irradiation in our experiments. Therefore, 250 mg/L [Cl2] was selected in the following experiments. HO• + HOCl → ClO• + H 2O
(23)
Cl• + HOCl → ClO• + H+ + Cl−
(24)
•
2ClO ↔ Cl 2O2
(25)
2Cl 2O2 + H 2O → ClO−3 + Cl 2O + OCl− + 2H+
(26)
Figure 5. Effect of NO inlet concentration on NO absorption rate. Conditions: ACC 250 mg/L [Cl2]; UV 16 W; temperature 293 K.
from 250 to 1250 ppm, the NO absorption rate significantly increased from 1.0 × 10−5 to 3.4 × 10−5 mol/(m2·s), increasing by 240%. According to the two−film theory,29 the mass transfer driving force of NO in gas−liquid two phase will increase with increasing NO inlet concentration (or NO partial pressure), thereby increasing the NO absorption rate. In addition, it can be observed that the NO absorption rate almost kept a linear relationship with NO inlet concentration, which might suggest that the absorption process is a fast reaction.30 3.1.5. Effect of SO2 Concentration. Figure 6 shows the effect of SO2 concentration on NO absorption rate. When the SO2 concentration increased from 200 to 1000 ppm, the NO absorption rate increased slightly from 2.9 × 10−5 to 3.0 × 10−5 mol/(m2·s). In addition, as SO2 was more soluble than NO, SO2 was completely removed from the simulated flue gas with the SO2 concentration varying from 200 to 1000 ppm (data not shown). Results showed that although SO2 would consume oxidants by competing with NO (eqs 27−30), concentrations of the UV−induced oxidants in electrolyzed seawater were sufficient to simultaneously absorb NO and SO2 in the photochemical bubble reactor.
3.1.3. Effect of UV Power. Figure 4 displays the effect of UV power on NO absorption rate. The UV power was regulated by changing the length of the covered alumina foil on the lamp, which was adopted to restrain the UV light irradiation. As shown in Figure 4, the NO absorption rate increased linearly with the increase of UV power. This phenomenon can be attributable to the increase of active free radicals generated from photodecomposition of active chlorine (eqs 12−15). According to the Beer−Lambert law, the photochemical reaction products are proportional to the UV irradiation intensity. As the UV lamp power is also proportional to the UV irradiation intensity in our experiments, more effective photons are emitted by increasing UV power. Hence, the free radicals yield produced from photodecomposition of active chlorine increased accordingly. 3.1.4. Effect of NO Inlet Concentration. The effect of NO inlet concentration on NO absorption rate is presented in Figure 5. As shown in Figure 5, when the NO inlet concentration increased
SO2 (g) ↔ SO2 (aq) D
(27) DOI: 10.1021/acs.energyfuels.7b00458 Energy Fuels XXXX, XXX, XXX−XXX
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electrolyzed seawater was converted to OCl− and ClO3− at high temperature, which caused a drop in the NO absorption rate by the UV/electrolyzed seawater. 2HOCl + OCl− → ClO3− + 2Cl− + 2H+ −
−
HOCl + OH ↔ H 2O + OCl
(28)
HSO3− + HO• ↔ SO4 2 − + 2H+
(29)
HSO3− + HOCl ↔ SO4 2 − + 2H+ + Cl−
(30)
pK a = 3000/T − 10 + 0.025T (32)
According to the Arrhenius law, one may expect that a straight line would be obtained between the logarithm of the NO absorption reaction rate constant (ln(km,n)) and 1/T. However, as shown in Figure 7b, the graph of ln(km,n) versus 1/T was found to be different in our experiments. As the extra consumption of active chlorine was accelerated by the side eqs 31 and 32, the NO absorption reaction rate constant was expected to decrease at 1/T = 0.00309/K (T = 323 K). However, it was remarkable to note that when the reaction temperature was in the range of 293−313 K, the ln(km,n) only fluctuated within less than 0.056. This could be explained that, on the one hand, in general, the primary photochemical reaction rates of free radicals formation (eqs 13 and 14) were almost independent of reaction temperature.34 On the other hand, most of the secondary reactions in NO removal process by the UV/electrolyzed seawater involved reactants of active free radicals, which usually had a relatively low activation energy.35 Therefore, it could be inferred that when the reaction temperature was below 313 K, the NO absorption rate constant was mainly dependent on UV−induced free radicals. While, at high temperature (>313 K), reaction pathways (eqs 31 and 32), which required relatively high activation energy, would have a significant impact on NO removal process. 3.2. Kinetics of NO Absorption. 3.2.1. Theory. As discussed above, the effective components for NO absorption in UV/electrolyzed seawater system at pH 6 were free radicals and HOCl. The total chemical reaction for NO removal by UV/ electrolyzed seawater process can be expressed as follows:
Figure 6. Effect of SO2 concentration on NO absorption rate. Conditions: NO 1000 ppm; ACC 250 mg/L [Cl2]; UV 16 W; temperature 293 K.
SO2 (aq) + H 2O ↔ HSO3− + H+
(31)
3.1.6. Effect of Reaction Temperature. Figure 7 displays the effect of reaction temperature (T) on NO absorption rate and NO absorption reaction rate constant (km,n). As can be seen in Figure 7a, when the reaction temperature was below 313 K, the NO absorption rate increased as the reaction temperature rose. However, when the reaction temperature further increased to above 313 K, the NO absorption rate decreased sharply. This might be due to the enhancement of the self-decomposition (eqs 31 and 32) of active chlorine in solution. According to the literature,31−33 the reaction rates of active chlorine decomposition (eqs 31 and 32) were remarkable under conditions of reaction temperature 323 K and pH about 7. Thus, more HOCl in
hv
2NO + 3HOCl + H 2O → 5H+ + 2NO3− + 3Cl−
(33)
21,34
According to the chemical reaction kinetic theory, NO absorption by UV/electrolyzed seawater can be regarded as a
Figure 7. (a) Effect of reaction temperature on NO absorption rate. (b) Effect of reaction temperature on NO absorption reaction rate constant. Conditions: NO 1000 ppm; ACC 250 mg/L [Cl2]; UV 16 W. E
DOI: 10.1021/acs.energyfuels.7b00458 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels partial j-order reaction for H2O, a partial m-order reaction for NO, and a partial n-order reaction for HOCl, respectively. The chemical reaction rate equation can be expressed as the following eq 34: NNO =
m n kj , m , nC Hi 2OC NO,i C HOCl
E=
(34)
Ha =
(
1 kNO,G
+
EHNO,LkNO,L
)
NO,G
NO,L NO,L
)
(44)
Through solving the above eqs 43 and 44, NO absorption rate could be expressed as the following eq 45: m+1 n ⎛ 2k D ⎞1/2 m , n NO,L C NO,i C HOCl ⎟⎟ NNO = ⎜⎜ m+1 ⎝ ⎠
(45)
The following eq 46 is established by making a logarithm of eq 45. (38)
n 1 2km , nD NO,L C HOCl m+1 ln + ln C NO,i 2 m+1 2
ln NNO =
The results in Figure 2, 3, and 4 indicated that NO was mainly removed by the oxidation of free radicals (e.g., HO•, Cl•) in UV/electrolyzed seawater system. As these active free radicals have strong oxidation ability, the chemical reaction rate of NO removal by UV/electrolyzed seawater would be fast.36,37 In such case, here, it was first supposed that the NO removal by UV/electrolyzed seawater was a fast reaction. This hypothesis would be validated in the following section 3.2.3. Based on two−film theory, NO was completely removed before it entered into the bulk liquid phase in a fast reaction, so that CNO,L = 0. Therefore, the NO absorption rate (eq 38) can be further simplified to the following eq 39: pNO,G NNO = 1 1 + EH k k
(
(43)
⎛ N ⎞ C NO,i = HNO,L⎜⎜pNO,G − NO ⎟⎟ kNO,G ⎠ ⎝
)
1
(42)
In which the NO interface concentration can be obtained by solving eqs 36 and 37:
(37)
HNO,L
D NO,L km , nC NO,i C HOCL
m−1 n ⎛ 2D ⎞1/2 NO,L k m , nC NO,i C HOCl ⎟⎟ = ⎜⎜ m+1 ⎝ ⎠
Then the NO absorption rate equation can be rearranged to the following eq 38 by combining the above eqs 36 and 37.
NNO =
m+1
HNO,L(pNO,G kNO,G − NNO)
where pNO,G is the partial pressure of NO in the bulk gas (Pa); pNO,i is the partial pressure of NO at gas−liquid interface (Pa); CNO,L is the NO concentration in the bulk liquid phase (mol/L); E is the chemical reaction enhancement factor. On the basis of Henry law, the equilibrium relationship at the gas−liquid interface can be obtained below:
C NO,L
HNO,L
NNOkNO,G
(36)
−
(41)
3.2.2. Reaction Order. To determine the reaction order m with respect to NO interface concentration, the NO absorption rate (eq 42) is transformed to the following eq 43:
NNO = kNO,G(pNO,G − pNO,i ) = EkNO,L(C NO,i − C NO,L)
NO,G
⎤1/2 1 ⎡ 2 m−1 n k D C C ⎢ m , n NO,L NO,i HOCl ⎥ ⎦ kNO,L ⎣ m + 1
NO,G
where km,n = kj,m,nCH2O is pseudo-(m + n)-order rate constant for NO and HOCl. Based on the two−film theory, the NO absorption rate can be presented by
(p
(40)
When the NO absorption process belongs to a fast reaction kinetic region (i.e., Ha > 3.0), there is E = Ha. Thus, the NO absorption rate (eq 39) can be further expressed as follows: pNO,G NNO = 1 1 + 2 m−1 n k
(35)
C NO,i = HNO,LpNO,i
tanh{Ha[(E i − E)/(E i − 1)]n /2 }
where Ei is the enhancement factor for the instantaneous reaction; Ha is defined as the ratio of the maximum possible conversion in the film to the maximum diffusion transport through the film, which is given by
where kj,m,n is (j + m + n) order rate constant for the overall eq 33; CH2O is H2O concentration in liquid phase (mol/L); CNO,i is the NO concentration at gas−liquid interface (mol/L); CHOCl is the ACC in bulk liquid phase (mol/L); j is partial reaction order for H2O; m is partial reaction order for NO; n is partial reaction order for HOCl. As H2O is the solvent of the bubbling liquid, CH2O is considered to be infinite. It can be regarded as a constant as compared to CNO,i and CHOCl. Thus, the chemical reaction of NO absorption can be regarded as a pseudo-m-order reaction for NO and a pseudo-n-order reaction for HOCl. The NO absorption rate equation can be simplified to the following eq 35: m n NNO = km , nC NO,i C HOCl
Ha[(E i − E)/(E i − 1)]n /2
(46)
The values of NNO and CNO,i are calculated using eqs 2 and 44. The relationship between the NNO and CNO,i is plotted in Figure 8. The results showed that ln(NNO) increased almost linearly with increasing ln(CNO,i). The slope, (m + 1)/2, was 0.6. The partial reaction order for NO, m, was 0.2, so the absorption reaction of NO by UV/electrolyzed seawater could be regarded as a partial-0.2-order reaction for NO when the NO inlet concentration changed from 250 to 1250 ppm. To determine the reaction order, n, with respect to active chlorine in electrolyzed seawater, eq 46 is rearranged to the following eq 47 by taking m = 0.2. ln
(39)
(NNO)5 3
(C NO,i)
=
5 ⎛ 5km , nD NO,L ⎞ 5n ln⎜ ln C HOCl ⎟+ 2 ⎝ 3 2 ⎠
(47)
The relationship between NNO/CNO,i and CHOCl is plotted in Figure 9. The results showed that ln(NNO)5 − ln(CNO,i)3
Furthermore, for an irreversible pseudo-m-order reaction, the chemical reaction enhancement factor E can be approximated by F
DOI: 10.1021/acs.energyfuels.7b00458 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels
In which the pseudo-(m + n)-order reaction rate constant km,n can be calculated by the following eq 50: km , n
−2 HNO,L ⎞ 3 −1 0.8 −0.4 ⎛ pNO,G HNO,L ⎟⎟ = D NO,L C NO,iC HOCl ⎜⎜ − 5 NNO kNO,G ⎠ ⎝
(50)
The main mass transfer reaction kinetic parameters are calculated and summarized in Table 2. The results indicate that Ha > 3.0 under all experiment conditions. These results verified that NO removal process by UV/electrolyzed seawater was a fast reaction. Therefore, the mass transfer rate instead of the chemical reaction rate is the main rate-determining step for the NO absorption by UV/electrolyzed seawater process. In addition, it can be seen from the NO absorption rate model (eq 48) that the NO absorption rate mainly depends on gas phase mass transfer coefficient, gas−liquid interface area, NO partial pressure, and chemical reaction rate, but is independent of the liquid phase mass transfer coefficient. Therefore, gas−liquid contact devices with a large specific surface area, such as spray tower, packed tower, and venturi reactor, will be more appropriate for enhancing the NO absorption by UV/electrolyzed seawater as compared to bubble column reactor. 3.2.4. Pseudo-0.6-Order Reaction Rate Constant and Validation of NO Absorption Rate Equation. The pseudo0.6-order reaction rate constant km,n under all experiments are calculated, and the results are shown in Table 2. It can be seen that the km,n increased with the increase of UV power and SO2 concentration, but decreased with the increase of ACC and NO inlet concentration. To facilitate the calculation of NO absorption rate, the km,n values are fitted into the following empirical eqs 51−54 with ACC, UV power, NO inlet concentration, and SO2 concentration, respectively.
Figure 8. Plot of ln(NNO) vs ln(CNO,i).
km , n , C HOCl = 0.361exp(C HOCl /574) + 36.9 R2 = 0.996, 250 ≤ C NO,inlet ≤ 1500 mg/L[Cl 2]
Figure 9. Plot of ln(NNO) − ln(CNO,i) vs ln(CHOCl). 5
3
km , n ,UV = 3.63UV − 11.6
increased almost linearly with increasing ln(CHOCl). The slope, 5n/2, was 1.1. The partial reaction order for HOCl, n, was 0.4, so the absorption reaction of NO by UV/electrolyzed seawater could be regarded as a partial-0.4-order reaction for HOCl when ACC concentration changed from 250 to 1500 mg/L [Cl2]. Based on the reaction order analysis, the NO absorption by UV/electrolyzed seawater can be regarded as a pseudo-0.6-order reaction. The NO absorption rate equation could be finally described by the following eq 48: pNO,G NNO = 1 1 + 5 0.4 −0.8 k NO,G
HNO,L
3
D NO,L km , nC NO,i
C HOCl
R2 = 0.976, 5.3 ≤ UV ≤ 16 W
1/2 1 ⎡5 −0.8 0.4 ⎤ ⎢⎣ km , nD NO,L C NO,iC HOCl ⎥⎦ kNO,L 3
(52)
km , n , C NO,inlet = 20.5exp( −C NO,inlet /720) + 50.8 R2 = 0.987, 250 ≤ C NO,inlet ≤ 1250 ppm
(53)
km , n , CSO = 2.19exp(CSO2 /664) + 53.4 2
2
R = 0.991, 200 ≤ CSO2 ≤ 1000 ppm
(54)
The comparison between the calculated values (by eqs 48, 51−54) and the experimental values of NO absorption rates under different experimental conditions are shown in Figure 10. It can be observed that the calculated values are in good agreement with the experimental values. The maximum error is 5.26% between the calculated values and the experimental values, which is acceptable in the study of gas−liquid reaction. Thus, NO absorption rate (eqs 48, 51−54) can be used to simulate the NO absorption process by wet scrubbing process using UV/electrolyzed seawater. These data will offer some meaningful guidance for the design and amplification of the reactor.
(48)
3.2.3. Mass Transfer Reaction Kinetic Parameters. As mentioned above in section 3.2.1, the NO removal process by UV/electrolyzed seawater was supposed to be a fast reaction, which meant the Ha should be large than 3.0 under all conditions. Based on the values of m and n, the previous Ha eq 41 can be further reduced to the following eq 49: Ha =
(51)
(49) G
DOI: 10.1021/acs.energyfuels.7b00458 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels Table 2. Kinetic Parameters of NO Absorption by UV/Electrolyzed Seawater NNO (×10−6 mol/m2·s)
km,n (mol0.4/(L0.4·s))
Ha
250 500 750 1000 1250 1500 5.3 10.7 16.0 250 500 750 1000 1250 200
25.7 28.2 29.7 30.9 31.9 33.0 15.1 22.2 27.8 10.0 17.5 23.6 29.1 34.3 29.2
37.4 37.9 38.4 38.9 40.1 41.9 9.0 24.7 47.7 65.2 61.6 57.7 55.7 54.7 56.2
15.3 18.4 20.5 22.3 24.1 25.9 6.7 11.9 17.8 45.7 30.1 23.3 19.7 17.4 19.8
400 600 800 1000
29.4 29.6 29.9 30.3
57.6 58.9 60.4 63.3
20.1 20.3 20.7 21.3
parameters CHOCl (mg/L [Cl2])
UV (W)
CNO,inlet (ppm)
CSO2 (ppm)
Figure 10. Comparison between calculated values and experimental values of NO absorption rate under NO inlet concentration (a), ACC (b), UV power (c), and SO2 concentration (d).
4. CONCLUSIONS An investigation of mass transfer reaction kinetics of NO absorption from simulated flue gas by UV/electrolyzed seawater at pH 6 was performed in a photochemical bubble reactor.
The results showed that the NO absorption rate increased gradually with the increase of UV power, ACC, and NO inlet concentration. Both SO2 concentration (200−1000 ppm) and reaction temperature (293−313 K) did not have an obvious H
DOI: 10.1021/acs.energyfuels.7b00458 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
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impact on NO absorption rate under conditions of ACC 250 mg/L [Cl2], UV 0.047 W/cm3, and NO inlet concentration 1000 ppm. The mass transfer reaction process analysis indicates that NO absorption rate mainly depends on gas phase mass transfer coefficient, gas−liquid interface area, NO partial pressure, and chemical reaction rate, but is independent of the liquid phase mass transfer coefficient. The total reaction of NO absorption by UV/electrolyzed may be regarded as a pseudo-0.6order reaction. The absorption process is a pseudo-0.2-order with respect to NO, and a pseudo-0.4-order with respect to active chlorine. NO absorption models (eqs 48, 51−54) are established, which are in a good agreement with the experimental values.
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AUTHOR INFORMATION
Corresponding Author
*Tel: +86 136 0411 6411. 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 National Natural Science Foundation of China (Grants 51402033 and 51479020), the Science and Technology Plan Project of China’s Ministry of Transport (Grant 2015328225150) and the Fundamental Research Funds for the Central Universities (Grant 3132016018), and the Scientific Research Fund of Liaoning Provincial Education Department of China (Grant L2014198).
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DOI: 10.1021/acs.energyfuels.7b00458 Energy Fuels XXXX, XXX, XXX−XXX
