New Experimental Results of NO Removal from Simulated Flue Gas

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New Experimental Results of NO Removal from Simulated Flue Gas by Wet Scrubbing Using NaClO Solution Zhitao Han, Shaolong Yang, Xin-xiang Pan, Dongsheng Zhao, Jingqi Yu, Yutang Zhou, Pengfei Xia, Dekang Zheng, Yonghui Song, and Zhijun Yan Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b03062 • Publication Date (Web): 23 Jan 2017 Downloaded from http://pubs.acs.org on February 7, 2017

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New Experimental Results of NO Removal from Simulated Flue Gas by Wet Scrubbing Using NaClO Solution Zhitao Han,* Shaolong Yang, Xinxiang Pan, Dongsheng Zhao, Jingqi Yu, Yutang Zhou, Pengfei Xia, Dekang Zheng, Yonghui Song, Zhijun Yan Marine Engineering College, Dalian Maritime University, Dalian 116026, P. R. China

ABSTRACT: Both cyclic and non-cyclic scrubbing experiments were conducted to remove NO from simulated flue gas in a lab-scale countercurrent spraying reactor. The effects of various operating parameters (initial solution pH, NaClO concentration, absorbent temperature, inlet NO and SO2 concentrations) on NO removal efficiencies were investigated in the non-cyclic scrubbing mode. The results showed that NO removal efficiency increased greatly with the decrease of the initial pH value. However, there was a drop in NO removal efficiency, possibly due to the NO2 absorption in the gas phase. NO removal efficiency increased gradually with the increase of the NaClO concentration. Complete removal of NO was achieved when the NaClO concentration was 24×10-3 mol·L-1. However, the NO removal efficiency obviously decreased with the absorbent temperature increasing from 303 to 343 K. This could be mainly ascribed to the decrease of the water solubility of NO2 and the absorption reaction of NO2 with water vapor. Furthermore, NO removal efficiency increased quickly with the increase of the inlet NO concentration. The coexisting SO2 in the simulated flue gas had little effect on the NO removal efficiency. Both NO and SO2 could be removed simultaneously with a SO2 removal efficiency of 100% and a NO removal efficiency of > 93 %. More importantly, the results of cyclic scrubbing experiments indicated that an average NO removal efficiency of 74 % had been obtained for the whole cyclic scrubbing duration. The utilization of NaClO oxidant 1 / 29

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was calculated to be approximately 83 %. The ion chromatographic analysis showed that there was no ON 2− in the spent liquor. The results demonstrate that NaClO is a low-cost high-efficiency oxidant for NO removal from exhaust gas and that it has great potential for industrial applications.

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1. INTRODUCTION During the past few decades, air pollution has been a hot topic that has increasingly attracted people’s attention. The emissions from the combustion of fossil fuels contain a large amount of air pollutants, such as SOx, NOx, and PMs.1,2 This has resulted in serious environmental pollution around the world, especially in the developing countries.3 With the introduction of stringent environmental laws and regulations, various air emission control technologies are developing rapidly. For the removal of SOx, the flue gas wet scrubbing method is one of the best choices for stationary and mobile sources.4 However, the question of how to reduce NOx emission effectively poses a major challenge. Among the numerous NOx reduction technologies, selective catalytic reduction (SCR) and selective non-catalytic reduction (SNCR) are two widely-used approaches. SCR can reduce NOx into N2 in the presence of a catalyst and a reductant (NH3 or urea), and its NOx removal efficiency can reach up to 80-95%.5-9 However, it usually requires a high operating temperature (300-600 ℃) for catalyst activation. It also requires a large installation space and incurs high investment costs. Ammonia slip is another concern that cannot be ignored. For SNCR, a very high reaction temperature of 850-1100 ℃ is required. Meanwhile, elaborate temperature control is crucial for avoiding ammonia slip.10-12 Although the combination technology of wet scrubbing flue gas desulfurization and SCR (or SNCR) denitrification can simultaneously remove SO2 and NOx, it still faces some obvious limitations on its extensive application due to the large and complex system, and the high capital and operating costs. In recent years, numerous researchers have shown interest in removing NOx from flue gas by wet scrubbing in order to simultaneously remove SOx and NOx, together with other pollutants.13,14 However, NO that accounts for more than 95% of NOx is of very 3 / 29


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low solubility.15 It requires oxidizing NO into higher-order NOx species that are soluble in water. From this point of view, various oxidants, including H2O2,16-18 KMnO4,19 O3,20-23 nonthermal plasma,24,25 oxone,26,27, Fe(II)-EDTA,28,29 Na2S2O8,30-34 NaClO2,35-38, NaClO,39-42 ClO2,43,44 and Ca(ClO)2,45 have been investigated in order to improve the NO removal efficiency of the wet scrubbing method. Compared with other oxidants, NaClO has some unique merits such as low cost, strong oxidation power, and easy storage and transportation. However, there is little research work on NO removal by wet scrubbing using the sole NaClO solution. Ghibaudi E et al. studied the reaction of NO with a NaClO solution over the pH range of 6-12.46 A fast chain reaction between NO and HClO was proposed for NO removal. The reaction of NO with ClO- was estimated to be 30-fold slower than that of NO with HClO. Chen L et al. claimed that NO did not react with ClO-, and they studied NO removal in a packed tower with a NaClO solution.39,47 The result showed that NaClO was an excellent oxidant at pH 5.3. Mondal M K and co-workers conducted experiments in a magnetic stirrer reactor to investigate the simultaneous removal of SO2 and NO using a NaClO solution.40,41 The effects of various operating parameters on removal efficiency were discussed. Guo R et al. studied the absorption kinetics of NO in a weakly acidic NaClO solution in a stirred tank reactor.48 The reaction was found to be first-order with respect to both NO and NaClO. Yang S et al. investigated the enhancement effect of UV irradiation on NO removal efficiency using NaClO in a spraying reactor.49 Some researchers are also interested in adopting a NaClO-based complex absorbent to remove NO and SOx simultaneously, aiming to keep a low operating cost and to obtain high removal efficiency.50,51 Therefore, NaClO as an efficient oxidizing agent shows great potential for practical application. However, research on the effects of critical process parameters on NO removal by wet scrubbing 4 / 29


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using a NaClO solution seems to be insufficient. In this study, non-cyclic scrubbing experiments were conducted to investigate the effects of various operating variables (initial solution pH, NaClO concentration, absorbent temperature, inlet NO and SO2 concentrations) on NO removal efficiency, and the possible reaction mechanism is discussed. Furthermore, cyclic scrubbing experiments were performed to evaluate the average NO removal efficiency and the oxidant utilization during the whole scrubbing process. The ion chromatography results were used to analyze the anion compositions in the spent liquor. Finally, the results of the present study are compared with the most-studied chlorinated oxidants in the literature. 2. EXPERIMENTAL SECTION 2.1. Experimental Apparatus. As shown in Figure 1, the experimental system consists of three parts: a simulated flue gas system, a countercurrent spraying system, and a gas analyzing system. The spraying system mainly consists of a countercurrent spraying column, a peristaltic pump and a water bath. The spraying column, made of polymeric methyl methacrylate (PMMA), is installed with a fine-droplet spray nozzle (B1/4TT-SS+TG-SS0.4, Spraying System Co.) at the upper part and a gas distributor at the lower part. The height and inner diameter of the column are 25 cm and 5 cm, respectively. A constant water bath (F-34, JULABO Labortechnick GmbH) is used to control the solution temperature. The solution temperature is measured using a mercury thermometer. A semiconductor condenser is used as a drier to dry the gas before it enters the gas analyzer. The gas analyzer (MGA-5, MRU GmbH) is used to determine the concentrations of NO, NO2, and SO2 in the simulated flue gas.

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Figure 1. Schematic diagram of the experimental system: (A) primary gas path and (B) gas bypass.

2.2 Experimental Procedures. NaClO solutions were prepared using a commercial NaClO solution (5% available chlorine concentration, Shanghai Aladdin Co.) and deionized water. The deionized water was prepared in a two-stage ELGA PURELAB Option R15 purification system and had a resistivity of 15 MΩ·cm. NaClO concentrations were in the range of 4×10-3 - 24×10-3 mol·L-1. The initial pH values of the scrubbing solution were adjusted by titrating 1 mol·L-1 HCl solution and were determinated using a pH meter (SevenCompact, Mettler-Toledo Co.). Three kinds of gases, N2 pure gas (purity of 99.999%), NO span gas (10% NO, 90% N2 as balance gas), and SO2 span gas (9.8% SO2, 90.2% N2 as balance gas), were purchased from Dalian Guangming Specialty Gas Co. They were provided from separate air bottles and metered through mass flow controllers (MFCs). The synthetic flue gas was obtained from the feed gases by blending with an online mixer, and then it was introduced into the countercurrent spraying column. The inlet concentrations of pollutants were measured using the flue gas analyzer through the gas bypass B. The 6 / 29


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solution temperatures were adjusted to the required ones by the constant temperature water bath and the mercury thermometer. When the solution temperatures and the inlet concentrations of the pollutants in the simulated flue gas reached the set values, the three-way valve was turned to allow the simulated flue gas to enter the spraying column through the primary route A. The simulated flue gas flowed through the column continuously, and the calculated residence time of the flue gas in the column was 19.6 s. When the system was running in a non-cyclic mode, a fresh solution was fed into the column by a peristaltic pump (WT-600, Baoding Lange Pump Co.) and then collected in a beaker below the column. The flow rate of the solution was approximately 0.3 L·min-1. The size of the liquid droplets sprayed from the nozzle was in the range of 80-100 µm. The outlet flue gas was dried by the semiconductor condenser and then entered the flue gas analyzer. Each experiment run was maintained for 20 min. The concentrations of the targeted pollutants were measured and recorded every 10 s. When the system was working in a cyclic mode, a 0.2 L solution with a NaClO concentration of 20×10-3 mol·L-1 was used. The sprayed solution was collected and returned to the baker in the water bath for cyclic use. The experiments ended when the NaClO in the solution was depleted completely and the outlet NO concentration recovered to the initial value. The spent solutions from the cyclic scrubbing runs were analyzed by an ion chromatograph (ICS-1500, Dionex) to determine the anions. 2.3 Removal Efficiency. The concentrations measured by bypass B are the inlet concentrations of the pollutants. The concentrations in the exit flue gas measured by the outlet gas of the column are the outlet concentrations of the pollutants. The removal efficiencies of the pollutants are calculated by the following equation:52

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η =

Cin − C out × 100% Cin

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(1)

where η is the removal efficiency of the targeted pollutant (%), and Cin and Cout are the inlet and outlet concentrations of the targeted pollutants (ppm). 3. RESUTLS AND DISCUSSION 3.1. Process Chemistry For NO removal, the effective compositions in the NaClO solution are the available chlorine or active chlorine species, mainly including HClO, ClO- and Cl2. They can be produced through the chlorine hydrolysis. The reaction chemistry of the chlorine hydrolysis is described as follows: 40 Cl2(g ) → Cl2(l)

(2)

Cl2(l) + H 2 O(l) → HClO(l)+ Cl −(l) + H(+l)

(3)

HClO(l) ↔ ClO −(l) + H +(l)

(4)

Cl2(l) + Cl −(l) ↔ Cl3−(l)

(5)

It is known that the fractional compositions of HClO, ClO- Cl2 and Cl3− in a NaClO solution depend largely on the solution pH. Figure 2 shows the change of factional compositions of various chlorine species as functions of pH in a NaClO aqueous solution.53 It clearly shows that when the pH of the NaClO solution is in the range of 3-7.5, HClO constitutes the majority of the chlorine species. Although all three chlorine species, HClO, ClO- and Cl2, exhibit oxidizing capacity to some extent, NO can obviously be oxidized by HClO and Cl2. Some relevant gas-phase, aqueous-phase, and interfacial reactions are summarized for NOx oxidation and absorption in Table 1.15,39,40

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Figure 2. Fractional compositions of chlorine species in NaClO aqueous solution as functions of pH. Conditions: solution temperature, 298 K; Cl − concentration, 0.1 mol·L-1; ambient pressure, 1 atm.

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Table 1. The main chemical reactions of NOx absorption in NaClO solution. Gas-phase Reactions 2NO 2(g ) ↔ N 2O 4(g )

(6)

NO(g) + NO 2(g) + H 2 O(g) ↔ 2HNO 2(g)

(7)

3NO 2(g ) + H 2 O(g ) → 2HNO 3(g ) + NO(g )

(8)

2NO(g ) + Cl2(g ) ↔ 2NOCl(g )

(9)

Interfacial and Liquid-phase Reactions NO(g ) + HClO(l) → NO 2(l) + HCl(l)

(10)

Cl2(l) → Cl2(g)

(11)

NOCl(g ) ↔ NOCl(l)

(12)

NOCl(l) + H 2 O(l) ↔ HNO 2(l) + HCl(l)

(13)

2NO 2(l) + H 2 O(l) ↔ HNO 3(l) + HNO 2(l)

(14)

NO 2(l) ↔ NO 2(g )

(15)

HNO 2(l) + ClO(−l) ↔ HNO 3(l) + Cl(−l)

(16)

HNO 2(l) + HClO(l) ↔ HNO 3(l) + HCl(l)

(17)

3HNO 2 ↔ HNO 3(l) + 2NO(l) + H 2 O(l)

(18)

N 2 O 4(g ) → N 2 O 4(l)

(19)

N 2 O 4(l) + H 2 O(l) → HNO 3(l) + HNO 2(l)

(20)

Due to its low solubility, NO in flue gas is oxidized into NO2 or NOCl in the gas phase or at the gas-liquid interface. Both NO2 and NOCl are much more water soluble than NO. NO2 will react in water to form HNO3 and HNO2. HNO2 in the solution might be further oxidized into nitrates. NOCl will react in water to quickly form 10 / 29


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HNO2. The total NO removed by the absorbing solution is in the form of NO2, N2O4, HNO2, NO 2− , HNO3, and NO 3− , with NO 3− being the major nitrogen species present in the liquid phase. 3.2. Effects of initial pH of NaClO solution on NO removal efficiency. The effects of solution pH on NO removal efficiency were studied, and the results are shown in Figure 3. It can be seen from Figure 3 that with the decrease of the solution pH from 10 to 7, the NO removal efficiency obviously increased. The NO removal efficiency was 91 % when the initial pH was at 7. When the solution pH was higher than 10, the active chlorine in NaClO existed in the form of ClO-. The reaction of NO with ClOwas very slow at the gas-liquid interface, resulting in a very low NO removal efficiency in the strong alkaline medium.47 The fractional concentration of HClO began to increase quickly with the decrease of solution pH. HClO could oxidize NO into NO2 effectively, so the NO removal efficiency greatly increased. However, there was a decline in NO removal efficiency as the solution pH decreased from 7 to 6. This might be ascribed to the NO2 absorption in the solution. At pH 7, the absorbed NO2 reacted with water and turned into HNO3 and HNO2, as shown in eq 14. When the initial pH decreased to 6, the fractional concentration of HClO became much higher compared with that at pH 7. At that moment, NO in the flue gas could be oxidized into NO2 more effectively. However, the weak acidic medium led to the reduction of NO2 absorption in the liquid phase. Part of the NO2 that escaped from the solution could react with water vapor in eq 8, resulting in the regeneration of NO in the exit gas. Although it has been reported that the relative oxidation power of these chlorine species is ClO- > HClO > Cl2 in neutral or acidic medium, there is no evidence that ClO- can react with NO effectively.54 Upon further decreasing the solution pH from 6 to 3, complete removal of NO had been achieved. This was because the fractional 11 / 29


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concentration of Cl2 increased quickly with the initial pH decreasing from 6 to 3. During the scrubbing process, Cl2 would easily be purged from the solution and then oxidize the regenerated NO effectively in the gas phase. Thus, complete NO removal could be achieved when the solution pH was below 5. The results suggest that it is easy to obtain high NO removal efficacy in acidic conditions. However, it was apparent that problems would arise if an acidic NaClO solution was used to remove NO from the flue gas. Excessive Cl2 might escape from the scrubbing reactor into the atmosphere when the solution pH is in the range of 3-5, resulting in secondary pollution. At the same time, it would reduce the NaClO utilization greatly. On the other hand, acidic solution is unfavorable for the absorption of acidic oxides of SO2 and NO2. One more stage for the simultaneous removal of SOx and NOx might be required. Thus, it is recommended to adjust the initial pH of the NaClO solution to 7 for the practical application, as it is relatively easy to obtain a high NO removal efficacy and to keep a low operating cost at the same time. Here, an initial NaClO pH of 7 was chosen for the subsequent experiments.

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Figure 3. Effect of initial pH of NaClO solution on NO removal efficiency and outlet NO2 concentration. Conditions: NO concentration, 1000 ppm; gas flow, 1.5 L·min-1; solution temperature, 298 K; NaClO concentration, 20×10-3 mol·L-1; non-cyclic scrubbing mode.

3.3. Effects of NaClO concentration on NO removal efficiency. Experiments were conducted to investigate the effects of NaClO concentration on NO removal efficiency. The results are shown in Figure 4. When the initial pH of the NaClO solution was 7, the NO removal efficiencies increased obviously with the increase of the NaClO concentrations in the aqueous solution. Complete removal of NO was achieved when the NaClO concentration was 24×10-3 mol·L-1. This indicates that a high NO removal efficiency could be achieved with a relatively high NaClO concentration at a neutral condition. This is mainly because the increase of the NaClO concentration would promote efficient liquid-phase mass transfer, which is beneficial for the oxidation of NO by HClO at the gas-liquid interface. For comparison, NaClO solutions without adjusting the solution pH were also used to scrub the flue gas. As shown in Figure 4, the NO removal efficiencies for scrubbing the solution with various NaClO concentrations were < 1%. Table 2 shows the change of the NaClO solution pH before and after the scrubbing process. It can be seen that the initial pH values of the NaClO solution before scrubbing were higher than 10.5. The available chlorine species existed in the form of ClO-. Therefore, the strong alkaline medium greatly suppressed the oxidation power of the NaClO solution, resulting in a very low NO removal efficiency.

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Figure 4. Effect of NaClO concentration on NO removal efficiency. Conditions: NO concentration, 1000 ppm; gas flow, 1.5 L·min-1; solution temperature, 298 K; non-cyclic scrubbing mode.

Table 2. The pH of the NaClO aqueous solution before and after the scrubbing process. Conditions: NaClO concentrations, 4×10-3-24×10-3 mol·L-1; solution temperature, 298 K. NaClO concentrations 4

8

12

16

20

24

Initial pH of the NaClO solution before scrubbing

10.57

10.83

11.02

11.15

11.27

11.35

The pH of the NaClO solution after scrubbing

10.48

10.74

10.92

11.02

11.12

11.23

(10-3 mol·L-1)

3.4. Effects of absorbent temperature on NO removal efficiency. A relatively low NaClO concentration might clearly and easily exhibit the effects of absorbent temperature on NO removal efficiency. A NaClO solution with an initial concentration of 4×10-3 mol·L-1 and an initial pH of 7 was adopted to scrub the flue gas. The

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experimental results are presented in Figure 5. With the absorbent temperature increasing from 303 to 343 K, the NO removal efficiency decreased from 53 to 9 %. Normally, with the increase of the solution temperature, the chemical reaction rate would be enhanced according to the Arrhenius equation, thereby promoting the NO removal efficiency. The oxidation of NO by NaClO absorbent involved mass transfer and gas-liquid reactions. The absorbent temperature had an important impact on the mass transfer of NOx between the bulk of the gas phase and the bulk of the liquid phase, which was governed by the diffusion, dissolution and ionization behaviors of the active species in the liquid phase.40 The increase in absorbent temperature would enhance the mass transfer, which was favorable for NO oxidation. However, the increase in solution temperature would also promote the ionization of HClO into ClO-, which would obviously decrease the NO oxidation because the reaction rate of NO and ClO- was much slower than that of NO and HClO, as mentioned above.46 But it was notable that the increase of absorbent temperature might not decrease the oxidant utilization to some extent. Furthermore, the increase of absorbent temperature would reduce the NOx solubility in the aqueous solution. It imposed a negative effect on the mass transfer at the gas-liquid interface. At that moment, more NO2 escaped from the liquid phase and reacted with water vapor in the gas phase to give off NO through eq 8. Another side reaction could also be taken into consideration. The increase of absorbent temperature might promote the decomposition of HNO2 into HNO3 and NO, as in eq 18. Thus, it was necessary to control the NaClO absorbent temperature during the scrubbing process in order to achieve a high NO oxidation efficiency.

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Figure 5. Effect of absorbent temperature of NaClO solution on NO removal efficiency and outlet NO2 concentrations. Conditions: NO concentration, 1000 ppm; gas flow, 1.5 L·min-1; NaClO concentration, 4×10-3 mol·L-1; initial solution pH, 7; non-cyclic scrubbing mode.

3.5. Effects of initial NO concentration on NO removal efficiency. The results of the experiments performed to test the effects of initial NO concentration on NO removal efficiency are illustrated in Figure 6. It shows that with the initial NO concentrations increasing from 200 to 1000 ppm, the NO removal efficiency increased quickly from 14 to 53 %, and the NO2 concentration in the outlet gas increased gradually from 6 to 147 ppm. The increase of NO removal efficiency could mainly be attributed to the enhancement of the gas-liquid mass transfer. According to the two-film theory, the absorption rate of NO can be expressed as:52 N NO = κ NO,G(p NO − p NO,i )

(21)

where NNO is the absorption rate of NO (mol·m-2·s-1), kNO,G is the gas-phase mass transfer coefficient of NO (mol·m-2·s-1·Pa-1), pNO is the NO partial pressure in the gas body (Pa), and pNO,i is the NO partial pressure in the phase interface (Pa). An increasing NO concentration (NO partial pressure) could raise the mass-transfer 16 / 29


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driving force of NO, thereby improving the NO removal efficiency.

Figure 6. Effect of initial NO concentration in simulated flue gas on NO removal efficiency and outlet NO2 concentrations. Conditions: gas flow, 1.5 L·min-1; solution temperature, 298 K; initial solution pH, 7; NaClO concentration, 4×10-3 mol·L-1; non-cyclic scrubbing mode.

3.6. Effects of initial SO2 concentration on NO removal efficiency. The effects of SO2 concentrations on NO removal efficiency were investigated, and the results are presented in Figure 7. It can be observed from Figure 7 that with the increase of inlet SO2 concentrations from 0 to 600 ppm, the NO removal efficiency was > 93% and was relatively stable. SO2 had been removed completely, indicating that the SO2 absorption was independent of initial SO2 concentrations. As to the SO2 absorption in the NaClO aqueous solution, the following reactions can be presumed:40 SO 2(g ) → SO 2(l)

(22)

SO 2(l) + H 2 O(l) → HSO 3−(l) + H(+l)

(23)

HSO 3−(l) ↔ SO 32(-l) + H(+l)

(24)

HSO 3−(l) + ClO(−l) ↔ SO 24(-l) + H(+l) + Cl(−l)

(25) 17 / 29


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SO 32(−l) + ClO(−l) ↔ SO 24(-l) + Cl(−l)

(26)

HSO3−(l) + HClO(l) ↔ SO 24(-l) + 2H(+l) + Cl(−l)

(27)

SO 32(−l) + HClO(l) ↔ SO 24(-l) + H(+l) + Cl(−l)

(28)

SO2 would be absorbed and hydrolyzed efficiently in a neutral medium. The active chlorine species (HClO and ClO-) in the NaClO solution would quickly oxidize the SO2 hydrolysis products of HSO 3− and SO32 − into SO24 − . This result suggests that the NaClO solution could simultaneously remove NO and SO2 from the flue gas with high efficiency. The initial SO2 concentration had little effect on the NO removal efficiency.

Figure 7. Effect of initial SO2 concentration in simulated flue gas on NO removal efficiency and SO2 removal efficiency. Conditions: NO concentration, 1000 ppm; gas flow, 1.5 L·min-1; solution temperature, 298 K; initial solution pH, 7; NaClO concentration, 20×10-3 mol·L-1; non-cyclic scrubbing mode.

3.7. NO removal by cyclic scrubbing using a NaClO solution. The experiments described above were conducted in a non-cyclic scrubbing mode. The results 18 / 29


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demonstrated that the NaClO solution was a highly efficient oxidizing agent that could be used to remove NO from simulated flue gas effectively. That the molar concentration of NaClO in fresh solution was much higher than that of NO in flue gas means there was still large amount of NaClO left in the sprayed solution. Aiming to examine the utilization of NaClO oxidant, cyclic scrubbing experiments were performed with a 0.2 L 20×10-3 mol·L-1 NaClO solution. The initial solution pH was adjusted to 7. The changes in NO removal efficiency and NO2 concentration in the outlet gas with the scrubbing duration are shown in Figure 8. It can be seen that at the start of the scrubbing process, the NO removal efficiency increased sharply to 96 % and then began to decrease gradually to 68 % after cyclic scrubbing for 20 min. The decrease of NO removal efficiency was mainly due to the decrease of the solution pH during the scrubbing process, as shown in Figure 9. With the initial pH decreasing from 7 to 6 at the beginning of scrubbing process, the NO removal efficiency began to decrease, which agrees well with the experimental results shown in Section 3.2. The solution pH decreased further as the scrubbing process proceeded. Complete NO removal was expected when the solution pH was below 5. At that moment, Cl2 escaping from the liquid phase would oxidize NO efficiently in the gas phase. After scrubbing for more than 40 min, the NO removal efficiency began to decrease gradually due to the significant decrease of NaClO oxidants. The NO concentration in the exit gas recovered to the initial level after scrubbing for > 70 min. This implies that the NaClO in the solution had been depleted at that moment.

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Figure 8. The changes of NO removal efficiency and outlet NO2 concentrations with scrubbing duration. Conditions: NO concentration, 1000 ppm; gas flow, 1.5 L·min-1; solution temperature, 298 K; initial solution pH, 7; NaClO concentration, 20×10-3 mol·L-1; NaClO solution volume, 0.2 L; cyclic scrubbing mode.

Figure 9. The change of the NaClO solution pH with scrubbing duration.

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Energy & Fuels

To better understand the process chemistry of NO oxidation and absorption, the anions in the spent liquor were determined using ion chromatography. The results of the ion chromatographic analysis are depicted in Figure 10. It can be seen that the major anions in the spent liquor were Cl− , NO 3− , and ClO 3− . Most of Cl− may have come from the fabrication process of NaClO rather than from the denitration reaction. That no NO 2− was found in the spent liquor suggests that the product of NO absorption by the NaClO solution was mainly in the form of NO 3− . According to the results of the ion chromatographic analysis, the molar quantity of NO 3− in the spent liquor was calculated to be approximately 0.72×10-3 mol. Since the concentration of NO in the outlet gas was measured every 10 s during the whole cyclic scrubbing process, which lasted for nearly 71 min, an average NO removal efficiency of approximately 74 % for the cyclic scrubbing process was calculated by averaging the measuring values. The molar quantity of NO that entered the scrubbing reactor during the whole cyclic scrubbing process was calculated to be 4.59×10-3 mol by multiplying the flow rate of the flue gas (1.5 L·min-1), the NO initial concentration (1000 ppm), and the scrubbing duration (71 min). Similarly, the molar quantities of NO and NO2 in the outlet gas were calculated by integrated calculation of the measuring points during the scrubbing process, which were approximately 1.27×10-3 mol and 0.98×10-3 mol, respectively. Thus, the NaClO utilization UNaClO might be calculated approximately as: U NaClO =

m in,NO − m out,NO m NaClO

× 100%

(9)

where m in,NaClO is the molar quantity of NO that entered the scrubbing reactor, m out,NO is the molar quantity of NO that escaped in the outlet gas, and m NaClO is the initial molar quantity of NaClO in aqueous solution. Here, UNaClO was calculated to be 83 %. 21 / 29


Energy & Fuels

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Therefore, both high NO removal efficiency and NaClO utilization had been achieved at the same time for cyclic scrubbing using the NaClO solution. The sums of the molar quantities of NO 3− in the spent liquor, NO and NO2 in the outlet gas were much lower than that of NO in the inlet gas. This suggests that there was a certain amount of NOx species (such as N2O4) other than NO and NO2 in the exit gas, according to the nitrogen balance. Thus, a subsequent treatment process might be required to remove NOx completely.

Figure 10. Ion chromatograms of the absorption solution.

3.8. Comparison of the results of the present study with some published works. The results of the present study were compared with some published works regarding to the NOx removal, as shown in Table 3. The latest prices of the oxidants were acquired from a number of Chinese suppliers. Here, the quotations with a unit of ¥·10-3 mol were adopted in order to compare the prices explicitly. It can be seen that the price of NaClO is much lower than that of NaClO2 or ClO2. Since the absorption conditions adopted by various researchers are quite different from each other, it was difficult to compare the NOx removal efficiencies of various chlorinated oxidants. The 22 / 29


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Energy & Fuels

experimental results of the present work show that complete removal of NO can be achieved during a non-cyclic scrubbing process. More importantly, the approximate calculation of oxidant utilization according to the cyclic-scrubbing experimental results indicates that ~ 83 % of the NaClO in the solution contributed to the oxidization of NO into NO2. This demonstrates that NaClO could be used as a low-cost high-efficiency oxidant for NO removal and that it has great potential for industrial applications, such as onshore power plants and ocean-going ships.

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Table 3 Comparison of the results of the present study with some published works Oxidant Price S. No.

Absorbent

(RMB ¥·10-3 mol)

Conditions

Maximum NOx Removal efficiency

Oxidant utilization

(%)

(%)

NO: 100

~ 83

the present study

NO: 92

-

[40]

NO: ~82

-

[39]

NO: 100

-

[55]

NO: 100

-

[56]

NOx: 75.9

-

[45]

NO: 100

-

[43]

Reference

Spray reactor Cyclic scrubbing 1

NaClO

480

1000 ppm NO 0.02 mol·L-1 NaClO pH=7 Magnetic stirrer reactor 816.67 ppm NO

2

NaClO

480

0.01 mol·L-1 NaClO pH=5.8 Packed column 100 ppm NO

3

NaClO

480 1.2 wt% NaClO pH=5.3 Bubbling reactor 760 ppm NO

4

NaClO2

1270

0.2 mol·L-1 NaClO pH=4 Swirl wet system 280 ppm NO

5

NaClO2

1270

0.2 mol·L-1 NaClO pH=6 Double bubbling reactor 728 ppm NO

6

Ca(ClO)2

390 0.1 wt% Ca(ClO)2 pH=12 Double bubbling reactor 728 ppm NO

7

ClO2

2700 0.1 wt% Ca(ClO)2 pH=12

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Energy & Fuels

4. CONCLUSION The effects of initial solution pH, NaClO concentration, absorbent temperature, inlet NO and SO2 concentrations on NO removal efficiency were investigated in a lab-scale scrubbing reactor in a non-cyclic scrubbing mode. The results showed that the solution pH had a great impact on the oxidation and absorption of NO. Complete removal of NO could be achieved using 20×10-3 mol·L-1 NaClO solution at a pH below 5. NO removal efficiency increased with the increasing of the NaClO concentration at pH 7. Complete removal of NO could also be obtained when the NaClO concentration was higher than 24×10-3 mol·L-1. With the increase of absorbent temperature, the NO removal efficiency decreased obviously, possibly due to the decrease of NOx solubility. With the initial NO concentrations increasing from 200 to 1000 ppm, the NO removal efficiency increased quickly from 14 to 53 %. In addition, the NaClO solution could remove NO and SO2 simultaneously with very high efficiency. When the SO2 concentration was in the range of 100-600 ppm, it did not obviously influence the NO removal efficiency. More importantly, cyclic scrubbing experiments were conducted using a 0.2 L 20 mmol·L-1 NaClO solution. The scrubbing process lasted for > 70 min, and the average NO removal efficiency was 74 %. The result of ion chromatographic analysis indicated that there was no NO 2− in the spent solution. The approximate calculation of molar quantities of nitrogen materials implied that there were certain amounts of other NOx species than NO and NO2 in the exit gas. The NaClO utilization was calculated to be approximately 83 %, according to the cyclic scrubbing experimental result. This demonstrates that NaClO could be used to remove NO with high efficiency and high utilization. Thus, there is great potential for wet scrubbing using a NaClO solution to remove NO effectively and economically for industrial applications. 25 / 29


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AUTHOR INFORMATION Corresponding Author *Telephone: +86-138 9869 2035. E-mail: [email protected] (Z.H.). Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS This study was financially supported by the National Natural Science Foundation of China (51402033, 51479020), the Science and Technology Plan Project of China's Ministry of Transport (2015328225150), the Scientific Research Fund of Liaoning Provincial Education Department of China (L2014198), and the Fundamental Research Funds for the Central Universities (3132016018, 3132016326). REFERENCES (1) Kan, H. D.; Chen R, J.; Tong, S. L. Environ. Int. 2012, 42, 10-19. (2) Skalska, K.; Miller, J.S.; Ledakowicz, S. Sci. Total. Enviro. 2010, 408, 3976-3989. (3) Wei, J. C.; Yu, P.; Cai, B.; Luo, Y. B.; Tan, H. Z. J. Ind. Eng. Chem. 2009, 15, 16-22. (4) Yang, L.; Bao, J.; Yan, J.; Liu, J. H.; Song, S. J.; Fan, F. X. Chem. Eng. J. 2010, 156, 25-32. (5) Kwak, J. H.; Tonkyn, R. G.; Kim, D. H.; Szanyi, J.; Peden, C. H. F. J. Catal. 2010, 275, 187-190. (6) Guan, B.; Zhan, R.; Lin, H.; Huang, Z. Appl. Therm. Eng. 2014, 66, 395-414. (7) Zhang, Q. M.; Song, C. L.; Lv, G.; Bin, F.; Pang, H. T.; Song, J. O. J. Ind. Eng. Chem. 2015, 24, 79-86. (8) Jiang, Y.; Xing, Z. M.; Wang, X. C.; Huang, S. B.; Liu, Q. Y.; Yang, J. S. J. Ind. Eng. Chem. 2015, 29, 43-47. (9) Hallquist, Å. M.; Fridell, E.; Westerlund, J, Mattias, H. Environ. Sci. Technol. 2012, 47, 773-780. (10) Farcy, B.; Abou-Taouk, A.; Vervisch, L.; Domingo, P.; Perret, N. Fuel, 2014, 118, 291-299. (11) Fan, W.; Zhu, T.; Sun, Y.; Lv, D. Chemosphere, 2014, 113, 182-187. 26 / 29


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New Experimental Results of NO Removal from Simulated Flue Gas

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