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Ag(I)-mediated electrochemical oxidation
Separation and Purification Technology 65 (2009) 156–163
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A sustainable environmentally friendly NOx removal process using Ag(II)/Ag(I)-mediated electrochemical oxidation Sang Joon Chung, K. Chandrasekara Pillai, Il Shik Moon ∗ Department of Chemical Engineering, Sunchon National University, # 315 Maegok Dong, Suncheon 540-742, Chonnam, Republic of Korea
a r t i c l e
i n f o
Article history: Received 28 March 2008 Received in revised form 19 October 2008 Accepted 21 October 2008 Keywords: NOx removal Wet scrubbing Electrochemical mediation Ag(II)/Ag(I) redox system Experimental parameters
a b s t r a c t This paper presents the results of investigations related to the development of a mediated electrochemical oxidation (MEO) process-based wet scrubbing method for NO and NO2 abatement from a simulated NOair flue gas mixture. The Ag(II) redox mediator in nitric acid medium generated in an electrochemical cell was circulated continuously through a packed-bed column by a closed loop, and the NOx gas was oxidatively removed by absorption/oxidation. The outlet gas from scrubber-I was subsequently admitted into scrubber-II for further continuous washings by 3 mol L−1 HNO3 . Numerous experimental runs were carried out in order to observe the performance of the wet scrubber-electrochemical cell integrated system as a function of the operating conditions: the initial NO concentration in the feed (100–400 mg L−1 ), gas flow rate (0.061–0.243 m s−1 ), liquid flow rate (0.012–0.049 m s−1 ), concentration of silver ion mediator (0.01–0.1 mol L−1 ), nitric acid concentration (3–6 mol L−1 ), and the temperature (15–45 ◦ C). Low initial NO feed, low gas flow rates, high liquid flow rates, high HNO3 acid concentration, Ag(I) concentration as low as 0.05 mol L−1 and low temperatures were most suitable for high removal performance. Total removal (100%) of NO and 80% removal of NOx were achieved in the oxidation with electrogenerated Ag(II) in a single stage gas scrubbing in scrubber-I. But, interestingly, a second stage gas scrubbing by a simple HNO3 (3 mol L−1 ) wash in scrubber-II led to a better removal of NOx with overall efficiency reaching 90% as well. This is an alternative green treatment process for zero emission of waste gases. Some NOx removal studies were also performed using Ce(IV)/Ce(III) redox mediator system, and the effect of nitric acid concentration and temperature examined, for a comparison with Ag(II)/Ag(I). © 2008 Elsevier B.V. All rights reserved.
1. Introduction Nitrogen oxide, NOx , is the generic term for a group of highly reactive gases, all of which contain nitrogen and oxygen in varying amounts. Nitrogen oxides are formed when fuel is burnt at high temperatures, as in a combustion process. The primary sources of NOx are motor vehicles, electric utilities, and other industrial, commercial, and residential sources that burn fuels. The main source of NOx emission is road transportation (61%), while industrial activities are responsible for 32% of the total emissions. The details on the many different sources of NOx are given in Fig. 1. Typically, 90–95% of nitrogen oxides by volume are emitted as NO and 5–10% as NO2 , although substantial variations are possible between various sources. In the normal conditions of atmospheric pressure and 25 ◦ C, NO is a colorless and odorless gas, while NO2 is a pungent reddish-brown gas. They are both noxious and directly responsible for large contributions to the formation of acid rain and resultant
∗ Corresponding author. Tel.: +82 61 7503581; fax: +82 61 7503581. E-mail addresses: [email protected], kc [email protected] (I.S. Moon). 1383-5866/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.seppur.2008.10.030
acidifications, photochemical smog and general atmospheric visibility degradation. For these reasons, the emissions of NO, NO2 and NOx from industrial processes are closely monitored and regulated. Processes for the NOx removal from stack gases emitted by stationary combustion sources can be generally characterized as either dry contact reduction processes or wet adsorption/absorption processes. Dry reduction processes may be either selective catalytic reduction or non-selective catalytic reduction. Selective catalytic reduction methods are characterized by reduction of nitrogen oxides in presence of oxygen at different catalysts and their consequent removal. The scarcity and expense nature of these catalyst materials have hindered wide-spread acceptance of this type of dry method for NOx control. Wet methods include wet scrubbing absorption, electron beam irradiation or plasma and so on. Among these technologies, wet scrubbing methods are economically the most competitive and have proven advantages [1]. NO2 can be effectively absorbed in some aqueous solutions [2–4] but NO is not. In industrial emission, the waste gases are mostly NO (more than 95%). Therefore, NO oxidation to NO2 is a crucial step followed by NO2 absorption for an effective chemical scrubbing system. Several wet chemical scrubbing processes have been devel-
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Fig. 1. Primary sources of NOx emission in global scenario.
oped for efficient removal of NOx , and these methods have been critically reviewed by Joshi and his coworkers [5,6]. Thus, strong oxidizing agents, like chlorine dioxide [7], ozone [8], or chlorine [9] are injected into the flue gas to improve the slow oxidation rate of NO in air, or oxidizing agents, such as, hydrogen peroxide [10], sodium chlorite [11,12], potassium permanganate [13], or sodium hypochlorite [14] are added into the scrubbing solution for oxidation. ClO2 has also been used recently for combined removal of NOx and SO2 from the flue gas mixture [15]. In most of the conventional chemical processes, the oxidizing agents have been used in lower concentration at which the oxidation power is high. However, the absorption is good at high concentration for the removal of waste gases. Electrochemical processes are potential alternatives to chemical methods providing the main advantage that the continuous usage of chemicals for oxidation can be avoided. In addition, the electrochemical methods prevent the production of secondary waste in the spent scrubbing solution [16,17]. The basic principle involved in the electrochemical gas purification is absorption of the waste gas component into a liquid phase, and its conversion to less harmful chemicals electrochemically by oxidation/reduction either directly at an electrode or using a redox mediator through a chemical process. Although, some work on direct reduction of NOx relating to flue gas has been carried out [18], the indirect mediated electrochemical oxidation (MEO) studies using redox mediator systems, like, Ce(IV)/Ce(III) [19,20], Co(III)/Co(II) [21], Mn(III)/Mn(II) [21] have shown that the MEO based technique is more suitable for flue gas treatment for NOx removal. The main advantage in such a process is that gases can be continuously scrubbed in an
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absorption device, e.g., packed-bed column, whereas the aqueous phase containing the redox mediator ions is circulated batch-wise in both the absorption tower and the electrochemical cell coupled in series. The continuous regeneration of the active component of the redox system in the electrochemical cell, and its recirculation through the scrubber column ensures its repeated reuse towards achieving high removal efficiency. Indeed, MEO based processes using redox systems, like Ag(II)/Ag(I), Co(III)/Co(II), Ce(IV)/Ce(III), Mn(III)/Mn(II), have been traditionally used to successfully accomplish complete mineralization of several organic pollutants to CO2 and H2 O [22–31]. Although MEO based processes using redox systems viz., Ce(IV)/Ce(III), Co(III)/Co(II), Mn(III)/Mn(II), have been studied to some extent, literature shows that the Ag(II)/Ag(I) redox system, which is expected to show much superior performance, since it has the highest standard redox potential (E0 = 1.98 V (NHE)) of all the known systems in MEO application, has not been studied. Thus, the present work has been undertaken to investigate Ag(II) as the redox mediator in the process of the removal of NOx from a simulated NO-air flue gas mixture with a view to rationalize the efficient application of MEO process for the removal of waste gas components from industrial flue gas mixtures. Some NOx removal studies have also been done using Ce(IV) as the mediator and the performances have been compared. 2. Experimental Removal of NOx was carried out in two scrubber columns. The Ag(II) redox mediator in nitric acid medium generated in an electrochemical cell was circulated through scrubber-I for continuous scrubbing of the NO-air gas mixture. The outlet gas from scrubber-I was subsequently admitted into scrubber-II for further continuous washings by 3 mol L−1 HNO3 . The experimental system was divided into two parts, i.e., Ag(II) redox mediator generation by electrochemical method and NOx gas treatment by wet scrubbing. A schematic diagram of the experimental system is shown in Fig. 2 and the experimental conditions are listed in Table 1. 2.1. Electrochemical system for Ag(II) mediator generation The electrochemical system consisted of an anolyte and a catholyte tank, each with a capacity of 2.0 L coupled to an electrochemical cell. The electrochemical cell was a plate and frame type of narrow gap flow cell configuration. The cell consisted of an anode and a cathode separated by a Nafion 324 membrane. The anode and
Fig. 2. Schematic diagram of experimental set-up.
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Table 1 Experimental conditions adopted in NOx removal experiments in scrubber-I and electrochemical cell. Parameters
Conditions and their range
Anolyte
(i) 3–6 mol L−1 HNO3 (ii) 0.01–0.1 mol L−1 AgNO3
Catholyte Current density Cell voltage Temperature NO feed concentration Superficial gas velocity Superficial liquid velocity
3 mol L−1 H2 SO4 8.9 A dm−2 2.8–3.0 V 15–45 ◦ C 100–400 mg L−1 0.061–0.243 m s−1 0.012–0.049 m s−1
the cathode were Pt-coated-Ti (Pt/Ti) and Ti mesh type electrodes, respectively, having a working area of 1.12 dm2 . A 6 mol L−1 nitric acid containing 0.1 mol L−1 Ag(I)NO3 was used as the anolyte, and 3 mol L−1 sulphuric acid as the catholyte. The anolyte and catholyte solutions were circulated through the cell using magnetic ceramic FHP pumps at 2 L min−1 flow rate. The anolyte and catholyte flow rates were maintained constant in all the experiments. The electrolysis for generation of Ag(II) redox mediator was conducted galvanostatically by applying a constant current of 10 A. 2.2. Scrubber system for NOx removal The NOx removal system was composed of an NO cylinder, an air compressor, two scrubbers (I and II), two liquid storage tanks one for scrubber-I and the other for scrubber-II, a data logging system and a gas analyzer. The scrubber-I (ID = 5 cm; height = 120 cm) was a glass vessel filled with raschig glass rings (10 mm) as packing material having an internal volume of 2.4 L. The liquid storage tank for scrubber-I was of 2 L capacity made of glass, and it was provided with a heat jacket surrounding the tank to maintain a constant temperature (±1 ◦ C) by hot-water circulation. Simulated flue gas mixtures of different composition were obtained by controlled mixing of air and NO using the mass flow controllers (MFC). The simulated gas was introduced at the bottom of the scrubber, and the scrubbing liquid (6 mol L−1 HNO3 containing Ag(II)/Ag(I)) was introduced at the top of the scrubber in a counter current flow pattern. The inlet and outlet gas concentrations for NO and NO2 were analyzed continuously. The outlet scrubbing solution containing the reduced form of silver, i.e., Ag(I), was passed through the electrochemical cell for Ag(II) regeneration, and was further recirculated. It must be mentioned that prior to commencing the gas removal experiments, electrolysis was carried out until 15% of Ag(II) conversion from the oxidation of 0.1 M Ag(I) was achieved, and only then the solution was allowed in to the scrubber column. Note that the chemical oxidation of NO and NO2 by Ag(II) ions to soluble HNO3 occurred in scrubber-I (reactions given in Section 3.1). However, some NO2 was found remaining in the outlet gas from scrubber-I, and it was removed further by continuous washings with 3 mol L−1 HNO3 in scrubber-II at a constant liquid flow rate = 0.049 m s−1 . The second stage scrubber-II (ID = 5 cm; height = 200 cm) made of PVC was packed with raschig glass rings. To achieve complete conversion of NO, the first stage scrubber unit was designed and operated with high scrubbing liquid recirculation rates, lower than normal gas superficial velocities, and packed with raschig rings as packing material. The NO and NO2 analyses were performed by Fuji ZSU and Teledyne Model No. 9110 gas analyzers, respectively. The inlet and outlet concentration of NO and NO2 were analyzed with respect to time of reaction. The removal efficiencies for NO and NOx were calculated based on the inlet and outlet concentration of gas feeds.
The destruction efficiency of NO was calculated as: NO destruction efficiency =
[NO]t − [NO]0 [NO]0
(1)
where [NO]0 and [NO]t were the NO gas concentration measured in the outlet gas initially at t = 0 and at any time t. Similarly, the NOx destruction efficiency was calculated from a similar equation: NOx destruction efficiency =
[NOx ]t − [NOx ]0 [NOx ]0
(2)
Here, the concentration of NOx was the sum of NO and NO2 concentrations measured in the system at any given time. All the measurements were conducted at atmospheric pressure and constant required temperatures. 3. Results and discussion A number of experiments with different NO-air compositions were studied to observe the effects of initial concentration of NO in the feed, gas flow rate, liquid flow rate, mediator ion concentration, scrubbing liquid (nitric acid) concentration on the performance of Ag(II) mediator in NOx gas removal process, represented by NO and NOx concentration in the scrubber-I outlet gas and their removal efficiency. The NOx removal in scrubber-II was also analyzed to understand the overall removal efficiency of the Ag(II) mediation coupled with two scrubber combination. In the case of Ce(III) mediator, the effect of mediator concentration, HNO3 acid concentration and the temperature was examined for a comparison with Ag(II) mediator. 3.1. Removal of NO and NOx in scrubber-I The NO concentration monitored at the scrubber-I outlet is shown in Fig. 3A, which also includes the NO removal efficiency. These data were collected with experimental conditions maintained constant at: gas flow rate = 0.121 m s−1 , liquid flow rate = 0.025 m s−1 , HNO3 = 6 mol L−1 and Ag(I) in electrochemical cell = 0.1 mol L−1 . One could see in Fig. 3A that 100 mg L−1 NO was removed rapidly, and it was complete within 5 min. The total removal efficiency was 100% for NO in scrubber-I, and it sustained for as long as 120 min. Fig. 3B shows the variation of the concentration and removal efficiency for NOx based on the total concentration of NO and NO2 measured at the inlet and outlet of scrubber-I. NOx concentration also showed a decrease with treatment time, but there was always some residual NOx even at longer treatment times, consequently showing lesser removal efficiencies, compared to NO removal. It was 88% for 100 mg L−1 . It is clear that a high degree of conversion and removal of nitrogen oxides was achieved by using Ag(II) redox mediator in nitric acid medium for the oxidation . Thus, the removal of nitrogen oxides (NOx ) containing NO and NO2 by means of a wet scrubbing technique using Ag(II)/Ag(I) redox system appears to be very engaging. Since the Ag(II)/Ag(I) redox couple has a higher standard reduction potential (E0 = 1.98 V (NHE)) with strong oxidizing power, the following reaction mechanism could be predicted for oxidation of NO and NO2 by Ag(II) ions in scrubber-I [19,21]: 2NO + 5Ag(II) + 3H2 O → HNO3 + NO2 + 5Ag(I) + 5H+ NO2 + Ag(II) + H2 O → HNO3 + Ag(I) + H
+
HNO2 + 2Ag(II) + H2 O → HNO3 + 2Ag(I) + 2H
(3) (4)
+
(5)
During the scrubbing treatment, the outlet gas from scrubber-I was found to contain no trace of NO gas, indicating that the total
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Fig. 3. (A) NO outlet concentration profiles and NO removal efficiencies as a function of time in scrubber-I. (B) NOx outlet concentration profiles and NOx removal efficiencies as a function of time in scrubber-I. [Experimental conditions: gas superficial velocity = 0.121 m s−1 ; liquid superficial velocity = 0.025 m s−1 ; Ag(I) concentration in cell = 0.1 M; HNO3 concentration = 6 M; T = 25 ◦ C.]
amount of NO charged into the scrubber-I was completely oxidized by the Ag(II) ions in scrubber-1 at the fixed gas and liquid velocities. This means NO was oxidized very rapidly to HNO3 by Ag(II) ions following the reaction step described in Eq. (3). Bringmann et al. [21] showed that when the Co(III)/Co(II) system in 3 mol L−1 H2 SO4 was used as a redox mediator, a removal efficiency of 36% was observed for NOx , and the low efficiency was attributed to kinetic inhibition. In contrast to the Co(III)/Co(II) system, the Mn(IV)/Mn(III) in 6 mol L−1 H2 SO4 was found by the same authors to give an efficiency of >90%. Aurousseau et al. [19] studied the oxidation of nitrogen oxides in ceric solution by the liquid-phase oxidation of nitrous acid, and it was shown that the oxidation rate of NOx by Ce(IV) was largely reduced by increasing the acid concentrations of both H2 SO4 and HNO3 . Kleifges et al. [18] examined direct and indirect NO reduction with some complexing agents and redox mediators . But in all these systems, the NOx conversion and removal efficiencies were low. The present work clearly indicate that the Ag(II)/Ag(I) redox mediator system was more stronger with quite a high removal of NO and NOx from waste gases. 3.2. Removal of NOx in scrubber-II The oxidation of NO in scrubber-I was always accompanied with a small amount of NO2 gas in the scrubber outlet gas. Note that the outlet gas from scrubber-I is free from NO. The unconverted NO2 was treated further with 3 mol L−1 HNO3 in scrubber-II (liquid flow rate = 0.049 m s−1 , gas flow rate = 0.121 m s−1 ), where it was removed by absorption in to aqueous nitric acid: NO2(gas)
H2 O, HNO3
−→
NO2(dissolved)
(6)
Table 2 lists the NOx concentrations measured at the inlet and out let of scrubber-II. The NOx removal efficiency in scrubber-II was found to be around 50%, except for the case of 100 mg L−1 NO concentration in the initial feed, indicating that only 50% of NOx was
removed by the washings by 3 mol L−1 , and the rest of NOx came out of the outlet. In Table 2 also included is the total removal efficiency for NOx across both scrubber-I and scrubber-II, calculated as per Eq. (2) taking in to account of the NO2 gas concentration present in the initial NO-air feed. Very effective removal of nearly 90% was evident for NOx removal by introducing the second stage scrubbing. It should be mentioned here that the initial NO-air mixture feed to scrubber-I always showed the presence of some quantities of NO2 around 10–15% of the NO concentration possibly due to air oxidation of NO. Thus, it becomes clear that when the NO-air gas mixture was treated in scrubber-I, by washing the gas with 6 mol L−1 HNO3 involving absorption/chemical oxidation reaction by electrogenerated Ag(II) oxidant, followed by an acid washing of the scrubbing-I outlet gas by 3 mol L−1 HNO3 in scrubber-II yielded very satisfactory results concerning the flue gas removal: 100% NO removal and 90% NOx removal. The process efficiency could be attributed to favorable conditions provided by scrubber-I, which functioned both as reactor and scrubber with fast chemical reaction between Ag(II) and NO along with simultaneously good absorption, further assisted by a thorough acid washings in scrubber-II functioning as a traditional scrubber column. 3.3. Effect of NO feed concentration Various concentrations of NO in the feed, in the range 100–400 mg L−1 , were studied to find out the effect of NO feed concentration on the removal performance of Ag(II) redox mediator. The results on the residual concentration in the scrubber-I gas outlet and the corresponding efficiencies for the removal of NO and NOx are shown in Fig. 3A and B, respectively. It is clear that with increase in NO feed concentration, the NO removal from the feed was slower (Fig. 3A). A comparison of the
Table 2 NOx removal efficiency in scrubber-II and in combined scrubber-I and scrubber-II for different initial NO-air feed mixtures. NO concentration in NO-air feed (mg L−1 )
100 200 300 400 a b
NOx removal efficiency in scrubber-IIb
NOx removal efficiency through scrubber-Ia and scrubber-IIb
NOx inlet to scrubber-II (mg L−1 )
NOx outlet of scrubber-II (mg L−1 )
Removal efficiency (%)
NOx inlet to scrubber-I (mg L−1 )
NOx outlet of scrubber-II (mg L−1 )
Removal efficiency (%)
14 36 59 89
13 21 31 40
7 42 47 55
119 232 338 457
13 21 31 40
89 91 91 91
In scrubber-I: scrubbing solution = 6 mol L−1 HNO3 , Ag(I) concentration = 0.1 mol L−1 , gas flow rate = 0.121 m s−1 , liquid flow rate = 0.025 m s−1 . In scrubber-II: scrubbing solution = 3 mol L−1 HNO3 , gas flow rate = 0.121 m s−1 , liquid flow rate = 0.049 m s−1 .
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residual NO concentration at 1 min clearly demonstrates that it was 3, 15, 29 and 78 mg L−1 for the initial NO feed concentrations 100, 200, 300 and 400 mg L−1 , respectively. However, in spite of such a difference in removal rate, complete degradation with almost 100% removal was attained even for higher feed concentrations at sufficiently longer treatment times beyond 5 min. The removal of NOx also showed a similar behavior (Fig. 3B); however, with somewhat lower efficiencies even when the treatment time was prolonged. Also, the feed concentration showed a definite inhibiting influence on the removal efficiency for NOx removal. For example, the NOx removal decreased from 88 to 81% when the feed concentration was increased from 100 to 400 mg L−1 . These observations point-out that Ag(II) was able to oxidize NO to NO2 (Eq. (3)) more effectively, but not NO2 to HNO3 (Eq. (4)) completely. Moreover, the added NO appeared to slowdown the NO2 oxidation by Ag(II), and the reasons for such a behavior is difficult to understand at present. 3.4. Effect of gas superficial velocity on NO and NOx removal in scrubber-I To evaluate the effect of the gas velocity on the removal efficiency of NO and NOx in scrubber-I, a series of experiments were performed at different gas superficial velocities in the range of 0.061–0.243 m s−1 at a fixed NO feed concentration of 400 ppm and superficial liquid velocity of 0.025 m s−1 . As shown in Fig. 4, the high removal efficiency for NO (100%) slightly decreased at higher gas flow rate possibly as a result of decrease in gas/liquid contact time at higher gas flow rates. The results of Fig. 4 indicate that the decreasing contact time exerted more influence on the removal of NOx , as it showed a decrease of efficiency, although smaller, from 80 to 76% when the gas flow rate was increased from 0.061 to 0.243 m s−1 . 3.5. Effect of liquid superficial velocity NO and NOx removal in scrubber-I Experiments were carried out to investigate the effect of the superficial velocity of the scrubbing solution (6 mol L−1 HNO3 containing 0.1 mol L−1 Ag(I) in cell) on NO and NOx removal efficiency in scrubber-I. The liquid superficial velocities were varied from 0.012 to 0.049 m s−1 keeping the gas velocity and NO feed concentration constant at 0.121 m s−1 and 400 mg L−1 , respectively. Fig. 5 shows
Fig. 5. NO and NOx removal efficiencies as a function of superficial liquid velocities in scrubber-I. [Experimental conditions: feed NO concentration = 400 ppm; superficial gas velocity = 0.121 m s−1 ; Ag(I) concentration in cell = 0.1 M; HNO3 concentration = 6 M; T = 25 ◦ C.]
the removal efficiencies for NO and NOx as a function of liquid velocity. It is clear that the removal of both NO and NOx increased with liquid flow rate: the NO removal from 98 to 100%, and the NOx removal from 76 to 81. Thus, within the experimental range studied in the present work, 0.025 m s−1 liquid flow rate was optimum for higher NO and NOx removal from NO-air flue gas mixture. 3.6. Effect of Ag(I) concentration Silver(I) ion concentration in the range 0.01–0.1 mol L−1 was chosen to study the effect of mediator ion concentration on NO removal. Low concentrations of silver ion were selected in these studies to achieve high removal efficiencies by establishing suitable experimental conditions (gas flow rate, liquid flow rate, nitric acid concentration, electrolysis current, electrolysis time, etc.) in view of the expensive nature of silver and its compounds. The other important reason to prefer low silver ion concentration in MEO based application was that the previous studies [29] indicated that the percentage conversion of Ag(II) formation in a batch electrochemical reactor with recirculation decreased upon increasing the concentration of Ag(I) in the higher concentration range 0.1–1.0 mol L−1 in 8 mol L−1 HNO3 at 25 ◦ C. Fig. 6 shows the residual NO concentration in the scrubber-I outlet gas for different Ag(I) concentrations from experiments at fixed NO feed (400 mg L−1 ), gas flow rate (0.121 m s−1 ), liquid flow rate (0.025 m s−1 ) and HNO3 concentration (6 mol L−1 ). The removal efficiencies are also plotted. The results indicate that the NO concentration in the scrubber-I gas outlet decreased more rapidly, leading to increasingly higher removal, with increase in the Ag(I) mediator ion concentration. The efficiency was 90% at the low Ag(I) concentration studied, i.e., 0.01 mol L−1 , and the maximum 100% attained when the Ag(I) concentration reached 0.05 mol L−1 . Thus, a low concentration of 0.05 mol L−1 Ag(I) was enough in the scrubber solution for the efficient removal of NO with removal degree reaching 100%. The NOx removal was also highest at 0.05 mol L−1 of Ag(I). 3.7. Effect of HNO3 concentration
Fig. 4. NO and NOx removal efficiencies as a function of superficial gas velocities in scrubber-I. [Experimental conditions: feed NO concentration = 400 ppm; superficial liquid velocity = 0.025 m s−1 ; Ag(I) concentration in cell = 0.1 M; HNO3 Concentration = 6 M; T = 25 ◦ C.]
The acid concentration is an important parameter in MEO based processes, since it directly influences the yield of the redox mediator at the electrode surface, which in turn can affect the gas removal efficiency [19,20,29]. Thus, the removal of NO by Ag(II)/Ag(I) redox
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Fig. 6. NO outlet concentration profiles and NO removal efficiencies as a function of Ag(I) concentration in scrubber-I. [Experimental conditions: feed NO concentration = 400 ppm; superficial gas velocity = 0.121 m s−1 ; superficial liquid velocity = 0.025 m s−1 ; HNO3 concentration = 6 M; T = 25 ◦ C.]
161
Fig. 7. NO outlet concentration profiles and NO removal efficiencies as a function of HNO3 concentration in scrubber-I. [Experimental conditions: feed NO concentration = 400 ppm; superficial gas velocity = 0.121 m s−1 ; superficial liquid velocity = 0.025 m s−1 ; Ag(I) concentration in cell = 0.1 M; T = 25 ◦ C.]
context of its NO and NOx removal by mediation. mediation in scrubber-I was investigated with various HNO3 concentrations (3–6 mol L−1 ). The results are presented in Fig. 7 for 400 ppm NO feed gas at gas flow rate = 0.121 m s−1 , liquid flow rate = 0.025 m s−1 with Ag(I) maintained at 0.1 mol L−1 . Note that the NO removal rate, and hence the removal efficiency, significantly increased as the HNO3 acid concentration increased. 92% removal efficiency at 3 mol L−1 HNO3 was seen to increase to 100% when HNO3 was at its highest value 6 mol L−1 used in the present work. The 6 mol L−1 HNO3 also caused highest removal of NOx as well. The increase in the performance of Ag(II)/Ag(I) redox system at higher HNO3 acid concentration towards NO and NOx removal might be correlated with the availability of more Ag(II) ions for mediation as a result of increased electrogeneration of Ag(II) from Ag(I) due to higher stability of [Ag(II)NO3 ]+ complex at higher acid concentrations [29]. A 45% conversion of Ag(I) (0.1 mol L−1 ) to Ag(II) in 8 mol L−1 HNO3 was reported [29], when the anolyte was circulated at relatively low circulation rates of 100 mL min−1 (0.0085 m s−1 ). The efficient performance of Ag(II)/Ag(I) system at high HNO3 acid concentrations (Fig. 7) is highly encouraging in the
3.8. Ag(II)/Ag(I) MEO system versus Ce(IV)/Ce(III) MEO system To highlight the efficient performance of Ag(II) mediator in NOx removal process, the previously studied Ce(IV) mediator [19,20] was investigated in the removal of NOx process and compared. Detailed work on Ce(IV)/Ce(III) redox mediator system towards MEO of several aqueous organic pollutants were carried out by the present authors [26–28,30], and 3 mol L−1 HNO3 acid was found to offer better conditions for efficient performance of this redox couple [30]. Thus, 3 mol L−1 HNO3 was used as the acid medium for Ce(IV)/Ce(III) in the comparative studies. 3.8.1. Effect of mediator ion concentration Three different concentrations of Ce(III) ca. 0.5, 0.75 and 1.0 mol L−1 in 3 mol L−1 HNO3 were used to generate Ce(IV) in the divided electrochemical cell, and circulated through scrubber-I at 0.025 m s−1 flow velocity. NO-air gas mixture of 400 mg L−1 NO feed concentration admitted in to scrubber-I at 0.121 m s−1 gas flow rate was used in this study. Fig. 8 shows the corresponding removal effi-
Fig. 8. NO and NOx removal efficiencies as a function of redox ion concentration in scrubber-I: (A) Ce(III) concentration in 3 M HNO3 for cerium MEO system; (B) Ag(I) concentration in 6 M HNO3 for silver MEO system. [Experimental conditions: feed NO concentration = 400 ppm; superficial gas velocity = 0.121 m s−1 ; superficial liquid velocity = 0.025 m s−1 ; T = 25 ◦ C.]
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Table 3 NO and NOx removal efficiencies for Ce(IV)/Ce(III) and Ag(II)/Ag(I) MEO systems at different temperatures and HNO3 acid concentrations in scrubber-I (gas flow rate = 0.121 m s−1 , liquid flow rate = 0.025 m s−1 , NO feed concentration = 400 mg L−1 ). Ce(IV)/Ce(III)a
Temperaturec (◦ C)
HNO3 d concentration (mol L−1 )
a b c d
Ag(II)/Ag(I)b
NO removal efficiency (%)
NOx removal efficiency (%)
NO removal efficiency (%)
NOx removal efficiency (%)
15 25 35 45
43 43 43 40
37 37 37 34
100 100 100 95
81 81 80 67
3 4 5 6
43 50 53
37 43 47
92 95 97 100
72 78 79 81
Ce(III) concentration = 0.5 mol L−1 . Ag(I) concentration = 0.1 mol L−1 . HNO3 concentration = 3 mol L−1 for Ce(IV)/Ce(III) system, 6 mol L−1 for Ag(II)/Ag(I) system. Temperature = 25 ◦ C for both Ce(IV)/Ce(III) and Ag(II)/Ag(I) systems.
ciency results for NO and NOx as a function of Ce(III) concentration at 25 ◦ C. It can be noticed that the removal of both NO and NOx increased with Ce(III) concentration. For a comparison, the NO and NOx removal results obtained with Ag(II)/Ag(I) redox system, collected under the same experimental conditions of gas and liquid flow rates and 400 mg L−1 NO feed, were also plotted. Two results are strikingly clear on comparison. The silver based MEO system was highly efficient showing complete (100%) removal of NO and 80% removal of NOx , which is quite contrast to only ≈46% for NO and ≈38% for NOx offered by cerium MEO system. The other point of interest about Ag(II)/Ag(I) ions in Fig. 8 is: these significantly higher removals were provided by silver ion concentrations in the range 0.01–0.1 mol L−1 , which were ten times lower than those of cerium ions, 0.1–1 mol L−1 . Thus the unique nature of silver based MEO system is evident.
3.8.2. Effect of temperature and HNO3 acid concentration The temperature is one of the important process parameters because flue gases from different sources are at different high temperatures. In the present work, temperature of the scrubbing solution was varied in the range 15 to 45 ◦ C (gas flow rate = 0.121 m s−1 , liquid flow rate = 0.025 m s−1 , NO feed concentration = 400 mg L−1 ). The removal efficiencies for NO and NOx in the scrubber-I are listed in Table 3 for both the redox systems. Note that the silver system was with Ag(I) concentration = 0.1 mol L−1 and HNO3 = 6 mol L−1 , while the cerium system was with Ce(III) at 0.5 mol L−1 and HNO3 at 3 mol L−1 . Table 3 shows that within this small temperature range, 15–45 ◦ C, almost no variation was observed for the cerium system in the removal efficiencies of both NO and NOx . The silver system also showed a similar behavior only towards NO, but there was a marginal decrease for NOx . Besides, in a similar attempt to examine the acid concentration effect, HNO3 concentration was changed from 3 to 6 mol L−1 (gas flow rate = 0.121 m s−1 , liquid flow rate = 0.025 m s−1 , NO feed concentration = 400 mg L−1 , T = 25 ◦ C) and the removal results were collected for silver system with Ag(I) concentration = 0.1 mol L−1 , and for cerium system with Ce(III) = 0.5 mol L−1 . Beneficial effect was observed with increase in acid concentration for both silver and cerium systems in their performance towards NO and NOx removal as shown in Table 3. Thus, the acid effect and temperature effect show that the response of both the systems was similar to the enforced process parameters; however, the strong catalytic effect of silver system always remained at all conditions.
4. Conclusions The following are the important findings of our study: (1) Ag(II) mediator based MEO system showed complete 100% removal of NO and 80% NOx removal from simulated NO-air flue gas mixture in a single stage gas scrubbing operation. Good improvement was shown in combination with a second stage gas scrubbing by a simple HNO3 wash, wherein the overall removal of even the NOx gas was improved to 90%. (2) The rate of removal of NO and NOx was slowed somewhat when the NO feed was increased. (3) While the gas velocity tended to decrease the NO and NOx removal efficiencies, the liquid flow rate assisted the removal process to some extent. (4) Low silver ion concentrations in the range 0.01–0.1 mol L−1 were found to be enough to cause extensive NO and NOx removal, and total (100%) NO removal was achieved when the concentration of Ag(I) in the flowing scrubbing liquid reached 0.05 mol L−1 . (5) Our detailed study involving NO and NOx removal by mediation clearly established that Ag(II)/Ag(I) based MEO system was superior in terms of low levels of mediator concentration showing several times higher removal efficiencies, compared to Ce(IV)/Ce(III) based MEO system. (6) In a narrow temperature range of 15 to 45 ◦ C, both silver and cerium systems showed almost no variation in NO removal; but both systems showed enhanced removal with increase in acid concentration; however, the strong catalytic effect of silver system always remained at all conditions. (7) A complete understanding of the underlying processes need detailed studies on determination of mass transfer parameters (interfacial area and mass transfer coefficient) of the scrubber, absorption of NOx into Ag(II) containing solutions, the kinetic constants of the liquid phase oxidation of NOx , etc. These and estimation of reaction stoichiometry needed for arriving at current efficiency, energy consumption, etc., are under progress, though at present we do not have data to report. Acknowledgements This work was supported by several funding agencies: the Korea Ministry of Environment as “The Eco-technopia 21 project”, the Ministry of Commerce, Industry and Energy (MOCIE) through Regional Innovation Centre (RIC) project, the Korea Research Foundation and the Korean Federation of Science and Technology Society Grant funded by the Korean Government (MOEHRD, Basic Research Promotion Fund). K. Chandrasekara Pillai wishes to thank the authorities of the University of Madras, Chennai 600005, India for granting sabbatical leave.
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[19] M. Aurousseau, F. Lapicque, A. Storck, Simulation of NOx scrubbing by ceric solutions: Oxidation of Nitrous acid by Ce(IV) species in acidic solutions, Ind. Eng. Chem. Res. 33 (1994) 191–196. [20] P. Hoffmann, C. Roizard, L. Lapicque, S. Venot, A. Maire, A process for the simultaneous removal of SO2 and NOx using Ce(IV) redox catalysis, Institution of chemical Engineers, Trans IchemE. 75B (1997) 43–53. [21] J. Bringmann, K. Ebert, U. Galla, H. Schmieder, Oxidative separations of nitrogen oxides from off-gases by electrochemical mediators, J. Appl. Electrochem. 27 (1997) 870–872. [22] J.C. Farmer, F.T. Wang, R.A. Hawley-Fedder, P.R. Lewis, L.J. Summers, L. Foiles, Electrochemical treatment of mixed and hazardous wastes: Oxidation of ethylene glycol and benzene by silver(II), J. Electrochem. Soc. 139 (1992) 654–662. [23] J. Bringmann, K. Ebert, U. Galla, H.J. Schimider, Electrochemical mediators for total oxidation of chlorinated hydrocarbons: Formation kinetics of Ag(II), Co(III) and Ce(IV), J. Appl. Electrochem. 25 (1995) 846–851. [24] U. Galla, P. Kritzer, J. Bringmann, H. Schmieder, Process for total degradation of organic wastes by mediated electrooxidation, Chem. Eng. Technol. 23 (2000) 230–233. [25] L. Kaba, G.D. Hitchens, J.O.M. Bockris, Electrochemical incineration of wastes, J. Electrochem. Soc. 137 (1990) 1341–1345. [26] S. Balaji, V.V. Kokovkin, S.J. Chung, I.S. Moon, Destruction of EDTA by mediated electrochemical oxidation process: Monitoring by continuous CO2 measurements, Water Res. 41 (2007) 1423-L1432. [27] S. Balaji, S.J. Chung, M. Matheswaran, V.V. Kokovkin, I.S. Moon, Destruction of organic pollutants by cerium(IV) MEO process: A study on the influence of process conditions for EDTA mineralization, J. Hazard. Mater. 150 (2007) 596–603. [28] M. Matheswaran, S. Balaji, S.J. Chung, I.S. Moon, Mineralization of phenol Ce(IV)mediated electrochemical oxidation in methanesulphonic acid medium: A preliminary study, Chemosphere 69 (2007) 325–331. [29] M. Matheswaran, S. Balaji, S.J. Chung, I.S. Moon, Silver mediated electrochemical oxidation: Production of silver(II) in nitric acid medium and in situ destruction of phenol in semi batch process, J. Ind. Eng. Chem. 13 (2007) 231–236. [30] M. Matheswaran, S. Balaji, S.J. Chung, I.S. Moon, Electrooxidation kinetics of cerium(III) in nitric acid using divided electrochemical cell for application in the mediated electrochemical oxidation of phenol, Bull. Korean Chem. Soc. 28 (2007) 1329–1334. [31] K. Chandrasekara Pillai, M. Matheswaran, S.J. Chung, I.S. Moon, Studies on promising cell performance with H2 SO4 as the catholyte for electrogeneration of Ag2+ from Ag+ in HNO3 anolyte in mediated electrochemical oxidation process, J. Appl. Electrochem. 39 (2009) 23–30.
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Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
A sustainable environmentally friendly NOx removal process using Ag(II)/Ag(I)-mediated electrochemical oxidation Sang Joon Chung, K. Chandrasekara Pillai, Il Shik Moon ∗ Department of Chemical Engineering, Sunchon National University, # 315 Maegok Dong, Suncheon 540-742, Chonnam, Republic of Korea
a r t i c l e
i n f o
Article history: Received 28 March 2008 Received in revised form 19 October 2008 Accepted 21 October 2008 Keywords: NOx removal Wet scrubbing Electrochemical mediation Ag(II)/Ag(I) redox system Experimental parameters
a b s t r a c t This paper presents the results of investigations related to the development of a mediated electrochemical oxidation (MEO) process-based wet scrubbing method for NO and NO2 abatement from a simulated NOair flue gas mixture. The Ag(II) redox mediator in nitric acid medium generated in an electrochemical cell was circulated continuously through a packed-bed column by a closed loop, and the NOx gas was oxidatively removed by absorption/oxidation. The outlet gas from scrubber-I was subsequently admitted into scrubber-II for further continuous washings by 3 mol L−1 HNO3 . Numerous experimental runs were carried out in order to observe the performance of the wet scrubber-electrochemical cell integrated system as a function of the operating conditions: the initial NO concentration in the feed (100–400 mg L−1 ), gas flow rate (0.061–0.243 m s−1 ), liquid flow rate (0.012–0.049 m s−1 ), concentration of silver ion mediator (0.01–0.1 mol L−1 ), nitric acid concentration (3–6 mol L−1 ), and the temperature (15–45 ◦ C). Low initial NO feed, low gas flow rates, high liquid flow rates, high HNO3 acid concentration, Ag(I) concentration as low as 0.05 mol L−1 and low temperatures were most suitable for high removal performance. Total removal (100%) of NO and 80% removal of NOx were achieved in the oxidation with electrogenerated Ag(II) in a single stage gas scrubbing in scrubber-I. But, interestingly, a second stage gas scrubbing by a simple HNO3 (3 mol L−1 ) wash in scrubber-II led to a better removal of NOx with overall efficiency reaching 90% as well. This is an alternative green treatment process for zero emission of waste gases. Some NOx removal studies were also performed using Ce(IV)/Ce(III) redox mediator system, and the effect of nitric acid concentration and temperature examined, for a comparison with Ag(II)/Ag(I). © 2008 Elsevier B.V. All rights reserved.
1. Introduction Nitrogen oxide, NOx , is the generic term for a group of highly reactive gases, all of which contain nitrogen and oxygen in varying amounts. Nitrogen oxides are formed when fuel is burnt at high temperatures, as in a combustion process. The primary sources of NOx are motor vehicles, electric utilities, and other industrial, commercial, and residential sources that burn fuels. The main source of NOx emission is road transportation (61%), while industrial activities are responsible for 32% of the total emissions. The details on the many different sources of NOx are given in Fig. 1. Typically, 90–95% of nitrogen oxides by volume are emitted as NO and 5–10% as NO2 , although substantial variations are possible between various sources. In the normal conditions of atmospheric pressure and 25 ◦ C, NO is a colorless and odorless gas, while NO2 is a pungent reddish-brown gas. They are both noxious and directly responsible for large contributions to the formation of acid rain and resultant
∗ Corresponding author. Tel.: +82 61 7503581; fax: +82 61 7503581. E-mail addresses: [email protected], kc [email protected] (I.S. Moon). 1383-5866/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.seppur.2008.10.030
acidifications, photochemical smog and general atmospheric visibility degradation. For these reasons, the emissions of NO, NO2 and NOx from industrial processes are closely monitored and regulated. Processes for the NOx removal from stack gases emitted by stationary combustion sources can be generally characterized as either dry contact reduction processes or wet adsorption/absorption processes. Dry reduction processes may be either selective catalytic reduction or non-selective catalytic reduction. Selective catalytic reduction methods are characterized by reduction of nitrogen oxides in presence of oxygen at different catalysts and their consequent removal. The scarcity and expense nature of these catalyst materials have hindered wide-spread acceptance of this type of dry method for NOx control. Wet methods include wet scrubbing absorption, electron beam irradiation or plasma and so on. Among these technologies, wet scrubbing methods are economically the most competitive and have proven advantages [1]. NO2 can be effectively absorbed in some aqueous solutions [2–4] but NO is not. In industrial emission, the waste gases are mostly NO (more than 95%). Therefore, NO oxidation to NO2 is a crucial step followed by NO2 absorption for an effective chemical scrubbing system. Several wet chemical scrubbing processes have been devel-
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Fig. 1. Primary sources of NOx emission in global scenario.
oped for efficient removal of NOx , and these methods have been critically reviewed by Joshi and his coworkers [5,6]. Thus, strong oxidizing agents, like chlorine dioxide [7], ozone [8], or chlorine [9] are injected into the flue gas to improve the slow oxidation rate of NO in air, or oxidizing agents, such as, hydrogen peroxide [10], sodium chlorite [11,12], potassium permanganate [13], or sodium hypochlorite [14] are added into the scrubbing solution for oxidation. ClO2 has also been used recently for combined removal of NOx and SO2 from the flue gas mixture [15]. In most of the conventional chemical processes, the oxidizing agents have been used in lower concentration at which the oxidation power is high. However, the absorption is good at high concentration for the removal of waste gases. Electrochemical processes are potential alternatives to chemical methods providing the main advantage that the continuous usage of chemicals for oxidation can be avoided. In addition, the electrochemical methods prevent the production of secondary waste in the spent scrubbing solution [16,17]. The basic principle involved in the electrochemical gas purification is absorption of the waste gas component into a liquid phase, and its conversion to less harmful chemicals electrochemically by oxidation/reduction either directly at an electrode or using a redox mediator through a chemical process. Although, some work on direct reduction of NOx relating to flue gas has been carried out [18], the indirect mediated electrochemical oxidation (MEO) studies using redox mediator systems, like, Ce(IV)/Ce(III) [19,20], Co(III)/Co(II) [21], Mn(III)/Mn(II) [21] have shown that the MEO based technique is more suitable for flue gas treatment for NOx removal. The main advantage in such a process is that gases can be continuously scrubbed in an
157
absorption device, e.g., packed-bed column, whereas the aqueous phase containing the redox mediator ions is circulated batch-wise in both the absorption tower and the electrochemical cell coupled in series. The continuous regeneration of the active component of the redox system in the electrochemical cell, and its recirculation through the scrubber column ensures its repeated reuse towards achieving high removal efficiency. Indeed, MEO based processes using redox systems, like Ag(II)/Ag(I), Co(III)/Co(II), Ce(IV)/Ce(III), Mn(III)/Mn(II), have been traditionally used to successfully accomplish complete mineralization of several organic pollutants to CO2 and H2 O [22–31]. Although MEO based processes using redox systems viz., Ce(IV)/Ce(III), Co(III)/Co(II), Mn(III)/Mn(II), have been studied to some extent, literature shows that the Ag(II)/Ag(I) redox system, which is expected to show much superior performance, since it has the highest standard redox potential (E0 = 1.98 V (NHE)) of all the known systems in MEO application, has not been studied. Thus, the present work has been undertaken to investigate Ag(II) as the redox mediator in the process of the removal of NOx from a simulated NO-air flue gas mixture with a view to rationalize the efficient application of MEO process for the removal of waste gas components from industrial flue gas mixtures. Some NOx removal studies have also been done using Ce(IV) as the mediator and the performances have been compared. 2. Experimental Removal of NOx was carried out in two scrubber columns. The Ag(II) redox mediator in nitric acid medium generated in an electrochemical cell was circulated through scrubber-I for continuous scrubbing of the NO-air gas mixture. The outlet gas from scrubber-I was subsequently admitted into scrubber-II for further continuous washings by 3 mol L−1 HNO3 . The experimental system was divided into two parts, i.e., Ag(II) redox mediator generation by electrochemical method and NOx gas treatment by wet scrubbing. A schematic diagram of the experimental system is shown in Fig. 2 and the experimental conditions are listed in Table 1. 2.1. Electrochemical system for Ag(II) mediator generation The electrochemical system consisted of an anolyte and a catholyte tank, each with a capacity of 2.0 L coupled to an electrochemical cell. The electrochemical cell was a plate and frame type of narrow gap flow cell configuration. The cell consisted of an anode and a cathode separated by a Nafion 324 membrane. The anode and
Fig. 2. Schematic diagram of experimental set-up.
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Table 1 Experimental conditions adopted in NOx removal experiments in scrubber-I and electrochemical cell. Parameters
Conditions and their range
Anolyte
(i) 3–6 mol L−1 HNO3 (ii) 0.01–0.1 mol L−1 AgNO3
Catholyte Current density Cell voltage Temperature NO feed concentration Superficial gas velocity Superficial liquid velocity
3 mol L−1 H2 SO4 8.9 A dm−2 2.8–3.0 V 15–45 ◦ C 100–400 mg L−1 0.061–0.243 m s−1 0.012–0.049 m s−1
the cathode were Pt-coated-Ti (Pt/Ti) and Ti mesh type electrodes, respectively, having a working area of 1.12 dm2 . A 6 mol L−1 nitric acid containing 0.1 mol L−1 Ag(I)NO3 was used as the anolyte, and 3 mol L−1 sulphuric acid as the catholyte. The anolyte and catholyte solutions were circulated through the cell using magnetic ceramic FHP pumps at 2 L min−1 flow rate. The anolyte and catholyte flow rates were maintained constant in all the experiments. The electrolysis for generation of Ag(II) redox mediator was conducted galvanostatically by applying a constant current of 10 A. 2.2. Scrubber system for NOx removal The NOx removal system was composed of an NO cylinder, an air compressor, two scrubbers (I and II), two liquid storage tanks one for scrubber-I and the other for scrubber-II, a data logging system and a gas analyzer. The scrubber-I (ID = 5 cm; height = 120 cm) was a glass vessel filled with raschig glass rings (10 mm) as packing material having an internal volume of 2.4 L. The liquid storage tank for scrubber-I was of 2 L capacity made of glass, and it was provided with a heat jacket surrounding the tank to maintain a constant temperature (±1 ◦ C) by hot-water circulation. Simulated flue gas mixtures of different composition were obtained by controlled mixing of air and NO using the mass flow controllers (MFC). The simulated gas was introduced at the bottom of the scrubber, and the scrubbing liquid (6 mol L−1 HNO3 containing Ag(II)/Ag(I)) was introduced at the top of the scrubber in a counter current flow pattern. The inlet and outlet gas concentrations for NO and NO2 were analyzed continuously. The outlet scrubbing solution containing the reduced form of silver, i.e., Ag(I), was passed through the electrochemical cell for Ag(II) regeneration, and was further recirculated. It must be mentioned that prior to commencing the gas removal experiments, electrolysis was carried out until 15% of Ag(II) conversion from the oxidation of 0.1 M Ag(I) was achieved, and only then the solution was allowed in to the scrubber column. Note that the chemical oxidation of NO and NO2 by Ag(II) ions to soluble HNO3 occurred in scrubber-I (reactions given in Section 3.1). However, some NO2 was found remaining in the outlet gas from scrubber-I, and it was removed further by continuous washings with 3 mol L−1 HNO3 in scrubber-II at a constant liquid flow rate = 0.049 m s−1 . The second stage scrubber-II (ID = 5 cm; height = 200 cm) made of PVC was packed with raschig glass rings. To achieve complete conversion of NO, the first stage scrubber unit was designed and operated with high scrubbing liquid recirculation rates, lower than normal gas superficial velocities, and packed with raschig rings as packing material. The NO and NO2 analyses were performed by Fuji ZSU and Teledyne Model No. 9110 gas analyzers, respectively. The inlet and outlet concentration of NO and NO2 were analyzed with respect to time of reaction. The removal efficiencies for NO and NOx were calculated based on the inlet and outlet concentration of gas feeds.
The destruction efficiency of NO was calculated as: NO destruction efficiency =
[NO]t − [NO]0 [NO]0
(1)
where [NO]0 and [NO]t were the NO gas concentration measured in the outlet gas initially at t = 0 and at any time t. Similarly, the NOx destruction efficiency was calculated from a similar equation: NOx destruction efficiency =
[NOx ]t − [NOx ]0 [NOx ]0
(2)
Here, the concentration of NOx was the sum of NO and NO2 concentrations measured in the system at any given time. All the measurements were conducted at atmospheric pressure and constant required temperatures. 3. Results and discussion A number of experiments with different NO-air compositions were studied to observe the effects of initial concentration of NO in the feed, gas flow rate, liquid flow rate, mediator ion concentration, scrubbing liquid (nitric acid) concentration on the performance of Ag(II) mediator in NOx gas removal process, represented by NO and NOx concentration in the scrubber-I outlet gas and their removal efficiency. The NOx removal in scrubber-II was also analyzed to understand the overall removal efficiency of the Ag(II) mediation coupled with two scrubber combination. In the case of Ce(III) mediator, the effect of mediator concentration, HNO3 acid concentration and the temperature was examined for a comparison with Ag(II) mediator. 3.1. Removal of NO and NOx in scrubber-I The NO concentration monitored at the scrubber-I outlet is shown in Fig. 3A, which also includes the NO removal efficiency. These data were collected with experimental conditions maintained constant at: gas flow rate = 0.121 m s−1 , liquid flow rate = 0.025 m s−1 , HNO3 = 6 mol L−1 and Ag(I) in electrochemical cell = 0.1 mol L−1 . One could see in Fig. 3A that 100 mg L−1 NO was removed rapidly, and it was complete within 5 min. The total removal efficiency was 100% for NO in scrubber-I, and it sustained for as long as 120 min. Fig. 3B shows the variation of the concentration and removal efficiency for NOx based on the total concentration of NO and NO2 measured at the inlet and outlet of scrubber-I. NOx concentration also showed a decrease with treatment time, but there was always some residual NOx even at longer treatment times, consequently showing lesser removal efficiencies, compared to NO removal. It was 88% for 100 mg L−1 . It is clear that a high degree of conversion and removal of nitrogen oxides was achieved by using Ag(II) redox mediator in nitric acid medium for the oxidation . Thus, the removal of nitrogen oxides (NOx ) containing NO and NO2 by means of a wet scrubbing technique using Ag(II)/Ag(I) redox system appears to be very engaging. Since the Ag(II)/Ag(I) redox couple has a higher standard reduction potential (E0 = 1.98 V (NHE)) with strong oxidizing power, the following reaction mechanism could be predicted for oxidation of NO and NO2 by Ag(II) ions in scrubber-I [19,21]: 2NO + 5Ag(II) + 3H2 O → HNO3 + NO2 + 5Ag(I) + 5H+ NO2 + Ag(II) + H2 O → HNO3 + Ag(I) + H
+
HNO2 + 2Ag(II) + H2 O → HNO3 + 2Ag(I) + 2H
(3) (4)
+
(5)
During the scrubbing treatment, the outlet gas from scrubber-I was found to contain no trace of NO gas, indicating that the total
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159
Fig. 3. (A) NO outlet concentration profiles and NO removal efficiencies as a function of time in scrubber-I. (B) NOx outlet concentration profiles and NOx removal efficiencies as a function of time in scrubber-I. [Experimental conditions: gas superficial velocity = 0.121 m s−1 ; liquid superficial velocity = 0.025 m s−1 ; Ag(I) concentration in cell = 0.1 M; HNO3 concentration = 6 M; T = 25 ◦ C.]
amount of NO charged into the scrubber-I was completely oxidized by the Ag(II) ions in scrubber-1 at the fixed gas and liquid velocities. This means NO was oxidized very rapidly to HNO3 by Ag(II) ions following the reaction step described in Eq. (3). Bringmann et al. [21] showed that when the Co(III)/Co(II) system in 3 mol L−1 H2 SO4 was used as a redox mediator, a removal efficiency of 36% was observed for NOx , and the low efficiency was attributed to kinetic inhibition. In contrast to the Co(III)/Co(II) system, the Mn(IV)/Mn(III) in 6 mol L−1 H2 SO4 was found by the same authors to give an efficiency of >90%. Aurousseau et al. [19] studied the oxidation of nitrogen oxides in ceric solution by the liquid-phase oxidation of nitrous acid, and it was shown that the oxidation rate of NOx by Ce(IV) was largely reduced by increasing the acid concentrations of both H2 SO4 and HNO3 . Kleifges et al. [18] examined direct and indirect NO reduction with some complexing agents and redox mediators . But in all these systems, the NOx conversion and removal efficiencies were low. The present work clearly indicate that the Ag(II)/Ag(I) redox mediator system was more stronger with quite a high removal of NO and NOx from waste gases. 3.2. Removal of NOx in scrubber-II The oxidation of NO in scrubber-I was always accompanied with a small amount of NO2 gas in the scrubber outlet gas. Note that the outlet gas from scrubber-I is free from NO. The unconverted NO2 was treated further with 3 mol L−1 HNO3 in scrubber-II (liquid flow rate = 0.049 m s−1 , gas flow rate = 0.121 m s−1 ), where it was removed by absorption in to aqueous nitric acid: NO2(gas)
H2 O, HNO3
−→
NO2(dissolved)
(6)
Table 2 lists the NOx concentrations measured at the inlet and out let of scrubber-II. The NOx removal efficiency in scrubber-II was found to be around 50%, except for the case of 100 mg L−1 NO concentration in the initial feed, indicating that only 50% of NOx was
removed by the washings by 3 mol L−1 , and the rest of NOx came out of the outlet. In Table 2 also included is the total removal efficiency for NOx across both scrubber-I and scrubber-II, calculated as per Eq. (2) taking in to account of the NO2 gas concentration present in the initial NO-air feed. Very effective removal of nearly 90% was evident for NOx removal by introducing the second stage scrubbing. It should be mentioned here that the initial NO-air mixture feed to scrubber-I always showed the presence of some quantities of NO2 around 10–15% of the NO concentration possibly due to air oxidation of NO. Thus, it becomes clear that when the NO-air gas mixture was treated in scrubber-I, by washing the gas with 6 mol L−1 HNO3 involving absorption/chemical oxidation reaction by electrogenerated Ag(II) oxidant, followed by an acid washing of the scrubbing-I outlet gas by 3 mol L−1 HNO3 in scrubber-II yielded very satisfactory results concerning the flue gas removal: 100% NO removal and 90% NOx removal. The process efficiency could be attributed to favorable conditions provided by scrubber-I, which functioned both as reactor and scrubber with fast chemical reaction between Ag(II) and NO along with simultaneously good absorption, further assisted by a thorough acid washings in scrubber-II functioning as a traditional scrubber column. 3.3. Effect of NO feed concentration Various concentrations of NO in the feed, in the range 100–400 mg L−1 , were studied to find out the effect of NO feed concentration on the removal performance of Ag(II) redox mediator. The results on the residual concentration in the scrubber-I gas outlet and the corresponding efficiencies for the removal of NO and NOx are shown in Fig. 3A and B, respectively. It is clear that with increase in NO feed concentration, the NO removal from the feed was slower (Fig. 3A). A comparison of the
Table 2 NOx removal efficiency in scrubber-II and in combined scrubber-I and scrubber-II for different initial NO-air feed mixtures. NO concentration in NO-air feed (mg L−1 )
100 200 300 400 a b
NOx removal efficiency in scrubber-IIb
NOx removal efficiency through scrubber-Ia and scrubber-IIb
NOx inlet to scrubber-II (mg L−1 )
NOx outlet of scrubber-II (mg L−1 )
Removal efficiency (%)
NOx inlet to scrubber-I (mg L−1 )
NOx outlet of scrubber-II (mg L−1 )
Removal efficiency (%)
14 36 59 89
13 21 31 40
7 42 47 55
119 232 338 457
13 21 31 40
89 91 91 91
In scrubber-I: scrubbing solution = 6 mol L−1 HNO3 , Ag(I) concentration = 0.1 mol L−1 , gas flow rate = 0.121 m s−1 , liquid flow rate = 0.025 m s−1 . In scrubber-II: scrubbing solution = 3 mol L−1 HNO3 , gas flow rate = 0.121 m s−1 , liquid flow rate = 0.049 m s−1 .
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residual NO concentration at 1 min clearly demonstrates that it was 3, 15, 29 and 78 mg L−1 for the initial NO feed concentrations 100, 200, 300 and 400 mg L−1 , respectively. However, in spite of such a difference in removal rate, complete degradation with almost 100% removal was attained even for higher feed concentrations at sufficiently longer treatment times beyond 5 min. The removal of NOx also showed a similar behavior (Fig. 3B); however, with somewhat lower efficiencies even when the treatment time was prolonged. Also, the feed concentration showed a definite inhibiting influence on the removal efficiency for NOx removal. For example, the NOx removal decreased from 88 to 81% when the feed concentration was increased from 100 to 400 mg L−1 . These observations point-out that Ag(II) was able to oxidize NO to NO2 (Eq. (3)) more effectively, but not NO2 to HNO3 (Eq. (4)) completely. Moreover, the added NO appeared to slowdown the NO2 oxidation by Ag(II), and the reasons for such a behavior is difficult to understand at present. 3.4. Effect of gas superficial velocity on NO and NOx removal in scrubber-I To evaluate the effect of the gas velocity on the removal efficiency of NO and NOx in scrubber-I, a series of experiments were performed at different gas superficial velocities in the range of 0.061–0.243 m s−1 at a fixed NO feed concentration of 400 ppm and superficial liquid velocity of 0.025 m s−1 . As shown in Fig. 4, the high removal efficiency for NO (100%) slightly decreased at higher gas flow rate possibly as a result of decrease in gas/liquid contact time at higher gas flow rates. The results of Fig. 4 indicate that the decreasing contact time exerted more influence on the removal of NOx , as it showed a decrease of efficiency, although smaller, from 80 to 76% when the gas flow rate was increased from 0.061 to 0.243 m s−1 . 3.5. Effect of liquid superficial velocity NO and NOx removal in scrubber-I Experiments were carried out to investigate the effect of the superficial velocity of the scrubbing solution (6 mol L−1 HNO3 containing 0.1 mol L−1 Ag(I) in cell) on NO and NOx removal efficiency in scrubber-I. The liquid superficial velocities were varied from 0.012 to 0.049 m s−1 keeping the gas velocity and NO feed concentration constant at 0.121 m s−1 and 400 mg L−1 , respectively. Fig. 5 shows
Fig. 5. NO and NOx removal efficiencies as a function of superficial liquid velocities in scrubber-I. [Experimental conditions: feed NO concentration = 400 ppm; superficial gas velocity = 0.121 m s−1 ; Ag(I) concentration in cell = 0.1 M; HNO3 concentration = 6 M; T = 25 ◦ C.]
the removal efficiencies for NO and NOx as a function of liquid velocity. It is clear that the removal of both NO and NOx increased with liquid flow rate: the NO removal from 98 to 100%, and the NOx removal from 76 to 81. Thus, within the experimental range studied in the present work, 0.025 m s−1 liquid flow rate was optimum for higher NO and NOx removal from NO-air flue gas mixture. 3.6. Effect of Ag(I) concentration Silver(I) ion concentration in the range 0.01–0.1 mol L−1 was chosen to study the effect of mediator ion concentration on NO removal. Low concentrations of silver ion were selected in these studies to achieve high removal efficiencies by establishing suitable experimental conditions (gas flow rate, liquid flow rate, nitric acid concentration, electrolysis current, electrolysis time, etc.) in view of the expensive nature of silver and its compounds. The other important reason to prefer low silver ion concentration in MEO based application was that the previous studies [29] indicated that the percentage conversion of Ag(II) formation in a batch electrochemical reactor with recirculation decreased upon increasing the concentration of Ag(I) in the higher concentration range 0.1–1.0 mol L−1 in 8 mol L−1 HNO3 at 25 ◦ C. Fig. 6 shows the residual NO concentration in the scrubber-I outlet gas for different Ag(I) concentrations from experiments at fixed NO feed (400 mg L−1 ), gas flow rate (0.121 m s−1 ), liquid flow rate (0.025 m s−1 ) and HNO3 concentration (6 mol L−1 ). The removal efficiencies are also plotted. The results indicate that the NO concentration in the scrubber-I gas outlet decreased more rapidly, leading to increasingly higher removal, with increase in the Ag(I) mediator ion concentration. The efficiency was 90% at the low Ag(I) concentration studied, i.e., 0.01 mol L−1 , and the maximum 100% attained when the Ag(I) concentration reached 0.05 mol L−1 . Thus, a low concentration of 0.05 mol L−1 Ag(I) was enough in the scrubber solution for the efficient removal of NO with removal degree reaching 100%. The NOx removal was also highest at 0.05 mol L−1 of Ag(I). 3.7. Effect of HNO3 concentration
Fig. 4. NO and NOx removal efficiencies as a function of superficial gas velocities in scrubber-I. [Experimental conditions: feed NO concentration = 400 ppm; superficial liquid velocity = 0.025 m s−1 ; Ag(I) concentration in cell = 0.1 M; HNO3 Concentration = 6 M; T = 25 ◦ C.]
The acid concentration is an important parameter in MEO based processes, since it directly influences the yield of the redox mediator at the electrode surface, which in turn can affect the gas removal efficiency [19,20,29]. Thus, the removal of NO by Ag(II)/Ag(I) redox
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Fig. 6. NO outlet concentration profiles and NO removal efficiencies as a function of Ag(I) concentration in scrubber-I. [Experimental conditions: feed NO concentration = 400 ppm; superficial gas velocity = 0.121 m s−1 ; superficial liquid velocity = 0.025 m s−1 ; HNO3 concentration = 6 M; T = 25 ◦ C.]
161
Fig. 7. NO outlet concentration profiles and NO removal efficiencies as a function of HNO3 concentration in scrubber-I. [Experimental conditions: feed NO concentration = 400 ppm; superficial gas velocity = 0.121 m s−1 ; superficial liquid velocity = 0.025 m s−1 ; Ag(I) concentration in cell = 0.1 M; T = 25 ◦ C.]
context of its NO and NOx removal by mediation. mediation in scrubber-I was investigated with various HNO3 concentrations (3–6 mol L−1 ). The results are presented in Fig. 7 for 400 ppm NO feed gas at gas flow rate = 0.121 m s−1 , liquid flow rate = 0.025 m s−1 with Ag(I) maintained at 0.1 mol L−1 . Note that the NO removal rate, and hence the removal efficiency, significantly increased as the HNO3 acid concentration increased. 92% removal efficiency at 3 mol L−1 HNO3 was seen to increase to 100% when HNO3 was at its highest value 6 mol L−1 used in the present work. The 6 mol L−1 HNO3 also caused highest removal of NOx as well. The increase in the performance of Ag(II)/Ag(I) redox system at higher HNO3 acid concentration towards NO and NOx removal might be correlated with the availability of more Ag(II) ions for mediation as a result of increased electrogeneration of Ag(II) from Ag(I) due to higher stability of [Ag(II)NO3 ]+ complex at higher acid concentrations [29]. A 45% conversion of Ag(I) (0.1 mol L−1 ) to Ag(II) in 8 mol L−1 HNO3 was reported [29], when the anolyte was circulated at relatively low circulation rates of 100 mL min−1 (0.0085 m s−1 ). The efficient performance of Ag(II)/Ag(I) system at high HNO3 acid concentrations (Fig. 7) is highly encouraging in the
3.8. Ag(II)/Ag(I) MEO system versus Ce(IV)/Ce(III) MEO system To highlight the efficient performance of Ag(II) mediator in NOx removal process, the previously studied Ce(IV) mediator [19,20] was investigated in the removal of NOx process and compared. Detailed work on Ce(IV)/Ce(III) redox mediator system towards MEO of several aqueous organic pollutants were carried out by the present authors [26–28,30], and 3 mol L−1 HNO3 acid was found to offer better conditions for efficient performance of this redox couple [30]. Thus, 3 mol L−1 HNO3 was used as the acid medium for Ce(IV)/Ce(III) in the comparative studies. 3.8.1. Effect of mediator ion concentration Three different concentrations of Ce(III) ca. 0.5, 0.75 and 1.0 mol L−1 in 3 mol L−1 HNO3 were used to generate Ce(IV) in the divided electrochemical cell, and circulated through scrubber-I at 0.025 m s−1 flow velocity. NO-air gas mixture of 400 mg L−1 NO feed concentration admitted in to scrubber-I at 0.121 m s−1 gas flow rate was used in this study. Fig. 8 shows the corresponding removal effi-
Fig. 8. NO and NOx removal efficiencies as a function of redox ion concentration in scrubber-I: (A) Ce(III) concentration in 3 M HNO3 for cerium MEO system; (B) Ag(I) concentration in 6 M HNO3 for silver MEO system. [Experimental conditions: feed NO concentration = 400 ppm; superficial gas velocity = 0.121 m s−1 ; superficial liquid velocity = 0.025 m s−1 ; T = 25 ◦ C.]
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Table 3 NO and NOx removal efficiencies for Ce(IV)/Ce(III) and Ag(II)/Ag(I) MEO systems at different temperatures and HNO3 acid concentrations in scrubber-I (gas flow rate = 0.121 m s−1 , liquid flow rate = 0.025 m s−1 , NO feed concentration = 400 mg L−1 ). Ce(IV)/Ce(III)a
Temperaturec (◦ C)
HNO3 d concentration (mol L−1 )
a b c d
Ag(II)/Ag(I)b
NO removal efficiency (%)
NOx removal efficiency (%)
NO removal efficiency (%)
NOx removal efficiency (%)
15 25 35 45
43 43 43 40
37 37 37 34
100 100 100 95
81 81 80 67
3 4 5 6
43 50 53
37 43 47
92 95 97 100
72 78 79 81
Ce(III) concentration = 0.5 mol L−1 . Ag(I) concentration = 0.1 mol L−1 . HNO3 concentration = 3 mol L−1 for Ce(IV)/Ce(III) system, 6 mol L−1 for Ag(II)/Ag(I) system. Temperature = 25 ◦ C for both Ce(IV)/Ce(III) and Ag(II)/Ag(I) systems.
ciency results for NO and NOx as a function of Ce(III) concentration at 25 ◦ C. It can be noticed that the removal of both NO and NOx increased with Ce(III) concentration. For a comparison, the NO and NOx removal results obtained with Ag(II)/Ag(I) redox system, collected under the same experimental conditions of gas and liquid flow rates and 400 mg L−1 NO feed, were also plotted. Two results are strikingly clear on comparison. The silver based MEO system was highly efficient showing complete (100%) removal of NO and 80% removal of NOx , which is quite contrast to only ≈46% for NO and ≈38% for NOx offered by cerium MEO system. The other point of interest about Ag(II)/Ag(I) ions in Fig. 8 is: these significantly higher removals were provided by silver ion concentrations in the range 0.01–0.1 mol L−1 , which were ten times lower than those of cerium ions, 0.1–1 mol L−1 . Thus the unique nature of silver based MEO system is evident.
3.8.2. Effect of temperature and HNO3 acid concentration The temperature is one of the important process parameters because flue gases from different sources are at different high temperatures. In the present work, temperature of the scrubbing solution was varied in the range 15 to 45 ◦ C (gas flow rate = 0.121 m s−1 , liquid flow rate = 0.025 m s−1 , NO feed concentration = 400 mg L−1 ). The removal efficiencies for NO and NOx in the scrubber-I are listed in Table 3 for both the redox systems. Note that the silver system was with Ag(I) concentration = 0.1 mol L−1 and HNO3 = 6 mol L−1 , while the cerium system was with Ce(III) at 0.5 mol L−1 and HNO3 at 3 mol L−1 . Table 3 shows that within this small temperature range, 15–45 ◦ C, almost no variation was observed for the cerium system in the removal efficiencies of both NO and NOx . The silver system also showed a similar behavior only towards NO, but there was a marginal decrease for NOx . Besides, in a similar attempt to examine the acid concentration effect, HNO3 concentration was changed from 3 to 6 mol L−1 (gas flow rate = 0.121 m s−1 , liquid flow rate = 0.025 m s−1 , NO feed concentration = 400 mg L−1 , T = 25 ◦ C) and the removal results were collected for silver system with Ag(I) concentration = 0.1 mol L−1 , and for cerium system with Ce(III) = 0.5 mol L−1 . Beneficial effect was observed with increase in acid concentration for both silver and cerium systems in their performance towards NO and NOx removal as shown in Table 3. Thus, the acid effect and temperature effect show that the response of both the systems was similar to the enforced process parameters; however, the strong catalytic effect of silver system always remained at all conditions.
4. Conclusions The following are the important findings of our study: (1) Ag(II) mediator based MEO system showed complete 100% removal of NO and 80% NOx removal from simulated NO-air flue gas mixture in a single stage gas scrubbing operation. Good improvement was shown in combination with a second stage gas scrubbing by a simple HNO3 wash, wherein the overall removal of even the NOx gas was improved to 90%. (2) The rate of removal of NO and NOx was slowed somewhat when the NO feed was increased. (3) While the gas velocity tended to decrease the NO and NOx removal efficiencies, the liquid flow rate assisted the removal process to some extent. (4) Low silver ion concentrations in the range 0.01–0.1 mol L−1 were found to be enough to cause extensive NO and NOx removal, and total (100%) NO removal was achieved when the concentration of Ag(I) in the flowing scrubbing liquid reached 0.05 mol L−1 . (5) Our detailed study involving NO and NOx removal by mediation clearly established that Ag(II)/Ag(I) based MEO system was superior in terms of low levels of mediator concentration showing several times higher removal efficiencies, compared to Ce(IV)/Ce(III) based MEO system. (6) In a narrow temperature range of 15 to 45 ◦ C, both silver and cerium systems showed almost no variation in NO removal; but both systems showed enhanced removal with increase in acid concentration; however, the strong catalytic effect of silver system always remained at all conditions. (7) A complete understanding of the underlying processes need detailed studies on determination of mass transfer parameters (interfacial area and mass transfer coefficient) of the scrubber, absorption of NOx into Ag(II) containing solutions, the kinetic constants of the liquid phase oxidation of NOx , etc. These and estimation of reaction stoichiometry needed for arriving at current efficiency, energy consumption, etc., are under progress, though at present we do not have data to report. Acknowledgements This work was supported by several funding agencies: the Korea Ministry of Environment as “The Eco-technopia 21 project”, the Ministry of Commerce, Industry and Energy (MOCIE) through Regional Innovation Centre (RIC) project, the Korea Research Foundation and the Korean Federation of Science and Technology Society Grant funded by the Korean Government (MOEHRD, Basic Research Promotion Fund). K. Chandrasekara Pillai wishes to thank the authorities of the University of Madras, Chennai 600005, India for granting sabbatical leave.
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