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Removal of NO from flue gas by aqueous chlorine-dioxidescrubbing solution in a lab-scale bubbling reactor

Bal Raj Deshwal a, Dong Seop Jin b, Si Hyun Lee b, Seung Hyun Moon b,Jong Hyeon Jung c, Hyung Keun Lee b,∗

a Department of Chemistry, A.I.J.H.M. College, Rohtak 124001, Haryana, Indiab Korea Institute of Energy Research, Daejon 305 600, South Korea

c Department of Environmental Engineering, Sorabol College, Kyungbuk – 780 711, South Korea

bstract

The present study attempts to clean up nitric oxide from the simulated flue gas using aqueous chlorine-dioxide solution in the bubbling reactor.hlorine-dioxide is generated by chloride–chlorate process. Experiments are carried out to examine the effect of various operating variables like

nput NO concentration, presence of SO2, pH of the solution and NaCl feeding rate on the NOx removal efficiency at 45 ◦C. Complete oxidation ofitric oxide into nitrogen dioxide occurred on passing sufficient ClO2 gas into the scrubbing solution. NO is finally converted into nitrate and ClO2

s reduced into chloride ions. A plausible reaction mechanism concerning NOx removal by ClO2 is suggested. DeNOx efficiency increased slightly

ith the increasing input NO concentration. The presence of SO2 improved the NO2 absorption but pH of solution showed marginal effect on NO2

bsorption. NOx removal mechanism changed when medium of solution changed from acidic to alkaline. A constant NOx removal efficiency ofbout 60% has been achieved in the wide pH range of 3–11 under optimized conditions.

eywords: Flue gas; Bubbling reactor; Chlorine-dioxide; NOx removal; Mechanism

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. Introduction

.1. Chlorine-dioxide: a new generation oxidant

Chlorine-dioxide is a novel neutral oxy-chlorine specieshich is powerful oxidant and disinfectant. In the recent years,

t has attracted significant commercial attention not only dueo environmental concern but also due to its wide applica-ions in the fields of bleaching, oxidation and disinfection. Its commercially used in textile, paper, fat and pulp bleach-ng, waste water treatment, water purification, removal of iron,anganese, H2S and phenols from industrial wastes, medical

reatment, sanitation, food processing, fumigation, deodoriza-

ion and aquiculture, etc. The chlorine-dioxide-based bleachingrocess, so-called ECF technology [1], has become quite popularn the production of bleached chemical pulps. Chlorine-dioxide

∗ Corresponding author. Tel.: +82 42 860 3647.E-mail address: hklee@kier.re.kr (H.K. Lee).

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s a very powerful biocide and rapidly inactivates most of micro-rganisms over a wide pH range. It helps in eliminating taste anddor problems from drinking waters.

Though chlorine-dioxide is a well known strong oxidantapable of oxidizing NO into NO2 yet removal of NOx fromhe flue gas using ClO2 has, to the best of our knowledge, nevereen reported in the literature. Therefore, treatment of industrialaste gases by aqueous chlorine-dioxide solution is the majorbjective of the present study.

Chlorine-dioxide is produced from acid solutions of eitherodium chlorite [2–4] or sodium chlorate [5–10]. Most of themall and medium scale generators use sodium chlorite as therecursor material. Other applications where large quantities ofhlorine-dioxide are needed, utilize sodium chlorate. Thoughhe conditions for the production of ClO2 from sodium chlorite

an be controlled better than from sodium chlorate but chlorite isore expensive and unstable chemical. So from industrial point

f view, sodium chlorate is the most suitable raw material forlO2 generation.

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sdTNbatscpwtpcSion chromatograph (IC) or titrated iodometrically using auto-titrator (Metrohm-Swiss). The potentiometric titration systemincluded a 670 titroprocessor, 730 sample changer, 665 dosimatand platinum electrode. The inlet and outlet NOx concentra-

Table 1The operating conditions of bubbling reactor and chlorine-dioxide generator

Bubbling reactor Chlorine-dioxide generator

Reactor volume 5 L Reactor volume 2.5 LTemperature 45 ◦C Temperature 45 ◦CpH 3–12 (3.5) N2 flow rate 2 L/minInput SO2 conc. 0–1800 ppm NaCl conc. 1–2 MInput NO conc. 150–1180 ppm H2SO4 conc. 10–12N

Chlorine-dioxide can be formed by reduction of chlorate withwide variety of organic and inorganic reducing agents in a

elatively concentrated acid solution as follows:

lO3− +2H+ + e− → ClO2 + H2O (1)

he optimum reaction conditions, by-products and the eco-omics of the process help in deciding the suitable reducinggent. In all chlorate based processes, chloride ion plays a cru-ial role. No chlorine-dioxide is formed if chloride ion is notresent in the reaction medium. With this view, we selectedhloride–chlorate process to generate ClO2 gas for further usen removing NO from the flue gas. The general stoichiometry ofhloride–chlorate process [9,10] can be expressed as:

H+ + 2ClO3− + 2Cl− → 2ClO2 + Cl2 + 2H2O (2)

heoretically, it gives euchlorine, which is a mixture of chlorine-ioxide and chlorine in molar ratio of 2:1. This process isxtremely simple to operate, responds immediately and givesighest yield of ClO2 at the lowest cost among all other com-ercial processes.

.2. Removal of NO

Though the flue gas from stationary sources such as powerlants, incinerators and boilers mainly contains NO and NO2ut the major component of NOx is NO (ca. 90%). Thenert nature of NO has posed a persistent problem in thendustry. Technologies for NOx removal include combustionontrol and post-combustion treatment. Combustion controlims at reducing the NOx formation during combustion of fossiluel. Post-combustion methods include selective non-catalyticeduction (SNCR), selective catalytic reduction (SCR), ther-al DeNOx and scrubbing, etc. Among these technologies,

crubbing methods are economically most competitive and havedvantage of controlling other acid gases and particulates at theame time [11].

In general, additives are added into scrubbing system to firstonvert relatively inert NO into NO2 which can be removed bylkaline absorbents. Aqueous solutions of numerous inorganicnd organic absorbents have been investigated to determine theirffectiveness in NOx removal [11–31]. Sodium chlorite [16–24]as proved the most efficient oxidant among various NO absorp-ion agents. However the problem with sodium chlorite is that itperates best under slightly acidic conditions. It has good oxidiz-ng ability at lower pH but good absorbing ability at higher pH.o pH is a crucial parameter to oxidize NO into NO2 and absorbO2 thereafter. Further, it is quite expensive chemical and it is

tself produced by reduction of chlorine-dioxide as follows:

ClO2 + H2O2 + 2NaOH → 2NaClO2 + O2 + 2H2O (3)

Keeping all these facts in mind, it seems reasonable to use

lO2 directly to oxidize NO. It will not only reduce the cost butlso solve the problem of pH adjustment. Therefore the purposef this study is to clean up NO from flue gas using aqueoushlorine-dioxide solution.

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. Experimental

The experimental system composed of two parts namelyhlorine-dioxide generation unit and flue gas cleansing unit.

.1. Chlorine-dioxide generation unit

ClO2 generation unit consisted of a reactor which is welltirred sealed vessel having total volume of 2.5 L. Concentratedodium chloride solution (2 M) was continuously injected intoeactor at a suitable flow rate by syringe pump. The reactoras filled with 1.5 L solution of sodium chlorate (0.4 M) in

oncentrated sulfuric acid (∼12N). Continuous stirring was pro-ided by a mechanical agitator. Temperature of the reactionessel was controlled within 45 ± 0.1 ◦C by water thermostat.he reactor was wrapped with an aluminium foil to avoid anyhoto-dissociation of ClO2. Analytical grade reagents and dis-illed de-ionized water were used throughout. Nitrogen gas wasurged through the reaction mixture using a bubbling devicet a flow rate of 2 L/min. ClO2 carried by N2 gas was furtherntroduced into bubbling reactor.

.2. Flue gas cleansing unit

Flue gas cleansing unit included simulated flue gas supplyystem, bubbling reactor, pH control system, ClO2 absorber,ata acquisition system, and sampling cum analysis system.he simulated flue gas was obtained by controlled mixing ofO, N2 and O2 using mass flow controllers (MFC). The bub-ling reactor is made up of acrylic material. The inner diameternd height of the reactor are 15 and 45 cm, respectively. Con-inuous stirring was provided by mechanical agitator with apeed of 250 rpm. The temperature of the bubbling reactor wasontrolled within 45 ± 0.1 ◦C, the most common operating tem-erature for wet scrubbing methods. The pH of reaction solutionas controlled by using an auto-pH control system by con-

inuous addition of NaOH (0.2 M) solution with the help oferistalsis pump. The ClO2 absorber (2 L vessel) consisted ofa. 2% carbonate buffered potassium iodide solution (1.5 L).amples from reactor and absorber were analyzed using Dionex

lO2 flow rate 2 L/min NaClO3 conc. 0.2–0.4 Mlue gas flow rate 45 L/min NaCl feeding rate 0.5–1.5 mL/minorced air flow rate 4 L/minSD 4–12 cm (6 cm)

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5NO + 2ClO2 + H2O → 5NO2 + 2HCl (oxidation) (4)

5NO2 + ClO2 + 3H2O → 5HNO3 + HCl (absorption) (5)

Fig. 1. A schematic diagram of bubbling r

ions were analyzed using the NOx analyzer (Chemiluminiscentype, Model: 42C, Thermo Environmental Instruments, USA)fter removing the moisture in the sample conditioner. The O2oncentrations were analyzed with help of dissolved oxygenDO) meter. The overall specifications of the experimental setupre given in Table 1. A schematic diagram of the experimentalystem is shown in Fig. 1.

. Results and discussion

Chlorine-dioxide has been extensively used for oxidation,isinfection and bleaching, but no reference is cited on usingqueous ClO2 solution in the removal of NO. There is nooubt about the oxidizing capability of ClO2. Standard oxida-ion potential of chlorine-dioxide in gas and solution phase is

0.95 and −1.27 V, respectively [32]. If it can oxidize NO intoO2, then it may prove useful for removal of NO from flue gas.ith this view, experiments for the removal of NO have been

onducted using aqueous chlorine-dioxide solution. Effect ofarious parameters, viz., input NO concentration, presence ofO2, NaCl feeding rate and pH of solution has been studied inlab-scale bubbling reactor at 45 ◦C.

.1. Removal of nitric oxide from simulated flue gas

Experiments were carried out at pH of 3.5 and input NOoncentration of 500 ppm in absence of SO2 at 45 ◦C to investi-ate the NOx removal using aqueous chlorine-dioxide scrubbingolution. Fig. 2 displays the variation of pH, NO and NO2 outlet

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for the removal of NO from the flue gas.

oncentration with the passage of time. NOx removal efficiencynd ion concentration are plotted versus time in Fig. 3. It wasbserved that ClO2 can oxidize NO into NO2 completely and aonsistent and reproducible NO2 absorption efficiency of about0% is achieved.

In the acidic medium, a plausible mechanism for the removalf NOx by aqueous chlorine-dioxide solution is as follows:

ig. 2. Variation in the outlet concentration of NO, NO2 and pH with the passagef time.

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ig. 3. NOx removal and ion concentration with passage of time (pH 3.5, inputO = 500 ppm, T = 45 ◦C).

he overall reaction for NOx removal may be written as:

NO + 3ClO2 + 4H2O → 5HNO3 + 3HCl (6)

hlorine gas produced along with chlorine-dioxide as suggestedn Eq. (2) is also a strong oxidant. Yang et al. [11] reportedhat chlorine is capable of oxidizing NO into NO2 and nitrate.toichiometry of reaction of chlorine with NO can be expresseds:

O + Cl2 + H2O → NO2 + 2HCl (7)

NO + 3Cl2 + 4H2O → 2HNO3 + 6HCl (8)

Formation of nitrate and chloride as suggested in the aboveechanism was confirmed by analyzing the sample from bub-

ling reactor using Dionex ion chromatograph. The variation ofon concentration with time is presented in Fig. 3.

.2. NOx removal in presence/absence of sulfur dioxide

NOx removal was also studied at input NO concentrationf 350 ppm, pH of 3.5 in absence as well as in presence ofO2. Fig. 4 displays the variation of pH, NO and NO2 outletoncentration with the passage of time. SO2 and NOx removal

ig. 4. Variation of NO and NO2 concentration with time (input NO = 350 ppm,nput SO2 = 500 ppm, pH 3.5, T = 45 ◦C).

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ig. 5. Simultaneous removal SO2 and NOx with time (input NO = 350 ppm,nput SO2 = 500 ppm, pH 3.5, T = 45 ◦C).

fficiencies are presented in Fig. 5. It is observed that ClO2an clean up both SO2 and NO quite efficiently. The maximumeSOx and DeNOx efficiency obtained at an optimum NaCl

2 M) feeding rate of 0.5 mL/min are about 100 and 60%, respec-ively. In the acidic medium, SO2 removal by chlorine-dioxideppears to take place by the following mechanism:

SO2 + 2ClO2 + 6H2O → 5H2SO4 + 2HCl (9)

.3. Effect of SO2 concentration on NOx removal

Experiments were carried out at pH of 3.5 and input NOoncentration of 350 ppm to investigate the effect of input SO2oncentration on NOx removal efficiency. Fig. 6 shows that NOx

emoval increased from 51 to 58% when input SO2 concentra-ion was increased from 200 to 700 ppm. SO2 is moderatelyoluble in aqueous solution and undergoes hydrolysis to form

− 2− 2−

20]. The equilibrium concentrations of all these species dependpon partial pressure of SO2 and pH of the solution. These S(IV)pecies are believed to catalyze the NO2 absorption [33,34].

ig. 6. Effect of input SO2 concentration on NOx removal efficiency (pH 3.5,= 45 ◦C, input NO = 350 ppm).

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ig. 7. Effect of input NO concentration on NOx removal efficiency in presencend absence of SO2 (T = 45 ◦C, pH 3.5).

.4. Effect of input no concentration on NOx removalfficiency

Effect of input NO concentration on the NOx removal haseen investigated at pH of 3.5. In absence of SO2 gas, it is foundhat NOx removal efficiency increased from 50 to 61% whenO input concentration is increased from 150 to 1150 ppm. Inresence of SO2 (500 ppm), NOx removal efficiency is foundetter than in absence of it as can be seen from Fig. 7. Thislight increase in NOx removal in presence of SO2 may be dueo reaction of S(IV) species with nitrogen dioxide as predictedy earlier researchers [33,34].

.5. Effect of NaCl feeding rate

We also performed some experiments to investigate the effectf NaCl feeding rate on NOx removal efficiency at pH of 3.5,nput NO concentration of 500 ppm in absence of SO2 at 45 ◦C.

t is apparent from Fig. 8 that NOx removal increases sharplyith increasing NaCl (2 M) feeding rate up to 0.4 mL/min and

hereafter it attains a steady state on achieving ∼60% DeNOx

fficiency. NO oxidation remained 100% complete throughout.

ig. 8. Effect of NaCl feeding rate on NOx removal in absence of SO2 (pH 3.5,nput NO = 500 ppm).

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he increasing NaCl feeding rate into ClO2 generator increasedhe rate of ClO2 generation (Eq. (2)), which subsequently facil-tated the NO oxidation as well as NO2 absorption. The excesshlorine-dioxide in the scrubbing solution did not improve theO2 absorption rate. Thus the optimum concentration and feed-

ng rate of sodium chloride solution can be easily decided on theasis of input NO concentration.

.6. Effect of pH

In the present study, pH of reaction medium was varied fromto 11 at constant input NO concentration of 500 ppm at 45 ◦C

nd its effect on NOx removal, output NO and NO2 concentra-ion is reported in Fig. 9. At constant NaCl (2 M) feeding rate of.5 mL/min, there occurred no change in the NOx removal effi-iency when pH is increased from 3 to 6 but thereafter removalfficiency decreased in the pH range of 6–8 and then increasedp to pH of 9 and attained a steady state. The sudden decrease inOx removal in the pH range of 6–8 and then increase in NOx

emoval in pH > 8 may be attributed to the different mechanismsnvolved in acidic and alkaline media.

In acidic medium (pH < 7), NOx removal mechanism is con-rolled by ClO2 (as proposed in Eqs. (4)–(6)). When pH reaches7, the disproportionation [3,35] of chlorine-dioxide slowly

tarts and speed up with increasing pH. It leads to formationf chlorite and chlorate as follows:

ClO2 + 2NaOH → NaClO2 + NaClO3 + H2O (10)

he 50% of ClO2 (from Eq. (10)) is thus converted into sodiumhlorite, which is again a strong oxidant and absorbent forO and NO2, respectively. In alkaline medium (pH > 7) NOx

emoval mechanism thus changed and is now controlled byaClO2 as follows:

NO + NaClO2 → 2NO2 + NaCl (oxidation) (11)

4NO2 + NaClO2 + 4NaOH

→ 4NaNO3 + NaCl + 2H2O (absorption) (12)

ig. 9. Effect of pH on NOx removal at different NaCl feeding rates (inputO = 500 ppm, T = 45 ◦C).

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4NO + 3NaClO2 + 4NaOH

→ 4NaNO3 + 3NaCl + 2H2O (13)

he above mechanism has been discussed in detail by severalorkers [16,18–20,22–24].The decrease in NOx removal efficiency at pH of ≥7 is

elieved due to insufficient chlorine-dioxide and chlorite in reac-ion solution. To confirm that NOx removal mechanism changesn the alkaline medium, the NaCl feeding rate was enhancedo that more chlorite is formed by disproportionation of unusedissolved ClO2. It was found that NOx removal attained a steadytate at higher NaCl feeding rate due to formation of sufficienthlorite. At NaCl (2 M) feeding rate of 1.5 mL/min, constantOx removal efficiency of around 60% was achieved in the wideH range of 3–11.

. Conclusions

Chlorine-dioxide has proved the most promising among var-ous additives used so far in removal of NOx. It has severaldvantages over sodium chlorite, which is supposed to be theest additive so far. The merits of chlorine-dioxide can be sum-arized as:

It exhibited 100% NO oxidation and ∼60% NOx removalefficiency which is comparable with that observed using otheradditives.Comparing the capital cost, chlorine-dioxide is much cheaperthan sodium chlorite as the later is produced itself by reductionof chlorine-dioxide. Hence it is more appropriate to utilizeClO2 instead of first converting it into chlorite and then usingit for NOx removal.When NaClO2 is used for NOx removal, pH of the scrubbingsolution needs a tight control. The pH should be low enoughto provide NaClO2 high oxidizing ability and high enough toallow absorption of NO2 thereafter. However in case of ClO2,there is no need of such pH adjustment. It is as effective atlow pH as at high pH.Sodium chlorite has high oxidizing power at low pHbut simultaneously it decomposes at lower pH producingchlorine-dioxide which is another secondary pollutant. How-ever if ClO2 is supplied from outside then it is easy to controlthe supply of ClO2 to avoid its excessive emission into atmo-sphere.Operation of the system is comparatively easier.It is also easy to handle the waste water by simply adjustingthe pH because the major components in the effluent are HCland HNO3.

Based upon the above merits, it would be reasonable to con-lude that chlorine-dioxide is a promising additive for NOx

emoval due to wide pH range, cost effectiveness and easyperation.

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cknowledgements

Bal Raj Deshwal is grateful to Korean Federation of Sci-nce and Technology (KOFST) and Korea Institute of Energyesearch (KIER) for inviting him as a Visiting Scientist underrain Pool Program.

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