pilot study of absorption of no2 with na2s aqueous solutions

Pilot Study of Absorptionof NO2 with Na2S AqueousSolutionsLuke Chen,a Kun-Fon Lin,a Chen-Lu Yangba Department of Water Resources and Environmental Engineering, Tamkang University, Tamsui, New Taipei City, Taiwan;[email protected] (for correspondence)b Advanced Technology and Manufacturing Center, University of Massachusetts, Dartmouth, Fall River, MA 02723

Published online 5 April 2011 in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/ep.10551

A pilot-scale study of sodium sulfide (Na2S) aque-ous solutions for NO2 absorption was conducted toobtain field data for process design and system opera-tions. Considered parameters include Na2S concen-trations in the scrubbing solutions, gas mass flowrate, liquid mass flow rate, liquid/gas ratio (L/G), pH,and gas residence time. The effects of these parame-ters on NO2 absorption in a packed tower were inves-tigated thoroughly. The height of transfer unit of thesystem was deducted as a function of the testedparameters that can be used for designing a NOx

scrubbing system. The absorption of NO2 is not sensi-tive to the inlet NO2 concentration. However, asexpected, the NO2 absorption increases with theincrease of L/G ratio. An L/G greater than 2.5 isessential to a practical NO2 scrubbing application.The pilot test suggested that the scrubbing solution iswith Na2S concentration greater than 0.0015 M atpH 9. The solution effectively removed more than90% of the incoming NO2 in the five-foot heightpacked bed scrubber. The study also indicates that agas residence time of 4.5 s is essential to achievemore than 80% NO2 absorption. � 2011 American Insti-tute of Chemical Engineers Environ Prog, 30: 632–639, 2011Keywords: NOx absorption, scrubber system,

sodium sulfide

INTRODUCTION

Oxides of nitrogen (NOx) are generated in high-temperature operations through fuel oxidation (fuel

NOx) and high-temperature fixation (thermal NOx).By manipulating peak temperature, residence time,mixing, and/or oxygen concentration in flame zone,the formation of NOx can be reduced to certainextent. Over the past two decades, intense researchand development efforts have led to a number ofsuccessful tactics in combustion modification for NOx

control. These tactics include low NOx burner, lowexcess air firing, off-stoichiometric combustion, fluegas recirculation, reduced air preheating, and reducedfiring rates. The drawbacks of these techniques arethe relatively low NOx reduction efficiencies and sig-nificant heat loss during the manipulation. Flue gastreatment technologies are used for cases that requirehigher removal efficiencies than those can beachieved by combustion modification. Among theflue gas treatment techniques, selective non-catalyticreduction (SNCR) and selective catalytic reduction(SCR) are widely adapted into industry for NOx emis-sion reduction. SNCR requires a narrow temperaturewindow to operate and is constantly causing ammo-nia slip. SCR, on the other hand, requires high opera-tion cost and considerable facility space. Scrubbingsystems are considered potential low-cost alternativesif the low solubility of nitric oxide (NO) can be over-come [1]. Two-stage chemical scrubbing systems aredesigned to oxidize NO in the first stage and absorbthe NO2 in a second stage. In the first stage, the slowoxidation reaction of NO in air can be promoted byinjecting strong oxidizing agents such as ozone (O3),chlorine dioxide (ClO2), or chlorine (Cl2) into theflue gas, or adding an oxidant such as sodium chlor-ite (NaClO2), hydrogen peroxide (H2O2), sodiumhypochlorite (NaClO) [2] to the scrubbing solution.� 2011 American Institute of Chemical Engineers

632 December 2011 Environmental Progress & Sustainable Energy (Vol.30, No.4) DOI 10.1002/ep

Recently, non-thermal plasmas are under intenseinvestigation for the oxidation of NO in the first stage[3]. After the NO is oxidized to NO2 in the first stage,a second stage for NO2 absorption is critical for thetechnology to be effective for NOx control.

The advantage of a wet scrubbing system is thatthe system removes NOx as well as sulfur dioxidein the flue gas [4–7]. However, the major challengefor the technology is the low solubility of NO. Usu-ally, the NO must be oxidized to NO2 in the flue gasbefore a reasonable degree of absorption can occurin an aqueous system [8–15]. A pilot-scale field testfor NO2 absorption with Na2SO3 was performed tocollect operational parameters for industrial applica-tion [16]. Since Na2SO3 is a derivative of Na2S, it isdesirable to understand the effectiveness of Na2S forNO2 absorption [17]. In this study, a pilot test wasperformed for the NO2 absorption with aqueous Na2Ssolution. The objective of this study is to establishpilot test data for the design and operation of theNOx scrubbing system. How the considered parame-ters, the concentrations of Na2S in the scrubbing solu-tion, liquid/gas ratio, pH of the aqueous solution, gasflow velocity, and residence time affected the NOx

absorption performance were thoroughly investi-gated. The heights of the transfer unit correspondentto the parameters for the design of the NOx absorp-tion system were also deducted.

PROCESS CHEMISTRY

The overall chemical reactions for the absorptionof NO2 by Na2S solution can be described by the fol-lowing reactions [18]:

S�2 þ H2OðlÞ $ HS� þOH� (1)

NO2 þ HS� $ NO�2 þ �HS (2)

2NOðgÞ þO2ðgÞ ! 2NO2ðgÞ (3)

N2O4ðgÞ ! N2O4ðlÞ (4)

N2O4ðlÞ þ H2OðlÞ $ HNO2ðlÞ þHNO3ðlÞ ! NO�2 þ NO�

3

(5)

The study indicated that at high pH condition,the reactions favor the absorption of NO2 in theNa2S aqueous solutions. More S22 ions are dissoci-ated from Na2S solution at pH 9 than that at neutralor acidic conditions. The dominating reactions ofNO2 absorption are reactions (1) and (2). The studyalso indicated that NO2 absorption is most likelythrough hydrolysis of N2O4 as presented in reactions(5).

CHEMICAL ABSORPTION

The column height, ZT of an absorption packtower, can be expressed as below:

ZT ¼ ðVAÞKya

Z b

a

dy

y � y�(6)

where y is gas molar fraction of absorbate, A is thecross-sectional area of absorption tower, V is gasmolar flow rate, Kya is the overall mass transfer coef-ficient, dZ is differential height, and a, b are the inte-gration location along the tower, respectively. Theintegral in Eq. 6 represents the change in vapor con-centration divided by the average driving force and isrepresented as the number of transfer units, NTU.The other part of the Eq. 6 has the unit of length andis represented as the height of transfer unit, HTU.The chemical reaction in the liquid phase reducedthe equilibrium partial pressure of the solute over thesolution, which generally increases the driving forcefor mass transfer. If the reaction is essentially irrever-sible at absorption conditions, the equilibrium partialpressure is zero, and the NTU can be calculated justfrom the change in gas composition. For y* 5 0,

NTU ¼Z b

a

dy

y � y�¼

Z b

a

dy

y¼ ln

ybya

(7)

where yb is the inlet NO2 concentration, ya is the out-let NO2 concentration.

The rate of absorption of NO2 can be evaluated bythe overall mass transfer coefficient Kya which isimplicitly included in HTU.

EXPERIMENT

The NO2 absorption tests were carried out at a pilotplant built by Kunstoff Manufacturer, Co. Figure 1shows the schematic of the pilot-scale flue gas scrub-bing system. The system consists of a gas blendingunit, a packed bed scrubber, a chemical injection andcontrol system, and a NOx monitoring unit. The gasblending unit is capable of producing a wide variety ofgas compositions by mixing air with high concentrationNO2 from cylinders. The NO2-containing air stream isthen passed through the scrubbing tower where theNO2 absorption takes place. Samples are taken to mon-itor the inlet and outlet concentrations of NO2 undervarious settings of parameters to determine the optimalconditions for NO2 absorption with the Na2S aqueoussolutions, and to obtain HTU and Kya information forthe absorption process design reference.

The gas blending system is capable of a total flowrate of 45 m3/min (1600 cfm). The NO2 concentra-tions are varied by injecting NO2 from a 5% gas cylin-der through a mass flow meter. The whole system ismade of fiberglass reinforced plastic, including theblower, except for the NO2 lines which are polypro-pylene tubing. After the NO2 is injected into the airstream, the whole stream is passed into a section ofTellerete Packing to have a better mixing. The well

Environmental Progress & Sustainable Energy (Vol.30, No.4) DOI 10.1002/ep December 2011 633

mixed NO2-containing air stream is then carried intothe gas scrubber where the NO2 absorption occurs.

The packed tower is constructed with a 5-m-longand 0.45-m-diameter polypropylene column with asection of 1.8-m packed bed which is made by ran-domly packed 3.25-inch, K-type Tellerete packing.The top of the column holds a demister head packedwith R-type Tellerete packing for removing entraineddroplets from the gas stream. The entire column sitson a vessel which serves as the scrubbing solutionreservoir.

The scrubbing solution is prepared by mixing Na2Swith tap water and is pumped into the reservoirbefore each experiment. A circulating pump with-draws the scrubbing solution from the reservoir andpumps it up to the top to be sprayed down on thepacked bed, countercurrent to the gas flow. Therough pumping rate is controlled by regulating therecirculation rate of the pump with the final adjust-

ment being made at the Signet 5500 flow meterdownstream from the pump. Experimental conditionsand parameters are presented in Table 1.

A chemiluminescent NOx analyzer is used to meas-ure NO and NO2 concentrations. Basically, the signalcomes from the light emitted from the chemilumines-cent gas phase reaction of NO and ozone (O3). Theozone is generated in situ by a high voltage arcozone generator. The resulting chemiluminescence ismonitored through an optical filter by a high-sensitiv-ity photomultiplier positioned at one end of the reac-tion chamber. Since the analysis is only sensitive toNO, to measure NOx concentrations, the sample gasis diverted through a high-temperature converterwhere the NO2 is converted to NO and the total NOx,NO plus NO2 is detected as NO. The NO2 concentra-tion is the difference between the two readings forNOx and NO. Signals from the NOx analyzer are con-tinuously recorded.

Figure 1. The schematic diagram of the pilot NO2 absorption system.

Table 1. Experimental parameters and operating conditions.

NO2

Concentration (ppm)Na2S(aq)

Concentration (M)Gas mass

rate (kg/m2-h)Liquid mass

rate (kg/m2-h) pH ORP (mV)

50 0.001 1296 2440 8.0 200 6 2050 0.0015 1944 4880 9.050 0.002 2592 7320 10.0

5184 9760 11.0


RESULTS AND DISCUSSION

A series of tests were first conducted to identify asuitable inlet concentration range for further parame-ters study. Various inlet concentrations, ranging from25 to 100 ppm were tested in the system to deter-mine their effect on the NO2 absorption. Within therange of 25 and 100 ppm, NO2 absorption was rela-tively stable at different inlet concentrations. The testsconfirmed that the system performance become lesssensitive to the change of inlet NO2 concentrationswhen the NO2 concentration is over 50 ppm asshown in Figure 2. Thus, an inlet concentration of50 ppm was chosen for further study.

A set of experiments was conducted to explore theeffect of gas and liquid mass flow rates on NO2 absorp-tion. Experiments were conducted at constant NO2 inthe gas phase and Na2S, pH in the scrubbing solutions.The gas mass rate was varied from 1296.1 to 5184.2kg/m2-h, and the liquid mass rate was tested in a rangebetween 2440 and 9760 kg/m2-h. The NO2 absorption

was found to drop significantly when liquid mass flowrate is reduced from 9760 kg/m2-h to 2440 kg/m2-h asshown in Figure 3. When comparing NO2 absorptionchange among different liquid mass flow rate, wefound the NO2 absorption becomes less sensitive toliquid mass flow rate as the rate is over 4880 kg/m2-h.At a aqueous mass rate of 4880 kg/m2-h, more than80% of the incoming NO2 was absorbed into the scrub-bing solution when the gas mass flow rate was kept at1944.2 kg/m2-h. Hence the gas mass flow rate of1944.2 kg/m2-h was chosen for further experiment.

With a liquid mass flow rate at 4880 kg/m2-h anda gas mass flow rate is below 2592 kg/m2-h, a 65%NO2 absorption was achieved as indicated in Figure3. There is a 15% decrease of the NO2 absorptionamong each increase of gas mass flow rate. A reason-able gas–liquid contact time in the packed bed isessential for the operation. To test extreme condi-tions, the least liquid mass flow rate of 2440 kg/m2-hwas pumped through the packed bed at the maxi-mum gas mass flow rate of 5184 kg/m2-h. A less 60%NO2 absorption was found in the test. This indicatesthat there is a lower limit for liquid mass flow rateand an upper limit for gas mass flow rate to be keptto maintain an acceptable NO2 absorption perform-ance in the scrubbing system. To achieve more than60% NO2 absorption, this study indicates that thelower limit for the liquid mass flow rate is 4880 kg/m2-h and the upper limit for the gas mass flow rate is2592 kg/m2-h. For design reference, the HTU of theNO2 absorption system under various gas and liquidmass rates with Na2S concentration of 0.002 M wasprovided in Figure 4.

To illustrate the effect of liquid and gas mass rate onNO2 conversion, a combined parameter, liquid-to-gasratio (L/G) can be introduced. The NO2 absorption atvarious pH with respect to L/G ratios are illustrated inFigure 5. As can be expected, NO2 absorption increaseswith the increasing of L/G ratio. For L/G 5 1, there is49% NO2 absorption and is independent of the pH inthe scrubbing solutions. For the condition of L/G 51.88, only when pH at 9 results in more than 60% NO2

absorption. At an L/G greater than 2.51 for pH at 9.0,

Figure 2. Effect of inlet NO2 concentration on NO2

absorption.

Figure 3. Effect of gas and liquid mass rate on NO2

absorption.

Figure 4. HTU for various gas and liquid mass mate.


more than 80% NO2 absorption was measured. At afixed L/G ratio, NO2 absorption increased from 75% to95% when pH increased from pH 8.0 to pH 9.0. How-ever, the absorption decreased from 95% to 65% whenthe pH in the solution was further increased to pH 10and 11. The optimized pH condition that results inmost effective NO2 absorption is at pH 9. Previous liter-ature [18] indicated that the condition of pH 9 is favor-able for Na2S hydrolysis to release HS2 anions whichare the active component for the NO2 absorption inthe scrubbing system. An L/G greater than 2.51 and atpH 9 are essential for the system to sustain a reason-able NO2 absorption. The HTU of the NO2 absorptionsystem for various L/G ratios and pH were provided inFigure 6 that indicates the required HTU of the absorp-tion system at pH 9.

To estimate chemical consumption in the opera-tion, the effect of Na2S concentration on NO2 absorp-tion was examined at various L/G ratios. A higherNa2S concentration in the solution resulted in ahigher NO2 absorption as expected from data shownin Figure 7. When the system was operated at an L/G 5 1.0 or 1.88, the NO2 absorption was about 40%.

Even with an increased Na2S concentration such as0.002 M in the solution, there was only 50% NO2

absorbed. A 90% NO2 absorption was achieved whena Na2S concentration was kept at 0.002 M and an L/Gratio at 2.51. The NO2 absorption with Na2S solutionof 0.002 M was 20% higher than that of a Na2S solu-tion with 0.0015 M. The more Na2S in the solution,the more HS2 ions are dissociated to react with NO2.Above specific L/G ratio, such as L/G 5 3.77, theeffect of Na2S in the solution on NO2 absorption isno more prevailing. When both NO2 absorption capa-bility and chemical consumption cost are considered,a 0.0015 M Na2S concentration is the optimal condi-tion for the system. The HTU of the NO2 absorptionsystem for various L/G ratios and Na2S concentrationswere provided in Figure 8.

The effect gas–liquid contact indicated by gas resi-dence time in the packed bed on NO2 absorptionwas illustrated in Figure 9. Gas velocity can be calcu-lated from gas flow rate and the dimension of thetower. With 1.5 s gas residence time, a 40% NO2

absorption were observed at all pH conditions. On

Figure 5. Effect of L/G Ratio on NO2 absorptionunder various pHs.

Figure 6. HTU for Various L/G Ratio and pH.

Figure 7. Effect of Na2S solution concentration onNO2 absorption.

Figure 8. HTU for various L/G ratios and Na2S con-centrations.


the other hand, when the gas residence time wasincreased to 4.1 s at pH 9, an 80% NO2 absorptionwas achievable. If a 60% NO2 absorption is requiredto comply with local regulations, a 4.1 s of gas resi-dence time is essential.

Gas velocity can be calculated from gas flow rate andthe dimension of the tower. The scrubber removed 60%of the incoming NO2 for a gas velocity at 0.45 m/s underdifferent pH conditions as shown in Fig. 10. However,when gas velocity was increased from 0.45 m/s to 1.19m/s, the NO2 absorption dropped below 45%. On theother hand, if gas velocity was at 0.3 m/s, less than 0.41m/s, 80% of the incoming NO2 was absorbed into thescrubbing solution. As stated before, the system requiresa sufficient contact time between gas and liquid phasesto complete the chemical absorption process. Since gasvelocity is closely related with the gas residence time, ascrubber volume has to be specified tomeet the requiredremoval efficiency. This usually depends on individualdesign consideration. In this pilot test, a gas velocity of0.45m/s or a residence time of 4.1 s is recommended.

A previous study suggested that Na2SO3 aqueoussolutions are good media for NO2 absorption as well[3]. Two systems for NO2 absorption are compared inFigure 11. The comparison indicates that Na2SO3

aqueous solutions are superior to Na2S aqueous solu-tions in NO2 absorption capability. At a certain L/Gratio, the NO2 absorption increased about 15% whenthe concentration of Na2SO3 was increased from 0.15M to 0.25 M. With the same concentration of 0.015 M,Na2SO3 solution is superior to Na2S solution in NO2

absorption. When the L/G ratio is greater than 3, ahigher NO2 absorption was measured with Na2S sys-tem; however, the difference between the two sys-tems is insignificant. Since the molecular weight ofNa2S is less than that of Na2SO3, when both NO2

absorption efficiency and the chemical consumptioncost are considered; Na2S system is competitive withNa2SO3 for NOx absorption.

The results from this study are also compared withthe results of a laboratory scale test performed by Shenand Rochelle [19]. Table 2 shows the differencebetween the two studies. The comparison indicates thatwider variations of NO2 absorption were conducted inthis pilot-scale test. The results from the pilot-scalestudy are comparable with that from a bench-scale.This study has lay out a solid foundation for scrubberdesign by providing all of the important parameterssuch as concentration and pH of scrubber solution,liquid and gas mass rate, and the gas residence time.

CONCLUSIONS

This pilot-scale study of NO2 absorption by Na2Saqueous solution is concluded with the followingfindings:� To maintain high NO2 absorption, the gas massflow rate needs to be kept at less than 1944.2 kg/m2-h and the liquid mass flow rate has to be over7320 kg/m2-h.

� When L/G ratio is greater than 2.51, the scrubbingsystem can achieve more than 90% of NO2

absorption.

Figure 9. Effect of gas residence time on NO2

absorption.

Figure 10. Effect of gas velocity on NO2 absorption.

Figure 11. Comparison of Na2S and Na2SO3 solutionon NO2 absorption.


� The Na2S aqueous solution with concentration of0.002 M performs the highest NO2 absorption inthis pilot test. When considering both NO2 absorp-tion capability and chemical consumption cost, a0.0015 M Na2S concentration is considered as theoptimal concentration.

� The NO2 absorption is very sensitive to the pH inthe Na2S aqueous solution. An optimal pH for NO2

absorption in the process is 9.0.� A gas residence time longer than 4.1 s or gasvelocity less than 0.45 m/s is critical to have a rea-sonable NO2 absorption in this pilot-scale scrub-bing system.

� Some HTU information regarding to the NO2

absorption with various concentration of Na2S sol-ution, L/G ratio, and pH are provided for designreference.This study is concluded with a set of optimal oper-

ation and design parameters listed in Table 3. Withthe test results from this study and one of the pre-vious studies on NO oxidation, a two-stage chemicalscrubbing system can be designed to operate at lowcost yet high efficiency for industrial application.

ACKNOWLEDGMENTS

The authors are grateful to Kunstoff Co. for provid-ing facility and instrument to implement the pilot testof this research.

LITERATURE CITED

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scrubbing, Pollution Engineering, 32–36.2. Chen, L., Hsu, C.-H., & Yang, C.L. (2005). Oxida-

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Table 2. Comparison of the test results between pilot-scale and lab scale experiments.

Shen & Richelle (lab scale) This study (pilot)

Inlet NO2 concentration 20, 200, 500 (ppm) 50 (ppm)Na2S(aq) concentration 0.005 � 0.1 (M) 0.001 � 0.002 (M)pH 9.0 6 0.2 8, 9, 10, 11Extra additives Na2S2O3, NaHCO3, NaOH NoneTemperature 558C 288CVolume of the scrubber 1.275 (L) 620 (L)NO2 absorption 90 � 95% 65 � 90%

Table 3. Conclusions of the optimal parameters setting in this pilot test.

Gas massrate (G)

Liquid massrate (L)

Liquid/Gasratio (L/G)

Na2Sconcentration

Residencetime

Gasvelocity pH ORP (mv)

1944.2 (kg/m2-h) 7320 (kg/m2-h) >3.77 >0.0015 (M) >4.1 (s) <0.45 (m/s) 9.0 6 0.2 200


13. Yang, C.-L., & Chen, L. (2000). Oxidation of nitricoxide in a two-stage chemical scrubber using DCcorona discharge, Journal of Hazardous Materials,B80, 135–146.

14. Adewuyi, Y.G., & Owusu, S.O. (2003). Aqueousabsorption and oxidation of nitric oxide withozone for the treatment of tail gases: Process fea-sibility, stoichiometry, reaction pathways, andabsorption rate, Industrial & Engineering Chemis-try Research, 42, 4084–4100.

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17. Brogren, C.T., Karlsson, H.T., & Bjerle, I. (1998).Absorption of NO2 in an aqueous solution of Na2S,Chemical Engineering and Technology, 21, 61–70.

18. Lee, Y.-N., & Schwartz, S.E. (1981). Reactionkinetics of nitrogen dioxide with liquid water atlow partial pressure, Journal of Physical Chemis-try, 85, 840.

19. Shen, C.H., & Rochelle, G.T. (1998). Nitrogendioxide absorption and sulfite oxidation in aque-ous sulfite, Environmental Science and Technol-ogy, 32, 1994–2003.


pilot study of absorption of no2 with na2s aqueous solutions

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