Energy Conversion and Management 52 (2011) 3083–3088

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Energy Conversion and Management

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Surfactants as additives for NOx reduction during SNCR process with urea solutionas reducing agent

Muhammad Ayoub a, Muhammad Faisal Irfan b,⇑, Kyung-Seun Yoo a

a Department of Environmental Engineering, Kwangwoon University, Republic of Koreab Department of Chemical Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia

a r t i c l e i n f o a b s t r a c t

Article history:Received 1 April 2010Received in revised form 18 March 2011Accepted 13 April 2011Available online 31 May 2011

Keywords:NOx reductionSNCR processReducing agentUrea solutionSurfactantsAdditives

0196-8904/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.enconman.2011.04.010

⇑ Corresponding author. Tel.: +60 3 7967 5292; faxE-mail address: muhammadfipk@um.edu.my (M.F

NOx reduction from gas stream by selective non-catalytic reduction (SNCR) using urea as a reducing agentwas performed in this study. A Pilot-scale experimental system was designed and constructed to evaluatethe NOx reduction efficiency and temperature window of the process. Particularly, different types of addi-tives were added during SNCR process to improve NOx reduction efficiency and enlarge temperature win-dow. The addition of additives was based on organic compounds like alcoholic group (CH3OH, C2H5OHand C3H7OH) and metallic compounds like alkali metals (NaOH, KOH and LiOH). Some newly introducedadditives, such as surfactants and different blends of alkali metal NaOH (1%) with organic group or sur-factants were also added to assess the effect of these mixed additives on NOx reduction efficiency andreaction temperature window during SNCR process. Main focus was laid on surfactants as an additivebecause of their cost effectiveness and availability. Basically, surfactants have both organic and metallicparts which provide AOH free radicals from both ends (organic and metallic) to enhance the reactionmechanism and improve the NOx reduction at low temperature. Different types of surfactants (anionic,cationic, amphitricha, long chain, short chain and with different functional groups attached to chains)were tested as an additive during SNCR process. Anionic surfactants (SPES, APS, LAS and SPS) gave max-imum efficiency for NOx reduction and provided maximum range for the temperature window.

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1. Introduction

Combustion processes are known to result in a number of nitro-gen species. Of these, the nitrogen species which are regulated arenitric oxide (NO), and nitrogen dioxide (NO2) which together areknown as NOx. These nitrogen compounds are responsible for ur-ban smog, including ozone, and some forms of acid precipitation[1,2]. Another nitrogen species of interest is nitrous oxide (N2O).Although currently not regulated, nitrous oxide has been classifiedas a greenhouse gas. Typically, nitrogen dioxide and nitrous oxideconstitute less than 1% of the primary nitrogen compounds fromcombustion sources. According to Californian Environmental Pro-tection Agency Air Resources Board report the limitation of N2Ois equivalent to 310 of CO2 making Global warming and GHG lim-itation is suggested by 427 CO2e MMT (including N2O).

There are logically three major categories of methods of reduc-ing nitric oxides: r treating the fuel (pre combustion), s minimiz-ing the formation of nitric oxides at the source (during combustion),and t removing nitric oxides by some means before expelling them

ll rights reserved.

: +60 3 7967 5319.. Irfan).

into the atmosphere (post combustion) [3]. The SNCR process is anexample of the post combustion technique and also there are somenew techniques for post combustion like hybrid SNCR–SCR process[4] and hybrid fast SCR process [5] for NOx reduction. The SNCR pro-cess is a useful method for NOx reduction by injecting amines(ANHA) or cyanides (ACNA) containing selective reducing agentssuch as NH3, urea and cyanuric acid into the flue gases [6–8]. Thisprocess could reduce nitric oxides to nitrogen and water rapidlyand effectively at 800–1100 �C [9]. It has been reported that injec-tion of some additives together with the reducing agent duringSNCR process can reduce and enlarge the optimum reaction tem-perature window for NOx reduction [10–13].

This paper presents a study on the NOx reduction efficiency andoptimum reaction temperature window for SNCR process with thehelp of different additives. These additives were alcoholic (CH3OH,C2H5OH, C3H7OH), metallic (NaOH, KOH, LiOH) along with surfac-tants. For improvement in the efficiency of NOx reduction duringSNCR process, different types of surfactants (SPES, APS, LAS, andSPS) and their blends with alkali metals were used as additives.Considering NOx reduction using SNCR process with any type ofadditive either organic or metallic compound, the basic process isinitiated by the reaction of AOH free radical with NH3 [14]. Thesetypes of additives get decomposed and provide AOH free radical

3084 M. Ayoub et al. / Energy Conversion and Management 52 (2011) 3083–3088

during the process. The more AOH free radicals are availableduring SNCR process, the more NO are to be removed since NO re-acts with NH2. Then NO value will rapidly decrease during SNCRprocess and overall NOx reduction will increase. Since surfactantsconsist of both organic and metallic parts as investigated by previ-ous researchers [15], therefore they provide free radicals from bothsides to react with NO and improve NOx reduction efficiency. Thusdifferent types of surfactants like nature wise (anionic, cationic,amphoteric), structure wise (long chain, short chain) and attachedfunctional group wise (ACOOH, ASO3, ANa, ACl, AN, ACOO, etc.)were used to enhance NOx reduction efficiency.

2. Experimental

The apparatus used to investigate the NOx reduction by SNCRprocess is shown as a sketch diagram in Fig. 1. All the experimentswere carried out using this pilot scale flow reactor (ideal tubularreactor) located horizontally and the reactor consists of combus-tion zone and reaction zone. The combustor and front of reactorwas made by refractory material due to a high temperature region.The circular combustor’s internal diameter 0.22 m, length 0.50 mand outside thickness 0.10 m while front of reactor’s internaldiameter 0.22 m, length 0.50 m and outside thickness 0.05 m.The rear reactor was made by stainless steel because of a low tem-perature region and non-corrosive metal for flue gas. The combus-tion zone was set up by a liquefied petroleum gas burner and athermal NOx generator. The gas burner could be operated at tem-perature up to 1500 �C at atmospheric pressure. Thermal NOx gen-erator consisted of commercially available LPG cylinder and pureoxygen cylinder. Fuel and oxygen mixture was introduced in com-bustion chamber to maintain initial NOx concentration of 200–350 ppm. Actually LPG burner produced very low amount of initialNOx concentration, therefore this thermal NOx generator was usedto add additional initial NOx in combustion chamber. The ureasolution was sprayed into flue gas through an atomizing nozzlein reaction region. The spraying angle of nozzle, which consistedof wide angle round tip included six holes, at 70� and the meandroplet size of urea solution through the nozzle was measuredapproximately 35 lm at 3 atmospheric pressure in solution andair pressure. The outside-wall of pilot scale flow reactor was insu-lated by thick layers of ceramic wool. The temperature inside the

combustionchamber

burner

TC

TC

LPGurea sol'nO2 AIR

thermal NOxgenerator

FM

FM

FM

cooling waterinlet

cooling outle

FM FM

air atomizingnozzle

1st castablereactor

2nd castablereactor

3rd carea

TC

Pb

FM : flowmeter, Pb : probe, TC : thermocouple, : ceramic wool

Fig. 1. Schematic diagram of the pilo

flow reactor was measured with a thermocouple. The gas temper-ature was varied between 750 �C and 1100 �C. The total flowthrough the reactor was maintained at 190 N m3/h for these exper-iments. However, this flow rate resulted in different volumetricflow rates at different reaction temperatures. Then, reaction resi-dence time was calculated between 0.33 s and 0.42 s at theseexperimental conditions. NOx and NO gas concentration were con-tinuously measured by nondispersive infrared analyzer (NDIR)type gas analyzers (Fuji Electric Instruments Co., ZKJ-3) and O2

was measured by NDIR analyzer (VIA-300, HORIBA INC.) respec-tively. The linearity of NDIR analyzer was ±1. Experiments werecarried out at different Normalized Stoichiometric Ratio (NSR) ofthe reducing agent urea (2 � [urea]/[NO]) and in the present exper-imental conditions 2.0 was the optimum value of NSR. The NSR([additives]/[NO]) of additives (CH3OH, C2H5OH and C3H7OH) var-ied from 0.2 to 2.0. The initial gas composition was NOx c.a. 200�350 ppm and O2 c.a. 14–20%. Actually reaction conditions weremeasured according to NOx produced by incinerators where mostlyNOx concentration and oxygen concentration was observed in thisrange.

3. Results and discussion

In most of studies at higher temperature the urea NOx reductionsystem can be broken into three parts: the urea decomposition,NH3/NO (NH2/NO) and HNCO (NCO/NO) system [16,17]. The basickinetic mechanism of the overall reaction is shown in Fig. 2. In allcases the process requires generation of radical species, usuallyAOH, AO or AH, which drive the overall reaction. The NO removaloccurs by reaction of NO in the combustion products with a nitro-gen compound to produce either N2 or N2O. Either N2O may subse-quently react with AH to form N2 or it may remain in thecombustion products and be emitted. At significantly higher tem-perature, the nitrogen radicals are oxidized to produce additionalNO. At low temperature, the reaction does not produce sufficientradicals to sustain the NO removal process; hence there is temper-ature window where the process is effective in reducing NO in thecombustion products.

The effect of the different concentration of urea solutions on theNOx reduction efficiency at a temperature 970 �C with O2 concen-tration 14–20 vol.% is shown in Fig. 3. The behavior of urea solution

TC TC TCwatert

stablector

stainless steel

reactor

heat exchanger

TC

TC

vent

Pb Pb

temperaturereadout unit

NOx analyzer

air atomizingnozzle

t-scale reactor for SNCR process.

N2O

(HOCN)3

CO(NH2)2

NH3

HNCO NCO

NH2 N2

+H

NNH

+OH

+H

+O,+OH NO NO

NO

NO

Fig. 2. Reaction path diagram illustrating the major steps in gas-phase NO-removalby reaction with urea [17].

NSR [NHi /NO]20 1 3 4 5 6

NO

x R

educ

tion

(%)

0

20

40

60

80

100

120Urea = 1.0%, Urea = 2.5%Urea = 5.0% Urea = 7.0% Urea = 10%

Fig. 3. Effect of urea solution concentration and flow rate of nitric oxide on NOx

reduction efficiency with 14–20% O2 at 970 �C.

Temperature [oC]700 800 900 1000 1100

NO

x Red

uctio

n [%

]

0

20

40

60

80

100

Urea onlyLiOH 1wt%NaOH 1wt%KOH 1wt%

Fig. 4. NOx reduction efficiency and optimum reaction temperature window forSNCR process with and without addition of alkali metals (NaOH, LiOH and KOH).

M. Ayoub et al. / Energy Conversion and Management 52 (2011) 3083–3088 3085

at different concentration levels was observed by injecting ureainto injection zone as a reducing agent. Urea with 5% concentrationexhibited excellent efficiency for NOx reduction during SNCR pro-cess. Actually, with the concentration of urea 5%, NOx reductionefficiency was maintained at 60% for a vast range of NSR value from1 to more than 4. At the same time, Fig. 3 also contains the effect ofdifferent NSR values on the NOx reduction efficiency at a tempera-ture 970 �C with 14–20% O2. This Fig. 3 contains two important fac-tors, one for urea solution concentration and another for NSR [NHi/NO] value for the reaction of SNCR process.

For the case of urea concentration solution, the best concentra-tion of urea solution was found to be 5% where maximum NOx

reduction efficiency was noted to be 68.5% with urea solution flowrate 100 ml/min. Because at concentration less than 5%, the effi-ciency of NOx reduction was decreased while at high concentrationmore than 5%, there was no more improvement in NOx reductionefficiency. Even though at urea solution concentration 10%, the effi-ciency of NOx reduction was 60% with flow rate of urea solution80 ml/min. This is because when the concentration of urea solutionis increased at a particular value of urea/NO ratio, the content ofwater in the reactor is also increased and that change in concentra-tion of urea solution causes a change in NO reduction due to chem-ical kinetic (effect of water). For the case of NSR value, theoptimum value was found to be 2 at 5% urea solution with flowrate of 100 ml/min. Because at NSR < 2, the NOx reduction effi-ciency was less than 60%, while at NSR P 2, the efficiency wasaround 68% and did not fluctuated much as it seemed to be leveledoff.

The basic NOx reduction efficiency during SNCR process withand without using any additive is shown in Fig. 4. The reaction con-ditions for this urea based SNCR process for NOx reduction were

used as follows: 5% urea solution; NSR = 2; 14–20% O2; urea flowrate at nozzle point 40 ml/min; NO flow rate at inlet 600 ml/min;temperature window 850–1050 �C. It is clear from Fig. 4 that max-imum efficiency of NOx reduction was noted to be 70% at temper-ature of 980 �C.

The same Fig. 4 also shows the effect of alkali metal (NaOH1 wt%, KOH 1 wt% and LiOH 1 wt%) on NOx reduction efficiencyduring SNCR process with urea reagent. It was noted that theNOx reduction efficiency reached at maximum and temperaturewindow was shifted to lower temperature region, when alkali me-tal NaOH was added as an additive during SNCR process. The alkaliLiOH 1 wt% and KOH 1 wt% was also effective in enlarging andshifting temperature range towards lower side, although the effi-ciency of the process was not increased, as compared to alkali me-tal NaOH 1 wt%. Actually, these alkali metals are connected withhydroxides, which are dissociated into OH radical and metal atomsat the place where nozzle is spraying the reagent solution at hightemperature region. The OH radical is an important material forthe reaction of NOx reduction. Thus, the main reaction can be pro-moted if OH radicals are increased during SNCR process. Then highefficiency of NOx reduction can be achieved at low temperaturewindow. Fig. 4 also shows the effect of LiOH 1 wt% on SNCRprocess. The results were much closed to those of NaOH 1 wt% asan additive.

The effect of alcoholic group (CH3OH, C2H5OH, and C3H7OH) onNOx reduction efficiency and optimum reaction temperature win-dow in SNCR process is shown in Fig. 5. With the addition of alco-holic additives ([CH3OH]/[NO]), ([C2H5OH]/[NO]) and ([C3H7OH]/[NO]) at constant NSR(2 � [urea]/[NO]) = 2 with different concen-tration 0.2 and 2.0, there was no immense effect on the NOx reduc-tion efficiency but reaction temperature window slightly shifted tolower temperature as compared to urea only used for NOx reduc-tion during SNCR process. It was observed that alcoholic additive[CH3OH]/[NO] = 0.2 gave maximum efficiency of 68% at 970 �Cand temperature window shifted slightly downside, while otheralcoholic additive efficiency was noted less than this value. Itwas also observed that with the increasing concentration of alco-holic additives from 0.2 to 2.0 at constant NSR = 2, the efficiencyof NOx reduction decreased from 68% to less than 20%. But at lowtemperature i.e. 850 �C [C2H5OH]/[NO] = 2.0 showed maximumefficiency 39% as compared to the others. The reason behind thisreduction of efficiency by increasing concentration of alcoholicadditive at constant NSR = 2 is that at high temperature alcoholicadditives decomposed and provided OH radical. The supplies ofOH radical concentration increased NH2 radical concentrationwhich reacts with NO to reduce it but in the presence of oxygen

Fig. 7. Synergy effect of NaOH 1 wt% with alcoholic additives (methanol andethanol) on NOx reduction efficiency and optimum reaction temperature window.

urea onlyNSR=2[CH3OH /NO] = 0.2

[C2H5OH /NO] = 0.2

[C3H7OH /NO] = 0.2

[CH3OH /NO] = 2

[C2H5OH /NO] = 2

[C3H7OH /NO] = 2

CH3OH by Rota and ZanoeloC2H5OH by Rota and Zanoelo

Temperature [oC]700 800 900 1000 1100

NO

x Red

uctio

n [%

]

0

20

40

60

80

100

Fig. 5. Effect of alcoholic group (CH3OH, C2H5OH and C3H7OH) on NOx reductionconditions (present study): 5% urea solution; [additives]/[NO] = 0.2–2; NO 600 ml/min; O2 �14%; nitrogen to balance; residence time 0.33 s; P 1 bar. (By Rota andZanoelo): NO 500 ppmv; urea 600 ppmv; H2O 19%; O2 1.7%; additive 60 ppmv;nitrogen to balance; residence time 0.2 s; P 1 bar.

3086 M. Ayoub et al. / Energy Conversion and Management 52 (2011) 3083–3088

NH2 radical produced HNO which reacts with excess OH radical toproduce NO again. That is why with the increase of alcoholic addi-tive concentration, NOx reduction efficiency decreases.

In the same figure, there is also a comparison between effi-ciency of alcoholic group at present work and experiments previ-ously discussed [18]. The efficiency trend of NOx reduction ofprevious work is almost same with present work except slightlyhigher maximum efficiency that may be due to difference in reac-tion conditions like humidity and oxygen. Overall efficiency andtemperature window trend for NOx reduction of previous and pres-ent work (in case of [additive]/[NO] = 0.2) is same and it may beconcluded that alcoholic additive can help for NOx reduction undercertain conditions like [additive]/[NO] = 0.2 and humidity concen-tration. Basically both alcoholic and metallic additives produce OHfree radicals during reduction process which react with NH2 radicalwhich further react with NO to reduce it during SNCR process. But,reaction mechanism of each type of additive to produce OH freeradical is different [17]. Therefore, it is logical to evaluate NOx

reduction efficiency separately when using different types of alco-holic and metallic additives. The reaction mechanism of urea andalcoholic additive to produce OH free radical is shown in Figs. 2and 6 respectively.

The co-effect of alkali metal and alcoholic additive for NOx re-moval efficiency was assessed. There was no considerable effectafter addition of this mixture on NOx reduction efficiency and tem-perature window for SNCR process, as shown in Fig. 7. Although,

CH3OH

CH3O

CH2OH

H

OH

HCO

O

CH2O+M,O2

+H,O,OH

+M

+O2

+H2O, O2,H

+H2O,CH2O+H,O,OH

OH

+M

+M

Fig. 6. Reaction path diagram illustrating the major steps of methanol to form a freeradical [6].

with the addition of mixture of alkali metal and alcoholic additivethe NOx reduction efficiency was increased and the temperaturewindow was expanded, as compared to process in absence of addi-tives. The results were far less efficient than those with addition ofalkali metal NaOH 1 wt%.

Different types of additives were used to enhance NOx reductionefficiency and enlarge its temperature window. It was found thatNOx reduction efficiency was maximized when NaOH 1 wt% wasused as an additive during SNCR process. But there is still a needfor special additive that improves NOx reduction efficiency byenlarging its temperature window towards lower side. Since ithas been investigated by previous researchers that surfactantsbuilt up with two different compounds (organic compound andmetallic compound) [15], when these were used as an additive,they are decomposed at high temperature and split up into bothorganic and metallic compounds. During the process they giveOH free radicals from both ends to enhance the reaction mecha-nism and subsequently improve the NOx reduction efficiency.Therefore different surfactants were tested as an additive to en-hance the efficiency of NOx reduction during SNCR process.

It is difficult to assess the different surfactants behavior at dif-ferent temperatures, because different surfactants build up withdifferent gradients and these gradients exhibit different propertiesat a large range of temperature window. Therefore, the activity ofall tested surfactant additives were evaluated at a fixed tempera-ture of 850 �C Table 1 shows a list of different types of surfactantswith their formulae and nature, which were used during experi-mental studies to observe their behavior toward NOx reductionefficiency at constant temperature of 850 �C.

It was observed that different surfactant additives with thesame concentration 10 wt% during SNCR process, showed fruitfulresults for NOx reduction efficiency at temperature of 850 �C com-pared with NaOH 1 wt% and urea only at NSR = 2 as shown in Fig. 8.It is clear that all different types of surfactants (anionic, cationic,amphoteric) improve SNCR process efficiency with nearly same le-vel at temperature 850 �C which was also a sign of shifting temper-ature window to downside. It was observed that with the additionof different surfactants as an additive at 850 �C, the NOx reductionefficiency was increased more than 40% as compared to SNCR pro-cess using urea only as a reducing agent. Meanwhile, at the sametemperature, the NOx efficiency was noted 54% in the presence ofan additive NaOH 1 wt% during SNCR process. Moreover, surfactantSPES showed maximum efficiency 48% for NOx reduction amongthe other three anionic surfactants at 850 �C. Similarly, the othertypes of surfactant CT-50 (cationic) and PB (amphoteric) showedmaximum efficiency 52% and 60% respectively. However, cationic

Table 1List of different surfactants tested for experiment.

Surfactants Common name Formula Nature

LAS Linear alkylbenzen sulfonic acid CH3(CH2)11C6H4SO3H Anionic surfactantsSPES Sodium lauryl ether sulfate CH3(CH2)10CH2(OCH2CH2)3OSO3Na Anionic surfactantsAPS Ammonium lauryl sulfate CH3(CH2)10CH2OSO3NH4 Anionic surfactantsTP Triethanolamine lauryl sulfate CH3(CH2)11OSO3N(CH2CH2OH)3 Anionic surfactantsSPS Sodium lauryl sulfate CH3(CH2)10CH2OSO3Na Anionic surfactantsCT-50 Cetyl trimethyl ammonium chloride CH3(CH2)15N(Cl)(CH3)3 Cationic surfactantsOx Lauryl dimethylamine oxide C14H31NO Cationic surfactantsBT Behenyl trimethylammonium chloride CH3(CH2)21N(Cl)(CH3)3 Cationic surfactantsST Stearyl trimethyl ammonium chloride CH3(CH2)17N(CH3)3CH2ACl Cationic surfactantsSB Stearyl dimethylbenzyl ammonium chloride CH3(CH2)17N(CH3)2CH2C6H5ACl Cationic surfactantsDDAC Distearyl dimethyl ammonium chloride [CH3(CH2)17]NCl(CH3)2 Cationic surfactantsPB Cocoamidopropyl betaine RCONH(CH2)3N + (CH3)2CH2COOA Amphoteric surfactants

Different surfactants [10% each]

SPESSPS

LASAPS TP

CT-50 SB ST Ox BT PB

NaOH 1w

t%

Urea only

NO

x R

educ

tion

at 8

50o C

[%]

0

10

20

30

40

50

60

70

Anionic Cationic

Amphoteric

Fig. 8. Effect of various surfactant species 10 wt% of each and alkali metal NaOH0.1 wt% for NOx reduction efficiency at 850 �C.

M. Ayoub et al. / Energy Conversion and Management 52 (2011) 3083–3088 3087

and amphoteric surfactants showed poor repeatability results andthey also found costly compared with anionic surfactants. There-fore they were not considered for further studies of surfactantsas an additive during SNCR process.

As anionic surfactants are cost effective, easily available surfac-tants and also SPES showed maximum NOx reduction. Hence in

Fig. 9. Effect of different concentration of anionic surfactant SPES for NOx reductionefficiency and optimum reaction temperature window.

Fig. 9 SPES was further studied at different concentrations (1 wt%,3 wt% and 10 wt%) for NOx reduction efficiency at 800–1050 �C. Itis clear from Fig. 9 that efficiency of NOx reduction slightly in-creased by increasing the concentration of surfactant SPES (from1 wt% to 3 wt%) at optimum temperature 960 �C but further addi-tion of surfactant SPES (from 3 wt%, to 10 wt%) NOx reduction effi-ciency decreased especially at high temperature greater than900 �C as compared to urea only. Therefore optimum value of con-centration for SPES anionic surfactant at temperature window 800–1050 �C was considered to be 3 wt%.

For further improvement in NOx reduction efficiency and opti-mum reaction temperature window, a commercially available an-ionic detergent for laundry washing named ‘‘Beat 1 wt%’’ wasused as an additive which showed an excellent performanceslightly up than additive NaOH 1 wt% for temperature window800–1050 �C. Therefore a co-effect of anionic surfactant ‘‘Beat1 wt%’’ and alkali metal NaOH 1 wt% as an additive was observedas shown in Fig. 10. The experimental results showed that due tothis synergy effect, the efficiency of NOx reduction slightly im-proved as compared to additive NaOH 1 wt% but equivalent to an-ionic surfactant ‘‘Beat 1 wt%’’ but on other hand, temperaturewindow slightly shifted towards lower side due to synergy effect.The synergy effect on shifting of temperature window was slightlyup as compared to additive NaOH 1 wt% and synergy effect on effi-ciency of NOx in comparison with NaOH 1 wt% was prominent attemperature of 850 �C. Actually anionic surfactant Beat commonlycontains sodium and ammonium lauryl sulfates which work to im-prove NOx reduction. It may possible that during synergy, maincomponents of Beat (sodium and ammonium lauryl sulfates) and

0

20

40

60

80

100urea onlyNaOH 1%Local surfactant Beat 1%+ NaOH 1%Local surfactantBeat 1%

Temperature [oC]700 800 900 1000 1100

NO

x Red

uctio

n [%

]

Fig. 10. Synergy effect of NaOH 1 wt% and anionic surfactant Beat 1 wt% on NOx

reduction efficiency and optimum reaction temperature window.

3088 M. Ayoub et al. / Energy Conversion and Management 52 (2011) 3083–3088

NaOH come together to enhance efficiency and enlarge the tem-perature window of SNCR process.

4. Conclusions

The characteristics of different additives for NOx reduction dur-ing SNCR process with reducing agent urea have been studied on pi-lot scale reactor. During a series of experiments with additives likealkali metal (NaOH, LiOH and KOH), alcoholic (CH3OH, C2H5OH andC3H7OH), and different type of surfactants, it was observed thatnewly searched additive anionic surfactants or a mixture additive(1 wt% anionic surfactant and 1 wt% NaOH) have a great impactand immense advantages to enhance NOx reduction efficiency andextend process temperature window due to having both organicand metallic parts which have ability to provide AOH free radicalson both sides. These AOH free radicals play an important role inNOx reduction process. Thus, the temperature window of the pro-cess can be easily enlarged and shifted toward lower side in orderto raise NOx reduction efficiency with the help of these anionic sur-factant additives or a mixture additive (1 wt% anionic surfactantand 1 wt% NaOH) as compared to conventional additives. It is con-cluded that anionic surfactant additives make SNCR process moreprecise, optimal, efficient and economical for NOx reduction.

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