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Energy Conversion and Management 52 (2011) 3083–3088
Contents lists available at ScienceDirect
Energy Conversion and Management
journal homepage: www.elsevier.com/locate/enconman
Surfactants as additives for NOx reduction during SNCR process with urea solution
as reducing agent
Muhammad Ayoub a, Muhammad Faisal Irfan b,⇑, Kyung-Seun Yoo a
a
b
Department of Environmental Engineering, Kwangwoon University, Republic of Korea
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
Article history:
Received 1 April 2010
Received in revised form 18 March 2011
Accepted 13 April 2011
Keywords:
NOx reduction
SNCR process
Reducing agent
Urea solution
Surfactants
Additives
a b s t r a c t
NOx reduction from gas stream by selective non-catalytic reduction (SNCR) using urea as a reducing agent
was performed in this study. A Pilot-scale experimental system was designed and constructed to evaluate
the NOx reduction efﬁciency and temperature window of the process. Particularly, different types of additives were added during SNCR process to improve NOx reduction efﬁciency and enlarge temperature window. The addition of additives was based on organic compounds like alcoholic group (CH3OH, C2H5OH
and C3H7OH) and metallic compounds like alkali metals (NaOH, KOH and LiOH). Some newly introduced
additives, such as surfactants and different blends of alkali metal NaOH (1%) with organic group or surfactants were also added to assess the effect of these mixed additives on NOx reduction efﬁciency and
reaction temperature window during SNCR process. Main focus was laid on surfactants as an additive
because of their cost effectiveness and availability. Basically, surfactants have both organic and metallic
parts which provide AOH free radicals from both ends (organic and metallic) to enhance the reaction
mechanism 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 maximum efﬁciency for NOx reduction and provided maximum range for the temperature window.
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
Combustion processes are known to result in a number of nitrogen species. Of these, the nitrogen species which are regulated are
nitric oxide (NO), and nitrogen dioxide (NO2) which together are
known as NOx. These nitrogen compounds are responsible for urban 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 classiﬁed
as a greenhouse gas. Typically, nitrogen dioxide and nitrous oxide
constitute less than 1% of the primary nitrogen compounds from
combustion sources. According to Californian Environmental Protection Agency Air Resources Board report the limitation of N2O
is equivalent to 310 of CO2 making Global warming and GHG limitation is suggested by 427 CO2e MMT (including N2O).
There are logically three major categories of methods of reducing nitric oxides: r treating the fuel (pre combustion), s minimizing the formation of nitric oxides at the source (during combustion),
and t removing nitric oxides by some means before expelling them
⇑ Corresponding author. Tel.: +60 3 7967 5292; fax: +60 3 7967 5319.
E-mail address: muhammadﬁ[email protected] (M.F. Irfan).
0196-8904/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.enconman.2011.04.010
into the atmosphere (post combustion) [3]. The SNCR process is an
example of the post combustion technique and also there are some
new techniques for post combustion like hybrid SNCR–SCR process
[4] and hybrid fast SCR process [5] for NOx reduction. The SNCR process is a useful method for NOx reduction by injecting amines
(ANHA) or cyanides (ACNA) containing selective reducing agents
such as NH3, urea and cyanuric acid into the ﬂue gases [6–8]. This
process could reduce nitric oxides to nitrogen and water rapidly
and effectively at 800–1100 °C [9]. It has been reported that injection of some additives together with the reducing agent during
SNCR process can reduce and enlarge the optimum reaction temperature window for NOx reduction [10–13].
This paper presents a study on the NOx reduction efﬁciency and
optimum reaction temperature window for SNCR process with the
help of different additives. These additives were alcoholic (CH3OH,
C2H5OH, C3H7OH), metallic (NaOH, KOH, LiOH) along with surfactants. For improvement in the efﬁciency of NOx reduction during
SNCR process, different types of surfactants (SPES, APS, LAS, and
SPS) and their blends with alkali metals were used as additives.
Considering NOx reduction using SNCR process with any type of
additive either organic or metallic compound, the basic process is
initiated by the reaction of AOH free radical with NH3 [14]. These
types of additives get decomposed and provide AOH free radical
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M. Ayoub et al. / Energy Conversion and Management 52 (2011) 3083–3088
during the process. The more AOH free radicals are available
during SNCR process, the more NO are to be removed since NO reacts with NH2. Then NO value will rapidly decrease during SNCR
process and overall NOx reduction will increase. Since surfactants
consist of both organic and metallic parts as investigated by previous researchers [15], therefore they provide free radicals from both
sides to react with NO and improve NOx reduction efﬁciency. Thus
different types of surfactants like nature wise (anionic, cationic,
amphoteric), structure wise (long chain, short chain) and attached
functional 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 SNCR
process is shown as a sketch diagram in Fig. 1. All the experiments
were carried out using this pilot scale ﬂow reactor (ideal tubular
reactor) located horizontally and the reactor consists of combustion zone and reaction zone. The combustor and front of reactor
was made by refractory material due to a high temperature region.
The circular combustor’s internal diameter 0.22 m, length 0.50 m
and outside thickness 0.10 m while front of reactor’s internal
diameter 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 temperature region and non-corrosive metal for ﬂue gas. The combustion zone was set up by a liqueﬁed petroleum gas burner and a
thermal NOx generator. The gas burner could be operated at temperature up to 1500 °C at atmospheric pressure. Thermal NOx generator consisted of commercially available LPG cylinder and pure
oxygen cylinder. Fuel and oxygen mixture was introduced in combustion chamber to maintain initial NOx concentration of 200–
350 ppm. Actually LPG burner produced very low amount of initial
NOx concentration, therefore this thermal NOx generator was used
to add additional initial NOx in combustion chamber. The urea
solution was sprayed into ﬂue gas through an atomizing nozzle
in reaction region. The spraying angle of nozzle, which consisted
of wide angle round tip included six holes, at 70° and the mean
droplet size of urea solution through the nozzle was measured
approximately 35 lm at 3 atmospheric pressure in solution and
air pressure. The outside-wall of pilot scale ﬂow reactor was insulated by thick layers of ceramic wool. The temperature inside the
ﬂow reactor was measured with a thermocouple. The gas temperature was varied between 750 °C and 1100 °C. The total ﬂow
through the reactor was maintained at 190 N m3/h for these experiments. However, this ﬂow rate resulted in different volumetric
ﬂow rates at different reaction temperatures. Then, reaction residence time was calculated between 0.33 s and 0.42 s at these
experimental conditions. NOx and NO gas concentration were continuously 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.) respectively. The linearity of NDIR analyzer was ±1. Experiments were
carried out at different Normalized Stoichiometric Ratio (NSR) of
the reducing agent urea (2 [urea]/[NO]) and in the present experimental conditions 2.0 was the optimum value of NSR. The NSR
([additives]/[NO]) of additives (CH3OH, C2H5OH and C3H7OH) varied 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 were
measured according to NOx produced by incinerators where mostly
NOx concentration and oxygen concentration was observed in this
range.
3. Results and discussion
In most of studies at higher temperature the urea NOx reduction
system can be broken into three parts: the urea decomposition,
NH3/NO (NH2/NO) and HNCO (NCO/NO) system [16,17]. The basic
kinetic mechanism of the overall reaction is shown in Fig. 2. In all
cases the process requires generation of radical species, usually
AOH, AO or AH, which drive the overall reaction. The NO removal
occurs by reaction of NO in the combustion products with a nitrogen compound to produce either N2 or N2O. Either N2O may subsequently react with AH to form N2 or it may remain in the
combustion products and be emitted. At signiﬁcantly higher temperature, the nitrogen radicals are oxidized to produce additional
NO. At low temperature, the reaction does not produce sufﬁcient
radicals to sustain the NO removal process; hence there is temperature window where the process is effective in reducing NO in the
combustion products.
The effect of the different concentration of urea solutions on the
NOx reduction efﬁciency at a temperature 970 °C with O2 concentration 14–20 vol.% is shown in Fig. 3. The behavior of urea solution
air atomizing
nozzle
FM
LPG
O2
urea
sol'n
FM
vent
heat exchanger
air atomizing
nozzle
AIR
TC
cooling water
inlet
TC
cooling water
TC
outlet
TC
TC
TC
burner
FM
combustion 1st castable
chamber
reactor
thermal NOx
generator
FM
2nd castable
reactor
stainless
steel
reactor
3rd castable
reactor
Pb
Pb
Pb
NOx analyzer
TC
FM
FM : flowmeter, Pb : probe, TC : thermocouple,
TC
: ceramic wool
temperature
readout unit
Fig. 1. Schematic diagram of the pilot-scale reactor for SNCR process.
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M. Ayoub et al. / Energy Conversion and Management 52 (2011) 3083–3088
100
HNCO
CO(NH 2 )2
+OH
NO
NCO
N2O
+H
80
NOx Reduction [%]
(HOCN)3
+H
NO
NH 2
NH3
+O,
+OH
N2
NNH
NO
NO
Fig. 2. Reaction path diagram illustrating the major steps in gas-phase NO-removal
by reaction with urea [17].
40
20
80
40
20
0
1
2
900
1000
1100
Fig. 4. NOx reduction efﬁciency and optimum reaction temperature window for
SNCR process with and without addition of alkali metals (NaOH, LiOH and KOH).
60
0
800
Temperature [ oC]
Urea = 1.0%,
Urea = 2.5%
Urea = 5.0%
Urea = 7.0%
Urea = 10%
100
NO x Reduction (%)
60
0
700
120
Urea only
LiOH 1wt%
NaOH 1wt%
KOH 1wt%
3
4
5
6
NSR [NH i /NO]
Fig. 3. Effect of urea solution concentration and ﬂow rate of nitric oxide on NOx
reduction efﬁciency with 14–20% O2 at 970 °C.
at different concentration levels was observed by injecting urea
into injection zone as a reducing agent. Urea with 5% concentration
exhibited excellent efﬁciency for NOx reduction during SNCR process. Actually, with the concentration of urea 5%, NOx reduction
efﬁciency was maintained at 60% for a vast range of NSR value from
1 to more than 4. At the same time, Fig. 3 also contains the effect of
different NSR values on the NOx reduction efﬁciency at a temperature 970 °C with 14–20% O2. This Fig. 3 contains two important factors, 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 concentration of urea solution was found to be 5% where maximum NOx
reduction efﬁciency was noted to be 68.5% with urea solution ﬂow
rate 100 ml/min. Because at concentration less than 5%, the efﬁciency of NOx reduction was decreased while at high concentration
more than 5%, there was no more improvement in NOx reduction
efﬁciency. Even though at urea solution concentration 10%, the efﬁciency of NOx reduction was 60% with ﬂow rate of urea solution
80 ml/min. This is because when the concentration of urea solution
is increased at a particular value of urea/NO ratio, the content of
water in the reactor is also increased and that change in concentration of urea solution causes a change in NO reduction due to chemical kinetic (effect of water). For the case of NSR value, the
optimum value was found to be 2 at 5% urea solution with ﬂow
rate of 100 ml/min. Because at NSR < 2, the NOx reduction efﬁciency was less than 60%, while at NSR P 2, the efﬁciency was
around 68% and did not ﬂuctuated much as it seemed to be leveled
off.
The basic NOx reduction efﬁciency during SNCR process with
and without using any additive is shown in Fig. 4. The reaction conditions for this urea based SNCR process for NOx reduction were
used as follows: 5% urea solution; NSR = 2; 14–20% O2; urea ﬂow
rate 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 maximum efﬁciency of NOx reduction was noted to be 70% at temperature of 980 °C.
The same Fig. 4 also shows the effect of alkali metal (NaOH
1 wt%, KOH 1 wt% and LiOH 1 wt%) on NOx reduction efﬁciency
during SNCR process with urea reagent. It was noted that the
NOx reduction efﬁciency reached at maximum and temperature
window was shifted to lower temperature region, when alkali metal NaOH was added as an additive during SNCR process. The alkali
LiOH 1 wt% and KOH 1 wt% was also effective in enlarging and
shifting temperature range towards lower side, although the efﬁciency of the process was not increased, as compared to alkali metal NaOH 1 wt%. Actually, these alkali metals are connected with
hydroxides, which are dissociated into OH radical and metal atoms
at the place where nozzle is spraying the reagent solution at high
temperature region. The OH radical is an important material for
the reaction of NOx reduction. Thus, the main reaction can be promoted if OH radicals are increased during SNCR process. Then high
efﬁciency of NOx reduction can be achieved at low temperature
window. Fig. 4 also shows the effect of LiOH 1 wt% on SNCR
process. The results were much closed to those of NaOH 1 wt% as
an additive.
The effect of alcoholic group (CH3OH, C2H5OH, and C3H7OH) on
NOx reduction efﬁciency and optimum reaction temperature window in SNCR process is shown in Fig. 5. With the addition of alcoholic additives ([CH3OH]/[NO]), ([C2H5OH]/[NO]) and ([C3H7OH]/
[NO]) at constant NSR(2 [urea]/[NO]) = 2 with different concentration 0.2 and 2.0, there was no immense effect on the NOx reduction efﬁciency but reaction temperature window slightly shifted to
lower temperature as compared to urea only used for NOx reduction during SNCR process. It was observed that alcoholic additive
[CH3OH]/[NO] = 0.2 gave maximum efﬁciency of 68% at 970 °C
and temperature window shifted slightly downside, while other
alcoholic additive efﬁciency was noted less than this value. It
was also observed that with the increasing concentration of alcoholic additives from 0.2 to 2.0 at constant NSR = 2, the efﬁciency
of NOx reduction decreased from 68% to less than 20%. But at low
temperature i.e. 850 °C [C2H5OH]/[NO] = 2.0 showed maximum
efﬁciency 39% as compared to the others. The reason behind this
reduction of efﬁciency by increasing concentration of alcoholic
additive at constant NSR = 2 is that at high temperature alcoholic
additives decomposed and provided OH radical. The supplies of
OH radical concentration increased NH2 radical concentration
which reacts with NO to reduce it but in the presence of oxygen
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100
urea only
NSR=2
[CH3OH /NO] = 0.2
NOx Reduction [%]
80
[C2H5 OH /NO] = 0.2
[C3H7OH /NO] = 0.2
[CH3OH /NO] = 2
60
[C2H5OH /NO] = 2
[C3H7OH /NO] = 2
40
CH3OH by Rota
and Zanoelo
C2H5OH by Rota
and Zanoelo
20
0
700
800
900
1000
1100
o
Temperature [ C]
Fig. 5. Effect of alcoholic group (CH3OH, C2H5OH and C3H7OH) on NOx reduction
conditions (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 and
Zanoelo): 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.
NH2 radical produced HNO which reacts with excess OH radical to
produce NO again. That is why with the increase of alcoholic additive concentration, NOx reduction efﬁciency decreases.
In the same ﬁgure, there is also a comparison between efﬁciency of alcoholic group at present work and experiments previously discussed [18]. The efﬁciency trend of NOx reduction of
previous work is almost same with present work except slightly
higher maximum efﬁciency that may be due to difference in reaction conditions like humidity and oxygen. Overall efﬁciency and
temperature window trend for NOx reduction of previous and present work (in case of [additive]/[NO] = 0.2) is same and it may be
concluded that alcoholic additive can help for NOx reduction under
certain conditions like [additive]/[NO] = 0.2 and humidity concentration. Basically both alcoholic and metallic additives produce OH
free radicals during reduction process which react with NH2 radical
which further react with NO to reduce it during SNCR process. But,
reaction mechanism of each type of additive to produce OH free
radical is different [17]. Therefore, it is logical to evaluate NOx
reduction efﬁciency separately when using different types of alcoholic and metallic additives. The reaction mechanism of urea and
alcoholic additive to produce OH free radical is shown in Figs. 2
and 6 respectively.
The co-effect of alkali metal and alcoholic additive for NOx removal efﬁciency was assessed. There was no considerable effect
after addition of this mixture on NOx reduction efﬁciency and temperature window for SNCR process, as shown in Fig. 7. Although,
OH
+H,O,OH
CH3 O
+M
+H 2O,
O2 ,H
CH3OH
H
OH
CH 2OH
+M
+H 2O,CH 2O
+O 2
O
+M
+M,O2
CH 2O
HCO
+H,O,OH
Fig. 6. Reaction path diagram illustrating the major steps of methanol to form a free
radical [6].
Fig. 7. Synergy effect of NaOH 1 wt% with alcoholic additives (methanol and
ethanol) on NOx reduction efﬁciency and optimum reaction temperature window.
with the addition of mixture of alkali metal and alcoholic additive
the NOx reduction efﬁciency was increased and the temperature
window was expanded, as compared to process in absence of additives. The results were far less efﬁcient than those with addition of
alkali metal NaOH 1 wt%.
Different types of additives were used to enhance NOx reduction
efﬁciency and enlarge its temperature window. It was found that
NOx reduction efﬁciency was maximized when NaOH 1 wt% was
used as an additive during SNCR process. But there is still a need
for special additive that improves NOx reduction efﬁciency by
enlarging its temperature window towards lower side. Since it
has been investigated by previous researchers that surfactants
built up with two different compounds (organic compound and
metallic compound) [15], when these were used as an additive,
they are decomposed at high temperature and split up into both
organic and metallic compounds. During the process they give
OH free radicals from both ends to enhance the reaction mechanism and subsequently improve the NOx reduction efﬁciency.
Therefore different surfactants were tested as an additive to enhance the efﬁciency of NOx reduction during SNCR process.
It is difﬁcult to assess the different surfactants behavior at different temperatures, because different surfactants build up with
different gradients and these gradients exhibit different properties
at a large range of temperature window. Therefore, the activity of
all tested surfactant additives were evaluated at a ﬁxed temperature of 850 °C Table 1 shows a list of different types of surfactants
with their formulae and nature, which were used during experimental studies to observe their behavior toward NOx reduction
efﬁciency at constant temperature of 850 °C.
It was observed that different surfactant additives with the
same concentration 10 wt% during SNCR process, showed fruitful
results for NOx reduction efﬁciency at temperature of 850 °C compared 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 level at temperature 850 °C which was also a sign of shifting temperature window to downside. It was observed that with the addition
of different surfactants as an additive at 850 °C, the NOx reduction
efﬁciency was increased more than 40% as compared to SNCR process using urea only as a reducing agent. Meanwhile, at the same
temperature, the NOx efﬁciency was noted 54% in the presence of
an additive NaOH 1 wt% during SNCR process. Moreover, surfactant
SPES showed maximum efﬁciency 48% for NOx reduction among
the other three anionic surfactants at 850 °C. Similarly, the other
types of surfactant CT-50 (cationic) and PB (amphoteric) showed
maximum efﬁciency 52% and 60% respectively. However, cationic
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Table 1
List of different surfactants tested for experiment.
Surfactants
Common name
Formula
Nature
LAS
SPES
APS
TP
SPS
CT-50
Ox
BT
ST
SB
DDAC
PB
Linear alkylbenzen sulfonic acid
Sodium lauryl ether sulfate
Ammonium lauryl sulfate
Triethanolamine lauryl sulfate
Sodium lauryl sulfate
Cetyl trimethyl ammonium chloride
Lauryl dimethylamine oxide
Behenyl trimethylammonium chloride
Stearyl trimethyl ammonium chloride
Stearyl dimethylbenzyl ammonium chloride
Distearyl dimethyl ammonium chloride
Cocoamidopropyl betaine
CH3(CH2)11C6H4SO3H
CH3(CH2)10CH2(OCH2CH2)3OSO3Na
CH3(CH2)10CH2OSO3NH4
CH3(CH2)11OSO3N(CH2CH2OH)3
CH3(CH2)10CH2OSO3Na
CH3(CH2)15N(Cl)(CH3)3
C14H31NO
CH3(CH2)21N(Cl)(CH3)3
CH3(CH2)17N(CH3)3CH2ACl
CH3(CH2)17N(CH3)2CH2C6H5ACl
[CH3(CH2)17]NCl(CH3)2
RCONH(CH2)3N + (CH3)2CH2COOA
Anionic surfactants
Anionic surfactants
Anionic surfactants
Anionic surfactants
Anionic surfactants
Cationic surfactants
Cationic surfactants
Cationic surfactants
Cationic surfactants
Cationic surfactants
Cationic surfactants
Amphoteric surfactants
70
NO x Reduction at 850 oC [%]
Amphoteric
60
Cationic
Anionic
50
40
30
20
10
t%
on
ly
1w
H
re
U
N
aO
a
ST
O
x
B
T
PB
50
SB
TP
C
T-
S
PS
A
LA
SP
ES
SP
S
0
Different surfactants [10% each]
Fig. 8. Effect of various surfactant species 10 wt% of each and alkali metal NaOH
0.1 wt% for NOx reduction efﬁciency at 850 °C.
and amphoteric surfactants showed poor repeatability results and
they also found costly compared with anionic surfactants. Therefore they were not considered for further studies of surfactants
as an additive during SNCR process.
As anionic surfactants are cost effective, easily available surfactants and also SPES showed maximum NOx reduction. Hence in
Fig. 9 SPES was further studied at different concentrations (1 wt%,
3 wt% and 10 wt%) for NOx reduction efﬁciency at 800–1050 °C. It
is clear from Fig. 9 that efﬁciency of NOx reduction slightly increased by increasing the concentration of surfactant SPES (from
1 wt% to 3 wt%) at optimum temperature 960 °C but further addition of surfactant SPES (from 3 wt%, to 10 wt%) NOx reduction efﬁciency decreased especially at high temperature greater than
900 °C as compared to urea only. Therefore optimum value of concentration for SPES anionic surfactant at temperature window 800–
1050 °C was considered to be 3 wt%.
For further improvement in NOx reduction efﬁciency and optimum reaction temperature window, a commercially available anionic detergent for laundry washing named ‘‘Beat 1 wt%’’ was
used as an additive which showed an excellent performance
slightly up than additive NaOH 1 wt% for temperature window
800–1050 °C. Therefore a co-effect of anionic surfactant ‘‘Beat
1 wt%’’ and alkali metal NaOH 1 wt% as an additive was observed
as shown in Fig. 10. The experimental results showed that due to
this synergy effect, the efﬁciency of NOx reduction slightly improved as compared to additive NaOH 1 wt% but equivalent to anionic surfactant ‘‘Beat 1 wt%’’ but on other hand, temperature
window slightly shifted towards lower side due to synergy effect.
The synergy effect on shifting of temperature window was slightly
up as compared to additive NaOH 1 wt% and synergy effect on efﬁciency of NOx in comparison with NaOH 1 wt% was prominent at
temperature of 850 °C. Actually anionic surfactant Beat commonly
contains sodium and ammonium lauryl sulfates which work to improve NOx reduction. It may possible that during synergy, main
components of Beat (sodium and ammonium lauryl sulfates) and
100
NOx Reduction [%]
80
urea only
NaOH 1%
Local surfactant
Beat 1%+ NaOH 1%
Local surfactant
Beat 1%
60
40
20
0
700
800
900
1000
1100
Temperature [ oC]
Fig. 9. Effect of different concentration of anionic surfactant SPES for NOx reduction
efﬁciency and optimum reaction temperature window.
Fig. 10. Synergy effect of NaOH 1 wt% and anionic surfactant Beat 1 wt% on NOx
reduction efﬁciency and optimum reaction temperature window.
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NaOH come together to enhance efﬁciency and enlarge the temperature window of SNCR process.
4. Conclusions
The characteristics of different additives for NOx reduction during SNCR process with reducing agent urea have been studied on pilot scale reactor. During a series of experiments with additives like
alkali metal (NaOH, LiOH and KOH), alcoholic (CH3OH, C2H5OH and
C3H7OH), and different type of surfactants, it was observed that
newly searched additive anionic surfactants or a mixture additive
(1 wt% anionic surfactant and 1 wt% NaOH) have a great impact
and immense advantages to enhance NOx reduction efﬁciency and
extend process temperature window due to having both organic
and metallic parts which have ability to provide AOH free radicals
on both sides. These AOH free radicals play an important role in
NOx reduction process. Thus, the temperature window of the process can be easily enlarged and shifted toward lower side in order
to raise NOx reduction efﬁciency with the help of these anionic surfactant additives or a mixture additive (1 wt% anionic surfactant
and 1 wt% NaOH) as compared to conventional additives. It is concluded that anionic surfactant additives make SNCR process more
precise, optimal, efﬁcient and economical for NOx reduction.
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