Transcript Control of Oxides of Nitrogen from Stationary Sources

Control of Oxides of Nitrogen
from Stationary Sources
Oxides of Nitrogen
• The gaseous oxides of nitrogen include:
– N2O:nitrous oxide
– NO:nitric oxide (free radical)
– N2O3: nitrogen trioxide
– NO2:Nitrogen dioxide (free radical)
– N2O5:nitrogen pentoxide
An unstable form NO3 also exists.
Only N2O,NO, and NO2 present in the
atmosphere in significant concentrations
NO and NO2 (NOx)
• NO is a colorless gas with average ambient
concentration of 0.5 ppm
• No adverse health effect at this concentration
but it is a precursor to the formation of NO2 and
active compound in photochemical smog
formation
• NO2 is a reddish brown gas. A concentration of
1 ppm can be detected by the eye. Adverse
health effect is primarily associated with the
pulmonary problems.On an annual basis air
ambient air level standart is 0.05 ppm (100
ug/m3)
Sources and Concentrations of
NOx
• Over 90% of all man-made nitrogen oxides entering the
atmosphere are from combustion sources
• For the US, 50% is from mobile sources
• For Turkey ambient NOx concentration is around xx
• At an emission source the concentration of oxided of
nitrogen is much higher than ambient values
• For example the NOx concentration in the flue gas from
the steam boiler of a power plant may reach 500 to
1000ppm
• From such combustion processes the NOx is the
exhaust stack gas would be 90% or more NO and the
rest NO2
NOx Control
• 1. Control over the reaction that produces
the pollutant
• 2. Remove NOx after it is formed
NOx Formation
Basic Chemistry,Thermodynamics and Kinetics of the
Formation Reactions
• There are two sources of N that contribute
to the formation of oxides of nitrogen
– 1. Fuel N (Coal and fuel oil composition, note
that no N in natural gas)(Fuel NOx)
– 2. Air N (78% of air is N)(Thermal NOx)
Thermodynamics of Thermal NOx
Formation
• It is essential to understand the thermodynamics
and kinetic of nitrogen-oxygen reactions,
especially at high temperatures
• NOx formation mechanism was first proposed by
Zeldovich (1946):
• N2+O⇋NO + N
• N+O2⇋NO +O
• N+OH⇋NO +H
(1)
(2)
(3)
Reaction 1 and 2 are the most important reactions
in the model.
Thermodynamics of Thermal NOx
Formation
• Let’s first consider only the NO and NO2
formation equilibrium reactions:
• N2+O2⇋2NO
• NO+1/2O2⇋NO2
2
(4)
(5)
( PNO )
K P1 
PN 2 PO2
K P2 
PNO2
( PNO )(PO2 )1/ 2
Equilibrium Constants:
Temperature
(K)
300
KP
N2+O22NO
10-30
1.4(10)6
500
2.7(10)-18
1.3(10)2
1000
7.5(10)-9
1.2(10)-1
1500
1.1(10)-5
1.1(10)-2
2000
4.0(10)-4
3.5(10)-3
NO+1/2O2NO2
Predicted Equilibrium
Concentrations
Temperature
(K)
300
Flue Gas (ppm)
NO
NO2
1.1(10)-10
3.3(10)-5
800
0.8
0.1
1400
250
0.9
1873
2000
1.8
Predicted Equilibrium
Concentrations
Flue
1. At flue gas exit T (420-590 K),
expect to observe very low NOx
Gas
(ppm)
(<1 ppm)
and primarily NOx in
the form NO2
NO2
Temperature
(K)
300
NO
1.1(10)-10
3.3(10)-5
800
0.8
0.1
1400
250
1873
2000
1. At flame zone T (1850-3500
K), expect to observe very high
0.9
NOx (up to 10000 ppm) and
primarily NOx in the form NO
1.8
Predicted Equilibrium
Concentrations
1. At flue gas exit T (420-590 K),
expect to observe very low NOx
(<1 ppm) and primarily NOx in
the form NO2
• At actual furnaces however typical flue gas
consists of 300-1200 ppm NOx and mostly
(90-95%) in NO form
• So there must be other factors other than
equilibrium to explain this:
Kinetics of Nitric Oxide Formation
in Combustion Process
• The rate of NO formation is the major
factor influencing NOx concentrations
• Considering reactions 1 to 3, the rate
expression can be written:
• rNO= k+1[N2][O]-k-1[NO][N]+k+2[O2][N]
-k-2[NO][O]+k+3[N][OH]-k-3[NO][H]
• rN= k+1[N2][O]-k-1[NO][N]-k+2[O2][N]
-k-2[NO][O]-k+3[N][OH]+k-3[NO][H]
Rate Constants
Rate constant k, m3/mol-s
Reaction
k+1
(1)N2+O⇋NO + N
k-1
k+2
(2)N+O2⇋NO +O
k-2
k+1=1.8(10)8e-38,370/T
k-1= 3.8(10)7e-425/T
k+2=1.8(10)4e-4680/T
k-2= 3.8(10)3e-20820/T
k+3=1.8(10)7e-450/T
(3)N+OH⇋NO +H
k-3
k-3= 3.8(10)8e-24560/T
k+3
T is in degrees Kelvin
Kinetics of Nitric Oxide Formation
in Combustion Process
• N atoms are much more reactive than NO,
we can assume quasi steady state for N and
an expression for [N]ss can be obtained:
• [N]ss= k+1[N2][O]+k-2[NO][O]+k-3[NO][H]/
k-1[NO]+k+2[O2]+k+3[OH]
And substituting this into rate equation for NO
• rNO= k+1[N2][O]-k-2[NO][O]-k-3[NO][H]+(
-k-1[NO]+k+2[O2]+k+3[OH])[N]ss
Kinetics of Nitric Oxide Formation
in Combustion Process
The interesting thing about above rate equation
is that the initial rate of formation of NO
(when NO concentrations are small) is just
equal to twice that of reaction 1. That is:
• rNO= k+1[N2][O]-k-2[NO][O]-k-3[NO][H]+(
-k-1[NO]+k+2[O2]+k+3[OH])[N]ss
N2+O⇋NO + N (1)
rNOinitial= 2k+1[N2][O]
Example 16.2
• Given a HC flame at 1870 C where the
mole fractions of N2 gas and O atoms are
0.75 and 9.5(10)-4 respectively,
• a) calculate the initial rate of NO formation
(in mole(m3-s)
• b) if this rate holds constant for 0.03
seconds, calculate the concentration in
ppm of NO in the gases leaving the flame
zone
Solution
• a) at T = 1870 C = 2143 K,
k+1=1.8(10)8e-38,370/2143=3.015 m3/mol-s
Assuming P= 1atm, the molar density of the
gases:
1atm
 M  P / RT 
8.206(10)
5 m3  atm
mol  K
2143K
 5.686m ol/ m3
[N2]=0.75x5.686=4.26 mol/m3
[O]=9.5(19)-4x5.686=0.0054 mol/m3
rNOinitial= 2k+1[N2][O]=2(3.015)(4.26)(0.0054)
=0.1388 mol/m3-s
Solution
• b) If this inital rate holds for 0.03 seconds
then
[NO]=0.1388(0.03)=4.16(10)-3 mol/m3
4.16(10) 3
[ NO] 
(10) 6  732ppm
5.686
Research on NOx Formation
• Experimental results in various studies
showed that NO concentrations in the
flame zone are significantly higher than
could have been formed by the Zeldovich
mechanism
• This may be due to “prompt” NO formation
• Prompt NO: NO formed in the first five
milliseconds (40-100 ppm)
Research on NOx Formation
• MacKinnon worked
with heated N2,O2, Ar
samples
• NO concentrations
increased rapidly with
time up to about 4-5
seconds, after no
further increases
observed
T (C )
NO conc.
<1600
<200 ppm
>1800
=1950
Several
thousands
12,000
1990
Peak value
>2000
decreases
Research on NOx Formation
• MacKinnon developed a model predicts the NO
concentration (in ppm) as a function of
temperature (in K), N and O concentrations, and
time (in s)
CNO  5.2(10) [exp 72,300/ T ] yN2 y t
17
1/ 2
O2
NOx Formation from Fuel Nitrogen
• When a fuel contains organically bound N,
the contribution of the fuel bound N to the
total NOx production is significant
Example 16.3
Strategies for Combustion Modification
 fuel   fuel 
Equivalentratio() : 
/

 air   air Stoichiometric
• Reduce peak
temperatures of
the flame zone
• Reduce gas
residence time
in the flame
zone
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Aerosol & Particulate Research
Lab
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Modification of Operating Conditions
• Off-Stoichiometric
Combustion/Staged
combustion: combusting the
fuel in two or more steps. Fuel
rich then fuel lean.
• Flue gas recirculation: reroute
some of the flue gas back to
the furnace; lower O2 and allow
NOx to proceed the “frozen”
reactions
• Water injection: reduce http://en.wikipedia.org/wiki/Staged_combustion_cycle_(roc
ket)
flame temperature; energy
2015/4/13
Aerosol & Particulate Research
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penalty
Lab
• Gas reburning: injection of natural gas into the
boiler above the main burner to create a fuel-rich
reburn zone; hydrocarbon radicals react with NOx to
reduce NOx to N2.
2015/4/13
Aerosol & Particulate Research
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http://www.lanl.gov/projects/cctc/factsheets/eerco/gasreburndemo.html
Lab
• Low-NOx burner: inhibit NOx formation
by controlling the mixing of fuel and air;
lean excess air and off-stoichiometric
combustion
2015/4/13
Aerosol & Particulate Research
Lab
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Case: Paşabahçe Glass Production
Flue Gas Treatment
• Selective Catalytic Reduction (SCR)
or V2 O5 supported catalyst
4 NO  4 NH 3  O2 TiO
2

 4 N 2  6 H 2O
or V2O5 supported catalyst
2 NO2  4 NH 3  O2 TiO
2

 3N 2  6 H 2O
Temperature ~ 300 - 400 oC
Q: Why is it called “selective”?
Also good for Hg emission control!!! But it creates SO3.
• Selective Noncatalytic Reduction (SNR)
4 NH 3  4 NO  O2  4 N 2  6 H 2O
Temperature ~ 800 - 1000 oC
4 NH 3  5O2  4 NO  6 H 2O
Above 1000 oC
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Aerosol & Particulate Research
Lab
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SCR
•
•
•
•
•
Removal efficiency is over 90%
Expenses from use of catalyst
High operation and capital cost
Large area requirement
Requires temperature control for
optiumum reduction
SNCR
• No catalyst cost
• High temperatures (850-1100)
• Low removal efficiency ( %30-%66 less than
SCR)
Other Control Methods
• Absorption
• Adsorption
• Biological
Biological Control Technologies
Under aerobic conditions, nitrification and
chemical oxidation leads to NOx oxidation to
nitrate.
İlk denemelerde, yüksek O2 konsantrasyonlu
aerobik şartlarda (>%17 Oksijen) NO
giderimi toluenle muamele edilmiş silika
pelet dolgulu bir biyofiltrede %97
mertebesinde başarılmıştır
Biological Control Technologies
Anoksik koşullarda ise NOx denitrifikasyon
prosesi yüksek bir verimle azot gazına
indirgenmektedir.
Toprak kompostu içeren laboratuar ölçekli bir
biyofiltre çalışmasında NO2 için %100’e
yaklaşan bir giderim verimi elde edilmiştir.
(Okuno et. al, 2000). Tüm bunlar BiyoDeNox
teknolojilerinin NOx kontrolü için gelecekte
de artan oranlarda kullanılacağını
göstermektedir.
Biological Control Technologies
Avantajlar
Biyofiltreler
Biyo-yıkama
Dezavantajlar
 Geleneksel dizaynı için geniş alan
ihtiyacı vardır
 Kurulum maliyeti düşüktür.
 İşletim maliyetleri genellikle  Yatakta pH ayarı veya besin eklemek
düşüktür.
için iç sıvı akışı sürekliliği yoktur
 Belirli bileşikler için yüksek  Tepesinin olmaması temsili numune
DREs oluşur
alımını zorlaştırır
 Yatak değişimi 2- 6 hafta sürebilir
 Arıtım
öncesi
gazı
nemlendirmek gerekli değildir  Kurulumu oldukça pahalıdır
 Aşırı biyokütle büyümesi tıkanmalara
neden olabilir
 Diğer biyoreaktörlere göre
daha az ayakizi vardır
 pH kontrolü ve besin besleme  İşletme maliyetleri diğer biyoreaktör
otomatikleştirilebilir
proseslerinden fazladır
 Asit üreten tesislerin atık  Pahalı ve kompleks besleme ve nötralize
gazları için idealdir
etme sistemleri gereksinimi vardır
 Partikül
madde
içeren  Toksik
ve
tehlikeli
bileşiklerin
emisyonların
arıtımına
envanterinin
tutulması
ve
uygundur
uzaklaştırılması gerekir
Biological Control Technologies
BioDeNOx Process
BioDeNOx Process
• First nitrosyl complex was formed by the
reactions 1 and 2:
• NO (g)  NO (aq) (1)
• NO (aq) + Fe(II)EDTA2- > Fe(II)EDTA −
NO2(2)
BioDeNOx Process
• To be able to regenarate the absorption
liquid, formed Fe(III)EDTA- needs to be
reduced to Fe(II)EDTA2- by the m/o
• Regeneration of Fe(II)EDTA2- is essential
for the system NO removal performance:
• 12 Fe(III)EDTA- + C2H5OH + 3 H2O 12
Fe(II)EDTA 2- + 2 CO2 + 12 H+
Jet-Loop Biyoreaktör
• JetLoop reaktörlerde sistem içinde draft tüpüne
püskürtülen gaz reaktörü terk etmeden önce draft tüpün
içerisinde birkaç kez devir yapmaktadır.
• Devir sayısı ve jet akımından kaynaklanan daha küçük
çaplı gaz oluşumu NO gazının ortama bir kelat ilave
etmeksizin daha fazla çözünmesini sağlamaktadır.
İTÜ XIII. Endüstriyel Kirlenme Kontrolü Sempozyumu
Pilot Tesis
• Toplam hacim 20 L, Çamur yaşı 50 gün,
1. Su girişi 2. Hava/gaz girişi 3. Hava/gaz çıkışı 4. JetLoop biyoreaktör
5. Su boşaltma vanası 6. Soğutucu 7. Su debimetresi 8. ÇO, Redox,
pH, Sıcaklık sensörleri haznesi 9. Pompa 10. Besleme girişi 11. MBR
sistemi, V: vana)
İTÜ XIII. Endüstriyel Kirlenme Kontrolü Sempozyumu
Pilot Tesis
İTÜ XIII. Endüstriyel Kirlenme Kontrolü Sempozyumu
Pilot Tesis İşletme Sonuçları
MLSS QGD
Besleme ORP
Deney
Q
/Q
Gaz
GD
(mg/L) (m3/sa)
Şekli
(mV)
Giriş Çıkış
Verim
NO
NO
(%)
(ppm) (ppm)
1
3000
1.0
0.12
Sürekli
-485
550
427
23
3*
2640
1.0
0.12
Sürekli
-476
1100
820
25
2
3000
1.8
0.07
Sürekli
-488
550
175
70
4*
2400
1.8
0.07
Sürekli
-475
1100
413
65
İTÜ XIII. Endüstriyel Kirlenme Kontrolü Sempozyumu