BUDAPEST
FACULTY
UNIVERSITY OF TECHNOLOGY AND ECONOMICS
OF CHEMICAL AND
BIOCHEMICAL ENGINEERING
DEPARTMENT
OF CHEMICAL AND
PROCESS
ENVIRONMENTAL
ENGINEERING
NITROGEN-OXIDES
Authors: Dr. Bajnóczy Gábor
Kiss Bernadett
The pictures and drawings of this
presentation can be used only for
education !
Any commercial use is prohibited !
Nitrogen oxides

In the atmosphere: NO, NO2, NO3, N2O, N2O3, N2O4, N2O5

Continuously : only NO, NO2, N2O
The others decay very quickly :


Into one of three oxides
Reaction with water molecule
NO
nitric oxide
colourless
odourless
toxic
nonflammable
NO2
nitrogen
dioxide
reddish
brown
strong
choking
odour
very toxic
nonflammable
N2O
nitrous oxide
colourless
sweet odour
non-toxic
nonflammable
Physical properties of NO, NO2 and N2O
Nitric oxide
NO
Molecular mass
Melting
oC
point
Boiling
oC
point
density
0
0 C, 101.3 kPa
25 0C, 101.3 kPa
Solubility in
water
0
0 C 101.3 kPa
Conversion
factors
0 0C, 101.3 kPa
Nitrogendioxide
NO2
Nitrous oxide
N2O
30
46
44
-164
-11
-91
-152
21
-89
1.250 g/dm3
1.145 g/dm3
73,4 cm3/ dm3
(97.7 ppmm)**
1 mg/m3 =
0.747 ppmv***
1 ppmv = 1.339
mg/m3
2.052 g/dm3
1,916 g/dm3
1,963 g/dm3
1.833 g/dm3
bomlik
decay
1305 cm3 /dm3
• NO2 under 0ºC colourless
nitrogen tetroxide (N2O4)
•NO2 natural background
– 9,4 μg/Nm3 (0,2 – 5 ppb)
• in urban area :
20 – 90 μg/Nm3
(0,01 – 0,05 ppm)
• sometimes : 240 – 850 μg/Nm3
(0,13 – 0,45 ppm)
• N2O background ~ 320 ppb
1 mg/m3 = 0.487
ppmv***
1 ppmv = 2,053
mg/m3
1 mg/m3 = 0,509
ppmv***
1 ppmv = 1,964
mg/m3
0,4
Nitrogen oxides


Environment: NO and NO2 acidic rain,
photochemical smog, ozone layer destroyer
N2O :
 stable
 No photochemical reactions in the troposphere ►
lifetime 120 year
 Natural background : 313 ppmv
 Rate of increase 0,5-0,9 ppmv/year
 Greenhouse effect showed itself recently
Natural sources of nitrogen oxides

Atmospheric origin of NO:

Electrical activity (lightning)
20 ppb NO
HNO3
transition → continuous sink
 Equilibrium concentration is kept by the
biosphere:
see: nitrogen cycle
~
Nitrogen-oxides (NO, N2O) from
bacterial activity
• NO emission by the soils 5-20 μg nitrogen/m2 hour, function of organic and
water content and temperature
• Natural N2O : oceans, rivers
Natural sources of nitrogen oxides
Electrical activity in the
atmosphere; lightning
N2 + O2 => 2 NO
Organic nitrogen
content of the soil is
decomposed by micro
organisms
Bottom of the river,
anaerobic condition,
microbiological activity
Anthropogenic sources of nitrogen
oxides
Transportation
Fuel combustion
Application of
nitrogen fertilizers
Anthropogenic sources of nitrogen
oxides


NO:
 Fossils fuel combustion: power plants and
transportation
 Agriculture: Nitrogen fertilizers increase the microbiological
activity resulting in NO emission
N2O:
 Agriculture: Nitrogen fertilizers increase the microbiological
activity resulting in N2O emission
 Transportation (three way catalyst system)
 Power plants (fluid bed boilers)
 Chemical industry (nitric acid)

0,2 % yearly increase in atmospheric content.
Formation of nitric oxide:
Thermal way
• N2 : strong bond in the molecule →
no direct chemical reaction with oxygen
Chain reaction: (Zeldovich, 1940)
O forms in
the flame
N2 + O = NO + N
N + O2 = NO + O
N + •OH = NO + H
→ rate limiting step
The concentration of atomic oxygen is the function of the flame
temperature.
▼
thermal way dominates above 1400 ºC
Rate limiting factors of
thermal NO
Temperature
[ 0C ]
NO concentration at
equilibrium
[ ppm ]
Time 500 ppm [ sec ]
27
1,1 x 10 -19
-
527
0,77
-
1316
550
1370
1538
1380
162
1760
2600
1,1
1980
4150
0,117
The amount of thermal NO is the function of
the flame temperature and the residence time
Formation of prompt NO
Fenimore, 1970:
low flame temperature
Hydrocarbons ▬▬▬▬▬▬▬▬▬►
1000 oC
The reactions
starts by the
alkyl radicals.
• CH + • CH2 + • CH3 + • •
+ N2 = HCN + N
→ rate determination step
• CH + N
•
2
2 = HCN + NH
• CH + N
•
3
2 = HCN + NH2
• CH
High temperature flame section:
HCN + O = NO + • CH
• NH + O = NO + H
• NH + • OH = NO + H
2
The prompt NO is slightly temperature dependent (approx: 5% of the total).
NO from the nitrogen content of
the fuel
• The bond energy of C-N in organic molecule : (150 – 750 kJ/mol), smaller
…than N-N in the nitrogen molecule → increased reactivity
• not sensitive to the flame temperature,
• sensitive to the air excess ratio
• in oxygen lean area (reduction zone) the HCN and NH3 are reduced to
…nitrogen
NO2 formation in the flame
Only a few % of NO2 can be found in the stack gas
NO2 starts to decompose above 150 °C and total decay: above 620 °C
At low flame temperature:
NO + •HO2 = NO2 + • OH
Formation of hydroperoxyl radicals:
At high flame temperature:
H + O2 + M = • HO2 + M
H + O2 = • OH + O
Significant part of NO2 returns back to the
higher flame temperature section :
• decays thermally
NO2 = NO + O
• chemical reaction transforms back to NO:
NO2 + H = NO + • OH
NO2 + O = NO + O2
Formation of N2O :
Low temperature combustion
~10-50% of the fuel N at 800 ºC – 900 ºC may transform to N2O.
In exhaust gas → 50 – 150 ppmv N2O
Thermal decay of coal → hydrogen cyanide formation
HCN + O = NCO + H
NCO + NO = N2O + CO
There is no N2O above 950 ºC , decays thermally above 900 ºC
N2O + M = N2 + O+ M
Increasing temperature favours the formation of hydrogen atoms → reduction
N2O + H = N2 + •OH
Fuels with low heat value (biomass) favours the formation of N2O
N2O formation by catalytic side reactions
•
•
Anthropogenic N2O source : automobiles equipped with catalytic converter
By products of three way catalytic converters:
1. NO reduction
2. CO oxidation
3. Oxidation of hydrocarbons
temperature
product of side reaction
increase
suppresses
the reaction
Adsorption, dissociation
On the surface of
catalyst
product of main reaction
N2O emission from automobiles
Catalyst type
mg/km
year
without
~ 10
1966 - 1972
Installation of catalysts
increases the N2O emission.
Two way system
(oxidation)
~27
1978 - 1982
The benefit > the drawback
~46
1983 - 1995
~19
1996 -
Three way system
(oxidation – reduction)
Three way system
(oxidation – reduction)
Diesel engine
~ 10
Summary of the nitrogen oxide
formation in the flame
Simplified reaction way
Above 1400 0C, strongly
temperature
dependent,
forms in the oxidation zone
Thermal NO
Above 1000 0C, slightly
temperature
dependent,
forms in the reduction zone
Prompt NO
NO from the
fuel
Thermal decay
Organic-N
Above 1000 0C, slightly
temperature
dependent,
forms in the oxidation zone.
Forms in the cooler part of the
flame, decays in warmer
parts
NO2
N2O
remark
Thermal decay
Organic-N
Forms in the range of 800 0C
– 900 0C, decays at higher
temperatures
NO → NO2 transformations in the
troposphere
Possible reaction with O2 → slow
Formation of hydroxyl radicals
NO oxidation by hydroxyl radicals
NO oxidation by methylperoxy radicals
The pure cycle of NO in the troposphere
The ozone molecule may react with another molecule
N2O in the atmosphere


Source: natural and anthropogenic
Very stable in the troposphere:


No reaction with the hydroxyl radicals
λ >260 nm → there is no absorption
Previously it was not considered polluting material.
Recently came to light: greenhouse effect gas
Fate of nitrogen oxides from the
atmosphere
Nitric oxide, nitrogen dioxide
• NO photochemically inert, no solubility in water, forms to NO2
• NO2 soluble in water:
Another way of NO2 elimination:
NO2 + H2O → HNO3 + HNO2
NO2 + O = •NO3
•NO3 + NO2 = N2O5
N2O5+ H2O = 2 HNO3
▼
Effect of light
slow
Only after sunset.
Nitrous oxide N2O
Transport from the troposphere to the stratosphere, here decays:
• oxidation:
•photochemical decay:
N2O + O = 2 NO
Detrimental effect:
decays the ozone layer:
 260 nm
N2O  

  N2 + O
The human activity continuously increases the N2O concentration of the atmosphere.
There is a 0,25% increase /year
Effect of nitrogen oxides on
Plants

Outspokenly harmful

In the atmosphere NO and NO2 together (NOx)

10 000 ppmv NO → reversible decrease of photosynthesis

NO2 → destruction of leaves
(formation of nitric acid), cell damages
Effect of nitrogen oxides on
Humans

NO2 is four times toxic than NO

Odor threshold: 1-3 ppmv

Mucos irritation: 10 ppmv
200 ppmv 1 minute inhaling → death!

Origin of death: wet lung




Nitric acid formation in the alveoli
Alveoli have semi permeable membrane (only gas exchange is
possible)
Nitric acid : destroys the protein structure of the membrane → the
alveoli is filled up by liquid
No more free surface for the gas exchange → death
Effect of nitrogen oxides on
constructing materials


Acid rain causes electrochemical corrosion
Surface degradation on limestone, marble
by the acidic rain.
Control of nitrogen oxides emission
Technological developments: only 15% decrease
(since 1980)
~90% of anthropogenic emission comes from



boilers
internal combustion engines
Control of emission:
 make conditions do not favor the formation
 elimination of the nitrogen oxides from the exhaust
gases
Control of nitrogen oxides emission

The NO formation in the flame depends on:

N content of the fuel

Flame temperature

Residence time in the flame

Amount of reductive species
The air excess ratio (n) has strong effect on the last three.
The air excess ratio can be adjusted globally or locally.
Control of nitric oxide (NO) emission, by
two stage combustion
Two stage combustion: the air input is shared to create different zones
in the flame → a./ reduction zone where the combustion starts
b./ oxidation zone where the combustion is completed.
oxidation zone
secondary
air
fuel
+ air
secondary
air
reduction zone
Control of nitric oxide (NO) emission by two
stage combustion
BOILER
Control of nitric oxide (NO) emission,
by three stage combustion
ZONES IN THE FLAME:
1. Perfect burning in the most inner part of the flame (oxidation zone).
2. Fuel input to reduce the NO (reduction zone).
3. Finally air input to oxidize the rest of hydrocarbons (oxidation zone).
burner
Control of nitric oxide (NO) emission by
three stage combustion
Control of nitric oxide (NO) emission, by
three stage combustion



1. zone
fuel (coal powder, oil) ( n>1)
2. zone 10..20% fuel input
n=0,9 temperature 1000°C
3. zone
air input, n>1, perfect
burning.
30..70% NO reduction
is available
Flue gas recirculation


Application:

oil and

gas boilers
The cooled flue gas has high
specific heat due to the water
content.

The recirculated flue gas decrease
the flame temperature.

Generally ~10% is recirculated

More than 20 % produces higher
CO and hydrocarbon emissions.
1. Mixed with air input (FGR: flue
gas recirculation)
2. Mixed with fuel input (FIR: fuel
induced recirculation)
Nitric oxide (NO) eliminations from the
exhaust gas
possibilities:
 Selective noncatalytic reduction SNCR
(thermal DENOx process)
 Selective catalytic reduction SCR
(catalytic DENOx process)
Reduction of NO emission by
selective non catalytic reduction
Ammonia is added to the NO contaminated fuel gas at 900 ºC:
4 NO + 4 NH3 + O2 = 4 N2 + 6 H2O
Danger of excess ammonia. Better solution is the urea
2 NH2▬CO▬NH2 + 4 NO + O2 = 4 N2 + 4 H2O + 2 CO2
• advantage: simplicity
• disadvantage: temperature sensitive.
• ammonia: 870 – 980 ºC, urea 980 – 1140 ºC
At higher temperature ammonia is oxidized to NO
At lower temperature ammonia remains in the fuel gas
Efficiency : 40 – 70 % at optimal condition.
Reduction of NO emission by selective
catalytic reduction
• better efficiency is available
• composition: V2O5 or WO3 on
titanium dioxide supporter
• Applied NH3 / NO rate ~0,8
(mol/mol),
Drawback:
• SO2 content of the fuel gas is
oxidized to SO3 → corrosion
• Ammonium-sulphate deposition on
the catalyst surface
•The method can not be applied over
0,75 % sulfur content in the stack gas
NO elimination from the exhaust gas of
internal combustion engines
Only the treatment of the exhaust gas is possible
Control methods applied to one pollutant often influence the output of
other pollutant
NO elimination from the exhaust gas of
internal combustion engines

NO from internal combustion engine is thermal origin.

NO elimination by selective catalytic reduction.

Discussed in details at hydrocarbons