1. BUDAPEST UNIVERSITY OF TECHNOLOGY AND ECONOMICS
FACULTY OF CHEMICAL AND BIOCHEMICAL ENGINEERING
DEPARTMENT OF CHEMICAL AND
ENVIRONMENTAL PROCESS ENGINEERING
NITROGEN-OXIDES
Authors: Dr. Bajnóczy Gábor
Kiss Bernadett

2. The pictures and drawings of this presentation can be used only for education ! Any commercial use is prohibited !

3. 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
non-flammable
NO2
nitrogen dioxide
reddish brown
strong choking odour
very toxic
N2O
nitrous oxide
sweet odour
non-toxic

4. Physical properties of NO, NO2 and N2O
Nitric oxide NO
Nitrogen-dioxide
NO2
Nitrous oxide
N2O
Molecular mass 30 46 44
Melting point oC
-164
-11
-91
Boiling point oC
-152 21
-89
density
0 0C, kPa
25 0C, kPa
1.250 g/dm3
1.145 g/dm3
2.052 g/dm3
1,916 g/dm3
1,963 g/dm3
1.833 g/dm3
Solubility in water
0 0C kPa
73,4 cm3/ dm3 (97.7 ppmm)**
bomlik
1305 cm3 /dm3
Conversion factors
1 mg/m3 = ppmv***
1 ppmv = mg/m3
1 mg/m3 = ppmv***
1 ppmv = 2,053 mg/m3
1 mg/m3 = 0,509 ppmv***
1 ppmv = 1,964 mg/m3
NO2 under 0ºC colourless nitrogen tetroxide (N2O4)
NO2 natural background ,4 – 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
decay

5. 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

6. Natural sources of nitrogen oxides
Atmospheric origin of NO:
Electrical activity (lightning)
~ 20 ppb NO HNO transition → continuous sink
Equilibrium concentration is kept by the biosphere:
see: nitrogen cycle

7. 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

8. Natural sources of nitrogen oxides
Bottom of the river, anaerobic condition, microbiological activity
Electrical activity in the atmosphere; lightning
N2 + O2 => 2 NO
Organic nitrogen content of the soil is decomposed by micro organisms

9. Anthropogenic sources of nitrogen oxides
Transportation
Fuel combustion
Application of nitrogen fertilizers

10. Anthropogenic sources of nitrogen oxides
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.

11. Formation of nitric oxide: Thermal way
N2 : strong bond in the molecule →
no direct chemical reaction with oxygen
Chain reaction: (Zeldovich, 1940)
N2 + O = NO + N
N + O2 = NO + O
N + •OH = NO + H
→ rate limiting step
O forms in the flame
The concentration of atomic oxygen is the function of the flame temperature. ▼
thermal way dominates above 1400 ºC

12. Rate limiting factors of thermal NO
Temperature
[ 0C ]
NO concentration at equilibrium
[ ppm ]
Time 500 ppm [ sec ] 27
1,1 x -
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

13. The reactions starts by the alkyl radicals.
Formation of prompt NO
Fenimore, 1970:
low flame temperature
Hydrocarbons ▬▬▬▬▬▬▬▬▬►
• CH + • CH2 + • CH3 + • •
1000 oC
• CH + N2 = HCN + N
• CH2 + N2 = HCN + • NH
• CH3 + N2 = HCN + • NH2
→ rate determination step
The reactions starts by the alkyl radicals.
High temperature flame section:
HCN + O = NO + • CH
• NH + O = NO + H
• NH + • OH = NO + H2
The prompt NO is slightly temperature dependent (approx: 5% of the total).

14. 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

15. 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:
H + O2 + M = • HO2 + M
At high flame temperature:
H + O2 = • OH + O
Significant part of NO2 returns back to the higher flame temperature section :
decays thermally
chemical reaction transforms back to NO:
NO2 = NO + O
NO2 + H = NO + • OH
NO2 + O = NO + O2

16. 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

17. N2O formation by catalytic side reactions
Anthropogenic N2O source : automobiles equipped with catalytic converter
By products of three way catalytic converters:
NO reduction
CO oxidation
Oxidation of hydrocarbons
temperature increase suppresses the reaction
product of side reaction
Adsorption, dissociation
On the surface of catalyst
product of main reaction

18. N2O emission from automobiles
Catalyst type
mg/km
year
without
~ 10
Two way system
(oxidation)
~27
Three way system
(oxidation – reduction)
~46
~19
1996 -
Diesel engine
Installation of catalysts increases the N2O emission.
The benefit > the drawback

19. Summary of the nitrogen oxide formation in the flame
Simplified reaction way
remark
Thermal NO
Above C, strongly temperature dependent, forms in the oxidation zone
Prompt NO
Above C, slightly temperature dependent, forms in the reduction zone
NO from the fuel
Above C, slightly temperature dependent, forms in the oxidation zone.
NO2
Forms in the cooler part of the flame, decays in warmer parts
N2O
Forms in the range of 800 0C – 900 0C, decays at higher temperatures
Thermal decay
Organic-N
Thermal decay
Organic-N

20. 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

21. The pure cycle of NO in the troposphere
The ozone molecule may react with another molecule

22. 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

23. 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:
NO2 + H2O → HNO3 + HNO2
slow
NO2 + O = •NO3
•NO3 + NO2 = N2O5
Another way of NO2 elimination:
Only after sunset.
N2O5+ H2O = 2 HNO3 ▼
Effect of light

24. Nitrous oxide N2O
Transport from the troposphere to the stratosphere, here decays:
oxidation:
N2O + O = 2 NO
Detrimental effect: decays the ozone layer:
photochemical decay:
N2O
N2 + O
The human activity continuously increases the N2O concentration of the atmosphere. There is a 0,25% increase /year

25. Effect of nitrogen oxides on Plants
Outspokenly harmful
In the atmosphere NO and NO2 together (NOx)
ppmv NO → reversible decrease of photosynthesis
NO2 → destruction of leaves
(formation of nitric acid), cell damages

26. 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

27. Effect of nitrogen oxides on constructing materials
Acid rain causes electrochemical corrosion
Surface degradation on limestone, marble by the acidic rain.

28. 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

29. 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.

30. 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

31. Control of nitric oxide (NO) emission by two stage combustion
BOILER

32. 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

33. Control of nitric oxide (NO) emission by three stage combustion

34. Control of nitric oxide (NO) emission, by three stage combustion
1. zone
fuel (coal powder, oil) ( n>1)
2. zone % fuel input
n=0,9 temperature 1000°C
3. zone
air input, n>1, perfect burning.
30..70% NO reduction is available

35. 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.
Mixed with air input (FGR: flue gas recirculation)
Mixed with fuel input (FIR: fuel induced recirculation)

36. Nitric oxide (NO) eliminations from the exhaust gas
possibilities:
Selective noncatalytic reduction SNCR (thermal DENOx process)
Selective catalytic reduction SCR (catalytic DENOx process)

37. 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▬NH 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.

38. 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

39. 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

40. 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