nitrogen-oxides
DESCRIPTION
BUDAPEST UNIVERSITY OF TECHNOLOGY AND ECONOMICS. DEPARTMENT OF CHEMICAL AND ENVIRONMENTAL PROCESS ENGINEERING. FACULTY OF CHEMICAL AND BIOCHEMICAL ENGINEERING. NITROGEN-OXIDES. Authors: Dr. Bajnóczy Gábor Kiss Bernadett. - PowerPoint PPT PresentationTRANSCRIPT
NITROGEN-NITROGEN-OXIDESOXIDES
Authors: Dr. Bajnóczy Gábor
Kiss Bernadett
BUDAPEST UNIVERSITY OF TECHNOLOGY AND ECONOMICS
DEPARTMENT OF CHEMICAL AND ENVIRONMENTAL PROCESS ENGINEERING
FACULTY OF CHEMICAL AND BIOCHEMICAL ENGINEERING
The pictures and drawings The pictures and drawings of this presentation can be of this presentation can be used only for education !used only for education !
Any commercial use is Any commercial use is prohibited !prohibited !
Nitrogen oxidesNitrogen 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 moleculeNO
nitric oxidecolourless odourless toxic non-
flammable
NO2
nitrogen dioxide
reddish brown
strong choking odour
very toxic non-flammable
N2O
nitrous oxide
colourless sweet odour non-toxic non-flammable
Physical properties of NO, NO2 and N2O
Nitric oxideNO
Nitrogen-dioxide
NO2
Nitrous oxideN2O
Molecular mass 30 46 44
Melting point oC -164 -11 -91
Boiling point oC
-152 21 -89
density0 0C, 101.3 kPa
25 0C, 101.3 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 101.3 kPa
73,4 cm3/ dm3 (97.7 ppmm)**
bomlik 1305 cm3 /dm3
Conversion factors0 0C, 101.3 kPa
1 mg/m3 = 0.747 ppmv***
1 ppmv = 1.339 mg/m3
1 mg/m3 = 0.487 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 0,4 – 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
Nitrogen oxidesNitrogen 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 Natural sources of nitrogen oxidesoxides
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, NNitrogen-oxides (NO, N22O) O) from bacterial activityfrom 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 Natural sources of nitrogen oxidesoxides
Electrical activity in the atmosphere; lightning
N2 + O2 => 2 NO
Bottom of the river, anaerobic condition, microbiological activity
Organic nitrogen content of the soil is decomposed by micro organisms
Anthropogenic sources of Anthropogenic sources of nitrogen oxidesnitrogen oxides
Transportation Fuel combustion
Application of nitrogen fertilizers
Anthropogenic sources of Anthropogenic sources of nitrogen oxidesnitrogen 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:Formation of nitric oxide: Thermal wayThermal way
• N2 : strong bond in the molecule → no direct chemical reaction with oxygenChain reaction: (Zeldovich, 1940)
N2 + O = NO + N
N + O2 = NO + O
N + •OH = NO + H
O forms in the flame
→ rate limiting step
The concentration of atomic oxygen is the function of the flame temperature.
▼
thermal way dominates above 1400 ºC
Rate Rate limitinglimiting factors of factors of thermal NOthermal 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
low flame temperature
Formation of prompt NOFormation of prompt NO
• CH + N2 = HCN + N• CH2 + N2 = HCN + • NH
• CH3 + N2 = HCN + • NH2
HCN + O = NO + • CH• NH + O = NO + H
• NH + • OH = NO + H2
Fenimore, 1970:
High temperature flame section:
→ rate determination step
The prompt NO is slightly temperature dependent (approx: 5% of the total).
The reactions starts by the alkyl radicals.
• CH + • CH2 + • CH3 + • •
Hydrocarbons ▬▬▬▬▬▬▬▬▬► 1000 oC
NO from the nitrogen content NO from the nitrogen content of the fuelof 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
NONO22 formation in the flame formation in the flame
NO + •HO2 = NO2 + • OH
H + O2 + M = • HO2 + M
H + O2 = • OH + O
NO2 + H = NO + • OHNO2 + O = NO + O2
At low flame temperature:
Formation of hydroperoxyl radicals:
At high flame temperature:
Significant part of NO2 returns back to the higher flame temperature section :
• decays thermally
• chemical reaction transforms back to NO:
Only a few % of NO2 can be found in the stack gasNO2 starts to decompose above 150 °C and total decay: above 620 °C
NO2 = NO + O
Formation of NFormation of N22O :O :
Low temperature combustionLow temperature combustion
HCN + O = NCO + H NCO + NO = N2O + CO
N2O + M = N2 + O+ M
N2O + H = N2 + •OH
~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
There is no N2O above 950 ºC , decays thermally above 900 ºC
Increasing temperature favours the formation of hydrogen atoms → reduction
Fuels with low heat value (biomass) favours the formation of N2O
NN22O formation by catalytic side O formation by catalytic side reactionsreactions
• Anthropogenic N2O source : automobiles equipped with catalytic
converter• By products of three way catalytic converters:
1. NO reduction2. CO oxidation3. Oxidation of hydrocarbons
product of main reaction
product of side reaction
temperature increase suppresses the reaction
Adsorption, dissociation
On the surface of catalyst
NN22O emission from O emission from automobilesautomobiles
Catalyst type mg/km year
without ~ 10 1966 - 1972
Two way system(oxidation)
~27 1978 - 1982
Three way system(oxidation – reduction)
~46 1983 - 1995
Three way system(oxidation – reduction)
~19 1996 -
Diesel engine ~ 10
Installation of catalysts increases the N2O emission.
The benefit > the drawback
Summary of the nitrogen oxide Summary of the nitrogen oxide
formation in the flameformation in the flame
Simplified reaction way remark
Thermal NOAbove 1400 0C, strongly temperature dependent, forms in the oxidation zone
Prompt NO
Above 1000 0C, slightly temperature dependent, forms in the reduction zone
NO from the fuel
Above 1000 0C, slightly temperature dependent, forms in the oxidation zone.
NO2
Forms in the cooler part of the flame, decays in warmer parts
N2OForms in the range of 800 0C – 900 0C, decays at higher temperatures
Organic-N
Thermal decay
Thermal decay
Organic-N
NO NO → NO→ NO22 transformations in transformations in the tropospherethe 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 The pure cycle of NO in the tropospheretroposphere
The ozone molecule may react with another molecule
NN22O in the atmosphereO 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 Fate of nitrogen oxides from the atmosphereatmosphere
N2O5+ H2O = 2 HNO3
NO2 + O = •NO3
•NO3 + NO2 = N2O5
NO2 + H2O → HNO3 + HNO2
Nitric oxide, nitrogen dioxide
• NO photochemically inert, no solubility in water, forms to NO2
• NO2 soluble in water: slow
Another way of NO2 elimination:
Only after sunset.
▼
Effect of light
Nitrous oxide NNitrous oxide N22OO
N2O + O = 2 NO
Transport from the troposphere to the stratosphere, here decays:
Detrimental effect: decays the ozone layer:
• oxidation:
•photochemical decay:
N2O N2 + O nm260
The human activity continuously increases the N2O concentration of the
atmosphere. There is a 0,25% increase /year
Effect of nitrogen oxides onEffect of nitrogen oxides on PlantsPlants
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 onEffect of nitrogen oxides on HumansHumans
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 onEffect of nitrogen oxides on constructing materialsconstructing materials
Acid rain causes electrochemical corrosion
Surface degradation on limestone, marble by the acidic rain.
Control of nitrogen oxides Control of nitrogen oxides emissionemission
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 Control of nitrogen oxides emissionemission
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.
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.
Control of nitric oxide (NO) Control of nitric oxide (NO) emission, by two stage combustionemission, by two stage combustion
oxidation zone
reduction zone
secondary
secondary
air
air
fuel
+ air
Control of nitric oxide (NO) emission by Control of nitric oxide (NO) emission by two stage combustiontwo stage combustion
BOILER
Control of nitric oxide (NO) Control of nitric oxide (NO) emission, by three stage emission, by three stage
combustioncombustion
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 Control of nitric oxide (NO) emission by three stage combustionthree stage combustion
Control of nitric oxide (NO) emission, Control of nitric oxide (NO) emission, by three stage combustionby three stage combustion
1. zone fuel (coal powder, oil) ( n>1)
2. zone 10..20% fuel inputn=0,9 temperature 1000°C
3. zoneair input, n>1, perfect burning.
30..70% NO reduction is available
FFluelue gas recirculation 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 Nitric oxide (NO) eliminations from the exhaust gasthe exhaust gas
possibilities: Selective noncatalytic reduction
SNCR (thermal DENOx process) Selective catalytic reduction SCR
(catalytic DENOx process)
Reduction of NO emission by Reduction of NO emission by selective non catalytic selective non catalytic
reductionreduction
4 NO + 4 NH3 + O2 = 4 N2 + 6 H2O
2 NH2▬CO▬NH2 + 4 NO + O2 = 4 N2 + 4 H2O + 2 CO2
Ammonia is added to the NO contaminated fuel gas at 900 ºC:
Danger of excess ammonia. Better solution is the urea
• advantage: simplicity
• disadvantage: temperature sensitive.
• ammonia: 870 – 980 ºC, urea 980 – 1140 ºC
At higher temperatureAt higher temperature ammonia is oxidized to NO
At lower temperatureAt lower temperature ammonia remains in the fuel gas
Efficiency : 40 – 70 % at optimal condition.Efficiency : 40 – 70 % at optimal condition.
Reduction of NO emission by Reduction of NO emission by selective catalytic reductionselective 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 NO elimination from the exhaust gas of internal combustion gas of internal combustion
enginesengines
Control methods applied to one pollutant often influence the output of other pollutant
Only the treatment of the exhaust gas is possible
NO elimination from the exhaust NO elimination from the exhaust gas of internal combustion gas of internal combustion
enginesengines
NO from internal combustion engine is thermal origin.
NO elimination by selective catalytic reduction.
Discussed in details at hydrocarbons