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