 

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

The pictures and drawings of this presentation can be used only for education !

Any commercial use is prohibited !

Nitrogen oxides

 In the atmosphere : NO, NO 2 , NO 3 , N 2 O, N 2 O 3 , N 2 O 4 , N 2 O 5  Continuously : only NO, NO 2 , N 2 O The others decay very quickly :   Into one of three oxides Reaction with water molecule

NO nitric oxide NO 2 nitrogen dioxide N 2 O nitrous oxide colourless reddish brown colourless odourless strong choking odour sweet odour toxic very toxic non-toxic non flammable non flammable non flammable

Physical properties of NO, NO

2

and N

2

O

Nitric oxide NO Nitrogen dioxide NO 2

46

Nitrous oxide N 2 O Molecular mass Melting o C point Boiling o C point density 0 0 C, 101.3 kPa 25 0 C, 101.3 kPa Solubility in water 0 0 C 101.3 kPa

-164 -152 1.250 g/dm 1.145 g/dm 73,4 cm 3 / dm 3 (97.7 ppmm) ** 3 3 30

Conversion factors 0 0 C, 101.3 kPa

2.052 g/dm 1,916 g/dm 3 3 -11 21 1,963 g/dm 1.833 g/dm 1305 cm 3 3 3 /dm 3 44 -91 -89 1 mg/m 3 = 0.747 ppmv *** 1 ppmv = 1.339 mg/m 3 1 mg/m 3 = 0.487 ppmv *** 1 ppmv = 2,053 mg/m 3 1 mg/m 3 ppmv *** = 0,509 1 ppmv = 1,964 mg/m 3 • NO

2

under 0ºC colourless nitrogen tetroxide (N 2 O 4 ) •NO 2 natural background 0,4 – 9,4 μg/Nm 3 (0,2 – 5 ppb) • in urban area : 20 – 90 μg/Nm 3 (0,01 – 0,05 ppm) • sometimes : 240 – 850 μg/Nm 3 (0,13 – 0,45 ppm) • N 2 O background ~ 320 ppb

Nitrogen oxides

  Environment: NO and NO 2 acidic rain, photochemical smog, ozone layer destroyer N 2 O :  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 HNO 3 transition → continuous sink Equilibrium concentration is kept by the biosphere: see: nitrogen cycle

Nitrogen-oxides (NO, N

2

O) from bacterial activity

• NO emission by the soils 5-20 μg nitrogen/m 2 water content and temperature • Natural N 2 O : oceans, rivers hour, function of organic and

Natural sources of nitrogen oxides

Electrical activity in the atmosphere; lightning N 2 + O 2 => 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 N 2 O:   Agriculture: Nitrogen fertilizers increase the microbiological activity resulting in N 2 O 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

• N

2 : strong bond in the molecule → no direct chemical reaction with oxygen Chain reaction:

(Zeldovich, 1940) → rate limiting step O forms in the flame

N 2 + O = NO + N N + O 2 = NO + O N + • OH = NO + H

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 [ 0 C ]

27 527 1316 1538 1760 1980

NO concentration at equilibrium [ ppm ] Time 500 ppm [ sec ]

1,1 x 10 -19 0,77 550 1380 2600 4150 1370 162 1,1 0,117 The amount of thermal NO is the function of

the flame temperature and the residence time

Formation of prompt NO

Fenimore, 1970: Hydrocarbons

low flame temperature

▬▬▬▬▬▬▬▬▬

1000 o C

► • CH + • CH2 + • CH3 + • • The reactions starts by the alkyl radicals .

• CH + N 2 • CH 2 + N 2 • CH 3 + N 2 = HCN + N = HCN + • NH = HCN + • NH 2 High temperature flame section: HCN + O = NO + • CH • NH + O = NO + H • NH + • OH = NO + H 2

→ rate determination step

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 NH 3 … nitrogen are reduced to

NO

2

formation in the flame

Only a few % of NO NO 2 2 can be found in the stack gas starts to decompose above 150 °C and total decay: above 620 °C

At low flame temperature: NO + •HO 2 = NO 2 + • OH Formation of hydroperoxyl radicals: H + O 2 + M = • HO 2 + M At high flame temperature: H + O 2 = • OH + O Significant part of NO 2 returns back to the higher flame temperature section :

• decays thermally

NO 2 = NO + O

• chemical reaction transforms back to NO:

NO 2 NO 2 + H = NO + • OH + O = NO + O 2

Formation of N

2

O :

Low temperature combustion

~10-50% of the fuel N at 800 ºC – 900 ºC may transform to N 2 O. In exhaust gas → 50 – 150 ppmv N 2 O Thermal decay of coal

→ hydrogen cyanide formation HCN + O = NCO + H NCO + NO = N 2 O + CO

There is no N 2 O above 950 ºC , decays thermally above 900 ºC

N 2 O + M = N 2 + O+ M

Increasing temperature favours the formation of hydrogen atoms → reduction

N 2 O + H = N 2 + •OH Fuels with low heat value (biomass) favours the formation of N 2 O

N 2 O formation by catalytic side reactions

• • Anthropogenic N 2 O source : automobiles equipped with catalytic converter By products of three way catalytic converters : 1. NO reduction 2. CO oxidation 3. Oxidation of hydrocarbons

product of side reaction

temperature increase suppresses the reaction

Adsorption, dissociation On the surface of catalyst product of main reaction

N

2

O emission from automobiles

Catalyst type

without Two way system (oxidation) Three way system (oxidation – reduction) Three way system (oxidation – reduction) Diesel engine

mg/km

~ 10 ~27 ~46 ~19 ~ 10

year

1966 - 1972 1978 - 1982 1983 - 1995 1996 Installation of catalysts increases the N 2 O emission.

The benefit

>

the drawback

Summary of the nitrogen oxide formation in the flame Simplified reaction way Thermal NO Prompt NO NO from the fuel Organic-N Thermal decay NO 2 N 2 O Organic-N Thermal decay remark

Above 1400 temperature 0 C, strongly dependent, forms in the oxidation zone Above 1000 temperature 0 C, slightly dependent, forms in the reduction zone Above 1000 temperature 0 C, slightly dependent, forms in the oxidation zone.

Forms in the cooler part of the flame, decays in warmer parts Forms in the range of 800 – 900 0 0 C C, decays at higher temperatures

NO → NO

2

transformations in the troposphere

Possible reaction with O 2 → 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

N

2

O 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 NO 2 • NO 2 soluble in water:

NO 2 + H 2 O → HNO 3 + HNO 2 slow

Another way of NO 2 elimination:

NO 2 •NO 3 + O = •NO + NO 2 = N 3 2 O 5 N 2 O 5 + H 2 O = 2 HNO 3 ▼

Effect of light

Only after sunset.

Nitrous oxide N

2

O

Transport from the troposphere to the stratosphere, here decays:

• oxidation:

N 2 O + O = 2 NO

Detrimental effect: decays the ozone layer : •photochemical decay:

N 2 O

   260  

N 2 + O

The human activity continuously increases the N 2 O concentration of the atmosphere. There is a 0,25% increase /year

Effect of nitrogen oxides on

Plants

 

Outspokenly harmful In the atmosphere NO and NO 2 together (NOx)



10 000 ppmv NO → reversible decrease of photosynthesis



NO 2 → destruction of leaves (formation of nitric acid), cell damages

Effect of nitrogen oxides on

Humans

   NO 2 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 NH 3 + O 2

= 4 N

2 + 6 H 2 O

Danger of excess ammonia. Better solution is the urea

2 NH 2 ▬CO▬NH 2 + 4 NO + O 2

= 4 N

2 + 4 H 2 O + 2 CO 2

• 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: V 2 O 5 or WO 3 titanium dioxide supporter on • Applied NH 3 (mol/mol), / NO rate ~0,8

Drawback:

• SO 2 content of the fuel gas is oxidized to SO 3 →

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