Presentation on theme: "NITROGEN-OXIDES Authors: Dr. Bajnóczy Gábor Kiss Bernadett BUDAPEST UNIVERSITY OF TECHNOLOGY AND ECONOMICS DEPARTMENT OF CHEMICAL AND ENVIRONMENTAL PROCESS."— Presentation transcript:
NITROGEN-OXIDES 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
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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 colourlessodourlesstoxicnon- flammable NO 2 nitrogen dioxide reddish brown strong choking odour very toxicnon- flammable N 2 O nitrous oxide colourlesssweet odournon-toxicnon- flammable
Physical properties of NO, NO 2 and N 2 O Nitric oxide NO Nitrogen- dioxide NO 2 Nitrous oxide N 2 O Molecular mass Melting point o C Boiling point o C density 0 0 C, kPa 25 0 C, kPa g/dm g/dm g/dm 3 1,916 g/dm 3 1,963 g/dm g/dm 3 Solubility in water 0 0 C kPa 73,4 cm 3 / dm 3 (97.7 ppmm) ** bomlik1305 cm 3 /dm 3 Conversion factors 0 0 C, kPa 1 mg/m 3 = ppmv *** 1 ppmv = mg/m 3 1 mg/m 3 = ppmv *** 1 ppmv = 2,053 mg/m 3 1 mg/m 3 = 0,509 ppmv *** 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 decay
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 hour, function of organic and water content and temperature Natural N 2 O : oceans, rivers
Natural sources of nitrogen oxides Electrical activity in the atmosphere; lightning N 2 + O 2 => 2 NO Bottom of the river, anaerobic condition, microbiological activity Organic nitrogen content of the soil is decomposed by micro organisms
Anthropogenic sources of nitrogen oxides TransportationFuel combustionApplication 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) N 2 + O = NO + N N + O 2 = 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 limiting factors of thermal NO Temperature [ 0 C ] NO concentration at equilibrium [ ppm ] Time 500 ppm [ sec ] 271,1 x , , ,117 The amount of thermal NO is the function of the flame temperature and the residence time
low flame temperature Formation of prompt NO CH + N 2 = HCN + N CH 2 + N 2 = HCN + NH CH 3 + N 2 = HCN + NH 2 HCN + O = NO + CH NH + O = NO + H NH + OH = NO + H 2 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 o C
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 are reduced to …nitrogen
NO 2 formation in the flame NO + HO 2 = NO 2 + OH H + O 2 + M = HO 2 + M H + O 2 = OH + O NO 2 + H = NO + OH NO 2 + O = NO + O 2 At low flame temperature: Formation of hydroperoxyl radicals: At high flame temperature: Significant part of NO 2 returns back to the higher flame temperature section : decays thermally chemical reaction transforms back to NO: Only a few % of NO 2 can be found in the stack gas NO 2 starts to decompose above 150 °C and total decay: above 620 °C NO 2 = NO + O
Formation of N 2 O : Low temperature combustion HCN + O = NCO + H NCO + NO = N 2 O + CO N 2 O + M = N 2 + O+ M N 2 O + H = N 2 + OH ~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 There is no N 2 O 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 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 main reaction product of side reaction temperature increase suppresses the reaction Adsorption, dissociation On the surface of catalyst
N 2 O emission from automobiles Catalyst typemg/kmyear without~ Two way system (oxidation) ~ Three way system (oxidation – reduction) ~ Three way system (oxidation – reduction) ~ Diesel engine~ 10 Installation of catalysts increases the N 2 O emission. The benefit > the drawback
Summary of the nitrogen oxide formation in the flame Simplified reaction wayremark 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. NO 2 Forms in the cooler part of the flame, decays in warmer parts N2ON2O Forms in the range of C – C, decays at higher temperatures Organic-N Thermal decay Organic-N
NO → NO 2 transformations in the troposphere Possible reaction with O 2 → slow Formation of hydroxyl radicals NO oxidation by hydroxyl radicalsNO 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 N 2 O 5 + H 2 O = 2 HNO 3 NO 2 + O = NO 3 NO 3 + NO 2 = N 2 O 5 NO 2 + H 2 O → HNO 3 + HNO 2 Nitric oxide, nitrogen dioxide NO photochemically inert, no solubility in water, forms to NO 2 NO 2 soluble in water: slow Another way of NO 2 elimination: Only after sunset. ▼ Effect of light
Nitrous oxide N 2 O N 2 O + O = 2 NO Transport from the troposphere to the stratosphere, here decays: Detrimental effect: decays the ozone layer : oxidation: photochemical decay: N2ON2O 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) 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.
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) emission, by two stage combustion oxidation zone reduction zone secondary air fuel + air
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 % fuel input n=0,9 temperature 1000°C 3. zone air input, n>1, perfect burning % 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 4 NO + 4 NH 3 + O 2 = 4 N H 2 O 2 NH 2 ▬CO▬NH NO + O 2 = 4 N H 2 O + 2 CO 2 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 temperature At higher temperature ammonia is oxidized to NO At lower temperature 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 on titanium dioxide supporter Applied NH 3 / NO rate ~0,8 (mol/mol), 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 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 gas of internal combustion engines NO from internal combustion engine is thermal origin. NO elimination by selective catalytic reduction. Discussed in details at hydrocarbons