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
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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
Physical properties of NO, NO2 and N2O • 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 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 oxides • 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, 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
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
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 • 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:Thermal way • N2 : strong bond in the molecule → • no direct chemical reaction with oxygen • Chain reaction: (Zeldovich, 1940) → 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
Rate limiting factors of thermal NO The amount of thermal NO is the function of the flame temperature and the residence time
Formation of prompt NO Fenimore, 1970: low flame temperature Hydrocarbons ▬▬▬▬▬▬▬▬▬► • CH + • CH2 + • CH3 + • • 1000 oC → rate determination step The reactions starts by the alkyl radicals. High temperature flame section: 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 NH3are reduced to …nitrogen
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
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 ppmvN2O 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
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
N2O emission from automobiles Installation of catalysts increases the N2O emission. The benefit > the drawback
Summary of the nitrogen oxide formation in the flame Thermal decay Organic-N Thermal decay Organic-N
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
The pure cycle of NO in the troposphere The ozone molecule may react with another molecule
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
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 Another way of NO2 elimination: Only after sunset. N2O5+ H2O = 2 HNO3 ▼ Effect of light
N2O N2 + O 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: The human activity continuously increases the N2O concentration of the atmosphere. There is a 0,25% increase /year
Effect of nitrogen oxides onPlants • 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 onHumans • 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 onconstructing 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 • 1. zone fuel (coal powder, oil) ( n>1) • 2. zone10..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. • Mixed with air input (FGR: flue gas recirculation) • 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 NH3 + O2= 4 N2 + 6 H2O Danger of excess ammonia. Better solution is the urea 2 NH2▬CO▬NH2 + 4 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.
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
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