Presentation on theme: "BUDAPEST UNIVERSITY OF TECHNOLOGY AND ECONOMICS"— Presentation transcript:
1 BUDAPEST UNIVERSITY OF TECHNOLOGY AND ECONOMICS FACULTY OF CHEMICAL AND BIOCHEMICAL ENGINEERINGDEPARTMENT OF CHEMICAL ANDENVIRONMENTAL PROCESS ENGINEERINGNITROGEN-OXIDESAuthors: Dr. Bajnóczy GáborKiss Bernadett
2 The pictures and drawings of this presentation can be used only for education ! Any commercial use is prohibited !
3 Nitrogen oxides In the atmosphere: NO, NO2, NO3, N2O, N2O3, N2O4, N2O5 Continuously : only NO, NO2, N2OThe others decay very quickly :Into one of three oxidesReaction with water moleculeNOnitric oxidecolourlessodourlesstoxicnon-flammableNO2nitrogen dioxidereddish brownstrong choking odourvery toxicN2Onitrous oxidesweet odournon-toxic
5 Nitrogen oxidesEnvironment: NO and NO2 acidic rain, photochemical smog, ozone layer destroyerN2O :stableNo photochemical reactions in the troposphere ► lifetime 120 yearNatural background : 313 ppmvRate of increase 0,5-0,9 ppmv/yearGreenhouse effect showed itself recently
6 Natural sources of nitrogen oxides Atmospheric origin of NO:Electrical activity (lightning)~ 20 ppb NO HNO transition → continuous sinkEquilibrium concentration is kept by the biosphere:see: nitrogen cycle
7 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 temperatureNatural N2O : oceans, rivers
8 Natural sources of nitrogen oxides Bottom of the river, anaerobic condition, microbiological activityElectrical activity in the atmosphere; lightningN2 + O2 => 2 NOOrganic nitrogen content of the soil is decomposed by micro organisms
9 Anthropogenic sources of nitrogen oxides TransportationFuel combustionApplication of nitrogen fertilizers
10 Anthropogenic sources of nitrogen oxides Fossils fuel combustion: power plants and transportationAgriculture: Nitrogen fertilizers increase the microbiological activity resulting in NO emissionN2O:Agriculture: Nitrogen fertilizers increase the microbiological activity resulting in N2O emissionTransportation (three way catalyst system)Power plants (fluid bed boilers)Chemical industry (nitric acid)0,2 % yearly increase in atmospheric content.
11 Formation of nitric oxide: Thermal way N2 : strong bond in the molecule →no direct chemical reaction with oxygenChain reaction: (Zeldovich, 1940)N2 + O = NO + NN + O2 = NO + ON + •OH = NO + H→ rate limiting stepO forms in the flameThe concentration of atomic oxygen is the function of the flame temperature.▼thermal way dominates above 1400 ºC
12 Rate limiting factors of thermal NO Temperature[ 0C ]NO concentration at equilibrium[ ppm ]Time 500 ppm [ sec ]271,1 x-5270,771316550137015381380162176026001,1198041500,117The amount of thermal NO is the function ofthe flame temperature and the residence time
13 The reactions starts by the alkyl radicals. Formation of prompt NOFenimore, 1970:low flame temperatureHydrocarbons ▬▬▬▬▬▬▬▬▬►• CH + • CH2 + • CH3 + • •1000 oC• CH + N2 = HCN + N• CH2 + N2 = HCN + • NH• CH3 + N2 = HCN + • NH2→ rate determination stepThe reactions starts by the alkyl radicals.High temperature flame section:HCN + O = NO + • CH• NH + O = NO + H• NH + • OH = NO + H2The prompt NO is slightly temperature dependent (approx: 5% of the total).
14 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 reactivitynot sensitive to the flame temperature,sensitive to the air excess ratioin oxygen lean area (reduction zone) the HCN and NH3 are reduced to …nitrogen
15 NO2 formation in the flame Only a few % of NO2 can be found in the stack gasNO2 starts to decompose above 150 °C and total decay: above 620 °CAt low flame temperature:NO + •HO2 = NO2 + • OHFormation of hydroperoxyl radicals:H + O2 + M = • HO2 + MAt high flame temperature:H + O2 = • OH + OSignificant part of NO2 returns back to the higher flame temperature section :decays thermallychemical reaction transforms back to NO:NO2 = NO + ONO2 + H = NO + • OHNO2 + O = NO + O2
16 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 ppmv N2OThermal decay of coal → hydrogen cyanide formationHCN + O = NCO + HNCO + NO = N2O + COThere is no N2O above 950 ºC , decays thermally above 900 ºCN2O + M = N2 + O+ MIncreasing temperature favours the formation of hydrogen atoms → reductionN2O + H = N2 + •OHFuels with low heat value (biomass) favours the formation of N2O
17 N2O formation by catalytic side reactions Anthropogenic N2O source : automobiles equipped with catalytic converterBy products of three way catalytic converters:NO reductionCO oxidationOxidation of hydrocarbonstemperature increase suppresses the reactionproduct of side reactionAdsorption, dissociationOn the surface of catalystproduct of main reaction
18 N2O emission from automobiles Catalyst typemg/kmyearwithout~ 10Two way system(oxidation)~27Three way system(oxidation – reduction)~46~191996 -Diesel engineInstallation of catalysts increases the N2O emission.The benefit > the drawback
19 Summary of the nitrogen oxide formation in the flame Simplified reaction wayremarkThermal NOAbove C, strongly temperature dependent, forms in the oxidation zonePrompt NOAbove C, slightly temperature dependent, forms in the reduction zoneNO from the fuelAbove C, slightly temperature dependent, forms in the oxidation zone.NO2Forms in the cooler part of the flame, decays in warmer partsN2OForms in the range of 800 0C – 900 0C, decays at higher temperaturesThermal decayOrganic-NThermal decayOrganic-N
20 NO → NO2 transformations in the troposphere Possible reaction with O2 → slowFormation of hydroxyl radicalsNO oxidation by hydroxyl radicalsNO oxidation by methylperoxy radicals
21 The pure cycle of NO in the troposphere The ozone molecule may react with another molecule
22 N2O in the atmosphere Source: natural and anthropogenic Very stable in the troposphere:No reaction with the hydroxyl radicalsλ >260 nm → there is no absorptionPreviously it was not considered polluting material.Recently came to light: greenhouse effect gas
23 Fate of nitrogen oxides from the atmosphere Nitric oxide, nitrogen dioxideNO photochemically inert, no solubility in water, forms to NO2NO2 soluble in water:NO2 + H2O → HNO3 + HNO2slowNO2 + O = •NO3•NO3 + NO2 = N2O5Another way of NO2 elimination:Only after sunset.N2O5+ H2O = 2 HNO3▼Effect of light
24 Nitrous oxide N2OTransport from the troposphere to the stratosphere, here decays:oxidation:N2O + O = 2 NODetrimental effect: decays the ozone layer:photochemical decay:N2ON2 + OThe human activity continuously increases the N2O concentration of the atmosphere. There is a 0,25% increase /year
25 Effect of nitrogen oxides on Plants Outspokenly harmfulIn the atmosphere NO and NO2 together (NOx)ppmv NO → reversible decrease of photosynthesisNO2 → destruction of leaves(formation of nitric acid), cell damages
26 Effect of nitrogen oxides on Humans NO2 is four times toxic than NOOdor threshold: 1-3 ppmvMucos irritation: 10 ppmv200 ppmv 1 minute inhaling → death!Origin of death: wet lungNitric acid formation in the alveoliAlveoli have semi permeable membrane (only gas exchange is possible)Nitric acid : destroys the protein structure of the membrane → the alveoli is filled up by liquidNo more free surface for the gas exchange → death
27 Effect of nitrogen oxides on constructing materials Acid rain causes electrochemical corrosionSurface degradation on limestone, marble by the acidic rain.
28 Control of nitrogen oxides emission Technological developments: only 15% decrease (since 1980)~90% of anthropogenic emission comes fromboilersinternal combustion enginesControl of emission:make conditions do not favor the formationelimination of the nitrogen oxides from the exhaust gases
29 Control of nitrogen oxides emission The NO formation in the flame depends on:N content of the fuelFlame temperatureResidence time in the flameAmount of reductive speciesThe air excess ratio (n) has strong effect on the last three.The air excess ratio can be adjusted globally or locally.
30 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 zonesecondaryairfuel+ airsecondaryairreduction zone
31 Control of nitric oxide (NO) emission by two stage combustion BOILER
32 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
33 Control of nitric oxide (NO) emission by three stage combustion
34 Control of nitric oxide (NO) emission, by three stage combustion 1. zonefuel (coal powder, oil) ( n>1)2. zone % fuel inputn=0,9 temperature 1000°C3. zoneair input, n>1, perfect burning.30..70% NO reduction is available
35 Flue gas recirculation Application:oil andgas boilersThe cooled flue gas has high specific heat due to the water content.The recirculated flue gas decrease the flame temperature.Generally ~10% is recirculatedMore than 20 % produces higher CO and hydrocarbon emissions.Mixed with air input (FGR: flue gas recirculation)Mixed with fuel input (FIR: fuel induced recirculation)
36 Nitric oxide (NO) eliminations from the exhaust gas possibilities:Selective noncatalytic reduction SNCR (thermal DENOx process)Selective catalytic reduction SCR (catalytic DENOx process)
37 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 H2ODanger of excess ammonia. Better solution is the urea2 NH2▬CO▬NH NO + O2 = 4 N2 + 4 H2O + 2 CO2advantage: simplicitydisadvantage: temperature sensitive.ammonia: 870 – 980 ºC, urea 980 – 1140 ºCAt higher temperature ammonia is oxidized to NOAt lower temperature ammonia remains in the fuel gasEfficiency : 40 – 70 % at optimal condition.
38 Reduction of NO emission by selective catalytic reduction better efficiency is availablecomposition: V2O5 or WO3 on titanium dioxide supporterApplied NH3 / NO rate ~0,8 (mol/mol),Drawback:SO2 content of the fuel gas is oxidized to SO3 → corrosionAmmonium-sulphate deposition on the catalyst surfaceThe method can not be applied over 0,75 % sulfur content in the stack gas
39 NO elimination from the exhaust gas of internal combustion engines Only the treatment of the exhaust gas is possibleControl methods applied to one pollutant often influence the output of other pollutant
40 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