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HYDROCARBONS AND PHOTOCHEMICAL OXIDANTS

BUDAPEST UNIVERSITY OF TECHNOLOGY AND ECONOMICS. DEPARTMENT OF CHEMICAL AND ENVIRONMENTAL PROCESS ENGINEERING. FACULTY OF CHEMICAL AND BIOCHEMICAL ENGINEERING. HYDROCARBONS AND PHOTOCHEMICAL OXIDANTS. Authors: Dr. Bajnóczy Gábor Kiss Bernadett.

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HYDROCARBONS AND PHOTOCHEMICAL OXIDANTS

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  1. BUDAPEST UNIVERSITY OF TECHNOLOGY AND ECONOMICS DEPARTMENT OF CHEMICAL AND ENVIRONMENTAL PROCESS ENGINEERING FACULTY OF CHEMICAL AND BIOCHEMICAL ENGINEERING HYDROCARBONS AND PHOTOCHEMICAL OXIDANTS Authors: Dr. Bajnóczy Gábor Kiss Bernadett

  2. The pictures and drawings of this presentation can be used only for education !Any commercial use is prohibited !

  3. Hydrocarbons: primary pollutants (saturated and unsaturated aliphatic hydrocarbons, terpenes, mono and polycondensed aromatic hydrocarbons) • Photochemical oxidants: secondary pollutants, forms from the primary pollutants e.g..: peroxyacylnitrates, ozone

  4. Hydrocarbons • 1 - 4 carbon atoms: gas in the troposphere • 4 < carbon atoms: steam or liquid/solid particles in the troposphere The unsaturated hydrocarbons photochemically are more active in the troposphere than the saturated ones.

  5. Terpenes • Significant amount in the troposphere • Unit: isoprene molecule CH2=C(CH3)-CH=CH2 • General structure: (C5H8)n • Monoterpenes: two unites of isoprene e.g. pinene, , camphor, menthol, limonene. Organic hydrocarbons (CH)x or (CxHy) Volatile organic hydrocarbons: VOC

  6. Polycyclic aromatic hydrocarbons in the atmosphere in form of gas phase • PAH (polycyclic aromatic hydrocarbons) • Two or more condensed aromatic rings • Some of them carcinogenic → strongest effect : benz[a]pyrene, ( BaP ) First three: in paints-, pesticides-industrial raw materials The others: in fuel gas of wood, coal, natural gas petroleum products

  7. Polycyclic aromatic hydrocarbons in the atmosphere in form of condensed or adsorbed phase

  8. Polycyclic aromatic hydrocarbons • Two groups have been defined (U.S. Environmental Protection Agency), (7-PAH) and (16-PAH). • All members of 7-PAH are carcinogenic. • In the 16-PAH the 7-PAH members and other non carcinogenic PAH materials are involved

  9. Photochemical oxidants • Source: oxidation of unsaturated hydrocarbons • Harmful, irritating molecules • Members: peroxyacyl nitrates and ozone • Only the following three can be found in the troposphere : peroxyacetyl nitrate : PAN, peroxypropionyl nitrate : PPN, peroxybenzoyl nitrate : PBzN

  10. Natural sources • Greatest amount: methane→ anaerobe decay of organic molecules • Natural background: • Methane: 1.0 – 1.5 ppm • Other hydrocarbons: < 0,1 ppm • Other hydrocarbons from natural sourcespl.: terpenes with pleasant odor emitted by different plants (e.g. pine tree ) • polycyclic aromatic hydrocarbons from natural sources: • Forest fires • Natural weathering of oily rocks • Natural leakage of crude oil • Peroxyacyl nitrates: • No direct natural sources • ozone • lightning, 20 – 30 ppbv,.

  11. Anthropogenic sources • Majority of the emissions: • Exhaust gases of burned fuel • Evaporation of organic solvents (toluene, xylene, alkanes, esters) • PAH emission: • Coal industry (coke manufacturing) • Mineral oil processing • Pyrolysis (soot, fuel oil from biomass) • Peroxyacyl nitrates and ozone indirect source: from hydrocarbons and nitric oxide

  12. Formation of hydrocarbons • Effective factors: air excess ratio (n), flame temperature and the residence time at high temperature • Main source: transportation (in spite of the optimal air excess ratio) • Reason: wall effect The cooler wall slows the rate of oxidation in the vicinity of it. The piston pushes out the exhaust gas earlier than the time needed for the completed combustion. Boilers with smaller firebox produces much more hydrocarbons, carbon monoxide and soot particles than the boilers with large firebox.

  13. Formation of polycyclic aromatic hydrocarbons I. Combustion of carbon content fuel, 500 – 800 0C → decay above Forms in the vicinity of cooler part of the burn => smaller fire box greater PAH emission 1. Additional reaction with acetylene and ethylene radicals resulting in ring closure. (Wang-Frenklach mechanism 1997) H2C=CH2 + H => H2C=CH• + H2 The addition of acetylene radical on the aromatic ring produces more and more condensed aromatic rings. (HACA mechanism : hydrogen adsorption and C2H2 addition) .

  14. Formation of polycyclic aromatic hydrocarbons II. 2. The polycondensed aromatic structure forms quickly by the addition of benzene rings (soot formation).

  15. Emissions of polycyclic aromatic hydrocarbons

  16. Formation of peroxyacyl nitrates The lifetime of aldehyde is short in the atmosphere. It decays by light or hydroxyl radicals to acyl radicals which forms peroxyalkyl radicals with oxygen. The alkyl radicals (alkilgyök) form alkylperoxy radicals (alkilperoxigyök) with the oxygen of air. The alkylperoxy radicals play a significant role in the oxidation of NO to NO2. The effect of oxygen on the alkoxy radicals (alkokszigyök) results in the formation of formaldehyde. Aldehyde formation is possible in the reaction of unsaturated hydrocarbons and ozone. The peroxyalkyl radicals may oxidize the NO or forms peroxyacyl nitrates by NO2. The hydroxyl radicals starts the process in hydrocarbon polluted air.

  17. Formation of peroxyacyl nitrates • Peroxyacyl nitrates concentration depends on: • Power of acyl radical formation of hydrocarbons • Ozone concentration • The rate of nitrogen-dioxide / nitric oxide formation in the polluted air Concentration of peroxyacyl nitrates in urban air 1960 years 60 – 65 ppb Nowadays smaller 10 ppb due to tree way catalysts in cars

  18. Ozone formation in the troposphere • Reaction with atomic oxygen O + O2 = O3 (1) • The atomic oxygen is served by photolytic dissociation of NO2 NO2 + hν = NO + O v2 = k2[NO2] (2) • Ozone may oxidize the nitric oxide to NO2 O3 + NO = NO2 = O2 v3 = k3[O3][NO] (3) The rate determining step is the photodissociation of NO2. ↓ No ozone formation in the troposphere after sunset, Concentration maximum in summer at noon.

  19. Decay of PAH compounds in the troposphere • Decay by hydroxyl radicals • No reaction with ozone • Light helps the decay • Lifetime: some hours in the troposphere especially in sunshine Decay of PAH compounds in the troposphere

  20. Elimination of peroxyacyl nitrates from the troposphere • Thermal decay by increasing temperature CH3C(O)OONO2 → CH3C(O)OO• + NO2 • Photochemical decay, longer lifetime during night

  21. Elimination of ozone from the troposphere • Strong oxidizing agent => lifetime: some days • Routes of decay NO + O3→ NO3• + O NO + O3→ NO2 + O2 R-CH=CH2 + O3→ RCHO + OH• O3 + hν → O + O2

  22. Formation of smog • The two types of smog: London and Los Angeles (photochemical) • LONDON type smog • Coal fire origin • In winter • Early morning • High humidity • No sunshine • Composition: hydrocarbons, soot, sulfur dioxide.

  23. The London smog

  24. Reasons of London smog • Emission of pollutants • Temperature inversion in the troposphere • During cloudless and windless night → strong infrared radiation towards the sky • The surface of soil cools down • The cool soil cools the air layer above it. • The upper layers remains warmer • The vertical mixture is limited • Quick increase of pollutant concentration

  25. Formation of photochemical smog (Los Angeles type) • The main reason is the transportation • Photochemical smog: • In summer, • Mainly at noon, • Low air humidity, • Strong sunshine. • Composition: secondary pollutants (ozone, aldehydes, NO2, PAN).

  26. Towns in photochemical smog Peking Torontó Denver

  27. Smog components in function of time hydrocarbons ozone concentration aldehydes hour hour hour hour hour hour hour Reddish brown dome above the town.

  28. Hydrocarbons, photochemical oxidants, effect onPlants • hydrocarbons: no effect • ozone and peroxyacyl nitrates: toxic Chronic effect above 40 ppb → yellow spots on the upper side of leaves

  29. Hydrocarbons, photochemical oxidants, effect on Plants • Peroxyacylnitrate : plant injury shows up as a glazing and bronzing of the lower leaf surfaces • The resistance depends on the concentration of antioxidants in the leaf.

  30. Hydrocarbons, photochemical oxidants, effect onHumans • Aliphatic hydrocarbons are not toxic at ambient concentrations. • Aromatic hydrocarbons are toxic: • Most dangerous ones : • benzene • PAH compounds e.g. benz(a)pyrene • Photochemical oxidants: • Eye, throat irritation • Chronic respiratory disease

  31. Control of hydrocarbon emission • Close connection between the hydrocarbon emission and the formation of photochemical oxidants. • Control of hydrocarbon emission means control of photocemical oxidants • Main source: incomplete burning • Hydrocarbon concentration: • Under the lower flammability limit → thermal or catalytic adsorption • Over the upper flammability limit → combustion with air and water

  32. Thermal afterburner I. • afterburner: auxiliary burner is applied to burn the hydrocarbon content of the stack gas, temperature 700 – 1000 0C, residence time : 0,5-1 sec., efficiency 99% • regenerative method: alternative streams of a hydrocarbon free and hydrocarbon polluted fuel gas through a heat storage material. Regenerative thermal afterburner Regenerative thermal afterburner in use

  33. Thermal afterburner without heat utilization II. • The hydrocarbon concentration must be between the lower and upper flammability limit. • Used in case of mixed hydrocarbon, e.g. oil industry • Water vapor addition to reduce the soot formation. C + H2O = CO + H2

  34. Thermal afterburner III. Recuperative process: the flue gas is reburned, and the heat content of the purified fuel gas is continuously transferred to the hydrocarbon contaminated fuel gas. Problem: increase in NO emission CHx free fuel gas burner Heat exchanger CHx contaminated fuel gas Recuperative afterburner Recuperative afterburner in use

  35. Catalytic afterburner • Oxidation at lower temperature (200 – 500 oC), efficiency ≈ 95%, lower NOx emission • Not recommended: • High soot content • Inorganic particles • Heavy metals (catalyst poisoning) • Coal, oil, biomass firing

  36. Catalytic afterburner • Success in cleaning of exhaust gas petrol based internal combustion engines (automobiles)

  37. Catalytic afterburner Two way system: oxidation of carbon monoxide and hydrocarbons on Pt catalyst Three way system: oxidation and reduction of nitrogen monoxide (Pd catalyst) oxidation conversion reduction Air excess ratio (n) n = 0,95 – 1,05 air excess ratio acceptable level of the conversion of (CH)x , CO and NO

  38. Catalytic afterburner • requirement: adjustment of air excess ratio. • lambda meter  measures the oxygen content of the exhaust gas continuously and regulates the air/fuel ratio. Adjustment of air fuel/ ratio electronics signal receiver Lambda meter inert emissions Engin with petrol fuel Harmful emissions catalyst compounds

  39. Catalytic afterburner • Works at 290 0C – optimum at 400 0C • Further bonus effect: • Unleaded fuel • Reduction of sulfur content of petrol

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