Presentation on theme: "HYDROCARBONS AND PHOTOCHEMICAL OXIDANTS Authors: Dr. Bajnóczy Gábor Kiss Bernadett 1.BUDAPEST UNIVERSITY OF TECHNOLOGY AND ECONOMICS 1.DEPARTMENT OF CHEMICAL."— Presentation transcript:
HYDROCARBONS AND PHOTOCHEMICAL OXIDANTS Authors: Dr. Bajnóczy Gábor Kiss Bernadett 1.BUDAPEST UNIVERSITY OF TECHNOLOGY AND ECONOMICS 1.DEPARTMENT OF CHEMICAL AND 2.ENVIRONMENTAL PROCESS ENGINEERING 1.FACULTY OF CHEMICAL AND BIOCHEMICAL ENGINEERING
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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..: peroxyacyl nitrates, ozone
Hydrocarbons 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. Hydrocarbons in urban air Los Angeles (1965) hydrocarbon(ppm) MethaneCH 4 3,22 TolueneC7H8C7H8 0,05 n-butaneC 4 H 10 0,06 i-pentaneC 5 H 12 0,04 EthaneC2H6C2H6 0,1 BenzeneC6H6C6H6 0,03 n-pentaneC 5 H 12 0,03 PropaneC3H8C3H8 0,05 ethyleneC2H4C2H4 0,06
Terpenes Significant amount in the troposphere Unit: isoprene molecule CH 2 =C(CH 3 )-CH=CH 2 General structure: (C 5 H 8 ) n Monoterpenes: two unites of isoprene e.g. pinene,, camphor, menthol, limonene. Organic hydrocarbons (CH) x or (C x H y ) Volatile organic hydrocarbons: VOC
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
Polycyclic aromatic hydrocarbons in the atmosphere in form of condensed or adsorbed phase
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
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
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 sources pl.: 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,.
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
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.
Formation of polycyclic aromatic hydrocarbons I. Combustion of carbon content fuel, 500 – C → 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) H 2 C=CH 2 + H => H 2 C=CH + H 2 The addition of acetylene radical on the aromatic ring produces more and more condensed aromatic rings. (HACA mechanism : hydrogen adsorption and C 2 H 2 addition).
Formation of polycyclic aromatic hydrocarbons II. 2. The polycondensed aromatic structure forms quickly by the addition of benzene rings (soot formation).
Emissions of polycyclic aromatic hydrocarbons PAH és BaP emission of boilers with different size. source: Huotari J., Vesterinnen R. (1995), Finland Household boilers with solid fuel boilers 1-5 MW boilers 5 – 50 MW boilers >50 MW PAH μg/MJ 1000 – (solid) < 5 (oil, gas) < 10< 5 BaP μg/MJ < 20< 0,1< 0,01
Formation of peroxyacyl nitrates The hydroxyl radicals starts the process in hydrocarbon polluted air. 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 NO 2. 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 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 peroxyalkyl radicals may oxidize the NO or forms peroxyacyl nitrates by NO2.
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
Ozone formation in the troposphere Reaction with atomic oxygen O + O 2 = O 3 (1) The atomic oxygen is served by photolytic dissociation of NO 2 NO 2 + hν = NO + O v 2 = k 2 [NO 2 ] (2) Ozone may oxidize the nitric oxide to NO 2 O 3 + NO = NO 2 = O 2 v 3 = k 3 [O 3 ][NO] (3) The rate determining step is the photodissociation of NO 2. ↓ No ozone formation in the troposphere after sunset, Concentration maximum in summer at noon.
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
Elimination of peroxyacyl nitrates from the troposphere Thermal decay by increasing temperature CH 3 C(O)OONO 2 → CH 3 C(O)OO + NO 2 Photochemical decay, longer lifetime during night
Elimination of ozone from the troposphere Strong oxidizing agent => lifetime: some days Routes of decay NO + O 3 → NO 3 + O NO + O 3 → NO 2 + O 2 R-CH=CH 2 + O 3 → RCHO + OH O 3 + hν → O + O 2
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.
The London smog
Reasons of London smog Emission of pollutants Emission of pollutants Temperature inversion in the troposphere Temperature inversion in the troposphere During cloudless and windless night → strong infrared radiation towards the sky During cloudless and windless night → strong infrared radiation towards the sky The surface of soil cools down The surface of soil cools down The cool soil cools the air layer above it. The cool soil cools the air layer above it. The upper layers remains warmer The upper layers remains warmer The vertical mixture is limited The vertical mixture is limited Quick increase of pollutant concentration Quick increase of pollutant concentration
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, NO 2, PAN).
Towns in photochemical smog 1. Peking Denver Torontó
Smog components in function of time Reddish brown dome above the town. hydrocarbons ozone aldehydes hour concentration
Hydrocarbons, photochemical oxidants, effect on Plants hydrocarbons: no effect ozone and peroxyacyl nitrates: toxic Ozone concentration: summer maximum near the soil ozone / ppb / Urban100 – 400 Rural50 – 120 Tropical forest20 – 40 Oceans fare from shore Chronic effect above 40 ppb → yellow spots on the upper side of leaves
Hydrocarbons, photochemical oxidants, effect on Plants Peroxyacyl nitrate : 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.
Hydrocarbons, photochemical oxidants, effect on Humans 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
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: 1. Under the lower flammability limit → thermal or catalytic adsorption 2. Over the upper flammability limit → combustion with air and water
Thermal afterburner I. afterburner: auxiliary burner is applied to burn the hydrocarbon content of the stack gas, temperature 700 – C, 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 in use Regenerative thermal afterburner
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 + H 2 O = CO + H 2
Thermal afterburner III. 1. 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. 2. Problem: increase in NO emission Recuperative afterburner in use Recuperative afterburner Heat exchanger burner CHx contaminated fuel gas CHx free fuel gas
Catalytic afterburner Oxidation at lower temperature (200 – 500 o C), efficiency ≈ 95%, lower NOx emission Not recommended: High soot content Inorganic particles Heavy metals (catalyst poisoning) Coal, oil, biomass firing
Catalytic afterburner Success in cleaning of exhaust gas petrol based internal combustion engines (automobiles) Composition of the exhaust gas from petrol based automobiles Gasconcentration hydrocarbons≈ 750 ppm Nitrogen oxides≈ 1050 ppm Carbon monoxide≈ 0,68 tf% Hydrogen≈ 0,23 tf% Carbon dioxide≈ 13,5 tf% Oxygen≈ 0,51 tf% water≈ 12,5 tf% Nitrogen≈ 72,5 tf%
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) n = 0,95 – 1,05 air excess ratio acceptable level of the conversion of (CH)x, CO and NO oxidation reduction Air excess ratio (n) conversion
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 Engin with petrol fuel electronics signalreceiver catalyst Lambda meter Harmful emissions inert emissions compounds
Catalytic afterburner Works at C – optimum at C Further bonus effect: Unleaded fuel Reduction of sulfur content of petrol