HYDROCARBONS AND PHOTOCHEMICAL OXIDANTS Authors: Dr. Bajnóczy Gábor Kiss Bernadett 1.BUDAPEST...
-
Upload
isaias-sheron -
Category
Documents
-
view
218 -
download
2
Transcript of HYDROCARBONS AND PHOTOCHEMICAL OXIDANTS Authors: Dr. Bajnóczy Gábor Kiss Bernadett 1.BUDAPEST...
HYDROCARBONS AND HYDROCARBONS AND PHOTOCHEMICAL PHOTOCHEMICAL
OXIDANTSOXIDANTS 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
The pictures and drawings The pictures and drawings of this presentation can be of this presentation can be used only for education !used only for education !
Any commercial use is Any commercial use is prohibited !prohibited !
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
HydrocarbonsHydrocarbons
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.
Hydrocarbons in urban air Los Angeles (1965)
hydrocarbon (ppm)
Methane CH4 3,22
Toluene C7H8 0,05
n-butane C4H10 0,06
i-pentane C5H12 0,04
Ethane C2H6 0,1
Benzene C6H6 0,03
n-pentane C5H12 0,03
Propane C3H8 0,05
ethylene C2H4 0,06
TerpenesTerpenes 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
Polycyclic aromatic hydrocarbons in the Polycyclic aromatic hydrocarbons in the atmosphere in form of gas phaseatmosphere 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 Polycyclic aromatic hydrocarbons in the atmosphere in form of condensed or atmosphere in form of condensed or
adsorbed phaseadsorbed phase
Polycyclic aromatic Polycyclic aromatic hydrocarbonshydrocarbons
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 oxidantsPhotochemical 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 sourcesNatural 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 sourcesAnthropogenic 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 hydrocarbonsFormation 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 Formation of polycyclic aromatic hydrocarbons Ihydrocarbons 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) .
Formation of polycyclic aromatic Formation of polycyclic aromatic hydrocarbons II.hydrocarbons II.
2. The polycondensed aromatic structure forms quickly by the addition of benzene rings (soot formation).
Emissions of polycyclic aromatic Emissions of polycyclic aromatic hydrocarbonshydrocarbons
PAH és BaP emission of boilers with different size.source: Huotari J., Vesterinnen R. (1995) , Finland
Household boilers with solid fuel
boilers1-5 MW
boilers5 – 50 MW
boilers>50 MW
PAHμg/MJ
1000 – 30002-10 (solid)< 5 (oil, gas)
< 10 < 5
BaPμg/MJ
< 20 < 0,1 < 0,01
Formation of peroxyacyl nitratesFormation 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 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 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 Formation of peroxyacyl nitratesnitrates
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 ppbNowadays smaller 10 ppb due to tree way
catalysts in cars
Ozone formation in the Ozone formation in the tropospheretroposphere
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.
Decay of PAH compounds in the Decay of PAH compounds in the tropospheretroposphere
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 Elimination of peroxyacyl nitrates from the tropospherefrom the troposphere
Thermal decay by increasing temperature
CH3C(O)OONO2 → CH3C(O)OO• + NO2
Photochemical decay, longer lifetime during night
Elimination of ozone from the Elimination of ozone from the tropospheretroposphere
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
Formation of smogFormation 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.
Reasons of London smogReasons of London smog
Emission of pollutantsEmission of pollutants
Temperature inversion in the troposphereTemperature inversion in the troposphere
During cloudless and windless night During cloudless and windless night →→ strong strong infrared radiation towards the skyinfrared radiation towards the sky
The surface of soil cools downThe 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 warmerThe upper layers remains warmer
The vertical mixture is limitedThe vertical mixture is limited
Quick increase of pollutant concentrationQuick increase of pollutant concentration
Formation of photochemical Formation of photochemical smog (Los Angeles type)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).
Smog components in function of Smog components in function of timetime
Reddish brown dome above the town.
hydrocarbons
ozone
aldehydes
hour hour hour hour hour hour hour
con
cen
trat
ion
Hydrocarbons, photochemical oxidants, Hydrocarbons, photochemical oxidants,
effect oneffect on PlantsPlants
hydrocarbons: no effect ozone and peroxyacyl nitrates: toxic
Ozone concentration: summer maximum near the soil
ozone / ppb /
Urban 100 – 400
Rural 50 – 120
Tropical forest 20 – 40
Oceans fare from shore 20 -40
Chronic effect above 40 ppb → yellow spots on the upper side of leaves
Hydrocarbons, photochemical oxidants, Hydrocarbons, photochemical oxidants, effect on effect on
PlantsPlants
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, Hydrocarbons, photochemical oxidants,
effect oneffect on HumansHumans
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 Control of hydrocarbon emissionemission
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.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.
reganeratív termikus utóégető
regeneratív termikus utó égető
Regenerative thermal
afterburner in useRegenerative thermal afterburner
Thermal afterburner without heat Thermal afterburner without heat utilization II.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
Thermal afterburner III.Thermal afterburner III.
rekuperatív utóégető
rekuperatív utóégető
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 useRecuperative afterburner
Heat exchanger
burner
CHx contaminated fuel gas
CHx free fuel gas
Catalytic afterburnerCatalytic 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
Catalytic afterburnerCatalytic afterburner
Success in cleaning of exhaust gas petrol based internal combustion engines (automobiles)
Composition of the exhaust gas from petrol based automobiles
Gas concentration
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 afterburnerCatalytic afterburnerTwo 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)
con
vers
ion
Catalytic afterburnerCatalytic 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
signal receiver
catalyst
Lambda meter
Harmful emissions
inert emissions
compounds