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.

The London smogThe London smog

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).

Towns in photochemical smogTowns in photochemical smog1. Pekin

g

Denver Torontó

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

Catalytic afterburnerCatalytic afterburner

Works at 290 0C – optimum at 400 0C

Further bonus effect:

Unleaded fuel

Reduction of sulfur content of petrol