Transcript Hydrocarbons

1.
1.
BUDAPEST UNIVERSITY OF TECHNOLOGY AND
ECONOMICS
FACULTY OF
CHEMICAL AND
BIOCHEMICAL
ENGINEERING
1.
2.
DEPARTMENT OF
CHEMICAL AND
ENVIRONMENTAL PROCESS
ENGINEERING
HYDROCARBONS AND
PHOTOCHEMICAL
OXIDANTS
Authors: Dr. Bajnóczy Gábor
Kiss Bernadett
The pictures and drawings of this
presentation can be used only for
education !
Any commercial use is prohibited !
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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
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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
Terpenes
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Significant amount in the troposphere
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Unit: isoprene molecule CH2=C(CH3)-CH=CH2
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General structure: (C5H8)n
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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 atmosphere
in form of gas phase
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PAH (polycyclic aromatic hydrocarbons)
Two or more condensed aromatic rings
Some of them carcinogenic → strongest effect : benz[a]pyrene, ( BaP )
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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
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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
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Source: oxidation of unsaturated hydrocarbons
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Harmful, irritating molecules
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Members: peroxyacyl nitrates and ozone
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Only the following three can be found in the troposphere :
peroxyacetyl nitrate : PAN, peroxypropionyl nitrate : PPN, peroxybenzoyl nitrate : PBzN
Natural sources
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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
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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
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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.
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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 – 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 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
1000 – 3000
2-10 (solid)
< 5 (oil, gas)
< 10
<5
< 0,1
< 0,01
PAH
μg/MJ
BaP
μg/MJ
< 20
Formation of peroxyacyl nitrates
TheThe
lifetime
of
aldehyde
is shortform
in the
atmosphere.
It decays by
alkyl
radicals
(alkilgyök)
alkylperoxy
radicals
The
effect
of
oxygen
on
the
alkoxy
radicals
(alkokszigyök)
results
innitrates
the formation
Aldehyde
formation
is radicals
possible
inoxidize
the reaction
of
unsaturated
hydrocarbons
and
The
peroxyalkyl
radicals
may
the
NO
or
forms
peroxyacyl
by NO2.
light
or
hydroxyl
to
acyl
radicals
which
forms
peroxyalkyl
(alkilperoxigyök)
with
the
oxygen
of
air.
The
alkylperoxy
radicals
of
formaldehyde.
ozone.
radicals
oxygen.
Theawith
hydroxyl
radicals
the process
in hydrocarbon
polluted air.
play
significant
role instarts
the oxidation
of NO
to NO2.
Formation of peroxyacyl nitrates
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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
Nowadays
60 – 65 ppb
smaller 10 ppb
due to tree way
catalysts in cars
Ozone formation in the troposphere
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Reaction with atomic oxygen
O + O 2 = O3
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The atomic oxygen is served by photolytic dissociation of NO2
NO2 + hν = NO + O
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(1)
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 troposphere
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Decay by hydroxyl
radicals
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No reaction with
ozone
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Light helps the decay
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Lifetime: some hours
in the troposphere
especially in sunshine
Decay of PAH compounds in the troposphere
Elimination of peroxyacyl nitrates from
the troposphere
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Thermal decay by increasing temperature
CH3C(O)OONO2 → CH3C(O)OO• + NO2
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Photochemical decay, longer lifetime during
night
Elimination of ozone from the troposphere
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Strong oxidizing agent => lifetime: some days
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Routes of decay
NO + O3 → NO3• + O
NO + O3 → NO2 + O2
R-CH=CH2 + O3 → RCHO + OH•
O3 + hν → O + O2
Formation of smog
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The two types of smog: London and Los Angeles
(photochemical)
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LONDON type smog
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Coal fire origin
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In winter
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Early morning
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High humidity
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No sunshine
Composition: hydrocarbons, soot, sulfur dioxide.
The London smog
Reasons of London smog
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Emission of pollutants
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Temperature inversion in the troposphere
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During cloudless and windless night → strong
infrared radiation towards the sky
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The surface of soil cools down
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The cool soil cools the air layer above it.
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The upper layers remains warmer
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The vertical mixture is limited
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Quick increase of pollutant concentration
Formation of photochemical smog
(Los Angeles type)
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The main reason is the transportation
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Photochemical smog:
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In summer,
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Mainly at noon,
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Low air humidity,
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Strong sunshine.
Composition: secondary pollutants (ozone, aldehydes,
NO2, PAN).
Towns in photochemical smog
1.
Denver
Peking
Torontó
Smog components in function of time
hydrocarbons
concentration
ozone
aldehydes
hour
hour
hour
hour
hour
hour
Reddish brown dome above the town.
hour
Hydrocarbons, photochemical oxidants, effect on
Plants
 hydrocarbons: no effect
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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, effect on
Plants
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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
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Aliphatic hydrocarbons are not toxic at ambient concentrations.
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Aromatic hydrocarbons are toxic:
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Most dangerous ones :
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benzene
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PAH compounds e.g. benz(a)pyrene
Photochemical oxidants:
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Eye, throat irritation
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Chronic respiratory disease
Control of hydrocarbon emission
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Close connection between the hydrocarbon emission and the formation
of photochemical oxidants.
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Control of hydrocarbon emission means control of photocemical oxidants
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Main source: incomplete burning
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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.
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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
Regenerative thermal afterburner in
use
regeneratív
termikus utó thermal
égető
Regenerative
afterburner
Thermal afterburner without heat utilization II.
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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.
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
CHx free fuel gas
Heat exchanger
CHx contaminated fuel gas
rekuperatív
utóégető
Recuperative
afterburner in use
rekuperatív
utóégető
Recuperative
afterburner
burner
Catalytic afterburner
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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 afterburner
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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 afterburner
Two way system: oxidation of carbon monoxide and hydrocarbons on Pt catalyst
oxidation
conversion
Three way system: oxidation and reduction of nitrogen monoxide (Pd catalyst)
reduction
Air excess ratio (n)
n = 0,95 – 1,05 air excess ratio acceptable level of the conversion of
(CH)x , CO and NO
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
Catalytic afterburner
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Works at 290 0C – optimum at 400 0C
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Further bonus effect:
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Unleaded fuel
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Reduction of sulfur content of petrol