1.
1.
FACULTY OF
CHEMICAL AND
BIOCHEMICAL
ENGINEERING
BUDAPEST UNIVERSITY OF TECHNOLOGY AND
ECONOMICS
1.
2.
DEPARTMENT OF
CHEMICAL AND
ENVIRONMENTAL PROCESS
ENGINEERING
Authors: Dr. Bajnóczy Gábor
Kiss Bernadett


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


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
Methane
Toluene n-butane i-pentane
Ethane
Benzene n-pentane
Propane ethylene
CH
C
7
C
4
H
4
H
8
10
C
5
H
12
C
2
H
6
C
6
H
6
C
5
H
12
C
3
H
8
C
2
H
4
(ppm)
3,22
0,05
0,06
0,04
0,1
0,03
0,03
0,05
0,06





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
Volatile organic hydrocarbons: VOC y
)

Polycyclic aromatic hydrocarbons in the atmosphere in form of gas phase



PAH ( p olycyclic a romatic h ydrocarbons)
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


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





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



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,.


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




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 – 800 0 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
BaP
μg/MJ
1000 – 3000
2-10 (solid)
< 5 (oil, gas)
< 10
< 20 < 0,1
< 5
< 0,01

The lifetime of aldehyde is short in the atmosphere. It decays by light or hydroxyl radicals to acyl radicals which forms peroxyalkyl play a significant role in the oxidation of NO to NO
2
.


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


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

 Thermal decay by increasing temperature
CH
3
C(O)OONO
2
→ CH
3
C(O)OO• + NO
2
 Photochemical decay, longer lifetime during night


Strong oxidizing agent => lifetime: some days

Routes of decay
NO + O
NO + O
R-CH=CH
O
3
2
3
3
+ O
→ NO
3
→ NO
3
•
+ O
2
+ O
2
→ RCHO + OH
•
+ hν → O + O
2


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.


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


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).

Denver
1.
Peking
Torontó

hydrocarbons ozone aldehydes hour hour hour hour hour hour
Reddish brown dome above the town.
hour

Hydrocarbons, photochemical oxidants, effect on
 hydrocarbons: no effect

ozone and peroxyacyl nitrates: toxic
Ozone concentration: summer maximum near the soil
Urban ozone / ppb
/
100 – 400
Rural
Tropical forest
50
– 120
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

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



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



afterburner: auxiliary burner is applied to burn the hydrocarbon content of the stack gas, temperature 700 – 1000 0 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

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

1.
2.
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
Heat exchanger burner
CHx contaminated fuel gas rekuperatív utóégető



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


Success in cleaning of exhaust gas petrol based internal combustion engines
(automobiles)
Composition of the exhaust gas from petrol based automobiles
Gas hydrocarbons concentration
≈ 750 ppm
Nitrogen oxides
≈ 1050 ppm
Carbon monoxide ≈ 0,68 tf%
≈ 0,23 tf% Hydrogen
≈ 13,5 tf%
Carbon dioxide
Oxygen ≈ 0,51 tf%
≈ 12,5 tf% water
≈ 72,5 tf% Nitrogen

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

• 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
Engin with petrol fuel catalyst inert emissions
Harmful emissions compounds



Works at 290 0 C – optimum at 400 0 C
Further bonus effect:


Unleaded fuel
Reduction of sulfur content of petrol