Air pollution, sources and characteristics

1. Chapter 2
Air Pollution, Sources and Characteristics
1 Introduction
Air pollution is the presence of substances in air in sufficient concentration and for
sufficient time, so as to be, or threaten to be injurious to human, plant or animal life, or
to property, or which reasonably interferes with the comfortable enjoyment of life and
property.
Air pollutants arise from both man made and natural processes. Pollutants are also
defined as primary pollutants resulting from combustion of fuels and industrial
operations and secondary pollutants, those which are produced due to reaction of
primary pollutants in the atmosphere. The ambient air quality may be defined by the
concentration of a set of pollutants which may be present in the ambient air we breath
in. These pollutants may be called criteria pollutants. Emission standards express the
allowable concentrations of a contaminant at the point of discharge before any mixing
with the surrounding air.
Table 1 lists names of some common air pollutants, their sources and classification.
This session will be limited to discussion of primary pollutants generated due to human
activity.
Table 1: Common pollutants and their sources.
Pollutants Sources
Suspended particulate Matter, SPMa
Chlorine
Fluoride
Sulphur dioxide
Lead
Oxides of nitrogen,3
NO, N02 (NOx)
Peroxyacetyl nitrate, PAN
Automobile, power plants, boilers,
Industries requiring crushing and
grinding such as quarry, cement.
Chlor-alkali plants.
Fertilizer, aluminum refining
Power plants, boilers, sulphuric
acid manufacture, ore refining,
petroleum refining.
Ore refining, battery
manufacturing, automobiles.
Automobiles, power plants, nitric
acid manufacture, also a
secondary pollutant
Secondary pollutant

2. Formaldehyde Secondary pollutant
Ozone'
a
Secondary pollutant
Carbon monoxide1a
Automobiles
Hydrogen sulphide Pulp and paper, petroleum
refining.
Hydrocarbons Automobiles, petroleum refining
Ammonia Fertilizer plant
a- Criteria pollutants
2 Combustion sources
By combustion sources is meant operations where primarily fossil fuels, coal, natural
gas, petrol, diesel and furnace oil are burnt to obtain energy. This includes power
plants, industrial boilers, domestic heating and automobiles.
Thermal power pants
Thermal power plants are major sources of SPM, S02 and NOx. Depending upon the
type of fuel used, emission of one or more of these pollutants may be of environmental-
significance.
A large amount of SPM as fly ash is emitted from coal fired plants, particularly if the ash
content of coal is high and a fly ash removal unit, such as, an electrostatic precipitation
(ESP) is not used.
Oxides of nitrogen formed in combustion processes are usually due to either thermal
fixation of atmospheric nitrogen in combustion air or to the conversion of chemically
bound nitrogen in the fuel. Thermal fixation occurs when combustion temperature is
above 1600°C. For natural gas and distillate oil nearly all NO results from thermal
fixation. For residue oil and coal, the contribution to NO emission from fuel bound
nitrogen may be significant.

3. The concentration of NOx formed increases with increase in excess oxygen maintained
in the combustion process and with the increase in temperature of the furnace. For coal
based thermal power plants in India, it ranges between 100 and 200 mg/Nm3
in the flue
gas. In the case of natural gas and liquid fuels, the emission limits for flue gases
prescribed in European countries is in the range of 200 - 400 mg/Nm3
.
Example 1
Calculate S02 concentration in flue gas when one mole of C7H13 containing 1 % sulphur is
burnt in presence of stochiometric amount of oxygen.
Solution
First we write stochiometric equation for combustion:
C7H13 + 1 0.2502 = 7 C02 + 6.5 H2 0
Since 02 is supplied through air which also contains nitrogen and in air each mole of
oxygen is accompanied by 3.76 mole N2, for 10.25 mole 02 , 38.54 mole N2 will be
supplied. Therefore we may write.
C7H13 + 1 0.2502 + 38.54 N2 = 7 C02 + 6.5 H2 0 +38.54N2
Therefore quantity of flue gas at STP = 45.54 mole
22.4 L
or 45.54 mole x =1020 L
1 mole
Since one mole C7H13 = 7 x 1 2 + 1 3 x 1 = 9 7 g , sulfur contents of fuel = 97 x 0.01 = 0.97g.
Therefore S02 produced = 1.94 g or 1940 mg/mole of fuel.
As an approximation, neglecting the volume of oxygen consumed in production of S02,
concentration of S02 = 1940 mg/1020 L = 1902 mg/m3
, at STP.
273
or 1902 x = 1742 mg/Nm3
298

4. Automobiles
In urban areas automobiles form a significant source of a number of air pollutants,
namely, particulates, NOx, hydrocarbons, carbon monoxide and lead. These pollutants
are produced when fuel is burnt under less than ideal conditions. Non-uniform oxygen
supply within the combustion chamber and lower flame temperature leads to incomplete
combustion releasing CO, HC and unburnt particles in the exhaust.
Tetraethyl lead, (C2H5)4 Pb, is added to petrol as anti-knock additive. Where such petrol
is used lead is emitted in the exhaust fumes as inorganic particulates.
3 Industrial sources
Only two sources are discussed here as illustrative examples.
Cement manufacture
Raw materials include lime, silica, aluminum and iron. Lime is obtained from calcium
carbonate. Other raw materials are introduced as sand, clay, shale, iron are and blast
furnace slag. The process consist of mining, crushing, grinding, and calcining in a long
cylindrically shaped oven or kiln. Air pollutants can originate at several operations as
listed below.
Source Emission
o Raw material crushing, grinding Particulates
o Kiln operation and cooling Particulates, CO, S 0 2 ,
NOx, HC
o Product grinding and packaging Particulates
Control of emission of particulate matter is economically viable as the cost of collected
dust (raw material and product) pays for control measures.
Sulphuric acid manufacture
Sulphuric acid is produced from sulphur, which is burnt to obtain S02. Sulphur dioxide is
converted to trioxide in presence of vanadium pentaoxide catalyst. The sulphur trioxide
is absorbed in recycling concentrated sulfuric acid. Unreacted S02 escapes with the flue
gas. New large plants now a days use double conversion double absorption (DCDA)
process realizing above 99 percent efficiency.

5. Example 2
A 250 T/d DCDA sulphuric acid plant burns 82T/d sulphur in the manufacturing
process. Flue gas containing 350 ppm S02 is discharged at the rate of 35 Nm /s,
What is the percent recovery of sulfur in the product.
Solution: -
350 x 64
350 ppm S02 = = 916 mg/Nm3
24.45
Therefore S02 discharged with flue gas
916 35Nm3
3600s 24h 1kg
x x x x x
Nm3
s 1h Id 106
mg
The quantity of sulphur in 2.77 T/d S02
2.77 T 32 g S
x =1.35 T/d
d 64 g S02
82-1.38
Therefore sulphur recovery= = 98.3 %
82
Note: DCDA plants are expected to give better than 99% recovery. Therefore the
reason for poor performance should be investigated and corrected.
4 Characteristics
Major effects of some of the common pollutants are described in this section.
Suspended particulate matter
Atmospheric particulate matter is defined to be any dispersed matter, solid or liquid,
smaller than, 500 pm. Under various conditions of their generation, they are also called
by other names such as dust, fume, smoke and mist. Dust usually
refers to particles in the range of 1 to 200 pm size. Fume is very fine solid or liquid
particles arising from chemical reactions or condensation of gases. Smoke refers to
finely divided particles resulting from incomplete combustion of substances such as
coal, petroleum, etc.
1T
= 2.77 T/d
103
kg

6. Particles in the range of 0.1 pm to 10|jm are of, most interest from health viewpoint.
Particulate matter of less then 10pm size (PM10) is classified as respirable dust. Larger
particles that enter the respiratory system are trapped by hairs and lining of nose or can
be captured by mucus in upper respiratory tract and worked back to the throat by cilia
and removed by spitting, Figure 1.
Particles between 0.5 and 10 pm, called thoracic particles, penetrate to the lungs and
are deposited there. These particles by themselves and or by carrying other air
pollutants adsorbed on them may cause the greatest harm. Elevated particulate
concentrations in the atmosphere, in the range of 400 pg/m3
and above especially in
conjunction with oxides of sulphur have been linked to respiratory infections, bronchitis,
asthma, pneumonia and the like. Many carbonaceous particles, especially those
containing polycyclic aromatic hydrocarbons (PAH) are suspected carcinogens.
Sulphur dioxide
Sulphur dioxide when released in the atmosphere can also convert to SO3, which leads
to production of sulphuric acid. When SO3 is inhaled it is likely to be absorbed in moist
passages of respiratory tract. When it is entrained in an aerosol, however, it may reach
far deeper into the lungs.
Figure 2 summarizes adverse health effects of S02. Sulphur dioxide can damage
vegetation and cause corrosion. Airborne sulfates reduce visibility. It is also the cause of
acid rain in some countries.
Nitrogen oxides
Almost all NOx emissions are in the form of NO, which has no known adverse health
effects in the concentrations found in atmosphere. However, NO can be oxidized to N02
in the atmosphere, which in turn may give rise to secondary pollutants, which are
injurious. N02 may also lead to formation of HN03, which is washed out of the
atmosphere as acid rain.
Carbon monoxide
Most of the CO emissions are from transportation sector. Peak concentrations occur at
street level in busy urban centers particularly when there is no atmospheric mixing as it
happens during winter season. Carbon monoxide interferes with blood's ability to carry
oxygen. With the blood stream carrying less oxygen, brain function is affected and heart
rate increases in an attempt to offset the oxygen deficit. Breathing between 20 to 35
ppm CO in air for 4 h results in impairment of time related response. Individuals with
heart condition may experience chest pain. Exposure to 100ppm concentration would
result in dizziness.

7. Lead
Lead released from motor vehicle exhaust may affect human populations by direct
inhalation, in which case people living nearest to highways are at greatest risk. Lead
can be ingested also after it is deposited on to foodstuffs.
Measurements made in exposed communities indicate that lead concentration of
1|u,g/m3
in ambient air results in an increase of about 1-2 ^ig per decilitre (|ig/dl) in blood.
Lead poisoning can cause destructive behavioural changes, learning disabilities and
permanent brain damage. Children and pregnant women are at greatest risk. Blood
levels of 50-60 i^g/dl are associated with neurobehavioural changes in children.

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