Chapter 2 Air Pollution, Sources and Characteristics

Transcription

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: Pollutants Common pollutants and their sources. Sources Suspended particulate Matter, SPM a Chlorine Fluoride Sulphur dioxide Lead Oxides of nitrogen, a NO, NO 2 (NO X ) 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 Peroxyacetyl nitrate, PAN Secondary pollutant 9

2 Formaldehyde Ozone a Carbon monoxide a Hydrogen sulphide Hydrocarbons Ammonia Secondary pollutant Secondary pollutant Automobiles Pulp and paper, petroleum refining. Automobiles, petroleum refining 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, SO 2 and NO x. Depending upon the type of fuel used, emission of one or more of these pollutants may be of environmentalsignificance. 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. The concentration of NO x 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/nm 3 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 mg/nm 3. 10

3 Example 1 Calculate SO 2 concentration in flue gas when one mole of C 7 H 13 containing 1 % sulphur is burnt in presence of stochiometric amount of oxygen. Solution First we write stochiometric equation for combustion: C 7 H O 2 = 7 CO H 2 O Since O 2 is supplied through air which also contains nitrogen and in air each mole of oxygen is accompanied by 3.76 mole N 2, for mole O 2, mole N 2 will be supplied. Therefore we may write. C 7 H O N 2 = 7 CO H 2 O N 2 Therefore quantity of flue gas at STP = mole 22.4 L or mole x =1020 L 1 mole Since one mole C 7 H 13 = 7 x x 1 = 97 g, sulfur contents of fuel = 97 x 0.01 = 0.97g. Therefore SO 2 produced = 1.94 g or 1940 mg/mole of fuel. As an approximation, neglecting the volume of oxygen consumed in production of SO 2, concentration of SO 2 = 1940 mg/1020 L = 1902 mg/m 3, at STP. 273 or 1902 x = 1742 mg/nm

4 Automobiles In urban areas automobiles form a significant source of a number of air pollutants, namely, particulates, NO x, 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, (C 2 H 5 ) 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 o Raw material crushing, grinding Emission Particulates o Kiln operation and cooling Particulates, CO, SO 2, NO x, 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 SO 2. Sulphur dioxide is converted to trioxide in presence of vanadium pentaoxide catalyst. The sulphur trioxide is absorbed in recycling concentrated sulfuric acid. Unreacted SO 2 escapes with the flue gas. New large plants now a days use double conversion double absorption (DCDA) process realizing above 99 percent efficiency. 12

5 Example 2 A 250 T/d DCDA sulphuric acid plant burns 82T/d sulphur in the manufacturing process. Flue gas containing 350 ppm SO 2 is discharged at the rate of 35 Nm 3 /s, What is the percent recovery of sulfur in the product. Solution: x ppm SO 2 = = 916 mg/nm Therefore SO 2 discharged with flue gas Nm s 24h 1kg 1T = x x x x x = 2.77 T/d Nm 3 s 1h Id 10 6 mg 10 3 kg The quantity of sulphur in 2.77 T/d SO T 32 g S = x = 1.35 T/d d 64 g SO 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 µm. 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 µm 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. Particles in the range of 0.1µm to 10µm are of, most interest from health viewpoint. Particulate matter of less then 10µm size (PM 10 ) 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 µm, called thoracic particles, penetrate to the lungs and 13

6 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 µg/m 3 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 SO 3, which leads to production of sulphuric acid. When SO 3 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 SO 2. 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 NO x 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 NO 2 in the atmosphere, which in turn may give rise to secondary pollutants, which are injurious. NO 2 may also lead to formation of HNO 3, 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. 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 µg/m 3 in ambient air results in an increase of about 1-2 µg per decilitre (µg/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 µg/dl are associated with neurobehavioural changes in children. 14

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