UTMUNIVERSITI TEKNOLOGI MALAYSIA

AIR POLLUTION

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PHYSICAL AND CHEMICAL FUNDAMENTAL

• IDEAL GAS LAW• ADIABATIC EXPANSION

COMPRESSION • UNIT OF MEASURE

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IDEAL GAS LAWAlthough polluted air may not be ‘’ideal’ from the biological point view, we may treat is behavior with respect to temperature and pressure as if were ideal.

We assume that at the same temperature and pressure, different kind of gases have different density proportional to their molecular weight may being writing in this form.

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Dalton’s Law of Partial Pressure

Dalton’s law form the basic for

calculation of the correlation factor.

Dalton found that the total pressure

exerted by a mixture of gases is equal

to the sum of the pressure that each

type of gas would exert if it alone

occupied the container.

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Adiabatic Expansion and Compression

Air pollution meteorology is, in part, a consequence of the thermodynamic processes of the atmosphere. One such process is adiabatic expansion and contraction. An adiabatic process is one that taken place with no addition or removal of heat and with sufficient slowness so that the gas can be considered to be in equilibrium at all time. As example, let consider the piston and cylinder in figure.

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Left side of the equation is zero (because of adiabatic process), the increase in thermal energy is equal to the work done. The increase in thermal energy is reflected by an increase in the temperature of the gases. If the gas is expanded adiabatically, its temperature will decrease.

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Unit measure

• Micrograms per cubic meter

• Parts per million

• The micron meter

Unit measure are used to indicate the concentration

of a gaseous pollutant.

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Converting

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Example Convert the following data to “ppm” at all temperature and pressure data.

material Part per million“ppm”

Temperature“Celsius”

Pressure“kPA”

Ppm

SO2 80 25 101.325 0.0305

NO2 0.55 -17.7 100.000

CO2 370 20 101.325

C (12g), O(16g), N(0.00g) and S(32.07g), R = 8.3143 J/K.mole and T (K) = X (Celsius) + 273K

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AIR POLLUTION METEREOLOGY• TURBULENCE • STABILITY• THE ATMOSPHERIC ENGINE• TERRAIN EFFCECTS

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TURBULENCE• MECHANICAL TURBULENCETurbulence is the addition of

fluctuations in the wind velocity, as compared to the average wind velocity.

It is caused by fact that the atmosphere is sheared as it moves.

This shearing occurs because the air actually sticks to the ground (even though we may not feel it) due to friction. Therefore the wind velocity at the earth's surface is zero.

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As the mass of air moves across the earth, the air on top moves faster than the air on the bottom and falls over the slower air. This "tumbling" creates a swirling motion.

The faster the average wind velocity, the more tumbling and swirling is created.

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• THERMAL TURBULENCEWhen the earths surface is heated by

the sun, it will also heat the air directly above it.

Since hot air is less dense than cool air, this heated air will rise from the earths surface to a higher elevation.

This movement forces a vertical rotation of the air because the cooler air sinks to the bottom as the warm air rises.

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In the evening, the opposite occurs. The cold ground cools the air that is above is, causing it to become more dense.

This dense air will feel heavy and will sink even closer to the ground. 

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STABILITY• Stability is defined as the atmospheres

ability to enhance or resist vertical motion.

• The stability of the atmosphere is affected by the wind speed and by the lapse rate (the change in air temperature with height) of the atmosphere.

• The atmosphere is classified as either stable, neutral, or unstable.

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• Neutral Stability the temperature of air parcel moving up

or down adjusts to that of its surrounding and the rate of cooling is the same as the adiabatic lapse rate of 1C/100m.

In other words, the temperature will drop by 1 degree C for every 100 meters we go up into the air. 

       

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UNSTABLE ATMOSPHERE When the rate of air cooling with altitude is

greater than >1C/100m, the air mass becomes unstable and rapid mixing and dilution of pollutants occurs.

If air is moving up, it is warmer than its surroundings and it will continue to climb

whilst conversely, if the air is moved down, it is cooler and denser than surroundings and it will continue to fall.

This steeper temperature gradient encourages greater thermal turbulence.

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STABLE ATMOSPHERE• However, if the rate of cooling with

altitude is slower than the adiabatic lapse rate of 1C/100m (ie<1C/100m ), the air will remain stable and pollutants will concentrate.

• This occurs commonly at night and during winter.

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PLUME TYPES

• The smoke trail or plume from a tall stack located on flat terrain has been found to exhibit a characteristic shape that is dependent on the stability of the atmosphere

• The 6 classical plumes are shown in figure along with the corresponding temperature profile

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LOOPING common in early afternoon Require windy conditions which cause the

plume can swirl up and down Moderate and strong winds are formed on

sunny days creating unstable conditions

CONING Happen at late morning Require moderate winds and overcast days wider than it is deep, and is elliptical in shape

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FANNING Common at night Require stable air and slow vertical movement

of the emission temperature inversion limits the rise of the

plume into the upper atmosphere

FUMIGATION Common in early morning occur when the conditions move from stable to

unstable unstable air causes the plume to move up and

down - can cause localised pollution

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LOFTING Common in late afternoon When plume is above the inversion layer (or

there is no inversion), it becomes a lofting plume

Normal wind direction and speed will disperse the plume into the atmosphere without effect from ground warming or cooling.

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ATMOSPHERIC ENGINE

• Atmospheric like an engine (continually expanding and compressing gases, exchanging heat and generally raising chaos)

• Driving energy comes from the sun

• Diff. in heat input between the equator and the poles provides initial overall circulation of the earth’s atmosphere.

• Rotation of the earth coupled with different heat conductivities of the ocean and land produce weather.

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• HIGHS AND LOWS:a)• Water: exert greater pressure

at greater depths• Atmosphere: Exerts more

pressure at the surface than it does a higher elevations

 • PICTURE (page 581,figure 7.13)

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b) • winds flows from the higher pressure

areas to the lower pressure area • Nonrotating planet: wind direction is

perpendicular to the isobars PICTURE (page 581, figure 7.14a) • Rotating planet: angular thrust called

the coriolis effect is added to this motion. The resultant wind direction in the northern hemisphere it is shown.

PICTURE (page 581,figure 7.14b)

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• Anticyclones system for high. Usually it associated with good weather

• Cyclones system for low. Usually associated with foul weather.

• Tornadoes and Hurricanes are the foulest than the cyclone.

 • Wind speed: When isobar close together, pressure

gradient be steep, wind speed high. When isobar well spread, wind are

light and nonexistent.

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TERRAIN EFFECT• HEAT ISLAND• LAND/SEA BREEZES• VALLEYS

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HEAT ISLAND

absorbs and reradiates heat at greater than surrounding area.

Causes moderate to strong vertical convection currents above the heat island. Can be nullified by strong wind

Industrial complexes and cities

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Depending upon location of the pollutant, it can be good or bad news.- good news: ground level sources such automobiles, the bowl of unstable air that forms will allow greater air volume for dilution of pollutant.-Bad news: stable conditions plume from tall stacks will carried out over countryUnstable conditionsheat island will mixes these plumes to the ground levels

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LAND/SEA BREEZES

Stagnant anticyclone strong circulation will develop

across the shorelineof large water bodies

During night The land cools more rapidly than the water. Cooling air over the land flows toward the water.`land breeze’(bayu darat)

During morningland heats faster than water. The air over the land become warm and rise. The rising air is replaced by air from over the water body.‘lake breeze’(bayu laut)

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• Effect of lake breeze on stability:-imposed a surface based inversion on the temperature profile

• Air moves from the water over the warm ground. Thus, stack plumes originating near the shoreline,stable lapse rate causes a fanning plume close to the stack. The lapse condition grows to the height of the stack as the air moves inland. At some point inland,a fumigation plume results

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Valleys• Moderate to strong winds, valleys oriented at an

acute angle to the wind direction channel the wind• The valleys peels off part of the wind and forces it

follow the direction of valley flow (page 588,figure 7.19)

• Valley will have its own circulation under stagnating cyclone.

Valley air will be warmed by warming valley walls. It become more bouyant and flow up. At night, wind will flow down.

• Valley walls protect the floor from radiative heating by sun. walls and floor are free to to radiate heat away to the cold night sky.

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Factor affecting

dispersion of air pollutants

Source characteristics

Downwind distance

- greater the distance between the point of

discharge and a ground level receptor

downwind, greater will the volume of air available for diluting

the contaminant discharge before it

reaches the receptor

Stability- the more unstable

the atmosphere, the greater the diluting power.

- turbulence: no diluting power

Wind speed and direction

- wind direction determines the direction in which

contaminated gas stream will move across local

terrain. Wind speed affect the plume rise and the rate

of mixing/dilution of the contaminated gases as they leave the discharge point.

When wind speed increase, plume rise will decreased,

the pollutant’s ground level concentration also

increased, rate of dilution of the effluent plume

increase

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DISPERSION MODELING

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What is a dispersion model?

Mathematical description of the meteorological transport and

dispersion process that is quantified in terms of source and meteorologic

parameters during a particular time

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Basic Point Source Gaussian Dispersion Model

• The model gives the ground level concentration (X) of pollutant at coordinate (x,y) downwind from a stack with an effective height (H)

• The equation model is as follows:

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Where;= Downwind concentration at

ground level, g/m³= emission rate of pollutant, g/s= plume standard deviations, m= wind speed, m/s= distances, m= exponential

x is the crosswind distance from the centerline of the plumey is the downwind distance along plume mean centerline from point source

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Value of effective stack height, H

Where h = physical stack height ∆H = plume rise

∆H may be computed from Holland’s formula as follows;

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Where;= stack velocity. m/s= stack diameter, m= wind speed, m/s= pressure, kPa= stack temperature, K= air temperature, K

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A – Extremely unstableB – Moderately unstableC – Slightly unstableD – NeutralE – Slightly stableF – Moderately stable

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A – Extremely unstableB – Moderately unstableC – Slightly unstableD – NeutralE – Slightly stableF – Moderately stable

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TABLE 7-8Key to stability categories

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Algorithm to express stability class lines developed by D.O Martin (1976)

where the constants a, c, d and f are defined in Table 7-9

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Table 7-9Values of a, c, d and f for calculating sy and sz

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Inversion Aloft

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Inversion Aloft• Vertical standard deviation,

Sz = 0.47(L – H)Where L = Height to bottom of inversion layer, m

H = Effective stack height, m

• When the distance is > 2XL, the centerline concentration of pollution may be estimated using equation below:-

-------- (7-25)

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Example 7.4

It has been estimated that the emission of SO2 from a coal-fired power plant is 1656.2 g/s (E). At 3km downwind on an overcast summer afternoon, what is the centerline concentration of SO2 if the wind speed is 4.50 m/s (u)? (Note: “centerline” implies y = 0)

Stack Parameters: Height, h = 120.0 mDiameter, d = 1.20 mExit velocity, vs = 10.0 m/sTemperature, T = 315ºC

Atmospheric conditions: Pressure, P = 95.0 kPaTemperature, Ta = 25.0ºC

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Solutioni) Determine effective stack height (H)

ii) Determine atmospheric stability class based on Table 7-8Since it is stated in the question that it has overcast condition, thus class D is used.

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iii) Determine plume standard deviations, Sy and Sz2 ways to determine: 1st Graphical method ( Figure 7-22 & Figure 7-23)

2nd Equation 7-22 & 7-23

Hence;

iv) Substitute all values into Eqn 7-19 to obtain centerline concentration of SO2

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Indoor Air Quality Model

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Mass balance model for indoor air pollution

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If we assume the contents of the box are well mixed:

Or

Where V = volume of box, m3

C = concentration of pollutant, g/m3

Q = rate of infiltration of air into and out of box, m3/s

Ca = concentration of pollutant in outdoor air, g/m3

E = emission rate of pollutant into box from indoor source g/s

k = pollutant decay rate or reaction rate coefficient, s-1

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Air Pollution Control of Stationary Sources

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Gaseous PollutantsAbsorption• Control devices based on the principle of

absorption attempt to transfer the pollutant from a gas phase to a liquid phase

• This is a mass transfer process in which the gas dissolves in the liquid

• Example: Spray chambers and towers or columns

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Gaseous PollutantsAdsorption• This is mass transfer process in which

the gas is bonded to a solid• The gas (adsorbate) penetrates into the

pores of the solid (the adsorbent)- exp: active carbon, silica gel

• Chemical bonding is by reaction with the surface

• All the adsorbents are subject to destruction at high temperature. Exp: 150C for active carbon, 400C for silica gel

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Gaseous PollutantsCombustion• Suitable when the contaminant in the gas

stream is oxidizable to an inert gas• CO and hydrocarbons • Commercial application: Direct flame

incineration and Catalytic incinerator

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Gaseous Pollutants

Flue Gas Desulfirization (FGD)

• Flue gas desulfurization systems fall into 2 broad categories:

Nonregenerative – reagent used to remove sulfur oxides is discarded

Regenerative – reagent used is recovered and reused

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Particulate PollutantsCyclones• For particle sizes greater than 10µm in

diameter• The efficiency of collection of various

particle sizes () can be determined from an empirical expression and efficiency graph (FIGURE 7-36)

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Whered0.5 = cut diameter, the particle size

for which the collection efficiency is 50%

µ = dynamic viscosity of gas, Pa.sB = width of entrance, mH = height of entrance, mρp = particle density, kg/m3sQg = gas flow rate, m3/s = effectiveness number of turns

made in traversing the cyclone

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Reverse flow cyclone

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Particulate Pollutants

Filters

• When high efficiency control of particle smaller than 5µm is required

• 2 types are in use– The deep bed filter– The bag house

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Mechanically cleaned (shaker) baghouse (a) and pulse-jet-clean baghouse (b)

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Particulate PollutantsLiquid Scrubbing• When the particulate matter to be

collected is wet, corrosive or very hot, the fabric filter may not work

• Use liquid scrubbing• Typical scrubbing applications include

control of emission – exp: talc dust, phosphoric acid mist

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Venturi scrubber

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Particulate Pollutants

Electrostatic Precipitation (EPS)• High efficiency, dry collection of particles

from hot gas streams can be obtained by electrostatic

• The EPS is usually constructed of alternating plates and wires

• A large direct current potential (30 – 75 kV) is established between the plates and wires

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Electrostatic precipitator