air pollution.pdf

Chapter 3

AIR POLLUTION

AIR POLLUTION Topics to be studied

Physical and chemical fundamentals Ideal gas laws

Dalton’s law of partial pressure

Adiabatic expansion and compression

Units of measure

Converting μg/m3 to ppm

Effects of air pollutants Effects on materials

Mechanism of deterioration

Effects on vegetation

Cell and leaf anatomy

Problems of diagnosis

AIR POLLUTION Effects on health

Susceptible population

Anatomy of the respiratory system

Inhalation and retention of particles

Chronic respiratory disease

Carbon monoxide (CO)

Hazardous air pollutants (HAPs)

Lead (Pb)

Nitrogen dioxide (NO2)

Photochemical oxidants

PM10

Sulphur oxides (SOx) and total suspended particles (TSP)

AIR POLLUTION Origin and fate of air pollutants

Carbon monoxide Hazardous Air Pollutants (HAPs) Lead Nitrogen dioxide Photochemical oxidants Sulphur oxides Particulates

Micro and macro air pollution Indoor air pollution Acid rain Ozone depletion Global warming

Scientific basis Impacts A rationale for action

AIR POLLUTION Air pollution metrology

The atmospheric engine

Highs and lows

Turbulence Mechanical turbulence

Thermal turbulence

Stability

Neutral stability

Unstable atmosphere

Stable atmosphere

Plume types

AIR POLLUTION Atmospheric dispersion

Factors affecting dispersion of air pollutants

Source characteristics

Downwind distance

Wind speed and direction

Stability

Indoor air quality control

AIR POLLUTION Air pollution control of stationary sources

Gaseous pollutants Absorption Adsorption Combustion Flue gas desulfurization (FGD) Non regenerative systems Control technologies of nitrogen dioxides Prevention Post combustion Particulate pollutants

Cyclones Filters Liquid scrubbing Electrostatic precipitation Control technologies for mercury

AIR POLLUTION Air pollution control of mobile sources

Engine fundamentals

The gasoline engine

The diesel engine

The jet engine

Effects of design and operating variables on emission

AIR POLLUTION Physical and chemical fundamentals

Ideal gas law

Assuming that at the same temperature and pressure, different kinds of gases have densities proportional to their molecular masses. This may be written as ρ = PM

RT

ρ = density of gas, kg/m3

M = molecular mass, grams/mole

T = absolute pressure, K

Universal gas constant

Ideal gas law Since density is mass per unit volume, hence

PV = nRT

Where V is the volume occupied by n moles of gas.

Dalton’s law of partial pressure Forms the basis for the calculation of the correction factor.

Dalton found that the total pressure exerted by the mixture of gases is equal to the sum of the pressures that each type of gas would exert if it alone occupied the container Pt = P1 + P2 + P3 +……..

Pt = total pressure of mixture

P1, P2, P3 = pressure of each gas if it were in container alone, partial pressure

Ideal gas law Dalton’s law also may be written in terms of the ideal

gas law: Pt = n1 RT + n2 RT + n3 RT +….

V V V

= (n1 + n2 + n3 + …) RT

V

Adiabatic expansion and compression Air pollution metrology is in part a consequence of the

thermodynamic processes of the atmosphere

One such process is adiabatic expansion and contraction

Ideal gas law Let us consider a piston and cylinder. The cylinder and the piston face are assumed to be

perfectly insulated. The gas is at pressure at P. A force, equal to PA must be

applied to the piston to maintain equilibrium. If the force is increased and the volume is compressed the

pressure will be increased and the work will be done on the gas by the piston.

Since no heat enters or leaves the gas, the work will go into increasing the thermal energy of the gas in accordance with the first principle of thermodynamics that is Heat added to gas = increase in thermal energy + external

work done by or on the gas

Ideal gas law Since the left side of the equation is zero (because it is

an adiabatic process), the increase in thermal energy is equal to the work done.

Increase in thermal energy is reflected by an increase in the temperature of the gas.

If the gas is expanded adiabatically, its temperature will decrease.

Units of measure The three basic units of measure used in reporting air

pollution data are micrograms per cubic meter (μg/m3), parts per million (ppm), and micron or micrometre (μm).

Converting (μg/m3) ppm The conversion between μg/m3 and ppm is based on the fact

that at standard condition (0oC and 101.325 KPa), one mole of an ideal gas occupies 22.414 L.

Thus we may write an equation that converts the mass of the pollutant Mp in grams to its equivalent volume Vp in liters at standard temperature and pressure.

Vp = Mp x 22.414 L/GM GMW where GMW is the gram molecular weight of the pollutant.

Converting (μg/m3) ppm

We use ideal gas law to make the correction

22.414 L/GM x T2 x 101.325 kPa

273K P2

Where T2 and P2 are the absolute temperature and absolute pressure at which the readings were made. Since ppm is a low volume ratio, we may write as

ppm = Vp

Va

Effects of air pollutants Effects in material (mechanism of deterioration)

Five mechanism of deterioration Abrasion

Deposition and removal

Direct chemical attack

Indirect chemical attack

Electrochemical corrosion

Solid particles of large enough size and travelling at high enough speed can cause deterioration by abrasion.

With the exception of soil particles in dust storm and lead particles from automatic weapons fire, most air pollutant particles either too small or travel at too slow a speed to be abrasive.

Effects of air pollutants Solubilization and oxidation/ reduction reactions typify

direct chemical attack.

Water must be present as a medium for these reactions to take place.

Sulphur dioxide and SO3 in the presence of water react with limestone (CaCO3) to form calcium sulphate (CaSO4) and gypsum (CaSO4.2H2O) are more soluble in water than CaCO3 and both are leached away when it rains.

The tarnishing of silver by H2S is a classic example of an oxidation/reduction reactions.

Effects of air pollutants Indirect chemical attack occurs when pollutants are absorbed

and then react with some component of the absorbent to form a destructive compound.

The compound maybe destructive because it forms an oxidant, reductant, or solvent.

A compound can be destructive by removing an active bond in some lattice structure.

Leather becomes brittle after it absorbs SO2, which reacts to form sulphuric acid because of the presence of minute quantities of iron.

The iron acts as a catalyst for the formation of the acid.

Oxidation/reduction reactions cause local chemical and physical differences on metal surfaces.

Effects of air pollutants Effects on vegetation (Cell and leaf anatomy)

Leaf is the primary indicator of the effects of air pollution on plants is should be understood how the leaf functions.

A typical plant has three main components The cell wall

The protoplast

The inclusions

Cell is in younger plants is thick and gradually thickens with age.

Protoplast is the terms used to describe the protoplasm of one cell.

It consists of water but it also includes protein, fat , carbohydrates.

The nucleus contains DNA which controls the operation of cell.

Protoplasm located outside the nucleus is called cytoplasm.

Effects of air pollutants Problems of diagnosis

Various factors make it difficult to diagnose actual air pollution damage.

Droughts, insects, disease, herbicide overdose, and nutrient deficiencies call can cause injury that resembles air pollution damage.

Combinations of pollutants that alone cause no damage are known to produce acute effects when combined.

This effect is known as synergism.

Effects of air pollutants Effects on health

Susceptible population It is difficult at best to assess the effects of air pollution on human

health. Personal pollution from smoking results in exposure to air pollutant

concentrations far higher than the low levels found in the ambient atmosphere.

Anatomy of respiratory system The respiratory system is the primary indicator of air pollution

effects in humans. The major organs of the respiratory system are the nose, pharynx,

larynx, trachea, bronchi, and lungs known as the upper respiratory tract (URT)

Lower respiratory tract (LRT) consists of the branching structures known as bronchi and the lung itself, composed of grape like clusters of sacs called alveoli.

Effects of air pollutants Inhalation and retention of particles

The degree of penetration of particles into the LRT is primarily a function of the size of the particles and the rate of breathing.

Particles greater than 5 to 10 μm are screened out by the hairs of the nose.

Sneezing also helps the screening process.

Particles in the 1 to 2 μm size range penetrate to alveoli.

These particles are small enough to bypass screening and deposition in the URT, how ever they are big enough that their terminal settling velocity allows them to deposit where they can do most damage.

Effects of air pollutants Inhalation and retention of particles

Particles that are 0.5μm in diameter do not have a large enough terminal velocity to be removed efficiently.

Smaller particles diffuse to the alveolar walls.

Carbon monoxide (CO) colourless, odourless, gas is lethal to humans within a few minutes

at concentrations exceeding 5,000 ppm. CO reacts with haemoglobin in the blood to form

carboxyhemoglobin (COHb). Haemoglobin has a greater affinity for CO than it does for oxygen. Hence the formation of COHb effectively deprives the body of

oxygen. At COHb levels of 5 to 10 percent, visual perception, manual

dexterity and ability to lean are impaired.

Effects of air pollutants Hazardous air pollutants (HAPs)

Exposure to air toxics in the workplace is generally much higher than the ambient air.

Asbestos, arsenic, benzene, coke oven emissions, and radionuclides may cause cancer.

Lead (Pb) Lead is a cumulative poison. Has the ability to ingested in food and water. Lead is measured in the urine and blood for diagnostic

evidence of lead poisoning. Chronic exposure to lead may result in brain damage

characterized by seizure, mental incompetence, and highly active aggressive behaviour.

Effects of air pollutants Nitrogen dioxide (NO2)

Exposure to NO2 concentrations above 5 ppm for 15 minutes results in cough and irritation of the respiratory tract.

Continued exposure may produce an abnormal accumulation of fluid in the lung.

The gas is reddish brown in concentrated form and gives a brownish yellow tint at lower concentrations.

Photochemical oxidants Photochemical oxidants include peroxyacetyl nitrate,

acrolein, aldehydes, and nitrogen oxides, the major oxidant is ozone (O3).

Ozone is commonly used as an indicator of the total amount of oxidant present.

Effects of air pollutants Sulphur oxides (SOx) and total suspended particulates

(TSP)

Sulphur oxides include sulphur dioxide, trioxide, their acids, and the salts of their acids.

Patients suffering from chronic bronchitis have shown an increase in respiratory symptoms when the TSP levels exceeds 350 μg/m3 and the SO2 level was above 0.095 ppm.

Origin and fate of air pollutants Carbon monoxide

Incomplete oxidation of carbon results in the production of carbon monoxide.

The natural formation of CO results in formation of an intermediate step in the oxidation of methane.

The hydroxyl radical (OH.) serves as the initial oxidizing agent.

it combines with CH4 to form an alkyl radical.

CH4 + OH. CH3. + H2O

This reaction is followed by a complex series of 39 reactions, which have been oversimplified to the following

CH3. + O2 + 2(hv) CO + H2 + OH.

Effects of air pollutants This says that CH3 and O2 are each zapped by a

photon of light energy (hv)

The symbol v stands for the frequency of the light. The h plank’s constant = 6.626 x 10-34 J/Hz.

Anthropogenic sources (those associated with the activities of human beings) include motor vehicles, fossil fuel burning for electricity and heat, industrial processes, solid waste disposal, and miscellaneous burning of such things as leaves and brush.

Effects of air pollutants Hazardous air pollutants (HAPs)

EPA has identified 166 categories of major sources and 8 categories of area sources for the HAPs.

The source categories represent a wide range of industrial groups such as Fuel combustion

Metal processing

Petroleum and natural gas production

Surface coating processes

Waste treatment

Disposal processes

Agricultural chemical production

Effects of air pollutants Lead

Volcanic activity and airborne soil are the primary sources of atmospheric lead.

Smelters and refining processes, as well as incineration of lead containing wastes are major point sources of lead.

Approximately 70 to 80 percent of the lead that used to be added to gasoline was discharged to the atmosphere.

Nitrogen dioxide Bacterial action in the soil releases nitrous oxide (N2O) to the

atmosphere. In the upper troposphere and stratosphere, atomic oxygen

reacts with the nitrous oxide to form nitric oxide (NO). N2O + O2 2NO

Effects of air pollutants Nitrogen dioxide

The atomic oxygen results from the dissociation of ozone. The nitric oxide reacts with ozone to form nitrogen dioxide (NO2)

NO + O3 NO2 + O2

Combustion processes account for 96 percent of the anthropogenic sources of the nitrogen oxides.

Although nitrogen and oxygen co exist in our atmosphere without reaction, their relationship is much less indifferent at high temperatures and pressures.

At temperature in excess of 1,600 K they react

N2 + O2 2NO (heat)

Effects of air pollutants Photochemical oxidants

Unlike the other pollutants, the photochemical oxidants result entirely from atmospheric reactions and are not directly attribute to either people or nature.

They are also known as secondary pollutants. Formed through a series of reactions initiated by the

absorption of a photon by an atom, molecule, free radical, or iron.

Ozone is the principal photochemical oxidant. It formation is usually attributed to the nitrogen dioxide

photolytic cycle. Hydrocarbons, nitrogen oxides, and ozone react and interact

to produce more nitrogen dioxide and ozone. Figure shows the cycle

Effects of air pollutants Sulphur dioxides

Sulphur dioxide may be both primary and secondary pollutants.

Power plants, industry, volcanoes, and the oceans emits SO2, SO3 and SO4

2-

Biological decay processes and some industrial sources emits H2S which is oxidized to form the secondary pollutant SO2.

The most important oxidizing reaction for H2S appears to be one involving ozone H2S + O3 H2O + SO2

The combustion of fossil fuels containing sulphur yields sulphur dioxide in direct proportion to the sulphur content of the fuel. S + O2 SO2

Effects of air pollutants Particulates

Sea salt, soil dust, volcanic particles, and smoke from forest account for 2.9 Pg of particulate emissions each year.

Secondary sources of particulates include the conversion of H2S, SO2, NOx, NH3 and hydrocarbons.

H2S and SO2 are converted to sulphates.

NOx and NH3 are converted to nitrates.

The hydrocarbons react to form products that condense to form particles at atmospheric temperatures.

Micro and macro air pollution Air pollution problem may occur on three scales

Micro, meso and macro

Micro scale problems range from those covering less than a centimetre to those the size of a house or slightly larger.

Meso scale air problems are those of few hectares up to the size of a city or county.

Macro scale problems extend from counties to states, nations and in the broad sense globe.

Indoor air pollution Carbon monoxide from improperly operating furnaces has long

been a serious concern. Chronic low level of CO pollution have also been recognized. Gas ranges, ovens, pilot lights, gas and kerosene space heaters and

cigarette smoke all contribute.

Effects of air pollutants Acid rain

Unpolluted rain is naturally acidic because CO2 from the atmosphere dissolves to a sufficient extent to form carbonic acid.

The equilibrium pH for pure rainwater is about 5.6. The average pH range in rain weighted by the amount of

precipitation over the United States in 1997 is shown in the figure. Chemical reactions in the atmosphere convert SO2, NOx, and the

volatile organic compounds (VOCs) to acidic compounds and associated oxidants.

The primary conversion of SO2 is through aqueous phase reaction with the hydrogen per oxide (H2O2) in clouds.

Nitric acid is formed by the reaction of NO2 with OH radicals formed photo chemically.

Ozone is formed and then protected by a series of reactions involving both NOx, and VOCs.


Concern about acid rain relates to potential effects of acidity on aquatic life, damage to crops and forests and damage to building materials.

Low pH values may effect fish directly by interfering with productive cycles or by releasing otherwise insoluble aluminium, which is toxic.

Ozone depletion Without ozone every living thing on the earth’s surface will be

incinerated. The presence of ozone in the upper atmosphere (20 to 40 km)

provides as barrier to ultraviolet (UV) radiation. Too much UV will cause skin cancer. Although oxygen also serves as a barrier to UV radiation, it absorbs

only over a narrow band cantered at a wavelength of 0.2 μm.


The photochemistry of these reactions is shown in figure.

Air pollution meteorology

The atmospheric engine

The atmosphere is somewhat like an engine.

It is continually expanding and compressing gases, exchanging heat, and generally raising chaos.

Sun provides the required energy for all processes to occur.

Effects of air pollutants Highs and lows

Since air has mass it exerts pressure on things under it.

The atmosphere exerts more pressure at the surface than it does at higher elevations.

The highs and lows depicted on weather maps are simply areas of greater and lower pressure.

The elliptical lines shown on more detailed weather maps are lines of constant pressure, or isobars.

A two dimensional plot of pressure and distance through a high or low pressure system would appear as shown in the figure.

Effects of air pollutants Highs and lows

The wind flows from the higher pressure areas to the lower pressure areas.

On a no rotating planet the wind direction would be perpendicular to the isobars.

Since the earth rotates, an angular thrust called the coriolis effect is added to this motion.

Technical names given to these systems anti cyclones for highs and cyclones for lows.

Anticyclones are associated with good weather. Cyclones on the other hand are associated with foul weather. Tornadoes and hurricanes are the foulest of the cyclones. Wind speed is in part a function of the steepness of the pressure

surface.

Effects of air pollutants Turbulence Mechanical turbulence

In its simplest terms, we may consider turbulence to be the addition of random fluctuations of wind velocity (that is, speed direction) to the overall average wind velocity.

These fluctuations are caused, in part, by the fact that the atmosphere is being sheared.

The shearing results from the fact that the wind speed is zero at the ground surface and rises with elevation to near the speed imposed by pressure gradient.

Shearing results in tumbling, tearing motion as the mass just above the surface falls over the slower moving air at the surface.

The swirls thus formed are known as eddies feeding the larger ones. Greater the mean wind speed, the greater the mechanical turbulence

which results in easier dispersion and spreading atmospheric pollutants.

Effects of air pollutants Thermal turbulence

Heating of the ground surface causes turbulence in the same fashion that heating the bottom of a beaker full of water causes turbulence.

At some point below boiling density currents rises off the bottom. If earth’s surface is heated strongly and in turn heats the air above it,

thermal turbulence will be generated.

Stability Tendency of the atmosphere to resist or enhance vertical motion is

termed stability. It is related to both wind speed and the change of air temperature with

height. There are three stability categories

Unstable Neutral Stable

Effects of air pollutants Neutral stability

The laps rate for a neutral atmosphere is defined by the rate of temperature increase (or decrease) experienced by a parcel of air that expands (or contracts) adiabatically (without the addition or loss of heat).

This rate of temperature decrease is called the dry adiabatic lapse rate designated by Greek letter gamma with a value of approximately -1.00oC/100m .

Unstable atmosphere If the temperature of the atmosphere falls at a rate greater

than gamma, the lapse rate is said to be super adiabatic and the atmosphere is unstable.

Using figure we can see that this is so. The actual lapse rate is shown by a solid line.

Effects of air pollutants Unstable atmosphere

If we capture a balloon full of polluted air at elevation A and adiabatically displace it 100 m to elevation B, the temperature of the air inside the balloon will decrease from 21.15 to 20.15oC.

At a lapse rate of -1.25OC/10o m, the temperature of the air outside the balloon will decrease from 21.15o to 19.9o.

The air inside the balloon will be warmer than the air outside, this temperature difference gives the balloon buoyancy.

It will behave as a hot gas and continue to rise without any mechanical support effort.

Thus mechanical turbulence is enhanced and the atmosphere is unstable.

Effects of air pollutants Unstable atmosphere

If we adiabatically displace the balloon downward to elevation C, the temperature inside the balloon would rise at the rate of the dry adiabat.

Thus in moving 100 m, the temperature will increase from 21.15 to 22.15oC.

The temperature outside the balloon will increase at the super adiabatic lapse rate to 22.40oC.

The air in the balloon will be cooler than the ambient air and the balloon will have a tendency to sink.

Again mechanical turbulence (displacement) is enhanced.

Effects of air pollutants Stable atmosphere

If the temperature of the atmosphere falls at a rate less than, it is called sub adiabatic, and the atmosphere is stable.

If we again capture a balloon ait at elevation A and adiabatically displace it vertically to elevation B, the temperature of the polluted air will decrease at a rate equal to the dry adiabatic rate.

Thus in moving 100 m, the temperature will decrease from 21.15 to 20.15oC as before.

Since the ambient lapse rate is -0.5oC/100 m, the temperature of the air outside the balloon will have dropped only 20.65oC.

Because the air inside the balloon is cooler than the air outside the balloon, the balloon will have the tendency to sink.


Hence the mechanical displacement (turbulence) is initiated.

In contrast if we displace the balloon adiabatically to elevation C, the temperature inside the balloon would increase to 22.15oC while the ambient temperature would increase to 21.65oC.

In this case, the air inside the balloon be warmer than the ambient air and the balloon would tend to rise.

Again the mechanical displacement would be inhibited.


There are two special causes of sub adiabatic lapse rate.

When there is no change of temperature with elevation, the lapse rat is called isothermal.

When the temperature increases with elevation, the lapse rate is called an inversion.

The inversion is the most severe form of a stable temperature profile.

Often associated with restricted air volumes that cause air pollution episodes.

Effects of air pollutants 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 dependant in the stability of the atmosphere.

Figure shows six classical plumes along with the temperature profiles.

In each case is given as a broken line to allow comparison with the actual lapse rate given as a solid line.

In the bottom three cases, particular attention should be given to the location of the inflection point with respect to the top of the stack.

Effects of air pollutants Terrain effects

Heat islands

A heat island results from mass of material, either natural or anthropogenic, that absorbs and reradiates heat at a greater rate than the surrounding area.

This causes moderate to strong vertical convection currents above the heat island.

Large industrial complexes and small to large cities are example of places that would have a heat island.

Because of the heat island effect, the atmospheric stability will be less over a city than it is over the surrounding countryside

Effects of air pollutants Land/sea breezes

Under a stagnating anticyclone, a strong local circulation pattern may develop across the shore line of large water bodies.

During the night, the land cools more rapidly than the water. The relatively cooler air over the land flows towards the water.

Suring morning the land heats faster than the water. The air over the land becomes relatively warm and begins to

rise. The rising air is replaced by air from over the water body. The effect of the lake on stability is to impose a surface based

inversion on the temperature profile. As the air moves from the water over the warm ground, it is

heated from below.

Effects of air pollutants Values

When the general circulation imposes moderate to strong winds, valleys that are oriented at an acute angle to the wind direction channel the wind.

The valley effectively peels off the part of the wind and forces it to allow the direction of the valley floor.

Under a stagnating anticyclone, the valley will setup its own circulation.

Warming of the valley walls will cause the valley air to be warmed.

It will become more buoyant and flow up the valley. At night the cooling process will cause the wind to flow down

the valley.

Atmospheric dispersion Factors affecting dispersion of air pollutants

The factors that affect the transport, dilution, and dispersion of air pollutants can generally be categorized in terms of the emission point characteristics, the nature of the pollutant, meteorological conditions, and effects of terrain and anthropogenic structures.

Source characteristics Most industrial effluents are discharged vertically into the open air

through a stack or duct.

As the contaminated gas stream leaves the discharge point, the plume tends to expand and mix with the ambient air.

Horizontal air movement will tend to bend the discharge plume toward the down wind direction.

The effluent plume will level off between 300 and 3,000m downwind.

Effects of air pollutants Source characteristics

While the effluent plume is rising, bending, and beginning to move in a horizontal direction, the gaseous effluents are being diluted by the ambient air surrounding the plume.

As the contaminated gases are diluted by larger and larger volumes of ambient air, they are eventually dispersed toward the ground.

The plume rise is affected by both the upward inertia of the discharge gas stream and by its buoyancy.

The vertical inertia is related to the exit gas velocity and mass.

The plume’s buoyancy is related to the exit gas mass relative to the surrounding air mass.

Effects of air pollutants Source characteristics

Increasing the exit velocity or the exit gas temperature will generally increase the plume rise.

The plume rise, together with the physical stack height, is called the effective stack height.

Downward distance The greater the distance between the point of discharge and a ground

level receptor downwind, the greater will be the volume of air available for diluting the contaminant discharge before it reaches the receptor.

Wind speed and direction The wind direction determines the direction in which the

contaminated gas stream will move across local terrain. Wind speed affects the plume rise and the rate of mixing or dilution of

the contaminated gases as they leave the discharge point. An increase in wind speed will decrease the plume rise by bending the

plume over more rapidly.

Effects of air pollutants Stability

The turbulence of the atmosphere follows no other factor in power of dilution.

The more instable the atmosphere, the greater the diluting power.

Inversions that are not ground based, but begin at some height above the stack exit act as a lid to restrict vertical dilution.

Air pollution control of stationary sources Gaseous pollutants

Absorption 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.

The dissolution may or may not be accompanied by a reaction with an ingredient of the liquid.

Mass transfer is a diffusion process where in the pollutant gas moves from points of higher concentration to points of lower concentrations.

Removal takes place in three steps

Diffusion of the pollutant gas to the surface of the liquid

Transfer across the gas/liquid interface (dissolution)

Diffusion of the dissolved gas away from the interface into the liquid.

Air pollution control of stationary sources

Air pollution control of stationary sources Absorption

Structures such as spray chambers and towers or columns are two classes of devices employed to absorb pollutant gases.

In scrubbers which are a type of spray chambers, liquid droplets are used to absorb the gas.

In towers a thin film of liquid is used as an absorption medium.

Regardless of the type of device, the solubility of the pollutant in the liquid must be relatively high.

If water is the solute it generally limits the application to a few inorganic gases such as NH3, Cl2 and SO2.

Scrubbers are relatively inefficient absorbers but have the advantage of being able to simultaneously remove particulates.

Towers are much more efficient absorbers but they become plugged by particulate matter.


The amount of absorption that can take place for a nonreactive solution is governed by the partial pressure of the pollutant.

For dilute solutions as we have in pollution control systems, the relationship between partial pressure and the concentration of the gas in solution is given by Henry’s law

Pg = KHCequil

Where Pg = partial pressure of gas in equilibrium with liquid, kPa

KH = henry’s law constant, Kpa.m3/g Cequil = concentration of pollutant gas in the liquid phase,

g/m3


The given equation implies that the partial pressure of the gas must increase as the liquid accumulates more pollutant or else it will come out of solution.

Since the liquid is removing pollutant from the gas phase, this means the partial pressure is decreasing as the gas is cleaned.

This is just the reverse what we want to happen.

The easiest way to get around this problem is to run the gas and liquid in opposite directions.

This is called counter current flow.

In this manner the high concentration gas is absorbed into a liquid with a high pollution concentration.


The lower concentration gas is absorbed by liquid with no pollutants in it.

A mass balance diagram of a counter current flow absorption column is shown in the figure. The mass balance equation is

Three variables of interest in the design of a packed tower are the Gas flow rate

Liquid flow rate

Height of the tower

Air pollution control of stationary sources Adsorption

Mass transfer operation in which the gas is bonded to a solid.

It is a surface phenomenon.

The gas (the adsorb ate) penetrates into the pores of the solid (the adsorbent) but not into the lattice itself.

The bond maybe physical or chemical.

Electrostatic forces hold the pollutant gas when physical bonding is significant.

Chemical bonding is by reaction with the surface.

Pressure vessels having a fixed bed are used to hold the adsorbent.


Activate carbon (activated charcoal), molecular sieves, silica gel, and activated alumina are the most common adsorbent.

Active carbon is manufactured from nut shells (coconuts are great) or coal subjected to heat treatment in a reducing atmosphere.

Molecular sieves are dehydrated zeolites (alkali metal silicates).

Sodium silica is reacted with sulphuric acid to make silica gel .

Activated alumina is a porous hydrated aluminium oxide.


the common property of these adsorbents is a large active surface per unit volume after treatment.

They are very effective for hydrocarbon pollutants.

In addition they can capture H2S and SO2.

One special form of molecular sieve can also capture NO2.

With the exception of active carbons, adsorbents have the drawback that they preferentially select water before any of the pollutants.

Thus water must be removed from the gas before it is treated.


All of the adsorbent are subjected to destruction at moderately high temperatures (150oC for active carbon, 600oC for molecular sieves, 400oC for silica gel, and 500oC activated alumina).

They are very insufficient at high temperatures. In fact their activity is regenerated at these temperatures! The relation between the amount of pollutant adsorbed and the

equilibrium pressure at constant temperature is called adsorption isotherm.

This relation can effectively be described by Langmuir W = aCR* 1 + bCg

*

Where W = amount of gas per unit mass of adsorbent, kg/kg a,b = constants determined by experiment Cg* = equilibrium concentration of gaseous pollutant, g/m3

Air pollution control of stationary sources Combustion

When the contaminant in the gas stream is oxidizable to an inert gas, combustion is a possible alternative method of control.

Typically, CO and hydrocarbons falls in this category.

Both direct and flame combustion by afterburners and catalytic combustion have been used in the commercial applications.

Direct flame incineration is the method of choice if two criteria are satisfied.

First the gas stream must have a net heating value (NHV) greater than 3.7 MJ/m3. At this NHV, the gas flame will be autogenously (self supporting after ignition).


Below this point supplementary fuel is required.

The second requirement is that none of the by products of combustion be toxic.

In some cases the combustion by product may be more toxic than the original pollutant gas.

For example combustion of trichloroethylene produces phosgene, which was used as a poison gas in World War I.

Direct flame incineration has been successfully applied to varnish cooking, meat smoke house, and paint bake-oven emissions.


Some catalytic materials enable oxidation to be carried out in gases that have an NHV of less than 3.7 MJ/m3.

Conventionally, the catalyst is placed in beds similar to absorption beds. Frequently the active catalyst is a platinum or palladium compound.

The supporting lattice is usually a ceramic.

Aside from expense, a major drawback of the catalysts is their susceptibility to poising by sulphur and lead compounds in trace amounts.

Catalytic combustion has successfully been applied to printing-press, varnish-cooking, and asphalt-oxidation emissions.


The fundamental problem is the design of a catalytic reactor is to determine the volume and dimensions of the catalytic bed for a given conversion and flow rate.

The catalyst increases the rate of reaction at lower temperatures than are required in direct flame incineration.

The reaction is assumed to be first order reaction while the reaction rate constant k may be estimated from the Arrhenius equation for flame incineration, the reaction rate constant for catalytic incineration is highly dependant on the catalyst.

Typical catalyst operating temperatures are in the range of 25o-550oC.

The actual residence time is estimated from the total gas flow rate (contaminated gas stream plus the combustion gases) at operating temperature of the catalyst.

Air pollution control of stationary sources Flue gas desulfurization (FGD)

Falls into two broad categories Non regenerative

Regenerative

Non regenerative means that the reagent used to remove the sulphur oxides from the gas stream is used and discarded.

Regenerative means that the reagent is recovered and reused.

In terms of the number and size of system installed, non regenerative systems dominate

Air pollution control of stationary sources Non regenerative systems

There are nine commercial non regenerative systems. All have reaction chemistries based on lime (CaO), caustic

soda (NaOH), soda ash (Na2CO3) or ammonia NH3. The SO2 removed in a lime/limestone based FGD system is

converted to sulphite. The overall reaction are generally represented as Although the overall reactions are simple, the chemistry is

quite complex and well not defined. The choice between the lime and the limestone, the type of

limestone and the method of calcining and slaking can influence the gas-liquid-solid reactions taking place in the absorber.


The principal types of absorbers used in the wet scrubbing systems include venturi scrubber/absorbers, static packed scrubbers, moving bed absorbers, tray towers, and spray towers.

Spray dryer based FGD system consists of one or more spray dryers and a particulate collector.

The reagent material is typically a slaked lime slurry or a slurry of lime and recycled material.

Although lime is the most common reagent, soda ash has also been used.

The reagent is injected in droplet form into the flue gas in the spray dryer.

The reagent droplet absorbs SO2 while simultaneously being dried.


Ideally the slurry or solution droplets are completely dried before they impact the wall of the dryer vessel.

The flue gas stream becomes more humidified in the process of evaporation of the reagent droplets, but it does not become saturated with water vapour.

This is the single most significant difference between spray dryer FGD and wet scrubber FGD.

The humidified gas stream and a significant portion of the particulate matter are carried by the flue gas to the particulate collector located downstream of the spray dry vessel.

Air pollution control of stationary sources Control technologies for nitrogen oxides

Almost all nitrogen oxide (NOx) air pollution results from combustion processes.

They are produced from the oxidation of nitrogen bound in fuel, from the reaction of molecular oxygen and nitrogen in the combustion air at temperatures above 1,600 K and from the reaction of nitrogen in the combustion air with hydrocarbon radicals.

Control technologies for NOx are grouped into two categories

Those that prevent the formation of NOx during the combustion

Those that convert the NOx formed during combustion into nitrogen and oxygen.

Air pollution control of stationary sources Prevention

The processes in this category employ the fact that the reduction of the peak flame temperature in the combustion zone reduces NOx formation.

Nine alternatives have been developed to reduce flame temperature. Minimizing operating temperatures Fuel switching Low excess air Flue gas recirculation Lean combustion Staged combustion Low NOx burners Secondary combustion Water/steam injection


Routine burner tune-ups and operation with combustion zone temperatures at minimum values reduce the fuel consumption and NOx formation.

Converting to a fuel with a lower nitrogen content or one that burns at a lower temperature will reduce NOx formation.

Low excess air and flue gas recirculation work on the principle that reduced oxygen concentrations lowest he peak flame temperatures. In contrast, in lean combustion, additional air is introduced to cool the flame.

In staged combustion and low Nox burners, initial combustion takes place in a fuel rich zone that is followed by the injection of air downstream of the primary combustion zone.

The downstream combustion is completed under fuel lean conditions at a lower temperature.


Staged combustion consists of injecting part of the fuel and all of the combustion air into the primary combustion zone.

Thermal NOx production is limited by the low flame temperatures that result from high excess air levels.

Water/steam injection reduces thermal NOx emissions by lowering the flame temperature.

Post combustion Three processes may be used to convert Nox to nitrogen gas :

Selective catalytic reduction

Non catalytic reduction

Non selective catalytic reduction


The SCR process uses a catalyst bed (usually vanadium-titanium, or platinum-based and zeolite) and anhydrous ammonia (NH3).

After the combustion process, ammonia is injected upstream of the catalyst bed.

The Nox reacts with the ammonia in the catalyst bed to form N2 and water.

In the SNCR process ammonia or urea is injected into the flue gas at an appropriate temperature (870 to 1,090oC).

The urea is converted to ammonia which reacts to reduce the Nox to N2 and water.


NSCR uses a three way catalyst similar to that used in automotive applications.

In addition to Nox control, hydrocarbons and carbon monoxide are converted to CO2 and water.

These systems require a reducing agent similar to CO and hydrocarbons upstream of the catalyst.

Larger boilers have post combustion Nox controls are generally equipped with SCR.

Typical reduction capabilities of the Nox techniques range from 30 to 60 percent for the prevention methods, 30 to 50 percent for SNCR and 70 to 90 percent for the SCR systems.

Air pollution control of stationary sources Particulate pollutants

Cyclones For particle size greater than about 10 micro meter in diameter, the

collector of the choice is cyclone (see figure)

This is and inertial collector with no moving parts.

The particulate laden is gas accelerated through spiral motion, which imparts a centrifugal force to the particles.

The particles are hurled out of the spinning gas and impact on the cylinder wall of the cyclone.

They then slide to the bottom of the cone.

Here they are removed through an airtight valving system.

The standard single barrel cyclone will have dimensions proportioned as shown in the figure.

Air pollution control of stationary sources Particulate pollutants

The efficiency of collection of various particle sizes can be determined from an empirical expression and graph developed by Lapple

The value θ may be determined approximately by the following Where L1 and L2 are the length of the cylinder and cone, respectively. As the diameter of the cyclone is reduced, the efficiency of collection is

increased. However the pressure drop also increases. This increases the power requirements for moving the gas through the

collector. Since an efficiency increase will result, even if the tangential velocity

remains constant, the efficiency may be increased without increasing the power consumption by using multiple cyclones in parallel (multiclones)

Air pollution control of stationary sources Filters

When high efficiency control of particles smaller than 5 micro meter is desired, a filter may be selected as the control method.

Two types are The deep bed filter The baghouse

The deep bed filter resembles a furnace filter. A packing of fibres is used to intercept particles in the gas stream. For relatively clean gases and low volumes, such as air conditioning

systems, these are quite effective. For dirty industrial gas with high volumes, the baghouse is preferable. The fundamental mechanisms of collection include screening or

sieving (where the particles are larger than the opening between the fibres), interception by the fibre themselves, and electrostatic attraction(because of the difference in static charge on the particle and fibre).


Once a dust cake begins to from on the fabric, sieving is probably the dominant mechanism.

A particulate matter collects on the bag, the collection efficiency process increases.

The build up of the dust cake also increases the resistance to gas flow.

at some point the pressure drop across the filter bags reduces the gas flow rate to an unacceptable level and the filter bags must be cleaned.

The three methods used to clean the bags are Mechanical shaking Reverse air flow Pulse jet cleaning


Mechanically cleaned baghouses operate by directing the dirty gas into the inside of the bag.

The particulate matter is collected on the inside of the bag much in the same manner as a vacuum cleaner bag.

The bags are hung on a frame that oscillates. They are shaken at periodic intervals, ranging from 30 minutes to

more than 2 hours. The bags are arranged in groups in separate compartments that are

taken off line during cleaning. In reverse air flow cleaning, a compartment is isolated and a large

volume of gas flow is forced counter current to normal operation. The dust cake is removed by collapsing or flexing the bag. The reverse flow combined with the inward collapse of the bag

causes the collected dust cake to fall into the hopper below.


Pulse jet baghouses are designed with frame structures, called cages, that support the bags.

In contrast to the other two cleaning methods, the particulate matter is collected on the outside of the bag instead of the inside of the bag.

The dust cake is removed by directing a pulsed jet of compressed air into the bag.

This causes a sudden expansion of the bag. Dust is removed primarily by inertial forces as the bag reaches maximum

expansion. The pulse of cleaning air is at such high pressure drop and short duration that

cleaning is normally accomplished with the baghouse in line. Cleaning occurs at 2 to 15 minute intervals. Extra bags, which are normally provided to compensate for the bags that are

required in the other cleaning schemes, are not required pulse jet baghouses.

Air pollution control of stationary sources Liquid scrubbing

When the particulate matter to be collected is wet, corrosive, or very hot, the fabric filter may not work.

Liquid scrubbing might. Typical scrubbing applications include control of emission of talc

dust, phosphoric acid mist, foundry cupola dust, and open hearth steel furnace.

Liquid scrubbers vary in complexity. Simple spray chambers are used for relatively coarse particle sizes. Fro high efficiency removal of particles, the combination of a

venturi scrubber followed by a cyclone would be selected The underlying principle of operation of the liquid scrubbers is that

a differential velocity between the droplets of collecting liquid and the particulate pollutant allows the particle to impinge onto the droplet.

Air pollution control of stationary sources Liquid scrubbing

Since the droplet particle combination is still suspended in the gas stream, an inertial collection device is placed downstream to remove it.

Because the droplet enhances the size of the particle, the collection efficiency of the inertial device is higher than it would be for the original particle without the liquid drop.

The most popular collection efficiency equation is that proposed by Johnstone, Field and Tassler.

Air pollution control of stationary sources Electrostatic precipitator

High efficiency, dry collection of particles from hot gas streams can be obtained by electrostatic precipitation of the particles.

The ESP is usually constructed of alternating plates and wires. A large direct current potential (30-75 KV) is established between the wire and

the plate, ions attach to the particles, giving them a negative charge. The particles then migrate toward the positively charged plate where they stick. The plates are rapped at frequent intervals and the agglomerated sheet falls to a

hopper. Unlike baghouse, the gas flow between plates is not stopped during cleaning. The gas velocity through the ESP is kept low (less than 1.5 m/s) to allow particle

migration. Thus the terminal settling velocity of the sheet is sufficient to carry it to hopper

before it exits the precipitation. The classic ESP efficiency equation is the one proposed by Deutsch


One operational problem of ESPs is of particular note. Fly ash is a generic term used to describe the particulate matter

carried in the effluent gases from furnaces burning fossil fuels. ESPs are often used to collect fly ash. The strongest force holding fly ash to the collection plate is

electrostatic and is caused by the flow of current through the fly ash.

The fly ash acts as a resistor, hence, resists the flow of current. This resistance to current flow is called the resistivity of the fly ash. It is measured in units of ohm.cm. If the resistivity is too low (less than 104 ohm.cm), not enough

charge will be retained to produce a strong face and the particles will not stick to the plate.


Conversely, and often more importantly, if the resistivity is too high (greater than 1010 ohm.cm), there is an insulating effect.

The layer of fly ash breaks down locally and a local discharge of current (back corona) from the normally passive collection electrode occurs.

This discharge lowers the spark over voltage and produces positive ions that decrease particle charging and hence, collection efficiency.


The presence of SO2 in the gas stream reduces the resistivity if the fly ash.

This makes particle collection relatively easy.

However, mandate to reduce SO2 emissions has frequently been satisfied by switching the low sulphur coal.

The result has been increased particulate emissions.

This problem can be resolved by adding conditioners such as SO3 and NH3 to reduce the resistivity or by building larger precipitators.

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