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AIR POLLUTION AND TREATMENT PROCESSES 1

INTRODUCTION

Composition of the Atmosphere

The earth’s atmosphere consists of an

abundance of chemical elements. Its main

constituents are nitrogen (volume ratio 78.1%)

and oxygen (21.0%). Water vapor is also highly

prevalent in the atmosphere though quantities

vary. Other than this, existing in extremely minor

quantities are trace gases.

Should wind shift in the atmosphere be weakly defined, gaseous components

would likely undergo a diffusive separation. This is due to the fact that individual

elements differ in mass from one another, resulting in heavier elements sinking to lower

regions, while lighter elements would rise to regions higher in the atmosphere.

However, in reality, due to the existence of wind conditions in the regions below

the tropopause, 11 kilometers above ground, the elements are well mixed. The mixing

ratio is relatively the same. However, traces gases, excluding noble gases, are not

equally distributed throughout the atmosphere by source or photochemical reaction

within the atmosphere. But rather, their concentrations and distribution differ, effected

by such factors as altitude, latitude, longitude, and time of year. Concentration also

differ depending on place, such as in urban areas.




AIR POLLUTION AND AIR POLLUTANTS

When the substances which have been emitted into the atmosphere exist

throughout the atmosphere, over and above the natural physical diffusion and

deposition functions, chemical elimination functions, and biological purification

functions, is dispersed throughout the atmosphere and their volume exceeds natural

conditions, they have a direct or indirect influence on living beings, including human

being. This is called air pollution, air pollutants are generally defined as those

substances which alter the composition of the natural atmosphere.

The Types of Air Pollutants

Air pollutants are divided into primary pollutants like carbon monoxide, sulfur

dioxide, hydrocarbon species, dust and soot, which are emitted directly by air pollutant

sources, and secondary pollutants like nitrogen dioxide, photochemical ozone, and

aerosols, which are created by chemical changes which occur in the atmospheric

environment. Physically, they are also divided into gas particles and particulate matter.

The Sources of Air Pollutants

Sources of air pollutants are divided into those

which derive from natural causes and those which are

brought about by human power. These are called natural

sources and anthropogenic sources. Some of the major

natural sources include volcanic emissions, forest fires,pollen scattering, sandstorms and yellow sand being

blown up from the surface of the earth by winds, sea salt

particles being blown up from the surface of the sea by

winds, and ozone which has subsided from the




stratosphere to the troposphere. Some of the major anthropogenic sources include

factories and thermal power stations, substances emitted due to the burning of fossil

fuel in engines like those of automobiles, gases and particulate matter created in the

production process, and suspended particulate matter and chemical substances

created in the process of waste disposal.

1. Thermal Power Station

The thermal power station is an

important facility to support our civilization, but

is also causing air pollution. The technology of

thermal power generation includes the steam

power station which makes electricity byrotating the electrical generator via steam turbines that are driven by high

pressure steam generated by the boiler, the electric generation which uses

internal combustion engines, and that which combines the former two

technologies. The sources of air pollutant are the former two systems which use

coal, oil and natural gas as a fuel.

2. Source of Dust

The pulverization or crushing which

fragments a solid material by mechanical

force and makes fine particles is the oldest

mechanical operation of humankind, By this

operation, the handling of solid becomes

easier and the rate of reaction and dissolution

and the catalyst reactivity are improves

because the surface area of solid is increased.These effects are readily seen in such processes as coal pulverization in

the pulverized coal combustion facility and ore processing in the puddling. Also,

the polishing is an operation to smooth out the surface of solid materials.

The particulate matter that is generated and scattered during smashing

or other mechanical operations, or associated with the accumulation of the




materials obtained through these processes, is called the dust and is attracting

attention as a source of air pollution.

The major source of dust includes the pulverizing equipment, polishing

machine, sieve, particulate transport facility, pile of ore or soil, and cokes oven.

Generally, the size of dust generated by the mechanical operation such as

pulverization or crashing is relatively large and is about a few µ m and its range is

rather broad, unlike the fine fume generated by the chemical reaction.

3. Chemical Process

The chemical industry is an essential factor for the prosperity of

humankind, nut it has been causing many air pollution problems because it deal

with various chemical substances, As this kind of chemical industry is diverse, onlysome typical examples of incidence of the emission of air pollutant will be

described below.

A. Cement Industry

Dust scattering from the cement

industry has been often a subject of

claims. Cement is a generic name of

inorganic colloid containing plaster,

lime, magnesia, volcanic ashes, etc.

There is a dry process and a wet

process in the manufacturing method of cement.

The rotary kiln is the largest source of air pollutant in the

cement industry, and generates air pollutants such as cement dust,

sulfur oxides, nitrogen oxides and heavy metal fumes.

The major air pollutant in this industry is the dust generated fromprocesses such as crushing and mixing, silo storage, transportation,

burning process, and drying, and the sulfur oxides and nitrogen oxides are

also generated from the rotary kiln and rotary dryer. Particularly, the

concentration of nitrogen oxides in the exhaust gas of the rotary kiln is




quite high. The pollutant that is attracting much attention recently, is the

particle, and it is presently handled by the electrostatic precipitator.

B. Sulfuric Acid Industry

In the production of sulfuric acid, contact process is adopted. In

this process, the elemental sulfur is oxidized by air to sulfur dioxide, then

the latter is oxidized to sulfur trioxide

using vanadium pentoxide as a catalyst.

Then it is made into contact with sulfuric

acid to obtain highly concentrated

sulfuric acid. Sulfur dioxides has been

obtained by burning iron sulfide before,but is now obtained by the method using elemental sulfur which is a

byproduct of desulfurization of the heavy oil.

The major air pollutants produced are sulfur dioxide and mist.

C. Nitric Acid Industry

In the production of nitric acid, a method using ammonia as a raw

material is adopted. In this process,

ammonia is oxidized by air in high

temperature to form nitrogen monoxide,

at the existence of platinum catalyst.

Nitrogen dioxide, obtained by oxidizing

the mixture of this nitrogen monoxide

and air, is then absorbs into the water to form nitric acid.

The major air pollutants emitted from this process are nitrogen

monoxide and nitrogen dioxide exhausted from the absorption tower.




D. Chlorine Production

Chlorine is produced by the

electrolysis of sodium chloride water

solution, and it is formed at the positive

electrode. High temperature chlorine

formed is cooled, and then washed by

nitric acid to dry. It is then liquefied

and shipped. Major cause of chlorine release is the discharge of gas that

is not liquefied.

4. Metal Refinery

A. Copper Refinery

Major air pollutants from the

copper refinery process are sulfur dioxide

and particles.

B. Zinc Refinery

The major ore containing zinc is zinc blende (ZnS), so zinc is also

mined as a sulfide ore like copper. In the refinery of zinc blende, the refined

ore obtained through the concentration process is roasted to remove sulfur

as sulfur dioxide. A sulfur dioxide is an air pollutant; it is used to manufacture

sulfuric acid.

C. Aluminum Refinery

This refinery work has discharged hydrogen fluoride in the molten

salt electrolytic process and caused air pollution problems. Carbonmonoxide, tar and alumina particles are also generated in this process.

D. Refinery of Titanium and Silicon

Titanium is a super heat resisting metal with an excellent anti-

corrosion characteristics, and its alloy is used as a material for gas turbine




blade. Also, silicon, germanium, and zirconium are quite useful as a material

for semiconductors and special purpose materials. Those elements are easily

combined with chlorine to form chloride, and the chlorides of these

elements are refined by hydrogen reduction after removing impurities

through distillation. Primary air pollutant emission by those refinery processes

is chlorine.

5. Petroleum Refinery

The sources of air pollutant in the oil

refinery to be noted are the storage tanks for

crude oil and its products. The tanks are

classified into a fixed roof type and a floatingroof type, and both types of tank release

hydrocarbon vapor as a result of the

evaporation of oil.

Furthermore, air pollutants are

generated in the combustion process of heating furnace (pipe still), catalyst

recovery furnace and boiler in the oil refinery. Particularly, carbon particles and

fine particulate metal salts could be contained in the exhaust gas of the catalyst

recovery furnace. The source substance of offensive odor in the oil refinery is

mainly hydrogen sulfide and mercaptane.

6. Transportation

Gasoline-engine vehicles

Exhaust emission is what after the

gasoline is burned in the engine, isdischarged from the muffler in the form of

CO2, NOx and H2O as the products of

complete combustion, and CO and HC as

the products of incomplete combustion.

Next is blow-by gas which leaks into the crankcase through a gap between the




piston and cylinder wall and contains unburned fuel, mostly HC. Also, there is the

evaporative emission. Because gasoline is a highly volatile fuel, it evaporates

from the fuel supply equipment such as the fuel tank and the carburetor,

polluting the atmosphere.

Diesel-powered vehicles

Because diesel fuel’s volatility is lower than gasoline’s, pollution of the air

from emission evaporation after the fuel is burned is remarkably low. Also, as only

air is pushed into the piston ring part in the early stages of burning, blow-by gas

emissions are low resulting in a remarkably low concentration of HC. Therefore,

most of the emissions from diesel engines that pollute the air are discharges from

the exhaust. An air-excess condition in burning occurs mostly and there is littledischarge of CO and HC. However, on the other side is much NOx emission, and,

also, black smoke and particulate matter cannot be avoided because of the

partial lack of oxygen in the latter part of the burn period. Moreover, because

there is a high percentage of sulfur in diesel fuel than in gasoline, SO2 is

discharged, too.

7. Sources of Offensive Odors

WORKSITES

Livestock and farming industry

Chemical processing plants

Paint factories

Print works

Food processing plants

Petrol refineries

Chemical plants

Sewage Treatment plants

Raw sewage treatment facilities

Craft pulp manufacturing




EFFECTS OF AIR POLLUTION

Health Effects on Human

The first target organs attacked by

air pollutants are respiratory system

organs. Of the air pollutants inhaled, the

larger particulate matters are caught in

the nasal cavity. However, so-called

suspended particulate matters,

measuring 10 µm or less in diameter pass

through the nasal cavity to reach the

trachea and/or the alveola. Of the

gaseous substances, sulfur dioxide,

which is soluble in water, is absorbed mainly in the upper airway, causing

bronchitis or asthma. On the other hand, ozone, nitrogen oxides and other

insoluble gaseous substances advance deep in the lungs, causing asthma or

chronic bronchitis or possibly pulmonary emphysema. Also, carbon monoxide,

when coming into contact with hemoglobin contained in the blood in alveola,

disturbs transportation of oxygen by the blood because the substance combines

with hemoglobin more easily than oxygen.

Effect on Vegetation

Plant damage resulting from air

pollution largely entails the visibledamage on leaf surfaces, etc. Although

visible damage usually appears after

short term exposure to high

concentrations of pollutant gases,

plant’s fundamental physiological functions including photosynthesis, respiration,




transpiration, etc., are affected and plant development stunted by long term

exposure to low concentrations of pollutant gases as well. Photosynthesis is

particularly sensitive to pollutant gases and is affected even by extremely low

concentrations. Hydrogen fluoride (HF) is most toxic followed by gaseous chlorine

(Cl2), ozone (O3), sulfur dioxide (SO2), and nitrogen dioxide (NO2).

Effects on materials and cultural properties

The effects of air pollution are not

only on people’s health and living things

such as plants, but also extend to man-made items such as materials like metals

and cultural properties. Hydrogen sulfide

greatly affects silver and copper, and

sulfuric acid has a large effect on iron,

steel and stone. Also, ozone has a large

effect on high molecule substances. In this

way, depending on the material and its

type, the degree of the effect of air pollutants will be different.

Visibility degradation

When visibility is degraded, the surrounding scenery becomes difficult to

see clearly compared with normal

conditions. People’s first experience ofair pollution is visibility degradation.

The light from the sun deteriorates

through its being absorbed and

scattered due to aerosols, absorption by




air pollutants and water vapor, scattering by airborne particles, and so forth. The

main cause of visibility degradation due to air pollution are aerosol and gases in

the atmosphere, but the visibility conditions can differ greatly due to

atmospheric conditions such as humidity, the optical characteristics of the target

matter, and the strength and distribution of the light at the time in question.

When air pollution is severe, the atmosphere appears to be colored but

the color can vary depending on the type of pollution. Air pollution appears

black when it is due to soot from the burning of fossil fuels, but has a whitish hue

when it is due to photochemical air pollution.




AIR POLLUTION MEASURING METHOD

Measurements are taken using collection-analysis (manual analysis),

concentration meters, continuous (automatic) analyzers, and the other simple

measuring equipment.

MEASURING METHOD FUNCTION

Manual Analysis Applicable to most of

compounds that can be

used with ordinary

equipment such as

spectrophotometers and gas

chromatographs, because

once has been collected at

fields, it has to be carried

back to the laboratory for

analysis

Concentration

meter

Obtains on-site values in a

portable form for the target

gas

Continuous analyzer Used for fixed point

observations, and records

both the time course and

the mean concentration

Simple Measuring

Equipment

Inexpensive, easy to operate

and aims to be able to takemeasurements on-site




AIR POLLUTION CONTROL

Reducing the emission of air pollutants into the atmosphere is

indispensable for air pollution control. Many technologies are applied toward this

purpose and the technology for the collection and treatment of pollutants such

as dust collector and exhaust gas desulfurization and denitrification equipment is

spreading widely. However, these are not the only means of air pollution control.

Among many other control techniques are the improvement of fuels,

combustion management, energy saving and process control.

Methods to prevent air pollution include:

1. Separation of damaged areas and pollution sources. (Aim at reduction by

widening the distance)

2. Heighten ventilation pipes and chimneys to avoid high concentrations on

ground level.

3. Attachment of dust collecting equipment and toxic gas treatment.

(Automobiles: three way catalytic converter)

4. Change over to better quality fuel. (Automobiles: low-pollution vehicles) as

well as many other measures.

Better fuel countermeasure

By the use of better fuel controls, the creation of air pollutants is reduced.

Ways to convert types of fuel (changing from burning coal to burning heavy oil

or gas) and the method of using the better quality fuels within the same fuel type

(switching from those with high sulfur content to low sulfur content) are being

adopted.

The effects of using better fuel

1. Gaseous fuels (natural gas, LPG, etc.)

a) Burns with little excess air, burns efficiently

b) Because they do not contain sulfur or ash, they do not create sulfur

oxides or soot.




c) Because they do not contain nitrogen they do not create fuel NOx

(however we must pay attention to Thermal NOx)

2. Liquid fuels (heavy oil, light oil, kerosene, etc.)

a) Heavy oils create soot, but not as much as coal.

b) Because heavy oils contain sulfur and nitrogen, they create sulfur

oxides and nitrogen oxides. In this case, flue gas desulfurization and

denitrification are performed.

c) Low-sulfur heavy oils, made by removing sulfur by desulfurization,

produce few sulfur oxides and nitrogen oxide.

3. Solid fuels (bituminous coal, anthracite coal, lignite, coke)a) Because they contain a lot of ash and sulfur, large quantities of soot

and sulfur oxides are produced.

b) Because they also contain nitrogen, they also produce nitrogen oxides.

c) Large furnaces use pulverized coal firing and fluidized combustion, and

by the exhaust treatment gas, soot and dust sulfur oxides and nitrogen oxides are

removed.

Combustion control

Combustion control refers to the operation of combustion furnaces so that

the combustion efficiency is improved by rationalizing the combustion method

and equipment maintenance.

Dust Collector

Dust collecting is the operation which collects and separates the particles

contained in processing gases. Generally, the dust collector is used in the last

step of the production process and it is called an air pollution control equipment.




However, it is used in various fields with divergent purposes such as preservation

of the work environment, purifying exhaust gas, recovery of useful particles and

improving the purity of processes. Generally, fine particles are generated during

mechanical operations such as pulverizing in addition to soot particles occurring

during the combustion process, and these are generally called “dust”.

Flue Gas Desulfurization Equipment

Flue gas desulfurization is categorized into two separate procedures, the

wet method and the dry process. The majority of equipment which s currently in

operation utilizes the wet met method.

Denitrification of Exhaust Gas

Air pollution resulting from nitrogen oxides (NOx) can be improved through

the use of the low NOX combustion and the flue gas denitrification.

Reaction Agent Method Application

Ammonia Catalytic

Reduction Process

NH3, (NH2)2CO Utilizes a catalyst,

causes a

reaction

between

ammonia and

NOx, and

separates the

non-harmful

nitrogen and

water vapor

Boilers, gas

turbine, diesel

garbage

incinerator and

heating furnaces

Non-catalytic

reduction Method

NH3, (NH2)2CO Uses ammonia as

a reductionagent, which is

then sprayed into

the exhaust gas,

reduces NOx into

N2 using a gas

phase non-

Petroleum

heating furnaces,garbage

incinerators




catalyst

Activated Carbon

(simultaneous

desulfurization –

denitrification)

NH3 Adsorbs SO2 from

the exhaust gas

using activated

carbon (or

activated cokes),

the NOx is

decomposed to

N2 by the NH3

agent utilizing

activated carbon

(activated cokes)

as its catalyst

Boilers,

incinerators

Electron Beam

Irradiation Method

NH3 NH3 is added to

the exhaust gas

and irradiated

with an electron

beam. The Sox is

converted into

ammonium

sulfate and the

NOx is converted

into ammonium

nitrate.

Still undergoes

testing for

verification of

the method

Oxidation ReductionMethod

O3, ClO2,NA2SO3

NO is oxidizedusing ozone or

chlorine dioxide

and adsorbed by

a soda solution.

Small boilers andheater furnaces

Toxic Gas Treatment

Toxic gas emissions come in diverse forms depending on the source of this

emission. When selecting the collection method for polluted gas and the

treatment methods, it is necessary to take into consideration the properties of

the raw gas, at the same time taking into account the facility site, surrounding

environment, and economical efficiency. The thermal combustion method ,



catalytic method , condensation method , adsorption method and absorption

method are just some of the processes found when categorizing the treatment

methods available.

Hydrocarbon Species Emission Control

Hydrocarbon species substances exist in the atmosphere as vapor or mist.

They are mainly fuel types and various solvents. Treatment methods of

hydrocarbon species are broadly divided into the condensation method,

absorber method, adsorption method, direct combustion method and catalyzer

oxidation method.

Offensive Odor Deodorization Method

Methods used for the removal of offensive odor include the thermal

combustion method, adsorption method, wetscrubber method, biological

deodorization method, ozone oxidation method, and the use of deodorants.

These methods and their respective devices are selected based upon the

physic-chemical qualities of the substance to the deodorized and the conditions

of the facilities exuding the odor.

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