ENVIRONMENTAL

ENGINEERING

BY

D.RAVIKUMAR

AIR POLLUTION

UNIT – I

Air pollution:

Air pollution is defined as “presence in the outdoor atmosphere of one are more contaminents such as dust ,fumes , gas ,mist ,odour , smoke or vapour, in quantities, of characteristics, and of duration such as injurious to human, plant or animal life or to property, or which unreasonably interfers with the comfortable enjoyment of life and property.

SOURCE OF POLLUTION

LINE AND AREAL SOURCE

NATURAL AND ARTIFICIAL SOURCES

STATIONARY SOURCES

MOBILE SOURCES

ARTIFICIAL SOURCES:

LINE SOURCES: Smoke from highways ,trains ,channel vessels comes under line sources.

AREAL SOURCES: The smoke and dust from domestic combustion of fuel in residential areas , offices, apartments, hotels, hospitals, and institutions. Incineration and open burnings comes under areal sources too.

STATIONARY SOURCES:

Industrial process chimneys, power plants, oil refineries, pulp and paper industries.

NATURAL SOURCES: Release from plants and animals, biological decay of fossil

fuels etc……., oceans

Forest fires , oxidation of CH4 and decay and synthesis of chlorophyll , photochemical oxidation.

Volcanoes , oceans and biological decay by sulphide bacteria, eruptions , biological action in swamps and marshes.

Lightning and electrical storms ,bacterial action in soil ,fossil fuels , biomass burning.

Salt particals from seas is also an important pollutant which damages the materials

EFFECTS ON HUMAN BEINGS

Exposure to air pollution is associated with numerous effects on human health, including pulmonary, cardiac, vascular, and neurological impairments.

The health effects vary greatly from person to person. High-risk groups such as the elderly, infants, pregnant women, and sufferers from chronic heart and lung diseases are more susceptible to air pollution.

Children are at greater risk because they are generally more active outdoors and their lungs are still developing.

Acute effects are usually immediate and often reversible when exposure to the pollutant ends. Some acute health effects include eye irritation, headaches, and nausea

CONTOL METHODS:

Source Control Technology :

Air quality management sets the tools to control air pollutant emissions.

Control measurements describes the equipment, processes or actions used to reduce air pollution.

The extent of pollution reduction varies among technologies and measures.

The selection of control technologies depends on environmental, engineering, economic factors and pollutant type.

Source correction methods:

oRaw material changes

oProcess changes

oEquipment modification & replacement

PARTICULATE CONTROL DEVICES

GRAVITTY-SETTING CHAMBERS

Used to remove particles with size greater than 50 μm.oVelocity of flue gas reduced in large chamber.oParticles settle under gravitational force.

Cyclones operate to collect relatively large size particulate matter from a gaseous stream through the use of centrifugal forces. Dust laden gas is made to rotate in a decreasing diameter pathway forcing solids to the outer edge of the gas stream for deposition into the bottom of the cyclone. Efficiencies of 90% in particle sizes of 10 microns or greater are possible.

Performance & Collection Efficiency

• Linear increases with: particle density, gas stream velocity, and rotational passes

• Linear decrease with fluid viscosity

• Exponential increase with particle diameter

CYCLONE

FABRIC FILTERS

Fabric filters, or bag houses, remove dust from a gas stream by passing the stream through a porous fabric. The fabric filter is efficient at removing fine particles and can exceed efficiencies of 99 percent in most applications.

The selection of the fiber material and fabric construction is important to baghouse performance. The fiber material from which the fabric is made must have adequate strength characteristics at the maximum gas temperature expected and adequate chemical compatibility with both the gas and the collected dust.

One disadvantage of the fabric filter is that high-temperature gases often have to be cooled before contacting the filter medium.

FABRIC FILTERS

An electrostatic precipitator (ESP) is a particle control device that uses electrical forces to move the particles out of the flowing gas stream and onto collector plates. The ESP places electrical charges on the particles, causing them to be attracted to oppositely charged metal plates located in the precipitator.

The particles are removed from the plates by "rapping" and collected in a hopper located below the unit. The removal efficiencies for ESPs are highly variable; however, for very small particles alone, the removal efficiency is about 99 percent.

ELECTROSTATIC PRECIPITATOR

ELECTROSTATIC PRECIPITATOR

VENTURI SCRUBBER:

A converging/diverging section of duct. The converging section accelerates the gas

stream to high velocity. When the liquid stream is injected at the throat, which is the point of maximum velocity, the turbulence caused by the high gas velocity atomizes the liquid into small droplets, which creates the surface area necessary for mass transfer to take place.

The higher the pressure drop in the venturi, the smaller the droplets and the higher the surface area.

The penalty is in power consumption. For simultaneous removal of SO2 and fly ash,

venturi scrubbers can be used.

VENTURI SCRUBBER:

PACKED BED SCRUBBER:

Consists of a tower with packing material inside in the shape of saddles, rings, or some highly specialized shapes designed to maximize contact area between the dirty gas and liquid.

Packed towers typically operate at much lower pressure drops than venturi scrubbers and are therefore cheaper to operate. They also typically offer higher SO2 removal efficiency.

The drawback is that they have a greater tendency to plug up if particles are present in excess in the exhaust air stream.

PACKED BED SCRUBBERS

SPRAY TOWERS:

The simplest type of scrubber.

It consists of a tower with spray nozzles, which generate the droplets for surface contact.

  Spray towers are typically used when

circulating a slurry .

The high speed of a venturi would cause erosion problems, while a packed tower would plug up if it tried to circulate a slurry.

SPRAY TOWERS:

GENERAL METHOD TO CONTROL GASEOUS POLLUTANTS

ADSORPTION

ABSORPTION

REDUCTION SYSTEM

CONDENSATION

Adsorption involves the interaction between gaseous contaminants and its surface of a solid adsorbent.

In this phenomenon molecules from a gas or liquid will be attached in a physical way to a surface. The binding to the surface is usually weak and reversible. The most common industrial adsorbents are activated carbon, silica gel, and alumina, because they have enormous surface areas per unit weight.

Activated carbon is the universal standard for purification and removal of trace organic contaminants from liquid and vapour Carbon adsorption uses activated carbon to control and/or recover gaseous pollutant emissions. In carbon adsorption, the gas is attracted and adheres to the porous surface of the activated carbon.

ADSORPTION:

ADSORPTION

Gaseous contaminants that are soluble in aqueous liquids can be removed in absorbers. This is one of the main mechanisms used for the removal of acid gas compounds (e.g., sulphur dioxide, hydrogen chloride, and hydrogen fluoride) and water soluble organic compounds (e.g., alcohols, aldehydes, organic acids).The contaminant gas or vapour is absorbed from the gas stream as it comes into contact with the liquid.

Biological treatment systems are termed either biological oxidation or biofilter systems. Regardless of the term, the fundamental processes involved is the collection of contaminants on the surface of a mediathat contains viable microorganisms. The contaminant is metabolized by the organism and carbon oxide and water vapour is re-emitted.

All absorption processes operate best when the gas and liquid temperatures are low. Gas and vapour phase contaminants are most soluble under cold conditions.

ABSORPTION

ABSORPTION

Reduction systems are used primarily for the destruction of NOx compounds emitted from combustion processes. These systems include selective noncatalytic reduction systems (SNCR) and selective catalyticreduction systems (SCR).

In both types of systems, a chemically reduced form of nitrogen is injected into the gas stream to react with the oxidized nitrogen compounds, namely NO and NO2. The reactionsbetween the reduced and oxidized forms of nitrogen result in molecular N2, the major constituent of clean air

SNCR and SCR systems are believed to be effective for NO and NO2. It is important to note that in themajority of combustion systems, 90% to 95% of the NOX in the gas stream is present as NO, and theremaining is present as NO2.

REDUCTION SYSTEM:

Condensation systems are used exclusively for the recovery of organic compounds present at moderate to high concentrations in industrial process effluent gas steams.

The most common type of condensers are those using cooling water in direct contact or indirect contact vessels. Refrigeration and cryogenic systems are used primarily for the high efficiency recovery of high value contaminants. Condensation systems reduce the contaminant concentration in the gas stream to a concentration equivalent to the vapor pressure of the compound at the operating temperature of the condenser.

Most condensers operate on systems with only a single contaminant compound or a mixture of compounds that does not usually require separation (i.e. gasoline).

CONDENSATION:

AIR EMISSION STANDARDS