WELCOME

A PRESENTATION ON

Techniques to controlGaseous & Particulate

Pollution

Prepared ByShristi Soni

Our Focus:- Introduction

Methods to control gaseous pollutant

Control devices to control gaseous pollution

Control devices for Particulate Pollution

Conclusion

INTRODUCTION1. A substance in the air which causes harm to humans and environment

is known as pollutant.

2. Pollutants such as sulphur dioxide, hydrogen sulphide, and nitrogen dioxide combine with moisture in the air to form acids that attack and damage library material are known as gaseous pollutants.

3. Smoking, cooking, and off-gassing from unstable materials (cellulose nitrate film, paint finishes, fire-retardant coatings, and adhesives) may also produce harmful gaseous pollutants.

4. Particulate pollutants, such as soot, dirt, and dust abrade, soil, and disfigure materials. Dust and dirt that have absorbed gaseous pollutants from the air become sites for harmful chemical reactions when they settle on library material. The pollution caused by this is known is particulate pollution.

GASEOUS SUBSTANCEGases and vapours can be categorized under gaseous pollutants.

DUST FUMES DROPLETS OF MIST

GASES VAPOURS

Cement Iron Oxide Sulphuric acid SO2 Gasoline

Coal Zinc Oxide Chromic acid NO2 Trichloroethylene

Ores Lead Oxide

Oil CO Perchloroethyl

Grains ------ Grease(From cooking and smoking of metals

H2S Toulene

AIRBORNE CONTAMINANTS

METHOD TO CONTROL GASEOUS

POLLUTANTSADSORPTIONABSORPTION

REDUCTION SYSTEMCONDENSATION

ADSORPTIONAdsorption 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

ABSORPTION 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

REDUCTION SYSTEMSReduction 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

CONDENSATIONCondensation 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 PROCESS

CONTROL DEVICES FOR GASEOUS POLLUTANTS

CYCLONESCyclones 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 PROCESS

INCINERATORSIncineration involves the high efficiency combustion of certain solid, liquid, or gaseous wastes. The reactions may be self sustaining based on the combustibility of the waste or require the addition of fuels.

They may be batch operations or continuous as with flares used to burn off methane from landfills; and, they may incorporate secondary control methods and operate at efficiency levels of 99.99%, as with hazardous waste incinerators. Combustion temperatures, contact time, and mass transfer are the major parameters affecting incineration performance.

• It has, High destruction efficiencies possible • Variations in fuel content of waste • Transition among wastes require significant control changes • Good for gases, liquids, and solids

INCINERATION PROCESS

CONTROL DEVICES FOR PARTICULATE

POLLUTION

ELECTROSTATIC PRECIPITATORSAn 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

FABRIC FILTERSFabric filters, or baghouses, 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

VENTURI SCRUBBERSVenturi scrubbers use a liquid stream to remove solid particles. In the venturi scrubber, gas laden with particulate matter passes through a short tube with flared ends and a constricted middle. This constriction causes the gas stream to speed up when the pressure is increased.

A water spray is directed into the gas stream either prior to or at the constriction in the tube. The difference in velocity and pressure resulting from the constriction causes the particles and water to mix and combine. The reduced velocity at the expanded section of the throat allows the droplets of water containing the particles to drop out of the gas stream. Venturi scrubbers are effective in removing small particles, with removal efficiencies of up to 99 percent.

One drawback of this device, however, is the production of wastewater.

VENTURI SCRUBBERS

CONCLUSIONThe best control measure, of course, is prevention. However, as long as there are fossil fuel emissions from our coal-burning factories and gas-burning automobiles, there will be air pollution.

The key to easing future impacts is control. The sources of air pollution are many, although most authorities identify sulphur dioxide, nitrogen dioxide, ozone and particulate matter as the major pollutants. Identification of sources of pollution offers opportunities for control