Air Pollution Control Measures:

Air pollution occurs when the air contains gases, dust, fumes or odour in harmful amounts.

The substances that cause air pollution are called pollutants.

5 Major Pollutants:

1.) Carbon Monoxide

2.) Sulfur Dioxide

3.) Nitrogen Dioxide

4.) Particulate Matter

5.) Ground Level Ozone

The atmosphere has several built-in self cleaning processes such as dispersion, gravitational settling, absorption, rain-washout, etc to cleanse the atmosphere.

However, control of contaminants at their sourcelevel is a desirable and effective method throughpreventive or control technologies.

To be able to identify and describe the different air pollution control devices which are used by different industries.

Flue-gas Desulfurization

To determine the effectiveness of this devices in controlling pollution.

To familiarize with the mechanisms or processes on how this modern technologies work.

Objectives:

Flue-gas

desulfurization (FGD):

Flue gas is the gasexiting to theatmosphere via a flue,which is a pipe orchannel for conveyingexhaust gases from afireplace, oven,furnace, boiler orsteam generator

Flue-gas desulfurization (FGD) is a set of technologies used to remove sulfur dioxide (SO2) from exhaust flue gases of fossil-fuel power plants, and from the emissions of other sulphur oxide emitting processes.

It is a control device that absorbs and react using the alkaline reagent to produce a solid compound.

The largest part of the flue gases from most fossil fuel combustion are..

Uncombusted Nitrogen

Carbon Dioxide (CO2)

Water Vapor (H20)

Nitrogen Oxides (NOx)

Sulfur Dioxide (SO2)

Particulate Matter

Flue-gas desulfurization (FGD):Mechanism of Pollutant Removal

How does it work?

•Wet scrubbing using a slurry of alkaline sorbent,

usually limestone or lime, or seawater to scrub gases.

•Spray-dry scrubbing using similar sorbent slurries.

oWet sulphuric acid process recovering sulfur in

the form of commercial quality sulphuric acid.

oSNOX Flue gas desulfurization removes sulfur

dioxide, nitrogen oxides and particulates from flue

gases.

oDry sorbent injection systems.

o Works via the contact of target compounds orparticulate matter with the scrubbing solution.Solutions may simply be water (for dust) orsolutions of reagents that specifically targetcertain compounds.

o A wet scrubber is used to clean air, fuel gas or other gases of various pollutants and dust particles.

Wet scrubbing

o Removal efficiency of pollutants is improvedby increasing residence time in the scrubber or bythe increase of surface area of the scrubbersolution by the use of a spray nozzle, packedtowers or an aspirator.

o Wet scrubbers may increase the proportion of water in the gas, resulting in a visible stack plume, if the gas is sent to a stack.

Fig.1 wet scrubber

Scrubber: a device that removes impurities from gases.Slurry: flowable suspension of small particles in liquid.

Dry scrubbing

No moisture is added, while in others only the

amount of moisture that can be evaporated in the

flue gas without condensing is added. Therefore,

dry scrubbers generally do not have a stack steam

plume or wastewater handling/disposal

requirements. Dry scrubbing systems are used to

remove acid gases (such as SO2 and HCl) primarily

from combustion sources.

Dry scrubbing systems

Often used for the removal of odorous and corrosive gases from wastewater treatment plant operations.

The medium used is typically an activated alumina compound impregnated with materials to handle specific gases such as hydrogen sulfide.

Wide range of removal for other odorous compounds such as methyl mercaptans, aldehydes, volatile organic compounds, dimethyl sulfide, and dimethyl disulfide.

Dry sorbent injection

Involves the addition of an alkaline material (usually hydrated lime or soda ash) into the gas stream to react with the acid gases.

The acid gases react with the alkaline sorbents to form solid salts which are removed in the particulate control device. These simple systems can achieve only limited acid gas (SO2 and HCl) removal efficiencies.

These devices have been used on medical waste incinerators and a few municipal waste combustors.

Spray Dryer Absorbers

•The flue gases are introduced into an absorbing tower (dryer) where the gases are contacted with a finely atomized alkaline slurry. Acid gases are absorbed by the slurry mixture and react to form solid salts which are removed by the particulate control device.

•The heat of the flue gas is used to evaporate all the water droplets, leaving a non-saturated flue gas to exit the absorber tower.

These devices have been used on industrial andutility boilers and municipal waste incinerators.

Absorber

The flue gas is passedthrough a cartridge which isfilled with one or severalabsorber materials and hasbeen adapted to thechemical properties of thecomponents to beremoved. The absorbermaterial has to be replacedafter its surface is saturated.

Mercury Removal

Usually addition of the halogens to the flue gas areused for this purpose. The type of coal burned as well asthe presence of a selective catalytic reduction unit bothaffect the ratio of elemental to oxidized mercury in theflue gas and thus the degree to which the mercury isremoved.

Mercury is a highly toxic element commonly found in coal and municipal waste. Wet scrubbers are only effective for removal of soluble mercury species, such as oxidized mercury.

System Description

The equipment necessary for SO2 emission reduction comes under four operations:

• Scrubbing or absorption - Accomplished with scrubbers,

holding tanks, liquidspray

nozzles, and circulation pumps.

• Lime handling and slurry preparation - Accomplished

with lime unloading

and storage equipment, lime processing and slurry

preparation equipment.

•Sludge processing - Accomplished with sludge clarifiers for

dewatering, sludge

pumps and handling equipment, and sludge solidifying

equipment.

•Flue-gas handling - Accomplished with inlet and outlet

ductwork, dampers, fans, and stack gas reheaters.

Flue-gas desulfurization (FGD): Advantages

•High SO2 removal efficiencies, from 50% up to 98%

•Products of reaction may be reusable

•Difficulty to retrofit is moderate to low

•Inexpensive and readily available reagents

Flue-gas desulfurization (FGD): Disadvantages:

High capital and O&M cost

Scaling and deposit of wet solids on absorber and downstream equipment

Wet system generates a water waste product and may result in visible plume

Cannot be used for waste gas SO2 concentrations greater than 2,000 ppm

Disposal of waste products significantly increases O&M costs