CONTROL OF GASEOUS POLLUTANTS IN AIR POLLUTION

A SMALL ASSIGNMENT FOR B.E 2NDYEAR 1 SEM IN E.S ON CONTROL OF GASEOUS POLLUTANTS IN AIR POLLUTION

Control of gaseous pollutants from stationary sources 

• The most common method for controlling gaseous pollutants is the addition of add-on control devices to recover or destroy a pollutant.
• There are four commonly used control technologies for gaseous pollutants: 
  
  
• 1. Absorption, 
• 2. Adsorption,
• 3. Condensation, and 
• 4.Incineration (combustion)

 1.Absorption 

The removal of one or more selected components from a gas mixture by absorption is probably the most important operation in the control of gaseous pollutant emissions. Absorption is a process in which a gaseous pollutant is dissolved in a liquid. As the gas stream passes through the liquid, the liquid absorbs the gas, in much the same way that sugar is absorbed in a glass of water when stirred. Absorbers are often referred to as scrubbers, and there are various types of absorption equipment. The principal types of gas absorption equipment include spray towers, packed columns, spray chambers, and venture scrubbers. In general, absorbers can achieve removal efficiencies grater than 95 percent. One potential problem with absorption is the generation of waste-water, which converts an air pollution problem to a water pollution problem.

2.Adsorption 

• When a gas or vapor is brought into contact with a solid, part of it is taken up by the solid. The molecules that disappear from the gas either enter the inside of the solid, or remain on the outside attached to the surface. The former phenomenon is termed absorption (or dissolution) and the latter adsorption.
• 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 vapor streams.
  Carbon adsorption systems are either regenerative or non-regenerative.
  - Regenerative system usually contains more than one carbon bed. As one bed actively removes pollutants, another bed is being regenerated for future use.
  - Non-regenerative systems have thinner beds of activated carbon. In a non-regenerative adsorber, the spent carbon is disposed of when it becomes saturated with the pollutant. 

 

 3.Condensation 

Condensation is the process of converting a gas or vapor to liquid. Any gas can be reduced to a liquid by lowering its temperature and/or increasing its pressure. Condensers are typically used as pretreatment devices. They can be used ahead of absorbers, absorbers, and incinerators to reduce the total gas volume to be treated by more expensive control equipment. Condensers used for pollution control are contact condensers and surface condensers. In a contact condenser, the gas comes into contact with cold liquid. In a surface condenser, the gas contacts a cooled surface in which cooled liquid or gas is circulated, such as the outside of the tube. Removal efficiencies of condensers typically range from 50 percent to more than 95 percent, depending on design and applications.

4. Incineration

• Incineration, also known as combustion, is most used to control the emissions of organic compounds from process industries.
• This control technique refers to the rapid oxidation of a substance through the combination of oxygen with a combustible material in the presence of heat.
  When combustion is complete, the gaseous stream is converted to carbon dioxide and water vapor.
• Equipment used to control waste gases by combustion can be divided in three categories:
  - Direct combustion or flaring,
  - Thermal incineration and
  - Catalytic incineration.

 Direct combustor 

• Direct combustor is a device in which air and all the combustible waste gases react at the burner. Complete combustion must occur instantaneously since there is no residence chamber.
• A flare can be used to control almost any emission stream containing volatile organic compounds. Studies conducted by EPA have shown that the destruction efficiency of a flare is about 98 percent.
  In thermal incinerators the combustible waste gases pass over or around a burner flame into a residence chamber where oxidation of the waste gases is completed. Thermal incinerators can destroy gaseous pollutants at efficiencies of greater than 99 percent when operated correctly.
  
  
  
  Thermal incinerator general case 
  
  Catalytic incinerators are very similar to thermal incinerators. The main difference is that after passing through the flame area, the gases pass over a catalyst bed. A catalyst promotes oxidation at lower temperatures, thereby reducing fuel costs. Destruction efficiencies greater than 95 percent are possible using a catalytic incinerator.