Control of air pollution | gaseous and particulate pollutants.

Control of air pollution (gaseous and particulate matter)

In the previous topic we know that air pollution causes 7 million deaths per year worldwide, hence for the environmentalists it become a matter of great concern to lower down this figure because every life matters. Here in this section we discuss various control measures of gaseous and particulate matter

Control of gaseous air pollutants

Gaseous air pollutants consist of carbon monoxide, carbon dioxide, sulfur oxides, nitrogen oxides, hydrocarbon, methane gas, organic and in organic acid gasses. We also know that these gaseous pollutants originate from a variety of sources such as point stationary sources and mobile sources as discussed in previous topic. similarly, here we discuss the control measures of gaseous air pollutants under two headings.

A. Control of gaseous air pollutants at stationary sources:

Environmental scientists devised a number of control measures for gaseous pollutants at stationary sources these include Absorption, adsorption, condensation and combustion.

1.Absorption

According to Davis and Cornwell 1991, absorption is a process in which the gaseous air pollutants are bring into intimate contact with a liquid absorbent in order to dissolve gases into that volatile liquid. This process is carried out under three steps:

·        Diffusion of the gaseous pollutants to the surface of liquid,

·        Transfer across the gas-liquid interface and

·        Diffusion of the dissolved gas away from the interface into the liquid

Different types of absorbers include spray chambers, plate or tray towers, packed tower and venture scrubbers. They provide a large surface area for liquid gas interactions. Absorption is efficient for the control of sulfur dioxide, hydrogen sulphide, hydrogen chloride, chlorine, ammonia and oxides of nitrogen. The efficiency of this method is 90-95% in removal of above pollutants.

2.Adsorption

Adsorption is a surface phenomenon in which a gaseous or liquid phase form a film of adsorbate (pollutants) on the surface of adsorbent. The adsorbent attract and hold the adsorbate by physical or chemical adsorption.

The physical adsorption is a reversible process due to weak Vander Walls force of attraction between the adsorbate (gaseous pollutants) and adsorbent. while chemical adsorption is an irreversible process due to strong chemical forces attraction between the adsorbate and adsorbent.

The common absorptive materials are activated carbon, alumina, manganese, bone char, silica gel. Whereas activated charcoal is one of the efficient adsorbent of gaseous air pollutants it absorbs hydrocarbon vapors and odors from the polluted air.

When polluted air passed through any of the above adsorbate the gaseous pollutant of the air gets absorbed on surface of these adsorbate and remain in the bed of the setup. The efficiency of this method is 100%.

3.Condensation:

According to USEPA condensers are used to convert condensable component of a gas stream to the liquid phase, this is achieved by reducing the temperature of gas stream. Condenser remove volatile gases. It is of two types surface and contact

In case of surface condenser, a shell-and-tube heat exchanger is used as cooling medium of air or water and a metal wall is used to separate the condensed vapor, the coolant flow through the tubes whereas the vapor with volatile gas is passed over a condenser on the outside of the tube.

Whereas in contact condenser the vapor is cooled by spraying liquid directly on the vapor stream to condensate it later on the water and condensate mixture ae removed, treated and disposed of properly.

4.Incineration:

As we know that under normal condition the process of combustion attribute to air pollution but under high temperature conditions combustion or incineration of hydrocarbons produce carbon dioxide and water which on contrary considered as lass harmful.

B. Control of gaseous air pollutants emitted from mobile sources

According to the Council of Environmental Quality (CEQ) 1982, transportation accounted for 55% air pollutants emitted to the atmosphere in 1980.

Gaseous air pollutants emitted from mobile sources can be controlled by replacing the internal combustion engine, or by following the following protocols.

1.Positive crankcase ventilation(PVC)

With the aid of this technology crank emission is controlled to a great extent, the hydrocarbon blow-by gases into the crank are recirculated to the combustion chamber for re-combustion.

2.Control of evaporative emission

Evaporative emission includes gasoline vapor emission from the hot engine in order to combat this activated charcoal filled canister is installed to absorb hydrocarbon emission.

3.Catalytic converters

Three types of catalytic converts are installed to lower the emission. These include

Oxidizing converter, reducing converter, and three-way converter.

Control of particulate matter

Environmentalists especially environmental engineers design and suggest various control devices to control particulate matter. These devices are as follows:

1.Gravity settling chambers

These are considered as one of the simplest devices it consists of a long chamber in which the polluted air with particulates is circulated and is allowed to settle in the chamber under the force of gravity. The settled particulate air matter is removed manually from the chamber. The efficiency of this control device is quite low and used to remove large sized particulate matter only.

                             Fig.Gravity Settling Chamber

2.Cyclone collectors /centrifugal separators

This control device work on centrifugal force generated by a spinning gas in order to separate the particulate matter. Particulate loaded air is circulated in a spiral fashion inside the tube, during this course of time the lager particles move outwards by centrifugal force after colliding with the walls, fall to the bottom of the cone where they are collected and removed. The particulate free air moves out from the top of the device this type of device is efficient for large sized particulate matter. The maintenance cost of this is very low. And are commonly used in industries like cement, textile and paper.

                                Fig. Cyclone Collecter

3.Electrostatic precipitators

This type of control device works on electrical force to separate the particulate from the air. The device is provided with electrodes the particulate matter gets ionized and migrate on appositively charged plate whereas the clean air rush out through the device the residue is removed from the hopper. This device is efficient in the removal of coarse and fine particles but it requires high temperature and low pressure. Depending on the mode of operation dry or wet, Electrostatic preceptors are of wo types dry electrostatic precipitator and wet electrostatic precipitator.

  Fig. Electrostatic Precipitator

4.Wet scrubbers:

This device is used to collect particulate matter from the air on liquid droplets. The operation of this device completes in four steps

• ·      Transportation of particulate containing air so as to bring it close to water droplets.
• ·        collision of the particulate with water droplets,
• ·        adhesion of particulate with water droplets and
• ·        precipitation is formation which is removed.

There are various types of wet scrubbers these include:

Spray towers, Venture scrubbers, Cyclone scrubbers, Packed scrubbers and Mechanical scrubbers.

                                                Fig. Wet Scrubber

5.Fabric filter or bag house filter

This is one of the most commonly used control device in which a stranded fiber made up of spun or a synthetic continuous monofilament is used in cylindrical tubes containing multiple rows of fiber bags. Particulate laden air is force through this setup which result in the accumulation of particulate matter on the fiber bags and a stream of particulate free air come out through the device. The accumulated particulate matter is removed from the bags by shaking or reversing the air flow. The efficiency of this device is 99% and is quite efficient in the removal of submicometer particles. However, they are costly and require high maintenance.

 

               Fig. Fabric Filter.