Fundamentals of prevention and control of air pollution:

Fundamentals of prevention and control of air pollution:

As mentioned in my earlier discussion, air pollutants can be gaseous or particulate matters. Different techniques for controlling these pollutants are discussed below:

A. Methods of controlling gaseous pollutants -

1. Combustion - This technique is used when the pollutants are in the form of organic gases or vapors. During flame combustion or catalytic process, these organic pollutants are converted into water vapor and relatively less harmful products, such as CO2.

2. Absorption - In this technique, the gaseous effluents are passed through scrubbers or absorbers. These contain a suitable liquid absorbent, which removes or modifies one or more of the pollutants present in the gaseous effluents.

3. Adsorption - The gaseous effluents are passed through porous solid adsorbents kept in suitable containers. The organic and inorganic constituents of the effluent gases are trapped at the interface of the solid adsorbent by physical adsorbent.

B. Methods to control particulate emissions:

1. Mechanical devices generally work on the basis of the following:

(a) Gravity: In this process, the particles settle down by gravitational force.

(b) Sudden change in direction of the gas flow. This causes the particles to separate out due to greater momentum.

2. Fabric Filters: The gases containing dust are passed through a porous medium. These porous media may be woven or filled fabrics. The particles present in the gas are trapped and collected in the filters. The gases freed from the particles are discharged.

3. Wet Scrubbers: Wet scrubbers are used in chemical, mining and metallurgical industries to trap SO2, NH3, metal fumes, etc.

4. Electrostatic Precipitators: When a gas or an air stream containing aerosols in the form of dust, fumes or mist, is passed between two electrodes, then, the aerosol particles get precipitated on the electrode.

C. Apart from the above, following practices also help in controlling air pollution.

(i) Use of better designed equipment and smokeless fuels, hearths in industries and at home.

(ii) Automobiles should be properly maintained and adhere to recent emission-control standards.

(iii) More trees should be planted along road side and houses.

(iv) Renewable energy sources, such as wind, solar energy, ocean currents, should fulfill energy needs.

(v) Tall chimneys should be installed for vertical dispersion of pollutants.

Dust in cement industry: Its prevention

Dust in cement industry: Its prevention and collection enhances environment standard:

The manufacturing of cement involves mining; crushing and grinding of raw materials (mostly limestone and clay); calcinating the material in rotary kiln; cooling the resulting clinker; mixing the clinker with Gypsum; and milling, storing and bagging the finished cement. The cement manufacturing process generates lot of dust, which is captured and recycled to the process. Gasses from clinker cooler are used as secondary combustion air. The process, using pre-heaters and pre-calciners, is both economically and environmentally preferable to wet process because of techno-economic advantages of the energy saving dry system over wet. Certain other solids such as pulverized fly ash from power plants, slag, roasted pyrite residue and foundry sand can be used as additives to prepare blended cement.

A. Dust generation: Generation of fine particulates and dust are inherent in the process; but most are recovered and recycled. The sources of dust emission include clinker cooler, crushers, grinders and material-handling equipments. Material-handling operations such as conveyors result in fugitive dust emission.

B. Prevention and control of dust: The priority in the cement industry is to minimize the increase in ambient particulate levels by reducing the mass load emitted from the stacks, from fugitive emissions, and from other sources. Collection and recycling of dust in the kiln gases in required to improve the efficiency of the operation and to reduce atmospheric emissions. Units that are well designed, well operated, and well maintained can normally achieve generation of less than 0.2 kilograms of dust per metric tonne (kg /t) of clinker, using dust recovery systems. For control of fugitive dust:

(a) ventilation systems should be used in conjunction with hoods and enclosures covering transfer points and conveyors;

(b) Drop distances should be minimized by the use of adjustable conveyors;

(c) Dusty areas such as roads should be wetted down to reduce dust generation;

(d) Appropriate stormwater and runoff control systems should be provided to minimize the quantities of suspended material carried off site.

C. Mechanical systems for controlling dust: Several mechanical equipments are used in cement manufacturing plant to control / collect dust. These are:

• Dust collector - A dust collector (bag house) is a typically low strength enclosure that separates dust from a gas stream by passing the gas through a media filter. The dust is collected on either the inside or the outside of the filter. A pulse of air or mechanical vibration removes the layer of dust from the filter. This type of filter is typically efficient when particle sizes are in the 0.01 to 20 micron range.
  

• Cyclone - Dust laden gas enters the chamber from a tangential direction at the outer wall of the device, forming a vortex as it swirls within the chamber. The larger articulates, because of their greater inertia, move outward and are forced against the chamber wall. Slowed by friction with the wall surface, they then slide down the wall into a conical dust hopper at the bottom of the cyclone. The cleaned air swirls upward in a narrower spiral through an inner cylinder and emerges from an outlet at the top. Accumulated particulate dust is deposited into a hopper, dust bin or screw conveyor at the base of the collector. Cyclones are typically used as pre-cleaners and are followed by more efficient air-cleaning equipment such as electrostatic precipitators and bag houses.
  

• Electrostatic Precipitator - In an electrostatic precipitator, particles suspended in the air stream are given an electric charge as they enter the unit and are then removed by the influence of an electric field. A high DC voltage (as much as 100,000 volts) is applied to the discharge electrodes to charge the particles, which then are attracted to oppositely charged collection electrodes, on which they become trapped. An electrostatic precipitator can remove particulates as small as 1 μm (0.00004 inch) with an efficiency exceeding 99 percent.

Control of Air pollution by controlling Particulate Matters

Airborne Particulate Matters (PM) - Pollution Prevention and Control:

Airborne particulate matters (PM) emissions can be minimized by pollution prevention and emission control measures. Prevention, which is frequently more cost-effective than control, should be emphasized. Special attention should be given to mitigate the effects, where toxics associated with particulate emissions may pose a significant environmental risk.

Measures such as improved process design, operation, maintenance, housekeeping, and other management practices can reduce emissions. By improving combustion efficiency in Diesel engines, generation of particulate matters can be significantly reduced. Proper fuel-firing practices and
combustion zone configuration, along with an adequate amount of excess air, can achieve lower PICs (products of incomplete combustion). Few following steps should be adhered to control PM:

1. Choosing cleaner fuels - Natural gas used as fuel emits negligible amounts of particulate matter.

2. Low-ash fossil fuels contain less noncombustible, ash-forming mineral matter and thus generate lower levels of particulate emissions.

3. Reduction of ash by coal cleaning reduces the generation of Particulate Matter (PM) emissions.

4. The use of more efficient technologies or process changes can reduce PIC emissions.

5. Advanced coal combustion technologies such as coal gasification and fluidized-bed combustion are examples of cleaner processes that may lower PICs by approximately 10%.

6. A variety of particulate removal technologies, are available – these are (a) Inertial or impingement separators, (b) Electrostatic precipitators (ESPs) , (c) Filters and dust collectors (baghouses), (d) Wet scrubbers that rely on a liquid spray to remove dust particles from a gas stream.

The principal methods for controlling the release of particulate matter are summarized here.

• Identify measures for improving operating and management practices.

• Consider alternative fuels such as gas instead of coal.

• Consider fuel-cleaning options such as coal washing, which can reduce ash content by up to 40%.

• Consider alternative production processes and technologies, such as fluidized bed combustion, that result in reduced PM emissions.

• Select optimal particulate removal devices such as ESPs and Baghouses.