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Concepts and technologies for air emission control in the textile & apparel industry

As the textile and apparel industry of Bangladesh is moving toward sustainable manufacturing to cope with the environmental concern of the consumers across the world, reducing the air pollution due to the production activities has become a matter of concern for the manufacturers.

Textile garments
Figure 1: The textile, garments, and footwear industry is a big contributor to the air pollution in Bangladesh.

Air is a must for our survival but when we inhale polluted air, it damages our health and could lead to death. The textile, garments, and footwear industry is a big contributor to the air pollution in Bangladesh. As the economy of Bangladesh is growing with a growing number of such industries, its negative impact on the air quality is also rising.

These industries are responsible for a significant amount of Sulphur oxides (SOx), Nitrogen oxides (NOx), Particulate matter, Greenhouse gases (CO2), Ozone-depleting substances (CFC), etc. All of these are responsible for damage to human health and our environment.

Air pollution is responsible for some life-threatening diseases like ischemic heart disease, stroke, chronic obstructive pulmonary disease (COPD), lung cancer, and acute lower respiratory infections in children. Air pollution also damages our water, soil, and the ecosystem which ultimately impacts the agriculture industry severely.

Figure 2:Cyclone collector.

Some certifications like Higg FEM put scores for implementing air pollution abatement technologies in the facilities. So, let’s have a look at some air pollution technologies to control stack air emissions and volatile organic substances (VOC).

In a manufacturing unit, the generators and boilers create polluted stack air, on the other hand, the processes like stenting, curing, drying, printing, and solvent dyeing cause the emission of VOCs also known as non-point or fugitive emissions in the indoor air, in many cases, these are vented outside. The control technologies vary for particulate matter and gaseous matter of the emitted air of both stack and non-point sources.

The following technologies are used to control the particulate matter of air emissions:

Cyclones

A cyclone removes particulates by causing the dirty airstream to flow in a spiral path inside a cylindrical chamber. Dirty air 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 particulates, because of their greater inertia, move outward and are forced against the chamber wall. owed 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 periodically removed from the hopper for disposal. These are best at removing relatively coarse particulates.

Scrubbers

Devices called wet scrubbers to trap suspended particles by direct contact with a spray of water or other liquid. In effect, a scrubber washes the particulates out of the dirty airstream as they collide with and are entrained by the countless tiny droplets in the spray.

Concepts and technologies 3
Figure 3: Wet scrubber.

Several configurations of wet scrubbers are in use such as spray-tower scrubbers, orifice scrubbers, venturi scrubbers, etc.

Electrostatic precipitators

Electrostatic precipitation is a commonly used method for removing fine particulates from airstreams. In an electrostatic precipitator, particles suspended in the airstream are given an electric charge as they enter the unit and are then removed by the influence of an electric field. The precipitation unit comprises baffles for distributing airflow, discharge, and collection electrodes, a dust clean-out system, and collection hoppers.

Concepts and technologies 4
Figure 4: Electric precipitator.

A high voltage of direct current (DC), 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. In a typical unit, the collection electrodes comprise a group of large rectangular metal plates suspended vertically and parallel to each other inside a boxlike structure.

Baghouse filters

One of the most efficient devices for removing suspended particulates is an assembly of fabric-filter bags, commonly called a baghouse. A typical baghouse comprises an array of long, narrow bags—each about 25 cm (10 inches) in diameter—that are suspended upside down in a large enclosure. Dust-laden air is blown upward through the bottom of the enclosure by fans.

Fabric filter
Figure 5: Fabric filter baghouse.

Particulates are trapped inside the filter bags, while the clean air passes through the fabric and exits at the top of the baghouse. A fabric-filter dust collector can remove very nearly 100 percent of particles as small as 1 μm and a significant fraction of particles as small as 0.01 μm.

The following technologies are used to control the gases of air emissions:

Absorption

In the context of air pollution control, absorption involves the transfer of a gaseous pollutant from the air into a contacting liquid, such as water. The liquid must be able either to serve as a solvent for the pollutant or to capture it by means of a chemical reaction.

Wet scrubbers and packed scrubbers:

Wet scrubbers similar to those described above for controlling suspended particulates may be used for gas absorption. Gas absorption can also be carried out in packed scrubbers, or towers, in which the liquid is present on a wetted surface rather than as droplets suspended in the air. A common type of packed scrubber is the countercurrent tower. After entering the bottom of the tower, the polluted airstream flows upward through a wetted column of light, chemically inactive packing material.

The liquid absorbent flows downward and is uniformly spread throughout the column packing, thereby increasing the total area of contact between gas and liquid. Thermoplastic materials are most widely used as packing for countercurrent scrubber towers. These devices usually have gas-removal efficiencies of 90–95 percent.

Flue gas desulfurization:

Sulfur dioxide in flue gas from fossil-fuel power plants can be controlled by means of an absorption process called flue gas desulfurization (FGD). FGD systems may involve wet scrubbing or dry scrubbing. In wet FGD systems, flue gases are brought in contact with an absorbent, which can be either a liquid or a slurry of solid material.

Flu gas desulfurization
Figure 6: Flu gas desulfurization.

The sulfur dioxide dissolves in or reacts with the absorbent and becomes trapped in it. In dry FGD systems, the absorbent is dry pulverized lime or limestone; once absorption occurs, the solid particles are removed by means of baghouse filters (described above). Dry FGD systems, compared with wet systems, offer cost and energy savings and easier operation, but they require higher chemical consumption and are limited to flue gases derived from the combustion of low-sulfur coal.

Adsorption

Gas adsorption, as contrasted with absorption, is a surface phenomenon. The gas molecules are adsorbed, attracted to, and held on the surface of a solid. Gas adsorption methods are used for odor control at various types of chemical manufacturing and food-processing facilities, in the recovery of a number of volatile solvents (e.g., benzene), and in the control of VOCs at industrial facilities.

Activated carbon (heated charcoal) is one of the most common adsorbent materials. It is very porous and has an extremely high ratio of surface area to volume. Activated carbon is particularly useful as an adsorbent for cleaning airstreams that contain VOCs and for solvent recovery and odor control. A properly designed carbon adsorption unit can remove gas with an efficiency exceeding 95 percent.

Adsorption systems are configured either as stationary bed units or as moving bed units. In stationary bed adsorbers, the polluted airstream enters from the top, passes through a layer, or bed, of activated carbon, and exits at the bottom. In moving bed adsorbers, the activated carbon moves slowly down through channels by gravity as the air to be cleaned passes through in a cross-flow current.

carbon adsorption
Figure 7:  Activated carbon adsorption.

Incineration

The process called incineration or combustion chemically, rapid oxidation can be used to convert VOCs and other gaseous hydrocarbon pollutants to carbon dioxide and water. Incineration of VOCs and hydrocarbon fumes usually is accomplished in a special incinerator called an afterburner. To achieve complete combustion, the afterburner must provide the proper amount of turbulence and burning time, and it must maintain a sufficiently high temperature.

Sufficient turbulence, or mixing, is a key factor in combustion because it reduces the required burning time and temperature. A process called direct flame incineration can be used when the waste gas is itself a combustible mixture and does not need the addition of air or fuel.

air emission
Figure 8: Incineration of air emission.

The best way to reduce the levels of carbon dioxide in the air is to use energy more efficiently and to reduce the combustion of fossil fuels by using alternative energy sources (e.g., nuclear, wind, tidal, and solar power). In addition, carbon sequestration can be used to serve the purpose. Carbon sequestration involves the long-term storage of carbon dioxide underground, as well as on the surface of Earth in forests and oceans. Carbon sequestration in forests and oceans relies on natural processes such as forest growth.

If anyone has any feedback or input regarding the published news, please contact: info@textiletoday.com.bd

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