Restricting use of fossil and using hydel power fitting smoke stacks to factories with electrostatic precipitators, fabric filters, scrubbers and inertial separators.
Answers
Answer:Devices called wet scrubbers 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.
Several configurations of wet scrubbers are in use. In a spray-tower scrubber, an upward-flowing airstream is washed by water sprayed downward from a series of nozzles. The water is recirculated after it is sufficiently cleaned to prevent clogging of the nozzles. Spray-tower scrubbers can remove 90 percent of particulates larger than about 8 μm.
In orifice scrubbers and wet-impingement scrubbers, the air-and-droplet mixture collides with a solid surface. Collision with a surface atomizes the droplets, reducing droplet size and thereby increasing total surface contact area. These devices have the advantage of lower water-recirculation rates, and they offer removal efficiencies of about 90 percent for particles larger than 2 μm.
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Venturi scrubbers are the most efficient of the wet collectors, achieving efficiencies of more than 98 percent for particles larger than 0.5 μm in diameter. Scrubber efficiency depends on the relative velocity between the droplets and the particulates. Venturi scrubbers achieve high relative velocities by injecting water into the throat of a venturi channel—a constriction in the flow path—through which particulate-laden air is passing at high speed.
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. 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.
electrostatic precipitator
electrostatic precipitator
Electrostatic precipitator, a common particle-collection device at fossil-fuel power-generating stations.
Encyclopædia Britannica, Inc.
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. There are often hundreds of plates having a combined surface area of tens of thousands of square metres. Rows of discharge electrode wires hang between the collection plates. The wires are given a negative electric charge, whereas the plates are grounded and thus become positively charged.
Particles that stick to the collection plates are removed periodically when the plates are shaken, or “rapped.” Rapping is a mechanical technique for separating the trapped particles from the plates, which typically become covered with a 6-mm (0.2-inch) layer of dust. Rappers are either of the impulse (single-blow) or vibrating type. The dislodged particles are collected in a hopper at the bottom of the unit and removed for disposal. An electrostatic precipitator can remove particulates as small as 1 μm with an efficiency exceeding 99 percent. The effectiveness of electrostatic precipitators in removing fly ash from the combustion gases of fossil-fuel furnaces accounts for their high frequency of use at power stations.
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. Particulates are trapped inside the filter bags, while the clean air passes through the fabric and exits at the top of the baghouse.
fabric-filter baghouse
fabric-filter baghouse
Baghouse employing an array of fabric bags for filtering the airstream.
Encyclopædia Britannica, Inc.
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. Fabric filters, however, offer relatively high resistance to airflow, which leads to substantial energy usage for the fan system. In addition, in order to prolong the useful life of the filter fabric, the air to be cleaned must be cooled (usually below 300 °C [570 °F]) before it is passed through the unit; cooling coils needed for this purpose add to the energy usage. (Certain filter fabrics—e.g., those made of ceramic or mineral materials—can operate at higher temperatures.)
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