Industrial Air Pollution and Cotol Flashcards
Primary industries
Provide raw material - diary, beef and sheep farming, forestry, fisheries and horticulture, coal mining, aggregate /mineral mining quarry’s
Secondary industries
Manufacturing products out of raw material - heat and dairy export, fish and other food products, timber and concrete, coal power (for local use).
What major industries are dust emitting
Coal miring, aggregate mineral mining, timber and concrete, coal power
4 major air polluting industries
Coal power plant, cement, mining, waste incineration (global)
Flue gas
Gas stream that carries the fire particles → need to separate these particles from the gas stream
Particle collection mechanism
An external force must be applied to enable the particle to develop a velocity that will displace it and for direct it to a collection or retrieval area. The particles should be retained in this area with forces strong enough to prevent re-entrainment. Collected recovered particles should be subsequently removed. These particles should be disposed recovered or reused.
Forces that can be used for particle recovery and collection / mechanisms to separate fine particles from flue gas streams.
Gravity (gravity settler), wet scrubbing (wet scrubber), filtration (bag filter), centrifugal action (cyclone separater), electrostatic attraction (electrostatic precipatator).
Gravity settler
Low efficiency, usually below 50%, effective for larger sized particles > 50 um. Low power requirement as it uses gravity for separation. Large space required as the separating chambers can be bulky.
Horizontal flow settling chamber
When gas stream enters the expansion section of the duct, the expansion of the gas stream causes the gas velocity to be reduced, and particles are separated by the force of gravity. Low velocity and law pressure drop maintained.
Multiple (Howard) tray settling chamber
Shortens the length of the settling path and improves collection efficiency. Diffulcut to clean and not suitable for high temperature applications.
Baffle chamber (inertial separator)
Baffles within the chamber enhance particle separation and collection. This happens by changing the direction of the gas velocity and imparting a downward motion to the particle. Particles as small as 20-40 um can be collected.
4 types of wet scrubber
Tray scrubber, packed tower, Venturi scrubber, quencher (spray tower)
Wet scrubber
Moderate efficiency 50-99%, works for fine and course particles 0.5 - um. Moderate to high power requirement, requires energy for pumps and fans.
Filter bag house
99%+ efficiency, effective for particles <10 um, high power requirement, fans and cleaning systems require energy, large space requirement.
Impaction and interception
Force applied in a bag filter to separate particles from gas stream
Important process variables for bag filters
Particle characteristics, gas characteristics, fabric properties. The most important design parameter is gas to cloth ratio. Most important operating parameter is the pressure drop across the filter (important to clean filter)
Types of filter bags cleaning
Shaker cleaning, reverse flow type
Cyclone separator
Moderate efficiency 70 - 90% effective for particles >10 um, low power requirement. The shape / curvature of the collector causes the gas stream to rotate in a spiral motion. Larger heavy particles are pushed outwards, pressure drop created, particles loose their kinetic energy and are separated from the gas stream. Clea air enters a second spiral movement and exit from the top.
Particle cut diameter
Size of a particle that you can collect with 50% efficiency using a cyclone separator e.g. 1 um 50% efficiency is really good.
Types of cyclone separators
High efficiency- have the highest pressure drop but fine particles are collected. High throughput - designed to treat large volumes of gas with a low pressure drop. Fine particles can escape
Electrostatic precipitator
Uses electrostatic attraction, charged particles migrate to an oppositely charged collection surface. Very efficient, > 99% efficiency, captures PM I0 and PM 2.5 fine particles takes up lots of space
Summary particle collection efficiency
Electrostatic precipitators and filter bag houses offer the highest efficiency, while gravity settlers have the lowest
Summary power requirement
Gravity settlers and cyclone separators generally have lower power demands compared to electrostatic precipitators and filter bag houses,
Summary space requirement
Gravity settlers and electrostatic precipitators usually require the most space due to their size and design, whereas wet scrubbers and cyclone separators are more compact
