Controlling air pollution
Reducing pollutants in the air is important for human health and the environment.
Poor air quality has adverse effects on human health, particularly respiratory and cardiovascular systems. These effects are often first noticed as wheezing, coughing, shortness of breath and a worsening of existing respiratory and cardiac conditions, particularly to sensitive portions of the population, including the very young, old or those with existing medical conditions, such as asthma. Pollutants also have harmful effects on the environment through damage to plants, buildings and a reduction in visual amenity from smoke or haze.
At the community level, reductions in air pollutants can be achieved by actively choosing to walk, cycle or take public transport rather than drive a car. Motor vehicles should be kept in good condition and driven correctly to minimise emissions.
Good urban design and planning can also reduce pollution by having cleaner, greener choices for the public. These may include increasing walking and cycling paths and by having urban 'hubs' where people work, play and shop, which reduce the need to travel. Purchasing items that have low energy requirements for their manufacture and use, or that can be recycled, also reduce the energy that needs to be generated.
For agriculture, improved practices, such as coordinating burning to times when impacts can be reduced, lessens the effects of hazard reduction burns, and moving to green sugar cane harvesting removes the associated smoke impact. However, uncontrolled bushfires and dust storms, still have a major impact on air quality.
Industry can be required to minimise emissions through licensing and regulation as well as by the use of pollution control devices, which remove pollutants by absorption, filtration or dilution by dispersion.
Simplified schematic diagrams of some of the more common industrial pollution control devices are shown below.
Diagram of a cyclone
A cyclone works on the principle of creating a high-speed spiral air flow.
The spiral motion exerts a centrifugal force on the particles and the inertia of the larger particles forces them to the outside walls of the cyclone, from where they fall to the bottom and are collected.
The conical section at the base of the cyclone gradually decreases the diameter of the spinning gas stream resulting in a more efficient removal of the smaller particles.
The overall efficiency is related to the velocity of the air flow and the diameter of the cyclone—the smaller the diameter the more efficient the particle removal.
However, in practice reducing the diameter increases the pressure drop across the cyclone and to combat this several units are clustered together in what is known as a 'multicyclone'.
Particle removal—Electrostatic precipitator
Diagram of an electrostatic precipitator
The electrostatic precipitator works by removing particles and smoke from a gas stream using an induced electrostatic charge.
Dust particles pass by wires that have a high DC voltage applied, which ionises the surrounding gas. This is a known as a 'corona discharge' from which the particles pick up a small electrostatic charge. These particles are then attracted to an oppositely charged plate where the charge is neutralised and the particles retained.
Particles are physically removed by regular tapping (rapping) of the plates.
In some situations, the particles may be pre-conditioned by introducing a water spray to reduce the electrical resistivity of the particles allowing them to accept the charge more easily.
Electrostatic precipitators are an efficient way of removing particles and do not cause a significant pressure drop across the unit.
Particle and gas removal—Packed wet scrubber
Diagram of packed wet scrubber
A wet scrubber is a device that removes gaseous and/or particle contaminants from a gas stream. It operates by bringing the gas stream into contact with a scrubbing liquid (usually water).
To increase the contact between the gas and scrubbing liquid and thus increase the removal efficiency some form of packing is often used. These are known as packed bed wet scrubbers.
Gaseous pollutants are removed by absorption into the scrubbing liquid (absorbers) while particles are removed by physical capture of the particles in the droplets.
While packed bed scrubbers can remove both gaseous pollutants and particles they are usually engineered to be more efficient at one or the other depending on the conditions.
Gaseous pollutant removal can also be enhanced by using specific scrubbing liquids. An example would be the removal of acidic pollutants using an alkaline liquid. Some gas streams may require pre-conditioning, either to reduce high temperatures or to remove very high dust loadings that would otherwise clog the packing material.
Other designs may use an entrainment separator to remove any scrubbing liquid droplets that may be carried along in the cleaned gas stream, or to recycle the scrubbing liquid either directly or after suitable treatment.
Particle removal—Bag filters
Diagram of typical bag filter
Bag filters, alternatively known as fabric filters or baghouses, use fabric filter bags to remove particles from dust-laden gas.
These filters can achieve high efficiencies for very fine particles due to the build up of particles on the surface of the bag.
Collection can occur on either the inside or outside of the bag depending on the design.
As particles build up, the pressure drop across the bag increases, so the filters must undergo a regular cleaning schedule, usually by having the filters running in tandem—while one is operating the other is undergoing cleaning.
Cleaning schedules are normally automated and the methods employed are mechanical shaking, the use of a reversed air flow, or a pulse of compressed air.
Each method is used to dislodge the cake of dust from the fabric surface, which then falls by gravity into a collection system.